Ag-Decorated ATaO3 (A = K, Na) Nanocube Plasmonic Photocatalysts

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Ag-Decorated ATaO3 (A = K, Na) Nanocube Plasmonic Photocatalysts with Enhanced Photocatalytic Water-Splitting Properties Dongbo Xu, Songbo Yang, Yu Jin, Min Chen, Weiqiang Fan, Bifu Luo, and Weidong Shi* School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date (Web): August 24, 2015 | doi: 10.1021/acs.langmuir.5b01294

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ABSTRACT: Tantalate semiconductor nanocrystals have been at the forefront of the photocatalytic conversion of solar energy to supply hydrogen owing to their favorable and tunable optical and electronic properties as well as advances in their synthesis. However, a narrow band gap is required for response to improve the efficiency of the photocatalysts. Here we propose an efficient enhancement of the H2 generation under simulated sunlight and visible light irradiation by a dispersion of Ag-decorated KTaO3 and NaTaO3 nanocubes. Xray diffraction and UV−vis diffuse reflectance spectra are used to characterize the products. Transmission electron microscope (TEM) and high-resolution high-angle annular darkfield scanning TEM (HAADF-STEM) images show that the Ag nanoparticles (NPs) are uniformly loaded on the surfaces of KTaO3 and NaTaO3. The photocatalytic water-splitting results over Ag-decorated KTaO3 and NaTaO3 show that the rate for H2 evolution from aqueous CH3OH solutions is up to 185.60 and 3.54 μmol/h·g under simulated sunlight and the rate for H2 evolution is more than 2 times than that of pure NaTaO3 and KTaO3 materials. However, under purely visible light illumination the highest H2 evolution of 25.94 and 0.83 μmol/h·g is observed in the case of Ag-decorated KTaO3 and NaTaO3 nanocubes. To the best of our knowledge, this is the first time that the photocatalytic water-splitting activity of the prepared Ag-decorated KTaO3 and NaTaO3 nanocubes has been reported. significant promise.20 It is known that controlling the noble metal nanoparticle size makes for a controllable quantum effect which can enhance light adsorption and photocatalytic activity significantly. The electrons are excited by SPR from the noble metal nanostructures directly injected into the semiconductor surface by enhancing the electron−hole separation rates at the semiconductor interface. It has been reported that composite photocatalysts such as Ag-TiO2,21 Au-TiO2,22 Au-CdS,23 AgSrTiO3,24 Ag-NaTaO3,25 and Ag-AgCl26 played a positive role in enhancing the photocatalytic activity. There have been a lot of reports that show that the composite plasmonic-noblemetal/semiconductor photocatalysts have achieved important higher reaction rates in various photocatalytic reactions compared to their pure semiconductor counterparts. In this study, a hydrothermal treatment was applied to synthesize KTaO3 and NaTaO3 nanocubes. KTaO3 or NaTaO3 nanocubes (0.2 g) were dispersed in a methanol aqueous solution (50 mL of methanol, 150 mL of distilled water) in an irradiation reaction cell under magnetic stirring. Different mass percentages of silver nanoparticles were deposited on the KTaO3 and NaTaO3 nanocube surfaces in a AgNO3 aqueous solution by an in situ photodeposition method. Therefore, two

1. INTRODUCTION Water splitting into H2 and O2 using sunlight on a photocatalyst semiconductor has attracted research interest since the pioneering work conducted by Honda and Fujishima on a photoelectrochemical.1,2 To achieve efficient solar water splitting, it is necessary to develop efficient photocatalysts. In 1980, Lehn,3 Sato and White,4 and Domen5 reported the stoichiometric evolution of hydrogen and oxygen by using particulate photocatalysts for water splitting. During the past few decades, niobates and tantalates have attracted significant research interest because of their wide applications in environmental fields such as air adsorbents, hazardous waste treatment, and water splitting to produce hydrogen.6−8 Niobates and tantalates such as KNbO 3 , 9 NaNbO 3 ,10 LiTaO3,11,12 KTaO3,11−14 NaTaO3,11−16 Sr2M2O7 (M = Nb, Ta),17,18 and RbNdTa2O719 have been reported. Therefore, the perovskite structure of niobates and tantalates plays a very important role in the photocatalysts. However, niobates and tantalates have low quantum efficiencies and are limited by high band gaps (3.0−4.7 eV and are excited only by ultraviolet (UV) light;1,9,12 the relatively high reduction potentials of niobates and tantalates generally lead to low photocatalytic reaction rates. Therefore, utilizing niobates and tantalates as highly active photocatalysts remains challenging. It is worth mentioning that the surface plasmon resonance (SPR) of noble metals (such as silver and gold) show © XXXX American Chemical Society

Received: April 15, 2015 Revised: August 13, 2015

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DOI: 10.1021/acs.langmuir.5b01294 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. TEM images of as-prepared Ag-KTaO3: (a) 0.025 wt % Ag-KTaO3, (b) 0.05 wt % Ag-KTaO3, (c) 0.1 wt % Ag-KTaO3, and (d) 0.2 wt % Ag-KTaO3. PerkinElmer LS 55 luminescence spectrometer. Electron spin resonance (ESR) analysis was performed on a Bruker EPR A30010/12 spectrometer.

Ag-decorated KTaO3 and NaTaO3 nanocube plasmonic photocatalysts were prepared. Meanwhile, the Ag nanoparticle SPR effect in terms of the Ag content under simulated sunlight and the visible light photocatalytic activity of the nanocomposite was studied for water splitting.

3. RESULTS AND DISCUSSION 3.1. Analysis. The XRD patterns of the pure cube KTaO3 crystalline powders and the Ag-KTaO3 nanocomposite are in Figure S1a, and similar Ag-NaTaO3 XRD patterns are shown in Figure S1b. All peaks of KTaO3 can be indexed to the orthorhombic phase of KTaO3 according to the JCPDS card (77-0918), and those of NaTaO3, to the JCPDS card (250863). Moreover, due to the fact that the Ag nanoparticles are dispersedly loaded on the surface of the cube KTaO3 and NaTaO3 nanocrystals, no signal about silver can be detected for the Ag element. In order to clarify the Ag NPs, the TEM image (Figure 1) shows a behavior for Ag deposition. When the weight ratio of Ag to KTaO3 is 0.025 wt % (Figure 1a), it can be seen that there are microcrystalline Ag particles on the surface of KTaO3. When the weight ratio of Ag to KTaO3 is added, more and more Ag nanoparticles are deposited and the particles of Ag are larger than 0.025 wt %, as shown in Figure 1b−d. Ag-NaTaO3 in the photocatalyst is determined by transmission electron microscope (TEM) spectroscopy as shown in Figure S2. The EDS is also used to further confirm the existence of Ag for AgNaTaO3 and Ag-KTaO3 photocatalysts (Figure S3a,b). There are also Ag nanoparticles on the surface of NaTaO3 and KTaO3. High-resolution high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure S4a) of Ag-KTaO3 also reveal the presence of Ag NPs. From the image we can see that

2. EXPERIMENTAL SECTION We synthesized KTaO3 and NaTaO3 nanocubes according to previously published procedures.16,27 The photocatalytic reactions were carried out in a 250 mL Pyrex round-bottomed flask. Before the photocatalytic reaction began, the opening of the round-bottomed flask was sealed with a silicone rubber septum. The catalyst powder (0.2 g) was dispersed in a methanol aqueous solution (150 mL of distilled water, 50 mL of methanol) by a magnetic stirrer in a quartz reaction cell. An aqueous solution of AgNO3 was added to the flask. The pH was about 7.1. A 300 W xenon arc lamp was used as a simulated sunlight source and was positioned 2 cm away from the reactor. Prior to light irradiation, the above suspension was thoroughly degassed to remove air completely, and the reaction temperature was below 5 °C. During the whole reaction process, vigorous agitation was performed to ensure the uniform irradiation of the photocatalyst suspension. The amount of evolved H2 was determined by using a gas chromatograph (GC, SP-7800A, thermal conductivity detector, 5 Å molecular sieve columns, and N2 carrier gas). After the reactions stopped, the pH was about 7.0, the samples were characterized by transmission electron microscopy (TEM), highresolution high-angle annular dark-field scanning HRTEM (HAADFSTEM, Tecnai-F20 operated at 200 kV), energy-dispersive spectrometry (EDS) analysis, and X-ray diffraction (XRD), and UV−vis diffuse reflectance spectra (DRS) of the samples were recorded with a UV− vis spectrophotometer (Shimadzu UV-3100) using BaSO4 as a reference. The photoluminescence properties were measured on a B


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Figure 2. High-angle annular dark-field scanning transmission election microscope patterns of 0.1 wt % Ag-KTaO3.

Figure 3. UV−vis diffuse-reflectance spectra of (a) naked KTaO3 and Ag-KTaO3 nanocomposites and (b) naked NaTaO3 and Ag-NaTaO3 nanocomposites.

Figure 4. Production rates of H2 of pure KTaO3 nanocubes under simulated sunlight (A) and under visible light (B): (a) pure KTaO3, (b) 0.025 wt % Ag-NaTaO3, (c) 0.05 wt % Ag-NaTaO3, (d) 0.1 wt % Ag-NaTaO3, and (e) 0.2 wt % Ag-NaTaO3.

(Figure S4b) and high-angle annular dark-field scanning transmission election microscope (Figure S5) images of the 0.1 wt % Ag-NaTaO3 nanocomposite also show the Ag NPs on the surface of NaTaO3 nanocrystals. Figure 3 shows the absorption spectra of the as-prepared AgKTaO3 and Ag-NaTaO3 samples converted from the diffuse reflection spectra data by the Kubelka−Munk (K-M) theory method.29 According to the spectrum, the pure KTaO3 sample presents the photoresponse property in the UV light region, which is due to the intrinsic band gap transition. The 0.2 wt % Ag-KTaO3 sample has an obviously broad peak from 400 to

there are two kinds of Ag nanoparticles which are 10 and 6 nm, respectively. The average size of Ag nanoparticles is from 6 to 10 nm. The lattice spacing measured for the Ag nanoparticles (0.23 nm) is consistent with the (111) lattice plane distance for elemental Ag (0.24 nm).28 Figure 2 is the high-angle annular dark-field scanning transmission election microscope image of Ag-KTaO3. Corresponding element mapping images indicate the presence of K, Ta, O, and Ag elements in single particles on the surface of KTaO3 nanocrystals, showing the successful combination of Ag NPs with KTaO3 nanocrystals, this being consistent with the result of HRTEM and EDS. The HRTEM C


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Figure 5. Comparison of the average rates of H2 production with different samples in 5 h under simulated sunlight (A) and under visible light (B): (a) pure NaTaO3, (b) 0.05 wt % Ag-NaTaO3, (c) 0.1 wt % Ag-NaTaO3, and (d) 0.2 wt % Ag-NaTaO3.

Figure 6. Photoluminescence spectra of the samples: (a) KTaO3 and 0.05 wt % Ag-KTaO3; (b) NaTaO3 and 0.1 wt % Ag-NaTaO3.

which are more than 2 times those of pure NaTaO3 and KTaO3 under simulated sunlight. At the same time, the photocatalytic water-splitting activity of the prepared Ag-KTaO3 and AgNaTaO3 for H2 production is obvious for pure NaTaO3 and KTaO3 under visible light. It is attributed to the surface plasmon resonance (SPR) effect of the Ag nanoparticles. We propose an efficient enhancement photocatalytic activity of KTaO3 and NaTaO3 by dispersing Ag nanoparticles. In the Ag-KTaO3 and Ag-NaTaO3 systems, some samples have a greater concentration of Ag than the samples which have the highest rate of water splitting, in which the rates are much slower. The reason is that further increasing the Ag content led to the aggregation of the Ag NPs and the Ag NPs are large, as shown in TEM images (Figure 1), limiting the transfer of photogenerated charge carriers. This indicates that the major factor contributing to an enhanced photocatalytic efficiency is the photogenerated charge carriers.33 It is well known that the electron−hole recombination and the release of the electron from the Coulomb drag of the hole for Ag-KTaO3 and AgNaTaO3 nanocomposites are key factors which affect the photocatalytic activity of the H2 production rate. The characteristic photoluminescence (PL) was used to measure the electron−hole recombination and release of the electron from the Coulomb drag of the hole. Figure 6a shows that the PL at 468 nm for the Ag-KTaO3 nanocomposite is weaker than for the KTaO3 nanocube, and in Figure 6b the PL at 472 nm for Ag-NaTaO3 is quenched and weaker than for the NaTaO3 nanocube. Therefore, the electron−hole recombination and release of the electron from the Coulomb drag of the hole for Ag-KTaO 3 and Ag-NaTaO 3 nanocomposites might be responsible for the higher efficiency. In this article, the synergetic effects of Ag nanoparticles on the photocatalytic activity of ATaO3 (A = K, Na) are illustrated

600 nm. For the addition of AgNO3 at low concentration, the peaks are not very obvious because the Ag nanoparticles are very small. This is consistent with the TEM images (Figure 1). In the UV−vis spectra for Ag-NaTaO3 photocatalysts (Figure 3b), the peaks are very obvious from 350 to 600 nm and are very broad. The prominent absorption in the visible light region could be attributed to the surface plasmon resonance (SPR) effect of the Ag nanoparticles.30−32 The SPR effect of the Ag nanoparticles may make a partial contribution to the photocatalytic activity of Ag-NaTaO3 by the Ag quantum-size effect. 3.2. Photocatalytic Activity. In order to compare the different Ag-KTaO3 photocatalysis properties with respect to weight ratio, the photocatalytic water splitting experiments of Ag-KTaO3 materials were carried out under the same conditions as for pure KTaO3. Figure 4A shows the profile of H2 evolution versus time over pure KTaO3 and different weight ratios of Ag-KTaO3 under simulated sunlight. After 7 h, the average rates of H2 production for pure KTaO3 and 0.025 wt % Ag-KTaO3, 0.05 wt % Ag-KTaO3, 0.1 wt % Ag-KTaO3, and 0.2 wt % Ag-KTaO3 are 98.48, 164.13, 185.60, 149.59, and 109.88 μmol/h·g (Figure S6), respectively. As shown in Figure 4B, we can see that the H2 evolution over pure KTaO3 nanocubes after 7 h is zero under visible light. However, Ag-KTaO3 exhibits a hydrogen evaluation rate under visible light. In Figure 5A, after 5 h, the average rates of H2 production for pure NaTaO3 and 0.05 wt % Ag-NaTaO3, 0.1 wt % Ag-NaTaO3, and 0.2 wt % AgNaTaO3 are 1.66, 2.74, 3.54, and 3.17 μmol/h·g. As shown in Figure 5B, the H2 production for pure NaTaO3 is zero and the highest H2 production is 0.83 μmol/h·g for 0.1 wt % AgNaTaO3 under visible light. We compared Ag-KTaO3 and AgNaTaO3 to pure KTaO3 and NaTaO3. The best photocatalytic water-splitting activities of the prepared Ag-KTaO3 and AgNaTaO3 for H2 production rates are 185.60 and 3.54 μmol/h·g, D


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in Figure 7. It has been reported that the excited electrons by SPR from large noble metal particles can transfer to the

Figure 8. ESR spectra of ·OH radical adducts with DMPO in 0.05 wt % Ag-KTaO3 suspensions.

Figure 7. Promotion effects of Ag particles on the photocatalytic activity of ATaO3 (A = K, Na) under UV−visible light irradiation.

semiconductive conduction band and then further transfer to the small noble metal particles due to the Fermi-level equilibration.34−38 The visible light have been harvested by large Ag nanoparticles (10 nm, Figure S4a) as surface plasmon resonance (SPR) which is excited and then transfer to the ATaO3 (A = K, Na) conduction band. Meanwhile, the Ag SPR which enhanced the local electric field for ATaO3 (A = K, Na) can promote the electron−hole pair separation under UV light irradiation. The small Ag nanoparticle (6 nm, Figure S4a) on ATaO3 (A = K, Na) act as an efficient cocatalyst and then trap the photoexcited electrons which are from the conduction band of ATaO3 (A = K, Na).37,38 So under UV−visible light irradiation, synergetic and promotion effects on the separation of the photogenerated electron−hole pairs on ATaO3 (A = K, Na) by Ag nanoparticles can be achieved.39 And the basic principle of the synergetic promotion effects of Ag nanoparticles on the surface of ATaO3 (A = K, Na) nanocubes in methyl alcohol and water solution is depicted in Figure S7. It shows that the hydroxyl anion present from water in large quantities diffuses to the ATaO3 (A = K, Na) nanocube surface where it is oxidized by the photoexcited holes to a hydroxyl radical. Subsequently, the hydroxyl radical is expected to oxidize the methanol molecule. The oxidation reaction is known to proceed at a high rate.40 Meanwhile, the photoexcited electron transfers from the conduction band of ATaO3 (A = K, Na) nanocubes to the Ag nanoparticles, and subsequently the electron reduces protons to hydrogen. 3.3. Detection of ·OH Radicals by ESR. The ESR spectra of DMPO spin-trapped adducts was further used to detect the presence of ·OH radicals in the 0.05 wt % Ag-KTaO3 photocatalytic reaction systems. Figure 8 shows that the black line is under the faint natural light and the red line is under the instrumental light. The four characteristic peaks with relative intensities of 1:2:2:1 from the DMPO-·OH adducts are the evidence that the hydroxyl radicals are formed during the photocatalytic reaction. The ·OH signal intensity under the instrumental light is obviously stronger than that under the natural light, suggesting that the ·OH radicals are generated under a strong light.

NaTaO3 nanocubes under simulated sunlight and visible light. This promising result can be ascribed to the synergistic effect of photocatalysts with different light absorbance by SPR. The Ag nanoparticle SPR effect may be of significant value in improving the quantum efficiency for photocatalytic water splitting. This endeavor is being undertaken to synthesize other metal− semiconductor nanocomposites for photocatalysis applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01294. Figures showing the XRD patterns, TEM, EDS, and HRTEM images, and additional data as well as the possible mechanism (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (21276116, 21477050, 21301076, 21303074, and 21201085), financial support from the Zhenjiang Industry Supporting Plan (GY2013023), the Chinese-German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of the Jiangsu Scientific Committee (BK20140011), the Open Project of the State Key Laboratory of Rare Earth Resource Utilizations (RERU2014010), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068), and Six Talents Peak Project in Jiangsu Province (XCL-025).

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REFERENCES

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4. CONCLUSIONS We have directly monitored that Ag-decorated KTaO3 and NaTaO3 nanocubes plasmonic led to a substantial improvement in water splitting compared to that of pure KTaO3 and E


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Ag-Decorated ATaO3 (A = K, Na) Nanocube Plasmonic Photocatalysts

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