Effects of deposited ions on the photocatalytic activity of TiO₂–Au nanospheres

Abstract

Photocatalytic TiO₂–Au nanospheres (TiO₂–Au NSs, 206 ± 23.7 nm) have been prepared and used as catalysts for the photo degradation of methylene blue (MB) and for the reduction of Cr⁶⁺ to Cr³⁺. TiO₂ NSs are firstly prepared from titanium isopropoxide (TIP) via a solvothermal method. The TiO₂ NSs are then sequentially modified with poly-(sodium-4-styreneulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC), which then interact with Au NPs (15 ± 1.3 nm) as seeds. Through reduction of HAuCl₄ by ascorbic acid, core–shell structures of TiO₂–Au NSs are prepared. Under UV irradiation, TiO₂–Au NSs provide highly catalytic activity for the degradation of MB and reduction of Cr⁶⁺ within 15 and 60 min, respectively. The TiO₂–Au NSs relative to commercial TiO₂ (P25) and TiO₂ NSs provide 1.8 and 1.2-fold activity higher for the photo degradation of MB, and 4.3 and 1.8-fold higher for the reduction of Cr⁶⁺. TiO₂–Au/Hg and TiO₂–Au/Ag NSs that are prepared from the deposition of Hg²⁺ and Ag⁺ onto TiO₂–Au NSs, respectively, allow degradation of MB within 10 min, with activities 4.2- and 3.3-fold greater than that of the TiO₂–Au NSs. The present study reveals that TiO₂–Au/Hg and TiO₂–Au/Ag NSs are effective for removal of organic pollutants, while TiO₂–Au NSs are useful for the reduction of Cr⁶⁺.

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Introduction

Environmental pollutants such as methylene blue (MB) and chromium(VI) (Cr⁶⁺) are a potential threat to the environment, mainly because they exhibit high toxicity, mutagenicity and carcinogenicity.^1,2 Today, close to 10 000 dyes are utilized for the color index as well as in the textile industries for dyeing and printing purposes.³ Organic dyes such as MB can cause skin and urine discoloration in addition to hemolysis and methemoglobin production. The maximum limit for MB is 4 mg kg⁻¹.⁴ On the other hand, Cr⁶⁺ which is used in the leather, dyeing and chemical manufacturing industries⁵ has been known to cause lung cancer, as well as liver, kidney and gastric damage.⁶ Relative to Cr³⁺, Cr⁶⁺ exhibits up to 500 times more toxicity, with the maximum limit of 0.05 mg L⁻¹ in waste water set by the Environment Protection Agency (EPA) of the United States.⁶ Hence it is critical to treat these two chemicals before discharging them into the environment. Various methods for removal of dyes and toxic metal ions have been reported using filtration, electrolysis, precipitation, ion exchange and adsorption properties.⁷ However, most of these methods are expensive and require frequent expenditure, in addition to sophisticated systems, making them impractical for real world usage.

Titanium oxide (TiO₂) nanomaterials possess advantages of high catalytic activity, ease of preparation, chemical stability, low toxicity and inexpensiveness, thereby becoming one of the most popular photocatalysts for degradation (conversion) of toxic pollutants and for preparation of various organic products.^1–3 The size and morphology have great effects on the surface area and energy and thus affect the activity of TiO₂ nanomaterials significantly. In addition to spherical structures, various morphologies of TiO₂ have been prepared, including nanorods,⁸ nanotubes⁹ and nanosheets,¹⁰ and used for degradation of pollutants. However, the wide band gaps of TiO₂ nanomaterials (e.g. 3.0 and 3.2 eV for rutile and anatase, respectively) require UV light for generations of hole/electron (h⁺/e⁻) pairs. In addition, recombination of as-generated h⁺/e⁻ pairs causing loss of the photocatalytic activity of TiO₂ nanomaterials sometimes occurs.⁸ To enhance photoreaction efficiency, many strategies have been demonstrated to prepare TiO₂ nanomaterials having smaller band gaps, including doping of various metals/nonmetals to TiO₂ nanomaterials⁹ and preparation of nanocomposites.^10,11 Doping with other nonmetals has some disadvantages such as small changes in the band gap and requirement of the dopants having close ionic radii to that of Ti⁴⁺.⁸ On the other hand, nanocomposites of TiO₂ with other metals, particularly noble metals such as Au, Ag and Pt, have become popular photocatalysts, mainly because of their smaller band gaps that allow efficient generation of h⁺/e⁻ pairs under irradiation.¹² For example, TiO₂ nanocomposites with various structures such as Au@TiO₂ core–shell nanocomposites,¹³ M@TiO₂ (M = Au, Ag) core–shell nanocomposites¹⁴ with a wedge-shaped morphology¹⁵ and mesoporous TiO₂–Au microspheres¹⁶ possessing high photoactivity have been demonstrated for the degradation of methyl orange,¹⁷ phenol,¹⁸ and malathion.¹⁹ Incorporation of the metals having high Fermi energy levels relative to TiO₂ further separates the generated h⁺/e⁻ pairs, leading to an increase in the interfacial charge-transfer process that inhibits the recombination of photoexcited h⁺/e⁻ and subsequently enhances the photocatalytic efficiency.²⁰

In this study, TiO₂–Au nanospheres (NSs) were prepared and used for the preparation of various TiO₂–Au/M NSs, in which M is either Hg (Hg²⁺) or Ag (Ag⁺). A simple solvothermal method was applied to prepare TiO₂ NSs, which with Au nanoparticles (NPs) were then used to prepare core–shell TiO₂–Au NSs.²¹ Through the d¹⁰–d¹⁰ interaction, the two metal ions were deposited onto TiO₂–Au NSs to form TiO₂–Au/M NSs.²² The TiO₂–Au NSs and TiO₂–Au/M NSs were both highly effective for the degradation of MB and reduction of Cr⁶⁺ to Cr³⁺ under UV irradiation. Factors such as pH, loading of the photocatalysts, reaction time and oxygen content were evaluated. The results show that TiO₂–Au/Hg NSs and TiO₂–Au/Ag NSs relative to TiO₂–Au NSs are more effective to remove the organic pollutants, while are slightly less effective for reduction of metal ions.

Experimental

Chemicals

Ascorbic acid, isopropyl alcohol and diethylenetriamine (DETA) were purchased from Acros (Geel, Belgium). Sodium citrate dihydrate, hydrogen tetrachloroaurate (HAuCl₄), titanium(IV) isopropoxide (TIP), poly(diallyldimethylammonium chloride) (PDADMAC), poly-(sodium-4-styreneulfonate) (PSS), sodium chloride, mercuric chloride and MB were purchased from Sigma Aldrich (St. Louis, MO, USA). Silver nitrate and cobalt sulphate were purchased from E. Merck (Darmstadt, Germany). 1,5-Diphenylcarbazide was purchased from Alfa Aesar (Ward Hill, MA, USA). Ethanol (99.5%) was purchased from Shimakyu's Pure Chemicals (Osaka, Japan). Potassium dichromate was purchased from Riedel-de Haen (Seelze, Germany). Lead nitrate was purchased from Wako Pure Chemicals (Richmond, VA, USA). Ultrapure water was obtained from a Milli-Q ultrapure (18.2 MΩ cm) system from Merck Millipore (Billerica, MA, USA).

Synthesis of Au NPs

Au NPs were prepared by reducing HAuCl₄ (0.1 M, 250 μL) using citrate solution (4 mM, 50 mL).²³ The final concentration of HAuCl₄ is 0.5 mM. Trisodium citrate solution was boiled in a round bottom flask fitted with a condenser under constant stirring. Aqueous HAuCl₄ was then added immediately and the solution was boiled for 3 min. When the solution turned from yellow to deep red, it was immediately transferred to an ice bath to terminate the reaction. The size and shape of the NPs were verified through transmission electron microscopy (TEM). The absorption (extinction) spectrum of the as-prepared Au NP solution was recorded. The particle concentration of the Au NPs (ca. 15 nM) was determined according to Beer's law, using an extinction coefficient of ca. 10⁸ M⁻¹ cm⁻¹ at 520 nm for the 13 nm Au NPs.

Synthesis of TiO₂ NSs

TiO₂ NSs was first prepared by a solvothermal process.²⁴ To 42 mL of isopropyl alcohol (99%), 0.3 mL of DETA (99%) was added. The solution was gently stirred for 5 min, following which 1.5 mL of TIP (97%) was added. The solution was then heated in a Teflon lined stainless steel autoclave for 24 h at 200 °C. The precipitate obtained was then washed with ethanol three times and dried overnight at 60 °C. The product obtained was then calcined at 400 °C for 2 h.

Synthesis of TiO₂–Au NSs, TiO₂–Au/Hg NSs and TiO₂–Au/Ag NSs

Gold nanoparticles were bound to the TiO₂ NSs that had been modified sequentially with PSS and PDADMAC through electrostatic interactions.²¹ PSS (1 mg mL⁻¹, 1 mL) and PDADMAC (1 mg mL⁻¹, 1 mL) were sequentially adsorbed onto the surface of TiO₂ NSs (5–15 mg mL⁻¹, 1 mL), providing a uniform, positively charged surface. To the PSS and PDADMAC treated TiO₂ NSs solutions (5–15 mg mL⁻¹, 1 mL), 1 mL of 13 nm Au NPs (3.75–15 nM, as seeds) were separately added and allowed to equilibrate for 30 min. Excess Au NPs were removed by centrifugation (16 100g for 10 min). Each of the precipitates was then re-dispersed in 2 mL of H₂O. To each of the solution, 0.1 mL of 0.5 mM HAuCl₄ and 0.1 mL of 0.34 mM ascorbic acid were added to form core–shell TiO₂–Au NSs.²¹ This was then washed with ultrapure water three times and stored for further use. For the synthesis of TiO₂–Au/Hg NSs, Hg²⁺ (0.05 to 0.5 mM) in tris–borate solution (5 mM, 1 mL, pH 7.0) was added to the core–shell TiO₂–Au NSs (10 mg mL⁻¹ that refers to the concentration of TiO₂ NSs loaded with 7.5 nM Au NPs, 1 mL), which was then stirred vigorously for 10 min. Similarly, TiO₂–Au/Ag NSs were prepared using Ag⁺ (0.05 to 0.5 mM) instead of Hg²⁺. As control, Co²⁺ (0.1 mM) in tris–borate solution (5 mM, 1 mL, pH 7.0) and Pb²⁺ (0.1 mM) in tris–borate solution (5 mM, 1 mL, pH 9.0) were used instead of Hg²⁺. The as-formed materials were calcined at 200 °C for 2 h prior to being used for conducting photocatalytic experiments.

Characterization of TiO₂–Au, TiO₂–Au/Hg and TiO₂–Au/Ag NSs

A double-beam UV-Vis spectrophotometer (Cintra 10e, GBC) was used to record the absorption spectra of the NSs. JEOL JSM-1230 and FEI Tecnai-G2-F20 transmission electron microscopes (TEMs) were used to measure the sizes and shapes of the as-prepared NSs. Similarly, a Hitachi S-2400 scanning electron microscope (SEM) (Hitachi High-Technologies, Tokyo, Japan) equipped with an energy dispersive X-ray spectrometer was used to characterize the nanomaterials. The re-dispersed NSs were separately placed on formvar/carbon film Cu grids (200 mesh; Agar Scientific) and dried at ambient temperature. X-ray diffraction (XRD) patterns of the NSs were measured using a PANalytical X'Pert PRO diffractometer (PANalytical B.V., EA Almelo, Netherlands) and Cu-Kα radiation (λ = 0.15418 nm); the samples were prepared on Si substrates. X-ray photoelectron spectroscopy (XPS) was performed using PHI 5000 VersaProbe (Physical Electronics, Eden Prairie, MN, USA).

Photocatalytic degradation of MB

Experiments were carried out in an apparatus specially designed for the photocatalytic reaction. A 500-W UV-Vis lamp (Newport, Oriel Instrumentation, Irvine, California, USA) with a cut off filter to block light with wavelengths above 420 nm was placed in a well-aerated empty chamber. The distance between the light source and the mouth of the glass bottle is 4 cm. The reactor temperature was maintained at 30 °C. MB (10 μM, 10 mL) and one of the TiO₂–Au/Hg, TiO₂–Au/Ag and TiO₂–Au NSs (10 mg mL⁻¹) in water was added to a wide mouthed glass bottle. The reaction solution was subjected to sonication for 10 min, and then purged with O₂ for 20 min and kept in the dark for 30 min.²⁵ The reaction solution was kept under UV illumination and aliquots (1 mL) of the solution were taken out at certain intervals of time. After being centrifuged at 10 000g for 10 min to remove the particles, the absorbance of each supernatant at the wavelength of 664 nm was recorded. The concentrations of MB in the solution at various reaction times were determined (n = 5) using a calibration curve of absorbance against standard MB solutions (1–25 μM).

Photocatalytic reduction of Cr⁶⁺

Experiments were carried out in the same apparatus designed for the photocatalytic degradation of MB. Cr⁶⁺ (34 μM, 10 mL) and one of TiO₂–Au/Hg, TiO₂–Au/Ag and TiO₂–Au NSs (10 mg mL⁻¹) in water were added to a wide mouthed glass bottle. The reaction solution was subjected to sonication for 10 min and kept in the dark for 30 min. The pH of the reaction solution was maintained at 4.0 (acetate buffer, 10 mM). The reaction solution was kept under UV illumination and aliquots (1 mL) of the solution were taken out at certain intervals of time. The solutions were centrifuged at 10 000g for 10 min and the supernatants were collected. To determine the reduction of Cr⁶⁺ to Cr³⁺ a standard method using 1,5-diphenylcarbazide was applied.²⁶

Reusability

The three types of NSs were subjected to multiple cycles (n = 5) of photocatalytic degradation of MB. After completion of each cycle, the photocatalyst was collected through centrifugation at RCF 12 000g for 20 min, followed by washing thoroughly with ultrapure water. The NSs were then used for the photocatalytic degradation of MB. The procedure was repeated for the second to fifth consecutive cycles. Similarly, reusability of the three types of NSs was carried out for the photocatalytic reduction of Cr⁶⁺.

Result and discussion

Formation and characterization of TiO₂–Au NSs, TiO₂–Au/Hg and TiO₂–Au/Ag NSs

TiO₂ NSs with positive charges on their surfaces were firstly modified with negative PSS and then with positive PDADMAC, which allows stronger electrostatic interactions with citrate-protected Au NPs. These bound Au NPs were then used as seeds to allow deposition of reduced Au atoms from Au³⁺ ions by ascorbic acid at pH 4.8, leading to the formation of core–shell structures of TiO₂–Au NSs. This simple synthetic strategy allowed preparation of homogeneous and stable TiO₂–Au NSs with great amounts of Au contents. Based on the fact that relative to Au NPs, Au/Hg and Au/Ag NPs have greater catalytic activity toward the reaction between Amplex UltraRed and hydrogen peroxide,²⁷ TiO₂–Au/Hg and TiO₂–Au/Ag NSs were prepared separately in this study. Through strong metallic Hg²⁺–Au⁺ and Ag⁺–Au⁺ interactions (d¹⁰–d¹⁰ interactions), Au/Hg and Au/Ag were formed on the Au shells, leading to the formation of TiO₂–Au/Hg and TiO₂–Au/Ag NSs, separately. The TEM image (Fig. 1A) displays that as-formed TiO₂–Au NSs have a mean diameter of 206 ± 24 nm (50 counts). Each of the NSs has a rough shell of Au NPs (15 ± 1.3 nm) on the surface of TiO₂ NSs. The EDX spectrum displayed in Fig. 1B confirms the presence of Au NPs on the surfaces of TiO₂ NSs. The SEM image of TiO₂–Au NSs as shown in the inset further supports their rough surfaces. XRD pattern (Fig. 1C) reveals a crystalline anatase structure of TiO₂ NSs from the corresponding (101), (004), and (200) diffractions around 2θ = 25.2°, 37.8°, and 47.61° (JCPDS card no. 21-1272).

Fig. 1 (A) TEM image (inset: high magnification TEM image), (B) EDX (inset: SEM image), (C) XRD pattern and (D) XPS spectrum of TiO₂–Au NSs.

Small peaks observed at around 2θ = 44.4°, 64.6° and 77.9° are indexed to the (200), (220), and (311) planes of the Au NPs (JCPDS card no. 002-1095).²⁸ The XPS of TiO₂–Au NSs (Fig. 1D) reveals Ti 2p_1/2 and Ti 2p_3/2 at 464 and 548 eV, respectively, and O 1s at 531 eV.²⁹ Along with the peak at 83.8 eV that is assigned for the Au 4f peak, the XPS data confirm the formation of the TiO₂–Au NSs.³⁰ The SEM image of TiO₂–Au/Hg NSs displayed in Fig. S1A† shows similar core–shell structures of TiO₂–Au NSs. The deposition of Hg or Ag onto Au NPs does not significantly change the morphology of Au NPs.²² The corresponding EDX spectrum and XRD pattern (Fig. S1B and C†) display intense peaks for Ti, O, Au and Hg. The SEM image of TiO₂–Au/Ag NSs (Fig. S1D†) shows their similar core–shell structures to that of TiO₂–Au NSs. The corresponding EDX spectrum and XRD pattern of TiO₂–Au/Ag NSs shown in Fig. S1E and F† confirm their successful formation. The anatase peak intensity and shape of the TiO₂ at 25.2° in the TiO₂–Au/Ag NSs decrease and become slightly broader, respectively, in comparison to that in the TiO₂–Au NSs, mainly due to a distortion in the crystal lattice of TiO₂–Au NSs as a result of the deposition of Ag atoms.³¹ Whereas, TiO₂–Au/Hg NSs exhibits a more sharper diffraction peak at 25.2° as the addition of Hg²⁺ leads to the crystallization of anatase TiO₂.³² Fig. S2† displays the UV-Vis absorption spectra of TiO₂–Au NSs (curve a), TiO₂–Au/Ag NSs (curve b), TiO₂–Au/Hg NSs (curve c), TiO₂ NSs (curve d) and P25 (curve e).

Photocatalytic activity of TiO₂–Au NSs, TiO₂–Au/Hg NSs and TiO₂–Au/Ag NSs

Irradiation of TiO₂–Au NSs under UV light leads to the generation of h⁺/e⁻ pairs, in which water with great capture capability for photoexcited holes acts as a hole scavenger to allow the free electron to be transferred from TiO₂ to Au.³³ The h⁺/e⁻ recombination is minimized by the introduction of Au NPs, mainly because of the generation of reactive oxidation species such as hydroxyl, peroxide and superoxide radical.³⁴ Au NPs also act as an electron sink preventing electrons in the conduction band (CB) from migrating to the surface of TiO₂ NS, which in turn facilitates the formation of oxygen superoxide radical anion.³⁵ The holes in the valence band (VB) of TiO₂ NSs form hydroxyl radicals and protons through their interaction with water molecules. Furthermore, Au NPs also absorb UV light, causing the transition of 5d electrons to the 6sp band (interband transition) that promotes charge trapping and favors charge migration to O₂ and thus increases the photocatalytic activity.³⁵ Protonation of superoxide anions creates ˙OOH to the formation of peroxide and hydroxyl radical that accelerate the degradation of pollutants.³³ TiO₂–Au/Hg NSs and TiO₂–Au/Ag NSs also exhibited enhanced catalytic properties. This can be attributed to the enhanced dispersion forces between closed shell metal atoms as a result of the interaction of Au (4f¹⁴ 5d¹⁰) with Ag (4d¹⁰ 5s¹) and with Hg (4f¹⁴ 5d¹⁰), respectively.³⁶ The metallic Hg²⁺ deposited acts as a catalyst for reduction, accelerating the reduction of MB.³⁷ On the other hand, Ag⁺ acts as electron traps due to the mixing of Ag 4d states and Ti 3d states, wherein they trap the photoinduced electrons, inhibiting the h⁺/e⁻ recombination and thus enhancing its photocatalytic property.³⁸

Photocatalytic degradation of MB

Degradation of MB in the presence of TiO₂–Au NSs was carried out under UV irradiation. The absorbance at 664 nm decreased upon increasing the reaction time (Fig. S3†). The solution was completely decolorized in less than 5 min while complete degradation was achieved in 15 min. Under the experimental condition (pH 10.0), adsorption of MB (positive charge) onto TiO₂–Au NSs (negative charge) is strong through their electrostatic interaction. As a result of MB on the surface of TiO₂–Au NSs, degradation of MB is efficient. The photo degradation of MB is more efficient (1.6 fold) in the solution purged with O₂, mainly because the electrons photogenerated on TiO₂–Au NSs are scavenged more efficiently in the presence of greater amount of O_2. Furthermore the presence of O₂ is necessary to suppress h⁺/e⁻ recombination in the course of the photocatalytic reaction (Fig. S4†).²⁵

The photo degradation efficiency of MB (10 μM) increased upon increasing the amount of TiO₂ NSs up to 10 mg mL⁻¹ at a constant amount of Au NPs as shown in Fig. 2A. This is largely attributed to the higher number of active sites for adsorption of MB and larger number of h⁺/e⁻ generation upon increasing concentration of TiO₂. When TiO₂ loading was further increased above 10 mg mL⁻¹, the solution became turbid and aggregation eventually occurred, resulting in the decrease of photon flux penetration and conversion efficiency.³⁹

Fig. 2 Effect of (A) amount of TiO₂, (B) Au NPs, and (C) Hg²⁺ and (D) Ag⁺ on the photocatalytic degradation of MB (10 μM). (A) Concentration of Au NPs is 7.5 nM (B) concentration of TiO₂ NSs is 10 mg mL⁻¹ (C) and (D) concentrations of TiO₂ NSs and Au NPs are 10 mg mL⁻¹ and 7.5 nM, respectively.

The optimal concentration of TiO₂ loading was found to be 10 mg mL⁻¹. Au NPs provided an electron sink leading to the prevention of electrons in the CB to migrate to its surface, thereby reducing the odds of h⁺/e⁻ recombination in TiO₂. Upon increasing the amount of Au NPs up to 7.5 nM in the TiO₂–Au NSs (Fig. 2B), the photocatalytic degradation efficiency of MB increased. The reduction efficiencies were 79%, 91.6%, and 99.7% in the presence of 3.7, 5.6, and 7.5 nM Au NPs, respectively.

The efficiency decreased slightly upon further increasing the concentration of Au NPs. The optimum loading for Hg²⁺ and Ag⁺ ions were both found to be 0.1 mM (Fig. 2C and D). Further increasing the loading of Hg²⁺ or Ag⁺ reduced the effective surface on the TiO₂–Au NSs for the reactant to be adsorbed, consequently reducing its photocatalytic activity.^37,40 Co²⁺ and Pb²⁺ were also tested as two controls.

They did not provide any significant enhancement on the photocatalytic activity of TiO₂–Au NSs towards MB, mainly due to inefficient deposition of Co²⁺ and Pb²⁺ onto the TiO₂–Au NSs surface, low reduction strength and/or slight changes in the band gap (separation of h⁺/e⁻).⁴¹

We further investigated the photocatalytic degradation of MB (Fig. 3) in the absence of any photocatalyst (curve a) and in the presence of Au NPs (curve b), P25 (curve c), TiO₂ NSs (curve d), TiO₂–Au NSs (curve e), TiO₂–Au/Ag NSs (curve f) and TiO₂–Au/Hg NSs (curve g).

Fig. 3 Degradation rates of MB (10 μM) in the (a) absence of any photocatalyst, and in the presence of (b) Au NPs (7.5 nM) (c) P25 (10 mg mL⁻¹) (d) TiO₂ NSs (10 mg mL⁻¹) (e) TiO₂–Au NSs (10 mg mL⁻¹) (f) TiO₂–Au/Ag NSs (10 mg mL⁻¹ loaded with 7.5 nM Au NPs and 0.1 mM Ag⁺) and (g) TiO₂–Au/Hg NSs (10 mg mL⁻¹ loaded with 7.5 nM Au NPs and 0.1 mM Hg²⁺). C_o and C are the concentrations of MB at reaction times 0 and t, respectively.

Negligible degradation was found either in the absence of any photocatalyst or in the presence of Au NPs. The degradation rate of TiO₂–Au NSs was 1.8 and 1.2-fold faster than that of P25 and TiO₂ NSs, respectively. TiO₂–Au/Ag NSs and TiO₂–Au/Hg NSs provided degradation rates of 3.3 and 4.2-fold faster than that of TiO₂–Au NSs, mainly because of enhanced dispersion forces between closed shell metal atoms as a result of the interaction of Au with Ag and with Hg. TiO₂–Au/Ag NSs and TiO₂–Au/Hg NSs degraded MB efficiently at room temperature within 10 min, due to higher number of active sites for adsorption of MB and Au NPs acting as an electron sink.

Furthermore, the activity increased due to the presence of the metal ions. The degradation efficiencies provided by the two NSs are higher than those reported by TiO₂ and ZnO.^42,43

Photoreduction of Cr⁶⁺ to Cr³⁺

To confirm the reduction of Cr⁶⁺ to Cr³⁺, 1,5-diphenylcarbazide was used to check the remaining Cr⁶⁺. Cr⁶⁺ and 1,5-diphenylcarbazide form a reddish pink colored complex that has a maximum absorption wavelength at 540 nm.⁴⁴ Thus the remaining amount of Cr⁶⁺ is proportional to the absorbance at 540 nm. By measuring the absorbance change at 540 nm at various pH values, the maximum reduction rate of highly toxic Cr⁶⁺ to less toxic Cr³⁺ was found to be at pH 4.0 (acetate buffer, 10 mM) when using TiO₂–Au NSs as the photocatalyst (Fig. S5†).⁴⁵ In addition to pH dependent reduction potential of Cr⁶⁺ to Cr³⁺ and catalytic activity of the catalysts, adsorption of Cr⁶⁺ onto the surface of the catalysts is another important factor. At pH values <2.0, H₂CrO₄ is the major species and positive charges on the surface of TiO₂–Au NSs, leads to weaker adsorption of chromium species and thus lower degradation rates. Upon increasing the pH values from 2.0 to 4.0, amounts of anions (HCrO₄⁻, CrO₄²⁻) increased, leading to their stronger adsorption onto TiO₂–Au NSs and thus increases in their degradation rate. At pH values <3.0, the TiO₂ surface has greater positive charges, leading to the neutralization of photogenerated electrons, and consequently decreasing its photocatalytic efficiency. However, at pH >4.0, TiO₂ surface has lower positive charge, thereby reducing the adsorption of chromium ions and subsequently slowing down the photocatalytic reaction.⁴⁶ Cr⁶⁺ is reduced to Cr³⁺ through the photogenerated electrons as shown in eqn (1) to (3):

TiO₂–Au NSs + hν → e⁻ + h⁺ (1)

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (2)

2H₂O + 4h⁺ → O₂ + 4H⁺ (3)

Fig. 4A displays the time-dependent changes in the absorbance at 540 nm of solutions (pH 4.0) containing TiO₂–Au NSs under UV irradiation, showing that the reduction was complete within 60 min. Fig. 4B displays the photocatalytic reduction of Cr⁶⁺ in the absence of any photocatalyst (curve a) and in the presence of Au NPs (curve b), P25 (curve c), TiO₂ NSs (curve d), TiO₂–Au/Hg NSs (curve e), TiO₂–Au/Ag NSs (curve f) and TiO₂–Au NSs (curve g). Negligible reduction was found either in the absence of any photocatalyst or in the presence of Au NPs. The reduction rate provided by the TiO₂–Au NSs was 4.3 and 1.8-fold faster than that of P25 and TiO₂ NSs, respectively. TiO₂–Au NSs relative to other materials provided a higher reduction rate of Cr⁶⁺, mainly because of the electrons generated in the conduction band of the catalyst.^45,46 The reduction time is shorter when using TiO₂–Au NSs than using as prepared TiO₂ and commercial TiO₂.^47,48 TiO₂–Au NSs relative to ZnO and BiVO₄ reduce Cr⁶⁺ to Cr³⁺ more rapidly (60 vs. 120 and 180 min).^49,50 TiO₂–Au/Ag NSs and TiO₂–Au/Hg NSs provided slightly lower reduction rates when compared to TiO₂–Au NSs, revealing that deposition of other metals onto TiO₂–Au NSs do not provide any benefit for the reduction of Cr⁶⁺ to Cr³⁺.

Fig. 4 (A) UV-Vis absorption spectra of Cr⁶⁺–1,5-diphenylcarabazide complex after photoreduction in the presence of TiO₂–Au NSs (final concentration: 10 mg mL⁻¹). (B) Reduction rates of Cr⁶⁺ (34 μM) in the (a) absence of any photocatalyst, and in the presence of (b) Au NPs (7.5 nM) (c) P25 (10 mg mL⁻¹) (d) TiO₂ NSs (10 mg mL⁻¹) (e) TiO₂–Au/Ag NSs (10 mg mL⁻¹ loaded with 7.5 nM Au NPs and 0.1 mM Ag⁺) (f) TiO₂–Au/Hg NSs (10 mg mL⁻¹ loaded with 7.5 nM Au NPs and 0.1 mM Hg²⁺) (g) TiO₂–Au NSs (10 mg mL⁻¹). C_o and C are the concentrations of Cr⁶⁺ at reaction times 0 and t, respectively.

Reusability

The reusability of the photocatalyst was evaluated over 5 cycles. The three NSs were all stable (activity loss less than 8%) over the course of 5 cycles of the photocatalytic degradation of MB as displayed in Table S1.† The loss in the photocatalytic activity of TiO₂–Au NSs for the reduction of Cr⁶⁺ was less than 3% over 5 cycles. Slightly higher loss in the case of MB than in Cr⁶⁺ indicates that adsorption of the organic residues on the surfaces of the NSs occurred. We point out that the loss of the photocatalytic materials through centrifugation/wash processes is also responsible for the results. The results revealed that the low-cost, efficient, and durable TiO₂–Au NSs hold great potential for removal of environmental toxicants.

Conclusion

TiO₂–Au NSs were prepared as photocatalysts for the degradation of MB and reduction of Cr⁶⁺ within 15 and 60 minutes, respectively. Deposition of Hg²⁺ or Ag⁺ onto TiO₂–Au NSs accelerated the degradation of MB (under 10 min), but caused slight loss of the activity for the reduction of Cr⁶⁺. To the best of our knowledge, TiO₂–Au/Hg and TiO₂–Au/Ag NSs provided the highest efficiency for the photocatalytic degradation of MB (100% within 10 min). The result suggests that TiO₂–Au/Hg and TiO₂–Au/Ag NSs hold great potential for the degradation of organic pollutants. Owing to the high toxicity of Hg²⁺, TiO₂–Au/Ag NSs are more suitable for removal of organic pollutants. On the other hand, TiO₂–Au NSs are effective for the reduction of Cr⁶⁺ to Cr³⁺. However, they are not ideal for removal of large scale of contaminants, mainly because of high costs of Au and Ag. In the future, less costly and higher efficient nanomaterials such as carbon dots and copper nanoclusters will be tested for removal of the contaminants.

Acknowledgements

This work was funded by the Ministry of Science and Technology (MOST), Taiwan (grant no: 103-2923-M-002-002-MY3). P.R. and A.P.P. are grateful to MOST and National Taiwan University for a postdoctoral fellowship under the contract number MOST 103-2811-M-002-002-054 and 101-R-4000, respectively. We thank Ms Ching-Yen Lin and Ms Ya-Yun Yang for their assistance for TEM measurement in the Instrumentation Center at National Taiwan University. We also thank Ms S.-J. Ji and C.-Y. Chien of Precious Instrument Center (National Taiwan University) for their assistance in SEM and EDX analysis.

Notes and references

1. J. Waterhouse, Br. J. Cancer, 1975, 32, 262.
2. C. D. Palmer and P. R. Wittbrodt, Environ. Health Perspect., 1991, 92, 25–40.
3. J. Joshi, J. Vora, S. Sharma, C. C. Patel and A. B. Patel, Asian J. Chem., 2004, 16, 1069–1075.
4. H. N. Ali and M. G. Ellabban, Ann. R. Coll. Surg. Engl., 2008, 90, 707–708.
5. P. Suksabye, P. Thiravetyan, W. Nakbanpote and S. Chayabutra, J. Hazard. Mater., 2007, 141, 637–644.
6. E. Petala, K. Dimos, A. Douvalis, T. Bakas, J. Tucek, R. Zbořil and M. A. Karakassides, J. Hazard. Mater., 2013, 261, 295–306.
7. K. Lee, N. H. Lee, S. H. Shin, H. G. Lee and S. J. Kim, Mater. Sci. Eng., B, 2006, 129, 109–115.
8. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
9. J. K. Zhou, L. Lv, J. Yu, H. L. Li, P.-Z. Guo, H. Sun and X. Zhao, J. Phys. Chem. C, 2008, 112, 5316–5321.
10. Y. Zhang, X. Yu, Y. Jia, Z. Jin, J. Liu and X. Huang, Eur. J. Inorg. Chem., 2011, 2011, 5096–5104.
11. N. Zhang, S. Liu, X. Fu and Y.-J. Xu, J. Phys. Chem. C, 2011, 115, 9136–9145.
12. V. Subramanian, E. Wolf and P. V. Kamat, J. Phys. Chem. C, 2001, 105, 11439–11446.
13. M. Murdoch, G. Waterhouse, M. Nadeem, J. Metson, M. Keane, R. Howe, J. Llorca and H. Idriss, Nat. Chem., 2011, 3, 489–492.
14. J. Z. Dengyu Pan, Z. Li, C. Wu, X. Yan and M. Wu, Chem. Commun., 2010, 46, 3681–3683.
15. X.-F. Wu, H.-Y. Song, J.-M. Yoon, Y.-T. Yu and Y.-F. Chen, Langmuir, 2009, 25, 6438–6447.
16. Z. Bian, J. Zhu, F. Cao, Y. Lu and H. Li, Chem. Commun., 2009, 3789–3791.
17. E. Murugan and R. Rangasamy, J. Biomed. Nanotechnol., 2011, 7, 225–228.
18. R. Su, R. Tiruvalam, Q. He, N. Dimitratos, L. Kesavan, C. Hammond, J. A. Lopez-Sanchez, R. Bechstein, C. J. Kiely and G. J. Hutchings, ACS Nano, 2012, 6, 6284–6292.
19. H. Yu, X. Wang, H. Sun and M. Huo, J. Hazard. Mater., 2010, 184, 753–758.
20. P. V. Kamat, J. Phys. Chem. C, 2007, 111, 2834–2860.
21. V. Salgueiriño-Maceira, M. A. Correa-Duarte, M. Farle, A. López-Quintela, K. Sieradzki and R. Diaz, Chem. Mater., 2006, 18, 2701–2706.
22. C.-W. Lien, Y.-C. Chen, H.-T. Chang and C.-C. Huang, Nanoscale, 2013, 5, 8227–8234.
23. S.-J. Chen, Y.-F. Huang, C.-C. Huang, K.-H. Lee, Z.-H. Lin and H. T. Chang, Biosens. Bioelectron., 2008, 23, 1749–1753.
24. J. S. Chen, Y. L. Tan, C. M. Li, Y. L. Cheah, D. Luan, S. Madhavi, F. Y. C. Boey, L. A. Archer and X. W. Lou, J. Am. Chem. Soc., 2010, 132, 6124–6130.
25. T. Zhang, T. Oyama, A. Aoshima, H. Hidaka, J. Zhao and N. Serpone, J. Photochem. Photobiol., A, 2001, 140, 163–172.
26. M. Bose, Anal. Chim. Acta, 1954, 10, 201–208.
27. C.-W. Lien, Y.-T. Tseng, C.-C. Huang and H.-T. Chang, Anal. Chem., 2014, 86, 2065–2072.
28. T. Zeng, Y. R. Ma, H. Y. Niu and Y. Q. Cai, J. Mater. Chem., 2012, 22, 18658–18663.
29. J. Guillot, F. Fabreguette, L. Imhoff, O. Heintz, M. Marco de Lucas, M. Sacilotti, B. Domenichini and S. Bourgeois, Appl. Surf. Sci., 2001, 177, 268–272.
30. J. Zhao, S. Sallard, B. M. Smarsly, S. Gross, M. Bertino, C. Boissière, H. Chen and J. Shi, J. Mater. Chem., 2010, 20, 2831–2839.
31. T.-D. Pham and B.-K. Lee, Int. J. Environ. Res. Public Health, 2014, 11, 3271–3288.
32. R. Mechiakh, N. Ben Sedrine and R. Chtourou, Appl. Surf. Sci., 2011, 257, 9103–9109.
33. H. Chang, C. Su, C.-H. Lo, L.-C. Chen, T.-T. Tsung and C.-S. Jwo, Mater. Trans., 2004, 45, 3334–3337.
34. C. Gomes Silva, R. Juárez, T. Marino, R. Molinari and H. García, J. Am. Chem. Soc., 2010, 133, 595–602.
35. A. Primo, A. Corma and H. Garcia, Phys. Chem. Chem. Phys., 2011, 13, 886–910.
36. J. Xie, Y. Zheng and J. Y. Ying, Chem. Commun., 2010, 46, 961–963.
37. M. Aguado, S. Cervera-March and J. Giménez, Chem. Eng. Sci., 1995, 50, 1561–1569.
38. R. Nainani, P. Thakur and M. Chaskar, J. Mater. Sci. Eng. B, 2012, 2, 52–58.
39. P. Roy, A. P. Periasamy, C.-T. Liang and H.-T. Chang, Environ. Sci. Technol., 2013, 47, 6688–6695.
40. M. J. Height, S. E. Pratsinis, O. Mekasuwandumrong and P. Praserthdam, Appl. Catal., B, 2006, 63, 305–312.
41. D. Chen and A. K. Ray, Chem. Eng. Sci., 2001, 56, 1561–1570.
42. A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard and J.-M. Herrmann, Appl. Catal., B, 2001, 31, 145–157.
43. Y. J. Jang, C. Simer and T. Ohm, Mater. Res. Bull., 2006, 41, 67–77.
44. H. Marchart, Anal. Chim. Acta, 1964, 30, 11–17.
45. J.-K. Yang and S.-M. Lee, Chemosphere, 2006, 63, 1677–1684.
46. Y. Ku and I.-L. Jung, Water Res., 2001, 35, 135–142.
47. C. R. Chenthamarakshan, K. Rajeshwar and E. J. Wolfrum, Langmuir, 2000, 16, 2715–2721.
48. G. Colón, M. C. Hidalgo and J. A. Navío, J. Photochem. Photobiol., A, 2001, 138, 79–85.
49. L. Khalil, W. Mourad and M. Rophael, Appl. Catal., B, 1998, 17, 267–273.
50. B. Xie, H. Zhang, P. Cai, R. Qiu and Y. Xiong, Chemosphere, 2006, 63, 956–963.

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Footnote

† Electronic supplementary information (ESI) available: SEM, EDX, XRD, UV-Vis absorption, effect of oxygen, effect of pH, reusability results. See DOI: 10.1039/c4ra10192h

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