Design,Synthesis And Performance Of Hydrogenated TiO₂ And Its Heterojunction Nanomaterials For Photocatalytic Water Splitting

The efficient conversion of solar energy to chemical energy via semiconductor-based photocatalytic materials for producing clean and renewable hydrogen is an important topic in energy field.TiO₂,a multi-functional n-type semiconductor metal oxide with wide band gap,has become the hotspot of photocatalytic materials in the field of energy conversion due to its good chemical stability,low cost and high resistance to photocorrosion.The research of TiO₂-based photocatalytic nanomaterials mainly focuses on improving the light absorption and transfer of carriers.Enormous efforts have been devoted to promote the photocatalytic performance through inducing the mid-gap states,narrowing the band gap,increasing the active sites and setting up the built-in electric fields.The concrete strategies include elemental doping,hydrogenation treatment and forming heterojunctions with other semiconductors.Recently,the hydrogenated TiO₂ nanomaterials treated under H₂ atmosphere exhibited excellent photocatalytic performance under solar illumination,which has triggered world-wide research interest.Hydrogenation can introduce disordered structure,Ti³⁺?oxygen vacancy?,interstitial hydrogen and hydroxyls on the surface of TiO₂,which is a crucial method in band engineering of metal oxide semiconductors.Meanwhile,a self-hydrogenated shell containing hydroxyls and reduced titanium ions is observed on the TiO₂ surface at the initial stage of H₂ evolution.The surface-to-subsurface H diffusion made H₂ generation kinetically feasible by lowering the energy barrier of hydrogen?H–H bond?generation in TiO₂.Therefore,it is meaningful and valuable to explore hydrogenated TiO₂ photocatalytic nanomaterials for expanding their applications in photocatalytic hydrogen evolution.This paper mainly focuses on the novel method of hydrogenated nano TiO₂,the construction of homophase junction and heterostructure by hydrogenation as well as the selective hydrogenation of crystal surface.We have optimized the hydrogenation process under wet chemical conditions and synthesized a series of hydrogenated TiO₂photocatalysts with different structural and function gradients.The chemical compositions and structures of the materials are changed to engineer the band structures.This paper will clarify the key role of the introduced hydrogen in the photocatalytic hydrogen evolution,reveal the inherent mechanism of enhancing the photocatalytic activity over the hydrogenated TiO₂ photocatalysts,explore the role of the hydrogenation in constructing junction structures,and finally obtain a series of high-performance photocatalysts.The fundamental research is constructive in promoting the practical application of TiO₂ photocatalytic materials in the field of solar-electric conversion.The research work of this paper is mainly divided into the following aspects:1.A new method for synthesizing hydrogenated TiO₂ nanocrystals using TiH₂ as the hydrogen source under wet chemical condition is developed.Hydroxyls are introduced into TiO₂ nanocrystals to engineer the band structure and reduce the energy barrier of hydrogen formation,which can enhance the photocatalytic performance.The amounts of hydroxyls are controlled by adjusting the molar ratio of titanium source to oxidant during the hydrothermal treatment.TiO₂ nanocrystals with different colors?yellow,light yellow,blue,and gray?are obtained.The presence of hydroxyls leads to the increase of lattice parameters and lattice distortion in TiO₂,which could effectively suppress the recombination of carriers and induce impurity energy levels as band tails,thus narrowing the band gap.TiO₂ nanocrystals with higher concentration of hydroxyl groups show stronger light absorption,narrower band gap,and lower recombination rate of electron-hole pairs,leading to better photocatalytic performance.The blue TiO₂ nanocrystals exhibit the best photocatalytic performance in H₂ evolution.The maximum hydrogen evolution rate under solar light illumination?10.0 mmol/h/g?is 4.6 times superior to that of P25nanoparticles.The introduction of abundant hydroxyls effectively optimizes the structures and properties of TiO₂ nanocrystals,which reduces the recombination rate of carriers and the energy barrier of hydrogen formation while facilitates the light absorption for promoting photocatalytic performance.2.The crystal growth in hydrogenation process under wet chemical condition is controlled to regulate the composition,morphology,and structure energy band of TiO₂,obtaining hydroxylated TiO₂ single crystals?SCs?,polycrystals?PCs?,and MCs.The pH of the precursor during hydrothermal treatment has a great influence on the morphologies,structures,hydroxyl concentrations and band structures of the products.The highly hydroxylated TiO₂ MCs are constructed from crystallographically oriented nanocrystals with abundant hydroxyls in the moderate alkaline condition.The unique superstructure and large surface area of MCs enable the effective charge separation and transport.Moreover,the high density of hydroxyls could reduce the recombination rate of photo-generated free charges and energy barrier for hydrogen formation while facilitate light absorption,resulting in highly efficient photocatalytic activity.MCs exhibit the highest photocatalytic activity in H₂ evolution.Specifically,the maximum hydrogen evolution rate under solar light illumination?22.8 mmol/h/g?is 10.5 times higher than that of P25.Moreover,the MCs exhibit excellent catalytic stability.The hydrogen evolution rate could be maintained at 20.8 mmol/h/g under solar illumination after five cycles of reactions.3.Ti³⁺concentration difference between the surface and the interior of TiO₂nanocrystal is well-designed by hydrogenation,which constructs the quasi-core-shell homophase structure and engineers the energy band structure of TiO₂,thus improving the photocatalytic performance.Diverse Ti³⁺concentration differences are obtained via regulating the mass ratio of Ti-source to H-source.The high Ti³⁺concentration in the interior of TiO₂ could induce vacancy-related electronic states under the conduction band maximum and break the lattice periodicity and octahedral symmetry of TiO₆,inducing the blueshift of valance band maximum.Therefore,the quasi-core region of TiO₂ exhibits narrower bandgap compared with the quasi-shell region.The nano anatase TiO₂ quasi-core-shell homophase junctions induced by Ti³⁺concentration difference exhibit improved photocatalytic hydrogen evolution.To be specific,the maximum hydrogen evolution rate of 50.02 mmol/h/g is 25.4 times superior to that of commercial anatase nanoparticles under solar illumination.Besides,the photocatalytic activity of TiO₂ homophase junction remains stable.H₂ evolution rate is 49.21 mmol/h/g after five cycles of catalytic test,corresponding to activity loss of<2%.The promoted photocatalytic activities are ascribed to the constitution of a built-in electrical field between the quasi-shell and quasi-core induced by the band bending,which accelerates the spatial charge separation and suppresses the recombination of carriers.Moreover,the atomic-level contact at the homophase junction interface provides smooth channels for carrier transfer,resulting in more effective separation and transfer of photo-generated electrons and holes.4.The selective hydrogenation of commercial anatase TiO₂ nanocrystals is conducted by partially protecting the crystal surface,thereby constructing the multi-junctions with continuous built-in electric fields.TiO₂ nano heterostructure exhibits extended light absorption and effective spatial charge separation.Pt nanoclusters are photodeposited to protect the crystal surface of TiO₂ nanocrystals.The exposed surface of crystalline TiO₂ is then hydrogenated to induce the disordered layers.Finally,TiO₂ nano heterostructure with multi-junctions?Pt-TiO₂-H-Ag?is formed by self-catalyzed photoreduction of Ag nanoparticles on the disordered layers.Other than the n-n⁺junction fabricated at the contact region between crystalline TiO₂?n-TiO₂?and disordered layers?n⁺-TiO₂?,the Schottky diode and Ohmic contact are formed on Pt/n-TiO₂ and Ag/n⁺-TiO₂ interfaces,respectively,leading to the formation of multiple continuous built-in electric fields,which can accelerate the spatial separation of charge carriers.Meanwhile,the disordered layers on TiO₂ surface can effectively enhance the light absorption capacity.The resulting nano heterostructure with multi-junctions exhibits remarkably promoted photocatalytic performance.The maximum hydrogen generation rate of Pt-TiO₂-H-Ag under solar illumination is18001.0?mol/h/g.Meanwhile,the H₂ generation rate under visible illumination is2382.7?mol/h/g.The corresponding apparent quantum efficiency of Pt-TiO₂-H-Ag is15.8%?420 nm?.The nano heterostructure with multi-junctions also exhibits excellent stability after five cycles,remaining hydrogen evolution rates of 15581.5 and 2211.4?mol/h/g under solar and visible illumination,respectively.