Facet-Dependent Electrical, Photocatalytic, and Optical Properties of

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15828 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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ACS Applied Materials & Interfaces

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 non-conductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. absorption wavelengths.

The crystal surfaces can even tune their light

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 1

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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 2


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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 general idea to explain their facet-dependent electrical and photocatalytic properties emerged.14,15 Optical facet effects, in which the exposed crystal faces tunes 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 photocatalytic 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 3



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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 nanocrystals are also presented.

Optical facet effects of Cu2O

The idea of this ultrathin surface layer is also useful

to understand the observed facet-specific 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, making electrical contacts to nanoscale 3-dimensional objects can be quite difficult, especially contacts need to be made on specific crystal faces. 4


For

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polyhedral crystals that are hundreds of nanometers in size, it is more convenient to use tungsten probes connected to a multi-probe 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} 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 5



a more typical semiconductor I–V curve has been recorded.

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Surprisingly, the {110}

facets of a Cu2O rhombic dodecahedron is completely non-conductive, 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 bended 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 with electrical 6


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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 7



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actual electronic or photocatalytic behavior of a specific surface, as such analysis cannot explain why a facet should be electrically insulating or photocatalytically 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 facet-dependent 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, while 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 8


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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 facets of Ag2O are most conductive.

Figure 2b shows the {111}

The {100} faces are moderately conductive,

but the {110} faces are nearly non-conductive. 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. current is higher for sub-micrometer-sized crystals.

Generally

This is another semiconductor

material showing strong facet-dependent electrical conductivity properties. 9



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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 corner-truncated 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 10


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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 non-doped, 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 responses have also been obtained.

Asymmetrical I–V

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 into and out of a crystal.

This idea is also useful to the explanation of

photocatalytic activities of semiconductor crystals.

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

11



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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

Since 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 12


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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 µmol h–1 g–1 vs. 3.7 µmol h–1 g–1). Electrochemical impedance spectroscopy (EIS) reveals smaller charge carrier transfer 13



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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 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. While the CeO2 nanorods adsorbed MO, the cubes were found not to adsorb MO due to the repulsive electrostatic interaction between the anionic MO dye and the negatively charge {100} surface of CeO2. 14


The

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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 photo-reactive.

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. CeO2 cubes.

The active {110} edges are also present on the

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 photocatalytic water oxidation reaction.

This charge separation should enhance 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 15



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 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 16


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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.

Since 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 17



properties.41–44

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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–Cu2O core–shell cubes remained inactive.48 also found photocatalytic inactivity of Cu2O cubes.49–52

Other studies have

However, a number of

reports still show that Cu2O cubes exhibit a moderate photocatalytic activity.14,53–55 18


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These contradicting results can be rationalized by the presence of a small amount of other crystal shapes, 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 Au-decorated

Cu2O rhombic dodecahedra and octahedra showed enhanced photocatalytic activities compared to the pristine ones, Au-deposited Cu2O cubes remained inactive (Figure 4a).18

Since hydroxyl (•OH) and superoxide anion (•O2–) 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. 19



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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 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 {100} faces because of a large barrier present, and this barrier is likely the same thin layer 20


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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 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

Since 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 21



behaviors.

Page 22 of 48

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 22


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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. qualitatively match with this assumption.19

Results from DFT calculations

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.

23



Page 24 of 48

4. FACET-DEPENDENT OPTICAL PROPERTIES The fact optical properties of Cu2O crystals are also facet-dependent was discovered when single-crystalline octahedral or cubic gold nanocrystals with sizes of ~ 50 nm or smaller were used as cores for the fabrication of Au–Cu2O core–shell cubes, octahedra, cuboctahedra, and rhombic dodecahedra (Figure 5a and 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 high-resolution 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. 24


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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 red-shifted than that of largerAu‒Cu2O octahedra and rhombic dodecahedra (Figure 5a and 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

non-uniform.

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 25



Page 26 of 48

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 red-shifting 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 ºC 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. 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,

26


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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 display slightly different colors.64

Nanocubes with widely tunable sizes also In addition to Cu2O and the above KNTO

example showing facet-dependent 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.

Since 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 27



Page 28 of 48

longer ignore the presence of facet effects affecting situations where charge transport through semiconductor surfaces or interfaces are involved.

Since 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.

Since 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 well defined 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 super-excited 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 28


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semiconductor structures or films are involved.

Knowing certain bad faces or

heterojunctions can detrimentally affect device performance should be important.

In

a sense, blue light-emitting 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. thermoelectric materials development.

Facet effects may also be relevant to Since 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.

Since 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 29



facet effects are fundamental properties of semiconductors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Michael H. Huang: 0000-0002-5648-4345 Notes The authors declare no competing financial interest.

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

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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. or at a thickness of 6.2Å, a gap appears near the Fermi level. believed to give rise to the observed facet effects. ref. 15.

With 5 (111) plane layers, This thin layer is

Reprinted with permission from

Copyright 2015 American Chemical Society.

43



Page 44 of 48

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). {111}, {100}, and {110} faces of Ag2O crystals. contacting facets.

(b) I‒V curves for the

The models illustrate the

(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. Reprinted with permission: Chemistry. & Co KGaA.

Inset shows the expanded I‒V curves.

Panel a from ref. 22.

Panel b and c from ref. 25. Panel d from ref. 28.

Copyright 2015 Royal Society of

Copyright 2017 Wiley-VCH Verlag GmbH


44


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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 45



and b from ref. 32. from ref. 33.


Page 46 of 48

Panels c and d

Copyright 2016 Elsevier B. V.

Panels e and f from ref. 34.


Panels g and h from ref. 36.


Panel i from ref. 38.

American Chemical Society.

Panel j from ref. 39.

Copyright 2014

Copyright 2017 American

Chemical Society.

Figure 4. (a) A plot of the extent of photodegradation of MOas 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. Extents of MO photodegradation as a function of time using Ag2O rhombic 46


(d)

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dodecahedra, octahedra, and cubes as the photocatalysts.

(e) A 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. b from ref. 18. from ref. 21.

Reprinted with permission: Panel a and

Copyright 2016 Wiley-VCH Verlag GmbH & Co KGaA. Copyright 2017 Royal Society of Chemistry.

Panel c

Panel d from ref. 24.

Copyright 2016 American Chemical Society. Panels e and f from ref. 19. Copyright 2017 Wiley-VCH Verlag GmbH & Co KGaA.

Figure 5. (a) UV–vis absorption spectra of octahedral, cuboctahedral, and cubic Au–Cu2O core–shell nanocrystals with50-nm octahedral Au cores. (b) UV‒vis absorption spectra of size-tunable Au‒Cu2O core‒shell rhombic dodecahedra, 47



octahedra, and cubes with 35-nm octahedral Au cores.

Page 48 of 48

(c) UV‒vis absorption

spectra of Au‒Cu2O rectangular nanobars with tunable Cu2O shell thicknesses synthesized from 57-nm Au nanorod cores.

(d) A 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 Verlag GmbH & Co KGaA. Chemistry.

Panel c from ref. 62.

Panel d from ref. 17.

Copyright 2016 Royal Society of

Copyright 2016 Wiley-VCH Verlag GmbH & Co

KGaA.

TOC Graphic

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Facet-Dependent Electrical, Photocatalytic, and Optical Properties of

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