Anticorrosion Coated Stainless Steel as Durable Support for C-N-TiO2 Photo Catalyst Layer

The development of durable photocatalytic supports resistant in harsh environment has become challenging in advanced oxidation processes (AOPs) focusing on water and wastewater remediation. In this study, stainless steel (SS), SS/Ti (N,O) and SS/Cr-N/Cr (N,O) anticorrosion layers on SS meshes were dip-coated with sol gel synthesised C-N-TiO2 photo catalysts pyrolysed at 350 °C for 105 min, using a heating rate of 50 °C/min under N2 gas. The supported C-N-TiO2 films were characterised by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman spectroscopy. The results showed that C-N-TiO2 was successfully deposited on anticorrosion coated SS supports and had different morphologies. The amorphous C and TiO2 were predominant in C-N-TiO2 over anatase and rutile phases on the surface of SS and anticorrosion supports. The C-N-TiO2 coated films showed enhanced photocatalytic activity for the decolouration of O.II dye under both solar and UV radiations. The fabricated C-N-TiO2 films showed significant antibacterial activities in the dark as well as in visible light. Herein, we demonstrate that SS/Ti(N,O) and SS/Cr-N/Cr(N,O) anticorrosion coatings are adequate photocatalytic and corrosion resistant supports. The C-N-TiO2 photo catalytic coatings can be used for water and wastewater decontamination of pollutants and microbes.


Introduction
The removal of recalcitrant organic pollutants such as dyes, pharmaceuticals, and personal care products in industrial wastewater effluents and sewage has been the subject of research in recent years [1][2][3][4][5][6][7]. On the other hand, the presence of micro-organisms in water and wastewater treatment effluents and distribution systems has resulted in a challenge to reclaim potable water from unconventional sources. The consumption of pathogen-contaminated water has exposed low-income populations to water-related sicknesses such as diarrhea, typhoid fever, cholera, giardia, dysentery, etc., which in return have devastated communities worldwide [8][9][10]. Various treatment methods such as adsorption on activated carbon, ozonation, reverse osmosis, ion exchange on synthetic adsorbent resins, flocculation, etc. have been developed [1,[11][12][13].
However, most of these methods have high operating costs and/or are inefficient due to the complexity of the aromatic structures of persistent organic pollutants [13]. Likewise, the resistance of pathogens and micro-organisms in treatment systems often requires the use of excessive amounts of disinfectants such as chlorine, which in turn incur high costs for their removal [14]. The existing methods developed for bulk water sterilisation still show some limitations. Typically, exposure of bacterial colonies to toxic compounds such as ethylene oxide or chlorine gas followed by high temperature and pressure are common techniques for the deactivation of micro-organisms [15]. However, previous reports claimed that some of these procedures could damage/destroy the equipment used to kill micro-organisms and often suffer from reduced efficacies [16]. Therefore, there is a need for the development of new and appropriate advanced treatment protocols that are capable of not only degrading organic pollutants but also are efficient at eliminating micro-organisms from water sources.
Advanced oxidation processes (AOPs) are considered as robust techniques capable of degrading organic contaminants in water and wastewater, converting them into harmless substances without any post-treatment processes needed [17][18][19][20][21][22][23][24]. AOPs are based on the generation of the hydroxyl radical, a strong oxidation agent that can completely degrade organic contaminants into forms such as CO 2 , water and simple salts [4,20,22,24,25]. AOPs induced by photo catalysis under solar and UV light are promising techniques for the removal of POPs and deactivation of pathogenic micro-organisms from water sources [26,27]. When irradiated, photo catalysis using semi-conductor catalysts such as TiO 2 , ZnO, etc., produces diverse reactive oxygen species, including O 2 − , O . , H 2 O 2 and mostly non-selective OH . that efficiently terminates POPs and micro-organisms [28]. In AOPs, heterogeneous photo catalysts such as TiO 2 have been used to accelerate the production of free radicals by both oxidation and reduction processes [4]. TiO 2 has been used as a convenient photo catalyst due to its low cost, high stability and exceptional photo catalytic effectiveness [29]. Consequently, various classes of water pollutants including azo dyes and microorganisms have been treated using TiO 2 -based photo catalysis [13,21,[29][30][31].
Research also supports that semiconductor catalysts can be doped to control the band gap and reduce the electron-hole recombination rate, which in turn may improve the activity of the catalyst [32][33][34][35][36][37]. Moreover, literature also states that semiconductor photo catalysts in their single or co-doped form can be deposited on particular supports to overcome the post-separation dilemma experienced with powder catalysts [38,39]. Even though previous studies reported that the coating process may decrease the specific surface area of the catalysts [40,41], coating of various material supports such as SS has been conducted [42][43][44][45]. Apart from these, Zhang and Wang [46] reported that prolonged exposure of stainless steel (SS) in oxidizing or acidic environments may result in its corrosion mostly when the Cr 2 O 3 passive layer is scratched and may cause metal rusting, loss of thickness and weight. This could lead to contamination of water effluents being treated. Hence, the corrosion of SS in the oxidizing environment needs to be overcome to achieve the desired removal of the pollutant and avoid undesired water toxicity. SS mesh (304 L grade) was coated with Ti and Cr transition metal-based nitrides and oxynitrides as both mono-and double-protective layers by cathodic arc evaporation (CAE) method [47,48]. The anticorrosion behaviour of the obtained SS/Ti(N,O) and SS/Cr-N/Cr(N,O) coatings was also investigated and were found to be corrosion resistant in acidic environments [49]. 532 nm laser excitation. The Raman spectra were collected from 50-1000 cm −1 using a TE cooled CCD camera (Horiba, Kyoto, Japan) attached to the monochromator of a spectrometer with 600 gr/mm. The spectra were obtained by collecting 10 acquisitions.

Photo Catalysis Investigation
The photo catalytic activity of the C-N-TiO2-coated supports including SS and anticorrosion meshes were determined by the decolouration of orange II sodium dye, under solar and UV light as described in Figure 1. Beforehand, the absorption behaviour of control and coated catalysts in O.II dye was verified by running experiments in the dark.
The C-N-TiO2-coated meshes (6 cm long and 2 cm large) were individually immersed in 500 mL of 5 mg/L orange II solution in a 1000 mL round glass vessel and consecutively irradiated with solar and UV light at the applied conditions as shown in Figure 1. The photo catalysis system was ice cooled around the vessel. The solution was sampled every 30 min for 2 h and taken for UV-vis analysis at a fixed wavelength of 485 nm. The absorbance recorded was further used to define the decolouration efficiency of orange II dye at each sampling time according to Equation (2).
Decolouration rate % = (Ao − At/Ao) × 100 (2) where Ao is O.II initial concentration at time t = 0 min, and At O.II is the concentration at sampling time t (min).  All meshes were tested in the same size of 2 cm wide by 6 cm long (2 cm × 6 cm and hole diameter: 1 mm). Marine Broth (Hi-media, India) and Marine agar (Hi-media, India) were used to prepare bacterial media for antimicrobial experiment with Bacillus subtilis (SQUMSF005). Bacillus subtilis is a Materials 2020, 13, 4426 4 of 27 marine biofouling bacteria and it was isolated from a reverse osmosis membrane of a desalination plant [51].

Experimental-Synthesis of C-N-TiO 2 Sol Gel, Dip Coating of Stainless Steel and Anticorrosion Meshes and Calcination under N 2 Gas
The Cr and Ti-based nitride and oxynitride (SS/Ti(N,O) and SS/Cr-N/Cr(N,O)) corrosion coatings were prepared by reactive cathodic arc evaporation (CAE) at the applied conditions according to Dinu et al. [49].
The C-N-TiO 2 sol-gel was synthesised by the dissolution of 8 g PAN in 100 mL of 99% DMF followed by the addition of 3 mL TiCl 4 and 3 mL 5% NH 4 NO 3 in a 200 mL capped borosilicate glass bottle that was stirred for 24 h at room temperature [52]. Stainless steel (SS) meshes and their anticorrosion coated meshes were carefully cleaned with acetone, ethanol and water and dried in an oven at 60 • C for 30 min prior to remove impurities.
About 40 mg of the prepared C-N-TiO 2 sol gel was loaded on the clean and dried stainless steel meshes by dip coating technique. The coated meshes were placed on clean and dried sample holders (crucibles), which were positioned at the centre of the heating zone. The samples were then calcined at a chosen temperature of 350 • C at a heating rate of 50 • C /min in a furnace for a holding time of 105 min under nitrogen gas at flow rate of 20 mL/min. The calcination temperature, ramping rate and holding time on the furnace were manually set following the instrument guidelines. The system was allowed to cool down under N 2 flow until dark annealed C-N-TiO 2 films were obtained. The SS mesh coated with anticorrosion SS/Ti (N,O) and SS/Cr-N/Cr (N,O) layers was dip coated with the C-N-TiO 2 sol-gel in a similar manner and calcined following the same procedure.

Characterisation of C-N-TiO 2 Nano Films
The elemental composition of uncoated SS and discs coated by anticorrosion SS/Ti (N,O) and SS/Cr-N/Cr (N,O) coatings were scrutinised by energy dispersive X-ray spectrometer (EDS) (Bruker, Billerica, MA, USA). To recall, the elemental composition of uncoated SS and anticorrosion SS/Ti (N,O) and SS/Cr-N/Cr (N,O) layers proving the presence of Ti, Cr, O, and N, etc was already reported in our previous investigations [49,50]. Alternatively, the EDS analysis of C-N-TiO 2 -coated SS meshes was conducted using the Oxford instruments (X-Max) detector and data were integrated by Oxford Aztec software suite. The detection of carbon, nitrogen, and titanium distribution in the C-N-TiO 2 films, mapping elemental images was learned in diverse areas of the sample surface. Images of the surface morphology for each sample were documented at both 30× and 100× magnifications.
X-ray diffraction method (XRD) was used to determine the phase composition of uncoated SS and (SS/Ti (N,O) and SS/Cr-N/Cr (N,O)) coatings (SmartLab diffractometer, Rigaku, Tokyo, Japan), with CuKα radiation (λ = 0.15405 nm). The measurements were taken from 20 • to 80 • , at a step size of 0.02 • . The phase structure of the C-N-TiO 2 films was studied using a multipurpose X-ray diffractometer D8-Advance from Bruker operated in a continuous theta-theta (θ-θ) scan in locked coupled mode with Cu-Kα radiation (λ = 0.15405 nm). The sample was mounted in the centre of the sample holder on a glass slide and levelled up to the correct height. The measurements run within a 2θ range of 20 • to 80 • with a typical step size of 0.034 • . A positioned sensitive detector, Lyn-Eye, was used to record diffraction data at a typical speed of 0.5 sec/step, which was equivalent to an effective time of 92 sec/step for a scintillation counter. Data were background subtracted so that the phase analysis is carried out for diffraction pattern with zero background after the selection of a set of possible elements from the periodic table. Phases were identified from the match of the calculated peaks with the measured ones until all phases were identified within the limits of the resolution of the results. The size of C-N-TiO 2 nano crystals was calculated using the Scherrer Equation (1) [35], and the outcomes are presented in Table 1.
where d is the nano crystal size; K ≈ 0.94 is a dimensionless shape factor; λ ≈ 0.15406 nm is the CuK α diffraction wavelength; B (2θ) is the line broadening at half the maximum intensity (FWHM), expressed in radians (after subtracting the instrumental line broadening); and θ is the Bragg angle in degrees. Raman spectroscopy (XploRA from Horiba, Kyoto, Japan) was used to elucidate the chemical binding of the prepared C-N-TiO 2 -coated films by probing on the sample surface with continuous 532 nm laser excitation. The Raman spectra were collected from 50-1000 cm −1 using a TE cooled CCD camera (Horiba, Kyoto, Japan) attached to the monochromator of a spectrometer with 600 gr/mm. The spectra were obtained by collecting 10 acquisitions.

Photo Catalysis Investigation
The photo catalytic activity of the C-N-TiO 2 -coated supports including SS and anticorrosion meshes were determined by the decolouration of orange II sodium dye, under solar and UV light as described in Figure 1. Beforehand, the absorption behaviour of control and coated catalysts in O.II dye was verified by running experiments in the dark.
The C-N-TiO 2 -coated meshes (6 cm long and 2 cm large) were individually immersed in 500 mL of 5 mg/L orange II solution in a 1000 mL round glass vessel and consecutively irradiated with solar and UV light at the applied conditions as shown in Figure 1. The photo catalysis system was ice cooled around the vessel. The solution was sampled every 30 min for 2 h and taken for UV-vis analysis at a fixed wavelength of 485 nm. The absorbance recorded was further used to define the decolouration efficiency of orange II dye at each sampling time according to Equation (2).
where A o is O.II initial concentration at time t = 0 min, and A t O.II is the concentration at sampling time t (min).

Kinetics Investigation
The degradation behaviour of O.II dye was mathematically investigated using the rate constant and half-life kinetic studies according to the following equations: Therefore, the half-life is

Antimicrobial Activity
To determine the antibacterial properties of the coatings, the duplicates of the catalyst-coated SS, SS/C-N-TiO 2 , SS/Ti(N,O)/C-N-TiO 2 and SS/Cr-N/Cr(N,O)/C-N-TiO 2 (size = 1cm × 1cm) were placed in separate wells of a 24 multi-well plate (Corning, New York city, USA). Uncoated stainless steel (SS) was used as a control. Each well was filled with 3 mL of freshly prepared bacterial culture of Bacillus subtilis (SQUMSF005). The initial concentration of the bacterial culture was~10 Colony Forming Unit per milliliter (~10 CFU/mL). Two similar sets of experiments were conducted; one of which was exposed to the visible light (~30-31.5 Klux, light experiment) and another one was covered with aluminium foil (dark experiment). In both experiments, the multi-well plates were incubated at 37 • C for 48 h. At the beginning (0 h), after 24 h and at the end of the experiment (48 h), 1 mL of the broth culture from each well (both under light and dark conditions) was collected and diluted for 50 times with sterile marine water to determine the number of CFUs. The experiment was conducted in triplicate (n = 3).

Statistical Analysis
The Analysis of Variance (ANOVA) followed by the Tukey post-hoc HSD test was used to test the effect of treatment on the number of CFUs of B. subtilis. Prior to analysis, Shapiro-Wilk's test was used to verify the normality of the data. In all cases, a significance level was p = 0.05. The calculations were performed using Statistica software version 11.0 (Stat Soft, Austin, TX, USA).

Scanning Electron Microscopy/Energy Dispersive Spectroscopy
Scanning electron microscopy coupled with electron dispersive spectroscopy (SEM-EDS) analysis was used in order to understand the morphological behaviour/patterns of C-N-TiO 2 nano-catalyst on SS and the anticorrosion coated metal supports.
The SEM micrographs of the films are presented in Figure 2. The selected rough surface of uncoated SS is shown in Figure 2a, whereas a region coated by C-N-TiO 2 nano composites is shown in Figure 2b. The SEM micrograph in Figure 2b shows that the morphology of C-N-TiO 2 appeared in condensed shape on SS substrate. The photo catalytic coating was confirmed by the presence of Ti and N in SS/C-N-TiO 2 mesh shown by the elemental composition in Table 1. From SEM and EDS results obtained on SS/C-N-TiO 2 sample, the sol-gel based coating of C-N-TiO 2 was adhering on SS support and is comparable to the highlights of Passalía et al. [53].  As for SS/Cr-N/Cr(N,O)/C-N-TiO 2 , the SEM micrographs in Figure 2e,f show that C-N-TiO 2 adhered well to the anticorrosion supports. The C-N-TiO 2 nano crystals that formed on SS/Cr-N/Cr(N,O) support surface exhibited a fine nano rod shape closer to the outcomes reported by Vijayalakshmi and Rajeswari [54].
The EDS results of uncoated discs and those of C-N-TiO 2 deposited on anticorrosion meshes by pyrolysis of the sol gel layer disclosed are shown in Table 1a,b. Nevertheless, the aim of the EDS analysis before and after immobilization of meshes with C-N-TiO 2 catalysts was to prove the presence of key elements such as Ti, Cr, O, C, and N in the prepared films. The data in Table 1a indicate that elements Cr, Ti, N, and O were detected in SS and SS/Cr-N/Cr (N,O) and SS/Ti(N,O) anticorrosion coatings, recalling that the corrosion resistance of these coatings has successfully been studied [49,50]. On the other hand, after dip coating immobilisation of C-N-TiO 2 on the aforementioned supports, the EDS data in Table 1b demonstrate that principal elements including Cr, Ti, O, C, and N were identified. This signified that C-N-TiO 2 was effectively polished on SS and anticorrosion coatings. The differences in atomic percentages of the elements in Table 1a,b could be attributed to the use of different EDS analytical equipment.
To provide a visual aid of the distribution of the C and N atoms in the TiO 2 matrix at the SEM-EDS percentages in Table 1, we conducted the energy dispersive spectroscopy (EDS) mapping of C-N-TiO 2 nano catalyst on the prepared SS/C-N-TiO 2 film, and the EDS micrographs are shown in Figure 3. The micrographs presented in Figure 3a-e show that the elements C, N, Ti, and O were all present in the fabricated C-N-TiO 2 film at percentages dictated by SEM-EDS analysis shown in Table 1b. The morphological changes observed in Figure 2 could be due to the temperature of 350 • C that falls within the temperature range 300 to 400 • C investigated by Tijani et al. [45], which impacted the physical and chemical properties of the synthesised C-TiO 2 nano composites. This was sustained by Pang et al. [55] who showed that a temperature ranges from 300 to 900 • C had an influence on nano tubes of TiO 2 morphologies. In summary, during the characterisation process, the SEM images of the coated supports showed that C-N-TiO 2 photo catalyst was well deposited on SS and the anticorrosion supports. The morphologies of the nano C-N-TiO 2 on the anticorrosion substrates differed from one another and SS probably only because of the different elemental compositions of each anticorrosion coating and the thermal coefficient of different layers. Hence, the properties of these photo catalytic coatings could also be evaluated.

X-ray Diffraction Analysis of the C-N-TiO 2 Coated Catalysts
The XRD analysis was used to define the phase composition and particle size of SS/C-N-TiO 2 , SS/Ti(N,O)/C-N-TiO 2 and SS/Cr-N/Cr(N,O)/C-N-TiO 2 films, and the results are shown in Figure 4. The XRD patterns in Figure 4a show that stainless steel (support) has a major peak with high intensity around 44.05 • , a mid-strength peak at 50.65 • and a minor peak at 74.03 • that could be assigned to Chromium Iron Nickel often referred to as 304-stainless steel (SS) grade. These peaks appear there is an amorphous hump at around 28 • from the carbon in C-N-TiO 2 nano catalysts that is broad and quite intense, which is typical for graphitic carbon [56][57][58]. The graphitic carbon is more structured in the case of SS/C-N-TiO 2 than for SS/Ti (N,O)/C-NTiO 2 and SS/Cr-N/Cr (N,O)/C-N-TiO 2 , and hence, it could be more graphitic. This strongly compliments the Raman spectroscopy results plotted in Figure 5. Indeed, the predominant Raman peaks at 143, 399 and 639 cm −1 in Figure 5 identified for SS/Cr-N/Cr(N,O)/C-N-TiO 2 coating were less featured in SS/Ti(N,O)/C-N-TiO 2 sample and almost invisible in SS/C-N-TiO 2 coating except for the peak persisting at 143 cm −1 , which became minimal and less intense. This trend is further consistent with the EDS outcomes earlier disclosed in Table 1b

X-ray Diffraction Analysis of the C-N-TiO2 Coated Catalysts
The XRD analysis was used to define the phase composition and particle size of SS/C-N-TiO2, SS/Ti(N, O)/C-N-TiO2 and SS/Cr-N/Cr(N, O)/C-N-TiO2 films, and the results are shown in Figure 4. The XRD patterns in Figure 4a show that stainless steel (support) has a major peak with high intensity around 44.05°, a mid-strength peak at 50.65° and a minor peak at 74.03° that could be assigned to Chromium Iron Nickel often referred to as 304-stainless steel (SS) grade. These peaks appear  4 indicate that C-N-TiO2 in the films contained both anatase (JCPDS, no 00-021-1272) and rutile phase (JCPDS, no 00-021-1276). The Rutile phase designated by a diffraction peak at 2θ = 44.05° (210), matched the lattice tetragonal shape of the rutile phase. While the anatase phase occurring at diffraction peak 2θ = 74.03° (107), as from JCPS, no 00-021-1272, is equivalent to the body-centred tetragonal lattice structure of the mineral anatase phase. In order to elucidate the emergence of phase composition of C-N-TiO2 nano films, we decided to subject the samples to Raman spectroscopy an al ysi s.    Furthermore, the Rutile and Anatase phases of C-N-TiO 2 expected to appear at 44.05 • and 74.03 • might have been hidden by the prominent graphitic carbon in C-N-TiO 2 nanocomposites [59]. Nevertheless, we believe that the overlapping of rutile and anatase peaks with SS at 2θ = 44.05 • (210) and 74.03 • (107) and their shadowing by graphitic carbon were responsible for the minimal and broadening of peak intensities as shown in Figure 4b. From this point of view, the outcomes in Figure 4 indicate that C-N-TiO 2 in the films contained both anatase (JCPDS, no 00-021-1272) and rutile phase (JCPDS, no 00-021-1276). The Rutile phase designated by a diffraction peak at 2θ = 44.05 • (210), matched the lattice tetragonal shape of the rutile phase. While the anatase phase occurring at diffraction peak 2θ = 74.03 • (107), as from JCPS, no 00-021-1272, is equivalent to the body-centred tetragonal lattice structure of the mineral anatase phase. In order to elucidate the emergence of phase composition of C-N-TiO 2 nano films, we decided to subject the samples to Raman spectroscopy analysis.
The XRD analysis in Figure 4  These results meant that C-N-TiO 2 films fabricated by sol-gel/pyrolysis route were present in both rutile and anatase phases, which may impact the photo catalytic applications of the films owing to the large crystalline size 146 nm, based on the Scherrer equation and enlarged band gap [11,34] compared to 5 nm crystals in anatase phase that were hidden by the graphitic carbon in the synthesised C-N-TiO 2 nano catalysts.

Raman Spectroscopy Characterisation of the Nano C-N-TiO 2 Films
The clarification of chemical binding of C-N-TiO 2 nano films was further investigated by Raman spectroscopy analysis, and the Raman vibrational modes recorded between 20 and 1000 cm −1 are shown in Figure 5.
The Raman spectra of irradiated C-N-TiO 2 nanocomposites in Figure 5 exhibit three major peaks discernable at 143 cm −1 , 399 cm −1 and 639 cm −1 and minimal/negligible bands around 198 cm −1 and 520 cm −1 , respectively, which are all characteristics of vibration modes of TiO 2 anatase phase [60]. The most dominant peak at 143 cm −1 is probably due to the symmetric stretching vibration of oxygen atoms in O-Ti-O structure, symmetric bending vibration of O-Ti-O and anti-symmetric bending vibration of ¬O-Ti-O assembly in C-N-TiO 2 nano catalyst as previously reported [61,62]. The vibrational frequency at 520 cm −1 could not be properly examined due to its poor intensity, nevertheless, its shoulder mode could result from the overlapping of two vibration bands that could be depicted at 512 cm −1 and 523 cm −1 , both indicating the anatase phase of TiO 2 material [63]. For all samples, the outcomes in Figure 5 show that the intensity of vibration peaks declined with the upsurge of Raman frequency, suggesting the progressive reduction of the film's crystallinity.
Likewise correspondingly. The redundancy of amorphous carbon in this sample order probably shadowed the occurrence of anatase and Rutile phases of C-N-TiO 2 , which were barely detectable by XRD analysis in Figure 4b. This aspect was attested by a few authors [64,65] who conveyed that the Raman spectrum of amorphous TiO 2 exhibits no preponderant peaks.
On the other hand, the broad frequency bands noticeable between 720 and 1000 cm −1 in SS/Cr-N/Cr(N,O)/C-N-TiO 2 coating ( Figure 5) could perhaps be ascribed to the rutile phase [66], which was previously identified in our XRD analysis though the Raman spectra in Figure 5, showing their total disappearance for SS/Ti(N,O)/C-N-TiO 2 and SS/C-N-TiO 2 films. The assignment of these modes between 720 and 1000 cm −1 to amorphous phase of TiO 2 has been previously reported [67,68]. The appearance of these Raman weak and expansive modes in this frequency range suggests the predominance of amorphous edifice over the crystalline one [59]. This corroborates the findings of Hardwick et al. [69], claiming that the shape of Raman spectra can be biased by numerous aspects including phonon confinement, non-stoichiometry due to oxygen deficits, or core strain in the nano-crystallites recalling that phonon captivity influence arises when the estimated grain size of the prepared films lies below 10 nm of those calculated in Table 2. Hence, the diagnostic characterisation of C-N-TiO 2 films by XRD and Raman spectroscopy reveals that C-N-TiO 2 on films was predominantly in anatase phase with tiny traces of rutile phase that were shadowed by amorphous carbon and TiO 2 in C-N-TiO 2 -prepared coatings.

Kinetics Trends for the Decolouration of Orange II Dye
The photo catalysis results previously discussed were complimented by kinetics investigation to further clarify the catalytic effectiveness of the C-N-TiO2 coatings. The kinetic results in Figure 7a,c showed that the photocatalytic decay of orange II dye over time followed a first order reaction rate as expressed in Equations (3)- (8).
In order to assess the kinetic behaviour for the decomposition of O.II over time, − (ln ( [ . ] [ . ] ) was plotted against time (t) for each photocatalytic system as presented in Figure 7 b,d.
The plot of the first order reaction rate for the decolouration of orange II under solar or UV light is presented in Figure 7b,d and linear trends were observed whose slope corresponded to the rate constant (min −1 ). The rate constant (kr) and correlation coefficient (R 2 ) of O.II dye decompositions of each oxidation process are presented in Table 3.  Figure 6b) within 120 min followed by 56, 46 and 32% achieved with SS/Cr-N/Cr(N,O)/C-N-TiO 2 , SS/C-N-TiO 2 , and UV light alone, correspondingly. These results were ascribed to the effectiveness of the C-N-TiO 2 coatings that achieved over 50% removal of O.II in the allocated time [52]. The photo catalytic efficiency of C-N-TiO 2 coating demonstrated in Figure 6a is closer to research previously reported [45].
Contrary to our previous investigation [70], we showed that SS, SS/Ti (N,O) and SS/Cr-N/Cr(N,O) anticorrosion coatings are effective catalytic supports. In this study, solar or UV light alone were used as controls. Thus, the results presented in Figure 6 demonstrate that coating SS, SS/Ti(N,O) and SS/Cr-N/Cr(N,O) with C-N-TiO 2 catalysts improved the removal of O.II about 15-fold more compared to the controls, hence by a ratio of 12:1, which is in accordance with the results reported by Bestetti et al. [71]. These results confirm that layers of C-N-TiO 2 are photo catalytically effective when deposited on the anticorrosion meshes, which thus can be used as excellent and durable supports. These corroborate the findings reported in previous studies [72][73][74].  (8), it could be noticed that from the first order decomposition of O.II, its halflife is independent of its initial concentration. Therefore, at t = 0 min, [O.II] = 10 mg/L decreased to ½ [O.II]o after further integration of ln2/kr. Consequently, Equation (9) was used to approximate the half-life of orange II dye during solar or UV illumination in the presence of the composite C-N-TiO2 catalysts on SS, SS/Ti(N, O) and SS/Cr-N/Cr(N, O) supports as shown in Table 3.

So, in Equation
The Furthermore, it would have taken 277 min and 210 min for O.II concentration to go down to 5 mg/L during its irradiation with solar or UV light alone (Figure 7a,c). Thus, the rate constants and half-lives recorded in Table 3 sustain that stainless steel supplemented with its anticorrosion layers could be used as a convenient photocatalytic support in advanced oxidation processes (AOPs), and its subsequent coating with doped heterogeneous nano photo catalysts can significantly improve the removal of persistent organic dye from wastewater, preferably before being discharged into the environment. The nature of the support had an effect on the catalyst due to its impact on the structure of the catalyst.
These results substantiated that the sol-gel/pyrolysis procedure for coating synthesised C-N-TiO2 on solid supports such as SS or SS protected with anticorrosion layers could further be utilised    The kinetic results in Figure 7a,c showed that the photocatalytic decay of orange II dye over time followed a first order reaction rate as expressed in Equations (3)- (8).
In order to assess the kinetic behaviour for the decomposition of O.II over time, −(ln (  The plot of the first order reaction rate for the decolouration of orange II under solar or UV light is presented in Figure 7b,d and linear trends were observed whose slope corresponded to the rate constant (min −1 ). The rate constant (k r ) and correlation coefficient (R 2 ) of O.II dye decompositions of each oxidation process are presented in Table 3. In comparison to the previous report on the low or non-existent photocatalytic performance of uncoated support or anticorrosion layers SS, SS/Ti(N,O) and SS/Cr-N/Cr(N,O) [70], the current outcomes substantiated that the immobilised coating of C-N-TiO 2 on SS support improved the decolouration rate of O.II by 15%, and it is in accordance with Bestetti et al. [71].
Moreover, these results show that the C-N-TiO 2 coating is photo catalytically effective, and the anticorrosion meshes were suitable to be used as excellent and durable supports, because SS in the absence of anticorrosion layers may corrode in oxidative environments [72][73][74].
These results further endorse that anticorrosion coatings SS/Cr-N/Cr(N,O) and SS/Ti(N,O) can successfully be used as supports for active coatings with the desired catalytic efficiencies in advanced oxidation systems illuminated by UV light or solar light [73]. Indeed, the morphology of C-N-TiO 2 on SS or on the anticorrosion meshes varied from well dispersed crystals, condensed nano crystals, to nano rod shapes with different surface areas, which all probably absorbed the UV and solar light differently. This consequently implied that the morphology of the catalyst can impact upon its photocatalytic activity [74,75]. Subtle differences in morphology were induced by the underlying anticorrosion coating, which should be further explored. The anticorrosion layers offer a route to prevent SS corrosion in the highly oxidative environment over time [76,77].
In addition to the rate constant discussed in Figure 7b Table 3. Furthermore, it would have taken 277 min and 210 min for O.II concentration to go down to 5 mg/L during its irradiation with solar or UV light alone (Figure 7a,c). Thus, the rate constants and half-lives recorded in Table 3 sustain that stainless steel supplemented with its anticorrosion layers could be used as a convenient photocatalytic support in advanced oxidation processes (AOPs), and its subsequent coating with doped heterogeneous nano photo catalysts can significantly improve the removal of persistent organic dye from wastewater, preferably before being discharged into the environment. The nature of the support had an effect on the catalyst due to its impact on the structure of the catalyst.
These results substantiated that the sol-gel/pyrolysis procedure for coating synthesised C-N-TiO 2 on solid supports such as SS or SS protected with anticorrosion layers could further be utilised as photocatalytic materials under solar light or UV light to enhance the generation of free radicals and hence the removal of the targeted pollutant in AOPs.

Antimicrobial Activity of C-N-TiO 2 Coated SS and Anticorrosion Meshes
The  Table 4.
The results in Figure 8 indicate that most of the coatings did not show any significant (ANOVA, Tukey, p > 0.05) reduction in the number of viable cells of B. subtilis in either light or dark conditions compared to the uncoated stainless steels (control). Nevertheless, the outcomes in Figure 8a show that in the dark, the coating SS/Cr-N/Cr(N,O)/C-N-TiO 2 followed by SS/C-N-TiO 2 considerably reduced the number of bacteria after 48 h (ANOVA, Tukey, p < 0.0005). This implies that either the bacteria were being absorbed by SS/Cr-N/Cr (N,O)/C-N-TiO 2 or SS/C-N-TiO 2 or these two C-N-TiO 2 coatings were toxic to the bacterium in the dark due to their ability to generate charge carriers (electrons and holes) in the dark that led to substantial decrease of B. subtilis colony in the absence of light irradiation.
Conversely  Table 4 suggests that both time and type of coating, as well as their combination, affected the number of viable cells of B. subtilis.
Altogether, we found that the three coatings could biologically be effective after extended treatment times, with SS/Cr-N/Cr (N,O)/C-N-TiO 2 being more toxic to bacteria in the dark while SS/C-N-TiO 2 responded in both the dark and slightly under visible light. In contrast, SS/Ti (N,O)/C-N-TiO 2 could be effective under visible light after prolonged exposure time.
Therefore, the C-N-TiO 2 composites-engineered anticorrosion coatings in this study represent adequate nano thin films that can be utilised in advanced oxidation processes (AOPs) to improve the decontamination of polluted water. These findings have not been reported elsewhere. after 24 h onwards. This further implies that SS/C-N-TiO2 and SS/Ti (N, O)/C-N-TiO2 could biologically be effective catalysts under visible light after extended illumination time. Moreover, with respect to control under visible light after 48 h, it can be observed that SS/C-N-TiO2 and SS/Cr-N/Cr (N, O)/C-N-TiO2 were slightly more active compared to SS/Ti (N, O)/C-N-TiO2 coating. The statistical analysis (ANOVA) displayed in Table 4 suggests that both time and type of coating, as well as their combination, affected the number of viable cells of B. subtilis.
Altogether, we found that the three coatings could biologically be effective after extended treatment times, with SS/Cr-N/Cr (N, O)/C-N-TiO2 being more toxic to bacteria in the dark while SS/C-N-TiO2 responded in both the dark and slightly under visible light. In contrast, SS/Ti (N, O)/C-N-TiO2 could be effective under visible light after prolonged exposure time.

Discussion
The mono and double-layered coatings, SS/Ti(N,O) and SS/Cr-N/Cr(N,O), were identified in our previous investigation as the most corrosion-resistant coatings in acidic environments [49,50]. Thus, the deposition of C-N-TiO 2 using sol-gel and pyrolysis procedure to form coatings on SS resulted in good adherence of C-N-TiO 2 coatings at the applied conditions ( Figure 2). This can be observed from SEM images shown in Figure 2b This inferred that the agglomeration of powder nanoparticle issues encountered during water treatment and the problem of particulate recovery after treatment could be overcome by the coating of C-N-TiO 2 catalysts on supports followed by carbonisation at convenient temperatures and calcination holding times [78][79][80][81][82]. The C-N-TiO 2 nanocomposites adopted different morphologies for each support. This was related to the physical, chemical and mechanical properties of solid supports SS, SS/Ti(N,O) and SS/Cr-N/Cr(N,O) and their interaction with the nano catalyst [55,77]. Even though the anticorrosion coatings used in this study have the same base/substrate SS, the anticorrosion layers Ti(N,O) and CrN/Cr(N,O) deposited by CAE on SS conferred different properties to the newly fabricated SS/Ti(N,O) and SS/Cr-N/Cr(N,O) coatings when compared to SS substrate [49,50]. So it is evident that deposition of the C-N-TiO 2 nanocomposites on SS and Ti, Cr nitrides and oxynitride anticorrosion-based layers resulted in various morphologies.
Also, the amorphisation of C and TiO 2 in C-N-TiO 2 catalyst, which is demonstrated in Figures 4 and 5, is consistent with SEM images in Figure 2b,d,f in which C-N-TiO 2 appeared in condensed shape, well-dispersed nano crystals, and fine nano rod shape when immobilised on SS, SS/Ti(N,O) and SS/Cr-N/Cr(N,O) meshes, respectively. This hence confirmed that C and TiO 2 in C-N-TiO 2 nano composites were more amorphous on SS than on Ti (N,O) and Cr-N/Cr (N,O) supports, correspondingly. Comparable studies involving the immobilisation of catalysts on supports using different supports and catalysts have been reported [83][84][85][86].
XRD patterns in Figure 4 show that C-N-TiO 2 coated on SS or anticorrosion coatings was detected as being in rutile phase at 2θ = 44.05 • (210), and to some extent the anatase phase was present with the peaks especially depicted at 2θ = 74.03 • (107), which is consistent with JCPDS no. 00-021-1276 and JCPDS no. 00-021-1272, respectively. Even though the diffraction peaks identified at 2θ = 44.05 • , 50.79 • , and 74.03 • could also be assigned to SS substrate (JCPDS no. 01-081-8770), the slight increase of peak intensities and their broadening suggest that two phases of C-N-TiO 2 were present on the supports, and hence, XRD complimented the EDS findings, which are shown in Table 1b and Figure 3. The XRD features of SS have already been discussed in previous studies [87,88], and the illustrated diffraction peaks of SS correspond to the XRD results discussed in Figure 4.
Hence, the large C-N-TiO 2 nano crystals with a size of 146 nm as shown in Table 2 indicate that all supports were fully covered with the catalyst. It should be noted that small particles/crystals of 5.1 nm size of anatase were barely depicted by XRD analysis, which is certainly due to the predominance of the amorphous C and TiO 2 in C-N-TiO 2 nano catalyst identified by the broad and intense XRD hump around 28 • and the Raman shift around 143 cm −1 , 399 cm −1 and 639 cm −1 , consistently. So, the amorphous C and TiO 2 in C-N-TiO 2 catalyst coupled with the dominant 146 nm rutile phase may have reduced the catalytic activity of C-N-TiO 2 , resulting in the reduced removal percentages of O.II shown in Figure 6 and slower kinetics trends disclosed in Figure 7. Hence, the SEM, EDS and XRD results discussed in this work show that the immobilisation of catalysts on the tested supports may lead to different crystal morphologies or phases that, in turn, may impact on the photocatalytic activity of the fabricated coatings. Thus, optimisation of the deposition and carbonisation process may be of interest to achieve the desired properties of the films and greater activity.
The O.II dye to CO 2 , H 2 O and simpler inorganic entities, as described in Figure 9.
Materials 2020, 13, x FOR PEER REVIEW 20 of 27 adherence due to the composition of the support, which in turn could be influenced by interfacial issues or thermal properties that could be further investigated.

Conclusions
This study showed successful coating of C-N-TiO2 nano composite layers upon supports, being obtained by pyrolysis of a sol-gel TiCl4/PAN/DMF dip coated onto the SS support or onto preprepared anticorrosion meshes. The C-N-TiO2 coating exhibited different morphologies and the crystal habit varied from well dispersed, condensed-shaped crystals to dispersed nano rods. The C-N-TiO2 immobilised on SS or onto anticorrosion meshes was predominantly in the rutile phase with a crystal size of 146 nm compared to a minor phase of anatase present with particle size of 5 nm. The photo catalytic efficiency of C-N-TiO2-coated catalysts for the removal of O.II dye was between 70% and 32% under both solar or UV light.
The decolouration of O.II at these percentages followed a first order reaction rate characterised by linear trends. The fabricated C-N-TiO2 films showed significant antifouling and antibacterial activities in both dark and visible light at prolonged treatment times. Herein, we proved that morphology, phase and crystal size of the catalyst immobilised on SS and anticorrosion supports impact on the photocatalytic capabilities of the coatings and were advantageous for the deactivation of microorganisms in the presence or absence of light. This is the first time that C-N-TiO2 nano catalyst synthesised by sol-gel method was immobilised on SS and various anticorrosion meshes that resulted in different morphologies with improved photo catalytic activities. The chemical coordination between the immobilised nanocatalyst and supports might have affected the optical and electronic properties of the films, which in return led to different photo catalytic activities. The C-N-TiO2 coatings engineered in this study can be used in water and wastewater treatment plants for the decomposition of POPs under both solar and UV light and for Sambandam et al. [92] reported that the anatase phase is photo catalytically more effective than the rutile phase due to rapid electron-hole recombination and probably lower surface activity of rutile phase.
This consequently led to a reduced number of microstates that in return resulted in quick electron-hole pair recombination (λ 1&2 ) as shown in Figure 9. This diminished the storing of e − on the catalyst surface and hence lowered reduction of O 2 to O 2 − and OH. This in turn decelerated the decolouration of O.II dye to percentages below 80%. Similar studies on sol-gel deposition of TiO 2 -doped or co-doped catalysts on SS have been conducted and high decolouration efficiencies of POPs were also achieved [45,[93][94][95][96]. SS has been proven unstable in acidic environments due to the erosion of its passive layer that often leads to its corrosion [45,54]. The new materials also offer a route to prevent SS corrosion in the highly oxidative environment over time [75].
The different photo catalytic activities of C-N-TiO 2 -coated nano films SS/C-N-TiO 2 ; Cr-N/Cr (N,O)/C-N-TiO 2 and SS/Ti (N,O)/C-N-TiO 2 observed in both solar and UV light, as shown in Figure 6 and discussed above, could be ascribed to the coordination chemistry that involves inorganic semiconductor-insulator, inorganic semiconductor-semiconductor, and inorganic semiconductor-metal interactions with doped semiconductor nanomaterials of C-N-TiO 2 that were earlier described by Li and Zhang [97]. These chemical interfaces consequently affected the optical and electronic properties of C-N-TiO 2 films, leading to different activities under solar and UV light illumination. The understanding of these chemical interactions in the current study requires full investigation and will be considered as part of our future studies. Besides the photocatalytic properties of C-N-TiO 2 coated on SS and anticorrosion meshes discussed above, investigation of antibacterial characteristics of the coating could be crucial to evaluate whether during decomposition of POPs in polluted effluents, the C-N-TiO 2 coating catalysts might eliminate microbes from effluents being remediated.
Hence, the second test of our coatings involved assessing antimicrobial activities towards the Bacillus subtilis in the dark and under visible light. Indeed, Chang et al. [98] and Cai et al. [99] demonstrated that Cr-N anticorrosion layers do not have any antibacterial activity. On the other hand, Li and Zhang [97] claimed that TiO 2 photo catalysts doped with C, N, S, or F anionic impurities exhibit great photocatalytic efficiencies, but they often lose their photocatalytic ability in the dark milieu because they cannot produce electron and hole-pairs. Nevertheless, the antimicrobial activity/properties/behaviour of SS/Cr-N/Cr (N,O)/C-N-TiO 2 and SS/C-N-TiO 2 films experienced in the dark in Figure 8a could likely be attributed to their capability to generate charge carriers (electrons and positive holes) as a consequence of optoelectronic coupling between C, N dopants and TiO 2 semiconductor, which promotes the charge carrier separation in C-N-TiO 2 nanocomposites. Previous research studies [100] highlighted that the ideal scenario would require the fabrication of photo catalysts with high activity under visible/UV light and when the photo excitation process is turned off so that contaminants such as bacteria can be cleaned up either in the presence or absence of light.
The photocatalytic mechanistic scenarios plotted in Figure 9 indicate that the irradiation of the co-doped C-N-TiO 2 nanocomposites with solar/UV light initiated the excitation of electrons from the valence band (VB) to the conduction band (CB), leaving behind positively charged empty holes (h + ) that both contributed to the production of reactive oxygen species (ROS). Gao et al. [101] and Ajiboye [102] recalled that the indirect inactivation of bacteria such as B. subtilis described in Figure 9 is often initiated by the damage of the plasmonic membrane, which may alter the bacteria metabolism, followed by the destruction of DNA sequence leading to its lysis.
Indeed, the electrons stored on the CB of C-N-TiO 2 film participated in the reduction of O 2 to superoxide anions (O 2 − ), which directly attacked the plasmonic membrane of B. subtilis leading to its deactivation and hence the slight reduction of its colony-forming unit counts observed in Figure 8b with SS/C-N-TiO 2 after 48 h. Similar investigations were carried out using different advance oxidations [103][104][105][106][107][108][109][110][111]. The extended inactivation time of B. subtilis suggests that OH oxidants originated from various chains of chemical reactions at low rates between O 2 − , H 2 O 2 and other species after 2 h and are in line with photo catalysis results discussed in Figures 5 and 6. This implies that in the case of bacteria resistance, prolonged deactivation time is required as observed in Figure 8. Besides, O 2 − oxidants also contributed to the generation of powerful non-selective OH radicals that are engaged in both decomposition of POP dye O.II and deactivation of B. subtilis. The decomposition of O.II in Figure 9 was probably initiated by OH, and the results agree with [112]. On the other hand, the empty hole charge carriers on the VB of C-N-TiO 2 nanocatalyst oxidised H 2 O molecules to OH . . These in return participated in both dye decolouration and inactivation of B. subtilis after extended irradiation times. Secondary species including O . and H 2 O 2 might have also been involved in reaction chains producing an O 2 − and OH radical during the photo catalysis process and during deactivation of B. subtilis [113,114]. In contrast, the mechanisms of action of C-N-TiO 2 -coated films on B. subtilis inactivation in visible light shown in Figure 9 may not only result from the ROS generated but also from the photo chemical irradiation of water that further induced water disinfection via sterilisation of Bacillus micro-organisms (pathogens). Furthermore, Ruddaraju et al. [115] noted that nanoparticles can modify the metabolic behaviour of bacteria when interacting directly with bacterial cells via electrostatic interaction, van der Waals forces, receptor-ligand, and hydrophobic contacts, which is in line with Choi et al. [116].
This study demonstrated that coating SS mesh with transition metals and non-metals in mono and double protective layers SS/Ti(N,O) and SS/Cr-N/Cr(N,O) contributed not only to the protection of stainless steel against corrosion in the oxidative photolytic environment but also made excellent photocatalytic supports. The morphology of the catalyst being deposited may vary according to its adherence due to the composition of the support, which in turn could be influenced by interfacial issues or thermal properties that could be further investigated.

Conclusions
This study showed successful coating of C-N-TiO 2 nano composite layers upon supports, being obtained by pyrolysis of a sol-gel TiCl 4 /PAN/DMF dip coated onto the SS support or onto pre-prepared anticorrosion meshes. The C-N-TiO 2 coating exhibited different morphologies and the crystal habit varied from well dispersed, condensed-shaped crystals to dispersed nano rods. The C-N-TiO 2 immobilised on SS or onto anticorrosion meshes was predominantly in the rutile phase with a crystal size of 146 nm compared to a minor phase of anatase present with particle size of 5 nm. The photo catalytic efficiency of C-N-TiO 2 -coated catalysts for the removal of O.II dye was between 70% and 32% under both solar or UV light.
The decolouration of O.II at these percentages followed a first order reaction rate characterised by linear trends. The fabricated C-N-TiO 2 films showed significant antibacterial activities in both dark and visible light at prolonged treatment times. Herein, we proved that morphology, phase and crystal size of the catalyst immobilised on SS and anticorrosion supports impact on the photocatalytic capabilities of the coatings and were advantageous for the deactivation of microorganisms in the presence or absence of light. This is the first time that C-N-TiO 2 nano catalyst synthesised by sol-gel method was immobilised on SS and various anticorrosion meshes that resulted in different morphologies with improved photo catalytic activities. The chemical coordination between the immobilised nanocatalyst and supports might have affected the optical and electronic properties of the films, which in return led to different photo catalytic activities. The C-N-TiO 2 coatings engineered in this study can be used in water and wastewater treatment plants for the decomposition of POPs under both solar and UV light and for the killing of bacteria in both dark and light.