Pristine TiO₂ and Sr-Doped TiO₂ Nanostructures for Enhanced Photocatalytic and Electrocatalytic Water Splitting Applications

Abstract

Pristine TiO₂ and Sr-doped TiO₂ (1%, 2.5% and 5%) nanoparticles were synthesized at low temperatures via an eco-friendly hydrothermal route for water-splitting applications. XRD, EDAX and Raman analysis were performed to analyze the crystallinity, purity and structure of the as-synthesized materials. TEM, SEM, BET and UV-DRS studies were carried out to elucidate the size, morphology, surface area and optoelectronic properties of the nanoparticles. High surface areas of 169, 182, 178 and 141.16 m² g⁻¹ for pristine TiO₂ (12 ± 0.6 nm) and 1% (11.1 ± 0.6 nm), 2.5% (12.1 ± 0.6 nm) and 5% (13 ± 0.7 nm) Sr-doped TiO₂ nanoparticles were obtained, respectively. One-percent Sr-doped TiO₂ nanoparticles were found to be active photocatalysts, as they showed higher hydrogen production (26.30 mmol$g_{cat}^{-1}$). Furthermore, electrocatalysis was investigated for HER and OER in 0.5 N H₂SO₄ and 0.1 N KOH electrolytic solutions using calomel as a reference electrode, revealing that 1% and 5% Sr-doped TiO₂ showed maximum current density for both HER (≈10 mA/cm²) and OER (≈2.49 mA/cm²), with an onset potential of 0.96 V for HER and 1.55 V for OER, and Tafel slopes of 84.09 and 91.60 mV/dec, respectively.

1. Introduction

Hydrogen production through photocatalytic water splitting is considered to be the most renewable and sustainable energy production technology, with almost negligible impact on the environment [1]. Hydrogen is considered to be the most promising and clean source of energy for the future, as the only combustion product is water. Furthermore, with the process of development, the demand for energy is also increasing. As a result, present energy resources cannot last long and fulfill our future requirements. Advanced functional nanomaterials have shown potential applications in nanocatalysis for H₂ generation to meet the global energy demand [2,3,4]. The utilization of non-renewable energy resources has become one of the major causes of environmental pollution due to the release of CO₂, which causes the greenhouse effect and global warming [5,6,7,8]. Hence, the researchers are trying to develop sustainable and environment-friendly energy resources with high gravimetric energy density that are easily accessible. There are several environment-friendly techniques for green fuel/energy on which researchers are working; among them, photo- and electrocatalytic techniques have attracted significant attention -due to their relatively decreased dangers to the environment [2,4,5,9,10,11]. In today’s era, hydrogen energy has become the center of attraction, and water-splitting via electro-catalysis or photo-catalysis is considered to be the best method for hydrogen generation, presenting a clean and renewable source of energy for fuel cells and batteries [12]. To store renewable electricity, electrochemical water-splitting is one of the most attractive ways of achieving hydrogen energy [13]. Nanosized metal oxide nanoparticles are widely used in the semiconductor industry as catalysts for hydrogen generation due to their low toxicity, high stability and low cost [14,15].

Transition metal-based oxides have wide band gap semiconducting properties, which are large enough for water splitting and charge transfer. This meets the thermodynamic requirement, showing excellent hydrogen evolution performance as photo- and electrocatalysts because of their exceptional size dependency [16,17]. These materials have extraordinary optoelectronic and chemical properties, along with advanced chemical and thermal stabilities with these processes encompassing charge carrier (e⁻/h⁺) transformation [18,19,20,21]. Recently, several metals and non-metal oxide-based catalysts have been explored as photo- and electrocatalysts for significant hydrogen evolution activity [22,23,24,25,26,27,28,29]. The use of metal oxides as catalysts was uncovered by the use of TiO₂ as a catalyst in photocatalytic and electrocatalytic water splitting. Honda and Fujishima (in 1972) used TiO₂ with a Pt electrode for the electrocatalysis of water [30]. However, it has very limited application as a photocatalyst, though due to it low cost, non-toxicity and excellent physical and chemical properties, TiO₂ was found to be very useful in the paint industry, cosmetics industry, gas sensors, optoelectronic devices, protective coating, white pigment, catalysis and electronics, etc [16,17,31,32,33,34,35].

The photocatalysis mechanism is based on e⁻ and h⁺ pair formation with the irradiation of a light source [36,37,38,39]. Due to wide bandgap (3.2 eV) energy, fast charge carrier recombination rate, and low utilization of lights, the photocatalytic activity of TiO₂ towards hydrogen evolution is limited [36,38]. To overcome these issues, researchers have tried to develop TiO₂ materials to reduce the band gap and charge separation enhancement in many ways, such as by coupling it with other semiconductors and doping with foreign elements [40]. Metal doping to TiO₂ host lattice causes surface plasmon resonance (SPR), which is free electrons on the metal-doped nano-catalyst. Owing to SPR, the charge transfer efficiency of metal-doped TiO₂ is enhanced through electron trapping via efficient absorption of solar radiation [41,42]. In addition, metal doping introduces interstitial site defects and oxygen vacancies by the formation of Ti³⁺ in the TiO₂ lattice. Furthermore, metal doping and heterostructured composites create an empty impurity band above the valence band (V.B.) and bridge most of the gaps of valence band maxima and conduction band (C.B.) minima in pristine TiO₂, with less charge (e⁻/h⁺) separation and charge transfer. This leads to the tailoring of the bandgap and hence enhances the photo- and electrocatalytic performance of the nanocatalysts [43,44,45,46]. For instance, iron, cobalt, molybdenum, strontium, magnesium or noble metals (Ag, Pt, Au) and non-metal elements (C, N, S) have been exploited as dopants to improve the photocatalytic and electrocatalytic performance of pristine TiO₂ [47,48,49,50]. Strontium is a significant alkaline earth metal extensively used in many research fields, electronics, the military industry, the chemical industry, optics, metallurgy, etc. In recently reported literature related to Sr and TiO₂ systems, most researchers reported Sr-doped composite materials or nanostructures. Film Sr-doped TiO₂ nanotube shows a 65% incident photon conversion efficiency. Sr-doping to TiO₂ enhances the visible light response and generates active site defects in the pristine TiO₂ host lattice; it thereby enhances the activity by a decrease in the band gap and suppresses the charge recombination rate [49,51]. The precious nature of metals (Ru, Ir, W, etc.) and oxides, which are the most efficient electro-active materials for OER, is becoming a bottleneck for commercial applications [52,53,54,55]. Since OER follows a complex multistep reaction mechanism, it involves four electrons in either an acidic or alkaline medium (OER, 2H₂O → O₂ + 4H⁺ + 4e⁻ in acidic solution, 4OH⁻ →O₂ + 2H₂O + 4e⁻ in alkaline solution); hence, OER reactions have high overpotential and sluggish kinetics even after using active electrocatalysts such as RuO₂ and IrO₂, which is hinders practical application of water electrocatalysis [56,57,58]. For OER, RuO₂ and IrO₂ are considered the best electrocatalysts owing to their high stability, excellent electrolytic activity and low overpotential [59,60].

The major concern of the current research is to develop highly efficient, cheap and earth-abundant electrocatalysts and photocatalysts for use in sustainable energy. Herein, we report a simple, environmentally friendly, low-temperature hydrothermal approach to the synthesis of pristine and Sr-doped TiO₂ nanocatalysts. As-synthesized material was further investigated with XRD, Raman, TEM, SEM, EDX, BET and UV-vis DRS. In addition, detailed photo and electrocatalytic water-splitting studies were carried out for the application of as-prepared Sr-doped TiO₂ nanoparticles for hydrogen production.

2. Experimental Section

2.1. Materials

The following chemicals and reagents were used without further purification: glacial acetic acid (Fisher Scientific, Heysham, Lancashire, UK), titanium isopropoxide (Alfa Aesar, 97%, Heysham, Lancashire, UK ), sodium hydroxide (97%, Merck Life Science Pvt. Ltd., Vikhroli, Mumbai, India), strontium acetate (Aldrich, Sigma-Aldrich Chemie GmbH, Steinheim, Germany), ethanol (Merck, 99.9%, Darmstadt, Germany), sodium sulfide (Na₂S, 60%, SRL Pvt. Ltd., Taloja, Maharashtra, India), sodium sulfite (Na₂SO₃, Alfa Aesar, 98%, Heysham, England), and double distilled water.

2.2. Synthesis of Pristine and Sr-Doped TiO₂ Nanoparticles (NPs)

Pristine TiO₂ NPs were synthesized through a hydrothermal route using a laboratory autoclave. 7.4 mL glacial acetic acid was taken in a round-bottom flask to prevent the direct oxidation of titanium isopropoxide to TiO₂, which helps to carry out the reaction in a controlled manner by forming stable hydroxyl titanium acylate [61,62]; then, 0.74 mL of titanium isopropoxide was added after making an inert atmosphere with continuous stirring. After, 16.86 mL of distilled water was added such that the total volume became 25 mL; the resulting solution is referred to as 0.1 M Ti⁴⁺ [61,62]. This solution was stirred on a magnetic stirrer for about 30 min with slow addition of 25 mL of 0.1 M NaOH. The solution was then kept in an autoclave and placed in a vacuum oven at 150 °C for 12 h. The solution was cooled to room temperature and then centrifuged, washed with distilled water and ethanol several times, and dried overnight at 60 °C. Then, the dried white precipitate was ground and white powders were obtained.

For the preparation of three different compositions, 25 mL of strontium acetate solution of 0.001 M was prepared by doping 1.0% of strontium in 0.1 M of TiO₂. In addition, a solution of titanium metal ions was prepared using a similar procedure to that stated above. These two solutions were mixed and stirred on a magnetic stirrer with slow addition of 45 mL of 0.1 M NaOH solution for about 30 min [29], followed by a normal synthesis procedure to obtain washed and dried precipitate, which was ground to obtain a white-yellowish powder. Similarly, the procedure for the doping of 2.5% and 5.0% of Sr in TiO₂ nanoparticles was performed separately with 0.0025 M strontium acetate solution for 2.5% doping and 0.005 M for 5.0% doping of Sr in TiO₂ nanoparticles. The reaction scheme was depicted schematically in Figure 1 below.

2.3. Characterizations

The powder X-ray diffraction (XRD) technique was employed to analyze the crystal structure and phase purity of the as-prepared samples by employing a Rigaku Japan D/max 2500 diffractometer operating at a scan rate of 5° min⁻¹ with Cu Kα radiation (1.5406 Å) with a step size of 0.05 °/s to carry out the XRD studies in the 2θ range of 20–80°. A RENISHAW inVia Raman microscope (Leica Microsystems CMS GmbH, Wetzlar, Germany) with a He–Ne laser and an output of 15 mW was used to analyze the Raman spectral studies using a 532 nm excitation wavelength. TEM analyses were carried out on a TALOS instrument operating at 200 kV to investigate the size and crystal structure of undoped pristine and Sr-doped TiO₂ nanoparticles. The samples for TEM analysis were prepared by making creating a suspension in ethanol by sonication and mounting the samples on a carbon-coated copper grid by employing a droplet of suspension. A Nova Nano SEM-450 microscope (FEI Company, Hillsboro, OR, USA) accelerated at 20 kV voltage combined with an energy-dispersive X-ray (EDAX) system was used to perform field emission scanning electron microscopy (FESEM) to evaluate the surface morphological features, elemental analysis and chemical composition. To analyze the FESEM analysis, a dry sample was mounted on an ultrathin layer of gold coated with carbon tape to prevent the surface charging effect. UV-visible DRS (UV-Vis diffused reflectance spectroscopy) studies were performed on PerkinElmer Lambda 365 spectrophotometer. The reflectance spectra of as-synthesized samples were recorded in the 200–800 nm wavelength range to determine the band gap. Furthermore, a Nova 2000e Instrument (Quantachrome Instruments Limited, USA) was used to determine the Brunauer–Emmett–Teller (BET) surface area and pore size distribution of pristine TiO₂ and Sr-doped TiO₂ nanoparticles.

2.4. Photocatalytic Hydrogen Evolution Measurements

The activity of the as-synthesized nanocatalyst towards photocatalysis of water for hydrogen production was estimated with 0.128 M Na₂S (500 mg) and 0.079 M Na₂SO₃ (500 mg) as sacrificial agents, which provide electron donor sites throughout photocatalytic reaction in ambient conditions irradiated under a UV visible light source of 200–2400 nm varying wavelength. In the hydrogen evolution activity, 20 mg of photocatalyst and sacrificial agents were dispersed in 50 mL double distilled water in an airtight photoreactor, which is a specialized cylindrical quartz cell with a bottleneck of 4 cm diameter and a flat base [63]. The prepared solution was stirred with the purging of nitrogen for 30 min in order to create an inert atmosphere and to eliminate gas impurities (such as dissolved oxygen). After stirring, the cylindrical quartz cell containing the solution mixture was fixed on the stirrer and its flat surface was placed 7 cm away from the light source; it was then exposed to the light source (200 W, Hg−Xe arc lamp, Newport, MODEL: 66906–200HXF-R15 Ozone-free), which irradiates at 170 W with constant stirring. Hydrogen production was quantified using a gas chromatography instrument procured from Perkin Elmer, model Clarus 590 GC, using a TCD detector and nitrogen as carrier gas, with sampling done after every 1 h time interval with a specialized airtight glass syringe. For further study, the photocatalysts from the reaction mixture were recovered by centrifugation, dried and then analyzed the recyclability, photostability, reusability and photocatalytic response showing maximum H₂ production by taking consecutive three cycles of 8 h long activity.

2.5. Electrode Preparation and Electrocatalytic Measurements

For the measurement of electrocatalytic activity of pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ nanostructures, all electrochemical measurements were taken with three electrode systems (i.e., working electrode, reference electrode and counter electrode) in 0.5 N H₂SO₄ (98%, Merck) and 0.1 N KOH (85%, Merck) electrolyte solution for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity, respectively. Measurements were performed using an Autolab PGSTAT204 instrument at room temperature. Here, Pt wire and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The working electrodes of as-synthesized pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ electrocatalysts for the electrochemical setup were prepared by the coating of catalyst dispersion on pretreated ITO substrates (sheet resistance < 10 ohm/sq, Vritra) of 1 × 1 cm² dimension. Before the drop-casting of the dispersion on the substrates, they were cleaned with ethanol (Merck, Emsure), acetone (99.5%, SRL) and isopropanol (99.5%, SRL) for 10 min in each solvent by ultra-sonication. Catalyst dispersions were prepared by making a suspension of 4 mg electrocatalysts, 400 μL isopropanol and 15 μL of Nafion (5 wt %, Alfa Aesar) as a polymer binder. The resulting mixtures were sonicated for about 30 min. 250 μL dispersions were deposited via drop casting on the conducting side (oxide layer side) of ITO substrate and then dried overnight at 60 °C in a vacuum oven, as per the procedure reported elsewhere [64]. For investigation of the electrochemical activity of the as-synthesized nanostructures, cyclic voltammetry (CV) measurements were carried out in the range of 10–100 mV/s, linear sweep voltammetry (LSV) measurements were performed at 100 mV/s and chronoamperometry (CA) measurements were obtained at −0.5 V and 0.5 V for HER and OER, respectively.

3. Results and Discussion

3.1. XRD Analysis

Powder X-ray diffraction was used to identify the phase purity and crystal structure of the as-synthesized nanoparticles. Figure 2 presents the XRD pattern, which was satisfactorily indexed with the anatase phase of TiO₂ with JCPDS card number 71-1167 [65]. It was observed that the synthesized material was pure and highly crystalline. Similarly, the 1%, 2.5% and 5% Sr-doped TiO₂ nanoparticles were indexed with the same anatase phase, as shown in Figure 2. From the XRD, it was observed that there is a slight shift in the peaks after Sr-doping, which shows the incorporation of Sr into the host lattice of TiO₂ nanoparticles. Further, the 2θ values shift to lower values with 1% doping of Sr, which then decrease with 2.5% and 5% doping. In addition, with the increase in Sr concentration in the TiO₂ host lattice, the peak intensity increases; sharpness was also observed, which reflects the increase in overall crystallinity. Since the Sr²⁺ (1.12 Å) ionic radius is much higher than Ti⁴⁺ (0.605 Å), on substitution, lattice expansions were observed. This also suggests that the solubility limit of Sr²⁺ over the TiO₂ host lattice is not high; hence, Sr-doped TiO₂ nanoparticles were fabricated at small concentrations. Due to the effect of Sr dopant, the crystallinity and crystallite size of Sr-doped TiO₂ were considerably reduced. It also causes the enhancement of point defects such as oxygen vacancy [49]. By using Scherrer’s equation, the average crystallite size of all samples was calculated [66] and was found to be 10.21 ± 0.5, 8.92 ± 0.5, 8.99 ± 0.5, and 9.85 ± 0.5 nm for pure TiO₂, 1%, 2.5%, and 5% Sr-doped TiO₂, respectively. The crystal structure of Sr-doped TiO₂ nanoparticles depicting the positions of Sr²⁺ and Ti⁴⁺ ions is shown in Figure 3.

3.2. TEM Analysis

TEM analysis showed the effect of Sr-doping on TiO₂ nanoparticles. Figure 4a–d presents the micrographs of pristine TiO₂ and Sr-doped TiO₂ nanoparticles. In the TEM micrographs, the appearance of rough cubes with little agglomeration could be seen, along with black spots, showing the appearance of strontium on the TiO₂ host nanoparticles. From the TEM images, particle size histograms were obtained, which revealed the average particle size to be in the range of 5–30 nm and the average particle size to be 12 ± 0.6, 11.1 ± 0.6, 12.1 ± 0.6 and 13 ± 0.7 nm for pristine TiO₂, 1%, 2.5% and 5% Sr-doped TiO₂ nanoparticles, respectively, as shown in Figure 4e–h. It was also observed that with the increase in the Sr concentration, the particle size increases. Furthermore, HRTEM micrographs depict the well-defined lattice fringes separated by the interplaner spacing of 0.35, 0.35, 0.35 and 0.35 nm for the pristine, 1%, 2.5%, and 5% Sr-doped TiO₂ nanoparticles, respectively which correspond to the (101) crystallographic plane of anatase tetragonal TiO₂, as shown in Figure 4i–l.

3.3. SEM Analysis

SEM analysis of pristine and Sr-doped TiO₂ nanoparticles was carried out to gather morphological information, as shown in Figure 5a–d. SEM images show the formation of dense nanoparticles with rough and irregular morphology. Careful observation shows that the aggregation and density increase with increasing dopant concentration. The EDAX spectra of pristine TiO₂ and Sr-doped TiO₂ nanoparticles represent the successful doping of Sr into the TiO₂ host lattice, as shown in Figure 5e–h. EDAX spectra show the presence of only the desired elements Sr, Ti and O in the as-synthesized nanoparticles; theoretically, the derived elemental composition was found to have a close agreement with the experimentally loaded composition. X-ray elemental mapping of pristine TiO₂ and Sr-doped TiO₂ also revealed that Sr was dispersed minutely and Ti and O were present in the synthesized material.

3.4. UV-Visible DRS Analysis

Pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ NPs were employed for the analysis of optical properties by UV–visible absorption spectroscopy, as shown in Figure 6a. Reflectance spectra are presented in Figure 6b. By using the following Kubelka–Munk equation and reflectance data, the optical band gap of the materials was calculated:

F(R) = α/s = (1 − R)²/2R

where the terms have their standard and usual meanings. Kubelka–Munk plots of pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ nanoparticles were drawn using [F(R∞)hv]^1/n Vs eV, and with the help of linear extrapolation of [F(R∞)hv]^1/n, their band gaps were determined [63,64]. The estimated band gap values were found to be 3.19, 3.16, 3.18 and 3.20 eV for pristine TiO₂ and 1%, 2.5% and 5% Sr-doped TiO₂ NPs, respectively. The small changes in the band gap could be due to the incorporation of impurities or vacancy defect levels between V.B. and C.B.; this may result in a change in the activity of the materials in terms of their photo- and electrocatalytic applications [46,67].

3.5. Raman Analysis

Raman spectroscopy is a characterization technique used to investigate the structure of complex metal oxides. This technique was used to explore the different vibrational modes of pristine and Sr-doped TiO₂ nanoparticles. For the TiO₂ anatase phase, 15 optical modes were found: 1A_1g + 1A_2u + 2B_1g + 1B_2u + 3E_g + 2E_u with normal vibrations; six of these are Raman active modes represented by 1A_1g, 2B_1g and 3E_g symmetries. The vibration modes A_1g, B_1g and E_g arise from antisymmetric, symmetric bending and symmetric stretching of O–Ti–O bending vibrations in the TiO₂ lattice, respectively. As shown in Figure 7, the Raman peaks at 143, 195, 393, 510 and 635 cm⁻¹ correspond to the E_g, E_g, B_1g, A_1g + B_1g and Eg modes, respectively [51,68,69]. Comparing the Raman spectrum of pristine and Sr-doped TiO₂, a small blue shift was found due to Sr doping. The principal Raman peak at the E_g (143) mode arises due to external vibration of the anatase structure, which is attributable to the formation of the anatase phase [51,70]. This Raman spectrum was found to be compatible with XRD results.

3.6. BET Surface Area Studies

The catalytic property of materials depends upon the surface area, as the greater surface area of the catalyst is, the greater the number of surface-active sites. Therefore, to examine the catalytic activity of as-prepared nanomaterials, the surface areas of the as-synthesized pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ nanoparticles were determined. BET results, as shown in Figure 8a for all the as-synthesized samples, have type-IV isotherms and a H3 hysteresis loop. With the help of BET surface area analysis, the specific surface areas of the as-synthesized pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ nanoparticles were determined and found to be 169, 182, 178 and 141.16 m²g⁻¹, respectively. The surface area results were associated with the grain size trend, as observed in TEM and SEM studies of Sr-doped TiO₂ NPs. The average pore size distributions calculated by BJH studies of the as-synthesized pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ nanoparticles were found to be 17.03 ± 0.85, 16.95 ± 0.85, 16.97 ± 0.85 and 16.94 ± 0.85 Å, respectively, as depicted in Figure 8b, which reveals the mesoporous nature of the samples. The pore radius was calculated by using the Dubinin and Astakov (DA) method and was found to be 7–30 Å; the onset of the plots was observed at 14.0, 14.2, 14.0 and 13.7 Å for the pristine TiO₂ and 1, 2.5 and 5% Sr-doped TiO₂ NPs, respectively, as shown in Figure 8c. The surface area investigation with the band gap of as-synthesized materials is presented in

Table 1. BET surface area (S_BET), average pore size, pore volume (V_total) and energy band gaps (Eg) of as-synthesized nanocatalysts.

Sample	SBET/(m2g−1)	Average Pore Size/Å	Vtotal/(cm3g−1)	Eg/eV
Pristine TiO2	169.9	17.02 ± 0.85	0.359	3.19
1% Sr-doped TiO2	182.2	16.95± 0.85	0.351	3.16
2.5% Sr-doped TiO2	178.3	16.97 ± 0.85	0.426	3.18
5% Sr-doped TiO2	141.2	16.94 ± 0.85	0.294	3.20

. Further, from the results of UV-DRS and BET surface area, it was observed that the increase in surface area with the decrease in the particle size causes a decrease in the band gap of the nanoparticles, as reported in the literature [53,63,64].

3.7. Photocatalytic Water Splitting for Hydrogen Generation

The catalytic activity of as-synthesized nanoparticles was investigated for photocatalytic hydrogen generation by using water-splitting studies. The production of hydrogen is measured as a function of time, as shown in Figure 9a. The total apparent hydrogen evolution after 8 h was found to be 26.3, 16.8, 9.8 and 12.4 mmol$g_{cat}^{-1}$ for 1%, 2.5%, 5% Sr-doped TiO₂ and pristine TiO₂ nanoparticles, respectively, as shown in Figure 9b. The order of photocatalysts on the basis of hydrogen evolution activity is 1% Sr-doped TiO₂ > 2.5% Sr-doped TiO₂ > PristineTiO₂ > 5% Sr-doped TiO₂. Furthermore, 1%, 2.5% and 5% Sr-doped TiO₂ and pristine TiO₂ photocatalysts evolve an average per hour hydrogen of 3.3, 2.1, 1.2 and 1.6 mmol$g_{cat}^{-1}$h⁻¹, respectively, as presented in Figure 9c and comparison of results of the photocatalytic performance of metal-doped TiO₂ synthesized by different methods with present work summarized in

Table 2. Comparison of the photocatalytic performance of metal-doped TiO₂ synthesized by different methods.

S.No.	Dopant	Synthesis Method	Parameters	Hydrogen Production	Ref.
1.	Bismuth, nitrogen	Sol-gel method	Solar light, methanol	1800 μmol/g	[71]
2.	Nitrogen	Solid state/calcination method	UV–Vis light irradiation, Na2S/Na2SO3	18 μmol	[72]
3.	Platinum, nitrogen	Photodeposition method	1150 mL Pyrex vessel, UV light	3200 μmol	[73]
4.	Strontium, silver	Sol–gel method,	500 W Xe arc lamp	49.4 μmol/h	[74]
5.	Pd/0.2%K+	Hydrothermal method	_	76.6 μmol h−1	[75]
6.	Strontium	Hydrothermal method	200 W, Hg−Xe arc lamp	3.3 mmol$g_{cat}^{-1}$h−1	In this work

. The photocatalytic activity of 1% Sr-doped TiO₂ was found to be maximum with respect to other compositions and pristine TiO₂ photocatalyst. The increase of the Sr dopant concentration to the host TiO₂ NPs influences the hydrogen evolution rate and decreased the photocatalytic activity after attaining the maximum, as shown in Figure 9b. This study concludes that Sr dopant shows the corrosive effect of increasing concentration towards hydrogen evolution. A similar trend of corrosiveness was previously reported for Ag-doped SnO₂ nanoparticles [25]. The reproducibility experiment was performed under the same experimental conditions to analyze the stability and repeating ability of the as-synthesized photocatalysts for an 8 h reaction time. Here, the quantitative analysis of hydrogen evolution was performed with a gas chromatography analyzer. The rate of hydrogen evolution, reproducibility and stability were examined for three consecutive cycles, as depicted in Figure 9d. The experimental results show that the first and second cycles have approximately no change in hydrogen evolution (26.30 mmol${\mathrm{g}}_{\mathrm{cat}}^{-1}$) in 8 h, but in the third cycle, a small decrease in hydrogen evolution (25.93 mmol${\mathrm{g}}_{\mathrm{cat}}^{-1}$) was observed.

3.8. Possible Photocatalytic Reaction Mechanism of Sr-doped TiO₂ Nanoparticles

The hydrogen evolution of pristine TiO₂ and Sr-doped TiO₂ NPs as photocatalysts is illustrated in Figure 10. Here, the photocatalytic efficiency of pristine TiO₂ was observed to be less due to the recombination tendency of photo-induced e⁻/h⁺. Furthermore, the Sr-doped TiO₂ shows enhanced hydrogen evolution due to a decrease in the recombination tendency by enlarging the band gap and by providing more catalytic active sites, which are generated by enhancing the active surface area [25,63,64]. The photocatalysis efficiency is increased not only by suppressing the e⁻/h⁺ recombination, but also because of more generation of hydroxyl radicals (^●OH). Since Sr-doped TiO₂ NPs involve the replacement of some Sr²⁺ ions by Ti⁴⁺ ions, which cause charge imbalances, OH⁻ ions might be adsorbed at the catalyst surface and accept photogenerated holes, generating ^●OH. This favors the suppuration of photo-induced charge carriers and suppresses the recombination rate. In addition, Sr-doped TiO₂ NPs in solution easily provide electrons to H⁺ ions, which facilitates the hydrogen evolution of the system [51,76]. Hence, the holes generated because of electron abstracting by H⁺ ions at the valence band of pristine and Sr-doped TiO₂ nanoparticles as photocatalysts were consumed by the sacrificial agents sodium sulphide and sodium sulphite. To summarize, strontium doping causes increased surface area, altering the band gap and inducing defects, which are the reasons for the photocatalytic efficiency of the Sr-doped TiO₂ photocatalysts due to their suppressing the photo-induced e⁻/h⁺ recombination rate. The possible multi-step reaction for hydrogen evolution based on active sites and adsorbed excited state HS⁻ species is as follows:

Path A:

H₂O + *(active sites) → *OH⁻ + H⁺

OH⁻ + h⁺ → *OH

$$2\mathrm{OH}\to H_2{O}_2\rightleftharpoons \mathrm{OOH}+H^{+}\rightleftharpoons \mathrm{O}\begin{array}{c}2-\\ 2\end{array}+2H^{+}$$

H₂O₂ (a) + 2h⁺ → O₂ + 2H⁺

OOH(a) + 2h⁺ → O₂ + H⁺

$$\mathrm{O}\begin{array}{c}2-\\ 2\end{array}(a)+2h^{+}\to O_2$$

2H + →H₂

Path B:

$${\mathrm{HS}}^{-}(\mathrm{aq})\to {\mathrm{HS}}^{-}(\mathrm{ads})\begin{array}{c}hv\\ \to \end{array}[{\mathrm{HS}}^{-}(\mathrm{ads})]*$$

[HS⁻(ads)]* → H* + S⁻(ads)

$${\mathrm{S}}^{-}(\mathrm{ads})+[{\mathrm{HS}}^{-}(\mathrm{ads})]*\to [HS\begin{array}{c}2-\\ 2\end{array}]†$$

$$[HS\begin{array}{c}2-\\ 2\end{array}]†\to \mathrm{H}*+S\begin{array}{c}2-\\ 2\end{array}(\mathrm{ads})$$

2H* → H₂

[HS⁻(ads)]* + H₂O → S⁰ (ads) + H₂ + OH⁻

Here, ads = adsorption, * = excited state species and † = species undergoing intramolecular charge transfer [1,77,78].

3.9. Electrocatalytic Water Splitting Studies

The electrocatalytic activity of as-synthesized pristine TiO₂ and 1%, 2.5%, 5% Sr-doped TiO₂ NPs as electrocatalysts towards HER (Hydrogen evolution reaction) and OER (Oxygen evolution reaction) performances were determined using a three-electrode configuration in 0.5 N H₂SO₄ and 0.1 N KOH electrolytic solutions under ambient conditions, as demonstrated in Figure 11 and Figure 12. Here, HER responses were investigated by linear sweep voltammetry (LSV) and cyclic voltammetry (CV), as shown in Figure 11a–c. With the increase in the Sr concentration in TiO₂ nanocatalyst, an increase in the overpotential was observed at 10 mA/cm⁻² cathode current density, which revealed that 1% Sr-doped TiO₂ has high electrocatalytic HER activity. The overpotential values of pristine TiO₂, 1%, 2.5% and 5% Sr-doped TiO₂ electrocatalysts at 10 mA cm⁻² were 1.00, 0.96, 1.07 and 1.08 V, respectively. The OER performances of pristine TiO₂ and 1%, 2.5% and 5% Sr-doped TiO₂ nanoparticles were evaluated by LSV and CV measurements, as depicted in Figure 13a–c. From the LSV results of OER, superior electrocatalytic activity was shown by 5 % Sr-doped TiO₂ nanostructures, with an onset potential of 0.8 V as shown in Figure 13a, and an increase in its anodic current density, which produced 2.49 mA/cm² at 1.55 V applied potential. Compared with the previously reported current density versus RHE generated by IrO₂, the present works results with 5 % Sr-doped TiO₂ at 1.55 V were ~3.01 and 2.49 mA cm⁻², respectively, showing that 5% Sr-doped TiO₂ electrocatalyst has approximately similar potential as IrO₂ electrocatalyst [53,79]. With the help of HER and OER responses, the reaction kinetics and mechanism for electrocatalytic activity were elucidated using Tafel analysis. The reaction kinetics and mechanism are completely correlated with characteristic properties of nanoparticles (such as size, surface area, morphology and optical properties) and orientation of electrocatalysts [4,39,53,80]. Figure 11d and Figure 12d illustrate the Tafel polarization plots of the as-synthesized pristine TiO₂ and 1%, 2.5% and 5% Sr-doped TiO₂ electrocatalysts for the water electrocatalysis. The observed electrodic parameters (HER and OER) are presented in

Table 3. HER and OER electrodic parameters of pristine and Sr-doped TiO₂ electrocatalysts.

S. No	Materials	HER		OER
		Overpotential (V) to Attain 10 mA/cm2	Tafel Slope (mV/dec)	Onset Potential (V)	Anodic Current Density (mA/cm2) at 1.55 V	Tafel Slope (mV/dec)
1.	Pristine TiO2	1.00	133.33	0.8	0.86	272.94
2.	1% Sr-doped TiO2	0.96	84.09	1.23	0.30	135.09
3.	2.5% Sr-doped TiO2	1.07	139.07	1.34	0.87	170.66
4.	5% Sr-doped TiO2	1.08	146.16	1.1	2.49	91.60

. The decrease in resistance on the applied potential window were monitored at different scan rates from 10–100 mV. Chronoamperometric (CA) measurements were taken to test the durability and stability of 1% Sr-doped TiO₂ electrocatalyst operating at a voltage of-0.5 V; there was no change found in current density, which was almost stable, as shown in Figure 13. The stability of 5% Sr-doped TiO₂ was also examined through CA at 0.5 V voltage by running for up to 8000 s; it was observed that the current density was stable throughout the measurement (Figure 13). With the help of CV measurement of optimum HER and OER, electrocatalysis of 1% Sr-doped TiO₂ and 5% Sr-doped TiO₂ was carried out, as shown in Figure 11d,e and Figure 12c, respectively.

4. Conclusions

Pristine anatase TiO₂ and Sr-doped TiO₂ nanocatalysts with highly crystalline and tetragonal structure were successfully synthesized with an eco-friendly low-temperature hydrothermal method. Due to the various reinforcing and synergistic factors of the cubical architectures of the TiO₂ and its dopants, such as increased surface area, the introduction of defects created by doping and the band gap tuning of the material resulted in an enhanced photocatalytic and electrocatalytic performance. One-percent Sr-doped TiO₂ showed the best photocatalytic and electrocatalytic HER activity and 5% Sr-doped TiO₂ displayed optimum electrocatalytic activity towards OER. The maximum H₂ production of 1% Sr-doped TiO₂ was found to be 26.3 mmol$g_{cat}^{-1}$. Furthermore, 1% and 5% Sr-doped TiO₂ electrocatalysts showed maximum current density for both HER (≈10 mA/cm²) and OER (≈2.49 mA/cm²), with an onset potential of 0.96 V for HER and 1.55 V for OER with Tafel slopes of 84.09 and 91.60 mV/dec, respectively. The increase in the photocatalytic and electrocatalytic performances of the photo/electrocatalysts was attributed to the high specific surface areas and the tuning of the band gap introduced by the incorporation of dopants, and resulted in an overall increase in performance. The study thus reveals that the synthesized compounds can act as efficient materials for photo/electrocatalysis for water-splitting hydrogen evolution and current density for HER and OER, which makes them potential candidates for green/renewable energy resources.