- Email: [email protected]
Surface plasmon resonance-induced photocatalysis by Au nanoparticles decorated mesoporous g-C3N4 nanosheets under direct sunlight irradiation
Materials Research Bulletin 75 (2016) 51–58
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Surface plasmon resonance-induced photocatalysis by Au nanoparticles decorated mesoporous g-C3N4 nanosheets under direct sunlight irradiation Surendar Tonda, Santosh Kumar1, Vishnu Shanker* Department of Chemistry, National Institute of Technology, Warangal, Warangal 506004, Telangana, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 May 2015 Received in revised form 5 November 2015 Accepted 8 November 2015 Available online 17 November 2015
In recent years, surface plasmon-induced photocatalytic materials with tunable mesoporous framework have attracted considerable attention in energy conversion and environmental remediation. Herein we report a novel Au nanoparticles decorated mesoporous graphitic carbon nitride (Au/mp-g-C3N4) nanosheets via a template-free and green in situ photo-reduction method. The synthesized Au/mp-gC3N4 nanosheets exhibit a strong absorption edge in visible and near-IR region owing to the surface plasmon resonance effect of Au nanoparticles. More attractively, Au/mp-g-C3N4 exhibited much higher photocatalytic activity than that of pure mesoporous and bulk g-C3N4 for the degradation of rhodamine B under sunlight irradiation. Furthermore, the photocurrent and photoluminescence studies demonstrated that the deposition of Au nanoparticles on the surface of mesoporous g-C3N4 could effectively inhibit the recombination of photogenerated charge carriers leading to the enhanced photocatalytic activity. More importantly, the synthesized Au/mp-g-C3N4 nanosheets possess high reusability. Hence, Au/mp-g-C3N4 could be promising photoactive material for energy and environmental applications. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors A. Nanostructures B. Optical properties C. X-ray diffraction D. Catalytic properties
1. Introduction As a clean and renewable energy source, solar energy can become a sustainable solution to the increasing global energy demands and environmental issues using efficient semiconductor photocatalysts [1–3]. Therefore, development of novel photocatalyst is the key strategy for direct use of solar energy. Since the discovery of photocatalytic water splitting for hydrogen production using TiO2 electrodes by Fujishima and Honda under ultraviolet (UV) light in 1972 [4], there has been significant progress in the design of efficient semiconductor photocatalysts for different kinds of applications such as photodecomposition of hazardous substances, water splitting for hydrogen fuel, artificial photosynthesis and photocatalytic conversion of CO2 to energyrich hydrocarbon fuels [5–7]. Unfortunately, most of the developed
* Corresponding author. Fax: +91 870 2459547. E-mail address: [email protected] (V. Shanker). 1 Present address: European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK. http://dx.doi.org/10.1016/j.materresbull.2015.11.011 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
photocatalysts containing metals can only work in the UV region with moderate performance. Recently, attention has been devoted to a new class of metalfree organic semiconductor, especially graphitic carbon nitride (gC3N4), which is considered to be a breakthrough in the field of visible light active photocatalysis due to its fascinated properties such as visible light response, suitable band gap, high chemical and physical stability, low cost, and nontoxicity [8,9]. Unfortunately, the practical applications of g-C3N4 are still limited due to the short lifespan of photogenerated electron–hole pairs, low specific surface area and poor quantum yield [10]. However, many attempts have been made to improve the performance of g-C3N4, such as doping with foreign elements, and coupling with other semiconductors or noble metals [11–15]. Among them, visible light active plasmonic photocatalysts are identified as one of the most promising alternatives to traditional photocatalysts, which can efficiently improve the efficiency of photocatalytic process [16]. Noble metal nanoparticles, such as Au, Pt, and Ag can strongly absorb visible light owing to their localized surface plasmon resonance (SPR) from the collective oscillation of the surface electrons and they exhibit a great potential to extend the lightabsorption range of semiconductors [17]. Maeda et al. [10]
52
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
reported the noble metals such as Pt, Au and Pd deposited g-C3N4 photocatalysts for the enhanced photocatalytic activity for H2 evolution. For instance, Au@g-C3N4 hybrids display a synergistic effect between semiconductor and plasmonic photocatalysis and exhibit superior photocatalytic activity for the degradation of methyl orange [18], but still the system efficiency is low due to low surface area and inadequate light harvesting capacity. On the other hand, materials with porous structure have been extremely involved in recently developed energy storage and conversion systems due to their large surface area, interfacial transport and shorten diffusion pathways [19,20]. Considering the advantages of porous materials, various synthetic strategies have been employed to introduce pores in g-C3N4. For example, Yan [21] reported the synthesis of mesoporous g-C3N4 by using a Pluronic P123 as a structure directing group for improved photocatalytic H2 evolution under visible light. But most of these methods require additional soft or hard templates and complicated post-treatments [22,23]. Therefore, the development of a template-free and green strategy is most economical with improved the photocatalytic properties. In this report, Au nanoparticles deposited mesoporous g-C3N4 nanosheets were synthesized by template free controlled heat treatment and in situ photo-reduction method. The photocatalytic activity of the synthesized Au/mp-g-C3N4 nanosheets was evaluated by photodegradation of rhodamine B (RhB) under sunlight irradiation. The effect of Au loading on the optical absorption, photoluminescence, electrochemical properties and photocatalytic performance were investigated in detail. The photocatalytic mechanism of Au/mp-g-C3N4 photocatalysts was proposed based on the surface plasmon effect of the Au nanoparticles. Our results demonstrated that Au/mp-g-C3N4 exhibited significantly enhanced photocatalytic activity for the degradation of RhB under sunlight irradiation, and this novel plasmon-induced photocatalyst was stable even after five cycling photocatalytic experiments.
material. Briefly, melamine was placed in a semi-closed aluminum boats to control the formation and escape rate of gas products in calcination process, and then heated at 550 C for 2 h in a tubular furnace under N2 atmosphere with a slow ramp rate. The obtained pale yellow colored product from the semi-closed system was grounded into fine powders, and resultant mesoporous g-C3N4 was named as mp-g-C3N4. For comparison, bulk g-C3N4 was synthesized according to our previous reported procedure [12]. Au nanoparticles loaded mesoporous g-C3N4 samples were synthesized by in situ photo-reduction method. In brief, 50 mg of the synthesized mp-g-C3N4 powder was added to 40 mL of methanol, and the suspension was sonicated for 1 h. Then, 0.01 mM HAuCl4 aqueous solution was added to suspension, which was stirred for 30 min at room temperature to complete adsorption of Au nanoparticles on the surface of mp-g-C3N4. After that the above suspension was irradiated for 1 h by a Xenon lamp with a 420 nm cutoff filter under constant stirring. The resulting solution turns to brown red color. The final product was obtained by washing with double distilled water and dried at 70 C for overnight. The resulting 1 mol% Au loaded mp-g-C3N4 photocatalyst was denoted as Au/mp-g-C3N4-1.0. Accordingly, 0.5 mol% and 2.0 mol% Au loaded mp-g-C3N4 photocatalysts were synthesized by the same method and denoted as Au/mp-g-C3N4-0.5 and Au/mp-g-C3N4-2.0, respectively. 2.3. Material characterization
Melamine (Sigma–Aldrich, 99.0%), chloroauric acid (Sigma– Aldrich, 99.9%) rhodamine B (Sigma–Aldrich, 95.0% dye content), terephthalic acid (Merck, AR grade), tert-butyl alcohol (Merck, AR grade), and ammonium oxalate (Merck, AR grade) were used as received. All other reagents used in this work were of analytically pure grade and used without further purification. All aqueous solutions were prepared with double distilled water.
X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 advance X-ray diffractometer using Ni filtered Cu Ka (l = 1.5406 Å) radiation in a 2u scan range between 10 and 80 . UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained on a Thermo Scientific Evolution 600 diffuse reflectance spectrophotometer, and BaSO4 was used as a reference. Raman studies were conducted on a PerkinElmer RamanStation 400 spectrometer with a 785 nm laser as the excitation source. The transmission electron microscopy (TEM) measurements were conducted on JEOL JSM6700F transmission electron microscope with an acceleration voltage of 200 kV. The chemical composition of the samples was investigated by energy dispersive spectroscopy (SEM–EDS; Oxford Instruments, INCAx-act). The surface area measurements were recorded using a Quanta chrome NOVA 1200e and specific surface area of the catalyst was estimated using Brunauer–Emmett–Teller (BET) method. UV–visible absorption spectra (UV–vis) were recorded on a Thermo Scientific Evolution 600 UV–vis NIR spectrophotometer. The photoluminescence (PL) spectra of the photocatalysts were recorded on TSC Solutions F96pro fluorescence spectrophotometer at an excitation wavelength of 365 nm.
2.2. Method
2.4. Evaluation of photocatalytic activity
The mesoporous g-C3N4 sample was synthesized by a simple and facile heating method using low cost melamine as the starting
Rhodamine B was chosen as a representative hazardous dye in the present work. The photocatalytic activities of the synthesized
2. Experimental 2.1. Materials
Scheme 1. Schematic representation for the synthesis of Au nanoparticles deposited mesoporous g-C3N4 nanosheets.
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
53
Fig. 1. XRD patterns of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets: (a) mp-g-C3N4, (b) Au/mp-g-C3N4-0.5, (c) Au/mp-g-C3N4-1.0 and (d) Au/mp-g-C3N4-2.0.
under similar conditions in May 2014 at NIT Warangal, where the fluctuation of the sunlight intensity (5 kWh/m2/day) is minimal during this month. In each experiment, 0.1 g of photocatalyst was added to 250 mL RhB solution with a concentration of 5 mg L1. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to achieve adsorption equilibrium between the RhB molecules and the photocatalyst. At given irradiation time intervals, the suspensions were periodically withdrawn and centrifuged to separate the photocatalyst particulates for analysis. The filtrates were analyzed by recording variations at the wavelength of maximal absorption in the UV–vis spectrophotometer. A blank test was also carried out on an aqueous RhB solution without photocatalyst under sunlight irradiation, i.e., photolysis of RhB, to examine the degradation efficiency of the photocatalyst. 2.5. Analysis of reactive species
Fig. 2. UV–vis diffuse reflectance spectra of the synthesized mp-g-C3N4 and Au/ mp-g-C3N4 nanosheets.
samples were evaluated by monitoring the photocatalytic degradation of RhB in an aqueous solution under sunlight irradiation. Photocatalytic experiments of all the samples were conducted
The effect of reactive species on the photodegradation of RhB over Au/mp-g-C3N4 photocatalysts was examined to understand the photocatalytic mechanism. Different type of scavengers was subjected into the RhB aqueous solution prior to addition of photocatalyst. The analysis method was similar to the photocatalytic degradation process. Furthermore, the generation of hydroxyl radicals (OH) on the surface of sunlight irradiated photocatalyst was detected by the photoluminescence (PL)
Fig. 3. Raman spectra of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets: (a) mp-g-C3N4, (b) Au/mp-g-C3N4-0.5, (c) Au/mp-g-C3N4-1.0 and (d) Au/mp-g-C3N4-2.0.
54
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
Fig. 4. (a) TEM image of the synthesized Au/mp-g-C3N4-1.0 nanosheets, bulk g-C3N4 (inset), (b) magnified image of Au/mp-g-C3N4-1.0 nanosheets and (c) corresponding HRTEM image of Au nanoparticles.
technique using terephthalic acid (TA) as a probe molecule. In a brief experimental procedure, 0.1 g of Au/mp-g-C3N4 sample was dispersed in 100 mL of a mixture of a 5 104 mol L1 aqueous TA solution and a 2 103 mol L1 NaOH solution at room temperature. The resulting suspension was magnetically stirred and exposed to sunlight. At every 5 min intervals, the suspension was periodically withdrawn and centrifuged to measure the maximum PL intensity using fluorescence spectrophotometer with an excitation wavelength of 365 nm.
working electrodes were as follows: 10 mg of photocatalyst was dispersed in 1 mL double distilled water to make slurry. Then, the obtained slurry was dripped into an indium tin oxide (ITO) glass with 1 cm 0.5 cm area. Finally, the electrode was dried at 120 C for 1 h.
2.6. Electrochemical impedance and photocurrent measurements
The synthesis strategy for mesoporous g-C3N4 nanosheets is according to Le Chatelier’s principle through a semi-closed system (Fig. S1). The semi-closed system was employed to control inherent decomposition of the polymer. In general, melamine is polymerized into melem at first when it is heated [11]. As temperature rises, finally leads to mp-g-C3N4, accompanying with release of ammonia in semi-closed system as shown in Scheme 1. As it is well known that, the decomposition of melem and carbon nitride polymer formation takes place during the heating process via generation of nitrogen and cyano fragments mainly release of NH3 [24]. Therefore, the decomposition of polymer can be controlled by adjusting the opened extent of the alumina boat with fixed heat rate and time. Further, Au nanoparticles decorated mp-g-C3N4 nanosheets were synthesized by in situ photo-reduction method using HAuCl4 and methanol. Initially, Au (III) ions from the source (HAuCl4 aqueous solution) were deposited on the surface of the synthesized mp-g-C3N4 and the suspension was irradiated under
Electrochemical impedance spectroscopy (EIS) and photocurrent response of the synthesized photocatalysts were carried out in a conventional three electrode quartz cell assembly using an electrochemical workstation (model: IM6e, Zahner, GmbH, Germany). The synthesized photocatalyst was used as the working electrode, a platinum wire was used as the counter electrode, and the reference electrode was Ag/AgCl (3 N KCl). The EIS experiments were performed at an open circuit potential of 0.24 V within a frequency range from 10 mHz to 100 kHz with a sinusoidal potential perturbation of 5 mV amplitude and 10 mM K3[Fe (CN)6] solution containing 0.1 M KCl was used as the electrolyte. The transient photocurrent response of the synthesized samples was investigated for several on-off cycles of irradiation by a Xe arc lamp through a UV-cutoff filter. Na2SO4 (0.01 mol L1) aqueous solution was used as the electrolyte. The preparation procedures of
3. Results and discussion 3.1. Formation mechanism of Au/mp-g-C3N4 nanosheets
Fig. 5. (a) Nitrogen adsorption–desorption isotherm plots and pore size distribution (inset) of the synthesized Au/mp-g-C3N4-1.0 nanosheets and bulk g-C3N4, (b) room temperature photoluminescence (PL) spectra of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts excited at 365 nm.
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
55
Fig. 6. (a) Photocurrent response and (b) EIS profiles of the synthesized mp-g-C3N4 and Au/mp-g-C3N4-1.0 photocatalysts.
visible light. Then, the Au (III) ions were reduced by electrons photogenerated from mp-g-C3N4, and the elemental Au nanoparticles were deposited on the surface of the mg-C3N4 [25,26]. 3.2. Characterizations of the Au/mp-g-C3N4 nanosheets The phase structure and composition of the synthesized samples were investigated by XRD studies. The typical diffraction patterns are shown in Fig. 1. The characteristic diffraction peak at 27.5 , corresponds to (0 0 2) plane of mp-g-C3N4 is due to the interlayer stacking of the conjugated aromatic systems [8]. The diffraction peaks at 2u values of 38.28 , 44.37, 64.63 and 77.74 are corresponding to (111), (2 0 0), (2 2 0) and (3 11) planes of Au nanoparticles, respectively. These reflections are well consistent with the cubic crystal phase of Au nanoparticles (JCPDS No. 040784). Moreover, the intensity of these diffraction peaks was increased with increasing Au content on mp-g-C3N4. Notably, the intensity of (0 0 2) peak of mp-g-C3N4 was gradually decreased with increasing Au loading, suggesting that the introduction of Au nanoparticles significantly inhibits the growth of the mp-g-C3N4 [12,13]. It can also be seen from the Fig. 1, the diffraction peak position for Au/mp-g-C3N4 samples shifted to a slightly lower 2u,
further confirming the successful deposition of the Au nanoparticles on the surface of mp-g-C3N4 [27]. The average crystallite size of Au in Au/mp-g-C3N4-0.5, Au/mp-g-C3N4-1.0 and Au/mp-gC3N4-2.0 nanosheets was estimated to be 10.2 nm, 12 nm and 15.6 nm, respectively, according to the Scherrer equation. UV–vis diffuse reflectance spectroscopy (DRS) was performed to justify the light-harvesting ability of the synthesized mp-gC3N4 and Au/mp-g-C3N4 nanosheets. As shown in Fig. 2, the synthesized mp-g-C3N4 exhibited a strong absorption edge in UV and visible-light regions, corresponding wavelength of 465 nm. Interestingly, Au/mp-g-C3N4 nanosheets exhibited improved light absorption in UV region when compare to mp-g-C3N4, which can be attributed to the characteristic UV light absorption of Au nanoparticles [28]. More remarkably, the synthesized Au/mp-gC3N4 nanosheets show a significant enhancement of light absorption in the visible range (500–600 nm), owing to the surface plasmon resonance effect of the Au nanoparticles. Moreover, the light absorption in this region was increased with increasing Au loading. The red shift of the absorption wavelength was also observed for Au/mp-g-C3N4 nanosheets compared to mp-g-C3N4. The surface plasmon resonance generated by Au nanoparticles can cause the strong local field enhancement
Fig. 7. (a) Comparison of photocatalytic activity for the degradation of RhB under sunlight irradiation over bulk g-C3N4, mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts and (b) the corresponding first-order kinetics plots of bulk g-C3N4, mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts.
56
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
around Au nanoparticles, which can improve the light absorption of the surrounding molecules [29,30]. The enhanced light harvesting efficiency can therefore provide more photo-induced charge carriers needed for the photocatalytic reactions, which can result in enhanced photocatalytic properties. The band gap energies of mp-g-C3N4, Au/mp-g-C3N4-0.5, Au/mp-g-C3N4-1.0 and Au/mp-g-C3N4-2.0 were calculated to be 2.65 eV, 2.52 eV, 2.48 eV and 2.45 eV, respectively, according to the Kubelka–Munk functions against the photon energy. Fig. 3 shows the Raman spectroscopic measurements of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets recorded with a 785 nm laser as the excitation source. The observed Raman peaks located at 710 and 988 cm1 for all the catalysts assigned to the characteristic breathing modes of aromatic s-triazine ring, which is present in the g-C3N4 crystal structure [12,31]. The peak observed at 1569 cm1 can be assigned to the C¼N stretching vibration of s-triazine ring [31]. After the loading of Au nanoparticles, there is no distinct change in the Raman pattern of Au/ mp-g-C3N4 nanosheets as compared with that of mp-g-C3N4, which indicates that the Au/mp-g-C3N4 nanosheets retain the same crystal structure of mp-g-C3N4. However, the absorption of the peaks of Au/mp-g-C3N4 nanosheets become broader and weaker and shifted to a higher wavenumber as the Au content is increased from 0.5 to 2.0 mol%, demonstrating increased crystalline defects within the framework. Such crystalline defects could be caused at the contact region of Au and mp-g-C3N4, which can strongly influence the characteristic vibrational frequency of the mp-g-C3N4 as shown in Fig. 3 [29,32]. These results are well consistent with the XRD observations which also confirm the broadening of diffraction peaks suggesting the Au nanoparticles induced lattice strain in the mp-g-C3N4 framework. In order to study the microstructure of the Au/mp-g-C3N4 nanosheets, transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) were performed. As shown in Fig. 4a, the bulk g-C3N4 exhibited aggregated layers and irregular shapes (inset of Fig. 4a), whereas Au/mp-g-C3N4-1.0 sample composed of a thin and flat sheet-like structure with numerous pores of nanometers in size, revealing the successful porousification in g-C3N4. It is also observed from the TEM image of Au/mp-g-C3N4-1.0 (Fig. 4b); the Au nanoparticles in
Fig. 8. Schematic illustration of the high photocatalytic activity of Au/mp-g-C3N4 nanosheets for the degradation of RhB under sunlight irradiation.
the range of 10–20 nm were generated on the surface of the mp-gC3N4 after irradiation of the light in presence of Au salt. These Au nanoparticles are approximately spherical in shape and anchored on the surface of the mp-g-C3N4. The HRTEM image of the Au/mpg-C3N4-1.0 photocatalyst exhibits the clear lattice fringes with an inter-planar distance of 0.236 nm corresponding to the (111) plane of metallic Au (Fig. 4c), which is further confirming the formation of Au nanoparticles [18]. These results are in good agreement with the XRD observations. In addition, X-ray energy dispersive spectroscopy (EDS) was conducted to determine the chemical compositions of the synthesized samples (Fig. S2). The loaded Au contents on the surface of Au/mp-g-C3N4-0.5, Au/mp-g-C3N4-1.0 and Au/mp-g-C3N4-2.0 were found to be 0.42, 0.98 and 1.75 mol%, respectively. The nitrogen adsorption–desorption isotherms and Barrett– Joyner–Halenda (BJH) pore-size distribution curves were measured to learn more about the porous nature and specific surface area of synthesized Au/mp-g-C3N4 nanosheets. Fig. 5a exhibits type IV adsorption–desorption isotherm with a hysteresis loop, suggesting the mesoporous nature of Au/mp-g-C3N4 nanosheets [33]. It can be found that the synthesized Au/mp-g-C3N4-1.0 sample shows a relatively large specific surface area (224.78 m2 g1), which is much higher than the previously reported porous g-C3N4 [34,35]. The pore volume of Au/mp-gC3N4 nanosheets is estimated to be 0.45 cm3 g1. And an average pore diameter of 12 nm for Au/mp-g-C3N4-1.0 can be calculated from the BJH pore size distribution (inset of Fig. 5a). Besides, the specific surface area and pore structures of bulk g-C3N4 are very poor (Fig. 5a). The higher specific surface area of Au/mp-g-C3N4 nanosheets indicated that the transmission distance and recombination rate of photogenerated charge carriers were reduced and the transportation efficiency was substantially raised along with the expanded pore volume [20]. The improved separation and migration of photogenerated charge carriers lead to the enhanced photocatalytic activity of Au/mp-g-C3N4 nanosheets. The photoluminescence (PL) studies are widely used to investigate the migration, transfer, and recombination processes of the photogenerated electron–hole pairs in semiconductor materials, since PL emission arises from the recombination of charge carriers [11,36]. Fig. 5b shows the PL spectra of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts recorded at room temperature with an excitation wavelength of 365 nm. From the Fig. 5b, it can be observed that there is a significant decrease in the PL intensity of Au/mp-g-C3N4 photocatalysts compared to that of pure mp-g-C3N4. The greater quenching of PL signal suggests that the dispersed Au nanoparticles on the surface of mp-g-C3N4 could effectively inhibit the recombination of photogenerated charge carriers leading to greater separation of photogenerated electron–hole pairs in Au/ mp-g-C3N4 nanosheets. Thus, both improved light harvesting and the faster separation of photogenerated charge carriers contribute to the enhanced photocatalytic activity of Au/mp-g-C3N4 photocatalysts. Fig. 6a shows the transient photocurrent responses of mp-gC3N4 and Au/mp-g-C3N4 electrodes under visible light irradiation, which may widely regarded as the most efficient evidence to demonstrate the light response and charge separation in semiconductor photocatalysts [37]. The electrodes of both the samples are prompt in generating photocurrent with a reproducible response to on-off cycles. The visible irradiated photocurrent density of the Au/mp-g-C3N4-1.0 photocatalyst is much higher than that of mp-g-C3N4 as shown in Fig. 6a. This is due to the fact that the embedded Au nanoparticles on the porous surface of gC3N4 can create the crystal defects within the frame work and such defects can act as traps to capture photoelectrons and then these trapped electrons may be de-trapped and transferred to the
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
57
Fig. 9. (a) Effects of different scavengers on the degradation of RhB in presence of Au/mp-g-C3N4-1.0 photocatalyst under sunlight irradiation and (b) OH trapping PL spectra of Au/mp-g-C3N4-1.0 with TA solution under sunlight irradiation.
3.3. Photocatalytic activity of Au/mp-g-C3N4 nanosheets
Fig. 10. Reusability of the Au/mp-g-C3N4-1.0 photocatalyst for the photocatalytic degradation of RhB during five successive experimental runs under sunlight irradiation.
collecting electrode for photocurrent generation [29,32]. Therefore, in the case of Au/mp-g-C3N4 photocatalysts, the separation and transfer of photogenerated electron–hole pairs are more efficient. Furthermore, electrochemical impedance spectroscopy (EIS) was also used to investigate the photogenerated charge separation process [14,38]. Fig. 6b shows the Nyquist plots of the synthesized mp-g-C3N4 and Au/mp-g-C3N4-1.0 nanosheets. These Nyquist plots are best fitted to the equivalent Randle circuit shown in the inset of Fig. 6b. The EIS results reveal that the diameter of the semicircle observed for Au/mp-g-C3N4-1.0 nanosheets is significantly smaller than that of mp-g-C3N4. A lower Rct value of Au/mp-g-C3N4 nanosheets suggests more efficient charge transfer across the electrode interface, reducing the possibility of charge recombination and thus enhancing the photocatalytic activity. These results are well consistent with the PL and photocurrent response studies.
The photocatalytic activity of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets was evaluated by studying the photocatalytic degradation of RhB under sunlight irradiation. The photolysis of RhB (without catalyst) and adsorption ability (Figs. S3 and S4) of catalysts were also examined, and the results suggest that both light and catalyst are necessary for efficient photocatalytic reaction. For comparison, photocatalytic degradation of bulk g-C3N4 was also studied, which showed rather poor photocatalytic activity owing to the low surface area, high recombination of photogenerated charge carriers and limited photo-response range. As shown in Fig. 7a, the synthesized Au/mpg-C3N4 nanosheets exhibit a much higher photocatalytic activity than mp-g-C3N4 and bulk g-C3N4. The photocatalytic results also demonstrate that the loading amount of Au has a strong influence on the photodegradation of RhB. Thus, the optimal Au content in mp-g-C3N4 nanosheets is 1 mol%, and then the photocatalytic activity decreases with increasing Au content. The decrease in the photocatalytic activity by loading with excess amounts of Au may be due to following reasons: (1) the large Au nanoparticles in Au/ mp-g-C3N4-2.0 may act as the recombination centers of photogenerated charge carriers and reduce quantum efficiency, rather than facilitate charge transport and reduce charge recombination and (2) the presence of a large number of Au nanoparticles within the pore channels may also slow down mass transport and reduce the reaction rate [32]. These results clearly demonstrating that the introduction of Au nanoparticles on the surface of mp-g-C3N4 plays an important role in the enhancement of RhB photodegradation. Moreover, the RhB photodegradation was found to follow pseudofirst order kinetics (Fig. 7b), which also indicates that the photocatalytic activity of Au/mp-g-C3N4-1.0 nanosheets is almost 6 times higher than that of bulk g-C3N4 and 3.5 times higher than that of mp-g-C3N4 nanosheets. Au nanoparticles modification greatly improved the photocatalytic RhB degradation performance of mp-g-C3N4 nanosheets under sunlight irradiation. This is due to the fact that the deposited Au nanoparticles on the surface of the mp-g-C3N4 nanosheets were charge polarized, actively absorbed and scattered the visible light in the solar spectrum because of their strong surface plasmon resonance (SPR) effect [13,39]. In general, surface plasmons are
58
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
collective oscillations confined to the surfaces of conducting materials and can strongly interact with light. The SPR of these Au nanoparticles can be tuned by size, shape, material and proximity to other nanoparticles. The noble metal Au Fermi level mainly exists between the conduction band (CB) and valance band (VB) of polar semiconductor g-C3N4 leading to the formation of plasmonic photocatalysts [17,40,41]. The photogenerated electrons excited to the CB of mp-g-C3N4 under sunlight irradiation are readily trapped by the Au nanoparticles which results the increment of electron density on it (Fig. 8). These trapped electrons can reduce the surface-chemisorbed O2 to produce O2, because ECB (1.3 eV vs. NHE) of g-C3N4 is more negative than E(O2/O2) (0.33 eV vs. NHE) [42]. This phenomenon has been proved by a N2 purging experiment which is used to examine the role of dissolved O2 in the solution. If N2 was bubbled into the reaction system, a rapid decrease in the degradation of RhB was observed compared with air-equilibrated conditions (no scavenger), which confirms that dissolved O2 plays an important role for the photodegradation process (Fig. 9a). Nevertheless, photogenerated holes cannot directly oxidize the adsorbed H2O molecules to OH on g-C3N4, due to EVB (+1.4 eV vs. NHE) of g-C3N4 is lower compared to E(OH/ H2O) (+2.68 eV vs. NHE) [36], which is also confirmed by adding the tert-butyl alcohol as a OH scavenger as shown in Fig. 9a. A small change in the degradation of RhB was observed after the addition of ammonium oxalate to the reaction solution, which is used as a hole scavenger. In addition, OH generation during the photocatalytic reaction was detected by a photoluminescence (PL) technique using terephthalic acid (TA) as a probe molecule, which readily reacts with OH to produce the highly fluorescent product, 2-hydroxyterephthalic acid (Fig. 9b) [43]. This result demonstrate that the OH only originate from the reaction of photogenerated electrons during the multistep reduction of dissolved O2. The stability and recyclability of a photocatalyst is important for its assessment and practical applications. To demonstrate the efficiency of the Au/mp-g-C3N4 photocatalyst, five successive experimental runs were carried out under same experimental conditions. From Fig. 10, it was found that the photocatalytic activity of Au/mp-g-C3N4-1.0 photocatalyst did not exhibit significant loss after five recycles for the photodegradation of RhB. Therefore, the Au/mp-g-C3N4 can be concluded as stable photocatalyst during the photocatalytic process. All the above aspects together contribute to make the Au/mp-g-C3N4 nanosheets as highly efficient and stable visible light active photocatalysts for the environmental applications. 4. Conclusions In summary, we have successfully synthesized a highly efficient visible-light-driven plasmonic photocatalyst Au/mp-g-C3N4 by a template-free and green in situ photo-reduction strategy. A strong visible light absorption and Raman blue shift of mp-g-C3N4 were observed in Au/mp-g-C3N4 photocatalysts owing to the characteristic surface plasmon resonance of Au nanoparticles. Moreover, the optimum photocatalytic activity at 1 mol% Au loaded mp-g-C3N4 nanosheets is almost 6 times higher than bulk g-C3N4 and 3.5 times higher than mesoporous g-C3N4 for the photodegradation of RhB under sunlight irradiation. The high photocatalytic performance of the Au/mp-g-C3N4 nanosheets is associated with the extended light-absorption range, high specific surface area, large pore volume, increased light harvesting capacity and efficient charge separation due to the surface plasmon resonance effect of Au nanoparticles. Very importantly, the reusability of the synthesized plasmonic photocatalysts was very high. Thus, the present study provides new insight into the design and development of advanced porous g-C3N4-based plasmonic photocatalysts for energy and environmental applications.
Acknowledgements Surendar Tonda thanks the Ministry of Human Resource Development, Government of India, for providing a fellowship. The authors also thank Prof. A. K. Ganguli, Indian Institute of Technology Delhi, for carrying out part of this work in his lab. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. materresbull.2015.11.011. References [1] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24 (2012) 229–251. [2] T.P. Yoon, M.A. Ischay, J. Du, Nat. Chem. 2 (2010) 527–532. [3] A. Kubacka, M. Fernandez-García, G. Colon, Chem. Rev. 112 (2012) 1555–1614. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [5] S.X. Ouyang, H. Tong, N. Umezawa, J. Cao, P. Li, Y. Bi, Y. Zhang, J. Ye, J. Am. Chem. Soc. 134 (2012) 1974–1977. [6] K.N. Wood, R. O’Hayre, S. Pylypenko, Energy Environ. Sci. 7 (2014) 1212–1249. [7] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [8] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [9] L. Ge, C. Han, J. Liu, J. Mater. Chem. 22 (2012) 11843–11850. [10] K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti, K. Domen, J. Phys. Chem. C 113 (2009) 4940–4947. [11] S.C. Yan, Z.S. Li, Z.G. Zo, Langmuir 26 (2010) 3894–3901. [12] T. Surendar, S. Kumar, S. Kandula, V. Shanker, J. Mater. Chem. A 2 (2014) 6772– 6780. [13] S. Samanta, S. Martha, K.M. Parida, ChemCatChem 6 (2014) 1453–1462. [14] S. Kumar, T. Surendar, B. Kumar, A. Baruah, V. Shanker, J. Phys. Chem. C 117 (2013) 26135–26143. [15] L. Ge, C. Han, X. Xiao, L. Guo, Int. J. Hydrogen Energy 38 (2013) 6960–6969. [16] Y.S. Xu, W.D. Zhang, ChemCatChem 5 (2013) 2343–2351. [17] W. Hou, S.B. Cronin, Adv. Funct. Mater. 23 (2012) 1612–1619. [18] N. Cheng, J. Tian, Q. Liu, C. Ge, A.H. Qusti, A.M. Asiri, A.O. Al-Youbi, X. Sun, ACS Appl. Mater. Interfaces 5 (2013) 6815–6819. [19] B. Kiskan, J. Zhang, X. Wang, M. Antonietti, Y. Yagci, ACS Macro Lett. 1 (2012) 546–549. [20] Y. Li, Z.Y. Fu, B.L. Su, Adv. Funct. Mater. 22 (2012) 4634–4667. [21] H. Yan, Chem. Commun. 48 (2012) 3430–3432. [22] Y. Wang, X.C. Wang, M. Antonietti, Y.J. Zhang, ChemSusChem 3 (2010) 435– 439. [23] M. Groenewolt, M. Antonietti, Adv. Mater. 17 (2005) 1789–1792. [24] B.V. Lotsch, W. Schnick, Chem. Mater. 18 (2006) 1891–1900. [25] J. Xue, S. Ma, Y. Zhou, Z. Zhang, M. He, ACS Appl. Mater. Interfaces 7 (2015) 9630–9637. [26] Y. Bu, Z. Chen, W. Li, Appl. Catal. B: Environ. 144 (2014) 622–630. [27] R.A. Naphade, M. Tathavadekar, J.P. Jog, S. Agarkar, S. Ogale, J. Mater. Chem. A 2 (2014) 975–984. [28] H. Zhu, X. Chen, Z. Zheng, X. Ke, E. Jaatinen, J. Zhao, C. Guo, T. Xie, D. Wang, Chem. Commun. (2009) 7524–7526. [29] Y. Li, H. Wang, Q. Feng, G. Zhou, Z.S. Wang, Energy Environ. Sci. 6 (2013) 2156– 2165. [30] E.D. Smolensky, M.C. Neary, Y. Zhou, T.S. Berquoa, V.C. Pierre, Chem. Commun. 47 (2011) 2149–2151. [31] P.V. Zinin, L.C. Ming, S.K. Sharma, V.N. Khabashesku, X. Liu, S. Hong, S. Endo, T. Acosta, Chem. Phys. Lett. 472 (2009) 69–73. [32] H. Li, Z.F. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 129 (2007) 4538– 4539. [33] B. Naik, S.M. Kim, C.H. Jung, S.Y. Moon, S.H. Kim, J.Y. Park, Adv. Mater. Interfaces 1 (2014) 1300018. [34] G. Dong, L. Zhang, J. Mater. Chem. 22 (2012) 1160–1166. [35] Y. Zhang, Z. Liu, G. Wu, W. Chen, Nanoscale 4 (2012) 5300–5303. [36] T. Surendar, S. Kumar, V. Shanker, Phys. Chem. Chem. Phys. 16 (2014) 728–735. [37] X. Bai, L. Wang, R. Zong, Y. Zhu, J. Phys. Chem. C 117 (2013) 9952–9961. [38] S. Kumar, A. Baruah, T. Surendar, B. Kumar, V. Shanker, B. Sreedhar, Nanoscale 6 (2014) 4830–4842. [39] H. Zhu, X. Chen, Z.F. Zheng, X. Ke, E. Jaatinen, J. Zhao, C. Guo, T. Xied, D. Wang, Chem. Commun. (2009) 7524–7526. [40] P. Wang, B. Huang, Y. Dai, M.H. Whangbo, Phys. Chem. Chem. Phys. 14 (2012) 9813–9825. [41] Y. Ide, F. Liu, J. Zhang, N. Kawamoto, K. Komaguchi, Y. Bandoa, D. Golberga, J. Mater. Chem. A 2 (2014) 4150–4156. [42] J. Jiang, H. Li, L.Z. Zhang, Chem. Eur. J. 18 (2012) 6360–6369. [43] K. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Electrochem. Commun. 2 (2000) 207–210.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Surface plasmon resonance-induced photocatalysis by Au nanoparticles decorated mesoporous g-C3N4 nanosheets under direct sunlight irradiation Surendar Tonda, Santosh Kumar1, Vishnu Shanker* Department of Chemistry, National Institute of Technology, Warangal, Warangal 506004, Telangana, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 May 2015 Received in revised form 5 November 2015 Accepted 8 November 2015 Available online 17 November 2015
In recent years, surface plasmon-induced photocatalytic materials with tunable mesoporous framework have attracted considerable attention in energy conversion and environmental remediation. Herein we report a novel Au nanoparticles decorated mesoporous graphitic carbon nitride (Au/mp-g-C3N4) nanosheets via a template-free and green in situ photo-reduction method. The synthesized Au/mp-gC3N4 nanosheets exhibit a strong absorption edge in visible and near-IR region owing to the surface plasmon resonance effect of Au nanoparticles. More attractively, Au/mp-g-C3N4 exhibited much higher photocatalytic activity than that of pure mesoporous and bulk g-C3N4 for the degradation of rhodamine B under sunlight irradiation. Furthermore, the photocurrent and photoluminescence studies demonstrated that the deposition of Au nanoparticles on the surface of mesoporous g-C3N4 could effectively inhibit the recombination of photogenerated charge carriers leading to the enhanced photocatalytic activity. More importantly, the synthesized Au/mp-g-C3N4 nanosheets possess high reusability. Hence, Au/mp-g-C3N4 could be promising photoactive material for energy and environmental applications. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors A. Nanostructures B. Optical properties C. X-ray diffraction D. Catalytic properties
1. Introduction As a clean and renewable energy source, solar energy can become a sustainable solution to the increasing global energy demands and environmental issues using efficient semiconductor photocatalysts [1–3]. Therefore, development of novel photocatalyst is the key strategy for direct use of solar energy. Since the discovery of photocatalytic water splitting for hydrogen production using TiO2 electrodes by Fujishima and Honda under ultraviolet (UV) light in 1972 [4], there has been significant progress in the design of efficient semiconductor photocatalysts for different kinds of applications such as photodecomposition of hazardous substances, water splitting for hydrogen fuel, artificial photosynthesis and photocatalytic conversion of CO2 to energyrich hydrocarbon fuels [5–7]. Unfortunately, most of the developed
* Corresponding author. Fax: +91 870 2459547. E-mail address: [email protected] (V. Shanker). 1 Present address: European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK. http://dx.doi.org/10.1016/j.materresbull.2015.11.011 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
photocatalysts containing metals can only work in the UV region with moderate performance. Recently, attention has been devoted to a new class of metalfree organic semiconductor, especially graphitic carbon nitride (gC3N4), which is considered to be a breakthrough in the field of visible light active photocatalysis due to its fascinated properties such as visible light response, suitable band gap, high chemical and physical stability, low cost, and nontoxicity [8,9]. Unfortunately, the practical applications of g-C3N4 are still limited due to the short lifespan of photogenerated electron–hole pairs, low specific surface area and poor quantum yield [10]. However, many attempts have been made to improve the performance of g-C3N4, such as doping with foreign elements, and coupling with other semiconductors or noble metals [11–15]. Among them, visible light active plasmonic photocatalysts are identified as one of the most promising alternatives to traditional photocatalysts, which can efficiently improve the efficiency of photocatalytic process [16]. Noble metal nanoparticles, such as Au, Pt, and Ag can strongly absorb visible light owing to their localized surface plasmon resonance (SPR) from the collective oscillation of the surface electrons and they exhibit a great potential to extend the lightabsorption range of semiconductors [17]. Maeda et al. [10]
52
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
reported the noble metals such as Pt, Au and Pd deposited g-C3N4 photocatalysts for the enhanced photocatalytic activity for H2 evolution. For instance, Au@g-C3N4 hybrids display a synergistic effect between semiconductor and plasmonic photocatalysis and exhibit superior photocatalytic activity for the degradation of methyl orange [18], but still the system efficiency is low due to low surface area and inadequate light harvesting capacity. On the other hand, materials with porous structure have been extremely involved in recently developed energy storage and conversion systems due to their large surface area, interfacial transport and shorten diffusion pathways [19,20]. Considering the advantages of porous materials, various synthetic strategies have been employed to introduce pores in g-C3N4. For example, Yan [21] reported the synthesis of mesoporous g-C3N4 by using a Pluronic P123 as a structure directing group for improved photocatalytic H2 evolution under visible light. But most of these methods require additional soft or hard templates and complicated post-treatments [22,23]. Therefore, the development of a template-free and green strategy is most economical with improved the photocatalytic properties. In this report, Au nanoparticles deposited mesoporous g-C3N4 nanosheets were synthesized by template free controlled heat treatment and in situ photo-reduction method. The photocatalytic activity of the synthesized Au/mp-g-C3N4 nanosheets was evaluated by photodegradation of rhodamine B (RhB) under sunlight irradiation. The effect of Au loading on the optical absorption, photoluminescence, electrochemical properties and photocatalytic performance were investigated in detail. The photocatalytic mechanism of Au/mp-g-C3N4 photocatalysts was proposed based on the surface plasmon effect of the Au nanoparticles. Our results demonstrated that Au/mp-g-C3N4 exhibited significantly enhanced photocatalytic activity for the degradation of RhB under sunlight irradiation, and this novel plasmon-induced photocatalyst was stable even after five cycling photocatalytic experiments.
material. Briefly, melamine was placed in a semi-closed aluminum boats to control the formation and escape rate of gas products in calcination process, and then heated at 550 C for 2 h in a tubular furnace under N2 atmosphere with a slow ramp rate. The obtained pale yellow colored product from the semi-closed system was grounded into fine powders, and resultant mesoporous g-C3N4 was named as mp-g-C3N4. For comparison, bulk g-C3N4 was synthesized according to our previous reported procedure [12]. Au nanoparticles loaded mesoporous g-C3N4 samples were synthesized by in situ photo-reduction method. In brief, 50 mg of the synthesized mp-g-C3N4 powder was added to 40 mL of methanol, and the suspension was sonicated for 1 h. Then, 0.01 mM HAuCl4 aqueous solution was added to suspension, which was stirred for 30 min at room temperature to complete adsorption of Au nanoparticles on the surface of mp-g-C3N4. After that the above suspension was irradiated for 1 h by a Xenon lamp with a 420 nm cutoff filter under constant stirring. The resulting solution turns to brown red color. The final product was obtained by washing with double distilled water and dried at 70 C for overnight. The resulting 1 mol% Au loaded mp-g-C3N4 photocatalyst was denoted as Au/mp-g-C3N4-1.0. Accordingly, 0.5 mol% and 2.0 mol% Au loaded mp-g-C3N4 photocatalysts were synthesized by the same method and denoted as Au/mp-g-C3N4-0.5 and Au/mp-g-C3N4-2.0, respectively. 2.3. Material characterization
Melamine (Sigma–Aldrich, 99.0%), chloroauric acid (Sigma– Aldrich, 99.9%) rhodamine B (Sigma–Aldrich, 95.0% dye content), terephthalic acid (Merck, AR grade), tert-butyl alcohol (Merck, AR grade), and ammonium oxalate (Merck, AR grade) were used as received. All other reagents used in this work were of analytically pure grade and used without further purification. All aqueous solutions were prepared with double distilled water.
X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 advance X-ray diffractometer using Ni filtered Cu Ka (l = 1.5406 Å) radiation in a 2u scan range between 10 and 80 . UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained on a Thermo Scientific Evolution 600 diffuse reflectance spectrophotometer, and BaSO4 was used as a reference. Raman studies were conducted on a PerkinElmer RamanStation 400 spectrometer with a 785 nm laser as the excitation source. The transmission electron microscopy (TEM) measurements were conducted on JEOL JSM6700F transmission electron microscope with an acceleration voltage of 200 kV. The chemical composition of the samples was investigated by energy dispersive spectroscopy (SEM–EDS; Oxford Instruments, INCAx-act). The surface area measurements were recorded using a Quanta chrome NOVA 1200e and specific surface area of the catalyst was estimated using Brunauer–Emmett–Teller (BET) method. UV–visible absorption spectra (UV–vis) were recorded on a Thermo Scientific Evolution 600 UV–vis NIR spectrophotometer. The photoluminescence (PL) spectra of the photocatalysts were recorded on TSC Solutions F96pro fluorescence spectrophotometer at an excitation wavelength of 365 nm.
2.2. Method
2.4. Evaluation of photocatalytic activity
The mesoporous g-C3N4 sample was synthesized by a simple and facile heating method using low cost melamine as the starting
Rhodamine B was chosen as a representative hazardous dye in the present work. The photocatalytic activities of the synthesized
2. Experimental 2.1. Materials
Scheme 1. Schematic representation for the synthesis of Au nanoparticles deposited mesoporous g-C3N4 nanosheets.
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
53
Fig. 1. XRD patterns of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets: (a) mp-g-C3N4, (b) Au/mp-g-C3N4-0.5, (c) Au/mp-g-C3N4-1.0 and (d) Au/mp-g-C3N4-2.0.
under similar conditions in May 2014 at NIT Warangal, where the fluctuation of the sunlight intensity (5 kWh/m2/day) is minimal during this month. In each experiment, 0.1 g of photocatalyst was added to 250 mL RhB solution with a concentration of 5 mg L1. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to achieve adsorption equilibrium between the RhB molecules and the photocatalyst. At given irradiation time intervals, the suspensions were periodically withdrawn and centrifuged to separate the photocatalyst particulates for analysis. The filtrates were analyzed by recording variations at the wavelength of maximal absorption in the UV–vis spectrophotometer. A blank test was also carried out on an aqueous RhB solution without photocatalyst under sunlight irradiation, i.e., photolysis of RhB, to examine the degradation efficiency of the photocatalyst. 2.5. Analysis of reactive species
Fig. 2. UV–vis diffuse reflectance spectra of the synthesized mp-g-C3N4 and Au/ mp-g-C3N4 nanosheets.
samples were evaluated by monitoring the photocatalytic degradation of RhB in an aqueous solution under sunlight irradiation. Photocatalytic experiments of all the samples were conducted
The effect of reactive species on the photodegradation of RhB over Au/mp-g-C3N4 photocatalysts was examined to understand the photocatalytic mechanism. Different type of scavengers was subjected into the RhB aqueous solution prior to addition of photocatalyst. The analysis method was similar to the photocatalytic degradation process. Furthermore, the generation of hydroxyl radicals (OH) on the surface of sunlight irradiated photocatalyst was detected by the photoluminescence (PL)
Fig. 3. Raman spectra of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets: (a) mp-g-C3N4, (b) Au/mp-g-C3N4-0.5, (c) Au/mp-g-C3N4-1.0 and (d) Au/mp-g-C3N4-2.0.
54
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
Fig. 4. (a) TEM image of the synthesized Au/mp-g-C3N4-1.0 nanosheets, bulk g-C3N4 (inset), (b) magnified image of Au/mp-g-C3N4-1.0 nanosheets and (c) corresponding HRTEM image of Au nanoparticles.
technique using terephthalic acid (TA) as a probe molecule. In a brief experimental procedure, 0.1 g of Au/mp-g-C3N4 sample was dispersed in 100 mL of a mixture of a 5 104 mol L1 aqueous TA solution and a 2 103 mol L1 NaOH solution at room temperature. The resulting suspension was magnetically stirred and exposed to sunlight. At every 5 min intervals, the suspension was periodically withdrawn and centrifuged to measure the maximum PL intensity using fluorescence spectrophotometer with an excitation wavelength of 365 nm.
working electrodes were as follows: 10 mg of photocatalyst was dispersed in 1 mL double distilled water to make slurry. Then, the obtained slurry was dripped into an indium tin oxide (ITO) glass with 1 cm 0.5 cm area. Finally, the electrode was dried at 120 C for 1 h.
2.6. Electrochemical impedance and photocurrent measurements
The synthesis strategy for mesoporous g-C3N4 nanosheets is according to Le Chatelier’s principle through a semi-closed system (Fig. S1). The semi-closed system was employed to control inherent decomposition of the polymer. In general, melamine is polymerized into melem at first when it is heated [11]. As temperature rises, finally leads to mp-g-C3N4, accompanying with release of ammonia in semi-closed system as shown in Scheme 1. As it is well known that, the decomposition of melem and carbon nitride polymer formation takes place during the heating process via generation of nitrogen and cyano fragments mainly release of NH3 [24]. Therefore, the decomposition of polymer can be controlled by adjusting the opened extent of the alumina boat with fixed heat rate and time. Further, Au nanoparticles decorated mp-g-C3N4 nanosheets were synthesized by in situ photo-reduction method using HAuCl4 and methanol. Initially, Au (III) ions from the source (HAuCl4 aqueous solution) were deposited on the surface of the synthesized mp-g-C3N4 and the suspension was irradiated under
Electrochemical impedance spectroscopy (EIS) and photocurrent response of the synthesized photocatalysts were carried out in a conventional three electrode quartz cell assembly using an electrochemical workstation (model: IM6e, Zahner, GmbH, Germany). The synthesized photocatalyst was used as the working electrode, a platinum wire was used as the counter electrode, and the reference electrode was Ag/AgCl (3 N KCl). The EIS experiments were performed at an open circuit potential of 0.24 V within a frequency range from 10 mHz to 100 kHz with a sinusoidal potential perturbation of 5 mV amplitude and 10 mM K3[Fe (CN)6] solution containing 0.1 M KCl was used as the electrolyte. The transient photocurrent response of the synthesized samples was investigated for several on-off cycles of irradiation by a Xe arc lamp through a UV-cutoff filter. Na2SO4 (0.01 mol L1) aqueous solution was used as the electrolyte. The preparation procedures of
3. Results and discussion 3.1. Formation mechanism of Au/mp-g-C3N4 nanosheets
Fig. 5. (a) Nitrogen adsorption–desorption isotherm plots and pore size distribution (inset) of the synthesized Au/mp-g-C3N4-1.0 nanosheets and bulk g-C3N4, (b) room temperature photoluminescence (PL) spectra of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts excited at 365 nm.
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
55
Fig. 6. (a) Photocurrent response and (b) EIS profiles of the synthesized mp-g-C3N4 and Au/mp-g-C3N4-1.0 photocatalysts.
visible light. Then, the Au (III) ions were reduced by electrons photogenerated from mp-g-C3N4, and the elemental Au nanoparticles were deposited on the surface of the mg-C3N4 [25,26]. 3.2. Characterizations of the Au/mp-g-C3N4 nanosheets The phase structure and composition of the synthesized samples were investigated by XRD studies. The typical diffraction patterns are shown in Fig. 1. The characteristic diffraction peak at 27.5 , corresponds to (0 0 2) plane of mp-g-C3N4 is due to the interlayer stacking of the conjugated aromatic systems [8]. The diffraction peaks at 2u values of 38.28 , 44.37, 64.63 and 77.74 are corresponding to (111), (2 0 0), (2 2 0) and (3 11) planes of Au nanoparticles, respectively. These reflections are well consistent with the cubic crystal phase of Au nanoparticles (JCPDS No. 040784). Moreover, the intensity of these diffraction peaks was increased with increasing Au content on mp-g-C3N4. Notably, the intensity of (0 0 2) peak of mp-g-C3N4 was gradually decreased with increasing Au loading, suggesting that the introduction of Au nanoparticles significantly inhibits the growth of the mp-g-C3N4 [12,13]. It can also be seen from the Fig. 1, the diffraction peak position for Au/mp-g-C3N4 samples shifted to a slightly lower 2u,
further confirming the successful deposition of the Au nanoparticles on the surface of mp-g-C3N4 [27]. The average crystallite size of Au in Au/mp-g-C3N4-0.5, Au/mp-g-C3N4-1.0 and Au/mp-gC3N4-2.0 nanosheets was estimated to be 10.2 nm, 12 nm and 15.6 nm, respectively, according to the Scherrer equation. UV–vis diffuse reflectance spectroscopy (DRS) was performed to justify the light-harvesting ability of the synthesized mp-gC3N4 and Au/mp-g-C3N4 nanosheets. As shown in Fig. 2, the synthesized mp-g-C3N4 exhibited a strong absorption edge in UV and visible-light regions, corresponding wavelength of 465 nm. Interestingly, Au/mp-g-C3N4 nanosheets exhibited improved light absorption in UV region when compare to mp-g-C3N4, which can be attributed to the characteristic UV light absorption of Au nanoparticles [28]. More remarkably, the synthesized Au/mp-gC3N4 nanosheets show a significant enhancement of light absorption in the visible range (500–600 nm), owing to the surface plasmon resonance effect of the Au nanoparticles. Moreover, the light absorption in this region was increased with increasing Au loading. The red shift of the absorption wavelength was also observed for Au/mp-g-C3N4 nanosheets compared to mp-g-C3N4. The surface plasmon resonance generated by Au nanoparticles can cause the strong local field enhancement
Fig. 7. (a) Comparison of photocatalytic activity for the degradation of RhB under sunlight irradiation over bulk g-C3N4, mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts and (b) the corresponding first-order kinetics plots of bulk g-C3N4, mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts.
56
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
around Au nanoparticles, which can improve the light absorption of the surrounding molecules [29,30]. The enhanced light harvesting efficiency can therefore provide more photo-induced charge carriers needed for the photocatalytic reactions, which can result in enhanced photocatalytic properties. The band gap energies of mp-g-C3N4, Au/mp-g-C3N4-0.5, Au/mp-g-C3N4-1.0 and Au/mp-g-C3N4-2.0 were calculated to be 2.65 eV, 2.52 eV, 2.48 eV and 2.45 eV, respectively, according to the Kubelka–Munk functions against the photon energy. Fig. 3 shows the Raman spectroscopic measurements of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets recorded with a 785 nm laser as the excitation source. The observed Raman peaks located at 710 and 988 cm1 for all the catalysts assigned to the characteristic breathing modes of aromatic s-triazine ring, which is present in the g-C3N4 crystal structure [12,31]. The peak observed at 1569 cm1 can be assigned to the C¼N stretching vibration of s-triazine ring [31]. After the loading of Au nanoparticles, there is no distinct change in the Raman pattern of Au/ mp-g-C3N4 nanosheets as compared with that of mp-g-C3N4, which indicates that the Au/mp-g-C3N4 nanosheets retain the same crystal structure of mp-g-C3N4. However, the absorption of the peaks of Au/mp-g-C3N4 nanosheets become broader and weaker and shifted to a higher wavenumber as the Au content is increased from 0.5 to 2.0 mol%, demonstrating increased crystalline defects within the framework. Such crystalline defects could be caused at the contact region of Au and mp-g-C3N4, which can strongly influence the characteristic vibrational frequency of the mp-g-C3N4 as shown in Fig. 3 [29,32]. These results are well consistent with the XRD observations which also confirm the broadening of diffraction peaks suggesting the Au nanoparticles induced lattice strain in the mp-g-C3N4 framework. In order to study the microstructure of the Au/mp-g-C3N4 nanosheets, transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) were performed. As shown in Fig. 4a, the bulk g-C3N4 exhibited aggregated layers and irregular shapes (inset of Fig. 4a), whereas Au/mp-g-C3N4-1.0 sample composed of a thin and flat sheet-like structure with numerous pores of nanometers in size, revealing the successful porousification in g-C3N4. It is also observed from the TEM image of Au/mp-g-C3N4-1.0 (Fig. 4b); the Au nanoparticles in
Fig. 8. Schematic illustration of the high photocatalytic activity of Au/mp-g-C3N4 nanosheets for the degradation of RhB under sunlight irradiation.
the range of 10–20 nm were generated on the surface of the mp-gC3N4 after irradiation of the light in presence of Au salt. These Au nanoparticles are approximately spherical in shape and anchored on the surface of the mp-g-C3N4. The HRTEM image of the Au/mpg-C3N4-1.0 photocatalyst exhibits the clear lattice fringes with an inter-planar distance of 0.236 nm corresponding to the (111) plane of metallic Au (Fig. 4c), which is further confirming the formation of Au nanoparticles [18]. These results are in good agreement with the XRD observations. In addition, X-ray energy dispersive spectroscopy (EDS) was conducted to determine the chemical compositions of the synthesized samples (Fig. S2). The loaded Au contents on the surface of Au/mp-g-C3N4-0.5, Au/mp-g-C3N4-1.0 and Au/mp-g-C3N4-2.0 were found to be 0.42, 0.98 and 1.75 mol%, respectively. The nitrogen adsorption–desorption isotherms and Barrett– Joyner–Halenda (BJH) pore-size distribution curves were measured to learn more about the porous nature and specific surface area of synthesized Au/mp-g-C3N4 nanosheets. Fig. 5a exhibits type IV adsorption–desorption isotherm with a hysteresis loop, suggesting the mesoporous nature of Au/mp-g-C3N4 nanosheets [33]. It can be found that the synthesized Au/mp-g-C3N4-1.0 sample shows a relatively large specific surface area (224.78 m2 g1), which is much higher than the previously reported porous g-C3N4 [34,35]. The pore volume of Au/mp-gC3N4 nanosheets is estimated to be 0.45 cm3 g1. And an average pore diameter of 12 nm for Au/mp-g-C3N4-1.0 can be calculated from the BJH pore size distribution (inset of Fig. 5a). Besides, the specific surface area and pore structures of bulk g-C3N4 are very poor (Fig. 5a). The higher specific surface area of Au/mp-g-C3N4 nanosheets indicated that the transmission distance and recombination rate of photogenerated charge carriers were reduced and the transportation efficiency was substantially raised along with the expanded pore volume [20]. The improved separation and migration of photogenerated charge carriers lead to the enhanced photocatalytic activity of Au/mp-g-C3N4 nanosheets. The photoluminescence (PL) studies are widely used to investigate the migration, transfer, and recombination processes of the photogenerated electron–hole pairs in semiconductor materials, since PL emission arises from the recombination of charge carriers [11,36]. Fig. 5b shows the PL spectra of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 photocatalysts recorded at room temperature with an excitation wavelength of 365 nm. From the Fig. 5b, it can be observed that there is a significant decrease in the PL intensity of Au/mp-g-C3N4 photocatalysts compared to that of pure mp-g-C3N4. The greater quenching of PL signal suggests that the dispersed Au nanoparticles on the surface of mp-g-C3N4 could effectively inhibit the recombination of photogenerated charge carriers leading to greater separation of photogenerated electron–hole pairs in Au/ mp-g-C3N4 nanosheets. Thus, both improved light harvesting and the faster separation of photogenerated charge carriers contribute to the enhanced photocatalytic activity of Au/mp-g-C3N4 photocatalysts. Fig. 6a shows the transient photocurrent responses of mp-gC3N4 and Au/mp-g-C3N4 electrodes under visible light irradiation, which may widely regarded as the most efficient evidence to demonstrate the light response and charge separation in semiconductor photocatalysts [37]. The electrodes of both the samples are prompt in generating photocurrent with a reproducible response to on-off cycles. The visible irradiated photocurrent density of the Au/mp-g-C3N4-1.0 photocatalyst is much higher than that of mp-g-C3N4 as shown in Fig. 6a. This is due to the fact that the embedded Au nanoparticles on the porous surface of gC3N4 can create the crystal defects within the frame work and such defects can act as traps to capture photoelectrons and then these trapped electrons may be de-trapped and transferred to the
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
57
Fig. 9. (a) Effects of different scavengers on the degradation of RhB in presence of Au/mp-g-C3N4-1.0 photocatalyst under sunlight irradiation and (b) OH trapping PL spectra of Au/mp-g-C3N4-1.0 with TA solution under sunlight irradiation.
3.3. Photocatalytic activity of Au/mp-g-C3N4 nanosheets
Fig. 10. Reusability of the Au/mp-g-C3N4-1.0 photocatalyst for the photocatalytic degradation of RhB during five successive experimental runs under sunlight irradiation.
collecting electrode for photocurrent generation [29,32]. Therefore, in the case of Au/mp-g-C3N4 photocatalysts, the separation and transfer of photogenerated electron–hole pairs are more efficient. Furthermore, electrochemical impedance spectroscopy (EIS) was also used to investigate the photogenerated charge separation process [14,38]. Fig. 6b shows the Nyquist plots of the synthesized mp-g-C3N4 and Au/mp-g-C3N4-1.0 nanosheets. These Nyquist plots are best fitted to the equivalent Randle circuit shown in the inset of Fig. 6b. The EIS results reveal that the diameter of the semicircle observed for Au/mp-g-C3N4-1.0 nanosheets is significantly smaller than that of mp-g-C3N4. A lower Rct value of Au/mp-g-C3N4 nanosheets suggests more efficient charge transfer across the electrode interface, reducing the possibility of charge recombination and thus enhancing the photocatalytic activity. These results are well consistent with the PL and photocurrent response studies.
The photocatalytic activity of the synthesized mp-g-C3N4 and Au/mp-g-C3N4 nanosheets was evaluated by studying the photocatalytic degradation of RhB under sunlight irradiation. The photolysis of RhB (without catalyst) and adsorption ability (Figs. S3 and S4) of catalysts were also examined, and the results suggest that both light and catalyst are necessary for efficient photocatalytic reaction. For comparison, photocatalytic degradation of bulk g-C3N4 was also studied, which showed rather poor photocatalytic activity owing to the low surface area, high recombination of photogenerated charge carriers and limited photo-response range. As shown in Fig. 7a, the synthesized Au/mpg-C3N4 nanosheets exhibit a much higher photocatalytic activity than mp-g-C3N4 and bulk g-C3N4. The photocatalytic results also demonstrate that the loading amount of Au has a strong influence on the photodegradation of RhB. Thus, the optimal Au content in mp-g-C3N4 nanosheets is 1 mol%, and then the photocatalytic activity decreases with increasing Au content. The decrease in the photocatalytic activity by loading with excess amounts of Au may be due to following reasons: (1) the large Au nanoparticles in Au/ mp-g-C3N4-2.0 may act as the recombination centers of photogenerated charge carriers and reduce quantum efficiency, rather than facilitate charge transport and reduce charge recombination and (2) the presence of a large number of Au nanoparticles within the pore channels may also slow down mass transport and reduce the reaction rate [32]. These results clearly demonstrating that the introduction of Au nanoparticles on the surface of mp-g-C3N4 plays an important role in the enhancement of RhB photodegradation. Moreover, the RhB photodegradation was found to follow pseudofirst order kinetics (Fig. 7b), which also indicates that the photocatalytic activity of Au/mp-g-C3N4-1.0 nanosheets is almost 6 times higher than that of bulk g-C3N4 and 3.5 times higher than that of mp-g-C3N4 nanosheets. Au nanoparticles modification greatly improved the photocatalytic RhB degradation performance of mp-g-C3N4 nanosheets under sunlight irradiation. This is due to the fact that the deposited Au nanoparticles on the surface of the mp-g-C3N4 nanosheets were charge polarized, actively absorbed and scattered the visible light in the solar spectrum because of their strong surface plasmon resonance (SPR) effect [13,39]. In general, surface plasmons are
58
S. Tonda et al. / Materials Research Bulletin 75 (2016) 51–58
collective oscillations confined to the surfaces of conducting materials and can strongly interact with light. The SPR of these Au nanoparticles can be tuned by size, shape, material and proximity to other nanoparticles. The noble metal Au Fermi level mainly exists between the conduction band (CB) and valance band (VB) of polar semiconductor g-C3N4 leading to the formation of plasmonic photocatalysts [17,40,41]. The photogenerated electrons excited to the CB of mp-g-C3N4 under sunlight irradiation are readily trapped by the Au nanoparticles which results the increment of electron density on it (Fig. 8). These trapped electrons can reduce the surface-chemisorbed O2 to produce O2, because ECB (1.3 eV vs. NHE) of g-C3N4 is more negative than E(O2/O2) (0.33 eV vs. NHE) [42]. This phenomenon has been proved by a N2 purging experiment which is used to examine the role of dissolved O2 in the solution. If N2 was bubbled into the reaction system, a rapid decrease in the degradation of RhB was observed compared with air-equilibrated conditions (no scavenger), which confirms that dissolved O2 plays an important role for the photodegradation process (Fig. 9a). Nevertheless, photogenerated holes cannot directly oxidize the adsorbed H2O molecules to OH on g-C3N4, due to EVB (+1.4 eV vs. NHE) of g-C3N4 is lower compared to E(OH/ H2O) (+2.68 eV vs. NHE) [36], which is also confirmed by adding the tert-butyl alcohol as a OH scavenger as shown in Fig. 9a. A small change in the degradation of RhB was observed after the addition of ammonium oxalate to the reaction solution, which is used as a hole scavenger. In addition, OH generation during the photocatalytic reaction was detected by a photoluminescence (PL) technique using terephthalic acid (TA) as a probe molecule, which readily reacts with OH to produce the highly fluorescent product, 2-hydroxyterephthalic acid (Fig. 9b) [43]. This result demonstrate that the OH only originate from the reaction of photogenerated electrons during the multistep reduction of dissolved O2. The stability and recyclability of a photocatalyst is important for its assessment and practical applications. To demonstrate the efficiency of the Au/mp-g-C3N4 photocatalyst, five successive experimental runs were carried out under same experimental conditions. From Fig. 10, it was found that the photocatalytic activity of Au/mp-g-C3N4-1.0 photocatalyst did not exhibit significant loss after five recycles for the photodegradation of RhB. Therefore, the Au/mp-g-C3N4 can be concluded as stable photocatalyst during the photocatalytic process. All the above aspects together contribute to make the Au/mp-g-C3N4 nanosheets as highly efficient and stable visible light active photocatalysts for the environmental applications. 4. Conclusions In summary, we have successfully synthesized a highly efficient visible-light-driven plasmonic photocatalyst Au/mp-g-C3N4 by a template-free and green in situ photo-reduction strategy. A strong visible light absorption and Raman blue shift of mp-g-C3N4 were observed in Au/mp-g-C3N4 photocatalysts owing to the characteristic surface plasmon resonance of Au nanoparticles. Moreover, the optimum photocatalytic activity at 1 mol% Au loaded mp-g-C3N4 nanosheets is almost 6 times higher than bulk g-C3N4 and 3.5 times higher than mesoporous g-C3N4 for the photodegradation of RhB under sunlight irradiation. The high photocatalytic performance of the Au/mp-g-C3N4 nanosheets is associated with the extended light-absorption range, high specific surface area, large pore volume, increased light harvesting capacity and efficient charge separation due to the surface plasmon resonance effect of Au nanoparticles. Very importantly, the reusability of the synthesized plasmonic photocatalysts was very high. Thus, the present study provides new insight into the design and development of advanced porous g-C3N4-based plasmonic photocatalysts for energy and environmental applications.
Acknowledgements Surendar Tonda thanks the Ministry of Human Resource Development, Government of India, for providing a fellowship. The authors also thank Prof. A. K. Ganguli, Indian Institute of Technology Delhi, for carrying out part of this work in his lab. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. materresbull.2015.11.011. References [1] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24 (2012) 229–251. [2] T.P. Yoon, M.A. Ischay, J. Du, Nat. Chem. 2 (2010) 527–532. [3] A. Kubacka, M. Fernandez-García, G. Colon, Chem. Rev. 112 (2012) 1555–1614. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [5] S.X. Ouyang, H. Tong, N. Umezawa, J. Cao, P. Li, Y. Bi, Y. Zhang, J. Ye, J. Am. Chem. Soc. 134 (2012) 1974–1977. [6] K.N. Wood, R. O’Hayre, S. Pylypenko, Energy Environ. Sci. 7 (2014) 1212–1249. [7] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [8] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [9] L. Ge, C. Han, J. Liu, J. Mater. Chem. 22 (2012) 11843–11850. [10] K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti, K. Domen, J. Phys. Chem. C 113 (2009) 4940–4947. [11] S.C. Yan, Z.S. Li, Z.G. Zo, Langmuir 26 (2010) 3894–3901. [12] T. Surendar, S. Kumar, S. Kandula, V. Shanker, J. Mater. Chem. A 2 (2014) 6772– 6780. [13] S. Samanta, S. Martha, K.M. Parida, ChemCatChem 6 (2014) 1453–1462. [14] S. Kumar, T. Surendar, B. Kumar, A. Baruah, V. Shanker, J. Phys. Chem. C 117 (2013) 26135–26143. [15] L. Ge, C. Han, X. Xiao, L. Guo, Int. J. Hydrogen Energy 38 (2013) 6960–6969. [16] Y.S. Xu, W.D. Zhang, ChemCatChem 5 (2013) 2343–2351. [17] W. Hou, S.B. Cronin, Adv. Funct. Mater. 23 (2012) 1612–1619. [18] N. Cheng, J. Tian, Q. Liu, C. Ge, A.H. Qusti, A.M. Asiri, A.O. Al-Youbi, X. Sun, ACS Appl. Mater. Interfaces 5 (2013) 6815–6819. [19] B. Kiskan, J. Zhang, X. Wang, M. Antonietti, Y. Yagci, ACS Macro Lett. 1 (2012) 546–549. [20] Y. Li, Z.Y. Fu, B.L. Su, Adv. Funct. Mater. 22 (2012) 4634–4667. [21] H. Yan, Chem. Commun. 48 (2012) 3430–3432. [22] Y. Wang, X.C. Wang, M. Antonietti, Y.J. Zhang, ChemSusChem 3 (2010) 435– 439. [23] M. Groenewolt, M. Antonietti, Adv. Mater. 17 (2005) 1789–1792. [24] B.V. Lotsch, W. Schnick, Chem. Mater. 18 (2006) 1891–1900. [25] J. Xue, S. Ma, Y. Zhou, Z. Zhang, M. He, ACS Appl. Mater. Interfaces 7 (2015) 9630–9637. [26] Y. Bu, Z. Chen, W. Li, Appl. Catal. B: Environ. 144 (2014) 622–630. [27] R.A. Naphade, M. Tathavadekar, J.P. Jog, S. Agarkar, S. Ogale, J. Mater. Chem. A 2 (2014) 975–984. [28] H. Zhu, X. Chen, Z. Zheng, X. Ke, E. Jaatinen, J. Zhao, C. Guo, T. Xie, D. Wang, Chem. Commun. (2009) 7524–7526. [29] Y. Li, H. Wang, Q. Feng, G. Zhou, Z.S. Wang, Energy Environ. Sci. 6 (2013) 2156– 2165. [30] E.D. Smolensky, M.C. Neary, Y. Zhou, T.S. Berquoa, V.C. Pierre, Chem. Commun. 47 (2011) 2149–2151. [31] P.V. Zinin, L.C. Ming, S.K. Sharma, V.N. Khabashesku, X. Liu, S. Hong, S. Endo, T. Acosta, Chem. Phys. Lett. 472 (2009) 69–73. [32] H. Li, Z.F. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 129 (2007) 4538– 4539. [33] B. Naik, S.M. Kim, C.H. Jung, S.Y. Moon, S.H. Kim, J.Y. Park, Adv. Mater. Interfaces 1 (2014) 1300018. [34] G. Dong, L. Zhang, J. Mater. Chem. 22 (2012) 1160–1166. [35] Y. Zhang, Z. Liu, G. Wu, W. Chen, Nanoscale 4 (2012) 5300–5303. [36] T. Surendar, S. Kumar, V. Shanker, Phys. Chem. Chem. Phys. 16 (2014) 728–735. [37] X. Bai, L. Wang, R. Zong, Y. Zhu, J. Phys. Chem. C 117 (2013) 9952–9961. [38] S. Kumar, A. Baruah, T. Surendar, B. Kumar, V. Shanker, B. Sreedhar, Nanoscale 6 (2014) 4830–4842. [39] H. Zhu, X. Chen, Z.F. Zheng, X. Ke, E. Jaatinen, J. Zhao, C. Guo, T. Xied, D. Wang, Chem. Commun. (2009) 7524–7526. [40] P. Wang, B. Huang, Y. Dai, M.H. Whangbo, Phys. Chem. Chem. Phys. 14 (2012) 9813–9825. [41] Y. Ide, F. Liu, J. Zhang, N. Kawamoto, K. Komaguchi, Y. Bandoa, D. Golberga, J. Mater. Chem. A 2 (2014) 4150–4156. [42] J. Jiang, H. Li, L.Z. Zhang, Chem. Eur. J. 18 (2012) 6360–6369. [43] K. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Electrochem. Commun. 2 (2000) 207–210.