Preparation and Photoelectrochemical Properties of Mo/N Co-Doped TiO 2 Nanotube Array Films

: Mo/N co-doped TiO 2 nanotube array ﬁlms were obtained by a combination of magnetron sputtering and anodization. The inﬂuences of doping concentration and nanotube morphology on the structure, morphology, elemental composition, light-absorption capacity, and optoelectronic properties of TiO 2 nanotubes were studied. The ﬁndings revealed that Mo was primarily incorporated into the TiO 2 lattice in the Mo 6+ valence state, while N was mainly embedded into the lattice as interstitial atoms. It was observed that when the sputtering power was 35 W for TiN target and 150 W for Mo-Ti target, the Mo/N co-doped TiO 2 nanotube array ﬁlms exhibited the best photovoltaic performance with a photogenerated current of 0.50 µ A/cm 2 , which was 5.5 times of that of Mo-doped TiO 2 . The enhanced photocatalytic efﬁciency observed in Mo/N co-doped TiO 2 nanotube array ﬁlms can be ascribed to three main factors: an increase in the concentration of photogenerated electrons and holes, a reduction in the band gap width, and intense light absorption within the visible spectrum.


Introduction
Photocatalysis is a promising and environmentally friendly process for converting solar energy into chemical energy.As an important photocatalyst, TiO 2 has the advantages of chemical stability, less pollution, and simple preparation.It has found utility in a wide range of environmental applications encompassing the purification of water and air [1].When TiO 2 is exposed to ultraviolet radiation with a wavelength shorter than 388 nm, electrons in the valence band become energized and move to the conduction band so that there is a spatial separation of the photoexcited electrons and the holes [2].Holes possess an oxidizing effect and can interact with OH -or H 2 O attached to the surface of TiO 2 particles to generate HO•.Electrons are reductive and can interact with O 2 to generate reactive oxygen species such as HO 2 and O 2 − .These highly reactive radicals act on the pollutants to achieve photocatalytic degradation [3].Nevertheless, the anatase and rutile phases of TiO 2 possess a wide band gap of 3.2 eV and 3.0 eV, respectively.As a result, the photo-response primarily lies in the ultraviolet range, and the charges generated by light absorption tend to recombine easily, thus constraining the photocatalytic performance of TiO 2 [4].
In order to address these limitations and enable TiO 2 to achieve improved photocatalytic performance, various strategies involving the incorporation of metal/non-metal elements and rare earth metals through the technique of doping have been explored [5].Recent investigations have focused on the doping of transition metal elements such as Mo [6], Fe [7], Mn [8], Cr [9], W [10], and Ni [11] to improve the photocatalytic efficiency of TiO 2 .Additionally, doping non-metals such as B [12], C [13], S [14], F [15], and N [16,17] has been observed to reduce the band gap and expand the photo-responsive range into the visible light spectrum.
In recent years, TiO 2 co-doped with metal and non-metal elements has attracted more attention due to the limitations of single-element doping [18].Zhang et al. [19] prepared N-Ni co-doped TiO 2 , and the results demonstrated that the incorporation of nitrogen atoms Coatings 2023, 13, 1230 2 of 13 facilitated visible light response, while nickel atoms were present as Ni 2 O 3 , effectively suppressing the recombination of charges generated by photons and enhancing carrier transfer efficiency.Co-doping has the capability to retard the transition from anatase to rutile and allow the anatase phase to become more thermally stable.Wang et al. [20] discovered that anatase-to-rutile phase transition occurred at 500 • C for TiO 2-x N x samples.However, the rutile phase of ZrO 2 /TiO 2-x N x was not observed even after being heated at 600 • C, and the average crystal size did not increase with increasing temperature as did N-TiO 2 .Therefore, the ZrO 2 -modified TiO 2-xNx exhibits better properties, such as higher porosity and larger specific surface area structurally as well as enhanced thermal stability.Kaveh et al. [21] found that the synergistic effect of Fe and N dopants leads to a remarkable improvement in the photocatalytic activity of Fe and N co-doped TiO 2 .The presence of nitrogen and iron extends absorption into the visible region.Additionally, Fe doping acts as a crucial element in efficiently separating the electron-hole pairs generated upon light absorption, thus inhibiting the recombination of photogenerated charges.
Mo is considered to be an excellent option for modifying TiO 2 because its atomic radius is similar to Ti [22].Yin [23] et al. drew conclusions from calculations that cations with 4d and 5d orbitals can bring high mobility of photogenerated electrons into the TiO 2 lattice, while anions with 2p or 3p orbitals can lift the upper edge of the valence band of TiO 2 .Therefore, Mo is one of the best donor elements, and N is one of the best acceptor elements.Moreover, in an effort to elevate the photocatalytic effectiveness of TiO 2 , it is possible to construct a micro-nano structure that has the potential to result in an expanded surface area of the TiO 2 photoanode.Among various nanostructures, nanotubes possess several advantages, such as larger surface area and volume of pores, efficient ion exchange, and rapid as well as long-range electron transport, which ultimately enhance the light absorption [24].
In our previous work [25], Mo-doped TiO 2 nanotube array films were prepared through a dual method involving magnetron sputtering and anodic oxidation.Mo cubes were placed on Ti targets, and the effect of Mo doping concentration was examined by varying the number of Mo cubes.In this study, we focused on the synergistic effect arising from the incorporation of N elements into the TiO 2 nanotube array films while maintaining a constant Mo content.The synthesis of Mo/N co-doped TiO 2 nanotube array films was achieved through a similar procedure.The Ti targets placed with two Mo cubes were sputtered together with TiN targets to obtain Mo/N-Ti films.Subsequently, Mo/N codoped TiO 2 nanotube array films were synthesized by subjecting the Mo/N-Ti films to oxidation.The N doping concentration was regulated by adjusting the sputtering power of the TiN target.The enhanced photovoltaic properties of the Mo/N co-doped TiO 2 nanotubes were demonstrated in characterization.The preparation process for the Mo/N co-doped TiO 2 nanotube array films is briefly depicted in Figure 1.

Preparation of Mo/N Co-Doped TiO2 Nanotube Films
Mo/N-Ti films were fabricated on titanium foils through magnetron sputtering using a vacuum deposition machine (SY-500, Beijing Shengdeyu Vacuum Technology Co., Bei-

Preparation of Mo/N Co-Doped TiO 2 Nanotube Films
Mo/N-Ti films were fabricated on titanium foils through magnetron sputtering using a vacuum deposition machine (SY-500, Beijing Shengdeyu Vacuum Technology Co., Beijing, China).Prior to sputtering, the titanium foil (0.1 mm thickness, 99.6% purity) underwent a cleaning process involving acetone, ethanol, and deionized water.To further remove surface impurities, the cleaned foil was immersed in a solution of HF (40%), HNO 3 (65%), and deionized water (in a ratio of 1:4:5) for one minute.Subsequently, the substrates were dried under a nitrogen (N 2 ) atmosphere.Two Mo cubes (2 mm × 2 mm × 2 mm, 0.5 g) were placed symmetrically on the pure Ti target (99.99%,Alluter, Shenzhen, China), and the power for DC sputtering of the Ti target remained constant at 150 W. The sputtering power of the TiN target (99.99%,Alluter, Shenzhen, China) was adjusted to different levels: 25 W, 35 W, 50 W, 75 W, and 100 W, respectively, to achieve the change of N doping concentration.Argon gas was used during sputtering with a constant flow rate of 50 sccm.The sputtering pressure remained at 0.25 Pa.During sputtering, the substrate continuously rotated at a consistent speed for 40 min.Then the Mo/N-Ti coating substrate was connected to the positive electrode of the DC voltage, while a graphite sheet was connected to the negative electrode.They were immersed in a 0.5 at.% HF solution to form an electrolytic cell.Anodization was carried out at a fixed voltage of 20 V for a duration of 60 min.The obtained nanotube films were annealed at 550 • C for 2 h.

Characterization of Morphology and Structure
Surface and cross-sectional views were obtained by scanning electron microscope (SEM) instrument (ZEISS Gemini 300).The length of the nanotubes can be measured by taking a cross-sectional view of the sheet sample by bending it at the breakage.The phase of the film was measured by X-ray diffraction (XRD).The Bruker D8 A25 powder diffractometer was used in this experiment, and the X-ray source was Cu-Kα radiation.Valence state and relative content of Mo, N, Ti, and O elements of the samples were analyzed by X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha).

Photoelectrochemical Testing
The diffuse reflectance absorption spectra of the films were measured by a UV-vis spectrophotometer with a scanning wavelength ranging from 300 to 800 nm.To characterize the optoelectronic properties, a visible light transient photocurrent test was conducted under dark conditions while intermittently illuminating the sample with the xenon lamp.An electrochemical workstation was used to record the transient photogenerated current density.A three-electrode system was utilized, consisting of a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and either pure or doped TiO 2 nanotube array films as the working (optical) electrode.A 0.5 mol/L Na 2 SO 4 solution was utilized as the electrolyte.The test involved cyclic irradiation of the sample by the xenon lamp (62 mW/cm 2 ) for 50 s and shielding (no irradiation) for the subsequent 50 s.The magnitude of the measured transient photocurrent density indicated the sample's photoelectric conversion ability.
Furthermore, the photolysis properties of the films were also evaluated with a UV-vis spectrophotometer.A 1 × 1 cm 2 sample was immersed in a 4 mg/L methylene blue solution and exposed to visible light at a distance of 30 cm, while 3 mL of the decomposed methylene blue solution was extracted every 10 min during the 40 min irradiation period.The UV spectrophotometer adopted the transmission mode, and the cuvette was used as the sample cell.The obtained curve represented a photodegradation curve, where a steeper slope indicated stronger photodegradation capacity of the sample.

Results and Discussion
The SEM images of the surface of Mo/N co-doped TiO 2 nanotube array films at various doping concentrations are shown in Figure 2. The nanotubes formed through 60 min of anodizing etching exhibited a well-organized and vertical arrangement on the substrate.It was found that with the increase of the concentration of N, the nanotube structure gradually experienced a collapsing tendency.When the Mo doping concentration was constant, an appropriate amount of N doping managed to partially preserve the nanotube morphology of the film [26].Notably, when the TiN sputtering power reached 35 W or higher, there was a reduction in the occurrence of notches and depressions along the upper edge of the nanotubes on the surface, resulting in smoother openings.The diameter of the hollow nanotubes also increased from 100 nm to approximately 150 nm with the introduction of N doping.
The diffraction patterns of the Mo/N-TiO 2 nanotube films with different nitrogen doping concentrations are presented in Figure 3.The XRD analysis revealed distinct diffraction peaks ascribed to both TiO 2 and the Ti substrate [27], suggesting that the fabricated TiO 2 nanotube film is characterized by a thin structure and strong adhesion to the substrate.After being annealed at 550 • C for 2 h, the TiO 2 films showed a mixed crystal structure.The peaks detected at angles of 25.3 • and 37.9 • corresponded to the diffraction peaks of the ( 101) and (004) crystal planes of anatase TiO 2 .The peaks at 27.6 • (110) and 56.6 • (220) were commonly considered as indicative peaks of the rutile phase.Based on the equation [28,29], the following is given: Coatings 2023, 13, x FOR PEER REVIEW 4 of 14 current density.A three-electrode system was utilized, consisting of a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and either pure or doped TiO2 nanotube array films as the working (optical) electrode.A 0.5 mol/L Na2SO4 solution was utilized as the electrolyte.The test involved cyclic irradiation of the sample by the xenon lamp (62 mW/cm 2 ) for 50 s and shielding (no irradiation) for the subsequent 50 s.The magnitude of the measured transient photocurrent density indicated the sample's photoelectric conversion ability.Furthermore, the photolysis properties of the films were also evaluated with a UVvis spectrophotometer.A 1 × 1 cm 2 sample was immersed in a 4 mg/L methylene blue solution and exposed to visible light at a distance of 30 cm, while 3 mL of the decomposed methylene blue solution was extracted every 10 min during the 40 min irradiation period.The UV spectrophotometer adopted the transmission mode, and the cuvette was used as the sample cell.The obtained curve represented a photodegradation curve, where a steeper slope indicated stronger photodegradation capacity of the sample.

Results and Discussion
The SEM images of the surface of Mo/N co-doped TiO2 nanotube array films at various doping concentrations are shown in Figure 2. The nanotubes formed through 60 min of anodizing etching exhibited a well-organized and vertical arrangement on the substrate.It was found that with the increase of the concentration of N, the nanotube structure gradually experienced a collapsing tendency.When the Mo doping concentration was constant, an appropriate amount of N doping managed to partially preserve the nanotube morphology of the film [26].Notably, when the TiN sputtering power reached 35 W or higher, there was a reduction in the occurrence of notches and depressions along the upper edge of the nanotubes on the surface, resulting in smoother openings.The diameter of the hollow nanotubes also increased from 100 nm to approximately 150 nm with the introduction of N doping.The diffraction patterns of the Mo/N-TiO2 nanotube films with different nitrogen doping concentrations are presented in Figure 3.The XRD analysis revealed distinct diffraction peaks ascribed to both TiO2 and the Ti substrate [27], suggesting that the fabricated TiO2 nanotube film is characterized by a thin structure and strong adhesion to the substrate.After being annealed at 550 °C for 2 h, the TiO2 films showed a mixed crystal structure.The peaks detected at angles of 25.3° and 37.9° corresponded to the diffraction peaks of the (101) and (004) crystal planes of anatase TiO2.The peaks at 27.6° (110) and 56.6° (220) were commonly considered as indicative peaks of the rutile phase.Based on the equation [28,29], the following is given: The proportion of the anatase and rutile phase was calculated and listed in Table 1.The integrated intensities of diffraction peak on crystal surface of the anatase (101) and rutile (110) were denoted as I a and I r .In the prepared samples, the ratio of the anatase phase to rutile phase was approximately 4:6; thus, the synergistic effect of the two phases can be expected during photocatalysis [30].In comparison to pure TiO 2 , the doping of Mo and N facilitated the development of anatase phase to some extent, and the proportion of anatase appeared to rise with the increase of nitrogen content.As shown in Table 2, the lattice parameters a and c were calculated, respectively, for anatase and rutile phases.For Mo-doped samples (P TiN = 0 W), the lattice parameters a and c exhibited smaller values compared to undoped TiO 2 for both phases due to the substitution of Mo 6+ ions (with an Coatings 2023, 13, 1230 6 of 13 ion radius of 0.041 nm) in place of Ti 4+ ions (with an ion radius of 0.061 nm) .As the doping concentration of nitrogen increased, a corresponding expansion in the lattice parameters could be observed, which was attributed to the larger size of N 3− ions (with a radius of 0.171 nm) compared to O 2− ions (with a radius of 0.132 nm) upon their incorporation into the TiO 2 lattice [31].To analyze the elemental composition and valence state of the Mo/N co-doped TiO 2 nanotube array film, the XPS spectra of the sample prepared with a TiN target sputtering power of 35 W are shown in Figure 4.The film was composed of five elements: Ti, O, Mo, N, and C. The presence of the C 1s peak (at 284.8 eV) was caused by carbon contamination during the sample-preparation process and was utilized for charge correction before further data analysis.The peak positions of Ti 2p, O 1s, Mo 3d, and N 1s corresponded to energies of 460 eV, 525 eV, 230 eV, and 400 eV, respectively.As shown in Figure 4b, the Mo 3d peaks were fitted, revealing characteristic peaks at 232.1 eV and 235.5 eV, which corresponded to the 3d 5/2 and 3d 3/2 orbitals of Mo [32].Additionally, there was residual partially oxidized Mo present in the +5 valence state.The area under the peaks of Mo 6+ and Mo 5+ accounted for 56.7% and 43.3%, respectively, suggesting that the predominant presence of Mo into the TiO 2 crystal lattice was observed as substitution doping, specifically with a valence state of +6.The absorption spectra of diffuse reflectance for Mo/N-TiO2 nanotube samples featuring various concentrations is illustrated in Figure 5.The sample prepared with a TiN target sputtering power of 35 W exhibited exceptional light-absorption capabilities in the visible range.The synergistic effect achieved through the co-doping of Mo and N reduced the band gap of TiO2 as expected, causing a significant enhancement in the efficiency of visible light response [39].In the UV light region, the primary absorption can be attributed to the inherent band gap of TiO2.The optical absorption observed in the visible range is associated with electron transitions facilitated by N doping, where electrons shift from the O 2p state to the Ti 3d state [40], as well as the transfer of electrons from the O 2p state to the Mo 4d state [41].In Figure 4c, the N 1s peaks were fitted.The peak at 399.7 eV corresponded to N-O bonds formed by nitrogen in the oxidized state, while the peak at 401.4 eV indicated N-O bonds formed directly between nitrogen and oxygen atoms adsorbed on the surface [33].In addition, characteristic peaks associated with Ti-N bonds near 396 eV were not observed, suggesting that nitrogen atoms mainly existed in the form of O-Ti-N bonds [34].
The spectrum of Ti 2p is depicted in Figure 4d.The binding energies of 458.5 eV and 464.3 eV corresponded to the Ti 2p 3/2 and Ti 2p 1/2 orbitals [35], with a peak intensity ratio of approximately 2:1.Titanium was found to exist in the +4 valence state within the TiO 2 lattice.
Figure 4e is the spectrum of O 1 s.After analysis, it was decomposed into two distinct peaks at 529.7 eV and 530.6 eV.The former peak originated from oxygen atoms present within the TiO 2 lattice, while the latter was related to the existence of hydroxyl functional groups on the surface of TiO 2 films and the residues of carbonate radicals [36,37].
Table 3 shows the elements composition of samples prepared at different power levels of the TiN target.The doping concentration of nitrogen tended to increase with elevated power, but such an increase became slow when the power exceeded 50 W.For thin films fabricated through sputtering using physical vapor deposition, it is common for highly disordered energy states to form.These energy states tend to recombine when supplied with additional energy.In the case of anodizing Mo/N co-doped titanium films, the nitrogen component is selectively removed, and Ti-O bonds are formed [38].The absorption spectra of diffuse reflectance for Mo/N-TiO 2 nanotube samples featuring various concentrations is illustrated in Figure 5.The sample prepared with a TiN target sputtering power of 35 W exhibited exceptional light-absorption capabilities in the visible range.The synergistic effect achieved through the co-doping of Mo and N reduced the band gap of TiO 2 as expected, causing a significant enhancement in the efficiency of visible light response [39].In the UV light region, the primary absorption can be attributed to the inherent band gap of TiO 2 .The optical absorption observed in the visible range is associated with electron transitions facilitated by N doping, where electrons shift from the O 2p state to the Ti 3d state [40], as well as the transfer of electrons from the O 2p state to the Mo 4d state [41].The Tauc analysis based on Kubelka-Munk formula could be utilized for estimating the band gap energy of the samples [42,43]: where α is the absorption coefficient, and A is a constant.The value of the index indicates the properties associated with the electron transition, where n is 1/2 for direct allowed transitions and 2 for indirect allowed transitions.Since TiO 2 is considered to be an indirect semiconductor, the relationship between (αhν) 1/2 and hν was plotted.As displayed in Figure 6, the band gap of the samples could be determined by evaluating the intercept of the tangent line on the photon energy axis.The calculated values of band gap are given in Table 4.Among the Mo/N co-doped TiO 2 nanotube films, the sample obtained with a sputtering power of 35 W for the TiN target exhibited the lowest band gap of 2.93 eV.
The effective incorporation of donor-acceptor energy levels led to a significant increase in the visible light-absorption capability of the co-doped TiO 2 nanotube film [39].The doping of Mo alters the electronic structure by introducing Mo 4d states below the conduction band and causing a shift in the Fermi level.Simultaneously, N doping results in the development of an isolated N 2p state above the top of the valence band [40].Compared to undoped or Mo/N mono-doped TiO 2 , Mo/N co-doped TiO 2 has a much larger number of impurity states, which implies a narrowed band gap and increased light absorption under visible light.The incorporation of the Mo 4d state transforms N 2p from the unoccupied state to the occupied state, which brings about a decrease in the recombination center and thus improves the transport ability of the carriers [44].Figure 7 shows the photogenerated current density curves of Mo/N co-doped TiO 2 nanotube array films with different doping concentrations when exposed to visible light.It is evident from the figure that the sample with TiN target sputtering power of 35 W excited more electronic transitions when exposed under visible light.The photogenerated carrier density exhibited a transient photogenerated current density of 0.50 µA/cm 2 , which was 5.5 times higher than that of the Mo-doped TiO 2 nanotube array film and 16.7 times higher than that of the undoped TiO 2 nanotube array film.The doping of Mo and N introduced donor-acceptor energy levels and altered the band structure.The narrowed band gap resulting from the doping of Mo and N could be recognized as the contributing factor to the enhanced photoelectrochemical performance.As the N content continued to elevate with increasing sputtering power, the photogenerated current density instead began to decrease, suggesting that there could be an optimum sputtering power to obtain the ideal N doping content.When the N doping concentration is at a high level, deep-level defects can be formed, which act as recombination centers [45].Moreover, the excessive N doping may exert an adverse influence on the structure of the nanotubes, which also reduces the carrier mobility.
where r is the rate of degradation of reactants, C is the concentration of the reactants, t stands for the time, k is the reaction rate constant, and K is the adsorption equilibrium constants for reactants on TiO 2 surfaces.In the case of a first-order reaction, suppose the initial concentration at the beginning of the reaction is C 0 , and upon completion of the reaction, the concentration is denoted as C.Then, the formula can be rewritten as follows: K' is the first-order kinetic constant of photocatalytic reaction.The correlation coefficient R 2 and the rate constant K' (absolute value of the slope) obtained by fitting the curves in Figure 8 are listed in Table 5.The correlation coefficients of all samples were above 0.98, indicating that the photocatalytic degradation of the samples was consistent with first-order reactions.The rate constant of doped TiO 2 nanotubes was more than twice that of undoped TiO 2 nanotubes, and the highest degradation rate of 0.02264 was achieved for Mo/N co-doped TiO 2 nanotubes.The co-doping of Mo and N in TiO 2 substantially boosts the absorption of visible light and leads to a greater generation of electrons and holes, which actively engage in the photocatalytic process.Furthermore, Mo 6+ can also inhibit the recombination of photo-induced carriers by serving as an electron-or hole-trapping site [48].Mo 6+ reacts with electrons and holes to form Mo 5+ and Mo 7+ , respectively, which are highly unstable, thus producing reactive radicals (O − 2 and OH) that significantly contribute to the photocatalytic process.the absorption of visible light and leads to a greater generation of electrons and holes, which actively engage in the photocatalytic process.Furthermore, Mo 6+ can also inhibit the recombination of photo-induced carriers by serving as an electron-or hole-trapping site [48].Mo 6+ reacts with electrons and holes to form Mo 5+ and Mo 7+ , respectively, which are highly unstable, thus producing reactive radicals (O 2 − and OH) that significantly contribute to the photocatalytic process.

Conclusions
A physical method of nitrogen doping using a solid nitrogen source was proposed.In the magnetron sputtering process, a TiN target was utilized as a nitrogen source, and a controllable number of Mo cubes were embedded on the Ti target.The doping content of N was regulated by the sputtering power of the TiN target.Mo/N co-doped TiO2 nanotube array films were obtained after co-sputtering and anodization.The technology is simple and eco-friendly.It was indicated that Mo was mainly introduced into the TiO2 lattice through Mo 6+ substitution, while N mainly occupied interstitial positions in the lattice, which changed the energy band structure and improved the photoelectric conversion efficiency.Furthermore, the construction of the nanotube structure provided a broad

Conclusions
A physical method of nitrogen doping using a solid nitrogen source was proposed.In the magnetron sputtering process, a TiN target was utilized as a nitrogen source, and a controllable number of Mo cubes were embedded on the Ti target.The doping content of N was regulated by the sputtering power of the TiN target.Mo/N co-doped TiO 2 nanotube array films were obtained after co-sputtering and anodization.The technology is simple and eco-friendly.It was indicated that Mo was mainly introduced into the TiO 2 lattice through Mo 6+ substitution, while N mainly occupied interstitial positions in the lattice, which changed the energy band structure and improved the photoelectric conversion efficiency.Furthermore, the construction of the nanotube structure provided a broad transport channel for photogenerated electronhole pairs and an extensive surface area with active sites for photocatalysis.The optimal sputtering power of the TiN target for fabricating Mo/N co-doped TiO 2 nanotube array films was determined to be 35 W. The prepared TiO 2 nanotube array film exhibited a band gap of 2.93 eV and a photogenerated current of 0.50 µA/cm 2 under visible light irradiation, surpassing the performance of Mo-doped TiO 2 nanotube array films by 5.5 times.The photocatalytic degradation rate constant for methyl blue of co-doped TiO 2 nanotube array film was 0.02264, which was also higher than 0.02062 of Mo-doped TiO 2 nanotube array film.

Figure 2 .
Figure 2. SEM images of surface morphology of Mo/N co-doped TiO 2 nanotube array films with different sputtering power of TiN target (the sputtering power of Mo-Ti target is 150 W, TiN target is 25 W for (a,b); 35 W for (c,d); 50 W for (e,f); 75 W for (g,h); 100 W for (i,j)).

Coatings 2023 , 14 Figure 4 .
Figure 4.The XPS spectrum of Mo/N co-doped TiO2 nanotube array films with a TiN target sputtering power of 35 W: full spectrum (a) and the high-resolution spectrum of Mo 3d (b), N 1s (c), Ti 2p (d) and O 1s (e).

Figure 4 .
Figure 4.The XPS spectrum of Mo/N co-doped TiO 2 nanotube array films with a TiN target sputtering power of 35 W: full spectrum (a) and the high-resolution spectrum of Mo 3d (b), N 1s (c), Ti 2p (d) and O 1s (e).

Table 4 .
Calculated band gap of Mo/N co-doped TiO 2 nanotube array films.

Figure 8
Figure 8 shows the catalyzed degradation of 40 mL of 4 mg/L methylene blue by Mo/N co-doped TiO 2 .The film prepared with TiN target sputtering power of 35 W and anodization time of 60 min was tested for degradability under visible light exposure, with pure TiO 2 and Mo-doped TiO 2 as comparisons.The kinetics of the photocatalytic reaction conform to the L-H model [46,47]: r = − dC dt = kKC 1 + KC(3)

Figure 8 .
Figure 8.The curve of the photodegradation of methylene blue by Mo/N-doped TiO2 nanotube array film under visible light irradiation.Table 5. Correlation coefficient R 2 and first-order kinetic constants of Mo/N-doped TiO2 nanotube array films.

Figure 8 .
Figure 8.The curve of the photodegradation of methylene blue by Mo/N-doped TiO 2 nanotube array film under visible light irradiation.

Table 1 .
Proportion of anatase and rutile phase of Mo/N co-doped TiO 2 nanotube array films.

Table 2 .
Lattice parameters a and c of Mo/N co-doped TiO 2 nanotube array films.

Table 3 .
Elements fractions of Mo/N co-doped TiO2 nanotube array films.

Table 3 .
Elements fractions of Mo/N co-doped TiO 2 nanotube array films.

Table 5 .
Correlation coefficient R 2 and first-order kinetic constants of Mo/N-doped TiO 2 nanotube array films.