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PREPARATION OF GAS SENSITIVE FILM BY DEPOSITION OFULTRAFINE TIN DIOXIDE PARTICLES*M. ADACHI, t K. OKUYAMA, ~ Y. KOUSAKA ~ and H. TANAKA ~tRadiation Center of Osaka Prefecture, Shinke-cho, Sakai, Osaka 593, Japan~Department of Chemical Engineering, University of Osaka Prefecture, Mozu-umemachi, Sakai, Osaka 591,Japan (Received 5 March 1987) : 

PREPARATION OF GAS SENSITIVE FILM BY DEPOSITION OFULTRAFINE TIN DIOXIDE PARTICLES*M. ADACHI, t K. OKUYAMA, ~ Y. KOUSAKA ~ and H. TANAKA ~tRadiation Center of Osaka Prefecture, Shinke-cho, Sakai, Osaka 593, Japan~Department of Chemical Engineering, University of Osaka Prefecture, Mozu-umemachi, Sakai, Osaka 591,Japan (Received 5 March 1987) An evaporation-reaction type aerosol generator for ultrafine tin dioxide particles has been developed, and the effect of its evaporating temperature on particle size distributions and number concentrations of the generated ultrafme tin dioxide particles has been studied. The particles are drawn with into a deposition chamber at high velocity with a pressure of about 400 Pa, and are deposited by inertia on a glass substrate, where they form a thin film. The electrical response of the film was evaluated in response to various gases as a function of operating temperature and applied voltage.

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GENERATION OF ULTRAFINE TIN DIOXIDE PARTICLES Figure 1 shows a schematic diagram of the particle generation system. The generator consists of an annular section where high purity tin wire (99.9 %) in a ceramic boat is vaporized into a nitrogen gas stream, and a mixing section where tin vapour is oxidized by oxygen gas to produce a supersaturated tin dioxide vapour. On reaching certain level of supersaturation The tin dioxide vapour starts forming ultrafine primary tin oxide clusters by homogeneous nucleation. Larger secondary particles are then successively formed by the >> agglomeration of clusters and by the >> simultaneous heterogeneous condensation of vapour molecules on clusters.

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This kind of generator may be called an evaporation reaction type generator. The nitrogen and oxygen gases used are passed through a drying column (containing silica gel, calcium sulphate, active anhydrous and magnesium perchlorate) to remove water molecules and, then, the nitrogen gas is treated by a deoxidizer (containing activated copper) to obtain dry and oxygen-free carrier gas. Thus, water and oxygen concentrations in the nitrogen are less than 1.0 ppm.  The volumetric flow rates of nitrogen and oxygen in the generator are 1.6 and 0.4 lpm, respectively  The conductance of the film increases with the decrease of film thickness and with the increase of the substrate temperature. The current-voltage relation of the film is ohmic.  This shows that the ultrafine tin oxide particle film can selectively detect the gases N~ and Oz and is very sensitive to H2.

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PREPARATION OF ULTRAFINE TIN DIOXIDE PARTICLE FILMS Figure 5 shows the deposition chamber used in the production of thin particle films. Tin dioxide particles are introduced through a critical nozzle of 0.8 mm diameter to the deposition chamber which is pumped down to about 4000 Pa (= 30 Torr). Particles are accelerated to about 200 m s- ~ at the critical nozzle and then deposited on the glass substrate by inertial force to form a thin film.

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The glass substrate has two silver-plated electrodes with a gap between them of 1 mm as shown in Fig. 6. The ultrafine particles are deposited over the gap, so as to convert the two electrodes. The width and length of the film are 2 and 3 mm, respectively, and the thickness is varied from 0.01 to 0.2 mm. Fig7 shows a scanning electron micrograph of such a tin dioxide film. It is seen that the thickness of the film is constant and the structure of the film is porous, enabling passage of a gas.

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ELECTRICAL PROPERTIES OF TIN DIOXIDE PARTICLE FILMS Figure 8 shows the chamber for testing electrical characteristics of produced ultrafine particle films. The electrical current flowing through the film was measured by an electrometer in hydrogen, nitrogen and oxygen atmospheres. The operating voltage and temperature are changed from 1.5 to 700 V d.c. and from 293 to 573 K, respectively. Water molecules from the hydrogen, nitrogen and oxygen removed by a dryer; oxygen molecules from the nitrogen by deoxidizer. The concentrations of water and oxygen vapours were lower than 1.0 ppm.

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Effects of substrate temperature and thickness of particle film on the electric current.   The current in nitrogen gas tends to increase with the increase of the substrate temp. whereas it decreases with the film thickness. The values of current in hydrogen gas are found to be higher than those in oxygen and nitrogen gases. this particle film has a high sensitivity for hydrogen gas of low concentration.   Time-response of the ultrafine tin dioxide particle film to H2 and O2 gases, Sensitivity to N2 gas-3 mins, Sensitivity to H2 gas- 2 mins   All results, show that ultrafine tin dioxide particle film is found to possess good properties as a material for a gas sensor and that ultrafine tin dioxide particle films can selectively detect hydrogen, nitrogen and oxygen gases and have good properties as a gas sensor.

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references Clark, A. 0970) The Theory of Adsorption and Catalysis. Academic Press, New York. Hartman, T. E. (1963) J. appl. Phys. 34, 943. Hayashi, C. [1985) Oyo Buturi 54, 687 (in Japanese). Honing, R. E. and Kramcr, D. A. (1969) RCA Rev. 30, 285. Ihokura, K. (1975) Electronic Ceramics 6, 9 (in Japanese). Jarzebski, Z. M. and Morton, J. P. (1976) J. Electrochem. Soc. 123, 299c. Kousaka, Y., Niida, T., Okuyama, K. and Tanaka, H. (1982) J. Aerosol Sci. 12, 231. Kousaka, Y., Okuyama, K. and Adachi, M. (1985) Aerosol Sci. Technol. 4,209. Leaver, K. D. and Chapman, B. N. (1971) Thin Films. Wykeham Publications, London. Ogawa, H., Abe, A., Nishikawa, M. and Hayakawa, S. (1981a) J. Electrochem. Soc. 128, 685. Ogawa, H., Abe, A., Nishikawa, M. and Hayakawa, S. (1981b) J. Electrochem. Soc. 128, 2020. Okuyama, K., Kousaka, Y. and Motouchi, T. (1984) Aerosol Sci. Technol. 3, 353. Shibata, Y. (1983) Thin Film Hand Book. Ohm, Tokyo (in Japanese).

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In sensors having an ultrafine particle gas sensing film, the sensitivity to various gases could be separated both by >> Sensor operating temperature, as also >> Depositing conditions of the ultrafine particle films.   And this has been achieved WITHOUT catalyst use. Evaporation of the metal in a low oxygen pressure atmosphere can result in ultrafine particles of metal oxide deposition in form of films with thickness in order of just a few tens of angstroms. The SnOe typical ultrafine particle film fabricated by using this technology has the porous columnar structure constructed from the ultrafine particles having the median particle size of several tens of angstroms (Compared to this sintered polycrystalline materials, have particle size more than several thousands of angstroms) Electrical Properties of Tin Oxide Ultrafine Particle Films Hisahito Ogawa, Atsushi Abe, Masahiro Nishikawa, and Shigeru Hayakawa Matsushita Electric Industrial Company Limited, Mater~aZ Research Laboratory, 1006 Kadoma, Osaka, 571 Japan

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The unique characteristics of SnO~ ultrafine particle films & their advantage for use in gas sensors are:

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The optimum structure of the ultrafine particle film to detect each gas can be obtained by selecting the film deposition condition to be suitable.

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Sample preparation.—Two different substrate materials, Silica glass (Corning No.7059) and silicon covered with SiO~ with the dimension of 2.0 • 2.0 • 0.2 mm were used. ( not much difference between the sensors fabricated on these two different substrates. ) ·    The sensing material, SnO2 ultrafine particle film, was deposited between the two gold-plated interdigital electrodes (the gap of electrode is 0.02 mm and the total length of electrode is 11.6 mm) on one side of the substrate in the following procedures (11). ·    Evaporating materials such as Sn, SnO, and SnO2, of purity higher than 99.99%, were placed in an evaporator boat. ·    The chamber was pumped down to approximately 2 • 10 -6 Tort and then oxygen (about 1 Torr) was introduced into the chamber

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·        The glow discharge of the oxygen was produced by applying the rf power. Then the heating power was supplied to the boat and, after the growth rate of the tin oxide ultrafine particles in the glow discharge of oxygen reached a stable condition, the shutter was opened to deposit the ultrafine particles on the substrate. ·        The deposition was allowed for a few minutes, depending on the thickness of the final ultrafine particle film. ·        Electrical contacts were made with 0.05 mm gold wires attached to the gold electrodes. The contact resistance between tin oxide ultrafine particle films and gold electrodes was measured at the operation temperature from 25 ~ to 500~ and was confirmed to be small enough compared with the resistance of the tin oxide ultrafine particle films themselves.

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The development of gas sensor for carbon monoxide monitoring using nanostructure of Nb–TiO2 T. Anukunpraserta,*, C. Saiwana, E. Traversab aThe Petroleum and Petrochemical College, Chulalongkorn University, Bangkok,Thailand bThe University of Rome “Tor Vergata”, Italy   Received 12 January 2005; revised 17 February 2005; accepted 17 February 2005

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Experimental   Nanosize TiO2: preparation and characterization   Pure TiO2 was prepared by a microemulsion technique according to a procedure described in literature [7]; ten g of the aqueous solution of 0.3 M TiCl4 was added to 90 g of 6 wt% AOT in n-heptane solution with rapid stirring. After thorough mixing, the solution was equilibrated at 30 degC for 2 hr. For Nb-doped TiO2, the procedure was slightly modified by adding NbCl5 in the aqueous phase before mixing. Resulting microemulsion was stable and separated for precipitation, which was carried out by bubbling air through concentrated NH4OH solution into the microemulsion. The as-synthesized TiO2 was ·        separated by high-speed centrifugation at 10,000 rpm. Then, washed sequentially with n-heptane, twice with ethanol and acetone followed by water to remove remaining surfactant from the as-synthesized particles. ·        dried and calcined for 5 h at various calcination temperatures.   After that, the characterization of the microstructure TiO2 was carried out by XRD, BET and TEM.

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Micro structural analysis of thick film sensor After calcination at 460 degC, the TiO2 powder was formed into a thick- film sensor. Thick-film sensors were fabricated using pastes obtained by adding each powder with an organic vehicle. The pastes were painted on an alumina substrate with an activated Au electrode. The sensor was fired in air at temperatures ranging from 550 to 850 degC and The sensor was subsequently characterized by XRD. The gas sensing characteristics were examined by fixing the concentration of CO at 1000 ppm and the operating temperature at 550 degC. The electrical responses of the films were observed. After gas was applied to the flow system, the oxidizing CO caused a dramatic decrease in resistance of the TiO2. Consequence was, a sudden increase in current can be detected. A step response was observed by switching the flow from air to gas and gas to air.

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TEM image of TiO2 powder calcined at 450 degC (a), HR-TEM of anatase structure (plane 101) of pure TiO2 calcined at 450 degC (b) and 3% Nb-doped TiO2 calcined at 850 deg (c).

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The effect of calcination temperature on the surface area and grain size Average grain size of powder and specific surface area are a function of calcination temperature. The pure TiO2 show large grain growth and a drastic drop of specific surface area where as the Nb-doped TiO2 has insignificant grain growth. This shows that the Nb addition can inhibit the grain growth while maintaining high surface area of the powder. Phase transformation from anatase to rutile The effect of temperature on the phase transformation from anatase to rutile for pure and Nb-doped-TiO2 by varying the calcination temperatures from 550 to 850degC shows that: The transformation temperature from anatase to rutile structure increased in presence of 3% Nb doping. At 850 degC, the Nb–TiO2 was still in pure anatase structure.

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The effect of Nb-doped TiO2 on sensing properties :   1. Nb imparts enhanced thermal stability. 2. The existence of the anatase phase at high temperature results in a better electrical signal of the film on CO. 3. Moreover, the resistance of the film became 10 times lower than that of the pure TiO2. Conclusion A nanostructure with the high thermal stability of niobium doped TiO2 (Nb–TiO2) was synthesized using the water-in-oil (w/o) microemulsion system of n-heptane/water/sodium bis (2-ethylhexyl) sulfosuccinate (AOT) surfactant.

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It was compared with undoped TiO2. It was found that 1. The Nb-doped TiO2 at 3–5 mole% clearly hinders the anatase-to-rutile phase transition and 2. inhibits the grain growth in comparison with pure TiO2. 3. The nanostructure of anatase could be maintained even after the powder was fired at 850 degC. 4. In a CO sensing study, it was found that the sensitivity of CO is significantly increased with an increase of thermal stability of Nb-doped TiO2. By comparison, undoped TiO2 did not behave this way. This shows that the nanostructure of Nb-doped TiO2 is promising for environmental monitoring.

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One-step synthesis of noble metal–titanium dioxide nanocomposites in a flame aerosol reactor Vinay Tiwari a,b,1, Jingkun Jiang a,1, Virendra Sethi b, Pratim Biswas a,* a Aerosol and Air Quality Research Laboratory, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, MO 63130, USA b Center for Environmental Science and Engineering, Indian Institute of Technology, Mumbai 400076, India Synthesis Noble metal–titanium dioxide nanocomposites (Pt/TiO2, Pd/TiO2 and bimetallic Pt-Pd/TiO2) were synthesized in one step using a flame aerosol reactor (FLAR). Process Control The specific surface area, crystal phase ,morphology and noble metal loading of the nanocomposites were controlled by adjusting operating parameters of reactant concentration and temperature–time history in the reactor.

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Characterization The synthesized nanomaterials were characterized using transmission electron microscopy (TEM), electron diffraction, X-ray diffraction (XRD) and nitrogen adsorption (BET). Nanocomposites with 0.5–3.0% (wt%) noble metal loading were synthesized. Nanosized noble metal particles (2–4 nm) were dispersed on the 30–40 nm TiO2 surface with an overall specific surface area in the range of 40–60 m2/g The specific surface area increased with increasing noble metal loading. For the chosen flame conditions, a mixture of anatase and rutile phase was obtained without noble metal addition. On incorporation of the noble metal, the formation of the rutile phase of titanium dioxide got suppressed.

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Photocatalytic Activity The synthesized nanocomposites were tested for the photocatalytic oxidation of methyl orange dye in an aqueous phase. Platinum particles dispersed on the TiO2 surface increased the photocatalytic activity more than in case of pristine TiO2. Experimentation shows that there exists an optimum platinum loading for the highest photocatalytic activity. This is approximately 0.5–1.0% Pt. Palladium addition is detrimental to photocatalytic activity of titanium dioxide Bimetallic noble metal catalysts (Pt-Pd/TiO2) showed enhanced photocatalytic activity compared to pristine titanium dioxide, but lower than platinum (only)–titanium dioxide nanocomposites.   Conventional liquid phase synthesis methods are multistep processes are multistep processes flame synthesis provides better control on resultant nanocomposite properties; and can produce such materials in a single step.   Based on these advantages and considering availability of Flame Aerosol Reactor in CSE,IIT-B synthesis of nano composites at CSE,IIT-B is being done by this method.

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NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition Ch.Y. Wang ∗, M. Ali, Th. Kups, C.-C. R¨ohlig, V. Cimalla, Th. Stauden, O. Ambacher Institute of Micro- and Nanotechnologies, Technical University Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany Received 30 July 2007; received in revised form 10 October 2007; accepted 11 October 2007 In order to detect a low concentration of NOx gases, metal oxide based sensors are used due to: Structural simplicity, high sensitivity, short response time, small size, low cost and good compatibility with the fabrication process for microelectronic devices [3–5]. In2O3 nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. (MOCVD)

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The response of 10-nm thick In2O3 particle containing layers to NOx and O2 gases showed ·        The lowest detectable NOx concentration is ∼200 ppb and the sensor performance is strongly dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NOx is found to be above 104 . Furthermore, the cross-sensitivity against O2 is very low, demonstrating that the In2O3 nanoparticles are very suitable for the selective Nox detection In2O3 in specific In2O3 nanostructures show very high sensitivity to oxidizing gases like NOx [8] and O3 [9,10]. Li et al. examined the NO2-sensitivity of In2O3 nanowires to find the lowest detectable concentration as 5 ppb [7].

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Therefore, In2O3 nanostructures are very suitable to be used as active material in NOx detectors, especially at sub-ppm levels. In2O3 nanostructures exhibit a very high sensitivity to oxidizing gases like Nox In2O3, having a variety of electrical [14] and structural properties (i.e. single crystalline [15], polycrystalline [16], and nanostructured [17]), can be obtained by metal organic chemical vapor deposition (MOCVD). In2O3 deposited at low substrate temperature (200 ◦C) forms a 10-nm thick In2O3 nanoparticle layer composed of mean crystallite diameter of 7 nm. The prime objective is to determine the response to low-concentration NOx and the selectivity against O2 of the In2O3 nanoparticles at low operating temperatures.

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Experimental Reactor Growth of the In2O3 layers was carried out in a horizontal MOCVD reactor (AIXTRON 200). Precursors Trimethylindium (TMIn) and H2O vapor were used as precursors for the indium and oxygen, respectively, and carrier N2 served as the carrier gas. Film deposition and characterization The In2O3 thin films deposited on sapphire (0 0 0 1) substrates by supplying TMIn and H2Ovapor at flowrates of 15 and 1160 _mol/min, respectively at constant substrate temperature of 200 degC. After growth, the In2O3 layers were ex situ characterized by means of high-resolution transmission electron microscopy (HRTEM). Sensitivity NOx of a concentration in a sub-ppm range (200 ppb) is detectable. There is a significant resistance increase with increasing the NOx concentration from 800 ppb to 4 ppm.   These results indicate that the In2O3 particle containing sensor can monitor the sub-ppm NOx.

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The operating temperature was varied from room temperature (RT) to 250 ◦C to obtain optimum sensor response S, which was defined by S = Rgas/Rvacuum, where Rvacuum is the film resistivity after gas removal in the vacuum and after 20 min of UV light illumination, and Rgas is the resistivity after 15 min of gas exposure. The response and recovery times of the sensor are defined as the times necessary for the In2O3 layer to get 90% of the total change in resistivity after exposure to NOx and to return to 90% above the original conductance after UV-irradiation, respectively. NOx response of In2O3 nanoparticles display gives the sensor response once NOx gas of concentrations from 0.3 to 0.8 ppm enters into the chamber. Increase in resistance change and decrease in response time occurs due to a rise of NOx concentration. Layer resistivity returns to its initial value after a few minutes. In2O3 nanostructures show very high sensitivity to oxidizing gases like Nox and hence can be used as active material in NOx detectors, in segment of sub-ppm levels. In2O3, having a variety of electrical [14] and structural properties (i.e. single crystalline [15], polycrystalline [16], and nanostructured [17]), can be obtained by metal organic chemical vapor deposition (MOCVD).

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Conclusion 1.      In2O3 nanoparticles with a mean diameter of 7 nm have been obtained by low-temperature MOCVD. 2.      The responses of 10-nm thick In2O3 particle containing layers to NOx (S>104) and O2 gases determined shows, NOx gas of a concentration down to 200 ppb could be detected by the In2O3 sensor. Its performance banks heavily on the gas partial pressure, from 200 ppb to 200 ppm, and the operating temperature. 3.      The optimum detection temperatures occurred at 200 ◦C and 100 ◦C for the detection of NOx of high and low concentrations, respectively. 4.      The response toO2 was very low, indicating that the sensor was very suitable for selective NOx detection. 5.      The cross-sensitivity to humidity, CO and hydrocarbons for In2O3 nanoparticle based NOx sensors should be examined.

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References [1] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Novel NO2 gas sensor based on cuprous oxide thin films, Sens. Actuator B 113 (2006) 468–476. [2] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films, Appl. Phys. Lett. 82 (2003) 961–963. [3] J.-B. Sanchez, F. Berger,M. Fromm, M.-H. Nadal, V. Eyraud, Tin dioxidebased gas sensors for detection of hydrogen fluoride in air, Thin Solid Films 436 (2003) 132–136. [4] H.H. Afify, I.K. Battisha, Oxygen interaction with CdS-based gas sensors by varying different preparation parameters, J. Mater. Sci. 1 (2000) 373–377. [5] G. Neri, A. Bonavita, G. Micali, G. Rizzo, S. Galvagno, M. Niederberger, N. Pinna, A highly sensitive oxygen sensor operating at room temperature based on platinum-doped In2O3 nanocrystals, Chem. Commun. 48 (2005) 6032–6034. [6] J. Shieh, H.M. Feng, M.H. Hon, H.Y. Juang, WO3 and W–Ti–O thin-film gas sensors prepared by sol–gel dip-coating, Sens. Actuator B 86 (2002) 75–80. [7] C. Li, D. Zhang, S. Han, X. Liu, T. Tang, B. Lei, Z. Liu, C. Zhou, Synthesis, electronic properties, and applications of indium oxide nanowires, Ann.NY Acad. Sci. 1006 (2003) 104–121. [8] A. Gurlo, M. Ivanovskaya, A. Pfau, U.Weimar,W.G¨opel, Sol–gel prepared In2O3 thin films, Thin Solid Films 307 (1997) 288–293 [9] G. Kiriakidis, M. Bender, N. Katsarakis, E. Gagoudaskis, E. Hourdakis, E. Douloufakis, V. Cimalla, Ozone sensing properties of polycrystalline indium oxide films at room temperature, Phys. Status Solidi A 185 (2001) 27–32. [10] Ch.Y. Wang, V. Cimalla, Th. Kups, C.-C. R¨ohlig, Th. Stauden, O. Ambacher, M. Kunzer, T. Passow, W. Schirmacher, W. Pletschen, K. K¨ohler, J. Wagner, Integration of In2O3 nanoparticle based ozone sensors with GaInN/GaN light emitting diodes, Appl. Phys. Lett. 91 (2007) (103509-1-3).

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[11] C.A. Pan, T.P. Ma, High-quality transparent conductive indium oxide films prepared by thermal evaporation, Appl. Phys. Lett. 37 (1980) 163–165. [12] S. Kasiviswanathan, G. Rangarajan, Direct current magnetron sputtered In2O3 films as tunnel barriers, J. Appl. Phys. 75 (1994) 2572–2577. [13] H. Imai, A. Tominaga, H. Hirashima, M. Toki, N. Asakuma, Ultravioletreduced reduction and crystallization of indium oxide films, J. Appl. Phys. 85 (1998) 203–207. [14] Ch.Y. Wang, V. Cimalla, H. Romanus, Th. Kups, M. Niebelschuetz, O. Ambacher, Tuning of electrical and structural properties of indium oxide films grown by metal organic chemical vapor deposition, Thin Solid Films 515 (2007) 6611–6614. [15] Ch.Y. Wang, V. Lebedev, V. Cimalla, Th. Kups, K. Tonisch, O. Ambacher, Structural studies of single crystalline bixbyte In2O3 films epitaxially grown on InN(0 0 0 1), Appl. Phys. Lett. 90 (2007) (221902-1-3). [16] N.H. Kim, J.H. Myung,H.W. Kim, C. Lee, Growth of In2O3 films on silicon by the metalorganic chemical vapor deposition method, Phys. Status Solidi A 202 (2004) 108–112. [17] H.W. Kim, N.H. Kim, C. Lee, A MOCVD route to In2O3 onedimensional materials with novel morphologies, Appl. Phys. A 81 (2005) 1135–1138. END

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EXTRA [18] Ch.Y. Wang, V. Cimalla, H. Romanus, Th. Kups, G. Ecke, Th. Stauden, M. Ali, V. Lebdev, J. Pezoldt, O. Ambacher, Phase selective growth and properties of rhombohedral and cubic indium oxide, Appl. Phys. Lett. 89 (2006) (011904-1-3). [19] M. Ali, Ch.Y.Wang, C.-C. R¨ohlig, V. Cimalla, Th. Stauden, O. Ambacher, NOx sensing properties of In2O3 thin films grown by MOCVD, Sens. Actuator B, in press. [20] M. Ali, V. Cimalla, V. Lebedev, Th. Stauden, Ch.Y. Wang, G. Ecke, V. Tilak, P. Sandvik, O. Ambacher, Reactively sputtered InxVyOz films for detection of NOx, D2, and O2, Sens. Actuator B 123 (2007) 779–783. [21] D. Zhang, C. Li, S. Han, X. Liu, T. Tang, W. Jin, C. Zhou, Ultraviolet photoreduction properties of indium oxide nanowires, Appl. Phys. A 77 (2003) 163–166. [22] M. Bender, N. Katsarakis, E. Gagaoudakis, E. Hourdakis, E. Douloufakis, V. Cimalla, G. Kiriakidis, Dependence of the photoreduction and oxidation behavior of indium oxide films on substrate temperature and film thickness, J. Appl. Phys. 90 (2001) 5382–5387. [23] Ch.Y.Wang, V. Cimalla, Th. Kups, C.-C. R¨ohlig, H. Romanus, V. Lebedev, J. Pezoldt, Th. Stauden, O. Ambacher, Photoreduction and oxidation behavior of In2O3 nanoparticles by metal organic chemical vapor deposition, J. Appl. Phys. 102 (2007) (044310-1-6).