A Novel Oxidation-Reduction Route for Layer-by-Layer Synthesis of Nanolayers and Investigation of Its Photocatalytical Properties
Layer-by-layer (LbL) synthesis of titanium dioxide was performed by an oxidation-reduction route using a Ti(OH)3 colloid and NaNO2 solutions. A model of chemical reactions was proposed based on the results of an investigation of synthesized nanolayers by scanning electron microscopy, electron microprobe analysis and X-ray photoelectron spectroscopy, and studying colloidal solution of Ti(OH)3 with laser Doppler microelectrophoresis. At each cycle, negatively charged colloidal particles of adsorbed onto the surface of substrate. During the next stage of treatment in NaNO2 solution, the particles were oxidized to Ti(OH)4. Photocatalytic activity was studied by following decomposition of methylene blue (MB) under UV irradiation. Sensitivity of the measurements was increased using a diffuse transmittance (DT) method. The investigation revealed strong photocatalytical properties of the synthesized layers, caused by their high area per unit volume and uniform globular structure.
Titanium dioxide is one of the most perspective photoactive materials. It is intensively used in the development and for manufacturing of photocatalysts, superhydrophilic and self-cleaning surfaces, solar elements, and so forth. Research in the area of thin layered structures opens vast opportunities for creation of new materials with unique properties. A number of techniques are used for this purpose, for example, Langmuir-Blodgett , sol-gel method [2–5], dip coating , liquid-phase deposition , chemical bath deposition , CVD , spray pyrolysis deposition , and magnetron sputtering .
Special attention is paid to the layer-by-layer (LbL) synthesis. Its main advantage is the possibility of obtaining layers on the surface of arbitrarily complex shapes with precisely specified composition and thickness. Dozens of articles are devoted to the synthesis of solid thin films of titanium dioxide by LbL method. These compositions are commonly obtained using easily hydrolyzed organic containing compositions of titanium (IV). For example, titanium isopropoxide [12, 13], salts of titanium (IV) (NH4)2TiO(C2O4)2H2O , and titanium(IV) bis(ammonium lactato) dihydroxide .
Titanium dioxide layers can be synthesized LbL using an oxidation-reduction route, which was proposed in our earlier work on the synthesis of nanolayers of hydrated oxides of Sn4+ [16, 17], Mn4+ , Fe3+ , and Ce4+ . In these papers, the LbL method was named the Successive Ionic Layer Deposition (SILD). The synthesis consists of sequential treatment of substrate in the solution of precursor in the lowest oxidation number, followed by removal of excess reagent with water. At the second stage, the sample is immersed in the solution of oxidizer and the excess is again removed. A similar approach was subsequently applied to the synthesis of layers of titanium dioxide (IV) colloidal solution from Ti(OH)3 [21, 22]. In this case, the method was named Successive Ionic Layer Adsorption and Reaction (SILAR) and Ti(OH)3 colloid was obtained by partial hydrolysis of TiCl3. At every cycle, after titanium particles adsorption, the substrate was immersed in NaOH or NH4OH solution, pH = 11. Under these conditions, oxidation of Ti3+ into Ti4+ by atmospheric oxygen takes place. Therefore, nanoparticles of (TiOH)4 appear on the surface of the substrate and convert into TinH2O after drying.
The above method is limited by the necessity to use alkali solution. This not only decreases variety of substrates that can be used for TiO2 deposition but also negates the possibility of obtaining layers constructed from minimum sized nanoparticles; dissolution-precipitation of the smallest particles into larger ones takes place within at high pH.
The aim of the research reported on here was to develop an LbL method for synthesis of photoactive layers of TiO2 using readily accessible low-cost reagents, at a pH close to neutral.
2. Experimental Section
2.1. Preparation of the Reagents
A colloidal solution of Ti(OH)3 was obtained by partial hydrolysis of 0.02 M Ti2(SO4)3 solution at pH 3.2. Adjustment of the pH of Ti2(SO4)3 solution was achieved by addition of 5% NH4OH solution. A solution of NaNO2 (C = 0.01 M, pH = 7.6) was prepared by dissolving dry salt in double-distilled water. All reagents were obtained from Vekton LLC. The substrates used are polished single crystalline silicon wafers with orientation and resistance of 30–40 Ohm mm and grinded quartz KU type mm.
2.2. Synthesis of the Layers
Computation of chemical equilibrium in 25°С solution was performed by Hydra-Medusa  software. It was found that, within current pH = 3.2, titanium dioxide existed in the form of colloidal particles of Ti(OH)3, which adsorb onto the surface of the substrate. On the second step, after washout of excess of deposited reagent, these particles transform into Ti(OH)4 during immersing in NaNO2 solution (oxidizer). Double-distilled water was used to flush excess of every reagent from the surface of substrate. Time of treatment in solutions of reagents was 30 seconds and in washout water 5 seconds. One cycle of the synthesis was formed by sequential treatment of the substrate in solutions of Ti(III), washout water, NaNO2 solution and washout water.
2.3. Surface and Structural Analysis
Synthesized layers were characterized with scanning electron microscopy (SEM). The composition was determined by electron probe microanalysis (EPMA) and XPS. These studies were made on a Zeiss EVO 40EP scanning electron microscope (Carl Zeiss Microscopy) with a LaB6 cathode, equipped with an energy dispersive microanalyzer (Oxford INCA 350) and a Si(Li) detector of 30 mm2. Accelerating voltage was 20 kV. XPS spectra were recorded on a photoelectron spectrometer (SPECS). Photoelectrons were excited by X-rays of the MgKα line. Radiograms of synthesized layers were made on an X-ray diffractometer DRON-3.0 (NPO Burevestnik, USSR) for CuKα emission. Electrophoretic mobility of colloidal particles was measured on a Zetasizer Nano analyzer (Malvern Instruments Ltd.) using M3-PALS technology.
2.4. Photocatalytical Properties Investigation
Photocatalytical properties of titanium dioxide layers synthesized on the surface of grinded quartz were studied applying standard method , which is based on analysis of degradation under UV irradiation of methylene blue adsorbed on the substrate. General characteristics of MB are presented in Table 1. Sample of quartz was immersed in 50 mg/L solution during 20 minutes to obtain a layer of adsorbed dye on the surface [25, 26]. Descent of value of optical density at maximum was used for calculating degree of MB decomposition and photocatalytic activity of synthesized layers of titanium dioxide on quartz surface accordingly.
To increase the value of intensity of adsorption bands in visible range spectra, a diffuse transmittance method (DT)  was applied. The samples of grinded quartz were placed in front of aluminum (Al) mirror for obtaining spectra of diffuse scattering (DS). In this set-up, the beam twice penetrates the plate, which is covered with titanium dioxide layers on both sides (Figure 1). The beam firstly penetrates the sample without scattering (minorly informative), then is reflected back by the Al mirror to the light receiver aperture (majorly informative). This latter diffusely scattered part of beam is reflected from the inner surface of the integrating sphere to the detector. Thus, the beam goes through the sample twice and increases bands’ intensity. Application of this method provides the increasing sensitivity of this Application of this method increases sensitivity of this spectrometry. It provides possibility to carry out investigations not only for the dispersed but also for bulk substances with a minimized thickness of photocatalytical layer All DT spectra were registered by Lambda-9 spectrophotometer (Perkin-Elmer).
Before investigation of photocatalytical activity, the sample was preheated in the open air for 1 hour at 500°С to remove molecules of water.
3. Results and Discussions
Analysis of solubility and hydrolysis of titanium salts with an oxidation number 3+ and an oxidation number 4+ showed (Figure 2) that the best reagent for synthesis of layers of TinH2O was a solution of Ti3+ salt. In this solution, the formation of titanium hydroxide proceeds under higher pH. The experimental results showed that stable colloidal solutions can be prepared at a pH of about 3.0. Ti2(SO4)3 was chosen for experiments because of its low cost and prevalence.
The thin film visually appeared on the surface of silicone substrate after multiple sequential treatment in Ti2(SO4)3 and NaNO2 solutions by the LbL technique. It became light blue after approximately 15 cycles.
From the micrographs obtained by SEM (Figure 3), it was evident that the synthesized layers consisted of globules of 10–20 nm diameters. Based on the energy-dispersive X-ray spectrum (Figure 4) and XPS data (Figures 5(a) and 5(b)), the thin film was composed of atoms of titanium and oxygen. The content of Na, S, and N atoms did not exceed 1–3%, which lies within the range of accuracy for the XPS method. The position of the characteristic band of Ti 2p electrons was at 459.0 eV, which indicates the oxidation number of Ti4+ in TiO2 . Based on the analysis by X-ray diffraction, the obtained layers were amorphous (X-ray graphs are not shown in the figures).
Another important result which explains details of the chemical reactions on the surface during the synthesis is the value of -potential of the Ti(OH)3 particles in solution. It was calculated by the Smoluchowski equation based on the electrophoretic mobility data. According to the measurements, the colloidal particles in the solution possessed a net negative charge and -potential of −4.21 mV. This can be explained by adsorption of the anions on the surface. Based on this result and the other experimental data, the following scheme for sequence of chemical reactions on the surface (Figure 6) was proposed.
Thus, at the first cycle of LbL, the substrate is immersed in colloidal solution [Ti(OH)3]H. These particles of titanium hydroxide (III) adsorb on silica surface which is literally zero charged at pH 3.2. At the next step, the excess of adsorbed on the substrate anions is removed by washing in distilled water. During subsequent treatment of the sample in NaNO2 solution, Ti3+ ions are oxidized to Ti4+ forming Ti(OH)4 nanoparticles layers. After removal of excess of oxidizing agent, the substrate is treated in Ti(OH)3 colloid. Hereby, the next cycle of LbL synthesis starts. It is to be noted that, at a solution pH 3.2, Ti(OH)4 does not dissolve. The charge of Ti(OH)4 nanoparticles layers, obtained after the first LbL cycle, is positive due to adsorption of the protons on the surface of substrate in acidic conditions. This facilitates adsorption of negatively charged [Ti(OH)3]H and causes formation of Ti(OH)4 layers. After drying in the air, the sample transforms into the layers of hydrated TinH2O. This layer does not dissolve in Ti(OH)3 colloid on the second and following steps of synthesis. Thus, irreversible conditions of synthesis were achieved.
Unfortunately, determination of the size of colloidal particles in Ti(OH)3 solution was not possible due to the effect of fluorescence of the solution during study by dynamic light scattering (DLS).
It should be noted that N anions can, possibly, adsorb onto the surface of Ti(OH)4 nanoparticles during immersion of sample in NaNO2 solution. They can partially transfer into Ti(OH)3 solution and, therefore, participate in reaction of oxidation of Ti3+ to Ti4+. Oxidation of colloidal particles of Ti(OH)3 is also possible by reaction with atmospheric oxygen. However, we believe that, during synthesis, the reaction of oxidation by atmospheric oxygen does not play a role, since control experiments of the synthesis of thin layers without treatment in NaNO2 solution revealed no reproducible growth of the layer thickness.
Results of the study of photocatalytic activity of synthesized layers are shown in Figures 7 and 8. Sample synthesis involved 20 cycles. After irradiation with UV light (Hg lamp), the intensity of the characteristic band of MB at 650 nm decreased in the spectrum of DT.
It follows from the results presented in Figure 8 that, after 20 minutes of exposure, the intensity of the band decreases by 80%. Similar results  have been reported for thin-layer structures of TiO2 and SiO2 obtained by the LbL technique. An equal decomposition ratio was achieved for thicker structures and longer irradiation time: 120 layers and 60 min correspondingly. Apparently, the effect achieved in our work is caused by higher specific surface area and uniform globular morphology of the layers, which consist of nanoparticles of 10–20 nm diameters.
LbL synthesis by an oxidation-reduction method, using a Ti(OH)3 colloid and NaNO2 solution on the surface of quartz substrate, resulted in formation of layers of nanostructured titanium dioxide(IV). It had amorphous structure with nanoparticles of approximately 10–20 nm diameters. Formation of these layers at each LbL cycle is originated by successive reactions of adsorption of colloidal particles of Ti(OH)3H and their subsequent oxidation in NaNO2 solution.
The method of diffuse transmittance in the Vis region of the spectrum can be effectively used to estimate photocatalytical activity of nanolayered catalysts deposed on the surface of bulk substrates; results are obtained by following photodegradation of MB dye.
Application of this method in current research has revealed relatively high photocatalytical activity of the layers of TiO2 synthesized by LbL technique. This can be explained by a high surface area of the layers and their uniform globular structure.
|DLS:||Diffuse light scattering.|
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by SPbSU Grant (no. 12.38.259.2014).
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