Facet‐Controlled Synthetic Strategy of Cu2O‐Based Crystals for Catalysis and Sensing

Shape‐dependent catalysis and sensing behaviours are primarily focused on nanocrystals enclosed by low‐index facets, especially the three basic facets ({100}, {111}, and {110}). Several novel strategies have recently exploded by tailoring the original nanocrystals to greatly improve the catalysis and sensing performances. In this Review, we firstly introduce the synthesis of a variety of Cu2O nanocrystals, including the three basic Cu2O nanocrystals (cubes, octahedra and rhombic dodecahedra, enclosed by the {100}, {111}, and {110} facets, respectively), and Cu2O nanocrystals enclosed by high‐index planes. We then discuss in detail the three main facet‐controlled synthetic strategies (deposition, etching and templating) to fabricate Cu2O‐based nanocrystals with heterogeneous, etched, or hollow structures, including a number of important concepts involved in those facet‐controlled routes, such as the selective adsorption of capping agents for protecting special facets, and the impacts of surface energy and active sites on reaction activity trends. Finally, we highlight the facet‐dependent properties of the Cu2O and Cu2O‐based nanocrystals for applications in photocatalysis, gas catalysis, organocatalysis and sensing, as well as the relationship between their structures and properties. We also summarize and comment upon future facet‐related directions.


Introduction
In addition to the shapes of nanocrystals (NCs), their surface conditions (surface energies and electronic structures) also determine their physical and chemical properties. [ 1 ] Facets with distinctive crystallographic feature possess different atomic terminated characters, which have shown big differences in cataly sis and sensing. [2][3][4][5][6][7][8][9] Over the past decades, the brief discussion of solution phase synthetic strategy of the three basic Cu 2 O NCs ( c -Cu 2 O, o -Cu 2 O and d -Cu 2 O) and Cu 2 O NCs enclosed by high-index planes, as well as the key role of CA for controlling their crystallographic facets. We then introduce in detail the three main facet-controlled synthetic strategies (deposition, etching and template) on the Cu 2 O NCs to fabricate Cu 2 O-based NCs with heterogeneous, etched, or hollow structures, and discuss in detail a number of important concepts involved in those facet-controlled routes, including the selective adsorption of CA for protecting special facets, and the impacts of surface energy and active sites on reaction activity trends. Finally, we summarize the exciting facet-dependent properties of Cu 2 O and Cu 2 O-based NCs for applications of photocatalysis, gas catalysis, organocatalysis and sensing, as well as the relationship between their structures and properties. We expect that this review will inspire facet-controlled methodologies, and more examples of these facet-dependent properties should be continuously explored, endowing nanomaterials with excellent properties for numerous applications. low-index facets of Cu 2 O crystals, it is well established that the surface energy is closely related to the density of under-coordinated Cu atoms. [ 75 ] The atomic arrangements along three lowindex facets of Cu 2 O are illustrated in Figure 1 b-1 d. Only O atoms are terminated in the {100} facet, leading to electric neutrality (Figure 1 b). [ 44 ] By contrast, Cu atoms at the {111} facet are coordinated unsaturated. Each two Cu atoms have a dangling bond perpendicular to the {111} facet illustrated by the pink circles in Figure 1 c, which make them positively charged. [ 7 ] Similarly, the {110} facet has the same terminated Cu atoms with dangling Cu atoms (illustrated by the pink circles in Figure 1 d), while the number of dangling Cu atoms on {110} plane per unit surface area is approximately 1.5 times higher than that on {111} plane. [ 75 ] Thus, the {110} facet should be more positively charged than the {111} facet, and the surface energies of Cu 2 O are in the following order: However, the conditions of high-index facets of Cu 2 ( Figure 2 ). [ 135 ] Therefore, compared to low index {100} and {111} facets, the numerous kinks and steps endow those highindex facets with higher surface energies.
Cu 2 O crystals with clean facets were primarily synthesized through solution phase synthesis (hydrothermal and solvothermal process), [ 4,7,8,17,44,75,101,107,122,124 ] because that route could delicately tailor the exposed facets of crystals, through controlling the nucleation and growth behaviours (especially growth rates in different directions) of crystals. [ 136,137 ] The Wulff construction determines the equilibrium or natural morphologies of crystals, because minimizing the total surface energies mainly lead the shape evolution of crystal. [ 123 ] Based on the Gibbs-Wulff's theorem, the facets with higher surface energies always grow rapidly and fi nally decrease or vanish from the ultimate morphologies, while the crystal facets with lower surface energies grow slowly and are preserved in the fi nal structure. [ 5 ] However, selective surface stabilization of appropriate organic or inorganic additives (molecules or ions) as CA can effectively decrease the surface energies and retard the crystal growth along their normal orientations ( Figure 3 ). [ 5,7 ] CAs tend to selectively adsorb on the surface with higher surface energy, which consequently lead to delicately tuning of the percentages of different facets of crystals. [ 7,138 ] To date, CAs have played an important role on shape-controlled synthesis of NCs, [137][138][139][140][141][142] and there are many successful examples in preparing Cu 2 O NCs. [ 7,8,75,107,122,125,143 ] We will introduce some classic synthetic routes of Cu 2 O NCs enclosed by low-index facets. For instance, by using the preferential adsorption of polyvinylpyrrolidone (PVP) on the {111} facets, our group [ 7 ] successfully achieved the systematic morphology evolution from c -Cu 2 O to o -Cu 2 O ( Figure 4 a), which was in accordance with the identical evolution in shapes of cubicstructured crystal depending on the ratio R (the growth rate ratio of <100> to <111>). [ 144 ] The negatively charged O atoms of " C = O" in PVP (Figure 4 b) would strongly interact with the positively charged dangling Cu atoms on {111} facet to stabilize the crystal surfaces. The ratio of the surface area of {111} to {100} could be controlled by increasing the concentration of PVP (Figure 4 c). It is worth noting that {110} facets could not be obtained by only using PVP as CA. The reason is that the relatively strong adsorption of PVP is not enough to reduce the growth rate of {110} facets. Interestingly, L. Gao et al. [ 125 ] reported that, by employing oleic acid with stronger adsorption ability as the CA, rhombic dodecahedron Cu 2 O NCs totally enclosed by {110} facets could be obtained. With the increasing concentrations of oleic acid, the morphologies  [ 135 ] Copyright 2012, Royal Society of Chemistry. Figure 3. Illustration of facet-control of crystal facets by solvent and additive/impurity molecules or ions. Reproduced with permission. [ 5 ]  This suggested that a slower growth rate contributed to the generation of d -Cu 2 O. Furthermore, a slower growth rate, namely a kinetic-controlled process is essential for obtaining high-index facets. C. Wang et al. [ 17 ]   polyhedral NCs depending of the ratio R (the ratio of the growth rate along <100> to that of <111>), and the corresponding 3D structures. Scale bar = 300 nm. b) The molecular formula of PVP. c) PVP adsorption during the growth process of Cu 2 O NCs. Reproduced with permission. [ 7 ] Copyright 2009, Royal Society of Chemistry. Figure 5. SEM images and the corresponding geometry models with shape evolution from c -Cu 2 O to d -Cu 2 O. Reproduced with permission. [ 75 ] Copyright 2012, American Chemical Society. concentration of copper salts as well as a weak reducing agent contributed to the kinetic-controlled process, and the decreased viscosities caused by the extra ethanol may improve the diffusions of the reactants. Those above factors fi nally contributed to the generation of the novel confi gurations. So far, another shape of 50-facet Cu 2 O architectures with {311}, {522}, {211} facets, [ 135 ] 50-facet and 74-facet Cu 2 O polyhedra with {211}, {522} and {744} facets [ 117 ] and 30-facet Cu 2 O polyhedra with {332} facets [ 121 ] could also be obtained through different kinetic-controlled process by changing the concentration of reactants.
To sum up, by using CA or kinetic-controlled process, Cu 2 O polyhedra with smooth surfaces could be easily obtained, which lays a solid foundation for further tailoring and investigation of the facet-dependent performance.

Facet-Controlled Deposition
Recently, numerous studies are focused on the formation of heterogeneous structures by rational growing supported substances (typically noble metal nanoparticles) on the support (typically metal oxides), since metal oxides can not only serve as a support for a better dispersibility of disperse metal nanoparticles (NPs), but also enhance the catalytic abilities by interacting with the metal NPs. [ 6,27,30,33,36,40,50,61,[145][146][147] Despite many successful examples on the synthesis of heterogeneous structures, it is noteworthy that the spatially controllable deposition of noble metal NPs on metal oxide support is a signifi cant topic. For example, R. G. Li et al. [ 27 ] demonstrated that for the monoclinic BiVO 4 enclosed by {110} and {010} facets, photogenerated holes and electrons were transferred to the {110} and {010} surfaces for oxidation and reduction reactions respectively, due to the different energy levels of the two facets. When MnO x (oxidation co-catalyst) and Pt (reduction co-catalyst) were preferentially deposited by light-induced deposition onto the {110} and {010} facets of BiVO 4 , the performances of photocatalytic water splitting were signifi cantly improved. They further optimized the experiments to design two highly effi cient photocatalyst systems (M/MnO x /BiVO 4 and M/Co 3 O 4 /BiVO 4 , where M stands for noble metals). [ 6 ] Besides the intrinsic nature of separation of charge between the two facets, the synergetic effect of those catalysts also played a signifi cant role in enhancing photocatalytic performances. So far, lots of Cu 2 O-based heterogeneous structures have been reported, [ 33,36,40,50,61,70,[76][77][78]89,108,148 ] and the synthetic routes mainly focused on light-induced deposition [ 33,70 ] or galvanic deposition. [ 28,36,40,50,76,89,148 ] In this section, we plan to discuss the site-selective deposition of noble NPs on the preferential faces, edges, or corners of Cu 2 O crystals.
K. S. Choi et al. [ 126 ] synthesized o -Cu 2 O by employing the preferential adsorption of SDS on {111} facets ( Figure 6 a, left). They then demonstrated that the selective adsorption of SDS could be used for preferentially blocking the nucleation of Au NPs on these planes (Figure 6 a, right). [ 133 ] In the presence of SDS, Au NPs only electrodeposited on the {100} facets of truncated octahedral Cu 2 O; however, Au NPs would form on both {100} and {111} facets in the absence of SDS.
By contrast, galvanic or light-induced process can control the site-selective deposition in the absence of CAs. X. W. Liu [ 28 ] reported that during the in situ reduction of AuCl 4 − precursors, a galvanic process occurred that Au NPs selectively grew on {111} facets of Cu 2 O truncated octahedra and cubooctahedra (Figure 6 b), which can be formulated as shown in Eq. ( 1) :  Figure 6. a) Using selective adsorption of SDS for controlling morphology (left) and for Au preferential deposition (right). Reproduced with permission. [ 133 ] Copyright 2009, American Chemical Society. b) Selective growth of Au NPs on {111} facets of Cu 2 O microcrystals. Reproduced with permission. [ 28 ] Copyright 2011, American Chemical Society. c) Illustration of the shape evolution of the preferential growth of Au NPs on o -Cu 2 O. Reproduced with permission. [ 148 ] The galvanic deposition selectively occured on the {111} facet of Cu 2 O since that metallic component prefers to nucleate on highly active surface sites or defects with a large curvature, in which the {111} facet is more active than {100} facet due to the higher surface energy. By changing the concentration of the AuCl 4 − precursor, the density and size of Au NPs can also be controlled. Unlike galvanic deposition, light-induced deposition can lead to a distinct selectivity. After light illumination, the photogenerated electrons preferred to transfer from bulk to {100} surface of c -Cu 2 O, which was contributed to reduce metal ions to pure metal. In contrast, the photoexcited holes mostly accumulated on the {111} facet that inhibited the reduction of metal ions. [ 33 ] Edges and corners with a large curvature also play a key role in selective growth. M. L. Du et al. [ 148 ]

Facet-Controlled Etching
Recently, much effort is dedicated to a so-called "top-down" engineering approach that delicately modifi es crystals to create more highly active sites by etching and crystal cut, for the purpose of improvement the physical and functional properties of crystals. [ 43,46,51,68,[149][150][151][152][153][154] (In this section, the "top-down" means crystal carving without phase transformation; while the "topdown" in the next section refers to total phase transformation from Cu 2 O to various hollow structures.) To date, various metal or alloys (Ag, [ 151 ] Rh, [ 152 ] Pd, [ 153 ] Pd-Pt, [ 139 ] Pt, [ 155 ] and Pt x Ni y [ 150 ] etc.) and metal oxide (Cu 2 O, [ 42,44,60,66,85,110,113,130,132 ] TiO 2 , [ 13,51 ]   Reproduced with permission. [ 114 ] Copyright 2008, American Chemical Society. b) High-magnifi cation SEM image of a Cu 2 O nanoframe with empty {100} faces. Reproduced with permission. [ 42 ] c) The preferential adsorption between ethanol molecules and o -Cu 2 O crystals: apexes (pink) > edges (yellow) > facets (orange / pea green / black)". Reproduced with permission. [ 130 ] Copyright 2011, Royal Society of Chemistry. d) SEM images of Cu 2 O jagged polyhedrons. Reproduced with permission. [ 44 ] e) The morphological evolution of uniform Cu 2 O NCs in a weak acetic acid solution. Reproduced with permission. [ 132 ] Copyright 2011, American Chemical Society.  [ 46 ] and ZnO [ 154 ] etc.) NCs with sophisticated structures have been fabricated through a chemical "top-down" route. The fi rst step is partial dissolution of the mother-crystal, namely via a surface etching process, in which the etching agent (ions or molecules) chelates to exposed facets by cations, and then leads the chelated surfaces to dissolve. [ 5 ] A subsequent step of surface recrystallization on the residual surfaces of motherparticles may occur, which make the mother-particles roughen or convert to more stable facets. [ 5 ] In other words, if the surface recrystallization process does not happen, the continuous surface etching process would contribute to the transformation from the mother-crystal particles to hollow [ 42,88,110,113,114,150 ] or branch [ 41,151,152,155 ] structures.
In the absence of CAs, when many kinds of facets are exposed on the surface of a precursor, the etching will proceed with facet selectivity beginning with the facet(s) with the highest active sites. Although Ag 2 O has an identical cuprite crystal structure, the order of facet stability for Ag 2 O to chemical etching by NH 3 and NaOH is {111} > {110} > {100}. [ 31 ] The drastically different facet stability is caused by the pH of the reaction system.   In order to obtain more delicate structures, a "pre-synthesis strategy" has been widely used to carve NCs involving a twostage route where NCs acted as precursors for subsequent etching. [ 41,42,46,150,151 ] H. B. Yang et al. [ 42 ] reported other Cu 2 O nanocages and nanoframes with empty {100} facets from truncated octahedral Cu 2 O precursors (Figure 7 b). The capping PVP preferentially adsorbed onto the {111} facets of the Cu 2 O polyhedra and "freezes" the {111} planes; thus, the subsequent oxidative etching selectively occurred on the {100} facets. Similarly, S. D. Sun et al. [ 130 ] reported the branching growth of Cu 2 O NCs via selective oxidative etching with ethanol solution (Figure 7

Sacrifi cial Templates
Due to the large surface area, low density, good surface permeability, and high loading capacity, the shape-controllable synthesis of hollow/cage-like nanostructures, even with non-spherical shapes and regular interiors, has received extensive attention in recent years because of their widespread applications. [ 38,47,156 ] A templateassisted synthetic strategy is straightforward for the preparation of nanocages and the possible creation of nonspherical nanostructures. [ 47,49,157 ] The following steps occur during the template synthesis of cage-like/hollow nanostructures: i) synthesizing template, ii) using template to create target structure, iii) removing template (if necessary). [ 157 ] Recently, one "top-down" synthetic route has been extensively studied by using the low-cost and highly chemically reactive Cu 2 O NCs   (a2) is the corresponding SAED pattern. Reproduced with permission. [ 49 ] Copyright 2010, American Chemical Society. b) Schematic illustration of the formation of noble metal alloy mesocages from c -Cu 2 O. Reproduced with permission. [ 134 ] Copyright 2011, American Chemical Society. c) Schematic presentation of the i) PVP-Cu 2 O and ii) non-PVP-Cu 2 O etching reaction behaviour by metal(II) ions. Reproduced with permission. [ 166 ] Copyright 2013, Royal Society of Chemistry.

Galvanic Replacement
Galvanic replacement is an electro-chemical process, in which the sacrifi cial template is oxidized and dissolved in the solution; meanwhile another metal ion with a higher reduction potential would be reduced and deposited on the surface of the template, and fi nally inherits the original structure. [ 39 ] For example, due to the lower standard reduction potential of Cu 2+ /Cu 2 O (0.203 V vs standard hydrogen electrode (SHE)) than that of the Fe 3+ /Fe 2+ pair (0.77 V vs SHE), Fe(III) ions could instantly oxidize a Cu 2 O template at room temperature. This redox reaction is showed in Eq. ( 2) [ 49 ] : Amorphous Fe(OH) x nanoboxes ( Figure 9 a1) with thin and smooth shells perfectly duplicated the shape of c-Cu 2 O templates. After an annealing process, polycrystalline α -Fe 2 O 3 nanoboxes were obtained (Figure 9 a2). Fe(OH) x box-in-box structures could be created through further redox etching of the Cu 2 O/Fe(OH) x core/shell (Figure 9 a3). Due to the higher standard reduction potential of Pd 2+ /Pd (0.987 V vs SHE) and PtCl 6 2− /Pt (0.735 V vs SHE) pairs, Cu 2 O polyhedra could also use for the preparation of nonsperical metal mesocages. F. Hong et al. [ 134 ] synthesized noble metal alloy mesocages (Pd, Pt and Pt/ Pd) with many morphologies (cube, octahedron, "star"). Figure 9 b illustrates the generation process of metal mesocages from c -Cu 2 O.
Galvanic replacement has facet selectivity when the surface of template possesses more than one type of facet. [ 139 ] Similar to the etching process, galvanic replacement also begins with the facet(s) with the highest surface energy. Certainly, the surface energies of facets can also be altered and even reverse their order via the adsorption of CA. [ 39 ] Using PVP to stabilize the {111} facets of Cu 2 O truncated octahedra, Y. S. Kang et al. [ 166 ] obtained metal oxide hollow structures by controlling the galvanic replacement occurring on the {100} facets (Figure 9 ci). In contrast, galvanic replacement and subsequent selective deposition would happen on the {111} facet of Cu 2 O crystals without the protection of PVP, leading to the formation of hierarchical structures (Figure 9 cii).

The Kirkendall Effect
The Kirkendall effect is defined as the migration of the boundary layer between two materials when the two materials have different interdiffusion rates. Due to the faster diffusion rate, voids would be formed in the inner component, which is the most defining feature of the Kirkendall effect. [ 39 ] Over the past decade, the Kirkendall effect has become a promising route for creating micro-nano materials with hollow structures. [ 167,168 ] Compared to the mono-stoichiometric Cu 2 O, copper sulfides (Cu x S) at room temperature possess at least five stable phases: i.e., chalcocite (Cu 2 S), djurlite (Cu 1.95 S), digenite (Cu 1.8 S), anilite (Cu 1.75 S), and covellite (CuS). [ 93 ] Their unique electrical and optical properties derive from the valence states and complicated structures. [ 93,96 ] The Cu 2 O-template route (Figure 8 b) is a facile and straightforward by adding sulfur sources (i.e., Na 2 S solution, thioacetamide, and thiourea) into the Cu 2 O suspension, in which Cu 2 O template is transformed into Cu 2 O/Cu x S core/shell structures at once because of the minimal solubility product constant K sp of Cu x S ( K sp ≈ 10 −48 ). [ 93 ] Finally, the Cu 2 O core is dissolved completely, and the Cu x S shell is kept to the formation of hollow structures.
By using Cu 2 O crystals as templates, D. S. Xu et al. [ 93 ] fi rst created non-spherical Cu x S mesocages (including cubic, octahedral and multi-pod) with single-crystalline shells. Reproduced with permission. [ 93 ] b) TEM images of double-walled Cu 7 S 4 nanoboxes. Inset of (b) is the SAED pattern of the single nanobox. Reproduced with permission. [ 160 ] Copyright 2009, Royal Society of Chemistry. c,d) SEM and TEM images of a individual 26-facet Cu 7 S 4 microcage, and e) the corresponding simulated structure. Reproduced with permission. [ Guided by the above mechanisms, W. X. Zhang et al. [ 160 ] synthesized double-walled Cu 7 S 4 nanoboxes by two consecutive cycles that repeatedly produced Cu 7 S 4 layers in Na 2 S solution and dissolved the Cu 2 O core in NH 3 solution (Figure 10 b). Using polyhedral 26-facet Cu 2 O microcrystals as the templates, S. D. Sun et al. [ 161 ]

Coordinating Eetching
Coordinating dissolution is commonly used for dissolving insoluble materials. For instance, by using certain ligands (CN − , SCN − , S 2 O 3 2− , Cl − or NH 3 etc.) coordinate Cu 2 O polyhedra, various transition metal hydroxides, or oxides with hollow structures could be obtained, perfectly imitating the geometry of the Cu 2 O template. Z. Y. Wang et al. [ 48 ] The strategy was well designed by using Na 2   Well-defi ned MH nanocages (Mn(OH) 2 , Fe(OH) 2 , Co(OH) 2 , Ni(OH) 2 , and Zn(OH) 2 ) could be produced according the CEP route ( Figure 11 ). The as-prepared MH nanocages kept the shape of the o -Cu 2 O template with an edge length of ≈500 nm (Figure 11 x 1 ; x = a-e), and small NPs consisted of the MH shell ( Figure 11 x 2 ). TEM images of MH nanocages (Figure 11 x 3 ) clearly illustrated their hollow characteristic, and the SAED patterns (Figure 11 x 4 ) demonstrated their amorphous in nature. According to this strategy, amorphous Ni(OH) 2 nanoboxes [ 52 ] ( Figure 12 a), Co(OH) 2 /reduced graphene oxide [ 45 ] (Figure 12 b) and Ni-Co amorphous double hydroxides [ 35 ] (Figure 12 c,d) can also be obtained by minor revised this method, and illustrate excellent performances in the realm of sensor and energy.

Applications in Catalysis and Sensing
To date, the applications of Cu 2 O have mainly been in the realms of catalysis and sensing. In this section, we will focus on the facetdependent performances of the three basic Cu 2 O NCs and such Cu 2 O-based NCs for applications of photocatalysis, gas catalysis, organocatalysis, and sensing as well as the relationship between their structures and properties.

Photocatalysis
The requirement of sustainable energy and reduction of environmental pollution has driven considerable research efforts in photodegradation of pollutants and water splitting by employing abundant solar energy. [ 169 ] Cu 2 O with bandgap of 2.1 eV are expected to be promising materials in visible-light photocatalytic degradation, [ 69 ] and great studies have been devoted to the controlled synthesis of Cu 2 O with their morphology-dependent photo catalytic activities. [ 33,36,44,73,114,147 ] During the photocatalytic process, one of the key factors for the catalyst is "catching" the organic pollutants, since that would offer the catalyst more opportunities to contact and catalyze those pollutants. [ 44,75 ] Our group [ 7 ] demonstrated that the adsorption ability of methyl orange (MO), one of the industrial pollutants, to the different shapes of Cu 2 O NCs followed the sequence of octahedra > cubooctahedra > cubes. The exposed {111} facets of o -Cu 2 O had positively charged "Cu" atoms that inclined to interact with the negatively charged groups -SO 3 − in MO molecules. This suggested that Cu 2 O {111} facets would strongly interact with the molecules possessing negatively charged groups, and then effectively photodecompose these molecules; while the {111} facets interact weakly with the positively charged molecules, and lead to a poor photodegradation activities. As expected, M. H. Huang et al. [ 128 ] verifi ed that the photocatalytic activity of o -Cu 2 O was higher than that of c -Cu 2 O. Furthermore, the photocatalytic activities of extended hexapods Cu 2 O NCs with more {111} facets were more effective and active than o -Cu 2 O ( Figure 13 a). Subsequently, they synthesized d -Cu 2 O NCs with only exposed {110} facets, [ 75 ] which exhibited an excellent photocatalytic activities for the photodegradation of MO because of the high density of Cu atoms on the surface (Figure 13 b). T. R. Zhang et al. [ 135 ]   Reproduced with permission. [ 52 ] b) Typical TEM image of Co(OH) 2 /rGO with secondary structures. Reproduced with permission. [ 45 ] Copyright 2014, American Chemical Society. c) A typical TEM image of NiCo 2.7 (OH) x double hydroxides nanocages, and d) their corresponding EDS measurements. Reproduced with permission. [ 35 ] Inset of a-c) is the SAED pattern of each hydroxides nanocages.
A more effi cient photogenerated electron-hole (e − /h + ) pair separation would contribute to the improvement of photocatalytic activity. Besides the strong interaction between MO and the {111} corners and {110} edges of Cu 2 O jagged polyhedron ( Figure 14 a,b), the OH − ions also selectively adsorb onto these corners and edges with higher energy. Thus, a faster e − /h + separation will accelerate the production of the ·OH free radicals and then enhance their photocatalytic activities. Compared to the precursor of Cu 2 O truncated octahedron, the Cu 2 O jagged polyhedron displayed a better photocatalytic performance in the degradation of MO (Figure 14 c). [ 44 ] After 75 min, MO was only degraded to 60% by the Cu 2 O precursor, while MO was even degraded to 82% by jagged Cu 2 O.
Furthermore, another important factor for photodecomposition reactions is the rapid transportation to the surfaces of photogenerated charges. A Schottky barrier could be formed at the metal-semiconductor interface that reduces the recombination of the photogenerated e − /h + pairs, and then improve photocatalytic effi ciency. [ 30,33,36 ] Y. J. Xiong et al. [ 33 ] designed a p-type metal-semiconductor (Pd-Cu 2 O) heterostructure, and demonstrated that the synergistic effect between charge spatial separation and Schottky barrier contributed to the effi cient hydrogen production from pure water ( Figure 15 ). Due to the low work function of {111} facet, no Schottky barrier is formed at the Pd-Cu 2 O{111} interface; instead, an anti-blocking layer would be formed at that interface that increase the recombination of e − /h + pairs (Figure 15 a). In contrast, since the high work function of {100} facet, e − /h + pairs would be well separated at the Cu 2 O{100}-Pd interface (Figure 15 b). The hydrogen production of Pd-Cu 2 O cubes with proper Pd load capacity over 4 h was 2.20 mmol g −1 , which was dramatically higher than other Cu 2 O counterparts (Figure 15 c).

Gas Catalysis
Cu 2 O crystals have been actively studied in gas catalysis, and showed facet-dependent catalytic performance. [ 2,3,59,170 ] W. X. Huang et al. [ 2 ] evaluated the CO oxidation of uniform c -Cu 2 O and o -Cu 2 O. HRTEM images ( Figure 16 a, W. X. Huang et al. [ 3 ] further reported the facet-dependent performances in catalyzing propylene oxidation by using CAfree c -Cu 2 O, o -Cu 2 O, and d -Cu 2 O. c -Cu 2 O enclosed by {100} facets were most selective for CO 2 ; o -Cu 2 O exposed {111} facets were most selective for acrolein; d -Cu 2 O enclosed by {110} facets were most selective for propylene oxide ( Figure 17 a). All the three Cu 2 O NCs became active at 170 ºC, and the conversion rate of C 3 H 6 rose with the increase of reaction temperature ( Figure 17 b1-b3). In addition, the effi ciency of C 3 H 6 conversion was followed the order o -  . "Cubo-oct" denotes cubo-octahedron, and the concentrations represent the molar ratio of Pd/Cu 2 O. Reproduced with permission. [ 33 ] with an adsorption energy ( E ads ) of -1. . The E ads for the two sites of (110) facet was similar.
In addition, different SI produced on each Cu 2 O NCs. On the Cu 2 O (111) surface, the distance of C=C bond ( d C=C ) in Cu CUS -C 3 H 6 (a) was calculated to be 1.37 Å, while d C=C in C 3 H 6 molecule was 1.34 Å. Therefore, the stable C=C bond of Cu CUS -C 3 H 6 (a) can be kept in the subsequent reactions, which was in favor of the generation of acrolein. On the Cu 2 O(100) surface, d C=C in O CUS ,O CUS -C 3 H 6 (a) was 1.59 Å; hence, the weakened C=C bond would be cleaved by propylene combusting. DFT calculation results ( Figure 18 a) [ 2 ] combustion, which was in accordance with the experiments of c -Cu 2 O. On the Cu 2 O(110) surface, d C=C in O CUA -C 3 H 6 (a) was 1.36 Å, which was considered to the generation of acrolein; while the weakened C=C bond in Cu CSA ,O CUS -C 3 H 6 (a) with a distance of 1.50 Å inclined to break during the reactions. DFT calculations results (Figure 18 b) suggested that the E a for the combustion of O CUA ,O CUS -C 3 H 6 (a) to adsorb C 3 H 6 O(a) and the disintegration of O CUA ,O CUS -C 3 H 6 (a) into adsorbed CH 2 (a) and CHCH 3 (a) was 1.28 and 2.08 eV, respectively. This result suggested that propylene was in favor of epoxidation, which was in accordance with the experiments of d - Compared to low-index facets, the high-index facets have higher catalytic activities due to the more atomic steps and kinks. C. Wang et al. [ 17 ] evaluated the CO oxidation of a series of

Organocatalysis
Numerous important products (optical devices, drugs, materials, etc.) commercialized or in the stage of development, have aromatic C N and aromatic C C bonds that can be coupled by organocatalysts through cross-coupling reactions. [ 171 ] Thus, scaling up production of these bonds with any novel and basic technology is greatly signifi cant for industry. [ 171 ] Over the last decade, the research focus on coupling of C N and C C bonds has gradually moved from the high-cost Pd-catalyst to the lowcost Cu-catalyst. [ 172,173 ] Recently, Cu 2 O (NC form or bulk) has been reported as excellent catalysts for cross-coupling reactions. [ 14,24,32,40,66 ] The facet-dependent organocatalysis activity of c -Cu 2 O, o -Cu 2 O, and d -Cu 2 O NCs was fi rstly evaluated by M. H. Huang et al. [ 24 ] based on the synthesis of 1,2,3-triazoles [ 14 ] and the regioselective synthesis of 3,5-disubstituted isoxazoles. [ 24 ] To compare the catalytic activities of each Cu 2 O NCs, all the three NCs were used with identical surface area ( Table 1 ). d -Cu 2 O displayed the most effi cient catalytic activity, with shortest reaction times and the highest product yields, followed by o -Cu 2 O and the least active c -Cu 2 O. These results demonstrate that delicate facet controlling of Cu 2 O NCs can greatly improve the organocatalytic effi ciency. Subsequently, M. H. Huang et al. [ 32 ] developed a capping-free synthetic approach for the synthesis of sub-100 nm Cu 2 O NCs with morphology evolution from c -Cu 2 O to o -Cu 2 O. All the Cu 2 O NCs illustrated high yields  ). White, red, gray, green and pink balls behalf H, O, C, coordinatively unsaturated Cu, and coordinatively saturated Cu, respectively. Reproduced with permission. [ 3 ] www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2015, 2, 1500140 within short reaction times. o -Cu 2 O was the most excellent catalyst that could catalyse the cycloaddition reaction in just 2 h with high yields.
L. L. Li et al. [ 66 ] employed monodisperse c -Cu 2 O, d -Cu 2 O, and octadecahedra to catalyse aerobic oxidative coupling of phenylacetylene and arylboronic acids. During the catalytic reaction, those NCs showed high yields but different crystal surface stability. After three catalytic cycles, c -Cu 2 O were seriously etched and aggregated beyond recognition, as well as their yield dropping from 94% to 32%. By contrast, no change was observed in the {110} facets of d -Cu 2 O and octadecahedra during the reaction. Interestingly, Cu 2 O octadecahedra had the best catalytic activity upon recycling, and the yield of aerobic oxidative coupling was increased from 89% to 97%. The reason was that the {100} facets of Cu 2 O octadecahedra were more prone to etching than the {110} facets during this catalytic reaction, and the Cu 2 O octadecahedra were gradually oxidized and etched to high-active concaves ( Figure 20 a). However, the indepth etching mechanism still requires further study. A metalmetal oxide interface formed after deposition of noble metal onto metal oxide, and the hybrid structure displayed superior catalytic performances to the physical mixtures or single dom ains. [ 30,33,36,40,50,145 ] L. L. Li et al. [ 40 ] further improved the experimental route by selective depositing noble metals on the concave Cu 2 O NCs. Pd atoms only grew on cavities (Figure 20 b, inset), but Ag 0 majorly nucleated on edges and vertices (Figure 20 c, inset). During the aerobic oxidative arylation of phenylacetylene, the hybrid nanoconcaves exhibited more excellent catalytic activities than the single component or physical mixtures (Figure 20 b,c). XPS spectra combined DFT calculation results verifi ed the improvement of catalytic activities attributed to the synergistic effect, in which e − migrated from the noble metal to Cu 2 O.

Sensing
H. C. Zeng et al. [ 8 ] evaluated the ethanol sensing ability of Cu 2 O self-assembled 3D superlattices (≈10 nm) and disassembled nanocubes (≈20 nm), and the corresponding TEM images were shown in Figure 21 a, b, respectively. The organized Cu 2 O illustrated a better sensing capability than the  [ 17 ] Copyright 2010, American Chemical Society. b) CO conversion of Cu 2 O microcrystals of different shapes. Reproduced with permission. [ 121 ] Copyright 2013, Royal Society of Chemistry. − , and O 2− are produced. A localized accumulation of holes were formed that separated from e − near the surfaces of Cu 2 O. When Cu 2 O was exposed to ethanol/air mixtures, e − was produced from the redox reactions between adsorbed O 2 on the surface and ethanol that would be transferred into conduction band of Cu 2 O. And then, e − and h + would be recombined that lead to decrease the concentration of carrier. Due to the higher ratio of surface to bulk, a greater carrier depletion layer would be formed of the self-assembled c -Cu 2 O when exposed to ethanol. That layer, in cooperation with a relatively small contact potential, would result in a distinct improvement of sensitivity (Figure 21 d).
Our group [ 60 ] evaluated the CO sensing performance of Cu 2 O-CuO composite microframes at the working temperature of 240 ºC. As the concentration of CO increased, the Cu 2 O-CuO composite microframes illustrated excellent CO sensing performance with highest sensitivity and shortest response time, followed by the pure CuO microcubes and the pure Cu 2 O microcubes ( Figure 22 ).

Conclusion and outlook
Through numerous examples, we have demonstrated that facetcontrolled synthetic strategies provide remarkably facile and convenient approaches to the preparation of Cu 2 O-based NCs with heterogeneous, etched, or hollow structures. These routes depend on the different surface atomic structure of Cu 2 O NCs, in which the selective adsorption of CAs could protect special Figure 21. TEM images of a) Cu 2 O self-assembled 3D supercrystals and b) disassembled nanocubes. c) Their corresponding sensitivities toward ethanol sensing measured under identical situations. d) The schematic diagrams of 1D array of c -Cu 2 O toward ethanol sensing, where E F , E V , and E C are Fermi energy, valence band energy, and conduction band energy, respectively. Reproduced with permission. [ 8 ] Copyright 2010, American Chemical Society. facets, and the surface energy and active sites would determine the reaction activity trend. The facet-dependent properties of the Cu 2 O NCs and such Cu 2 O-based NCs have been investigated, especially in the realm of photocatalysis, gas catalysis, organocatalysis and sensing. Due to different crystal surface structures, the Cu 2 O NCs exhibit distinct facet-dependent properties; a subsequently rational design and synthesis of Cu 2 O-based NCs could tailor and optimize their facet-dependent performances. Although the controllable synthesis of NCs and their derivatives has seen considerable progress and development in the last decade, the next requirements for NCs with excellent performance are the development of more simple and convenient synthetic routes to tailor NCs with ideal components and structures.
The progress we have summarized also opens the door for in-detail studies in catalysis and sensing. Due to their well-defi ned facets, shape-controlled NCs can provide smooth surfaces for further delicate carving, modifying, or transforming; a deep insight into the relationship between structures and properties will be obtained by combining with theoretical calculations and simulations of the catalysis and sensing process. This perception will enable further delicate tailoring of NCs, and bridge the gap between structures and properties, so that traditional trial-and-error pattern to obtain functional NCs would be instead replaced by ingenious design and controllable synthesis.
Finally, it is should be envisaged that the facet-dependent properties of Cu 2 O-based NCs could also apply to other realms. For instance, our group [ 92 ] found that o -Cu 2 O NCs displayed a higher oxidative stress to D. magna than that of c -Cu 2 O due to its higher reactivity. M. H. Huang et al. [ 9 ] investigated the facet-dependent electrical properties of the three basic Cu 2 O NCs. o -Cu 2 O is highly conductive, c -Cu 2 O is moderately conductive, and d -Cu 2 O is nonconductive. A thin surface layer having different degrees of band bending contributed to the different conductivities. Interestingly, a diode-like response was obtained when electrical connection was made on two different facets of a rhombicuboctahedron. D. F. Xue et al. [ 25 ] evaluated Li-ion battery anode performances and showed that c -Cu 2 O had the highest capacity among Cu 2 O polyhedra (the sequence of electroactivity is {110} < {111} < {100}), because {100} facets had high electroactivities toward redox reactions. Thus, more instances of the facet-dependent properties should be continuously explored, endowing nanomaterials with excellent performances for numerous applications.   [ 29 ]