Electrodeposited multi-walled carbon nanotubes on Ag-loaded TiO₂ nanotubes/Ti plates as a new photocatalyst for dye degradation

Abstract

MWCNTs/Ag/TiO₂NTs plates were synthesized via electrochemical reduction of functionalized multiwalled carbon nanotubes (MWCNTs) on Ag/TiO₂NTs. The loading of silver nanoparticles was carried out by electroless reduction of Ag¹⁺ onto TiO₂ nanotubes previously formed by anodizing titanium. The SEM analysis revealed that MWCNTs were loaded significantly on the as-prepared Ag/TiO₂NTs, where nanoparticles of Ag had grown on the walls of TiO₂NTs. The photocatalytic activity of the synthesized plates was evaluated by the photodegradation of Methyl Orange (MO) dye under UV light irradiation. The results indicated that the MWCNTs/Ag/TiO₂NTs demonstrated improved efficiency for MO photodegradation in comparison with Ag/TiO₂NTs and TiO₂NTs. This is attributed to the efficient separation and transfer of photogenerated electron–hole pairs. The chemical oxygen demand of an MO dye solution as measured at specified time intervals provides a good idea about the degradation of MO.

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1. Introduction

TiO₂ is one of the most important photocatalysts because of its high durability, very low toxicity, chemical inertness and corrosion resistance. TiO₂ has been applied as a photocatalyst for the removal of organic contaminants in water or air and for the formation of hydrogen through water splitting.^1,2 There are some disadvantages with the use of powder TiO₂ like stirring during the reaction and separation of the catalyst from the treated water after each run. Some systems have used sedimentation, centrifugation and filtration to recover highly dispersed and suspended catalyst particles as it is easy for smaller particles to stay suspended in water, penetrate through filtration materials and clog filter membranes.³ Among various types of TiO₂, TiO₂NTs/Ti plate fabricated by anodizing titanium can be used as a suitable photocatalyst because of its high surface area, thermal stability, controlled pore structure, relatively low cost and especially the excellent reusability relative to other powdery TiO₂ nanomaterials.^4,5 However, the photocatalytic efficiency of TiO₂ and also TiO₂NTs/Ti plate for degradation of dyes decrease substantially due to the high recombination ratio of photoinduced electrons (e⁻) and holes (h⁺) produced when irradiated under ultraviolet (UV) light.^6,7 Therefore, in order to retard the recombination of the photogenerated electrons and holes, various strategies have been designed, including doping TiO₂ with metals or nonmetals.^8–11 MWCNTs or CNTs are recently discovered material with cylindrical structures that possess high mechanical strength, large specific surface areas and favorable electronic properties.^12–14 These can prevent the recombination of electron–hole pairs by serving as an electron sink when used in conjunction with semiconductor materials.^7,15,16 Moreover, it has been known that the deposition of Ag nanoparticles on TiO₂ reduces the possibility of electron–hole recombination, enhances the absorption of visible light and increases the rate of electron transfer to the oxidant, resulting in the photocatalytic activity improvement of TiO₂.^4,17 It seems plausible that the growth of MWCNTs on Ag/TiO₂ might lead to a synergetic effect on photodegradation process. Therefore, it is rationally desirable to prepare new catalyst based on Ag, MWCNTs and TiO₂.

In this work, MWCNTs/Ag/TiO₂NTs/Ti plates were fabricated via electrodeposition of Ag onto the previously formed TiO₂ nanotubes followed by the electrochemical reduction of functionalized MWCNTs onto its surface. The properties of fabricated plates were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The photocatalytic activity was examined by the degradation of Methyl Orange (MO) under UV light irradiation and the experimental results were discussed. To the best of our knowledge, this work may be the first report about electro-deposition of functionalized MWCNTs on the substrate for photocatalytic applications.

2. Experimental

2.1. Reagents and apparatus

Raw MWCNTs (purity > 95%, surface area 198 m² g⁻¹) were purchased from Aldrich. Methyl orange (MO) (MW = 327.33 g mol⁻¹, CI = 13 025) provided by Iran Colour Research Centre, was used without further purification and selected as a probe material to study the photocatalytic performance of the fabricated catalysts. The chemical structure of MO dye is shown in Fig. 1.

Fig. 1 Chemical structure of MO.

Morphological studies were carried out by scanning electron microscope (Philips, Model XL30). Chemical compositions were determined by EDX in a scanning electron microscope (VEGA\\Tescan). The crystallographic structures of the materials were determined by X-ray diffraction (XRD). Impedance measurements were performed by Voltalab10 electrochemical system.

2.2. Preparation of the plates

2.2.1. Preparation of the Ag/TiO₂NTs/Ti plates. Plates of Ti (99% purity) with an area of 15 cm² were used for the anodic growth of titanium dioxide nanotubes. Plates were polished with emery papers no. 400 to 3000 to obtain a mirror finish and subsequently cleaned in acetone and de-ionized (DI) water in an ultrasonic bath. The cleaned samples were anodized at 20 V for 3 h in a solution of glycerol/water (75 : 25, vol%) + 0.5 wt% NH₄F at room temperature in a two-electrode cell using platinum foil as a counter electrode. The anodized samples were further washed with DI water and ultrasonically cleaned for 60 s in DI water to remove surface debris and subsequently dried in air. Finally, the anodized samples were annealed at 450 °C for 2 h to form the anatase phase.

Ag was deposited onto TiO₂NTs/Ti plates by the electroless procedure at room temperature in an ultrasonic bath. Firstly, the TiO₂NTs/Ti plates were immersed into an ethanol bath containing 0.25 mM AgNO₃ for 1 min and were subsequently transferred to a reducing aqueous bath containing 5 M NH₃ and 0.02 M hydrazine hydrate for 2 min.

2.2.2. Preparation of the MWCNTs/Ag/TiO₂NTs/Ti plates. Functionalization of MWCNTs was carried out in a mixture of sulfuric and nitric acid (50 mL, 3 : 1 v/v) under reflux condition according to the previous work.^18,19 Prior to the process, purified MWCNTs were sonicated in the acid solution for 2 h to open the agglomeration of nanotube and anchoring acid solution uniformly on the carbon surface. Thereafter, homogenized carbon solution was oxidized under reflux condition at 100 °C for 6 h to introduce functional groups. Five-fold dilution was then applied to the carbon solution to stop oxidation reaction. Stirring and decantation were consecutively conducted for five times and lastly washed with DI water by filtration until the pH of the resultant solution was approximately at 7. The obtained precipitates were finally dried in a vacuum oven at 60 °C and denoted as f-MWCNTs. The f-MWCNTs dispersed solutions were prepared by adding given amounts of f-MWCNTs powder to 50 mL of DI water and sonicated for 1 h to enhance exfoliation to separate f-MWCNTs particles from each other. Deposition of f-MWCNTs onto the Ag/TiO₂NTs/Ti plates was carried out galvanostatically at a current density of −10 mA cm⁻² for the duration of 3000 s where the electrolyte was 1.0 M KNO₃ + 0.3 g L⁻¹ exfoliated f-MWCNT solution.

2.3. Photocatalytic degradation of MO

MO was selected as the probe molecule to investigate the photocatalytic activity of the prepared catalysts because MO is chemically stable and persistent nitrogen containing dye pollutant. The photocatalytic experiments were carried out by vertically placing the prepared plates in a Pyrex reactor containing 100 mL of 10.0 mg L⁻¹ MO equipped with a magnetic stirrer. A 125 W mercury lamp served as the source of UV light to trigger the photocatalytic reaction. This reactor set-up was surrounded by a circulating water jacket to keep the temperature constant within ±1.0 °C. Prior to irradiation, MO solution in the presence of prepared plates was stirred for 30 min in a darkroom to establish the adsorption/desorption equilibrium of MO dye on the photocatalyst surface to exclude the effect of the dye adsorption in the photodegradation process and then photocatalytic reaction was initiated. At specified time intervals (15 min), 3 mL samples were removed and MO concentration was determined from the absorbance change at the wavelength of 463 nm using spectrophotometric method. To further amplify the studies, the extent of degradation was also followed by chemical oxygen demand (COD) measurements via titration of MO solution with KMnO₄ solution at specified time intervals.

3. Results and discussion

3.1. Structural and morphological characterization

The FT-IR spectra of both MWCNTs and functionalized MWCNTs are shown in Fig. 2. The absorption band at 1600 cm⁻¹ is ascribed to the C=C bond of the hexagonal network in MWCNTs. After the oxidation in acidic media, new bands appear. The peaks at 1730 cm⁻¹, 3400 cm⁻¹ and bands from 1380–1460 cm⁻¹ are attributed to the stretching of C=O bond, O–H and the stretching modes of the C=OH bonds of carboxylic acid, respectively. Hence, the presence of carbonyl and carboxylic acid groups on the surface of MWCNTs confirms the efficiency of oxidizing process. These functional groups can be efficiently reduced electrochemically at room temperature where no extra chemicals are employed and the thickness and amount of MWCNT film can also be best controlled. The chemical states of elements in the Ag particles coated TiO₂ nanotube were analyzed by X-ray photoelectron spectroscopy (XPS). Fig. 3 shows the survey XPS spectrum of the Ag nanoparticles coated TiO₂ nanotube and high-resolution XPS spectrum of Ag 3d. The survey spectrum (Fig. 3a) clearly indicates that three major sets of peaks from the O 1s, Ti 2p and Ag 3d states exist in the Ag nanoparticle coated TiO₂ nanotube sample by electroless deposition. A high-resolution XPS spectrum confined to the Ag window (Fig. 3b) gave binding energies of Ag 3d doublet peaks located at 376.4 (Ag 3d_3/2) and 371.0 (Ag 3d_5/2) eV. This reveals that the Ag exists in a metallic form.^20,21 The SEM images in Fig. 4 show significant differences in the morphologies of the investigated coatings. Fig. 4a exhibits the SEM image of bare TiO₂NTs where distinct and well resolved nanotubes are visible. The inner diameter of TiO₂NTs is 80–120 nm and the wall thickness is 20–30 nm. Fig. 4b displays SEM micrograph of the Ag-loaded TiO₂NTs where Ag nanoparticles grown on the walls of TiO₂NTs. The wall thickness of the Ag/TiO₂NTs plates is 60–80 nm. Fig. 4c shows Ag/TiO₂NTs structure coated well with f-MWCNTs by using the electro-deposition method. As seen in Fig. 4c, f-MWCNTs have been deposited on the surface of Ag/TiO₂NTs. In order to probe the chemical compositions of prepared plates, the plate material was further characterized by EDX. Fig. 4d shows the EDX spectrum of the MWCNT/Ag/TiO₂NTs/Ti plates that confirms the presence of both Ag and MWCNTs on TiO₂NTs/Ti. As seen from Fig. 4d, Ti and C are the major elements which are originating from TiO₂NTs/Ti and MWCNT, respectively and small amounts of elemental Ag identified at 2.984 keV. Fig. 5 displays XRD patterns of annealed TiO₂NTs/Ti and MWCNTs/Ag/TiO₂NTs/Ti plates prepared by the mentioned procedures. The peaks with 2θ of 25.3° and 48.2° are assigned to anatase crystal structure of TiO₂NTs/Ti while lines at 40.4° and 38.5° correspond to the crystalline Ti metal. After deposition of MWCNTs and Ag, the pattern exhibits new peaks. The diffraction peaks at 44.5° and 64.5° correspond to Ag (200) and (220), respectively. It is clear that there are no diffraction peaks of MWCNTs in Fig. 5. This might be attributed to the presence of small amounts of non-crystalline MWCNTs thin films deposited on the Ag/TiO₂NTs/Ti plates.

Fig. 2 FT-IR spectra of (a) MWCNTs and (b) functionalized MWCNTs.

Fig. 3 XPS analysis of the Ag/TiO₂NTs/Ti: (a) the survey XPS spectrum and (b) the high-resolution spectrum for the Ag 3d states.

Fig. 4 SEM images of bare (a) TiO₂NTs/Ti, (b) Ag/TiO₂NTs/Ti, (c) MWCNTs/Ag/TiO₂NTs/Ti and (d) EDX spectra obtained of MWCNTs/Ag/TiO₂NTs/Ti plate.

Fig. 5 XRD patterns of TiO₂NTs/Ti and MWCNTs/Ag/TiO₂NTs/Ti plates.

Electrochemical impedance spectroscopy (EIS) was used to study the role of MWCNTs in the photocatalytic performance of Ag/TiO₂NTs/Ti. Fig. 6 shows the Nyquist plots of the prepared plates at the open circuit potential (OCP) in the frequency range of 100 kHz to 10 mHz in aqueous solution containing 10 mM K₃[Fe(CN)₆] : K₄[Fe(CN)₆] (1 : 1) and 0.1 M KCl. The plots consist of an overall distorted semicircle which can be attributed to the existence of two double layers in the interface of Ag/TiO₂NTs and MWCNTs/Ag/TiO₂NTs with electrolyte. The impedance at high frequencies and the diameter of the semicircle represent ohmic resistance (R_s) and total charge transfer resistance (R_ct) in the plates, respectively. As seen from Fig. 6, MWCNTs/Ag/TiO₂NTs/Ti plate possesses lower R_ct and R_s than Ag/TiO₂NTs/Ti plate, which is due to the conductive network formed by MWCNTs. The low values of R_s and R_ct for MWCNTs/Ag/TiO₂NTs/Ti plate indicate MWCNTs were chemically bonded on Ag/TiO₂NTs/Ti. Also, both the electron accepting and transporting properties of MWCNT in the composite could contribute to the suppression of charge recombination, and thereby a higher rate in the photocatalysis would be achieved.

Fig. 6 Impedance Nyquist plots at OCP voltage in the presence of 10 mM K₃[Fe(CN)₆] : K₄[Fe(CN)₆] (1 : 1) mixture containing 0.1 M KCl as the supporting electrolyte for the Ag/TiO₂NTs/Ti and MWCNTs/Ag/TiO₂NTs/Ti plates.

3.2. Photocatalytic experiments

3.2.1 Evaluation of the photocatalytic activity. The photocatalytic activities of the fabricated samples were comparatively evaluated using the photodegradation of MO in aqueous solution under UV light illumination as a model reaction and the results are displayed in Fig. 7. Degradation conversion for the MO mineralization was calculated from eqn (1):where X is the degradation efficiency of MO dye, C₀ and C are MO concentration initially and after degradation at time (t), respectively.

Fig. 7 Photocatalytic degradation efficiency of MO under UV light irradiation.

The enhanced photocatalytic activity is governed by three main factors: (1) improved adsorbing affinity toward organic molecules, (2) increased light absorption and (3) effective charge carrier transfer and separation. Surface area, separation and transfer of charge carriers are the most influential factors in controlling both the rate and efficiency of photocatalysis.^22–25

The results presented in Fig. 7 indicated that the modified MWCNTs/Ag/TiO₂NTs/Ti displays significant improvement compared to the pure TiO₂NTs/Ti and Ag/TiO₂NTs/Ti samples under UV light irradiation. This could be rationalized on two grounds. Firstly, the interactions between MO and the MWCNTs may lead to the higher adsorption capacity of MWCNTs/Ag/TiO₂NTs/Ti via higher π-conjugations. Secondly, the transportation and mobility of photogenerated electrons in the MWCNTs are very rapid in the π-conjugated of MWCNT structures; thus the efficient electron transfer facilitates the separation of photogenerated e⁻/h⁺ pairs. Facilitating their usage in catalytic process contribute to the superior photocatalytic activity of the modified MWCNTs/Ag/TiO₂NTs/Ti sample.

For the Ag/TiO₂NTs/Ti sample, the photocatalysis results also showed that it was more active than pure TiO₂NTs/Ti. The higher photocatalytic efficiency may be ascribed to the silver particles that can reduce e⁻/h⁺ recombination rate by accommodating photogenerated electrons. Silver nanoparticle (Ag) has a higher work function than TiO₂, 4.65 versus 4.2 eV.²⁶ So, Ag deposited on the TiO₂ nanotubes could markedly act as electron trappers to reduce the e⁻/h⁺ recombination rate and improve the photocatalytic activity. The reason for higher activity of TiO₂NTs/Ti loaded with Ag nanoparticles can be explained via the Schottky barrier formed at the metal/semiconductor interface.^26,27 It plays a key role as an effective electron trap to prevent e⁻/h⁺ recombination in the photocatalysis process. Fig. 8 presents the consumption of MO in the presence of MWCNTs/Ag/TiO₂NTs/Ti with time as signified by the light absorption.

Fig. 8 Typical absorbent spectra of the MO solution illuminated by UV light at different times in the presence of MWCNTs/Ag/TiO₂NTs/Ti.

To further investigate the photocatalytic activity of the fabricated samples, the COD test was carried out. COD has been extensively used as an efficient technique to estimate the organic strength of wastewater. The COD test enables measurement of waste in terms of the total oxygen quantity needed for the oxidation of organic molecules to CO₂ and H₂O. The results demonstrate that the most active photocatalyst was MWCNTs/Ag/TiO₂NTs/Ti as displayed in Fig. 9. The decrease in the COD values of the treated MO solution shows that the mineralization of MO occurs along with the colour removal. A maximum of 84% of the degradation efficiency was gained by MWCNTs/Ag/TiO₂NTs/Ti photocatalyst in the present study, which is acceptable.

Fig. 9 COD removal and photocatalytic degradation efficiency of the MO solution.

3.2.2. Photostability. The preliminary stability of the MWCNTs/Ag/TiO₂NTs/Ti sample was evaluated by the repeated photocatalytic tests on the degradation of MO using recycled samples under the same reaction conditions. As shown in Fig. 10a and b, MWCNTs/Ag/TiO₂NTs/Ti activity exhibits a slight decline after five cycling tests; 92.57% of the original MO was degraded, whereas it was 100% in the first run, proving that the MWCNTs/Ag/TiO₂NTs/Ti sample is stable enough during the photocatalytic reaction. Therefore, MWCNTs/Ag/TiO₂NTs/Ti was effective, stable and was not effectively photocorroded during the photocatalytic degradation of MO under UV light irradiation.

Fig. 10 (a) Cyclic degradation curve and (b) the degradation efficiency of MO under UV light irradiation for the MWCNTs/Ag/TiO₂NTs/Ti photocatalyst.

3.2.3 Kinetic analysis. The photocatalytic degradation kinetic was studied; the linear simulation of MO photodegradation under UV light illumination can be accounted for by a pseudo-first-order model, called the Langmuir–Hinshelwood (L–H) kinetics model.^22,25in which r is the degradation rate (mg L⁻¹ min⁻¹), C is MO concentration after various intervals of time (mg L⁻¹), t is illumination time (min), k is rate constant (min⁻¹) and K is the adsorption coefficient of MO dye (L mg⁻¹). At low MO concentration, The L–H kinetic model is well established for heterogeneous photocatalysis.²⁸ Therefore, eqn (2) is reduced to eqn (3) as below:k_app was obtained for MO degradation from plotting ln(C₀/C_t) versus t (Fig. 11a and b). In order to make a more useful comparison, the pseudo-first-order rate law was applied to the photocatalytic systems for studying the MO degradation kinetics in the presence of prepared photocatalyst samples. The obtained results present a good correlation with pseudo-first-order reaction kinetics. As seen in the Fig. 11b, the modified MWCNTs/Ag/TiO₂NTs/Ti displayed the fastest rate of MO degradation under UV light illumination.

Fig. 11 (a) Kinetic fit and (b) rate constant for the degradation of MO under UV light irradiation with TiO₂NTs/Ti, Ag/TiO₂NTs/Ti and MWCNTs/Ag/TiO₂NTs/Ti.

4. Conclusions

The novel samples developed in this study are new photocatalysts working under UV light irradiation. The prepared photocatalysts demonstrate high photocatalytic activity with respect to MO in aqueous solution. Evidence indicates that Ag/TiO₂NTs/Ti is photoactive component and that MWCNTs species are probably contributing to facilitate both electron transfer and electrical conductivity. Due to the superior specific properties, new MWCNTs/Ag/TiO₂NTs/Ti promotes its practical application in addressing various environmental issues. To the best of our knowledge, this is the first example in which MWCNTs/Ag/TiO₂NTs/Ti plates have been used as the photocatalysts for MO degradation.

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