In(OH)3 Composite Photocatalysts with Adjustable Surface

Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), ... under 540 ± 12 and 420 ± 12 nm monochromatic irradiat...
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Ag3PO4/In(OH)3 Composite Photocatalysts with Adjustable SurfaceElectric Property for Efficient Photodegradation of Organic Dyes under Simulated Solar-Light Irradiation Jianjun Guo,†,‡,§ Shuxin Ouyang,*,‡ Han Zhou,‡,§ Tetsuya Kako,†,‡ and Jinhua Ye*,†,‡,§,∥ †

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, Japan Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, Japan § International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, Japan ∥ TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, P. R. China ‡

S Supporting Information *

ABSTRACT: A new composite photocatalyst Ag3PO4/In(OH)3 was successfully synthesized via in-situ precipitation method and applied to eliminate Rhodamine B under the irradiation of solar simulator. The composite photocatalysts exhibited higher activities than that of individual Ag3PO4 and In(OH)3, and the highest activity (the rate constant kapp = 1.75 min−1) was observed over the Ag3PO4/In(OH)3 with a molar ratio of 1.65:1.00. The further mechanism study and material characterizations indicated that the photocatalytic activity is closely related to the surface-electric property of the composite photocatalyst. Moreover, the surface-electric property could be continually adjusted by changing the content of In(OH)3.The photosensitization and the intrinsic photocatalytic degradation of Rh B were investigated under 540 ± 12 and 420 ± 12 nm monochromatic irradiation, respectively. The results indicated that the intrinsic photocatalytic degradation of Rh B dominated the overall degradation under the solar light irradiation. The energy-band structure of the composite photocatalyst was also investigated and considered as a reason for the enhanced multielectron reactions. Here, the composite photocatalysts with adjustable surface-electric property and suitable energy-band structure reveal a material design concept of exploiting a new photocatalyst based on the reaction kinetics and thermodynamics. posite construction.26,27 Recently, some composite photocatalysts based on Ag3PO4, such as AgX/Ag3PO4 (X = Cl, Br, I),28 Ag3PO4/TiO2,29 Fe3O4/Ag3PO4,30 Ag3PO4/SnO2,31 and graphene oxide/Ag3PO432 have been successfully synthesized and exhibited enhanced photocatalytic activities. However, the efficiency of Ag3PO4 in photodecomposition of gaseous organic contaminants still needs to be improved. Lately, we reported a new heterojunction Ag3PO4/Cr-SrTiO3 photocatalyst toward efficient elimination of gaseous organic pollutants under visible light irradiation.33 The photocatalytic activity of the composite is about 33 times higher than that of pure Ag3PO4. The work reveals that fabricating heterojunctions with proper hand structure to establish a new chemical reaction process is an effective strategy to enhance photocatalytic efficiency and attain new photocatalytic application. In most cases, the energy-band structure in composite photocatalyst is considered as the primary reason for the

1. INTRODUCTION The photocatalytic degradation of organic pollutants in the presence of semiconductors has attracted increasing interest because it is a promising, environmental, and cost-effective technology for the treatment of contaminated groundwater and wastewater.1−4 To better utilize visible light accounting for about 43% of solar energy, great efforts have been made to exploit visible-light-sensitive photocatalysts. Besides some conventional methods such as cation ion or anion ion doping,5−7 heterojunctions,8−10 band engineering,11−13 dye sensitization,14,15 and so forth, some multiple-metal oxides, such as Ag-based,16,17 Bi-based,18,19 In-based semiconductors,20 have been fabricated. Among these photocatalysts, silver orthophosphate (Ag3PO4) as a novel photocatalyst with promising efficiency in water oxidization and photodecomposition of organic dyes was reported recently.21 The quantum yield of the generation of O2 gas by water oxidation using Ag3PO4 is nearly 90% under visible light, which is significantly higher than that of most photocatalysts (nearly ∼20%). Since then, a series of efforts have been focused on morphology modification,22−24 electronic-structure calculation,25 and com© 2013 American Chemical Society

Received: June 26, 2013 Revised: July 23, 2013 Published: July 29, 2013 17716

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2. EXPERIMENTAL SECTION 2.1. Synthesis of In(OH)3 Nanorods. The In(OH)3 nanorods were synthesized via a hydrothermal treatment method. Typically, 0.3 g of InCl3·4H2O was dissolved in 100.0 mL of distilled water with stirring for 30.0 min. Then, 3.0 g of urea was added into the solution and kept stirring for 30.0 min. After that, 100.0 mL of the mixture was poured into a Teflon-lined stainless steel autoclave and heated up to 130.0 °C for 13.0 h. The obtained precipitates were washed several times with distilled water until neutral solution pH and then dried at room temperature overnight. 2.2. Synthesis of Ag3PO4/In(OH)3 Composite Photocatalyst. The deposition of Ag3PO4 nanoparticles onto the In(OH)3 was carried out by an in-situ precipitation method. Typically, amount of as-prepared In(OH)3 was dispersed in 100.0 mL of distilled water and sonicated for 10.0 min. Meanwhile, some amount of Ag(NO)3 was dissolved in 50.0 mL of distilled water and stirred for 10.0 min. Then, a 10.0% ammonia solution was added into above solution to obtain the silver-ammine complex. After that, the silver-ammine solution was mixed with the suspension of In(OH)3 nanorods and stirred for several hours. Finally, 5.0 mmol/L Na2HPO4 was added into the mixture drop by drop and kept stirring for 20.0 h. The obtained precipitates were washed several times with distilled water and then dried at room temperature overnight. Ag3PO4/In(OH)3 particles (the molar ratios of Ag3PO4 to In(OH)3 are 3.0:1.0, 1.65:1.0, 0.65:1.0, 0.3:1.0, and 0.15:1.0) were synthesized by the same method. For comparison, pure Ag3PO4 particles were also prepared under the same conditions without the In(OH)3 nanorods. 2.3. Photocatalyst Characterization. X-ray diffraction patterns were characterized with a Rigaku Rint-2000 X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.54178 Å). Scanning electron microscopy images were recorded with a JEOL 6700F field emission scanning electron microscope. UV−visible diffuse reflectance spectra were recorded on a Shimadzu UV-2500 spectrophotometer and converted to absorption spectra by the standard Kubelka−Munk method. The surface area measurements were carried out on a surface area analyzer (BELSORP II). Light intensity in the photocatalytic reaction was monitored using a spectroradiometer (USR-40; Ushio Inc., Japan). The surface electronegativity was characterized via zeta potential measurement (Delsa Nano C, Beckman Co.). Changes in total organic carbon (TOC) were determined by using a total organic carbon analyzer (Model TOC-VCPH). The concentration of H2O2 generated in the photocatalytic reaction was determined by a photometric method, and more experimental details are introduced in the Supporting Information. 2.4. Photocatalytic Reaction. In all photocatalytic experiments, 0.2 g of the as-prepared Ag3PO4, In(OH)3, and Ag3PO4/In(OH)3 samples was dispersed in 100.0 mL of 8.0 mg/L Rhodamine B (Rh B) solution. The light source was a solar light simulation lamp with 30.5 mW/cm2 illumination intensity. The photocatalytic degradation of organic dyes was monitored by measuring the changes of UV−vis absorption spectra as a function of irradiation time. For the contrast experiments under monochromatic irradiation, a 300.0 W Xe arc lamp (7 A imported current, focused through a 45 × 45 mm shutter window) equipped with a band-pass filter (540 nm ± 12 nm and 420 nm ± 12 nm, HOYA Co., Japan) and a water

enhanced activity. As illustrated in Scheme 1, the separation of electron/hole pairs is enhanced which contributes to the higher Scheme 1. Schematic Diagram of the Photocatalytic Mechanism of Composite Photocatalysts in Dye Degradation

photoactivity. However, besides the band gap structure, the surface-electric property of the semiconductor is also an important factor for photocatalytic activity because of its influence on the adsorption of organic dyes, which is relevant to the reaction dynamics. As previously reported, the surfaceelectric property can be adjusted in two ways: one is adjusting the pH values of the photocatalyst suspension, and another one is composing with other materials as a composite. For instance, Bourikas and co-workers34 reported that more positively charged surfaces could absorb more AO7 molecules, which is closely related to the photocatalytic activity. Therefore, they used HNO3 to adjust the pH values of the TiO2 suspension and found that the highest adsorption of AO7 on the TiO2 was obtained when the pH value was 2.0. Besides the pH adjustment, Kim and co-workers35 used phosphate to adjust the surface charge characteristic of TiO2 and obtained enhanced photoactivity in dye degradation. To the best of our knowledge, there is no report on the surface-electric property of Ag3PO4. And unfortunately, Ag3PO4 cannot be stable in either acid or alkaline solution due to the hydrolyzation of Ag+ and PO43‑ ions, which means it is unavailable to adjust the surface-electric property of the Ag3PO4 via pH adjustment. Therefore, composing Ag3PO4 with suitable materials is a possible approach to obtain adjustable surfaceelectric property for enhanced photocatalytic efficiency. In(OH)3 with a wide band gap of 5.15 eV has attracted increasing attention because of its higher hole oxidation potential to decompose benzene and stronger redox ability of conduction band (CB) electrons for H2 evolution or CO2 photoreduction under UV-light irradiation.36−38 Moreover, due to its unique physical and chemical properties, it also has potential applications in various fields such as nonlinear optics,39 nanoelectronics,40 and gas sensing.41 Herein, a new composite photocatalyst based on Ag3PO4 and In(OH)3 was designed to attain the efficient elimination of Rhodamine B under solar-light simulated irradiation. Surface-electric properties of the composite photocatalysts with different molar ratios were investigated via zeta-potential characterization. Moreover, the photosensitization and the intrinsic photocatalytic degradation of Rh B were investigated under 540 ± 12 and 420 ± 12 nm monochromatic irradiation, respectively, which were used to confirm the dominated pathway of Rh B degradation. The energy-band structure of the Ag3PO4/In(OH)3 composite was also investigated to help understand the transfer of photoelectrons between the two semiconductors. 17717

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filter was used as the light source. The spectra of the light sources are shown in Figure S1 in the Supporting Information.

In(OH)3 −Ag(NH3)+ (ad) + 1/3HPO4 2 −(aq) + 1/3OH− + 2/3H 2O → Ag 3PO4 /In(OH)3 + NH3·H 2O

3. RESULTS AND DISCUSSION 3.1. The Formation Mechanism of Ag3PO4/In(OH)3. Ag3PO4/In(OH)3 was synthesized by an in-situ precipitation method in the silver-ammine solution with pH of about 12.0. The surface-electric property of In(OH)3 nanorods was characterized via zeta potential measurement. The growth process of Ag3PO4 onto In(OH)3 is illustrated in Figure 1.

3.2. Characteristics of the In(OH)3/Ag3PO4 Composite Photocatalyst. The X-ray diffraction patterns of as-prepared In(OH)3, Ag3PO4, and Ag3PO4/In(OH)3 (molar ratio = 0.15:1) are shown in Figure S2 in the Supporting Information. The indexed diffraction peaks can be ascribed to Ag3PO4 and In(OH)3, respectively. Peaks related to other materials are not detected in the synthesized samples, indicating that the Ag3PO4 did not react with the In(OH)3. Morphologies of the asprepared samples were observed with SEM. The Ag3PO4 contains rhombic dodecahedral and cubic with sizes from several hundred nanometers to several micrometers (see Figure 3a). In contrast, the In(OH)3 consists of microrods with uniform size of about 1 μm in length and 100 nm in diameter (see Figure 3b,c). Furthermore, the morphology of the composite (Ag3PO4/In(OH)3 = 0.15:1, molar ratio) is shown in Figure 3c,d. Ag3PO4 was formed on the surface of In(OH)3 nanorods. Therefore, part of the nanorod was covered by the Ag3PO4 particle. As shown in Figure 3d, the TEM image indicates that the Ag3PO4 is well connected with the In(OH)3, which promotes the electron transfer between them. Figure 4 shows the UV−vis absorption spectra of Ag3PO4, In(OH)3, and Ag3PO4/In(OH)3 composite photocatalyst (molar ratio = 1.65:1). The absorption spectrum of Ag3PO4 indicates that it can absorb solar energy with a wavelength shorter than ∼530 nm, corresponding to 2.5 eV of band gap energy. This result coincides with our previous report.21 Meanwhile, the absorption spectrum of In(OH)3 shows that the optical absorbance edge is about 240 nm and the band gap is 5.17 eV, which are in agreement with previous work.36 Thus, in the absorption spectra of the Ag3PO4/In(OH)3 composite, besides the absorption band edge (around 500−530 nm) in the visible light range, another band edge for pure In(OH)3 appears in the deep UV light range. Moreover, the UV−vis light absorptions of the heterojunctions are related with the molar ratios between Ag3PO4 and In(OH)3 (see Figure S3, Supporting Information), such as the more content of In(OH)3 in the composite results in the lower absorption of light. 3.3. Photocatalytic Activities of Ag3PO4, In(OH)3, and Ag3PO4/In(OH)3. In this study, Rhodamine B (Rh B) solution was chosen as a model pollutant for the evaluation of photocatalytic activities of the Ag3 PO4, In(OH)3, and Ag3PO4/In(OH)3. The photocatalytic degradation of Rh B was carried out under the simulation solar irradiation (30.5 mW/cm2 illumination intensity). In general, Rh B was decomposed into smaller colorless organics first, and then the colorless organics would be mineralized further into inorganics. Figure 5 depicts the photocatalytic degradation of Rh B based on its concentration changes which could be evaluated by the light absorption spectra (see Figure S4, Supporting Information). It could be observed that 98.0% of the Rh B was decomposed within 1.5 min of irradiation (see Figure 5a) over the composite photocatalyst Ag3PO4/In(OH)3. In contrast, the photocatalytic activities of the Ag3PO4 and In(OH)3 were extremely lower than that of Ag3PO4/In(OH)3 composite photocatalyst. The Ag3PO4 spent 9 min to decolorize 95% of the Rh B solution, while the In(OH)3 exhibited even negligible activity. In the absence of photocatalyst, only 6.7% of the Rh B was degraded after 2 h of irradiation (see Figure S4, Supporting Information).

Figure 1. Schematic diagram of the growth process of Ag3PO4/ In(OH)3.

When In(OH)3 was suspended in a solution with pH higher than 10.7, the surfaces of In(OH)3 nanorods were negatively charged (see Figure 2). As a result, positively charged silver-

Figure 2. Zeta potentials of In(OH)3 suspension with different pH values.

ammine ions (Ag(NH3)+) could be adsorbed onto the surface of In(OH)3. After the NaHPO4 solution was dropped into the mixture, the Ag+ (ad) would react with the HPO42‑ on the surface of In(OH)3 to form the composite Ag3PO4/In(OH)3. The synthetic process of the Ag3PO4/In(OH)3 composite photocatalyst can be summarized as follows: In(OH)3 (s) + Ag(NH3)+ (aq) → In(OH)3 −Ag (NH3)+ (ad),

pH = 11.0−12.0 17718

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Figure 3. SEM and TEM images of (a) Ag3PO4, (b) In(OH)3, and (c, d) Ag3PO4/In(OH)3 composite photocatalyst (molar ratio = 0.15:1).

suspensions (curve b), indicating that the extent of decomposition of the organic substrate in the Rh B/Ag3PO4 suspensions (Δ% TOC = 29.4%) is much lower than that in the Rh B/composite photocatalyst suspensions (Δ% TOC = 40.0%). Moreover, the TOC decreased quickly in the first 4 min of reaction for the Rh B/composite photocatalyst suspensions and in the first 10 min of reaction for the Rh B/ Ag3PO4 suspensions. However, the decreases of TOC concentrations were very limited in another 2 h of irradiation, indicating that completed decomposition of the dye proceeded slowly. In order to investigate the photosensitization of Rh B and the intrinsic photocatalytic degradation of Rh B, the photoactivities of Ag3PO4, In(OH)3, and Ag3PO4/In(OH)3 under monochromatic irradiation (540 ± 12 nm, and 420 ± 12 nm) were also evaluated. The light intensity and the apparent rate constant of samples are listed in Table 1. Under 540 nm monochromatic irradiation, the intrinsic photocatalytic degradation of Rh B could be excluded because Ag3PO4 or the In(OH)3 cannot be excited by 540 nm monochromatic light. As shown in Figure 6a and b, all of the samples exhibited activities. In particular, the highest efficiency was obtained on the composite photocatalyst Ag3PO4/In(OH)3 (molar ratio = 1.65:1). More than 98.0% of the Rh B was degraded within 40 min irradiation, and the apparent rate constant is about kapp = 0.071 min−1. In comparison, the photoactivities of the Ag3PO4 and the In(OH)3 were much lower than that of Ag3PO4/In(OH)3. There was about 70% of Rh B degraded after 120 min irradiation over the Ag3PO4 photocatalyst. For the In(OH)3, it took 14 h to degrade 80% of the Rh B solution. As listed in Table 1, the apparent rate constants of the Ag3PO4 and the In(OH)3 are kapp = 0.011 and 0.0012 min−1, respectively. On the other hand, under 420 nm monochromatic irradiation, the photosensitization of Rh B could be inhibited as the Rh B molecules cannot be sensitized by 420 nm monochromatic light. As shown in Figure 6c, more than 98.0% of the Rh B was decolorized within 7 min irradiation over the Ag3PO4/In(OH)3 composite photocatalyst and the

Figure 4. UV−vis absorption spectra of Ag3PO4, Ag3PO4/In(OH)3 (molar ratio = 1.65:1), and In(OH)3.

The kinetics of these photocatalytic reactions can be described using the first order reaction for low concentrations of Rh B solutions (see Figure 5b). The apparent rate constant (Kapp, min−1) determined from regression curves of −ln(C/C0) versus irradiation time is 1.75 min−1 for Rh B degradation over the Ag3PO4/In(OH)3 composite photocatalyst (molar ratio = 1.65:1). In comparison, the apparent rate constant of Ag3PO4 is about 0.35 min−1. In a word, the Ag3PO4/In(OH)3 composite photocatalyst has notably enhanced activity in photocatalytic degradation of Rh B. Moreover, measurements of total organic carbon (TOC) concentrations were adopted to assess the completed decomposition extent of an organic substrate during the irradiation period. As shown in Figure 5c, TOC concentrations of both the irradiated Rh B/Ag3PO4 suspensions (curve a) and the irradiated Rh B/composite photocatalyst suspensions (curve b) decrease with increasing irradiation time. The initial TOC concentration of curve b is lower than that of curve a which may be due to the higher adsorption of Rh B molecules over the Ag3PO4/In(OH)3 (to be discussed later). The TOC concentrations of the Rh B/Ag3PO4 suspensions (curve a) are greater than those of the Rh B/composite photocatalyst 17719

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Figure 5. (a) Changes in Rh B concentration over Ag3PO4, In(OH)3, and Ag3PO4/In(OH)3 (molar ratio = 1.65:1) under solar simulating irradiation. (b) Regression curves of of −ln(C/C0) versus irradiation time. (c) Changes in TOC during the course of photocatalytic degradation of Rh B.

Figure 6. (a) Self-photosensitized degradation of Rhodamine B (Rh B) over Ag3PO4, and Ag3PO4/In(OH)3 (molar ratio = 1.65:1) under monochromatic irradiation 540 nm. (b) Self-degradation of Rh B without photocatalysts and self-photosensitized degradation over In(OH)3. (c) Photocatalytic degradation of Rh B over Ag3PO4 and Ag3PO4/In(OH)3 (molar ratio = 1.65:1) under monochromatic irradiation 420 nm.

apparent rate constant is about kapp = 0.42 min−1. For the individual photocatalysts, the Ag3PO4 needs more than 20 min to degrade the Rh B completely, while the In(OH)3 exhibits even negligible activity. Compared with the results under 540 nm monochromatic irradiation, both of the Ag3PO4 and the Ag3PO4/In(OH)3 composite photocatalyst exhibit higher activity under 420 nm monochromatic irradiation (as listed in Table 1). Contrast experiment without photocatalyst was also carried out under the same condition to exclude the self-photocatalysis of Rh B. After 20 h irradiation, the extent of degradation of Rh B was about only 4.8% under 540 nm monochromatic irradiation and 1.7% of the Rh B under 420 nm monochromatic irradiation, respectively. The apparent rate constants are very

low that can be ignored. That difference of degradation may be due to the sensitization of Rh B. 3.4. Mechanism of the Photodegradation over the Ag3PO4/In(OH)3 Composite Photocatalyst. 3.4.1. Two Possible Pathways of Degradation of Rh B. Degradation of the Rh B in solution undergoes two possible pathways: intrinsic photocatalysis and photosensitization.42 As shown in Scheme 2B1, the photocatalysis pathway is mainly considered to be controlled by the following processes: (1) the light absorption of the semiconductor catalyst, (2) the generation of photogenerated electron and hole, (3) the transfer of charge carriers, and (4) the utilization of the charge carriers by the reactants.

Table 1. Apparent Rate Constants (kapp) of Photocatalysts under Different Irradiationsa under 540 nm monochromatic irradiation (light intensity: 5.106 mW/cm−2) under 420 nm monochromatic irradiation (light intensity: 5.292 mW/cm−2) a

Ag3PO4

In(OH)3

Ag3PO4/In(OH)3

0.011 min−1 0.15 min−1

0.0012 min−1

0.071 min‑1 0.42 min‑1

The kapp of self-photodegradation of Rh B without photocatalysts is too low that it can be ingnored. 17720

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Scheme 2. Comparison of the Photocatalytic Mechanism (B1) under Visible Light Irradiation (B2) for the Self-Photosensitized Pathway, and (B3) the Ag3PO4/In(OH)3 Composite Photocatalysts with Self-Photosensitization

As the above results, both the Ag3PO4 and the Ag3PO4/ In(OH)3 composite photocatalysts exhibited much higher activity under 420 nm monochromatic irradiation. Moreover, the intensities of monochromatic lights were almost the same. Therefore, the intrinsic photocatalytic degradation of Rh B is much stronger than the photosensitization of Rh B, which means the intrinsic photocatalytic degradation of Rh B dominated the overall degradation under the solar light irradiation. 3.4.2. Effects of surface-Electric Property on the Enhanced Activity. Since the photocatalytic degradation of organic dyes undergoes the processes both to capture dyes from the solution and to convert it into final products, the adsorption of dyes onto the surface of photocatalyst is the crucial process. Because the Rh B molecule is positively charged in solution (see Figure S5, Supporting Information), more negative zeta potential could promote the adsorption of Rh B on the surface of photocatalyst. As listed in Table 2, the surface-electric

Meanwhile, Scheme 2B2 summarizes some of the accepted features of the photosensitization pathway: Rh B adsorbed on the photocatalyst surface is excited by the visible light (eq 1) and injects electrons into the conduction band of the semiconductor to form Rh B cationic radicals (eq 2). The electrons in the conduction band of the semiconductor undergo the similar transformation to eq 3−6. Finally, Rh B is photocatalytic degraded to end products (eq 7).42−44 Rh B + hv → Rh B* ;

− 1.09 V (vs NHE)

(1)

Rh B* + Sem. → Rh B+ + Sem.(e−)

(2)

2e− + O2 + 2H+ → H 2O2 ;

(3)

0.695 V

O2 + H 2O + 2e− ↔ HO2− + OH−; O2 + e− ↔ O2•− ;

−0.33 V

O2 + H+ + e− ↔ HO2 ; +

+

Rh B + h + •OH + •O2

−0.046 V

−0.076 V

(4) (5) (6)

Table 2. Experimental Data of Ag3PO4/In(OH)3 with Different Molar Ratios

−

→ CO2 + H 2O + mineral acids

(7)

The intrinsic photocatalytic pathway of Rh B degradation was confirmed over the Ag3PO4 under 420 nm monochromatic irradiation. The UV−vis light absorption spectrum indicated that this dye exhibits negligible absorption around 420 nm, but the Ag3PO4 could be excited by the light shorter than 500 nm. Thus, the photocatalytic degradation of Rh B was attributed to the intrinsic photocatalysis of the Ag3PO4. The photosensitization of Rh B was detected under 540 nm monochromatic irradiation. The UV−vis light adsorption spectrum of Rh B solution indicated that Rh B molecular can be excited by 540 nm irradiation. Moreover, the potentials of the conduction bands of Ag3PO4 and In(OH)3 are +0.45 V (vs NHE) and −0.93 V (vs NHE), respectively, which are more positive than the redox potential of E0 (Rh B*/Rh B•+) (−1.09 V vs NHE).45 Therefore, it leads to electron injection from the adsorbed Rh B* species to the conduction bands of photocatalysts. The electrons injected from excited dyes are primarily used in the formation of H2O2 via two-electron reduction of dioxygen adsorbed on the photocatalysts (eq 3).46 Accordingly, the photosensitization pathway of Rh B should be responsible for the activity under monochromatic irradiation with 540 nm. Under the irradiation of solar simulator, both Ag3PO4 and Rh B molecular could be excited; therefore, two pathways were included in the whole photocatalytic degradation.

molar ratio (Ag3PO4 to In(OH)3)

zeta potential (mV)

adsorption of Rh B (mg/g)

degradation of Rh B (Kapp/min)

surface area (m2/g)

0:1 0.15:1 0.3:1 0.65:1 1.65:1 3:1 1:0

17.4 −41.4 −45.2 −51.4 −60.2 −44.5 −1.78

0.40 0.59 0.63 0.64 0.68 0.61 0.26

1.1 × 10−4 1.12 1.24 1.71 1.75 1.73 0.35

24.2 13.4 8.3 4.8 4.6 4.1 2.1

properties of photocatalysts were evaluated by zeta potentials at pH 7.5−8.0. The zeta potential of the composite photocatalyst Ag3PO4/In(OH)3 is about −60.2 mV which is more negative than that of Ag3PO4 (−1.78 mV) and In(OH)3 (+17.4 mV). Therefore, the composite photocatalyst Ag3PO4/ In(OH)3 can adsorb the Rh B molecule more efficiently, which has been confirmed by contrast experiments carried out in dark. As shown in Table 2, the adsorption amount of Rh B over the composite Ag3PO4/In(OH)3 is about 0.68 mg/g, which is much higher than that of Ag3PO4 (0.40 mg/g) and In(OH)3 (0.26 mg/g). The reason for the different surface-electric properties could be attributed to the hydrolyzation of In3+ and PO43‑ ions as follows: In 3 + + 3H 2O ↔ In(OH)3 + 3H+(ad) 17721

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That means the surface-electric property of the composite photocatalysts is really related to the adsorption and degradation of the dyes. 3.4.3. Effects of Composite Energy-Band Structures on Enhanced Activity. As in previous reports, the energy-band structure of the composite photocatalysts is also responsible for the enhanced photocatalytic activity. As previously mentioned, the potential of conduction band of Ag3PO4 cannot match the redox potentials of the active oxygen species which are necessary in the degradation of various organic substrates.47−49 Therefore, the multielectron reaction (2e− + O2 + 2H+→ H2O2, 0.695 V vs NHE) over Ag3PO4 is a more important way to obtain the oxygen species for Rh B photocatalytic degradation.50 Hydrogen peroxide (H2O2) can be detected by a photometric method, which is based on the horsedish peroxide (POD)-catalyzed oxidation by H2O2 of N,N-diethyl-pphenylenediamine (DPD).51,52 As shown in Figures S6 and S7 in the Supporting Information, the results indicate that the multielectron reaction is really carried out over the Ag3PO4 and the Ag3PO4/In(OH)3 composite under light irradiation (300 nm < λ < 800 nm), and the composite exhibits higher activity in the multielectron reaction. Especially, the concentration of H2O2 could be improved with the existence of Rh B. The reason could be attributed to the dye photosensitization promoting the multielectronic reaction. As plotted in Scheme 2B3, due to the relative positions of the conduction bands of Ag3PO4 (+0.45 eV) and In(OH)3 (−0.93 eV) and the redox potential of self-sensitized Rh B (Rh B*/Rh B•+, −1.09 V vs NHE), the generated electrons could be injected to the In(OH)3 and some of them could even be transferred to Ag3PO4, which results in enhanced multielectron reaction. The contrast experiment over a mixture of Ag3PO4 and In(OH)3 without calcination was carried out under the same conditions (see Figure S8, Supporting Information). The apparent rate constant of the mixture is about 0.29 min−1 which is much lower than that of the composite (kapp = 1.75 min−1). Therefore, besides the surface-electric property, the energyband structure of the Ag3PO4/In(OH)3 composite is also advantageous for the enhanced photocatalytic activity because of improving the multielectron reaction (2e− + O2 + 2H+ → H2O2, 0.695 V vs NHE).

(9)

HPO4 2 − + H+ ↔ H 2PO4 −

(10)

OH− + H+ ↔ H 2O

(11)

or 3+

The hydrolyzation of In in solution results in more H+ adsorbed on the surface of In(OH)3, which made the photocatalyst positively charged (as shown in reaction 8). As the same reason, the individual Ag3PO4 was negatively charged because of the hydrolyzation of PO43‑ (as shown in reaction 9). After constructing composite photocatalysts, the HPO4 2− or OH− could be consumed by H+, which would promote the shown in eq 9 to produce more OH− and result in more negative surface-electric property. As above analysis, the adsorption of Rh B is related to the surface-electric property of the photocatalyst. Herein, we successfully adjusted the surface-electric properties of the Ag3PO4/In(OH)3 composite by changing the content of In(OH)3. As shown in Table 2, all the zeta potentials of composite samples with different molar ratios are more negative than those of individual photocatalysts. In particular, the most negatively charged surface was obtained when the molar ratio of Ag3PO4 to In(OH)3 was 1.65:1.0. The adsorption of Rh B under dark conditions and the photocatalytic degradation over the composite photocatalysts with different surface-electric properties under irradiation of solar simulator are also investigated. As shown in Figure 7a, the variation tendency of the adsorption of Rh B is in accordance with that of the zeta potential of the composite photocatalysts. The same phenomenon was also found in the variation tendency of the photocatalytic activities, as shown in Figure 7b.

4. CONCLUSIONS A new composite photocatalyst Ag3 PO 4/In(OH) 3 was successfully synthesized via an in-situ precipitation method in the silver ammine solution for the first time. The results of photocatalytic Rh B degradation reveal that the Ag3PO4/ In(OH)3 composite photocatalyst is more efficient than the individual materials. More particularly, the highest activity (the rate constant kapp = 1.75 min−1) was obtained over the composite photocatalyst with a molar ratio of 1.65:1.0 (Ag3PO4 to In(OH)3) with which is significantly higher than that of the individual Ag3PO4 (kapp = 0.35 min−1). The surface-electric properties of the Ag3PO4/In(OH)3 composite photocatalysts could be continually adjusted by changing the In(OH)3 content and the variation tendency of zeta potentials was in accordance with that of photocatalytic activities, which indicates that surface-electric property is responsible for the enhanced photoactivity. The composite photocatalysts with adjustable surface-electric property reveals a material design concept of exploiting a new photocatalyst based on considering both the reaction kinetics and thermodynamics. The results and

Figure 7. (a) Adsorption of Rh B over Ag3PO4/In(OH)3 composite photocatalysts with different zeta potentials. (b) Photocatalytic activity of Ag3PO4/In(OH)3 composite photocatalysts with different zeta potentials. 17722

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discussion herein supply useful information for the further development of other composite photocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.O.); Jinhua.YE@ nims.go.jp (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by World Premier International Research Center Initiative on Material Nanoarchitectonics (WPI-MANA), MEXT, Japan.

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REFERENCES

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