Photocatalytic activity of Ag nanoparticle-dispersed N-TiO₂ nanofilms prepared by magnetron sputtering

Abstract

Using magnetron sputtering, pure TiO₂ nanofilms and Ag/TiO₂ bilayer nanofilms doped with nitrogen were deposited on glass substrates. Heat treatment of the Ag/N-TiO₂ nanofilms at 400 °C led to the formation of Ag nanoparticles, which were dispersed inside the TiO₂ films as well as on the free surface of the TiO₂ films. The photocatalytic activity of the Ag/N-TiO₂ nanofilms with dispersed Ag nanoparticles was examined by UV-vis diffuse reflectance spectroscopy. The rate constants for the photodegradation of methylene blue (MB) in aqueous solutions of MB by N-TiO₂-based nanofilms are about one order larger than those for self-degradation or for pure TiO₂ nanofilms. The rate constants for the photodegradation of aqueous solutions of MB by the Ag/N-TiO₂ nanofilms are larger than those for the N-TiO₂ nanofilms. The Ag nanoparticles improve the photocatalytic activity of TiO₂ films, possibly through the surface plasmon absorption effect of Ag nanoparticles, which activates photo-generated charge carriers through the transfer of plasmonic energy.

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1. Introduction

TiO₂-based nanostructures have potential in various applications, including photocatalytic processes,^1–4 lithium-ion batteries,^5–8 solar cells,^9–11 and supercapacitors.¹² At the heart of the photocatalytic processes of TiO₂-based nanostructures is the activity that leads to the formation of photo-induced electron–hole pairs to activate a series of chemical reactions, resulting in the mineralization of pollutants.^13,14 However, the fast recombination of the photogenerated electron–hole pairs,¹⁵ large optical-band gap (3.2 eV for anatase and 3.0 eV for rutile),¹⁶ low electron mobility¹⁷ and small diffusion length of the minority carrier (hole) (10–100 nm)^18,19 have limited the applications of TiO₂.

To increase the potential applications of TiO₂-based nanostructures, various methods, including the doping of non-metallic or metallic elements,^20–23 have been developed to extend the work spectrum of TiO₂ to the visible (VIS) and infrared (IR) regions, and to enhance the electron–hole separation. Noble metal nanoparticles, such as gold (Au) and silver (Ag) have been used to improve the photocatalytic efficiency under the irradiation of visible light. Kannaiyan et al.²⁴ used amphiphilic poly(styrene-block-ethylene oxide) diblock copolymer (PS-b-PEO) micelles loaded with AgNO₃ and TiO₂ sol–gel precursors as templates to fabricate hybrid Ag/TiO₂ nanodot arrays and observed that the Ag/TiO₂ nanodot arrays significantly increased photocatalytic degradation of methylene blue (MB) in comparison with pure TiO₂ nanoparticle arrays. Xu et al.²⁵ found an enhancement of the light absorption of the TiO₂/SiO₂ bilayer with the implantation of Ag nanoparticles in the SiO₂ layer in comparison with TiO₂ films, which is likely due to the plasmonic effect near the interface of TiO₂ and silica glass.

TiO₂-based composites with nitrogen doping have been intensively studied. Yang et al.²⁶ investigated the photocatalytic activity and stability of N-doped anatase TiO₂, using the decomposition of MB and methyl orange (MO) as model reactions under the irradiation of visible light, and showed that nitrogen has a significant effect on the optical absorption of TiO₂. They suggested that both the degree of N doping and number of oxygen vacancies contribute to the visible light absorption of the N-doped TiO₂ materials. Li et al.²¹ showed the good photocatalytic activity and high stability of hierarchical N-doped TiO₂ microspheres with exposed (001) facets [N-TiO₂-(001)] under visible light irradiation in comparison with the commercially available P25 TiO₂. Such good photocatalytic activity for N-doped TiO₂ is likely due to the formation of intermediate energy levels in the band gap from either the mixing of N 2p with O 2p states or the presence of localized states.^27,28

Currently, the preparation of doped TiO₂ structures has been mainly based on solution-based techniques. There are various issues that need to be improved, including reproducibility, impurities, and the removal of residuals. To fully utilize the good photocatalytic activity of N-doped TiO₂ nanostructures and limit the concerns involving the use of solution-based techniques, different techniques need to be developed to prepare N-doped TiO₂ nanostructures. It is known that sputtering a metal of high purity on the surface of a substrate can lead to the formation of nanoparticles under certain conditions. This work explores the possibility of preparing N-doped TiO₂ nanofilms with dispersed Ag nanoparticles, using the magnetron sputtering technique. This approach offers several advantages over solution-based techniques, including scalability, high purity, accurate control of metal loading, and a solution-free synthesis. It limits the use of a reducing agent such as sodium borohydride, washing treatment to remove residuals, or a high temperature reduction treatment. The characterization of the microstructures of the prepared N-doped TiO₂ nanofilms was performed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The photocatalytic activity of the prepared N-doped TiO₂ nanofilms was investigated by measuring the degradation behavior of the MB organic compound.

2. Experimental detail

Preparation of thin films

Ag nanofilms on glass substrates were prepared at room temperature (25 °C) by magnetron sputtering in a multifunctional magnetron sputtering instrument (JGP560B) under high vacuum conditions, using an Ag target. Prior to sputtering, the glass substrates (2 × 2 cm²) were ultrasonically cleaned with acetone, ethyl alcohol, and deionized water for 15 min, respectively. Argon gas was flowed into the sputtering chamber after the pressure of the sputtering chamber reached 8 × 10⁻⁴ Pa. Pre-sputtering for 3 min was performed to remove surface residues. During the sputtering, the flow rate of argon gas was 30 sccm, and the chamber pressure was 6 Pa. The distance between the glass substrate and the target was 30 mm, and the thicknesses of Ag nanofilms were controlled by the sputtering time.

After the deposition of Ag nanofilms, both argon gas and nitrogen gas were flowed simultaneously into the sputtering chamber to prepare N-doped TiO₂ nanofilms after the pressure of the sputtering chamber reached 8 × 10⁻⁴ Pa. The volume ratio of Ar to N₂ was 1 : 1, and the flow rate of each individual gas was 15 sccm. The Ag/TiO₂ nanofilms without nitrogen were also prepared, using a similar approach. The prepared nanofilms were heat-treated at 400 °C for 3 h at a ramping rate of 2 °C min⁻¹ in a tube furnace in air.

Materials characterization

The surface topology of the prepared nanofilms was examined using Field Emission Scanning Electron Microscopy (FESEM) (Tescan MIRA 3 LMH) at 10 kV; the composition of the nanofilms was determined using Energy Dispersive Spectroscopy (EDS) (OXFORD X-Max 20). The crystal structures of the nanofilms were analyzed with the Cu Kα line on a Rigaku D/max 2500 X-ray diffractometer with patterns recorded in a range of 20–80°. Optical absorption of the nanofilms in the wavelength range of 300–800 nm was measured using a UV-vis spectrophotometer (HITACHI: U-3900). The banding energy was determined using X-ray photoelectron spectroscopy (XPS) (Escalab 250).

Characterization of photocatalytic activity

The photocatalytic activity of the prepared TiO₂-based nanofilms was evaluated by measuring the degradation behavior of the MB organic compound in an aqueous solution under the irradiation visible light. 50 mL of the aqueous MB solution (5 mg L⁻¹) was placed in a shallow, round glass vessel with a diameter of 4.5 cm and stirred in the dark at room temperature for 30 min. 3 square pieces of 20 × 20 mm² glass slide containing the nanofilms were placed in the vessel, which were suitable for the experimental conditions as they didn’t disturb the visible light entering the glass vessel. The glass vessel was placed in an ice bath to maintain a constant temperature during the characterization of the photocatalytic activity. A 300 W tungsten lamp (Philips Halogen) with a cutoff filter (λ ≧ 420 nm) at a distance of 20 cm above the solution was used to irradiate the aqueous MB solutions. The adsorption of MB on the surface of the TiO₂ nanofilms led to a decrease of MB concentration in the aqueous solution and a change in the absorbance of the MB aqueous solutions under the irradiation of visible light. To eliminate the effect of the background, the photo-induced reactions were performed after mixing the TiO₂ nanofilms with the MB solution in the dark for half an hour to reach a steady state. All the samples for the experiment of photocatalytic activity were calcined at 400 °C.

It is known that the irradiation of visible light can cause the reduction of the cationic form of MB and a color change from blue to colorless through accepting photo-catalytically generated electrons.²⁹ The photo-induced MB degradation was thus determined from the absorbance of the MB peak at a wavelength of 664 nm, using an UV/vis spectrophotometer (Perkin-Elmer, Lambda 35), from which the MB concentration, c, was estimated as,

c = A₀/A (1)

where A₀ is the absorbance of MB at the beginning of the irradiation of visible light, and A is the absorbance of MB at time t. The rate constant, k, is calculated as,

ln(c₀/c) = ln(A/A₀) = kt (2)

where c₀ is the concentration of the MB solution without the irradiation of visible light.

3. Results and discussion

X-ray diffraction analysis

X-ray diffraction (XRD) was carried out on Ag/N-TiO₂ nanofilms with Ag film thicknesses of 20, 50, and 80 nm, which are referred to as Ag 20 nm/N-TiO₂, Ag 50 nm/N-TiO₂ and Ag 80 nm/N-TiO₂, respectively. Fig. 1 shows the XRD patterns of the Ag/N-TiO₂ nanofilms calcined at 400 °C. All the nanofilms clearly display the peaks of TiO₂, including the planes of (101), (004), (200), (204), and (220), at 2θ values of ca. 25.3, 37.8, 48.1, 62.8, and 68.7°, respectively. This result suggests that there is only the anatase phase present in the Ag/N-TiO₂ nanofilms calcined at 400 °C.

Fig. 1 XRD patterns of Ag/N-TiO₂ nanofilms calcined at 400 °C.

The peaks at 44.3 and 77.4° of the 2θ values are associated with the (200) and (311) diffractions of the cubic phase of Ag with a lattice constant of 4.0861 Å. This result suggests that Ag was present in the Ag/N-TiO₂ nanofilms. The peaks at 38.2 and 64.5° of the 2θ values of face-centered cubic Ag overlap the diffraction peaks of (004) and (204) at 37.8 and 62.6° of the 2θ values of TiO₂, respectively. It is difficult to detect the (111) and (204) planes of Ag from the XRD spectra.

XPS analysis

The chemical state of Ag in the Ag/N-TiO₂ nanofilms was characterized by XPS. Fig. 2a shows the Ag 3d fine XPS spectra of Ag/N-TiO₂ nanofilms. The binding energies for Ag 3d_5/2 and Ag 3d_3/2 levels are found to be ca. 367.5 and 373.5 eV, respectively, which suggests that the state of Ag is metal Ag(0)^30,31 and no silver oxide is present. Such a result is further supported by the O 1s spectra (Fig. 2b), in which no new peaks appear except the peaks at ca. 529.6 and 531.4 eV. The peak at ca. 529.6 eV is ascribed to the Ti–O bonds in the TiO₂ lattice, and the peak at ca. 531.4 eV is related to the oxygen in the surface hydroxyl groups (H–O bonds).^32,33 It is evident that the peaks of Ag 3d shift to lower positions in comparison with 368.3 eV for Ag 3d_5/2 and 374.3 eV for Ag 3d_3/2 of bulk Ag,³⁴ which suggests the migration of electrons from the TiO₂ nanofilms to the metallic Ag, and a strong interaction between the Ag nanostructures and TiO₂ nanofilms at the interface of the nano-heterostructures.

Fig. 2 XPS spectra of the Ag/N-TiO₂ nanofilms; (a) Ag 3d, (b) O 1s, (c) N 1s, and (d) Ti 2p.

It is known that nitrogen in TiO₂ plays a key role in changing the band gap, which determines the photocatalytic activity of TiO₂-based materials. Fig. 2c shows the XPS spectra of N 1s of pure TiO₂ and Ag 50 nm/N-TiO₂ nanofilms. The intensity of N of the Ag 50 nm/N-TiO₂ nanofilms is much stronger than that of the pure TiO₂ nanofilms, suggesting the presence of nitrogen with a binding energy of ca. 400 eV in the N-doped TiO₂ nanofilms. Nitrogen atoms occupy interstitial sites and form either Ti–N–O or Ti–O–N oxynitride bonds. Fig. 2d shows the Ti 2p spectra of the Ag/N-TiO₂ nanofilms. The two peaks at 458.5 and 464.2 eV correspond to the binding energies of the Ti 2p_3/2 and Ti 2p_1/2 levels, revealing the presence of the Ti(IV) state. For the pure TiO₂ nanofilms, the peaks in the Ti 2p spectrum associated with the binding energies of the Ti 2p_3/2 and Ti 2p_1/2 levels are at 457.8 and 463.5 eV,³⁵ slightly less than the corresponding binding energies of the Ag/N-TiO₂ nanofilms, suggesting the presence of the Ti(IV) state. This result confirms a lower electron density of the TiO₂ surface with the presence of Ag nanoparticles. There is a strong interaction between metallic Ag nanoparticles and TiO₂ in the Ag/TiO₂ nanofilms.

SEM and EDS analyses

Back-scattered electron (BSE) imaging was performed to identify the phases present in the Ag/N-TiO₂ nanofilms. Fig. 3 shows the BSE images, secondary electron images, and the corresponding EDS spectrum of the Ag/N-TiO₂ nanofilms. The EDS patterns, as shown in Fig. 3, display the peaks of Ti, O, Ag and N, confirming the presence of Ag and N elements in all the Ag/N-TiO₂ nanofilms.

Fig. 3 Back-scattered electron images, secondary electron images, and the corresponding EDS patterns of the Ag/N-TiO₂ nanofilms; (a) Ag 20 nm/N-TiO₂, (b) Ag 50 nm/N-TiO₂, and (c) Ag 80 nm/N-TiO₂.

In the BSE images, the white nanoparticles are Ag and the relatively dark nanoparticles are TiO₂. Ag nanoparticles are also present on the free surface of all the Ag/N-TiO₂ nanofilms, i.e. Ag atoms migrated through the TiO₂ nanofilm, moving from the Ag nanofilm between the glass substrate and the TiO₂ nanofilm to the free surface of the TiO₂ nanofilm. For the Ag 20 nm/N-TiO₂ nanofilms, the particle size of the Ag nanoparticles is in the range of 30–50 nm and the surface is relatively smooth. There is no observable difference in the particle sizes between the Ag 20 nm/N-TiO₂ nanofilms and the Ag 50 nm/N-TiO₂ nanofilms; while increasing the fraction of Ag leads to an increase in agglomeration of the Ag nanoparticles, the number of small Ag nanoparticles on the surface of the Ag 20 nm/N-TiO₂ nanofilms is less than that on the surface of the Ag 50 nm/N-TiO₂ nanofilms. Larger Ag nanoparticles are present on the surface of the Ag 80 nm/N-TiO₂ nanofilms, which can become the recombination center of electrons and holes and become detrimental to the photocatalytic activity. It is necessary to control the size of Ag nanoparticles in order to limit the recombination of electrons and holes.

Fig. 4 shows the back-scattered electron image, secondary electron image, and the corresponding EDS pattern of a scraped Ag 50 nm/N-TiO₂ nanofilm after heat treatment at 400 °C. It is evident that Ag nanoparticles were well dispersed in the TiO₂ nanofilm, which qualitatively supports the observation that Ag nanoparticles are present on the free surface of all the Ag/N-TiO₂ nanofilms. The heat treatment causes the migration of Ag atoms through the TiO₂ nanofilm. The EDS pattern shows the peaks of Ti, O, N and Ag, and no atoms from the glass substrate diffuse into the TiO₂ nanofilm during the heat treatment.

Fig. 4 Back-scattered electron image, secondary electron image, and the corresponding EDS pattern of a scraped Ag 50 nm/N-TiO₂ nanofilm after heat treatment at 400 °C.

Okumu et al.³⁶ observed the formation of Ag nanoparticles in TiO₂ films from TiO₂/Ag/TiO₂ sandwiched structures, which were prepared by direct current magnetron sputtering. They suggested that the formation of Ag nanoparticles in TiO₂ films is controlled by the impinging, energetic oxygen ions formed during the sputtering of the second layer of TiO₂ and is driven by the Ag/TiO₂ interfacial energy. Okumu et al.³⁷ later suggested that the formation of Ag nanoparticles in TiO₂ films involves three steps, (1) the oxidation of Ag upon reactive sputter deposition, (2) the dissociation of the oxide at high temperature, and (3) the formation of Ag nanoparticles by aggregation. However, they did not discuss the driving force for the migration of Ag atoms or Ag oxide in a TiO₂ film.

It is known that the diffusion flux of atoms or molecules is proportional to the gradient of chemical potential.^38,39 The chemical potential consists of the contribution of the concentration of a substance, the interfacial free energy and the mismatch strain energy. The interfacial free energy is proportional to the interface area, and the mismatch strain energy is proportional to the volume of the material and the square of the mismatch strain. During the magnetron sputtering, the mismatch strain can likely be introduced on the interface between the glass substrate and the Ag nanofilm, and on that between the Ag nanofilm and the TiO₂ nanofilm. The combination of the concentration gradient and the mismatch strain energy, if the contribution of the interfacial free energy is negligible, will cause the migration of Ag atoms from the Ag-rich region to the Ag-free region through the TiO₂ nanofilm. In addition, the TiO₂ nanofilm formed by the magnetron sputtering is amorphous. There are many open spaces, which allow the fast motion of Ag atoms and the nucleation and formation of Ag nanoparticles during heat treatment.

Photoluminescence (PL) analysis

It is known that the photoluminescence (PL) emission spectrum can be used to evaluate the efficiency of the trapping, migration, and transfer of charge carriers in order to understand the fate of electron–hole pairs in semiconductor particles.^40,41 Fig. 5 shows the PL emission spectra of the TiO₂-based nanofilms, in which the Ag/N-TiO₂ nanofilms were calcined at 400 °C. The positions of the emission peaks of the Ag/N-TiO₂ nanofilms in the PL spectra are similar to those of the TiO₂ nanofilms, while the emission intensity of the N-Ag/TiO₂ nanofilms is much smaller than that of the TiO₂ nanofilms. Such behavior indicates the efficient transfer of interfacial electrons from the conduction band of N-TiO₂ to the Ag nanoparticles, which act as electron sinks and suppress the recombination of photoinduced carriers.⁴² From Fig. 5, one can note that the Ag 50 nm/N-TiO₂ nanofilms have the lowest PL emission intensity. Thus, the electron–hole recombination rate in the Ag 50 nm/N-TiO₂ nanofilms is the smallest, since the recombination rate of the photoinduced electrons and holes is proportional to the emission intensity of photoluminescence.

Fig. 5 Photoluminescence emission spectra of TiO₂-based nanofilms (the Ag/N-TiO₂ nanofilms were calcined at 400 °C).

Optical properties of Ag/N-TiO₂ nanofilms

The UV-vis absorption of the TiO₂-based nanofilms was analyzed with a UV-vis spectrophotometer (HITACHI: U-3900). Fig. 6 shows the diffuse reflectance spectra of the TiO₂ nanofilm, N-TiO₂ nanofilm, Ag/TiO₂ nanofilm, and Ag/N-TiO₂ nanofilms, in which all the materials were calcined at 400 °C. The small fluctuations in the absorption spectra are likely due to the interferences occurring at the interface of the thin film and the glass substrate, as discussed by Li et al.⁴³ The pure TiO₂ nanofilms show the typical absorption of anatase with an intense transition in the UV region, which is due to the excitation of electrons from the valence band to the conduction band. The N-TiO₂ nanofilms possess both enhanced UV and visible light absorption in comparison with the TiO₂ nanofilms. The cut off of the absorption edge is at ∼375 nm (∼3.3 eV) for the TiO₂ nanofilms and ∼410 nm (∼3.1 eV) for the N-TiO₂ nanofilms. There is a shift in the band gap of the N-TiO₂ nanofilms towards the visible spectrum, which can be attributed to the presence of nitrogen.⁴⁴

Fig. 6 Diffuse reflectance spectra of the TiO₂ nanofilm, N-TiO₂ nanofilm, Ag/TiO₂ nanofilm, and Ag/N-TiO₂ nanofilms (all the materials were calcined at 400 °C).

Notably, all of the Ag/N-TiO₂ nanofilms exhibit a broad absorption within the visible spectrum from 400 to 700 nm. Such behavior is likely due to the surface plasmon resonance (SPR) absorption of Ag nanoparticles. The SPR of Ag nanoparticles extends the light absorption of the N-Ag/TO₂ nanofilms to longer wavelengths, increases the light scattering and activates photo-generated charge carriers through the transfer of plasmonic energy from Ag nanoparticles to the N-TiO₂ nanofilms.⁴⁵

In general, the doping of Ag in Ag–TiO₂ composites can cause a decrease of the band gap of TiO₂ due to the presence of localized energy levels,⁴⁶ which allows the excitation of electrons of lower energies from the valence band to these energy levels instead of to the conduction band. The decrease of the band gap can improve the conversion efficiency of solar energy and assist the formation of reactive oxygen species under the irradiation of visible light.⁴⁷ From Fig. 6, one can note that Ag/N-TiO₂ nanofilms exhibit stronger absorption of visible light than N-TiO₂ nanofilms, which confirms that nitrogen narrows the band gap of TiO₂. Such a decrease in the band gap will likely improve the photocatalytic efficiency. However, Ag nanoparticles as shown in Fig. 3 were formed inside and on the surface of the TiO₂ nanofilms. It is very difficult to ascertain the doping of Ag in TiO₂ nanofilms. The Ag nanoparticles likely act as electron scavengers to inhibit the motion of the electrons. It is worth mentioning that the Ag 50 nm/N-TiO₂ nanofilms exhibit strong light absorption in both UV and visible light regions. The Ag 50 nm/N-TiO₂ nanofilms will likely have better photocatalytic performance in both UV and visible light regions. Note that the Ag 50 nm/N-TiO₂ nanofilms also exhibit stronger absorption at ∼420 nm. The mechanism for such behavior is not clear; it might be due to the combination effect of the N doping and the localized surface plasmon resonance (LSPR) from a large amount of Ag nanoparticles of small sizes, as suggested by Wu et al.⁴⁸

As shown in Fig. 6, there is a broad absorption band at ∼580 nm. There are previous reports on a broad absorption band in the range of 400–800 nm. Yang et al.⁴⁹ observed a broad absorption in the range of 490–800 nm with a summit at ∼540 nm, and suggested that the LSPR effect of surface-deposited Ag(0) likely contributes to the enlargement of the optical absorption. They also suggested that the increase of the LSPR absorbance at ∼540 nm is due to the increase of the Ag loading amount and aggregation degree. Wu et al.⁴⁸ observed a broad absorption ranging from 400 to 800 nm with a peak at about 425 nm and ascribed this phenomenon to the surface plasmon resonance (SPR) effect of Ag. In general, it is expected that the Ag nanoparticles on the free surface of the TiO₂ films will introduce a local SPR effect, leading to a broad absorption band in the range of 400–800 nm, as observed in this work. The wavelength of the broad absorption band might depend on the size and surface characteristics of the Ag nanoparticles.

Photocatalytic activity

The photocatalytic activity of the TiO₂-based nanofilms was investigated by measuring the degradation behavior of the MB organic compound in an aqueous solution under the irradiation of visible light. Fig. 7 shows the temporal evolution of the normalized concentration of the MB organic compound in aqueous solution with various TiO₂-based nanofilms under the irradiation of visible light. Note that all the materials were calcined at 400 °C. In general, the use of TiO₂ nanofilms has improved the photo-degradation of the MB organic compound in aqueous solution under the irradiation of visible light as a result of the photocatalytic effect of TiO₂ and the photosensitization of MB.

Fig. 7 Temporal evolution of the concentration of the MB organic compound in aqueous solution with various TiO₂-based nanofilms under the irradiation of visible light (all the materials were calcined at 400 °C).

Under the experimental conditions, the temporal evolution of the concentration of the MB organic compound follows eqn (2). The photo-induced degradation of MB is a first-order reaction. From the regression of experimental data, the rate constant, k, can be determined. Fig. 8 shows the rate constants for the photo-induced degradation of MB in aqueous solution with different TiO₂-based nanofilms. It is evident that the rate constants for the degradation of aqueous solutions with N-TiO₂-based nanofilms are about one order larger than that for self-degradation and that for pure TiO₂ nanofilms. The photocatalytic activities of N-TiO₂ nanofilms are improved due to the decrease of the band gap induced by nitrogen. However, the rate constants for the degradation of MB aqueous solutions with the Ag/N-TiO₂ nanofilms are larger than that for N-TiO₂ nanofilms, suggesting that silver further improves the photocatalytic activity of TiO₂ nanofilms, possibly through the SPR effect of Ag nanoparticles which activates photo-generated charge carriers through the transfer of plasmonic energy. In addition, Ag can act as an e⁻ trap, reducing the recombination rate of h⁺/e⁻ pairs, which benefits the photocatalytic activity.

Fig. 8 Rate constants in the unit 10⁻³ min⁻¹ for the photo-induced degradation of MB in aqueous solution with different TiO₂-based nanofilms (all nanofilms were calcined at 400 °C).

Xu et al.⁵⁰ studied the effect of particle size of TiO₂ particles on the photo-induced degradation of MB in a suspended aqueous solution, and observed an increase of the adsorption rate and adsorbability of MB on suspended TiO₂ particles with the decrease in the particle size of TiO₂. In studying the size effect of Ag nanoparticles on the plasmonic photocatalytic behavior of TiO₂ thin films with Ag nanoparticle embedded-SiO₂ films, Oh et al.⁵¹ found that the TiO₂ films with a 7 nm thick Ag film has the best decomposition rate. From Fig. 8, one can note that the reaction constant for the photo-induced degradation of MB in aqueous solution with Ag 80 nm/N-TiO₂ nanofilms is less than that with Ag 50 nm/N-TiO₂ nanofilms. Such a result reveals the size effect of Ag nanoparticles on the photo-induced degradation of MB, since larger Ag nanoparticles are present on the surface of the Ag 80 nm/N-TiO₂ nanofilms (as shown in Fig. 3c) than those on the surfaces of the Ag 20 nm/N-TiO₂ and Ag 50 nm/N-TiO₂ nanofilms. The larger Ag nanoparticles enhance the recombination rate of electrons and holes and lead to the decrease of the photocatalytic activity of Ag 80 nm/N-TiO₂ nanofilms, which is qualitatively in accord with the size effect of Ag nanoparticles observed by Oh et al.⁵¹

To examine the stability of the TiO₂-based nanofilms in assisting the photodegradation of MB in aqueous solution under the irradiation of visible light, the TiO₂-based nanofilms were used repeatedly, and each cycle lasted 4 h. After each cycle, the residual concentration of MB was measured at a wavelength of 664 nm, using an UV/vis spectrophotometer (Perkin-Elmer, Lambda 35). Before starting each new cycle, the remaining solution was replaced with 50 mL fresh aqueous MB solution (5 mg L⁻¹).

The degradation rate of the TiO₂-based nanofilms is defined as (1 − c_f)/c₀, in which c_f is the residual concentration of MB after the test. Fig. 9 shows the variation of the degradation rate with the number of cycles. The degradation rate decreases with the number of cycles, suggesting that the efficiency of the TiO₂-assisted photodegradation of MB decreases with time, which is likely due to the interaction between the MB aqueous solution and the surface of the TiO₂ nanofilms. A significant drop in the photodegradation of MB in the MB aqueous solution with TiO₂ nanofilms after 4 cycles is observed. The degradation rates of the N-TiO₂ nanofilms, Ag 50 nm/N-TiO₂ nanofilms, and Ag 50 nm/TiO₂ nanofilms in the MB aqueous solutions are much larger than those of the TiO₂ nanofilms, and the Ag 50 nm/N-TiO₂ nanofilms have the largest degradation rate.

Fig. 9 Dependence of the degradation rate of the TiO₂-based nanofilms on the number of cycles for the photodegradation of MB (all nanofilms were calcined at 400 °C).

Table 1 lists the rate constants for the photo-induced degradation of MB in aqueous solution with different TiO₂-based nanofilms for different numbers of cycles. The rate constants decrease with the increasing number of cycles, qualitatively in accord with the behavior of the degradation rate of the TiO₂-based nanofilms in assisting the photodegradation of the MB aqueous solutions. However, the Ag 50 nm/N-TiO₂ nanofilms still have the largest rate constant for the photo-induced degradation of MB in aqueous solution in the 4^th cycle.

Table 1 Rate constants for the photo-induced degradation of MB in aqueous solution with different TiO₂-based nanofilms for different numbers of cycles

Number of cycles	Rate constant (k) (min^−1)
	Ag 50 nm/N-TiO2	Ag 50 nm/TiO2	N-TiO2	TiO2
1	6.39 × 10^−3	4.16 × 10^−3	3.37 × 10^−3	7.53 × 10^−4
2	5.12 × 10^−3	3.84 × 10^−3	2.86 × 10^−3	7.53 × 10^−4
3	3.91 × 10^−3	2.69 × 10^−3	1.85 × 10^−3	6.87 × 10^−4
4	2.66 × 10^−3	2.53 × 10^−3	1.63 × 10^−3	4.60 × 10^−4

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It is known that TiO₂ is an N-type semiconductor. The reaction in the MB aqueous solution under the irradiation of visible light involves the excitation of electrons from MB molecules adsorbed on the surface of the TiO₂ nanofilms from the valence band into the conduction band of the TiO₂ semiconductor.^52,53 The transferred electrons, which can be subsequently trapped by molecular oxygen and adsorbed on the surface of the TiO₂ film, form O˙⁻ to generate highly active ˙OOH and ˙OH radicals. The reaction of the ˙OOH and ˙OH radicals with the radical cation of MB leads to the formation of intermediates or completely mineralized product and the decomposition of MB. The results shown in Fig. 9 and Table 1 imply that there exist significantly irreversible reactions on the surface of the TiO₂ nanofilms, which reduces the number of excited electrons from MB molecules adsorbed on the surface of the TiO₂ nanofilms, which jump from the valence band into the conduction band of the TiO₂ semiconductor. The synergistic effect of N and Ag inhibits the irreversible reactions on the surface of the TiO₂ nanofilms, and there is a much lower decrease in the number of excited electrons from MB molecules adsorbed on the surface of the Ag 50 nm/N-TiO₂ nanofilms, jumping from the valence band into the conduction band of the TiO₂ semiconductor, than that on the surface of the TiO₂ nanofilms.

4. Conclusion

Pure TiO₂ nanofilms and Ag/TiO₂ bilayer nanofilms doped with N were deposited on glass substrates by magnetron sputtering. The heat treatment of the Ag/N-TiO₂ nanofilms at 400 °C led to the formation of Ag nanoparticles, which were dispersed inside the TiO₂ nanofilms as well as on the free surface of the TiO₂ nanofilms. The XRD patterns reveal that only the anatase phase in the Ag/N-TiO₂ nanofilms was formed at 400 °C. The XPS analysis suggests that there are migration of electrons from the TiO₂ nanofilms to the metallic Ag nanoparticles and a strong interaction between the Ag nanostructures and the TiO₂ nanofilms at the interface of the nano-heterostructures. Nitrogen reduces the electron–hole recombination rate in the Ag/N-TiO₂ nanofilms, which improves the photocatalytic activity of the TiO₂ nanofilms. The rate constants for the photo-induced degradation of the MB aqueous solutions with N-TiO₂-based nanofilms are about one order larger than that for self-degradation and that for pure TiO₂ nanofilms. The photocatalytic activities of N-TiO₂ and Ag/N-TiO₂ nanofilms are improved due to the decrease of the band gap induced by nitrogen and the immobilization of electrons by Ag nanoparticles. The rate constants for the photo-induced degradation of the MB aqueous solutions with the Ag/N-TiO₂ nanofilms are larger than those for N-TiO₂ nanofilms, suggesting that Ag nanoparticles further improve the photocatalytic activity of TiO₂ nanofilms, possibly through the SPR effect of Ag nanoparticles which activates photo-generated charge carriers through the transfer of plasmonic energy.

Acknowledgements

LW is grateful for the financial support from the National Natural Science Foundation of China (51301118, 51175363) and the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi province (2013108). FY is grateful for the support from the “Hundred-People-Plan” Program of Shanxi (2014). YL is grateful for the financial support from the Scientific and Technological Programs of Shanxi province (2015021066).

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