Effect of Spray Conditions on Formation of One-Dimensional Fluorine-Doped Tin Oxide Thin Films

This paper describes the preparation and characterization of optically-transparent thin films of fluoride-doped tin oxide (FTO) nanoparticles, nanotubes and nanorods grown using purpose-built, novel and advanced version of spray pyrolysis technique, known as Rotational, Pulsed and Atomized Spray Pyrolysis. Uniform and crack-free FTO1-D nanostructured thin films over 50 mm × 50 mm soda lime glass substrate have been routinely achieved. This technique allows a perfect control of morphology of nanostructures of FTO layer simply by adjusting the spray conditions. Formed 1-D FTO nanostructures on the glass substrate show an excellent optical transparency in the visible light range. XRD (x-ray diffraction) and SEM (scanning electron microscope) data show excellent correlations.


Introduction
Thin films have diverse applications in many technologies.Optically-transparent (OT) and electrically-conductive (EC) thin films have the further advantages typical with such properties for versatile applications in opto-electronic (OE) device fabrications [1].Usually used OTEC thin films contain transparent, conducting oxide (TCO) semiconducting materials such as Sn 4+ -doped In2O3 (ITO), F-doped SnO2 (FTO), Al 3+ -doped ZnO (AZO), B 3+ -doped ZnO (BZO), F --doped ZnO (FZO), Ga 3+ -doped ZnO (GZO), Nb 5+ -doped TiO2 (TNO), and so on [2].In these materials, their intrinsic stoichiometry has been distorted due to the replacement of a very few percentage of a cation with a higher valence than that of the usual cation of the solid, or an anion with a valence less than that of the usual anion of the solid.This introduces free electrons within the solid to attain its electrical neutrality.As such, some impurity levels containing extra electrons are introduced [3], energetically, just below the conduction band (CB) of the solid, thus contributing to the increase of n-type conductivity of the material.Although, n-type transparent, conducting oxides (TCOs) are very common in technological applications, research is been also focused on making p-type, TCO materials also [4,5].Although, this research is in its early stage, Li+-doped Cr2MnO4 is a material that has shown promising results.Among different TCO materials ITO has been widely used in optoelectronics.However, the long-term use of ITO has severe limitations [6].Compared to ITO, fluorine doped tin oxide (FTO) is low cost, indium free, stable at high temperatures, and in acidic and hydrogen environments.Therefore, FTO has been widely used for devices that require high fabrication temperature and hydrogen containing environments such as organic light emitting diodes, organic solar cells, inorganic thin film photovoltaics, and dye-sensitized solar cells (DSSCs) [7].One-dimensional nanostructured FTO thin film is important strategy to improve DSSCs performance.1-D nanostructured FTO may provide a direct scheme to enhance light absorption, interfacial surface area, and hence power conversion efficiency of DSSCs [8,9].SnO2 1-D nanostructures have been prepared by various research groups, [10,11] but there are only a few publications on FTO 1-D nanostructured thin films.For the first time, Russo and Cao fabricated FTO nanorods with the template base method.Also, Cho et al. fabricated FTO nanorods for gas sensing applications [12].Recently, Devinda Liyanage et al. synthesized FTO nanorods with ethylene glycol assisted precursor solution [13].There are various techniques to fabricate FTO films.In their broadest sense, these techniques include Physical Deposition (PD) and Chemical Deposition (CD) processes.Physical Vapor Deposition (PVD) [14], Cathodic Arc Deposition (CAD) [15], Electron Beam Physical Vapor Deposition (EBPVD) [16], Pulsed Laser Deposition (PLD) [17], and Sputter Techniques (ST) [18], belong to PD processes.PD techniques utilize some form of energy and/or vacuum techniques to evaporate the material and to deposit from its solid or liquid phase and the gas flume of evaporated material to deposit on the surface of the substrate.Since the processes involve the conversion of solid/liquid → vapor → solid, which are mere phase transformations, which do not involve any chemical reactions, all these processes belong to the category of Physical Processes.It is interesting to note that Michael Faraday used PVD techniques back in 1838 [19].Some CD processes essentially contain components of PVD but the precursor is present usually as an aerosol to undergo necessary chemical reactions to form the required material and to deposit it on the substrate surface.Chemical Vapor Deposition (CVD) [20], Spray Pyrolysis Deposition (SPD) [21], Electro-or Electroless-depositions (EDs) [22], Sol-Gel Processes [23], etc. belong to CD processes.Out of these processes, SPD techniques are versatile yet very simple.In this study, we describe a novel and improved version of SPD that can be used to fabricate wide range of TCO nanostructures on ordinary soda lime glass surfaces.This technique is versatile and there are several advantages when we compared with conventional SPD technique.Different spraying parameters such as rotational and non-rotational mode, rotational speed, and spray angle, spray duration, nozzle distance to the substrate, spray pulses and pressure can be controlled.In this research, we maintained 2s on and 13s off pulses to control the substrate temperature and low angle spraying to the substrate is mandatory to form the one dimensional nanostructures.Also the rotational mode provides a big advantage to fabricate homogeneous FTO thin films and large area coatings.We have chosen to synthesize and characterize various nanostructures of FTO and we found that this improved version of SPD which we call Rotational, Pulsed and Atomized Spray Pyrolysis (RPASP) can be utilized to fabricate FTO nanoparticle (NP), nanotube (NT) and nanorod (NR) structures on normal glass surfaces simply by changing amount of the precursor solution.Such diverse nanostructures may find innumerable applications in many electronic and opto-electronic devices.

Experimental
FTO precursor solution was prepared by mixing tin (IV) chloride penta-hydrate (SnCl4.5H2O98%, Wako Chemicals) and Ammonium fluoride (NH4F 98%, Aldrich Chemicals) in deionized water with addition of 8% propanone ((CH3)2CO 97.7% Wako Chemicals).The concentration of SnCl4.5H2O was fixed at 0.20M and NH4F was controlled to be 0.80M.FTO precursor solution was sprayed on to soda lime glass substrate with 50mm * 50mm size by using rotational, pulsed and atomized spray pyrolysis deposition technique and air as a carrier gas.The glass substrate was put onto the hot plate and heated at 480 0 C for 10 min before coating.The precursor solution materials are transferred to the glass substrate by atomizing the solution and required nanostructures form after evaporation of the solvent.The spray pressure of the precursor solution was 0.20 MPa and spray process carried out at low angle to the substrate.The deposition volume was varied from 25 ml to 75ml.The thin layers formed at each spray volume were subjected to Scanning Electron Microscopic (JEOL JSM-6320F) , X-Ray Diffraction (XRD, Rigaku RINT Ultima-III, Cu Kα, λ = 1.541836Å) and UV-Visible Transmission (JASCO V630).

Results and Discussion
Fig. 1 shows the XRD patterns of FTO thin films prepared by spraying the precursor solution with various volumes.The results show that the nanostructured thin films were observed to be cassiterite type with the tetragonal rutile structure.As expected, during the smallest spray amount, the materials formed has 0-D geometry(NP) and XRD peaks from 2θ 26.59° (110), 33.89° (101), 37.96° (200) and 51.79° (211) all characteristic of SnO2 with the standard PDF card no 01-072-1147 of PDXL XRD analysis software.As shown in Fig. 1(a).The FTO sample prepared with 25ml of precursor solution is well crystallized and (110) plane became the preferred orientation.In the SnO2 crystal structure, the (110) plane is the thermodynamically most stable plane due to the lowest surface energy.Therefore, FTO thin film showed the preferential orientation of ( 110 Further spray of 75ml FTO solution gradually converted the nanotubes to almost vertically aligned nanorods as revealed in Fig. 2 (c).The diameter of the nanorods is between 40nm to 80nm in size range.The formation of FTO nanostructure in aqueous solution is achieved by the hydrolysis of SnCl4.5H2O.During that process, Cl -ions are replaced by OH -ions and produce Sn-OH, which then turns into Sn(OH)2.The final product SnO2 is formed with the pyrolysis process.Spraying at low angle to the substrate is very important to the growth of one dimensional nanostructure.The density of the aerosol spray of the precursor solution can be increased when it sweeps along the substrate surface.As a result, SnO2 nanostructures grow along vertical direction.As depicted from Fig. 2 (a).Vertically aligned FTO nanoparticles are formed in the initial stage of the spray.However, the increase of the spray volume is involved to generate almost vertically aligned FTO nanotubes and nanorods.Also the SEM images confirmed that the prepared FTO thin films are crack-free and uniform.UV-Visible transmittance spectra depicted in Fig. 3 (a) for FTO nanoparticles, (b) for nanotubes and (c) almost vertically align nanorods formed at 480 0 C clearly show that all three samples are highly transparent in the entire visible range with percentage of transmittance values of 85.0%, 84.0% and 76.0% respectively.The FTO NP has over 85% optical transmittance in the visible range as its higher crystallinity and transmittance of the NR has decreased due to increase of the films thickness with the deposition volume.The light absorption in the region of UV region is attributed to optical band gap energy of FTO thin film.The band gap energy is related to light absorption due to the electron-interband transitions from the valence to the conduction band, like this equation, Eg∝(αһν) 2 .Even though, the visible light transmission of the all FTO thin films showed higher values, the conductivity is thought to be not better in NT and NR thin film due to lack of contact in between nanostructures.Therefore, further research need to be carried out in order to enhance the conductivity of the 1-D nanostructured FTO thin films.

Conclusion
A novel and improved SPD technique, known as RPASP, has been developed to prepare different nanostructures of FTO on soda lime glass surfaces.The rotational method of this technique was important to enhance the homogeneity of the FTO thin films.Proper control of the kinetics and spray amount enables the synthesis of FTO nanoparticles, nanotubes and nanorods layers firmly attached to glass surfaces.Nanomaterials prepared have excellent optical transparency in the visible range.

Fig. 1 .
Fig.1shows the XRD patterns of FTO thin films prepared by spraying the precursor solution with various volumes.The results show that the nanostructured thin films were observed to be cassiterite type with the tetragonal rutile structure.As expected, during the smallest spray amount, the materials formed has 0-D geometry(NP) and XRD peaks from 2θ 26.59° (110), 33.89° (101), 37.96° (200) and 51.79° (211) all characteristic of SnO2 with the standard PDF card no 01-072-1147 of PDXL XRD analysis software.As shown in Fig.1(a).The FTO sample prepared with 25ml of precursor solution is well crystallized and (110) plane became the preferred orientation.In the SnO2 crystal structure, the (110) plane is the thermodynamically most stable plane due to the lowest surface energy.Therefore, FTO thin film showed the preferential orientation of (110) in the initial crystal formation process, but (101) becomes the preferential orientation with deposition volume increased.The change in preferred orientation from (110) to (101) may be due to the growth of nanocrystalline structure of FTO from 0-D geometry to 1-D nanostructured geometry.As evident from Fig. 1(b) and 1(c), samples prepared with 50ml and 75 ml precursor solutions have only the 33.89° (101) peak corresponding to 1-D nanostructures of FTO species.Preferred orientation of the film, implying an enhancement in the number of grains of 1-D nanostructures along the (101) plane.There are no features of fluoride in the patterns of SnO2: F films, providing experimental evidence of the incorporation of fluorine atoms into the SnO2 lattice.Peaks for SnO or Sn phases were not observed, indicating that the films were fully oxidized.The intensity of (101) preferred orientation of the 1-D nanostructures thin films increased with amount of deposition volumes.It indicates that the crystallinity of the 1-D nanostructured FTO thin films has further increased along (101) plane with deposition amounts.The SEM images corresponding to the NP, NT and NR are shown in Fig. 2(a), (b) and (c), respectively.These images clearly show that the smallest spray amount of 25ml produces FTO nanoparticles of 30nm to 60nm size range.These nanoparticles also vertically oriented and appeared as needle-like shapes crystals.Increasing the spray amount until 50ml produces FTO nanotubes as clearly visible from Fig. 2(b).The dimeter of the Nanotubes varied from 40nm to 70nm.