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Article

Tungsten Trioxide and Its TiO2 Mixed Composites for the Photocatalytic Degradation of NOx and Bacteria (Escherichia coli) Inactivation

1
Dipartimento di Chimica, Università degli Studi di Milano, Via Camillo Golgi 19, 20133 Milano, Italy
2
Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via Roberto Cozzi, 55, 20125 Milano, Italy
3
Dipartimento di Scienze Farmacologiche e Biomolecolari, Università degli Studi di Milano, Via Giuseppe Balzaretti, 9, 20133 Milan, Italy
4
Dipartimento di Fisica, Università degli Studi di Milano, Via Giovanni Celoria, 16, 20133 Milano, Italy
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(8), 822; https://doi.org/10.3390/catal12080822
Submission received: 5 July 2022 / Revised: 22 July 2022 / Accepted: 22 July 2022 / Published: 26 July 2022
(This article belongs to the Special Issue Advanced Oxidation Catalysts)

Abstract

:
The increased air pollution and its impact on the environment and human health in several countries have caused global concerns. Nitrogen oxides (NO2 and NO) are principally emitted from industrial activities that strongly contribute to poor air quality. Among bacteria emanated from the fecal droppings of livestock, wildlife, and humans, Escherichia coli is the most abundant, and is often associated with the health risk of water. TiO2/WO3 heterostructures represent emerging systems for photocatalytic environmental remediation. However, the results reported in the literature are conflicting, depending on several parameters. In this work, WO3 and a series of TiO2/WO3 composites were properly synthesized by an easy and fast method, abundantly characterized by several techniques, and used for NOx degradation and E. coli inactivation under visible light irradiation. We demonstrated that the photoactivity of TiO2/WO3 composites towards NO2 degradation under visible light is strongly related to the WO3 content. The best performance was obtained by a WO3 load of 20% that guarantees limited e/h+ recombination. On the contrary, we showed that E. coli could not be degraded under visible irradiation of the TiO2/WO3 composites.

Graphical Abstract

1. Introduction

Due to growing industrialization, urban environments have faced chronic air pollution issues in the last decades. Exhaust gases and burning fuels from factories represent the primary sources of air pollutants on a global scale, causing a significant impact on human health, animal and plant life, and climate [1]. Although natural sources responsible for air pollutants production, such as broad forest fires, volcanic eruptions, and soil erosion, can play a role in air pollution, the emissions resulting from human activities, such as motor vehicle exhaust, combustion of fossil fuels, and industrial processes, are the most active and concerning cause of air quality decline [2].
Nitrogen monoxide (NO) and nitrogen dioxide (NO2), known as nitrogen oxides (NOx), are relevant pollutants whose emissions are directly related to human health problems [3], as they affect respiratory and immune systems [4], to the production of tropospheric ozone, acid rains, and in general to global air pollution.
Over the years, different techniques have been developed for NOx abatement. Among the traditional techniques, selective catalytic reduction (SCR) with ammonia in the presence of oxygen is the most used, mainly applied to reduce NOx emission from combustion processes [5], as well as absorption, adsorption, or electrical discharge processes [6]. However, all these methods are characterized by several limitations and disadvantages that make actual application hard. Moreover, the growing environmental constraints invoke restrictions regarding NOx emission, requiring more efficient techniques for NOx abatement.
In addition, awareness about the importance of supplying adequate drinking water has recently increased. In 2012, the United Nations estimated that nearly 11% of the world’s population did not have access to improved drinking water sources. African water resources indeed contain high levels of microbial pathogens, including bacteria, viruses, and protozoa, as well as chemical contaminants. Escherichia coli and related bacteria constitute approximately 0.1% of gut flora, and fecal–oral transmission is the primary route through which pathogenic strains of the species cause disease. For that reason, new disinfection technologies are currently in development to fulfill the WHO Guidelines for drinking-water quality (World Health Organization, 2008). The traditional disinfection methods lead to chloro-organic disinfection by-products (DBPs) with carcinogenic and mutagenic effects.
In both study cases, using a TiO2 semiconductor as a catalyst under UV or visible irradiation seems the most promising method.
Titania (TiO2) has been considered the most efficient photocatalyst for a wide variety of applications, such as pollution abatement [7,8], water and air purification [9], antimicrobial applications [10], and energy conversion [11]. However, TiO2 in its photoactive anatase phase has a wide band gap of 3.2 eV, limiting the photoactivity of the semiconductor only under UV irradiation [12]. Moreover, because of its suspected carcinogenic nature [13], researchers are willing to replace TiO2 with new low-cost and visible-light-active smart materials.
Though many studies have focused on using TiO2 [14,15,16,17,18], WO3 and its composites have been poorly investigated to date [19,20,21,22].
WO3 is a cheap, physiochemically stable, and mechanically robust semiconductor with a narrow band gap energy (2.4–2.8 eV), making it a visible-light-responsive photocatalyst for different applications [23,24,25,26,27,28,29,30]. Therefore, WO3 represents a suitable choice for photocatalytic degradation under visible light irradiation.
As described in Figure 1, because the VB (valence band) edge potential of WO3 is lower than that of TiO2, upon photon absorption, electrons can be transferred from the conduction band of TiO2 to WO3, whereas photogenerated holes move in the opposite direction from electrons.
The transfer of photogenerated carriers is accompanied by consecutive W6+ reduction into W5+ by capturing photogenerated electrons at trapping sites in WO3. In addition, W5+ ions on the surface of WO3 are reoxidized into W6+. However, the reduction potential value for the photogenerated electrons in the conduction band is not high enough for the single electron reduction in O2 (Figure 1) [30]. The holes accumulated in the TiO2 VB take part in the oxidation process to make OH or OH hydroxyl radical reactive species. These processes in WO3/TiO2 heterostructures restrain the recombination of electron–hole pairs significantly.
Different approaches have been developed to produce highly active TiO2/WO3 composites aiming to optimize the WO3 content [31]. It has been demonstrated that the influence of WO3 on TiO2 photoactivity depends on several factors, such as crystal phase, electrons accumulation ability of WO3, type of pollutants, and degradation pathways involved [31].
Yang et al. investigated the role of amorphous WOx species, demonstrating that they are more active than the crystalline ones toward methylene blue degradation [32].
Other experiments by Žerjav et al. explained the correlation between the photocatalytic performance of TiO2/WO3 and their shallow and deep electron trapping states [33].
However, concerning WO3 and its mixed oxides, the obtained results are conflicting because in the same cases, the presence of WO3 seems to positively affect the photoactivity and the performances of TiO2. In other cases, the results worsen [19,31,32,34].
Regarding NOx degradation, Luévano-Hipólito et al. demonstrated that WO3 with a polyhedral shape leads to 50% NO oxidation to NO2 [19]. On the other hand, Yu and coworkers noticed for the first time the photo-transformation of NO2 into NO in the presence of N2 on the surface of a WO3 photocatalyst under UV/visible light irradiation [20].
Recently, Mendoza et al. proposed TiO2/WO3 composites as efficient materials for NOx abatement under visible light, leading to 90% of photodegradation in 1 h [22], whereas Paula and coworkers observed the decay of the photocatalytic activity of TiO2/WO3 heterostructures as a function of the W(VI) content [31].
Jawwad A. Darr et al. reported the easy disinfection of water by TiO2/WO3 mixed composites, which induce bacterial inactivation after 30 min of photo-irradiation [35].
In order to clarify the behavior of WO3 and TiO2/WO3 heterostructures in the photocatalytic degradation of NOx under visible light irradiation, in this work, WO3 and a series of TiO2/WO3 composites were synthesized by a fast and cost-effective chemical procedure and tested for the photodegradation of NOx and the inactivation of E. coli under visible light irradiation.
The role of the calcination temperature in the TiO2/WO3 preparation has been investigated and critically discussed, as well as the effect of the WO3 loading in the final composites. Differently from the recent literature, the results proved that high calcination temperatures could cause complete or partial WO3 sublimation with adverse effects on the activity of the TiO2/WO3 heterostructures.
Finally, while the synthesized catalysts were active in NO2 photodegradation, they were inert to the antibacterial activity under visible light irradiation, in line with the scientific literature [36].

2. Results

2.1. Materials Characterization

Figure 2 shows the XRD patterns of all the synthesized materials.
TiO2 exhibits the characteristics of diffraction peaks of anatase, as confirmed by the peaks at 25.3°, 37.7°, 48.0°, 53.8°, and 55.0°, with (101), (004), (200), (105), and (211) diffraction planes, respectively. The XRD pattern of WO3 shows a crystalline phase characterized by diffraction peaks at 23.1°, 23.6°, 24.4°, and 34.2°, corresponding to the (002), (020), (200), and (202) crystal planes of monoclinic phase.
As expected, in the WO3@TiO2 composites, the intensity of the diffraction peaks of WO3 declines by decreasing the percentage of WO3; on the contrary, anatase peaks intensity increases or appears with the higher concentration of TiO2 in each composite.
Based on its XRD pattern, WO3@TiO2_20* exhibits higher crystallinity degree if compared to the others and the appearance of new diffraction peaks can be observed.
As reported in the literature [37], it is directly correlated to the high temperature (600 °C) used for the calcination of this material. The degree of crystallinity increases with the temperature, and in the case of TiO2-based compounds, at 600 °C the phase conversion from anatase to rutile starts.
According to the literature [22,34], when the WO3 content in the composite materials is 20% or lower, the diffraction peaks of this semiconductor are undetected. Some authors justify this result with the presence of highly dispersed WO3 small particles in TiO2/WO3 composites, which makes it hard to detect them by this technique [22,34]. In addition, Yang et al. demonstrated that when the loading amount of WO3 was below 3 mol%, it exists in highly dispersed amorphous species that do not respond to XRPD. However, accurate quantification of WO3 loading on TiO2 after calcination is necessary to verify unequivocally the WO3/TiO2 composite formation rather than a superficial W doping on the TiO2 surface. In the present work, this was easily carried out by the reaction yield calculation (Equation (2)) and by EDS analysis (Table S1) for two composites with a nominal WO3 load of 20% (WO3@TiO2_20 and WO3@TiO2_20*) calcined at two different temperatures (400 °C and 600 °C). From the results of the reaction yield, the WO3@TiO2_20* composite calcined at 600 °C exhibits a mass loss of approximately 40%, unlike the same sample calcined at 400 °C, obtained with a yield of 94.8%. Since WO3 is a low-temperature sublimation material [38], it cannot be excluded that by increasing the calcination temperature, a complete or partial WO3 sublimation can occur, as also confirmed by the EDS results (Table S1), where the measured percentage of WO3 in the final composite is lower than 3%, whereas the material calcined at 400 °C shows a 27% of WO3.
Figure 3 displays the FT-IR for all the synthesized composites calcined at 400 °C.
The FT-IR spectrum of WO3 exhibits characteristic vibration bands, such as those at approximately 3433 cm−1 and 1635 cm−1, that can be associated with the symmetric stretching vibrations of WO3 and intercalated water molecules and the deformation vibrations of H–O–H bonds of the adsorbed water molecules, respectively, and the signal at 805 cm−1, attributed to O–W–O stretching modes of WO3 [39].
On the other hand, the FT-IR spectrum of TiO2 nanoparticles is characterized by several peaks. The OH stretching mode of the hydroxyl groups is responsible for the broad band observed in the range of 3600–3000 cm−1, indicating the presence of moisture in the sample. The band at approximately 1605 cm−1 is due to the OH bending vibrations of the absorbed water molecules. Finally, the broadband between 1000 and 500 cm−1 can be related to the Ti–O stretching and Ti–O–Ti bridging stretching modes [40].
As expected, in the FT-IR spectra of the composites, the WO3 characteristic bands are covered by the more intense ones of TiO2.
The optical properties of the synthesized WO3/TiO2 series calcined at 400 °C, as well as of single-phase semiconductors, were investigated by UV–Vis scanning spectrophotometry (Figure 4).
The main absorption edges of the samples are all around 400 nm, attributing to the excitation of electrons from the valence band to the conduction band. As reported in the literature [41], the empty orbitals of W6+ (W 5d) are closed to the Ti 3d orbitals of the conduction band. Therefore, the O2−→W6+ charge transfer transitions are overlapped with the O2→Ti4+ charge transfer transitions. Increasing the WO3 loading, the absorption edge of photocatalysts red-shifts. If this is only slightly noticeable up to 10%, increasing the WO3 percentage to 50% and above, the effect is much more evident, and the absorption edges of these materials are shifted at higher wavelengths.
The band gaps of the materials, estimated by the Kubelka–Munk function, are summarized in Table 1, and the Tauc plots are reported in Figure S2.
Samples Eg decreases, increasing the tungsten content, due to the formation of defective energy levels within the forbidden band gap of WO3. On the other hand, by increasing the TiO2 content, the total band gap of the photocatalyst decreases [42].
According to the shape of nitrogen adsorption–desorption isotherms reported in Figure 5 and the IUPAC classification [43], all the photocatalysts calcined at 400 °C can be classified as mesoporous materials of type IV, as confirmed by the values of CBET in Table 1, containing other quantitative data.
Figure 6 displays the SEM images and the elemental mapping of Ti and W for the synthesized composites, whereas the EDX spectra are reported in the (Supplementary Information Table S1).
The elemental maps of W and Ti by X-ray energy dispersion (EDX) in the composites demonstrate that TiO2 and WO3 are well dispersed in each material. All the WO3@TiO2 show a globular-like morphology with particle sizes ranging from 60 to 5 nm. The particles are aggregated by sharing corners or edges that probably involve the formation of Ti–O–W bonds [44]. The same information was obtained by TEM investigations (Figure 7) showing nanoparticles of 12–35 nm that gradually aggregate with the WO3 load, reaching up to 60 nm in size.

2.2. Photocatalytic and Biological Activities of Catalysts

2.2.1. NOx Photocatalytic Degradation

The photocatalytic activity of the TiO2/WO3 composites, as well as single-phase photocatalysts, referred to both NOx (NO + NO2) and NO2 conversion under visible light irradiation (Figure 8).
According to the literature [22,31,45], for this type of material the photocatalytic degradation of NOx consists of a photo-oxidation process, where both NO and NO2 species are first adsorbed on the surface of the heterostructures and then converted into the corresponding oxidation product (NO3) under light irradiation. Although the formation of the oxidation products was well documented in the literature [46,47], an analytical confirmation was not performed in the present work due to the low NOx concentrations used during the test.
For all the experiments, the initial concentration of NOx was approximately 500 ppb. The NOx photodegradation results are summarized in Figure 8.
As expected, despite its extraordinarily high surface area, TiO2 shows poor activity towards NOx conversion under visible light irradiation, leading to NOx and NO2 degradation in 25% and 20%, respectively. On the other hand, according to the band gap value, regardless of its low surface area, pristine WO3 exhibits a good photoactivity towards NO2 photodegradation (72%) in 3 h, whereas the NOx abatement is only 46%. In fact, as reported in the scientific literature [19,20,21,22], WO3 can remarkably reduce NO2 into NO in the presence of N2. This is confirmed by the results reported in Figure S3, showing for the WO3 sample an increase in the NO concentration during the reaction. This makes pristine WO3 not efficient in the NOx abatement, because, as it is known, in air NO is immediately reoxidized to NO2. In this regard, from the pioneering investigations of Yu et al., carried out under nitrogen atmosphere and UV irradiation, a 20% conversion of NO2 into NO can be inferred [20]. The present results demonstrate that even under visible light irradiation, the percentage of NO formation from NO2 is of the same order (24%), calculated by Equation (1):
NO   produced   ( % ) = NOt NOi NOt
where [NOt] is the NO concentration at the end of the reaction (after 3 h of light irradiation), [NOi] is the NO concentration before light irradiation.
If compared to the single-phase photocatalysts (TiO2 and WO3), the photoactivity of WO3/TiO2 heterostructures strongly depends on their composition. More in detail, the activity of the catalysts gradually increases with the WO3 load, reaching the highest photodegradation efficiency by WO3@TiO2_20 (54.4% NOx conversion and 56.4% NO2 abatement), whereas it decreases for a percentage of WO3 > 20. These results are in line with the pioneering investigations of Balayeva et al., who tested the photocatalytic activity of TiO2/WO3 composites towards NO degradation under UV irradiation, obtaining a ca. 35% of conversion for heterostructures characterized by a 1% and 2.5% of WO3 load [48].
For a very high amount of WO3 (WO3@TiO2_80), these latter composites maintain the photoactive capability of pristine WO3, converting NO2 to NO. The different photoactivity of the materials may be due to a combination of factors.
First of all, it can be assumed that for a WO3 load < 20%, TiO2 and WO3 only play their own photocatalytic role, and coupled photocatalysts are not formed. In this case, the low activity of TiO2 prevails because it is the major component. In contrast, for a large amount of WO3, the fast e/h+ recombination of the WO3 component predominates.
On the contrary, the absence of WO3 peaks in the XRD spectra of the WO3@TiO2_20 sample suggests that its increased photoactivity is not related to the formation of crystalline tungsten oxide but is probably due to the presence of WO3 centers on the surface of TiO2 acting as electrons/holes separators [49]. When the test was carried out using WO3@TiO2_20*, in order to observe the effect of calcination temperature on the photoactivity of the material, the percentage of NOx and NO2 degradation dropped to ca. 20%, confirming that the thermal treatment acts by reducing the WO3 content in the WO3@TiO2 heterostructure and as a consequence of its activity.
It is known that the quantum efficiency of photocatalytic reactions carried out by heterogeneous photocatalysts depends on the competition between the recombination of photogenerated electrons and holes and the transfer of both electrons and holes at the interface of the material. Extending the electrons and holes recombination time and increasing the transfer rate of electrons at the interface enhance the quantum efficiency positively. As reported in the literature [31], the formation of TiO2/WO3 heterostructures leads to enhanced charge carrier lifetimes, due to the transfer of photogenerated electrons in the TiO2 to WO3 CB, and at the same time to the entrapment of the photogenerated holes within the TiO2 particle. Both these phenomena make charge separation more efficient.
Finally, the effect related to the different surface area values cannot be ignored. As for the photocatalytic activity, the surface area values also seem to be correlated to the WO3 load and the most active catalyst (WO3@TiO2_20) is also the one with the highest surface area (Figure 8, Table 1). At first glance, the results obtained by the WO3@TiO2_20 photocatalyst seem to contrast with those of Mendoza et al. [22], who report very high NO conversion values under visible light irradiation in similar conditions. The different photocatalytic activity of the WO3@TiO2_20 sample compared to those reported by Mendoza and coworkers can be reasonably attributed to the real WO3 content in the synthesized composites that it is not specified in the work of the author [22].

2.2.2. E. coli Photoinactivation

The antibacterial activity of WO3@TiO2 composites, as well as single-phase photocatalysts, was assessed by determining the percentage of E. coli cell survival following exposure to visible light. According to standard methods (ASTM E2149, 2001), values of survival ≤ 90% indicate the antibacterial activity of a given photocatalytic film.
As shown in Figure 9, none of the tested samples displays antibacterial activity under visible light irradiation, in agreement with what was reported in the literature for the WO3/TiO2 catalyst [36].
Different catalysts (e.g., sulfur-doped carbon quantum dots loaded hollow tubular g-C3N4) give degradation of E. coli cells instead, under visible light [50].
Based on the morphology of nanoparticles (Figure 7), we speculate that the nanoparticle aggregation of TiO2/WO3 hinders a suitable surficial interaction with the bacteria and the catalyst cytotoxicity.

3. Materials and Methods

3.1. Chemicals

Tungstic acid (H2WO4, 99% Merck), AMT 100 TiO2 (Tayca Corporation, WP0097, Osaka, Japan), ammonium hydroxide solution (ACS reagent, 28.0–30.0% Merck & Co., St. Louis, MO, USA), hydrochloric acid (HCl 36%, Suprapur®, Supelco, Belfont, PA, USA) were used as received.

3.2. Synthesis of TiO2/WO3 Series

To synthesize 1 g of TiO2/WO3 composite, a proper amount of TiO2 was dispersed in 25 mL of 2 M ammonium hydroxide solution under constant stirring (solution A). The WO3 precursor solution was prepared by dissolving a proper amount of tungstic acid H2WO4 in 25 mL of 2 M ammonium hydroxide solution under constant stirring (solution B). The quantity of TiO2 and H2WO4 used is reported in Table 2. The two solutions were stirred for 30 min at room temperature, then solution B was added to solution A and the stirring was continued for another 2 h at room temperature. Then, the solvent was evaporated, heating the mixture at 110 °C. The white-yellow powder was treated with 0.5 M hydrochloric acid solution and dried again. The final powder was washed with deionized water abundantly, dried at 100 °C overnight, and calcinated in the air at 400 °C for 2 h (heating rate 8 °C·min−1). The synthesized samples containing different TiO2/WO3 w/w ratios (95:5, 90:10, 80:20, 20:80) were properly characterized and tested for NOx photodegradation under visible light irradiation.
WO3 was synthesized by the same procedure using a TiO2-free solution A.
An aliquot of the TiO2/WO3 composite with an 80:20 w/w ratio was calcined at 600 °C.
Table 2 reports a list of synthesized composites with the corresponding label, WO3/TiO2 w/w ratio, calcination temperature, and reaction yield, calculated by Equation (2):
Yield   % = g   H 2 WO 4   ×   m o l a r   m a s s WO 3   m o l a r   m a s s   H 2 WO 4 + g   T i O 2 g   f i n a l   p r o d u c t

3.3. Characterization Methods

X-Ray Diffraction (XRD) measurements investigated the crystalline structure on a PW3830/3020 X’Pert diffractometer (PANalytical, Almelo, The Netherlands) working Bragg–Brentano, using the Cu Kα1 radiation (k = 1.5406 Å).
FT-IR spectra were recorded in the range of 400–4000 cm−1 with a resolution of 0.5 cm−1 by anPerkin-Elmer spectrometer (Perkin Elmer, Waltham, MA, USA) dispersing a few milligrams of each material in anhydrous KBr. The morphology of the catalysts was inspected employing high-resolution electron transmission microscopy (HR-TEM), using a JEOL 3010-UHR instrument (Musashino Akishima, Japan; acceleration potential: 300 kV; LaB6 filament), and by scanning electron microscopy (SEM), using a Zeiss LEO 1525 field emission microscope (Jena, Germany). The samples were ‘‘dry’’ dispersed on lacey carbon Cu grids for TEM analyses, whereas SEM analyses were carried out without any pre-treatment of the samples.
For the band gap determinations, diffuse reflectance spectra of the powders were collected on a UV–Vis diffuse reflectance spectra using a scanning spectrophotometer PerkinElmer, Lambda 35 (Perkin Elmer, Waltham, MA, USA), which was equipped with a diffuse reflectance accessory. A thin film of each sample was placed in the sample holder on an integrated sphere for the reflectance measurements. A KBr pill was used as the reference material. Data were elaborated using the Kubelka–Munk function (Equation (3)), which expresses the adsorbance as a function of reflectance (F(R)) [51]:
F(R) = (1 − R)2/2R
where R = reflectance of the powder.
The band gap values were determined by performing the first derivative of the Kubelka–Munk function (Equation (4)):
dF(R)/dλ
where λ = wavelength of the incident radiation. The energy of the radiation at which the first derivative dF(R)/d λ shows the maximum was taken to estimate the band gap values.
Specific surface area and porosity distribution were determined by processing N2 adsorption–desorption isotherms at 77 K (Coulter SA3100 instrument, Beckman Life Sciences, Los Angeles, CA, USA) with Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda analyses. Before the analysis, samples were heat-treated (T = 150 °C, 4 h, N2) to remove adsorbed foreign species.

3.4. NOx Photodegradation Tests

A photocatalytic film of each sample was deposited by drop-casting on glass supports as follows: a suspension of 0.050 ± 0.001 g of photocatalyst in 5 mL of isopropanol was deposited on a glass plate (230 × 19 mm). Once the solvent was evaporated, the photocatalyst was placed inside a 20 L Pyrex glass cylindrical batch reactor for the photocatalytic tests. The photocatalytic tests were performed by a mixture of NO and NO2 in air. The starting inlet gas contains only NO2, but the chemical equilibrium between NO and NO2 is established as it is exposed to air. An LED lamp (350 mA, 9–48 V DC, 16.8 W) with emissions in the 400–700 nm range was used as the light source. The luminous intensity (lux) was measured using an illumination meter (Delta Ohm photo/radiometer HD 2102.2) and was 2900 lx to estimate the light intensity. It was then converted to the irradiance unit (in mW/cm2) [19], obtaining a light intensity of 3.24 mW/cm2.
The NOx initial concentration was 500 ± 50 ppb. A chemiluminescence analyzer measured the NOx concentration after 30, 60, and 180 min of exposure to light irradiation (ENVEA AC32e).

3.5. Antibacterial Assay

Cultures of E. coli MG1655 [52] were grown at 37 °C in Luria–Bertani (LB) medium (10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCl) or LB-agar medium (LB medium with 10 g/L agar). Bacteria (200 mL) were collected by 10 min centrifugation at 5000 rpm, washed in PBS 1X (Merck & Co., St. Louis, MO, USA), and resuspended in the same volume of PBS 1X. Stationary phase cultures of E. coli were diluted up to optical density at 600 nm (OD600) of 0.05 and then grown aerated up to 0.6.
Films of the synthesized heterostructures were prepared as follows. A total of 100 mg of each material was dispersed in 8 mL of isopropanol and deposited on a Petri dish (90 mm in diameter) by drop-casting and air-dried.
10 mL of bacterial cells were added to Petri dishes. The plates were irradiated with visible light (2900 lux obtained with an LED lamp) and removed from light at different time points (60, 120, and 240 min). As controls, bacterial cells were deposited onto empty Petri dishes and irradiated (CTR under LED) or incubated at 37 °C under dark (CTR). Viable bacteria expressed as CFU/mL (colony-forming unit /mL) were enumerated at t = 0, 60, 120, and 240 min by plating suitable dilutions onto LB-agar plates following incubation at 37 °C for 18 h.
The percentage of bacterial survival is expressed as follows:
(average of viable bacteria at a given time/average of viable bacteria CTR 37° at t = 0 min) × 100.
The average of viable bacteria is calculated from at least three independent experiments. Values of survival ≤90% indicate the antibacterial activity of a given photocatalytic film.

4. Conclusions

In this study, we investigated the photoactivity of WO3 and TiO2/WO3 composites towards NO2 degradation under visible light, in order to clarify the numerous conflicting data reported in the literature so far. It was demonstrated that the photoactivity of TiO2/WO3 heterostructures are strongly related to their composition. For WO3@TiO2 materials characterized by low tungsten trioxide content (<20%), TiO2 and WO3 are present as separate phases, each playing their own photocatalytic role, whereas coupled photocatalysts are not formed.
The composite with a WO3 load of 20% was the most efficient photocatalyst, extending the electrons and holes recombination time and promoting the transfer rate of electrons at the interface. The high activity of the material can be explained with its high surface area value and with the presence of WO3 centers on the surface of TiO2 acting as electrons/holes separators. However, if the WO3 load is higher than 20%, a fast e/h+ recombination can occur and the ability of tungsten trioxide to reduce NO2 to NO prevails over the composites’ capability to photo-oxidize NO2 to NO3. Moreover, the photodegradation activity of the heterostructures can be attributed to the oxidizing effect of holes. Moreover, it was demonstrated that high-temperature calcination leads to a partial sublimation of the WO3 component that causes a decrease in heterostructure activity. As for the lack of the bacteria degradation, we tentatively suggest that the aggregation of nanoparticles hinders an efficient surface contact between bacteria and catalyst.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12080822/s1: Table S1: EDS analysis, Figure S1: EDX spectrum of (A) WO3@TiO2_20* and (B) WO3@TiO2_20, Figure S2: Tauc plot of TiO2 (A), WO3@TiO2_5 (B), WO3@TiO2_10 (C), WO3@TiO2_20 (D), WO3@TiO2_50 (E), WO3@TiO2_80 (F), WO3 (G), Figure S3: Dependence of WO3 content (%) in the composites versus and NO production.

Author Contributions

Conceptualization, C.L.B., F.M. and A.P.; methodology, C.L.B. and A.P.; investigation, E.F. and F.D.V.; data curation, C.L.B., F.M. and A.P.; writing—original draft preparation, E.F., F.M., C.L.B. and A.P.; writing—review and editing, E.F.; supervision, F.M.; funding acquisition, I.R.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Fondazione di Comuntà Milano, Fondo Ignazio Renato Bellobono Letizia Stefanelli.

Data Availability Statement

The data that support the plots within this paper are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photocatalytic mechanism of WO3−loaded TiO2 under light irradiation.
Figure 1. Photocatalytic mechanism of WO3−loaded TiO2 under light irradiation.
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Figure 2. XRD patterns of the samples of Table 1 (* peaks of TiO2 anatase, • peaks of TiO2 rutile).
Figure 2. XRD patterns of the samples of Table 1 (* peaks of TiO2 anatase, • peaks of TiO2 rutile).
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Figure 3. FT−IR spectra of the samples calcined at 400 °C.
Figure 3. FT−IR spectra of the samples calcined at 400 °C.
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Figure 4. UV–Vis absorption spectra of the WO3/TiO2 composites series calcined at 400 °C.
Figure 4. UV–Vis absorption spectra of the WO3/TiO2 composites series calcined at 400 °C.
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Figure 5. Nitrogen adsorption–desorption isotherms for the synthesized samples.
Figure 5. Nitrogen adsorption–desorption isotherms for the synthesized samples.
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Figure 6. SEM images and elemental mapping of the synthesized materials. (left) sample analyzed, ((middle), red-colored) titanium map, ((right), green-colored) tungsten map.
Figure 6. SEM images and elemental mapping of the synthesized materials. (left) sample analyzed, ((middle), red-colored) titanium map, ((right), green-colored) tungsten map.
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Figure 7. TEM images of WO3@TiO2_5 (A), WO3@TiO2_10 (B), WO3@TiO2_20 (C), WO3@TiO2_50 (D), WO3@TiO2_80 (E).
Figure 7. TEM images of WO3@TiO2_5 (A), WO3@TiO2_10 (B), WO3@TiO2_20 (C), WO3@TiO2_50 (D), WO3@TiO2_80 (E).
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Figure 8. Dependence of NOx and NO2 photodegradation on WO3 content and specific surface area (SSA) of the materials.
Figure 8. Dependence of NOx and NO2 photodegradation on WO3 content and specific surface area (SSA) of the materials.
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Figure 9. Percentage of E. coli cell survival after exposure to visible light in the presence of the synthesized heterostructures.
Figure 9. Percentage of E. coli cell survival after exposure to visible light in the presence of the synthesized heterostructures.
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Table 1. Energy of band gap (eV), specific surface area, CBET, Vm, and mean pore diameter of the WO3/TiO2 composites series calcined at 400 °C. * Surface area by BET equation (2-parameters), ** mean pore diameter by BJH model from isotherm desorption branch (0.3 < p/p0 < 0.95).
Table 1. Energy of band gap (eV), specific surface area, CBET, Vm, and mean pore diameter of the WO3/TiO2 composites series calcined at 400 °C. * Surface area by BET equation (2-parameters), ** mean pore diameter by BJH model from isotherm desorption branch (0.3 < p/p0 < 0.95).
SampleBand Gap (eV)* Specific Surface Area (m2/g)CBETVm (cm3/g)** Mean Pore Diameter (nm)
WO32.394.0075.750.9421.17
WO3@TiO2_802.6342.78123.229.928.6
WO3@TiO2_503.05110.6598.1125.876.4
WO3@TiO2_203.14179.7875.5043.266.0
WO3@TiO2_103.26139.4794.733.156.8
WO3@TiO2_53.20111.08112.8731.299.1
TiO23.29318.0084.4875.504.70
Table 2. Labels of the WO3/TiO2 composites, WO3/TiO2 w/w ratio, calcination temperature, and reaction yield.
Table 2. Labels of the WO3/TiO2 composites, WO3/TiO2 w/w ratio, calcination temperature, and reaction yield.
LabelTiO2 (g)H2WO4 (g)WO3/TiO2 (w/w Ratio)Calcination
Temperature (°C)
Yield (%)
WO30.001.08100:040095.2
WO3@TiO2_800.200.8680:2040095.7
WO3@TiO2_500.500.5550:5040095.1
WO3@TiO2_200.800.2220:8040094.8
WO3@TiO2_100.900.1110:9040096.2
WO3@TiO2_50.950.055:9540095.7
WO3@TiO2_20*0.800.2220:8060060.9
TiO21.000.000:10040098.5
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Falletta, E.; Bianchi, C.L.; Morazzoni, F.; Polissi, A.; Di Vincenzo, F.; Bellobono, I.R. Tungsten Trioxide and Its TiO2 Mixed Composites for the Photocatalytic Degradation of NOx and Bacteria (Escherichia coli) Inactivation. Catalysts 2022, 12, 822. https://doi.org/10.3390/catal12080822

AMA Style

Falletta E, Bianchi CL, Morazzoni F, Polissi A, Di Vincenzo F, Bellobono IR. Tungsten Trioxide and Its TiO2 Mixed Composites for the Photocatalytic Degradation of NOx and Bacteria (Escherichia coli) Inactivation. Catalysts. 2022; 12(8):822. https://doi.org/10.3390/catal12080822

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Falletta, Ermelinda, Claudia Letizia Bianchi, Franca Morazzoni, Alessandra Polissi, Flavia Di Vincenzo, and Ignazio Renato Bellobono. 2022. "Tungsten Trioxide and Its TiO2 Mixed Composites for the Photocatalytic Degradation of NOx and Bacteria (Escherichia coli) Inactivation" Catalysts 12, no. 8: 822. https://doi.org/10.3390/catal12080822

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