Facet-Dependent Electrical, Photocatalytic, and Optical Properties of

Spotlight on Applications Cite This: ACS Appl. Mater. Interfaces 2018, 10, 4−15

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Facet-Dependent Electrical, Photocatalytic, and Optical Properties of Semiconductor Crystals and Their Implications for Applications Michael H. Huang,* Gollapally Naresh, and Hsiang-Sheng Chen Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT: Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as Cu2O, Ag2O, TiO2, PbS, and Ag3PO4 exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of Cu2O crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows good understanding of the observed phenomena. Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, we should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photocatalytic efficiency or design electrical components utilizing the facet effects. There should be many opportunities to advance applications of semiconductor nanocrystals and nanostructures with continued research on the facet-dependent properties of various semiconductor materials. KEYWORDS: electrical conductivity, facet-dependent properties, nanocrystals, photocatalytic activity, semiconductors

1. INTRODUCTION Progress in the preparation of semiconductor crystals with tunable shapes and sharp faces has naturally lead to the exploration of crystal facets to their various properties.1−5 For facet-dependent studies, the crystal surfaces should be clean from surfactant and adsorbed molecules, so that the issue of adsorbate-induced band bending can be avoided, unless the effect of adsorbed molecules is the subject of investigation. A significant challenge hindering the popularity of facet-related studies is the growth of crystals with perfect and tunable shapes and sometimes varying sizes. That is why this important and exciting area of research has not seen rising activities. The story probably begins with the successful synthesis of semiconductors such as Cu2O and TiO2 with shape variation and thus control of the exposed crystal faces. Investigations of their photocatalytic and organocatalytic activities then follow to see how the exposed surfaces affect their catalytic performance.6−13 In the case of Cu2O crystals, it was actually through electrical conductivity measurements on single crystals that a © 2017 American Chemical Society

general idea to explain their facet-dependent electrical and photocatalytic properties emerged.14,15 Optical facet effects, in which the exposed crystal faces tune the light absorption wavelengths and thus band gaps of the semiconductor nanocrystals, came more recently.16,17 Gradually, we are able to understand why these phenomena are observed through band bending at the crystal faces. This review first introduces electrical conductivity measurements on single polyhedral semiconductor crystals to show clearly the effects of contacting facets to current flow and the need to modify band diagram to address the observed electrical responses. The knowledge gained from these electrical conductivity measurements is helpful to understand facet-dependent photocatalytic behaviors as the process also involves charge transport. Many semiconductor materials have shown facet-dependent photoReceived: October 18, 2017 Accepted: December 11, 2017 Published: December 11, 2017 4

DOI: 10.1021/acsami.7b15828 ACS Appl. Mater. Interfaces 2018, 10, 4−15

Spotlight on Applications

ACS Applied Materials & Interfaces

Figure 1. (a) Adjusted band diagram of Cu2O with consideration of relative band edge energies of different crystal surfaces. (b) I−V curves for the {100}, {110}, and {111} faces of Cu2O crystals. Inset gives the expanded I−V curves. (c) I−V measurements on a single Cu2O crystal with tungsten probes contacting two different facets. (d) Density of state plots for the (111) planes of Cu2O consisting of different numbers of plane layers counting from the crystal surface. With 5 (111) plane layers, or at a thickness of 6.2 Å, a gap appears near the Fermi level. This thin layer is believed to give rise to the observed facet effects. Reprinted with permission from ref 15. Copyright 2015 American Chemical Society.

making electrical contacts to nanoscale 3-dimensional objects can be quite difficult, especially contacts need to be made on specific crystal faces. For polyhedral crystals that are hundreds of nanometers in size, it is more convenient to use tungsten probes connected to a multiprobe nanomanipulator installed inside a scanning electron microscope (SEM) to make electrical contacts. However, fabrication of sharpened probe tips, manipulation of the probes, and the current shock to crystal’s compositional and positional stability are challenges that require skill and patience to obtain useful I−V curves, in addition to the availability of the nanomanipulator. These obstacles explain why very few reports on the facet-dependent electrical properties of semiconductor nanocrystals are available. Cu2O crystals with and without gold nanocrystal cores were first used for electrical conductivity measurements.14 Although a Au core can significantly increase the electrical conductivity of a Cu2O octahedron, the metal core only raises the current of a weakly conductive Cu2O cube slightly. Recognizing the use of Au cores is not necessary to observe facet effects, the need for a more complete examination of all three facets of Cu2O crystals, and the potential for the observation of asymmetric I−V responses with electrical contacts to different facets simultaneously, single Cu2O cubes, octahedra, and rhombic dodecahedra were tested for their electrical properties.15 Figure 1b shows the {111}

catalytic activities. Strategies such as deposition of metal nanoparticles and graphene on crystal surfaces, or formation of semiconductor heterostructures, to facilitate charge transport and electron−hole pair separation have been frequently employed to improve catalytic performance, but the influence of the contacting faces on the efficiency of charge migration is less systematically examined.3 We find the power of facet effect on photocatalysis is most dramatically displayed when a photocatalytic material becomes completely inactive due to the exposed crystal faces.18,19 Optical facet effects of Cu2O nanocrystals are also presented. The idea of this ultrathin surface layer is also useful to understand the observed facetspecific optical properties of semiconductor nanostructures. Recognizing that the chemical and physical properties of semiconductors are significantly tuned by the crystal facets and contacting faces, applications of semiconductor materials should take into account of this fact to explain the observed results and design systems with optimized functions.

2. FACET-DEPENDENT ELECTRICAL CONDUCTIVITY PROPERTIES OF SEMICONDUCTOR CRYSTALS 2.1. Cu2O Crystals. Electrical conductivity measurements on single semiconductor crystals are extremely useful to demonstrate the presence of facet effects and the need to modify their band diagrams (see Figure 1a). Unlike nanowires, 5




Figure 2. (a) Facet-dependent electrical conductivity measurements on the {101} faces of a TiO2 nanocrystal (red), the {101} and {001} faces of a TiO2 nanocrystal (blue), and the {001} faces of a TiO2 nanocrystal (black). (b) I−V curves for the {111}, {100}, and {110} faces of Ag2O crystals. The models illustrate the contacting facets. (c) SEM images showing tungsten probe contact combinations of the {110}/{111} faces, {110}/{100} faces, and {100}/{111} faces of a Ag2O rhombicuboctahedron and the measured I−V curves. Scale bars are equal to 1 μm. (d) Schematic illustrations of electrical conductivity measurements on single PbS nanocrystals and the measured I−V curves. Inset shows the expanded I−V curves. Reprinted with permission: Panel a from ref 22. Copyright 2015 Royal Society of Chemistry. Panel b and c from ref 25. Copyright 2017 Wiley−VCH. Panel d from ref 28. Copyright 2016 American Chemical Society.

with electrical contacts on the most conductive {111} facet and the insulating {110} facet, showing the potential of using simply the electrical facet effects to design operating diode or other electronic components. To provide evidence for the existence of the ultrathin surface layer, density of states (DOS) plots for tunable number of Cu2O lattice planes have been calculated. For few (111) lattice planes, the DOS plots show initially continuous energy band structures (Figure 1d). A clear gap develops with 5 (111) planes at a thickness of 6.2 Å, signifying a transition from an unusual metallic band structure back to a semiconductor band structure. For Cu2O {110} and {100} facets, the layer thicknesses causing the facet effects have been determined to be 4.5 and 11.7 Å, respectively.15 This thin surface layer is not detectable using current surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) because the same spectra were collected regardless of the particle shapes.21 Band diagrams constructed using more surface-sensitive ultraviolet photoelectron spectroscopy (UPS) and diffuse reflectance UV−visible spectra look similar for Cu2O cubes, octahedra, and rhombic dodecahedra, so they cannot correctly explain the observed facet-dependent electrical properties of Cu2O crystals.21 However, slight differences in the energies of the valence band maximum and work function for different facets of Cu2O are detectable by UPS characterization.21 Another difficulty is that comparison of surface atomic arrangements or electron density map cannot correctly predict the actual electronic or photocatalytic behavior of a specific surface, as such analysis cannot explain why a facet should be electrically insulating or photocatalyti-

facets of a Cu2O octahedron are so conductive that the conductivity of an octahedron resembles that of a metal (see inset of Figure 1b). The {100} facets of a Cu2O cube are only moderately conductive, and a more typical semiconductor I−V curve has been recorded. Surprisingly, the {110} facets of a Cu2O rhombic dodecahedron is completely nonconductive, even though they exhibit a superior photocatalytic activity.7 Since the idea that the presence of an ultrathin surface layer having different band structures and varying degrees of band bending for these crystal facets can explain the observed phenomena, it is natural to construct a modified band diagram of Cu2O as shown in Figure 1a. The band bending is drawn downward due to the p-type semiconductivity of Cu2O. The {110} facet is drawn most deviated from the normal band energies of bulk Cu2O to represent its largest barrier to charge transport exiting this surface or entering the crystal. Hence, the most conductive {111} facet is only slightly bent to signify its lowest barrier. The moderately conductive {100} facet bends to a greater extent than that for the {111} facet. Obviously, if Cu2O can be simultaneously conductive and insulating, the band bending can only occur at the crystal surface, or it is not possible to draw a rational band diagram. Because of the large difference in the electrical conductivity behaviors of these facets, asymmetrical current rectifying I−V curves similar to that of p−n junctions can be obtained for any combination of crystal facets (Figure 1c).20 Here current entering Cu2O through a facet with a low barrier facilitates charge transport, but current flow into the crystal through a facet with a high barrier is more difficult. The most useful combination is one 6




Figure 3. (a) Hydrogen evolution rates for KNTO octahedra and circular nanosheets under UV light irradiation in CH3OH/H2O solution. (b) Top and side views of the {111} and {101} faces of KNTO. (c) Photocatalytic oxygen evolution from α-Fe2O3 cubes and octahedra in NaIO3 and Na3PO4 aqueous solution. (d) Schematic drawing of the reaction. (e) Proposed mechanism for MO degradation by CeO2 nanocrystals under UV light and Au-decorated CeO2 under UV illumination and visible light excitation. (f) Photodegradation of MO in the presence of CeO2 nanocubes (NCs), nanorods (NRs), and 11.6 wt % Au-decorated CeO2 nanorods. (g) Schematic diagram of band structure and expected charge migration of Ag3PO4/SrTiO3 composite under visible-light irradiation. (h) Photocatalytic oxygen evolution for Ag3PO4/SrTiO3 composite with different molar ratios. (i) Current versus voltage curves of the polyhedral In2O3 microcrystal photoanodes obtained from a 1 M aqueous NaOH solution under illumination and comparison of photocurrent densities at 0.22 V of different In2O3 polyhedra. (j) Photocatalytic performances of Pt/PbS catalysts with different PbS nanocrystal morphologies for hydrogen evolution reaction and the proposed band bending at the interface. Reprinted with permission: Panels a and b from ref 32. Copyright 2013 American Chemical Society. Panels c and d from ref 33. Copyright 2016 Elsevier B. V. Panels e and f from ref 34. Copyright 2015 American Chemical Society. Panels g and h from ref 36. Copyright 2014 American Chemical Society. Panel i from ref 38. Copyright 2014 American Chemical Society. Panel j from ref 39. Copyright 2017 American Chemical Society.

cally inactive.15 These results illustrate why conventional analytical methods are inadequate to reveal the presence of large semiconductor facet effects, and the existence of this ultrathin surface layer remains conceptual but very useful to our understanding. 2.2. TiO2, Ag2O, PbS and Other Crystals. Anatase TiO2 bipyramids bound by the {101} facets, square plates with {001} square faces, and tapered rectangular bars with largely {010} side facets have been synthesized and used for facetdependent electrical conductivity measurements (Figure 2a).22,23 In addition to showing the benefit of using selected crystal facets of TiO2 to enhance a photoelectrochemical water splitting reaction and the performance of lithium ion batteries through better charge transport/transfer, the facet-specific electrical conductivity measurements have yielded results

similar to those observed in Cu2O crystals. The {101} facets of TiO2 are highly conductive with current reaching 18 μA at an applied voltage of 2 V.22 The {001} facets are less conductive with a current of 11 μA at 2 V, whereas the {010} facets are comparatively slightly conductive (∼0.3 μA at 2 V).23 When tungsten probes contacted the {101} and {001} facets simultaneously, a current-rectifying asymmetric I−V curve was also obtained, suggesting this phenomenon is generally observable in many semiconductor materials. Silver oxide has the same body-centered cubic crystal structure as that for Cu2O. After the successful synthesis of diverse Ag2O crystal morphologies including cubes, octahedra, rhombic dodecahedra, and rhombicuboctahedra, electrical conductivity measurements were conducted on these crystals.24,25 Figure 2b shows the {111} facets of Ag2O are most 7




into and out of a crystal. This idea is also useful to the explanation of photocatalytic activities of semiconductor crystals.

conductive. The {100} faces are moderately conductive, but the {110} faces are nearly nonconductive. The conductivity order is the same as that found for Cu2O, but clearly the {111} facets of Ag2O are not quite as conductive as the {111} facets of Cu2O. By measuring a rhombicuboctahedron exposing all three crystal faces, current-rectifying asymmetric I−V curves have been collected for all combinations of different crystal faces (Figure 2c). The {111}/{110} combination is the best for electronic operation application with zero current extending to −5 V. Because the {111} face is more conductive, electron flow into a silver oxide crystal from this facet sees a smaller energy barrier, so this direction produces a much larger current. The low current for the forward bias or the positive voltage side should be attributed to the comparatively lower electrical conductivity of the {111} faces of Ag2O than that of Cu2O and the large crystal size. Generally, current is higher for submicrometer-sized crystals. This is another semiconductor material showing strong facet-dependent electrical conductivity properties. By heating an aqueous solution of cetyltrimethylammonium bromide (CTAB), nitric acid, lead acetate, and thioacetamide (TAA) and paying attention to the reagent introduction sequence, small PbS nanocubes and octahedra with sizes of tens to hundreds of nanometers can all be synthesized.26,27 Using the large PbS protruded cubes, edge- and cornertruncated cubes, and octahedra with sizes over 200 nm to make electrical contacts, their electrical conductivity properties were also examined.28 The edge-truncated cube provides the {110} faces. Figure 2d shows the {110} faces of PbS are highly conductive, and are similarly or more conductive than the {100} faces. However, current does not rise immediately but requires a voltage greater than 1 V. In contrast, the {111} facets are barely conductive. Asymmetric I−V curves have also been obtained with tungsten probes contacting the {110} and {111} facets of a truncated cube. Large facet-dependent electrical conductivity differences are also observable in n-type PbS crystals. Through density functional theory (DFT) calculations, tunable numbers of silicon (111) and (112) planes have recently been shown to possess metal-like band structures with continuous density of states going from the valence band to the conduction band.29 In contrast, the Si (100) and (110) planes always show a band gap with tunable number of plane layers. DFT calculations also show deviation in Si−Si bond length and bond distortion within 6 Si (111) and (112) surface planes, as well as obviously different 3s and 3p electron energy distributions in this ultrathin surface layer.29 The combined message is that the facet effect is quantum mechanical in nature, and can be considered as frontier orbitals within the Si (111) and (112) surface layer looking slightly different from those in the crystal interior in terms of electron energy distribution and orbital geometry. This work suggests that silicon should possess facet-dependent electrical conductivity properties, and functional electronic components can be fabricated utilizing the facet effects. Very recently, such electrical conductivity measurements on intrinsic, or nondoped, Si wafers have been reported, confirming poor electrical conductivity properties of the Si {100} and {110} faces, but the {111} face is much more conductive and the {112} face is highly conductive.30 Asymmetrical I−V responses have also been obtained. It is now clear that the facet-dependent electrical conductivity properties are widely observable in many semiconductor materials due to different degrees of band bending at various crystal surfaces, affecting charge transport

3. FACET-DEPENDENT PHOTOCATALYTIC ACTIVITY PROPERTIES 3.1. Various Semiconductor Materials Showing Large Photocatalytic Activity Differences due to the Exposed Crystal Faces. A general goal of photocatalyst design should be the improvement of photocatalytic activity for photodegradation or other reactions. When semiconductor crystals can be prepared with different morphologies and thus exposed facets, it is natural to compare their photocatalytic performance and explain the cause for the catalytic differences. Different explanations including surface energy and molecular interactions with crystal surfaces through charge consideration, as well as surface atomic arrangements and trap states/defects within a crystal, have been proposed.31−35 As we will see, these plausible explanations have difficulty accounting for the observed photocatalytic inactivity of some crystal faces and the huge facet-induced differences in the electrical conductivity responses, especially when a crystal exposing different faces can be simultaneously highly conductive and nearly insulating. For example, {111}-terminated K1.9Na0.1Ta2O6·2H2O (KNTO) octahedra with edge lengths of 100 to 400 nm and {101}bound circular disks with a diameter of ∼100 nm were used for photocatalytic hydrogen evolution from an ethanol−water solution under UV light irradiation, showing octahedra are more than 3 times higher in activity than the disks (Figure 3a).32 Because crystallinity was found not to be a key factor to the activity difference, it was concluded that the main difference between the two crystal shapes is the exposed facets with dissimilar surface atomic structures as shown in Figure 3b. In addition, UV−vis diffuse reflectance spectra revealed sharp absorption edge of these crystals with a red-shift from 251 nm (4.94 eV) for the octahedra to 257 nm (4.82 eV) for the nanosheets. The difference in electronic structure has been attributed to the exposure of different facets with unique surface atomic arrangements. With a higher conduction band minimum (CBM) for the octahedra, it can be expected that they possess a stronger reduction power, which is beneficial for the photocatalytic reaction. This example illustrates how optical properties can influence photocatalytic activity, but the reason for such absorption edge shift was not provided. The absorption shift should be related to the exposed facets. The weakness with using surface atomic arrangement to discuss facet differences is its inability to predict observed photocatalytic or electrical conductivity differences, as we will see that Cu2O and Ag2O have very different facet-dependent photocatalytic properties even though they have the same crystal structure.18,21,24 Such analysis also cannot predict photocatalytic inactivity or insulating electrical response. In another study, α-Fe2O3 nanocubes exposing {012} and {104} facets were found to facilitate the reduction of IO3− and promote water oxidation forming oxygen.33 α-Fe2O3 octahedra exposing {101} and {111} facets, on the other hand, are very inefficient at these reactions (Figure 3c and d). Oxygen evolution rate is 84 time higher for cubes than for octahedra (309.4 vs 3.7 μmol h−1 g−1). Electrochemical impedance spectroscopy (EIS) reveals smaller charge carrier transfer resistance for cubes, so more photogenerated electrons and holes participate in the redox reactions for cubes. This is a good example of crystal facets strongly enhancing or greatly 8




planes, including some bad alignment with unfavorable charge transfer from Ag3PO4 to SrTiO3. In other words, band alignment analysis using simply valence and conduction band energies of semiconductors in a heterostructure is not reliable because facet effects tuning the interfacial band structures and alignment has been neglected. Strongly facet-dependent photoelectrochemical reactions can also be observed in In2O3 and PbS crystals deposited on an electrode.38,39 In the case of In2O3 microcrystals, In2O3 cubes, cuboctahedra, octahedra, and other shapes were grown on a silicon substrate using a high-temperature chemical vapor deposition approach.38 The In2O3 crystal-loaded Si substrate was used as the electrode for photoelectrochemical (PEC) oxygen evolution with holes transferring to water to produce oxygen. In2O3 cubes exhibited the best PEC water splitting reaction yielding the highest current density (Figure 3i). Crystals exposing some fractions of the {001} facets also gave substantial reactivity, but {111}-bound In2O3 octahedra were essentially inactive. Because holes are depicted to transfer through the crystal surface to the adsorbed water in the dissociated H+ and OH− forms, photocatalytic activity should be considered as the efficiency in a surface or interface charge transport process. PbS nanocubes, octahedra, and other intermediate shapes loaded with Pt nanoparticles were examined for PEC hydrogen evolution.39 PbS octahedra gave the highest current, but cubes barely showed any photocurrent. Nanocrystals with mixed facets displayed intermediate activities. Because PEC reactions and solar cells also involve surface and/or interfacial charge transfer, these results profoundly demonstrate the importance of semiconductor interface facets to the performance of photocatalysts and solar cells. Interestingly, a thin film of PbS nanocubes has also been reported to exhibit high photocurrent enhancement while octahedra barely give photocurrent.40 Various other semiconductors also exhibit facet-dependent photocatalytic activity properties.41−44 Many of them possess two or more crystal facets.35,45−47 For example, BiVO4 square plates and truncated bipyramids have {010} basal faces and {110} lateral faces.35 ZnO plates and rods have both end and side facets with significant proportion for each facet.45 Anatase TiO2 square plates have {001} basal faces and {101} lateral facets, and additional {010} side facets for tapered rectangular bars yielding tunable photocatalytic activity.46,47 The imperfect control of crystal shapes and mixed facets makes the resulting photocatalytic differences less pronounced and harder to determine conclusively.35 3.2. Facet-Dependent Photocatalytic Properties of Cu2O and Ag2O Crystals. We have long observed strikingly different photocatalytic activities of Cu2O cubes, octahedra, and rhombic dodecahedra toward the photodegradation of MO.6,7 {110}-bound Cu2O rhombic dodecahedra are highly photocatalytically active, but {100}-bound cubes appear to be inactive. Octahedra exposing {111} facets are moderately active compared to that of rhombic dodecahedra. Use of Au cores to form Au−Cu2O core−shell octahedra led to photocatalytic enhancement due to photoexcited electron transfer to the metal core and thereby better electron−hole separation, but Au−Cu2 O core−shell cubes remained inactive.48 Other studies have also found photocatalytic inactivity of Cu2O cubes.49−52 However, a number of reports still show that Cu2O cubes exhibit a moderate photocatalytic activity.14,53−55 These contradicting results can be rationalized by the presence of a small amount of other crystal shapes,

suppressing photocatalytic reactions. If a mixture of both particle shapes were used without the knowledge of their huge catalytic differences, only average catalytic performance can be obtained. However, bulk band structure diagrams for the two crystal shapes constructed from valence band XPS spectra and UV−vis diffuse reflectance spectra look very similar, and thus cannot explain the observed large facet effect on the water oxidation reaction. {100}-bound CeO2 nanocubes and {110}-bound rectangular nanorods have been reported to give sharply different photocatalytic activities toward the photodegradation of methyl orange (MO).34 Figure 3e is the proposed photocatalytic mechanism of fluorite-structured CeO2 under UV and visible light irradiation. Figure 3f gives the photocatalysis results. The CeO2 cubes are essentially photocatalytically inactive under UV irradiation. The nanorods are active, and deposition of Au nanoparticles on the rod surface greatly enhances their photocatalytic performance resulting from better charge separation with photoexcited electrons migrating to the Au cocatalyst. Although the CeO2 nanorods adsorbed MO, the cubes were found not to adsorb MO because of the repulsive electrostatic interaction between the anionic MO dye and the negatively charge {100} surface of CeO2. The variation in low-coordinate surface cerium cations between the {110} and {100} facets has been attributed to cause the difference in photocatalytic activity, rendering the {110} facets more photoreactive. However, facet-specific coordination difference is part of the nature of crystal surface, and materials having the same facet coordination situation can still display completely different photocatalytic behaviors as we will see in Cu2O and Ag2O. The active {110} edges are also present on the CeO2 cubes. Both the cubes and rods have identical XPS spectra and similar band gap energies (3.10 eV for the cubes and 2.90 eV for the nanorods). This example again shows band diagrams constructed using these analytical techniques cannot reveal large facet effects. A typical way to enhance photocatalytic activity is to prepare semiconductor heterostructures with Z-scheme band alignment of constituting components. According to the band structure diagram of Ag3PO4 and SrTiO3 seen in Figure 3g, photogenerated holes produced in Ag3PO4 under visible light irradiation should migrate to SrTiO3, and photoexcited electrons should exit the Ag3PO4 surface or surface-deposited Ag nanoparticles.36 This charge separation should enhance photocatalytic water oxidation reaction. Because SrTiO3 absorbs in the UV light region, photoexcitation does not occur using visible light. Initial rates of oxygen evolution indeed increase with some SrTiO3 deposition, but further increase in SrTiO3 growth on polyhedral Ag3PO4 microcrystals exposing different uncharacterized facets leads to gradual and eventually significant suppression of photocatalytic activity (Figure 3h). The attribution of catalytic activity decline with more SrTiO3 decoration to less effective contacts is not supported by evidence and not understandable. Such reversed or fluctuating photocatalytic responses with increased heterojunction formation have been reported occasionally but have not been carefully examined. 37 As we will see, such unpredictable photocatalytic behaviors not revealed in a band diagram originates from the negative effects of some contacting facets in the heterojunctions. As SrTiO3 nanoparticles are deposited on various Ag3PO4 crystal faces, several combinations of interfacial band alignment different from that of bulk materials are possible depending on the contacting crystal 9




Figure 4. (a) Plot of the extent of photodegradation of MO as a function of time for the various Cu2O crystals and Au-decorated Cu2O heterostructures. (b) EPR spectra of DMPO−OH present in photoirradiated Cu2O cubes, octahedra, and rhombic dodecahedra. (c) Band diagram of Cu2O with consideration of different degrees of band bending for different crystal surfaces and drawings showing different photocatalytic responses for Cu2O cubes, octahedra, and rhombic dodecahedra. (d) Extents of MO photodegradation as a function of time using Ag2O rhombic dodecahedra, octahedra, and cubes as the photocatalysts. (e) Plot of extent of MO photodegradation as a function of irradiation time for various Cu2O crystals and Cu2O−ZnO heterostructures with and without placement of a filter blocking UV light. (f) Adjusted band diagram at the interface between Cu2O and ZnO with consideration of relative band edge energies of different Cu2O crystal surfaces and drawings showing different photocatalytic responses for Cu2O−ZnO heterostructures over Cu2O cubes, octahedra, and rhombic dodecahedra. Reprinted with permission: Panel a and b from ref 18. Copyright 2016 Wiley−VCH. Panel c from ref 21. Copyright 2017 Royal Society of Chemistry. Panel d from ref 24. Copyright 2016 American Chemical Society. Panels e and f from ref 19. Copyright 2017 Wiley−VCH.

are the major radical species formed in a photocatalytic reaction, a spin-trapping agent DMPO can be added in the solution during photoirradiation of Cu2O crystals. Electron paramagnetic resonance (EPR) spectra of the solution can reveal presence of these radical species and their relative percentages.56 Figure 4b clearly reveals greater production of hydroxyl and superoxide anion radicals from the photoirradiated Cu2O rhombic dodecahedra than from the irradiated octahedra. Cu2O cubes are photocatalytically inactive because no radicals are generated upon photoexcitation. The next experiment is to use electron and hole scavengers to understand the photocatalytic differences among these particle shapes (Figure 4c). The scavenger experiments showed efficient transfer of photogenerated electrons and holes migrating to the crystal surfaces of rhombic dodecahedra to

slight edge truncation of the cubes, and/or the formation of surface CuO. A tiny amount of CuO on Cu2O crystals cannot be detected by XRD or SEM analysis, but XPS characterization can reveal its presence giving some photocatalytic activity.18,19 Thus, the surface composition of Cu2O cubes should be verified with XPS spectroscopy if they display some photocatalytic activity. To confirm photocatalytic inactivity of Cu2O cubes, a common catalytic activity enhancement approach can be used by decorating Cu2O cubes, octahedra, and rhombic dodecahedra with a small amount of Au nanoparticles. While Audecorated Cu2O rhombic dodecahedra and octahedra showed enhanced photocatalytic activities compared to the pristine ones, Au-deposited Cu2O cubes remained inactive (Figure 4a).18 Because hydroxyl (•OH) and superoxide anion (•O2−) 10




Figure 5. (a) UV−vis absorption spectra of octahedral, cuboctahedral, and cubic Au−Cu2O core−shell nanocrystals with 50 nm octahedral Au cores. (b) UV−vis absorption spectra of size-tunable Au−Cu2O core−shell rhombic dodecahedra, octahedra, and cubes with 35 nm octahedral Au cores. (c) UV−vis absorption spectra of Au−Cu2O rectangular nanobars with tunable Cu2O shell thicknesses synthesized from 57 nm Au nanorod cores. (d) Plot summarizing the volume variation of Cu2O octahedra and cubes with respect to their UV−vis absorption band positions. Reprinted with permission: Panel a from ref 16. Copyright 2014 Royal Society of Chemistry. Panel b from ref 58. Copyright 2014 Wiley−VCH. Panel c from ref 62. Copyright 2016 Royal Society of Chemistry. Panel d from ref 17. Copyright 2016 Wiley−VCH.

{100} faces because of a large barrier present, and this barrier is likely the same thin layer producing the facet-dependent electrical conductivity behaviors. Remember that both properties involve charge transfer into and out of a crystal through its surfaces, and photocatalytic activity is really about the efficiency of carrier transport through the crystal surface or interface. One should also be mindful that the barrier height experienced by charge carriers for a particular surface can be different for these two processes, because photocatalysis is concerned with charge transfer out of a crystal, but electrical conductivity considers current flow into a crystal. The charge carriers (electrons, for example) also have different energies for the two processes before meeting a particular crystal surface. Facet-dependent photocatalytic activity comparison of Au nanoparticle-deposited Ag2O cubes, octahedra, and rhombic dodecahedra shows the best activity for cubes and lowest activity for octahedra.57 When pristine Ag2O cubes, octahedra, and rhombic dodecahedra were employed for photocatalytic degradation of methyl orange, cubes were most active and

react with water or dissolved oxygen to form hydroxyl and superoxide anion radicals.21 For octahedra, it is mainly electron transfer providing the needed radical species for photocatalysis, so photocatalytic activity of octahedra is inferior to that of rhombic dodecahedra. Cu2O cubes remained photocatalytically inactive in the presence of electron and hole scavengers. The results can be interpreted as lack of photoexcited electrons and holes at the crystal surfaces and thus no production of radical species. Using a modified band diagram to illustrate the observed facet effects on photocatalysis, the band edge bending should be very steep presenting an insurmountable barrier for electrons and holes to reach the {100} surface (Figure 4c). The barrier is much smaller for the {111} face of Cu2O, and little or no upward bending for the {110} face of Cu2O. This model is also consistent with the experimental observations of most red-shifted absorption edge for Cu2O cubes, and most blue-shifted absorption edge for rhombic dodecahedra.21 The cause for the photocatalytic inactivity of Cu2O cubes should be the unavailability of photogenerated electrons and holes at the 11



ACS Applied Materials & Interfaces rhombic dodecahedra were least active (Figure 4d).24 EPR spectra agree with the photocatalytic results, showing weak but definite EPR signals for Ag2O rhombic dodecahedra.24 Because both Cu2O and Ag2O have the same crystal structure and hence the same facet atomic arrangement, their relative electron density and possibly surface energy order for the {100}, {111}, and {110} faces should be the same, but they exhibit completely opposite photocatalytic activity behaviors. Also the least active {110} facets of Ag2O have a moderate photocatalytic activity. Surface energies of Cu2O are in the order of γ{100} < γ{111} < γ{110}.4 DOS plots for tunable number of surface lattice planes look quite different for the two materials, so the idea of a thin surface layer with different degrees of band bending for respective facets is still useful to understand the dissimilar photocatalytic properties between Cu2O and Ag2O.25 This example highlights that the use of surface atomic arrangement/structure, electron density, and possibly surface energy to explain facet-specific photocatalytic activity cannot be broadly applied and hence cannot be the correct explanations. In fact, surface energy comparison of different facets would not predict complete photocatalytic inactivity. As we have seen, photocatalytic inactivity happens more often than we think, but such examples have generally not been reported or analyzed as researchers are more interested in photocatalytic activity enhancement. The truth is that we probably learn more about the nature of semiconductors when they do not work than when they work as expected. Recognizing the photocatalytic inactivity of Cu2O cubes, one can further demonstrate such dramatic facet effects on photocatalysis by forming semiconductor heterostructures and see if the cubes stay inactive. When spiny ZnO structures were grown on the Cu2O cubes, the composite cubes remained photocatalytically inactive when irradiated with visible light (Figure 4e).19 Photocatalytic inactivity can be understood because of the large barrier height at the Cu2O {100} surface preventing electron transport to ZnO. ZnO-decorated Cu2O rhombic dodecahedra showed the expected photocatalytic enhancement because of the favorable band alignment between Cu2O and ZnO to facilitate electron transfer to the ZnO side for effective electron−hole separation (Figure 4f). Surprisingly, Cu2O octahedra became photocatalytically inactive after ZnO deposition. High-resolution TEM analysis of the interfacial region between the {111} surface of Cu2O and the contacting planes of ZnO reveals preferential epitaxial growth of the (101) lattice planes of ZnO (Figure 4f). Assuming a significant upward band bending for the {101} surface of ZnO above the conduction band energy of the bended {111} surface, suddenly electron transfer from Cu2O to ZnO becomes unfavorable. Results from DFT calculations qualitatively match with this assumption.19 Thus, strong photocatalytic facet effects to the point of inactivity does not occur only on single crystals, but bad heterojunctions with unfavorable band alignment of the contacting planes can also produce photocatalytic suppression despite apparent favorable bulk band alignment. Given the multiple contacting facet combinations, one can imagine that total photocatalytic inactivity may be more common than we expect.

were used as cores for the fabrication of Au−Cu2O core−shell cubes, octahedra, cuboctahedra, and rhombic dodecahedra (Figure 5a, b).5,16,58 The Cu2O shell should have sharp faces and simple geometric shapes and be single-crystalline to see this optical effect. Amazingly, our synthetic method used to make Cu2O crystals with tunable shapes can accommodate all kinds of Au particle shapes and still give single-crystalline Cu2O shells with no evidence of defects as judged from highresolution TEM and electron diffraction pattern images.48,59 The metal core and the Cu2O shell have an exact lattice orientation relationship, so that the {111} faces of the Au core align parallel to the (111) planes of Cu2O shell. When the polyhedral crystals are less than 250 nm and preferably below 200 nm in size to avoid strong light scattering interference, the surface plasmon resonance (SPR) absorption band of the Au cores, red-shifted by more than 200 nm when coated with Cu2O, have different peak positions depending on the shape or the exposed facets of the Cu2O shells. The Au SPR band remains stationary despite large changes in the shell thickness. Cubes have the most red-shifted SPR absorption band, and rhombic dodecahedra give a more blue-shifted SPR band than that of octahedra. Cuboctahedra having both {100} and {111} facets show a SPR band between those of cubes and octahedra. In addition, the Cu2O absorption band (the lower wavelength band) of smaller Au−Cu2O cubes is consistently more redshifted than that of larger Au−Cu2O octahedra and rhombic dodecahedra (Figure 5a, b). These unusual optical phenomena suggest the inherent facet-dependent optical properties of Cu2O, since plasmonic metal particles do not show such optical behavior.5 Because the ultrathin surface layer has different band structures for the {100}, {111}, and {110} faces and can be considered as composed of different materials with dissimilar refractive indices, the plasmonic field of the metal cores can feel these environmental changes and respond by tuning their plasmonic band positions. The notion that this ultrathin layer is more than just a monolayer of surface atoms but also not so thick as it is invisible supports the need of a finite layer thickness to induce sufficient optical shifts. Cu2O shell thickness does not matter so much as long as it is not too thin or nonuniform. The same facet-dependent SPR and Cu2O absorption properties are observable when cubic Pd and Au− Ag core−shell cubes are used as the plasmonic cores.60,61 If short gold nanorods are used as cores to form {100}-bound Cu2O rectangular bars with tunable sizes, the longitudinal SPR band can be easily shifted to cover the near-infrared region. Figure 5c shows fixed transverse and longitudinal SPR bands of 57 nm Au nanorods at respectively 593 and 1390 nm despite changes in the shell thickness.62 It is important to recognize that the smallest Au−Cu2O bars just 73 nm long directly shift the Au SPR band to ∼1400 nm, benefiting from the optical facet effects of Cu2O with its {100} faces being the most redshifting surface. These nanobars with tunable aspect ratios can cover the entire near-infrared light spectrum for efficient photothermal activity. A solution of Au@Ag−Cu2O cuboctahedra and octahedra with Ag SPR band at around 800 nm can reach over 90 °C when irradiated with light from an 808 nm laser.61 Photothermal activity examinations on Au−Cu2O and Au@Ag−Cu2O polyhedra have also suggested facet-dependent heat transmission efficiency across different Cu2O surfaces.61,62 This can be understood as the ultrathin surface layer behaving like different materials can have slightly different heat transmission efficiency.

4. FACET-DEPENDENT OPTICAL PROPERTIES The fact optical properties of Cu2O crystals are also facetdependent was discovered when single-crystalline octahedral or cubic gold nanocrystals with sizes of ∼50 nm or smaller 12




semiconductor structures or films are involved. Knowing certain bad faces or heterojunctions can detrimentally affect device performance should be important. In a sense, blue-lightemitting diodes may be benefiting from facet effects as band alignment tuning requires GaN layers with specific surfaces to optimize charge migration. It is just that we do not think of its existing application this way. For electrical conductivity, diodes fabricated utilizing the facet effects may represent a possible field effect transistor design. Facet effects may also be relevant to thermoelectric materials development. Because thermoelectric power is related to electrical conductivity and thermal conductivity, and both properties may be facet-dependent, especially the electrical conductivity term, it may be possible to simply fabricate thermoelectric materials exposing different crystal faces to tune ZT values instead of keep changing the material composition. Because the optical band gap is tunable with crystal size and exposed facets, emissive semiconductor materials such as perovskite and CdS/CdSe can capitalize on this fact to extend their photoluminescence range and test the limitation of the generally accepted quantum confinement effects. Photothermal activity also benefits from facet effects as shown in the case of Au−Cu2O nanobars to capture broad near-infrared light while keeping the particles relatively small. More demonstrations of possible applications of facet-dependent effects from diverse topics should eventually make us realize that facet effects are fundamental properties of semiconductors.

With the continuous development of new synthetic methods to make ultrasmall Cu2O nanocubes and octahedra, cubes with tunable edge lengths of 9 to 87 nm and octahedra with opposite corner distances of 52 to 157 nm have been synthesized.17,63 Taking UV−vis absorption spectra of these samples and expressing the particle sizes in terms of their volumes, a plot of Cu2O absorption band positions versus particle volumes is constructed (Figure 5d). Cu2O cubes consistently show a more red-shifted absorption than octahedra of similar volumes by ∼15 nm, confirming the existence of optical facet effects in Cu2O crystals. Actually Cu2O cubes, octahedra, and rhombic dodecahedra also exhibit somewhat different colors and hence different band gap values, but such visual differences have not been well-recognized and understood until recently.8,17,21,53,63 Nanocubes with widely tunable sizes also display slightly different colors.64 In addition to Cu2O and the above KNTO example showing facetdependent optical properties, Ag3PO4, CeO2, TiO2, α-Fe2O3, and Bi25GaO39 crystals, as well as ultrathin Si nanowires, have also presented notable facet- and/or size-dependent optical effects, indicating that this optical phenomenon is a general semiconductor property.33−35,65−70

5. IMPLICATIONS AND APPLICATIONS Recognizing the various semiconductor properties described using band diagrams are affected by their exposed crystal facets, and these effects can be understood through the presence of an ultrathin surface layer with different band structures for the various surfaces causing tunable band bending, the new understandings of semiconductor materials are extremely helpful to analyze experimental data from a more comprehensive perspective. Because the various crystal surfaces have very different barrier heights to charge transport, as evidenced by their displayed wide range of electrical conductivity and photocatalytic activity behaviors from highly conductive to insulating and from highly photocatalytically active to completely inactive, we can no longer ignore the presence of facet effects affecting situations where charge transport through semiconductor surfaces or interfaces are involved. Because optical properties of semiconductor nanomaterials have facet and size components, utilizing the same idea of an ultrathin surface layer and recognizing flaws to our understanding of quantum confinement effects, naturally band gaps can be adjustable through particle shapes and extended sizes. Because these new insights of semiconductor materials still have not been widely perceived, researchers need to show more examples of semiconductor facet effects to establish their existence and significance. Such demonstrations can be quite difficult, as nanostructure preparation with sharp faces is very challenging, but true science emerges when a system is welldefined and characterized. Simply said, ionic crystals are naturally faceted; using spherical models to describe ionic crystals without regard to facet presence would not truly advance our understating of semiconductor materials. How can we be insensitive to the fact that a semiconductor crystal can simultaneously behave like a metal, a semiconductor, and an insulator, and not see the opportunities the phenomenon holds, but be superexcited when conducting polymers and graphene were first reported? At this early stage of facet effect development, their myriad applications may still be limited by our imagination. The immediate impact is on vast photocatalytic processes and solar cell and even battery design in which charge transport through

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael H. Huang: 0000-0002-5648-4345 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (MOST 104-2119-M-007-013MY3,105-2633-M-007-003, 106-2811-M-007-004, and 1062811-M-007-028).

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REFERENCES

(1) Chiu, C.-Y.; Huang, M. H. Achieving Polyhedral Nanocrystals Growth with Systematic Shape Control. J. Mater. Chem. A 2013, 1, 8081−8092. (2) Huang, M. H.; Rej, S.; Hsu, S.-C. Facet-Dependent Properties of Polyhedral Nanocrystals. Chem. Commun. 2014, 50, 1634−1644. (3) Bai, S.; Wang, L.; Li, Z.; Xiong, Y. Facet-Engineered Surface and Interface Design of Photocatalytic Materials. Adv. Sci. 2017, 4, 1600216. (4) Shang, Y.; Guo, L. Facet-Controlled Synthetic Strategy of Cu2OBased Crystals for Catalysis and Sensing. Adv. Sci. 2015, 2, 1500140. (5) Huang, M. H.; Rej, S.; Chiu, C.-Y. Facet-Dependent Optical Properties Revealed through Investigation of Polyhedral Au−Cu2O and Bimetallic Core−Shell Nanocrystals. Small 2015, 11, 2716−2726. (6) Ho, J.-Y.; Huang, M. H. Synthesis of Submicrometer-Sized Cu2O Crystals with Morphological Evolution from Cubic to Hexapod Structures and Their Comparative Photocatalytic Activity. J. Phys. Chem. C 2009, 113, 14159−14164. (7) Huang, W.-C.; Lyu, L.-M.; Yang, Y.-C.; Huang, M. H. Synthesis of Cu2O Nanocrystals from Cubic to Rhombic Dodecahedral Structures and Their Comparative Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134, 1261−1267. 13



ACS Applied Materials & Interfaces (8) Chanda, K.; Rej, S.; Huang, M. H. Facet-Dependent Catalytic Activity of Cu2O Nanocrystals in the One-Pot Synthesis of 1,2,3Triazoles by Multicomponent Click Reactions. Chem. - Eur. J. 2013, 19, 16036−16043. (9) Chanda, K.; Rej, S.; Huang, M. H. Investigation of Facet Effects on the Catalytic Activity of Cu2O Nanocrystals for Efficient Regioselective Synthesis of 3,5-Disubstituted Isoxazoles. Nanoscale 2013, 5, 12494−12501. (10) Wen, C. Z.; Jiang, H. B.; Qiao, S. Z.; Yang, H. G.; Lu, G. Q. Synthesis of High-Reactive Facets Dominated Anatase TiO2. J. Mater. Chem. 2011, 21, 7052−7061. (11) Liu, S.; Yu, J.; Jaroniec, M. Anatase TiO2 with Dominant HighEnergy {001} Facets: Synthesis, Properties, and Applications. Chem. Mater. 2011, 23, 4085−4093. (12) Liu, G.; Yu, J. C.; Lu, G. Q.; Cheng, H.-M. Crystal Facet Engineering of Semiconductor Photocatalysts: Motivations, Advances and Unique Properties. Chem. Commun. 2011, 47, 6763−6783. (13) Pan, L.; Zou, J.-J.; Wang, S.; Liu, X.-Y.; Zhang, X.; Wang, L. Morphology Evolution of TiO2 Facets and Vital Influences on Photocatalytic Activity. ACS Appl. Mater. Interfaces 2012, 4, 1650− 1655. (14) Kuo, C.-H.; Yang, Y.-C.; Gwo, S.; Huang, M. H. FacetDependent and Au Nanocrystal-Enhanced Electrical and Photocatalytic Properties of Au−Cu2O Core−Shell Heterostructures. J. Am. Chem. Soc. 2011, 133, 1052−1057. (15) Tan, C.-S.; Hsu, S.-C.; Ke, W.-H.; Chen, L.-J.; Huang, M. H. Facet-Dependent Electrical Conductivity Properties of Cu2O Crystals. Nano Lett. 2015, 15, 2155−2160. (16) Yang, Y.-C.; Wang, H.-J.; Whang, J.; Huang, J.-S.; Lyu, L.-M.; Lin, P.-H.; Gwo, S.; Huang, M. H. Facet-Dependent Optical Properties of Polyhedral Au−Cu2O Core−Shell Nanocrystals. Nanoscale 2014, 6, 4316−4324. (17) Ke, W.-H.; Hsia, C.-F.; Chen, Y.-J.; Huang, M. H. Synthesis of Ultrasmall Cu2O Nanocubes and Octahedra with Tunable Sizes for Facet-Dependent Optical Property Examination. Small 2016, 12, 3530−3534. (18) Yuan, G.-Z.; Hsia, C.-F.; Lin, Z.-W.; Chiang, C.; Chiang, Y.-W.; Huang, M. H. Highly Facet-Dependent Photocatalytic Properties of Cu2O Crystals Established through the Formation of Au-Decorated Cu2O Heterostructures. Chem. - Eur. J. 2016, 22, 12548−12556. (19) Wu, S.-C.; Tan, C.-S.; Huang, M. H. Strong Facet Effects on Interfacial Charge Transfer Revealed through the Examination of Photocatalytic Activities of Various Cu2O−ZnO Heterostructures. Adv. Funct. Mater. 2017, 27, 1604635. (20) Tan, C.-S.; Hsiao, C.-H.; Wang, S.-C.; Liu, P.-H.; Lu, M.-Y.; Huang, M. H.; Ouyang, H.; Chen, L.-J. Sequential Cation Exchange Generated Superlattice Nanowires Forming Multiple p-n Heterojunctions. ACS Nano 2014, 8, 9422−9426. (21) Chu, C.-Y.; Huang, M. H. Facet-Dependent Photocatalytic Properties of Cu2O Crystals Probed by Electron, Hole and Radical Scavengers. J. Mater. Chem. A 2017, 5, 15116−15123. (22) Kim, C. W.; Yeob, S. J.; Cheng, H.-M.; Kang, Y. S. A Selectively Exposed Crystal Facet-Engineered TiO2 Thin Film Photoanode for the Higher Performance of the Photoelectrochemical Water Splitting Reaction. Energy Environ. Sci. 2015, 8, 3646−3653. (23) Liu, G.; Yin, L.-C.; Pan, J.; Li, F.; Wen, L.; Zhen, C.; Cheng, H.M. Greatly Enhanced Electronic Conduction and Lithium Storage of Faceted TiO2 Crystals Supported on Metallic Substrates by Tuning Crystallographic Orientation of TiO2. Adv. Mater. 2015, 27, 3507− 3512. (24) Chen, Y.-J.; Chiang, Y.-W.; Huang, M. H. Synthesis of Diverse Ag2O Crystals and Their Facet-Dependent Photocatalytic Activity Examination. ACS Appl. Mater. Interfaces 2016, 8, 19672−19679. (25) Tan, C.-S.; Chen, Y.-J.; Hsia, C.-F.; Huang, M. H. FacetDependent Electrical Conductivity Properties of Silver Oxide Crystals. Chem. - Asian J. 2017, 12, 293−297. (26) Wu, J.-K.; Lyu, L.-M.; Liao, C.-W.; Wang, Y.-N.; Huang, M. H. Fast Synthesis of PbS Nanocrystals in Aqueous Solution with Shape

Evolution from Cubic to Octahedral Structures and Their Assembled Structures. Chem. - Eur. J. 2012, 18, 14473−14478. (27) Chen, H.-S.; Wu, S.-C.; Huang, M. H. Direct Synthesis of SizeTunable PbS Nanocubes and Octahedra and the pH Effect on Crystal Shape Control. Dalton Trans. 2015, 44, 15088−15094. (28) Tan, C.-S.; Chen, H.-S.; Chiu, C.-Y.; Wu, S.-C.; Chen, L.-J.; Huang, M. H. Facet-Dependent Electrical Conductivity Properties of PbS Nanocrystals. Chem. Mater. 2016, 28, 1574−1580. (29) Tan, C.-S.; Huang, M. H. Metal-like Band Structures of Ultrathin Si {111} and {112} Surface Layers Revealed through Density Functional Theory Calculations. Chem. - Eur. J. 2017, 23, 11866−11871. (30) Tan, C.-S.; Hsieh, P.-L.; Chen, L.-J.; Huang, M. H. Silicon Wafers Revealing Facet-Dependent Electrical Conductivity Properties. Angew. Chem., Int. Ed. 2017, 56, 15339−15343. (31) Li, C.; Koenigsmann, C.; Ding, W.; Rudshteyn, B.; Yang, K. R.; Regan, K. P.; Konezny, S. J.; Batista, V. S.; Brudvig, G. W.; Schmuttenmaer, C. A.; Kim, J.-H. Facet-Dependent Photoelectrochemical Performance of TiO2 Nanostructures: An Experimental and Computational Study. J. Am. Chem. Soc. 2015, 137, 1520−1529. (32) Li, Y.; Chen, S.; He, H.; Zhang, Y.; Wang, C. Tuning Activities of K1.9Na0.1Ta2O6·2H2O Nanocrystals in Photocatalysis by Controlling Exposed Facets. ACS Appl. Mater. Interfaces 2013, 5, 10260− 10265. (33) Wang, Y. L.; Li, Y. H.; Wang, X. L.; Hou, Y.; Chen, A. P.; Yang, H. G. Effects of Redox Mediators on α-Fe2O3 Exposed by {012} and {104} Facets for Photocatalytic Water Oxidation. Appl. Catal., B 2017, 206, 216−220. (34) Lei, W.; Zhang, T.; Gu, L.; Liu, P.; Rodriguez, J. A.; Liu, G.; Liu, M. Surface-Structure Sensitivity of CeO2 Nanocrystals in Photocatalysis and Enhancing the Reactivity with Nanogold. ACS Catal. 2015, 5, 4385−4393. (35) Liu, J.; Lu, W.; Tian, B.; Hu, B.; Jin, L.; Shi, Y.; Li, L.; Wang, Z. Shape-Controlled Synthesis and Facet-Dependent Performance of Single-Crystal Bi25GaO39 Photocatalysts. CrystEngComm 2016, 18, 7715−7721. (36) Guan, X.; Guo, L. Cocatalytic Effect of SrTiO3 on Ag3PO4 toward Enhanced Photocatalytic Water Oxidation. ACS Catal. 2014, 4, 3020−3026. (37) Huang, M.; Yan, Y.; Feng, W.; Weng, S.; Zheng, Z.; Fu, X.; Liu, P. Controllable Tuning Various Ratios of ZnO Polar Facets by Crystal Seed-Assisted Growth and Their Photocatalytic Activity. Cryst. Growth Des. 2014, 14, 2179−2186. (38) Meng, M.; Wu, X.; Zhu, X.; Yang, L.; Gan, Z.; Zhu, X.; Liu, L.; Chu, P. K. Cubic In2O3 Microparticles for Efficient Photoelectrochemical Oxygen Evolution. J. Phys. Chem. Lett. 2014, 5, 4298−4304. (39) Liu, Y.; Gu, X.; Qi, W.; Zhu, H.; Shan, H.; Chen, W.; Tao, P.; Song, C.; Shang, W.; Deng, T.; Wu, J. Enhancing the Photocatalytic Hydrogen Evolution Performance of a Metal/Semiconductor Catalyst through Modulation of the Schottky Barrier Height by Controlling the Orientation of the Interface. ACS Appl. Mater. Interfaces 2017, 9, 12494−12500. (40) Zha, C.; Ji, C.; Zhang, J.; Shen, L.; Zhang, X.; Dong, S.; Bao, N. Facet Engineering of Monodisperse PbS Nanocrystals with Shapeand Facet-Dependent Photoresponse Activity. RSC Adv. 2016, 6, 107151−107157. (41) Liu, J.; Lu, W.; Tian, B.; Hu, B.; Jin, L.; Shi, Y.; Li, L.; Wang, Z. Shape-Controlled Synthesis and Facet-Dependent Performance of Single-Crystal Bi25GaO39 Photocatalysts. CrystEngComm 2016, 18, 7715−7721. (42) Li, J.; Bai, H.; Yi, W.; Liu, J.; Li, Y.; Zhang, Q.; Yang, H.; Xi, G. Synthesis and Facet-Dependent Photocatalytic Activity of Strontium Titanate Polyhedron Nanocrystals. Nano Res. 2016, 9, 1523−1531. (43) Zheng, B.; Wang, X.; Liu, C.; Tan, K.; Xie, Z.; Zheng, L. Highly-Efficiently Visible Light-Responsive Photocatalysts: Ag3PO4 Tetrahedral Microcrystals with Exposed {111} Facets of High Surface Energy. J. Mater. Chem. A 2013, 1, 12635−12640. 14




(61) Yang, K.-H.; Hsu, S.-C.; Huang, M. H. Facet-Dependent Optical and Photothermal Properties of Au@Ag−Cu2O Core−Shell Nanocrystals. Chem. Mater. 2016, 28, 5140−5146. (62) Wang, H.-J.; Yang, K.-H.; Hsu, S.-C.; Huang, M. H. Photothermal Effects from Au−Cu2O Core−Shell Nanocubes, Octahedra, and Nanobars with Broad Near-Infrared Absorption Tunability. Nanoscale 2016, 8, 965−972. (63) Tsai, Y.-H.; Chanda, K.; Chu, Y.-T.; Chiu, C.-Y.; Huang, M. H. Direct Formation of Small Cu2O Nanocubes, Octahedra, and Octopods for Efficient Synthesis of Triazoles. Nanoscale 2014, 6, 8704−8709. (64) Kuo, C.-H.; Chen, C.-H.; Huang, M. H. Seed-Mediated Synthesis of Monodispersed Cu2O Nanocubes with Five Different Size Ranges from 40 to 420 nm. Adv. Funct. Mater. 2007, 17, 3773− 3780. (65) Martin, D. J.; Umezawa, N.; Chen, X.; Ye, J.; Tang, J. Facet Engineered Ag3PO4 for Efficient Water Photooxidation. Energy Environ. Sci. 2013, 6, 3380−3386. (66) Bi, Y.; Ouyang, S.; Umezawa, N.; Cao, J.; Ye, J. Facet Effect of Single-Crystalline Ag3PO4 Sub-microcrystals on Photocatalytic Properties. J. Am. Chem. Soc. 2011, 133, 6490−6492. (67) Liu, G.; Sun, C.; Yang, H. G.; Smith, S. C.; Wang, L.; Lu, G. Q.; Cheng, H.-M. Nanosized Anatase TiO2 Single Crystals for Enhanced Photocatalytic Activity. Chem. Commun. 2010, 46, 755−757. (68) Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q.; Cheng, H.M. Titanium Dioxide Crystals with Tailored Facets. Chem. Rev. 2014, 114, 9559−9612. (69) Ye, L.; Jin, X.; Ji, X.; Liu, C.; Su, Y.; Xie, H.; Liu, C. FacetDependent Photocatalytic Reduction of CO2 on BiOCl Nanosheets. Chem. Eng. J. 2016, 291, 39−46. (70) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Control of Thickness and Orientation of Solution-Grown Silicon Nanowires. Science 2000, 287, 1471−1473.

(44) Wang, H.; Yang, J.; Li, X.; Zhang, H.; Li, J.; Guo, L. FacetDependent Photocatalytic Properties of AgBr Nanocrystals. Small 2012, 8, 2802−2806. (45) Huang, M.; Weng, S.; Wang, B.; Hu, J.; Fu, X.; Liu, P. Various Facet Tunable ZnO Crystals by a Scalable Solvothermal Synthesis and Their Facet-Dependent Photocatalytic Activities. J. Phys. Chem. C 2014, 118, 25434−25440. (46) Liu, C.; Han, X.; Xie, S.; Kuang, Q.; Wang, X.; Jin, M.; Xie, Z.; Zheng, L. Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet-Induced Spontaneous Separation of Photogenerated Electrons and Holes. Chem. - Asian J. 2013, 8, 282− 289. (47) Ong, W.-J.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T.; Mohamed, A. R. Facet-Dependent Photocatalytic Properties of TiO2-Based Composites for Energy Conversion and Environmental Remediation. ChemSusChem 2014, 7, 690−719. (48) Wang, W.-C.; Lyu, L.-M.; Huang, M. H. Investigation of the Effects of Polyhedral Gold Nanocrystal Morphology and Facets on the Formation of Au−Cu2O Core−Shell Heterostructures. Chem. Mater. 2011, 23, 2677−2684. (49) Zhang, Y.; Deng, B.; Zhang, T.; Gao, D.; Xu, A.-W. Shape Effects of Cu2O Polyhedral Microcrystals on Photocatalytic Activity. J. Phys. Chem. C 2010, 114, 5073−5079. (50) Xu, H.; Wang, W.; Zhu, W. Shape Evolution and SizeControllable Synthesis of Cu2O Octahedra and Their MorphologyDependent Photocatalytic Properties. J. Phys. Chem. B 2006, 110, 13829−13834. (51) Sun, L.; Wu, X.; Meng, M.; Zhu, X.; Chu, P. K. Enhanced Photodegradation of Methyl Orange Synergistically by Microcrystal Facet Cutting and Flexible Electrically-Conducting Channels. J. Phys. Chem. C 2014, 118, 28063−28068. (52) Su, Y.; Nathan, A.; Ma, H.; Wang, H. Precise Control of Cu2O Nanostructures and LED-Assisted Photocatalysis. RSC Adv. 2016, 6, 78181−78186. (53) Liu, L.; Yang, W.; Sun, W.; Li, Q.; Shang, J. K. Creation of Cu2O@TiO2 Composite Photocatalysts with p−n Heterojunctions Formed on Exposed Cu2O Facets, Their Energy Band Alignment Study, and Their Enhanced Photocatalytic Activity under Illumination with Visible Light. ACS Appl. Mater. Interfaces 2015, 7, 1465−1476. (54) Pu, Y.-C.; Chou, H.-Y.; Kuo, W.-S.; Wei, K.-H.; Hsu, Y.-J. Interfacial Charge Carrier Dynamics of Cuprous Oxide-Reduced Graphene Oxide (Cu2O-rGO) Nanoheterostructures and Their Related Visible-Light-Driven Photocatalysis. Appl. Catal., B 2017, 204, 21−32. (55) Kandjani, A. E.; Sabri, Y. M.; Periasamy, S. R.; Zohora, N.; Amin, M. H.; Nafady, A.; Bhargava, S. K. Controlling Core/Shell Formation of Nanocubic p-Cu2O/n-ZnO Toward Enhanced Photocatalytic Performance. Langmuir 2015, 31, 10922−10930. (56) Barbieriková, Z.; Dvoranová, D.; Sofianou, M.-V.; Trapalis, C.; Brezová, V. UV-Induced Reactions of Mg2+-Doped Anatase Nanocrystals with Exposed {001} Facets: An EPR Study. J. Catal. 2015, 331, 39−48. (57) Wang, G.; Ma, X.; Huang, B.; Cheng, H.; Wang, Z.; Zhan, J.; Qin, X.; Zhang, X.; Dai, Y. Controlled Synthesis of Ag2O Microcrystals with Facet-Dependent Photocatalytic Activities. J. Mater. Chem. 2012, 22, 21189−21194. (58) Hsu, S.-C.; Liu, S.-Y.; Wang, H.-J.; Huang, M. H. FacetDependent Surface Plasmon Resonance Properties of Au−Cu2O Core−Shell Nanocubes, Octahedra, and Rhombic Dodecahedra. Small 2015, 11, 195−201. (59) Kuo, C.-H.; Hua, T.-E.; Huang, M. H. Au Nanocrystal-Directed Growth of Au−Cu2O Core−Shell Heterostructures with Precise Morphological Control. J. Am. Chem. Soc. 2009, 131, 17871−17878. (60) Rej, S.; Wang, H.-J.; Huang, M.-X.; Hsu, S.-C.; Tan, C.-S.; Lin, F.-C.; Huang, J.-S.; Huang, M. H. Facet-Dependent Optical Properties of Pd-Cu2O Core-Shell Nanocubes and Octahedra. Nanoscale 2015, 7, 11135−11141. 15


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