Composite of NaBiO3-loaded WO3 with a mixing ratio of 10:100 was prepared for photocatalytic harmful-organic-contaminant decomposition. The composite properties were measured using X-ray diffraction, ultraviolet-visible spectrophotometer (UV-Vis), and valence band-X-ray photoelectron spectroscope (VB-XPS). The results exhibited that the potentials for top of the valence band and bottom of conduction band for NaBiO3 can be estimated, respectively, as +2.5 V and -0.1 to 0 V. Furthermore, WO3, NaBiO3, and the composite showed IPA oxidation properties under visible-light irradiation. Results show that the composite exhibited much higher photocatalytic activity about 2-propanol (IPA) decomposition into CO2 than individual WO3 or NaBiO3 because of charge separation promotion and the base effect of NaBiO3.

Our living environment is surrounded by numerous and diverse harmful compounds. To enjoy a safe and comfortable life, it is extremely important to remove such harmful compounds. Removal by adsorbents such as activated carbon and by oxidation using ozone is effective methods.1,2 In addition, photocatalytic oxidation of harmful compounds is regarded as an effective method.3,4

The most frequently used photocatalyst is TiO2. This oxide is used mainly for outdoor applications. However, TiO2 technology is rarely introduced into indoor applications because TiO2 is sensitive only to UV light and the UV light intensity in indoor illumination is extremely weak. Indoor illumination includes much larger amounts of visible light. Therefore, for indoor applications, visible-light sensitive photocatalysts of many kinds have been investigated.5–18 Among them, WO3-modified photocatalysts, such as Pt-loaded WO3, have attracted many interests. Pt-loaded WO3 shows high photocatalytic activity,19 but Pt is an expensive precious metal. Therefore, development of a cheaper material is often necessary.

Recently, we found that NaOH-loaded WO3 is a visible-light sensitive photocatalyst with relatively high photocatalytic activity.20 This result suggests that WO3 loaded with Na-containing oxide with a base might exhibit good photocatalytic activity. However, NaOH is water-soluble and a strong base. Its use as a photocatalyst is therefore extremely restricted. Therefore, we sought some other Na-containing oxide-loaded WO3 photocatalyst. Among Na-containing oxides, NaBiO3 was selected as a candidate because it is a visible-light sensitive semiconductor21 and is only weakly basic. Results show that a composite consisting of 10 wt. % of NaBiO3 and WO3 is a promising visible-light sensitive photocatalyst, showing much higher photocatalytic activity than either WO3 or NaBiO3 alone.

NaBiO3 was pretreated by heating of commercially available NaBiO3 (Wako Co., Japan) at 413 K for 5 h in an oven.21 Next, the NaBiO3 and WO3 were mixed in a mortar for over 30 min. Then, the mixed powder was dried at 343 K for 4–5 h, producing the composite sample. The mixing weight ratio of NaBiO3 to WO3 was 10:100. Since the pH of an aqueous NaBiO3 suspension with concentration of 0.02 mol/l was about 10, NaBiO3 was regarded as functioning as a weak base. As a reference, a simple mixture sample was also prepared.22,23

The samples were measured using an X-ray diffraction meter (XRD, X’pert Pro, PANalytical Co., Netherlands) with Cu Kα radiation for checking of the formation of impurity phases in the sample. For the evaluation of visible-light absorption properties, reflectance spectra of the samples were first measured using a UV–Vis spectrophotometer (UV-2500PC, Shimadzu Co., Japan); BaSO4 was used as a reference sample. Subsequently, the optical absorption spectra were obtained by conversion of the reflectance spectra by using the Kubelka-Munk relation. The relative potential for the top of the valence band (TVB) for the sample was evaluated using an X-ray photospectroscope (AXIS-HS, Shimadzu-Kratos Analytical Co., Japan) with an X-ray source of monochromatic Al. Binding energy was calibrated using C 1s peak with the binding energy of 284.5 eV. The specific surface areas of the samples were measured with a surface area analyzer (Gemini 2360, Micromeritics Co., USA) by a Brunauer–Emmett–Teller (BET) method at 77 K.

Photocatalytic activity was evaluated from 2-propanol (IPA) oxidation into CO2 via acetone because IPA is a frequently used volatile organic compound (VOC) and because gaseous organic decomposition has been reported only rarely for the NaBiO3-containing photocatalyst.24,25 Photocatalyst of 0.4 g was put on a glass dish having bottom area of about 8 cm2. Then, the dish was set on the bottom of 500 cm3 glass reactor. Next, the atmosphere of the reactor interior was replaced with artificial air (N2:O2 = 80:20). Subsequently, IPA-concentrated gas was injected into the reactor. The IPA gas concentration was confirmed to be about 500–800 ppm in the reactor. After the IPA gas injection, the reactor was kept in the dark until the IPA gas reached the adsorption–desorption equilibrium state on the photocatalyst. Then, visible light was irradiated on the photocatalyst in the reactor. Visible light (400 nm < λ < 530 nm) was emitted from 300 W of a Xe lamp equipped with a water filter and glass filters of three types: UV-cutoff filter (Y-44), blue filter (B390), and IR cutoff filter (HA-30) (Hoya, Co., Japan). The light intensity was about 1 mW/cm2. The IPA and acetone concentrations were measured with a gas chromatograph (GC-14B, Shimadzu Co., Japan) equipped with a flame-ionized detector (FID). The CO2 concentration was evaluated using the gas chromatograph with a methanizer and FID.

A test for photocatalytic-activity stability was also done: the following four steps were repeated four times. (i) The reactor was replaced with pure air. (ii) Concentrated IPA gas was injected into the reactor; then (iii) visible light was irradiated on the reactor. (iv) Thereby, IPA was oxidized gradually by the photocatalysis. Finally, we confirmed that IPA was oxidized completely into CO2 from CO2 concentration measurements.

Figures S1 and S222 depict XRD patterns of WO3 and the composite, NaBiO3-loaded WO3. The XRD patterns of the composite and mixture can be indexed into monoclinic WO3 and NaBiO3 standard XRD patterns (PDF No. 43-1035 and PDF No. 30-1160, respectively).20,21 The results indicated that the composite and mixture were composed mainly of pure WO3, with a small amount of ilmenite NaBiO3 exists in them; no impurity phase was generated during preparation.

Figure 1 presents optical absorption spectra of pure WO3, pure NaBiO3, and the composite, which was composed of WO3 and NaBiO3 with the mixing ratio of 100:10. Actually, NaBiO3 can absorb visible light greater than 600 nm. The absorption property for light of over 500 nm on NaBiO3 is not high, but NaBiO3 can absorb visible light at wavelengths of 400–500 nm. The bandgap of NaBiO3 was estimated to be about 2.5–2.6 eV from the onset of absorption edge. A WO3 semiconductor also absorbs less than 480 nm of light. The bandgap was 2.6 eV.

FIG. 1.

Optical absorption spectra of WO3, NaBiO3, the composite, and mixture.

FIG. 1.

Optical absorption spectra of WO3, NaBiO3, the composite, and mixture.

Close modal

On the other hand, the absorption of the composite was slightly blue-shifted in comparison with that of pure WO3 and NaBiO3. This shift may be derived from changes of the amounts of functional groups, such as OH group and adsorbed water.26,27 A similar phenomenon was also observed for NaOH-loaded WO3.20 The absorption of the mixture is sum of the absorption of WO3 and NaBiO3. These results indicated that the contact and interaction between WO3 and NaBiO3 on the composite may be stronger than that on the mixture.

Photocatalytic activity is dependent on the band structure. Therefore, the band structure was investigated. The potentials for the TVB were first evaluated using VB-XPS. The TVB potentials for NaBiO3 and composite were estimated as the following. The tangent of the VB-XPS spectrum with binding energy of around 0 eV was extrapolated. Furthermore, the binding energy at the intersection of the extended line and X axis was the relative TVB energy, which corresponds to relative TVB potential. Figure S322 presents XPS spectra of the samples at around 0 eV in the binding energy. The VB-XPS result showed that the TVB potential for NaBiO3 is about 0.5 V smaller than that for WO3. Because the potential for bottom of conduction band (BCB) for WO3 is reportedly +0.4 V (vs. standard hydrogen electrode (SHE))28 and because the TVB potential for WO3 is calculated to be 3.0 V (vs SHE), the TVB for NaBiO3 can be estimated as 2.5 V (vs. SHE). Furthermore, the TVB potential for the composite consisting of NaBiO3 and WO3 (ratio, 1:10) was also estimated as 2.5 V using the same method.

From the potential for TVB and band gap for NaBiO3, the potential for the BCB can be calculated. The potentials for BCB for NaBiO3 and WO3 were −0.1 to 0 V and +0.4 V, respectively. The calculated potential (−0.1 to 0 V) for BCB of NaBiO3 was close to the potential (−0.21 V) reported in an earlier paper.25 So, the band structure of NaBiO3 can be drawn in Figure 2.

FIG. 2.

Schematic illustration of band structures for WO3 and NaBiO3.

FIG. 2.

Schematic illustration of band structures for WO3 and NaBiO3.

Close modal

IPA is decomposed into CO2 via acetone by photocatalysis. At the initial reaction stage, the increase of CO2 concentration is nearly zero by most of photocatalysis. The acetone concentration increases with irradiation time. Therefore, acetone evolution was monitored at first, as shown in Figure S4.22 For pure WO3, IPA can be oxidized steadily into acetone between 0 min and 40 min. This oxidation is regarded as proceeded by photogenerated holes, and photogenerated electrons are consumed by two electron O2 reduction and formation of H2O2 on the surface (Equation (2)). However, after over 40 min of light irradiation, the activity decreased. This decrease of activity is expected to be derived from coverage of formed H2O2 on the surface of WO3 and the poor consumption of photogenerated electrons at this stage.20 However, for pure NaBiO3, no such decrease of activity was observed and IPA was oxidized into acetone. Furthermore, for the composite consisting of NaBiO3 and WO3 (10:100), its acetone evolution rate was higher than those for pure NaBiO3 and pure WO3. Although the composite is composed of less than 10 wt. % of NaBiO3, the composite showed much higher activity than pure WO3.

Further visible-light irradiation, intermediate acetone was reoxidized into CO2 over the composite. Figure 3 presents the irradiation time dependence of change of CO2 evolution concentrations over pure WO3, pure NaBiO3, the composite, and mixture. Although the CO2 evolution amount is extremely small for pure WO3 and NaBiO3, the composite showed a much greater amount of CO2 evolution. The effect of the surface area difference on the activity is extremely small because the BET surface area of the composite (5.8 m2/g) was close to those of NaBiO3 (5.0 m2/g) and WO3 (5.8 m2/g). Therefore, this higher activity of the composite might derive from the composite’s synergistic effects, which were promotion of charge separation and H2O2 consumption. Because the BCB potential for WO3 is greater than that for NaBiO3 (Figure 2), photogenerated electrons on the composite can be migrated from the NaBiO3 to the WO3. As a result, charge separation was promoted on the composite. However, for the WO3 in the composite, the electrons were consumed via two electron O2 reduction and formation of H2O2. If the composite does not contain the NaBiO3, H2O2 is expected to accumulate on the surface,29–32 leading to deactivation.20 However, because H2O2 and NaBiO3 are a weak acid and a weak base, respectively, H2O2 will be readily transformed into more reactive OOH (Equation (3)) over NaBiO3-loaded WO3 and more easily consumed (Equation (4)).20,33–35 Because of high reactivity of OOH, the OOH generated from alkali-H2O2 is known to be useful for bleaching of hair and pulp in industry.36,37 Therefore, this transformation and consumption is expected to suppress the H2O2 accumulation on the WO3 in the composite (Equations (3)–(6)). Photogenerated electrons were kept consuming via two-electrons O2 reduction (Equations (2) and (5)). As a result, the enhanced charge separation and H2O2 consumption are regarded as enhancing the composite photocatalysis performance,

O 2 + H + + e = OOH ( 0 . 046 V vs SHE ) ,
(1)
O 2 + 2 H + + 2 e = H 2 O 2 ( + 0 . 68 V vs SHE ) ,
(2)
2 H 2 O 2 = 2 H + + 2 OOH ( in base condition ) ,
(3)
2 OOH = O 2 + 2 OH ,
(4)
OOH +  H 2 O + 2 e = 3 OH ( + 0 . 87  V vs SHE ) ,
(5)
H + + OH = H 2 O.
(6)
FIG. 3.

IPA decomposition into CO2 under visible light irradiation in the presence of WO3, NaBiO3, composite, or mixture.

FIG. 3.

IPA decomposition into CO2 under visible light irradiation in the presence of WO3, NaBiO3, composite, or mixture.

Close modal

The activity of the mixture was slightly lower than that of the composite. This is possibly because the contact and interaction between NaBiO3 and WO3 on the mixture may be weaker.

Stability is an important characteristic for a photocatalyst. To ascertain its stability, repeated IPA decomposition was also done, as shown in Figure S5.22 The CO2 evolution rate for every run changed little, indicating that the composite was regarded as a stable photocatalyst.

Results showed that NaBiO3-loaded WO3 composite exhibited much higher activity for IPA decomposition into CO2 under visible-light irradiation than either WO3 or NaBiO3. Because the surface area of the composite (5.8 m2/g) was close to those of NaBiO3 (5.0 m2/g) and WO3 (5.8 m2/g), the surface area difference only slightly affects improvement of the activity for the composite. The composite showed higher activity probably because of charge separation promotion and the base effect of NaBiO3. The consumption of photogenerated H2O2 via O2 two-electron reduction on the WO3 is expected to be promoted by the base of NaBiO3. Recombination between holes and electrons was suppressed because of the smaller BCB potential for the NaBiO3. However, for pure WO3, H2O2 was covered with the surface of WO3 and saturated for lengthy light irradiation, leading to difficulty of electron consumption and decreased photocatalytic activity.

Compared with the strong base NaOH, NaBiO3 has some benefits. The advantage of NaBiO3 loading is that it is a weak base and not water soluble. For NaOH-loaded WO3, when we try to prepare the slurry for coating films, loaded NaOH will be dissolved in solutions including water and is easily removed from the WO3 surface. The loss of NaOH on the WO3 will lead to lower activity. However, NaBiO3 is not soluble in water. Therefore, the NaBiO3-loaded WO3 will maintain its activity even in preparation of slurry for coating.

This work was partially supported by JSPS KAKENHI Grant No. 15K00591, Japan.

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Supplementary Material