Preparation And Properties Of Graphitic Carbon Nitride-based Photocatalysts

Hydrogen is regarded as the cleanest renewable energy and can be used to solve the increasingly serious energy crisis and environmental pollution problems.Photocatalytic water splitting is considered as an effective means of hydrogen genneration.Water splitting reaction need the high efficent and stable catalyst.Among numerous photocatalysts,graphite carbon nitride(g-C3N4)attracted wide concern of all over the world with its unique two-dimensional layered structure.g-C3N4 owns the following advantages,a suitable band gap,low cost,simple preparation,good stability,and environmentally friendliness,and has been used in photocatalytic degradation of pollutants,photocatalytic hydrogen generation,CO2 reduction etc.However,its high recombination rate and low efficiency limited its futher application.In this paper,we studied the improvement of the photocatalytic activity of g-C3N4 from three aspects:loaded co-catalyst,construction of heterojunction and structural optimization.And,the morphology and structure,the photocatalytic performance of samples and the mechanism of photocatalytic water splitting were studied,evaluated and analyzed.The detailed research contents are as follows:1.Loading black phosphorous quantum dots(BPQD)to improve the separation of g-C3N4 photogenerated carriers.BPQD owns good ultraviolet-visible light absorption performance.However,its stability in water and air is poor,it can be oxidized easily and lost catalytic activity.In this paper,BPQD was loaded on the layered g-C3N4 prepared by urea to form heterojunction by means of light-assisted vacuum stirring.The structure effectively prevents the oxidation of BPQD.When incident photons irradiated the heterojunction,the photogenerated electrons in BPQD would migrate to g-C3N4 through the heterojunction,which effectively improved the carrier separation rate of BPQD and promoted the water splitting reaction.The highest hydrogen generated rate of 7%BPQD-C3N4 are 190,133,90 and 10.4 ?mol h-1 under simulated sunlight,LED-405,LED-420,LED-550 nm irradiation,respectively,which are 3.5,3.6,3,and 3 times of g-C3N4.2.Construction of heterojunction with ?-Fe2O3 to improve the activity of g-C3N4 photocatalytic oxidation water.?-Fe2O3 has the advantages of high thermodynamic stability and low cost,and is widely used as a visible light catalyst.However,?-Fe2O3 also has the shortcomings of short carrier life and short hole migration distance.In this paper,with FeOOH and g-C3N4 as precursors,under nitrogen atmosphere and 580?,FeOOH was dehydrated and converted into ?-Fe2O3,the edge of g-C3N4 was carbonized.Finally,they are combined to form the Fe2O3/C-C3N4 with tight heterojunction.The carbon layer and the tight heterojunction structure are the key to the improvement of photocatalytic performance.The different migration rates of electrons and holes on the surface of carbon promote the separation and migration of carriers,while the tight heterojunction has a small migration resistance to carriers,which is beneficial to the improvement of carrier migration rate.Without any co-catalyst,the oxygen evolution rate of Fe2O3/C-C3N4 under LED-420 nm irradiation was 22.3 mol h-1,which is 3,6,and 30 times of Fe2O3/C3N4-r,?-Fe2O3 and g-C3N4,respectively.3.Introducting-N=C=O groups at the edge to improve the migration and separation of g-C3N4 photogenerated carriers,and loading MoS2 to improve the activity of g-C3N4 photocatalytic hydrogen generation.MOS2 owns a low water reduction overpotential and is one of the photocatalysts with better performance.In this paper,by introducing CO2 gas during the thermal polymerization of urea,the hydrogen atoms of-NH2 on the edge of g-C3N4 are replaced to form-N=C=O groups,forming CO-C3N4.The presence of-N=C=O group increases the photocatalytic hydrogen production rate of CO-C3N4 to 1.85 times than that of pure g-C3N4.The CO-C3N4 was combined with MoS2,and finally form a MoS2/CO-C3N4 heterojunction by light-assisted vacuum stirring.The heterojunction structure of MoS2/CO-C3N4 effectively promotes the migration of photogenerated electrons in CO-C3N4 to the surface of MoS2 and improves the separation rate of photogenerated carriers.The 10%MoS2/CO-C3N4 samples exhibited the best photocatalytic performance under ?>400 nm and ?>420 nm irradiation with hydrogen production rates of 1990 and 1440 mol/(g*h),respectively.The sample also showed excellent photocatalytic activity of 44.3 mol/(g*h)under LED-600 nm irradiation.