Hydrothermal synthesis and photocatalytic properties of layered La2Ti2O7 nanosheets

I NSTITUTE OF P HYSICS P UBLISHING N ANOTECHNOLOGY Nanotechnology17(2006)4863–4867doi:10.1088/0957-4484/17/19/014Hydrothermal synthesis and photocatalytic properties of layeredLa2Ti2O7nanosheetsKunWei Li,Yan Wang,Hao Wang1,Mankang Zhu and Hui YanThe College of Materials Science and Engineering,Beijing University of Technology,Beijing100022,People’s Republic of ChinaE-mail:haowang@Received17July2006,infinal form1August2006Published11September2006Online at /Nano/17/4863AbstractLayered La2Ti2O7nanosheets were prepared through a one-step hydrothermalmethod at low temperature.The concentration of NaOH mineralizer plays animportant role in the synthesis.The scanning electron microscopy(SEM)andtransmission electron microscopy(TEM)images show that the thickness ofevery nanosheet is about5–10nm,while the planar dimension is more than1μm.The photo-catalytic activities of the nanosheets were characterized by thedecolourization of methyl orange solution and the evolution rate of H2.Theresults demonstrated that the La2Ti2O7nanosheets possess significantlyimproved photocatalytic properties in water purification and evolution rate ofH2from water–ethanol solution compared with those of samples prepared byconventional solid-state reaction.1.IntroductionLanthanide titanate(La2Ti2O7),as one of the layered compounds,has attracted widespread attention[1,2]in the photocatalytic domain due to its unique layered structure and chemical activity[3].Hwang et al have reported that La2Ti2O7 loaded with Ni–NiO has good photocatalytic activity in the water-splitting reaction[4]and the destruction of volatile organic compounds(CH3Cl)[5].However,they obtained La2Ti2O7through the conventional solid-state reaction(SSR) or the polymerizable complex method[6].Both of these methods need high temperatures and produce materials with low specific surface areas,nonuniform particle sizes and low phase purity,which lead to a relatively low photocatalytic activity.Zhang et al[7]obtained nanopowders of La2Ti2O7 by a metallorganic decomposition method.However,no photocatalytic properties were reported in their study.The desired catalyst should have high specific surface area,uniform particle sizes and pure phase.Therefore,if we control the experimental parameters to synthesize the layered compounds with a few layers,that is to say,nanosheets,the products will better answer our desire for a catalyst.The use of solution-based soft chemical methods to prepare nanocrystalline materials is expected to result in 1Author to whom any correspondence should be addressed.chemically homogeneous and phase-pure specimens,a narrow particle size distribution,and low crystallization temperatures of the materials.Among many methods,hydrothermal synthesis is particularly promising for low-cost production of advanced catalysts on a large scale.Because crystalline powders are directly produced in a hydrothermal process, the needs for high-temperature calcination and milling procedures are eliminated.Chen and Xu[8]have prepared La2Ti2O7powders through the hydrothermal treatment of La2O3·2TiO2·n H2O coprecipitation,but this method needs two steps to get thefinal product,and they did not get the La2Ti2O7 nanosheets.Here,we report the simple hydrothermal synthesis of La2Ti2O7two-dimensional(2D)nanosheets.The products were used as photocatalysts for the decolourization of methyl orange solution and the evolution of H2from water–ethanol solution.2.Experimental details2.1.SynthesisAll reagents were of analytical grade and purchased from Beijing Chemical Reagent Ltd without further purification. The exact quantity of La in the lanthanum nitrate hydrate,0957-4484/06/194863+05$30.00©2006IOP Publishing Ltd Printed in the UK4863K Li et alLa(NO3)3·6H2O,was determined by thermo-gravimetric analysis.The equivalent molar quantities of La(NO3)3·6H2O and Ti(SO4)2were dissolved in deionized water to form the clear aqueous solution.Then some amount of NaOH solution was dropped into the above solution to form white precipitation mixtures with different nominal NaOH concentrations.The mixture was stirred at ambient temperature for10min and then sealed in a50ml Teflon lined stainless-steel autoclave and allowed to heat at temperatures ranging from140to220◦C for different reaction times.After reaction,the resulting powders were repeatedly washed by centrifugation and decantation with deionized water to remove undesirable anions such as Na+,SO2−4and NO−3,and then dried at80◦C for6h to get theproducts.2.2.CharacterizationThe structure of the products was examined by an x-ray diffractometer(XRD,Bruker Advance D8)using Cu Kαradiation(λ=1.5406˚A).Transmission electron microscopy(TEM)and selected area electron diffraction (SAED)were taken on a JEOL-JEM2010F transmission electron microscope,using an accelerating voltage of200kV. The specific surface areas of the powders were determined using a Micromeritics ASAP2020specific surface area and porosity analyser in the method of Brunauer–Emmett–Teller (BET)nitrogen adsorption and desorption.Optical absorption studies were carried out using an ultraviolet–visible–near-infrared(UV–vis–NIR)spectrophotometer(Shimadzu UV-3101PC).The decolourization of methyl orange solution was carried out in our home-made instruments.Atfirst,0.2g of product was dispersed intofive beakers which werefilled with100ml of10mg l−1methyl orange solution separately,and irradiated under ultraviolet light by using a light resource(400W high-pressure Hg lamp)for different times,and then characterized by UV–vis spectroscopy.The evolution of H2from water–ethanol solution was carried out at room temperature and atmospheric pressure in a closed gas circulation system containing air using a high-pressure Hg lamp(400W)placed in an inner irradiation-type quartz reaction cell.The catalyst(1g)was suspended in1:5 ethanol/water solution(500ml).The amounts of H2and O2were determined by a Shimadzu14C gas chromatograph (molecular sieve5˚A column and Ar carrier)equipped with TCD.3.Results and discussionIn order to obtain suitable NaOH concentrations,samples were synthesized under different NaOH concentrations(a:0M;b: 1M;c:3M)at200◦C for24h.As can be seen from the XRD patterns infigure1,three kinds of materials were formed at different NaOH concentrations.Figure1(a)was ascribed to TiO2with a tetragonal system(JCPDS78-2486)while figure1(c)was attributed to lanthanium hydroxide La(OH)3 with the hexagonal system(JCPDS83-2034).Figure1(b) shows pure monoclinic phase La2Ti2O7with a perovskite structure conforming to the P21space group(JCPDS81-1066).It could be seen that the concentration of NaOHplayed Figure1.XRD of La2Ti2O7samples synthesized at200◦C for24h in different concentration of NaOH(a:0,b:1,c:3mol l−1).a critical role in the formation of phase-pure La2Ti2O7by the hydrothermal method.According to the above results and the related experi-ments,we conclude that the following reactions might occur in this process:Ti4++3H2O−→←−H2TiO3+4H+(1)H2TiO3heat−→←−TiO2↓+H2O.(2) La3++3OH−−→La(OH)3↓(3) 2La(OH)3+2H2TiO3−→La2Ti2O7↓+5H2O(4) H2TiO3+2OH−+H2O−→Ti(OH)2−6.(5) It is well known that the mineralizers such as KOH or NaOH play a fundamental part in the hydrothermal process[9,10]. When there was no NaOH in the solution,the La(NO3)3and Ti(SO4)2were ionized.Due to the stronger hydrolysis effect of Ti4+ions,equation(1)occurred and led to the formation of colloidal deposition H2TiO3.When these reactants were sealed in an autoclave and heated,TiO2deposits were formed from the decomposition equation(2).However,La3+ions were still kept in the solution and were washed away after reaction.When the concentration of NaOH was1M,the H+ions formed from the strong hydrolysis effect of Ti4+ ions were neutralized by NaOH.The neutralization of H+ led to the break of the equilibrium of equation(1)and the formation of colloidal H2TiO3.At the same time,La(OH)3 precipitates were formed through equation(3).Then,during the hydrothermal process,La2Ti2O7precipitates were formed from the neutralization equation(4)between La(OH)3and H2TiO3,because the solubility of La2Ti2O7was lower than that of La(OH)3under this condition.However,when the concentration of NaOH was3M,the La(OH)3(formed by equation(3))was very stable and could not be dissolved,as was shown infigure1(c).In the meantime,Ti species were washed away after hydrothermal reaction according to equation(5).Figure2shows the XRD patterns for the samples synthesized using1M NaOH for24h under different temperatures.Fromfigure2(a),we can see that the products4864Hydrothermal synthesis and photocatalytic properties of layered La 2Ti 2O 7nanosheetsFigure 2.XRD for the samples synthesized using 1M NaOH under different temperatures (a:140◦C;b:160◦C;c:180◦C;d:200◦C;e:220◦C)by the hydrothermal method for 24h.were amorphous when the synthesis temperature was 140◦C.With an increase in temperature,the main peaks (004,211and 212)of La 2Ti 2O 7gradually became obvious.As can be seen from figures 2(d)and (e),when the temperature was above 200◦C,phase-pure La 2Ti 2O 7crystals were obtained.The following measurements were based on the sample synthesized at 200◦C using 1M NaOH as the mineralizer.In figure 3,the SEM and TEM image show the nanosheet morphology of the La 2Ti 2O 7synthesized by the hydrothermal method.The SAED pattern was obtained by aligning the electron beam perpendicular to the face of this plate.The regular spot pattern indicates single-crystalline structure.The clear lattice fringes shown in the high-resolution (HRTEM)image of the selected area marked by the square in figure 3(b)indicates that the nanosheet presented perfect crystal structure without observed defects.The spacing of 0.5091nm corresponds to the (110)planes of La 2Ti 2O 7.Interestingly,although the planar dimension is more than 1μm,the plate is transparent under the electron beam,indicating that the plate is very thin.It can be estimated from the SEM image that the thickness is about 5–10nm.The formation of a thin nanosheet may be decided by the structure of La 2Ti 2O 7and the solution environment for crystal growth.In the structure of La 2Ti 2O 7with a monoclinic cell [11],the spacing of layers results in a feeble binding power and it is favourable to grow in a layered structure.In the hydrothermal condition,the ions can freely assemble according to the relative specific surface energies associated with the facets of this crystal [12].However,in the solid-state reaction,the obvious diffusion of ions occurred only at high temperature.At the same time,the rapid growth of particle size at high temperature made it difficult to form nanoparticles.To the best of our knowledge,there have been no reports on the La 2Ti 2O 7nanosheets to date.The specific surface area of the La 2Ti 2O 7sample synthesized by the hydrothermal method was approximately 69.8m 2g −1.However,that of the La 2Ti 2O 7sample synthesized by the solid-state method at 1050◦C was approximately 1.2m 2g −1,which was consistent with that of [13].This indicates that the hydrothermal method significantly enlarges the specific surface area of thesample.Figure 3.SEM and TEM images of the La 2Ti 2O 7prepared at 200◦C by using the hydrothermal method:(a)SEM;(b)TEM (SAEDinserted);and (c)HRTEM images of the selected area marked by the square in figure 2(b).The higher specific surface area would affect the photocatalytic efficiency.In figure 4,the steep shape of the spectra indicated that the visible light absorption was not due to the transition from the impurity level but was due to the band-gap transition [14].Compared to the SSR sample,the absorption of La 2Ti 2O 7nanosheets appeared to red-shift obviously.The band gap of the nanosheets was estimated to be 2.92eV from the onset of the absorption edge,which was smaller than the SSR sample (3.31eV).This can be attributed to the nanosize effect.As is well known,the red-shift phenomenon mainly resulted from the surface effect which occurs widely in the absorption of nanoparticle size.In figure 5,curve (a)shows the decolourization curve of absolute methyl orange solution without any La 2Ti 2O 7sample irradiated under ultraviolet light for 50min,while other curves denote methyl orange solution with a 2g l −1La 2Ti 2O 7sample irradiated for different times (b:10min,c:20min,d:30min,e:40min,f:50min).Seen from the figure,the solutions with the La 2Ti 2O 7sample decoloured gradually4865K Li etalFigure4.The UV–vis spectra of La2Ti2O7sample synthesized(a:by the hydrothermal method at200◦C;b:by the solid-state method at 1050◦C).Figure5.Decolourization curves of methyl orange solutions irritated under ultraviolet light(a)without La2Ti2O7for50min,(b)withLa2Ti2O7for10min,(c)with La2Ti2O7for20min,(d)withLa2Ti2O7for30min,(e)with La2Ti2O7for40min,and(f)withLa2Ti2O7for50min.and were almost decoloured totally when the irradiation time was up to50min.Obviously,the La2Ti2O7sample played the role of a photocatalyst.We have compared the XRD patterns of the La2Ti2O7sample before and after photocatalytic measurements.It confirmed that the photocatalytic reaction did not change the crystal structure of the catalyst.So the catalysts can be recycled and used again.As can be seen fromfigure6,the solutions decoloured gradually with prolonged irradiation.After50min,the decolourization degree of methyl orange solution with La2Ti2O7sample synthesized by the hydrothermal method reached98%,while it is still lower than that with TiO2,whose rate is up to nearly100%.In spite of this,the La2Ti2O7 sample synthesized by the hydrothermal method mayfind potential application due to its chemical and structural stability in solution[15].Comparatively,the photocatalytic abilities of the La2Ti2O7sample synthesized by the solid-state methodare Figure6.The degree of decolourization of methyl orange solution with the role of La2Ti2O7samples and Degussa P25(a:hydrothermal;b:SSR;c:P25).Figure7.The rates of H2evolution with the role of La2Ti2O7 samples synthesized by(a)hydrothermal method;(b)solid state method;and(c)P25under ultraviolet light for different times. weaker;its decolourization degree is just20%after50min. This may result from the reduction in the sample specific surface area and the decrease in the active sites on the surface of the samples.Fromfigure7,the rates of H2evolution from water–ethanol solution with P25and hydrothermal La2Ti2O7were about750μmol g−1h−1at160min.However,the rate with the sample synthesized by SSR was only about20μmol g−1h−1. All the samples were not loaded with nickel or other metals. The rate of H2evolution on hydrothermal La2Ti2O7was about 37.5times larger than that of the SSR sample.As was shown by BET measurements,the specific surface areas of the hydrothermal samples is about60times larger than that of the SSR sample.It was noted that the photocatalytic rate is not proportional to the surface area.The reasons for this phenomenon could be explained by the following: the specific surface areas of the powders in this study were determined by nitrogen adsorption and desorption.In this vapour atmosphere during the measurement,the surface of the samples could be adequately exposed and detected.However,4866Hydrothermal synthesis and photocatalytic properties of layered La2Ti2O7nanosheetsin the case of photocatalytic measurement,the samples were dispersed in the solution with magnetic stirring;even so,only the surface that the light irradiated could produce electrons and holes.Due to the agglomeration and stacking of some powders,one could not ensure that the light could irradiate the entire surface of the sample.As a result,parts of surface were‘lost’during the photocatalytic measurements,and the quantity of photocatalytic results is not proportional to the results of surface areas.Similar results have also been reported in the literature[16–18].However,after all,it could be seen that the photocatalytic properties of hydrothermal-synthesized La2Ti2O7samples have been significantly improved compared with the SSR samples.The reason for the high H2evolution with hydrothermal La2Ti2O7may lie in the following two aspects.Firstly,the nanosheet thickness is about5nm,which is equivalent to that of a few layers of TiO6octahedron.Photo-produced electrons and holes will rapidly diffuse to the surface of a nanosheet,which decreases the combination of electrons and holes.Secondly,the larger specific surface area will supply more chance for contact between photo-produced carriers and water or organics.4.ConclusionsIn summary,layered La2Ti2O7nanosheets have been prepared through the one-step hydrothermal method at200◦C for 24h when the NaOH concentration equals1M.The XRD and the SAED patterns confirm that the products are well-crystallized single crystals.The TEM images show samples with a nanosheet shape whose width is about150nm and length is near to300nm.The specific surface area of the La2Ti2O7sample synthesized by the hydrothermal method was approximately69.8m2g−1,which was about60times that of the La2Ti2O7sample synthesized by the solid-state method.Photocatalytic measurements through the photo-decolourization of methyl orange show that the La2Ti2O7 nanosheets possess superior photocatalytic properties in water purification and mayfind potential application in relatedfields. The rate of H2evolution on the hydrothermal La2Ti2O7 without loading of nickel was about750μmol g−1h−1from water–ethanol solution,which was remarkably improved compared with that of the SSR sample.The high H2evolution rate may result from the unique nanosheet structure and higher specific surface area.A change of material synthesis method may be an effective route to improve the material properties. AcknowledgmentThe authors are grateful to the Project of New Star of Science and Technology of Beijing forfinancial support. 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