Constructing High-Efficiency Photocatalysts Based On Bonding Structure Differences

Photocatalytic water splitting using semiconductor photocatalysts can convert renewable solar energy into chemical energy stored in hydrogen with high energy density and environmental friendliness,thus addressing the challenges related to energy supplies and environmental degradation.The low utilization of visible light,weak charge separation ability and poor catalytic activity of photocatalysts,all of which are determined by the bonding structures of materials,limit the solar energy conversion efficiency.Aiming at improving intrinsic properties of photocatalysts,this thesis takes the advantage of the bonding structure differences in photocatalysts on the types and strength as the breakthrough point,with materials of typical bonding variety,i.e.graphite carbon nitride(GCN),layered perovskite material BaLa4Ti4O15 and multi-metal oxide LaTaO4 as the subjects of study,by means of thermal treatment,element doping and hydrothermal etching as the methods,to adjust the atomic structure,electronic structure and phase composition of photocatalysts.Details are listed as below.An amorphous carbon nitride photocatalyst with greatly extended visible-light-responsive range for photocatalytic hydrogen generation was constructed through the thoroughly breaking of weak interaction.Graphite carbon nitride(GCN)as a layered material has strong C-N covalence bonds and weak hydrogen bonds in the layers,and Van der Waals forces between the layers.By thermal treatment at 620? in argon,the weak interaction hydrogen bonds and Van der Waals forces were both destroyed and the strong chemical bonds were retained,resulting in the long-range disordered and short-range ordered amorphous carbon nitride photocatalytic material(ACN).The amorphization changed the state density as well as the distribution of local energy levels within the bandgap,leading to decreased bandgap of ACN from pristine 2.82 eV to 1.90 eV,and the extended absorption edge from pristine 460 nm to 682 nm.The photocatalytic hydrogen production activity under visible light was improved by an order of magnitude,and the spectral response range was extended to 650 nm.A highly active porous carbon nitride for photocatalytic water splitting was synthesized by selectively breaking the hydrogen bonds.Theoretical calculation showed that the intralayer hydrogen bonds in the GCN layer enhanced the localization of photogenerated carriers within the melon chains,thus limiting the transfer of carriers,and inhibiting the photocatalytic activity.By precisely controlling the treatment temperature in argon,the hydrogen bonds in GCN layers were selectively destroyed.The breaking of intralayer hydrogen bonds between melon chains resulted in porosity though out the GCN structure,which inhibited the recombination of photogenerated carriers,shortened the carrier diffusion distance,and provided abundant surface active sites.Besides,the incomplete amorphization in the layers extended the light response range of the GCN.These characteristics enhanced the photocatalytic activity by nearly 35 times under visible light irradiation.Homogeneous nitrogen doping of layered BaLa4Ti4O15 was realized based on the interlayer ions with strong metallicity and large ion radius.In BaLa4Ti4O15,the metallicity of alkaline earth metal Ba is obviously stronger than that of transition metal Ti.Therefore,compared with bulk TiO2,the ionicity of Ba-O layers in the layered BaLa4Ti4O15 is stronger than that of Ti-O octahedron layers.As the metal-oxygen interaction of Ba-O is weaker,it is easier for nitrogen source to diffuse into the bulk.Taking Ba-O layer as diffusion channel is beneficial for homogeneous nitrogen doping under high temperature in NH3 atmosphere.The homogeneous nitrogen distribution results in the transition of local state to continuous state,and the integral band-to-band red shift of the absorption edge from 361 to 636 nm.The photocatalytic reaction response range of the material was broadened to 650 nm.LaTaO4/Ta2O5 heterostructure was in-situ constructed through hydrothermal etching of the metal-oxygen bonds with higher ionicity.Theoretical calculations in LaTaO4 showed that the ionicity of La-O bonds is higher than that of Ta-O bonds,and the strength of the former is weaker than that of the latter.Under the same condition,La-O bonds are easier to be destroyed and dissolved out,leading the in-situ formation of Ta2O5 and construction of LaTaO4/Ta2O5 heterojunction.By means of hydrothermal treatment with HF,selective dissolution of La3+ions occurred,and the residual Ta-O grew into Ta2O5 nanorods on the surface of LaTaO4.A built-in electric field forming at the interface of LaTaO4/Ta2O5 promoted charge separation between them and improved the photocatalytic activity.Further experimental results indicated that the methods of in-situ constructing heterostructures based on the metal-oxygen bond differences of the multi-metal oxides was universal.