Low temperature synthesis of Ag@anatase TiO₂ nanocomposites through controlled hydrolysis and improved degradation of toxic malachite green under both ultra-violet and visible light

Abstract

The nanocomposite material of titania coated silver nanoparticles (AgNPs) was prepared by employing a combination of two different synthetic routes. The proposed strategy demonstrates the utilization of a radiation chemical route to synthesize natural biopolymer gum acacia capped AgNPs at a very low pH followed by the controlled hydrolysis of Ti-tetra isopropoxide (TiPP) at low temperature for better growth of the titania shell on the AgNPs. The formation of the hybrid Ag–TiO₂ nanocomposites was confirmed through UV-Vis spectroscopic analysis, which showed a red shift in the surface plasmon resonance (SPR) peak of the Ag NPs (about 15 nm) and a blue shift in the case of TiO₂ (about 10 nm) with concomitant reduction in the intensity of peak of the AgNPs at 410 nm. In addition, the synthesized materials were characterized by dynamic light scattering (DLS), Fourier transform infra-red (FTIR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD). The as-synthesized TiO₂ NPs and Ag@TiO₂ nanocomposites were subsequently applied to the photochemical degradation of the toxic dye molecule, malachite green (MG), chosen as a model pollutant. The apparent photocatalytic degradation rate constants in regard to the Ag@TiO₂ and TiO₂ nanomaterials were calculated to be 0.25 and 0.05 min⁻¹, respectively. The photocatalytic degradation rates of MG by the Ag@TiO₂ nanocomposites under visible light illumination were found to be nearly 42 times higher than that of the TiO₂ NPs implicating its great promise for the improved degradation of toxic materials such as azo dyes using visible solar light.

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Introduction

In recent times, environmental problems, such as air and water pollution, have made the impetus for sustained fundamental and applied research in the field of environmental remediation. In particular, referring to the treatment of dye-containing wastewater, there are many processes, such as incineration, biological treatment, ozonation, and adsorption on solid phases^1–3 that have been extensively carried out. The advanced oxidation process (AOP) is characterized by the formation of strong oxidizing OH˙ radicals onto the catalyst surface, which promote quantitative mineralization of a variety of organic micro-pollutants to carbon dioxide and water. However, these processes have some limitations. The incineration can generate toxic volatile compounds. The biological methods (oxic and anoxic) needs longer treatment times and in many cases leads to foul odours. Moreover, sometimes these compounds may inhibit bacterial development thus reducing their efficiency.^4–6 Ozonation processes deals with short ozone stability and the adsorption process results in phase transfer with the production of sludge. Amongst the others, photocatalytic degradation represents a promising area for engineered process investigation.

TiO₂-based materials (anatase, rutile and various titanates) are promising photocatalysts that show activity in the UV region and stability over a wide range of conditions. It is a well-established fact that the anatase phase of TiO₂ shows high photocatalytic activity towards the oxidative degradation of a variety of contaminants in water, wastewater, and air.^7–9 In addition, nanosized TiO₂ has been widely used due to its specific optical, electronic properties and chemical stability^10–12 because catalytic applications demand titania with a high surface area-to-volume ratio. Moreover, for photo-applications, the catalyst should absorb and not block or scatter incident radiation and generate charge carriers by band gap excitation. This is possible only with nano-sized semiconductor materials with suitable band gap energy. Therefore, owing to the enhanced molecular transport properties at their surface, it is evident that nano-sized materials are beneficial as photocatalysts.

Even though TiO₂ is currently being used as the most popular photocatalyst, its use has a few shortcomings. TiO₂ absorbs only 2–3% of the solar light impinging on the Earth's surface as it can be excited only under UV irradiation with a wavelength of less than 390 nm. This low quantum yield prevents its practical use under solar irradiation.^13–16 Moreover, a high rate of electron–hole recombination in TiO₂ nanoparticles results in low photocatalytic efficiency.^17,18

To overcome the drawbacks of TiO₂ nanoparticles for its important photocatalytic applications, numerous efforts have been made to modify the structural, optical, and electrical properties of TiO₂-based materials. Especially, studies have been recently performed to decelerate the electron–hole recombination rate and to extend the absorption range of TiO₂ into the visible range. For instance, some strategic methods have been developed to make the lifetime of photoexcited charge carriers much longer and narrow the band gap of TiO₂, including doping metal ions into the TiO₂ lattice,^19,20 doping of non-metal elements,^21,22 loading of noble metals,^23–25 deposition of noble metals^26,27 and dye photosensitisation on the TiO₂ surface.^28,29

Noble metals, such as Ag, Au, Pt, and Pd, deposited on a TiO₂ surface can enhance the photocatalytic efficiency because they act as an electron trap promoting interfacial charge transfer processes in the composite systems.³⁰ However, this type of catalyst structure, though effective, results in exposing the noble metal to reactants and the surrounding medium. Metals on the surface of the semiconductor will be easily corroded and dissolved. In addition, it is difficult to prevent the aggregation of Ag particles and keep uniform structures during the deposition process. An efficient way to overcome these drawbacks is to exploit a core–shell-type structure in which the noble metal particles are introduced as cores and the semiconductor such as TiO₂ as shells.³¹

It is believed that semiconductor-based hetero-structure photocatalysts are excellent candidates for the degradation of various organic pollutants. In the present study, malachite green (MG) (Basic Green 4; CI 42 000) was taken as a model pollutant. Recently, it has been reported that MG has carcinogenic and mutagenic activity on warm blooded animals. The US Food and Drug Administration has banned its use. However, because of its low cost, ready availability and effectiveness, this dye is still being used in many parts of the world.³² Barring extensive use in textile industry, it also has widespread use in the pharmaceutical industry. It is reported that during the manufacturing process, dyes are lost in wastewater and the loss can vary from 2–50%.³³ As a result, MG is found in high concentrations in MG related industrial wastewater. With facile biodegradation being impossible, MG is found in relatively low concentrations in surface and ground water. Thus, it is of particular concern to remove MG from wastewater in an efficient and cost-effective way before its release into the environment.

Nano-composites of semiconductor and optically active metallic nanostructures represent a promising alternative to conventional photocatalysts. Among metal–titania nanocomposites, there are several reports on Au@TiO₂ type synthesis,³⁴ but from economic point of view Ag@TiO₂ nanocomposites are more suitable materials for mass applications. Several methods can be found for the preparation of Ag modified TiO₂ nanohybrids in the literature.³⁵ However, these conventional synthetic methods still possess some disadvantages. For instance, the hydrothermal method and ultrasonic-assisted sol–gel techniques³⁶ require a long synthesis times, the addition of large amounts of acid, as well as high calcination temperatures, which results in the aggregation of particles. The photo-reduction method has a drawback as it cannot make a satisfactory dispersion of Ag particles in TiO₂ media, which markedly decreases the amount of active photocatalytic sites on the TiO₂ surface.³⁷ The major shortcoming of the microwave synthetic method³⁸ is the phase transformation of TiO₂ from anatase to rutile.

Herein, we present a synthesis strategy combining the radiation-induced production of gum acacia capped silver NPs at low pH and the controlled hydrolysis of a titania precursor at low temperature. It has also been shown that there was no leakage of Ag⁺ ions from the AgNPs during the evolution of the TiO₂ shells around the silver particles. Furthermore, the as-synthesized nanocomposite material seems very effective in the degradation of toxic dye molecules under visible light.

Experimental section

Materials

Titanium tetra-isopropoxide (TiPP) was obtained from Sigma-Aldrich, USA. Nitric acid (HNO₃), gum arabic (gum acacia) (>99% of purity), AgNO₃, and 2-propanol (isopropanol) of analytical reagent quality were purchased from Merck, India. Milli-Q water was used for all the preparations. All reagents were used as received without any further purification.

Synthetic procedure for the Ag@TiO₂ nanocomposites

To compare the photocatalytic activity of the TiO₂ NPs and Ag@TiO₂ nanocomposites, one should employ the same procedure in the synthesis of both the TiO₂ and Ag@TiO₂ nanomaterials. Herein, the Ag–TiO₂ nanohybrids were synthesized by a combination of a radiation chemical approach and controlled hydrolysis methods. In the first step, gum acacia capped Ag nanoparticles were synthesized following our gamma irradiation method.³⁹ Typically, silver nanoparticles of 4.5 nM concentration were prepared by irradiating a solution containing 0.2 mL of aqueous AgNO₃ (0.1 wt%) solution mixed with 2 mL of gum acacia solution (0.01 wt%) and 0.25 mL of 2-propanol (13 M) at a dose rate of 4.2 kGy h⁻¹ for 45 minutes. The method adopted in this study for coating the as-synthesized AgNPs with TiO₂ material utilizes a controlled hydrolysis process with modification.⁴⁰ Typically, the hydrolysis was carried out by adding 0.05 mL of TiPP dissolved in 4.5 mL of 2-propanol on 30 mL of the γ-radiolytically pre-synthesized aqueous AgNPs at pH 1.5 under a temperature of 2 °C with constant stirring overnight. This allows the TiO₂ shells to grow around the silver NPs avoiding the separate formation of TiO₂ NPs. Herein, the gum acacia made the AgNPs stable even at low pH 1.5 during the growth of the titania shell and acted as non-conductive barrier between the metal and semiconductor blocks.

Instrumentation

A Co-60 irradiation chamber obtained from the Board of Radiation and Isotope Technology, India (Model no. GC 1200) was used for uniform and homogeneous irradiation with gamma ray photons with an average energy of 1.25 MeV. The strength of the radioactive Co source in this chamber was about 2.3 kCi having an absorbed radiation dose rate of 4.2 kGy h⁻¹ at the sample compartment. UV-Vis spectra of the as-prepared Ag@TiO₂ nanocomposite were obtained on an UV-1601 PC (Shimadzu) spectrophotometer. The un-irradiated solution was taken as the blank and all samples were diluted six times before recording the measurements. The photocatalytic degradation experiments were carried out by mixing freshly synthesized TiO₂ NPs or Ag–TiO₂ nanocomposite samples with a solution of malachite green taken in a quartz cell and placed in the cell holder. Photons entered the sample chamber after passing through the grating monochromator with bandpass of 2.0 nm. For FTIR measurements, the precipitates of TiO₂ and Ag@TiO₂ nanoparticles were freeze-dried and pelletized along with dry KBr powder. The FTIR spectra of the samples were obtained on a Perkin-Elmer spectrometer (Spectrum GX) with a resolution of 2 cm⁻¹ over a scan range of 4000–400 cm⁻¹.

X-ray diffraction (XRD) measurements were carried out in reflection mode on a Rigaku diffractometer ULTIMA-III (Tokyo, Japan) operated at a voltage of 40 kV and a current of 30 mA with Cu K_α radiation (λ = 0.1546 nm). The morphology and particle sizes were also determined by transmission electron microscopy (TEM). The TEM images were taken on transmission electron microscope (FEI, Technai S-twin) with an accelerating voltage of 200 kV. A drop of the as-prepared sample was placed on a carbon coated copper grid (300 mesh) and dried before placing it into the TEM sample chamber. The particle size and distribution were also determined by dynamic light scattering (DLS) setup (Model DLS-nano ZS, Zetasizer, Nanoseries, Malvern instruments). The pH was measured with a Jenway 3345 pH cum ion meter.

Results and discussion

Surface plasmon resonance (SPR) originating from the coherent oscillation of the conduction band electrons induced by the interaction with the alternating field of electromagnetic radiation is characteristic of many metal nanoparticles such as silver, gold, and copper. Thus, the SPR band is a unique feature utilized in optical spectroscopic characterization. Herein, the UV-Vis spectroscopic results illustrate a red shift in the SPR peak of the AgNPs and a blue shift with regard to the TiO₂ characteristic peak with simultaneous reduction in the absorption of the Ag peak at 410 nm. This indicates the formation of the hybrid Ag@TiO₂ nanocomposites. We have obtained two different peaks in the UV-Vis spectra, one is the surface plasmon peak at ∼405 nm for the Ag NPs and another is at ∼290 nm for TiO₂. The building blocks are separated only by the non-conductive gum acacia capping agent that surrounds the Ag. Consequently, we observe that the spectral profile on the absorption of Ag NPs (Fig. 1) is not affected significantly by the titania layer due to the small direct contact area between the silver core and titania shell particles. The biopolymer gum acacia plays a pivotal role in the stabilization of the radiolytically synthesized Ag NPs under very low pH conditions followed by the controlled hydrolysis of Ti-tetra isopropoxide at low temperature for the improved growth of the titania shell on the silver nanoparticles during the synthesis process of the composite materials.

Fig. 1 The absorption spectra of the Ag NPs and titania coated Ag nanocomposites.

For spherical metal particles, plasmon absorption peak are influenced by two major factors:⁴¹ (i) the dielectric constant of the medium and (ii) the density of electrons in the metal cluster. However, it is difficult to obtain an accurate estimate of the electron density in a colloidal particle. We assumed the density of electrons per nanoparticle is constant because the silver nanoparticles reported in our earlier method are monodisperse. Therefore, the observed red-shift is mainly accounted for the presence of TiO₂ shell over the silver core metal (Fig. 2).

Fig. 2 The absorption spectra of bare TiO₂ NPs and the Ag@TiO₂ nanocomposites with two different concentrations of titania precursor.

The position of the plasmon absorption band can be discussed within the framework of the Drude model.^42,43 According to the Drude model, the surface plasmon peak position for a spherical particle depends on the refractive index of the surrounding medium⁴⁴ and the relationship between them can be can be expressed by the following equations.⁴⁵

λ_peak² = λ_p²(ε^∞ + 2ε_m) (1)

λ_p² = 4π²c²mε₀/Ne² (2)

where λ_p² is the bulk plasma wavelength in terms of electron mass, ε₀ is the vacuum permittivity, m is the effective mass of the free electron of the metal, e is the electron charge, N is the electron density, ε^∞ is the high frequency dielectric constant due to inter-band and core transitions (for silver ε^∞ = 4.9 ± 0.3) and ε_m is the medium dielectric constant, which is numerically equal to the square of the refractive index of the solvent (for water it is 1.78).

For example, the introduction of a TiO₂ shell would reduce the surface charge density, N on the Ag core. The decrease in N will lead to an increase in λ_p² (eqn (2)). It can be found from eqn (1) that the value of λ_peak increased following the enhancement of λ_p. Thus, the peak of the Ag core coated with TiO₂ shows a red shift when compared to those of the uncoated samples. Otherwise, the intensity of the absorption band of the Ag@TiO₂ colloid would reduce gradually with a decreasing proportion of AgNPs.

The typical absorption spectra due to the variation of titania precursor concentration over the Ag NPs are shown in Fig. 2. From an earlier report,⁴⁶ it may be reasonably assumed that an optimum loading of silver clusters is required for synthesizing the Ag@TiO₂ nanocomposites.

Otherwise, the excess silver islands can act as centres for the recombination of electron–hole pairs in the TiO₂ material.⁴⁷ Because of this reason, we maintained the Ag NPs concentration (6 mM) the same during the synthesis of the Ag@TiO₂ nanocomposites while the concentration of Ti-precursor was varied. It may be noted that Ag⁺ ions when added into bare TiO₂ NPs prepared by the abovementioned method cause turbidity in the mixture. Therefore, in our present synthesis method, the absence of turbidity in the resulting nanocomposite solution suggests that there was no leakage of Ag⁺ ions from the AgNPs during the controlled hydrolysis process. This hereby confirms that the AgNPs synthesized at pH 1.5 remain intact during the formation of the nanocomposite materials. To optimize the silver loading in the TiO₂ material, we varied the Ti-precursor concentrations. The optimization of the TiO₂ layer was carried out based on the UV-Vis spectroscopic measurements in terms of the peak ratio analysis and DLS data ​(Fig. 3). Measurement of the shell thickness was also in agreement with the TEM results. Thus, we found that the optimal impregnation of the AgNPs in TiO₂ material is achieved by maintaining the titania precursor concentration at around 5 mM. This is an essential and crucial step in synthesizing good quality Ag@TiO₂ nanocomposites to maximize the photocatalytic efficiency.

Fig. 3 The DLS histogram showing the sizes of AgNPs and the Ag@TiO₂ nanocomposites for the corresponding samples as described in Fig. 1.

However, upon increasing the amount of Ti-tetra isopropoxide, the precursor of the TiO₂ NPs, a red-shift of the peak at a wavelength of 290 nm occurred. It is well known that the introduction of the TiO₂ shell would reduce the surface charge density on the Ag core. Therefore, it is expected that with an increasing amount of Ti-tetra isopropoxide, the absorption of the plasmon peak at 405 nm would decrease. We observed a similar trend up to a critical amount of Ti-tetra isopropoxide. This suggests that for the growth of a larger band gap semiconductor layer on the surface of the metal nanoparticle, the growth conditions must be controlled such that no homogeneous nucleation would occur, but growth proceeds only on the surface of the nanoparticles. Therefore, the concentration of the growth species needs to be controlled so that the supersaturation is not high enough for nucleation, but high enough for growth.⁴⁸ The photocatalytic activity decreases with an increase in the thickness of the TiO₂ layer over the silver core.⁴⁹ The absorption coefficient (90 cm⁻¹) and refractive index (2.19) of TiO₂ at 380 nm indicates that thick TiO₂ layer appears to be opaque.⁵⁰ Thus, an appropriate thickness of TiO₂ layer on the Ag core is very essential to maximize the photocatalytic efficiency.

To control the supersaturation of the growth species, temperature can be a controlling parameter during the growth process.⁵¹ It was observed that addition of the titania solution at room temperature resulted in turbidity in the reaction mixture. Thus, in our present method, the precursor was added slowly at low temperature (around 2 °C) dropwise onto the Ag core nanoparticle solution. Lower temperature is also preferred for the growth on smaller nanoparticles, because the solubility and the supersaturation depend on the surface curvature. Herein, it may be emphasized that the rate of addition of the Ti-precursor solution onto the core nanoparticle solution is a very crucial step, because fast addition leads to the formation of TiO₂ particles separately instead of creating a layer on the surface of the AgNPs. The TEM image of the Ag@TiO₂ nanocomposites is shown in Fig. 4. The dark spots of the metal core show that these colloids are well separated by the capping layer of TiO₂.

Fig. 4 The typical TEM image of the Ag@TiO₂ nanocomposites.

When NaCl solution was added into the solution of the synthesized composite solution, no precipitation of AgCl was detected. This shows that more or less all the Ag colloids were stable within the TiO₂ layer.

To look into the attached layers of TiO₂ in the composite materials, FTIR spectral measurements were carried out. Fig. 5 shows the FTIR spectra of TiO₂ and the Ag@TiO₂ nanocomposites. The absorption between 500 and 900 cm⁻¹ originates from the Ti–O–Ti stretching vibration in titanium dioxide. The strong and broadband around 3450 cm⁻¹ signifies the stretching vibration of the hydroxyl groups of Ti–OH on the surface and the weak sharp peak at about 1625 cm⁻¹ was associated with the deformation vibration of the H–O–H bonds of the physically adsorbed water on TiO₂ layer. Moreover, the peaks at 2920, 2840 and 1160 cm⁻¹ correspond to the –CH and –COH groups of iso-propanol adsorbed on the surface of the TiO₂ shells.

Fig. 5 FTIR spectra of TiO₂ NPs and the Ag@TiO₂ nanocomposites.

To understand the phases of the starting materials and synthesized nanocomposite, X-ray diffraction analysis was undertaken. The XRD pattern of the as-synthesized Ag NPs, TiO₂ NPs and Ag@TiO₂ nanocomposite samples are shown in Fig. 6. The characteristic peaks at scattering angles (2θ) of about 38°, 44°, 64° and 78° correspond to the scattering from the (111), (200), (220) and (311) planes (JCPDS Card no. 04-0783) in the radiolytically synthesized AgNPs, respectively.⁴¹ These diffraction peaks represent the face centered cubic (fcc) crystalline phase of the silver NPs.^52,53 The XRD pattern of anatase TiO₂ NPs shows characteristic peaks at scattering angles (2θ) of about 26°, 38°, 49°, 55°, 56°, 63° and 71° corresponding to the scattering from the (101), (004), (200), (105), (211), (204) and (116) planes (JCPDS Card no. 75-1537), respectively.⁵⁴ The XRD pattern of the Ag@TiO₂ nanocomposites contains two phases, one is of the anatase phase of the TiO₂ shell and the other arises from the fcc phase of metal Ag core.⁵⁵ The broad anatase diffraction peaks suggest that the sizes of the TiO₂ NPs are small. However, the respective peaks are somewhat narrow in the case of the Ag@TiO₂ nanocomposites, which may indicate an increase in the shell thickness. It may be noted that the unknown peaks at (2θ) scattering angles of 27°, 33° and 47° disappeared when the sample was calcined at 400 °C.

Fig. 6 XRD pattern of Ag NPs, TiO₂ NPs and the Ag@TiO₂ nanocomposites.

Most applications require the titania well crystallized anatase phase (E_BG = 3.2 eV) compared to rutile (E_BG = 3.0 eV) due to its high photoactivity and photovoltaic properties.^56–58 This is because the conduction band position of anatase TiO₂ is more negative when compared to rutile, which results in the higher reducing power of anatase.⁵⁹

Determination of the photocatalytic activity

The as-synthesized TiO₂ NPs and Ag@TiO₂ nanocomposites were subsequently applied to look into their efficacy in the photochemical degradation of a toxic dye molecule, namely, malachite green (MG) as a model pollutant. The MG degradation studies were performed in a UV-Vis spectrometric arrangement at a given excitation wavelength at room temperature and the photon energy absorbed by the sample was determined using the Hatchard–Parker actinometry method.⁶⁰ The energy absorbed by the sample was 2.7 mJ cm⁻² s⁻¹ and 17.9 mJ cm⁻² s⁻¹ upon irradiation with photons at 285 nm and visible photons at 405 nm, respectively.

In the absence of the TiO₂ NPs and Ag@TiO₂ nanocomposites, malachite green itself does not degrade under the illumination of UV and visible light. Furthermore, we observed that there was no significant change in the concentration of MG in the presence of the TiO₂ and Ag–TiO₂ nanoparticles under dark conditions. Therefore, the necessary prerequisites for the efficient degradation of MG are the presence of UV-visible illumination and the TiO₂ or Ag@TiO₂ nanocomposites. The characteristic absorption peak of MG at λ = 617.5 nm was chosen to monitor the photocatalytic degradation process.

Fig. 7 shows the degradation of malachite green (MG) with respect to time of photo-irradiation with 265 nm light in the absence and in the presence of TiO₂ NPs and the Ag@TiO₂ nanocomposites. The degradation efficiency of the Ag@TiO₂ nanocomposite material was significantly higher when compared to that of the TiO₂ NPs. At a relatively low concentration of TiO₂ NPs or Ag–TiO₂ nanocomposites (about 1.08 mg mL⁻¹), the percentage degradation of MG is nearly 35% in case of the TiO₂ NPs and 44% in case of the Ag@TiO₂ nanocomposites under UV photo-irradiation for a given time of 30 min. At a relatively higher concentration of NPs (∼5.9 mg mL⁻¹), the degradation of MG reaches to 52% in the case of TiO₂ and 84% in the case of the Ag@TiO₂ nanocomposites under the same UV photo-irradiation for a time of 30 min. Thus, the synthesized composite material seems to be very effective in the degradation of toxic malachite green.

Fig. 7 The photocatalytic degradation of malachite green (MG) in the presence of TiO₂ or the Ag@TiO₂ nanocomposites.

It is observed that the degradation of organic dye molecules by the as-synthesized TiO₂ or Ag@TiO₂ photocatalysts follows pseudo-first order kinetics, which indicates the linear relationship between ln(C₀/C) versus time (eqn (3)).

ln(C₀/C) = kt + A (3)

where C is the concentration of dye at time t, C₀ is the initial concentration of dye molecules, k is the slope and the apparent reaction rate, and the intercept A is the value of ln(C₀/C) when irradiation was initiated. If there was no obvious dark adsorption on the surface of the photocatalysts, A would be zero.

Although the mechanism for the enhanced photocatalytic efficiency of these Ag@TiO₂ nano-hybrid systems compared to pure TiO₂ NPs under irradiation with ultra-violet photons is not yet well established, it may be thought of plasmon-mediated transfer of energy from Ag NPs to titania layer to increase the concentration of e⁻/h⁺ pairs in the system.⁶¹

Furthermore, we have investigated the degradation of MG irradiated with visible light (Fig. 8). At a relatively higher concentration of NPs (∼5.9 mg mL⁻¹), the degradation of MG reached 18.7% in the case of TiO₂ and 58.6% in the case of Ag@TiO₂ under photo-irradiation with 405 nm photons for a given time of 3 hours. The apparent photocatalytic degradation rate constants in the case of the Ag@TiO₂ and TiO₂ nanomaterials were calculated to be 0.25 and 0.05 min⁻¹, respectively.

Fig. 8 The photocatalytic degradation of malachite green (MG) by TiO₂ NPs and the Ag@TiO₂ nanocomposites under visible illumination at 405 nm.

In contrast to the observed degradation of MG with a high concentration (5.9 mg mL⁻¹) of TiO₂ and Ag@TiO₂, the degradation efficiency of Ag@TiO₂ under visible light is estimated to be 41.2 times higher than the TiO₂ nanoparticles, when a relatively low concentration (1.08 mg mL⁻¹) of the material were used. In general, it is always advisable to use a lower quantity of photocatalyst in the degradation process in view of its harmful effect on the environment, if any. Thus, it appears that the as-synthesized Ag@TiO₂ nanocomposite material can be a very suitable candidate for carrying out degradation on a large scale.

The property of surface plasmon resonance of the silver core facilitates the generation of photo-excited electrons at the core–shell interface of the nanocomposite material. From the surface of silver nano-core, the photo-generated electrons may excite the conduction band electrons of the TiO₂ nanoshell material. Subsequently, these electrons diffuse throughout the surrounding environment leading to the reactions that help degrade the toxic chemicals into simpler innocuous molecules.^62–64 This possible pathway has led to the efficient catalytic degradation of MG in the visible light region.

Conclusions

In the present study, we have shown the utilization of a radiation chemical method to synthesize natural biopolymer gum acacia capped AgNPs at a very low pH and its encapsulation with titania shell through the controlled hydrolysis of Ti-tetra iso-propoxide at low temperature. Herein, we intend to stress that the rate of addition of the growth solution onto the core nanoparticle solution is a crucial step for the formation of the core–shell nanocomposites. Furthermore, it has been demonstrated that there was no leakage of Ag⁺ ions in the synthesized material and this confirms that the AgNPs retain their structure in the proposed synthesis methodology. Basic understanding of the charge transfer process of the metal core@semiconductor shell systems is likely to benefit the development of next generation visible photocatalysts. We believe that similar strategies can be used with semiconducting materials that absorb in the visible region. A similar synthesis strategy may be extended to the preparation of complex functional nanostructures with controlled physicochemical properties for a variety of applications such as wastewater treatment.

Note added after first publication

This article replaces the version published on 18th May 2016, in which editorial errors caused Fig. 2 and 3 to be interchanged.

Acknowledgements

Transmission electron microscopy and XRD measurements were carried out at the Saha Institute of Nuclear Physics and Variable Energy Cyclotron Centre, Kolkata, respectively.

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