Enhanced photocatalytic activity and stability of the reduced graphene oxide loaded potassium niobate microspheres for hydrogen production from water reduction

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Enhanced photocatalytic activity and stability of the reduced graphene oxide loaded potassium niobate microspheres for hydrogen production from water reduction Zhen Hong a, Xiangqing Li a,*, Shi-zhao Kang a, Lixia Qin a, Guodong Li b, Jin Mu a,* a School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China b State Key Laboratory of Inorganic Synthesis Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

article info

abstract

Article history:

A facile approach to synthesize reduced graphene oxide (RGO) loaded potassium niobate

Received 2 April 2014

microspheres was reported. The composition, microstructure and electron-transfer prop-

Received in revised form

erties of the obtained product were characterized. Compared to pure potassium niobate

4 June 2014

microspheres and commercial P25 TiO2, the as-prepared potassium niobate microspheres/

Accepted 12 June 2014

RGO composite showed much higher photocatalytic activity for generating hydrogen under

Available online 9 July 2014

UV irradiation. It was ascribed to the enhanced separation efficiency of electron/hole pairs as testified by electrochemical impedance spectrum and fluorescence spectrum. Impor-

Keywords:

tantly, the composite photocatalyst was stable and easy to recycle, and the amount of

Potassium niobate microspheres

hydrogen evolution did not decrease after six recycles. The results are potentially appli-

Reduced graphene oxide

cable to a range of semiconductors useful in water reduction as well as other areas of

Photocatalysis

heterogeneous photocatalysis.

Hydrogen evolution

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The depletion of fossil fuel reserves is one of the most urgent issues in modern society. Using clean, abundant and sustainable energy without fossil fuel consumption and CO2 emission is of great significance. As a kind of storable and environmentally benign fuel, hydrogen energy is considered as an ideal energy source [1]. An important technique of

producing hydrogen is the electrolysis of water. Although electrolysis of water would be preferred to produce for highquality hydrogen, this technique is costly [2]. It is found that using solar energy to produce clean hydrogen through photocatalysis is a good choice [3]. It is the key to design and synthesize ideal catalysts for photocatalytic hydrogen evolution. Due to their electronic features, some semiconductor materials are mostly used as photocatalysts. An ideal

* Corresponding authors. Tel.: þ86 21 60873061; fax: þ86 21 64253317. E-mail addresses: [email protected] (X. Li), [email protected] (J. Mu). http://dx.doi.org/10.1016/j.ijhydene.2014.06.075 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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semiconductor photocatalyst for hydrogen evolution should be highly active, stable and low cost. So, it is significantly meaningful to design and prepare a cheap and efficient semiconductor photocatalyst. Among a diverse set of photocatalysts, the active materials being employed are mainly TiO2 and titanates [4]. Therefore, further development of the photocatalytic materials for water splitting is indispensable. Recently, for their availability and low toxicity, some niobates with perovskite structure have been intensively investigated in pollutant degradation and water splitting, and the relationship between the crystal structure and photocatalytic activities was also discussed [5]. However, photocatalytic water splitting cannot achieve a desired level due to fast recombination of photo-generated electron/hole pairs. In order to optimize the photocatalytic efficiency and provide a sufficient base for the future design and development of high performance photocatalysts, great efforts have been made to probe the underlying principles and photocatalytic performance of materials [6,7]. Jonker et al. found that efficient electrical injection of spin-polarized carriers can be achieved from a non-lattice-matched magnetic contact into a semiconductor heterostructure [8]. Besides, morphology has an important influence on properties of potassium niobates [9e12]. Especially, the morphology has a direct influence on the photoactivity by affecting the specific surface area, the hydrophilic character or the availability of charge carrier by modifying the internal electric field close to the surface. Therefore, enlarging surface area and increasing the number of active sites is an efficient way to improve the separation efficiency of electron/hole pairs through adjusting the morphology of photocatalysts [13]. Zhou et al. prepared the porous K4Nb6O17 microspheres via a simple homogeneous precipitation method, and the microspheres show a high activity for generating hydrogen [14]. It was mainly attributed to the porous structure and high specific surface area of the K4Nb6O17 microspheres. Ma et al. found that photocatalytic activity of hydrogen evolution over the exfoliated-scrolled K4Nb6O17 is obviously higher than that of the synthetic K4Nb6O17 crystals. It is demonstrated that the unmodified nanoscrolls are better catalysts for UV light driven hydrogen evolution from aqueous methanol solutions [15]. Mobility of the charges affects the probability of electrons reaching the reaction sites on the surface of the photocatalyst, so it affects the photocatalytic activity and is important to a photocatalyst. Integrated with noble-metal with low over potential (Pt, Pd, Au or Ag) is an efficient way to improve the charge mobility of niobates [16,17]. But noble-metals are very expensive for industrial applications. Nowadays, it is found that the mobility of charge can be improved by integrated with graphene [18], or carbon nanotube [19]. Owing to its unique structure and electronic properties, graphene has drawn the attention of scientists in recent years [20]. Since the reduced graphene oxide with huge specific surface area and high mobility of charge carriers exhibits many physical properties similar to those of graphene, it is being considered in electronic, sensor, and catalytic applications [21]. Here, we propose a green and facile way to synthesize reduced graphene oxide (RGO) loaded potassium niobate microspheres. The purpose is to improve the separation efficiency of electron/ hole pairs of the potassium niobate microspheres by using

RGO served as a cheap cocatalyst. The UV photocatalytic activity for hydrogen evolution over the potassium niobate microspheres/RGO is evaluated. In addition, the mechanism of electron transfer in the potassium niobate microspheres/RGO is also investigated.

Experimental section Preparation of RGO loaded potassium niobate microspheres RGO was prepared according to the published procedure [22]. The potassium niobate microspheres were synthesized using hydrothermal method [14]. All chemicals were used without further purification. Without other pre-modifications to potassium niobate microspheres and RGO, the RGO loaded potassium niobate microspheres was prepared only by simple mixing a certain amount of potassium niobate microspheres and RGO. In a typical procedure, 100 mg of potassium niobate microsphere and 5.3 mg of RGO were dispersed into 100 mL of deionized water, and then slowly stirred for 24 h. The solid obtained by centrifugation was thoroughly washed with water, and followed by drying at 60  C overnight.

Preparation of working electrodes Indiumetin oxide (ITO) glasses were cleaned by sonication in ethanol, acetone, chloroform, and deionized water for 15 min, respectively, and then dried in the air. Various working electrodes were prepared via impregnation and subsequent calcination method. In brief, 10 mg of sample (potassium niobate microspheres or potassium niobate microspheres/ RGO) was added into 2 mL of alcohol, and the obtained mixture was sonicated for 30 s. After that, the ITO glass (1  1.5 cm2) was soaked into the slurries for 2 min, and heattreated at 100  C for 1 h.

Photoelectrochemical behavior measurement The photoelectrochemical measurements were carried out at room temperature using Chenhua CHI 660E computercontrolled electrochemical analyzer with a standard threeelectrode system (Chenhua Instruments Co., Shanghai, China). A platinum wire electrode was used as the counter electrode and Ag/AgCl electrode was used as the reference electrode. The electrolyte was 1 mol L1 of NaCl aqueous solution. The scan rate was 50 mV s1. Electrochemical impedance spectra (EIS) were tested in the potentiostatic mode with the frequency range of 1 to 1 MHz, and the bias potential was 0.5 V. The amplitude was 5 mV. The light source was a 150 W Xe lamp. The distance between light source and the working electrode was 7 cm. The air in the solution was removed by purging nitrogen for 15 min.

Photocatalytic experiment The photocatalytic water reduction for hydrogen evolution was performed using a CEL-SP2N water splitting system with an outer-irradiation type quartz cell. In a typical process,

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100 mg of catalyst was dispersed into 100 mL of aqueous solution containing 20 vol% methanol. The light source was a 300 W Xe lamp equipped with a filter to remove light with wavelengths above 400 nm. The reactor was evacuated by a vacuum pump to remove air before irradiation. The amount of hydrogen produced was measured with a continuous on-line inspection gas chromatography (AULTT Co., Beijing) with thermal conductivity detector. N2 gas with ultrahigh purity was used as the carrier.

Characterization The surface morphology of the sample was observed by a NOVA Nano SEM450 ultrahigh resolution field emission scanning electron microscope (FESEM) operated at an accelerating voltage of 15 kV (Japan). The chemical composition of the sample was determined by energy dispersive X-ray (EDX) spectroscopy. The samples were diluted with ethanol and dried on an aluminum foil and sputtered with platinum prior to examinations (Japan). Crystal structure identification of the samples was carried out using a PAN analytical Xpert Pro MRD X-ray diffractometer (XRD) using Cu Ka radiation (l ¼ 0.154056 nm) (Netherlands). The samples for XRD were supported on glass substrates. The element maps were taken with FEI Tecnai G2 20 high resolution transmission electron microscope (USA). The TEM samples were prepared by placing drops of the sample dispersion on a carbon-coated copper grid and dried at room temperature. X-ray photoelectron spectra (XPS) were carried out on a Thermo ESCALAB 250 X-RAY photo electron spectrometer with a monochromatic X-ray source (Al Ka hn ¼ 1486.6 eV) (USA). The energy scale of the spectrometer was calibrated using Au 4f7/2, Cu 2p3/2, and Ag 3d5/2 peak positions. Raman spectra were measured by the Thermo Scientific DXR Raman microscope with a 532 nm DPSS laser and a 50 objective (NA ¼ 0.42) (USA). The spot is 1.1 mm. The incident laser power is 1 mW and the exposure time is 3 s to avoid laser-induced thermal effects or damage. Solid diffuse reflectance UVevisible spectra were recorded at room temperature using a SHIMADZU 3600 spectrophotometer (Japan). Fine BaSO4 powder was used as a standard. The fluorescence spectra were achieved using a HITACHI F-4600 spectrophotometer (Japan).

Results and discussion The XRD peaks of the pure potassium niobate microspheres (Fig. 1(a)) are indexed to K4Nb6O17 (JCPDS card 53-0780), which is in good agreement with the literature [14]. As can be seen in Fig. 1(b), the diffraction peaks of the potassium niobate microspheres/5% RGO are very similar to those of the pure potassium niobate microspheres. It is demonstrated that the loading of RGO do not destroy the structure of potassium niobate. However, the peak intensity of the potassium niobate microspheres is slightly decreased after loaded with RGO, which indicates that the RGO could be loaded on the potassium niobate microspheres. In addition, the diffraction peaks of the RGO were not noticeable in the XRD pattern of the potassium niobate microspheres/5% RGO. It may be due to high dispersion and low loading amount (5%) of RGO on the

Fig. 1 e XRD patterns of potassium niobate microspheres (a), potassium niobate microspheres/5% RGO (b) and RGO (c). Peaks ascribed to RGO and potassium niobate microspheres are marked with - and C, respectively.

microspheres [23]. Existence of RGO on the potassium niobate microspheres will be further characterized by EDX spectrum, XPS spectrum, TEM maps and Roman spectra followed. To further investigate the morphology and structure of the samples, ultrahigh resolution field emission scanning electron microscope is performed operated at an accelerating voltage of 15 kV (Fig. 2). It can be seen that, the potassium niobate microspheres are formed by a lot of nanosheets intercrossed with each other (Fig. 2(A)). Upon introduction of RGO, the surface of microspheres is partly blurred, and some aggregation among sheets can be observed (Fig. 2(B)). The composition of the as-prepared composite is reflected by EDX spectrum (Fig. 2(C)) and XPS survey spectrum (Fig. 2(D)). In Fig. 2(C) and (D), the peaks of Nb, K, C and O elements are clearly observed. The Nb element and K element come from the potassium niobate microspheres, C element corresponds to the RGO, and O element is owed to RGO and the potassium niobate microspheres. The occurrence of Nb, K, C and O signals confirms the presence of the RGO and the potassium niobate in the composite. To further clarify the component of the composite and the distribution of the RGO, energy-filtered TEM of the as-prepared potassium niobate microspheres/5% RGO is taken with high resolution transmission electron microscope operated at an accelerating voltage of 200 kV. Fig. 3(A) shows the element maps of the as-prepared potassium niobate microspheres/5% RGO. It can be seen that there exist C, O, Nb and K elements in the sample. Moreover, the maps of C, O, Nb and K shown in Fig. 3(A) are exactly corresponding to the TEM image shown in Fig. 3(B). For the substrate used in TEM measurement is the carbon-coated copper grid, the signal of C can also be seen in the other region of the map. But its density is higher around potassium niobate. It should be caused by the RGO coated on the surface of the potassium niobate. In general, Raman spectroscopy is more sensitive to lattice perturbations in the local structure in comparison with XRD.

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Fig. 2 e FESEM images of the pure potassium niobate microspheres (A) and the as-prepared potassium niobate microspheres/5% RGO (B); EDX spectrum (C) and XPS survey spectrum (D) of the as-prepared potassium niobate microspheres/5% RGO.

Therefore, the potassium niobate microspheres/5% RGO is characterized by Raman spectra. The Raman peaks of pure potassium niobate microspheres (Fig. 4(a)) are located at 244 cm1, 656 cm1 and 898 cm1. But the intensity of these

peaks decreases after introduction of RGO. It is indicated that RGO has been loaded onto the potassium niobate microspheres [24], which coincides with the result of XRD. Raman spectrum of the potassium niobate microspheres/5% RGO also

Fig. 3 e Energy-filtered TEM images of the as-prepared potassium niobate microspheres/5% RGO. (A) Maps of C, O, Nb and K; (B) zero-loss map of K, Nb, O and C.

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Fig. 4 e Raman spectra of the pure potassium niobate microspheres (a), the potassium niobate microspheres/5% RGO (b) and RGO (c).

provides an evidence for the presence of RGO. The characteristic peaks attributed to D band and G band of RGO appear at 1343 cm1 and 1584 cm1, respectively. However, the G band of RGO in the potassium niobate microspheres/RGO is blue-shifted 14 cm1, which demonstrates that there exists stronger interaction between potassium niobate microspheres and RGO by an electron transfer driven by the work function difference [25]. In addition, the intensity ratio of the D band and G band (ID/IG) for RGO deceases a little after loaded on the potassium niobate microspheres, which is probably due to the restoration of the aromatic structures in RGO by repairing defects [26]. XPS is employed to study the element constitution and the chemical binding state of the potassium niobate microspheres/RGO. To identify the components of carbon-based atom, the high resolution XPS spectrum of C1s is examined. The deconvoluted XPS spectrum of C1s for the potassium niobate microspheres/RGO is displayed in Fig. 5(A). Here we can see three components of carbon-based atom in the composite: the main C1s peak is dominated by non-oxygenated CeC at 284.8 eV, attributed mainly to sp2 hybridized carbon

284.8 eV C-C

282

286

288

Binding energy (eV)

1240 l

Eg is the band-gap energy, l is the wavelength, which is 337 nm and 346 nm for potassium niobate microspheres/RGO and potassium niobate microspheres, respectively. Obviously, band-gaps for the two samples (>3.0 eV) are wide, which indicates that both samples should be UV response. The fluorescence spectrum is used to investigate the formation of photo-induced charge carriers. Fig. 6(B) shows

O1s

A

286.4 eV C-O 288.0 eV C=O

284

Eg ¼

290

B

530.7 eV Nb-O

Intensity counts)

Intensity (a.u.)

C1s

of the RGO [27]. One weak peak at 286.4 eV is assigned to the oxygen bound species CeO, and another very weak peak at 288.0 eV is attributed to C]O of the RGO. The weakened peaks for CeO and C]O could be caused by the reduction of GO [27]. To identify the components of the oxygen-based atom, the high resolution XPS spectrum of O1s is examined (Fig. 5(B)). The peaks with binding energies located at 530.7 eV, 531.3 eV, 532.7 eV and 533.7 eV, are assigned to the NbeO, C]O, NbeOH and CeOH species, respectively [28]. Compared to the binding energies of pure K4Nb6O17 reported in the literatures [29,30], the binding energies of NbeO and NbeOH for the potassium niobate microspheres/RGO are slightly higher. It could be concluded that the CeOeNb bond is formed in the potassium niobate microspheres/RGO [31]. The XPS results of C1s and O1s demonstrate that the RGO is loaded into the potassium niobate microspheres and there exists some interaction between the potassium niobate microspheres and RGO in the composite, which is favor to the charge transfer from potassium niobate microspheres to RGO upon light excitation [28]. To study the optical absorption property of the as-prepared samples, solid UVevis diffuse reflectance spectra are measured, and the results are shown in Fig. 6(A). It is observed that an enhanced background absorption in the range of 350e800 nm for the potassium niobate microspheres/5% RGO besides an absorption peak at 267.5 nm. It is due to the dark color of the sample and the enhanced surface charges of potassium niobate [32]. Besides, the hydroxyl interaction between potassium niobate microspheres and RGO also attributes to the light absorption [33]. Compared to pure potassium niobate microspheres, absorption onset of potassium niobate microspheres/RGO shifts slightly towards the shorter wavelength range, giving rise to a small increase in band-gap energy to 3.68 eV (pure potassium niobate microspheres: 3.58 eV), as calculated by the following equation [34]:

531.3 eV C=O 532.7 eV Nb-OH 533.7 eV C-OH

526

528

530

532

Binding energy (eV)

Fig. 5 e XPS spectra of C1s (A) and O1s (B).

534

536

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600

0.2

450 300

337nm

0.1

B

a Intensity

Absorbance

0.3

750

A

b a

b

150

346nm

0

0.0 200 300 400 500 600 700 800 Wavelength (nm)

300

330 360 390 Wavelength (nm)

420

Fig. 6 e (A) Solid diffuse reflectance UVevis absorbance spectra and (B) Fluorescence spectra. lex ¼ 230 nm. Pure potassium niobate microspheres (a) and potassium niobate microspheres/5% RGO (b).

1500

A

1200 900 600 300 0 0

microspheres is obviously higher than that over P25 TiO2. It is indicated that the potassium niobate microspheres are the better photocatalyst for hydrogen evolution. In addition, photocatalytic activity of the potassium niobate microspheres is further increased after loading a small amount of RGO, and exceeds those of pure potassium niobate microspheres and P25 TiO2 catalysts by a factor of 1.3 and 4.8 at the same conditions, respectively. The enhancement of photocatalytic activity for the potassium niobate microspheres/5% RGO is mainly caused by excellent electron transfer of RGO which can suppress the recombination of photo-generated electron/ hole pairs of potassium niobate microspheres. Besides, the interaction between potassium niobate microspheres and RGO is also beneficial to electron transfer and light absorption as displayed in Figs. 5 and 6. With the potassium niobate microspheres/RGO as the photocatalyst, the rate of hydrogen evolution was compared to some traditional data [26,36e38]. As shown in Table 1, under similar conditions, the photocatalytic activity for hydrogen evolution over the potassium niobate microspheres/RGO is the highest. It is indicated that the potassium niobate microspheres/RGO is a highly efficient photocatalyst for hydrogen evolution.

H 2 evolution (umol)

H 2 evolution (umol)

fluorescence spectra of the pure potassium niobate microspheres and potassium niobate microspheres/5% RGO. The pure potassium niobate microspheres exhibit a strong peak around 380 nm, which indicates that electron/hole pairs generate when potassium niobate microspheres are irradiated. However, the emission intensity declines drastically upon loading of RGO. It is explained that RGO acted as electron traps can accept photo-generated electrons produced by potassium niobate microspheres. Therefore, recombination of electron/holes pairs in potassium niobate microspheres is prohibited [35]. With a 300 W Xe lamp equipped with a filter to remove light with wavelength above 400 nm, the rate of photocatalytic water reduction for hydrogen production is evaluated for the pure potassium niobate and potassium niobate microspheres/ 5% RGO using methanol as sacrificial agent. As shown in Fig. 7(A), it is observed that the amount of hydrogen evolved is approximately linear with the time. The hydrogen evolution stops when the light is turned off, showing that the reaction is induced by the absorption of light. The activity of hydrogen production is in the order of potassium niobate microspheres/ RGO > pure potassium niobate microspheres > P25 TiO2. The activity of hydrogen production over the potassium niobate

2

4

Time (h)

6

8

1200

B

900 600 300 0 0

6

12

18

24

Time (h)

30

36

Fig. 7 e (A) Time course of hydrogen evolution over potassium niobate microspheres/5% RGO (C), pure potassium niobate microspheres (-), P25 TiO2 (;) under UV irradiation and over potassium niobate microspheres/5% RGO without irradiation (:). (B) Hydrogen evolution over the recycled potassium niobate microspheres/5% RGO photocatalyst under UV irradiation.

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Table 1 e The comparison of our results with the references. Catalysts

Amount of catalyst

Ref. [36]

ZnIn2S4/RGO

50 mg

Ref. [26]

TiO2/RGO

50 mg

Ref. [37] Ref. [38] Our result

TiO2/CuO BiPO4/RGO Potassium niobate microspheres/RGO

100 mg 100 mg 100 mg

Long-term stability is an important factor to evaluate the performance of a photocatalyst. So it is necessary to investigate the stability and repeat ability of the potassium niobate microspheres/5% RGO in photocatalytic hydrogen evolution. The photocatalytic stability of the potassium niobate microspheres/5% RGO is evaluated by performing the recycle experiments of the photocatalyst under same conditions. Fig. 7(B) shows the rate of photocatalytic hydrogen evolution over the potassium niobate microspheres/5% RGO under 6 times of repeated irradiation for 6.0 h each. After evacuating the reaction system and re-running the experiment, almost identical hydrogen is achieved in the second as well as the other runs. It can be seen that, after six recycles, the amount of hydrogen evolution over potassium niobate microspheres/ 5% RGO does not exhibit an obvious loss, which indicates that the composite photocatalyst is stable. More than 6646 mmol hydrogen is evolved during the course of a 36 h experiment, suggesting that the photocatalytic activity of the potassium niobate microspheres/5% RGO is high and stable under UV irradiation. The photocatalytic mechanism is analyzed with respect to the transfer of photo-induced charge carriers in the potassium niobate microspheres/RGO composite. Schematic illustration for the charge transfer and separation in the potassium niobate microspheres/RGO is shown in Fig. 8. Photo-induced electrons/hole pairs are generated over potassium niobate microspheres under UV irradiation. The photo-induced electrons in potassium niobate microspheres easily transfer to RGO due to a slightly lower graphene/graphene,- redox potential (0.08 V vs. NHE) as compared to the conduction band (CB) of potassium niobate microspheres, while the photoinduced holes remain in the valence band (VB) of potassium niobate microspheres [20,36]. Therefore, the photo-induced electron/hole pairs are efficiently separated. In the presence of sacrificial agent (methanol), the photo-induced holes accumulated on the VB of potassium niobate microspheres are consumed, and meanwhile the photo-induced electrons can rapidly reduce H2O to H2 on RGO [39]. Cyclic voltammetry is an electrochemical technique suitable for investigating the mechanism and kinetic parameters of the reaction. Apparently, the current of potassium niobate microspheres improves due to introduction of RGO (Fig. 9(A)). Besides, EIS of the as-prepared potassium niobate microspheres/5% RGO and potassium niobate microspheres

Irradiation condition A 300 W Xe lamp equipped with a 420 nm cutoff filter to provide the visible light irradiation A 500 W Xe lamp with a water filter to remove the infrared part of the spectrum A 150 W of 2 metal halide light bulbs A 125 W high-pressure Hg lamp A 300 W Xe lamp equipped with a filter to remove light with wavelengths above 400 nm

Rate of H2 evolution (mmol h1 g1) 280 mmol h1 g1

120 mmol h1 g1

139 mmol h1 g1 306 mmol h1 g1 1530 mmol h1 g1

electrodes are measured in order to further confirm our suggestion. As shown in Fig. 9(B), the diameter of the semicircle for the potassium niobate microspheres/RGO electrode is smaller than that of the potassium niobate microspheres electrode, which indicates that the resistance of the potassium niobate microspheres/RGO electrode is lower than that of the pure potassium niobate microspheres electrode. It is attributed to the introduction of RGO, which enhance the electron transfer of the potassium niobate microspheres electrode [40]. So the recombination of electron/hole pairs is hindered and the activity of hydrogen evolution is remarkably improved. The work on influence factors of photocatalytic activity for potassium niobate microspheres/RGO is being undertaken in our lab and it will be reported elsewhere.

Conclusions A highly efficient photocatalyst was prepared by simple mixing potassium niobate microspheres and RGO. Compared to pure potassium niobate microspheres, the photocatalytic activity for hydrogen evolution over the potassium niobate microspheres can be improved after introducing a small amount of RGO. The higher separation efficiency of photo-

Fig. 8 e Schematic illustration for the charge transfer and separation in the potassium niobate microspheres/RGO.

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0.06

a

a

0.00 -0.06

1500

b

1000 500

-0.12 -0.9

B

2000 Z"/ohm

Current (mA)

0.12

2500

A

b

-0.6

-0.3 0.0 0.3 Voltage (V)

0.6

0.9

0

0

2000

4000 6000 Z'/ohm

8000

Fig. 9 e Cyclic voltammograms (A) of the ITO/potassium niobate microspheres electrode (a) and ITO/potassium niobate microspheres/5% RGO electrode (b) under illumination. Nyquist plots of EIS (B) for potassium niobate microspheres (a) and potassium niobate microspheres/5% RGO (b). Scaning rate: 50 mV s¡1; supporting electrolyte: 1 mol L¡1 NaCl aqueous solution.

generated carriers was conducive to the improvement of photocatalytic activity. Moreover, the potassium niobate microspheres/RGO showed a stable photocatalytic activity. The amount of hydrogen evolution did not exhibit obvious loss after six recycles. The results were potentially useful in other areas of heterogeneous photocatalysis, such as wastewater treatment and air purification. The photocatalytic activity of potassium niobate microspheres/RGO is expected to be further enhanced with optimization of parameters such as the content of photocatalyst, pH value of the solution, loading amount of RGO.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21301118, 21305092, 21371070 and 21071060), and the Top Disciplines Construction Foundation of Shanghai City (No. 405ZK120017001 and No. 405ZK120021001).

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