Good dispersion of the Au nanoparticles deposited on the STO surface was ... loading, and Au-support interface interaction as well as metal-oxide support.
Mater. Res. Soc. Symp. Proc. Vol. 1509 © 2013 Materials Research Society DOI: 10.1557/opl.2013 .729
Gold Nanoparticles Supported on SrTiO3 by Solution Plasma Sputter Deposition for Enhancing UV- and Visible-light Photocatalytic Efficiency Gasidit Panomsuwan 1, Nobuyuki Zettsu 1,3, and Nagahiro Saito 13 1 Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 2 EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 3 Green Mobility Collaborative Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
ABSTRACT Gold (Au) nanoparticles were synthesized and deposited on the perovskite SrTiO3 (STO) via a one-step solution plasma sputter deposition (SPSD) without any reducing reagents at ambient condition. Good dispersion of the Au nanoparticles deposited on the STO surface was clearly observed. The synthesized Au nanoparticles were well-crystallized with a spherical shape and preferably exhibited multiply twinned structure. An average diameter of Au nanoparicles was estimated to be 6.1 1.4 nm by transmission electron microscopy. Enhanced photocatalytic activity was found for the Au-STO when compared to the pure STO, as investigated from the degradation of methylene blue solution under ultraviolet and visible light irradiation. The SPSD seems to be a rapid and facile approach to prepare the Au nanoparticles supported on the metal oxide for photocatalytic applications.
INTRODUCTION Gold (Au) nanoparticles supported on metal oxide has been received an enormous attention as effective catalysts for the promotion of a wide variety of reactions such as H2 production [1], CO oxidation [2], and oxidative decomposition of organic compounds [3]. Their catalytic properties were strongly dependent on the particle size and shape of Au, amount of Au loading, and Au-support interface interaction as well as metal-oxide support. The SrTiO3 (STO) is selected as support in this study because it is a prominent one due to its thermal stability, chemical inertness, availability, and non-toxicity. Its band gap is also comparable to the conventional TiO2 (Eg 3.2 eV). Over last decade, several preparation methods have been developed to prepare the Au nanoparticles supported on metal oxide such as depositionprecipitation (DP) [4], photo-deposition (PD) [5], and wet impregnation [6]. These preparation methods commonly use the HAuCl4 as the precursor, which inevitably requires reducing agents (e.g. NaBH4, N2H4, and citric acid) and stabilizers (e.g. thiol compound and polymer) for the particle formation. This may lead to some impurities remaining in the final product. In addition, some limitations were also reported; for example, only half of Au in starting materials could be deposited on supports and the particles size is very sensitive to the concentration of precursor for the DP method. The PD can be applied for only semiconductor support and take a long irradiation time [7,8]. Therefore, a suitable method plays a key role for the preparation of a highly active Au nanoparticles supported on metal-oxide catalysts.
Very recently, solution plasma provides great potential in the field of nanoparticle synthesis due to its several advantages such as fast synthesis, simplicity, atmospheric pressure, no further purification, inexpensive equipment, and ability to scale up process [9]. There are two main routes for the synthesis of Au nanoparticles by solution plasma. The first is to use the reduction of HAuCl4 aqueous solution via hydrogen radicals generated by solution plasma [10]. The second relies on the sputtering of Au electrode, namely solution plasma sputtering (SPS). The synthesized Au nanoparticles from the SPS showed good crystalline quality, narrow size distribution, and high purity. In addition, they exhibited an excellent stability in water for a long period of time [11], which is well-suited for further studies in the field of catalysis. Inspired by this approach, we have further developed the SPS process to the preparation of the Au nanoparticles supported on the STO for photocatalytic applications in this study. It would be expected that the Au nanoparticles can be sputtered from electrode and then suddenly deposited on the STO surface in one-step processing, namely solution plasma sputter deposition (SPSD). Here, we report a rapid and facile preparation of the Au nanoparticles supported on STO by SPSD. The samples were characterized using transmission electron microscopy, diffuse reflectance spectroscopy, and X-ray diffraction. The photocatalytic activity was also investigated by the degradation of methylene blue dye.
EXPERIMENTAL DETAILS Figure 1a depicts the self-designed reactor system used in this study. The dimension size of glass reactor is 50 mm in diameter and 100 mm in height. The 1 mm-diameter Au wire (Nilaco Co. Ltd., purity: 99.99%,) was used as electrodes and placed at the center of the reactor. The electrodes were shielded by an insulating ceramic tube. The distance between the electrodes was set at 0.5 mm. The 0.3 g of STO powders (Aldrich; particle size ~ 100 nm, purity 99.5%) was dispersed in 80 mL of the ultra pure water. The glow discharge plasma was generated in the aqueous suspension of STO powders at the area between the Au electrodes by applying a high voltage of 2000 V from a bipolar pulsed power supply. The pulse width and frequency were fixed at 2 s and 15 kHz, respectively (figure 1b). After discharge under vigorous stirring, the samples were separated by centrifugation and then dried at 100 C. The color appearance of the powder products was changed from white to purple after plasma discharge, as illustrated in figure 1c. The amount of Au loading in the STO was determined by weight loss of the Au electrodes after discharge. The ~1 wt% Au-STO was approximately obtained after discharge for 3 min. Phase structure was confirmed by X-ray diffraction (XRD, Rigaku SmartLab) equipped with monochromatic Cu K radiation ( = 1.54184 Å). UV-vis diffuse reflectance spectroscopy (DRS) was examined using a Shimadzu UV-3600 spectrophotometer. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2500SE with an accelerating voltage of 200 kV. Photocatalytic activity was carried out by monitoring the degradation of methylene blue (MB) dye in aqueous solution (10 ppm, 2.7 105 M). Aqueous solution of the MB dye was stirred in the dark for 1 hr in order to obtain absorption-desorption equilibrium. The catalysts were suspended in the MB solution with the concentration of 0.1g/L. The MB solution was then exposed to the 200 W HgXe lamp (Supercures UVF-203S). The samples were collected at a given time interval (20 min), centrifuged to separate photocatalyst powder from the solution. Then, the absorption of separated solution was measured by UV-vis spectrophotometer.
Figure 1. (a) Schematic illustrations of the self-designed reactor system for the SPSD in this study, (b) bipolar pulsed voltage supplied to the Au electrodes, and (c) the color appearance of the products before (white) and after SPSD (purple).
RESULTS AND DISCUSSION Figure 2a shows a bright field TEM image of the STO after SPSD. It was clearly seen that the spherical Au nanoparticles were uniformly deposited on the STO surface without any agglomeration and abnormal growth. The Au nanoparticles were in the range between 2 and 10 nm with an average diameter size of 6.1 1.4 nm. The particle-size distribution deduced from 500 particles is shown in the inset of figure 2a. The size of the Au nanoparticles was found to be dependent on the discharge conditions such as discharge voltage, discharge current, pulse width, frequency, electrode gap, solvent conductivity and solvent temperature. However, these effects on the particle size are beyond the scope of this study. The shape and microstructure of the Au nanoparticles were investigated by a high-resolution TEM image, as revealed in figure 2b. The Au nanoparticles were well-crystallized and mostly exhibited multiply twinned structure (MTS) consisting of single tetrahedral crystals with {111}Au planes in twin relation with each others. It has been reported that the Au nanoparticles with MTS were normally observed in case of the particle size below 10 nm since the MTS preferably formed at the initial stage of Au particle growth [11,12]. From this result, it was evidence that the Au nanoparticles were successfully synthesized and deposited on the STO surface by the SPSD method. To further confirm the existence of the Au nanoparticles on the STO supports, the XRD and DRS measurements were carried out. A representative XRD pattern of the Au-STO is shown in figure 3a. The sharp and intense diffraction peaks corresponding to the cubic-perovskite STO phases confirmed a crystalline nature of the STO. The diffraction peaks corresponding to the 111Au (2 = 38.208) and 200Au (2 = 44.387) were also detected; however, other diffraction peaks could not be seen due to their low intensity. The occurrence of Au diffraction peaks indicated the formation of Au with good crystallinity, which was consistent with aforementioned high-resolution TEM result. The DRS spectra were recorded in reflectance mode and transformed into the Kubelka-Munk function:
K (1 R ) 2 , (1) S 2R where R is equal to Rsample/Rstandard, K is the absorption coefficient, and S is the scattering coefficient (assume to be a constant value). UV-vis DRS spectra of the STO and the Au-STO are demonstrated in figure 3b. A broad absorption centered at the wavelength of 560 nm was apparently observed to cover the range between 400 and 800 nm in the spectrum of the Au-STO. This was attributed to the surface plasmon resonance (SPR) effect of the Au nanoparticles. However, it was not found a significant shift of the absorption edge for the Au-STO when compared to the pure STO. F (R )
Figure 2. (a) Bright-field TEM image (the inset shows particle-size distribution of Au nanoparticles) and (b) high-resolution TEM image of the Au nanoparticle deposited on the STO surface with clear interface.
Figure 3. (a) XRD pattern of the Au-STO and (b) UV-vis DRS spectra of the STO and the AuSTO.
The Au-STO was expected to have higher phocatalytic activity than the pure STO. Compared with the pure STO, the MB solution in the presence of the Au-STO showed a faster degradation rate. In case of the Au-STO, 90% degradation of MB solution was observed within 180 min, whereas only 65% degradation was found for the pure STO at the same irradiation time (figure 4a). The photocatalytic activity for the STO and the Au-STO was quantitatively evaluated using the respective pseudo first-order rate constant (k) from the following relation: C ln t kt , (2) C0 where C0 and Ct are the initial concentration and the reaction concentration of the MB solution at time t, respectively. The rate constant could be obtained by a slope of linear fitting curve, as revealed in figure 4b. Rate constant of the Au-STO was estimated to be 1.18 102 min1, which was about two times higher than that of the pure STO (5.58 103 min1). This suggested that the Au-STO had a higher photocatalytic activity than the pure STO. Higher photocatalytic efficiency of the Au-STO was described by an active charge transfer transition. Under ultraviolet and visible light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB) of the STO and the hole is created in the VB. In the absence of the Au nanoparticles, most of these charges rapidly recombine in a sub-nanosecond time scale. Thus, only a small number of electrons and holes are trapped and allowed for the photocatalytic reactions, leading to poor catalytic activity. On the other hand, for the Au-STO, the excited electrons in the CB can be transferred to the Au nanoparticles that deposited on the STO surface, leading to chargeseparation enhancement, better stabilization, and recombination suppression. The schematic representation of mechanism of enhanced photocatalytic activity for Au-STO is illustrated in figure 5.
Figure 4. (a) Absorption change plots and (b) pseudo first-order plots comparing the photocatalytic degradation of MB solution in the presence of the STO and the Au-STO at various irradiation times.
Figure 5. Schematic representation of mechanism of the photocatalytic reaction for MB by AuSTO. CONCLUSIONS The Au nanoparticles supported on the STO were successfully prepared via a one-step SPSD without any reducing agents in a short processing time. The Au nanoparticles were observed to be well-dispersed on the STO surface with an average diameter of 6.1 1.4 nm. The photocatalytic activity of the Au-STO was significantly enhanced about two times when compared to the pure STO due to the effect of charge transfer and charge separation at the AuSTO interface. According to our results, the SPSD presents a facile and powerful method to prepare clean Au nanoparticles supported on the metal-oxide for photocatalytic applications. This has also opened up a new route for the preparation of other heterogeneous catalyst systems for advanced photocatalysts in the near future.
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