Introduction

Titanium dioxide (TiO2) is the most widely investigated photocatalyst due to its good photocatalytic activity, high chemical and thermal stability, nontoxicity, low cost, and excellent degradation capacity1,2,3. However, a large band gap (3.2 eV) of TiO2 has limited its practical applications since it can be only activated by the illumination of ultra-violet light, which only makes up 4–5% of the solar spectrum4. In order to utilize a wider solar spectrum, it is highly desirable that the TiO2-based photocatalysts can work under visible light. Therefore, considerable efforts have been devoted for TiO2 to facilitate its visible light absorption. There are several ways can be applied to achieve this goal, such as element doping5,6,7 and coupling with metal or other semiconductors8,9,10,11. Among various approaches, non-metal doping of TiO2 has shown great promise in enhancing visible light active photocatalysis, with nitrogen doping being the most promising dopant3,12,13. N-doped TiO2 nanomaterials have been synthesized successfully by various methods, such as hydrolysis of TTIP in a water/amine mixture, post-treatment of the TiO2 sol with amines, ball milling of TiO2 in a NH3 water solution14,15,16. N-doped TiO2 nanomaterials could also be obtained by annealing TiO2 under NH3 flux at high temperature17. In addition, several film deposition techniques including sputtering18, chemical vapour deposition19, atomic layer deposition (ALD)20, have also been applied to prepare N-doped TiO2 film. The visible light photocatalytic activity of N-doped TiO2 nanomaterials has been explored thoroughly. Although the effect of N doping on photocatalytic enhancement of TiO2 is still debated, it is well accepted that N doping can cause the red shift absorption threshold of TiO2, improving the visible light photocatalytic activity21,22.

ALD is a novel and promising thin film deposition technique based on sequential self-limited and complementary surface chemisorption reactions, which is able to deposit ultrathin, uniform, and conformal layers, and it’s especially suitable for coating 3D complex structures. In recent years, ALD has attracted increasing attention in synthesis and surface engineering of complex nanostructures in recent years23,24,25,26. ALD has shown great prospects in various applications, such as lithium ion batteries27,28, supercapacitors29,30,31, catalysis32,33, and solar energy conversions34. Plasma enhanced ALD (PEALD), employing plasma as one precursor, has shown some merits over conventional thermal ALD (T-ALD), such as higher film density, lower impurity, higher growth rate, better electronic properties. Moreover, less energy is required to drive the surface reaction because of the high reactivity of plasma species, resulting in a lower deposition temperature35.

Various N-doped TiO2 nanomaterials, which exhibit highly visible light photocatalytic performance, have been successfully synthesized. However, the effect of ultrathin N-doped TiO2 surface coating/modification on visible light photocatalysis of TiO2 has not been well researched. Herein, PEALD was adopted to deposit ultrathin TiN film on TiO2 powders. The deposited TiN film would be oxidized into TiOxNy (N doped TiO2) when exposed to the air, achieving TiOxNy coated TiO2 after PEALD TiN coating. This ultrathin TiOxNy coating can facilitate the visible light absorption of TiO2. Correspondingly, the TiOxNy coated TiO2 powders exhibit significantly enhanced visible light photocatalytic activity towards methyl orange (MO) and phenol degradation.

Results

Bui et al. have reported that the surface of deposited TiN film would be oxidized when exposed to the air36. Therefore, the XPS spectra of the TiN film deposited by PEALD on silicon were firstly conducted to explore the surface chemistry of as-deposited TiN, as shown in Fig. S1. Both Ti-O and Ti-N bonding can be detected in Ti 2p spectra, confirming the formation of TiOxNy on the PEALD TiN surface, in consistent with reported literature36. Therefore, it can be speculated that TiOxNy coated TiO2 composite can be achieved here after coating ultrathin PEALD TiN on TiO2 surface.

Next, the surface chemical nature of PEALD TiN coated TiO2 catalyst was also characterized by XPS. XPS spectra were fitted with Gaussian-Lorentzian (G-L) functions after smart-type background subtraction. Figure 1a shows the N 1 s spectra of pristine TiO2 and PEALD TiN coated TiO2. It can be found that pristine TiO2 only exhibits a weak peak at ~400.1 eV, which can be assigned to absorptive nitrogen molecules37,38. After PEALD TiN coating, there appears a new peak at 396.5 eV, corresponding to the formation of N-Ti bonding37,38. And the intensity of N-Ti is enhanced with increasing the PEALD TiN cycles. In Ti 2p spectra of pristine TiO2 (Fig. S2a), the doublet at 464.4 and 458.7 eV can be assigned to Ti 2p1/2 and Ti 2p3/2 peaks of Ti-O bonds with the spin orbit splitting energy of 5.7 eV, consistent with the value of TiO211. Besides, there are two weak doublet peaks at 462.9 eV and 457.2 eV can also be detected, which can be assigned to Ti3+ defects on the surface39. More Ti3+ can be introduced onto the surface of TiO2 after PEALD TiN coating. Therefore, the surface Ti3+/Ti ratio increases with increasing PEALD deposition cycles, as shown in Fig. 1d. All the samples show the similar O 1s spectra with main O-Ti bonds of TiO2 at 529.9 eV, as shown in Fig. 1c and Fig. S2, the peak at 532.4 eV related to the -OH on the surface can also be detected7. Figure 1d illustrates the plots of surface Ti3+/Ti ratio and N content versus the PEALD TiN cycles, it can be seen that both surface Ti3+ and N content increase with PEALD TiN cycles. Combined with XPS data of PEALD TiN film on Si (Fig. S1), it can be concluded that TiOxNy coating layer was formed on TiO2 surface after ultrathin PEALD TiN coating. It has been proved in previous literatures that both Ti3+ and N sites can narrow the band gap of TiO240,41. Therefore, it can be speculated that a much smaller band gap can be achieved for the TiOxNy layer, promoting the visible light absorption.

Figure 1
figure 1

(a) N 1s XPS spectra of pristine TiO2 and TiOxNy coated TiO2 powders, (b) Ti 2p and (c) O1s spectra of TiO2@100TiN, (d) Ti3+/Ti ratio and N content verse TiN coating cycles.

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Figure 2 shows the Raman spectra of pristine TiO2 and TiO2@50TiN prepared by PEALD. According to the previously reported data42, the anatase phase of TiO2 has six Raman bands at 144 cm−1 (Eg), 197 cm−1 (Eg), 399 cm−1 (B1g), 513 cm−1 (A1g), 519 cm−1 (B1g) and 639 cm−1 (Eg), and the rutile phase has four Raman bands at 143 cm−1 (B1g), 447 cm−1 (Eg), 612 cm−1 (A1g), and 826 cm−1 (B2g). Both samples here present Raman spectra the same as the pure anatase phase, with no peaks related to the rutile phase. More importantly, the most remarkable feature is that the predominant peak position (Eg) undergoes a blue shift from 141.1 cm−1 to 144.0 cm−1 after TiOxNy modification. Previous literatures have demonstrated that the N doping in TiO2 can result in the blue shift for Eg mode43. Therefore, the slight blue shift here can be ascribed to the small amount TiOxNy formation on the TiO2 surface.

Figure 2
figure 2

Raman spectra of pristine TiO2 and TiO2@50TiN powders.

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FESEM was performed to observe the morphology and crystal size of TiO2 and TiOxNy coated TiO2 powders, as shown in Fig. S3. It can be found that the pristine TiO2 powders show well dispersed sphere of around 10–30 nm and aggregate together. After PEALD deposition, it can be seen that ultrathin TiOxNy coating has no obvious effect on the morphology and crystal size of TiO2. All the samples exhibit the similar morphology. In order to thoroughly characterize the microstructure change of TiO2 after surface coating, high resolution transmission electron microscopy (HRTEM) was also applied to observe the microstructure of TiO2 and TiO2@50TiN. It can be found that pristine TiO2 exhibits good crystallinity with a sharp well-ordered surface (Fig. 3a). After 50 cycles TiN coating, there is an amorphous layer formed on the TiO2 surface of ~1 nm (Fig. 3b). It is supposed to be the ultrathin TiOxNy coating formed after PEALD TiN deposition. Besides, both samples show a lattice spacing of 0.35 nm, which corresponds to the (101) planes of anatase TiO2. Therefore, it can be concluded from XPS spectra, Raman spectra, and HRTEM images that an amorphous ultrathin TiOxNy was formed on the TiO2 surface.

Figure 3
figure 3

TEM images of (a) pristine TiO2 and (b) TiOxNy coated TiO2 (TiO2@50TiN).

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The corresponding XRD patterns of pristine TiO2 and TiOxNy coated TiO2 powders are shown in Fig. 4. All the samples exhibit the similar characteristic diffraction peaks at 25.4°, 37.9°, 48.0°, 54.1°, 63.0° etc., indicating good agreement with standard anatase TiO2 (JCPDS No. 71-1168). Besides, there are no other peaks such as Ti-N detected in the samples. In addition, the average crystal size and lattice parameters of different samples can be determined by XRD using Scherrer equation, as listed in Table 1. It can be found that the crystal size of all the samples is estimated to be around 19 nm, in agreement with SEM images. In addition, all the samples show nearly the same lattice parameters, indicating that ultrathin TiOxNy surface coating does not change the crystal size and average unit cell dimension. Moreover, Nitrogen adsorption-desorption isotherms were also performed to measure the surface area of TiO2 powders, it can be found that all the samples exhibit nearly the same BET surface area of around 113 m2/g (Table 1 and Fig. S4).

Figure 4
figure 4

XRD patterns of (a) pristine TiO2, (b) TiO2@20TiN, (c) TiO2@50TiN, and (d) TiO2@100TiN.

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Table 1 Parameters of pristine TiO2 and TiOxNy coated TiO2 powders.
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Therefore, it can be concluded that ultrathin TiOxNy coating can be formed on the surface of TiO2 powders. And this ultrathin surface coating doesn’t show obvious change in the morphology, crystal size, lattice parameters, and surface area of TiO2 nano powders. However, it can be clearly seen that there is a vivid color change of TiO2 powders from white to yellow after ultrathin TiOxNy surface modification, as shown in Fig. 5a,b. Hence, UV-Vis diffuse reflectance spectra were conducted to explore the influence of ultrathin TiOxNy surface coating on the visible light absorption of TiO2 powders, as shown in Fig. 5c. For comparison, the spectrum of pristine TiO2 powder is also illustrated. The absorption edge of pristine TiO2 is approximately 371 nm and does not show noticeable absorption in the visible region. However, all the TiOxNy coated TiO2 samples exhibit distinct and meaningful absorption in the visible range from 390 to 500 nm, consistent with previous experimental results20,44. Furthermore, more TiOxNy coating can induce more visible light absorption. For the indirect bandgap semiconductor, the relation between the absorption edge and the photon energy (hν) can be written as follows: (αhν)1/2 = A(hν - Eg), where A is the absorption constant of the indirect band gap semiconductor material. The absorption coefficient (α) is determined from the scattering and reflectance spectra according to Kubelka-Munk theory. The indirect bandgap energies estimated from the intercept of the tangents to the plots are presented in Fig. 5d. The bandgap of pristine TiO2 powders is determined to be 3.24 eV. TiOxNy coated TiO2 (100 cycles TiN) exhibits two bandgaps. The larger bandgap of 3.18 should be related to the TiO2 supporters. Besides, a smaller band gap of 1.64 eV can be assigned to the band gap value of TiOxNy coating layer. Therefore, it can be concluded that ultrathin TiOxNy surface modification layer with smaller band gap can facilitate the visible light absorption of TiO2 powders.

Figure 5
figure 5

Optical photos of (a) pristine TiO2 and (b) TiO2@100TiN. (c) UV-Vis diffuse reflectance spectra of pristine TiO2 and TiOxNy coated TiO2 powders. (d) The corresponding band gaps determination plots of pristine TiO2 and TiO2@100TiN.

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The photocatalytic activity of TiOxNy coated TiO2 has been investigated carefully through degrading methyl orange (MO) under visible light irradiation, as shown in Fig. 6. All the samples exhibit negligible adsorption capacity of MO, as shown in Fig. S5. Meanwhile, almost no degradation of MO is observed in the absence of catalyst, indicating that MO is stable under visible light irradiation. As shown in Fig. 6a, pristine TiO2 shows very limited photocatalytic activity of ~4.4% in 120 min under visible light irradiation due to its large band gap. However, after ultrathin TiOxNy coating with only 20 cycles PEALD TiN, a much-improved photocatalytic activity of ~57.3% is achieved. Moreover, the photocatalytic activity improves with increasing the TiN coating cycles, with the TiO2@100TiN exhibiting the highest degradation efficiency of ~96.5%. The experimental results were also fitted to the pseudo-first-order kinetics. At low initial pollutant concentration, the rate constant k was given by In(Ct/C0) = −kt. Here, k and t represent the first-order rate constant (h−1), and the irradiation time (h), respectively. C0 is the initial concentration of MO, and Ct is the concentration at reaction time of t. The corresponding plots of -In(Ct/C0) versus the irradiation time for photodegradation of MO are shown in Fig. 6b. A linear relation between -In(Ct/C0) and the irradiation time has verified that the photodegradation of MO using TiOxNy coated TiO2 catalyst follows the first-order kinetics. TiOxNy coated TiO2 exhibit the kinetic constants of 1.62 h−1 (TiO2@100 TiN), 0.82 h−1 (TiO2@50 TiN), and 0.45 h−1 (TiO2@20 TiN), which are much larger than pristine TiO2 of 0.023 h−1. Apparently, ultrathin TiOxNy coating can greatly improve the visible light photodegradation efficiency of MO due to its absorption in visible light spectrum. In order to evaluate the stability of the photocatalyst, the recycling experiments about MO photodegradation were performed with TiO2@50TiN catalyst. As shown in Fig. 6c, the photocatalytic activity of TiO2@50TiN exhibits an extremely limited decline for three times. The degradation efficiency of MO solution is nearly the same for three recycling experiment under 120 min irradiation, exhibiting wonderful recycling ability.

Figure 6
figure 6

(a) Photocatalytic degradation of MO by using TiO2 and TiOxNy coated TiO2 catalysts prepared by PEALD under visible-light irradiation, (b) the corresponding -ln(Ct/C0) vs. time curves, (c) three cycles of MO degradation for TiO2@50TiN in 120 min, (d) photocatalytic degradation of phenol by using TiO2 and TiO2@50TiN catalysts under visible-light irradiation.

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Moreover, colorless phenol was also adopted to evaluate the visible photocatalytic performance of TiO2@50TiN. As shown in Fig. 6d, it can be seen that both pure TiO2 and TiO2@50TiN exhibit negligible absorption for phenol molecule in the darkness. Pure TiO2 powder shows no photocatalytic activity towards degrading phenol molecule. There is hardly any degradation of phenol for TiO2 with 2 h visible irradiation. However, after modification with 50 cycles of TiN, the TiO2@50TiN powders exhibit visible photocatalytic activity for phenol, around 34.3% of phenol can degrade in 2 h. Therefore, it can also be demonstrated that surface modification with PEALD TiN can greatly improve the visible photocatalytic activity of TiO2.

As reported previously, visible light active photocatalytic N-doped TiO2 can be achieved by annealing TiO2 under NH3 flux at high temperature17. Thus, a control experiment using TiO2 photocatalyst treated by NH3 plasma at 360 °C was also performed, as shown in Fig. 7. It can be seen that NH3 plasma treatment can only slightly improve the photocatalytic activity of TiO2, the photocatalytic activity is much lower than the TiOxNy coated sample. It can be concluded that, in order to achieve highly visible light active N-TiO2 based photocatalyst, it is easier and more effective to coat TiOxNy thin film on TiO2 than replacing O with N under NH3 flux at high temperature.

Figure 7
figure 7

Photocatalytic degradation of MO by using NH3 plasma treated TiO2 and TiOxNy coated TiO2 catalysts under visible-light irradiation.

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The photocatalytic mechanism of TiOxNy coated TiO2 is also proposed. There are a large number of reports focusing on the photocatalytic activity mechanism of N-doped TiO2. It has been demonstrated that both N doping and Ti3+ can contribute to narrowing the band gap of TiO221, the band gap alignment and charge transfer of TiO2@TiOxNy is shown in Fig. 8. It is widely accepted that N doping can form a new substitution N 2p band above the O 2p valance band. While the Ti3+ sites exhibit the 3d orbital in the band gap, which is found to below the bottom of the conduction band21. Therefore, TiOxNy coated TiO2 exhibits a small band gap value of ~1.64 eV here, which can absorb the visible light. Upon visible light irradiation, electrons can transfer into the conduction band of TiO2 and Ti3+ sites, reducing O2 to form O2. radicals. The holes (h+) formed in the valance band and N doping sites would react with H2O to produce OH. radicals. Both radicals are responsible for the degradation of MO under visible light, as shown in Fig. 8. It should be noted that only an ultrathin TiOxNy coating here can significantly improve the visible light photocatalytic activity of commercial TiO2 powders. Therefore, maybe it’s needless to synthesize the monolithic N-doped TiO2 composites, adopting ultrathin TiOxNy coating can be an effective approach to prepare visible light active photocatalysts. In addition, surface coating or modification using ALD technology can be easily extended to other supporters, such as porous materials, nanowires, and so on.

Figure 8
figure 8

Proposed mechanism of TiOxNy coated TiO2 for the degradation of MO under visible light irradiation.

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Conclusions

In summary, a novel and facile approach to prepare ultrathin TiOxNy coated TiO2 composite by PEALD has been developed to promote the application of TiO2 photocatalyst under visible light. An ultrathin TiOxNy film can be formed perfectly on the surface of TiO2 powders using PEALD. Introducing ultrathin TiOxNy coating with smaller bandgap of ~1.64 eV can facilitate the absorption of TiO2 in visible light spectrum. As a result, this ultrathin TiOxNy coating can extraordinarily improve the photocatalytic activity of commercials TiO2 powders towards degrading both MO and phenol under visible light. TiO2@100TiN prepared by PEALD photocatalyst could nearly degrade MO completely (~96.5%) in 120 min under visible light irradiation, while pristine TiO2 shows very weak photoactivity of only 4.4%. Moreover, TiOxNy coated TiO2 photocatalyst is quite stable and reusable. Therefore, this surface modification using PEALD is an extremely promising route that could also be extended to other supporters to prepare visible light active photocatalysts. These results presented in this work could open a new window to the future design and synthesis of visible light photocatalysts.

Methods

Chemicals

In ALD process, Titanium tetrachloride (TiCl4) (5N, Suzhou Fornano Corporation Ltd.) and NH3 plasma were used as Ti precursor and Nitrogen sources, respectively. High pure N2 (5N) and Ar (5N) were used as carrier and purge gas. Commercial anatase TiO2 powders (Nanjing Haitai nano materials Co) with diameter of ~20 nm were used as supporters. Methyl Orange (MO, C14H14N3NaO3S, J&K Scientific) and phenol was prepared into 4 mg L−1 with Milli-Q water.

Preparation of TiOxNy modified TiO2 powder

TiO2 powders were loaded into a special powder container with porous mesh. The schematic diagram of coating TiO2 powders by PEALD TiN is shown in Fig. S6. TiCl4 and NH3 plasma were used as precursors for TiN deposition. Plasma power and NH3 gas flow rate were 2500 W and 160 sccm, respectively. And it is a remote plasma source. Pure N2 (5N) and Ar (5N) were used as carrier/purge gas for TiCl4 and NH3 plasma, respectively. Various cycles of TiN were deposited onto TiO2 surface at 360 °C, where one cycle consisted of 2 s TiCl4 injection, 10 s purging, 24 s NH3 plasma injection, and 6 s purging. Long dosing/purging time was applied to gain conformal coating on nano powders. In this work, the samples coated by 20, 50, 100 cycles of TiN are termed as TiO2@20TiN, TiO2@50TiN and TiO2@100TiN, respectively. As a control experiment, TiO2 powders were treated by NH3 plasma at 360 °C for 20 min, which is equal to the NH3 plasma injection time of 50 cycles of PEALD TiN.

Characterization

The chemical feature was investigated by X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha) with standard Al Kα (1486.7 eV) X-ray source. The binding energies were calibrated with respect to the signal from the adventitious carbon (binding energy = 284.6 eV). Raman spectra of TiO2 were collected by a confocal Raman microscope (LabRAM HR Evolution, Horiba) with excitation laser wavelength of 632.8 nm. An objective lens is employed to focus the excitation laser on the substrate and collect the Raman signal. The microstructure and morphology were examined by filed effect scanning electron microscopy (FESEM, Ultra55, ZEISS) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20 S-Twin, FEI). Crystallinity and phase structures of powders were analyzed by a Rigaku-D/MAX 2000X-ray diffraction (XRD) system with Cu Kα radiation. The Brunauer-Emmett-Teller (BET) surface area was estimated by a surface area apparatus (TriStar-3000, Micromeritics). UV-visible absorption spectra were recorded by a UV-vis-NIR spectrophotometer (UV-3600, Shimadzu).

Photocatalytic activity

The photocatalytic activity of as-prepared photocatalysts was evaluated via the degradation of methyl orange (MO) or phenol in aqueous solution. A solar simulator (300 W Xe lamp, MircoSolar300, PerfectLight) with a 420 nm cut-off filter provides the visible-light irradiation. The lamp was located at 15 cm away from the reaction solution. 100 mg catalyst and 100 ml of aqueous solution containing 4 mg L−1 MO or phenol were placed in a glass reactor with continuous stirring at 500 rpm. Prior to irradiation, the pollutant solutions suspended with photocatalysts were stirred in absence of light for 30 min (MO) or 2 h (phenol) to attain the equilibrium adsorption/desorption between photocatalyst powders and organic molecules. During the reaction, the temperature was maintained at 25 ± 1 °C using cooling water. For each given irradiation time, about 3 mL of the reacted solution was withdrawn and centrifuged at 10,000 rpm for 10 min to remove the photocatalyst. Then, the concentration of the centrifuged solution was determined by a UV-vis-NIR spectrophotometer, measuring the maximum absorption of MO at 464 nm and phenol at 270 nm.

Stability test of photocatalysts

In order to evaluate the stability of the photocatalysts, a recycled usage experiment was carried out. 100 mg TiO2@50TiN photocatalyst was suspended in a 100 mL of 4 mg L−1 solution of MO and irradiated under Xe lamp for 120 min. The photocatalysts were collected and washed by distilled water and ethanol, then dried in the oven at 100 °C for 12 h. Finally, the photocatalyst was reused again for the second cycle of degradation with a fresh dye solution. This process was about to repeat up to 3 times of application.