Effect Of Defect Control On Photoelectric Properties Of G-C₃N₄ And ZnO Materials

With the development of economy and society,environmental pollution and energy shortages have become increasingly prominent,and it is extremely urgent to find a sustainable and clean energy source.Semiconductor photocatalytic technology drives a series of important chemical reactions through sunlight to convert low-density solar energy into high-density chemical energy or directly degrade and mineralize organic pollutants.At the same time,photocatalytic decomposition of water to decompose water into hydrogen and oxygen is a green,effective,safe,environmentally friendly and promising method for generating renewable energy from sunlight.In recent years,among many semiconductor photocatalyst materials,non-metal graphite phase carbon nitride?g-C₃N₄?has gradually attracted the attention of the scientific research community.As a new type of photocatalytic material with graphite phase structure,carbon nitride is widely used in the field of photocatalysis with its narrow band gap,suitable valence band position and catalytic stability.In this paper,the g-C₃N₄material has insufficient visible light absorption and high photoelectron-hole recombination rate,and the photoelectric properties are improved by adjusting the doping concentration of hetero atoms to introduce defects.At the same time,the traditional zinc oxide photoelectrode was modified by simple and economical electrochemical reduction.The main contents are summarized as follows:The first part:Constructing isotype g-C₃N₄g-C₃N₄ heterojunction is an approach to improve the efficiency of g-C₃N₄ towards solar-assisted oxidation of water.Such functional configuration can effectively overcome the intrinsic drawback of rapid charge recombination of g-C₃N₄.Here,a modified g-C₃N₄,with homogeneous phosphorus doping,is prepared in this work through a phosphide-involved gas phase reaction.The resulting P-g-C₃N₄ displays altered electronic structure,including upshifted band edge potential,narrowed band gap and improved electronic conductivity.These features allow P-g-C₃N₄ as an outstanding candidate to form isotype junction with pristine g-C₃N₄.As expected,the accelerated charge separation and migration in target junction is validated by various measurements.The optimized isotype g-C₃N₄/P-g-C₃N₄ heterojunction achieves a photocurrent as high as 0.3 mA·cm^-2 at 1.23 V vs RHE(AM 1.5G,100 mW·cm^-2),representing 8-fold's enhancement compared with pristine g-C₃N₄.The present strategy for constructing g-C₃N₄-based isotype heterojunction networks is found effective for large-scale manufacturing.In the second part:Graphitic carbon nitride?g-C₃N₄?has been widely explored as photocatalyst for water splitting.The anodic water oxidation reaction?WOR?remains the major obstacle for such process,with particular issues on low surface area of g-C₃N₄,poor light absorption as well as low charge transfer efficiency.In this work,such longtime concerned issues have been partially addressed with band gap and surface engineering of nanostructured graphitic C₃N₄.Specifically,surface area and charge transfer efficiency are significantly enhanced via architecturing g-C₃N₄ on nanorod TiO₂ to avoid the aggregation of layered g-C₃N₄.Moreover,a simple phosphide gas treatment of TiO₂/g-C₃N₄ configuration not only narrows the band gap of g-C₃N₄ by0.57 eV into visible range,but also in-situ generates a metal phosphide?M=Fe,Cu?water oxidation cocatalyst.This TiO₂/g-C₃N₄/FeP configuration significantly improves charge separation and transfer capability.As a result,our non-noble metal photoelectrochemical system yields outstanding visible light?>420 nm?photocurrent:ca.0.3 mA·cm^-2 at 1.23 V and 1.1 mA·cm^-2 at 2.0 V vs RHE,the highest using g-C₃N₄as photoanode.We expect that our TiO₂/g-C₃N₄/FeP configuration generating via simple phosphide gas treatment will bring in new insight for robust g-C₃N₄ for water oxidation.The third part:Recently,a un-element doping protocol i.e,treating the oxygen containing semiconductor with highly reducing agents,is popular for improving the photoelectrochemical?PEC?performance of electrodes.However,such reduction method has not yet shown impressive success for ZnO photoanode.In this work,a simple and safe electrochemical method is explored to reduce ZnO nanowire arrays at-1.0 V vs SCE in 0.5 M Na₂SO₄.Following this treatment,ZnO have a remarkably enhanced photocurrent.Compared to successful example of other materials with chemical treatment,such mild electrochemical reduction method is also capable of creating oxygen vacancies in ZnO but with less concentration.Unobservable change in color for reduced samples,together with the results of optical spectra indicates the preservation of electronic structures of ZnO during reduction.Although this protocol is not so effective for extending working spectrum of ZnO,it dramatically improves the light absorption in UV region.Without doubt,this method affords new avenues for modifying semiconductor feature of ZnO,and may hold huge potential application in other inorganic semiconductors.