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

Ajith Bandara1*, R.M.G. Rajapakse2, Masayuki Okuya3, Masaru Shimomura3 and Kenji Murakami3*

1Graduate School of Science and Technology, Shizuoka University, Hamamatsu 432-8011,

Japan 2 Department of Chemistry, University of Peradeniya, Peradeniya 20400, Sri Lanka 3Graduate School of Integrated Science and Technology, Shizuoka University,

Hamamatsu 432-8011, Japan

E-mail: h.m.u.g.a.bandara.14@shizuoka.ac.jp; rskmura@ipc.shizuoka.ac.jp

(Received October 8, 2015)

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.

1. 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 Sn4+-doped

In2O3 (ITO), F-doped SnO2 (FTO), Al3+-doped ZnO (AZO), B3+-doped ZnO (BZO), F--doped ZnO

(FZO), Ga3+-doped ZnO (GZO), Nb5+-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

JJAP Conf. Proc. (2016) 011102©2016 The Japan Society of Applied Physics

4Proc. 14th Int. Conf. on Global Research and Education, Inter-Academia 2015

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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.

2. Experimental

FTO precursor solution was prepared by mixing tin (IV) chloride penta-hydrate (SnCl4.5H2O 98%,

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 0C 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 = 1.541836 Å) and UV-Visible Transmission (JASCO V630).

011102-2JJAP Conf. Proc. (2016) 0111024

3. 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) 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.

(a) (b)

(c)

Fig. 1. X-Ray Diffractogrammes of FTO samples prepared at (a) 25ml, (b) 50ml and (c) 75ml

volumes of the precursor solution.

011102-3JJAP Conf. Proc. (2016) 0111024

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.

Fig. 2. SEM images of FTO thin films deposited from (a) 25ml, (b) 50ml and (c) 75ml volumes of the

precursor solution.

(a) (b)

(c)

011102-4JJAP Conf. Proc. (2016) 0111024

UV-Visible transmittance spectra depicted in Fig. 3 (a) for FTO nanoparticles, (b) for nanotubes

and (c) almost vertically align nanorods formed at 480 0C 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.

Fig. 3. UV-visible transmittance spectra of FTO samples prepared with (a) 25ml, (b) 50ml and (c) 75ml

volumes of the precursor solution.

4. 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.

Acknowledgment

We would like to express our sincere gratitude to Dr. Davinda Liyanage and Dr. Viraj Jayaweera for

their designing of rotational spraying apparatus and continuous supports.

(a) (b)

(c)

011102-5JJAP Conf. Proc. (2016) 0111024

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