TiO₂–Au nanocomposite materials embedded in polymer matrices and their application in the photocatalytic reduction of nitrite to ammonia

Abstract

Titanium dioxide–gold nanocomposite materials ((TiO₂–Au)_nps) were prepared and embedded in methyl functionalized silicate sol–gel (MTMOS) and Nafion (Nf) matrices. The nanocomposite materials were characterized by UV-vis absorption spectra, scanning electron micrographs (SEM) and high resolution transmission electron micrographs (HRTEM). When gold nanoparticles were deposited on TiO₂ ((TiO₂–Au)_nps) and immobilized in MTMOS silicate sol–gel and Nafion matrices, the band-gap (E_bg) of the TiO₂ shifted to lower energy. During the photocatalytic experiment, the (TiO₂–Au)_nps incorporated polymer matrices improved the photocatalytic reduction of nitrite ions to ammonia, owing to the effective interfacial charge transfer process in the presence of a hole scavenger, oxalic acid. Further increase in the photocatalytic reduction of nitrite to ammonia was observed by incorporating a [Ni(teta)]²⁺ complex into the MTMOS/(TiO₂–Au)_nps or Nf/(TiO₂–Au)_nps film. The immobilization of (TiO₂–Au)_nps in a functionalized silicate sol–gel or Nafion matrix is advantageous for the preparation of solid-phase photocatalyst film and to design a solid–solution system leading to the physical separation of the catalyst from the solution and the products in contrast compared to the colloidal photocatalyst system. The Au_nps deposited on TiO₂ act as an e⁻ sink which promotes efficient interfacial electron transfer from TiO₂ to the substrate upon irradiation. The ultimate contact between (TiO₂–Au)_nps and Ni(II) complex in the film efficiently promotes the electron transfer from Au_nps to Ni(II) complex leading to the formation of an intermediate Ni(I) complex, which facilitates the efficient catalytic reduction of nitrite to ammonia.

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1. Introduction

Semiconductors and metal nanoparticles are attracting much attention because of their shape and size dependent optical, electronic and catalytic properties.^1–4 The properties of metal nanoparticles are significantly influenced by the external interactions with solid support materials, and they acquire new composite materials with novel catalytic properties.^5,6 Photocatalysis using semiconductor–metal nanocomposite materials and an advanced oxidation or reduction process have been developed in recent years.⁷ Guczi and coworkers⁸ reported the effective oxidation of carbon monoxide (CO) at the TiO₂-decorated Au/SiO₂ catalyst. Overbury and coworkers⁹ developed an effective method for the modification of silica mesoporous surfaces with TiO₂ and Au nanoparticles in a layer-by-layer arrangement and this material was found to be highly active for the catalytic oxidation of CO.⁹ The Au_nps act as an electron sink during the photoinduced charge separation process at the TiO₂ by promoting the interfacial charge transfer process and reducing the charge recombination rate. Hence, the TiO₂–Au nanocomposite materials find a wide range of applications in the field of photocatalysis,^10,11dye-sensitized solar cells,¹² photoelectrochemical cells^13,14 and sensors.¹⁵ Kamat and Dawson¹⁶ reported the preparation of gold-capped TiO₂ core–shell nanoparticles. These (TiO₂–Au)_nps promote the photocatalytic oxidation of thiocyanate ions due to the enhanced interfacial charge-transfer process.

The reduction of nitrite ions (NO₂⁻) is of considerable importance in the remediation of environmental pollutants and nitrogen fixation.¹⁷Nitrites are present in high concentrations in caustic radioactive waste from nuclear plants, and their reduction to gaseous products would very much diminish the volume of such waste.^18,19 Excess nitrite ions are detrimental to the human body and the higher nitrite concentration level in drinking water is fatal to infants under 6 months of age. In the human body, nitrite combines with hemoglobin in the blood to form methaemoglobin, and leads to a condition commonly known as “blue baby syndrome”. Further, nitrite is also converted into nitrosamine, which can cause cancer and hypertension. Studies on the photocatalytic reduction of nitrite are important and we have selected nitrite for the photocatalytic investigation.^20–22 In artificial photosynthesis, the aim is to mimic the ability of green plants and other photosynthetic organisms in their use of sunlight to make high energy chemicals. Ammonia (NH₃) is a key component in chemical fertilizers and also noted as an energy carrier for alkaline fuel cells. In the present situation of both food and energy crises, greater demand for NH₃ is forecasted. The reduction of nitrite to ammonia is of significant interest as a means of mimicking the nitrogen fixation system of green plants.²³ However, reports on the photocatalytic reduction of nitrite are scarce.^23–34 The reduction of nitrite has been studied electrochemically by using metalloporphyrin and B-doped diamond electrode,^24,25 biocatalytically²⁶ and photoelectrochemically by using Ag and CdS,^27,28 photocatalytically by using ZnS,²⁹TiO₂,^30–32[Ru(bpy)₃]²⁺³³ and metal phthalocyanines (MPCs).³⁴ Guan and coworkers³⁵ reported the photocatalytic reduction of nitrite to nitrogen using Ag-doped TiO₂. Transition metals such as Fe, Cr, Pd, Pt, Rh, Ru and Ag doped TiO₂ nanoparticles and sacrificial donors like methanol, ethanol, EDTA, oxalic acid, sodium oxalate, formic acid, triethanolamine, sucrose and humic acid are commonly employed to improve the photocatalytic reduction of nitrite to ammonia.^29–32,35 Among the above photocatalytic systems, many works have been reported on the physical separation of a catalyst from the substrate.^36–39 In a colloidal photocatalytic system, the separation of the catalyst, reactant and the product becomes complicated. The solid-phase photocatalyst film has several advantages over colloidal photocatalytic systems, such as no need to use filtration/centrifugation for the removal of the colloidal catalyst. Physical separation of a photocatalyst from the reaction solution can easily be achieved by using the solid-phase film photocatalyst and the product yield can also be improved.^36–39 The suitable use of polymer matrix film brings about preconcentration of the substrates in the solvent swollen film achieving the close proximity of the reactant with the catalyst.

In this paper, the preparation of (TiO₂–Au)_nps and [Ni(teta)]²⁺ (teta = C-meso 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) embedded in methyl functionalized silicate sol–gel and Nafion films and their photocatalytic activity towards the reduction of nitrite to ammonia are reported. Experiments demonstrated the effective interfacial charge transfer process and enhanced photocatalytic reduction of nitrite ions at the (TiO₂–Au)_nps nanocomposite when [Ni(teta)]²⁺ was incorporated into the film.

2. Experimental

2.1. Chemical reagents

Nafion (5 wt%, in a mixture of lower aliphatic alcohols and H₂O), methyl trimethoxy silane (MTMOS) and hydrogen tetrachloroaurate trihydrate (HAuCl₄·3H₂O) were received from Aldrich. Titanium(IV) isopropoxide was obtained from Sigma–Aldrich. Sodium nitrite, oxalic acid, 1,2-diaminoethane, perchloric acid, n-butanol, acetone, sodium borohydride, potassium hydroxide and nickel chloride trihydrate were purchased from Merck and used as-received. All other reagents are of analar grade and used as-received. All the solutions were prepared by using double distilled water.

2.2. Preparation of (TiO₂–Au)_nps nanocomposite materials

Five different suspensions of gold-capped TiO₂ nanoparticles ((TiO₂–Au)_nps) were prepared¹⁶ by keeping the TiO₂ concentration constant at 10 mM while varying the gold (HAuCl₄) concentration from 0.1 to 0.5 mM. Briefly, the TiO₂ nanoparticles were prepared by dropwise addition of 103 μL of 10% titanium isopropoxide in 2-propanol to 2 mL of double distilled water with vigorous stirring. Prior to addition of titanium isopropoxide the pH of water was adjusted approximately to 1.5 with HClO₄. The (TiO₂–Au)_nps suspensions containing [TiO₂] : [Au] molar ratios of 20 : 1, 25 : 1, 33 : 1, 50 : 1 and 100 : 1 were prepared. The (TiO₂–Au)_nps were prepared by adding a desired amount of HAuCl₄ solution to the colloidal TiO₂ suspension in water while stirring vigorously. The negatively charged [AuCl₄]⁻ adsorbed strongly on the positively charged surface of the TiO₂ nanoparticles. The solution was stirred for an additional 5 min and the reduction of [AuCl₄]⁻ was achieved by the dropwise addition of NaBH₄ (8–10 mM) until the color of the solution changed to wine red.

2.3. Preparation of silicate sol–gel and Nafion matrix embedded (TiO₂–Au)_nps

The (TiO₂–Au)_nps nanomaterials dispersed in a methyl functionalized silicate sol–gel (MTMOS) or Nafion (Nf) matrix were obtained by the following procedure. A homogeneous MTMOS silicate sol–gel solution was prepared according to the reported procedure⁴⁰ by using a mixture of EtOH/MTMOS/0.1 M HCl in the ratio 7.5 : 3.75 : 1.0 (v/v). A known volume of methyl functionalized silicate sol–gel (MTMOS) or 0.5% Nafion solution was mixed with a known amount of colloidal (TiO₂–Au)_nps by stirring for 5 min. The mixture solution immediately turned to purple color indicating the dispersion of (TiO₂–Au)_nps in the MTMOS silicate sol–gel or Nafion matrix.

2.4. Characterization techniques

Absorption and diffuse reflectance spectra of the TiO₂ and (TiO₂–Au)_nps were recorded using an Agilent 8453 diode array UV-visible spectrophotometer fitted with a Labsphere RSA-HP-8453 reflectance accessory. High resolution transmission electron microscopic (HRTEM) images of TiO₂ and (TiO₂–Au)_nps were recorded using a JEOL high resolution EM (JEOL 3010 model) operating at an accelerating voltage of 300 kV. The surface morphology of TiO₂ and (TiO₂–Au)_nps dispersed in MTMOS silicate sol–gel or Nafion films was analyzed using a HITACHI (Model S-3400) scanning electron microscope (SEM).

2.5. Photocatalytic studies

MTMOS/TiO₂, MTMOS/(TiO₂–Au)_nps or Nf/TiO₂, Nf/(TiO₂–Au)_nps were prepared by casting a known volume of MTMOS/(TiO₂–Au)_nps or Nf/(TiO₂–Au)_nps solution on a glass plate (1 cm²). The solvent was evaporated in air at room temperature for 4 h and the film was then immersed in water for 10 min. The thickness of MTMOS silicate sol–gel and Nafion films on the glass plate was calculated as 1 μm.^40,41 The [Ni(teta)]²⁺ (teta = C-meso 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) complex was prepared according to the reported procedure.⁴² The [Ni(teta)]²⁺ was adsorbed irreversibly onto the film and the amount of incorporated complex was calculated by measuring the decrease in the absorbance of [Ni(teta)]²⁺ (λ_max = 460 nm and ε₄₆₀ = 40 dm³ mol⁻¹ cm⁻¹)⁴³ solution before and after dipping the film into the solution. The MTMOS/(TiO₂–Au)_nps/Ni(II) or Nf/(TiO₂–Au)_nps/Ni(II) film was used as a photocatalyst for the photocatalytic reduction of nitrite to ammonia. Oxalic acid was used as a sacrificial electron donor.³¹

The MTMOS/TiO₂, MTMOS/(TiO₂–Au)_nps and MTMOS/(TiO₂–Au)_nps/Ni(II) or Nf/TiO₂, Nf/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps/Ni(II) film coated glass plates were dipped in a quartz cell containing a mixture of 1 mM sodium nitrite and 5 mM oxalic acid and then irradiated using UV-visible light. Before starting the illumination, nitrogen gas was purged into the reaction mixture for 30 min. The reaction mixture was slowly stirred at constant speed under illumination. A 450 W xenon lamp was used as the light source with a water filter cell (6 cm path length with pyrex glass windows) to cut off IR and UV-B radiations. This water filter cell transmitted light from ∼340 nm onwards. The distance between the light source and the photocatalyst coated glass plate was 60 cm. After 60 min of irradiation, the cell solution was tested for ammonia, hydroxylamine and hydrazine.⁴⁴ Only ammonia was found as the product during nitrite reduction. The ammonia concentration was determined using Nessler's reagent by careful spectroscopic analysis in the 400–430 nm wavelength range. A calibration curve was obtained by dissolving a standard amount of NH₄Cl in KOH and using Nessler's reagent (detection limit is 0.1 mg L⁻¹).⁴⁵

3. Results and discussion

3.1. Absorption spectra of (TiO₂–Au)_nps nanocomposite materials

The absorption spectra recorded for the colloidal TiO₂ and (TiO₂–Au)_nps solutions with various amounts of Au deposited on TiO₂ are shown in Fig. 1. The absorption spectra of (TiO₂–Au)_nps showed the characteristic surface plasmon resonance band (SPB) around 520 nm due to the presence of gold nanoparticles when compared to the bare TiO₂.^16,46 The absorption spectra of the colloidal (TiO₂–Au)_nps dispersed in MTMOS silicate sol–gel and in 0.5% Nafion solutions are shown in Fig. 2. The (TiO₂–Au)_nps in MTMOS silicate sol–gel and Nafion matrices showed the SPB around 523 and 525 nm, respectively. When the (TiO₂–Au)_nps were dispersed in MTMOS silicate sol–gel and Nafion matrices, a small red shift in the SPB was observed for (TiO₂–Au)_nps (Fig. 2) due to the interaction of (TiO₂–Au)_nps with the silicate sol–gel and Nafion polymer matrices. Diffuse reflectance spectra were recorded for the MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps films coated on a glass plate (with different amounts of Au on TiO₂) and are shown in Fig. S1 (ESI†). The SPB was observed around 525 nm due to the dispersed (TiO₂–Au)_nps in the matrix film and no aggregation was observed in the film state. This clearly reveals that the particle size in the MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps films was maintained both in solution and film states.

Fig. 1 Absorption spectra of (TiO₂–Au)_nps nanocomposite materials in aqueous solution. TiO₂ core concentration was maintained constant and the Au shell concentration was varied. The molar ratios of TiO₂ : Au are: 100 : 0 (a), 100 : 1 (b), 100 : 2 (c), 100 : 3 (d), 100 : 4 (e) and 100 : 5 (f).

Fig. 2 Absorption spectra of colloidal solutions of TiO₂ and (TiO₂–Au)_nps dispersed in an MTMOS silicate sol–gel network (A) and Nafion (B). The molar ratios of TiO₂ : Au are: 100 : 0 (a), 100 : 1 (b), 100 : 2 (c), 100 : 3 (d), 100 : 4 (e) and 100 : 5 (f).

3.2. HRTEM images of TiO₂ and (TiO₂–Au)_nps

The HRTEM images of TiO₂ and (TiO₂–Au)_nps are shown in Fig. 3. Well-dispersed and isolated TiO₂ particles with sizes typically in the range of 10 to 15 nm were observed (Fig. 3(A)). The well-dispersed particles with particle diameters of 5–20 nm were observed for the (TiO₂–Au)_nps with a TiO₂ : Au molar ratio of 33 : 1 (Fig. 3(B)). The HRTEM images revealed that both TiO₂ and (TiO₂–Au)_nps were spherical in shape. The lattice resolved HRTEM image of the TiO₂ nanoparticles clearly showed an interlayer spacing of 0.35 nm, which corresponds to the (1 0 1) plane of anatase TiO₂¹¹ (Fig. S2(A), ESI†). The size range of the gold particles was estimated using the TEM and lattice resolved TEM images of (TiO₂–Au)_nps (Fig. S2(B and C), ESI†) as 5–10 nm. The selected area electron diffraction (SAED) pattern of the (TiO₂–Au)_nps indicated the presence of both TiO₂ and Au in the nanoparticles (Fig. S2(D), ESI†).

Fig. 3 HRTEM images of TiO₂ (A) and (TiO₂–Au)_nps (B) nanomaterials.

3.3. Surface morphology of the photocatalyst films

The SEM images of TiO₂ and (TiO₂–Au)_nps dispersed in MTMOS and Nafion films are recorded and shown in Fig. 4. Fig. 4(A and B) shows the clear SEM images of MTMOS silicate sol–gel and Nafion films in the absence of TiO₂ and (TiO₂–Au)_nps. The MTMOS silicate sol–gel and Nafion matrices provide the necessary support to anchor the nanoparticles with homogeneous dispersion of the nanoparticles. Fig. 4(C and D) shows the dispersions of TiO₂ nanoparticles in MTMOS silicate sol–gel and Nafion films. Relatively larger sized particles were observed when TiO₂ nanoparticles were dispersed in silicate sol–gel film (Fig. 4C) when compared to the colloidal TiO₂ nanoparticles. Fig. 4D shows the highly dispersed TiO₂ nanoparticles in a Nafion matrix. The (TiO₂–Au)_nps dispersed in methyl functionalized silicate sol–gel showed rod like structure and highly porous structure was observed in the Nafion film (Fig. 4(E and F)). The interaction between the nanoparticles, when they are dispersed into the functionalized silicate sol–gel matrix film, will lead to the formation of larger sized particles than their primary building blocks (Fig. 4E). These aggregates are typically of sizes ranging in between 30 and 150 nm and represent the smallest particles which actually form the silicate sol–gel matrix. The situation is complicated further because the aggregates of the smaller particles together form the loosely-bound agglomerates as a result of solid-phase. The observed difference in the morphology (Fig. 4(C and D)) suggests the different environment provided by the methyl functionalized silicate sol–gel and the polyelectrolyte Nafion polymer to the TiO₂ and (TiO₂–Au)_npsnanoparticles. The uniform distribution with high density of nanoparticles was obtained by employing a different nature of solid support polymer matrices. We have estimated the porosity of the photocatalyst films by a ‘liquid-uptake method’.^47,48 The liquid-uptake method was employed to understand the penetration of substrates into the polymer film. In the present systems, the plain polymer films (MTMOS and Nafion) showed higher liquid uptake percentage, whereas a decrease in the liquid uptake percentage was observed for the TiO₂ dispersed in polymer films (MTMOS/(TiO₂)_nps and Nf/(TiO₂)_nps) (Table S1, ESI†). When the (TiO₂–Au)_nps nanomaterials were incorporated into the MTMOS or Nafion film, the liquid uptake percentage increased again. This observation indicates that the porosity of the film was maintained when the (TiO₂–Au)_nps were dispersed in the matrix film.

Fig. 4 SEM images of MTMOS silicate sol–gel (A), Nafion (B), MTMOS/TiO₂ (C), Nf/TiO₂ (D), MTMOS/(TiO₂–Au)_nps (E) and Nf/(TiO₂–Au)_nps (F) films.

3.4. Influence of the Au shell on the band-gap energy of TiO₂

The band-gap energies of TiO₂, (TiO₂–Au)_nps and silicate sol–gel and Nafion matrices embedded TiO₂ and (TiO₂–Au)_nps were calculated by extrapolating the linear portion of the plot of (αhν)^1/2vs. hν.^49,50A band-gap energy of 3.35 eV for TiO₂ and the band-gap energies of 3.26, 3.15, 3.02, 2.94 and 2.86 eV for (TiO₂–Au)_nps nanocomposites with TiO₂ : Au molar ratios of 100 : 1, 50 : 1, 33 : 1, 25 : 1 and 20 : 1, respectively, were calculated as shown in Fig. 5. The calculated E_bg values showed a decrease with higher capping of Au on TiO₂ for the (TiO₂–Au)_nps (Fig. 5(a)). The band-gap energy of TiO₂ nanoparticles dispersed in MTMOS silicate sol–gel (3.39 eV) showed a gradual decrease i.e. 3.31, 3.24, 3.13, 3.03 and 2.96 eV with increasing the amount of Au on TiO₂ (Fig. 5(b)). It has been reported that the interaction of silica with TiO₂ nanoparticles increases the band-gap energy from 3.15 eV to 3.42 eV.^49,50 This is mainly due to the interaction of (TiO₂–Au)_nps with the polymer matrix. It has already been reported^51,52 that the polymer matrix could influence the electronic properties of the TiO₂ catalyst such as the quantum size effect and solid support matrix effect. Lassaletta et al. reported⁵⁰ that the band-gap energy of the titania–silica catalyst is influenced due to the combination of the quantum size effect and support matrix effect. The band-gap energy of TiO₂ nanoparticles dispersed in the Nafion matrix (3.36 eV) showed a gradual decrease i.e. 3.27, 3.18, 3.08, 2.98 and 2.89 eV with different amounts of Au (Fig. 5(c)). A small blue shift in the band-gap energy was observed for the TiO₂ and (TiO₂–Au)_nps dispersed in the silicate sol–gel or Nafion matrix due to the interaction of the polymer matrix with TiO₂ and (TiO₂–Au)_nps. The E_bg of the TiO₂ nanoparticles decreased with increased deposition of the Au content on TiO₂. In TiO₂, the conduction band is curved upwards, the valence band is curved downwards and the Fermi level is located at the mid band-gap. Deposition of Au_nps on TiO₂ particles leads to the formation of donor states just below the conduction band. At Au_nps concentrations higher than a certain value, free electron properties are exhibited and shift the conduction band downwards and the valence band upwards.^53,54 Hence, the electron–hole pair can be generated under irradiation of longer wavelengths. The band-gap energies and their corresponding wavelengths of (TiO₂–Au)_nps, MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps are given in the ESI† (Table S2). The (TiO₂–Au)_nps dispersed in a silicate sol–gel or Nafion membrane were used to design the solid-phase photocatalytic system.

Fig. 5 Band-gap energies (E_bg) of (TiO₂–Au)_nps (a), MTMOS/(TiO₂–Au)_nps (b) and Nf/(TiO₂–Au)_nps (c). The molar ratios of TiO₂ : Au are: 100 : 0, 100 : 1, 50 : 1, 33 : 1, 25 : 1 and 20 : 1.

3.5. Photocatalytic reduction of nitrite to ammonia

It is desirable to use a solid-phase catalyst due to its ease of separation from solution, substrate and the reaction product. The use of TiO₂ nanoparticles for the photocatalytic reduction of NO₂⁻ has attracted considerable attention.^30–32 In the present investigation, the photocatalytic activity of the (TiO₂–Au)_nps dispersed in MTMOS silicate sol–gel and Nafion matrices and their application towards the reduction of nitrite to ammonia were investigated. The schematic representation of the photocatalytic reduction of NO₂⁻ using (TiO₂–Au)_nps is shown in Fig. S3 (ESI†). The photocatalytic study was carried out by dipping the MTMOS/(TiO₂–Au)_nps or Nf/(TiO₂–Au)_nps film into a solution containing nitrite and oxalic acid. Fig. 6 shows the amount of ammonia formed during the photocatalytic reduction of NO₂⁻ at TiO₂ and (TiO₂–Au)_nps immobilized in MTMOS silicate sol–gel or Nafion films upon irradiation. The photocatalytic reduction of nitrite ions to ammonia depends on a multitude of factors such as light irradiation time, concentration of catalyst, concentration of substrate and nature of the solid-support. The amounts of ammonia were estimated for the MTMOS silicate sol–gel and Nafion matrix supported TiO₂ and (TiO₂–Au)_npscatalysts and the results are summarized in Table S3 (ESI†).

Fig. 6 Amounts of NH₄OH obtained at different concentrations of Au on TiO₂ when dispersed in MTMOS silicate sol–gel (A) and Nafion (B) films. The molar ratios of TiO₂ : Au are given in the figure.

3.6. Influence of Au_nps on the photocatalytic activity of TiO₂ nanoparticles

Fig. 6 shows the catalytic activity of the (TiO₂–Au)_nps when compared to that of the TiO₂. It is reported that the doping of noble metals in TiO₂ in the colloidal form enhances the photocatalytic reduction of nitrite and nitrate ions.^30,35 In the present system the Au_nps shell enhanced the charge separation at the (TiO₂–Au)_nps by accumulating the electrons thereby decreasing the charge recombination process. The larger amount of Au_nps shell might enrich more electrons which become new recombination centers for the photogenerated electrons and holes.¹⁶ The Nf/(TiO₂–Au)_nps photocatalyst exhibits higher photocatalytic activity towards the reduction of nitrite to ammonia. This may be due to the solvent swollen porous structure of the Nf/(TiO₂–Au)_nps film. The combination of the microheterogeneous nature of the Nafion membrane and the catalytic properties of the nanocrystalline (TiO₂–Au)_nps helps to develop the solid–solution interfacial photocatalytic system.⁵⁵ The relatively lower catalytic activity found with the MTMOS/(TiO₂–Au)_nps film when compared to the Nf/(TiO₂–Au)_nps film (Table S3, ESI†) may be attributed to the larger size of particles formed in the silicate sol–gel film. Photocatalytic reduction of nitrite to ammonia was carried out using both solid-state film photocatalysts (MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps) as well as the same photocatalysts in the colloidal form. The same amounts of (TiO₂–Au)_nps were taken in both colloidal solution as well as in film. The film photocatalysts showed better photocatalytic activity (twice higher) when compared to the colloidal photocatalysts towards the nitrite reduction to ammonia. When TiO₂ is capped with gold nanoparticles, the amount of ammonia formed is expected to increase due to the catalytic activity of Au_nps and efficient interfacial electron transfer process at the (TiO₂–Au)_nps. The literature shows that the excess loading of Au on TiO₂ leads to the light shield effect and the Au_nps act as new recombination centers for the photogenerated electrons and holes.⁵⁶ The existence of Au(I) and Au(0) together at the TiO₂ boundary in the (TiO₂–Au)_nps is advantageous at lower Au_nps concentration levels.¹⁴ The maximum amount of ammonia was found at the (TiO₂–Au)_nps with a TiO₂ : Au molar ratio of 33 : 1 dispersed in MTMOS silicate sol–gel and Nafion films.

3.7. [Ni(teta)]²⁺ catalyzed nitrite reduction at the solid-phase (TiO₂–Au)_nps photocatalyst

The macrocyclic nickel(II) complex, [Ni(teta)]²⁺, is used as an electron relay in the electrocatalytic and photocatalytic reduction of nitrite.^33,57 In the present investigation, the [Ni(teta)]²⁺ complex was incorporated into the MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps films (MTMOS/(TiO₂–Au)_nps/Ni(II) and Nf/(TiO₂–Au)_nps/Ni(II)) and used for the photocatalytic reduction of nitrite. Upon the irradiation of TiO₂ and (TiO₂–Au)_nps/Ni(II) films, the photogenerated charge separation (e⁻⋯h⁺) leads to the efficient electron transfer from TiO₂ to Ni(II) through Au_nps. The Au_nps act as a sink for photoexcited electrons and then transfer the electrons to the Ni(II) complex leading to the formation of the Ni(I) complex. The Ni(I) complex on reaction with NO₂⁻ produces the [Ni(O⁻–N⁻–O⁻)] intermediate.^33,57 This intermediate [Ni(O⁻–N⁻–O⁻)] species undergoes successive reduction with the photoexcited electrons and finally produces [Ni(teta)]²⁺ and NH₄OH (Fig. 7).^33,57 The oxalic acid acts as a sacrificial electron donor by scavenging the photoexcited holes generated at the TiO₂. Under the given experimental conditions, the amounts of NH₄OH produced by the photocatalytic reduction of NO₂⁻ at the (TiO₂–Au)_nps/Ni(II)/NO₂⁻ system upon irradiation are summarized in Table S3 (ESI†).

Fig. 7 Schematic representation of the proposed photocatalytic reduction of NO₂⁻ at the MTMOS/(TiO₂–Au)_nps/Ni(II) and Nf/(TiO₂–Au)_nps/Ni(II) films. Ni(II) = [Ni(teta)]²⁺ and OA = oxalic acid.

The effect of the amount of Au_nps deposited on the TiO₂ nanoparticles on the photocatalytic reduction of nitrite to ammonia at the MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps films is shown in Fig. 8. The increase in the amount of Au_nps on TiO₂ increases the amount of ammonia formed and reaches a maximum at the TiO₂ : Au molar ratio of 33 : 1. The presence of the Ni(II) complex in the (TiO₂–Au)_nps film further increases the amount of ammonia formed (Fig. 6 and 8). Fig. 8 shows that the Nafion film acts as a better solid-support material for the photocatalytic reduction of nitrite to ammonia due to the microheterogeneous ionic environment coupled with the large pinholes present in the Nafion film. When compared to the metal doped TiO₂ colloidal photocatalyst,^30,32 the present polymer/(TiO₂–Au)_nps film showed better photocatalytic activity in reduction of nitrite to ammonia (Table S4, ESI†).

Fig. 8 Amounts of NH₄OH observed at different amounts of Au_nps deposited on TiO₂ in MTMOS/(TiO₂–Au)_nps/Ni(II) (A) and Nf/(TiO₂–Au)_nps/Ni(II) (B) films upon irradiation.

The turn over number (TON) of the Ni(II) catalyst in the film was calculated by knowing the amounts of Ni(II) complex in the film and NH₄OH as shown in Fig. 9. The TON was calculated by using eqn (1).

Fig. 9 Amounts of NH₄OH observed with different amounts of Ni(II) complex incorporated in the MTMOS/(TiO₂–Au)_nps/Ni(II) (A) and Nf/(TiO₂–Au)_nps/Ni(II) (B) films and the corresponding TONs.

All the [Ni(teta)]²⁺ catalysts may not involve in the photocatalytic reduction of nitrite to ammonia. However, the total concentration of [Ni(teta)]²⁺ in the film was considered for the calculation of TON (Fig. 9) and the actual TON of the Ni(II) catalyst might be much higher under the given experimental conditions. Fig. 9 clearly shows that the distribution of particular concentration of Ni(II) in the film is sufficient to control the electron transfer process leading to the efficient reduction of nitrite to ammonia.

4. Conclusion

The (TiO₂–Au)_nps nanocomposite materials embedded in functionalized MTMOS silicate sol–gel and Nafion matrices were prepared and characterized by absorption spectra, HRTEM and SEM. The influence of concentration of Au_nps on the band-gap energy of TiO₂ was studied in the absence and presence of silicate sol–gel and Nafion matrices. The (TiO₂–Au)_nps nanocomposite materials embedded in MTMOS and Nafion films were used as solid-phase photocatalysts for the reduction of nitrite to ammonia in the presence of oxalic acid as a hole scavenger. The photocatalytic activity increased with increasing amounts of Au_nps on TiO₂. A higher amount of ammonia (6.36 μmol) was observed when the Ni[(teta)]²⁺ complex was incorporated into the Nf/(TiO₂–Au)_nps film. The deposition of Au_nps on TiO₂ nanoparticles promotes an efficient interfacial electron transfer process and the Au_nps act as an electron sink thereby decreasing the charge recombination process. The introduction of a Ni[(teta)]²⁺ catalyst into the MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps films enhances the photocatalytic reduction of nitrite to ammonia. The present solid–solution photocatalytic system may also find potential applications in other photocatalytic reduction and oxidation reactions.

Acknowledgements

The financial support from the Department of Science and Technology (DST), New Delhi, is gratefully acknowledged. APK is a recipient of CSIR-Senior Research Fellow fellowship.

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Footnote

† Electronic supplementary information (ESI) available: The diffuse reflectance spectra of MTMOS/(TiO₂–Au)_nps and Nf/(TiO₂–Au)_nps films, lattice resolved HRTEM image of a TiO₂ nanoparticle, (TiO₂–Au)_nps and selected area electron diffraction pattern (SAED) of core–shell (TiO₂–Au)_nps, schematic representation of photocatalytic reduction of nitrite at core–shell (TiO₂–Au)_nps in the absence of [Ni(teta)]²⁺. See DOI: 10.1039/c1cy00298h

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