| Splitting water into hydrogen and oxygen using sunlight is a promising approach to achieve chemical synthesis of clean fuel from solar energy.The main challenge of this approach is to ensure that photogenerated holes efficiently oxidize water to O2,which involves a four-electron transfers and encounters a low-efficient PEC conversion.Transition metal oxides have been extensively studied as photoanode materials due to excellent photostability and high water-oxidation activity.Nevertheless,much lower valence band maximum(VBM)level mainly dominated by O 2p orbitals significantly enlarges their bandgaps and thus restricts their utilization for solar energy.Various approaches have been developed to increase the visible light photoactivity of metal oxides,but heteroatom doping is currently the most commonly used method.Herein,we report dopant activation study on the typical Ti O2 model and the detailed contents include the following:(1)We have demonstrated the activation of oxygen vacancies in N doped Ti O2photoanode model.Experimental characterization and DFT calculations consistently confirmed the oxygen vacancy incorporation made the hybridization of N 2p and O2p orbitals more sufficiently and thus increased the delocalization of N impurity states,which leaded to the significantly extended lifetime of photoexcited carriers and high-efficient charge separation efficiency in PEC water-splitting.As a result,the N/Ovac-Ti O2 delivered an unprecedented visible-light IPCE,far exceeding the values in N-Ti O2.Moreover,we further demonstrated this strategy is a general and facile approach to remarkably improve the PEC water splitting of other metal-oxides including anatase Ti O2,Zn O,Bi VO4 and other anion dopants,like carbon and sulfur doping,and even for photocatalytic degradation of organic pollutants.(2)We also have confirmed that Cr implantation and subsequent vacuum annealing process can greatly improve the PEC performance of Ti O2 photoanodes and achieve unprecedented visible light photoactivity.The optimized Cr-Ti O2-vac without any co-catalysts an IPCE of 6.8%at 450 nm,which is one of the best results reported for metal ion doped Ti O2 photoanodes.Notably,Cr-Ti O2-vac also showed almost no photocurrent decay after testing for 100 hours under visible light illumination.The combination of electrochemical studies and DFT calculation revealed that the remarkable visible light photoactivity of Cr-Ti O2-vac samples is due to the synergistic interaction between Cr-dopants and oxygen vacancies.Specifically,the Cr-dopants enhance visible light absorption by introducing impurity levels within the band gap of Ti O2,while oxygen vacancies function as shallow donors that increase carrier density and facilitate the separation and transport of photoinduced carriers.(3)we have successfully achieved the Midas touch transformation of pristine Ti O2 electrodes in visible light photoactivity via carbon/nitrogen co-implantation.Systematic study and analysis demonstrate that the excellent PEC performance of C/N-Ti O2 samples is attributed to the synergistic effects between doped C and N atoms,which not only greatly enhance the visible light absorption capability compared to mono doped samples,but also markedly facilitate the separation and transfer of photogenerated carriers.Therefore,the optimal designed C/N co-implanted Ti O2 NW arrays yield a photocurrent density of 0.76 m A/cm2 under visible light(>420 nm)which is about 253 times higher than that of the pristine Ti O2,and an IPCE of 14.8%at 450 nm without any other co-catalysts or hole scavengers,which is one of the best results among all Ti O2 based photoelectrodes reported to date.We believe that the effectiveness of ion implantation doping approaches and co-doping strategies could enable further improvement of the PEC water-splitting performance of many other semiconductor photoelectrodes. |
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