Synthesis and Investigation of Na-Zeolite-Decorated Black TiO₂ Hybrid Material with Enhanced Properties

Abstract

Enhancing the photocatalytic efficiency of oxide materials under Vis light remains a significant challenge within the scientific community. Natural zeolite–metal oxide composites exhibit enhanced properties, especially due to the zeolite’s large active surface area, which facilitates the incorporation of metal oxide nanoparticles into its structure, thereby significantly increasing photocatalytic efficiency. The present study presents the synthesis of Na-zeolite-decorated black-TiO₂ by the impregnation method, in order to improve the structural characteristics to absorb into visible light. The experimental protocol involves two main steps: first, the synthesis of black-TiO₂ and white-TiO₂ nanocrystals using the sol-gel method, and second, the preparation of hybrid materials, consisting of Na-zeolite decorated with black-TiO₂ and white-TiO₂, through impregnation followed by thermal treatment. The morpho-structural and optical properties of the as-synthesized materials were investigated using XRD, SEM/EDX, FTIR, and DRUV-VIS analysis. The characterization results indicated that natural zeolite has a good thermal stabilization, the lamellar texture of natural zeolite and spherical form of anatase-TiO₂ materials being highlighted by SEM. In the case of Na-zeolite-decorated black-TiO₂, the adsorption edge is slightly shifted to the visible range, while Na-zeolite-decorated white-TiO₂ absorbs only in the ultraviolet region. The above results showed that these hybrid materials are adequate for application in photocatalytic processes.

1. Introduction

Industrial development in recent decades has brought many benefits to increasing the quality of life, but also many disadvantages, especially for environmental pollution. Water contamination with multiple harmful chemical agents causes serious disruptions in the functioning of ecosystems. Water contamination has numerous and extremely severe effects on human health, as is widely known and proven. In this context, there are significant issues about promoting several water decontamination techniques. In the last years, there has been great interest regarding titanium dioxide (TiO₂) since it has been considered the best semiconductor due to its exceptional photocatalytic activity, combined with its affordability, nontoxicity, and chemical stability. However, the wide electronic band gap of TiO₂, between 3.0 and 3.2 eV, restricts its optical absorption to the ultraviolet spectrum. This limitation significantly reduces its efficiency in capitalizing solar energy, resulting in utilization rates of less than 5% [1,2]. Thereby, to improve TiO₂’s optical absorption properties and photocatalytic activities, it is required to increase its surface area and number of active sites while also reducing its electron–hole recombination [3]. One of the most often used techniques for increasing the TiO₂ optical response in the visible light range is metal and non-metal doping. Recently, researchers underlined that metal doping in TiO₂ generated secondary impurities that decreased the crystallinity of TiO₂ and reduced the efficiency of the material’s photoactivity. In addition, non-metal TiO₂ doping exhibited minimal or no absorption in the solar spectrum’s infrared region [4]. Currently, there is significant interest in the hydrogen modification of TiO₂ to create various defect states within the band gap. This process results in a significant color change, commonly referred to as “black-TiO₂” [5]. Chen et al. reported for the first time, in 2011, that the hydrogenation of white-TiO₂ (W-TiO₂) to produce black-TiO₂ (B-TiO₂) could extend the light response into the visible and near-infrared regions. Further, this process enhanced visible light catalytic performance by introducing surface disorders in TiO₂ [6]. Many synthesis processes, including hydrogenation, aluminum reduction, and chemical reduction, have been developed to obtain B-TiO₂ nanomaterials. Thus, numerous researchers have used Al [7], Mg [8], Zn [9], H₂ [10], NaBH₄ [11], imidazole [12], and ascorbic acid [13] as reducing agents to convert W-TiO₂ to B-TiO₂. B-TiO₂ has garnered significant interest across different applications, such as photocatalytic pollutant degradation [14,15,16], hydrogen generation via water splitting [17,18], photocatalytic CO₂ reduction [19,20], supercapacitors [21,22], dye-sensitized solar cells [23], lithium-ion batteries [24,25], and various medical uses [26], achieving remarkable success. Research indicates that while W-TiO₂ is a photocatalyst primarily active in the UV range, B-TiO₂ exhibits a narrower band gap of approximately 3.0 eV, which is lower than that of W-TiO₂. This distinction arises from an increased presence of Ti³⁺ species and oxygen vacancies, enhancing photon energy absorption (below the direct band gap). Additionally, studies reveal that oxygen vacancies (Ov) and Ti³⁺ surface defects effectively reduce the band gap of TiO₂, facilitating charge carrier trapping and, consequently, extending the lifetime of electron–hole pairs [27].

Due to some issues such as TiO₂’s adsorption capacity and high aggregation tendency, many researches have focused on supporting TiO₂ nanoparticles on various materials such as activated carbon [28,29], clay [30,31], zeolite [32,33], and silica [34,35,36]. Numerous studies have demonstrated that these supports exhibit excellent pollutant adsorption and diffusion capabilities, along with consistent pore and channel sizes, leading to superior photocatalytic performance [37,38,39]. Natural zeolites with uniform pore size, unique topology, and high structural stability are widely employed in a variety of catalytic/photocatalytic processes, as well as separation and purification techniques. Zeolites are well-known for their remarkable catalytic properties, but they cannot perform every desired reaction on their own, which is why they are frequently used as substrates for metal nanoparticles to increase functionality. Therefore, zeolites are commonly used to support various metal particles, improving their functionality [40]. One of the most important characteristics of natural zeolite is the ion exchange property, which allows the resultant hybrid materials to be used in numerous applications such as catalytic/photocatalytic, medical, antibacterial, and so on. TiO₂ supported on zeolite induces a synergetic effect because of the photocatalytic activity of TiO₂ and the adsorption properties of zeolite, resulting in an enhanced photocatalytic efficiency. Natural minerals like zeolite and montmorillonite have long been utilized to manufacture adsorption–photocatalyst hybrid materials, owing to their large surface area, excellent thermal and chemical stability, enhanced active surface, and ability to prevent catalyst aggregation [41]. Many studies have demonstrated that zeolite’s unique physicochemical properties make them highly efficient supports for metal oxides, with a high potential in photocatalytic applications [42,43,44]. As shown in our previous studies [37,38,45,46,47], composite materials composed of natural zeolite and white-TiO₂ display enhanced photocatalytic efficiency in degrading and mineralizing organic pollutants in water. Furthermore, in a recent study, M. R. Pour et al. obtained a functionalized commercial H-type zeolite with black-TiO₂ using the sono-deposition method. The obtained hybrid material was applied in photocatalytic processes for the degradation of azo pollutants (methylene blue) under visible light. The valuable results of this study highlighted the advantage of using composite materials with significantly enhanced photocatalytic properties in the degradation of organic pollutants present in wastewater [48]. Our prior research work has focused on synthesizing hybrid materials based on natural zeolite and undoped/doped TiO₂ using methods such as sol-gel [46], solid-state reactions under microwave-assisted hydrothermal conditions [37,45,47], and fast hydrothermal techniques [38].

Taking into account the previous results, the progress and novelty of this study consist in the development of new hybrid materials based on Romanian natural zeolite decorated with black-TiO₂ by the impregnation method, followed by thermal treatment in a controlled atmosphere containing a gas mixture of 98% Ar and 2% O₂. This research opens new paths for the synthesis of hybrid materials by an easy and reproducible impregnation approach, offering potential for the development of photocatalysts based on natural zeolite and TiO₂, applicable in catalytic or photocatalytic degradation processes.

2. Materials and Methods

2.1. Regents

Commercial reagents, such as titanium (IV) isopropoxide (TTIP, 98%), polyethylene glycol 400 (PEG400), and nitric acid (HNO₃), were purchased from the Sigma Aldrich Company (St. Louis, MO, USA) and used without any further treatment. The natural zeolite used for the synthesis of the hybrid material was Romanian Zeolitic Mineral from the Mirsid Area, supplied by the Cemacon Company, Salaj county, Romania. The mineral was powdered and sieved with a Multilab sieve shaker, with grain diameter sizes of about 315 and 500 µm and a mineralogical composition of clinoptilolite (SiO₂ 68.75%), volcanic glass (Al₂O₃ 11.35%), plagioclase (Fe₂O₃ 2.10%), SiO₂ (CaO 2.86%), and other minerals (MgO 1.18%, Na₂O+K₂O 3.99%, P.C. 9.77%).

2.2. Synthesis of TiO₂ Nanocrystals

TiO₂ nanocrystals were synthesized by the sol-gel method, where 0.5 g of PEG400 was dispersed in 30 mL of ethanol, and afterward under continuous stirring at 40 °C, 6 mL of TTIP was added. The pH of the solution was adjusted with HNO₃, and finally 1 mL of distilled water was added in drops to obtain the TiO₂ gel. Before thermal treatment, the gel was washed with distilled water and dried at 60 °C, for 7 h. In order to obtain white-TiO₂ (code name W-TiO₂), the sample was treated in a furnace (Nabertherm GmbH, Lilienthal, Germany) in an air atmosphere at 550 °C for 5 h. Black-TiO₂ (code name B-TiO₂) was synthesized under an identical temperature and time conditions using an oven tube (GSL1100X Tube Furnace Vacuum System, MTI Corporation, Richmond, CA, USA) with a controlled atmosphere of a gas mixture of 98% Ar and 2% O₂.

2.3. Synthesis of Hybrid Materials

Hybrid materials composed of natural zeolite and TiO₂ nanocrystals were successfully synthesized using the impregnation method. The Romanian natural zeolite, for a more efficient ion exchange, was chemically modified to the sodium form (Na-zeolite), as described in our previous paper [37]. Therefore, an amount of Na-zeolite was mixed with B-TiO₂, respectively, W-TiO₂ nanocrystals with a mass ratio of 1:0.5, in a 40 mL ethanolic solution under continuous stirring for 4 h. After mixing, both types of the hybrid materials, Na-zeolite decorated with B-TiO₂ (code name Na-zeolite@B-TiO₂) and Na-zeolite decorated with W-TiO₂ (code name Na-zeolite@W-TiO₂), were washed with distilled water and dried at a temperature of 60 °C for 6 h. The final stage comprised treatment at 500 °C for 5 h in air atmosphere conditions in the case of Na-zeolite@W-TiO₂, respectively, in a controlled atmosphere furnace of 98% Ar and 2% O₂ for Na-zeolite@B-TiO₂. A schematic representation of the synthesized materials is shown in Figure 1.

2.4. Characterization Methods

The structural properties of the as-synthesized materials were investigated using an X-ray diffraction spectrometer (XRD, PANalytical X’Pert PRO MPD Diffractometer, Almelo, The Netherlands) with Cu-Kα radiation (2theta = 20–60°). The Fourier transform infrared (FTIR) spectra were recorded in the range of 400–4000 cm⁻¹ for all samples in KBr pellet form with a JASCO FT/IR 430 spectrometer, and the optical properties were studied through UV-VIS diffuse reflectance spectroscopy (PerkinElmer Lambda 950 UV/Vis, Shelton, WA, USA) in the range of 300–800 nm. Morphological and elemental characterizations were achieved with a scanning electron microscope (SEM), the Inspect S PANalytical model using an Everhart Thornley Detector (ETD), coupled with an energy dispersive X-ray analysis detector (EDX) with a 30 kV voltage.

3. Results

Figure 2 shows the XRD patterns of as-synthesized materials. For B-TiO₂ and W-TiO₂ (Figure 2a), the patterns reveal the pure anatase phase of TiO₂ corresponding to specific peaks presented at 2theta: 25.3°, 37°, 37.8°, 38.6°, 48°, 54°, and 55.18° (JCPDS-01-078-2486) [49]. The X-ray patterns from Figure 2b show that the natural zeolite used in the experiment is mostly clinoptilolite, corresponding to peaks at 2theta: 10.00°, 22.5°, and 30.00° (JCPDS-00-025-1349) [49]. It can be seen that the main peak positions of natural zeolite (clinoptilolite) are unchanged, indicating that the structure of natural zeolite has a good thermal stabilization after TiO₂ loading. The presence of anatase TiO₂ peaks in the XRD spectra of the hybrid material certified that Na-zeolite was successfully functionalized with TiO₂ nanocrystals.

The absorption spectra via DRUV-Vis analysis of the as-synthesized materials are presented in Figure 3. Because the band gap determination is a crucial characterization for photocatalytic materials, as it plays a significant role in the generation of electron–hole pairs and, respectively, in light absorption, in Figure 3a is presented the spectra for Na-zeolite, Na-zeolite@B-TiO₂, and Na-zeolite@W-TiO₂ in the range of 300–800 nm. The band gap energy values were calculated, and a slight shift was observed due to the decoration with TiO₂ nanoparticles. Moreover, the absorbance spectra for B-TiO₂ (Figure 3b) and W-TiO₂ (Figure 3c) particles are presented, and based on Kubelka–Munk theory, the Eg optical band gap energy derived from the intersection of the straight line with the hν-axis was calculated to be about 1.80 eV for B-TiO₂ (inset of Figure 3b) and 3.56 eV for W-TiO₂ (inset of Figure 3c). It is well-known that W-TiO₂ absorbs in the UV domain, an aspect certified also by the energy band gap of about 3.56 eV, while B-TiO₂ clearly absorbs in the Vis region [38]. According to M. R. Pour et al., the increased visible light absorption of TiO₂ can be attributed to the narrowing of the band gap caused by defect-induced energy states (Ov and Ti³⁺) near the top and bottom of the VBM and CBM, respectively [48].

Figure 4 shows the comparison of the FT-IR transmittance spectra of as-prepared TiO₂ nanocrystals (Figure 4a) and, respectively, for the Na-zeolite@B-TiO₂ and Na-zeolite@W-TiO₂ hybrid materials (Figure 4b). From Figure 4a, it can be observed that both samples, B-TiO₂ and W-TiO₂, exhibited similar absorption bands from 400 to 1200 cm⁻¹, the main characteristic features being the presence of a band at ~710 cm⁻¹ specific for the stretching vibrations of the Ti-O-Ti, and the peak at ~3418 cm⁻¹ attributed to hydroxyl and absorbed water [50]. Figure 4b elucidates that the mid-infrared spectrum, extended in the range of 400–2000 cm⁻¹, reveals the characteristic chemical bond specific to zeolite. Notably, the infrared peaks around ~1060 cm⁻¹ and ~1100 cm⁻¹ are related to vibrations of the Al-O bond and Si-O-Si asymmetric stretching. Compared to Na-zeolite, a new weak band at 960 cm⁻¹ was observed, which is generally attributed to Ti-O-Si vibration. Other researchers have reported that the band intensity is proportional to the amount of TiO₂ particles present in the zeolite framework, which are fixed on the surface of Na-zeolite with the force of a chemical bond [51].

The morphology of the synthesized materials, TiO₂ nanocrystals, and hybrid materials, Na-zeolite@B-TiO₂ and Na-zeolite@W-TiO₂, highlighted by SEM analysis and EDX spectra are depicted in Figure 5. As can be observed in Figure 5a, the representative morphology of clinoptilolite, a stacked lamellar structure, is observed for both the hybrid materials, Na-zeolite@W-TiO₂ and Na-zeolite@B-TiO₂. The surface morphology of the W-TiO₂ and B-TiO₂ samples synthesized by the sol-gel method (Figure 5b,c) reveals that the particles have a spherical shape with a good dimensional dispersion. Spherical TiO₂ nanoparticles attached to the surface of Na-zeolite and exhibited obvious agglomerates due to the high-temperature treatment. Using the Debye–Scherrer formula [52], the average crystallite size was calculated to be about 17.91 nm for W-TiO₂, with a slightly lower value of 12.87 nm for B-TiO₂ materials. From the images presented in Figure 5d,e, it can be seen that the W-TiO₂ and B-TiO₂ particles were successfully incorporated into the natural zeolite support, obtaining the Na-zeolite@W-TiO₂ and Na-zeolite@B-TiO₂ hybrid materials. The morphology of zeolite remains unchanged, and the TiO₂ particles were distributed over a large surface of the natural zeolite forming conglomerates; the place of TiO₂ on the zeolite surface being marked within the images. These aspects were also demonstrated by M. R. Pour et al. [48], who observed the same agglomeration of the TiO₂ particles on the surface of zeolite.

In addition, the heating treatment applied to the hybrid materials did not affect the structure of the Na-zeolite or the TiO₂ nanocrystals. To confirm the purity of the as-synthesized materials, EDX analysis was performed. From the spectra, only the specific elements Na, Si, Al, Ca, K, Mg, Ti, and O corresponding to Na-zeolite and TiO₂ particles were observed, this aspect confirming the presence of the TiO₂ on the hybrid material surface (Figure 5f). The EDX quantifications are presented in the Supplementary Materials as Figure S1. Moreover, the major peak of Ti at ~4.5 keV seen on the EDX spectrum for TiO₂ nanocrystals represents the binding energy for the Ti⁴⁺ oxidation present in the anatase-TiO₂ phases, which confirms the XRD analysis for TiO₂.

4. Conclusions

The new as-synthesized hybrid materials, Na-zeolite@B-TiO₂ and Na-zeolite@W-TiO₂,were successfully prepared by decorating B-TiO₂ and W-TiO₂ on a Na-zeolite surface via the impregnation method. Firstly, B-TiO₂ and W-TiO₂ were synthesized by the sol-gel method in this study, and according to the XRD and EDX analysis, only the pure anatase form was presented as the major crystalline phase. Moreover, the Debye–Scherrer formula was used to calculated the average crystallite size, and the obtained dimensions were about 17.91 nm for W-TiO₂ and 12.87 nm for the B-TiO₂ materials. FT-IR spectroscopy was used to demonstrate the chemical bond between TiO₂ and natural zeolite through the detection of a weak absorption peak near 960 cm⁻¹, which was assigned to the asymmetrical stretching vibration of the Ti-O-Si bond. Also, the absorption edge of Na-zeolite@B-TiO₂ was slightly shifted to the visible range, with the optical band gap energy calculated to be about 1.80 eV for B-TiO₂ and 3.56 eV for W-TiO₂. The morphology investigations presented a spherical shape with a good dimensional dispersion of the TiO₂ samples, highly agglomerated, embedded on the natural zeolite support. Based on the morphological, structural, and optical characterization results, these hybrid materials based on natural zeolite decorated with black-TiO₂ offer new perspectives for applications as photocatalysts in catalytic or photocatalytic processes for water treatment.