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Plasmonic Pd-Sb nanosheets for photothermal CH4 conversion to HCHO and therapy
Ultrathin Pd-Sb nanosheets for methane conversion
https://orcid.org/0000-0001-8553-253X
Wang Mengjun Conceptualization Data curation Formal analysis Investigation Methodology Project administration Resources Supervision Validation Visualization Writing - original draft Writing - review & editing 1 2 †
https://orcid.org/0000-0001-6632-5906
Jia Jun Conceptualization Data curation Formal analysis Funding acquisition Investigation Methodology Project administration Resources Software Supervision Validation Visualization Writing - original draft Writing - review & editing 1 3 †
Meng Zhaodong Formal analysis Methodology Software Visualization Writing - original draft 1 4
https://orcid.org/0000-0001-6092-1344
Xia Jing Methodology Validation 5
https://orcid.org/0009-0004-8779-3708
Hu Xinyan Investigation Resources 1
https://orcid.org/0000-0002-7779-4932
Xue Fei Formal analysis Methodology 1
Peng Huiping Conceptualization Data curation Investigation Methodology Resources Validation Writing - original draft Writing - review & editing 1
https://orcid.org/0000-0002-1667-2632
Meng Xiangmin Investigation Methodology 5
https://orcid.org/0000-0003-2186-6615
Yi Jun Methodology Project administration Resources Software 1 4 6
Chen Xiaolan Data curation Formal analysis Investigation Methodology Project administration Resources Validation 1
https://orcid.org/0000-0001-9307-1809
Li Jun Data curation Formal analysis Investigation Methodology Resources Software Supervision Validation Visualization Writing - review & editing 1
https://orcid.org/0000-0001-9224-3816
Guo Yuzheng Data curation Formal analysis Funding acquisition Methodology Project administration Resources Software Supervision Validation Visualization Writing - review & editing 3 *
https://orcid.org/0000-0002-2525-7086
Xu Yong Conceptualization Funding acquisition Methodology Supervision Validation 2 *
https://orcid.org/0000-0003-3219-4316
Huang Xiaoqing Conceptualization Data curation Formal analysis Funding acquisition Investigation Methodology Project administration Resources Software Supervision Validation Visualization Writing - original draft Writing - review & editing 1 4 *
1 State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
2 i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS), 398 Ruoshui Road, Suzhou 215123, China.
3 School of Electrical Engineering and Automation, Wuhan University, Hubei 430072, China.
4 Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen University, Xiamen 361005, China.
5 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry Chinese Academy of Sciences, Beijing 100190, China.
6 College of Electronic Science and Engineering, Fujian Key Laboratory of Ultrafast Laser Technology and Applications, Xiamen University, Xiamen 361005, China.
* Corresponding author. Email: yguo@whu.edu.cn (Y.G.); yxu2023@sinano.ac.cn (Y.X.); hxq006@xmu.edu.cn (X.Hua)
† These authors contributed equally to this work.

06 9 2024
04 9 2024
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30 7 2024
Copyright © 2024 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
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https://creativecommons.org/licenses/by-nc/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

Photothermal catalysis effectively increases catalytic activity by using the photothermal effect of metal nanomaterials; however, the combination of strong light absorption and high catalytic performance remains a challenge. Here, we demonstrate hexagonal ~5-nanometer-thick palladium antimony (chemical formula as Pd8Sb3) nanosheets (NSs) that exhibit strong light absorption within full spectral and localized surface plasmon resonance (LSPR) effects in the visible region. Such LSPR features lead to strong photothermal effects, and Pd8Sb3 NSs aqueous dispersion enables enhanced photothermal methane (CH4) conversion to formaldehyde (HCHO) under full-spectrum light irradiation at 1.7 watts per square centimeter, leading to selectivity of ~98.7%, productivity of ~665 millimoles per gram of catalyst, ~700 times higher than that of Pd NSs. Mechanism investigations suggest that different radicals were generated on Pd8Sb3 (·OH) and Pd NSs (·O2−), where Pd8Sb3 NSs displays stronger adsorption strength to CH4 and facilitates CH4 oxidation to HCHO. Besides, the strong light absorption ability of Pd8Sb3 NSs enables photothermal therapy for breast cancer.

Plasmonic and catalytic Pd-Sb nanosheets exhibit a superior performance of photothermal CH4 conversion to HCHO.

http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 22025108 http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China U21A20327 http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 22121001 http://dx.doi.org/10.13039/501100004739 Youth Innovation Promotion Association of the Chinese Academy of Sciences 2020026 http://dx.doi.org/10.13039/501100011152 Suzhou Institute of Nanotechnology, Chinese Academy of Sciences N/A National Key Research and Development Program of China 2022YFA1504500 National Key Research and Development Program of China 2021YFF0704705 Guangdong Provincial Natural Science Fund for Distinguished Young Scholars 2021B1515020081 Scientific Equipment Development Project of Chinese Academy of Sciences N/A License OptionCC BY-NC
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pmcINTRODUCTION

As a clean and renewable energy source, solar energy has been widely used in various fields, among which photothermal conversion is the most direct way of using solar energy (1–4). However, due to the narrow spectral absorption range and thermal dissipation in the infrared (IR) region, most traditional photocatalytic processes still have relatively low efficiency in using sunlight. Recently, photothermal effects have emerged as an efficient strategy in catalysis, since the transformation of light to heat can intrinsically promote the performance of C1 molecules including CO, CO2, and CH4 (5–10). Over the past decades, substantial efforts have been devoted to CH4 oxidation, and various chemicals including CH3OOH, CH3OH, HCHO, and HCOOH were successfully produced from CH4 oxidation (11–15). Nevertheless, great challenges remain for precisely balancing the conversion and selectivity during CH4 oxidation, as a result of low selectivity to the target product. It is of great importance to develop efficient catalytic processes to realize the selective oxidation of CH4 to target products with solar light.

In principle, ideal materials for photothermal catalysis should be capable of absorbing light, especially in near-IR regions and exposing active sites. Pd-based catalysts have been widely used as active sites for C─H activation (16–20); however, their localized surface plasmon resonance (LSPR) properties are generally inferior to those of Au and Ag (21–24). It has been reported that the structural engineering of Pd nanoparticles, for instance, the ultrathin Pd nanosheets (NSs), endows enhanced LSPR properties in near-IR regions and displays excellent photothermal therapy performance (25). However, the photothermal catalysis of Pd NSs has not been revealed yet. Therefore, the ultrathin Pd nanostructures with near-IR region absorption properties increase the opportunity to design a Pd-based photothermal catalyst with an LSPR effect for C─H activation. Designing Pd-based nanocatalysts with enhanced absorption to visible and near-IR light is formidably challenging, which is vital for converting solar energy to heat.

Here, we fabricated freestanding ultrathin hexagonal Pd8Sb3 NSs with strong light absorption in the full spectral range and strengthened photothermal stability via a wet-chemical approach. Owing to the excellent photothermal properties, Pd8Sb3 NSs can serve as an efficient catalyst for CH4 selective oxidation to HCHO. Impressively, the productivity of HCHO reaches ~665 millimoles per gram of catalyst (mmol/gcat) with a high selectivity of ~98.7% under the irradiation of simulated solar light at a light intensity of 1.7 W/cm2 over Pd8Sb3 NSs, which is ~700 times higher than Pd NSs, showing an unprecedented catalytic performance of CH4 oxidation to HCHO. Mechanism studies imply that the photothermal oxidation of CH4 is triggered by radicals. In particular, Pd8Sb3 NSs display stronger adsorption strength to CH4 and facilitate CH4 oxidation to HCHO with the assistance of ·OH radical. In addition, the promising application of Pd8Sb3 NSs has been demonstrated in the photothermal therapy of breast cancer. We believe this work will attract the rapid interest of researchers in diverse fields, including materials science, catalysis, chemistry, bioscience, and beyond.

RESULTS

Synthesis and structural characterizations

Pd-Sb NSs were synthesized by adding palladium acetylacetonate [Pd(acac)2], triphenylantimony [(C6H5)3Sb], 2-methylimidazole (C4H6N2), and polyvinyl pyrrolidone [(C6H9NO)n, PVP] into N-methylpyrrolidone (C5H9NO, NMP), and heated at 160°C for 5 hours (see Materials and Methods for details) (Fig. 1A). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and TEM images reveal that the hexagonal and flake feature of Pd-Sb NSs with average diagonal length of ~525 nm (Fig. 1, B to D). The thickness of an individual NS was measured to be ~5 nm on the basis of atomic force microscopy (AFM) image (Fig. 1E). Elemental mapping shows that Pd and Sb are uniformly distributed in Pd-Sb NSs (Fig. 1F), which was further validated by the line-scan analysis across the diagonal direction (fig. S1). X-ray diffraction (XRD) was carried out to investigate the structure of the Pd-Sb NSs. The characteristic peaks in the XRD pattern were ascribed to hexagonal Pd8Sb3 [powder diffraction files (PDF): 44-0825] (Fig. 1G and fig. S2). Through drop coating the ethanol dispersion of Pd8Sb3 NSs on a silicon substrate, Pd8Sb3 NSs would horizontally stack on silicon, leading to the formation of (001)-orientated arrays. Besides, the atomic ratio of Pd/Sb was calculated to be ~2.80 (73.7/26.3) in Pd8Sb3 NSs, which is close to the value from inductively coupled plasma optical emission spectrometer (ICP-OES) (~2.50) (fig. S3 and table S1).

Fig. 1. Preparation and structural characterizations of Pd8Sb3 NSs.

(A) Scheme for the synthesis of Pd8Sb3 NSs. (B) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, (C) TEM image, (D) size distribution, (E) atomic force microscopy image, (F) HAADF-STEM image with element mappings, and (G) x-ray diffraction pattern of Pd8Sb3 NSs. Inset in (E) is the corresponding height profile of Pd8Sb3 NSs. Inset in (G) shows the schematic diagram of randomly arranged Pd8Sb3 NSs on a Si substrate. a.u., arbitrary units.

Aberration-corrected HAADF-STEM images were collected to further reveal the atomic structure of Pd8Sb3 NSs. Figure 2A displays the HAADF-STEM image of an individual Pd8Sb3 NS when adjusting the sample to the position of the positive belt axis. The selected area electron diffraction (SAED) pattern in Fig. 2B exhibits one set of sixfold symmetry diffraction spots, confirming the single-crystalline nature of NS with a hexagonal structure. Besides, the SAED pattern was indexed to the [001] axis of Pd8Sb3 with R3-CH #161 space group. Figure 2C displays a bright-field high-resolution TEM (HRTEM) image of the enlarged edge position of Pd8Sb3 NSs, where the lattice fringe that parallel to the edge direction is 0.25 nm, corresponding to the two-thirds of the layer spacing d (110) planes (Fig. 2C). It is found that the edge of Pd8Sb3 NSs is perpendicular to the [110] direction. The surface atomic arrangement that was viewed from the [001] direction of the unit-cell atomic model of Pd8Sb3 was shown in Fig. 2D. The [110] direction can be referred to as the growth orientation that is perpendicular to the (110) plane. As we superimpose the atomic arrangement model viewed from [001] direction on the high-resolution HAADF-STEM images, the atomic columns with brightness contrast are observed in Fig. 2E, in agreement with the atomic model of Pd8Sb3. Furthermore, the atomic resolution elemental mappings measured in the middle area from NSs also displayed the ordered hexagonal phase of the even distribution of Pd and Sb (Fig. 2, F to H). However, the dim atomic imaging appears as analyzed from the position near the edges of Pd8Sb3 NSs (Fig. 2I). To further verify the atomic arrangement at the edge area, we rotated the unit cell of Pd8Sb3 toward [100] direction, where we can clearly observe the atomic situation parallel to the electron beam (Fig. 2J). The results indicated that the dim atomic imaging has no concern with the absence of Pd or Sb atoms at the edge but may be the contrast difference caused by the thin edges of the NSs morphology, which was in accordance with the atomic resolution elemental mappings from the edge location (Fig. 2, K to M). Therefore, the Pd8Sb3 NSs contain two {001} and six {110} planes and grow along the 110,1¯10,100,1¯00,010 , and 01¯0 directions (Fig. 2N).

Fig. 2. Atomic structure characterizations of Pd8Sb3 NSs.

(A) HAADF-STEM and (B) selected area electron diffraction images of Pd8Sb3 NSs. (C) HRTEM image of the edge area of Pd8Sb3 NSs. (D) The unit-cell atomic arrangement viewed along [001] direction. (E) High-resolution HAADF-STEM image and (F to H) the corresponding atomic resolution elemental mappings taken from the middle area of Pd8Sb3 NSs. (I) High-resolution HAADF-STEM images taken from the edge area of Pd8Sb3 NSs. (J) The unit-cell atomic arrangement viewed along [100] direction. (K to M) The atomic resolution elemental mappings taken from the edge area of Pd8Sb3 NSs. (N) Schematic of the structure and growth direction of Pd8Sb3 NSs.

Spectroscopy characterization

The optical absorption properties of Pd8Sb3 NSs were characterized by ultraviolet-visible–near-IR (UV-Vis–NIR) spectroscopy (Fig. 3A). It is found that Pd NSs display absorption in the UV-Vis–NIR region, while Pd8Sb3 NSs exhibit much stronger absorption than that of Pd NSs even its concentration is only 1/4 of Pd NSs (Fig. 3A), revealing the excellent light absorption ability of Pd8Sb3 NSs throughout the entire spectral region. Moreover, an individual Pd8Sb3 NS was analyzed by using dark-field light scattering spectroscopy to further understand the optical properties (Fig. 3B and fig. S4). Note that the Pd8Sb3 NSs enable strong absorbing and scattering light waves, with a surface plasmon resonance at ~580 nm. In addition, the numerical simulation of Pd8Sb3·NSs theoretically confirms the absorption feature in the NIR region (Fig. 3C). Moreover, it is found that the laser power density has a substantial impact on the average temperature of the Pd8Sb3 NSs (fig. S5). The simulation result gives the spatial distribution of enhanced electric field |E/E0|, volumetric heat source density q(r), and the increased temperature ΔT at a power density of 1 mW/μm2 (Fig. 3, D to F). The Pd8Sb3 NS represents the strong enhanced electric field on the edges and thermal power generation on the surface with the excitation of the NIR light, accompanied by the increase of temperature by 100 K. Consequently, Pd8Sb3 NSs can efficiently convert light to thermal energy.

Fig. 3. Photothermal effects of Pd8Sb3 NSs.

(A) Ultraviolet-visible–near-IR spectra of aqueous dispersions of different samples. (B) Dark-field scattering spectrum of a single Pd8Sb3 NS (the inset indicated that the plane wave propagates along the z axis with the polarization direction parallel to the y axis). (C) Simulation spectra of Pd8Sb3 NSs. (D) Electric field distribution of Pd8Sb3 NSs under different excitation wavelengths. (E) Thermal power generation and (F) spatial distribution of temperature rise that excited at an incident power density of 1 mW/μm2. Scale bars, 200 nm.

Photothermal catalytic CH4 oxidation performance

Owing to the superior photothermal efficiency of Pd8Sb3 NSs, CH4 photothermal oxidation was selected as the proof-of-concept model (Fig. 4A), and Pd NSs were used as a reference. The photothermal CH4 oxidation measurements were carried out in a high-pressure sealed reaction chamber with an optical window under the concentrated light of a solar simulator. The gas-solid-liquid system was implemented under a mixture of O2/CH4 of 3.5-MPa pressure with 10 ml of H2O and 200 μl of H2O2 (30 wt %). Screening experiments on the catalytic performance of Pd-Sb NSs with different stoichiometric ratios (figs. S6 and S7) and thicknesses (figs. S8 and S9) show that Pd8Sb3 with a mean size of ~5 nm displays the highest productivity of HCHO and therefore was selected as the optimal catalyst for CH4 oxidation. When Pd8Sb3 NSs were used as the catalyst for CH4 photothermal oxidation, the yield of HCHO reached ~665 mmol/gcat with a selectivity of ~98.7% after 2 hours (fig. S10 to S12). In contrast, when Pd NSs were used as the catalyst for CH4 photothermal oxidation, CH3OH (0.762 mmol/gcat) and HCHO (0.947 mmol/gcat) were dominant in the products, with a selectivity of ~45 and ~55%, respectively, after 2 hours (Fig. 4B). The above results suggest that Pd8Sb3 NSs can serve as highly active and selective catalyst for CH4 photothermal oxidation to HCHO. To investigate the mechanism of photothermal CH4 oxidation, control experiments were conducted without light irradiation or CH4 (Fig. 4C). No products were detected in dark, suggesting that CH4 oxidation to HCHO was triggered by light irradiation (Fig. 4C). Given that the apparent temperature was ~70°C for Pd8Sb3 NSs after light irradiation, CH4 oxidation was performed without light irradiation but with heating to ~70°C. As shown in Fig. 4C, the absence of HCHO in products implies the significance of photothermal catalysis. Besides, no products were observed when CH4 was replaced by N2, suggesting that the formation of HCHO was ascribed to CH4 oxidation. Therefore, the performance of photothermal CH4 oxidation exhibits a strong dependence on light irradiation. Note that the yield and selectivity of HCHO gradually increase with the increased light intensities (Fig. 4D), suggesting the crucial role of the photothermal effect toward catalytic performance. Given that the photothermal effect may induce the hot electrons, we detected the photoinduced current with light irradiation. The light irradiation on Pd8Sb3 NSs leads to a substantial increase in photocurrent, suggesting the formation of hot electrons (fig. S13A). Besides, Nyquist plots were used to verify the enhanced migration of hot electrons with light irradiation (fig. S13B). The smaller radius of Pd8Sb3 NSs with light irradiation than that without light irradiation suggests that plasmonic excitation can facilitate the transfer of hot electrons. Moreover, we put the reaction system in an ice bath to retard the transfer of hot electrons. The productivity of HCHO decreased by 32% (fig. S13C), further confirming that hot electrons can enhance the CH4 photooxidation to HCHO. Furthermore, analysis on x-ray photoelectron spectroscopy (XPS) implies that the ratios of metallic Pd and Sb on the surface of the spent Pd8Sb3 NSs increase after light irradiation, which is attributed to the hot electrons (fig. S14). In addition, when the Pd8Sb3 NSs catalyst was irradiated with different single wavelength light sources of 380, 520, and 650 nm, HCHO was dominant in liquid products, further confirming that Pd8Sb3 NSs could absorb a broad range of light to trigger the oxidation of CH4 to HCHO (Fig. 4E). The apparent quantum efficiency values are ~0.24, ~0.015, and ~ 0.008% at 380, 520, and 650 nm, respectively, indicating the importance of light and thermal contributions toward CH4 oxidation reaction (fig. S15). Therefore, the satisfactory photothermal effect of Pd8Sb3 NSs at 1.7 W/cm2 resulted in a promising catalytic performance of CH4 oxidation to HCHO as compared with many previously reported catalysts (Fig. 4F and table S2). Given the promising performance toward photothermal catalytic CH4 oxidation, the stability of Pd8Sb3 was investigated. We recycled the Pd8Sb3 NSs for 10 consecutive cycles, and the steady yield of product in 10 cycles suggests the superior stability of Pd8Sb3 NSs for CH4 photothermal catalytic oxidation (fig. S16). Moreover, the gradual increase of product in 48 hours further confirms the superior long-term stability of Pd8Sb3 NSs for photothermal catalytic CH4 oxidation (fig. S17). Furthermore, no obvious Pd was observed in the solution based on ICP-OES measurement, suggesting that the leaching of the catalyst during the reaction is negligible. Besides, the enhanced stability of Pd8Sb3 NSs was further validated by the maintained hexagonal morphology and structure after catalysis (figs. S18 and S19).

Fig. 4. Catalytic performance toward CH4 oxidation.

(A) Schematic diagrams of photothermal CH4 oxidation on Pd8Sb3 NSs. (B) Investigations on the catalytic activities toward CH4 oxidation of noble metal aqueous dispersions. (C) Control experiments using Pd8Sb3 NSs catalyst in dark, dark with external heat at 70°C, and N2 atmosphere, respectively. (D) Solar-driven catalytic activities over Pd8Sb3 NSs under different light intensities. (E) Catalytic activities of Pd8Sb3 NSs at varied monochromatic wavelengths. (F) Comparison of catalytic performance of CH4 oxidation to HCHO with previously reported catalysts in the gas-solid-liquid system in the presence of H2O2.

Mechanism analysis

The above results show that Pd8Sb3 NSs can strongly absorb UV-Vis–NIR light and covert it to heat to trigger the oxidation of CH4 to HCHO (Fig. 5A). To reveal the reaction pathways of CH4 oxidation, in situ electron paramagnetic resonance (EPR) spectroscopy was used to track the reactive oxygen species (ROS) generated on the surface of samples with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical-trapping agent. As shown in Fig. 5B, two sets of radical signals were observed in the EPR spectrum of Pd8Sb3 NSs, where the signals could be assigned to ·OH radicals (26, 27) and ·CH3 radicals (28, 29), respectively. In contrast, the peaks of ·OH were absent in the EPR spectrum of Pd NSs (fig. S20), and the presence of ·O2− peaks suggested that CH4 oxidation on Pd NSs was different from that of Pd8Sb3 NSs (Fig. 5C) (30, 31). To further reveal the mechanism, an isotopic experiment was conducted using H218O and H216O2 for CH4 oxidation. The product consists of 46% HCH16O and 54% HCH18O (fig. S21), suggesting that the presence of H2O also contributes to HCHO production. Moreover, in situ IR spectra were collected to monitor the surface reaction intermediates on Pd8Sb3 NSs (fig. S22A). In IR spectra, the peaks at ~2700 and 2300 cm−1 were assigned to the stretching vibration of the C─H bond of the aldehyde group and CO2, respectively (fig. S22B) (32, 33). Note that the intensity of the C─H peak gradually increases with the increased irradiation time, suggesting the formation of HCHO. In contrast, the peak intensity of CO2 remained unchanged after 40 min (fig. S22C), which was consistent with the experimental observation of CH4 selective oxidation to HCHO.

Fig. 5. Mechanism analysis of CH4 oxidation.

(A) Schematic diagrams of CH4 catalytic oxidation on Pd8Sb3 NSs. (B) In situ electron paramagnetic resonance (EPR) spectra of Pd8Sb3 NSs obtained by dissolving CH4, O2, and H2O2 in water under light illumination. (C) In situ EPR spectra of Pd NSs obtained by dissolving CH4, O2, and H2O2 under light illumination in methanol. (D and E) Reaction pathways for the formation of ·OH + ·OH, ·OH + H+, ·O2− on the surface of Pd8Sb3 and Pd NSs. (F) Spin density maps for the ·O2−, ·OH + H+, and ·OH + ·OH on the surfaces of Pd8Sb3 and Pd NSs. (G and H) Possible reaction pathways and free energy diagrams for the production of HCHO and CH3OH during CH4 oxidation on the surface of Pd8Sb3 and Pd NSs. The blue shading indicates the rate-determining steps. The inset shows the optimized intermediates and transition state structures. Color scheme: cyan for Pd, purple for Sb, red for O, white for H, and gray for C.

In addition, the reaction pathways of CH4 oxidation were further revealed through density functional theory (DFT) calculations (34–36). Considering that CH4 oxidation on Pd8Sb3 and Pd NSs involves ·OH and ·O2−, respectively, Gibbs free energy calculations for the generation of ·OH and ·O2− on Pd8Sb3 and Pd NSs were conducted (Fig. 5, D and E, and fig. S23). It is found that H2O2 → 2·OH on Pd8Sb3 NSs is a spontaneous process with an energy barrier of 0.08 eV, much lower than that of H2O → ·OH + H+ (1.79 eV) and O2 → ·O2− (2.75 eV). In contrast, on the surface of Pd NSs, the barrier for O2 → ·O2− (0.10 eV) is lower than that for H2O2 → 2·OH (0.22 eV) and H2O → ·OH + H+ (1.04 eV). Considering that the much lower barrier for H2O2 → 2·OH (0.08 eV) than H2O → ·OH + H+ (1.79 eV) on Pd8Sb3 NSs, in principle, H2O2 should act as the major oxidant for CH4 oxidation comparing with H2O, which seems to conflict with experimental observation (i.e., 54% of HCHO from H2O). Therefore, the consumption of H2O2 was evaluated with titanium oxide sulfate spectrophotometry (fig. S24). Note that ~99.7% of H2O2 was consumed in the first 1 hour when Pd8Sb3 NSs were used as catalysts (fig. S25 and table S3). Once most H2O2 was consumed, H2O would act as the secondary oxidant for CH4 oxidation, as a result of the presence of HCH18O in the final product during the isotopic experiment. Further calculations were conducted to evaluate the interaction of ·O2− and ·OH on the surfaces of Pd8Sb3 and Pd NSs. The optimized bond lengths of O─O/O─H on Pd8Sb3 and Pd NSs are 1.37/0.98 and 1.34/0.98 Å, respectively (fig. S26), in line with the values in previous reports (36, 37). Spin electronic density calculations indicate that ·O2− on the surfaces of Pd8Sb3 and Pd NSs exhibits a side-on binding mode (π bond), while ·OH shows an end-on binding mode (σ bond) (Fig. 5F). In addition, the impact of different structural coordination environments on the work function values results in differences in the work function between ·O2− and ·OH radicals (fig. S27). Specifically, Pd NSs have a higher work function (5.25 eV) than Pd8Sb3 NSs (4.56 eV), indicating that Pd NSs are more prone to losing electrons and binding with ·O2−. Conversely, Pd8Sb3 NSs are more likely to bind with electron acceptors, namely, ·OH. The Bader charges of ·O2− and ·OH on the surfaces of Pd8Sb3 and Pd NSs are −0.77/−0.52, −0.47, and −0.49/−0.46 |e|, respectively (fig. S26C), indicating that Pd atoms on the surfaces of Pd8Sb3 and Pd NSs act as active centers for oxidation. Besides, the electron localization function results further highlight the substantial ionic bonding components arising from electron delocalization between Pd and ·OH and ·O2− radicals (fig. S28). Furthermore, DFT calculations were performed to reveal the possible pathways for the oxidation of CH4 to HCHO and CH3OH (Fig. 5, G and H). Compared to Pd NSs, Pd8Sb3 NSs exhibit stronger adsorption ability to CH4 (fig. S29), which is further confirmed by the density of states analysis (fig. S30). For Pd8Sb3 NSs, the strong coupling between the p orbitals of Sb and the d orbitals of Pd reduces the energy level distance between the d band center of Pd and the p band center (εd − εp = 2.84 eV), indicating the enhanced orbital interactions and strengthened adsorption. In contrast, Pd NSs have a larger value of εd − εp (3.22 eV), that is, a weaker orbital coupling and CH4 adsorption strength (fig. S30). On Pd8Sb3 NSs, the adsorbed *CH4 will convert into *·CH3, which further couples with *·OH into *CH3OH with a barrier of 1.72 eV (Fig. 5G). Afterward, the formed *CH3OH will further convert into HCHO due to the substantially higher desorption barrier of CH3OH (0.58 eV) compared with the formation barrier of HCHO (0.41 eV) (Fig. 5G and fig. S31A). For Pd NSs, the rate-determining step is *·CH3 + ·O2− → *·CH3O + *·O2−, with an energy barrier of 2.18 eV (Fig. 5H). Note that the desorption energy of HCHO (0.52 eV) is close to that of CH3OH formation (0.60 eV) on Pd NSs, as a result of the copresence of HCHO and CH3OH in the final product, which is consistent with experimental observations (Fig. 5H and fig. S31B).

Photothermal therapy

Last, Pd8Sb3 NSs were used for the photothermal therapy of cancer cells (Fig. 6A). Specifically, Pd8Sb3 NSs were irradiated by an 808-nm laser with a power density of 3 W/cm2 for 15 min, and the temperature increases were recorded by a thermodetector. It was found that temperature increased from ~20° to ~62°C within 15 min at a concentration of 100 μg/ml (Fig. 6B), which was much higher than that of pure water under the same irradiation (fig. S32). Moreover, Pd8Sb3 NSs exhibit excellent photothermal stability under continuous irradiation with an 808-nm laser (3 W/cm2) (fig. S33), with a photothermal conversion efficiency of ~10.5% (fig. S34). Furthermore, we use 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays in PC12 cells to assess the cytotoxicity of Pd8Sb3 NSs. No obvious toxicity of Pd8Sb3 NSs to normal cells was observed in the concentration range of 0 to 125 μg/ml (Fig. 6C). To investigate the therapeutic effect, 4T1 cells were coincubated with Pd8Sb3 NSs ( 50 μg/ml) for 12 hours, followed by irradiation at a power density of 3 W/cm2 for different durations. It was observed that the mortality of the cells gradually increased with the irradiation time, and the cell viability remained only 20% after 6 min (Fig. 6D). To observe the condition of cells, the cytotoxicity was further visualized through staining with calcein AM and propidium iodide (PI). The cells in the NIR group showed red fluorescence with the increase of irradiation time, indicating that the Pd8Sb3 NSs could enter the cells and kill cancer cells through near-IR light irradiation (green and red fluorescence–labeled live and dead cells, respectively; Fig. 6E). It was found that the cells were near-completely killed by laser irradiation after 6 min (Fig. 6E). Besides, the promising application of Pd8Sb3 NSs for tumor hyperthermia has been validated by killing 4T1 cell (figs. S35 and S36).

Fig. 6. Photothermal therapy of Pd8Sb3 NSs.

(A) Schematic of the photothermal therapy process. (B) A time-dependent temperature increase of Pd8Sb3 NSs with different concentrations. (C) The cell viability of PC12 with different concentrations of Pd8Sb3 NSs. (D) The cell viability with different irradiation times. (E) Bright-field (BF) image, calcein AM, propidium iodide (PI) staining, and merge images in different groups. Scale bars, 20 μm.

DISCUSSION

In summary, we have fabricated freestanding hexagonal Pd8Sb3 NSs with strong absorption to UV-Vis–NIR light. Owing to the excellent photothermal efficiency, Pd8Sb3 NSs can serve as efficient catalysts for the photothermal oxidation of CH4 to HCHO. Impressively, the productivity of HCHO reaches ~665 mmol/gcat with a HCHO selectivity of ~98.7% at a light intensity of 1.7 W/cm2, which is ~700 times higher than that of Pd NSs under the same conditions. Mechanism investigations suggest that the oxidation of CH4 on Pd8Sb3 and Pd NSs is triggered by radical processes. The Pd8Sb3 NSs with stronger CH4 adsorption strength can effectively promote CH4 activation, and with the assistance of ·OH radicals, Pd8Sb3 NSs can selectively oxidize CH4 to HCHO. Conversely, the ·O2− radicals created higher reaction energy barriers to produce CH3OH and HCHO, leading to low activity and selectivity of CH4 oxidation on Pd NSs. In addition, the excellent photothermal properties of Pd8Sb3 NSs endow the photothermal therapy for breast cancer. This work shows unique plasmonic noble metal alloys with full spectral absorption and efficient photothermal conversion capabilities and is expected to become potential candidates in photothermal conversion and beyond.

MATERIALS AND METHODS

Chemicals

Palladium acetylacetonate [Pd(acac)2; ≥99.0%] was purchased from J&K. Triphenylantimony [(C6H5)3Sb; >95.0%] was obtained from TCI. 2-Methylimidazole (C4H6N2; ≥98.0%) was purchased from Aladdin Biochemical Technology Co. Ltd. (Shanghai, China). Polyvinyl pyrrolidone [(C6H9NO)n, PVP; molecular weight (Mw) = 58,000] was obtained from Energy Chemical Co. Ltd. N-methylpyrrolidone (C5H9NO, NMP; ≥99.0%), N, N-dimethylacetamide (C4H9NO, DMAC; ≥99.5%), formaldehyde (HCHO; 37 to 40 wt %), acetic acid (CH3COOH; ≥99.5%), and hydrogen peroxide (H2O2; 30 wt %) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ammonium acetate (CH3COONH4; 97%) was obtained from Senrise Technology Co. Ltd. (Anhui, China). Acetylacetone (C5H8O2; 99%) was purchased from Acmec Biochemical Co. Ltd. (Shanghai, China).

Synthesis of Pd8Sb3 and Pd NSs

In a typical preparation of Pd8Sb3 NSs, Pd(acac)2 (7.6 mg), (C6H5)3Sb (4.4 mg), C4H6N2 (17 mg), and PVP (100 mg) were dispersed in NMP (10 ml), and the mixture was ultrasonicated for 1 min, and then the homogeneous mixture was heated at 160°C for 5 hours in an oil bath. After cooling to room temperature, the products were collected by centrifugation and washed three times with ethanol, followed by fully washing with ultrapure water. For the synthesis of Pd NSs, 7.6 mg of Pd(acac)2,100 mg of PVP, and 10 ml of DMAC were homogeneously mixed in a pressure-resistant glass bottle under ultrasonication. After being charged with high-purity CO gas (99.999%) to 0.15 MPa, the vessel was heated at 150°C for 2 hours. The products were collected by centrifugation and washed with ultrapure water three times.

Characterizations

TEM was performed on a JEM-1400 microscope with an accelerating voltage of 100 kV. HAADF-STEM, line-scan analysis, elemental mappings, HRTEM, and SAED were conducted on an aberration-corrected JEM-ARM300F microscope operating at 300 kV. AFM images were recorded on a Bruker dimension icon. The powder XRD was tested on an Ultima-IV x-ray diffractometer using Cu Kα radiation. Scanning electron microscopy energy-dispersive spectrometer was performed on a Hitachi S-4800 scanning electron microscopy. The content of metal was measured by the ICP-OES conducted with iCAP 7000 Thermo Fisher ICP-OES (Thermo Fisher Scientific, Waltham, MA USA) in axial mode. UV-Vis–NIR spectroscopy was collected on a JASCO-V-780 spectrometer. XPS analyses were conducted with Thermo Fisher Scientific K-Alpha. The isotope labeling experiments of HCHO were detected with gas chromatography–mass spectrometry (QP2020, Shimadzu Co. Ltd) equipped with the Cap WAX column to identify the existence of H218O.

Photothermal catalytic CH4 oxidation measurement

The photothermal CH4 oxidation was carried out in a stainless steel autoclave equipped with a light entry window. First, 0.4 mg of Pd8Sb3 NSs was dispersed in 10 ml of ultrapure water, and then we added a certain volume of H2O2 (30 wt %) as an oxidant. After the reactor was sealed, it was flushed with high-purity O2 (99.999%) for 20 min and then pressured to 0.5 MPa. Afterward, high-purity CH4 (99.999%) was pressured into the reactor to a total pressure of 3.5 MPa. After continuous stirring for 15 min, a xenon lamp source (emission wavelength of 300 to 2500 nm, PLS-SXE300+, Perfect Light) was to trigger the CH4 oxidation. The light intensities were sensed by a light intensity meter, and bandpass filters were used to tune the wavelength of light.

Product analysis

CH3OOH and CH3OH were analyzed by 1H nuclear magnetic resonance spectroscopy using dimethyl sulfoxide (DMSO) as the internal standard on the Bruker Advance III 500 MHz instrument. Typically, the 500-μl liquid product was mixed with 100 μl of DMSO (100 parts per million in D2O). The productivity of HCHO was determined by the acetylacetone color method. In particular, 25 g of CH3COONH4 was dissolved in 10 ml of ultrapure water, followed by adding 3 ml of CH3COOH and 250 μl of C5H8O2. Afterward, 70 ml of ultrapure water was added under continuous stirring until the formation of a homogeneous solution. Different concentrations of HCHO aqueous solutions (2 ml) were reacted with the chromogenic agents (3 ml) for 2 hours at room temperature. After plotting the HCHO content versus absorbance intensity at 413 nm by UV spectroscopy, the productivity of HCHO can be quantified by comparing the absorbance intensity against the calibration curve. The gaseous products were analyzed by a gas chromatograph (Agilent) equipped with a methanator and flame ionization detector and thermal conductivity detector.

In situ EPR spectra for the detection of radicals

In situ EPR spectra were measured on Bruker EMX-10/12 EPR spectrometer (300 K, 100 kHz) at ambient temperature. DMPO was used as the trapping agent for monitoring the ROS including ·O2− and ·OH radicals. For the detection of ·OH radicals, 0.4 mg of Pd8Sb3 NSs was dispersed in 10 ml of ultrapure water and 200 μl of H2O2, followed by adding 50 μl of DMPO. Afterward, the mixture of CH4 and O2 was introduced into the stainless steel autoclave. After irradiating with light for 10 min, the solution was immediately taken out and injected into a capillary tube for the EPR test. The detection of ·O2− radicals was similar to that of ·OH, except for changing the reaction solution to methanol.

In situ IR spectra for the detection of reaction intermediates

In situ IR spectra were collected on Mettler-Toledo ReactIR 15. Typically, Pd8Sb3 NSs were dispersed in ultrapure water and put into the cell, and a mixture of CH4 and O2 was continuously introduced under continuous stirring for 60 min. After that, 200 μl of H2O2 was added into the solution. Before collecting the in situ IR spectra, we first collected the spectra in the dark until a stable signal was obtained.

Numerical simulation method

We demonstrate the finite element method simulation via COMSOL Multiphysics. Both Maxwell’s equations and thermal conduction equations are solved in one model, in which one piece of the hexagonal Pd8Sb3 NSs is illuminated by the linear polarized plane wave as the heat source. The incident direction of the plane wave is perpendicular to the planar NS (fig. S4), with a power density of 1 mW/μm2. The lateral size and thickness of Pd8Sb3 NS are 525 and ~5 nm, respectively, in line with the experimental results. Since the permittivity of NSs is critical to the solution of Maxwell’s equations, we assume the permittivity of Pd8Sb3 alloy as the composition-weighted average of the bulk Pd’s and Sb’s permittivity (38–40). The permittivity of alloy can be written asεPdxSb1−xω=xεPdω+1−xεSbω

where x is set as 0.71 according to the element analysis.

After Maxwell’s equations are solved, the volumetric heat source density q(r) generated inside the alloy is given byqr=ω2ImεPdxSb1−xEr2

where ω is the angular frequency of the incident light and E(r) is the electric field in the NS. The thermal conductivity of water is set as 0.6 W/(mK).

Calculation details

In this study, all DFT calculations were performed using the Vienna Ab-initio Simulation Package (VASP) (41). The calculations involved the use of the generalized gradient approximation with the Perdew-Burke-Ernzerhof functional and the ion cores were described using the projector augmented wave method (42, 43). The plane-wave basis set for valence electrons had a kinetic energy cutoff set precisely at 400 eV. The Kohn-Sham orbitals were partially occupied using a Gaussian smearing method with a smearing width set to 0.05 eV to ensure self-consistent convergence of electronic energy below 10−5 eV. Geometric optimization was considered converged when the change in forces was less than 0.03 eV/Å, using a 3 × 3 × 1 Monkhorst-Pack grid for the Brillouin zone sampling.

To account for van der Waals dispersion interactions between intermediates and the catalyst, the DFT + D3 method incorporating Grimme’s D3 dispersion correction was used (44, 45). On the basis of the results of HRTEM, the (001) facet of Pd8Sb3 and the (111) facet of Pd were selected for simulation. In addition, we used the dipole correction method to ensure precision in the calculation of work functions. For comparative analysis across different materials, the vacuum level was set uniformly to 0 eV for all materials. Furthermore, to accurately compute the structure and electronic properties of radicals, spin polarization was considered, and localization settings were applied to the radical electrons. In parallel, the effect of the solvent on the catalytic reaction was studied using the implicit solvation model VASPsol (46), with a dielectric constant set to that of water (ε = 78.4). The calculation of Gibbs free energy was based on the formula: ΔG = ΔE(DFT) + ΔE(ZPE) – TΔS (47), where ΔE(DFT) represents the total energy change obtained from VASP, ΔE(ZPE) represents the change in zero-point energy, and TΔS is the product of temperature and entropy change. The climbing image nudged elastic band method (48) was used to determine the energy barriers and transition states along the reaction path, confirming that each transition state had only one imaginary (negative) vibrational mode. The activation energy Ea was calculated using the formula: Ea = E(TS) − E(IS), where E(TS) and E(IS) represent the energies of the transition state and initial state, respectively.

Photothermal efficiency

The investigation of the photothermal conversion efficiency of Pd8Sb3 NSs was conducted on FJ808X13. Temperature variations of aqueous solutions containing different concentrations of Pd8Sb3 NSs (25, 50, 100, and 200 μg/ml) were monitored with a time interval of 30 s during uniform laser irradiation. The stability and the photothermal conversion efficiency were used to evaluate the performance (49–51).

Cell culture

All cell lines were obtained from the cell bank of type culture collection of the Chinese Academy of Sciences and propagated in Dulbecco’s modified Eagle’s media (DMEM) supplemented with 10% fetal bovine serum (Hyclone) and antibiotics (1% penicillin/streptomycin). The cells were cultured in a humidified atmosphere containing 5% CO2. Cell media were replenished every 2 or 3 days.

In vitro cytotoxicity assay

The MTT assay was used to evaluate the cytotoxicity of Pd8Sb3 NSs in PC12 cells. Cells were seeded onto 96-well plates at a density of 6 × 103 cells per well and allowed to incubate in 200 μl of DMEM medium for 24 hours. Then, cells were exposed to various concentrations of Pd8Sb3 NSs, which had been dispersed in DMEM medium, and incubated for 12 hours. The cytotoxic effect of Pd8Sb3 NSs on PC12 cells was determined by measuring the cell survival rate using the MTT method.

Live/dead cell staining assay

4T1 cells were seeded onto confocal culture dishes at a density of 5 × 105 cells per well and cultured in 2 ml of DMEM medium. After 24 hours of incubation, the cells were treated with different groups and coincubated for 12 hours. The medium was then replaced with a fresh medium, and the cells were irradiated with 808-nm lasers of the same power intensities for different durations. Untreated cells were used as controls. All cells were subsequently treated with the calcein AM/PI cell activity, and a cytotoxicity detection kit obtained from Biyuntian was used to label live and dead cells and incubated at 37°C for 30 min. Subsequently, the cells were subjected to a thorough cleansing using phosphate-buffered saline, followed by a meticulous inspection through the employment of a confocal laser microscope. After performing the same procedures on the 96-well plate, the cell viability was assessed using the MTT assay.

Acknowledgments

Funding: We thank the financial supports by the National Key R&D Program of China (2022YFA1504500 to X.Hua.), the National Natural Science Foundation of China (22025108, U21A20327, and 22121001 to X.Hua.), Guangdong Provincial Natural Science Fund for Distinguished Young Scholars (2021B1515020081 to Y.X.), start-up support from Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS; to Y.X.), National Key Research and Development Program of China (no. 2021YFF0704705 to X.Hua.), Scientific Equipment Development Project of Chinese Academy of Sciences, and Youth Innovation Promotion Association Project of Chinese Academy of Sciences (2020026 to X.Hua.).

Author contributions: X.Hua. and Y.X. designed and supervised this work. M.W. carried out the synthesis/characterizations/photothermal catalytic experiments and wrote the manuscript. X.Hua. and Y.X. revised the manuscript. J.J. and Y.G. conducted the DFT simulation section. J.X. and X.M. measured the AC-STEM section. X. Hu and X.C. contributed to photothermal therapy. Z.M. and J.Y. carried out Raman spectra and numerical simulation. F.X. and H.P. participated in experimental discussions. All authors contributed to the general discussion. M.W. and J.J. contributed equally to this work.

Competing interests: The authors declare that they have no competing interest.

Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

Supplementary Materials

This PDF file includes:

Figs. S1 to S36

Tables S1 to S3

References
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