Hollow Cu x O ( x = 2, 1) micro/nanostructures: synthesis, fundamental properties and applications

Shaodong Sun; Qing Yang; Shuhua Liang; Zhimao Yang

doi:10.1039/C7CE01530E

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DOI: 10.1039/C7CE01530E (Highlight) CrystEngComm, 2017, 19, 6225-6251

Hollow Cu_xO (x = 2, 1) micro/nanostructures: synthesis, fundamental properties and applications

Shaodong Sun *^a, Qing Yang ^a, Shuhua Liang *^a and Zhimao Yang *^b
^aShaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, ShaanXi, People's Republic of China. E-mail: sdsun@xaut.edu.cn; liangsh@xaut.edu.cn
^bSchool of Science, State Key Laboratory for Mechanical Behavior of Materials, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Center of Suzhou Nano Science and Technology, Xi'an Jiaotong University, Xi'an 710049, ShaanXi, People's Republic of China. E-mail: zmyang@mail.xjtu.edu.cn

Received 23rd August 2017 , Accepted 25th September 2017

First published on 25th September 2017

Abstract

Hollow micro/nanostructures possess fascinating physicochemical properties that would generate great application potential in the fields of catalysis, sensing and energy conversion. In the past decades, much more efforts have been employed in the exploration and enrichment of copper-based binary oxides, Cu_xO (x = 2, 1); thus a number of Cu_xO micro/nanostructures with well-controlled ingredients and morphologies have been rationally synthesized and applied. However, a thorough summary on hollow Cu_xO materials has been lacking so far and it is urgently necessary to further promote the progress in novel function-directed Cu_xO-based micro/nanostructures. In this review, we will comprehensively summarize the important advances in hollow Cu_xO micro/nanostructures with tailored morphologies, including the universal synthesis strategies and formation mechanisms, the interfacial Cu–O atomic structures as well as the intrinsic properties, and their potential applications. Remarks on emerging issues and promising research directions are also discussed.

Shaodong Sun

Shaodong Sun (S. D. Sun) received his BS and MS degrees from Xi'an University of Technology in 2004 and 2007, respectively, and his PhD degree in materials science and engineering from Xi'an Jiaotong University in 2011. Then he joined the School of Science at Xi'an Jiaotong University (2011–2014). From 2015 to 2016, he worked as a research fellow in the Department of Chemistry at National University of Singapore. He is now a professor in the School of Materials Science and Engineering at Xi'an University of Technology. His main research interest focuses on the designated synthesis of novel functional nanostructures for photocatalytic applications.

Qing Yang

Qing Yang received his BE and ME degrees from Zhengzhou University in 2004 and 2007, respectively, and his PhD degree in optoelectronics and nanostructure science from Shizuoka University in 2010. He is now a professor in the School of Materials Science and Engineering at Xi'an University of Technology. His main research interest focuses on metal oxide nanostructures for catalytic applications.

Shuhua Liang

Shuhua Liang received her BS and MS degrees from Xi'an University of Architecture and Technology in 1991 and 1994, respectively, and her PhD degree in materials science and engineering from Xi'an Jiaotong University in 2004. Now she is a full professor in Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, at Xi'an University of Technology. Her research interests focus on copper-based electrical materials and photocatalysts.

Zhimao Yang

Zhimao Yang is currently a full professor in the School of Science, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter and State Key Laboratory for Mechanical Behavior of Materials at Xi'an Jiaotong University. He received his PhD degree in materials science and engineering from Xi'an Jiaotong University in 1996. His research interests focus on material chemistry, technology for nanomaterial synthesis and electrode materials.

1. Introduction

As a promising architecture, hollow micro/nanostructures with large specific surface areas, low density and superior charge/mass transport and high permeability have been attracting much attention in the past years.^1–9 In regard to the potential application, hollow micro/nanostructures generally act as supports for catalysts/drugs or are directly used as active components in some fields. In particular, the active components supported onto/into hollow supports would exhibit excellent performances compared to the unsupported counterparts.^10–13 As a result, to accurately regulate the size, shape, crystallinity, composition, defects, building blocks and hybridization of hollow micro/nanostructures is imperative.

Copper-based binary oxides Cu_xO (x = 2, 1), as typical p-type semiconductors, have been widely investigated for a long time because of their unusual optical and electrical properties in the fields of catalysis, sensing, energy conversion and chemical templates,^14–22 which have employed tremendous endeavors on the rapid development of Cu_xO with controllable microstructures. To date, various Cu_xO with tailored morphologies have been developed. As for Cu₂O, morphological micro/nanostructures, including three-dimensional solid architectures with tailored surfaces,^23–82 hollow and porous structures,^83–122 and low-dimensional nanostructures,^123–130 have been successfully synthesized. As for CuO, the configurations are mainly nanowires,^131–142 nanobelts,^143–146 and hierarchical^147–162 and hollow architectures.^163–180 Among them, hollow Cu_xO micro/nanostructures with controllable features are still one of the hottest and challenging topics for both fundamental studies and technological applications.

Optimizing the interfacial atomic structures of Cu_xO micro/nanostructures can provide distinct abilities for improved performances. Great efforts have been devoted to the modification of Cu–O interfaces, which is favorable to the design and synthesis of hybrid Cu_xO-based heterogeneous architectures with tunable performances.¹⁸¹ Inspired by this viewpoint, selective construction of function-directed hybrid Cu_xO-based hollow micro/nanostructures can obviously bring enhanced performances for practical applications.^182–186 Therefore, the development of synthesis methodologies of various hollow Cu_xO micro/nanostructures and their hybridizing forms would provide a good pathway to comprehend the intrinsic principles of the formation mechanisms as well as the fundamental theories of the interface-dependent performances. Additionally, hollow Cu_xO can interestingly act as sacrificial templates and/or substrates for in situ synthesizing of the desired hollow or hybridizing architectures. In particular, a Cu₂O-templated strategy for the synthesis of definable hollow architectures has been demonstrated previously.⁹ However, the hollow Cu_xO template-based synthetic strategy for preparing novel hybrid copper-based nanocomposites is still in its infancy.

Although several references have reviewed the advancements in Cu₂O and CuO,^{14–19,22,138} a thorough review on hollow Cu_xO micro/nanostructures has been rarely reported so far and it is urgently necessary to further develop the novel function-directed copper-based oxides. Therefore, it is imperative to individually summarize the promising hollow Cu_xO micro/nanostructures. Herein, this review is devoted to comprehensively highlighting the advancements in hollow Cu_xO with tailored morphologies (see Fig. 1). We will start with a summary on the general synthetic strategies and formation mechanisms of hollow Cu_xO micro/nanostructures. Next, the intrinsic properties and the interfacial Cu–O atomic structures of hybridizing hollow Cu_xO micro/nanostructures (such as CuO/Cu₂O and Cu/Cu₂O) will be highlighted. Additionally, the potential applications of hollow Cu_xO micro/nanostructures are reviewed. Remarks on emerging issues and promising investigation directions are also proposed.


	Fig. 1 Summary of the recent advancements in hollow Cu_xO.

2. Synthetic strategy and growth mechanism

In general, the synthetic strategies for preparing hollow architectures are mainly divided into two categories according to the formation mechanisms, the template-assisted and the template-free approach.^5–9

The template-assisted synthetic strategy has been extremely practiced to synthesize various hollow architectures,⁵ which can effectively control the morphologies, sizes, compositions and spatial arrangements of the as-synthesized products. Typically, hollow architectures can be synthesized by the following three-step process, namely, (i) the construction of templates, (ii) the direct synthesis of target architectures using the above templates, and (iii) the removal of templates (if necessary). So far, two types of templates have been developed to synthesize hollow architectures: (i) hard template (physical directing solid particle, such as metal–organic frameworks (MOFs),^187,188 polystyrene spheres (PS),^189–196 carbon nanotubes (CNTs),^197–202 block copolymers (BCPs),^203–205 molecular sieves,^206,207 silica,^208–210 anodic aluminum oxides (AAOs),^211–218 and Cu₂O (ref. 219–228)); and (ii) soft template (molecular assembly, such as emulsion droplets, surfactants, supramolecular micelles, polymer vesicles, polymer aggregates and gas bubbles).^{1,3,229–232}

However, some drawbacks should be overcome in using hard-template strategies.³ For example, it is difficult to remove the hard templates by either a chemical or a thermal route, and the synthetic process is complex and high-cost. Soft templates are usually thermodynamically unstable, leading to uniform sizes and shapes, but the organic molecular assembly might affect the purity of the final products. Therefore, a green synthetic strategy with low cost, environmentally friendly, mild synthetic conditions and potential large-scale production should be greatly considered. The template-free approach is an alternative promising strategy to synthesize hollow architectures, which is principally of two types: self-template and self-aggregation.^1,3

So far, the formation mechanisms involved in the synthesis of hollow architectures mainly include inside–out Ostwald ripening, oriented attachment, Kirkendall effect, oxidative etching and their combined strategies.^1,3,6–8

Inside–out Ostwald ripening generally refers to a coarsening and recrystallization process in an almost saturated solution-phase system.^233,234 Briefly, it involves the “dissolution of small nanocrystals or sol nanoparticles and the re-deposition of the dissolved species on the surfaces of larger crystals or sol nanoparticles, finally leading to decrease in the total Gibbs energy”.²³⁵ Thus crystallites located in the center regions dissolve and relocate on the surfaces of the aggregates, resulting in the formation of hollow interior spaces.²³⁶ Accordingly, the inside–out Ostwald ripening mechanism could act as a general strategy to prepare various hollow micro/nanostructures, which can be divided into the following two types.⁹⁴ The first one is based on a three-dimensional (3D) random aggregation of initial nanocrystallites and core hollowing by the inside–out Ostwald ripening process, as shown in Fig. 2a. The second case is the combined “oriented attachment” and “inside–out Ostwald ripening” mechanism, in which random crystallites undergo an “oriented attachment” process to form a well-defined geometrical structure under a different set of reaction parameters, and then solid voids occur in the central part of the shape-defined aggregate through Ostwald ripening, leading to the generation of non-spherical (faceted) structures, as shown in Fig. 2b. Herein, oriented attachment often occurs before the Ostwald ripening process for the formation of a hollow architecture. In regard to the oriented attachment process, individual crystallites acting as primary subunits can directly merge with their adjacent crystallites to form a mesostructure with a similar crystallographic orientation to minimize the interfacial energy.^237,238


	Fig. 2 (a) 3D oriented attachment for solid nanocubes and creation of hollow interiors by Ostwald ripening. (b) 2D oriented attachment for formation of thin crystal planes and construction of hollow octahedra in a plane-by-plane manner. Hashed areas indicate the solid parts of nanostructures, and dark areas represent interior spaces. Reprinted with permission from ref. 94. Copyright ACS 2006.

The Kirkendall effect is a classical atomic diffusion phenomenon for the construction of hollow nanoparticles,^239,240 which can be attributed to an unbalanced mutual interdiffusion occurring through an interface of two different components. Vacancies can be generated at the interface and then move in the region of the fast-diffusing component. The coalescence of excess vacancies further gives rise to voids located at the initial interface and within the fast-diffusing element. Finally, the confined voids can condense and form a single hole, resulting in the formation of a hollow nanostructure.

Oxidative etching is a special route for achieving the structure evolution of Cu₂O from the solid state to hollow cages/frames, in which both the etching agent and the dissolved oxygen play a crucial role in determining the final hollow features. The reaction process can be described by the following reaction equation (see eqn (1)).


2Cu₂O + O₂ + 4H₂O → 4Cu²⁺ + 8OH⁻	(1)

As expected, these strategies have been developed to synthesize hollow Cu_xO micro/nanostructures.¹⁸¹ However, an in-depth overview on the synthetic strategies and formation mechanisms of hollow Cu_xO with various morphologies is less reported. In this section, we will discuss the reported synthetic strategies for synthesizing hollow Cu₂O and CuO micro/nanostructures and summarize the corresponding growth mechanisms.

In addition, spray pyrolysis is an effective strategy to obtain hollow materials, whereby precursor solutions are atomized and decomposed thermally to form particles. Notably, in order to generate the hollow architecture, an in situ blowing agent is imperative. For example, the introduction of sucrose and hydrogen peroxide to the copper nitrate starting solution is significant to form a hollow CuO structure with small nanoparticles as building blocks.¹⁷⁸ The composition of the hollow Cu_xO structure can be controlled by the reaction temperature and species of solvent.^121,122

2.1 Hollow Cu₂O

In the past decades, extensive efforts have been employed to synthesize various hollow Cu₂O micro/nanostructures by soft-template and template-free synthetic strategies based on the solid-state precursor transformation (such as Cu₂O, CuO and CuCl evolution).^83–113 Therefore, it is imperative to summarize the emerging growth mechanisms and develop new methods to achieve the morphology-tuned synthesis and expand the practical applications. In this subsection, we will highlight the previous synthetic strategies and growth mechanisms of hollow Cu₂O micro/nanostructures with different morphologies.

2.1.1 Zero-dimensional (0D) nanodots. As a unique class of nanostructures, quantum dots (QDs) can exhibit promising chemical properties.¹²³ However, the hollow 0D Cu₂O nanodots are rarely reported. According to the Kirkendall principle, hollow metal oxide colloidal nanoparticles can be synthesized by the following steps: (i) synthesis of colloidal metal nanoparticles and (ii) surface oxidation (leading to the formation of core/shell nanostructures) and vacancy coalescence induced by outward diffusion of the metal atoms (resulting in the formation of hollow structures). Therefore, the surface oxidation of metal nanoparticles under different reaction conditions can determine the compositions, sizes and shapes of the final hollow metal oxide. The high diffusion rate of Cu in copper oxides can be considered as an ideal case for studying the Kirkendall effect; thus hollow Cu₂O nanodots were prepared by Yang and coworkers.⁸³ Oxidation and aging of the colloidal Cu in different solution-phase systems can generate different monodisperse nanostructures, as shown in Fig. 3.⁸³ Notably, hollow Cu₂O nanodots were formed in a chloroform environment. However, the stability of the hollow Cu₂O nanodots might limit their potential applications.


	Fig. 3 Oxidation of Cu nanoparticles to generate different monodisperse nanostructures. RT = room temperature. Reprinted with permission from ref. 83. Copyright Wiley-VCH 2010.

2.1.2 One-dimensional (1D) nanotubes. In 2003, Hu and coworkers found that reducing [Cu(OH)₄]²⁻ with hydrazine hydrate and glucose in the presence of a structure-directing surfactant at room temperature can achieve the formation of Cu₂O nanotubes and nanorods, respectively. A lower concentration of precursor favored the preparation of tubular architectures, whereas a higher concentration generally resulted in the formation of rodlike-nanostructures.¹¹³ Similarly, highly ordered arrays of hierarchical nanotubes constructed from Cu₂O hollow nanospheres have been prepared by using an array of Cu(OH)₂ nanorods as a sacrificial template (see Fig. 4).⁸⁴ The synthesis of 1D Cu₂O nanotubes was attributed to the Kirkendall effect, while the evolution of Cu₂O hollow nanospheres from solid nanospheres in the walls was considered to be the result of the Ostwald ripening process. Moreover, large-scale single-crystalline hexagonal Cu₂O nanotube arrays have been prepared through the electrodeposition of oriented Cu₂O nanorods and a subsequent dissolution technique along the c-axis to form a tubular structure, which can be attributed to the selective etching of solid 1D Cu₂O nanostructures from the interior to outside.¹¹² However, the preparation of perfect hollow nanotubes with high surface area and good diffusion is still an important challenge in the field of controlled synthesis of Cu₂O nanostructures.


	Fig. 4 (a) A schematic illustration of the formation process of arrays of hierarchically 1D Cu₂O nanotubes. Reprinted with permission from ref. 84. Copyright ACS 2009.

2.1.3 Hollow sphere with controllable shells. It has been widely proved that the inside–outside Ostwald ripening is a general mechanism for the synthesis of hollow Cu₂O spheres with controllable shells. During a typical Ostwald ripening process, smaller nanoparticles were gradually dissolved to form larger ones through a multistep solid–solution–solid recrystallization process. So far, various hollow Cu₂O spheres with single-shell, double-shell and multi-shell have been successfully synthesized based on the modified Ostwald ripening process, and the overview will be discussed as follows.
Single shell. Zeng and coworkers have demonstrated that hollow Cu₂O with polycrystalline single shells can be synthesized by the reduction of aggregated CuO nanocrystals.⁸⁵ Typically, the hollow Cu₂O products were prepared by using a mixture of Cu(NO₃)₂/N,N-dimethylformamide (DMF)/H₂O under solvothermal conditions. The detailed formation process can be described as follows: (i) the synthesis of CuO nanoparticles; (ii) the aggregation of spherical CuO crystals; (iii) the reduction of CuO precursors to form spherical Cu₂O; and (iv) the formation of hollow Cu₂O spheres via the inside–out Ostwald ripening mechanism. A schematic of the formation process and color change in this case is shown in Fig. 5.⁸⁵ Similarly, Su and coworkers have reported that hollow Cu₂O microspheres with two active {111} and {110} facets have been synthesized in a Cu(NO₃)₂/ethylene glycol (EG)/H₂O system through a fast solvothermal route in only 1 h at 180 °C.¹¹¹ In this case, EG acted as a structure-directing reagent that can adsorb on the surface of the primary Cu₂O nanoparticles and then direct the formation of a cross-linked polyhedron with two active {110} and {111} facets. Thus the formation mechanism can be attributed to a self-transformation process of the metastable nanoparticle aggregation accompanied by localized Ostwald ripening. Furthermore, poly(ethylene glycol 400) (PEG-400) molecules self-assemble to form micelles that can act as both a structure-directing and a reducing agent for the preparation of hollow Cu₂O spheres with mesoporous single shells by a solvothermal approach.¹¹⁰


	Fig. 5 A schematic formation process and color change in the formation of hollow Cu₂O spheres via the inside–out Ostwald ripening mechanism. Reprinted with permission from ref. 85. Copyright ACS 2005.

The above solvothermal route for the synthesis of hollow Cu₂O spheres involves a higher reaction temperature. Hence, it is necessary to develop a facile and fast synthetic strategy. Wang and coworkers have demonstrated that the Ostwald ripening of Cu₂O can be generated at room temperature under ambient air through which solid Cu₂O nanospheres were gradually evolved into thick nanoshells, thin nanoshells, and eventually collapsed shell structures over a time period of about 2 h (see Fig. 6a–f). In a typical synthesis, the hollow Cu₂O spheres were formed in a mixture solution of Cu(NO₃)₂/polyvinylpyrrolidone (PVP)/hydrazine. PVP acted as a structure-directing agent to mediate the nuclei aggregation through its physical adsorption to form the solid spheres at the initial stage of the process. The detailed formation process can be described as follows: (i) the assembly of Cu₂O nuclei to solid spherical aggregates by the reduction of Cu²⁺ ions by hydrazine; (ii) the interior cavities of the solid spheres through Ostwald ripening; (iii) the tailoring of the shell thickness by tuning the aging time. A schematic of shell evolution is illustrated in Fig. 6g, and it can be seen that the shell thickness continuously decreased as the Ostwald ripening continued until the shell structure began to collapse eventually.⁸⁶ A similar Ostwald ripening of hollow Cu₂O nanospheres has also occurred in Zhu and coworkers' work.¹⁰⁹


2Cu²⁺ + N₂H₄ → 2Cu + N₂ + 4H⁺	(2)


	Fig. 6 (a)–(f) The time-dependent formation of hollow Cu₂O nanospheres with single shells based on the Oswald ripening mechanism. (g) A schematic of the shell-evolution process. Reprinted with permission from ref. 86. Copyright ACS 2011.

A facile and surfactant-free gas bubble direct aggregation route has been employed to prepare hollow Cu₂O spheres (see Fig. 7a and b) in a simple system of a mixture of CuSO₄/hydrazine/H₂O by Wang and coworkers.⁸⁷ A schematic representation of the gas bubble assisted formation mechanism of hollow Cu₂O spheres is shown in Fig. 7c. First, the Cu²⁺ ions were reduced by hydrazine to form Cu nanoparticles, which immediately evolved to Cu₂O nanoparticles with oxidation by dissolved oxygen, as illustrated in step I. Then these Cu₂O nanoparticles have a tendency to aggregate around the microbubbles of N₂ gas produced in the reaction (see eqn (2) and step II). Finally, hollow Cu₂O nanospheres aggregated from small nanoparticles were formed driven by the minimization of gas–liquid interfacial energies (step III). Accordingly, the idea of gas bubbles providing aggregation centers is a novel and effective strategy to synthesize hollow micro/nanospheres.


	Fig. 7 (a) and (b) SEM images of the hollow Cu₂O nanospheres synthesized by a surfactant-free gas bubble direct aggregation route. (c) A schematic representation of the gas bubble assisted formation mechanism of Cu₂O hollow spheres. Reprinted with permission from ref. 87. Copyright RSC 2010.

Double shell. Based on Wang and coworkers' report on the synthesis of hollow Cu₂O with a single shell,⁸⁶ the authors have also developed a multistep Ostwald ripening approach for the preparation of shell-in-shell (double-shell) Cu₂O.⁸⁸ First, Cu₂O single-layer nanoshells were obtained from the first step, the Ostwald ripening process,⁸⁶ as schematically illustrated in Fig. 8a. However, it can be found that more Cu₂O nanocrystallites were produced after the introduction of additional reactants into the reaction mixture and further assembled and packed new nanocrystals both inside and outside the first shell to form a thicker nanoshell (see Fig. 8b). Subsequently, a second Ostwald ripening process occurred; thus both the inner and the outer shells would gradually become thinner, resulting in the formation of double-layer nanoshell structures (see Fig. 8c and d). Interestingly, this multistep Ostwald ripening of hollow Cu₂O spheres can be expanded by the addition of reactant in a different reaction stage, and it can also be effectively stopped once the particles were separated from the reaction mixture through centrifugation and re-dispersion in ethanol. Moreover, as a capping surfactant, cetyltrimethylammonium bromide (CTAB) can form self-assembled bilayers (multilayer) micelles, which act as soft templates to direct the synthesis of hollow Cu₂O with a double shell. Wang and coworkers have developed a large-scale synthesis of double-shell Cu₂O hollow spheres with the assistance of surfactant CTAB at room temperature.¹⁰⁵ A typical synthesis process involves the reduction of a mixture of CuSO₄/CTAB/H₂O by using hydrazine, which is different from their previous gas-bubble-assisted synthesis of single-shell Cu₂O hollow nanospheres.⁸⁷ This synthesis mainly comprises two processes: (i) the formation of the single-shell Cu₂O hollow spheres by the oxidation of Cu nanoparticles that were reduced from Cu²⁺ ions by hydrazine and (ii) the formation of the CTAB molecule layer strongly adsorbed on the surfaces of the single-shell Cu₂O hollow spheres, resulting in the formation of double-shell Cu₂O hollow spheres by a self-transformation process of the metastable aggregated small Cu₂O particles accompanied by Ostwald ripening.


	Fig. 8 A multistep Ostwald ripening approach for tunable preparing Cu₂O shell-in-shell (double-shell) hollow nanospheres. (a) Single-shell Cu₂O hollow nanosphere; (b) multi-shell Cu₂O hollow nanosphere; (c) Cu2O shell-in-shell structure with thick shell; (c) Cu₂O shell-in-shell structure with thin shell. Reprinted with permission from ref. 88. Copyright ACS 2011.

The template-free synthetic strategy has been attracting much attention for the synthesis of double-shell Cu₂O hollow spheres. For example, Geng and coworkers have developed a template-free reduction reaction to prepare homogeneous Cu₂O core–shell hollow nanostructures at room temperature without any surfactant. Typically, the reactants were only copper acetate (Cu(CH₃COO)₂) and ascorbic acid (AA); the Cu₂O nucleation clusters were formed quickly and subsequently aggregated to form solid colloids through a general process of Ostwald ripening The resulting core–shell solid colloids have two parts with the same components, resulting in the formation of hollow Cu₂O spheres with a double shell.¹⁰⁶

Multi-shell. Although various hollow Cu₂O spheres with multi-layer shells have been synthesized previously, the poly-crystalline structural shells generally possess some drawbacks, such as limiting electronic conductivity and mobility. Therefore, the integration of hollow and single-crystalline multi-shell/high-active facets in Cu₂O crystals still remains a challenge.

As mentioned above, CTAB can form self-assembled micelles and bilayer (even multilayer) aggregates, including micelles, vesicles and multi-lamellar vesicles with increasing CTAB in aqueous solution. Inspired by this viewpoint, Wang and coworkers have further proved that a multi-lamellar vesicle assisted synthesis of multi-shell Cu₂O hollow spheres can be achieved in a CuSO₄/CTAB/AA/NaOH system at 60 °C, in which the attachment process was combined with Ostwald ripening to prepare multi-shell hollow structures with single-crystalline shells.⁸⁹ The formation of different Cu₂O hollow structures in the presence of a CTAB template is shown in Fig. 9a. The layers of the multi-lamellar vesicle acting as growth sites would lead to the formation of multi-shell hollow spheres, and the shell structures (single-, double- and triple-shelled, as shown in Fig. 9b and c) of these hollow Cu₂O spheres can be controllably synthesized by adjusting the concentration of the CTAB. Based on the above synthesis, the authors have also found that the much smaller Cu₂O nanoparticles first assembled into the well-crystallized porous shelled hollow spheres by oriented attachment, and subsequently the porous shells were further changed by Ostwald ripening to produce well-crystallized hollow spheres (see Fig. 10).⁹⁰ In addition, various multiple-shell core–shell and yolk–shell nanostructures of (Cu₂O@)n-Cu₂O (n = 1–4) were prepared by a seed-mediated approach with successive Ostwald ripening; typical TEM images of various multiple-shell Cu₂O hollow nanospheres are shown in Fig. 11a–d.⁹¹ The structure formed in a previous step can act as a former-generation seed for the subsequent shell growth; thus a total of 20 typical architectures have been produced by this step-by-step strategy (schematic shown in Fig. 11e),⁹¹ which is similar to that which occurred in Huang's work.⁷²


	Fig. 9 (a) A schematic of the formation process of hollow Cu₂O structures with different shells in the presence of a CTAB template. (b) Single-shelled hollow cages. (c) Double-shelled hollow cages. (d) Triple-shelled hollow cages. Reprinted with permission from ref. 89. Copyright Wiley-VCH 2007.


	Fig. 10 Schematic illustration of the time-dependent evolution of multi-shelled Cu₂O hollow cages synthesized based on oriented attachment and Ostwald ripening mechanisms. Reprinted with permission from ref. 90. Copyright ACS 2008.


	Fig. 11 (a)–(d) TEM images of multiple-shell core–shell and yolk–shell nanostructures of (Cu₂O@)n-Cu₂O (n = 1–4). (e) 20 typical architectures produced in a step-by-step manner. Reprinted with permission from ref. 91. Copyright ACS 2012.

2.1.4 Hollow cubes. Although solid-state Cu₂O exposed with various crystalline-index facets have been synthesized, the synthesis of polyhedral hollow architectures (such as nanocages and nanoframes) is relatively difficult. So far, a few studies have achieved the preparation of hollow polyhedral Cu₂O with specific exposed facets,^{92–94,96–99} which further enriched the fundamental study of the facet-dependent property and potential application of Cu₂O crystals. A hollow Cu₂O cube with 6 exposed {100} facets is the common morphology of hollow polyhedral Cu₂O.^92–94,99

A room-temperature CuCl-hydrolysis strategy has been proved to be facile for the synthesis of hollow Cu₂O nanocubes, which was determined by the pH value of the medium solution and involved the absence of any reducers or surfactants. It has been demonstrated that the behavior of CuCl in aqueous solution was known to be pH sensitive; thus various products (such as HCuCl₂, CuCl and Cu₂O) can be obtained by increasing the pH value. Consequently, the transformation from CuCl to uniform hollow Cu₂O nanocubes can be designed based on the chemical behavior of CuCl in water media with different pH values. First, the pH-controlled precipitation of CuCl at very low pH values (eqn (3)) was used to prepare uniform CuCl nanocubes. Next, the alkaline hydrolysis reaction from in situ CuCl nanocubes to hollow Cu₂O nanocubes was generated based on the Kirkendall effect. According to the above-mentioned reaction mechanism, Du and coworkers have investigated the role of the pH value in the hydrolysis of dissolved CuCl precursor. In a typical synthesis, commercial CuCl powder (Fig. 12a) was first dissolved in a dilute HCl solution with a low pH value (Fig. 12b, pH <2.4). Then pure water was introduced to the solution to raise the pH value, leading to an immediate precipitation of uniform CuCl nanoparticles (pH >5.0). Finally, uniform CuCl nanocubes were formed (see Fig. 12c) and further served as self-sacrificial templates to prepare hollow Cu₂O nanocubes (see Fig. 12e) through alkaline hydrolysis (see eqn (3)) along with further raising the pH value (see Fig. 12d).⁹²


2CuCl + 2OH⁻ → 2Cl⁻ + Cu₂O + H₂O	(3)


	Fig. 12 A schematic illustration of the one-pot synthetic route from CuCl micro-powder to fine hollow Cu₂O nanocubes with uniform sizes. (a) A schematic illustration of CuCl micro-powder; (b) a schematic illustration of HCuCl₂ solution; (c) a schematic illustration of CuCl nanocube; (d) a schematic illustration of Cu₂O hollow nanocube; (e) TEM images of Cu₂O hollow nanocube. Reprinted with permission from ref. 92. Copyright RSC 2013.

Similarly, Xie and coworkers have reported a surfactant-free synthesis of hollow Cu₂O nanocubes by reducing Cu²⁺ precursors using Cl⁻ ions as the shape-regulator at room temperature.⁹³ Typically, hollow Cu₂O nanocubes with an average length of 100 nm were synthesized in a mixture solution of Cu²⁺/NaCl/NaOH/ascorbic acid (AA). In this process, sufficient Cl⁻ ions can be beneficial to prepare cubic CuCl self-sacrificial templates, resulting in the evolution into hollow Cu₂O nanocubes. To further verify the CuCl-hydrolysis strategy for the synthesis of hollow Cu₂O nanocubes, hollow Cu₂O nanocubes can be synthesized when NaCl was replaced by NaBr, in which the crystal structure and chemical feature of CuBr was very similar to that of CuCl. Significantly, the authors have demonstrated that the formation mechanism of the as-prepared hollow Cu₂O nanocubes was highly dependent on the amount of NaCl and NaOH in the reaction. When a small amount of NaOH was added into CuCl₂ aqueous solution, a Cu₂Cl(OH)₃ precursor can be formed and subsequently evolve into cubic CuCl intermediates owing to the reduction of AA. Finally, hollow Cu₂O nanocubes were successfully prepared through a hydrolysis reaction-induced Kirkendall process with CuCl nanocubes as self-sacrificial templates, and the reaction pathway is shown in Fig. 13a. However, solid Cu₂O nanocubes were synthesized in a medium with a high concentration of NaOH and low concentration of Cl⁻ ions because ascorbate was more easily oxidized under stronger alkaline conditions; thus higher reaction kinetics from Cu(OH)₂ was achieved by the decreasing of CuCl precursors, finally resulting in the formation of solid-state products (see Fig. 13b).


	Fig. 13 A schematic illustration of the synthesis of (a) hollow and (b) solid Cu₂O nanocubes. Reprinted with permission from ref. 93. Copyright RSC 2013.

Interestingly, Jiang and coworkers have found that slower reaction kinetics could be achieved when NaOH was replaced by weaker alkalinity LiOH (namely in a Cu²⁺/LiOH/L-ascorbic acid reaction system), which can weaken the oxidation of AA and might promote the formation of CuCl precursors. Thus the presence of Cl⁻ ions would effectively control the oriented aggregation of primary Cu₂O crystallites toward mesocrystalline Cu₂O hollow nanocubes.⁹⁹

From the above examples, it can be found that both the pH value (namely the concentration of H⁺ ions) and the concentration of Cl⁻ ions were the key parameters for the construction of hollow Cu₂O nanocubes. Therefore, balancing the hydrolysis and recrystallization rates of the CuCl precursors through precisely adjusting the experimental parameters (the concentration of the H⁺ and/or Cl⁻ ions) was very crucial to the controllable synthesis of hollow Cu₂O nanostructures. Based on this formation mechanism, Guo and coworkers have demonstrated a recrystallization-induced self-assembly for the preparation of hollow Cu₂O superstructures.⁷⁷ In the synthesis process, P123-decorated CuCl precursors were first synthesized in a mixture solution of CuCl₂/NH₃·H₂O/P123/AA at room temperature. Afterwards, these CuCl precursors would transform into cube-like hollow Cu₂O superstructures in another mixture solution of P123/KCl/AA/NH₃·H₂O. NH₃·H₂O was introduced to adjust the pH value and KCl was employed to adjust the [Cl⁻] concentration. P123 acted as a structure-directing agent for the oriented aggregation, which covered the surface of the CuCl precursors via the coordination of oxygen atoms in P123 to copper atoms in CuCl, preventing CuCl from undergoing rapid hydrolysis to build the hollow Cu₂O superstructures. Hence, the development of a recrystallization-based process for the guided construction of 3D superstructures should further be explored, which might inspire us to synthesize new structures for expanding the practical applications.

A combination of oriented attachment with Ostwald ripening has been employed in the formation of hollow Cu₂O nanocubes. For example, the synthesis of hollow Cu₂O nanocubes through reductive self-assembly of CuO nanocrystals has been successfully achieved in a Cu(NO₃)₂/DMF/H₂O system under hydrothermal conditions by Zeng and coworkers (see Fig. 14a and b).⁹⁴ In this case, the Cu²⁺ ions can be controllably converted to oxide phases of CuO and Cu₂O (see Fig. 14c). Instead of generating polycrystalline Cu₂O nanospheres, primary CuO nanoparticles can undergo the “oriented attachment” mechanism to construct well-defined cubic shapes under a different set of process parameters, and then solid evacuation takes place in the central part of the shape-defined aggregates owing to Ostwald ripening, resulting in the formation of single-crystalline hollow Cu₂O non-spherical (faceted) nanostructures (see Fig. 14d).


	Fig. 14 (a) TEM image and (b) selected area electronic diffraction (SAED) pattern of the hollow Cu₂O nanocube synthesized reductive self-assembly of CuO nanocrystals. (c) The reaction equations. (d) A schematic illustration of the formation process. Reprinted with permission from ref. 94. Copyright ACS 2006.

Additionally, in situ light-induced photochemical synthesis of Cu₂O microcubes can be achieved by utilizing CuWO₄ as the precursor.¹⁰³ Typically, Cu²⁺ ions were first released from CuWO₄ under a side irradiated photoreactor containing an aqueous solution of glucose and NaOH. Then, Cu²⁺ ions reacted with NaOH to form copper hydroxides. Subsequently, the copper hydroxides were further reduced by glucose to form hollow Cu₂O cubes by a self-aggregation mechanism. Similarly, Zn-doped hollow Cu₂O microcubes can also be synthesized using Zn-doped CuWO₄ precursors under otherwise the same conditions.¹⁰³ However, it remains a great challenge to develop feasible methods for the synthesis of doped Cu₂O hollow structures.

2.1.5 Hollow octahedra. So far, references to hollow octahedral Cu₂O architectures have been less reported. A Cu₂O self-template strategy has been proved by reducing a copper–citrate complex with glucose. First, octahedral Cu₂O solid-state particles were synthesized under hydrothermal conditions. Then, Cu₂O can be further reduced to Cu when the glucose was sufficient. Finally, hollow Cu₂O octahedral particles were obtained after removing the Cu nanoparticles.¹¹⁵

Octahedral nanowire-assembled Cu₂O superstructures can be prepared through a facile hydrothermal approach by reducing cupric acetate with o-anisidine or o-phenetidine in the presence of carboxylic acids at 180 °C, as shown in Fig. 15a and b.⁹⁵ It can be seen that the products have polyhedral outlines and contain highly ordered nanowire subunits. The growth of the Cu₂O nanowire polyhedra was tuned by the orientation and growth rates of the nanowire branches which can be adjusted by addition of carboxylic acid doped polymer chains. Additionally, concave octahedron and hexapod superstructures can be formed by further controlling the reaction parameters. Inspired by this synthesis pathway, reduced graphene oxide (rGO)-conjugated Cu₂O nanowire mesocrystals were successfully synthesized by a non-classical crystallization mechanism in the presence of GO and o-anisidine under hydrothermal conditions, which displayed a higher sensitivity toward NO₂ at room temperature.¹¹⁸


	Fig. 15 (a) and (b) SEM images of typical octahedral nanowire-assembled Cu₂O superstructures. Reprinted with permission from ref. 95. Copyright Springer 2011. (c) and (d) TEM images of typical hollow octahedral Cu₂O nanocages synthesized based on a catalytic solution route. Reprinted with permission from ref. 96. Copyright Wiley-VCH 2005.

Moreover, typical hollow octahedral Cu₂O nanocages can be synthesized by a self-catalytic etching strategy.⁹⁶ Qi and coworkers have prepared hollow octahedral Cu₂O nanocages with single-crystalline shells by reducing Fehling's solution (copper tartrate complex) with glucose at 75 °C in the presence of PdCl₂ catalyst.⁹⁶ Mechanistic investigation suggested that the formation of these octahedral Cu₂O nanocages can be divided into the following two steps. (i) Octahedral solid nanocrystals were prepared by the Pd⁰-catalyzed reduction of the copper tartrate complex with glucose. (ii) The trace oxygen dissolved in solution would gradually oxidize Cu₂O into Cu²⁺ in the presence of the Pd⁰ catalyst (oxygen-engaged oxidative etching, see eqn (1)). Finally, octahedral Cu₂O nanocages with etching apexes were obtained after an aging process (see Fig. 15c and d).⁹⁶ The selective oxidative etching on the apexes of the primary solid octahedral Cu₂O nanoparticles might be attributed to the difference of the surface energies in different exposed facets.⁴⁸ Based on the selective oxidative etching mechanism, it is believed that more novel hollow Cu₂O micro/nanostructures might be facilely synthesized.

2.1.6 Hollow nanoframes. The selective etching on faceted Cu₂O can occur along a specific crystallographic orientation towards different resistances to the corrosion in the reaction medium. Inspired by the oxidative-etching mechanism, a cheap synthetic route for synthesizing Cu₂O nanoframes and nanocages with single-crystal shells was developed by Yang and coworkers.⁹⁷ In this synthetic process, 14-facet Cu₂O particles were first prepared by adding a weak reducing agent (glucose) to a solution of copper citrate complex with polyvinylpyrrolidone (PVP) as capping agent, and then Cu₂O nanoframes and nanocages were obtained by selective oxidative-etching at room temperature. The reaction mechanism of oxygen etching was similar to that occurring in the above Qi's report. Moreover, the PVP capping agent played a significant role in the selective etching. In a typical Cu₂O {111} facet, the Cu atom has a dangling bond perpendicular to the {111} facets, so the coordinatively unsaturated Cu in the {111} facet is obviously more active for interaction with PVP. Thus the PVP-capped {111} facets would freeze the {111} facets and thereby avoid oxidative etching, finally leading to the formation of truncated octahedral Cu₂O nanoframes composed of {111}-faceted skeletons and empty {100} facets (see Fig. 16).⁹⁷


	Fig. 16 SEM (A and B) and TEM (C and D) images of the truncated octahedral Cu₂O nanoframes composed of {111}-faceted skeletons and empty {100} facets. Reprinted with permission from ref. 97. Copyright Wiley-VCH 2010.

Huang and coworkers have demonstrated a facile chemical etching route for the synthesis of truncated polyhedral Cu₂O nanocages and nanoframes in an aqueous solution containing CuCl₂, sodium dodecyl sulfate (SDS), NH₂OH·HCl, HCl, and NaOH.¹⁰² The Cu₂O nanostructures can be grown into nanoframes and nanocages by aging the reaction mixture under different reaction conditions, in which the introduction of HCl is crucial for promoting the etching process. Moreover, SDS may play a role in protecting the Cu₂O nanocrystals from direct etching by HCl. A schematic illustration of the growth and etching mechanisms in the synthesis of the nanoframes and nanocages is shown in Fig. 17.⁹⁸ Rapid seed-particle aggregation and surface reconstruction of the intermediate structure led to the formation of type I nanoframes, which have only {110} skeleton facets and empty {100} facets (see Fig. 18a–f),⁹⁸ whereas selective acidic etching on the {110} facets of the nanocages through the addition of ethanol followed by sonication of the solution resulted in the formation of type II nanoframes (see Fig. 18g–l),⁹⁸ which have elliptical pores on the {110} facets. Random etching of the nanocages can be obtained at a slow rate without addition of ethanol. Therefore, the strategy of oxidative etching combined with self-template of the precursors can be beneficial to the morphology-controllable synthesis of Cu₂O with selective etching facets. However, the intrinsic feature of different crystallographic facets of Cu₂O should attract attention in designing an etching hollow architecture. The anisotropic features on different facets will be thoroughly discussed in subsequent section 3.


	Fig. 17 A schematic illustration of the growth and etching mechanisms in the synthesis of the nanoframes and nanocages. Reprinted with permission from ref. 98. Copyright ACS 2008.


	Fig. 18 (a)–(f) TEM images and corresponding configurations of the type I Cu₂O nanoframes with only {110} skeleton facets and empty {100} facets. (g)–(l) TEM images and corresponding configurations of the type II Cu₂O nanoframes with elliptical pores on the {110} facets. Reprinted with permission from ref. 98. Copyright ACS 2008.

2.1.7 Hierarchically hollow architectures. Generally, hierarchically hollow 3D materials assembled from the structure unit with 2D nanosheets have a high value in the field of catalysis and sensing. For example, a template-free synthesis of complicated double-shell Cu₂O hollow spheres (see Fig. 19) can be achieved in a mixture of Cu(NO₃)₂/ethanol–glycerol–water/urea under a solvothermal route. The formation process involves three steps: (1) the reduction of Cu₂(OH)₂CO₃ particles to Cu₂O, (2) the formation of scabbled Cu₂O microspheres by assembly of dispersed small Cu₂O spherical particles, and (3) the formation of double-shell Cu₂O hollow spheres by Ostwald ripening.¹⁰⁴ Additionally, hollow microspheres of Cu₂(OH)₃NO₃ were synthesized initially because of the Ostwald ripening and then reduced by glutamic acid; the resultant Cu₂O nanocrystals were deposited on the hollow intermediate microspheres and finally organized into multi-shell structures. Thus, hierarchical assembly of hollow Cu₂O microspheres with nanocrystal-composed porous multi-shells was successfully achieved.¹⁰⁷ The hierarchically hollow Cu₂O microspheres, which showed much higher sensitivity than solid Cu₂O microspheres, can be used to fabricate a gas sensor.


	Fig. 19 (a–d) SEM images with different magnifications and (e) XRD pattern of the Cu₂O powder which was synthesized in an ethanol solution containing copper nitrate and glutamic acid after solvothermal treatment for 48 h. Reprinted with permission from ref. 107. Copyright Wiley-VCH 2007.

Briefly, the formation of novel hollow Cu₂O is expected to offer new opportunities to further explore the fundamental understanding as well as to enhance the practical applications. The synthesis methods, growth mechanisms and morphologies of the reported hollow Cu₂O micro/nanostructures are summarized in Table 1. However, the development of simple synthetic strategies for controllably synthesizing uniform hollow Cu₂O nanostructures wholly exposed with multi-index facets is still in its infancy.

Table 1 Synthesis methods, formation mechanisms and morphologies of hollow Cu₂O micro/nanostructures

Precursor	Synthetic method	Mechanism	Morphology	Ref.
Copper(I) acetate/trioctylamine/tetradecylphosphonic acid	Oxidization of copper	Kirkendall effect	Hollow nanodots	83
Cu(OH)₂/ascorbic acid	Liquid-phase reduction	Kirkendall effect	Hierarchical nanotubes	84
Cu foil/Cu(NO₃)₂/NH₄Cl/KCl	Electrodeposition	Oxidative etching	Nanotube arrays	112
CuCl₂/NaOH/CTAB/glucose/N₂H₄·H₂O	Hydrothermal approach	Kirkendall effect	Nanotubes	113
CuSO₄/CTAB/N₂H₄·H₂O	Liquid-phase reduction	Ostwald ripening	Double-wall hollow spheres	105
Cu(CH₃COO)₂/ascorbic acid	Liquid-phase reduction	Ostwald ripening	Double-wall hollow spheres	106
Cu(NO₃)₂/glutamic acid/ethanol	Hydrothermal approach	Ostwald ripening	Multi-shell hollow spheres	107
Cu(NO₃)₂/DMF/water	Hydrothermal approach	Ostwald ripening	Single-shell hollow spheres	85
Cu(NO₃)₂/N₂H₄·H₂O/PVP	Liquid-phase reduction	Ostwald ripening	Single-shell hollow spheres	86
CuSO₄/N₂H₄·H₂O	Liquid-phase reduction	Ostwald ripening	Single-shell hollow spheres	87, 108
Cu(NO₃)₂/N₂H₄·H₂O	Liquid-phase reduction	Multistep Ostwald ripening	Shell-in-shell hollow spheres	88
CuSO₄/CTAB/NaOH/ascorbic acid	Liquid-phase reduction	Ostwald ripening	Single-, binary-, triple- and quadruple-shell hollow spheres	89
CuSO₄/CTAB/NaOH/ascorbic acid	Liquid-phase reduction	Ostwald ripening	Triple-shell hollow spheres	90
Cu₂O/Cu(NO₃)₂/NH₄NO₃/NaOH/ascorbic acid	Liquid-phase reduction	Ostwald ripening	Core–shell and yolk–shell (Cu₂O@)_nCu₂O (n = 1–4)	91
CuSO₄/N₂H₄·H₂O/PVP	Liquid-phase reduction	Ostwald ripening	Single-shell hollow spheres	109
Cu(NO₃)₂/ethylene glycol	Hydrothermal approach	None	Single-shell hollow spheres	111
CuSO₄/PVP/sodium citrate/glucose	Liquid-phase reduction	Oxidative etching	Nanoframes with empty {100} facets	97
CuSO₄/PVP/KOH/glucose/PdCl₂	Liquid-phase reduction	Oxidative etching	Hollow octahedron without {100} facets	96
CuCl₂/NaOH/SDS/NH₂OH·HCl/HCl	Liquid-phase reduction	Oxidative etching	Nanoframes with empty {110} and {100} facets	98, 102
CuCl/KCl/NH₃·H₂O	Hydrolysis of CuCl	Aggregation-based recrystallization	Hollow cubes	77
CuCl/HCl or NaOH	Hydrolysis of CuCl	Kirkendall effect	Hollow cubes	92
CuCl₂/NaCl/NaOH/ascorbic acid	Liquid-phase reduction	Kirkendall effect	Hollow cubes	93
Cu(NO₃)₂/DMF/ethanol	Hydrothermal approach	Ostwald ripening	Hollow cubes	94
CuCl₂/LiOH/ascorbic acid	Liquid-phase reduction	Oriented aggregation	Hollow cubes	99
CuCl₂/NaOH/PEG-200	Hydrothermal approach	Ostwald ripening	Hollow cubes	101
CuWO₄/glucose	Liquid-phase reduction	None	Hollow cubes	103
CuSO₄/ammonium citrate/glucose	Hydrothermal approach	Oxidative etching	Hollow octahedra	115
Copper nitrate solution	Spray pyrolysis approach	None	Hollow particles	121
Copper hydroxy nitrate/ethanol	Spray pyrolysis approach	None	Hollow particles	122

2.2 Hollow CuO

It has also been found that the Kirkendall effect and Ostwald ripening as well as oriented attachment were generally used for the synthesis of hollow CuO.^163,164 Hence, to overview the growth mechanisms and develop novel synthetic strategies are necessary for achieving the shape-tailored synthesis and expanding the potential applications. In this subsection, we will summarize the previous synthetic strategies and growth mechanisms of hollow CuO micro/nanostructures with different morphologies.

2.2.1 Hollow nanotubes. The hard-template method is effective for the synthesis of hollow CuO nanotubes. For instance, Xu and coworkers have introduced a novel and simple route to synthesize porous hollow CuO nanofibers (see Fig. 20a and b ¹⁶⁵) through single-spinneret electrospinning of PVP/Cu(CH₃COO)₂ solution followed by annealing and reduction. A possible formation mechanism of porous CuO hollow nanotubes can be proposed as follows (see Fig. 20c).¹⁶⁵ Initially, composite nanofibers combined with Cu(CH₃COO)₂ and PVP were prepared by electrospinning. Afterwards, Cu(CH₃COO)₂ located at the surface of the composited nanofibers started to oxidatively decompose to form CuO nanoparticles by heat treatment at about 230 °C, but most of the Cu(CH₃COO)₂ in the core region of nanofibers cannot react due to lack of oxygen. Thus, a concentration gradient of Cu(CH₃COO)₂ was formed, which will drive Cu(CH₃COO)₂ to diffuse to the surface; at the same time, another concentration gradient of CuO also exited from the surface to the interior of the nanofibers. Finally, the formation of a vacancy flux and subsequent void would occur by the Kirkendall process because the diffusion speed of melting Cu(CH₃COO)₂ was faster than that of solid CuO.¹⁶⁵


	Fig. 20 (a) SEM image and (b) TEM image of hollow CuO nanotubes synthesized by single-spinneret electrospinning of PVP/Cu(CH₃COO)₂ solution followed by annealing and reduction. (c) A possible formation mechanism of these porous CuO hollow nanotubes. Reprinted with permission from ref. 165. Copyright RSC 2011.

To further improve the performance of CuO in practical applications, well-aligned CuO nanotube arrays with controllable diameters were synthesized using porous anodic alumina films (PAA) as templates. Typically, PAA templates were first prepared, and then the PAA was filled with a high concentration of cupric nitrate to form a mixture of PAA/cupric nitrate. Subsequently, the formation of PAA/CuO nanotubes was achieved by thermal decomposition of copper nitrate at 500 °C. Finally, CuO nanotube arrays were prepared by immersing the PAA/CuO nanotubes in sodium hydroxide solution to remove the PAA templates.¹⁶⁶ Additionally, highly ordered hollow CuO nanotube arrays can be formed by the oxidization of Cu arrays through the Kirkendall effect.^163,164

2.2.2 Hierarchically hollow spheres. The hollow sphere is the most common shape of hollow CuO micro/nanostructures. So far, a number of studies have reported on the controllable-synthesis of hollow CuO spheres with various subunits. For example, carbon microspheres can be used as sacrificial hard templates for the synthesis of hollow CuO spheres.¹⁷⁴ However, it is more perspective to develop a novel template-free or soft-template strategy for fabricating hierarchically hollow CuO spheres with high performance.

Tan and coworkers have developed a template-free method for preparing a hierarchically porous CuO hollow microsphere assembly with oriented 2D nanosheet subunits (see Fig. 21a–d) by mixing the CuSO₄/KOH/NH₃·H₂O solution at 68 °C, which can be attributed to the direct transformation from [Cu(NH₃)₄]²⁺/Cu(OH)₂ precursors to CuO primary nanocrystallites that occurred according to eqn (III) and/or eqn (IV) as shown in Fig. 21e.¹⁶⁷ A two-step mechanism was proposed for the formation of these hierarchically porous CuO hollow microspheres. (i) The “oriented attachment” step, in which CuO nuclei was formed immediately and then a large number of primary CuO nanograins were produced. Subsequently, the fast oriented attachment of these CuO nanocrystals might lead to the formation of CuO nanosheet bundles driven by surface energy minimization. (ii) The “Ostwald ripening” step, in which the CuO crystallites located in the inner cores possessed relatively high surface energies possibly because of the defects of mismatch generated from the high growth rate at the early oriented attachment stage. Thus the unstable inner cores dissolved gradually along with prolonging the reaction time, finally resulting in the formation of hollow interiors in the CuO spheres.


	Fig. 21 Hierarchically porous CuO hollow microsphere assembly by oriented nanosheet subunits. (a) A typical low-magnification SEM image; (b) a typical SEM image of an individual particle; (c) a high-magnification SEM image of the nanosheet subunits; (d) a typical TEM image of a nanosheet subunit, and the inset is the corresponding SAED pattern; (e) the corresponding chemical reaction equations. Reprinted with permission from ref. 167. Copyright ACS 2012.

Similar hierarchical CuO hollow microspheres composed of 2D nanosheet subunits have also been synthesized by using CuSO₄/organic solvent/H₂O under a hydrothermal approach.^168,169 Moreover, hierarchical CuO hollow microspheres assembled with nanorods have been synthesized by a template-free hydrothermal strategy based on the combined mechanism of oriented attachment and Ostwald ripening.¹⁶⁹ Uniform CuO hollow microspheres were successfully achieved from non-uniform MOFs by using a template-free method, which can be attributed to aggregation and Ostwald ripening.¹⁷⁰ In addition, a gas-bubble and/or droplet assisted strategy is also used for the synthesis of hollow CuO spheres with nanosized building blocks.^171,172

2.2.3 Hollow polyhedra. The success of the construct engineering on hollow cages has been demonstrated by a Cu₂O-template strategy. This Cu₂O-templated growth of hollow CuO cages is interesting and has wide utility. For example, Song and co-workers have found that Cu₂O nanocubes (see Fig. 22a) can act as sacrificial templates to synthesize hollow CuO nanocages by a sequential oxidative dissolution–precipitation mechanism.¹⁷⁷ In the synthesis process, CuO can be produced in a mixture solution of Cu₂O/NH₃·H₂O/ethanol/H₂O at room temperature, and the morphologies of the hollow nanostructures would be tailored by adjusting the concentration of the aqueous ammonia solution. It can be attributed to CuO being the most stable phase among CuO and ammine complexes at pH 10–11 in the presence of aqueous ammonia solution at room temperature according to the Pourbaix diagram, which suggested that the oxidation of Cu₂O would occur to form the more thermodynamically stable CuO. The detailed chemical reactions can be described as follows (eqn (4)–(7)):


Cu₂O + 1/2O₂ + 2H₂O + 8NH₃ → 2[Cu(NH₃)₄]²⁺ + 4OH^–	(4)


[Cu(NH₃)₄]²⁺ + 2OH^– → Cu(OH)₂ + 4NH₃	(5)


Cu(OH)₂ + 2OH^– → [Cu(OH)₄]^2–	(6)


[Cu(OH)₄]^2– → CuO + 2OH^– + H₂O	(7)


	Fig. 22 (a) TEM image of the Cu₂O nanocubes. (b)–(d) TEM images of the hollow cubes, spheres and urchin-like CuO nanoparticles, respectively. Reprinted with permission from ref. 177. Copyright Wiley-VCH 2009.

First, Cu₂O was oxidized by oxygen molecules to produce a copper(II)–amine complex (reaction (4)) in an aqueous ammonia solution. The copper(II) amine slowly evolved to divalent copper hydroxide (Cu(OH)₂) in the solution-phase system (reaction (5)), whereas the tetrahydroxo-cuprate(II) anion [Cu(OH)₄]²⁻ would be generated from the copper hydroxide under a strong basic environment (reaction (6)) because of the Jahn–Teller stabilization in square-planar copper(II) complexes.²⁴¹ Finally, thermodynamically stable CuO was produced by the decomposition of [Cu(OH)₄]²⁻ species (reaction (7)) due to its low solubility in water. From the above reactions, it can be proposed that a high ammonia concentration would result in the fast dissolution of Cu₂O as in reaction (4), and thus the hydroxide ion concentration also increased the total reaction rates through reactions (5) and (6). However, the [Cu(OH)₄]²⁻ species were highly stable at high OH⁻ concentrations, which can decrease the precipitation rate of CuO through the reverse process of reaction (7). Therefore, various CuO nanostructures (such as hollow cubes, spheres and urchin-like nanoparticles, see Fig. 22b–d) were finally synthesized by the introduction of different NaOH concentrations.

Based on the intrinsic principles of the above synthesis, Yang and coworkers have synthesized 26-facet hollow CuO microstructures by a modified approach, as shown in Fig. 23a. The formation of the hollow CuO 26-facet polyhedron can be attributed to the Kirkendall effect. In this case, a CuO shell was initially formed on the surface of Cu₂O. Subsequently, the oxygen and ammonia in the solution diffused inward, while the Cu⁺ in the Cu₂O core diffused outward. The different diffusion rates resulted in the hollow structure.¹⁷⁹ Notably, it is proposed that different CuO micro/nanocages can be easily prepared by full oxidation of the morphologically hollow Cu₂O cages (as discussed in section 2.1).


	Fig. 23 (a) 26-facet hollow CuO microstructures synthesized by a modified Cu₂O-templated approach. Reprinted with permission from ref. 179. Copyright RSC 2014. (b) Octahedral hollow CuO microstructures synthesized by a MOF-templated approach. Reprinted with permission from ref. 180. Copyright RSC 2013.

MOFs acting as sacrificial templates can also be employed to prepare the hollow CuO architectures because the metal ions or clusters of MOFs can evolve into metal oxide through a proper thermal treatment process. Huang and coworkers have reported a simple and scalable synthesis of porous CuO hollow octahedra by annealing the [Cu₃(btc)₂] (Cu-btc, btc = benzene-1,3,5-tricarboxylate) MOF templates at 300 °C (see Fig. 23b).¹⁸⁰ The synthesis strategies and morphologies of the reported hollow CuO micro/nanostructures are summarized in Table 2.

Table 2 Synthesis methods, formation mechanisms and morphologies of hollow CuO micro/nanostructures

Precursor	Synthetic method	Mechanism	Morphology	Ref.
Cu nanowire arrays	Thermal annealing	Kirkendall effect	Nanotube arrays	163, 164
Cu(CH₃COO)₂/PVP	Electrospinning + annealing	None	Hollow nanofibers	165
Cu(NO₃)₂/PAA	Electrodeposition	Alumina template	Nanotubes	166
CuSO₄/KOH/NH₃·H₂O	Liquid-phase hydrolysis	Oriented attachment and Ostwald ripening	Hierarchically hollow spheres	167
Cu(CH₃COO)₂/EG/H₂O	Hydrothermal approach	Self-assembly	Hierarchically hollow spheres	168
Cu(CH₃COO)₂/urea/H₂O	Hydrothermal approach + annealing	Self-assembly	Hierarchically hollow spheres	169
Cu(NO₃)₂/benzene-1,3,5-tricarboxylic acid/DMF/EtOH/H₂O	Thermal annealing	MOF template	Hierarchically hollow spheres	170
Cu(NO₃)₂/sucrose/hydrogen peroxide	Spray pyrolysis method	None	Hollow spheres	171
CuSO₄/NH₃·H₂O/tyrosine	Hydrothermal approach	Self-organization	Hierarchically hollow spheres	173
CuCl₂/NaOH/carbon	Thermal annealing	Carbon sphere template	Hollow spheres	174
Cu(HCOO)2/sodium alginate/NH₃·H₂O	Hydrothermal approach	None	Hollow spheres	175
CuSO₄/glutamine	Hydrothermal approach	None	Hierarchically hollow spheres	176
Cu₂O/NaOH/ethanol/H₂O	Liquid-phase reaction	Kirkendall effect	Hierarchically hollow cubes, spheres and urchins	177, 178
Cu₂O/NaOH/ethanol/H₂O/glucose	Liquid-phase reaction	Kirkendall effect	Hierarchically hollow polyhedra	179
[Cu₃(btc)₂]	Thermal annealing	None	Hierarchically hollow octahedra	180
Cu(HCOO)₂/hexylamine/ethyl cellulose/1-methyl-2-pyrrolidinone	Thermal annealing	Kirkendall effect	Hollow spheres	231

3. Interfacial Cu–O atomic structures and their intrinsic properties

As a cuprite structure, Cu₂O is a cubic close packing of Cu atoms, with O atoms in tetrahedral sites at positions (3/4, 3/4, 3/4) and (1/4, 1/4, 1/4) of the cubic lattice, in which the coordination of the Cu atoms is half that of the O atoms (see Fig. 24a).¹⁶⁰ Consequently, Cu atoms in the bulk crystal are twofold coordinated by O atoms; each O atom is surrounded by a tetrahedron of Cu atoms, and each Cu atom has two O atom neighbors, as described by its unit cell model. Meanwhile, the CuO crystal is a monoclinic structure with two molecules per primitive cell (see Fig. 24b) where each Cu atom is surrounded by four O atoms.¹⁶⁰


	Fig. 24 Crystallographic structures of the Cu₂O lattice (a) and the CuO lattice (b). Reprinted with permission from ref. 160. Copyright RSC 2013.

When the Cu–O bonds of Cu₂O encounter O molecules, the O atoms insert the lattice of the Cu₂O unit cell through different atom rearrangements and reconstructions of the lattice/unit cell; thus the monoclinic unit cell of CuO is formed at a higher temperature, finally resulting in the synthesis of a Cu₂O–CuO heterogeneous interface. While the Cu–O bonds of Cu₂O encounter a specific reducer, the O atoms are released from the Cu₂O unit cells, leading to the formation of Cu species on their surfaces, and thus a Cu₂O–Cu heterogeneous interface is formed.

Briefly, the construction of Cu₂O–CuO and Cu₂O–Cu heterogeneous architectures is beneficial to the investigation of the intrinsic properties of interfacial Cu–O atomic structures, which can be used to direct the design and synthesis of hybrid Cu_xO micro/nanostructures for expanding their potential applications. In this section, we will mainly discuss the intrinsic properties of different interfacial Cu–O atomic structures and then summarize the advancements in controllable synthesis of hybrid Cu_xO micro/nanostructures (including hollow Cu₂O–CuO and Cu₂O–Cu).

3.1 Intrinsic properties of interfacial Cu–O atomic structures

In general, surface Cu atoms of Cu₂O are coordinatively unsaturated, resulting in the intrinsic surface atomic structures being different.^14,15,61 For example, both Cu and O atoms predominate in the {111} and {110} facets of Cu₂O, and surface Cu atoms with dangling bonds can make them positively charged. The {110} facets contain rows of twofold-coordinated Cu atoms and onefold-coordinated Cu atoms with one dangling bond perpendicular to the surface, and the same terminated Cu atoms with dangling bonds are existing in the {111} facet, but the distance between two the Cu atoms is about twice that in the {110} facet, suggesting that the number and density of Cu dangling bonds in the {111} facets are lower than those of the {110} facets.⁶¹ In addition, the {100} facet is constituted by Cu or O atoms only, and thus the Cu atoms on the {100} surface are in saturated coordination, resulting in an electrically neutral state.⁶¹ Hence, new Cu–O interfaces generated from various Cu₂O surfaces will lead to the difference in their intrinsic properties and thus the underlying intrinsic principle of interfacial Cu–O atomic structures is significant to develop novel hybridizing architectures with improved performances.

3.1.1 Interfacial Cu–O atomic structures of hybrid Cu₂O–CuO. Surface restructuring to in situ prepare new CuO thin films on Cu₂O surfaces can be achieved at a higher reaction temperature under a specific atmosphere. The new interfacial Cu–O atomic structures of the hybrid Cu₂O–CuO can be determined by the pristine surface of Cu₂O. For instance, during a catalytic CO oxidation, CO oxidation catalyzed by the CuO thin film on Cu₂O{111} and CO oxidation catalyzed by the CuO thin film on Cu₂O{100} involve different types of catalytically active sites and proceed with different reaction mechanisms. Based on density functional theory (DFT) calculations, the CuO overlayer on the Cu₂O{111} facet is terminated with three-coordinated Cu (Cu_3c) and three-coordinated O atoms (O_3c), while the CuO overlayer on Cu₂O{100} facet is terminated with only O_2c atoms. The new interfacial Cu–O atomic structures on the Cu₂O{111} and Cu₂O{100} facets are shown in Fig. 25.¹⁸¹ Thus CuO–Cu₂O{111} possessed a lower apparent activation energy than that catalyzed by CuO–Cu₂O{100}, which agreed well with the experimental results, that the catalytic performance of CuO–Cu₂O{111} was better than that of the CuO–Cu₂O{100}.


	Fig. 25 The interfacial Cu–O atomic structures of Cu₂O{111} and Cu₂O{100} facets. Reprinted with permission from ref. 181. Copyright Wiley-VCH 2011.

Moreover, the formation of the Cu₂O–CuO heterogeneous interface can promote the spatial charge carrier (electrons and holes) separation and suppress charge recombination. As an example of a photocatalyst, the Cu₂O–CuO heterojunction is in agreement with the type II mechanism. Under solar-light irradiation, the photogenerated electrons from Cu₂O with a higher conduction band (CB) edge can migrate to CuO with a lower CB edge. Furthermore, the photogenerated holes from CuO with a lower valence band (VB) edge will be transported to Cu₂O with a higher VB edge. Thus, the hybrid Cu₂O–CuO heterogeneous catalysts can exhibit an enhanced photocatalytic performance compared to the single-component ones.²⁴²

Therefore, the construction of highly active Cu–O atomic structures on Cu₂O surfaces in reactive environments (namely anisotropic surface restructuring) would open a new door for optimizing and designing efficient catalysts.

3.1.2 Interfacial Cu–O atomic structures of hybrid Cu₂O–Cu. A copper-rich or oxygen-deficient surface can generate Schottky barriers by building a Cu₂O–Cu structure.²⁴³ The presence of Schottky barriers in Cu₂O–Cu structures can effectively promote the lifetime of photo-excited charge carriers through increasing the rate of electron–hole separation and limiting the reverse current flowing from Cu to the Cu₂O semiconductor, thus enhancing the photocatalytic performances. DFT calculations were employed to understand the surface structures of {111}, {110} and {100}, indicating that the bond length between the O atom and the Cu atom in the {111} facet was larger than that of the {111} and {100} facets (see Fig. 26a).¹⁸² Therefore, the Cu₂O{111} facet was easily reduced than the {110} and {100} surfaces based on the reaction, as shown in the inset of Fig. 26b,¹⁸² leading to the selective growth of Cu nanoparticles on Cu₂O{111} surfaces (see Fig. 26b). The as-prepared Cu–Cu₂O heterogeneous architectures displayed much better adsorption and photodegradation of methyl orange dyes than the pristine Cu₂O (see Fig. 26c),¹⁸² which can be due to the Schottky barriers on the Cu–Cu₂O interfaces.¹⁸² This finding not only enriches the new purposive design of interfacial Cu–O atomic structures possible but also provides a good opportunity to understand the foundational Schottky barriers for improving the photocatalytic activity.


	Fig. 26 (a) DFT calculations of the surface structures of {111}, {110} and {100}. (b) SEM image of the Cu–Cu₂O synthesized by an in situ reduction route. (c) Photodegradation curves of Cu–Cu₂O and pristine Cu₂O. Reprinted with permission from ref. 182. Copyright RSC 2012.

3.2 Synthesis of interfacial Cu–O atomic structures

As discussed above, the construction of different interfacial Cu–O atomic structures can effectively develop novel Cu_xO micro/nanostructures with improved performances. In this subsection, we will highlight the advances in controllable synthesis of hybrid Cu_xO micro/nanostructures, including hollow Cu₂O–CuO and Cu₂O–Cu.

3.2.1 Hollow Cu₂O–CuO. Uniform Cu₂O–CuO microcubes can be prepared by oxidation etching of Cu₂O microcubes under hydrothermal conditions by using only a Cu₂O and NaOH aqueous solution (see Fig. 27a).¹⁸³ Under this hydrothermal condition, the surface of Cu₂O was initially oxidized into divalent copper hydroxide (Cu(OH)₂) in the presence of H₂O and dissolved oxygen (see eqn (8)). However, the tetrahydroxo-cuprate(II) anion [Cu(OH)₄]²⁻ might be formed from the Cu(OH)₂ under a strong basic environment (see eqn (6)), and then CuO was produced by the decomposition of [Cu(OH)₄]²⁻ species (see eqn (7)), as discussed in section 2.2.3. Finally, CuO gradually precipitated on the inner wall of the hollow Cu₂O frames, which can be attributed to the synergistic effect of Kirkendall effect and oxidation etching.


2Cu₂O + O₂ + 4H₂O → 4Cu(OH)₂	(8)


	Fig. 27 (a) Uniform Cu₂O–CuO microcubes prepared by oxidation etching of Cu₂O microcubes under hydrothermal conditions. Reprinted with permission from ref. 183. Copyright RSC 2013. (b) Hollow Cu₂O–CuO truncated octahedral microstructures synthesized by annealing of the truncated octahedral [Cu₃(btc)₂]_n. Reprinted with permission from ref. 184. Copyright RSC 2013. (c) Hollow Cu₂O–CuO octahedral microstructure assembly of many nanoparticles prepared by annealing of the octahedral [Cu₃(btc)₂]_n. Reprinted with permission from ref. 185. Copyright RSC 2015. (d) Hollow Cu₂O–CuO microspheres synthesized by template-free hydrothermal treatment of copper precursors. Reprinted with permission from ref. 186. Copyright ACS 2007.

Although it is a general strategy for preparing uniform hollow cages of metal hydroxides by coordinating etching of Cu₂O templates at room temperature and then obtaining hollow metal oxide cages by simple thermal treatment of the as-prepared metal hydroxides, it is not ensured that hollow CuO or Cu₂O–CuO cages can be synthesized by this approach.²⁴⁴

In order to obtain hybrid Cu₂O–CuO nanostructures with high specific surface areas, a MOF-templated strategy is useful to achieve this purpose. For example, hollow Cu₂O–CuO truncated octahedral microstructures with porous shells can be synthesized by thermal decomposition of the coordination compound [Cu₃(btc)₂]_n (see Fig. 27b).¹⁸⁴ Furthermore, a hollow Cu₂O–CuO octahedral microstructure assembly of many nanoparticles was also prepared by annealing of the octahedral [Cu₃(btc)₂]_n (see Fig. 27c).¹⁸⁵ However, there is still much room for progress with regard to the development of new hollow Cu₂O–CuO cages based on this method. Additionally, a template-free hydrothermal treatment of copper precursors can be used to synthesize hollow Cu₂O–CuO microspheres (see Fig. 27d).¹⁸⁶

3.2.2 Hollow Cu₂O–Cu. Combining the advantages of the good conductivity of Cu with the semiconducting property of Cu₂O to prepare different hybrid Cu₂O–Cu composites can effectively enhance their electrochemical and photochemical properties. Although a number of hybrid Cu₂O–Cu composites have been synthesized in the past decades,^245–247 the synthesis of hollow Cu₂O–Cu still needs much more efforts. Hybrid Cu₂O–Cu hollow spheres with submicron diameters (see Fig. 28a) were fabricated by a microwave-assisted hydrothermal method using Cu(CH₃COO)₂, PVP and AA solution as precursors.²⁴⁸ Feng and coworkers have successfully developed an effective route to prepare hollow Cu₂O–Cu (see Fig. 28b) through over-reducing copper(II) precursors with EG by a facile solvothermal method. The morphology transformation from solid microcubes to hollow microspheres and composition changes from single Cu₂O crystals to hybrid Cu₂O–Cu can be achieved by increasing the reaction temperature and time.²⁴⁹ However, an in-depth investigation of the intrinsic properties of interfacial Cu–O atomic structures for improving the final performances is still imperative.


	Fig. 28 (a) Hybrid Cu₂O–Cu hollow spheres fabricated by a microwave-assisted hydrothermal method. Reprinted with permission from ref. 248. Copyright RSC 2015. (b) Hollow Cu₂O–Cu synthesized by solvothermal reduction of copper(II) precursors. Reprinted with permission from ref. 249. Copyright RSC 2012.

4. Applications

The synthesis of hollow Cu_xO micro/nanostructures with high porosities, uniform and well-defined morphologies, and mono-dispersed sizes would expand their potential applications, which can be attributed to higher specific surface areas and surface activities. Furthermore, the optimization of interfacial Cu–O atomic structures of hybrid Cu_xO can further offer a new pathway to improve their performances. Additionally, thin shells and hollow interiors can decrease the diffusion path lengths relative to conventional bulk ones. Consequently, the enhanced performances of hollow Cu_xO micro/nanostructures can afford many potential opportunities for their application in the fields of catalysis and sensing and as lithium-ion battery anode materials. In this section, we will review the detailed applications of hollow Cu_xO micro/nanostructures (including Cu₂O, CuO, Cu₂O–CuO as well as Cu₂O–Cu) in the above mentioned fields.

4.1 Catalysts

4.1.1 Photocatalytic degradation. As for the photocatalytic degradation of pollutants, hollow Cu_xO micro/nanostructures have been demonstrated to be low-cost and efficient because of the occurrence of strong oxidative species under visible-light irradiation. Moreover, hollow architectures would provide a platform for generating higher activity, in which the interiors and the surface porous shells can enhance the adsorption–desorption of reactants, charge-carrier mobility, and the redox potentials of photogenerated electrons and holes. Therefore, the photocatalytic activities of hollow Cu_xO micro/nanostructures are highly dependent on their larger specific surface areas and special exposed surfaces.

Mesopores in hollow photocatalysts can bring about larger specific surface areas and accelerate the adsorption of pollutants onto the catalysts, and shorten the diffusion path of free photogenerated charge carriers from the bulk phase to the surface, thereby increasing the separation efficiency of photogenerated charge carriers.^95,115 For example, a hollow Cu₂O octahedral assembly of nanowire building blocks (as shown in Fig. 15a and b) could display improved photocatalytic activities for dye degradation under visible light compared with the solid ones, probably because of the high-density photogenerated charge carriers from the branched nanowires with their special structures.⁹⁵ Notably, Cu₂O has been found to possess a much stronger adsorption ability for methyl orange (MO) than for methylene blue (MB), since MO experienced a greater decrease in absorption intensity with respect to that of MB under the same conditions after the adsorption–desorption equilibrium was reached. Hence, the rapid dynamic process of the adsorption of MB and the desorption of its oxidized form can partially be attributed to the higher rate of MB degradation compared with that of MO (see Fig. 29a and b).


	Fig. 29 Plots of concentration percentage of (a) MB and (b) MO in an aqueous solution against irradiation time in the presence or absence of Cu₂O photocatalyst samples. Reprinted with permission from ref. 95. Copyright Springer 2011. (c) Photocatalytic hydrogen production under visible light irradiation over Zn-doped Cu₂O. Reprinted with permission from ref. 103. Copyright ACS 2014. (d) Optimization of click reaction catalyzed by various CuO(II) nanostructures. Reprinted with permission from ref. 178. Copyright RSC 2010.

As reported, the {111} facets of Cu₂O possess surface copper atoms with dangling bonds, so that the {111} facets are higher in surface energy and expected to be more photocatalytically active than the {100} facets.⁶¹ As for a truncated octahedral architecture, the presence of more {111} facets can contribute to more efficient photocatalysis. However, a truncated octahedral Cu₂O is generally enclosed by 6 {100} and 8 {111} facets; the low-activity {100} facets would limit the wholly catalytic activity. A hollow Cu₂O truncated octahedral nanoframe composed of 8 {111} facets but without {100} facets (as shown in Fig. 16) is a typical example for achieving the construction of a catalyst with useful and active exposed surfaces. The photocatalytic degradation of the rhodamine B (RhB) activity of this hollow particle was higher than that of the solid particles, which can be attributed to the hollow structure with a larger surface area, because the hollow structure can allow multiple reflections of light within the interior and result in more efficient use of light and enhanced photocatalytic activity.⁹⁷ Similarly, the hollow CuO nanospheres with higher surface areas also exhibited superior photocatalytic degradation activities than the solid ones.¹⁷⁵

4.1.2 Photocatalytic hydrogen production. A p-type Cu₂O semiconductor has a CB edge of about −1.35 V (vs. Ag/AgCl, pH = 7), which is sufficiently more negative than the reduction potential of H₂O of about −0.61 V (vs. Ag/AgCl, pH = 7). This means a large overpotential is available for hydrogen production.^250–252 However, a direct report displaying photocatalytic hydrogen evolution from illuminated p-type Cu₂O has been far less than theoretically expected.²⁵³ Therefore, a major drawback of the Cu₂O photocatalyst is its photo-instability in aqueous solution during light illumination. In order to obtain stable Cu₂O photocatalysts, Guo and coworkers have developed a light-induced photochemical synthesis of hollow Cu₂O microcubes utilizing CuWO₄ as a precursor.¹⁰³ This synthetic strategy simulated the photocatalytic reaction conditions in which Cu₂O was used as a photocatalyst. The as-prepared hollow Cu₂O synthesized in such a strong redox photochemical reaction environment has the inherent advantage of withstanding photocorrosion in a kinetically more favorable way (see Fig. 29c). Furthermore, the modification of the band structure of hollow Cu₂O by zinc doping can be achieved to make it thermodynamically more favorable for photocatalytic hydrogen evolution.

Remarkably, the optical responses of hollow Cu₂O with controllable nanoshells can be fine-tailored in the visible spectral region by tuning the inner and outer radii of the spherically symmetric nanoshells, which was believed to be important to the optimization of Cu₂O-based photonic devices for photovoltaic and photocatalytic applications.^86,88 Therefore, it is necessary to carry out the synthesis/modification of hollow Cu₂O with fine-tuned shells. Briefly, hollow Cu_xO structures can effectively shorten the distance as well as the time for charge carrier transfer from the bulk to the surface and increase the specific surface areas to enhance the active sites to optimize the photocatalytic performances.

4.1.3 Organic synthesis catalyst. Previously, it has been demonstrated that Cu_xO micro/nanostructures were a good catalyst for organic synthesis. However, organic synthesis involving hollow Cu_xO catalysts are less reported. For example, hollow CuO nanostructures used as catalysts in a cycloaddition reaction of benzyl azide and phenylacetylene with in H₂O/t-BuOH afforded 1,4-disubstituted 1,2,3-triazoles as a single regioisomer. As shown in Fig. 29d, CuO hollow nanostructures have been used for the catalytic [3 + 2] cycloaddition of azides with terminal alkynes to provide the products in good yields with high regioselectivity.¹⁷⁸

4.2 Sensing

4.2.1 Gas sensing. Hollow Cu_xO micro/nanostructures have been widely investigated as a typical low-cost and high-sensitivity electrode material in gas sensing.^167,183,248 The conductivity of the p-type Cu_xO semiconductor is mainly attributed to the hole carrier density of the intergranular contract region.

For example, ethanol molecules can be adsorbed on the surface of hollow Cu_xO micro/nanostructures at higher reducing gas temperature, and then the injected electrons from ethanol molecules neutralized the holes and decreased the carrier density in the surface charge layer of the hollow Cu_xO micro/nanostructures. Thus, the adsorption of ethanol molecules would cause a marked increase of the resistance of Cu_xO sensors. Therefore, the surface accessibility and shell thinness are significant to keep the high sensitivity for the hollow architecture. Typically, the hierarchical Cu₂O microspheres with hollow and multilayered configuration are very suitable to sensing for ethanol molecules.¹⁰⁷ Similarly, hierarchical CuO microspheres with controllable building blocks also show suitable performances for the application in gas sensing. For instance, hierarchically porous CuO hollow spheres (see Fig. 20) can display an excellent sensing activity toward H₂S, such as low detection limit of 2 ppb, high sensitivity at parts per billion level concentration, broad linear range, short response time of 3 s, and recovery time of 9 s. The excellent performance was due to the unique hierarchically hollow architecture: the quasi-micropores provided active sites for effective sensing, the mesopores promoted the molecular diffusion kinetics, and the macropores acted as gas reservoirs and minimized diffusion length, while the good conductivity of the quasi-single-crystal nanosheet building blocks can be beneficial to fast charge transportation, finally leading to the high sensitivity, quick response and recovery of the H₂S gas (see Fig. 30a).¹⁶⁷


	Fig. 30 (a) Sensor responses obtained for the hierarchically porous CuO spheres at the working temperature toward 1 ppm H₂S and 100 ppm CH₄, NH₃, N₂, H₂, CO, and O₂. Reprinted with permission from ref. 167. Copyright ACS 2012. (b) The calibration curves of the sensor response versus CO concentration at the working temperature of 240 °C when using the Cu₂O–CuO microframes, the CuO microcubes and the Cu₂O microcubes. Reprinted with permission from ref. 183. Copyright RSC 2013. (c) Amperometric responses upon successive injections of glucose to 0.1 M NaOH at the mesocrystalline Cu₂O hollow nanocube modified glass carbon electrode. (d) Amperometric responses at the mesocrystalline Cu₂O hollow nanocube modified glass carbon electrode upon the periodic addition of H₂O₂ to 0.1 M PBS (pH 7.0) at −0.3 V. Reprinted with permission from ref. 99. Copyright RSC 2012.

As expected, the modification of interfacial Cu–O atomic structures toward hollow Cu_xO micro/nanostructures can be an effective strategy for improving the gas sensing performance. For example, the cubic Cu₂O–CuO hollow microframes (see Fig. 26a) exhibited enhanced gas sensing activities towards CO with a lower detection limit, higher sensitivity and shorter response time than those of the pure CuO microcubes and Cu₂O microcubes at the working temperature of 240 °C (see Fig. 30b), which was possibly attributed to the additional depletion layer formed between CuO and Cu₂O as well as the structural advantages of the hollow microframes.¹⁸³ The heterogeneous Cu₂O–Cu hollow spheres with a large amount of macropores and mesopores (see Fig. 27a) can offer sufficient space for the interaction between the spheres and the test gas, which was similar to that occurring in the above hierarchical porous hollow architectures.²⁵⁴ When the metal Cu came into contact with the semiconductor Cu₂O at thermal equilibrium, the electrons flowed from Cu to Cu₂O until the Fermi levels aligned at equilibrium because of their different work functions, which can lead to an excess of negative charge on the Cu₂O surface and the formation of a positive charge depletion zone in Cu near its surface. The depletion zone can promote the diffusion of holes from the Cu₂O to the Cu, which can be favorable to the oxidation of the NO₂ gas. In addition, the negative charges on the Cu₂O surface could facilely react with NO₂, resulting in the formation of adsorbed NO²⁻ and O⁻. Therefore, these reactions can decrease the electron concentration on the surface of Cu₂O and increase the hole conductivity in the Cu₂O semiconductor, which lead to a decrease of the resistance. This would benefit the fast adsorption and diffusion of NO₂ molecules and result in effective sensing properties.²⁴⁸

4.2.2 Biosensing. In the past decades, more and more attention has been devoted to the electrochemical biosensor because of its importance for reliable and fast determination in clinical diagnostics and the food industry. The multiple oxidation states of the Cu components in Cu_xO will provide intriguing biosensing performances to the redox reactions. In particular, hollow Cu_xO micro/nanostructures with excellent physicochemical properties are very beneficial to optimizing the biosensing performances.

As for hollow Cu₂O, the multi-shell hollow Cu₂O nanospheres with fine-tuned shells also possessed enhanced sensitivity and activity for non-enzymatic glucose sensing.⁹¹ The mesocrystalline Cu₂O hollow nanocubes can display high electrocatalytic activities for non-enzymatic amperometric detection of hydrogen peroxide and glucose (see Fig. 30c and d).⁹⁹ The double-shell Cu₂O hollow nanospheres exhibited high efficiency and convenience toward the detection of dopamine.¹⁰⁶ As for hollow CuO, the hollow CuO polyhedral 26-facet architectures exhibited high sensitivity, low detection limit, good stability and fast response towards the non-enzymatic oxidation of glucose.¹⁷⁹ Hierarchical hollow mesoporous CuO microspheres showed an excellent recyclable catalytic activity for the decomposition of hydrogen peroxide, and the catalytic efficiency was improved with the increasing special surface areas of the hollow microspheres.¹⁶⁹ However, the inherent conductivity of the Cu_xO semiconductor is not very good, which is unfavorable for charge separation and transfer and might affect the electrochemical performances for biosensors. Hybrid Cu₂O–Cu hollow spheres with conductive Cu composites can effectively improve the electrochemical performances toward the non-enzymatic oxidation of glucose,²⁴⁹ in which the intrinsic principle for enhanced performance might be similar to that occurring in a previous reference.²⁴⁸

So far, the interfacial chemistry of the Cu–O system on sensing properties is still controversial and further study is imperative. The design and synthesis of new hybrid Cu_xO-based hollow micro/nanostructures is necessary.

4.3 Anode material for lithium-ion battery

The use of CuO as electrode materials for rechargeable lithium-ion batteries (LIBs) has been intensively investigated due to its high theoretical capacity (670 mA h g⁻¹), high level of safety, non-toxicity and low cost. However, the large volume variation during the lithium uptake/release process and low conductivity will result in severe mechanical strains and very rapid capacity decay during the cycling process. Hence, to overcome such drawbacks and enhance the electrochemical performances, great effort has been made to develop hollow structural electrodes with outstanding performances and having properties such as shorter ion diffusion and larger contact areas between the electrode and the electrolyte compared to their bulk companions.

Currently, hierarchically hollow CuO nanostructures have been considered as promising candidates because of their high surface-to-volume ratio and the synergistic effect between the nanoscale building blocks and the porous pathways, which not only can keep the high activity of the CuO materials, but also offer an effective structure to relieve the volume variation during Li⁺ uptake/removal. For example, the urchin-like CuO nanoparticles can maintain a charge capacity above 560 mA h g⁻¹ at a current density of 150 mA g⁻¹ for up to 50 cycles (see Fig. 31a).¹⁷⁷ Pompon-like hierarchical CuO hollow microspheres assembled from porous nanorods exhibited excellent performance as anodes for LIBs, which can deliver a capacity of 683.7 mA h g⁻¹ at a current density of 67 mA g⁻¹ after 50 charge–discharge cycles (see Fig. 31b).¹⁷⁶


	Fig. 31 Cycling performance of the (a) urchin-like CuO, (b) pompon-like hollow CuO and (c) CuO/Cu₂O hollow polyhedron, respectively. Reprinted with permission from ref. 177. Copyright Wiley-VCH 2009. Reprinted with permission from ref. 176. Copyright RSC 2014. Reprinted with permission from ref. 184. Copyright RSC 2013.

Binary-component CuO–Cu₂O often integrates two types of functional materials that can display a strong synergistic effect, which will improve the intrinsic properties of each component such as electrical/ionic conductivity, electrochemical reactivity, and mechanical stability.^186,255 For example, hollow polyhedral Cu₂O–CuO with porous shells exhibited a reversible lithium storage capacity as high as 740 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 250 cycles even if the charge–discharge process was stopped for one week during the test time (see Fig. 31c).¹⁸⁴ The superior capacity might be attributed to the particular micro/nanostructures of hollow Cu₂O–CuO polyhedra as follows. First, the porous shells were convenient for the intercalation of Li⁺ ions into the active species and accelerating their diffusion velocity. Second, the hollow structures gave structural integrity with enough void spaces for buffering the volume variation during the redox reaction, thus alleviating the pulverization problem and achieving a long cycling life. Third, the hierarchical micro/nanostructures could reduce side reactions with the electrolyte, ensuring excellent electrical properties. Finally, as discussed in previous reports, the synergistic effect between the CuO and Cu₂O components can also be beneficial to the high capacity.

Based on the above discussion, it can be highlighted that hollow Cu₂O is a promising candidate for application in the fields of photocatalysis and sensing, while hollow CuO is widely used in the fields of organic synthesis catalysis and sensing and as anode material for LIBs.

5. Conclusion and perspectives

In summary, various kinds of hollow Cu_xO micro/nanostructures have been investigated and applied in the fields of photocatalysis and sensing and as LIBs owing to the large specific surface area and surface activity, the special morphology (thin shells and hollow interior), superior charge/mass transport and high permeability. The intrinsic property of the interfacial Cu–O atomic structure has also become an attractive field in Cu_xO-based studies in the past years. In this review, a comprehensive summary of the universal synthesis strategies and formation mechanisms, the interfacial Cu–O atomic structures as well as intrinsic properties, and their potential applications, have been specially demonstrated for different types of hollow Cu_xO micro/nanostructures. The recent advances in the controllable synthesis of hollow Cu_xO micro/nanostructures offer a good opportunity for designing and constructing high-performance architectures by fine-controlling the shell structures as well as the surface atomic/or interfacial properties.

In regard to the synthesis of new hollow Cu_xO micro/nanostructures, a significant task is to rationally design and tune the morphologies, surface atomic arrangements and intrinsic atomic/electronic structures. As reported, a facet-dependent property, especially introducing more high-activity exposed facets, could provide a top priority for improving the catalytic performances of inorganic crystals.^14–16 However, it has rarely been considered for the hollow Cu_xO micro/nanostructures with facet-dependent performances. Consequently, more efforts should be employed to facilely synthesize novel hollow Cu_xO micro/nanostructures with facet-dependent properties to uncover the relationship between intrinsic surface atomic structures and catalytic performances. In addition, using metal and/or nonmetal-doped copper precursors might be an effective way for designing and achieving the synthesis of hollow doped-Cu_xO micro/nanostructures. Notably, the stability of interfaces and the photo-corrosion of Cu₂O photocatalysts should be optimized and avoided. An appropriate protective layer coating on hollow Cu₂O micro/nanostructures is an effective approach for overcoming these drawbacks. Furthermore, light-induced photochemical synthesis of hollow Cu₂O micro/nanostructures is worth further developing, in which the as-synthesized Cu₂O would withstand photocorrosion in a kinetically more favorable way to improve the stability.

The application of hollow Cu_xO micro/nanostructures as hard templates or nanoreactors for preparing other hollow inorganic nanoparticles should be highlighted. In addition, hybrid hollow Cu_xO templates combined with a new template-based synthetic approach can provide a good idea to obtain a broad range of novel binary and ternary heterogeneous architectures, which would further enrich the template-synthesis of hybrid copper-based hollow nanostructures.

Concerning the modifications on the intrinsic properties of interfacial Cu–O atomic structures, in situ hybridizing (such as the oxidization or reduction of Cu_xO) is an effective strategy. Illuminating the interface-dependent intrinsic property to substantially optimize the potential performance is imperative. Selective hybridizing of active species to specific facets of hollow Cu_xO micro/nanostructures should attract intensive attention. Significantly, doped Cu_xO hollow micro/nanostructures with novel electronic structures acting as supports are promising for preparing highly effective architectures, but it is still in its infancy.

Briefly, the comprehensive and in-depth highlight on the universal synthesis strategies and formation mechanisms, the interfacial Cu–O atomic structures as well as intrinsic properties, and their potential applications, will promote the development of novel function-oriented Cu_xO-based hollow micro/nanostructures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation of China (NSFC No. U1502274, 51302213, 51272209, 51471132 and 51202191), the National High Technology Research and Development Program of China (863 Program, No. 2015AA034304), the Pivot Innovation Team of Shaanxi Electric Materials and Infiltration Technique (2012KCT-25), the Hundred Talent Program of Shaanxi Province, the Science Foundation of Jiangsu Province of China (No. BK20161250), and the Zhejiang Public Welfare Technology Research Industry Project (No. 2016C31G4181807).

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Hollow CuxO (x = 2, 1) micro/nanostructures: synthesis, fundamental properties and applications