Electrodeposited CZTS loaded titania nanotubes for photocatalytic water splitting

Copper Zinc Tin Sulphide (Cu2ZnSnS4 (CZTS)) is a promising semiconductor material with optimum direct band gap of 1.4-1.6 eV, large light absorption coefficient (> 104 cm-1) and p-type electrical conductivity. In addition, its earth abundant and non-toxic elemental constituents make it a good candidate to replace the CdTe and CIGS absorber layers used in thin film solar cells. Also, it has properties that make it useful as a buffer layer that helps to reduce the effective band gap of TiO2 when used as an anode for photocatalytic splitting of water. We prepared titania nano tube (TNT) arrays by anodization of titanium foil. Raman spectrum of TNT confirmed the presence of pure anatase phase of TiO2. Field emission scanning electron microscopy images were obtained to analyse the surface morphology of TNT. CZTS sensitized titania nano tube arrays were then prepared by the electrodeposition of CZTS thin films over TNT arrays. FESEM images of the CZTS sensitized titania nano tube arrays showed that the electrodeposited CZTS layer had a fibrous structure. A photo electro chemical (PEC) cell was set up with CZTS sensitized titania nanotubes as anode. An anodic current density of 0.19 mA/cm2 was shown by the PEC cell when the anode was irradiated with blue light of wavelength 430 nm at power 50 mW/cm2. The incident photon to current conversion efficiency was 1.1 %.


Introduction
Hydrogen is an ideal fuel for the future because it is sustainable, energy-dense and has eco-friendly properties.Greenhouse gas emissions could be reduced by using hydrogen as a fuel [1].Photocatalytic water splitting has great potential for low cost and clean hydrogen production [2].In 1972, Honda and Fujishima discovered electrochemical water splitting over a TiO 2 electrode [3].Titania with properties like low cost, high stability, non-toxicity, biological inertness, redox ability and environmental merits has been extensively studied.TiO 2 is an n-type semiconductor which shows optical absorption in the UV region of solar spectrum with lowest conduction band level of -0.3 eV which is more negative than the reduction potential of hydrogen (H + /H 2 0 eV vs NHE) and a valence band maximum level of 2.9 eV which is more positive than the oxidation potential of water (O 2 /H 2 O 1.23 eV vs NHE [4].Nanostructured TiO 2 such as nanoparticles, nanowires, nano tubes show enhancement in photoelectrochemical performance.One-dimensional (1D) TiO 2 nanostructures act as direct pathways of electrons and holes and cause light to be more easily scattered and trapped compared to bulk counterpart.However, the large band gap which results in inefficient utilization of solar light, fast recombination of photogenerated electron-hole pairs and photocorrosion are some disadvantages which reduce the scope of TiO 2 in this application [5].TiO 2 can be modified using various methods 1291 (2023) 012006 IOP Publishing doi:10.1088/1757-899X/1291/1/012006 2 like doping, co-doping, modification with inorganic acids, semiconductor coupling, constructing heterojunctions, sensitization etc.
In inorganic thin film photovoltaic technology, CZTS is one of the very popular light absorbing materials consisting entirely of non-toxic, cheap and earth abundant elemental constituents.It has an optimum band gap (1.5 eV) and has a kesterite crystal structure (KS) with I 4 − space group symmetry [6].It is a good substitute for platinum counter electrode of dye sensitized solar cell due to its catalytic property and p-type conductivity.For photocatalytic water splitting, CZTS has been utilized to form heterojunctions with TiO 2 because of its band gap matching with the optimum value required for photocatalyst.The electronic and optical behaviour of the semiconductor have a prime contribution in this application because the redox reactions that occur on the catalyst are responsible for hydrogen generation.For this, the conduction band bottom edge must be more negative than the water reduction potential (0V versus NHE) and valence band top edge should be more positive than the water oxidation potential (1.23V versus NHE) [1].The energy band gap of bulk CZTS is suitable for solar energy conversion but it is not sufficient for splitting water because bulk CZTS with a band gap range of 1.4-1.6 eV, has a valence band maximum slightly higher than water oxidation potential.By size reduction, the band gap is widened.CZTS when combined with the large area wide band gap material TiO 2 nano tube arrays to form heterostructure is a better photocatalyst for water splitting.During past years, CZTS thin films have been developed using several vacuum and non-vacuum techniques like sputtering, thermal and electron beam evaporation, chemical bath deposition, spin coating, successive ionic layer adsorption and reaction, nano crystal ink approaches, electrodeposition and hydrazinebased methods.Co-electro deposition technique is a simple, flexible, fast, easy and energy efficient method to deposit CZTS films using non-toxic precursors, safe solvents and cheap equipment compared to the other methods [7].
In this paper we report how we decorated titania nano tubes by electrodepositing CZTS fibres over it and investigated the possibility of photocatalytic hydrogen evolution in a water splitting reaction.We noticed that CZTS sensitized titania nano tube arrays possessed improved photoactivity properties.

Preparation of titania nanotubes
For preparing highly ordered titania nano tube (TNT) arrays, anodization of titanium foil was done in a two-electrode system using Ti foil as anode and Pt mesh as cathode.Initially, before anodization titanium foils (0.3mm thickness, 99.7% purity (from Sigma Aldrich), and 2 ⨯ 2 cm 2 area) were washed in de-ionized water, sonicated in isopropanol and acetone and dried.Anodization was carried out at 55 V for 4 hours.The electrolyte was ethylene glycol containing 0.3 wt.% NH 4 F and 0.3 volume % water.To remove the organic compounds, as-grown titania nano tube (TNT) arrays were cleaned by sonicating in isopropanol for 5 minutes and dried in air.To crystallize the TNTs, it was annealed at 500 ⁰C for 2 h in air [8].

Deposition of CZTS thin films
Using an ordinary DC power supply converted into a potentiostat in conventional three electrode mode with Ag/AgCl standard electrode as the reference electrode, Pt wire for counter electrode and TNT coated titanium foil as the working electrode, the electrodeposition of CZTS was done over the TNT layer.An aqueous solution of 0.01 M CuSO 4 , 0.03 M ZnSO 4 .7H 2 O, 0.02M SnCl 2 .2H 2 O and 0.01 M Na 2 S 2 O 3 .5H 2 O was used as the electrolyte.100 mM trisodium citrate was added to the bath as complexing agent.In order to maintain the pH of the electrolyte in the range 4-5, 20 mM tartaric acid was added.Deposition of CZTS thin films over TNT arrays was done at -2.5 V for 5 minutes at room temperature.The films were rinsed with de-ionized water and dried in air [7].Thereafter the structure was annealed at 200 o C for an hour in order to form a heterojunction.

Characterization
Raman spectrum of titania nano tubes was acquired using Horiba Labram HR Evo Raman spectrometer with an excitation wavelength of 532 nm.FESEM images of TNT and CZTS sensitized titania nanotube arrays were acquired using JEOL JSM 6390.

Photoelectrochemical cell
To measure the photoactivity properties, a photo electro chemical cell (PEC) was set up using a cubical quartz vessel with titanium foil coated with CZTS sensitized titania nanotubes as anode, Pt mesh as cathode and aqueous solution of sodium sulphate (1M) as electrolyte.Photocurrents were measured using Keithley SMU 2450 set in the ammeter mode.

Results and discussion
FESEM image of the surface of titania nanotubes grown on titanium foils prepared by the electro deposition process is shown in figure 1(a).An average tube diameter of 86 nm can be seen on the FESEM image of cross section of the anodized Ti foil. Figure 1(b) shows the longitudinal view of the well aligned and densely formed nanotubes.Figure 1(c) shows the FESEM image of the CZTS coated titania nanotubes from the surface.We can see that the deposited CZTS has a fibrous structure.The side view in figure 1(d) shows the approximate length of the TNT as 12.6 microns.Upon anodizing Ti sheet for 4 hours, we find that length of nano tubes formed is 12.6 µm.This implies that the oxidation/etching process takes place only on the surface which is only 5 % of the thickness of the sheet.Rest of the thickness remains as Ti metal and acts as a substrate which supports the TNT and as an electrode that provides electrical contact.The length of the titania nanotubes depend on the time for which anodization is done.
Raman spectrum of TNT shows peaks corresponding to the active modes at 142.79 (E g ), 197.33(E g ), 394.21 (B 1g ), 512.013 (A 1g ) and 636.87(E g ) cm -1 , which confirms the presence of pure anatase phase of TiO 2 .The active mode E g refers to O-Ti-O symmetric stretching vibrations, B 1g relates to O-Ti-O symmetric bending vibrations and A 1g refers to anti-symmetric bending vibrations in anatase crystal.Though the aim was to grow a thin film of CZTS over the TNT surface, the FESEM images show that the deposited layer has a fibrous structure.Optical absorption could be higher in the fibrous structure and the electron-hole pairs generated will produce a current if they can be separated with the help of the barrier potential at the heterojunction formed at the interface of the n-type TiO 2 and p-type CZTS (Figure 3  Due to the large band gap of titania (> 3.2 eV) no current could be obtained in the PEC with bare titania nano tubes when the anode was irradiated with light from blue, green and red lasers at power 50 mW.The wide bandgap makes it unable to utilize visible range of the spectrum.For photocatalytic water splitting, it can utilize only UV light.But when coated with CZTS, we observed that a current flows in the external circuit when the anode was illuminated.The PEC using CZTS sensitized TNT as anode showed a current density of 0.19 mA/cm 2 when the anode was irradiated with blue light of wavelength 430 nm at power 50 mW/cm 2 .The number of carriers generated per incident photon is the incident photon to current conversion efficiency (IPCE) or quantum yield.The IPCE value obtained for blue light was 1.1 %.This shows that though small, there is a distinct possibility of effecting the water splitting hydrogen evolution reaction through photocatalytic method using the CZTS/TNT combination.
In photocatalytic water splitting, when the photocatalyst is irradiated with light of energy greater than its band gap, electron-hole pairs are formed.This is due to the excitation of electron from its valence band (VB) to conduction band (CB) thereby forming holes in the VB.The photo generated electrons and holes migrate to the surface and act as reducing and oxidizing agents respectively.Water splitting is a highly endothermic process which requires a Gibbs free energy of 1.23 eV per electron.This corresponds to light with a wavelength of 1008 nm [3].A photocatalyst of band gap > 1.23 eV is required, or else the electrons will not have enough energy to start the reaction.For water splitting the reduction and oxidation potentials of water should lie within the band gap of the photocatalyst.TiO 2 is an n-type semiconductor which shows optical absorption in UV-region of solar spectrum.Its CB level (-0.3 eV) is more negative and suitable for the redox potential of H + /H 2 (0 eV) and its VB level (2.9 eV) is more positive than the standard redox potential of O 2 /H 2 O (1.23 eV) vs. normal hydrogen electrode.But due to its wide band gap, it is unable to utilize the visible light of solar spectrum [3].We observed that no current could be obtained in the PEC with bare titania nano tubes when the anode was irradiated with light from blue, green and red lasers at power 50 mW.The wide bandgap makes it unable to utilize visible range of the spectrum.For photocatalytic water splitting, it can utilize only UV light.The formation of p-n junction of CZTS and TiO 2 reduces the effective band gap since CZTS is having an optimum band gap of 1.4-1.6 eV.When blue light of wavelength 430 nm at power 50 mW/cm 2 is irradiated over CZTS/TiO 2 junction, electron-hole pairs are formed in CZTS.We observed that a current flowed in the external circuit.The PEC using CZTS sensitized TNT as anode showed a current density of 0.19 mA/cm 2 when the anode was irradiated with blue light of wavelength 430 nm at power 50 mW/cm 2 .The electric field at the p-n junction helps in separation of the two by attracting them in opposite directions.Photo generated electrons transfer from CZTS to TiO 2 and the electrons are easily drawn into the Ti metal electrode through the channel provided by the TNT nanotubes.They travel through the external circuit, reach the platinum electrode and reduce water molecules releasing hydrogen at the Pt cathode of the PEC cell.The holes are attracted in the opposite direction because of the barrier potential at the p-n junction and reach the surface of CZTS layer where they oxidise water to oxygen [3].
The current in the circuit of the PEC is negligible in the absence of light and is found to increase upon incidence of light from the blue laser.This clearly indicates that CZTS/TiO2 junction behaves as a photovoltaic p-n junction, where photons produce e-h pairs.A photo current of 0.19 mA/cm 2 indicates the possibility of water splitting process.The number of carriers generated per incident photon is the incident photon to current conversion efficiency (IPCE) or quantum yield.The IPCE value obtained for blue light was 1.1 %.This shows that though small, there is a distinct possibility of effecting the water splitting hydrogen evolution reaction through photocatalytic method using the CZTS/TNT combination.

Conclusion
The possibility of modifying TiO 2 in order to engineer its band gap to a value where it can absorb visible light has been a promising prospect of research.Its application in the field of photo catalytic water splitting reaction has been explored by us using the p-type semiconductor CZTS.The electrodeposition method which is simple, efficient and low cost has been employed for the fabrication of both titania nano tubes as well as the CZTS layer.The efficiency of the PEC reported is not very 1291 (2023) 012006 IOP Publishing doi:10.1088/1757-899X/1291/1/0120066 high, but the photocurrent clearly indicates the possibility of the process and shows that better values and more efficient hydrogen generation can be achieved by improving the interface properties and by reducing recombination of the photogenerated carriers.Several factors like length of the titania nanotubes which depends on the anodization time and presence of binary and ternary defect phases in the CZTS layer could affect the conversion efficiency of the PEC.These effects need to be investigated in future work and formation of the catalytic heterojunction for water splitting optimized accordingly.

Figure 1 .
Figure 1.FESEM images of (a) bare TNT when viewed from the surface (b) longitudinal view of the titanium nanotubes showing dense and regular growth (c) surface of the CZTS layer electro deposited over TNT (d) side view of the TNT used to measure the length of the titania nano tubes.Fibrous CZTS deposited on top of the TNT can also be seen.

Figure 2 .
Figure 2. Raman spectrum of TiO 2 nanotubes (a)).A schematic of the anode developed and the PEC set up are shown in figure 2 (b) and (c).

Figure 3 .
Figure 3. (a) Energy level diagram showing photo generation of electron hole pair and transport of electron and holes across the CZTS-TNT junction (b) schematic of the anode and (c) photo electro chemical cell.Due to the large band gap of titania (> 3.2 eV) no current could be obtained in the PEC with bare titania nano tubes when the anode was irradiated with light from blue, green and red lasers at power 50