Industrial processes must minimize heat-treatment to reduce energy consumption and CO2 emissions during the manufacture of products. In particular, fossil fuels should be conserved for the next generation. However, the use of heat-treatment is essential in the manufacturing processes of many highly functional materials.
In many cases, the materials’ functions depend on their surfaces. From this point of view, modification by thin film fabrication on various substrates, as opposed to manufacturing the entire body with the functional material, can generally save resources. The molecular precursor method (MPM) that was developed in our study is a wet chemical process for fabricating metal oxide and phosphate thin films [1-11]. This method requires heat-treatment to eliminate organic ligands from metal complexes involved in spin-coated precursor films and to fabricate thin films of crystallized metal oxides or phosphates. We emphasize the importance of heat-treatment by describing recent results obtained using the MPM, which show great potential for the development of nanoscience and nanotechnology tools and materials.
This chapter focuses on the transparent thin film fabrication of both a visible (Vis)-light-responsive anatase thin film having enhanced UV sensitivity and an unprecedented Vis-responsive rutile thin film on glass substrates. These photoreactive thin films were easily fabricated using the MPM. Heat-treatment under controlled conditions produced these attractive thin films. Thin film fabrication of a highly conductive Ag nanoparticles/titania composite and several metal oxides, including Cu2O, will be also discussed, illustrating the broad utility of the MPM and the importance of heat-treatment in this novel wet process.
2. Solution-based thin film formation
Novel thin films are an active area of research and are widely used in industry. Most of the thin films have thicknesses ranging from monolayer to nanometer levels up to several micrometers. Due to their relatively high hardness and inertness, ceramic coatings are of particular interest for the protection of substrate materials against corrosion, oxidation, and wear resistance [12-17]. The electronic and optical properties of thin films are used in many electronic and optical devices [18-20]. The wide range of materials, techniques for preparation, and range of applications make this an interdisciplinary field. Many different methods are used to fabricate thin films, including physical techniques and chemical processes. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are the two most common types of thin film formation methods. PVD methods such as thermal evaporation and sputtering involve atom by atom, molecule by molecule growth, or ion deposition on various materials in a vacuum system [21-23]. CVD and sol–gel methods are less expensive than PVD [24, 25]. The heat-treatment of thin films in these methods is generally important for the formation of crystallized metal oxides.
The sol–gel method is a versatile technology in which metal/organic polymers are used to produce ceramics and glasses [26-34]. This technology can be used to manufacture thin films in a relatively cheap way compared with PVD. In a typical sol–gel protocol (Figure 1), the process starts with a solution consisting of metal compounds, such as a metal alkoxide, acetylacetonate, carboxylate, and soluble inorganic species as the source of cations in the target oxide. Additional reactants include water as the hydrolysis agent, alcohols as the solvent, and an acid or base as a catalyst. Metal compounds undergo hydrolysis and polycondensation near room temperature, giving rise to a sol in which polymers or colloidal particles are dispersed without precipitation. Further reaction connects the fine particles, solidifying the sol into a wet gel, which still contains water and solvents. Vaporization of solvents and water produces a dry gel. Heating the gel to higher temperatures, where the organic constituents and residues are removed, gives rise to microstructures of inorganic–inorganic composites or hybrids, and glasses and ceramics. In Figure 2, the structural changes to metal oxides from the corresponding hydrolysed polymers by heat treatment of the as-deposited gel. The processes accompanied with dehydration can be categorized into two reactions; (A) the intrachain and (B) the interchain condensation.
3. Principle of MPM
The MPM is a wet process for the formation of thin films of various metal oxides, including titania or calcium phosphate compounds [1-11]. This method is based on the design of metal complexes in coating solutions with excellent stability, homogeneity, miscibility, coatability,
To the best of our knowledge, the crystallite size of the oxide particles in the resultant thin films fabricated by the MPM is generally smaller than those prepared by the conventional sol–gel method. The smaller size of the crystallites obtained using the MPM may be related to the nucleation process of the crystallized metal oxides. In the nucleation process in the sol–gel method, the polymer chains themselves are rearranged by heat-treatment (Figure 2). The polymer chains should move to produce the core structure of the metal oxide, especially during interchain condensation. In contrast, the nucleation of metal oxides occurs more easily during the MPM. When coupled with elimination of the organic ligands via heat-treatment, a vast number of crystallites can be rapidly formed. It is consequently feasible that the crystallite sizes of metal oxides fabricated using the MPM are smaller than those obtained using the sol–gel method.
4. Heat-treatment for titania thin film fabrication
4.1. Thin film fabrication and basic properties of TiO2
Titania is a popular industrial material that has been used as a white pigment for paints, cosmetics, and foodstuffs . The photoreactivity of titania is known from observations of phenomenon such as the chalking of white paints containing titania after long-term outdoor exposure. The photoreactivity of titania was first observed by Honda and Fujishima in 1967, and reported in the literature in 1972 . When the surface of a titania electrode (under an appropriate bias between a Pt counter electrode) was irradiated with light of energy shorter than its band gap, 3.0 eV, a photocurrent flowed from the Pt electrode to the titania electrode through the external circuit. The oxidation reaction occurs at the titania electrode, and the reduction reaction occurs at the Pt electrode. This observation showed that water molecules could be split into oxygen and hydrogen using UV light in a sulfuric acid electrolyte. The energy-conversion process that occurs on the titania surface is termed the Honda–Fujishima effect. When titania particles absorb UV radiation, they produce pairs of electrons and holes inside the particles. Because the photoinduced electrons and holes can be incorporated into redox reactions on the titania surface before spontaneous recombination, the surface states of the titania particle are quite important for photoreactivity [37-52].
Titania has three polymorphs: anatase, rutile, and brookite. Anatase thin film deposition on a glass substrate has been achieved using the MPM. A water-resistant coating solution was prepared by the reaction of a neutral [Ti(H2O)(EDTA)] complex (EDTA is ethylenediamine-
5. Vis-responsive anatase thin film fabricated using the MPM
Many researchers study the fabrication and photoreactivity of Vis-responsive thin films by physical and/or chemical modification of anatase films because of the importance of Vis-photoreactive materials [53-61]. However, there is little information on the enhancement of UV sensitivity of Vis-responsive anatase films.
The implantation of various transition-metal ions such as V5+, Cr3+, and Cu2+ into the lattice of Ti4+ in anatase thin films was investigated by Anpo
The MPM forms transparent titania thin films using the ethanol solution obtained as a coating solution (SED) by the reaction of alkylamine with a titanium complex of EDTA as the ligand [1, 2, 4-9]. According to single-crystal structural analysis, this Ti complex contains Ti–N bonds (Figure 3). If the Ti–N bonds in this complex can be preserved in the anatase thin film obtained by heat-treatment after coating, a partially nitrided anatase film can be directly formed. However, XRD and X-ray photoelectron spectroscopy (XPS) confirmed that the precursor film formed on a glass substrate using the ethanol solution is transformed into a nitrogen-free anatase thin film through heat-treatment for 30 min in air at a temperature of 450C or higher. Based on these results, the heat-treatment of molecular precursor films spin-coated with SED on ITO glass substrates was examined in an Ar gas flow of 0.1 L min−1 at 500C for 30 min. The XRD pattern indicated that the spin-coated precursor film crystallized to anatase through heat-treatment at a temperature of 500C or higher under atmospheric conditions. Thus, the anatase form was created even if oxygen was not supplied externally to remove organic residues in the metal complex. Furthermore, the chemical bond toward the Ti4+ ion from both oxygen and nitrogen atoms can be observed in the XPS spectra of the resultant thin films. The binding energies of Ti 2p3/2 attributed to Ti–O and Ti–N had typical values of 459.1 and 455.3 eV, respectively (Figure 4) [5, 70-72]. Importantly, the binding energy of N 1s was 396.7 eV, and the existing nitrogen was only in the oxygen-substituted form, not in the chemisorbed form [73, 74]. Thus, the heat-treatment of the precursor films in an Ar gas flow was effective in preserving nitrogen atoms in the complex. However, the photoreactivity of the thin film was not observed after Vis irradiation with a weak fluorescent lamp. Because the partially nitrided film obtained by the MPM alone did not respond to irradiation with a fluorescent lamp, a precursor solution, SOX, with no Ti–N bonds, in complexes whose ligand was oxalic acid (OX), was freshly prepared and the layering of the anatase films was achieved using the two precursor solutions.
Two types of three-layer film,
The XRD patterns of both films indicated that anatase was formed. The field-emission scanning electron microscopy (FE-SEM) data of both films show even surfaces without cracks or pinholes. These surfaces were too smooth to detect the roughness by measuring with a stylus profilometer, whose detective limit is ca. 10 nm. The XPS depth profiles of these films are shown in Figure 7. The depth profile for
The photoreactivities of the films were tested using the decolorizing reaction of methylene blue (MB) aqueous solution [5-8, 75-77]. The decoloration rate of 0.01 mol L–1 MB solution by the photoreaction with both multilayered thin films under UV or Vis irradiation are summarized in Table 1. The
The Vis-responsive property of the
|under UV irradiation||under VIS irradiation|
6. Formation of O-deficient anatase thin film
To clarify the factors for designing an anatase thin film with a higher photoreactivity under UV irradiation, the relationship between the photoreactivity and O deficiency of anatase thin films fabricated with the heat-treated precursor films under regulated conditions was examined. Thin films were formed by heat-treating precursor films spin-coated onto FTO glass substrates with SED and SSG (sol–gel solution) under Ar or air.
The spin-coating method at ambient temperature was used for forming precursor films using a double-step method. The first step used 500 rpm for 5 s and the next step was 2000 rpm for 30 s, in all cases. The precursor films were pre-heated in a drying oven at 70 °C for 10 min and then heat-treated at 500°C for 30 min in a 0.1 L min–1 Ar gas flow. A tubular furnace made of quartz was employed for the heat-treatment. Thin films, ED and SG, were formed by applying the precursor solutions SED and SSG, respectively, before annealing in air. The film EDair was fabricated by firing the precursor film spin-coated with SED in air at 500°C for 30 min.
When the concentration of titanium was 0.4 mmol g–1 for SED, the film thickness was 100 nm. An SSG of 0.5 mmol g–1 was stirred for 3 days at ambient temperature to fabricate an anatase film of thickness 100 nm. The post-annealing treatment for the ED, EDair, and SG thin films was carried out in air at 500C for 5, 10, 15, 20, and 30 min. The number in the notation used for post-annealed films indicates the annealing time (min). For example, ED-PA5 indicates an ED film post-annealed for 5 min. The photoreactivities of the thin films are presented in Table 2. Each value was calculated as the difference between the decoloration rate under UV-light irradiation and the corresponding value measured for each thin film in the dark. The maximum photoreactivity of ED-PA15 produced by the MPM is twice that of SG-PA10 prepared by a conventional sol–gel procedure.
|Annealing Time (min)|
It is generally accepted that the main factors to consider when designing enhanced photoreactivity of anatase are (1) higher crystallinity, (2) larger surface area, and (3) decreased impurities. The crystallite size is an indicator of crystallinity [78, 79]. Among the crystallite sizes of the three anatase thin films, ED, EDair and SG, the SG thin film had the largest value and the ED film had the smallest (Table 3). These values for the anatase crystallites in EDair and SG thin films were not affected by post-annealing treatment in air. The thin film ED-PA15 (whose crystallite size was the smallest) showed the highest photoreactivity in the decoloration of an MB aqueous solution among the various thin films formed in this study. The specific surface areas of the thin films were not measured quantitatively because of the difficulties involved. However, the degrees of adsorption of MB molecules in aqueous solution were nearly equal among the thin films, including those formed by the sol–gel method. Therefore, the differences in the photoreactivity among these thin films should be due to other factors than the specific surface area. The XPS spectra suggested that the thin films SG and SG-PA
A coordination skeleton of (TiO4N2) or (TiO5N2) can be assumed in the EDTA complex as a precursor molecule from the structural study of a Ti complex [Ti(H2O)(EDTA)] 1.5H2O reported by Fackler
7. O deficiency in rutile thin film
Rutile is the most stable crystal form of titania. Since Nishimoto
Because the band edge of a rutile single-crystal is 3.0 eV, rutile has the potential to respond to Vis light. Using this knowledge and the results of previous experiments on anatase responses to Vis light, this section describes an attempt to achieve direct fabrication of O-deficient rutile thin films with high photoreactivity using a MPM. The first Vis-light-responsive thin film created from O-deficient rutile is discussed here. This material works without application of an electric potential, due to its unprecedentedly high photosensitivity under UV-light irradiation. The present findings should facilitate widespread practical use of rutile in light-related applications.
The thin films were formed by heat-treating the precursor films after spin-coating onto a quartz glass substrate. SED and SSG were applied in an Ar gas flow. The transparent precursor films formed by spin-coating the solutions and pre-heating in a drying oven at 70 °C for 10 min were heat-treated at 700 °C for 30 min in a furnace made from a quartz tube with an Ar gas flow rate of 0.1 L min–1. When SED was used, a transparent rutile thin film R was formed. When SSG was used, a transparent anatase thin film A was formed. The film thickness was 100 nm in both cases.
Each structure was characterized using XRD, Raman spectroscopy, and transmission electron microscopy (TEM). The selectivity was due to the O-vacant sites in the oxide thin films formed at different levels due to the differences between the amounts of oxygen in the two precursors. In this case, the oxygen source required to structure titania was available only in the precursor films when these thin films were fabricated. Therefore, crystallization into rutile, which has many O-vacant sites, and the accompanying rapid elimination of organic residues from the R precursor film, occurred because of the heat-treatment.
In contrast, the amount of oxygen available to Ti4+ in titanoxane polymers, though significant, was insufficient to develop stoichiometric TiO2 from A. The oxygen defects in an anatase lattice generally lower the temperature of the phase transformation from anatase to rutile [84, 85]. Thus, selective formation occurred according to the differing degrees of O deficiency.
The photoreactivities of the thin films were evaluated by the decoloration rates of MB solutions, which served as a model for organic pollutants in water. The results measured under Vis- and UV-light irradiation are summarized in Table 4, along with those measured under dark conditions (reference values). The data show the effects of adsorption on the samples, vessels, and self-decoloration of MB under each condition. Moreover, the photoreactivity of R was extremely high under UV irradiation and higher than the photoreactivity of A. This is without precedent.
|under visible light||under UV light|
The photosensitivities of R and A were also examined by measuring the effects of Vis and UV irradiation on the water contact angle for the surfaces of the thin films. The results are shown in Figure 9 . The rutile thin film R exhibited Vis-light-induced hydrophilicity with a fluorescent light, even though high-energy light below 400 nm was eliminated. In contrast, Vis light alone did not reduce the contact angle on A under the same conditions. Furthermore, a rapid decrease in the water contact angle for R was observed with weak UV-light irradiation. The super-hydrophilic property of R appeared after only 1 h. When fluorescent light with a UV component was employed, the contact angle on R rapidly reduced and the values reached 38 and 10 after irradiation for 1 and 24 h respectively.
It is noteworthy that the simple fabrication of a Vis-responsive rutile film with high photoreactivity could be attained. Thus, the O defects in titania are also effective at providing photoreactivity of rutile, which is usually insensitive to both UV and Vis light.
8. Crystal orientation and photoluminescence of rutile in thin film
PL emission has been widely used to investigate the efficiency of charge carrier trapping, migration, and transfer, and to understand the fate of electron–hole pairs in semiconductor particles . It is therefore helpful to examine the position and intensity of the PL bands of semiconductor particles to understand the photoreactivities of the particles . In this section, we report the changes in the PL and photoreactive properties of the Vis-responsive rutile thin film fabricated by the MPM, which are effected by annealing in air at 700C. The relationship between O deficiency and PL emission was examined to understand the incredibly high photoreactivity of the rutile thin film. Furthermore, the level of crystal orientation of the rutile thin film was quantitatively evaluated on the basis of data from XRD analyses. The amount of oxygen supplied during the annealing process was analyzed by XPS measurements. The growth of crystals and particles was also investigated by crystallite-size measurements and SEM observations. The heat-treatment of the fabricated O-deficient rutile R thin film was carried out in air at 700C for 15, 30, and 60 min. The number in the notation of the post-annealed films indicates the annealing time (min). For example, R-PA15 indicates that post-annealing treatment of the R thin film was carried out for 15 min. The extent orientation factor (
The extent orientation could be estimated from the XRD peak intensity by using the Lotgering method . The terms
In this study, each
The definitions of the terms
The Lotgering orientation factor
After 15 min of heat-treatment in air, the crystallite size of the R thin film increased, while those of the R-PA
Previously, the peak position of the PL emission band obtained for rutile crystals was observed at ca. 450 nm . However, PL emission bands of the R and R-PA
For the O-deficient rutile thin film R with high photoreactivity, no PL emission was observed in the range 190–850 nm. Thus, as previously suggested, the O-defect sites on the rutile thin film may suppress recombination of the photoinduced electron–hole pairs by electron trapping. In contrast, the PL emission from rutile thin films after annealing in air may be due to the oxide ions that are supplied to the O-defect sites on the film surface, because they function as recombination centers. As a result, the lattice oxygens of titania, especially in rutile thin films, function as recombination centers for the photoinduced electron–hole pairs.
The reduction of rutile surfaces by heated hydrogen activates the photoreactivities of these surfaces . 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
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 . 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 .
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 25C 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 (
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 600C 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
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 forming well-dispersed Ag nanoparticles in the titania matrix. This present Ag-nanoparticle/titania composite thin film is useful for fabrication of highly conductive electrodes for devices such as solar cells.
The importance of heat-treatment in the MPM was demonstrated through fabrication of thin films of anatase and rutile with unprecedentedly high photoreactivities. This is due to a photoreactive mechanism via O deficiency in the oxide thin films. Based on the excellent miscibilities of the molecular precursors in the SrTiO3 thin film fabrication, heat-treatment was shown to be an essential step. It eliminates organic ligands in the precursor metal complexes and provides important functions to the metal oxides in the chemical fabrication of Ag-nanoparticle/titania composite thin films with high conductivity.
The chemical fabrication of the first p-type Cu2O transparent thin film was also recently achieved using the MPM, although the heat-treatment of the spin-coated substrates resulted in the deposition of a large amount of powder on the substrates in previous sol–gel studies . The electrical and optical properties of the Cu2O thin film fabricated using the MPM were consistent with those of similar thin films fabricated using physical procedures. It is very interesting that the charge on copper changes stepwise from +2 to +1 through 0 during heat-treatment of the precursor film involving a Cu–EDTA complex in an Ar gas flow. Based on these results, a transparent dry-type solar cell of area 20 × 20 mm2 with a combination of Vis-responsive anatase thin films was examined. This film is mentioned in the section Vis-responsive anatase thin film fabricated using the MPM. The structure of the solar cell was FTO electrode/n-Vis-responsive anatase/p-Cu2O/conductive polymer/Ag on a glass substrate, and the photovoltaic nature of the solar cell could be successfully measured under the light from a solar simulator. Thus, the present MPM is useful for fabricating Vis-responsive dry solar cells. The MPM coupled with heat-treatment of various precursor films allows transparent thin films of metal oxides such as Co3O4, ZnO, Ga2O3, and ZrO2