- Email: [email protected]
Zn1−xAgxO nanocomposite in visible-light region
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced photoelectrochemical response of reduced-graphene oxide/Zn1¡xAgxO nanocomposite in visible-light region Sumant Upadhyay*, Samira Bagheri, Sharifah Bee Abd Hamid Nanotechnology and Catalysis Research Center (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia
article info
abstract
Article history:
Graphene based nanocomposites have the potential to work as efficiently, multifunctional
Received 23 January 2014
materials for energy conversion & storage. These composites may exhibit better photo-
Received in revised form
catalytic properties by the improvement of their electronic and structural properties than
5 May 2014
pure photocatalysts. In the present work, reduced graphene oxide (rGO) & ZnO nano-
Accepted 15 May 2014
composite with 0e5 atom% Ag doping was prepared by electrodeposition method and
Available online 13 June 2014
characterized by XRD, Raman spectroscopy, FE-SEM, EDX, UVeVis spectroscopy and final photoelectrochemical activity was assessed under 1.5 AM solar simulator in 1 M NaOH as
Keywords:
electrolyte. Significant changes in the Raman spectrum for the nanocomposite suggest the
Ag-doping
possible electronic interaction between rGO and ZnO nanocomposite and its successful
Nanocomposite
fabrication, which improves the charge separation and enhanced photoelectrochemical
Photoelectrochemical water
activity in the nanocomposite. We find a red-shift of 0.35 eV in the UVevis spectrum and
splitting
therefore an enhanced photoelectrochemical activity in the visible range on Ag doping in
Graphene oxide
rGO/ZnO nanocomposite. Nanocomposite with 1 atom% Ag doping showed the highest photocurrent density of 2.48 mA/cm2 at 0.8 V vs Ag/AgCl over other samples, which was almost five times higher than that for undoped rGO/ZnO composite. Calculated Flat-band potential and donor densities using MotteSchottky data also supported the better photoelectrochemical response for Ag doping in nanocomposite. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Supply and demand of energy determine the course of global development in every sphere of human activity. The world's energy need is sure to increase with growth in population. We now use 4.1 1020 J of energy per year, which is equivalent to 13 TW of power. This consumption may more than double to ~30 TW by the year 2050 and then triple to 46 TW by the end of
the century. Thus, finding viable alternative sources of clean energy to satisfy the anticipated demand is one of the real challenges faced by Scientists at large [1]. Solar energy offers a clean, abundant, affordable, sustainable, renewable, and an ideal source of energy and efficient conversion of solar energy into chemical fuels (e.g. Hydrogen) is one of the “Holy Grails” of the 21st century. Hydrogen is widely regarded as a promising energy carrier in the future. It has high energy density of 140 MJ kg1, far more than gasoline and coal, also its
* Corresponding author. E-mail address: [email protected] (S. Upadhyay). http://dx.doi.org/10.1016/j.ijhydene.2014.05.094 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
11028
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
combustion byproduct is water. There are several methods for utilizing solar radiation in splitting water for hydrogen generation [2,3]. However, photovoltaic electrolysis of water uses the expensive proton exchange membranes [4] and alkaline electrolytes [5]. Solar to thermochemical water splitting needs the high temperature of 700e1000 C [6]. Photobiological water splitting has to overcome the natural short-term nature of biospecies hydrogen production [7]. Thus, much attention has been focused on splitting water for hydrogen generation by photoelectrochemical (PEC) or photocatalytic reactions because they are cost-effective, simple and convenient, and have huge potential for further development [8]. Hydrogen generation via photoelectrochemical (PEC) water splitting using solar light is believed to be a clean and efficient way to overcome the global energy and environmental problems. Ideally, PEC water splitting requires electrode materials that have: (1) a proper band gap (~1.6e2.2 eV); (2) band edge alignment that straddles the H2 and O2 reaction potentials; (3) high energy conversion efficiency; (4) stability in the working environment; and (5) low production cost and an abundant natural source which is desirable for large-scale applications [9]. Emerging nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened new ways to utilize renewable energy resources because of large surface areas, abundant surface states, and diverse morphologies compared to their corresponding bulk materials. Among them, graphene, a two-dimensional (2D) network of hexagonal sp2-hybridized carbon atoms, has stimulated research interest in various energy conversion applications. Graphene exhibits many outstanding properties, such as fast room-temperature mobility of charge carriers (200,000 cm2 V1 s1), exceptional conductivity (106 S cm1), large theoretical specific surface area (2630 m2 g1), and excellent optical transmittance (~97.7%) [10]. These unique properties indicate that graphene has great potential to be an ideal construction component of PEC electrodes or photocatalytic materials for hydrogen production. By incorporating two or more catalyst particles on a single graphene or reduced graphene oxide sheet, it should be possible to carry out selective catalytic processes at separate sites. In addition, graphene's ability to store and shuttle electrons will be an important parameter in dictating catalytic activity. Thus, proper design of a catalyst material can provide greater versatility in carrying out selective catalytic or sensing processes [11]. Wurtzite-structured ZnO, a wide-band-gap (3.37 eV) semiconducting oxide with a large exciton binding energy (60 eV), has versatile properties that are important for applications in optoelectronics, solar cells, and sensors [12,13]. The tremendous interests have been shown by ZnO for the last two decades because of its multifunctional character, which can be varied by controlling its morphology and crystallinity. However, the major drawback with ZnO is its wide band gap, resulting in its limited activity under visible light due mainly to the rapid recombination of photogenerated electrons/holes, which limit the photodegradation reaction under normal conditions [14]. To fabricate a Photocatalyst that works under visible light, which covers 43% of sunlight, has been a challenging problem [15]. Doping in ZnO with selective elements
offers an effective method to adjust their electrical, optical, and magnetic properties, which is crucial for their practical applications. Studies on ZnO with Ag doping are relatively less. Previously, researchers have reported the ionization potential of Ag as 0.4 eV and shown its potential application in ZnO [16]. Georgekutty, R, et al. [17], showed the photocatalytic activity of Ag-doped ZnO photocatalyst five times higher than the unmodified one using sunlight. Some researchers have also shown the effects of Ag on photoactivity degradation activity in ZnO [18e20]. In ZnOegraphene hybrid materials, ZnO acts as a photocatalyst, to excite the electrons from the valence band to the conduction band and create electronehole pairs, which can migrate and initiate redox reactions with water and oxygen, and then degrade organic molecules or reduce metal ion absorbed on the surface of ZnO [21e23]. Graphene acts as an excellent electron-transport material in the process of photocatalysis, so that the hybridization of ZnO with graphene can hinder the recombination of charge carriers and increase the photocatalytic performance [24]. Taking into account the excellent properties of ZnO and graphene, their nanocomposite can enable versatile properties with competence far beyond those of the individual members [25]. In the present work, 0e5 atom% Ag-doped ZnO nanostructured thin films were coated over pre-deposited, reduced-graphene oxide (RGO) set using electrodeposition method on ITO glass substrate, and samples were characterized for XRD, Raman Spectroscopy, SEM-EDX, UVeVis spectroscopy, and Photoelectrochemical performance.
Experimental All reagents used were of analytical grade.
Preparation of reduced graphene oxide (rGO) nanosheet Graphene oxide was prepared using a modified Hummer's method [26]. Graphite oxide was obtained by oxidation of 1 g of graphite flakes with 120 ml of H2SO4 (60%), 13 ml of H3PO4 (60%) and 6 g of KMnO4. The mixture was stirred for three days to complete oxidation of the graphite. During oxidation, the color of the mixture changed from dark purplish-green to dark brown. To stop the oxidation, 7 ml of H2O2 (30%) solution and 150 g of ice were added and the color of the mixture changed to bright yellow, indicating a high oxidation level of graphite. The formed graphite oxide was washed three times with 1 M of aqueous HCl solution and repeatedly with deionized water until a pH of 4e5 was achieved. The washing process was carried out using a simple decantation of the supernatant with a centrifugation technique. During the washing process with deionized water, the graphite oxide underwent exfoliation, which resulted in thickening of the graphene oxide solution, forming graphene oxide gel. Electrodeposition of rGO over ITO glass substrate was performed using cyclic Voltammetry in a three-electrode potentiostat system [Autolab Potentiostat/Galvanostat (Netherlands)]. ITO glass substrate (SPIE, USA) was used as working electrode. One third part of conducting glass
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
substrate was initially covered with transparent tape to establish the electrical contact. Before performing electrodeposition, substrate plates were cleaned by soaking for ~2 min in detergent solution, followed by washing with doubledistilled deionized water. Subsequently, these were trickled for ~10 s with 0.1 M HCl, rinsed with water and acetone, and air-dried. ITO substrate, used as working electrode in conjunction with platinum as counter electrode were hanged vertically in an aqueous electrolyte consists of 7 mg L1 of GO as prepared and 0.1 M phosphate buffer (K2HPO4 and KH2PO4) at pH ~7.0. Potential applied was in a range from 0 to 1.5 V at a scan rate of 1 mV/s. The as-synthesized rGO nanosheets were washed in a gentle flow of water and dried at 50 C over a hot plate surface.
Preparation of nanocomposite (ZnO/rGO) ZnO/rGO nanocomposite films were obtained by depositing ZnO over pre-deposited rGO nanosheets using electrodeposition at 1.16 V (vs. Ag/AgCl) in the same three-electrode potentiostat system. The electrolyte used consists of an aqueous solution of Zinc Nitrate (0.1 M), Potassium Chloride (KCl, 0.1 M), ethylene diammine (EDA, 0.01 M) and a calculated amount of AgNO3 was added for Ag-doped ZnO thin films. Electrolyte solution was kept at ~80 C under constant stirring during the deposition process. The as-prepared films were kept for sintering at 600 C for crystallization.
Materials characterization X-ray diffraction patterns of samples were obtained on a D8 Advance X-ray diffractometer (Bruker, Germany) using CuKa radiation of 1.5416 Å at a scan rate of 0.02 2q s1. The surface morphology of as prepared samples was examined by using field-emission electron microscope (Carl Zeiss SUPRA 35VP). The Raman spectra of the samples were recorded using a Renishaw In-Via Raman spectrometer (Renishaw, UK). To determine the absorption band edge, and band gap energy, optical study was carried out using absorption data of samples recorded by UVeVisible spectrophotometer (Perkin Elmer, Lambda 35).
Photoelectrochemical performance Before performing photoelectrochemical study electrical contacts on all thin film samples were made through a fine copper wire loop attached to the undeposited area of the ITO glass with conducting silver paint. The area of contact was later covered and sealed with epoxy resin (Hysol, Singapore) which was non-conducting and opaque. The entire structure was heated in an oven at 70e80 C to ensure complete drying. A three-electrode potentiostat system was used to measure the photocurrent, which allowed measurement of the electron and hole pair formation as a function of the externally applied potential necessary for water splitting. Photoelectrodes were placed in the electrolyte (0.1 M, NaOH), while potentiostatic control was maintained with an Autolab Potentiostat, PGSTAT 302N, Netherlands, at a scan rate of 20 mV/s. Platinum wire mesh was used as the counter electrode and Ag/AgCl as the reference electrode. The working
11029
surface area of all electrodes was 1.0 cm2. The light source was simulated sunlight from a 150 W xenon solar simulator (Model 94306 A, Oriel) through an Air Mass filter (AM1.5 Global, 81094) with a measured intensity equivalent to standard AM1.5 sunlight (100 mW/cm2) at the sample face. In this work the MotteSchottky approach was used to determine the electronic properties of the ZnO/rGO nanocomposite with and without Ag-doping. The analysis was conducted in a 0.1 M NaOH, using a potentiostat/galvanostat Autolab PGSTAT 320N with a frequency response analyzer (FRA) module. A three-electrode cell configuration was used for the measurements with the rGO, ZnO/rGO and Ag-doped ZnO/rGO as the working electrode, Ag/AgCl as reference and a platinum wire as the counter-electrode. Capacitance measurements were performed at 1 kHz. The working electrodes were polarized in the cathodic direction in successive steps of 20 mV from þ1.0 V up to 1.0 V. From the intercept of these MotteSchottky curves flatband potential, donor density and depletion layer width were calculated for all the samples using the following relation [27]. 1 C2 ¼ ð1=2qε0 εNÞ$ Vapp VFB kT q
(1)
where ε0 is the permittivity of the vacuum, N is the donor density, Vapp is the applied potential, VFB is the flat band potential, ε is the dielectric constant of the semiconductor, q is the electronic charge, kT/q is the temperature dependent term. The intercept of linear plot at C2 ¼ 0 gives the flatband potential. The donor density (N) was calculated from the slope of the plot (1/C2 versus electrode potential) using the relation in Equation (1).
Results & discussion XRD studies The XRD patterns of rGO, rGO/ZnO and rGO/Ag-doped ZnO (0.5, 1.0, 3.0 & 5.0 atom%) samples is shown in Fig. 1. Sharp diffraction peaks indicate that the samples are well crystallized. It is shown of Fig. 1A that the peak of rGO at around 2q ¼ 10.2 corresponds to the (001) reflection. The peaks at 2q ¼ 31.7 , 34.4 , 36.2 , 47.5 , 56.5 , 62.8 , 69.1 , corresponds to the hexagonal wurzite phase of ZnO, which are consistent with the values in the standard card (JCPDS No. 36-1451). In the XRD patterns of rGO, rGO/ZnO and rGO/Ag-doped ZnO samples (BeF) the characteristic peak of graphene oxide (GO)/ graphite is not observed, which indicates the exfoliation of layered graphene sheets to the growth of ZnO nanoparticles. After the formation of the nanocomposite, the restacking of carbon sheets is prevented by the nanoparticles [25]. No peak corresponding to mixed oxide or impurity phase was identified. As calculated by the DebyeeScherrer equation, for the hexagonal phase of (101) peak crystalline diameters as a function of Ag substitution are listed in Table 1. At lower concentrations of Ag-doping (0.5, 1.0, 3.0 atom%) the crystallite size is decreased, however, in the region of 5 atom% we found an increase in crystallite size. A slight variation in the crystal lattice parameters of 0.008% with doping concentration was observed.
11030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Table 1 e Showing the crystallite size, lattice parameter and band gap values for all samples. Samples
(rGO) ZnO/rGO 0.5 atom% AgeZnO/rGO 1.0 atom% AgeZnO/rGO 3.0 atom% AgeZnO/rGO 5.0 atom% AgeZnO/rGO
Fig. 1 e X-ray diffraction patterns for all samples (A) rGO, (B) rGO/ZnO, (C) rGO/0.5 atom% AgeZnO, (D) rGO/1.0 atom% AgeZnO, (E) rGO/3.0 atom% AgeZnO (F) rGO/5.0 atom% AgeZnO.
Raman spectroscopy Raman spectroscopy is a powerful tool to investigate the crystallization, structure, and defects in the nanostructure materials. Fig. 2AeC shows a typical spectrum of rGO, rGO/ ZnO and rGO/Ag-doped ZnO samples. We observed three characteristic peaks of rGO, the G-band, which is due to the E2g vibrational mode of sp2 bonded carbon and is observed at 1586 cm1, and the D-band at 1345 cm1 is due to the A1g mode breathing vibrations of six-membered sp2 carbon rings and 2D-band at 2740 cm1. The 2D-band is the second order of the D-band, sometimes referred to as an overtone of the D-band. It is the result of a two photon lattice vibrational process, but unlike the D-band, it does not need to be activated by proximity to a defect. As a result the 2D-band is always a strong band in graphene even when no D-band is present, and it does not represent defects. The intensity of the Raman 2D-band can be infinite even if the D-band Raman intensity is negligible. ZnO crystallizes mainly in the hexagonal wurtzite structure with C6v point group symmetry. Group theory predicts that there are two E2, two E1, two A1 and two B1 symmetry phonon modes in wurtzite structure crystals and among these modes the two B1 modes are not Raman active [28]. Therefore, the frequencies of the fundamental optical modes in ZnO are E2 (low) ¼ 101 cm1, E2 (high) ¼ 437 cm1, A1 (TO) ¼ 380 cm1, A1 (LO) ¼ 574 cm1, E1 (TO) ¼ 407 cm1, E1 (LO) ¼ 583 cm1 [29]. Fig. 2B shows that the Raman spectrum of the rGO/ZnO with three Raman shifts at ~331.4, ~437, and ~587 cm1. The strong Raman shift at ~437 cm1 is assigned to E2 mode of ZnO E2 mode of ZnO crystal, which is consistent with the Raman peak of bulk ZnO crystals [30]. The two small peaks at ~331 cm1 and ~587 cm1 are designated to the second order Raman spectrum arising from zone-boundary phonons 3E2H e E2L for wurtzite hexagonal ZnO single crystals and E1 (LO) mode of ZnO associated with oxygen
Lattice parameters (Å)
Crystallite size (nm)
Band gap energy (eV)
a
c
e 3.2606 3.2599
e 5.2186 5.2180
42 39 30
3.15 3.05 2.86
3.2596
5.2179
22
2.70
3.2598
5.2168
20
2.68
3.2589
5.2164
37
2.94
deficiency in ZnO nanomaterials respectively [31]. The second-order modes are located at 1050e1200 cm1 [10]. In the present work, the D-band disappeared for rGO/ZnO and rGO/ Ag-doped ZnO in the Raman spectrum, a possible reason for this is the D-band is known as the disorder band or the defect band. It represents a ring breathing mode from sp2 carbon rings; although to be active the ring must be adjacent to a graphene edge or a defect. The band is the result of a one phonon lattice vibrational process. The band is typically very weak in graphite and is typically weak in graphene as well. If the D-band is significant, it indicates that there are a lot of defects in the material. The intensity of the D-band is directly proportional to the level of defects in the sample. Thus, the disappearance of D-band in rGO/ZnO & rGO/Ag-doped ZnO samples suggest the deposition of the high crystalline nature of ZnO over rGO with negligible or no defects in the nanocomposite. It is estimated that IG/ID of rGO is 0.754 which significantly decreases to zero because of the disappearance of D-band in case of rGO/ZnO & rGO/Ag-doped ZnO thin film samples. However, there is a band up-shifting in 2D-band for rGO/ZnO & rGO/Ag-doped ZnO samples, which are generally
Fig. 2 e Raman spectra for rGO, rGO/ZnO and rGO/1 atom% AgeZnO samples.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
evidences of chemical doping of carbon materials, suggesting charge transfer between rGO and ZnO in the nanocomposite [32]. Our Raman results indicate that ZnO firm interacts with RGO and the nanocomposite is successfully fabricated. Thus, our result suggests the possible electronic interaction between ZnO and graphene contact, which improves the charge separation and enhanced photocatalytic activity in the nanocomposite.
FE-SEM & EDX study Fig. 3 shows the Field Emission-Scanning Electron Microscopy (FE-SEM) images of rGO, rGO/ZnO & rGO/Ag-doped ZnO nanocomposite thin film samples. Fig. 3a shows a layered structured, irregular folding and a curled morphology for electrodeposited rGO nanosheets. FE-SEM images (Fig. 3bec) shows that the ZnO nanoparticles are uniformily distributed on the surface of rGO nanosheet. The particle size calculated for rGO/ZnO and rGO/Ag-doped ZnO thin film samples are in good agreement with the size calculated using XRD data. For rGO/ZnO the particle size estimated from FE-SEM is ~40 nm and for rGO/Ag-doped ZnO particle sizes decreased for low atomic concentration of Ag with a value of ~20 nm, however, we found an agglomeration (Inset of Fig. 3d) in case of 5.0 atom % Ag-doped sample (Fig. 3d), resulting in an increase in particle size. The reason for this could be that the large amount of dopant concentration in ZnO, may force to recrystallize the ZnO, leading to bigger crystallite size. Also, formation of additional sites by the dopant may lead to nucleation or random size distribution of crystallite size. To confirm the presence of Ag in the rGO/ZnO thin films, the elemental composition of the samples was investigated by the EDX spectra. Fig. 4(aeb) shows the EDX measurement for the rGO/ZnO & rGO/1 atom% Ag-doped ZnO thin films. The result reveals that Ag atoms were indeed incorporated in the
11031
ZnO films and its peak was at about 3.0 KeV. The same atomic percentage of ‘O’ obtained for both the samples indicates the uniform doping of Ag in the ZnO lattice without forming any other impurity phase. Elemental composition of 1 atom% Ag doped sample as obtained from EDX was 1.22%.
UVeVis spectrum & band-gap measurements The UVeVis spectra (Fig. 5) for all samples were measured as a function of wavelength from 200 to 800 nm using a UVeVis spectrophotometer. The rGO, rGO/ZnO shows an absorption at 380 nm and the rGO/Ag-doped ZnO samples show absorption in the visible light region. It is noticeable that there is an enhancement in absorption with the Ag-doping in rGO/ZnO composite samples towards the visible light region. The optical band gap values for all samples were calculated from Tauc-Plots (Table 1). The best fit was found for the direct band gap transition with the values of 3.15 eV, 3.05 eV and 2.70 eV for rGO, rGO/ZnO & rGO/1 atom% AgeZnO samples respectively (Fig. 6). The estimates for band gap values have been given in Table 1. The decrease in band gap values for 0.5, 1.0 & 3.0 atom% Ag-doping shows that Ag-doping significantly changes the band-gap energy, which may be due to the intermediate level formed by Ag within the energy level of ZnO. However, we find an increase in band gap of 5.0 atom% Agdoped ZnO sample.
Photoelectrochemical performance The MotteSchottky (MS) plots are presented in Fig. 7 for rGO, rGO/ZnO & rGO/1 atom% AgeZnO samples and to estimate flatband potential (VFB), the linear part of the data was extrapolated to 1/C2 ¼ 0. The VFB for rGO, rGO/ZnO & rGO/ 1 atom% AgeZnO estimated from MS plots were þ0.310, 0.460 & 0.780 respectively. The slope of the curves in the MS
Fig. 3 e FE-SEM images (a) rGO (b) rGO/ZnO (c) rGO/1 atom% AgeZnO (d) rGO/5 atom% AgeZnO.
11032
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Fig. 6 e Tauc-Plots for rGO, rGO/ZnO and rGO/1 atom% AgeZnO.
Fig. 4 e EDX spectra for (a) rGO/ZnO and (b) rGO/1 atom% AgeZnO.
Fig. 5 e UVeVis absorption curves for all samples.
plots are negative for rGO (Fig. 7, Inset) and positive for rGO/ ZnO & rGO/1 atom% AgeZnO saples indicating p-type and ntype nature of the samples. Shifting of negative values of the VFB in nanocomposite samples indicates the better ability of the semiconductor films to facilitate the charge separation in PEC application. Donor density (ND) was also found to increase with increase in Ag-doping concentration; however, we found a decrease in ND at 5 atom% Ag-doped sample. The higher value of ND & VFB for the 1 atom% Ag-doped sample over other samples may be responsible for the significantly higher photocurrent density (Table 2). Thus, 1 atom% Ag-doped sample turns out to be an optimal dopant level for the rGO/ ZnO nanocomposite for enhanced Photoelectrochemical response in the visible light region. On the other hand, higher concentration of dopant may have provided more defectscattering recombination centers inhibiting the increase charge separation efficiency [33].
Fig. 7 e Motteschottky plots for rGO, rGO/ZnO and rGO/ 1 atom% AgeZnO.
11033
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Table 2 e Measured parameters calculated using Mott-Schottky and IeV data for all samples. Samples
rGO rGO/ZnO rGO/0.5 atom% rGO/1.0 atom% rGO/3.0 atom% rGO/5.0 atom%
AgeZnO AgeZnO AgeZnO AgeZnO
Open circuit photovoltage, VOC
Flatband potential, VFB
0.234 0.682 0.710 0.850 0.734 0.658
þ0.310 0.460 0.530 0.780 0.692 0.390
The current (I)evoltage (V) measurements were done for all samples using AM 1.5 solar simulator with 100 mW/cm2 power. The externally applied potential was varied from 1.0 V vs Ag/AgCl (cathodic bias) to þ1.0 V vs Ag/AgCl (anodic bias). The photocurrent density of all the samples was calculated by subtracting dark current from the current under illumination. Fig. 8 shows the photocurrent density versus applied potential for all samples. Increase in photocurrent density with an increase of anodic bias confirms the n-type nature of undoped & Ag-doped nanocomposite samples. However, rGO sample shows p-type nature (Fig. 8, inset). The calculated value of dark current was insignificant, suggesting a well-formed depletion layer, which is a typical characteristic of ZnO. The open-circuit photovoltage (VOC) under illumination was also calculated for all the samples (Table 2). The nanocomposite with 1 atom% Ag-doping yielded the highest photocurrent density, the effect may be largely attributed to the expected increase in photogenerated charge carriers and their swift separation/immigration across the depletion layer and in the bulk of the semiconductor, facilitated by the efficient absorption of light and significantly reduced electrical resistivity [34]. The highest value of Voc recorded for this sample also suggests the higher photocurrent density value.
Conclusions The rGO/ZnO nanocomposites with & without Ag-doping have been successfully prepared by using electrodeposition
Fig. 8 e Photocurrent density for all the samples.
Donor density, ND (cm3) 0.98 1.52 2.70 3.11 2.96 1.31
1016 1017 1018 1018 1018 1018
Photocurrent density (mA cm2) at 0.8 V vs Ag/AgCl 0.22 0.54 1.07 2.48 2.18 1.10
method over conducting glass (ITO) substrate and their structural, spectroscopic & photoelectrochemical activity were studied. Our result of Raman, XRD & FE-SEM study reveals the successful fabrication of the nanocomposite and suggests the better interaction between the nanocomposite & efficient charge separation at the electrode/electrolyte interface, thereby enhanced the photoelectrochemical response. The successful incorporation of Ag into the ZnO matrix as confirmed by EDX suggests the better ability of Ag-doped ZnO for improvement in Photoelectrochemical response of nanocomposite in visible light and reduced recombination rate of photogenerated electron and hole pairs induced by Ag doping. Further studies of using graphing based nanocomposite structures, with rational bandgap and doping design, may lead to a variety of opportunities for optimizing transition metal oxide-based PEC conversion of solar energy.
Acknowledgments This work was supported by the University of Malaya through HIR Grant (No. H-21001-F0032) & NND research project grant.
references
[1] Lewis NS. Toward cost-effective solar energy use. Science 2007;315:798e801. [2] Turner JA. A realizable renewable energy future. Science 1999;285:687e9. [3] Akhavan O. Graphene nanomesh by ZnO nanorod photocatalysts. ACS Nano 2010;4:4174e80. [4] Millet P, Andolfatto F, Durand R. Design and performance of a solid polymer electrolyte water electrolyzer. Int J Hydrogen Energy 1996;21:87e93. [5] Kreuter W, Hofmann H. Electrolysis: the important energy transformer in a world of sustainable energy. Int J Hydrogen Energy 1998;23:661e6. € ller S. [6] Pregger T, Graf D, Krewitt W, Sattler C, Roeb M, Mo Prospects of solar thermal hydrogen production processes. Int J Hydrogen Energy 2009;34:4256e67. [7] Ni M, Leung DYC, Leung MKH, Sumathy K. An overview of hydrogen production from biomass. Fuel Process Technol 2006;87:461e72. [8] Walter MG, et al. Solar water splitting cells. Chem Rev 2010;110:6446e73. [9] Zhang Z, Wang P. Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy. J Mater Chem 2012;22:2456e64.
11034
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
[10] Chen Yan-Li, et al. Zinc oxide/reduced graphene oxide composites and electrochemical capacitance enhanced by homogeneous incorporation of reduced graphene oxide sheets in zinc oxide matrix. J Phys Chem C 2011;115:2563e71. [11] Kamat PV. Graphene-based nanoarchitectures. Anchoring semiconductor and metal nanoparticles on a twodimensional carbon support. J Phys Chem Lett 2010;1:520e7. [12] Minne SC, Manalis SR, Quate CF. Parallel atomic force microscopy using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators. Appl Phys Lett 1995;67:3918. [13] Huang MH, et al. Room-temperature ultraviolet nanowire nanolasers. Science 2001;292:1897e9. [14] Srikant V, Clarkea DR. On the optical band gap of zinc oxide. J Appl Phys 1998;83:5447e51. [15] Wang F, Zhao D, Xu Z, Zheng Z, Zhang L, Shena D. Monochromatic visible light-driven photocatalysis realized on 2D ZnO shell arrays. J Mater Chem A 2013;1:9132e7. [16] Volnianska O, Boguslawski P, Kaczkowski J, Jakubas P, Jezierski A, Kaminska E. Theory of doping properties of Ag acceptors in ZnO. Phys Rev B 2009;80:245212. [17] Georgekutty R, Seery MK, Pillai SC. A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism. J Phys Chem C 2008;112:13563e70. [18] Wang R, et al. The characteristics and photocatalytic activities of silver doped ZnO nanocrystallites. Appl Surf Sci 2004;227:312e7. [19] Chao HE, Yun YU, Xiangfang HU, Larbot A. Effect of silver doping on the phase transformation and grain growth of solgel titania powder. J Eur Ceram Soc 2003;23:1457e64. [20] Stathatos E, Petrova T, Lianos P. Study of the efficiency of visible-light photocatalytic degradation of basic blue adsorbed on pure and doped mesoporous titania films. Langmuir 2001;17:5025e30. [21] Mekasuwandumrong O, Pawinrat P, Praserthdam P, Panpranot. Effects of synthesis conditions and annealing post-treatment on the photocatalytic activities of ZnO nanoparticles in the degradation of methylene blue dye. J Chem Eng 2010;164:77e84.
[22] Chakrabarti S, Chaudhuri B, Bhattacharjee S, Ray AK, Dutta BK. Photo-reduction of hexavalent chromium in aqueous solution in the presence of zinc oxide as semiconductor catalyst. Chem Eng J 2009;153:86e93. [23] Liua Xinjuan, et al. UV-assisted photocatalytic synthesis of ZnO-reduced graphene oxide composites with enhanced photocatalytic activity in reduction of Cr(VI). Chem Eng J 2012;183:238e43. [24] Li BJ, Cao HQ. ZnO@graphene composite with enhanced performance for the removal of dye from water. J Mater Chem 2011;21:3346e9. [25] Kavitha MK, John H, Gopinath P, Philip R. Synthesis of reduced graphene oxide-ZnO hybrid with enhanced optical limiting properties J. Mater Chem C 2013;1:3669e76. [26] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [27] Turner John A. Energetics of the semiconductor-electrolyte interface. J Chem Educ 1983;60:327e9. [28] Youn CJ, Jeong TS, Han MS, Kim JH. Optical properties of Znterminated ZnO bulk. J Cryst Growth 2004;261:526e32. [29] Damen TC, Porto SPS, Tell B. Raman effect in zinc oxide. Phys Rev 1966;142:570e4. [30] Roy C, et al. Correlation of Raman and X-ray diffraction measurements of annealed pulsed laser deposited ZnO thin films. Thin Solid Films 2003;436:273e6. [31] Exarhos Gregory J, Sharma Shiv K. Influence of processing variables on the structure and properties of ZnO films. Thin Solid Films 1995;270:27e32. [32] Guo J, et al. Synthesis of WO3@Graphene composite for enhanced photocatalytic oxygen evolution from water. RSC Adv 2012;2:1356e63. [33] Hu Yong-Sheng, Kleiman-Shwarsctein Alan, Stucky Galen D, McFarland Eric W. Improved photoelectrochemical performance of Ti-doped a-Fe2O3 thin films by surface modification with fluoride. Chem Mater 2008;20:3803e5. [34] Singh N, Choudhary S, Upadhyay S, Satsangi Vibha R, Dass S, Shrivastav R. Nanocrystalline Zn1xAgxOy thin films evolved through electrodeposition for photoelectrochemical splitting of water. J Solid State Electrochem; 2013. http://dx.doi.org/ 10.1007/s10008-013-2285-y.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced photoelectrochemical response of reduced-graphene oxide/Zn1¡xAgxO nanocomposite in visible-light region Sumant Upadhyay*, Samira Bagheri, Sharifah Bee Abd Hamid Nanotechnology and Catalysis Research Center (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia
article info
abstract
Article history:
Graphene based nanocomposites have the potential to work as efficiently, multifunctional
Received 23 January 2014
materials for energy conversion & storage. These composites may exhibit better photo-
Received in revised form
catalytic properties by the improvement of their electronic and structural properties than
5 May 2014
pure photocatalysts. In the present work, reduced graphene oxide (rGO) & ZnO nano-
Accepted 15 May 2014
composite with 0e5 atom% Ag doping was prepared by electrodeposition method and
Available online 13 June 2014
characterized by XRD, Raman spectroscopy, FE-SEM, EDX, UVeVis spectroscopy and final photoelectrochemical activity was assessed under 1.5 AM solar simulator in 1 M NaOH as
Keywords:
electrolyte. Significant changes in the Raman spectrum for the nanocomposite suggest the
Ag-doping
possible electronic interaction between rGO and ZnO nanocomposite and its successful
Nanocomposite
fabrication, which improves the charge separation and enhanced photoelectrochemical
Photoelectrochemical water
activity in the nanocomposite. We find a red-shift of 0.35 eV in the UVevis spectrum and
splitting
therefore an enhanced photoelectrochemical activity in the visible range on Ag doping in
Graphene oxide
rGO/ZnO nanocomposite. Nanocomposite with 1 atom% Ag doping showed the highest photocurrent density of 2.48 mA/cm2 at 0.8 V vs Ag/AgCl over other samples, which was almost five times higher than that for undoped rGO/ZnO composite. Calculated Flat-band potential and donor densities using MotteSchottky data also supported the better photoelectrochemical response for Ag doping in nanocomposite. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Supply and demand of energy determine the course of global development in every sphere of human activity. The world's energy need is sure to increase with growth in population. We now use 4.1 1020 J of energy per year, which is equivalent to 13 TW of power. This consumption may more than double to ~30 TW by the year 2050 and then triple to 46 TW by the end of
the century. Thus, finding viable alternative sources of clean energy to satisfy the anticipated demand is one of the real challenges faced by Scientists at large [1]. Solar energy offers a clean, abundant, affordable, sustainable, renewable, and an ideal source of energy and efficient conversion of solar energy into chemical fuels (e.g. Hydrogen) is one of the “Holy Grails” of the 21st century. Hydrogen is widely regarded as a promising energy carrier in the future. It has high energy density of 140 MJ kg1, far more than gasoline and coal, also its
* Corresponding author. E-mail address: [email protected] (S. Upadhyay). http://dx.doi.org/10.1016/j.ijhydene.2014.05.094 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
11028
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
combustion byproduct is water. There are several methods for utilizing solar radiation in splitting water for hydrogen generation [2,3]. However, photovoltaic electrolysis of water uses the expensive proton exchange membranes [4] and alkaline electrolytes [5]. Solar to thermochemical water splitting needs the high temperature of 700e1000 C [6]. Photobiological water splitting has to overcome the natural short-term nature of biospecies hydrogen production [7]. Thus, much attention has been focused on splitting water for hydrogen generation by photoelectrochemical (PEC) or photocatalytic reactions because they are cost-effective, simple and convenient, and have huge potential for further development [8]. Hydrogen generation via photoelectrochemical (PEC) water splitting using solar light is believed to be a clean and efficient way to overcome the global energy and environmental problems. Ideally, PEC water splitting requires electrode materials that have: (1) a proper band gap (~1.6e2.2 eV); (2) band edge alignment that straddles the H2 and O2 reaction potentials; (3) high energy conversion efficiency; (4) stability in the working environment; and (5) low production cost and an abundant natural source which is desirable for large-scale applications [9]. Emerging nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened new ways to utilize renewable energy resources because of large surface areas, abundant surface states, and diverse morphologies compared to their corresponding bulk materials. Among them, graphene, a two-dimensional (2D) network of hexagonal sp2-hybridized carbon atoms, has stimulated research interest in various energy conversion applications. Graphene exhibits many outstanding properties, such as fast room-temperature mobility of charge carriers (200,000 cm2 V1 s1), exceptional conductivity (106 S cm1), large theoretical specific surface area (2630 m2 g1), and excellent optical transmittance (~97.7%) [10]. These unique properties indicate that graphene has great potential to be an ideal construction component of PEC electrodes or photocatalytic materials for hydrogen production. By incorporating two or more catalyst particles on a single graphene or reduced graphene oxide sheet, it should be possible to carry out selective catalytic processes at separate sites. In addition, graphene's ability to store and shuttle electrons will be an important parameter in dictating catalytic activity. Thus, proper design of a catalyst material can provide greater versatility in carrying out selective catalytic or sensing processes [11]. Wurtzite-structured ZnO, a wide-band-gap (3.37 eV) semiconducting oxide with a large exciton binding energy (60 eV), has versatile properties that are important for applications in optoelectronics, solar cells, and sensors [12,13]. The tremendous interests have been shown by ZnO for the last two decades because of its multifunctional character, which can be varied by controlling its morphology and crystallinity. However, the major drawback with ZnO is its wide band gap, resulting in its limited activity under visible light due mainly to the rapid recombination of photogenerated electrons/holes, which limit the photodegradation reaction under normal conditions [14]. To fabricate a Photocatalyst that works under visible light, which covers 43% of sunlight, has been a challenging problem [15]. Doping in ZnO with selective elements
offers an effective method to adjust their electrical, optical, and magnetic properties, which is crucial for their practical applications. Studies on ZnO with Ag doping are relatively less. Previously, researchers have reported the ionization potential of Ag as 0.4 eV and shown its potential application in ZnO [16]. Georgekutty, R, et al. [17], showed the photocatalytic activity of Ag-doped ZnO photocatalyst five times higher than the unmodified one using sunlight. Some researchers have also shown the effects of Ag on photoactivity degradation activity in ZnO [18e20]. In ZnOegraphene hybrid materials, ZnO acts as a photocatalyst, to excite the electrons from the valence band to the conduction band and create electronehole pairs, which can migrate and initiate redox reactions with water and oxygen, and then degrade organic molecules or reduce metal ion absorbed on the surface of ZnO [21e23]. Graphene acts as an excellent electron-transport material in the process of photocatalysis, so that the hybridization of ZnO with graphene can hinder the recombination of charge carriers and increase the photocatalytic performance [24]. Taking into account the excellent properties of ZnO and graphene, their nanocomposite can enable versatile properties with competence far beyond those of the individual members [25]. In the present work, 0e5 atom% Ag-doped ZnO nanostructured thin films were coated over pre-deposited, reduced-graphene oxide (RGO) set using electrodeposition method on ITO glass substrate, and samples were characterized for XRD, Raman Spectroscopy, SEM-EDX, UVeVis spectroscopy, and Photoelectrochemical performance.
Experimental All reagents used were of analytical grade.
Preparation of reduced graphene oxide (rGO) nanosheet Graphene oxide was prepared using a modified Hummer's method [26]. Graphite oxide was obtained by oxidation of 1 g of graphite flakes with 120 ml of H2SO4 (60%), 13 ml of H3PO4 (60%) and 6 g of KMnO4. The mixture was stirred for three days to complete oxidation of the graphite. During oxidation, the color of the mixture changed from dark purplish-green to dark brown. To stop the oxidation, 7 ml of H2O2 (30%) solution and 150 g of ice were added and the color of the mixture changed to bright yellow, indicating a high oxidation level of graphite. The formed graphite oxide was washed three times with 1 M of aqueous HCl solution and repeatedly with deionized water until a pH of 4e5 was achieved. The washing process was carried out using a simple decantation of the supernatant with a centrifugation technique. During the washing process with deionized water, the graphite oxide underwent exfoliation, which resulted in thickening of the graphene oxide solution, forming graphene oxide gel. Electrodeposition of rGO over ITO glass substrate was performed using cyclic Voltammetry in a three-electrode potentiostat system [Autolab Potentiostat/Galvanostat (Netherlands)]. ITO glass substrate (SPIE, USA) was used as working electrode. One third part of conducting glass
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
substrate was initially covered with transparent tape to establish the electrical contact. Before performing electrodeposition, substrate plates were cleaned by soaking for ~2 min in detergent solution, followed by washing with doubledistilled deionized water. Subsequently, these were trickled for ~10 s with 0.1 M HCl, rinsed with water and acetone, and air-dried. ITO substrate, used as working electrode in conjunction with platinum as counter electrode were hanged vertically in an aqueous electrolyte consists of 7 mg L1 of GO as prepared and 0.1 M phosphate buffer (K2HPO4 and KH2PO4) at pH ~7.0. Potential applied was in a range from 0 to 1.5 V at a scan rate of 1 mV/s. The as-synthesized rGO nanosheets were washed in a gentle flow of water and dried at 50 C over a hot plate surface.
Preparation of nanocomposite (ZnO/rGO) ZnO/rGO nanocomposite films were obtained by depositing ZnO over pre-deposited rGO nanosheets using electrodeposition at 1.16 V (vs. Ag/AgCl) in the same three-electrode potentiostat system. The electrolyte used consists of an aqueous solution of Zinc Nitrate (0.1 M), Potassium Chloride (KCl, 0.1 M), ethylene diammine (EDA, 0.01 M) and a calculated amount of AgNO3 was added for Ag-doped ZnO thin films. Electrolyte solution was kept at ~80 C under constant stirring during the deposition process. The as-prepared films were kept for sintering at 600 C for crystallization.
Materials characterization X-ray diffraction patterns of samples were obtained on a D8 Advance X-ray diffractometer (Bruker, Germany) using CuKa radiation of 1.5416 Å at a scan rate of 0.02 2q s1. The surface morphology of as prepared samples was examined by using field-emission electron microscope (Carl Zeiss SUPRA 35VP). The Raman spectra of the samples were recorded using a Renishaw In-Via Raman spectrometer (Renishaw, UK). To determine the absorption band edge, and band gap energy, optical study was carried out using absorption data of samples recorded by UVeVisible spectrophotometer (Perkin Elmer, Lambda 35).
Photoelectrochemical performance Before performing photoelectrochemical study electrical contacts on all thin film samples were made through a fine copper wire loop attached to the undeposited area of the ITO glass with conducting silver paint. The area of contact was later covered and sealed with epoxy resin (Hysol, Singapore) which was non-conducting and opaque. The entire structure was heated in an oven at 70e80 C to ensure complete drying. A three-electrode potentiostat system was used to measure the photocurrent, which allowed measurement of the electron and hole pair formation as a function of the externally applied potential necessary for water splitting. Photoelectrodes were placed in the electrolyte (0.1 M, NaOH), while potentiostatic control was maintained with an Autolab Potentiostat, PGSTAT 302N, Netherlands, at a scan rate of 20 mV/s. Platinum wire mesh was used as the counter electrode and Ag/AgCl as the reference electrode. The working
11029
surface area of all electrodes was 1.0 cm2. The light source was simulated sunlight from a 150 W xenon solar simulator (Model 94306 A, Oriel) through an Air Mass filter (AM1.5 Global, 81094) with a measured intensity equivalent to standard AM1.5 sunlight (100 mW/cm2) at the sample face. In this work the MotteSchottky approach was used to determine the electronic properties of the ZnO/rGO nanocomposite with and without Ag-doping. The analysis was conducted in a 0.1 M NaOH, using a potentiostat/galvanostat Autolab PGSTAT 320N with a frequency response analyzer (FRA) module. A three-electrode cell configuration was used for the measurements with the rGO, ZnO/rGO and Ag-doped ZnO/rGO as the working electrode, Ag/AgCl as reference and a platinum wire as the counter-electrode. Capacitance measurements were performed at 1 kHz. The working electrodes were polarized in the cathodic direction in successive steps of 20 mV from þ1.0 V up to 1.0 V. From the intercept of these MotteSchottky curves flatband potential, donor density and depletion layer width were calculated for all the samples using the following relation [27]. 1 C2 ¼ ð1=2qε0 εNÞ$ Vapp VFB kT q
(1)
where ε0 is the permittivity of the vacuum, N is the donor density, Vapp is the applied potential, VFB is the flat band potential, ε is the dielectric constant of the semiconductor, q is the electronic charge, kT/q is the temperature dependent term. The intercept of linear plot at C2 ¼ 0 gives the flatband potential. The donor density (N) was calculated from the slope of the plot (1/C2 versus electrode potential) using the relation in Equation (1).
Results & discussion XRD studies The XRD patterns of rGO, rGO/ZnO and rGO/Ag-doped ZnO (0.5, 1.0, 3.0 & 5.0 atom%) samples is shown in Fig. 1. Sharp diffraction peaks indicate that the samples are well crystallized. It is shown of Fig. 1A that the peak of rGO at around 2q ¼ 10.2 corresponds to the (001) reflection. The peaks at 2q ¼ 31.7 , 34.4 , 36.2 , 47.5 , 56.5 , 62.8 , 69.1 , corresponds to the hexagonal wurzite phase of ZnO, which are consistent with the values in the standard card (JCPDS No. 36-1451). In the XRD patterns of rGO, rGO/ZnO and rGO/Ag-doped ZnO samples (BeF) the characteristic peak of graphene oxide (GO)/ graphite is not observed, which indicates the exfoliation of layered graphene sheets to the growth of ZnO nanoparticles. After the formation of the nanocomposite, the restacking of carbon sheets is prevented by the nanoparticles [25]. No peak corresponding to mixed oxide or impurity phase was identified. As calculated by the DebyeeScherrer equation, for the hexagonal phase of (101) peak crystalline diameters as a function of Ag substitution are listed in Table 1. At lower concentrations of Ag-doping (0.5, 1.0, 3.0 atom%) the crystallite size is decreased, however, in the region of 5 atom% we found an increase in crystallite size. A slight variation in the crystal lattice parameters of 0.008% with doping concentration was observed.
11030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Table 1 e Showing the crystallite size, lattice parameter and band gap values for all samples. Samples
(rGO) ZnO/rGO 0.5 atom% AgeZnO/rGO 1.0 atom% AgeZnO/rGO 3.0 atom% AgeZnO/rGO 5.0 atom% AgeZnO/rGO
Fig. 1 e X-ray diffraction patterns for all samples (A) rGO, (B) rGO/ZnO, (C) rGO/0.5 atom% AgeZnO, (D) rGO/1.0 atom% AgeZnO, (E) rGO/3.0 atom% AgeZnO (F) rGO/5.0 atom% AgeZnO.
Raman spectroscopy Raman spectroscopy is a powerful tool to investigate the crystallization, structure, and defects in the nanostructure materials. Fig. 2AeC shows a typical spectrum of rGO, rGO/ ZnO and rGO/Ag-doped ZnO samples. We observed three characteristic peaks of rGO, the G-band, which is due to the E2g vibrational mode of sp2 bonded carbon and is observed at 1586 cm1, and the D-band at 1345 cm1 is due to the A1g mode breathing vibrations of six-membered sp2 carbon rings and 2D-band at 2740 cm1. The 2D-band is the second order of the D-band, sometimes referred to as an overtone of the D-band. It is the result of a two photon lattice vibrational process, but unlike the D-band, it does not need to be activated by proximity to a defect. As a result the 2D-band is always a strong band in graphene even when no D-band is present, and it does not represent defects. The intensity of the Raman 2D-band can be infinite even if the D-band Raman intensity is negligible. ZnO crystallizes mainly in the hexagonal wurtzite structure with C6v point group symmetry. Group theory predicts that there are two E2, two E1, two A1 and two B1 symmetry phonon modes in wurtzite structure crystals and among these modes the two B1 modes are not Raman active [28]. Therefore, the frequencies of the fundamental optical modes in ZnO are E2 (low) ¼ 101 cm1, E2 (high) ¼ 437 cm1, A1 (TO) ¼ 380 cm1, A1 (LO) ¼ 574 cm1, E1 (TO) ¼ 407 cm1, E1 (LO) ¼ 583 cm1 [29]. Fig. 2B shows that the Raman spectrum of the rGO/ZnO with three Raman shifts at ~331.4, ~437, and ~587 cm1. The strong Raman shift at ~437 cm1 is assigned to E2 mode of ZnO E2 mode of ZnO crystal, which is consistent with the Raman peak of bulk ZnO crystals [30]. The two small peaks at ~331 cm1 and ~587 cm1 are designated to the second order Raman spectrum arising from zone-boundary phonons 3E2H e E2L for wurtzite hexagonal ZnO single crystals and E1 (LO) mode of ZnO associated with oxygen
Lattice parameters (Å)
Crystallite size (nm)
Band gap energy (eV)
a
c
e 3.2606 3.2599
e 5.2186 5.2180
42 39 30
3.15 3.05 2.86
3.2596
5.2179
22
2.70
3.2598
5.2168
20
2.68
3.2589
5.2164
37
2.94
deficiency in ZnO nanomaterials respectively [31]. The second-order modes are located at 1050e1200 cm1 [10]. In the present work, the D-band disappeared for rGO/ZnO and rGO/ Ag-doped ZnO in the Raman spectrum, a possible reason for this is the D-band is known as the disorder band or the defect band. It represents a ring breathing mode from sp2 carbon rings; although to be active the ring must be adjacent to a graphene edge or a defect. The band is the result of a one phonon lattice vibrational process. The band is typically very weak in graphite and is typically weak in graphene as well. If the D-band is significant, it indicates that there are a lot of defects in the material. The intensity of the D-band is directly proportional to the level of defects in the sample. Thus, the disappearance of D-band in rGO/ZnO & rGO/Ag-doped ZnO samples suggest the deposition of the high crystalline nature of ZnO over rGO with negligible or no defects in the nanocomposite. It is estimated that IG/ID of rGO is 0.754 which significantly decreases to zero because of the disappearance of D-band in case of rGO/ZnO & rGO/Ag-doped ZnO thin film samples. However, there is a band up-shifting in 2D-band for rGO/ZnO & rGO/Ag-doped ZnO samples, which are generally
Fig. 2 e Raman spectra for rGO, rGO/ZnO and rGO/1 atom% AgeZnO samples.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
evidences of chemical doping of carbon materials, suggesting charge transfer between rGO and ZnO in the nanocomposite [32]. Our Raman results indicate that ZnO firm interacts with RGO and the nanocomposite is successfully fabricated. Thus, our result suggests the possible electronic interaction between ZnO and graphene contact, which improves the charge separation and enhanced photocatalytic activity in the nanocomposite.
FE-SEM & EDX study Fig. 3 shows the Field Emission-Scanning Electron Microscopy (FE-SEM) images of rGO, rGO/ZnO & rGO/Ag-doped ZnO nanocomposite thin film samples. Fig. 3a shows a layered structured, irregular folding and a curled morphology for electrodeposited rGO nanosheets. FE-SEM images (Fig. 3bec) shows that the ZnO nanoparticles are uniformily distributed on the surface of rGO nanosheet. The particle size calculated for rGO/ZnO and rGO/Ag-doped ZnO thin film samples are in good agreement with the size calculated using XRD data. For rGO/ZnO the particle size estimated from FE-SEM is ~40 nm and for rGO/Ag-doped ZnO particle sizes decreased for low atomic concentration of Ag with a value of ~20 nm, however, we found an agglomeration (Inset of Fig. 3d) in case of 5.0 atom % Ag-doped sample (Fig. 3d), resulting in an increase in particle size. The reason for this could be that the large amount of dopant concentration in ZnO, may force to recrystallize the ZnO, leading to bigger crystallite size. Also, formation of additional sites by the dopant may lead to nucleation or random size distribution of crystallite size. To confirm the presence of Ag in the rGO/ZnO thin films, the elemental composition of the samples was investigated by the EDX spectra. Fig. 4(aeb) shows the EDX measurement for the rGO/ZnO & rGO/1 atom% Ag-doped ZnO thin films. The result reveals that Ag atoms were indeed incorporated in the
11031
ZnO films and its peak was at about 3.0 KeV. The same atomic percentage of ‘O’ obtained for both the samples indicates the uniform doping of Ag in the ZnO lattice without forming any other impurity phase. Elemental composition of 1 atom% Ag doped sample as obtained from EDX was 1.22%.
UVeVis spectrum & band-gap measurements The UVeVis spectra (Fig. 5) for all samples were measured as a function of wavelength from 200 to 800 nm using a UVeVis spectrophotometer. The rGO, rGO/ZnO shows an absorption at 380 nm and the rGO/Ag-doped ZnO samples show absorption in the visible light region. It is noticeable that there is an enhancement in absorption with the Ag-doping in rGO/ZnO composite samples towards the visible light region. The optical band gap values for all samples were calculated from Tauc-Plots (Table 1). The best fit was found for the direct band gap transition with the values of 3.15 eV, 3.05 eV and 2.70 eV for rGO, rGO/ZnO & rGO/1 atom% AgeZnO samples respectively (Fig. 6). The estimates for band gap values have been given in Table 1. The decrease in band gap values for 0.5, 1.0 & 3.0 atom% Ag-doping shows that Ag-doping significantly changes the band-gap energy, which may be due to the intermediate level formed by Ag within the energy level of ZnO. However, we find an increase in band gap of 5.0 atom% Agdoped ZnO sample.
Photoelectrochemical performance The MotteSchottky (MS) plots are presented in Fig. 7 for rGO, rGO/ZnO & rGO/1 atom% AgeZnO samples and to estimate flatband potential (VFB), the linear part of the data was extrapolated to 1/C2 ¼ 0. The VFB for rGO, rGO/ZnO & rGO/ 1 atom% AgeZnO estimated from MS plots were þ0.310, 0.460 & 0.780 respectively. The slope of the curves in the MS
Fig. 3 e FE-SEM images (a) rGO (b) rGO/ZnO (c) rGO/1 atom% AgeZnO (d) rGO/5 atom% AgeZnO.
11032
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Fig. 6 e Tauc-Plots for rGO, rGO/ZnO and rGO/1 atom% AgeZnO.
Fig. 4 e EDX spectra for (a) rGO/ZnO and (b) rGO/1 atom% AgeZnO.
Fig. 5 e UVeVis absorption curves for all samples.
plots are negative for rGO (Fig. 7, Inset) and positive for rGO/ ZnO & rGO/1 atom% AgeZnO saples indicating p-type and ntype nature of the samples. Shifting of negative values of the VFB in nanocomposite samples indicates the better ability of the semiconductor films to facilitate the charge separation in PEC application. Donor density (ND) was also found to increase with increase in Ag-doping concentration; however, we found a decrease in ND at 5 atom% Ag-doped sample. The higher value of ND & VFB for the 1 atom% Ag-doped sample over other samples may be responsible for the significantly higher photocurrent density (Table 2). Thus, 1 atom% Ag-doped sample turns out to be an optimal dopant level for the rGO/ ZnO nanocomposite for enhanced Photoelectrochemical response in the visible light region. On the other hand, higher concentration of dopant may have provided more defectscattering recombination centers inhibiting the increase charge separation efficiency [33].
Fig. 7 e Motteschottky plots for rGO, rGO/ZnO and rGO/ 1 atom% AgeZnO.
11033
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
Table 2 e Measured parameters calculated using Mott-Schottky and IeV data for all samples. Samples
rGO rGO/ZnO rGO/0.5 atom% rGO/1.0 atom% rGO/3.0 atom% rGO/5.0 atom%
AgeZnO AgeZnO AgeZnO AgeZnO
Open circuit photovoltage, VOC
Flatband potential, VFB
0.234 0.682 0.710 0.850 0.734 0.658
þ0.310 0.460 0.530 0.780 0.692 0.390
The current (I)evoltage (V) measurements were done for all samples using AM 1.5 solar simulator with 100 mW/cm2 power. The externally applied potential was varied from 1.0 V vs Ag/AgCl (cathodic bias) to þ1.0 V vs Ag/AgCl (anodic bias). The photocurrent density of all the samples was calculated by subtracting dark current from the current under illumination. Fig. 8 shows the photocurrent density versus applied potential for all samples. Increase in photocurrent density with an increase of anodic bias confirms the n-type nature of undoped & Ag-doped nanocomposite samples. However, rGO sample shows p-type nature (Fig. 8, inset). The calculated value of dark current was insignificant, suggesting a well-formed depletion layer, which is a typical characteristic of ZnO. The open-circuit photovoltage (VOC) under illumination was also calculated for all the samples (Table 2). The nanocomposite with 1 atom% Ag-doping yielded the highest photocurrent density, the effect may be largely attributed to the expected increase in photogenerated charge carriers and their swift separation/immigration across the depletion layer and in the bulk of the semiconductor, facilitated by the efficient absorption of light and significantly reduced electrical resistivity [34]. The highest value of Voc recorded for this sample also suggests the higher photocurrent density value.
Conclusions The rGO/ZnO nanocomposites with & without Ag-doping have been successfully prepared by using electrodeposition
Fig. 8 e Photocurrent density for all the samples.
Donor density, ND (cm3) 0.98 1.52 2.70 3.11 2.96 1.31
1016 1017 1018 1018 1018 1018
Photocurrent density (mA cm2) at 0.8 V vs Ag/AgCl 0.22 0.54 1.07 2.48 2.18 1.10
method over conducting glass (ITO) substrate and their structural, spectroscopic & photoelectrochemical activity were studied. Our result of Raman, XRD & FE-SEM study reveals the successful fabrication of the nanocomposite and suggests the better interaction between the nanocomposite & efficient charge separation at the electrode/electrolyte interface, thereby enhanced the photoelectrochemical response. The successful incorporation of Ag into the ZnO matrix as confirmed by EDX suggests the better ability of Ag-doped ZnO for improvement in Photoelectrochemical response of nanocomposite in visible light and reduced recombination rate of photogenerated electron and hole pairs induced by Ag doping. Further studies of using graphing based nanocomposite structures, with rational bandgap and doping design, may lead to a variety of opportunities for optimizing transition metal oxide-based PEC conversion of solar energy.
Acknowledgments This work was supported by the University of Malaya through HIR Grant (No. H-21001-F0032) & NND research project grant.
references
[1] Lewis NS. Toward cost-effective solar energy use. Science 2007;315:798e801. [2] Turner JA. A realizable renewable energy future. Science 1999;285:687e9. [3] Akhavan O. Graphene nanomesh by ZnO nanorod photocatalysts. ACS Nano 2010;4:4174e80. [4] Millet P, Andolfatto F, Durand R. Design and performance of a solid polymer electrolyte water electrolyzer. Int J Hydrogen Energy 1996;21:87e93. [5] Kreuter W, Hofmann H. Electrolysis: the important energy transformer in a world of sustainable energy. Int J Hydrogen Energy 1998;23:661e6. € ller S. [6] Pregger T, Graf D, Krewitt W, Sattler C, Roeb M, Mo Prospects of solar thermal hydrogen production processes. Int J Hydrogen Energy 2009;34:4256e67. [7] Ni M, Leung DYC, Leung MKH, Sumathy K. An overview of hydrogen production from biomass. Fuel Process Technol 2006;87:461e72. [8] Walter MG, et al. Solar water splitting cells. Chem Rev 2010;110:6446e73. [9] Zhang Z, Wang P. Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy. J Mater Chem 2012;22:2456e64.
11034
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 0 2 7 e1 1 0 3 4
[10] Chen Yan-Li, et al. Zinc oxide/reduced graphene oxide composites and electrochemical capacitance enhanced by homogeneous incorporation of reduced graphene oxide sheets in zinc oxide matrix. J Phys Chem C 2011;115:2563e71. [11] Kamat PV. Graphene-based nanoarchitectures. Anchoring semiconductor and metal nanoparticles on a twodimensional carbon support. J Phys Chem Lett 2010;1:520e7. [12] Minne SC, Manalis SR, Quate CF. Parallel atomic force microscopy using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators. Appl Phys Lett 1995;67:3918. [13] Huang MH, et al. Room-temperature ultraviolet nanowire nanolasers. Science 2001;292:1897e9. [14] Srikant V, Clarkea DR. On the optical band gap of zinc oxide. J Appl Phys 1998;83:5447e51. [15] Wang F, Zhao D, Xu Z, Zheng Z, Zhang L, Shena D. Monochromatic visible light-driven photocatalysis realized on 2D ZnO shell arrays. J Mater Chem A 2013;1:9132e7. [16] Volnianska O, Boguslawski P, Kaczkowski J, Jakubas P, Jezierski A, Kaminska E. Theory of doping properties of Ag acceptors in ZnO. Phys Rev B 2009;80:245212. [17] Georgekutty R, Seery MK, Pillai SC. A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism. J Phys Chem C 2008;112:13563e70. [18] Wang R, et al. The characteristics and photocatalytic activities of silver doped ZnO nanocrystallites. Appl Surf Sci 2004;227:312e7. [19] Chao HE, Yun YU, Xiangfang HU, Larbot A. Effect of silver doping on the phase transformation and grain growth of solgel titania powder. J Eur Ceram Soc 2003;23:1457e64. [20] Stathatos E, Petrova T, Lianos P. Study of the efficiency of visible-light photocatalytic degradation of basic blue adsorbed on pure and doped mesoporous titania films. Langmuir 2001;17:5025e30. [21] Mekasuwandumrong O, Pawinrat P, Praserthdam P, Panpranot. Effects of synthesis conditions and annealing post-treatment on the photocatalytic activities of ZnO nanoparticles in the degradation of methylene blue dye. J Chem Eng 2010;164:77e84.
[22] Chakrabarti S, Chaudhuri B, Bhattacharjee S, Ray AK, Dutta BK. Photo-reduction of hexavalent chromium in aqueous solution in the presence of zinc oxide as semiconductor catalyst. Chem Eng J 2009;153:86e93. [23] Liua Xinjuan, et al. UV-assisted photocatalytic synthesis of ZnO-reduced graphene oxide composites with enhanced photocatalytic activity in reduction of Cr(VI). Chem Eng J 2012;183:238e43. [24] Li BJ, Cao HQ. ZnO@graphene composite with enhanced performance for the removal of dye from water. J Mater Chem 2011;21:3346e9. [25] Kavitha MK, John H, Gopinath P, Philip R. Synthesis of reduced graphene oxide-ZnO hybrid with enhanced optical limiting properties J. Mater Chem C 2013;1:3669e76. [26] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [27] Turner John A. Energetics of the semiconductor-electrolyte interface. J Chem Educ 1983;60:327e9. [28] Youn CJ, Jeong TS, Han MS, Kim JH. Optical properties of Znterminated ZnO bulk. J Cryst Growth 2004;261:526e32. [29] Damen TC, Porto SPS, Tell B. Raman effect in zinc oxide. Phys Rev 1966;142:570e4. [30] Roy C, et al. Correlation of Raman and X-ray diffraction measurements of annealed pulsed laser deposited ZnO thin films. Thin Solid Films 2003;436:273e6. [31] Exarhos Gregory J, Sharma Shiv K. Influence of processing variables on the structure and properties of ZnO films. Thin Solid Films 1995;270:27e32. [32] Guo J, et al. Synthesis of WO3@Graphene composite for enhanced photocatalytic oxygen evolution from water. RSC Adv 2012;2:1356e63. [33] Hu Yong-Sheng, Kleiman-Shwarsctein Alan, Stucky Galen D, McFarland Eric W. Improved photoelectrochemical performance of Ti-doped a-Fe2O3 thin films by surface modification with fluoride. Chem Mater 2008;20:3803e5. [34] Singh N, Choudhary S, Upadhyay S, Satsangi Vibha R, Dass S, Shrivastav R. Nanocrystalline Zn1xAgxOy thin films evolved through electrodeposition for photoelectrochemical splitting of water. J Solid State Electrochem; 2013. http://dx.doi.org/ 10.1007/s10008-013-2285-y.