Global development in any society is based on demand and supply of energy. It has been realized that existing sources of energy derived from fossil fuels are limited will not be able to fulfill the increasing demand for future generations. Apart from that, decarburizing our energy sources is also an important issue because their continuous use creates well-known hazards that threaten human health and are associated with global climate change, and are posing a great danger for our environment and eventually for the life on our planet (Vayssieres, 2009). World over innovations on finding sustainable and green alternate sources of energy are ongoing relentlessly to reach the level of generating “zero-carbon fuel Hydrogen”. Hydrogen has emerged out as one of the most promising energy carriers in recent times and also gaining increasing attention from the scientific community by virtue of its potency as efficient and carbon-neutral fuel of the future (Joy et al. 2018, Walter et al. 2010, Watanabe et al 2020, Palmer et al. 2020, Dawood et al. 2020).
Photoelectrochemical (PEC) technology is an ideal and promising way to produce hydrogen along with oxygen by harnessing solar energy from water splitting. Scientists world over are pursuing research on the design and synthesis of the most efficient electrode with characteristics like optical function for maximal absorption of solar energy, bandgap lying between 1.6-2.0 eV, corrosion resistance, straddling band edges with H2O redox potentials, long lifetime of charge carriers, long-term stability in aqueous electrolyte, etc. but so far all these desired characteristic has not been achieved in a single semiconductor(Choudhary et al. 2012).
TiO2 has been extensively explored for its application in hydrogen generation via photoelectrochemical and photocatalytic pathways referring to its properties like economic viability, environmentally friendly, stability, non-corrosive nature, abundance, etc (Hashimato et al. 2005, Naseri et al. 2010). Owing to a high bandgap of 3.2 eV, TiO2 absorbs primarily the ultraviolet portion of the solar spectrum and limits its use in the visible spectrum. To enhance its absorption in the visible region various modifications have been adopted like doping, dye sensitization, ion implantation, surface modification, the formation of composite system with other materials including plasmonic nanoparticles (Choi 2006, Subramanyam et al. 2020, Cao et al. 2020).
There are many reports which confirm that the presence of Au nanoparticles enhances the light absorption capability of TiO2 (Cheng et al. 2020, Barczuk et al. 2020, Li et al. 2014, Nguyen et al. 2021, Abed et al. 2021, Atabaev 2021). Das (2020) sensitized TiO2 with Au nanoparticles with their different concentrations (0.5-1 wt.%) and obtained the highest photocurrent of 10 µA cm− 2 with 0.7 wt.%. The presence of Au nanoparticles enabled their photoactivity in the visible region. Xu (2017) obtained 2500 µA cm− 2 photocurrent density on modifying the nanorod array of TiO2 with Au nanoparticles at 1.23V vs. RHE on account of increased charge carrier density and light absorption due to plasmonic Au nanoparticles. F. Su (2013) attached plasmonic gold nanoparticles on the branched nanorod array of TiO2 and obtained photocurrent of 125 µA cm− 2 under visible light (≥ 420 nm). Due to plasmonic Au nanoparticles, enhancement in light absorption was found as well as promoted charge carrier separation and mobility. Zhan (2014) investigated two different configurations of TiO2 viz. Au embedded in TiO2 and Au sitting on TiO2. They found that Au embedded in TiO2 outperforms in light absorption and photocatalytic response.Naik(2019) reported high photocatalytic activity in nitrogen-doped [email protected]2 core-shell structures fabricated by the hydrothermal method. A hydrogen evolution rate of 4800 µmol per gram per hour was achieved under xenon light irradiation for a time duration of 240 min. The N doping does not bring crystallinity changes in TiO2 while due to plasmonic Au impurity band is created between the valence band and conduction band of TiO2. The localized surface plasmon resonance band demonstrated by gold nanoparticles present in the core was found to be responsible for reduced electron-hole pair recombination rate. Theoretical simulations by Gelle (2017) have revealed that a more intense local electrical field is created within the semiconductor near plasmonic metal nanoparticles in Au in TiO2 configuration in comparison to Au on TiO2 systems. Thus, an increase in segregation of charge carriers and extended visible light absorption is feasible in plasmonic Au nanoparticles embedded in the semiconductor matrix. High water splitting efficiency of amorphous black TiO2 was reported by Shi (2018) on the use of plasmonic Au nanoparticles which resulted in photocurrent density of 2.82 mA/cm2 at 1.23 V vs RHE which is significantly higher than unmodified photoelectrode of Au-TiO2. The intermediate energy level in amorphous black TiO2 improves the charge carrier separation resulting in high photoresponse. Mataresse (2019) designed hierarchial Au/TiO2 nanostructures in different configurations viz. Au nanoparticles (NPs) as a bottom layer, top layer as well as in integrated assemblies for their application in water splitting and bisphenol A oxidation. They depicted the critical role of Au nanoparticles present at the bottom layer in exploiting the radiations through scattering effect and in turn the photoactivity of TiO2. J. Jun (2019) fabricated a large-area 3D moth-eye Au NP/TiO2/Au hierarchical structure by direct printing method and deposition process which showed 2–3 times high photocurrent density in comparison to 2D Au NP/TiO2/Au absorber in the visible region. Atef (2021)investigated the tuning of Au nanoparticles deposited by a sputtering method on titania nanotube arrays exhibiting high surface area. Au nanoparticles with an optimized diameter of 7–25 nm were distributed on titania nanotube arrays by adjusting the sputtering current. A computational study confirmed the redshift on account of localized surface plasmon resonance exhibited by Au nanoparticles. An eighty-six percent increase in photocurrent was observed in comparison to bare titania nanotube arrays. Liu (2021) fabricated ZnS-PbS quantum dot loaded Au/TiO2 photocatalysts and obtained absorption from UV to infrared region. The extended absorption reduced charge recombination and hydrogen production rate of 5011 µmol g− 1h− 1 was achieved in the presence of a sacrificial reagent. An increase in hydrogen production rate by virtue of enhanced charge separation and conductivity has been demonstrated in ternary CdS/Au/TiO2 photoanodes by Zhang et al(2021). A photocurrent of 2.78 mA/cm2 at 1.23 V vs RHE was achieved which was approximately over twice more than the pristine sample. Verma et al (2016) reported the influence of irradiation by low energy ion beams on Au/TiO2 thin films. Six times enhancement in photocurrent density was achieved compared with pristine sample on account of the reduced bandgap, charge transfer resistance, and higher value of Voc (open-circuit voltage).
The investigation on the role of depositing Au nanoparticles of different thicknesses by a sputtering method as a subjacent layer below TiO2 and its effect on the photoelectrochemical response has not been done so far to the best of our knowledge.