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. 2025 Jun 16;25(25):10169–10177. doi: 10.1021/acs.nanolett.5c02177

Unraveling Charge Transport in Heterostructured Nanomotors for Efficient Photocatalytic Motion

Yufen Chen †, Chunyu Li ‡, Rebeca Ferrer Campos †,§, María José Esplandiu ∥,*, Jordi Fraxedas ∥, Nicoletta Liguori ‡,*, Katherine Villa †,*
PMCID: PMC12203641  PMID: 40518840

Abstract

Photocatalytic micro/nanomotors have emerged as promising tools for environmental remediation, biosensing, and targeted delivery. To enhance their light-driven propulsion, significant efforts have focused on engineering semiconductor heterostructures, which promote charge separation. However, a clear understanding of how these architectures govern photocatalytic mechanisms and influence motion performance remains limited. Here, we design a visible light-responsive nanomotor based on a Fe2O3-Pt-TiO2 trilayered heterostructure, combining narrow-bandgap α-Fe2O3 and wide-bandgap TiO2 with an intermediate Pt layer. Remarkably, Fe2O3-TiO2 nanomotors without the Pt layer exhibit only modest propulsion under visible light, whereas the inclusion of Pt significantly enhances their motility. Through advanced techniques, including in situ synchrotron radiation-based near-ambient pressure X-ray photoelectron spectroscopy and transient absorption spectroscopy, we reveal that Pt serves as an efficient electron mediator, enabling directional charge transfer across the heterojunction. This study provides fundamental insights into charge transport in multicomponent nanomotors and introduces a rational strategy for designing efficient photoactive systems.

Keywords: photocatalytic nanomotors, heterojunction structure, electron transfer, near ambient pressure X-ray photoelectron spectroscopy, transient absorption spectroscopy

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Micro/nanomotors capable of autonomous motion have gained significant attention over the past two decades due to their ability to perform targeted cargo transport, sensing and catalytic reactions in environmental, analytical and biomedical applications. Such tiny artificial devices are capable of self-propulsion by converting external energy into mechanical motion. − Among the energy resources, light stands out due to its greenness, abundance and remote controllability. The motion activation of light-driven nanomotors typically relies on photothermal effects, photocatalysis, photon momentum transfer or optical forces. In particular, photocatalytic nanomotors offer tunable material properties, from inorganic to organic semiconductors, diverse activation wavelengths, and versatile bandgap engineering strategies for motion modulation. ,

Heterojunction engineering of photocatalytic systems is an attractive approach to enhance charge separation and boost catalytic efficiency. − The formation of a built-in electric field at the interface between a semiconductor and a secondary component (metal or another semiconductor) promotes the separation and migration of photogenerated carriers, thereby improving the photocatalytic performance. , For instance, semiconductor/metal heterostructured micromotors, such as TiO2/Au, TiO2/Au/Pt, BiOI-Au, ZnO/Pt, α-Fe2O3/Au Janus structures, have demonstrated efficient motion through self-electrophoresis, where the metal caps serve as the electron acceptors. However, semiconductor/semiconductor heterojunctions remain underexplored in nanomotor design, despite their potential to broaden light absorption, improve catalytic efficiency, and significantly boost propulsion performance. So far, the reported heterojunction-based photocatalytic micro/nanomotors either primarily rely on UV light irradiation, − or require complex polarization setups, limiting their practical implementation. More importantly, a comprehensive understanding of charge transfer mechanisms in heterostructured photocatalytic nanomotors, particularly in relation to their optical properties and motion dynamics, is still lacking.

Herein, we introduce rod-shaped Fe2O3-Pt-TiO2 nanomotors as a visible-light-driven heterojunction system. α-Fe2O3, with a bandgap of ca. 2.2 eV, is a promising photocatalyst for visible light harvesting, yet its performance is limited by rapid electron–hole recombination. A common strategy to mitigate this drawback involves constructing heterojunctions with complementary semiconductors, which can facilitate charge separation and extend carrier lifetimes. In our design, α-Fe2O3 functions as the visible-light absorber due to its narrow bandgap, TiO2 serves as the electron transport layer, and an intermediate Pt layer acts as an electron acceptor to facilitate charge separation and enhance motion. Although Fe2O3/TiO2 nanomotors alone showed negligible motion improvement, the incorporation of a Pt interlayer significantly enhanced their propulsion, confirming its role in facilitating electron transfer. To unravel the underlying charge transfer mechanisms in Fe2O3-Pt-TiO2 nanomotors, various cutting-edge techniques have been implemented, including in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation, as well as transient absorption spectroscopy (TAS) measurements. These advanced techniques provide direct insights into electron transfer dynamics and photocatalytic activity, establishing a fundamental framework for the future design of efficient heterojunction-based photocatalytic nanomotors. As proof of concept, the performance of Fe2O3-Pt-TiO2 nanomotors was tested for the photocatalytic degradation of methylene blue (MB) as a model reaction, which is a major contributor to aquatic ecosystem disruption, eutrophication, and aesthetic pollution. −

The fabrication process of the rod-like α-Fe2O3-based nanomotors is illustrated in Figure a. The nanorod-like α-Fe2O3 structure was obtained through a facile hydrothermal synthesis, and then the Pt and TiO2 layers were further introduced via a sputtering procedure. Field emission scanning electron microscope image (FESEM, Figure b) shows that the obtained α-Fe2O3 nanorods exhibited an average length of ca. 700 ± 20 nm and an average diameter of ca. 116 ± 10 nm. Brunauer-Emmett-Teller (BET) analysis indicates that the specific surface area of as-prepared α-Fe2O3 nanorods is 17.72 m2/g (Table S2). High-resolution transmission electron microscopy (HRTEM) and the corresponding energy-dispersive X-ray (EDX) mapping (Figure c) confirm the intended structure of Fe2O3-Pt-TiO2, with Pt forming the middle layer and TiO2 in the outer layer, with thicknesses of approximately 10 nm and 10–15 nm, respectively. In addition, we observed that the as-deposited TiO2 layer is present in an amorphous phase, as presented in Figures d, e and S1a. Figure f shows that the Fe2O3-Pt-TiO2 nanomotors exhibit a broad absorption peak at around 400–550 nm, due to the optical properties of the α-Fe2O3 (Figure S1b). The bandgap of Fe2O3-Pt-TiO2, estimated from its Tauc plot (Figure g), indicates that the incorporation of Pt and TiO2 does not alter the bandgap energy of the underlying α-Fe2O3 nanorods (Figure S1c), which remains around 2.1 eV and is favorable for visible light absorption. For comparison, UV–vis absorption spectra of Fe2O3-TiO2 and Fe2O3-Pt are also shown in Figures S1d and S1e. The crystalline phases of Fe2O3-Pt-TiO2 nanomotors were analyzed by X-ray diffraction (XRD), and Raman spectroscopy was used to confirm phase composition. XRD patterns (Figures h and S1f and S1g) show characteristic reflections of α-Fe2O3 along with Pt, while no TiO2-related peaks were detected due to its low crystallinity. It should be noted that the diffraction patterns from the FTO substrate (SnO2) are also observed. Upon calcination, crystallinity improves, revealing the formation of anatase TiO2, confirming that TiO2 was initially amorphous in the as-prepared Fe2O3-Pt-TiO2 heterostructure (see the Figures S1h and S1i for details). The results also confirm that the iron oxide phase remains unchanged after sputtering. Raman spectrum (Figure i) displays peaks at 221 and 496 cm–1 (A1g modes), 242, 290, 407, and 608 cm–1 (Eg modes), and a 661 cm–1 peak corresponding to a longitudinal optical phonon mode, all consistent with the hematite structure. −

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Synthesis and characterization of Fe2O3-Pt-TiO2 nanomotors. (a) Schematic illustration of the synthesis of Fe2O3-Pt-TiO2 nanomotors. (b) FESEM images of as-prepared α-Fe2O3 nanorods. (c) HRTEM image and the corresponding EDX mapping of Fe2O3-Pt-TiO2 nanomotors. (d) and (e) are the enlarged images of the orange dashed frame in (c). (f) UV–vis absorption spectrum of Fe2O3-Pt-TiO2. (g) Tauc plots of (F­(R∞)­E)2 versus E­(eV) for Fe2O3-Pt-TiO2. Due to the amorphous nature of TiO2, the presence of Pt, and possible interfacial states, the Fe2O3-Pt-TiO2 heterostructure presents additional complexities that complicate the determination of the true absorption onset. To address this, we adopted a more robust approach, which involves fitting two linear regions: one below and one above the absorption edge. The intersection of these two lines provides an estimate of the optical bandgap. (h) XRD profile of Fe2O3-Pt-TiO2. α-Fe2O3 (JCPDS 01-087-1165), Pt (JCPDS 00-004-0802). (i) Raman spectrum of Fe2O3-Pt-TiO2.

The motion characterization of bare α-Fe2O3, Fe2O3-TiO2, Fe2O3-Pt, and Fe2O3-Pt-TiO2 was evaluated under both dark and blue light (475 nm, 333 mW/cm2) conditions. The representative trajectories (Figures a and b) show that upon light illumination, the motion of bare α-Fe2O3 and Fe2O3-TiO2 did not significantly differ from their behavior under dark conditions. In contrast, the motion of Fe2O3-Pt and Fe2O3-Pt-TiO2 was significantly enhanced under light irradiation (Videos S1 and S2). Due to the small size of the nanomotors, it is impossible to distinguish their motion orientation. Based on previous reports, we propose that the propulsion of Fe2O3-Pt-TiO2 nanomotors under visible light follows a self-electrophoretic mechanism, characteristic of metal–semiconductor heterostructures, as illustrated in the conceptual scheme in Figure c. ,, The MSD plots (Figures d–g) confirm that the motion dynamics of the α-Fe2O3-based nanomotors are of the diffusion type. Remarkably, the Fe2O3-Pt-TiO2 displays the steepest MSD slope, indicating the highest effective diffusion coefficient (De = MSD/(4Δt)), which is attributed to its unique heterojunction structure formed between α-Fe2O3, Pt and TiO2. It is worth mentioning that such heterojunction (n-n type) is also theoretically formed in the Fe2O3-TiO2 sample, however, its motion efficiency under light irradiation remained negligible. This observation prompted a closer investigation into the role of the Pt interlayer in facilitating charge transfer and enhancing photocatalytic propulsion in Fe2O3-Pt-TiO2 nanomotors.

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Motion characterization of α-Fe2O3-based nanomotors. Representative tracking trajectories of the as-synthesized nanomotors recorded over 20 s under dark (a) and 475 nm light irradiation (b). (c) Schematic illustration of the proposed propulsion mechanism of Fe2O3-Pt-TiO2 nanomotors in H2O2 under 475 nm blue light. (d–g) Plots of averaged MSD versus Δt analyzed from tracking trajectories of α-Fe2O3, Fe2O3-TiO2, Fe2O3-Pt, and Fe2O3-Pt-TiO2 nanomotors. Results are shown as the mean ± standard error of the mean, N = 20 nanomotors. (h) and (i) Schematic illustration of charge-transfer processes under blue light in different α-Fe2O3-based heterostructured nanomotors. ΦB represents the Schottky barrier height. Since the work functions of metal and semiconductors were not measured in this work, the mechanism is proposed based on previous literature. −

Most previous studies on Fe2O3-TiO2-based micromotors rely on UV light to activate both semiconductors. However, under visible light, where only α-Fe2O3 is active, this heterojunction has shown negligible contribution to propulsion and remains poorly understood. Consistent with a previous study, the motion of Fe2O3-TiO2 in this work did not show noticeable enhancement under visible light (Figure e). This limited performance may be due to the fact that, although a small amount of photoexcited charges can transfer from α-Fe2O3 to adjacent TiO2, the low carrier mobility and short hole diffusion length (ca. 2–4 nm) result in rapid electron–hole recombination (Figure h). As a result, the ability of Fe2O3-TiO2 to generate propulsive force is significantly hindered. In contrast, the Fe2O3-Pt-TiO2 nanomotors contain a Pt interlayer that acts as a metallic mediator, promoting efficient charge separation and transport between the α-Fe2O3 and TiO2 components.

In principle, the different work functions of Pt (4.5 eV), α-Fe2O3 (5.5 eV) and TiO2 (5.3 eV), , lead to the formation of Schottky barriers at the Fe2O3/Pt and TiO2/Pt interfaces under equilibrium conditions, as illustrated in Figures i and S2a. Upward band bending at a Schottky contact typically creates a potential barrier that inhibits electron migration from the semiconductor to the metal. However, under blue illumination, a fraction of electrons can acquire enough energy (thermal or kinetic energy) to overcome the barrier toward Pt. Moreover, the Schottky barrier, in turn, could play a beneficial role in suppressing electron backflow from Pt to α-Fe2O3, thereby reducing recombination and indirectly enhancing charge separation. These electrons accumulated at the Pt interface may then transfer into mid-gap states in TiO2 before being injected into the liquid interface. Additionally, due to the heterogeneous surface and asymmetric shape of nanomotors, the generated photocatalytic species would be unevenly distributed, contributing to their motion behavior.

For the Fe2O3-Pt sample (Figure f), light illumination results in enhanced diffusive motion, consistent with previous findings on noble metal-capped micro/nanomotors. Nevertheless, it is important to note that the diffusion coefficient is still lower than that of Fe2O3-Pt-TiO2, likely due to faster electron–hole pair recombination, which reduces the number of electrons available to generate reactive species (Figure S2b). It is well-known that the rate of a photocatalytic reaction depends on the number of charge carriers that successfully reach the surface. However, charge recombination often competes with surface reactions, occurring on a much faster time scale (10–9 s versus 10–8–10–3 s). As a result, although some electrons can transfer to the adjacent Pt layer, the short diffusion length leads to partial recombination, reducing propulsion efficiency. Additionally, the exposed Pt surface may directly catalyze H2O2 decomposition, triggering competing reactions that could act in opposite directions and further limit the motion of the nanomotors.

To further evaluate the role of the heterojunction architecture, we tested the motion behavior of various α-Fe2O3-based control nanomotors with systematically modified structures: (1) an insulating end layer (Fe2O3-Pt-SiO2), (2) thermally enhanced TiO2 crystallinity (Fe2O3-Pt-TiO2-Cal and Fe2O3-TiO2-Cal; Cal denotes calcination), and (3) altered Pt/TiO2 positioning (Fe2O3-TiO2-Pt). None of these configurations showed a significant improvement in propulsion under light irradiation. In particular, the insulating SiO2 layer in Fe2O3-Pt-SiO2 hindered electron transfer from Pt to the outer surface (Figure S2c), promoting charge recombination and limiting photocatalytic reactions. In the case of Fe2O3-Pt-TiO2-Cal and Fe2O3-TiO2-Cal samples (Figure S3), the annealing process could increase the crystallinity of TiO2 and eliminate the internal bands/defects, which in fact reduced the electron trapping ability of TiO2.

To gain deeper insight into the enhanced photocatalytic activity observed in Fe2O3-Pt-TiO2, we first performed the steady-state photoluminescence (PL) and time-resolved PL spectroscopy on all α-Fe2O3-based nanomaterials. The steady-state PL spectra (Figure S4) show that the Fe2O3-Pt-TiO2 heterojunction exhibits the lowest emission intensity, indicating the most efficient suppression of electron–hole recombination among all samples. Moreover, the slowest decay profile in Figure a confirms that Fe2O3-Pt-TiO2 exhibits the longest excited carrier lifetime compared to that of other samples. , In addition, the electrochemical impedance spectra (EIS, Nyquist plots) under blue light irradiation were collected to investigate the intrinsic photoelectrochemical properties of α-Fe2O3-based nanomotors (Figure b). Evidently, the Fe2O3-Pt and Fe2O3-Pt-TiO2 structures resulted in a great reduction of charge transfer resistance, wherein the Fe2O3-Pt-TiO2 exhibited the smallest arc radius, which implies the lowest electron transport resistance and therefore the highest charge separation efficiency. Moreover, the photocurrent responses (Figure c) demonstrate that the presence of the Pt layer greatly enhances charge carrier transfer between α-Fe2O3 and TiO2, well in line with the observations from EIS analysis. These results further validate our hypothesis that the Fe2O3-Pt-TiO2 nanomotor exhibits superior photocatalytic performance owing to its unique metal-assisted heterojunction structure.

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Charge carrier dynamics and surface chemical states of α-Fe2O3-based nanomotors under light irradiation. (a) Time-resolved photoluminescence decay profiles, λem = 420 nm, (b) Nyquist plots and (c) transient photocurrent response of α-Fe2O3, Fe2O3-Pt-TiO2, Fe2O3-TiO2, and Fe2O3-Pt nanomotors under blue light irradiation, λ = 460 nm. XPS Fe 2p (d), Ti 2p (e), and Pt 4f (f) spectra of Fe2O3-Pt-TiO2, Fe2O3-TiO2, and Fe2O3-Pt samples using 1600 eV photons and conducted under white light illumination (100 mW/cm2) at 3.1 mbar of water pressure.

In-situ NAP-XPS analysis was performed to investigate the charge transfer mechanisms and oxidation states of Fe2O3-Pt-TiO2 nanomotors under dark and illuminated conditions. Fe 2p XPS spectra and their fitted results (Figures d under illumination and S5a in the dark) show two main peaks located at 710.7 and 724.3 eV in all samples, corresponding to the Fe 2p3/2 and 2p1/2 orbital peaks of α-Fe2O3, respectively, , providing robust evidence of the formation of α-Fe2O3 in all measured nanomaterials. In the Ti 2p region (Figures e under illumination and S5b in the dark), two different peaks centered at 458.5 and 464.2 eV are observed in all samples, corresponding to the 2p3/2 and 2p1/2 of Ti4+ state of n-type TiO2, respectively. − Pt 4f XPS data are illustrated in Figures f (under illumination) and S5c (in the dark). The results reveal that in Fe2O3-Pt-TiO2, Pt is predominantly present in metallic state (Pt0 at 71.0 eV), along with a small fraction of oxidized Pt species (denoted as Ptδ+). In contrast, only metallic Pt0 was detected in the Fe2O3-Pt samples. Note that the Pt deposition procedure was conducted in an argon atmosphere, without the involvement of oxygen. Hence, we suggest that the small amount of oxidized Pt in Fe2O3-Pt-TiO2 might be due to the strong electronic interaction formed between Pt/Fe2O3 and Pt/TiO2 interfaces. More interestingly, we found that upon light irradiation, the metallic Pt species in Fe2O3-Pt-TiO2 get slightly increased compared with the dark condition, from 88.6% to 93.2%, as shown in Figure S6, implying that the Pt species received electrons during the photocatalytic reaction process. This is direct evidence of the electron transfer that occurs in heterostructured nanomotors upon light illumination. It is worth noting that since the white light LED source used in the in situ XPS measurements was not optimized, only a slight increase could be detected. In contrast, for the Fe2O3-Pt sample, no difference was observed from the Pt XP spectra in the absence and presence of light irradiation. Taken together, from the in situ NAP-XPS analysis, we can conclude that in the Fe2O3-Pt-TiO2 configuration, the Pt layer plays an essential role in promoting electron transfer mediated by Schottky barriers at the interface between α-Fe2O3 and TiO2 semiconductors, which in turn leads to enhanced photocatalytic motion performance. Whereas this effect was not observed in the Fe2O3-Pt and Fe2O3-TiO2 samples, consistent with the motion behavior previously shown in Figure .

Furthermore, in order to probe nonemissive states in photoexcited α-Fe2O3-based heterojunction structured nanomotors, transient absorption spectroscopy (TAS) was employed to study the charge-transfer dynamics at heterojunction interfaces over femtosecond to nanosecond time scales. As depicted in Figures a and b, upon photoexcitation, three major TAS regions were observed in the visible range, where a negative ΔA signal peaking at ca. 550 nm (area I) was mainly attributed to ground-state bleach (GSB). The positive TAS signal can be divided into two regions: (II) a characteristic positive excited-state absorption (ESAI) band centered at ca. 600 nm, and (III) a broad tail ESAII extending into the near-IR region. Further, the TAS spectral evolution was compared for α-Fe2O3-based photocatalytic nanomotors (Figure c), and the results demonstrated that the fs-TAS signals of the heterojunction systems primarily originated from the α-Fe2O3 component, as no discernible spectral features were observed when α-Fe2O3 was coupled with TiO2 and/or Pt. Notably, a relative reduction in the intensity of the strong ESAI peak at approximately 600 nm was observed in Fe2O3-TiO2, Fe2O3-Pt, and Fe2O3-Pt-TiO2 compared to that of bare α-Fe2O3, suggesting enhanced interfacial charge transfer facilitated by the strong electron coupling between α-Fe2O3 and TiO2/Pt. This is in agreement with the chemical quenching and spectral analysis results of the α-Fe2O3 suspension in the presence of Ag+ as an electron scavenger (see the discussion in SI and Figures S7–S10). Figures d and e show that no significant kinetic changes are observed at 550 nm; however, at 600 nm, a clear heterojunction-induced effect is evident in the α-Fe2O3-based samples, characterized by a buildup of the TAS signal reaching its peak within 100 ps. Considering that the polaron formation in metal oxides typically occurs on faster time scales (∼1 ps), the observed rise kinetics are most likely attributed to the accumulation of photogenerated holes within the α-Fe2O3 layer as a result of rapid electron extraction toward Pt and TiO2, particularly in the case of Pt. − However, it is of interest to note that while the presence of Pt can effectively enhance interfacial charge carrier separation and promote hole accumulation, as discussed in XPS analysis, this accumulation may lead to undesirable reverse recombination of charge carriers, thereby reducing the overall photocatalytic activity. , On the contrary, by coupling TiO2, which acts as an electron acceptor and active catalytic surface to suppress the recombination of electron–hole pairs, a more effective transfer of photogenerated carriers can be achieved (Fe2O3-Pt-TiO2). Indeed, following a similar early time TAS signal buildup for Fe2O3-Pt-TiO2 compared to Fe2O3-Pt, the decay kinetics decelerate after coupling with TiO2, likely due to the suppression of electron–hole recombination. , However, it should also be noted that, based on the data available, we cannot exclude contributions to the TAS signal from thermal-induced lattice expansion, which may obscure the dynamics of charge carriers. − Based on the above analyses, the enhanced motion performance of Fe2O3-Pt-TiO2 with a Pt interlayer can be attributed to the effective suppression of charge carrier recombination, which is well supported by motion tracking, photoelectrochemical measurements and XPS analysis.

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Transient absorption spectroscopy (TAS) analysis of α-Fe2O3-based nanomotors. (a) 2D TAS contour map of α-Fe2O3 upon photoexcitation at 500 nm. (b) TAS spectra of α-Fe2O3 plotted at selected pump–probe delay times. (c) Comparison of TAS spectra normalized to -1 at the negative maximum of bare α-Fe2O3 and α-Fe2O3-based heterojunction nanomotors at different delay times and their corresponding TAS kinetics monitored at (d) 550 and (e) 600 nm. A pump wavelength of 500 nm was employed to selectively excite the α-Fe2O3 support.

Furthermore, the mechanistic study was verified by using the photocatalytic degradation of organic pollutant MB as a model reaction in the presence of the as-prepared samples. The most intense absorption peaks, located at around 663 nm and a shoulder peak at 613 nm, are associated with an MB monomer and dimer, respectively. As shown in Figure , after 2 hours of light irradiation, Fe2O3-TiO2 and Fe2O3-Pt removed only 25% and 48% of MB, respectively. In contrast, Fe2O3-Pt-TiO2 nanomotors achieved a remarkable 91% MB removal within the same time, after which the degradation rate remains almost unchanged (Figure S11), demonstrating significantly higher photocatalytic efficiency compared to the reference samples. Noteworthy, a very small amount of MB degradation was also observed in the absence of any α-Fe2O3-based nanomaterials under light irradiation, which is because MB undergoes photolysis in the presence of H2O2 and decomposes itself under visible light (Figures d and S11d). The MB solution with and without H2O2 under light irradiation was also tested as control experiments (Figures S11e and S11f), both showing negligible degradation. A reusability test was conducted for the Fe2O3-Pt-TiO2 nanomotors, showing that their photocatalytic activity and structural integrity remained stable after three consecutive cycles (Figures S11h, S11i, S12, S13, and S14).

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Photocatalytic MB degradation of α-Fe2O3-based nanomotors. (a–c) Time-dependent UV–visible spectra of methylene blue solution (containing 0.1% H2O2) in the presence of Fe2O3-TiO2, Fe2O3-Pt, and Fe2O3-Pt-TiO2 under blue light illumination, respectively. (d) Degradation rate curve for MB solution with and without α-Fe2O3-based nanomotors. The inset included in (d) is a photograph of MB solution after photodegradation over various samples: (i) Fe2O3-Pt-TiO2, (ii) Fe2O3-Pt, (iii) Fe2O3-TiO2, and (iv) only H2O2.

In a typical H2O2-assisted photocatalytic degradation of MB, the photogenerated carriers react with H2O2 to produce •OH species, which are highly oxidative and responsible for breaking down MB molecules (Figure S11g). − The MB degradation results are consistent with the observed motion performance (Figure ). A comparison of the photocatalytic efficiency of Fe2O3-Pt-TiO2 nanomotors with previously reported Fe2O3-based materials is summarized in Table S1, demonstrating equal or superior MB degradation performance despite significantly lower catalyst loading and reduced light intensity. In summary, this study demonstrates that Fe2O3-Pt-TiO2 heterostructured nanomotors exhibit enhanced photocatalytic motion under visible light due to the formation of Schottky barriers mediated by the presence of Pt at the semiconductor-semiconductor interface. This heterojunction, which has been rarely used for nanomotor design, provides unique charge separation, a key factor in achieving improved propulsion. In contrast, Fe2O3-TiO2 nanorods did not show the expected enhancement, as revealed by in situ NAP-XPS, TAS, PL, and photoelectrochemical measurements. Therefore, our findings provide fundamental insights into the design of photocatalytic micro/nanomotors, particularly through metal-assisted semiconductor/semiconductor coupling, and emphasize the critical role of mechanistic understanding in refining fabrication strategies for advanced light-driven nanosystems.

Supplementary Material

nl5c02177_si_001.docx (22.8MB, docx)
Download video file (9.9MB, mp4)
Download video file (9MB, mp4)

Acknowledgments

The research was funded by the European Union (ERC, PhotoSwim, 101076680). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. This publication is also part of Grants PID2022-136886OA-I00 and PID2021-124568NB-I00 financed by MCIN/AEI/10.13039/501100011033/FEDER, UE. K.V. acknowledges the support from the Spanish Ministry of Science (MCIN/AEI/10.13039/501100011033) and the European Union (Next Generation EU/PRTR) through the Ramón y Cajal grant, RYC2021-031075-I. Y.C. acknowledges the support from “Juan de la Cierva Grant” JDC2023-051508-I, funded by MICIU/AEI/10.13039/501100011033. This work was partly funded by a BIST Ignite Programme grant from the Barcelona Institute of Science and Technology-Project NANOLYMPICS. ICIQ is supported by the Ministerio de Ciencia e Innovación (MICIU/AEI/10.13039/501100011033) through the Severo Ochoa Excellence Accreditation CEX2019-000925-S and ICN2 by CEX2021-001214-S and by the CERCA Programme/Generalitat de Catalunya. J.F. acknowledges funding from Generalitat de Catalunya through the 2021-SGR-00644 project. We thank Diamond Light Source for access to beamline B07 VERSOX, branch C TPOT (Proposal SI36056) that contributed to the results presented here and to Drs. D. Grinter and G. Held for technical assistance and fruitful discussions. M.J.E. and J.F. also acknowledge the scientific exchange and support of the Centre for Molecular Water Science (CMWS). N.L. acknowledges the financial support by “la Caixa” Foundation (ID 100010434) with the fellowship code LCF/BQ/PI22/11910032. C.L. acknowledges the funding from “Juan de la Cierva Grant” JDC2022-049604-I, funded by MCIN/AEI/10.13039/501100011033 and the European Union’s Horizon 2023 research and innovation programme under the Marie Sklodowska-Curie Grant 101152468.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.5c02177.

  • Experimental methods, FT analysis of HRTEM image, UV–visible spectra of α-Fe2O3-based nanomaterials, XRD profiles, schematic illustration of band structure of different nanomotors, MSD plots, photoluminescence spectra, XPS under dark condition, atomic ratios of Pt obtained from XPS data, TAS discussion, TAS spectra, time-dependent UV–visible spectra of MB solution over different photocatalytic nanomotors and kinetic fitting results of MB degradation over Fe2O3-Pt-TiO2, characterizations of Fe2O3-Pt-TiO2 after photocatalytic degradation of MB, summary of recently reported Fe2O3-based photocatalysts for MB degradation under light irradiation, and BET analysis of α-Fe2O3 nanorods (DOCX)

  • Video S1, the motion of bare α-Fe2O3 nanorod under dark and blue light illumination in 0.1% H2O2 (MP4)

  • Video S2, the motion of Fe2O3-Pt-TiO2 under dark and blue light illumination in 0.1% H2O2 (MP4)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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