Synthesis and Characterization of TiO₂ Thin Films Modified with Anderson-Type Polyoxometalates (Ni, Co, and Fe)

Abstract

In this work, TiO₂ and Anderson-type polyoxometalates (Ni, Co, and Fe) thin-film composites were fabricated. The composites were characterized by FTIR and Raman spectroscopy, diffuse reflectance, and scanning electronic microscopy. The methylene blue (MB) photocatalytic degradation on the composites under UV irradiation was studied. Spectroscopic results verified the modification of TiO₂ thin films. Optical and morphological properties changed after TiO₂ modification. The largest change in the optical band gap was observed for the FePOM/TiO₂ system, which reported a value of 3.05 eV. The POM/TiO₂ systems were more efficient in methylene blue (MB) adsorption than bare TiO₂. Furthermore, the modified films were more efficient than bare TiO₂ during MB photodegradation tests. The NiPOM/TiO₂ and the CoPOM/TiO₂ were the most efficient in the MB adsorption, reaching ~20%. The NiPOM/TiO₂ and the CoPOM/TiO₂ composites were the most efficient in the photodegradation process, reaching ~50% of MB removal. The stability tests indicated that composite films were moderately stable after the three performed reusability cycles. Thus, these results suggest that POM modification of TiO₂ can improve the adsorption and photodegradation capacity of semiconductors.

1. Introduction

The pollution of the environment (e.g., water, air, and land) is a topic of great interest around the world, especially for developing of a sustainable society [1]. Development and industrialization are the most influential factors in water use, producing recalcitrant polluting waste, and many of the pollutants end up in surface water reservoirs [2]. Most recalcitrant compounds (e.g., hydrocarbons, phenols, nitrogenous compounds, and dyes) come from the industrial production and processing of various goods, such as oils, polymers, textiles, pesticides, and pharmaceuticals [3]. Among these, dyes are used in many industries (food, paper, rubber, plastics, cosmetics, and textiles) to color their products. However, because of their stability and bioaccumulation even in drinking water sources, there is now great concern about the adverse effects of these compounds on environmental, ecological, and human health [4,5]. These compounds are continuously released into the aquatic environment due to their high consumption and insufficient removal by conventional treatment methods [6]. Advanced oxidation processes offer an alternative to traditional methods for the removal of these types of contaminants [7]. Among these, heterogeneous photocatalysis (HP) has demonstrated its potential for treating various emerging contaminants. This technique provides new ideas for the development of traditional catalysis. During HP, the action of solar energy on photocatalysts can generate reactive oxygen species (ROS). These ROS can react with different emergent pollutants [8,9]. Currently, TiO₂ is one of the most studied photocatalysts in the field. However, this semiconductor has two drawbacks: (i) a high band gap energy value and (ii) low quantum yield to generate charge carriers [10]. In recent years, different strategies have been implemented to improve both the photoactivity of TiO₂ in the visible range of the electromagnetic spectrum and the quantum yield to generate the charge carriers (e.g., metal and non-metal doping [11,12], quantum dots [13], natural and synthetic sensitization [14,15], nano composites with metals [16], and heterostructures [17]). Among these, the TiO₂ heterostructures with polyoxometalates (POMs) have received attention due to the special physical chemical properties of POMs. The heterostructures can be obtained by combining two or more dissimilar semiconductors, and this process can change the band structure near the interface, affecting the recombination process of the charge carriers [18]. The heterostructure generation has been applied for the removal of other pollutants under UV irradiation [19]. The POMs can accept electrons without changing their structure, reducing the recombination process of charge carriers; this property is valuable in photocatalytic applications [20,21,22]. POMs are nanoclusters of metal–oxygen anions formed primarily by the condensation of tungstates or molybdates with or without the involvement of other elements. Among POMs, there are three main structures, known as Keggin, Anderson–Evans, and Wells–Dawson. Anderson-type POMs have a general formula of [H_y(XO₆)M₆O₁₈]ⁿ⁻ (y = 0–6, n = 2–8, M = Mo, X = heteroatom). They have a flat structure consisting of an XO₆ central octahedral structural unit surrounded by six MO₆ octahedral structural units that share edges. These compounds can incorporate a great variety of central heteroatoms, which has a significant impact on their electronic and catalytic properties [23,24]. As POMs can act as mediators in the dynamics of photocatalytic electron transfer from the TiO₂ conduction band, they can form a heterostructure with TiO₂ for photocatalytic applications. Liu et al. modified TiO₂ with Anderson-type POM through the sol–hydrothermal method. They reported that the system was 100% effective in the photooxidation of dibenzothiophene under visible irradiation [25]. Tang et al. reported improvement in the photocatalytic removal efficiency of bisphenol A after the modification of TiO₂ with Fe-tungstates, showing a 100% bisphenol A removal as the best result [26]. Recently, Diaz et al. reported the synergic effect of ZnPOM and CuPOM anchored to TiO₂ thin films in the removal of MB from aqueous solutions, showing 82% MB removal as the best result [27].

In this work, TiO₂ composites with three Anderson-type POMs (Fe, Co, and Ni) were fabricated. All materials were studied in the MB removal and degradation under UV irradiation.

2. Materials and Methods

2.1. Synthesis and Characterization

In the synthesis of TiO₂, first, titanium isopropoxide (TTIP) (0.030 mol) and 2-propanol (0.150 mol) were mixed. Next, distilled water (550 μL) was added to the resulting mixture, and it was stirred at 200 rpm for 2 h. Then, 150 μL of 3 M HCl was added to adjust the pH to 5.5. After that, the resulting mixture was heated in a stainless steel reactor at 200 °C for 2 h [28]. The Anderson-type POMs with heteroatoms (Fe³⁺, Co²⁺, and Ni²⁺) were synthesized by the co-precipitation method using (NH₄)₆Mo₇O₂₄•4H₂O (0.180 mol) and the heteroatom salt (0.030 mol in each case), Fe(NO₃)₃•9H₂O (Merck ≥ 99%), Co(NO₃)₂•6H₂O (Merck ≥ 99%), and Ni(NO₃)₂•6H₂O (Merck ≥ 99%) [29]. The (NH₄)_6−n[XMo₆O₂₄H₆]^{− 6+n} were obtained, where X presents the heteroatom and n is the charge of the heteroatom X. The POM-modified TiO₂ composites were fabricated, incorporating the POMs during the TiO₂ synthesis. To generate the POM/TiO₂ composite, POMs (0.011 mol) were added to the mixture of TTIP (0.015 mol) and 2-propanol (0.077 mol), and the same procedure described above to synthesize TiO₂ was followed. After composites were obtained, the thin films were fabricated using the Doctor Blade method [30,31]. The thickness of the TiO₂ thin films was between 5.5 and 6.7 μm. The thickness of the films was determined by a cross-section SEM ×8000 image (see Figure S2 in the Supplementary Materials).

Spectroscopic properties were determined by Fourier transform infrared spectroscopy on a Nicolet Summer FT-IR spectrometer (Waltham, MA, USA), in the region of 4000 cm⁻¹ to 400 cm⁻¹ with a resolution of 4 cm⁻¹. Furthermore, the synthesized films were studied through Raman spectroscopy analysis using a Raman spectrometer (Thermo Scientific DXR3, Waltham, MA, USA, device equipped with a 780 nm laser). Morphology and elemental surface assay were carried out by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX), and the samples were analyzed using the SEM instrument model QUANTA FEG 650 with operation at an acceleration potential of 25.0 kV with a magnification of ×5000. Furthermore, the pore area size for each material was then studied with the ImageJ software v. 1.45 [32]. Finally, optical thin film properties were determined by diffuse reflectance using a Perkin Elmer Lambda 4 spectrometer (Waltham, MA, USA) equipped with an integrating sphere.

2.2. Adsorption and Photocatalytic Study

The adsorption properties of the composites in the removal of methylene blue (MB) from aqueous samples was studied. The adsorption of MB on the composites was carried out in darkness. A solution of MB (100 mL; 20 ppm) was mixed with the composite, and the mixture was stirred for 1 h. The kinetics of the process was followed by spectrophotometry every 5 min (λ = 665 nm). The photocatalytic study was carried out in a batch reactor using a PHILIPS tubular lamp as a UV radiation source (arc length 161 mm, tube diameter 16 mm, 7 W, 15 μW/cm² at 1 m). The lamp (mercury/argon) has a strong band between 240 nm and 270 nm with a maximum emission of 260 nm. Before irradiation, adsorption–desorption equilibrium was reached. The system was irradiated for 2 h, and every 5 min, a sample was extracted to determine the MB concentration.

3. Results and Discussion

3.1. FTIR Characterization

Figure 1 shows the FTIR spectra of the materials fabricated in this work. TiO₂ shows a characteristic band between the range 600–900 cm⁻¹; this band is attributed to the Ti-O bond stretching. Another band in the region of 3000–2400 cm⁻¹ was observed related to the hydroxyl groups (-OH) of water adsorbed on the TiO₂ surface [33]. The FePOM/TiO₂ composite showed bands at 950–1400 cm⁻¹ corresponding to the bonds (Mo=O), (Ti-O-Ti), (Fe-Mo=O), and (Ti-O-Mo), indicating the adsorption of the POM. Hydroxyl group bands were also observed at 2340–2400 cm⁻¹. These changes, which include shifts and variations in band intensity, suggest the formation of new bonds such as (Mo=O), (-OH), and (Fe-O=Mo). For CoPOM/TiO₂, bands in the range 500–1600 cm⁻¹ were identified and attributed to bond interactions (-NH), (Co-O=Mo), (Ti-O-Ti), and (Mo=O). Finally, for the NiPOM/TiO₂, the bands located at 800–1650 cm⁻¹ were associated with the bonds (Mo=O), (Ti-O-Ti), (-NH), and (Ni-Mo=O), respectively [34].

3.2. Raman Characterization

Figure 2 shows the vibrational modes of the thin-film TiO₂ Raman spectra for FePOM/TiO₂, CoPOM/TiO₂, and NiPOM/TiO₂. All the spectra were normalized from 0 to 1 for comparative purposes. Bare TiO₂ films report six active modes in Raman spectroscopy: A_1g + 2B_1g + 3E_1g [35]. Figure 2 shows five of the six expected vibrational modes located at 148 cm⁻¹ (E_g), 200 cm⁻¹ (Eg₍₁₎), 400 cm⁻¹ (B_1g), 522 cm⁻¹ (A_1g), and 645 cm⁻¹ (Eg₍₁₎). The presence of a strong and dominant Raman mode at 148 cm⁻¹ is indicative that the TiO₂ films were in the anatase crystalline and no other peaks of rutile phase [36]. The Raman modes at 645 cm⁻¹ (E_g(1)) and 522 cm⁻¹ (A_1g, B_1g) were the Ti-O stretch vibrations, and the modes at 400 cm⁻¹ (B_1g), 200 cm⁻¹ (E_g(1)), and 148 cm⁻¹ (E_g) were the Ti-O bending vibrations of the TiO₂. All of the modified films showed at least four of the six characteristic vibrational modes of TiO₂. No one spectrum showed signals of other phases or compounds, suggesting that TiO₂ was the main compound of the modified catalysts. This is in accordance with the low atomic percentages of Mo and Co, Ni, and Fe elements determined by EDX assay (see Section 3.4).

3.3. Optical Characterization

Figure 3a shows the thin-film diffuse reflectance spectra of TiO₂, FePOM/TiO₂, CoPOM/TiO₂, and NiPOM/TiO₂. All the spectra were normalized from 0 to 100 for comparative purposes. The spectrum obtained for TiO₂ showed a minimum reflectance close to 370 nm in length; this result is in agreement with previous reports in the literature for this semiconductor in the anatase phase. Figure 3 shows a significant shift in the minimum reflectance value for POM-modified films compared to bare TiO₂, reaching minimum reflectance values for wavelengths of 350–380 nm. This change in spectral response is associated with the interaction of semiconductor bands. Modified thin films show Urbach-like tails associated with a sub-band gap energy state due to the presence of defects as a result of the interface interaction between both semiconductors [37].

From the diffuse reflectance spectra shown in Figure 3, the value of the bandgap energy was calculated using the Kubelka–Munk remission function [38]:

$${\displaystyle \frac{k}{s}}=F\left(R_{\infty }\right)={\displaystyle \frac{{\left(1-R_{\infty }\right)}^2}{2R_{\infty }}}$$

(1)

where $R_{\infty }$ is the reflectance value of the material and F(R_∞) represents the ratio of the absorption and scattering coefficients (k/s); F(R_∞) is proportional to the absorption constant of the material. From Equation (1) and the curves shown in Figure 3, an analog to the Tauc graphs (F(R_∞)hv)^1/2was constructed against the photonic energy (eV), according to

$${\left(F\left(R_{\infty }\right)hv\right)}^{{\displaystyle \frac{1}{2}}}=A\left(hv-E_g\right)$$

(2)

Figure 3b shows the plots of (F(R_∞)hv)^1/2 vs. hv for the reflectance spectra shown in Figure 3a. The band gap was determined by extrapolating the linear portion of the graph on the x-axis after applying the baseline correction (see Figure 3b) [39]. Inside the figure, the baseline correction is shown (dotted line for each curve). Furthermore,

Table 1. Energy band gap values for the composites fabricated in this work.

Compound	Energy Band Gap 1 (eV)	Pore Area (μm)
TiO2	3.22	210
FePOM/TiO2	3.17	900
CoPOM/TiO2	3.30	555
NiPOM/TiO2	3.41	351

¹ Values were obtained from Figure 3b.

lists the results of the band gap for each composite. TiO₂ had a band gap of 3.22 eV, which is consistent with reports found in the literature. Table 1 shows changes in the value of the forbidden energy gap for POM/TiO₂ systems. The largest change was observed for the FePOM/TiO₂ system, which reported a value of 3.05 eV. The results show that the presence of a species such as POM adsorbed on the TiO₂ changed the optical properties of the material. In the case of FePOM/TiO₂, the composite showed a small reduction in the band gap value. This behavior is associated with the interaction of the TiO₂ electron bands with the bands of the POMs [40]. The presence of the POMs might create oxygen vacancies (O_v) that could generate intra-gap states reducing the TiO₂ band gap, thus improving light absorption at longer wavelengths. Furthermore, in Anderson-type POMs, the heteroatom plays an important role in the band gap value between the conduction and valence of the semiconductor [41]. For the samples CoPOM/TiO₂ and NiPOM/TiO₂, there was a little increase in the bad gap value compared to bare TiO₂ (see Table 1). A similar trend was reported by Singh et al. They reported that the increase in the band gap value followed a regular trend in W-based POMs after changing the central metal from left to right in the periodic table (Mo, Co, Ni, Zn) [42]. Finally, the presence of the POM in the composite can facilitate the dynamics of the transfer of photogenerated charge carriers, reducing recombination processes and assisting the photocatalytic processes [43].

3.4. Morphological Characterization

Figure 4 shows the SEM images of the fabricated composites. The SEM images show that the coatings presented a heterogeneous and structurally complex morphology. In the case of TiO₂, Figure 4a shows that TiO₂ had aggregated particles of irregular shapes and varied sizes. The material presented empty spaces between a cluster of particles in certain regions, presenting a porous texture. The morphological composition of the material showed the characteristic conformation of mesoporous TiO₂—this is an expected result for TiO₂ synthesized by the sol–gel method. When measuring the 300-pore area of the material, TiO₂ showed a normal-type distribution to the pore area value. This thin film showed a pore area of 0.210 μm². Table 1 lists the pore area for each material by using ImageJ software.

In the case of CoPOM/TiO₂, the SEM image shows the aggregates of material particles with different sizes. Furthermore, their aggregates had both greater uniformity and greater porosity compared with bare TiO₂ thin films. For the FePOM/TiO₂ films, Figure 4c shows the formation of several aggregates that differed greatly in size. Furthermore, FePOM/TiO₂ film had larger cavities than bare TiO₂. Finally, the NiPOM/TiO₂ thin film shows the formation of aggregates of particles of different sizes with a porosity similar to the CoPOM material. According to the results of the EDX assay (see Figure S1 in the Supplementary Materials), the atomic ratios between the heteroatoms and the Mo (Mo/X) for each of the POMs in the films were close to 6 (see

Table 2. Quantitative results of the EDX assay for POM/TiO₂ thin films fabricated in this work.

Element	CoPOM/TiO2	NiPOM/TiO2	FePOM/TiO2
	Atomic (%) 1	Atomic (%)	Atomic (%)
O	78.01	64.1	71.1
Ti	8.13	24.90	11.3
Mo	0.28	1.19	0.69
Co	0.05	0.0	0.0
Ni	0.0	0.2	0.0
Fe	0.0	0.0	0.13
C	12.8	7.8	12.2

¹ Values were obtained from Figure S1.

), and all values were close to the theoretical value.

3.5. Removal and Degradation Study

Before carrying out the photodegradation tests, the system (contaminant and photocatalyst) was allowed to reach the adsorption–desorption equilibrium. In this work, we measured the amount of contaminant removed from the solution during this stage and compared it with the amount of contaminant removed during the photodegradation process. Figure 5a shows the percentage results of MB removal by adsorption and by photodegradation for each used material. These results show an improvement in the removal efficiency of MB for the POM/TiO₂ materials compared to bare TiO₂. Furthermore, Figure 5a shows the stability test for the synthesized materials. Conventionally, the adsorption–desorption equilibrium stage was not studied in detail, whereas only the system was kept under agitation for 60 or 90 min before the photo-degradation process began. Figure 5a indicates that the MB removed from the solution during this stage can vary between 5 and 20%. After three consecutive cycles, the TiO₂ films and the modified materials did not significantly change its adsorption capacity. This could be associated with the great stability of the TiO₂ thin film.

Most of the studies reported in the field of photocatalysis used catalyst as powders in suspension. In these systems, it is always necessary to add a separation stage to recover the photocatalyst. The great advantage of using the photocatalyst as a thin film is that the additional steps of catalyst recovery are avoided (which may require additional time and energy), allowing it to be reused in new photo-degradation processes [44,45]. Furthermore, some reports show the viability of thin films over powder semiconductors as photocatalysis. Thus, the immobilization of the catalysts as thin films allows for better long-term performance, simply tuning its material properties and providing miniaturization of the devices [46]. In the case of POM/TiO₂ composites, the stability test (Figure 6a) showed a decrease in adsorption, with differences on the first and third cycles from 10% and 50%, suggesting that the composites are moderately stable for MB removal by means of the adsorption process.

We applied the Langmuir–Hinshelwood (L-H) kinetic model to determine the kinetic constant of the process. This model is expressed by the following equation [47,48]:

$$\left[MB\right]=\left[{MB}_0\right]e^{-kt}$$

(3)

where [MB] corresponds to the concentration of methylene blue as a function of time, [MB_o] is the initial methylene blue concentration, and k is the kinetic constant of the degradation process. Figure 6b and

Table 3. Kinetic fitting applying the L-H kinetic model ¹.

Compound	Photocatalytic Efficiency (%)	k (min−1) × 10−4	R2	k/kTiO2
TiO2	18.3	6.5	0.972	-
FePOM/TiO2	46.2	22	0.969	3.4
CoPOM/TiO2	50.1	24	0.988	3.7
NiPOM/TiO2	49.4	23	0.975	3.5

¹ Values were obtained from Figure 6b.

list the fitting results. As can be observed, the modified catalysts greatly improved the rate constant value of the photodegradation process. CoPOM/TiO₂ and NiPOM/TiO₂ thin films showed the best results after 300 min of irradiation. The best test showed a k value of 24 × 10⁻⁴ min⁻¹, with this value being 3.5 times higher than the value obtained for bare TiO₂ thin films (6.5 × 10⁻⁴ min⁻¹, see Table 3). Figure 6a shows the photocatalytic behavior of the composites fabricated in the degradation of MB under UV irradiation. The results shown in this figure are comparable with other previously reported studies (see

Table 4. Photocatalytic efficiency of different semiconductors employed as photo-catalysts under UV irradiation.

Semiconductor	Pollutant/Time Irradiation	Degradation Efficiency (%)
TiO2-chitosan/TF ¨ [51]	Phenol/300 min	35
ZnO:Co/TF [52]	CV +*/210 min	90
Cu:TiO2/reduced GO */TF [53]	MB **/180 min	63
Bi2VO5.5/Bi2O3/TF [54]	MB/300 min	90
BCTi +/powder [55]	MB/180 min	90
CTi ++/powder [56]	MB/120 min	98
CoPOM/TiO2 (this work)	MB/300 min	50.1

¨ Thin film, ⁺* crystal violet, * graphene oxide, ** methylene blue, ⁺ brookite–rutile TiO₂, ⁺⁺ carbonaceous TiO₂.

). According to the optical results, the Eg values did not change significantly compared with TiO₂. These results suggest that significant improvements in the photocatalytic activity of the POM/TiO₂ systems can be associated with the redox properties of the POMs. Due to their redox properties, the POMs can act as electron acceptors, reducing the recombination of the charge carriers and favoring the photocatalytic processes [49,50].

After the composite synthesis, a heterojunction might be formed between the POM and the TiO₂. The difference between the redox potential of the POMs and the TiO₂ conduction bands (CB) might assist the charge carrier separation [57]. Thus, the presence of this heterojunction might reduce the charge carrier recombination process after the UV irradiation, assisting the ROS generation and finally also assisting the photocatalytic process [58]. Despite both compounds being only active under UV irradiation, the heterojunction might improve the photocatalytic efficiency of the composite [59]. Finally, the stability tests of the POM/TiO₂ composites in their photocatalytic activity showed a moderate diminution after the third photocatalytic cycle. In the case of the FePOM/TiO₂ and CoPOM/TiO₂ films, a larger stability was observed, showing photodegradation reductions of 14% and 18%, respectively. However, the NiPOM/TiO₂ thin films reduced its photocatalytic activity 20% after the third photocatalytic cycle. This behavior might be related to the reduction in MB adsorption capacity after each cycle. Generally, it was observed that the MB adsorbed amount onto recycled POM/TiO₂ thin films was reduced gradually after each photocatalytic cycle, and the largest reduction was found for NiPOM/TiO₂ films.

4. Conclusions

In this work, we synthesized and characterized POM/TiO₂ thin-film composites. The spectroscopic results verified that the TiO₂ surface was modified by the incorporation of POMs. Morphological characterization indicated that the composite films corresponded to mesoporous materials. POM/TiO₂ systems showed a better performance in MB removal by adsorption. The removal capacities of the composites followed the following trend: CoPOM/TiO₂ (10%) > NiPOM/TiO₂ (19%) > FePOM/TiO₂ (14%) > TiO₂ (5.4%). On the other hand, the photocatalytic efficiency was 50.1% for CoPOM/TiO₂, 49.4% for NiPOM/TiO₂, 46.2% for FePOM/TiO₂, and 18.3% for TiO₂ thin films. These results were associated with a reduction in the recombination process by the presence of the POMs on the TiO₂ surface. The photodegradation kinetics of MB on the surface of the materials fit the pseudo-first-order model, and the material with the best performance was NiPOM/TiO₂, with a rate constant value of 6.2 × 10⁻³ min⁻¹, being significantly higher than the value obtained for the bare TiO₂ films. Finally, the interaction of the POMs with TiO₂ changed the morphological, optical, and adsorption properties of the semiconductor, generating a synergistic effect on the photocatalytic activity of the composites.