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Photoelectrochemical behavior of bimetallic Cu–Ni and monometallic Cu, Ni doped TiO2 for hydrogen production
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 5 ) 1 e8
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Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production Norani Muti Mohamed a,c,*, Robabeh Bashiri b, Fai Kait Chong c, Suriati Sufian b, Saeid Kakooei d a
Centre of Innovative Nanostructures & Nanodevices, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia b Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia c Fundamental & Applied Sciences Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia d Center for Corrosion Research & Mechanical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia
article info
abstract
Article history:
Photocatalyst is the heart of the photoelectrochemical (PEC) cell that is used to generate
Received 10 January 2015
hydrogen by water splitting from solar energy. Thus improving the photoelectrochemical
Received in revised form
properties will result in better conversion efficiency. This paper presents the investigation
7 July 2015
of the photoelectrochemical behavior of the synthesized nanostructured CueNi doped TiO2
Accepted 13 July 2015
compared to monometallic Ni and Cu doped TiO2 and also TiO2 photocatalysts. The pho-
Available online xxx
toelectrochemical properties of photoanode in the PEC cell showed that 5Cu-5Ni doped
Keywords:
water, photocurrent density of 2.29 mA/cm2 at 0.24 V, photoconversion efficiency of 4.33%,
Cu-Ni/TiO2 thin film
electron life time of 217.71 ms with the flat band and donor density from MotteSchottky of
Electrochemical impedance
0.93 V and 2.14 1021 cm3, respectively. This better performance compared to other
spectroscopy
photocatalysts is attributed to the synergetic effect of two metals as charge carriers traps,
Photoconversion efficiency and
more electronehole separation, longer electron lifetime, more negative flat band potential
hydrogen
for water splitting and higher electron donor density.
TiO2 thin film produced the highest amount of hydrogen (5.3 ml) from photosplitting of
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Modern society is habituated to a high degree of mobility, fast communication, and daily comfort, all of which require
considerable energy input [1]. Energy dense fossil fuels (oil, coal and natural gases) are the most coveted fuel that has ever been discovered. Since the oil crisis of 1973 considerable progress has been made in the search for alternative energy sources [2]. Exploring renewable energy resource is an
* Corresponding author. Centre of Innovative Nanostructures & Nanodevices, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. Tel.: þ60 5 368 8220; fax: þ60 5 3655903. E-mail address: [email protected] (N.M. Mohamed). http://dx.doi.org/10.1016/j.ijhydene.2015.07.064 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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important research concern due to high energy demand and limited fossil fuels resource in the 21 century [3]. Therefore, access to affordable and reliable energy drawn from renewable, environmentally acceptable and cheap sources of supply is an important feature of sustainable development. Hydrogen as a next-generation energy carrier can be considered as a desirable alternative energy source with higher energy content per weight than coal and gasoline [4,5]. Hydrogen production methods are based on the hydrogen separation from hydrocarbon feedstock or water source whilst the most hydrogen produced currently is from methane steam reforming by producing CO2 [6]. However, one of the most promising desirable technologies for the hydrogen production is to use solar energy to split water into hydrogen and oxygen in a photo-electrochemical (PEC) cell [7]. Photoelectrolysis of water using an n-type semiconductor (TiO2) and harvesting solar energy by the flowing of excited electrons through the PEC cell was established by Fujishima and Honda in 1972 [8,9]. A number of requirements as shown in Table 1 are needed to produce hydrogen in PEC Cell. Many research worked carried out to obtain photoanode, having characteristic of efficient sun light absorption, high photoconversion efficiency, proper bandgap energy, practical durability and low cost in the ideal PEC cell [11]. Oxide materials like TiO2, ZnO, BaTiO3 and WO3 exhibit sustainable performance as the PEC cell photoelectrode. TiO2 as an attractive n-type semiconductor has been used widely due to high chemical stability, easily accessible, environmentally safe and suitable conduction band position for water splitting. Nevertheless, its main drawbacks are low efficiency, large bandgap energy (3.2 eV) and fast recombination of photo-generated charge carriers within nanosecond [10,12,13]. Numerous efforts have been conducted to enhance the efficiency of TiO2 including: metal ion doping with electronic configuration dn (0 < n < 10), spectral sensitization, and controlling valence band using orbitals p of anion and s of pblock metal ions [4]. Doping TiO2 with metal ion has been known as a simple method, cost-effective and repeatable preparing pathway of modified TiO2 [14]. It is notable that loading 3d transition metals as dopants (Cr, Fe, Ni, V, Mn and Cu) with low-cost and largely available are popular to extend the light absorbance to visible region through the formation of mid-gap states [15,16]. In recent years, many researchers have reported that TiO2 co-doping with metals such as Ni and Cu [17e19], Au and Cu [20], Ni and N [21], Sb and Cr [22] and Fe and Ni [23] can cause higher photocatalytic activity than that of single doped TiO2. Cu and Ni can be considered as the
appropriate dopants for TiO2 in PEC cell because the oxides form of copper (CuO and Cu2O) displayed photocatalytic activity comparable to the noble metals due to their narrow bandgap energies (1.4 and 2.2 eV), negative conduction positions (0.96 and 0.22 V) and high light absorption coefficient [24]. On the other hand, Ni as a fairly costly transition metal has been proposed for improving the photocatalytic activity of some semiconductors by improving the thermal stability and controlling the morphology of mesoporous photocatalysts [17]. In this study, a novel photoanode of bimetallic (CueNi/TiO2) and monometallic doped TiO2 (Cu/ TiO2 and Ni/TiO2) was synthesized as the photoanodes in PEC cell. In addition, their photoelectrochemical behavior was investigated in the PEC cell using electrochemical impedance spectroscopy (EIS), IeV characteristics analysis and MotteSchottky analysis (MS).
Experimental descriptions Paste preparation All photocatalysts CueNi/TiO2, Cu/TiO2, Ni/TiO2 (total metal loading 10 mol% and 5:5 Cu:Ni molar ratio) and TiO2 were synthesized with solegel associated hydrothermal method. Titania precursor solution was prepared by mixing Titanium tetraisopropoxide (TTIP), absolute ethanol and glacial acetic acid in the controlled environment in the gloves box. Then, solutions of ethanol, Cu (NO3)2 and Ni (NiO3)2 were added dropwise to the titania precursor solution. The resulted sol was transferred to a Teflon-lined autoclave for hydrothermal treatment at 180 C for 12 h. The product was separated via centrifuging, then rinsed and dried at 105 C overnight and calcined at 450 C for 2 h. More detail of the synthesis process was given in our previous work [25]. All synthesized photocatalysts and commercial TiO2 (P25) were used to prepare the paste for coating the FTO glass substrate. The exact amount of each component in order to obtain a printable paste, free from cracking, peeling off and not easily evaporated are listed in Table 2. The powder was ground for 10 min with intermittent addition of acetic acid drop-wise to improve the dispersiblity and homogeneity of the paste. Then 0.6 ml of water was added followed by 0.5 ml of carbowax solution which is a mixture of PEG2000, ethanol and water. Then, 10 ml ethanol was added to photocatalyst dispersions and transferred to a mixture of
Table 2 e Chemical composition of screen printable paste. Table 1 e Main PEC water requirements [10].
Component
Material
Amount
Condition
Semiconductor Binder
Photocatalysts a PEG 20000
Rheological agent Dispersant Solvent/binder Acidification agent/ surface modifier
Ethylcellulose aeTerpineol Ethanol Acetic acid
1.2 g 0.5 ml of 10 v% PEG in Ethanol 1.0 g in 10 ml EtOH 6.6 ml 10.0 ml 0.2 ml
Main requirements
Water splitting Minimum potential required Practical potential (þover potential &losses)
H2O(l) þ 2hn / 1/2O2(g) þ H2(g) EoH2 O (25 C)min ¼ 1.23 V EoH2 O (25 C)prac ¼ 1.6e2.0 V, Ebandgap > EoH2 O
For efficient utilization of sunlight Band edges requirement
UV > hn (Vis) > IR, hn Ebandgap Cband edge < EoH2 =Hþ , Vband < EoO2 =H2 O
edge
a
Polyethylene glycol.
Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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ethylcellulose and aeTerpineol in ethanol. Ultrasonic homogenisation was conducted using ultrasonic bath for 2 min followed by continuous stirring until the paste became thicker and printable.
Photoanode fabrication The cleaned FTO (Fluorine doped Tin Oxide) glass with 2.5 mm thickness and resistance 20 U/square was sintered at 400 C to remove the organic compound from the glass surface. A layer of prepared paste was screen printed onto the FTO substrate with the tested thickness of 6 mm with active surface area of 2 cm2. Then the printed paste was dried and sintered in the furnace at 400 C. The printing process was repeated 4 times to obtain a film thickness of 24 mm. After the final sintering, uncoated part of FTO substrate and copper wire assembly were finally covered with non-conductive and non-corrosive epoxy resin sealant for protection from the electrolyte.
PEC cell setup Fig. 1a represents a schematic diagram of experimental setup for PEC cell to measure the photoelectrochemical performance of prepared photoanode. The PEC cell includes the coated multiple layers photocatalysts on the FTO substrate as a photoanode or working electrode (WE), platinum (Pt) rod was fitted in the electrode holder as counter electrode (CE), Ag/ AgCl/saturated KCl as a reference electrode. All electrodes were immersed in a 150 ml electrolyte, mixing of 1 M KOH (pH ¼ 13) aqueous solution and 10 v/v% glycerol as the hole scavenger in a Pyrex cell. All of electrodes of this cell were connected to Autolab PGSTAT302N (Metrohm) for the photoelectrochemical study. In order to expose light to the thin film, PEC setup was put in the dark cabinet of small solar simulator with a 150 W xenon lamp with AM 1.5G filter for restriction of the incident of UV region (wavelength < 400 nm) with intensity of 1 Sun (100 mW/cm2). The photoelectrochemical
properties measurement were conducted by a conventional three electrode configuration. The hydrogen production measurements were conducted at the same conditions via water displacement method integrated to the counter electrode (Pt) as shown in Fig. 1b.
Photoelectrochemical characteristic Electrochemical impedance spectroscopy (EIS) experiments were conducted with a computer-controlled potentiostat which was equipped with a frequency response analyzer (FRA) module and supported by NOVA software. All impedance measurements were recorded at frequency range of 0.1e100 kHz with AC amplitude of 10 mV. Values of photocurrent density were collected by applying external potential with linear Sweep voltammetry (LSV) procedure for data acquisition at potential from - 0.4 to 0.4 V vs. Ag/AgCl with the scan rate 20 mV/s. The efficiency of light energy conversion to the chemical energy in the presence of an external applied potential (Eapp) under illumination condition is named photoconversion efficiency (h) which is subsequently calculated from Eq. (1). h% ¼ ðjp ½ð Eorev Eapp Þ=Io Þ 100
(1)
The standard water splitting potential is Eorev ¼ 1.23 V, jEappj is the absolute value of the applied potential and Io (mW/cm2) describes light power intensity. jEappj can be calculated using Eq. (2), Emeas is the working electrode potential at measured photocurrent and Eaoc is the potential of working electrode at open-circuit condition. Also the measured potential (Eapp) with respect to reference electrode Ag/AgCl was converted to the reversible hydrogen electrode (RHE) by Eq. (3) [26,27]. Eapp ¼ Emeans Еaoc
(2)
ERHE ¼ EAgCl þ EoAgCl þ 0:059 pH
(3)
Fig. 1 e (a) Schematic diagram of the PEC cell with the label: (1) 150 W xenon lamp, (2) UV filter, (3) Pt Rod, (4) Ag/AgCl reference electrode (5) 5Cu-5Ni/TiO2 thin film, (6) electrolyte and (7) potentiostat and (b) experimental setup for hydrogen collection. Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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MotteSchottky analysis (MS) can be conducted on EIS procedure to measure the capacitance as a function of released potential in the system under depletion condition [28]. The capacitance of space charge of the depleted semiconductor electrode in the photoelectrode/electrolyte interface is approximated from equation (4) at an applied frequency of 1 kHz in the dark condition. 00
C ¼ 1=2pfZ
(4)
(ND ¼ 2/e3 o3 m) where e is the electronic charge unit (1.6 1019 C), 3 0 is the permittivity of the free space charge (8.86 1012 F/m) and 3 is dielectric constant for the anatase TiO2 (48 Fm) [29].
Results and discussion Electrochemical impedance spectroscopy
2
The extrapolation of the MS plot on V-axis in which C is zero equalize to the flat band potential (VFb). Whilst the donor density is calculated from the slope m of MS plot by means of
Fig. 2a, b, c, and d present typical Nyquist plots of the photoelectrochemical system for all prepared photocatalysts and
Fig. 2 e Nyquist plots of (a and b) P25, TiO2, (c and d) monometallic and bimetallic Cu and Ni doped TiO2 in the dark and under illumination from solar simulator, (e) Schematic diagram of an equivalent circuit and (f) pen junction at the interface of metal oxide (NiO or CuO) and TiO2.
Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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commercial TiO2 (P25) in dark and simulated sunlight condition in the PEC cell. Here, X-axis and Y-axis denote the real part of measured impedance (Z0 ) and imaginary part of measured impedance (Z00 ), respectively. All plots illustrate a single capacitive loop (semicircle) in dark and light. It can be observed that the semicircle arc and the loop diameter decreased in the all Nyquist plot under light irradiation. Fig. 2a and b illustrate that the loop size of the prepared TiO2 is much reduced compared to P25. This is due to higher absorption of radiation by the larger surface area, better penetration by bigger pore size, and the presence of highly conductive anatase phase of TiO2 [30]. Fig. 2e describes an equivalent circuit model consisting of resistance (Rs) in series with a parallel of resistance Rp (space charge layer) and Csc (double-layer capacitance) with the doublelayer capacitance replaced by constant phase element (CPE). The impedance of CPE was defined by ZCPE ¼ 1/juCn where 0 < n > 1 is the empirical constant with no real meaning (n ¼ 1, CPE is an ideal capacitor and n ¼ 0, CPE is an ideal resistor). The fitted data of equivalent circuit model given in Table 3 shows that the values of all recombination resistance were reduced in light condition compared to dark whilst CPE and electron lifetime (tn ¼ Rb* CPE) increased [30]. Therefore, smaller loop diameter indicates less internal resistance with better electron transfer which is reflected in the better photoelectrochemical performance of the photocatalyst [31]. It is notable that Cu can trap both excited electrons and holes, a better way to reduce the value of recombination resistance (depicted in Fig. 2c and d) compared to Ni as the only hole trapping center [16]. In addition, the presence of NiO and CuO as p-type semiconductors in TiO2 (n-type) structure can improve the conductivity of photoanode by the formation of different nep junctions as shown in Fig. 2f. The formation of inner electric field at the equilibrium caused electron-hole pairs to be effectively separated in the PEC cell, thus enhanced the photocatalytic performance of photocatalysts [32e35].
Photocurrent density and photoconversion efficiency Fig. 3a and b illustrate the photocurrent density (jp) curves and the corresponding photoconversion efficiency as the function of applied potential for various TiO2 photoanodes. Photocurrent density under light irradiation is higher than the one in the dark for TiO2 because of more generation and transfer of photoinduced electrons from the photoanode to the counter
electrode through an external circuit. At the initial bias potential, photocurrent gradually increases until the potential of the water splitting (0.247 vs Ag/AgCl), corresponding to a low separation of photogenerated charge carriers. Whilst at the higher bias potential, photocurrent increases sharply after this point due to the increase in the space charge layer at the semiconductor/electrolyte interface and the faster electron transfer rate [36]. Maximum jp recorded by 5Cue5Ni/TiO2 cell at potential 0.24 V is around 2.29 and 3.89 mA/Cm2 at 0.4 V. Significantly, there is no saturation of photocurrent observed in all studied metal doped TiO2 at more positive potential, which indicates efficient charge separation in the illumination condition [37]. This better performance of 5Cue5Ni/TiO2 in jp may be attributed to the shift of titania bandgap towards the visible region compared to other photocatalysts, less recombination resistance and higher electron lifetime which allow for more migration of electrons to the semiconductor/electrolyte interface [15,25]. The 5Cue5Ni/TiO2 exhibits maximum photoconversion efficiency of 3.25% at 0.109 V vs Ag/AgCl. It is noted that only 75% of the internal energy may be converted into chemical energy, while the remaining 25% constitute to the entropy-related losses. Thus, the corrected efficiency of 5Cue5Ni/TiO2 is 4.33% [38]. This result is higher than the reported values for the undoped TiO2 in the literature [9]. Such differences in the photoconversion efficiencies reflect the improvement of the overall photoelectron efficiency in the generation, separation, and transportation in the doped TiO2 fabricated in different conditions [39].
MotteSchottky (MS) study Fig. 4 illustrates the MS plots (1/C2 (capacitance) vs applied potential) for all prepared samples. MS plots with positive slope for TiO2 and metal-doped TiO2 are as expected for the ntype semiconductors. The slope of the plot decreases from P25 to 5Cue5Ni/TiO2, which indicates a significant increase of charge carrier densities and a negative shift of the flat band. The calculated flat band (VFB) and donor densities (ND) from MS curves are tabulated in Table 4. The value of VFB is found to be more negative from P25 (0.6 V) to 5Cue5Ni/TiO2 (0.93 V). For water splitting, VFB should be more negative than the reduction potential of hydrogen (Eh), which varies as a function of pH value and can be quantified by the equation: Eh ¼ 0e0.059 pH. As expected, the observed VFB for all prepared
Table 3 e Quantitative data of EIS measurements and simulated with an equivalent electrical circuits. Photocatalyst
Condition
RS (U)
Rp(U)
n
tn (ms)
06
CPE (F)
P25 TiO2 10Ni/TiO2 10Cu/TiO2 5Cue5Ni/TiO2
Dark
37.51 42.76 36.27 47.95 47.00
12,159 4000.5 170.66 134.52 108.06
2.38 8.66 2.78 1.03 1.31
10 1006 1004 1003 1003
0.98 0.98 0.75 0.77 0.73
28.95 34.64 47.49 138.22 141.55
P25 TiO2 10Ni/TiO2 10Cu/TiO2 5Cue5Ni/TiO2
150 W xenon lamp
37.13 40.58 37.07 33.23 34.69
2587.20 1703.2 80.58 65.12 54.73
1.87 1.71 1.34 2.38 3.98
1005 1005 1003 1003 1003
0.95 0.96 0.82 0.81 0.80
48.25 29.16 108.36 154.92 217.71
Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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Fig. 4 e MotteSchottky plots for all photocatalysts at the frequency of 1 kHz.
Table 4 e Donor density (ND) and flat band potential (VFB) values obtained from MS plot. Photocatalysts P25 TiO2 10 Cu/TiO2 10 Ni/TiO2 5Ni/TiO2
ND (cm3) 3.47 4.71 8.07 7.48 2.14
20
10 1020 1020 1020 1021
VFB (V) 0.62 0.83 0.87 0.91 0.93
Fig. 3 e (a) Photocurrent density (jp) and (b) photoconversion efficiency for all photocatalysts under simulated solar illumination and TiO2 under dark condition.
photoanodes are more cathodic than the calculated minimum of VFB of 0.76 V. The negative and large VFB for the n-type semiconductor is the thermodynamic ability to reduce more water to hydrogen. The photocatalytic performance was determined for an applied external potential corresponding to the photoconversion efficiency. As illustrated in Fig. 5, the highest
Fig. 5 e The hydrogen evolution from all prepared photocatalysts and P25.
Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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 5 ) 1 e8
hydrogen production during the 2 h period is shown by 5Cue5Ni/TiO2 with total volume of 5.3 ml, followed by 10Cu/ TiO2 with 4.5 ml, 10Ni/TiO2 with 3.1 ml, TiO2 with 1.8 ml and lastly, P25 with 1.2 ml. The outcome implies that hydrogen production from water splitting through the PEC cell depends on charge carrier generation, separation, crystalline phase, flat band potential and charge carrier density that migrate to the surface of photocatalyst and combine with Hþ at Pt electrode for hydrogen generation [40].
Conclusion New bimetallic and monometallic doped TiO2 photoanodes were coated onto FTO glass using screen printing method in the PEC cells for solar water splitting. Photoelectrochemical analyses reveal that the incorporation of both Ni and Cu ions on TiO2 matrix has produced a synergetic effect, resulting in lower recombination resistance, longer electron lifetime, better electronehole separation, and easier charge carriers transfer to the surface of TiO2 as compared to 10Cu/TiO2 and 10Ni/TiO2. All these improved properties resulted in the bimetallic 5Cue5Ni/TiO2 having higher current density, photoconversion efficiency (4.43% at 0.109 V) compared to other photocatalysts.
Acknowledgment Authors wish to thank Universiti Teknologi PETRONAS and Ministry of Education (Higher Education Department) for the Exploratory Research Grant Scheme (No: ERGS/1/2012/TK05/ UTP/01/04).
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Sol-Gel hydrothermal. Adv Mater Res 2014;925(1):396e400. Mishra PR, Shukla PK, Srivastava ON. Study of modular PEC solar cells for photoelectrochemical splitting of water employing nanostructured photoelectrodes. Int J Hydrogen Energy 2007;32(12):1680e5. Chiang Chia-Ying, K.A., Franson Nicholas, Satsangi Vibha Rani, S.E., Dass Sahab. Copper oxide nanoparticle made by flame spray pyrolysis for photoelectrochemical water splitting e Part II. Photoelectrochemical study. Int J Hydrogen Energy 2011;36(24):5519e15526. Lee MS, Cheon IC, Il Kim Y. Photoelectrochemical studies of nanocrystalline TiO2 film electrodes. Bull Korean Chem Soc 2003;24(8):1155e62. Boschloo GK, Goossens A, Schoonman J. Photoelectrochemical study of thin anatase TiO2 films prepared by metallorganic chemical vapor deposition. J Electrochem Soc 1997;144(4):1311e7. Park K, Zhang Q, Myers D, Cao G. Charge transport properties in TiO2 network with different particle sizes for dye sensitized solar cells. ACS Appl Mater Interfaces 2013;5(3):1044e52. Zhang M, Yuan S, Wang Z, Zhao Y, Shi L. Photoelectrocatalytic properties of Cu2þ-doped TiO2 film under visible light. Appl Catal B Environ 2013;134e135(0):185e92. Jinfeng Z, Yunguang Y, Wei L. Preparation, characterization, and activity evaluation of CuO/F-TiO2 photocatalyst. Int J Photoenergy 2012;2012(1):1e9. Yildirim N, Serin T, Serin N. Investigation of a p-CuO/n-TiO2 thin film heterojunction fabricated by the sol-gel process. J Optoelectron Adv Mater 2010;12(5):1153e6.
[34] Tsai T-Y, Chang S-J, Hsueh T-J, Hsueh H-T, Weng W-Y, Hsu C-L, et al. p-Cu2O-shell/n-TiO2-nanowire-core heterostucture photodiodes. Nanoscale Res Lett 2011;6(1):1e7. [35] Yan-Qing C, Zhe L, Qi Wang, Yi Z, Qian X, Fang-Xia F. Enhanced photoeletrocatalytic activity of TiO2 nanotube arrays modified with simple transition metal oxides(Fe2O3,CuO,NiO). Acta Phys Chim Sin 2012;28(6):1489e96. [36] Lee WJ, Shinde PS, Go GH, Ramasamy E. Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation. Int J Hydrogen Energy 2011;36(9):5262e70. [37] Yang X, Wolcott A, Wang G, Sobo A, Fitzmorris RC, Qian F, et al. Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting. Nano Lett 2009;9(6):2331e6. [38] Bak T, Nowotny J, Rekas M, Sorrell CC. Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects. Int J Hydrogen Energy 2002;27(10):991e1022. [39] Lai CW, Sreekantan S. Photoelectrochemical performance of smooth TiO2 nanotube arrays: effect of anodization temperature and cleaning methods. Int J Photoenergy 2012;2012:1e11. [40] Shen Q, Arae D, Toyoda T. Photosensitization of nanostructured TiO2 with CdSe quantum dots: effects of microstructure and electron transport in TiO2 substrates. J Photochem Photobiol A Chem 2004;164(1e3):75e80.
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Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production Norani Muti Mohamed a,c,*, Robabeh Bashiri b, Fai Kait Chong c, Suriati Sufian b, Saeid Kakooei d a
Centre of Innovative Nanostructures & Nanodevices, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia b Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia c Fundamental & Applied Sciences Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia d Center for Corrosion Research & Mechanical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia
article info
abstract
Article history:
Photocatalyst is the heart of the photoelectrochemical (PEC) cell that is used to generate
Received 10 January 2015
hydrogen by water splitting from solar energy. Thus improving the photoelectrochemical
Received in revised form
properties will result in better conversion efficiency. This paper presents the investigation
7 July 2015
of the photoelectrochemical behavior of the synthesized nanostructured CueNi doped TiO2
Accepted 13 July 2015
compared to monometallic Ni and Cu doped TiO2 and also TiO2 photocatalysts. The pho-
Available online xxx
toelectrochemical properties of photoanode in the PEC cell showed that 5Cu-5Ni doped
Keywords:
water, photocurrent density of 2.29 mA/cm2 at 0.24 V, photoconversion efficiency of 4.33%,
Cu-Ni/TiO2 thin film
electron life time of 217.71 ms with the flat band and donor density from MotteSchottky of
Electrochemical impedance
0.93 V and 2.14 1021 cm3, respectively. This better performance compared to other
spectroscopy
photocatalysts is attributed to the synergetic effect of two metals as charge carriers traps,
Photoconversion efficiency and
more electronehole separation, longer electron lifetime, more negative flat band potential
hydrogen
for water splitting and higher electron donor density.
TiO2 thin film produced the highest amount of hydrogen (5.3 ml) from photosplitting of
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Modern society is habituated to a high degree of mobility, fast communication, and daily comfort, all of which require
considerable energy input [1]. Energy dense fossil fuels (oil, coal and natural gases) are the most coveted fuel that has ever been discovered. Since the oil crisis of 1973 considerable progress has been made in the search for alternative energy sources [2]. Exploring renewable energy resource is an
* Corresponding author. Centre of Innovative Nanostructures & Nanodevices, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. Tel.: þ60 5 368 8220; fax: þ60 5 3655903. E-mail address: [email protected] (N.M. Mohamed). http://dx.doi.org/10.1016/j.ijhydene.2015.07.064 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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important research concern due to high energy demand and limited fossil fuels resource in the 21 century [3]. Therefore, access to affordable and reliable energy drawn from renewable, environmentally acceptable and cheap sources of supply is an important feature of sustainable development. Hydrogen as a next-generation energy carrier can be considered as a desirable alternative energy source with higher energy content per weight than coal and gasoline [4,5]. Hydrogen production methods are based on the hydrogen separation from hydrocarbon feedstock or water source whilst the most hydrogen produced currently is from methane steam reforming by producing CO2 [6]. However, one of the most promising desirable technologies for the hydrogen production is to use solar energy to split water into hydrogen and oxygen in a photo-electrochemical (PEC) cell [7]. Photoelectrolysis of water using an n-type semiconductor (TiO2) and harvesting solar energy by the flowing of excited electrons through the PEC cell was established by Fujishima and Honda in 1972 [8,9]. A number of requirements as shown in Table 1 are needed to produce hydrogen in PEC Cell. Many research worked carried out to obtain photoanode, having characteristic of efficient sun light absorption, high photoconversion efficiency, proper bandgap energy, practical durability and low cost in the ideal PEC cell [11]. Oxide materials like TiO2, ZnO, BaTiO3 and WO3 exhibit sustainable performance as the PEC cell photoelectrode. TiO2 as an attractive n-type semiconductor has been used widely due to high chemical stability, easily accessible, environmentally safe and suitable conduction band position for water splitting. Nevertheless, its main drawbacks are low efficiency, large bandgap energy (3.2 eV) and fast recombination of photo-generated charge carriers within nanosecond [10,12,13]. Numerous efforts have been conducted to enhance the efficiency of TiO2 including: metal ion doping with electronic configuration dn (0 < n < 10), spectral sensitization, and controlling valence band using orbitals p of anion and s of pblock metal ions [4]. Doping TiO2 with metal ion has been known as a simple method, cost-effective and repeatable preparing pathway of modified TiO2 [14]. It is notable that loading 3d transition metals as dopants (Cr, Fe, Ni, V, Mn and Cu) with low-cost and largely available are popular to extend the light absorbance to visible region through the formation of mid-gap states [15,16]. In recent years, many researchers have reported that TiO2 co-doping with metals such as Ni and Cu [17e19], Au and Cu [20], Ni and N [21], Sb and Cr [22] and Fe and Ni [23] can cause higher photocatalytic activity than that of single doped TiO2. Cu and Ni can be considered as the
appropriate dopants for TiO2 in PEC cell because the oxides form of copper (CuO and Cu2O) displayed photocatalytic activity comparable to the noble metals due to their narrow bandgap energies (1.4 and 2.2 eV), negative conduction positions (0.96 and 0.22 V) and high light absorption coefficient [24]. On the other hand, Ni as a fairly costly transition metal has been proposed for improving the photocatalytic activity of some semiconductors by improving the thermal stability and controlling the morphology of mesoporous photocatalysts [17]. In this study, a novel photoanode of bimetallic (CueNi/TiO2) and monometallic doped TiO2 (Cu/ TiO2 and Ni/TiO2) was synthesized as the photoanodes in PEC cell. In addition, their photoelectrochemical behavior was investigated in the PEC cell using electrochemical impedance spectroscopy (EIS), IeV characteristics analysis and MotteSchottky analysis (MS).
Experimental descriptions Paste preparation All photocatalysts CueNi/TiO2, Cu/TiO2, Ni/TiO2 (total metal loading 10 mol% and 5:5 Cu:Ni molar ratio) and TiO2 were synthesized with solegel associated hydrothermal method. Titania precursor solution was prepared by mixing Titanium tetraisopropoxide (TTIP), absolute ethanol and glacial acetic acid in the controlled environment in the gloves box. Then, solutions of ethanol, Cu (NO3)2 and Ni (NiO3)2 were added dropwise to the titania precursor solution. The resulted sol was transferred to a Teflon-lined autoclave for hydrothermal treatment at 180 C for 12 h. The product was separated via centrifuging, then rinsed and dried at 105 C overnight and calcined at 450 C for 2 h. More detail of the synthesis process was given in our previous work [25]. All synthesized photocatalysts and commercial TiO2 (P25) were used to prepare the paste for coating the FTO glass substrate. The exact amount of each component in order to obtain a printable paste, free from cracking, peeling off and not easily evaporated are listed in Table 2. The powder was ground for 10 min with intermittent addition of acetic acid drop-wise to improve the dispersiblity and homogeneity of the paste. Then 0.6 ml of water was added followed by 0.5 ml of carbowax solution which is a mixture of PEG2000, ethanol and water. Then, 10 ml ethanol was added to photocatalyst dispersions and transferred to a mixture of
Table 2 e Chemical composition of screen printable paste. Table 1 e Main PEC water requirements [10].
Component
Material
Amount
Condition
Semiconductor Binder
Photocatalysts a PEG 20000
Rheological agent Dispersant Solvent/binder Acidification agent/ surface modifier
Ethylcellulose aeTerpineol Ethanol Acetic acid
1.2 g 0.5 ml of 10 v% PEG in Ethanol 1.0 g in 10 ml EtOH 6.6 ml 10.0 ml 0.2 ml
Main requirements
Water splitting Minimum potential required Practical potential (þover potential &losses)
H2O(l) þ 2hn / 1/2O2(g) þ H2(g) EoH2 O (25 C)min ¼ 1.23 V EoH2 O (25 C)prac ¼ 1.6e2.0 V, Ebandgap > EoH2 O
For efficient utilization of sunlight Band edges requirement
UV > hn (Vis) > IR, hn Ebandgap Cband edge < EoH2 =Hþ , Vband < EoO2 =H2 O
edge
a
Polyethylene glycol.
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ethylcellulose and aeTerpineol in ethanol. Ultrasonic homogenisation was conducted using ultrasonic bath for 2 min followed by continuous stirring until the paste became thicker and printable.
Photoanode fabrication The cleaned FTO (Fluorine doped Tin Oxide) glass with 2.5 mm thickness and resistance 20 U/square was sintered at 400 C to remove the organic compound from the glass surface. A layer of prepared paste was screen printed onto the FTO substrate with the tested thickness of 6 mm with active surface area of 2 cm2. Then the printed paste was dried and sintered in the furnace at 400 C. The printing process was repeated 4 times to obtain a film thickness of 24 mm. After the final sintering, uncoated part of FTO substrate and copper wire assembly were finally covered with non-conductive and non-corrosive epoxy resin sealant for protection from the electrolyte.
PEC cell setup Fig. 1a represents a schematic diagram of experimental setup for PEC cell to measure the photoelectrochemical performance of prepared photoanode. The PEC cell includes the coated multiple layers photocatalysts on the FTO substrate as a photoanode or working electrode (WE), platinum (Pt) rod was fitted in the electrode holder as counter electrode (CE), Ag/ AgCl/saturated KCl as a reference electrode. All electrodes were immersed in a 150 ml electrolyte, mixing of 1 M KOH (pH ¼ 13) aqueous solution and 10 v/v% glycerol as the hole scavenger in a Pyrex cell. All of electrodes of this cell were connected to Autolab PGSTAT302N (Metrohm) for the photoelectrochemical study. In order to expose light to the thin film, PEC setup was put in the dark cabinet of small solar simulator with a 150 W xenon lamp with AM 1.5G filter for restriction of the incident of UV region (wavelength < 400 nm) with intensity of 1 Sun (100 mW/cm2). The photoelectrochemical
properties measurement were conducted by a conventional three electrode configuration. The hydrogen production measurements were conducted at the same conditions via water displacement method integrated to the counter electrode (Pt) as shown in Fig. 1b.
Photoelectrochemical characteristic Electrochemical impedance spectroscopy (EIS) experiments were conducted with a computer-controlled potentiostat which was equipped with a frequency response analyzer (FRA) module and supported by NOVA software. All impedance measurements were recorded at frequency range of 0.1e100 kHz with AC amplitude of 10 mV. Values of photocurrent density were collected by applying external potential with linear Sweep voltammetry (LSV) procedure for data acquisition at potential from - 0.4 to 0.4 V vs. Ag/AgCl with the scan rate 20 mV/s. The efficiency of light energy conversion to the chemical energy in the presence of an external applied potential (Eapp) under illumination condition is named photoconversion efficiency (h) which is subsequently calculated from Eq. (1). h% ¼ ðjp ½ð Eorev Eapp Þ=Io Þ 100
(1)
The standard water splitting potential is Eorev ¼ 1.23 V, jEappj is the absolute value of the applied potential and Io (mW/cm2) describes light power intensity. jEappj can be calculated using Eq. (2), Emeas is the working electrode potential at measured photocurrent and Eaoc is the potential of working electrode at open-circuit condition. Also the measured potential (Eapp) with respect to reference electrode Ag/AgCl was converted to the reversible hydrogen electrode (RHE) by Eq. (3) [26,27]. Eapp ¼ Emeans Еaoc
(2)
ERHE ¼ EAgCl þ EoAgCl þ 0:059 pH
(3)
Fig. 1 e (a) Schematic diagram of the PEC cell with the label: (1) 150 W xenon lamp, (2) UV filter, (3) Pt Rod, (4) Ag/AgCl reference electrode (5) 5Cu-5Ni/TiO2 thin film, (6) electrolyte and (7) potentiostat and (b) experimental setup for hydrogen collection. Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064
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MotteSchottky analysis (MS) can be conducted on EIS procedure to measure the capacitance as a function of released potential in the system under depletion condition [28]. The capacitance of space charge of the depleted semiconductor electrode in the photoelectrode/electrolyte interface is approximated from equation (4) at an applied frequency of 1 kHz in the dark condition. 00
C ¼ 1=2pfZ
(4)
(ND ¼ 2/e3 o3 m) where e is the electronic charge unit (1.6 1019 C), 3 0 is the permittivity of the free space charge (8.86 1012 F/m) and 3 is dielectric constant for the anatase TiO2 (48 Fm) [29].
Results and discussion Electrochemical impedance spectroscopy
2
The extrapolation of the MS plot on V-axis in which C is zero equalize to the flat band potential (VFb). Whilst the donor density is calculated from the slope m of MS plot by means of
Fig. 2a, b, c, and d present typical Nyquist plots of the photoelectrochemical system for all prepared photocatalysts and
Fig. 2 e Nyquist plots of (a and b) P25, TiO2, (c and d) monometallic and bimetallic Cu and Ni doped TiO2 in the dark and under illumination from solar simulator, (e) Schematic diagram of an equivalent circuit and (f) pen junction at the interface of metal oxide (NiO or CuO) and TiO2.
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commercial TiO2 (P25) in dark and simulated sunlight condition in the PEC cell. Here, X-axis and Y-axis denote the real part of measured impedance (Z0 ) and imaginary part of measured impedance (Z00 ), respectively. All plots illustrate a single capacitive loop (semicircle) in dark and light. It can be observed that the semicircle arc and the loop diameter decreased in the all Nyquist plot under light irradiation. Fig. 2a and b illustrate that the loop size of the prepared TiO2 is much reduced compared to P25. This is due to higher absorption of radiation by the larger surface area, better penetration by bigger pore size, and the presence of highly conductive anatase phase of TiO2 [30]. Fig. 2e describes an equivalent circuit model consisting of resistance (Rs) in series with a parallel of resistance Rp (space charge layer) and Csc (double-layer capacitance) with the doublelayer capacitance replaced by constant phase element (CPE). The impedance of CPE was defined by ZCPE ¼ 1/juCn where 0 < n > 1 is the empirical constant with no real meaning (n ¼ 1, CPE is an ideal capacitor and n ¼ 0, CPE is an ideal resistor). The fitted data of equivalent circuit model given in Table 3 shows that the values of all recombination resistance were reduced in light condition compared to dark whilst CPE and electron lifetime (tn ¼ Rb* CPE) increased [30]. Therefore, smaller loop diameter indicates less internal resistance with better electron transfer which is reflected in the better photoelectrochemical performance of the photocatalyst [31]. It is notable that Cu can trap both excited electrons and holes, a better way to reduce the value of recombination resistance (depicted in Fig. 2c and d) compared to Ni as the only hole trapping center [16]. In addition, the presence of NiO and CuO as p-type semiconductors in TiO2 (n-type) structure can improve the conductivity of photoanode by the formation of different nep junctions as shown in Fig. 2f. The formation of inner electric field at the equilibrium caused electron-hole pairs to be effectively separated in the PEC cell, thus enhanced the photocatalytic performance of photocatalysts [32e35].
Photocurrent density and photoconversion efficiency Fig. 3a and b illustrate the photocurrent density (jp) curves and the corresponding photoconversion efficiency as the function of applied potential for various TiO2 photoanodes. Photocurrent density under light irradiation is higher than the one in the dark for TiO2 because of more generation and transfer of photoinduced electrons from the photoanode to the counter
electrode through an external circuit. At the initial bias potential, photocurrent gradually increases until the potential of the water splitting (0.247 vs Ag/AgCl), corresponding to a low separation of photogenerated charge carriers. Whilst at the higher bias potential, photocurrent increases sharply after this point due to the increase in the space charge layer at the semiconductor/electrolyte interface and the faster electron transfer rate [36]. Maximum jp recorded by 5Cue5Ni/TiO2 cell at potential 0.24 V is around 2.29 and 3.89 mA/Cm2 at 0.4 V. Significantly, there is no saturation of photocurrent observed in all studied metal doped TiO2 at more positive potential, which indicates efficient charge separation in the illumination condition [37]. This better performance of 5Cue5Ni/TiO2 in jp may be attributed to the shift of titania bandgap towards the visible region compared to other photocatalysts, less recombination resistance and higher electron lifetime which allow for more migration of electrons to the semiconductor/electrolyte interface [15,25]. The 5Cue5Ni/TiO2 exhibits maximum photoconversion efficiency of 3.25% at 0.109 V vs Ag/AgCl. It is noted that only 75% of the internal energy may be converted into chemical energy, while the remaining 25% constitute to the entropy-related losses. Thus, the corrected efficiency of 5Cue5Ni/TiO2 is 4.33% [38]. This result is higher than the reported values for the undoped TiO2 in the literature [9]. Such differences in the photoconversion efficiencies reflect the improvement of the overall photoelectron efficiency in the generation, separation, and transportation in the doped TiO2 fabricated in different conditions [39].
MotteSchottky (MS) study Fig. 4 illustrates the MS plots (1/C2 (capacitance) vs applied potential) for all prepared samples. MS plots with positive slope for TiO2 and metal-doped TiO2 are as expected for the ntype semiconductors. The slope of the plot decreases from P25 to 5Cue5Ni/TiO2, which indicates a significant increase of charge carrier densities and a negative shift of the flat band. The calculated flat band (VFB) and donor densities (ND) from MS curves are tabulated in Table 4. The value of VFB is found to be more negative from P25 (0.6 V) to 5Cue5Ni/TiO2 (0.93 V). For water splitting, VFB should be more negative than the reduction potential of hydrogen (Eh), which varies as a function of pH value and can be quantified by the equation: Eh ¼ 0e0.059 pH. As expected, the observed VFB for all prepared
Table 3 e Quantitative data of EIS measurements and simulated with an equivalent electrical circuits. Photocatalyst
Condition
RS (U)
Rp(U)
n
tn (ms)
06
CPE (F)
P25 TiO2 10Ni/TiO2 10Cu/TiO2 5Cue5Ni/TiO2
Dark
37.51 42.76 36.27 47.95 47.00
12,159 4000.5 170.66 134.52 108.06
2.38 8.66 2.78 1.03 1.31
10 1006 1004 1003 1003
0.98 0.98 0.75 0.77 0.73
28.95 34.64 47.49 138.22 141.55
P25 TiO2 10Ni/TiO2 10Cu/TiO2 5Cue5Ni/TiO2
150 W xenon lamp
37.13 40.58 37.07 33.23 34.69
2587.20 1703.2 80.58 65.12 54.73
1.87 1.71 1.34 2.38 3.98
1005 1005 1003 1003 1003
0.95 0.96 0.82 0.81 0.80
48.25 29.16 108.36 154.92 217.71
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Fig. 4 e MotteSchottky plots for all photocatalysts at the frequency of 1 kHz.
Table 4 e Donor density (ND) and flat band potential (VFB) values obtained from MS plot. Photocatalysts P25 TiO2 10 Cu/TiO2 10 Ni/TiO2 5Ni/TiO2
ND (cm3) 3.47 4.71 8.07 7.48 2.14
20
10 1020 1020 1020 1021
VFB (V) 0.62 0.83 0.87 0.91 0.93
Fig. 3 e (a) Photocurrent density (jp) and (b) photoconversion efficiency for all photocatalysts under simulated solar illumination and TiO2 under dark condition.
photoanodes are more cathodic than the calculated minimum of VFB of 0.76 V. The negative and large VFB for the n-type semiconductor is the thermodynamic ability to reduce more water to hydrogen. The photocatalytic performance was determined for an applied external potential corresponding to the photoconversion efficiency. As illustrated in Fig. 5, the highest
Fig. 5 e The hydrogen evolution from all prepared photocatalysts and P25.
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hydrogen production during the 2 h period is shown by 5Cue5Ni/TiO2 with total volume of 5.3 ml, followed by 10Cu/ TiO2 with 4.5 ml, 10Ni/TiO2 with 3.1 ml, TiO2 with 1.8 ml and lastly, P25 with 1.2 ml. The outcome implies that hydrogen production from water splitting through the PEC cell depends on charge carrier generation, separation, crystalline phase, flat band potential and charge carrier density that migrate to the surface of photocatalyst and combine with Hþ at Pt electrode for hydrogen generation [40].
Conclusion New bimetallic and monometallic doped TiO2 photoanodes were coated onto FTO glass using screen printing method in the PEC cells for solar water splitting. Photoelectrochemical analyses reveal that the incorporation of both Ni and Cu ions on TiO2 matrix has produced a synergetic effect, resulting in lower recombination resistance, longer electron lifetime, better electronehole separation, and easier charge carriers transfer to the surface of TiO2 as compared to 10Cu/TiO2 and 10Ni/TiO2. All these improved properties resulted in the bimetallic 5Cue5Ni/TiO2 having higher current density, photoconversion efficiency (4.43% at 0.109 V) compared to other photocatalysts.
Acknowledgment Authors wish to thank Universiti Teknologi PETRONAS and Ministry of Education (Higher Education Department) for the Exploratory Research Grant Scheme (No: ERGS/1/2012/TK05/ UTP/01/04).
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Please cite this article in press as: Mohamed NM, et al., Photoelectrochemical behavior of bimetallic CueNi and monometallic Cu, Ni doped TiO2 for hydrogen production, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.064