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8НАНОСТРУКТУРЫ
NANOSTRUCTURES
Статья поступила в редакцию 07.08.12. Ред. рег. № 1397 The article has entered in publishing office 07.08.12. Ed. reg. No. 1397
УДК 537.622.6; 539.231: 539.216.2
СВОЙСТВА И СТРУКТУРА ТОНКИХ ПЛЕНОК ФЕРРИТА И МНОГОПЛЕНОЧНЫХ СИСТЕМ, ВЫРАЩЕННЫХ ПИРОЛИЗОМ ПУЛЬВЕРИЗОВАННОГО СЛОЯ
11 1 2 А. Кнокс , И. Дирба , Я. Клеперис , М. Майоров
1Институт физики твердого тела при Латвийском университете Латвия, Рига, LV-1063, ул. Кенгарага, д. 8 Тел.: (+371)67187816; (+371)67132778, e-mail: ainars.knoks@gmail.com 2Институт физики при Латвийском университете Латвия, Саласпилс, LV-2169, ул. Миера, д. 32 E-mail: maiorov@sal.lv
Заключение совета рецензентов: 20.08.12 Заключение совета экспертов: 25.08.12 Принято к публикации: 30.08.12
В работе используется простая система для пиролиза пульверизованного слоя, позволяющая напылять 2 различных раствора прекурсоров (нитраты, металл-органика и т.д.) из 2 пушек. Таким образом можно получить тонкие пленки из одного материала или многослойные пленки из двух различных материалов. В данной публикации рассмотрены моно-слойные пленки из феррита кобальта и многослойные системы TiO2/CoFe2O4. Анализ морфологии и структуры показал, что ключевым параметром для получения стабильных и оптически качественных образцов является температура подложки. Для определения свойств образцов использованы измерения электрических, магнетических, оптических параметров и фотоиндуцированного потенциала.
Ключевые слова: CoFe2O4, тонкие пленки феррита, многослойные системы, пиролиз пульверизованного слоя, магнетизм, оптические свойства.
PROPERTIES AND STRUCTURE OF THIN FERRITE FILMS AND MULTI-FILM SYSTEMS GROWN IN SPRAY PYROLYSIS PROCESS
A. Knoks1, I. Dirba1, J. Kleperis1, M. Maiorov2
'Institute of solid State Physics, University of Latvia 8 Kengaraga str., Riga, LV-1063, Latvia Tel.: (+371)67187816; (+371)67132778, e-mail: ainars.knoks@gmail.com 2Institute of Physics, University of Latvia 32 Miera str., Salaspils, LV-2169, Latvia E-mail: maiorov@sal.lv
Referred: 20.08.12 Expertise: 25.08.12 Accepted: 30.08.12
System is set up for simple spray pyrolysis technology method, which allows spray two different precursor solutions (nitrates, metal-organics, etc) from two guns. Using this system thin films from single material or sandwich-type multi-layer films from two different materials can be obtained. Results about thin Co ferrite film and multi-layer TiO2/CoFe2O4 system are reported. Morphology and structural results of thin films shows that substrate temperature is key-parameter to obtain optically qualitative and stable thin films. Electric, magnetic, optical parameters and photo-induced potential value are measured to characterize obtained thin films and film-systems.
Keywords: CoFe2O4, thin ferrite films, multi-layer systems, pyrolysis of spraying layer, magnetism, optical properties.
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
Organization(s): Faculty of Physics and Mathematics, University of Latvia. Education: Bachelor degree from University of Latvia, 2012.
Experience: Engineer at Institute of Solid State Physics, University of Latvia from 2012. Main range of scientific interests: renewable energy technologies, material physics, medieval traditions and fight.
Ainars Knoks
Introduction
Functionalised magnetic nanoparticles is technologically important materials with a wide range of applications such as magnetic memory elements and microwave devices, gas sensors for identification of biological substances and medical diagnoses [1, 2]. Nanocrystalline ferrites have attracted considerable interest both in research and technology due their unique magnetic properties, where the dominant role is to the surface magnetic atoms, which differ significantly from their bulk analogues [2]. Particularly to ferrites very important are structural parameters (cation distribution between tetrahedral and octahedral sub-lattice sites, particle size) which determine the physical and chemical properties of ferrite films [2]. Synthesis and/or deposition methods are very sensitive to structural parameters of ferrite materials. Different methods are applied to deposit ferrite films - magnetron sputtering, ion beam sputtering, Liquid injection metal organic chemical vapour deposition, chemical vapour deposition, sol-gel method and others [1-7]. These methods are complicate; require special expensive equipment, skill personnel and sometimes high processing temperatures above 500 °C. Recently to obtain spinel ferrite thin films, spray pyrolysis method were used [8-10]. The spray pyrolysis method is simple and allows easy to change the crystallite size and composition [8-10]. However, to obtain optically good spinel ferrite thin film, the deposition with spray pyrolysis method almost is not used.
Semiconductor photo-electrode systems are studied intensively in the last decade with applications to solar energy harvesting [12, 13]. The application of TiO2 electrode to split water in sunlight was for the first time demonstrated by Fujishima and Honda in 1972 [14], but not yet found a cheap and good materials to make this idea practical. However, titanium dioxide is characterized by the absorption edge around 4 eV, and expressed photo-conductivity only in the UV light spectrum [15]. The main requirement for effective photoelectrode is the presence of intensive electron transitions (usually corresponding to the absorption edge of material) in the region of visible light, providing the potential corresponding to the electrical splitting of
water (at least 1.6 eV). Ferrites with the absorption edge little above 2 eV fulfill this condition and has been applied as an impurity in TiO2 [15].
In this article we report results about thin Co ferrite films obtained from single gun system and multi-layer TiO2/CoFe2O4 thin film sandwich obtained from system with two guns in spray-pyrolysis method.
Experimental part
Iron nitrate (Fe(NO3)3-9H2O), cobalt nitrate Co(NO3)2-6H2O and titanium tetrabutoxide Ci6H36O4Ti (TTB) were obtained from Aldrich, dissolved in distilled water with desired concentration. To obtain thin CoFe2O4 films cobalt nitrate and iron nitrate solutions in relation 1:2 were poured in one spray gun and 10 shoots were sprayed on hot substrate with interval 1 minute. The nanoparticles of cobalt ferrite were synthesized using coprecipitation reaction method [16], firstly generating metal-oleate complex from the thermal decomposition of a (C5H5)CoFe2(CO)9 and further oxidizing to CoFe2O4 which was dispersed in decane solution. Prepared solutions were atomized on a glass substrate via one or two pneumatic spray guns (Star EVO-T mini, Taiwan) under nitrogen pressure 0.3 MPa using step by step film growth (Fig. 1).
Рис. 1. Схема для пиролиза пульверизованного слоя с одной пушкой Fig. 1. Setup of spray pyrolysis method with single spray gun
Glass substrate (Thermo Scientific Menzel-Gläser, Germany, and Pilkington NSG-TEC-glass) was mounted on a sample holder with heater (ap to 550 °C). Pre-set temperature of the hot plate was controlled with chromel-alumel thermocouple. To recover the pre-set temperature and complete ferrite crystallization before next deposition step, the time interval 1 min was used between subsequent steps. All film samples after deposition were cooled at rate ~ 15 °C/min because fast cooling promotes the detachment of thin films. One spray gun was used to spray solution from iron and cobalt nitrates in ratio 2:1; and two spray guns connected to nitrogen gas cylinder were used to obtain multi-layer thin film structures from titanium tetrabutoxide (one gun) and iron nitrate + cobalt nitrate mixture (second gun).
The deposited films were characterizes with XRD (Rigaku Ultima+, Japan) and SEM (Tescan Mira/Lmu, Czech Republic). Thermogravimeter SHIMATZU TG-TDA was used to measure the change of mass and specific heat for iron and cobalt precursor mixture during heating from room temperature to +500 °C (at constant N2 gas flow 50 ml/min). X-ray fluorescence spectrometer Eagle III with micro-focused X-ray excitation and energy-dispersive detector was used for elemental analysis. Surface morphology was analyzed with Scanning Probe AFM "SMENA" NT-MDT microscope; optical microscope ECLIPSE L150 equipped with Color Matrix CCD and PC; Dektak 150 Surface Profile Measuring System. DC electrical resistivity was measured with four probe method by using Solartron SI 1287 device. Magnetic properties of thin ferrite films were determined with Lake Shore Vibrating Sample Magnetometer Model 7404 at Institute of Physics, University of Latvia. To measure optical properties of samples, the instrument UV VIS Spectrophotometer SPECORD® 210 (Analytik Jena, Germany) was used. Measurements were performed for wavelengths varying from 200 up to 1100 nm and clean glass was used as reference. Photo-EDS of multi-layer sample onto ITO/glass substrate was measured with potentiostat VoltaLab 40 PGZ301 in three electrode cell with 1 M Na2SO4 solution, calomel reference electrode and Pt counter electrode. The samples were exposed to light sources - 120 W mercury lamp and halogen 300W lamp, placed 5 cm away from quartz window of cell.
Results and discussion
The mixture of cobalt nitrate and iron nitrate precursor solutions (0.1M) in ratio 1:2 accordingly with total mass 29.9 mg was poured in Al crucible and heated in nitrogen flow at thermogravimeter with rate 10 deg/min (Fig. 2).
As it is seen from Fig. 2, quick evaporation starts above 50 °C and up to 100 degrees almost all water is evaporated; remaining 0,1 mass % corresponds to the product of reaction:
Co(NO3)26H2O + 2Fe(NO3)36H2O = = CoFe2O4 + 12H2O + 8NO2 + 2O2.
Рис. 2. Дифференциальный термический (DTA) и дифференциальный гравиметрический (DTG) анализ
раствора прекурсора для получения CoFe2O4 Fig. 2. Differential thermal (DTA) and differential gravimetric (DTG) analysis of precursor solution to obtain CoFe2O4
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Рис. 3. Морфология поверхности пленок CoFe2O4 на стеклянной подложке: оптический микроскоп (а) и АСМ (b) Fig. 3. Surface morphology of thin CoFe2O4 film on glass substrate: optical microscope (a) and AFM (b)
Thin CoFe2O4 thin films were examined in optical microscope (Fig. 3, a). Only foots from drops are seen in Fig. 3, a where overlap drop contours suggests that the droplets are collapsed on top of each other. It can be assumed that crystallization occurs before the next spraying cycle and applied time between two successive
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
spray cycles (1 minute in our case) is sufficient to form homogeneous thin ferrite film. Film thickness and average surface roughness were estimated from profilometer and AFM measurements - accordingly ~300-500 nm and ~25-250 nm (Fig. 3, b).
Таблица 1
Элементарный состав тонких пленок, полученных из растворов с различными концентрациями прекурсоров
Table 1
Elemental composition of thin films obtained from precursor solutions with different concentrations
M T, °C Fe/Co
0,05 281,4 1,88
0,1 382,2 1,59
0,1 362,8 1,92
0,1 382,4 1,87
0,1 345,3 1,95
0,1 335,3 1,82
0,1 351,1 2,10
0,25 337,8 1,78
0,3 350,8 2,14
1 382,8 2,51
Elemental composition of thin films was determined with X-ray fluorescence spectrometer and results only for thin film material (glass substrate elements are subtracted) are shown in Table 1. Molar concentration of precursor solutions is shown in first column, temperature of substrate during film grown - in the middle and relation between atomic concentrations of iron and cobalt, determined by X-ray fluorescence method - in third column. As it is seen, relation is close to stoichiometric compound CoFe2O4, only two samples differ more.
XRD analysis confirmed that sputtered thin films are only poorly crystalline CoFe2O4 (Fig. 4). From cobalt ferrite thin films better crystallinity was for the sample obtained from disperse solution of cobalt ferrite nano-grains in decane. Reference spectrum of single phase CoFe2O4 is shown at the bottom and as it is seen only thin film obtained from nanoparticle dispersion in decane has almost all characteristic spinel ferrite peaks. Wide and sloping maximum between 20-30 degrees correspond to amorphous glass substrate.
Optical properties of cobalt ferrite thin films and TiO2/CoFe2O4 film system on glass/ITO substrates were studied using UV-VIS spectrophotometer. The absorbance was measured by subtracting the absorbance of the glass/ITO substrate, which was taken as reference in measurements. The UV-VIS absorbance spectra were used to obtain band gap energy (Eg) of the thin film. Direct band gap energies can be obtained from the dependencies (aho)2 on ho, where a is absorption coefficient of thin film, but ho is photon energy in eV [17] - see Fig. 5 and 6.
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Рис. 4. Спектры ДРА для тонких пленок CoFe2O4, полученных пиролизом пульверизованного слоя Fig. 4. XRD spectra of thin CoFe2O4 thin films, obtained by spray pyrolysis method
Рис. 5. Интервал поглощения света с прямым переходом для тонких пленок CoFe2O4, полученных из растворов с различными концентрациями прекурсоров Fig. 5. Direct transition light absorption gap of CoFe2O4 thin films obtained from precursor solutions with different concentrations
Рис. 6. Интервал поглощения света с прямым переходом для тонких пленок CoFe2O4, полученных из дисперсных растворов различной концентрации наночастиц в декане Fig. 6. Direct transition light absorption gap of CoFe2O4 thin films obtained from nanoparticle dispersion
in decane with different concentrations
As it is seen, the absorption edge of thin CoFe2O4 films obtained as from precursor solutions as well from nanoparticle dispersion in decane is around 2.5-2.7 eV. Generally the band gap value is influenced by crystallite size, lattice parameter, film thickness, purity, stoichiometry, charge carrier concentration and lattice strains [18].
The spray of titanium tetrabutoxide (TTB) on hot (around 360 °C) substrate results in poorly crystallized TiO2 (no diffraction peaks in XRD spectra), likely as spray of Co and Fe nitrate solution gives poorly crystallized CoFe2O4. Mixed layer structure was obtained, using step-by-step spray of TTB and cobalt/iron nitrate solutions accordingly (Fig. 7).
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
Рис. 7. Многослойная система из тонких пленок TiO2 и CoFe2O4, полученных пиролизом пульверизованного слоя, используя две пушки Fig. 7. Multi-layer system of thin TiO2 and CoFe2O4 films obtained by spray pyrolysis method using two spray guns
If direct absorption edge of TiO2 is around 4 eV and cobalt ferrite thin film is at 2.5 eV, the absorption edge of multi-layer films (Fig. 8) decreases with increase the number of layers and concentration of solution (Table 2).
Light induced potential change was measured for multilayer system obtained on glass substrate coated with conductive transparent electrode - ITO (indium-tin oxide). Measurements were performed in 3-electrode cell using calomel as reference electrode and Pt foil as counter electrode.
Рис. 8. Оптический спектр и край полосы поглощения многослойной системы из тонких пленок TiO2 и CoFe2O4, полученных пиролизом пульверизованного слоя, используя две пушки Fig. 8. Optical spectra and absorption edge of multi-layer thin TiO2 and CoFe2O4 films obtained by spray pyrolysis method
using two spray guns
Таблица 2
Край полосы поглощения для различных слоев тонких пленок
Table 2
Light absorption edge of the different layers of thin films
Thin film systems Eg(d), eV
CoFe2O4 G,G5M lGx 34G °C 3,2
TiO2/CoFe2O4 mix 5x 354 °C G,lM 3,6
TiO2/CoFe2O4 mix 6x 37G °C G,lM 3,5
TiO2/CoFe2O4 mix lGx 362 °C l,GM 2,98
The samples were exposed to two different light sources - 120 W mercury lamp (mostly UV light radiation) and halogen 300 W lamp (mostly visible light radiation), placed 5 cm away from quartz window of cell. As it is seen (Fig. 9), multilayer system has enhanced light sensitivity in visible light region, comparing to separate TiO2 thin film obtained with spray pyrolysis method.
Рис. 9. Фотоиндуцированный потенциал многослойной системы из тонких пленок - ртутная и галогеновая лампы
как источники света Fig. 9. Photo-induced potential for multi-layer thin film system -illumination with mercury lamp (UV) and halogen lamp
Using a vibrating sample magnetometer, the saturation magnetic moments and coercivity was measured for obtained cobalt ferrite samples. As it is
seen from Fig. 10 and 11, only sample obtained from CoFe2O4 nanoparticles dispersion in decane solution has magnetic properties.
Authors [19] found that Co-ferrite 50 nm thin films on SiO single-crystal substrates has high coercivity (9.3 kOe) at room temperature. The magnetic properties were strongly dependent on heat treatment condition (authors [19] annealed samples at 900 °C) and film thickness. Such large magnetic anisotropy and high coercivity is explained with relatively large lattice strain in Co-ferrite thin films, because the Co-ferrite phase was formed at high temperature - 900 °C [19]. Authors [20] investigated magnetic properties of CoFe2O4 thin films epitaxially grown on (100) MgO by pulsed layer deposition, and found that films obtained at 300 °C has strong dependence of magnetic anisotropy on thickness (the stress in film structure is dominant). However for films obtained at 800 °C the magnetic properties are less sensitive to film thickness suggesting that magnetocrystalline is the only prevailing form of anisotropy [20].
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Рис. 10. Измерения магнитной коэрцитивности для тонких пленок CoFe2O4, полученных из дисперсных растворов различной концентрации наночастиц в декане Fig. 10. Magnetic coercivity measurements of thin layers of CoFe2O4 obtained with spray pyrolysis method from nanoparticles dispersion in decane
V Л $0x, B»0 2x,t>0 Sample hole
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Рис. 11. Измерения магнитной коэрцитивности для тонких пленок CoFe2O4, полученных из растворов
с различными концентрациями прекурсоров Fig. 11. Magnetic coercivity measurements of thin layers of CoFe2O4 obtained with spray pyrolysis method from precursor solutions
Conclusions
It is demonstrated that homogeneous poor-crystallized nanostructured spinel cobalt ferrite films can be deposited with different thickness by spray pyrolysis using aqueous solution of metal nitrates. It is observed from AFM images that the film covered the substrate completely and layers are fixed with a flat surface with roughness around 25-250 nm and thickness below 500 nm. The direct band gap energy of CoFe2O4 thin films obtained from different starting materials (solution from precursors or nanoparticle dispersion) is determined around 2.5 eV. Only films from nanoparticle dispersion were with noticeable crystallinity and low magnetic properties.
Thin film multi-layer system is obtained from titanium oxide and cobalt ferrite using spray pyrolysis from two spray-guns. Optical absorption measurements indicated the shift of absorption edge to visible region with increasing the number of layers. Also photo-induced potential measurements proved increased sensitivity to halogen lamp comparing with UV lamp.
Acknowledgements
Author ID acknowledge ESF within the project "Support for Doctoral Studies at University of Latvia" for the research scholarship. JK acknowledge National Research Program in Energy & Environment "LATENERGI" for financial support.
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