(Pt,Fe2O3) nanocomposites

International Journal of Inorganic Materials 1 (1999) 253–258

Synthesis and photocatalytic properties of HTaWO 6 /(Pt,TiO 2 ) and HTaWO 6 /(Pt,Fe 2 O 3 ) nanocomposites a b b b b, Jihuai Wu , Satoshi Uchida , Yoshinobu Fujishiro , Shu Yin , Tsugio Sato * a

Institute of Material Physical Chemistry, Huaqiao University, Quanzhou, 362011, China b Institute for Chemical Reaction Science, Tohoku University, Sendai 980 -8577, Japan Received 16 March 1999; accepted 10 August 1999

Abstract HTaWO 6 /(Pt,TiO 2 ) and HTaWO 6 /(Pt,Fe 2 O 3 ) nanocomposites were synthesized by successive intercalation reactions of HTaWO 6 with [Pt(NH 3 ) 4 ]Cl 2 aqueous solution, n-C 3 H 7 NH 2 /n-heptane mixed solution and acidic TiO 2 colloid solution or [Fe 3 (CH 3 CO 2 ) 7 (OH)(H 2 O) 2 ]NO 3 aqueous solution followed by UV light irradiation. The gallery heights of HTaWO 6 /(Pt,TiO 2 ), HTaWO 6 / TiO 2 , HTaWO 6 /(Pt,Fe 2 O 3 ) and HTaWO 6 / Fe 2 O 3 was less than 0.51 nm. The host HTaWO 6 was white possessing band gap energy of 3.1 eV, whereas HTaWO 6 / Pt, HTaWO 6 /(Pt,TiO 2 ), HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ) were yellow and showed broad reflection over 400–600 nm together with that corresponding to the host layer. Photocatalytic activities of HTaWO 6 / TiO 2 and HTaWO 6 / Fe 2 O 3 were superior to those of unsupported TiO 2 and Fe 2 O 3 and were greatly enhanced by co-incorporation of Pt. HTaWO 6 / Pt, HTaWO 6 /(Pt,TiO 2 ), HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ) showed photocatalytic activity.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. layered compounds; A. microporous materials; A. semiconductors; B. intercalation reactions; D. catalytic properties

1. Introduction Photocatalytic reactions of semiconductors, such as splitting of water and reduction of carbon dioxide, have received special attention because of their possible application for the conversion of solar energy into chemical energy. Many studies have been carried out to enhance the photochemical activities of these catalysts. It is to be expected that the photoactivity of a semiconductor increases with the decrease of particle size since, in such a system, the distance which the photoinduced holes and electrons have to diffuse before reaching the interface decreases. Consequently, the holes and electrons can be effectively captured by the electrolyte in the solution [1]. Incorporation of semiconductor particles via chemical reactions in the interlayer region of a lamellar compound is a promising method for the fabrication of a nanocomposite consisting of host layers with ultrafine particles in the *Corresponding author. Tel.: 181-22-217-5597; fax: 181-22-2175599. E-mail address: [email protected] (T. Sato)

interlayer. Yamanaka et al. [2], Enea and Bard [3], Yoneyama and co-workers [4–6], and Sato and co-workers [7–10] have reported the incorporation of extremely small particles of Fe 2 O 3 , TiO 2 , CdS and CdS–ZnS mixtures, ,1 nm in thickness, into the interlayer of layered compounds, such as montmorillonite, layered double hydroxides, layered niobate and layered titanate. As expected, the photocatalytic activities of the incorporated semiconductors were much higher than that of the unsupported semiconductors. In previous papers [7–10], we investigated the synthesis and photoactivity of H 2 Ti 4 O 9 , H 4 Nb 6 O 17 and HNbWO 6 layered nanocomposites incorporating Fe 2 O 3 and TiO 2 and reported that the incorporation of semiconductor particles in the interlayer of a semiconductor was more efficient to enhance the photocatalytic activity than when an insulator was used as a host material. Therefore, it is expected that the photocatalytic activity of the layered compound / semiconductor would change depending on the electrical property of the layered compound. In continuation of our study, new layered nanocomposites, HTaWO 6 /(Pt,TiO 2 ) and HTaWO 6 / (Pt,Fe 2 O 3 ) were synthesized using n-type semiconductor,

1466-6049 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 99 )00038-0

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J. Wu et al. / International Journal of Inorganic Materials 1 (1999) 253 – 258

HTaWO 6 as a host material and their photocatalytic activities were evaluated.

HTaWO 6 / TiO 2 . The sample obtained was designated as HTaWO 6 /(Pt,TiO 2 ).

2. Experimental

2.3. Incorporation of Fe2 O3 and Pt into the interlayer of HTaWO6

2.1. Chemicals HTaWO 6 was prepared by the ion exchange reaction of LiTaWO 6 with 3 M HNO 3 at room temperature for 72 h with one intermediate replacement of the acid in 32 h. LiTaWO 6 was obtained by calcining a stoichiometric mixture of Li 2 CO 3 , WO 3 and Ta 2 O 5 at 9008C in air for 24 h with one intermediate grinding [11]. [Fe 3 (CH 3 COO) 7 (OH)(H 2 O) 2 ]NO 3 was synthesized by the reaction of Fe(NO 3 ) 3 ?9H 2 O with acetic anhydride in ethanol as reported [2]. Unsupported TiO 2 (Deggusa P-25) was commercially obtained and used without further purification. Unsupported Fe 2 O 3 was prepared by adding 1 M Fe(NO 3 ) 3 aqueous solution (50 ml) to 5 M NH 3 aqueous solution (500 ml) at room temperature and washing the precipitate with water until free of NH 3 followed by drying at 1208C.

Fe 2 O 3 was incorporated into the interlayer of HTaWO 6 by irradiating the [Fe 3 (CH 3 COO) 7 (OH)(H 2 O) 2 ] 1 exchanged compound with UV light from a 450-W high-pressure mercury lamp at 508C for 12 h. The exchanged compound was obtained by ion-exchange reaction with HTaWO 6 /n-C 3 H 7 NH 2 (2 g) and [Fe 3 (CH 3 COO) 7 (OH)(H 2 O) 2 ]NO 3 (20 g) in 400 ml water at 508C for 72 h. The sample obtained was designated as HTaWO 6 / Fe 2 O 3 . Fe 2 O 3 was incorporated into HTaWO 6 / Pt by successive intercalating reactions with 20 vol.% n-C 3 H 7 NH 2 /n-heptane mixed solution, [Fe 3 (CH 3 COO) 7 (OH)(H 2 O) 2 ]NO 3 aqueous solution followed by photodecomposition of [Fe 3 (CH 3 COO) 7 (OH)(H 2 O) 2 ] 1 in a similar manner as that for preparing HTaWO 6 / Fe 2 O 3 . The sample obtained was designated as HTaWO 6 /(Pt,Fe 2 O 3 ).

2.4. Analysis 2.2. Incorporation of TiO2 and Pt into the interlayer of HTaWO6 TiO 2 acidic sol was made by adding titanium tetraisopropoxide to 1 M HCl solution with the TiO 2 / HCl molar ratio of 0.25. HTaWO 6 /n-C 3 H 7 NH 2 was obtained by stirring HTaWO 6 (1 g) in 40 ml of 20 vol.% nC 3 H 7 NH 2 /n-heptane mixed solution under reflux at 508C for 72 h. HTaWO 6 /n-C 3 H 7 NH 2 was added to TiO 2 acidic sol solution with the TiO 2 / HTaWO 6 molar ratio of 20. The suspension was continuously stirred for 6 h at room temperature so as to incorporate TiO 2 into the interlayer of HTaWO 6 . After being filtered and washed with water, the specimen was dispersed in water and irradiated with UV light from a 450-W high-pressure mercury lamp at 608C for 12 h so as to decompose any n-C 3 H 7 NH 2 remaining in the interlayer of HTaWO 6 . The sample obtained was designated as HTaWO 6 / TiO 2 . [Pt(NH 3 ) 4 ] 21 was incorporated in the interlayer of HTaWO 6 by stirring HTaWO 6 (5 g) in 0.6 mM [Pt(NH 3 ) 4 ]Cl 2 aqueous solution (1000 ml) at room temperature for 72 h. After being filtered and washed with water, the specimen was dispersed in water and irradiated with UV light from a 450-W high-pressure mercury lamp at room temperature for 5 h to deposit Pt particles in the interlayer of HTaWO 6 . The product obtained thus was designated as HTaWO 6 / Pt. After that TiO 2 was incorporated into the interlayer of HTaWO 6 / Pt by successive intercalating reactions with 20 vol.% n-C 3 H 7 NH 2 /n-heptane mixed solution, TiO 2 acidic sol solution and irradiation with UV light in a similar manner as that for preparing

The crystalline phases of the products were identified by X-ray diffraction (Rigaku Denki Geiger-flex 2013) using graphite monochromatized Cu Ka radiation. The chemical compositions of the products were determined by TG-DTA analysis (Rigaku Denki TAS 200 TG-DTA) and by inductively coupled plasma atomic emission spectroscopy (Seiko SPS-1200A) by dissolving the samples in water after mixing 0.1 g samples with 4 g Na 2 CO 3 and calcining at 9008C for 4 h. The band gap energies of the products were determined from the onset of diffuse reflectance spectra of the powders measured by using a Shimadzu Model UV-2000 UV-VIS spectrophotometer. The specific surface areas of samples were determined by nitrogen gas adsorption method (Quantachrome Autosorb-1).

2.5. Photocatalytic reaction Photocatalytic reaction was carried out in a Pyrex reactor of 1250 ml capacity attached to an inner radiation type 450-W mercury lamp. The inner cell had thermostat water flowing through a jacket between the mercury lamp and the reaction chamber. The inner cell was constructed of Pyrex glass which served to filter out UV emission the mercury arc below 290 nm. UV emission of the mercury arc below 400 nm was filtered out by flowing 1 M NaNO 2 solution between the mercury lamp and the reaction chamber. The photocatalytic activities of the samples were determined by measuring the volume of hydrogen gas evolved with a gas burette when the suspensions of samples were irradiated.

J. Wu et al. / International Journal of Inorganic Materials 1 (1999) 253 – 258

3. Results and discussions

3.1. Intercalation of TiO2 and Fe2 O3 into the interlayer of HTaWO6 Fig. 1 shows the X-ray powder diffraction patterns of (a) HTaWO 6 , (b) HTaWO 6 /n-C 3 H 7 NH 2 , (c) HTaWO 6 / Pt, (d) HTaWO 6 / TiO 2 , (e) HTaWO 6 /(Pt,TiO 2 ) (f) HTaWO 6 / Fe 2 O 3 and (g) HTaWO 6 /(Pt,Fe 2 O 3 ). The peak positions, corresponding to (110) of HTaWO 6 , of samples (b)–(g), are almost the same as HTaWO 6 (a), but those corre-

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sponding to (002) changed significantly depending on the species in the interlayer. These results suggest that layered structure of HTaWO 6 was retained after intercalation of n-C 3 H 7 NH 2 , TiO 2 , Fe 2 O 3 and Pt, although the distance of the interlayer changed. Compared with sample (a), (002) diffraction peak of sample (b) shifted significantly to lower 2u angle, which indicates the expansion of the interlayer by incorporation of n-C 3 H 7 NH 2 . The gallery height of HTaWO 6 /n-C 3 H 7 NH 2 determined by subtracting the HTaWO 6 layer thickness (0.76 nm) [11,12], was 1.03 nm. Since the length of n-C 3 H 7 NH 2 is ca. 0.5 nm, it is suspected that two molecules of n-C 3 H 7 NH 2 are vertically arranged in the interlayer of HTaWO 6 . The TG curve of HTaWO 6 /n-C 3 H 7 NH 2 (shown in Fig. 2) indicated about 11% of weight loss until 6508C, which agreed with the calculated value (11%) according to the following reaction. C 3 H 7 NH 3 TaWO 6 1 9 / 2O 2 → 1 / 2Ta 2 O 5 1 WO 3 1 3CO 2 1 NH 3 1 7 / 2H 2 O Therefore, it was confirmed that the molar ratio of HTaWO 6 /n-C 3 H 7 NH 2 was 1:1. These result were similar to those in H 2 Ti 4 O 9 , H 4 Nb 6 O 17 and HNbWO 6 systems [7,8,10]. The diffuse reflection spectra of (a) HTaWO 6 , (b) HTaWO 6 / Pt, (c) HTaWO 6 / TiO 2 , (d) HTaWO 6 /(Pt,TiO 2 ), (e) HTaWO 6 / Fe 2 O 3 and (f) HTaWO 6 /(Pt,Fe 2 O 3 ) are shown in Fig. 3. The spectra of HTaWO 6 and HTaWO 6 / TiO 2 were almost the same, indicating the onset at ca. 400 nm (3.1 eV). On the other hand, HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ) show broad reflection spectra over 400–600 nm. The absorption of visible light above 400 nm might be caused by iron oxide incorporated. The different phenomena that occurred on incorporation of Fe 2 O 3 and TiO 2 might be due to the difference in band gap energies, i.e., the difference in the band gap energy of TiO 2 and HTaWO 6 is not large, but that of Fe 2 O 3 and HTaWO 6 is large. Similar phenomena were also observed in H 2 Ti 4 O 9 / TiO 2 , H 4 Nb 6 O 17 / TiO 2 , HNbWO 6 / TiO 2 , H 2 Ti 4 O 9 /

Fig. 1. Powder X-ray diffraction patterns of (a) HTaWO 6 , (b) HTaWO 6 / n-C 3 H 7 NH 2 , (c) HTaWO 6 / Pt, (d) HTaWO 6 / TiO 2 , (e) HTaWO 6 / (Pt,TiO 2 ), (f) HTaWO 6 / Fe 2 O 3 , and (g) HTaWO 6 /(Pt,Fe 2 O 3 ).

Fig. 2. TG curve of HTaWO 6 /n-C 3 H 7 NH 2 .

J. Wu et al. / International Journal of Inorganic Materials 1 (1999) 253 – 258

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systems, since both samples were irradiated with UV light for 5 and 12 h for the photodeposition of Pt and photodecomposition of N–C 3 H 7 NH 3 1 in the interlayer, respectively (see Section 2.2). So that, the onset at 400 and 550 nm might be attributed to the HTaWO 6 remaining and WO 3 formed by the photo-induced phase transformation. Since no noticeable transformation was observed in HTaWO 6 / TiO 2 , which was irradiated with UV light for 12 h, it is suspected that Pt promoted the photo-induced phase transformation of HTaWO 6 . The gallery heights, amounts of Ti, Fe and Pt elements incorporated, band gap energies and specific surface areas of products are summarized in Table 1. The amounts of Ti, Fe and Pt elements intercalated are 4.08–6.66, 7.05–10.50 and 0.46–0.63 wt.%, respectively. Since the gallery heights of TiO 2 and Fe 2 O 3 pillared HTaWO 6 was less than 0.51 nm, the thickness of TiO 2 and Fe 2 O 3 intercalated was suggested to be less than 0.51 nm. The specific surface areas of HTaWO 6 / TiO 2 , HTaWO 6 /(Pt,TiO 2 ), HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ) were four to six times greater than that of HTaWO 6 which further indicates the formation of the Fe 2 O 3 and TiO 2 pillars. (Table 1)

3.2. Photocatalytic properties Fig. 4 shows the amount of hydrogen gas produced from 1250 ml of 10 vol.% methanol solution containing 1 g of dispersed unsupported TiO 2 (P-25), unsupported Fe 2 O 3 , HTaWO 6 , HTaWO 6 / Pt, HTaWO 6 / TiO 2 , HTaWO 6 / (Pt,TiO 2 ), HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ) at 608C for 5 h under irradiation at (a) l .290 nm and (b) l .400 nm from a 450-W mercury lamp. All samples showed photocatalytic activity to evolve hydrogen gas under irradiation above 290 nm. The amount of hydrogen gas evolved increased in the sequence, unsupported Fe 2 O 3 ,unsupported TiO 2 ,HTaWO 6 ,HTaWO 6 / Fe 2 O 3 5HTaWO 6 / TiO 2 ,HTaWO 6 /(Pt,Fe 2 O 3 ), HTaWO 6 / Pt
Fig. 3. Reflection spectra of (a) HTaWO 6 , (b) HTaWO 6 / Pt, (c) HTaWO 6 / TiO 2 , (d) HTaWO 6 /(Pt,TiO 2 ), (e) HTaWO 6 / Fe 2 O 3 , and (f) HTaWO 6 / (Pt,Fe 2 O 3 ).

Fe 2 O 3 , H 4 Nb 6 O 17 / Fe 2 O 3 and HNbWO 6 / Fe 2 O 3 systems [7,8,10]. It was also notable that although HTaWO 6 and HTaWO 6 / TiO 2 were white, both HTaWO 6 / Pt and HTaWO 6 /(Pt,TiO 2 ) showed yellow coloring and two onsets at ca. 400 and 550 nm. It was reported that HTaWO 6 caused photo-induced phase transformation to Ta 2 O 5 and WO 3 on laser irradiation [13]. Therefore, it was suspected that similar photo-induced phase transformation occurred in both HTaWO 6 / Pt and HTaWO 6 /(Pt,TiO 2 )

Table 1 Gallery heights, element contents, band gap energies and surface areas of the products Product

HTaWO 6 HTaWO 6 / Pt HTaWO 6 / TiO 2 HTaWO 6 /(Pt,TiO 2 ) HTaWO 6 / Fe 2 O 3 HTaWO 6 /(Pt,Fe 2 O 3 )

Gallery height

Content (wt.%)

(nm)

Ti

Fe

0.28 0.30 0.44 0.51 0.37 0.37

0 0 4.08 6.6 0 0

0 0 0 0 7.05 10.50

Band gap energy

Specific surface area

Pt

(eV)

(m 2 g 21 )

0 0.63 0 0.46 0 0.51

3.1 3.1, 3.1 3.1, 3.1, 3.1,

2.2 2.3 2.1 2.1

4.22 4.98 18.5 27.1 33.4 22.1

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Fig. 4. Amount of hydrogen gas produced from 1250 ml of 10 vol.% methanol solution containing 1 g of dispersed unsupported TiO 2 (P-25), unsupported Fe 2 O 3 , HTaWO 6 , HTaWO 6 / Pt, HTaWO 6 / TiO 2 , HTaWO 6 /(Pt,TiO 2 ), HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ) at 608C for 5 h under irradiating of a 450-W mercury arc. (a) Under irradiating light, l .290 nm; (b) under irradiating light, l .400 nm.

HTaWO 6 / Pt and HTaWO 6 /(Pt,TiO 2 ) is about 0.5, 0.7, 1.1, 1.2, 3.6, 4.8 and 110 times larger than that from unsupported TiO 2 (P-25). On the other hand, as expected from their wide band gap energies (.3 eV), no hydrogen gas evolution was observed under visible light ( l .400 nm) irradiation in the presence of P-25, HTaWO 6 and HTaWO 6 / TiO 2 . It is notable that the amount of hydrogen gas produced in the presence of HTaWO 6 / Fe 2 O 3 was twice that of unsupported Fe 2 O 3 and was greatly enhanced by co-intercalation of Pt with Fe 2 O 3 . The enhancement of hydrogen gas evolution by incorporating Fe 2 O 3 in the interlayer of HTaWO 6 indicates that the electrons and holes photoinduced from intercalated Fe 2 O 3 can be effectively used for the reduction of water and oxidation of methanol, but the electrons and holes produced from unsupported Fe 2 O 3 rapidly recombine. The depression of the recombination of electrons and holes might be due to electron transfer from Fe 2 O 3 to host HTaWO 6 and / or incorporated Pt. Similar results were observed in H 4 Nb 6 O 17 , H 2 Ti 4 O 9 and HNbWO 6 systems [7,8,10]. It was also notable that both

HTaWO 6 / Pt and HTaWO 6 /(Pt,TiO 2 ) show photocatalytic activities even under visible light irradiation. Since HTaWO 6 , Ta 2 O 5 and TiO 2 are wide band gap semiconductors, it is suspected that WO 3 formed in these samples by photo-induced phase transformation played an important role in the hydrogen gas evolution under visible light irradiation. The photocatalytic activity under visible light irradiation is in the order, HTaWO 6 /(Pt,TiO 2 )4 HTaWO 6 /(Pt,Fe 2 O 3 ).HTaWO 6 / Pt.HTaWO 6 / Fe 2 O 3 . unsupported Fe 2 O 3 . It was notable that HTaWO 6 / (Pt,TiO 2 ) nanocomposite showed exceptionally excellent photocatalytic activity under irradiation at both l .290 nm and l .400 nm. Further study is necessary to clear the mechanism for the high photocatalytic activity of HTaWO 6 /(Pt,TiO 2 ) nanocomposite.

4. Conclusions From the results of the tests described, the following conclusions may be drawn. (1) TiO 2 and Fe 2 O 3 together

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with Pt could be intercalated into the interlayer of HTaWO 6 by successive reaction of HTaWO 6 with [Pt(NH 3 ) 4 ]Cl 2 aqueous solution, n-C 3 H 7 NH 2 /n-heptane mixed solution, acidic TiO 2 colloid solution or [Fe 3 (CH 3 CO 2 ) 7 (OH)(H 2 O) 2 ]NO 3 aqueous solution followed by UV light irradiation. (2) The thickness of TiO 2 and Fe 2 O 3 was less than 0.51 nm. (3) The photocatalytic activity of HTaWO 6 / TiO 2 and HTaWO 6 / Fe 2 O 3 nanocomposites was superior to that of unsupported TiO 2 (P-25) and Fe 2 O 3 , respectively. (4) The hydrogen gas production activities of HTaWO 6 / TiO 2 and HTaWO 6 / Fe 2 O 3 nanocomposites were enhanced by co-intercalation of Pt. (5) Not only unsupported Fe 2 O 3 , HTaWO 6 / Fe 2 O 3 and HTaWO 6 /(Pt,Fe 2 O 3 ), but also HTaWO 6 / Pt and HTaWO 6 /(Pt,TiO 2 ) showed photocatalytic activity under visible light ( l .400 nm) irradiation.

Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture.

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