0 Copyright 1992 American Chemical Society
JANUARY 1992 VOLUME 8, NUMBER 1
Letters Rectification Effect by a p-n Junctioned Oxide Film N. Uekawa, T. Suzuki, S. Ozeki, and K. Kaneko* Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Chiba-shi 260, Japan Received May 29,1991. I n Final Form: September 30, 1991 The electrical conductivity of the NiO film was controlled by doping Li ions. The junctioned oxide film of p-type Li-doped NiO and n-type Ti02 was prepared by the sol-gel method. The p-n junctioned oxide film showed clear rectification effect.
Introduction Nonstoichiometric transition-metal oxides exhibit interesting semiconductivity, leading to characteristic surface activities.'-3 Ti02 is one of representative n-type transition-metal oxides and the photocatalysis is wellk n ~ w n . ~On , ~the other hand, NiO is a p-type semiconductive oxide and doping of Li ions enhances the electrical conductivity.6 Transition-metal oxides have narrow energy bands compared with covalent semi~onductivity.~ Also the valence and conduction bands of n- or p-type fine oxide particles bend upward or downward near their surfaces, respectively.s Such band bending causes marked charge separation of electrons and holes, which is deeply associated with catalysis of transition-metal oxides. It is difficult to prepare homogeneous valence controlled oxides by doping using coprecipitation and firing at high temperature. The recently developed sol-gel method can provide a homogeneous and uniform thin film of metal o x i d e ~ . ~InJ particular, ~ the sol-gel process is suitable for preparing the film of multicomponent oxides such as high Tc superconducting oxides, compared with the coprecip(1) Kiselev, V. F.;Krylov, 0. V. In Adsorption and Catalysis on Transition Metals and Their Oxides; Springer-Verlag: Berlin, 1989. (2) Kung, H.H.In Transition Metal Oxides; Elsevier: Amsterdam, 1989. (3) Kaneko, K.; Mataumoto, A. J . Phys. Chem. 1989, 93, 8090. (4) Fujishima, A.; Honda, K. Nature 1972,238, 37. (5)Duonghong, D.; Borgarello,E.;Gratzel, M. J . Am. Chem. SOC.1984, 106,4685. (6) Bosman, A. J.; Crevecoeur,C. J . Phys. Chem. Solids 1976,29,109. (7)Adler, D. J. Solid State Chem. 1976, 12, 332. (8)Low,J.T. In Semiconductors; Hannay, N. B., Ed.; Reinhold: New York, 1959; Chapter 16. (9) Sakka,S.In Science in Sol-CelMethod;Agne: Tokyo, 1988 Chapter 6. (10) Segal, D. In Chemical Synthesis of Advanced Ceramic Materials; Cambridge University Press: Cambridge, 1989.
itation method, because homogeneous multicomponent systems can be easily obtained by mixing the molecular precursor solutions." The microporous and valencecontrolled oxide film is expected to play an important role in adsorption, separation, and catalysis. In this paper, the microporous Ti02 and Li-doped NiO, and Li-doped NiO/TiOz junctioned films was prepared by the sol-gel method and their microporosity and optical and electrical properties were examined. The rectification effect of the junctioned oxide film will be described.
Experimental Section The Li-doped NiO films were prepared by spin coating of the mixed solution of Ni(N0&-6HzO,LiN03, citric acid, and ethylene glycol, predrying at 413 K for 10 min, and heating 773 K for 10 min in air. The Ti02 film was prepared from the mixed solution of titanium tetraisopropoxide (Ti(o-iPr)r)and diethanolamine. The p-n junctioned film was prepared by successive coatingsof componentTi02 and Li-dopedNiO films. The quartz and Pyrex glasses were used as substrates. The X-ray diffractionpatterns of the 20 or 18 coated Ti02 and Li-doped NiO films were obtained by both a powder X-ray diffractometer (Rigaku Denki 2028) by use of Cu Ka at 35 kV and 10 mA and a thin film X-ray diffractometer (MAC ScienceMXP) using low-angleCu Ka radiation from an 18-kW generator. The thin-film X-ray diffractometer has a sensitivity of 1 nm. The electronic spectra of the prepared oxide films were examined at wavelengths of 200-900 nm with the aid of a UV spectrometer (Hitachi,200-10). The thickness of the oxide film was determined by the multiple reflection interference method with an optical microscopeat 539 nm. The nitrogen adsorption isotherm of NiO powder prepared in a similar way to the oxide without the coating was gravimetrically determined at 77 K by a computer-aided apparatus after preevacuation at 393 K and 1 mPa for 2 h.12 (11) Livage, J.;Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259.
0743-746319212408-0001$03.00/0 0 1992 American Chemical Society
Letters
2 Langmuir, Vol. 8,No. 1, ,1992
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Top view
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Side viewm iN O -iyLfri-r Pyrex glass
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Figure 1. Configuration of the oxide film and electrodes. 200
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Figure 3. Electronic spectra of Li-doped NiO samples.
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Figure 2. Thin-filmX-ray diffraction patterns of the 18 coated NiO and 10% Li-doped NiO. The dc electrical resistance was measured under 8 V in vacuo from 303 to 393 K using a surface conductive type cell and an electronic micrometer (Keithley 427). A1 electrodes of 25 mm width were evaporated 25 mm apart. The configuration of the oxide film and electrodes is shown in Figure 1. Cu leads were connected to the electrodes. Current-voltage characteristicswere measured in the voltage range of 0.8-100 V. The rectification of the stacked film of Li-doped NiO and Ti02 was examined at 373 K from -100 to 100 V.
Results and Discussion X-ray diffraction results showed the formation of Ti02 and NiO. The powder diffractometer gave the diffraction pattern of the NiO lattice with high background for all Li-doped NiO samples; the NiO lattice was formed even in the Li-doped samples. Such NiO lattice formation was confirmed by thin-film X-ray diffraction patterns. Figure 2 shows thin-film X-ray diffraction patterns of the 18 coated NiO and 10% Li-doped NiO. Both patterns are closed to each other and all five sharp peaks are assigned to the NiO lattice. As the intensities of the strongest 111 and 200 peaks are converted by the Li doping, morphology of the NiO polycrystals in the film possibly changes with the Li doping. (12) Kakei, K.; Ozeki, S.;Suzuki, T.;Kaneko, K. J.Chem. SOC.,Faraday Trans. I 1990,86, 371.
The adsorption isotherms of NiO powders prepared in the same way as the film are type II.13 It had marked uptake at low relative pressure, indicating the presence of micropores. Two lines of the t plot for the Nz adsorption isotherm intersected at 0.55 nm, suggesting that the micropore width is less than 1.1 nm. The total surface area and external surface area were 7.1 and 4.3 m2 g-l, respectively. Although the microporosity of the film itself was not determined yet, the prepared film probably has similar microporosity. The film thicknesses of the NiO and Ti02 films per single coating were 15 and 50 nm, respectively. The thickness grew with the coating turn. The UV absorbance increased with the film thickness so sensitively that a thickness of less than 2 nm can be examined. The lateral uniformity of the film was confirmed at 5 mm resolution of the light spot area by UV examinations. Figure 3 shows the electronic spectral change of the NiO film with Li doping. There is a clear change between 5% and 10% dopings. The absorption band near 300 nm has double maxima. The higher energy peak grows with Li doping. This electronic transition arises from the ligand-to-metal charge transfer e x i t ~ n s . ' ~Possibly J~ the double maxima structure is associated with the mixed valence state of Ni2+and Ni3+. The weak band at 608 nm due to the spinforbidden tzg-e, transition15 hardly changes with the Li doping. The electrical resistance of the NiO film decreases steeply with Li doping, as shown in Figure 4. Li doping of more than 10 % decreases the resistance by an order of more than 2. In earlier works,16J7less than 1 % doping of Li more remarkably lowers the electrical resistivity; Li doping enhances markedly the electrical conductivity. Possibly marked enhancement of electrical conductivity of NiO with doping of a slight amount of Li by the coprecipitation method arises from the formation of the (13) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; p 4. (14) Powell, R. J.; Spicer, W. E. Phys. Reu. B: Solid State 1970, 2, 2182. (15)Johnson, K. H.; Messmer, R. P.; Connolly, J. W. D. Solid State Commun. 1973, 12, 313. (16) Bosman, A. J.; Creveoueur, C. Phys. Reu. 1966, 144, 763. (17) Adler, P. Solid State Phys. 1968, 21, 1.
Langmuir, Vol. 8, No. 1, 1992 3
Letters
;"" 0
;Kl
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W
0
10
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(6)
Figure 5. Changes in the logarithm of electrical resistance (A) and in the activation energy (B) with the reciprocal of the intercarrier distance (lid).
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lo
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Ti02
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Figure 4. Change in the electrical resistance of the NiO film with Li doping. conducting path. This is because the doped Li atoms should be segregated near the surface of grains. On the contrary, such segregation does not occur in the doped film prepared by the sol-gel process. The inter-Li-ion distance reaches 0.7 nm at the doping of 10%. When the inter-hole distance is close to the order of the lattice constant, the hopping energy in the mobility term should be significantly reduced. The activation energy EA for hopping conductivity can be expressed by the small polaron hopping energy w~~~ EA= W, = e2 1 - 1
-(< --) 4%ff
where l/t,ff = 116- 1/60, t and €0are the high frequency and static permittivities, ro is the radius of the potential well of the small polaron, and a is the interion distance. The hopping energy decreases due to the repulsion caused by the nearest carrier interaction. If d is the intercarrier distance, the repulsion term of -ce2/d should be added to eq 1. Here c is a constant and d can be determined by eq 2 using the carrier concentration n d = 2[3/(4~n)]l/~ (2) Thus, the intercarrier repulsion gives rise to the electrical resistivity decrease with the doping. Figure 5 shows changes in the logarithm of the electrical resistivity and in the activation energy EA with the reciprocal of the intercarrier distance (114. Both relationships are briefly expressed by the lines, indicating good applicability of eqs 1 and 2. Consequently, Ni3+ should be produced by Li doping, leading to the hole hopping between Ni3+ and Ni2+. The increase of Ni3+ lowers the activation energy for hopping conduction due to the intercarrier repu1~ion.l~ Li-doped NiO and Ti02 films exhibit the current-voltage (I-V) characteristics without rectification behavior. The stacked film of Li-doped NiO (300nm) and Ti02 (350nm) showed a remarkable rectification effect, as shown in Figure 6. The I-V characteristics of the forward direction is concave against the abscissa, whereas that of the reverse (18)Mott, N. F. J. Non-Cryst. Solids 1968, I , 1.
0
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Figure 6. Forward and reverse current-voltage characteristics of the p-type Li-doped NiO and n-type Ti02 oxide film: solid line, observed; dotted line, calculated by eq 3. direction is slightly convex. The I-V characteristics are similar to that of a p-n junction of covalent semiconductors.20 The rectification behavior of the p-n junctioned covalent semiconductor can be described by
I = I,[exp(aeV/kn - 11 (3) The theoretical rectification curve (I,= 8.0 X a = 6.0 X is shown by the dotted line in Figure 6. The observed I-Vcharacteristics of the stacked oxide film can be roughly described by the rectifier theory for the covalent semiconductor. However, there should be an energy barrier at the interface between p-type and n-type oxide films, which is ascribed to the small a value. Acknowledgment. We are indebted to Drs. F. Mizukami and Maeda for the thin-film X-ray diffraction measurements. This work was partially supported by a grant from Tokuyama Soda Corp. (19)Kaneko, K.;Inouye, K. J. Chem. SOC.,Faraday Trans. 1 1976,72, 1258. (20)Schockley, W. In Electrons and Holes in Semiconductors; Van Nostrand: New York, 1966;p 91.
