Influence of Metal Oxide Particles on Bandgap of 1D Photocatalysts Based on SrTiO3/PAN Fibers

This paper deals with the study of the optical properties of one-dimensional SrTiO3/PAN-based photocatalysts with the addition of metal oxide particles and the determination of their bandgaps. One-dimensional photocatalysts were obtained by the electrospinning method. Particles of metals such as iron, chromium, and copper were used as additives that are capable of improving the fibers’ photocatalytic properties based on SrTiO3/PAN. The optimal ratios of the solutions for the electrospinning of fibers based on SrTiO3/PAN with the addition of metal oxide particles were determined. The transmission and reflection of composite photocatalysts with metal oxide particles were measured in a wide range of spectra, from the ultraviolet region (185 nm) to near-infrared radiation (3600 nm), to determine the values of their bandgaps. Thus, the introduction of metal oxide particles resulted in a decrease in the bandgaps of the obtained composite photocatalysts compared to the initial SrTiO3/PAN (3.57 eV), with the following values: −3.11 eV for SrTiO3/PAN/Fe2O3, −2.84 eV for SrTiO3/PAN/CuO, and −2.89 eV for SrTiO3/PAN/Cr2O3. The obtained composite photocatalysts were tested for the production of hydrogen by the splitting of water–methanol mixtures under UV irradiation, and the following rates of hydrogen evolution were determined: 344.67 µmol h−1 g−1 for SrTiO3/PAN/Fe2O3, 398.93 µmol h−1 g−1 for SrTiO3/PAN/Cr2O3, and 420.82 µmol h−1 g−1 for SrTiO3/PAN/CuO.


Introduction
Photocatalysis is a well-researched method for renewable energy production in the forms of solar energy and high-purity chemical fuel (H 2 , CH 4 /CH 2 OH) [1,2]. Photocatalytic water splitting occurs upon the solar light irradiation of a semiconductor photocatalyst and results in the formation of hydrogen (H 2 ) and oxygen (O 2 ) [3]. An advantage over conventional energy sources, like fossil fuels, is the lack of carbon monoxide production, which, in light of ongoing climate change debates, is a great benefit for the environment [4].

Electrospinning of SrTiO 3 /PAN-Based Fibers with the Addition of Metal Oxide Particles
SrTiO 3 was obtained as described in [27]. A precursor for the electrospinning of fibers based on SrTiO 3 and metal oxide particles was prepared as follows: PAN was used to create the polymer solution by its dissolution in dimethylformamide under constant stirring for 30 min. Then, SrTiO 3 powder and FeCl 3 , Cr 2 (SO 4 ) 3 , or CuO were added to the polymer solution at different ratios and stirred until the mixture became homogeneous. The obtained suspension was used as a precursor for obtaining fibers based on SrTiO 3 with metal oxide particles by pulling under high voltage. Fiber electrospinning was carried out at room temperature with a voltage of 16 kV and a flow rate of 1.5 mL/h. The collector was located at a distance of 15 cm from the needle. Aluminum foil with a 20 cm diameter was used as a collector, which was replaced every 1.5 h throughout the entire process. The obtained fibers were then thermally stabilized at 185 • C for 15 min and calcined at 500 • C for 30 min in an argon medium. The resulting samples were designated as SrTiO 3 /PAN/Fe 2 O 3 , SrTiO 3 /PAN/CuO, and SrTiO 3 /PAN/Cr 2 O 3 depending on the added metal.

X-ray Diffraction Analysis of Samples
The X-ray diffraction (XRD) analysis was carried out on a Dron-4-type X-ray diffractometer (Omsk, Russian Federation) with a range of rotation angles for diffraction unit detection from −100 • to 168 • . The minimum step for moving the detection unit is 0.001 • . The permissible connection of the detection unit from a given rotation angle is ± 0.015 • . The transport rate of the goniometer is 820 • /min. The main error in measuring the pulse count of the X-ray measurement was not more than 0.4%.

Scanning Electron Microscope Characterization of the Surface Morphology of Samples
The surface morphology of the obtained photocatalytic fibers was studied using a Quanta 3D 200i (Waltham, MA, USA) scanning electron microscope (SEM) under an accelerating 15 kV voltage.

Measurement of the Transmission and Reflection of SrTiO 3 /PAN Fibers with the Addition of Metal Oxide
Particles in a Wide Spectral Region from Ultraviolet (185 nm) to Near-Infrared Radiation (3600 nm) The transmission and reflection measurements of photocatalytic fibers with the addition of metal oxide particles in a wide spectral region, from ultraviolet (185 nm) to near-infrared radiation (3600 nm), were carried out on a Shimadzu UV-3600 spectrophotometer (Moscow, Russian Federation) equipped with three detectors: a photoelectron multiplier for operation in the ultraviolet and visible spectral range, a semiconductor InGaAs, and cooled PbS detectors for near-infrared operation.

Investigation of the Activity of Photocatalysts Based on SrTiO 3 /PAN Fibers with the Addition of Metal Oxide Particles
The activity of photocatalysts based on SrTiO 3 /PAN fibers with the addition of metal oxide particles was tested by measuring the output of hydrogen during the water-methanol mixture splitting under UV radiation. The mixture, containing a photocatalyst, water, and methanol in different ratios, was loaded into a quartz tube reactor, which was previously purged with inert gas (argon) and exposed to UV irradiation with a wavelength of 320 nm and a power source of 40 W. As a result of the photocatalytic reaction of the water-methanol mixture splitting, the evolved mixture of gases was accumulated in a sealed sampler. The qualitative and quantitative composition of the evolved mixture of gases was analyzed by gas chromatography on a Chromos 1000 chromatograph (Dzershinsk, Russian Federation) with three packed 3 mm columns filled with NAX and PORAPAK Q phases, allowing for the identification of the leading gases: hydrogen, nitrogen, oxygen, carbon monoxide, and carbon dioxide.

The Synthesis of Fibers Based on SrTiO 3 /PAN with the Addition of Metal Oxide Particles and a Study of Their Physicochemical Properties
To achieve a highly efficient photocatalyst based on SrTiO 3 fibers, it is necessary to create its composites with metal oxide particles. The addition of metal oxide particles allows for a narrowing of the bandgap of SrTiO 3 , leading to the possible use of a wide spectrum of visible light. It also contributes to the improvement of redox reactions that occur during the absorption of light. The experimentally selected optimal ratios of the solution components for obtaining fibers with the required characteristics are 1:9:2:88 SrTiO 3 :PAN:FeCl 3 :solvent, 0.5:10:1.5:88 SrTiO 3 :PAN:Cr 2 (SO 4 ) 3 :solvent, and 1.5:8:2.5:88 SrTiO 3 :PAN:CuO:solvent. Figure 1 presents SEM images of the obtained polymer fibers based on SrTiO 3 /PAN with metal oxide particles added. Polymer fibers based on SrTiO 3 /PAN with the addition of metal oxide particles have a continuous cylindrical shape without defects and are randomly arranged. The samples have the typical structure of fibers obtained by electrospinning, in which they are in contact with each other, forming a three-dimensional polymer network [28]. The average diameter of the obtained fibers is in the range from 200 to 400 nm, which is directly proportional to the viscosity of the solution used for electrospinning and the high voltage applied [29]. According to the SEM images, the presence of metal oxide particles and SrTiO 3 does not affect the morphological characteristics of the forming polymer fibers, which is also confirmed by the results obtained in [30], in which the effect of the composition of the electrospinning solution on the diameters of such fibers was studied. For all types of obtained fibers, the size of the agglomerates of SrTiO 3 and metal oxides ranges from 1 to 4 µm. have a continuous cylindrical shape without defects and are randomly arranged. The samples have the typical structure of fibers obtained by electrospinning, in which they are in contact with each other, forming a three-dimensional polymer network [28]. The average diameter of the obtained fibers is in the range from 200 to 400 nm, which is directly proportional to the viscosity of the solution used for electrospinning and the high voltage applied [29]. According to the SEM images, the presence of metal oxide particles and SrTiO3 does not affect the morphological characteristics of the forming polymer fibers, which is also confirmed by the results obtained in [30], in which the effect of the composition of the electrospinning solution on the diameters of such fibers was studied. For all types of obtained fibers, the size of the agglomerates of SrTiO3 and metal oxides ranges from 1 to 4 µm. The inclusion of metal oxides does not cause a corresponding change in the fiber diameter. On the one hand, the metal particle addition increases the viscosity of the solution, but on the other hand, this effect is balanced by an increase in the electrical conductivity of the initial solution, which contributes to the formation of thinner fibers due to an increase in the charge density on the electrospinning jet, and this, in turn, leads to an elongation of the jet along its axis [31,32].
To confirm the presence of metal oxide particles in the calcined fiber structure based on SrTiO3/PAN, an XRD analysis of the samples was performed. Figure 2 presents the X-ray diffraction patterns of calcined fibers based on SrTiO3/PAN and their composites with metal oxide particles.  The inclusion of metal oxides does not cause a corresponding change in the fiber diameter. On the one hand, the metal particle addition increases the viscosity of the solution, but on the other hand, this effect is balanced by an increase in the electrical conductivity of the initial solution, which contributes to the formation of thinner fibers due to an increase in the charge density on the electrospinning jet, and this, in turn, leads to an elongation of the jet along its axis [31,32].
To confirm the presence of metal oxide particles in the calcined fiber structure based on SrTiO 3 /PAN, an XRD analysis of the samples was performed. Figure 2 presents the X-ray diffraction patterns of calcined fibers based on SrTiO 3 /PAN and their composites with metal oxide particles.

Investigation of the Transmission and Reflection Spectra of the Obtained Photocatalytic Fibers
The photocatalysis mechanism of SrTiO3 is based on the formation of electron-hole pairs under UV irradiation, where they have sufficiently high energy for the formation of radicals with a high oxidation ability. To study the possible use of a wider spectrum, including visible light, for SrTiO3/PAN-based photocatalysts with the addition of metal oxide particles, their transmission and reflection spectra were determined. Analysis of the transmission spectra allows for the calculation of the bandgap of SrTiO3-based photocatalysts with the addition of metal oxides. For crystalline semiconductors, the following equation is valid for the relationship between the absorption coefficient and the incident photon's energy: where Egap is the optical bandgap, B is a constant, hϑ is incident photon energy, and α(υ) is an absorption coefficient, which is in accordance with the law of Beer-Lambert, equal to where d is film thickness and Ab(λ) is the film absorption coefficient. For a more accurate determination of α, a correction that accounts for the reflection spectrum for the absorption coefficient must be made. To calculate the bandgap of SrTiO3/PAN-based photocatalysts with the addition of metal oxide particles, it is necessary to rewrite Equation (1): where λg is the wavelength corresponding to the bandgap, h is the Planck constant, and c is the speed of light. Using the Beer-Lambert law, Equation (3) can be rewritten as follows: where B1 is the constant that takes into account the reflection spectrum.

Investigation of the Transmission and Reflection Spectra of the Obtained Photocatalytic Fibers
The photocatalysis mechanism of SrTiO 3 is based on the formation of electron-hole pairs under UV irradiation, where they have sufficiently high energy for the formation of radicals with a high oxidation ability. To study the possible use of a wider spectrum, including visible light, for SrTiO 3 /PAN-based photocatalysts with the addition of metal oxide particles, their transmission and reflection spectra were determined. Analysis of the transmission spectra allows for the calculation of the bandgap of SrTiO 3 -based photocatalysts with the addition of metal oxides. For crystalline semiconductors, the following equation is valid for the relationship between the absorption coefficient and the incident photon's energy: where E gap is the optical bandgap, B is a constant, hϑ is incident photon energy, and α(υ) is an absorption coefficient, which is in accordance with the law of Beer-Lambert, equal to where d is film thickness and Ab(λ) is the film absorption coefficient. For a more accurate determination of α, a correction that accounts for the reflection spectrum for the absorption coefficient must be made. To calculate the bandgap of SrTiO 3 /PAN-based photocatalysts with the addition of metal oxide particles, it is necessary to rewrite Equation (1): Nanomaterials 2020, 10, 1734 6 of 9 where λ g is the wavelength corresponding to the bandgap, h is the Planck constant, and c is the speed of light. Using the Beer-Lambert law, Equation (3) can be rewritten as follows: where B 1 is the constant that takes into account the reflection spectrum. Using Equation (4), the optical bandgap can be calculated by fitting the absorption spectrum without considering the film thickness. To determine the bandgap (E gap ), the absorption coefficient's dependence on the incident radiation energy was plotted and a linear approximation was carried out. Figure 3 presents the absorption and reflection spectra ( Figure 3a) and a graph of the absorption coefficient's dependence on the incident radiation energy for SrTiO 3 /PAN-based photocatalysts with the addition of metal oxide particles (Figure 3b).
Using Equation (4), the optical bandgap can be calculated by fitting the absorption spectrum without considering the film thickness. To determine the bandgap (Egap), the absorption coefficient's dependence on the incident radiation energy was plotted and a linear approximation was carried out. Figure 3 presents the absorption and reflection spectra ( Figure 3a) and a graph of the absorption coefficient's dependence on the incident radiation energy for SrTiO3/PAN-based photocatalysts with the addition of metal oxide particles (Figure 3b).
As a result, the bandgap for SrTiO3/PAN-based photocatalysts with Cr2O3 particles added is determined to be 2.89 eV (Figure 3b, red line). Thus, it is found that the addition of Cr2O particles to SrTiO3-based fibers narrows the bandgap to 2.89 eV, making it possible to use a wide radiation spectrum. The determined value of the bandgap for a SrTiO3/PAN-based photocatalyst with Cr2O3 particles added can be explained by the occupied level of Cr 3+ cations, which is 1.0 eV above the valence band. The bandgap of the SrTiO3/PAN-based photocatalyst with CuO particles added is 2.84 eV (Figure 3b, blue line). The determined bandgap of the SrTiO3/PAN-based photocatalyst with the addition of Fe2O3 particles is 3.11 eV (Figure 3b, black line). A high bandgap value for a photocatalyst with Fe2O3 particles added is associated with a low Fe2O3 content in calcined fibers, confirmed by XRD (Figure 1b, black line). In turn, the calculated bandgap of the photocatalyst based on the initial SrTiO3/PAN fibers without metal oxide particles added is 3.57 eV.

Investigation of the Activity of Photocatalysts Based on SrTiO3/PAN Fibers with the Addition of Metal Oxide Particles by the Output of Hydrogen during the Splitting of Water-Methanol Mixture
After determining the bandgaps of photocatalysts based on SrTiO3/PAN fibers with the addition of metal oxide particles, their photocatalytic efficiencies in the splitting of water-methanol mixtures with the production of hydrogen were studied. As seen in Table 1, the composition of the photocatalyst significantly affects the efficiency of hydrogen evolution. As reported in previous work [26], the photocatalytic hydrogen evolution rate from the splitting of the water-methanol mixture using SrTiO3/PAN-based fibers at a 40W UV radiation is 305.96 µmol/h. In turn, the results of the measurement of the average rate of hydrogen evolution during the splitting of the water-methanol mixture at 400W UV irradiation of the composite SrTiO3/PAN-based fibers with the addition of metal oxides are the following: 344.67 µmol/h for the SrTiO3/PAN/Fe2O3 photocatalyst, 398.93 µmol/h for the SrTiO3/PAN/Cr2O3 photocatalyst, and 420.82 µmol/h for the SrTiO3/PAN/CuO photocatalyst. The higher rates of hydrogen evolution for photocatalysts based on SrTiO3/PAN fibers with metal oxides added are explained by the fact that the optical, thermal, and electrochemical properties of metal oxide particles, which also highly depend on their sizes, allow not only for a narrowing of the bandgap of the semiconductors used as a photocatalyst but also for an improvement of their ultraviolet absorption ability. After determining the bandgaps of photocatalysts based on SrTiO 3 /PAN fibers with the addition of metal oxide particles, their photocatalytic efficiencies in the splitting of water-methanol mixtures with the production of hydrogen were studied. As seen in Table 1, the composition of the photocatalyst significantly affects the efficiency of hydrogen evolution. As reported in previous work [26], the photocatalytic hydrogen evolution rate from the splitting of the water-methanol mixture using SrTiO 3 /PAN-based fibers at a 40W UV radiation is 305.96 µmol/h. In turn, the results of the measurement of the average rate of hydrogen evolution during the splitting of the water-methanol mixture at 400W UV irradiation of the composite SrTiO 3 /PAN-based fibers with the addition of metal oxides are the following: 344.67 µmol/h for the SrTiO 3 /PAN/Fe 2 O 3 photocatalyst, 398.93 µmol/h for the SrTiO 3 /PAN/Cr 2 O 3 photocatalyst, and 420.82 µmol/h for the SrTiO 3 /PAN/CuO photocatalyst. The higher rates of hydrogen evolution for photocatalysts based on SrTiO 3 /PAN fibers with metal oxides added are explained by the fact that the optical, thermal, and electrochemical properties of metal oxide particles, which also highly depend on their sizes, allow not only for a narrowing of the bandgap of the semiconductors used as a photocatalyst but also for an improvement of their ultraviolet absorption ability. A comparison of the photocatalytic activity of different photocatalysts for hydrogen production shows that the composition of the photocatalyst and the type and intensity of irradiation significantly influence the intensity of hydrogen evolution. The rate of photocatalytic hydrogen evolution from a water-organic alcohol mixture using SrTiO 3 /PAN-based fibers with the addition of metal oxide particles during UV radiation with 40 W power is several times higher than that of reference analogs [32][33][34][35][36]. Moreover, irradiation with a power of 300 to 400 W was used in these works, which is not economically profitable, in contrast to lamps with a power of 40 W. In [34], TiO 2 doped with platinum particles was used as a photocatalyst. The power of ultraviolet radiation was 125 W, and the rate of photocatalytic hydrogen evolution using this photocatalyst with platinum was 523.71 µmol/h, which is still comparable to the hydrogen evolution rate for the SrTiO 3 /PAN/CuO-based photocatalyst, while the cost and applied irradiation power required to conduct photocatalysis are much lower.

Conclusions
One-dimensional photocatalysts based on SrTiO 3 /PAN fibers with the addition of metal oxide particles were obtained by the electrospinning method. A study of the transmission and reflection spectra showed that the addition of Cr 2 O 3 , CuO, and Fe 2 O 3 particles led to decreases in the bandgaps of SrTiO 3 /PAN-based photocatalysts to 2.89, 2.84, and 3.11 eV, respectively. As a result of the addition of metal oxide particles to the initial SrTiO 3 /PAN-based photocatalyst, the rate of hydrogen evolution in the photocatalytic splitting of a water-methanol mixture increased to 344.67 µmol h −1 g −1 for the photocatalyst based on SrTiO 3 /PAN/Fe 2 O 3 , 398.93 µmol h −1 g −1 for the photocatalyst based on SrTiO 3 /PAN/Cr 2 O 3 , and 420.82 µmol h −1 g −1 for the photocatalyst based on SrTiO 3 /PAN/CuO. We believe that this article's proposed approach for increasing the efficiency of photocatalysts by the addition of non-expensive materials, allowing for a reduction in their bandgap, is a promising method for the further development of technologies for efficient solar hydrogen production.