Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger

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 x x x ( 2 0 1 4 ) 1 e1 2

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Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger Muhammad Ibadurrohman, Klaus Hellgardt* Faculty of Engineering, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom

article info

abstract

Article history:

Graphene-modified TiO2 (G-TiO2) photoanode films were successfully prepared by a simple,

Received 16 July 2014

versatile, and low-cost spray pyrolysis deposition method. The effects of graphene incor-

Received in revised form

poration on the relevant properties of TiO2 films were investigated by means of XRD, SEM,

24 August 2014

UVevis absorbance spectroscopy, and photoelectrochemistry-related measurements. Bias-

Accepted 28 August 2014

dependant efficiency calculated from linear-sweep voltammograms shows that the pres-

Available online xxx

ence of graphene within the film networks, despite its low content, could promote a substantial improvement in maximum photoconversion efficiency from 0.39% (at
0.27 V

Keywords:

vs HgOjHg) to 0.65% (at
0.35 V vs HgOjHg). This improvement is attributable to the

TiO2

enhancement of the electron-transferring ability upon the insertion of graphene, as

Graphene

confirmed by transient photocurrent analysis and Electrochemical Impedance Spectros-

Photoelectrochemical cell

copy (EIS). The effectiveness of the photoelectrochemical cell employing G-TiO2 as an

Hole scavenger

excellent photoanode was further examined by running it in the presence of glycerol as a

Glycerol

hole scavenger. Glycerol plays an important role as an effective sink for the photogenerated holes so that the surface charge recombination can be significantly suppressed and, subsequently, the photocurrent is enhanced. The additional photocurrent due to glycerol introduction into the cell depends upon the initial concentration of glycerol according to a model resembling a LangmuireHinshelwood isotherm. Based upon the results of the present study, further improvements in terms of graphene content, surface morphology modification or the use of other organic wastes as hole scavengers may be important for future investigation. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction It is more than four decades ago that Fujishima and Honda demonstrated the first photo-driven water cleavage producing

H2 and O2 over rutile TiO2 single crystals [1]. TiO2-based materials are thermodynamically able to drive water cleavage, are stable under a wide range of experimental conditions, are not affected by photo-corrosion, are non-toxicity, and readily available at reasonable price [2]. In spite of these advantages,

* Corresponding author. Tel.: þ44 (0) 20 7594 5577. E-mail address: [email protected] (K. Hellgardt). http://dx.doi.org/10.1016/j.ijhydene.2014.08.142 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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TiO2 suffers from rapid carrier recombination, which occurs within the order of 1 ns, significantly faster than the time required for these carriers to react with suitable redox partners (10 nse10 ms) [3,4]. Moreover, the large band gap of TiO2 (typically 3.2 eV for anatase and 3.0 eV for rutile [5]) allows it to respond solely to UV irradiation (l < 400 nm), which accounts for only about 4% of the solar spectrum. Enormous efforts have been expended to tailor the band structure of TiO2 such that it can be effective under visible light illumination, including band gap narrowing by anion doping [6], generation of impurity levels by cation doping [7], formation of lattice defects by hydrogen treatment [8], or combinations thereof [9e11]. Despite many successful reports, several problems associated with the altered electronic band structure have also arisen, and the most common one revolves around the increase of recombination rate of the carriers (post-excitation process) [12,13]. Therefore, it is of fundamental importance to find a technique capable of impeding the charge carrier recombination. Inspired by the pioneering work of Kamat et al. [14,15], many reports regarding retardation of charge carrier recombination by graphene-based modification have been emerging on regular basis [16]. Comprehensive reviews on the eminence of graphene-based photocatalyst composites are also available [17e19]. 2D-featured graphene offers exceptional properties particularly for photonic and electronic applications owing to its outstanding thermal conductivity, remarkable mobility of charge carriers, excellent mechanical strength, high transparency, large specific surface area and adjustable structure [16]. The combinations of graphene-based materials and TiO2 have shown remarkable performances in the field of photocatalytic degradation of organic pollutants and dyes [20e31] by performing several modes of action, e.g. improving the electron-conducting ability [20,26,27,32e34], red-shifting the spectral photo-response [21,26,27,29], or increasing the affinity towards organics [25,29,33,34]. However, reports on graphene-modified TiO2 for the production of hydrogen and oxygen from water, more particularly in thin film electrode form, are relatively limited. Song et al. reported a successful preparation of graphene oxide (GO) modified TiO2 nanotubes by simple impregnation, resulting in a 15-fold increase in maximum photoconversion efficiency compared to that of unmodified nanotubes [21]. Bell et al. incorporated reduced GO (RGO) into commercial TiO2 nanoparticles via UV-assisted photo-deposition, resulting in the better electron-conducting ability of the composite film [32]. Lee et al. synthesised TiO2 nanorods-decorated graphene sheets with observed redshifts in the absorbance spectra and a 6-fold photocurrent improvement compared to pristine nanorods [24]. In-situ electrochemical reduction of graphene oxide and subsequent photo-deposition of graphene into TiO2 was conducted by Liu et al., resulting in remarkable photoelectrolytic activity [25]. Although a sceptical opinion has also been raised by Xu et al. who implied that the effects of graphene on the photocatalytic-related behaviour of titania is not singularly superior to that of other carbon-based modifiers (e.g. carbon nanotubes, fullerene, or activated carbon) [30,31], all of the aforementioned reports suggest a promising application of graphene-TiO2 composites in photoelectrochemical systems,

owing to the unique virtues associated with the incorporation of graphene. In order to further promote the reaction rate and photoconversion efficiency by disrupting the carrier recombination in photocatalytic or photoelectrochemical systems, sacrificial hole scavengers have been introduced. Many organic compounds, which are thermodynamically more favourable towards hole-driven photo-oxidation than water, have been utilised as electron donors, including methanol [35e39], ethanol [40], sugars [41], organic acids [42], and glycerol [43]. Cheng et al. has successfully incorporated reduced graphene oxide (RGO) onto P25 via a solvothermal method with an excellent photocatalytic performance for hydrogen production in the presence of methanol as a sacrificial agent [35]. Good results were also obtained by Xiang et al. who prepared graphene-modified titania nanosheets by a hydrothermal method and observed a 40-fold enhancement in hydrogen production from a water-methanol mixture [36]. Other semiconductors have also been modified by graphene to drive photo-production of hydrogen from aqueous methanol solution, resulting in notable enhancements in reaction rates [37e39]. These experiments, however, were conducted over powdered photocatalysts in colloidal systems which often have to deal with the photocatalyst recovery and separation issues [44]. Photocatalysts in the form of films are arguably more desirable since they enable convenient photocatalyst recovery. Furthermore, the use of methanol as a sacrificial agent in photo-assisted energy production is questionable since methanol has its own value as fuel. In this paper, we report a synthesis of graphene-modified TiO2 (G-TiO2) photoanode films by a spray pyrolysis deposition method which is known for its simplicity, versatility, low cost, and short processing time. In terms of application, we combine the photo-driven energy conversion and photoassisted degradation of pollutants by introducing glycerol as an anodic sacrificial agent [45e48] for the photoelectrochemical hydrogen production. Glycerol is of special interest mainly because it is massively produced in the biodiesel industry as an undesired product [49], and has been considered as waste.

Experimental section Materials The chemicals used in this study were of analytical grades and used as received without further purification. For the spray pyrolysis deposition, titanium isopropoxide (TTIP, 97%, SigmaeAldrich) was used as a TiO2 precursor, acetylacetone (AcAc, ReagentPlus®, 99.5%, SigmaeAldrich) as a stabiliser, and ethanol absolute (AnalaR NORMAPUR®, VWR) as a solvent. Commercially available graphene solution (Graphene Laboratories Inc., USA) was used as the graphene source. The graphene solution contains 1 mg/L ultrapure graphene flakes (no oxidation) with ethanol as the solvent. For the photoelectrochemical measurements, sodium hydroxide pellets (99.0e100.5%, AnalaR NORMAPUR®, VWR) and glycerol solution (86e89%, Sigma) were used to make electrolyte solutions and as a sacrificial agent, respectively.

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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Preparation of photoanode films The precursor solutions of G-TiO2 were prepared by diluting TTIP and AcAc (3:2 volumetric ratio) into 50 ml of the commercial graphene solution in a 100-ml volumetric flask, and top them up with ethanol absolute to make 0.2 M of TTIP. The prepared solutions were then subjected to subsequent ultrasonication and vigorous stirring for 0.5 h and 4 h, respectively. The above procedure results in a solution containing 0.05 mg graphene flakes per 0.02 mol of TTIP, which corresponds to 0.021% mole of graphene in TiO2. The same procedure was applied for the fabrication of pure (unmodified) TiO2 films by excluding the graphene solution during the preparation process. Fig. 1 depicts a set-up of the spray pyrolysis system that was used for the fabrication of photoanode films. The automated computer interface of this system is capable of controlling the spraying pattern and the film deposition area, while other parameters, such as substrate temperature, solution flow rate, or the pressure of the carrier gas can be manually, yet conveniently, adjusted. Fluorine-doped tin oxide (FTO, Hartford Glass Inc., USA) with sheet resistance of 8 U/sq was used as the underlying substrate. Operating parameters were kept constant in order to maintain reproducibility. Typically, the precursor solution was delivered by a syringe pump to a nozzle (TQþ quartz nebuliser, Meinhard®, USA) at a flow rate of ~1 ml/min with 50-psig pressurised air as the carrier gas. The sprayed solution hits the pre-heated FTO substrate (at ca. 350  C), on which the growth of TiO2 particles takes place. The deposition was done for 40 layers resulting in a film thickness of about 1.1 mm. The as-deposited films were then annealed at 500  C for 2 h under N2-flowing atmosphere.

Characterisation of the films X-ray diffractograms were recorded using a desktop D8 Bruker XRD machine with Cu Ka irradiation as a photon source ˚ ) and Ni filter, operating at 40 kV and 40 mA. All (l ¼ 1.5406 A samples were analysed within the range of 2q ¼ 5 e70 at a scanning rate of 2 /min. The spectral absorbance of the prepared films was measured by Agilent Technologies 8453 UVeVis Spectrophotometer G1103A controlled by ChemStation software. The morphology and the thickness of the

Fig. 1 e Schematic diagram of the spray pyrolysis deposition to produce photoanode films.

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prepared films were analysed using a Zeiss LEO Gemini 1525 High-resolution Field Emission Gun Scanning Electron Microscope (FEGSEM), operating at an EHT of 5 kV.

Photoelectrochemical (PEC) measurements PEC measurements were conducted in a conventional threeelectrode cell with platinised Ti employed as the counter electrode and HgOjHg (þ0.1077 V vs NHE in 1 M NaOH at room temperature and pressure [50]) as the reference electrode. A concentration of 1 M of sodium hydroxide (NaOH) solution (pH ¼ 12.7) was used as the electrolyte with or without glycerol (0e275 mM). The cell was connected to a potentiostat (AUTOLAB PGSTAT302N) and controlled by NOVA software. A 10-mV s
1 scan rate was used in all linear-sweep voltammetry measurements. A 150-W Xe lamp (LOT-Oriel GmbH&Co.KG) was used as the light source to provide broad-spectrum illumination, equipped with a TLS1509-X150 monochromator/ spectrograph (Omni-l1509, Zolix). The light intensity (ca. 120 W m
2) was measured by a USB-connected StellarNet EPP2000C UVeVis spectrometer with concave holographic grating. Unless specified otherwise, all experiments were conducted at room temperature and pressure.

Results and discussion TiO2 and graphene-modified TiO2 films As depicted in Fig. 2(a), the produced TiO2 thin films show regular and dense features of nanoparticles with well-defined grain boundaries. The surface morphology of the graphenemodified TiO2 film is shown in Fig. 2(b) in which it is clearly seen that the graphene flakes are attached onto the film surface while the morphology remains similar to that of the unmodified TiO2 film. It is also worth pointing out that the smaller graphene flakes favour the grain boundaries which are known to be the sites in which the carrier recombination is most likely to take place [51]. Therefore, the presence of highly conductive graphene in these boundaries is very beneficial in order to enhance the charge transfer process. Fig. 2(c) shows the cross sectional view of the TiO2 film, displaying a film thickness of ca. 1.1 mm. The distribution of TiO2 particle size is provided in Fig. 2(d), indicating that most of those nanoparticles are 400e600 nm in size. Neither film thickness nor particle size distribution is significantly affected by the incorporation of graphene into the films. X-ray diffractograms of the as-deposited and annealed films are illustrated in Fig. 3(a). Characteristic peaks for anatase are observed at 2q of 25.3 and 47.8 in both TiO2 and G-TiO2 samples which conform to the indices of (1 0 1) and (2 0 0) planes, respectively (JCPDS card no. 21-1272). XRD patterns of the as-deposited films show less developed characteristic peaks, indicating that anatase crystallite growth was established in two steps: firstly during the spray pyrolysis deposition process at a temperature of 350  C and secondly during the annealing process at a temperature of 500  C. Three different phases have been recognised to be attributable to the TiO2 crystal structure, namely anatase, rutile, and brookite [52]. Nevertheless, due to difficulties in preparation and

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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Fig. 2 e SEM images of the surface morphology of TiO2 (a) and G-TiO2 (b); cross-sectional micrographs of a TiO2 film (c); and particle size distribution of a typical film (d).

stability issues, brookite is the least studied TiO2 compared to its counterparts [53]. Thus, photocatalytic and/or photoelectrolytic applications almost exclusively utilise anatase or rutile. In our films, however, no characteristic peak of rutile (e.g. (1 1 0) plane at 27.4 , according to JCPDS card no. 21-1276) is observed in both TiO2 and G-TiO2 samples because rutile has been reported to be generated at 610  C or above [54], which is much higher than the annealing temperature applied in this study. Moreover, there is no significant difference in the shape and the relative intensity of the TiO2 characteristic peaks upon graphene addition, nor is there a shift in 2q since the amount of graphene used in this study is quite small. The band gap (Eg) is one of the most important properties of a semiconductor since it determines the spectral energy threshold by which electronehole pairs can be photogenerated. One of the most common techniques to measure the band gap of an indirect semiconductor, such as TiO2, is by constructing Tauc plots [55]. As depicted in Fig. 3(b), (ahv)1/2 is plotted against hv according to the well known relationship: 1=2

ðahvÞ

¼ k hv
Eg




(1)

where h is the Planck constant, v is the frequency of incident radiation, and a is the absorption coefficient calculated from the measured absorbance which is given by the following relationship: a¼

2:303A d

(2)

where A is the measured absorbance and d is the film thickness in cm. Extrapolating the plot of equation (1) to intercept the x-axis can be directly translated to the band gap value [55e57]. In spite of the straightforwardness of this method, limitation could come from the arbitrary way of constructing the tangent line which may result in subjective interpretations [58].The band gap measurements of TiO2 and GTiO2 samples give values of 3.2 and 3.15 eV, respectively, as illustrated in Fig. 3(b). It should also be noted that the absolute absorbance of G-TiO2 is slightly higher than that of TiO2 within the range of 350e550 nm (inset of Fig. 3(b)). Red shifts of TiO2 band gap due to graphene incorporation have also been observed by other researchers [14,20,23,28,59,60]. This observation can be rationalised by considering two facts [26]: the intrinsic ability of graphene to absorb visible light, and the presence of TieOeC bonds established between TiO2 and graphene which is believed to be responsible for the photoexcitation upon visible light irradiation.

Photoelectrochemical measurements Linear-sweep voltammograms of TiO2 and G-TiO2 photoanodes under dark and illuminated systems are illustrated in Fig. 4(a). In a typical measurement, the potential of the working electrode with respect to the reference electrode is swept linearly with time in order to record the corresponding current. Both TiO2 and G-TiO2 give similar shapes of IeV plots in which the dark reaction shows negligible anodic currents.

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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favourable [16,23,66]. Furthermore, these captured electrons are then spontaneously delivered to the FTO underlying substrate whose work function is ca. 5.0 eV [67]. This photoresponse enhancement, in spite of the small amount of graphene used in this study, indicates that the presence of a recombination inhibitor in a TiO2-driven photoelectrochemical system is very sensitive to the photocurrent response. In a photoelectrolytic system, the photoconversion efficiency can be expressed as follows [68,69]: hð%Þ ¼

total power output
electrical power input  100 incident light power (3)

hð%Þ ¼

Fig. 3 e XRD patterns (a) and Tauc plots (inset: absorbance spectroscopy) (b) of the as-prepared films.

In the illuminated system, the photocurrents start to be detectable at potentials of
0.66 V and
0.68 V vs HgOjHg for TiO2 and G-TiO2 photoanodes, respectively. These potentials, at which the onset of the photo-induced current occurs, can be translated to flat-band potential (EFB) [61]. Starting from EFB towards more positive potentials, the photocurrent increases and more photo-generated charges can be transferred due to larger potential difference between the photoanode and the counter electrode. At a sufficiently positive potential, the photocurrent reaches saturation, which indicates that the band bending is greatly developed so that all the photocharges are consumed by their respective redox partners and, provided that the redox reactions are distinctively fast, the photoelectrolytic process is controlled by the photoexcitation of charge carriers [61]. It is inferred from Fig. 4(a) that the incorporation of graphene leads to a significant improvement in photocurrent responses due to its extensive two-dimensional pep conjugation network [14,23,26,31,33,35] with a high electron mobility that allows photoinduced electrons from TiO2 CB to be captured and promptly transferred to the external circuit. In addition, since the work function of graphene is ca. 4.42e4.45 eV [62e64] and the CB position of anatase is ca.
4.21 eV [65] with respect to Vacuum, the movement of electrons from TiO2 CB to the graphene is energetically


Ip Erev
Eapp  100 PA

(4)

where Ip is the measured photocurrent in ampere (A), Erev is the reversible potential of the overall water splitting (1.229 V), Eapp is the applied potential in volts (V), P is the incident light power density in watt per metre square (W m
2), and A is the active area of the photoanode in m2. There has been a dispute among researchers in defining Eapp. In most reported studies, Eapp is presented as the difference between the potential of the photoanode and its open circuit potential on the same electrolyte and illuminated condition [56,70,71]. However, Murphy et al. denoted that such definition may lead to an overestimated value of photoconversion efficiency [72]. Therefore, in this report, Eapp is expressed as EWEeECE where EWE is the potential of the working electrode (photoanode) and ECE is the potential of the counter electrode [73]. Assuming that there is no overpotential of hydrogen evolution in the counter electrode, ECE is then equal to
0.86 V vs HgOjHg (thermodynamic potential of hydrogen evolution at the experimental pH of 12.7). A typical parabolic correlation of photoconversion efficiency as a function of the applied potential is depicted in Fig. 4(b). Starting from the open circuit potential under illuminated condition, photoconversion efficiency continues to rise and reach an optimum value. As the potential scan goes towards more positive potentials, the efficiency decreases and reaches zero at 0.37 V vs HgOjHg, which is the thermodynamic potential of water oxidation to generate molecular oxygen at the experimental condition. This is the maximum potential that can be imposed to the photoanodes in order for the cell to still be able to claim a net gain of free energy from the photons [73]. As clearly shown in Fig. 4(b), the graphene-modified film gives better efficiency (0.65%) compared to its counterpart (0.39%). It is also noteworthy that the maximum efficiency in the use of G-TiO2 is achieved at a more negative value (
0.35 V vs HgOjHg) than that of TiO2 (
0.27 V vs HgOjHg), suggesting that the insertion of graphene into TiO2 films is an effective technique not only to boost the photoconversion efficiency but also to reduce the external energy required by a photoelectrochemical cell to reach its maximum photoconversion. In addition, Fig. 4(c) depicts the time course of photocurrent density generated from the cell employing G-TiO2. Within the first few seconds of illumination (<25 s), we observe a slight photocurrent decay, the nature of which will be discussed later in this paper. Otherwise, as clearly visualised, the

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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Fig. 4 e Linear sweep voltammograms (a) and photoconversion efficiency (b) of the prepared photoanode films; and the prolonged photoelectrochemical performance of G-TiO2 film at 0 V vs HgOjHg (c).

photocurrent density is well maintained throughout the experiment period, and there is no sign of deactivation even after more than 6000 s of illumination. This finding confirms the robustness and the excellent stability of the G-TiO2 film. The photoelectrochemical enhancement of TiO2 photoanodes associated with graphene insertion in this study is relatively small (ca. 70%) compared to previous reported works. Song et al. reported a 15-fold photocurrent enhancement upon attaching graphene oxide to TiO2 nanotube arrays [21]. However, the absolute values of the maximum photoconversion efficiency were rather low (0.0033% for the unmodified nanotubes and 0.0487% for the graphene-oxidemodified nanotubes). Since our unmodified film has given a decent value of photoconversion efficiency, the photocurrent enhancement upon graphene insertion is expected to be moderate. Besides, only photoelectrochemical measurements under visible light illumination were reported in their study; thus no direct comparison can be made. Bell et al. reported photo-assisted deposition of reduced graphene oxide on TiO2, resulting in a 12-fold photocurrent enhancement [32]. Similarly, remarkable photoelectrochemical performances of TiO2-graphene composites have also been achieved by other researchers [23,27,74]. Nevertheless, these works were carried out under UV illumination hence they are not directly comparable with our system. For practical application, photoelectrochemical studies under broad-spectrum illumination are favourable. Min et al. synthesised graphene-TiO2 composites via a sonochemical-assisted method which enhance the photoanodic response up to 6 times compared to pristine TiO2 films under 100 mW cm
2 AM 1.5G illumination [29]. In spite of this significant improvement, the generated photocurrent is relatively low (0.6 mA cm
2). Moreover, the weight ratio of graphene oxide used in their study is particularly high (8.5%wt). Since the production and purchase costs of

graphene are generally high [75], the use of small amounts of graphene is desired. A 40-fold photoactivity enhancement upon the introduction of graphene into TiO2 photocatalysts was also achieved by Xiang et al. [36]. This work, however, was done in a colloidal system over powdered photocatalysts. The advantage of this system is that it provides a large number of active sites for the hydrogen photoproduction to take place, as opposed to the electrode system in which the active sites are limited to those available on the film surface. Therefore, a significant improvement due to graphene insertion is expected in such a system. Yet, the colloidal system is arguably not very practical for long term applications since it is often complicated to separate the photocatalyst powders from the solution after reactions, as we mentioned in the Introduction section. Fig. 5(a) illustrates the photocurrent generated by TiO2 and G-TiO2 photoanodes upon the illumination of monochromatic light (l ¼ 320 nm) at 0 V vs HgOjHg as a function of irradiation time. Sudden irradiation leads to rapid initial generation of electronehole pairs and a great photocurrent excitement at the early stage of the illumination [32,76]. The deterioration of the photocurrent over time is observed and attributed to the charge recombination within the bulk of the film as the holes diffuse towards the surface, to combine with their redox partners, and the electrons move in the opposite direction, i.e. the FTOjphotoanode back contact [32,76]. Once the rates of photocharge generation and recombination reach an equilibrium, the photocurrent levels off at a steady state current [32,76]. As represented in the inset of Fig. 5(a), TiO2 undergoes more severe photocurrent decay as compared to G-TiO2, indicating that the charge transfer within the interior of the graphene-modified film takes place more effectively. Several authors have quantitatively expressed the carrier recombination rate of a photoelectrochemical cell by a term

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Fig. 5 e Transient photo-current responses of TiO2 and G-TiO2: chronoamperometry plots (inset: normalised photocurrents) (a); definitions of relevant parameters (b); and calculation of transient time constants (c). Photocurrents are recorded under incident wavelength of 320 nm at 0 V vs HgOjHg.

called the transient time constant (t) which can be calculated from a chronoamperometry plot as follows [32,57,76e78]: D¼

IðtÞ
IðstÞ IðinÞ
IðstÞ

    t 1 D ¼ exp
/ln D ¼
t t t

(5)

(6)

where I(t) is the photocurrent generated at time t, I(st) is the steady state photocurrent and I(in) is the initial photocurrent, as described in Fig. 5(b). Mathematical expressions in Equations (5) and (6) suggest that a large extent of recombination dominating the charge transfer would give a relatively short transient time constant, and vice versa. Fig. 5(c) displays linear correlations between ln D and time as expressed by Equation (6), giving transient time constants of 2.2 s and 3.4 s for the use of TiO2 and G-TiO2, respectively. These results signify that the addition of graphene leads to better photocharge separation and transfer compared to that of the unmodified film. The rise of transient time constant upon graphene addition is not of particular significance possibly due to a combination of the small amount of graphene and the relatively high external bias applied. The intrinsic competition of the carrier separationerecombination processes plays a more important role at relatively low external bias while, at relatively high external bias, the lifetime of photocharges is mainly governed by the imposed electric field [32,76].The improvement in chargetransferring capability of the films upon graphene insertion is further supported by Electrical Impedance Spectroscopy (EIS) results. EIS Nyquist plots of the prepared films are illustrated in Fig. 6(a) which shows that the G-TiO2 film exhibits a smaller radius of a typical semicircle, i.e. lower phase angle, in

comparison to TiO2. According to previous reports, the arc radius of a Nyquist plot is associated with the ability of the corresponding film to conveying charge carriers across the solidesolid interfacial network within the film and the solideliquid junction on the surface of the film [20,22,32,33,37]. The smaller the Nyquist arc radius, the better the chargeconveying capability of the film is. In addition, Fig. 6(b) depicts the frequency response analysis of the film admittance (i.e. the inverse of impedance), a complex number consisting of the conductance as a real part and the susceptance as an imaginary part. It is clearly shown that G-TiO2 possesses slightly better admittance within the frequency range of interest (10 mHze10 kHz). Within the frequency of 1 mHze1 kHz, the admittance is attributable to the chargeconveying capacity within the film while, at ultralow (1 mHz) and very high frequency (>1 kHz), the admittance are attributed to the FTO-film interface contact and filmelectrolyte junction, respectively [32]. Therefore, we conclude that attaching graphene flakes to TiO2 films, even in a relatively small amount, leads to higher mobility of the photocharges across the film network and consequently boosts the photocurrent response. Based upon these findings, we suppose that the effect of graphene content on the photoelectrochemical behaviour of TiO2 films can be one of the most important issues to be tackled in future study [42].

Glycerol as a sacrificial hole scavenger In addition to hydrogen production via water splitting, degradation of organic pollutants is one of the most attractive applications in photocatalytic-related research. In spite of similar fundamentals governing both applications, they are

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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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 x x x ( 2 0 1 4 ) 1 e1 2

Fig. 6 e Frequency Response Analysis of TiO2 and G-TiO2 at ¡0.6 V vs HgOjHg: Nyquist plots (a), Admittance plots (b).

very often studied and discussed separately [2]. Organic compounds, the most common one being formic acid, have been utilised to evaluate the mechanisms of advanced oxidation process [79]. In this report, however, we introduce glycerol into the photoelectrochemical cell to act as a sink for the photogenerated holes and to evaluate its effect on the photooxidation process. With an important role played by glycerol [43,45e47,80e83], the carrier separation is expected to be further enhanced hence improving the photoelectrochemical response of the cell and, at the same time, consuming glycerol in an irreversible manner. The effect of glycerol as a hole scavenger on the generated photocurrent is illustrated in Fig. 7(a). At relatively low concentrations of glycerol, the onset of photoanodic current slightly shifts towards less positive potentials while, at high concentrations (>20 mM), the shift becomes more significantly noticeable in both the dark (inset of Fig. 7) and the illuminated system. Furthermore, monotonous increases in current density are also observed upon increasing the glycerol concentration, particularly at sufficiently positive anodic bias. These observations indicate that glycerol plays a major role in the hole-driven photo-oxidation process and acts as an effective hole scavenger to prevent the recombination of electronehole pairs. Additional current density due to glycerol introduction at the potential of 0.37 V vs HgOjHg under dark (Djd) and illuminated (Djp) conditions as a function of glycerol concentration is represented in Fig. 7(b). As clearly depicted in Fig. 7(b), Djd is negligible at low concentrations of glycerol (<100 mM) and gradually increases at higher concentrations. In contrast, Djp dramatically rises with the introduction of glycerol in low concentrations and starts to levelling-off at higher concentrations. It should also be noted that the significance of darkdriven additional current density on Djp increases as the glycerol concentration increases. This observation leads to a conjecture that, at very high concentrations of glycerol, additional photocurrent density is actually dominated by the dark-driven electro-oxidation of glycerol. Nevertheless, even at the highest concentration of glycerol studied in this report (ca. 275 mM), the additional current in illuminated systems is still reasonably superior compared to that of the dark system. For comparison purposes, (DjpeDjd) parameters resulting from both samples are plotted against the glycerol concentration. The parameter (DjpeDjd) is used in order to exclude the effect of dark-driven glycerol oxidation as well as to eliminate the difference in photocurrent response of both samples in

the absence of glycerol. As clearly observed in Fig. 7(c), the GTiO2 gives much better response to the introduction of glycerol, i.e. pushing the (DjpeDjd) to saturate at a higher value than that of the TiO2 sample. The effects of glycerol concentration ([Glycerol]) on (DjpeDjd) are expressed by fitting the experimental data with a model resembling a LangmuireHinshelwood isotherm model as follows:     K½Glyerol (7) Djp
Djd ¼ Djp
Djd max 1 þ K½Glycerol Plotting the inverses of ðDjp
Djd Þ and [Glycerol] leads to a linear relationship as follows: 

1 Djp
Djd

¼

1 Djp
Djd

 max

1  þ Djp
Djd

max

1 ½Glycerol K

(8)

Linear regression of Equation (8) gives ðDjp
Djd Þmax values of 74.6 and 30.1 mA/cm2 for G-TiO2 and TiO2, respectively. It is generally known that the effect of the initial pollutant concentration on the photodegradation rate or, similarly, the effect of the initial concentration of hole scavengers on the rate of hydrogen photoproduction follows LangmuireHinshelwood adsorption models [84,85] as observed in the case of 2-chlorophenol [86], chloroacetic acid [87], oxalic acid [88], methanol [89], glucose [90], and glycerol [83]. This indicates that the reaction rate is limited by the saturation of active sites available on the surface of photocatalysts/photoanodes. Nonetheless, this probably does not explain the real effect of increasing the glycerol concentration in our case. Since no stirring was employed during the photoelectrochemical measurements, mass transfer resistance could be substantial and hinder the glycerol molecules reaching the film surface. Therefore, increasing the bulk glycerol concentration does not necessarily mean increasing occupied sites of the film surface by glycerol molecules. Yet, by increasing the glycerol concentration, the driving force for the glycerol molecules to diffuse towards the film surface is increased and the overall current density should increase. However, a different postulation may also be constructed on the basis of the bulk and the surface recombination. In the absence of a hole scavenger, photocurrents are limited by the recombination occurring both on the bulk and on the surface of the photoanode. Introduction of glycerol into the electrolyte improves the electronehole separation on the surface of the photoanode, while the bulk recombination remains intact due to the great mass transfer limitation within the interior region of the film which does not allow glycerol molecules to diffuse

Please cite this article in press as: Ibadurrohman M, Hellgardt K, Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.08.142

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 x x x ( 2 0 1 4 ) 1 e1 2

9

Fig. 7 e Effects of glycerol as a hole scavenger on the photoelectrochemical performance of G-TiO2: LSV under illumination of white light (120 W m¡2) in 1 M NaOH, inset: dark scan (a), additional generated current due to glycerol introduction at the potential of 0.37 V vs HgOjHg under dark and illuminated conditions (b), Langmuirian behaviour of glycerol as a sacrificial hole scavenger (c).

in. Gradual increase in glycerol concentration further reduces the surface recombination until, at a sufficiently high concentration of glycerol, the resistance that limits the photocharge extraction on the surface is totally overcome. At this point, the bulk recombination becomes the rate-determining step for the overall reaction rate, hence saturating the value of (DjpeDjd). In any case, the use of glycerol as a hole scavenger is an effective method to boost the photoelectrochemical performance of the cell. Nevertheless, as mentioned earlier, the significance of glycerol effects is restricted to the holescavenging process on the photoanode surface. We believe that the effects of glycerol as a hole scavenger will be more significant by improving the surface reactivity and/or increasing the specific surface area of the photoanodes. Therefore, surface/morphology-related modification of the films can be an important subject for future study.

Concluding remarks Our conclusions can be summarised as follows: 1. Graphene can be successfully incorporated onto TiO2 films via a spray pyrolysis method, resulting in improved photocharge separation thereby enhancing the efficiency of the photo-assisted water splitting process. Graphenemodified TiO2 films also exhibit excellent stability during prolonged photo-electrochemical operation. 2. Glycerol is an effective sacrificial agent to consume photogenerated holes, judging by the increase of photocurrents as well as the shift of their onsets to more negative potentials upon the addition of glycerol into the electrolyte.

Furthermore, introduction of graphene into TiO2 photoanode films enhances the additional photocurrent associated with the presence of glycerol in the electrolyte. 3. The additional photocurrent density due to the introduction of glycerol depends upon the concentration of glycerol according to a model resembling a LangmuireHinshelwood isotherm. 4. The current photoconversion system still leaves a wide scope for improvements (e.g. in terms of graphene content or surface reactivity and area) in order to increase the efficiency further. In addition, a wide range of organic wastes, (e.g. humic acid, formic acid, lactic acid, or phenols) could also be utilised as sacrificial hole scavengers. Considering these factors for future study may lead to a promising photoconversion system in which energy is produced and organic pollutants are photodegraded.

Acknowledgement This research is supported by the Directorate General of Higher Education, Indonesian Ministry of Education and Culture, via a doctoral scholarship for the first author (568/E4.4/K/2012).

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