Efficient Photodegradation of Rhodamine B by Fiber-like Nitrogen-Doped TiO2/Ni(OH)2 Nanocomposite under Visible Light Irradiation

N-TiO2/Ni(OH)2 nanofiber was successfully prepared by combining the electrospinning and solvothermal method. It has been found that under visible light irradiation, the as-obtained nanofiber exhibits excellent activity for the photodegradation of rhodamine B, and the average degradation rate reaches 3.1%/min−1. Further insight investigations reveal that such a high activity was mainly due to the heterostructure-induced increase in the charge transfer rate and separation efficiency.


Introduction
Back in 2001, Asahi et al. [1,2] identified that the tactic of non-metal-doping can narrow the bandgap of TiO 2 (anatase, 3.2 eV). Both experimental results and the relevant density functional theory (DFT) theoretical calculations revealed that nitrogen doping could significantly extend the absorption range of TiO 2 into the visible region. Since then, numerous investigations focus on non-metal-doped (N, S, P, B) TiO 2 photocatalysts [3,4]. Through the non-metallic dopants and their combinations, the corresponding photocatalytic activity of derived TiO 2 could be improved, thus exhibiting outstanding photocatalytic performances in various applications, such as photocatalytic water splitting [5,6], CO 2 reduction [7,8], removal of organic compounds [9,10], sterilization [11], and anti-viral applications [12]. Owing to these excellent properties, N-doped TiO 2 and cocatalyst-supported TiO 2 have been commercialized and widely used in many practical applications [13]. However, the photocatalytic reaction rate of N-doped TiO 2 is still very low, which is believed to be caused by the following two reasons: one is its weak absorption in the visible range, while the other is the limited rate of the photocatalytic reactions by the nitrogen-induced oxygen vacancies, which is supposed to act as the carrier traps and/or recombination centers.
To enhance the photocatalytic performance of nitrogen-doped TiO 2 (N-TiO 2 ) in the visible range, tremendous efforts have been devoted to it during the past decades [14][15][16]. It has been revealed that transition metal nickel hydroxide and NiS could act as cocatalysts to significantly strengthen the photocatalytic activity of TiO 2 , which is believed to be attributed to the inhibition of charge carriers' recombination [17][18][19]. Zhang et al. [20] prepared three-dimensional flower-like TiO 2 @Ni(OH) 2 core-shell heterostructures, exhibiting a sixfold activity enhancement for hydrogen photogeneration. Meng et al. [21] synthesized a hierarchical TiO 2 /Ni(OH) 2 composite photocatalyst via decorating Ni(OH) 2 nanosheets on the electrospun TiO 2 nanofibers by a simple wet method, and the results indicate that, compared to pristine TiO 2 fibers, its activity of CO 2 photoreduction was significantly improved, which was ascribed to the increased charge separation efficiency and higher CO 2 capture capacity owing to the presence of Ni(OH) 2 . Chen et al. [22] reported the construction of a photo-supercapacitor with high specific capacitance and ultra-fast chargedischarge response, in which TiO 2 was used as a photoelectric conversion material and Ni(OH) 2 was used as an energy storage material in the TiO 2 /Ni(OH) 2 composite material, offering a simple fabrication process for efficient direct solar energy storage. These findings inspired us to develop a heterojunction between N-TiO 2 and nickel hydroxide to increase the charge separation efficiency thereby improving its corresponding photocatalytic activity.
To our knowledge, only a few studies were focused on the photocatalytic applications of N-TiO 2 /Ni(OH) 2 composite. In this work, we first prepared N-TiO 2 nanofibers by electrospinning technique followed by decoration of Ni(OH) 2 via the hydrothermal method to obtain the high-quality N-TiO 2 /Ni(OH) 2 heterostructures. The designated composite exhibits a much-enhanced photocatalytic ability for the degradation of rhodamine B (RhB) compared to N-TiO 2 under visible light irradiation. Further detailed analyses reveal the relevant enhanced activity is ascribed to the increased charge separation efficiency induced by the delicate heterostructure.

Synthesis of Ni(OH) 2 Nanosheets
N-TiO 2 nanofibers were prepared by the electrospinning method according to the previous work [23]. Typically, 5 mL of acetic acid and 5 mL of butyl titanate were added into 10 mL ethanol under vigorous stirring at room temperature (RT). After~10 min., 0.2 g of urea was introduced, followed by another 20 min stirring. Meanwhile, another solution was obtained by introducing 1.5 g of polyvinylpyrrolidone into 10 mL ethanol. Then, the two above solutions were homogeneously mixed and further heated at 70 • C for 1 h under vigorous stirring to obtain the precursor solution for electrospinning. At a static voltage of 15 kV, a capillary tube with an inner diameter of 0.5 mm at a 15 cm distance from the collector was used to prepare the electrospun film. The flow rate was set at 1 mL/h. Afterward, the fiber film was peeled off and placed in a crucible that was sealed with tin foil and kept at 550 • C for 3 h in a muffle furnace to obtain N-TiO 2 fibers. For comparison, TiO 2 nanofibers were prepared by a similar procedure and the only exception is without introducing urea.

Preparation of N-TiO 2 /Ni(OH) 2 Nanocomposite
A total of 100 mg of the as-prepared N-TiO 2 nanofibers were added into 40 mL ultrapure water followed by sonication for 10 min to obtain a homogeneous suspension. Next, 3.1 mg nickel nitrate hexahydrate and 1.7 mg hexamethylenetetramine were introduced and the corresponding solution was further stirred for 1 h at RT. Then, the solution was transferred to the Teflon-lined autoclave and heated at 120 • C for another 6 h. The obtained precipitation was alternatively washed with ethanol and water several times and subsequently dried in a vacuum oven at 80 • C for 4 h. Samples with 0.5 and 1.5 wt% Ni(OH) 2 contents were prepared by the same protocol with different amounts of nickel nitrate hexahydrate and hexamethylenetetramine introduced.

Characterization
The crystal structure of samples was investigated on an X-ray diffractometer (Rigaku D/Max 2550, Japan Rigaku Co., Ltd., Tokyo, Japan) at 40 kV and 40 mA with copper Kα radiation (λ = 0.154056 nm). The morphological and structural information was characterized via field-emission scanning electron microscopy (SEM, JSM-7610F, Tokyo, Japan) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, FEI, Valley City, ND, USA). X-ray photoelectron spectroscopy (XPS) measurements were recorded by a photoelectron spectrometer (ESCALAB MKII, VG Scientific, Waltham, MA, USA). The relevant photoluminescence results were obtained on an FLS-1000 fluorescence spectrophotometer (Edinburgh Instruments Ltd., Livingston, UK). The transient photocurrent signals were measured by an electrochemical workstation (CHI-660, CH Instruments, Austin, TX, USA). UV-Vis diffuse reflectance spectra of the samples, and the absorbance of Rh B solution was measured on a Shimadzu-2600 spectrophotometer (Shimadzu, Kyoto, Japan).

Measurement of Photocatalytic Reactions
The RhB photodegradation experiments were performed in a glass flask. Typically, 25 mg of the relevant photocatalyst was dispersed into a 100 mL Rh B aqueous solution (10 mg/L). Before the photoreaction, the solution was stirred for 30 min in dark to exclude the adsorption effect of the sample. Then, the solution was illuminated by a 300-W Xenon lamp with a filter (>420 nm). A total of 2 mL of the solution was taken at regular intervals (30 min), then subjected to UV-Vis measurements after filtrating the photocatalyst.

Results
The crystal structural information of TiO 2 , N-TiO 2 , and N-TiO 2 /Ni(OH) 2 nanofibers were obtained by X-ray diffraction (XRD) technique. As shown (Figure 1 The results demonstrate that under the current calcination condition, the obtained TiO 2 nanofibers are in the mixed phase of anatase and rutile. Interestingly, when nitrogen is introduced into TiO 2 , the rutile phase of TiO 2 seems to be significantly inhibited. Furthermore, for N-TiO 2 /Ni(OH) 2 nanocomposite, no peak belonging to Ni(OH) 2 was observed, which is considered to be owing to the low Ni(OH) 2 content [18]. The morphological and structural information of the sample was obtained by SEM and TEM measurements. Figure 2a displays the SEM result of TiO2 nanofibers. It is seen that the TiO2 was in a fibrous structure with a diameter of ~500 nm and length over 10 µm, while the introduction of nitrogen did not induce any observable morphological change (Figure 2b). Further decoration of Ni(OH)2 onto N-TiO2 nanofibers could induce the two- The morphological and structural information of the sample was obtained by SEM and TEM measurements. Figure 2a displays the SEM result of TiO 2 nanofibers. It is seen that the TiO 2 was in a fibrous structure with a diameter of~500 nm and length over 10 µm, while the introduction of nitrogen did not induce any observable morphological change (Figure 2b). Further decoration of Ni(OH) 2 onto N-TiO 2 nanofibers could induce the two-dimensional structural growth on the surface of the corresponding samples, (Figure 2c), which manifests the successful deposition of Ni(OH) 2 . TEM image of the N-TiO 2 /1.0 wt% Ni(OH) 2 nanofibers indicates the diameter of the relevant sample in~500 nm (Figure 2d), and higher magnification TEM result clearly demonstrates a 50-nm-thick layer was homogenously coated onto the outside surface of the N-TiO 2 fibers, which is considered to be Ni(OH) 2 ( Figure 2e). Furthermore, HRTEM measurement confirms our assumption. As is shown (Figure 2f), the clear granular nanostructure ascribed to (110) crystal plane of Ni(OH) 2 was found in the corresponding outside layer area of the sample, and the green circle represents Ni(OH) 2 nanocrystalline particles in Figure 2f. The morphological and structural information of the sample was obtained by SEM and TEM measurements. Figure 2a displays the SEM result of TiO2 nanofibers. It is seen that the TiO2 was in a fibrous structure with a diameter of ~500 nm and length over 10 µm, while the introduction of nitrogen did not induce any observable morphological change (Figure 2b). Further decoration of Ni(OH)2 onto N-TiO2 nanofibers could induce the twodimensional structural growth on the surface of the corresponding samples, (Figure 2c), which manifests the successful deposition of Ni(OH)2. TEM image of the N-TiO2/1.0 wt% Ni(OH)2 nanofibers indicates the diameter of the relevant sample in ~ 500 nm (Figure 2d), and higher magnification TEM result clearly demonstrates a 50-nm-thick layer was homogenously coated onto the outside surface of the N-TiO2 fibers, which is considered to be Ni(OH)2 ( Figure 2e). Furthermore, HRTEM measurement confirms our assumption. As is shown (Figure 2f), the clear granular nanostructure ascribed to (110) crystal plane of Ni(OH)2 was found in the corresponding outside layer area of the sample, and the green circle represents Ni(OH)2 nanocrystalline particles in Figure 2f.  To acquire the chemical state of each element in the samples, XPS measurements were performed. Figure 3 shows the high-resolution Ti 2p, O 1s, N 1s, and Ni 2p XPS spectra of the N-TiO 2 /1.0 wt% Ni(OH) 2 . As is shown for Ti 2p, peaks at 458.5 and 464.3 eV could be ascribed to Ti 2p 3/2 and Ti 2p 1/2 of TiO 2 , respectively, indicating the existence of Ti 4+ in the sample [24][25][26]; while for O 1s, signals of lattice oxygen (titanium-oxygen-titanium, 529.7 eV) and hydroxyl oxygen (Ti-OH and Ni-OH, 531.4 eV) are observed [27] (Figure 3b). A weak characteristic peak (N1s) at 399.9 eV attributes to the signal combination of weakly charged nitrogen species with C, H, or O atoms [28] (Figure 3c), and demonstrates the successful diffusion of nitrogen into the TiO 2 lattice at interstitial positions [29,30]. It is noted that nitrogen had a weak positive charge in the Ni 2p region (Figure 3d) due to the bonding with O atoms of TiO 2 [30][31][32]. In addition, the peaks at 855.3 and 873.0 eV correspond to Ni 2p 3/2 and Ni 2p 1/2 , respectively, indicating the existence of nickel hydroxide [33]. In addition, the two shoulder peaks were ascribed to the satellite peaks of Ni 2p 3/2 and Ni 2p 1/2 in the vicinity of 861.7 and 880.3 eV, respectively [20]. charged nitrogen species with C, H, or O atoms [28] (Figure 3c), and demonstrates the successful diffusion of nitrogen into the TiO2 lattice at interstitial positions [29,30]. It is noted that nitrogen had a weak positive charge in the Ni 2p region (Figure 3d) due to the bonding with O atoms of TiO2 [30][31][32]. In addition, the peaks at 855.3 and 873.0 eV correspond to Ni 2p3/2 and Ni 2p1/2, respectively, indicating the existence of nickel hydroxide [33]. In addition, the two shoulder peaks were ascribed to the satellite peaks of Ni 2p3/2 and Ni 2p1/2 in the vicinity of 861.7 and 880.3 eV, respectively [20]. The absorption property of different samples was measured by UV-Vis diffuse reflectance absorption spectroscopy. As indicated (Figure 4), Ni(OH)2 control sample exhibits an obvious absorption at 450 and 600-800 nm, which is due to the d-d transition of Ni [34]. In addition, compared to TiO2, the absorption of N-TiO2 and different N-TiO2/Ni(OH)2 samples in the visible region are significantly enhanced. It should be noted that, in the range of 400-550 nm, the absorption intensity of N-TiO2/Ni(OH)2 nanofibers is increased with the increase in Ni(OH)2 content. The absorption property of different samples was measured by UV-Vis diffuse reflectance absorption spectroscopy. As indicated (Figure 4), Ni(OH) 2 control sample exhibits an obvious absorption at 450 and 600-800 nm, which is due to the d-d transition of Ni [34]. In addition, compared to TiO 2 , the absorption of N-TiO 2 and different N-TiO 2 /Ni(OH) 2 samples in the visible region are significantly enhanced. It should be noted that, in the range of 400-550 nm, the absorption intensity of N-TiO 2 /Ni(OH) 2 nanofibers is increased with the increase in Ni(OH) 2 content.  The photocatalytic activity on the degradation of RhB of different samples was then evaluated. As shown (Figure 5a), all the employed samples exhibit the similar physical adsorption property for RhB. However, interestingly, once illuminated, all the N-TiO2/Ni(OH)2 nanocomposites show a much higher ability for the photodegradation of RhB compared to that of a relevant single component. Especially, N-TiO2/Ni(OH)2 nanocomposite possesses the best performance, and ~97% of RhB would be photodegraded in 90 min. To illustrate the decomposition rate more clearly, the Langmuir-Hinshelwood kinetics equation was employed [35]. As shown in Figure 5b, the activities of all the nanocomposites were higher than those of a single component, indicating an apparent synergistic effect between N-TiO2 and Ni(OH)2, which could also be reflected by the rate constant (Figure 5c). Our results unambiguously manifest the vital role of forming the heter- The photocatalytic activity on the degradation of RhB of different samples was then evaluated. As shown (Figure 5a), all the employed samples exhibit the similar physical adsorption property for RhB. However, interestingly, once illuminated, all the N-TiO 2 /Ni(OH) 2 nanocomposites show a much higher ability for the photodegradation of RhB compared to that of a relevant single component. Especially, N-TiO 2 /Ni(OH) 2 nanocomposite possesses the best performance, and~97% of RhB would be photodegraded in 90 min. To illustrate the decomposition rate more clearly, the Langmuir-Hinshelwood kinetics equation was employed [35]. As shown in Figure 5b, the activities of all the nanocomposites were higher than those of a single component, indicating an apparent synergistic effect between N-TiO 2 and Ni(OH) 2 , which could also be reflected by the rate constant (Figure 5c). Our results unambiguously manifest the vital role of forming the heterostructure for significantly enhancing the RhB photodegradation ability. Further optimization of the addition of Ni(OH) 2 in the composite showed that when the amount of Ni(OH) 2 in the composite is 1.0-wt%, the best performance of 0.0310 min −1 could be achieved (Figure 5b), which was comparable to the recent benchmark test results (Table 1). The stability of photocatalyst is a critical issue for the potential real applic Hence, the relevant stability of N-TiO2/1.0-wt%Ni(OH)2 was evaluated (Figure 5d). each round of photocatalytic reaction, the sample was collected via centrifugation washed with ultrapure water several times before further usage. As shown, only a activity decrease (~3.7%) was observed after five cycles demonstrating its excellent s ity for long-term use.
To elucidate the underlying mechanism of the heterostructure-induced photo lytic activity enhancement, photoluminescence (PL) measurements were first empl As shown in Figure 6a, N-TiO2 gives an obvious PL emission in the range of 400-55 while for that of Ni(OH)2, hardly any PL signal could be observed. Interestingly, fo nanocomposite sample, the relevant PL intensity is gradually decreased with the gr increase in Ni(OH)2 content, which is considered to be ascribed to charge transfer fro TiO2 to Ni(OH)2 [35]. To further verify the separation and migration behavior of c carriers, photocurrent response tests were performed. As shown in Figure 6b, all the posite samples give higher photocurrent responses than that of the N-TiO2 or Ni(  The stability of photocatalyst is a critical issue for the potential real application. Hence, the relevant stability of N-TiO 2 /1.0-wt% Ni(OH) 2 was evaluated (Figure 5d). After each round of photocatalytic reaction, the sample was collected via centrifugation and washed with ultrapure water several times before further usage. As shown, only a slight activity decrease (~3.7%) was observed after five cycles demonstrating its excellent stability for long-term use.
To elucidate the underlying mechanism of the heterostructure-induced photocatalytic activity enhancement, photoluminescence (PL) measurements were first employed. As shown in Figure 6a, N-TiO 2 gives an obvious PL emission in the range of 400-550 nm, while for that of Ni(OH) 2 , hardly any PL signal could be observed. Interestingly, for the nanocomposite sample, the relevant PL intensity is gradually decreased with the gradual increase in Ni(OH) 2 content, which is considered to be ascribed to charge transfer from N-TiO 2 to Ni(OH) 2 [35]. To further verify the separation and migration behavior of charge carriers, photocurrent response tests were performed. As shown in Figure 6b, all the composite samples give higher photocurrent responses than that of the N-TiO 2 or Ni(OH) 2 , and N-TiO 2 /1.0 wt% Ni(OH) 2 exhibits the highest response, which is consistent with the trend of their photocatalytic performance. This strongly signifies the introduction of Ni(OH) 2 is beneficial for increasing charge separation efficiency, and thus enhances the relevant photocatalytic activity. Based on the above results, the plausible underlying mechanism for the enhanced photocatalytic activity of N-TiO2/Ni(OH)2 composite has been proposed (Figure 7). After the absorption of the incident light, both N-TiO2 and Ni(OH)2 were excited. Thereafter, the electrons and holes are photogenerated in each component, and electrons are excited into the conduction band (CB) while the holes are left at the valence band (VB). Owing to the potential of Ni 2+ /Ni (0.23 eV) being slightly lower than that of anatase TiO2 (~0.26 eV), photogenerated electrons will transfer from CB of TiO2 into CB of Ni(OH)2, while holes remain in VB of TiO2. This will significantly promote charge separation and inhibit the recombination of photogenerated electrons and holes. Subsequently, electrons could react with adsorbed oxygen to generate superoxide free radicals. It is noted the formation of free radicals can not only effectively inhibit electron-hole recombination, but also destroy the bonds of RhB and therefore degrade them, which finally exhibits enhanced photocatalytic activity. Based on the above results, the plausible underlying mechanism for the enhanced photocatalytic activity of N-TiO 2 /Ni(OH) 2 composite has been proposed (Figure 7). After the absorption of the incident light, both N-TiO 2 and Ni(OH) 2 were excited. Thereafter, the electrons and holes are photogenerated in each component, and electrons are excited into the conduction band (CB) while the holes are left at the valence band (VB). Owing to the potential of Ni 2+ /Ni (0.23 eV) being slightly lower than that of anatase TiO 2 (~0.26 eV), photogenerated electrons will transfer from CB of TiO 2 into CB of Ni(OH) 2 , while holes remain in VB of TiO 2 . This will significantly promote charge separation and inhibit the recombination of photogenerated electrons and holes. Subsequently, electrons could react with adsorbed oxygen to generate superoxide free radicals. It is noted the formation of free radicals can not only effectively inhibit electron-hole recombination, but also destroy the bonds of RhB and therefore degrade them, which finally exhibits enhanced photocatalytic activity. photogenerated electrons will transfer from CB of TiO2 into CB of Ni(OH)2, while holes remain in VB of TiO2. This will significantly promote charge separation and inhibit the recombination of photogenerated electrons and holes. Subsequently, electrons could react with adsorbed oxygen to generate superoxide free radicals. It is noted the formation of free radicals can not only effectively inhibit electron-hole recombination, but also destroy the bonds of RhB and therefore degrade them, which finally exhibits enhanced photocatalytic activity.

Conclusions
In this work, the heterostructured N-TiO2/Ni(OH)2 nanofibers were successfully constructed and further applied for the enhanced photodegradation of RhB compared to that of N-TiO2 under visible light irradiation (>420 nm). Our results indicate that the elaborated design could significantly enhance the corresponding photocatalytic activity and the one

Conclusions
In this work, the heterostructured N-TiO 2 /Ni(OH) 2 nanofibers were successfully constructed and further applied for the enhanced photodegradation of RhB compared to that of N-TiO 2 under visible light irradiation (>420 nm). Our results indicate that the elaborated design could significantly enhance the corresponding photocatalytic activity and the one with 1.0 wt% Ni(OH) 2 exhibits the best performance. Further detailed investigations revealed that the enhanced activity is mainly ascribed to the efficient charge transfer and separation in N-TiO 2 induced by the integration of Ni(OH) 2 . It is anticipated that our tactics and results would be beneficial for the future design of high-performance photocatalysts.