Photocatalysis with hybrid Co-coated WS2 nanotubes

25 ice | science Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. Pages 25–34 http://dx.doi.org/10.1680/nme.12.00019 Themed Issue Research Paper Received 23/06/2012 Accepted 17/10/2012 Published online 20/10/2012 Keywords: Hybrid materials and structures/ photosynthesis/ nanomaterial/ semiconductors ICE Publishing: All rights reserved 1. Introduction In recent years, much effort was devoted to reduce the damage that has been afflicted to water supplies by wastewater pollutants pro- duced from textile, house-holding and other industrial processes. A major environmental problem arose as a consequence of the inten- sive azo dyes used in the textile industry. Thus, effluent streams were found to contain toxic or even worse carcinogenic dye resi- dues and their by-products. Photocatalytic processes with semiconductor nanoparticles (NP) have been successfully used for the removal of dyes and organic pollutants from wastewater.1 The fundamentals of this process has been discussed extensively in the literature.2 The most common studied semiconductor particles for photocatalysis are TiO 2 and ZnO with a wide band gap close to 3·2 and 3·0 eV, respectively.3–6 Such a wide band gap entails an absorption edge within the UV range. As follows from these studies, these wide band gap mate- rials can degrade the organic pollutants only under UV irradia- tion and show almost no reactivity within the visible spectrum. Nonetheless, it is worth noting that some studies have shown the photodegradation of several dyes under visible light illumination in aqueous TiO 2 dispersions. In this case, in addition to the formed exited electrons in TiO 2 , a TiO 2 /H 2 O 2 surface complex is formed during the dye degradation. The excited electrons of the dyes are transferred onto the TiO 2 surface and are depleted principally for the formation of H 2 O 2 . This process is called “TiO 2 -mediated dye degradation.”7 Both experiment8 and density functional tight binding calculations showed that MoS 2 and WS 2 nanotubes are semiconductors,9–11 which exhibit a small indirect (~1·3 eV) and a moderate (~1·8 eV) direct gap.12 Thus, their absorption edge lies within the visible region and could lead to potential applications in the field of photocatalysis.13 Compounds with layered structure, such as MoS 2 and WS 2 , are made of covalently bonded atoms within the layer. The layers are stacked together via weak van der Waals forces. NPs of these inorganic compounds, having a layered structure, are also known to form closed-cage layered NPs, known as inorganic fullerene- like structures (IF) and inorganic nanotubes (INT).14,15 The recent Photocatalysis with hybrid Co-coated WS2 nanotubes Y. Tsverin MSc Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel T. Livneh Dr.# Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel R. Rosentsveig Dr. Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel A. Zak Dr. Faculty of Science, Holon Institute of Technology, Holon, Israel I. Pinkas Dr. Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, Israel R. Tenne Dr.* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel Inorganic fullerene-like (IF) nanoparticles (NP) and inorganic nanotubes (INT) of layered compounds, such as WS2, have been of particular interest due to their unique structural characteristics. Recently, the catalytic decomposition of thiophene using INT of WS2 decorated with Co NP was demonstrated. This finding also suggests that these materials could be also suitable for the photocatalytic treatment of pollutants in wastewaters. In the present work, the photo- catalytic decomposition of methyl orange (MO) in aqueous solution using Co-coated INT-WS2 as well as other NP was investigated. The photocatalytic reactivity under visible light illumination of this photocatalyst was measured and com- pared with that of various IF and INT and TiO2 (P25). The Co NP-coated INT-WS2 exhibited the highest photodegradation of MO among the studied NP. The significant enhancement in the photoactivity of the hybrid nanostructure can be attributed to the combination of the metallic Co NP and the semiconducting WS2 nanotubes. The hybrid nanostructure enables the efficient light absorption by the INT and the subsequent charge separation of the hybrid semiconductor- metal NP under visible light illumination. In addition, Raman spectroscopy technique was used to verify that the MO was decomposed by Co-coated nanotubes and not adsorbed in large amounts on the hybrid NP surface. #On sabbatical leave from Department of Physics, Nuclear Research Center Negev, Beer Sheva, Israel *Corresponding author e-mail address: [email protected] Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 26 scaling-up of the INT-WS 2 synthesis16 paved the way to numerous studies including the present one, where the decomposition of the azo dye-methyl orange (MO) was investigated. Recently, hybrid structures combining metal NPs and semiconduc- tor materials have been reported to increase the photocatalytic- reactivity of several environmentally important processes.17,18 In these systems, defects or/and metal NPs on the surface of the semi- conductor materials served to promote the charge separation and transfer to the redox reaction. Therefore, they suppress the recom- bination of the electron–hole pairs, which were formed by photoex- citation of the semiconductor NPs.19–22 Thus, the deposited metal NPs/defects can lead to efficient separation of the photo-generated carriers and enhance of photocatalytic activity. Interestingly, the photocatalytic properties of the TiO 2 were also improved by using the heterostructured powders. For example, heterostructures such as PbTiO 3 –TiO 2 core–shell particles23 and FeTiO 3 /TiO 2 ,24 were found to enhance the photocatalytic activity with respect to organic compounds under visible light illumination. The improved photoactivity of PbTiO 3 –TiO 2 particles was attributed to the absorption of visible light by the PbTiO 3 core, the charge separation by the internal fields at the interface and finally the dye degradation on the nanostructured TiO 2 shell. The enchantment of reactivity of the FeTiO 3 /TiO 2 hetrostructure was attributed to hole transfer between the energetically close valance bands of FeTiO 3 and TiO 2 . Herein, the authors demonstrate a new hybrid system composed of INT-WS 2 coated with Co NPs for photocatalysis. The photocatalytic properties of Co-coated nanotubes were investigated using MO as a model molecule for decolorization process by pure photocatalysis. The Co-coated system was found to be the most pristine photo- catalytically active system among various materials such as IF and INT and TiO 2 NP. Pure INT-WS 2 , notwithstanding, were found to be almost photocatalytically inactive under visible light irradiation. It was suggested that the Co particles facilitate the charge transfer improving thereby the charge separation and catalyzing the reac- tion. This thesis was verified by femtosecond transient absorption (TA) technique. Ultrafast TA spectroscopy has been proven to be a valuable tool for investigation of energy transfer dynamics in com- posite systems.25,26 In addition, Raman spectroscopy was used to verify the MO degradation and exclude the possibility of the dye adsorption to the surface of the Co NP-coated nanotube. 2. Experimental 2.1 Co NP coating of INT-WS2 The pure multiwall INT-WS 2 (WS 2 nanotubes) were obtained from NanoMaterials Ltd., Israel. They were synthesized according to a published procedure.16,27 The nanotubes were typically 30–100 nm in diameter and 1–20-µm long. These nanotubes were coated with Co NPs using electroless plating method. Prior to Co deposition, the relatively inert surface of the nanotubes was activated using a palladium seeding process. These activated nanotubes served as a catalytic substrate for electroless plating of Co. The details of this procedure are published elsewhere.28 In addition to the published procedure, the Co NP-coated nanotubes were annealed for 30 min at 400°C. The annealing is executed under inert atmosphere, in order to reduce the thin Co oxide layer on top of the Co NPs and stabilize their crystalline structure. 2.2 Other inorganic materials The IF-MoS 2 29 and IF-WS 2 NP were prepared according to a pub- lished procedure.30 TiO 2 P25 NPs were obtained from Evonik; 2H-WS 2 powder (99% pure) with average grain size Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 27 inVia Raman Microscopy with He-Ne laser excitation wavelength of 632·8 nm [1·96 eV]). For the Raman spectroscopy, the samples were prepared by placing the powder of the measured material on a clean glass slide and measuring the Raman spectra. The materials that were measured by Raman are as follows: pure MO powder, the pristine WS 2 nanotubes, Co-coated WS 2 nanotubes and Co-coated WS 2 nanotubes, which were used in the photocatalysis experiments. These coated nanotubes were retrieved from the photocatalytic solu- tion by centrifugation, and then the nanotubes were dried. 2.5 Femtosecond TA The TA set-up used for this study is described in detail elsewhere.32 Briefly, the sample was excited by a 400-nm ultrashort pulse of light produced by an amplified Ti:Sapphire system (Spitfire Ace, Spectra-Physics, Santa Clara, CA, USA) and was probed at varying delays by a white-light continuum, covering the visible-near infra- red (VIS-NIR) region. Transients were acquired with subpicosec- ond resolution up to 2 ns and analyzed for the decay kinetics. 3. Results and discussion 3.1 Photocatalytic activity The photocatalytic activity of different inorganic materials was evaluated by the degradation of MO in aqueous solution illumi- nated by visible light. The absorbance of pure MO solution at λ max of 464 nm was measured by a UV-Vis spectrophotometer. The decrease in absorbance value of the MO solution at λ max after illumination for a certain time interval was used as a measure of the MO decomposition and is expressed as the level of decoloriza- tion. The decolorization level of the MO solution can be evaluated according to equation 1 given below: 1. Decolorization level% A A A 1000 t 0 = − × Where, A 0 is the initial absorbance and A t is the absorbance of the sample at time t, the illumination time of the solution. By using equation 1, the decolorization level of the MO was evaluated for the different inorganic materials (Figure 1). The inorganic materials such as IF and INT-WS 2 , shown in Figure 2, are indirect semiconductors, which can absorb light in the visible range. Therefore, it was suggested that these mate- rials will show a good photocatalytic activity. However, from Figure 1b, it can be seen that these materials exhibit a moder- ate activity, that is, only 5–25% of the MO decomposed after 120-min illumination (see also Table 1). As was expected, the TiO 2 that absorbs only within the UV range, had low reactivity (13%) with respect to the decomposition of MO under visible light. The best photocatalytic reactivity was achieved using the hybrid system. For better understanding of the results, SEM and TEM images of IF-WS 2 (Figure 2a and Figure 2d), INT-WS 2 coated with Co NPs (Figure 2b and Figure 2e) and INT-WS 2 (Figure 2c and Figure 2f) are shown. In addition, SEM analysis of the hybrid photocatalyst as well as other used inorganic NPs after 120-min reaction did not reveal any significant changes in the structure or composition of these materials. The photocatalytic experiments under visible light illumination, carried out with Co-coated nanotubes yielded the best result for MO decomposition. Figure 3a depicts the UV-Vis spectral absorb- ance of MO solution resulting from photocatalysis by Co-coated INT-WS 2 . As can be seen the MO undergoes rapid degradation, that is, after only 15 min of illumination approximately 65% of the MO molecules were decomposed. By using these hybrid struc- tures the MO solution undergoes almost complete degradation after 120 min of illumination, that is, up to almost 91%. In contrast to the hybrid photocatalyst, the absorbance of the MO solution changed insignificantly by using pristine WS 2 nanotubes (Figure 3b) for the photocatalytic reaction representing only ̴10% degradation (Table 1). The decomposition of the MO solution could be followed qualitatively also by the bare eye after various periods of times by the change of the solution color (Figure 3c). The BET specific area of the nanotubes increased meagerly from 7·78 m2g−1 for the pristine INT-WS 2 to 10·08 m2g−1 for the Co-coated INT-WS 2 . However, the significant enhancement in the photocata- lytic MO degradation can be attributed to the combination of Co NPs and the WS 2 nanotubes. In such systems, the metal NPs act as electron traps, leading to efficient charge separation of the photo- generated carriers.22,33 The photocatalytic process involves excitation of the electron from the valance band to the conduction band of the WS 2 nanotube and generation of an electron (e–)/hole (h+) pair. The excited electron is then transferred to the Co NPs. In order to verify this assumption, TA spectroscopy was used. 3.2 Femtosecond TA The kinetics and the dynamics of the interparticle charge trans- fer between the semiconductor (INT-WS 2 ) and metal NPs (Co) was investigated in detail using femtosecond TA. In order to bet- ter understand the interaction between the WS 2 and Co NPs, two sets of experiments were conducted: for pristine nanotubes and for Co-coated nanotubes. Figure 4 shows the TA spectra, resulting from excitation of uncoated (Figure 4a) and coated WS 2 nanotubes (Figure 4b) with 400-nm laser pulses. In both cases, the WS 2 nano- tubes gave rise to two characteristic transient bleaching maxima at 660 and 555 nm (1·88 and 2·23 eV), which corresponds to exci- tons A and B, respectively, at the direct band gap in the K point of the Brillouin zone.34 These characteristic transient bleaches corre- spond also with the A and B excitons position in the optical absorp- tion spectra of the INT-WS 2 (Figure 4c) and Co-coated INT-WS 2 (Figure 4d). The two bleached bands show a blue shift at longer delays indicating charge transfer or charge separation. The surface plasmon absorption band of the Co lies in the UV range; therefore, one does not expect to see any traces of it in the TA spectra of Co-coated samples. Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 28 Figure 1. The total decolorization level of methyl orange, calculated by equation 1, (a) for different inorganic materials and (b) the enlarged area for moderately active inorganic materials. It is worth noting that most of the experiments were repeated several times and the average standard deviation was estimated to be about 2%. 100 25(b)(a) 20 15 10 5 0 90 80 70 60 50 40 30 D ec o lo ri za ti o n le ve l ( % ) D ec o lo ri za ti o n le ve l ( % ) 20 0 0 20 40 60 80 100 120 0 20 40 60 80 Time (min) 100 120 TiO2 2H-MoS2 2H-WS2 INT-WS2 INT-WS2/Co IF-MoS2 IF-WS2 Time (min) 10 TiO2 2H-MoS2 2H-WS2 INT-WS2 IF-MoS2 IF-WS2 Figure 2. SEM images of (a) IF-WS2, (b) INT-WS2 coated with Co nanoparticles, (c) INT-WS2. TEM images of (d) IF-WS2, (e) INT-WS2 coated with Co nanoparticles and (f) INT-WS2. SEM, scanning electron microscopy; TEM, transmission electron microscopy. (a) (b) (c) (d) (e) (f) 100 nm 10 nm 5 nm 5 nm 100 nm 200 nm Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 29 The recovery of both bleached states (A and B excitons) in the WS 2 system is composed of a fast component and a much slower one, such that after 1·8 ns only half of the initial excited carriers have returned to the ground state. As can be seen from Figure 4e, the WS 2 excitons are very stable and long living, probably because they are trapped on the nanotube surface. The TA spectra of pure WS 2 system shows an absorption band at the wavelengths longer than 680 nm (1·82 eV), which can be attributed to the photo-induced absorption (PA) of excited carriers in the WS 2 nanotubes. However, the Co-coated nanotubes transient behavior is different. The recovery of the transient bleaching in the coated system is faster (Figure 4f), such that after 1·8 ns approximately 80% of carrier population is recovered from the excited state. In addition, it is clearly seen from Figure 4b that the excited state absorption that was evident in the uncoated system beyond 680 nm is suppressed. The faster decay of the A and B excitons and PA suppression in the Co-coated system can be attributed to the transfer of the car- riers from the INT-WS 2 to the Co NPs. For the photo-generated electrons in the WS 2 nanotubes to be transferred quickly to the Co NPs, the Fermi level of Co has to be within the indirect band gap of INT-WS 2 . Figure 5 shows a scheme that is constructed from the following three consecutive processes: (a) excitation of the direct optical transition at the K point of the Brillouin zone (A and B excitons), (b) intraband relaxation to band minimum (which is fast enough to avoid photoluminescence in indirect band gap 2H-WS 2 ) and (c) transfer of the electron to the Co NP. Since the work func- tion of Co is 5 eV and the indirect band gap of 2H-WS 2 is 1·3 eV, E VB (Γ), the energy of the valance band at Γ point of the Brillouin zone, which is the valance band maximum, has been measured to be a few hundreds of meV higher in energy with respect to E VB Figure 3. UV-Vis spectral changes in the absorption of methyl orange solution with (a) Co-coated INT-WS2 (b) pristine INT-WS2 and (c) pictures of MO aqueous solution after different periods of time. MO, methyl orange. 1·5 (a) (b) INT-WS2 INT-WS2 INT-WS2/Co INT-WS2/Co (c) 1·2 0·9 A b s A b s 0·6 0·3 0 1·4 1·2 1 0·8 mo 15 30 45 60 75 90 105 120 0·6 0·4 0·2 0 240 300 360 420 480 540 Wavelength (nm) 240 0 120Increased illumination time (min) 300 360 420 480 540 Wavelength (nm) mo 15 30 45 60 75 90 105 120 Material Decolorization rate (%) after 120 min INT-WS2/Co 91 INT-WS2 10 2H-WS2 25 IF-WS2 5 IF-MoS2 7 2H-MoS2 9 TiO2 13 Table 1. The decolorization rate of the methyl orange after 2 h of the illumination using different inorganic materials. Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 30 (Κ)35 and is therefore less than 6·3 eV below the “vacuum level.” The relative position of E VB (Γ) with respect to the “vacuum level” has not yet been measured for 2H-WS 2 . However, two photoemis- sion studies on 2H-MoS 2 show that the maximum in the density of states of the conduction band is at about 5·0 eV above the valance band edge36 and that the “vacuum level” is at about 6 eV above the valance band edge.37 If the authors assume close resemblance between the corresponding energy shifts in 2H-WS 2 and 2H-MoS 2 , it is likely that E CB indir – E F (Co) > 0 and that the scheme the authors proposed in Figure 5 is indeed applicable. Figure 4. Transient absorption spectra received after excitation at 400 nm for (a) uncoated (b) Co-coated WS2 nanotubes in water; UV-visible-normalized absorption of (c) uncoated (d) Co-coated WS2; decay kinetics for (e) uncoated (f) Co-coated WS2 nanotubes at different absorption peaks. 0·0003 ps 2·5 ps 7·5 ps 614 ps 1294 ps 1754 ps 1·1 1 0·9 0·8 480 540 600 660 Wavelength (nm) A b s (n o rm al iz ed ) 720 (d) 1·1 1 0·9 0·8 480 540 0·012 0·009 0·006 0·003 0·000 0·006 0·004 0·002 0·000 ∆A ∆A −0·003−2 198 398 598 798 998 1198 1398 1598 555 nm 620 nm 660 nm 725 nm 1798 −0·006 −0·009 −0·012 0·009 0·006 0·003 0·000 ∆A −0·003 −0·006 −0·009 −0·012 −0·002 −0·004 600 660 Wavelength [nm] A b s (n o rm al iz ed ) 720 480 540 570510 600 630 690 720660 Wavelength (nm) 750 480 540 570510 600 630 690 720660 Wavelength (nm) 750 (c) (a) 0·003 0·002 0·001 0·000∆A −0·001 −0·002 −0·003 (b) (e) (f) INT-WS2 INT-WS2 INT-WS2 Time (ps) INT-WS2/Co Time (ps) INT-WS2/Co INT-WS2/Co 555 nm 620 nm 660 nm 725 nm −2 198 398 598 798 998 1198 1398 1598 1798 0 ps 2·54 ps 7·55 ps 594 ps 1254 ps 1754 ps Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 31 According to Figure 5, the electron transfer to the Co creates the unbound electron and hole, leading to efficient charge separation so that the electron resides on the Co NPs and the hole at the surface of the WS 2 nanotubes. The increase of electron/hole pair separation enhances the photocatalytic activity by increasing the generation of superoxide anion radical and hydroxyl radical species. The mech- anism of the photodegradation of MO by TiO 2 photocatalyst has been discussed in the literature extensively.38,39 The photocatalytic degradation of methylene blue by a hybrid NPs system of MoS 2 (WS 2 ) clusters deposited onto TiO 2 NPs was discussed as well.39 It was argued that the potent OH radical (potential +2·8 eV)38 is able to decompose the MO. The plausible photocatalytic mechanism can be presented by the following sequence of reactions:39 2. WS Co h e h2 Cb Vb/ + → + − +ν 3. H O H OH2 ↔ + + − 4. e O OCb 2 2 − −+ → ⋅ 5. ⋅ ⋅O H H O22 − ++ → 6. 2H O H O O2 2 2 2⋅ → + 7. H O e OH OH2 2 Cb+ → + −− ⋅ 8. h OH OHVb + −+ → ⋅ 9. ⋅ ⋅ OH O methyl orange degradation products CO H O2 2 + + → + + − 2 Where, eCb − and hVb + stand for electron in the conduction band and hole in the valance band, respectively. Figure 5. Representative energy scheme of the WS2 interaction with Co during photoexcitation. Filled and empty circles denote electron and hole, respectively. WS2 Co ECB dir. EVBdir. ECB EF (Co) −5 eV AB K Γ 1·3 eV indir. Figure 6. A comparison of the Raman spectra measured from Co-coated INT-WS2 before and after photocatalysis and pristin INT- WS2. The Raman spectra of MO with dominant peaks identification are also shown. MO, methyl orange. 1 MO INT-WS2 INT-WS2/CO before photocatalysis INT-WS2/CO after photocatalysis Mo dominant peaks δ(C–H) ν(Ph–N) δ(C–H) ν(C–C) δ(C–C) ν(C–C) δ(C–H) ν(C–N)Me ν(C–C) δ(C–H) ν(N=N) ν(C–C) δ(C–H) ν(C–C) 0·8 0·6 In te n si ty ( n o rm al iz ed ) 0·4 0·2 0 100 400 700 1000 1300 1600 1900 Raman shift/cm−1 Nanomaterials and Energy Volume 2 Issue NME1 Photocatalysis with hybrid Co-coated WS2 nanotubes Tsverin et al. 32 Comparing the rate of photodegradation observed in this study and in some of the previous works,38,39 it is clear that though the pho- ton flux is not that different, the rates are much higher. Using a first-order kinetics, the rate constant for the photobleaching of MO observed with the Co-activated WS 2 nanotubes can be estimated at 0·004 s−1. 3.3 Raman spectroscopy Figure 6 shows a comparison of the Raman spectra of Co-coated INT-WS 2 nanotubes before and after the photocatalytic reaction, with that of pristine INT-WS 2 . The authors note that the spectrum of the Co-coated nanotubes “before” was taken before the intro- duction of MO. The spectrum “after” was taken using INT that were extracted from a suspension after the termination of the pho- tocatalytic reaction. All the Raman peaks of the Co-coated INT- WS 2 before and after the photocatalytic reaction were found to be similar to the ones measured from the pristine INT-WS 2 . In order to clarify that no MO is adsorbed on the Co-coated nano- tube’s surface after the photocatalysis, the authors measured its spectrum in the 900–2000 cm−1 spectral range. The authors show it with enhanced intensity after subtracting the background, which stems from the characteristic fluorescence of MO as it is excited by He–Ne laser at the wavelength of 632·8 nm. The indicated assignments of the dominant bands of MO are consistent with that of Ref. 40.40 It is evident that peaks, which belong to the MO moiety (i.e. –N=N– functional groups benzene ring and –C–N–), were neither detected in the INT spectrum after the photocatalytic reaction nor did the authors detect the evidence of characteris- tic MO fluorescence.41 Furthermore, the authors also found no traces of amorphous carbon on the Co-coated nanotubes after the photocatalysis. In conclusion, it was found that the decolorization of MO solution was produced by photo-induced degradation of the MO moiety. The MO decomposed without leaving significant amounts (within the limit of the detection) of traces on the surface of the Co-coated nanotubes. 4. Conclusions In summary, the photocatalytic properties of several inorganic sys- tems, consisting of INT and IF NPs, were investigated. It was found that among these systems only the hybrid system has high photo- catalytic activity, whereas others have only moderate photocatalytic activity under visible light illumination. The highest level of MO degradation was received using Co-coated nanotubes with 91% of decolorization (after 120-min irradiation). Femtosecond TA spectra following excitation with a 400-nm laser pulse revealed that the photo-generated electron–hole pair is separated by electron transfer from the semiconductor (INT-WS 2 ) to the metal NP (Co). Thus, enhancing the photocatalytic activity of the Co-coated INT-WS 2 . It was verified by Raman spectroscopy that the decolorization of the MO solution was produced by the degradation of the dye molecule rather than by its adsorption onto the surface of coated nanotubes. Acknowledgements The authors are grateful to Dr. Ronit Popovitz-Biro from the Electron Microscopy Unit of the Weizmann Institute of Science for her high-resolution TEM images and to Dr. D. Oron for the enlightening discussions. This research was supported by the Israel Science Foundation and the ERC INTIF grant. RT holds the Drake Family Chair in Nanotechnology and is the director of the Helen and Martin Kimmel Center for Nanoscale Science. The authors are grateful to the support of the Harold Perlman Foundation and the Irving and ChernaMoskowitz Center for Nano and Bio-Nano Imaging. REfEREnCEs 1. Matthews, R. W. Photooxidative degradation of coloured organics in water using supported catalysts. TiO 2 on sand. Water Research 1991, 25(10), 1169–1176. 2. Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO 2 surfaces: principles, mechanisms, and selected results. Chemical Reviews 1995, 95(3), 735–758. 3. Li, P.; Wei, Z.; Wu, T.; Peng, Q.; Li, Y. Au−ZnO hybrid nano- pyramids and their photocatalytic properties. Journal of the American Chemical Society 2011, 133(15), 5660–5663. 4. Rizzo, L.; Koch, J.; Belgiorno, V.; Anderson, M. A. 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