In situ growth of TiO₂ on TiN nanoparticles for non-noble-metal plasmonic photocatalysis

Abstract

Plasmonic photocatalysis could provide a promising solution to the two fundamental problems of current TiO₂-based visible-light photocatalysis on low photocatalytic efficiency and low usage of solar illumination. But till now, most plasmonic photocatalysts have relied on noble metal nanostructures of Au or Ag due to their easy synthesis and efficient absorption of visible light. In this study, a TiN/TiO₂ nanocomposite photocatalyst was synthesized by the in situ growth of TiO₂ nanoparticles on TiN nanoparticles with a fluorine-free, vapor-phase hydrothermal process. In this composite photocatalyst system, the desirable visible light absorption could be attributed to the LSPR effect of a nanostructured TiN phase. Thus, a plasmonic photocatalyst without noble-metal components was developed, and its good visible light photocatalytic activity was demonstrated by both the photodegradation of organic pollutants of RhB and 4-NP and the disinfection of microorganisms of E. coli. From the energy alignment analysis, hot electrons were expected to be completely injected from TiN to TiO₂ once they were excited above the Fermi energy level of TiN because no barrier existed, resulting in better electron injection efficiency than previous reported noble-metal-based plasmonic photocatalysts.

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1 Introduction

Since the discovery of the photocatalytic water splitting on TiO₂ electrodes in 1972,¹ semiconductor-based photocatalysts have been extensively investigated for both solar energy conversion and environmental applications.^2–10 Among them, TiO₂ has been demonstrated as the leading candidate for potential applications on an industrial level due to its good photoactivity, nontoxicity, high chemical stability, and relatively low cost.¹¹ However, the activation of TiO₂ is limited only to ultraviolet light (λ > 387 nm, accounting for just 5% of the solar spectrum) due to its relatively wide band gap (ca. 3.2 eV for anatase TiO₂), so its solar efficiency was severely limited. Various approaches had been investigated to develop TiO₂-based photocatalysts with visible light (∼43% of the solar spectrum) activation for better solar energy utilization, including doping with anions, metals, or their co-dopings,^12–15 forming heterojunctions with proper semiconductors of smaller band gaps,^10,16,17 and crystal facet engineering.^18,19

Recently, the creation of plasmonic photocatalysts had been demonstrated as a promising approach to largely enhance the photocatalytic performance for various applications.^20,21 By introducing nanostructure components (mostly noble metal nanoparticles, like Au and Ag) that could possess a localized surface plasmon resonance (LSPR) effect into semiconductor photocatalysts, both the photo-induced electron–hole pair separation efficiency could be enhanced and the responsive illumination range could be expanded to visible and near-infrared light regions.^22,23 By modulating the nanostructure size, shape and composition, the frequency of LSPR could be further tuned to meet the needs of various applications.^24,25 Furthermore, LSPR could create the electric field localization and subsequently induce an intensive local electric field upon the proper design of the nanostructure curvature and distribution to favor photocatalytic reactions.^26–28 Thus, plasmonic photocatalysis could provide a promising solution to the two fundamental problems of current TiO₂-based visible-light photocatalysis on low photocatalytic efficiency and low usage of solar illumination.

Till now, most plasmonic photocatalysts relied on noble metal nanostructures of Au or Ag due to their easy synthesis and efficient absorption of visible light.^22,29,30 However, their rarity, high cost, low melting point, low thermal stability, and easy dissolution (especially for Ag) upon the exposure to air or humidity largely limited their potential for practical applications. Thus, novel plasmonic photocatalysts without noble metal components should be developed to overcome these problems, which could have great potentials for various environmental applications. Titanium nitride (TiN) is a hard material with gold-like optical properties, which is commonly used as coatings for various substrates due to its high melting temperature, strong corrosion resistance, and non-toxicity/bio-compatibility. Recently, it was reported that TiN could possess a plasmonic resonance absorption peak located in the visible and near-infrared light range.^31–35 In the recent work of Boltasseva and co-workers,^36–38 it was demonstrated that TiN nanoparticles could provide comparable field enhancement and better absorption efficiency when compared to Au nanoparticles, and their extinction peak was similar to that of Au nanoparticles, but with a broader width. Furthermore, TiO₂-based photocatalysts had been successfully synthesized by oxidizing TiN with various approaches.^39,40 Thus, a novel plasmonic photocatalyst without noble metal components could be developed if nanostructured TiN/TiO₂ composite with good contact could be created. In this novel plasmonic photocatalyst, the LSPR effect from the component of TiN could introduce visible light photocatalytic activity to TiO₂, which could avoid the intrinsic problem of massive charge carrier recombination brought by nitrogen-doping to TiO₂.⁴¹

In this work, a novel plasmonic photocatalyst of TiN/TiO₂ was created by in situ growth of TiO₂ nanoparticles on TiN nanoparticles with a fluorine-free vapor-phase hydrothermal (VPH) process, which demonstrated its effectiveness in various visible-light-activated photocatalysis reactions. In our approach, TiN nanoparticles were synthesized through a chemical vapor deposition (CVD) process, and TiO₂ was grown in situ based on these TiN nanoparticles to form TiN/TiO₂ nanocomposite by a VPH process with HNO₃ as the oxidant. By replacing the commonly used oxidant of HF in the VPH technique to create various metal oxide nanostructures,^42,43 a fluorine-free VPH approach was developed which avoided the use of highly corrosive chemicals. TiN/TiO₂ composite photocatalyst demonstrated the desirable visible light absorption, and their good visible light photocatalytic activity was demonstrated by both the photodegradation of organic pollutants and the disinfection of microorganism.

2 Experimental

2.1 Chemicals and materials

Titanium tetrachloride (TiCl₄, AR, 99% Chemical Reagent Co., Ltd., Beijing, P. R. China) was used as the Ti source in the synthesis of TiN nanoparticles, nitric acid (HNO₃, 65%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P. R. China) was used as the oxidizing agent in the vapor-phase hydrothermal process, deionized water (DI) and ethyl alcohol (EtOH, 99.7%, Beijing Yili Fine Chemicals Co. Ltd., Beijing, P. R. China) was used to wash the products. Rhodamine B (RhB, AR, Chemical Reagent Co., Shenyang, P. R. China) and 4-nitrophenol (4-NP, 99%, Alfa Aesar Chemical Co., Ltd., Tianjin, P. R. China) were used as the target compounds to investigate the visible-light photocatalytic performance, Degussa P25 TiO₂ nanoparticles (Evonik Industries, Germany) were used for the comparison with TiN nanoparticles and TiN/TiO₂ nanocomposites on their visible-light photocatalytic performance. Commercial titanium oxide nanoparticles (anatase, 25 nm, 99.8%, Aladdin Co., Shanghai, P. R. China) and titanium nitride nanoparticles (TiN, 20 nm, 99.9%, Aladdin Co., Shanghai, P. R. China) were used for the preparation of the TiN/TiO₂ mixture sample. Tert-butyl alcohol (TBA, 98%, Sinopharm Chemical Reagent Co., Ltd. Shanghai, P. R. China) and benzoquinone (BQ, 98%, Sinopharm Chemical Reagent Co., Ltd. Shanghai, P. R. China) were used as radical scavengers in the photocatalytic degradation experiments.

2.2 Synthesis of TiN nanoparticles

TiN nanoparticles were synthesized by a modified CVD process.⁴⁴ In a typical experiment, 2 mL of TiCl₄ solution was dropped into an alumina crucible. Then, the crucible was placed in a quartz tube furnace, and both ends of the quartz tube were plugged with silicone plugs. Argon gas was used as the protective gas during the CVD process. After the temperature in the quartz tube rose to 100 °C, ammonia gas was introduced to the tube with a flow rate of 80 mL min⁻¹. The heating rate was set as 10 °C min, and it was then kept at 800 °C for 30 min. Finally, the furnace was cooled down to room temperature with the protection of argon gas, and black powders were observed in the surface of the crucible. After being collected, these black powders were washed with DI water for several times to remove impurities until no Cl⁻ ion could be detected by AgNO₃ precipitation.

2.3 Synthesis of TiN/TiO₂ nanocomposite photocatalyst

TiN/TiO₂ nanocomposite photocatalyst was synthesized by a vapor-phase hydrothermal (VPH) method. In a typical experiment, 25 mg of TiN nanoparticles were first put in a Teflon-lined container. Then, the container was put above a 15 mL of 0.1 M HNO₃ aqueous solution in a Teflon-lined stainless steel autoclave, and the VPH process was conducted at 180 °C for different treatment times of 1 h, 2 h, 3 h, 5 h, and 6 h, respectively. Direct contact between TiN nanoparticles and HNO₃ solution was avoided so that TiN nanoparticles were only treated by the vapor hydrothermal process. The treated sample was collected, washed several times with DI water to remove the impurities until neutral pH, and dried at 60 °C for 12 h in air to obtain the final product. The sample was named as TiN/TiO₂-X, where X h was the VPH treatment time.

2.4 Material characterization

The crystal structures of the samples were examined by X-ray diffraction (XRD) on a D/MAX-2004 X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu K_α radiation (λ = 0.154178 nm). The morphologies of the as-prepared samples were observed on a JEOL 2100 TEM (JEOL Ltd., Tokyo, Japan) operated at 200 kV with point-to-point resolution of 0.28 nm. High-resolution transmission electron microscopy (HRTEM) images were recorded on a FEI Tecnai G2 F30 microscope (FEI, Acht, The Netherlands). X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo Fisher, Waltham, USA). The UV-vis spectra of samples were measured on a UV-2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan), and their optical absorbances were determined by the diffuse reflectance spectra (DRS) measurements using BaSO₄ as the reference.

2.5 Photocatalytic degradation of rhodamine B and 4-nitrophenol under visible light illumination

The photocatalytic performances of these samples were evaluated by examining their photocatalytic degradation of rhodamine B (RhB) and 4-Nitrophenol (4-NP) under visible light irradiation (λ > 400 nm). 20 mg of the photocatalyst was added into 40 mL of RhB (5 mg L⁻¹) or 4-NP solution (10 mg L⁻¹) in a 250 mL glass reactor. A water bath was used to minimize the temperature increase in the solution under the light irradiation. The solution was stirred for 30 min to ensure equilibrium adsorption in the dark and then exposed to visible light irradiation. The light source was a 300 W Xe arc lamp (PLS-SXE300, Beijing Perfect Light Technology Co., Ltd., Beijing, P. R. China) with a UV cutoff filter (λ > 400 nm). Before the experiment, the lamp was warmed up for half an hour. The light intensity was determined at ∼70 mW cm⁻² by a FZ-A optical radiometer (Photoelectric Instrument Factory of Beijing Norman University, Beijing, P. R. China). At each time intervals, 3 mL of the solution was sampled. Photocatalysts were separated by centrifugation at 10 000 rpm for 10 min, and the concentration of RhB or 4-NP was determined by measuring the absorption maximum at 553 nm and 317 nm, respectively, with a UV-vis spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan). P25 TiO₂ nanoparticles and TiN nanoparticles were also used in the photocatalytic degradation experiments for comparison purpose under the same experimental conditions. The photocatalytic degradation of RhB experiment was also conducted under ∼700 nm visible light illumination with the same experimental setup. The light source was the 300 W Xe arc lamp with a 700 nm band-pass filter, and the light intensity was determined at ∼3 mW cm⁻². The ˙O₂⁻ scavenger (BQ, 1 mg) and ˙OH scavenger (TBA, 2 mL) were used as radical scavengers in the photocatalytic degradation of RhB in order to distinguish which active radical plays the critical role of in the photocatalytic process.

2.6 Photocatalytic disinfection of bacteria Escherichia coli (E. coli) under visible light illumination

Wild-type E. coli AN 387 (ATCC 15597, the American Type Culture Collection, Manassas, VA, USA) was used for evaluating the sample's photocatalytic disinfection performance. After overnight culture, E. coli cells were diluted to a cell suspension (ca. 10⁷ cfu mL⁻¹) in the buffer solution (0.05 M KH₂PO₄ and 0.05 M K₂HPO₄, pH 7.0) for photocatalytic disinfection experiments. All solid or liquid materials had been autoclaved for 30 min at 121 °C before use. The same water bath and visible light source were used as that used in the photocatalytic degradation experiment. In the photocatalytic disinfection of E. coli bacteria experiment, 10 mL E. coli cell suspension was pipetted onto a sterile 60 × 10 mm petri dish with 10 mg photocatalyst sample placed at the bottom with magnetic stirring. At each time intervals, 100 μL of the treated cell suspensions were withdrawn in sequence. After appropriate dilutions in buffer solution, aliquot of 100 μL was spread onto an agar medium plate and incubated at 37 °C for 24 h. The number of viable cells in terms of colony-forming units was counted. The survival ratio was determined by N(t)/N(t₀), where N(t) was the number of colony-forming units at the treatment time t, while N(t₀) was the number of colony-forming units without treatment. All the analyses were conducted in triplicate. The photocatalytic disinfection of E. coli experiment was also conducted under ∼700 nm visible light illumination with the same experimental setup, in which the influence of oxygen was investigated by comparing the survival ratio of E. coli cells in air and in enriched N₂ atmospheres. All the experiment was conducted at room temperature.

3 Results and discussion

3.1 The synthesis of TiN nanoparticles

Fig. 1a shows the XRD pattern of the as-synthesized TiN nanoparticles, which clearly demonstrated that all of the diffraction peaks could be indexed as the face-centered cubic structure of TiN (JCPDS no. 38-1420). Their crystallite size could be obtained from the strongest diffraction peak (at 2θ ∼42°) by the Scherrer's formula:⁴⁵

D = 0.9λ/β cos θ (1)

where λ is the average wavelength of the X-ray radiation, β is the line-width at half-maximum peak position, and θ is the XRD peak. Their average crystallite size was determined at ∼17 nm. Fig. 1b shows the TEM image of the as-synthesized TiN nanoparticles. They had an average size of ∼20 nm, which was in accordance with the XRD analysis result.

Fig. 1 (a) XRD patterns and (b) TEM image of the as-synthesized TiN nanoparticles.

3.2 The creation of TiN/TiO₂ nanocomposite photocatalyst

A vapor-phase hydrothermal (VPH) process was developed to create TiN/TiO₂ nanocomposite photocatalyst from these TiN nanoparticles. Fig. 2a shows the XRD patterns of as-synthesized TiN/TiO₂ nanocomposite photocatalysts with a series of VPH treatment time from 0 to 6 h. When the treatment time was 1 h, all diffraction peaks still belonged to TiN and their intensity was similar to that of TiN nanoparticles without the VPH treatment, which indicated that the oxidation of TiN was still in the initial stage. When the treatment time was 2 h and over, strong diffraction peaks belonged to anatase TiO₂ (JCPDS no. 21-1272) occurred, which indicated the oxidation of TiN to TiO₂ and subsequent crystallization. With the increase of the VPH treatment time, the intensity of diffraction peaks belonged to TiN decreased, while that of diffraction peaks belonged to TiO₂ increased. Besides that of anatase TiO₂, a diffraction peak belonged to brookite TiO₂ (JCPDS no. 29-1360) could also be observed, which indicated that a small amount of brookite TiO₂ was formed during the VPH treatment of TiN nanoparticles.

Fig. 2 (a) XRD patterns of as-synthesized TiN/TiO₂ nanocomposite photocatalysts with a series of VPH treatment time from 0 to 6 h. (b) HRTEM image of TiN/TiO₂ sample obtained by a 5 h VPH treatment.

The TiO₂/TiN weight ratio (ω₁/ω₂) of these samples could be estimated by the reference intensity method using eqn (2):

where I₁ and I₂ are the intensities of the TiO₂ and TiN diffraction peaks, respectively, RIR₁ and RIR₂ are the reference intensities of the TiO₂ and TiN phases compared to the α-Al₂O₃ standard, respectively, and the sum of ω₁ and ω₂ is 1. From the XRD analysis results, the TiO₂/TiN weight ratio (ω₁/ω₂) of these samples could be approximately estimated, which was summarized in Table 1. It demonstrated clearly that the ω₁/ω₂ increased quickly during the first 3 h VPH treatment from 0/1 to 0.82/0.18 and then it increased to 0.91/0.09 during the next 3 h VPH treatment up to 6 h with a slower increase speed. The slowdown of the TiO₂ phase increase could be attributed to the reactant decrease of both TiN and HNO₃ with the prolonged VPH process.

Table 1 Summary of the weight ratio (ω₁/ω₂) of the as-synthesized TiN/TiO₂ nanocomposite photocatalysts with a series of VPH treatment time from 1 to 6 h

Samples	TiN (wt%)	TiO2 (wt%)
TiN/TiO2-1	100	0
TiN/TiO2-2	37	63
TiN/TiO2-3	18	82
TiN/TiO2-5	12	88
TiN/TiO2-6	9	91

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Fig. 2b shows the HRTEM image of the TiN/TiO₂-5 sample obtained by a 5 h VPH treatment. Sets of lattice planes with the d-spacing at ∼0.35 nm could be clearly identified on the surface of the central nanoparticle as marked by dot lines, which sample was 0.88/0.12. In the center part of this nanoparticle, lattice planes got blurred, which may reflect the remaining TiN core after the 5 h VPH treatment. The in situ oxidation of TiN could create heterojunctions with good contact between TiN and TiO₂, beneficial to electron transfer between them.

Fig. 3a shows the XPS survey spectrum of the TiN/TiO₂-5 sample, which clearly demonstrated the existence of Ti, O, and N in this sample. The obvious C 1s peak could be attributed to the widespread presence of carbon in the environment. Fig. 3b and c show the high resolution XPS scans over Ti 2p peak of the original sample and the same sample after 100 s Ar⁺ sputtering, respectively. Before Ar⁺ sputtering, the binding energies of Ti 2p_1/2 peak and Ti 2p_3/2 peak were determined at ∼464.2 eV and ∼458.5 eV, respectively. This observation was in agreement with the values reported for anatase TiO₂,^46,47 which suggested that the sample surface was composed mainly by TiO₂. After 100 s Ar⁺ sputtering, an obvious shift to the lower bonding energy was observed for Ti 2p peak, indicating a change of its chemical environment. The Ti 2p peak could be best fitted into a combination of three sets of Ti 2p peaks.⁴⁶ Ti 2p_1/2 peak at ∼464.3 eV and Ti 2p_3/2 peak at ∼459.1 eV were for TiO₂,⁴⁷ Ti 2p_1/2 peak at ∼462.6 eV and Ti 2p_3/2 peak at ∼457.0 eV may belong to an “intermediate” state between TiN and TiO₂,⁴⁶ while Ti 2p_1/2 peak at ∼461.0 eV and Ti 2p_3/2 peak at ∼455.3 eV were for TiN.⁴⁶ This observation suggested that TiO₂ in the sample was created by the oxidation of TiN, and the remaining TiN mainly existed in the inside part of the sample and was exposed by the Ar⁺ sputtering.

Fig. 3 (a) XPS survey spectrum of TiN/TiO₂-5 sample. (b) The high resolution XPS scans over Ti 2p peak of the original sample and (c) the same sample after 100 s Ar⁺ sputtering, respectively. (d) The high resolution XPS scans over N 1s peak of the original sample and (e) the same sample after 100 s Ar⁺ sputtering, respectively.

Fig. 3d and e show the high resolution XPS scans over N 1s peak of the original sample and the same sample after 100 s Ar⁺ sputtering, respectively. Without Ar⁺ sputtering, the binding energy of N 1s peak was determined at ∼400.2 eV, which could be attributed to molecularly chemisorbed γ-N₂.^12,46 After 100 s Ar⁺ sputtering, 2 peaks appeared in the spectrum, among which the one at ∼400.2 eV could be attributed to molecularly chemisorbed γ-N₂ and the other one at ∼397 eV could be assigned to atomic β-N.^46,48 Thus, in the inside part of the sample, Ti–N existed, which could be in the form of N–Ti–N or O–Ti–N. From XRD, HRTEM, and XPS analysis results, it was clear that TiN/TiO₂ nanocomposite photocatalyst was created from the in situ oxidation of TiN nanoparticles by the VPH process we developed, and contact was formed between TiN and TiO₂, which was beneficial for electron transfer between them.

3.3 Optical properties of TiN/TiO₂ nanocomposite photocatalyst

The optical properties of the TiN nanoparticles and TiN/TiO₂ nanocomposite photocatalyst were investigated by measuring their diffuse reflectance spectra. From the reflectance data, the optical absorbance could be approximated by the Kubelka–Munk function, as given by eqn (3):where R is the diffuse reflectance.⁴⁹ Fig. 4 shows the light absorbance (in term of Kubelka–Munk equivalent absorbance units) of the TiN/TiO₂-5 sample, compared with that of TiN nanoparticles and Degussa P25 TiO₂ nanoparticles. Degussa P25 TiO₂ nanoparticles demonstrated their characteristic spectrum with the fundamental absorbance stopping edge at ∼400 nm. TiN nanoparticles, however, demonstrated a nearly all light absorption from UV to the NIR region, which was in agreement with its metallic nature.³¹ A broad absorption peak at ∼680 nm could be observed for TiN nanoparticles from their surface plasmon resonance effect, which was very similar to a recent report by Guler et al. on TiN nanoparticles.³⁸ The TiN/TiO₂-5 sample exhibited two distinctive light absorption behaviors. The one close to that of Degussa P25 TiO₂ nanoparticles came from the TiO₂ phase of the composite photocatalyst, while the other one was from the absorption of TiN phase of the composite photocatalyst. Thus, visible light absorption capability was introduced to TiN/TiO₂ samples from the surface plasmon resonance effect of TiN nanostructures.

Fig. 4 UV-vis light absorbance spectrum of TiN/TiO₂-5 photocatalysts, compared with that of the TiN and Degussa P25 TiO₂ nanoparticles.

3.4 Photocatalytic degradation of rhodamine B and 4-nitrophenol under visible light illumination

The photocatalytic activities of TiN/TiO₂ composite photocatalysts were demonstrated by their degradation effect on rhodamine B (RhB) and 4-nitrophenol (4-NP) under visible light illumination. For comparison, the photocatalytic degradations of RhB and 4-NP were also conducted on pure TiN, P25 TiO₂ nanoparticles, nitrogen-doped TiO₂ nanoparticles, and the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample under the same experimental setup. Fig. 5a summarizes the residue RhB concentration vs. treatment time by various treatments under same conditions. When there was no photocatalyst presence, RhB solution kept its initial concentration under visible light illumination, which indicated that visible light irradiation itself could not degrade RhB. Before the photocatalytic degradation experiment under visible light illumination, all samples were mixed with the RhB solution in the dark for 30 min to establish the adsorption–desorption equilibrium. As expected, Degussa P25 TiO₂ nanoparticles showed a weak photocatalytic RhB degradation effect under visible light illumination due to the well-known mixed phase effect of P25 with the co-existence of anatase and rutile phases.⁵⁰ After 2 h treatment under visible light illumination by P25, the residual RhB concentration was still ∼60%. Nitrogen-doped TiO₂ (TiON) nanoparticles were obtained from a complete VPH oxidation process and their N/Ti molar ratio was ∼2%, typical for nitrogen doping of TiO₂. The residual RhB concentration was ∼50% after 2 h treatment by them under visible illumination, which could be attributed to their visible light absorption from nitrogen doping. Under visible illumination, TiN nanoparticles (1 h VPH treatment sample) showed no photocatalytic degradation effect on RhB because of their metallic nature.

Fig. 5 (a) Residual rhodamine B and (b) residual 4-nitrophenol percentage vs. treatment time under visible light illumination treated by TiN/TiO₂-X samples, compared with that without photocatalyst presence, by Degussa P25 TiO₂ nanoparticles, and by the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample, and by TiON (c) survival ratio of E. coli cells vs. treatment time under visible light illumination by a series of TiN/TiO₂-X sample, compared with that without photocatalyst presence, by the TiN/TiO₂-5 sample in the dark, by Degussa P25 TiO₂ nanoparticles, and by the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample.

For TiN/TiO₂ composite photocatalysts, they demonstrated different photocatalytic RhB degradation effects under visible light illumination due to their different TiN/TiO₂ ratio. When the VPH treatment time increased from 2 h to 5 h, their photocatalytic RhB degradation performance enhanced under visible light illumination, while it decreased when the VPH treatment time further increased from 5 h to 6 h. In this photocatalyst system, the component of TiN served as the visible light absorber for the creation of the LSPR effect to generate hot electrons, which could then be injected into the conduction band of TiO₂ and react with oxygen to produce ˙O₂⁻ radicals in aqueous solution and subsequently produce ˙OH radicals. Thus, an optimal TiN/TiO₂ ratio must exist to balance the light absorption and radical production to create the optimal photocatalytic performance. In this series of photocatalytic degradation of RhB experiments, the optimized performance was observed on the TiN/TiO₂-5 sample, which was composed of 12% TiN and 88% TiO₂. After 2 h treatment under visible light illumination by the TiN/TiO₂-5 sample, the residual RhB concentration was only ∼10%, much lower than that treated by either Degussa P25 TiO₂ nanoparticles or nitrogen-doped TiO₂ nanoparticles. For comparison purpose, we also examined the photocatalytic degradation effect of the mixture of TiN/anatase TiO₂ nanoparticles, which showed a weak photocatalytic RhB degradation effect under visible light illumination although it had the same TiN/TiO₂ ratio as the optimized TiN/TiO₂-5 sample. The mixture of TiN nanoparticles with anatase TiO₂ nanoparticles could not create close contacts between TiN and anatase TiO₂, so it was difficult for hot electrons created by TiN from the LSPR effect to transfer to the conduction band of TiO₂. Thus, the mixture of TiN/anatase TiO₂ with the same TiN/TiO₂ ratio could not compete with the TiN/TiO₂-5 sample on the photocatalytic performance under visible light illumination.

To exclude the potential photosensitization effect from RhB (maximum absorption at λ ∼552 nm), 4-NP was also used as a model organic pollutant to further examine the photocatalytic degradation, which has no absorption in the visible light region. Fig. 5b summarizes the residue 4-NP concentration vs. treatment time by various treatments under same conditions, which clearly demonstrated a similar trend as that for the photocatalytic RhB degradation. The optimized performance was also observed on the TiN/TiO₂-5 sample, which further confirmed that the photocatalytic performance of the TiN/TiO₂ composite photocatalyst could be modulated by changing the ratio of TiN to TiO₂ in this composite photocatalyst. As expected, the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample only demonstrated a weak photocatalytic degradation effect on 4-NP.

3.5 Photocatalytic disinfection of Escherichia coli bacteria under visible light illumination

The photocatalytic performance of the TiN/TiO₂ composite photocatalyst was further demonstrated by its photocatalytic disinfection of E. coli cells, which was conducted by exposing the cells suspended in buffer solution with the TiN/TiO₂-5 sample under visible light illumination (λ > 400 nm) for varying time intervals. Fig. 5c shows the survival ratio of E. coli cells by various treatments under same conditions. No obvious change was found for the survival ratio of E. coli cells under visible light illumination when no photocatalyst was present, which indicated that visible light itself could not disinfect E. coli cells. When treated by the TiN/TiO₂-X samples without visible light illumination (the result by the TiN/TiO₂-5 sample was shown in Fig. 5c as an example), the survival ratio of E. coli cells also showed no obvious change. Thus, the TiN/TiO₂-X samples did not have the bactericidal effect by themselves. When both the TiN/TiO₂-X samples and the visible light illumination were in presence, the survival ratio of E. coli cells dropped continuously with the increase of the treatment time. Similar to results of the photocatalytic degradation of RhB and 4-NP, the TiN/TiO₂-5 sample showed the best performance among these TiN/TiO₂-X samples. After 2 h treatment by the TiN/TiO₂-5 sample, the survival ratio of E. coli dropped to ∼20% under visible light illumination, which clearly demonstrated its good photocatalytic disinfection capability. As expected, the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample only demonstrated a weak photocatalytic disinfection effect on E. coli bacteria.

3.6 Photocatalytic activity of TiN/TiO₂ composite photocatalysts under 700 nm visible light illumination

The oxidation of TiN had been explored as an effective approach to synthesize nitrogen-doped TiO₂.^39,40 In our approach, the TiN/TiO₂ composite photocatalyst was created from a partial oxidation of TiN by HNO₃ in the VPH process, and the remaining TiN part was designed to create LSPR effect for its photocatalytic activity under visible light illumination. During this VPH process, nitrogen-doped TiO₂ might also be produced, especially on the interface between TiO₂ and TiN. Thus, it is critical to examine if the TiN/TiO₂ composite photocatalyst could possess the visible-light-activated photocatalytic activity when the potential nitrogen-doping effect was excluded. A 700 nm band-pass filter was chosen to provide a visible light illumination with wavelength at 700 ± 15 nm, which was far away from the visible light absorbance of nitrogen-doped TiO₂ (usually less than 500 nm).¹² RhB was used as the model organic pollutant, which could not have the photosensitization effect under 700 nm visible light illumination. Fig. 6a summarizes the residue RhB concentration vs. treatment time by the TiN/TiO₂-5 sample, compared with the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample under 700 nm visible light illumination. When treated by the TiN/TiO₂-5 sample, RhB concentration continuously decreased with a stable degradation rate and over 40% RhB were degraded after 5 h treatment. Thus, the TiN/TiO₂-5 sample clearly demonstrated the photocatalytic activity under 700 nm visible light illumination, which should come from the LSPR effect of TiN and could not happen from the nitrogen-doping effect. The slower degradation rate demonstrated in this experiment may be attributed to the much lower light intensity of ∼3 mW cm⁻². When treated by the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample, less than 5% RhB were degradaded after 5 h treatment, which clearly demonstrated the close contact between TiN and TiO₂ was critical to obtain an efficient TiN/TiO₂ composite photocatalyst.

Fig. 6 (a) Residual RhB concentration vs. treatment time under 700 nm visible light illumination by the TiN/TiO₂-5 sample and by the mixture of TiN/anatase TiO₂ nanoparticles with the same TiN/TiO₂ ratio as the TiN/TiO₂-5 sample. (b) Survival ratio of E. coli cells vs. treatment time under 700 nm visible light illumination in air and in enriched N₂ atmosphere by the TiN/TiO₂-5 sample, respectively.

Fig. 6b shows the survival ratio of E. coli cells vs. treatment time by the TiN/TiO₂-5 sample under 700 nm visible light illumination in air and in enriched N₂ atmospheres, respectively, to examine the role of oxygen in the photocatalytic activity of the TiN/TiO₂ composite photocatalyst. In the air atmosphere, the E. coli survival ratio dropped to ∼60% after 2 h treatment, while it only dropped to ∼80% in enriched N₂ atmosphere under the same experimental conditions. This result clearly demonstrated that oxygen played an important role in the production of radicals for the photocatalytic disinfection of E. coli cells. It is well known that photo-generated electrons could react with oxygen to form ˙O₂⁻ and subsequently ˙OH.⁵¹ This result provided a further evidence for the LSPR effect of TiN, which produced hot electrons to be injected into the conduction band of TiO₂ and react with oxygen to produce radicals for the subsequent photocatalytic reactions.

3.7 Mechanism of the photocatalytic activity of TiN/TiO₂ composite photocatalysts under visible light illumination

Fig. 7a shows the proposed mechanism of the photocatalytic activity of TiN/TiO₂ composite photocatalyst under visible light illumination. As demonstrated by both theoretical and experimental studies, TiN nanostructures could serve as the plasmonic component in this composite photocatalyst to absorb visible light illumination and produce hot electrons by the LSPR effect. As an n-type semiconductor, TiO₂ has a good electron-accepting capability due to the high density of states in its conduction band. In our synthesis approach, TiO₂ nanoparticles were grown in situ on TiN nanoparticles to form good contact between TiN and TiO₂, which was beneficial for electron transfer between them. Thus, these hot electrons could then be injected quickly into the conduction band of TiO₂, and oxygen could react with these hot electrons to produce ˙O₂⁻ radicals in aqueous solution and subsequently produce ˙OH radicals. These highly reactive radicals could effectively degrade organic pollutants and disinfect microorganisms.

Fig. 7 (a) The proposed mechanism of the photocatalytic activity of TiN/TiO₂ composite photocatalyst under visible light illumination. (b) The residue RhB concentration vs. treatment time under visible light illumination by the TiN/TiO₂-5 sample when the ˙O₂⁻ scavenger (BQ) and ˙OH scavenger (TBA) were present in the reaction solution, respectively. (c) The schematic diagram of the energy alignment between TiN and TiO₂.

Fig. 7b summarizes the residue RhB concentration vs. treatment time under visible light illumination by the TiN/TiO₂-5 sample when the ˙O₂⁻ scavenger (BQ) and ˙OH scavenger (TBA) were present in the reaction solution, respectively.⁵² After the addition of BQ (1 mg) into the reaction solution, the photocatalytic degradation of RhB by the TiN/TiO₂-5 sample under visible light illumination was largely inhibited. After the addition of TBA (2 mL) into the reaction solution, the photocatalytic degradation of RhB by the TiN/TiO₂-5 sample under visible light illumination was weakly decreased. This observation suggested that both ˙O₂⁻ and ˙OH radicals were produced during the photocatalytic process as we proposed, and ˙O₂⁻ radicals played the major role in the photocatalytic process by these TiN/TiO₂-X composite photocatalysts under visible light illumination.

Fig. 7c shows the schematic diagram of the energy alignment between TiN and TiO₂, and the Fermi energy of Au was also presented for comparison purpose. When metal and n-type semiconductor contact, a barrier will usually form at their interface. The energy of the barrier could be calculated as φ = φ_M − χ_S, where φ_M is the work function of the metal and χ_S is the electron affinity of the semiconductor. Electrons in the metal could be injected into the semiconductor if their energy is higher than φ.^53,54 For example, the work function of Au is ∼5.1 eV,⁵⁵ while TiO₂ has a work function of ∼4.50 eV and an electron affinity of ∼4.2 eV.^56,57 So the calculated energy of the barrier formed between Au and TiO₂ is ∼0.9 eV. Thus, to overcome this barrier to be injected into TiO₂, hot electrons in Au should has an energy of at least 0.9 eV above its Fermi energy level, which means that part of excited electrons could not be injected into TiO₂ because they may not have energy of 0.9 eV or more over its Fermi energy level.⁵³ The work function of TiN is ∼3.7 eV,⁵⁸ so the calculated energy barrier formed between TiN and TiO₂ is negative (−0.5 eV), which is quite different with previous reported noble-metal-based plasmonic photocatalysts. Thus, the mechanism of the visible light photocatalytic activity of the TiN/TiO₂ composite photocatalyst was verified to the LSPR effect of nanostructured TiN, while no energy barrier was found to exist between TiN and TiO₂. Once electrons in TiN are excited above the Fermi energy level, these hot electrons could be completely injected to TiO₂, resulting in better electron injection efficiency than previous reported noble-metal-based plasmonic photocatalysts. This could still be likely to hold even when nitrogen-doped TiO₂ was formed on the TiO₂/TiN interface because nitrogen-doping usually formed additional bands from N 2p states into the band gap of TiO₂ and did not change the conduction band position of TiO₂.

4 Conclusions

In summary, a novel plasmonic TiN/TiO₂ composite photocatalyst with visible-light-activity was synthesized successfully by a fluorine-free, vapor-phase hydrothermal process to in situ grow TiO₂ on TiN nanoparticles with HNO₃ as the oxidant. TiN nanoparticles could server as the plasmonic component to harvest visible light, generate hot electrons, and inject them into the conduction band of TiO₂ for the observed good visible light photocatalytic activities on both the photodegradation of organic pollutants and the disinfection of microorganism. The close contact between TiN and TiO₂ phases in this composite photocatalyst was beneficial to electron transfer between them. Hot electrons could be completely injected to TiO₂ from TiN once they were excited above the Fermi energy level of TiN. Thus, besides its much lower cost from its non-noble metal nature and its high stability, better electron injection efficiency could be achieved in this novel TiN/TiO₂ composite photocatalyst than previous reported noble-metal-based plasmonic photocatalysts. This study suggested that novel plasmonic photocatalysts without noble metal components could be developed to possess features that traditional noble-metal-based plasmonic photocatalysts could not have, which have great potentials for various environmental applications.

Acknowledgements

The authors would like to thank Ms Mian Song and Ms Shuang Jiao of Environment Functional Materials Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences for the experimental assistance on E. coli culture. This study was supported by the National Natural Science Foundation of China (Grant no. 51102246), the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant no. Y2N5711171), the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant no. Y4N56R1161), and the Young Scholar Program of Shenyang National Laboratory for Materials Science (Grant No. Y5N56F2161).

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Footnote

† These authors contributed equally to this work.

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