J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7528-8

Facile preparation of nanocrystalline ­TiO2 thin films using electrophoretic deposition for enhancing photoelectrochemical water splitting response Bao Lee Phoon1 · Guan‑Ting Pan3 · Thomas C.‑K. Yang3 · Kian Mun Lee1 · Chin Wei Lai1 · Joon Ching Juan1,2 

Received: 3 May 2017 / Accepted: 12 July 2017 © Springer Science+Business Media, LLC 2017

Abstract  Titanium oxide (­TiO2) nanocrystalline particles have been successfully deposited on fluorine-doped tin oxide (FTO) glass using an electrophoretic deposition (EPD) with a good uniformity. This study aims to optimize the heat treatment temperature, applied electric field, and deposition time to produce a good uniformity nanocrystalline ­TiO2 layer coating on FTO glass for better photoelectrochemical water splitting hydrogen generation performance under UV irradiation. In the present study, EPD technique allows controlling of T ­iO2 nanocrystalline layer characteristics on FTO glass, including smoothness and thickness. Characterisation like FESEM revealed that nanocrystalline ­TiO2 with a 14.6  μm thickness with good uniformity and minimal cracking surface exhibited the highest photocurrent density (2.12  mA/ cm2) among samples. Besides, Raman and XRD analyses showed that nanocrystalline ­TiO2 thin coating form successfully. Based on our experiments, we proposed that the optimum condition to form nanocrystalline T ­ iO2 thin film layer using EPD technique is 30 V, 60 s of deposition and post-annealing of the thin film at 400 °C based on the ­jp–V characteristic. Under illumination of 100  W UV light, the * Chin Wei Lai [email protected] * Joon Ching Juan [email protected] 1

Nanotechnology & Catalysis Research Centre (NANOCAT), Level 3, IPS Building, University of Malaya, 50603 Kuala Lumpur, Malaysia

2

School of Science, Monash University, Sunway Campus, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor Darul Ehsan, Malaysia

3

Department of Chemical Engineering, National Taipei University of Technology, Taipei City, Taiwan, ROC







photoconversion efficiency can be up to 2.49%. In addition, hydrogen production was tested with the production rate is up to 0.45 mL/cm2/h and the photoelectrode can be re-used and remained stable for five cycles continuously.

1 Introduction With ever increasing energy usage, the exploitation of renewable resources becomes urgent to alleviate the effects of rising energy requirement in the sustainable-energy societies. Efficient utilization of renewable resources is a significant scientific challenge in the twenty-first century. At present, most of the hydrogen is utilized by chemical and refinery industries [1]. In future, hydrogen could be used for generating power by fuelling gas turbines, fuel cells and combustion engines [2]. About 0.1 gigatonnes of hydrogen is produced commercially per year [3] and up to 95% is derived from fossil fuels. The majority of industries are using natural gas reforming and coal gasification process [4], which involve burning fossil fuels that emit harmful greenhouse gasses. Various initiatives have been made to seek for renewable energy system to produce hydrogen using electrical energy [5], thermal energy [6], photonic energy [7] and biochemical energy [8]. Among the viable renewable hydrogen production approaches, the use of photoelectrochemical (PEC) water splitting system is one of the most promising techniques with high potential in the hydrogen economy to assure sustainable supply of recyclable and clean hydrogen energy. PEC water splitting is a hybrid cell by integration of the photovoltaic system with an electrolyzer to generate clean and portable hydrogen energy carrier. The energy can be stored as fuel cell, which can then be efficiently converted into electrical energy and to be available at all

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times. The usage of titanium dioxide (­TiO2) as photoelectrode in photochemical water splitting was first successfully reported by Fujishima and Honda in [9]. TiO2 photocatalyst has emerged as leading candidate in variety of applications including purification of water [10], photocatalyst [11], and photovoltaic [12] because of its good light absorption ability, high corrosive resistance, high thermal stability and suitable band edge position for water splitting process [13]. Thus, pristine ­TiO2 is a good model photocatalyst to study photocatalytic reactions. There are some miscellaneous methods to prepare uniform thin film on FTO glass, such as paste coating [14], sputter deposition [15] and spin coating [16]. Each of these techniques has its own advantages and disadvantages. For example, paste coating has low production cost requires meticulous process to control the thickness and uniformity. Sputter deposition can produce a highly uniform thin film but the ion bombardment or UV generated by plasma may cause substrate damages and material degradation, especially for organic materials. Among those techniques, EPD technique is arguably the most feasible since the coating exhibits high compatibility ­TiO2 network compared to conventional paste coating. This compatibility of T ­ iO2 can directly influence the electron transport dynamics during PEC water splitting process [14]. Besides that, the thin film thickness able to control accurately through certain parameters such as deposition time. This technique has been introduced by Russian scientist Reuss noticed an electric field-induced movement of clay particles in water in 1808 [17]. The basic phenomenon of EPD is well known and has been subjected to extensive theoretical and experimental research. The EPD of ceramics was first studied in the 1980s by Hamaker [18]. This is a technique to process the material which charged T ­ iO2 nanoparticles in a suspension that contain electrolytes moving towards an oppositely charged electrode and are then deposited on a substrate by supplying direct current (d.c.) electric field [19]. In other words, EPD is a combination of electrophoresis Table 1  The different applications of EPD

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and deposition [20]. One of the major drawbacks is EPD can only undergo deposition on conductive surfaces such as metals and conductive glasses. Even so, it has the advantage of being low-cost, relatively fast, reproducible and potential for use in continuous processing. It is a technique in controlling the assembly of nanoparticles under room temperature and atmospheric pressure. To date, EPD can be prepared efficiently with uniform thickness, high smoothness and most importantly, it is reproducible. The applications of EPD for material processing are in extensive use in fabrication of variety materials as shown in Table 1. In 2014, Fakhouri et  al. [29] applied similar application by synthesizing T ­ iO2 thin films but produced only 567 μA/cm2 of photocurrent density. In 2015, Binetti et al. [30] prepared thin films using magnetron sputtering process and sintering process in 300 °C. The thin films are applied to PEC water splitting and achieved 0.99 mA/cm2 of photocurrent density under illumination. To the best of our knowledge, photocatalyst thin film is still limited in improving the PEC water splitting performance. Despite significant study on PEC water splitting response, the technique of utilizing T ­ iO2 nanocrystalline thin film on FTO glass via EPD technique to improve the PEC water splitting response is seldom discussed.

2 Experiment 2.1 Preparation of ­TiO2 thin film TiO2 (21  nm, Degussa P25) and fluorine-doped tin oxide glass (FTO, 7  Ω/cm2, Aldrich) were used as the substrate for the photo-electrodes. The FTO glass was cut into the desired dimension (1 × 2  cm). The electrophoretic suspension used in the deposition was composed of nanocrystalline ­TiO2 particles in isopropyl alcohol (IPA, 99.5%, Merck) containing 1­0−4  M Mg(NO3)2·6H2O (99.5%, Aldrich). Two FTO glass substrates were used as cathodic substrate and counter electrode, the distance between the

Material

Substrate

Application

Ref.

TiO2 (P-25) TiO2 (P-25) TiO2 (P-25) Cadmium sulphide Iron oxide δ-alumina and titania TiO2 Albumin coating

FTO glass ITO glass ITO-PET Aluminium plates Carbon plate Carbon fiber tows Stainless steel Stainless steel

In this study [21] [22] [23] [24] [25] [26] [27]

Nano-zirconia

AZ91D magnesium alloy

PEC water splitting Dye-sensitized solar cells Dye-sensitized solar cells Degradation of tetracycline antibiotic Tap water purification Corrosion and oxidation protection Antimicrobial Improving corrosion resistance and bioactivity of titanium implants Bio-corrosion control purposes

[28]

J Mater Sci: Mater Electron

mercury lamp (Sylvania) with 100  mW/cm2 was transmitted by a quartz glass as the UV lamp shone on the working electrode. The photocurrent density ­(jp) applied was plotted versus the potential (V). Hydrogen production was measured for 60 min and hydrogen gas was collected using the water displacement technique. Hydrogen gas was produced at counter electrode in the water splitting set up. The volume of hydrogen gas was measured by directly reading the variation of the electrolyte level in the burette at sequential times. The schematic representation of the PEC water splitting setup is shown in Fig. 2. Fig. 1  Schematic diagram of the experimental set-up for EPD

anode and cathode was fixed at 0.70  cm. Nanocrystalline ­TiO2 powder was deposited on the FTO glass using EPD under the following conditions: (1) at various applied electric field using a DC power supply and (2) nanocrystalline ­TiO2 suspension at room temperature. A schematic representation of the EPD setup is shown in Fig. 1. The deposited thin film was rinsed with distilled water and subsequently undergoes heat treatment at desired temperature for 2 h (10 °C/min). 2.2 Characterisation Field emission scanning electron microscopy (FE-SEM) analysis was carried out using a JEOL JSM-7600F FESEM to observe the morphologies and microstructures of the characteristics of the cross-section of the nanocryatlline ­TiO2 thin film. The crystalline phases were obtained by using an inVia Raman microscope (Renishaw) with laser (514 nm, 5 mW) focused onto the micro-sized spot (1 μm). The crystal structure of the nanocrystalline ­TiO2 thin film was analyzed by using Bruker D8 Advanced Diffractometer operate at 40  kV and 30  mA with Cu Kα radiation (λ = 0.15406  nm) for phase identification through a scanning range from 2θ = 20°–90° at a step size of 0.02°  s−1. Zeta potential measurements were carried out by Zetasizer Nano ZS90 (Malvern) to observe the colloidal stability of the suspension in a different concentrations of Mg(NO3)2·6H2O additive.

3 Result and discussion 3.1 Colloidal stability In order to maintain a uniform and high surface charge of the suspended particles, measurement of Zeta potential is essential in EPD process. The purpose of Zeta potential measurement in this study is to indicate the stability of nanocrystalline ­TiO2 as a suspension in IPA. Zeta potential has three roles in EPD (1) to stabilize the suspension by determining the intensity of repulsive interaction between particles, (2) to determine the direction and migration velocity of the particle during EPD, (3) to determine the green density of the deposits [31]. Zarbov et  al. [32] suggested additives can affect the ionic conductivity of suspension, which become a driving force and transfer particles to the electrode. In Fig. 3, the zeta potential value increased when the Mg salt concentration increased from ­ 10−6 to ­ 10−4  M. The maximum

2.3 Photocatalytic reaction The PEC measurement was performed using μAutolab III with three electrode configurations connected to potentiostat: working electrode (­ TiO2/FTO), platinum wire counter electrode, and Ag/AgCl reference electrode. For photocurrent measurements, the electrodes were immersed in the solution of 1  M KOH (pH 14). The ­TiO2/FTO electrode was scanned from −1.2 to +0.6  V (vs. Ag/AgCl electrode) at a rate of 0.1  V/s. A 100  W high-pressure

Fig. 2  Schematic diagram of the experimental set-up for PEC water splitting

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Fig. 3  Zeta potentials of nanocrystalline ­TiO2 as a function of concentration of Mg salt in the IPA

value of zeta potential is 32.2  mV by adding ­10−4  M of Mg(NO3)2·6H2O. Jeon et  al. [33], suggested this is because ­MgNO3+ would attach on the surface of T ­ iO2 nanoparticles and cause the suspension to have more positive charge and lead to high colloidal stability. However, further increment of salt concentration would lower down the zeta potential value because the ­TiO2 particle surface is saturated for ion adsorption and produce correspondingly high concentrations of counter ions in diffuse layer. Meanwhile, the electrical double layer from ­TiO2 nanoparticles structure is compressed and caused the van der Waals force to become predominant. The net force of nanoparticles is attractive and this is the reason that ­TiO2 particles undergo sedimentation. Zeta potential would affect the deposition rate and coagulation. This result is consistent with earlier reports [21, 22].

J Mater Sci: Mater Electron

Fig. 4  The ­jp–V curves of nanocrystalline T ­ iO2 thin film undergo heat treatment in different temperatures, a without calcinatin, b 200 °C, c 400 °C, d 600 °C

the coated T ­ iO2 thin films is to improve the necking of nanoparticles and this could affect the electron conveying ability of T ­ iO2 nanoparticles. When the temperature is increased to 600 °C, the reading dropped to 1.60  mA/ cm2. Referring Raman spectrum shown in Fig.  5, ­TiO2 that calcined under 600 °C shows rutile peaks at 446 and 612  cm−1. Yet, for as-annealing ­TiO2, 200, and 400 °C, the ­TiO2 shows signals at 197, 394, 516, and 637  cm−1, which are anatase peak [35]. This demonstrates that ­TiO2 undergo phase transformed at 600 °C and it was reported that anatase can be better photocatalytic activity because anatase possesses an indirect band gap while the rutile has a direct band gap. Photocatalysts that exhibit indirect

3.2 Parameters study 3.2.1 Heat treatment The purpose of heat treatment parameter in this study is to control the thin film quality after conducting the EPD technique. The effect of heat treatment temperature is performed in four different conditions, which are the asannealing sample (without calcination), 200, 400, and 600 °C of calcination. Figure  4 shows that when the asannealed ­TiO2 sample exhibits poor photocurrent density among samples (0.20  mA/cm2) whereas the sample that underwent heat treatment at 400 °C shows the highest photocurrent density (2.12 mA/cm2). This finding can be explained by post-annealing enhancement of the attachment between ­TiO2 and FTO substrate. It was reported earlier by Hsiao and Teng [34], the intention for sintering

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Fig. 5  Raman spectrum of electrophoretically deposited nanocrystalline ­TiO2 on FTO glass that undergo different temperature of heat treatment in the range of 190–700 cm−1

J Mater Sci: Mater Electron

band gap would have longer charge carrier life due to the minimum in the conduction band is away from the maximum in the valence band and this enables the excited electron to stabilize at the lower level in the conduction band [36]. According to Hamaker equation [Eq. (1)], changing the time and electric field affects the deposition yield.

dY = f 𝜇cES dt

(1)

which the yield is Y (g); the t (s); the electrophoretic mobility μ ­(m2/V/s); the electric field E (V/m); concentration of powder in suspension c (g/L); electrode surface area S ­(m2) and a dimensionless factor f which takes into account will be eventually deposited (f ≤ 1). The influence of applied voltage on the nanocrystalline ­TiO2 deposition is discussed in this part of the studies. Five different voltages are applied in between 10 and 50 V. In order to gain insight into the correlation between the surface morphology of the ­TiO2 thin film and the PEC water splitting properties, all samples are applied as the photoelectrode for their PEC water splitting performance evaluation. In Fig.  6, the amount of deposits increases in mass by increasing applied potential from 10 to 50  V. This is because when the higher electric field is applied, the more negative charge is available to attract particles towards cathode by electrostatic force. So, T ­ iO2 nanoparticles would deposit quickly when a higher voltage is supplied. There are two steps involved in the EPD process. First, charged particles in the suspension migrate towards electrode under electric field supply. Second, the particles deposit on the electrode to form a thin film [37]. In Fig. 7, The photocurrent density increases in between 10 and 30 V. The photocurrent density peaked (2.12  mA/cm2) when

Fig. 6  Deposition mass of T ­ iO2 nanocrystalline by EPD in different voltage

Fig. 7  The ­jp–V curves of nanocrystalline T ­ iO2 deposited in different voltages, a 10 V; b 20 V; c 30 V; d 40 V; e 50 V

30 V is applied for EPD. However, the photocurrent density fell to 1.94 and 1.57 mA/cm2 when the thin films deposited under 40 and 50  V of electric field respectively. This can be explained by the fast moving particles might have insufficient time to attach in a good position to form a compactly packed film. Thus, higher voltage deposition would cause the ­TiO2 nanoparticles deposit in a loose packing structure as illustarted in Fig.  8. Thin films formation on the electrode is a kinetic phenomenon, the packing behavior in the coating can be affected by the nanoparticles deposition rate [38]. Figure  9 illustrates the electron flows in both close and loose packing structure of thin film, these voids and holes gradually obstruct to the electron transport pathway. This justify the drop in the photocurrent density when 40 and 50  V of electric fields are applied for nanocrystalline ­TiO2 deposition, due to poor interparticle contact of the thin film. Moreover, this effect is also observed by Basu et  al. [38] in 2001 and they proposed the turburlence in ­TiO2 nanoparticles suspension may be caused by higher electric field and the coating may be interrupted by flows in the surrounding medium. In 2010, Liou et al. [14] reported that loose packing nanocrystalline ­TiO2 contains numerous voids and these voids are consequence of electron diffusion. We investigated the deposition time by EPD technique from 30 to 180 s in a fixed voltage of 30 V, and heat treatment of 400 °C. In this manner, deposition time could control the thickness of thin film significantly. Figure 10 shows the nanocrystalline ­TiO2 thin film thickness as well as the photocurrent density in different deposition time, respectively. The relationship of thin film thickness is directly proportional to the deposition time. The FE-SEM analysis by thin film cross section as shown in Fig. 11 in which the 30  s of deposition with the thin film thickness of 6.7  μm generates only 0.45  mA/cm2 of photocurrent density,

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Fig. 8  The FESEM analysis of T ­ iO2 arrangement

Fig. 9  Schematic showing the electron pathway in difference in packing density of thin film: a closely packed, b loosely packed

power output to the incoming light intensity that strikes the cell as shown in equation below Eq. (2).

Total power output − Electrical power output × 100% Light power input [( )] | | o − |Eapp | jp Erev | | = × 100 % Io (2)

𝜂=

Fig. 10  TiO2 film thickness and photocurrent density in different deposition time

whereas 60 s of deposition time gives the best result in photocurrent density with 14.7 μm of thin film thickness. This result is consistent with Liou et al. [14]. The photocurrent density dropped when further increment of the thickness of the thin film in extended the deposition time. We also determine the photocurrent efficiency, η to estimate the correlation of light absorption quantitatively. The conversion efficiency defined as the ratio of its electric

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where η is photocurrent efficiency (%); j­p is photocurrent density (mA/cm2), both j­p|Eapp| and ­jpEorev are total power output and electrical power input respectively; ­Io is the incident light power density (mW/cm2); ­Eorev is the standard reversible potential (1.23 V/SHE); E ­ app is E ­ mean − Evoc. ­Emean is the working electrode potential (vs. Ag/AgCl electrode) under illumination and E ­ voc is the open circuit potential (vs. Ag/AgCl electrode) of the working electrode. Figure  12 illustrates the photoconversion efficiency. We found the highest value 2.49% for 60 s of deposition whereas the lowest is about 0.56% for 30  s of deposition. This suggests that 60  s of deposition is optimum. The efficiency dropped after of 120  s, possibly due to relatively poor interconnection of ­TiO2, such as voids and cracks which could affect electron transferring of the thin film. On the other hand, the uniformity and thickness of nanocrystalline ­TiO2 film affects photoconversion

J Mater Sci: Mater Electron Fig. 11  FE-SEM micrographs of electrophoretically deposited nanocrystalline ­TiO2 on FTO glass in cross section view, a 30 s; b 60 s; c 120 s; d 180 s

decreases when nanocrytalline ­TiO2 thin films deposited in 120 and 180 s. Furthermore, the XRD patterns of the thin films are shown in Fig. 14. Pattern (a) shows the pristine nanocrystalline ­TiO2 peaks and pattern (b) shows the S ­ nO2 structure of FTO conducting glass with peaks at 2θ = 26°, 34°, and 52° (JCPDS No. 01-077-0452). Pattern (c), (d), (e), and (f) represents nanocrystalline T ­ iO2 thin film in different deposition times. Based on the pattern (c) to (f), we found ­TiO2 peaks at 2θ = 25°, 38° and 48° for anatase structure (JCPDS No. 01-071-1166) as well as 2θ = 27°, 54°, and 69° for rutile structure (JCPDS No. 00-012-2176). According to these XRD patterns, there were no impurity phases obtained and this indicates that T ­ iO2 successfully formed on FTO glass. Fig. 12  Calculated photoconversion efficiency for the T ­ iO2 film in different deposition time, a 30 s; b 60 s; c 120 s; d 180 s

3.3 PEC water splitting

efficiency of the PEC water splitting, non-uniform thin film will cause charge recombination and the conversion efficiency will be reduces [39]. We also found more cracks on the thin film surface when the thin film is getting thicker as shown in Fig. 13. Apart from that, cracks formation may be caused by solvent selection for electrophoretic suspension. In this case, IPA is an organic solvent which has low boiling point and high evaporation rate at room temperature. Cracks normally form before sintering, and these cracks would affect the adherence between ­TiO2 thin film and FTO substrate [21]. Therefore, it is not surprising that thin film with 180 s of deposition time has the most visible cracks and non-uniform surface. Since cracks influenced the adhesion between photocatalyst and FTO substrate, photocurrent density

Based on the parameters mentioned previously, we found the optimum condition for the nanocrystalline ­TiO2 thin film formation by using EPD technique was found to be 30  V, 60  s of deposition and post-annealing of the thin film at 400 °C. Hydrogen production is one of the useful applications of PEC water splitting. We calculated the hydrogen generation rate as per Eq. (3). The evolution rate of hydrogen gas generated from the water splitting process is evaluated as data collected from Fig.  15. We recorded the readings every 10 min and it showed a linear relation. Photocatalyst typically can be reused to minimize processing cost. We repeated several cycles to test its recyclability. As shown in Fig. 15, 0.45 mL of hydrogen gas evolved in 1  h for the first cycle. The hydrogen gas evolution is maintained at approximately 0.40 mL/cm2/h in the following cycles and the thin film can be reused up to five times.

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Fig. 13  FE-SEM micrographs of electrophoretically deposited nanocrystalline ­TiO2 on FTO glass, a empty FTO glass; b nanocrystalline ­TiO2 powder; c 30 s; d 60 s; e 120 s; f 180 s

However, we did observe a slight drop in hydrogen production (~0.05 mL, 10%). We suspected some ­TiO2 was peeled off after immersed in high alkalinity electrolyte for some time. We will conduct further studies to extend the reusability of the thin film. Hydrogen generation rate =

4 Conclusion

Fig. 14  XRD patterns for samples: (a) empty FTO glass; (b) ­TiO2 powder; (c) 30 s; (d) 60 s; (e) 120 s; (f) 180 s

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Volume of hydrogen gas generated (mL) Area of sample (cm2 ) × Time (h)

(3)

In this study, we demonstrated that nanocrystalline ­TiO2 is successfully coated on thin film via EPD technique and its capability to attain good performance in PEC water splitting. The results suggest that the photocatalyst post-annealing temperature, the electric field applied and deposition time can significantly affect the thickness and the packing of the ­TiO2 thin film that subsequently can influence the PEC water splitting performance. The electric field applied

J Mater Sci: Mater Electron Fig. 15  The hydrogen evolution of optimised ­TiO2 thin film under UV illumination at 0.6 V vs. Ag/AgCl in several cycles

and deposition time influences the amount of thickness, smoothness, and packing which is clearly determined by FE-SEM analysis. We demonstrated that appropriate control on the quality of the thin film by avoiding voids and cracks can enhance photocurrent density and photoconversion efficiency. We found the optimum condition to synthesize good uniformity T ­ iO2 nanocrystalline layer on FTO glass to be 30 V, 60 s of deposition and post-annealing of the thin film at 400 °C. We determined the highest photocurrent density to be 2.12  mA/cm2 and achieved highest photoconversion efficiency of 2.49%. We achieved hydrogen production rate of 0.45  mL/cm2/h under illumination of UV light and the photoelectrode can be reused for five times. With these findings, we conclude, electrophoretically deposited T ­ iO2 nanoparticles have the potential in PEC water splitting by controlling the thickness and packing structure. Acknowledgements  This work is supported by the Ministry of Science, Technology, and Innovation (MOSTI-Science Fund, 3-01-03SF1032), Postgraduate Research Grant (PPP) Grant (PG287-2016A), Transdisciplinary Research Grant Scheme (TR001B-2015A)  and SATU Grant (RU018B-2016).

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