Catalytic activity of gold nanoparticles supported on KNbO3 microcubes

Catalytic activity of gold nanoparticles supported on KNbO3 microcubes

G Model CATTOD-8755; No. of Pages 7 ARTICLE IN PRESS Catalysis Today xxx (2013) xxx–xxx Contents lists available at ScienceDirect Catalysis Today j...

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G Model CATTOD-8755; No. of Pages 7

ARTICLE IN PRESS Catalysis Today xxx (2013) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Catalytic activity of gold nanoparticles supported on KNbO3 microcubes Lisha Yan a,b , Tingting Zhang b , Wanying Lei b , Quanlong Xu b , Xuemei Zhou b , Peng Xu b , Yinshu Wang a,∗ , Gang Liu b,∗∗ a b

Department of Physics, Beijing Normal University, Beijing 100875, China National Center for Nanoscience and Technology, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 3 August 2013 Received in revised form 30 October 2013 Accepted 2 November 2013 Available online xxx Keywords: Catalysis by gold Potassium niobate Hydrogen peroxide Methylene blue Heterogeneous catalysis

a b s t r a c t Potassium niobate (KNbO3 ) microcubes with cubic crystalline phase were hydrothermally prepared and deposited with gold nanoparticles. The structural and electronic properties of the as-prepared samples were studied by X-ray diffraction, X-ray photoelectron spectroscopy, diffuse reflectance UV–visible spectroscopy, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The catalytic property of Au/KNbO3 was evaluated toward hydrogen peroxide (H2 O2 ) decomposition in dark and methylene blue (MB) degradation under visible-light ( > 420 nm), respectively. At the same Au contents, increasing reaction pH from 7 to 12 can dramatically promote H2 O2 decomposition by a factor of ca. 18. With the increase of Au particle size from 4.2 to 10.0 nm at the same pH, the reaction rate constant was increased as well. The underlying mechanism responsible for the observed catalytic performance was discussed in terms of Au particle size, reaction pH, the interaction between Au and KNbO3 , and the catalytically active sites at the interface between Au and KNbO3 . Likewise, MB degradation was shown to be Au particle size-dependent. Increasing Au particle size from 4.2 to 10.0 nm can enhance MB degradation that is mediated by visible-light-induced surface plasmon resonance on Au nanoparticles. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years, semiconductor materials have received a great deal of attention with their promising applications in environment remediation [1–7]. In this regard, the synthesis of nano- and microcrystalline catalytic materials in terms of size and shape control is one of the approaches for the development of highly efficient heterogeneous catalysts [8–10]. In general, catalytic decomposition of hydrogen peroxide (H2 O2 ) can generate hydroxyl radicals and hydroxylions (i.e., H2 O2 → • OH + OH− ). As a powerful oxidant, • OH dictates catalytic activity in wastewater treatment [11,12]. Therefore, developing high-performance catalytic materials, aiming to completely decompose and/or activate H2 O2 , is a key issue. On the other hand, H2 O2 produced at the cathode of polymer electrolyte fuel cells (PEFCs) as a by-product can result in the deterioration of constituent materials [13]. A variety of catalysts, such as metals (e.g., Pt, Cu, Co) and oxides (e.g., Al2 O3 , TiO2 , CeO2 ) are currently utilized for H2 O2 decomposition [13–15]. In particular, metal nanoparticles (NPs), e.g., Au NPs, supported on a

∗ Corresponding author. Tel.: +86 10 58806921. ∗∗ Corresponding author. Tel.: +86 10 82545613; fax: +86 10 62656765. E-mail addresses: [email protected] (Y. Wang), [email protected] (G. Liu).

number of particular oxides often show enhanced catalytic activity for H2 O2 decomposition. Owing to the formation of Schottky junction at the metal/semiconductor interface, the conduction electrons of metal oxides are often transferred to noble metals with the Fermi energy (EF ) lower than the semiconductor conduction band potential and these electrons are effectively involved in catalysis [16–18]. On the other hand, it is well known that Au NPs are able to absorb visible-light via surface plasmon resonance (SPR) that in turn enhances the photocatalytic reactivity [19–21]. In heterogeneous photocatalysis, there are growing interests toward the abatement of harmful species such as organic dyes by nano- and microcrystals [22–24]. Organic dyes are often used in printing, textile and paper industries. A significant amount of dyes is lost in the dying processes and released into waters. In many situations, dye molecules are nonbiodegradable and in part responsible for water resource contamination. Therefore, it is highly desirable to mineralize dye molecules in aqueous solutions using photocatalysts driven by renewable sunlight. Among novel photocatalysts, perovskites are environmentally friendly and have many technologically promising applications [25]. While the vast majority of studies on heterogeneous photocatalysis by niobate-based materials are focused on water splitting for hydrogen generation [26–29], far less research is available regarding photocatalysis, in particular those modified with gold NPs.

0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.11.033

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In this work, we investigate catalytic decomposition of H2 O2 in dark and photocatalytic degradation of methylene blue (MB) under visible-light by Au NPs supported on potassium niobate (KNbO3 ) micro-cubes (MCs) with cubic crystalline phase. KNbO3 is a perovskite with high chemical stability under light illumination. Recently, we assessed photocatalytic water splitting for hydrogen generation by a series of KNbO3 crystals and found that cubic KNbO3 has the highest symmetry in the bulk structure and thus present greater photocatalytic performance than the orthorhombic and tetragonal counterparts, as well as commercially available powered KNbO3 [30,31]. To the best of our knowledge, the current study is the first report on KNbO3 MCs modified with Au NPs toward H2 O2 decomposition and MB degradation.

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2 Theta (deg.) 2. Experimental 2.1. Sample synthesis For the synthesis of cubic phase KNbO3 MCs, 3.57 g Nb2 O5 (puratronic 99.9985%, Alfa Aesar) and 37.69 g KOH (85% min, K2 CO3 2.0% max, Alfa Aesar) were added into 19 mL Milli-Q water (18 M cm, Millipore) which was stirred vigorously for 30 min at room temperature. Then the reaction mixture was sealed in a Teflon-lined stainless steel autoclave and heated at 160 ◦ C for 12 h. The asobtained products were washed with Milli-Q water (18 M cm, Millipore) and ethanol several times, and then dried at 80 ◦ C overnight. Gold NPs supported on KNbO3 submicrostructures were prepared using a deposition–precipitation method. First, 0.5 g of KNbO3 was dispersed in 100 mL Milli-Q water (18 M cm, Millipore). Then 2 mL of NH3 ·H2 O was added into the suspension, and the pH was adjusted to be ca. 11. Afterwards, a certain amount of the aqueous solution of hydrogen tetrachloroaurate (III) trihydrate (HAuCl4 ·3H2 O, ACS, 99.99%, Alfa Aesar) with an Au3+ concentration of 10 mg mL−1 was added. The suspension was vigorously stirred at 80 ◦ C for 1 h and then washed, dried, and calcined at 200 ◦ C in air for 1 h. 2.2. Sample characterization Powder X-ray diffraction (XRD) data were collected using a Shimadzu X-ray diffractometer (XRD-6000) with Cu-K␣ radiation ( = 0.154178 nm). Measurement was in the 2 range of 10–80◦ with a scanning step of 10◦ min−1 . Scanning electron microscopy (SEM) was carried out on a field emission Hitachi S-4800 microscope. Transmission electron microscopy (TEM) imaging was conducted using a Tecnai G2 20 S-TWIN microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) data were obtained using an ESCALab 250 electron spectrometer from Thermo Scientific Corporation. Monochromatic Al K␣ radiation (150 W) was utilized with a pass energy of 30 eV. Low-energy electrons were used to neutralize the samples by charge compensation. The binding energies were referenced to the adventitious C1s line at 284.8 eV. Diffuse reflectance ultraviolet and visible (DRUV–vis) spectra were obtained using a PerkinElmer Lambda 950 UV–vis spectrometer and fine BaSO4 powder was used as a reference. The Brunauer–Emmett–Teller (BET) specific surface area was measured using a Micromeritics ASAP 2020 apparatus. 2.3. H2 O2 decomposition The as-prepared samples (50 mg) for either Au/KNbO3 or KNbO3 were dispersed in 50 mL Milli-Q water (18 M cm, Millipore) and stirred vigorously for 10 min. By dropwise adding NaOH with a concentration of 1 mol dm−3 , the suspension pH was adjusted to a certain value. H2 O2 with a concentration of 4.5 × 10−3 mol dm−3 was then added to the suspension. Then the suspension was stirred

Fig. 1. Powder XRD patterns of c-KNO3 MCs and Au NPs supported on c-KNO3 MCs. a, b and c refer to c-KNbO3 MCs, 3 wt% Au/c-KNbO3 MCs and 6 wt% Au/c-KNbO3 MCs, respectively.

at 25 ◦ C in dark. At certain time intervals, about 4 mL aliquots were sampled and centrifuged. The H2 O2 concentration as a function of time was determined by reduction–oxidation titration using KMnO4 . 2.4. MB degradation Film-like fixed bed catalysts were prepared by coating an aqueous suspension (20 mL) of Au/KNbO3 powders onto a petri dish with a diameter of 5.0 cm and then dried at 80 ◦ C for 2 h. The photocatalyst weight for each experiment was 0.05 g. And 20 mL MB with a concentration of 4.0 × 10−5 M was put into the petri dish. The petri dish was placed in dark for 1 h to reach a complete adsorption–desorption equilibrium, and then illuminated with a 300 W xenon lamp located at ca. 35 cm away from the petri dish. During the irradiation, the reaction temperature was kept at room temperature. In case of visible-light experiment, a wave filter was utilized to allow visible-light ( > 420 nm) to transmit and a water filter was placed between the sample and the light source to eliminate infrared irradiation. The light intensity in the position of the center of the petri dish was measured to be ca. 180 mW cm−2 using Newport optical meter (842-PE). At certain time intervals, about 3 mL aliquots were sampled, then re-poured into the petri dish after measurements. The absorption intensity of MB at 664 nm was measured to determine the dye concentration on a PerkinElmer Lambda 950 UV–vis spectrometer. 3. Results and discussion 3.1. Sample characterization Fig. 1a shows that all the XRD peaks are well indexed to the ¯ cubic phase KNbO3 (denoted as c-KNbO3 , space group Pm3m) with a lattice constant of a = 0.4022 nm (JCPDS 08-0212). In case of Au/KNbO3 , the characteristic diffraction peak of Au (1 1 1) is enhanced with increasing Au weight percentage from 3 to 6 wt%, as shown in Fig. 1b and c. To further identify the elemental compositions and chemical state of the as-prepared samples, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Fig. 2A shows that Nb 3d5/2 and Nb 3d3/2 are located at 206.5 and 209.3 eV, respectively. Upon depositing with Au NPs, Nb 3d core-level binding energy increases by +0.4 eV. The origin of Nb 3d positive shift is attributed to electron transfer from KNbO3 MCs to gold NPs. On the other hand, Fig. 2B shows Au 4f7/2 core-level binding energy is ca. 83.4 eV for the as-prepared Au/KNbO3 samples. Keeping in mind that the binding energy of Au 4f7/2 for metallic gold is 84.0 eV. The decrease in the Au 4f binding energy for

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Fig. 2. High-resolution XPS spectra of (A) Nb 3d core-level, (B) Au 4f core-level, and (C) valence band. a, b and c refer to c-KNbO3 MCs, 3 wt% Au/c-KNbO3 MCs and 6 wt% Au/c-KNbO3 MCs, respectively.

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Wavelength (nm) Fig. 3. DRUV–vis spectra of KNbO3 MCs and Au NPs supported on KNbO3 MCs. (a) KNbO3 MCs. (b) 3 wt% Au/KNbO3 MCs. (c) 6 wt% Au/KNbO3 MCs.

supported Au NPs was also reported in prior studies [32,33] and ascribed to the electron transfer from metal oxides to gold. Our results prove that electron transfer, closely related to band bending on the metal/semiconductor heterojunctions [34], indeed occurs from KNbO3 to gold. Valence band spectra (Fig. 2C) show that the lineshape for bare KNbO3 is similar to that reported previously [35–37]. Nevertheless, the corresponding lineshape for Au/KNbO3 is significantly altered upon Au deposition. Additionally, the edge of the valence band is largely shifted to the Fermi level. Interestingly, K 3p peak is shifted to higher binding energy by 0.4 eV when Au is deposited on KNbO3 . This trend is consistent with that of Nb 3d core-level and again indicates the electron transfer from KNbO3 to Au. Overall, the interplay between Au and KNbO3 and the associated electronic modifications at the Au–KNbO3 interface is observed and expected to play a role in catalytic performance. Fig. 3 shows the UV–vis spectra of the as-prepared samples. The bare KNbO3 sample exhibits absorption only at wavelength less than 400 nm due to the band gap excitation. Loading Au NPs onto KNbO3 gives rise to an increase in the baseline of photoabsorption, evidenced by a change from white to purple in sample color. In the spectra of 3 wt% Au/KNbO3 MCs and 6 wt% Au/KNbO3 MC samples,

the respective photoabsorption centroid was located at around ca. 550 and 570 nm, which is attributed to the intrinsic SPR effects of Au NPs. The observed red-shift from 3 to 6 wt% Au/KNbO3 MC samples is caused by the Au particle size difference as shown in the following section. The morphology and structure of the as-prepared KNbO3 and Au/KNbO3 samples were characterized using SEM, high-resolution TEM and selected area electron diffraction (SAED). SEM image as shown in Fig. 4A confirms cube-like morphology of KNbO3 . Fig. 4B displays that the average side length is ca. 700 nm. The results of high-resolution TEM imaging (Fig. 4C) indicate that the measured d-spacings are 0.411 and 0.407 nm, which are well ascribed to the respective (0 0 1) and (1 0 0) planes of cubic phase KNbO3 . Additionally, fast Fourier transform (FFT) patterns (inset of Fig. 4C) display that two sets of d-spacings (0.411 and 0.407 nm) fit well to the (0 0 1) and (1 0 0) planes, respectively. The morphologies and size distribution of Au NPs supported on KNbO3 MCs are shown in Fig. 4D and E. The respective mean size and standard deviation of Au NPs is (4.2 ± 0.6) nm and (10.0 ± 1.3) nm for 3 wt% and 6 wt% Au supported on KNbO3 MCs. In case of Au/KNbO3 , nearly spherical Au NPs are highly dispersed on the KNbO3 MCs surface. High-resolution TEM images shown in Fig. 4F reveal that the typical d-spacing exhibited by Au NPs is 0.205 nm, which is well ascribed to the Au (1 1 1) plane.

3.2. Catalytic decomposition of H2 O2 The catalytic activity of the as-prepared samples was evaluated toward H2 O2 decomposition in dark. The changes of relative H2 O2 concentration as a function of reaction time are shown in Fig. 5. To evaluate the reactivity of KNbO3 MCs and Au/KNbO3 catalysts quantitatively, the turnover number (TON) was calculated, and the results are shown in Table 1. For the blank experiment without catalysts at pH = 10, no apparent change of H2 O2 concentration is seen. Likewise, the reactivity in the presence of pure KNbO3 MCs is also negligible. In case of Au/KNbO3 , the observed catalytic performance is significant and shown to be dependent on Au particle

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Fig. 4. Representative morphologies and structures of KNbO3 and supported Au NPs. (A) SEM image of cubic phase KNbO3 MCs and (B) the corresponding length distribution. (C) High-resolution TEM image of cubic phase KNbO3 . (D) TEM image and particle size distribution for 3 wt% Au/KNbO3 MCs. (E) TEM image and particle size distribution for 6 wt% Au/KNbO3 MCs. (F) High-resolution TEM image of Au NPs on KNbO3 MCs. Table 1 Au particle size d, reaction pH and TON for H2 O2 decomposition on Au/KNbO3 . Catalyst

d/nm

3 wt% Au/KNbO3 6 wt% Au/KNbO3 6 wt% Au/KNbO3 6 wt% Au/KNbO3

4.2 10.0 10.0 10.0

a

± ± ± ±

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TON (t = 100 min)

Ratioa

10 10 7 12

67.2 101.0 9.0 164.2

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TON ratios: with respect to that of 6 wt% Au/KNbO3 (pH = 10).

size. With the increase of Au contents from 3 to 6 wt%, the particle size changes from 4.2 to 10 nm and the observed catalytic reactivity is greatly enhanced. To investigate the origin of reactivity, the turnover number (TON, the number of H2 O2 decomposed/the number of exposed surface Au atoms in Au/KNbO3 ) at 100 min was determined and shown in Table 1. The number of exposed surface Au atoms can be estimated based on the following equation, NS = NT × D = NT × 6

m /am dVA

(1)

where NS , NT , D, m , ˛m and dVA represent the respective number of surface exposed Au atoms, the number of total Au atoms in surface and bulk, dispersion, the volume occupied by an Au atom in the bulk form of Au, and the surface area occupied by an atom on a polycrystalline surface and the volume–area mean diameter. For Au, m and ˛m is 16.94 A˚ 3 and 8.75 A˚ 2 , respectively [38]. The respective dVA for 3 wt% Au/KNbO3 and 6 wt% Au/KNbO3 was calculated to be 4.3 and 11.4 nm. Accordingly, NS for 3 wt% Au/KNbO3 and 6 wt% Au/KNbO3 is 1.0 × 1018 and 0.8 × 1018 , respectively. Regarding the active site on nanosized gold catalysts, Yates et al. recently reported that the active site for Au/TiO2 catalysts is located at the dual Ti–Au sites at the Au/TiO2 interface in catalyzing CO oxidation [39] and partial oxidation of acetic acid [40]. Herein, it is reasonable to hypothesize that the active site located at the Au/KNbO3 interface plays an indispensable role for H2 O2 decomposition. In order to probe the effects of pH on the catalytic activity, we examined 6 wt% Au/KNbO3 to catalyze H2 O2 decomposition under different pH. Fig. 5B shows that increasing the solution pH from 7 to 12 can dramatically promote H2 O2 decomposition by a factor of

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between Au NPs and semiconductor materials can cause electron transfer. As aforementioned XPS results in this study (Fig. 2), there is interfacial electron transfer from KNbO3 to Au NPs. Accordingly, H2 O2 adsorption and further decomposition into OH− ions and • OH radicals is promoted. On the other hand, it was suggested that the Fermi level of semiconductors in aqueous solutions can be shifted to a relatively high level as the solution pH increases [41]. Thus, with increasing pH, the Fermi level of KNbO3 (Efs ) and Ef was increased and the electron transfer from KNbO3 to Au NPs was probably promoted. Recently, Tada et al. [17] reported that the catalytic reactivity toward H2 O2 decomposition by Au/TiO2 was rationalized as the combination of the Ef change and catalytically active sites available on the surface of Au NPs [17]. With the increase of Au particle size from 2 to 4 nm, the Ef of Au/TiO2 increases. The resulting Ef could promote the electron transfer from Au to H2 O2 and thus give rise to greater catalytic performance.

3.3. Photocatalytic degradation of MB

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Fig. 5. (A) The change of H2 O2 concentration as a function of time over Au/KNbO3 MCs with different Au contents at pH = 10. (a) Blank. (b) KNbO3 MCs. (c) 3 wt% Au/KNbO3 MCs. (d) 6 wt% Au/KNbO3 MCs. (B) The change of H2 O2 concentration over 6 wt% Au/KNbO3 as a function of time with various pH values.

Fig. 6. Energy band diagrams of Au and KNbO3 for Au/KNbO3 .

ca. 18 (i.e., TON changes from 9.0 to 164.2). To illustrate the underlying mechanism governing the catalytic performance, the band structure of Au and KNbO3 , the Fermi energy levels and a possible reaction scheme is displayed in Fig. 6. As an n-type semiconductor, the Fermi energy of KNbO3 (denoted as Efs ) is presumably located at an energy level slightly lower than its conduction band ECB . Nevertheless, the Fermi level of Au NPs (Efm ) is much lower than Efs . When KNbO3 and Au NPs contact with each other, interfacial electron transfer occurs until these two systems reach an equilibrium and form a new Fermi level (Ef ). In general, the strong interaction

The temporal visible spectral changes of MB aqueous solutions in the process of photodegradation are displayed in Fig. 7. The absorption maxima wavelength at 664 nm reflecting the absorbance of n→* transition in MB was used for analysis. The initial pH value of the MB solution is 6.0. It is seen that with increasing irradiation time, the major absorbance was monotonically decreased, with more pronounced change exhibited by 6 wt% Au/KNbO3 MCs. Fig. 8 shows the change of MB relative concentrations as a function of irradiation time. For either blank experiment or KNbO3 MCs, the rate of MB degradation is much slower than that of Au/KNbO3 MCs. Furthermore, with the increasing size of Au NPs from 4.2 to 10.0 nm, the reaction rate constant k increases. To evaluate the reactivity of Au/KNbO3 catalysts quantitatively, the reaction rate constants were calculated by using a pseudo firstorder kinetic model, and the results are shown in Table 2. Keeping in mind that compared to suspended photocatalyst counterparts, fixed bed photocatalysts often display relatively low photoreactivity due to the decrease in specific surface area, albeit the merit of them lies in the fact that the post-reaction treatment involving catalyst separation from reaction solutions is not necessary. Using in situ electron paramagnetic resonance (EPR) spectroscopy, Brückner et al. [42] directly monitored the electron transfer from Au NPs to TiO2 under visible-light for Au/TiO2 photocatalysts, and found that those electrons are trapped in vacancies in close vicinity to the Au–TiO2 interface. The underlying electron transfer is stimulated by two different electron excitation pathways within the Au NPs, involving d-sp interband transitions and SPR transitions under the lower and higher wavelength illumination of visible-light, respectively [42]. Similarly, the intraband transitions of 6sp electrons driven by SPR within Au NPs were also reported in previous work [36,43,44]. In case of Au/KNbO3 , the inherent bandgap is ca. 3.2 eV for KNbO3 samples, visible-light is exclusively absorbed by metallic nanogold. The d-sp interband transitions and SPR transitions of Au NPs can generate energetic electrons that are transferred to the conduction band of KNbO3 . Those electrons on KNbO3 can be captured by the oxygen molecules to generate catalytically powerful oxidants like HO2 • or • OH radicals, which are responsible for MB

Table 2 The MB adsorption capacity, reaction rate constants and corresponding regression coefficients r2 for MB degradation on KNbO3 MCs and Au/KNbO3 based on pseudo-first-order kinetic model. Catalyst

MB adsorption (×10−3 mmol g−1 )

Reaction rate constant k (min−1 )

Regression coefficients r2

Blank KNbO3 3 wt% Au/KNbO3 6 wt% Au/KNbO3

– 2.51 3.17 3.91

0.0031 0.0036 0.0091 0.0115

0.99 0.97 0.99 0.98

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Fig. 7. The temporal UV–vis spectral changes of MB aqueous solutions as a function of irradiation time. (A) Blank. (B) KNbO3 MCs under visible-light. (C) 3 wt% Au/KNbO3 MCs under visible-light. (D) 6 wt% Au/KNbO3 MCs under visible-light.

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photocatalytic degradation of MB on Au/KNbO3 is shown to be size-dependent. Increasing Au particle size from 4.2 to 10.0 nm promotes the reactivity through stronger SPR and associated effects.

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The authors acknowledge the support of this work from National Natural Science Foundation of China (51272048, 10974017) and Fundamental Research Funds for the Central Universities.

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Fig. 8. Photocatalytic degradation of MB as a function of time on KNbO3 MCs and Au/KNbO3 composite photocatalysts under visible-light. (a) Blank. (b) KNbO3 MCs. (c) 3 wt% Au/KNbO3 MCs. (d) 6 wt% Au/KNbO3 MCs.

degradation. The resulting oxidized Au NPs with photogenerated holes can capture electrons from donors (i.e., photosensitized dyes) and either directly degrade MB or oxidize H2 O to form • OH radicals. As Fig. 3 shows, the red-shift in the DRUV–vis spectra caused by the different loading from 3 to 6 wt% Au/KNbO3 MC samples is clearly observed. And the absorption for 6 wt% Au/KNbO3 is apparently more pronounced than that of 3 wt% Au/KNbO3 . The above difference could in turn result in the reactivity difference between 3 wt% to 6 wt% Au/KNbO3 . 4. Conclusions H2 O2 decomposition on Au NPs supported on cubic KNbO3 MCs in dark is strongly influenced by Au particle size as well as reaction pH. Increasing pH from 7 to 12 can dramatically promote H2 O2 decomposition by a factor of ca. 18 in terms of TON. Additionally, with the increase of Au particle size from 4.2 to 10.0 nm, the reactivity was increased. Electron transfer from KNbO3 to Au evidenced by XPS is important for enhancing the catalytic activity. Additionally, the catalytically active site located at the Au/KNbO3 interface presumably plays an indispensable role dictating the observed catalytic performance. Visible-light ( > 420 nm) induced

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