Solid-Phase Photochemical Growth of Composition-Variable AuâAg

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Solid-Phase Photochemical Growth of CompositionVariable Au-Ag Alloy Nanoparticles in AgBr Crystal Shin-ichi Naya, Yoshihiro Hayashido, Ryo Akashi, Kaoru Kitazono, Tetsuro Soejima, Musashi Fujishima, Hisayoshi Kobayashi, and Hiroaki Tada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04531 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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The Journal of Physical Chemistry

Solid-Phase Photochemical Growth of Composition-Variable Au-Ag Alloy Nanoparticles in AgBr Crystal Shin-ichi Naya,a Yoshihiro Hayashido,b Ryo Akashi,b Kaoru Kitazono,b Tetsuro Soejima,b Musashi Fujishima,b Hisayoshi Kobayashi,c Hiroaki Tadaa,b * a

Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. b Graduate School of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. c Emeritus professor of Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan. ABSTRACT: Silver-silver halides (Ag-AgX, X = Cl, Br, I) have emerged as a new type of visible-light photocatalyst for solar-to-chemical transformations. The key to improving the activity of the “plasmonic photocatalyts” is the compatibility of local electric field enhancement (LEFE) and effective utilization of the sunlight as the energy source. Ag nanoparticle (NP) possesses intense LEFE effect, while the absorption peak is situated near the blue edge of the visible region. On the other hand, Au NP has absorption well matching the solar spectrum, but the LEFE is much smaller than that of Ag NP. The optical property of Aux-Ag1-x alloy NP varies between those of Ag and Au NPs depending on the composition x, and thus, Aux-Ag1-x alloy NP-incorporated AgX (Aux-Ag1-x@AgX) can be a promising plasmonic photocatalyst. At the first step, gold ion-doped AgBr NPs are formed on mesoporous TiO2 film by the successive ionic layer adsorption and reaction (SILAR) method. At the second step, UV-light irradiation (λ > 320 nm) of the sample in methanol yields Aux-Ag1-x alloy particles having diameter of ~5 nm in the interior of AgBr with crystallite size of ~50 nm. The peak wavelength for the localized surface plasmon resonance can be tuned in the range between 500 nm to 600 nm through the alloy composition. On the basis of the experimental and density functional theory calculation results, we propose a plausible reaction mechanism.

in water were calculated by finite-difference time-domain (FDTD) technique.13 In the absorption spectra shown in Figure 1A, the LSPR weakens and the LSPR peak wavelength (λp,LSPR) redshifts with an increase in x. A maximum in the local electric field (Emax) exists near the surface of the metal NPs. Figure 1B shows plots of the λp,LSPR and the maximum local electric field enhancement factor (EFmax) defined by Eq. 1 as a function of x.

INTRODUCTION In virtue of energy and environmental issues, there is growing interest in heterostructures consisting of plasmonic metal nanoparticles (NPs) and semiconductors as a new type of visible-light photocatalysts for solar-to-chemical transformations.1-3 The so-called “plasmonic photocatalysts” rely on the intense visible absorption due to the localized surface plasmon resonance (LSPR) of the metal NPs.4 Gold NP-loaded TiO2 (Au/TiO2) has been studied as the prototype of the plasmonic photocatalysts5-8 since the discovery of the LSPR excitation-induced electron transfer from Au NP to TiO2 by Tian and Tatsuma.9 Recently, silver-silver halides (Ag-AgX, X = Cl, Br, I) have appeared as a new material.10 Many studies on the visible-light-induced reactivity of twoand three-component Ag-AgX systems have been reported for the degradation of model water pollutants10 and the reduction of carbon dioxide.11 Also, we have prepared Ag NP-incorporated AgBr (Ag@AgBr) by the successive ionic layer adsorption and reaction (SILAR)-photoreduction (PR) technique, showing that it works as a local electric field-enhanced (LEFE) plasmonic photocatalyst.12 The prerequisite for enhancing the photocatalytic activity is the compatibility of the LEFE (requirement 1) and effective utilization of sunlight (requirement 2). For the latter to be achieved, the plasmonic photocatalyst should respond to the visible light of which energy occupies ca. 45% of the solar energy. The optical properties of spherical Aux-Ag1-x alloy NPs with a particle diameter of 5 nm and varying composition (x)

EFmax = (Emax/E0)2 where E0 is the amplitude of incident electric field.

A)

(1)

B)

Figure 1. (A) FDTD-calculated absorption spectra for Aux-Ag1-x NPs in water. (B) LSPR-peak wavelength and local electric field enhancement factor as a function of x. The NP size was fixed at 5 nm.

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were used to build the calculation models. To study an electric field distribution around NPs, total-field scattered-field (TFSF) sources were used. In the simulation, non-polarized plane wave with the wavelength from 300 to 800 nm (f = 375 ~ 1,000 THz) was injected to the model from the normal direction and was focused to the immediate vicinity of the model. To enhance the resolution of field mapping images, a mesh override region was set around NPs. The simulation region was set to be 405 × 405 × 405 nm3 with perfectly matched layer (PML) absorbing boundary condition. To minimize the computational time, the simulation was performed on the quarter of each calculation model using symmetric and anti-symmetric boundary conditions. The simulation time and temperature were set to be 1,000 fs and 300 K, respectively. The electric field intensity enhancement factor (EF) represented by Eq. 1 was calculated at the spatial coordinate where the local field intensity was maximum for each frequency point.

As a result of the increase in x from 0 to 1, the LSPR peak wavelength (λp,LSPR) of Aux-Ag1-x alloy NPs shifts from 387 nm to 533 nm, while the EFmax of Aux-Ag1-x NPs remarkably decreases from ~103 to ~101 (Figure S1, Table S1). The EFmax at x = 1 is comparable with the value for Ag NP previously reproted.14 Thus, if Aux-Ag1-x clusters can be generated in AgX crystal with the composition finely tuned, the Aux-Ag1-x@AgX should be highly promising as the plasmonic photocatalyst reconciling requirements 1 and 2. There have been many reports on liquid-phase synthesis of Aux-Ag1-x colloids,15-18 and also, solid-phase photochemical generation of Ag NPs in AgBr has intensively been studied because of the importance for photography19 and photochromisim.20 However, there is no report on the solid-phase photochemical generation of Au-Ag alloy NPs in AgBr crystal. Here we report the preparation of Aux-Ag1-x@AgBr particles on mesoporous TiO2 film (mp-TiO2) by the SILAR-PR technique. The alloy composition can be widely controlled by varying the concentration of the Au source in the SILAR process.

DFT calculation. DFT calculations with the periodic boundary conditions were carried out using a plane wave based program, Castep.23 The Perdew-Burke-Ernzerhof (PBE) functional24,25 was used together with the ultrasoft-core potentials. 26 The basis set cutoff energies were set to 280 eV for geometry optimization and 300 eV for the post energy calculation. The electron configurations of the atoms were Br: 4s24p5, Ag: 4d105s1, and Au: 5d106s1. Geometry optimization was carried out with respect to all atomic coordinates, and the lattice constants were fixed. Fig. S1 shows the unit cells used for calculations. Two types of Frenkel defect models were employed, and they were twice and eight times super unit cells of the conventional cubic unit cell with a = b = c =5.772 Å, and α = β = γ = 90°. For models of Frenkel defect, one Ag atom in the ab-base-centered site was removed, and Ag or Au atom was put to the center of (AgBr)4 cube.

EXPERIMENTAL SECTION Preparation of Aux-Ag1-x@AgBr/mp-TiO2. To prepare mp-TiO2, An anatase TiO2 paste (mean particle size = 20 nm, PST-18NR, JGC C&C) was coated with sample area = 4 × 2.5 cm2 on FTO-film coated glass substrates (sheet resistance = 10 Ω/square) by the doctor blade, and heated at 773 K for 1 h. On mp-TiO2, Au ion-doped AgBr crystals are formed by the SILAR method. The mp-TiO2 film was immersed into a solution of AgNO3 (100 mM) in MeOH for 1 min, and rinsed with MeOH. Then, the film was pored into a solution of KBr (100 mM) with HAuCl4 as the Au source with the concentration (CAu) varied at 0 ≤ CAu ≤ 2.4 mM. After adsorbing for 1 min, the sample was washed with MeOH. The cycle was repeated 15 times to obtain AgBr:Au+/TiO2 film. The film was immersed into MeOH, and illuminated by UV-light (Hg lamp, λ > 320 nm, I310-420 = 4 mW cm-2) to obtain Aux-Ag1-x@AgBr/mp-TiO2. Aux-Ag1-x@AgBr/mp-TiO2 was dispersed into H2O (10 mL) containing NaCl (7.5 M) and trimethylstearylammonium chloride (40 mM), and heated at 70oC for 0.5 h to obtained Aux-Ag1-x colloid solution.

RESULTS AND DISCUSSION 1. Preparation Aux-Ag1-x@AgBr/mp-TiO2. mp-TiO2 was used as a support for AgBr nanocrystals in this study. The SILAR-PR technique12 has been modified to synthesize Aux-Ag1-x@AgBr/mp-TiO2. Scheme 1 illustrates the experimental procedures for preparing Aux-Ag1-x@AgBr/mp-TiO2. A paste containing anatase TiO2 particles with mean size of 20 nm was coated on glass substrate with fluorine-doped SnO2 (FTO) film using a doctor blade. The sample was heated at 773 K for 1 h to form mp-TiO2. Au ion-doped AgBr crystals are formed by the SILAR method using HAuCl4 as the Au source with the

Characterization. X-ray diffraction (XRD) measurements were performed by a Rigaku SmartLab X-ray diffractometer operating at 40 kV and 100 mA. The data in the range from 20 to 90 o (2θ) were collected by the use of Cu Kα radiation (λ= 1.545 Å). Diffuse reflectance UV-Vis spectra of the samples were recorded by a Hitachi U-4000 spectrometer with an integrating sphere. The reflectance (R∞) was recorded by using BaSO4 as reference, and the Kubelka-Munk function [F(R∞)] expressing the relative absorption coefficient was calculated by the equation F(R∞) = (1 - R∞)2/2R∞. The sample morphology was determined by transmission electron microscopy at an applied voltage of 200 kV (JEM-2100F, JEOL). FDTD calculation. The local electric field analysis for Aux-Ag1-x alloy, Au, and Ag NPs in water was performed by 3D finite difference time domain (FDTD)-method Maxwell solver using FDTD Solutions (Lumerical Solutions, Inc.). Experimentally measured particle size (5 nm) and optical constants (refractive index and extinction coefficient) 21,22

Scheme 1. Preparation of the Aux-Ag1-x@AgBr NPs on mesoporous TiO2 nanocrystalline film (N = 15). Methanol was used as the solvent.


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concentration (CAu) varied at 0 ≤ CAu ≤ 2.4 mM. AuCl4- ion has absorption peaks at 227 nm and 325 nm in methanol (Figure S2). When it was mixed with 0.1 M KBr solution, new peaks of AuBr4- appear at 258 nm, 396 nm and 473 nm, while the AuCl4- absorption disappears. Further, the AuBr4absorption disappears by the addition of 0.1 M AgNO3 solution with a new shoulder around 266 nm due to AuBr2-.27 These results indicate that Ag+ ion is adsorbed on mp-TiO2 (Ag+ad) from solution A, and AuBr4- is reduced to AuBr2- by the reaction with Ag+ad in solution B (Eq. 2) since the Gibbs energy for the reaction (∆rG0) is -230 kJ mol-1. In this case, the formation of AgBr crystal (∆rG0 = -140.1 kJ mol-1) provides a driving force for the reaction.

Next, solid-phase photochemical reaction in AgBr cystal without and with Au+-doping on TiO2 was studied. Figure 3A shows XRD patterns for AgBr:Au+/mp-TiO2 after UV-light irradiation. Besides the diffraction peaks of AgBr, a new peak appears at 2θ = 38.1° due to diffraction from the (111) crystal plane of Ag, Au or Au-Ag alloy. UV-light irradiation decreased the AgBr crystallite size to 48.5 nm at CAu = 0, 51.6 nm at CAu = 0.24 mM, and 45.8 nm at CAu = 2.4 mM. Figure 3B shows UV-visible absorption spectral change for AgBr:Au+(CAu = 2.4 mM)/mp-TiO2 with UV-light irradiation. The LSPR of Au NPs appears around λ = 600 nm, intensifying with increasing irradiation time. The λp,LSPR of plasmonic metal NPs redshifts as the dielectric constant of surrounding medium increases.30,31 The λp,LSPR in this system much larger than the value of 510 nm for aqueous Au NP colloids with diameter of ~5 nm32 indicates that most Au NPs are generated in the interior of AgBr crystal with a dielectric constant of ε = 12.50.33 The same conclusion was drawn in the Ag@AgBr/mp-TiO2 system prepared by the SILAR-PR technique.12

AuBr4- + 2Ag+ad + CH3OH → AuBr2- + 2AgBr + OPs (2) where OPs denote the oxidation products of methanol. After the SILAR cycle (N) was repeated 15 times, the resulting sample was irradiated by UV-light in methanol. 2. Photoinduced generation of Aux-Ag1-x clusters in AgBr crystal. Figure 2A shows X-ray diffraction (XRD) patterns for AgBr:Au+(CAu)/mp-TiO2 before UV-light irradiation. Regardless of Au+ ion-doping, every XRD pattern has clear diffraction peaks at 2θ = 26.8°, 31.0°, 44.4°, 55.1°, 64.5° and 73.2° indexed as the diffraction from the (111), (200), (220), (222), (400), and (331) crystal planes of AgBr, respectively. The AgBr crystallite size was determined to be 71.7 nm at CAu = 0, 68.7 nm at CAu = 0.24 mM, and 66.0 nm for CAu = 2.4 mM by the Scherrer equation from the half-width of the (200) peak. Figure 2B shows UV-visible absorption spectra of the samples, and unmodified mp-TiO2 for comparison. While the absorption edge of mp-TiO2 is ~390 nm, AgBr(CAu = 0)/mp-TiO2 possesses broad absorption at λ < 650 nm with a shoulder around 500 nm. AgBr is known to have a significant amount of the Frenkel defects or interstitial Ag+ ions (Agi+).28 The absorption results from the LSPR of Ag NPs because the Agi+ ions in AgBr undergo reduction even by weak ambient light.12 In the spectra for the samples prepared at CAu ≥ 0.024 mM, the LSPR disappears, and the interband transition of AgBr is observed at λ < 450 nm without the absorption by Au3+ ion.29 These results suggest that Agi+ ions are replaced by the Au+ ions doped into AgBr crystal (AgBr:Au+) (Eq. 3). Agi+ + AuBr2- → Aui+ + AgBr + Br-

3. Characterization of Aux-Ag1-x@AgBr/mp-TiO2. A)

B)

Kubelka-Munk

4

2 1

400 500 600 700 Wavelength / nm

800

Figure 3. (A) XRD patterns for AgBr:Au+/mp-TiO2 after UV irradiation (λ > 320 nm) in methanol. (B) UV-visible absorption spectral change of AgBr:Au+(CAu = 2.4 mM)/mp-TiO2 with the UV-light irradiation.

Optical properties for AgBr:Au+(0 ≤ CAu ≤ 2.4 mM)/mp-TiO2 irradiated by UV-light in methanol were examined. Figure 4A shows the UV-visible absorption spectral change with irradiation. Every spectrum has only one LSPR-peak. Figure 4B(a) shows plots of λp,LSPR vs. CAu for Au-Ag@AgBr/mp-TiO2. Interestingly, the λp,LSPR continuously increases from 495 nm to 573 nm with an increase in CAu.

(3)

Kubelka-Munk

B) 4 3

B) Au-Ag@AgBr / mp-TiO2 CAu = 0 mM

3 2 1 mp-TiO2

+ / mp-TiO AgBr:Au AgBr:Au / mp-TiO 2 2 n+

2

tp = 5 min

300

4

A)

3

0

A)

Kubelka-Munk

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2.4 mM

0 300

400 500 600 700 Wavelength / nm

800

1 mp-TiO2

AgBr / mp-TiO2

AgBr / mp-TiO2

Figure 4. (A) UV-visible absorption spectra for AgBr:Aun+(0.024 ≤ CAu ≤ 2.4 mM)/mp-TiO2 after UV-light irradiation in methanol. (B) Plots of λpLSPR vs. CAu for Au-Ag@AgBr/mp-TiO2 (a) and Au-Ag colloids (b), and plots of λpLSPR vs. alloy composition (x) FDTD-calculated for Aux-Ag1-x aqueous colloids (c).

0 300

400 500 600 700 Wavelength / nm

800

Figure 2. XRD patterns (A) and UV-visible absorption spectra (B) for as-prepared AgBr:Au+/mp-TiO2.


The Journal of Physical Chemistry Further, to directly characterize the Aux-Ag1-x NPs in AgBr crystal, the AgBr matrix was selectively dissolved by 7.5 M NaCl aqueous solution containing 40 mM trimethylstearylammonium chloride. Figure 5 shows TEM image (A) and energy dispersive X-ray (EDX) analysis (B, C) for a metal particle obtained from Au-Ag(CAu = 0.24 mM)@AgBr/mp-TiO2 after UV-light irradiation. TEM observation for many other particles confirmed the particle size is ~5 nm although the particle size is fairly scattered (Figure S3). Among them, a large particle of ~30 nm was selected to obtain clear EDX image indicating that Au and Ag are uniformly distributed in the metal NP. Figure 5D shows UV-visible absorption spectra for the Aux-Ag1-x colloids, and the plot of the λp,LSPR vs. CAu is also shown in Figure 4B(b). As a result of the increase in CAu, the λp,LSPR varies from 412 nm to 534 nm. The curve parallels with the FDTD-calculated data for the Aux-Ag1-x aqueous colloid system (Figure 4B(c))). Evidently, Aux-Ag1-x alloy NPs are formed in the interior of AgBr crystal in this solid-phase photochemical reaction, and the composition can be widely controlled by CAu. A)

B) Au

Ag

Figure 6. PDOS for Ag atoms (except for interstitial site) (top), Ag atom at interstitial site (center), and Br atoms (bottom) for Ag31(Ag)Br32 unit cell.

C)

crystal is 0.707 eV, which is significantly smaller than the experimental value of 2.6 eV.22 The bandgaps of semiconductors and insulators are known to be underestimated by the current level of DFT calculations. The purpose of this work is not to discuss the band gap, and the GGA+U method was not used. The introduction of Ag+ and Au+ Frenkel defects reduces the apparent bandgap to 0.655 eV and 0.550 eV, respectively. Figures 6 and 7 show the projected density of states (PDOS) for the AgBr with interstitial Ag and Au ions, respectively. The PDOS for prefect AgBr is shown in Figure S6. The PDOS for Ag atoms (except for interstitial site) and Br atoms are almost the same as those for the perfect AgBr. It is noticeable that the PDOS of interstitial Ag+ ion locate in the lower energy region in the Ag4d band. Similar characteristic feature is seen for the PDOS of interstitial Au+ ion. DFT calculations were also conducted for anionic super cells, Ag31(Ag)Br32 and Ag31(Au)Br32, and their PDOS are shown in Figures S7 and S8, respectively. The figures clearly indicate that the donor levels are generated below the conduction band (CB), and they are mainly composed of the Ag5s and Au6s orbitals at the interstitial site. By taking into the DFT calculation results for model AgBr clusters without and with Agi+ and Aui+ ions, we propose a reaction mechanism on the photochemical formation of Aux-Ag1-x alloy NPs in AgBr crystal. The Au+ ions doped into AgBr occupy the interstitial sites (Aui+) by substituting Agi+ ions. The mole ratio of the Aui+ ions (m) to Agi+ ions (n) increases with increasing CAu. Upon UV-light irradiation of AgBr, the electrons in the valence band (VB) are excited to the CB (Eq. 4). The VB-holes (h+VB) oxidize methanol (Eq. 5). Thus, the CB-electrons (e-CB) can be trapped by the levels of the interstitial Agi+ ions or Aui+ ions (Eq. 6). The resulting

D) CAu = 0 mM CAu = 0.024 mM CAu = 0.12 mM CAu = 0.24 mM CAu = 0.36 mM CAu = 0.48 mM CAu = 2.4 mM

0.14

Au

Absorbance

0.12 Intensity / a.u.

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0.10 0.08 0.06 0.04 0.02

Ag (x10)

0.00 300

400 500 600 700 Wavelength / nm

800

Figure 5. TEM image (A) and EDX analysis (B,C) for a metal particle obtained from UV-irradiated AgBr:Au+(CAu = 0.24 mM) by dissolving AgBr. (D) UV-visible absorption spectra for Au-Ag aqueous colloids obtained from AgBr:Au+(0 ≤ CAu ≤ 2.4 mM) by dissolving AgBr.

4. Mechanism on the generation of Aux-Ag1-x clusters in AgBr crystal. The effect of the Frenkel defects on the AgBr band structure was studied by density functional theory (DFT) calculations (Supporting Information). Twice and eight times super cells of the cubic conventional unit cell were employed (Figure S4). Energies for the generation of the Frenkel defects were estimated to be 1.068 eV for Agi+ ions and 1.003 eV for Aui+ ions with (AgBr)8 model, whereas they became to 0.754 eV and -0.289 eV, respectively, with (AgBr)32 model (Table S1). The band structures and density of states (DOS) for perfect AgBr, and Frenkel defects with interstitial Ag+ and Au+ ions are shown in Figure S5. The indirect bandgap for perfect AgBr


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metal atom (M) with large electronegativity attracts e-CB to generate metal anion (Eq. 7).34 Ag+ ions easily diffuse in AgBr crystal with a low activation energy of ~4 kJ mol-1 because of the Frenkel defects at 298 K,28 and thus, the Agi+ and Aui+ ions can move to the Ag and Au metal anions to yield diatomic metal cluster (Eq. 8). The repetition of these processes yield Aux-Ag1-x alloy clusters in AgBr crystal, and the composition (x = m/(m + n)) can be controlled by the doping amount of Au+ ions (m) or CAu (Eq. 9). mp-TiO2 only works as a support for AgBr nanocrystals, and can be replaced by any other materials. Further, this method would be applicable for the preparation of support-free Aux-Ag1-x@AgBr crystals. AgBr + hν → e-CB + h+VB

(4)

h+VB + CH3OH

(5)

→ OPs

Mi+ + e-CB → M where M denotes Ag or Au metal atom.

(6)

M + e-CB → M-

(7)

M- + Mi+ → M-M

(8)

M-M + Mi+ + e-CB → ··· → Aum-Agn

(9)

CONCLUSIONS This study has presented a technology for preparing Ag1-x alloy clusters in AgBr nanocrystal with the composition (x) varied (Aux-Ag1-x@AgX). A plausible reaction mechanism is proposed on the basis of the experimental and density functional theory calculation results. We anticipate that fine-tuning the composition of Aux-Ag1-x@AgBr leads to the improvement in the performances as the plasmonic photocatalyst, further widening the applications to the plasmon-based devices for solar-to-chemical transformations.

Figure 7. PDOS for Ag atoms (top), Au atom at interstitial site (center), and Br atoms (bottom) for Ag31(Au)Br32 unit cell.

ACKNOWLEDGMENT The authors acknowledge I. Tatsumi for experimental assistance. This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654, and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

ASSOCIATED CONTENT REFERENCES AUTHOR INFORMATION

(1)

Corresponding Author (2)

Tel +81-6-6721-2332; Fax +81-6-6727-2024; E-mail: [email protected].

(3)

Supporting Information (4)

Supporting Information Available: FDTD calculation model for Au-Ag NPs (Figure S1); UV-visible absorption spectra of Au complexes (Figure S2); TEM images of metal nanoparticles (Figure S3); Unit cells for DFT calculations (Figure S4); Band structures and DOSs for perfect AgBr, Ag31(Ag)Br32, and Ag31(Au)Br32 (Figure S5); PDOS for perfect AgBr (Figure S6); PDOS for anionic Ag31(Ag)Br32 unit cell (Figure S7); PDOS for anionic Ag31(Au)Br32 unit cell (Figure S8); Parameters for maximum EFs (Table S1); Energies for defect formation (Table S2)

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TOC Graphic

cb e-

Br Ag Au-Ag Alloy Nanoparticle Ag Br

Ag

Ag

Ag

Br

Ag

Au++ Ag Br

Ag

Br

Br

Ag

Ag

Br

Br

Ag

Ag

Br

hv MeOH Ox

Br

Ea = 4 kJ mol-1 h+

vb

Br

Ag

Br

Br

Ag

Ag+

Ag+ Ag

Ag++ Au Br

Br

Ag


Solid-Phase Photochemical Growth of Composition-Variable AuâAg

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