Rapid Deposition of Photocatalytically Enhanced TiO2 Film by Atmospheric SPPS Using Ar/N2-Vortex Plasma Jet

Dickson Kindole; Ifeanacho Anyadiegwu; Yasutaka Ando; Yoshimasa Noda; Hideya Nishiyama; Satoshi Uehara; Tomoki Nakajima; Oleg P. Solonenko; A.V. Smirnov; A.A. Golovin

doi:10.2320/matertrans.T-M2017853

Abstract

In this study, as cost effective and an environmentally friendly film deposition technology, Atmospheric Solution Precursor Plasma Spray (ASPPS) was utilized for the deposition of the photo-catalytic titanium oxide (TiO₂) film for the fabrication process of photovoltaic devices for rural electrification in developing countries. In addition, ethanol-diluted titanium tetra-iso-butoxide (TTIB: Ti(OC₄H₉)₄) was used as a feedstock. N₂-dominant Ar/N₂ was also utilized as plasma working gas as well as for the elevation of the thermal plasma energy. By controlling deposition distances and temperature, using vortex generation anode nozzle operated at 1 kW, photo-catalytic TiO₂ film was deposited and its crystallinity was confirmed by X-ray diffraction. Besides, the photo-catalytic properties of the film were confirmed by the methylene blue decolorization and the surface wettability tests. Surface morphologies of the TiO₂ film was evaluated using optical micrographs. Furthermore, the film thickness and strength were measured using micro screw gauge and pencil scratch tester respectively. Lastly, when this Photo-catalytic TiO₂ film was applied to photovoltaic devices, the device generated an open circuit voltage of 146.7 mV with solar irradiance intensity of 574 W/m². From these results it was confirmed that, the ASPPS technology equipped with high cooling efficiency vortex anode nozzle is available for deposition of TiO₂ film for the fabrication process of low-cost photovoltaic devices for rural areas in developing countries.

1. Introduction

Recently, renewable energy technologies have become important alternative resources from the dependence of non-renewable energy technologies. This is due to rapid population growth and the rising demand for the energy worldwide mainly in developing countries. Fossil fuel is still preferred owing to its low price and ease in their usage^1–3). However, since the fossil fuel reserves are rapidly decreasing, alternative energy resources which are environmental friendly with low-cost is tremendously recommended. Large population, particularly in developing countries live in rural areas far away from the national grid^4–6). Most of the rural areas and non-electrified sites, extension of utility grid lines experiences a number of problems such as high capital investment, low load factor, and frequent power supply interruptions. Moreover, there is a greater transmission line losses and poor reliability of electrical supply. As an alternative method to address this energy demand, renewable energy technologies such as small-scale wind energy, bio-energy and solar energy have been investigated intensively owing to their eco-friendliness and abundant availabilities evenly in rural areas. In addition, solar energy technology has been among of the environmental friendly utilized to harness the sun's energy into direct usable energy^7–10).

Photovoltaic (PV), commonly known as solar cell, has been one of the inexpensive means for converting sunlight into useful energy. Although sunlight can last up to roughly six hours in many areas, solar charged battery systems have been frequently used to provide electric power for a complete twenty-four hours in a day¹¹⁾. The electricity generated from this PV can be utilized directly to power household utilities and can be stored in the battery for later usage. As for the semiconductor materials for the fabrication of this cells, silicon(Si) and Cadmium Telluride (CdTe)-based PV solar cells have been mass-produced and utilized in many applications^12,13). However, these PV solar cells have several inevitable drawbacks such as high production cost, power efficiency limit and their production process are associated with unfriendly environmental activities.

Recently, as an alternative to the expensive silicon based photovoltaic devices, TiO₂-based photovoltaic devices have been among of the most investigated solar cells^14–17). This is owing to great promise of semiconductor TiO₂ film that has shown such as photo-stability, environmental friendly, and high solar energy conversion efficiency^18,19). Furthermore, TiO₂ film is among of the wide bandgap oxide semiconductor with band energy of 3.0–3.2 eV tolerating to absorb even ultraviolet light^20,21). TiO₂-based PV has emerged as a promising option for harnessing sun's energy owing to its low cost, production simplicity, and environmental friendliness than the conventional Si and CdTe-based PVs.TiO₂-based PV, commonly known as dye sensitized solar cell (DSSC), utilizes an organic dye extracted from plants to emulate the manner in which plants convert sunlight into useful energy. Furthermore, in the DSSC which was invented by Gratzel in 1991, the TiO₂ film-electrodes can be assembled with the graphite electrode, electrolyte and dye sensitizers to convert solar light into direct current electricity^22,23). Dye-sensitizer plays a role for light harvesting and electrons trasportation^24,25). As for the film deposition technologies, various technologies have been practically utilized for the deposition of photo-catalytic TiO₂ film such as metal organic chemical vapor and sputtering^26–28). However, there are several problems associated with these techniques such as low deposition rate and high initial cost due to vacuum equipment. On the other hand, atmospheric solution precursor plasma spray (ASPPS) which is a thermal spray technique has been used for deposition of photo-catalytic TiO₂ film to overcome some of these problems.

In this study, ASPPS technology was utilized owing to its possibility for depositing unique TiO₂ film structure comprising many of the required properties for the application performance including particle size, crystal structure and morphology of the film. However, the conventional ASPPS using free jet plasma has some disadvantages such as thermal damage of the substrate during film deposition due to the direct plasma jet irradiation to the substrate, and insufficient activation of the feedstock due to high flight speed (low resident time) of feedstock in the plasma jet. Therefore, in order to overcome these disadvantages, ASPPS utilizing vortex plasma jet (vortex ASPPS), which can control feedstock flight speed and deposit film without direct irradiation of plasma jet, was developed and consequently photo-catalytic TiO₂ film could be successfully deposited. Nevertheless, Ar is expensive and difficult to obtain regularly in the non-grid connected rural areas in developing countries. On the other hand, since N₂ can be obtained from ambient air by using commercial nitrogen generator, vortex ASPPS using N₂ dominant working gas was developed.

In this study, in order to develop a TiO₂-based PV manufacturing and repairing process using ASPPS for the rural areas, Vortex ASPPS equipment utilizing Ar/N₂ as a working gas was developed and a photo-catalytic TiO₂ film deposition was carried out.

2. Experimental Procedures

2.1 Preparation of solution precursor for TiO₂ film

Ethanol-diluted Titanium tetra-iso-butoxide was used as feedstock for the film deposition. The precursor (TTIB) was diluted with the ethanol at a ratio of 1:20, i.e. (TTIB: Ti(OC₄H₉)₄). Figure 1 shows flow chart for the preparation of the feedstock and the deposition process of a crystalline TiO₂ film.

Fig. 1

Flow chart for preparation of feedstock and TiO₂ film deposition.

2.2 Deposition of photo-catalytic TiO₂ film

Figure 2 shows schematic diagram of the atmospheric SPPS equipment with improved water cooling vortex generation anode nozzle and substrate holder. TiO₂ film were deposited on the grit-blasted 15 mm × 15 mm × 1 mm^t 304 stainless steel plate substrates. The feedstock was quantitatively fed by commercial air brush (Anest-Iwata MX2900).

Fig. 2

Schematic diagram of the ASPPS film deposition equipment.

Table 1 shows TiO₂ film deposition conditions used in this study. The distance between the anode nozzle outlet of the plasma torch and the surface of the substrate was controlled between 40–80 mm and the deposition time was fixed at 7 minutes. N₂-dominant Ar/N₂ was used as the working gas.

Table 1 Deposition conditions for the TiO₂ film.

Plasma working gas	Ar/N₂
N₂ gas flow rate	2.5 L/min
Ar gas flow rate	1.5 L/min
Discharge power	50 A/20 V
Deposition distance	40 – 80 mm
Deposition time	7 min
Feed stock materials	C₂H₅OH diluted TTIB
TTIB/Ti (OC₄H₉)	1:20
Feedstock feed rate	100 ml/hr.
Substrates	304 SS/ITO glass
TTIB: Titanium tetra iso butoxide

In order to confirm the applicability of the deposited TiO₂ film to photovoltaic devices, the film was deposited on a 40 × 20 × 2^t mm Indium Tin Oxide(ITO) coated glass substrate. In each deposition case, the substrates were fixed on the water-cooled substrate holder to protect from the thermal damage.

2.3 Characterization of photo-catalytic TiO₂ film

The crystallinity and crystal structure of each TiO₂ film were investigated using X-ray diffraction (CuKa, 40 kV/100 mA). The surface morphologies and cross-sections of crystallized TiO₂ film were evaluated using scanning electron micrograph (SEM) (Hitachi S-4200). Moreover, the methylene blue decolorization tests (Methylene blue droplet decolorization test and methylene blue solution decolorization test) and surface wettability tests were carried out in order to confirm the photo-catalytic properties of the deposited film. Respectively, a 0.05 ml methylene blue droplet (methylene blue concentration: 0.026 mol/l) and 30 ml methylene blue water solution (methylene blue concentration: 12.8 µmol/l) were used^29,30). For the measurement of methylene blue concentration in the water solution after the decolorization test, absorptiometer (Coper Electronic Co. PE-01) was used.

The open circuit voltage of the DSSC and solar irradiance were measured using a digital multimeter (Custom M-04 12V A23) and a solar power meter (CEM LA-1017), respectively. The film strength was evaluated using a pencil scratch tester (Coating tester kogyo pencil scratch tester No. 850).

3. Results and Discussion

3.1 Influence of N₂-dominant plasma working gas to the film

Figure 3 shows the appearance of N₂-dominant Ar/N₂ plasma jets before and after feedstock (TTIB/ethanol) injection using vortex flow generation nozzle. In our previous study using vortex ASPPS equipment³⁰⁾, although Ar/N₂ working was tried to be used, continuous vortex plasma could not be generated. This is due to melting down of the vortex generation nozzle which occurred during the operation owing to poor cooling system. On the other hand, in this study by using the vortex generation nozzle with improved water cooling system, even on the condition of N₂-dominant Ar/N₂ as working gas, the vortex plasma jet could be generated continuously over 5 minutes without electrodes erosion. Moreover, even in the case of vortex plasma generation utilizing N₂ as the working gas, the following results were confirmed. The plasma jet was not generated continuously when the flow rate is set over 2.5 l/min. A plasma jet was continuously generated when the flow rate is set below 2.5 l/min. However, even in this condition, the plasma jet was not stable and intensive electrode erosion occurred.

Fig. 3

Appearances of vortex flow generation nozzle and vortex Ar/N₂ plasma. (a) Front view, (b) Side view.

Figures 4 and 5 respectively, shows the appearance and the X-ray diffraction (XRD) patterns of the samples with the film deposited at distances (d) of 40, 60, and 80 mm. As shown in Fig. 4, a white colored film could be deposited in each deposition distance. As seen from the XRD patterns, strong peaks of the rutile and anatase phases were observed in each deposition distances. The sample deposited at d = 80 mm showed few and weaker peaks of the crystalline rutile and anatase phases than those deposited at d = 40 and 60 mm. The reasons why the patterns and intensity were not similar could be related to the variation of the degree of crystallinity of the film, which were deteriorated with increasing deposition distance³¹⁾. Other reasons are related to the nucleation of the feedstock material in the plasma jet which occurred during flight as well as the variation of deposition temperature which was reduced by the carrier gas^32,33).

Fig. 4

Appearance of deposited film on the stainless-steel substrates. (d: Deposition distance). (a) d = 80 mm, (b) d = 60 mm, (c) d = 40 mm.

Fig. 5

Results of the X-ray diffraction patterns of the samples. (○: Anatase, △: Rutile, ■: Fe-Cr (Substrate))

Figures 6 and 7 respectively, shows the optical micrographs of the surface morphologies and SEM images of cross-sections of the TiO₂ film deposited at various distances. Although the thickness of the film deposited at d = 40 and 60 mm were 38 μm and 36 μm respectively, the film deposited at d = 80 was 28 μm. The thickness of the deposited film was decreasing with increasing deposition distance^32–34). The surface wettability and methylene blue decolorization tests were carried out in order to confirm the photo-catalytic properties of TiO₂ film. In the case of the surface wettability test, the following results were confirmed. For the films-samples deposited at d = 40 mm, the film showed high wettability with surface contact angles of over 26 degree. In contrast, the film deposited at d = 60 and 80 mm showed intermediate and low wettability with contact angles of over 48 degree and 82 degree, respectively. Thus, it was confirmed that the photo-catalytic properties of the film could be controlled by varying deposition distances. However, during wettability test, it was noted that, wettability of the film was affected by the surface morphology and the porosity of the film.

Fig. 6

Optical micrographs of the surface morphologies of the TiO₂ films. (a) d = 80 mm, (b) d = 60 mm, (c) d = 40 mm.

Fig. 7

SEM images of cross-sections of the TiO₂ film. (a) d = 80 mm, (b) d = 60 mm, (c) d = 40 mm.

Figures 8 and 9 shows the appearance of the sample during methylene blue droplet decolorization test and the concentration variation curve of methylene blue solution decolorization as a function of ultraviolet irradiation time for each TiO₂ film-sample deposited at distances of 40, 60, and 80 mm. It was noted that, for the methylene blue droplet decolorization test, the films deposited at the d = 40 and 60 mm started to decolor the methylene blue droplets after approximately 6 hours ultraviolet irradiation. In particular, the droplet was decolored completely after 24 hours ultraviolet irradiation in the case of the film deposited at d = 60 mm. Moreover, it was also noted that the absorption rate of the methylene blue concentration was minimal even after 24 hours in the ultraviolet irradiation in the case of film deposited at d = 80 mm. However, the decolorization rate for the film at d = 60 mm was much higher than that for 40 mm. The reason why, is related to the fact that the film at d = 60 mm was more porous than that at d = 40 mm. That is, ultraviolet could not irradiate the methylene blue infiltrated in the porous film for the droplet decolorization test. While, ultraviolet can irradiate almost all the methylene blue by convection of the solution during operation in the case of the solution decolorization test. Therefore, although the film at d = 60 mm had higher photo-catalytic property than that at d = 40 mm, decolorization rate of the film at d = 60 mm was much lower than that at d = 40 mm. As for the result that the film deposited on the condition of d = 60 mm had higher photo-catalytic property than those on the conditions of d = 40 and 80 mm, the following reasons can be considered.

Fig. 8

Appearances of the samples during methylene-blue droplet decolorization test.

Fig. 9

Variation curve of the methylene blue concentration decolored by the TiO₂ film as a function of UV irradiation time for methylene blue solution decolorization test. (*Reagent: Methylene blue solution (Methylene blue concentration: 12.8 μmol/l.) 12.8 μmol/l methylene blue solution is regarded as 100% concentration of the reagent.)

1) Since the degree of crystalline of the TiO₂ film was increased with decreasing the deposition distance from d = 80 mm to 40 mm, photo catalytic property of the film was increased.
2) Since the rutile concentration of the TiO₂ film were increased with decreasing the deposition distance from d = 80 mm to 40 mm, photo catalytic property of the film was decreased.

On the subject of the film strength and bonding strength, the results of the pencil scratch test showed that the photo-catalytic TiO₂ film deposited at d = 60 mm had sufficiency strength to withstand 2H pencils scratch. On the other hand, the film deposited at d = 40 and 80 mm showed weaker strength and can only with stand up to 1H and 1B respectively. From these results, it was proved that the degree of crystallinity, photo-catalytic properties and the strength of the film were increased and improved at 60 mm.

3.2 Application of TiO₂ film to photovoltaic devices

Figure 10 shows the appearance of the ITO glass substrate before and after TiO₂ film deposition. The TiO₂ film deposited on the ITO glass substrate was used as the anode to the photovoltaic device. As the cathode, graphite film deposited on the ITO glass substrate was used. Furthermore, in order to enhance sun light harvesting capacity of the device, the dye extracted from hibiscus was infiltrated into the photo-catalytic TiO₂ film before assembling.

Fig. 10

Appearance of the ITO film coated glass before and after TiO₂ film deposition. (a) Before TiO₂ film deposition, (b) After TiO₂ film deposition.

Figure 11 shows the appearance and schematic diagram of photovoltaic device fabricated by utilizing the photocatalytic TiO₂ film deposited in this study. Moreover, Fig. 12 shows the image of the photovoltaic device and the resulting output voltage (V_OC) during testing. The photovoltaic device had an effective area of approximately 740 mm² and was able to generate an open circuit voltage of 146.7 mV with solar irradiance intensity of 574 W/m². From the best of our knowledge, so far there is no DSSC that has been fabricated using ASPPS. However, there are some researchers that has fabricated different DSSC using different methods with setbacks described in the introduction section. These methods include Hydrothermal^35,36), Electrochemical^37,38), and Spuerting^39,40). Our result is in good agreement with the commercial TiO₂ photovoltaic devices that was fabricated by Nishinoda denko Co. Ltd.

Fig. 11

Appearance and schematic diagram of the photovoltaic device. (a) Appearance, (b) Schematic diagram.

Fig. 12

Image and test results of photovoltaic device.

Table 2 shows the photovoltaic test results of the commercial device and the device developed in this study. From these results, the ASPPS technology was found to have higher potential for high-rate TiO₂ film deposition with high crystallinity and photo-catalytic properties. As the improvement method for the photocatalytic properties and film strength, uniform film deposition and post heat treatment with sintering are thought to be effective. As the future work, the effectiveness and efficiency of these method needs further consideration. In addition, the efficiency, fill factor and current density of the TiO₂ photovoltaic device need to be confirmed.

Table 2 Results of photovoltaic test (in V).

TiO₂ Photovoltaic Device	Testing condition
TiO₂ Photovoltaic Device	Without sunlight irradiation	With sunlight irradiation
Developed device	0.00	0.14
Commercial device	0.00	0.15

4. Conclusion

Photo-catalytic TiO₂ films have been deposited by 1 kW – ASPPS utilizing N₂-dominant Ar/N₂ plasma working gas. Consequently, the obtained results in the present study are summarized as follows;

(1) By using vortex flow generation nozzle with improved cooling system, vortex plasma could be generated continuously without erosion of electrodes even in the case of N₂-dominant Ar/N₂ plasma working gas.
(2) Even in the case of N₂-dominant Ar/N₂ plasma working gas, porous TiO₂ film including anatase and rutile could be deposited in each deposition distance.
(3) The degree of the crystallinity and crystal structure (anatase-rutile ratio) can be controlled by changing deposition distances.
(4) The TiO₂ film deposited at d = 40 mm had fairly sufficient photo-catalytic property to decolor methylene blue droplet by 24 hours in the ultraviolet irradiation. Besides, the film deposited at d = 60 mm had enough photo-catalytic property to show hydrophilic and almost perfectly decolor methylene blue solution by 6 hours in the ultraviolet irradiation. On the other hand, the film deposited at d = 80 mm had slight photo-catalytic property.
(5) The DSSC with the TiO₂ film deposited by this developed vortex ASPPS equipment could generate an open circuit voltage up to 146.7 mV.

Acknowledgments

This study has been conducted as General Collaborative Research Project and Discretionary Collaborative Research Project of the Institute of Fluid Science, Tohoku University, Japan (Grant No. J14H005, J15041, J16030, J17L024). The authors appreciate Prof. Shigeaki Kobayashi of Ashikaga Institute of Technology providing the SEM observation results on the as-sprayed titania coatings.

REFERENCES

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