**2. Materials & Methods**

Chemicals: titanium oxide mix phase nanofiber (TiO<sup>2</sup> MP, 99%), titanium oxide nanofiber type 1 (TiO<sup>2</sup> nanofiber type 1, 99%), titanium oxide nanofiber type 2 (TiO<sup>2</sup> nanofiber type 2, 99%), copper sulfide nanosheets (CuS), star-like BiVO4, flower-like Bi2WO6, and platinum (IV) chloride (PtCl4, 98%) were purchased from commercial sources and used without further purification.

Photodeposition of Pt: the photodeposition of Pt nanoparticles was taken under ultraviolet (UV) light illumination. The photocatalyst materials were dispersed in deionized water and mixed with PtCl<sup>4</sup> aqueous solution. The mixture, with continuous magnetic stirring, was irradiated with ultraviolet light (λ = 365 nm) from a UV lamp. The light intensity on the sample was 134 mW/cm<sup>2</sup> . For comparison, the reactions without irradiation were also prepared.

Characterization: transmission electron microscopes (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) were carried out on a JEOL 2100F electron microscope (JEOL, Tokyo, Japan), operated with an accelerating voltage of 200 kV. Energy-dispersive X-ray (EDX) analysis and corresponding elemental mapping data were taken with the X-ray spectroscopy (Oxford X-Max 80, Oxford Instruments, Abingdon, UK) attached to the TEM instrument. To prepare the TEM specimens, a drop of nanocrystals, dispersed in ethanol, was dropped on the surface of a

lacey formvar/carbon 200-mesh Cu grid. X-ray diffraction analysis (XRD) patterns were collected on a Rigaku SmartlabSE X-ray diffractometer (Rigaku, Tokyo, Japan) by using Cu Kα radiation. terns were collected on a Rigaku SmartlabSE X-ray diffractometer (Rigaku, Tokyo, Japan) by using Cu Kα radiation. **3. Results** 

with an accelerating voltage of 200 kV. Energy-dispersive X-ray (EDX) analysis and corresponding elemental mapping data were taken with the X-ray spectroscopy (Oxford X-Max 80, Oxford Instruments, Abingdon, UK) attached to the TEM instrument. To prepare the TEM specimens, a drop of nanocrystals, dispersed in ethanol, was dropped on the surface of a lacey formvar/carbon 200-mesh Cu grid. X-ray diffraction analysis (XRD) pat-

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### **3. Results** In this work, the photodeposition of Pt nanoparticles were employed on different

In this work, the photodeposition of Pt nanoparticles were employed on different kinds of photocatalyst materials, including TiO2, Bi2WO6, BiVO4, and CuS. The reaction times and the amount of PtCl<sup>4</sup> were summarized in Table 1. All the photocatalyst materials in this work were prepared by hydrothermal method. The TiO<sup>2</sup> nanomaterials used in this work included three types: pure phase TiO<sup>2</sup> nanofiber type 1 (TiO<sup>2</sup> t1), pure phase TiO<sup>2</sup> nanofiber type 2 (TiO<sup>2</sup> t2), and mixed phase TiO<sup>2</sup> (TiO<sup>2</sup> MP). The TiO<sup>2</sup> MP were synthesized in a one-pot reaction. The crystal structure of the purchased TiO<sup>2</sup> were characterized by the XRD patterns in Figure 1. It can be observed that the XRD peaks of TiO<sup>2</sup> t1 coincided well with the brookite phase (JCPDS No. 46-1238), and the XRD peaks of TiO<sup>2</sup> t2 were indexed to the anatase phase (JCPDS No. 21-1272) without any secondary phase. The broadened XRD peaks should be correlated with the nano-scale particles. The XRD peaks of the TiO<sup>2</sup> MP sample at 14.2◦ , 24.9◦ , 28.6◦ , 43.5◦ , and 44.5◦ were ascribed to the brookite phase, while the peaks at 25.3◦ , 37.8◦ , and 48.0◦ were attributed to the anatase phase. It confirmed that the brookite and anatase phases coexisted in the TiO<sup>2</sup> MP sample. kinds of photocatalyst materials, including TiO2, Bi2WO6, BiVO4, and CuS. The reaction times and the amount of PtCl4 were summarized in Table 1. All the photocatalyst materials in this work were prepared by hydrothermal method. The TiO2 nanomaterials used in this work included three types: pure phase TiO2 nanofiber type 1 (TiO2 t1), pure phase TiO2 nanofiber type 2 (TiO2 t2), and mixed phase TiO2 (TiO2 MP). The TiO2 MP were synthesized in a one-pot reaction. The crystal structure of the purchased TiO2 were characterized by the XRD patterns in Figure 1. It can be observed that the XRD peaks of TiO2 t1 coincided well with the brookite phase (JCPDS No. 46-1238), and the XRD peaks of TiO2 t2 were indexed to the anatase phase (JCPDS No. 21-1272) without any secondary phase. The broadened XRD peaks should be correlated with the nano-scale particles. The XRD peaks of the TiO2 MP sample at 14.2°, 24.9°, 28.6°, 43.5°, and 44.5° were ascribed to the brookite phase, while the peaks at 25.3°, 37.8°, and 48.0° were attributed to the anatase phase. It confirmed that the brookite and anatase phases coexisted in the TiO2 MP sample. **Table 1.** Summary of the crystal structure of Pt deposited nanocrystals and the reaction condi-

**Table 1.** Summary of the crystal structure of Pt deposited nanocrystals and the reaction conditions. tions.


**Figure 1.** XRD patterns of TiO2 MP, TiO2 t1, and TiO2 t2 samples. The peaks marked by pound sign and asterisk are dependent to anatase phase (JCPDS No. 21-1272) and brookite phase (JCPDS No. 46-1238), respectively. **Figure 1.** XRD patterns of TiO<sup>2</sup> MP, TiO<sup>2</sup> t1, and TiO<sup>2</sup> t2 samples. The peaks marked by pound sign and asterisk are dependent to anatase phase (JCPDS No. 21-1272) and brookite phase (JCPDS No. 46-1238), respectively.

Figure 2 showed the XRD results of Pt-deposited Bi2WO6, BiVO4, and CuS photocatalyst materials. The crystal structure of Bi2WO6, BiVO4, and CuS nanomaterials could be illustrated by XRD patterns in Figure 2. The XRD patterns of Bi2WO6, BiVO4, and CuS were well indexed to the russellite phase (JCPDS No. 39-0256), clinobisvanite phase (JCPDS No. 14-0688), and covellite phase (JCPDS No. 06-0464), respectively. These XRD results proved that these nanomaterials had a good crystallization crystal structure and the

expected stoichiometric composition. The broadened XRD peaks should be correlated with the nano-scale particles. Since the experiments were attended to load Pt nanoparticles on these photocatalyst materials, typical XRD peaks, located at 39.8◦ and 46.2◦ of platinum (JCPDS No. 04-0802), were also marked in the Figure 2. Disappointingly, none of the Pt XRD peaks were detected in these four Pt-deposited photocatalyst materials. This might be because of the small size and low content of Pt nanoparticles. with the nano-scale particles. Since the experiments were attended to load Pt nanoparticles on these photocatalyst materials, typical XRD peaks, located at 39.8° and 46.2° of platinum (JCPDS No. 04-0802), were also marked in the Figure 2. Disappointingly, none of the Pt XRD peaks were detected in these four Pt-deposited photocatalyst materials. This might be because of the small size and low content of Pt nanoparticles.

Figure 2 showed the XRD results of Pt-deposited Bi2WO6, BiVO4, and CuS photocatalyst materials. The crystal structure of Bi2WO6, BiVO4, and CuS nanomaterials could be illustrated by XRD patterns in Figure 2. The XRD patterns of Bi2WO6, BiVO4, and CuS were well indexed to the russellite phase (JCPDS No. 39-0256), clinobisvanite phase (JCPDS No. 14-0688), and covellite phase (JCPDS No. 06-0464), respectively. These XRD results proved that these nanomaterials had a good crystallization crystal structure and the expected stoichiometric composition. The broadened XRD peaks should be correlated

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**Figure 2.** XRD patterns of Pt-deposited TiO2 MP nanofiber, Pt-deposited Bi2WO6 nanoflower, Ptdeposited BiVO4 nanostar, and Pt-deposited CuS nanosheet. **Figure 2.** XRD patterns of Pt-deposited TiO<sup>2</sup> MP nanofiber, Pt-deposited Bi2WO<sup>6</sup> nanoflower, Pt-deposited BiVO<sup>4</sup> nanostar, and Pt-deposited CuS nanosheet.

In order to confirm the existing of Pt nanoparticles, TEM, HRTEM, SAED, and EDX analyses were carried out. Figure 3 showed the TEM images of Pt-deposited TiO2 MP (a), Bi2WO6 (b), BiVO4 (c), CuS (d), TiO2 t1 (e), and TiO2 t2 (f). The black dots on the particles, with the size of 1–2 nm, were considered as the Pt nanoparticles. The Pt nanoparticles were homogeneously distributed on the surface of the photocatalyst matrix. In addition, the deposited Pt nanoparticles increased with the increasing PtCl4 solution. For instance, the amount of Pt4+ used in reactions was 1.2 wt% in TiO2-based materials and 2.9 wt% for bismuth and copper compounds in Figure 3. It can also be seen that the amount of Pt particles on bismuth and copper particles were much higher than those on TiO2 particles. In order to confirm the existing of Pt nanoparticles, TEM, HRTEM, SAED, and EDX analyses were carried out. Figure 3 showed the TEM images of Pt-deposited TiO<sup>2</sup> MP (a), Bi2WO<sup>6</sup> (b), BiVO<sup>4</sup> (c), CuS (d), TiO<sup>2</sup> t1 (e), and TiO<sup>2</sup> t2 (f). The black dots on the particles, with the size of 1–2 nm, were considered as the Pt nanoparticles. The Pt nanoparticles were homogeneously distributed on the surface of the photocatalyst matrix. In addition, the deposited Pt nanoparticles increased with the increasing PtCl<sup>4</sup> solution. For instance, the amount of Pt4+ used in reactions was 1.2 wt% in TiO2-based materials and 2.9 wt% for bismuth and copper compounds in Figure 3. It can also be seen that the amount of Pt particles on bismuth and copper particles were much higher than those on TiO<sup>2</sup> particles. This result could correspond well with the added amount of PtCl<sup>4</sup> solution in the reaction system. Therefore, it can be inferred that the 1.2 wt% Pt4+ loading amount is not enough for TiO<sup>2</sup> nanomaterials. In other words, more Pt nanoparticles would be deposited on the surface of TiO<sup>2</sup> nanomaterials if more PtCl<sup>4</sup> solution was added during the reaction.

This result could correspond well with the added amount of PtCl4 solution in the reaction system. Therefore, it can be inferred that the 1.2 wt% Pt4+ loading amount is not enough for TiO2 nanomaterials. In other words, more Pt nanoparticles would be deposited on the surface of TiO2 nanomaterials if more PtCl4 solution was added during the reaction.

**Figure 3.** TEM images of Pt-deposited TiO2 MP (**a**), Bi2WO6 (**b**), BiVO4 (**c**), CuS (**d**), TiO2 t1 (**e**), and TiO2 t2 (**f**). **Figure 3.** TEM images of Pt-deposited TiO<sup>2</sup> MP (**a**), Bi2WO<sup>6</sup> (**b**), BiVO<sup>4</sup> (**c**), CuS (**d**), TiO<sup>2</sup> t1 (**e**), and TiO<sup>2</sup> t2 (**f**).

To further verify the existence of the loaded Pt particles, SAED and EDX analyses were applied on the Pt-deposited TiO2 nanofibers (Figure 4). In Figure 4b, a pair of diffraction spots, labelled by a red circle with a *d*-spacing of 2.26 Å, was observed. This diffraction data was correlated well with (111) crystal plane of Pt, which finally confirmed the presence of Pt in this Pt-deposited TiO2 nanomaterial. Additionally, the scanning TEM (STEM), and the corresponding EDX mapping signal, authenticate uniform distribution of elemental Ti and O throughout the TiO2 nanofibers, as well as random distribution of the elemental Pt on the fiber's surface. To further verify the existence of the loaded Pt particles, SAED and EDX analyses were applied on the Pt-deposited TiO<sup>2</sup> nanofibers (Figure 4). In Figure 4b, a pair of diffraction spots, labelled by a red circle with a *d*-spacing of 2.26 Å, was observed. This diffraction data was correlated well with (111) crystal plane of Pt, which finally confirmed the presence of Pt in this Pt-deposited TiO<sup>2</sup> nanomaterial. Additionally, the scanning TEM (STEM), and the corresponding EDX mapping signal, authenticate uniform distribution of elemental Ti and O throughout the TiO<sup>2</sup> nanofibers, as well as random distribution of the elemental Pt on the fiber's surface. *Materials* **2022**, *15*, x FOR PEER REVIEW 6 of 13

**Figure 4.** TEM image of Pt deposited TiO2 MP (**a**) and the corresponding SAED patterns (**b**); STEM image of Pt deposited TiO2 MP (**c**), and the corresponding EDX mapping images of elemental Ti (**d**), O (**e**), and Pt (**f**). **Figure 4.** TEM image of Pt deposited TiO<sup>2</sup> MP (**a**) and the corresponding SAED patterns (**b**); STEM image of Pt deposited TiO<sup>2</sup> MP (**c**), and the corresponding EDX mapping images of elemental Ti (**d**), O (**e**), and Pt (**f**).

Above results revealed the successful deposition of Pt nanoparticles on various photocatalyst nanomaterials, including TiO2-based, bismuth-based, and copper-based mate-

also indicated that the Pt particles did not show any selectivity on deposition position; they deposited very randomly. This is considered to be the result of a surface defectsdriven electron escape mechanism. In our experiments, only photocatalyst nanoparticles were presented together with the Pt source (PtCl4) in aqueous. Under the illumination, electrons generated and migrated to the particle surface to react with adsorbed Pt4+ ions. Pt4+ ions should be reduced at the electron escaping sites, resulting in the formation of Pt nanoparticle-deposited photocatalyst nanomaterials. As mentioned before, surface defects played important roles for electron migration and escape in monodispersed semiconductor nanoparticles. In general, defects distributed on the surface randomly. This information explained that Pt nanoparticles randomly deposited on semiconductor nanoparticles, which confirmed that the Pt deposition is a result of surface defects engineering. Meanwhile, with different amounts of PtCl4 used in the reaction, the amount of Pt nanoparticles deposited on nanofibers changed. As shown in Table 2, the Pt4+ loading amount increased from 1.2 wt% to 12.0 wt%, and the Pt-deposited amount increased, linearly, from 0.48 wt% to 7.97 wt%. This could also be evidence that the Pt deposition on monodisperse nanoparticles mainly occurred due the surface defects-driven electron escape

**Table 2.** Summary of Pt4+ loading amounts and actual Pt-deposited amounts, analyzed by EDX, for

1.2 wt% 0.76 wt% 2.3 wt% 2.05 wt% 5.8 wt% 4.07 wt% 12.0 wt% 7.96 wt%

**(Analyzed by EDX)** 

**Basic Materials Pt4+ Loading Amount Pt Deposited Amount** 

TiO2 t2 1.2 wt% 0.31 wt%

mechanism.

TiO2-based materials.

TiO2 t1

Above results revealed the successful deposition of Pt nanoparticles on various photocatalyst nanomaterials, including TiO2-based, bismuth-based, and copper-based materials; the Pt particles almost occupied the entire surface of core particles. The TEM results also indicated that the Pt particles did not show any selectivity on deposition position; they deposited very randomly. This is considered to be the result of a surface defects-driven electron escape mechanism. In our experiments, only photocatalyst nanoparticles were presented together with the Pt source (PtCl4) in aqueous. Under the illumination, electrons generated and migrated to the particle surface to react with adsorbed Pt4+ ions. Pt4+ ions should be reduced at the electron escaping sites, resulting in the formation of Pt nanoparticle-deposited photocatalyst nanomaterials. As mentioned before, surface defects played important roles for electron migration and escape in monodispersed semiconductor nanoparticles. In general, defects distributed on the surface randomly. This information explained that Pt nanoparticles randomly deposited on semiconductor nanoparticles, which confirmed that the Pt deposition is a result of surface defects engineering. Meanwhile, with different amounts of PtCl<sup>4</sup> used in the reaction, the amount of Pt nanoparticles deposited on nanofibers changed. As shown in Table 2, the Pt4+ loading amount increased from 1.2 wt% to 12.0 wt%, and the Pt-deposited amount increased, linearly, from 0.48 wt% to 7.97 wt%. This could also be evidence that the Pt deposition on monodisperse nanoparticles mainly occurred due the surface defects-driven electron escape mechanism.

**Table 2.** Summary of Pt4+ loading amounts and actual Pt-deposited amounts, analyzed by EDX, for TiO<sup>2</sup> -based materials.


In order to understand the reaction mechanism well, comparison experiments and replenish analyses have been done. Firstly, besides the studies on the precipitate phase, Pt deposited TiO<sup>2</sup> nanomaterials during the solution phase, which was supposed to contain excess PtCl<sup>4</sup> and some small photocatalyst nanoparticles, has also been investigated by TEM and EDX analyses. TEM results revealed that there are no free Pt nanoparticles observed, while the EDX mapping signal of Pt has only been detected on residual TiO<sup>2</sup> nanofibers. Secondly, a comparison experiment, based on a TiO<sup>2</sup> MP nanofiber sample, was taken in the dark. EDX data shows 0.98 wt% Pt signal. However, the sample prepared with the same condition as the comparison experiment showed a 7.26 wt% Pt signal after illumination for 30 min. Combined with TEM and SAED results, there are neither small particles vision nor a Pt diffraction signal (a d-spacing of 2.26 Å). Such a small amount of Pt loading is regarded as Pt4+ ions adsorbed on particle surface. These comparison experiment results further prove the reaction mechanism: Pt4+ ions adsorbed on a particle's surface and reacted with escaped electrons to form Pt nanoparticles, which means Pt nanoparticles were formed and deposited, directly on the particle surface, at electron escaping sites. This mechanism makes it possible to identify electron escaping conditions, directly, by the visual detection of Pt nanoparticles.

Another part of our work was the study on heterostructured TiO<sup>2</sup> MP nanofibers. As verified by XRD analysis, the TiO<sup>2</sup> MP sample contained both brookite and anatase phases. The HRTEM images, as shown in Figure 5, also revealed the co-existence of the two phases' particles in this sample. In the HRTEM picture of the left nanofiber (Figure 5b), two lattice fringes, with *d* spacing 6.26 Å and 3.69 Å, were correlated well with that of the (001) and (201) planes of the brookite phase (JCPDS No. 46-1238), respectively. For the right counterpart (Figure 5c), lattice fringes, with a *d* spacing 3.54 Å, could be indexed to the (101) planes of the anatase phase (JCPDS #21-1272). The HRTEM results were in accordance with the XRD analysis. *Materials* **2022**, *15*, x FOR PEER REVIEW 8 of 13

**Figure 5.** (**a**) TEM images of Pt-deposited TiO2 MP sample (with 1.2 wt% Pt4+ loading amount). HRTEM images of (**b**) brookite TiO2 nanofibers and (**c**) anatase TiO2 nanofibers. **Figure 5.** (**a**) TEM images of Pt-deposited TiO<sup>2</sup> MP sample (with 1.2 wt% Pt4+ loading amount). HRTEM images of (**b**) brookite TiO<sup>2</sup> nanofibers and (**c**) anatase TiO<sup>2</sup> nanofibers.

Interestingly, the Pt nanoparticles preferred to be reduced on the brookite TiO2 nanofibers instead of the anatase TiO2 nanofibers, which could be seen from Figure 5a. As analyzed in the above TEM results (Figure 3e,f), in pure TiO2 t1 and t2 samples, most of the particles show obvious Pt deposition in both brookite and anatase TiO2 nanofibers when 1.2 wt% PtCl4 was added. Considering that the Pt nanoparticles were only photodeposited at the electron escape site, the electron escape sites should exist in both brookite and anatase TiO2 nanofibers. However, when the same amount of PtCl4 was used to react with TiO2 MP nanoparticles, a different phenomenon was observed. In the mixed phase sample, Pt nanoparticles were deposited on partial nanofibers. By analyzing TEM, it seems that Pt nanoparticles were preferred to deposit on brookite phase nanofibers (Figure 5b) compared to anatase fibers (Figure 5c), when the Pt4+ loading amount is 1.2 wt%. This can also be evidenced by the EDX analysis data in Table 2. Compared to the pure t1 and t2 samples, mixed phase TiO2 nanofibers show almost the same Pt deposition amount. This means the Pt nanoparticles were mainly deposited on one kind of TiO2 nanomaterial Interestingly, the Pt nanoparticles preferred to be reduced on the brookite TiO<sup>2</sup> nanofibers instead of the anatase TiO<sup>2</sup> nanofibers, which could be seen from Figure 5a. As analyzed in the above TEM results (Figure 3e,f), in pure TiO<sup>2</sup> t1 and t2 samples, most of the particles show obvious Pt deposition in both brookite and anatase TiO<sup>2</sup> nanofibers when 1.2 wt% PtCl<sup>4</sup> was added. Considering that the Pt nanoparticles were only photodeposited at the electron escape site, the electron escape sites should exist in both brookite and anatase TiO<sup>2</sup> nanofibers. However, when the same amount of PtCl<sup>4</sup> was used to react with TiO<sup>2</sup> MP nanoparticles, a different phenomenon was observed. In the mixed phase sample, Pt nanoparticles were deposited on partial nanofibers. By analyzing TEM, it seems that Pt nanoparticles were preferred to deposit on brookite phase nanofibers (Figure 5b) compared to anatase fibers (Figure 5c), when the Pt4+ loading amount is 1.2 wt%. This can also be evidenced by the EDX analysis data in Table 2. Compared to the pure t1 and t2 samples, mixed phase TiO<sup>2</sup> nanofibers show almost the same Pt deposition amount. This means the Pt nanoparticles were mainly deposited on one kind of TiO<sup>2</sup> nanomaterial instead of

instead of not being deposited. Furthermore, this phenomenon became more obvious when the Pt4+ loading amount decreased to 0.70 wt% and even to 0.23 wt%. By studying a

more evidently, with or without loading Pt particles. It confirmed that nanofibers with the brookite phase showed higher reaction activities for the photocatalytic metal deposition compared to anatase phase particles. This can be attributed to the difference of work function. Referring to Vera and co-workers' results [37], work function of brookite TiO2 is a little bit smaller than in the anatase phase. Although work function value is variable for different testing methods, in different testing environments, or even based on different facets, work function of the brookite phase is always smaller than anatase under the same

not being deposited. Furthermore, this phenomenon became more obvious when the Pt4+ loading amount decreased to 0.70 wt% and even to 0.23 wt%. By studying a relatively low Pt4+ loading amount sample, we were able to distinguish these particles more evidently, with or without loading Pt particles. It confirmed that nanofibers with the brookite phase showed higher reaction activities for the photocatalytic metal deposition compared to anatase phase particles. This can be attributed to the difference of work function. Referring to Vera and co-workers' results [37], work function of brookite TiO<sup>2</sup> is a little bit smaller than in the anatase phase. Although work function value is variable for different testing methods, in different testing environments, or even based on different facets, work function of the brookite phase is always smaller than anatase under the same conditions. EDX analysis results also present the same trend, in which pure TiO<sup>2</sup> t2 samples have a higher Pt deposited amount. *Materials* **2022**, *15*, x FOR PEER REVIEW 9 of 13 conditions. EDX analysis results also present the same trend, in which pure TiO2 t2 samples have a higher Pt deposited amount.

> Except for the two kinds of pure phase particles, a two phase co-existence structure was fabricated side by side on one single particle. In Figure 6, a heterostructure TiO<sup>2</sup> nanofiber, with brookite and anatase phases, was observed. It is obvious that a distinct interface between the two phases was observed, in one particle, by the bright-field TEM image. Figure 6b showed the high resolution image of the upper side, in which the lattice fringes, with a *d* spacing 2.31 Å, was identified. This area was correlated with (112) the crystal plane of the anatase TiO<sup>2</sup> phase. The bottom part, with *d* spacing 5.88 Å lattice fringes, has been decided as the brookite phase. TEM results showed that the two phases co-existed, and a special heterostructure containing the brookite and anatase phases was constructed in one particle. Except for the two kinds of pure phase particles, a two phase co-existence structure was fabricated side by side on one single particle. In Figure 6, a heterostructure TiO2 nanofiber, with brookite and anatase phases, was observed. It is obvious that a distinct interface between the two phases was observed, in one particle, by the bright-field TEM image. Figure 6b showed the high resolution image of the upper side, in which the lattice fringes, with a *d* spacing 2.31 Å, was identified. This area was correlated with (112) the crystal plane of the anatase TiO2 phase. The bottom part, with *d* spacing 5.88 Å lattice fringes, has been decided as the brookite phase. TEM results showed that the two phases co-existed, and a special heterostructure containing the brookite and anatase phases was constructed in one particle.

**Figure 6.** TEM images of a nanoparticle with a heterostructure (**a**) in the Pt-deposited TiO2 MP sample (with 1.2 wt% Pt4+ loading amount). HRTEM images of (**b**) upper and (**c**) lower areas, as indicated in (**a**). **Figure 6.** TEM images of a nanoparticle with a heterostructure (**a**) in the Pt-deposited TiO<sup>2</sup> MP sample (with 1.2 wt% Pt4+ loading amount). HRTEM images of (**b**) upper and (**c**) lower areas, as indicated in (**a**).

materials have an electrical junction that could promote the separation of the charge carrier and drive the electrons across the junction. In this mixed phase sample, the electrons mainly escaped from the brookite phase. Thus, it is inferred that the brookite phase was performed as an electron attractor component. The existence of heterojunction efficiently causes the migration of electrons across the junction interface, and then, they come out from the surface of the counterpart with the electron attractor component. It resulted in

Meanwhile, TEM results also revealed that almost all the Pt nanoparticles are loaded on the bottom part, which is the brookite phase. This phenomenon declared that the elec-

Meanwhile, TEM results also revealed that almost all the Pt nanoparticles are loaded on the bottom part, which is the brookite phase. This phenomenon declared that the electrons mainly escaped from the surface of the brookite TiO2. As discussed previously, there are two effects that could be used to explain this phenomenon. First, some heterostructure materials have an electrical junction that could promote the separation of the charge carrier and drive the electrons across the junction. In this mixed phase sample, the electrons mainly escaped from the brookite phase. Thus, it is inferred that the brookite phase was performed as an electron attractor component. The existence of heterojunction efficiently causes the migration of electrons across the junction interface, and then, they come out from the surface of the counterpart with the electron attractor component. It resulted in the higher content of escaped electrons from brookite TiO<sup>2</sup> particles compared with the pure brookite TiO<sup>2</sup> sample. Therefore, the Pt nanoparticles, attached on the brookite TiO<sup>2</sup> particles in the heterojunction sample, were more than that in the pure brookite TiO<sup>2</sup> sample. Second, the different work functions of these two phases would be other driving forces for the different Pt deposition densities. *Materials* **2022**, *15*, x FOR PEER REVIEW 10 of 13 the higher content of escaped electrons from brookite TiO2 particles compared with the pure brookite TiO2 sample. Therefore, the Pt nanoparticles, attached on the brookite TiO2 particles in the heterojunction sample, were more than that in the pure brookite TiO2 sample. Second, the different work functions of these two phases would be other driving forces for the different Pt deposition densities. Figure 7 shows the TEM images of Pt-deposited TiO2 MP nanofibers with a Pt4+ load-

Figure 7 shows the TEM images of Pt-deposited TiO<sup>2</sup> MP nanofibers with a Pt4+ loading amount from 2.3 wt% to 12.0 wt%. From Figure 7a,b, a clear viewer of the different Pt deposited densities, on two sides of the heterostructures, was observed even if the Pt4+ loading amount increased to 5.8 wt%. The work functions, different between brookite phase (around 3.72 eV) and anatase phase (around 3.84 eV) [37], may not be big enough to result in such different Pt-deposited densities, especial at higher Pt4+ loading amounts. Thus, we believe that both the heterojunction driven force and work function difference contribute to the Pt deposited density difference between brookite and anatase phases in heterostructured nanoparticles. In Figure 7c, Pt nanoparticles abundantly deposited on both sides, which could be attributed to the saturation of active sites on the brookite component surface. ing amount from 2.3 wt% to 12.0 wt%. From Figure 7a,b, a clear viewer of the different Pt deposited densities, on two sides of the heterostructures, was observed even if the Pt4+ loading amount increased to 5.8 wt%. The work functions, different between brookite phase (around 3.72 eV) and anatase phase (around 3.84 eV) [37], may not be big enough to result in such different Pt-deposited densities, especial at higher Pt4+ loading amounts. Thus, we believe that both the heterojunction driven force and work function difference contribute to the Pt deposited density difference between brookite and anatase phases in heterostructured nanoparticles. In Figure 7c, Pt nanoparticles abundantly deposited on both sides, which could be attributed to the saturation of active sites on the brookite component surface.

**Figure 7.** TEM images of Pt-deposited TiO2 MP nanofibers with Pt4+ loading amount 2.3 wt% (**a**), 5.8 wt% (**b**), and 12.0 wt% (**c**), respectively. ture materials, electrons are driven to migrate to the electron attractor component, due to **Figure 7.** TEM images of Pt-deposited TiO<sup>2</sup> MP nanofibers with Pt4+ loading amount 2.3 wt% (**a**), 5.8 wt% (**b**), and 12.0 wt% (**c**), respectively.

From another point of view, the Pt deposit sites directly point out the electron escape site. In monodisperse nanomaterials, Pt nanoparticles are loaded randomly. This attrib-

From another point of view, the Pt deposit sites directly point out the electron escape site. In monodisperse nanomaterials, Pt nanoparticles are loaded randomly. This attributes to the abundant surface defects of photocatalyst materials. Surface defects are considered as the main electron escape driving force. Different from this, in some heterostructure materials, electrons are driven to migrate to the electron attractor component, due to the existence of heterojunction structure and the difference of work functions. Thus, although Pt4+ ions are already adsorbed on the entire surface, only those ions located at the electron escape side are successfully reduced. Our results correspond well with those.
