1 Introduction

Climate change is a serious global issue that is mainly driven by the rise in greenhouse gas emissions, mainly carbon dioxide (CO2). This is creating problems of global warming, rising sea levels, ocean acidification, biodiversity loss, extreme weather events and health effects. To lessen these climate-related issues, it is critical to reduce carbon emissions and transition to a low-carbon, sustainable energy arrangement. This includes executing plans such as transitioning to renewable energy sources, growing energy efficiency, accepting sustainable land-use practices, and upholding carbon capture and storage technologies [1]. The International Paris Agreement aims to tie nations to fight against climate change by setting targets for dipping greenhouse gas emissions. Addressing carbon emissions and mitigating climate variation is a complex and crucial challenge that needs cooperative global efforts to defend the environment and confirm a sustainable future by creating green and sustainable energy.

Among various forms of alternative sustainable energies green hydrogen is measured as the best one owing to its high energy values, easiness of preparation and low cost. As the world seeks to change to a low-carbon, sustainable energy system, green hydrogen is important as a valuable resource for attaining these goals [2]. The most significant benefits of producing and using green hydrogen are decarbonizing energy, energy storing, sector integration, transport, heavy industry, fuel for power generation, energy independence, job creation, international trade and collaboration, environmental welfare and long-term sustainability. There are numerous methods for green hydrogen manufacture. The most significant include electrolysis, biomass gasification, thermochemical water splitting, biological hydrogen production, nuclear hydrogen production, hydrogen from seawater, photo-electro-chemical (PEC) water splitting and direct radiation water splitting. The last method of direct radiation water splitting is the most significant as it requires very low-cost raw materials like freely available water and radiation. Keeping the reputation of the green hydrogen mandate and the availability of low-cost water and radiation energy, attempts are made to discuss hydrogen production by direct radiation water splitting methods. Besides, efforts are also made to deliberate the challenges and future perspectives. This article will be a strength for academicians, researchers, industrial personnel and Government authorities to make more advancements in the direct radiation water-splitting method of hydrogen production.

2 Updates in direct radiation water-splitting methods of hydrogen production

All the updates in this area are discussed in the following sub-sections.

2.1 Direct water splitting by visible radiations

Generally, water splitting by visible light is denoted as photo-electro-chemical (PEC) water splitting. This is a procedure that utilizes sunlight to split water into hydrogen and oxygen in an electrolyzer. The electrolyzer has two electrodes comprising the supply of the electric current. Direct solar water splitting uses advanced materials, such as semiconductors and catalysts. The direct sunlight hits the photo-catalyst and water molecules on the photo-catalyst surface and gets water molecules split into hydrogen and oxygen. The core of the process is a photo-catalyst, which is naturally a semiconductor material precisely designed to absorb visible light. The normal photo-catalyst materials for visible-light-driven water splitting comprise tungsten oxide (WO3), bismuth vanadate (BiVO4), and modified titanium dioxide (TiO2). The photo-catalyst absorbs visible light, exciting electrons from the valence band to the conduction band, and creating electron–hole pairs. The photo-excited electrons contribute to the reduction reaction, which converts water into hydrogen gas (H2).

The visible light (400 to 800 nm) has more than 50% solar energy and about 16% solar-to-hydrogen energy conversion efficiency (STH), which may be harvested by utilizing an appropriate photo-catalyst. Hence, development of photo-catalysts in long-wavelength is an important area of research [3]. In 2002, Hitoki et al. [4] prepared Ta3N5 and used it as a photo-catalyst. This was because this material absorbs strongly at 600 nm wavelength. This material was modified by Pan, et al. [5] and the authors prepared LaMg1/3Ta2/3O2N and used for water splitting at 600 nm. The authors reported a good amount of hydrogen evolution. Maeda et al. [6,7,8,9,10] carried out a remarkable work in water splitting. The authors reported the preparation of Ga1-xZnx)(N1-xOx photo-catalyst for water splitting [6, 7]. Furthermore, the same group [8] prepared oxy-sulfides and oxy-nitrides of transition metal ions for water splitting because of their excellent water-splitting features. Again this group reported the combination of ZrO2/TaON and Pt/WO3 and got 6.3% AEQ at 420 nm [9]. In 2013, the same workers [10] described direct water splitting for hydrogen at λ > 400 nm by a modified TaON photo-catalyst. The modification of a less-defective TaON (ZrO2/TaON) with core/shell-structured RuOx/Cr2O3 nanoparticles and colloidal IrO2 was observed to be indispensable for getting good hydrogen production. The value of 6.3% AEQ at 420 nm was augmented by 6.8% Chen et al. [11] reported the preparation of MgTa2O6-xNy/TaON hetero-structure and used to produce hydrogen. The authors used MgTa2O6–xNy/TaON as HEP and PtOx-WO3 as the OEP. The authors claimed 6.8% as apparent quantum efficiency (AQE) at 420 nm.

Zou et al. [12] reported the direct water splitting for hydrogen production by using a series of indium-tantalum-oxides [In1-xNixTaO4 (x = 0–0.2)] with nickel. The task was achieved by direct visible light irradiation. The authors reported 0.66% as the quantum yield of hydrogen production. Furthermore, the authors suggested that solar energy with suitable photo-catalysts can provide a viable source of green hydrogen by increasing surface area and suitable modifications of the semiconductors [(In1-xNixTaO4 (x = 0–0.2)]. In 2014, Martin et al. [13]; first of all; reported g-C3N4 semiconductor for water splitting. The authors reported 36 and 18 μmol/h.g H2 and O2 in 14 h. Li et al. [14] used Pd as a co-catalyst on GaN-ZnO for hydrogen production with a good amount of hydrogen production. Tao et al. [15] reported a series of photo-catalysts i.e. bismuth tantalum oxyhalide, Bi4TaO8X (X = Cl, Br), with valence band and conduction band placed at ≈ − 0.70 and ≈1.80 eV. These materials were found to be effective for both water reduction and oxidation under visible light irradiation. Wang et al. [16] prepared and used Y2Ti2O5S2 with a band gap of 1.9 eV. The hydrogen evolution was augmented by adding IrO2 and Rh/Cr2O3 as oxygen and hydrogen evolution co-catalysts. Oshima et al. [17] used an artificial Z-scheme for water splitting by using metal oxide nano-sheets. The authors used HCa2Nb3O10 nano-sheets sensitized by a Ru(II) tris-diimine type photo-sensitizer, in a grouping of WO3 for water splitting. The authors reported 2.4% as an apparent quantum yield.

Basically, the Z-scheme is an idea used to clarify the procedure of water splitting in photosynthesis, which occurs in the thylakoid membranes of chloroplasts in plant cells. In this scheme, the oxidation and reduction alter in the photosynthesis light reactions. The electrons are removed from the water and then given to non‐excited oxidized P680. The Z-scheme is so named as the graphical representation of the electron transfer reactions involved looks like the letter Z. Fujito et al. [18] reported a good amount of hydrogen evolution by using Bi4NbO8Cl. The authors described Z-scheme water splitting by coupling with Rh-doped SrTiO3. Ma et al. [19] also described the Z scheme of water splitting by using oxysulfides (La5Ti2CuS5O7 and La6Ti2S8O5) as photo-catalysts. Qi et al. [20] prepared and used Ta3N5 for hydrogen generation following the Z scheme. The same Z-type scheme was adopted by Wang et al. [21] for producing hydrogen generation from water by using aza-fused microporous polymers. The authors reported a satisfactory performance. Sun et al. [22] also described the application of La5Ti2CuS5O7 photo-catalyst for hydrogen production following Z-scheme. Song and co-workers [23] described Z-scheme water splitting with Pt/NiS-loaded LTA as hydrogen and PtOx-WO3 as oxygen evolution photo-catalysts. Pan et al. [24] prepared carbon nitride/reduced graphene oxide/Fe2O3 and used for water splitting. This Z-scheme system proved to be talented for photocatalytic water splitting under sunlight.

Recently, many workers have been working in the area of water splitting for hydrogen production. Cai et al. [25] prepared ZnIn2S4 modified with separated dual co-catalysts, which exhibited encouraging photocatalytic activity for hydrogen generation. Sun et al. [26] fabricated in-situ ultrathin ZnIn2S4 nanosheets for water splitting without co-catalysts in visible light irradiation. The average photo-catalytic hydrogen and oxygen evolution rates of the resultant ultrathin Zni-ZIS nano-sheets were 42.80 and 19.10 µmol/g.h in visible light irradiation, with an apparent quantum yield (AQE) of 1.51% at 420 nm without noble-metal co-catalyst. Sindhu et al. [27] prepared co-doped V@S-Ta3N5/PANI composite materials for water splitting for hydrogen production. The authors reported remarkably high photocatalytic activity of 3.8 times higher than pure Ta3N5 at 98.4 mmol/g.h. Wu et al. [28] designed a ternary hetero-junction of rhombic phase In2O3 (rh-In2O3)/cubic phases In2O3 (c-In2O3) homo-junction combined CdIn2S4 (rh/c-IO/CIS), which was used to split water (4.7 mmol·g.h). This was 3.6 and 7.8 times higher than that of CdIn2S4 and rh/c-In2O3, separately. This work provided a new tack for coherent fabrication and design of photo-catalysts with improved photo-catalytic hydrogen evolution. Nowadays, many workers are working in the area of water splitting for hydrogen production. It is not possible to describe all papers herein. Interested readers should consult some available reviews on visible-light-driven water splitting [29,30,31,32,33].

2.2 Direct water splitting by UV radiations:

UV radiation in the range of 200–400 nm is being used for water water-splitting phenomenon. The procedure includes the utilization of photo-catalysts, which are materials that can absorb UV light and enable the splitting of water molecules. The simple steps of the water splitting in this procedure are photon absorption, redox reactions and the collection of products. When a photo-catalyst is exposed to UV light, it absorbs photons, producing excited electrons (e−) and holes (h+). In the redox reaction, the excited electrons can reduce water (H2O) to make hydrogen ions (H+), while the holes can oxidize water to produce oxygen gas (O2). However, most photo-catalysts function with ultraviolet light, which accounts for only 4% of the incoming solar energy and, thus, makes the overall process unfeasible. Scientists have been working on refining the efficiency of photo-catalysts to make the process more applied since 1980 [34,35,36,37,38]. Among some photo-catalysts, SrTiO3 is found to show good UV absorption features. On the other hand, aluminium-doped SrTiO3 has also shown remarkably high stability and efficiency [39,40,41,42].

Onsuratoom et al. [43] enhanced the photo-catalytic power of TiO2 by preparing Ag-loaded mesoporous assembled TiO2-ZrO2. These photo-catalysts showed good hydrogen evolution properties. Machín et al. [44] incorporated different amounts of gold nanoparticles (1–10 wt.%) on zinc oxide nanowires (ZnO NWs), and zinc oxide nanoparticles (ZnO NPs) surfaces for water splitting. The authors reported 559 to 853 μmol/h.g as the highest amount of hydrogen evolution at 400 nm. Furthermore, the authors reported many folds of increment for hydrogen production by incorporating gold metal. In 2020, scientists in Japan effectively split water into hydrogen and oxygen utilizing light [45] and accurately designed catalysts, and they did so at the maximum with almost no loss [45]. Rani et al. [46] prepared reduced graphene oxide (rGO; 1–10 wt%) coated with TiO2 and Ag (1.0 wt%)-TiO2 core–shell nano-composites and used for water splitting. The amount of hydrogen evolution increased 5 to 14 mmol after rGO(1–3 wt%) coating over TiO2. Not much work has been carried out because of only 5% availability of UV radiation in the solar spectrum on the earth. This limitation makes UV radiation technology of little use and, hence, researchers are not focusing on UV radiation-mediated water splitting. Some other papers describing the direct water splitting under visible and UV radiations are summarized in Table 1.

Table 1 Hydrogen generation from water splitting using visible and UV radiations
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2.3 Mechanism of photo water splitting

The direct water splitting by photo radiations includes numerous steps. This procedure is enabled by specialized materials called photo-catalysts [1]. The photo-catalytic water-splitting reaction can be brief as follows:

$$ {\text{2H}}_{{2}} {\text{O }}\left( {{\text{water}}} \right) \, + {\text{hv }}({\text{photon}}) \to {\text{2H}}_{{2}} \left( {{\text{hydrogen}}} \right) + {\text{O}}_{{2}} ({\text{oxygen}}) $$
(1)

The mechanism can be, generally, divided into the following processes.

Absorption of light: The photo-catalyst absorbs light energy, which begins the water-splitting process.

Generation and separation of charge carriers: When the photo-catalyst absorbs light, electrons are excited from the valence band to the conduction band, making electron–hole pairs (e−/h+). These charge carriers then separate, with electrons moving to the surface of the photo-catalyst and holes moving to the bulk.

Redox reactions: The separated holes react with water molecules, causing them to dissociate into oxygen and hydrogen ions (oxidation reaction). Then electrons reduce hydrogen ions to form hydrogen molecules (reduction reaction). In this way, two molecules of H2O are splitted into 1 molecule of O2 and 2 molecules of H2. The complete reactions are shown below.

$$ {\text{Photo}} - {\text{catalyst }} + {\text{ hv}} \to {\text{e}}^{ - } + {\text{ h}}^{ + } $$
(2)
$$ {\text{Oxidation reaction}}:{\text{ 2H}}_{{2}} {\text{O }} + {\text{4h}}^{ + } \to {\text{O}}_{{2}} + {\text{4H}}^{ + } $$
(3)
$$ {\text{Reduction reaction}}:{\text{ 2H}}^{ + } + {\text{ 2e}}^{ - } \to {\text{H}}_{{2}} $$
(4)
$$ {\text{Overall reaction}}:{\text{ 2H}}_{{2}} {\text{O }} + {\text{ 2e}}^{ - } \to {\text{2H}}_{{2}} \left( {\text{g}} \right) + {\text{O}}_{{2}} \left( {\text{g}} \right) $$
(5)

Photo-catalyst Regeneration: These species must be able to regenerate by accepting another photon to continue the process. This recurring process can occur as long as there is a supply of visible or UV light.

There are two types of procedures for photo-catalytic water splitting i.e. one-step and two-step ones. In the first-step process, a single photo-catalyst is accountable for both water oxidation and reduction; demanding a suitable thermodynamic potential for both reactions. In the second-step process; stirred by the natural Z-scheme; two different photo-catalysts are used i.e. one for water oxidation and the other for water reduction. The schematic representation of one-step and two-step processes of water splitting is shown in Fig. 1.

Fig. 1
figure 1

Photo-water splitting, a direct and b two-step processes

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2.4 Water radiolysis

Water radiolysis is a procedure in which water (H2O) is disintegrated into its constituents i.e. hydrogen (H2) and oxygen (O2), by the stroke of ionizing radiation. This process comprises the breaking of chemical bonds in water molecules owing to the energy deposited by high-energy radiation, usually ionizing radiation such as gamma rays, X-rays, or high-energy particles (e.g., electrons, protons, neutrons). Water radiolysis has been studied and utilized in numerous applications, including hydrogen generation for nuclear and space exploration.

Debierne was the first worker to hypothesize that the radiolysis of water takes place via the generation of H• atom and the OH• radical [69]. Since then many workers attempted to use this technique for several products; especially green energy hydrogen. Yamada et al. [70] reported hydrogen production of acidic (0.4 M H2SO4) water radiolysis having alumina as the catalyst. The authors reported increased hydrogen amount by augmenting the absorbed dose rate in the region of 1–5 kGy/h. Crumière et al. [71] studied the effect of gamma and cyclotrons effect on hydrogen production. The values of Linear Energy Transfer (LET) were in the range of 0.23 and 151.5 keV/μm. Southwortha et al. [72] reported ZrO2 nanopowders for water radiolysis to produce green hydrogen. The authors optimized hydrogen production with a good amount of hydrogen produced. McGrady et al. [73] reported the water radiolysis on ZrO2 nanoparticles' surface using gamma radiation. The authors reported increased yields of H2 and O2 on increasing nanoparticle surface area. Kumar et al. [74] reported a Monte Carlo simulation study of water radiolysis for hydrogen generation. The authors described that gamma water radiolysis created excited hydrogen atoms; leading to hydrogen production.

Two groups i.e. one in India and another in Azerbaijan are working together a lot in this area. These authors used various catalysts to produce hydrogen from water splitting by gamma radiation irradiation. The most important catalysts used are alumina [75, 76], zirconium [77, 78] and niobium alloy [79]. These authors used a metro-system of pure water, sea water and water with organic solvent in the presence of the above-cited catalysis. These authors carried out radiolysis in thermal, radiation and radiation-thermal modes. In general, the yields of hydrogen were in the order of radiation-thermal > radiation > thermal modes. The effect of time, temperature, radiation dose, amount of catalysts and the particle size of the catalysts were studied. The nano-sized catalysts were found to be effective in producing a good amount of hydrogen. Overall, in their efforts, these authors reported a good amount of hydrogen production during their experiments. Recently, these authors described [2] the modeling of hydrogen production by taking a mixture of water and hexane. The authors reported various rates of production of H2, CH4, C2H6, C3H8, C4H10, and C5H12 as 3, 0.12, 0.56, 0.57, 0.3 and 0.06 × 10–14 molecule/s. Monte Carlo modeling recognized the kinetics of the increase of radicals and radiolysis products. It was established that the reactions happened in spurs concerning the hydrated electron eaq, H•, H+, OH•, H2, OH−, and H2O2, and the number of reactions comprising these particles was 10. The same group presented an interesting review article on the role of radiation on water splitting. The authors discussed various radiations for water splitting along with the use of different catalysis [80]. Briefly, these two groups are very active in hydrogen production using the radiolysis method.

Water radiolysis has been utilized in nuclear reactors to create hydrogen for numerous applications, including cooling and as a potential energy carrier. In space exploration, it has also been assumed that creating hydrogen from water resources on celestial bodies like the Moon or Mars, where nuclear reactors could be used as a radiation source. Nevertheless, the challenges and safety concerns associated with water radiolysis are of main concern in specialized applications.

Still, radiolysis is not successful in producing hydrogen at a large scale. It needs the optimization of the radiolysis process. The most important parameters that need to be optimized are radiation source, radiation dose, and radiolytic byproducts. There is a great need for high-energy radiation sources like particle accelerators or nuclear reactors and controlling careful doses of both radiation and catalysts. There is also a great need to develop more advanced catalysis; preferably nanoparticles for a good amount of hydrogen production. There is also a great demand to separate green hydrogen from other radiolysis products (H2, HO., H., H2O., H3O+, OH−, H2O2, etc.). Minimizing and managing the production of these byproducts is essential. Safety concerns are also important because the radiation of high energy is used. The good quality of hydrogen can be prepared by using pure water. Therefore, water quality is also important in this process. Some other papers describing direct water splitting under nuclear radiations are summarized in Table 2.

Table 2 Hydrogen generation from water splitting by nuclear radiations
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2.5 Mechanism of water radiolysis

The mechanism of water radiolysis comprises the breakdown of water (H2O) into hydrogen (H2) and oxygen (O2), as well as the formation of various radical species, owing to the interactions of ionizing radiation with water molecules. The mechanism of water radiolysis is well documented as many workers attempted to explain it [85,86,87]. The complete reaction of water hydrolysis is written as shown below.

$$ {\text{H}}_{{2}} {\text{O }} + {\text{ Radiation}} \to {\text{e}}^{ - } + {\text{ H}}_{{2}} ,{\text{ HO}}^{.} ,{\text{ OH}}^{ - } ,{\text{ H}}.,{\text{ H}}_{{2}} {\text{O}}^{.} ,{\text{ HO}}_{{2}}^{.} ,{\text{ H}}_{{2}} {\text{O}}_{{2}} ,{\text{ H}}_{{3}} {\text{O}}^{ + } ,{\text{ etc}}. $$
(6)

Basically, this reaction is carried out in three steps i.e. physical, physico-chemical and chemical stages. The physical stage is attained in about 1.0 fs after the starting matter-ionizing radiation interactions and contains energy deposition followed by a fast relaxation phenomenon. This conveys the creation of ionized water molecules (H2O+), excited water molecules (H2O*) and sub-excitation electrons (e−). The physico-chemical stage (10–15 to 10–12 s) comprises numerous processes to occur; including ion–molecule reaction, dissociative relaxation, auto-ionization of excited states, thermalization of sub-excitation electrons (solvation of electrons), hole diffusion, etc. The reactions occurring in this stage are shown below.

$$ {\text{H}}_{{2}} {\text{O}}^{ + } + {\text{ H}}_{{2}} {\text{O}} \to {\text{H}}_{{3}} {\text{O}}^{ + } + {\text{ OH}}^{*} $$
(7)
$$ {\text{H}}_{{2}} {\text{O}}^{*} + {\text{ HO}}_{{2}} \to {\text{OH}}^{*} {\text{ + H}}^{*} $$
(8)
$$ {\text{e}}^{ - } \to {\text{e}}^{ - }_{{{\text{aq}}}} $$
(9)

The third chemical stage (10–12 to 10–6 s) includes the interactions of various species followed by their diffusion. Thus, these react with each other along with neighboring molecules. The path of the particles enlarges because of the diffusion of radicals and their succeeding chemical reactions. These three stages are shown in Fig. 2.

Fig. 2
figure 2

Mechanism of water radiolysis

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The radiolysis process of water in heterogeneous systems is strongly influenced by the solid–liquid boundary, and all three (physical, physico-chemical and chemical) stages of the process in those systems differ from pure water to other liquid media. It should be noted that when ionizing rays pass through the substance, energy transfer takes place via interactions among the atoms or molecules. This leads to the formation of electron–ion pairs, electron-excitation, active intermediate particles and other species. The presence of the catalysts plays a crucial role in all these interactions and fragmentations. The electrons and holes generated in the catalyst due to radiation are captured by volume defects. The holes migrate to the surface and are captured by the surface –OH group and form H+:

$$ h^{ + } + - OH \to - O^{*} + H^{ + } $$
(10)

Subsequently, H + proton captures the electrons (e−) captured by the surface and an H atom is formed:

$$ H^{ + } + e_{t}^{ - } \to H^{*} $$
(11)

Only a few authors have paid attention to the study of electrons, which are formed by the impact of ionizing rays on heterogeneous systems, which play a major role in the formation of molecular hydrogen. Regardless of the mechanism of acquisition, two hydrogen atoms combine to form molecular hydrogen:

$$ H^{*} + H^{*} \to H_{2} $$
(12)

2.6 Catalysis for water splitting

Catalysis plays a vital role in the process of photo-water splitting, also known as photo-catalytic water splitting. This procedure utilizes a photo-catalyst to assist the splitting of water into hydrogen (H2) and oxygen (O2) when irradiated to light, naturally ultraviolet (UV) or visible light. The catalysts augment the efficiency of the reaction by dropping the activation energy and quickening the rate of the reaction. Effective photo-catalytic water splitting trusts the design and collection of appropriate photo-catalyst materials, catalysts, and reaction conditions. The selection of photo-catalyst can considerably affect the reaction's efficiency, with common materials including titanium dioxide (TiO2), strontium titanate (SrTiO3), and more advanced materials like bismuth vanadate (BiVO4) and tungsten trioxide (WO3). Investigators are continuously discovering new materials, co-catalysts, and strategies to improve the performance of photocatalytic water splitting systems, as this procedure has the talent to be a sustainable and environmentally friendly method for hydrogen manufacture using solar energy. Refining the efficiency and stability of photo-catalysts and co-catalysts is essential for enhancing hydrogen production [88, 89].

The band gap of a photo-catalyst is a crucial parameter that controls its ability to aid water splitting through the absorption of light and the creation of electron–hole pairs. The band gap signifies the energy difference between the valence band (VB), where electrons are bound to atoms, and the conduction band (CB), where electrons are free for movement. In the situation of water splitting, the band gap of a photo-catalyst is important because it affects the absorption of light and generation of electron–hole pairs [90, 91]. In the situation of water splitting, ideal photo-catalysts have band gaps that meet the band gap corresponding and redox potential arrangement criteria. The investigators are dynamically discovering and developing new materials with tailored band gaps and electronic structures to improve their presentation for water splitting under various light sources.

The catalysis in water radiolysis plays a dynamic role in hydrogen production. The catalyst can be in the form of a solid, liquid, or gas, and it can be made of numerous materials, such as metals, semiconductors, or even biological catalysts like enzymes. The selection of catalyst depends on the specific reaction circumstances, such as type of ionizing radiation, pH of the water, and temperature. The use of catalysts can significantly enhance the efficiency and controllability of this process. The catalysis offers the surface for water decomposing supplementing the hydrogen product. The most significant catalysts used in radiolysis are alumina, zirconium, niobium and their alloys.

2.7 Challenges

Photo-water splitting for hydrogen production is an encouraging area of research in the field of renewable energy and hydrogen generation. Nevertheless, it also faces several challenges, and issues, which need to be addressed [92]. Some of the major challenges are efficacy enhancement, catalyst development, material sturdiness, resource accessibility, scaling up, storage and transportation, etc. The efficiency of the current methods is not quite good, which requires enhancement in the quantum efficiency. This problem is because of the reasonable working capacity of the present photo-catalysts with restriction of band gaps and vulnerability to degradation. Some materials are expensive and cannot be used for catalysis. The scaling of hydrogen, storage and transport are the big glitches [93].

The challenges in water radiolysis for hydrogen production comprise numerous issues like separation of hydrogen and oxygen, methods of fair efficiency, limited photo-catalysis capacity, incorporation with nuclear reactors, etc. Water radiolysis can yield by-products such as HO., OH−, H., H2O., HO2., H2O2, H3O+, etc. The managing and control of the formation of these by-products is vital to confirm high purity of the produced hydrogen. Also, separation methods are desired to get pure hydrogen. Maximizing the efficiency of the water radiolysis process to upsurge the yield of hydrogen is another challenge. The collection of suitable catalysts for the hydrogen evolution reaction (HER) and for suppressing the oxygen evolution reaction (OER) is significant for optimizing the procedure but absolute high-quality catalysis is not obtainable. The assimilating water radiolysis systems within nuclear reactors while ensuring safe and dependable operation is a complex task that requires engineering proficiency. For space exploration, developing compact and consistent radiolysis systems for generating hydrogen from accessible water resources on celestial bodies, such as the Moon or Mars, presents a unique set of challenges.

2.8 Future perspectives

As the request for clean energy and hydrogen grows, the development of a hydrogen economy can offer new chances for photo-water splitting applications in transportation, industry, and energy storage. Hydrogen generation by photo-water splitting has a great future but it needs to be produced at a large scale economically. The finding of new materials with tailored band gaps, enhanced charge separation, and improved stability is crucial for advancing photo water splitting. The developing materials, such as perovskite compounds, and graphene derivatives should be developed and used as photo-catalysts. The tandem solar cells, which combine multiple layers of semiconductors with different band gaps, can capture a broader spectrum of sunlight; leading to more effectual water splitting. The developing tandem cell configurations are future viewpoints. Researchers are discovering artificial photosynthetic systems that mimic natural photosynthesis. These systems use biomimetic methods to capture and convert sunlight into hydrogen and other energy carriers. The merging of photo water splitting with renewable energy sources such as solar and wind can offer a reliable and clean energy supply; particularly for areas with ample access to sunlight. Government policies and private sector investments in research, development, and technical development skills can considerably drive progress in this field [94, 95].

The future perspective for photo water splitting for hydrogen production is quite hopeful, as it denotes a sustainable and environmentally friendly method of generating hydrogen, a clean energy carrier. It is very significant to minimize the various by-products formed by tuning the amount of catalysis, radiation and temperature. The mixing of water with suitable reagents may help in this area. The more advanced catalysis should be used; particularly the nanomaterials of varied sizes. Water radiolysis is a key technology for making hydrogen and oxygen on other celestial bodies. Research and development in this area will be essential for future lunar and Martian missions. Hydrogen produced through water radiolysis can be combined into a broader energy ecosystem, including applications in energy storage, transport, and industrial procedures. Efforts to decrease the cost of hydrogen production through water radiolysis will make this technology more inexpensive than other hydrogen manufacturing methods. The progressions in safety protocols and reactor design will continue to make water radiolysis safer and more effectual, mainly within the nuclear industry. The Governments of various nations and international organizations may play an important role in determining policies, rules, and standards for the safe and effective use of water radiolysis technology.

3 Conclusion

Green hydrogen is a key component in the transition to a low-carbon and sustainable energy system, and ongoing research and development efforts are aimed at making these production methods more effectual and cost-effective. Green hydrogen is being produced by using many methods but the generation of this sort of energy by direct hit of radiations to water on suitable catalysis (direct radiation water splitting methods) is a method of choice due to their economic nature. In this category, water is generated by direct photo-water splitting and water radiolysis. The choice of method depends on factors like the availability of renewable resources, infrastructure, cost, and the specific goals of the hydrogen production project. During the write-up of this article, it was realized that both these methods i.e. direct photo water splitting and water radiolysis are not fully developed and under their development stages. There are many challenges but the future is quite bright because both methods involve inexpensive energy sources i.e. solar and nuclear radiation. It is realized that these methods may be the choice in the future to obtain pure green hydrogen at a large scale economically.