Effect of Mo on pha transition temperature of VO2nanopowders
Y.Chen*,W.D.Liang and H.Xu
A facile method to prepare Mo doped VO2nanopowders was introduced.Using V2O5as a raw material and molybdenum acid as a dopant,the precursor was obtained by redox.Then,doped VO2nanopowders were produced through heating the precursor at500u C under an argon atmosphere.The as obtained samples were characterid with powder XRD,X-ray photoelectron spectroscopy,TEM,differential scanning calorimetry and the temperature dependence of infrared spectrum.The analys indicated that the products were Mo doped VO2,and its pha transition temperature decread by molybdenum acid doping,but the efficiency of the dopant was weakened with the increa in its content.After doping,the optical property of VO2did not change.
Keywords:VO2,Doping,Pha transition temperature,Optical property
Introduction
喜马拉雅山脉地图
Since Morinfirst reported a reversible miconductor–metallic transition,68u C in VO2,which associated with a structure pha transformation from mono-clinic to tetragonal,1VO2has been theoretically studie
d by many scientists.This pha transition is accompanied by dramatic changes in electrical,optical and magnetic properties.2,3Therefore,VO2has poten-tial applications and can be ud in different areas,for example,energy efficiency windows,optical switching devices,temperature nsing devices and optical data storage medium.4–10However,the pha transition at 68u C confines applications of VO2.For extending its applications,how it decreas the pha transition temperature of VO2is one of the focus in the VO2 rearch.
Usually,doping high valent cations,such as W6z, Mo6z,Nb5z and Ta5z,can lower the pha transition temperature.11–13Some physical methods to prepare doped VO2films,such as magnetron sputtering,vacuum evaporation,etc.have been reported.However,the equipment which the methods need is expensive,and the oxidation state of vanadium is not easy to control. At the same time,becau the volume of bulk VO2 changes1%after pha transition,which brings stress at break,the lifetime of thinfilms is shortened.14 However,the powdery VO2can obviously reduce the stress,and the degree of dramatic changes in electrical, optical and magnetic properties is improved.Therefore, the methods of preparing doped VO2nanopowders attract more and more attention.The conventional doping methods are sol–gel,hydrothermal method and solid state react,while the process of the methods is complex to a certain extent.
The authors reported W doped VO2nanopowders.15 In that paper,a facile method to prepare doped VO2 powders was introduced.Using V2O5as raw material and molybdenum acid as a dopant,the doped VO2 nanopowders were obtained.The advantages of this method are that the reaction can be achieved at lower temperatures;the reaction time is short,and the procedure is really convenient.Furthermore,its pha transition temperature is decread by molybdenum acid doping,and the optical property of VO2is still maintained after doping.
Experimental
With0?5g V2O5added into20mL heavy hydrochloric acid,the mixture was stirred and heated to80–90u C. After,2h,the suspension was changed from henna to transparent green.Then,a few drops of formaldehyde were added to the solution and reacted after,30min. The colour of the solution became blue,which indicated that V5z was reduced to V4z.Later on,1mol.-% molybdenum acid as a dopant was added to the solution.At the beginning,the solution was turbid; then,for a moment,it turned blue and transparent again.Thirty minutes later,after stopping the reflux,the solution was concentrated until dark blue precursor appeared.Then,it was put into the quartz boat and heated at500u C under highly pure argon atmosphere for 5h.In the end,dark blue powders were obtained. Powder XRD(D/MAX-2400)with Cu K a radiation was ud to identify the crystalline pha of the doped sam
ples.X-ray photoelectron spectroscopy(XPS)data were obtained by a V.GESC Lab.The morphology of the samples was obrved using a TEM(Hitachi H-800). The pha transition temperature of samples was tested by differential scanning calorimetry(PE sapphire DSC).
School of Petrochemical Technology,Lanzhou University of Technology, Lanzhou730050,China
*Corresponding author,email
ßW.S.Maney&Son Ltd.2010
The samples were also analyd by the temperature dependence of infrared.
Results and discussion
Figure 1a shows XRD pattern of VO 2nanopowders after doping.The diffraction peaks are in good agreement with monoclinic VO 2structure (JCPDS file no.43-1051)and have no peaks of impurities,which indicate that the dopant entered into the VO 2crystal lattice,and there was no crystal pha of molybdenum oxide.
To further understand whether Mo 6z entered into the crystal lattice of VO 2,the authors did a compa
rison experiment by taking some precursors of VO 2and adding it into 1mol.-%molybdenum acid according to molar percentage.After grinding for a few minutes,the mixture was heated at 500u C for 5h under a highly pure argon atmosphere;then,dark blue powders were obtained.Figure 1b shows XRD pattern of the reference sample.Except for the diffraction peaks of monoclinic VO 2(JCPDS file no.43-1051),there are four diffraction peaks which agree with MoO 3(JCPDS file no.21-0569).The result implies that VO 2and molybdenum acid only mix together through simply grinding and
succeeding
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1X-ray diffraction patterns of a doped sample and b reference
sample
2X-ray photoelectron spectra of a O1s and V2p in sample and b Mo3d in
sample
3Images (TEM)of sample heated at 500u C for 5h
heat treatment.At the same time,the comparison experiment indicates that the method chon makes Mo 6z enter into the VO 2crystal lattice,or the diffractions of MoO 3will appear at the XRD pattern of the sample.
莲藕山药汤Valence analysis was investigated by the XPS spectrum.Figure 2a shows enlarged XPS spectrum between 505and 532?5eV.The binding energy of O1s appears at 529?8eV.The peaks at 516?1and 523?8eV are attributed to V2p 3/2and V2p 1/2,which can be indexed to V 4z 2p 3/2and V 4z 2p 1/2according to the literatures.11,16It shows that the valence of vanadium ion in the samples is z 4,and the samples are certainly VO 2.
Figure 2b shows XPS spectrum of Mo3d in the samples.The binding energy of Mo3d 3/2and Mo 3d 5/2in the samples is located at 235?9and 232?8eV,which agree with the binding energy in MoO 3.17It indicates that the valence of molybdenum ion in the dopant does not change and is still z 6.
A TEM image of the samples can be en in Fig.3.It is obrved that the crystallites display sphere-like morphology with diameters about 30–50nm,and some grains agglomerated becau of little water on the samples’surface.Electron diffraction pattern suggests that the powder is polycrystalline.
The differential scanning calorimetry curve of the samples is depicted in Fig.4,which aimed to examine the influence of Mo 6z upon VO 2nanopowders.It can
be en from Fig.4a ,which contains 1at-%Mo 6z ,that there is an obvious endothermic peak reprenting the pha transition temperature of the sample at 58u C between room temperature and 100u C.Compared with the undoped VO 2,the pha transition temperature of doped VO 2was decread ,10u C,which agrees with the literature.11Figure 4b shows that the pha transition temperature of the sample,which contains 2at-%Mo 6z ,is at 54?8u C.Improving the content of Mo 6z ,the magnitude of the VO 2pha transition temperature change compared with the sample,which contains 2at-%Mo 6z ,is reduced.The results indicate that the pha transition temperature of Mo doped VO 2nanopowders prepared by the method is decread,but the magnitude of the VO 2pha transition temperature change is reduced with the content of Mo 6z improved.
The mechanism of dopant decreasing the pha transition of VO 2is still being rearched.It is generally thought that Mo 6z as solute penetrates into the crystal lattice of VO 2and substitutes the V 4z ion,so the donor-like defects are formed,and a doping energy emerges among the energy gap of VO 2.This results in the band gap of VO 2decreasing;then,electrons can easily transit between the band gap and conduction band.15,17This implies that doped VO 2can carry out a miconductor–m
etallic transition at low temperatures.
However,this does not mean that the more the dopants,the better.On the contrary,more donor defects forming in the VO 2crystal configuration will cau vere lattice distortion,which has a negative impact on the electronic transmission.In other words,although the number of electronic transition is theoretically incread by dopant,the resistance resulting from lattice distortion will impede the transition,so with the increa in its content in the samples,the efficiency of Mo 6z dopant is weakened.
With a structure from monoclinic to tetragonal at pha transition temperature in VO 2,its optical trans-mittance has an abrupt change in the infrared.After doping,the authors cho the sample containing 1at-%Mo 6z with pha transition temperature at 58u C to analy whether the feature is still remaining with the temperature dependence of infrared.The result can be en in Fig.5.Infrared spectra at room temperature are reprented by line a,and tho at 60u C are reprented by line b.It can be en that the optical transmittance of the sample has a remarkable change in the
infrared
5Infrared spectra of 1at-%Mo doped sample at a room
temperature and b 60u
C
4Differential scanning calorimetry curve of a 1at-%and b 2at-%Mo doped samples
during the pha transition.This result indicates that the optical property of VO2was not changed after doped Mo6z.
Conclusion
The Mo doped VO2nanopowders are successfully prepared by the facile method.The pha transitio
n temperature obviously decread,but the efficiency of the dopant is weakened with the increa in its content. After doping,the optical property of VO2still remained. References
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