Highly efficient tandem Z-scheme heterojunctions for visible light-based photocatalytic oxygen evolution reaction

Yi Lu ,Xing-ki Cui ,Cheng-xio Zho ,Xio-fei Yng ,,*

a College of Science,Nanjing Forestry University,Nanjing 210037,China

b School of Materials Science and Engineering,Jiangsu University,Zhenjiang 212013,China

Received 3 September 2020;accepted 12 November 2020

Available online 10 December 2020

Abstract Oxygen is important in maintaining a clean and reliable water environment.Designing heterojunction photocatalysts that can evolve oxygen from water splitting through an artificial Z-scheme pathway is a promising strategy for solving environmental problems.In this study,flower-like MoS2 nanostructures were fabricated via a simple hydrothermal process,and the electrostatic-based assembly ion-exchange method was used to construct a tandem Ag3PO4/MoS2/g-C3N4(AMC)heterojunction.The as-synthesized photocatalyst exhibited significant improvements in harvesting visible light and transporting charge carriers.Moreover,the catalyst that was similar to the Z-scheme with intimate interface contact exhibited a strong oxygen evolution performance.The oxygen evolution activity of the optimal AMC-10 catalyst was approximately 11 times that of the pristine Ag3PO4.The results indicated that addition of a small amount of the flower-like MoS2 could significantly enhance the efficiency of oxygen evolution by the heterojunction.The findings in this study provide an alternative pathway for rationally designing efficient oxygen-evolving photocatalysts in order to improve the quality of water and rehabilitate the water environment.© 2020 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Z-scheme;Graphitic carbon nitride;MoS2;Ag3PO4;Oxygen evolution reaction;Water splitting

1.Introduction

Water does not contain much oxygen,particularly warm water.Water bodies receive oxygen from the atmosphere and form aquatic plants.Dissolved oxygen concentrations usually have a significant effect ondecomposition rates of plant litter,and thus,they are considered important aquatic ecosystem health indicators.During the treatment of wastewater,onsite generation of oxygen may increase productivity and reduce the operating costs,thereby providing an incentive to seek for efficient oxygenevolving materials that can produce oxygen in situ in order to improve the quality of water.Employing semiconductor-based photocatalysts to realize visible light-based oxygen evolution is a promising approach to solving the issues of severe environmental contamination and energy crisis(Wang et al.,2018a,2018b;Lin et al.,2019;Mishra et al.,2019).However,a singlecomponent semiconductor is unable to meet all the criteria in terms of band structure,visible light utilization,and redox potential.Thus,to realize high-performance solar-based water splitting,rationally designing and managing the synthesis of visible-light-responsive composite photocatalysts with a hybridization technique is necessary.Generally,heterojunction photocatalysts can be designed by combining different semiconductors in a manner similar to conventional type-II heterojunctions with specific Z-scheme configuration(Di et al.,2019;Fajrina and Tahir,2019;Faraji et al.,2019;Hisatomi and Domen,2019;Yi et al.,2019;Zhang et al.,2019).Inspired by the photosynthesis process in green plants,there has been increased interest in developing artificial Z-scheme heterostructured photocatalysts in recent decades,in particular solar-based mediator-free Z-scheme photocatalytic systems(Miseki and Sayama,2019;Ren et al.,2019;Yang et al.,2019).

Notably,photocatalytic water splitting depends on various fundamental processes,including photon absorption,exciton separation,and carrier diffusionand transport,and italsodepends on catalytic efficiency.Compared with conventional type-II heterojunctions and mediated Z-scheme photocatalysts,the advantages of a direct Z-scheme photocatalytic system are as follows:(a)the path for electron-hole separation and charge-carrier migration is shortened by the unique tandem architecture,and thus more efficient charge transport and higher redox capability can be achieved;(b)the band energy required for redox reactions is reduced to the utmost extent;and(c)undesirable backward reactions can be avoided without an electron mediator(Huang et al.,2019;Wang et al.,2019b;Xue et al.,2020;Yi et al.,2020;Zhao et al.,2020).In recent years,silver orthophosphate(Ag3PO4)has been considered a promising visible-light oxygenevolving photocatalyst candidate because of its suitable band gap(2.45 eV)and valence band potential(2.9 eV),which are higher than the thermodynamic water oxidation potential(1.23 eV versus normal hydrogen electrode(NHE))(Tang et al.,2020).However,it suffers from severe photocorrosion and fast recombination of photoexcited charge carriers under light illumination.The fabrication of Ag3PO4-based Z-scheme heterojunction photocatalysts has proven effective in accelerating the charge separation,enhancing the photo stability,and boosting water oxidation efficiency.

Our previous work has demonstrated that the photocatalytic oxygen-evolving efficiency of the Ag3PO4-based nanohybrids under light illumination could be significantly increased via a specific Z-scheme pathway for charge transfer(Cui et al.,2018;Tian et al.,2019;Zhao et al.,2019).The critical roles of different electron mediators such as silver,graphene,and indirect Z-scheme configuration in regulating the solar-based photocatalytic oxygen evolution have been revealed.As a representative graphene-like lamellar two-dimensional(2D)material,a single or few-layered molybdenum disulfide(MoS2),with its unique electronic and chemical properties,is of particular interest to researchers.In recent years,there have been intensive investigations into the use of MoS2as a cocatalyst for photo-and electro-catalytic hydrogen evolution reaction(HER).However,exploring the 2D MoS2material as a real catalyst rather than a co-catalyst or conductive substrate in Z-scheme heterojunction water splitting systems in experiments remains a challenge.In addition to our previously published work on Ag3PO4-based Z-scheme oxygen-evolving photocatalysts,in this study we used the coupling of the 2D MoS2nanosheets with a suitable band structure into the indirect Z-scheme Ag3PO4/g-C3N4heterojunctions to obtain dual Z-scheme Ag3PO4/MoS2/g-C3N4(AMC)heterostructures.Notably,the MoS2-bridged tandem Z-scheme heterostructure can facilitate the directional transport of photogenerated charge carriers,remove the electrons from the conduction band(CB),and keep oxidative holes on the valence band of the Ag3PO4for highly improved visible lightbased water splitting.

2.Experimental section

2.1.Synthesis of modified g-C3N4 and MoS2

All chemical reagents were of analytical grade,and they were used without further purification.Modified g-C3N4nanosheets were prepared according to previous studies(Peng et al.,2018;Tian et al.,2018;Yang et al.,2019).To synthesize the flower-like MoS2nanosheets,(NH4)Mo7O24.4H2O(2.47 g)and CH4N2S(3.8 g)were dissolved in deionized water(60 mL)and stirred for 0.5 h.The mixture was then transferred into a Teflon-lined autoclave(100 mL)and subsequently heated at 180°C for 24 h in a muffle furnace.After cooling naturally,the samples were subjected to high-speed centrifugation.The precipitates were collected and then washed with deionized water and ethanol repeatedly.Finally,black powders were obtained through vacuum drying at 60°C for 12 h.

2.2.Synthesis of AMC composites

In a typical synthesis,the modified g-C3N4powder(200 mg)was dispersed into deionized water(100 mL)to obtain an exfoliated g-C3N4nanosheet dispersion via sonication for 30 min.AgNO3(9 mmol)was ultrasonically dissolved in deionized water(20 mL)and then added to the g-C3N4nanosheet dispersion and gently stirred overnight.Subsequently,the flower-like MoS2was dispersed and ultrasonicated in deionized water(20 mL)for 2 h,and then was mixed with the Ag+/g-C3N4dispersion and continuously stirred for 12 h.Finally,a Na3PO4.12H2O aqueous solution(9 mmol)was added,leading to a color change from black to dark green.The samples were subjected to high-speed centrifugation,and the precipitates were collected and washed with deionized water and ethanol three times,then dried in a vacuum at 60°C overnight.The samples obtained through hybridizing with MoS2with amounts of 10 mg,20 mg,50 mg,100 mg,and 200 mg were denoted as AMC-10,AMC-20,AMC-50,AMC-100,and AMC-200,respectively.In addition,the Ag3PO4,Ag3PO4/modified g-C3N4,and Ag3PO4/MoS2samples were synthesized for comparisons.

2.3.Photocatalytic oxygen evolution experiments

The oxygen-evolving performances of the as-synthesized photocatalysts were monitored using an O2sensor(PreSens GmbH,Fibox 3)in a sealed flask connected to a water-cooling system.Before light illumination,the O2sensor was calibrated using oxygen-free water and pure water as a standard correction of oxygen sensor.Under light-emitting diode(LED)irradiation,the temperature of the system was kept constant by recycling cold water.In a typical procedure,a mixture of the photocatalyst(0.3 g)and AgNO3(1.0 g)was ultrasonically dispersed in deionized water(100 mL)for 30 min to obtain strong dispersity.Thereafter,the mixture was illuminated under an LED light(30 W).The photocatalytic oxygenproducing efficiencies of different nanohybrids were determined by measuring the amount of oxygen evolved from water at regular time intervals.

3.Results and discussion

3.1.Morphology characterizations

Field emission scanning electron microscopy(FE-SEM)was conducted to analyze the morphology of the catalyst.Fig.1(a)shows that the pristine g-C3N4was porous,and it consisted of curved layered nanosheets with a thickness of approximately 10 nm.Moreover,the porous flakes were observed to originate from partial interconnection and overlapping of the curved nanosheets.For the pure Ag3PO4,the pellet structure looked like the inconsistent crystal sugar,but the surface was smooth(Fig.1(b)).The MoS2exhibited a micro flower-like morphology(Fig.1(c)).Notably,as shown in Fig.1(d),the AMC-10 composite prepared with the selfassembled hydrothermal method exhibited a distinct morphology.The Ag3PO4surfaces were randomly deposited by the g-C3N4nanosheets with crumpled-shell nanostructures,and they were embedded with the flower-like MoS2,leading to the appearance of a rough surface.The elemental mapping results reveal the well-defined spatial distribution of silver(Ag),oxygen(O),phosphorus(P),carbon(C),nitrogen(N),molybdenum(Mo),and sulphur(S)in the heterojunction(Fig.1(e)),confirming the successful preparation of the AMC hybrid photocatalyst.

Fig.1.SEM micrographs.

3.2.Characterizations of phases and spectral properties

X-ray photoelectron spectroscopy(XPS)was performed to study the chemical compositions and elemental states of the optimal sample.From Fig.2,it is evident that C,N,O,P,Ag,S,and Moweredetectedinthe AMC-10 composite.Asshowninthe high-resolution XPS spectrum of C 1s(Fig.2(b)),the peaks fitted at 284.8 eV,286.6 eV,and 288.2 eV could be ascribed to the adsorbed carbon species,C-C bond,and C=C bond,respectively.The high-resolution XPS spectrum of N 1s for the composite is shown in Fig.2(c).The asymmetrical N 1s XPS signal can be fitted into three peaks located at 398.6 eV,399.7 eV,and 401.6 eV,which represent the C=N-C bond,N-(C)3bond,and C-N-H bond in the g-C3N4,respectively(Fageria et al.,2017;Lu et al.,2018).For the high-resolution XPS spectrum of O 1s,there are two peaks at 530.5 eVand 532.5 eV(Fig.2(d)),which represent the oxygen in the Ag3PO4and the C-O bond,respectively.In addition,the peak at 133.6 eV represents P 2p in Fig.2(e).In Fig.2(f),the peaks at 368.4 eV and 374.3 eV are ascribed to Ag 3d5/2and Ag 3d3/2for Ag+in Ag3PO4,respectively(Shao et al.,2019;Li et al.,2020).The S 2p spectrum in Fig.2(g)shows doublet peaks located at 168.5 eVand 162.4 eV,corresponding to the S 2p1/2and S 2p3/2in MoS2,respectively.Moreover,the peaks at 234.1 eVand 236.7 eVare ascribed to the Mo6+in MoO3and Mo4+in MoS2,respectively.

Furthermore,light-harvesting properties of the photocatalysts were evaluated via UV-visible diffuse reflectance spectroscopy(UV-Vis DRS)(Fig.3(a)).The pure g-C3N4and Ag3PO4exhibited absorption edges at approximately 450 nm and 550 nm,respectively.The Ag3PO4/g-C3N4hybrids exhibited increased absorption intensity at a wavelength(λ)ranging from 400 nm to 800 nm.This is attributed to the quantum confinement and light scattering effects of the nanosheet structure.Fig.3(b)shows the UV-Vis DRS spectra of the composite doped with different contents of MoS2.The absorption intensity of the composite at a wavelength above 500 nm increased with increasing loading amounts of MoS2.The results imply that addition of MoS2can increase the intensity of the UV-Vis absorption of the composite.Therefore,the hybrid heterojunction can absorb and utilize more light to produce sufficient photo-induced charge carriers,thus improving the photocatalytic activity.

3.3.Oxygen evolution performance

Fig.4(a)shows the time-dependent photocatalytic oxygen evolution for the pure Ag3PO4,Ag3PO4/MoS2,Ag3PO4/g-C3N4,and AMC-10 under LED visible-light irradiation.The oxygen evolution efficiency of the pure Ag3PO4was significantly attenuated after 5 min of irradiation,reaching 6.48μmol/L.The Ag3PO4/g-C3N4and Ag3PO4/MoS2exhibited oxygen evolution efficiencies of 36.1μmol/L and 33.9μmol/L,respectively.Notably,the AMC-10 composite exhibited the highest oxygen evolution efficiency of 69.6μmol/L within the initial 40 min of irradiation,which may be attributed to the excellent visible-light response.Fig.4(c)shows the calculated oxygen evolution rates of these catalysts.It is evident that the oxygen evolution performance of the AMC-10 composite was better compared to those of the pure Ag3PO4,Ag3PO4/MoS2,and Ag3PO4/g-C3N4.Additionally,Fig.4(b)and(c)shows the oxygen evolution performance of the Ag3PO4/g-C3N4doped with MoS2.The oxygen evolution performance gradually decreased with the increasing MoS2content,which is consistent with the fact that MoS2is a superior electronic transmission medium,and can be used as such instead of serving as a half-redox reaction solid-state Z-scheme catalyst.

Fig.2.XPS survey spectra and high-resolution XPS spectra of all elements in AMC-10 composite(blue,green,and purple lines represent fitting peaks at different binding energies).

Fig.3.UV-Vis DRS spectra of all synthesized nanohybrids.

Theoretically,the photoluminescence(PL)intensity indicates theamountof effective carriers participatingin thephotoelectron recombination in the photocatalytic process.Fig.5(a)shows the PL intensities of the g-C3N4,Ag3PO4/g-C3N4,and AMC-10 catalysts.The high PL intensity of g-C3N4demonstrates effective recombination of charge carriers.However,the low PL intensity of the Ag3PO4/g-C3N4composite suggests that the formation of the nano-heterostructure would weaken the photoelectron recombination.Furthermore,the AMC-10 catalyst exhibits a relatively lower PL intensity than that of the Ag3PO4/g-C3N4.The decrease in PL intensity is ascribed to the formation of an effective Z-scheme pathway,which facilitates the charge transport between g-C3N4and Ag3PO4,thus suppressing fast recombination of photoelectrons.Moreover,in principle,the restrained charge carrier recombination could be verified by the time-dependent PL decay of the samples.The transient photocurrents of the g-C3N4,Ag3PO4/g-C3N4,and AMC-10 catalysts at 0.75 V were recorded during various ON/OFF cycles(Fig.5(b)).All samples exhibited prompt and reproducible photocurrent responses.The transient photocurrent density of AMC-10 was above 1.85μA,which was higher than that of the pure g-C3N4(below 0.2μA)and Ag3PO4/g-C3N4(below0.4μA).AsshowninFig.5(c),theaveragePLlifetimefor the AMC-10 and g-C3N4catalysts were 0.884 and 0.975 ns,respectively.Notably,the significant shortening of the PL lifetime is attributed to the efficient exciton dissociation in the heterostructured photocatalyst that facilitates the transfer and yield of hot charge carriers.Additionally,as shown in Fig.5(d),electrochemical impedance spectroscopy(EIS)was conducted to evaluate the photocatalyst electronic performance.Z′andZ′′were real and imaginary parts of the impedance,respectively.The resistance of AMC-10 was lower compared to those of g-C3N4and the Ag3PO4/g-C3N4composite,which can be attributed to the specific geometric structure of the nanosheet that enables a larger contact area and a shorter diffusion distance to reaction sites of the photogenerated charges.These results demonstrate that the flower-like MoS2served as the electron transport medium to reduce the resistance of the electron-hole separation,providing a vital channel for the persistent and effective photolysis of the catalysts.

Fig.4.Photocatalytic oxygen evolution performance under LED irradiation.

Fig.5.Photoelectrochemical measurements of g-C3N4,Ag3PO4/g-C3N4,and AMC-10 samples.

Electron spin resonance(ESR)spectroscopy is considered a powerful tool for identifying and quantifying the different types of short-lived free radicals.This tool can be employed to directly detect the photo-generated electron,holes,and other active species(Li et al.,2019;Wang et al.,2019a;Zhou et al.,2019).In this study,2,2,2,6-tetramethylpiperidine-1-oxyl(TEMPO)with unpaired electrons was used as a trapping agent for capturing the photo-induced electron/holes.As shown in Fig.6(a)and(b),the Ag3PO4,Ag3PO4/g-C3N4,and AMC-10 catalysts exhibit significantly decreased ESR signals after being illuminated for 10 min,indicating the generation of various photogenerated electrons/holes.The weak ESR signal of the TEMPO observed in the AMC-10 catalyst suggests that the photo-induced electron/holes are generated on the surface of AMC-10.Additionally,5,5-dimethyl-1-pyrroline-Noxide(DMPO)was used to verify the generation of hydroxyl radicals(.OH-)and superoxide(.O-2)in the dispersions via the ESR signals of the adducts formed between DMPO and.OH/.OOH.As shown in Fig.6(c)and(d),the significant ESR signals associated with DMPO-.OH/.OOH were observed,confirming the formation of.OH/.O-2in all the systems under simulated light illumination.An apparent signal detected in the AMC-10 catalyst suggests the easy formation of.OH/.O-2.Moreover,the increasing amplitude of the DMPO-.OH signal was more evident than that of the DMPO-.OOH signal in the AMC-10 suspension,implying that.OH species are more easily formed.To further reveal the redox potentials of these composites,the CB and valence band(VB)positions of MoS2and g-C3N4were determined using VB XPS spectra and Mott-Schottky plots,respectively.Fig.7(a)and(b)shows that the VB potentials of the pure MoS2and g-C3N4are located at 1.79 and 1.56 eV,respectively.In Fig.7(c)and(d),whereCis the interfacial capacitance,the CB potentials of MoS2and g-C3N4(versus NHE)are located at-0.26 and-1.17 V,respectively.

Fig.6.ESR spectra of radical adducts trapped by DMPO and TEMPO for Ag3PO4,Ag3PO4/g-C3N4,and AMC-10.

Fig.7.VB XPS spectra and Mott-Schottky plots of MoS2 and g-C3N4.

4.Conclusions

In this study,the artificial heterogeneous structures of the AMC composite were successfully constructed with g-C3N4and flower-like MoS2anchoring on the surface of the Ag3PO4microspheres.Additionally,their phase,structural,and photocatalytic properties were studied.The AMC-10 composite exhibited a significantly high oxygen evolution rate of 232.1μmol.L-1.g-1.h-1,which was higher compared to that of the pure Ag3PO4,indicating that an all-solid-state Z-scheme photocatalytic system had formed.Moreover,the MoS2deposited on the surface of the Ag3PO4microspheres in the AMC-10 composite played an important role as an electron transport medium,contributing to the enhancement of oxygen precipitation activity and durability of the catalyst.Therefore,the AMC composite,with a strong oxygen evolution performance,could be employed as an in situ oxygen evolution material to purify and replenish water resources and help manage the water ecosystem.

Declaration of competing interest

The authors declare no conflicts of interest.