**9. Highly conductive Ag-nanoparticle/titania composite thin films**

An excellent perovskite-type SrTiO3 thin film was fabricated using a mixed precursor solution from a titania precursor solution containing a Ti complex of EDTA and an SrO precursor solution containing a Sr complex of EDTA [4]. The metal complex ions dissolve independently in each precursor solution and the homogeneity of the mixed solution can be kept at the molecular level. In fact, a mixed precursor solution containing exact amounts of Ti and Sr can be easily prepared due to the excellent miscibility of the solutions. This is the essential difference between the MPM and conventional sol–gel methods in which the hydrolyzed polymers are heterogeneous because of the different rates of hydrolysis of each metal ion. On the basis of this excellent miscibility in the MPM, Ag-nanoparticle/titania (Ag-NP/TiO2) composite thin films with a wide range of volumetric fractions of Ag in the titania matrix were developed using the titania precursor **SED** [9].

Many researchers have tried to incorporate metal nanoparticles into semiconductor materials to improve the conductivity of the semiconductor. The TiO2 film's relatively high resistivity of 1012 Ω cm at 25°C can be reduced by incorporating metal nanoparticles into the TiO2 matrix. Electrically conducting particles can be randomly distributed within a semiconductor matrix to form a composite. This composite sample is non-conducting until the volume fraction of the conducting phase reaches the so-called percolation threshold. It has been experimentally and analytically shown that in a conductor/semiconductor composite with the conductor at or above a given volume fraction (*φi*), a network of conducting particles is established and thus the composite resistivity suddenly decreases. The most widely applied technique for the preparation of metal/TiO2 composite materials is the sol–gel process. This has been applied to the fabrication of an Ag-NP/TiO2 system by Li *et al*. [91]. They prepared a solution for fabricating Ag-NP/TiO2 composite thin films by mixing a sol–gel solution of titania for thin film fabrication with an 18 mol% silver nitrate solution; a 6 mol% ethanol solution of silver nitrate was also employed. The electrical resistivity of the resultant composite thin film with the highest concentration (18 mol%) of Ag nanoparticles was of the order of 103 Ω cm. Because the sol–gel method used in these studies involves metalloxane polymer formation in the medium, and because poor miscibility of each component is inevitable, Li *et al*. reported that it was difficult to obtain a homogeneous solution for silver concentrations above 18 mol%. Therefore, a lower electrical resistivity of the composite thin film may be attained in the event that a solution with an even higher volumetric fraction of Ag nanoparticles can be homogeneously mixed with the titania precursor solution. The electrical conductivity of the resultant films is largely dependent on the volumetric fraction, size, and connectivity of the Ag nanoparticles, and the homogeneity of the dispersed silver in the dielectric titania matrix [92-95].

314 Heat Treatment – Conventional and Novel Applications

The reduction of rutile surfaces by heated hydrogen activates the photoreactivities of these surfaces [36]. As a result, the formation of an oxygen deficiency provides photoreactivity. The present study revealed that the unprecedentedly high photoreactivity of the **R** thin film is suppressed by oxygen supply during the annealing process. However, the high levels of photoreactivity of these films could be maintained even after oxygen supply to the surface by the post-annealing treatment (Table 5). These results indicate that the enhanced photoreactivity is related not only to the surface but also to the inner part of the thin films as a result of an interparticle electron transfer (IPET) effect, as proposed in our previous paper.

**9. Highly conductive Ag-nanoparticle/titania composite thin films** 

matrix were developed using the titania precursor **SED** [9].

An excellent perovskite-type SrTiO3 thin film was fabricated using a mixed precursor solution from a titania precursor solution containing a Ti complex of EDTA and an SrO precursor solution containing a Sr complex of EDTA [4]. The metal complex ions dissolve independently in each precursor solution and the homogeneity of the mixed solution can be kept at the molecular level. In fact, a mixed precursor solution containing exact amounts of Ti and Sr can be easily prepared due to the excellent miscibility of the solutions. This is the essential difference between the MPM and conventional sol–gel methods in which the hydrolyzed polymers are heterogeneous because of the different rates of hydrolysis of each metal ion. On the basis of this excellent miscibility in the MPM, Ag-nanoparticle/titania (Ag-NP/TiO2) composite thin films with a wide range of volumetric fractions of Ag in the titania

Many researchers have tried to incorporate metal nanoparticles into semiconductor materials to improve the conductivity of the semiconductor. The TiO2 film's relatively high resistivity of 1012 Ω cm at 25°C can be reduced by incorporating metal nanoparticles into the TiO2 matrix. Electrically conducting particles can be randomly distributed within a semiconductor matrix to form a composite. This composite sample is non-conducting until the volume fraction of the conducting phase reaches the so-called percolation threshold. It has been experimentally and analytically shown that in a conductor/semiconductor composite with the conductor at or above a given volume fraction (*φi*), a network of conducting particles is established and thus the composite resistivity suddenly decreases. The most widely applied technique for the preparation of metal/TiO2 composite materials is the sol–gel process. This has been applied to the fabrication of an Ag-NP/TiO2 system by Li *et al*. [91]. They prepared a solution for fabricating Ag-NP/TiO2 composite thin films by mixing a sol–gel solution of titania for thin film fabrication with an 18 mol% silver nitrate solution; a 6 mol% ethanol solution of silver nitrate was also employed. The electrical resistivity of the resultant composite thin film with the highest concentration (18 mol%) of Ag nanoparticles was of the order of 103 Ω cm. Because the sol–gel method used in these studies involves metalloxane polymer formation in the medium, and because poor miscibility of each component is inevitable, Li *et al*. reported that it was difficult to obtain a homogeneous solution for silver concentrations above 18 mol%. Therefore, a lower electrical resistivity of the composite thin film may be attained in the event that a solution with an Using the MPM, mixed precursor solutions for fabricating Ag-NP/TiO2 composite thin films could be easily prepared. As a result, Ag-NP/TiO2 composite thin films of Ag volumetric fractions from 0.03 to 0.68 were fabricated with heat-treatment of the mixed precursor films at 600°C in air. To obtain quantitative information about the effects of Ag nanoparticles on the electrical properties, the nanostructures of the films were examined by TEM. The TEM images films with *φ*Ag of 0.26, 0.30, and 0.55 are shown in Figures 12 (A), (B), and (C) respectively [9]. The presence and distribution of Ag nanoparticles (black dots) inside the TiO2 film can be clearly seen. The percolation threshold of Ag nanoparticles in the titania thin film was found to be *φ*Ag 0.30. It is near the percolation threshold, when Ag particles are still not totally connected, that increasing the *φ*Ag by adding a small amount of Ag nanoparticles helps to build the conductive network and reduce the resistivity of the composite. Therefore, the decrease in resistivity was attributed to a change in the Ag nanoparticles` size, shape, and center-to-center distance between the Ag nanoparticles. As the Ag volumetric fraction increased further from 0.27 to 0.55, the electrical resistivity decreased from 10−2 to 10−<sup>5</sup> Ω cm, respectively. At *φ*Ag 0.61 to 0.68, the resistivity increased from 10−5 to 10−<sup>3</sup> Ω cm due to the inevitable increase in resistivity caused by agglomeration of the Ag particles. This study shows that the MPM, which offers excellent miscibility of the silver and titania precursor solutions, is effective at overcoming the miscibility limitations of the conventional sol–gel method and is necessary for fabricating composite thin films with large *φ*Ag values.

**Figure 12.** TEM images of the Ag-NP/TiO2 composite thin films at Ag volumetric fractions, φAg, of (A) 0.26, (B) 0.30, and (C) 0.55, respectively [9].

The excellent miscibilities of the precursor complexes in the MPM overcame the limitations of the extremely low Ag volumetric fraction in the previous sol–gel process. Therefore, the percolation threshold for the electrical resistivity of the composite film could be examined for a wide range of Ag fractions. Heat-treatment plays an important role in the production of Ag nanoparticles by reducing Ag+ ions in the precursor film and

#### 316 Heat Treatment – Conventional and Novel Applications

forming well-dispersed Ag nanoparticles in the titania matrix. This present Agnanoparticle/titania composite thin film is useful for fabrication of highly conductive electrodes for devices such as solar cells.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 317

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