Ni-doped rutile TiO₂ nanoflowers: low-temperature solution synthesis and enhanced photocatalytic efficiency

Abstract

Easily recoverable photocatalysts with high activities are desirable in photocatalytic wastewater treatment. In this paper, we reported a low-temperature solution synthesis of Ni-doped flower-like rutile TiO₂, which possessed an activity four times that of the commercial Degussa P25 TiO₂ nanoparticles when utilized to assist photodegradation of rhodamine B in water under the illumination of a Xe lamp. More importantly, the micrometer-sized aggregations facilitate the subsequent recovery from the slurry. Hydrogen titanate nanowires were firstly achieved by interactions between metallic Ti and a H₂O₂ aqueous solution at 80 °C. A subsequent immersing at 80 °C of the titanate nanowires in a H₂SO₄ aqueous solution containing NiSO₄ transformed the nanowires to rutile TiO₂ nanoflowers, which were assembled by single-crystalline nanorods. The transformation proceeded through dissolution–precipitation in the acidic environment, which in sequence led to the growth of rutile nanorods that assemble the nanoflowers, via an oriented attachment mechanism. When utilized to assist photodegradation of rhodamine B in water under Xe lamp illumination, the Ni-free rutile TiO₂ nanoflowers exhibited an activity double that of P25. The appropriate doping of Ni further improved the efficiency to four times that of P25. The enhanced photocatalytic activity can be attributed to both the high specific surface area of ca. 118 m² g⁻¹, and the appropriate Ni-doping that favors both the light harvesting and the charge separation.

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Introduction

Among the various techniques emerging to deal with the environmental problems encountered nowadays, photocatalysis stands out as the most promising one.¹ Increasing attention has been paid to photocatalysts with high performances.² Titanium oxide (TiO₂), which also finds wide applications in dye-sensitized solar cells,³ Li-ion batteries,⁴ sensors,⁵ and so forth, has been extensively investigated as a stable photocatalyst with excellent photocatalytic activity.⁶ For wastewater treatments, the utilization of TiO₂ thin films avoids the subsequent recovery procedure; yet the efficiency is much lower when compared with TiO₂ powders. Commercially available Degussa P25 TiO₂ powders exhibit a small particle size of averagely 30 nm, which well dispersed in slurry to assure a striking photocatalytic performance; unfortunately, it is a nuisance to recover nanoparticles from water. It remains a great challenge to synthesize TiO₂ powders that can be easily recycled yet still maintain a high photocatalytic activity.

TiO₂ powders with distinct morphologies of nanowires,⁷ nanorods,⁸ nanobelts,^9a,b nanosheets,^9c and nanoflowers,¹⁰ together with various doping^9b,11 and compositing tactics,¹² have been adopted to achieve a high photocatalytic activity. Many literatures report modified TiO₂ powders with activities superior to that of P25 under visible light illumination,¹³ which are relatively easy to achieve because unmodified TiO₂ like P25 is a wide band gap semiconductor adsorbing only UV light. Recently, several micrometer-sized aggregates of TiO₂ composited with graphene,¹⁴ g-C₃N₄,¹⁵ ZnO^12a have been reported to exhibit efficiencies superior to that of P25 under UV or UV + Vis light illuminations.

TiO₂ powders are synthesized by either the direct synthesis¹⁶ or the precursor-transformed route.^17,18 Two steps are involved in the precursor-transformed synthesis, that is, the synthesis of precursors and their subsequent transformations to crystallized TiO₂. Precursors like titanates are often fabricated through hydrothermal routes.^18,19 The techniques utilized to transfer precursors to TiO₂ include but not limited to calcinations^20,21 and hydrothermal approaches.^22,23 The high temperature involved in the calcination process leads to grain growth and in turn a reduction in specific surface area; whilst hydrothermal approaches demand high pressure apparatus, which bring along additional safety concerns.

Except for the calcination and hydrothermal techniques above-mentioned, some effective tactics have also been reported for the titanate (precursors) transformations under a mild environment. Titanate nanowires (H₂Ti₆O₁₃) were transferred to crystallized TiO₂ of dumbbell-shaped rutile, rod-like rutile, and quasi-octahedral anatase by acid-treating at 60–70 °C for 7 days in 2 M HCl, 2 M HNO₃ and 1 M H₂SO₄.²⁴ Our previous study also revealed that, thin films of nanowires achieved by a Ti–H₂O₂ interaction transformed to crystallized TiO₂ arrays of nanorods and nanoflowers, respectively, when immersed at 80 °C in HCl²⁵ and H₂SO₄ (ref. 26) aqueous solutions. The HCl treatment induced the conversion of titanate nanowires to rutile nanorods, which exhibited high dye-adsorption capacities.²⁷

TiO₂ can be activated only under UV light due to its wide band gap of 3.0–3.2 eV, greatly limiting its efficiency under natural sun light. Doping TiO₂ with metal elements such as Ni is found to be effective to extend light absorption in TiO₂ to the visible region and improve the charge separation efficiency, which in turn enhanced the photocatalytic performance.^28–31 Ni-doped TiO₂ can be achieved by co-precipitation,³² dip-coating,³³ anodization,³⁴ and hydrothermal synthesis.³⁵

In this work, we reported a low-temperature solution approach to Ni-doped flower-like rutile TiO₂ through simply immersing hydrogen titanate nanowires in an aqueous H₂SO₄ solution containing NiSO₄. The powders possessed a high specific surface area of ca. 117.6 m² g⁻¹ and exhibited an efficiency four times that of P25 nanoparticles when utilized to assist photodegradation of rhodamine B in water under the UV + Vis light illumination.

Experimental section

Synthetic procedures

The titanate nanowires (H₂Ti₅O₁₁) synthesis can be found in our previous work.²⁷ Typically, in a beaker, 90 mg of metallic Ti powders (200 mesh) were added to 300 mL 30 wt% H₂O₂ aqueous solution which contained HNO₃ (0.29 M) and melamine (2.4 mM). Bath sonication was carried out for 10 min. Subsequently, the beaker was sealed with a polyethylene film and placed in an oven maintained at 80 °C for 48 h. The precipitates were collected by centrifugation, washed in sequence with deionized water and ethanol for three times, and dried in air at 80 °C overnight. The collected titanate nanowires (62.5 mg) were then dispersed in 250 mL 0.05 M H₂SO₄ aqueous solution at 80 °C for 72 h. To achieve the Ni-doping, 1 mM NiSO₄·6H₂O was added into the H₂SO₄ solution.

Characterizations

The powder morphologies were examined by a field emission scanning electron microscopy (FESEM, Hitachi, S-4800) and a transmission electron microscopy (TEM, FEI, Tecnai G2 F20 S-TWIN). The X-ray diffraction (XRD) tests were performed using a XRD-6000 diffractometer (SHIMADZU) with a CuKα radiation, operated at 40 kV, 40 mA (λ = 0.15406 nm). The Brunauer–Emmett–Teller (BET) approach using adsorption data was utilized to determine the specific surface area. The sample was degassed at 80 °C for 25 h to remove physisorbed gases prior to the measurement. The X-ray photoelectron spectra (XPS) characterization was carried out on an Escalab 250Xi system (Thermo Fisher Scientific). The ambient photoluminescence (PL) emission spectra were collected by a fluorescence spectrophotometer (HITACHI F-4500) with an excited wavelength of 360 nm. The UV-Vis diffuse reflectance spectra were collected using a UV-Vis near-infrared spectrometer (UV-3600, SHIMADZU).

Photocatalytic test

The photocatalytic performance of the powders was evaluated via photodegradations of rhodamine B (RhB) in water. Typically, 10 mg powders were dispersed into 50 mL RhB solution with an initial concentration of 0.02 mM, which was firstly stirred in dark for 30 min to reach an adsorption–desorption equilibrium and then subjected to a 500 W Xe-lamp irradiation for additional 60 min. The light intensity reaching the solution level was measured to be 4.5 and 190 mW cm⁻², respectively, for UV and visible light (Model: UV-A, 320–400 nm with a peak wavelength of 365 nm, and FZ-A, 400–1000 nm, Beijing Normal University, China). For the visible light illumination (intensity 200 mW cm⁻²), a filter was used to cut off light with wavelength less than 420 nm. The UV light was provided by an 18 W UV light (intensity 5.0 mW cm⁻²). At certain intervals, 3 mL suspension was taken out for sampling. After removing the powders by centrifugation, the dye concentration was determined with a UV-Vis spectrophotometer (UV-1800 PC, Shanghai Mapada Instruments Co. Ltd, Shanghai, China) at the wavelength of 553 nm. The photodegradation reaction was maintained at ambient temperature via a water cooling system. The cycling performance test was carried out under the same conditions except that the catalyst loading was increased to 0.5 g L⁻¹ to alleviate the influence caused by the loss of catalyst. The Aeroxide P25 TiO₂ nanoparticles served as a benchmark was supplied by Sigma-Aldrich. A commercial available phase pure anatase (purity > 99%) nanoparticles 5–10 nm in size were also utilized for comparison (provided by Zhoushan Mingri, Zhejiang, China).

Results and discussion

Fig. 1 shows that, hydrogen titanate nanowires (refer to Fig. S1, ESI,† for the additional XRD pattern and Fig. S2† for the SEM images) transformed to mainly rutile TiO₂ upon immersing in the H₂SO₄ solution at 80 °C for 72 h, under the atmospheric pressure. Fig. 1b indicates that, trace amounts of anatase appeared along with the rutile phase and some titanates retained after the acid treatment. With the additive of NiSO₄·6H₂O, the similar phase change was observed (Fig. 1c).

Fig. 1 XRD patterns of (a) the as-synthesized titanate nanowires, (b) un-doped rutile TiO₂, and (c) Ni-doped rutile TiO₂.

Fig. 2a shows the SEM image of the Ni-doped rutile TiO₂. Micrometer-sized aggregates can be seen, which are assembled with flower-like nanorods that are ca. 800 nm in length and 150 nm in width. Rutile TiO₂ achieved in the Ni-free H₂SO₄ solution exhibits a similar morphology (Fig. S3†). The additive of 0.5–4 mM NiSO₄·6H₂O in the H₂SO₄ solution altered neither the nanoflower structure (Fig. S4†) nor the rutile-dominated phase composition (Fig. S5†), which suggests that the resultant morphology and phase composition were not affected by the Ni-doping. In addition, the corresponding TEM images demonstrate further the flower-like nanostructures consisted of nanorods (Fig. 2b). The average length and width of the trunk nanorods estimated from the TEM observation is 800 nm and 120 nm, respectively, which is in accordance with the SEM observations. The HRTEM image of a typical branched nanorod is displayed in Fig. 2c. It can be seen that the nanorod was a single-crystalline. The inter-plane space of ca. 0.25 nm can be assigned to the (101) crystal plane of rutile TiO₂. The trunk nanorod also displayed an inter-plane space of ca. 0.25 nm that can be indexed to rutile (101) crystal plane (Fig. 2d). The corresponding selected area electron diffraction (SAED) pattern of the trunk nanorod further demonstrates its single-crystalline nature (inset in Fig. 2d). The EDS mapping suggests a homogeneous distribution of Ti, O, N, S, and Ni elements throughout the whole nanorod (Fig. 2), which confirms the successful doping of Ni into the single-crystalline rutile nanorods. As will be further revealed by XPS later, the N-doping comes from the reagent melamine during fabrications of the titanate nanowires; and the sulphur-incorporation is a result of the subsequent H₂SO₄ treatment.

Fig. 2 SEM (a), TEM (b) and HRTEM ((c) the branch and (d) the trunk) images of the Ni-doped rutile TiO₂ nanoflowers, and the corresponding EDS mapping images of Ti, O, N, S, Ni. The inset in (a) is the low magnification SEM image and the inset in (d) presents the corresponding SAED pattern.

The low temperature (77 K) nitrogen adsorption–desorption isotherms of the un-doped and Ni-doped rutile TiO₂ nanoflowers are illustrated in Fig. 3. The specific surface area of the as-synthesized titanate nanowires was 60.6 m² g⁻¹ [ref. 27]. The transformation of titanate nanowires to rutile nanoflowers via the H₂SO₄ treatment greatly increases the specific surface area to ca. 114.7 m² g⁻¹, which increases slightly to ca. 117.6 m² g⁻¹ upon the Ni-doping.

Fig. 3 The low-temperature N₂ adsorption–desorption isotherms of the un-doped and Ni-doped rutile TiO₂ nanoflowers.

The high-resolution XPS spectra of Ti 2p, O 1s, N 1s, S 2p, and Ni 2p for the un-doped and Ni-doped rutile TiO₂ nanoflowers are shown in Fig. 4. The peak locations (binding energy) of Ti 2p, O 1s, N 1s and S 2p are the same for the two samples. There are two peaks in the Ti 2p region, located at 464.2 and 458.5 eV, which correspond to Ti 2p_1/2 and Ti 2p_3/2, respectively. The splitting between Ti 2p_1/2 and Ti 2p_3/2 is 5.7 eV, which is characteristic of Ti⁴⁺ in TiO₂ lattice.³⁶ The O 1s spectrum can be fitted into two peaks. The lower binding energy (BE) peak located at 530.0 eV originated from the lattice oxygen and the higher BE peak located at 531.8 eV can be assigned to hydroxyl groups (–OH).³⁷ Nitrogen and sulfur elements were also detected (Fig. 4c and d). The binding energy of N 1s located at around 399.6 eV, which corresponds to N–O, N–N, or N–C, resulting from the decomposition of the reagent melamine during fabrications of the titanate nanowires.³⁸ Sulfate ions were incorporated into rutile TiO₂ nanoflowers through the H₂SO₄ treatment, as demonstrated by the binding energy of S 2p at 168.7 eV.³⁹ Fig. 4e shows the Ni 2p spectrum, of which a Ni 2p_3/2 peak at 854.7 eV and a Ni 2p_1/2 peak at 873.5 eV, although with quite a low signal-to-noise ratio due to the low Ni content, can be discerned for Ni-doped TiO₂ nanoflowers, which correspond to the Ni–O bond;⁴⁰ yet for un-doped TiO₂ nanoflowers, such peaks cannot be detected. For the Ni-doped TiO₂ nanoflowers, the atomic ratios of N/Ti, S/Ti, and Ni/Ti are determined to be 0.17, 0.10, and 0.027 by the XPS measurements. The XPS results thus demonstrate that Ni was successfully doped into TiO₂, which are in good accordance with the EDS mapping results in TEM (Fig. 2). The doping amount was small so that the chemical environment of TiO₂ stays the same.

Fig. 4 High-resolution XPS spectra of (a) Ti 2p, (b) O 1s, (c) N 1s, (d) S 2p, and (e) Ni 2p for the un-doped and Ni-doped rutile TiO₂ nanoflowers.

The transformation from titanate nanowires to rutile nanoflowers can be interpreted by a dissolution–precipitation procedure during the H₂SO₄ treatment. The titanate nanowires firstly reacted with H₂SO₄ to produce Ti(SO₄)₂,⁴¹ which would hydrolyze predominantly to Ti(OH)₂²⁺ under an acidic solution with pH 1.⁴² Owing to the limited solubility of Ti(IV) ions in water, they saturated easily and precipitated in the solution to form tiny TiO₂ nanocrystals.^42,43 Eqn (1)–(3) depict the transformation route.

H₂Ti₅O₁₁·3H₂O + H₂SO₄ → Ti(SO₄)₂ + H₂O (1)

Ti(SO₄)₂ + 2H₂O → Ti(OH)₂²⁺ + 2SO₄²⁻ + 2H⁺ (2)

Ti(OH)₂²⁺ → TiO₂ + 2H⁺ (3)

The phase composition of the precipitated TiO₂ nanocrystals is determined by the arrangement of the TiO₆ octahedra, which is the fundamental structural unit of both anatase and rutile. Both the pH value and the anion ions in the solution affect readily the TiO₆ octahedra stacking and in turn the final crystal phase. It has been reported that F⁻ and SO₄²⁻ favor the formation of anatase; whilst the additive of Cl⁻ helps the formation of rutile.⁴⁴ Hong et al. reported that, alkali-hydrothermally synthesized titanate nanowires (H₂Ti₆O₁₃) transferred to crystallized TiO₂ of dumbbell-shaped rutile, rod-like rutile, and quasi-octahedral anatase by acid-treating at 60–70 °C for 7 days in 2 M HCl, 2 M HNO₃ and 1 M H₂SO₄.²⁴ The formation of anatase is contributed to the large steric block effect of SO₄²⁻ during the arrangement of the TiO₆ octahedra, leading to the skewed chains instead of linear chains.²⁴ A mixture of anatase and rutile was obtained by an acid-hydrothermal treatment of titanate (H₂Ti₂O₅) nanowire film in 0.02 M H₂SO₄ at 100 °C for 4 h.⁴¹ Shen et al. also reported that, when titanate (H₂Ti₄O₉) fibers were hydrothermally treated at 150 °C for 24 h with 0.1–7 M H₂SO₄, a lower H₂SO₄ concentration favored the formation of rutile; whilst anatase was favoured by a high H₂SO₄ concentration.²³ In the current investigation, the concentration of SO₄²⁻ (0.05 M) is remarkably lower than that adopted by Hong et al.,²⁴ which is not high enough to block the formation of linear chains of TiO₆ octahedra, as a result, titanate nanowires transformed to mainly rutile, with trace anatase incorporated (Fig. 1).

The tiny rutile nanocrystals that precipitated from the acidic solution further grew to rutile nanorods, via an oriented attachment mechanism.^10a,43 The branched growth of such nanorods thus led to the rutile nanoflowers,^10a which tended to aggregate together to give a size in several to tens of micrometers (the inset in Fig. 2a). The aggregation to such a large size surely reduces the photocatalytic activity of the rutile nanoflowers; yet it is beneficial for the subsequent recovery from the slurry system, which is of importance for the practical application in wastewater treatments.

Fig. 5a shows the UV-Vis diffuse reflectance spectra of un-doped and Ni-doped rutile nanoflowers. Assuming an indirect transition between the valance and conduction bands, the band gap is estimated to be 2.87 and 2.72 eV, respectively, for the un-doped and Ni-doped rutile nanoflowers (Fig. 5b). The reduced band gap of the un-doped rutile nanoflowers when compared with bulk rutile TiO₂ (3.0 eV) is attributed to the N doping,^7a which is achieved because melamine involved in the fabrication history. Compared with un-doped rutile nanoflowers, the absorption edge of Ni-doped rutile nanoflowers further red-shifted (Fig. 5). The origin of the red-shift of Ni-doped rutile is due to the formation of a dopant energy level within the band gap of TiO₂. The electronic transitions from the valence band to the dopant level or from the dopant level to the conduction band can effectively cause the red shift in absorption edge.²⁸ In addition, the generation of oxygen vacancies by metal-ion doping may couple with the generation of new energy levels due to the injection of impurities within the band gap to contribute to the observed visible absorption.²⁹

Fig. 5 (a) UV-Vis diffuse reflectance spectra of the un-doped and Ni-doped rutile TiO₂ nanoflowers. (b) Re-plotting of (a) in an (αhν)^1/2–hν coordinate to evaluate the corresponding band gap.

Fig. 6a shows the photodegradation curves of RhB in the presence of the un-doped and Ni-doped rutile nanoflowers under the UV + Vis light illumination. The RhB degradation curves assisted by the commercial anatase and P25 nanoparticles are also included for reference. The blank test revealed no RhB degradation under solely the Xe-lamp illumination. In the presence of TiO₂ powders, remarkable RhB photodegradation can be discerned (Fig. 6a). All the degradation procedures can be fitted well by a pseudo-first order kinetic, as displayed in Fig. 6b. The reaction rate constants are determined to be 0.0056, 0.011, 0.024, and 0.047 min⁻¹ for pure anatase, P25, un-doped, and Ni-doped rutile nanoflowers, respectively. P25 exhibited much higher activity than that of phase pure anatase, which can be contributed to the phase junction of anatase/rutile that enhances the charge separation.⁴⁵ The un-doped rutile nanoflowers exhibited an activity more than two folds that of P25; whilst the Ni-doped rutile nanoflowers exhibited an activity more than four folds that of P25. More importantly, the rutile nanoflowers synthesized in the current investigation subside within 1 min when stop stirring the slurry, which is beneficial for the subsequent recovering procedure in practice. On the contrary, the P25 nanoparticles were well dispersed in the slurry even after 24 h without stirring (Fig. S6†).

Fig. 6 Photodegradation curves of rhodamine B in water in the presence of the un-doped and Ni-doped rutile TiO₂ nanoflowers, pure anatase, and P25 TiO₂ nanoparticulate powders under illumination of (a) UV + Vis light, (c) visible light and (e) UV light (initial RhB concentration: 0.02 mM; catalyst load: 0.2 g L⁻¹). The corresponding fitting results assuming a pseudo-first order reaction were shown in (b), (d), and (f); (g) the cycling performance of the Ni-doped rutile TiO₂ nanoflowers under UV + Vis light illumination (initial RhB concentration: 0.02 mM; catalyst load: 0.5 g L⁻¹; illumination time: 45 min); (h) the rate constant k derived from photodegradation curves of rhodamine B in water under UV + Vis light illumination in the presence of commercial P25 TiO₂ nanoparticles, and rutile TiO₂ nanoflowers achieved with various concentrations of NiSO₄·6H₂O in the aqueous H₂SO₄ solution.

Exposure of the dye solution to solely visible light for 120 min, 16%, 72%, and 97% rhodamine B molecules were degraded for P25, un-doped, and Ni-doped rutile nanoflowers, respectively (Fig. 6c). Both un-doped and Ni-doped rutile nanoflowers demonstrated much increased reaction rate constants than that of P25 under the visible light illumination (Fig. 6d), because of the reduced band gaps resulting from the N-doping and co-doping of N and Ni (Fig. 5), which guarantee their visible light harvesting.

Fig. 6e shows that, under the solely UV light illumination for 90 min, the dye removals are 63%, 79% and 89%, respectively, for P25, un-doped, and Ni-doped rutile nanoflowers. The beneficial effect of Ni-doping can also be discerned (Fig. 6f), although not so significant as that under the visible light illumination. It thus concludes that the remarkable enhanced photocatalytic activity of the Ni-doped rutile nanoflowers under the UV + Vis light illumination can be mainly contributed to the increased visible light harvesting. The much improved photocatalytic activity of the Ni-doped rutile nanoflowers when compared with P25 is also confirmed for degradations of RhB with a much higher initial concentration of 0.1 mM (Fig. S8†).

The stability of the Ni-doped rutile nanoflower powders was confirmed by repetitively RhB degradations under the illumination of the Xe-lamp for 5 cycles, as illustrated in Fig. 6g. After each cycle, the powders were recovered by centrifugation only, which resulted in slightly catalyst loss and in turn led to the reduced removal after the initial 3 cycles.

The higher activity of rutile nanoflower powders when compared with P25 can be firstly attributed to the much higher specific surface area (115 and 118 vs. 50 m² g⁻¹). Considering the similar specific surface area, and also the fact that Ni doping altered neither the resultant morphology (Fig. S2–4†) nor the phase structure (Fig. 1), the further enhanced activity of the Ni-doped powders when compared with the un-doped one can be ascribed solely to the successful doping of Ni throughout the nanorods, as demonstrated by the EDS mapping (Fig. 2) and XPS results (Fig. 4e). The Ni-doping expanded the optical absorption range (Fig. 5) and enhanced the charge separation efficiency, both of which contribute to the enhanced photocatalytic activity.^30,31

As a photocatalyst, anatase TiO₂ is more active than rutile TiO₂, because of its combined effects of lower charge recombination rate and stronger surface adsorption ability.⁴⁵ It's generally believed that, a synergistic effect exists between rutile and anatase; as a result, TiO₂ with appropriate mixtures of anatase and rutile displays an enhanced photocatalytic performance over pure anatase or rutile.^46,47 Typically, Degussa P25 that consists of 80% anatase and 20% rutile exhibits higher activity than either anatase or rutile.⁴⁵ The coexistence of small amount of anatase in the rutile nanoflowers synthesized here (Fig. 1) may also contribute to the enhanced photocatalytic performance.

The ambient PL measurement confirmed the enhanced charge separation arising from the Ni-doping. Fig. 7 indicates the PL spectra collected from the rutile nanoflowers with and without the Ni-doping. The strong but broad peak at ca. 390 nm is attributed to the band–band PL phenomenon, which is related to the separation of photogenerated charge carriers.⁴⁸ The Ni-doped rutile nanoflowers exhibited much weaker band–band PL intensity than that of un-doped rutile nanoflowers, implying a lower charge recombination rate, which can be attributed to the additional energy level generated by the Ni-doping. The excitonic PL peak located at ca. 620 nm is dependent of the surface defects, oxygen vacancies and surface states, which cannot directly reflect the separation efficiency of photogenerated charge carriers.⁴⁸

Fig. 7 Room temperature photoluminescence emission spectra of the un-doped and Ni-doped rutile TiO₂ nanoflowers.

The amounts of Ni-doping can be controlled simply by adjusting NiSO₄·6H₂O concentrations in the H₂SO₄ solution. Fig. S7† shows that, the Ni/Ti atomic ratio of the Ni-doped TiO₂ nanoflowers, which is estimated by the EDS analysis, increased roughly with increasing NiSO₄·6H₂O concentrations. It should be noted that the Ni/Ti atomic ratio estimated by EDS and XPS differed significantly, because of the uncertainty brought about by the relatively low content. Fig. 6h shows that, with the increasing Ni-doping, the photocatalytic activity of the resultant rutile nanoflowers first increased, and then decreased. The appropriate dosage of NiSO₄·6H₂O that achieved the best photocatalytic performance is ca. 1 mM, corresponding to a Ni/Ti atomic ratio of 0.027 (determined by XPS) in the resultant powders. TiO₂ with too many Ni dopants resulted in a decreased efficiency because Ni dopants also serve as recombination centers of photogenerated electrons and holes, which affects negatively the photocatalytic activity.³¹

Conclusions

Rutile TiO₂ nanoflowers self-assembled with single-crystalline nanorods were fabricated via an acid treatment of the precursor titanate nanowires, which were synthesized through a low temperature interaction between metallic Ti powders and H₂O₂ solution. The transformation proceeded in a dissolution precipitation route, accompanied by an oriented attachment to grow the rutile nanorods that further assemble to the nanoflowers. The micrometer-sized nanoflower aggregates possessed a high specific surface area of 114.7 m² g⁻¹. The efficiency to assist photodegradation of rhodamine B in water under the UV + Vis light illumination is twice that of commercial Degussa Aeroxide P25 TiO₂ nanoparticles. The spontaneous Ni-doping was achieved during formations of the rutile TiO₂ nanoflowers. When compared with the un-doped one, the Ni-doping with a Ni/Ti atomic ratio of ca. 0.027 (determined by XPS) induced negligible changes in both the morphology and the phase composition, increased slightly the specific surface area to 117.6 m² g⁻¹; however, a photocatalytic activity that is nearly four times that of P25 was achieved. The present work develops a TiO₂ photocatalyst with a photocatalytic activity far exceeding that of P25 and at the same time can be easily recovered from a slurry system, which may find practical applications in wastewater treatments.

Acknowledgements

This work is supported by the Natural Science Foundation of China (Project No. 51502065) and Department of Science Technology of Zhejiang Province under Contract No. 2015C31034.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional XRD pattern, FESEM images, optical photos, SEM-EDS plot, and photodegradation curves to support the discussion. See DOI: 10.1039/c6ra01752e

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