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7НАНОСИСТЕМЫ: СИНТЕЗ, СВОЙСТВА, ПРИМЕНЕНИЕ
NANOSYSTEMS: SYNTHESIS, PROPERTIES, AND APPLICATION
Статья поступила в редакцию 07.08.12. Ред. рег. № 1391 The article has entered in publishing office 07.08.12. Ed. reg. No. 1391
УДК 539.216.2; 541.145
НАНОСТРУКТУРИРОВАННЫЕ ПЛЕНКИ TiO2 ДЛЯ ПРОИЗВОДСТВА ВОДОРОДА
Л. Гринберга, Я. Линитис, Я. Клеперис
Институт физики твердого тела при Латвийском университете Латвия, Рига, LV-1063, ул. Кенгарага, д. 8 Тел.: (+371)67262145; (+371)67132778, e-mail: Liga.Grinberga@cfi.lu.lv, yoto2@inbox.lv
Заключение совета рецензентов: 20.08.12 Заключение совета экспертов: 25.08.12 Принято к публикации: 30.08.12
Двуокись титана в форме анатазы привлекает внимание исследователей своей фотоактивностью, что позволяет использовать ее в качестве фотокатализатора и в фотогальванических элементах. В данной работе описываются условия анодирования для получения стабильной тонкой пленки TiO2, состоящей из вертикально ориентированных нанотрубок высотой приблизительно 358 нм, внутренним диаметром 48 нм и толщиной стенок 20 нм, межцентровое расстояние 100 нм. После отжига при температуре 500 °С двуокись титана в основном переходит в анатазу, ДРА показывает также наличие примеси рутила. Полученные пленки обладают фоточувствительностью в основном к УФ части спектра.
Ключевые слова: нанотрубки, пленка TiO2, фотокатализ.
NANOSTRUCTURED TiO2 LAYERS FOR HYDROGEN PRODUCTION
L. Grinberga, J. Linitis, J. Kleperis
Institute of solid State Physics, University of Latvia 8 Kengaraga str., Riga, LV-1063, Latvia Tel.: (+371)67262145, (+371)67132778; e-mail: Liga.Grinberga@cfi.lu.lv, yoto2@inbox.lv
Referred: 20.08.12 Expertise: 25.08.12 Accepted: 30.08.12
Titania with anatase structure is investigated due to its photo-active properties that can be used in the water photocatalysis applications and in the organic photovoltaic devices. In this work the anodization conditions are described to obtain stable thin film TiO2 layers formed from vertically oriented nanotubes with approximate height 358 nm, inner tube diameter 48 nm and wall thickness 20 nm, but centre to centre distance 100 nm. Annealed at 500 °C TiO2 layer mostly consists from oxide with anatase structure, though XRD spectroscopy shows rutile impurities as well. Obtained nanotube layers are sensitive mostly to UV light.
Keywords: nanotubes, TiO2 layer, photocatalysis.
Organization: Faculty of Chemistry, University of Latvia. Education: University of Latvia, Faculty of Chemistry, BSc (2009-2012).
Experience: Institute of Solid State Physics, University of Latvia: Engineer at Laboratory of Hydrogen Energy Materials; participated in 2 Scientific research projects.
Publications: J. Linitis et al 2011 IOP Conf. Ser.: Mater. Sci. Eng. 23 012010.
Janis Linitis
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
Introduction
The key to hydrogen economy is development of efficient processes for the carbon-free hydrogen production. Promising approach is to use sunlight for water photo-electrolysis. The defined the target of the department of Energy (USA) for 2013 is to achieve solar-to-fuel conversion efficiency 12% [1]. The photocatalytic water electrolysis process is based on hydrogen/oxygen evolution on metal/oxide electrodes in the water cell under illumination of the Sun. So far the photocatalytic water splitting has been criticized as being uneconomical compared with other hydrogen production systems, due to its inherently low efficiency [2]. In order to resolve these problems and make photocatalytic hydrogen production feasible, continuous efforts have been made to improve the activity of the photo-catalysts employed. Titanium dioxide TiO2 is n-type wide band gap semiconductor and exists in three distinct crystalline polymorphs: rutile, anatase, and brookite [3]. Rutile is the most studied and well known natural form of titania. Anatase is rare in abundance but of great interest, particularly for its key role in the injection and transport of electrons in photovoltaic devices. Brookite is the rarest form of the mineral and not easily obtained synthetically. Anatase structure is photo-active, therefore it has been successfully utilised in photocatalysis, splitting of water by light and also in organic photovoltaic devices, like dye-sensitised solar cells [4]. In recent years a wide range of scientists focused their attention on TiO2 nanotubes due to their greater surface area per unit mass, a better adsorption capacity, as well as higher fotoactivity compared with TiO2 powder [5]. Hydro thermal synthesis of TiO2 nanotubes have been widely used, but they faced with problems caused by difficult reuse them, as well as the application this technology is limited. Nowadays this problem is solved with obtaining nanotubes directly on the substrate. One of the cheapest and easiest is an electrochemical anodizing titanium substrate [6]. It has been shown that the morphology of TiO2 nanotubes layer, obtained by anodic oxidation of titanium substrate, is affected by the anodising voltage, anodizing time, electrolyte composition and temperature [7]. There are distinguishes four generations of TiO2 nanotubes layer synthesis with anodic oxidation method [2]:
• The first generation - growth using an aqueous solution of electrolytes;
• The second generation - growth using the buffered electrolytes;
• Third generation - growth using polar organic electrolytes;
• The fourth generation - growth in electrolytes which do not contain fluoride.
Gong et al [8] on 2001 firstly reported on anodised titania nanotubes, grown using a dilute solution of hydrofluoric acid.
The goal of current work was to find electrolyte where it is possible to repeat anodization experiments and achieve the same quality TiO2 nanotube coatings. Satisfactory results are obtained and nano-structured thin TiO2 films are characterized with SEM, XRD and Raman spectroscopy methods.
Experimental part
Titanium foil 0.2 mm (Aldrich, purity 99.5%, 2.5 cm x 2 cm) was used as an anode and Pt foil (Aldrich, 99%, 5 cm x 5 cm) was used as a cathode in anodization process. Before anodization the Ti foil was polished with sandpaper (P600), ultrasonically cleaned in deionised water for 5 min, cleaned with methanol and dried. Different electrolytes (1% HF solution, 0.1 M NaF + 1 M H2SO4; 1M H3PO4 + 1 M NaOH + 0.5 wt% HF etc) and anodization regimes (15-50 V) were tested, but no satisfactory coatings obtained. After numerous experiments it was found that electrolyte consisting of 0.14 M NaF (Fluka) and 0.5 M H3PO4 (Aldrich) and supplied anodization potential 20 V gives the best results. Repeatedly performed anodization provided equal quality TiO2 nanotube coatings in these conditions. The growth of nanotubes can be described as a competition between a number of electrochemical and chemical reactions, including the formation and dissolution of anodically formed oxide:
Ti + 2H2O ^ TiO2 + 4H+ + 4e-;
TiO2 + 6F- +4H+ ^ [TiF6]2- + 2H2O
and diffusion of titanium ions through the oxide layer to an interface with electrolyte:
Ti4+ + 6F- ^ [TiF6]2-.
The anodic oxidation was carried out at stabilized temperature (23 °C) and constant 20 V potential for 1h using power unit N5751A (Agilent Technologies) - see Fig. 1 for experimental setup.
Рис. 1. Схема для анодного выращивания слоя нанотрубок TiO2 Fig. 1. Experimental setup to grow TiO2 nanotubes layer anodically
The potential was raised from open circuit potential to 20 V at a rate 3 V per second. The current was registered with PCS10/K8047 (Vellman instruments). The step-by-step rise of the voltage at the beginning of oxide layer formation significantly impacts the quality of coating and its stability. As it is seen from current-time relationship (Fig. 2), in first seconds as potential is applied to Ti electrode, the current sharp declines (Region 1), then rise again but very little (Region 2) and then stabilizes (Region 3).
Рис. 2. Типичная зависимость тока от времени при анодной поляризации в Ti электроде Fig. 2. Typical current-time relationship in anodic polarization of Ti electrode
Рис. 3. Ячейка для фотоэлектрохимических измерений Fig. 3. Cell for photoelectrochemical measurements
In the first region which characterizes with rapid current drop the formation of oxide layer is observed; in the second region the selective dissolution of oxide layer starts - current is slowly increasing, but in the third region the pore formation in oxide structure is reached, and the oxide nanotubes grows in length.
During experiments electrolyte solution was continuously stirred with a magnetic stirrer. Obtained samples with nanostructured layers were rinsed several times in deionized water and annealed at 500 °C in air for 1 hour. Morphology and structure of TiO2 layers were inspected with scanning electron microscope SEM EVO 50 XVP (Carl Zeiss), X-ray Diffractometer X'Pert
Pro MPD and Near-Infrared Raman spectrometer Advantage 785 (DeltaNu). Photo-activity of samples was measured with potentiostat VoltaLab 40 PGZ301 in three electrode cell (made from black poliacetal POM) with 1 M Na2SO4 solution, calomel reference electrode and Ni counter electrode (Fig. 3).
The samples were exposed to light source (120 W mercury and 300 W halogen lamps) placed 25 cm away from quartz window of cell. UV (YFS-2) and yellow (ZC-9) filters were used to select only UV light and visible light accordingly from full spectra of halogen lamp.
Results and discussion
The TiO2 films obtained after anodizing process looked smooth and coloured in greenish - yellow colour depending from the duration of the anodization process. In order to observe the TiO2 nanotubes in scanning electron microscopy (SEM), it was adjusted carefully precisely perpendicularly to the SEM electron beam, because even a small deviation gives a completely different terrain images.
In obtained SEM images it is seen that thin layer of vertically oriented nanotubes is formed on the surface of the Ti foil (Fig. 4).
b
Рис. 4. Результаты исследований СЭМ для вертикально ориентированного слоя TiO2 нанотрубок, полученных
в процессе анодизации Ti фольги (1 час) Fig. 4. SEM images from vertically oriented TiO2 nanotube layer obtained in anodization process of Ti foil (1 hour)
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
The average nanotube layer thickness after 1 hour anodization reaches 350 nm (Fig. 4, a). Inner tube diameter is around 50 nm, wall thickness is about 20 nm and centre to centre distance is close to100 nm (Fig. 4, b).
The structure measurements of obtained annealed thin films with Raman spectroscopy confirm that thin oxide layer is formed mostly of anatase structure modification (Fig. 5). According to factor group analysis [9], anatase has six Raman active modes: 144 cm-1 (Eg),
197 cm-1 (Eg), 399 cm-1 (B1g), 513 cm-1 (A1g), 519 cm-1 (Big) and 639 cm-1 (Eg). Three of them are clearly noticeable for our samples (Fig. 2) at 399 cm-1, 518 cm-1 and 639 cm-1.
X-Ray diffraction analysis shows the presence not only of the anatase phase but also impurities of the rutile phase in an annealed sample (Fig. 6). Titanium peaks from cubic lattice phase are observed in XRD spectra, because the thickness of oxide film is only 0.4
Рис. 5. Рамановский спектр отожженного TiO2 слоя на Ti фольге Fig. 5. Raman spectra of annealed thin TiO2 layer onto Ti foil
5000
4000
3000
<л с
(D
2000
1000
■ - Titanium Ti - Hexagonal A - Anatase
4 - Titanium Ti - Cubic • - Rutile
■
■ 1
A ■ , A ■ j ; . i:
20
30
40 50
20 (degrees)
60
70
80
Рис. 6. Рентгеновский спектр тонкой пленки TiO2, анодно выращенной на субстрате из фольги Ti Fig. 6. X-Ray spectra of thin TiO2 film grown anodically onto Ti foil substrate
Рис. 7. Фотоиндуцированные изменения потенциала (EDS) в образцах слоя нанотрубок Ti/TiO2
в зависимости от условий роста и отжига Fig. 7. Photo-induced changes of potential (EDS) of Ti/TiO2 nanotube layer samples in dependence from their growth and annealing procedures
Photo-potential was measured as difference between light-induced potential of working electrode - Ti foil with thin TiO2 nanotybe layer, and potential of this electrode in darkness. The value of photo-induced potential at UV light (full output from Hg-lamp) changed in dependence from sample preparation (electrolyte) and next annealing procedures (Fig. 7).
nm) is selected using filter YFS-2, the change of photopotential is same as it is with a full spectra of halogen lamp, but if UV radiation of the halogen lamp is cut by yellow filter ZC-9 (>380 nm), the change of photopotential reduces dramatically. It means that obtained TiO2 nanotube layers are selective mostly to UV light.
Photo-induced current was measured in short-circuit scheme when potential applied between Ti/TiO2 electrode and reference electrode is zero. Very sharp photo-induced current peaks were observed when UV light from mercury lamp is switched on and switched off (Fig. 8), indicating on small life time of photo-induced charge carriers.
From obtained results the light-current conversion efficiency is calculated, using next equation:
n = 100%%;
n = -
P
0,322 mV• 35 ^A 1,37 mW
100% = 0,S2%,
Рис. 8. Фотоиндуцированные пики тока короткого замыкания Fig. 8. Photo-induced short-circuit current peaks
As it is seen, the UV light (source mercury lamp 120 W) arises fast potential changes with long relaxation tail (Fig. 7). Measurements with the full spectrum of halogen light source (lamp 300 W) induce smaller changes of potential. If only UV light of the halogen lamp (<360
where q - efficiency (%), P - the density of light power (mW/cm2) measured with PMA 2200 (Solar Light Co.), AEDS - induced photo-potential (mV), i - the density of induced photo-current (^A/cm2). Obtained value 0.82% is comparable with published data [2].
Next step is to increase the light conversion efficiency. It was performed etching (chemical, mechanical) of titanium foil, but obtained results show that no increase of photo-activity is observed.
7G
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
Conclusions
Anodization conditions are found to obtain stable thin film TiO2 layers formed from vertically oriented nanotubes with average height of 350 nm (anodization time 1 hour), inner tube diameter around 50 nm and wall thickness 20 nm, but centre to centre distance about 100 nm. Annealed at 500oC TiO2 layer mostly consists from oxide with anatase structure, however small rutile phase impurities are observed as well. Obtained nanotube layers are sensitive to UV light with conversion efficiency of ~ 0.82%.
Acknowledgements
Authors acknowledge the National Research Program in Material Sciences and Information and Communications 'IMIS' for financial support. K.Kundzins and G.Cikvaidze are acknowledged for SEM and RAMAN experiments accordingly.
References
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