Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet

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Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet-Dependent Optical Property, Photocatalytic Activity and Electrical Conductivity Examinations Meng-Shan Hsieh, Huang-Jen Su, Pei-Lun Hsieh, Yun-Wei Chiang, and Michael H. Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13941 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Applied Materials & Interfaces

Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet-Dependent Optical Property, Photocatalytic Activity and Electrical Conductivity Examinations

Meng-Shan Hsieh,† Huang-Jen Su,‡ Pei-Lun Hsieh,§ Yun-Wei Chiang,† and Michael H. Huang*,†

†

Department of Chemistry, ‡Interdisciplinary Program of Sciences, and §Department

of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT:

This work has developed conditions for the synthesis of Ag3PO4

cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods by tuning the amount of NH4NO3, NaOH, AgNO3, and K2HPO4 solutions added.

Use of a lesser amount of AgNO3 solution can form much smaller rhombic

dodecahedra and tetrahedra.

Sub-micrometer-sized Ag3PO4 cubes and rhombic

dodecahedra with sizes larger than 300 nm do not exhibit optical size effect, but ~290 nm rhombic dodecahedra show a smaller band gap value than that for larger cubes, and tetrahedra show the most blue-shifted absorption edge. present in Ag3PO4 crystals.

Optical facet effect is

Ag3PO4 cubes are more photocatalytically active than 1

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rhombic dodecahedra toward photodegradation of methyl orange, but tetrahedra are inactive, showing clear presence of photocatalytic facet effects.

Electron

paramagnetic resonance (EPR) results confirm much higher production of hydroxyl radicals from photoirradiated Ag3PO4 cubes than from rhombic dodecahedra, while tetrahedra yield essentially no radicals.

A modified band diagram showing different

degrees of band edge bending can explain these observations.

All these Ag3PO4

crystals show poor electrical conductivity properties, but the {110} faces are slightly more conductive than the {100} faces.

As a result, current rectifying I‒V curves

have been obtained, demonstrating facet-dependent electrical properties are broadly observable in many semiconductor materials.

This work reveals again that

facet-dependent optical, photocatalytic, and electrical conductivity properties are intrinsic semiconductor properties.

KEYWORDS:

electrical conductivity, facet-dependent properties, photocatalytic

activity, semiconductor, silver phosphaste

INTRODUCTION

Recent demonstrations of facet-dependent photocatalytic,1‒8 electrical 2


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conductivity,9‒14 and light absorption properties of Cu2O, Ag2O, PbS, TiO2 crystals and other nanostructures have shown that control of crystal surfaces and interfaces are highly important to fully understand semiconductor properties.15‒20 transmission can have facet effect.19,20

Even heat

The general idea is that there exists an

ultrathin surface layer with different degrees of surface band bending for various crystal faces giving rise to these interesting phenomena.1‒3,9‒11

This layer affects

how charge carriers, photons, and heat enter and exit the semiconductor crystals. For example, Cu2O rhombic dodecahedra are appreciably more photocatalytically active than octahedra, while cubes are inactive.1‒3

Ag2O and Cu2O octahedra are

much more electrically conductive than their cubic crystals, but their {110}-bound rhombic dodecahedra are simply not conductive.9,10

To show these properties are

generally observable in many semiconductors, it is necessary to examine other materials such as silver phosphate crystals.

Ag3PO4 is an excellent choice to extend

facet effect examinations because their various crystal morphologies with sharp faces have been synthesized.21‒27

For example, {110}-bound Ag3PO4 rhombic

dodecahedra can be synthesized by mixing silver acetate and Na2HPO4 solutions, while {100}-bound cubes have been prepared by mixing AgNO3, NH3, and Na2HPO4 solutions.21

Ag3PO4 tetrahedra exposing {111} facets can be obtained by reacting

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silver nitrate with H3PO4 in ethanol solution.22 In another report, the chemicals and temperature used to make Ag3PO4 crystals with these shapes are also not consistent.24 It is desirable to develop synthetic conditions to grow Ag3PO4 crystals with tunable shapes using the same reagents.

With successful synthesis of Ag3PO4 cubes,

tetrahedra, and rhombic dodecahedra, their facet-dependent photocatalytic activity and electrical conductivity properties can be examined.

Electrical conductivity

measurements have not been performed on single Ag3PO4 crystals.

In this study, we have developed aqueous synthetic conditions to make Ag3PO4 cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods using the same reagents in the absence of any capping agent.

Some level

of size tunability has been achieved to evaluate crystal size and facet factors on their band gap values.

Use of the Ag3PO4 crystals for the photodegradation of methyl

orange reveals moderate performance for cubes and rhombic dodecahedra but shows surprisingly photocatalytic inactivity for tetrahedra.

In the literature, all Ag3PO4

particle shapes have been reported to exhibit good to excellent photocatalytic properties.21‒26

Electron paramagnetic resonance (EPR) spectra were taken to

confirm the photocatalysis results. observations have been considered.

Possible explanations for these contradictory Electrical conductivity measurements have been 4


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performed on single Ag3PO4 particles exposing different crystal faces for the first time. Although all crystal facets are poorly conductive, a slightly current rectifying effect has been recorded with electrodes making contacts to the {100} and {110} facets of a truncated rhombic dodecahedron.

EXPERIMENTAL SECTION

Chemicals.

Ammonium nitrate (NH4NO3, 99%, Showa), silver nitrate (AgNO3,

99.8%, Sigma-Aldrich), sodium hydroxide (NaOH, 99%, Sigma-Aldrich), potassium hydrogen phosphate (K2HPO4, Hayashi Pure Chemical), ethanol (EtOH, 99.8%, Sigma-Aldrich), methyl orange (C14H14N3NaO3S, Hayashi Pure Chemical), and spin trapping reagent DMPO (5,5-dimethyl-1-pyrroline N-oxide, ≥ 97%, Sigma-Aldrich) were used without further purification. Ultrapure distilled and deionized water (18.2 MΩ) was used for all solution preparations.

Synthesis of Ag3PO4 Crystals with Tunable Shapes. synthesized with room light turned off.

All Ag3PO4 crystals were

To make Ag3PO4 cubes, rhombic

dodecahedra, and {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods, 8.92, 8.42, 8.12, 1.8874 and 1.8874 mL of deionized water were added to respective 5



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vials, and then 100, 600, 600, 218.8, and 281.3 µL of 0.4 M NH4NO3 solution were respectively introduced.

Next, 180, 180, 180, 393.8, and 506.3 µL of 0.2 M NaOH

solution and 400, 400, 550, 500, and 500 µL of 0.05 M AgNO3 solution were added to the respective vials.

The solutions were stirred vigorously for 10 min to complete

the formation of [Ag(NH3)2]+ complex.

Finally, 400, 400, 550 µL of 0.1 M K2HPO4

solution were added to grow Ag3PO4 cubes, rhombic dodecahedra, and {100}-truncated rhombic dodecahedra, and the solution color turned from colorless to light yellow.

To form Ag3PO4 tetrahedra and tetrapods, 7.0 mL of 0.7 M K2HPO4

solution was introduced to each vial. growth.

The solutions were stirred for 2 min for crystal

Scheme S1 and Table S1 in the Supporting Information provide the

synthetic process and the exact reagent amounts used. centrifuged at 7500 rpm for 5 min.

The solutions were then

The top solution containing unreacted chemicals

was carefully removed, and the Ag3PO4 precipitate was dispersed in deionized water and 95% ethanol with 1:1 volume ratio and centrifuged twice more.

Finally, the

Ag3PO4 particles were kept in 99.8% ethanol for subsequent characterization and photocatalysis experiments.

To prepare smaller Ag3PO4 rhombic dodecahedra and tetrahedra, the volumes of 0.4 M NH4NO3, 0.2 M NaOH and 0.1 or 0.7 M K2HPO4 solutions are the same as 6


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those used to make regular-sized Ag3PO4 rhombic dodecahedra and tetrahedral, but the volume of 0.05 M AgNO3 solution added was decreased to 325 µL for both shapes. Precipitate was not observed after adding the K2HPO4 solution.

Again stirring for 10

min before K2HPO4 introduction and 2 min after K2HPO4 addition was required. Next, the solution was poured into a centrifuge tube with 5 mL of 95% ethanol.

The

solution turned light yellow gradually, indicating the formation of Ag3PO4 crystals. After 2 min, the same washing process followed as depicted above.

See Scheme S2

and Table S2 for the reaction procedure and the exact reagent amounts used.

Photocatalytic Experiments.

The synthesized Ag3PO4 particles were dried,

weighted, and dispersed in deionized water with a concentration of 1 mg/mL.

For

Ag3PO4 cubes, rhombic dodecahedra, and tetrahedra, 2.45, 1.77, and 1.22 mL of the solutions having the same total particle surface area (0.029 m2) were respective added to a cubic quartz vial. Then methyl orange (MO) in deionized water was added to the vial with a final concentration of 15 ppm and a total solution volume of 12 mL. Before light illumination, the solution was stirred continuously in the dark for 30 min for molecular adsorption equilibrium. 500-W Xenon lamp placed 30 cm away.

Then the solution was illuminated by a The average light intensity reaching to the

solution was 436 mW/cm2 measured using a power meter. 7


A filter blocking light


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wavelengths below 400 nm was placed between the vial and Xenon lamp to reduce Ag3PO4 absorption.

An aliquot of 800 µL was withdrawn from the solution at

regular time intervals and centrifuged immediately at 6500 rpm for 2 min to remove the Ag3PO4 particles for spectral measurements.

Instrumentation.

Scanning electron microscopy (SEM) images were taken

using a JEOL JSM-7000F scanning electron microscope.

X-ray diffraction (XRD)

patterns were collected using a Bruker D2 PHASER desktop diffractometer with Cu Kα radiation.

UV–vis diffuse reflectance spectra were obtained by a JASCO V-570

spectrophotometer equipped with a solid sample holder.

A X500 Xenon lamp from

Blue Sky Technologies was used as light source in photocatalysis.

EPR spectra were

collected at ambient temperature on a Bruker ELEXSYS E580-400 EPR spectrometer.

RESULTS AND DISCUSSION

Precipitation reaction proceeds instantly.

From what has been learned in the

synthesis of Ag2O crystals with systematic shape evolution, formation of [Ag(NH3)2]+ complex can slow down the precipitation reaction to yield crystals with morphology

8


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control.28 crystals.

The same synthetic strategy has been employed here to make Ag3PO4 The following reactions are involved.

3 Ag+ + PO43‒ ⇌ Ag3PO4

(1)

NH4NO3 + NaOH → NH3 + H2O + Na+ + NO3‒

(2)

AgNO3 + 2 NH3 ⇌ [Ag(NH3)2]+ + NO3‒

(3)

3 [Ag(NH3)2]+ + 6 K2HPO4 ⇌ Ag3PO4 + 6 NH4+ + 5 PO43‒ + 12 K+

(4)

Equation (1) has an equilibrium constant K of 5.56 × 1017 (inverse of Ksp of Ag3PO4 at 1.8 × 10‒18) for the formation of Ag3PO4 by directly mixing AgNO3 and a soluble phosphate source.

Equations (3) and (4) can be considered to have an overall

equilibrium constant K of 3.27 × 1010 (1/(Kf of [Ag(NH3)2]+ at 1.7 × 107) (Ksp of Ag3PO4)), recognizing the proton on HPO42‒ and the dissociated PO43‒ complicates the calculation.

Formation of [Ag(NH3)2]+ complex before reaction with phosphate

can significantly reduce the driving force toward Ag3PO4 production, allowing better shape control of the resulting crystals.

To make Ag3PO4 cubes, an almost equimolar NH4NO3 and NaOH mixture was first prepared to give the ammonia solution, and then AgNO3 solution was added to

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form the [Ag(NH3)2]+ complex.

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Next, a K2HPO4 solution was introduced. Molar

ratios of NH4NO3:NaOH:AgNO3:K2HPO4 are 2:1.8:1:2.

To form Ag3PO4 rhombic

dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra and tetrapods, the molar ratios of NH4NO3 and AgNO3 were adjusted to tune the crystal growth rate. For example, a molar ratio of NH4NO3:AgNO3 at 6:1 yields rhombic dodecahedra; an excess amount of NH4+ ions favors the reverse reaction and lower the reaction rate. Using molar ratios of NH4NO3:AgNO3:K2HPO4 at 3.5:1:196 and 4.5:1:196, Ag3PO4 tetrahedra and tetrapods can be synthesized.

Figure 1 shows SEM images of the synthesized Ag3PO4 cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedral, and tetrapods. the samples are highly uniform in size and shape, and appear to have sharp faces.

All A

{100}-truncated rhombic dodecahedron has 6 square {100} facets and 12 hexagonal {110} facets.

The tetrahedra have a somewhat curved contour because of their tips

are shorter than those in a perfect tetrahedron. be expose mostly {111} faces.22,24

The tetrahedra and tetrapods should

Figure S1 and Table S3 give the size distribution

histograms, average sizes, and standard deviations of these Ag3PO4 crystals.

In

particular, the rhombic dodecahedra have an average opposite face distance of 1057 nm, while the cubes and tetrahedra have respective average edge lengths of 534 and 10


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1283 nm.

XRD patterns of these Ag3PO4 crystals are displayed in Figure 2.

All

samples give XRD patterns well matched to the standard pattern of Ag3PO4. Because of their large dimensions and random orientations, enhanced peak intensities reflecting the exposed surface facets are less pronounced.

Nevertheless, the rhombic

dodecahedra and {100}-truncated rhombic dodecahedra show a very strong (110) reflection peak.

Similarly, the Ag3PO4 cubes exhibit an enhanced (200) peak.

The

(111) peak of tetrahedra and tetrapods are not particularly enhanced, but the tetrapods do show strong (222) peak compared to the (111) and (200) peaks.

Similar (222)

peak intensity has been observed for Ag3PO4 tetrahedra.22

Crystal morphology control is generally related to the particle growth rate and is observable from the solution color changes.5

For this reason, images of the reaction

mixture was taken from the instant K2HPO4 solution was added.

Figure S2 offers

photographs at selected time points in the growth of Ag3PO4 cubes, rhombic dodecahedra, and tetrapods.

The initially colorless solutions in the growth of cubes

and tetrapods changed to light yellow in just 1 and 10 sec, respectively.

Clearly

cubes are produced at the fastest rate, and the solution color remains unchanged after just 4 sec of reaction.

Tetrapods take about 20 sec to complete crystal growth.

Ag3PO4 rhombic dodecahedra took 30 sec to develop slightly yellow tint and 60 sec to 11



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complete crystal growth, so they have the slowest growth rate among these particle shapes.

Because the synthesized Ag3PO4 crystals are microcrystals except the cubes, attempts have been made to reduce the dimensions of rhombic dodecahedra and tetrahedra.

We found reducing the amount of AgNO3 introduced and adding 7.5 mL

of ethanol in the last step is effective.

Since ethanol was added 2 min after the

introduction of K2HPO4 solution, complete crystal growth can be longer than 2 min for ethanol to play a role.

The initially clear solution gradually turned yellow upon

ethanol introduction, signifying precipitate formation.

Figure 3 gives SEM images

of the synthesized smaller Ag3PO4 rhombic dodecahedra and tetrahedra.

They have

sharp faces and perfect shapes necessary for facet-dependent property examinations. Figure S3 provides their size distribution histograms. standard deviations are listed in Table S4.

Average crystal sizes and their

The rhombic dodecahedra have an

average opposite face distance of 417 nm, while tetrahedra have an average edge length of 648 nm.

Recently it has shown that the light absorption positions of Cu2O nanocrystals are both size- and facet-dependent by taking UV-vis absorption spectra of size-tunable Cu2O cubes below 100 nm and octahedra smaller than 200 nm.18 12


As particle size

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increases, the absorption band red-shifts steadily.

The absorption edge of 600 nm

Ag3PO4 rhombic dodecahedra have been shown to red-shift by 30 nm than that of similarly-sized Ag3PO4 cubes.21

To further evaluate size and facet effects to the

optical properties of Ag3PO4 crystals, cubes with tunable edge lengths of 365, 462, 530, and 648 nm, as well as rhombic dodecahedra with sizes of 289, 643, 993, and 1314 nm, were synthesized.

Table S5 lists their synthetic conditions.

Figure S4

presents SEM images, diffuse reflectance spectra, and Tauc plots of the size-tunable Ag3PO4 cubes for the determination of their indirect optical band gaps.

The cube

solutions all have a greenish color. Because of their similar absorption edge, the cubes have indirect band gaps of 2.33 (365 and 462 nm cubes), 2.36 (530 nm cubes), and 2.37 eV (648 nm cubes).

It is possible that for fairly large Ag3PO4 cubes beyond

300 nm, optical size effect is not observable. size-tunable rhombic dodecahedra.

Figure S5 gives SEM images of the

All samples show a yellow solution color except

the 289 nm rhombic dodecahedra with a greenish yellow hue.

The UV‒vis diffuse

reflectance spectra of Ag3PO4 rhombic dodecahedra and their Tauc plots are available in Figure 4.

The 289 nm rhombic dodecahedra have a band gap of 2.25 eV, while

the larger rhombic dodecahedra have a band gap of 2.34‒2.35 eV.

It is unclear why

the smaller 289 nm rhombic dodecahedra yield a lower band gap value. Again

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optical size effect is not observable, suggesting much smaller Ag3PO4 nanocrystals are needed to better evaluate their optical properties.

Figures S4 and S5 also express

these Ag3PO4 cubes and rhombic dodecahedra in terms of their volumes for better comparison of the crystal sizes.

For large cubes and rhombic dodecahedra, they

have very similar band gap values.

Previously these two particle shapes have also

exhibited the same absorption edge.24

Figure S6 offers SEM image, the solution

color, diffuse reflectance spectrum, and the corresponding Tauc plot for Ag3PO4 tetrahedra having an average edge length of 673 nm (volume of 3.6 × 107 nm3). solution has a greenish yellow color. is 2.38 eV (or 521 nm).

The

The determined band gap for these tetrahedra

Comparing the 289 nm rhombic dodecahedra (volume of

1.7 × 107 nm3) having a band gap of 2.25 eV (or 551 nm) to larger 365 nm cubes (volume of 4.8 × 107 nm3) with a band gap of 2.33 eV and that of 673 nm tetrahedra, rhombic dodecahedra show a more red-shifted light absorption edge, while tetrahedra display the most blue-shifted absorption edge (Figure S6d).

For these particles with

similar sizes, their absorption edges differ by 30 nm (Figure S6e), showing optical facet effects should be present even for such large Ag3PO4 crystals.

Previously

Ag3PO4 tetrahedra have also been shown to give a slightly large band gap than cubes and rhombic dodecahedra, although the sizes of these particle shapes are quite

14


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different.24

The facet-dependent photocatalytic activities of Ag3PO4 crystals have been investigated using 512 nm cubes, 371 nm rhombic dodecahedra, and 625 nm tetrahedra as the photocatalysts.

Due to the fast crystal growth rates, actual particle

sizes must be determined even though the same synthetic conditions have been followed. study.

Figure S7 shows SEM images of the crystals used for the photocatalysis

For photocatalytic activity comparison, the total particle surface area was

fixed at 0.00245 m2.

From their surface areas and volumes (see Figure S8), the

weight of a single particle can be obtained from the density of Ag3PO4 (6.37 g/cm3). The ratios of surface area per unit mass for the cube, rhombic dodecahedron, and tetrahedron are 1:1.38:2.01, so 2.45, 1.77, and 1.22 mg of Ag3PO4 cubes, rhombic dodecahedra, and tetrahedra were used.

Figure S9 offers UV‒vis absorption spectra

of methyl orange (MO) as a function of irradiation time in the presence of Ag3PO4 cubes, rhombic dodecahedra, and tetrahedra. summarized in Figure 5.

The photocatalysis results are

Before illumination, the solution was stirred for 30 min in

the dark to establish molecular adsorption equilibrium.

After illumination, the MO

absorption band centered at 463 nm shows different photodegradation efficiencies for these particle morphologies, although all of them are not good photocatalysts. 15


The


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Ag3PO4 cubes are most photocatalytically active; MO absorption dropped 34% after 90 min of light irradiation.

The extent of MO photodecomposition is only 17% in

the presence of rhombic dodecahedra.

Surprisingly, the Ag3PO4 tetrahedra are

completely inactive after 90 min of reaction.

Strongly facet-dependent

photocatalytic activity of Ag3PO4 crystals has been observed.

Interestingly, in

addition to photocatalytic inactivity of Cu2O {100} faces, the {111} faces of Ag3PO4 crystals are also lacks photocatalytic activity.

However, literature reports have

shown all Ag3PO4 morphologies to be good photocatalysts, and the tetrahedral crystals are more photocatalytically active than rhombic dodecahedra and cubes.23,24 Our observed trend is exactly opposite of what have been reported before, although one report has shown that Ag3PO4 tetrahedra are less photocatalytically active than aggregated Ag3PO4 nanoparticles.29

To verify our photocatalysis results, EPR spectra detecting the formation of hydroxyl and other radicals were collected.

Hydroxyl radicals added to DMPO, a

spin trapping agent, form DMPOX with characteristic EPR hyperfine coupling constants.30

Purified DMPO was added to a solution containing Ag3PO4 crystals and

MO molecules for 5 min of photocatalysis experiment, and the solution was immediately sent for EPR measurements.

Figure S10 shows SEM images of the 16


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various Ag3PO4 crystals for EPR characterization. obtained.

Figure 6 is the EPR spectra

The purified DMPO without adding Ag3PO4 crystals gave no EPR signals.

Ag3PO4 cubes indeed showed the strongest EPR peaks.

The spectrum matches

exactly with theoretical parameters of DMPOX (AN = 7.2 G, Aβ-2H = 4.0 G, and g = 2.0062).

The analysis indicates the photogenerated radicals are essentially ·OH

radicals.

Ag3PO4 rhombic dodecahedra showed the same EPR profile, but the peak

intensities are much lower.

Remarkably, tetrahedra gave essentially no EPR signals,

meaning no radicals are produced from the {111} faces of Ag3PO4 crystals upon photoexcitation.

This happens presumably because no photoexcited electrons and

holes reach the {111} faces of Ag3PO4 to react with water or dissolved oxygen forming radical species.

The EPR data confirm our photocatalysis results.

The

question remains why various Ag3PO4 morphologies have been reported to show good photocatalytic properties. We suspect possible formation of a tiny amount of Ag nanoparticles during the course of photodegradation experiment.

Metal

nanoparticles and other conductive materials have been inadvertently or intentionally introduced to Ag3PO4 crystals to enhance photogenerated charge separation for improved photocatalytic performance.30,31

The morphologies of our Ag3PO4 crystals

are maintained after the photocatalysis experiment (Figure S7), although some small

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nanoparticles can be seen on the surfaces of cubes and rhombic dodecahedra.

Page 18 of 35

XRD

patterns of the most active Ag3PO4 cubes before and after the photodegradation experiment are shown in Figure S11.

Only Ag3PO4 peaks are present, showing no

metallic Ag particles have been formed.

This outcome makes sense, because

formation of Ag nanoparticles during photoirradiation should greatly enhance photocatalytic activity of the cubes, but this is not the case.

Similarly, any formation

of Ag through surface etching would make Ag3PO4 tetrahedra photocatalytically active. We also consider MO concentrations versus catalyst concentrations in our and other studies (8 ppm MO and 2 mg/mL catalyst; 10 ppm methylene blue and 0.13 mg/mL catalyst).21,24

In our case, MO and catalyst concentrations are 15 ppm and

0.1‒0.2 mg/mL, respectively. transmit only visible light.23

Light source is all Xe lamps with a filter placed to These differences cannot explain the observed

photocatalytic inactivity of Ag3PO4 tetrahedra. One thing worthy to mention is that our Ag3PO4 crystals were synthesized in a dark room to minimize the possibility of any Ag formation, while such precautionary measure was not followed in other studies.

On the basis of the observed facet-dependent photocatalytic results, an adjusted band diagram of Ag3PO4 is constructed (Figure 7).

A similar band diagram for Cu2O

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has been presented to explain its facet effects on photocatalytic activity.2

The {111}

surface is drawn to bend up most steeply and deviate from the normal valence and conduction band positions to indicate the highest barrier for electron transport to the crystal surface.

The photoexcited electrons then combine with holes resulting in

photocatalytic inactivity of Ag3PO4 tetrahedra. The {100} surface is drawn to have the least degree of upward bending to represent its best electron transfer to surface-adsorbed water or oxygen for more efficient production of hydroxyl radicals. Upward bending for the {110} face should be higher than that for the {100} face to signify less efficient photoexcited electron transport to the crystal surface for radical generation.

Figure 7b‒d show the three cases of photocatalytic process for Ag3PO4

crystals.

We next probed electrical conductivity properties of a single Ag3PO4 cube, rhombic dodecahedron, and tetrahedron.

Figure 8 shows SEM images of two

tungsten probes making contacts to a single Ag3PO4 crystal and the obtained I‒V curves.

Additional I‒V curves for each particle shape are available in Figure S12.

The asymmetrical I‒V curves should be attributed to unequal contact areas of the tungsten probes.

Previous electrical conductivity measurements on polyhedral Cu2O,

Ag2O, PbS, and TiO2 crystals have recorded current in the nA to µA range.9‒13 19



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However, ultrasmall current (pA) has consistently been measured for Ag3PO4 crystals, showing Ag3PO4 is poorly conductive.

This poor electrical conductivity property

may arise from the large energy band gaps for bulk and the different surfaces of Ag3PO4.24 Despite the poor conductivity behaviors and current variations from one measurement to another, the {110} faces of a rhombic dodecahedron are generally more conductive than the {100} faces of a cube. The fact the bigger rhombic dodecahedron is still more conductive than the much smaller cube also supports better electrical conductivity for the {110} faces of Ag3PO4.

To further test this slight

conductivity difference, I‒V measurements were performed on a {100}-truncated rhombic dodecahedron with tungsten probes contacting simultaneously its {100} and {110} facets (Figure 8e and Figure S12d).

Current rectifying effect has been

recorded with higher current flowing in the direction of {110} to {100}, suggesting Ag3PO4 crystals also possess facet-dependent electrical conductivity properties.

CONCLUSIONS

In this study, Ag3PO4 cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods have been synthesized by tuning the amounts

20


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of NH4NO3, NaOH, AgNO3, and K2HPO4 aqueous solutions added. cubes, all other particles are micro-crystals. tetrahedra have also been prepared.

Other than the

Much smaller rhombic dodecahedra and

Optical size effect is not observable for Ag3PO4

cubes and rhombic dodecahedra with sizes greater than 300 nm.

The smallest 289

nm rhombic dodecahedra show a more red-shifted absorption edge than that for larger cubes, while similarly-sized tetrahedra display the most blue-shifted absorption edge, so optical absorption of Ag3PO4 crystals also exhibits facet effect.

Ag3PO4 cubes are

more photocatalytically active than rhombic dodecahedra toward MO degradation, while tetrahedra display no photocatalytic activity.

EPR measurements detecting

photogenerated radicals agree well with photocatalysis results.

A modified band

diagram has been presented to explain the observed photocatalytic properties.

All

particle shapes are poorly electrically conductive, but the {110} faces are generally more conductive than the {100} faces.

Hence, slight electrical facet effects should

still be present in Ag3PO4 crystals, showing facet-dependent electrical conductivity properties are broadly observable in many semiconductor materials.

ASSOCIATED CONTENT

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Supporting Information

This Supporting Information is available free of charge via the Internet on the ACS Publications website at DOI:

Additional experimental procedures, particle size distribution histograms, photographs showing solution color changes during crystal synthesis, SEM images of Ag3PO4 cubes and rhombic dodecahedra with tunable sizes and diffuse reflectance spectra of the cubes, SEM images of Ag3PO4 crystals after the photocatalysis experiment, UV‒vis spectra, XRD patterns, and additional I‒V curves.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ORCID

Michael H. Huang: 0000-0002-5648-4345

Notes

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We thank the Ministry of Science and Technology of Taiwan for support of this research (MOST 104-2119-M-007-013-MY3 and 105-2633-M-007-003).

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Figure 1.

SEM images of the synthesized Ag3PO4 (a) cubes, (b) rhombic

dodecahedra, (c) {100}-truncated rhombic dodecahedra, (d) tetrahedra, and (e) tetrapods.

Insets show magnified SEM images with scale bars equal to 500 nm.

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Figure 2.

(a) A standard XRD pattern of Ag3PO4.

crystals with various shapes.

(b–f) XRD patterns of Ag3PO4

The stars indicate enhanced peak intensities due to the

uniformed particle shapes.

Figure 3. tetrahedra.

SEM images of smaller Ag3PO4 (a) rhombic dodecahedra and (b) Insets scale bars equal to 200 nm.

31



Figure 4.

(a) UV-vis diffuse reflectance spectra of Ag3PO4 rhombic dodecahedra. (b)

Tauc plot of (αhν)1/2 vs. energy (hν) for Ag3PO4 rhombic dodecahedra to determine their indirect optical band gap.

100 90 80 70

C/C0 (%)

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60 50 40 30 Tetrahedra Rhombic Dodecahedra Cubes

20 10 0 -30 -20 -10

0

10 20

30 40 50 60 70 80

90

Time (min)

Figure 5.

Extents of methyl orange photodegradation as a function of reaction time

using Ag3PO4 cubes, rhombic dodecahedra, and octahedra as the photocatalysts.

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DMPO Tetrahedra Rhombic Dodecahedra Cubes Fit

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


3460

3480

3500

3520

Magnetic field [G] Figure

6.

EPR spectra of DMPO-OH in the presence of photoirradiated Ag3PO4

crystals. The purified DMPO without any Ag3PO4 catalysts gives no EPR signal.

a

b

Electron-hole recombination

Figure 7.

c

Moderate electron transfer

d

High electron transfer

(a) Adjusted band diagram of Ag3PO4 with different degrees of surface

band bending for the {111}, {110}, and {100} facets.

In the diagram, qX is the

33



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Page 34 of 35

semiconductor electron affinity, Eg is the semiconductor band gap, Ec is the conduction band energy, Ev is the valence band energy, and Ef is the Fermi level. (b‒d) Drawings showing different photocatalytic responses of Ag3PO4 (b) tetrahedra with {111} facets, (c) rhombic dodecahedra with {110} facets, and (d) cubes with {100} facets.

Figure 8.

(a‒c) SEM images of a single Ag3PO4 (a) cube, (b) rhombic

dodecahedron, and (c) tetrahedron for electrical conductivity measurements with tungsten probes contacting a crystal.

(d) The measured I-V curves.

(e) SEM image

showing tungsten probes contacting the {110} and {100} faces of a {100}-truncated rhombic dodecahedron and the measured I‒V curves. 34


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100

Tetrahedra Rhombic Dodecahedra Cubes

90 80

Intensity (a. u.)

70

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


60 50 40 30 20

Tetrahedra Rhombic Dodecahedra Cubes

10 0 0

10

20

30

40

50

60

70

80

90

3460

Time (min)

3480

3500

Magnetic field [ G]

TOC Graphic

35


3520

Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet

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