Environmentally sustainable biogenic fabrication of AuNP decorated-graphitic g-C3N4 nanostructures towards improved photoelectrochemical performances

Noble-metal gold (Au) nanoparticles (NPs) anchored/decorated on polymeric graphitic carbon nitride (g-C3N4), 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 Ω 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 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 opens up new ways of producing photoactive Au-g-C3N4 nanostructures for potential practical applications, such as visible light-induced photonic materials for real device development.


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 cutting-edge technological areas. [4][5][6][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 spectrum 10,11 owing to their special effect of surface plasmon resonance (SPR), which can be adjusted by varying their size and shape. [12][13][14][15] The size and shape-dependent optical and electronic properties of metal nanoparticles have made them attractive for interfacial charge transfer in semiconductormetal nanostructures. 14,16,17 Plasmonic AuNPs work as a visible light absorber and a thermal redox active center. [18][19][20][21] Considering the advantages of AuNPs, it is probable that the photoelectrochemical performance of g-C 3 N 4 can be improved further aer the successful anchoring of AuNPs. 22 Polymeric graphitic carbon nitride (g-C 3 N 4 ) with a band gap of 2.7 eV and long range p-p conjugation is a stable allotrope with a stacked two-dimensional structure under ambient conditions. [23][24][25] Compared to its inorganic semiconductor counterparts, g-C 3 N 4 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 g-C 3 N 4 nanostructures exhibited photoactivity for H 2 production, considerable efforts have been made to synthesize g-C 3 N 4 through the heat treatment of numerous nitrogen-rich organic precursors. 26,27 Metal-free p-conjugated g-C 3 N 4 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][29][30] In the present study, a novel, simple and biogenic/green synthesis approach was applied for the fabrication of Au-g-C 3 N 4 nanostructures. The successful anchoring of AuNPs onto the sheet-like structure of g-C 3 N 4 was optimized using HAuCl 4 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-C 3 N 4 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-C 3 N 4 nanostructures highlight its potential as good quality plasmonic-based electronic material for energy storage and conversion applications for real device fabrication.

Materials
Hydrogen tetrachloroaurate(III) hydrate (HAuCl 4 $nH 2 O; n ¼ 3.7) from Kojima Chemicals, Japan. Urea (98.0%), ethyl cellulose, and a-terpineol (C 10 H 18 O) were acquired from KANTO Chemical Co., Japan. Sodium acetate (CH 3 COONa) and sodium sulfate (Na 2 SO 4 ) were obtained from Duksan Pure Chemicals Co. Ltd., South Korea. Fluorine-doped transparent conducting oxide glass (FTO; F-doped SnO 2 glass; 7 U sq À1 ) 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.

Methods
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-C 3 N 4 and Au-g-C 3 N 4 nanostructures samples were obtained using an ultraviolet-visible-near infrared (UV-VIS-NIR) double beam spectrophotometer (VARIAN, Cary 5000, USA) equipped with a diffuse reectance accessory. A given amount of the g-C 3 N 4 and Au-g-C 3 N 4 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 3d 5/2 ) 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 3d 5 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-C 3 N 4 and Au-g-C 3 N 4 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 (Na 2 SO 4 ) 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 cm 2 . 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.
2.3. Development and fabrication of single strain biolm on carbon foam 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 N 2 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-C 3 N 4 nanostructures.
2.4. Single strain developed biolm synthesis of Au-g-C 3 N 4 nanostructures (1 mM, 3 mM and 6 mM) Graphitic g-C 3 N 4 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 À1 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-C 3 N 4 (ref. 33 and 34) (Scheme 1).
Three setup arrangements of 200 mL of aqueous suspensions of pure g-C 3 N 4 and 1 mM, 3 mM, and 6 mM Au 3+ were prepared. The mixture of pure g-C 3 N 4 and HAuCl 4 (Au 3+ ions) was stirred for 15 min to allow the adsorption of Au 3+ ions onto the sheet-like g-C 3 N 4 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 (N 2 ) 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 Au 3+ to Au 0 . Finally, purple to light purple precipitates were obtained in the 1 mM, 3 mM, and 6 mM HAuCl 4 cases, respectively. The reaction mixtures were centrifuged and the powdered Au-g-C 3 N 4 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-C 3 N 4 aqueous suspensions (200 mL) were prepared. In the rst controlled synthesis, an aqueous solution containing a 0.2 g sodium acetate and 1 mM HAuCl 4 was added. In the second controlled synthesis, only a 3 mM HAuCl 4 aqueous solution was added. Both reaction mixtures were sparged with N 2 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-C 3 N 4 nanostructures.

Photoelectrochemical studies of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures as a photoelectrode performance
The photoelectrochemical performance of the pure g-C 3 N 4 and Au-g-C 3  investigate the photoresponsivity of nanostructures using the xenon lamp with specic wavelength lters to select the required wavelength of light.

Results and discussion
3.1. Standard characterization of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures 3.1.1. Structural, purity and phase conrmation analysis of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures. X-Ray diffraction was performed to explore the crystal structure, phase, and purity of pure g-C 3 N 4 , as shown in Fig. 1(a). The XRD pattern of pure g-C 3 N 4 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-C 3 N 4 (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 g-C 3 N 4 samples, the XRD patterns revealed four separate reections at 38.1 (111), 44.4 (200), 64.8 (220) and 77.6 (311) for the AuNPs, in addition to the peaks for g-C 3 N 4 . The observed reections were well matched with the AuNPs in the prepared nanostructures corresponding to the reported JCPDS values (04-0784). 37 The intensity of the peaks for the AuNPs increased gradually with increasing loading of Au 3+ ions onto the sheetlike structure of g-C 3 N 4 . The four peaks conrmed the anchoring of AuNPs onto the g-C 3 N 4 surface, which was clearly absent in the pure g-C 3 N 4 sample; no other extra/impurity peaks were found in the as-fabricated samples. The presence of both Au planes and g-C 3 N 4 conrmed the successful formation of the Au-g-C 3 N 4 nanostructures using the green/biogenic synthesis approach.
The mean crystallite size of the g-C 3 N 4 and Au-g-C 3 N 4 nanostructures were calculated using the Scherrer's formula, 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-C 3 N 4 , which was observed at 27.43 2q. The calculated crystallite size of bare g-C 3 N 4 from the most intense peak at 27.43 2q was 6.6 nm and the calculated crystallite size of the Au-g-C 3 N 4 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-C 3 N 4 nanostructures increased because of the anchoring of AuNPs on to the g-C 3 N 4 sheets compared to pure g-C 3 N 4 . These increased crystallite values further conrmed the successful fabrication of the Au-g-C 3 N 4 nanostructures. 3.1.2. Optical and photoluminescence analysis of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures. Fig. 2(a and b) shows the optical absorbance and photoluminescence analysis of the pure g-C 3 N 4 and Au-g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 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-C 3 N 4 nanostructures compared to that of pure g-C 3 N 4 . The inset in Fig. 2(a) shows that the AuNPs decorated onto g-C 3 N 4 , in the 1 mM sample displayed a purple color, which was a clear indication of the successful reduction of Au 3+ to Au 0 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-C 3 N 4 nanostructures, which further conrmed the successful formation of Au-g-C 3 N 4 nanostructures. Fig. 2(b) shows the photoluminescence (PL) spectra of the pure g-C 3 N 4 and Au-g-C 3 N 4 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-C 3 N 4 and Au-g-C 3 N 4 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-C 3 N 4 and Au-g-C 3 N 4 nanostructures for the photoelectrode performance. 13,40 As the PL intensity is inversely related to the charge recombination between the photogenerated electronhole pairs, the anchoring/decoration of the AuNPs onto the sheet-like structure of g-C 3 N 4 could prevent charge recombination between the opposite charge carriers, leading to improved photoelectrochemical performance. 40,41 The overall PL studies of the Au-g-C 3 N 4 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-C 3 N 4 nanostructures.
3.1.3. High resolution transmission electron microscopy (HR-TEM) image of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures. The surface structures, morphology, and particle size of the asfabricated samples Au-g-C 3 N 4 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-C 3 N 4 . The as-synthesized Au-g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 sheet. Fig. 3(d) clearly showed the interfacial interaction of AuNPs with the sheet-like structure of g-C 3 N 4 , 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-C 3 N 4 sheet. The Fig. 2 (a) UV-Vis absorbance spectra, and (b) photoluminescence spectra of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures. 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 g-C 3 N 4 . Fig. 3(h) shows the elemental composition of the Au-g-C 3 N 4 nanostructures without any other elemental peak. Fig. S2(a-d) † presents HR-TEM images of the Au-g-C 3 N 4 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-C 3 N 4 and Au-g-C 3 N 4 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-C 3 N 4 and Au-g-C 3 N 4 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-C 3 N 4 and Au-g-C 3 N 4 (1 mM, 3 mM, and 6 mM). Fig. 4(a) displayed the elemental composition of pure g-C 3 N 4 , 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) attached to the surface of g-C 3 N 4 . No impurity peak was observed. 42 The survey scan spectrum ( Fig. 4(b)) of Au-g-C 3 N 4 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-C 3 N 4 . 41 The C 1s peaks were observed at 285 eV and 288.3 eV (Fig. 4(c)), 42 which were assigned to the sp 2 -hybridized carbon atom and the carbon atom bonded to three nitrogen atoms -C(N 3 ) of g-C 3 N 4 , 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-(C 3 ) 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 4f 7/2 and 4f 5/2 electrons of the metallic behavior of gold. 41,44 Therefore, the Au 3+ ions were reduced to the Au 0 oxidation state on the sheet like surface of g-C 3 N 4 . 45,46 Fig. 4(f) shows the combined C 1s spectrum of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructure. In case of AuNPs, the peak intensity is decreased (Fig. 4(f)) with the little shi in the binding energy. 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 Au 3+ to Au 0 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-C 3 N 4 and Au-g-C 3 N 4 nanostructures. N 2 -BET (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-C 3 N 4 and Au-g-C 3 N 4 nanostructures (1 mM, 3 mM, and 6 mM) were 31.0116 AE 0.3652 m 2 g À1 , 31.9655 AE 0.1336 m 2 g À1 , 34.9131 AE 0.3450 m 2 g À1 , and 41.1593 AE 0.4697 m 2 g À1 , respectively. In Fig. 5, the surface area of Au-g-C 3 N 4 (6 mM) increased with increasing amount of precursor, which was much higher than that of pure g-C 3 N 4 . The higher specic 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-C 3 N 4 nanostructures could be improved greatly due to the higher specic surface area.
The nitrogen adsorption-desorption isotherm of pure g-C 3 N 4 displays a hysteresis loop, suggesting the existence of mesopores. 42 The AuNPs-loaded g-C 3 N 4 exhibited much higher specic surface areas that of pure g-C 3 N 4 ( Table 1). The 6 mM AuNPs decorated g-C 3 N 4 had a specic surface area of up to 41.15 m 2 g À1 . This shows that the optimal amount of AuNPs decorated g-C 3 N 4 could provide more adsorption sites and photochemical reaction sites to improve the photoelectrochemical performance. Fig. 4 (a and b) XPS survey scan spectra of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures, (c, d and e) fitted spectra of C 1s, N 1s and Au 4f, and, (f) combined spectra of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures.

Photoelectrochemical studies
4.1. Photoelectrochemical studies of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures using LSV, EIS and CV measurements Studies of the photoelectric behavior of pure g-C 3 N 4 and Au-g-C 3 N 4 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 Na 2 SO 4 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 À1 over the applied potential range, (À1 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-C 3 N 4 nanostructures (1-6 mM) displayed an enhanced photocurrent compared to pure g-C 3 N 4 ( 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-C 3 N 4 . 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 electrons amassed in the conduction band of g-C 3 N 4 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-C 3 N 4 and Au-g-C 3 N 4 as a photoelectrode material, as shown in Fig. 6(b). In general, the complex impedance plot is normally presented as Z 0 /ohm vs. ÀZ 00 /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 Z 0 /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-C 3 N 4 was obtained compared to pure g-C 3 N 4 , which clearly reveals a rapid electron-transfer process in the case of the Au-g-C 3 N 4 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-C 3 N 4 . The performance of the as-fabricated nanostructure was better than that of pure g-C 3 N 4 .
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, R ct and C dl 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 R ct for Au-g-C 3 N 4 nanostructures decrease from 1 mM to 6 mM. The higher concentration of AuNPs exhibit the small R ct value which was much lower than that of pure-g-C 3 N 4 , which clearly suggested that the charge-transfer resistance is signicantly reduced by anchoring of AuNPs onto sheet like structure of g-C 3 N 4 . The C dl values displayed the opposite tendency as that of R ct values. The low R ct and high C dl values indicate high electron transfer efficiency which further supports the improved photoelectrochemical performance of Au-g-C 3 N 4 nanostructures.
The cyclic voltammogram (Fig. 7) of pure g-C 3 N 4 and Au-g-C 3 N 4 nanostructures were obtained in the dark and under visible light irradiation at a scan rate of 0.05 mV s À1 . The CV plot of the Au-g-C 3 N 4 nanostructures showed an improved positive and negative sweep, indicating their pseudo capacitive nature. The peak current of the Au-g-C 3 N 4 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-C 3 N 4 nanostructures under visible light irradiation, which also reveals better capacitive performance of the as-fabricated nanostructures.  Consequently, the improved capacitive performance of the Au-g-C 3 N 4 nanostructures can be attributed to its improved charge loading ability and the synergistic effect of AuNPs and g-C 3 N 4 under visible light irradiation. 46,49,50

Incident photon-to-current conversion efficiency (IPCE) test
To investigate the photoresponse of pure-g-C 3 N 4 and Au-g-C 3 N 4 (1-6 mM) nanostructures, IPCE measurements at 1.2 eV    vs. Ag/AgCl as the reference electrode are presented in Fig. 8.
The IPCE can be expressed as follows: 51,52 where l, 4 and I p denote the wavelength of the incident light (nm), the irradiation power (mW cm À2 ), and the photocurrent density (A cm À2 ) measured at the corresponding wavelength, respectively. Fig. 8 of IPCE tests shows a visible light response in case of higher Au-g-C 3 N 4 nanostructures. The absorption threshold of g-C 3 N 4 is approximately 460 nm, with a lower IPCE value. Anchoring of AuNPs onto sheet like structure of g-C 3 N 4 results in substantial enhancement of the IPCE values for Aug-C 3 N 4 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-C 3 N 4 , the IPCE performance was very less (2.2%) without any hump in visible region as compared to Aug-C 3 N 4 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-C 3 N 4 nanostructures conrmed the successful anchoring of AuNPs onto the sheet-like structure of g-C 3 N 4 . The improved photocurrent performance revealed the interfacial interaction and charge transfer between the AuNPs and g-C 3 N 4 , which could explain the enhanced photoelectrochemical performance of the Au-g-C 3 N 4 nanostructures.

Proposed electron transfer mechanism of Au-g-C 3 N 4 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-C 3 N 4 nanostructures excited and electrons (e À ) 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-C 3 N 4 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-C 3 N 4 nanostructures 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-C 3 N 4 nanostructures sample showed higher photocurrent performance (1-6 mM) because of its tuned optical properties compared to the bare g-C 3 N 4 . 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-g-C 3 N 4 nanostructures. This journal is © The Royal Society of Chemistry 2018

Conclusions
This paper reported a facile, green, and competent approach for the fabrication of Au-g-C 3 N 4 nanostructures with spherical and uniform sized AuNPs with a high surface area (41.1593 m 2 g À1 ) and improved photoelectrochemical performance. A single strain developed biolm was used as a tool to reduce Au 3+ to Au 0 and Au-g-C 3 N 4 nanostructures (1 mM, 3 mM and 6 mM) were fabricated. The anchoring of AuNPs onto the sheet-like structure of g-C 3 N 4 produced promising photoelectrode material for real photonic devices. The boosted photoelectrochemical performance of Au-g-C 3 N 4 nanostructures compared to that of pure g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 , which inhibited the charge recombination process and increased the photocurrent performance.

Conflicts of interest
The authors declare no competing nancial interests.