Co-catalyst Modification Of One-Dimensional Semiconductor Nanoarrays For Enhanced Photoelectrochemical Performance

Energy crisis and environmental pollution are two of the most difficult problems for mankind so far.Photoelectrochemical water splitting for hydrogen generation is promising to address the above issues by using the free and abundant sunlight as a clean power source.Among the semiconductor catalytic materials,monocrystalline silicon and hematite are intensively used for photoelectrochemical water splitting because of their low cost,non-toxicity,chemical stability and narrow band gaps.Because of the high surface area and the special one-dimensional morphology,one-dimensional nanoarrays can provide not only more active sites for water splitting,but also fast and direct charge carrier transport pathways to enhance the separation efficiency of the electron-hole pairs.Therefore,preparation of one-dimensional silicon nanowire（SiNWs）arrays and iron oxide nanorod（Fe2O3 NRs）arrays is an effective way to improve the photoelectrochemical activity of monocrystalline silicon and iron oxide.However,the silicon nanowire arrays and the iron oxide nanorod arrays have large kinetic over-potential and low separation efficiency of electron-hole pairs for water splitting.Herein,we prepared SiNWs array and Fe₂O₃ NRs array,and modificed them by loading co-catalysts to improve their photoelectrochemical activity.The following results are mainly achieved:Firstly,silicon nanowire arrays were prepared and then modified by electrochemically reducing graphene oxide onto the SiNWs to obtain the reduced grapheneoxide/siliconnanowire（rGO/SiNWs）arrayphotocathodes.Photoelectrochemical measurements were performed in a solution of 0.1 mol·L^-1H₂SO₄ and 0.5 mol·L^-1 K₂SO₄（pH=2）.Compared to the bare SiNWs,the obtained rGO20/SiNWs photocathode exhibits the highest photocurrent density of-5.22mA·cm^-2 at-0.5 V vs.RHE,which is 29.5 times of the bare SiNWs(0.18 mA·cm^-2 at-0.5 V vs.RHE).Moreover,the onset potential of the rGO20/SiNWs is 0.25 V vs.RHE,representing a positive shift by 0.66 V in comparison with that of SiNWs（-0.41 V vs.RHE）,because of the high electrical conductivity of rGO and improved separation of the photogenerated electron-hole pairs.This work shed light on the application of the facile electrochemical reduction approach to enhance the performance of solar water splitting devices involving silicon and graphene-based materials.Secondly,using NiCo layered double hydroxide（NiCo LDH）as the co-catalyst,NiCo/Fe2O3 NRs photoanode was fabricated on FTO-glass substrate by a two-step hydrothermal method.Photoelectrochemical measurements were performed in 1mol·L^-1 NaOH solution（pH=13.6）.NiCo/Fe₂O₃ NRs photoelectrode shows the highest photocurrent density with 1.3 mA·cm^-2 at 1.23 V vs.RHE,which is 3 times the bare Fe₂O₃ NRs photoelectrode.Moreover,the photocurrent density of NiCo/Fe2O3 NRs photoelectrode is larger than that of single metal hydroxide modified electrodes of Ni/Fe2O3 NRs and Co/Fe2O3 NRs.Furthermore,the onset potential of the NiCo/Fe₂O₃ NRs photoelectrode is 0.85 V vs.RHE,representing a negetive shift by 0.15 V in comparison with that of bare Fe2O3 NRs photoelectrode because of the synergistic effect of the NiCo LDH and Fe2O3,which allows electron-holes to be quickly and efficiently separated for water splitting.In addition,the enhanced photoelectrochemical activity of NiCo/Fe2O3 NRs photoelectrode was investigated by transient photocurrent test,Mott-Schottky test and impedance spectroscopy.The results show that the co-catalyst of NiCo LDH can reduce the interfacial charge transfer resistance of Fe2O3,improve the lifetime of photogenerated carriers,promote the effective separation of photoelectron-hole pairs,increase the carrier concentration and promote the carrier migration,consequently leading to the enhanced photoelectrochemical performance of Fe2O3 NRs.