Construction Of Cellulose/BiOCl 2D Lamellar Membrane And Its Mechanism For Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen evolution（PHE）technology uses water as a reactant to convert solar energy into green chemical energy.Rapid surface reaction is the key factor to achieve efficient PHE performance.Increasing the reactive active sites of the photocatalyst and activating water molecules are effective ways to promote the surface reaction of PHE.In the practical application process,it is necessary to disperse the photocatalyst evenly into the solution reaction system to make the catalyst particles fully contact with water.In order to obtain a large surface area to improve the reactive active sites in contact with water,most photocatalysts are presented as nano-sized particles.However,nano photocatalysts are difficult to disperse,recycle and reuse.In this dissertation,the design strategy of bottom-up self-assembly of cellulose nanofiber（CNF）and bismuth oxychloride（Bi OCl）nanosheet（BN）photocatalysts was proposed to construct two-dimensional lamellar membranes（2DLMs）with abundant nanochannels,and the mechanism of water transport and confined water molecules promoting surface reaction to improve PHE performance was discussed.The main research contents are highlighted as follows:Firstly,using BN as a demo photocatalyst,a Bi OCl 2DLM（BOCM）was designed and constructed.The relationship between water transport in the nanochannels of 2DLM and PHE was studied,and the construction strategy of 2DLM and the enhancement mechanism of PHE were discussed.The experimental results show that the PHE rate of BOCM-30 is 6.75μmol·g^-1·h^-1,which is 2.5-fold that of BN particles dispersed in an aqueous solution.The rate of PHE remained unchanged even after 10 discontinuous cycles for the total time of 60 hours,indicating that BOCM-30 showed outstanding stability.Further theoretical calculations revealed that it is conducive to the rapid transfer of water when the size of the nanochannel is 2.95 nm,and the capillary force provides the power for the water transport in the nanochannels.The rapid water transport of the 2DLM promotes the surface reaction between water molecules and photocatalysts,thus improving the PHE performance.This strategy of using nano photocatalyst to prepare 2DLM not only improves the PHE performance,but also contributes to the development of recyclable stable membrane photocatalyst.Secondly,based on the polyhydroxy structure and excellent hydrophilicity of cellulose,a composite 2DLM（BOC-CNFM）was constructed by CNF and BN.The relationship between the confined water molecules of CNF and PHE was studied,and the effects of CNF on the construction of 2DLM and its PHE performance were investigated.When CNF content is 3.2 wt.%,the PHE rate of BOC-CNFM is 12.49μmol·g^-1·h^-1,which is 4.7-fold that of raw BN particles dispersed in an aqueous solution alone and 1.85-fold that of the BOCM-30.Combined with molecular dynamics simulation and experimental results,it is confirmed that the addition of CNF can effectively regulate the hydrogen bond network of confined water molecules and increase the proportion of intermediate water.The confined water molecules of CNF reduces the free energy barrier of water molecules self-dissociation to H⁺,and thus improved the efficiency of H₂O conversion to H₂ under the action of photocatalyst.Moreover,the addition of CNF also regulates the nanochannels of the 2DLM and accelerates the water transport.It can improve the PHE performance by regulating the state of confined water molecules to improve the reactant activity.Thirdly,based on the modifiability of the functional group of cellulose,the functional groups of CNF were regulated by chemical modification.The relationship between the functional groups of CNF and the PHE was studied,and the effect of the functional groups of CNF on water molecule state was discussed.CNF with cationic group（ch CNF）was obtained by modification of 3-chloro-2-hydroxypropyltrimethyl ammonium chlorid,and CNF with anionic group（so CNF₁ and so CNF₂）was obtained by oxidation sulfonation and direct sulfonation.Combined with the results of water evaporation efficiency,Raman spectrum and molecular dynamics simulation of CNF with different types functional groups,it is confirmed that CNF with sulfonic group is more beneficial to adjust the hydrogen bond network structure of water molecules and promote water molecules to form intermediate water with weak hydrogen bonds.In addition,the above modified CNF was used as a one-dimensional material to prepare a composite 2DLM with BN,and the PHE rate of the 2DLM of so CNF_2-2and BN is up to 13.97μmol·g^-1·h^-1.Combined with the influence of functional groups of CNF on the state of water molecules,the mechanism of cellulose regulating the water molecule state to enhance the PHE performance was further clarified.This work enriches the methods of activating water molecules to improve the performance of PHE from the perspective of improving the state of water molecules by functional groups.Fourthly,based on the above results,a composite 2DLM with carbon nanotubes（CNT）to enhance PHE performance was prepared by using BN and one-dimensional assembly CNF@CNT.The relationship between CNT and enhanced PHE was studied,and the synergistic mechanism of PHE and photothermal water evaporation（PWE）was discussed.The experimental results show that the PHE rate of the composite 2DLM of BN and CNF@CNT（BOC-CNF@CNTM）is 25.42μmol·g^-1·h^-1,which is1.99-fold that of the BOC-CNFM and 1.42-fold that of the composite2DLM of BN and CNT（BOC-CNTM）.Under 1 sun irradiation,the surface temperature of the BOC-CNF@CNT can rapidly reach 46.6°C,and the PWE rate is up to 2.05 kg·m^-2·h^-1.The function of one-dimensional material in 2DLM was revealed.On the one hand,CNT promotes the transport of photogenerated carriers and acts as the active site of PHE,and CNF regulates the hydrogen bond network of confined water molecules,thus improving the PHE performance.On the other hand,CNT promotes light absorption and light-to-thermal conversion,and the confined water molecules of CNF reduce the enthalpy of water evaporation,thus improving the PWE performance.The bi-function of PHE and PWE on the same material has been simultaneously realized,providing a new idea for the multiple utilization of solar energy.Finally,based on the different functions of CNF and CNT,a bi-layered2DLM consisting of BOC-CNF and BOC-CNF@CNT was prepared.The relationship between the bifunctional layer and the PHE was studied,and the effect of water transport layer and light absorption layer on the PHE performance was also investigated.The thickness of the bifunctional layers was adjusted and optimized.The experimental results show that when the ratio of BOC-CNF@CNT layer to BOC-CNF layer is 1:1 and the BOC-CNF@CNT layer is oriented towards the light source,the PHE performance is up to 39.81μmol·g^-1·h^-1.Combined with the hydrophilicity,optical properties,PWE and PHE of the bifunctional layer,it is confirmed that the BOC-CNF with excellent hydrophilicity can be used as the water transport layer,the BOC-CNF@CNT with unique optical properties can be used as the light absorption layer and the photocatalysis layer,and the functional separation can be realized by taking advantage of the difference properties between the two layers.In addition,a preliminary study on the functionalization of the bi-layered 2DLM was conducted,and the PWE and photocatalytic degradation properties of bi-layered 2DLM for Rh B solution and phenol solution were tested.The PWE test of Rh B solution in outdoor environment shows that the light intensity was approximately 866W·m^-2 at 12:00,and the water evaporation rate reaches the maximum(1.28kg·m^-2·h^-1)at 12:00-13:00.The construction of bi-layered 2DLM will broaden their application scenarios,and provide a new scheme for the promotion and application of photocatalysis and 2DLM technology.