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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

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Photocatalytic Activity Suppression of AgPODeposited CuO Octahedra and Rhombic Dodecahedra 2

Gollapally Naresh, An-Ting Lee, Vandana Meena, Moru Satyanarayana, and Michael H. Huang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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The Journal of Physical Chemistry

Photocatalytic Activity Suppression of Ag3PO4-Deposited Cu2O Octahedra and Rhombic Dodecahedra Gollapally Naresh,† An-Ting Lee,† Vandana Meena,‡ M. Satyanarayana,§ 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 Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667,

India §Center

for Condensed Matter Sciences, National Taiwan University, Taipei 10617,

Taiwan

ABSTRACT:

Ag3PO4 nanoparticles were lightly deposited on Cu2O cubes,

octahedra, and rhombic dodecahedra for facet-dependent photocatalytic degradation of methyl orange.

Cu2O cubes remain inactive after loading with Ag3PO4.

Drastic

photocatalytic activity suppression occurs for Ag3PO4-decorated Cu2O octahedra. Surprisingly, total deactivation results for Cu2O rhombic dodecahedra after Ag3PO4 deposition.

Electron paramagnetic resonance (EPR) spectra and electrochemical

impendence (EIS) measurements support the experimental observations. Ag3PO4 lattice planes have been identified.

Interfacial

A modified band diagram is constructed

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to understand photocatalytic activity decay or deactivation due to large interfacial band bending to create an unfavorable situation for charge carrier transport across the heterojunctions.

This study further illustrates that photocatalytic suppression can be

often observed in many semiconductor heterojunctions, and multiple interfacial plane combinations mean photocatalytic outcomes are unpredictable.

INTRODUCTION Cu2O, Ag2O, Ag3PO4 and PbS nanocrystals with cubic, octahedral, rhombic dodecahedral and other intermediate structures have manifested highly facet-dependent photocatalytic,1–12 electrical conductivity,12–15 and optical properties.2,9,12,16–22

Recently, Si and Ge have also been shown to possess

surface-dependent band structures and electrical conductivity properties.23–26

For

example, {110}-bound Cu2O rhombic dodecahedra are more photocatalytically active than {111}-bound octahedra, while cubes exposing entirely {100} facets are photocatalytically inactive.1,2

The above studies have revealed that the emergence of

these semiconductor facet effects originates from the presence of an ultrathin surface layer with dissimilar density-of-states structures for different crystal faces and thus varying degrees of conduction band and valence band bending for different crystal surfaces. 2 ACS Paragon Plus Environment

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For photocatalysis studies, normally the goal is to enhance photocatalytic activity through crystal facet control, graphene or metal particle deposition, or formation of semiconductor heterojunctions having matched band energy alignment.1,12,27–32

Such

performance-driven mentality means researchers are usually not interested in understanding why certain crystal shapes have much inferior photocatalytic activities than others, and why notable photocatalytic suppression for favorable heterostructures can occur in some samples.33

Crystal face energies and molecular interactions with

particle surfaces are often used to explain the observed facet-related photocatalytic activities.34–36

Such interpretations are not broadly applicable, since Cu2O and Ag2O

cubes, octahedra, and rhombic dodecahedra having the same crystal structures exhibit opposite photocatalytic trends.1,2,7

In addition, Cu2O–CdS and Cu2O–ZnO

heterostructures with favorable band energy alignments can still display complete photocatalytic activity suppression for some interfaces.5,6

Obviously, molecular

interactions with interfacial planes are impossible. Since photocatalytic activity suppression for semiconductor heterostructures is rarely discussed, it is necessary to examine more heterostructure examples to see how broadly observable such cases are.

Here we have lightly deposited Ag3PO4 particles

on the surfaces of Cu2O cubes, octahedra, and rhombic dodecahedra for facet-dependent photodegradation of methyl orange.

Remarkably,



Ag3PO4-deposited Cu2O cubes and rhombic dodecahedra become essentially photocatalytically inactive, while the photocatalytic activity of Ag3PO4-loaded Cu2O octahedra is drastically reduced.

Electron paramagnetic resonance (EPR) data

confirmed the experimental results.

Nyquist impedance plots also support the

observation of photocatalytic activity suppression after Ag3PO4 particle deposition. High-resolution transmission electron microscopy (HR-TEM) analysis has identified specific Ag3PO4 lattice planes preferentially grown on these Cu2O polyhedral particles.

A modified band diagram of the heterostructures is therefore constructed

to show how interfacial band bending of the various contacting planes strongly affect charge transport across the interface.

This example further illustrates that

photocatalytic deactivation is generally observable, and interfacial band bending must be considered to understand experimental observations.

EXPERIMENTAL METHODS Chemicals.

Copper(II) chloride anhydrous (CuCl2, 98.0%, Alfa Aesar),

hydroxylamine hydrochloride (NH2OH⋅HCl, 99.0%, Sigma-Aldrich), sodium hydroxide (NaOH, ≥98.0%, Sigma-Aldrich), sodium dodecyl sulfate (SDS, ≥99.0%, J. T. Baker), silver nitrate (AgNO3, ≥99.8%, Sigma-Aldrich), disodium hydrogen phosphate (Na2HPO4, 99.0%, Alfa Aesar), DMPO (5,5-dimethyl-1-pyrolin-N-oxide, 4 ACS Paragon Plus Environment

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≥97%, Sigma-Aldrich), ethanol (EtOH, ≥99.8%, Sigma-Aldrich), and methyl orange (C14H14N3NaO3S, Hayashi Pure Chemical) were used as received.

Distilled and

deionized water was used throughout the sample preparation and photodegradation experiments. Synthesis of Cu2O–Ag3PO4Heterostructures.

Cu2O cubes, octahedra, and

rhombic dodecahedra were synthesized following our reported procedures (see the Supporting Information).37,38 The prepared Cu2O cubes, octahedra, and rhombic dodecahedra were decorated with Ag3PO4 nanoparticles.

Previously total surface

areas of Cu2O cubes, octahedra, and rhombic dodecahedra were measured to be 2.84, 0.56, and 1.35 m2g–1, respectively.39

For photocatalytic activity comparison, the

particles should have the same total particle surface area.

Cu2O cubes, octahedra,

and rhombic dodecahedra with weight ratios of 1:5:2 were used for Ag3PO4 deposition.

First, a solution with 1 mg mL–1 of Cu2O crystals suspended in 99.8%

ethanol was prepared for each particle shape.

Then 3.75, 18.75, and 7.5 mL of the

solutions with cubic, octahedral, and rhombic dodecahedral particles were taken in 50-mL sample vials containing 24.25, 9.25, and 20.50 mL of ethanol, respectively. Next, 1.0 mL of 4.5 mM AgNO3 solution was added dropwise into each sample vial under vigorous stirring at room temperature.

After stirring for 1 h, 1.0 mL of 1.5

mM Na2HPO4 solution was added dropwise and continued stirring for 1 h. 5 ACS Paragon Plus Environment

All the


loading experiments were carried out in a dark room.

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The obtained products were

washed with 1:1 volume of water and ethanol twice and finally with 99.8% ethanol. Photocatalytic Activity Experiments.

For the photodegradation studies, 1 mg

mL–1 of Cu2O–Ag3PO4 heterostructures were suspended in deionized water.

Next,

3.75, 18.75, and 7.5 mL of CCA, COA, and CRA solutions were, respectively, added to an aqueous methyl orange solution to fill the home-made square quartz cell up to 45 mL with a dye concentration of 15 ppm.

For adsorption–desorption equilibrium

between dye and catalyst, the suspension was magnetically stirred for 30 min, and the suspension was then exposed to light from a 500-W xenon lamp placed 28 cm away. To avoid UV light irradiation, a long-pass Y-43 cutoff filter was placed between the light source and the quartz cell.

To monitor the extent of photodegradation, 1-mL

aliquots was withdrawn at regular time intervals, centrifuged at 8000 rpm to separate the catalyst particles, and UV–vis absorption spectra of the filtrate was recorded. Electrochemical Measurements.

Electrochemical impedance spectroscopy

(EIS) of the photocatalysts was measured using a Zehner Zemium electrochemical workstation.

The electrochemical cell consists of photocatalyst (nearly 3 mg in

ethanol) coated on an ITO glass as the working electrode, Pt wire as the counter electrode, Ag/AgCl as the reference electrode, and 10 mM K3[Fe(CN)6] solution containing 0.1 M KCl as the electrolytic solution.

The potential was systematically


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changed between –1.4 and +1.4 V with a frequency of 50 Hz. Instrumentation. electron microscope.

SEM images were taken using a JEOL JSM-700F scanning XRD patterns were collected on a Shimadzu XRD-6000

diffractometer with Cu Kα radiation ( = 1.5406 Å).

TEM images and SAED

patterns were obtained with the aid of a JEOL JEM-3000F electron microscope with an acceleration voltage of 300 kV.

UV–vis diffuse reflectance and absorption

studies were performed on a JASCO V-670 spectrophotometer.

XPS spectra were

produced using a PHI Quantera high-resolution X-ray photoelectron spectrometer. A X500 Xenon lamp (Blue Sky Technologies) was used as the light source for the photocatalysis experiments.

EPR spectra were measured on a Bruker ELEXSYS

E580-400 EPR spectrometer with 1.6 G of modulation amplitude, 9.77 GHz of microwave frequency, 10 mW of microwave power, 100 kHz of modulation frequency, 3450 G center field, 2 × 104 receiver gain, and 100 G sweep width. DMPO was used to detect hydroxyl radicals generated during the photocatalytic experiment with the help of the EPR spectrometer.

RESULTS AND DISCUSSION Cu2O cubes, octahedra, and rhombic dodecahedra were synthesized following our reported procedures (see the Supporting Information).37,38 7 ACS Paragon Plus Environment

Figures S1 and S2 give


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scanning electron microscopy (SEM) images and size distribution histograms of the synthesized Cu2O cubes, octahedra, and rhombic dodecahedra with average edge length or opposite corner/face distances of 237, 415, and 273 nm, respectively. particles have uniform shapes and sharp faces for facet effect investigation.

The

Figure 1

shows SEM images of Ag3PO4-decorated Cu2O cubes, octahedra, and rhombic dodecahedra.

For convenience, CCA refers to Ag3PO4-deposited Cu2O cubes, COA

represents Ag3PO4-decorated Cu2O octahedra, and CRA is for Ag3PO4-deposited Cu2O rhombic dodecahedra.

Light deposition of Ag3PO4 particles on the Cu2O

crystals is important because we do not want to form core–shell structures.

X-ray

diffraction (XRD) patterns of the heterostructures are provided in Figure 2.

All

samples give the same XRD patterns presumably due to the random orientation of the composite particles.

Because of the small amounts of silver phosphate deposited,

only Cu2O peaks are present in the XRD patterns of all samples.

To confirm the

deposition of Ag3PO4 particles on Cu2O cubes, the amounts of AgNO3 and Na2HPO4 used were increased by 10-fold (CCA*10).

Ag3PO4 peaks dominate the XRD

pattern of CCA*10, confirming its formation (Figure S3). Figure S4 displays UV–vis diffuse reflectance spectra of the synthesized Cu2O cubes, octahedra, rhombic dodecahedra, and Ag3PO4 particles for the determination of their band gaps.

This information is included in the construction of the band 8 ACS Paragon Plus Environment

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diagram of Cu2O–ZnS heterostructures.

The converted Tauc plots give the

determined band gaps of 2.17, 1.98, and 2.09 eV for Cu2O cubes, octahedra, and rhombic dodecahedra, respectively.

Semiconductor band gaps have been

demonstrated to depend on both particle size and shape (or exposed facets).16,17

For

separately prepared Ag3PO4 particles, the obtained band gap value is 2.38 eV. Figure S5 also presents UV–vis diffuse reflectance spectra of the heterostructures. Due to sparse Ag3PO4 deposition, the heterostructures have essentially the same spectra as the pristine Cu2O polyhedra. To further analyze the heterostructures, their X-ray photoelectron spectra (XPS) were taken (Figures S6 and S7). look identical.

XPS spectra of the CCA, COA, and CRA samples

The characteristic Cu 2p3/2 and Cu 2p1/2 peaks of Cu(I) oxidation

state are located at 932.6 and 952.4 eV, respectively. Moreover, two tiny Cu2O satellite peaks at 943.9 and 946.6 eV are also observed. Absence of satellite peaks of CuO in the Cu 2p region indicates CuO was not formed in the Cu2O–Ag3PO4 heterostructures.

The Ag 3d5/2 and 3d3/2 peaks are located at 367.7 and 373.6 eV,

respectively. These peak positions match with the Ag(I) oxidation state.40

The P 2p

peak appears at 133.2 eV. Results of photodegradation of methyl orange using various photocatalysts are shown in Figure 3.

Figure S8 gives UV–vis absorption spectra of methyl orange as a 9 ACS Paragon Plus Environment


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function of irradiation time for Cu2O cubes, octahedra, rhombic dodecahedra, CCA, COA, and CRA heterostructures.

For complete comparison, UV–vis absorption

spectra of methyl orange as a function of irradiation time for Ag3PO4 particles are available in Figure S9. Ag3PO4 decoration.

Cu2O cubes remain photocatalytically inactive even after

Cu2O cubes are photocatalytically inactive because charge

carriers cannot move past the {100} faces of Cu2O as verified by EPR measurements and various scavenger tests.1,2

Since Ag3PO4 particles also absorb visible light and

have moderate photocatalytic activity as seen in Figure 3, complete photocatalytic deactivation is not entirely expected for the CCA sample.

This interesting result

suggests photogenerated holes from Ag3PO4 should first migrate toward the heterojunctions because of the favorable band energy alignment.

However, the

heterojunction presents an insurmountable barrier to hole transport across the interface; the photogenerated electrons and holes then recombine to yield no photocatalytic activity for the CCA particles.

If charge carriers migrate directly to

the Ag3PO4 particle surfaces unaware of the presence of the heterojunctions, some photocatalytic activity should be recorded. Interestingly, the good photocatalytic activity of Cu2O octahedra is greatly suppressed after silver phosphate deposition.

Amazingly, the most

photocatalytically active Cu2O rhombic dodecahedra become totally inactive after 10 ACS Paragon Plus Environment

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decorating with some Ag3PO4 nanoparticles.

In contrast, it is the ZnO-deposited

Cu2O octahedra exhibiting this sudden deactivation, while ZnO growth on Cu2O rhombic dodecahedra showed the expected photocatalytic enhancement.5

For

CdS-decorated Cu2O octahedra and rhombic dodecahedra, both samples displayed complete photocatalytic suppression with sufficient CdS loading.6

These examples

illustrate the unpredictability of photocatalytic outcomes at semiconductor heterojunctions; conventional wisdom considering only bulk band energy alignment of the contacting semiconductors cannot be correct.

A plausible explanation must be

that band bending at the semiconductor interfaces can be very different for various combinations of the contacting planes. In this case, Cu2O {100}, {111}, and {110} faces should already have dissimilar surface band bending levels.

And depending on

the contacting lattice planes of Ag3PO4 with its similarly tunable interfacial band bending, unfavorable band alignment can frequently happen at the Cu2O–Ag3PO4 interfaces.

This should be the general scenario for various semiconductor interfaces.

For example, perfectly epitaxial GaN growth is stringently required for blue light-emitting diode (LED) fabrication to facilitate charge transport.41

Figures S10

and S11 offer SEM images and XRD patterns of all the heterostructures after the photocatalysis experiments.

The particle morphologies are preserved, and only

Cu2O diffraction peaks were recorded. 11 ACS Paragon Plus Environment


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To further confirm the obtained photocatalysis results, EPR spectra of DMPO-OH generated from light-irradiated Cu2O cubes, octahedra, rhombic dodecahedra, and the Cu2O–Ag3PO4 heterostructures were taken (see Figure 4). EPR signals indicate production of essentially hydroxyl radicals.

The

The amounts of

particles introduced were adjusted so that the total particle surface area was kept constant for all samples for fair comparison of EPR peak intensities.

Actually the

amounts of samples used for EPR measurements were same as those for the photocatalytic experiments. results.

Remarkably, EPR data fully support the photocatalysis

As expected, no radicals were produced from Cu2O cubes and the CCA

cubes upon photoexcitation.

EPR signals of Cu2O octahedra have reduced

drastically after Ag3PO4 deposition, indicating decreased photocatalytic activity for the COA particles.

Consistent with the experimental observations, Cu2O rhombic

dodecahedra have considerably higher EPR peak intensities than those of octahedra, supporting the best photocatalytic performancefor Cu2O rhombic dodecahedra. Remarkably, EPR peaks of CRA have nearly disappeared, confirming sudden photocatalytic deactivation after Ag3PO4 deposition. Electrochemical impendence results provide additional support to the facet-dependent photocatalysis observations.

Figure 5 is the Nyquist impedance

plots for the Cu2O polyhedral, Cu2O–Ag3PO4 heterostructures, and pure Ag3PO4 12 ACS Paragon Plus Environment

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crystals.

The diameters of the Nyquist semicircles are much smaller for pristine

Cu2O rhombic dodecahedra and octahedra, moderate for CRA and COA heterostructures and significantly larger for Cu2O cubes and CCA heterostructures. Ag3PO4 crystals have a semicircle similar to the size for Cu2O cubes.

The

corresponding charge transfer resistance values (RCT) are given in Table S1.

A

sample with a large semicircle means charge transfer across the particles meets greater resistance.

Pristine Cu2O cubes and CCA heterostructures have much higher

RCT values due to much less flow of photogenerated charge carriers than that in Cu2O rhombic dodecahedra and octahedra.

Formation of COA and CRA heterostructures

also leads to notable increase in charge transport resistance. To know the contacting Ag3PO4 planes grown on Cu2O crystals, HR-TEM images of the interfacial regions were collected (see Figure 6).

After inspection at

several interfacial regions, Ag3PO4 (211) planes have been found to align parallel to the Cu2O (200) and (110) surface planes.

Due to Ag3PO4 sensitivity to electron

beam irradiation, it was difficult to check multiple interfacial regions to establish this preferential Ag3PO4 plane alignment. The Ag3PO4 (220) planes run somewhat parallel to the (111) planes of Cu2O.

Epitaxial growth of other Ag3PO4 planes on

these Cu2O crystals is also possible.

Nevertheless, the limited interfacial lattice

information is still useful.

Figure S12 presents TEM images of Ag3PO4-decorated 13 ACS Paragon Plus Environment


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Cu2O cubes, octahedra, and rhombic dodecahedra and their SAED patterns. Diffraction spots of Cu2O and Ag3PO4 are both observed, but further analysis of the lattice orientation relationship between the two materials is not possible due to sparse silver phosphate decoration.

It should be noted that the lattice constants of Cu2O and

Ag3PO4 are 4.27 and 6.004 Å, respectively.42,43

Such large lattice mismatch may

explain the observation of less preferential epitaxial growth of Ag3PO4 planes on Cu2O, and the lack of lattice orientation relationship in the SAED patterns. On the basis of the experimental results, it is necessary to construct a band diagram to understand the observations of photocatalytic suppression in Cu2O– Ag3PO4 heterostructures.

Figure 7 shows the adjusted band diagram.

Based on the

flat-band potentials of all Cu2O crystals, the calculated valence band potentials of Cu2O cubes, octahedra, and rhombic dodecahedra were taken from our previous work.6

The conduction band potentials were calculated from their optical band gaps

(Figure S4).

The valence band and conduction band potentials of Ag3PO4 were

taken from the literature.44,45

Clearly such minor differences in the measured

valence and conduction band potentials of the various Cu2O crystals cannot explain the observed inactivity of Cu2O cubes and the sudden photocatalytic deactivation of the Cu2O–Ag3PO4 heterostructures.

Because charge carriers cannot move past the

{100} faces of Cu2O as demonstrated by EPR spectra and scavenger tests, a very 14 ACS Paragon Plus Environment

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large degree of downward band bending is drawn for the {100} surface to signify a huge barrier present for hole migration to this surface.

Photoexcited electrons and

holes then recombine giving no photocatalytic activity.

With this scenario, Cu2O

cubes remain inactive even after Ag3PO4 deposition, unless photocatalytic activity comes from the Ag3PO4 side.

In contrast, the {110} face should have a normal and

least degree of downward band bending to match with the upward band bending of Ag3PO4 at the heterojunction.

The {111} face should bend to a greater extent than

that of the {110} face to represent the less efficient photocatalytic activity of Cu2O octahedra than rhombic dodecahedra.

Of course, the degrees of band bending for the

two Cu2O surfaces can also be drawn closer together in this case since the valence and conduction band potentials of Ag3PO4 differ widely from those of Cu2O.

As

mentioned above, charge carriers from Ag3PO4 must also migrate toward the heterojunction, or some photocatalytic activity would be observed in the heterostructures.

For complete photocatalytic suppression to occur, the upward band

bending of Ag3PO4 (211) and (200) planes should reach to more positive potentials than those of the Cu2O (110) and (111) contacting planes, respectively, such that photoexcited electrons from the Cu2O side see a barrier at the interface.

Similarly,

holes from Ag3PO4also confront with a barrier due to unmatched interfacial band alignment.

Drastic or total photocatalytic activity suppression then results. 15 ACS Paragon Plus Environment

If


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considering Cu2O as a p-type semiconductor, photogenerated holes also cannot move to Ag3PO4 with this interfacial band alignment.

Please note that this diagram is a

rational picture to assist understanding of sudden photocatalytic inactivity created at the heterojunctions, since it is not possible to experimentally determine the real band bending at the interfaces. transfer situations.

Figure 7 also summarizes the various interfacial charge

In all the cases, photocatalytic suppression occurs regardless of

the exposed Cu2O surfaces.

CONCLUSIONS Despite a large number of publications on photocatalysis enhancement using various materials design strategies, it is surprising to learn that reports on facet-dependent photocatalysis of semiconductor heterojunctions is very rare.

This Cu2O–Ag3PO4

heterostructure example illustrates that photocatalytic suppression is unavoidable despite their favorable bulk band alignment.

EPR spectra and EIS measurements

support the observations of sudden photocatalytic activity suppression after Ag3PO4 deposition.

However, researchers never consider this unusual outcome is possible.

This means our understanding of charge transfer at semiconductor heterojunctions is seriously flawed. A possible reason this happens is because of our lack of interest to understand instances of photocatalytic suppression when they are observed, or such 16 ACS Paragon Plus Environment

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unexpected results are simply not reported because the results are incomprehensible. This indifference to true experimental results can lead to initial unacceptance when experimental facts are presented.

We need to remind ourselves that ionic solids are

in principle faceted, so presentation of semiconductors as circles or spheres can be misleading.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis conditions for Cu2O crystals, SEM images, size distribution histograms, diffuse reflectance spectra, XPS spectra, UV–vis spectra, XRD patterns, charge transfer resistance values, and TEM analysis.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Michael H. Huang: 0000-0002-5648-4345 17 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

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

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Figure 1.

SEM images of Ag3PO4-decorated Cu2O (a) cubes, (b) octahedra, and (c)

rhombic dodecahedra.

Figure 2.

XRD patterns of (a, b) standard Ag3PO4 and Cu2O, (c) Ag3PO4-deposited

Cu2O rhombic dodecahedra, (d) Ag3PO4-deposited Cu2O octahedra, and (e) Ag3PO4-deposited Cu2O cubes.



Figure 3.

Extents of photodegradation of methyl orange with irradiation time for

various Cu2O crystals and Cu2O–Ag3PO4heterostructures.

Figure 4.

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

octahedra, rhombic dodecahedra, and the Cu2O–Ag3PO4 heterostructures.


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Figure 5.

Nyquist impedance plots for various Cu2O crystals and Cu2O–Ag3PO4

heterostructures.

The equivalent circuit is provided.



Figure 6.

TEM and HR-TEM images of Ag3PO4-deposited Cu2O (a, b) cubes, (c, d)

octahedra, and (e, f) rhombic dodecahedra for interfacial lattice plane analysis.


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Figure 7.

(a) Adjusted band diagram at the interface between Cu2O and Ag3PO4

with considerations of relative band edge energies of different Cu2O crystals (dark blue for cubes, orange for octahedra, and green for rhombic dodecahedra) and Ag3PO4 contacting planes.

In the diagram, qX is semiconductor electron affinity, Ec is

conduction band energy, Ev is valence band energy, and Evac is the vacuum level energy.

Eg is band gap of the semiconductor. (b–d) Drawings showing different

photocatalytic responses for Ag3PO4-deposted Cu2O (b) cubes, (c) octahedra, and (d) rhombic dodecahedra.



TOC Graphic


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Photocatalytic Activity Suppression of Ag3PO4 ... - ACS Publications

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