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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Photocatalytic Activity Suppression of CdS NanoparticleDecorated CuO Octahedra and Rhombic Dodecahedra 2

Jing-Yi Huang, Pei-Lun Hsieh, Gollapally Naresh, Hsin-Yi Tsai, and Michael H. Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03609 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

Photocatalytic Activity Suppression of CdS Nanoparticle-Decorated Cu2O Octahedra and Rhombic Dodecahedra

Jing-Yi Huang,† Pei-Lun Hsieh,‡ Gollapally Naresh†, Hsin-Yi Tsai†, 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

ABSTRACT:

Wurtzite CdS nanoparticles have been lightly deposited on Cu2O

cubes, octahedra, and rhombic dodecahedra to examine facet effects on interfacial charge transfer in a photocatalytic reaction.

Instead of an expected photocatalytic

activity enhancement on the basis of a favorable band alignment at the heterojunction, CdS-decorated Cu2O octahedra and rhombic dodecahedra show drastically reduced photocatalytic activities.

Further increasing CdS deposition amount leads to

complete suppression of photocatalytic activity. after CdS deposition.

Cu2O cubes remain inactive even

Transmission electron microscopy (TEM) analysis reveals 1

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epitaxial growth of the (101) planes of CdS on the (110) planes of a Cu2O rhombic dodecahedron, whereas the (110) planes of CdS align parallel to the (111) planes of a Cu2O octahedron.

Because facet-dependent photocatalytic activity can be

understood from different degrees of band bending at the crystal surfaces, significantly upward bending for the CdS contacting planes can explain the observed photocatalytic inactivity.

This work demonstrates that strong facet effects tuning the

band energies of both semiconductors at the heterojunctions make predictions of an enhanced photocatalytic activity simply through bulk band energy alignment analysis highly unreliable.

INTRODUCTION

Cu2O, Ag2O, Ag3PO4 and PbS nanocrystals with cubic, octahedral, rhombic dodecahedral and other intermediate structures have revealed strongly facet-dependent photocatalytic,1–12 electrical conductivity,12–15 and light absorption properties.2,8,12,16–22 Recently, Si has also been shown to possess surface-dependent band structures and electrical conductivity properties.23,24

For example, the

photocatalytic activity of Cu2O rhombic dodecahedra is superior to that of octahedra,

2


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but cubes are inactive because photoexcited electrons and holes see a large energy barrier at the Cu2O {100} faces, and thus do not reach the surface to produce hydroxyl and superoxide anion radicals for photodegradation of dye molecules.1,2 Electron paramagnetic resonance (EPR) spectra and electron, hole, and radical scavenger tests have confirmed these experimental observations.1,2

Optical

properties of Cu2O nanocrystals also display size and facet effects because optical band gaps are tunable with particle size and shape.16

Although not widely

recognized, these interesting phenomena are related and should originate from the presence of an ultrathin surface layer having varying degrees of band bending and band structures for different crystal faces.

Among these semiconductor properties,

photocatalytic activity of semiconductor nanocrystals has been most extensively studied with the general goals of achieving enhanced catalytic performance through exposed crystal facets, metal particle or graphene deposition, or formation of semiconductor heterojunctions with favorable band energy alignment.1,3,22,25‒29 Surface energy of crystal faces and strength of molecular interactions with crystal surfaces have often been used to explain the relative photocatalytic activities.10,30,31 However, photocatalytic process first involves photoexcited electrons and holes migrating to the crystal surfaces or interfaces before transferring the charges to water

3



or dissolved oxygen to generate radicals.2

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In the case of heterojunctions, it is about

charge transport across the interface to reach the other surface before redox reactions occur.

Molecular adsorption itself does not lead to photodegradation.

Energy

barrier to charge transport at the crystal surfaces or interfaces should be better used to account for the observed facet-dependent photocatalytic and electrical conductivity properties.

Previously growing ZnO nanostructures on Cu2O cubes, octahedra, and rhombic dodecahedra has shown the resulting cubes remained photocatalytically inactive, whereas rhombic dodecahedra displayed an enhanced activity due to a favorable band alignment for the two semiconductors.2 Remarkably, ZnO-decorated Cu2O octahedra became photocatalytically inactive.

The (101) planes of ZnO were found

to grow preferentially on the {111} surfaces of a Cu2O octahedron. We have proposed that a sharp upward conduction band bending for the {101} surface of ZnO above the conduction band energy of the Cu2O {111} surface inhibits photoexcited electrons from migrating to the ZnO side.

Electrons and holes near the interface

then recombine leading to the loss of photocatalytic activity.

Density functional

theory calculations of the interfaces qualitatively agree with the experimental results. Since this sudden photocatalytic inactivity of ZnO-decorated Cu2O octahedra is quite 4


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surprising and interesting, we would like to know if deposition of CdS nanoparticles on Cu2O crystals can also produce the same effect.

The purpose is to see if complete

photocatalytic inhibition is a rare occurrence, or the phenomenon is more frequently observable than we are aware of.

Such kind of study has profound significance, as

interfacial charge transfer is involved in the design of solar cells, catalysts, and light-emitting diode devices.

In this study, CdS nanoparticles have been sparsely grown on Cu2O cubes, octahedra, and rhombic dodecahedra for the photodecomposition of methyl orange. CdS with a band gap of 2.4 eV absorbing visible light should enhance photocatalytic performance of the composites using a conventional analysis.32,33

Surprisingly,

moderate CdS deposition already causes substantial decreases in the photocatalytic activities of Cu2O octahedra and rhombic dodecahedra.

A higher loading of CdS

nanoparticles inhibits their photocatalytic activity entirely. cubes are always photocatalytically inactive.

CdS-decorated Cu2O

CdS lattice planes preferentially grown

on Cu2O octahedra and rhombic dodecahedra have been identified.

These results

demonstrate that photocatalytic activity suppression arising from band bending at the crystal faces may be more prevalent than we think, and the outcomes of charge transfer cannot be safely predicted using simply conventional band diagrams without 5



regard to the specific contacting planes at the heterojunctions.

EXPERIMENTAL METHODS Chemicals.

Anhydrous copper (II) chloride (CuCl2, 97%, Sigma-Aldrich),

hydroxylamine hydrochloride (NH2OH·HCl, 99%, Alfa Aesar), sodium hydroxide (NaOH, 98.2 %, Mallinckrodt), sodium dodecyl sulfate (C12H25SO4Na, Mallinckrodt), cadmium chloride (CdCl2, anhydrous, 99.0%, Alfa Aesar), thioacetamide (CH3CSNH2, 99%, Alfa Aesar), ethanol (EtOH, 99.8 %, Sigma-Aldrich), and methyl orange (C14H14N3NaO3S, Hayashi Pure Chemical) were used without further purification. Ultrapure distilled and deionized water (18.2 MΩ) was used for all solution preparations.

Preparation of Cu2O‒CdS Heterostructures.

Cu2O cubes, octahedra, and rhombic

dodecahedra were synthesized following our reported procedures to fabricate Cu2O‒CdS heterostructures (see Scheme S1).3 For photocatalysis activity comparison, the Cu2O crystals used should have the same total surface area. According to our previous report, surface areas of the synthesized Cu2O cubes, octahedra, and rhombic dodecahedra were measured to be 2.84, 0.56 and 1.35 m2g–1,

6


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respectively, so the amounts of Cu2O crystal needed have a ratio of 1:5:2.34 Cu2O crystals were dried and weighted.

The

For each mg of Cu2O crystals, 1 mL of

ethanol was added to give the same concentration of 1 mg/mL of ethanol.

To

fabricate Cu2O‒CdS cubes, octahedra, and rhombic dodecahedra, 23.25, 8.25, and 19.50 mL of ethanol were filled into vials and then 3.75 mL of Cu2O cubes, 18.75 mL of octahedra, and 7.50 mL of rhombic dodecahedra were added, respectively. 1.5 mL of 0.0125 M cadmium chloride solution was injected into each vial.

Next, After 5

min of stirring, 1.5 mL of 0.03325 M thioacetamide (TAA) solution was added to the vials under stirring in a 75 ºC water bath for 2 h.

The total volume is 30 mL.

the reaction, the solutions were centrifuged at 8500 rpm for 5 min.

After

The particles

were centrifuged in 1:1 volume ratio of water and ethanol twice, and kept in 99.8% ethanol for subsequent characterization and photocatalysis experiments.

The

resulting samples are called 1.5 CdCl2/TAA Cu2O‒CdS heterostructures.

We also tried to deposit more CdS nanoparticles on Cu2O crystals.

To fabricate

3.0 CdCl2/TAA Cu2O‒CdS cubes, octahedra and rhombic dodecahedra, 20.25 mL, 5.25 mL and 16.5 mL of ethanol were added to vials, and 3.75 mL of Cu2O cubes, 18.75 mL of octahedra, and 7.5 mL of rhombic dodecahedra were introduced, respectively.

Then 3 mL of 0.0125 M cadmium chloride solution was injected into 7



each vial.

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After 5 min of stirring, 3 mL of 0.03325 M TAA solution was added to

the vials and stirred in a 75 ºC water bath for 2 h. washed as depicted above.

After the reaction, the sample was

Scheme S2 provides the procedure and reagent amounts

used to synthesize CdS-deposited Cu2O cubes, octahedra and rhombic dodecahedra.

Photocatalysis Experiments.

The prepared Cu2O‒CdS heterostructures dispersed

in ethanol were re-dispersed in pure water. mg/mL.

The particle concentration remains at 1

Next, 3.75, 18.75, and 7.5 mL of solutions containing respectively

Cu2O‒CdS cubes, octahedra and rhombic dodecahedra were introduced to a cubic quartz cell with an inner edge length of 3.7 cm and a small capped opening at the top with a volume capacity of 45 mL. cell.

Pure water and methyl orange were added to the

The concentration of methyl orange is 15 ppm in the final 45 mL of solution.

Before illumination, the solution was stirred in the dark to achieve molecular adsorption equilibrium.

After 10 min, the solution was illuminated by a 500-W

xenon lamp placed 28 cm away.

The light intensity reaching the cell was measured

to be 300 mW/cm2 by using a power meter.

During the experiment, aliquots (1 mL)

were withdrawn from the turbid solution at regular time intervals. centrifuged to remove particles.

The solution was

Extent of photodecomposition of methyl orange

was monitored by taking UV‒vis spectra. 8


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Electrochemical Measurements.

The flat-band potentials were calculated with

the help of Mott-Schottky plots using a Zahner Zennium electrochemical workstation. The electrochemical cell consists of a photocatalyst-coated ITO working electrode, a counter electrode (Pt wire), a Ag/AgCl reference electrode, and a 0.1 M Na2SO4 electrolytic solution.

For the preparation of working electrode, nearly 3 mg of Cu2O

powder suspended in ethanol was coated on the conductive side of the ITO substrate, and dried at the room temperature for overnight. The potential was systematically varied between –1.0 and +1.0 V with a frequency of 100 Hz.

Instrumentation.

SEM images of the synthesized Cu2O nanocrystals were

obtained using a JEOL JSM-7000F scanning electron microscope.

TEM

characterization was performed on a JEOL JEM-ARM200FTH electron microscope operating at 200 kV.

XRD patterns were collected using a Shimadzu XRD-6000

diffractometer with Cu Kα radiation.

UV–vis absorption spectra were acquired with

the use of a JASCO V-570 spectrophotometer.

Photocatalysis irradiation source is

an X500 xenon lamp from Blue Sky Technologies.

A PHI Quantera high-resolution

X-ray photoelectron spectrometer was used for surface analysis.

Photoluminescence

spectra were performed on a FLS920 Edinburgh fluorescence spectrometer.

RESULTS AND DISCUSSION

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Cu2O cubes, octahedra, and rhombic dodecahedra have been synthesized following reported procedures by preparing an aqueous mixture of CuCl2, NaOH, and NH2OH·HCl.3

Tuning the amounts of reagents used enables the formation of these

particle morphologies.

Figure S1 shows SEM images of the synthesized Cu2O cubes,

octahedra, and rhombic dodecahedra.

The crystals have good size and shape

uniformity and possess sharp faces necessary for facet-dependent photocatalytic property examination. The average edge length of cubes is 238 nm.

Octahedra

have an average opposite corner distance of 415 nm, while the average opposite face distance for rhombic dodecahedra is 274 nm (see Figure S2 for the particle size distribution histograms).

The Cu2O crystals having the same total particle surface

area were dispersed in ethanol in the presence of 1.5 or 3.0 mL of 0.0125 M CdCl2 solution for 5 min, and then 1.5 or 3.0 mL of 0.03325 M TAA solution as the sulfur source was added.

The solution was heated at 75 ºC for 2 h to form CdS

nanoparticles on polyhedral Cu2O crystals.

Figure 1 presents SEM images of the

prepared Cu2O cubes, octahedra, and rhombic dodecahedra decorated with a sparse amount of CdS nanoparticles.

The 3.0 CdCl2/TAA Cu2O‒CdS heterostructures

generally have a higher loading of CdS nanoparticles on the Cu2O crystals than the 1.5 CdCl2/TAA Cu2O‒CdS particles have.

Sparse CdS deposition is desirable

10


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because the exposed Cu2O crystals can be irradiated to produce excited electrons and holes for photocatalytic activity examination.

Figure 2 shows XRD pattern of the

3.0 CdCl2/TAA Cu2O‒CdS rhombic dodecahedra. Cu2O peaks are present.

At this CdS loading amount, only

To confirm the formation of CdS nanoparticles, the

precursor amount of CdS used was increased 4-fold.

Figure 2 indicates that

reflection peaks from hexagonal wurtzite CdS are now visible (see the XRD pattern for Cu2O-CdS*4).

However, for actual photocatalytic activity experiments, 1.5 and

3.0 CdCl2/TAA Cu2O‒CdS heterostructures were used.

Pristine Cu2O cubes, octahedra, rhombic dodecahedra, as well as 1.5 and 3.0 CdCl2/TAA Cu2O‒CdS cubes, octahedra, and rhombic dodecahedra, have been employed for photocatalytic activity comparison.

Figure 3 gives the methyl orange

photodegradation results using various Cu2O and Cu2O‒CdS photocatalysts.

Figure

S3 provides UV‒vis absorption spectra of methyl orange as a function of irradiation time using these photocatalysts.

Consistent with previous observations, Cu2O

rhombic dodecahedra are the best photocatalyst for methyl orange degradation, taking only 45 min to complete the reaction.

Octahedra showed a comparatively moderate

photocatalytic activity, taking around 75 min to complete the reaction.

As expected,

Cu2O cubes are photocatalytically inactive even after 120 min of photoirradiation 11



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because photogenerated electrons and holes cannot move past the {100} faces to react with water and dissolved oxygen producing radical species.1,2

Previously EPR

results have shown highest hydroxyl radical generation from photoirradiated Cu2O rhombic dodecahedra, followed by octahedra, but no production of radicals from photoexcited Cu2O cubes.1

Cubic Cu2O‒CdS samples also displayed no photocatalytic activity.

This

observation is not entirely expected, although ZnO-decorated Cu2O cubes have also been shown to be photocatalytically inactive.3

This is because CdS absorbs visible

light, so both Cu2O and CdS should generate photoexcited electrons and holes. Apparently the large conduction and valence band bending at the {100} faces of Cu2O prevents not only migration of charge carriers to the {100} surfaces, but it also inhibits charges from CdS from exiting through the CdS surfaces.

The favorable

band alignment means photogenerated holes from CdS should migrate toward the Cu2O–CdS interface rather than exiting from the CdS surface.

The large downward

bending of the Cu2O {100} valence band presents a significant barrier to hole transport, so electrons and holes from CdS simply recombine giving no photocatalytic activity to CdS-decorated Cu2O cubes.

For the 1.5 CuCl2/TAA Cu2O‒CdS octahedra

and rhombic dodecahedra, their photocatalytic activities have decreased substratially. 12


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Such outcome is unexpected; the favorable Cu2O and CdS band alignment should facilitate charge separation and their visible light absorption predicts enhancement of photocatalytic activity through heterojunction formation.

Surprisingly, further

increasing the CdS deposition to form 3.0 CuCl2/TAA Cu2O‒CdS octahedra and rhombic dodecahedra led to total supression of photocatalytic activity.

It is worthy

to note that the same experiment was conducted twice, and the results were unchanged.

Furthermore, after the photocatalysis test using the 3.0 CuCl2/TAA

Cu2O–CdS cubes, octahedra, and rhombic dodecahedra, the particles were centrifuged and the solution was removed for two more cycles of photocatalysis experiments using the same particles and fresh MO solution (Figure S4). consistently showed no photocatalytic activity.

All samples

Clearly the conventional energy

band alignment used by researchers to design superior composite photocatalysts fails completely in this case. To further confirm the photocatalytic inactivity of the 3.0 CuCl2/TAA rhombic dodecahedra, the same photocatalysis reaction was performed in the presence of 12.5 mL of isopropanol (IPA) serving as a hydroxyl radical scavenger.2

Figure S5 also

shows no photocatalytic activity for the rhombic dodecahedra after 120 min of photoirradiation.

Because no or few hydroxyl radicals are produced, adding IPA

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either have no function or all the produced hydroxyl radicals are entirely removed, making the photocatalyst to appear photocatalytically inactive.

To detemine if the

photocatalytic inactivity of CdS-decorated Cu2O rhombic dodecahedra arises from the absence of radical species, just 10% of the original IPA volume was used in another photocatalysis experiment.

The idea is that if a detectable amount of radicals is

generated, a vey small amount of IPA added would not completely remove all the radicals and some photocatalytic activity should be observable.

Figure S6 shows

there is still no photocatalytic activity, meaning radicals are essentially not produced. Again the photocatalytic results suggest that the favorable band alignment between bulk Cu2O and CdS driving charge migration across the interface to achieve charge separation can fail completely because the interfacial band energies are not at the same levels as the bulk components.

Band bending occurs on both the Cu2O and

CdS sides and is dependent on the contacting faces, making simple analysis based on bulk band alignment unreliable.

Figures S7 and S8 offer SEM images of 1.5 and 3.0

CuCl2/TAA Cu2O‒CdS cubes, octahedra, and rhombic dodecahedra after 120 min of photoirradiation.

The particle shapes appear maintained, although some degree of

particle aggregation seems present.

To further analyze the cubic, octahedral, and

rhombic dodecahedral heterostructures, their XPS spectra were taken before the

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photocatalysis experiments and are shown in Figure S9.

The exact peak positions

for the Cu 2p, Cd 3d, and S 2p peaks are available in Figure S10.

All samples give

the same XPS profile, so XPS data cannot reveal facet differences.

This is because

the depth of X-ray penetration is far beyond the ~ 1 nm thickness yielding the facet effects for Cu2O.

No satellite peaks of CuO were recorded in the Cu 2p peak region,

indicating CuO was not formed during the heterojunction preparation.

Furthermore,

photoluminescence spectra of Cu2O cubes, octahedra, rhombic dodecahedra, and the 3.0 CuCl2/TAA Cu2O‒CdS heterostructures were taken (Figure S11).

Significant

increase in the photoluminescence intensity has been recorded for all Cu2O‒CdS heterostructure samples.

Recombination of photogenerated electrons and holes in

these photocatalytically inactive heterostructures leads to pronounced emission intensity enhancement. To address the abrupt decline and even complete suppression in photocatalytic activity, the interfacial region between Cu2O and CdS should be examined.

The idea

is that some preferential lattice planes of CdS grown on the {111} and {110} faces of Cu2O cause the unfavorable charge migration. CdS-deposited Cu2O rhombic dodecahedron.

Figure 4 provides TEM analysis on a The high-resolution TEM (HR-TEM)

analysis shows the (101) planes of CdS grow preferentially and epitaxially over the

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(110) planes of Cu2O, as these lattice planes are parallel to each other at the interfaces. The obtained selected-area electron diffraction (SAED) pattern gives diffraction spots for both Cu2O and CdS, but only the (002) spot from CdS was found to align along the same direction as the (110) spots from Cu2O.

The HR-TEM results are more

important because the images directly reveal the contacting planes at the heterojunctions.

Similar TEM analysis for a CdS-deposited Cu2O octahedron is

presented in Figure 5.

The (110) planes of CdS were found to align parallel to the

(111) planes of the Cu2O octahedron at or near the interface. planes may also grow over the (111) planes of Cu2O.

However, other CdS

The recorded SAED pattern

shows only parallel lattice alignment between the (200) planes of Cu2O and the (110) planes of CdS due to a lack of diffraction spots from Cu2O. characterization of a CdS-deposited Cu2O cube.

Figure S12 gives TEM

No preferred lattice planes of CdS

have been found to grow on the {100} face of Cu2O. From the TEM analysis, we can construct an adjusted band diagram of the interface between Cu2O and CdS as shown in Figure 6 to explain the observed facet-dependent interfacial charge transfer effects on photocatalysis.

The band gap

and work function magnitudes are literature values.13,35 The flat-band potentials for Cu2O crystals were determined from the intercepts of the Mott–Schottky plots

16


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recorded in an aqueous Na2SO4 solution.

The ﬂat-band potential for Cu2O octahedra,

cubes, and rhombic dodecahedra are estimated to be 0.40, 0.43, and 0.49 V vs. Ag/AgCl, respectively (Figure S13).

The calculation details of band edge potentials

are given in the Supporting Information and valence and conduction band potentials of different Cu2O crystals are provided in Table S1.

The obtained valence band

potentials of Cu2O crystals are in excellent agreement with a previous study.36 valence and conduction potentials of CdS were taken from literature.37

The

Figure 6

shows the differences in the valence and conduction band energies of various Cu2O crystals are fairly small and unable to explain the observed photocatalytic deactivation. In accordance with the existence of an ultrathin surface layer having varying degrees of valence band and conduction band bending for different crystal faces, the {100} surface is drawn most deviated from the normal valence band and conduction band energies to signify an insurmountable barrier for either excited electrons or holes. The charge carriers then recombine, making Cu2O cubes photocatalytically inactive. The {110} surface bends only slightly, facilitating charge migration out of Cu2O. The {111} surface bends to a greater degree, but should be closer to the {110} side because of its comparatively high photocatalytic activity. CdS side.

Now let’s consider the

If the heterojunctions are comprised of mixed CdS lattice planes, the CdS

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interface should bend upward moderately as the dash line suggests.

Under this

condition, charge transport is favorable for Cu2O octahedra and rhombic dodecahedra, and an enhanced photocatalytic activity should be observed.

However, no

photocatalytic activity was recorded with sufficient deposition of CdS nanoparticles on Cu2O octahedra and rhombic dodecahedra.

This implies the CdS valence and

conduction bands in the interfacial region are significantly bent for the (101) and (110) planes, and possibly to the extent that their conduction band (as well as valence band) energies are higher than those of the (110) and (111) surface planes of Cu2O as depicted in Figure 6. With such interfacial band alignment, electron transport from Cu2O to CdS and hole transport from CdS to Cu2O becomes unfavorable.

One needs

to be mindful that other CdS planes may also have sharp interfacial band bending when contacted with Cu2O.

It is also important to recognize that the band energies

of Cu2O and CdS at the interface cannot be determined by taking UV–vis absorption and ultraviolet photoelectron spectra (UPS).

This is because only the absorption

band of Cu2O is recorded at such a low CdS loading amount, and UPS probes only binding energies at the surface, rather than the interface inside the heterostructure. Thus, it is impossible to obtain a semiconductor heterojunction band diagram to know the exact degrees of band edge bending, and the diagram shown is currently a simple

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and useful way to rationalize the experimental observations. the three cases of Cu2O–CdS heterojunctions.

Figure 6 also illustrates

In all the cases, the bulk band

alignment drives the charge carriers toward the interface, but the unfavorable interfacial band alignment results in charge recombination and loss of photocatalytic activity.

If charge carriers were not initially migrated to the interfacial region, some

photocatalytic activity should be recorded.

This work demonstrates that the

facet-dependent semiconductor band bending can be so dramatic to partially or totally quench catalytic activity, but this fact is largely not recognized before.

CONCLUSIONS

CdS nanoparticles have been sparsely deposited on the surfaces of Cu2O cubes, octahedra, and rhombic dodecahedra to evaluate facet effects on interfacial charge transfer in a photocatalytic process.

Remarkably, light CdS deposition leads to

substantial decreases, instead of enhancement, in the photocatalytic activities of Cu2O octahedra and rhombic dodecahedra.

Further increasing the amount of CdS

deposition causes complete suppression of photocatalytic activity for Cu2O octahedra and rhombic dodecahedra. without CdS decoration.

Cu2O cubes remain photocatalytically inactive with and The (101) planes of CdS have been found to grow 19



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epitaxially on the (110) planes of Cu2O, while the (110) planes of CdS align parallel to the (111) planes of Cu2O.

It is proposed that the significant band bending at the

interfacial region creates unfavorable band alignment and prevents charge carriers from moving past the interface to achieve efficient charge separation.

This work

demonstrates that strong facet effects on interfacial charge transfer are more frequently observable in semiconductor heterostructures than we think, and even the most photocatalytically active Cu2O {110} facets can become totally inactive if attached to another semiconductor with unfavorable contacting planes.

It seems

clear that the large combination of different contacting lattice planes in semiconductor–semiconductor heterojunctions makes simple heterostructure design to improve photocatalytic performance highly unreliable.

ASSOCIATED CONTENT Supporting Information This Supporting Information is available free of charge on the ACS Publication website at DOI: Experimental details, SEM images, size distribution histograms, UV–vis absorption spectra, XPS spectra, and high-resolution TEM images. 20


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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.

ACKNOWLEDGMENTS

This work was funded by Ministry of Science and Technology of Taiwan (MOST 104-2119-M-007-013-MY3 and 105-2633-M-007-003).

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a

d

100 nm

100 nm

e

b

100 nm

100 nm

f

c

100 nm Figure 1.

Page 30 of 36

100 nm

SEM images of the synthesized (a‒c) 1.5 and (d‒f) 3.0 CdCl2/TAA

Cu2O‒CdS heterostructures using Cu2O (a, d) cubes, (b, e) octahedra, and (c, f) rhombic dodecahedra.

30


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Figure 2.

XRD patterns of 3.0 CdCl2/TAA Cu2O‒CdS rhombic dodecahedra and

Cu2O‒CdS rhombic dodecahedra prepared by adding 4 times more CdCl2 and TAA amount.

Standard XRD patterns of Cu2O and wurtzite CdS are provided. Weaker

CdS peaks are not shown.

Figure 3.

Extents of photodegradation of MO using various Cu2O and Cu2O‒CdS

photocatalysts. 31



Figure 4.

(a) TEM image of a Cu2O‒CdS rhombic dodecahedron.

images of the (b) upper and (c) lower boxed regions. interfaces.

(d) SAED pattern of this heterostructure.

Page 32 of 36

(b, c) HR-TEM

The dash lines indicate the The viewing zone axis is

indicated.

32


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Figure 5.

(a) TEM image of a Cu2O‒CdS octahedron.

the (b) upper and (c) lower boxed regions.

(b, c) HR-TEM images of

The dash lines indicate the interfaces.

(d) SAED pattern of this heterostructure.

33



Page 34 of 36

E (V) vs. RHE -5.0

a Cu2O

CdS

Evac

-4.0 qX (Cu2O) = 3.2 eV

-3.0

qX (CdS) = 4.5 eV {101}

-2.0 {110} {111}

EC -1.0

{110}

Eg (Cu2O) 2.17 eV 0.0 1.0

{100} EV

Eg (CdS) = 2.4 eV

2.0

Figure 6.

(a) Adjusted band diagram of the interface between Cu2O and CdS with

consideration of relative band edge energies of different Cu2O crystal surfaces.

The

band positions for Cu2O octahedra, cubes, and rhombic dodecahedra are shown in orange, dark blue, and green colors, respectively.

In the diagram, Evac is the vacuum

level energy, qX is semiconductor electron affinity, Ec is conduction band energy, Ev is valence band energy, and Eg is band gap of the semiconductor. 34


Absolute valence and

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conduction band energies of Cu2O and CdS are shown. expanded view.

The square gives the

The dash curves represent the normal surface band bending of CdS

in contact with Cu2O. Drawings showing different photocatalytic responses for Cu2O–CdS heterostructures over Cu2O (b) cubes, (c) octahedra, and (d) rhombic dodecahedra.

35



TOC Graphic

36


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Photocatalytic Activity Suppression of CdS ... - ACS Publications

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