Preparation of High-Transparency, Superhydrophilic Visible Photo-Induced Photocatalytic Film via a Rapid Plasma-Modiﬁcation Process

: In this study, different amounts of SiO 2 nanoparticles (7 nm) were added to simultaneously reach high transmittance, high hardness, and high adhesion for TiO 2 ﬁlm prepared by the sol–gel method and coated on glass through a dip-coating technique. For the ﬁlm to achieve self-cleaning, anti-fogging, superhydrophilicity, and visible photo-induced photocatalysis, TiO 2 -SiO 2 ﬁlm was modiﬁed via a rapid microwave plasma-nitridation process for efﬁcient N-doping by various N 2 containing gases (N 2 , N 2 /Ar/O 2 , N 2 /Ar). Through nitrogen plasma, the content of N atom reached 1.3% with the ratio of O/Ti atom being 2.04. The surface of the thin ﬁlms was smooth, homogeneous, and did not crack, demonstrated by the root mean square (RMS) roughness of ﬁlm surface being 3.29–3.94 nm. In addition, the ﬁlms were composed of nanoparticles smaller than 10 nm, with a thickness of about 100 nm, as well as the crystal phase of the thin ﬁlm being anatase. After the plasma-nitridation process, the visible-light transmittance of N-doped TiO 2 -SiO 2 ﬁlms was 89.7% (clean glass = 90.1%). Moreover, the anti-fogging ability was excellent (contact angle < 5 ◦ ) even without light irradiation. The degradation of methylene blue showed that the photocatalytic performance of N-doped TiO 2 -SiO 2 ﬁlms was apparently superior to that of unmodiﬁed ﬁlms under visible-light irradiation. Moreover, the pencil hardness and adhesion rating test of the thin ﬁlms were 7H and 5B, respectively, indicating that the obtained coatings had great mechanical stability.


Introduction
Titanium oxide (TiO 2 ) is a very important material due to its multifunctional application in photocatalysis, hydrophilic or hydrophobic material, gas sensors, biosensors, photovoltaic cells, photochromic/electrochromic/optical devices, corrosion protection, and bactericide [1][2][3]. Superhydrophilic and self-cleaning TiO 2 film on glass can be used to decompose oily stains or air pollutants, which could have promising potential applications in industry. Moreover, through the production and modification of light-sensitive TiO 2 thin films, mirrors or windows with high performance of self-cleaning and anti-fogging functions could be achieved [4][5][6].
TiO 2 has a high refractive index (n = 2.52 for anatase; 2.76 for rutile), resulting in poor transmittance, no adsorption bands in the visible wavelengths, and the formation of white surface coatings [7,8]. Hence, refractive index grading at the interface of two materials is required for reaching efficient transmission of a normal-incident wave for some particular wavelengths. Since SiO 2 provides a refractive index that is intermediate between that of air (index of refraction, n = 1) and that of glass (n = 1.55), SiO 2 (n = 1.46), it is generally used DI water flushing. Then, the TiO 2 /SiO 2 films on dry glass were applied via dip-coating technology at various withdrawal speeds (WS = 5, 8, 10, 20, 40 mm/s) to examine the effects on transmittance, adhesion, and hardness of the films on glass. Consequently, the prepared thin film was calcined by a furnace (in air, 2.3 kW, Tender F-12, TenDer Co. Ltd., Kaohsiung, Taiwan) at 500 • C for 1 h with a heating rate of 2 • C/min for transforming amorphous crystallization into the anatase phase.
To induce the self-cleaning ability of the film at visible light, the sensitive photocatalyst thin films with high transmittance and superhydrophilic attributes were modified by Ndoping the film via the microwave (MW) plasma-nitridation procedure using different reaction gases, including N 2 , N 2 /Ar(4.7%)/O 2 (2%), and N 2 /Ar(6.7%) at a total flow rate of 12 L/min (N 2 is balanced gas) and an applied power of 900 W with a nitridation temperature of 500 • C for 1 min. The pressure used in the MW plasma system during plasma-nitridation process was at atmospheric pressure. Table 1 presents the parameters and conditions of sample preparation in this study. The 2.45 GHz microwave (MW) plasma apparatus was applied to modify the thin films. The general scheme of the system is similar to the previous study [28]. The plasma system was assembled with a commercially available magnetron (YJ-1600, National Electronics, La Fox, IL, USA), with a stationary power of 5 kW (maximum) and operated in continuouswave mode by passing microwaves through a circulator and a waveguide with a three-stub tuner, then reaching a cavity. An arc was used to ignite the plasma. A quartz tube reactor with a diameter of 2.9 cm intersected the waveguide (ASTEX WR340, MKS, Wilmington, MA, USA), and a resonator was placed perpendicular to it.
A glass dish was used for the photocatalytic experiments. The dish was cleaned by using DI water and ethanol solution, separately, with an ultrasonic oscillator for 30 min at room temperature. Then, the SiO 2 -TiO 2 film/glass was located at the bottom of dish. The photocatalytic performance of the samples was assessed by adding 15 mL methyl blue (MB) (15 ppm) solution to the dish under visible light (Philips, T5-6WA, max. peak at 610 nm, Amsterdam, The Netherlands). The effect of N-doping on the catalysis performance of methylene blue via visible photo-induced process was examined and discussed.

Analytical Methods
The transmittance of the produced TiO 2 /SiO 2 thin film was measured by using a UV-Vis spectrophotometer between 400 and 800 nm (Perkin Elmer/Lambda 35, Waltham, MA, USA). The thickness, morphology, and topology of thin films were characterized by scanning with an electron microscope (SEM, S3000N, Hitachi, Krefeld, Germany). The crystal structure of the synthesized TiO 2 -SiO 2 film was determined using X-ray diffraction (XRD, RINT-2000, Rigaku, Austin, TX, USA) with CuKα radiation in the scan range of 15 • to 85 • (2θ). The superhydrophilic capacity of films was examined by a contact-angle analyzer (Digidrop/R&D, GBX, Dublin, Ireland). The roughness of films was analyzed by an atomic force microspore (AFM, NanoMan NS4 + D3100, Digital Instruments, Bresso, Italy). In the glow discharge zone, an optical emission spectrometer (OES, Ocean Optics, HR 4000CG, Kent, UK) was used to detect the active species involved in the plasma-nitridation process.

Average Transmittance
After the cleaning procedure of glass substrate, the average transmittance of dry glass was 90.5% for the visible light region between 400 and 800 nm. Then, TiO 2 thin films were prepared by the gel prepared from the TTIP/AcAc/EtOH/HCl/H 2 O solution via the dip-coating process. As a result, the average transmittance of double-sided TiO 2 film/glass (without SiO 2 addition) was apparently reduced. At a 5 mm/s withdrawal speed (WS), the average transmittance was only 68.3%. When WS was increased from 8 to 40 mm/s, the average transmittance was between 71.6% and 72.5% (Figure 1), significantly lower than that of original glass due to the high refractivity of TiO 2 (n = 2.52 for anatase; 2.76 for rutile) compared to that of glass (n = 1.55) with a low surface roughness, which led to a reduction in transmittance for visible light.
RINT-2000, Rigaku, Austin, TX, USA) with CuKα radiation in the scan range of 15° to 85° (2θ). The superhydrophilic capacity of films was examined by a contact-angle analyzer (Digidrop/R&D, GBX, Dublin, Ireland). The roughness of films was analyzed by an atomic force microspore (AFM, NanoMan NS4 + D3100, Digital Instruments, Bresso, Italy). In the glow discharge zone, an optical emission spectrometer (OES, Ocean Optics, HR 4000CG, Kent, UK) was used to detect the active species involved in the plasma-nitridation process.

Average Transmittance
After the cleaning procedure of glass substrate, the average transmittance of dry glass was 90.5% for the visible light region between 400 and 800 nm. Then, TiO2 thin films were prepared by the gel prepared from the TTIP/AcAc/EtOH/HCl/H2O solution via the dipcoating process. As a result, the average transmittance of double-sided TiO2 film/glass (without SiO2 addition) was apparently reduced. At a 5 mm/s withdrawal speed (WS), the average transmittance was only 68.3%. When WS was increased from 8 to 40 mm/s, the average transmittance was between 71.6% and 72.5% (Figure 1), significantly lower than that of original glass due to the high refractivity of TiO2 (n = 2.52 for anatase; 2.76 for rutile) compared to that of glass (n = 1.55) with a low surface roughness, which led to a reduction in transmittance for visible light.  In order to elevate the transmittance of TiO 2 -SiO 2 thin films, the nanoparticles of SiO 2 (0.1~3 wt.%) were added to the TiO 2 gel. It is a similar technique with the production of anti-reflection optical film by adding the material of a lower index of refraction [29]. When the additional amount of SiO 2 was in the range of 0.1~3 wt.%, the average transmittances were significantly higher than those lacking the additional SiO 2 (71.6%); a secondary result yielded a slight increase from increasing the withdrawal speed from 8 mm/s to 40 mm/s ( Figure 1). The high visible-light transmittance (88.0% to 90.6%) of as-prepared film/glass could be achieved (90.5% for clear glass) when SiO 2 nanoparticles were added; this new value was higher than those without the addition of SiO 2 due to the reduction of the index of refraction. In addition, a higher withdrawal speed led to an increase of either film thickness or roughness, resulting in higher transmittance. Due to the higher transmittance for 0.1 wt.% added SiO 2 than that for other amounts of added SiO 2 at a withdrawal speed of 8~40 mm/s, an additional amount of SiO 2 was fixed at 0.1% for the subsequent experiments.

Superhydrophilic Characteristics
The angle of contact (θ) was about 10 • for the glass substrate after cleaning procedures. However, it was easily dirtied when absorbing the air pollutants or organics which existed in the atmospheric environment. When the TiO 2 film was coated on the glass, θ decreased from 18 • to 7 • when the withdrawal speed was reduced from 40 to 5 mm/s due to the elevation of the roughness of the surface, leading to an increase in hydrophilicity [30]. After 20 min of UV radiation, the angle of contact could decrease to under 5 • , producing a superhydrophilic property of the film. However, θ reverted to its original status after 24 h of storage time in the dark.
When factoring in the addition of SiO 2 , θ decreased with the increase in the SiO 2 addition, the decrease in withdrawal speed, and the UV irradiation time. At 0.1% SiO 2 , θ decreased from 10 • to 5 • by decreasing WS from 40 mm/s to 5 mm/s under the condition of no UV irradiation ( Figure 2) due to the elevation of roughness. Moreover, after UV irradiation for 20 min, θ decreased to 1.5 •~5• (Figure 2a), achieving superhydrophilic and anti-fogging properties, as well as reverting to its original status after 24 h of storage time in the dark ( Figure 2b). Furthermore, when the addition of SiO 2 was increased from 0.1 wt.% to 3 wt.%, θ only decreased 1~3 • due to similar roughness [31].    Figure 3 shows the profile and thickness of the TiO 2 film at SiO 2 = 0.1 wt.%. The faster withdrawal speed in the dip-coating process led to a thicker film [32]. The thickness of the thin film was only 70, 90, and 110 nm at withdrawal speeds of 5, 8, and 10 mm/s, respectively.   From the morphology analysis, SEM images ( Figure 4) showed that the films comprised well-distributed nanoparticles and the primary particle size was uniform and smaller than 10 nm. As the various withdrawal speeds increased, the particle size and porosity gradually became smaller, resulting in the roughness being reduced ( Figure 4). The surface of the films was exceptionally smooth, and the nanoparticles were arranged compactly. Moreover, there was no aggregation or agglomeration of particles found, even after sintering at 500 • C.    The analysis of the atomic force microscope indicated that the root mean square roughness of the surface was only 0.25 nm for clean glass. After the coating of TiO2 with 0.1 wt.% SiO2, excluding the film thickness, the roughness was affected by withdrawal speed, reaching 5.73 nm at a WS of 5 mm/s, then decreasing to 0.61 nm at a WS of 40 mm/s ( Table 2). In order to access the adhesion between the TiO2-SiO2 (0.1%) film and clean glass, the standard test method (ASTM D3359) [33] was used by applying and removing pressuresensitive tape over cuts made in the film. The results showed that the adhesion forces of films prepared for all withdrawal speeds are ASTM class 5B, indicating no film pull-off and representing the highest level of adhesion.

Rapid Plasma-Nitridation of Films by Different N2-Containing Gases
The superior experimental conditions when preparing the high-adhesion, transparent, and superhydrophilic TiO2-SiO2 film on glass were achieved by adding 0.1 wt.% of SiO2 at a WS of 8 mm/s. However, the photocatalytic performance should be carried out The analysis of the atomic force microscope indicated that the root mean square roughness of the surface was only 0.25 nm for clean glass. After the coating of TiO 2 with 0.1 wt.% SiO 2 , excluding the film thickness, the roughness was affected by withdrawal speed, reaching 5.73 nm at a WS of 5 mm/s, then decreasing to 0.61 nm at a WS of 40 mm/s ( Table 2). In order to access the adhesion between the TiO 2 -SiO 2 (0.1%) film and clean glass, the standard test method (ASTM D3359) [33] was used by applying and removing pressuresensitive tape over cuts made in the film. The results showed that the adhesion forces of films prepared for all withdrawal speeds are ASTM class 5B, indicating no film pull-off and representing the highest level of adhesion.

Rapid Plasma-Nitridation of Films by Different N 2 -Containing Gases
The superior experimental conditions when preparing the high-adhesion, transparent, and superhydrophilic TiO 2 -SiO 2 film on glass were achieved by adding 0.1 wt.% of SiO 2 at a WS of 8 mm/s. However, the photocatalytic performance should be carried out under UV irradiation. To produce a visible photo-induced photocatalytic TiO 2 -SiO 2 film, the prepared films were modified by doping N atoms via the rapid microwave plasmanitridation process in 1 min by inducing different N 2 -containing gases (N 2 , N 2 /Ar/O 2 , N 2 /Ar).

Surface Composition Analyses
The surface compositions and functional groups of the modified films were measured by XPS analysis. The results of chemical composition analysis showed that the content of N atoms was 1.3, 0.9, and 0.1 wt.% in the TiO 2 -SiO 2 film by using N 2 , N 2 /Ar/O 2 , and N 2 /Ar plasma gases, respectively, indicating that more N atoms could be doped by N 2 plasma.
In Figure 6,  (Figure 6a,b) [34][35][36][37]. However, by using N 2 /Ar as plasma gases (Figure 6c), it is similar to unmodified films (Figure 6d), lacking an apparent N peak region that can be deconvoluted into surface functional group contributions, which may be caused by the reduction in density of high energetic N-containing species due to easier energy transfer to Ar than to N 2 molecules in the discharge zone. The main functional groups for the plasma N-doped films showed that O can be substituted rapidly for the N in TiO 2 via the formation of oxygen vacancy [38] by using an atmospheric-pressure MW plasma coupled with plasma-nitridation gases. under UV irradiation. To produce a visible photo-induced photocatalytic TiO2-SiO2 film, the prepared films were modified by doping N atoms via the rapid microwave plasmanitridation process in 1 min by inducing different N2-containing gases (N2, N2/Ar/O2, N2/Ar).

Surface Composition Analyses
The surface compositions and functional groups of the modified films were measured by XPS analysis. The results of chemical composition analysis showed that the content of N atoms was 1.3, 0.9, and 0.1 wt.% in the TiO2-SiO2 film by using N2, N2/Ar/O2, and N2/Ar plasma gases, respectively, indicating that more N atoms could be doped by N2 plasma.
In Figure 6, the N1S peak regions were deconvoluted into surface functional group contributions. The peaks of N1s observed with the chemical bonding energies around 398.2~399.3 eV, 400.6~401.7 eV, 402.7~403.7 eV, 407.1~407.3 eV, and 409.1 eV when using N2 or N2/Ar/O2 gases indicate the possible presence of N-TiO2 or N-O bonds (TiNxOy), adsorbed N2, TiO-N, N2O adsorption, and NO 3− (surface nitrate), respectively (Figure 6a,b) [34][35][36][37]. However, by using N2/Ar as plasma gases (Figure 6c), it is similar to unmodified films (Figure 6d), lacking an apparent N peak region that can be deconvoluted into surface functional group contributions, which may be caused by the reduction in density of high energetic N-containing species due to easier energy transfer to Ar than to N2 molecules in the discharge zone. The main functional groups for the plasma N-doped films showed that O can be substituted rapidly for the N in TiO2 via the formation of oxygen vacancy [38] by using an atmospheric-pressure MW plasma coupled with plasma-nitridation gases.

Photocatalytic Performance of As-Modified Films under Visible-Light Irradiation
Under the condition of UV-light irradiation, the TiO 2 -SiO 2 films will be superhydrophilic (θ < 5 • ). However, for utilization under a visible photo-induced environment, for example, indoors or a cloudy day, the superhydrophilic visible-induced TiO 2 -SiO 2 (0.1 wt.%) films on glass with high adhesion, transparent, and anti-fogging ability are prepared via the rapid (1 min) plasma N-doping process by different plasma gases, including N 2 , N 2 /Ar/O 2 , and N 2 /Ar.
The analysis of UV-visible spectra of the non-doped and N-doped TiO 2 -SiO 2 films (Figure 7) indicated that nitrogen plasma modification promoted visible-light absorption and red shift. The non-doped TiO 2 -SiO 2 film shows the absorbance edge around 380 nm. A red shift in the absorption edge toward the visible-light region (λ is about 410 nm) is evident in the N-doped TiO 2 -SiO 2 film modified by N 2 plasma at 0.9 kW for 1 min due to the incorporation of nitrogen atoms into the lattice of TiO 2 .

Photocatalytic Performance of As-Modified Films under Visible-Light Irradiation
Under the condition of UV-light irradiation, the TiO2-SiO2 films will be superhydrophilic (θ < 5°). However, for utilization under a visible photo-induced environment, for example, indoors or a cloudy day, the superhydrophilic visible-induced TiO2-SiO2 (0.1 wt.%) films on glass with high adhesion, transparent, and anti-fogging ability are prepared via the rapid (1 min) plasma N-doping process by different plasma gases, including N2, N2/Ar/O2, and N2/Ar.
The analysis of UV-visible spectra of the non-doped and N-doped TiO2-SiO2 films (Figure 7) indicated that nitrogen plasma modification promoted visible-light absorption and red shift. The non-doped TiO2-SiO2 film shows the absorbance edge around 380 nm. A red shift in the absorption edge toward the visible-light region (λ is about 410 nm) is evident in the N-doped TiO2-SiO2 film modified by N2 plasma at 0.9 kW for 1 min due to the incorporation of nitrogen atoms into the lattice of TiO2. The removal efficiency of methylene blue (MB) in a glass reactor was carried out using visible-light irradiation, to evaluate the performance of the individual N-doped films [39]. The results of degradation for the photocatalytic activity of the TiO2-SiO2 (0.1 wt.%) films modified by different plasma gases are compared in Figure 8. Under visible-light degradation, the orders of conversion of MB are the films modified by N2 (84.3%) > N2/Ar/O2 > N2/Ar >> without nitridation (14.8%) after a visible-light irradiation of 500 min, indicating that the photocatalytic performance of TiO2-SiO2 films modified by N2 plasma was slightly higher than that of those by N2/Ar/O2 or N2/Ar plasma; however, this was much higher than that of those without N-doped film. The removal efficiency of methylene blue (MB) in a glass reactor was carried out using visible-light irradiation, to evaluate the performance of the individual N-doped films [39]. The results of degradation for the photocatalytic activity of the TiO 2 -SiO 2 (0.1 wt.%) films modified by different plasma gases are compared in Figure 8. Under visible-light degradation, the orders of conversion of MB are the films modified by N 2 (84.3%) > N 2 /Ar/O 2 > N 2 /Ar >> without nitridation (14.8%) after a visible-light irradiation of 500 min, indicating that the photocatalytic performance of TiO 2 -SiO 2 films modified by N 2 plasma was slightly higher than that of those by N 2 /Ar/O 2 or N 2 /Ar plasma; however, this was much higher than that of those without N-doped film.
The results revealed that a higher removal efficiency of methylene blue for visible-light induced degradation was found at a higher content of N atoms in TiO 2 -SiO 2 film, because the red shift of N-doped films into the visible light absorption range was achieved. The results also indicate that the plasma-nitridation process can be used for rapid N-doped modification of films. Moreover, the photocatalytic reaction of MB is usually fitted with the pseudo-first-order reaction rate under visible light irradiation [28]. The results revealed that a higher removal efficiency of methylene blue for visiblelight induced degradation was found at a higher content of N atoms in TiO2-SiO2 film, because the red shift of N-doped films into the visible light absorption range was achieved. The results also indicate that the plasma-nitridation process can be used for rapid N-doped modification of films. Moreover, the photocatalytic reaction of MB is usually fitted with the pseudo-first-order reaction rate under visible light irradiation [28].

Characterization of Optical Emission Spectra
An MW plasma can be successfully used to rapidly dope N atoms into TiO2-SiO2 films in 1 min of residence time. Resulting from active or energetic N-containing species, including atomic N, excited N2, N ions, and N2 ions, are generated from nitrogen and ammonia gas in the glow discharge zone [40,41]. These energetic N-containing species can thermodynamically react with TiO2 more easily than N2 molecules, forming TiO2−xNx. Figure 9 shows the optical emission intensity of intermediate species that were detected in the glow discharge zone at 0.9 kW. For nitrogen plasma, the active or energetic species can be produced via the complex plasma-chemical reactions, such as electron-impact processes, dissociative ionization, ionization, excitation reactions, and Penning ionization reaction. The characteristic emission lines of N2 from the first positive band (B 3 Πg  A 3 Σu + transition, at 550-900 nm) and the second positive band (C 3 Πu  B 3 Πg transition, at 300-400 nm), and the emission lines of N2 + from the first negative band (B 2 Σu +  X 2 Σg + transition, at 381-470 nm) [40][41][42], and nearly all the peaks correspond to molecular nitrogen and ionic molecular nitrogen for N2 or N2/Ar plasma gases (Figure 9) can be identified. However, the atomic nitrogen metastable molecule and ionic atomic nitrogen can not to be observed in the discharge zone. Moreover, weak representative optical emission spectra of Ar in the range of 700~850 nm can also be identified. Except for N2 or N band, Figure  9 also shows that N-O bands ( system, 226.9, 237.0, 247.9, 259.6, 272.2, 286.0 nm) and very weak O peaks (777.2, 777.4 nm) can be observed in the O2 − containing (N2/Ar/O2) MW plasma [43,44].

Characterization of Optical Emission Spectra
An MW plasma can be successfully used to rapidly dope N atoms into TiO 2 -SiO 2 films in 1 min of residence time. Resulting from active or energetic N-containing species, including atomic N, excited N 2 , N ions, and N 2 ions, are generated from nitrogen and ammonia gas in the glow discharge zone [40,41]. These energetic N-containing species can thermodynamically react with TiO 2 more easily than N 2 molecules, forming TiO 2−x N x . Figure 9 shows the optical emission intensity of intermediate species that were detected in the glow discharge zone at 0.9 kW. For nitrogen plasma, the active or energetic species can be produced via the complex plasma-chemical reactions, such as electronimpact processes, dissociative ionization, ionization, excitation reactions, and Penning ionization reaction. The characteristic emission lines of N 2 from the first positive band (B 3 Π g → A 3 Σ u + transition, at 550-900 nm) and the second positive band (C 3 Π u → B 3 Π g transition, at 300-400 nm), and the emission lines of N 2 + from the first negative band (B 2 Σ u + → X 2 Σ g + transition, at 381-470 nm) [40][41][42], and nearly all the peaks correspond to molecular nitrogen and ionic molecular nitrogen for N 2 or N 2 /Ar plasma gases (Figure 9) can be identified. However, the atomic nitrogen metastable molecule and ionic atomic nitrogen can not to be observed in the discharge zone. Moreover, weak representative optical emission spectra of Ar in the range of 700~850 nm can also be identified. Except for N 2 or N band, Figure 9 also shows that N-O bands ( [43,44].

Characteristics of TiO2-SiO2 Films After Plasma-Nitridation
θ was about 10° for the clean glass substrate, 7~18° for TiO2 film/glass, 5~10° for TiO2-SiO2 (0.1 wt.%) film/glass, and lower than 5° after UV radiation of 20 min for TiO2-SiO2 (0.1 wt.%) film. The TiO2-SiO2 (0.1 wt.%) film was prepared via modification by a rapid (1 min) microwave plasma-nitridation process using different N2-containing gases, resulting in θ being lower than 5° without visible-light irradiation, as shown in Table 2. After the plasma-nitridation process, the regular, porous, and smooth surface was formed, leading to an increase in hydrophilicity [45]. Moreover, θ was still decreased to about 3° after visible-light irradiation for 20 min, regardless of what kind of plasma gases were utilized ( Table 3), revealing that the modified superhydrophilic and anti-fogging films could be used in a visible photo-induced environment. After the plasma-nitridation process, the RMS roughness of the film surface slightly decreased when compared to that without plasma-nitridation, from 3.94 nm (unmodified) to 3.54 nm (N2 plasma), 3.50 nm (N2/Ar/O2 plasma), and 3.29 nm (N2/Ar) (Table 3). Hence, the RMS roughness of films was not affected significantly by plasma-nitridation. Visiblelight transmittance was also unchanged by the different plasma gases, more so than that of the unmodified TiO2-SiO2 film or clean glass, reaching a range of 89.8~90.1% (Table 3). Both the film being modified at low temperature (500 °C) and a short modification time (1 min) resulted in the roughness not being significantly changed, and the film surface was porous and smooth with a high visible-light transmittance.
In order to assess the hardness of the TiO2-SiO2 (0.1%) film on the glass, the standard test method (ASTM D3363-05) [46] was carried out by drawing with pencil leads of known

Characteristics of TiO 2 -SiO 2 Films after Plasma-Nitridation
θ was about 10 • for the clean glass substrate, 7~18 • for TiO 2 film/glass, 5~10 • for TiO 2 -SiO 2 (0.1 wt.%) film/glass, and lower than 5 • after UV radiation of 20 min for TiO 2 -SiO 2 (0.1 wt.%) film. The TiO 2 -SiO 2 (0.1 wt.%) film was prepared via modification by a rapid (1 min) microwave plasma-nitridation process using different N 2 -containing gases, resulting in θ being lower than 5 • without visible-light irradiation, as shown in Table 2. After the plasma-nitridation process, the regular, porous, and smooth surface was formed, leading to an increase in hydrophilicity [45]. Moreover, θ was still decreased to about 3 • after visible-light irradiation for 20 min, regardless of what kind of plasma gases were utilized (Table 3), revealing that the modified superhydrophilic and anti-fogging films could be used in a visible photo-induced environment. After the plasma-nitridation process, the RMS roughness of the film surface slightly decreased when compared to that without plasma-nitridation, from 3.94 nm (unmodified) to 3.54 nm (N 2 plasma), 3.50 nm (N 2 /Ar/O 2 plasma), and 3.29 nm (N 2 /Ar) (Table 3). Hence, the RMS roughness of films was not affected significantly by plasma-nitridation. Visible-light transmittance was also unchanged by the different plasma gases, more so than that of the unmodified TiO 2 -SiO 2 film or clean glass, reaching a range of 89.8~90.1% (Table 3). Both the film being modified at low temperature (500 • C) and a short modification time (1 min) resulted in the roughness not being significantly changed, and the film surface was porous and smooth with a high visible-light transmittance.
In order to assess the hardness of the TiO 2 -SiO 2 (0.1%) film on the glass, the standard test method (ASTM D3363-05) [46] was carried out by drawing with pencil leads of known hardness. This ranges from 9H (hardest), 8H, 7H, . . . 7B, 8B to 9B (softest). The results showed that the pencil hardness of all prepared films for various plasma nitridation gases was 7H (Table 4), indicating good sol-gel coatings on glass (9H) due to the in situ prepared nanoparticles with only 0.1% SiO 2 nanoparticles due to the nature of networking [47].

Conclusions
This study successfully developed procedures to optimize the operational conditions for the sol-gel dip-coating method combined with a rapid plasma-nitridation technique to produce N-doped TiO 2 -SiO 2 nano film on glass, while simultaneously achieving a high transmittance (~90%), high hardness (7H), superhydrophilicity (θ < 5 • ), and good adhesion (5B) for the applications of self-cleaning and anti-fogging. Moreover, the high photocatalytic performance of the film/glass can be easily accomplished under visible photo-induced irradiation after the rapid plasma-nitridation process.