AuPd bimetallic nanoparticles decorated graphitic carbon nitride for highly efficient reduction of water to H2 under visible light irradiation

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

AuPd bimetallic nanoparticles decorated graphitic carbon nitride for highly efficient reduction of water to H2 under visible light irradiation Changcun Han

a,b

, Linen Wu b, Lei Ge

a,b,* ,

Yujing Li b, Zhen Zhao

a

a State Key Laboratory of Heavy Oil Processing, China University of Petroleum Beijing, No. 18 Fuxue Rd., Beijing 102249, People’s Republic of China b Department of Materials Science and Engineering, College of Science, China University of Petroleum Beijing, No. 18 Fuxue Rd., Beijing 102249, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Article history:

In this work, a new AuPd bimetallic cocatalyst decorated graphitic carbon nitride (g-C3N4)

Received 5 January 2015

photocatalysts with high H2 evolution activity was synthesized via an in situ chemical

Accepted 24 February 2015

deposition method. The physical and photophysical properties of the as-prepared AuPd/

Available online 2 March 2015

g-C3N4 were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscope (TEM), Ultraviolet–visible diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) and surface photovoltage spectroscopy (SPV). The photocatalytic H2 evolution experiments indicate that the AuPd bimetallic co-catalysts can effectively promote the separation efficiency of photo-generated charge carriers in g-C3N4, and consequently enhance the H2 evolution activity. The 0.5 wt% AuPd/g-C3N4 catalyst shows the highest catalytic activity, and corresponding H2 evolution rate is 326 lmol h

1

g 1, which are enhanced by 3.5 and 1.6

times compared with that of pristine Au/g-C3N4 and Pd/g-C3N4 under visible light irradiation. The photocatalyst can maintain photocatalytic activity after 4 cycles. A possible photocatalystic mechanism of AuPd bimetallic nanoparticles (NPs) on the enhancement of visible light performance is proposed to guide further improvement for other desirable functional materials. Ó 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Semiconductor photocatalytic materials have attracted considerable attention due to their potential application in solving current issues such as globe warming, energy conservation and environmental pollution [1–4]. Among various

semiconductor photocatalysts, titanium dioxide (TiO2) is considered as one of the most promising photocatalysts in H2 production [5–7], water purification [8,9] and air purification [10,11] owing to its excellent photocatalytic performance, high stability, low cost and nontoxicity. However, the light response range and the photocatalysis activity of TiO2 are limited, owing

* Corresponding author at: State Key Laboratory of Heavy Oil Processing, China University of Petroleum Beijing, No. 18 Fuxue Road, Beijing 102249, People’s Republic of China. E-mail address: [email protected] (L. Ge). http://dx.doi.org/10.1016/j.carbon.2015.02.070 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved.

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to its large band gap (3.2 eV). Therefore, designing and exploring photocatalysts with higher efficiency has attracted intensive efforts in the field of photocatalysis. Recently, graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst has attracted interest for its hopeful applications in H2 production, decomposition of organic pollutants, and photo-synthesis under visible light irradiation [12,13]. However, high recombination rate of photoexcited charge carriers and low separation efficiency are limiting the application of g-C3N4. Many attempts have been made to improve the photocatalytic performance of g-C3N4, such as metal deposition (Au [14], Ag [15], Pd [16], CuTCPP [17]), nonmetal doping (S [18], g-PAN [19], CdS [20]), semiconductor synergic effect (BiWO6 [21], LaVO4 [22], Bi2O2CO3 [23], BiPO4 [24], ZnO [25]) and preparation of lamellar structure g-C3N4 [26–28]. Recently, efforts have been devoted to design of carbon nitride, which can control electronic structure and enhanced optoelectronic conversion [29–30]. Among various strategies, the rapid separation-transfer-transformation of photo-generated charge carriers is a key issue which should be addressed. It has been suggested that the presence of monometallic NPs might effectively capture photogenerated electrons, which potentially prolongs the lifetime of radicals and may effectively improve the catalytic efficiency [14–16]. Compared to monometallic nanocatalysts, bimetallic nanomaterials show greater potential in catalytic applications due to their unique microstructures and enhanced photocatalytic performance. Bimetallic catalytic systems can potentially achieve chemical transformations that can hardly be accomplished by monometallic catalysts, which can be attributed to the reason that different components of the catalysts have a particular function in the overall reaction mechanism [31,32]. It is observed that the activity, selectivity and resistance to poisoning of the metal catalysts can be drastically influenced by the presence of a second metal component [33]. Various bimetallic alloy NPs with improved properties have been reported, such as AuPt [34], PtNi [35], AuCu [36] and PtSn [37]. Among them, AuPd bimetallic NPs have been explored as catalytic materials for a variety of reactions, including formic acid dehydrogenation [38], photocatalytic benzyl alcohol oxidation [39], photocatalytic Suzuki-coupling reaction [40], benzene oxidation [41], phenol photodecomposition [42] and CO oxidation [43]. In this study, for the first time we report the synthesis of AuPd/g-C3N4 photocatalyst via facile impregnation method for photocatalytic H2 evolution under visible light irradiation. The activity of g-C3N4 can be significantly enhanced by adding the monodisperse AuPd bimetallic NPs. The 0.5 wt% AuPd/ gC3N4 shows the highest H2 evolution activity of 326 lmolÆh 1 g 1, which is 3.5 times higher than that of Au/gC3N4 and 1.6 times than that of Pd/g-C3N4 under the same conditions. The effects of AuPd alloy NPs contents on the light absorption, charge transfer process and photocatalytic activity were investigated in detail, and the photocatalytic mechanism for enhanced H2 evolution activity of this composite was also discussed. The work may provide more insight into synthesizing novel hybrid photocatalytic materials with high activities applications in solar energy conversion and utilization.

2.

Experimental

2.1.

Synthesis of the photocatalysts

All chemicals were analytical grade and used without further treatment. In a typical procedure, 1 mM HAuCl4Æ3H2O and PdCl2 aqueous solution were prepared as stock solutions. Fresh aqueous solutions of 0.1 M NaBH4 and poly vinyl alcohol (PVA) (1 wt% aqueous solution, Aldrich, MW = 10,000, 80% hydrolyzed) were also prepared. The AuPd alloy NPs were prepared according to references with slight modification [44]. The PdCl2 and HAuCl4 stock solution were mixed in the desired ratio and the required amount (1 wt%) of a PVA solution was added (PVA/ (Au + Pd)=1.2, weight ratio), the fresh NaBH4 solution (NaBH4/(Au + Pd) = 5, molar ratio) was then added to form a dark-brown solution. The solution was stirred for 30 min. In the same, AusPdc and AucPds nanoparticles were also prepared by varying the addition sequence during the colloidal preparation. The g-C3N4 was prepared though the reported method of our group [45,46]. The metal-free g-C3N4 powders were prepared by heating cyanamide in an alumina combustion boat to 550 °C for 4 h at a heating rate of 2 °C min 1. The products was collected and ground into a powder. A light yellow powder of g-C3N4 was obtained. The preparation of AuPd/g-C3N4 composite photocatalysts is described as follows: the as prepared g-C3N4 was added to solution containing an appropriate amount of AuPd NPs. After stirring another 30 min, the product was dried in a vacuum environment at 40 °C. The resulting powder was collected and calcined in air at 300 °C for 1 h in a muffle furnace. Then, AuPd/g-C3N4 with different amounts of AuPd bimetallic was obtained. The weight percentages of AuPd in the initial photocatalyst precursors were 0.1 wt%, 0.3 wt%, 0.5 wt%, 1.0 wt%, 3.0 wt%, 5.0 wt%, respectively. The product was centrifuged, washed with water and ethyl alcohol, and then dried in a vacuum oven at 40 °C for 24 h. For comparison, 0.5 wt% Au/g-C3N4, 0.5 wt% Pd/gC3N4 and 0.5 wt% (Au + Pd)/ g-C3N4 were prepared.

2.2.

Characterization

The crystal structure of the sample was investigated using Xray diffraction (XRD; Bruker D8 Advance, X-ray diffractometer) with CuKa radiation at a scan rate of 4 min 1. The acceleration voltage and the applied current were 40 kV and 40 mA, respectively. The morphology of the samples was examined by High resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20; accelerating voltage = 200 kV). UV– vis diffuse reflection spectroscopy (DRS) was performed on a Shimadzu UV-4100 spectrophotometer using BaSO4 as the reference material. The X-ray photoelectron spectroscopy (XPS) was measured in a PHI 5300 ESCA system. The beam voltage was 3.0 eV, and the energy of Ar ion beam was 1.0 keV. The binding energies were normalized to the signal for adventitious carbon at 284.8 eV. The electron spin resonance (ESR) signals of spin-trapped oxidative radicals were obtained on a Bruker model ESR JES-FA200 spectrometer equipped with a

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quanta-Ray Nd:YAG laser system as the light source with a UV-cutoff filter (k P 400 nm).

2.3.

Photocatalytic activity

The photocatalytic H2 evolution experiments were performed in Perfectlight Labsolar IIIAG system with a 300 ml quartz reactor at 4 °C. The reactor is connected to a low-temperature thermostat bath. PLS-SXE 300UV Xe arc lamp with a UV-cutoff (P400 nm) filter was used as the light source. The light intensity employed was 35 mW/cm2. In a typical photocatalytic experiment, 50 mg of photocatalyst powder was suspended in a 100 mL of aqueous solution containing 10 vol.% TEOA. Before photocatalytic experiments, the reaction vessel was evacuated for 30 min to remove the dissolved oxygen to ensure that the vacuum conditions. The products were analyzed by chromatography (Beifen 3420A, high purity Argon as a carrier gas, 99.999%) equipped with a thermal conductivity detector.

2.4.

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SPV measurements

The surface photovoltage (SPV) measurement was carried out on the basis of the lock-in amplifier [47]. The measurement system consists of a source of monochromatic light, a lockin amplifier (SR830, Stanford research systems, Inc.) with a light chopper (SR540, Stanford research systems, Inc.), and a simple chamber. The monochromatic light is provided by passing light from a 500 W xenon lamp (CHFXQ500 W, global xenon lamp power) though a grating mono-chromator (Omni-5007, No.09010, Zolix), which chopped with a frequency of 24 Hz. All the measurements were operated under ambient pressure at room temperature.

3.

Results and discussion

3.1.

Characterization of Au-Pd/g-C3N4 composite samples

The X-ray diffraction patterns of as-prepared Au/g-C3N4, Pd/g-C3N4, AuPd/g-C3N4 photocatalysts are shown in Fig. 1.

The pure polymeric g-C3N4 (Fig. 1a) has two distinct peaks at 13.1° and 27.4°, which can be indexed for graphitic materials as the (1 0 0) and (0 0 2) peaks in JCPDS 87-1526. The enlarged view of XRD patterns (from 32° to 42°) reveal the presence of face-centered cubic (fcc) Au nanocrystals (JCPDS, No. 65-2870) and Pd nanocrystals (JCPDS, No. 652867) in the samples. The sample exhibit diffraction peaks at approximately 38.2° (Fig. 1d, inset) and 40.1° (Fig. 1e, inset), which can be indexed to the Au (1 1 1) and Pd (1 1 1) diffraction planes, respectively. However, the (1 1 1) diffraction peak of AuPd bimetallic NPs is located at 38.9° (Fig. 1c, inset) [48]. With the increase of the AuPd bimetallic NPs doping amount, the diffraction peaks gradually become more obvious at 38.9° (Fig. 1a–c). This result indicates that the prepared NPs are homogeneous AuPd bimetallic alloys. HRTEM was further applied to investigate the microstructure of composite samples. Fig. 2(a–c) illustrates HRTEM images with high magnifications for the 0.5 wt% AuPd/gC3N4 photocatalyst. After the chemical reduction deposition of AuPd species on g-C3N4, the AuPd bimetallic species was deposited as nanoparticles on the g-C3N4 with high dispersion. The monodisperse AuPd bimetallic NPs are decorated on the surface of g-C3N4 to form intimate interfaces facilitating charge transfer between the AuPd and g-C3N4. As for the as-prepared nanoparticles, AuPd NPs are found to be aggregating in Fig. 2d. However, AuPd NPs are highly dispersed on the surface of g-C3N4. The AuPd alloy NPs was prepared by controlling the metal sate solution identity and adding sequences. Fig. 2d shows the arrangement of the AuPd NPs via HRTEM, where AuPd NPs is seen as having various orientations and lattice spacing. By carefully measuring the lattice parameters using a Digital micrograph and comparing with the data in JCPDS, five different kinds of lattice fringes were clearly observed. Only one interplanar crystal spacing with d = 0.236 nm can be ascribed to the (1 1 1) crystallographic plane of Au NPs (JCPDS 65-2870). There is only a trace of Au NPs decorated on the surface of g-C3N4. However, no diffraction peaks corresponding to Pd NPs can be observed in Fig. 2d. Therefore, it is deduced that the rest of intervals belong to AuPd alloys. It is expected that the AuPd NPs can

C3N4(002) (e) 0.5%Pd (d) 0.5%Au

C3N4(100)

(c) 3.0%AuPd 32

34

36 38 40 2θ(degree)

42

(a) 0%AuPd

AuPd

(b) 0.5%AuPd

(c) 3.0%AuPd

Au(111)

(d) 0.5%Au

Pd(111)

(e) 0.5%Pd

10

20

30

40

50

60

2θ (Degree)

Fig. 1 – XRD patterns of AuPd/g-C3N4 composite photocatalysts. (A color version of this figure can be viewed online.)

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Fig. 2 – HRTEM images of the as-prepared (a–c) AuPd/g-C3N4 and (d) AuPd NPs samples.

improve the separation of photogenerated charge carriers and the photocatalytic activity. The UV–vis diffuse reflectance spectra of as-prepared AuPd/g-C3N4 composite samples were investigated and shown in Fig. 3. The pure g-C3N4 sample shows absorption

0.8

Relative intensity (a.u.)

3.0% AuPd 1.0% AuPd

0.6

0.5% Pd 0.5% Au

0.4

0.2

0.0

0.5% AuPd 0% AuPd

450 500 550 600

350

400

450

500

550

600

Wavelength (nm) Fig. 3 – UV–vis diffuse reflectance spectra of pure g-C3N4, Au/ g-C3N4, Pd/g-C3N4 and AuPd/g-C3N4 composite samples (inset: Au NPs absorption position). (A color version of this figure can be viewed online.)

from the UV through the visible range up to 462 nm, the steep shape of the spectrum indicates that the visible light absorption is ascribed to the band gap transition, corresponding to a band gap of 2.7 eV for pure g-C3N4 [12]. After introducing AuPd bimetallic NPs, the AuPd/g-C3N4 samples show the similar absorption edge in shape and slightly enhance light absorption compared with the pure g-C3N4 in the visible region. The absorption intensity of the as-prepared samples strengthen with increasing AuPd NPs contents, which agrees with the color of the prepared samples that vary from light yellow to dark gray. Compared with pure g-C3N4 photocatalyst, the Au/g-C3N4 composite materials exhibit a slight light absorption centered at 550 nm which could be attributed to the surface plasmon resonance (SPR) effect of Au species, while the Pd/g-C3N4 sample does not illustrate clear SPR signals in the absorption spectrum. The DRS results also indicate that the chemical deposited AuPd bimetallic NPs could improve the visible light absorption, and hence is expected to increase photocatalytic performance. In order to further verify the deposition of AuPd bimetallic alloy NPs in the g-C3N4, the 0.5 wt% AuPd/g-C3N4 sample was examined by X-ray photoelectron spectroscopy (XPS) analysis. Fig. 4a and b presents the high-resolution XPS spectra of C1s sand N1s in the AuPd/g-C3N4 sample. The C1s shows two distinct peaks at 284.8 and 288.4 eV. The first peak can be ascribed to carbon species adsorbed on the surface of gC3N4; the second binding energy peak belongs to carbon

399.0 eV

288.4 eV

(a) C1s

284.8 eV

282

285

288

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Relative intensity (a.u.)

Relative intensity (a.u.)

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291

294

(b) N1s 399.13 eV

398.53 eV 400.58 eV

393

396

399

402

Relative intensity (a.u.)

Relative intensity (a.u.)

(c) Au4f 83.1 eV 86.7 eV

81

84

87

90

405

Binding energy (eV)

Binding energy (eV)

93

Binding energy (eV)

(d) Pd3d

Pd 3d5/2

334.3 eV

335.1 eV

Pd 3d 3/2 340.1 eV

339.8 eV

332

334

336

338

340

342

344

Binding energy (eV)

Fig. 4 – XPS spectra of AuPd/g-C3N4 sample: (a) C 1s; (b) N 1s; (c) Au 4f; (d) Pd 3d. (A color version of this figure can be viewed online.)

atoms in g-C3N4 lattice [49]. The N1s XPS binding energy can be deconvoluted into three peaks located at 398.53, 399.13 and 400.58 eV, which represent nitrogen atoms in the C@NAC, NA(C)3 and CANAH functional groups in the polymeric gC3N4 structures [50]. As shown in Fig. 4c, the Au 4f7/2 and Au 4f5/2 peaks in the spectrum of AuPd/g-C3N4 locate at 83.1 and 86.4 eV, respectively, which exhibit slight downshift as compared to that of Au/TiO2 [48]. Furthermore, the Pd 3d spectra of AuPd/g-C3N4 could be fitted into asymmetric peaks, suggesting the existence of two states of Pd species. The Pd 3d5/2 peak at 334.3 eV and Pd 3d3/2 peak at 339.8 eV are attributed to the metallic Pd0, where the binding energy peaks shown at 335.1 and 340.1 eV are originated form of Pd2+, respectively [51]. The result indicated that the oxide species formed on the surface of Pd. The binding energy peaks of Pd 3d of AuPd/g-C3N4 showed a slight downshift compared to the peaks in the Pd/TiO2 sample. The shift of binding energy for Au and Pd in the AuPd/g-C3N4 samples may be ascribed to the charge transfer between them due to the formation of AuPd bimetallic alloy.

3.2.

Photocatalytic H2 evolution activity

The photocatalytic activities of the AuPd/g-C3N4 composite photocatalysts were evaluated by H2 evolution via water splitting in triethanolamine (TEOA) solutions under visible light irradiation. Fig. 5 displays the H2 evolution histogram over AuPd/g-C3N4 photocatalysts with different AuPd NPs

Fig. 5 – Photocatalytic H2 evolution over AuPd/g-C3N4 composite samples with different AuPd NPs contents under visible light: (a) 0.1%; (b) 0.3%; (c) 0.5%; (d) 1.0%; (e) 3.0%; and (f) 5.0%. (A color version of this figure can be viewed online.) contents. The pure g-C3N4 sample without AuPd loading shows distinctly lower H2 production rate than other samples. After introducing 0.1 wt% of AuPd NPs, the catalytic activity of

Rate of H2 evolution (μmol·h-1·g-1 )

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(d) (c) 326 316 (e) 288.2

350 300 250 200 150

(b) 78.2

100

(i) (h) 282.8 294.4 AucPds AusPdc (f) (j) 226.4 202.4 Pd (l) (g) 131.4 155.8 (k) (Au+Pd) 94.6 Au

50 0

(a) (0 0.1 0.3 0.5 1.0 3.0 5.0) 0.5 0.5 0.5 0.5 0.5 AuPd/g-C3 N4 Samples

Fig. 6 – Rate of H2 evolution over AuPd/g-C3N4 composite samples with different AuPd contents, Pd/g-C3N4 and Au/gC3N4 under visible light: (a) 0%; (b) 0.1%; (c) 0.3%; (d) 0.5%; (e) 1.0%; (f) 3.0%; (g) 5.0%; (h) 0.5% AusPdc; (i) 0.5% AucPds; (j) 0.5% Pd; (k) 0.5% Au and (l) 0.5% (Au + Pd). (A color version of this figure can be viewed online.)

the separation efficiency of the photogenerated electron–hole pairs. However, a further increase of AuPd NPs content leads to a decrease of H2 evolution activity. The decrease can be related to the increase in the opacity of the composite samples (Fig. 3). The introduction of higher ratio of black AuPd NPs can lead to shielding of the active sites on the photocatalyst surface, and also block the transmission of light through the reaction solution. As a consequence, a suitable content of AuPd NPs is crucial for optimizing the photocatalytic performance of AuPd/g-C3N4 composites. To further examine the role of AuPd Alloys in the catalytic process, the photocatalytic activities of Au/g-C3N4 and Pd/gC3N4 samples were conducted and compared. Fig. 6 illustrates the rate of H2 evolution over different samples. After Au and Pd NPs separately loaded on g-C3N4, the catalytic activities were significantly enhanced, and reached to 94.6 and 202.4 lmol h 1 g 1. As shown in Figs. 5 and 6, the 0.5 wt% AuPd/g-C3N4 sample shows the highest photocatalytic performance with H2 evolution rate of 326 lmol h 1 g 1, about 3.5 and 1.6 times higher than 0.5 wt% Au/g-C3N4 and 0.5 wt% Pd/g-C3N4, which indicates that the AuPd bimetallic nanoparticles play an important role in enhancement of H2 evolution activity in the AuPd/g-C3N4 composite photocatalysts. For comparison, the photocatalytic activity of 0.5 wt% (Au + Pd)/

(a)

320

Relative intensity (a.u.)

240 160

Dark 80 0

Light

-80 -160 319

321

323

325

327

Magnetic (mT)

320

H2 evolution is significantly increased to 78.2 lmol h 1 g 1. With continue to increase the AuPd doping amount, the photocatalytic H2 evolution on AuPd/g-C3N4 is further enhanced. The 0.5 wt% AuPd/g-C3N4 shows the highest H2 evolution rate of 326 lmol h 1 g 1. The higher photocatalytic activity of AuPd/g-C3N4 sample may be attributed to the reason that the interfaces between AuPd and g-C3N4 effectively promote

240

Relative intensity (a.u.)

Fig. 7 – Cycling runs for the photocatalytic H2 evolution in the presence of 0.5 wt% AuPd/g-C3N4 composite sample under visible light irradiation (>400 nm); photocatalytic H2 evolution over 0.5 wt% AusPdc, 0.5 wt% AucPds, 0.5 wt% Pd/ g-C3N4, 0.5 wt% Au/g-C3N4 and 0.5 wt% (Au + Pd)/g-C3N4. (A color version of this figure can be viewed online.)

(b)

160

Dark

80 0

Light

-80 -160 319

321

323

325

327

Magnetic (mT) Fig. 8 – ESR spectra of AuPd/g-C3N4 photocatalysts: (a) O2 (b) OH . (A color version of this figure can be viewed online.)

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g-C3N4 was also examined in the same system under the same conduction, and the sample only had an activity of 131.4 lmol h 1 g 1 which was much lower than AuPd/g-C3N4. Due to the importance of the stability of a photocatalyst for its practical application, the photocatalytic stability of the 0.5 wt% AuPd/g-C3N4 catalyst was further investigated via photocatalytic H2 evolution cycling experiments. Fig. 7(1–4) presents the H2 evolution curve in cycling photocatalytic run. This results show that the composite sample does not display obvious decrease of H2 production activity after irradiated for 24 h, indicating that the AuPd/g-C3N4 photocatalyst has sufficient stability for photocatalytic H2 production.

3.3.

Photocatalytic mechanism investigation

To verify the photocatalytic mechanism, the electron spin resonance (ESR) technique was applied in our experiments. The ESR technique can be used to detect free radicals in reaction systems. To elucidate the main reactive species responsible for the photocatalytic reaction over the g-C3N4 photocatalyst, a series of quenchers were employed to scavenge the relevant reactive species. Typically, DMPO (5,5dimethyl-1-dimethy N-oxide) was generally used as a radical scavenger due to the generation of stable free radicals, DMPOO2 or DMPO-OH . Fig. 8 shows ESR spectra measured with light irradiation over the 0.5 wt% AuPd/g-C3N4 photocatalyst at room temperature in air. As shown in Fig. 8, there is no ESR signal in the dark, a gradual evolution of ESR peaks for DMPO-O2 adducts was observed under visible light irradiation. In contrast, no signals of DMPO-OH adducts were detected under otherwise identical conditions. Bai et al. [26] reported a

37

similar ESR results that only superoxide radical was produced by the photo-activated g-C3N4 nanorods under visible light irradiation. Refer to our previous experimental data [52], the ESR results confirm that O2 radicals exist in the AuPd/gC3N4 system under visible light irradiation. Based on the results of the structure characterizations and the visible light photocatalytic activities of AuPd/g-C3N4 samples, a possible mechanism for photocatalytic H2 evolution on AuPd/g-C3N4 catalyst is proposed and illustrated in Fig. 9. Under visible light irradiation, the polymeric semiconductor g-C3N4 absorbs photons and excites electron–hole pairs. However, the photogenerated electrons and holes are likely to recombine without co-catalyst. The significant enhancement of H2 evolution can be attributed to synergistic effect between g-C3N4 and AuPd alloys. By introducing AuPd alloys, the two materials closely bound together and form interfaces. The AuPd alloys show higher electron-capture capability and can stimulate electron transfer from g-C3N4 towards AuPd alloys surface due to the lower Fermi level. Moreover, the CB and VB edge potentials of polymeric g-C3N4 are determined at 1.13 and +1.57 eV [46]. Therefore, in the AuPd/g-C3N4 system, the photogenerated electrons in the CB of the g-C3N4 transfer to AuPd co-catalysts via contacting interfaces, endowing the conduction band electrons higher mobilities and promoting the separation of electron–hole pairs. The holes in the VB of g-C3N4 are consumed by TEOA sacrificial regents. Therefore, the recombination process of the electron–hole pairs is effectively inhibited, resulting in obviously improvement of H2 production for the AuPd/g-C3N4 photocatalyst. In order to verify the above analysis, it is necessary and important to fully understand the behavior of photogenerated

Fig. 9 – The schematic illustration for electron charge transfer and H2 evolution mechanism under visible light irradiation. (A color version of this figure can be viewed online.)

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Photovoltage ( μV)

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sample

conducng layer

ITO

insulated rubber tape

24

0.5%AuPd 1.0%AuPd

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0.1%AuPd 0% AuPd 3.0%AuPd

8

0 320

370

420

470

520

Wavelength (nm) Fig. 10 – SPV spectra of AuPd/g-C3N4 photocatalyst with different AuPd NPs content. Inset: Schematic diagram of SPV measurement configuration. (A color version of this figure can be viewed online.)

NPs by a mixing and heating method. The doping of AuPd nanoparticles did not affect the morphology and crystal structure of g-C3N4 photocatalysts. The composite photocatalysts exhibited enhanced photocatalytic activity in the presence of small AuPd NPs, and the highest efficiency was observed with 0.5 wt% AuPd/g-C3N4 sample. Superoxide radicals (O2 ) are the main oxidative species for AuPd/g-C3N4 sample, and the presence of AuPd could increase the interfacial charge transfer and inhibit the recombination of electron– hole pairs. A possible photocatalytic mechanism is proposed based on the experimental results. The surface photovoltage technique can be applied to investigate he photoelectric processes of AuPd/g-C3N4 effectively, leading to a better understanding of photocatalytic behavior to exploration of novel composite photocatalysts with advanced functions. Therefore, the AuPd bimetallic NPs are a promising co-catalyst material which can be potentially used for photocatalytic hydrogen evolution.

Acknowledgements charges at interfaces [47]. The surface photovoltage spectroscopy (SPV) technique was employed to reveal the kinetic behaviors of the photogenerated charge carriers in the as-prepared AuPd/g-C3N4 samples, which could be beneficial for understanding the effect of AuPd nanoparticles. The signal of surface photovoltage (SPV) can be attributed to the variations of surface potential barriers during the light irradiation, which can identify the light-responsive wavelength range and the separation efficiency of the electron–hole pairs in the photocatalysts. Fig. 10 illustrates the SPV spectra of AuPd/g-C3N4 with different AuPd bimetal contents. The SPV signal ranging from 300 to 450 nm is observed for pure g-C3N4. This is representative feature of n-type semiconductor in SPV, where positive charges from inner semiconductor migrate to the surface. Compared to the pure g-C3N4, the AuPd/g-C3N4 (0.5 wt%) exhibits stronger SPV signal, indicating that the photogenerated electron–hole pairs are separated more effectively. The enhanced separation efficiency of electron–hole pairs can be attributed to the efficient charge transfer between the interfaces of AuPd NPs and g-C3N4 in the composite samples. The SPV signal intensity varies as the AuPd NPs content in the AuPd/g-C3N4 sample from 0 wt% to 3.0 wt%, with an optimal signal obtained at 0.5 wt%. The enhanced SPV signal intensity illustrates that the introduction of a suitable content of AuPd NPs is beneficial to the separation of photogenerated electron–hole pairs in g-C3N4. Nevertheless, if excessive AuPd NPs are introduced, the light absorption by g-C3N4 is reduced as more AuPd NPs prevent the light from reaching the g-C3N4 surface, leading to the decrease of SPV signal in comparison to that for 0.5 wt% AuPd/g-C3N4 sample. More importantly, the SPV spectra are consistent with the results of photocatalytic H2 evolution, which can explain the origin of enhanced separation efficiency of photogenerated electron–hole pairs in the AuPd/g-C3N4 composites.

4.

Conclusions

In summary, novel visible light induced AuPd/g-C3N4 photocatalysts were synthesized via introducing AuPd bimetallic

This work was financially supported by the National Science Foundation of China (Grant No. 21003157 and 21273285), Beijing Nova Program – China (Grant No. 2008B76), and Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-06-20).

R E F E R E N C E S

[1] Asahi R, Morikawa T, Ohwaki T, Aoki K, Tage Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001;293:269–71. [2] Zhang RY, Elzatahry AA, Al-Deyab SS, Zhao DY. Mesoporous titania: from synthesis to application. Nano Today 2012;7:344–66. [3] Qu YQ, Duan XF. Progress challenge and perspective of heterogeneous photocatalyst. Chem Sov Rev 2013;42:2568–80. [4] Juan CC, Rafael L. Heterogeneous photocatlaytic nanomaterials: prospects and challenges in selective transformations of biomass-derived compounds. Chem Sov Rev 2014;43:765–78. [5] Andreev YG, Panchmatia PM, Liu Z, Parker SC, Islam MS, Bruce PG. The shape of TiO2-B NPs. J Am Chem Soc 2014;136:6306–12. [6] Chen XB, Shen SH, Guo LJ, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110:6503–70. [7] Seger B, Pedersen T, Laursen AB, Vesborg P, Hansen O, Chorkendorff I. Using TiO2 as a conductive protective layer for photocathodic H2 evolution. J Am Chem Soc 2013:1057–64. [8] Kim J, Monllor-satoca D, Choi W. Simultaneous production of hydrogen with the degradation of organic pollutants using TiO2 photocatalyst modified with dual surface components. Energy Environ Sci 2012;5:7647–56. [9] Su JW, Zhang YX, Xu SC, Wang SA, Ding HL, Pan SS, et al. Highly efficient and recyclable triple-shelled Ag@Fe3O4@SiO2@TiO2 photocatalysts for degradation of organic pollutants and reduction of hexavalent chromium ions. Nanoscale 2014;6:5181–92. [10] Asahi R, Morikawa T, Irie H, Ohwaki T. Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst:

CARBON

[11] [12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

9 2 (2 0 15 ) 3 1–40

designs, developments, and prospects. Chem Rev 2014;114(19):9824–52. Sang LX, Zhao YX, Burda C. TiO2 NPs ad functional building blocks. Chem Rev 2014;114:9283–318. Wang XC, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson JM, et al. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat Mater 2009;9:76–80. Zheng Y, Lin LH, Ye XJ, Guo FS, Wang XC. Helical graphitic carbon nitrides with photocatalytic and optical activities. Angew Chem Int Ed 2014;53:11926–30. Chen LC, Zeng XT, Si P, Chen YM, Chi YW, Kim DH, et al. Gold-nanoparticle-graphite like C3N4 nanosheet nanohybrids used for electrochemiluminescent immunosensor. Anan Chem 2014;86:4188–95. Bai XJ, Zong RL, Li CX, Liu D, Liu YF, Zhu YF. Enhancement of visible photocatalytic activity via Ag@C3N4 core-shell plasmonic composite. Appl Catal B Environ 2014;147:82–91. Chang C, Fu Y, Hu M, Wang CY, Shan GQ, Zhu LY. Photodegradation bisphenol A by highly stable palladiumdoped mesoporous graphite carbon nitride (Pd/mpg-C3N4) under simulated solar light irradiation. Apppl Catal B Environ 2013;142–143:553–60. Chen DM, Wang KW, Hong WZ, Zong RL, Yao WQ, Zhu YF. Visible light photoactivity enhancement via CuTCPP hydridized g-C3N4 nanocomposite. Appl Catal B Environ 2015;166–167:366–73. Liu G, Niu P, Sun CH, Smith SC, Chen ZG, Lu GQ, et al. Unique electronic structure induced photoreactivity of sulfur-doped graphitic C3N4. J Am Chem Soc 2010;132:11642–8. Ge L, Han CC, Liu J. In situ synthesis and enhanced visible light photocatalytic activities of novel PANI/g-C3N4 composite photocatalysts. J Mater Chem 2012;22(23):11843–50. Ge L, Zuo F, Liu JK, Ma Q, Wang C, Sun DZ, et al. Synthesis and efficient visible light photocatalytic hydrogen evolution of polymeric g-C3N4 coupled with CdS quantum dots. J Phys Chem C 2012;116:13708–14. Tian YL, Chang BB, Lu JL, Fu J, Xi FN, Dong XP. Hydrothermal synthesis of graphitic carbon nitride-Bi2WO6 heterjunction with enhanced visible light photocatalytic activities. ACS Appl Mater Interfaces 2013;5:7079–85. He YM, Cai J, Zhang LH, Wang XX, Lin HJ, Teng BT, et al. Comparing two new composite photocatalysts, t-LaVO4/gC3N4 and m-LaVO4/g-C3N4 for their structures and performances. Ind Eng Chem Res 2014;53:5905–15. Tian N, Huang HW, Guo YX, He Y, Zhang YH. A g-C3N4/ Bi2O2CO3 composite with high visible-light-driven photocatalytic activity for rhodamin B degradation. App Surf Sci 2014;322:149–54. Pan CS, Xu J, Wang YJ, Li D, Zhu YF. Dramatic activity of C3N4/ BiPO4 photocatalyst with core/shell structure formed by selfassembly. Adv Funct Mater 2012;22:1518–24. Chen DM, Wang KW, Xiao DG, Zong RL, Yao WQ, Zhu YF. Significantly enhancement of photocatalytic performances via core-shell structure of ZnO@mpg-C3N4. Applied Catal B Environ 2014;147:554–61. Yang SB, Gong YJ, Zhang JS, Zhan L, Ma LL, Fang ZY, et al. Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Adv Mater 2013;25:2452–6. Huang ZA, Sun Q, Lv KL, Zhang ZH, Li M, Li B. Effect of contact interface between TiO2 and g-C3N4 on the photoreactivity of g-C3N4/TiO2 photocatalyst: (0 0 1) vs (1 0 1) facets of TiO2. Appl Catal B Environ 2015;164:420–7. Bai XJ, Wang L, Zong RL, Zhu YF. Photocatalytic activity enhanced via g-C3N4 nanoplates to nanorods. J Phys Chem C 2013;117:9952–61.

39

[29] Zhang YJ, Mori T, Niu L, Ye JH. Non-convalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion. Energy Environ Sci 2011;4:4517–21. [30] Zhang XM, Wang AZ, Zhao MW. Spin-gapless semiconducting graphitic carbon nitrides: a theoretical design from first principles. Carbon 2015;84:1–8. [31] Liu XW, Wang DS, Li YD. Synthesis and catalystic properties of bimetallic nanomaterials with various architectures. Nano Today 2012;7:448–66. [32] Mu RT, Fu Q, Xu H, Zhang H, Huang YY, Jiang Z, et al. Synergetic effect of surface and subsurface Ni species at PtNi bimetallic catalysts for CO oxidation. J Am Chem Soc 2011;133:1978–86. [33] Sneed BT, Young AP, Jalalpoor D, Gloden MC, Mao SJ, Jiang Y, et al. Shaped Pd-Ni-Pt core-sandwich-shell NPs: influence of Ni sandwich layers on catalytic electrooxidations. ACS Nano 2014;8(7):7239–50. [34] Xu JB, Zhao TS, Liang ZX, Zhu LD. Facile preparation of AuPt alloy nanoparticles for organometallic complex precursor. Chem Mater 2008;20:1688–90. [35] Liang GF, He LM, Arai M, Zhao FY. The Pt-enriched PtNi alloy surface and its excellent catalytic performance in hydrolytic hydrogenation of cellulose. ChemSusChem 2014;7:1415–21. [36] Motl NE, Bondi JF, Schaak RE. Synthesis of Colloidal Au-Cu2S heterodimers via chemically triggered phase segregation of AuCu nanoparticles. Chem Mater 2012;24:1552–4. [37] Kong C, Min SX, Lu GX. Robust Pt–Sn alloy decorated graphene nanohybrid cocatalyst for photocatalytic hydrogen evolution. Chem Comm 2014;50:9281–3. [38] Mentin O, Sun XL, Sun SH. Monodisperse gold–palladium alloy nanoparticles and their composition-controlled catalysis in formic acid dehydrogenation under mild conditions. Nanoscale 2013;5:910–2. [39] Jiang TT, Jia CC, Zhang LC, He SR, Sang YH, Li HD, et al. Gold and gold–palladium alloy nanoparticles on heterostructured TiO2 nanobelts as plasmonic photocatalysts for benzyl alcohol oxidation. Nanoscale 2015;7:209–17. [40] Xiao Q, Sarina S, Jaatinen E, Jia JF, Arnold DP, Liu HW, et al. Efficient photocatalytic Suzuki cross-coupling reactions on Au–Pd alloy nanoparticles under visible light irradiation. Green Chem 2014;16:4272–85. [41] Tabakova T, Ilieva L, Petrova P, Venezia AM, Acdeev G, Zanella R, et al. Complete benzene oxidation over mono and bimetallic Au-Pd catalysts supported on Fe-modified ceria. Chem Eng J 2015;260:133–41. [42] Su R, Tiruvalam R, He Q, Dimitratos N, Kesavan L, Hammond C, et al. Promotion of phenol photodecomposition over TiO2 using Au, Pd, and Au-Pd nanoparticles. ACS Nano 2012;6:6284–97. [43] Gao F, Wang YL, Goodman W. CO oxidation over AuPd (1 0 0) from ultrahigh vacuum to near-atmospheric pressures: the critical role of contiguous pd atoms. J Am Chem Soc 2009;131:5734–5. [44] Su R, Tiruvalam R, Logsdail AJ, He Q, Downing CA, Jensen MT, et al. Designer Titania-supported AuPd nanoparticles for efficient photocatalytic hydrogen production. ACS Nano 2014;8:3490–7. [45] Ge L, Han CC, Xiao XL, Guo LL, Li YJ. Enhanced visible photocatalytic hydrogen evolution of sulfur-doped polymeric g-C3N4 photocatalysts. Mater Res Bull 2013;48:3919–25. [46] Ge L, Han CC, Xiao XL, Guo LL. In situ synthesis of cobaltphosphate (Co–Pi) modified g-C3N4 photocatalysts with enhanced photocatalytic activities. Appl Catal B Environ 2013;142–143:414–22. [47] Zhang X, Zhang LZ, Xie TF, Wang DJ. Low-temperature synthesis and high visible-light-induced photocatalytic

40

CARBON

9 2 ( 2 0 1 5 ) 3 1 –4 0

activity of BiOI/TiO2 heterostructures. J Phys Chem C 2009;113:7371–8. [48] Cybula A, Priebe JB, Pohl MM, Sobczak JW, Schneider M, Jurek AZ, et al. The effect of aclcination temperature on structure and photocatalytic properties of Au/Pd nanoparticles supported on TiO2. Appl Catal B Environ 2014;152–153:202–11. [49] Ge L, Han CC. Synthesis of mwnts/g-C3N4 composite photocatalysts with efficient visible light photocatalytic hydrogen evolution activity. Appl Catal B Environ 2012;117– 118:268–74. [50] Ge L, Han CC, Xiao XL, Guo LL. Synthesis and characterization of composite visible light photocatalysts

MoS2-g-C3N4 with enhanced hydrogen evolution activity. Int J Hydrogen Energy 2013;38:6960–9. [51] Li TQ, Zhou HH, Huang JQ, Yin JL, Chen ZX, Liu D, et al. Facile preparation of Pd–Au bimetallic nanoparticles via in-situ self-assembly in reverse microemulsion and their electrocatalytic properties. Colloid Surface A 2014;463:55–62. [52] Han CC, Ge L, Chen CC, Li YJ, Xiao XL, Zhang YN, et al. Novel visible light induced Co3O4-g-C3N4 heterojunction photocatalysts for efficient degradation of methyl orange. Appl Catal B Environ 2014;147:546–53.