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WO3 nanocatalysts for efficient pollutant degradation using visible light irradiation
Chemical Engineering Journal 180 (2012) 323–329
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CuO/WO3 and Pt/WO3 nanocatalysts for efficient pollutant degradation using visible light irradiation Hendri Widiyandari a , Agus Purwanto b , Ratna Balgis c , Takashi Ogi c,∗ , Kikuo Okuyama c a b c
Department of Physics, Faculty of Mathematic and Natural Science, Diponegoro University, Jl. Prof. H. Soedarto SH, Semarang, Central Java, 50275, Indonesia Department of Chemical Engineering, Faculty of Engineering, Sebelas Maret University, Jl. Ir. Sutami 36 A, Surakarta, Central Java, 57126, Indonesia Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan
a r t i c l e
i n f o
Article history: Received 9 July 2011 Received in revised form 26 October 2011 Accepted 31 October 2011 Keywords: Visible-light-driven photocatalyst Nanoparticles Flame assisted spray pyrolysis Tungsten oxide Co-catalyst Photodegradation activity
a b s t r a c t CuO/WO3 and Pt/WO3 nanocatalysts with a nanorod morphology were used for photodegradation of organic compounds using visible light irradiation. Both nanocatalysts were prepared using flame assisted spray pyrolysis method (FASP). The prepared nanocatalysts were mechanically stable during the agitation treatment for the photodegradation test due to good interconnection between WO3 and co-catalysts. The enhancement of photocatalytic activity was observed after the addition of CuO and Pt as co-catalysts. The addition of CuO would change the morphology of WO3 from nanorods to cubic. The optimal concentration of the CuO addition was 0.33 wt.%. A low Pt concentration (0.12 wt.%) was required for optimal photocatalytic activity of the Pt/WO3 nanocomposite. The addition of Pt affected neither the morphology nor the crystallite structure of WO3 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tungsten oxide (WO3 ) is a visible-light-driven photocatalyst which has many potential applications: energy conversion devices [1–3], virus deactivation [4], and harmful pollutant degradation [5,6]. The optical band gap of WO3 is approximately 2.7–2.8 eV, which allows it to absorb light that ranges from ultraviolet to blue ( < 455 nm) [7,8]. In comparison to titania (band gap 3.0–3.2 eV) [7,9], WO3 shows better photoabsorption in visible-light irradiation due to its smaller band gap. This ability opens the application of WO3 for indoor pollutant treatment of volatile organic compound (VOC) gases using domestic light sources where ultraviolet light is limited. In outdoor applications, WO3 has the potential to use sunlight as energy for the treatment of harmful pollutants in industrial waste-water, and has the potential for hydrogen production as well. Bare WO3 shows very low photocatalytic activity under visible light irradiation. Many attempts have been made to improve the photocatalytic activity of WO3 . The addition of a co-catalyst, such as platinum (Pt), gold (Au), and silver (Ag), enhances the photodegradation activity of WO3 [5,6,10–12]. Thus far, Pt has been the most powerful co-catalyst for high-activity WO3 . Typically, electrons will be excited when a semiconductor catalyst is irradiated by light. Pt serves as a pool for electrons, thereby catalyzing the
∗ Corresponding author. Tel.: +81 82 424 7850; fax: +81 82 424 7850. E-mail address: [email protected] (T. Ogi). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.10.095
reduction of O2 via a multi-electron route. As a result, an abundant amount of hydroxyl radicals (OH• ) are available. These hydroxyl radicals (OH• ) effectively degrade the target organic compounds. Thus, the effective excited electron consumption of multi-electron reduction results in an efficient charge separation and ensures high activity for Pt/WO3 . Unfortunately, the high cost of Pt is a primary impediment for a scale-up process. To replace Pt as a co-catalyst, metal oxide and metal ion have been intensively investigated. Metal oxides of CuBi2 O4 [13], CaFe2 O4 [14], TiO2 [15] and CuO [16,17] were prepared as a composite with WO3 . Also, the grafting of Cu [18] and Cu ion [19] onto the surface of WO3 has been conducted. Among those methods, the utilization of CuO and CuBi2 O4 as co-catalysts was most interesting. CuO and CuBi2 O4 mix simply with WO3 and create an efficient photocatalyst composite for volatile gas decomposition [13,16,17]. However, a problem occurs when photodegradation of organic components in aqueous solution is conducted. The composite of WO3 and oxide co-catalysts disintegrates due to extensive stirring treatment during the photodegradation test. As a result, the co-catalyst effect is no longer observed. Therefore, an interconnected structure for co-catalyst-WO3 composites that will be strong enough to resist mechanical treatment is highly desirable. Herein, is shown the fabrication of CuO/WO3 and Pt/WO3 nanocatalysts using a flame assisted spray pyrolysis (FASP) method. This is a fast, continuous, and controlled method for the production of both nanoparticles and nanocomposites [8,20]. The prepared
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Fig. 1. Schematic diagram of flame assisted spray pyrolysis (FASP).
nanocatalysts have strong interconnection WO3 and co-catalysts which were effective for organic compound degradation under visible light irradiation. In addition, the FASP method also was effective in the preparation of a Pt/WO3 photocatalyst using a low Pt concentration. A low concentration of Pt is important in the practical application of a Pt/WO3 photocatalyst.
Fig. 2. Time–concentration chart of amaranth degradation using (a) CuO/WO3 nanocomposite prepared by simple mixing of nanoparticles, the side reaction of organic compound oxidation produced Cu ions (inset) and (b) CuO/WO3 nanocomposite prepared by FASP in different CuO concentrations.
2.2. Nanocatalysts characterization 2. Experimental 2.1. Preparation of CuO/WO3 and Pt/WO3 nanocomposites The CuO/WO3 and Pt/WO3 nanocatalysts were prepared using FASP as shown in Fig. 1. FASP is comprised of three main parts: precursor atomizer, diffusion burner, and particle collector. The precursor was atomized using an ultrasonic nebulizer (NE-U17, OMRON, Japan) to produce droplets (average size 5 m). The droplets were then transported to the flame zone using a carrier gas (O2 or N2 ). The prepared powder was collected using a bag filter and was used for further characterization. The preparation of both composites was carried out under optimal conditions. The CuO/WO3 composite was prepared using a methane flow rate of 1 L/min (oxygen flow rate of 2.5 L/min) with an oxygen carrier gas flow rate of 2 L/min. In the case of Pt/WO3 , the flame was generated using a methane gas flow rate of 0.5 L/min (oxygen flow rate of 1.1 L/min) with a nitrogen carrier gas flow rate of 1 L/min. The precursor chemicals were 5(NH4 )2 O·12WO3 ·5H2 O (ammonium pentahydrate, 88–90%, Kanto Kagaku, Tokyo, Japan), H2 PtCl6 ·6H2 O (98.5%, Kanto Kagaku, Tokyo, Japan), and CuO nanoparticles (11 nm, Nishin Engineering, Tokyo, Japan). The precursor was prepared by dilution of 5(NH4 )2 O·12WO3 ·5H2 O with pure water with the concentration 0.01 M. The Pt and CuO were added to the solution with a calculated weight fraction that ranged from 0 to 0.5 wt.%. To facilitate better dispersion of CuO in the solution, homogenization was conducted using an ultrasonic cleaner for approximately 2 h. The prepared precursor was then used to prepare the nanocatalyst in the FASP method.
The as-prepared nanocatalysts were characterized using fieldemission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), X-ray diffractometer (XRD), Ultraviolet-visible Spectroscopy (UV–vis) and photoluminescence (PL). FE-SEM observations were carried out at 20 kV using an S-5000 (Hitachi Ltd., Tokyo, Japan). The characterization of the annealed powder was used to calculate particle size diameter. The calculation was conducted for about 1000 particles with the aid of Perfect Screen Ruler software. The crystalline nature of WO3 was observed using TEM (JEM-3000F, JEOL, Tokyo, Japan) operated at 300 kV. The crystal structure of WO3 was examined using an Xray diffractometer (XRD, RINT 2200 V, Rigaku-Denki Corp., Tokyo, Japan). The XRD measurements were carried out using nickelfiltered Cu K␣ radiation ( = 0.154 nm) at 40 kV and 30 mA with a scan step of 0.02◦ and a scan speed of 4◦ /min. Photoabsorption characterization was conducted using a UV–vis spectrophotometer (UV 2450, Shimadzu, Kyoto, Japan). The PL measurement was conducted using a Shimadzu RF-5300PC. The PL intensity was measured at an excitation wavelength of 270 nm using a xenon lamp at a resolution of 0.2 nm. The PL graph give an information regarding the correlation between nanocatalysts composition and electron–hole recombination process. 2.3. Photodegradation test To evaluate the photodegradation action of WO3 nanocatalysts, amaranth (C20 H11 N2 Na3 O10 S3 , 90%, Sigma–Aldrich, Germany) was used. The amaranth (C20 H11 N2 Na3 O10 S3 ) was chosen for the probe because it’s resistance to self-degradation under light irradiation. The photocatalytic reaction was carried out in a pyrex glass
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equipped with a cooling system to control the reaction temperature at 25 ◦ C. Continuous agitation was conducted using a magnetic stirrer to provide a homogeneous concentration inside the reactor. 10 ppm of amaranth was mixed with 200 mg nanocatalyst in a 100 mL solution. The solution was aerated using 200 mL/min of oxygen gas. A solar simulator (PEC-L11, Peccell Technologies, Inc., Japan) was used to simulate sunlight conditions at AM 1.5G (100 mW/cm2 ). To provide saturation conditions for the adsorption of dye-photocatalysts, the solution was stirred in the dark for 2 h. Solar-simulated illumination was conducted for 2 h. The solution was sampled and centrifuged at 15,000 rpm for 5 min to separate the dye solution and photocatalyst. The dye degradation was tracked by measuring the dye concentration using a UV–vis spectrophotometer (UV 2450, Shimadzu, Kyoto, Japan). The absorbance was evaluated in the wavelength range of 400–700 nm and was measured at room temperature (the amaranth peak was 520 nm). Dye concentration was calculated from its absorbance using a calibration curve (R2 = 0.9997, Supporting information Fig. S1).
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3. Results and discussion
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Reportedly, CuO and WO3 can be simply mixed to form an efficient photocatalyst composite for organic gas degradation [16,17]. To evaluate this method for organic compound degradation in an aqueous media, 2 wt.% of CuO (optimal conditions in the previous report) [16] was mixed with WO3 and annealed at 500 ◦ C for 30 min. CuO and WO3 nanoparticles (particle size 12 and 11 nm, respectively) were used for this experiment. The annealed CuO/WO3 composite was then evaluated for photocatalytic testing. The concentration–time evolution indicated that the amaranth did not degrade under solar-simulated irradiation (Fig. 2a). This result showed that the simple mixing of WO3 and CuO was insufficient to produce a photocatalyst composite for aqueous pollutant degradation. It is worth noting that the addition of 2 wt.% of CuO exhibited a good adsorption of amaranth (56.6%). In addition, the
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Fig. 3. (a) Diffuse-reflectance spectra of pure WO3 (CuO = 0 wt.%) and CuO/WO3 (CuO = 0.33 wt.%) nanocatalysts, and (b) photoluminescence spectra of CuO/WO3 in different CuO concentrations.
Fig. 4. FE-SEM and TEM images of as-prepared CuO/WO3 in different CuO concentration. (a) FE-SEM image of pure WO3 , (b) TEM image of pure WO3 , (c) FE-SEM image of CuO/WO3 (CuO = 0.33%), and (d) TEM image of CuO/WO3 (CuO = 0.33%).
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Fig. 5. XRD pattern of CuO/WO3 nanocomposite in different CuO concentrations. Fig. 6. Time–concentration chart of amaranth degradation using Pt/WO3 nanocomposite in different Pt concentrations.
CuO surface. The oxygen available in the solution may have oxidized the Cu(I) to Cu(II). The electron consumption reduced the electron/hole recombination, thereby enhancing the efficiency of the charge separation [16,17]. Beside of CuO, Pt addition as co-catalyst was also investigated. Pt is a well-known efficient co-catalyst for the WO3 system [5,6,21–23]. Many pollutants in both gas and aqueous solutions
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side reaction of organic compound oxidation produced Cu ions, which was indicated by the changing of the solution color from red to brownish (Fig. 2a, inset). Since the simple mixing of CuO/WO3 showed no photocatalytic activity, the preparation of the CuO/WO3 composite was then carried out using the FASP method. CuO nanoparticles of various weight fractions were mixed with 0.005 M ammonium tungstate pentahydrate (5(NH4 )2 O·12WO3 ·5H2 O) and were used as a precursor. The photodegradation test of the as-prepared CuO/WO3 nanocatalyst was conducted using amaranth as a pollutant probe. Fig. 2b shows the time–concentration evolution of amaranth photodegradation under solar-simulated irradiation. The as-prepared pure WO3 showed no appreciable photodegradation effect after 90 min of irradiation. In contrast, the CuO/WO3 nanocatalyst with a CuO content of 0.33 wt.% showed the highest photodegradation activity. The amaranth was degraded by 41% after 90 min irradiation. From diffusive-reflectance spectra (Fig. 3a), it is shown that the addition of CuO improves slightly light absorption of nanocatalyst. However, the addition of CuO did not show significant effect on PL intensity indicated by PL spectra in Fig. 3b. It is worth to noting that the FASP-made CuO/WO3 nanocatalyst did not disintegrate after extensive stirring during photodegradation testing. To evaluate the morphology of the as-prepared CuO/WO3 nanocatalyst, FE-SEM characterization was conducted as shown in Fig. 4. Pure WO3 (without CuO addition) has a nanorod shape with a diameter of from 5 to 20 nm and a length of from 30 to 50 nm. From the literature, the reported CuO/WO3 composite was on the order of a submicron to a micron size [16,17]. The HR-TEM image (Fig. 4b, inset) shows highly crystalline WO3 nanorods with a lattice spacing of 3.9 A˚ (lattice orientation (0 0 2)). In contrast, the addition of CuO propagated the formation of a cubic shape. The SEM image of the as-prepared particle with a CuO addition of 0.33 wt.% is shown in Fig. 4c. The change in morphology was verified by XRD characterization, as shown in Fig. 5, which showed that the (2 0 0) orientation peak was strongest with the addition of 0.33 wt.% CuO. The XRD spectra indicated that WO3 was monoclinic and that it fit well with JCPDS no. 72–0677 (unit cell parameters: a = 7.301, ˚ ˇ = 90.89; space group: P21 /n). In addition, the b = 7.539, c = 7.690 A, absence of CuO peaks in the composite was due to the small amount of added CuO. CuO is a p-type semiconductor that may act as a photocatalyst. However, the quantity of CuO in the composite was too low, and, thus, the possibility that CuO acted as a photocatalyst was negligible. In addition, an experimental test of pure CuO showed no photocatalytic activity. The role of CuO in enhancing photocatalytic activity was suggested from the efficient charge separation during photon irradiation. The excited electrons were transferred to the CuO and facilitated the reduction of Cu(II) to Cu(I) on the
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Wavelength [nm] Fig. 7. (a) Diffuse-reflectance spectra of pure WO3 (Pt = 0 wt.%) and Pt/WO3 (Pt = 0.12 wt.%) nanocatalysts, and (b) photoluminescence spectra of Pt/WO3 in different Pt concentrations.
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can be decomposed completely in a relatively short amount of time. The remaining problem with the application of Pt as a cocatalyst is the price. Thus, a reduction in the quantity of the Pt co-catalyst would be one way to reduce the catalyst cost. A popular method for the deposition of Pt onto the WO3 surface is photodeposition [5,6,12,21,23]. In this process, the Pt ions are reduced using photo-excited electrons from the catalyst semiconductor. The batch process is time consuming, and it requires a holes scavenger (i.e. methanol). The optimal Pt content for a highly active Pt/WO3 photocatalyst prepared by this method is from 0.5 to 1 wt.% [5,6,21]. To examine the possibility of lowering the Pt content in a Pt/WO3 photocatalyst, a FASP method was used. In this method, H2 PtCl6 ·6H2 O was mixed with 5(NH4 )2 O·12WO3 ·5H2 O (ATP) in aqueous solution to produce a homogeneous precursor. A thorough mixing of the Pt and W components was expected to produce a better distribution of Pt in the Pt/WO3 nanocatalyst. The preparation of Pt/WO3 was conducted with a CH4 flow rate of 0.5 L/min, an O2 flow rate of 1.1 L/min, and a nitrogen carrier gas flow rate of 2 L/min. The ATP concentration was 0.01 M, and the Pt content was varied from 0 to 0.5 wt.%. Fig. 6 shows the concentration–time evolution of amaranth degradation by Pt/WO3 at various levels of Pt content under solar simulated irradiation. The optimal level of Pt addition for high photocatalytic activity was 0.12 wt.%. This concentration was much lower compared with other methods (0.5–1 wt.%) [7,12]. Fig. 7a shows the diffuse reflectance spectra of pure WO3 (Pt = 0 wt.%) and Pt/WO3 (Pt = 0.12 wt.%) nanocatalysts. The Pt/WO3 (Pt = 0.12 wt.%) sample exhibited only a slightly elevated adsorption ability in visible light, at less than 550 nm. However, the PL graph for Pt concentration of 0, 0.12, and 0.5 wt.% in Fig. 7b shows that PL intensity was decreased with an increase in Pt concentration. This result shows that a higher amount of Pt affected the optical properties of the nanocatalyst. A lower PL intensity (at = 367 nm) is an indirect indicator of low recombination process. XRD characterization was conducted to evaluate the effect of Pt addition on the crystal structure of the as-prepared Pt/WO3 .
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0.2 wt% 0.12 wt% 0 wt% 20
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Fig. 8 shows the XRD patterns of the as-prepared Pt/WO3 for various amounts of Pt content (0–0.5 wt.%). The spectra indicated that WO3 was monoclinic. Unlike the CuO co-catalyst addition, the Pt addition did not induce a lattice orientation of WO3 . The characteristic peaks of Pt were not detected in the spectra due to the low fraction of Pt in the nanocatalyst. The crystallite sizes of Pt/WO3 were 16, 17, 17, and 16 nm for Pt fractions of 0, 0.12, 0.2, and 0.5 wt.%, respectively. These relatively similar crystallite sizes indicated that the presence of Pt did not affect the crystal growth process. Fig. 9 shows the FE-SEM images of Pt/WO3 in different Pt fractions and the HR-TEM images of Pt/WO3 (Pt = 0.12 wt.%). The as-prepared particles were nanorods with aspect ratios ranging from 2 to 5 (diameter 10–20 nm and length 20–50 nm). The HRTEM images of Pt/WO3 (Pt = 0.12 wt.%) shown in Fig. 9d do not clearly represent the existence of Pt particles, which may have been due to the small amount of Pt. Further characterization indicated
Fig. 9. (a, b) FE-SEM images of Pt/WO3 nanocomposite in different Pt concentrations, and (c, d) TEM images of Pt/WO3 nanocomposite (Pt = 12 wt.%).
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electrons from pure WO3 to reduce O2 . This is because the conduction band edge of WO3 (+0.3 to +0.5 VNHE ) is higher than the O2 reduction (E◦ (O2 /O2 −• ) = −0.33 VNHE or E◦ (O2 /HO2 • ) = −0.55 VNHE ) [22,25–27]. When Pt is used as the co-catalyst, the excited electrons can react with O2 producing H2 O2 and then react with electrons producing OH• radicals. The reactions are as follows: [21] 2e− cb + O2 + 2H+ → H2 O2 H2 O2 + e− cb → OH− + OH• The reduction of O2 to H2 O2 (E◦ (O2 /H2 O2 ) = +0.695 VNHE ) using Pt as a catalyst needs two electrons to complete the reaction [5]. The production of OH• radicals via H2 O2 reduction is dominant compared to the generation of OH• radicals via holes and water reaction [21]. This implies that there is an enhancement of photocatalytic activity in Pt-loaded WO3 . Our results showed that FASP-made Pt/WO3 nanocatalyst required a lower Pt fraction for an efficient photodegradation process compared to the well-known photodeposition method. This lower Pt fraction resulted from a stacking fault of Pt in the WO3 matrix, as shown in Fig. 10. In the photodeposition technique, Pt was deposited onto the WO3 surface. However, the reduced Pt was not all deposited onto the WO3 surface. This implies that a higher concentration of Pt is needed for optimal photocatalytic activity. Thus, FASP offers a facile method to produce a CuO/WO3 and Pt/WO3 nanocatalyst which is mechanically strength and a low fraction of Pt for efficient dye photodegradation. 4. Conclusion
Fig. 10. Stacking fault of Pt/WO3 nanocomposite at Pt = 12 wt.%.
the Stacking-fault in the HR-TEM images (Fig. 10), which indirectly implied the existence of Pt. Pt serves as a pool for electrons which lead to the effective excited electron consumption. This results in an efficient charge separation and ensures high activity for Pt/WO3 . This investigation suggests that the optimal condition for Pt/WO3 is reached at a Pt content of 0.12 wt.%. Organic compound degradation during the photocatalytic process resulted from the oxidation reaction of the organic compound with hydroxyl radicals (OH• ) or holes. Therefore, the presence of OH• produced from the reaction between the photocatalyst and absorbed water controlled the photodegradation rate. If pure WO3 is used as the photocatalyst, the possible reactions are as follows: WO3 + h → e− cb + h+ vb H2 O + h+ vb → OH• + H+ OH− + h+ vb → OH• OH• + organic compound → degradation of organic compound A + e− cb → A− · The hydroxyl radical and radical anions are the oxidizing species for organic compound oxidation [24]. However, the photocatalytic activity of pure WO3 is low, due to the inability of the excited
In the present study, FASP was used to prepare CuO/WO3 and Pt/WO3 nanocatalysts that exhibit high-performance photocatalytic activity. The addition of CuO enhanced the photocatalytic activity of WO3 for dye photodegradation under visible light irradiation. The morphology of WO3 was changed from nanorods to cubic following the addition of CuO. The optimal concentration of CuO for high photocatalytic activity was 0.33 wt.%. The FASP-made CuO/WO3 was found to be mechanically stable during agitation treatment during a photodegradation test. The Pt/WO3 nanocatalyst showed optimal photocatalytic activity with a low Pt concentration. This optimal Pt concentration was 0.12 wt.%, which is much lower compared with the reported optimal concentrations using a photodeposition technique (0.5–1 wt.%). In conclusion, the FASP method is a prospective route for the production of a lowcost WO3 -based photocatalyst that will degrade aqueous organic pollutants. Acknowledgments We thank Mr. Kozo Watanabe for assisting with the experimental work and Dr. Eishi Tanabe from the Hiroshima Prefectural Institute of Industrial Science and Technology for helping with the TEM analysis. We acknowledge the DGHE-Ministry of National Education of Indonesia for providing a collaborative research grant (H.W, A.P.; contract no. 495/SP2H/PL/Dit.Litabmas/VII/2011). The work was also supported by (MEXT)-KAKENHI grant-in-aid for Scientific Research A (no. 22246099). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2011.10.095.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
CuO/WO3 and Pt/WO3 nanocatalysts for efficient pollutant degradation using visible light irradiation Hendri Widiyandari a , Agus Purwanto b , Ratna Balgis c , Takashi Ogi c,∗ , Kikuo Okuyama c a b c
Department of Physics, Faculty of Mathematic and Natural Science, Diponegoro University, Jl. Prof. H. Soedarto SH, Semarang, Central Java, 50275, Indonesia Department of Chemical Engineering, Faculty of Engineering, Sebelas Maret University, Jl. Ir. Sutami 36 A, Surakarta, Central Java, 57126, Indonesia Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan
a r t i c l e
i n f o
Article history: Received 9 July 2011 Received in revised form 26 October 2011 Accepted 31 October 2011 Keywords: Visible-light-driven photocatalyst Nanoparticles Flame assisted spray pyrolysis Tungsten oxide Co-catalyst Photodegradation activity
a b s t r a c t CuO/WO3 and Pt/WO3 nanocatalysts with a nanorod morphology were used for photodegradation of organic compounds using visible light irradiation. Both nanocatalysts were prepared using flame assisted spray pyrolysis method (FASP). The prepared nanocatalysts were mechanically stable during the agitation treatment for the photodegradation test due to good interconnection between WO3 and co-catalysts. The enhancement of photocatalytic activity was observed after the addition of CuO and Pt as co-catalysts. The addition of CuO would change the morphology of WO3 from nanorods to cubic. The optimal concentration of the CuO addition was 0.33 wt.%. A low Pt concentration (0.12 wt.%) was required for optimal photocatalytic activity of the Pt/WO3 nanocomposite. The addition of Pt affected neither the morphology nor the crystallite structure of WO3 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tungsten oxide (WO3 ) is a visible-light-driven photocatalyst which has many potential applications: energy conversion devices [1–3], virus deactivation [4], and harmful pollutant degradation [5,6]. The optical band gap of WO3 is approximately 2.7–2.8 eV, which allows it to absorb light that ranges from ultraviolet to blue ( < 455 nm) [7,8]. In comparison to titania (band gap 3.0–3.2 eV) [7,9], WO3 shows better photoabsorption in visible-light irradiation due to its smaller band gap. This ability opens the application of WO3 for indoor pollutant treatment of volatile organic compound (VOC) gases using domestic light sources where ultraviolet light is limited. In outdoor applications, WO3 has the potential to use sunlight as energy for the treatment of harmful pollutants in industrial waste-water, and has the potential for hydrogen production as well. Bare WO3 shows very low photocatalytic activity under visible light irradiation. Many attempts have been made to improve the photocatalytic activity of WO3 . The addition of a co-catalyst, such as platinum (Pt), gold (Au), and silver (Ag), enhances the photodegradation activity of WO3 [5,6,10–12]. Thus far, Pt has been the most powerful co-catalyst for high-activity WO3 . Typically, electrons will be excited when a semiconductor catalyst is irradiated by light. Pt serves as a pool for electrons, thereby catalyzing the
∗ Corresponding author. Tel.: +81 82 424 7850; fax: +81 82 424 7850. E-mail address: [email protected] (T. Ogi). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.10.095
reduction of O2 via a multi-electron route. As a result, an abundant amount of hydroxyl radicals (OH• ) are available. These hydroxyl radicals (OH• ) effectively degrade the target organic compounds. Thus, the effective excited electron consumption of multi-electron reduction results in an efficient charge separation and ensures high activity for Pt/WO3 . Unfortunately, the high cost of Pt is a primary impediment for a scale-up process. To replace Pt as a co-catalyst, metal oxide and metal ion have been intensively investigated. Metal oxides of CuBi2 O4 [13], CaFe2 O4 [14], TiO2 [15] and CuO [16,17] were prepared as a composite with WO3 . Also, the grafting of Cu [18] and Cu ion [19] onto the surface of WO3 has been conducted. Among those methods, the utilization of CuO and CuBi2 O4 as co-catalysts was most interesting. CuO and CuBi2 O4 mix simply with WO3 and create an efficient photocatalyst composite for volatile gas decomposition [13,16,17]. However, a problem occurs when photodegradation of organic components in aqueous solution is conducted. The composite of WO3 and oxide co-catalysts disintegrates due to extensive stirring treatment during the photodegradation test. As a result, the co-catalyst effect is no longer observed. Therefore, an interconnected structure for co-catalyst-WO3 composites that will be strong enough to resist mechanical treatment is highly desirable. Herein, is shown the fabrication of CuO/WO3 and Pt/WO3 nanocatalysts using a flame assisted spray pyrolysis (FASP) method. This is a fast, continuous, and controlled method for the production of both nanoparticles and nanocomposites [8,20]. The prepared
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Fig. 1. Schematic diagram of flame assisted spray pyrolysis (FASP).
nanocatalysts have strong interconnection WO3 and co-catalysts which were effective for organic compound degradation under visible light irradiation. In addition, the FASP method also was effective in the preparation of a Pt/WO3 photocatalyst using a low Pt concentration. A low concentration of Pt is important in the practical application of a Pt/WO3 photocatalyst.
Fig. 2. Time–concentration chart of amaranth degradation using (a) CuO/WO3 nanocomposite prepared by simple mixing of nanoparticles, the side reaction of organic compound oxidation produced Cu ions (inset) and (b) CuO/WO3 nanocomposite prepared by FASP in different CuO concentrations.
2.2. Nanocatalysts characterization 2. Experimental 2.1. Preparation of CuO/WO3 and Pt/WO3 nanocomposites The CuO/WO3 and Pt/WO3 nanocatalysts were prepared using FASP as shown in Fig. 1. FASP is comprised of three main parts: precursor atomizer, diffusion burner, and particle collector. The precursor was atomized using an ultrasonic nebulizer (NE-U17, OMRON, Japan) to produce droplets (average size 5 m). The droplets were then transported to the flame zone using a carrier gas (O2 or N2 ). The prepared powder was collected using a bag filter and was used for further characterization. The preparation of both composites was carried out under optimal conditions. The CuO/WO3 composite was prepared using a methane flow rate of 1 L/min (oxygen flow rate of 2.5 L/min) with an oxygen carrier gas flow rate of 2 L/min. In the case of Pt/WO3 , the flame was generated using a methane gas flow rate of 0.5 L/min (oxygen flow rate of 1.1 L/min) with a nitrogen carrier gas flow rate of 1 L/min. The precursor chemicals were 5(NH4 )2 O·12WO3 ·5H2 O (ammonium pentahydrate, 88–90%, Kanto Kagaku, Tokyo, Japan), H2 PtCl6 ·6H2 O (98.5%, Kanto Kagaku, Tokyo, Japan), and CuO nanoparticles (11 nm, Nishin Engineering, Tokyo, Japan). The precursor was prepared by dilution of 5(NH4 )2 O·12WO3 ·5H2 O with pure water with the concentration 0.01 M. The Pt and CuO were added to the solution with a calculated weight fraction that ranged from 0 to 0.5 wt.%. To facilitate better dispersion of CuO in the solution, homogenization was conducted using an ultrasonic cleaner for approximately 2 h. The prepared precursor was then used to prepare the nanocatalyst in the FASP method.
The as-prepared nanocatalysts were characterized using fieldemission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), X-ray diffractometer (XRD), Ultraviolet-visible Spectroscopy (UV–vis) and photoluminescence (PL). FE-SEM observations were carried out at 20 kV using an S-5000 (Hitachi Ltd., Tokyo, Japan). The characterization of the annealed powder was used to calculate particle size diameter. The calculation was conducted for about 1000 particles with the aid of Perfect Screen Ruler software. The crystalline nature of WO3 was observed using TEM (JEM-3000F, JEOL, Tokyo, Japan) operated at 300 kV. The crystal structure of WO3 was examined using an Xray diffractometer (XRD, RINT 2200 V, Rigaku-Denki Corp., Tokyo, Japan). The XRD measurements were carried out using nickelfiltered Cu K␣ radiation ( = 0.154 nm) at 40 kV and 30 mA with a scan step of 0.02◦ and a scan speed of 4◦ /min. Photoabsorption characterization was conducted using a UV–vis spectrophotometer (UV 2450, Shimadzu, Kyoto, Japan). The PL measurement was conducted using a Shimadzu RF-5300PC. The PL intensity was measured at an excitation wavelength of 270 nm using a xenon lamp at a resolution of 0.2 nm. The PL graph give an information regarding the correlation between nanocatalysts composition and electron–hole recombination process. 2.3. Photodegradation test To evaluate the photodegradation action of WO3 nanocatalysts, amaranth (C20 H11 N2 Na3 O10 S3 , 90%, Sigma–Aldrich, Germany) was used. The amaranth (C20 H11 N2 Na3 O10 S3 ) was chosen for the probe because it’s resistance to self-degradation under light irradiation. The photocatalytic reaction was carried out in a pyrex glass
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equipped with a cooling system to control the reaction temperature at 25 ◦ C. Continuous agitation was conducted using a magnetic stirrer to provide a homogeneous concentration inside the reactor. 10 ppm of amaranth was mixed with 200 mg nanocatalyst in a 100 mL solution. The solution was aerated using 200 mL/min of oxygen gas. A solar simulator (PEC-L11, Peccell Technologies, Inc., Japan) was used to simulate sunlight conditions at AM 1.5G (100 mW/cm2 ). To provide saturation conditions for the adsorption of dye-photocatalysts, the solution was stirred in the dark for 2 h. Solar-simulated illumination was conducted for 2 h. The solution was sampled and centrifuged at 15,000 rpm for 5 min to separate the dye solution and photocatalyst. The dye degradation was tracked by measuring the dye concentration using a UV–vis spectrophotometer (UV 2450, Shimadzu, Kyoto, Japan). The absorbance was evaluated in the wavelength range of 400–700 nm and was measured at room temperature (the amaranth peak was 520 nm). Dye concentration was calculated from its absorbance using a calibration curve (R2 = 0.9997, Supporting information Fig. S1).
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Reportedly, CuO and WO3 can be simply mixed to form an efficient photocatalyst composite for organic gas degradation [16,17]. To evaluate this method for organic compound degradation in an aqueous media, 2 wt.% of CuO (optimal conditions in the previous report) [16] was mixed with WO3 and annealed at 500 ◦ C for 30 min. CuO and WO3 nanoparticles (particle size 12 and 11 nm, respectively) were used for this experiment. The annealed CuO/WO3 composite was then evaluated for photocatalytic testing. The concentration–time evolution indicated that the amaranth did not degrade under solar-simulated irradiation (Fig. 2a). This result showed that the simple mixing of WO3 and CuO was insufficient to produce a photocatalyst composite for aqueous pollutant degradation. It is worth noting that the addition of 2 wt.% of CuO exhibited a good adsorption of amaranth (56.6%). In addition, the
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Fig. 3. (a) Diffuse-reflectance spectra of pure WO3 (CuO = 0 wt.%) and CuO/WO3 (CuO = 0.33 wt.%) nanocatalysts, and (b) photoluminescence spectra of CuO/WO3 in different CuO concentrations.
Fig. 4. FE-SEM and TEM images of as-prepared CuO/WO3 in different CuO concentration. (a) FE-SEM image of pure WO3 , (b) TEM image of pure WO3 , (c) FE-SEM image of CuO/WO3 (CuO = 0.33%), and (d) TEM image of CuO/WO3 (CuO = 0.33%).
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Fig. 5. XRD pattern of CuO/WO3 nanocomposite in different CuO concentrations. Fig. 6. Time–concentration chart of amaranth degradation using Pt/WO3 nanocomposite in different Pt concentrations.
CuO surface. The oxygen available in the solution may have oxidized the Cu(I) to Cu(II). The electron consumption reduced the electron/hole recombination, thereby enhancing the efficiency of the charge separation [16,17]. Beside of CuO, Pt addition as co-catalyst was also investigated. Pt is a well-known efficient co-catalyst for the WO3 system [5,6,21–23]. Many pollutants in both gas and aqueous solutions
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side reaction of organic compound oxidation produced Cu ions, which was indicated by the changing of the solution color from red to brownish (Fig. 2a, inset). Since the simple mixing of CuO/WO3 showed no photocatalytic activity, the preparation of the CuO/WO3 composite was then carried out using the FASP method. CuO nanoparticles of various weight fractions were mixed with 0.005 M ammonium tungstate pentahydrate (5(NH4 )2 O·12WO3 ·5H2 O) and were used as a precursor. The photodegradation test of the as-prepared CuO/WO3 nanocatalyst was conducted using amaranth as a pollutant probe. Fig. 2b shows the time–concentration evolution of amaranth photodegradation under solar-simulated irradiation. The as-prepared pure WO3 showed no appreciable photodegradation effect after 90 min of irradiation. In contrast, the CuO/WO3 nanocatalyst with a CuO content of 0.33 wt.% showed the highest photodegradation activity. The amaranth was degraded by 41% after 90 min irradiation. From diffusive-reflectance spectra (Fig. 3a), it is shown that the addition of CuO improves slightly light absorption of nanocatalyst. However, the addition of CuO did not show significant effect on PL intensity indicated by PL spectra in Fig. 3b. It is worth to noting that the FASP-made CuO/WO3 nanocatalyst did not disintegrate after extensive stirring during photodegradation testing. To evaluate the morphology of the as-prepared CuO/WO3 nanocatalyst, FE-SEM characterization was conducted as shown in Fig. 4. Pure WO3 (without CuO addition) has a nanorod shape with a diameter of from 5 to 20 nm and a length of from 30 to 50 nm. From the literature, the reported CuO/WO3 composite was on the order of a submicron to a micron size [16,17]. The HR-TEM image (Fig. 4b, inset) shows highly crystalline WO3 nanorods with a lattice spacing of 3.9 A˚ (lattice orientation (0 0 2)). In contrast, the addition of CuO propagated the formation of a cubic shape. The SEM image of the as-prepared particle with a CuO addition of 0.33 wt.% is shown in Fig. 4c. The change in morphology was verified by XRD characterization, as shown in Fig. 5, which showed that the (2 0 0) orientation peak was strongest with the addition of 0.33 wt.% CuO. The XRD spectra indicated that WO3 was monoclinic and that it fit well with JCPDS no. 72–0677 (unit cell parameters: a = 7.301, ˚ ˇ = 90.89; space group: P21 /n). In addition, the b = 7.539, c = 7.690 A, absence of CuO peaks in the composite was due to the small amount of added CuO. CuO is a p-type semiconductor that may act as a photocatalyst. However, the quantity of CuO in the composite was too low, and, thus, the possibility that CuO acted as a photocatalyst was negligible. In addition, an experimental test of pure CuO showed no photocatalytic activity. The role of CuO in enhancing photocatalytic activity was suggested from the efficient charge separation during photon irradiation. The excited electrons were transferred to the CuO and facilitated the reduction of Cu(II) to Cu(I) on the
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can be decomposed completely in a relatively short amount of time. The remaining problem with the application of Pt as a cocatalyst is the price. Thus, a reduction in the quantity of the Pt co-catalyst would be one way to reduce the catalyst cost. A popular method for the deposition of Pt onto the WO3 surface is photodeposition [5,6,12,21,23]. In this process, the Pt ions are reduced using photo-excited electrons from the catalyst semiconductor. The batch process is time consuming, and it requires a holes scavenger (i.e. methanol). The optimal Pt content for a highly active Pt/WO3 photocatalyst prepared by this method is from 0.5 to 1 wt.% [5,6,21]. To examine the possibility of lowering the Pt content in a Pt/WO3 photocatalyst, a FASP method was used. In this method, H2 PtCl6 ·6H2 O was mixed with 5(NH4 )2 O·12WO3 ·5H2 O (ATP) in aqueous solution to produce a homogeneous precursor. A thorough mixing of the Pt and W components was expected to produce a better distribution of Pt in the Pt/WO3 nanocatalyst. The preparation of Pt/WO3 was conducted with a CH4 flow rate of 0.5 L/min, an O2 flow rate of 1.1 L/min, and a nitrogen carrier gas flow rate of 2 L/min. The ATP concentration was 0.01 M, and the Pt content was varied from 0 to 0.5 wt.%. Fig. 6 shows the concentration–time evolution of amaranth degradation by Pt/WO3 at various levels of Pt content under solar simulated irradiation. The optimal level of Pt addition for high photocatalytic activity was 0.12 wt.%. This concentration was much lower compared with other methods (0.5–1 wt.%) [7,12]. Fig. 7a shows the diffuse reflectance spectra of pure WO3 (Pt = 0 wt.%) and Pt/WO3 (Pt = 0.12 wt.%) nanocatalysts. The Pt/WO3 (Pt = 0.12 wt.%) sample exhibited only a slightly elevated adsorption ability in visible light, at less than 550 nm. However, the PL graph for Pt concentration of 0, 0.12, and 0.5 wt.% in Fig. 7b shows that PL intensity was decreased with an increase in Pt concentration. This result shows that a higher amount of Pt affected the optical properties of the nanocatalyst. A lower PL intensity (at = 367 nm) is an indirect indicator of low recombination process. XRD characterization was conducted to evaluate the effect of Pt addition on the crystal structure of the as-prepared Pt/WO3 .
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Fig. 8 shows the XRD patterns of the as-prepared Pt/WO3 for various amounts of Pt content (0–0.5 wt.%). The spectra indicated that WO3 was monoclinic. Unlike the CuO co-catalyst addition, the Pt addition did not induce a lattice orientation of WO3 . The characteristic peaks of Pt were not detected in the spectra due to the low fraction of Pt in the nanocatalyst. The crystallite sizes of Pt/WO3 were 16, 17, 17, and 16 nm for Pt fractions of 0, 0.12, 0.2, and 0.5 wt.%, respectively. These relatively similar crystallite sizes indicated that the presence of Pt did not affect the crystal growth process. Fig. 9 shows the FE-SEM images of Pt/WO3 in different Pt fractions and the HR-TEM images of Pt/WO3 (Pt = 0.12 wt.%). The as-prepared particles were nanorods with aspect ratios ranging from 2 to 5 (diameter 10–20 nm and length 20–50 nm). The HRTEM images of Pt/WO3 (Pt = 0.12 wt.%) shown in Fig. 9d do not clearly represent the existence of Pt particles, which may have been due to the small amount of Pt. Further characterization indicated
Fig. 9. (a, b) FE-SEM images of Pt/WO3 nanocomposite in different Pt concentrations, and (c, d) TEM images of Pt/WO3 nanocomposite (Pt = 12 wt.%).
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electrons from pure WO3 to reduce O2 . This is because the conduction band edge of WO3 (+0.3 to +0.5 VNHE ) is higher than the O2 reduction (E◦ (O2 /O2 −• ) = −0.33 VNHE or E◦ (O2 /HO2 • ) = −0.55 VNHE ) [22,25–27]. When Pt is used as the co-catalyst, the excited electrons can react with O2 producing H2 O2 and then react with electrons producing OH• radicals. The reactions are as follows: [21] 2e− cb + O2 + 2H+ → H2 O2 H2 O2 + e− cb → OH− + OH• The reduction of O2 to H2 O2 (E◦ (O2 /H2 O2 ) = +0.695 VNHE ) using Pt as a catalyst needs two electrons to complete the reaction [5]. The production of OH• radicals via H2 O2 reduction is dominant compared to the generation of OH• radicals via holes and water reaction [21]. This implies that there is an enhancement of photocatalytic activity in Pt-loaded WO3 . Our results showed that FASP-made Pt/WO3 nanocatalyst required a lower Pt fraction for an efficient photodegradation process compared to the well-known photodeposition method. This lower Pt fraction resulted from a stacking fault of Pt in the WO3 matrix, as shown in Fig. 10. In the photodeposition technique, Pt was deposited onto the WO3 surface. However, the reduced Pt was not all deposited onto the WO3 surface. This implies that a higher concentration of Pt is needed for optimal photocatalytic activity. Thus, FASP offers a facile method to produce a CuO/WO3 and Pt/WO3 nanocatalyst which is mechanically strength and a low fraction of Pt for efficient dye photodegradation. 4. Conclusion
Fig. 10. Stacking fault of Pt/WO3 nanocomposite at Pt = 12 wt.%.
the Stacking-fault in the HR-TEM images (Fig. 10), which indirectly implied the existence of Pt. Pt serves as a pool for electrons which lead to the effective excited electron consumption. This results in an efficient charge separation and ensures high activity for Pt/WO3 . This investigation suggests that the optimal condition for Pt/WO3 is reached at a Pt content of 0.12 wt.%. Organic compound degradation during the photocatalytic process resulted from the oxidation reaction of the organic compound with hydroxyl radicals (OH• ) or holes. Therefore, the presence of OH• produced from the reaction between the photocatalyst and absorbed water controlled the photodegradation rate. If pure WO3 is used as the photocatalyst, the possible reactions are as follows: WO3 + h → e− cb + h+ vb H2 O + h+ vb → OH• + H+ OH− + h+ vb → OH• OH• + organic compound → degradation of organic compound A + e− cb → A− · The hydroxyl radical and radical anions are the oxidizing species for organic compound oxidation [24]. However, the photocatalytic activity of pure WO3 is low, due to the inability of the excited
In the present study, FASP was used to prepare CuO/WO3 and Pt/WO3 nanocatalysts that exhibit high-performance photocatalytic activity. The addition of CuO enhanced the photocatalytic activity of WO3 for dye photodegradation under visible light irradiation. The morphology of WO3 was changed from nanorods to cubic following the addition of CuO. The optimal concentration of CuO for high photocatalytic activity was 0.33 wt.%. The FASP-made CuO/WO3 was found to be mechanically stable during agitation treatment during a photodegradation test. The Pt/WO3 nanocatalyst showed optimal photocatalytic activity with a low Pt concentration. This optimal Pt concentration was 0.12 wt.%, which is much lower compared with the reported optimal concentrations using a photodeposition technique (0.5–1 wt.%). In conclusion, the FASP method is a prospective route for the production of a lowcost WO3 -based photocatalyst that will degrade aqueous organic pollutants. Acknowledgments We thank Mr. Kozo Watanabe for assisting with the experimental work and Dr. Eishi Tanabe from the Hiroshima Prefectural Institute of Industrial Science and Technology for helping with the TEM analysis. We acknowledge the DGHE-Ministry of National Education of Indonesia for providing a collaborative research grant (H.W, A.P.; contract no. 495/SP2H/PL/Dit.Litabmas/VII/2011). The work was also supported by (MEXT)-KAKENHI grant-in-aid for Scientific Research A (no. 22246099). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2011.10.095.
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