Photocatalytic water splitting technology based on TiO₂ semiconductor is a promising strategy for clean, low-cost, and environmental friendly H₂ production by utilizing solar energy. The whole process includes three crucial steps:solar light harvesting to excite electron from VB to CB of TiO₂, charge separation and transportation, and the catalytic reduction of H⁺ to H₂ evolution reaction. Many progresses are achieved on the first two steps, while much less researches are concentrated on the third step to improve catalytic activity with the utilization of the cocatalysts. Noble metal Pt as cocatalyst can obviously promote the H₂-producing rate in TiO₂ photocatalytic system. However, it has been restricted seriously due to its limited sources and high cost. Thus, developing cheap, earth-abundant, and high active noble-metal-free cocatalysts is very significant for high efficient photocatalytic water splitting. This review summarizes the research progress on noble-metal-free cocatalysts for TiO₂-based H₂-evolution half reaction, including transition metals, transition metal compounds, nanocarbon materials and nanocarbon-based composites. The role of different cocatalysts in catalytic performance improving, such as, content, structure, particle size, surface area, dispersity, synthesis method of the cocatalysts, and so on, was discussed in detail. And the challenges and perspectives of the research directions are also remarked.

Contents
1 Introduction
2 Fundamentals of photocatalytic water splitting and the role of cocatalyst
2.1 Fundamental of TiO₂-based photocatalyst
2.2 Process of H₂ production from water splitting
2.3 The roles of cocatalysts for H₂-evolution
2.4 Factors influencing the performance of cocatalysts
3 Noble-metal-free cocatalyst for H₂-evolution half reactions
3.1 Transition metal cocatalysts
3.2 Transition metal compounds
3.3 Nanocarbon-based cocatalysts
3.4 Catalytic mechanism
4 Conclusion and outlook