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Carbon functionalized TiO2 nanofibers for high efficiency photocatalysis

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Carbon functionalized TiO2 nanofibers for high efficiency photocatalysis Kakarla Raghava Reddy, Vincent G Gomes and Mahbub Hassan School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia E-mail: [email protected] Received 16 October 2013, revised 18 November 2013 Accepted for publication 17 December 2013 Published 30 January 2014 Materials Research Express 1 (2014) 015012 doi:10.1088/2053-1591/1/1/015012

Abstract

TiO2 nanofibers (30–50 nm diameter), fabricated by the electro-spinning process, were modified with organo-silane agents via a coupling reaction and were grafted with carbohydrate molecules. The mixture was carbonized to produce a uniform coating of amorphous carbon on the surface of the TiO2 nanofibers. The TiO2@C nanofibers were characterized by high resolution electron microscopy (HRTEM), x-ray diffraction (XRD), x-ray photoelectron (XPS), Fourier transform infrared (FTIR) and UV-vis spectroscopy. The photocatalytic property of the functional TiO2 and carbon nanocomposite was tested via the decomposition of an organic pollutant. The catalytic activity of the covalently functionalized nanocomposite was found to be significantly enhanced in comparison to unfunctionalized composite and pristine TiO2 due to the synergistic effect of nanostructured TiO2 and amorphous carbon bound via covalent bonds. The improvement in performance is due to bandgap modification in the 1D co-axial nanostructure where the anatase phase is bound by nano-carbon, providing a large surface to volume ratio within a confined space. The superior photocatalytic performance and recyclability of 1D TiO2@C nanofiber composites for water purification were established through dye degradation experiments. Keywords: one-dimensional TiO2, carbon nanomaterial, functionalization, photocatalysis 1. Introduction

Recent interest in one-dimensional (1D) nanostructured materials is due to their potential for fabricating nano-devices, catalysts and for opto-electronic and nanomedicine applications. Their unique physico-chemical and optical properties [1–5] stem from their outstanding surface to volume ratio and the quantum confinement effects of 1D nanomaterials. Among the inorganic Materials Research Express 1 (2014) 015012 2053-1591/14/015012+15$33.00

© 2014 IOP Publishing Ltd

Mater. Res. Express 1 (2014) 015012

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nanomaterials, titania (TiO2) has been receiving considerable attention owing to its advantage of wide compatibility, unique optical characteristics and semiconducting properties with large exciton binding energy. These desirable attributes make titania a potential candidate for applications in solar cells, sensors, Li-ion batteries, field-effect transistors, UV lasers, photodetectors and photocatalysts [6–9]. The use of TiO2 for degrading environmental pollutants has stimulated interest due to its high efficiency, nontoxic nature and low cost [6, 8]. In particular, nano-structured TiO2 has potential for high photocatalytic activity due to its chemisorptive sites and relatively large surface to volume ratio relative to bulk titania. Carbon nanomaterials are finding diverse applications as dispersions in polymers, paints, surface coatings, ceramics, metals and metal oxides for improvement of corrosion resistance, thermal stability, enhancement of mechanical, rheological and electrical properties, and biocompatibility [10–20]. Carbon nanostructures with novel functionality have been prepared from inexpensive, naturally occurring precursors by carbonization of biomass and polymers [21–24]. These novel carbonaceous materials have shown promising applications in areas such as electronics, biological materials and for templates. Properties of TiO2 nanomaterials are tunable due to their physiolocal conditions, and functionalized 1D nanostructures with a TiO2 core could enhance effectiveness relative to TiO2 alone. Nano-carbon is a versatile, conductive, low work function material, stable under corrosive conditions; hence, in a nanocomposite it can help enhance the thermal, optical, electrical and electrochemical properties of TiO2. The synergistic effect of carbon with TiO2 has the potential to tune recombination of photo-induced electrons and holes for photocatalysis, help modify electrochemical properties and enhance environmental stability. To synthesize nanocomposites from carbon nanomaterial and TiO2, available methods include solvo-thermal, chemical vapor deposition (CVD), flame oxidation, ultrasonic spray deposition and sputtering [25–30]. These techniques suffer from disadvantages such as: (i) requirement of specialized, expensive equipment; (ii) reactions at high temperatures; (iii) formation of blended products with large agglomerates, rather than well-defined nanostructures. Also, weak interactions between TiO2 and carbon may lead to limited functionalization of TiO2 and marginal application advantage. In contrast, a dimensionally confined nanostructured system with functionalized TiO2 may be used in a range of applications involving nanoelectronics and nanomedicine; however, information on synthesis and properties of such systems is either limited or unavailable. Here we describe a new method for coating TiO2 nanofibers (NFs) with carbon in a multistep process. The synthetic strategy is shown schematically in figure 1. At the first stage, TiO2 nanofibers were prepared by the electro-spinning process. At the second stage, the surface of TiO2 NFs were modified with –NH2 groups through silanization in the presence of 3aminopropyltrimethoxysilane (APTMS) which acts as a coupling agent. Subsequently, glucose molecules were grafted onto –NH2 terminated TiO2 NFs through a reaction between the aminogroups on the TiO2 surface and aldehyde groups of glucose molecules. Finally, TiO2-grafted glucose NFs were carbonized to induce the transformation of glucose into carbon nanomaterial. The synthesized TiO2@C NFs were characterized for structural, morphological and photocatalytic properties, including tests for effects of pH and re-usability of photocatalysts.

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Figure 1. Schematic diagram describing fabrication steps of functional TiO2@C core-

sheath composite nanofibers.

2. Experimental section 2.1. Materials and procedure

Polyvinylpyrrolidone (PVP), glucose (C6H12O6) and 3-aminopropyltrimethoxysilane (APTMS) were used as received (Aldrich, USA). Tetrabutyl titanate (TBT), acetic acid, toluene, glycerol and methanol were purchased from Kanto Chem Co., Japan. Milli-Q purified water was used for all experiments. A procedure for the fabrication of TiO2 nanofibers and their functionalization with carbon is described below. Following the fabrication of the composite TiO2 catalysts, the characterization and photocatalytic tests were conducted as described below. 2.1.1. Fabrication of TiO2 nanofibers.

Initially, TiO2 NFs were fabricated by the electrospinning process conducted under ambient conditions. Weighed quantity (1.5 g) of TBT was mixed with 3 mL acetic acid and 3 mL ethanol in a single round neck flask, and stirred for 30 min. The mixed TBT solution was added to 7.5 mL ethanol containing 0.45 g polyvinylpyrrolidone and mixed by magnetic stirring for 1 h. The mixture was placed into a plastic syringe equipped with an 18-cm needle, which was connected to a high voltage (17 kV) power supply. Electro-spinning was performed to produce electrospun fibers, which were subsequently pyrolyzed in air for 3 h at 500 °C. The temperature was elevated at a rate of 10 °C min−1, to obtain high quality uniform TiO2 nanofibers, which we denote as TiO2 NFs.

2.1.2. Stages of functionalization for TiO2 nanofibers.

The functionalization of TiO2 nanofibers with carbon was conducted according to the following stages: (i) fabrication of TiO2 NFs with –OH groups on their surface and formation of –NH2 modified TiO2 NFs with covalent linkage via silane coupling reaction; (ii) grafting of glucose molecules on modified TiO2 NFs, followed by carbonization of TiO2-grafted glucose sites to produce nanocarbon on the surface of TiO2 NFs. The stages for preparing the TiO2@C NFs are described below. 3


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Stage 1: Amino-functionalized TiO2 nanofibers A weighed amount of TiO2 NFs was mixed with 20 mL freshly distilled toluene to form a colloidal dispersion under ultrasonication. The dispersion was then transferred to a three-necked flask equipped with a reflux condenser. After dispersing the TiO2 NFs in toluene, 8.36 mM of APTMS was added to the mixture and refluxed for 6 h at 110 °C under nitrogen gas flow to complete the silanization reaction. APTMS acts as a coupling agent, where silanization takes place on the surface of NFs bearing the hydroxyl groups. The amino-functionalized TiO2 NFs (referred to as TiO2-NH2) were separated from the solution by membrane-filtration, washed several times with absolute ethanol and dried under vacuum. Stage 2: Deposition of carbon on functionalized TiO2 NFs The synthesized TiO2-NH2 NFs were dispersed in 10 mL ethanol containing 0.2 g glucose. The suspension was stirred for 3 h at 60 °C to promote the functionalization of TiO2-NH2 NFs with glucose. Next, the solids were collected and washed three times with ethanol to remove the by-products and impurities. Thus, TiO2 NFs grafted on to glucose molecules were obtained. The product was transferred to a single-neck flask filled with 25 mL glycerol which acts as a solvent and thermal medium. The viscosity adjustment allowed the TiO2 NFs to be well dispersed in the solvent and to prevent aggregation of the carbonized product. After purging with nitrogen for 10 min, the solution was quickly transferred to a Teflon lined autoclave and sealed for thermal treatment at 150 °C for 6 h. During this process, the solid synthetic product changed its color from an initial white to a brownish hue, suggesting the presence of carbon on the product. No white solids were observed subsequently. After the reaction, the product was cooled to room temperature, filtered using membrane filter and washed several times with distilled water and ethanol. The substance was dried under vacuum at room temperature for 24 h to obtain the carbon functionalized TiO2 nanofibers, which we denote as functional TiO2@C NFs. For comparison, mixed and unfunctionalized TiO2 fiber–carbon composite was prepared under the same conditions using the same proportion of glucose that was used for synthesizing functionalized TiO2@C composites, but without using any silane agent. The size and morphology of the materials were determined by field transmission micrographs produced from a Topcon EM-002B high resolution transmission electron microscope (HRTEM). The samples for TEM analysis were prepared by sonication in ethanol to produce a uniform dispersion. A few drops of the mixture were transferred by a pipette onto a carbon-coated copper grid. The phase compositions and x-ray diffraction (XRD) patterns of the products were recorded using a Rigaku RINT 1500 diffractometer using Cu K radiation (λ = 1.54 Å). The presence of Ti and carbon in the composites was analyzed using x-ray photoelectron spectroscopy (XPS) with non-monochromated Mg-Kα radiation as the excitation source. Fourier transform infrared (FTIR) spectra were recorded on a JASCO 6100 spectrometer using KBr pellets. The ultraviolet and visible spectra of the samples were recorded using a UV-vis (U-2450, Shimadzu) spectrometer.

2.1.3. Characterization.

2.1.4. Photocatalytic tests.

The photoreactor was designed with an internal light source surrounded by a water-cooling quartz jacket which contained the UV light source. The distance between the sample and the lamp was 10 cm. The photocatalytic activity of the prepared samples was evaluated by their ability to decompose an organic dye, methyl orange (MO), in aqueous suspension (5 mg L−1) under UV-light at room temperature. In a typical process, 0.12 g of the prepared photocatalysts were dispersed in 50 mL dye solution in a reactor under 4


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magnetic stirring. Dye degradation was monitored at regular intervals by measuring the absorption of the dye solution at 464 nm using the UV-Vis spectrometer. Prior to photocatalytic reaction, the aqueous dye solutions containing photocatalysts (TiO2 NFs, functional TiO2@C NFs, and mixed TiO2-carbon composites) separately were kept in the dark-room for 2 h to establish an adsorption-desorption equilibrium. We found a negligible amount of dye adsorption (85% for 40 min). For a first order process, the rate constant of dye degradation of functional TiO2@C NFs was estimated to be 0.0412 min−1. To evaluate the photocatalytic activity of the functional TiO2@C catalyst in acidic and basic conditions, we carried out experiments for photocatalytic degradation of MO at variable pH values ranging from 3–12. We found that the photocatalytic activity of functional TiO2@C NF was practically unchanged at various pH values (figure 10(a)). This is due to the improved chemical and environmental stability arising from the uniform coating of nano-carbon on the TiO2 NF surface, and the high separation efficiency of photogenerated electrons at the TiO2–nanocarbon interface. 10


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Figure 7. UV-Vis absorption spectra during photodegradation of MO under UV-

illumination at regular time intervals with (a) TiO2 NFs, (b) functional TiO2@C NFs.

Applications involving multiple cycles while maintaining high photocatalytic activity are desirable for long-term use of a catalyst. The stability of functional TiO2@C NFs in degrading MO was examined during a series of cyclic multiple applications at pH 7 (figure 10(b)) using the same amount of catalyst for all experiments. The catalysts were washed thoroughly with water and dried after each cycle. We found that the TiO2@C NF catalysts were effective for degrading MO under UV light, and that the photocatalyst could be re-used without any measurable change in photocatalytic activity. On re-using the composite photocatalyst over several cycles to decompose the pollutant, its activity did not diminish due to the covalently bonded nano-structure with high aspect ratio, an arrangement that provided catalytic sites and also promoted electron-hole pair separation efficiently. Further, the catalyst did not exhibit deactivation and maintained its stability during multiple tests. The results at pH 7 are similar to those obtained after acid/base treatment. The tests confirm that the functional TiO2@C composite structures possess excellent acid/base resistance and chemical stability. Thus, the

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Figure 8. Normalized MO concentration as a function of time for photocatalytic

degradation of MO under UV-light: (a) without catalyst, (b) TiO2 NFs, (c) mixed TiO2 and carbon composite, (d) functionalized TiO2@C NFs.

Figure 9. Images of filtrate MO solution degradation with functional TiO2@C NFs

catalyst at regular time intervals: (a) 0 min, (b) 20 min, (c) 40 min, and (d) 120 min.

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Figure 10. (a) Degradation profiles of MO over functional TiO2@C NFs at variable

pH values of: (i) 3 (acidic), (ii) 7 (neutral) and iii) 12 (basic); (b) Photocatalytic activity of functional TiO2@C NFs for MO degradation with application cycles at pH 7: (i) cycle 1; (ii) cycle 2 after regeneration and (iii) cycle 3 after regeneration.

variable pH and cyclic tests demonstrate pollutant abatement capability under environmental stress, and potential use in removing pollutants from acid/base waste water systems. 3.4. Photocatalysis mechanism

The higher photocatalytic activity of functional TiO2@C NFs is due to the synergistic effect of carbon and TiO2. It is known that carbon nanomaterials are good electron acceptors and semiconducting TiO2 materials are good electron donors under UV radiation. Carbon as a low work function (3.5–4.0 eV) material when grafted on TiO2 is able to facilitate the flow of photogenerated electrons and holes to occur in the opposite direction, and thereby help prolong the electron lifetime and utilize light with greater efficiency. Another important factor in enhancing photoactivity is the large nanofiber surface area and the hybrid TiO2@C structure that is able to expose reactive sites. The mechanism underpinning the photocatalytic 13


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degradation of MO is shown by the following reaction steps:

TiO2@Carbon NFs + hυ (400 nm > λ > 100 nm) → + e − (carbon interface) + h+ (TiO2NFs)

e− + O2 → O2 •−

O2 •− + H 2O → HO2 • + OH − HO2 • + H 2O → H 2O2 + OH •

H 2O2 → 2OH • h+ + OH → OH • OH • + organic pollutant (MO) → degradation products + CO2 + H 2O The reaction steps show that the photogenerated electrons produced via UV light irradiation are freely transferred from the conduction band of anatase TiO2 NFs to the surface of nano-carbon which binds with conjugated MO dye molecules via π-π stacking. Meanwhile, the photogenerated holes remain in the valence band of TiO2 NFs. Subsequently, the holes (h+) are trapped by the surface hydroxyl groups of the fibrous catalyst surface to yield OH• radicals. Dissolved oxygen molecules react with the surface of the carbon electrons (e−) to yield superoxide radical anions (O2•−), which on protonation generate the hydroperoxy radicals (HO2•). These hydroxyl radicals act as strong oxidizing agents in decomposing organic pollutants, thereby enhancing the photocatalytic activity of carbon-functionalized TiO2 NFs. 4. Conclusions

We developed an efficient method for functionalizing TiO2 NFs with carbon via sequential synthetic steps comprising silanization, grafting and carbonization. The method produced a special cable structured functional TiO2-carbon nanohybrid with strong interactions between the two components. The structural, morphological and photocatalytic degradation properties of the synthesized nano-fibrous photocatalysts were systematically characterized. For functional TiO2@C NFs, the a thin carbon nanolayer was found to be uniformly distributed on the surface of TiO2 NFs to form stable and high-quality surface of TiO2@C nano-fibrous photocatalyst. The novel heterostructures of fibrous photocatalyst exhibit higher photocatalytic activities for the degradation of MO dye compared to pristine TiO2 NFs under UV-light. They also showed excellent stability for cycling and re-use under variable pH conditions. Further, the photocatalytic activity of functional TiO2@C composite synthesized through the grafting method were found to be higher than that of mixed TiO2 and carbon prepared without a silane agent. The enhanced catalytic activity is due to both the morphology of the carbon and the incorporation of nanocarbon in TiO2 NFs. Owing to their unique structural features and physiochemical properties, the carbon-functionalized titanium oxide hybrid nanofibers have potential for future applications in waste water purification. Acknowledgements

The authors greatly acknowledge the support from an ARC Linkage grant and The University of Sydney Research Fellowship. 14


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