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Photoelectrochemical characteristics of CuO films with different electrodeposition time
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Photoelectrochemical characteristics of CuO films with different electrodeposition time Asad Mahmood*, Fatih Tezcan, Gulfeza Kardas‚** C¸ukurova University, Science and Letters Faculty, Chemistry Department, 01330, Adana, Turkey
article info
abstract
Article history:
This paper explores the effect of electrodeposition time on microstructure, optical, and
Received 1 February 2017
photoelectrochemical properties of CuO films. CuO films were electrochemically deposited
Received in revised form
on tin-doped indium oxide (ITO) substrates using a Cu2O electrodeposition method fol-
8 May 2017
lowed by annealing at 550 C for 2 h. The electrochemical deposition was carried out at
Accepted 1 June 2017
different times (300, 600, 1200, and 1800 s) utilizing a copper sulfate pentahydrate and lactic
Available online xxx
acid solution. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to perform phase and microstructure analysis. Photoluminescence (PL)
Keywords:
studies confirmed an increase in emission intensities with increasing deposition time. In
CuO
addition, a significant change was observed in photoelectrochemical properties of the film
Photoelectrodes
by varying the deposition time. The film deposited for 600 s showed a high photocurrent
Electrodeposition
density of 0.55 mA cm2 at 0.5 V. Moreover, a lowest resistance from electrochemical
FESEM
impedance spectroscopy (EIS) was recorded for the films electrochemically deposited for
Photocurrent
600 s.
Water splitting
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photoelectrochemical water splitting is an effective way to produce and store hydrogen as a clean energy source and to overcome the energy crisis and environmental hazards such as global warming and acid rain those are associated with the hydrogen production from the sources such as coal and natural gas [1,2]. For this purpose a range of photoelectrode materials has been reported, i.e., Cu2O, CuO, p-doped Si, CuRhO2, Cu2ZnSnS4, Fe3O4 and CuBi2O4 [3,4]. Another technique is electrolysis of alkaline water, which has been investigated in depth for the hydrogen evaluation reactions (HER) [5,6]. However, adapting the laboratory practices into industry for the commercial production of hydrogen is still hindered by
several physical and chemical barriers, such as stability and economical feasibility. To address these issues, attempts have been reported to increase the stability of photocathodes in photocatalytic water splitting reactions and utilization of earth-abundant materials (copper based oxides, Fe2O3, ZnO, TiO2, BiVO4) instead of using expensive and heavy metals such as platinum, iridium, and ruthenium [7,8]. Among these, owing to its narrow band gap (1.21e2.1 eV), CuO is considered as one of the most promising semiconductor material for applications in catalysis, batteries, gas sensing, and photoelectrochemical cells (PECs) [9,10]. The CuO films and powders exhibiting different shapes and sizes have been processed by various synthetic procedures [11e13], such as thermal decomposition of Cu foil, hydrothermal method, sol-gel method, solvothermal method, self-catalytic growth, and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A. Mahmood), [email protected] (G. Kardas‚). http://dx.doi.org/10.1016/j.ijhydene.2017.06.003 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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electrodeposition [14e18], and reference therein. Chiang et al. [19] synthesized CuO nanoparticles utilizing flame spray pyrolysis. The processing parameters such as precursor concentration, gas flow, and temperature were studied in detail. The variation in precursor concentration from 0.5 to 35% w/w resulted in a corresponding increase in particles diameter from 7 ± 2 to 20 ± 11 nm. In addition, the particles grown in the low gas flow showed a significant increase in diameter, which was associated with more retention time in high-temperature zone. Shaislamov et al. [20] developed CuO/ZnO photoelectrode utilizing a two-step synthesis route. The films morphology and PEC performance were studied in depth. In the initial step, CuO nanorods were developed by thermal oxidation of the already electrodeposited CuO films. Next, ZnO nanobranches were grown on the surface of the initially deposited CuO films using a hydrothermal synthesis route. The durability test showed an increase in stability of the electrode up to 90%. Similar studies suggest that microstructure plays an important role in materials' performance. For example, a high hydrogen storage capacity of 220 and 180 mAh/g has been reported for Cu(OH)2 and CuO nanoribbons [21]. Among these, electrodeposition has been considered as the best technique to develop homogeneous thin films with precise control over film thickness and microstructure. Further, the method is economical due to the use of inexpensive precursor sources and simple experimental setup [14,22,23]. The microstructure of film, grain morphology, and surface topography plays an important role in the photoinduced processes [22,24]. The surface features affect the interaction of light with film surface and consequently, the electron transport mechanism; therefore, it is important to optimize the microstructure-property relationship in these optoelectronic devices. The properties of CuO have been extensively reported; however, no attempt has been reported to investigate the effect of electrodeposition time on the CuO film microstructure and the PEC performance to our best knowledge. In this paper, we have studied the effect of electrodeposition time on surface morphology of ITO/CuO films and the corresponding effect on the photoelectrochemical properties. For efficient absorption of light, an optimum value of film thickness is required for better PEC performance. The XRD and FESEM were used for the phase and microstructure investigation, respectively. Furthermore, the optical properties and photocatalytic response were studied to investigate the CuO films' electrodeposition time on the photocurrent in water splitting reactions.
Experimental procedure Copper sulfate pentahydrate (CuSO4$5H2O; Aldrich), lactic acid (C3H6O3; Aldrich) and deionized water were used as the starting materials and solvent. The CuO films were deposited on ITO substrates by using a galvanostatic deposition method. Prior to deposition, ITO substrates were washed with a hot detergent solution, acetone, and ethanol for 10 min each. Then, the substrates were dried with a nitrogen blow. The precursor solution was prepared by using a 0.5 M CuSO4$5H2O and 1.3 M lactic acid. During the electrodeposition, the
temperature was kept constant at 60 C, while the pH was maintained around 10e12 by using a 5.0 M NaOH solution. The electrodeposition was carried out for 300, 600, 1200 and 1800 s which corresponds to notation S1, S2, S3, and S4, respectively, by applying a current density of 0.3 mA cm2. A 50.0 mL of fresh precursor solution was used for each experiment. After deposition, the Cu2O films were washed with distilled water and subsequently dried in an oven at 60 C for 24 h. The dried Cu2O films were annealed at 550 C for 2 h in air in order to achieve CuO films. Thin film deposition was carried out using Gamry Instrument (Galvaniostatic-potentiostatic; Gamry Interface 1000). Thermo-FTIR spectrometer (Model: smart UR diamond attenuated total reflection (ATR)) was used to determine the phase and the nature of the atomic bond in the films. UVevisible NIR (Model: Cary 7000 Universal Measurement Spectrophotometer (UMS)) was used to study the absorption behavior of the thin films. The surface morphology was investigated by using a FESEM (Model: Zeiss/Supra 55 VP). The phase of CuO films was studied by using X-ray diffraction (XRD, Model: Rigaku Miniflex 600) with a step size of 5 min1. The PEC characteristics of films were measured by using a CHI analyzer (Model: CHI 660D electrochemical). The scan rate linear sweep voltammetry (LSV) of 5 mV s1 was applied between 0 and 0.5 V range. The EIS measurements were performed in the frequency range of 105 Hz and 103 Hz at open circuit potential and with an amplitude of 5 mV. A 0.5 M Na2SO4 solution was used to study the photocatalytic activity of CuO films on water splitting. A solar simulator (SunliteTM Solar Simulators; M-SLSS) was used to illuminate the samples. The PEC performance was measured in three electrode system using Ag/AgCl (3.0 M KCl solution) as reference electrode, a platinum sheet (2 cm2) as counter electrode and CuO film samples as the working electrode.
Results and discussions Fig. 1(a) shows XRD profiles for the ITO/CuO films annealed in air at 550 C for 2 h. The peak profiles were matched to the monoclinic CuO phase (JCPDS card: 048-1548), where the diffraction peaks at 2q ¼ 32.48, 35.74, 38.96, 48.75, 53.02 and 58.64 were indexed to (110), (111), (111), (202), (020) and (202) crystal planes, respectively [23,25]. No impurities such as Cu2O and Cu3O4 were observed, which further confirmed that the Cu2O was converted to CuO on ITO substrates. The relative intensities were observed to increase with increasing deposition time, which might be due to enhanced crystallinity related to the high deposition time [26]. Fig. 1(b) shows FT-IR spectrum of the ITO/CuO films annealed at 550 C for 2 h. The peaks in the wave number range from 420 to 610 cm1, which can be resolved in three modes, i.e., Au and Bu modes, and other Bu modes of the CuO. The strong peaks around 597 cm1 and 699 cm1 are the characteristic Cu (II)eO stretching vibration peaks. The peaks around 538 cm1 and 607 cm1 were assigned to CueO stretching vibration [14,27]. Similar peaks were observed for all the samples. Fig. 2 shows PL spectra of the ITO/CuO films annealed at 550 C for 2 h. So far the origin of luminescence in CuO has not been fully established. The near band edge emission for the
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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 7 ) 1 e8
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Fig. 1 e (a) XRD and (b) FTIR for the ITO/CuO films annealed at 550 C for 2 h in air. The (*) represent peaks associated with the substrates. where a is absorption coefficient, hv is the photo energy, A is constant, and r represents an indirect transition. The value of absorption can be calculated by Eq. (2). a ¼ 2; 303
Fig. 2 e Photoluminescence spectra for the ITO/CuO films annealed at 550 C for 2 h in air.
CuO has been reported at different wavelengths (l) by various researchers, i.e., 365, 300, 395 and 467 nm [28] and references therein. This characteristic peak of the band emission was observed around 386 nm in the current study. All the samples showed a second emission peak at 780 nm, which were associated with the near band emission. The photoluminescence peak for the Cu2O around 626 nm was not observed, which further suggested absence of the Cu2O phases [29]. The relative intensities of the emission peaks were observed to increase for the samples S1 to S4 with increasing electrodeposition time. UV-NIR absorption measurements were performed to study the absorption mechanism of electrodes (Fig. 3(a) and (b)). The optical band gap (Eg) was calculated using Eq. (1) [30]:
(ahu)r ¼ A(hn Eg)
(1)
absorbance d
(2)
where d is thickness of the CuO film on ITO. The band gap values were calculated by extrapolation of the curve to the xaxis. Sample S4 showed a high absorbance value in contrast to the rest of the samples, which is associated with the high thickness value. The calculated band values were 1.41 eV, 1.40 eV, 1.43 eV and 1.44 eV for S1, S2, S3 and S4, respectively. The lowest band gap value of S2 suggests more active sites on the electrode surface which can efficiently utilize light for water splitting reaction. Fig. 4 shows surface morphology of the ITO/CuO films annealed at 550 C for 2 h. It has been established that Cu2O deposits in the form of pyramid structures in the given experimental conditions. At high temperature (500 C), Cu2O is converted to CuO and the color of film changes from reddish brown to black [31]. All the samples showed a heterogeneous grain growth. No considerable defects were observed on the film's surface, which is generally associated with the solution based processed films. Sample S1 showed an empty surface area among the particles due to an incomplete surface coverage related with the limited deposition time. The surface was covered completely by increasing the deposition time for samples S2, S3, and S4. In addition, a small particle size was observed for the film deposited for 300 s. The particle size was observed to increase with increasing deposition time such as 600, 1200, and 1800 s. This behavior might be due to an increase in volume diffusion and coalescence, which resulted in bigger particles and more compactness. The corresponding average thickness for samples S1, S2, S3, and S4 were measured to be 233, 627, 816, and 980 nm, respectively (Fig. 5). Initially, the rate of deposition is high as evident from the respective thicknesses of sample S1 and S2; however, the films' thickness did not increase significantly with increasing deposition time (S3, S4). This may be associated with the decreasing electrolyte concentration. The first step in electrodeposition is the formation of Cu(I)[C3H6O3] in the
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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Fig. 3 e (a) UV-NIR absorption spectra and (b) Tauc plot of the ITO/CuO films annealed at 550 C for 2 h in air.
Fig. 4 e FESEM surface micrograph for the ITO/CuO films (S1, S2, S3, S4) annealed at 550 C for 2 h in air. The average grain size diameter has been presented by the digital scale from the instrument software. precursor solution by Eq. (3), which in the basic media, Cu(I) [C3H6O3] is further converted to Cu2O film structures in the second step (Eq. (4)). The second step is critical in determining the size of the pyramid shaped structures, which are formed as result of the mass migration of the Cu(I)[C3H6O3] from the solution boundary to the electrode surface. Furthermore, the average particle (agglomerates) size increased with increasing deposition time, which might be due to high mass migration with increasing deposition time. CuO is the most stable phase compared to Cu2O, which is achieved at high temperatures (Eq. (5)) [32].
Cu(II) [C3H6O3]2 þ e / Cu(I) [C3H6O3] þ [C3H5O3]
(3)
2Cu(I) [C3H6O3] þ 2OH / Cu2O þ H2O þ 2[C3H5O3] 550 C
Cu2 O ! CuO
(4) (5)
Fig. 6(a) shows Nyquist plots of the ITO/CuO films at room temperature in 0.5 M Na2SO4 solution as electrolytes under illumination. All the samples showed a single semicircular arc except sample S2 (two semicircles), which can be used to study the resistive and capacitive nature of materials. The nature of the semicircular arcs was also confirmed from the phase angle diagrams (Fig. 6(c)). The behavior of the semicircle can be used to interpret the mobility of electrons in the bulk of material as well as at the interface of electrode and electrolyte [24, 33]. Sample S1 showed the highest resistance while sample S2 showed a low resistance. From these results, it can
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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 7 ) 1 e8
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Fig. 5 e FESEM surface micrograph of the cross sections for the ITO/CuO films (S1, S2, S3, S4) annealed at 550 C for 2 h in air. The vertical line bar demonstrate the average thickness of the films.
Fig. 6 e Complex impedance analysis of the ITO/CuO films in 0.5 Na2SO4 solutions at room temperature; (a) Nyquist plots for the samples S1, S2, S3, S4, (b) and (c) shows jZj vs. logf and Phase angle vs. frequency for the ITO/CuO films, respectively.
be inferred that the electron mobility at the electrodeelectrolyte interface increases with increasing deposition time [34, 35]. This decrease in charge transfer resistance might be due to an increase in the number of carrier species accumulated at the electrode-electrolyte interface with increasing deposition time or thickness. In addition, the lowest resistance observed in the sample S2 might be associated with the optimum particle size and film thickness. Generally, electrons from the bulk of particles move to the surface of grains, which further contribute to the reaction; thus, bigger particles
suggest that more time is required for the electrons to come to the grain surface in contrast to the smaller particles. Moreover, bigger particles might exhibit a high number of defects such as oxygen vacancies, which act as electron sinks and are involved in electron hoping mechanism, which results in low conductivity in contrast to sample S2. Also, the two semicircles observed in sample S2 indicates the catalytic surface for hydrogen evaluation reaction. This behavior was further studied by the jZj vs. log frequency (log f) plots in Fig. 6(b) Here, jZj is the complex number which has both the real (Z0 ) and
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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imaginary (Z00 ) components of impedance. The sample S2 showed a low magnitude for the jZj compared to the rest of the samples. Fig. 6(c) presents the phase angle vs. log f plot. All the samples showed a broad phase angle around the frequency range from 104 to 103 Hz. Generally, the peak in the lowfrequency regime belongs to the electron diffusion within the electrolytes while the peaks at high frequency are associated with the resistance in semiconductors [24]. Sample S2 showed resolved peaks, which clearly identified the electron transfer phenomena dependence on both within the electrode and electrolyte interface and diffusion in electrolytes. The phase angle was observed to decrease from 80 to 60 . The less negative phase angles for the CuO thin films compared to the perfect capacitor (90 ) confirmed a low resistance to charge mobility in the vicinity of the semiconductor and at electrolyte and electrode interface [36]. Fig. 7 shows the Mott-Schottky plots for the CuO films electrochemically deposited on the ITO substrates and subsequently annealed at 550 C for 2 h in air. The flat band potential (Vfb) was measured using Mott-Schottky equation in order to understand the intrinsic properties of CuO films, which were conducted utilizing 5 mV AC amplitude at 500 Hz frequency. The calculated capacitance of CuO at electrode/ electrolyte interface at various potentials depend on the following equation: 1 2 ¼ q 20 2s ND C2
V Vfb
kB T q
(6)
where εs is the dielectric constant of CuO, ε0 is the permittivity of free space, q is an electronic charge, T is the absolute temperature, and kB is the Boltzman's constant [37]. The curve is extrapolation on the x-axis to define Vfb values of electrodes. The Mott-Schottky plot of CuO in 0.5 M Na2(SO4) demonstrate a negative slope, providing proof of CuO as a p-type semiconductor. Vfb values of þ0.6634 V, þ0.536 V, þ0.685 V, and þ0.611 V were obtained at different deposition times including 300 s, 600 s, 1200 s and 1800 s, respectively [38, 39]. Fig. 8 shows photocurrent density as a function of voltage for the ITO/CuO films. The photocatalytic performances of
Fig. 7 e Mott-Schottky plots of ITO/CuO films electrochemically deposited for different times.
Fig. 8 e Variation of current density vs. reference electrode (Ag/AgCl) for the ITO/CuO films in light. The inset figure presents current density vs. AgAgCl reference electrode in the dark.
the samples were measured in a three-electrode cell system. The measurements were conducted in the light of an artificial solar illumination with a voltage that varied from 0 to 0.5 V. The photocurrent density was observed to increase for all the samples in light. A high photocurrent density in light compared to the dark confirmed the well photocatalytic activity for water splitting as a photocathode. The photocurrent density is associated with the proton reduction and photogeneration of electrons. Further, it also confirmed the ptype semiconductive nature of the CuO [40]. The sample S1 showed a low photocurrent response compared to rest of the samples. Generally, the photogenerated electrons are responsible for the hydrogen production which must be sufficiently available to reduce hydrogen. However, the electrons transfer mechanism in a semiconductor is a complex phenomenon. In the current experimental scenario, it is assumed that the microstructure played a major role in variation of the photocurrent response in CuO films exhibiting different thicknesses. An increase in thickness and average grain size might be responsible for high absorption due to increasing number of CuO particles which provides a greater number of mobile electrons and surface roughness, respectively. However, in order to reduce hydrogen, the electrons must be available on the grain surface, which depends on the movement of electrons from the bulk of grains to the surface of grains. The current study suggests that the larger grains exhibit some defects such as oxygen vacancies and porosity which diminished the flow of electrons in contrast to the CuO films deposited at 600 s. In addition, the possibility of oxygen vacancies in the larger grains is high which might act as sinks for electrons and hence decrease the mobility of electrons. A high photocurrent density of 0.55 mA cm2 was recorded for the sample S2 at 0.5 V. In addition, sample S2 showed a 91.9% increase in the PEC efficiency in light compared to the dark. These IeV results are in good agreement with the EIS results.
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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Conclusion Copper oxide films were successfully electrodeposited on ITO substrates by varying the deposition time from 300 to 1800 s and subsequently annealed at 550 C for 2 h. The XRD pattern confirmed the monoclinic phase for CuO films. FTIR studies showed Au and Bu modes, and other Bu modes for the CuO films. The average grain growth was observed to increase with increasing deposition time. The optical band gap was observed to increase with increasing deposition time while the sample S2 showed the lowest band gap value (Eg ¼ 1.4 eV). In addition, the Mott Schottky results suggested the p-type semiconductor nature of the CuO films irrespective of their deposition time. According to EIS results, sample S2 exhibited the lowest total resistance. Moreover, S2 showed the highest current density (0.55 mA cm2) at 0.5 V at the LSV measurements, which further confirmed the EIS results. In the current study, 600 s (S2) proved to be an optimum electrodeposition time for the better photocurrent density of the ITO/CuO films as a photocathode for water splitting reaction via solar light irradiation.
Acknowledgement The authors acknowledge the financial support by the Scientific and Technological Research Council of Turkey (TUBITAK Project No: 116C035) under the 2236-Co-Funded Brain Circulation Scheme.
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[34] Masudy-Panah S, Siavash Moakhar R, Chua CS, Tan HR, Wong TI, Chi D, et al. Nanocrystal engineering of sputtergrown CuO photocathode for visible-light-driven electrochemical water splitting. ACS Appl Mater Interfaces 2016;8:1206e13. [35] Xiang JY, Tu JP, Qiao YQ, Wang XL, Zhong J, Zhang D, et al. Electrochemical impedance analysis of a hierarchical CuO electrode composed of self-assembled nanoplates. J Phys Chem C 2011;115:2505e13. [36] Messaoudi O, assaker IB, Gannouni M, Souissi A, Makhlouf H, Bardaoui A, et al. Structural, morphological and electrical characteristics of electrodeposited Cu2O: effect of deposition time. Appl Surf Sci 2016;366:383e8. [37] Windisch CF, Exarhos GJ. MotteSchottky analysis of thin ZnO films. J Vac Sci Technol A Vac Surf Films 2000;18:1677e80. [38] Yang L, Zhang M, Zhu K, Lv J, He G, Sun Z. Electrodeposition of flake-like Cu2O on vertically aligned two-dimensional TiO2 nanosheet array films for enhanced photoelectrochemical properties. Appl Surf Sci 2017;391:353e9. [39] Huang M-C, Wang T, Chang W-S, Lin J-C, Wu C-C, Chen IC, et al. Temperature dependence on p-Cu2O thin film electrochemically deposited onto copper substrate. Appl Surf Sci 2014;301:369e77. [40] Al-Ghamdi AA, Khedr MH, Ansari M Shahnawaze, Hasan PMZ, Abdel-wahab MS, Farghali AA. RF sputtered CuO thin films: structural, optical and photo-catalytic behavior. Phys E Low Dimens Syst Nanostruct 2016;81:83e90.
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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Photoelectrochemical characteristics of CuO films with different electrodeposition time Asad Mahmood*, Fatih Tezcan, Gulfeza Kardas‚** C¸ukurova University, Science and Letters Faculty, Chemistry Department, 01330, Adana, Turkey
article info
abstract
Article history:
This paper explores the effect of electrodeposition time on microstructure, optical, and
Received 1 February 2017
photoelectrochemical properties of CuO films. CuO films were electrochemically deposited
Received in revised form
on tin-doped indium oxide (ITO) substrates using a Cu2O electrodeposition method fol-
8 May 2017
lowed by annealing at 550 C for 2 h. The electrochemical deposition was carried out at
Accepted 1 June 2017
different times (300, 600, 1200, and 1800 s) utilizing a copper sulfate pentahydrate and lactic
Available online xxx
acid solution. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to perform phase and microstructure analysis. Photoluminescence (PL)
Keywords:
studies confirmed an increase in emission intensities with increasing deposition time. In
CuO
addition, a significant change was observed in photoelectrochemical properties of the film
Photoelectrodes
by varying the deposition time. The film deposited for 600 s showed a high photocurrent
Electrodeposition
density of 0.55 mA cm2 at 0.5 V. Moreover, a lowest resistance from electrochemical
FESEM
impedance spectroscopy (EIS) was recorded for the films electrochemically deposited for
Photocurrent
600 s.
Water splitting
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photoelectrochemical water splitting is an effective way to produce and store hydrogen as a clean energy source and to overcome the energy crisis and environmental hazards such as global warming and acid rain those are associated with the hydrogen production from the sources such as coal and natural gas [1,2]. For this purpose a range of photoelectrode materials has been reported, i.e., Cu2O, CuO, p-doped Si, CuRhO2, Cu2ZnSnS4, Fe3O4 and CuBi2O4 [3,4]. Another technique is electrolysis of alkaline water, which has been investigated in depth for the hydrogen evaluation reactions (HER) [5,6]. However, adapting the laboratory practices into industry for the commercial production of hydrogen is still hindered by
several physical and chemical barriers, such as stability and economical feasibility. To address these issues, attempts have been reported to increase the stability of photocathodes in photocatalytic water splitting reactions and utilization of earth-abundant materials (copper based oxides, Fe2O3, ZnO, TiO2, BiVO4) instead of using expensive and heavy metals such as platinum, iridium, and ruthenium [7,8]. Among these, owing to its narrow band gap (1.21e2.1 eV), CuO is considered as one of the most promising semiconductor material for applications in catalysis, batteries, gas sensing, and photoelectrochemical cells (PECs) [9,10]. The CuO films and powders exhibiting different shapes and sizes have been processed by various synthetic procedures [11e13], such as thermal decomposition of Cu foil, hydrothermal method, sol-gel method, solvothermal method, self-catalytic growth, and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A. Mahmood), [email protected] (G. Kardas‚). http://dx.doi.org/10.1016/j.ijhydene.2017.06.003 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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electrodeposition [14e18], and reference therein. Chiang et al. [19] synthesized CuO nanoparticles utilizing flame spray pyrolysis. The processing parameters such as precursor concentration, gas flow, and temperature were studied in detail. The variation in precursor concentration from 0.5 to 35% w/w resulted in a corresponding increase in particles diameter from 7 ± 2 to 20 ± 11 nm. In addition, the particles grown in the low gas flow showed a significant increase in diameter, which was associated with more retention time in high-temperature zone. Shaislamov et al. [20] developed CuO/ZnO photoelectrode utilizing a two-step synthesis route. The films morphology and PEC performance were studied in depth. In the initial step, CuO nanorods were developed by thermal oxidation of the already electrodeposited CuO films. Next, ZnO nanobranches were grown on the surface of the initially deposited CuO films using a hydrothermal synthesis route. The durability test showed an increase in stability of the electrode up to 90%. Similar studies suggest that microstructure plays an important role in materials' performance. For example, a high hydrogen storage capacity of 220 and 180 mAh/g has been reported for Cu(OH)2 and CuO nanoribbons [21]. Among these, electrodeposition has been considered as the best technique to develop homogeneous thin films with precise control over film thickness and microstructure. Further, the method is economical due to the use of inexpensive precursor sources and simple experimental setup [14,22,23]. The microstructure of film, grain morphology, and surface topography plays an important role in the photoinduced processes [22,24]. The surface features affect the interaction of light with film surface and consequently, the electron transport mechanism; therefore, it is important to optimize the microstructure-property relationship in these optoelectronic devices. The properties of CuO have been extensively reported; however, no attempt has been reported to investigate the effect of electrodeposition time on the CuO film microstructure and the PEC performance to our best knowledge. In this paper, we have studied the effect of electrodeposition time on surface morphology of ITO/CuO films and the corresponding effect on the photoelectrochemical properties. For efficient absorption of light, an optimum value of film thickness is required for better PEC performance. The XRD and FESEM were used for the phase and microstructure investigation, respectively. Furthermore, the optical properties and photocatalytic response were studied to investigate the CuO films' electrodeposition time on the photocurrent in water splitting reactions.
Experimental procedure Copper sulfate pentahydrate (CuSO4$5H2O; Aldrich), lactic acid (C3H6O3; Aldrich) and deionized water were used as the starting materials and solvent. The CuO films were deposited on ITO substrates by using a galvanostatic deposition method. Prior to deposition, ITO substrates were washed with a hot detergent solution, acetone, and ethanol for 10 min each. Then, the substrates were dried with a nitrogen blow. The precursor solution was prepared by using a 0.5 M CuSO4$5H2O and 1.3 M lactic acid. During the electrodeposition, the
temperature was kept constant at 60 C, while the pH was maintained around 10e12 by using a 5.0 M NaOH solution. The electrodeposition was carried out for 300, 600, 1200 and 1800 s which corresponds to notation S1, S2, S3, and S4, respectively, by applying a current density of 0.3 mA cm2. A 50.0 mL of fresh precursor solution was used for each experiment. After deposition, the Cu2O films were washed with distilled water and subsequently dried in an oven at 60 C for 24 h. The dried Cu2O films were annealed at 550 C for 2 h in air in order to achieve CuO films. Thin film deposition was carried out using Gamry Instrument (Galvaniostatic-potentiostatic; Gamry Interface 1000). Thermo-FTIR spectrometer (Model: smart UR diamond attenuated total reflection (ATR)) was used to determine the phase and the nature of the atomic bond in the films. UVevisible NIR (Model: Cary 7000 Universal Measurement Spectrophotometer (UMS)) was used to study the absorption behavior of the thin films. The surface morphology was investigated by using a FESEM (Model: Zeiss/Supra 55 VP). The phase of CuO films was studied by using X-ray diffraction (XRD, Model: Rigaku Miniflex 600) with a step size of 5 min1. The PEC characteristics of films were measured by using a CHI analyzer (Model: CHI 660D electrochemical). The scan rate linear sweep voltammetry (LSV) of 5 mV s1 was applied between 0 and 0.5 V range. The EIS measurements were performed in the frequency range of 105 Hz and 103 Hz at open circuit potential and with an amplitude of 5 mV. A 0.5 M Na2SO4 solution was used to study the photocatalytic activity of CuO films on water splitting. A solar simulator (SunliteTM Solar Simulators; M-SLSS) was used to illuminate the samples. The PEC performance was measured in three electrode system using Ag/AgCl (3.0 M KCl solution) as reference electrode, a platinum sheet (2 cm2) as counter electrode and CuO film samples as the working electrode.
Results and discussions Fig. 1(a) shows XRD profiles for the ITO/CuO films annealed in air at 550 C for 2 h. The peak profiles were matched to the monoclinic CuO phase (JCPDS card: 048-1548), where the diffraction peaks at 2q ¼ 32.48, 35.74, 38.96, 48.75, 53.02 and 58.64 were indexed to (110), (111), (111), (202), (020) and (202) crystal planes, respectively [23,25]. No impurities such as Cu2O and Cu3O4 were observed, which further confirmed that the Cu2O was converted to CuO on ITO substrates. The relative intensities were observed to increase with increasing deposition time, which might be due to enhanced crystallinity related to the high deposition time [26]. Fig. 1(b) shows FT-IR spectrum of the ITO/CuO films annealed at 550 C for 2 h. The peaks in the wave number range from 420 to 610 cm1, which can be resolved in three modes, i.e., Au and Bu modes, and other Bu modes of the CuO. The strong peaks around 597 cm1 and 699 cm1 are the characteristic Cu (II)eO stretching vibration peaks. The peaks around 538 cm1 and 607 cm1 were assigned to CueO stretching vibration [14,27]. Similar peaks were observed for all the samples. Fig. 2 shows PL spectra of the ITO/CuO films annealed at 550 C for 2 h. So far the origin of luminescence in CuO has not been fully established. The near band edge emission for the
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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Fig. 1 e (a) XRD and (b) FTIR for the ITO/CuO films annealed at 550 C for 2 h in air. The (*) represent peaks associated with the substrates. where a is absorption coefficient, hv is the photo energy, A is constant, and r represents an indirect transition. The value of absorption can be calculated by Eq. (2). a ¼ 2; 303
Fig. 2 e Photoluminescence spectra for the ITO/CuO films annealed at 550 C for 2 h in air.
CuO has been reported at different wavelengths (l) by various researchers, i.e., 365, 300, 395 and 467 nm [28] and references therein. This characteristic peak of the band emission was observed around 386 nm in the current study. All the samples showed a second emission peak at 780 nm, which were associated with the near band emission. The photoluminescence peak for the Cu2O around 626 nm was not observed, which further suggested absence of the Cu2O phases [29]. The relative intensities of the emission peaks were observed to increase for the samples S1 to S4 with increasing electrodeposition time. UV-NIR absorption measurements were performed to study the absorption mechanism of electrodes (Fig. 3(a) and (b)). The optical band gap (Eg) was calculated using Eq. (1) [30]:
(ahu)r ¼ A(hn Eg)
(1)
absorbance d
(2)
where d is thickness of the CuO film on ITO. The band gap values were calculated by extrapolation of the curve to the xaxis. Sample S4 showed a high absorbance value in contrast to the rest of the samples, which is associated with the high thickness value. The calculated band values were 1.41 eV, 1.40 eV, 1.43 eV and 1.44 eV for S1, S2, S3 and S4, respectively. The lowest band gap value of S2 suggests more active sites on the electrode surface which can efficiently utilize light for water splitting reaction. Fig. 4 shows surface morphology of the ITO/CuO films annealed at 550 C for 2 h. It has been established that Cu2O deposits in the form of pyramid structures in the given experimental conditions. At high temperature (500 C), Cu2O is converted to CuO and the color of film changes from reddish brown to black [31]. All the samples showed a heterogeneous grain growth. No considerable defects were observed on the film's surface, which is generally associated with the solution based processed films. Sample S1 showed an empty surface area among the particles due to an incomplete surface coverage related with the limited deposition time. The surface was covered completely by increasing the deposition time for samples S2, S3, and S4. In addition, a small particle size was observed for the film deposited for 300 s. The particle size was observed to increase with increasing deposition time such as 600, 1200, and 1800 s. This behavior might be due to an increase in volume diffusion and coalescence, which resulted in bigger particles and more compactness. The corresponding average thickness for samples S1, S2, S3, and S4 were measured to be 233, 627, 816, and 980 nm, respectively (Fig. 5). Initially, the rate of deposition is high as evident from the respective thicknesses of sample S1 and S2; however, the films' thickness did not increase significantly with increasing deposition time (S3, S4). This may be associated with the decreasing electrolyte concentration. The first step in electrodeposition is the formation of Cu(I)[C3H6O3] in the
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Fig. 3 e (a) UV-NIR absorption spectra and (b) Tauc plot of the ITO/CuO films annealed at 550 C for 2 h in air.
Fig. 4 e FESEM surface micrograph for the ITO/CuO films (S1, S2, S3, S4) annealed at 550 C for 2 h in air. The average grain size diameter has been presented by the digital scale from the instrument software. precursor solution by Eq. (3), which in the basic media, Cu(I) [C3H6O3] is further converted to Cu2O film structures in the second step (Eq. (4)). The second step is critical in determining the size of the pyramid shaped structures, which are formed as result of the mass migration of the Cu(I)[C3H6O3] from the solution boundary to the electrode surface. Furthermore, the average particle (agglomerates) size increased with increasing deposition time, which might be due to high mass migration with increasing deposition time. CuO is the most stable phase compared to Cu2O, which is achieved at high temperatures (Eq. (5)) [32].
Cu(II) [C3H6O3]2 þ e / Cu(I) [C3H6O3] þ [C3H5O3]
(3)
2Cu(I) [C3H6O3] þ 2OH / Cu2O þ H2O þ 2[C3H5O3] 550 C
Cu2 O ! CuO
(4) (5)
Fig. 6(a) shows Nyquist plots of the ITO/CuO films at room temperature in 0.5 M Na2SO4 solution as electrolytes under illumination. All the samples showed a single semicircular arc except sample S2 (two semicircles), which can be used to study the resistive and capacitive nature of materials. The nature of the semicircular arcs was also confirmed from the phase angle diagrams (Fig. 6(c)). The behavior of the semicircle can be used to interpret the mobility of electrons in the bulk of material as well as at the interface of electrode and electrolyte [24, 33]. Sample S1 showed the highest resistance while sample S2 showed a low resistance. From these results, it can
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Fig. 5 e FESEM surface micrograph of the cross sections for the ITO/CuO films (S1, S2, S3, S4) annealed at 550 C for 2 h in air. The vertical line bar demonstrate the average thickness of the films.
Fig. 6 e Complex impedance analysis of the ITO/CuO films in 0.5 Na2SO4 solutions at room temperature; (a) Nyquist plots for the samples S1, S2, S3, S4, (b) and (c) shows jZj vs. logf and Phase angle vs. frequency for the ITO/CuO films, respectively.
be inferred that the electron mobility at the electrodeelectrolyte interface increases with increasing deposition time [34, 35]. This decrease in charge transfer resistance might be due to an increase in the number of carrier species accumulated at the electrode-electrolyte interface with increasing deposition time or thickness. In addition, the lowest resistance observed in the sample S2 might be associated with the optimum particle size and film thickness. Generally, electrons from the bulk of particles move to the surface of grains, which further contribute to the reaction; thus, bigger particles
suggest that more time is required for the electrons to come to the grain surface in contrast to the smaller particles. Moreover, bigger particles might exhibit a high number of defects such as oxygen vacancies, which act as electron sinks and are involved in electron hoping mechanism, which results in low conductivity in contrast to sample S2. Also, the two semicircles observed in sample S2 indicates the catalytic surface for hydrogen evaluation reaction. This behavior was further studied by the jZj vs. log frequency (log f) plots in Fig. 6(b) Here, jZj is the complex number which has both the real (Z0 ) and
Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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imaginary (Z00 ) components of impedance. The sample S2 showed a low magnitude for the jZj compared to the rest of the samples. Fig. 6(c) presents the phase angle vs. log f plot. All the samples showed a broad phase angle around the frequency range from 104 to 103 Hz. Generally, the peak in the lowfrequency regime belongs to the electron diffusion within the electrolytes while the peaks at high frequency are associated with the resistance in semiconductors [24]. Sample S2 showed resolved peaks, which clearly identified the electron transfer phenomena dependence on both within the electrode and electrolyte interface and diffusion in electrolytes. The phase angle was observed to decrease from 80 to 60 . The less negative phase angles for the CuO thin films compared to the perfect capacitor (90 ) confirmed a low resistance to charge mobility in the vicinity of the semiconductor and at electrolyte and electrode interface [36]. Fig. 7 shows the Mott-Schottky plots for the CuO films electrochemically deposited on the ITO substrates and subsequently annealed at 550 C for 2 h in air. The flat band potential (Vfb) was measured using Mott-Schottky equation in order to understand the intrinsic properties of CuO films, which were conducted utilizing 5 mV AC amplitude at 500 Hz frequency. The calculated capacitance of CuO at electrode/ electrolyte interface at various potentials depend on the following equation: 1 2 ¼ q 20 2s ND C2
V Vfb
kB T q
(6)
where εs is the dielectric constant of CuO, ε0 is the permittivity of free space, q is an electronic charge, T is the absolute temperature, and kB is the Boltzman's constant [37]. The curve is extrapolation on the x-axis to define Vfb values of electrodes. The Mott-Schottky plot of CuO in 0.5 M Na2(SO4) demonstrate a negative slope, providing proof of CuO as a p-type semiconductor. Vfb values of þ0.6634 V, þ0.536 V, þ0.685 V, and þ0.611 V were obtained at different deposition times including 300 s, 600 s, 1200 s and 1800 s, respectively [38, 39]. Fig. 8 shows photocurrent density as a function of voltage for the ITO/CuO films. The photocatalytic performances of
Fig. 7 e Mott-Schottky plots of ITO/CuO films electrochemically deposited for different times.
Fig. 8 e Variation of current density vs. reference electrode (Ag/AgCl) for the ITO/CuO films in light. The inset figure presents current density vs. AgAgCl reference electrode in the dark.
the samples were measured in a three-electrode cell system. The measurements were conducted in the light of an artificial solar illumination with a voltage that varied from 0 to 0.5 V. The photocurrent density was observed to increase for all the samples in light. A high photocurrent density in light compared to the dark confirmed the well photocatalytic activity for water splitting as a photocathode. The photocurrent density is associated with the proton reduction and photogeneration of electrons. Further, it also confirmed the ptype semiconductive nature of the CuO [40]. The sample S1 showed a low photocurrent response compared to rest of the samples. Generally, the photogenerated electrons are responsible for the hydrogen production which must be sufficiently available to reduce hydrogen. However, the electrons transfer mechanism in a semiconductor is a complex phenomenon. In the current experimental scenario, it is assumed that the microstructure played a major role in variation of the photocurrent response in CuO films exhibiting different thicknesses. An increase in thickness and average grain size might be responsible for high absorption due to increasing number of CuO particles which provides a greater number of mobile electrons and surface roughness, respectively. However, in order to reduce hydrogen, the electrons must be available on the grain surface, which depends on the movement of electrons from the bulk of grains to the surface of grains. The current study suggests that the larger grains exhibit some defects such as oxygen vacancies and porosity which diminished the flow of electrons in contrast to the CuO films deposited at 600 s. In addition, the possibility of oxygen vacancies in the larger grains is high which might act as sinks for electrons and hence decrease the mobility of electrons. A high photocurrent density of 0.55 mA cm2 was recorded for the sample S2 at 0.5 V. In addition, sample S2 showed a 91.9% increase in the PEC efficiency in light compared to the dark. These IeV results are in good agreement with the EIS results.
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Conclusion Copper oxide films were successfully electrodeposited on ITO substrates by varying the deposition time from 300 to 1800 s and subsequently annealed at 550 C for 2 h. The XRD pattern confirmed the monoclinic phase for CuO films. FTIR studies showed Au and Bu modes, and other Bu modes for the CuO films. The average grain growth was observed to increase with increasing deposition time. The optical band gap was observed to increase with increasing deposition time while the sample S2 showed the lowest band gap value (Eg ¼ 1.4 eV). In addition, the Mott Schottky results suggested the p-type semiconductor nature of the CuO films irrespective of their deposition time. According to EIS results, sample S2 exhibited the lowest total resistance. Moreover, S2 showed the highest current density (0.55 mA cm2) at 0.5 V at the LSV measurements, which further confirmed the EIS results. In the current study, 600 s (S2) proved to be an optimum electrodeposition time for the better photocurrent density of the ITO/CuO films as a photocathode for water splitting reaction via solar light irradiation.
Acknowledgement The authors acknowledge the financial support by the Scientific and Technological Research Council of Turkey (TUBITAK Project No: 116C035) under the 2236-Co-Funded Brain Circulation Scheme.
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Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003
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Please cite this article in press as: Mahmood A, et al., Photoelectrochemical characteristics of CuO films with different electrodeposition time, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.003