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A facile approach for high surface area electrospun TiO2 nanostructures for photovoltaic and photocatalytic applications T. A. Arun, Asha Anish Madhavan, Daya K. Chacko, G. S. Anjusree, T. G. Deepak, Sara Thomas, Shantikumar V. Nair and A. Sreekumaran Nair* A rice-shaped TiO2–ZnO composite was prepared by electrospinning a mixture comprising the precursors of TiO2 and ZnO in polyvinyl acetate polymer dissolved in N,N-dimethyl acetamide. The electrospun nanofibers upon heat treatment in air resulted in collapse of the continuous fiber morphology and the formation of the rice-shaped TiO2–ZnO composite. The TiO2–ZnO composite was then treated with dilute acetic acid under hydrothermal conditions to etch ZnO from the TiO2–ZnO composite to get coral-shaped anisotropic TiO2. The structural anisotropy of TiO2 produced by the selective etching of ZnO resulted in a high surface area of 148 m2 g−1 for the TiO2. The initial and final materials were characterized
Received 5th October 2013, Accepted 6th January 2014 DOI: 10.1039/c3dt52780h www.rsc.org/dalton
1.
by scanning electron microscopy, transmission electron microscopy, Raman and XPS spectroscopies, powder X-ray diffraction and BET surface area measurements. The utility of the anisotropic TiO2 in photovoltaics and photocatalysis was explored. Dye-sensitized solar cells fabricated using the TiO2 showed a conversion efficiency of 6.54% as against 4.8% for a control experiment with the rice-shaped TiO2. The anisotropic TiO2 also showed good photocatalysis in the degradation of methyl orange dye and phenol.
Introduction
Nanostructured TiO2 materials have found widespread applications in many areas such as the ceramic industry,1 dye/ quantum dot-sensitized solar cells2 (DSCs/QDSCs), water splitting,3 Li-ion batteries,4 photonic crystals,5 sensors,6 water and air purification,7 self-cleaning coatings,8 etc. Generally, for the applications mentioned above, high surface area anatase TiO2 is essential. Various routes are available for synthesizing TiO2 such as sol–gel methods,9 hydrothermal routes,10 electrospinning,11 etc. Electrospinning is a versatile method for getting TiO2 in the one-dimensional (1D) form of nanofibers which have many appealing aspects for applications in energy and environmental areas such as semi-directed electron transport,12 the presence of straight pores which facilitate intercalation/de-intercalation of electrolyte species,13 etc. Despite these appealing aspects, the electrospun TiO2 materials usually have low surface areas in the range of 40–60 m2 g−1 (ref. 14) unless they are post-treated.15 The titanate route has been found to be a means of increasing the surface area of electrospun and other TiO2 by chemically transforming them into sodium/potassium titanate and subsequent conversion of the titanate into TiO2; overall this is a two-step
hydrothermal process which is expensive and time consuming.16 However, simple approaches for getting high surface area TiO2 (>100 m2 g−1) are lacking in the literature, especially for electrospun TiO2. Here, we present a facile approach for fabricating a composite of two metal oxides (TiO2–ZnO) and subsequent etching of ZnO as a means of creating structural anisotropy and high surface area for the electrospun TiO2. To test this methodology, we have chosen rice-shaped electrospun nanostructures as a model though the methodology can work well with other metal oxide nanostructures of any morphology. A rice-shaped TiO2–ZnO composite was fabricated by electrospinning a mixture of TiO2 and ZnO precursors in the presence of polyvinyl acetate (PVAc). Subsequent sintering of the TiO2–ZnO–PVAc composite resulted in polymer degradation and formation of a rice-shaped TiO2–ZnO composite. Etching of ZnO from the composite using dil. acetic acid (since the TiO2–ZnO composite is porous, complete etching of the ZnO was possible using an acid bath of acetic acid) under hydrothermal conditions resulted in coral-shaped TiO2 of higher surface area ( porosity) than the starting material. The utility of the material was explored in DSCs and photocatalysis.
2. Experimental details 2.1.
Amrita Centre for Nanoscience & Molecular Medicine, Amrita Institute of Medical Sciences, AIMS Ponekkara PO, Kochi 682041, Kerala, India. E-mail: [email protected]
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Synthesis of a rice-shaped TiO2–ZnO composite
Rice-shaped TiO2–ZnO composites were fabricated by electrospinning followed by a sintering process. The procedure is briefly
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outlined as follows: 1.2 g of polyvinyl acetate (PVAc, Mw = 500 000, Sigma Aldrich, Germany) was dissolved in 10 mL of N,N-dimethyl acetamide (DMAc, 99%, LR grade, Nice Chemicals, India) under stirring. To this solution, 2 mL of glacial acetic acid (AcOH, 99%, Nice Chemicals, India), 0.75 mL of titanium(IV) isopropoxide (TiP, 99%, Sigma Aldrich, Germany) and 0.25 g of zinc acetate (99%, Nice Chemicals, India) were added. This mixture was stirred for about 12 h for getting homogeneity and viscosity for the purpose of electrospinning. The mixture was electrospun at 15 kV with a flow rate of 1 mL h−1 using a home-made electrospinning setup (Nano Electrospinning Unit, Holmark, Optomechatronics, Kerala, India). The distance between the needle tip and the collector (a rotating drum) was 12 cm. The humidity level inside the electrospinning chamber was maintained at ∼55%. The electrospun fiber mats were peeled-off from the collector and kept for sintering at 500 °C for 1 h for the complete removal of the polymer, leaving the rice-shaped TiO2– ZnO composite. It must be noted that electrospinning TiO2 in the presence of PVAc produces a rice-shaped morphology while that in the presence of polyvinyl pyrrolidone (PVP) gives nanofibrous TiO2.17 The TiO2–ZnO composite was characterized by spectroscopy, microscopy and BET surface area measurements. 2.2.
Selective etching of ZnO from the TiO2–ZnO composite
About 100 mg of the rice grain-shaped TiO2–ZnO flakes was chemically treated using excess of dilute acetic acid (0.5 M) at 180 °C for 24 h in a steel-lined autoclave to completely etch the ZnO. We have chosen acetic acid for the purpose as it is a slow etchant for ZnO in comparison to the use of mineral acids. The acid-treated material was then washed in Millipore water 5 times and finally with methanol. The resulting anisotropic TiO2 thus obtained was then dried at 80 °C in an oven. The etched sample (TiO2) was characterized by spectroscopy, microscopy and BET surface area measurements. 2.3. Characterization of the as-synthesized TiO2–ZnO composite The rice-shaped sintered TiO2–ZnO composite and the etched TiO2 material were characterized by spectroscopy and microscopy. The scanning electron microscopy (SEM) of the samples was carried out using a JSM 6490 LA (JEOL, Tokyo, Japan) machine at an operating voltage of 15 kV. The sample was made electrically conductive by sputter-coating gold over it using a sputter coating unit (JEOL, Tokyo, Japan). The sample was coated on a sticky carbon tape and was fixed onto the Ti stub. TEM analysis of the sample was done using a JEOL 3010 machine operated at 300 kV. The sample was dispersed in methanol by sonication and a drop of the suspension was cast on a carbon-coated Cu grid and dried under vacuum. The surface area and pore size analyses of the samples were carried out using a Brunauer–Emmett–Teller (BET) surface area analyzer (Tristar II 3020 Surface Area Analyzer of Micromeritics, USA). The powder XRD was performed using the instrument X’pert pro PAN Analytical operated at data intervals of 0.03°
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and a current of 30 mA with a voltage of 40 kV. The X-ray Photoelectron Spectroscopy (XPS) (Kratos Analytical, UK) was done under a standard protocol for getting the survey spectrum for elemental analysis and high-resolution spectra for getting the oxidation state of the elements. Raman spectra were measured using a Witec confocal Raman-300 AR instrument using an excitation laser of 488 nm wavelength and power of 0.6 µW. The spot size was >2 µm. 2.4.
Fabrication of dye-sensitized solar cells (DSCs)
The DSC was fabricated from the anisotropic TiO2 by means of the screen printing technique. A comparison study has also been demonstrated with the electrospun rice-shaped TiO2. About 100 mg of the respective samples were mixed with 100 µL of the polyester. Polyester was prepared as per previous reports.14c This mixture was sonicated for 24 h for making a paste with the right rheology for doctor-blading. The paste was screen printed to a thickness of 15 µm on clean fluorinedoped tin oxide (FTO) plates (sheet resistance of 6–8 Ω per □) and kept in a vacuum for relaxation of the TiO2 pastes. The TiO2 films were then sintered at 450 °C for 1 h for making uniform porous TiO2 films by degradation of the polymer from the mixture. The thickness of the electrodes was now reduced to 11 µm (due to the removal of the polymer from the mixture). The electrode was immersed in 0.5 mM N3 dye solution (in a 1 : 1 acetonitrile–tert-butanol mixture) for 24 h to have saturable chemisorption of the sensitizer to the TiO2 on an active area of 0.16 cm2. The dye-anchored TiO2 electrodes were washed in absolute methanol for removal of the physisorbed dyes. These photoelectrodes were dried in a vacuum and sealed against a Pt counter electrode with a piece of parafilm as the spacer and I3−/I− as the electrolyte. The current– voltage (I–V) behavior of the as-fabricated cells was characterized using a Keithley 2420 digital source meter under an illumination of 1 Sun (Newport Oriel class A-Solar simulator, USA) and the incident photon-to-electron conversion efficiency (IPCE) was measured using an Oriel Newport (QE-PV-SI/QE) IPCE Measurement kit (USA). 2.5.
Photocatalysis experiments
The photocatalytic activity measurement of the anisotropic TiO2 fabricated through selective etching was carried out against the electrospun rice-shaped TiO2 for comparison. About 1 g of both the samples was made into a smooth paste with the polyester polymer ( paste preparation was similar to that in the DSC application mentioned above). A thin film of the anisotropic TiO2 sample and the rice-shaped TiO2 with an optimum thickness of 30 µm was coated on two glass plates with an exposed area of 2 cm2. The samples were sintered at 450 °C for 3 h for retaining the crystallinity. The experiment was carried out by adding two drops of methyl orange dye (MO, 0.05 mM) on the exposed area of the sintered samples. The samples were kept exposed under UV light (Sankyo Denki, Japan, model G15T8, power 15 W) for 20 min with intervals of 5 min and digital photographs were taken. The degradation of MO and phenol was also analyzed by a solution phase study. About 100 mg of
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both the samples were dispersed in 0.002 mM phenol solution in 250 mL water taken in a glass beaker. The solutions under constant stirring were UV irradiated. For the photocatalytic degradation studies of MO, about 100 mg of the coral- and rice-shaped TiO2 samples were dispersed in 0.1 mM MO dye solution taken in 250 mL water and the experiment was carried out as above. UV-Vis spectra of the test solutions were taken at 20 min intervals and that of the MO were taken at 10 min intervals. The absorption spectra were measured using a UV 1700 Pharma Spec UV-Vis spectrophotometer (Shimadzu, Japan).
3. Results and discussion 3.1. Morphology of the electrospun nanofibers and the sintered nanocomposites The morphology of the electrospun TiO2–ZnO–PVAc composite nanofibers is shown in the SEM image in Fig. 1. The fibers were smooth and continuous with an average diameter of 150 nm. The inset of Fig. 1 shows a resolved image. The fibers after sintering gave rise to nearly rice-shaped TiO2–ZnO structures (Fig. 2A and B, respectively). Electrospinning pure TiO2 gives rise to a perfect rice-like morphology17 for the TiO2 on sintering and the small distortion from the rice-like morphology for the present composite may be because of the incorporation of a ZnO precursor along with that of TiO2 in the electrospinning solution. The average dimensions of ricelike structures were ∼500 nm in length and ∼250 nm in breadth, respectively. Fig. 2C shows the TEM image of a few rice-like structures (the morphology destruction is because of the sonication of the material to get a fine dispersion for casting onto the TEM grids). The TEM image in Fig. 2D shows the lattice-resolved image showing the prominent (101) and (211) lattice planes of TiO2 with an interplanar spacing of 0.35 nm and 0.17 nm, respectively. The interplanar spacings of 0.24 nm and 0.26 nm, respectively, correspond to (101) and
Fig. 1 SEM image of the TiO2–ZnO–PVAc as-spun composite fibers. The inset shows an enlarged view.
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Fig. 2 Large area (A) and resolved (B) SEM images of the rice-shaped TiO2–ZnO composite. (C) A TEM image revealing that the rice-shaped composite is actually made of small particles ∼4–5 nm in size. (D) A lattice-resolved image showing the prominent lattice planes of TiO2 and ZnO. The composite of two metal oxides is polycrystalline in nature (SAED in the inset of D).
Fig. 3 Powder XRD (A) and Raman (B) spectra of the TiO2–ZnO composite showing the relevant peaks corresponding to the metal oxides.
(002) lattice planes of ZnO in the TiO2–ZnO composite. The SAED pattern shown in the inset of Fig. 2D indicates the polycrystalline nature of the metal oxide composite. Fig. 2C shows that the rice-like structures were actually made up of small spherical grains of ∼5 nm size. Fig. 3A shows the XRD pattern of the TiO2–ZnO composite. Distinct peaks of ZnO and TiO2 (anatase phase) could be seen in the spectrum (the respective peaks are indexed in the spectrum itself using the JCPDS file numbers 80-0075 for ZnO and 86-1157 for TiO2). The Raman spectrum (Fig. 3B) of the TiO2– ZnO composite indicates the prominent TiO2 and ZnO peaks. The peaks at 171 cm−1 and 646 cm−1, respectively, correspond to the Eg modes and that at 414 cm−1 and 526 cm−1 denote B1g and A1g modes, respectively. Raman peaks corresponding to the ZnO at 374 cm−1 and 455 cm−1 represent the A1 (TO) and the E2 (high) modes, respectively.18 Fig. 4A shows the XPS survey spectrum of the TiO2–ZnO nanocomposite. Fig. 4B–D, respectively, show the high-resolution spectra of the elements Ti, O and Zn. The peak positions of Ti 2p3/2 and Ti 2p1/2 with the respective binding energies centered at 459 eV and 465 eV show a spin–orbit coupling of
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Fig. 6 Powder XRD (A) and Raman (B) spectra of the TiO2 obtained from the TiO2–ZnO composite.
Fig. 4 XPS spectrum of the TiO2–ZnO composite (A – survey spectrum; B, C & D – high-resolution spectra of Ti, O and Zn).
∼6 eV. The de-convoluted peaks of O1s show two peaks at 529.5 eV and 531 eV which correspond to the oxygen atoms in TiO2 and ZnO, respectively.19 The Zn 2p3/2 and Zn 2p1/2 states have binding energies of 1025.2 eV and 1048.5 eV, respectively, with a spin–orbit coupling of 23.5 eV. 3.2.
Morphology of the etched material
Fig. 5A and B, respectively, show the SEM images of the TiO2 obtained by selective etching of ZnO from the TiO2–ZnO nanocomposite. From the images, it is clear that the initial rice-like morphology of the sintered sample was completely changed to a new mesoporous morphology (similar to that of coral reefs) for TiO2 upon etching the composite. The TEM image of the coral-shaped TiO2 is shown in Fig. 5C and a lattice-resolved image is shown in Fig. 5D, respectively. The TEM image in Fig. 5C shows that the coral-shaped TiO2 is
Fig. 7 XPS spectrum of the TiO2 obtained from the TiO2–ZnO composite (A – survey spectrum; B, C & D – high resolution spectra of the elements Ti, O and Zn; please note the near complete absence of Zn because of its etching from the composite).
actually made up of small spherical particles ∼5 nm in size. The lattice resolved image in Fig. 5D shows an interplanar spacing of 0.35 nm corresponding to the anatase phase TiO2. An SAED image given in the inset shows its polycrystalline nature. The XRD pattern and Raman spectra (Fig. 6A and B, respectively) of the etched sample confirm the complete removal of ZnO from the binary composite showing the dominant peaks of TiO2. This was additionally confirmed by EDAX measurements as well. The XPS of the coral reef-shaped TiO2 is shown in Fig. 7. It can be seen from the XPS spectra that the distinct peaks of Zn have vanished after the acid treatment implying the complete etching of the ZnO from the composite. 3.3.
Fig. 5 SEM (A & B) and TEM (C & D) images of the TiO2 obtained from the TiO2–ZnO composite by selective etching of ZnO. (D) A lattice resolved image showing the (101) lattice orientation of TiO2. The crystalline nature of TiO2 is evident from the SAED pattern.
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BET surface area analysis
The selective etching of ZnO from the TiO2–ZnO composite resulted in structural anisotropy and we anticipated that this will result in high surface area for the TiO2. Therefore, BET surface area analysis was done. The surface area of the former was found to be 40 m2 g−1 with a pore volume and pore size,
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respectively, of 0.084 cm3 g−1 and 75.6 Å. However, for the etched sample (TiO2), the BET surface area was found to be 148 m2 g−1 with the average pore volume and pore size, respectively, of 0.096 cm3 g−1 and 84.2 Å. Thus, through this facile approach, the surface area of TiO2 was enhanced by ∼3.5 times. It must be noted that the BET surface areas of electrospun TiO2 nanostructures are only in the range of 40–60 m2 g−1.14 3.4.
Effect of acid concentration on the TiO2 morphology
As the acetic acid used was an etchant of ZnO, we believed that the concentration of acetic acid will have some role in the morphology of the TiO2 and there will be a threshold concentration below which the etching process may not be so significant. This investigation revealed that when the concentration of acetic acid was lower than 0.05 M, the rice-like morphology was more or less retained which implies that etching of the ZnO from the composite was not significant. When the concentration of the acetic acid was increased to 0.5 M and above, the rice-shaped composite underwent complete etching, resulting in coral reef-shaped TiO2. 3.5.
Application in DSCs
Porous and high surface area coral-shaped TiO2 was used in DSCs as a photoanode with I3−/I− as the redox couple and Pt as the counter electrode. The active area of the DSC was 0.16 cm2 with a thickness of 11 µm and was square shaped. A comparison of the photovoltaic performance of the coral reefand rice-shaped TiO2 is shown in Fig. 8. The former has shown a current density ( Jsc) of 12.9 mA cm−2, an open-circuit voltage (Voc) of 0.75 V, a fill factor (FF) of 65.7% and an overall conversion efficiency of 6.54% under 1 Sun (AM 1.5G conditions) whereas the rice-shaped TiO2 has shown a Jsc of 9.65 mA cm−2, a Voc of 0.75 V, an FF of 64.9% and an overall conversion efficiency of 4.8% under the same conditions. Upon comparison of the two sets of photovoltaic parameters, it became obvious that the major difference is only in the Jsc value which was responsible for the 36.2% increase in
Fig. 8 TiO2.
I–V Traces of DSCs fabricated using rice- and coral-shaped
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Fig. 9
IPCE traces of rice- and coral-shaped TiO2 DSC devices.
efficiency. This 1.4 times (36.2%) increase in efficiency could be mainly attributed to the increase in surface area of the former, achieved through the selective etching process which resulted in an increased dye-loading (1.91 × 10−7 mol cm−2 for the coral-shaped TiO2 vs. 1.60 × 10−7 mol cm−2 for the riceshaped TiO2). It must be noted that the efficiency achieved by the coral reef-shaped TiO2 is one of the impressive values achieved from electrospun TiO2 material-based DSC devices.14c,d,20 From the IPCE spectra in Fig. 9, it is clear that the coral reef-shaped TiO2 shows an incident photon-to-electron conversion efficiency maximum of 63% (at 540 nm corresponding to the absorption maximum of the dye) whereas the rice-shaped TiO2 showed only 45%. The effect of enhanced scattering by the anisotropic coral reef-shaped TiO2 is also evident from the IPCE spectra. 3.6.
Application in photocatalysis
The photocatalytic degradation of organic dyes (say methyl orange, MO) by TiO2 in the presence of UV light has been well known, and the efficiency of photocatalysis depends on the surface area, crystallinity and phase purity of the TiO2. Though the exact mechanism of the degradation (such as the chemical nature of the intermediates formed, final products, etc.) is still not fully resolved,21 it is widely believed that the degradation proceeds in the following manner: UV irradiation produces photoexcited electrons (in the conduction band) and holes (in the valence band) in TiO2, which migrate to its surface and react with O2 and H2O, respectively, producing highly reactive superoxide (O2−) and hydroxyl (•OH) radicals. These radicals subsequently react with MO resulting in peroxylated (reduction product of MO with O2−)/hydroxylated (oxidation product of MO with •OH) intermediates which eventually get degraded to the products (CO2, H2O, and other products). We have tested the photocatalytic degradation of MO dye on the thin films of rice- and coral-shaped TiO2 (having the same area and thickness fabricated by the screen-printing technique). The degradation of MO was faster on the film
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Fig. 12 Panels (A–C) show a comparison of the photocatalytic degradation of phenol by coral- and rice-shaped TiO2. Fig. 10 A comparison of the photocatalytic activities of coral- (labeled 2) and rice-shaped (labeled 1)- TiO2.
phenol (in water) was monitored at time intervals of 20 min for up to 100 min for both the samples (Fig. 12A and B, respectively). In this case as well, faster degradation of phenol was observed in the case of the coral-shaped TiO2. The degradation in the presence of coral-shaped TiO2 was 72% after 100 min of irradiation whereas the same was only 48% (Fig. 12C) for the rice-shaped TiO2. This further supports better photocatalytic activity of the TiO2 obtained after the etching process.
4.
Fig. 11 Panels (A–C) show a comparison of the photocatalytic degradation of MO by coral- and rice-shaped TiO2. Panel (D) shows a digital photograph of the MO dye solution (left) after irradiation (50 min) in the presence of rice- (center) and coral-shaped (right) TiO2, respectively.
fabricated using coral-shaped TiO2 than that on the riceshaped one under the same UV irradiation durations (Fig. 10). It can be inferred that the faster degradation of MO dye on the coral-shaped TiO2 is because of its high surface area which supported the adsorption of more dye molecules on its surface and hence a faster degradation. In the solution phase experiment as well, the rate of degradation of MO was faster with the coral-shaped TiO2 compared to the rice-shaped one (Fig. 11A and B, respectively). About 92% degradation was observed for the former when compared to 75% for the latter after UV irradiation for 50 min (Fig. 11C). We have also probed the degradation of phenol by the respective TiO2 under UV light. The absorption spectrum of
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Conclusions
A porous coral reef-shaped TiO2 was fabricated from an electrospun TiO2–ZnO composite by selective etching of ZnO using dilute acetic acid by a hydrothermal process. The initial and final materials were characterized by spectroscopy, microscopy and BET surface area measurements. The anisotropic TiO2 exhibited high surface areas (148 m2 g−1) compared to the asfabricated TiO2–ZnO composite (38 m2 g−1). The coral reefshaped TiO2 when employed in the dye-sensitized solar cell (cells of the same area and thickness) showed an efficiency of 6.5% in comparison to 4.8% for the rice-shaped TiO2. The TiO2 also exhibited good photocatalytic properties in the degradation of MO and phenol.
5. Conflict of interest The authors declare no conflict of interest.
Acknowledgements The authors acknowledge financial assistance from the Ministry of New and Renewable Energy (MNRE), Government of India.
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Notes and references 1 (a) U. Diebold, Surf. Sci. Rep., 2003, 48, 53; (b) X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959. 2 (a) B. O’Regan and M. Grätzel, Nature, 1991, 353, 737–740; (b) K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69–74; (c) G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Nano Lett., 2006, 6, 215– 218; (d) M. Y. Song, D. K. Kim, K. J. Ihn, S. M. Jo and D. Y. Kim, Nanotechnology, 2004, 15, 1861; (e) A. S. Nair, Y. Shengyuan, Z. Peining and S. Ramakrishna, Chem. Commun., 2010, 46, 7421–7423; (f ) P. Zhu, A. S. Nair, S. Yang, S. Peng and S. Ramakrishna, J. Mater. Chem., 2011, 21, 12210–12212; (g) Y. Shengyuan, A. S. Nair, R. Jose and S. Ramakrishna, Energy Environ. Sci., 2010, 3, 2010–2014; (h) A. J. Nozik, Physica E, 2002, 14, 115–120; (i) P. V. Kamat, J. Phys. Chem. C, 2008, 112, 18737–18753; ( j) S. Rühle, M. Shalom and A. Zaban, ChemPhysChem, 2010, 11, 2290– 2304. 3 (a) M. Ni, M. K. H. Leung, D. Y. C. Leung and K. Sumathy, Renew. Sustain. Energ. Rev., 2007, 11, 401–425; (b) A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278; (c) V. J. Babu, M. K. Kumar, A. S. Nair, T. L. Kheng, S. I. Allakhverdiev and S. Ramakrishna, Int. J. Hydrogen Energy, 2012, 37, 8897–8904; (d) V. J. Babu, M. K. Kumar, R. Murugan, M. M. Khin, R. P. Rao, A. S. Nair and S. Ramakrishna, Int. J. Hydrogen Energy, 2013, 38, 4324– 4333; (e) S. U. M. Khan, M. Al-Shahry and W. B. Ingler Jr., Science, 2002, 297, 2243–2245. 4 (a) P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930–2946; (b) A. R. Armstrong, G. Armstrong, J. Canales, R. Garcia and P. G. Bruce, Adv. Mater., 2005, 17, 862–865; (c) P. Zhu, Y. Wu, M. V. Reddy, A. S. Nair, B. V. R. Chowdari and S. Ramakrishna, RSC Adv., 2012, 2, 531–537; (d) D. Deng, M. G. Kim, J. Y. Lee and J. Cho, Energy Environ. Sci., 2009, 2, 818–837; (e) S. Y. Huang, L. Kavan, I. Exnar and M. Grätzel, J. Electrochem. Soc., 1995, 142, L142–L144. 5 (a) X. D. Wang, C. Neff, E. Graugnard, Y. Ding, J. S. King, L. A. Pranger, R. Tannenbaum, Z. L. Wang and C. J. Summers, Adv. Mater., 2005, 17, 2103–2106; (b) S. Kanehira, S. Kirihara and Y. Miyamoto, J. Am. Ceram. Soc., 2005, 88, 1461–1464. 6 (a) Q. Zheng, B. Zhou, J. Bai, L. Li, Z. Jin, J. Zhang, J. Li, Y. Liu, W. Cai and X. Zhu, Adv. Mater., 2008, 20, 1044–1049; (b) C.-C. Lu, Y.-S. Huang, J.-W. Huang, C.-K. Chang and S.-P. Wu, Sensors, 2010, 10, 670–683; (c) S.-J. Bao, C. M. Li, J.-F. Zang, X.-Q. Cui, Y. Qiao and J. Guo, Adv. Funct. Mater., 2008, 18, 591–599. 7 (a) V. A. Ganesh, H. K. Raut, A. S. Nair and S. Ramakrishna, J. Mater. Chem., 2011, 21, 16304–16322; (b) v.-P. Parkin and R. G. Palgrave, J. Mater. Chem., 2005, 15, 1689–1695; (c) B. Liu, K. Nakata, M. Sakai, H. Saito, T. Ochiai, T. Murakami, K. Takagi and A. Fujishima, Catal. Sci. Technol., 2012, 2, 1933–1939; (d) J. Zhang, Y. Zhang, Y. Lei and C. Pan, Catal. Sci. Technol., 2011, 1, 273–278.
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8 (a) V. Rome′as, P. Pichat, C. Guillard, T. Chopin and C. Lehaut, New J. Chem., 1999, 23, 365–374; (b) G. K. Mora, M. A. Carvalhoa, O. K. Varghese, M. V. Pishko and C. A. Grimes, J. Mater. Res., 2004, 19, 628–634; (c) A. Nakajima, K. Hashimoto, T. Watanabe, K. Takai, G. Yamauchi and A. Fujishima, Langmuir, 2000, 16, 7044– 7047; (d) V. A. Ganesh, A. S. Nair, H. Kumar Raut, T. M. Walsh and S. Ramakrishna, RSC Adv., 2012, 2, 2067– 2072; (e) V. A. Ganesh, S. S. Dinachali, A. S. Nair and S. Ramakrishna, ACS Appl. Mater. Interfaces, 2013, 5, 1527– 1532. 9 (a) A. S. Nair, Z. Peining, V. J. Babu, Y. Shengyuan and S. Ramakrishna, Phys. Chem. Chem. Phys., 2011, 13, 21248– 21261; (b) M. Liu, L. Piao, W. Lu, S. Ju, L. Zhao, C. Zhou, H. Li and W. Wang, Nanoscale, 2010, 2, 1115–1117. 10 (a) Z. Sun, J. H. Kim, Y. Zhao, F. Bijarbooneh, V. Malgras, Y. Lee, Y.-M. Kang and S. X. Dou, J. Am. Chem. Soc., 2011, 133, 19314–19317; (b) A. K. Sinha, S. Jana, S. Pande, S. Sarkar, M. Pradhan, M. Basu, S. Saha, A. Pal and T. Pal, Cryst. Eng. Commun., 2009, 11, 1210–1212. 11 (a) D. Li and Y. Xia, Nano Lett., 2003, 3, 555–560; (b) A. Kumar, R. Jose, K. Fujihara, J. Wang and S. Ramakrishna, Chem. Mater., 2007, 19, 6536–6542; (c) D. Li, J. T. McCann, Y. Xia and M. Marquez, J. Am. Ceram. Soc., 2006, 89, 1861–1869; (d) W. E. Teo and S. Ramakrishna, Nanotechnology, 2006, 17, R89. 12 (a) K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69–74; (b) F. Sauvage, F. D. Fonzo, A. L. Bassi, C. S. Casari, V. Russo, G. Divitini, C. Ducati, C. E. Bottani, P. Comte and M. Grätzel, Nano Lett., 2010, 10, 2562–2567; (c) J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki and A. B. Walker, J. Am. Chem. Soc., 2008, 130, 13364–13372. 13 (a) G. Binitha, M. S. Soumya, A. A. Madhavan, P. Praveen, A. Balakrishnan, K. R. V. Subramanian, M. V. Reddy, S. V. Nair, A. S. Nair and N. Sivakumar, J. Mater. Chem. A, 2013, 1, 11698–11704; (b) C. A. Grimes, J. Mater. Chem., 2007, 17, 1451–1457. 14 (a) S. Chuangchote, T. Sagawa and S. Yoshikawa, Appl. Phys. Lett., 2008, 93, 033310; (b) M. Y. Song, Y. R. Ahn, S. M. Jo, D. Y. Kim and J.-P. Ahn, Appl. Phys. Lett., 2005, 87, 113113; (c) Y. Shengyuan, Z. Peining, A. S. Nair and S. Ramakrishna, J. Mater. Chem., 2011, 21, 6541–6548; (d) A. S. Nair, R. Jose, Y. Shengyuan and S. Ramakrishna, J. Colloid Interface Sci., 2011, 353, 39–45. 15 (a) A. S. Nair, Z. Peining, V. J. Babu, Y. Shengyuan, P. Shengjie and S. Ramakrishna, RSC Adv., 2012, 2, 992– 998; (b) A. S. Nair, P. Zhu, V. J. Babu, S. Yang, T. Krishnamoorthy, R. Murugan, S. Peng and S. Ramakrishna, Langmuir, 2012, 28, 6202–6206. 16 (a) T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino and K. Niihara, Langmuir, 1998, 14, 3160; (b) J. Li, H. Yang, Q. Li and D. Zu, Cryst. Eng. Commun., 2012, 14, 3019–3026; (c) Z.-Y. Yuan, X.-B. Zhang and B.-L. Su, Appl. Phys. A, 2004, 78, 1063–1066. 17 (a) Z. Peining, A. S. Nair, Y. Shengyuan, P. Shengjie, N. K. Elumalai and S. Ramakrishna, J. Photochem.
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Photobiol., A, 2012, 231, 9–18; (b) G. S. Anjusree, A. Bhupathi, A. Balakrishnan, S. Vadukumpully, K. R. V. Subramanian, N. Sivakumar, S. Ramakrishna, S. V. Nair and A. S. Nair, RSC Adv., 2013, 3, 16720–16727. 18 (a) T. Ohsaka, F. Izumi and Y. Fujiki, J. Raman Spectrosc., 1978, 7, 321–324; (b) T. Beuvier, M. Richard-Plouet and L. Brohan, J. Phys. Chem. C, 2009, 113, 13703–13706; (c) H. C. Choi, Y. M. Jung and S. B. Kim, Vib. Spectrosc., 2005, 37, 33–38. 19 (a) Y. Wang, W. Jia, T. Strout, A. Schempf, H. Zhang, B. Li, J. Cui and Y. Lei, Electroanalysis, 2009, 21, 1432–1438; (b) D. Barreca, E. Comini, A. P. Ferrucci, A. Gasparotto, C. Maccato, C. Maragno, G. Sberveglieri and E. Tondello, Chem. Mater., 2007, 19, 5642–5649; (c) R. Liu, H. Ye, X. Xiong and H. Liu, Mater. Chem. Phys., 2010, 121, 432–439. 20 (a) K. Onozuka, B. Ding, Y. Tsuge, T. Naka, M. Yamazaki, S. Sugi, S. Ohno, M. Yoshikawa and S. Shiratori,
This journal is © The Royal Society of Chemistry 2014
Paper
Nanotechnology, 2006, 17, 1026–1031; (b) S. Cavaliere, S. Subianto, I. Savych, D. J. Jones and J. Roziere, Energy Environ. Sci., 2011, 4, 4761–4785; (c) T. Krishnamoorthy, V. Thavasi, G. M. Subodh and S. Ramakrishna, Energy Environ. Sci., 2011, 4, 2807–2812; (d) D. Sabba, N. Mathews, J. Chua, S. S. Pramana, H. K. Mulmudi, K. Hemant, Q. Wang and S. G. Mhaisalkar, Scr. Mater., 2013, 68, 487– 490; (e) A. A. Madhavan, S. Kalluri, D. K. Chacko, T. A. Arun, S. Nagarajan, K. R. V. Subramanian, A. S. Nair, S. V. Nair and A. Balakrishnan, RSC Adv., 2012, 2, 13032– 13037. 21 (a) L. Yu, J. Xi, M.-D. Li, H. T. Chan, T. Su, D. L. Phillips and W. K. Chan, Phys. Chem. Chem. Phys., 2012, 14, 3589– 3595; (b) Y. Su1, Y. Yang, H. Zhang, Y. Xie, Z. Wu, Y. Jiang, N. Fukata, Y. Bando and Z. L. Wang, Nanotechnology, 2013, 24, 295401; (c) F. Chen, W. Zou, W. Qu and J. Zhang, Catal. Commun., 2009, 10, 1510–1513.
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A facile approach for high surface area electrospun TiO2 nanostructures for photovoltaic and photocatalytic applications T. A. Arun, Asha Anish Madhavan, Daya K. Chacko, G. S. Anjusree, T. G. Deepak, Sara Thomas, Shantikumar V. Nair and A. Sreekumaran Nair* A rice-shaped TiO2–ZnO composite was prepared by electrospinning a mixture comprising the precursors of TiO2 and ZnO in polyvinyl acetate polymer dissolved in N,N-dimethyl acetamide. The electrospun nanofibers upon heat treatment in air resulted in collapse of the continuous fiber morphology and the formation of the rice-shaped TiO2–ZnO composite. The TiO2–ZnO composite was then treated with dilute acetic acid under hydrothermal conditions to etch ZnO from the TiO2–ZnO composite to get coral-shaped anisotropic TiO2. The structural anisotropy of TiO2 produced by the selective etching of ZnO resulted in a high surface area of 148 m2 g−1 for the TiO2. The initial and final materials were characterized
Received 5th October 2013, Accepted 6th January 2014 DOI: 10.1039/c3dt52780h www.rsc.org/dalton
1.
by scanning electron microscopy, transmission electron microscopy, Raman and XPS spectroscopies, powder X-ray diffraction and BET surface area measurements. The utility of the anisotropic TiO2 in photovoltaics and photocatalysis was explored. Dye-sensitized solar cells fabricated using the TiO2 showed a conversion efficiency of 6.54% as against 4.8% for a control experiment with the rice-shaped TiO2. The anisotropic TiO2 also showed good photocatalysis in the degradation of methyl orange dye and phenol.
Introduction
Nanostructured TiO2 materials have found widespread applications in many areas such as the ceramic industry,1 dye/ quantum dot-sensitized solar cells2 (DSCs/QDSCs), water splitting,3 Li-ion batteries,4 photonic crystals,5 sensors,6 water and air purification,7 self-cleaning coatings,8 etc. Generally, for the applications mentioned above, high surface area anatase TiO2 is essential. Various routes are available for synthesizing TiO2 such as sol–gel methods,9 hydrothermal routes,10 electrospinning,11 etc. Electrospinning is a versatile method for getting TiO2 in the one-dimensional (1D) form of nanofibers which have many appealing aspects for applications in energy and environmental areas such as semi-directed electron transport,12 the presence of straight pores which facilitate intercalation/de-intercalation of electrolyte species,13 etc. Despite these appealing aspects, the electrospun TiO2 materials usually have low surface areas in the range of 40–60 m2 g−1 (ref. 14) unless they are post-treated.15 The titanate route has been found to be a means of increasing the surface area of electrospun and other TiO2 by chemically transforming them into sodium/potassium titanate and subsequent conversion of the titanate into TiO2; overall this is a two-step
hydrothermal process which is expensive and time consuming.16 However, simple approaches for getting high surface area TiO2 (>100 m2 g−1) are lacking in the literature, especially for electrospun TiO2. Here, we present a facile approach for fabricating a composite of two metal oxides (TiO2–ZnO) and subsequent etching of ZnO as a means of creating structural anisotropy and high surface area for the electrospun TiO2. To test this methodology, we have chosen rice-shaped electrospun nanostructures as a model though the methodology can work well with other metal oxide nanostructures of any morphology. A rice-shaped TiO2–ZnO composite was fabricated by electrospinning a mixture of TiO2 and ZnO precursors in the presence of polyvinyl acetate (PVAc). Subsequent sintering of the TiO2–ZnO–PVAc composite resulted in polymer degradation and formation of a rice-shaped TiO2–ZnO composite. Etching of ZnO from the composite using dil. acetic acid (since the TiO2–ZnO composite is porous, complete etching of the ZnO was possible using an acid bath of acetic acid) under hydrothermal conditions resulted in coral-shaped TiO2 of higher surface area ( porosity) than the starting material. The utility of the material was explored in DSCs and photocatalysis.
2. Experimental details 2.1.
Amrita Centre for Nanoscience & Molecular Medicine, Amrita Institute of Medical Sciences, AIMS Ponekkara PO, Kochi 682041, Kerala, India. E-mail: [email protected]
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Synthesis of a rice-shaped TiO2–ZnO composite
Rice-shaped TiO2–ZnO composites were fabricated by electrospinning followed by a sintering process. The procedure is briefly
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outlined as follows: 1.2 g of polyvinyl acetate (PVAc, Mw = 500 000, Sigma Aldrich, Germany) was dissolved in 10 mL of N,N-dimethyl acetamide (DMAc, 99%, LR grade, Nice Chemicals, India) under stirring. To this solution, 2 mL of glacial acetic acid (AcOH, 99%, Nice Chemicals, India), 0.75 mL of titanium(IV) isopropoxide (TiP, 99%, Sigma Aldrich, Germany) and 0.25 g of zinc acetate (99%, Nice Chemicals, India) were added. This mixture was stirred for about 12 h for getting homogeneity and viscosity for the purpose of electrospinning. The mixture was electrospun at 15 kV with a flow rate of 1 mL h−1 using a home-made electrospinning setup (Nano Electrospinning Unit, Holmark, Optomechatronics, Kerala, India). The distance between the needle tip and the collector (a rotating drum) was 12 cm. The humidity level inside the electrospinning chamber was maintained at ∼55%. The electrospun fiber mats were peeled-off from the collector and kept for sintering at 500 °C for 1 h for the complete removal of the polymer, leaving the rice-shaped TiO2– ZnO composite. It must be noted that electrospinning TiO2 in the presence of PVAc produces a rice-shaped morphology while that in the presence of polyvinyl pyrrolidone (PVP) gives nanofibrous TiO2.17 The TiO2–ZnO composite was characterized by spectroscopy, microscopy and BET surface area measurements. 2.2.
Selective etching of ZnO from the TiO2–ZnO composite
About 100 mg of the rice grain-shaped TiO2–ZnO flakes was chemically treated using excess of dilute acetic acid (0.5 M) at 180 °C for 24 h in a steel-lined autoclave to completely etch the ZnO. We have chosen acetic acid for the purpose as it is a slow etchant for ZnO in comparison to the use of mineral acids. The acid-treated material was then washed in Millipore water 5 times and finally with methanol. The resulting anisotropic TiO2 thus obtained was then dried at 80 °C in an oven. The etched sample (TiO2) was characterized by spectroscopy, microscopy and BET surface area measurements. 2.3. Characterization of the as-synthesized TiO2–ZnO composite The rice-shaped sintered TiO2–ZnO composite and the etched TiO2 material were characterized by spectroscopy and microscopy. The scanning electron microscopy (SEM) of the samples was carried out using a JSM 6490 LA (JEOL, Tokyo, Japan) machine at an operating voltage of 15 kV. The sample was made electrically conductive by sputter-coating gold over it using a sputter coating unit (JEOL, Tokyo, Japan). The sample was coated on a sticky carbon tape and was fixed onto the Ti stub. TEM analysis of the sample was done using a JEOL 3010 machine operated at 300 kV. The sample was dispersed in methanol by sonication and a drop of the suspension was cast on a carbon-coated Cu grid and dried under vacuum. The surface area and pore size analyses of the samples were carried out using a Brunauer–Emmett–Teller (BET) surface area analyzer (Tristar II 3020 Surface Area Analyzer of Micromeritics, USA). The powder XRD was performed using the instrument X’pert pro PAN Analytical operated at data intervals of 0.03°
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and a current of 30 mA with a voltage of 40 kV. The X-ray Photoelectron Spectroscopy (XPS) (Kratos Analytical, UK) was done under a standard protocol for getting the survey spectrum for elemental analysis and high-resolution spectra for getting the oxidation state of the elements. Raman spectra were measured using a Witec confocal Raman-300 AR instrument using an excitation laser of 488 nm wavelength and power of 0.6 µW. The spot size was >2 µm. 2.4.
Fabrication of dye-sensitized solar cells (DSCs)
The DSC was fabricated from the anisotropic TiO2 by means of the screen printing technique. A comparison study has also been demonstrated with the electrospun rice-shaped TiO2. About 100 mg of the respective samples were mixed with 100 µL of the polyester. Polyester was prepared as per previous reports.14c This mixture was sonicated for 24 h for making a paste with the right rheology for doctor-blading. The paste was screen printed to a thickness of 15 µm on clean fluorinedoped tin oxide (FTO) plates (sheet resistance of 6–8 Ω per □) and kept in a vacuum for relaxation of the TiO2 pastes. The TiO2 films were then sintered at 450 °C for 1 h for making uniform porous TiO2 films by degradation of the polymer from the mixture. The thickness of the electrodes was now reduced to 11 µm (due to the removal of the polymer from the mixture). The electrode was immersed in 0.5 mM N3 dye solution (in a 1 : 1 acetonitrile–tert-butanol mixture) for 24 h to have saturable chemisorption of the sensitizer to the TiO2 on an active area of 0.16 cm2. The dye-anchored TiO2 electrodes were washed in absolute methanol for removal of the physisorbed dyes. These photoelectrodes were dried in a vacuum and sealed against a Pt counter electrode with a piece of parafilm as the spacer and I3−/I− as the electrolyte. The current– voltage (I–V) behavior of the as-fabricated cells was characterized using a Keithley 2420 digital source meter under an illumination of 1 Sun (Newport Oriel class A-Solar simulator, USA) and the incident photon-to-electron conversion efficiency (IPCE) was measured using an Oriel Newport (QE-PV-SI/QE) IPCE Measurement kit (USA). 2.5.
Photocatalysis experiments
The photocatalytic activity measurement of the anisotropic TiO2 fabricated through selective etching was carried out against the electrospun rice-shaped TiO2 for comparison. About 1 g of both the samples was made into a smooth paste with the polyester polymer ( paste preparation was similar to that in the DSC application mentioned above). A thin film of the anisotropic TiO2 sample and the rice-shaped TiO2 with an optimum thickness of 30 µm was coated on two glass plates with an exposed area of 2 cm2. The samples were sintered at 450 °C for 3 h for retaining the crystallinity. The experiment was carried out by adding two drops of methyl orange dye (MO, 0.05 mM) on the exposed area of the sintered samples. The samples were kept exposed under UV light (Sankyo Denki, Japan, model G15T8, power 15 W) for 20 min with intervals of 5 min and digital photographs were taken. The degradation of MO and phenol was also analyzed by a solution phase study. About 100 mg of
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both the samples were dispersed in 0.002 mM phenol solution in 250 mL water taken in a glass beaker. The solutions under constant stirring were UV irradiated. For the photocatalytic degradation studies of MO, about 100 mg of the coral- and rice-shaped TiO2 samples were dispersed in 0.1 mM MO dye solution taken in 250 mL water and the experiment was carried out as above. UV-Vis spectra of the test solutions were taken at 20 min intervals and that of the MO were taken at 10 min intervals. The absorption spectra were measured using a UV 1700 Pharma Spec UV-Vis spectrophotometer (Shimadzu, Japan).
3. Results and discussion 3.1. Morphology of the electrospun nanofibers and the sintered nanocomposites The morphology of the electrospun TiO2–ZnO–PVAc composite nanofibers is shown in the SEM image in Fig. 1. The fibers were smooth and continuous with an average diameter of 150 nm. The inset of Fig. 1 shows a resolved image. The fibers after sintering gave rise to nearly rice-shaped TiO2–ZnO structures (Fig. 2A and B, respectively). Electrospinning pure TiO2 gives rise to a perfect rice-like morphology17 for the TiO2 on sintering and the small distortion from the rice-like morphology for the present composite may be because of the incorporation of a ZnO precursor along with that of TiO2 in the electrospinning solution. The average dimensions of ricelike structures were ∼500 nm in length and ∼250 nm in breadth, respectively. Fig. 2C shows the TEM image of a few rice-like structures (the morphology destruction is because of the sonication of the material to get a fine dispersion for casting onto the TEM grids). The TEM image in Fig. 2D shows the lattice-resolved image showing the prominent (101) and (211) lattice planes of TiO2 with an interplanar spacing of 0.35 nm and 0.17 nm, respectively. The interplanar spacings of 0.24 nm and 0.26 nm, respectively, correspond to (101) and
Fig. 1 SEM image of the TiO2–ZnO–PVAc as-spun composite fibers. The inset shows an enlarged view.
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Fig. 2 Large area (A) and resolved (B) SEM images of the rice-shaped TiO2–ZnO composite. (C) A TEM image revealing that the rice-shaped composite is actually made of small particles ∼4–5 nm in size. (D) A lattice-resolved image showing the prominent lattice planes of TiO2 and ZnO. The composite of two metal oxides is polycrystalline in nature (SAED in the inset of D).
Fig. 3 Powder XRD (A) and Raman (B) spectra of the TiO2–ZnO composite showing the relevant peaks corresponding to the metal oxides.
(002) lattice planes of ZnO in the TiO2–ZnO composite. The SAED pattern shown in the inset of Fig. 2D indicates the polycrystalline nature of the metal oxide composite. Fig. 2C shows that the rice-like structures were actually made up of small spherical grains of ∼5 nm size. Fig. 3A shows the XRD pattern of the TiO2–ZnO composite. Distinct peaks of ZnO and TiO2 (anatase phase) could be seen in the spectrum (the respective peaks are indexed in the spectrum itself using the JCPDS file numbers 80-0075 for ZnO and 86-1157 for TiO2). The Raman spectrum (Fig. 3B) of the TiO2– ZnO composite indicates the prominent TiO2 and ZnO peaks. The peaks at 171 cm−1 and 646 cm−1, respectively, correspond to the Eg modes and that at 414 cm−1 and 526 cm−1 denote B1g and A1g modes, respectively. Raman peaks corresponding to the ZnO at 374 cm−1 and 455 cm−1 represent the A1 (TO) and the E2 (high) modes, respectively.18 Fig. 4A shows the XPS survey spectrum of the TiO2–ZnO nanocomposite. Fig. 4B–D, respectively, show the high-resolution spectra of the elements Ti, O and Zn. The peak positions of Ti 2p3/2 and Ti 2p1/2 with the respective binding energies centered at 459 eV and 465 eV show a spin–orbit coupling of
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Fig. 6 Powder XRD (A) and Raman (B) spectra of the TiO2 obtained from the TiO2–ZnO composite.
Fig. 4 XPS spectrum of the TiO2–ZnO composite (A – survey spectrum; B, C & D – high-resolution spectra of Ti, O and Zn).
∼6 eV. The de-convoluted peaks of O1s show two peaks at 529.5 eV and 531 eV which correspond to the oxygen atoms in TiO2 and ZnO, respectively.19 The Zn 2p3/2 and Zn 2p1/2 states have binding energies of 1025.2 eV and 1048.5 eV, respectively, with a spin–orbit coupling of 23.5 eV. 3.2.
Morphology of the etched material
Fig. 5A and B, respectively, show the SEM images of the TiO2 obtained by selective etching of ZnO from the TiO2–ZnO nanocomposite. From the images, it is clear that the initial rice-like morphology of the sintered sample was completely changed to a new mesoporous morphology (similar to that of coral reefs) for TiO2 upon etching the composite. The TEM image of the coral-shaped TiO2 is shown in Fig. 5C and a lattice-resolved image is shown in Fig. 5D, respectively. The TEM image in Fig. 5C shows that the coral-shaped TiO2 is
Fig. 7 XPS spectrum of the TiO2 obtained from the TiO2–ZnO composite (A – survey spectrum; B, C & D – high resolution spectra of the elements Ti, O and Zn; please note the near complete absence of Zn because of its etching from the composite).
actually made up of small spherical particles ∼5 nm in size. The lattice resolved image in Fig. 5D shows an interplanar spacing of 0.35 nm corresponding to the anatase phase TiO2. An SAED image given in the inset shows its polycrystalline nature. The XRD pattern and Raman spectra (Fig. 6A and B, respectively) of the etched sample confirm the complete removal of ZnO from the binary composite showing the dominant peaks of TiO2. This was additionally confirmed by EDAX measurements as well. The XPS of the coral reef-shaped TiO2 is shown in Fig. 7. It can be seen from the XPS spectra that the distinct peaks of Zn have vanished after the acid treatment implying the complete etching of the ZnO from the composite. 3.3.
Fig. 5 SEM (A & B) and TEM (C & D) images of the TiO2 obtained from the TiO2–ZnO composite by selective etching of ZnO. (D) A lattice resolved image showing the (101) lattice orientation of TiO2. The crystalline nature of TiO2 is evident from the SAED pattern.
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BET surface area analysis
The selective etching of ZnO from the TiO2–ZnO composite resulted in structural anisotropy and we anticipated that this will result in high surface area for the TiO2. Therefore, BET surface area analysis was done. The surface area of the former was found to be 40 m2 g−1 with a pore volume and pore size,
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respectively, of 0.084 cm3 g−1 and 75.6 Å. However, for the etched sample (TiO2), the BET surface area was found to be 148 m2 g−1 with the average pore volume and pore size, respectively, of 0.096 cm3 g−1 and 84.2 Å. Thus, through this facile approach, the surface area of TiO2 was enhanced by ∼3.5 times. It must be noted that the BET surface areas of electrospun TiO2 nanostructures are only in the range of 40–60 m2 g−1.14 3.4.
Effect of acid concentration on the TiO2 morphology
As the acetic acid used was an etchant of ZnO, we believed that the concentration of acetic acid will have some role in the morphology of the TiO2 and there will be a threshold concentration below which the etching process may not be so significant. This investigation revealed that when the concentration of acetic acid was lower than 0.05 M, the rice-like morphology was more or less retained which implies that etching of the ZnO from the composite was not significant. When the concentration of the acetic acid was increased to 0.5 M and above, the rice-shaped composite underwent complete etching, resulting in coral reef-shaped TiO2. 3.5.
Application in DSCs
Porous and high surface area coral-shaped TiO2 was used in DSCs as a photoanode with I3−/I− as the redox couple and Pt as the counter electrode. The active area of the DSC was 0.16 cm2 with a thickness of 11 µm and was square shaped. A comparison of the photovoltaic performance of the coral reefand rice-shaped TiO2 is shown in Fig. 8. The former has shown a current density ( Jsc) of 12.9 mA cm−2, an open-circuit voltage (Voc) of 0.75 V, a fill factor (FF) of 65.7% and an overall conversion efficiency of 6.54% under 1 Sun (AM 1.5G conditions) whereas the rice-shaped TiO2 has shown a Jsc of 9.65 mA cm−2, a Voc of 0.75 V, an FF of 64.9% and an overall conversion efficiency of 4.8% under the same conditions. Upon comparison of the two sets of photovoltaic parameters, it became obvious that the major difference is only in the Jsc value which was responsible for the 36.2% increase in
Fig. 8 TiO2.
I–V Traces of DSCs fabricated using rice- and coral-shaped
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Fig. 9
IPCE traces of rice- and coral-shaped TiO2 DSC devices.
efficiency. This 1.4 times (36.2%) increase in efficiency could be mainly attributed to the increase in surface area of the former, achieved through the selective etching process which resulted in an increased dye-loading (1.91 × 10−7 mol cm−2 for the coral-shaped TiO2 vs. 1.60 × 10−7 mol cm−2 for the riceshaped TiO2). It must be noted that the efficiency achieved by the coral reef-shaped TiO2 is one of the impressive values achieved from electrospun TiO2 material-based DSC devices.14c,d,20 From the IPCE spectra in Fig. 9, it is clear that the coral reef-shaped TiO2 shows an incident photon-to-electron conversion efficiency maximum of 63% (at 540 nm corresponding to the absorption maximum of the dye) whereas the rice-shaped TiO2 showed only 45%. The effect of enhanced scattering by the anisotropic coral reef-shaped TiO2 is also evident from the IPCE spectra. 3.6.
Application in photocatalysis
The photocatalytic degradation of organic dyes (say methyl orange, MO) by TiO2 in the presence of UV light has been well known, and the efficiency of photocatalysis depends on the surface area, crystallinity and phase purity of the TiO2. Though the exact mechanism of the degradation (such as the chemical nature of the intermediates formed, final products, etc.) is still not fully resolved,21 it is widely believed that the degradation proceeds in the following manner: UV irradiation produces photoexcited electrons (in the conduction band) and holes (in the valence band) in TiO2, which migrate to its surface and react with O2 and H2O, respectively, producing highly reactive superoxide (O2−) and hydroxyl (•OH) radicals. These radicals subsequently react with MO resulting in peroxylated (reduction product of MO with O2−)/hydroxylated (oxidation product of MO with •OH) intermediates which eventually get degraded to the products (CO2, H2O, and other products). We have tested the photocatalytic degradation of MO dye on the thin films of rice- and coral-shaped TiO2 (having the same area and thickness fabricated by the screen-printing technique). The degradation of MO was faster on the film
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Fig. 12 Panels (A–C) show a comparison of the photocatalytic degradation of phenol by coral- and rice-shaped TiO2. Fig. 10 A comparison of the photocatalytic activities of coral- (labeled 2) and rice-shaped (labeled 1)- TiO2.
phenol (in water) was monitored at time intervals of 20 min for up to 100 min for both the samples (Fig. 12A and B, respectively). In this case as well, faster degradation of phenol was observed in the case of the coral-shaped TiO2. The degradation in the presence of coral-shaped TiO2 was 72% after 100 min of irradiation whereas the same was only 48% (Fig. 12C) for the rice-shaped TiO2. This further supports better photocatalytic activity of the TiO2 obtained after the etching process.
4.
Fig. 11 Panels (A–C) show a comparison of the photocatalytic degradation of MO by coral- and rice-shaped TiO2. Panel (D) shows a digital photograph of the MO dye solution (left) after irradiation (50 min) in the presence of rice- (center) and coral-shaped (right) TiO2, respectively.
fabricated using coral-shaped TiO2 than that on the riceshaped one under the same UV irradiation durations (Fig. 10). It can be inferred that the faster degradation of MO dye on the coral-shaped TiO2 is because of its high surface area which supported the adsorption of more dye molecules on its surface and hence a faster degradation. In the solution phase experiment as well, the rate of degradation of MO was faster with the coral-shaped TiO2 compared to the rice-shaped one (Fig. 11A and B, respectively). About 92% degradation was observed for the former when compared to 75% for the latter after UV irradiation for 50 min (Fig. 11C). We have also probed the degradation of phenol by the respective TiO2 under UV light. The absorption spectrum of
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Conclusions
A porous coral reef-shaped TiO2 was fabricated from an electrospun TiO2–ZnO composite by selective etching of ZnO using dilute acetic acid by a hydrothermal process. The initial and final materials were characterized by spectroscopy, microscopy and BET surface area measurements. The anisotropic TiO2 exhibited high surface areas (148 m2 g−1) compared to the asfabricated TiO2–ZnO composite (38 m2 g−1). The coral reefshaped TiO2 when employed in the dye-sensitized solar cell (cells of the same area and thickness) showed an efficiency of 6.5% in comparison to 4.8% for the rice-shaped TiO2. The TiO2 also exhibited good photocatalytic properties in the degradation of MO and phenol.
5. Conflict of interest The authors declare no conflict of interest.
Acknowledgements The authors acknowledge financial assistance from the Ministry of New and Renewable Energy (MNRE), Government of India.
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Notes and references 1 (a) U. Diebold, Surf. Sci. Rep., 2003, 48, 53; (b) X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959. 2 (a) B. O’Regan and M. Grätzel, Nature, 1991, 353, 737–740; (b) K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69–74; (c) G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Nano Lett., 2006, 6, 215– 218; (d) M. Y. Song, D. K. Kim, K. J. Ihn, S. M. Jo and D. Y. Kim, Nanotechnology, 2004, 15, 1861; (e) A. S. Nair, Y. Shengyuan, Z. Peining and S. Ramakrishna, Chem. Commun., 2010, 46, 7421–7423; (f ) P. Zhu, A. S. Nair, S. Yang, S. Peng and S. Ramakrishna, J. Mater. Chem., 2011, 21, 12210–12212; (g) Y. Shengyuan, A. S. Nair, R. Jose and S. Ramakrishna, Energy Environ. Sci., 2010, 3, 2010–2014; (h) A. J. Nozik, Physica E, 2002, 14, 115–120; (i) P. V. Kamat, J. Phys. Chem. C, 2008, 112, 18737–18753; ( j) S. Rühle, M. Shalom and A. Zaban, ChemPhysChem, 2010, 11, 2290– 2304. 3 (a) M. Ni, M. K. H. Leung, D. Y. C. Leung and K. Sumathy, Renew. Sustain. Energ. Rev., 2007, 11, 401–425; (b) A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278; (c) V. J. Babu, M. K. Kumar, A. S. Nair, T. L. Kheng, S. I. Allakhverdiev and S. Ramakrishna, Int. J. Hydrogen Energy, 2012, 37, 8897–8904; (d) V. J. Babu, M. K. Kumar, R. Murugan, M. M. Khin, R. P. Rao, A. S. Nair and S. Ramakrishna, Int. J. Hydrogen Energy, 2013, 38, 4324– 4333; (e) S. U. M. Khan, M. Al-Shahry and W. B. Ingler Jr., Science, 2002, 297, 2243–2245. 4 (a) P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930–2946; (b) A. R. Armstrong, G. Armstrong, J. Canales, R. Garcia and P. G. Bruce, Adv. Mater., 2005, 17, 862–865; (c) P. Zhu, Y. Wu, M. V. Reddy, A. S. Nair, B. V. R. Chowdari and S. Ramakrishna, RSC Adv., 2012, 2, 531–537; (d) D. Deng, M. G. Kim, J. Y. Lee and J. Cho, Energy Environ. Sci., 2009, 2, 818–837; (e) S. Y. Huang, L. Kavan, I. Exnar and M. Grätzel, J. Electrochem. Soc., 1995, 142, L142–L144. 5 (a) X. D. Wang, C. Neff, E. Graugnard, Y. Ding, J. S. King, L. A. Pranger, R. Tannenbaum, Z. L. Wang and C. J. Summers, Adv. Mater., 2005, 17, 2103–2106; (b) S. Kanehira, S. Kirihara and Y. Miyamoto, J. Am. Ceram. Soc., 2005, 88, 1461–1464. 6 (a) Q. Zheng, B. Zhou, J. Bai, L. Li, Z. Jin, J. Zhang, J. Li, Y. Liu, W. Cai and X. Zhu, Adv. Mater., 2008, 20, 1044–1049; (b) C.-C. Lu, Y.-S. Huang, J.-W. Huang, C.-K. Chang and S.-P. Wu, Sensors, 2010, 10, 670–683; (c) S.-J. Bao, C. M. Li, J.-F. Zang, X.-Q. Cui, Y. Qiao and J. Guo, Adv. Funct. Mater., 2008, 18, 591–599. 7 (a) V. A. Ganesh, H. K. Raut, A. S. Nair and S. Ramakrishna, J. Mater. Chem., 2011, 21, 16304–16322; (b) v.-P. Parkin and R. G. Palgrave, J. Mater. Chem., 2005, 15, 1689–1695; (c) B. Liu, K. Nakata, M. Sakai, H. Saito, T. Ochiai, T. Murakami, K. Takagi and A. Fujishima, Catal. Sci. Technol., 2012, 2, 1933–1939; (d) J. Zhang, Y. Zhang, Y. Lei and C. Pan, Catal. Sci. Technol., 2011, 1, 273–278.
4836 | Dalton Trans., 2014, 43, 4830–4837
Dalton Transactions
8 (a) V. Rome′as, P. Pichat, C. Guillard, T. Chopin and C. Lehaut, New J. Chem., 1999, 23, 365–374; (b) G. K. Mora, M. A. Carvalhoa, O. K. Varghese, M. V. Pishko and C. A. Grimes, J. Mater. Res., 2004, 19, 628–634; (c) A. Nakajima, K. Hashimoto, T. Watanabe, K. Takai, G. Yamauchi and A. Fujishima, Langmuir, 2000, 16, 7044– 7047; (d) V. A. Ganesh, A. S. Nair, H. Kumar Raut, T. M. Walsh and S. Ramakrishna, RSC Adv., 2012, 2, 2067– 2072; (e) V. A. Ganesh, S. S. Dinachali, A. S. Nair and S. Ramakrishna, ACS Appl. Mater. Interfaces, 2013, 5, 1527– 1532. 9 (a) A. S. Nair, Z. Peining, V. J. Babu, Y. Shengyuan and S. Ramakrishna, Phys. Chem. Chem. Phys., 2011, 13, 21248– 21261; (b) M. Liu, L. Piao, W. Lu, S. Ju, L. Zhao, C. Zhou, H. Li and W. Wang, Nanoscale, 2010, 2, 1115–1117. 10 (a) Z. Sun, J. H. Kim, Y. Zhao, F. Bijarbooneh, V. Malgras, Y. Lee, Y.-M. Kang and S. X. Dou, J. Am. Chem. Soc., 2011, 133, 19314–19317; (b) A. K. Sinha, S. Jana, S. Pande, S. Sarkar, M. Pradhan, M. Basu, S. Saha, A. Pal and T. Pal, Cryst. Eng. Commun., 2009, 11, 1210–1212. 11 (a) D. Li and Y. Xia, Nano Lett., 2003, 3, 555–560; (b) A. Kumar, R. Jose, K. Fujihara, J. Wang and S. Ramakrishna, Chem. Mater., 2007, 19, 6536–6542; (c) D. Li, J. T. McCann, Y. Xia and M. Marquez, J. Am. Ceram. Soc., 2006, 89, 1861–1869; (d) W. E. Teo and S. Ramakrishna, Nanotechnology, 2006, 17, R89. 12 (a) K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69–74; (b) F. Sauvage, F. D. Fonzo, A. L. Bassi, C. S. Casari, V. Russo, G. Divitini, C. Ducati, C. E. Bottani, P. Comte and M. Grätzel, Nano Lett., 2010, 10, 2562–2567; (c) J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki and A. B. Walker, J. Am. Chem. Soc., 2008, 130, 13364–13372. 13 (a) G. Binitha, M. S. Soumya, A. A. Madhavan, P. Praveen, A. Balakrishnan, K. R. V. Subramanian, M. V. Reddy, S. V. Nair, A. S. Nair and N. Sivakumar, J. Mater. Chem. A, 2013, 1, 11698–11704; (b) C. A. Grimes, J. Mater. Chem., 2007, 17, 1451–1457. 14 (a) S. Chuangchote, T. Sagawa and S. Yoshikawa, Appl. Phys. Lett., 2008, 93, 033310; (b) M. Y. Song, Y. R. Ahn, S. M. Jo, D. Y. Kim and J.-P. Ahn, Appl. Phys. Lett., 2005, 87, 113113; (c) Y. Shengyuan, Z. Peining, A. S. Nair and S. Ramakrishna, J. Mater. Chem., 2011, 21, 6541–6548; (d) A. S. Nair, R. Jose, Y. Shengyuan and S. Ramakrishna, J. Colloid Interface Sci., 2011, 353, 39–45. 15 (a) A. S. Nair, Z. Peining, V. J. Babu, Y. Shengyuan, P. Shengjie and S. Ramakrishna, RSC Adv., 2012, 2, 992– 998; (b) A. S. Nair, P. Zhu, V. J. Babu, S. Yang, T. Krishnamoorthy, R. Murugan, S. Peng and S. Ramakrishna, Langmuir, 2012, 28, 6202–6206. 16 (a) T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino and K. Niihara, Langmuir, 1998, 14, 3160; (b) J. Li, H. Yang, Q. Li and D. Zu, Cryst. Eng. Commun., 2012, 14, 3019–3026; (c) Z.-Y. Yuan, X.-B. Zhang and B.-L. Su, Appl. Phys. A, 2004, 78, 1063–1066. 17 (a) Z. Peining, A. S. Nair, Y. Shengyuan, P. Shengjie, N. K. Elumalai and S. Ramakrishna, J. Photochem.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 07 January 2014. Downloaded by University of Southern California on 06/04/2014 10:33:34.
Dalton Transactions
Photobiol., A, 2012, 231, 9–18; (b) G. S. Anjusree, A. Bhupathi, A. Balakrishnan, S. Vadukumpully, K. R. V. Subramanian, N. Sivakumar, S. Ramakrishna, S. V. Nair and A. S. Nair, RSC Adv., 2013, 3, 16720–16727. 18 (a) T. Ohsaka, F. Izumi and Y. Fujiki, J. Raman Spectrosc., 1978, 7, 321–324; (b) T. Beuvier, M. Richard-Plouet and L. Brohan, J. Phys. Chem. C, 2009, 113, 13703–13706; (c) H. C. Choi, Y. M. Jung and S. B. Kim, Vib. Spectrosc., 2005, 37, 33–38. 19 (a) Y. Wang, W. Jia, T. Strout, A. Schempf, H. Zhang, B. Li, J. Cui and Y. Lei, Electroanalysis, 2009, 21, 1432–1438; (b) D. Barreca, E. Comini, A. P. Ferrucci, A. Gasparotto, C. Maccato, C. Maragno, G. Sberveglieri and E. Tondello, Chem. Mater., 2007, 19, 5642–5649; (c) R. Liu, H. Ye, X. Xiong and H. Liu, Mater. Chem. Phys., 2010, 121, 432–439. 20 (a) K. Onozuka, B. Ding, Y. Tsuge, T. Naka, M. Yamazaki, S. Sugi, S. Ohno, M. Yoshikawa and S. Shiratori,
This journal is © The Royal Society of Chemistry 2014
Paper
Nanotechnology, 2006, 17, 1026–1031; (b) S. Cavaliere, S. Subianto, I. Savych, D. J. Jones and J. Roziere, Energy Environ. Sci., 2011, 4, 4761–4785; (c) T. Krishnamoorthy, V. Thavasi, G. M. Subodh and S. Ramakrishna, Energy Environ. Sci., 2011, 4, 2807–2812; (d) D. Sabba, N. Mathews, J. Chua, S. S. Pramana, H. K. Mulmudi, K. Hemant, Q. Wang and S. G. Mhaisalkar, Scr. Mater., 2013, 68, 487– 490; (e) A. A. Madhavan, S. Kalluri, D. K. Chacko, T. A. Arun, S. Nagarajan, K. R. V. Subramanian, A. S. Nair, S. V. Nair and A. Balakrishnan, RSC Adv., 2012, 2, 13032– 13037. 21 (a) L. Yu, J. Xi, M.-D. Li, H. T. Chan, T. Su, D. L. Phillips and W. K. Chan, Phys. Chem. Chem. Phys., 2012, 14, 3589– 3595; (b) Y. Su1, Y. Yang, H. Zhang, Y. Xie, Z. Wu, Y. Jiang, N. Fukata, Y. Bando and Z. L. Wang, Nanotechnology, 2013, 24, 295401; (c) F. Chen, W. Zou, W. Qu and J. Zhang, Catal. Commun., 2009, 10, 1510–1513.
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