Titania Thin Film Coated Glass for Simultaneous Ammonia Degradation and UV Light Blocking Layer in Photovoltaics

Abstract

In this work, we have investigated the potential dual application of TiO₂ thin films as a photocatalyst for ammonia degradation, and as a UV light blocking layer in c-Si photovoltaics. For this purpose, we deposited a series of TiO₂ thin films on a glass substrate by reactive magnetron sputtering and analysed the influence of the deposition parameters (O₂/Ar working gas content and pressure) on the structural, optical and photocatalytic properties. All samples are nanocrystalline anatase TiO₂ and have a uniform surface (RMS roughness < 5 nm) in a wide range of magnetron sputtering deposition parameters. They are transparent in the Vis/NIR spectral range and strongly absorb light in the UV range above the optical bandgap energy (3.3 eV), which makes them suitable for the use as UV blocking layers and photocatalysts. The photocatalytic properties were studied in a mini-photocatalytic wind tunnel reactor by examining ammonia degradation. A kinetic study was performed to estimate the reaction rate constants for all samples. The intrinsic reaction rate constant confirmed the crucial role of surface morphology in ammonia decomposition efficiency.

1. Introduction

Titanium dioxide (TiO₂) is one of the most intensively studied compounds in materials science due to its advantageous optical, electrical, mechanical and chemical properties [1,2,3,4]. It can be found in nature as a mineral in three polymorphic forms: rutile, anatase and brookite. Most of the research and application is focused on the polymorphs anatase and rutile, as pure brookite is difficult to obtain [5]. TiO₂ is a wide bandgap semiconductor (E_g ≈ 3 eV) whose band edge positions are suitable for solar cell application as an electron transport layer [6] and for a wide range of uses: hydrogen production by water splitting, photocatalysis, degradation of air pollutants, self-cleaning, etc. [7]. It is also known as a non-toxic, environmentally friendly and corrosion resistant material. The properties of TiO₂ depend largely on its microstructure and crystallographic phase.

Another benefit is that TiO₂ can be tailored in terms of size and shape. Nanostructured TiO₂ has a higher surface-to-volume ratio and distinct physical and chemical characteristics in comparison to bulk TiO₂, as well as longer diffusion lengths and lifetimes of photogenerated charge carriers, all of which contribute to increased photocatalytic efficiency [8]. Furthermore, the crystal shape determines the number of atoms at the surface, which means nanostructured TiO₂ contains more atoms on its surface, resulting in a greater number of active sites [9].

In the form of thin films, TiO₂ can be synthesised by various chemical and physical methods: sol-gel syntheses followed by spin and deep coating, chemical vapour deposition (CVD), plasma-enhanced CVD (PECVD), magnetron sputtering, electron beam evaporation, electrochemical deposition, pulsed laser deposition, atomic layer deposition etc. [10]. Magnetron sputtering and PECVD techniques generally allow deposition at a low substrate temperature, but usually, postdeposition annealing is required to obtain crystalline TiO₂ at temperatures above 450 °C [11]. In this work, for TiO₂ thin films deposition, we have used reactive magnetron sputtering [12]. Reactive magnetron sputtering of thin films is intensively investigated because the sputtering of metallic targets in the presence of reactive gas makes it possible to easily form compound films, such as nitrides, oxides, carbides or their combinations [13]. For oxide thin films, such as TiO₂ and ZnO, an argon + oxygen gas mixture could be used during deposition.

Due to its very high transparency in the visible range of the solar spectrum, TiO₂ finds an important application in photovoltaics. The new generation of solar cells, dye-sensitised solar cells and perovskite solar cells, are based on a porous TiO₂ layer, using TiO₂ as an electron transport layer. For this purpose, different morphological forms of TiO₂ are being tested: nanocrystals, nanowires, nanotubes, nanorods [14].

It can also be used as a UV blocker and self-cleaning layer in second generation solar cells based on mono- and polycrystalline silicon [15]. It is well-known that the performance of silicon solar cells degrades under UV light. This is mostly related to the degradation of ethylene vinyl acetate copolymer (EVA) foil encapsulant under UV light. TiO₂ has a wide optical band gap and efficiently absorbs light in the UV spectral range above the band gap energy and can be potentially used as UV light blocking layer in c-Si photovoltaics. TiO₂ is also transparent in the visible region of the spectrum, where the spectral sensitivity of silicon solar cells is highest.

On the other hand, the high absorption in the UV range of the solar spectrum makes TiO₂ very attractive for use as a photocatalyst for the degradation of air and water pollutants [16,17].

The NH₃ degradation over TiO₂-based photocatalysts has been thoroughly investigated [18,19,20,21,22]. Complimentary to the generally accepted oxidation pathway initialised by ^•OH radicals, Yuzawa et al. [21] proposed a tentative reaction pathway for NH₃ degradation over TiO₂ loaded with metal co-catalysts. The reaction pathway included oxidation of NH₃ to hydrazine via amide radicals. Hydrazine was eventually degraded to N₂ and H₂, with ammonium ions as by-products. Several studies confirmed the role of humidity in ammonia oxidation [21,22] and references therein, since it ensures the continuous progress of the reaction driven by the ^•OH radical formed on the TiO₂ surface, while the presence of water (vapour) restricts the accumulation of the undesirable ammonium ions on the TiO₂ surface. Disregarding the dominant degradation pathway, the role of incident irradiation and photocatalyst optical properties play a crucial role in terms of NH₃ degradation efficiency. Therefore, any intervention into photocatalysts surface morphology that will change the absorption and scattering coefficients should be thoroughly studied [20].

Most of the recent publications are related to photocatalytic reactions in liquid mediums such as low concentration water pollutants. There are few publications mentioning photocatalytic degradation of air pollutants, especially NH₃ [22]. Furthermore, a theoretical model of zero-order kinetics is very rarely used to analyse the results of photocatalytic experiments, which we have applied in our manuscript instead of the commonly used first-order kinetics.

The aim of this work is to investigate the influence of reactive magnetron sputtering deposition parameters (working gas O₂/Ar ration and pressure) on the structural and optical properties of the TiO₂ thin films, and to discuss their potential simultaneous use as UV blocking layer in solar cells and photocatalyst for air pollutant degradation. For structural analysis Raman spectroscopy (RS), grazing-incidence X-ray spectroscopy (GIXRD) and atomic force microscopy (AFM) were used. For characterisation of optical properties, we used UV-Vis transmittance/reflectance technique. All prepared samples were tested for ammonia photocatalytic degradation.

2. Materials and Methods

The TiO₂ thin film samples were prepared by DC reactive magnetron sputtering using a gas mixture of argon and oxygen as the working gas. The O₂/Ar flow rate ratio was varied in the range of 0.05 to 0.50. The Ar and O₂ gas flows were regulated and controlled by two mass flow controllers. The base pressure in the magnetron vacuum chamber was 7.0 × 10⁻⁷ mbar. Two values were used for the working gas pressure: 0.67 × 10⁻³ and 1.33 × 10⁻² mbar. The Ar/O₂ mixture was introduced into the magnetron vacuum chamber near the titanium target. The magnetron source Thorus 2 HV (Kurt J. Lesker Company, Jefferson Hills, PA, USA) was used for the TiO₂ deposition. The deposition was done at room temperature, but during the deposition the substrate temperature had increased up to 60 °C due to the interaction between plasma and the glass substrate. A target made of pure Ti (99.999%) with a diameter of 50.8 mm was used, and the distance between target and substrate was 140 mm. The deposition rate (Figure 1) for TiO₂ with the O₂/Ar mixture as the working gas is slightly lower than for Ti deposition with pure Ar working gas. It also decreases with the increase in the working gas O₂/Ar flow rate ratio because the lighter oxygen ions are less efficient in the sputtering process.

The TiO₂ thin films were deposited on a 1 mm thick glass substrate. Before deposition, they were cleaned according to the standard cleaning protocol: ultrasound sonication for 8 min in acetone; then 8 min in isopropyl alcohol, after which they were washed with mili-Q water and dried in a nitrogen flow, and finally treated for 15 min in an UV ozone cleaner to remove any residual organic solvents. After deposition, the TiO₂ thin film samples were annealed in a tube furnace (air atmosphere) at 450 °C for 1 h (heating rate 5 °C/min) to obtain nanocrystalline anatase TiO₂ thin films.

The content of the working gas plasma during deposition by magnetron sputtering was monitored by optical emission spectroscopy (OES). OES spectra were recorded using the HR4000 spectrometer (Ocean Optics Inc., Dunedin, FL, USA), which was connected to a quartz window (viewing window) of the magnetron sputtering chamber via optical fibre. A focusing lens was used for efficient light collection. A total of 100 single exposures (200 ms exposure time) were averaged for one scan, and the collection rate was 1 image (scan) per minute. The spectral resolution was 0.27 nm. The OES spectrum of gas plasma close to the substrate was recorded because the space close to the target is shielded by the dark space shield of the magnetron source.

The morphology and structure of the obtained TiO₂ thin films were analysed using AFM, GIXRD and RS.

N’tegra Prima SPM Atomic Force Microscope (NT-MDT, Moscow, Russia) was used for the TiO₂ thin film samples surface mapping in contact scanning mode. Dimensions of the scanned area were 2.5 μm × 2.5 μm. Before quantitative analysis, raw AFM images were corrected by mean plane and polynomial background subtraction. Moreover, larges voids or spikes at the sample surface were masked for surface roughness analysis (root mean square roughness).

GIXRD measurement were obtained using synchrotron X-ray radiation at the MCX beamline at synchrotron Elettra (Trieste, Italy) in grazing angle geometry with the a wavelength of 0.155 nm (8 keV) [23]. The diffraction patterns were obtained at several values of the angle of incidence slightly above the critical angle for total external reflection for TiO₂ in order to probe at different depths below the surface. The scattered intensity was collected in a 2θ angular range of 20°–75° with a step-size of 0.05°.

RS measurements were done using Horiba Jobin Yvon’s T64000 confocal micro-Raman spectrometer (Horiba, Kyoto, Japan) equipped with a solid-state laser operated at 532 nm for excitation, an 50× magnification objective. The measurements were conducted with a large working distance. The power of the laser was 20 mW and the confocal slit hole was 200 μm.

Optical properties (absorption coefficient, optical gap) were obtained by UV-Vis transmittance and reflectance measurements. Halogen and tungsten light source in combination with Ocean Optics HR4000 UV-Vis spectrometer (Ocean Optics Inc., Dunedin, FL, USA) was used for UV-Vis transmission detection in a wavelength range of 250–1000 nm. From obtained transmission data, the layer thickness and dielectric constants (index of refraction and extinction) were calculated using a point-wise unconstrained optimisation approach and approximating sample geometry with one thin film layer with plan-parallel boundaries on thick low-absorbing substrate, as described previously [24]. The optical gap was calculated using standard Tauc formula [25,26].

All photocatalytic experiments were conducted in a mini-photocatalytic wind tunnel (MPWT) reactor. The reactor was made of laboratory glass (DURAN^®) that transmits UV radiation larger than 305 nm. The shape of the reactor is cylindrical, 45 mm in diameter and 155 mm long, and it is assembled from two halves for easier insertion of the photocatalyst. More details about experimental setup for photocatalytic degradation of ammonia in air can be found in [22]. The MPWT reactor is placed under the lamps, so its entire surface is illuminated. The photocatalyst was placed in the middle of the photoreactor, at 7 cm distance from the lamps. The source of irradiation were linear full spectrum lamps (Terra Exotica-Sunray UVB 6.0) with an enhanced UVB effect simulating solar radiation. The lamps are 120 cm long and 36 W and are coated with a high-efficiency reflective surface in the shape of a parabolic mirror to direct radiation on the photoreactor surface. Average UVA intensities measured on the photocatalyst surface were 0.571 mW/cm², and UVB intensities 0.678 mW/cm². The air pump (Fluval Q2, Rolf C. Hagen Ltd., Castleford, UK) was used to volatilise ammonia from an evaporation chamber filled with ammonia solution (25% p.a., Kemika), C_(NH3,aq)(0) = 100 ppm. The maximum flow rate of 240 L/min was used with a resulting air flow velocity of 3.4 cm/s. Ammonia in the air stream enters the reactor in which the photocatalyst is located. Mean residence time in MPWT was τ = 4.41 s. Two outlets lead from the reactor: one that drains the excess gas into the Rettberg flusher filled with distilled water, and the other that is connected to the Geotech GA5000 (QED Environmental Systems Ltd., Coventry, UK) gas sensor for NH₃ concentration in outlet air stream monitoring at desired time intervals. Additional blank experiments were made to monitor the ammonia concentration at the entrance of the reactor and to estimate the trendline during desired time interval; the T-valve was used at the entrance to bypass the reactor and measure ammonia directly.

3. Results and Discussion

3.1. Optical Emission Spectroscopy (OES)

Figure 2 compares the OES spectra as a function of the working gas mixture O₂/Ar flow rate ratio. The most intense OES lines characteristic for Ar, Ti and O are assigned by comparison with data from the NIST Atomic Spectra Database [27]. All OES spectra presented in Figure 2 are dominated by Ar emission lines in the 600–900 nm range. The most intensive emission lines characteristic for Ti are clearly observed in the 350–550 nm range only when pure argon is used as the working gas. When O₂ is added to the working gas mixture, the surface of the Ti target is immediately oxidised and the bare Ti emission lines are attenuated. Emission lines of possible impurities present in the working gas, such as hydrogen, nitrogen, carbon or ^•OH, were not observed in the OES spectra.

According to NIST Atomic Spectra Database, several emission lines characteristic for oxygen ions can be observed (777 nm, 844 nm, 926 nm) as shown in Figure 2. The most intensive one, around 777 nm (Figure 3), was used to check the O₂/Ar flow rate ratio in the working gas mixture. The ratio of the integrated intensity of the oxygen line (777 nm) and the closest Ar emission line (772 nm) are presented in Figure 4. The OES line intensity ratio is linear dependent on the O₂/Ar flow rate ratio in the whole measured range and is not dependent on the total working gas mixture pressure in the magnetron sputtering chamber.

3.2. Structural Properties and Surface Morphology

The Raman spectra of all samples (Figure 5) confirmed that, by annealing at 450 °C for 1 h, we have obtained the anatase crystalline phase. In the sample prepared with the lowest O₂/Ar flow rate ratio, only the bands characteristic for anatase (at 144, 197, 399 and 519 cm⁻¹) can be observed [28], while in the other samples only the most prominent band around 144 cm⁻¹ was visible. The absence of other characteristic bands, as well as the broadness of this band, indicates the poor crystallinity of the samples.

Quantitative analysis of the E_g band around 144 cm⁻¹ shows a peak-shift to lower frequencies (

Table 1. Results of structural and optical analysis of TiO₂ thin film obtained by RS, GIXRD, AFM and UV-Vis transmittance.

O2/Ar Flow Rate Ratio	O2/Ar Pressure (Pa)	Raman Eg Band Position (cm−1)	GIXRD DScherrer (nm)	AFM RMS Roughness (nm)	UV-Vis Optical Gap Eg (eV)
0.05	0.67	144.25 ± 0.05	4.32 ± 0.05	3.96	3.27 ± 0.01
0.20	0.67	142.9 ± 0.1	4.44 ± 0.05	1.70	3.34 ± 0.01
0.50	0.67	142.99 ± 0.09	3.87 ± 0.05	2.12	3.35 ± 0.02
0.05	1.33	144.1 ± 0.1	4.71 ± 0.05	4.66	3.32 ± 0.02
0.20	1.33	142.5 ± 0.1	4.60 ± 0.05	4.36	3.36 ± 0.02
0.50	1.33	142.5 ± 0.1	-	2.80	3.36 ± 0.02

) compared to the value of bulk material taken from the literature [28]. The shift is more pronounced for samples deposited with a higher O₂/Ar flow rate ratio. This could be due to the presence of tensile strain in the TiO₂ thin film layers [29].

GIXRD diffractograms of the as deposited and the annealed TiO₂ thin film samples, deposited with the various O₂/Ar flow rate ratios and the working gas pressure Pa, are presented in Figure 6. It can be seen that the as deposited TiO₂ thin films are amorphous. Diffraction maxima characteristic for the anatase polymorph were observed. In comparison to other samples, the sample deposited with the highest contribution of oxygen in the working gas mixture (O₂/Ar = 0.50) and the higher working gas pressure (1.33 Pa) had the rutile phase as dominant (Figure 6). A very weak and wide background peak below 2θ = 30° is correlated to the glass substrate.

The average size of the nanocrystals is estimated from the width (FWHM) of the anatase (101) diffraction maximum by using the simple Scherrer formula [30], and it is between 3.5 and 5.0 nm for all samples (Table 1). The TiO₂ layers deposited with higher working gas pressure consist of somewhat larger nanocrystals. Moreover, nanocrystal size slightly decreases for the higher values of the working gas O₂/Ar flow rate ratio.

The surface morphology was analysed by atomic force microscopy (Figure 7). For all prepared samples, the surface is very flat with only a few very large spikes or grains. If these very large grains are excluded from the analysis, the RMS roughness is around or below 5 nm for all analysed samples (Table 1). RMS roughness is highest for the sample deposited with the lowest O₂/Ar working gas flow ratio. Samples with a higher average roughness are also deposited at a higher working gas pressure.

As expected, the RMS roughness values are comparable to average nanocrystal sizes estimated from GIXRD by using the Scherrer formula (Table 1). This indicates that the surface of the samples is dominated by the upper surfaces (endings) of the nanocrystals. A higher RMS roughness also means a larger effective surface area, which could improve the efficiency of the photocatalytic process and diffuse light scattering.

3.3. Optical Properties

The optical properties of the deposited TiO₂ samples were investigated by measuring the optical transmittance and reflectance in the UV, visible and NIR parts of the spectrum (Figure 8).

In the visible part of the spectrum, the TiO₂ samples are mostly transparent with a transmittance greater than 80%. In the NIR range, the transmittance of samples deposited with a higher working gas pressure and lower oxygen content almost reaches the transparency of glass substrates (92%). A significantly lower transmittance within the Vis/NIR range is only observed for the sample with O₂/Ar = 0.05 and p = 0.67 Pa. This may be due to deposition conditions (low working gas pressure and oxygen content) that are very close to the so-called metallic regime of reactive magnetron sputtering process [12]. In the UV range, below 350 nm, the transmittance of all samples decreases rapidly and approaches zero, indicating the existence of an optical bandgap in this wavelength range. The samples deposited at the higher values of working gas pressure have a higher transmittance compared to the samples deposited at the same O₂/Ar ratio and a lower working gas pressure. Interference fringes in the transmittance and reflectance data are not observed because the TiO₂ layers are very thin (about 50 nm). The interference fringes are only visible in the sample deposited with the smallest O₂/Ar ratio (0.05) and lowest pressure (0.67 Pa).

The transmittance of the EVA foil (Lushan EV1050G2), which is commonly used as an encapsulation material in c-Si photovoltaic (PV) modules between glass and solar cells, is added in Figure 8 for comparison. For the transmittance experiment, the EVA foil sample was laminated between two glass slides (the same as the substrate used for the TiO₂ thin film deposition). In the Vis/NIR range, the transmittance of all TiO₂ samples is comparable to the transmittance of the EVA foil while in the UV range, the TiO₂ samples start to absorb (block) light at higher wavelengths than the EVA foil. In this way, using the TiO₂-coated glass instead of the bare glass substrate in front of the EVA foil encapsulant will prevent UV light from reaching the EVA foil, being absorbed there and causing the EVA foil degradation problems mentioned in the Introduction. However, this also reduces the incidence of light into the c-Si solar cells and lowers the initial efficiency of the solar cells. The reflectance is very low in the Vis/NIR range and is slightly higher than the reflectance of the glass substrate (8%) for all prepared samples. The variations of the reflectance in the Vis/NIR range are comparable to the variations of the Vis/NIR transmittance. Samples with higher transmittance have lower reflectance in the Vis/NIR range. In the UV range, an increase in reflectance of up to 50% can be observed for samples deposited with a lower working gas pressure and a higher oxygen content in the working gas mixture.

The spectral distribution of the absorption coefficient calculated from the transmitance/reflectance data is shown in Figure 9. The sharp decrease in the transmittance below 400 nm is associated with the decrease in the absorption coefficient below the energy of the optical bandgap. In accordance with the variation of transmittance and reflectance, the samples deposited with a higher working gas pressure and a smaller O₂/Ar ratio have a higher absorption coefficient.

The width of the optical bandgap is calculated using the Tauc relation [31] and the absorption coefficient data is presented in Figure 9. The calculated values for all analysed samples are in a range of 3.27 to 3.36 eV (Table 1). These values are very close to the value for the EVA encapsulant, which is optimal for application in c-Si solar cells. In Ref. [32] Singh showed that c-Si solar cells encapsulated with the EVA foil with a cut-off wavelength of 360 nm (3.44 eV) have the best performance. For two samples deposited at a higher working gas pressure (1.33 Pa) and a lowest O₂/Ar flow rate ratio (0.05), the absorption edge is slightly shifted towards lower energies (Table 1). The energy of the bandgap can be varied by the deposition parameters in a limited range close to the values characteristic for bulk TiO₂.

For the application as a UV blocking layer, it is mandatory that the sample absorbs and reflects the UV radiation as much as possible (maximal attenuation of UV radiation), in order to minimise the influence on solar cells efficiency (degradation of EVA foil encapsulation layer and active part of solar cell). At the same time in the Vis/NIR range, the transmittance of the UV blocking layer should be maximal without interference fringes.

Because the variations in the optical bandgap are not significant, the transmittance in the Vis/NIR range has the greatest impact on solar cell efficiency. As a result, using a TiO₂ layer deposited at a higher working gas pressure and oxygen concentration as a UV blocking layer is ideal.

3.4. Photocatalysis

The results of the photocatalytic degradation of ammonia are shown in Figure 10.

The rate of ammonia degradation was given as:

$$\frac{dC}{d\tau }=r$$

(1)

The MPWT was considered as an ideal steady state plug flow reactor (PFR), while the reaction rate was fitted to zero-order kinetics. Per single pass in MPWT, a decrease in ammonia concentration was simplified to

$$\mathsf{\Delta }C=-k_{\mathrm{z}}\tau$$

(2)

where k_z (ppm s⁻¹) represents the apparent zero-order reaction rate constant and τ is the mean residence time. The zero-order kinetics is common to heterogeneous catalysis in the gas phase when only a small fraction of pollutant molecules is in a favourable position to react, i.e., NH₃ adsorbed to the surface near oxidising radicals. The reacting fraction of pollutants molecules at the photocatalyst surface continuously replenished from the “saturated” gas phase making the reaction rate independent on the pollutant concentration. Having in mind experimentally calculated absorption coefficients, α (cm⁻¹), an equation was modified to:

$$\mathsf{\Delta }C=-k_{\mathrm{intrinsic}}\left[{\alpha }_{\mathrm{UVA}}{\left(I_{0,\mathrm{UVA}}\right)}^{0.5}+{\alpha }_{\mathrm{UVB}}{\left(I_{0,\mathrm{UVB}}\right)}^{0.5}\right]\tau$$

(3)

where k_intrinsic (ppm cm^2.5 mW^−0.5 s⁻¹) represents the intrinsic zero-order reaction rate constant for degradation of ammonia over thin films of photocatalysts, α are average absorption coefficients in UVA and UVB region and I₀ is the incident irradiation at the photocatalyst surface (mW cm⁻²).

The ammonia degradation follows the established zero-order kinetics in all experiments. The apparent zero-order reaction rate constant k_z is similar for most samples (Figure 11 and

Table 2. Results of ammonia photodegradation kinetic analysis using a model for steady state PFR and zero-order kinetics. α_UVB and α_UVA are mean values of the absorption coefficient in UVA (315–400 nm) and UVB range (280–315 nm).

O2/Ar Ratio	p (Pa)	kz (ppm s−1)	αUVB (105 cm−1)	αUVA (105 cm−1)	kintrinsic 10−9 ppm cm2.5 mW−0.5 s−1
0.05	0.67	0.00220	6.333	1.124	3.63
0.20	0.67	0.00225	4.316	0.705	5.51
0.50	0.67	0.00210	3.878	0.457	5.94
0.05	1.33	0.00281	3.993	0.706	7.35
0.20	1.33	0.00277	3.408	0.436	8.83
0.50	1.33	0.00221	3.364	0.460	7.09

). Only the samples deposited with a higher pressure (p = 1.33 Pa) and a lower O₂/Ar flow rate ratio of the working gas (O₂/Ar = 0.05 and 0.20) show a slightly higher k_z constant.

On the other hand, the variations are significantly greater in the intrinsic reaction rate constant k_intrinsic, which is calculated from k_z by normalising to the mean absorption coefficient in the UVA and UVB spectral range for each sample. This variation of k_z can be correlated with the surface morphology of the prepared TiO₂ thin films. A comparison with the results presented in Table 1 shows the large influence of the RMS surface roughness on the k_intrinsic constant. The greatest intrinsic reaction rate is obtained for the samples with the greatest RMS surface roughness, i.e., effective surface area. According to Coto et al. [33], this can be explained by the reduction of the thickness of the hydrodynamic boundary layer and a more efficient interaction between pollutant and rough catalytic surface.

As expected, our TiO₂ thin films deposited by DC reactive magnetron sputtering are less efficient in ammonia degradation compared to the results of the TiO₂ P25 nanopowder samples tested in the same type of photocatalytic reactor [22].

4. Conclusions

We have successfully prepared a series of anatase TiO₂ thin films by DC reactive magnetron sputtering using Ar + O₂ mixture as the working gas, and a pure Ti target. The prepared samples have a homogeneous surface with a surface roughness in the nanometre range (<6 nm). We have shown that the structural and optical properties of the obtained thin films can be varied in a limited range by deposition conditions, such as Ar/O₂ ratio in the gas mixture. The prepared TiO₂ thin films are transparent in the visible region of the spectrum and absorb light in the UV region below the energy of the optical band gap (3.3 eV), making them suitable for the use in UV light blocking layers in c-Si photovoltaics. In addition, the very high absorption coefficient in the UV range enables the use of TiO₂ thin films for photocatalytic degradation of ammonia. We showed that the surface morphology of the TiO₂ layer has a great influence on the kinetics of the photocatalytic process. The optimal candidates for the dual application, UV blocking layer and photocatalytic degradation of ammonia are the samples with the highest surface roughness deposited with a higher working gas pressure and lower O₂/Ar ratios.