Optical and structural properties of ZnO + Zn2TiO4 thin ... - Springer Link

6 downloads 0 Views 437KB Size Report
Alfredo Cruz-Orea Æ Julio G. Mendoza Alvarez Æ. Orlando Zelaya-Angel. Received: 9 January 2007 / Accepted: 12 March 2007 / Published online: 20 April ...
J Mater Sci: Mater Electron (2007) 18:1127–1130 DOI 10.1007/s10854-007-9267-8

Optical and structural properties of ZnO + Zn2TiO4 thin films prepared by the sol–gel method Sandra A. Maye´n-Herna´ndez Æ Gerardo Torres-Delgado Æ Rebeca Castanedo-Pe´rez Æ Mario Gutie´rrez Villarreal Æ Alfredo Cruz-Orea Æ Julio G. Mendoza Alvarez Æ Orlando Zelaya-Angel

Received: 9 January 2007 / Accepted: 12 March 2007 / Published online: 20 April 2007  Springer Science+Business Media, LLC 2007

Abstract ZnO + Zn2TiO4 thin films were obtained by the sol–gel method using precursor solutions with different Ti/Zn ratios in the 0.18–2.13 range. The films were deposited on glass substrates and annealed in an open atmosphere at 550 C. The oxide was characterized by X-ray diffraction and photoacoustic (PA) spectroscopy. The films were constituted of polycrystalline ZnO for the lowest Ti/Zn ratio (0.18), polycrystalline Zn2TiO4 for the 0.70 and 1.0 ratios, and mixes of both oxides for the intermediate ratios (0.32 and 0.50). For the highest ratios studied (1.44 and 2.13), the films were amorphous. The energy band gap (Eg) values were determined from optical absorption spectra, measured by means of the PA technique spectra. Eg varied in the 3.15 eV (ZnO) to 3.70 eV (Zn2TiO4) range.

1 Introduction Many intense activities in material chemistry and physics are involved in the fabrication of titanium dioxide based S. A. Maye´n-Herna´ndez  G. Torres-Delgado  R. Castanedo-Pe´rez (&) Centro de Investigacio´n y de Estudios Avanzados del I. P. N., Unidad Quere´taro, A.P. 1-798, Queretaro Qro. 76001, Mexico e-mail: [email protected] M. G. Villarreal Centro de Investigacio´n en Quı´mica Aplicada, Boulevard Enrique Reyna No.140, Saltillo Coah. 25100, Mexico A. Cruz-Orea  J. G. M. Alvarez  O. Zelaya-Angel Depto. de Fı´sica, Centro de Investigacio´n y de Estudios Avanzados del I.P.N, A.P. 14-740, Mexico, D.F. 07360, Mexico

systems with peculiar physical–chemical properties for industrial applications. With this purpose different titanium dioxide based systems such as Al2O3–TiO2 [1], In2O3– TiO2 [2], IrO2–TiO2 [3], CaCO3–TiO2 [4] have been prepared by employing different techniques. In particular, the ZnO–TiO2 mixed oxide is useful in gas-sensing [5], photocatalysis [6], chemical sorbing [7], among other applications. In the photocatalytic process the forbidden energy band gap (Eg) of the semiconductor oxide plays an important role, because its value determines the range of wavelength used to illuminate the oxide. In this work we report the preparation of ZnO + Zn2TiO2 thin films by means of the sol–gel technique. The precursor solution is prepared by mixing two precursors, one of them used to prepare TiO2 films, and the other ones to prepare ZnO films. Seven solutions with different Zn–Ti proportions were prepared. The following notation (ZnO)1–x + (TiO2)x, where x is the proportion of the solution used to prepare TiO2, and 1 – x is the equivalent proportion used to prepare ZnO, is employed. It was found that for x = 0.5, Eg experiences an abrupt change from 3.10 to 3.70 eV. For ZnO (x = 0) Eg is equal to 3.15 eV, and for Zn2TiO4 (x = 0.5) it is equal to 3.70 eV.

2 Experimental 2.1 The solutions The TiO2 precursor solution was prepared starting from the following reagents: For 1 mol of titanium isopropoxide (TIP), 36.46 mol of ethanol, 0.4692 mol of nitric acid, and 1 mol of de-ionized water were used. Ethanol was divided into two equal parts, one used for the TIP dissolution and the other one was mixed with both the water and the acid.

123

1128

J Mater Sci: Mater Electron (2007) 18:1127–1130

After that, both solutions were mixed at room temperature (RT) under stirring. The ZnO precursor solution was obtained following a procedure reported in detail by the authors elsewhere [8]. The reagents used were Zn(CH3COO)2•2H2O, ethylene glycol, glycerol, 1-propanol and triethylamine. Finally, both Ti and Zn solutions were mixed at RT under stirring, at different volumetric proportions in order to obtain seven Ti/Zn ratios: 0.18, 0.32, 0.50, 0.70, 1.00, 1.44 and 2.13 (x = 0.15, 0.24, 0.33, 0.41, 0.50, 0.59, and 0.68, respectively). All the solutions were transparent, free of particles, and had gelation times longer than one month.

Table 1 The concentrations of Ti, Zn and the relative concentration of Ti/Zn ratio, and the band gap for all the samples in the entire 0 £ x £ 1 range, are showed

2.2 The films

3 Results and discussion The films prepared from the ZnO(1–x) + TiO2(x) solutions were transparent to the naked eye and they were strongly adhered to the substrate, which was checked by means of the tape-method. They also had high resistance to abrasion which was proved with a diamond tip. No scratch on the surface was observed. Listed in Table 1 is the summary of the Ti and Zn concentrations in solution, and the Ti/Zn ratios and Eg values obtained for all the samples studied. EDS (Electron Dispersive Spectroscopy) measurements (Fig. 1) reveal that the Ti/Zn ratio in the film is a linear function of the Ti/Zn ratio in the precursor solution (nominal concentration), with a slope of 0.86. This means that the relative concentration of Ti with respect to Zn is 86% of the ones in the precursor solution. In this work, x will indicate the nominal concentration.

123

1– x [Zn]

x/1 – x Ti/Zn

Eg (eV)

0

1.0

0

3.15

0.15

0.85

0.18

3.20

0.24

0.76

0.32

3.23

0.33

0.67

0.50

3.22

0.41

0.59

0.70

3.10

0.50

0.50

1.00

3.70

0.59 0.68

0.41 0.32

1.44 2.13

3.40 3.30

1.0

0

-

3.10

2.0 1.6

Ti/Zn by EDS

The films were grown on glass substrates by the dipping method, 24 h after the final solution (Ti + Zn) was prepared. The withdrawal speed was 1.8 cm/min. The samples were thermally pre-heated at 100 C for 60 min and, after that, sintered at a temperature (Ts) of 550 C for 60 min. Both, pre-heating and sintering treatments were done in an open atmosphere. Afterwards, the layers were kept inside the oven until the RT was reached. All the films studied were obtained by a six-dipping procedure. The XRD patterns were registered using a Rigaku D/max-2100 dif˚ ), using a thin film fractometer (CuKa1 radiation, 1.5406 A attachment. In order to avoid the interference of the absorption in the glass substrate on the optical absorption spectra of the films, photoacoustic (PA) spectroscopy was employed to measure the optical absorption of the films. The spectra were obtained with a conventional PA spectrometer. The modulation of the incident light beam was 17 Hz. All the characterization was carried out at room temperature (RT).

x [Ti]

1.2 0.8 0.4 0.0

Ti/Zn Nominal Fig. 1 The Ti/Zn ratio measured by EDS in the films versus the nominal Ti/Zn ratio in the precursor solution

The X-ray patterns indicate that pre-heated films are amorphous. The samples become polycrystalline after an annealing in air at 550 C. Diffractograms of polycrystalline samples for the 0 < x £ 0.5 range are displayed in Fig. 2(A) and 2(B). The XRD patterns of ZnO (x = 0) and TiO2 (x = 1) [9] films, prepared by the sol gel method are included for a comparison. For high Zn concentration (x = 0.15) the films only show the phase corresponding to hexagonal ZnO (whose peaks are indicated with an asterisk in the diffractograms). For x = 0.24 and 0.33, hexagonal ZnO and cubic Zn2TiO4 [9] crystalline phases appear mixed, but for x = 0.33 the cubic phase is the dominant structure. When x = 0.41 and 0.50, practically only the ternary Zn2TiO4 is present in the XRD pattern, with no preferential orientation (powder-like type). For the highest Ti concentration (x = 0.59 and 0.68) the amorphous character predominates, no crystallization is observed. Ti/Zn ratios lower than one can favor the formation of Zn2TiO4 and the Zn2TiO4 + ZnO phases in accordance with the ZnO-TiO2 phase diagram published by Dulin and Rase [10].

J Mater Sci: Mater Electron (2007) 18:1127–1130

30

(440)

(422) (511)

(311)

20

0.9 0.6 ZnO

0.3

40

(422) (511)

50

(440)

Zn2TiO4 (400) (331)

(222)

(220)

( 1 0 3 )*

60

1.2

PAS Normalized

A(200)

0.41

(311) (400)

(220)

0.0

( 1 1 0 )*

50

0.50

Intensity (a. u.)

(440)

0.0 0.0

4.0



60

2θ (Degrees)

2θ (Degrees)

0.8

(eV 2.4 )

coefficient, which is proportional to the PAS, thus, (ahm)  (PAShm). We have assumed n = 2 as a first approximation because ZnO has a direct band gap [11]. The inset of Fig. 5 illustrates a typical (PAShm)2 versus hm plot for the Eg calculation. The linear regression shown by this inset, calculated for x = 0.50, suggests that a high probability exists that Eg in Zn2TiO4 be direct. In the case of the TiO2 an indirect energy band gap is considered [12] (n = 1/2). For amorphous films the Tauc model was employed:

3.7 3.6

2

The average grain size for ZnO and Zn2TiO4 was calculated by means of the Debye-Scherrer formula, as a function of x and is plotted in Fig. 3. The optical absorbance measured by means of the PA spectroscopy, is illustrated in Fig. 4 for all the values of x studied. The shift of the absorption edge to higher energies when x passes from 0.41 to 0.50 is clearly observed. From the photoacoustic signal (PAS) as a function of the photon energy (hm), and the relationship (ahm)n = A(Eg–hm) for the calculation of the band gap energy (Eg) (where n = 2 for direct Eg and A is a constant), Eg was determined for the crystalline films with x £ 0.5. a is the absorption

Fig. 4 The PAS optical absorbance of the films is shown as a function of the photon energy (hm) for all the values of x studied

(a. u.)

Fig. 2 The XRD patterns. (A) for x = 0.0, 0.15, 0.24, and 0.33. (B) for Zn2TiO4 powder, and for x = 0.41, 0.55 and 1.0. The asterisks point out the reflections pertaining to the hexagonal phase of ZnO. The pattern for x = 1 corresponds to TiO2 in its anatase phase

Grain Size (Å)

ZnO TiO2

200

E g (eV)

3.5

250

X

1.2

1.6

(B)

-2

(A)

0.4

TiO 2

3.2

(PAS*h ν) X 10

40

1.0

A(105) A(211)

A(101)

(422) (400) (101)*

(102)*

(002)*

(220)

(311)

30

X

0.24

0.15

(100)*

I nt e n s i t y ( a . u . ) 20

A(004)

0.33 (511)

X

1129

4 X = 0.50 Eg = 3.7 eV 3 2 1 0 1

3.4

2 3 4 Eg (eV)

3.3

150

3.2 ZnO Zn2TiO4

100 0.0

0.2

0.4

0.6

0.8

3.1

1.0

X Fig. 3 The average grain size of both components observed in the films as a function of x. For a comparison, the grain size of ZnO and TiO2 are also included

0.0

0.25

0.5

0.75

1.0

X Fig. 5 Energy band gap (Eg) values are shown for the entire interval of x, from x = 0.0 to x = 1.0. The inset shows (PAS*hm)2 as a function of hm, for the Eg calculation when x = 0.5

123

1130

(PAShm)1/2 versus hm was plotted in order to determine the Eg values. In Fig. 5, the Eg versus x graph is displayed, where the values for x = 0 (ZnO) and x = 1 (TiO2) have been included as a reference. These two samples are the same ones which were included in Fig. 2. Clearly, in Fig. 5, an abrupt change in Eg is shown for 0.41 < x < 0.50. For x £ 0.30, the experimental points resemble the Eg value of ZnO, whereas for x > 0.50 Eg tends gradually to approach the value of TiO2. The maximum value of Eg is 3.7 eV, which is reached when x = 0.5 (Ti/Zn = 1), i.e., for the Zn2TiO4. To the best of our knowledge, this is the first time that the Eg value for Zn2TiO4 has been reported. ZnO and Zn2TiO4 do not form solid solutions [10]. In solid solutions of two semiconductors the Eg of the solid changes gradually between the Eg values of both materials. The experimental points in Fig. 5 could indicate that for x £ 0.41 the band gap of ZnO dominates the optical absorption spectra of the films. For x = 0.50 it is the Zn2TiO4 which dominates the spectra. For high x values (amorphous samples) Zn2TiO4 and TiO2 probably coexist both in the amorphous phase. It has been observed that TiO2 is soluble in Zn2TiO4 up to approximately 33% in a relative molar proportion [13]. For x > 0.50, the band gap changes gradually from Eg = 3.7 (for Zn2TiO4) to Eg = 3.1 (for TiO2), probably due to an amorphous solid solution of Zn2TiO4 and TiO2.

4 Conclusions ZnO(1 – x) + TiO2(x) precursor solutions were used to prepare ZnO + Zn2TiO4 thin films by the sol–gel method within the interval 0.15 £ x £ 0.68 on glass substrates. As prepared samples are amorphous, but become crystalline after annealing them in an open atmosphere at a temperature of 550 C. The films crystallize in the

123

J Mater Sci: Mater Electron (2007) 18:1127–1130

hexagonal phase of ZnO for x = 0.15; in both the hexagonal phase of ZnO and cubic phase of Zn2TiO4 for x = 0.24 and 0.33; in the cubic phase of Zn2TiO4 for x = 0.41 and 0.5; and the films are amorphous for x > 0.5. The band gap energy (Eg) as a function of x was analyzed, and a maximum Eg = 3.7 eV was observed when x = 0.5, i.e., for Zn2TiO4 Acknowledgements This work was supported by Secretarı´a del Medio Ambiente y Recursos Naturales (SEMARNAT) and Consejo Nacional de Ciencia y Tecnologı´a (CONACYT) under project Semarnat-2002-C01-1416/A-1. The authors wish to thank C. I. Zu´n˜iga Romero and Ing. Jose´ Eleazar Urbina Alvarez for their technical assistance.

References 1. S.P. Walvekar, A.B. Halgeri. Proc. Indian Nat. Sci. Acad. 41, 117 (1975) 2. K.S. Chandra Babu, D. Singh, O.N. Srivastava, Semic. Sci. Technol. 5, 364 (1990) 3. J. Kristof, J. Liszi, P. Szabo, A. Barbieri, A. de Battisti, J. Appl. Electrochem. 23, 615 (1993) 4. V. Berbenni, A. Marini, J. Mater. Sci. 39, 5279 (2004) 5. B.L. Zhu, C.S. Xie, W.Y. Wang, K.J. Huang, J.H. Hu, Mater. Lett. 58, 624 (2004) 6. G. Marci, V. Augugliario, M.J. Lo´pez-Mun˜oz, C. Martin, L. Palmesano, V. Rives, M. Schiavello, R.J.D. Tilley, A.M. Venezia, J. Phys. Chem. B 105, 23 (1997) 7. E. Garcia, S. Cilleruelo, J.V. Ibarra, M. Pineda, J.M. Palacios, Thermochim. Acta 306, 23 (1997) 8. R. Castanedo-Pe´rez, O. Jime´nez-Sandoval, S. Jime´nez-Sandoval, J. Ma´rquez-Marı´n, A. Mendoza-Galva´n, G. Torres-Delgado, A. Maldonado-Alvarez, J. Vac Sci. Technol. A 17, 1811 (1999) 9. JCPDS Cards: ZnO, 38-1451. TiO2,73-1764. Zn2TiO4, 77-0014 10. J. Yang, J. H. Swisher, Mater. Charact. 37 (1996) 153–159. F.H. Dulin, D.E. Rase, J.Am.Ceramic Soc. 43, 125 (1960) 11. S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98, 013505–1 (2005) 12. S-D. Moand, W.Y. Ching, Phys. Rev. B. 51, 13023 (1995) 13. H.T. Kim, Y. Kim, M. Valant, D. Suvorov, J. Am. Ceram. Soc. 84, 1081 (2001)