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Article

Heterogeneous Photocatalytic Degradation of Selected Pharmaceuticals and Personal Care Products (PPCPs) Using Tungsten Doped TiO2: Effect of the Tungsten Precursors and Solvents

by
Kunyang Li
,
Jing Li
,
Fengying Luo
,
Yuhua Yu
,
Yepeng Yang
and
Yizhou Li
*
Yunnan Key Laboratory of Metal-Organic Molecular Materials and Device, School of Chemistry and Chemical Engineering, Kunming University, Kunming 650214, China
*
Author to whom correspondence should be addressed.
Molecules 2024, 29(17), 4164; https://doi.org/10.3390/molecules29174164
Submission received: 11 July 2024 / Revised: 27 August 2024 / Accepted: 29 August 2024 / Published: 3 September 2024
(This article belongs to the Special Issue Feature Papers in Photochemistry and Photocatalysis)
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Abstract

:
Pharmaceuticals and personal care products (PPCPs) which include antibiotics such as tetracycline (TC) and ciprofloxacin (CIP), etc., have attracted increasing attention worldwide due to their potential threat to the aquatic environment and human health. In this work, a facile sol-gel method was developed to prepare tungsten-doped TiO2 with tunable W5+/W6+ ratio for the removal of PPCPs. The influence of solvents in the synthesis of the three different tungsten precursors doped TiO2 is also taken into account. WCl6, ammonium metatungstate (AMT), and Na2WO4●2H2O not only acted as the tungsten precursors but also controlled the tungsten ratio. The photocatalyst prepared by WCl6 as the tungsten precursor and ethanol as the solvent showed the highest photodegradation performance for ciprofloxacin (CIP) and tetracycline (TC), and the photodegradation performance for tetracycline (TC) was 2.3, 2.8, and 7.8 times that of AMT, Na2WO4●2H2O as the tungsten precursors and pristine TiO2, respectively. These results were attributed to the influence of the tungsten precursors and solvents on the W5+/W6+ ratio, sample crystallinity and surface properties. This study provides an effective method for the design of tungsten-doped TiO2 with tunable W5+/W6+ ratio, which has a profound impact on future studies in the field of photocatalytic degradation of PPCPs using an environmentally friendly approach.

Graphical Abstract

1. Introduction

Pharmaceuticals and personal care products (PPCPs), as a comprehensive category of emerging pollutants, have received increasing attention in recent years due to their diverse potential impacts on the dynamics of the natural environment and human health [1,2]. Generally, PPCPs include a broad range of medicinal and consumer chemicals such as antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), blood lipid regulators (BLRs), and fragrances. Tetracycline (TC) and ciprofloxacin (CIP) as a broad-spectrum antibiotic is widely used as a medicine in treatments [3]. Residues of TC and CIP enter the aquatic environment due to incomplete absorption and degradation. It is imperative to efficiently degrade the TC and CIP by employing advanced technology [4,5,6].
For the environmentally friendly treatment of TC and CIP, some sustainable methods such as adsorption [3], coagulation [4], biological treatment [5], and filtration [6] have attracted attention in recent years, but have been found to be ineffective. TiO2-based materials have been identified as promising candidates for the photocatalytic degradation of PPCPs in aquatic environments [7]. Nevertheless, the large bandgap energy (3.2 eV) and the accompanying suppression limit its practical applicability for natural solar applications [8,9,10,11]. In view of this, the doping of elements such as P [12], S [13], N [14], Fe [15], Cr [16], and Co [17] into pristine TiO2 has been investigated for the photocatalytic degradation of TC and CIP under natural solar light [18,19].
Transition metals have attracted extensive attention in the field of photocatalysis due to their unique photophysical-chemical properties [20]. Currently, tungsten (W) is widely used in photocatalysis [21], sensing [22], water decomposition [23], and photoelectrochemical properties [24]. Among these applications, it has shown high activity and environmental friendliness in photocatalytic degradation. TiO2 has also been doped with tungsten in order to enhance photocatalytic which optoelectrical properties are achieved by doping with different oxidation states (W4+, W5+ and W6+) [25]. Compared to W6+ doping, W4+ and W5+ doping has been studied less frequently. However, it has been proved that composites prepared with the participation of tungsten or tungsten compounds formed by W5+ and W4+ doping can be used to adjust the expected photocatalytic activity of tungsten-doped TiO2 [26,27]. W5+ and W4+ doped tungsten compounds can easily act as electron acceptors, thus improving the efficiency of photogenerated carrier separation [26,27].
The tungsten precursors are also essential for the production of tungsten-doped TiO2. In general, organic and inorganic tungsten compounds, such as sodium tungstate dehydrate (Na2WO4●2H2O), ammonium metatungstate (AMT), tungsten hexachloride (WCl6), and tungsten (VI) hexa-ethoxide (W(OC2H5)6 [28,29,30], have been widely used as tungsten precursors for the preparation of tungsten-doped TiO2. Different tungsten precursors have significant effects on the performance and properties of the prepared tungsten-doped TiO2 composite photocatalysts. Different tungsten precursors affect the distribution, particle size, and surface properties of tungsten during the reaction process, which in turn affect the activity of the photocatalysts [19].
It can be concluded that W5+ and W6+ co-doped TiO2 prepared by varying the tungsten dopants could further improve the photocatalytic activity of tungsten-doped TiO2. For example, Sanjayan Sathasivam et al. have demonstrated that the inclusion of tungsten in TiO2 materials in low quantities can enhance the photocatalytic activity by reducing the carrier mobility [25]. Raul Quesada-Cabrera et al. prepared W5+/W6+ coexistence WO3/TiO2 heterojunction films that exhibited unusual electron transfer from WO3 to TiO2 [31]. Tungstate-doped TiO2-SiO2 aerogels were prepared by the sol-gel method which contained W5+, W6+, and preferentially photodegraded methamphetamine [32]. Nevertheless, up to now there is little information available on the synergistic effects of the W5+/W6+ co-doping of TiO2, let alone the tuning of the W5+/W6+ ratios.
The solvent is also an important factor in the preparation of tungsten-doped TiO2. In general, considering factors such as solubility and hygroscopicity, methanol and ethanol are the most commonly used solvents employed in synthesis due to their water miscibility and compatibility with other compounds [33]. Less attention has been paid in the literature to the use of lower alcohols, especially in the sol-gel method for the synthesis of TiO2-based materials [34]. However, attention should be paid to the potential significance of the molecular configuration of the solvent in the sol-gel method. Previous studies also shown that the crystallinity, surface morphology, and optical properties of materials are strongly dependent on the solvent [34]. Zainab Yousif Shnain et al. have shown that the solvent has a significant effect on the particle size and morphology of the synthesized nanoparticles [35]. In the sol-gel process, the optical properties of the material are significantly affected by varying the solvent ratio of water to ethanol [36]. Pu-Xian Gao et al. selected six organic compounds as solvents for the synthesis of TiO2 by Solvotherma. The study implied that the configuration of the organic solvents can have a significant impact on the microstructures and properties of the final products [37]. Obviously, the complex nature of solvents, such as polarity and hydrogen bonding, can affect the kinetics of the sol-gel reaction and the properties of the material. There is a lack of reports regarding the effect of solvents during the synthesis of TiO2-based materials using the sol-gel method.
In this study, tungsten-doped TiO2 photocatalysts with varying W5+ and W6+ ratios were prepared via the sol-gel method at a low synthesis temperature. In the dark and photoreaction stages, the effects of different tungsten precursors and solvents on the adsorption and photocatalytic activity of TC and CIP were investigated. These results demonstrated that the photocatalytic activity of all W5+/W6+ co-doped TiO2 is superior to that of pristine TiO2. Furthermore, the photocatalytic activity of W5+/W6+ co-doped TiO2 under simulated sunlight increased with increasing W5+/W6+ by varying the tungsten precursors. Besides, the tungsten-doped TiO2 prepared with ethanol as a solvent showed enhanced photocatalytic activity compared to the samples prepared with DMF. We hypothesized that this could be attributed to the hydroxyl groups of the solvent molecules and the elemental electronegativity.

2. Results and Discussion

2.1. Characterization

XRD was used to analyse the crystalline phases of the prepared samples. As shown in Figure 1, the diffraction angles of the anatase phases corresponding to (101), (004), (200), and (211) of anatase crystals are 25.3°, 37.9°, 48.1°, and 55.1°, respectively, which proves that anatase structures are present in all TiO2-based materials. The diffraction peaks of the samples with dimethylformamide (DMF) as the solvent (diffraction peaks at 2 θ = 25.3°) are broader and weaker than those of the samples with ethanol (Et) as the solvent, which may indicate that the samples dissolved in ethanol have a better crystallisation effect compared to DMF. In addition, as shown in Table 1, we calculated the crystallinity of the samples and the results also indicate that the samples dissolved in ethanol have higher crystallinity. This revealed a preference for ethanol as a solvent in the crystallization of tungsten-doped TiO2 powders, over DMF. It is well known that crystallinity is crucial for photocatalytic activity. Materials with elevated crystallinity generally exhibit improved properties in photocatalysis attributed to the well-ordered and uniform structures, which facilitate charge transfer from the center to the surface [38]. The analysis indicates that tungsten-doped TiO2 synthesized utilizing WCl6 as the tungsten precursor potentially exhibits enhanced photocatalytic performance, attributed to its superior crystallinity, compared with Na2WO4 and AMT. For all samples, no phases other than the anatase phase were detected. The formation of WO3 was not evidenced by the characteristic peaks typically observed in XRD spectra, as noted by other authors. This may be due to the low concentration of WO3, which was insufficient for detection by XRD [29,39]. The absence of a characteristic WO3 peak in the XRD pattern of tungsten-doped TiO2 suggests that tungsten ions either formed W–O–Ti bonds within the lattice or occupied interstitial sites [40]. Several studies have shown that tungsten ions successfully substitute titanium ions within the TiO2 crystal lattice due to the similar ionic radius (W6+ at 0.060 nm and Ti4+ at 0.0605 nm) [40].
The grain size of the prepared material was calculated using the Scherrer formula. The results were presented in Table 1. As illustrated in Table 1, the average grain size of the tungsten-doped TiO2 samples is smaller than that of the un-doped TiO2 samples, indicating that doping with various tungsten precursors affected the crystallinity of materials and thus hinders the crystal growth.
The FTIR spectra of the pristine TiO2 and the tungsten-doped TiO2 materials between 4000 cm−1 and 1000 cm−1 are shown in Figure 2. There are only two distinct absorption bands around 3450 cm−1 and 1630 cm−1 in Figure 2, which represent the stretching vibrations of the water and hydroxyl groups, respectively [41]. As illustrated in Figure 2, the introduction of tungsten results in the slight enhancement of the peaks at 3450 cm−1 and 1630 cm−1, suggesting an increase in the presence of water and hydroxyl groups, respectively. This observation implies that the addition of tungsten may lead to a higher concentration of hydroxyl groups within the tungsten-doped TiO2. The Lewis surface acidity of tungsten-doped TiO2 increases with the addition of tungsten [42], making it easier to adsorb water and form surface hydroxyl groups, while the –OH groups can capture photogenerated holes (h+) and convert them into active •OH radicals [43], thus improving the photocatalytic performance of the material.
The SEM and EDS mapping images of the pristine TiO2 and tungsten-doped TiO2 are shown in Figure 3. It can be found that pristine TiO2 exhibits textural characteristics with a dominant presence of irregularly shaped aggregates accompanied by interparticle voids [44]. This observation highlights the agglomeration tendency of pristine TiO2 [20]. Tungsten-doped TiO2 with different tungsten precursors of WCl6, Na2WO4●2H2O, and AMT have been displayed in Figure 3b–d. Compared with Figure 3a, the tungsten-doped TiO2 samples were looser, which suppressed the tendency of agglomeration to some extent. Some studies suggest that this could prove advantageous for both adsorption and photocatalysis processes [45,46]. Compared with Figure 3b–d, W3-TiO2-Et exhibits a loosely structured morphology characterized by enhanced porosity, which potentially contributes to a significantly larger specific surface area compared to other tungsten-doped TiO2. The microscopic chemical ingredient analysis of the tungsten-doped TiO2 materials has been shown in Figure 3e. The patterns show the presence of titanium (Ti), oxygen (O), and tungsten (W) without any other element. The elements of Ti, O, and W are identified, confirming the presence of measured atomic percentage of 26.99, 71.61, and 1.40%, respectively. It can be clearly seen that the atomic percentage of oxygen is 2.7 times that of titanium, which may be due to the presence of oxygen functionalities remaining on the surface of the tungsten-doped TiO2 [35]. Figure 3f–h shows the elemental mapping of oxygen (O), titanium (Ti), and tungsten (W) in different colors on W1-TiO2-Et. The homogeneous distribution of three elements within the W1-TiO2-Et inferred the successful synthesis of tungsten-doped TiO2. This homogeneity suggests a well-integrated structure on materials which is crucial for the photocatalysis [29].
TEM and HRTEM techniques were used to analyse the morphology and microstructure of TiO2-ET and W1-TiO2-Et nanoparticles. In the TEM analysis (Figure 4), it was observed that the morphology of tungsten-doped TiO2 and pristine TiO2 nanoparticles is similar. Both materials are characterized by the presence of spherical nanoparticles, which aggregate to form larger clusters. The particle size distribution is between 6~10 nm, which is consistent with the calculation results of Scherrer’s equation (Table 1). The HRTEM image in Figure 4c revealed a fringe spacing of approximately 0.352 nm, which is consistent with the crystal growth direction of the anatase TiO2 (1 0 1) plane, as evidenced by XRD measurements of the sample.
As shown in Figures S2 and S3, the N2 adsorption and desorption isotherm curves were recorded to study the specific surface area and corresponding pore size distribution of the tungsten doped samples [47]. The IUPAC classification of the nitrogen adsorption and desorption isotherms for the examined samples revealed a type V pattern, indicating the mesoporous properties of the prepared material. Meanwhile, the pore size distribution of all the samples was mainly distributed in 0~50 nm, which also indicated the mesoporous properties of the prepared materials. As shown in Table 1, except for the tungsten source with Na2WO4●2H2O as the precursor, doping with other tungsten precursors increased the specific surface area, pore size, and pore volume of the materials and provided more active sites. The material with Na2WO4●2H2O as the precursor had little effect on the specific surface area, but the pore size of the material with Na2WO4●2H2O as the precursor was significantly increased compared to the pristine TiO2, which may improve the adsorption and photocatalytic ability of the materials. In Table 1, it was found that W3-TiO2-DMF had the largest specific surface area of 350.39 m2/g, while the pore volume and pore size of the samples with ethanol as a solvent were larger than those of the samples with DMF as a solvent.
The crystalline phase of as-prepared samples was analyzed by X-ray photoelectron spectroscopy (XPS). In the XPS spectrum of Ti 2p, two XPS signals appear with binding energies of 458.8 and 464.5 eV, which are contributed by Ti 2p1/2 and Ti 2p3/2 and originate from the Ti4+ (Figure S4) [48,49,50]. Figure S4 shows that the binding energy at 529.88 eV corresponds to the crystal lattice oxygen of Ti–O or W–O, suggesting that W–O and Ti–O share O 1s orbitals in the W–O–Ti bond. The binding energy at 531.2 eV originates from hydroxyl groups bonded to Ti or W at the surface, and the binding energy near 532.0 eV originates from adsorbed water (H2Oads) or oxygen bonded to carbon (C–O) [29,51]. The O 1s binding energies of pristine TiO2 and tungsten-doped TiO2 materials were investigated in Figure S5, and it was found that the tungsten-doped TiO2 materials were shifted towards a higher binding energy in the O 1s orbital, which may be because the electronegativity of W (2.36) is greater than the electronegativity of Ti (1.54). The doping of TiO2 with tungsten can alter the electron cloud density of oxygen, leading to a slight shift to the binding energy towards to higher place.
As illustrated in Figure 5, the visual representation revealed that the samples synthesized using a range of tungsten precursors exhibited a co-doping pattern, characterized by the presence of both W5+ and W6+ ions. However, a different ratio of W5+ to W6+ was discerned among these samples. The disparity in the W 4f region between W1-TiO2-Et (WCl6), W2-TiO2-Et (Na2WO4●2H2O), and W3-TiO2-Et (AMT) was employed as an illustrative example. The W 4f peak of W1-TiO2-Et (WCl6) was deconvoluted into four peaks, W6+ 4f5/2 at 37.55 eV, W6+ 4f7/2 at 35.58 eV, W5+ 4f5/2 at 36.70 eV, and W5+ 4f7/2 at 34.62 eV [52,53], while the XPS spectrum of pristine TiO2 is flat in this region. This is consistent with the expected characteristic signature of pristine TiO2. The analysis reveals that the signals of W2-TiO2-Et (Na2WO4●2H2O) and W3-TiO2-Et (AMT) can be explicitly decomposed into four distinct peaks, each exhibiting comparable positions within the examined region. The W5+/W6+ ratios of W1-TiO2-Et, W2-TiO2-Et, W3-TiO2-Et, W1-TiO2-DMF, W2-TiO2-DMF, and W3-TiO2-DMF were 1:4.6, 1:5.2, 1:7.0, 1:7.0, 1:7.8, and 1:8.9, respectively. The study’s outcomes demonstrate the substantial influence of diverse tungsten precursors on the co-doping process involving W5+ and W6+. The investigation of tungsten precursors is crucial for understanding the physicochemical properties and photocatalytic performance of tungsten-doped TiO2. However, the intricacies of the mechanism remain unexplored, necessitating comprehensive investigation for a clearer understanding.

2.2. Photocatalytic Degradation of TC and CIP

The photocatalytic activity of the tungsten-doped TiO2 was evaluated by photodegradation of TC (50 mg/L) and CIP (50 mg/L). In contrast, the degradation of the TC and CIP solutions showed consistent results for the prepared samples. The activities of all W5+/W6+ co-doped samples were increased compared to the undoped TiO2. As shown in Figure 6a,b, the highest removal of TC and CIP was achieved by the prepared W1-TiO2-Et composites, and the removal rate of W1-TiO2-Et (77.24%, 80%) was more than 2.5 times that of pure TiO2 (31%, 32%). Meanwhile, in the dark, it was found that different tungsten precursors had a great influence on the adsorption effect, and high adsorption of TC and CIP by W3-TiO2-DMF was observed.
The kinetic analyses of TC and CIP degradation were based on the fitting of pseudo-first order equations with kinetic constants as shown in Figure 7. During the photocatalytic degradation of TC and CIP, the highest K values of 0.0039 min−1 (Table 2) and 0.0044 min−1 (Table 3) were obtained for W1-TiO2-Et, respectively. The increase in K value after the addition of tungsten precursors indicates that the co-doping of W6+ and W5+ accelerated the degradation of TC and CIP in the composites. The co-doping promotes carrier migration and reduces the photogenerated electron-hole complexation efficiency [31].
In addition, we made comparisons with previous work. As shown in Table 4 and Table 5, a variety of photocatalysts for TC and CIP degradation are listed. By comparison, it was found that the larger concentration of TC and CIP was selected as the degradants in this work, and the prepared W5+/W6+ co-doped TiO2 composites showed high removal rates for both degradants.
The effect of different W contents on the photocatalytic properties of tungsten-doped TiO2 composites was investigated as shown in Figure 8. Figure 8a shows the removal curves of CIP, and Figure 8b shows the apparent first-order rate constant k (min−1) for CIP. Figure 8a shows that the highest CIP removal rate (80%) of tungsten-doped TiO2 composites was achieved when Ti/W = 3:0.1, but when the W content was further increased, the removal rate decreased instead. Figure 8b also shows that the composites have the maximum K value for Ti/W = 3:0.1, indicating that Ti/W = 3:0.1 is the optimum W loading. The photocatalytic activity of tungsten-doped TiO₂ composites was observed to increase when the optimum tungsten loading was reached, beyond which certain surface reaction sites of photocatalytic activity may be hindered, thus limiting the reaction rate [29].
In summary, the photocatalyst with ammonium metatungstate (AMT) as the tungsten precursor and DMF as the solvent exhibited the strongest adsorption capacity for ciprofloxacin (CIP) and tetracycline (TC) in the dark (52% and 48% in 2 h). However, the photocatalyst with WCl6 as the tungsten precursor and ethanol as the solvent (W1-TiO2-Et) showed the highest photodegradation performance for ciprofloxacin (CIP) and tetracycline (TC). The photodegradation performance to tetracycline (TC) on W1-TiO2-Et was 2.3, 2.8, and 7.8 times that of AMT, Na2WO4●2H2O as the tungsten precursors and pristine TiO2, respectively. The optimum tungsten loading was also investigated in this work, and it was found that Ti/W = 3:0.1 is the optimum tungsten loading.

2.3. Mechanism

It is worth highlighting some facts about tungsten-doped TiO2 which exhibited higher photocatalytic efficiency for the degraded TC and CIP, though the mechanism is far from understood.
As illustrated in Figure 7, the photocatalytic efficiencies of the different tungsten-doped TiO2 samples are significantly different. XPS analysis (Figure 5) revealed that these materials exhibited different ratios of W5+ and W6+. The photocatalytic efficiency of the prepared samples may be related to the ratio of W5+ and W6+, and co-doping promotes carrier migration and reduces the photogenerated electron-hole complex efficiency. To confirm this hypothesis, electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) analyses have been performed, and the results are shown in Figure 9a,b.
The findings indicated that tungsten-doped TiO2 exhibited a diminished photogenerated carrier radius and an extended photogenerated carrier lifetime in comparison to pristine TiO2. This suggests that the separation rate and migration of photogenerated carriers in the W5+/W6+ co-doped composites are more rapid than in pristine TiO2. W1-TiO2-Et has the smallest photogenerated carrier radius and the longest photogenerated carrier lifetime, which confirms that the W1-TiO2-Et has the highest photocatalytic activity. Meanwhile, it can be seen from Figure 5 that the W5+/W6+ ratio of W1-TiO2-Et is the largest; the higher percentage of W5+ indicates the higher oxygen vacancy concentration [53], which accelerates the migration rate of photogenerated carriers. The higher concentration of oxygen vacancies also makes it easier to adsorb water to form surface hydroxyls, which promotes the photocatalytic efficiency [53]. It is believed that the phenomenon should be influenced by the molecular configuration of the tungsten precursors. Tungsten in AMT and Na2WO4●2H2O is surrounded by oxygen and hydroxyl groups/H2O, respectively. In contrast, tungsten in tungsten (VI) chloride (WCl6) is also in the +6 oxidation state but is coordinated by chlorine atoms. The difference in coordination environment between these tungsten compounds results in the diverse ways in which tungsten can be bonded in the synthesis of tungsten-doped TiO2, influenced by the steric hindrance effect and surface hydroxyl group. The combined effects underscore the efficiency of photocatalytic degradation in the removal of TC and CIP.
The solvent also impacted the photocatalytic efficiency of the synthesized samples obviously. Figure 7 shows that tungsten-doped TiO2 synthesized with ethanol exhibited enhanced photocatalytic efficiency compared to those synthesized in DMF. This phenomenon may be attributed to the varying conformations exhibited by ethanol and DMF molecules. As mentioned earlier, compared to DMF, using ethanol as solvent can increase the crystallinity, pore size, and pore volume of the tungsten-doped TiO2 materials. This facilitates charge transfer from the center to the surface and increases the active sites, thus improving the photocatalytic activity. In addition, the gel formation process was faster in the DMF solvent than in the ethanol solvent during the preparation process, which may be due to the larger polarity and dielectric constant of DMF compared to ethanol. The reaction was slower when ethanol was used as a solvent and the nanoparticles nucleated uniformly in ethanol solvent [60]. Ethanol can be used as a very dispersive solvent by the reaction and ethanol as a solvent plays an important role in controlling the crystal growth [60].
Figure 10 shows the UV-vis absorption spectra of tungsten-doped TiO2 with different tungsten precursors and solvents. In general, the absorption of tungsten-doped TiO2 was stronger than that of undoped TiO2 in visible light region. The red shift was observed with the incorporation of tungsten into TiO2 materials. The band gaps of the prepared tungsten-doped TiO2 are estimated by the Tauc plot as shown in Figure S7. Pristine TiO2 exhibited the Eg values about 3.3 eV, which is consistent with the value reported in literature [61]. It is obvious that doping TiO2 with tungsten slightly modifies the TiO2 absorption edge in the visible region. This observed phenomenon has been elucidated by the quantum confinement effect [34,62]. It is true that tungsten doping in TiO2 has been observed to reduce the band gap of photocatalysts, which is attributed to create new energy levels within the band gap [49]. However, smaller particle size often leads to higher Eg values [13]. As the size of the particles decreases, the degree of quantization of energy levels is stronger, leading to an increased band gap [45]. That is, the narrow band gap and red-shifted adsorption edges of the samples analysed by UV-visible diffuse reflectance spectroscopy may indicate a better photocatalytic activity of the tungsten-doped TiO2 samples [29].
For the purpose of analysing the adsorption mechanism of the tungsten-doped TiO2 samples in the dark, the zeta potential of the tungsten-doped TiO2 samples have been measured (Figure S8), focusing on their surface charge characteristics.
The pka1, pka2, and pka3 of TC are 3.3, 7.7, and 9.7, respectively [63]. Positive charges dominate when the pH is below 3.3, both positive and negative charges are present when the pH is below 7.7 but above 3.3, and negative charges dominate when the pH is above 7.7 [60]. CIP has two pKa of 5.9 and 8.9 at pH values between 5.9 and 8.9 [64], at pH values below 5.9 the cation of CIP is dominant, and at pH values above 8.9, the anion of CIP is dominant [65,66]. This work is based on 50 ppm TC and 50 ppm CIP solutions at pH 3.2 and 5.3, so positive charges dominate in both 50 ppm TC and 50 ppm CIP solutions.
As shown in Table 6, the samples with AMT as the tungsten precursors had less positive surface charge in TC and CIP solutions (pH = 3.2, 5.3). According to the principle of anisotropic attraction and anisotropic repulsion, the sample with AMT as the tungsten precursor has a better adsorption effect on ciprofloxacin solution in the dark. From Table 1, it can be seen that the specific surface area of W3-TiO2-DMF is larger than the other samples; therefore, combined with the BET potential and zeta potential, it can be concluded that W3-TiO2-DMF has a stronger adsorption effect on the 50 ppm TC and 50 ppm CIP solutions, which is also consistent with the results in Figure 6.
Free radical trapping tests were carried out on W1-TiO2-Et as shown in Figure 11a,b to determine which are the active species in the photocatalytic process of W1-TiO2-Et [67]. The photodegradation efficiency decreased when IPA, EDTA-2Na, and BQ were added to the TC and CIP solutions. IPA (71.3%, 76.24%), EDTA-2Na (56.6%, 63.49), and BQ (43%, 51.86). This result indicates that ·O2− and h+ are the main active species of W1-TiO2-Et photocatalysts during the photodegradation of CIP and TC [68].
To assess the reusability of the photocatalysts, the W1-TiO2-Et photocatalysts were subjected to four cycles of reusability tests. The results are shown in Figure 12: the photocatalyst exhibits excellent photocatalytic efficacy for TC and CIP degradation in all reusability tests. This proves that the sample is an excellent photocatalyst for reusability.

3. Materials and Methods

3.1. Chemicals and Reagents

Sodium tungstate dihydrate (Na2WO4●2H2O, ≥98%) and titanium tetraisopropoxide (TTIP, ≥98%) were purchased from Adamas-beta (Shanghai, China). Tungsten (VI) chloride (WCl6, 99%), ammonium metatungstate hydrate (AMT, 99.5%), N,N-dimethylformamide (DMF, 99.5%), ethanol (Et, ≥99.7%), nitric acid (HNO3, 65–68%), p-benzoquinone (BQ, 99%), ciprofloxacin (CIP, 98%), and tetracycline hydrochloride (TC, 99%) were purchased from Merck (Shanghai, China). Thylenediaminetetraacetic acid disodium salt (EDTA-2Na, ≥99%) and isopropanol (IPA, 99.7%) were purchased from Tianjin ZhiYuan Reagent Co., Ltd. (Tianjin, China). All chemicals were used without further purification.

3.2. Catalyst Preparation

In a typical procedure, 0.44 g of WCl6 and 3 drops of nitric acid were added to 38.6 mL of DMF/ethanol and stirred until the WCl6 was completely dissolved, and then 10 mL of TTIP and 2.5 mL deionized water were added slowly drop by drop to hydrolyze the TiO2 completely. The sealed sol-gel was aged at room temperature for 48 h, then calcined in a high temperature and pressure reactor at 265 °C for 2 h. Then, it was washed with deionized water and ethanol, dried, and collected. Composites featuring sodium tungstate (Na2WO4) and AMT as the tungsten precursors were synthesized using identical methodologies. Employing WCl6 as the tungsten precursor, the composite’s synthesis involved a systematic investigation using two kinds of solvents, W1-TiO2-DMF and W1-TiO2-Et. Similarly, for sodium tungstate (Na2WO4), the samples were denoted W2-TiO2-DMF and W2-TiO2-Et, and for AMT, W3-TiO2-DMF and W3-TiO2-Et, respectively. All tungsten-doped TiO2 materials referenced above exhibit a Ti: W molar ratio of 3:0.1. The sample synthesis flowchart is shown in Figure S1.
In order to study the effect of tungsten doping, other samples with different Ti: W = 0.03, 0.05, 0.07, 0.1, 0.13 composites were synthesized by the similar method using WCl6 as the tungsten precursors and ethanol as the solvent, and the samples were named as 0.03-W1-TiO2-Et, 0.05-W1-TiO2-Et, 0.07-W1-TiO2-Et, 0.1-W1-TiO2-Et, and 0.13-W1-TiO2-Et. For comparison, two TiO2 materials using ethanol and DMF as solvents which denoted as TiO2-Et and TiO2-DMF were prepared by similar method.

4. Conclusions

The successful synthesis of tungsten-doped TiO2 samples with tunable W5+/W6+ ratio using a simple sol-gel method. To explore the impact of tungsten precursors and solvents in the synthesis of tungsten-doped TiO2 on the photocatalytic degradation of selected pharmaceuticals and personal care products (PPCPs), tetracycline (TC), and ciprofloxacin (CIP) were chosen as targets. The increase in photocatalytic efficiency may be due to the synergistic effect of crystallinity, surface properties, molecular configuration of the solvent, and W5+/W6+ ratio of the samples, in which the co-doping of W5+/W6+ may play a more important role. The co-doping of W5+/W6+ accelerated the photogenerated carrier migration rate and increased the photogenerated carrier lifetime, resulting in a higher photocatalytic efficiency of the tungsten-doped TiO2 samples than that of the pristine TiO2. In addition, the distinct molecular configurations of ethanol and DMF as solvents during the synthesis process resulted in variations in the gel formation rate, polarity, and dielectric constant of the tungsten-doped TiO2. These differences ultimately affect the photocatalytic degradation of TC and CIP on the tungsten-doped TiO2. The radical trapping assay revealed that ·O2− and h+ were found to be the main reactive species in the degradation process of TC and CIP. Repeated experiments showed that W1-TiO2-Et was a stable catalyst. In summary, the TC and CIP removal efficiency of tungsten-doped TiO2 suggest that it may be a promising candidate for the treatment of PPCPs under simulated sunlight.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29174164/s1, S1: Instruments and equipment; S2: Measurement of photocatalytic activity; Figure S1: Sample synthesis flowchart; Figure S2: Surface area analysis of W-TiO2 composites. N2 adsorption/desorption isotherms of (a) TiO2-DMF, (b) W1-TiO2-DMF, (c) W2-TiO2-DMF, (d) W3-TiO2-DMF; Figure S3: Surface area analysis of W-TiO2 composites. N2 adsorption/desorption isotherms of (a) TiO2-Et, (b) W1-TiO2-Et, (c) W2-TiO2-Et, (d) W3-TiO2-Et; Figure S4: XPS spectra of O 1S and Ti 2p region for W1-TiO2-Et; Figure S5: XPS spectra of O 1S region for W1-TiO2-DMF, W1-TiO2-Et, TiO2-Et, W3-TiO2-Et and W2-TiO2-Et; Figure S6: Adsorption/desorption curves in the dark (a) Tc, (b) CIP; Figure S7: plots of (ahv)2−hυ (a–d); Figure S8: Zeta potential of as-prepared samples (a,b).

Author Contributions

Conceptualization, Y.L.; methodology, Y.L. and K.L.; software, Y.Y. (Yepeng Yang) and Y.L.; validation, K.L., J.L. and F.L.; formal analysis, K.L. and Y.Y. (Yuhua Yu); investigation, J.L. and F.L.; resources, K.L. and Y.Y. (Yuhua Yu); data curation, K.L. and Y.L.; writing—original draft preparation, K.L. and Y.L.; writing—review and editing, Y.L. and K.L.; visualization, K.L. and J.L.; supervision, F.L. and Y.Y. (Yuhua Yu); project administration, Y.L. and Y.Y. (Yepeng Yang); funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Yunnan Fundamental Research Projects (No. 202101AU070042), the Scientific Research Fund of Yunnan Education Department (No. 2022J0627) and the Program of Introducing Talents of Kunming University (No. YJL2116), for financial support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The experimental data used to support the results of this study are available in the article and in the Supplementary Materials.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could appear to influence the work reported in this paper.

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Molecules 29 04164 g001
Figure 1. XRD patterns of pristine TiO2 and tungsten-doped TiO2.
Figure 1. XRD patterns of pristine TiO2 and tungsten-doped TiO2.
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Figure 2. FTIR spectra of pristine TiO2 and tungsten-doped TiO2.
Figure 2. FTIR spectra of pristine TiO2 and tungsten-doped TiO2.
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Figure 3. SEM images of pristine TiO2-Et (a), SEM images of W1-TiO2-Et (b), SEM images of W2-TiO2-Et (c), SEM images W3-TiO2-Et (d), EDS spectrum of W1-TiO2-Et (e), EDS mapping images of O (f), Ti (g), and W (h) on W1-TiO2-Et.
Figure 3. SEM images of pristine TiO2-Et (a), SEM images of W1-TiO2-Et (b), SEM images of W2-TiO2-Et (c), SEM images W3-TiO2-Et (d), EDS spectrum of W1-TiO2-Et (e), EDS mapping images of O (f), Ti (g), and W (h) on W1-TiO2-Et.
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Figure 4. TEM images of TiO2-Et (a) and W1-TiO2-ET (b), HRTEM images of W1-TiO2-Et (c).
Figure 4. TEM images of TiO2-Et (a) and W1-TiO2-ET (b), HRTEM images of W1-TiO2-Et (c).
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Figure 5. XPS spectra of W 4f region for (a) W1-TiO2-DMF and W1-TiO2-Et, (b) W1-TiO2-Et, W2-TiO2-Et, and W3-TiO2-Et.
Figure 5. XPS spectra of W 4f region for (a) W1-TiO2-DMF and W1-TiO2-Et, (b) W1-TiO2-Et, W2-TiO2-Et, and W3-TiO2-Et.
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Figure 6. Removal curves of TC (50 mg/L) (a) and CIP (50 mg/L) (b).
Figure 6. Removal curves of TC (50 mg/L) (a) and CIP (50 mg/L) (b).
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Figure 7. Apparent first order rate constant k (min−1) for TC (50 mg/L) (a) and CIP (50 mg/L) (b) photocatalytic degradation over the as-prepared samples.
Figure 7. Apparent first order rate constant k (min−1) for TC (50 mg/L) (a) and CIP (50 mg/L) (b) photocatalytic degradation over the as-prepared samples.
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Figure 8. CIP removals curves under different W doping content (a), apparent first order rate constant k (min−1) for CIP photocatalytic degradation (b).
Figure 8. CIP removals curves under different W doping content (a), apparent first order rate constant k (min−1) for CIP photocatalytic degradation (b).
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Figure 9. PL spectra of as-prepared samples (a), EIS Nyquist plots of pure TiO2 and W-TiO2 composites (b).
Figure 9. PL spectra of as-prepared samples (a), EIS Nyquist plots of pure TiO2 and W-TiO2 composites (b).
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Molecules 29 04164 g010
Figure 10. UV-vis diffuse reflectance spectra, (a) W1-TiO2-DMF, W2-TiO2-DMF and W3-TiO2-DMF, (b) W1-TiO2-Et, W2-TiO2-Et and W3-TiO2-Et.
Figure 10. UV-vis diffuse reflectance spectra, (a) W1-TiO2-DMF, W2-TiO2-DMF and W3-TiO2-DMF, (b) W1-TiO2-Et, W2-TiO2-Et and W3-TiO2-Et.
Molecules 29 04164 g010
Molecules 29 04164 g011
Figure 11. Free radical capture experiment TC (a), CIP (b).
Figure 11. Free radical capture experiment TC (a), CIP (b).
Molecules 29 04164 g011
Molecules 29 04164 g012
Figure 12. Recycling tests of degradation TC (a) and CIP (b) using W1-TiO2-Et.
Figure 12. Recycling tests of degradation TC (a) and CIP (b) using W1-TiO2-Et.
Molecules 29 04164 g012
Table 1. Physicochemical properties of tungsten-doped TiO2 samples.
Table 1. Physicochemical properties of tungsten-doped TiO2 samples.
SamplesAverage Crystalline Size (nm)SBET (m2/g)Band Gap Energy (eV)Pore Volume (cm3/g)Pore Size (A)
TiO2-DMF10.2145.123.320.297.18
TiO2-Et10.8143.233.320.3310.45
W1-TiO2-DMF8.2205.603.130.2234.01
W1-TiO2-Et9.6202.253.170.4464.94
W2-TiO2-DMF9.9148.633.260.2958.01
W2-TiO2-Et10.1147.983.290.3770.69
W3-TiO2-DMF6.4350.393.240.3737.08
W3-TiO2-Et7.9200.823.210.5458.21
Table 2. The corresponding kinetic constant of TC degradation (except for adsorption).
Table 2. The corresponding kinetic constant of TC degradation (except for adsorption).
SamplesTiO2-DMFTiO2-EtW1-TiO2-DMFW1-TiO2-EtW2-TiO2-DMFW2-TiO2-EtW3-TiO2-DMFW3-TiO2-Et
K (min−1)0.00010.00020.00300.00390.00120.00140.00160.0017
Table 3. The corresponding kinetic constant of CIP degradation (except for adsorption).
Table 3. The corresponding kinetic constant of CIP degradation (except for adsorption).
SamplesTiO2-DMFTiO2-EtW1-TiO2-DMFW1-TiO2-EtW2-TiO2-DMFW2-TiO2-EtW3-TiO2-DMFW3-TiO2-Et
K (min−1)0.00010.00020.00310.00440.00110.00140.00130.0016
Table 4. Lists a variety of photocatalysts for TC degradation.
Table 4. Lists a variety of photocatalysts for TC degradation.
SampleDosage (g/L)Concentration (mg/L)Time (min)Removal Rate (%)YearAuthorReference
Bi/BiVO40.5106074.72019Nianjun Kang[54]
2D/2D BiOCl/g-C3N40.1103097.12020Yuwei Sun[55]
Bi2O3/Ti3+—TiO20.21020096.52020Tao Tang[56]
This work0.45036077.22024--------
Table 5. Lists a variety of photocatalysts for CIP degradation.
Table 5. Lists a variety of photocatalysts for CIP degradation.
SampleDosage (g/L)Concentration (mg/L)Time (min)Removal Rate (%)YearAuthorReference
Ti3+/N-TiO20.430.5701002020Mansour Sarafraz[57]
Carbon-doped TiO2150360352015Jian-Wen Shi[58]
Fe-N-TiO20.620360702020Totsaporn Suwannaruang[59]
This work0.450360802024--------
Table 6. Zeta potential of as- prepared samples.
Table 6. Zeta potential of as- prepared samples.
pHW1-TiO2-DMF (mV)W1-TiO2-Et (mV)W2-TiO2-DMF (mV)W2-TiO2-Et (mV)W3-TiO2-DMF (mV)W3-TiO2-Et (mV)
321.622.15.818.54.673.86
50.668−2.13−12.4−10.6−12.2−13.1
7−18.3−19.3−18.2−18.8−19.2−20.1
9−23.7−23.4−22.1−23.1−23.3−24.2
11−29.9−30−28.7−30−31.6−32.4
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Li, K.; Li, J.; Luo, F.; Yu, Y.; Yang, Y.; Li, Y. Heterogeneous Photocatalytic Degradation of Selected Pharmaceuticals and Personal Care Products (PPCPs) Using Tungsten Doped TiO2: Effect of the Tungsten Precursors and Solvents. Molecules 2024, 29, 4164. https://doi.org/10.3390/molecules29174164

AMA Style

Li K, Li J, Luo F, Yu Y, Yang Y, Li Y. Heterogeneous Photocatalytic Degradation of Selected Pharmaceuticals and Personal Care Products (PPCPs) Using Tungsten Doped TiO2: Effect of the Tungsten Precursors and Solvents. Molecules. 2024; 29(17):4164. https://doi.org/10.3390/molecules29174164

Chicago/Turabian Style

Li, Kunyang, Jing Li, Fengying Luo, Yuhua Yu, Yepeng Yang, and Yizhou Li. 2024. "Heterogeneous Photocatalytic Degradation of Selected Pharmaceuticals and Personal Care Products (PPCPs) Using Tungsten Doped TiO2: Effect of the Tungsten Precursors and Solvents" Molecules 29, no. 17: 4164. https://doi.org/10.3390/molecules29174164

APA Style

Li, K., Li, J., Luo, F., Yu, Y., Yang, Y., & Li, Y. (2024). Heterogeneous Photocatalytic Degradation of Selected Pharmaceuticals and Personal Care Products (PPCPs) Using Tungsten Doped TiO2: Effect of the Tungsten Precursors and Solvents. Molecules, 29(17), 4164. https://doi.org/10.3390/molecules29174164

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