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Optimized photoactive coatings prepared with functionalized TiO2
international journal of hydrogen energy xxx (xxxx) xxx
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Optimized photoactive coatings prepared with functionalized TiO2 Yaowapa Treekamol a,b,*, Mauricio Schieda c, Iris Herrmann-Geppert d, Thomas Klassen c a
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002, Thailand Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), NANOTEC -KKU RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen University, Khon Kaen, 40002, Thailand c Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502, Geesthacht, Germany d Leibniz Institute for Crystal Growth, Forschungsverbund Berlin e.V., Rudower Chaussee 17, 12489, Berlin, Germany b
highlights Silane functionalized TiO2 electrode for solar driven water splitting. Simple spin coating technique used to prepare electrodes. Photoelectrochemical performance is three times better than the pristine one. Decrease in flat band potential indicating lower charge recombination loss.
article info
abstract
Article history:
Functionalization of TiO2 nanoparticles with silane coupling agents was investigated
Received 12 July 2019
aimed at low-temperature photoelectrode manufacturing for solar driven water splitting
Received in revised form
application. Different silanes were grafted on the surface of TiO2 in toluene solvent under
6 October 2019
mild condition. The electrodes were prepared with spin coating by dispersing functional-
Accepted 11 October 2019
ized particles in DMAc onto FTO glass and dried under vacuum atmosphere at low tem-
Available online xxx
perature. UVeVis spectroscopy of TiO2 powder and its electrodes was studied, and it was found that the spectrum of the modified TiO2 slightly shifted to higher wavelengths. The
Keywords:
electrode prepared with functionalized TiO2 showed photocurrent density of up to
Water splitting
0.14 mA cm2 compared to 0.04 mAcm2 for pristine TiO2 at 1.23 V, in the water oxidation
Functionalized TiO2
reaction. The increase in photocurrent density was due to better binding of the TiO2 par-
Hydrogen production
ticles to the substrate resulting in better charge collection observed under SEM. To enhance
Photoelectrochemical cell
the photoelectrochemical efficiency, heat treatment was performed and 300 C was found to be the best heat treatment temperature. The incident photon to current efficiency measurement exhibited an external quantum efficiency up to 4.9% for this heat-treated electrode. Mott-Schottky was plotted to examine the flat band potential. The result showed that the modification resulted in a decrease in the flat band potential suggesting that the charge recombination loss is lower compared to neat TiO2 electrode. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002 Thailand. E-mail address: [email protected] (Y. Treekamol). https://doi.org/10.1016/j.ijhydene.2019.10.085 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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international journal of hydrogen energy xxx (xxxx) xxx
Introduction With energy consumption continually on the increase and availability of conventional power sources on a downward trajectory, scientists have been looking for ways to garner energy from renewable sources. Alternative sustainable systems such as wind, solar and hydrogen energy have been subjected to intensive study in order to solve our energy crisis [1,2]. Solar energy is one of the most effective sources owing to its most abundant availability. A dropping cost has brought about a rise in the use of solar electricity in recent years [3]. However, the natural solar source is not always reliable being weather-dependent. Of late, hydrogen technology is considered as the high efficiency energy storage system due to its high energy density of 39 kWh/kg which is more than 100 times higher than batteries system [4e7]. This zero-emission technology is applicable in fuel cells for many other uses for example domestics and portable electric devices [8]. Nowadays hydrogen is produced from two main pathways, a reformation of natural fuel sources and an electrolysis of water by outsource electricity. The direct conversion of solar energy into hydrogen via photoelectrochemical (PEC) cell is a potential system with an advantage in that it can compensate the stability of solar energy using storable hydrogen [9e15]. The PEC is one of the most effective systems with maximum efficiency reportedly as high as 30% [16]. However, this technology is still challenging and there remains a lack of research. In principle, one of the main components of the PEC cell is a photoanode that most often made from semiconductor which converts incident photons to electronehole pairs. These photogenerated electrons reduce water to form hydrogen gas at a counter-electrode, while holes oxidize water to form oxygen gas at the semiconductor/electrolyte interface [17]. One of the main challenges of this system is finding a suitable catalyst for the electrode. A number of catalysts have been reported [18], most of which show either poor stability or rely on rare or expensive elements [19,20]. A small group of earth-abundant semiconducting oxide photocatalysts show significant stability in electrochemical conditions. These catalysts including TiO2, WO3 and Fe2O3, have been extensively studied [21e25]. Unfortunately, they are characterized by relatively low activity towards the water splitting reaction. Their catalytic properties need to be improved so that they can be deployed for practical hydrogen generation on a large scale. Several research groups demonstrated the viability of organic-inorganic hybrid and surface functionalized inorganic materials with their electrochemical property improvement relevance to photoelectrochemical applications [26e33]. Bansal and coworkers reported that with surface functionalization the alkyl groups bonded covalently to the Si surface were able to prevent the oxidation of Si and showed excellent electrochemical properties when used as photoelectrodes [34e36]. Adsorption of cations or anions at the solid/liquid interface has been shown to shift semiconductor band edges [37,38]. Similarly, Hila et al. [39] were able to control the interfacial kinetics of n-GaAs/liquid junctions by chemically anchoring positively charged species (Mn (III)-porpyrin complexes) to the semiconductor surface.
Increasing a number of positive charges per molecule shifted the band edges towards positive potentials. The onset potential, photovoltage, and conversion efficiency were also enhanced. Another significant factor that affects a conversion efficiency of PEC cell is charge transport either across semiconductor/semiconductor, semiconductor/electrolyte or semiconductor/back contact interface [40]. This factor might be controlled by its catalyst property or fabrication process. Conducting polymers adsorbed on the catalyst surface have been shown to improve the open circuit potential and stability of semiconductors at the solid liquid interface [41]. The composite and doping approach have been widely studied in order to increase quantum efficiency of electrodes [51e56]. The composite films based on P25/N-doped and B-doped hollow carbon spheres were employed. It was showed that the efficiency was increased up to four times in case of B-doped one [51]. The transition-metal doping treatment was investigated and found that the photocurrent density of FeeTiO2 significantly improved with bias voltage reached five times higher than that of the undoped TiO2 [52]. Surface treatment by thermal or plasma energy is also used to improve catalytic property in the oxidation reaction. The WO3 films was treated with hydrogen plasma, which created substoichiometric WO3x catalyst films with increasing photocurrent density and stability of the photoanode [42]. Treekamol et al. [43] reported that by modifying the surface of catalyst, an agglomeration of the particle was reduced resulting in better packing of the particles on the back contact and to the particles themselves. An approach of surface modification by silane coupling agent is not only simple but also requires low temperature. Moreover, the fact that silane can be further reacted with other active molecules thereby achieving higher efficiency makes this approach interesting in this application. In this work surface functionalization of TiO2 nanoparticles with silane coupling agents was investigated aimed at low-temperature electrode fabrication using 3-chloropropyl trimethylsilane and 3-aminopropyltriethoxysilane. Spin coating was used to enable homogeneous film deposition of semiconductor particles. The prepared electrodes were studied by UVeVis spectroscopy and SEM, and their photoelectrochemical efficiency was measured in water oxidation reaction. Moreover, to enhance their photocatalytic property, heat treatment was employed at different temperatures. Finally, the flat band potential of the electrodes was investigated by Mott-Schottky plot in basic electrolyte.
Experimental Chemical and materials 3-chloropropyltrimethylsilane (CPTMS) and 3-aminopropyltriethoxysilane (APTES) obtained from Gelest, Inc. were used as surface modifying agents in this study. Titanium (IV) oxide nanopowder (P25), with a particle size 21 nm supplied from Aldrich, fluorine-doped tin oxide lime glass, dimethylacetamide, potassium hydroxide, sulfuric acid and toluene were used as received.
Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Functionalization of TiO2 TiO2 nanoparticles (P25) were functionalized with silanes. Firstly, 0.250 g of the TiO2 catalyst was stirred for 5 min in a 1 mM potassium hydroxide solution to introduce hydroxyl groups onto its surface. After that the particles were filtered and rinsed with ultrapure H2O to remove residual basic solution. They were then dried overnight in vacuum oven at 110 C. Later, the alkaline-treated TiO2 was dispersed in 20 mL toluene for 15 min in ultrasonic disruptor and 5 mmol of silanes was added in the dispersion. The reaction temperature was set to 60 C for 72 h. Afterward, the functionalized particle was filtered and washed with toluene to eliminate the unreacted silane. Finally, the modified particles were dried in vacuum oven for 24 h at 80 C. The TiO2 nanoparticles which were functionalized with CPTMS and APTES were identified as Cl-P25 and AP-P25 respectively.
Electrode preparation and photoelectrochemical characterization After TiO2 nanoparticles were functionalized, a suspension of nanoparticle in DMAc was spin coated onto a FTO substrate. The glass was cleaned with isopropanol in sonication bath for 15 min twice, then rinsed and left at room temperature to dry out the solvent. Different thicknesses were obtained by varying the solid content of the suspension. The electrodes were dried in 80 C vacuum oven overnight to remove the solvent. An average height difference of coated and non-coated area was measured by using 3D Laser scanning microscope VKX200 Keyence (surface mode with 50x optical lens). The photoelectrodes with 2.54 cm2 active area were electrochemically characterized in an aqueous solution of 0.5 M H2SO4 together with a silver chloride reference electrode and a platinum ring as counter electrode in the Zahner cell. The electrolyte solution was purged with nitrogen to eliminate oxygen before testing. Current-voltage curves were collected by the Zahner PP211 potentiostat under dark, illumination via the solar simulator (AM 1.5 G), and chopped illumination using Rigol DG 1022 function/arbitrary waveform generator. The CV curves were measured from open circuit potential to 1.8 V using scan rate of 25 mV/s.
Results and discussion Functionalization of TiO2 was carried out basically through silanation reaction under a mild condition as described in Fig. 1. The hydroxyl groups on TiO2 surface were displaced by alkoxy groups of the silane creating Si-O covalent bond giving methyl alcohol as a byproduct. After that the functionalized titanium oxide particle was characterized by FTIR and TGA techniques (see Fig. 2). FTIR spectra of functionalized TiO2 show additional peaks at 2850 and 2975 cm1 corresponding to the symmetric and asymmetric vibrations of eCH2e and eCH3 groups, respectively. Additionally, the peaks at 1130 cm1 and 1040 cm1 are due to the vibration absorption of Si-O-C and Si-O-Si,
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respectively [44]. Regarding the thermal gravity analysis, both types of modified TiO2 show two decomposition steps concerning the 1st derivative analysis. The first one at around 100 C which relates to the water adsorbed on the surface of the particles. The second peak was found at 250 and 400 C in the cases of Cl-P25 and AP-P25, respectively. These indicate the decomposition temperature of silanes. The thickness of TiO2 layers was measured using laser scanning microscope (LSM) determining a difference in average height of coated and non-coated area. Fig. 3 shows the LSM images of TiO2 and modified TiO2 coated electrodes with different thicknesses. The electrode with functionalized catalyst obviously showed an improvement of particle distribution resulting in homogeneous films on FTO substrate compared to that of neat TiO2. As seen in the top view SEM image (Fig. 4.), due to the smaller aggregation obtained by functionalization, the better contact between the particles and uneven surface substrate was achieved. This explained that modified TiO2 provided better photocurrent due to an improvement of the TiO2 particles to the substrate binding causing higher electrons collection from back contact. However, it showed clearly that the electrode prepared by neat TiO2 had non-covered area which also resulted in poor photocatalytic activity.
UVevis spectroscopy Fig. 5 showed UVeVis spectra of P25 and Cl-P25. It is seen that the spectral onset red-shifts, due to aggregation [45]. The UVeVis spectra of pristine and modified TiO2 with different coating thickness on FTO substrate were shown in Fig. 6. It was found that the maximum absorption peaks of all electrodes were in the range of 320e370 nm. However due to the inhomogeneous TiO2 film, the different energy absorption of different thickness layer was observed as a result of a FTO exposure. In case of Cl-P25, the homogenous film showed rather an identical absorption energy with different film thickness.
Photoelectrochemical characteristics The photoelectrochemical (PEC) water oxidation reaction of the electrodes in a half cell was measured in a 0.5 M H2SO4 aqueous solution as an electrolyte. There is no significant redox response was observed in a range of open circuit voltage to 1.8 V vs. NHE on cyclic voltammogram in the dark condition. However, in case of illumination (solar simulator AM 1.5 G, 100 mWcm2) a photoanodic current was observed. As shown in Fig. 7 it was found that a photocurrent under chopped illumination of Cl-P25 were significantly greater than that of P25 which was 0.15 mA at 1.23 V(NHE) for the oxygen evolution reaction. The chemical composition of silane on PEC property of electrodes was also studied, CPTMS and APTES were used. The CPTMS was found to be the optimal coupling agent for TiO2. It might be that the CPTMS is the most active silane due to chloro group is least bulky so titanium oxide can be fully covered compared to other silanes as shown in TGA
Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Fig. 1 e Functionalization of titanium dioxide by the silane coupling agent.
Fig. 2 e FTIR spectra (left) and TGA thermograph (right) of P25, Cl-P25 and AP-P25.
Fig. 3 e Laser-scanning micrographs of electrodes with P25 (left) and Cl-P25 (right) on FTO glass.
Fig. 4 e SEM micrographs of the prepared electrodes on the FTO substrate, P25 (left) and Cl-P25 (right). Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Fig. 5 e UVeVis spectra of P25 and Cl-P25 measured in powder form. thermogram. We also studied the effect of thickness on a photocurrent (Fig. 8.), in case of P25 electrodes the photocurrent trend to decrease when the thickness is higher (from 0.2 to 1.5 mm). The optimum thickness of neat P25 electrode was around 250 nm while that of Cl-P25 was 700 nm due to it
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compromised an effect of light harvesting and film resistance [46]. Moreover the functionalized TiO2 electrodes showed stable photocurrents under illumination @ 0.7 V (NHE) up to at least 1 h under continuous illumination. In order to enhance the photoelectrochemical efficiency of the electrodes, a heat treatment applied with different temperatures (80, 300 and 500 C), the CV and an incident photonto-current efficiency (IPCE) curves were shown in Fig. 9. As 300 C is applied at atmospheric condition, the photocurrent obtained from this electrode is higher than that of one without heat treatment. The current was significantly improved from 0.3 mA to 0.4 mA @ 1.0 V it can be explain by better the contact between particle and substrate resulting in higher currents collected at the back contact. However when the temperature was increased up to 500 C, the photocurrent decreased due to the tranforming from anatase to rutile phase of titaniumoxide [47]. An external quantum efficiency (EQE) is equal to IPCE [50] which is about 4.9% for the electrode with 300 C heat treatment. This is comparable to the work from Ranganathan which also used P25, however, in their study the composite films based on P25/hollow carbon spheres were further used. The IPCE was increased to 12% and 20% in case of N-doped and B-doped hollow carbon spheres respectively [51].
Fig. 6 e UVeVis spectra of electrodes prepared with P25 and Cl-functionalized TiO2 on FTO glass substrate.
Fig. 7 e Voltammetry under chopped illumination (AM1.5 G) of photoanodes prepared with P25 (left) and Cl-P25 (right) with optimum thicknesses measured in H2SO4 0.5 M, 10 mV s¡2. Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Fig. 8 e Right: Stability measurement under illumination (AM1.5G) of P25 and Cl-P25 at 0.7 V vs NHE with optimum thicknesses measured in H2SO4 0.5 M, 10 mV s¡2. Left: effect of thickness on the photocurrent of electrodes prepared with pristine and Cl-P25.
Fig. 9 e CV and IPCE of electrode prepared by Cl-P25 (700 nm) with heat treatment temperature: 80 C, 300 C, 500 C for 6 h.
1 2 ¼ C2 εε0 A2 eND V Vfb keB TÞ where C and A are the interfacial capacitance and area of an electrode, respectively. ND is the number of donors, V the applied voltage, kB Boltzmann’s constant, T the absolute temperature, and e the electronic charge. The flat band potential (Vfb) can be obtained by extrapolating the intercepts of the Y axis from a plot of 1/C2 against E as shown in Fig. 10. Electrochemical impedance spectroscopy as a function of applied voltage was employed to examine the flat band potential. As in a previous work [49] the flat band potential had been found to vary with pH, thus we kept the pH constant using a pH 14 electrolyte (1 M KOH). The negative flat band potential of this neat and modified TiO2 system was found to be in the range of 0.7e0.9 vs NHE as shown in Table 1. The result showed that with different catalyst thicknesses, thicker TiO2 films negatively shifted the flat band potential
0,03
P25_A_1KHz_L
(C/A)-2/ 1012 F-2cm4
The flat band potential (Vfb) which affects the charge transfer and the recombination probability [48] of the electrodes can be determined from the Mott-Schottky plot according to the following equation;
0,02
0,01
0,00 -1,0
-0,5
0,0
0,5
E/V Fig. 10 e MotteSchottky plots for P25 at 1 KHz in 1 M KOH.
value. According to the result from Radecka et al., [21] doping TiO2 with 7.6 at % Cr increased the absolute value of Vf while it shortened the recombination time of generated electron and hole. Regarding our chlorosilane functionalized electrode, the
Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Table 1 e A flat band potential of P25 and Cl- P25 with different thicknesses. Electrode P25 P25 Cl-P25 Cl-P25 Cl-P25
Thickness (nm)
Vfb @ 1 KHz
Vfb @ 10 KHz
249 593 594 688 1354
0.859 0.876 0.715 0.761 0.871
0.855 e 0.722 0.796 0.8215
absolute flat band potential values significantly decreased implying that the charge recombination loss is lower compared to neat TiO2 electrode. This explanation agrees with our photoelectrochemical investigation that the Cl-P25 showed higher photocurrent than pristine catalyst.
Conclusion The catalyst material and fabrication for photoanode in solardriven photoelectrochemical water splitting cell were investigated. Surface functionalization by silane coupling agents was applied to TiO2 photocatalyst in order to assist particle distribution in spin coating process. It can also significantly improve contact between the particles and the electrically conductive substrate, contributing to better electron collection efficiency. Additionally, the flat band potential of these modified electrodes was also found to be lower meaning that charge recombination loss is minor compared to neat TiO2 electrode. These three reasons contribute to an improvement of PEC performance which is a photocurrent up to three times. Not only did they provide providing higher photocurrent, but they also showed good stability under continued illumination. Furthermore, an efficiency of the modified electrodes could be enhanced by sintering resulting in better a quantum efficiency. Although, the photocurrent obtained in this approach is still relatively limited due to low temperature fabrication process, the silanated P25 has high potential to be further modified with other active species such as electron donating groups or dyes to enhance their efficiency, which is another advantage of this work.
Acknowledgement The authors gratefully acknowledge the funding provided by the German Federal Ministry of Education and Research, junior research group project (BMBF Nachwuchsgruppe) “FocusH2”. This study has been supported in part by research and technology transfer affairs (KKUS60_001). Additionally, this work has been partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research and Innovation and Khon Kaen University, Thailand. YT acknowledges the Development and Promotion of Science and Technology Talent Project (DPST) for the financial support of this project (010/2559). YT also acknowledges Assoc. Prof. Dr. Rapee Utke from Institute of Science, Suranaree University of Technology for providing an academic mentoring.
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Optimized photoactive coatings prepared with functionalized TiO2 Yaowapa Treekamol a,b,*, Mauricio Schieda c, Iris Herrmann-Geppert d, Thomas Klassen c a
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002, Thailand Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), NANOTEC -KKU RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen University, Khon Kaen, 40002, Thailand c Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502, Geesthacht, Germany d Leibniz Institute for Crystal Growth, Forschungsverbund Berlin e.V., Rudower Chaussee 17, 12489, Berlin, Germany b
highlights Silane functionalized TiO2 electrode for solar driven water splitting. Simple spin coating technique used to prepare electrodes. Photoelectrochemical performance is three times better than the pristine one. Decrease in flat band potential indicating lower charge recombination loss.
article info
abstract
Article history:
Functionalization of TiO2 nanoparticles with silane coupling agents was investigated
Received 12 July 2019
aimed at low-temperature photoelectrode manufacturing for solar driven water splitting
Received in revised form
application. Different silanes were grafted on the surface of TiO2 in toluene solvent under
6 October 2019
mild condition. The electrodes were prepared with spin coating by dispersing functional-
Accepted 11 October 2019
ized particles in DMAc onto FTO glass and dried under vacuum atmosphere at low tem-
Available online xxx
perature. UVeVis spectroscopy of TiO2 powder and its electrodes was studied, and it was found that the spectrum of the modified TiO2 slightly shifted to higher wavelengths. The
Keywords:
electrode prepared with functionalized TiO2 showed photocurrent density of up to
Water splitting
0.14 mA cm2 compared to 0.04 mAcm2 for pristine TiO2 at 1.23 V, in the water oxidation
Functionalized TiO2
reaction. The increase in photocurrent density was due to better binding of the TiO2 par-
Hydrogen production
ticles to the substrate resulting in better charge collection observed under SEM. To enhance
Photoelectrochemical cell
the photoelectrochemical efficiency, heat treatment was performed and 300 C was found to be the best heat treatment temperature. The incident photon to current efficiency measurement exhibited an external quantum efficiency up to 4.9% for this heat-treated electrode. Mott-Schottky was plotted to examine the flat band potential. The result showed that the modification resulted in a decrease in the flat band potential suggesting that the charge recombination loss is lower compared to neat TiO2 electrode. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002 Thailand. E-mail address: [email protected] (Y. Treekamol). https://doi.org/10.1016/j.ijhydene.2019.10.085 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Introduction With energy consumption continually on the increase and availability of conventional power sources on a downward trajectory, scientists have been looking for ways to garner energy from renewable sources. Alternative sustainable systems such as wind, solar and hydrogen energy have been subjected to intensive study in order to solve our energy crisis [1,2]. Solar energy is one of the most effective sources owing to its most abundant availability. A dropping cost has brought about a rise in the use of solar electricity in recent years [3]. However, the natural solar source is not always reliable being weather-dependent. Of late, hydrogen technology is considered as the high efficiency energy storage system due to its high energy density of 39 kWh/kg which is more than 100 times higher than batteries system [4e7]. This zero-emission technology is applicable in fuel cells for many other uses for example domestics and portable electric devices [8]. Nowadays hydrogen is produced from two main pathways, a reformation of natural fuel sources and an electrolysis of water by outsource electricity. The direct conversion of solar energy into hydrogen via photoelectrochemical (PEC) cell is a potential system with an advantage in that it can compensate the stability of solar energy using storable hydrogen [9e15]. The PEC is one of the most effective systems with maximum efficiency reportedly as high as 30% [16]. However, this technology is still challenging and there remains a lack of research. In principle, one of the main components of the PEC cell is a photoanode that most often made from semiconductor which converts incident photons to electronehole pairs. These photogenerated electrons reduce water to form hydrogen gas at a counter-electrode, while holes oxidize water to form oxygen gas at the semiconductor/electrolyte interface [17]. One of the main challenges of this system is finding a suitable catalyst for the electrode. A number of catalysts have been reported [18], most of which show either poor stability or rely on rare or expensive elements [19,20]. A small group of earth-abundant semiconducting oxide photocatalysts show significant stability in electrochemical conditions. These catalysts including TiO2, WO3 and Fe2O3, have been extensively studied [21e25]. Unfortunately, they are characterized by relatively low activity towards the water splitting reaction. Their catalytic properties need to be improved so that they can be deployed for practical hydrogen generation on a large scale. Several research groups demonstrated the viability of organic-inorganic hybrid and surface functionalized inorganic materials with their electrochemical property improvement relevance to photoelectrochemical applications [26e33]. Bansal and coworkers reported that with surface functionalization the alkyl groups bonded covalently to the Si surface were able to prevent the oxidation of Si and showed excellent electrochemical properties when used as photoelectrodes [34e36]. Adsorption of cations or anions at the solid/liquid interface has been shown to shift semiconductor band edges [37,38]. Similarly, Hila et al. [39] were able to control the interfacial kinetics of n-GaAs/liquid junctions by chemically anchoring positively charged species (Mn (III)-porpyrin complexes) to the semiconductor surface.
Increasing a number of positive charges per molecule shifted the band edges towards positive potentials. The onset potential, photovoltage, and conversion efficiency were also enhanced. Another significant factor that affects a conversion efficiency of PEC cell is charge transport either across semiconductor/semiconductor, semiconductor/electrolyte or semiconductor/back contact interface [40]. This factor might be controlled by its catalyst property or fabrication process. Conducting polymers adsorbed on the catalyst surface have been shown to improve the open circuit potential and stability of semiconductors at the solid liquid interface [41]. The composite and doping approach have been widely studied in order to increase quantum efficiency of electrodes [51e56]. The composite films based on P25/N-doped and B-doped hollow carbon spheres were employed. It was showed that the efficiency was increased up to four times in case of B-doped one [51]. The transition-metal doping treatment was investigated and found that the photocurrent density of FeeTiO2 significantly improved with bias voltage reached five times higher than that of the undoped TiO2 [52]. Surface treatment by thermal or plasma energy is also used to improve catalytic property in the oxidation reaction. The WO3 films was treated with hydrogen plasma, which created substoichiometric WO3x catalyst films with increasing photocurrent density and stability of the photoanode [42]. Treekamol et al. [43] reported that by modifying the surface of catalyst, an agglomeration of the particle was reduced resulting in better packing of the particles on the back contact and to the particles themselves. An approach of surface modification by silane coupling agent is not only simple but also requires low temperature. Moreover, the fact that silane can be further reacted with other active molecules thereby achieving higher efficiency makes this approach interesting in this application. In this work surface functionalization of TiO2 nanoparticles with silane coupling agents was investigated aimed at low-temperature electrode fabrication using 3-chloropropyl trimethylsilane and 3-aminopropyltriethoxysilane. Spin coating was used to enable homogeneous film deposition of semiconductor particles. The prepared electrodes were studied by UVeVis spectroscopy and SEM, and their photoelectrochemical efficiency was measured in water oxidation reaction. Moreover, to enhance their photocatalytic property, heat treatment was employed at different temperatures. Finally, the flat band potential of the electrodes was investigated by Mott-Schottky plot in basic electrolyte.
Experimental Chemical and materials 3-chloropropyltrimethylsilane (CPTMS) and 3-aminopropyltriethoxysilane (APTES) obtained from Gelest, Inc. were used as surface modifying agents in this study. Titanium (IV) oxide nanopowder (P25), with a particle size 21 nm supplied from Aldrich, fluorine-doped tin oxide lime glass, dimethylacetamide, potassium hydroxide, sulfuric acid and toluene were used as received.
Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Functionalization of TiO2 TiO2 nanoparticles (P25) were functionalized with silanes. Firstly, 0.250 g of the TiO2 catalyst was stirred for 5 min in a 1 mM potassium hydroxide solution to introduce hydroxyl groups onto its surface. After that the particles were filtered and rinsed with ultrapure H2O to remove residual basic solution. They were then dried overnight in vacuum oven at 110 C. Later, the alkaline-treated TiO2 was dispersed in 20 mL toluene for 15 min in ultrasonic disruptor and 5 mmol of silanes was added in the dispersion. The reaction temperature was set to 60 C for 72 h. Afterward, the functionalized particle was filtered and washed with toluene to eliminate the unreacted silane. Finally, the modified particles were dried in vacuum oven for 24 h at 80 C. The TiO2 nanoparticles which were functionalized with CPTMS and APTES were identified as Cl-P25 and AP-P25 respectively.
Electrode preparation and photoelectrochemical characterization After TiO2 nanoparticles were functionalized, a suspension of nanoparticle in DMAc was spin coated onto a FTO substrate. The glass was cleaned with isopropanol in sonication bath for 15 min twice, then rinsed and left at room temperature to dry out the solvent. Different thicknesses were obtained by varying the solid content of the suspension. The electrodes were dried in 80 C vacuum oven overnight to remove the solvent. An average height difference of coated and non-coated area was measured by using 3D Laser scanning microscope VKX200 Keyence (surface mode with 50x optical lens). The photoelectrodes with 2.54 cm2 active area were electrochemically characterized in an aqueous solution of 0.5 M H2SO4 together with a silver chloride reference electrode and a platinum ring as counter electrode in the Zahner cell. The electrolyte solution was purged with nitrogen to eliminate oxygen before testing. Current-voltage curves were collected by the Zahner PP211 potentiostat under dark, illumination via the solar simulator (AM 1.5 G), and chopped illumination using Rigol DG 1022 function/arbitrary waveform generator. The CV curves were measured from open circuit potential to 1.8 V using scan rate of 25 mV/s.
Results and discussion Functionalization of TiO2 was carried out basically through silanation reaction under a mild condition as described in Fig. 1. The hydroxyl groups on TiO2 surface were displaced by alkoxy groups of the silane creating Si-O covalent bond giving methyl alcohol as a byproduct. After that the functionalized titanium oxide particle was characterized by FTIR and TGA techniques (see Fig. 2). FTIR spectra of functionalized TiO2 show additional peaks at 2850 and 2975 cm1 corresponding to the symmetric and asymmetric vibrations of eCH2e and eCH3 groups, respectively. Additionally, the peaks at 1130 cm1 and 1040 cm1 are due to the vibration absorption of Si-O-C and Si-O-Si,
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respectively [44]. Regarding the thermal gravity analysis, both types of modified TiO2 show two decomposition steps concerning the 1st derivative analysis. The first one at around 100 C which relates to the water adsorbed on the surface of the particles. The second peak was found at 250 and 400 C in the cases of Cl-P25 and AP-P25, respectively. These indicate the decomposition temperature of silanes. The thickness of TiO2 layers was measured using laser scanning microscope (LSM) determining a difference in average height of coated and non-coated area. Fig. 3 shows the LSM images of TiO2 and modified TiO2 coated electrodes with different thicknesses. The electrode with functionalized catalyst obviously showed an improvement of particle distribution resulting in homogeneous films on FTO substrate compared to that of neat TiO2. As seen in the top view SEM image (Fig. 4.), due to the smaller aggregation obtained by functionalization, the better contact between the particles and uneven surface substrate was achieved. This explained that modified TiO2 provided better photocurrent due to an improvement of the TiO2 particles to the substrate binding causing higher electrons collection from back contact. However, it showed clearly that the electrode prepared by neat TiO2 had non-covered area which also resulted in poor photocatalytic activity.
UVevis spectroscopy Fig. 5 showed UVeVis spectra of P25 and Cl-P25. It is seen that the spectral onset red-shifts, due to aggregation [45]. The UVeVis spectra of pristine and modified TiO2 with different coating thickness on FTO substrate were shown in Fig. 6. It was found that the maximum absorption peaks of all electrodes were in the range of 320e370 nm. However due to the inhomogeneous TiO2 film, the different energy absorption of different thickness layer was observed as a result of a FTO exposure. In case of Cl-P25, the homogenous film showed rather an identical absorption energy with different film thickness.
Photoelectrochemical characteristics The photoelectrochemical (PEC) water oxidation reaction of the electrodes in a half cell was measured in a 0.5 M H2SO4 aqueous solution as an electrolyte. There is no significant redox response was observed in a range of open circuit voltage to 1.8 V vs. NHE on cyclic voltammogram in the dark condition. However, in case of illumination (solar simulator AM 1.5 G, 100 mWcm2) a photoanodic current was observed. As shown in Fig. 7 it was found that a photocurrent under chopped illumination of Cl-P25 were significantly greater than that of P25 which was 0.15 mA at 1.23 V(NHE) for the oxygen evolution reaction. The chemical composition of silane on PEC property of electrodes was also studied, CPTMS and APTES were used. The CPTMS was found to be the optimal coupling agent for TiO2. It might be that the CPTMS is the most active silane due to chloro group is least bulky so titanium oxide can be fully covered compared to other silanes as shown in TGA
Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Fig. 1 e Functionalization of titanium dioxide by the silane coupling agent.
Fig. 2 e FTIR spectra (left) and TGA thermograph (right) of P25, Cl-P25 and AP-P25.
Fig. 3 e Laser-scanning micrographs of electrodes with P25 (left) and Cl-P25 (right) on FTO glass.
Fig. 4 e SEM micrographs of the prepared electrodes on the FTO substrate, P25 (left) and Cl-P25 (right). Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Fig. 5 e UVeVis spectra of P25 and Cl-P25 measured in powder form. thermogram. We also studied the effect of thickness on a photocurrent (Fig. 8.), in case of P25 electrodes the photocurrent trend to decrease when the thickness is higher (from 0.2 to 1.5 mm). The optimum thickness of neat P25 electrode was around 250 nm while that of Cl-P25 was 700 nm due to it
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compromised an effect of light harvesting and film resistance [46]. Moreover the functionalized TiO2 electrodes showed stable photocurrents under illumination @ 0.7 V (NHE) up to at least 1 h under continuous illumination. In order to enhance the photoelectrochemical efficiency of the electrodes, a heat treatment applied with different temperatures (80, 300 and 500 C), the CV and an incident photonto-current efficiency (IPCE) curves were shown in Fig. 9. As 300 C is applied at atmospheric condition, the photocurrent obtained from this electrode is higher than that of one without heat treatment. The current was significantly improved from 0.3 mA to 0.4 mA @ 1.0 V it can be explain by better the contact between particle and substrate resulting in higher currents collected at the back contact. However when the temperature was increased up to 500 C, the photocurrent decreased due to the tranforming from anatase to rutile phase of titaniumoxide [47]. An external quantum efficiency (EQE) is equal to IPCE [50] which is about 4.9% for the electrode with 300 C heat treatment. This is comparable to the work from Ranganathan which also used P25, however, in their study the composite films based on P25/hollow carbon spheres were further used. The IPCE was increased to 12% and 20% in case of N-doped and B-doped hollow carbon spheres respectively [51].
Fig. 6 e UVeVis spectra of electrodes prepared with P25 and Cl-functionalized TiO2 on FTO glass substrate.
Fig. 7 e Voltammetry under chopped illumination (AM1.5 G) of photoanodes prepared with P25 (left) and Cl-P25 (right) with optimum thicknesses measured in H2SO4 0.5 M, 10 mV s¡2. Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Fig. 8 e Right: Stability measurement under illumination (AM1.5G) of P25 and Cl-P25 at 0.7 V vs NHE with optimum thicknesses measured in H2SO4 0.5 M, 10 mV s¡2. Left: effect of thickness on the photocurrent of electrodes prepared with pristine and Cl-P25.
Fig. 9 e CV and IPCE of electrode prepared by Cl-P25 (700 nm) with heat treatment temperature: 80 C, 300 C, 500 C for 6 h.
1 2 ¼ C2 εε0 A2 eND V Vfb keB TÞ where C and A are the interfacial capacitance and area of an electrode, respectively. ND is the number of donors, V the applied voltage, kB Boltzmann’s constant, T the absolute temperature, and e the electronic charge. The flat band potential (Vfb) can be obtained by extrapolating the intercepts of the Y axis from a plot of 1/C2 against E as shown in Fig. 10. Electrochemical impedance spectroscopy as a function of applied voltage was employed to examine the flat band potential. As in a previous work [49] the flat band potential had been found to vary with pH, thus we kept the pH constant using a pH 14 electrolyte (1 M KOH). The negative flat band potential of this neat and modified TiO2 system was found to be in the range of 0.7e0.9 vs NHE as shown in Table 1. The result showed that with different catalyst thicknesses, thicker TiO2 films negatively shifted the flat band potential
0,03
P25_A_1KHz_L
(C/A)-2/ 1012 F-2cm4
The flat band potential (Vfb) which affects the charge transfer and the recombination probability [48] of the electrodes can be determined from the Mott-Schottky plot according to the following equation;
0,02
0,01
0,00 -1,0
-0,5
0,0
0,5
E/V Fig. 10 e MotteSchottky plots for P25 at 1 KHz in 1 M KOH.
value. According to the result from Radecka et al., [21] doping TiO2 with 7.6 at % Cr increased the absolute value of Vf while it shortened the recombination time of generated electron and hole. Regarding our chlorosilane functionalized electrode, the
Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Table 1 e A flat band potential of P25 and Cl- P25 with different thicknesses. Electrode P25 P25 Cl-P25 Cl-P25 Cl-P25
Thickness (nm)
Vfb @ 1 KHz
Vfb @ 10 KHz
249 593 594 688 1354
0.859 0.876 0.715 0.761 0.871
0.855 e 0.722 0.796 0.8215
absolute flat band potential values significantly decreased implying that the charge recombination loss is lower compared to neat TiO2 electrode. This explanation agrees with our photoelectrochemical investigation that the Cl-P25 showed higher photocurrent than pristine catalyst.
Conclusion The catalyst material and fabrication for photoanode in solardriven photoelectrochemical water splitting cell were investigated. Surface functionalization by silane coupling agents was applied to TiO2 photocatalyst in order to assist particle distribution in spin coating process. It can also significantly improve contact between the particles and the electrically conductive substrate, contributing to better electron collection efficiency. Additionally, the flat band potential of these modified electrodes was also found to be lower meaning that charge recombination loss is minor compared to neat TiO2 electrode. These three reasons contribute to an improvement of PEC performance which is a photocurrent up to three times. Not only did they provide providing higher photocurrent, but they also showed good stability under continued illumination. Furthermore, an efficiency of the modified electrodes could be enhanced by sintering resulting in better a quantum efficiency. Although, the photocurrent obtained in this approach is still relatively limited due to low temperature fabrication process, the silanated P25 has high potential to be further modified with other active species such as electron donating groups or dyes to enhance their efficiency, which is another advantage of this work.
Acknowledgement The authors gratefully acknowledge the funding provided by the German Federal Ministry of Education and Research, junior research group project (BMBF Nachwuchsgruppe) “FocusH2”. This study has been supported in part by research and technology transfer affairs (KKUS60_001). Additionally, this work has been partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research and Innovation and Khon Kaen University, Thailand. YT acknowledges the Development and Promotion of Science and Technology Talent Project (DPST) for the financial support of this project (010/2559). YT also acknowledges Assoc. Prof. Dr. Rapee Utke from Institute of Science, Suranaree University of Technology for providing an academic mentoring.
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Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085
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Please cite this article as: Treekamol Y et al., Optimized photoactive coatings prepared with functionalized TiO2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.085