Growth Process,Photoluminescence Property and Photocatalytic Activity of SnO2Nanorods

ZHAN Hong-QuanDENG CeLIU QuanLI Xiao-HongXIE Zhi-PengWANG Chang-An

(1School of Materials Science and Engineering,Jingdezhen Ceramic Institute,Jingdezhen,Jiangxi 333403,China)

(2School of Materials Science and Engineering,Tsinghua University,Beijing 100084,China)

Abstract:In this work,under the regulation of citric acid,SnO2nanorods were synthesized by hydrothermal method.The structural characteristics of SnO2samples during the growth process was investigated by using high-resolution transmission electron microscope,X-ray diffraction,infrared spectra Fourier transform spectra,Brunauer-Emmett-Teller nitrogen physisorption,UV-visible diffuse reflection spectrum,and photoluminescence spectra.The results reveal that the overall crystal growth can further be divided into two stages:the oriented attachment of SnO2nanocrystals at the early stage and subsequently a slow crystal growth along the[001]direction following the Ostwald ripening mode.Photoluminescence property and photocatalytic activity of SnO2nanoparticles at the different stages was also investigated,which show a similar changing trend with the crystal growing:greatly increased at the former stage and gradually decreased at the latter stage.

Keywords:SnO2;oriented attachment;Ostwald ripening;photocatalytic activity;photoluminescence

0 Introduction

Tin dioxide(SnO2)is a direct wide band gap semiconductor(Eg=3.6 eV)with excellent photoelectronic properties,gas sensitivity,and superior chemical stabil-ity[1-2],which has already been used in sensors[3-5],solar cells[6-7],lithium-ion batteries[8-9].It has aroused great interest to study SnO2nanostructures in recent years.Re-searchers have already realized that the physicochemical properties are closely associated with the sizes,surface states,and lattice defects[10-15].Among these factors,the defect is regarded as a crucial factor in the photocatalytic activity and photoluminescence(PL)property of SnO2nanocrystals.However,universal knowledge about how to control the formation and disappearance of defects is absent.

Over the past decades,considerable efforts have been made to improve the properties of SnO2nanomaterials.(1)Doping method:N-doped SnO2nanoparticles have been prepared by Wang et al.for lithium storage properties[16].(2)Morphological control:The octahedral SnO2particles were synthesized by Han et al.and show excellent gas-sensing performance due to the high chemical activity of the exposed(221)facets[17].Five atomic-layer-thick SnO2sheets were successfully synthesized for remarkably improved CO catalytic performances[18].Uniform SnO2nanorod arrays have been produced for the highly selective H2sensors[19].(3)Heterojunction hybridization:It was reported by Li et al.that 3D hierarchical electrode composed of porous SnO2nanosheets on flexible carbon cloth was used for the electroreduction of CO2to formate[20].(4)Growth mechanism:The aggregation-induced fast crystal growth mechanism was proposed to explain the steep growth mode of SnO2nanocrystals[21].A mixed growth mode was also discovered by Zhang et al.[22].In recent years,researchers have already realized that PL property and photocatalytic activity are closely associated with the defects and structure.In fact,both defects and structure are closely related to the growth process.Although some researchers have done the works about the growth trajectory of SnO2nanocrystals,there are few reports about the correlation between property,defects and growth evolution of SnO2nanorods.

In this work,through the effect of citric acid,the growth process of SnO2nanorods under the hydrothermal condition was investigated.It was discovered that the growth behavior of SnO2nanorods followed an oriented attachment(OA)mode at the beginning stage and then occurs an Ostwald ripening(OR)mode in the subsequent growth process.With the growth mode changing,the defects concentration also changed with the growth process and resulted in the different PL property and photocatalytic activity.

1 Experimental

1.1 Synthesis

SnO2nanorods were prepared by a hydrothermal method with stannic chloride(SnCl4·5H2O,analytical grade).The method was as follows:3.15 g SnCl4·5H2O were dissolved in 30 mL deionized water,and subsequently added 1 g citric acid to keep the pH value at～0.5.The suspension was uniformly stirred and finally transferred into a 60 mL Teflon-lined autoclave for consequence hydrothermal reaction.The hydrothermal reaction was conducted at 220℃in an oven for different durations,ranging from 2 h to 7 days.When the reaction was completed,the autoclave was removed from the oven and cooled down to room temperature.The samples were centrifugally separated and sufficiently washed by deionized water and ethanol to remove the residual impurities.Finally,the products were placed into a desiccator for drying at 60℃for 24 h.

1.2 Characterization

The structural characteristic of as-synthesized products was performed using X-ray diffraction(XRD)by CuKαradiation(λ=0.154 1 nm)on a Rigaku D/MAX 2200 VPC diffractometer,operating at 40 kV and 20 mA,with steps of 0.02°at 10(°)·min-1in a 2θrange from 15°to 75°.Transmission electron microscopic(TEM)and high resolution transmission electron microscopy(HRTEM)(TEM,JEM-2010HR electron microscope equipped with a Gatan GIF system)with an acceleration voltage of 200 kV were used to determine the morphological,structural,and chemical characteristics of synthesized products.The Brunauer-Emmett-Teller(BET)nitrogen physisorption experiments were carried out on a Micromeritics ASAP 2010 system.The pore size distributions of synthesized products were determined by using the Barrett-Joyner-Halenda(BJH)algorithm according to the desorption data of the N2isotherms.For photoluminescence measurements,using a Renishaw micro-Raman model via Reflex spectrograph with the excitation wavelength of 325 nm,the spectrum range was extended to 380～800 nm.The UV-Vis absorption was recorded using Lamda850 spectrophotometer in total reflection mode by the integration sphere in the region of 200～800 nm.Infrared spectra Fourier transform(FT-IR)spectroscopy was performed using a Nicolet 560 spectrophotometer.

1.3 Photocatalytic activity evaluation

The photocatalytic activity for the degradation of methyl orange(MO)was performed in a Pyrex reactor in which 200 mg of the photocatalysts was suspended in 200 mL of MO aqueous solution(10 mg·L-1).A 250 W mercury lamp(Beijing Perfect Co.Ltd.,the wave-length within the range of 365 nm)as an ultraviolet light source was positioned inside a cylindrical Pyrex vessel.Prior to irradiation,the suspension of the photocatalyst in MO aqueous solution was stirred in the dark for 30 min to secure an adsorption/desorption equilibrium.At the given irradiation time intervals of 5 min,4 mL of reaction suspension was sampled and separated by centrifugation.The absorption spectra of the centrifuged reaction solution were measured on a T6 spectrophotometer(Beijing Purkinje General Instrument Co.Ltd.).The concentration of MO was determined by monitoring the change in the absorbance at 464 nm.

2 Results and discussion

2.1 XRD analysis

The powder X-ray diffraction(XRD)pattern of the prepared products by hydrothermal method with different reaction time is shown in Fig.1.As can be seen,the peaks correspond to the characteristic diffraction of(110),(101)and(211)crystal planes respectively,which can be indexed as tetragonal rutile structure of SnO2(PDF No.41-1445).With the extension of time,the diffraction peaks of SnO2products became obvious-ly stronger.After the nucleation incubation time about 2 h,the weak diffraction peak appeared and SnO2nanocrystalline phase were gradually formed.With the hydrothermal reaction continuing,the intensity of characteristic diffraction peak was gradually enhanced and the width of the diffraction peak became narrower,which reveal that the nanoparticles size is enlarged and the crystallization degree is increased.

Fig.1 XRD patterns of SnO2samples synthesized at 220℃with different reaction times ranging from 2 to 168 h

2.2 FT-IR analysis

As shown in Fig.2,FT-IR spectra were used to analyze the surface absorbed citric acid and the structure of SnO2nanoparticles.The FT-IR spectra of the samples showed strong absorption occurring between 400 and 750 cm-1with two main maxima observed near 660 and 520 cm-1.These features are typical of SnO2nanoparticles and are related to asymmetric and symmetric Sn-O stretching vibrations[15].The two strong bands around 3 430 and 1 630 cm-1were attributed to the O-H stretching vibrations of H2O absorbed in the samples,indicating the presence of adsorbed H2O.With the reaction proceeding,the two bands were gradually decreased,suggesting that the adsorbed water reduced.The absorption band at 1 380 cm-1was assigned to the symmetric stretching motions of the carboxylate group and the weaker band around 1 200 cm-1corresponded to the coupled C-(OH)stretching and C-O-H bending motions[23].These spectra provide useful information aboutadsorbed citric acid onto SnO2nanoparticles.After 2 h,the bands at 1 380 and 1 200 cm-1have disappeared from the spectrum and were no longer present.Obviously,the adsorbed citric acid desorbed from the surface of SnO2nanoparticles.

Fig.2 FT-IR spectra of SnO2samples synthesized at 220℃with different reaction times ranging from 2 to 168 h

2.3 Morphology characterizations

In order to further reveal the growth mode and crystal structure of SnO2samples,TEM and HRTEM were characterized.Fig.3 shows the TEM and HRTEM images of the typical products synthesized from different reaction times,and the corresponding fast Fourier transform(FFT)patterns and the selected area electron diffraction(SAED)pattern are given in the insets.As the crystallizing time continuing,the morphology and growth of SnO2samples also changed.Fig.3a is the TEM image of the sample synthesized for 2 h,from which the nanoparticles about 2 nm were loosely aggregated together.These nanoparticles characteristics including the irregular morphology,rough surface,and weak crystallization are further illustrated by HRTEM and FFT in Fig.3e.Under prolonging the crystallization time to 6 h,the crystal morphology was changed from irregular sphere to an elongated shape described in Fig.3b.HRTEM image in Fig.3f shows the detailed microstructure of the sample reacting for 6 h.Clearly,the particle was composed of several sub-nanocrystals.The sub-nanocrystals have smooth(110)plane and attach together along[001]crystallographic orientation.The FFT pattern in the upper right corner of Fig.3f reveals that the aggregated-nanocrystal had singlecrystal feature,which agrees well with the OA growth[24-25].Due to OA between sub-nanocrystals,the OA-induced interface defect such as dislocation,misorientations,and interface defects can be obviously formed.When the reaction time reached to 12 h(Fig.3c),SnO2nanorods with length(～40 nm)and diameter(～8 nm)begin to appear.As shown in Fig.3g,the nanorods have enhanced crystallization and smooth surface.The continuous lattice fringes discovered that the OA-induced defects almost disappeared,which is ascribed to the self-integration between nanocrystals.Therefore,at this stage,the growth mode is different from OA and accords well with OR.As shown in Fig.3d,with the reaction going on,the perfect SnO2nanorods can be obtained,which have～100 nm length and～15 nm diameter.The SAED and FFT patterns in Fig.3d and 3h discovered that the nanorods have smooth(110)plane and grow along[001]direction.

Fig.3 TEM(a～d)and HRTEM(e～h)images of SnO2 samples synthesized at 220℃for different reaction duration:(a,e)2 h,(b,f)6 h,(c,g)12 h,(d,h)168 h

According to the analysis,SnO2crystal growth is based on OA in the early stage,While OR plays a dominant role during the latter period.The surface charge of nanoparticles plays a key role in the interactions between nanoparticles.Previous study indicated that the isoelectric point of SnO2nanocrystals was～3.1[26].In our results,the pH value of reaction solution was～0.5 and the OA of SnO2nanoparticles should be easy to do.According to the previous reports,SnO2spherical aggregated-nanopartilces were easily produced under OA[27].However,SnO2nanorods can be obtained in this result.So,it is sure that citric acid could play a critical role on the formation of SnO2nanorods.The surface energies of SnO2in different crystallographic orientations have been reported by several groups[28].Their calculations exhibited the same general trends,i.e.,in order of increasing energy,the planes form the sequence(110)＜(100)＜(101)＜(001).Since the(110)and(001)planes are suggested to have the lowest and the highest surface energies,respectively,the(110)crystal plane is easy to appear and the[001]direction is the favored growth direction.As soon as the SnO2nanocrystals formed,citric acid can rapidly absorb on its(110)plane.The absorption reduced the surface energy of(110)plane and accelerated the OA between SnO2nanocrystals along the[001]direction.Through the latter ripening,SnO2nanorods could be formed.

2.4 Schematic diagram

Based on the above analysis,the structural evolution of SnO2nanorod is illustrated in Fig.4.In the initial stage of hydrothermal reaction,citric acid can rapidly adsorb on the surface of SnO2crystal nucleus(as shown in Fig.4a).Under the low surface charge(due to the acid solution)and the action of citric acid,SnO2nanocrystals easily attached together along the[001]direction when the reaction time continues.As shown in Fig.4b,a lot of OA-induced crystal defects can be detected,which were good for PL and photocatalytic activity.As the reaction progresses,due to the selfintegration between SnO2nanocrystals,the OA-induced defects almost disappear and SnO2nanorods come into being,as shown in Fig.4c.Finally,after a long time of OR,the perfect SnO2nanorod along the[001]direction is shown in Fig.4d.

Fig.4 Schematic drawing to show the growth evolution of SnO2nanorods

2.5 BET surface area analysis

To evaluate the specific surface of the synthesized SnO2,the isothermal curve and BJH pore-size distribution are used,and the results are shown in Fig.5.As can be seen,the nitrogen adsorption/desorption isotherms reflect the typical Ⅳ-model adsorption isotherms,and the adsorption capacity improves with increasing the relative pressure.The sorption data also indicated the systematic decrease of BET surface area(SBET)from 174 to 23 m2·g-1with increasing the reaction time.The results show that the samples produced at 2 h have not only the largest surface area and small-er particle size but also a mesoporous structure centered at 2.0 nm.

Fig.5 Nitrogen adsorption/desorption isotherms and BJH pore size distribution plots of SnO2samples synthesized at 220℃for different reaction durations

2.6 UV-Vis absorption

As shown in Fig.6a,the UV-visible diffuse reflection spectra were used to understand the optical response of SnO2samples.The strong absorption peak around 280 nm represents the characteristic absorption of SnO2in UV-Vis spectrum.With the hydrothermal reaction proceeding,the absorption bands tend to be broadened and red-shifted,which is maybe the result of the increasing of particle size and the enhancing of crystallization.The curve of(αhν)2versus photon energy are described in Fig.6b and the corresponding band gap of SnO2became narrow with the reaction time,which can be calculated by Wood and Tauc method[29].The result means that the excitation energy required for the electron transition is gradually reduced,which is favorable for light absorption and photocatalysis.

Fig.6 (a)UV-visible diffuse reflection spectra of SnO2samples synthesized at 220℃for different reaction durations;(b)Band gap(Eg)of SnO2samples that estimated by the corresponding curve of(αhν)2versus photon energy

2.7 Photoluminescence spectra

Fig.7 is the PL spectra of SnO2samples under excitation at 325 nm.It is well-known that PL properties are attributed to the imperfections degree of the crystal structure[11,30-31].As can be seen,the PL behaviors can change with the crystal growing,which indicates that the different structure defects produced during the growth process.According to the PL results shown in Fig.7,the entire process can be divided into two stages.At the first stage from 2 to 6 h,PL intensity was greatly enhanced with the reaction time increasing.When the reaction time was 2 h,the PL curve of the sample was very flat and there were no any peaks appeared,which is due to the poor crystallization of SnO2samples.When the time reached 4 h,the distinct emission peak around 500 nm was observed,which was attributed to the high crystallinity and the structural defects such as dislocation,or dangling bonds produced during the crystallization process.With increasing time to 6 h,the strongest emission peak around 515 nm also appeared and had the obvious red-shift.During the period,the aggregation growth of more SnO2nanoparticles can result in a high concentration of OA-induced defects such as dislocation,misorientations,and interface defects.Therefore,the PL intensity is substantially improved by these OA-induced defects.The shoulder peaks at～580 nm was also revealed in the spectrum of the sample reacting for 6 h,which was derived from the oxygen vacancies defects formed in SnO2surface[11].At the second stage(after 6 h),with the crystallization going on,the nanoparticles grow perfectly and the reduced defects decreased the PL emission.In brief,during the SnO2growth,PL intensity increased rapidly at the initial stage and then decreased slowly at the later stage.

Fig.7 (a)Evolution characteristics of the PL spectra of SnO2nanocrystals synthesized at 220℃for different reaction durations;(b)Comparison of PL intensities of the typical SnO2samples

2.8 Photocatalytic analysis

As depicted in Fig.8,the photocatalytic properties of the SnO2nanocrystals are evaluated in terms of the photodegradation of MO in aqueous solution at ultravio-let light excitation.It can be seen that SnO2nanoparticles at different stages exhibited obvious different photocatalytic activities:In the first stage(2～6 h),the degradation ability to MO rapidly increased with prolonging the reaction time.While in the second stage(6～168 h),the photocatalytic activity of SnO2samples show a slow decreasing with increasing the time.Obviously,the photocatalytic results of SnO2samples show a clear corresponding relationship and similar changing trend with the PL analysis.

It is well noted that the photocatalytic properties depend on the surface area,defects and microstructure of SnO2nanocrystals[10,15].As described in Fig.5,the 2 h sample had the largest specific surface area which can provide more adsorption of reactant molecule.Especial-ly,the special mesoporous structure of the sample reacting for 2 h can accelerate the charge separation and migration of photogenerated carriers[29].Therefore,although having low crystallization,the the sample reacting for 2 h showed better photocatalytic activity than the the sample reacting for 168 h with high crystallization.With the crystallization going on,the surface area of SnO2nanoparticles decreased,while the OA-induced defects concentration increased rapidly,which can facilitate the separation of the e-/h+pairs and improve the photocatalytic activity.Finally,the sample reacting for 6 h with the highest concentration of OA-induced defects presents the best catalytic performance.After reacting for 6 h,due to the selfintegration and ripening of SnO2crystals,the surface area and the defect concentration gradually decreased,and the degradation rate of MO was seriously weakened.

Due to OA-induced defects,the photoinduced charge carriers can be effectively separated,and accordingly their recombination was slowed down.The efficient charge separation can increase the lifetime of the charge carriers and enhance the efficiency of the interfacial charge transfer to adsorbed substrates[32-34].So,it is reasonable that the SnO2nanocrystals with the highest concentration of OA-induced defects have higher photocatalytic efficiencies.The cycling experiments for the photocatalytic degradation of MO were further carried out and the results suggested the photocatalytic stability of the prepared SnO2samples.

Fig.8 (a)Photocatalytic activities of SnO2samples synthesized at 220℃;(b)Degradation rate of MO in the presence of SnO2nanocrystals synthesized for different reaction durations

3 Conclusions

SnO2nanorods were synthesized by the simple one-step hydrothermal method.The results reveal that the growth of SnO2nanorods follows“Oriented Attachment(OA)+Ostwald Ripening(OR)”.The adsorbed citric acid promoted the oriented attachment of SnO2nanocrystals at the early stage and subsequently a slow Ostwald ripening resulted in nanorod-like crystal along the[001]direction.The photocatalytic degradation of methyl orange indicates that SnO2nanocrystals at different stages exhibited obvious different photocatalytic activities.Especially,the aggregated-nanoparticles synthesized at 6 h showed the best catalytic performance and the strongest PL emission,which is ascribed to a lot of OA-induced defects.This study provides a new idea for the excellent performance photocatalyst.

Acknowledgments:This work was financially supported by National Natural Science Foundation of China(Grant No.21661017),Foundation of Jiangxi Provincial Department of Education(Grants No.GJJ190708,GJJ180718).