Ag-doped ZnO nanorods embedded reduced graphene oxide nanocomposite for photo-electrochemical applications

In this paper, the Ag-doped zinc oxide nanorods embedded reduced graphene oxide (ZnO:Ag/rGO) nanocomposite was synthesized for photocatalytic degradation of methyl orange (MO) in the water. The microstructural results confirmed the successful decoration of Ag-doped ZnO nanorods on rGO matrix. The photocatalytic properties, including photocatalytic degradation, charge transfer kinetics and photocurrent generation, are systematically investigated using electrochemical impedance spectroscopy (EIS), photocurrent transient response (PCTR) and open circuit voltage decay (OCVD). The results of photocatalytic dye degradation measurements indicated that ZnO:Ag/rGO nanocomposite is more effective than pristine ZnO to degrade the MO dye, and the degradation rate reached 40.6% in 30 min. The decomposition of MO with ZnO:Ag/rGO nanostructure followed first-order reaction kinetics with a reaction rate constant (Ka) of 0.01746 min−1. The EIS, PCTR and OCVD measurements revealed that the Ag doping and incorporation of rGO could suppress the recombination probability in ZnO by the separation of photo-generated electron–hole pairs, which leads to the enhanced photocurrent generation and photocatalytic activity. The photocurrent density of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO are 206, 121.4 and 88.8 nA cm−2, respectively.


Introduction
In day-to-day life, water contamination is one of the serious issues for humankind and the environment. Water contamination occurs

Synthesis of ZnO:Ag nanorods
The Ag-doped zinc oxide nanorod was prepared by hydrothermal method. To synthesize the Ag-doped ZnO nanorods, the aqueous solution of 0.5 M zinc nitrate (Zn(NO 3 ) 2 . 6H 2 O) and 0.05 M silver nitrate (AgNO 3 ) were mixed under constant stirring for 1 h at ambient temperature. To maintain the pH value of 9.5, few drops of ammonium hydroxide were slowly added into the solution. The final solution was then transferred into Teflon-lined autoclave followed by vigorous stirring (for 30 min). Thereafter, the autoclave was sealed and heated up to 2008C for 6 h. The final product was washed several times with ethanol and DI water and dried at 608C in a hot air oven overnight. The EDAX and SEM analyses were conducted to study the elemental composition and morphology of synthesized samples.

Synthesis of ZnO:Ag/rGO nanocomposite
The GO was synthesized by modified Hummer's method which is given in the electronic supplementary material. The nanocomposite was prepared in such a way that a 100 mg of GO was initially dispersed in 50 ml DI water under ultra-sonication for 20 min. The solution was constantly stirred for 1 h and ZnO:Ag solution (20 mg/40 ml) was added dropwise. Thereafter, the final solution was vigorously stirred and subsequently transferred to the Teflon-lined autoclave which was sealed and heated up to 4008C for 4 h. The final product was collected after the centrifugation at 4000 r.p.m. for 10 min, the obtained product was repeatedly washed with ethanol and DI water and dried at 458C for 12 h. The structural and micro-structural, electrochemical properties of the as-prepared nanocomposite were investigated.

Characterization
The structural analysis of the samples was carried out by using powder X-ray diffractometer (PAN analytical X'Pert PRO) equipped with X'Celerator position sensitive detector using Cu Ka radiation of wavelength l ¼ 1.5401 Å . Microstructural analysis of the samples was characterized by using JEOL (SEM and TEM). The UV-vis absorption spectra were obtained between 200 and 800 nm using Shimadzu UV-1800 spectrophotometer, equipped with quartz cuvettes with optical path lengths of 1 cm. The Raman spectra of the samples were recorded by using EZ Raman-N-785 spectrometer. The DC I -V characteristics of samples were analysed between 25 and þ5 V using a Keithley 2636B source meter.

Photo-electrochemical measurements
To understand the charge transportation kinetics, photocurrent generation and recombination kinetics of the samples, the electrochemical studies such as EIS, PCTR and OCVD measurements were carried out by using Biologic VSP-300 electrochemical workstation with the standard three-electrode configuration (working, reference and counter electrode). The photo anode was used as a working electrode and it was prepared by coating the nanocomposite film (an active area of 1 cm 2 ) on the FTO substrate (sheet resistance ¼ 7 V sq 21 ) using doctor blade technique and dried at 3008C for 30 min. The standard calomel electrode and platinum wire are used as the reference and counter electrode, respectively. 0.1 M of Na 2 SO 4 was used as an electrolytic medium for the electrochemical analysis. The electrochemical impedance spectral (EIS) properties of the samples were analysed in the frequency range from 500 mHz to 1 MHz under dark and UV light irradiation (300 W mercury lamp). The PCTR measurements were done under UV irradiation (300 W mercury lamp) with 5 min ON/OFF cycle at an input voltage of 0.5 V. The OCVD characteristics were measured as a function of time in which the voltage growth was monitored under UV irradiation for 60 s and then the voltage decay was recorded in the dark for 90 s. The photocatalytic degradation activity of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO was monitored by measuring the percentage degradation of MO dye. The photocatalytic solutions were prepared by using a photo-reactor, in which a 300 W mercury lamp was used as a UV source. A 50 mg of photocatalyst was dispersed in 100 ml MO solution (10 mg l 21 ). Prior to the UV irradiation, the suspensions were sonicated for 15 min in the dark to maintain the adsorptiondesorption equilibrium. To achieve the homogeneity during the photocatalytic measurement, the photo-reactor was placed on a magnetic stirrer in the dark. A 3 ml of suspension was taken out from the photo-reactor at regular time intervals and then the absorption of MO was monitored under royalsocietypublishing.org/journal/rsos R. Soc. open sci. 6: 181764 visible light (at l ¼ 480 nm) by using Shimadzu UV-1800 spectrophotometer. The percentage of photocatalytic degradation was calculated from the following equation [27]: where A 0 is the absorbance of MO before UV irradiation.

Result and discussion
The X-ray diffraction spectra of ZnO:Ag/rGO, ZnO/rGO, rGO and GO are presented in figure 1a. The characteristic diffraction peak of graphene oxide (GO) is observed at 11.38 corresponding to the d-spacing of 0.76 nm, which is 2.3 times larger than the d-spacing of graphite (0.33 nm). The enlarged d-spacing of GO originated from the intercalation of oxygen-containing functional groups (discussed and given in the electronic supplementary material, §4) between the graphitic interlayers distance due to the strong chemical oxidation treatment on graphite. After the oxidation, a very small fraction of unreacted graphitic phases also presents in GO, it can be clearly observed at 42.58. The thermal reduction at 4008C on GO causes large inter-planar spacing due to the elimination of oxygen-containing functional groups from the carbon interlayers. Therefore, the diffraction peak of GO shifted from 11.38 to 25.48. Hence from the diffraction spectra of GO and rGO, the peak shifting towards lower angle reveals the successful modification of sp 2 (graphite) into sp 3 (GO) hybridized carbon structure. But, the peak shifting towards the higher angle confirms the transformation of GO into rGO. The diffraction peaks of ZnO are observed in ZnO/rGO and ZnO:Ag/rGO as shown in figure 1a. All these peaks are indexed to be the hexagonal Wurtzite structure of ZnO and agreed well with standard data sheet (JCPDS-36-1451) [28]. Figure 1b shows the Raman spectra of ZnO:Ag/rGO, ZnO/rGO and GO. There are two prominent peaks (G-band and D-band) for all samples. The G-band observed at 1592 cm 21 is attributed to the presence of sp 2 hybridized carbon structure, whereas, the D-band at 1318 cm 21 occurs from the localized vibrational modes of oxygen functionalities [29,30]. Raman spectroscopy is a useful method to characterize the degree of structural disorder in the samples by correlating the intensity ratio (ID/IG). Therefore, the degree of disorder is proportional to the I D /I G value. The estimated I D /I G values from the Raman spectra are 1.10, 0.99 and 1.01 respectively, it reveals that the structural disorder of ZnO:Ag/rGO and ZnO/rGO is less than that of GO. The reason behind this is that during the thermal reduction on ZnO:Ag/rGO and ZnO/rGO, the regeneration occurs in the carbon structure from sp 3 into sp 2 , which leads to regaining the order in the carbon structure, as a result the intensity of the D-band is less than that of G-band. The typical TEM images of ZnO:Ag/rGO are shown in figure 2a-d. It is identified from the image in low magnification (figure 2a,b) that the zinc oxide nanorods are randomly distributed on rGO sheets. The ZnO nanorods are observed very clearly from the high magnification images (figure 2c,d). It has been estimated that the diameter and the length of the nanorods are about 50 + 5 and 300 + 10 nm, respectively. TEM images reveal that the rGO sheets are with the ZnO nanorods. Furthermore, the rGO sheets overlap with each other and form a three-dimensional network structure on ZnO    . The optical bandgap of the samples is calculated by using ahn ¼ A(hn 2 E g ) 1/2 , where a is an absorption coefficient, hn is the energy of the photon, A is a constant, E g is the bandgap energy of the sample and 1/2 defines the direct allowed transition. The optical bandgap of the samples is calculated by the extrapolation of a linear portion in the plot of (ahn) 2 against hn presented in Figure 5b. The calculated band gaps of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO are 2.97, 2.90 and 2.87 eV, respectively, indicating that the bandgap of ZnO is found to increase after the addition of rGO. This is because the thermal reduction on ZnO/GO composite can create oxygen vacancies in ZnO [31]. After Ag doping, a blue shift is observed in the bandgap energy of ZnO/rGO, attributed to the phenomenon called Moss -Burstein effect caused by the generation of electrons due to oxygen vacancies [32]. The substitution of Ag 2þ in Zn 2þ lattice, thus creating more oxygen vacancies and high electron density due to electronegativity and variation in the ionic radius between Ag 2þ and Zn 2þ . This high electron density leads to the lifting of  pristine ZnO. The impedance spectra of the samples were recorded under both dark and UV irradiation, the respective Nyquist plots and its equivalent circuit are shown in figure 6a. The equivalent circuit consists of a series resistance (R s ), a charge transfers resistance (R ct ), Warburg impedance (Z w ) and a capacitor. The electrochemical charge transfer resistances of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO calculated from the semicircle of the impedance curve in the high-frequency region are 0.17, 0.38 and 0.46 MV, respectively. As observed from figure 6a, the radius of the semicircle of the impedance curve decreases in the order of ZnO . ZnO/rGO . ZnO:Ag/rGO. The smaller the R ct of the ZnO:Ag/rGO leads to the better charge transportation at the interface between the working electrode (photo anode) and electrolyte. As the presence of rGO and Ag in ZnO:Ag/rGO nanocomposite promotes the mobility of charge carriers in the working electrode, the Ohmic resistance at the interface between the working electrode and electrolyte is reduced. In addition, the series resistance (R s ) determined from the Nyquist curve illustrates that the intrinsic conductivity of ZnO is enhanced by the incorporation of rGO and Ag. Figure 6b,c represents the photo-dependent electrochemical impedance spectra of ZnO/rGO and ZnO:Ag/rGO. The photo-dependent impedance spectra were recorded under the illumination of UV light for 1 to 5 min. It is observed from figure 6b that the radius of the semicircle of the impedance curve is reduced when the working electrode was exposed by the UV irradiation and the corresponding R ct is decreasing from 0.3 (dark) to 0.16 MV (UV), indicating that the presence of rGO is enhancing the mobility of photo-generated charge carrier in the photo anode (ZnO/rGO). The opto-electrochemical property of ZnO/rGO is further improved by the Ag doping. The representative impedance curves of ZnO:Ag/rGO (figure 6c) show the enhanced optical response under the UV irradiation, in which the R ct value decreases from 0.18 (dark) to 0.09 MV (UV). It emphasize that the ZnO:Ag/rGO can be used as a promising material for the optoelectrochemical applications. The DC I -V characteristics of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO films were analysed in the dark and UV with an interval of 5 min. The measurements were recorded between the sweep voltage of 25 and þ5 V. Figure 6d represents that the photocurrent as well as the dark current properties of ZnO is improved due to the addition of rGO and Ag doping with ZnO. The current density of the ZnO:Ag/rGO is found to increase from 120 to 192 mA cm 22 after the illumination of UV for 5 min. Besides light absorption, the photo-induced charge carrier transportation and separation are also considered to be the key factors, which will enhance the photocurrent generation and photocatalytic activity of ZnO. Many researchers also reported that the photocurrent and photocatalytic performance of ZnO can be improved by the incorporation of rGO [36][37][38][39][40][41][42]. Graphene is an excellent acceptor candidate due to its unique two-dimensional p-conjugation structure [36], it results in the fast transportation of photo-induced charge carriers from semiconductor into graphene via percolation mechanism [43]. But, photo-electrochemical properties can also be enriched by the addition of impurities (such as Ag, Fe, Ni and Pd) into the intrinsic ZnO via the doping process [27,[44][45][46]. The charge carrier concentration of doped atom (impurity) and the interstitial defects which are caused by the doped atom will improve the photocurrent and photocatalytic efficiency of a semiconductor material. This study reveals that the photocurrent properties of ZnO can be markedly improved by the addition of rGO and Ag doping.
To explore the photocurrent properties of ZnO:Ag/rGO photocatalyst, the PCTR measurement was carried out by applying 0.5 V between working and reference electrodes and the current has been measured between working and counter electrodes. For comparison, the similar measurement was It can be clearly seen that the photocurrent response of ZnO/rGO is increased due to the addition of rGO; this result agreed well with previous works [39]. Significantly, the ZnO:Ag/rGO shows superior photocurrent generation, compared to ZnO/rGO and pristine ZnO, indicating that the photo-conversion efficiency of ZnO/rGO is further improved after Ag doping. The excess carrier concentration, owing to the ionization of donor (Ag), leads to an increase in the n-type conductivity as well as photocurrent density of ZnO/rGO. In addition to that, the lattice defects caused by the dopant ions are also contributing to the photocurrent generation [27,46]. The photocurrent density of ZnO:Ag/rGO is 206 nA cm 22 , which is approximately 2.3 times higher than that of pristine ZnO. This enhancement in the photocurrent density of ZnO:Ag/rGO reveals that the photo-generated charge carrier separation efficiency of the sample is promoted after Ag doping along with the addition of rGO, resulting in a decrease in its recombination probability. The evidence for the low recombination property of ZnO:Ag/rGO is confirmed from the OCVD measurement (figure 7c). In a semiconductor, the photo-induced electrons in the conduction band and holes in the valence band are developing a potential (open circuit voltage). This potential growth occurs due to the excitation of charge carriers, whereas the potential drop occurs due to the recombination of photo-induced charge carriers. Therefore, the magnitude of the potential drop is proportional to the lifetime of the photo-generated charge carriers in the semiconductor. Compared to pristine ZnO and ZnO/rGO, the ZnO:Ag/rGO is displaying a lower potential drop (DV ¼ V growth 2 V decay ) (23.63 mV). It represents that the photoinduced charge carriers in ZnO are quickly trapped by rGO, which leads to the minimization of potential drop via reducing the carrier recombination rate and thereby increasing the lifetime of photo-induced charge carriers [47,48]. The improved lifetime of photo-generated charge carrier in ZnO:Ag/rGO is calculated from the time-correlated single photon count (TCSPC) measurement. As observed from figure 7b, the excited carrier lifetime of ZnO/rGO is found to increase up to 0.168 ns where K a defines the reaction rate constant. The photocatalytic activity can be estimated through kinetic rate constants of MO degradation. The obtained results (figure 8b,c, inset) show that the photocatalytic degradation rate of ZnO:Ag/rGO is found to be 0.01746 min 21 , which is 2.7 times higher than reaction rate of pristine ZnO, suggesting that the recombination probability of excited charge carrier in ZnO is suppressed by the incorporation of Ag and rGO, which results in the improvement of photocatalytic efficiency. These results are in good agreement with transient photocurrent response of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO.

Conclusion
The facile hydrothermal method was employed for the successful preparation of ZnO:Ag/rGO nanocomposites. The photo-electrochemical properties of ZnO:Ag/rGO were systematically investigated and results were compared with pristine ZnO. The Ag-doped ZnO nanorods embedded rGO composite is found to exhibit remarkable photo-electrochemical properties compared with that of  Figure 8. pristine ZnO. The ZnO:Ag/rGO showed a high photocurrent generation of 206 nA cm 22 and excellent MO degradation (40.6%) under UV irradiation with the high reaction rate of 0.01746 min 21 , because the addition of Ag and rGO improved the lifetime of photo-generated charge carrier by separating the electron -hole pair, which suppressed the carrier recombination in ZnO. The evidence for the enhanced carrier lifetime, carrier transportation and recombination was verified by time-correlated single photon counting, electrochemical impedance spectroscopic and OCVD measurements. The present study reveals that the ZnO:Ag/rGO nanocomposite can be used as an excellent photo anode for the electrochemical applications.
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