g-C3N4 composite photocatalysts with enhanced visible light driven activity

Applied Surface Science 301 (2014) 428–435

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

In2 O3 /g-C3 N4 composite photocatalysts with enhanced visible light driven activity Lu-Ya Chen, Wei-De Zhang ∗ School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 6 February 2014 Accepted 17 February 2014 Available online 25 February 2014 Keywords: g-C3 N4 In2 O3 Photocatalyst Rhodamine B 4-Nitrophenol

a b s t r a c t Novel In2 O3 /g-C3 N4 composite photocatalysts were prepared by a simple thermal polymerization process. The samples were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy with energy dispersive X-ray spectrometer, UV–vis diffuse reflection spectroscopy, Brunauer–Emmett–Teller surface area and thermogravimetric analysis. The prepared photocatalysts exhibited significantly enhanced photocatalytic performance towards the degradation of rhodamine B and 4-nitrophenol under visible light. The enhanced photocatalytic activity was generated from the matching energy levels of g-C3 N4 and In2 O3 . It was found that holes (h+ ) and superoxide (• O2 − ) were the main reactive species in the photocatalytic degradation of rhodamine B. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Photocatalysis based on semiconductor catalysts is considered as an effective way to tackle environmental pollutants. As a typical photocatalyst, TiO2 has drawn much attention because of its excellent photochemical stability, non-toxicity, and high photocatalytic activity. However, its limited utilization of sunlight and high recombination rate of the photogenerated electron–hole pairs have restricted its applications. Hence, to explore a new type of narrow bandgap semiconductors with visible light activity seems to be a promising way to utilize the solar energy. The semiconductors with narrow bandgaps, such as CdS [1,2], BiVO4 [3–5], Bi2 WO6 [6], Bi2 MoO6 [7,8], WO3 [9–11] alone usually exhibit poor photocatalytic activity due to the recombination of photogenerated carriers, although they have excellent absorption in visible light region. Designing semiconductor-based heterostructures is considered to be one of the most feasible methods to suppress the recombination of photogenerated electrons and holes. Recently, polymeric graphitic carbon nitride (g-C3 N4 ), similar to graphene, has been introduced as a prospective photocatalyst for water reduction [12–15] and oxidation [16] due to its unique electronic band structure. g-C3 N4 only contains elements C, N and a small amount of H as the residual –NH2 group which is

∗ Corresponding author. Tel.: +86 20 8711 4099/+86 13 7107 21660; fax: +86 20 8711 4099. E-mail addresses: [email protected], [email protected] (W.-D. Zhang). http://dx.doi.org/10.1016/j.apsusc.2014.02.093 0169-4332/© 2014 Elsevier B.V. All rights reserved.

detectable by elemental analysis [17] and FT-IR [18,19]. g-C3 N4 can be easily prepared by using an inexpensive starting precursors, such as dicyandiamide [20–22], melamine [23,24], urea [25,26], thiourea [27,28] and so on. Because of its frame of tri-s-triazine ring structure and high degree of condensation, g-C3 N4 possesses high thermal and chemical stability. In addition, g-C3 N4 can absorb visible light (bandgap: 2.7 eV) to split water to O2 and H2 . However, the photocatalytic activity of bulk g-C3 N4 was not satisfied due to the small surface area and the high recombination rate of photogenerated electron–hole pairs. So far, various approaches have been introduced to improve the photocatalytic activity for polymeric graphitic carbon nitride, such as increasing the surface area [29,30], loading metals as a co-catalyst [31,32], doping [21,23,33,34], and coupling g-C3 N4 with other semiconductors [35,36], polymers [37], metal ions [38], CNTs [39], graphene [40] and so on. The asprepared bulk carbon nitride usually has a very small surface area of ca. 10 m2 /g. Mesoporous g-C3 N4 was obtained by hard template such as AAO and SBA-15 with controlled morphology and large surface area. Wang and his colleagues reported the mesoporous g-C3 N4 with enhanced photocatalytic activity for hydrogen production [29]. Chen and his co-workers also found that g-C3 N4 with large surface area exhibited much higher photocatalytic activity towards degradation of MB when the pristine g-C3 N4 was treated by H2 O2 in a hydrothermal process. Jing et al. synthesized porous g-C3 N4 by pyrolysing dicyandiamide with urea, and the resulted porous g-C3 N4 displayed efficient photocatalytic performance for the degradation of MB and phenol under visible light [41]. Therefore, increasing the specific surface area can effectively improve

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435

the catalytic activity of g-C3 N4 . It was reported that the g-C3 N4 based heterostructured photocatalysts can effectively suppress the high recombination rate of photo-generated electron–hole pairs. To optimize this promising method, researchers have coupled g-C3 N4 with various semiconductors, such as TiO2 [42,43], ZnO [35,44,45], ZnWO4 [46], Ag3 PO4 [47–49], CdS [50] and Fe2 O3 [51] to strengthen the separation efficiency of photogenerated charge carriers. Therefore, it is promising to prepare composite photocatalysts with enhanced photocatalytic activity by using g-C3 N4 with large surface area to modify metal oxide semiconductors. As it is known to all, indium oxide (In2 O3 ), an important n-type semiconductor with a direct bandgap of about 3.7 eV and indirect bandgap of about 2.8 eV, has been widely used for solar cells, flat displays, gas sensors. In2 O3 usually exists in three phases: cubic (cIn2 O3 ), hexagonal (h-In2 O3 ) and hexagonal corundum (rh-In2 O3 ). Cubic In2 O3 was the most stable phase at ambient conditions. However, as a photocatalyst, In2 O3 exhibits very poor photocatalytic activity though it is visible light active. It has been found that the energy levels of graphitic carbon nitride and In2 O3 are wellmatched. Here, we report the In2 O3 /g-C3 N4 composite catalysts with higher photocatalytic performance compared to bare g-C3 N4 and In2 O3 . 2. Experimental 2.1. Chemicals Melamine was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. In(NO3 )3 ·4.5H2 O was purchased from Sinopharm Chemical Reagent Co., Ltd. All other chemicals are of reagent grade and deionized water was used throughout the work. 2.2. Synthesis of photocatalyst The bulk g-C3 N4 powders were synthesized by heating melamine in a muffle furnace. Typically, the melamine was placed in an alumina crucible with a cover, and then heated to 520 ◦ C and held for 2 h. Further deammoniation treatment was performed at 550 ◦ C for another 3 h. The bulk g-C3 N4 was ground to powders, and was then put into a crucible and annealed at 500 ◦ C for another 4 h in air. The obtained sample was labeled as g-C3 N4 (HT). Typical preparation of In2 O3 /g-C3 N4 (HT) composites were as follows: g-C3 N4 (HT, 0.20 g) and different amount of In(NO3 )3 ·4.5H2 O were added into a mortar and grounded for 5 min with some alcohol, and then the powders were transferred to a crucible and heated at 300 ◦ C for 6 h. In order to make clarity, the In2 O3 /g-C3 N4 (HT) photocatalysts with 0.0459, 0.0687, 0.137, 0.275 and 0.550 g In(NO3 )3 ·4.5H2 O were synthesized and named as In2 O3 /g-C3 N4 (HT)-1, In2 O3 /g-C3 N4 (HT)-2, In2 O3 /g-C3 N4 (HT)-3, In2 O3 /g-C3 N4 (HT)-4, and In2 O3 /g-C3 N4 (HT)-5, respectively. 2.3. Characterization XRD patterns of the as-prepared samples were collected on an X-ray powder diffractometer (X’Pert PRO MRD, PANalytical, the ˚ UV–vis diffuse Netherlands) using a Cu-K␣ target ( = 1.5418 A). reflectance spectra (DRS) were recorded on a UV–vis spectrophotometer (UV-3010, Hitachi, Japan). The thermogravimetric analysis (TGA) was performed using a thermal analyzer (SDT Q600, USA) at a rate of 10 ◦ C/min from room temperature to 800 ◦ C in air. The degradation of RhB and 4-NP in aqueous solutions was tracked by UV–vis spectroscopy (UV-3900, Hitachi, Japan). The surface morphology of the samples was examined by field emission scanning electron microscopy (SEM, JEOL-6000, Japan). Transmission electron microscopy (TEM, JSM-2010F, Japan) was used to characterize

429

Fig. 1. XRD patterns of In2 O3 , g-C3 N4 (HT) and In2 O3 /g-C3 N4 (HT) samples.

the fine morphology of the samples with an acceleration voltage of 200 kV. 2.4. Measurement of photocatalytic properties The photodegradation of RhB was conducted in 250 mL cylindrical glass vessel with a 300 W halogen tungsten lamp as light source ( ≥ 400 nm). In a typical photocatalytic experiment, 0.10 g catalyst was dispersed in 100 mL RhB solution (1 × 10−5 M) under ultrasonication for 2 min. Then, the mixture was put into a dark environment under stirring for 1 h to get adsorption–desorption equilibrium. 4 mL of suspension was extracted every 20 min and was centrifuged to separate the photocatalyst particles. Photodegradation of 4NP was evaluated under a 500 W Xe lamp (long-arc xenon lamp, 290–800 nm) with NaNO2 as a filter liquid. The obtained light wavelength is 400–800 nm. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of pure g-C3 N4 (HT), In2 O3 and In2 O3 -modified g-C3 N4 . The XRD pattern of the In2 O3 can be assigned to cubic structure (JCPDS card No. 06-0416) and the characteristic peaks at 30.58◦ , 35.47◦ and 51.04◦ are attributed to (2 2 2), (4 4 0) and (4 0 0) crystal planes of In2 O3 . The main diffraction peaks of g-C3 N4 (HT) were observed at around 13◦ and 27◦ , corresponding to the in-plane structural repeating motifs of aromatic systems and the interlayer reflection of the graphitic structure, respectively. Compared to pure g-C3 N4 (HT), most peaks for In2 O3 /g-C3 N4 (HT) were readily indexed to the structure of the c-In2 O3 . Due to the presence of In2 O3 , the peaks of g-C3 N4 (HT) became weaker. However, g-C3 N4 (HT) can still be found in the composites because of the presence of the peak at 27◦ in the XRD. The peak intensity of In2 O3 increases gradually with increasing In(NO3 )3 ·4.5H2 O. Therefore, the composites were composed of c-In2 O3 and g-C3 N4 . 3.2. SEM, TEM and EDS analysis The morphology and structure of the g-C3 N4 (HT), In2 O3 and In2 O3 /g-C3 N4 (HT)-3 were showed in Fig. 2. The g-C3 N4 (HT) sample displays a typical aggregated morphology with a large size and lamellar structure. It seems that annealing treatment did not have much effect on the morphology of g-C3 N4 (HT). In2 O3 was prepared

430

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435

Fig. 2. SEM images of (A) g-C3 N4 (HT), (B) In2 O3 , (C) and (D) In2 O3 /g-C3 N4 (HT)-3.

by annealing In(NO3 )3 ·4.5H2 O in a muffle furnace. It is consist of small and agglomerates of particles. When the g-C3 N4 (HT) was modified with In2 O3 , the surface of the samples became rough and some particles were found distributed on the surface of g-C3 N4 (HT). The cube-like particle with size smaller than 100 nm was In2 O3 , as is confirmed by XRD result. The fine structure of In2 O3 /gC3 N4 (HT)-3 was examined by TEM. Fig. 3 shows the TEM images of In2 O3 /g-C3 N4 (HT)-3 composite. It can be seen that In2 O3 /g-C3 N4 (HT)-3 was in layered structure and it can be inferred that In2 O3 was dispersed on the surface of g-C3 N4 (HT). Interestingly, the size of In2 O3 in the composite was significantly smaller than that of the neat In2 O3 . In this case, the large g-C3 N4 (HT) sheets act as excellent supports and stabilizers for the formation of In2 O3 nanoparticles.

Usually, the bulk g-C3 N4 shows a strong absorption edge at about 460 nm due to its bandgap of 2.70 eV. The optical properties of the bulk g-C3 N4 , g-C3 N4 (HT), In2 O3 and In2 O3 /g-C3 N4 (HT)-3 composite were examined by UV–vis diffuse reflectance spectroscopy. After thermal treatment, significant blue shift of DRS was achieved, as illustrated in Fig. 5. This can be attributed to the quantum confinement effect according to Niu’s report. g-C3 N4 (HT) and In2 O3 /g-C3 N4 (HT)-3 showed similar absorption spectra. The bandgaps were calculated to 2.7 eV for bulk g-C3 N4 , 2.75 eV for In2 O3 , 2.9 eV for g-C3 N4 (HT) and In2 O3 /g-C3 N4 (HT)-3. The result indicates that all of them can be driven by visible light. The BET surface areas are 10 m2 /g for bulk g-C3 N4 , 178.6 m2 /g for g-C3 N4 (HT) and 65.22 m2 /g for the composite.

3.3. FT-IR spectra

3.5. Photocatalytic activity

Fourier-transform infrared spectra (FT-IR) of In2 O3 /g-C3 N4 (HT)-3 composite and g-C3 N4 treated at 500 ◦ C were recorded, as indicated in Fig. 4. In2 O3 /g-C3 N4 (HT)-3 composite showed the similar spectrum to g-C3 N4 (HT), which is attributed to the low concentration of In2 O3 and the strong IR response of g-C3 N4 (HT). All characteristic absorptions associated with g-C3 N4 (HT) can be found. The peaks in the range of 1200 to 1500 cm−1 correspond to the typical stretching modes of the CN heterocycles. The typical band at ca. 805 cm−1 corresponds to triazine units. A wide band between 3100 and 3500 cm−1 corresponds to the stretching vibration of O–H of the absorbed water molecule and the residual –NH2 group attached to the sp2 hybridized carbon.

RhB was chosen as the typical probe molecule to evaluate the photocatalytic activity of the samples under visible light using a 300 W halide lamp with a 400 nm cutoff filter. The photocatalytic activity of g-C3 N4 (HT) was comparatively low. Only 90% RhB was degraded when the irradiation time lasted for 2 h, as shown in Fig. 6A. Moreover, pure In2 O3 showed even lower activity than pure g-C3 N4 . Most of the In2 O3 /g-C3 N4 (HT) composites displayed much higher photocatalytic activity than g-C3 N4 (HT) and In2 O3 except In2 O3 /g-C3 N4 (HT)-1. In order to find the optimal composition of the composite photocatalysts, samples with different weights of In2 O3 were prepared. According to the diagram, the photocatalytic activity was greatly influenced by the ratio between In2 O3 and gC3 N4 (HT). Upon increasing the weight of In2 O3 , the photocatalytic activity first increased and then decreased. In2 O3 /g-C3 N4 (HT)-3 showed the highest photocatalytic activity among all composites. The removal rate of RhB was about 99% for In2 O3 /g-C3 N4 (HT)-3 after 40 min. The temporal evolution of the absorption spectra of RhB catalyzed by In2 O3 /g-C3 N4 (HT)-3 and g-C3 N4 (HT) is illustrated in Fig. 6B and C. For In2 O3 /g-C3 N4 (HT)-3, the main peak

3.4. Light absorption and BET surface areas It is noted that the structure and defects affect the absorption edge of the bulk g-C3 N4 prepared by different methods. The band structure can also be influenced when the bulk g-C3 N4 is treated with acid, alkali, hydrothermal or heat treatment process.

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435

431

Fig. 3. TEM images and energy dispersive spectrum (EDS) of In2 O3 /g-C3 N4 (HT)-3.

of RhB blue-shifted from 554 to 498 nm during the first 40 min. As the time goes on, the absorption band at 498 nm decreased, and the color of the suspension became sequentially colorless. The experimental phenomenon was according to two competitive processes occurred in the degradation of RhB. The improved photocatalytic activity of the composites with an appropriate In2 O3 ratio is attributed to (1) the large surface area of g-C3 N4 (HT) treated at 500 ◦ C and (2) the matching band structures of the In2 O3 and gC3 N4 (HT). The heterojunction formed at the interface of In2 O3 and g-C3 N4 (HT) bred the effective separation of photo-generated electrons and holes, which contributes to the improved photocatalytic reactivity.

Moreover, 4-nitrophenol was chosen as a colorless target molecule for photocatalytic degradation. In2 O3 /g-C3 N4 (HT)-3 exhibited higher activity towards 4-nitrophenol degradation than g-C3 N4 (HT), as shown in Fig. 6D. As a reference, the g-C3 N4 obtained at 500 ◦ C displayed low photocatalytic activity, about 20% 4-nitrophenol was degraded under visible light irradiation for 5 h, while 40% 4-nitrophenol was degraded over In2 O3 /gC3 N4 (HT)-3 under the same condition. The degradation rate is twice that of over g-C3 N4 . This result indicates that the synergistic effect between In2 O3 and g-C3 N4 (HT) enhances the photocatalytic activity efficiently under visible light irradiation.

432

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435 Table 1 The weight of In2 O3 in the composites based on TGA. Catalysts In2 O3 /g-C3 N4 In2 O3 /g-C3 N4 In2 O3 /g-C3 N4 In2 O3 /g-C3 N4 In2 O3 /g-C3 N4

(HT)-1 (HT)-2 (HT)-3 (HT)-4 (HT)-5

Raw ratio of In2 O3 (wt%)

Real weight of In2 O3 (wt%)

14.3 20.0 33.3 50.0 66.7

3.60 11.3 19.0 31.5 49.8

3.6. TGA

Fig. 4. FT-IR spectra of g-C3 N4 (HT) and In2 O3 /g-C3 N4 (HT)-3.

The TGA was employed to investigate the thermal stability of gC3 N4 (HT) and In2 O3 /g-C3 N4 (HT) composites in air. As indicated in Fig. 7, the graphitic carbon nitride was stable in air and the starting temperature of weight loss was recorded at about 600 ◦ C. A sharp weight loss peak appeared at 750 ◦ C, which can be attributed to the decomposition of g-C3 N4 (HT). The stability of all In2 O3 /g-C3 N4 (HT) samples was lower than that of g-C3 N4 (HT). The initial decomposition temperature shifted to lower temperature. It is because In2 O3 is able to activate oxygen at elevated temperature, and serves as a catalyst to promote the decomposition of g-C3 N4 (HT). Based on TGA result, the real weight of In2 O3 was calculated and listed in Table 1. 3.7. Stability of the catalyst The stability of photocatalysts was a crucial factor for their assessment and practical applications. In2 O3 /g-C3 N4 (HT)-3 photocatalyst with the highest performance was tested by five times’ recycling reaction towards degradation of RhB. After every 2 h of visible light irradiation, the sample was separated and washed with deionized water and ethanol. As displayed in Fig. 8, the photocatalytic performance of In2 O3 /g-C3 N4 (HT)-3 did not decrease obviously after five consecutive experiments. Therefore, the In2 O3 /g-C3 N4 (HT)-3 photocatalyst showed high stability during the photocatalytic reaction. 3.8. Possible photocatalytic mechanism The high photocatalytic activity of the composite catalyst can be partly attributed to the adsorption of contaminant molecules. The residual concentration of RhB after dark adsorption is shown in Fig. 9A. Compared to bulk g-C3 N4 , g-C3 N4 (HT) and In2 O3 /gC3 N4 (HT)-3 samples both showed a good adsorption capacity due to their large surface area. The enhancement of photocatalytic performance of the photocatalyst is mainly attributed to the

Fig. 5. (A) Typical UV–vis spectra of (a) the bulk g-C3 N4 , (b) g-C3 N4 (HT), (c) In2 O3 /gC3 N4 (HT)-3 and (d) In2 O3 ; (B) Plot of (h)1/2 versus energy (h) for the bandgap energy of (a) the bulk g-C3 N4 , (b) g-C3 N4 (HT), (c) In2 O3 /g-C3 N4 (HT)-3 and In2 O3 (inset). Scheme 1. Electron–hole separation and transportation at the interface of visible light driven composite photocatalyst.

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435

433

Fig. 6. (A) Photocatalytic degradation efficiency of RhB over g-C3 N4 (HT), In2 O3 , and In2 O3 /g-C3 N4 (HT) composites; (B) and (C) Changes in UV–visible absorption spectra of RhB over In2 O3 /g-C3 N4 (HT)-3 and g-C3 N4 (HT); (D) Photocatalytic degradation efficiency of 4-NP over g-C3 N4 (HT) and In2 O3 /g-C3 N4 (HT)-3.

synergistic effect of In2 O3 and g-C3 N4 (HT) with efficient separation of electron–hole pairs. The high separation efficiency can be ascribed to the matching energy levels of In2 O3 and g-C3 N4 . Density functional theory (DFT) calculation suggests that the CB and VB potentials of g-C3 N4 are −1.12 and 1.57 eV, respectively [52]. According to DRS, the CB and VB of In2 O3 are −0.62 and 2.18 eV, respectively. The CB of In2 O3 can be determined by the

Fig. 7. TGA curves of the as-prepared g-C3 N4 (HT) and In2 O3 /g-C3 N4 (HT) photocatalysts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

equation: EVB = X − Ee − 0.5Eg , where X is 5.28 eV, Eg is 2.8 eV and Ee is 4.5 eV. The photoexcited electron–hole separation process at the In2 O3 /g-C3 N4 (HT) interface is illustrated in Scheme 1. Both In2 O3 and g-C3 N4 (HT) can be excited by visible light. Since the CB position of g-C3 N4 (HT) is more negative than that of the In2 O3 , the photo-generated electron on g-C3 N4 (HT) can directly transfer to In2 O3 . Similarly, the holes on the surface of In2 O3 can move to g-C3 N4 (HT). In this way, the recombination of electron–hole was

Fig. 8. Cycling runs in the photocatalytic reaction process.

434

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435

4. Conclusions In summary, we have developed a simple method to synthesize In2 O3 /g-C3 N4 (HT) composites. The composite catalysts displayed higher photocatalytic performance towards the decomposition of RhB and 4-NP, which can be attributed to superior adsorption from g-C3 N4 and the synergy effect between In2 O3 and g-C3 N4 . The highest photocatalytic activity of In2 O3 /g-C3 N4 (HT) composite was obtained with 19.0 wt% In2 O3 content. The In2 O3 /g-C3 N4 (HT) composite catalysts also exhibited excellent stability. This study sheds light on developing high performance composite photocatalysts based on g-C3 N4 supported semiconductors for environmental remediation.

Acknowledgements The authors thank The National Natural Science Foundation of China (no. 21273080 and no. 21043005) for the financial support.

References

Fig. 9. (A) Bar plots showing the remaining RhB in the solutions dispersed with gC3 N4 (HT), In2 O3 , or In2 O3 /g-C3 N4 (HT)-3 in the dark after 60 min stirring. (B) The effects of scavengers on the degradation efficiency of 2.0 × 10−5 M RhB.

hindered, resulting in the enhanced photocatalytic activity of the heterostructures. 3.9. Active species It is accepted that the process of RhB degradation is an oxidation reaction and the main active species include hydroxyl radicals (• OH), superoxide (• O2 − ) and the holes. In order to elucidate the reaction mechanism of RhB degradation, different scavengers such as tertiary butanol (TBA) and triethanolamine (TEOA) were added. The results are illustrated in Fig. 9. It can be seen that the degradation efficiency of RhB decreases significantly upon addition of TEOA (a hole scavenger). On the contrary, adding TBA (a quencher of • OH) has little impact on the photocatalytic activity. Moreover, the sample shows lower photocatalytic activity when N2 is purged to reduce the absorbed O2 . The result indicates that • O2 − and holes play important roles in the oxidation of RhB. The process can be described as follows: hv

In2 O3 /g − C3 N4 −→In2 O3 (e− + h+ )/g − C3 N4 (e− + h+ ) In2 O3 (e− + h+ )/g − C3 N4 (e− + h+ ) → In2 O3 (e− )/g − C3 N4 (h+ ) e− + O2 → • O2− h+ + • O2− + RhB → CO2 + H2 O

[1] A. Hernández-Gordillo, A.G. Romero, F. Tzompantzi, R. Gómez, Kinetic study of the 4-nitrophenol photooxidation and photoreduction reactions using CdS, Appl. Catal., B 144 (2014) 507–513. [2] J. Jin, J. Yu, G. Liu, P.K. Wong, Single crystal CdS nanowires with high visiblelight photocatalytic H2 -production performance, J. Mater. Chem. A 1 (2013) 10927–10934. [3] M. Zhou, H.B. Wu, J. Bao, L. Liang, X.W. Lou, Y. Xie, Ordered macroporous BiVO4 architectures with controllable dual porosity for efficient solar water splitting, Angew. Chem. Int. Ed. 52 (2013) 8579–8583. [4] P. Madhusudan, M. Kumar, T. Ishigaki, K. Toda, K. Uematsu, M. Sato, Hydrothermal synthesis of meso/macroporous BiVO4 hierarchical particles and their photocatalytic degradation properties under visible light irradiation, Environ. Sci. Pollut. Res. 20 (2013) 6638–6645. [5] H.W. Jeong, T.H. Jeon, J.S. Jang, W. Choi, H. Park, Strategic modification of BiVO4 for improving photoelectrochemical water oxidation performance, J. Phys. Chem. C 117 (2013) 9104–9112. [6] Z. Liu, F. Chen, Y. Gao, Y. Liu, P. Fang, S. Wang, A novel synthetic route for magnetically retrievable Bi2 WO6 hierarchical microspheres with enhanced visible photocatalytic performance, J. Mater. Chem. A 1 (2013) 7027–7030. [7] W.-D. Zhang, L. Zhu, Construction of hierarchical nanostructured TiO2 /Bi2 MoO6 heterojunction for improved visible light photocatalysis, J. Nanosci. Nanotechnol. 12 (2012) 6294–6300. [8] N. Li, L. Zhu, W.-D. Zhang, Y.-X. Yu, W.-H. Zhang, M.-F. Hou, Modification of TiO2 nanorods by Bi2 MoO6 nanoparticles for high performance visible-light photocatalysis, J. Alloys Compd. 509 (2011) 9770–9775. [9] G. Xi, J. Ye, Q. Ma, N. Su, H. Bai, C. Wang, In situ growth of metal particles on 3D urchin-like WO3 nanostructures, J. Am. Chem. Soc. 134 (2012) 6508–6511. [10] F. Wang, C. Di|Valentin, G. Pacchioni, Rational band gap engineering of WO3 photocatalyst for visible light water splitting, ChemCatChem 4 (2012) 476–478. [11] J. Kim, C.W. Lee, W. Choi, Platinized WO3 as an environmental photocatalyst that generates OH radicals under visible light, Environ. Sci. Technol. 44 (2010) 6849–6854. [12] K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti, K. Domen, Photocatalytic activities of graphitic carbon nitride powder for water reduction and oxidation under visible light, J. Phys. Chem. C 113 (2009) 4940–4947. [13] X.-H. Li, X. Wang, M. Antonietti, Mesoporous g-C3 N4 nanorods as multifunctional supports of ultrafine metal nanoparticles: hydrogen generation from water and reduction of nitrophenol with tandem catalysis in one step, Chem. Sci. 3 (2012) 2170–2174. [14] H. Yan, Soft-templating synthesis of mesoporous graphitic carbon nitride with enhanced photocatalytic H2 evolution under visible light, Chem. Commun. 48 (2012) 3430–3432. [15] F. Chang, Y. Xie, C. Li, J. Chen, J. Luo, X. Hu, J. Shen, A facile modification of gC3N4 with enhanced photocatalytic activity for degradation of methylene blue, Appl. Surf. Sci. 280 (2013) 967–974. [16] A.B. Jorge, D.J. Martin, M.T.S. Dhanoa, A.S. Rahman, N. Makwana, J. Tang, A. Sella, F. Corà, S. Firth, J.A. Darr, P.F. McMillan, H2 and O2 evolution from water halfsplitting reactions by graphitic carbon nitride materials, J. Phys. Chem. C 117 (2013) 7178–7185. [17] T. Sano, S. Tsutsui, K. Koike, T. Hirakawa, Y. Teramoto, N. Negishi, K. Takeuchi, Activation of graphitic carbon nitride (g-C3 N4 ) by alkaline hydrothermal treatment for photocatalytic NO oxidation in gas phase, J. Mater. Chem. A 1 (2013) 6489–6496. [18] Z. Hong, B. Shen, Y. Chen, B. Lin, B. Gao, Enhancement of photocatalytic H2 evolution over nitrogen-deficient graphitic carbon nitride, J. Mater. Chem. A 1 (2013) 11754–11761.

L.-Y. Chen, W.-D. Zhang / Applied Surface Science 301 (2014) 428–435 [19] Q. Li, L. Zong, Y. Xing, X. Wang, L. Yu, J. Yang, Preparation of g-C3 N4 /TiO2 nanocomposites and investigation of their photocatalytic activity, Sci. Adv. Mater. 5 (2013) 1316–1322. [20] Y. Zhang, T. Mori, L. Niu, J. Ye, Non-covalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion, Energy Environ. Sci. 4 (2011) 4517– 4521. [21] G. Liu, P. Niu, C. Sun, S.C. Smith, Z. Chen, G.Q. Lu, H.-M. Cheng, Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3 N4 , J. Am. Chem. Soc. 132 (2010) 11642–11648. [22] H. Ji, F. Chang, X. Hu, W. Qin, J. Shen, Photocatalytic degradation of 2,4,6trichlorophenol over g-C3 N4 under visible light irradiation, Chem. Eng. J. 218 (2013) 183–190. [23] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation of rhodamine B and methyl orange over boron-doped g-C3 N4 under visible light irradiation, Langmuir 26 (2010) 3894–3901. [24] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3 N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. [25] J. Liu, T. Zhang, Z. Wang, G. Dawson, W. Chen, Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity, J. Mater. Chem. 21 (2011) 14398–14401. [26] Y. Zhang, J. Liu, G. Wu, W. Chen, Porous graphitic carbon nitride synthesized via direct polymerization of urea for efficient sunlight-driven photocatalytic hydrogen production, Nanoscale 4 (2012) 5300–5303. [27] J. Hong, X. Xia, Y. Wang, R. Xu, Mesoporous carbon nitride with in situ sulfur doping for enhanced photocatalytic hydrogen evolution from water under visible light, J. Mater. Chem. 22 (2012) 15006–15012. [28] F. Dong, Y. Sun, L. Wu, M. Fu, Z. Wu, Facile transformation of low cost thiourea into nitrogen-rich graphitic carbon nitride nanocatalyst with high visible light photocatalytic performance, Catal. Sci. Technol. 2 (2012) 1332– 1335. [29] X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou, X. Fu, M. Antonietti, Polymer semiconductors for artificial photosynthesis: hydrogen evolution by mesoporous graphitic carbon nitride with visible light, J. Am. Chem. Soc. 131 (2009) 1680–1681. [30] J. Xu, H.-T. Wu, X. Wang, B. Xue, Y.-X. Li, Y. Cao, A new and environmentally benign precursor for the synthesis of mesoporous g-C3 N4 with tunable surface area, Phys. Chem. Chem. Phys. 15 (2013) 4510–4517. [31] C. Chang, Y. Fu, M. Hu, C. Wang, G. Shan, L. Zhu, Photodegradation of bisphenol A by highly stable palladium-doped mesoporous graphite carbon nitride (Pd/mpg-C3 N4 ) under simulated solar light irradiation, Appl. Catal., B 142–143 (2013) 553–560. [32] L. Ge, C. Han, J. Liu, Y. Li, Enhanced visible light photocatalytic activity of novel polymeric g-C3 N4 loaded with Ag nanoparticles, Appl. Catal., A 409–410 (2011) 215–222. [33] G. Dong, K. Zhao, L. Zhang, Carbon self-doping induced high electronic conductivity and photoreactivity of g-C3 N4 , Chem. Commun. 48 (2012) 6178–6180. [34] J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, A facile approach to synthesize novel oxygen-doped g-C3 N4 with superior visible-light photoreactivity, Chem. Commun. 48 (2012) 12017–12019. [35] Y. Wang, R. Shi, J. Lin, Y. Zhu, Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C3 N4 , Energy Environ. Sci. 4 (2011) 2922–2929.

435

[36] L. Huang, H. Xu, R. Zhang, X. Cheng, J. Xia, Y. Xu, H. Li, Synthesis and characterization of g-C3N4/MoO3 photocatalyst with improved visible-light photoactivity, Appl. Surf. Sci. 283 (2013) 25–32. [37] L. Ge, C. Han, J. Liu, In situ synthesis and enhanced visible light photocatalytic activities of novel PANI-g-C3 N4 composite photocatalysts, J. Mater. Chem. 22 (2012) 11843–11850. [38] X. Chen, J. Zhang, X. Fu, M. Antonietti, X. Wang, Fe-g-C3 N4 -catalyzed oxidation of benzene to phenol using hydrogen peroxide and visible light, J. Am. Chem. Soc. 131 (2009) 11658–11659. [39] Y. Xu, H. Xu, L. Wang, J. Yan, H. Li, Y. Song, L. Huang, G. Cai, The CNT modified white C3 N4 composite photocatalyst with enhanced visible-light response photoactivity, Dalton Trans. 42 (2013) 7604–7613. [40] Q. Xiang, J. Yu, M. Jaroniec, Preparation and enhanced visible-light photocatalytic H2 -production activity of graphene/C3 N4 composites, J. Phys. Chem. C 115 (2011) 7355–7363. [41] M. Zhang, J. Xu, R. Zong, Y. Zhu, Enhancement of visible light photocatalytic activities via porous structure of g-C3 N4 , Appl. Catal., B 147 (2014) 229–235. [42] B. Chai, T. Peng, J. Mao, K. Li, L. Zan, Graphitic carbon nitride (g-C3 N4 )-Pt-TiO2 nanocomposite as an efficient photocatalyst for hydrogen production under visible light irradiation, Phys. Chem. Chem. Phys. 14 (2012) 16745–16752. [43] J. Yu, S. Wang, J. Low, W. Xiao, Enhanced photocatalytic performance of direct Z-scheme g-C3 N4 -TiO2 photocatalysts for the decomposition of formaldehyde in air, Phys. Chem. Chem. Phys. 15 (2013) 16883–16890. [44] J.-X. Sun, Y.-P. Yuan, L.-G. Qiu, X. Jiang, A.-J. Xie, Y.-H. Shen, J.-F. Zhu, Fabrication of composite photocatalyst g-C3 N4 -ZnO and enhancement of photocatalytic activity under visible light, Dalton Trans. 41 (2012) 6756–6763. [45] W. Liu, M. Wang, C. Xu, S. Chen, Facile synthesis of g-C3 N4 /ZnO composite with enhanced visible light photooxidation and photoreduction properties, Chem. Eng. J. 209 (2012) 386–393. [46] Y. Wang, Z. Wang, S. Muhammad, J. He, Graphite-like C3 N4 hybridized ZnWO4 nanorods: synthesis and its enhanced photocatalysis in visible light, CrystEngComm 14 (2012) 5065–5070. [47] S. Kumar, T. Surendar, A. Baruah, V. Shanker, Synthesis of a novel and stable g-C3 N4 -Ag3 PO4 hybrid nanocomposite photocatalyst and study of the photocatalytic activity under visible light irradiation, J. Mater. Chem. A 1 (2013) 5333–5340. [48] F.-J. Zhang, F.-Z. Xie, S.-F. Zhu, J. Liu, J. Zhang, S.-F. Mei, W. Zhao, A novel photofunctional g-C3 N4 /Ag3 PO4 bulk heterojunction for decolorization of Rh.B, Chem. Eng. J. 228 (2013) 435–441. [49] Z. Xiu, H. Bo, Y. Wu, X. Hao, Graphite-like C3 N4 modified Ag3 PO4 nanoparticles with highly enhanced photocatalytic activities under visible light irradiation, Appl. Surf. Sci. 289 (2014) 394–399. [50] L. Ge, F. Zuo, J. Liu, Q. Ma, C. Wang, D. Sun, L. Bartels, P. Feng, Synthesis and efficient visible light photocatalytic hydrogen evolution of polymeric g-C3 N4 coupled with CdS quantum dots, J. Phys. Chem. C 116 (2012) 13708–13714. [51] S. Ye, L.-G. Qiu, Y.-P. Yuan, Y.-J. Zhu, J. Xia, J.-F. Zhu, Facile fabrication of magnetically separable graphitic carbon nitride photocatalysts with enhanced photocatalytic activity under visible light, J. Mater. Chem. A 1 (2013) 3008–3015. [52] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80.