1 Introduction
the world's energy demand continues to increase,and its main source is fossil fuels,especially oil,coal and natural gas.However,the reserves of fossil fuels are limited.Therefore,the rational and efficient use of solar energy is the future development trend of energy utilization.However,the efficient conversion of solar radiation into chemical energy or energy-intensive liquid energy is the key to achieving large-scale solar energy output
[1,2]。 At present,the light energy conversion efficiency is low due to the constraints of solar energy absorption,photogenerated carrier separation efficiency,catalytic reaction effect and other factors.Because the photogenerated electrons and holes are easy to recombine and lose energy,the key to improve the photoconversion efficiency is to continuously improve the construction system of high-efficiency and low-cost photocatalytic materials
[3,4]。
metal-organic frameworks(MOFs)are a class of porous crystalline materials structured by inorganic metal nodes(metal ions or metal clusters)and bridging organic linkers,which are widely used in sensing,gas storage,separation,and catalysis
[5][6,7][8,9][10,11]。 MOFs have highly porous structures and open channels,which make the active sites easily accessible and greatly facilitate the transport of products
[12⇓⇓⇓~16]。 MOFs directly used as photocatalysts have low photocatalytic activity and poor cycle stability,while MOFs-derived metal sulfides and their composites have shown great potential in the field of photocatalysis(Table 1)。
in this paper,the literature reports on sulfides and their composites derived from MOFs In the past five years are systematically reviewed,including monometallic sulfides,multicomponent metal sulfides,multicomponent metal sulfids,oxide-sulfide composites,carbon-sulfide composites and carbon-nitrogen-sulfide composites derived from MOFs。
2 Preparation Strategy of MOFs Derived Metal Sulfide
Using MOFs as precursors or templates,the developed sulfides derived from MOFs retain the original structural characteristics of the parent MOFs,with large surface area,dispersed nano-sized subunits and abundant active sites,after simple inert gas pyrolysis,air annealing,solvent thermal dissolution,ion exchange or in-situ sulfidation(Figure 1).it overcomes the limitation of raw material stability of MOFs at high temperature and harsh chemical environment.Compared with oxides,It has a narrower band gap,which extends Its optical absorption range to the visible region.its porous nature may provide an additional pathway for the migration of photoexcited electrons,thereby facilitating charge carrier separation。
3 MOFs derived monometallic sulfide
The synthesis method of MOFs-derived monometallic sulfides is simple,the synthesis path is easy to control,and the original structural characteristics of MOFs(with larger specific surface area and more active site pore structure)are retained.The metal element or morphology of metal sulfides can be precisely controlled by screening appropriate MOFs precursors and heat treatment methods.However,due to the great influence of material properties,there are few reports,and some redox active metal sulfides,such as ZnS,In2S3and SnS2,are mainly used in the field of photocatalysis 。
Li et al.Used thioacetamide(TAA)to sulfurize rod-like ZIF-L to obtain porous tubular ZnS,and further studied the effects of different sulfurization time on the photocatalytic reduction of Cr(Ⅵ)and dye degradation by ZnS
[17]。 The results showed that when the sulfidation time was 3 H,the catalytic performance of ZnS was the best,that is,ZnS-3 could achieve 100%reduction of Cr(Ⅵ)within 30 min under UV irradiation,and the highest degradation rate of reactive red(X-3B)was 94.6%within 20 min.The effects of different reaction conditions on the photocatalytic performance were studied,and the results showed that ZnS-3 showed good photocatalytic performance for the reduction of Cr(Ⅵ)under weak acidic and neutral conditions.The position of the conduction band(CB)is higher than the redox potential of the O
2/·O
2−,and the generated superoxide radical（·O
2−）can promote the reduction of Cr(Ⅵ)to Cr(Ⅲ)and the oxidation of X-3B.The valence band(VB)hole and the H
2O are oxidized to generate hydroxyl radical(·OH),which reduces the probability of electron-hole recombination and promotes the photocatalytic reaction 。
Fang et al.Prepared invaginated hexagonal In
2S
3nanorods with hollow structure by liquid phase sulfidation method using MIL-68(In)as precursor
[18]。 The effect of different curing time on the photodegradation of dye by In
2S
3was further studied,and the results showed that the In
2S
3-8 h exhibited excellent photodegradation performance,and the degradation rate of anionic dye methyl orange(MO)was 97%(within 120 min)under visible light.The lowest unoccupied molecular orbital(LUMO)and the highest occupied molecular orbital(HOMO)of MO molecule are localized in the semiconductor band gap（E
g）of In
2S
3due to the good adsorption ability of In
2S
3,forming a cross-band arrangement(type I).The transfer of photogenerated electrons and holes from In
2S
3to MO molecules is beneficial to the generation of more free radicals under the action of contact electric field,h
+can directly oxidize pollutants,and photogenerated electrons can reduce O
2to generate·O
2−active substances and further degrade pollutants to improve the photodegradation performance 。
Li et al.Prepared ZnS@C and CdS@C composites by one-pot sulfidation method using Zn-MOF and Cd-MOF as precursors,respectively,and obtained pure ZnS and CdS catalysts by removing carbon substances by high temperature pyrolysis
[19]。 One-pot sulfidation of The MOF precursor can suppress the sintering phenomenon during the stepwise carbonization-sulfidation process.the results showed that about 90.2%and 91.6%of cationic dye methylene blue(MB)was degraded by ZnS and CdS,respectively,after 100 min。
Dai et al.Used Sn-MOF as precursor to prepare SnS
2-SnS composite by pyrolytic sulfidation method
[20]。 The ratio of SnS
2to SnS was adjusted by changing the ratio of MOF to sulfur powder and the pyrolysis temperature.The reduction rate of Cr(Ⅵ)reached 91.51%after the catalyst was irradiated by visible light(400~800 nm)for 3 H.The higher catalytic activity is mainly due to the fact that the CB of SnS
2is more negative than that of SnS and the VB of SnS
2is more positive than that of SnS,and the photogenerated carriers generated by illumination will be transferred from the SnS
2wide band gap semiconductor to the SnS narrow band gap material,thus promoting the separation of photogenerated carriers 。
The butterfly wing(BW)has a porous hierarchical structure that exhibits efficient light harvesting capability,and the wing scales can use internal reflection to harvest light,which means that the unique structure of BW is an effective solar collector.The combination of BW with the abundant exposed metal sites of MOFs further improved the light absorption ability and suppressed the aggregation of active sites during sulfuration.Zhong et al.Deposited cadmium carbonate(NC)doped with cadmium nitrate on the surface of BW to obtain BW@NC,and then directly grew Cd-based MOF(CdIF-3)on the surface of BW@NC to prepare BW@NC@CdIF-3 with a network hierarchical structure(Fig.2),which was used as a sacrificial template to prepare BW@SS deposited with CdS/Cd
2SO
4（OH）
2nanoparticles through a simple vulcanization process
[21]。 The effect of co-catalyst on the photocatalytic activity of BW@SS was further investigated.The results show that the hydrogen evolution rate of BW@SS under visible light(room temperature)with Pt as the cocatalyst is 18420μmol·h
-1·g
-1,and the hydrogen evolution rate without the cocatalyst is 3376μmol·h
-1·g
-1.The excellent performance of BW@SS photocatalyst can be attributed to the enhanced light harvesting,higher conduction band,and efficient transfer of photogenerated charge carriers.The doped Cd
2SO
4(OH)
2can be reduced to metal Cd as an electron trap,thereby improving the charge separation efficiency and further enhancing the hydrogen production performance of CdS 。
At present,MOFs-derived monometallic sulfide materials are basically prepared by solvothermal vulcanization,which has more advantages in photocatalytic degradation system,but the cycle stability in photocatalytic reaction still needs to be improved.Due to the limited variety,it may be necessary to combine other metal materials to improve its photocatalytic performance。
4 MOFs derived multicomponent metal sulfide
Multicomponent metal sulfides,（MxNyS）,templated by multimetallic(M=Co,Zn,Cd,Cu,etc.)MOFs usually have the advantages of more active sites,larger specific surface area,higher conductivity,and unique mesoporous structure(shortened electron transport path).Multicomponent metal sulfides derived from MOFs are generally synthesized by sulfidation of homogeneous bimetallic MOFs formed by introducing different metal ions into a single framework.Multicomponent metal sulfides refer to metal sulphides with two or more different metal cations,which is different from the physical mixture of two or more metal sulphides 。
Homogeneous bimetallic MOFs formed by introducing different metal ions into a single framework have a large surface area,which can combine multiple active sites,thereby improving the photocatalytic performance.Mu et al.Used ZnCd-ZIFs obtained by introducing Cd into ZIF-8 as a self-template to prepare mesoporous Cd
xZn
1−xS polyhedra by in situ sulfurization
[22]。 The Cd/Zn ratio was adjusted by changing the Cd content,and the CZS showed the optimal hydrogen evolution rate（4.10 mmol·h
-1·g
-1）under visible light when Cd/Zn=0.42/0.58.The balance between CB potential and band gap width,the fast carrier separation and migration ability,and the high surface area of the photocatalyst facilitate the photocatalytic reaction 。
Bimetallic sulfide photocatalysts with porous hollow structure and enhanced photocatalytic performance can be prepared by using appropriate MOFs as templates and exogenous metals because of their unique inherent voids,huge surface area and surface permeability.Doping the second metal to MOFs can increase the catalytic active sites and improve the electrical conductivity,thereby improving the photocatalytic activity.Wu et al.Prepared 3D hollow structured Cd
0.5Zn
0.5S（CZS-X）nanocages using rhombohedral dodecahedron ZIF-8 as precursor and thioacetamide(TAA)as sulfur source at different sulfurization times
[23]。 The results show that CZS-3 with a curing time of 3 H exhibits the best photocatalytic activity under visible light,and has good recycling performance and stability.In the photocatalytic reduction of Cr(Ⅵ),CZS-3 showed the highest reduction efficiency(100%in 12 min),and in the photocatalytic MO oxidation,CZS-3 also showed the highest efficiency(94.2%in 20 min and 94.5%in 30 min).The CB redox potential of CZS-3 is higher than that of O
2/·O
2-,so it can produce·O
2-radicals to reduce Cr(Ⅵ).H
2O
2reacts with e
-to produce·OH,which can degrade MO.The Cd
0.5Zn
0.5S from ZIF-8 has larger specific surface area and more exposed active sites,so it has outstanding photocatalytic performance 。
Ma et al.Used the bimetallic Cu/Co-MOFs obtained by introducing Cu into ZIF-67 as the precursor to synthesize the nanoflower hybrid structure Cu
xCo
（3−x）S
4@MoS
2composite material in situ through hydrothermal reaction(Fig.3),and further studied the effect of different Cu ion doping amounts on the photocatalytic performance
[24]。 The results indicated that the composite exhibited efficient catalytic hydrogen production at a rate of 40 156μmol·h
-1·g
-1under visible light without additional noble metals and semiconductors as co-catalysts.Upon photoelectron excitation(420 nm),electrons are excited from the VB of the Cu
0.9Co
2.1S
4@MoS
2to the CB,and from the HOMO of the EY molecule to the LUMO.Triethanolamine(TEOA)reduction quenches the excited EY(*EY)to produce EY
•−.Electrons are transferred from EY
•−to the CB of Cu
0.9Co
2.1S
4@MoS
2,and H
+is reduced to form H
2 。
The heterostructure can effectively inhibit the recombination of photogenerated charge carriers and reduce the photocorrosion phenomenon in the presence of supported photocatalysts.Bai et al.Used ZIF-8 as a precursor to synthesize Zn
1−xCd
xS/CdS quantum dot heterojunction materials by low temperature sulfurization and cation exchange
[25]。 The Zn/Cd ratio was adjusted by changing the amount of CdS added.The results show that the photocatalytic hydrogen production rate of the Zn
0.7Cd
0.3S/CdS quantum dot material under visible light irradiation is optimal to reach 2.7 mmol·h
-1·g
-1without any noble metal co-catalyst.The VB photogenerated holes of CdS migrate to the local acceptor state of Zn
0.7Cd
0.3S,while the CB photogenerated electrons of Zn
0.7Cd
0.3S transfer to CdS.The photoexcited carriers formed by the Zn
0.7Cd
0.3S/CdS heterostructure can significantly inhibit the electron-hole recombination,which originates from the irreversible hole-driven oxidation reaction in CdS photocatalyst,which can effectively inhibit the photocorrosion of photogenerated carriers,thus promoting the photocatalytic reaction 。
Broader solar absorption and more efficient photoexcited charge separation can be achieved by fine-tuning the band gap.Solid solution catalysts have stronger electronic attraction and tighter interface,which can achieve higher interfacial charge transfer efficiency and thus improve the photocatalytic hydrogen evolution activity.Liu et al.Used Zn and Cu-doped Cd-MOF as precursors to obtain uniformly distributed solid solution（Zn
0.95Cu
0.05）
1−xCd
xS by solvothermal method and high temperature calcination
[26]。 On the basis of a certain molar ratio of Zn and Cu,the composition of（Zn
0.95Cu
0.05）
1−xCd
xS can be well controlled by changing the proportion of Cd,so as to fine-tune the conduction band edge to obtain the optimal hydrogen evolution rate.Under visible light,the optimized（Zn
0.95Cu
0.05）
0.6Cd
0.4S showed high stability,and its photocatalytic hydrolysis hydrogen production rate was 4150.1μmol·h
-1·g
-1 。
At present,MOFs-derived multi-metal sulfide materials are basically prepared by doping a second metal ion in a single MOFs sulfurization derivation process,and have higher catalytic activity and better cycle stability compared with single metal sulfides.the synergistic effect of two or more metals makes the multi-metal sulfides have rich redox active site mesoporous structure,and the synergistic effect is also reflected in the significant inhibition of electron-hole recombination。
5 MOFs derived multicomponent metal sulfides
In recent years,researchers at home and abroad have prepared multi-component metal sulfide nanocomposites for the synthesis of photocatalysts to improve their photocatalytic performance.Multicomponent metal sulfides（MxS/NyS）templated by metal(M=Co,Zn,Cd,Cu,Ni,In,etc.)MOFs generally integrate two or more functional materials with strong synergistic effect,which enhances the photocatalytic activity and structural stability of each component 。
The inherited porous structure of MOFs-derived metal sulfides can be used as a heterostructure constructed by combining guests or hosts with other metals or inorganic nanoparticles,and the regulation of energy band position is beneficial to the photocatalytic reaction.Li et al.Used ZIF-67 as a synthesis template to synthesize rhombic dodecahedron Co
3S
4@Mo
2S
3composite with hollow structure by one-step continuous hydrothermal method after introducing S and Mo elements
[27]。 The effects of the amount of ZIF-67,the amount of Na
2MoO
4·2H
2O and TAA solution and different reaction conditions on the activity of the catalyst were studied.The results showed that the Co
3S
4@Mo
2S
3-9（S
BET=47 m
2·g
-1）with 9 mL Na
2MoO
4·2H
2O and 9 mL TAA solution had the best photocatalytic hydrogen evolution performance(375.37μmol,5 H)under 420 nm light irradiation and pH=9.The contact between the Co
3S
4and the Mo
2S
3causes band bending,which further promotes the recombination of holes at the interface of the Co
3S
4and electrons on the CB of the Mo
2S
3.The simultaneous formation of a built-in electric field from Co
3S
4to Mo
2S
3can facilitate electron transfer.The S-type heterojunction formed by Co
3S
4,Mo
2S
3can maintain a more effective redox potential 。
Huang et al.Prepared a series of Z-type NiS/Co
3S
4/ZnCdS photocatalysts by sulfidation modification using two-dimensional Ni-ZIF-67 as a template,and discussed the effects of Ni-ZIF-67:S powder ratio,annealing temperature,and the addition of NiS/Co
3S
4on the performance of the catalysts,respectively.The results showed that under illumination(>420 nm),the best hydrogen evolution rate of NiS/Co
3S
4/ZnCdS was 98.6 mmol·h
-1·g
-1when the Ni-ZIF-67:S powder ratio was 1:2,the annealing temperature was 600°C,and the addition of NiS/Co
3S
4was 20 mg
[28]。 The synergistic effect between the heterojunction formed by ZnCdS and NiS/Co
3S
4is beneficial to electron transfer and carrier generation,which inhibits the agglomeration of ZnCdS nanoparticles and exposes more active sites,and the combination of NiS/Co
3S
4and ZnCdS narrows the band gap and improves the utilization of light absorption 。
Su et al.Synthesized NiS/CdS composite using Ni-MOF and cadmium salt as raw materials by one-step hydrothermal vulcanization
[29]。 The effect of Ni-MOF synthesized with different organic ligands on the catalytic activity of NiS/CdS was further studied,and the results showed that the NiS/CdS composite derived from Ni(TPA)-MOF with terephthalic acid(TPA)as the organic ligand had the best hydrogen evolution rate,which could reach 57.93 mmol·h
-1·g
-1under 420 nm light irradiation.The better catalytic activity is due to the fact that the organic ligand group and the electronegativity of Ni-MOF provide effective attachment sites for Cd precursor and CdS;The dynamic coordination interaction between Ni-MOF and Cd ions/CdS plays a role in regulating the uniformity and heterojunction of NiS/CdS composites,and leads to the change of conduction band position,which in turn improves the photocatalytic performance 。
Zhao et al.Fabricated heterojunctions by solvothermal sulfidation and thermal annealing using Zn and Ni-doped Cd-MOF as precursors NiS/Zn
xCd
1−xS（Fig.4 )
[30]。 By adjusting the concentration of doped metal in MOFs,the chemical composition and band gap of the heterojunction can be fine-tuned to further optimize its light absorption capacity and photocatalytic activity.The results show that the optimal hydrogen evolution rate of NiS/Zn
0.5Cd
0.5S is 16.78 mmol·h
-1·g
-1under visible light irradiation(λ>420 nm),and it has high stability and recyclability.In charge transport,the Zn
0.5Cd
0.5S region potential is higher and the NiS region potential is lower,which makes the photogenerated charge transfer from Zn
0.5Cd
0.5S to NiS,providing electrons to reduce the adsorbed H
+ 。
Nagata et al.Prepared Cu,In-doped ZnS(CIZS)by hydrothermal method using Cu,In-doped ZIF-8 as precursor(Fig.5)
[31]。 The effect of different amounts of Cu and In on the photocatalytic activity of CIZS was further studied.The results showed that the photocatalyst prepared by adding 1%Cu and 1%In in the synthesis process had the highest activity.Under 360 nm light irradiation,the hydrogen evolution rate is 74.8μmol·h
-1·g
-1,and the AQY ofα-cellulose photoreforming reaction is as high as 28%.The CIZS band gap is narrowed by Cu and In doping so that the redox potential is sufficient for the photoreforming reaction 。
The formation of that heterostructure can generate a strong built-in electric field to promote charge separation;The construction of hierarchical nanostructures can increase the specific surface area of the catalyst,enhance the light harvesting ability of the catalyst,and thus improve the photocatalytic activity.Song et al.Prepared nanocomposite ZnS/ZnIn
2S
4heterojunction by sulfidation of ZIF-8 and in-situ precipitation of ZnIn
2S
4in a one-pot hydrothermal method
[32]。 The mass ratio of ZIF-8 to ZnIn
2S
4was adjusted by changing the amount of ZIF-8 added.The results showed that the ZnS/ZIS-20 exhibited excellent photocatalytic activity for hydrogen production under xenon light irradiation at a ZIF-8 to ZnIn
2S
4mass ratio of 20%,and the optimal hydrogen production rate was 453.4μmol·h
-1·g
-1.The photogenerated electron on the CB of ZnIn
2S
4easily migrates to the CB of ZnS due to its more negative potential;At the same time,the holes on the VB of ZnS can flow to the VB of ZnIn
2S
4due to its large positive potential,and this heterojunction effective interface transfer is the main reason for the improvement of photocatalytic efficiency of ZnS/ZIS 。
Wang et al.Prepared a series of hierarchical Cd-Ni-MOF micro/nanostructures assembled by nanosheets and nanorods by a two-step solvothermal method through partial substitution of Cd
2+for Ni
2+in the Ni-MOF-74 structure
[33]。 Hierarchical Cd-Ni-MOF-T/CdS/NiS hybrid micro/nanostructured materials were then prepared by sulfidation of Cd-Ni-MOF.The effect of ethanol∶DMF on the catalytic performance was further studied.The results showed that when the ratio of ethanol to DMF was 1∶1,the prepared catalyst showed the best photocatalytic hydrogen evolution performance,which was about 40.08 mmol·h
-1·g
-1.The p-n heterojunction between CdS and NiS facilitates the transfer of photogenerated electrons from the CB of NiS to that of CdS,and the heterojunction between CdS and Cd-Ni-MOF-T further promotes the transfer of electrons from the CB of CdS to that of Cd-Ni-MOF-T,resulting in the effective separation of photogenerated carriers 。
Tan et al.Impregnated Mn
2+In MIL-68(In)submicron rods to make MnS/In
2S
3nanosheet composite photocatalyst by further sulfurization process
[34]。 By controlling the amount of Mn
2+injected into MIL-68(In),a series of MnS/In
2S
3p-n heterojunctions with tunable mass percentage can be fabricated.The photocatalytic activity of MnS/In
2S
3increases gradually with the increase of MnS mass fraction,and at MnS/In
2S
3（30 wt%）,the optimal CO formation rate is 58μmol·h
-1·g
-1.The better reduction rate of CO
2is due to the dispersion of Mn
2+and In
3+in the template,and a large number of MnS/In
2S
3small p-n heterojunctions can be simultaneously generated in each layered particle.Under light irradiation,the In
2S
3side will accumulate photogenerated electrons for CO
2reduction,while the photogenerated holes will accumulate on the MnS side and be consumed by TEOA in turn,thus establishing a static charge balance in the photocatalytic system 。
Su et al.Sulfided and calcined ZIF-67 as a precursor to obtain Co
9S
8,and Co
9S
8and Mn
0.05Cd
0.95S were used to prepare Co
9S
8-MCS composite catalyst by hydrothermal method（S
BET=13 m
2·g
-1）（Fig.6 )
[35]。 The effect of different Co
9S
8addition on the photocatalytic performance was further studied,and the results showed that when Co
9S
8∶Mn
0.05Cd
0.95S=1∶10,the hydrogen evolution rate of Co
9S
8-MCS composite catalyst was the best,which was 13.369 mmol·h
-1·g
-1under simulated sunlight.Beside,that Co
9S
8-MCSstill has good hydrogen evolution stability after four cycle.The CB and VB of Co
9S
8are between those of MCS,so the Co
9S
8-MCS composite catalyst has a suitable redox ability for photocatalytic reaction.Co
9S
8-MCS have strong light harvesting ability,abundant reaction sites and heterostructure to promote charge separation and transfer 。
Wang et al.Prepared ZnIn
2S
4@Co
3S
4（S
BET=90.4 m
2·g
−1）by hydrothermal method using ZIF-67 as a sacrificial template
[36]。 The effect of the amount of Co
3S
4on the photocatalytic performance was further studied,and the results showed that when the amount of Co
3S
4was 5%,the hydrogen production rate of ZnIn
2S
4@Co
3S
4was the best（4261μmol·h
-1·g
-1） 。
Xi et al.Prepared ZnIn
2S
4@CoS
2heterostructure（S
BET=48~60 m
2·g
−1）by a facile hydrothermal method using ZIF-67-derived Co
3O
4as precursor
[37]。 The molar ratio of Co
3O
4to ZnIn
2S
4was adjusted by controlling the amount of Co
3O
4added.Under simulated solar irradiation,the ZnIn
2S
4@CoS
2heterojunction with 5%CoS
2content(ZC-5)showed the optimal photocatalytic hydrogen evolution performance with a hydrogen production rate up to 2768μmol·h
-1·g
-1.Heterostructure was formed at the interface between ZnIn
2S
4and CoS
2,Co
3S
4,which promoted the electron transfer and inhibited the recombination of electron-hole pairs,thus improving the photocatalytic hydrogen production activity of the composite 。
Bimetallic MOFs can be used as precursors/templates for the preparation of photocatalysts due to their adjustable composition and diverse structures,while they can also form tight heterojunctions to promote the separation of photoexcited charge carriers and further improve the photocatalytic hydrogen evolution rate.Liu et al.Prepared MOF-74-Ni/Cd by introducing cadmium ions into MOF-74-Ni through ion exchange method,and used it as a precursor to synthesize CdS@NiS photocatalyst by one-step hydrothermal method
[38]。 Further comparing the catalytic activity of CdS@NiS derived from MOFs with that of CdS-NiS and CdS/NiS prepared from pure CdS,the results show that the photocatalytic hydrogen evolution rate of CdS@NiS at 450 nm is 42.7 mmol·h
-1·g
-1and the AQY is 13.23%,while the hydrogen evolution rate of CdS-NiS and CdS/NiS is only 1.727 and 2.258 mmol·h
-1·g
-1.The heterointerface formed by the effective combination of CdS and NiS in CdS@NiS(interface synergy effect)promotes the separation and migration of charge carriers,thus greatly accelerating the proton reduction reaction 。
Zhao et al.Used ZIF-8 as a precursor to sulfurize and synthesize hollow ZnS polyhedral cages,which were further coated with SnS
2nanosheets
[39]。 The obtained ZnS@SnS
2heterostructure cage is decorated by CdS quantum dots to obtain the ZnS@SnS
2/CdS material（S
BET≈86.5 m
2·g
−1）.The theoretical weight percentage of ZnS：ZnS@SnS
2was adjusted by changing the amount of ZnS added,and the effect of soaking time of ZnS@SnS
2in CdCl
2solution on the catalytic performance was further studied.The results showed that when the ZnS∶ZnS@SnS
2was 35%and the immersion time was 10 min,the properties of the ZnS@SnS
2/CdS composite were the best,the CO generation rate was 155.57μmol·h
-1·g
-1,and the selectivity was 80.4%.Suitable energy level matching among ZnS,SnS
2,and CdS can enable two-channel charge transfer and generate more electrons to participate in the CO
2photoreduction reaction 。
The formation of effective heterostructure,the introduction of cocatalyst,the addition of sacrificial agent and the formation of sulfur vacancy are important means to improve the photocatalytic activity.Lin et al.Coated Cd-MOF on the surface of MoS
2flower to construct a core-shell structure MoS
2@Cd-MOF
[40]。 Using TAA as sulfur source,MoS
2@Cd-MOF was sulfurized to form CdS/MoS
2heterojunction.The molar ratio of CdS was adjusted by changing the CdS loading on the surface of MoS
2.The results showed that under UV-Vis irradiation,when CdS：MoS
2=0.6∶1,the hydrogen evolution performance of CdS/MoS
2was the best（5587μmol·h
-1·g
-1）.The conduction band minimum(CBM)of MoS
2in the heterojunction is lower than that of CdS,and photoexcited electrons are transferred from CdS to MoS
2.MoS
2as a co-catalyst can capture CdS photoexcited electrons for photocatalytic hydrogen evolution 。
Huang et al.Prepared Co
4S
3nanosheets from flaky two-dimensional Co-MOF by sulfurization and annealing,and synthesized Co
4S
3/CdS by hydrothermal method
[41]。 The effect of the amount of Cd（CH
3COO）
2·2H
2O on the performance of the catalyst was further investigated.When the dosage of Cd（CH
3COO）
2·2H
2O is 2 mmol,the optimal hydrogen evolution rate of Co
4S
3/CdS is 5892.6μmol·h
-1·g
-1under the irradiation of 420 nm light.A typical typeⅡheterojunction structure was formed between Co
4S
3and CdS,which effectively promoted the separation and migration of charge carriers,and the sacrificial agent lactic acid captured the holes of CdS in VB and inhibited the charge recombination,thus enhancing the photocatalytic activity 。
Using ZIF-8 as a sacrificial template,Liu et al.Synthesized Sv-ZnS/ZnIn
2S
4heterostructures with sulfur vacancies by in situ sulfurization
[42]。 The as-prepared Sv-ZnS/ZIS has an AQY of 19.8%±1.0%at 420 nm,a hydrogen evolution rate of 2912.3±185.9μmol·h
-1·g
-1,a CO yield of 2075.7±63.0μmol·h
-1·g
-1,and a selectivity of 93.0%±0.8%.When Sv-ZnS and ZnIn
2S
4form a close contact interface,electrons will spontaneously flow from Sv-ZnS to ZIS,and this charge redistribution leads to the bending of the band edges of Sv-Zn and ZIS and the formation of an IEF with an orientation pointing from ZIS to Sv-Zn.IEF,band edge bending and Coulomb force accelerate the recombination of electrons in CB in SV-ZnS with holes in VB in ZIS under visible light irradiation 。
Shi et al.Successfully prepared MoS
2/CdS composite photocatalyst by in situ sulfidation method using Mo-MOF and Cd-MOF as dual precursors
[43]。 The effect of the weight percentage of MoS
2on the catalytic performance of MoS
2/Cd was further investigated,and the results showed that the best hydrogen evolution rate of 1%MoS
2/CdS was 106 729μmol·h
-1·g
-1.A large number of sulfur vacancies are introduced into the composite material to capture electrons as sulfur vacancy energy levels,thereby increasing the number of active sites for photocatalytic reaction 。
The morphology control of photocatalyst and the addition of sacrificial agent are effective ways to improve the activity of photocatalyst.Cheng et al.Used ZIF-67 as a template to synthesize lollipop-like Co
9S
8/CdS nanocomposites by direct annealing
[44]。 The effect of different annealing temperatures on ZCC was further investigated in nitrogen atmosphere.The results show that ZCC-600 has the best hydrogen production rate 1852μmol·h
-1·g
-1under visible light at 600°C annealing temperature.Under illumination,Co
9S
8and CdS simultaneously produce excited state electrons that can be transferred from VB to CB,leaving holes in VB,and the electrons in the CB of CdS can migrate to the VB of Co
9S
8,accelerating the separation of electrons and holes between the two bands.The holes accumulated in the VB of CdS can be continuously oxidized and consumed,further promoting carrier separation 。
Kumar et al.Used Co-MOF as a precursor to prepare hollow structure Co
4S
3/CdS by a simple synthesis method
[45]。 The effects of the mass fraction of Co
4S
3and the concentration of lactic acid in the Co
4S
3/CdS on the catalytic performance were further studied.When Co
4S
3is 6.0 wt%and lactic acid reaches 3 mL in 15 mL reaction solution,the optimal level is obtained,and the hydrogen evolution rate Co
4S
3/CdS under visible light is 12360μmol·h
-1·g
-1.The semiconductor CdS in Co
4S
3/CdS is excited by sunlight to generate electron-hole pairs in CB and VB,respectively,and the excited electrons are transferred to the Co
4S
3and effectively separate the photogenerated holes,followed by the reduction of protons to H
2.The sacrificial agent(lactic acid)is oxidized by photogenerated holes in the VB of CdS,inhibiting electron-hole recombination 。
The photoinduced electron transfer and separation can be promoted by the addition of photogenerated electron capture agent,the construction of electron traps and the introduction of co-catalyst,which can improve the photocatalytic activity.Xiong et al.Obtained nanomaterials Cu/ZnS-SS and Cu/ZnS-TAA by reacting sodium sulfide(SS)and TAA with ZIF-8 doped with Cu
2+at different Cu/Zn molar ratios under solvothermal conditions
[46]。 The results of catalytic experiments show that Cu/ZnS-20-SS（S
BET=43 m
2·g
-1）has a good photocatalytic degradation rate within 1 H,the degradation rate can reach more than 90%,and it can be reused for 4 times without obvious loss of activity.The adsorption study showed that Cu/ZnS-TAA（S
BET=129~135 m
2·g
-1）could rapidly adsorb MB and MO,with a high adsorption capacity of about 260 mg/G in 10 min,and could reach a saturation adsorption capacity of about 300 mg/G.Since the photocathode reaction is accelerated by the addition of the photogenerated electron capture agent,the efficiency of photogenerated electron-hole separation is further improved,and the formation of photogenerated holes is accelerated(Fig.7),thus improving the photocatalytic reaction 。
Using CuZn-MOF as precursor,Lu et al.Successfully prepared porous CuS/ZnS microspheres（S
BET=22.3 m
2·g
−1）through rational design and low-cost sulfidation process
[47]。 The ratio of Cu/Zn was adjusted by changing the addition of Cu(NO
3)
2·3H
2O.The results showed that when the theoretical molar ratio of Cu/Zn was 3 mol%,the hydrogen production rate of CuS/ZnS under visible light reached the optimum of 6208.6μmol·h
-1·g
-1,and the apparent quantum yield(AQY)at 420 nm was 8.5%.The excellent photocatalytic hydrogen evolution activity is attributed to the fact that under visible light irradiation,electrons are directly excited from the VB of ZnS into the CuS cluster,and the transferred electrons lead to partial reduction of CuS to Cu
2S.The CuS/Cu
2S cluster can be used as an electron trap and a co-catalyst to promote the transfer and separation of photoinduced electrons.The photogenerated electrons in the CuS/Cu
2S cluster transferred from the VB of ZnS can reduce the adsorbed H
+to H
2 。
MOFs-derived multi-component metal sulfide materials have a hybrid structure of two or more metal sulfides,which usually have rich mesoporous structure and strong coordination.the photocatalytic performance of the heterojunction structure formed by MOFs derived multi-component metal sulfides is greatly improved compared with that of the single-component structure,which is attributed to the energy level difference between the two structures,which enhances the absorption of visible light and promotes the separation of photogenerated charges.MOFs-derived multi-component metal sulfide partial materials(such as Co-MOFs-derived multi-component metal sulfides)have the problem of poor cycle stability without adding sacrificial agents。
6 MOFs derived oxide-sulfide composite
The MOFs derived oxide-sulfide composite material uses MOFs as a precursor to simultaneously retain two components of metal oxides and metal sulfides by controlling carbonization conditions to form MxS/MyO nanomaterials;MxS/NyO nanocomposites can also be formed by doping metals in MOFs by controlling the calcination conditions.The oxide-sulfide composite material integrates the advantages of the two,the introduction of the sulfide improves the dispersion of the oxide and exposes more active sites,and the heterojunction formed by the oxide/sulfide can effectively broaden the light absorption range,thereby improving the photocatalytic performance 。
The multi-component system formed by the heterojunction with the highly porous morphology of the genetic parent MOFs and the cocatalyst and conductive materials can promote the separation of photogenerated carriers,obtain more catalytic sites,and improve the charge transport rate.Wang et al.Prepared a CeO
2/ZnS heterojunction using Ce-doped ZIF-8 as a precursor,and then used an in situ cation exchange method to load the co-catalyst CuS onto the CeO
2/ZnS composite to obtain CeO
2/ZnS-CuS（S
BET=156.076 m
2·g
-1）
[48]。 The effect of Ce content on the catalytic performance was further studied,and the results showed that when the Ce content was 10 wt%,the photocatalytic hydrogen production performance of CeO
2/ZnS-CuS was the best,which could reach 13.47 mmol·h
-1·g
-1.Because the CB of CeO
2is more negative than that of ZnS,when the sunlight is irradiated,the electron on the CB of CeO
2is transferred to ZnS,the electron on the CB of ZnS is captured by the co-catalyst CuS,and the electron reacts with H
+to produce H
2.The addition of the supported cocatalyst CuS and the heterojunction form a ternary system,and the regular porous dodecahedron structure of the genetic ZIF-8 provides larger specific surface area,more active sites and more effective electron transport for the photocatalytic reaction 。
Zhao et al.Designed and synthesized ternary noble metal-free TiO
2-Ti
3C
2-CoS
xheterostructures with nanocrystal size by a simple solvothermal method using ZIF-67 as a template and Ti
3C
2as a conductive material
[49]。 The effects of the molar ratio of CoS
xand the content of Ti
3C
2on the activity of the catalyst were further studied,and the results showed that when the molar ratio of CoS
xreached 1 wt%,When the content of Ti
3C
2is 0.5 wt%,the best hydrogen evolution performance of TiO
2-Ti-C
2-CoS
xis 0.95 mmol·h
-1·g
-1.The co-loading of CoS
xand Ti
3C
2showed a synergistic effect in promoting the photocatalytic hydrogen production activity.That is,the introduction of the CoS
ximproves the dispersion of the TiO
2and increases the number of exposed active sites;CoS
xcan promote charge separation and provide active sites for hydrogen production due to its lower CB and electrocatalytic HER overpotential;Ti
3C
2can improve the conductivity of the heterostructure and facilitate the charge transport 。
Wang et al.Used the designed and synthesized bimetallic ZnCo-ZIF@ZIF-8 hybrid as a precursor to prepare Co
3O
4-ZnO@ZnS/Pt catalyst by simple calcination,sulfidation,and photodeposition methods
[50]。 Under the irradiation of 300 W xenon lamp,the hydrogen production activity of Co
3O
4-ZnO@ZnS/Pt using sacrificial agent Na
2S-Na
2SO
3was significantly improved,and the hydrogen production rate could reach 3269.3μmol·h
-1·g
-1.The structural analysis shows that Co
3O
4is highly dispersed in the ZnO@ZnS heterostructure,which effectively collects the photogenerated holes and greatly suppresses the photogenerated charge recombination,while the porous structure of Co
3O
4-ZnO@ZnS is beneficial to charge transfer and reduces the reactant diffusion limit 。
The nanoheterojunction structure can reduce the mean free path of charge carriers and improve the charge separation efficiency,thus significantly improving the HER activity of the catalyst under visible light.Zhao et al.Used MOF-5 as the precursor and TAA as the sulfur source,and the formation of ZnO/ZnS nanoheterojunction(Fig.8)could be adjusted by controlling the calcination temperature and time.The results showed that the optimal hydrogen evolution performance of ZnOS-30 with a treatment time of 30 min was 415µmol·h
-1·g
-1,and it had high stability and cyclicity without any co-catalyst
[51]。
The formation of p-n heterojunction can effectively broaden the range of light absorption,inhibit electron-hole recombination,and accelerate the rate of charge transfer,thereby improving the activity of photocatalytic reduction of CO
2.Sun et al.Anchored Bi
4O
5Br
2nanosheets on the surface of Mo-MOF-derived 1D NPC-MoS
2nanorods to prepare a series of hierarchical MOF-derived NPC-MoS
2@Bi
4O
5Br
2heterostructure（S
BET=48 m
2·g
−1）by solvothermal method
[52]。 The effect of NPC-MoS
2loading on the photocatalytic performance was further investigated,and the results showed that the optimal loading of NPC-MoS
2was 5 wt%,at which the yield of CO and CH
4reached 95.8 and 159.9μmol·g
-1,respectively,within 5 H.At the same time,the catalyst still showed good stability and regenerability after four cycles.Upon illumination,the photoinduced electrons of NPC-MoS
2and Bi
4O
5Br
2are excited to the CB,while the holes remain on their VB.Due to the internal electric field of the p-n heterojunction,the photoelectron is transferred to the CB of Bi
4O
5Br
2,which is more negative than the reduction potential of CO
2/CO and CO
2/CH
4.At the same time,holes are retained on the VB of NPC-MoS
2,which has a higher oxidation potential than（CH
3）
2CHOH/CH
3COCH
3.Therefore,isopropanol can be oxidized to produce H
+and acetone.Finally,H
+reduces CO
2to CO and CH
4 。
MOFs-derived oxide-sulfide composites overcome the disadvantages of large band gap and photocorrosion of oxides,and improve the light absorption range and photocatalytic performance by modification or modification of sulfides.the heterostructure formed at the same time can not only effectively control the energy band of the oxide,but also accelerate the transfer of electrons at the interface,inhibit the recombination of photogenerated electron-hole pairs,and enhance the activity and stability of the photocatalytic reaction。
7 MOFs-derived carbon-sulfide, carbon-nitrogen-sulfide composites
the MOFs material can also be used as a precursor for preparing a carbon-based sulfide composite material,the MOFs is both a carbon source and a metal source,and the two components can be simultaneously retained by controlling the carbonization condition,To obtain MOFs-derived carbon-sulfide composites,the metal nodes of MOFs can be transformed into metal nanoparticles during high temperature treatment,while the organic ligands can be transformed into porous carbon materials
[53]。 Graphene has good conductivity and excellent electron transfer ability,and loading on MOFs-derived sulfides can promote the separation of photogenerated charge carriers and the improvement of transport efficiency。
Graphite-like carbon nitride（g-C
3N
4）I s the most stable allotrope of carbon nitride with a graphite-like layered structure.As an inorganic nonmetallic semiconductor material with a band gap of∼2.7 eV,it has become a hot topic in the field of photocatalysis due to its simple preparation method,chemical stability,unique electronic structure,and response to visible light
[54,55]。 As a result,the researchers mixed MOFs sulfide with g-C
3N
4to obtain carbon-nitrogen-based sulfide composite materials,which have greatly improved photocatalytic performance compared with single-component MOFs sulfide and g-C
3N
4,which is attributed to the energy level difference between the two components enhancing the absorption of visible light and promoting the separation of photogenerated charges 。
Multi-component MOFs are ideal platforms for the in situ growth of semiconductor nanoparticles,which can be simultaneously deposited on various carbon-rich scaffolds,and the enhanced interfacial interaction of particles grown in situ on carbonaceous supports can facilitate the transport of photogenerated carriers,thereby improving the photocatalytic degradation performance.Cao et al.Prepared a novel nitrogen-doped carbon-supported cadmium sulfide(CdS/NC-T)composite by a simple in-situ carbonization method using Cd-MOFs as the precursor(Fig.9)
[56]。 the effect of calcination temperature on the catalytic performance of CdS/NC-T was further studied.the results showed that the optimal photocatalytic degradation efficiency of CdS/NC-500 for tetracycline(TC)was 83%(1 H)when the calcination temperature was 500℃.Due to the in situ growth of CdS nanoparticles on the carbonaceous support,the interfacial synergy is enhanced,resulting in the efficient generation and transport of photogenerated carriers。
MOFs-derived sulfides overcome the poor mechanical and chemical stability of pure MOFs,improve the stability and catalytic activity of catalysts,and inherit the porosity and multiple active centers of MOFs,which is conducive to the high turnover frequency of catalytic reactions and promote the smooth progress of photocatalytic reactions.Liu et al.Prepared the MoS
2/CN composite catalyst by solvothermal reaction in one step from a mixture of Mo-MOF/g-C
3N
4[57]。 The effect of Mo-MOF addition on the catalytic performance was further investigated.The results showed that the best hydrogen production efficiency of MoS
2/CN with 2 mg Mo-MOF addition was 1695.76μmol·g
-1（4 h）under visible light.Under visible light,CN is excited to generate electrons and holes,and TEOA can react with the photogenerated holes of CN to inhibit electron-hole recombination,while the MoS
2co-catalyst loaded on the surface of CN accelerates the transmission of photogenerated electrons,and the photocatalytic hydrogen evolution activity is greatly improved 。
MOFs derivatives are considered as potential high-performance co-catalysts because of their high specific surface area and high electrical conductivity,and ion exchange can simultaneously perform facile and effective structural and compositional transformations,and can avoid framework collapse during the conventional pyrolysis synthesis of MOFs materials.Chen et al.Prepared flower-like amorphous CoS
x-C/N assembled by C/N-rich two-dimensional(2D)nanosheets by photoinduced anion exchange using ZIF-67 as a sacrificial template,and loaded it on a one-dimensional（1D）Cd
0.5Zn
0.5S（CZS）catalyst to form a unique 2D@1D composite photocatalyst with a high and low dimensional cross structure CoS
x-C/N
y@CZS
[58]。 Further study on the effect of different CoS
x-C/N promoter content on the catalyst performance showed that the best hydrogen evolution activity of CoS
x-C/N
2@CZS with 3%CoS
x-C/N promoter under visible light was 18.57 mmol·h
-1·g
-1,and the AQY reached 21.05%.The catalytic reaction mechanism shows that the photogenerated electrons are transferred to the two-dimensional catalytic active center CoS
xthrough the Co-S-C/N charge channel for hydrogen evolution,while the photogenerated holes are left on the one-dimensional light absorption center CZS,which is eliminated by the sacrificial agent 。
The formation of heterostructure and the addition of rGO can promote the separation of photogenerated carriers and increase the transport rate,thus promoting the effective photocatalytic reaction.Si et al.Used ZIF-8 as precursor and TAA as sulfur source to react with g-C
3N
4nanosheets to synthesize g-C
3N
4/ZnS heterojunction
[59]。 The effect of different weight ratios on the catalytic performance was further studied.The results showed that g-C
3N
4/ZnS-8∶1 had the best degradation effect on MO,and the degradation efficiency was 74.40%(160 min).The heterojunction formed by g-C
3N
4,ZnS enhances the photocatalytic degradation activity,and the flat surface of ZnS provides a close contact interface with g-C
3N
4,through which the photogenerated electron-hole pairs are effectively transferred and separated 。
Xu et al.Prepared Cu
0.5Zn
0.5In
2S
4quaternary hollow materials using CuZn-ZIF@MIL-68(In)layered MOFs as templates
[60]。 Highly dispersed g-C
3N
4-rGO 2D-2D nanosheets were ingeniously inserted into the hollow tube wall to form Cu
0.5Zn
0.5In
2S
4-rGO-g-C
3N
4heterojunction.The effect of rGO addition on the catalytic performance was further investigated,and the results showed that the Cu
0.5Zn
0.5In
2S
4-rGO-g-C
3N
4with 0.5 wt%rGO exhibited the best hydrogen evolution rate（11.60 mmol·h
-1·g
-1）under visible light.The heterostructure formed by Cu
0.5Zn
0.5In
2S
4-rGO-g-C
3N
4and the high conductivity of rGO promote the separation of charge carriers and improve the transport efficiency 。
MOFs with permanent porosity can usually synthesize PCMs(porous carbon materials)with appropriate surface area and pore volume,which can promote the electron transfer rate by building the synergy between molecules and frameworks,molecules and molecules,thereby enhancing the photocatalytic activity.Choi et al.Successfully prepared In
6S
7NPs（multifunctional metal-derived nanoparticles)intercalated functional PCMs with S and N co-dopants by simple one-pot carbonization using In-MOF as a precursor
[61]。 The effect of different preparation temperatures on the catalytic performance of PCM was further studied.The results showed that PCM-900（S
BET=448 m
2·g
−1）prepared at 900°C exhibited the best photocatalytic activity with 88%MO degradation(160 min).Photocatalytically active semiconductor indium sulfide NPs with small band gap,Lewis basic S and N co-doped in carbon matrix,promote the interaction between CO
2molecules and carbon framework 。
By changing the intrinsic activity of the active center,the ternary heterogeneous metal compound prepared by doping an element in the binary matrix can obtain higher photocatalytic activity.Qi et al.Prepared g-C
3N
4/C@Ni
3S
4/Ni
2P by in situ sulfidation and phosphating of g-C
3N
4/Ni-MOF
[62]。 The effect of phosphating time on the catalytic performance was further studied.The results show that the best hydrogen evolution performance of g-C
3N
4/C@Ni
3S
4/Ni
2P under visible light is 14.49 mmol·h
-1·g
-1when the phosphating time is 30 min.The electronic interaction between Ni
2P and Ni
3S
4,which synergistically increases the electron transfer rate,and the staggered band alignment of excited state EY,g-C
3N
4,Ni
3S
4,and Ni
2P,improve the photocatalytic hydrogen evolution activity 。
MOFs and graphene-based materials can integrate the advantages of a single component,and then play a role in improving stability,modifying the structure,expanding the specific surface area,reducing aggregation,enhancing dispersion and improving catalytic activity.Yusuf et al.Synthesized a series of bimetallic MOFs using Cu and Ni as metal centers and carboxylate-rich pyromellitic acid（H
4BTEC）as ligands by a facile method
[63]。 Subsequently,mixed core-shell nanostructures of bimetallic Cu,Ni sulfides encapsulated on porous C were prepared from MOFs derivatives,and the MOF(Cu:Ni-BTEC/GO)was directly carbonized and sulfurized to prepare bimetallic sulfide Cu
2S：NiS
2@C/rGO.Cu
2S：NiS
2@C/rGO promoted phenylboronic acid and imidazole photocatalytic C-N coupling was applied to various substrates with conversion and selectivity greater than 90%.This indicates that Cu
2S：NiS
2@C/rGO possesses significant functional-group selective photocatalytic activity under mild conditions.The synergistic effect of non-noble metal Cu and Ni in the multifunctional structure of bimetallic sulfides enhances the photocatalytic activity of N-derivative compounds synthesized by oxidative coupling reaction 。
the MOFs-derived carbon-sulfide and carbon-nitrogen-sulfide composite materials Combine the advantages of each single component and retain the properties of high porosity,high specific surface area and multiple active sites of MOFs materials.combine that advantages of good electrical conductivity of carbon-based,visible light response of carbon-nitrogen base materials and stable chemical property,the problems of high recombination rate of photogenerated electron-hole pairs,low quantum yield and poor cycle stability exist in the photocatalytic reaction process are improved。
8 Conclusion and outlook
In this paper,the preparation and structure of MOFs-derived sulfides and their composites,as well as their applications in the field of photocatalysis,such as hydrogen evolution by hydrolysis,carbon dioxide reduction,and sewage treatment,are reviewed.MOFs-derived monometallic sulfides have better potential for photocatalytic degradation reactions,and MOFs-derived multi-component metal sulfides have more advantages for photocatalytic hydrogen evolution from water and CO2reduction,such as Ni-MOFs derived multi-component metal sulfides,which have the best hydrogen evolution rate,but there are few reports on photocatalytic CO2reduction.As a new type of photocatalytic material,sulfides derived from MOFs have many satisfactory advantages:(1)sulfides derived from MOFs retain the original structural characteristics of parent MOFs,with large surface area,dispersed nano-sized subunits and abundant active sites;(2)solving the problem of material stability of the parent MOFs in a high-temperature and harsh chemical environment;(3)narrower band gap and higher utilization rate of visible light;(4)Having a porous nature may provide an additional pathway for the migration of photoexcited electrons,thereby facilitating charge carrier separation.Although good results have been achieved in recent years,it should be recognized that the current research is still in the initial stage of laboratory research,and the application of MOFs-derived sulfides in the field of photocatalysis mainly has the following problems:(1)Some MOFs-derived sulfides materials have general or poor catalytic performance and still need to be modified;(2)The effect of the size and shape of MOFs-derived metal sulfides on the photocatalytic performance of composites is still not well studied;(3)The types of MOFs used in this field are generally limited to MIL-68,ZIF-8,ZIF-67,MOF-5 and MOF-74,and other MOFs with excellent photocatalytic activity still need to be expanded;(4)The effect of electron donors on the photocatalytic activity of MOFs-derived sulfide composites is still insufficient,and the use of sacrificial electron donors is generally limited to TEOA,lactic acid,methanol,and Na2S-Na2SO3;(5)Some MOFs-derived metal sulfides have poor cycle stability,such as Co-MOFs-derived multi-component metal sulfides,which need to add sacrificial agents to improve stability;(6)MOFs-derived metal sulfide supported photocatalysts have serious photocorrosion,which can be reduced by loading MOFs-derived sulfide semiconductors on other hole cocatalysts or layered oxide materials and mesoporous molecular sieves to form heterojunctions(Fig.10)and improve the stability of photocatalysts 。
Although many advances have been made in this field,more research is still needed before these photocatalysts can be applied on an industrial scale.(1)On the premise that light harvesting is sufficient for the smooth progress of photocatalytic reaction,molecular control methods such as halogen doping,nitrogen and carbon co-modification,and vacancy construction can control the energy band and promote charge separation;(2)The introduction of other semiconductor materials combined with MOFs-derived sulfides to form various effective heterostructures is a widely studied method to effectively improve the carrier behavior and improve the catalyst cycle performance;(3)To further study the preparation of high-performance dye photosensitizers that can be tightly adsorbed on the surface of wide band gap semiconductors to improve the chemical reaction stability of photocatalysts under long-term illumination;(4)Strengthen the research on the immobilization and modification methods of photocatalyst.The metal carrier which is easy to be processed into a network can meet the requirements of carrier stability and large surface area,and also enable the attached photocatalyst to obtain more light and adapt to static or dynamic catalytic environment.(5)Further research and development of other low-cost,easily available and sustainable sacrificial electron donors;(6)In addition to the addition of sacrificial agents,partial oxidation can also reduce photocorrosion by consuming holes,and further research can combine photocatalytic hydrogen evolution with oxidation to produce H2and valuable substances at the same time;(7)Since the VB potential is not positive enough,MOFs-derived metal sulfides are often used as reductive photocatalysts for hydrogen production,CO2reduction and Cr(Ⅵ)reduction,and it is also important to apply MOFs-derived metal sulfides to photocatalytic oxidation.To this end,more efforts need to be put into the design improvement of MOFs-derived sulfides in the future to solve complex problems.Gratifyingly,the great potential of unique MOFs-derived sulfides provides a vast field for further development.Through unremitting efforts,it is expected to be implemented in practical production applications in the future 。
Guangdong Science and Technology Innovation Strategy Program(2023S005054)
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