Synthesis and photoelectrochemical performance of Co doped SrTiO3 nanostructures photoanode

It is pertinent to realize that scientific research indicates that the most promising method for producing H2 is photo electrochemical water splitting through a photo anode. Cobalt‐doped SrTiO3 (Co‐SrTiO3) composite nanostructures were created in this study via hydrothermal synthesis. The impact of cobalt concentration change on Co‐SrTiO3 has been identified using morphological, structural, and photo electrochemical research. Surface morphology of pure SrTiO3 nanoparticles using SEM and TEM reveals that the particles are intermittently agglomerated. The inclusion of Cobalt lowered the particle size of the nanostructures to 23 nm than pure SrTiO3 (41 nm). In addition, the peak profile has been influenced by cubic phase also identified from the x‐ray diffraction analysis. The purity and composition of the materials were revealed by XPS analysis. The Co‐SrTiO3 composite's produced the best charge transfer and recombination capabilities at 3% Co doping, according to electrochemical chemical impedance (EIS) spectroscopy. At 0.2 V applied potential, the obtained 3% Co‐doped SrTiO3 photoanode system displays a photocurrent density of around 3.45 mA/cm2. The outcomes show that a promising application for the Co‐doped SrTiO3 photoanode in photoelectrochemical water splitting.


| INTRODUCTION
Despite beginning in the 1990s, the EU adopts a hydrogen economy policy around 2010, followed by third-world economies, and active exploitation of renewable sources (Solar, Wind, and Hydro) of energy are the only solution for global energy crisis and environmental problems. 1In this context photoelectrochemical water splitting (PEC) has been found most promising process to convert solar energy in the form of hydrogen. 2However, synthesis of nanomaterial with controlled size and high photochemical properties using various semiconductors has been always challenging.In this context SrTiO 3 materials with perovskite structure are very useful owing to their valence and vacancy control, thermal stability, and lower photocorrosion. 3However, due to charge recombination produced by surface states, SrTiO 3 has a low photogenerated current density and poor light absorption due to a wide band gap (3.2 eV).To address this issue, several approaches have been proposed, including nanostructure manipulation, elemental doping, and cocatalyst loading.Doping is one of the most common method to increase the photo conductivity and carrier concentration of SrTiO 3.   4   It has been reported that the photoconductivity is increased with the increase of platinum doping and the presence of platinum also decreasing the crystallite size.In a recent research Lanthanum and Rhodium doped SrTiO 3 have demonstrate high photo catalytic activity under phosphate buffered electrolyte solution. 4,5Addition of Lathanum and Rhodium on the SrTiO 3 surface and surface modification accelerates the supply of reactants to active sites and is more effective to enhancing the photocatalytic activity.The tungsten doping in SrTiO 3 has been done using sonication assisted solid state method and it has shown very high crystalline nature of synthesized material. 6When tungsten was doped into SrTiO 3 surface, the band gap was decreased compared to the pure SrTiO 3 sample and its electrical conductivity was increased, which can be utilized for the solar cell devices.Palladium (Pd) nanoparticle doping in SrTiO 3 has been performed using photochemical deposition and it has been improved visible and near infrared charge separation efficiency significantly and it generated more active radicals, which led to an enhanced photocatalysis performance. 7It has been proven that transition metal doping improves optoelectronic properties of perovskite materials by modifying their electrocatalytic capabilities and resultant materials are suitable for electrochemical reactions with molecular oxygen, 8 when Sr, Fe, and Co doped with non-noble metal perovskite LaNiO 3 and LaFeO 3 produced higher oxygen evolution reaction (OER). 9Flores et al. modified ABX 3 structure of perovskite and shown that large cation A were found to predominate at the surface of the catalyst; nevertheless, strong substitution with cobalt leads to enrichment of the cations in their respective higher oxidation states (A 4+ and Co 3+ instead of A 3+ and Co 2+ ) at the surface. 10In this work, the transition metal cobalt was doped with SrTiO 3 perovskite.
The hydrothermal process offers a number of benefits, one of which is a reduced need for energy due to the low temperature needs (less than 250 C).In addition, the amount of transformation as well as the size of the newly manufactured crystal may be controlled by the amount of reaction time as well as the temperature during the hydrothermal process.Recently Tezcan et  with different mol % of Co ions (1%, 3%, and 5%) were prepared by hydrothermal method. 16The Sr and Co precursors were dissolved in deionized water.Then, Ti precursor was dissolved in ethanol.Both solutions are stirred for 2 h until a homogeneous solution was obtained.The pH is adjusted at 13 by adding the KOH solution (0.1 M).The whole solution after stirring was transferred to an autoclave at a temperature of 180 C for 24 h.Finally, the resultant material was filtered and the precipitate was washed in distilled water and ethanol.The final powder was obtained after drying at 80 C for 6 h and its corresponding schematic synthesis process is illustrated in  No.-893697). 17The diffraction peaks can be indexed as cubic SrTiO 3 .
Although some raw powders can still be seen in the final product as a second phase, it has been found that the main impurities are rutile

| FE-SEM
The morphology of pure SrTiO

| TEM analysis
The morphology of the synthesized SrTiO  The peaks at 795.9 and 780.2 assigned to Co 3+ and the rest two

| Photo electrochemical properties
The Photo electrochemical (PEC) water splitting of the pure SrTiO 3 as well as (1%, 3%, and 5%) Co-doped SrTiO vacancy 23 which provides very accessible sites for catalytic activities.
In our case, the narrowing band gap and surface oxygen vacancy have an effect on the photoelectrochemical properties.

TiO 2 and
SrCO 3 impurities.This demonstrates that drying causes more species to enter the pores, converting titania particles into anatase or rutile after calcination, while more Sr salt converts to SrCO 3 by absorbing CO 2 from the atmosphere.The XRD peaks intensities are decreased when the Co content was in a range of 1-3 wt %.An exception was that; a peak intensity was increased as the Co content was doped at 5 wt %.It was evident that different amounts of Co can be incorporated into the SrTiO 3 lattice.This may be attributed to the Co-induced change in the internal stress and surface energy during the growth of SrTiO3.Also, Co ions may gradually occupy the regular as well as interstitial sites of SrTiO3 ions at the Co doping concentrations from 1 to 3 wt %.At high concentration, 5 wt % of Co doping, Co ions would have occupied some additional interstitial sites, which are otherwise unoccupied.This effect would have led to the observed changes in the peak positions. 18For (110) plane Scherrer equation has been used to calculate crystallite size of pure SrTiO 3 , which has been obtained around 41 nm when it is doped with 3% Cobalt ions, further decreases the crystallite size to 23 nm.It was observed that the crystallite size of the Co-doped SrTiO 3 nanoparticles decreased with an increasing content of Co doping except for the SrTiO 3 doping with 5 wt % Co.The decrease in the crystallite size of Co-doped SrTiO 3 nanoparticles is mainly attributed to the formation of Co-O-SrTi on the surface of the Co-doped SrTiO 3 nanoparticles.This can inhibit the crystal growth.
3 and Co doped SrTiO 3 has been investigated using scanning electron microscopy.The synthesized pure SrTiO 3 nanoparticle shown in Figure 3 a are cubic in shape and partially agglomerated.FE-SEM images of 1%, 3%, and 5% Co-doped SrTiO 3 samples are shown in Figure 3 b-d.It has been evident from the images that introduction of cobalt improves the perovskite structure of the SrTiO 3 nanoparticles and suppress agglomeration.
3 and Co doped SrTiO 3 has been investigated TEM as well and shown in Figure 4.All the samples F I G U R E 1 Schematic representation of Co-doped SrTiO 3 synthesis.F I G U R E 2 XRD analysis of pure SrTiO 3 and 1%, 3%, and 5% Codoped SrTiO 3 samples.have shown agglomerated cubic in morphology.The introduction of Cobalt has reduced the particle size of the nanoparticle when compared with pure SrTiO 3 .The mean diameter of pure SrTiO 3 and Co/SrTiO 3 is approximately 40 and 25 nm respectively, this values are in good agreement with the crystallite size, which has been calculated from Scherrer equation with XRD results.F I G U R E 3 FE-SEM images of (a) pure SrTiO 3 (b) 1% Co-doped SrTiO 3 , (c) 3% Co-doped SrTiO 3 , (d) 5% Co-doped SrTiO 3 .F I G U R E 4 TEM images of (a) pure SrTiO 3 (b) 1% Co-doped SrTiO 3 , (c) 3% Co-doped SrTiO 3 , (d) 5% Co-doped SrTiO 3.

3. 4 |
XPS analysisFor confirming surface chemical composition states of the SrTiO 3 perovskite and cobalt doped nano powder, XPS analysis has been performed.The wide survey spectrum of Figure 5 a indicates composition of nanocrystals including Sr, Ti, O in SrTiO 3 samples and Co for doped samples without other impurities.Figure 5 b indicates the Sr 3d 5/2 and 3d 3/2 peaks at 132.5 and 134.4 eV respectively, hence indicate presence of Sr 2+ state in all samples. 19The Ti matched with two peaks at 458.2 and 463.7 eV belongs with Ti element Ti 2p 3/2 and Ti 2p 1/2 .The magnifying curve of cobalt shows two clearly visible peaks at 795 and 779, which contributed due to binding energies of Co 2p1/2 and Co 2p3/2 respectively.Furthermore these two peaks further split into individual two peaks around 795.9, 795.2, 780.2, and 779.1 eV.20

F I G U R E 5
XPS spectra of pure SrTiO 3 and Co-doped SrTiO 3 samples (a) Wide survey spectra (b) Sr 3d, (c) Ti 2p, (d) Co 2p, and (e) O 1 s.peaks at 795.2 and 779.1 has been assigned to Co 2+ which proves that Co has been successfully doped to SrTiO 3 and the ion Co 2+ and Co 3+ both present in SrTi 1-x Co x O 3.21 The O 1 s high resolution XPS has been fitted with three Gaussian peaks around 529, 532.5, and 536 which can be described as, oxygen due to metal oxide, and presence of hydroxyl group at the surface and chemisorbed oxygen.223.5 | Optical AnalysisUV-vis spectrophotometer was used to record the light absorption performance of samples.The optical absorption characteristics of pure SrTiO 3 and Co-doped SrTiO 3 are analyzed using UV-Vis diffuse reflectance spectra (DRS), as shown in Figure 6 a. Figure 6 b shows the spectra of Tauc's plot.Both pure and co-doped SrTiO 3 have exhibited absorption bands in the UV region at about 340 nm.The absorbance of Co-doped SrTiO 3 samples are shifted toward higher wavelength than pure SrTiO 3 .There is a significant change for absorbance due to the introduction of cobalt ions into SrTiO 3 lattice.As illustrated in Figure 5 b, the band gap of the prepared samples is calculated using Tauc's plots.The optical energy bandgap can be calculated from the x-axis intercept of extrapolating linear fit of a plot of In(αhν) 1/2 versus hν.The calculated energy band gap values for pure SrTiO 3 and 1%, 3%, 5% Co-doped SrTiO 3 are found to be 3.29, 3.07, 2.90, and 3.20 eV.It is evident that as co-doping concentrations increase, the band gap slightly reduces.The change of SrTiO 3 lattice distortion may lead the band gap to shift.This could increase the number of defects in the SrTiO 3 lattice, which would alter light absorption and photocatalytic activity.
3 samples were tested by linear sweep voltammetry (LSV) under simulated sunlight irradiation.As shown in Figure 7 a, it is clear that the photocurrent density is increased from 0.42 mA/cm 2 of pure SrTiO3 to 3.45 mA/cm 2 for Co-doped SrTiO 3 photoanodes, indicating that the addition of cobalt can greatly improve the photoelectrochemical activity for the photoanodes.Due to the samples' increased active area and light absorption, the photocurrent density of the doped samples was increased.The cobalt doped SrTiO 3 samples has a stronger ability to capture visible light than the pure SrTiO 3 photocatalyst due to its narrow band gap compared to SrTiO 3 sample.An important factor that influences the sample's PEC performance is a surface defect such an oxygen

F
I G U R E 6 (a, b) Absorption spectra and Tauc's plot for the prepared samples.F I G U R E 7 (a) LSV Polarization curve and (b) EIS spectra a of pure SrTiO 3 Codoped SrTiO 3 (Inset: RC Circuit used for the fitting of EIS raw data).The electrochemical behaviors of the composites have been studied using EIS technique.The charge transfers and recombination mechanisms have been explained using the equivalent RC circuit (Figure 7 b inset).Figure 7 b shows the Nyquist curve of the pure SrTiO 3 , (1%, 3%, and 5%) Cobalt doped SrTiO 3 in 1 M KOH solution under light during the frequency range of 0.1 Hz to 100 kHz.The Nyquist curve shows two semi circles which belongs to charge transfer resistance (R ct ) and the parameter is shown in

4 | 26 Fe 2 O 3 /
photo catalysts have been characterized for morphological study using FESEM and TEM.The SEM and TEM images of pure SrTiO 3 show nanoparticles are partially agglomerated.The introduction of Cobalt has reduced the particle size of the nanoparticle when compared with pure SrTiO 3 .The mean diameter of pure SrTiO 3 and Co/SrTiO 3 is approximately 41 and 23 nm respectively and has been clearly shown through XRD that the profile of the peaks has been influenced by cubic as well as tetragonal phase.XPS analysis revealed purity and composition of the materials.The Electrochemical chemical impedance (EIS) spectroscopy showed that the prepared Co-SrTiO 3 composite have best charge transfer and recombination properties at 3% doping.
Strontium acetate (C 4 H 6 O 4 Sr), titanium (IV) n-butoxide ((C 16 H 36 O 4 ) Ti), Cobalt acetate (C 4 H 6 CoO 4 ) were purchased from Sigma-Aldrich.ethanol (C 2 H 5 OH) was purchased from Sigma Aldrich.All the chemicals were of analytical grade and used without further purifications.2.2 | Preparation of SrTiO 3 and Co-doped SrTiO 3 photocatalystIn a typical procedure, the Ti Source is titanium (IV) n-butoxide (1 mL) was added dropwise into ethanol (25 mL) under magnetic stirring at room temperature.The suspension was added into 25 mL of an aque-SrTiO 3, required amount of C 4 H 6 O 4 Sr and C1 6 H 3 6O 4 and C 4 H 6 CoO 4 strated that the resultant exhibited a greater photocurrent density than the pure SrTiO 3 .2|EXPERIMENTAL PROCEDURE2.1 | Materials ous solution containing 5 mmol Sr is from Strontium acetate.Then, the suspension was poured into a stainless steel Teflon-lined autoclave for hydrothermal treatment.For the synthesis of Co-doped

Table 1
0 V. From the Figure 8 b, the photocurrent density of the as-prepared 3% Co-SrTiO 3 photonoade initially drops and then it is relatively constant over the extended period of time indicating good long-term stability.A comparison of the photoelectrochemical properties of present work with literature reports on the various photonode is given in Table2.