Preparation Of Mxene/semiconductor Composite Photocatalyst And Its Photocatalytic Water Splitting

With the rapid development of human society,excessive consumption of fossil energy has created serious environmental problems.The searching and the production of alternative renewable energy has become a top priority.Photocatalytic water splitting into hydrogen can convert solar energy directly into chemical energy,which has become a research hotspot in the energy field.The key to photocatalytic technology lies in photocatalysts.High-efficiency photocatalysts should have a narrow bandgap and a suitable band structure to meet the kinetic and thermodynamic requirements for photocatalytic water splitting.A single photocatalyst has a high recombination rate of photo-generated electron-holes,which affects the efficiency of the photocatalytic reaction.The use of co-catalysts can effectively promote the separation of electrons and holes.The development of high-efficiency,inexpensive,non-precious metal co-catalysts is one of the effective strategies for the large-scale application of photocatalysts.Two-dimensional transition metal carbides,nitrides,or carbonitrides(MXene)can be used as cocatalysts for photocatalysts due to their good conductivity,hydrophilicity,and suitable hydrogen adsorption free energy.In this paper,Nb2O5/C/Nb2C,Ti3C2/TiO2 and 2D/2D Ti3C2/g-C3N4 composite photocatalysts were prepared by CO2 one-step oxidation or electrostatic self-assembly methods,and the activity of photocatalytic water splitting to hydrogen were studied.Firstly,by controlling the reaction time,and Nb2C(MXene)was used as a research model,Nb2O5/C/Nb2C composite photocatalysts with different Nb2O5/Nb2C ratios were prepared by CO2 one-step oxidation method,and used for photocatalytic water splitting to hydrogen.It was found that the surface of Nb2C was oxidized to form Nb2O5,and the resulting Nb2O5 had a close contact interface with Nb2C.The close contact interface not only promotes the photo-generated electrons to transfer from Nb2O5 to Nb2C rapidly,inhibits the recombination of photo-generated electrons and holes,but also enables the Nb2O5/C/Nb2C to have good stability.The Nb2O5/C/Nb2C was characterized by XRD,Raman,PL,EIS,SEM,TEM,etc.And the results revealed that the oxidized Nb2C was composed of Nb2O5,C,and Nb2C.Nb2O5 is in the outer layer,Nb2C is in the inner layer,and C species is between the Nb2O5 and Nb2C.Since both C and Nb2C have good conductivity,C can act as a fast transport channel for photo-generated electrons,while Nb2C acts as a receptor for photo-generated electrons,accumulates photo-generated electrons and uses it in the photocatalytic water splitting to hydrogen.When the oxidation time of Nb2C was 1.0 h,Nb2O5/C/Nb2C had the best photo-generated electrons-hole separation efficiency.Its photocatalytic hydrogen production rate was 7.81 ?mol h-1·gcat-1,which were 5 and 4 times higher than the physically mixed Nb2O5/Nb2C and Nb2O5.In this Nb2O5/C/Nb2C,the role of Nb2O5 is to absorb photons to generate photo-generated electrons and holes,while Nb2C acts as cocatalyst to trap photo-generated electrons,promote the separation of photo-generated electrons and holes,and water molecules can be reduced to hydrogen on the Nb2C.Secondly,in order to study the effect of monolayer and multilayer MXene on the photocatalytic activity of TiO2 semiconductor photocatalysts Monolayers Ti3C2 and multilayer Ti3C2 were prepared and used as co-catalysts for TiO2 to investigate the effects of monolayer and multilayer Ti3C2 on the photocatalytic activity of TiO2.The thickness of the prepared monolayer Ti3C2 was found to be?1 nm by TEM and AFM,and the multilayer Ti3C2 was a few micron-sized particles.When monolayer Ti3C2 was used as co-catalyst and the content was 5%,the photocatalytic hydrogen production rate of monolayer Ti3C2/TiO2 composite photocatalyst was 2.65 mmol h-1·gcat-1,which is 2.9 times and 9.1 times higher than that of multilayer Ti3C2/TiO2 and single TiO2.Since the Ti3C2 monolayer has a thickness of only?1 nm,the size of the bottom surface is about several hundred nanometers,and the size of the TiO2 particle is about 25 nm,the TiO2 particles can be supported on the bottom surface of a single layer of Ti3C2 to form a close contact interface with Ti3C2.When photogenerated electrons are generated from TiO2,they can be quickly transferred to Ti3C2.Because Ti3C2 has a thin thickness and suitable hydrogen adsorption free energy,the accumulated photogenerated electrons in Ti3C2 can rapidly migrate to the surface and participate in the photocatalytic water splitting reaction.The reaction not only promotes the separation of photoelectron-holes,but also greatly increases the photocatalytic hydrogen production rate from water.Finally,we synthesized the visible-light-responsive g-C3N4 nanosheets by thermal condensation and thermal stripping.A 2D/2D Ti3C2/g-C3N4 composite photocatalyst was prepared by electrostatic self-assembly method using a monolayer of Ti3C2 as a co-catalyst.Using the XRD,SEM,TEM and Raman spectroscopy to characterize the 2D/2D close contact interface formed between the monolayer MXene and g-C3N4 nanosheets.The effect of 2D/2D close contact interface on the photocatalytic performance of g-C3N4 was investigated.When the amount of monolayer Ti3C2 is 3%,the photocatalytic hydrogen production rate of 2D/2D Ti3C2/g-C3N4 reaches 2.3 umol·h-1·gcat-1 which is 3.5 times higher than that of 2D/2D graphene/g-C3N4 with 3%graphene content.In the 2D/2D Ti3C2/g-C3N4 photocatalyst system,based on the two-dimensional structure of the g-C3N4 and Ti3C2,the large and close contact interface between g-C3N4 and Ti3C2.under visible light irradiation,photo-generated electrons can quickly migrate from g-C3N4 to Ti3C2,the photogenerated electrons on Ti3C2 can be transferred to the surface involved in the photocatalytic water splitting reaction,greatly improving the separation efficiency of the photo-generated electron-hole and the photocatalytic activity of g-C3N4.