Photocatalytic Properties of WO3/TiO2 Core/Shell Nanofibers ...

DOI: 10.1002/cvde.201207037

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Photocatalytic Properties of WO3/TiO2 Core/Shell Nanofibers prepared by Electrospinning and Atomic Layer Deposition** By Imre Miklo´s Szila´gyi,* Eero Santala, Mikko Heikkila¨, Viljami Pore, Marianna Kemell, Timur Nikitin, Georg Teucher, Tama´s Firkala, Leonid Khriachtchev, Markku Ra¨sa¨nen, Mikko Ritala, and Markku Leskela¨ Core WO3 nanofibers (140–300 nm in diameter, several hundred mm long) are made by a novel, water-based electrospinning process using ammonium metatungstate (AMT) and polyvinylpyrrolidone (PVP) as precursors. TiO2 shells (1.5–20 nm) are grown by atomic layer deposition (ALD) using TiCl4 and water at 2508C. The WO3/TiO2 composite fibers are analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM)-energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Raman spectroscopy (RS), UV-Vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). The optimal photocatalytic conversion under visible light is reached by the WO3/1.5 nm TiO2 nanofibers, which have higher activity compared to bare WO3 and Degussa TiO2. Thicker TiO2 layers fill the pores of the nanowires and reduce the specific surface area, weakening the photocatalytic activity. Keywords: ALD, Electrospinning, Nanofibers, Photocatalysis, WO3

1. Introduction Metal oxide semiconductors are widely used as photocatalysts to decompose organic contaminations with solar energy.[1] The most studied photocatalyst is TiO2 as its valence and conductance energy levels are suitable for both oxidation and reduction of water molecules (water splitting).[2–6] The only drawback of TiO2 is that it absorbs only in the UV range. One major option to overcome this [*] Dr. I. M. Szila´gyi Technical Analytical Chemistry Research Group of the Hungarian Academy of Sciences, Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Budapest, Szt. Gelle´rt te´r. 4., H-1111 (Hungary) E-mail: [email protected] Dr. I. M. Szila´gyi, E. Santala, M. Heikkila¨, Dr. M. Kemell, T. Nikitin, Prof. L. Khriachtchev, Prof. M. Ra¨sa¨nen, Prof. M. Ritala, Prof. M. Leskela¨, Dr. V. Pore Department of Chemistry, University of Helsinki, Helsinki, P.O. Box 55, FI-00014 (Finland) Dr. V. Pore Present address: ASM Microchemistry Oy Vaïno¨ Auerin katu 12 A, 00560 Helsinki (Finland) G. Teucher Chemnitz University of Technology, Faculty of Natural Sciences 09107 Chemnitz, Reichenhainer Straße 70 (Germany) T. Firkala Institute of Materials and Environmental Chemistry, Research Centre of Natural Sciences, Hungarian Academy of Sciences, H–1025 Budapest, Pusztaszeri u´t 59–67 (Hungary) [**] I.M.S. gives thanks for a Marie Curie Intra-European Fellowship (PIEF-GA-2009-235655) and a Jańos Bolyai Research Fellowship of the Hungarian Academy of Sciences. Raman studies were supported by the Finnish Centre of Excellence in Computational Molecular Science and the University of Helsinki Research Funds (HENAKOTO). The Finnish Centre of Excellence in Atomic Layer Deposition, funded by the Academy of Finland, is also acknowledged.

Chem. Vap. Deposition 2013, 19, 149–155

problem is to make composites of TiO2 using Vis-active metal oxide materials.[7–9] The Vis-active semiconductor oxides can absorb UV and also visible light, and transfer an electron or hole generated by a visible light photon to TiO2. An attractive way to prepare metal oxide semiconductor (MOS) composite materials is to combine electrospinning and ALD.[10] Electrospinning is a convenient method of choice to prepare 1D nanostructures. It produces fibers which are quite long (up to several hundreds of micrometers) and have uniform diameters ranging between tens of nanometers to several micrometers. In electrospinning, a high static voltage is applied to a polymer or polymer/ inorganic solution. The strong electrostatic force ejects the solution from a nozzle. The charged jet is accelerated by the electric field, dries, and is deposited in the form of nanofibers onto a grounded substrate.[11,12] ALD is based on successive, alternating surface-controlled reactions from the gas phase to produce highly conformal and uniform thin films, with thickness control of sub-nanometer precision. Thus, ALD provides new strategies in modifying the properties of nanometer-scaled materials and new synthetic routes to novel nanostructures.[13,14] In this study, we combine the benefits of two metal oxide photocatalysts, i.e., TiO2 and WO3, to improve the photocatalytic properties. WO3, an n-type semiconductor similar to TiO2, can absorb UV and also visible light, which makes it the second most studied metal oxide photocatalyst.[15–17] Thus WO3 is an ideal candidate to prepare a composite with TiO2.[18,19] To prepare a WO3 nanofiber as core material, we developed a new, completely water-based electrospinning process, which uses different precursors than the previous

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studies,[3,5,20,21] and thus broadens the possibilities to prepare WO3 nanofibers by electrospinning. This offers a new alternative to the few previously reported WO3 electrospinning methods that all used organic solvents to dissolve the polymer and the inorganic precursors.[3,5,20,21] In contrast to previous studies, which reported mostly a simple recipe, we studied systematically how the properties of the electrospun fibers can be controlled by the concentration of precursors. Then we deposited TiO2 shell layers on the WO3 nanofibers by ALD, and modified the photocatalytic properties by controlling the TiO2 film thickness. Visible light can reach the WO3 core through the thin transparent TiO2 shell. Holes and electrons generated in the WO3 core are efficiently separated between the WO3 core and the TiO2 shell. This is expected to suppress the recombination of electrons and holes, and strongly increase the photocatalytic activity. We note that, to the best of our knowledge, there is no report on depositing any ALD films on a WO3 substrate; thus our study is the first such one. Recently, it was reported that various WO3 polymorphs (monoclinic or hexagonal) have different surface OH group density.[15] The different surface OH group concentrations of WO3 polymorphs make WO3 a good candidate to study how ALD thin films can be grown on different polymorphs of the same compound. It was unclear whether TiO2 deposited by ALD would form uniform films on WO3 or would nucleate as particles.

Table 1. Properties of PVP/AMT and WO3 nanofibers obtained by electrospinning and consecutive annealing. AMT [wt.-%] 16.7 50 16.7 50 16.7 50

PVP [wt.-%]

d (polymer fiber) [nm]

d (WO3 fiber) [nm]

15 15 20 20 25 25

150–200 400–600 250–350 350–450 500–700 1400–1600

140 (fibers, beads, ribbons) 350–400 (rods) 160 (fibers) 250 (fibers) 300 (fibers) 900 (ribbons)

For the lowest PVP and AMT concentrations (15 wt.-%, 16.7 wt.-%, respectively), nanoribbons and beads were also visible in addition to WO3 fibers, which showed that the electrospinning conditions were not satisfactory (Figs. 1a,b). For low PVP concentration (15 wt.-%) and high AMT concentration (50 wt.-%), no WO3 ribbons and beads were detected; however, after annealing, only shorter, few mm long WO3 nanorods were produced. For a PVP concentration of 20 wt.-%, several hundred mm long WO3 nanofibers were

2. Results and Discussion 2.1. Electrospinning of WO3 Nanofibers A new, water-based electrospinning process was elaborated for preparing WO3 nanofibers using aqueous solutions of PVP and AMT. The influence of PVP (15, 20, and 25 wt.%) and AMT (16.7 and 50 wt.-%) concentrations on the fiber properties was thoroughly studied (Table 1). The needle voltage was 14.4 kV and the fibers were collected on a rotating drum within 15 cm from the needle. The as-spun fibers had a diameter between 150 and 1600 nm, which could be controlled by the PVP and AMT concentrations. In order to obtain WO3 nanofibers, the PVP/AMT fibers were annealed at 5508C in air using a very slow heating rate, i.e., 1 K min1. The low heating rate was needed to maintain the fibrous structure of the electrospun oxide materials. Recently it has been shown that annealing of polymer/ inorganic electrospun fibers at high heating rates leads to disintegration of the oxide fibers into particles.[22] All the WO3 nanofibers obtained after annealing at 5508C were made up by 20 - 60 nm WO3 particles that were aggregated to form the fibers. This structure was beneficial for photocatalysis as the open structure of the fibers meant high specific surface area. 150

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Fig. 1. SEM images of (a) nanofibers electrospun from 1:4 mixture of 15 wt.-% PVP and 16.7 wt.-% AMT solutions and (b) annealed at 5508C; (c) nanofibers electrospun from 1:4 mixture of 25 wt.-% PVP and 50 wt.-% AMT solutions and (d) annealed at 5508C; (e) nanofibers electrospun from 1:4 mixture of 20 wt.-% PVP and 50 wt.-% AMT solutions and (f,g) annealed at 5508C.



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obtained with both AMT concentrations. With the higher AMT concentration (50 wt.-%), the WO3 fibers had somewhat larger diameter (350 - 450 nm instead of 250 - 350 nm). At the highest PVP concentration (25 wt.-%) and at 16.7 wt.-% AMT, WO3 wires were still obtained, but they were much thicker (500–700 nm), than in the case of 20 wt.-% PVP and 50 wt.-% AMT. For high PVP and AMT concentrations (25 wt.-% and 50 wt.-%, respectively), instead of fibers, WO3 ribbons appeared with a width of 900 nm and thickness of 100 nm (Figs. 1c,d). Based on these results, the optimal solution for preparing thin WO3 nanofibers in an efficient way contained a 1:4 mixture of 20 wt.-% PVP and 50 wt.-% AMT (Figs. 1e–g). In this case, the WO3 fibers were much thinner than in the case of 25 wt.-% PVP. In addition, a three times larger amount of WO3 fibers could be produced in one batch compared to the case of 20 wt.-% PVP and 16.7 wt.-% AMT. This is important, as in the case of 20 wt.-% PVP and 50 wt.-% AMT solutions, a typical electrospinning of 1 mL PVP/AMT mixture took 6 h, and after annealing at 5508C yielded 50 mg WO3 nanofibers. This WO3 electrospinning method is a new alternative, since it uses only water as solvent, without the need of any organic solvents (ethanol, acetone, etc.), which are otherwise widely used in electrospinning. The process is highly reproducible at a given temperature. The temperature of the solutions has to be monitored, as their viscosity can change significantly if their temperature, i.e., the temperature of the laboratory, changes by 58C. In this case it may happen that the process optimized for WO3 nanofibers produces nanoribbons instead.

2.2. Deposition of TiO2 Nanofilms onto WO3 Nanofibers by ALD TiO2 films with thicknesses between 1.5 and 20 nm were grown onto the WO3 nanofibers by ALD. XPS (Fig. 2a), with an average probing depth of 10 nm, measured the chemical composition near the surface, while the EDX analysis, which has a probing depth of 0.5 - 1 mm, gave information about the bulk composition (Table 2). The presence of Ti was already detected for 1.5 nm TiO2 shells. As the TiO2 thickness increased, the Ti/W concentration ratio increased, supported by both XPS and EDX data. XPS revealed that W atoms in all samples were in the þ6 oxidation state (35.6 and 37.7 eV for the W 4f7/2 and W 4f5/2 peaks), corresponding to WO3[23] (Fig. 2b), while Ti atoms were in the þ4 oxidation state (458.7 and 464.4 eV for the 2p3/2 and 2p1/2 peaks), corresponding to TiO2[24,25] (Fig. 2c). The XRD study (Fig. 3a) revealed that WO3 was present in its monoclinic phase (ICDD 43-1035). Since XRD was not as sensitive to the presence of TiO2 as XPS or EDX, TiO2 was detected by XRD only from the 10 nm TiO2 thickness. According to XRD, TiO2 was present in the anatase form (ICDD 21-1272). Chem. Vap. Deposition 2013, 19, 149–155

Fig. 2. (a) Overview XPS spectrum of the WO3/20 nm TiO2 composite. XPS spectra of the (b) W 4f region of the bare WO3 nanofibers and (c) Ti 2p region of the WO3/20 nm TiO2 composite.

Raman spectroscopy (Fig. 3b) also confirmed that WO3 was in the monoclinic phase. The most intense bands at 806 and 715 cm1 were the O–W–O stretching vibrations. The bands at 326 and 271 cm1 were assigned to the O–W–O deformation vibrations, while the band at 134 cm1 originated from a lattice mode.[26] In the Raman spectra, the peaks of TiO2 could again only be observed from the 10 nm TiO2 thickness. The bands at 144, 403, 520, and 642 cm1 can be assigned to the Eg, B1g, A1g or B2g, and Eg modes of the anatase phase, respectively.[27] The SEM images showed that, while the WO3/1.5 nm TiO2 composite had basically the same surface morphology as the bare WO3 nanofibers, thicker TiO2 layers started to fill the pores of the WO3 nanowires (i.e., the voids between the aggregated WO3 nanoparticles) and reduced the specific surface area (Fig. 4).

Table 2. Cation % of Ti and W in the WO3/TiO2 samples measured by XPS and EDX. ALD cycle

TiO2 [nm]

Cation% XPS (surface 10 nm)

50 100 300 500

1.5 3.1 10 19.3

EDX (bulk)

Ti

W

Ti

W

53 76 86 95

47 24 14 5

6 16 36 51

94 84 65 49


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Fig. 3. (a) XRD and (b) Raman spectroscopy patterns of the WO3/TiO2 samples.

The TEM images confirmed that the TiO2 layer was uniform already on the WO3/1.5 nm TiO2 composite. Based on the high resolution (HR)TEM image of the WO3/1.5 nm TiO2 composite, both the WO3 core and the TiO2 shell were highly crystalline (Fig. 5). The observed lattice distances matched the (202) plane of WO3 and the (200) plane of TiO2. The measured thickness of the TiO2 layer corresponds to the estimated thickness (i.e., 1.5 nm). The bandgaps were calculated from the absorption edges in the UV-Vis spectra (Fig. 6). The absorption edges and bandgaps of bare WO3 nanofibers (506.4 nm and 2.45 eV) and Degussa P25 TiO2 nanoparticles (402.7 nm and 3.08 eV) match the literature values.[15,28] The TiO2 layers introduce a small blue shift (ca. 15 nm) compared to the absorption of the core WO3 nanofibers, however all composites have significant absorption in the visible spectrum. As the TiO2 layer thickness increased, the absorption edges and bandgap energies of the WO3/TiO2 composites changed only slightly, i.e., 493.0 nm and 2.51 eV for WO3/1.5 nm TiO2; 492.1 nm and 2.52 eV for WO3/20 nm TiO2.

Fig. 5. HRTEM image of the WO3/1.5 nm TiO2 composite.

2.3. Photocatalysis with WO3/TiO2 Core/Shell Nanofibers At first, the photocatalytic experiments were done with a LED lamp emitting both in the UV and Vis region (peak maximum at 402 nm) (Fig. 7a). In this case, the WO3/1.5 nm TiO2 sample was the best photocatalyst among the WO3/ TiO2 composites. Based on the SEM images, this can be explained by filling of the pores of the nanofibers by thicker TiO2 layers, which reduces the specific surface area, resulting in lower photocatalytic activity. The bare WO3 nanofiber was as active a photocatalyst as the best WO3/TiO2 composite, i.e., the WO3/1.5 nm TiO2 nanofiber, however as we studied the adsorption of methylene blue on the surface WO3, it turned out that the adsorption was not complete after 1 h, which was the default adsorption time for every sample. Unlike other samples, at least 12 h were needed for the dye to reach an adsorption equilibrium on the surface of WO3. When 12 h adsorption time was used for WO3, its photocatalytic activity was significantly reduced, much below the activity of the

Fig. 4. SEM images of the (a) WO3/1.5 nm TiO2 composite and (b) WO3/20 nm TiO2 composite.

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more active photocatalyst that the WO3 and WO3/TiO2 nanofibers. Nevertheless, even with this lamp, the covering of WO3 with thin TiO2 layers increased the photocatalytic activity compared to the bare WO3 nanofibers – provided that in the case of WO3 long enough dye adsorption time was used before starting the lamp. With a LED lamp emitting mainly in the visible spectrum (peak maximum at 449 nm), the WO3/1.5 nm TiO2 composite showed better photocatalytic activity compared to the bare WO3 and the reference Degussa P25 TiO2 nanoparticles as well (Fig. 7b).

3. Conclusions

Fig. 6. Diffuse reflectance UV-Vis spectra of (a) Degussa P25 TiO2 nanoparticles, (b) electrospun pure WO3 nanofibers, (c) WO3/1.5 nm TiO2 composite, and (d) WO3/20 nm TiO2 composite.

WO3/1.5 nm TiO2 composite. The considerable difference in the adsorption capacity of WO3 and the WO3/TiO2 composites can be explained by a higher acidity of the surface of WO3 compared TiO2,[29,30] and thus there are more adsorption sites for methylene blue on WO3. With the 402 nm LED lamp, the Degussa P25 TiO2 was a much

A new, water-based electrospinning process for WO3 nanofibers was developed using AMT and PVP as precursors. By using various AMT and PVP concentrations, the properties of the electrospun WO3 nanofibers can be varied to a large extent. The optimal fiber quality and thickness were reached by applying a 1:4 mixture of 20 wt.-% PVP and 50 wt.-% PVP solutions. The obtained WO3 fibers were 250 nm in diameter, several hundred mm long, and were composed of 20–60 nm particles. On these core nanofibers made up by monoclinic WO3 nanoparticles, anatase shell layers were deposited by ALD with thicknesses between 1.5 and 20 nm. The study on the photocatalytic activity of the nanofibers reveals that the WO3/1.5 nm TiO2 composite is the best photocatalyst among the tested WO3/TiO2 core/shell nanofibers, because thicker TiO2 layers fill the pores of

Fig. 7. Photocatalytic activity of the WO3/TiO2 composites using (a) 403 nm and (b) 449 nm LED light sources. (WO3 (no ads. equilibrium) means that the adsorption equilibrium of the dye on WO3 was not reached after the 1 h default adsorption time. WO3 (12 h ads.) or WO3 (18 ads.) mean that 12 h or 18 h adsorption time was used, i.e., the WO3 samples dispersed in the dye solution were kept without illumination for 12 h or 18 h before the photocatalytic reaction started).



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the nanofibers, reducing their specific surface area and hence the photocatalytic activity. It is found that, since the surface of WO3 is more acidic than TiO2, ca. ten times more time is needed prior to starting the photocatalysis to reach adsorption equilibrium of methylene blue on WO3 than on the WO3/TiO2 composites. With visible light, the WO3/1.5 nm TiO2 composite is a better photocatalyst, compared to the bare WO3 nanofibers and also Degussa P25 TiO2. This means that we have reached our research goal, i.e., we have prepared such a WO3/TiO2 composite which combines the benefits of WO3 and TiO2, and surpasses the photocatalytic activity of both materials. The outstanding activity of the WO3/1.5 nm TiO2 core/ shell nanofibers can be rationalized by the fact that it has basically the same porous, high surface area as the WO3 nanofibers, but the deposition of TiO2 layer enhances the separation of the photon-generated holes and electrons between WO3 and TiO2. Thus the lower recombination possibility of holes and electrons in the WO3/1.5 nm TiO2 composite means higher photocatalytic activity compared to bare WO3. The WO3 core of the WO3/1.5 nm TiO2 composite significantly increases the absorption in the visible spectrum leading to higher visible light photocatalytic activity, compared to TiO2. Our study shows that 1–2 nm functional ALD layers on a nanopatterned material significantly change the properties of the substrate. Thus ALD is a unique tool to programme the surface properties of nanostructured systems.

XPS spectra were measured using a Physical Electronics Quantum 2000 instrument using Al Ka X-ray source (calibrated against the carbon 1s line at 284.5 eV). Diffuse reflectance UV–Vis absorption spectra of solid samples were recorded on a Jasco 570 UV–Vis spectrometer, equipped with an integrating sphere for solid phase characterization. The absorption edge was determined as the intersection point of the baseline and the tangent drawn at the inflection point. The band gap was calculated from the wavelength of the absorption edge using Planck’s equation. The Raman spectra were recorded with a LabRam HR confocal microscope (Horiba Jobin Yvon) using excitation at 488 nm of an Ar-ion laser (0.1–1 mW at the sample), 100 objective and spectral resolution 2 cm1. The photocatalytic activity of the samples was tested in the aqueous phase by decomposing methylene blue (MB). For this, 30 mg fiber sample was put into 150 mL aqueous 0.03 mM MB solution, kept in the dark for 1 h to adsorb dye, then illuminated either by a LED lamp emitting both in the UV and Vis region (emission peak at 402 nm with 34 nm FWHM, 50 mW cm2 at 1 cm distance) or by a LED lamp emitting mainly in the Vis region (emission peak at 449 nm with 34 nm full-width-at-half-maximum; 128 mW cm2 at 1 cm distance). The UV-Vis spectra of samples (4 mL) taken from the MB solution during the photocatalytical reactions were measured regularly after centrifuging using a HP 8453 UV-Vis spectrometer. The absorbance values, which were proportional to the MB concentrations (according to the Lambert-Beer law), were determined at 665 nm. On Fig. 7 the relative absorbance (A/A0) values are plotted against the photocatalysis time. We note that the MB absorption determination had ca. 1 - 5% relative error, which originated from the traces of WO3 and WO3/TiO2 powders which were still present in the MB solutions after centrifuging, could cause diffuse light scattering during the UV-Vis measurements. To overcome this we have centrifuged each sample taken from the MB solutions several times, and those absorbance values were recorded where the baselines were the lowest.

Received: November 17, 2012 Revised: February 25, 2013

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4. Experimental

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The WO3 core nanowires were made by electrospinning 1:4 mixtures of aqueous solutions of PVP ((C6H9NO)n), 15, 20, or 25 wt.-%) and AMT ((NH4)6[H2W12O40] nH2O, 16.7 or 50 wt.-%) (Table 1). AMT (Fluka, 99% purity) and PVP (Alfa Aesar, Mw: 1 300 000) were used as-received. The needle voltage was 14.4 kV. The polymer fibers were collected on a rotating drum, made of steel wires, 15 cm from the needle. WO3 fibers were obtained by heating the as-prepared fiber mats with a rate of 1 K min1 to 5508C, and maintaining that temperature for 1 h.[22] TiO2 shell nanolayers were grown by ALD at 2508C in a Picosun Sunale R150 reactor using TiCl4 (0.2 s pulse and 15 s purge) and H2O (0.4 pulse and 15 s purge) as reactants (purging with N2). The operating pressure during the ALD reaction was around the 10 hPa, which is typical for this type of reactor. The fiber substrates (50–100 mg in one batch) were placed in folded envelopes made of stainless steel meshes standing on small legs in the ALD reactor, which ensured that the fibers were not sucked away by the vacuum pump and that the precursor gases could reach the fibers from all directions. In all ALD runs, Si wafers were placed under the envelopes to check whether the pulses were long enough to saturate both the fibers and the Si wafer. The deposited TiO2 film thicknesses were measured on reference Si wafers by X-ray reflectometry (Bruker D8 Advance). The ALD cycle numbers and film thicknesses of the TiO2 films are presented in Table 2; the same growth rates were assumed on WO3 nanofibers. The XRD patterns were recorded by a PANalytical X’pert Pro MPD X-ray diffractometer using Cu Ka radiation and grazing incidence angle mode. The morphology and composition of the samples was studied by a Hitachi S-4800 field emission (FE)SEM equipped with an Oxford INCA 350 EDX. SEM images were recorded with a secondary electron detector. The acceleration voltage at the SEM-EDX measurements was 5 kV. TEM images were recorded on a JEM-2200FS instrument using 200 kV acceleration voltage.

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