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FEATURE ARTICLE

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Facet-dependent properties of polyhedral nanocrystals Michael H. Huang,* Sourav Rej and Shih-Chen Hsu Syntheses of metal and oxide nanocrystals with cubic crystal structures and well-controlled polyhedral morphologies such as cubic, octahedral, and rhombic dodecahedral shapes exposing, respectively, {100}, {111}, and {110} surfaces enable a more accurate determination of their facet-dependent properties. So far molecular adsorption, photocatalytic, organocatalytic, and electrical conductivity properties have been

Received 8th November 2013, Accepted 10th December 2013 DOI: 10.1039/c3cc48527g

demonstrated to be surface-related or facet-dependent. Chemical etching and metal nanoparticle deposition can also be face-selective. Examples of these surface properties are presented. In general, ionic solids such as Cu2O nanocrystals exhibit more sharply different surface properties than those seen in metal nanoparticles. A better understanding of these facet-dependent properties is necessary to prepare

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nanomaterials with enhanced properties such as their catalytic activities.

Introduction Advances in the synthesis of polyhedral nanocrystals with excellent shape control, particularly methods enabling the growth of nanocrystals with systematic shape evolution, have offered insights into factors which are effective in tuning the particle morphology.1 Various polyhedral metal nanocrystals including Au, Ag, Pd, and Pt have been synthesized.2–8 For metal oxide and semiconductor systems, Cu2O, Ag2O, PbS, and PbSe Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: [email protected]

Michael H. Huang obtained his BA degree in chemistry from Queens College in 1994, and his PhD degree from the Department of Chemistry and Biochemistry at UCLA in 1999. After postdoctoral research at UC Berkeley and UCLA, he joined the Department of Chemistry at NTHU in 2002. He was promoted to associate professor in 2006, and then to professor in 2010. His current research focus is on the Michael H. Huang shape-controlled synthesis of nanocrystals and the examination of their facet-dependent properties. He has received a number of awards, including the Outstanding Research Award from the National Science Council of Taiwan in 2012.

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nanocrystals with systematic shape control have been achieved.9–18 Nanoparticles of these face-centered cubic metals, as well as metal oxides and chalcogenides with a cubic crystal structure, expose different surface facets depending on the particle shapes. Cubic, octahedral, and rhombic dodecahedral crystals are bound by the {100}, {111}, and {110} low-index facets, respectively. If nanocrystals exposing only a single set of surface facets are available, facet-dependent properties can be evaluated more accurately. Particles exposing two or more surface planes such as cuboctahedra and truncated octahedra can also be examined, when their properties are compared to the monofaceted particles. The most widely studied facet-dependent

Sourav Rej

Sourav Rej received a BSc degree from Burdwan University, India, in 2009 and a MSc degree from Indian Institute of Technology Roorkee, India, in 2011. He is currently pursuing his PhD degree from National Tsing Hua University, Taiwan, under the supervision of Prof. Michael H. Huang. His research interests include shape-controlled synthesis of nanocrystals and their catalytic applications.

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property is the catalytic activity of nanoparticles toward a particular reaction.19 This is understandable because catalytic properties of nanocrystals are highly related to their exposed surfaces, and such experiments also show the importance of fine particle morphology control. In addition to the catalytic properties, molecular adsorption also responds differently to different particle surfaces, largely due to differences in the surface charges of metal oxide crystals.10,11,13 There are also reports addressing face-selective etching and deposition of polyhedral nanocrystals, showing that surface conditions such as charges and preferential molecular adsorption on selected faces can be harnessed to form unique hollow structures and composite materials.20–24 Another interesting and significant material property discovered recently is the facet-dependent electrical conductivity behavior observed in cuprous oxide crystals.25 Most of these facet-dependent properties have been discovered as a result of successful synthesis of nanocrystals with a series of well-defined morphologies. More examples of these facet-dependent properties should continue to emerge with research along this direction, offering materials with superior properties for various applications. In this feature article, we briefly describe the syntheses of metal and semiconductor nanocrystals with systematic shape control. Since several facet-dependent properties of nanocrystals are linked to molecular adsorption on the particle surfaces, the observed facet-dependent molecular adsorption effects are introduced first. Facet-dependent photocatalytic and organocatalytic activities of nanocrystals are presented next. Discussion on face-selective etching and metal deposition reactions on polyhedral nanocrystals follows. Lastly, novel facet-dependent electrical conductivity properties of Cu2O crystals are presented. A concluding remark on possible future research directions is also given.

Synthesis of nanocrystals with systematic shape evolution Syntheses of metal, metal oxide, and metal sulfide nanocrystals with systematic shape evolution by adjusting only the amount of a single reagent or tuning the reagent molar ratios are highly desirable, because the nanocrystals are prepared under essentially the same solution conditions, and this facilitates the understanding

Shih-Chen Hsu received his BS degree in 2007 and MS degree in 2009 in chemistry from Chung Yuan Christian University. He is currently pursuing his PhD degree under the supervision of Prof. Michael H. Huang at National Tsing Hua University. His research involves shape-controlled synthesis of nanocrystals and the examination of their optical, electrical, and photocatalytic properties. Shih-Chen Hsu

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of factors controlling the particle morphologies.1,26 The identical particle surface environment such as the solvent and the surfactant/ capping agent used also reduces the possibility of such differences affecting the accurate evaluation of facet-dependent properties, if various particle shapes are obtained via different synthetic routes. Au,2–4,27 Ag,5,6 Pd,7,28 and bimetallic core–shell nanocrystals29–34 have been synthesized with systematic shape evolution. For metal oxide and chalcogenide nanocrystals synthesized with systematic shape evolution, Cu2O, Ag2O, and PbS are the best demonstrated systems as mentioned earlier. Interestingly, Au–Cu2O core–shell nanocrystals with systematic shape control from cubic to octahedral structures synthesized using octahedral gold cores have also been prepared.35,36 Two examples of the synthesis of nanocrystals with systematic shape evolution are illustrated here. These synthesized nanocrystals have been used for the facet-dependent catalytic activity studies. Fig. 1 shows a schematic drawing of the procedure used to make gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures and the SEM images of the synthesized gold nanocrystals. The nanocrystals were prepared using a seed-mediated synthesis approach. Seed particles were synthesized first and transferred to growth solutions for development into particles with well-defined final morphologies. By adjusting the volume of KI solution added, Au nanocrystal shapes can be systematically changed to expose entirely {110} or {111} facets. Au nanocubes can be synthesized by using a similar synthetic method.2 The nanocrystals have excellent size and shape uniformity, such that they readily self-assemble into ordered packing arrangements. At high nanocrystal and sufficiently high surfactant concentrations, the particles can form supercrystals with polyhedral shapes on substrates.37 It is worth noting that the slight changes in the volume of dilute KI solution added are actually effective in controlling the final particle morphologies. The stronger binding of iodide than chloride to the metal ions should lead to partial ligand replacement on the Au precursor, and this causes changes in the reduction potential of the Au precursor and hence the reduction rate or particle growth rate.1 This is the role of iodide in the formation of Au nanocrystals with systematic shape evolution. Fig. 2 presents the synthetic procedure for the growth of Cu2O nanocrystals with morphology changes from cubic to edge- and corner-truncated octahedral and finally rhombic dodecahedral structures and their SEM images.11,38 Cu(OH)2 should form first and is reduced to give Cu2O crystals. By only adjusting the volume of reducing agent NH2OH
HCl introduced, Cu2O nanocrystals with systematic shape evolution were achieved. By visually monitoring the solution color changes, cubes are formed at a much faster rate than rhombic dodecahedra; the solution turns from blue to orange in less than 1 min in the growth of cubes, but the same solution color change takes around 20 min in the preparation of rhombic dodecahedra. The results again show that the formation of different particle morphologies are linked to their different growth rates.11 Sharp-faced Cu2O cubes, octahedra, and hexapods can be obtained using the same reagents and synthetic procedure, but the volume of CuCl2 solution added is less than that shown in Fig. 2.10 Both the

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Fig. 1 Schematic drawing of the procedure used to make gold nanocrystals with a systematic shape evolution from rhombic dodecahedral to octahedral structures by decreasing the volume of KI solution added. Approximate solution colors are displayed. Amounts of reagents used in the preparation of growth solutions are listed. AA and CTAC refer to ascorbic acid and cetyltrimethylammonium chloride. SEM images of the Au nanocrystal products formed are also shown. Reprinted with permission from ref. 3. Copyright 2011 Wiley-VCH.

octahedra and hexapods are bound by the {111} facets. These Cu2O nanocrystals are important for the illustration of facetdependent properties because semiconductor crystals normally exhibit more drastically different surface properties than those seen in metal nanoparticles. Sub-micrometer-sized Ag2O crystals with systematic shape evolution from cubic to octahedral and hexapod structures can be synthesized by fixing the molar ratios of AgNO3, NH4NO3, and NaOH at 1 : 2 : 11.8, while changing their concentrations in the solution.13 By lowering the amount of AgNO3 added and raising the volume of NaOH used, interesting octapods can be made. The Ag2O octapods are derived from cubes with perpendicularly crossed depressions over each of the six faces of a cube and a greater depression in the central region of each {100} face (see Fig. 3). Because each corner of a cube can be considered as a protrusion, the particles are called octapods. They are also primarily enclosed by the {100} facets. Face-etched Cu2O cubes with an octapod morphology have also been synthesized.39

Interactions of charged molecules with metal oxide crystals exposing different surface facets Interesting facet-dependent surface properties of metal oxide crystals were uncovered when Ag2O and Cu2O crystals were dispersed in solutions containing positively or negatively

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charged molecules.10,11,13 Very different dispersion behaviors of the crystals were observed depending on the exposed surfaces of the oxide crystals and the molecular charges. When Ag2O cubes, octapods, octahedra, and hexapods are dispersed in a solution containing positively charged methylene blue and stirred for some time, cubes and octapods enclosed by the {100} faces stay in the solution, but octahedra and hexapods bound by the {111} facets floated to the top of the solution (Fig. 3). When these Ag2O crystals are dispersed in a solution of negatively charged methyl orange, this floating phenomenon was not seen. A similar particle floating behavior was observed when Cu2O octahedra, hexapods, and rhombic dodecahedra were dispersed in a methylene blue solution (Fig. 3). The effects can be explained by considering the surface charges of the different facets of Ag2O and Cu2O. Cu2O and Ag2O have the same cuprite crystal structure, in which copper or silver atoms form a body-centered cubic unit cell lattice, and oxygen atoms occupy half of the tetrahedral sites (Fig. 4).40 The (110) plane contains fully exposed copper or silver atoms, while the (111) plane contains some exposed and some sub-surface copper or silver atoms. These planes are therefore more positively charged. On the other hand, the (100) plane is terminated by oxygen atoms, although the (100) plane can also be presented to expose terminal Cu atoms.41 This renders the (100) plane neutral or slightly negatively charged. The repulsive electrostatic interactions between the positively charged methylene blue molecules and Ag2O/Cu2O particles bound by the {111} or {110} facets are quite strong that the particles cannot be well dispersed in this

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Fig. 2 Schematic illustration of the procedure used to grow Cu2O nanocrystals with various shapes. SDS refers to sodium dodecyl sulfate. SEM images and the schematic drawings of the Cu2O nanocrystals synthesized with shape evolution from cubes to rhombic dodecahedra upon increasing the amount of NH2OH
HCl added to the reaction mixture are shown. Reprinted with permission from ref. 11. Copyright 2012 American Chemical Society.

solution, but float to the top of the solution. This is a manifestation of the dramatic facet-dependent surface properties. In addition to the surface effects from electrostatic interactions, a particular molecule can also have different binding energies to different surface planes of a material.42,43 Thiolphenol was calculated to show substantially different binding energies to (100), (110), and (111) planes of gold.42 Thiolphenol has the largest binding energy to the (110) plane of gold. At extremely low thiolphenol concentrations, preferential molecular adsorption on selected planes can be important. When cubic, octahedral, and rhombic dodecahedral gold nanocrystals were used as substrates

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for surface-enhanced Raman scattering (SERS) detection of thiophenol, rhombic dodecahedra were found to be the most sensitive substrates reaching a detection limit of 10 8 M.

Facet-dependent photocatalytic and organocatalytic activities of nanocrystals The successful preparation of metal and metal oxide nanocrystals with simple polyhedral shapes and preferably nanocrystals

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Fig. 3 Photographs of methylene blue solutions taken after dispersing Ag2O (a) octapods, (b) cubes, (c) octahedra, and (d) hexapods with stirring for 60 min. (e) Photograph of a methylene blue solution mixed with rhombic dodecahedral Cu2O nanocrystals and stirred for 30 min. SEM images of the respective particles are shown below the photographs. Scale bars are all equal to 100 nm. Remarkably, Ag2O octahedra and hexapods and Cu2O rhombic dodecahedra have floated to the top of the solution due to repulsive electrostatic interactions between the positively charged methylene blue molecules and the positively charged {111} and {110} faces of Ag2O and Cu2O crystals. Reprinted with permission from ref. 11 and 13. Copyright 2012 American Chemical Society and 2010 Wiley-VCH.

exposing only a single set of surface planes allows the examination of their facet-dependent photocatalytic and organocatalytic activities with greater accuracy. It is especially desirable to compare all three low-index facets if these particles are available for better understanding and explanation of the observed relative catalytic activities. For a reaction to take place, molecules need to adsorb on the particle surfaces. Surface energy, surface charge, the number of active catalytic sites, and binding energy of molecules to a particular surface are relevant factors to consider in evaluating the relative catalytic activities of different crystal faces. For photodegradation and photo-catalyzed reactions, the efficient transport of photogenerated charge carriers to the particle surfaces should be important. Facet-dependent photodegradation of negatively charged methyl orange is most widely studied for the photocatalytic activity investigation of Cu2O crystals.9–11,25,36,44–47 In some of these studies, particles with all three low-index facets and highindex facets have been examined.45–47 With mixed facets, it is not possible to clearly identify the most active crystal face. Particles with high-index facets generally show an enhanced catalytic activity. The synthesized Cu2O cubes, octahedra, rhombic dodecahedra, and Au–Cu2O core–shell cubes and octahedra with sharp faces were used to examine their relative photocatalytic activity toward photodegradation of methyl orange by irradiating the solution with a mercury lamp.10,11,36 Fig. 5 summarizes the experimental results. Note that the amounts of crystals and methyl orange used are not always the same in these studies. Octahedra and hexapods enclosed by the {111} surfaces are moderately photocatalytically active, while cubes bound by the {100} faces are simply inactive. Cuboctahedra exposing both {111} and {100} facets give an intermediate performance. Rhombic dodecahedra are the best catalysts, completing the photodegradation of methyl orange efficiently. The results clearly demonstrate that photocatalytic activity is

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Fig. 4 Crystal structure models of cuprite Cu2O showing the (a, b) {100}, (c, d) {111}, and (e, f) {110} surfaces. Oxygen atoms are shown in red, while copper atoms are shown in white. Two different viewing angles are presented for each surface to more clearly show the number of surface atoms. For panel (b), the uppermost layer of oxygen atoms has been removed to clearly show the number of surface copper atoms. Panel (d) shows the {111} planes rotated 631 with respect to the model shown in panel (c). The blue triangle in panel (d) encloses the same area as that shown in panel (c). The cubic unit cell parameter is a. Reprinted with permission from ref. 39. Copyright 2013 Wiley-VCH.

highly dependent on the exposed surface facets. When dispersing the Cu2O crystals in a methylene blue solution, all particles were inactive. The inactivity of octahedra is reasonable because they floated to the top of the solution, but cubes and cuboctahedra can be dispersed in the methylene blue solution. Upon photoirradiation, the photogenerated electrons and holes in the bulk of a Cu2O semiconductor crystal should migrate to the surface and react with adsorbed molecules to produce radical species and decompose the molecules. It can be assumed that the charge carriers reach the particle surfaces and are transferred to the adsorbed molecules most efficiently if the surfaces are terminated with the (110) planes. The (111) planes are less efficient in this charge transport process, so photocatalytic activity of octahedra is moderate. While in principle electrons and holes should migrate to the surfaces of Cu2O cubes, and molecules should still adsorb on the cubic crystal faces, the inactivity of the cubes suggests that electrons and holes have difficulty reaching the {100} surfaces. Results from the electrical conductivity measurements on single Cu2O cubes and octahedra described later imply that the {111} and {100} faces have different

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Fig. 5 (a–c) Plots of the extent of photodegradation of (a, c) methyl orange and (b) methylene blue vs. time for the various Cu2O nanostructures. The blank sample did not contain Cu2O crystals but only the methyl orange or methylene blue solution. (d) A plot of the extent of photodegradation of methyl orange vs. time using various pristine Cu2O nanocrystals and Au–Cu2O core–shell heterostructures. Pristine cubes and octahedra are denoted as pCube and p-Oct. For the core–shell face-raised cubes and core–shell octahedra (cs-FR Cube and cs-Oct), rhombic dodecahedral Au nanocrystal cores were used. For the core–shell cubes and core–shell face-raised octahedra (cs-Cube and cs-FR Oct), trisoctahedral Au nanocrystal cores were employed. Cross-sectional TEM images of a face-raised core–shell cube and a face-raised core–shell octahedron are also shown. Reprinted with permission from ref. 10, 11 and 36. Copyright 2009, 2011, and 2012 American Chemical Society.

surface band structures and hence present different barrier heights for electron transport. In addition, the (110) and (111) planes are positively charged, while the (100) planes are slightly negatively charged. The interface or surface charge state may be important to consider in the explanation of the observed facetdependent photocatalytic activity. The surface barrier height and repulsive electrostatic interaction of the photogenerated electrons and the negatively charged {100} face contribute to the photocatalytic inactivity of the Cu2O cubes. The inactivity of cubes and octahedra toward the photodegradation of positively charged methylene blue infers that photocatalysis depends on the intrinsic surface properties of the material itself, rather than on the charges of the adsorbed molecules. Interestingly, when Au–Cu2O core–shell cubes and octahedra synthesized using polyhedral gold cores were employed for the photocatalytic decomposition of methyl orange, the core– shell octahedra showed an enhanced catalytic performance, while the core–shell cubes remained inactive (Fig. 5d). The photogenerated

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electrons should migrate to the gold core, leading to more efficient charge separation and reducing the extent of charge recombination. Plasmon-induced charge transfer from metal particles to the attached semiconductor can also enhance photocatalytic activity.48 Better charge separation improves photocatalytic activity, but only for Cu2O octahedra possessing {111} facets. The cubes having inactive {100} faces cannot benefit from this effect because electrons and holes cannot efficiently reach the particle surface. Facet-dependent organocatalytic activity studies have become possible, especially ones comparing the catalytic activity among metal and oxide particles bound exclusively by the {100}, {111}, and {110} facets, as a result of the successful syntheses of these polyhedral nanocrystals. Various reactions, including nitroaniline reduction,43 Suzuki coupling,29,49,50 CO oxidation,51–53 styrene oxidation,54 and hydrogenation,55–61 have been examined for facet-dependent organocatalysis using metal nanocrystal catalysts.62–66 Electrocatalysis such as methanol oxidation

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reaction, oxygen reduction reaction, and formic acid oxidation using metal nanocrystal catalysts exposing specific surface facets is another popular research direction.67–74 Such reactions are potentially important for fuel cell applications. Generally particles exposing high-index facets give better catalytic performance. For Cu2O nanocrystals, [3+2] cycloaddition,40,75 cross coupling,76–78 water splitting,79 and CO oxidation80,81 reactions have been explored. Examination of metal nanocrystal-catalyzed reactions showed that the relative catalytic activities of the {100} and {111} faces can differ from one reaction to another. This means that it is not possible to predict the order of catalytic activity with respect to crystal facets; experiments must be performed to know this. The face-dependent catalytic activity study of gold nanocubes, octahedra, and rhombic dodecahedra toward 4-nitroaniline reduction by NaBH4 offers an interesting example to illustrate this (Fig. 6).43 Surfactant and other solution species such as iodide ions have been mostly removed prior to the catalysis experiments. By following the spectral changes as a function of reaction time, and carry out the same experiment at different temperatures, reaction rate constants and activation energies can be obtained. At 25 1C, the order of catalytic efficiency

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is rhombic dodecahedra > nanocubes > octahedra. However, the rate constant of octahedra approaches that of cubes at 36 1C. Remarkably, octahedra become more catalytically efficient than cubes at 40 1C. {110}-bound gold rhombic dodecahedra are consistently the best catalysts over the temperature range of 25 to 40 1C examined, but the rate constant differences are narrow for all these particle shapes at 40 1C. Thus, the catalytic efficiency of different surfaces of metal nanocrystals can be highly temperature-dependent, and at higher temperatures the difference becomes smaller. Compared to metal particles, Cu2O crystals can display greatly different facet-related catalytic activities, as seen in photocatalytic reactions. Recently, facet-dependent catalytic activity of Cu2O nanocrystals in the synthesis of 1,2,3-triazoles by multi-component click reactions of alkynes, organic halides, and NaN3 has been reported.40 Surfactant-removed Cu2O cubes, octahedra, and rhombic dodecahedra having the same total surface area were used for the catalytic activity comparison. Table 1 indicates that rhombic dodecahedral Cu2O nanocrystals are consistently much more catalytically active than Cu2O octahedra for the three reactions examined, whereas Cu2O nanocubes exhibit the lowest catalytic efficiency. The high catalytic activity of Cu2O rhombic dodecahedra is related to the fully exposed surface Cu atoms on the {110} facet as shown in Fig. 4. A large number of 1,4-disubstituted 1,2,3-triazoles have been made in excellent yields using rhombic dodecahedral Cu2O catalysts, including the synthesis of rufinamide, an antiepileptic drug. The highest catalytic activity of Cu2O rhombic dodecahedra has also been observed in the [3+2] cycloaddition reaction for the regioselective synthesis of 3,5-disubstituted isoxazoles.75 These examples illustrate the great potential of Cu2O nanocrystals for a wide range of organic coupling and cycloaddition reactions generating diverse products. Since rhombic dodecahedral Au, Pd, and Cu2O nanocrystals have become synthetically accessible, their inclusion in the catalytic activity comparison should be important because they are highly active catalysts.

Face-selective etching and deposition processes on the surfaces of nanocrystals

Fig. 6 ln[4-NA] versus time plots using gold nanocubes, octahedra, and rhombic dodecahedra as catalysts for the reduction of 4-nitroaniline by NaBH4 carried out at (a) 25 and (b) 40 1C. Reprinted with permission from ref. 43. Copyright 2012 American Chemical Society.

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Face-selective etching and deposition processes on Cu2O nanocrystals have been studied to a great extent.20,82–85 Ag2O nanocrystals were also examined for the face-selective etching process.21 Face-selective etching is facet-dependent because an acid or ammonia reacts preferentially on a certain crystal plane when etchant concentration is precisely tuned to yield a controlled etching reaction. Frequently the initially perfect shapes of Cu2O crystals become roughened or transform to exhibit more stable facets. For example, treatment of polyhedral Cu2O nanocrystals in an acetic acid solution environment leads to different degrees of oxidative dissolution depending on the exposed crystal planes.83 The {100} faces are stable and remain, but less stable {111} and {110} faces are converted to form new {100} facets. An application of rapid but controlled HCl or NH3

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Table 1

Comparison of the catalytic activity of different Cu2O nanocrystals for different 1,3-dipolar cycloaddition reactions40

Entry

Alkynea

Time (h)/Yieldb (RD)

Time (h)/Yieldb (OH)

Time (h)/Yieldb (Cube)

1

1/96

4.5/90

7/88

2

1.5/92

5/88

7/80

1

2/90

5.5/90

8/77

a

Organic halidesa

Product

Reagents and conditions: 1 (0.25 mmol), 2 (0.25 mmol), NaN3 (0.38 mmol) in EtOH (3 mL) at 55 1C.

etching of oxide is the formation of cubic and octahedral Cu2S and Ag2S cages from Cu2O–Cu2S and Ag2O–Ag2S core–shell structures.86,87 Nicely, nanoframes with empty {100} or {110} faces can also be achieved to produce novel hollow nanostructures. By using synthetic conditions similar to those shown in Fig. 2 to grow Cu2O nanocrystals, but adding HCl solution and a less amount of CuCl2, Cu2O truncated rhombic dodecahedral nanoframes composed of {110} skeleton faces and empty {100} faces are formed first.20 Further reaction fills the {100} faces and yields nanocages. Selective HCl etching over the {110} faces of the nanocages via the addition of ethanol, followed by sonication of the solution to temporarily remove

b

Isolated yields.

the surface-adsorbed surfactant for better acid etching, results in the formation of nanoframes with elliptical pores on the {110} faces (Fig. 7a). The whole process involves crystal growth and etching. Edge-truncated cubic nanoframes with empty {110} edges can also be directly prepared by growing Cu2O nanocrystals in the presence of a HCl etchant (Fig. 7b).82 These results show that the {110} faces of Cu2O are most susceptible to acid etching compared to the {111} and {100} faces. Faceselective etching of Ag2O nanocubes, rhombicuboctahedra, octahedra, and extended hexapods has been performed to establish the relative stability of different facets of Ag2O.21 Precise volumes of NH3 solution were injected into a mixture

Fig. 7 (a) SEM image showing the formation of two types of Cu2O nanoframes with empty {100} faces (type I) and {110} faces (type II) and nanocages resulting from crystal growth and face-selective etching processes. (b) SEM image of hollow Cu2O cubes with empty {110} edges. (c) SEM image of edgetruncated octahedral Ag2O crystals after the controlled etching by NH3 to yield etched {100} corners and {110} edges. The inset shows rhombicuboctahedral Ag2O crystals with etched square {100} faces. (d) SEM image of rhombicuboctahedral Cu2O microcrystals with Cu nanoparticles selectively deposited on the {111} faces. Reprinted with permission from ref. 20, 21, 82 and 92. Copyright 2008, 2011 and 2013 American Chemical Society and 2012 Royal Society of Chemistry.

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of Ag2O nanocrystals and NaOH to enable this face-selective etching. NaOH stabilizes Ag2O for a more controlled etching process. The following reactions lead to release of silver ions from Ag2O. Ag2O + H2O 2 2AgOH

(1)

AgOH + 2NH3 2 Ag(NH3)2+ + OH

(2)

The order of facet stability was found to be {111} > {110} > {100}. Fig. 7c shows that {100} and {110} faces of Ag2O crystals are etched, but the {111} facets remain intact. Hydroxide ions are likely less effective in adsorbing on the {100} faces of Ag2O containing terminal oxygen atoms, so these faces are more susceptible to etching by ammonia. Another interesting facet-dependent or site-selective property of metal and oxide nanocrystals is the preferential deposition of metal nanoparticles on the selective faces or edges of metal or Cu2O crystals.88–94 For example, Pt nanoparticles can be selectively deposited on the edges of Au decahedra by first reacting Au polyhedra with KI to etch Au atoms on the edges.88 For metal nanoparticle deposition on Cu2O polyhedra, two studies have shown that the preferential adsorption of the SDS surfactant on the {111} faces of truncated octahedral Cu2O microcrystals leads to the plane-selective deposition of gold nanoparticles on only the {100} faces of the crystals.90,91 In the absence of surfactant capping, Cu nanoparticles can form exclusively on the {111} faces of rhombicuboctahedral Cu2O microcrystals through a reaction between the Cu2O crystals and hydrazine (Fig. 7d).92 2Cu2O + N2H4 - 4Cu + N2 + 2H2O

(3)

It is presumed that copper atoms on the {111} faces are more easily reduced. Introduction of HAuCl4 into poly(vinyl pyrrolidone)-capped truncated octahedral Cu2O microcrystals resulted in selective Au nanoparticle deposition on the {111} faces.91,93 Au nanoparticles can also form selectively on the edges of Cu2O octahedra by reacting a relatively small volume of HAuCl4 solution with Cu2O crystals.94 Again these examples involve a process of Cu2O etching followed by metal deposition. Since the source of copper nanoparticles comes from Cu2O, it is not clear if metal deposition would still occur preferentially on the {111} faces or on edges when a separate copper source is introduced. As shown in these studies, face-selective metal deposition on Cu2O crystals can lead to enhanced photocatalytic activity of the particles.

Facet-dependent electrical conductivity of Cu2O nanocrystals Another dramatic display of facet-dependent properties comes from the electrical conductivity measurements of single Cu2O nanocrystals. Fig. 8 gives the I–V curves of a single Cu2O cube and corner-truncated octahedron performed with two tungsten probes contacting two opposite faces of a crystal.25 The cube is essentially non-conductive even at an applied voltage of 3 V. On the contrary, the octahedron is very conductive with a metallike electrical behavior (Fig. 8b). The results reveal that the

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Fig. 8 (a, b) I–V curves of a single pristine Cu2O octahedron and a nanocube measured using the same set of pre-annealed tungsten probes. (c) I–V curves of the same Cu2O corner-truncated octahedron with the probes contacting either only the {111} faces or only the {100} corners. Reprinted with permission from ref. 25. Copyright 2011 American Chemical Society.

{100} faces of Cu2O are not electrically conductive, while the {111} faces are highly conductive. Because Cu2O has a cubic crystal structure and no preferred direction for electron transport, the observed sharp electrical conductivity difference is considered to be surface-related. The same facet-dependent electrical properties can also be observed in a single corner-truncated octahedron by having the probes contacting its {100} or {111} faces (Fig. 8c). The explanation for this effect is that the band structure of the surface copper and oxygen atoms (possibly just 2 to 3 atomic layers thick)

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is tuned depending on the exposed surface plane. Within this surface layer on the {100} surface, the band bending is significant to create a large barrier to block electron transport. This band bending is small at the {111} interface, so electrons can easily move from the tungsten probe to the interior of the Cu2O crystal from this surface. Electrical conductivity measurements on the single Au–Cu2O core–shell cube and the octahedron were also performed. A significant improvement in conductivity was recorded for core– shell octahedra, but core–shell cubes showed little improvement. The results are similar to that seen in the photocatalytic activity of Au–Cu2O cubes and octahedra, so these facet-dependent phenomena must be related and considered together. Au enhancement of electrical conductivity is possible only when electrical current passes through the {111} surfaces of a Cu2O crystal. Since this type of facet-dependent electrical property can be envisioned to fabricate single crystal transistors and other functional electronic components, continued studies exploring the presence of similar properties in other materials are necessary.

Conclusions The ability to synthesize metal and semiconductor nanocrystals with systematic shape evolution facilitates the examination of their facet-dependent properties. Attractive or repulsive adsorption interactions of charged molecules with different planes of Cu2O and Ag2O crystals can lead to the pronounced crystal floating effect and influence interfacial photocatalytic activity of the crystals. Strongly facet-dependent electrical conductivity behaviors and photocatalytic activities of Cu2O nanocrystals suggest significant interfacial tuning of band structure depending on the exposed surface planes. Rhombic dodecahedral Au and Cu2O nanocrystals have been found to be most active for organocatalysis compared to cubes and octahedra. Chemical etching and metal nanoparticle deposition processes are also face-selective. Thus, facet-dependent properties can be manifested in many ways and should continue to be studied. A more complete facet-dependent examination of any property should include all three low-index faces. Because facet effects can be quite dramatic, especially for ionic solids such as Cu2O, material properties can be enhanced by synthesizing particles with the most desirable morphology. We have seen the value of using Cu2O rhombic dodecahedra to catalyze cycloaddition reactions with high product yields and a much less reaction time. Looking into the future, one may explore the facet-dependent electrical properties of other materials with a cubic crystal structure to establish that the observed phenomenon is a general intrinsic property of materials. Other properties, such as the possibility of optical properties of Cu2O crystals being facetdependent, should be very interesting and should be explored to expand our knowledge of facet effects of materials.

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