Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O

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Surfaces, Interfaces, and Applications

Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment Gollapally Naresh, Pei-Lun Hsieh, Vandana Meena, Shih-Kuang Lee, Yi-Hsuan Chiu, Mahesh Madasu, An-Ting Lee, Hsin-Yi Tsai, Ting-Hsuan Lai, Yung-Jung Hsu, Yu-Chieh Lo, and Michael H. Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19197 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 3, 2019

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ACS Applied Materials & Interfaces

Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment Gollapally Naresh,† Pei-Lun Hsieh,‡ Vandana Meena,§ Shih-Kuang Lee,# Yi-Hsuan Chiu,# Mahesh Madasu,† An-Ting Lee,† Hsin-Yi Tsai,† Ting-Hsuan Lai,# Yung-Jung Hsu,# Yu-Chieh Lo,*,# and Michael H. Huang*,† †Department

of Chemistry and Frontier Research Center on Fundamental and Applied

Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan ‡Department

of Materials Science and Engineering, National Tsing Hua University,

Hsinchu 30013, Taiwan §Department

of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667,

India #Department

of Materials Science and Engineering, National Chiao Tung University,

Hsinchu 30010, Taiwan

ABSTRACT:

ZnS particles were grown over Cu2O cubes, octahedra, and rhombic

dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu2O cubes stay photocatalytically inactive.

ZnS-decorated

Cu2O octahedra show enhanced photocatalytic activity resulting from better charge carrier separation upon photoexcitation.

Surprisingly, Cu2O rhombic dodecahedra 1

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give greatly suppressed photocatalytic activity after ZnS deposition.

Electron

paramagnetic resonance (EPR) spectra agree with these experimental observations. Time-resolved photoluminescence (TRPL) profiles provide charge transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu2O (110)/ZnS (200) plane interface.

A modified Cu2O–ZnS band diagram is presented.

Density functional

theory (DFT) calculations generating plane-specific band energy diagrams of Cu2O and ZnS match well with the experimental results, showing charge transfer across the Cu2O (110)/ZnS (200) plane interface would not happen.

This example further

illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.

KEYWORDS: band alignment, cuprous oxide, facet-dependent properties, heterojunctions, interfacial charge transfer, zinc sulfide

INTRODUCTION Cu2O nanocrystals with different shapes have shown strongly facet-dependent electrical, photocatalytic, optical, and likely heat transport properties.1–13

These

interesting phenomena can be attributed to the existence of a thin surface layer having 2 ACS Paragon Plus Environment

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tunable surface band bending for different crystal surfaces supported by DFT calculations.1

For photocatalysis, this ultrathin surface layer affects the barrier

height photogenerated electrons and holes encounter at the crystal surface, such that carrier migration to the Cu2O {100} faces is inhibited as verified by electron paramagnetic resonance (EPR) measurements and various scavenger tests.5,6

Similar

facet-dependent properties have also been recorded in other semiconductors including Ag2O, TiO2, PbS and Ag3PO4, so these are general semiconductor properties.14‒19 More recently, certain Si and Ge surfaces have revealed metal-like band structures through DFT calculations, and Si and Ge wafers indeed exhibit facet effects in electrical conductivity.20‒23

DFT results reveal notable differences in frontier orbital

electron distribution, bond geometry, and bond length within this ultrathin surface layer between semiconducting and metal-like Si and Ge faces. If semiconductor crystals of different shapes can be synthesized, their facet-dependent photocatalytic activity is most widely studied with the aim to improve photocatalytic performance.5,14,24–29

Relative crystal surface energies and

the degree of molecule-surface interactions are frequently considered to account for the face-related photocatalytic properties.30–32

Such interpretations are likely

incorrect, when one realizes that Cu2O and Ag2O cubes, rhombic dodecahedra, and octahedra having the same crystal structures and exposing the same surface planes 3 ACS Paragon Plus Environment


actually display opposite photocatalytic trends.5,6,16 be adsorbed between semiconductor interfaces.

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Substrate molecules also cannot

Furthermore, semiconductor

heterojunctions with properly located band energies, such as Cu2O–ZnO and Cu2O– CdS heterostructures, can still give total photocatalytic activity inhibition for certain interfacial plane combinations.33,34

These results suggest that facet-dependent

photocatalytic properties should be explained the same way as their facet-dependent electrical conductivity behaviors, that an ultrathin surface layer at the crystal surface or at the heterojunction tunes the surface band energies to create very different barrier heights to charge transport depending on the exiting or interfacial planes. Because photocatalytic activity suppression is rarely reported and discussed in the literature, it is necessary to examine more semiconductor heterojunctions to show that such events occur more often than we think.

In this study, we have lightly

deposited ZnS nanoparticles on Cu2O cubes, rhombic dodecahedra, and octahedra to form heterostructures with a favorable band alignment for improved photocatalytic activity.

ZnS-decorated Cu2O cubes remain photocatalytically inactive.

Interestingly, while ZnS–Cu2O octahedra showed enhanced photocatalytic activity, ZnS deposition on Cu2O rhombic dodecahedra led to drastic suppression. measurements agree with the photocatalysis results.

EPR

Transmission electron

microscopy (TEM) analysis of the interfacial regions provides the contacting planes. 4 ACS Paragon Plus Environment

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The interfacial band alignment should be unfavorable to charge transport arising from significant interfacial band bending at the ZnS (200)/Cu2O (111) contacting planes. DFT calculation-generated plane-specific band diagrams for Cu2O and ZnS also support the experimental observations.

Again precise control of interfacial regions

reveals that photocatalytic enhancement, suppression, and total inactivity can all happen to the same heterostructures but with different contacting planes.

EXPERIMENTAL SECTION Preparation of ZnS-Decorated Cu2O Crystals.

Cu2O polyhedra were prepared

following reported procedures described in the Supporting Information.35,36

Cu2O

cubes, rhombic dodecahedra, and octahedra have been measured to possess surface areas of 2.84, 1.35 and 0.56 m2g‒1, respectively.37

For comparison of photocatalytic

activity, the same total particle surface area should be used, so Cu2O cubes, octahedra, and rhombic dodecahedra with weight ratios of 1:5:2 were used.

Initially, 1 mg/mL

of Cu2O crystals in 100% ethanol was prepared individually for all the three shapes. Next, 3.75, 7.5 and 18.75 mL of cubic, rhombic dodecahedral, and octahedral Cu2O solutions were added to 50-mL sample vials containing 21.75, 18.0, and 6.75 mL of ethanol, respectively.

Then 2.25 mL of 12.5 mM ZnCl2 solution was added

dropwise into each vial placed in a 32 ºC water bath.

After stirring for 1 h, 2.25 mL

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of 12.5 mM TAA solution was added and raised the temperature to 60 ºC.

After 1 h

of reaction at 60 ºC, the solutions were cooled and centrifuged at 7000 rpm for 5 min. The obtained products were washed with ethanol and water at a volume ratio of 1:1 for several times.

For convenience, CCZ, CRZ, and COZ refer to Cu2O cubes,

rhombic dodecahedra, and octahedra with ZnS deposition, respectively. Electron Paramagnetic Resonance Measurements. capture photogenerated hydroxyl radicals.

DMPO was employed to

A 0.1 M DMPO solution was prepared by

purification with activated carbon (30 mg mL–1) for 5 times to remove contamination peaks in EPR spectra.

Here 0.1, 0.2, and 0.5 mL of the 1 mg/mL Cu2O cube,

rhombic dodecahedron, and octahedron solutions with and without ZnS loading were dispersed in 1.0 mL of this DMPO solution and illuminated with light for 3 min from a 300-W xenon lamp.

After illumination, EPR of the solutions were measured

immediately using a Bruker ELEXSYS E580-400 EPR spectrometer. Photoluminescence (PL) Measurements.

The PL measurement procedures

followed those reported in the previous studies.38–41

See the Supporting Information

for instrumentation detail.

RESULTS AND DISCUSSION Cuprous oxide cubes, octahedra, and rhombic dodecahedra have been prepared using 6 ACS Paragon Plus Environment

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reported methods.35,36

Figure S1 gives SEM images of the produced Cu2O cubes,

octahedra, and rhombic dodecahedra and plots of their distribution of sizes.

The

sharp-faced crystals are highly monodisperse in dimensions with average sizes of 234, 276, and 412 nm for cubes, rhombic dodecahedra, and octahedra.

ZnCl2 and TAA as

the sulfur source were added to the Cu2O crystals having approximately the same total particle surface area and heated at 60 ºC for 1 h to form ZnS particles lightly deposited on the Cu2O crystal surfaces.

Figure 1 shows scanning electron

microscopy (SEM) images of the produced ZnS-deposited Cu2O cubes (CCZ), rhombic dodecahedra (CRZ), and octahedra (COZ).

ZnS deposition was sparse

because we intended to make heterostructures instead of core–shell particles. Figure S2 provides a transmission electron microscopy (TEM) image of a CCZ cube, its elemental line scan, and EDS spectrum.

While enhanced signals from sulfur have

been recorded at the two ends of the cube, zinc signals are quite weak.

This happens

possibly because several Cu and Zn X-ray line energies are quite close to each other. X-ray diffraction (XRD) patterns of the different ZnS–Cu2O heterostructures and ZnS particles synthesized from ZnCl2 and TAA under the same reaction conditions are shown in Figure 2.

Due to light ZnS deposition, only Cu2O diffraction peaks

were recorded for the heterostructures.

The XRD pattern of pure ZnS particles

matches with a standard pattern of ZnS with broad peak widths. 7 ACS Paragon Plus Environment

To confirm the


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formation of ZnS particles on Cu2O cubes, ZnCl2 and TAA amounts were increased 10-fold (CCZ*10).

This time diffraction peaks from ZnS were observed.

However, for photocatalysis experiments, heterostructures with a normal ZnS deposition amount was used.

Figure S3 offers diffuse reflectance spectra of the

synthesized Cu2O cubes, rhombic dodecahedra, octahedra, and ZnS particles for the determination of their band gaps.

This information will be useful in the construction

of the band diagram of Cu2O–ZnS heterostructures.

The determined band gap

values of Cu2O cubes, rhombic dodecahedra, octahedra, and ZnS particles are 2.17, 2.09, 1.98, and 3.57 eV, respectively.

Due to light ZnS deposition on these Cu2O

polyhedra, essentially no change to the diffuse reflectance spectra has been recorded after ZnS decoration (Figure S4).

ZnS absorption in the ultraviolet light region

appears when the deposition amount is greatly increased (Figure S4). XPS spectra were recorded to further analyze these heterostructures (Figure S5). All three samples have the same full XPS spectra, so their surface states are identical. Figure 3 gives the spectra in the Cu 2p, Zn 2p, and S 2p regions.

The Cu 2p3/2 and

Cu 2p1/2 peaks at respectively 932.5 and 952.4 eV are observed in all three samples. Two small satellite bands at 943.8 and 946.7 eV are barely visible. These are the characteristic peaks of Cu2O.

No CuO satellite peaks were observed in the Cu 2p

peak area, demonstrating CuO was not generated in the heterostructure formation 8 ACS Paragon Plus Environment

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process.

All samples have Zn 2p3/2 and Zn 2p1/2 peaks at 1022.1 eV and 1045.1 eV,

respectively, due to the Zn(II) oxidation state. 161.1 eV and 162.2 eV.

The S 2p3/2 and 2p1/2 peaks are at

XPS data also confirm the presence of ZnS particles on

Cu2O crystals. Figure 4 shows the results of photocatalytic methyl orange decomposition using various Cu2O crystals and ZnS-deposited Cu2O heterostructures as the catalysts. UV‒vis spectra of methyl orange with respect to irradiation time are provided in Figures S6 and S7, showing the photocatalysis results are highly reproducible. Expectedly, rhombic dodecahedra are more photocatalytically active for this reaction than octahedra to fully decompose methyl orange in 40 min, but Cu2O cubes are inactive.5,6

Electrons and holes are efficiently transported to the Cu2O {110}

surfaces to produce radical species, while mainly electrons are available at the {111} faces of octahedra upon light irradiation, explaining the higher photocatalytic activity of Cu2O rhombic dodecahedra.6

Charge carriers are unavailable at the Cu2O {100}

surfaces due to a high energy barrier, so Cu2O cubes are photocatalytically inactive.6 Photocatalytic activities of the heterostructures have been performed with and without placing a filter to block UV light. generated from Cu2O.

When a UV filter is placed, excitons can only be

After ZnS deposition, Cu2O cubes remain photocatalytically

inactive due to significant band bending at the Cu2O {100} faces, halting charge 9 ACS Paragon Plus Environment


carriers from reaching to the Cu2O surfaces.

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Cu2O octahedra showed the expected

photocatalytic enhancement from ZnS deposition due to facile charge carrier separation.

The observed moderate activity decrease with the use of a UV filter is

caused by a lower light intensity reaching to the reaction solution.

Remarkably, the

Cu2O rhombic dodecahedra’s photocatalytic activity is drastically suppressed after light ZnS deposition.

Methyl orange decomposition only reached around 50% after

2 h of photoirradiation on the CRZ sample.

Figure S8 provides a plot ln(C/C0) vs.

time for all the samples and their rate constants, showing the photodegradation process follows first-order reaction kinetics.

After the photocatalysis experiments,

the CCZ, CRZ, and COZ heterostructures appear to maintain their morphologies (Figure S9).

XRD patterns of all the heterostructures obtained after the

photocatalysis experiments give only Cu2O diffraction peaks, so there should be no compositional change to the photocatalysts (Figure S10). EPR spectra were taken on Cu2O cubes, rhombic dodecahedra, octahedra, and their corresponding ZnS-deposited heterostructures to verify the photocatalysis results (Figure 5).

The recorded EPR signals show dominant production of hydroxyl

radicals upon light irradiation.

Consistent with the photocatalysis observations, no

radicals are produced after light illumination on Cu2O cubes and CCZ composites. This occurs because photoexcited charge carriers cannot migrate to the Cu2O {100} 10 ACS Paragon Plus Environment

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surfaces to react with water and form radicals, and charge carriers are not produced from the ZnS side with visible light irradiation.

Hydroxyl radicals are generated in

photoirradiated Cu2O octahedra, and enhanced production of radicals in the COZ sample gives the heterostructures an improved photocatalytic activity.

In contrast,

the most photocatalytically active Cu2O rhombic dodecahedra with maximum hydroxyl radical formation show significant reduction in radical production upon ZnS decoration.

In fact, radical production from the CRZ heterostructures is notably less

than that for the COZ sample.

The results demonstrate strong effects of

semiconductor heterojunctions on photocatalytic activity. The interfacial charge dynamics of the ZnS-decorated Cu2O crystals has been studied with PL spectroscopy.

The steady-state PL spectra reveal a broad emission

band spanning from 450–700 nm for the six samples (Figure S11).

The blue and

green bands below 550 nm can be assigned to the electronic transitions from the sub-levels of conduction band to the Cu d-shells of the valence band.42,43

The

yellow (580 nm) and red emissions (680 nm) are respectively assigned to the near band edge emission and defect-related emission of Cu2O. To further probe the charge transfer dynamics at the Cu2O/ZnS interface, time-resolved PL spectra were taken and analyzed.

Note that these kinetics profiles

were obtained by recording the repetitively emissive photons of Cu2O component at 11 ACS Paragon Plus Environment


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580 nm, an emissive event directly linked to the excitonic band-to-band near edge emission.

The thus obtained kinetics information can reflect the inherent charge

transfer behavior for the ZnS-decorated Cu2O samples.

As Figure 6 shows, the three

Cu2O samples all exhibited fastened PL decay kinetics upon ZnS particle decoration, implying charge separation has occurred at the Cu2O/ZnS interface.

The kinetics

profiles are fitted with a bi-exponential function to better analyze the data.38–40

As

summarized in Table S1, two decay components, 𝜏1 and 𝜏2, along with the corresponding amplitude contributions, A1 and A2, can be obtained from the fitting results.

Here, 𝜏1 and 𝜏2 respectively represent the lifetimes of charge carriers

undergoing radiative and non-radiative recombination pathways.

By assuming that

the non-radiative pathway dictates the possible charge transfer from Cu2O to ZnS,41 the rate constant of interfacial charge transfer (kct) for the ZnS-decorated Cu2O 1

1

samples could be computed by the equation 𝑘𝑐𝑡 = 𝜏2(Cu2O ― ZnS) ― 𝜏2(Cu2O).

The

computed kct values are 0.79 × 108, 0.89 × 108 and 5.91 × 108 s–1 for CCZ, CRZ and COZ, respectively.

Importantly, COZ showed an almost one order of magnitude

larger kct value than CCZ and CRZ samples, signifying that much pronounced charge separation is prevalent in the COZ heterostructures.

The rate constant of radiative

recombination (krec) is estimated from the reciprocal of 𝜏1, which offers a comparative index to assess the effectiveness of charge separation. 12 ACS Paragon Plus Environment

The kct of COZ

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is around 3 times the value of krec, reaffirming the occurrence of pronounced charge separation.

On the contrary, the kct of CCZ and CRZ are substantially lower than the

corresponding krec, suggesting that the interfacial charge transfer from Cu2O to ZnS is kinetically unfavorable for these two cases.

These observations further support the

experimental observations for the three ZnS-decorated Cu2O samples. To better explain the observed photocatalytic suppression of the Cu2O–ZnS heterostructures, interfacial TEM characterization of CCZ, COZ, and CRZ particles were performed.

The idea is that some heterojunction combinations are unfavorable

for interfacial charge migration due to significant surface band bending for one or both components at the junction.

Figure 6 offers TEM characterization of the

interfacial region of a ZnS-decorated Cu2O octahedron.

High-resolution TEM

images show the ZnS (220) lattice planes are grown roughly parallel to the Cu2O (111) planes.

Other planes can also deposit on the Cu2O (111) planes, since some

less clear interfacial lattice planes are also observed.

This happens presumably

because of the large lattice mismatch between Cu2O (a = 4.27 Å) and ZnS (a = 5.42 Å).

The selected-area electron diffraction (SAED) pattern yields single-crystalline

diffraction spots of Cu2O and discontinued spots of ZnS because of its light deposition.

Under this more heterogeneous ZnS epitaxial growth, normal

photocatalysis enhancement was observed.

The interfacial TEM analysis of a



ZnS-decorated Cu2O rhombic dodecahedron is shown in Figure 7.

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In this case, the

ZnS (200) planes were observed to grow epitaxially over the (110) planes of the Cu2O rhombic dodecahedron.

The SAED pattern shows fewer and more oriented ZnS

spots, suggesting more controlled epitaxial ZnS growth on the {110} faces of Cu2O. This lattice preference can explain photocatalytic suppression in the CRZ particles if this heterojunction combination leads to unfavorable interfacial band bending to hinder charge carrier migration.

In the case of ZnS-deposited Cu2O cubes, both ZnS

(200) and (220) planes align parallel to the Cu2O (200) planes (Figure S12).

The

ZnS spots seen in the SAED pattern show some order but is less so than in the CRZ case, suggesting less ZnS epitaxial plane preference over the Cu2O {100} face. A modified band diagram of the Cu2O–ZnS heterostructure is constructed to explain the recorded photocatalytic activity suppression and the facet-dependent effects (see Figure 8).

The flat-band potentials for Cu2O crystals were determined

from the intercept of the Mott–Schottky plots obtained with aqueous Na2SO4 electrolyte (Figure S13).

For Cu2O cubes, rhombic dodecahedra and octahedra, the

ﬂat band potentials are 0.41, 0.47, and 0.39 V vs. Ag/AgCl, respectively.34

From

these numbers, the valence band maxima of Cu2O cubes, rhombic dodecahedra, and octahedra are calculated to be at 0.68, 0.72, and 0.67 V vs. RHE (reversible hydrogen electrode), respectively (see Figure S14).

The conduction band maxima of Cu2O 14

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cubes, rhombic dodecahedra, and octahedra are respectively at –1.49, –1.37, and – 1.31 V vs. RHE using the band gaps determined in Figure S3.

The valence and

conduction band potentials of Cu2O crystals agree well with reported results.44 valence band potential of ZnS is referred to the literature.45,46

The

The obtained band gap

of ZnS particles is used to give the conduction band potential. The {100} surface is drawn to have the largest downward band bending, signifying the difficulty in charge carrier migration to the {100} face irrespective of the band potentials on the ZnS side.

The extent of band bending for the {111}

surface is larger than that of the {110} surface to indicate the best photocatalytic activity of the Cu2O rhombic dodecahedra.

Under normal situation with

heterogeneous ZnS loading over Cu2O crystals, the extent of ZnS band bending may be like that of the dash line or that for the ZnS (220) planes, such that excited electrons generated in Cu2O crystals should migrate to the ZnS side.

However,

significant upward conduction band bending for the {200} face of ZnS can result in an unfavorable interfacial band alignment for electrons, presenting a barrier for excited electrons to move past the Cu2O {110}/ZnS {200} interface.

With such an

interfacial band alignment, photocatalytic activity of the CRZ heterostructures becomes suppressed.

The three cases of photocatalytic activity behaviors are also

illustrated in Figure 8b–d. 15 ACS Paragon Plus Environment


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To further support the proposed interfacial band bending leading to unfavorable charge carrier transport from Cu2O to ZnS, we have performed DFT calculations for different Cu2O and ZnS surfaces to estimate individual work functions, bulk band structures, and their corresponding density of states.

Details for the DFT

calculations and results are available in the Supporting Information (Figures S15– S21).

Separate determination of the electronic structures of Cu2O and ZnS is

necessary, because it is still difficult to directly probe extents of band bending at the Cu2O and ZnS interfaces from the technical aspect of DFT calculations.

Moreover,

the electronic structure of a misfit-layer material is almost a superposition of the electronic structures of the two components.47

One can reasonably understand the

photocatalytic suppression of the Cu2O–ZnS heterostructures from the relative positions of conduction band minima of Cu2O and ZnS, which represent a minimum barrier for charge diffusion from one surface plane to another.

From the obtained

band edge energies, the plane-specific band diagrams of Cu2O and ZnS aligned to their respective vacuum levels are presented in Figure 9.

Despite the unchanged

band gap values (1.92 eV for Cu2O and 3.23 eV for ZnS), different surface planes give widely different work functions and hence valence band and conduction band energies.

As one can see that the work functions of different Cu2O surfaces follow

the order: (111) at 4.55 eV < (110) at 5.73 eV < (100) at 6.85 eV. 16 ACS Paragon Plus Environment

Work functions of

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ZnS are 4.58 eV for the (200) planes and 5.98 eV for the (220) planes.

Under such

scenario, photoexcited electron transfer from the Cu2O {111} surface to the interfacial ZnS (220) planes is favorable due to a small energy difference of 0.12 eV between the conduction bands.

But an incredibly large energy barrier of 2.46 eV exists for

electron transfer from the {110} surface of Cu2O to the interfacial (200) planes of ZnS.

Similarly, electron transport from the {100} surface of Cu2O to any interfacial

ZnS planes would not happen because of huge energy barriers.

The DFT results

agree nicely with experimental observations, demonstrating interfacial charge transport for semiconductors is truly dependent on the contacting planes.

CONCLUSIONS The facet-dependent photocatalytic behaviors of sparse deposition of ZnS particles on Cu2O cubes, rhombic dodecahedra, and octahedra have been studied through methyl orange degradation.

As expected, Cu2O cubes stay inactive after ZnS deposition due

to the very large band bending of the Cu2O {100} faces to prevent charge carriers from migrating to the {100} surfaces.

The ZnS-deposited Cu2O octahedra showed

the generally expected enhancement in photocatalytic activity resulting from better separation of charge carriers with a favorably matched band alignment between ZnS and Cu2O.

Remarkably, ZnS growth results in significant deactivation in the 17 ACS Paragon Plus Environment


photocatalytic activity of Cu2O rhombic dodecahedra.

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The (200) planes of ZnS

grow preferentially on the (110) planes of Cu2O, which has an unfavorable interfacial band alignment.

EPR measurements confirm the photocatalysis results.

calculations also support the experimental observations.

DFT

This example further

illustrates that photocatalytic activity suppression in semiconductor heterostructures having a favorable bulk band energy alignment can still be observable, because significant interfacial band bending for one or both contacting semiconductors can make the resulting interfacial band positions look very different from that of the bulk. One should keep in mind that interfacial charge transfer in semiconductors is strongly facet-dependent.

ASSOCIATED CONTENT Supporting Information Experimental details, synthesis conditions for Cu2O crystals, SEM images, size distribution histograms, UV‒vis diffuse reflectance spectra, XPS spectra, XRD patterns, time-resolved photoluminescence data, DFT calculation method and the obtained energy diagrams.

AUTHOR INFORMATION 18 ACS Paragon Plus Environment

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Corresponding Authors *[email protected]. (M. H. H.) *[email protected] (Y.-C. Lo) ORCID Michael H. Huang: 0000-0002-5648-4345 Yu-Chieh Lo: 0000-0002-7117-9154 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We thank the Ministry of Science and Technology of Taiwan for support of this research (MOST 104-2119-M-007-013-MY3, 106-2811-M-007-004, 106-2811-M-007-028, and 106-2221-E-009-074).

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Figure 1.

SEM images of the prepared ZnS-decorated Cu2O (a) cubes, (b) rhombic

dodecahedra, and (c) octahedra.

Figure 2.

XRD patterns of (a, b) standard ZnS and Cu2O, (c) ZnS particles, (d)

ZnS-deposited Cu2O cubes, (e) ZnS-deposited Cu2O rhombic dodecahedra, (f) ZnS-deposited Cu2O octahedra, and (g) the CCZ*10 sample with 10 times more ZnCl2 and TAA used to grow ZnS.



Figure 3.

XPS spectra of (a) Cu 2p, (b) Zn 2p, and (c) S 2p peaks of Cu2O‒ZnS

heterostructures recorded before photocatalysis. 30 ACS Paragon Plus Environment

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Figure 4.

Extents of photodegradation of methyl orange with irradiation time for

various Cu2O crystals and Cu2O–ZnS heterostructures.

The filter blocks UV light

from the Xe lamp.

Figure 5.

EPR spectra of DMPO-OH generated from photo-irradiated Cu2O cubes,

rhombic dodecahedra, octahedra, and the Cu2O–ZnS heterostructures. 31 ACS Paragon Plus Environment


Figure 6.

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Time-resolved PL spectra for the Cu2O cubes, octahedra, rhombic

dodecahedra, and the ZnS-decorated Cu2O heterostructures. response function (IRF) is also included.


The instrumental

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Figure 7.

(a) TEM and (b, c) interfacial HR-TEM images of the square regions of a

ZnS-deposited Cu2O octahedron.

(d) Corresponding SAED pattern of the TEM

image in panel (a). The arrowed dots come from ZnS.



Figure 8.

(a) TEM and (b, c) interfacial HR-TEM images of the square regions of a

ZnS-deposited Cu2O rhombic dodecahedron. the TEM image in panel (a).

(d) Corresponding SAED pattern of

The arrowed dots come from ZnS.


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Figure 9.

(a) Adjusted band diagram at the interface between Cu2O and ZnS with

consideration of relative band edge energies of different Cu2O crystals (dark blue for cubes, blue for rhombic dodecahedra, and orange for octahedra). conduction band energies are indicated. Eg is the band gap.

Valence and

(b–d) Drawings

showing different photocatalytic responses for Cu2O–ZnS heterostructures over Cu2O (b) cubes, (c) rhombic dodecahedra, and (d) octahedra. downward refers to electron–hole recombination.


The arrow pointing


Vacuum level

Figure 10.

DFT calculation-determined band energies of different Cu2O and ZnS

surfaces.

1.0 0.8

hν

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.0 -20

0

{200}

{110}

h+ h+

Cubes Cube-ZnS RD RD-ZnS Oct Oct-ZnS

0.2

e– e–

ZnS

Cu2O

20

40

60

80

100

120

Time (min)

Unfavorable band alignment at the Cu2O (110)/ZnS (200) interface

TOC Graphic


Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O

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