Tantalum Oxynitride Thin Films: Assessment of the Photocatalytic Efﬁciency and Antimicrobial Capacity

nanomaterials

Article Tantalum Oxynitride Thin Films: Assessment of the Photocatalytic Efﬁciency and Antimicrobial Capacity

Daniel Cristea 1 , Luis Cunha 2 , Camelia Gabor 1 , Ioana Ghiuta 1 , Catalin Croitoru 1,* , Alexandru Marin 3, Laura Velicu 4 , Alexandra Besleaga 4 and Bogdan Vasile 5 1 Materials Science and Engineering Faculty, Transilvania University, Eroilor 29, 500036 Bras, ov, Romania; [email protected] (D.C.); [email protected] (C.G.); [email protected] (I.G.) 2 Physics Center, Minho University, Gualtar Campus, 4710-057 Braga, Portugal; lcunha@ﬁsica.uminho.pt 3 Institute for Nuclear Research Pitesti, Str. Campului Nr. 1, POB 78, 115400 Mioveni, Arges, Romania; [email protected] 4 Faculty of Physics, Alexandru Ioan Cuza University, 11 Carol I Blvd, 700506 Iasi, Romania; [email protected] (L.V.); [email protected] (A.B.) 5 University Politehnica of Bucharest, National Research Center for Micro and Nanomaterials, Gh. Polizu Street No.1-7, 011061 Bucharest, Romania; [email protected] * Correspondence: [email protected]; Tel.: +40-748126598

Received: 28 February 2019; Accepted: 18 March 2019; Published: 23 March 2019

Abstract: Tantalum oxynitride thin films have been deposited by reactive magnetron sputtering, using a fixed proportion reactive gas mixture (85% N2 + 15% O2). To produce the films, the partial pressure of the mixture in the working atmosphere was varied. The characteristics of the produced films were analyzed from three main perspectives and correspondent correlations: the study of the bonding states in the films, the efficiency of photo-degradation, and the antibacterial/antibiofilm capacity of the coatings against Salmonella. X-ray Photoelectron Spectroscopy results suggest that nitride and oxynitride features agree with a constant behavior relative to the tantalum chemistry. The coatings deposited with a higher reactive gas mixture partial pressure exhibit a significantly better antibiofilm capacity. Favorable antibacterial resistance was correlated with the presence of dominant oxynitride contributions. The photocatalytic ability of the deposited films was assessed by measuring the level of degradation of an aqueous solution containing methyl orange, with or without the addition of H2O2, under UV or VIS irradiation. Degradation efficiencies as high as 82% have been obtained, suggesting that tantalum oxynitride films, obtained in certain configurations, are promising materials for the photodegradation of organic pollutants (dyes).

Keywords: tantalum oxynitride; magnetron sputtering; surface roughness; photocatalytic activity; antibioﬁlm capacity

1. Introduction Transitional metal oxynitride thin solid films have found a wide application in industrial environments, due to interesting electrical, optical, mechanical, and other properties in association with a relatively low cost and ease of manufacturing. Films from this class of compounds (oxynitrides) are promising candidates to be implemented in practical applications, due to their main feature, which is the possibility to tune firstly, the metallic/non-metallic ratio, and secondly, the non-metallic content ratio (O/N), by closely controlling the deposition parameters. Tantalum oxynitride thin films have already been studied for their potential in tribological and/or mechanical applications [1,2], electrical applications [3–8], and optical applications [9,10]. One promising direction for the tantalum oxynitride film system is that of materials with photocatalytic properties. Several reports can be found in the literature, certifying their potential for

Nanomaterials 2019, 9, 476; doi:10.3390/nano9030476 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 476 2 of 22 this type of application, under ultraviolet radiation, but more importantly, under visible radiation. However, to the best of our knowledge, only a limited number of reports are related to tantalum oxynitrides under thin film form. As reported by Hsieh et al. [11,12], a nitridation and oxidation process, implemented on the pure tantalum film, prepared by DC magnetron sputtering, was used to create tantalum oxynitride films at elevated temperatures and different oxygen flow rates. The photocatalytic activity of the obtained tantalum oxynitride films was investigated by the photodegradation test under visible light. In this case, the decomposition of methylene blue on tantalum oxynitrides under visible light (λ = 610 nm) irradiation, leading to the reduction of optical absorbance, was observed. The photodegradation of methylene blue was presented in [13], and it was observed that the co-deposition of Ag in TaOxNy films can enhance the photocatalytic behavior of the composite films. The reason for this increased photodegradation capacity is thought to be due to the increased number of electron–hole pairs [14]. Another report [15] mentions the deposition of TaOxNy photo-electrodes by DC reactive sputtering at room temperature. The authors motivate the nitrogen or oxygen incorporation within oxides or nitrides in order to modify the band gap of the starting material, so that the resulting band gap will exhibit optimal absorption properties, in relation to the requirements of the application. In the same report, it is mentioned that an increase in the oxygen partial pressure affects the maximum absorption domain, which is shifted to lower wavelengths for higher oxygen partial pressures. Dabirian et al. [16] report the manufacture of stable TaOxNy films, obtained in several stages: first, the deposition of a tantalum layer on top of a titanium layer (its role: adhesion layer), followed by an oxidation period of 5 h at 650 ◦C in air and a subsequent nitridation stage. The photocatalytic capacity was assessed by photoelectrochemical (PEC) experiments, performed under AM 1.5 illumination. The authors suggest that higher nitridation temperatures can be expected to give lower defect concentrations and higher photocurrents during PEC experiments. A comparison of TaOxNy and Ta3N5 [17], regarding their capacity to degrade atrazine, revealed that TaOxNy samples were more efficient than Ta3N5 samples. This higher photocatalytic activity of TaOxNy was explained by a higher separation efficiency of electrons and holes of TaOxNy, which could be related to the larger specific surface area. Furthermore, it was reported [18] that under visible light irradiation (420 nm ≤ λ ≤ 700 nm), tantalum (oxy)nitrides + + can oxidize water to O2 and reduce H to H2 in the presence of sacrificial reagents (Ag and methanol). The excellent corrosion resistance, fracture toughness, and biocompatibility of tantalum-based materials recommend their use as biomaterials in several applications: vascular clips, flexible vascular stents, and dental implants, as well as orthopedic components, as acetabular cups, or other trabecular metal components [19,20]. Bulk Ta is highly unreactive and biocompatible in the body. Ta does not exhibit toxicity to surrounding cells or inhibit local cell growth of surrounding bone. Moreover, it was found that pure tantalum exhibits a lower or similar S. aureus and S. epidermidis adhesion when compared with commonly used materials in orthopedic implants [21]. The biocompatibility, antibacterial properties, and cytocompatibility characteristics of tantalum-based coatings (mostly oxides) are presented, to date, in a limited number of references. Chang et al. [22] report the deposition of Ta and amorphous tantalum oxide coatings by magnetron sputtering. A hydrophilic crystalline β-Ta2O5 coating was obtained by rapid thermal annealing of the deposited tantalum oxide coating at 700 ◦C. It was observed that the antibacterial effect is more obvious for the deposited amorphous Ta2O5 coating than for the uncoated and other coated specimens. Moreover, Huang et al. [23] reported that tantalum nitride (TaN) coatings with a hydrophobic surface (contact angle = 92.0 ± 1.2◦, mean ± SD) exert antibacterial effects against Staphylococcus aureus. To the best of our knowledge, to date, there are no reports presenting results concerning the antibacterial/antibiofilm capacity of tantalum oxynitride materials, either in bulk, powder, or coating form. This work presents our results concerning the dye photodegradation and antibiofilm capacity of magnetron sputtered tantalum oxynitride thin films, corelated to the surface morphology, bonding states, and structural features. Nanomaterials 2019, 9, 476 3 of 22

2. Materials and Methods

2.1. Sample Preparation

TaOxNy thin films were deposited on silicon (100) wafers, stainless steel (AISI 316L), and glass substrates by DC reactive magnetron sputtering, using a laboratory-size deposition chamber, containing a tantalum target (99.6% purity), with the dimensions 200 × 100 × 6 mm, positioned at 70 mm relative to the substrate holder. The gas atmosphere during deposition was composed of argon as the working gas and a mixture of nitrogen and oxygen (fixed proportion of 15% O2 + 85% N2) as reactive gases. The argon flow during deposition (70 sccm) was kept constant during all depositions, while the overall N2 + O2 gas mixture flow was varied between 2.5 and 30 sccm, corresponding to partial pressures ranging from 0.02 Pa to 0.24 Pa (registered before the plasma ignition), according to Table1.

Table 1. Deposition parameters for the tantalum oxynitride ﬁlms.

Reactive Gas Flow Partial Pressure N + Sputtering Pressure Sample 2 (sccm) O2 (Pa) (Pa) B2 2.5 0.02 0.41 B3 5 0.04 0.43 B4 10 0.08 0.47 B5 15 0.13 0.50 B6 20 0.17 0.54 B7 22.5 0.20 0.57 B8 25 0.22 0.59 B9 30 0.24 0.64

The substrate holder was electrically grounded (GND), and the temperature was kept at 100 ◦C for all depositions. A more detailed presentation for the deposition protocol for the samples included in this work is presented elsewhere [24].

2.2. Structural and Morphological Characterization X-ray Photoelectron Spectroscopy (XPS) measurements were performed on silicon substrate samples, with an Escalab 250 system (Thermo Scientific, East Grinstead, United Kingdom) equipped with a −8 monochromated Al Kα (1486.6 eV) X-ray source and a 10 Pa base pressure in the analysis chamber. The acquired spectra were calibrated with respect to the C1s line of surface adventitious carbon at 284.8 eV. An electron flood gun has been used to compensate for the charging effect in insulating samples. A gentle removal of the contaminants confined on the outermost surface layers of the thin films was performed prior to the XPS surface chemical analysis. An Ar+ ion gun operated with a low energy ion beam of 1.0 keV was rastered over a (3 × 3) mm2 area and a 0.5 min sputter time was used for surface etching. The reflectance of the coatings deposited on silicon was measured on a UV-Vis-NIR spectrophotometer (Shimadzu UV-Vis-NIR 2505) in the spectral range of 250-800 nm. These measurements were used to find the type of energy transition and the optical band gap of the Ta-based phases (Tauc plots). Transmission electron microscopy (TEM) analyses were performed on selected samples (coatings deposited on silicon wafers, prepared by erosion of the substrate) using a Tecnai G2 F30 STWIN electron microscope from Thermo Fisher Scientific (Former FEI) operated at an acceleration voltage of 300 kV. The images were obtained in normal BFTEM (Bright Field Transmission Electron Microscopy) mode and HRTEM (High Resolution Transmission Electron Microscopy) mode. Topological characteristics of both coated and uncoated glass samples were analyzed in terms of root mean square (RMS) surface roughness, by Atomic Force Microscopy (AFM), using an NT-MDT Solver Pro-M system. All the measurements were performed in non-contact mode, under ambient conditions, over multiple random (10 × 10) µm2 areas. Nanomaterials 2019, 9, 476 4 of 22

2.3. Photoactivity Testing

The photodegradation behavior of the TaOxNy thin films was tested using two model azo-dyes, namely anionic methyl orange (C14H14N3SO3Na) (MO) and cationic methylene blue (C16H18N3SCl) (MB). The reason for choosing methyl orange as the photodegradation medium is represented by its high stability to degradation in comparison with other model dyes, such as methylene blue, thus allowing the photoefficiency response of the thin films to persistent organic pollutants to be simulated. Methylene blue, on the other hand, is often used in the reference literature to assess the type of photodegradation kinetic for various photocatalysts, especially under visible light irradiation conditions [25]. The MO photodegradation ability of the eight different TaOxNy thin films deposited on glass substrates (according to Table1), presenting a photoactive area of 2.5 cm 2, was assessed under ultraviolet (UV) and visible (VIS) irradiation conditions by using an open-air photodegradation reactor. The uncoated glass substrate (without any deposited film) has been used as the reference. For the UV irradiation experiments, three F18W/T8 black light tubes (Philips) (UVA light, typically 340–400 nm, with peak emission at λmax = 365 nm), placed annularly in relation to the photoreactor stand, were used for photolysis. For the VIS irradiation experiments, three TLD Super 80 18 W/865 white light tubes (Philips, λ > 400 nm) were used, annular to the photoreactor stand. The thin film samples and glass substrate reference were submerged in 25 mL of dye solution (MO and MB, respectively), with an initial concentration of c0 = 0.0125 mM, ensuring a constant irradiation power density of 60 mW/cm2. Before starting the photocatalysis experiments, the films were firstly equilibrated in the dye solution in the dark for 5 min, to ensure an adsorption-desorption equilibrium of MO or MB on the surface of the samples. The photo-response of the sputtered samples using MO as the model dye was evaluated under four different conditions: (i) ultraviolet (UV) irradiation; (ii) UV irradiation with 100 µL of 30% wt. H2O2 added to the volume of MO solution in each flask; (iii) visible (VIS) irradiation; and (iv) VIS irradiation with 100 µL of 30% wt. H2O2 added to the MO solution in each flask. The 30% wt. H2O2 solution was added as the oxidizing agent (electron acceptor) during the photocatalysis reactions. The effective H2O2 concentration in each flask, before photocatalysis, was 6.50 mg/L. This concentration was efficient in enhancing the MB removal rate, also providing an increased economic efficiency of the process (low amount of hydrogen peroxide), as found in other research [26,27]. The total duration of the photoirradiation experiments has been chosen at 6 h, considering that, at this period, photodegradation equilibrium occurs in the case of MO. The methyl orange removal efficiency (discoloration efficiency) η has been determined with the help of Equation (1), based on the absorption maxima for this dye (λmax = 463 nm, determined with a Perkin Elmer Lambda 25 spectrophotometer): A − A η = 0 × 100 (1) A0 where A0 is the absorbance of the initial dye solutions in contact with the films, and A is the absorbance of dye solutions in contact with the TaOxNy thin film samples, determined at λmax = 463 nm, after the 6 h period of illumination. The same formula has been used to determine the removal efficiency for the reference samples. Moreover, the photocatalytic activity of the TaOxNy films deposited onto glass substrates in comparison with the reference (uncoated glass substrate) was evaluated by determining the removal efficiency and type of degradation kinetic for the MB dye under visible light irradiation conditions, without the addition of H2O2. The absorbances of the MB solutions at their absorption maxima (λmax = 664 nm, measured with a Thermo Scientific Evolution 300 spectrophotometer) in contact with the samples were measured at determined time intervals until reaching equilibrium (total irradiation time of 2 h). The methylene blue removal efficiency at equilibrium was calculated with the help of Equation (1). Nanomaterials 2019, 9, 476 5 of 22

The photoactive behavior of the TaOxNy thin films and reference glass substrate was also evaluated by water contact angle measurements (θ), under UV irradiation. The measurements were conducted at room temperature, irradiating the samples by a UV lamp (Philips TUV T8) at a power Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 23 density of 1.8 mW/cm2 and analyzing the height profiles of small sessile drops (1 µL) of deionized water, depositedconducted at on room the sampletemperature, surfaces, irradiating with thethe helpsamples of theby a ImageJ UV lamp software. (Philips TUV T8) at a power density of 1.8 mW/cm2 and analyzing the height profiles of small sessile drops (1 μL) of deionized 2.4. Antibiofilmwater, deposited Capacity on Assay the sample surfaces, with the help of the ImageJ software. Salmonella The2.4. freeze-dried Antibiofilm Capacity strain Assay of was recovered by culturing the bacteria in solid medium, containing 1 g/L yeast extract, 18 g/L agar-agar, 5 g/L sodium nitrate, and 0.2 g/L glucose, supplied by ScharlauThe Chemicals. freeze-dried The strain third of generation Salmonella was of Salmonella recovered bystrain culturing wasused the bacteria for further in solid studies. medium, containing 1 g/L yeast extract, 18 g/L agar-agar, 5 g/L sodium nitrate, and 0.2 g/L glucose, supplied With a design to assay the antimicrobial/antibiofilm potential of the TaO N coatings, 1 µL of by Scharlau Chemicals. The third generation of Salmonella strain was used for furtherx y studies. Salmonella strain (1 × 10−9 CFU L−1) was inoculated into 50 mL liquid media, consisting of 0.6 g/L With a design to assay the antimicrobial/antibiofilm potential of the TaOxNy coatings, 1 μL of yeast extract,Salmonella 1 g/Lstrain sodium (1 × 10−9 nitrate,CFU L−1) and was 3inoculated g/L glucose. into 50 The mL coatedliquid media, samples, consisting deposited of 0.6 g/L on stainlessyeast steel, andextract, the control1 g/L sodium (uncoated nitrate, stainless-steel and 3 g/L glucose. disk, The mirror coated polished), samples, deposited were placed on stainless in distinct steel, Berzelius and glasses.the Thereafter, control (uncoated the samples stainless-steel were sterilized disk, mirror inan polished), autoclave were at placed 126 ◦C in for distinct 0.5 h Berzelius to ensure glasses. their safe handlingThereafter, in antimicrobial the samples testing. were sterilized in an autoclave at 126 °C for 0.5 h to ensure their safe handling Ain volume antimicrobial of 50 µ testing.L from the liquid suspension containing bacteria was placed on the surface of each sterileA sample, volume followedof 50 μL from by incubation the liquid suspension at 33 ◦C for containing 48 h. After bacteria the incubation was placed period,on the surface the samples of each sterile sample, followed by incubation at 33 °C for 48 h. After the incubation period, the samples were washed twice with PBS (phosphate-buffered saline), similarly with the method described by were washed twice with PBS (phosphate-buffered saline), similarly with the method described by Liu et al. [28]. The specimens were subsequently submerged in PBS solution and then exposed to Liu et al. [28]. The specimens were subsequently submerged in PBS solution and then exposed to ultrasoundsultrasounds for 0.5 for h, 0.5 using h, using an ultrasonican ultrasonic cleaner cleaner to to detach detach thethe adherent bacteria bacteria from from the the surface surface of of the samples.the samples. Simultaneously, Simultaneously, Petri Petri dishes dishes containing containing solid solidmedia media were were prepared prepared in inorder order to inoculate to inoculate ◦ the solutionthe solution obtained obtained through through ultrasonication. ultrasonication. The The plates plates were were incubatedincubated at at 33 33 °CC for for 48 48h, after h, after which which the inherentthe inherent properties properties of tantalum of tantalum in antibacterial/antibiofilmin antibacterial/antibiofilm activity activity were were evaluated evaluated by bycounting counting the bacterialthe bacterial colonies colonies and and assessing assessing the the area area of of dispersion dispersion [ 20[20,28].,28]. The antimicrobial/antibiofilm antimicrobial/antibiofilm assessmentassessment was carried was carried out threeout three times times as aas verification a verification of of the the replicabilityreplicability of of the the experiment. experiment.

3. Results3. Results and Discussion and Discussion

3.1. Spectral3.1. Spectral Characterization Characterization of the of Thinthe Thin Films Films XPS was used to access the bonding state of atoms across the surface layers and, after XPS was used to access the bonding state of atoms across the surface layers and, after quantitative quantitative analysis, to find the elements’ relative concentration and corresponding chemical states. analysis, toXPS ﬁnd spectra the elements’ revealed relative the characteristic concentration XPS andtransitions corresponding of tantalum, chemical oxygen, states. and nitrogen, XPSproving spectra the revealed chemical theinteraction characteristic between XPS metal transitions atoms and of tantalum,the reactive oxygen, gas mixture. and nitrogen,No impurities proving the chemicalor contaminants interaction were between detected metal in all atoms samples and analyzed the reactive in this gas study. mixture. The relative No impurities atomic or contaminantsconcentration were detectedof the films, in allas a samples function analyzed of the partial in this pressure study. of The the relativereactive atomicgases (P concentration(N2 + O2)), is of the ﬁlms,depicted as a function in Figure of 1. the partial pressure of the reactive gases (P(N2 + O2)), is depicted in Figure1.

65 60 O (1s) 55 N (1s) 50 Ta (4f) 45 40 35

30 25 20 15 10 Concentration [at%] Concentration 5 0 0.00 0.04 0.08 0.12 0.16 0.20 0.24 P(N +O ) [Pa] 2 2 FigureFigure 1. Atomic 1. Atomic concentration concentration of theof the ﬁlms, films, as as a a function function ofof the reactive reactive mixture mixture partial partial pressure, pressure, obtainedobtained from Ofrom (1s), O N(1s), (1s), N (1s), and and Ta (4f)Ta (4f) of XPSof XPS spectra. spectra.

Nanomaterials 2019, 9, 476 6 of 22

Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 23 To probe the surface chemistry of the films, O1s, N1s, and Ta4f XPS high resolution spectra wereTo probe acquired. the surface The surface chemistry chemical of the response films, O1s, of tantalum, N1s, and as Ta4f a function XPS high of the resolution nitrogen/oxygen spectra were acquired.flow ratio, The issurface portrayed chemical in Figure response2. The of tantalum tantalum, chemical as a function states wereof the assessed nitrogen/oxygen after curve-fitting flow ratio, is portrayedof the complexin Figure Ta4f 2. The envelops. tantalum Figure chemical1 summarizes states were the assessed relative chemicalafter curve-fit concentrations.ting of the complex It is very Ta4f envelops.important Figure to mention 1 summarizes that the the data relative in the literature chemical and concentrations. databases are spreadIt is very over important a rather wide to mention binding that the dataenergy in range.the literature and databases are spread over a rather wide binding energy range.

FigureFigure 2. XPS 2. XPS deconvoluted deconvoluted Ta4f Ta4f spectrum spectrum for for samples obtainedobtained in in different different reactive reactive gas gas mixture mixture partialpartial pressures. pressures.

The feature located at 22.0 eV can be ascribed to metallic Ta [7,29–32], while the peak at 23.5 eV The feature located at 22.0 eV can be ascribed to metallic Ta [7,29–32], while the peak at 23.5 eV corresponds to tantalum nitride [7,30,31,33–35]. The photoelectron binding energy peaks located at corresponds to tantalum nitride [7,30,31,33–35]. The photoelectron binding energy peaks located at 25.0 eV and 26.5 eV can be attributed to tantalum oxynitride [35,36] and to the Ta5+ oxidation state 25.0 eV and 26.5 eV can be attributed to tantalum oxynitride [35,36] and to the Ta5+ oxidation state corresponding to the tantalum pentoxide Ta2O5, respectively [29,30,32,33,37]. Chun and Ishihara correspondingreported 25.8 to eV the for tantalum the binding pentoxide energy corresponding Ta2O5, respectively to tantalum [29,30,32,33,37]. oxynitride bonding, Chunwhich and Ishihara is in reportedclose agreement25.8 eV for with the previous binding ﬁndings energy incorrespondi the literatureng [7to,32 tantalum]. oxynitride bonding, which is in close agreementFrom Figure with3, oneprevious can notice find aings dramatic in the decrease literature of [7,32]. metallic Ta with the increase of the reactive gasFrom mixture Figure partial 3, one pressure, can notice from a dramatic ~32% to decrease ~6% when of metallicP(N2 + O Ta2) with increases the increase from 0.02 of andthe reactive 0.08 Pa, gas mixturewhich partial completely pressure, vanishes from for ~32% higher to ~6% values when of P (NP(N2 +2 O+ 2O).2 This) increases is a consequence from 0.02 ofand the 0.08 increasing Pa, which completelynitrogen vanishes relative concentration for higher values (Figure of1 )P in(N association2 + O2). This with is thea consequence corresponding of increasethe increasing of tantalum nitrogen relativenitride concentration content (Figure (Figure2). Although 1) in association the oxygen with decreasing the corresponding trend was increase accompanied of tantalum by a decreasingnitride content (Figureoxynitride 2). Although contribution, the tantalumoxygen decreasing presents a fullytren oxidizedd was accompanied state for 0.13 Paby and a decreasing higher reactive oxynitride gas contribution,partial pressures. tantalum presents a fully oxidized state for 0.13 Pa and higher reactive gas partial pressures.

Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 23

90 Nanomaterials 2019, 9, x FOR PEER REVIEW Ta 7 of 23 Nanomaterials 2019, 9, 476 7 of 22 80 TaN TaO N 70 x y

Ta O 60 2 5 90 50 Ta 80

TaN 40 TaO N 70 x y 30 Ta O 60 2 5 20 50 10 40 Ta oxidation states [at%] 30 0 20 0.0010 0.05 0.10 0.15 0.20 0.25 Ta oxidation states [at%] 0 P(N +O ) [Pa] 2 2 0.00 0.05 0.10 0.15 0.20 0.25 P(N +O ) [Pa] Figure 3. Tantalum chemical states relative concentrations2 2 based on the fitting process of Ta 4f XPS peak. Figure 3. Tantalum chemical states relative concentrations based on the ﬁtting process of Ta 4f XPS peak. A completelyFigure 3. opposite Tantalum chemical chemical states behavior relative concentrations was observed based onin the the fitting case process of films of Ta prepared4f XPS peak. with high

partialA completelypressureA completely conditions. opposite opposite chemical Thus, chemical a behaviordecreasing behavior was was tendency observed observed of in in nitridethe the case case ofcontribution offilms films prepared prepared followed with high with by high an partialincrease pressure partialof oxynitride pressure conditions. feature conditions. Thus, was aThus, decreasingestablished a decreasing tendency by increasing tendency of nitride of P (Nnitride contribution2 + O contribution2) from 0.17 followed followed to 0.24 by Pa,by an an increasein good ofagreement oxynitrideincrease with feature ofelement oxynitride was establishedrelative feature concentrations was by established increasing by(FP igureincreasing(N2 + 1). O 2The )P from(N 2peculiar + O 0.172) from to chemical 0.240.17 Pa,to 0.24 inbehavior goodPa, in good agreement of Ta2O5 withcan be element agreementexplained relative with by concentrationselement the occurrence relative concentrations (Figure of 1a). threshold The(Figure peculiar 1). Thepressure chemicalpeculiar value chemical behavior at behavior 0.13 of TaPa of 2TaOin25O can5which be explaineddiffusion/segregationcan by be the explained occurrence processes by ofthe a thresholdtakeoccurrence place pressure ofthat a thresholdhave value an at pressureinfluence 0.13 Pa value in on which theat 0.13 diffusion/segregationsurface Pa in chemistry which of diffusion/segregation processes take place that have an influence on the surface chemistry of processesdeposited takelayers. place that have an influence on the surface chemistry of deposited layers. deposited layers. Figure4 4 showsFigure shows 4 theshowsthe N1sN1s the superimposedsuperimposed N1s superimposed corecore core levellevel level spectraspectra spectra forfor all allall mixed mixedmixed reactive reactivereactive gas gasatmospheres,gas atmospheres,atmospheres, indicatingindicatingindicating a similarsimilar a similar chemicalchemical chemical behaviorbehavior behavior withwith with smallsma smallll quantitativequantitative quantitative differences differencesdifferences regarding regardingregarding the nitrogen thethe nitrogennitrogen content. Incontent. addition, In addition, spectral spectral deconvolution deconvolution of of N1s N1s (Figure (Figure 55)5)) ofofof thethe the samples samples samples produced produced produced with with withP(N2 +PP (N(N22 + O2) = 0.02 Pa and P(N2 + O2) = 0.24 Pa reinforce the assessment of metallic nitride and oxynitride films O22) = 0.02 0.02 Pa Pa and and P(N(N2 ++ O O22) )= = 0.24 0.24 Pa Pa reinforce reinforce the the assessment assessment of of metallic metallic nitride nitride and and oxynitride oxynitride filmsfilms formation,formation,formation, inin goodgood in agreementagreement good agreement with with tantalumwith tantalum tantalum chemistry chemis chemistrytry (Figure (Figure(Figure2 ).2). 2). Therefore, Therefore, Therefore, the the theobserved observed observed features features features in in the nitrogen spectra are located at 396.5 eV and 397.2 eV, respectively, with the former being in the nitrogen spectra are located at 396.5 eV and 397.2 eV, respectively, with the former being the nitrogenascribed spectra to metal are located nitrides at[7,30–33] 396.5 eV and and the 397.2latter eV,to metal respectively, oxynitrides with [32]. the Moreover, former being Figure ascribed 6 toascribed metal nitridesrepresentsto metal [ 7 the,nitrides30 O1s–33 XPS] and [7,30–33] superimposed the latter and to spectrathe metal latte fo oxynitridesr rdifferent to metal reactive [ 32oxynitrides]. Moreover,gas partial [32]. pressures. Figure Moreover,6 represents Figure the 6 O1srepresents XPS superimposed the O1s XPS spectrasuperimposed for different spectra reactive for different gas partial reactive pressures. gas partial pressures.

Figure 4. N1s XPS superimposed spectra for different reactive gas partial pressures.

Figure 4. N1s XPS superimposed spectra for differentdifferent reactive gas partialpartial pressures.pressures.

The UV-VIS spectra of the films, shown in Figure7, present a reflectance decrease (absorption) at ~408 nm, which could be attributed to a O2− (2p) →Ta5+ (5d) charge transfer process [38]. These absorption maxima tend to shift to higher wavelengths as the partial pressure of reactive gases increases from P(N2 + O2) = 0.08 Pa, due to the presence of Ta2O5 on the surface of the samples. The samples containing the Ta (V) oxide, namely those obtained at partial pressures of 0.08, 0.13, 0.20, and 0.22 Pa, evidence higher reflectance values than the oxynitride-rich samples, as also determined in other research [12]. Another weak absorption mode can be evidenced at ~420 nm, which could be attributed to the NV+ (2p) → O2−(2p) orbital charge transfer process in TaON [39]. It is evident that this transition is prevalent in the case of the thin films containing a high amount of TaOxNy (0.02, 0.04, 0.08, 0.20, 0.22 Pa partial pressure). Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 23

Nanomaterials 2019, 9, 476 8 of 22 Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 23

Figure 5. XPS deconvoluted N1s spectrum for samples obtained with different reactive gas mixture Figure 5. FigureXPS deconvoluted 5. XPS deconvoluted N1s N1s spectrum spectrum for for samplessamples ob obtainedtained with with different different reactive reactive gas mixture gas mixture partial pressures: (a) 0.02 Pa; (b) 0.24 Pa. partial pressures:partial pressures: (a) 0.02 (a) Pa; 0.02 ( bPa;) 0.24(b) 0.24 Pa. Pa.

Figure 6. O1s XPS superimposed spectra for different reactive gas partial pressures.

The UV-VIS spectra of the films, shown in Figure 7, present a reflectance decrease (absorption) at ~408Figure nm, 6. which O1sO1s XPScould XPS superimposed superimposed be attributed to spectra a O2− (2p) for →different Ta5+ (5d) reactive charge transfer gas partial process pressures. [38]. These absorption maxima tend to shift to higher wavelengths as the partial pressure of reactive gases The spectra of the films produced with partial pressures in the range of 0.20–0.24 Pa are transparent The increasesUV-VIS fromspectra P(N 2of + Othe2) = films,0.08 Pa, shown due to the in presenceFigure 7,of presentTa2O5 on thea reflectance surface of the decrease samples. The(absorption) to visiblesamples light and containing show the characteristic Ta (V) oxide, interferencenamely those obtained fringes at in partial the visible pressures domain. of 0.08, 0.13, One 0.20, of the films at ~408 nm, which could be attributed to a O2− (2p) →Ta5+ (5d) charge transfer process [38]. These containingand higher 0.22 Pa, amountsevidence higher of metallic reflectance Ta values (produced than the withoxynitride-rich partial pressuresamples, as = also 0.04 determined Pa) presents the absorptionin othermaxima research tend [12]. to Another shift toweak higher absorption wavele modengths can be as evidenced the partial at ~420 pres nm,sure which of could reactive be gases highest absorption maximum among all the films, as was also found in previous research [40]. increases from P(N2 + O2) = 0.08 Pa, due to the presence of Ta2O5 on the surface of the samples. The The UV-Vis spectra were also used to find the type of energy transition and the optical band gap samples containing the Ta (V) oxide, namely those obtained at partial pressures of 0.08, 0.13, 0.20, of the Ta species, using the Tauc plots, considering that the optical absorption coefficient α depends on and 0.22 Pa, evidence higher reflectance values than the oxynitride-rich samples, as also determined the photon energy (hν) according to Equation (2): in other research [12]. Another weak absorption mode can be evidenced at ~420 nm, which could be ln(αhυ) = lnα0 + n × ln hυ − Eg (2) where α0 is the band tailing parameter and n is the power factor of the transition mode (for direct allowed transitions, n = 0.5, for the forbidden transitions n = 1.5, while for indirect allowed transitions, Nanomaterials 2019, 9, 476 9 of 22 n = 2). To determine the type of transitions for each type of film, ln(αhν) was plotted as a function of n · ln(hν − Eg), giving (for a certain region of the spectrum) a straight line with the slope equal to the power factor n [41]. By plotting (αhν)1/2, or (αhν)2, as a function of the photon energy, for indirect or direct transitions, respectively, the resulting dependency will have a linear part, whose intersection with the hν axis gives the values for the optical band gap energy, Eg (eV) [42]. The valence band and the conduction band edge potentials (EVB, respectively ECB) have been calculated with Equation (3) andNanomaterials (4): 2019, 9, x FOR PEER REVIEW 9 of 23 EVB = X − 4.5 + 0.5·Eg (3) attributed to the NV+ (2p) → O2−(2p) orbital charge transfer process in TaON [39]. It is evident that ECB = X − 4.5 − 0.5·Eg (4) this transition is prevalent in the case of the thin films containing a high amount of TaOxNy (0.02, 0.04, where0.08, X0.20,is the0.22 absolute Pa partial electronegativity pressure). of the semiconductor species, determined according to Renuka et al. [43] as 5.47 for TaN and 5.98 for TaOxNy. The results are depicted in Table2.

6.4 2.5 0 .02 a 5 0.04 0.02 Pa 10 0.08 0.04 Pa 15 0.13 6.0 20 0.17 0.08 Pa 22.5 0.20 0.13 Pa 5.6 25 0.22 30 0.24 0.17 Pa

5.2 0.20 Pa 0.22 Pa 4.8 0.24 Pa

4.4 4.0 Reflectance (%) 3.6 3.2 2.8 400 500 600 700 800 Wavelength [nm]

(a)

36 b 42 0.02 Pa 0.04 Pa 0.08 Pa 1/2

1/2 36 1/2 36 30 30 (eV/cm) (eV/cm) 30 (eV/cm) 1/2 1/2 ν 1/2 ν 24 ν h

h 24 h α α 24 α

123451234512345 hν (eV) hν (eV) hν (eV) 36 36 0.13 Pa 0.17 Pa 36 0.20 Pa 1/2 1/2 1/2 30 30 30 (eV/cm) (eV/cm) (eV/cm) 1/2

1/2 24 ν 1/2 24 ν ν h h

24 h α α α 18 18 123451234512345 ν 4x106 ν hν (eV) 36 h (eV) h (eV)

6 0.22 Pa 2 3x10 0.24 Pa 1/2 30 2x106 (eV/cm) 2 (eV/cm)

ν 6

h 1x10 1/2 α

ν 24 h α 0

1234512345 ν ν h (eV) h (eV) (b)

FigureFigure 7. 7.(a )(a UV-Vis) UV-Vis spectra; spectra; (b )(b Tauc) Tauc plots plots of theof the ﬁlms. films.

The spectra of the films produced with partial pressures in the range of 0.20–0.24 Pa are transparent to visible light and show characteristic interference fringes in the visible domain. One of the films containing higher amounts of metallic Ta (produced with partial pressure = 0.04 Pa) presents the highest absorption maximum among all the films, as was also found in previous research [40]. The UV-Vis spectra were also used to find the type of energy transition and the optical band gap of the Ta species, using the Tauc plots, considering that the optical absorption coefficient α depends on the photon energy (hν) according to Equation (2):

ln𝛼ℎ𝜐 =𝑙𝑛𝛼 +𝑛×lnℎ𝜐−𝐸 (2)

Nanomaterials 2019, 9, 476 10 of 22

Table 2. Parameters associated with the Tauc plots. The column titled EU refers to Urbach energy.

Edge Potentials Sample n R2 Eg (eV) EU (meV) EVB (eV) ECB (eV)

0.02 Pa 1.863 0.996 2.265 TaOxNy 147.43 2.61 0.35

0.04 Pa 1.947 0.993 2.665 TaOxNy 175.67 2.81 0.15

0.08 Pa 1.817 0.964 2.508 TaOxNy 259.47 2.73 0.22 1.913 0.979 2.411 TaO N 2.68 0.27 0.13 Pa x y 182.19 1.921 0.962 1.429 TaN 1.69 0.25 1.828 0.983 1.979 TaO N 2.46 0.49 0.17 Pa x y 331.61 1.835 0.967 1.864 TaN 1.90 -0.03

0.20 Pa 2.008 0.997 1.956 TaOxNy 485.96 2.46 0.51

0.22 Pa 1.996 0.984 1.951 TaOxNy 484.16 2.45 0.50

0.24 Pa 0.501 0.970 1.436 TaOxNy 499.97 2.19 0.76

It can be seen from Table2 that the Eg values of the thin films range from 1.43 to 2.67 eV. The reported values for Eg of tantalum oxynitrides typically range from 1.9 to 2.5 eV [44,45], while tantalum nitrides (TaNx) present optical band gaps in the 1.88–2.90 eV range. The Eg values for tantalum nitrides (when co-present with TaOxNy) tend to be slightly lower than in the case of TaOxNy [46,47]. In the samples obtained at 0.13 and 0.17 Pa, where both the nitride and the oxynitride species are present in a significant amount (Table2), two band gaps are observed, with the lower value being ascribed to TaN and the higher one being owed to TaOxNy. In the samples obtained with a 0.02, 0.04, 0.08, 0.20, 0.22, and 0.24 Pa partial pressure, where the oxynitride content is prevalent, only one Eg value has been registered. Nearly all the transitions are of an indirect type (n ≈ 2), while for the sample obtained at 0.24 Pa, a direct type of transition has been registered. The decreasing of the Eg values for these last three samples, as the partial pressure increases (and the O:N ratio, in the films composition, increases), could be due to the presence of lattice defects promoted by the higher oxygen content, as determined in other research [48]. For the chemical systems that present only one value of Eg (other extrapolations of linear domains to the photon energy axis are forbidden, i.e., n ≈ 1.5), this value could be ascribed to tantalum oxynitride. It may not be ruled out that the nitride/oxynitride phases may form inter-associations (i.e., they may possibly form solid solutions in certain regions of the sample surface, with localized areas of a high density of stacking defects, possibly induced by Ta2O5)[48]. As localized states extended in the bandgap could often be found in disordered and amorphous materials, the Urbach energy (EU), i.e., the width of the tail of localized states in the bandgap, has been determined for each film type, through linearly extrapolating the ln(α) dependence on photon energies to low hν values [49]. The results depicted in Table2 show an increase in the EU values with the increase in reactive gas flow, respectively TaOxNy content, which could lead to an increase in the disordered atoms and in the defects in the structural bonding [50,51]. The lack of a significant rise in the reflectance values of the films above their Eg values may also show a high concentration of defect states in these materials [52].

3.2. Transmission Electron Microscopy (TEM) The HRTEM micrographs for the two representative samples which showed a good photodegradation ability (the ones deposited with 0.02 and 0.04 Pa reactive gas mixture partial pressure) are illustrated in Figure8. These samples show the presence of both metallic Ta ( β − Ta) and nitride phases. In the sample obtained at 0.02 Pa, both crystalline and amorphous regions could be seen. Nanomaterials 2019, 9, x FOR PEER REVIEW 11 of 23 the reflectance values of the films above their Eg values may also show a high concentration of defect states in these materials [52].

3.2. Transmission Electron Microscopy (TEM) The HRTEM micrographs for the two representative samples which showed a good photodegradation ability (the ones deposited with 0.02 and 0.04 Pa reactive gas mixture partial pressure) are illustrated in Figure 8. These samples show the presence of both metallic Ta (β − Ta) and Nanomaterials 2019, 9, 476 11 of 22 nitride phases. In the sample obtained at 0.02 Pa, both crystalline and amorphous regions could be seen.

Figure 8. HRTEM images of the samples produced with P(N2 + O2) = 0.02 Pa and P(N2 + O2) = 0.04 Pa. Figure 8. HRTEM images of the samples produced with P(N2 + O2) = 0.02 Pa and P(N2 + O2) = 0.04 Pa. The average spacing between the crystallographic planes is 2.64 to 3.55 Å in the sample produced with P(N2 + O2) = 0.02 Pa, consistent with the TaN (110) and (01-1) crystallographic planes spacing. In The averagethe spacing sample obtained between at P(N the2 + O crystallographic2) = 0.04 Pa, several in-plane planes randomly-oriented is 2.64 to 3.55 domains Å in could the sample be produced observed, with possible alternation of crystalline and amorphous phases. In the case of the P(N + O ) with P(N2 + O2) = 0.02 Pa, consistent with the TaN (110) and (01-1) crystallographic2 planes2 spacing. In = 0.04 Pa sample, the amorphous phase is significantly less present, and some grains correspond to the sample obtainedβ-Ta ((011) at andP(N (100)2 + with O2) 2.22 = Å0.04 and 2.45Pa, Åseveral interplanar in-plane distance). Therandomly-oriented remaining samples, obtained domains could be with higher partial pressure values, are amorphous, as demonstrated by X-Ray Diffraction in our observed, with possible alternation of crystalline and amorphous phases. In the case of the P(N2 + O2) previous work [24]. = 0.04 Pa sample, the amorphous phase is significantly less present, and some grains correspond to β-Ta ((011) and 3.3.(100) Surface with Morphology 2.22 Å (AFM) and 2.45 Å interplanar distance). The remaining samples, obtained with higher partialThe pressure surface morphology values, hasare aamorphou pronounced effects, as on demonstrated the photocatalytic activity.by X-Ray For highDiffraction in our photocatalytic activity, the thin film should have a high effective surface area, and thus an increased previous work [24].surface roughness. This increases the number of active sites and the number of defects. An increased surface roughness can be obtained by choosing a substrate with a rough surface, but can also be promoted by generating loosely packed small grains. Quantitative measurements of roughness and 3.3. Surface Morphologysurface area, (AFM) obtained using AFM, suggest that the deposition parameters do not significantly influence the surface roughness of the tantalum oxynitride thin films presented in this paper. The surface morphology has a pronounced effect on the photocatalytic activity. For high photocatalytic activity, the thin film should have a high effective surface area, and thus an increased surface roughness. This increases the number of active sites and the number of defects. An increased surface roughness can be obtained by choosing a substrate with a rough surface, but can also be promoted by generating loosely packed small grains. Quantitative measurements of roughness and surface area, obtained using AFM, suggest that the deposition parameters do not significantly influence the surface roughness of the tantalum oxynitride thin films presented in this paper. Figure 9 presents the RMS roughness values for some of the as-deposited samples and uncoated substrates. The samples were analyzed on 10 × 10 μm2 areas. From the RMS roughness values, and also considering the surface features inherited from the substrate, one can conclude that the

Nanomaterials 2019, 9, 476 12 of 22

Figure9 presents the RMS roughness values for some of the as-deposited samples and uncoated Nanomaterials 2019, 9, x FOR PEER REVIEW 12 of 23 substrates. The samples were analyzed on 10 × 10 µm2 areas. From the RMS roughness values, and alsodeposition considering process the variable surface featuresparameter inherited (the reactive from the gas substrate, partial pre onessure) can conclude does not that have the a depositionsignificant processinfluence variable on the parameter surface roughness, (the reactive and, gas furthermor partial pressure)e, that doesthe preparation not have a significant stage of the influence substrate on thecould surface be used, roughness, at least and,in this furthermore, particular thatcase, the to preparationincrease the stageeffective of the surface substrate area could of the be tantalum used, at leastoxynitride in this particularfilms, due case, to the to increasefact that the the effective films fo surfacellow the area irregularities of the tantalum of oxynitridethe substrate films, surface. due to theHowever, fact that all the the films RMS follow roughness the irregularities values can be of therelatively substrate low, surface. which However,translates allinto the a RMSrelatively roughness small valueseffective can surface be relatively area of low, the which material. translates Higher into surface a relatively roughness small effective values surfacewould areaallow of thefor material.a better Higheradsorption surface of dye roughness molecules values on the would surface allow of for the a films better and adsorption subsequent of dye a faster molecules degradation on the surface of the ofmolecules. the films Thus, and subsequent the surface a roughness faster degradation would be of one the of molecules. the first parameters Thus, the surface to be improved roughness in would order beto oneobtain of thea higher first parameters photocatalytic to be improvedactivity for in orderthe tantalum to obtain oxynitride a higher photocatalytic films deposited activity using for the tantalumconditions oxynitride presented films herein. deposited using the conditions presented herein.

Figure 9.9. 3D3D AFMAFM imagesimages ofof TaOTaOxxNNyy thin ﬁlmsfilms depositeddeposited onon glassglass substrates,substrates, with theirtheir respectiverespective RMS values.

3.4. Photodegradation EfﬁciencyEfficiency All the films ﬁlms could could act act as as visible visible light light harvesters harvesters (due (due to totheir their low low EgEg values),values), but, but, in their in their as- as-obtainedobtained form, form, the the alignment alignment of oftheir their conduction conduction and and valence valence bands bands is is less less favorable for thethe degradation ofof organic organic compounds compounds (i.e., (i.e., the conductionthe conduction bands bands lie below lie thebelow oxygen the reductionoxygen reduction potential atpotential−0.22 eV, at − Figure0.22 eV, 10 Figure), or due 10), to or fast due recombination to fast recombination of the charge of the carriers charge [53 carriers,54]. [53,54]. The main photodegradation mechanism in this case would be the oxidation of the organic dyes with OH• radicals, generated from hydroxyl groups (-OH-) by photoneutralization with holes (h+)

Nanomaterials 2019, 9, 476 13 of 22 at the catalyst-solution interface, or through direct oxidation by a reaction with the photogenerated holes [55,56]:

+ − + + (H2O ⇔ H + OH ) + h → H + OH• (free radicals photogeneration) (5)

Dye + OH• → R• + H2O (dye degradation) (6) Dye + h+ → R+• → Degradation products (direct dye degradation) (7) Nanomaterials 2019, 9, x FOR PEER REVIEW 13 of 23

Figure 10. Band edge positions for the thin films. Figure 10. Band edge positions for the thin films. Figure 11 exhibits the methylorange decolorization/degradation efficiency after 6 h of contact. The main photodegradation mechanism in this case would be the oxidation of the organic dyes The MO decolorization/degradation efficiency in the case of UV irradiation ranges from 1.16 to 1.74% with OH• radicals, generated from hydroxyl groups (-OH-) by photoneutralization with holes (h+) at (Figure 12), increasing with the amount of oxygen in the reactive gas flow, causing an increase in the catalyst-solution interface, or through direct oxidation by a reaction with the photogenerated TaO N content of the films (Figure2). The use of this source of energy for photocatalysis in the holesx y[55,56]: case of the TaOxNy films seems inefficient, due to the fast charge carriers’ recombination. It could be seen from Figure (H112 Othat ⇔ theH+ films+ OH− are) + h able+ → H to+ + use OH• the (free visible radicals light photogeneration) in a more efficient manner, (5) a fact seen from the higher efficiencies (6.67 to 7.94%), per the optical band gaps of each system. The film Dye + OH• → R• + H2O (dye degradation) (6) obtained at 0.24 Pa has the lowest photodegradation efficiency, due to the unfavorable positioning of TaO N valence and conduction band potentials. The highest VIS efficiency for MO degradation x y Dye + h+ → R+• → Degradation products (direct dye degradation) (7) is reached for the samples obtained at 0.13 Pa and 0.20 Pa. In the case of the sample obtained with a 0.13 PaFigure partial 11 pressure, exhibits athe relatively methylorange high RMS decolorizati roughnesson/degradation has been recorded efficiency compared after 6 to h theof contact. remaining samples,The MO as decolorization/degradation determined from the AFM efficiency measurements, in the case which of UV determinesirradiation ranges an increase from 1.16 in theto 1.74% specific surface(Figure area 12), of increasing this sample, with when the amount in contact of withoxygen the in methylorange the reactive gas aqueous flow, causing solution. an Additionally, increase in a TaOxNy content of the films (Figure 2). The use of this source of energy for photocatalysis in the case heterojunction between TaOxNy and TaN could be seen (Figure 10), which may lead to a more efficient of the TaOxNy films seems inefficient, due to the fast charge carriers’ recombination. It could be seen charge carriers separation. After hydrogen peroxide addition, the efficiency of the photodegradation from Figure 11 that the films are able to use the visible light in a more efficient manner, a fact seen process is increased, due to the decrease in the charge carriers recombination (H O is an electron from the higher efficiencies (6.67 to 7.94%), per the optical band gaps of each system.2 2 The film acceptor), as well as due to an increased production in hydroxyl radicals (H O could be photoreduced obtained at 0.24 Pa has the lowest photodegradation efficiency, due to the unfavorable2 2 positioning of by the electrons at the conduction band or it could photocleave by UV radiation) [57]. TaOxNy valence and conduction band potentials. The highest VIS efficiency for MO degradation is reached for the samples obtained at 0.13 Pa and 0.20 Pa. In the case of the sample obtained with a 0.13 H O + e− → OH• + OH− (8) Pa partial pressure, a relatively high RMS2 2 roughness has been recorded compared to the remaining samples, as determined from the AFM measurements, which determines an increase in the specific H O ν → 2 OH• (9) surface area of this sample, when in contact2 with2 (h )the methylorange aqueous solution. Additionally, a heterojunction between TaOxNy and TaN could be seen (Figure 10), which may lead to a more efficient charge carriers separation. After hydrogen peroxide addition, the efficiency of the photodegradation process is increased, due to the decrease in the charge carriers recombination (H2O2 is an electron acceptor), as well as due to an increased production in hydroxyl radicals (H2O2 could be photoreduced by the electrons at the conduction band or it could photocleave by UV radiation) [57].

H2O2 + e− → OH• + OH− (8)

Nanomaterials 2019, 9, x FOR PEER REVIEW 14 of 23

Nanomaterials 2019, 9, 476 14 of 22

H2O2 (hν) → 2 OH• (9)

TheThe addition addition of of hydrogen hydrogen peroxide peroxide to to the the aqueous aqueou MOs MO solution solution has has a a significant significant influence influence on on the the photodegradationphotodegradation efficiency. efficiency. Photodegradation Photodegradation efficiencies efficiencies as as high high as as ~82% ~82% after after the the UV UV irradiation, irradiation, andand close close to to 34% 34% after after VISVIS irradiation,irradiation, can be observ observeded in in Figure Figure 11. 11 .The The highest highest performance performance in inthis thiscase case is exhibited is exhibited by bythe the lower lower partial partial pressure pressure samples, samples, namely namely the the ones ones obtained obtained with with 0.02 0.02 and and 0.04 0.04Pa. Pa. For For the the0.22 0.22 Pa partial Pa partial pressure pressure sample, sample, the thehigh high photodegradation photodegradation rate ratecould could be attributed be attributed to the toslight the slight increase increase in surface in surface roughness roughness (thus (thus a abigge biggerr effective effective surface surface area),area), whenwhen compared, compared, for for example,example, with with the the samples samples obtained obtained at at the the next next lower lower and and next next higher higher partial partial pressures: pressures:P P(N(N22 ++ O2) = =0.20 0.20 Pa Pa −− RMSRMS = =10.4 10.4 nm, nm, P(NP(N2 +2 O+2) O =2 )0.22 = 0.22 Pa − Pa RMS− RMS = 34.1 = nm, 34.1 P nm,(N2 P+(N O2) += 0.24 O2) =Pa 0.24 − RMS Pa − = 3.9RMS nm =(Figure 3.9 nm (Figure9). The9 ).sample The sample obtained obtained with with a 0.24 a 0.24 Pa Pa partial partial pressure pressure presents oneone ofof the the lowest lowest photocatalyticphotocatalytic activities, activities, despite despite its narrowits narrow band band gap. Beinggap. aBeing direct a semiconductor, direct semiconductor, the charge the carriers’ charge recombinationcarriers’ recombination rate is higher rate than is higher in the than case ofin thethe othercase of indirect the other semiconducting indirect semiconducting samples [58 ].samples As it can[58]. be seenAs it in can Figure be seen12a, thein VISFigure light-mediated 12a, the VIS methylene light-mediated blue photodegradation methylene blue efficienciesphotodegradation of the filmsefficiencies range from of the 29 films to 34%, range while from for 29 the to 34%, reference, while thefor photodegradationthe reference, the photodegradation efficiency was negligible efficiency (1.34%—notwas negligible pictured). (1.34%—not pictured).

90 UV 80 UV+H2O2 70 VIS VIS+H2O2 60 50

[%] 40 η 30 20 10 0 0.02 0.04 0.08 0.13 0.17 0.20 0.22 0.24 P(N +O ) [Pa] 2 2 Nanomaterials 2019, 9, x FOR PEER REVIEW 15 of 23 FigureFigure 11. 11.Variation Variation of theof the photodegradation photodegradation efﬁciency efficiency at equilibrium at equilibrium for the for tantalum the tantalum oxynitride oxynitride thin ﬁlmsthin as films a function as a function of the partialof the part pressureial pressure of the reactiveof the reactive gas. gas.

0.5 The40 dependence of the methylene blue concentration on 0.02 irradiation Pa time has been modelled 0.02 Pa 0.04 Pa 0.04 Pa 35 0.08 Pa against the pseudo-first 0.08 Pa order kinetic model (simplification0.4 of the Langmuir–Hinshelwood mechanism 0.13 Pa 0.13 Pa 30 0.17 Pa for heterogenous 0.17 Paphotocatalysis), shown by Equation (10) in linearized logarithmic form [59]: 0.20Pa 0.20 Pa

25 ) 0.3 0.22 Pa 0.22 Pa 𝑐 t 0.24 Pa /c 0.24 Pa 𝑙𝑛 =𝑘⋅𝑡0 20 (10) 𝑐

ln(c 0.2 η [%] 15 where c0 represents the initial concentration of methylene blue (0.0125 mM), ct represents the MB 10 concentration at different photoirradiation periods “t”,0.1 and k (min−1) is the pseudo-first-order rate 5 constant. a b 0 0.0 0 20406080100120 0 20406080100120 Time [min] Time [min]

Figure 12. (a) Photodegradation efficiencies of the tantalum oxynitride samples; (b) graphical determinationFigure 12. (a of) Photodegradation the reaction rate of efficiencies the photocatalytic of the degradationtantalum oxynitride of MB. samples; (b) graphical determination of the reaction rate of the photocatalytic degradation of MB. The dependence of the methylene blue concentration on irradiation time has been modelled againstIt thecan pseudo-firstbe seen, from order Table kinetic 3 and model Figure (simplification 13, that the photodegradation of the Langmuir–Hinshelwood rate dependsmechanism on the TaN forand heterogenous TaOxNy content photocatalysis), and on the surface shown byroughness Equation of (10) the infilms. linearized The sample logarithmic obtained form at [P59(N]:2 + O2) = 0.24 Pa presents the highest photodegradation rate and highest MB photodegradation/discoloration ct efficiency from the analyzed sample set, correlatedln = withk·t the highest content in TaOxNy (Figure 13).(10) c0

Table 3. MB photodegradation and contact angle kinetic parameters.

MB Photodegradation Kinetic Contact Angle Kinetic Sample k (min−1) R2 θ0 (°) kθ (min−1) R2 0.02 Pa 0.00304 0.998 84.18 0.0303 0.982 0.04 Pa 0.00349 0.998 71.84 0.0185 0.977 0.08 Pa 0.00270 0.991 70.77 0.0062 0.942 0.13 Pa 0.00210 0.993 71.79 0.0054 0.972 0.17 Pa 0.00291 0.998 67.52 0.0030 0.960 0.20 Pa 0.00297 0.999 61.86 0.0032 0.879 0.22 Pa 0.00315 0.999 61.33 0.0017 0.865 0.24 Pa 0.00355 0.996 68.43 0.0101 0.990

The samples obtained with P(N2 + O2) = 0.02 Pa and P(N2 + O2) = 0.04 Pa also exhibit high k values, due to the relatively high roughness, compared to the remaining samples (the former case) or high TaOxNy content (the latter case). The samples with a higher content of Ta2O5 on the surface present the lowest photodegradation rates and lower photodegradation efficiencies, in accordance with other research [60]. Good wettability (decreased contact angle) is essential for a high photocatalysis reaction rate, ensuring a rapid equilibration of the contact angles at lower values, thus maximizing the contact surface between the photoactive surface and various diluted organic pollutant solutions.

Nanomaterials 2019, 9, 476 15 of 22

where c0 represents the initial concentration of methylene blue (0.0125 mM), ct represents the MB concentration at different photoirradiation periods “t”, and k (min−1) is the pseudo-first-order rate constant. It can be seen, from Table3 and Figure 13, that the photodegradation rate depends on the TaN and TaOxNy content and on the surface roughness of the films. The sample obtained at P(N2 + O2) = 0.24 Pa presents the highest photodegradation rate and highest MB photodegradation/discoloration efficiency from the analyzed sample set, correlated with the highest content in TaOxNy (Figure 13).

Table 3. MB photodegradation and contact angle kinetic parameters.

MB Photodegradation Kinetic Contact Angle Kinetic −1 2 ◦ −1 2 Sample k (min )R θ0 ( ) kθ (min )R 0.02 Pa 0.00304 0.998 84.18 0.0303 0.982 0.04 Pa 0.00349 0.998 71.84 0.0185 0.977 0.08 Pa 0.00270 0.991 70.77 0.0062 0.942 0.13 Pa 0.00210 0.993 71.79 0.0054 0.972 0.17 Pa 0.00291 0.998 67.52 0.0030 0.960 0.20 Pa 0.00297 0.999 61.86 0.0032 0.879 0.22 Pa 0.00315 0.999 61.33 0.0017 0.865 Nanomaterials 2019, 9, x FOR PEER REVIEW 16 of 23 0.24 Pa 0.00355 0.996 68.43 0.0101 0.990

0.0038

0.0035 0.0036 0.0034 0.0032 ] 0.0030 -1 ] 0.0030 -1

min

[ 0.0028 min k 0.0025 [

k 0.0026 0.0024 0.0020 0.0022 0.0020 10 15 20 25 30 35 40 45 50 55 40 45 50 55 60 65 70 75 80 85 90 c [%] c [%] TaN TaON (a) (b)

FigureFigure 13. 13.(a) ( Variationa) Variation of theof the rate rate constant, constant,k, of k, MB,of MB, with with the the concentration concentration of TaNof TaN and and (b) ( TaOb) TaOxNyxNy phasesphases in thein the ﬁlms, films, detected detected by by XPS. XPS.

TheAs samples the RMS obtained roughness with ofP the(N 2films+ O2 presents) = 0.02 Palow and values,P(N2 the+ O contact2) = 0.04 angles Pa also were exhibit not corrected high k values,for roughness. due to the All relatively the deposited high roughness, films present compared a hydrophilic to the character, remaining essential samples for (the photocatalysis former case) in oraqueous high TaO environments.xNy content (the The latter initial case). values The samplesof the contact with a higherangles content(at the ofbeginning Ta2O5 on of the the surface wetting presentprocess) the span lowest between photodegradation 61 and 86° rates(Figure and 14), lower bein photodegradationg significantly higher efﬁciencies, than in in the accordance case of the withreference other research glass substrate [60]. (4.3°—not shown). Good wettability (decreased contact angle) is essential for a high photocatalysis reaction rate, ensuring a rapid equilibration of the contact angles at lower values, thus maximizing the contact surface between the photoactive100 surface and various diluted organic pollutant solutions.

As the RMS roughness90 of the films presents low values, the contact 0.02 anglesPa were not corrected 0.04 Pa for roughness. All the deposited films present a hydrophilic character, essential 0.08 Pa for photocatalysis in 80 0.13 Pa aqueous environments. The initial values of the contact angles (at the beginning 0.17 Pa of the wetting process) ◦ 70 0.20 Pa span between 61 and 86 (Figure 14), being significantly higher than in the0.22 Pa case of the reference glass substrate (4.3◦—not shown).60 0.24 Pa

] With the increase ofο the50 reactive gas ﬂow, the ﬁlms become less metallic and the contact angles [

tend to decrease, due to theθ formation of more hydrophilic oxygen-rich species on the surface of 40 the samples (TaOxNy, Ta2O5). For all the ﬁlms, the contact angles decrease considerably after UV 30

0 0 20406080100120 Time [min]

Figure 14. Contact angle kinetic of the films under UV irradiation.

With the increase of the reactive gas flow, the films become less metallic and the contact angles tend to decrease, due to the formation of more hydrophilic oxygen-rich species on the surface of the samples (TaOxNy, Ta2O5). For all the films, the contact angles decrease considerably after UV irradiation (with 40–73%), possibly due to the formation of oxygen vacancies on the surface of the materials, which in turn can photocleave the adsorbed water molecules, leading to transient surface hydroxyl groups, of the Ta-OH type [61–64]. The contact angle kinetic was fitted against an exponential dependency of time, described by Equation (11):

⋅ 𝜃=𝜃 ⋅𝑒 (11)

where θ0 is the contact angle at the beginning of the wetting process (i.e., at t = 0) and kθ is the wetting rate of the surface (related to the drop spreading on the surface of the sample). The fitting parameters are expressed in Table 3. The highest wetting rates are recorded for the samples obtained with partial

Nanomaterials 2019, 9, x FOR PEER REVIEW 16 of 23

0.0038

0.0035 0.0036 0.0034 0.0032 ] 0.0030 -1 ] 0.0030 -1

min

[ 0.0028 min k 0.0025 [ 0.0026 Nanomaterials 2019, 9, 476 k 16 of 22 0.0024 0.0020 0.0022 0.0020 irradiation (with10 15 40–73%), 20 25 30 possibly 35 40 45 due 50 to 55 the formation of oxygen40 45 50 vacancies 55 60 65 on 70 the 75 surface 80 85 90 of the materials, which in turn canc [%] photocleave the adsorbed water molecules, leadingc [%] to transient surface TaN TaON hydroxyl groups, of the Ta-OH(a) type [61–64]. The contact angle kinetic was ﬁtted(b) against an exponential dependency of time, described by Equation (11): Figure 13. (a) Variation of the rate constant, k, of MB, with the concentration of TaN and (b) TaOxNy

phases in the films, detected by XPS. −kθ ·t θ = θ0·e (11)

As the RMS roughness of the films presents low values, the contact angles were not corrected where θ0 is the contact angle at the beginning of the wetting process (i.e., at t = 0) and kθ is the wetting ratefor of roughness. the surface All (related the deposited to the drop films spreading present ona hydrophilic the surface character, of the sample). essential The for ﬁtting photocatalysis parameters in areaqueous expressed environments. in Table3. The The highest initial wetting values rates of the are contact recorded angles for the (at samples the beginning obtained of with the partial wetting pressuresprocess) of span the reactive between mixture 61 and of 86° 0.02, (Figure 0.04, and 14), 0.24 bein Pa,g forsignificantly which good higher photodegradation than in the ratescase of and the efﬁcienciesreference haveglass been substrate registered. (4.3°—not This shown). could imply a higher amount of hydroxyl groups formation on the surfaces of these samples, containing a higher amount of metallic Ta [52,65].

100

90 0.02 Pa 0.04 Pa 0.08 Pa 80 0.13 Pa 0.17 Pa 70 0.20 Pa 0.22 Pa 60 0.24 Pa

] ο 50 [

θ 40

0 0 20406080100120 Time [min]

Figure 14. Contact angle kinetic of the ﬁlms under UV irradiation. 3.5. Antibacterial/AntibioﬁlmFigure Capacity 14. Contact angle kinetic of the films under UV irradiation.

FigureWith 15 the exhibits increase results of the obtained reactive on gas selected flow, the samples, films become concerning less themetallic antimicrobial/ and the contact antibiofilm angles capacitytend toof decrease, the TaO duexNy tocoatings the formation against ofSalmonella more hydrophilic. During oxygen-rich the incubation species period on ofthe the surface samples of the coveredsamples with (TaO liquidxNy media, Ta2O containing5). For all Salmonellathe films,, somethe contact of the grown angles bacterial decrease cells considerably have adhered after to the UV surfaceirradiation of the (with samples. 40–73%), By rinsing possibly the samples due to withthe form PBSation (Phosphate-buffered of oxygen vacancies saline), on the the non-adherent surface of the bacteriamaterials, have which been removed.in turn can Following photocleave ultrasonication, the adsorbed the water adherent molecules, bacteria leading has also to transient been removed surface fromhydroxyl the surface groups, and immersedof the Ta-OH in PBS. type [61–64]. The contact angle kinetic was fitted against an exponentialAfter inoculation dependency and incubation of time, described of the solution by Equation containing (11): adherent bacteria, it could be observed that some bacterial colonies grew on agar plates after inoculation. The result of interest was the 𝜃=𝜃 ⋅𝑒⋅ (11) relative surface area of the bacterial colonies, compared to the control sample (uncoated AISI 316L steel).where Evans θ0 is andthe contact Sheel have angle reported at the beginning that thin of film the coatings wetting broadenprocess (i.e., the stainless-steelat t = 0) and kθ applicationis the wetting field,rate due of the to theirsurface potential (related of to improving the drop spreading the self-cleaning on the surface property of the ofthe sample). material. The Thisfitting effect parameters takes placeare expressed due to a reduction in Table 3. in The bacterial highest adhesion wetting rates and colonization,are recorded for which the samples are mainly obtained affected with by partial the surface roughness [66,67]. On the other hand, some studies mention that the antimicrobial/antibiofilm potential of thin films could be negatively influenced by the oxide component [68,69]. Contrary to this observation, a higher antimicrobial/antibiofilm activity of the coatings was exhibited by the samples deposited with reactive mixture higher partial pressure, as shown in Figure 16. It has been shown that the antimicrobial performance of tantalum oxide coatings is predominantly exhibited by the amorphous structure, instead of the crystalline alternative, which has also outperformed human skin fibroblast cellular biocompatibility [22]. Nanomaterials 2019, 9, 476 17 of 22 Nanomaterials 2019, 9, x FOR PEER REVIEW 18 of 23 Nanomaterials 2019, 9, x FOR PEER REVIEW 18 of 23

Figure 15. Bacterial colonies grown on agar plates. Figure 15. Bacterial colonies grown on agar plates.

100 Relative area 100 Relative area 85 Contact angle 85 80 Contact angle 80 80 angle [ Contact 80 angle [ Contact 60 60 75 75

40 70 40 70 o ]

20 o

Relative area [%] area Relative 65 20 ]

Relative area [%] area Relative 65

0 60 0.000 0.04 0.08 0.12 0.16 0.20 0.24 60 0.00 0.04 0.08P(N 0.12+O ) [Pa] 0.16 0.20 0.24 P(N2 +O2 ) [Pa] 2 2 Figure 16. Relative surface area of the bacterial colonies and contact angle, as a function of the Figure 16. Relative surface area of the bacterial colonies and contact angle, as a function of the partial partialFigure pressure.16. Relative surface area of the bacterial colonies and contact angle, as a function of the partial pressure. pressure. Chang and his co-authors have argued that a certain method which can ensure controlling Chang and his co-authors have argued that a certain method which can ensure controlling bacterialChang adhesion and his on co-authors the coatings, have because argued ofthat the a multitude certain method of factors which that can inﬂuence ensure thecontrolling process, bacterial adhesion on the coatings, because of the multitude of factors that influence the process, startingbacterial fromadhesion surface on propertiesthe coatings, to bacteriabecause particularities,of the multitude has of not factors yet beenthat developedinfluence the [22 process,]. If we starting from surface properties to bacteria particularities, has not yet been developed [22]. If we starting from surface properties to bacteria particularities, has not yet been developed [22]. If we consider the observation that tantalum does not have an intrinsic antibacterial activity, as consider the observation that tantalum does not have an intrinsic antibacterial activity, as

Nanomaterials 2019, 9, 476 18 of 22 consider the observation that tantalum does not have an intrinsic antibacterial activity, as demonstrated elsewhere [20], the results obtained on the sample deposited with the lowest partial pressure should be as expected. The potential causes for this behavior are the high RMS roughness (RMS = 37.7 nm), as well as the high metallic Ta content. Contrary to the expectations, the dependence between the relative surface area of the bacterial colonies and the contact angle is, in this case, inversely proportional. It was demonstrated that superhydrophobic surfaces (i.e., with higher contact angles) are much more suited to be used as antibacterial surfaces, due to their anti-adhesive capacity [70]. Anti-adhesive surfaces should reduce the adhesion force between bacteria and a solid surface to enable the easy removal of bacteria [71]. When discussing the surface roughness, higher values for Ra or RMS roughness do not necessarily mean better adhesion of the bacterial colonies, hence a poorer antibacterial capacity. Cells are easily removed from lower roughness surfaces, but they may be retained within features approximating in size to that of the cells. In larger features, the cells may again be relatively easily removed. Hence, the distance between surface features plays an important role. Micron-sized features may favor bacterial adhesion, whereas nano-sized features may create difficult surface conditions for attachment of the bacterial cells [72]. These observations are implying that there are more factors governing the antibacterial/ antibiofilm activity. Even if the mechanism related to the antibacterial capacity is not entirely understood, tantalum-based (oxides) thin films have been tested, and they have antimicrobial potential against several microorganisms, such as Staphylococcus aureus, Staphylococcus epidermidis, Actinobacillus actinomycetemcomitans, and Streptococcus mutans [19,22]. The research in this area is necessary due to the reported high potential of tantalum to stimulate osseointegration. Moreover, if trabecular structures coated with Ta-based materials would be used, their geometry would make the adhesion of microorganisms to the surface difficult [20].

4. Conclusions A set of tantalum oxynitride thin films were produced by DC reactive magnetron sputtering. Several configurations have been obtained by modifying the partial pressure of the reactive gases mixture (15% O2 + 85% N2) between 0.02 Pa and 0.24 Pa, while keeping the other deposition parameters identical (GND substrate, 100 ◦C substrate temperature, 1A target current, 1h deposition time, etc.). X-ray Photoelectron Spectroscopy investigations highlighted tantalum film formation and surface chemical changes during reactive gas partial pressure exposure. The assessed tantalum chemical species provide information on the chemical reactions occurring at different deposition gas pressures. A dramatic decrease of metallic Ta was noticed with the increase of the reactive gas mixture partial pressure. Furthermore, a decreasing trend of nitride contribution accompanied by an increase of oxynitride feature was accomplished by increasing the deposition ambient partial pressure. The enhanced oxynitride content was associated with the favorable antibacterial properties of Ta films related to Salmonella. Transmission electron microscopy results from selected samples reveal both crystalline and amorphous regions, mainly composed of TaN and β-Ta. AFM measurements showed relatively low RMS roughness values, regardless of the deposition parameters, for the as-deposited samples. Photocatalytic experiments show promising results after dye photodegradation under several conditions: ultraviolet irradiation, with and without H2O2 added to the methyl orange solution; visible irradiation, with and without H2O2 added to the methyl orange solution. The photodegradation performance could be improved by increasing the sample roughness, either by changing the deposition parameters or using a high roughness substrate. The antibacterial/antibiofilm capacity of the coatings was assessed against Salmonella. The result of interest was the relative surface area of the bacterial colonies, after inoculation, compared to the bare substrate. The coatings deposited with a higher reactive gas mixture partial pressure exhibit a significantly better antibiofilm capacity. Nanomaterials 2019, 9, 476 19 of 22

Author Contributions: Conceptualization, D.C., and L.C.; Data Curation, D.C., C.G., I.G, A.M., L.V., A.B. and B.V.; Formal Analysis, D.C., I.G., A.M., L.V., A.B., and B.V.; Investigation, D.C., and C.C.; Resources, L.C., A.M., L.V. and B.V.; Writing—Original Draft Preparation, D.C., C.G., I.G, C.C., and A.M.; Writing—Review & Editing, D.C., L.C., A.M., C.C., and L.V. Funding: This research was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011. Daniel Cristea, Camelia Gabor, Ioana Ghiuta, and Catalin Croitoru acknowledge the structural funds project PRO-DD (POSCCE, O.2.2.1., ID 123, SMIS 2637, ctr. no 11/2009) for providing some of the infrastructure used in this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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