Organic-inorganic composite of g-C3N4–SrTiO3:Rh photocatalyst for improved H2 evolution under visible light irradiation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 1 6 0 2 e1 1 6 1 0

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Organic-inorganic composite of g-C3N4eSrTiO3:Rh photocatalyst for improved H2 evolution under visible light irradiation Hyun Woo Kang, Sung Nam Lim, Dongsu Song, Seung Bin Park* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea

article info

abstract

Article history:

The organic-inorganic composite g-C3N4eSrTiO3:Rh was prepared for the first time as

Received 25 January 2012

a photocatalyst for hydrogen production and the resulting hydrogen evolution rate under

Received in revised form

visible light irradiation from aqueous methanol solution was measured. A high hydrogen

2 May 2012

evolution rate of 223.3 mmol h
1 was achieved by using 0.1 g of as-prepared photocatalyst

Accepted 5 May 2012

powder comprised of 20 wt.% g-C3N4 80 wt.% SrTiO3:Rh (0.3 mol%). The hydrogen evolution

Available online 5 June 2012

rate was greater than that obtained by SrTiO3:Rh (0.3 mol%) by a factor of 3.24. The quantum efficiency of as-prepared composite photocatalyst was 5.5% at 410 nm for hydrogen evolu-

Keywords:

tion. The high activity of the composite photocatalyst for hydrogen evolution stemmed from

Photocatalyst

its electronehole separation and transportation capabilities due to the hetero-junctions of

Hydrogen production

the organic-inorganic composite materials. The proposed mechanism for the electronehole

Visible light

separation and hydrogen evolution of the g-C3N4eSrTiO3:Rh composite under visible light

Spray pyrolysis

irradiation featured the reduced recombination of the photo-generated charge carriers. The

Composite

doping of Rh ions into the SrTiO3 has contributed to the high photocatalytic activity by forming a donor level from the valance band to the conduction band. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Hydrogen has become recognized as a potential clean energy source to solve future energy limitations in non-renewable fuels. Furthermore, the energy and raw materials required for hydrogen production must be renewable in order to optimize the process both economically and environmentally. Research has therefore focused on the development of a hydrogen production process utilizing solar energy for hydrogen evolution via the photocatalytic conversion of water. Various photocatalysts have been studied to maximize the hydrogen evolution rate under stable chemical conditions [1e9].

A composite material is a potential candidate for effectively enhancing the hydrogen evolution rate because it can improve the visible light absorption and separation efficiency of photo-generated electronehole pairs by taking advantage of the individual characteristics of each constituent comprising the composite. A composite of organic carbon and inorganic metal oxide has been suggested as a promising catalyst for enhancing the hydrogen evolution from an aqueous solution [10,11]. Recently, polymeric photocatalysts such as graphitic carbon nitrides (g-C3N4) have demonstrated high catalytic activity under visible light for the efficient production of hydrogen [12e15]. The catalytic ability of these nitrides can be

* Corresponding author. Tel.: þ82 42 350 3928; fax: þ82 42 350 3910. E-mail address: [email protected] (S.B. Park). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.020

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 1 6 0 2 e1 1 6 1 0

improved by combining them with SrTiO3 to enhance the hydrogen production rate, since the former can be used to adjust the electronic properties, while the latter can decompose water into hydrogen and oxygen without the application of an external bias potential [16e19]. Although the conduction band is sufficiently negative for the production of hydrogen, SrTiO3 has to be modified because its excessively large band gap energy prevents the effective absorption of the visible light. To overcome this drawback of SrTiO3, various transition and noble metals such as Rh, Cr, Sb, Mn, Ru and Ir have been utilized as dopants to reduce the band gap energy [1,8,9,19e27]. The preparation method of the photocatalyst can affect its photocatalytic activities, since its lattice structure, electron configuration and surface morphology are directly related to the preparation method [28e32]. Spray pyrolysis, with its ability to produce powders with uniform size and uniformly distributed constituents, can be used to prepare and modify the heterogeneous photocatalysts and create new functional powders [33e37]. In the present study, a composite of g-C3N4eSrTiO3:Rh was prepared and tested as a photocatalyst for the first time. The characteristics of the organic-inorganic heterogeneous composite powders with various amounts of polymeric compound (g-C3N4) in the as-prepared composite were analyzed. The amount of polymeric compound was optimized in terms of maximizing the photocatalytic activity for enhancing the hydrogen production rate under visible light irradiation. The mechanism for the photocatalytic activity of the as-prepared composite is also discussed.

2.

Experimental section

2.1.

Preparation of the photocatalyst

spray pyrolysis and of g-C3N4 powder prepared by heating melamine were mixed and then calcined at 673 K for 1 h in a muffle furnace. For comparison, as-prepared SrTiO3:Rh powder prepared by spray pyrolysis with no polymeric component was also calcined at 673 K for 1 h in a muffle furnace. In addition, SrTiO3:Rh (0.3 mol%) was also prepared by solid state reaction. The starting materials were SrCO3 (Aldrich, 98%), TiO2 (Junsei, 98.5%) and RhCl3 (Aldrich, 99.98%). These materials were mixed homogeneously in the prescribed ratio. The mixture was calcined at 1423 K for 20 h in air. Pt co-catalyst from aqueous H2PtCl6 (Aldrich, 99.9%) solution was loaded by photo-deposition onto the surfaces of all the photocatalyst powders.

2.2.

Characterization

The formation of the composite powders and inclusion of the dopant into the lattice were confirmed by X-ray diffraction (XRD, Rigaku, D/MAX-IIIC). The absorption band of the composite powder was analyzed by using a Fourier transform infrared (FT-IR) spectrometer (Bruker, Alpha-P). Diffuse reflectance spectra (DRS) of each sample were obtained by UVeViseNIR spectrophotometry (Shimadzu, UV-3101PC). BET surface areas, pore sizes, and pore volumes of each sample were measured from nitrogen adsorptionedesorption isotherms (Micromeritics, ASAP 2010). Surface morphologies and particle sizes were characterized by field-emission scanning electron microscopy (FE-SEM; FEI Company, Magellan400). The microstructures of samples were inspected by transmission electron microscopy (TEM; FEI Company, Tecnai G2 F30 S-TWIN).

2.3.

The precursor solution for SrTiO3:Rh was prepared from the starting materials of Sr(NO3)2 (Aldrich, 99.0%), Ti[OCH(CH3)2]4 (Aldrich, 97%) and RhCl3 (Aldrich, 99.98%). These starting materials were dissolved in distilled water with excess nitric acid to prepare the aqueous colloid solution. The amount of Rh ions doped into SrTiO3 was 0.3 mol%, which has been reported as the optimum amount for maximizing the hydrogen evolution rate [38]. SrTiO3:Rh powder was prepared by spray pyrolysis process using the precursor solution. A vertical quartz tube, with diameter and height of 30 mm and 1.2 m, respectively, was used as a reaction chamber for the pyrolysis of the liquid droplets. The droplets were generated from the precursor solution by using an ultrasonic atomizer (Dong-Lim Eng.) with a frequency of 1.7 MHz. The liquid droplets were transported into the pyrolysis chamber by air as the carrier gas at a flow rate of 5.0 L min
1. The reaction temperature in the pyrolysis chamber was maintained at 1173 K. The powders were collected by a thimble filter. The g-C3N4 polymer was prepared by heating melamine (Aldrich, 99%) as a precursor at 873 K in a muffle furnace for 4 h in a flow of Ar gas. After the reaction, the furnace was cooled down to room temperature while maintaining the Ar flow. For the preparation of the g-C3N4eSrTiO3:Rh composite, the designated amounts of SrTiO3:Rh powder prepared by

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Photocatalytic reaction

The hydrogen production rate was measured in a closed gas circulation reactor with a Pyrex top window at room temperature under atmospheric pressure. A 0.1 g sample of the photocatalyst composite powder was dispersed in aqueous methanol solution (20 vol.%) in the cell to give a powder concentration of 5 g L
1. As a visible light source, a 300 W Xe lamp (Newport, 6258) with a cut-off filter (Newport, 59472) was used to adjust the wavelength of the incident light longer than 415 nm. Additionally, the experiments were conducted under monochromatic light to observe the wavelength dependence of the quantum efficiency using a monochromator (Newport, 77250) and the 300 W Xe lamp [39]. The hydrogen production in the cell was measured by a gas chromatograph (HP 5890) equipped with a TCD detector in on-line system.

3.

Results and discussion

3.1.

Characterization of the photocatalyst

Fig. 1 shows the XRD patterns of the g-C3N4 and SrTiO3:Rh (0.3 mol%) powders and the g-C3N4eSrTiO3:Rh (0.3 mol%) composite with various g-C3N4 amounts with a range of gC3N4 content from 0 wt.% to 100 wt.%. The patterns confirmed the absence of any impurity phase. The inset of Fig. 1 shows

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Fig. 1 e X-ray diffraction patterns of g-C3N4eSrTiO3:Rh (0.3 mol%) composite with a variation of g-C3N4 amount: (a) 100 wt.% g-C3N4, (b) 80 wt.% g-C3N4, (c) 60 wt.% g-C3N4, (d) 40 wt.% g-C3N4, (e) 30 wt.% g-C3N4, (f) 20 wt.% g-C3N4, (g) 10 wt.% g-C3N4, (h) 0 wt.% g-C3N4. The inset shows the magnification of the diffraction peaks around 32.4 : (i) SrTiO3, (j) SrTiO3:Rh (0.3 mol%).

Transmittance (a.u.)

a

3140 1640 1557 1403 1320 1240 808

4000

3000

2000

1000

-1 Wavenumber ( cm )

b 1640

3140

Transmittance (a.u.)

a magnified image of the diffraction peaks for the (110) plane. The peak was shifted toward a higher angle by the doping of Rh ions, which confirmed the incorporation of the Rh ions into the SrTiO3 crystal lattice. In Fig. 1, two peaks were formed in gC3N4, corresponding to angles of 27.4 and 13.1 , due to the tris-triazine building blocks of the g-C3N4 structure [13,40]. The strong peak at 27.4 , indexed for graphitic materials as the (002) peak, was attributed to the inter-planar striking of the aromatic compound. The weak peak at 13.1 , indexed as (001), was associated with the interlayer striking. The presentation of the g-C3N4eSrTiO3:Rh sample as a two-phase material confirmed its formation as a composite. Fig. 2 shows the FT-IR spectra of g-C3N4 (Fig. 2(a)) and 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) (Fig. 2(b)). In Fig. 2(a), the spectra are similar to those in the literature [13,15,41,42] in that the absorption bands near 1640 cm
1, in the 1200e1400 cm
1 range and near 808 cm
1 correspond to the CeN stretching, aromatic CeN stretching and out-of plane bending modes of CeN hetero-cycles, respectively. A broad band near 3140 cm
1 is attributed to the NH stretching vibration modes [13,43]. The absorbance band of 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol %) composite shows a similar typical band to that of g-C3N4 (Fig. 2(b)), except for the reduced band intensity. This indicated that 20 wt.% g-C3N4 was well mixed with 80 wt.% SrTiO3:Rh (0.3 mol%) in the composite material. The residual hydrogen atoms binding to the edges of the graphene-like CeN sheet were reduced by calcination at 673 K. Fig. 3 shows the optical properties of the composite as analyzed by DRS. The spectra of the composite, which were transformed to absorbance intensity through the KubelkaeMunk method, showed a shift to a longer wavelength as the amount of g-C3N4 in the composite was increased. This spectral shift to the longer wavelength reflected the activity of the composite photocatalyst under visible light irradiation. The band gaps of g-C3N4 and SrTiO3:Rh (0.3 mol%) were

1557

4000

3000

2000

808 1240 1320 1403

1000

-1 Wavenumber ( cm )

Fig. 2 e The Fourier transform infrared spectra of (a) g-C3N4 and (b) 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite.

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(a) (b) (c)

(d) (e) (f) (g)

(h)

450 500 Wavelength (nm)

550

600

Fig. 3 e Diffuse reflectance spectra of g-C3N4eSrTiO3:Rh (0.3 mol%) composite with a variation of g-C3N4 amount: (a) 100 wt.% g-C3N4, (b) 80 wt.% g-C3N4, (c) 60 wt.% g-C3N4, (d) 40 wt.% g-C3N4, (e) 30 wt.% g-C3N4, (f) 20 wt.% g-C3N4, (g) 10 wt.% g-C3N4, (h) 0 wt.% g-C3N4.

estimated to be 2.75 and 2.3 eV, respectively, indicating the potential for hydrogen production under visible light irradiation. Fig. 4 shows the nitrogen adsorptionedesorption isotherms for the particles of each composite over the experimental range of g-C3N4 content. In Fig. 4(a), all the composite particles show hysteresis loops due to their micropores and the shapes of their pore size distributions are almost identical with a dominant peak at around 25e32 nm, as shown in Fig. 4(b). The pore volume in the composite particles gradually decreased with increasing g-C3N4 content in the gC3N4eSrTiO3:Rh composite photocatalyst. The BET surface area is listed in Table 1, over the experimental range of g-C3N4 content in the composite. Fig. 5 shows the field-emission SEM images of g-C3N4 (Fig. 5(a)), SrTiO3:Rh (0.3 mol%) (Fig. 5(b)) and the 20 wt.% gC3N4e80 wt.% SrTiO3:Rh (0.3 mol%) (Fig. 5(c)) composite. Although the g-C3N4 powder was a flexibly shaped polymer material, the SrTiO3:Rh prepared by spray pyrolysis was comprised of porous particles with unique surface morphology. Fig. 5(c) shows that the porous SrTiO3:Rh (0.3 mol %) particles with wrinkled surface were comprised of a composite material with g-C3N4 polymeric material. The high magnification of the SEM image enabled the surface of the composite photocatalyst to be visualized, as shown in Fig. 5(d). The surface of the g-C3N4eSrTiO3:Rh (0.3 mol%) composite was porous, and the unique surface morphology of each material was maintained. The SEM image reveals the extended active sites containing electrons and holes that facilitated the hydrogen transportation capabilities. Fig. 6 shows the TEM images of 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite (Fig. 6(a)) and distribution of Pt in the g-C3N4 powder (Fig. 6(b)). In Fig. 6(a), a typical structure of SrTiO3:Rh crystalline is wrapped by layer of g-C3N4 [14,15,19]. The size of Pt load in the composite was in the range of 2e3 nm as shown in Fig. 6(b). Fig. 7 shows the composite powder with Pt loading under high resolution TEM, indicating the smooth but distinct

C N 0 wt.% 3 4

C N 10 wt.%

80

3 4

C N 20 wt.% 3 4

C N 30 wt.%

60

3 4

C N 100 wt.% 3 4

40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P P0-1)

b

0.0020

C N 0 wt.% 3 4

C N 10 wt.% 3 4

Pore volume ( cm3 g-1)

400

100 Quantity adsorbed ( cm3 g-1 STP)

Absorbance (a.u.)

a

C N 20 wt.%

0.0015

3 4

C N 30 wt.% 3 4

C N 100 wt.% 3 4

0.0010

0.0005

0.0000 0

20

40 60 Pore diameter (nm)

80

Fig. 4 e (a) Nitrogen adsorptionedesorption isotherms and (b) pore size distribution profiles of g-C3N4eSrTiO3:Rh (0.3 mol%) composite with a variation of g-C3N4 amount.

interfaces between g-C3N4 and SrTiO3:Rh. At the interfaces between the two materials, a lattice plane separation is observed. This is due to the different inter-layer structure of each component comprising a hetero-junction. A lattice fringe spacing at the hetero-junction is ascribed to the different inter-layer striking of each component. The crystalline face with a lattice fringe of g-C3N4 belongs to (002), while that of SrTiO3:Rh to (110), as shown in Fig. 1. This smooth heterojunction can contribute to form a feasible field for separating electrons and holes effectively.

3.2.

Photocatalytic activity for H2 production

Fig. 8 shows the amount and rate of hydrogen evolution by photocatalytic reaction under visible light irradiation over the g-C3N4eSrTiO3:Rh (0.3 mol%) composite. The time course of hydrogen evolution revealed that the induction period of hydrogen evolution was almost 1 h, irrespective of the g-C3N4 content in the composite photocatalyst (Fig. 8(a)). The g-C3N4 content in the composite photocatalyst affected the maximum hydrogen evolution rate. At a g-C3N4 content of 20 wt.% this evolution rate was maximized at 223.3 mmol h
1 by using 0.1 g of photocatalyst, which is 3.24 times higher than that of the SrTiO3:Rh (0.3 mol%) photocatalyst in the same conditions, as summarized in Table 1. This indicated that at g-

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Table 1 e Hydrogen evolution rate under visible light irradiation and BET surface area of photocatalyst.a Photocatalyst g-C3N4 g-C3N4eSrTiO3:Rh (0.3 mol%) g-C3N4eSrTiO3:Rh (0.3 mol%) g-C3N4eSrTiO3:Rh (0.3 mol%) SrTiO3:Rh (0.3 mol%)

Amount of g-C3N4 (wt.%)

H2 evolution rate (mmol h
1)

BET surface area (m2 g
1)

100 30 20 10 0

10.7 81.0 223.3 92.6 68.9

6.0 12.2 16.2 17.5 22.4

a Catalyst: 5 g L
1, Pt (0.5 wt.%)-loaded; reactant solution: 20 vol.% methanol; light source: 300 W Xe lamp attached with a cut-off filter (l > 415 nm).

C3N4 contents up to 20 wt.%, an increasing content resulted in rapid transfer of holes from SrTiO3:Rh to g-C3N4 and of electrons from g-C3N4 to SrTiO3:Rh through the hetero-junction formed in the composite. However, further increase in gC3N4 content above 20 wt.% retarded the transportation of holes and electrons through the hetero-junctions of the organic (g-C3N4) and inorganic (SrTiO3:Rh) composite material (Fig. 8(b)). This can be ascribed to the internal electric fields induced by the spatial redistribution of electrons and holes in the g-C3N4 and SrTiO3:Rh particles. The photocatalytic reaction can occur under visible light irradiation when the potential at the hetero-junction electric field is larger than the potential barrier in the interfacial depletion layer [14]. In the present study, although the g-C3N4 contributed to the photocatalytic activity for hydrogen evolution by increasing the efficiencies of the separation and transportation of electrons and holes, the increasing g-C3N4

content in the composite decreased the number of micropores (Fig. 4). Thus, there is an optimum g-C3N4 content for maximizing the hydrogen evolution. Under the present experimental conditions, the hydrogen evolution rate was maximized at 223.3 mmol h
1 under visible light irradiation over g-C3N4eSrTiO3:Rh (0.3 mol%) composite photocatalyst with a g-C3N4 content of 20 wt.%. Fig. 9 shows the dependences of quantum efficiency and diffuse reflectance on the wavelength of as-prepared composite photocatalyst. The variation trends of two spectra agree well with each other, and the spectra are consistent with those reported in the literature [12,23]. The quantum efficiency of 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite was 7.2% at 380 nm and 5.5% at 410 nm. In spite of Rh doping into SrTiO3 and forming composite including organic and inorganic materials, the quantum efficiency of as-prepared photocatalyst exhibited the higher values comparing with those of

Fig. 5 e Field-emission SEM images: (a) g-C3N4, (b) SrTiO3:Rh (0.3 mol%), (c) 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite, (d) 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite, high magnification.

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Fig. 6 e TEM images: (a) 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite, (b) g-C3N4 loaded with Pt (0.5 wt.%).

other photocatalysts reported in the literature [1,17,23]. This can be due to the effective separation of electrons and holes through the smooth but distinct hetero-junction formed in the composite material. In addition, the doping of Rh ions into SrTiO3 can provide it with a donor lever to the conduction band [23]. These represent the reason why the high hydrogen evolution rate can be achieved by using as-prepared composite photocatalyst.

For comparison, the quantum efficiencies under different wavelength of SrTiO3:Rh (0.3 mol%) photocatalysts prepared by spray pyrolysis and solid state reaction and g-C3N4 photocatalyst were measured and depicted in Fig. 10. As-prepared SrTiO3:Rh (0.3 mol%) photocatalyst prepared by spray pyrolysis was calcined at 673 K for 1 h before measuring the quantum efficiency, for comparison with the efficiencies of the composite photocatalyst and g-C3N4 photocatalyst. In

Fig. 7 e TEM images: (a) (b) 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite loaded with Pt (0.5 wt.%), (c) (d) high resolution TEM images for the interface between g-C3N4 and SrTiO3:Rh (0.3 mol%) of 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite loaded with Pt (0.5 wt.%).

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Amount of H2 evolved (µ mol )

a

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 1 6 0 2 e1 1 6 1 0

700 C3N4 0 wt.%

600

C3N4 10 wt.% C3N4 20 wt.%

500

C3N4 30 wt.%

400

C3N4 40 wt.% C3N4 60 wt.%

300

C3N4 80 wt.%

200

C3N4 100 wt.%

100 0

H2 evolution rate (µ mol h-1)

b

0

1

0

20

2 Time (h)

3

4

250 200 150 100 50 0 40 60 80 Amount of C3N4 (wt.%)

100

Fig. 8 e Photocatalytic hydrogen evolution under visible light irradiation over g-C3N4eSrTiO3:Rh (0.3 mol%) composite: (a) time course of hydrogen evolution, (b) dependence of the photocatalytic activity upon the amount of g-C3N4. Catalyst: 5 g LL1, Pt (0.5 wt.%)-loaded; reactant solution: 20 vol.% methanol; light source: 300 W Xe lamp attached with a cut-off filter (l > 415 nm).

Fig. 10, the quantum efficiency of SrTiO3:Rh (0.3 mol%) photocatalyst prepared by spray pyrolysis (Fig. 10(a)) is lower than that of the composite photocatalyst (Fig. 10(b)). The inset of Fig. 10 shows that the quantum efficiencies of g-C3N4 photocatalyst (Fig. 10(c)) and SrTiO3:Rh (0.3 mol%) photocatalyst prepared by solid state reaction (Fig. 10(d)) are much lower than that of the composite photocatalyst (Fig. 10(b)). The quantum efficiency of composite photocatalyst is not negligible under the wavelength of 530 nm, and it fades out for the longer wavelength than 600 nm. Fig. 11 presents the suggested mechanism for the electronehole separation of the g-C3N4eSrTiO3:Rh (0.3 mol%) composite under visible light irradiation. The conduction band (CB) and valence band (VB) of SrTiO3 are mainly composed of Ti 3d and O 2p, respectively, indicating a band gap of 3.2 eV. However, by adding g-C3N4 to the composite, the band gap was reduced effectively for the transfer of electrons and holes with the aid of Rh3þ doping. That is, the difference of VB between g-C3N4 and SrTiO3 can be effectively reduced by the doping of Rh ions into the SrTiO3, because the Rh3þ ion can provide a host material with a donor level to the CB [23,38]. This facilitates a transfer of holes from the donor level of SrTiO3:Rh to the VB of g-C3N4 [12e15], since the VB gap between g-C3N4 and SrTiO3:Rh is reduced from 1.43 eV to 0.53 eV. The CB of g-C3N4 is more negative (
1.12 eV) than that of SrTiO3:Rh (
0.2 eV), which eases the transfer of the photo-generated electrons on the surface of g-C3N4 to the surface of SrTiO3:Rh and thereby assists the hydrogen production at the surface of SrTiO3:Rh with the aid of platinum [12]. This effective separation of electrons and holes, arising from the internally rebuilt electric fields between the two semiconductors, significantly increases the electron and hole densities at the surface of SrTiO3:Rh and gC3N4, respectively, which in turn enhances the hydrogen production attained by using the g-C3N4eSrTiO3:Rh composite photocatalyst.

Quantum efficiency Diffuse reflectance

6

Absorbance (a.u.)

Quantum efficiency (%)

8

4

2

0 400

450 500 Wavelength (nm)

550

600

Fig. 9 e Dependences of quantum efficiency and diffuse reflectance on the wavelength of 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite.

Fig. 10 e Dependence of quantum efficiency on the wavelength of photocatalyst: (a) SrTiO3:Rh (0.3 mol%), (b) 20 wt.% g-C3N4e80 wt.% SrTiO3:Rh (0.3 mol%) composite, (c) g-C3N4, (d) SrTiO3:Rh (0.3 mol%) prepared by solid state reaction.

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Fig. 11 e The mechanism for electronehole separation of g-C3N4eSrTiO3:Rh composite under visible light irradiation.

4.

Conclusion

A composite photocatalyst of g-C3N4eSrTiO3:Rh was developed for the first time by combing organic (g-C3N4) and inorganic (SrTiO3:Rh) materials for increasing the visible-lightdriven hydrogen evolution from aqueous methanol solution. The hydrogen evolution rate was enhanced by a factor of 3.24e223.3 mmol h
1, compared to that obtained by SrTiO3:Rh (0.3 mol%) photocatalyst. The optimum amount of organic material (g-C3N4) in the composite for maximizing the hydrogen evolution rate was 20 wt.% under the present experimental conditions. The quantum efficiency of asprepared composite photocatalyst was 5.5% at 410 nm for hydrogen evolution. This enhanced rate was mainly ascribed to the activity of the composite photocatalyst for electronehole separation and transportation through the hetero-junctions of the organic and inorganic materials. A mechanism for the electronehole separation of the g-C3N4eSrTiO3:Rh composite under visible light irradiation was suggested. The doping of Rh ions into the SrTiO3 lattice structure formed a donor level from the VB to the CB, which eased the transfer of the photo-induced holes at the SrTiO3:Rh surface to the g-C3N4 surface and of the photo-induced electrons at the g-C3N4 surface to the SrTiO3:Rh surface, thereby preventing the recombination of electronehole pairs.

Acknowledgement This work was supported by the National Research Foundation of Korea (2012R1A1B3003889) and the Advanced Biomass R&D Center of Korea (2010-0029728) grant funded by the Ministry of Education, Science and Technology.

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