The effect of In doping concentration on the optical band gap of nano-SnO2 is investigated as a function of calcination temperature. Changes in the band gap explain the room temperature H-2 gas sensing of doped nano-SnO2. The band gap was found to be lower than those reported for SnO2 (3.6 eV) from 2.55 to 3.43 eV and may be explained by the presence of nonequilibrium oxygen vacancies in the oxide lattice and band bending effects at the nanoscale

Drake, C.

Seal, S.

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2007 Band gap energy modifications observed in trivalent In substituted nanocrystalline SnO2 C. Drake University of Central Florida S. Seal University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Drake, C. and Seal, S., "Band gap energy modifications observed in trivalent In substituted nanocrystalline SnO2" (2007). Faculty Bibliography 2000s. 7069. https://stars.library.ucf.edu/facultybib2000/7069 Appl. Phys. Lett. 90, 233117 (2007); https://doi.org/10.1063/1.2746407 90, 233117© 2007 American Institute of Physics.Band gap energy modifications observed intrivalent In substituted nanocrystalline Cite as: Appl. Phys. Lett. 90, 233117 (2007); https://doi.org/10.1063/1.2746407Submitted: 05 March 2007 . Accepted: 11 May 2007 . Published Online: 11 June 2007C. Drake, and S. SealARTICLES YOU MAY BE INTERESTED INStructural and optical properties of Cu doped SnO2 nanoparticles: An experimental anddensity functional studyJournal of Applied Physics 113, 233514 (2013); https://doi.org/10.1063/1.4811374Atomic nitrogen doping and p-type conduction in Applied Physics Letters 95, 222112 (2009); https://doi.org/10.1063/1.3258354Enhancing the low temperature hydrogen sensitivity of nanocrystalline  as a function oftrivalent dopantsJournal of Applied Physics 101, 104307 (2007); https://doi.org/10.1063/1.2732498Band gap energy modifications observed in trivalent In substitutednanocrystalline SnO2C. Drake and S. SealaSurface Engineering and Nanotechnology Facility (SNF) Laboratory, University of Central Florida,4000 Central Florida Blvd., Orlando, Florida 32826; Advanced Materials Processing and Analysis Center,University of Central Florida, 4000 Central Florida Blvd., Orlando, Florida 32826;Mechanical Materials and Aerospace Engineering, University of Central Florida, Eng 1, Room No. 381,4000 Central Florida Blvd., Orlando, Florida 32826; and Nanoscience and Technology Center,University of Central Florida, 4000 Central Florida Blvd., Orlando, Florida 32826Received 5 March 2007; accepted 11 May 2007; published online 11 June 2007The effect of In doping concentration on the optical band gap of nano-SnO2 is investigated as afunction of calcination temperature. Changes in the band gap explain the room temperature H2 gassensing of doped nano-SnO2. The band gap was found to be lower than those reported for SnO23.6 eV from 2.55 to 3.43 eV and may be explained by the presence of nonequilibrium oxygenvacancies in the oxide lattice and band bending effects at the nanoscale. © 2007 American Instituteof Physics. DOI: 10.1063/1.2746407Nanocrystalline SnO2 is an important transparent mate-rial due to its potential use in many applications, includinggas sensors. Doping of the SnO2 system has been a point ofinterest due to the ability to tailor its electrical and micro-structural properties.1–6 In the area of sensors and other elec-tronic applications, tailoring of the electronic structure innanocrystalline materials has become pivotal.7–9SnO2 is a wide band gap semiconductor with a band gapat 3.6 eV and higher at 300 K.10–15 The space charge layercontrol of nanostructures, including nano-SnO2, makes themparticularly interesting since conduction can change drasti-cally with expansion and contraction of the layer in the pres-ence of different gases.16,17 The energy required for electronsor other charged species from adsorbed gas molecules toconduct through the material can be greatly impacted by themagnitude of the band gap and whether or not any energylevel exists within the forbidden gap. This letter will focuson changes in the direct optical band gap of nano-SnO2doped with trivalent In substitution that deviate from what islargely reported.Indium doped SnO2 was prepared using a sol-gelmethod.17 A sol of tin isopropoxide in isopropanol72 vol %  and toluene 18 vol %  and indium isopropoxidewas prepared. A polymer, hydroxypropyl cellulose, wasadded to control the grain size during the gelling reaction.The gel is dried at 150 °C for 1 h in air. Powders were thencalcined at 500, 600, 800, and 1000 °C for 1 h in air. Thesubsequent drying and pyrolysis treatments of the coatedpolymer nanostructures then result in the decomposition ofthe polymer structure, leaving behind In–SnO2 nanocrystal-lites. The following are the sol-gel reactions during the dry-ing and pyrolysis procedures:171 SnOC3H74+4H2O→ In–SnOH4+4C3H7OH at25 °C,2 SnOH4→SnO2amorphous+2H2O at 150 °C, and3 SnO2amorphous→SnO2 crystalline at 400 °C.Structural verification of the SnO2 rutile cassiteritestructure by x-ray diffraction and high-resolution transmis-sion electron microscopy Fig. 1 is published elsewhere.18Ultraviolet-visible UV-Vis measurements were carried outon a Cary 1 UV-Vis spectrophotometer from Varian, Inc.Doped SnO2 nanocrystalline powders were compressed andscanned in the range of 200 to 800 nm using a Labsphereintegrating sphere.The size of the In–SnO2 nanoparticles based on calcina-tion temperature and amount of In doping is shown in TableI. It is observed that the particles retain their nanocrystallinenature even at the higher calcination temperatures. It is alsoworth noting that the size of nanocrystalline In–SnO2 at thelower calcination temperatures is nearly independent of theamount of doping.For crystalline SnO2, optical transitions have beenshown to be direct.19 The variation in the absorption coeffi-cient as a function of photon energy for allowed direct tran-sitions is given by = Ah − Eg1/2, 1where  is the absorption coefficient, A is a constant, h isPlanck’s constant,  is the frequency, and Eg is the band gapenergy. The absorption coefficient  is obtained from Beer’slaw,I = I0 exp− x . 2In the above equation, x is the thickness of the measuredsample. A plot of 2 versus photon energy was used to obtainthe value of the direct band gap by extrapolating the linearportion of the curves to zero absorption.The overall values obtained for all dopings and calcina-tion temperatures are lower than the reported values forSnO2, usually reported around 3.6 eV. Plots, to obtain bandgap values for 1% In doped samples, are shown in Fig. 2a.The highest band gap value was obtained for the samplecalcined at 500 °C at 3.1 eV. The band gap values decreaseduntil a calcination temperature of 1000 °C, where it slightlyaElectronic mail: sseal@mail.ucf.eduAPPLIED PHYSICS LETTERS 90, 233117 20070003-6951/2007/9023/233117/3/$23.00 © 2007 American Institute of Physics90, 233117-1increased from the 800 °C sample, 2.52–2.65 eV. For the3% doped samples Fig. 2b the same trend is followed asthat of the 1% doped samples. The highest value for the 3%samples is for the one calcined at 500 °C with a value of3.05 eV. The values again decrease until a calcination tem-perature of 1000 °C, where the value slightly increases to2.5 eV. For the 6.5% doped samples, a similar trend is noted,with a decrease in the band gap values with increasing cal-cination temperature until 1000 °C, where the value in-creases Fig. 2c. Unlike the 1% and 3% doped samples,the highest value of the band gap is obtained at a calcinationtemperature of 1000 °C at 3.43 eV. For the 9% dopedsamples, the trend is different from the other dopingsFig. 2d. Between 500 and 600 °C, the band gap decreasesfrom 3.14 to 2.62 eV but then increases at800 °C to 3.16 eV. At 1000 °C the band gap decreases.In the case of In doped samples, the band gap drops withinitial increase in doping amount 1%–3% and then in-creases at 6.5% Fig. 1b. A similar trend was reported inCo doped SnO2.20Full doping of SnO2 with indium will lead to substitu-tion of In3+ onto a Sn4+ site and the creation of oxygen va-cancies to retain charge neutrality within the cassiterite struc-ture. In the Kroger-Vink notation, charge modification upondoping occurs asO0x→ 12O2g + VO00 + 2e−, 32In3+ + 2SnSnx + OOx → 2InSn + VO00 + Snsurface. 4For 1% and 3% dopings, if complete doping were to occur, alarge amount of oxygen vacancies would be created in thelattice. The thermodynamic solution limit for indium in theSnO2 system has been reported to occur around 3%.21 Thislowering of the band gap gives an indication of the stoichio-metric deviation and degeneracy of the doped SnO2 and theincrease in oxygen vacancies within the lattice. It has beenpreviously shown that at low temperatures as low as roomtemperature, a nonequilibrium amount of oxygen vacanciesare maintained.22 At room temperature, oxygen vacancy con-centrations of 1023/m3 have been observed.In order for band bending of the conduction band tooccur towards the valence band in a semiconductor, a nega-tive space charge must develop. This usually occurs by anexcess of electrons or negative charge. In order to developthis negative charge with excess oxygen vacancies, surfacespecies must adsorb onto the surface of the material anddonate electrons. These donated electrons can then associatethemselves with positively charged oxygen vacancies. Infact, it has been previously observed by the author’s groupthat the major free carriers in sol-gel derived nanocrystallineIn–SnO2 are, in fact, monoionized oxygen vacancies.22 Dueto the fact that these nanoparticles are small and comparablein size to their Debye length, the space charge region willcompletely occupy the material and control the bandstructure.23 At the nanoscale, this surface induced band bend-ing then characterizes the band structure of the material.Another explanation for the ability of a material to de-crease its band gap energy is that the density of surface statesinduced by chemisorbed oxygen species decreases with de-creasing particle size, which would lead to a lesser degree ofFermi pinning.24 This would allow the surface barrier to un-dergo larger changes and control the ability of charged spe-cies to move through the material. This decrease in densityof surface states is directly caused by the curvature of thenanoparticles. It has been shown using scanning tunnelingspectroscopy for nanoparticles 10 and 30 nm that a surfaceband gap of around 2.5 eV exists.24 It is likely that for theIn–SnO2 nanoparticles sizes ranging from 3 to 23 nm, acombination of the decrease in the density of surface statesTABLE I. Variation in crystallite size of indium doped SnO2 ±5% error.Calciningtemperature °CCrystallite sizes nm1% In 3% In 6.5% In 9% In500 4 4 3 3600 5 5 4 4800 8 8 6 101000 13 14 19 23FIG. 1. Color online a High-resolution transmission electron microscopyof 6.5% In–SnO2 and b band gap changes as a function of calcinationtemperature for In–SnO2.FIG. 2. a 1% In–SnO2 samples calcined at 1 500 °C, Eg=3.1 eV,2 600 °C, Eg=2.9 eV, 3 800 °C, Eg=2.52 eV, and 4 1000 °C,Eg=2.65 eV; b 3% In–SnO2 samples calcined at 1 500 °C,Eg=3.05 eV, 2 600 °C, Eg=2.74 eV, 3 800 °C, Eg=2.55 eV, and 41000 °C, Eg=2.5 eV; c 6.5% In–SnO2 samples calcined at 1 500 °C,Eg=3.1 eV, 2 600 °C, Eg=3.04 eV, 3 800 °C, Eg=2.75 eV, and 41000 °C, Eg=3.43 eV; d 9% In–SnO2 samples calcined at 1 500 °C,Eg=3.14 eV, 2 600 °C, Eg=2.62 eV, 3 800 °C, Eg=3.16 eV, and 41000 °C, Eg=2.95 eV.233117-2 C. Drake and S. Seal Appl. Phys. Lett. 90, 233117 2007and the large amount of oxygen vacancies retained in thematerial explains the departure in the band gap values fromwhat has been reported.22 This also partially explains theenhanced ability for nanocrystalline In–SnO2 to be used ingas sensor applications at room temperature, since the poten-tial barrier required for charge to move between grains isnow reduced with a lower band gap energy. Author’s group25has recently shown that In doped SnO2 coated gas sensor atroom temperature has a response time of 20 s for 900 ppm ofH2.The question arises as to why the 6.5% and 9% samplesexhibited slightly different behaviors than the 1% and 3%doped samples. First, both the 6.5% and 9% are above thethermodynamically predicted indium doping amount in SnO2of 3%.21 For the 6.5% In–SnO2 nanocrystalline samples, ithas been shown that using sol-gel techniques and at lowcalcination temperatures, doping amounts above what is ther-modynamically predicted can occur.18 From previous Fouriertransform infrared studies, it has been demonstrated that,when exposed to H2 gas, the 6.5% samples do not respond toH2 in the same manner that the 3% doped samples respond.18In the case of 3% In–SnO2, surface species such as CO2 andhydroxyl groups changed with regards to the magnitude oftheir peaks after exposure to H2. Contrary, the 6.5%In–SnO2 sample after exposure to hydrogen showed littleimpact on these surface species. This indicates a differencein the nature of the same adsorbed surface species on thesurface of these two different dopings of SnO2. Because ofthis, surface effects such as band bending and surface densityof states would be different, and it would be expected thatthese two materials would have different electronic behav-iors at the nanoscale. This helps us to explain the enhancedgas sensing behavior of 6.5% In doped SnO2 at low tempera-tures over other similar material systems. The 6.5% In dopedsample likely has the best trade-off between surface speciesretained after synthesis and band bending effects.As gas sensor applications move towards lower workingtemperatures, knowledge of what can be utilized at thenanoscale becomes very important. The ability to understandthe effect of doping and its impact on the surface of nano-materials is paramount as the surface dominates many fea-tures at the nanoscale. For room temperature applications,reduction of the activation energy required for the gas sens-ing reaction is just as important as the surface reaction. Bychanging the band gap values, the energy barrier required forspecies to move through the space charge region can bemodified making room temperature applications more of areality. This can have even wider implications as materialsscience moves towards better understanding at the nanoscale,fine tuning materials better to specific applications.For nanocrystalline chemically synthesized In dopedSnO2, it has been shown that the optical band gap values fordirect transitions are much lower than the expected valuesreported. The lowest value of the band gap was achieved inthe 3% doped sample calcined at 800 °C, while the highestvalue of the band gap was achieved in the 6.5% dopedsample calcined at 1000 °C. Because of this reduction inband gap, room temperature gas sensing is possible with areduction in the potential barrier required for charge move-ment in nanocrystalline 3% and 6.5% doped In–SnO2 cal-cined at 500 °C, when exposed to a reducing gas.The authors would like to thank NASA-Glenn FSEC:NAG:32751, ASRC Corporation, NSF-CTS for fundingnanotechnology and sensor research, and NSF GK-12 Fel-lowship.1O. Wurzinger and G. Reinhardt, Sens. Actuators B 103, 104 2004.2H. Yang, X. Song, X. Zhang, W. Ao, and G. Qiu, Mater. Lett. 57, 31242003.3A. Maciel, P. Lisboa-Filho, E. Leite, C. Paiva-Santos, W. Schreiner, Y.Maniette, and E. Longo, J. Eur. Ceram. Soc. 23, 707 2003.4H. Yang, S. Han, L. Wang, I. Kim, and Y. Son, Mater. Chem. Phys. 56,153 1998.5H. Ahn, H. Choi, K. Park, S. Kim, and Y. Sung, J. Phys. Chem. B 108,9815 2004.6G. Xu, Y. Zhang, X. Sun, C. Xu, and C. Yan, J. Phys. Chem. B 109, 32692005.7P. Singh, A. K. Chawla, D. Kaur, and R. Chandra, Mater. Lett. 61, 20502007.8G. Sanon, R. Rup, and A. Mansingh, Phys. Rev. B 44, 11 1991.9Y. Ma, F. Zhou, L. Lu, and Z. Zhang, Solid State Commun. 130, 3132004.10Z. Nabi, A. Kellou, S. Mecabih, A. Khalfi, and N. Benosman, Mater. Sci.Eng., B 98, 104 2003.11S. Shanthi, C. Subramanian, and P. Ramasamy, Cryst. Res. Technol. 34,1037 1999.12A. E. Taverner, C. Rayden, S. Warren, A. Gulino, P. A. Cox, and R. G.Egdell, Phys. Rev. B 51, 11 1995.13J. Kang, S. Tsunekawa, and A. Kasuya, Appl. Surf. Sci. 174, 306 2001.14J-H. Chung, Y-S. Choe, and D-S. Kim, Thin Solid Films 349, 126 1999.15H. Kim and A. Pique, Appl. Phys. Lett. 84, 218 2004.16S. Shukla, S. Seal, L. Ludwig, and C. Parish, Sens. Actuators B 97, 2562004.17S. Shukla, S. Patil, S. C. Kuiry, Z. Rahman, T. Du, L. Ludwig, C. Parish,and S. Seal, Sens. Actuators B 96, 343 2003.18C. Drake, A. Amalu, J. Bernard, and S. Seal, J. Appl. Phys. 101, 1043072007.19J. Robertson, J. Phys. C 12, 4767 1979.20J. Hays, A. Punnoose, R. Baldner, M. H. Engelhard, J. Peloquin, and K.M. Reddy, Phys. Rev. B 72, 075203 2005.21F. Sensato, R. Custodio, M. Calatayud, A. Beltran, J. Andres, J. Sambrano,and E. Longo, Surf. Sci. 511, 408 2002.22C. Drake, S. Deshpande, and S. Seal, Appl. Phys. Lett. 89, 1 2006.23N. Yamazoe, Sens. Actuators B 5, 7 1991.24T. G. G. Maffeis, G. T. Owen, C. Malagu, G. Martinelli, M. K. Kennedy,F. E. Kruis, and S. P. Wilks, Surf. Sci. 550, 21 2004.25S. Deshpande, P. Zhang, N. Posey, J. Cho, and S. Seal, Appl. Phys. Lett.90, 073118 2007.233117-3 C. Drake and S. Seal Appl. Phys. Lett. 90, 233117 2007

Band gap energy modifications observed in trivalent In substituted nanocrystalline SnO2

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Band gap energy modifications observed in trivalent In substituted nanocrystalline SnO2

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