cuprous oxide

Noble metal-free cuprous oxide/reduced graphene oxide for enhanced photocatalytic hydrogen evolution from water reductionHonglei Xu a ,Xiangqing Li a ,*,Shi-zhao Kang a ,Lixia Qin a ,Guodong Li b ,Jin Mu b ,*aSchool of Chemical and Environmental Engineering,Shanghai Institute of Technology,100Haiquan Road,Shanghai 201418,China bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry,College of Chemistry,Jilin University,Changchun 130012,Chinaa r t i c l e i n f oArticle history:Received 27April 2014Received in revised form 22May 2014Accepted 25May 2014Available online 19June 2014Keywords:Graphene Co-catalystHydrogen evolution Visible light Charge separationa b s t r a c tCu 2O loaded reduced graphene oxide (Cu 2O/RGO)was prepared via a one-step in-situ reduction position and structure of the Cu 2O/RGO were characterized by X-ray diffraction,high resolution transmission electron microscope and X-ray photoelectron spectroscopy.With eosin Y (EY)and rose bengal (RB)as co-sensitizers,the activity of hydrogen evolution over the Cu 2O/RGO dramatically increased and achieved a maximum when the loading amount of Cu on the RGO was about 3wt.%.It exceeded that of RGO and Cu 2O by a factor of 7.3and 4.2at the same conditions,respectively.It could be even comparable to that of the Pt/RGO under the same reaction conditions.This work showed a possibility of utilizing Cu 2O as an alternative for noble metals (such as Pt)due to its low cost and high performance in photocatalytic hydrogen production.Copyright ©2014,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rightsreserved.IntroductionAs a new,clean fuel source,hydrogen energy has attracted great attention recently.Photocatalytic hydrogen production from water splitting is an attractive and challenging issue in the conversion of solar energy into chemical energy [1,2].Herein,it is vital to design and prepare a cheap and stable photocatalyst for photocatalytic hydrogen evolution.Graphene,which offers an excellent electron transport property and possesses an extremely high specific surface area,is desirable for use as a two-dimensional catalystsupport to provide adsorption and reaction sites and to suppress the recombination of photogenerated electron-hole pairs [3,4].The potential application of graphene-based photocatalysts to boost the efficiency of solar en-ergy conversion has been explored [5,6].Dye-sensitization is an effective route to enhance the visible response of the RGO-based catalysts [7,8].Unfortunately,even though the electrons can transfer from excited dye to RGO sheets,they may recombine with surface oxidized dye species if the trapped electrons are not quickly transferred.Therefore,it is important to accelerate the electron transfer on the*Corresponding authors .Tel.:þ862160873061;fax:þ862164253317.E-mail addresses:xqli@ (X.Li),mujin@ (J.Mu).Available online at ScienceDirectjournal homepage:/locate/hei n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 39(2014)11578e 11582/10.1016/j.ijhydene.2014.05.1560360-3199/Copyright ©2014,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights reserved.surface of RGO in order to obtain higher photocatalytic efficiency.The graphene decorated with metal nanoparticles have been shown to exhibit enhanced electrocatalytic and photocatalytic performances[9].The graphene matrix is found to not only in-crease the active surface area and stability of the catalysts for the electrocatalytic reaction but also modulate the electronic struc-ture of support metal nanoparticles[10,11].For example,noble metal Pt nanoparticles immobilized on graphene sheets have displayed a high activity and stability in methanol and hydrogen fuel cells[10].In addition,as the co-catalysts,noble metals can not only promote the separation of photoexcited electrons and holes and serve as the active sites,but also improve the stability of the photocatalyst by timely consuming of the holes,which play an essential role in the production of H2and O2[12].How-ever,noble metals are rare and expensive,which restrain their practical application.Therefore,replacing noble metals may be a promising method in clean and renewable energy issues.It is found that cheap transition metals or their oxides such as Cu species are rarely used as the cocatalysts in photocatalytic hydrogen production even though some Cu species have demonstrated high activity in heterogeneous catalysis[13,14].Herein,by means of a facile process,Cu2O loaded RGO nanocomposites are prepared.With EY and RB(ER)as co-sensitizers,activity of the Cu2O/RGO for photocatalytic hydrogen production from water is investigated under visible light.ExperimentalWith graphite oxide and CuCl2as the reactants,NaBH4as the reductant,various Cu2O/RGO composites were prepared via one-step in-situ chemical reduction method.In addition,2% Pt/RGO and3%Pt/RGO composites were also prepared under same conditions.The experimental details were described in the Supporting Information(SI).Results and discussionFig.1A shows the XRD patterns of the RGO and the composite. Compared to that of the RGO(Fig.1A(a)),some new peaks (Fig.1A(b))at2q¼36.3 ,42.3 ,61.3 and73.4 are observed and indexed to(111),(200),(220)and(311)crystalline planes of the cubic phase Cu2O(JCPDS No.05-0667),respectively[15].The sharp and strong peaks suggest high crystallization of the Cu2O in the composite.The peaks assigned to C,O and Cu elements are observed in the EDX spectrum of the composite(Fig.S1).The C peak is resulted from RGO,Cu peak is from Cu2O species,and O peak comes from Cu2O and RGO.The results of XRD and EDX confirm the existence of Cu2O on the RGO.Further,the intensity ratio of D band(1346.6cmÀ1)to G band(1587cmÀ1)(Fig.S2) enhances,which indicates the presence of unrepaired defects after reduction of graphene oxide.It is consistent with the literature reported[16,17].The TEM image shows some nano-particles evenly loaded onto the RGO sheets(Fig.1B).The size of the nanoparticles ranges from4to24nm,and is mostly10nm, as illustrated in Fig.1D.The absence of isolation and apparent aggregation of Cu species in the image reveals a strong inter-action between RGO and Cu2O.In addition,the obvious lattice fringe assigned to(111)and(200)crystalline planes of cubic Cu2O can be observed in the HRTEM image of the Cu2O/RGO (Fig.1C).It is in good agreement with the result of XRD(Fig.1A).To further identify the surface chemical composition and the valence state of copper species in the composite,theFig.1e(A)XRD patterns of the RGO(a)and Cu2O/RGO composite(b);TEM image(B),HRTEM image(C)and size distribution histograms(D)for the Cu2O/RGO.The loading amount of Cu in the RGO was3wt.%.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y39(2014)11578e1158211579survey X-ray photoelectron spectrum (XPS)and the high-resolution XPS spectrum of Cu 2p are displayed in Fig.2.The survey spectrum (Fig.2A)shows the peaks of Cu,O and C,which is consistent with the result of EDX.Furthermore,the Cu 2p3/2bank (Fig.2B)can be divided into two peaks with binding energy at 932.7eV and 934.5eV,respectively,which indicates that there exists two kinds of Cu on the surface of the composite [13,18,19].The binding energy at 932.7eV can be assigned to Cu þ,and the binding energy at 934.5eV and the weak shake-up satellite at 942eV e 947eV are attributed to Cu 2þ[13,18].As aforementioned,XRD analysis and TEM image reveal the existence of Cu 2O in the composite,and no char-acteristic diffraction peaks for CuO are found in the XRD pattern of the composite.However,the peaks of Cu 2þare detected on the surface of the composite according to XPS analysis.It is deduced that tiny amounts of Cu 2O on the sur-face could be oxidized to CuO when the surface was exposed in air with humidity [19].It is reasonable that these tiny CuO on the surface could be detected by XPS,but not by XRD because XPS is a surface technique while XRD is a bulk char-acterization technique.Recent studies on dye-sensitized photocatalysis indicate that the combination of two dyes results in an extended light response,ensuring the efficient utilization of incident light energy [20].Here,with ER as the cosensitizers,the hydrogen evolution activity of the composite photocatalysts is evalu-ated using triethanolamine (TEOA)as the sacrificial agent.It is observed that the amount of hydrogen evolved is approxi-mately linear with time (Fig.S3).Fig.3A shows the amount of hydrogen evolution over various Cu 2O/RGO photocatalysts,together with those over pure RGO,Cu 2O and Pt/RGO for a comparison.It is obvious that the introduction of Cu 2O has a great effect on hydrogen evolution activity of the RGO (Tiny amounts of CuO on the surface of the Cu 2O/RGO can be reduced by H 2evolved and should have little effect on the activity of Cu 2O/RGO photocatalysts).With increasing the loading amount of Cu on the RGO,the rate of hydrogen evo-lution over the Cu 2O/RGO is dramatically increased.When the2004006008001000AC u 2pO 1sC 1sC u 3pI n t e n s i t y (a .u .)Binding energy (eV)I n t e n s i t y (a .u .)Binding energy (eV)Fig.2e (A)XPS spectrum of the Cu 2O/RGO composite and (B)high resolution XPS spectrum of the Cu 2p.The loading amount of Cu in the RGO was 3wt.%.| Ζ″| (Ω)Ζ′Ω100200300400AC u 2OH 2)l o m μ(n o i t u l o v e 2%P t 3%P t 5%C u 4%C u 3%C u 2%C u 1%C u R G O Fig.3e (A)The amount of hydrogen evolved over ER sensitized various photocatalysts under visible light irradiation for 6h (l >420nm).Reaction conditions:the concentration of ER is 3.0£10¡4mol L ¡1(molar ratio of EY to RB was 1:1);the amount of photocatalyst was 10mg;80mL of 15%TEOA aqueous solution (pH 9,adjusted by dilute hydrochloric acid).(B)Nyquist plots of electrochemical impedance spectra (EIS)for RGO and Cu 2O/RGO electrodes (The loading amount of Cu in the RGO was 3wt.%).The EIS measurements were performed in 0.1mol L ¡1Na 2SO 4aqueous solution.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 39(2014)11578e 1158211580loading amount of Cu is3%,the H2evolution rate achieves a maximum(4.53mmol hÀ1gÀ1)and exceeds that of RGO (0.618mmol hÀ1gÀ1)and Cu2O(1.09mmol hÀ1gÀ1)by a factor of7.3and4.2at the same conditions,respectively.It is indi-cated that there exists synergy effect between Cu2O and RGO. The appearance of a maximum in activity with an optimum loading of co-catalyst has also been observed for those loaded other co-catalysts[21].It could be related with the amount of active sites,which can not only trap photogenerated electrons to promote the charge separation,but also provide hydrogen evolution sites[12,14].The decreased activity(loading beyond 3%Cu)may be due to the aggregation of Cu species on the RGO sheets(Fig.S4),which reduces the amount of active sites and promotes recombination between the photogenerated elec-trons and the oxidized dye on the surface of the Cu2O/RGO [8,14,22].Because of its high work function and low Fermi energy,Pt is usually used as an efficient co-catalyst and the active sites for hydrogen generation[23].For comparison,2wt.%and 3wt.%of Pt were,respectively,loaded on RGO with similar method(Figs.S5and S6).As shown in Fig.3A,the amount ofhydrogen produced by the3%Pt/RGO(or2%Pt/RGO)can almost match with that of the Cu2O/RGO under same condi-tions.The results indicate that the Cu2O can collect effectively the electrons coming from RGO and reduce the over potential in hydrogen production from water.It is evidenced by the result of Nyquist plots.As shown in Fig.3B,the semicircle becomes smaller after the introduction of Cu2O,which in-dicates a lower resistance for the charge transfer in the interface of the Cu2O and the RGO.It is profitable to prevent electrons and holes from recombining and improve the ac-tivity of hydrogen evolution[4].So Cu2O is a highly efficient co-catalyst for the RGO in dye-sensitized photocatalytic hydrogen evolution system.In addition,the strong interaction between ER and RGO is also important for RGO to accept and transfer the photogenerated electrons produced by ER (Fig.S7).With ER as the cosensitizers,the stability of the Cu2O/RGO photocatalyst is investigated.As shown in Fig.4,the rate of hydrogen generation is the maximum(4.53mmol hÀ1gÀ1)in thefirst run,and then declines slightly in the consecutive runs.The rate of hydrogen generation can still reach 2.43mmol hÀ1gÀ1after consecutive5runs,which is still higher than those of single RGO(0.618mmol hÀ1gÀ1)or Cu2O (1.09mmol hÀ1gÀ1).Due to the EY molecules having less heavy halogen substituents than RB,it is reasonable that the EY possess lower stability than that of RB and is responsible for slightly decreased stability observed in the cosensitized system[24].Moreover,it is really observed that rate of hydrogen generation declines evidently in the consecutive runs with EY as the sensitizer(Fig.S8).In addition,using monochromatic light at l¼520nm,the time courses of hydrogen evolution over ER cosensitized Cu2O/RGO photo-catalyst is measured(Fig.S9).According to the Eq.(1)(SI,1.4), the quantum yield of hydrogenðF H2Þcalculated is about4.4%. Consequently,an efficient and stable photocatalytic hydrogen evolution system is constructed on the ER cosensitized Cu2O/ RGO photocatalytic system.The work on influence factors of photocatalytic activity for the Cu2O/RGO is being undertaken in our lab and it will be reported elsewhere.ConclusionsIn summary,the noble metal-free Cu2O/RGO nanocomposite was prepared via one-step in-situ chemical reduction strat-egy.The activity of RGO was increased up to7.3times when loaded with only3wt.%Cu,and it was even comparable to that of Pt/RGO under same conditions.A small amount of Cu2O on RGO sheets can efficiently trap the accumulated electrons to promote the separation of electrons and holes pairs.Thus the photocatalytic activity of hydrogen production enhanced.The study clearly demonstrated that choosing cheap transition metal oxide to replace noble metal as the co-catalyst was a promising method for photocatalytic hydrogen production.It is anticipated to open a new possibility in the investigation of RGO-based composites and promote their practical application in clean and renewable energy issues.AcknowledgmentsThis work wasfinancially supported by the National Natural Science Foundation of China(No.21301118,No.21305092,No. 21371070and No.21071060),and the Top Disciplines Con-struction Foundation of Shanghai City(No.405ZK120017001 and No.405ZK120021001).Appendix A.Supplementary dataSupplementary data related to this article can be found at /10.1016/j.ijhydene.2014.05.156r e f e r e n c e s[1]Maeda K,Takata T,Hara M,Saito N,Inoue Y,Kobayashi H,et al.GaN:ZnO solid solution as a photocatalyst for visible-H2evolution(μmol)Time (h)Fig.4e Cycling measurements of hydrogen evolution through photocatalytic water reduction using the Cu2O/ RGO nanocomposite as the photocatalyst under visible light irradiation(l>420nm).Reaction conditions were same as those in Fig.3.The loading amount of Cu in the RGO was3wt.%.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y39(2014)11578e1158211581light-driven overall water splitting.J Am Chem Soc2005;127:8286e7.[2]Mor GK,Prakasam HE,Varghese OK,Shankar K,Grimes CA.Vertically oriented Ti-Fe-O nanotube allayfilms:toward auseful material architecture for solar spectrum waterphotoelectrolysis.Nano Lett2007;7:2356e64.[3]Xiang QJ,Yu JG,Jaroniec M.Graphene-based semiconductorphotocatalysts.Chem Soc Rev2012;41:782e96.[4]Zhang J,Yu JG,Jaroniec M,Gong JR.Noble metal-free reducedgraphene oxide-Zn x Cd1Àx S nanocomposite with enhancedsolar photocatalytic H2-production performance.Nano Lett 2012;12:4584e9.[5]Zhang N,Zhang YH,Xu YJ.Recent progress on graphene-based photocatalysts:current status and future perspectives.Nanoscale2012;4:5792e813.[6]Kamat PV.Graphene-based nanoassemblies for energyconversion.J Phys Chem Lett2011;2:242e51.[7]Min SX,Lu GX.Dye-sensitized reduced graphene oxidephotocatalysts for highly efficient visible-light-driven water reduction.J Phys Chem C2011;115:13938e45.[8]Min SX,Lu GX.Dye-cosensitized graphene/Pt photocatalystfor high efficient visible light hydrogen evolution.Int JHydrogen Energy2012;37:10564e74.[9]Choi SM,Seo MH,Kim HJ,Kim WB.Synthesis of surface-functionalized graphene nanosheets with high Pt-loadings and their applications to methanol electrooxidation.Carbon 2011;49:904e9.[10]Yoo EJ,Okata T,Akita T,Kohyama M,Nakamura J,Honma I.Enhanced electrocatalytic activity of Pt subnanoclusters on graphene nanosheet surface.Nano Lett2009;9:2255e9. 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