Ball milling is a widely employed top-down approach for reducing bulk TiO₂ into nanosized particles，thereby increasing its surface area and enhancing the availability of active sites for photocatalytic reactions （Fig.2a）. Additionally，this process induces surface oxidation，leading to the formation of a dense Ti oxide layer that limits oxygen penetration，effectively slowing further oxidation and improving the material’s stability. Notably，ball milling is a simple，cost-effective，and environmentally friendly technique. Compared with the direct mixing of Cu₂O and TiO₂，ball milling facilitates a more intimate contact between the two components，thereby maximizing H₂ generation. However，excessive milling time and speed can significantly reduce the H₂ production rate，highlighting the importance of optimizing these parameters （Fig.2b）^[45].

In another study，Pd clusters supported on TiO₂ were synthesized via ball milling^[29]. Compared with samples prepared using conventional wet chemical methods，the Pd/TiO₂-BM photocatalyst exhibited a substantially enhanced photocatalytic H₂ production rate. The predominant Pd species generated through ball milling were in the metallic state，which promoted the effective separation of photogenerated electron-hole pairs. Under UV irradiation，a unique interaction between Pd clusters and TiO₂ was observed，playing a critical role in stabilizing the photocatalytic reaction. Importantly，the ball milling technique enables the formation of stable metal clusters on oxide supports without requiring organic ligands for active site stabilization. Thus，compared with traditional synthesis routes，ball milling represents a viable and effective alternative for fabricating visible-light-active composite photocatalysts^[29].

Laser ablation is an effective technique for fabricating TiO₂ nanostructures by irradiating bulk material with high-energy laser pulses，allowing precise control over nanoparticle size and morphology （Fig.2c）. This method can be categorized into two main approaches：laser irradiation of conventional TiO₂ powder dispersed in liquid，which results in partial oxygen loss，and pulsed laser ablation of a bulk metallic Ti target in liquid^[32].

In recent years，pulsed laser ablation in liquid （PLAL） has gained increasing attention for synthesizing a variety of catalysts，including those for photocatalytic H₂ production. Studies on TiO₂ synthesis via PLAL have examined key ablation processes，including the physical and chemical transformations occurring in the plasma cloud，target surface，and surrounding solution. Using PLAL，researchers have successfully synthesized and optimized dark TiO₂ for the photocatalytic H₂ evolution reaction （HER）^[32]. Notably，dark TiO₂ exhibits strong absorption across the entire visible light spectrum and demonstrates enhanced photocatalytic efficiency. The highest activity was observed in samples annealed at 400 ℃，characterized by narrowly dispersed TiO₂ with defective anatase crystals. Furthermore，modification with low concentrations of Pt further enhanced photocatalytic performance，increasing the H₂ evolution rate by a factor of five. This enhancement was attributed to the increased photogenerated carrier density and reduced electron transfer resistance，indicating that Pt incorporation facilitates more efficient charge carrier dynamics.

Chemical etching employs acids or other chemical agents to selectively remove material from bulk TiO₂，generating nanostructured surfaces that enhance light absorption and reaction kinetics （Fig.2d，e）^[46-47]. This method is particularly effective for improving photocatalytic performance by increasing surface roughness and active sites. A novel acid-etched CFA/TiO₂ nanocomposite was successfully synthesized using HCl etching and the sol-gel method，with CFA serving as the raw material^[35]. The photocatalytic degradation efficiency of the acid-treated CFA/TiO₂ nanocomposite was significantly higher than that of the untreated CFA/TiO₂. HCl etching increased the surface roughness of CFA，allowing UV light to undergo multiple scattering within the sample，thereby enhancing photocatalytic efficiency. The combination of low cost and high degradation efficiency makes CFA a promising support material for TiO₂ nanoparticles （NPs），expanding its potential for high-value applications.

Recently，researchers utilized a honeycomb aluminum plate as a metal substrate for TiO₂ powder deposition，aiming to facilitate the practical application of TiO₂ photocatalysts in toluene oxidation. To enhance the stability of the TiO₂ coating，surface etching was performed using TiCl₄ as a binder^[36]. Compared to anodic oxidation etching，acid etching of the metal substrate exhibited superior load stability and photocatalytic activity. These improvements were attributed to the increased surface roughness of the metal substrate and the strong chemical adsorption between TiO₂ and TiCl₄，leading to enhanced catalyst-substrate interactions.

Sol-gel synthesis is a widely used method for producing TiO₂ NPs. This process involves the hydrolysis and condensation of Ti alkoxides to form a gel，which is subsequently heated to induce crystallization. However，high-temperature annealing，often required to complete crystallization，can lead to unintended phase transitions. Specifically，anatase and brookite，both metastable phases，tend to irreversibly transform into rutile under prolonged thermal treatment^[37]. Additionally，extended high-temperature exposure may cause the collapse of porous structures and tunnels，reducing the material’s surface area and catalytic efficiency^[48]. Another critical challenge is uncontrolled nanoparticle aggregation，which can diminish the photocatalytic performance by reducing the available active sites. To mitigate this，surfactants are commonly employed as surface-directing or capping agents，reducing surface tension and enhancing solubility to prevent particle agglomeration^[49].

Ionic liquids （ILs） have emerged as effective soft templates for the controlled synthesis of TiO₂-based photocatalysts due to their broad liquid temperature range，high thermal stability，and solubility in both organic and inorganic compounds. More importantly，their self-assembly properties facilitate the design of well-ordered nanostructures. For instance，Choi et al.^[50] synthesized mesoporous nano-TiO₂ by hydrolyzing tetraisopropyl titanate （TTIP） in 1-butyl-3-methylimidazolium hexafluorophosphate （[Bmim][PF₆]） at room temperature. The resulting TiO₂ nanocrystals exhibited excellent crystallinity and thermal stability，maintaining their phase composition even after exposure to 800 °C. The IL-assisted sol-gel method enables the synthesis of nano-TiO₂ at relatively low temperatures by leveraging the self-assembly properties of ILs to guide mesoporous structure formation and control crystal growth （Fig.3a）. Additionally，ILs enhance the thermal stability of anatase and inhibit its transition to rutile during heat treatment. This stabilization effect is primarily attributed to the π-π stacking interactions of the imidazole ring，which facilitate the polycondensation and crystallization of TiO₂ nuclei，thereby improving phase stability and structural integrity.

The hydrothermal method utilizes high-temperature and high-pressure conditions to facilitate the formation of TiO₂ nanostructures. This approach is relatively simple，requiring fewer preparation steps，and is advantageous due to its environmental friendliness，low cost，and high tunability. It enables the synthesis of well-defined nanostructures with controlled morphologies，such as nanorods，nanotubes （NTs），and hierarchical structures （Fig.3b）^[51]. These structural features increase the surface area of the catalyst，enhance light absorption efficiency，and improve charge carrier extraction^[52]. For instance，TiO₂ photoanodes synthesized via the hydrothermal method exhibit enhanced charge transport performance due to the formation of highly crystalline structures and the suppression of electron-hole recombination.

The hydrothermal synthesis process typically involves dissolving a Ti precursor，such as TiCl₄，in a solvent and subjecting the solution to high temperature and pressure within a sealed container. These conditions enable precise control over the size，morphology，and crystal structure of the resulting TiO₂ NPs^[51]. Despite its advantages，the hydrothermal method also presents certain limitations，including scalability and reproducibility challenges，relatively low crystallinity in some cases，and the tendency for particle aggregation，which can reduce the photocatalytic efficiency^[41].

CVD is a gas-phase synthesis method that involves the decomposition of volatile precursors onto a substrate to form a thin film of TiO₂. This technique is commonly used to create uniform，high-quality coatings of TiO₂ on various surfaces. Moreover，the precise deposition of TiO₂ films during the preparation of photoanodes can improve charge transport and reduce recombination loss，thereby improving photoelectrochemical （PEC） performance^[53]. The doping technology in the CVD growth process can control the doping concentration and design the band gap，thereby enhancing light absorption in a wider solar spectrum. In addition，CVD promotes the growth of complex nanostructures with a high aspect ratio and large surface area，promoting light absorption and efficient charge carrier extraction，thereby improving PEC efficiency. The ZnO nanowires and ZnO-TiO₂ core-shell nanowires developed by Jeong et al.^[54] can precisely control the thickness of the shell （Fig.3c）^[41]. However，CVD still has some limitations. One limitation is the high cost and complexity of the equipment required for CVD synthesis，and the challenge of preparing high-quality films. In addition，the use of toxic and harmful precursor gases requires careful treatment and safety measures.

PVD is a process in which the solid is converted from a condensed state to a gas phase and then condensed into a film^[55]，which can be used to produce high-quality TiO₂ films. Advanced PVD technology can provide precise control of the surface morphology of TiO₂ photoanodes，allowing the generation of nanostructures or textured surfaces，thereby increasing active sites and improving light absorption efficiency^[56]. In addition，the composition of TiO₂ thin films can be controlled by adjusting the deposition parameters and material selection so that the band gap engineering can expand the absorption range of solar radiation and improve the utilization of solar energy.

Sputtering is a variant of PVD technology^[41]. TiO₂ films can be rapidly deposited on different substrates by RF magnetron sputtering. This method allows tailoring film properties in an economical，efficient and scalable manner，providing new possibilities for the production of advanced high-performance materials that are in direct contact with the final carrier. However，due to the high vacuum environment and temperature requirements of PVD，as well as the complex system architecture，it has a higher cost and more difficult operation than CVD. This greatly reduces the scalability of the PVD process.

Doping TiO₂ with metal elements is one of the most effective strategies for narrowing its band gap and enhancing its visible light absorption. Precious metals such as Pt and Au can serve as co-catalysts to facilitate charge separation. Pt and Au are more stable and durable than base metals，providing high activity for prolonging photocatalytic function^[57]. In an experiment，TiO₂ NSs with exposed （001） facets were prepared in a Ti（OC₄H₉）₄-HF-H₂O mixed solution. Then，under the irradiation of a xenon lamp，Pt NPs were deposited on TiO₂ NSs by the photochemical reduction method to obtain Pt/TiO₂ nano-photocatalyst （Fig.4a）^[58]. The researchers found that when the Pt/TiO₂ nanophotocatalyst was placed in an ethanol aqueous solution，the Pt loaded on the TiO₂ NSs increased the photocatalytic H₂ production rate，and even 2 wt% deposited Pt showed the highest catalytic activity. However，the disadvantage of Pt is that it is rare and too expensive. In addition，when methanol is decomposed during the photocatalytic process on the Pt/TiO₂ catalyst，CO，H₂ and other gases are produced. CO with a concentration of about 2.7 vol% was observed in H₂. This is a problem that needs to be solved because a very small amount of CO will poison the catalyst and thus affect the catalytic activity.

Au can also be used as a doping component on TiO₂ semiconductors to prepare photocatalysts. Researchers used an ultralow concentration of CO to produce H₂ by photocatalytic decomposition of methanol on an Au/TiO₂ catalyst （Fig.4b）^[58]. It is found that the H₂ production rate is significantly improved when the size of Au particles is reduced from 10 nm to less than 3 nm. In addition，with the decrease of Au particle size，the absorption of CO will decrease （Fig.4c）. This is because when the intermediate product formic acid is decomposed to form methanol，it mainly produces CO. When the Au particle size decreases，the decomposition of formic acid stops，and the main product CO stops producing；only H₂ and CO₂ are produced as by-products. However，Bamwenda et al.^[59] compared the hydrogen production activity of Au/TiO₂ and Pt/TiO₂ catalysts. The hydrogen production activity of the Pt sample was 30% higher than that of the Au sample. This is because Au is highly dependent on the preparation method compared to Pt. When the calcination temperature increases from 573 K，the hydrogen production of the Au sample is lower than that of the Pt sample.

Non-precious metal doping offers a cost-effective and environmentally benign strategy to enhance the photocatalytic properties of TiO₂. Such dopants not only suppress the recombination of photogenerated charge carriers—thereby improving photocatalytic and PEC performance—but also contribute to band gap narrowing，which facilitates broader visible-light absorption^[64]. In addition to their low cost and chemical stability，non-precious metal elements often introduce defect states or modulate the electronic structure of TiO₂，enabling more efficient utilization of solar energy. For example，Cu as a catalyst improves charge separation and enhances the photocatalytic activity of TiO₂ for H₂ production. Under visible light irradiation，Cu-TiO₂ exhibits an H₂ evolution rate comparable to that of Au-TiO₂^[65]. This can be attributed to the plasma surface effect of Cu-NPs and Au-NPs. Since the direct excitation of TiO₂ requires UV light，the activity of these TiO₂ materials can be achieved by the excitation of Cu and Au plasmon bands. In another experiment，Cu-doped mesoporous TiO₂ hybrid nanostructured composites were used for photocatalytic H₂ production from methanol-containing water under 300 W artificial light irradiation （Fig.4d）^[66]. The co-catalyst properties of Cu metal and the Schottky junction formed between Cu metal and TiO₂ n-type semiconductor play an important role in the separation and transmission of electron-hole pairs，the prevention of recombination and the improvement of HER potential （Fig.4e）. Among them，the H₂ evolution potential of TiO₂-Cu-2.5% is 23 times that of a pure TiO₂ catalyst. According to X-ray photoelectron spectroscopy （XPS） analysis，it can be inferred that the doping of Cu on TiO₂ follows interstitial doping. The higher photocurrent density of the TiO₂-Cu-2.5% catalyst indicates that Cu particles are beneficial to the separation and transport of photogenerated electron-hole pairs. Electrochemical impedance spectra （EIS） confirmed that the TiO₂-Cu-2.5% photocatalyst followed a small semi-circular pattern，indicating that the material had a medium-low charge transfer resistance and effective interface charge transfer potential. Similarly，the relatively low PL intensity of the TiO₂-Cu-2.5% catalyst indicates that the material has greater potential to inhibit electron and hole recombination. Therefore，based on the aforementioned optical properties，Cu-doped mesoporous TiO₂ hybrid nanostructured composites can provide a high HER rate. However，the performance of a single non-precious metal cocatalyst is quite limited and exhibits poor long-term stability due to its easy oxidation. The proper combination of non-precious metals can optimize the overall performance of metals and may exceed the durability of precious metals^[67].

Precious metals like Pt and Au have high doping efficiency but high cost，and need to solve the problem of CO by-products. The doping of non-precious metal Cu is economical and environmentally friendly，and the performance is close to that of precious metal，which is a more promising alternative. Future research can focus on optimizing metal particle size，loading and doping methods to balance activity，stability and cost.

The use of non-metal-doped TiO₂ can reduce its band gap，which is an effective way to enhance its visible light absorption capacity and H₂ evolution ability. The addition of non-metallic dopants to the TiO₂ matrix may cause structural defects in the material，form impurity energy levels，narrow the band gap，and help overcome the shortcomings of common carrier capture and thermal instability of metal NPs^[68]. N is one of the most studied non-metallic dopants because it has a similar atomic size to O，forming a metastable energy state，and there is a considerable overlap between N2p and O2p states，resulting in a narrowing of the band gap and the smooth excitation of electrons between the valence band and the conduction band^[69]. The results show that compared with pure TiO₂，the optical properties，valence band characteristics，and band position of N-doped TiO₂ samples have changed. Moreover，compared with pure TiO₂，N-doped TiO₂ samples show charge separation and an increase in charge carrier density.

C doping is also an effective method to modify TiO₂. It has been reported that C can not only reduce the band gap by forming an intermediate energy level in the band gap of TiO₂ but also promote the adsorption of organic matter and the transport of photogenerated carriers inside TiO₂^[70]. Studies have shown that the addition of C and Nd dopants can increase the light absorption intensity of TiO₂ and broaden the light absorption range to visible light^[68]. The absorption spectrum of pure TiO₂ is not extended to the visible light region，but the absorption range of TiO₂ is extended to the visible light region by C doping and C，Nd co-doping，which improves the photocatalytic efficiency of TiO₂ （Fig.4f）. In another study，the photodegradation of 4-chlorophenol under UV and visible light showed that C-doped TiO₂ exhibited better catalytic activity under visible light because the C-containing compounds on the surface of the catalyst were removed under UV light，which weakened the catalytic synergy between the C-containing substances on the surface of the catalyst and the crystalline C^[70]. An appropriate amount of dopant is very important to improve the catalytic activity of TiO₂. This is because，on the one hand，excessive dopants will lead to a large number of defects in the TiO₂ lattice，inhibiting the separation of photogenerated carriers；on the other hand，too many carbides will be formed on the surface of TiO₂，which will affect the light absorption，hide the active sites and reduce the photocatalytic activity.

Non-metal elements N and C doping is an effective means to regulate the band structure of TiO₂，which can significantly improve the visible light response and catalytic activity. However，it is necessary to optimize the doping concentration and method to balance the band gap narrowing and defect suppression to avoid side effects. In the future，more non-metal element combinations can be explored to further optimize the photocatalytic performance of TiO₂.

Graphene is the first 2D material with outstanding advantages，such as a high specific surface area and good electron transfer ability^[76]，which minimizes the reconsolidation of photogenerated charges and improves the photocatalytic yield of the material. Graphene has no band gap，while TiO₂ has a larger band gap. Therefore，the combination of graphene and TiO₂ is considered to be the perfect combination for photocatalytic applications^[77]. The simultaneous doping of metal ions and graphene into TiO₂ is a continuous research topic to improve photocatalytic H₂ production activity. Lang et al.^[78] prepared Ag-rGO-TiO₂ composites by depositing Ag nanocubes and TiO₂ nanolayers on the surface of reduced graphene oxide （Fig.6a~c）. In addition，the author also prepared Ag-TiO₂ as a comparison. Under visible light irradiation，the H₂ production rate per unit mass of Ag-rGO-TiO₂ was 0.53 μmol/（g·h） in methanol aqueous solution，while Ag-TiO₂ and TiO₂ did not show photocatalytic activity （Fig.6d）. This enhanced activity is considered to be the performance of rGO as a conductive bridge to transfer electrons from Ag to TiO₂. In addition，the Schottky barrier formed on the surface of rGO-TiO₂ enhances the diffusion of hot electrons from rGO to TiO₂. For the Ag-TiO₂ structure，the hot electrons and holes are recombined due to the absence of a barrier.

The co-doping of metal Ag and graphene can synergistically enhance the photocatalytic activity of TiO₂，in which the electron conduction and interface barrier regulation of graphene are crucial. The future direction is to explore other metal-graphene composite systems and optimize the interface structure to further improve the charge separation efficiency.

Metal oxide has excellent chemical stability and corrosion resistance，and the formation of complexes with TiO₂ can prolong the life of the catalyst^[82]. Moreover，metal oxide has the characteristics of enhancing photocatalytic activity，which can expand the light response range of the TiO₂ photocatalyst and inhibit electron-hole recombination. Encouragingly，metal oxide/TiO₂ is a TiO₂-based composite with superior HER performance. Taking ZnO as an example，it has the advantages of high photochemical stability，a wide band gap，good photosensitivity and non-toxicity，and has been widely studied in photocatalytic applications. Therefore，the composite of ZnO and TiO₂ is used in the field of photocatalytic HER^[77]. For example，Xie et al.^[79] mixed TiO₂ and ZnO in different proportions before depositing Pt as a co-catalyst for H₂ evolution （Fig.6e）. The results showed that 0.5% Pt/TiO₂-ZnO exhibited the best photocatalytic H₂ production performance，and the H₂ production rate per unit mass was 2150 μmol/（h·g） under visible light irradiation. The H₂ production rates per unit mass of TiO₂ and ZnO are 68 and 3.0 μmol/（h·g），respectively. In addition，the durability of the catalyst was tested for H₂ production. Compared with the first-day test，the H₂ production rate on the 7th and 14th days was only reduced by 12% and 23%，respectively （Fig.6f）.

A promising method for preparing metal oxide/TiO₂ composites is to use metal-organic frameworks （MOFs） as precursors. MOFs，known as the first members of the cage-like porous material family，are constructed by the combination of metal clusters and organic compounds. MOFs have outstanding properties such as high porosity，large surface area and adjustable chemical structure，and have many uses，including being used as sacrificial templates to prepare metal oxide/TiO₂ composites^[77]. Mondal and Pal^[80] used Cu-based MOF as a sacrificial template to prepare composites. The yield of the optimized Cu/CuO/TiO₂ hybrid nanocomposites under sunlight irradiation is much better than that of the traditional CuO/TiO₂ hybrid system，which is due to the formation of a small heterojunction and the loading of Cu into the TiO₂ matrix （Fig.6g）. Dekrafft et al.^[83] prepared a Fe₂O₃@TiO₂ core-shell structure using Fe-based MOF and then deposited Pt on its surface for photocatalytic H₂ production. Compared with Fe₂O₃，TiO₂ and their mixtures，the catalyst showed a faster H₂ production rate.

Metal oxide/TiO₂ composites have high activity and stability，and the MOF template method provides a new way for the synthesis of composite materials. In the future，it is necessary to solve the cost problem of complex preparation processes and explore more efficient and inexpensive metal oxide combinations.

The direct interaction between sulfide and TiO₂ has a short band gap and is easy to use in visible light^[84]. Their good response to sunlight is due to the presence of conduction bands at high positions compared to metal oxides. In addition，metal sulfides have good adsorption capacity，and this effective adsorption reduces their oxidation capacity，making them poor reagents in water decomposition. Therefore，these systems require a certain sacrificial agent to produce H₂ under the condition of photocatalytic reduction of water.

The band gap of CdS is 2.42 eV，and the smaller band gap makes it have a high application value by photocatalytic decomposition of water under visible light. However，it has a faster electron-hole pair recombination rate，which makes its photocatalytic efficiency lower. On the contrary，TiO₂ has good electron-hole pair recombination，making it a more effective photocatalyst，but its band gap is 3.42 eV，which limits its application in the visible region. Therefore，researchers began to focus on the study of CdS/TiO₂ composites to overcome the obstacles of photocatalytic decomposition of TiO₂ and CdS to produce H₂^[85]. Researchers have used different methods such as WI，CR，and photo deposition （PD） to platinum the CdS/TiO₂ composite photocatalyst to produce H₂ in the visible light region （Fig.6h，i）^[86]. To this end，the metal Pt is implanted on the composite material. Because of the extra high and low states generated by the WI，CR and PD processes，Pt becomes more important so that the efficiency of the CdS/TiO₂ composite photocatalyst is higher. On the other hand，the photocatalysts of WI and PD are composed of electron-deficient Pt. In WI，the formation of Pt-Ti fills this defect because the recombination of electron-hole pairs takes a longer time for two electron-deficient photocatalysts，resulting in higher efficiency of water splitting for H₂ production. In another experiment，the researchers used titanate NTs to realize the variable diameter systematization of CdS/TiO₂ composites through a simple ion change and vulcanization process at medium temperature^[86]. The use of this composite as a photocatalyst increased the yield of H₂. In this process，an almost 43.4% yield was obtained in the visible light region with a wavelength of about 525 nm. This may be due to the synergistic effect between TiO₂ NTs and CdS NPs，as well as the uniform distribution and quantum effect of CdS particles.

After the combination of sulfide CdS and TiO₂，the visible light hydrogen production efficiency is significantly improved by matching the energy band structure. Pt loading （such as the electron-deficient Pt-Ti bond formed by the WI/PD method） and nanostructure control （such as TiO₂ NTs and CdS NPs composite） can optimize charge separation and transport，and the hydrogen production rate is increased. In the future，it is necessary to combine new sulfides，atomic interface engineering and in situ characterization techniques to promote the development of efficient，stable and low-cost photocatalytic systems.