RSC Advances View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. PAPER View Journal | View Issue Cite this: RSC Adv., 2018, 8, 13898 Environmentally sustainable biogenic fabrication of AuNP decorated-graphitic g-C3N4 nanostructures towards improved photoelectrochemical performances† Mohammad Ehtisham Khan, and Moo Hwan Cho *a *a Mohammad Mansoob Khan *b Noble-metal gold (Au) nanoparticles (NPs) anchored/decorated on polymeric graphitic carbon nitride (gC3N4), as a nanostructure, was fabricated by a simple, single step, and an environmentally friendly synthesis approach using single-strain-developed biofilm as a reducing tool. The well deposited/ anchored AuNPs on the sheet-like structure of g-C3N4 exhibited high photoelectrochemical performance under visible-light irradiation. The Au-g-C3N4 nanostructures behaved as a plasmonic material. The nanostructures were analyzed using standard characterization techniques. The effect of AuNPs deposition on the photoelectrochemical performance of the Au-g-C3N4 nanostructures was examined by linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), incident photon-to-current efficiency (IPCE) and cyclic voltammetry (CV) in the dark and under visible-light irradiation. The optimal charge transfer resistance for Au-g-C3N4 nanostructures (6 mM) recorded at 18.21  1.00 U cm 2 and high electron transfer efficiency, as determined by EIS. The improved photoelectrochemical performance of the Au-g-C3N4 nanostructures was attributed to the synergistic Received 23rd January 2018 Accepted 8th April 2018 effects between the conduction band minimum of g-C3N4 and the plasmonic band of AuNPs, including high optical absorption, uniform distribution, and nanoscale particle size. This simple, biogenic approach DOI: 10.1039/c8ra00690c opens up new ways of producing photoactive Au-g-C3N4 nanostructures for potential practical rsc.li/rsc-advances applications, such as visible light-induced photonic materials for real device development. 1. Introduction Green chemistry focuses mainly on the reduction, recycling or removal of toxic and hazardous chemicals in various fabrication processes by nding creative, alternative routes for producing the desired products with a less adverse impact on the environment and human health. Green chemistry is a more ecofriendly green alternative to conventional chemistry practices.1 The development of environmentally friendly methodologies in material synthesis is of great importance to expand their visible light-induced applications in electrochemical analysis.2,3 Currently, noteworthy research efforts have been devoted to the realization of efficient, economical, and green sources for the fabrication of nanoparticles with a well-dened chemical composition, size, and morphology for applications in many a School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, South Korea. E-mail: mhcho@ynu.ac.kr; mehtishamkhan1@gmail.com; Fax: +82-53-810-4631; Tel: +82-53-810-2517, b Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, BE1410, Brunei Darussalam. E-mail: mmansoobkhan@yahoo.com † Electronic supplementary 10.1039/c8ra00690c information 13898 | RSC Adv., 2018, 8, 13898–13909 (ESI) available. See DOI: cutting-edge technological areas.4–7 Single strain developed biolms is one of the positive hopes for the fabrication of carbon-based metal nanostructures.8 In general, biolms form on solid surfaces by different kinds of micro-organism for their mutual benets. Here, a biolm was developed using a single strain Shewanella oneidensis, which is an electrochemically active microorganism that can be used to control reactions in a range of elds, such as chemical/biological synthesis and bioremediation.8,9 Nanoparticles of noble metals, such as Au, Ag, Pt, and Pd can strongly absorb visible light from the solar spectrum10,11 owing to their special effect of surface plasmon resonance (SPR), which can be adjusted by varying their size and shape.12–15 The size and shape-dependent optical and electronic properties of metal nanoparticles have made them attractive for interfacial charge transfer in semiconductor– metal nanostructures.14,16,17 Plasmonic AuNPs work as a visible light absorber and a thermal redox active center.18–21 Considering the advantages of AuNPs, it is probable that the photoelectrochemical performance of g-C3N4 can be improved further aer the successful anchoring of AuNPs.22 Polymeric graphitic carbon nitride (g-C3N4) with a band gap of 2.7 eV and long range p–p conjugation is a stable allotrope with a stacked two- This journal is © The Royal Society of Chemistry 2018 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Paper RSC Advances dimensional structure under ambient conditions.23–25 Compared to its inorganic semiconductor counterparts, g-C3N4 is a sustainable and environmentally friendly organic semiconductor material that consists of carbon and nitrogen, which are among the most abundant elements on Earth. Since Wang et al. rst reported that novel molecular photo-based material gC3N4 nanostructures exhibited photoactivity for H2 production, considerable efforts have been made to synthesize g-C3N4 through the heat treatment of numerous nitrogen-rich organic precursors.26,27 Metal-free p-conjugated g-C3N4 nanostructures have interesting electronic properties as well as high thermal and chemical stability, making them valuable materials for visible light-driven electrochemical analysis.26,28–30 In the present study, a novel, simple and biogenic/green synthesis approach was applied for the fabrication of Au-gC3N4 nanostructures. The successful anchoring of AuNPs onto the sheet-like structure of g-C3N4 was optimized using HAuCl4 precursor (1 mM, 3 mM, and 6 mM), and it was found that anchoring with up to 6 mM of AuNPs resulted in improved photoelectrochemical performance. The effects of small amounts of AuNPs (1 mM, 3 mM, and 6 mM) anchored successively onto sheet-like structures of g-C3N4 to improve the visible-light absorption performance and separate the photogenerated electron–hole pairs were studied. The as-fabricated nanostructures exhibited improved photocurrent performance under the visible-light irradiation. The photoelectrochemical performance was tested based on the SPR effects of AuNPs, lower band gap energy, low photoluminescence intensity, excellent visible-light absorption, and superior photocurrent generation. The charge transfer properties in the Au-g-C3N4 nanostructures highlight its potential as good quality plasmonic-based electronic material for energy storage and conversion applications for real device fabrication. 2. Experimental section 2.1. Materials Hydrogen tetrachloroaurate(III) hydrate (HAuCl4$nH2O; n ¼ 3.7) from Kojima Chemicals, Japan. Urea (98.0%), ethyl cellulose, and a-terpineol (C10H18O) were acquired from KANTO Chemical Co., Japan. Sodium acetate (CH3COONa) and sodium sulfate (Na2SO4) were obtained from Duksan Pure Chemicals Co. Ltd., South Korea. Fluorine-doped transparent conducting oxide glass (FTO; F-doped SnO2 glass; 7 U sq 1) was acquired from Pilkington, USA. The bacterial culture medium was purchased from Becton Dickinson and Company (NJ, USA). All other chemicals were of analytical grade and used as received. The solutions were prepared from DI water obtained using a PURE ROUP 30 water purication system. NIR) double beam spectrophotometer (VARIAN, Cary 5000, USA) equipped with a diffuse reectance accessory. A given amount of the g-C3N4 and Au-g-C3N4 nanostructure powder was pressed uniformly in the sample holder, which was then placed at the integrating sphere for the absorbance/reectance measurements. The photoluminescence (PL, Kimon, 1 K, Japan) of the samples was recorded over the scanning range, 300–800 nm, using an excitation wavelength of 325 nm. The BET specic surface area of the samples was measured using a Belsorp II-mini (BEL, Japan Inc.). The microstructure was examined by eld emission transmission electron microscopy (FE-TEM, Tecnai G2 F20, FEI, USA) operating at an accelerating voltage of 200 kV. Selected-area electron diffraction (SAED) and high angle annular dark eld (HAADF) observations were carried out on the same transmission electron microscope. Quantitative analysis was performed by energy dispersive spectrometry (EDS) attached to the transmission electron microscope. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS System, Thermo Fisher Scientic U.K.) was conducted using the following X-ray source: monochromated Al Ka radiation, hn ¼ 1486.6 eV; X-ray energy, 15 kV; 150 W; spot size, 500 mm; take-off angle, 90; pass energy, 20 eV; BE resolution, 0.6 eV (calibrated using Ag 3d5/2) at the Center for Research Facilities, Yeungnam University, South Korea. XPS tting was done using “AVANTAGE” soware by a Shirley subtraction and the shape of the peaks used for the deconvolution was Gaussian–Lorentzian shapes. The sensitivity factor used for Au 3d5 was 30.5. Photoelectrochemical analyses, such as linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV), were performed using a potentiostat (Versa STAT 3, Princeton Research, USA) comprised of a standard three-electrode system. Ag/AgCl (3 M KCl), a Pt gauge, and FTO glass coated with the pure g-C3N4 and Au-gC3N4 nanostructures were used as the reference, counter and working photoelectrode, respectively. The experiment LSV and EIS were performed in a 0.2 M sodium sulphate (Na2SO4) solution as the supporting electrolyte at room temperature and CV was performed in a 0.2 M phosphate buffer solution (pH 7; 0.2% PBS). The projection area of the photoelectrode was 1 cm2. The working electrodes were prepared as follows: 100 mg of each sample was mixed thoroughly by adding ethyl cellulose as a binder and a-terpineol as the solvent. The mixture was stirred and heated on a hot plate with a magnetic stirrer until a thick paste was obtained. The paste obtained was then coated on a FTO glass substrate using the doctor-blade method and kept drying overnight under a 60 W lamp; the electrode was later used as a photoelectrode for the photoelectrochemical measurements. 2.2. Methods 2.3. Development and fabrication of single strain biolm on carbon foam X-Ray diffraction (XRD, PANalytical, X'pert-PRO MPD) was performed using Cu Ka radiation (l ¼ 0.15405 nm). The diffuse absorbance/reectance ultraviolet-visible spectra (DRS) of the powder pure g-C3N4 and Au-g-C3N4 nanostructures samples were obtained using an ultraviolet-visible-near infrared (UV-VIS- Carbon foam was prepared using a melamine sponge (Dae Han Co. Ltd, Korea) as the template.31 The biolm on the carbon foam was prepared using the procedure reported elsewhere.21,32 Carbon foam (without wet proof, Fuel Cell Earth LLC) with a size of 2.5  4.5 cm2 was used as an anode electrode instead of This journal is © The Royal Society of Chemistry 2018 RSC Adv., 2018, 8, 13898–13909 | 13899 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. RSC Advances carbon paper. In the anode chamber, Luria Broth (LB) medium was inoculated with overnight cultures of Shewanella oneidensis at a ratio of 1 : 100. The LB media was purged with N2 gas for 10 min to remove the environmental oxygen and maintain the anaerobic conditions. The fully developed biolm on the carbon foam was conrmed using a microbial fuel cell by obtaining the appropriate voltage. The living biolm formed on the carbon foam specimens was used to synthesize the series of Au-g-C3N4 nanostructures. 2.4. Single strain developed biolm synthesis of Au-g-C3N4 nanostructures (1 mM, 3 mM and 6 mM) Graphitic g-C3N4 was prepared using a facile single pot method by the modest heating of urea at 550  C in a muffle furnace for 4 h with a ramping rate of 20  C min 1 under air ow conditions. The resulting material was then naturally cooled to room temperature, the whitish yellow color powder was extract as a sheet-like structure of pure g-C3N4 (ref. 33 and 34) (Scheme 1). Three setup arrangements of 200 mL of aqueous suspensions of pure g-C3N4 and 1 mM, 3 mM, and 6 mM Au3+ were prepared. The mixture of pure g-C3N4 and HAuCl4 (Au3+ ions) was stirred for 15 min to allow the adsorption of Au3+ ions onto the sheet-like g-C3N4 structure. Subsequently, the optimal amount of sodium acetate (0.2 g) was added individually to the suspension as an electron contributor. The reaction mixtures were sparged with nitrogen (N2) gas for 5 min to sustain an anaerobic environment. The single strain developed biolm were hung individually in a reaction bottle and the setup was sealed and le for magnetic stirring at 30  C. The reaction mixture setups were stirred for a further 6 h to complete the reaction. In each case, the initial white color changed to a dark pink color within 30 min, which was the sign of the reduction of Au3+ to Au0. Finally, purple to light purple precipitates were obtained in the 1 mM, 3 mM, and 6 mM HAuCl4 cases, respectively. The reaction mixtures were centrifuged and the Scheme 1 Paper powdered Au-g-C3N4 nanostructures were isolated for further characterization and photoelectrochemical studies. Two precise syntheses were performed to examine the role of the single strain developed biolm and sodium acetate. Two 5 mM g-C3N4 aqueous suspensions (200 mL) were prepared. In the rst controlled synthesis, an aqueous solution containing a 0.2 g sodium acetate and 1 mM HAuCl4 was added. In the second controlled synthesis, only a 3 mM HAuCl4 aqueous solution was added. Both reaction mixtures were sparged with N2 gas for 5 min to sustain the anaerobic environment. The developed biolm were hung in the second controlled synthesis only. Both systems were sealed and stirred with a magnetic stirrer at 30  C. No variations were detected, even aer 48 h. These long-established reaction steps conrmed that the biolm and sodium acetate are essential to complete the synthesis of the Au-g-C3N4 nanostructures. 2.5. Photoelectrochemical studies of pure g-C3N4 and Au-gC3N4 nanostructures as a photoelectrode performance The photoelectrochemical performance of the pure g-C3N4 and Au-g-C3N4 nanostructures was examined by LSV, EIS and CV under ambient conditions in the dark and under visible light irradiation. The LSV and EIS experiments were performed in 50 mL of an aqueous 0.2 M Na2SO4 solution in the dark and under visible light irradiation at room temperature. The photocurrent response was examined by LSV in the dark and under visible light irradiation at a scan rate of 50 mV s 1 over the applied potential range, 1.0 to 1.0 V. EIS which were conducted using a 400 W lamp under visible light irradiating intensity of 31.0 mW cm2 (3M, USA) with frequencies ranging from 1 to 104 Hz at 0.0 V vs. Ag/AgCl in potentiostatic mode. The CV experiments were conducted in 50 mL of 0.1 M PBS (phosphate buffer solution) in the dark and under visible light irradiation. CV analysis was performed at a scan rate of 50 mV s 1. The incident photon-to-current efficiency (IPCE), used to Projected schematic model for the biogenic synthesis of the Au-g-C3N4 nanostructures using an environmentally friendly approach. 13900 | RSC Adv., 2018, 8, 13898–13909 This journal is © The Royal Society of Chemistry 2018 View Article Online Paper RSC Advances investigate the photoresponsivity of nanostructures using the xenon lamp with specic wavelength lters to select the required wavelength of light. 3. Results and discussion Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. 3.1. Standard characterization of pure g-C3N4 and Au-g-C3N4 nanostructures 3.1.1. Structural, purity and phase conrmation analysis of pure g-C3N4 and Au-g-C3N4 nanostructures. X-Ray diffraction was performed to explore the crystal structure, phase, and purity of pure g-C3N4, as shown in Fig. 1(a). The XRD pattern of pure g-C3N4 showed two peaks at 13.1 and 27.3 2q. The small peak at 13.1 2q was assigned to the (100) plane with d ¼ 0.676 nm and the other strong peak at 27.3 2q corresponded to d ¼ 0.324 nm due to the long-range interplanar stacking of the aromatic arrangement and it is recognized as the (002) plane of pure g-C3N4 (JCPDSD 87-1526).35 Additionally, the two additional weak diffractions peaks appeared at 43 and 58 which can be attributed to the planes of graphitic carbon nitride. This outcome from the denser packing or a distortion of the mesopores structure in which every second the arrangement of mesopores sheet is displaced.36 Notably, the mentioned peaks were disappeared in case of 3 mM and 6 mM while small peak appeared in case 1 mM because there was small concentration of Au ions. In the case of (1 mM, 3 mM and 6 mM) of AuNPs-decorated gC3N4 samples, the XRD patterns revealed four separate reections at 38.1 (111), 44.4 (200), 64.8 (220) and 77.6 (311) for Fig. 1 the AuNPs, in addition to the peaks for g-C3N4. The observed reections were well matched with the AuNPs in the prepared nanostructures corresponding to the reported JCPDS values (040784).37 The intensity of the peaks for the AuNPs increased gradually with increasing loading of Au3+ ions onto the sheetlike structure of g-C3N4. The four peaks conrmed the anchoring of AuNPs onto the g-C3N4 surface, which was clearly absent in the pure g-C3N4 sample; no other extra/impurity peaks were found in the as-fabricated samples. The presence of both Au planes and g-C3N4 conrmed the successful formation of the Au-g-C3N4 nanostructures using the green/biogenic synthesis approach. The mean crystallite size of the g-C3N4 and Au-g-C3N4 nanostructures were calculated using the Scherrer's formula, D ¼ kl/b cos q (1) where k is the shape factor and has a typical value of 0.9, l is the wavelength (Cu Ka ¼ 0.15405 nm), b is the full width at half maximum of the most intense peak (in radians), and q is the main peak of g-C3N4, which was observed at 27.43 2q. The calculated crystallite size of bare g-C3N4 from the most intense peak at 27.43 2q was 6.6 nm and the calculated crystallite size of the Au-g-C3N4 nanostructures from the most intense peak was 12.2, 22.9, and 27.9 nm, respectively. This shows that the crystallite size of the Au-g-C3N4 nanostructures increased because of the anchoring of AuNPs on to the g-C3N4 sheets compared to pure g-C3N4. These increased crystallite values further conrmed the successful fabrication of the Au-g-C3N4 nanostructures. Representative XRD patterns of (a) pure g-C3N4, (b, c and d) 1 mM, 3 mM and 6 mM of Au-g-C3N4 nanostructures. This journal is © The Royal Society of Chemistry 2018 RSC Adv., 2018, 8, 13898–13909 | 13901 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. RSC Advances 3.1.2. Optical and photoluminescence analysis of pure gC3N4 and Au-g-C3N4 nanostructures. Fig. 2(a and b) shows the optical absorbance and photoluminescence analysis of the pure g-C3N4 and Au-g-C3N4 nanostructures. The present spectrum showed a high absorbance value in the range, 475–525 nm, because of the SPR band characteristics of the AuNPs, which showed that the AuNPs had been anchored successfully onto the g-C3N4 samples and showed the improved visible light absorption of AuNPs.38,39 In addition to the connement effect, the interparticle coupling contributes to the SPR broadening of AuNPs decorated g-C3N4 nanostructures as well. It is due to the particle interactions which increase in local eld uctuations, giving rise to an extensive range of photon energies for plasmon resonance to take place.17 From the absorbance spectra in Fig. 2(a), there was a red shi in the absorbance band of the Aug-C3N4 nanostructures compared to that of pure g-C3N4. The inset in Fig. 2(a) shows that the AuNPs decorated onto g-C3N4, in the 1 mM sample displayed a purple color, which was a clear indication of the successful reduction of Au3+ to Au0 and the fabrication of AuNPs. Fig. S1† presents the typical reectance spectra from 360–780 nm wavelengths, showing improved reectance in the case of the Au-g-C3N4 nanostructures, which further conrmed the successful formation of Au-g-C3N4 nanostructures. Fig. 2(b) shows the photoluminescence (PL) spectra of the pure g-C3N4 and Au-g-C3N4 nanostructures. These spectra are very helpful for examining the migration, transfer of charge carriers, and separation and recombination processes of the photogenerated electron–hole pairs.13,18 The PL intensity is exceedingly reliant on the electron–hole pair recombination processes. The PL intensity is dependent on electron–hole pair recombination processes. The as-fabricated pure g-C3N4 and Au-g-C3N4 nanostructures materials showed only one type of PL intensity in the recorded spectra. The broad luminescence peak at 455 nm was assigned to the band–band PL phenomenon with a light energy approximately equal to the band gap energy of the g-C3N4 and Au-g-C3N4 nanostructures for the photoelectrode performance.13,40 As the PL intensity is inversely related to the charge recombination between the photogenerated electron– hole pairs, the anchoring/decoration of the AuNPs onto the Fig. 2 Paper sheet-like structure of g-C3N4 could prevent charge recombination between the opposite charge carriers, leading to improved photoelectrochemical performance.40,41 The overall PL studies of the Au-g-C3N4 nanostructures clearly showed higher charge transfer ability, which could be responsible for the improved photoelectrochemical performance. On the other hand, the inset in Fig. 2(b) shows that there is no shi in the emission wavelength of 455 nm. In addition, two emission centers were observed in the shorter excitation wavelength (436.0 nm and 458.8 nm) which was in contrast to that observed for longer excitation wavelengths. The PL intensity decreased gradually with increasing wavelength as the corresponding excitation energy is reduced in the case of (1 mM to 6 mM) of Aug-C3N4 nanostructures. 3.1.3. High resolution transmission electron microscopy (HR-TEM) image of pure g-C3N4 and Au-g-C3N4 nanostructures. The surface structures, morphology, and particle size of the asfabricated samples Au-g-C3N4 nanostructures were investigated by TEM and HR-TEM. As shown in Fig. 3(a), the particles with a dark color can be assigned to AuNPs and the sheet like gray color area was assigned to the sheet-like structure of g-C3N4. The as-synthesized Au-g-C3N4 nanostructures displayed a sheet like morphology with a few layered structures. The sheets consisted mainly of graphitic planes with a conjugated aromatic system.13 The at surface of the g-C3N4 sheet acts as a visiblelight absorber. The TEM images (a, b and c) show that the number of AuNPs increases with increasing concentration of the Au precursor. The SAED pattern of the nanostructure showed a series of bright concentric rings, suggesting that the as-fabricated sample is polycrystalline in nature (inset of Fig. 3(a–c)). The mean diameter of the AuNPs was in the range, 12– 15 nm, and the nanoparticles were clearly attached to the surface and edges of the g-C3N4 sheet. Fig. 3(d) clearly showed the interfacial interaction of AuNPs with the sheet-like structure of g-C3N4, which also covered the intact surface area of the sheet uniformly. The lattice fringes of the Au 0.23 nm (111) plane for metallic Au indicated the crystalline behavior of the samples, which further conrmed the presence of the AuNPs and the good interaction at the interface of the g-C3N4 sheet. The (a) UV-Vis absorbance spectra, and (b) photoluminescence spectra of pure g-C3N4 and Au-g-C3N4 nanostructures. 13902 | RSC Adv., 2018, 8, 13898–13909 This journal is © The Royal Society of Chemistry 2018 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Paper RSC Advances (a, b and c) TEM images of the 1 mM, 3 mM, and 6 mM of Au-g-C3N4 nanostructures shows the uniform presence of AuNPs onto g-C3N4 (the inset shows the SAED crystal ring pattern); (d) HR-TEM image showing the interface between the AuNPs and g-C3N4, lattice fringes of AuNPs onto the g-C3N4; (e, f and g), showing elemental mapping of C (yellow color), N (green color), and Au (metallic gold color); (h) shows the elemental composition of the Au-g-C3N4 nanostructures. Fig. 3 elemental mapping presented in Fig. 3(e, f and g) shows C (yellow), N (green), and Au (metallic gold), which provides strong evidence for the existence of carbon, nitrogen, and AuNPs anchored successfully onto the sheet-like structure of gC3N4. Fig. 3(h) shows the elemental composition of the Au-gC3N4 nanostructures without any other elemental peak. Fig. S2(a–d)† presents HR-TEM images of the Au-g-C3N4 nanostructures. The average particle size distribution graph of Fig. S3(a–c)† screening the average particle size is ranging between 12–15 nm. 3.1.4. XPS of pure g-C3N4 and Au-g-C3N4 nanostructures. XPS is a surface-specic characterization tool that can be used to conrm the chemical environment and elemental oxidation state. XPS was carried out on the pure g-C3N4 and Au-g-C3N4 nanostructures in the region, 0–1000 eV (Fig. 4). Consequently, XPS was used to determine the formal oxidation state of all the elements present in the pure g-C3N4 and Au-g-C3N4 (1 mM, 3 mM, and 6 mM). Fig. 4(a) displayed the elemental composition of pure g-C 3N4, in which two major peaks were assigned to C and N and a small peak for oxygen at 531 eV, which might be some hydroxyl groups (–OH) This journal is © The Royal Society of Chemistry 2018 attached to the surface of g-C3N4. No impurity peak was observed.42 The survey scan spectrum (Fig. 4(b)) of Au-g-C3N4 conrmed the presence of an Au peak at 84 eV along with C and N, which veried the successful attachment of AuNPs onto the sheet-like structure of g-C3N4.41 The C 1s peaks were observed at 285 eV and 288.3 eV (Fig. 4(c)),42 which were assigned to the sp2-hybridized carbon atom and the carbon atom bonded to three nitrogen atoms –C(N3) of g-C3N4, respectively. The broad tted peak of N 1s was observed at 398.5 eV (Fig. 4(d)),43 which were assigned to the nitrogen atom bonded to two carbon atoms (C–N–C) and the other small tted peaks were attributed to nitrogen atoms bonded to the environment of three carbon atoms N–(C3) and to N–H bonding, respectively.43 The tted spectrum of Au 4f (Fig. 4(e)) showed two peaks at 84.19 eV and 87.87 eV, which originated from the Au 4f7/2 and 4f5/2 electrons of the metallic behavior of gold.41,44 Therefore, the Au3+ ions were reduced to the Au0 oxidation state on the sheet like surface of g-C3N4.45,46 Fig. 4(f) shows the combined C 1s spectrum of pure g-C3N4 and Au-g-C3N4 nanostructure. In case of AuNPs, the peak intensity is decreased (Fig. 4(f)) with the little shi in the binding energy. RSC Adv., 2018, 8, 13898–13909 | 13903 View Article Online Paper Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. RSC Advances Fig. 4 (a and b) XPS survey scan spectra of pure g-C3N4 and Au-g-C3N4 nanostructures, (c, d and e) fitted spectra of C 1s, N 1s and Au 4f, and, (f) combined spectra of pure g-C3N4 and Au-g-C3N4 nanostructures. Therefore, its mainly related to a change of oxidation state of the element, here the shiing of binding energy relates to the changes of Au3+ to Au0 oxidation state. This analysis was supported by XRD, XPS, BET, and HR-TEM studies. 3.1.5. Brunauer–Emmett–Teller, specic surface area analysis of the pure g-C3N4 and Au-g-C3N4 nanostructures. N2BET (Nitrogen Adsorption Brunauer–Emmett–Teller) was performed to detect the changes in the specic surface area of the as-fabricated samples. The measured specic surface areas of the pure g-C3N4 and Au-g-C3N4 nanostructures (1 mM, 3 mM, and 6 mM) were 31.0116  0.3652 m2 g 1, 31.9655  0.1336 m2 g 1, 34.9131  0.3450 m2 g 1, and 41.1593  0.4697 m2 g 1, respectively. In Fig. 5, the surface area of Au-g-C3N4 (6 mM) increased with increasing amount of precursor, which was much higher than that of pure g-C3N4. The higher specic 13904 | RSC Adv., 2018, 8, 13898–13909 surface area provides larger spaces to accommodate more charge storage and expose more active sites for the photochemical reaction. These results suggest that the visible light photoelectrochemical performance of the Au-g-C3N4 nanostructures could be improved greatly due to the higher specic surface area. The nitrogen adsorption–desorption isotherm of pure gC3N4 displays a hysteresis loop, suggesting the existence of mesopores.42 The AuNPs-loaded g-C3N4 exhibited much higher specic surface areas that of pure g-C3N4 (Table 1). The 6 mM AuNPs decorated g-C3N4 had a specic surface area of up to 41.15 m2 g 1. This shows that the optimal amount of AuNPs decorated g-C3N4 could provide more adsorption sites and photochemical reaction sites to improve the photoelectrochemical performance. This journal is © The Royal Society of Chemistry 2018 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Paper RSC Advances Nitrogen adsorption–desorption isotherm of the pure g-C3N4 and Au-g-C3N4 nanostructures. Fig. 5 Specific surface area measured from BET analysis of the pureg-C3N4 and Au-g-C3N4 nanostructures Table 1 Sample name BET surface area (m2 g 1) Pure g-C3N4 Au-g-C3N4 (1 mM) Au-g-C3N4 (3 mM) Au-g-C3N4 (6 mM) 31.0116 31.9655 34.9131 41.1593  0.3652  0.1336  0.3450  0.4697 4. Photoelectrochemical studies 4.1. Photoelectrochemical studies of pure g-C3N4 and Au-gC3N4 nanostructures using LSV, EIS and CV measurements Studies of the photoelectric behavior of pure g-C3N4 and Au-g-C3N4 nanostructures as a photoelectrode were performed using a standard three-electrode system. The measurements were taken under ambient conditions in the dark and under visible light irradiation in a 50 mL, a 0.2 M aqueous Na2SO4 solution as an electrolyte at room temperature. LSV and EIS were rst performed in the dark and then under visible light irradiation (l $ 400 nm) at a scan rate of 50 mV s 1 over the applied potential range, ( 1 to 1 V).41 LSV is a voltammetry process, where the current at a working electrode is measured while the potential between the working electrode and reference electrode is swept linearly with time. LSV was performed in the dark and under visible light irradiation to provide evidence of the visible light-induced performance. The Au-g-C3N4 nanostructures (1–6 mM) displayed an enhanced photocurrent compared to pure g-C3N4 (Fig. 6(a)). The results in Fig. 6(a) showed that the photocurrent density depends basically on AuNPs deposition onto the sheet-like structure of g-C3N4. The current density increased signicantly with increasing amount of AuNPs deposition. This higher increment in photocurrent density can be explained by the improved visible light absorption behavior of the material. The photocurrent depends largely on the number of photogenerated electrons; a higher number of electrons generated will improve the photocurrent density.43,47 The large number of This journal is © The Royal Society of Chemistry 2018 electrons amassed in the conduction band of g-C3N4 resulted in a higher amount of photocurrent generation.43,44,48 The interfacial charge transfer rate is essential for improving the photoelectrode performance. Electrochemical impedance spectroscopy (EIS) was performed in the dark and under visible light irradiation to understand the charge separation process and transport properties of pure g-C3N4 and Au-g-C3N4 as a photoelectrode material, as shown in Fig. 6(b). In general, the complex impedance plot is normally presented as Z0 /ohm vs. Z00 /ohm, which initiates from the resistance and capacitance component of the electrochemical cell. A representative Nyquist plot includes one or more semicircular arcs with the diameter along the Z0 /ohm axis.41 The semicircular arcs observed in the high and low frequency regions correspond to an electron transfer process, and its diameter represents the electron transfer or charge transfer resistance. In the present graph, a half circle arc with a reduced diameter for the Au-g-C3N4 was obtained compared to pure g-C3N4, which clearly reveals a rapid electron-transfer process in the case of the Au-g-C3N4 nanostructures under visible light irradiation. Generally, the small radius of the arc in the EIS spectra indicated lower electron transfer resistance at the surface of the photoelectrode, which is usually associated with the faster interfacial charge transfer. The concentration was increased from 1 mM to 6 mM under visible light irradiation; the EIS spectrum displayed a smaller arc radius of Au-g-C3N4. The performance of the as-fabricated nanostructure was better than that of pure g-C3N4. Based on the EIS data (Fig. 6(b)), an equivalent circuit (Fig. 6(c)) tted by the Zsimp Win 3.20d program with ne accuracy was obtained. Basically the equivalent circuit is used to analyze the measured impedance data. As shown in the circuitry, Rct and Cdl represent the charge transfer resistance and double layer capacitance, and L describes the diffusion behavior at low frequencies, respectively. Table 2 shows the EIS tting data obtained from the tting of the equivalent circuits and the experimental values obtained from the impedance data. The tting values of Rct for Au-gC3N4 nanostructures decrease from 1 mM to 6 mM. The higher concentration of AuNPs exhibit the small Rct value which was much lower than that of pure-g-C3N4, which clearly suggested that the charge-transfer resistance is signicantly reduced by anchoring of AuNPs onto sheet like structure of g-C3N4. The Cdl values displayed the opposite tendency as that of Rct values. The low Rct and high Cdl values indicate high electron transfer efficiency which further supports the improved photoelectrochemical performance of Au-g-C3N4 nanostructures. The cyclic voltammogram (Fig. 7) of pure g-C3N4 and Au-g-C3N4 nanostructures were obtained in the dark and under visible light irradiation at a scan rate of 0.05 mV s 1. The CV plot of the Au-gC3N4 nanostructures showed an improved positive and negative sweep, indicating their pseudo capacitive nature. The peak current of the Au-g-C3N4 nanostructures from 1 mM to 6 mM increased linearly in the dark and under visible light irradiation with a positive shi of the cathodic peak and a negative shi of the anodic peak.41,45 The improved anodic and cathodic peak veried the amended current transfer ability of the Au-g-C3N4 nanostructures under visible light irradiation, which also reveals better capacitive performance of the as-fabricated nanostructures. RSC Adv., 2018, 8, 13898–13909 | 13905 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. RSC Advances Paper Fig. 6 Visible light-induced photoelectrochemical performance (a) LSV, (b) EIS Nyquist plot of the pure g-C3N4 and Au-g-C3N4 as a photoelectrode in the dark and under visible light irradiation, and (c) equivalent circuit from the Nyquist plots. Fitting circuit values from equivalent circuit to analyze the Nyquist plots Table 2 Sample Rct (U cm 2) Cdl  108 (F cm 2) Pure-g-C3N4 Au-g-C3N4 (1 mM) Au-g-C3N4 (3 mM) Au-g-C3N4 (6 mM) 21.82  1.12 21.42  0.96 19.26  0.89 18.21  1.00 1.39  0.14 2.01  0.19 2.16  0.21 2.49  0.29 Consequently, the improved capacitive performance of the Au-gC3N4 nanostructures can be attributed to its improved charge loading ability and the synergistic effect of AuNPs and g-C3N4 under visible light irradiation.46,49,50 4.2. Incident photon-to-current conversion efficiency (IPCE) test To investigate the photoresponse of pure-g-C 3N4 and Au-gC3N4 (1–6 mM) nanostructures, IPCE measurements at 1.2 eV Cyclic voltammetry profile of pure g-C3N4 and Au-g-C3N4 nanostructures in a 0.2 M phosphate buffer (pH ¼ 7) solution at 25  C at a scan rate of 0.05 mV s 1. Fig. 7 13906 | RSC Adv., 2018, 8, 13898–13909 Fig. 8 IPCE graph for pure-g-C3N4 and Au-g-C3N4 (1–6 mM) nanostructures at 1.2 eV vs. Ag/AgCl in 0.2 M Na2SO4. This journal is © The Royal Society of Chemistry 2018 View Article Online Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Paper RSC Advances Fig. 9 Proposed schematic charge transfer mechanism for the photocurrent performance over the visible light-induced Au-g-C3N4 nanostructures as a photoelectrode. vs. Ag/AgCl as the reference electrode are presented in Fig. 8. The IPCE can be expressed as follows:51,52 IPCEð%Þ ¼ 1241  Ip  100 l  4 (2) where l, 4 and Ip denote the wavelength of the incident light (nm), the irradiation power (mW cm 2), and the photocurrent density (A cm 2) measured at the corresponding wavelength, respectively. Fig. 8 of IPCE tests shows a visible light response in case of higher Au-g-C3N4 nanostructures. The absorption threshold of g-C3N4 is approximately 460 nm, with a lower IPCE value. Anchoring of AuNPs onto sheet like structure of g-C 3N4 results in substantial enhancement of the IPCE values for Aug-C3N4 nanostructures (1–6 mM) as follows: 14.6%, 10.5%, and 7.4% under visible light irradiation respectively. In addition, the small hump appeared in the visible region which is caused primarily by the SPR effect of AuNPs. While in the case of bare g-C3N4, the IPCE performance was very less (2.2%) without any hump in visible region as compared to Aug-C3N4 which further conrms the role of AuNPs with spatial effect of SPR. This result indicates the anchored AuNPs shows SPR effect which helps to improve the photoelectrochemical performance of nanostructures. The progressive visible light-induced photoelectrochemical performance using Au-g-C3N4 nanostructures conrmed the successful anchoring of AuNPs onto the sheet-like structure of g-C3N4. The improved photocurrent performance revealed the interfacial interaction and charge transfer between the AuNPs and g-C3N4, which could explain the enhanced photoelectrochemical performance of the Au-g-C3N4 nanostructures. This journal is © The Royal Society of Chemistry 2018 5. Proposed electron transfer mechanism of Au-g-C3N4 nanostructures under visible-light irradiation Generally, in case of a semiconducting material, visible-light irradiation plays a signicant role in the excitation of electrons from the valence band (VB) to the conduction band (CB). In presence of visible-light irradiation (l $ 400 nm) g-C3N4 nanostructures excited and electrons (e ) from the VB transfer to the CB, leaving the holes (h+) in the VB, thereby forming the electron–hole pairs.21,53–57 The photogenerated electrons can rapidly transfer the AuNPs due to their intimate interfacial contact between g-C3N4 and AuNPs, resulting in a signicantly improved lifetime of the photogenerated electron–hole charge carrier.41,56 Fig. 9 shows a schematic diagram of the probable procedure for the charge separation in Au-g-C3N4nanostructures under visible-light irradiation. As shown in Fig. 9 visible-light irradiation was focused on the as-prepared electrode on FTO glass, which was dipped in the electrolyte solution. The electrolyte solution acts as a donor or acceptor to contribute or receive electrons from the electrodes. The Au-g-C3N4 nanostructures sample showed higher photocurrent performance (1–6 mM) because of its tuned optical properties compared to the bare gC3N4. The counter and reference electrode measure the photocurrent with the help of the electrolyte solution and nally we recorded the improved performance of photocurrent form Au-gC3N4 nanostructures. RSC Adv., 2018, 8, 13898–13909 | 13907 View Article Online RSC Advances Open Access Article. Published on 16 April 2018. Downloaded on 16/04/2018 07:55:02. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. 6. Conclusions This paper reported a facile, green, and competent approach for the fabrication of Au-g-C3N4nanostructures with spherical and uniform sized AuNPs with a high surface area (41.1593 m2 g 1) and improved photoelectrochemical performance. A single strain developed biolm was used as a tool to reduce Au3+ to Au0 and Au-g-C3N4 nanostructures (1 mM, 3 mM and 6 mM) were fabricated. The anchoring of AuNPs onto the sheet-like structure of g-C3N4 produced promising photoelectrode material for real photonic devices. The boosted photoelectrochemical performance of Au-g-C3N4 nanostructures compared to that of pure g-C3N4 were explained based on the strong visible-light absorption, superior photocurrent generation, surface plasmon effect of AuNPs, and lower photoluminescence intensity. The spherical shape, size and uniform dispersion of the AuNPs over the g-C3N4 sheet were valuable for increasing the photocurrent performance. These ndings were attributed mainly to the higher visible-light absorption by AuNPs ensuring the formation of a large number of photogenerated electron–hole pairs. This large number of exciton was transferred immediately through the polar-semiconductor–noble-metal interface to thesheet like structure of g-C3N4, which inhibited the charge recombination process and increased the photocurrent performance. 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