Surface Modification of Titanium Dioxide Thin Films via Manganese Doping∗

Mn-doped TiO2 thin films were produced by spin coating on FTO glass substrates to investigate the effects of Mn concentration on the structural, morphological, and optical properties of the films. Titanium isopropoxide and manganese chloride tetrahydrate were used as titanium and manganese sources respectively, in the precursor solution with isopropanol. The solution was added drop-wise on the substrate which was spun at 2000 rpm; annealing was done in air at 500◦C for 5 h. Films with very low dopant concentrations (≤ 1 wt%) were composed of anatase; those with moderate concentrations (3−5 wt%) showed the presence of anatase and rutile; while those with high Mn concentrations (≤ 7 wt% Mn) showed rutile formation. Increasing the dopant concentrations resulted in a slight increase in the surface roughness, except for the sample with 15 wt% Mn, that showed surface dimple formation. The films containing ≤ 10 wt% Mn showed similarity in grain sizes (∼ 10 nm) and thickness (∼ 300 nm). All the films showed relatively high transparencies, with the absorption edges shifting to longer wavelengths with increasing Mn levels. The optical indirect band gaps of the films decreased from 3.32 eV to 2.86 eV, and this data suggests that the formation of shallow trapping sites and reduced rate of electron-hole recombination are the result of the variable valence of Mn. [DOI: 10.1380/ejssnt.2012.103]


I. INTRODUCTION
Titanium dioxide (TiO 2 , titania) is used widely in several applications, including photovoltaic cells [1], dyesensitised solar cells (DSSCs) [2], water splitting [3], water and air purification [4], and self-cleaning and selfsterilising surfaces [5].TiO 2 is used widely in thin-film form rather than the bulk form owing to advantages, including high surface areas, ease of processing, and low costs.However, TiO 2 is a wide-band-gap semiconductor with an optical indirect band gap ranging from 3.0 eV to 3.9 eV for anatase [6,7].This implies that TiO 2 can be activated under ultraviolet (UV; ≤ 400 nm or 3.1 eV) radiation.Moreover, the surface roughness and surface area of a film are key parameters for consideration when using this material for water splitting, water and air purification, and for self-cleaning and self-sterilisation applications.
One of the most effective techniques to modify the optical indirect band gap and surface topology is to dope with transition metals.Several elements have been used as doping agents, including V [8], Cr [9], Mn [10], Fe [11], and Co [12].Of the preceding, Mn is a promising candidate since it exhibits three valence states (Mn 2+ , Mn 3+ , and Mn 4+ ) and so it has the potential for electron and/or hole trapping in the TiO 2 band structure, which is believed to prevent electron-hole recombination [13], and thereby enhancing the photocatalytic efficiency.
Mn-doped TiO 2 thin films can be synthesised by many methods, including pulsed laser deposition (PLD) [11], atomic layer deposition (ALD) [14], plasma-assisted molecular beam epitaxy [15], radio frequency (RF) magnetron co-sputtering [16], and spin coating [17].Spin coating is one of the simplest techniques because it does not require a vacuum system and so the process can be done in air.
The aims of the present work are to (i) deposit Mndoped TiO 2 on F-doped SnO 2 -coated (FTO) glass substrates by spin coating, and (ii) investigate the effect of the dopant concentration on the film properties (mineralogy, surface topology, and optical properties).

II. EXPERIMENTAL PROCEDURE
Precursor solutions were made from titanium isopropoxide (TIP, Reagent Grade, 97 wt%, Sigma-Aldrich) dissolved in isopropanol (Reagent Plus, ≤ 99 wt%, Sigma-Aldrich) at a Ti concentration of 0.1 M (metal basis).The solutions were mixed by stirring in Pyrex beakers by hand.The Mn dopant concentrations used were 1, 3, 5, 7, 10, and 15 wt% Mn (metal basis relative to titanium) by adding MnCl 2 •4H 2 O (Reagent Plus, ≤ 99 wt%, Sigma-Aldrich) to the solution while hand stirring.Spin coating (Laurell W S − 65052) was done by rapidly depositing ∼ 0.2 mL (ten drops) of solution onto an FTOcoated glass substrate (WuHan Geao Instruments Science & Technology Co., Ltd., China) spun at 2000 rpm in air.The films were dried by spinning in air for an additional 15 s.This process was repeated fourteen more times in order to obtain a film thickness of ∼ 300 nm in all cases.Annealing was done in a muffle furnace at 500 • C for 5 h (heating rate 5 • C/min, natural cooling).
The mineralogies of the films were analysed by glancing angle X-ray diffraction (GAXRD, Phillips X'pert Materials Research Diffraction, CuK α , 45 kV, 40 mA, step size 0.02 • 2θ, speed 6 • /min 2θ) and laser Raman microspectroscopy (Renishaw inVia, 514 nm Ar laser).The surface topologies were analysed using atomic force microscopy (AFM, trapping mode, Veeco Dimension 3000).The film thicknesses were determined using single-beam focussed ion beam (FIB) milling (FEI XP200) following the application of a ∼ 20 nm thickness gold (Au) coating by sputtering.Gallium ions (Ga + ) were used to erode a square hole in the film and an image of the cross-section of the layers was viewed at an angle of 45 • .The optical transmittances of the films were assessed using a UV-VIS dual-beam spectrophotometer (Perkin Elmer Lambda 35).

III. RESULTS AND DISCUSSION
Figure 1 shows that the films were highly transparent and, while the undoped film was colorless, the Mn-doped films were yellow in color, with the intensities' increasing with increasing dopant levels.Since the films were of similar grain size and thickness (discussed subsequently).It is clear that the color derived from the effect of the dopant rather than the microstructure.The change in color indicated that the absorption edge of the films shifted towards longer wavelengths (red shift) and this was confirmed by UV-VIS data (discussed subsequently).
The GAXRD patterns of the different films are given in Fig. 2. The undoped film and that with 1 wt% Mn consisted of anatase; the films with 3 wt% and 5 wt% Mn consisted of anatase+rutile; and those with 4 ≥ 7 wt% Mn consisted of rutile only.Since the thicknesses of the films were similar, they were fully dense, and the roughnesses varied only slightly, as shown in Table I, the low intensi- ties of the rutile are attributed to poor crystallinity.The differentiation between anatase and rutile is supported by the laser Raman microspectroscopy data shown in Fig. 3.These data demonstate the onset of rutile formation at 3 wt% Mn.
From previous work by the authors [18], it is known that annealing at 500 • C of a film with 7 wt% Mn results in the presence of both Mn 3+ and Mn 4+ valences.The sixfold-coordinated ionic radius of Ti 4+ (the case for both anatase and rutile [19]) is 0.075 nm while the equivalent radii for Mn 3+ are 0.079 nm (high spin) and 0.072 nm (low spin) and that of Mn 4+ is 0.067 nm [20].The similarities in sizes and valences suggest that substitutional solid solution formation is likely, giving rise to the attendant lattice disorder caused by structural stress and the generation of point defects.Substitution by Mn 3+ would require the formation of oxygen vacancies for charge balance [21] and these are known to enhance photoactivity [22].
It may be noted that the anatase→rutile phase transformation occurred at the annealing temperature of 500 • C. In contrast, anatase generally is considered to transform to rutile at temperatures above > 600 • C [21] in bulk materials and at ∼ 700-800 • C in thin films [6,7].However, Mn is known to promote the transformation to rutile [21,[23][24][25], which explains the present results.
The AFM images of the topologies of the films shown in  rutile were changing; the grain size at 15 wt% was not immediately apparent.These images and Table I indicate that the surface roughnesses increased slightly on the subnanometer scale from ∼ 2.012 nm (undoped) to 2.374 nm (10 wt% Mn).At 15 wt% Mn, dimples of ∼ 100 nm diameter and ≤ 20 nm depth dominated the nanostructure.This probably resulted from the exceeding of the solubility of limit of Mn in the TiO 2 structure, which was also observed by Sharma et al. [26] in samples of ∼ 15 wt% Mn spray pyrolysed at 450 • C. The UV-VIS transmission spectra shown in Fig. 5 and the data in Table I indicate that, with increasing Mn dopant levels, the absorption edge shifted toward longer wavelengths (viz., red shift), the optical indirect band gap (calculated as described elsewhere [6]) decreased significantly, and the transmittance (at ∼ 550-800 nm) decreased slightly.The interference fringes confirm the AFM data in that the films were relatively smooth.The retention of these even at 15 wt% Mn indicates that the film surface between the dimples dominates the spectrum.The decreasing optical band gap with increasing Mn dopant levels can be attributed to one or both of the following two mechanisms: 1) If substitutional (or even interstitial) Mn 3+ and Mn 4+ are present in the doped films [18], they can generate shallow trapping sites at the donor and acceptor levels [27] through valence change (Mn 2+ ↔ Mn 3+ ↔ Mn 4+ ).Increasing Mn dopant levels then would result in a progressive decrease in the optical indirect band gaps of the films.That is, the consequent electron and/or hole trapping may explain the shift in the absorption edge toward longer wavelengths through reduction in the rate of electron-hole recombination [28].
2) The anatase → rutile phase transformation changes the optical indirect band gap since the respective values are 3.2 eV [29] and 3.0 eV [29] and the overall band gap of a mixed-phase material tends to be a weighted balance between the phases [6,17].However, this is considered to be unlikely because amorphous titanium dioxide is not photocatalytic and so the observed transformation from crystalline anatase to poorly crystalline rutile is not considered to be likely to increase the photoactivity through reduction of the band gap.

IV. SUMMARY AND CONCLUSIONS
Mn-doped TiO 2 films on FTO substrates were prepared by spin coating and annealing at 500 • C; their surface characteristics and optical properties then were investigated.The major findings are as follows: • The addition of Mn promoted the phase transformation from anatase to rutile.
• The dissolution of ≥ 3 wt% Mn in the TiO 2 lattice resulted in the formation of partially amorphous rutile.
• The solubility limit of Mn in the TiO 2 lattice under the present processing conditions was between 10 wt% and 15 wt% Mn.
• The addition of ≤ 10 wt% Mn very slightly increased the surface roughness; the addition of 15 wt% Mn resulted in the formation of pronounced dimples on the surface.
• The addition of Mn shifted the absorption edge toward longer wavelengths, significantly decreased the optical indirect band gap from 3.32 eV to 2.86 eV, and slightly decreased the transmittance.
• The optical properties suggested that shallow electron and/or hole trapping and the associated reduction in the rate of recombination were responsible for the observed changes.

TABLE I :
Summary of the analytical data for undoped and Mn-doped TiO2 thin films.
FIG.4: AFM images of the undoped and Mn-doped TiO2 thin films.