Keywords

Substitutional N; Band-to-band excitation; Oxygen vacancies; Active O₂•^- radicals

Introduction

Waste water treatment being the active focus for ongoing research areas, have emerged several techniques to cope with the problem. As one of the most attracted field, photocatalysis fascinated a range of scientific and technological interest for using in water purification and splitting fields. It provides easy and reckless removal of water contaminations using only solar energy in the presence of an active material. A lot of work being done on these active materials to manipulate the band gaps, so that the material can be effectively utilized in the presence of sunlight, an abundant energy source. Most of the time, TiO₂ is considered as active material for photocatalysis owing to certain features and desirable properties (environmental friendly, facile synthesis, and high chemical stability), many research have focused on different structures of TiO₂ [1-2]. However, actual applications are restricted due to: (i) The relatively large band gap of anatase TiO₂ (around 3.2 eV) only requires UV light; (ii) Low separate rate of the photoproduced electron-hole pairs [3]. Hence, many scientists took certain modifications in TiO₂ structures, for example doped with various transition-metal cations, nonmetals and coupled with a sensitizing dye or short band-gap semiconductor for improving the visible light performance [4-6]. In addition, many new visible light driven photocatalysts have been exploited.

Besides typical and modified semiconductor metal oxides, layered titanates (KTiNbO₅) have now been considered as new type of photocatalysts [7-9]. These inorganic layered compounds possess unique properties due to electrostatic interactions between laminates and interlayer ions that are comparable with TiO₂ (anatase) crystals in few reverences. Firstly, special interlayer galleries amongst sheets which contained TiO₆ octahedra structure (with four edges) [10], secondly they have the ability of ion-exchange by H+ ions and obtained original layered structure of interlayers titanium niobate (for HTiNbO₅). And the third important characteristic is, an excellent pathway facilitated the nitriding treatment which happens due to the layered galleries and weak binding energy of hydrogen bond [11]. However, HTiNbO₅ usually a wide bandgap semiconductor photocatalyst and can only be activated under UV light. UV light accounts for 5% of the solar energy spectrum as compared to visible light that occupies 46% of the whole electromagnetic spectrum[12], it is of great worth to synthesize a visible light-responsive HTiNbO₅ photocatalyst.

Generally, N-doping not only results into intermediary energy levels in the bandgap as consequences of either mingling N 2p with O 2p states or by introduction of localized states [13], but can also act as trapping centers for photogenerated charge carriers thereby effectively reduce their recombination [14-15]. The oxygen vacancies produced in the process of nitriding under ammonia gas would promote the absorption of visible light to a large extent. In the present work, subsequent calcinationsion- exchange-nitrogen doping method has been used to synthesize layered N doped titanioniobate with 6.15 atom % N content successfully. The samples were carefully characterized and enhanced performance was observed in photocatalytic degradation of Rhodamine B under visible light irradiation. Further, brief analysis of active radicals in terms of their oxidative potentials has also been discussed.

Experimental Method

Materials

All chemicals were analytical reagents and used without further purification. The anatase TiO₂ (99%, aladdin), Niobium (V) oxide (Nb₂O₅, Sinopharm, China), Potassium carbonate (99%, K₂CO₃, Sinopharm, China), absolute ethanol and deionized water were supplied by the Sigma-Aldrich, China. Nitric acid, Rhodamine B (AR grade, Sinopharm, China) was used as model organic pollutants for assessing the activity of synthesized materials. In all experiments, doubly distilled water was used.

Synthesis of layered KTiNbO₅ and protonated form HTiNbO₅

Layered KTiNbO₅ was synthesized by thermally treating a mixture of K₂CO₃, TiO₂ and Nb₂O₅ in a stoichiometric molar ratio of 1: 2: 1 at 650°C for 1 h and then at 1200°C for 24 h. The protonated form (HTiNbO₅) was obtained by curing the KTiNbO₅ with 5 M HNO3 aqueous solution at room temperature for 3 days. During the proton exchange reaction, the acidic solution was replaced with a fresh one every 24 h.

Synthesis of KTiNbO_5-xN_x and HTiNbO_5-xN_x

The resulting layered KTiNbO₅ sample was subjected to calcination under ammonia gas flow at 500°C, 600°C and 700°C at 4°C/min ramp respectively for 2 h. The horizontal tube furnace with tight end-caps was used for nitriding treatment of the samples. Initially Argon gas flows for 15 min to ensure all the air is removed from quartz tube, then ammonia gas flow started subsequently raising the temperature to desired value. The product was cooled down to room temperature and Argon gas flows again to remove ammonia remaining in the tube. The HTiNbO₅ sample were treated at 250°C, 350°C and 450°C in ammonia gas for 30 min, respectively. In this paper, KTiNbO_5-xN_x (500°C, 2 h) stands for KTiNbO₅ sample and nitriding treatment at 500°C for 2 h. Other samples are represented as the same.

Characterization

Crystal phase analysis were made by X-ray diffraction (XRD) using a Miniflex 600 X-ray diffractometer with monochromatic Cu Kα radiation (λ=0.1542 nm, accelerating voltage 40 kV, applied current 15 mA) at scanning speed of 1°/min. Morphological analysis and compositions were confirmed using scanning electron microscopy (SEM) instrument (JSM-7800F, Japan). Hitachi U-3900 spectrophotometer was used to get UV-Vis Diffuse Reflectance Spectra in the range of 200–850 nm, using BaSO4 standard as the reference. XPS measurements were carried out on an X-ray photoelectron spectrometer (ESCALAB250Xi) using Al Kα (1486.6 eV) X-rays as the excitation source. C 1s (284.5 eV) was chosen as the reference. Horiba FL3-111 is used to analyze photoluminescence spectra of all the samples.

Photocatalytic experimental procedure: degradation of Rhodamine B (RhB)

For the photocatalytic degradation reaction, an open jacketed glass vessel has been used keeping the temperature constant at atmospheric conditions. 15 ppm solution (200 ml) of RhB containing 120 mg of photocatalyst was used as initial conditions. Before light irradiation, the suspension was equilibrated for 60 min in dark after which irradiated under continuous stirring using a visible light (300 W Microsolar 300 UV-Xe lamp with a UV-cutoff filter (420 nm) and was positioned 15 cm far from the reactor (removing the effect of light on photocatalysis). Sampling was performed every 10 min for evaluation of reaction kinetics with a UV-Vis absorption (Hitachi U-3900 spectrophotometer) at the characteristic absorption peak of 554 nm (λ_RhB) for RhB.

Results and Discussion

Structural and morphological analysis

Titanate samples (KTiNbO₅ and HTiNbO₅) usually have a layered structure. Figure 1 shows the XRD patterns of as-prepared KTiNbO₅ samples before and after treatment. It can be seen that all patterns of KTiNbO₅ matches well with the standard data (JCPDS No. 54-1155) and exhibited good crystallinity [10]. Supplementary Figure S1(a) represents a lattice structure of KTiNbO₅, as it is composed of corrugated sheets structure oxides with MO₆ octahedra (M=Ti, Nb). These kinds of structure have a low layer charge density and extraordinary ion exchange capacity [16]. After proton-exchange (HTiNbO₅), the sample can be well indexed according to standard data (JCPDS No. 54-1157 in Figure 1). Conferring to the position of the characteristic ((002) surface, 2θ=10.56°) reflection of the layered structure (Supplementary Figure S1(b)), the interlayer distance of HTiNbO₅ is calculated to be 8.57 Å. Further, the diffraction peak intensity at low angle (2θ ≈ 10.56°) was observed to decrease for HTiNbO_5-xN_x (350°C, 30 min) sample after nitriding by ammonia gas with some obvious peak shift towards large angle. For N-doped KTiNbO₅ samples (Supplementary Figure S2) synthesized at 500, 600 and 700°C, respectively, there is no significant change in peaks thus ensures that the layered structure is reserved but the intensity of the peaks was observed to reduce. It is mainly due to the defect induced by nitriding treatment. XRD patterns of N-doped HTiNbO₅ samples with different nitriding conditions are shown in Supplementary Figure S3. The lower nitriding temperatures have caused obvious change of peak pattern. With the increase of nitriding temperature, some peaks observed to shift for higher diffraction angles even reduced or disappeared. This is caused by the decrease of interplanar spacing and some defect that are induced by nitrogen doping. Totally, an obvious shift of peaks for N doped HTiNbO₅ sample at low nitriding temperatures can be seen clearly, but almost no change was observed for N doped KTiNbO₅ in high nitriding temperature except the intensity of peaks (Supplementary Figures S2 and S3).

Figure 1: XRD patterns of KTiNbO₅, HTiNbO₅ and HTiNbO₅ after nitrogen doping at 350°C samples.

As shown in Figure 2a, the layered metal oxides (KTiNbO₅) were made up of tabular particles with the size of 2 -5 μm. Figure 2b shows the topography of protonated HTiNbO₅ sample. It is observed that curing does not affect the lamellar structure of the virgin HTiNbO₅, as plates are stacked together in ordered form yielding a sheet like structure in micrometer range. After nitriding treatment at 350°C for 30 min under ammonia gas, the resulting sample showed a similar morphology to HTiNbO₅. However, it should be noted that layered HTiNbO_5-xN_x (350°C, 2h) (Figures 2c and 2d) consist of smaller particles and some lamellar character was not obvious, which could be ascribed to the nitriding conditions employed in the preparation of the products. Morphological analysis is well in accordance with structural analysis of the samples obtained from XRD.

Figure 2: Typical FESEM of (a) KTiNbO₅, (b) HTiNbO₅, (c and d) of HTiNbO₅ after nitrogen doping at 350°C.

XPS analysis and diffused reflectance spectra

In order to confirm the effect of nitrogen doping on the electronic states of the elements present, XPS binding energies have been observed for all the prepared samples. Figure 3 represents the elemental survey spectrum of Ti 2p, O 1s, Nb 3d, K 2p and N 1s for KTiNbO₅, KTiNbO_5-XN_X (500°C, 2 h), HTiNbO₅ and HTiNbO_5-XN_X (350°C, 30 min) samples. Generally, the peak at around 530 eV is assigned to the Ti-O bonds [17] and the peak around 458 eV (Ti 2P_3/2 levels) corresponds to Ti4+ [18]. In KTiNbO₅ and KTiNbO₅- XNX (500°C, 2 h) samples, the binding energies of Ti 2p_3/2, O 1s and Nb 3d_5/2 are shifted from 458.38, 530.15 and 206.76 eV to 458.26, 530.12 and 206.71 eV, respectively. The amplified surface electron densities of Ti, O and Nb by relieving lattice O atoms with N atoms (smaller electronegativity) have caused the phenomenon. However, no binding energy shift was observed for K 2p after nitrogen doping. Compared with the peaks of HTiNbO₅ sample, the binding energy shift of Ti, O and Nb in protonated HTiNbO₅ after nitrogen doping is only 0.03-0.08 eV. This small shift of the binding energy is mainly due to that the substitutional N have declining effect on the outer electron densities of Ti, O and Nb when associated with H which has strong electronegativity. Another noteworthy characteristic is the clear increase in Ti 2p, O 1s and Nb 3d peak intensity after nitrogen doping in HTiNbO_5-XN_X (350°C, 30 min) that can be ascribed probably to the establishment of the more Ti-N bonds [19]. Generally, the peak around 397 eV represents the existence of N-Ti-O bonds (substitution of N ion for O ion). The XPS peak at 400 eV can be allocated to interstitial N [20-21]. Therefore, peaks at the binding energies of 395.07, 398.68 eV and 396.35, 398.73 eV for KTiNbO_5-XN_X (500°C, 2 h) and HTiNbO_5-XN_X (350°C, 30 min) samples, respectively apportioned to substitutional N legitimately. As for the state of nitrogen species at 401.2 eV in HTiNbO_5-XN_X (350°C, 30 min), we tend to believe that it is originated from adsorbed nitrogen species or interstitial N. The total amount of N in HTiNbO_5-XN_X (350°C, 30 min) sample is 6.15 atom %, where substitution N occupies the major percentage of total. Although the nitriding temperature is high (500°C), the amount of doped N for KTiNbO_5-XN_X (500°C, 2 h) is only 1.87 atom %. As we have explored previously that, the substitutional N plays a key role in mixing N 2p with O 2p states and the band gap narrowing.

Figure 3: XPS spectra for (a) Ti(2p), (b) O(1s), (c) Nb(3d), (d) K(2p) and (e) N(1s) signals of (A) KTiNbO₅, (B) KTiNbO₅- XNX(500°C, 2 h), (C) HTiNbO₅ and (D) HTiNbO_5-XN_X (350°C, 30 min).

The above results are well in accordance with UV-Vis DRS of the samples. Before proton-exchange, KTiNbO₅ samples were treated by ammonia gas at 500°C, 600°C and 700°C for 2 h, respectively. In contrast to the pristine layered KTiNbO₅, the absorbance threshold of KTiNbO_5-xN_x (500°C, 600°C and 700°C, 2 h) samples have almost no change (Supplementary Figure S4). With an increase in the nitriding temperature, higher absorption intensity was perceived which is mainly due to the oxygen vacancies produced for maintaining the electroneutrality [22]. As shown in Supplementary Figure S5, the UV-Vis DRS for both KTiNbO₅ and HTiNbO₅ are almost same indicating that proton-exchange have no impact on the absorbance property of the materials. But in contrast to the primitive layered HTiNbO₅, the absorbance thresholds of HTiNbO_5-xN_x (250°C, 350°C and 450°C, 30 min) samples displayed an extraordinary band-to-band red shift from 380 nm to 420, 460 and 480 nm, respectively (Figure 4).

Figure 4: UV-Vis DRS of HTiNbO₅ and HTiNbO_5-xN_x samples.

In addition, the absorption intensity increases with the increase in nitriding temperature. This behavior is much diverse from the small absorption edge shift in KTiNbO_5-xN_x samples, as shown in Supplementary Figure S4. It is important to note that the bandto- band absorption characteristic of the samples is not relevant to the temperature of nitriding treatment and the layered structure. As shown in Supplementary Figure S1, both KTiNbO₅ and HTiNbO₅ possess similar crystal structure (orthorhombic), belongs to space group pnma but the bond energy of K-O is stronger than hydrogen, therefore at the same nitriding treatment temperature, KTiNbO₅ sample is hard for N doping.

As we known, the redox power of carriers plays a key role in determining the photocatalytic ability of samples, the energy levels of Valence Band Maximum (VBM) and conduction band minimum (CBM) were necessary to be confirmed. The XPS valence band spectra of KTiNbO₅ and HTiNbO₅ were measured (Figures 5a and 5b). And it can be seen clearly the different effect of nitrogen doping on the CBM and VBM between KTiNbO₅ and HTiNbO₅. Compared with the HTiNbO₅, an obviously upward shift of VBM of HTiNbO_5-xN_x (350°C, 30 min). But the upward shift was not obvious for KTiNbO_5-xN_x (500°C, 2 h). After nitrogen doping, the valence band maximum of KTiNbO₅ and HTiNbO₅ samples are shifted from 2.2 and 2 eV to 2.26 and 0 eV, respectively. As reported by Hou et al. [15], when test in the condition of normal hydrogen electrode (NHE) at pH 7, VBM of HTiNbO₅ located at +2.41 eV. In addition, for •OH/OH^- and O₂/O₂•^-, the redox potentials maintained at +1.99 and -0.33 eV [23,24]. Compared with the VBM value of HTiNbO₅ (+2.41 eV), the electronic potentials for the other samples can be determined (Figure 5c). The potential of holes in HTiNbO_5-xN_x(350°C, 30 min) are higher than the potential of •OH/OH^- that caused by nitrogen doped HTiNbO₅. So it cannot oxidize OH^- into •OH. And for KTiNbO₅, KTiNbO_5-xN_x (500°C, 2 h) and HTiNbO₅ samples, the oxidation ability is not strong enough. Moreover, the level of CBM of all samples is negative enough to allow the single-electron reduction reaction of oxygen (O₂+ e=•O₂^- (aq), -0.33 V vs. NHE). But the bandgap of KTiNbO₅, KTiNbO_5-xN_x (500°C, 2 h) and HTiNbO₅ are too wide to precede the photocatalysis reaction. Thus, with proper bandgap and more negative level of CBM than the redox potentials of single-electron reduction of oxygen, HTiNbO_5-xN_x (350°C, 30 min) is more suitable for using in photocatalytic reaction than other three.

Figure 5: XPS valence band spectra of (a) KTiNbO₅ and KTiNbO_5-xN_x (500°C, 2 h), (b) HTiNbO₅ and HTiNbO_5-xN_x (350°C, 30 min) samples, (c) Band alignments of samples KTiNbO₅-a, KTiNbO_5-xN_x (500°C, 2 h)-b, HTiNbO₅-c, HTiNbO_5-xN_x (350°C, 30 min)-d.

Photocatalytic analysis and photoluminescence spectra

Figure 6a shows the percentage degradation of RhB under visible light irradiation in the presence of prepared titanate samples. The photocatalyst and dye was equilibrated for 60 min prior to light irradiation. No change was observed in maximum absorbance (554 nm) in the absence of light. The percentage degradation rate of RhB was calculated using (PDE=(C₀-C_t)/C₀). KTiNbO_5-xN_x (500°C, 2 h) sample shows no photocatalytic activity as can be seen in the Figure 6a. Owing to large band gap (3.65 eV), KTiNbO_5-xN_x (500°C, 2 h) cannot be activated by visible light. The small degradation can be accorded to either dye adsorbed on the surface of the catalyst or self-sensitization. After proton-exchange and nitriding treatment, the photocatalytic degradation efficiency of RhB for HTiNbO_5-xN_x (350°C, 30 min) was found to be 73 %. The enhanced visible light photocatalytic response of HTiNbO_5-xN_x (350°C, 30 min) can be clearly attributed to its certainly N doping ability, band to band visible light excitation, more •O₂^- radical and proper amount of oxygen vacancies. Figure 6b shows representative timely reduction in the maximum absorbance of RhB (λ_max=554 nm) under visible light irradiation for the HTiNbO_5-xN_x (350°C, 30 min) sample. The characteristic absorption of RhB decreases obviously as the irradiation time increases, the hypsochromic shift of the absorption maximum was obvious, indicating that N-deethylation and a dominant cleavage of the whole conjugated chromophore structure produced at the same time. Although HTiNbO_5-xN_x (450°C, 30 min) sample absorb more visible light, but the photocatalytic performance of this sample is much lower than HTiNbO_5-xN_x (350°C, 30 min), this can be ascribed possibly to the higher density of oxygen vacancies which served as recombination centers leading to a decrease in the photocatalytic activity. Photoluminescence (PL) spectra of undoped and nitrogen doped KTiNbO₅ and HTiNbO₅ samples with excitations at 300 nm were recorded and presented in Figure 7. The broad emission in the spectral range from 320 to 450 nm was observed, which is known as charge transfer emission band. While we observed low energy band range from 450 to 500 nm, these bands are attributed to oxygen vacancies or defects sites. Therefore, emissions likely originated from surface defects, such as ionizable oxygen vacancies and the recombination of self-trapped excitons (STEs) of metal oxide semiconductors which are localized within metal oxide lattice sites [25-26].

Figure 6: (a) Time-dependent photocatalytic degradation of RhB solution upon exposure to visible light using the obtained KTiNbO₅, HTiNbO₅, HTiNbO_5-xN_x (350°C, 30 min) and HTiNbO_5-xN_x (450°C, 30 min) samples, Representative variations of the characteristic absorption of RhB (b) under visible light irradiation by using HTiNbO_{5- x}N_x (350°C, 30 min) (-60 to 0 as absorb time, 0-120 as photocatalysis time).

Figure 7: Photoluminescence spectra of KTiNbO₅, KTiNbO_5-xN_x (500°C, 2 h), HTiNbO₅, HTiNbO_5-xN_x (350°C, 30 min) and HTiNbO_5-xN_x (450°C, 30 min) samples at room temperature.

Moreover the PL intensities resulting from efficient charge separation was altered for KTiNbO₅ and HTiNbO₅ doped with nonmetal. The lower PL intensities were also reported to exhibit higher photocatalytic activity. From our observation PL intensity was decreased for N doped KTiNbO₅ and HTiNbO₅ samples than pristine KTiNbO₅ and HTiNbO₅, therefore we can consider that, the N doped KTiNbO₅ and HTiNbO₅ samples presumes good photocatalytic activity than pristine KTiNbO₅ and HTiNbO₅. Since HTiNbO_5-xN_x (450°C, 30 min) sample had low PL intensity but controversial photocatalytic behavior was observed. It indicates that large number of oxygen vacancies or defects couldn't influence the photocatalytic activity. HTiNbO_5-xN_x (350°C, 30 min) sample had small PL intensity, it means reasonable oxygen vacancies or surface defects and showed efficient photocatalytic activity among all other samples.

Conclusion

Ion-exchange and nitrogen doping was realized in layered titanates (KTiNbO₅), which leads to extraordinary band-to-band visible light excitation and high visible light photocatalytic activity. This efficient doping strategy distinctly decreases the bandgap of HTiNbO₅ from 3.58 to 2.56 eV. The nitrogen doped form and band alignments as evidenced by photoelectron spectroscopy, confirmed to be responsible for the upshift of valence band maximum of HTiNbO_5-xN_x (350°C, 30 min). The much stronger reductive potential of generated electron in HTiNbO_5-xN_x (350°C, 30 min) upon visible light excitation around 2.08 eV than O₂/ O₂•^-, narrow bandgap and oxygen vacancies induced by nitrogen doping contributes to its high visible-light photocatalytic activity in decomposing organic molecules. At low temperature nitriding treatment by ammonia gas, the N doped content in HTiNbO₅ sample reach to 6.15%. It is mainly due to the weak binding energy of hydrogen bond. This finding is of significant importance not only because a highly active visible-light photocatalyst is developed using a simple doping route but it advances the understanding of the more effective nitrogen doping in layered structure photocatalysts after ion-exchange by H+ ions.

Acknowledgements

This work is supported by National Natural Science Foundation of China through grant 21471147 and Liaoning Provincial Natural Science Foundation through grant 2014020087. Yang M, would like to thank for the National "Thousand Youth Talents" program of China.

Conflict of Interest

This manuscript has been read and approved by all the authors. The criteria for authorship have been met. The authors also do not have any financial interest or any other conflict of interests. Lastly, this paper or any part of this study has not been submitted to any publication yet.

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