Preparation of Bi@Ho3+:TiO2/Composite Fiber Photocatalytic Materials and Hydrogen Production via Visible Light Decomposition of Water

Abstract

Using electrospun nanofibers doped with TiO₂ and rare-earth ion Ho³+ as the matrix, and sodium gluconate as the reducing agent, Bi(NO₃)₃ was reduced using hydrothermal technology to produce Bi@Ho³⁺:TiO₂ composite fiber materials. The materials’ phase, morphology, and photoelectric properties were characterized using various analytical testing methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), and transient photocurrent (IP). During the hydrothermal process, it was confirmed that Bi³⁺ was reduced by sodium gluconate to form pure Bi nanoparticles, which combined with Ho³⁺:TiO₂ nanofibers to form heterojunctions. By leveraging the surface plasmon resonance (SPR) effect of metallic Bi and the abundant energy level structure and 4f electron transition properties of rare-earth Ho³⁺, the TiO₂ nanofibers underwent dual modification, effectively enhancing the photocatalytic activity and stability of TiO₂. Under visible light irradiation, the rate of hydrogen production through water decomposition reached 43.6 μmol·g⁻¹·h⁻¹.

1. Introduction

With the development of modern industry, environmental pollution and the energy crisis have become global challenges faced by all countries. Semiconductor photocatalysis, utilized to split water into clean and pollution-free hydrogen energy, is a cutting-edge technology with great development potential. TiO₂ has become the first and most widely used semiconductor photocatalytic material due to its stability, non-toxicity, acid and alkali resistance, lack of photodissolution or photocorrosion, and high availability at a low cost. However, its practical applications are limited by the high recombination rate of photogenerated electron–hole pairs and the excessively high activation energy of the hydrogen production reaction. To improve the photocatalytic activity of TiO₂, some researchers have broadened its spectral response range through ion doping or photosensitization, while others have improved the separation efficiency of photogenerated carriers and inhibited the recombination of photogenerated electrons and holes by means of semiconductor composites and noble metal deposition.

The introduction of surface plasmon resonance (SPR) as a novel optical field modulation technology into the field of photocatalysis, by adjusting factors such as the composition, morphology, size, and medium environment of metal nanoparticles, can help address the spectral response range of the catalytic system, thus opening up new directions and ideas for the development of efficient photocatalysts, and advancing the development of photocatalysis technology from a brand-new perspective [1,2,3,4]. For example, Kominami et al. [5] loaded Au nanospheres on the surface of TiO₂, which broadened the absorption spectrum of TiO₂ to 625 nm, and the apparent quantum efficiency reached 7.2%. Cheng et al. [6] modified g-C₃N₄ with gold nanoclusters, and the decomposition rate of aquatic hydrogen reached 230 μmol·g⁻¹·h⁻¹; Kumari et al. [7] explored the SPR effect of a single Ag nanoparticle, and reduced CO₂ under visible light irradiation to realize the recycling of carbon energy. At present, precious metals such as Au, Ag and Pt are the main metals studied as surface plasmon polaritons [8,9], and their high price and scarcity seriously limit their practical application in the field of photocatalysis. Therefore, it has become a research hotspot to find and develop non-precious metal plasmons with an SPR effect that are cheap and easy to obtain [10,11,12,13]. For example, Bi/g-C₃N₄ composites prepared by Dong et al. [14] showed excellent visible photocatalytic performance, and the mechanism of the enhancement of photocatalytic activity via SPR effect of non-noble metal Bi nanoparticles was put forward through a theoretical simulation of the Maxwell equation and a free radical capture experiment; Qu et al. [15] prepared carbon-coated Bi/Bi₂O₃ composites via the one-step hydrothermal method with sodium gluconate as a reducing agent, and the visible light degradation rate of methyl blue was 6.5 times higher than that of Bi₂O₃.

Rare-earth elements (REs) have a unique 4f electron orbital. Through the electron transition between f-f and f-d orbitals, they can absorb visible light with a larger wavelength and generate more highly active photogenerated electrons to make up for the low light absorption efficiency of the catalyst [16]. The changeable valence state (RE³⁺/RE⁴⁺) can change the migration path of photo-generated carriers, inhibit the recombination of photo-generated electron–hole pairs, and improve photocatalytic efficiency [17,18]. Therefore, doping with rare-earth ions has become an important method for modifying the photocatalytic performance of semiconductor oxides. Rare-earth ion doping has become an important means of photocatalytic modification of semiconductor oxides. Among them, the light-rare-earth ion doping represented by lanthanum and cerium is more studied [19,20], while the heavy-rare-earth ion doping represented by dysprosium and holmium is relatively less studied [21,22]. However, there have been no studies on Bi metal-modified rare-earth-ion-doped TiO₂ photocatalysis that has been reported.

In this work, nanofibers doped with rare-earth Ho³⁺ in TiO₂ (Ho³⁺:TiO₂) were prepared as a matrix using electrospinning technology combined with a high-temperature calcination process; by using a one-step hydrothermal method, metal Bi nanoparticles were deposited on the surface to prepare a Bi-composite Ho³⁺:TiO₂ nanofiber (Bi@Ho³⁺:TiO₂) photocatalyst material, aimed at enhancing the visible light photocatalytic activity of TiO₂ and improving the efficiency of photocatalytic hydrogen production. Additionally, the mechanism of photocatalytic water splitting for hydrogen production of Bi@Ho³⁺:TiO₂ was initially investigated, providing a reference for the synergistic enhancement of semiconductor photocatalyst activity using non-precious and rare-earth metals.

2. Results and Discussion

2.1. XRD Analysis

Figure 1 shows the XRD pattern of TiO₂ nanofibers and their composite fibers. As shown in Figure 1a, six obvious diffraction peaks were observed at the positions of 2θ = 25.5°, 37.9°, 48.2°, 54.1°, 55.0° and 62.5° for TiO₂ nanofibers. Compared with the standard card (JCPDS 21-1272), these diffraction peaks match crystal planes (101), (004), (200), (105), (211), and (204) of anatase TiO₂, indicating that the prepared fibers are of the anatase phase. After doping with rare-earth Ho³⁺, there was no change in the position of the diffraction peaks of TiO₂, but the peak shape broadened significantly. This was mainly due to the doping of rare-earth Ho³⁺ into the TiO₂ lattice, leading to lattice distortion. After the hydrothermal reaction, as shown in Figure 1b, the composite samples BHT2, BHT4, BHT6, and BHT8 exhibited four new diffraction peaks at 2θ = 27.3°, 38.3°, 39.4°, and 45.1°. Comparing with the standard card (JCPDS 85-1329), these new diffraction peaks belong to the (012), (104), (110), and (015) crystal planes of metallic Bi. This indicates that elemental Bi was reduced during the hydrothermal process.

2.2. SEM Analysis

Figure 2 shows the SEM image of the TiO₂ nanofibers and their composite fibers. It can be seen from Figure 2a that the length of TiO₂ nanofibers reached order of millimeters, their diameter was about 180 nm, their aspect ratio was large, their dispersibility was good, their surface was relatively smooth, and no other species were attached. After rare-earth Ho³⁺ doping, as shown in Figure 2b, the fiber morphology was well preserved, the diameter was slightly increased, and the surface became rough. After hydrothermal reaction, fine nanoparticles were formed on the surface of the composite fiber, and the content, particle size, and morphology of metal Bi changed with the increase in sodium gluconate dosage. When the dosage of sodium gluconate was 2 mg, as shown in Figure 2c, the nanoparticles on the surface of BHT2 were round, with a particle size of about 10 nm. When the dosage of sodium gluconate was 4 mg, as shown in Figure 2d, the number of nanoparticles on the surface of BHT4 increased, the diameter of particles increased, and the morphology became irregular. When the dosage of sodium gluconate was 6 mg, as can be seen from Figure 2e or Figure 3a, the nanoparticles on the surface of sample BHT6 were transformed into an octahedral morphology, which were uniformly dispersed on the fiber surface with a particle size of about 30 nm. When the dosage of sodium gluconate increased to 8 mg, as shown in Figure 2f, the surfaces of the BHT8 fiber began to accumulate, agglomerate, and even completely wrap the fiber.

Figure 3 shows a TEM, HRTEM, and EDS diagram of composite fiber sample BHT6. It can be observed from Figure 3a that polyhedral nanoparticles with different sizes were uniformly dispersed on the surface of the composite fiber, which is consistent with the results observed via SEM. The microstructure of the sample was further observed via HRTEM, and as shown in Figure 3b, two groups of lattice stripes with different widths were obtained. After analysis, it was confirmed that the lattice stripe with a width of 0.35 nm corresponded to the (101) crystal plane of anatase TiO₂, while the lattice stripe with a width of 0.27 nm corresponded to the (012) crystal plane of metal Bi, which clearly shows that the nanoparticles generated on the fiber surface were metal Bi. Figure 3c shows the distribution of elements on the surface of the composite fiber sample. As can be seen from the figure, the sample was composed of four elements, namely Ho, Bi, Ti, and O, and each element was evenly distributed around the fiber. According to EDS (energy dispersion spectrum) analysis and further analysis of elemental content, the metal Bi content on the surface of BHT2, BHT4, BHT6, and BHT8 samples was 15%, 18%, 20%, and 22%, respectively. This is related to the increasing amount of sodium gluconate in the reaction system, which shows that sodium gluconate plays a key role in the formation of metal Bi in hydrothermal reactions. The content of rare-earth holmium was 3.68%, 3.55%, 3.51%, and 3.47%, indicating that the hydrothermal process was basically unchanged for the composition of Ho³⁺:TiO₂.

2.3. XPS Analysis

Figure 4 is the XPS energy spectrum of composite fiber sample BHT6. It can be seen from the full spectrum in Figure 4a that four elements (Bi, Ti, O, and C) were on the surface of the composite fiber (among which the peak of element C came from the hydrocarbon of XPS instrument itself). Figure 4b–d are high-resolution XPS spectra of Bi 4f, Ti 2p, and O 1s, respectively. As shown in Figure 4b, the photoelectron peak of Bi 4f_7/2 appeared at 157.5 eV, suggesting the existence of metallic Bi, while the photoelectron peak of Bi 4f_5/2 was at 162.7 eV, and the binding energy between Bi 4f_5/2 and Bi 4f_7/2 was 5.2 eV, which further proves that the metal Bi [23,24] in the simple state was compounded on the composite fiber. In Figure 4c, two photoelectron peaks of Ti 2p_3/2 and Ti 2p_1/2 appear at 458.6 eV and 464.2 eV, respectively, indicating that Ti existed in the form of +4 valence in the sample [25]. In Figure 4d, there are two asymmetric photoelectron peaks in the binding energy range of 529~532 eV, indicating that oxygen existed in two states at this time. The photoelectron peak at the binding energy of 529.7 eV belonged to the lattice oxygen (O_latt) of the sample [26], while the photoelectron peak at the binding energy of 531.7 eV belonged to adsorbed oxygen (O_ads) on the fiber surface [27]. No XPS peak of Ho 4d was detected in this sample. On the one hand, there were many 4f unpaired spin electrons on the valence layer of rare-earth ion Ho³⁺, which were coupled with the inner shell 4D layer formed via photoionization, so in the XPS spectrum of Ho 4D, multiple splitting phenomena appear, and the photoelectron spectrum is complicated. On the other hand, because the position of the photoelectron peak of Ho 4D is in the range of 159.59~161.83 eV [28], this peak position overlaps with that of Bi 4f (157.5~162.7 eV).

2.4. Photoelectric Performance Analysis

Figure 5 shows a UV-vis DRS diagram of the TiO₂ nanofibers and their composite fibers. It can be seen from the figure that the absorption boundary of TiO₂ nanofibers is at 387.5 nm, which is consistent with the intrinsic absorption of anatase TiO₂. After doping with rare-earth Ho³⁺, the absorption boundary was red-shifted, indicating that the forbidden band width of TiO₂ was narrowed [29]. However, the composite fiber samples had strong absorption in the visible region, and with the increase in metal Bi content on the fiber surface, the absorption peak of the samples in the range of 520~750 nm gradually increased. McMahon et al. [30] used MIE (Mie scattering theory) theory to simulate the change in SPR of spherical bismuth in the size of nanoparticles in a vacuum, and camel-like absorption bands were observed at the wavelengths of 360 nm and 700 nm. The absorption band observed by Wang et al. [31] at 425~575 nm is considered to have been caused by the SPR effect of Bi nanoparticles. Based on this, we believe that the strong absorption peak observed in the range of 520~750 nm was caused by the SPR effect of metal Bi nanoparticles on the surface of composite fibers [14]. The change in absorption peak intensity shows that the SPR effect is influenced by the size and shape of metal particles, the dielectric constant of the medium, and the surrounding environment [32].

Figure 6 is a PL diagram of the TiO₂ nanofibers and their composite fibers. It can be seen from the figure that the PL signal peak of TiO₂ nanofibers was the strongest, indicating that the recombination probability of photogenerated electrons and holes was the highest. When doped with rare-earth Ho³⁺, the PL signal peak intensity decreased rapidly, indicating that the recombination probability of photogenerated electrons and holes decreased. The peak strength of the PL signal in composite metal Bi samples decreased further, and the fiber morphology changed with the increase in metal Bi content and Bi particle size, while the peak strength of the PL signal decreased with the increase in Bi content. However, the content of Bi in sample BHT8 was the highest, while the strength of the PL signal peak was not the weakest. It showed an increasing trend. This shows that the recombination of metal Bi is beneficial to the separation of photogenerated electrons and holes, while Bi itself is a semi-metallic element with certain conductivity, and a large amount of aggregation constitutes a new binding point of carriers, which makes the recombination probability of photogenerated electrons and holes increase.

Figure 7 shows the instantaneous photocurrent response of TiO₂ nanofibers and their composite fibers. As shown in the figure, TiO₂ nanofibers did not produce a photocurrent, indicating that pure anatase TiO₂ could not be excited by visible light to produce photogenerated electrons and holes. Under irradiation by visible light, the doped sample Ho³⁺:TiO₂ had a photocurrent, but the intensity was weak. According to the change in the photocurrent intensity of composite fiber samples being similar to that shown in the PL test results, with the increase inf Bi content and Bi particle size on the fiber surface, the photocurrent intensity shows a trend of first increasing and then decreasing. This is because when the Bi content is small on the fiber surface, photogenerated electrons can be transferred to TiO₂ through the heterojunction, so that the electron holes can be separated better. However, when the Bi content is large, the phenomenon of accumulation, agglomeration, or even wrapping on the fiber surface occurs, resulting in the rapid recombination of photogenerated electron holes, and the photocurrent intensity begins to decline. This is why the photocurrent intensity of BHT8 was lower than that of BHT6. At the same time, the photocurrent signal of all samples slowly dropped to zero when the light source was cut off. This shows that metal Bi nanoparticles can absorb visible light of a large wavelength and generate excited electrons with high energy through the SPR effect.

2.5. Photocatalytic Performance Evaluation

Figure 8 shows the hydrogen production rate of TiO₂ nanofibers and their composite fibers. The visible photocatalytic activity of different samples was compared. As shown in the figure, TiO₂ nanofibers do not generate H₂ under the irradiation of visible light, which corresponds to the observation that TiO₂ cannot be excited by visible light to generate photogenerated electrons and holes. After being doped with rare-earth Ho³⁺, the hydrogen production rate of Ho³⁺:TiO₂ nanofibers reached 8.6 μmol·g⁻¹·h⁻¹ under visible light for 5 h. This is because rare-earth Ho³⁺ is rich in 4f electrons and has different transition orbits, and doped into TiO₂ lattice, it resulted in impurity energy levels between the valence band and conduction band of TiO₂, which led to the narrowing of the band gap of TiO₂ and the ability to be excited by visible light to generate photogenerated electrons. After the hydrothermal reaction, the photocatalytic hydrogen production rate of composite fiber samples was similar to that shown by the test results of the change in photocurrent intensity. With the increase in Bi content and Bi particle size on the fiber surface, the photocatalytic hydrogen production rate showed a trend of first increasing and then decreasing. This is because when the Bi content on the surface of the fiber was small, photogenerated electrons were better able to separate the electron–hole pairs, and when the Bi content was large, the agglomeration phenomenon occurred, which made the electron–hole pairs rapidly recombine, while the hydrogen production rate began to decline, which is the reason why the photocatalytic hydrogen production rate first increased and then decreased. The hydrogen production rate of sample HTB6 was the largest, at 43.6 μmol·g⁻¹·h⁻¹, which is about five times that of Ho³⁺:TiO₂.

2.6. Visible Photocatalytic Hydrogen Production Mechanism

Based on the analysis of the above characterization results, we infer the mechanism of hydrogen decomposition of Bi@Ho³⁺:TiO₂ composite fiber under visible light. As shown in Figure 9, according to the SPR effect of metal Bi, when visible light irradiates at the interface between metal Bi and Ho³⁺:TiO₂, light waves oscillate with free electrons on the surface of metal Bi nanoparticles, effectively transforming light energy into the vibration energy of free electrons on the surface of metal Bi nanoparticles, and making some electrons excited from a low-energy state to a high-energy state into photogenerated excited electrons. These high-energy electrons are confined to a very small area such as Bi nanoparticles, forming an SPR electric field. Because TiO₂ has a low conduction band position [33], under the action of the SPR electric field, these photogenerated excited electrons migrate from the surface of Bi nanoparticles to the TiO₂ conduction band, thus increasing the lifetime of the photogenerated electrons. However, due to the Schottky barrier at the interface between metal Bi and TiO₂, the passage of photoinduced electrons generated by metal Bi is blocked, and only a small number of photoinduced electrons with high energy can migrate to the TiO₂ conduction band, so the electron migration process is limited. When rare-earth Ho³⁺ is doped, Ho³⁺ replaces Ti⁴⁺ and enters the TiO₂ lattice, resulting in an imbalance in charges in the TiO₂ crystal, forming impurity defects [34], resulting in a large number of unsaturated O on the surface of the composite fiber, which is strongly coupled with metal Bi, thus effectively reducing the Schottky barrier at the interface between metal Bi and TiO₂, allowing photogenerated electrons with lower energy to migrate to the conduction band of TiO₂ through the interface, and making metal Bi nanoparticles a supply station for electrons needed for photocatalysis. These high-energy electrons react with H⁺ adsorbed on the TiO₂ surface to generate H₂, while the surplus photogenerated holes react with the sacrificial agent TEOA in solution to generate oxide species. Finally, the performance of hydrogen production via the photolysis of water under visible light is promoted. When the content of metal Bi is too large, although more photogenerated electrons can be generated, the active sites on the fiber surface are covered by a large number of Bi nanoparticles, and the recombination probability of photo-generated electrons and holes is further increased, which leads to a decrease in the photocatalytic hydrogen production rate. This shows that there is an optimal value of metal Bi recombination.

3. Experiment

3.1. Materials and Methods

We used an X’Pert3 Powder X-ray powder diffractometer (XRD, Billerica, MA, USA); an SU8010 field emission scanning electron microscope (SEM, Hitachi, Tokyo, Japan), equipped with an X-ray energy spectrometer (EDS); JEM-2010 High Resolution Transmission Electron Microscope (HRTEM 2010 JEOL LaB6 microscope 200 kV, Tokyo, Japan); a PHI-5000 VersaProbe X-ray photoelectron spectrometer (XPS, ULVAC-PHI Company, Tokyo, Japan); a Lambda35 ultraviolet-visible spectrophotometer (UV-vis DRS, Perkin Elmer Company, Hopkinton, MA, USA); a Hitachi F-4500 fluorescence spectrometer (PL, Hitachi, Tokyo, Japan); and a CHI660 electrochemistry Learning workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China).

Polyvinylpyrrolidone (PVP, Ms = 1,300,000, AR) was supplied by Sigma-Aldrich (St. Louis, MO, USA); tetrabutyl titanate (AR) was supplied by Sigma-Aldrich (St. Louis, MO, USA); bismuth nitrate and holmium nitrate (AR) were supplied by Sigma-Aldrich (St. Louis, MO, USA); polyvinyl alcohol (PEG, Ms = 2000) and sodium gluconate (AR) were supplied by Sigma-Aldrich (St. Louis, MO, USA); ethylene glycol, anhydrous ethanol, and glacial acetic acid (AR) were supplied by Sigma-Aldrich (St. Louis, MO, USA).

3.2. Sample Preparation

Preparation of Ho³⁺:TiO₂ nanofibers: We added 1.0 g of PVP (polyvinylpyrrolidone) into 10 mL of absolute ethanol, and stirred the mixture for 4 h with magnetic force to prepare polymer solution A. We dissolved 1 mL of tetrabutyl titanate in 6 mL of a mixed solution of anhydrous ethanol and glacial acetic acid in an equal volume ratio, stirred it for 30 min to prepare solution B, slowly dropped solution B into solution A, vigorously stirred the mixture for 2 h, added 1 mL of 0.15 mol/L Ho (NO₃)₃·5H₂O solution, continued stirring for 30 min, and then transferred the mixture into an electrospinning syringe. At a voltage of 10 kV, the receiving distance was 12 cm, and the sample fiber felt was prepared via electrospinning for 5 h. After vacuum drying it for 24 h, it was placed in a muffle furnace at a heating rate of 0.5 °C/min, heated to 500 °C, and calcined at a constant temperature for 4 h, so as to prepare the Ho³⁺-doped TiO₂ nanofiber, which was labeled as HT. In order to compare experiments, pure TiO₂ nanofibers were prepared without adding Ho(NO₃)₃.

Preparation of Bi@Ho³⁺:TiO₂ composite fiber: An amount of 1.923 g of Bi (NO₃)₃·5H₂O was dissolved in 80 mL of ethylene glycol, stirred with magnetic force for 30 min, and divided into four parts; 2, 4, 6 and 8 mg of sodium gluconate were added, the mixture was continuously stirred to completely dissolve it, and then 10 mg of Ho³⁺:TiO₂ nanofibers was added. We adjusted the pH of the solution to 12 with 2 mol/L NaOH, transferred it into a 30 mL autoclave, and carried out the reaction at 180 °C for 24 h. We cooled it to room temperature, washing it with ethanol and deionized water for times, and then dried at 60 °C for 12 h to prepare aBi@Ho³⁺:TiO₂ sample with different Bi contents, which were labeled as BHT2, BHT4, BHT6, and BHT8.

3.3. Photoelectric Performance Test

Preparation of working electrode: Briefly, 0.03 g of PEG (polyvinyl alcohol) and 0.5 mL of ethanol were added to the sample, and the mixture was fully mixed and ground into slurry in a mortar. The slurry was evenly dispersed on the surface of FTO (fluorine-doped tin oxide) glass via the scraper method, and the dispersed film area was about 1 cm²; then, it was treated in an oven at 100 °C for 60 min. The photocurrent was measured using the standard three-electrode system CHI660E electrochemical workstation. Ag/AgCl (saturated KCl) was used as a reference electrode, Pt wire as a counter electrode, 0.5 mol·L⁻¹ of Na₂SO₄ solution as an electrolyte, and a 350 W xenon lamp as the light source. The light source switch interval was 50 s.

3.4. Performance Evaluation of Hydrogen in Photolysis Water

The experiment of photocatalytic water decomposition to produce hydrogen was carried out by using the photocatalytic water decomposition system produced by Beijing Pofilai Technology Co., LTD, Beijing, China. Amounts of 95 mL of H₂O and 5 mL of Triethanolamine (TEOA) were put into a special quartz photoreactor, and 50 mg of catalyst was added. After ultrasonic treatment for 20 min, they were put into a photocatalytic water decomposition system. Under normal temperature and pressure conditions, a 350 W xenon lamp with a filter (λ > 400 nm) was used to filter out ultraviolet light with a wavelength less than 400 nm. The light intensity at the reaction liquid surface was approximately 100 W/cm². The amount of hydrogen produced was detected using the thermal conductivity detector (TCD) of a gas chromatograph, with a 5A molecular sieve column and high-purity N₂ as the carrier gas.

4. Conclusions

In this work, nanofibers doped with rare-earth Ho³⁺ in TiO₂ (Ho³⁺:TiO₂) were prepared as the matrix using electrospinning technology combined with a high-temperature calcination process; by using a one-step hydrothermal method, metal Bi nanoparticles were deposited on the surface to prepare a Bi-composite Ho³⁺:TiO₂ nanofiber (Bi@Ho³⁺:TiO₂) photocatalyst material. In the process of photocatalytic decomposition of water to produce hydrogen, metal Bi nanoparticles become the supply station of electrons needed for photocatalytic hydrogen production, and rare-earth Ho³⁺ doping effectively reduces the Schottky barrier at the interface between metal Bi and TiO₂, which is beneficial for the rapid migration of electrons from the Bi to TiO₂ conduction band. The synergistic SPR effects of metal Bi and the special electronic structure of rare-earth elements effectively improve the separation efficiency of photogenerated electrons and holes, broaden the spectral response range, modify the absorption capacity of TiO₂ for visible light, and improve the photocatalytic activity of composite fiber, which provides new research ideas and feasible synthetic routes for developing efficient photocatalytic materials.

Author Contributions

Conceptualization, T.C. and Y.G.; methodology, T.C. and Y.G.; formal analysis, T.C. and Y.G.; resources, T.C. and Y.G.; writing—original draft preparation, T.C., Y.G. and W.X.; writing—review and editing, T.C., Y.G. and X.Q. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

Figure 1. (a) XRD patterns of Ho3+:TiO2 nanofibers and TiO2 nanofibers. (b) XRD patterns of the composite samples BHT2, BHT4, BHT6, BHT8, and HT.

Figure 2. SEM images of (a) TiO2 nanofibers and (b) Ho3+:TiO2 nanofibers; (c) composite fibers BHT2, (d) BHT4, (e) BHT6, and (f) BHT8.

Figure 3. (a) TEM, (b) HRTEM, and (c) EDS diagram of composite fiber sample BHT6.

Figure 4. (a) Full spectrum of composite fiber sample BHT6; (b) high-resolution XPS spectra of Bi 4f, (c) Ti 2p, and (d) O 1s.

Figure 5. UV-vis DRS of TiO2 nanofibers and their composite fibers.

Figure 6. PL diagram of TiO2 nanofibers and their composite fibers.

Figure 7. Transient photocurrent response of TiO2 nanofibers and their composite fibers.

Figure 8. Hydrogen production rate of TiO2 nanofibers and their composite fibers.

Figure 9. Schematic diagram showing the energy band structure and electron–hole pair separation of composite fiber sample Bi@Ho3+:TiO2.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14090588/s1. Figure S1: Yunfan (Tianjin) YFSP-T type electrostatic spinning machine. Figure S2: Labsolar-H2II photocatalytic water degradation system produced by Beijing Porfilet Technology Co., LTD. Figure S3: Low power SEM image of TiO₂ nanofibers. Figure S4: High power SEM image of Bi@Ho³⁺:TiO₂ composite fiber. Figure S5: High resolution XPS diagram of O1s in Ho³⁺:TiO composite fiber.

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