Synthesis of Diverse Ag2O Crystals and Their Facet-Dependent

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Synthesis of Diverse Ag2O Crystals and Their FacetDependent Photocatalytic Activity Examination Ying-Jui Chen, Yun-Wei Chiang, and Michael H. Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04686 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Synthesis of Diverse Ag2O Crystals and Their Facet-Dependent Photocatalytic Activity Examination Ying-Jui Chen, Yun-Wei Chiang, and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT:

Sub- to micrometer-sized Ag2O cubes, great rhombicuboctahedra,

cuboctahedra, corner-truncated octahedra, octahedra, and rhombic dodecahedra have been synthesized at room temperature using simple molar ratios of NH4NO3, NaOH, and AgNO3 solutions with a short reaction time.

In addition, tuning the

concentration of NH3 in the solution can provide more particle morphologies including edge- and corner-truncated cubes, small rhombicuboctahedra, and edge-truncated octahedra to enrich Ag2O shape diversity.

X-ray photoelectron

spectroscopy (XPS) spectra indicate surface composition of various crystals as pure Ag2O.

Diffuse reflectance spectra have been used to determine the band gap of

Ag2O cubes.

Ag2O cubes, octahedra, and rhombic dodecahedra having the same

total particle surface area were used for facet-dependent photocatalytic activity comparison in the degradation of methyl orange.

Cubes are comparably highly

active for this reaction, while octahedra and rhombic dodecahedra give moderate and low catalytic activities, respectively.

Electron paramagnetic resonance (EPR) 1

ACS Paragon Plus Environment


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measurements confirm this reactivity order.

Although all Ag2O samples show

significant etching during photocatalysis, metallic silver is not produced. KEYWORDS: electron paramagnetic resonance spectroscopy, facet-dependent properties, nanocrystals, photocatalysis, silver oxide

1. INTRODUCTION Cuprous oxide (Cu2O) nanocrystals with diverse shapes including cubic, octahedral, and rhombic dodecahedral structures have been demonstrated to exhibit various facet-dependent properties such as electrical conductivity, photocatalytic activity, optical absorption, and possibly heat transmission.1‒6

PbS nanocrystals with a NaCl

crystal structure also display facet-dependent responses to molecular charges and electrical conductivity properties.7,8 To expand the examination of facet-dependent properties to other semiconductors, Ag2O is an attractive material to prepare because it has the same body-centered cubic crystal structure as that of Cu2O, allowing a large variety of particle morphologies to be synthesized.

Ag2O films have been reported

to show a band gap of 1.46 eV for possible photovoltaic applications, while a band gap of 2.25 eV absorbing visible light is also known.9,10

Growth of Ag2O nano- and

microcrystals with well-defined shapes has been accomplished through formation of a Ag ion complex such as Ag(NH3)2+ before its reaction with NaOH.11‒15 2


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Face-selective etching can also be employed to generate additional Ag2O morphologies.15,16 For facet-dependent photocatalytic activity comparison, it is most desirable that sharp-faced Ag2O cubes, octahedra, and rhombic dodecahedra bound by respective {100}, {111}, and {110} surfaces can be exclusively synthesized.

Our

previously developed conditions mixing NH4NO3, AgNO3, and NaOH solutions at 35 ºC for 30 min enable the formation of Ag2O cubes and octahedra but not rhombic dodecahedra.12

Although Ag2O rhombic dodecahedra have been prepared in two

reports, the exact synthetic condition is either unclear or requires the use of harmful pyridine.13,14 In addition, great and small rhombicuboctahedra exposing all three low-index facets have not been synthesized before, which may be useful for possible electrical conductivity measurements.

It should be nice to expand the shape

diversity of Ag2O crystals using a simpler synthetic condition.

Furthermore, truly

facet-dependent photocatalytic activity comparison using pristine Ag2O cubes, octahedra, and rhombic dodecahedra is still not available; mixed-faced Ag2O and Ag@Ag2O particles were employed previously for such examination.14,15,17 In this study, we have used simple reagent amounts for Ag2O crystal growth at room temperature in just 12 min.

Nine distinct particle shapes including cubic,

octahedral, rhombic dodecahedral, and novel great and small rhombicuboctahedral structures were synthesized.

Ultrafast crystal growth rates have been captured for 3



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the precipitation reaction.

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Ag2O cubes, octahedra, and rhombic dodecahedra having

the same total surface area were used for facet-dependent photocatalytic degradation of methyl orange for the first time, showing cubes as the best catalyst. measurements confirmed the photocatalysis results.

EPR

The Ag2O crystals were

analyzed after photocatalysis for structure and composition stability.

2. EXPERIMENTAL SECTION Chemicals.

Silver nitrate (AgNO3, 99.8%, Sigma), ammonium nitrate

(NH4NO3, 99%, Showa), and sodium hydroxide (NaOH, 99%, Sigma) were used without further purification.

Ultrapure distilled and deionized water was used for all

solution preparations. Synthesis of Various Ag2O Crystals. in Table 1.

Exact reagent amounts used are available

First, 0.4 M NH4NO3, 2.0 and 0.2 M NaOH, and 0.1 M AgNO3 solutions

were prepared.

A different volume of deionized water was added to each vial to

adjust the final total volume at 5 mL.

Then different volumes of NH4NO3, 2.0 M

NaOH, and AgNO3 solutions were introduced depending on the shape of Ag2O particles to synthesize.

After stirring the mixture for 10 min at room temperature, 1

mL of 0.2 M NaOH solution was added (for rhombic dodecahedra, it is 1 mL of 2.0 M NaOH solution).

The solution was stirred for 2 min with its color changing from 4


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colorless to brown. Ag2O particles.

The sample was centrifuged at 3500 rpm for 3 min to collect

The top solution was removed and the precipitate was washed twice

more with ethanol and centrifuged at 3500 rpm for 3 min to clean the particles. Finally, the particles were dried at room temperature. Photocatalytic Activity Examination of Ag2O Crystals.

Based on the particle

sizes, the amounts of Ag2O cubes, octahedra, and rhombic dodecahedra having the same total particle surface area for the photocatalytic activity experiment were determined to be 1.7, 3.0, and 8.2 mg, respectively (see calculations in the Supporting Information and Figure S1). vial.

The weighted amount of Ag2O crystals was added to a

The vial was filled to 5 mL with deionized water.

orange (MO) solution was introduced into the vial.

1 mL of 15 ppm methyl

The vial was stirred in the dark

for 30 min to reach molecular adsorption equilibrium.

Then a 300-W Xenon lamp

placed 20 cm away from the vial was turned on to start the photodegradation reaction. The light intensity reaching the vial is 65 mW/cm2 as measured by a power meter. Each 1 mL solution withdrawn from the vial was centrifuged at 5000 rpm for 3 min to remove Ag2O particles, and UV–visible absorption spectra were taken as a function of reaction time. Electron Paramagnetic Resonance (EPR) Measurements on Ag2O Crystals. DMPO (5,5-dimethyl-1-pyrolin-N-oxide) was used to catch radicals photogenerated 5



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from Ag2O crystals.

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A DMPO solution was filtered by activated carbon for 3 times

to ensure good EPR spectra can be obtained.

The solution containing 1.0 mL of 0.1

M DMPO and 1 mL of Ag2O crystal solution was irradiated with light from a 300-W Xenon lamp for 5 min at room temperature. EPR measurements.

The solution was sent for immediate

EPR signals of radicals spin-trapped by DMPO were recorded

at ambient temperature.

The typical EPR instrumental settings were: center field

3450 G, microwave frequency 9.77 GHz, microwave power 10.1 Mw, sweep width 100 G, modulation frequency 100 kHz, modulation amplitude 0.5 G, receiver gain 2 × 104, time constant 81.92 ms, conversion time 40.96 ms, and total scan time 40 s. Each sample was scanned for three times with an interval time of 1 min to check the time dependence of the recorded signals.

The EPR spectral simulations and the

nonlinear least squares fits were performed using the EasySpin program.18 Instrumentation.

Scanning electron microscopy (SEM) images of synthesized

Ag2O crystals were taken using a JEOL JSM-7000F scanning electron microscope. X-ray diffraction (XRD) patterns were obtained from a Shimadzu XRD-6000 diffractometer with Cu Kα radiation.

X-ray photoelectron spectroscopy (XPS)

spectra were obtained with the use of an ULVAC-PHI XPS spectrometer.

UV–vis

diffuse reflectance spectra were taken on a JASCO V-570 spectrophotometer equipped with a solid sample holder. 6


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3. RESULTS AND DISCUSSION Compared to direct mixing of AgNO3 and NaOH, formation of Ag(NH3)2+ and its reaction with NaOH can dramatically reduce the equilibrium constant toward Ag2O formation, making shape control of Ag2O crystals possible.

The following equations

give the reactions involved in the synthesis of Ag2O crystals with shape tunability.16,19 NH4NO3 + NaOH → NH3 + H2O + Na+ + NO3–

(1)

AgNO3 + 2NH3 ↔ Ag(NH3)2+ + NO3–

(2)

Ag(NH3)2+ + NaOH ↔ AgOH + 2NH3 + Na+

(3)

2AgOH ↔ Ag2O + H2O

(4)

Following the equations, equimolar NH4NO3 and NaOH solution were first mixed in the preparation of all particle morphologies shown in Figure 1.

Then AgNO3

solution was introduced giving a simple molar ratio of NH4NO3:NaOH:AgNO3 at 2:2:1 in the growth of Ag2O cubes, corner-truncated octahedra, and octahedra. Tuning of the NH4NO3:AgNO3 molar ratio is needed to obtain rhombicuboctahedral and cuboctahedral structures. yields the final products.

Final addition of a large amount of NaOH solution

A NH4NO3:AgNO3 molar ratio of 6:1 and 10 times more

final NaOH addition is necessary to obtain Ag2O rhombic dodecahedra.

Crystal

growth is complete 2 min after final NaOH addition, so this procedure is simple and 7



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fast and thus superior compared to the previous condition used.12 Figure 1 shows SEM images of cubic, great rhombicuboctahedral, cuboctahedral, corner-truncated octahedral, octahedral, and rhombic dodecahedral Ag2O crystals synthesized with increasing addition of AgNO3 solution except the rhombic dodecahedra.

The particles all appear to have sharp faces necessary for

facet-dependent property studies and are uniform in size and shape.

The great

rhombicuboctahedron is a new Ag2O morphology exposing 6 octagonal {100} face, 8 hexagonal {111} faces, and 12 square {110} faces. distribution histograms for the 6 particle shapes. deviations are given in Table S1. sub-micrometer in size.

Figure S2 provides size

Average sizes and their standard

Cubes (454 nm) and cuboctahedra (848 nm) are

Great rhombicuboctahedra (1317 nm), corner-truncated

octahedra (1262 nm), octahedra (1378 nm), and rhombic dodecahedra (2174 nm) are all microcrystals. Figure 2 gives XRD patterns of these Ag2O crystals. the standard pattern of Ag2O.

All patterns match with

Cubes show an exceptionally strong (200) peak

resulting from their {100} faces and preferred orientation of deposition on a substrate loaded with a small amount of particles.

Great rhombicuboctahedra give a strong

(220) peak due to significant exposed {110} surfaces.

The (111) peak gradually

increases in intensity relative to that of (200) peak from cuboctahedra to truncated 8


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octahedra and becomes dominant in octahedra. For rhombic dodecahedra, the (220) peak is so strong that even the (110) peak can be seen.

These XRD patterns indicate

high particle shape uniformity in all the samples. Consistent with previous observations, the formation of Ag2O crystals with different shapes results from their different growth rates.20

To illustrate this, movies

of the solution color changes during the growth of cubic, octahedral, and rhombic dodecahedral Ag2O crystals were recorded, and some snapshots are shown in Figure S3.

Both cubes and octahedra completed their growth in just 0.6 and 0.4 sec,

respectively.

Both particle shapes are formed extremely rapidly because this is a

precipitation reaction, yet octahedra should be generated at a slightly faster rate than that for cubes. their large sizes.

Rhombic dodecahedra can take 12 sec to complete the growth due to Please note that the solution color darkened, even though the

solution did not turn brown due to a much less product yield.

This observation

shows that rhombic dodecahedra were produced at a much slower rate, even though a much larger amount of NaOH solution was introduced to make these particles. Due to the large particle sizes, diffuse reflectance spectra are more suitable to obtain their light absorption profiles.

Figure 3 presents diffuse reflectance spectra of

Ag2O cubes, octahedra, and rhombic dodecahedra with a broad-band feature centered at 448, 523, and 590 nm, respectively.

Because light scattering can contribute to the 9



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absorption feature for microcrystals, the spectrum for cubes having smaller sizes was used to produce a graph of (αhv)2 vs. hv for the determination of energy band gap of Ag2O crystals.

A band gap of ~1.4 eV was obtained, which is close to a reported

value for Ag2O films at 1.46 eV.9 By fixing the volumes of AgNO3 and final NaOH solutions added and keeping the NH4NO3 and initial NaOH solutions introduced equimolar, raising the NH4NO3:AgNO3 molar ratio from standard 2:1 to 2.6:1, 3:1, and 3.5:1 (equivalent to adding extra 0, 30, 50, and 75 µmol of NH3), the reaction kinetics may be tuned to produce other distinct Ag2O crystal morphologies. experimental conditions.

Table S2 provides the exact

Edge- and corner-truncated cubes, great

rhombicuboctahedra, small rhombicuboctahedra, and edge-truncated octahedra were synthesized (see Figure 4).

Here the experimental condition used for making great

rhombicuboctahedra is the same as before.

Figure S4 and Table S3 give their size

distribution histograms, average sizes and their standard deviations.

Edge- and

corner-truncated cubes (average edge length of 627 nm) are sub-micrometer in size, while small rhombicuboctahedra (average particle size of 2122 nm) and edge-truncated octahedra (average opposite corner distance of 4136 nm) are microcrystals.

Again they also display high shape and size uniformity. The small

rhombicuboctahedra resemble those of great rhombicuboctahedra but have closer to 10


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triangular {111} faces.

These new Ag2O morphologies are important, because they

greatly enrich the variety of Ag2O particle shapes achievable using this simple synthetic approach. For facet-dependent property investigations of Ag2O crystals, it is useful to know their surface chemical states.

XPS spectra of washed Ag2O cubes, octahedra, and

rhombic dodecahedra are available in Figure S5.

All three shapes have the same

binding energies for the Ag 3d5/2 peak at 367.9 eV and the 3d3/2 peak at 373.9 eV. These peak positions can be used as standard binding energies for Ag2O crystals because the particles are clean and surfactant-free.21

All samples give the same O 1s

peak at 259.2–259.3 eV and another oxygen peak at 530.7–531.0 eV, which match with the expected oxygen peak positions of Ag2O.22,23

The oxygen peak at

530.7–531.0 eV may be attributed to adsorbed oxygen or water.

This means that the

surface chemical state for the {100}, {111}, and {110} faces is essentially the same. With the successful synthesis of Ag2O rhombic dodecahedra, it is now possible to examine the facet-dependent photocatalytic properties of Ag2O cubes, octahedra, and rhombic dodecahedra.

For best comparison of the catalytic effects of different

faces, the total surface area of Ag2O particles used should be kept the same.

The

corresponding amounts of Ag2O rhombic dodecahedra (8.2 mg), octahedra (3.0 mg), and cubes (1.7 mg) having the same total particle surface area were used for the 11



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photocatalysis experiment.

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UV–vis spectra of methyl orange (MO) photodegraded

by cubic, octahedral, and rhombic dodecahedral Ag2O crystals are shown in Figure 5. To verify reproducibility of the recorded catalytic activities, three photocatalysis experiments were performed for each particle shape, and the results are shown in Figure S6.

The photodegradation rates have been found to be quite reproducible.

In the first 30 min in the dark with the MO/Ag2O solution being stirred continuously, the absorbance of MO stayed constant for all samples, showing that MO was not decomposed in this period.

After the solution was illuminated, the MO absorption

band at 463 nm decreased gradually.

However, there is an obvious difference in the

rate of photodegradation using various photocatalysts.

As shown in Figure 5d, Ag2O

cubes are most catalytically efficient giving a photodegradation extent of 85% in 90 min, while octahedra photodegraded only 35% of MO and rhombic dodecahedron just degraded 15% of MO in the same time period.

Quantum yields or efficiencies for

the Ag2O cubes, octahedra, and rhombic dodecahedra have been calculated to be ~0.28, 0.11 and 0.05, respectively (see Table S4).24

Strongly facet-dependent

photocatalytic activities of Ag2O crystals have been demonstrated.

This suggests the

efficiency of photoexcited charge carriers migrating to the surface and transferring the electrons or holes to species in the solution is not the same for different surfaces. However, the relative degree of photocatalytic activity of Ag2O crystals is different 12


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from those of Cu2O crystals.

Cu2O cubes are inactive toward this reaction, while

rhombic dodecahedra are highly active.1

Octahedra are moderately active.

Here

the trend is reversed and the least active Ag2O rhombic dodecahedra still show some catalytic activity.

Such surprising results demonstrate that even for materials with

the same crystal structure and most similar composition, one cannot safely predict the outcome of their facet-dependent physical and chemical properties; experiments need to be conducted to find out. For photodegradation to occur, photogenerated electrons and holes should migrate to the particle surfaces and transfer the charges to water or oxygen to generate reactive radicals, which attack adsorbed molecules such as MO.25,26

The amount of

radicals produced upon photoirradiation of Ag2O crystals can be directly correlated to the observed photocatalytic activity.

EPR spectra of a spin-trapping agent DMPO

added to a solution of Ag2O crystals can reveal the identity and percentage of radical species formed.

After light illumination on a DMPO-Ag2O solution, EPR

measurements were conducted immediately.

Figure 6 shows EPR signals of the

photoirradiated Ag2O cubes, octahedra, and rhombic dodecahedra.

The same EPR

profile was obtained for all samples, and the radical intensities match with the photocatalytic results of respective Ag2O particles.

Cubes gave the highest EPR

signal, whereas rhombic dodecahedra produced the lowest EPR signal. 13


Analysis


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performed on the cube sample shows that the photogenerated radicals consist of 50% •OH radicals, 35% •O2– radicals, and 15% unidentified radicals (see Figure S7). The EPR data confirm the photocatalysis results are correct, and facet-dependent photocatalytic activity order is indeed different for Cu2O and Ag2O crystals. Photostability of Ag2O crystals during photocatalysis can be a problem.

In-situ

formation of Ag metal on the surface of Ag2O particles through reduction of lattice Ag+ by photoexcited electrons has been proposed.17

Figure S8 presents SEM images

of Ag2O cubes, octahedra, and rhombic dodecahedra before and after the photocatalysis experiment.

All particle shapes have experienced a significant extent

of surface etching, particularly for the octahedral crystals. on octahedra and rhombic dodecahedra. Ag2O cubes.

Pits and holes are visible

Sheetlike structures are more prevalent for

Apparently all exposed facets of Ag2O crystals are unstable to

photoirradiation.

Since Ag2O is photosensitive and unstable to long-term storage in

water, light irradiation may accelerate the conversion of Ag2O to AgOH and then Ag. To clarify the composition of the Ag2O cubes and the sheetlike or flakelike structures produced during photocatalysis, XRD and XPS characterization was conducted on the most active Ag2O cubes after the photodegradation experiment (see Figure 7).

XRD

patterns of Ag2O cubes look the same before and after photocatalysis, and the Ag (200) peak does not appear.

This analysis suggests that no metallic silver particles have 14


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been formed.

To assist the analysis, XPS spectrum of Ag2O cubes after

photocatalysis was collected to check possible presence of metallic silver.

The Ag

3d5/2 and 3d3/2 peaks have shifted by only 0.1 eV to 368.0 and 374.0 eV, respectively. Bulk silver has known Ag 3d peaks at 368.2 and 374.2 eV.27

Ag nanoparticles have

also been reported to give Ag 3d peaks at 367.4 and 373.4 eV.28

The obtained peak

positions are closer to Ag2O, so it can be concluded that metallic Ag particles were not produced, and the observed flakelike structures should still be Ag2O.

Thus, although

the various crystals are not structurally stable under the photocatalysis condition, the observed facet-dependent photocatalytic activities are still attributed to Ag2O material.

4. CONCLUSION In this work, we have developed simple and fast synthetic conditions for the growth of sub- to micrometer-sized Ag2O crystals with diverse morphologies by mixing NH4NO3, AgNO3, and NaOH solutions.

Cubes, great rhombicuboctahedra,

cuboctahedra, corner-truncated octahedra, octahedra, and rhombic dodecahedra have been synthesized.

Tuning the effective solution NH3 concentration yields edge- and

corner-truncated cubes, small rhombicuboctahedra, and edge-truncated octahedra. Movies recording the particle synthesis process show that shape control is linked to their growth rates.

Diffuse reflectance spectra give a band gap size of ~1.4 eV for 15



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the smaller-sized Ag2O cubes. various particle shapes as Ag2O.

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XPS spectra verify the surface composition of Clean Ag2O cubes, octahedra, and rhombic

dodecahedra having the same total particle surface area were employed for facet-dependent photodegradation reaction of methyl orange.

Cubes are

comparatively highly active toward this reaction, while octahedra and rhombic dodecahedra show moderate and low photocatalytic activities, respectively. spectra confirm this relative reactivity.

EPR

Although significant etching has been

observed on these crystals after performing the photodegradation reaction, no metallic silver has been detected.

These Ag2O crystals with sharp faces should be important

for further facet-dependent property examinations.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Catalytic amount calculations, size distribution histograms, photographs showing solution color changes during crystal growth, experimental conditions for Ag2O crystal growth, XPS spectra, EPR spectrum analysis, and SEM images of Ag2O crystals after photocatalysis. 16


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AUTHOR INFORMATION Corresponding author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for support of this research (MOST 101-2113-M-007-018-MY3 and 104-2119-M-007-013-MY3).

REFERENCES (1) Huang, M. H.; Rej, S.; Hsu, S.-C. Facet-Dependent Properties of Polyhedral Nanocrystals. Chem. Commun. 2014, 50, 1634‒1644. (2) Zhang, Y.; Deng, B.; Zhang, T.; Gao, D.; Xu, A.-W. Shape Effects of Cu2O Polyhedral Microcrystals on Photocatalytic Activity. J. Phys. Chem. C 2010, 114, 5073‒5079. (3) Tan, C.-S.; Hsu, S.-C.; Ke, W.-H.; Chen, L.-J.; Huang, M. H. Facet-Dependent Electrical Conductivity Properties of Cu2O Crystals. Nano Lett. 2015, 15, 17



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2155‒2160. (4) Huang, M. H.; Rej, S.; Chiu, C.-Y. Facet-Dependent Optical Properties Revealed through Investigation of Polyhedral Au‒Cu2O and Bimetallic Core‒Shell Nanocrystals. Small 2015, 11, 2716‒2726. (5) Rej, S.; Wang, H.-J.; Huang, M.-X.; Hsu, S.-C.; Tan, C.-S.; Lin, F.-C.; Huang, J.-S.; Huang, M. H. Facet-Dependent Optical Properties of Pd‒Cu2O Core‒Shell Nanocubes and Octahedra. Nanoscale 2015, 7, 11135‒11141. (6) Wang, H.-J.; Yang, K.-H.; Hsu, S.-C.; Huang, M. H. Photothermal Effects from Au‒Cu2O Core‒Shell Nanocubes, Octahedra, and Nanobars with Broad Near-Infrared Absorption Tunability. Nanoscale 2016, 8, 965‒972. (7) Chen, H.-S.; Wu, S.-C.; Huang, M. H. Direct Synthesis of Size-Tunable PbS Nanocubes and Octahedra and the pH Effects on Crystal Shape Control. Dalton Trans. 2015, 44, 15088‒15094. (8) Tan, C.-S.; Chen, H.-S.; Chiu, C.-Y.; Wu, S.-C.; Chen, L.-J.; Huang, M. H. Facet-Dependent Electrical Conductivity Properties of PbS Nanocrystals. Chem. Mater. 2016, 28, 1574–1580. (9) Ida, Y.; Watase, S.; Shinagawa, T.; Watanabe, M.; Chigane, M.; Inaba, M.; Tasaka, A.; Izaki, M. Direct Electrodeposition of 1.46 eV Bandgap Silver(I) Oxide Semiconductor Films by Electrogenerated Acid. Chem. Mater. 2008, 20, 18


Page 18 of 28

Page 19 of 28

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


1254‒1256. (10) Yan, Z.; Bao, R.; Chrisey, D. B. Generation of Ag2O Micro-/Nanostructures by Pulsed Excimer Laser Ablation of Ag in Aqueous Solutions of Polysorbate 80. Langmuir 2011, 27, 851‒855. (11) Wang, X.; Wu, H.-F.; Kuang, Q.; Huang, R.-B.; Xie, Z.-X.; Zheng, L.-S. Shape-Dependent Antibacterial Activities of Ag2O Polyhedral Particles. Langmuir 2010, 26, 2774‒2778. (12) Lyu, L.-M.; Wang, W.-C.; Huang, M. H. Synthesis of Ag2O Nanocrystals with Systematic Shape Evolution from Cubic to Hexapod Structures and Their Surface Properties. Chem.‒Eur. J. 2010, 16, 14167‒14174. (13) Kim, M.-J.; Cho, Y.-S.; Park, S.-H.; Huh, Y.-D. Facile Synthesis and Fine Morphological Tuning of Ag2O. Cryst. Growth Des. 2012, 12, 4180‒4185. (14) Wang, G.; Ma, X.; Huang, B.; Cheng, H.; Wang, Z.; Zhan, J.; Qin, X.; Zhang, X.; Dai, Y. Controlled Synthesis of Ag2O Microcrystals with Facet-Dependent Photocatalytic Activities. J. Mater. Chem. 2012, 22, 21189‒21194. (15) Harn, Y.-W.; Yang, T.-H.; Tang, T.-Y.; Chen, M.-C.; Wu, J.-M. Facet-Dependent Photocatalytic Activity and Selective Etching of Silver(I) Oxide Crystals with Controlled Morphology. ChemCatChem 2015, 7, 80‒86. (16) Lyu, L.-M.; Huang, M. H. Investigation of Relative Stability of Different Facets 19



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

of Ag2O Nanocrystals through Face-Selective Etching. J. Phys. Chem. C 2011, 115, 17768‒17773. (17) Wang, X.; Li, S.; Yu, H.; Yu, J.; Liu, S. Ag2O as a New Visible-Light Photocatalyst: Self-Stability and High Photocatalytic Activity. Chem.–Eur. J. 2011, 17, 7777–7780. (18) Stoll, S.; Schweiger, A. Easy Spin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42‒55. (19) Lyu, L.-M.; Huang, M. H. Formation of Ag2S Cages from Polyhedral Ag2O Nanocrystals and their Electrochemical Properties. Chem. Asian J. 2013, 8, 1847–1853. (20) Chiu, C.-Y.; Huang, M. H. Achieving Polyhedral Nanocrystal Growth with Systematic Shape Control. J. Mater. Chem. A 2013, 1, 8081–8092. (21) Yu, H.; Liu, R.; Wang, X.; Wang, P.; Yu, J. Enhanced Visible-Light Photocatalytic Activity of Bi2WO6 Nanoparticles by Ag2O Cocatalyst. Appl. Catal., B 2012, 111, 326–333. (22) Wei, W.; Mao, X.; Ortiz, L. A.; Sadoway, D. R. Oriented Silver Oxide Nanostructures Synthesized through a Template-Free Electrochemical Route. J. Mater. Chem. 2011, 21, 432–438. (23) Yang, Z.-H.; Ho, C.-H.; Lee, S. Plasma-Induced Formation of Flower-like Ag2O 20


Page 20 of 28

Page 21 of 28

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Nanostructures. Appl. Surf. Sci. 2015, 349, 609–614. (24) Shidpour, R.; Vossoughi, M.; Simchi, A. R.; Micklich, M. Extended Quantum Yield: A Dimensionless Factor Including Characteristics of Light Source, Photocatalyst Surface, and Reaction Kinetics in Photocatalytic Systems. Ind. Eng. Chem. Res. 2014, 53, 11973–11978. (25) Xiong, J.; Li, Z.; Chen, J.; Zhang, S.; Wang, L.; Dou, S. Facile Synthesis of Highly Efficient One-Dimensional Plasmonic Photocatalysts through Ag@Cu2O Core–Shell Heteronanowires. ACS Appl. Mater. Interfaces 2014, 6, 15716–15725. (26) Pan, M.; Zhang, H.; Gao, G.; Liu, L.; Chen, W. Facet-Dependent Catalytic Activity of Nanosheet-Assembled Bismuth Oxyiodide Microspheres in Degradation of Bisphenol A. Environ. Sci. Technol. 2015, 49, 6240–6248. (27) Liang, Y.; Guo, N.; Li, L.; Li, R.; Ji, G.; Gan, S. Fabrication of Porous 3D Flower-like Ag/ZnO Heterostructure Composites with Enhanced Photocatalytic Performance. Appl. Surf. Sci. 2015, 332, 32–39. (28) Kandula, S.; Jeevanandam, P. Sun-Light-Driven Photocatalytic Activity by ZnO/Ag Heteronanostructures Synthesized via a Facile Thermal Decomposition Approach. RSC Adv. 2015, 5, 76150–76159.

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Table 1.

Figure 1.

Exact reagent amounts used in the synthesis of various Ag2O crystals.

SEM images of (a) cubic, (b) great rhombicuboctahedral, (c) 22


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cuboctahedral, (d) corner-truncated octahedral, (e) octahedral, and (f) rhombic dodecahedral Ag2O crystals.

9000 8000

a

All scale bars are equal to 1 µm.

(200)

b

700

(111)

600

7000 6000

500

5000

400

4000

(200)

300

(220)

3000

200

2000

(111)

1000

(220)

(311)

(222)

100 0

15 20 25 30 35 40 45 50 55 60 65 70 75 80

15

20

25

30

400 350

c

35

40

45

50

55

60

65

70

75

80

2θ (degree)

2θ (degree) 700

(111)

300

(111)

d

600 500

250

(200)

400

200

300

150

(220)

50 0

(200)

200

100

(311)

(220)

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80 1 5

20

25

30

2θ (degree) 2500

e

(311) (222)

100 35

40

45

50

55

60

65

70

75

80

60

65

70

75

80

2θ (degree)

(111)

600

(220)

f

500 2000

400 1500

300 1000

200

(110)

500

(200)

(220)

(222)

0

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (degree)

Figure 2.

(111)

100

15

20

25

30

35

40

45

50

55

2θ (degree)

XRD patterns of (a) cubic, (b) great rhombicuboctahedral, (c)

cuboctahedral, (d) corner-truncated octahedral, (e) octahedral, and (f) rhombic dodecahedral Ag2O crystals.

23



1.0

448 nm

590 nm

Cube RD OCT

0.9 523 nm

0.8 0.7 Absorbance

0.6 0.5 0.4 0.3 0.2 0.1 0.0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm)

2

2

(αhν) (eV/cm )

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

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 hν (eV)

Figure 3.

UV‒vis diffuse reflectance spectra of Ag2O cubes, octahedra, and

rhombic dodecahedra and the corresponding graph of (αhv)2 vs. hv for Ag2O cubes to determine their band gap size.

24


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Figure 4.

SEM images of additional Ag2O crystal morphologies synthesized: (a)

edge- and corner-truncated cubes, (b) great rhombicuboctahedra, (c) small rhombicuboctahedra, and (d) edge-truncated octahedra.

25



a

1.2

Dark 0 min 9 min 18 min 36 min 63 min 90 min

Intensity (a. u.)

1.0 0.8 0.6 0.4 0.2

300

c

400 500 Wavelength (nm)

600 Dark 0 min 18 min 36 min 54 min 72 min 90 min

1.0 0.8 0.6 0.4

700

0.8 0.6 0.4

0.0 200

1.0

300

d

400

600

700

0.6 0.4 0.2

300

400

500

600

0.0 -30 -20 -10 0

700

10 20 30 40 50 60 70 80 90

Wavelength (nm)

Figure 5.

500

Wavelength (nm)

0.8

0.2 0.0 200

Dark 0 min 18 min 36 min 54 min 72 min 90 min

0.2

C/C0 (%)

0.0 200 1.2

b

1.0 Intensity (a. u.)

1.2

Intensity (a. u.)

Time (min)

UV–vis absorption spectra showing the extent of MO photodegradation

by (a) cubic, (b) octahedral, and (c) rhombic dodecahedral Ag2O crystals as a function of reaction time.

(d) Summary of MO photodegradation extent versus reaction time.

Intensity (a. u.)

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

Page 26 of 28

3460

3480

3500

3520

3540

Field [G]

Figure 6.

EPR signals of photoirradiated Ag2O cubes, octahedra, and rhombic

dodecahedra. 26


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Ag 3d

After

Before Intensity (a. u.)

Ag 3p3 Ag 4p Ag 3p1 Ag 4s Ag 3s

0

100 200 300 400 500 600 700 800 900 1000 1100

Binding energy (eV)

3d5/2 368.0 eV

3d3/2

367.9 eV

After

O KLL

Intensity (a. u.)

O 1s C 1s

Intensity (a. u.)

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


(111)

Ag2O

(200)

(220)

(110)

374.0 eV 373.9 eV

(311) (222)

(111) Ag Before After

(200) (220)

15 20 25 30 35 40 45 50 55 60 65 70 75 80

364 366 368 370 372 374 376 378 380

2θ (degree)

Binding Energy (eV)

Figure 7.

(310)

(left) Full XPS spectrum of Ag2O cubes after photocatalysis and the

expanded XPS spectra comparing the Ag 3d peak positions before and after photocatalysis.

(right) XRD patterns of Ag2O cubes before and after photocatalysis.

Standard Ag2O and Ag patterns are also shown.

27



1.0 0.8 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

0.6 0.4 0.2 0.0 0

10

20

30

40

50

60

70

80

90

Time (min)

TOC Graphic

28


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Synthesis of Diverse Ag2O Crystals and Their Facet-Dependent

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