Synthesis of MagnetiteâSemiconductorâMetal Trimer Nanoparticles

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Synthesis of Magnetite-Semiconductor-Metal Trimer Nanoparticles through Functional Modular Assembly: A Magnetically Separable Photocatalyst with Photothermic Enhancement for Water Reduction Fei Pang, Ruifang Zhang, Dengpeng Lan, and Jianping Ge ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17046 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

Synthesis of Magnetite-Semiconductor-Metal Trimer Nanoparticles through Functional Modular Assembly: A Magnetically Separable Photocatalyst with Photothermic Enhancement for Water Reduction Fei Pang, Ruifang Zhang, Dengpeng Lan, Jianping Ge* Shanghai Key Laboratory of Green Chemistry and Chemical Processes School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200062, China E-mail: [email protected] Abstract: Hybrid nanoparticles have intrinsic advantages to achieve better activity in photocatalysis compared to single component materials, as it can synergistically combine functional components which promote light absorption, charge transportation, surface reaction and catalyst regeneration. Through functional modular assembly, a rational and stepwise approach has been developed to construct Fe3O4-CdS-Au trimer nanoparticles and its derivatives as magnetically separable catalysts for photothermo-catalytic hydrogen evolution from water. In a typical step-by-step synthetic process, Fe3O4-Ag dimers, Fe3O4-Ag2S dimers, Fe3O4-CdS dimers and Fe3O4-CdS-Au trimers were synthesized by seeding growth, sulfuration, ion-exchange and in-situ reduction consequently. Following the same reaction route, a series of derivative trimer nanoparticles with alternative semiconductor and metal were obtained for water-reduction reaction. The experimental results show that the semiconductor acts as an active component for photocatalysis and the metal nanoparticle acts as a co-catalyst for enhancement of charge separation, while the Fe3O4 component renders the catalysts convenient separation in magnetic field and improved photocatalytic activity under NIR illumination due to photothermic effect.

Keywords: Photocatalyst; Hybrid nanocrystal; Water reduction; Photothermic assistance ; Magnetic separation

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Introduction Colloidal hybrid nanoparticles have been proved as highly tunable and multifunctional materials for various applications such as biomedical imaging1-2, solar-energy harvesting3, heterogeneous catalysis4-5, medical diagnostics and treatment6 and photoelectrical devices2, etc. Among these applications, much attention has been paid to their potential in photocatalysis.7-9 Compared to nanoparticles with only one chemical composition or concentric core-shell structure whose surface chemistry is merely determined by the shell component, hybrid nanoparticles allow integration of distinct components into a one-particle system.10-11 Therefore, each subunit can provide surface functional moieties due to the hybrid structure and the resulted new topological properties can be used in photocatalysis. It is known that the efficiency of photocatalytic system depends on three independent parameters, namely the ability to absorb photons, the efficiency in production of electron-hole pairs and the catalytic activity of surface redox sites. It is difficult to promote these factors at the same time for single-component material. However, the hybrid nanoparticles will have intrinsic advantages to achieve better performance by solving the above problems. Therefore, the construction of hybrid nanoparticles following the strategy of functional modular assembly has been proposed recently to integrate components with different functions into one-particle system. Recent works did reveal the feasibility of functional modular assembly method in construction of hybrid nanoparticles and demonstrate their advantages in photocatalysis. For instance, Schaak et al.12 showed a total-synthesis framework for the construction of high-order oligomers of nanocrystals including M-Pt-Fe3O4 (M=Au, Ag, Ni, Pd) heterotrimers and MxS-Au-Pt-Fe3O4 (M=Pb, Cu) heterotetramers. Weng et al.13 developed a general synthetic method to achieve hierarchical control of

high-order

non-centrosymmetric

Pt-AuAg-CdSe

hybrid

nanostructures

with 14.8-fold

enhancement of photocatalytic efficiency than conventional photocatalysts, which uncovered a hot plasmonic electron-driven photocatalysis mechanism. Zhang et al.7, 14 developed a facile synthetic route to obtain metal-semiconductor (Au-CdS) hybrid nanostructures with fine-tuned symmetry by maneuvering interfacial strain during growth process. It was found that asymmetric Au-CdS heterodimer nanoparticles had much higher photocatalytic activity than symmetric ones because the synergetic effect promoted the enrichment of electron on Au tips for efficient water reduction. In some cases, the metal part in metal-semiconductor dimers contributes to the enhancement of activity -2-


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both in plasmonic resonance absorption and charge separation promotion, such as multi-segmented CdS-Au nanorods arrays developed by Wang15 and the Pt-tipped CdSe@CdS nanorods synthesized by Kalisman16. The introduction of iron oxide nanoparticles can not only bring convenience to separation and cycling utilization of catalysts, but also can promote the photocatalytic activity with photothermic assistance. Similar to homogenous catalysts, these nanocatalysts with high catalytic activity are usually well dispersed in aqueous solution, which makes fast separation hard to achieve by traditional sedimentation and filtration at the same time. A possible solution to this problem is the introduction of magnetic materials to produce magnetic separable nanoparticles (MNPs). Since Fe3O4 nanoparticles have superparamagnetic characteristics, the composite nanoparticles strongly respond to the external field during magnetic separation, and redisperse back into the reaction solution after withdrawing the magnetic flied.17 Another problem to be solved is the utilization of solar energy. Because most photocatalysts can absorb ultraviolet (UV) light and visible light, but not near infrared rays (NIR) which accounts for about 46 % of overall solar spectrum.18 Therefore, the utilization of NIR light in photocatalytic system is seldom reported.19 The introduction of photothermic materials, such as Cu3BiS320, Cu9S521, Cu7S422, Ti8O1523, may be a feasible plan. Among these materials, Fe3O4 nanoparticles have attracted lots of attention in photothermic application because it shows a list of pgreat properties for efficient and stable photothermic reactions, including strong intraparticle bonds, strong bonds to many surfactants, environmental friendly and economic efficiency. 24 Although some progress has been achieved during the study of magnet-metal-semiconductor hybrid nanostructures, there is still a big challenge to integrate these three functional subunits into a one-particle system. Herein, we developed a universal and stepwise approach for constructing a series of trimer nanoparticles which integrated three functional components (magnet, semiconductor, metal) into a one-particle system by functional modular assembly. First of all, Fe3O4-Ag2S dimers were prepared by a seeding growth of Ag nanocrystals onto cubic Fe3O4 nanocrystal followed by sulfuration. Based on Fe3O4-Ag2S dimers, a series of Fe3O4-semiconductor dimers were synthesized by ion-exchange. Then, metal nanoparticles were anchored onto the surface of semiconductor by in-situ reduction from metal salt precursors. For the as-prepared trimer nanoparticles, the chemical -3-



composition of semiconductor and metal can be changed in synthesis. In water reduction reaction, these trimer nanoparticles showed high activity in water splitting reaction as well as great convenience in separation and reutilization. The photocatalytic activity of different catalysts with tunable components were also investigated and compared. Furthermore, the introduction of Fe3O4 can not only bring the convenience to recycle catalysts, but also promote the activity with photothermic assistance in NIR light irradiation. The functional modular assembly can incorporate magnetic, catalytic, plasmonic and semiconductor functions through programmable composition, dimensional and structural arrangement, which provide an ideal system to investigate the relationships between activities and structures.

Results and discussion Synthesis and characterization of magnetite-semiconductor-metal trimers The multistep synthesis of Fe3O4-CdS-Au trimer nanoparticles and its derivative usually include three continuous reactions. (Figure 1) In step 1, the Ag component of Fe3O4-Ag dimer could be sulfurized to form Fe3O4-Ag2S nanoparticles without changing the structure of Fe3O4 nanocubes. Here, Fe3O4 cubes were synthesized by thermal decomposition of the iron-oleate complex in 1-octadecene in the presence of oleic acid as capping agents.25 TEM images show the size of Fe3O4 cubes is uniform and the average edge length of cubes is about 10 nm. Since Ag species have partial positive charge, they will be attracted and enriched around the OA-capped negatively charged Fe3O4 nanocrystals, which were reduced to form Ag crystal domains. (Figure S1) A further growth of Ag precursor on initial Ag domains gives rise to Fe3O4-Ag dimer nanoparticles. Using similar method, Fe3O4-Ag multimers with two or more Ag domains can be produced in an acidic environment with weak reducing ability where Fe3O4-OA seeds are replaced by Fe3O4-OA/TOPO seeds. (Figure S1) In the presence of elementary S precursor, Fe3O4-Ag dimer nanoparticles were eventually converted into Fe3O4-Ag2S dimers by sulfuration. In step 2, a cation exchange process based on the theory of hard-soft acid was used to transform Fe3O4-Ag2S to Fe3O4-CdS dimers domain. It was found that the ion exchange process was promoted in the presence of TBP, because it behaved as a phase-transfer agent to transport metal ions (Cd2+) to the surface of Ag domains and dissolve Ag into solution through coordination bonding to the Ag+ -4-


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ions at the same time. Recently, Horst Weller’s group26 and Zhang’s group27 uncovered this process at molecular level for the first time, which showed that Cd2+ ions proceeded exclusively through the interstitial lattice positions with a subsequent “kick out” to remove Ag+ ions from lattice sites without the formation of vacancies.26 This process can be expressed in the following chemical equation. Ag2S + [Cd2+ − (TBP)x] → CdS + [2Ag+ − (TBP)x] + ∆G In step 3, Au nanocrystals as co-catalyst could be selectively deposited onto the surface of CdS to form Fe3O4-CdS-Au trimer nanoparticles. Due to the strong interaction between the Au species and the S atoms on the surface of CdS component, Au nanoparticles will be preferentially deposited on the surface of CdS particles.28-29 During the synthesis, the loading amount of Au nanoparticles and their sizes can be effectively tuned by the concentration of Au precursor and surfactant, respectively. The flexible regulation and control made it possible to find the optimal trimer nanocatalysts for photoreaction. In the functional modular assembly of trimer nanoparticles, the components, size or dosage of every functional part can be flexibly turned according to the demands, which provide a rational and universal approach for constructing magnet-semiconductor-metal trimer nanoparticles. Except for the morphological study of heterostructures in synthesis, the change of particle size also provides solid proofs for the conversion. As shown in Figure S2, we have carefully monitored the edge length of Fe3O4 nanocubes and the diameter of Ag, Ag2S and CdS particles in synthesis. It is found that the edge length of the Fe3O4 keeps at about 10 nm, which indicates that the Fe3O4 nanocrystals are chemically stable in the whole conversion process. When the Fe3O4-Ag dimers convert into Fe3O4-Ag2S dimers, S atoms will implant into the lattice of Ag nanocrystals, so that the average diameter of Ag particles increase from 5 nm to 7.5 nm for Ag2S particles accordingly. When the Ag+ ions are further replaced by Cd2+ ions, the sulfide particles diameter decreases from 7.5 nm to 6.5 nm. It might be explained by the following considerations that the ionic radius of Cd2+ (97 Å) is much smaller than Ag+ ion (126 Å) and two Ag+ ions are replaced with one Cd2+ ion during the conversion. These changes in particle size are consistent with the conversion process, which verify the sulfuration and ion-exchange processes. The multistep preparation of Fe3O4-CdS-Au hybrid nanoparticles is also confirmed by their XRD patterns and UV-Vis absorption. (Figure 2) The XRD patterns of all the samples contains -5-



diffraction peaks at 35.4, 43.1 and 62.5° (PDF#19-0629), which corresponds to fcc-structured Fe3O4 nanocubes. Small diffraction peaks around 38.1 and 64.4° (PDF#04-0783) appears when Ag nanoparticles are grown on Fe3O4, and these peaks change to a new set of peaks around 25.7, 36.8 and 45.3° (PDF#04-0774) as Fe3O4-Ag was sulfurized to Fe3O4-Ag2S dimer. Through ion-exchange process, Fe3O4-Ag2S was converted to Fe3O4-CdS dimer, which was confirmed by the typical diffraction peaks of wurtzite CdS at around 24.8, 26.5 and 28.2° (PDF#41-1049). For the final Fe3O4-CdS-Au trimer, a new diffraction peak at 38.2° (PDF#04-0784) attributed to Au nanoparticle can be observed in its XRD patterns. The weak and broad diffraction of peak indicates that both the loading amount and the domain size of Au are small. The UV-Vis absorption spectrum also witnesses the conversion among these hybrid nanoparticles. The black Fe3O4 nanocrystals can absorb light in visible range. For Fe3O4-Ag dimers, the absorption peak located at 410 nm can be attributed to the surface plasmon resonance (SPR) effect of Ag nanoparticles. Fe3O4-Ag2S dimers can absorb more visible light than Fe3O4-CdS dimers because the bandgap of Ag2S is smaller than that of CdS. The broad spectrum peak of Fe3O4-CdS-Au trimers can be attributed to the absorption ability of CdS nanoparticles for visible light as well as the enhancement from Au domains, both of which ensure the good photocatalytic activity in visible light. In the following discussion, the as-prepared hybrid nanoparticles would be used as photocatalyst for water splitting to investigate the relationship between structure and activity as well as the function of each component in hybrid nanostructures. Before being used as catalysts in aqueous solution, the as-prepared trimer nanoparticles synthesized in toluene phase need to be transferred to aqueous phase to get them well dispersed in water. According to the research work of Uri Banin et al, polyethyleneimine (PEI) was chosen as a phase-transfer agent due to a more efficient charge separation in PEI-coated hybrid nanoparticles than nanoparticles coated by other surfactants. After phase-transfer, the trimer nanoparticles were dispersed into an aqueous solution of electron sacrificial agent (Na2S/Na2SO3), which was used to test the catalytic activity under simulated light in the visible range provided by a Xe lamp.

Synergistic enhancement of photocatalysis by semiconductor-metal heterostructures. First of all, the influence of Au particle numbers upon the photocatalytic activity is studied based -6-


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on samples with different Au dosage. For Fe3O4-CdS-Au trimer nanoparticles, Au precursors were exclusively nucleated at high-energy defect sites on CdS nanocrystals and further grown to form Au domains in the reduction reaction. Because these sites are isolated from each other, the number of Au nanocrystals attached to one Fe3O4-CdS particle is dependent upon the concentration of Au precursor. When the amount of CdS is fixed at 0.125 mmol and the addition of Au precursor increases from 2.1 µmol, 6.4 µmol to 19.2 µmol, the average number of Au dots on every Fe3O4-CdS-Au trimer increases from 2, 4 to 6 on average, accordingly. (Figure 3, S3) These three samples are labeled as Sample A, B and C and the absorption of corresponding colloidal dispersions were measured by UV-Vis spectrometer. The results indicate that the light absorption increases with the Au loading increasing from 2 to 4, while it is unexpected that more Au domains loading will not result in more light absorption. It suggests that an appropriate loading amount of Au nanocrystals may absorb more light. In the water-splitting experiments, the H2 evolution in 6 hours decrease in the sequence of Sample B, Sample A and Sample C, which shows the catalytic activity is significantly dependent upon the dosage of Au nanoparticles. Except for the number of attached Au particles, their sizes also have great influence upon the photocatalytic activity. (Figure S4) By adjusting the dosage of surfactant, Fe3O4-CdS-Au trimers with Au particle size varying from 1 nm, 2 nm to 3 nm were obtained, which are labeled as Sample D, E and F, respectively. All three samples were used to catalyze the water reduction to produce hydrogen under visible light. Their catalytic performance indicates that 77, 51 and 39 µmol of H2 was generated after 6-hour irradiation for Sample D, E and F, respectively. It shows that smaller Au particles results in higher activity in H2 generation, which is consistent with previous reports.30 The reason might be that smaller Au tips have a superiority in gathering electrons from CdS and benefit to the transfer of electrons to water and generation of hydrogen. An interesting question about the grafting of Au cocatalyst is that why does not the activity of Fe3O4-CdS-Au trimer nanoparticles increase monotonously with the loading of Au particles? For semiconductor-metal dimer nanoparticles as photocatalysts, like CdS-Au in this trimer system, there are two mechanisms from the literature reports.8, 31 In one case, the electrons of metal particles are excited preferentially because of surface plasmon resonance (SPR) effect. The generated electron will then transfer from Au dots to CdS, where they will be consumed for the reduction of water to -7-



produce H2. If the current process follows this mechanism, trimer particles with more Au dots should offer higher photocatalytic activity, because more Au deposition means more electrons can be used to reduce water to produce hydrogen. Unfortunately, the volcano trend of catalytic activity does not support this mechanism. In another case, visible light irradiation leads to the generation of electrons in the CdS nanoparticle and then transfer to Au tips, where the hydrogen molecules will be generated. At the same time, the holes migrate to the surface of CdS, which are consumed by the sacrificial reagents of S2- and SO32- ions to form S22- and SO42-, respectively. The reason for the decrease of the activity with an excess Au loading may be that it hinders the reaction of hole and further generation of electron-hole pairs. That gives a reasonable explanation to the experimental result in Figure 3e. Here, the Fe3O4-CdS-Au trimers with about four Au dots for a trimer nanoparticle show the best photocatalytic performance, which indicates the synergistic effect between CdS and Au nanoparticles. Enhancement of charge separation rather than SPR effect of Au nanoparticles is believed to play a more important role in this photocatalytic reaction. In order to verify the proposed mechanism, Pt and Ni cocatalysts were also combined to Fe3O4-CdS dimer particles, and the comparison of their catalytic activity to that of Fe3O4-CdS-Au shows the existence of proper metal cocatalyst will promote both charge separation and surface reduction due to the slowed electron-hole recombination and the low hydrogen overpotential, respectively. (Figure 4) Similar to the synthesis of Fe3O4-CdS-Au, Fe3O4-CdS-Pt was prepared by decomposition of Pt precursor followed by deposition on the surface of CdS particles. Fe3O4-CdS-Ni trimers was prepared by in-situ photocatalytic reduction under UV-Vis irradiation. Among the three photocatalysts, the activity of Fe3O4-CdS-Pt is the highest and that of Fe3O4-CdS-Ni is the lowest. According to the mechanism, the difference of activity in water splitting may be related to the easiness of H2 evolution on the surface of different metals. The H2 overpotentials of Pt, Au and Ni with a current density of 1 A·cm-2 are 0.048, 0.24 and 0.56 V, respectively, which suggests it’s easy to generate H2 on Pt but difficult on Ni under the same condition. Therefore, the difference in activity is consistent with the efficiency of surface redox reaction for these metals. On the other hand, the difference in activity may be related to the electron-hole separation and recombination, either. Here, photoluminescence (PL) decay curves were measured to evaluate the recombination life-time of electrons and holes. A longer recombination lifetime is attributable to a slower recombination of -8-


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trapped holes at the spatially localized states in nanoparticles that result from holes trapped at deeper states near the valence band.32 According to the PL curves, the τ1 and τ2 were calculated to be 0.852 and 3.089 ns for Fe3O4-CdS-Au, 1.236 and 9.451 ns for Fe3O4-CdS-Pt and 0.482 and 2.208 ns for Fe3O4-CdS-Ni. The τ2 of Fe3O4-CdS-Pt is 3.06 times than that of Fe3O4-CdS-Au and 4.28 times than that of Fe3O4-CdS-Ni. It shows that the attachment of Pt particles will enhance the electron-hole separation of CdS, which improves the photocatalytic activity accordingly. It should be noted that the H2 evolution speed for Fe3O4-CdS-Pt will slow down with the progress of water splitting, probably because the electron flow from Pt tips to solvent is rather fast and a large amount of holes generate in CdS bulk phase can’t be effectively used for oxidation, which coincides with some previous reports.33 As the key component to generate photo electrons, the semiconductor module of hybrid nanostructure can be replaced under the same synthetic framework, which brings more flexibility to optimize the activity in the follow-up works. (Figure 5) In addition to Fe3O4-CdS dimer, Fe3O4-CdSe and Fe3O4-ZnS can be prepared by similar recipe, in which Se and Zn precursor were used instead of S and Cd. In theory, many other Fe3O4-semiconductor dimer nanoparticles, such as Fe3O4-PbS and Fe3O4-CdTe, can also be synthesized by the same processes. The TEM images of Fe3O4-CdS, Fe3O4-ZnS and Fe3O4-CdSe dimer nanoparticles show that all three nanostructures are constituted by two recognizable parts, which are cubic Fe3O4 and spherical semiconductor nanoparticles. The XRD patterns (Figure S5) show they are all well crystallized, and the patterns are well consistent with the standard diffraction patterns of Fe3O4 and wurtzite CdS, CdSe and ZnS, respectively. The UV-Vis absorption spectrums of three dimer nanoparticles indicate that Fe3O4-CdS can absorb more light than the other two and Fe3O4-ZnS dimers absorb the least light. All three samples were decorated with the same amount of Au nanoparticles and further used to for water splitting under visible light. The catalytic activity for H2 evolution decreases in order of Fe3O4-CdS-Au, Fe3O4-CdSe-Au and Fe3O4-ZnS-Au, which is same as the order of ability to absorb light. With more light be absorbed, more photons are harvested to promote electron-hole separate, and more photo electrons will transfer from semiconductor to Au dots for hydrogen reduction.

Magnetic separation and photothermic enhancement realized by Fe3O4 component -9-



The apparent quantum yields (AQY) of water splitting as a function of incident light wavelength was also measured to determine the active sites of Fe3O4-CdS-Au trimers. (Figure 6) For semiconductor photocatalyst, the efficient light absorption is a precondition for high photocatalytic activity prior to electron-hole separation efficiency and surface chemical reactions. Due to the electronic bandgap of semiconductor, only photons with energy higher than the bandgap can induce the production of photoelectrons and holes to catalyze the water reduction. While the bandgap of semiconductor is usually determined by the absorption edge. Therefore, the longest-wavelength monochromic light capable of inducing water reduction should coincide with the absorption edge of the photocatalyst.34 In our experiment, the monochromatic photocatalysis indicates that the AQY decreases from 18.3% to 2.63% with wavelength of monochromatic incident light increasing from 400 to 500 nm and further decreases to nearly zero as the wavelength increases to 600 nm. It seems to disobey the aforementioned consistency between adsorption and AQY curves, since Fe3O4-CdS-Au photocatalyst can still absorb light (A = 40%) at 600 nm. A possible explanation is that the light absorbed by inactive Fe3O4 cannot be effectively used for the photocatalytic reaction. In order to verify the hypothesis, CdS-Au nanoparticles were prepared by dissolving the Fe3O4 component with diluted hydrochloric acid and sent for parallel characterization. Its UV-Vis absorption spectrum is in good consistence with the wavelength-dependent AQY of water splitting reaction. These results suggest that the light absorbed by CdS-Au can effectively produce photoelectrons, and CdS-Au is the active component for photocatalysis. While, the extra light absorbed by black Fe3O4 may not contribute to the production of photoelectrons, so that Fe3O4 is an inactive component for photocatalysis. Although the Fe3O4 nanoparticles are inactive under visible light, they can bring the convenience to the separation and reusability of catalysts, which is a practical problem to be solved in nanocatalysis. (Figure 7) For colloidal form nanocatalysts, good dispersibility means thorough access to reactants and high activity. However, their separation will be very difficult due to the small particle size and high colloid stability. The introduction of Fe3O4 brings magnetic response to trimer nanostructures, so that they can be conveniently separated by magnetic field. Figure 7a shows the magnetic hysteresis loops of Fe3O4 and Fe3O4-CdS-Au nanoparticle at room temperature. The absence of remanence and coercivity indicates that these particles have a typical superparamagnetic - 10 -


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response, which is essential to magnetic separation and catalyst redispersion. Although the saturated magnetization of Fe3O4-CdS-Au is 8.91 emu·g-1, which is half of that of Fe3O4 nanoparticle, it’s strong enough to be readily separated from a liquid suspension using a magnetic field. As shown in Figure 7b, nanoparticles dispersed in solution move to the vessel wall within less than three minutes along the increasing of magnetic gradient. In practice, the separation process will last about 10 minutes for complete separation. In order to verify the effectiveness of magnetic separation and the chemical stability of photocatalyst, Fe3O4-CdS-Au trimers were used as catalyst in five successive water-splitting reactions with reaction time of 6 hours for each round. After magnetic separation from solution, the trimer nanocatalysts were dried under vacuum for several minutes and used for next catalytic round directly. In 5 continuous reactions, the production of hydrogen decreases from about 40 µmol to 37 µmol possibly due to the deactivation of partial trimer nanoparticles. The decrease of H2 evolution rate is less than 10%, which proves that Fe3O4-CdS-Au trimer nanocatalysts can be reused without significant loss in activity. In addition to the convenience of catalyst separation from the reaction solution, the Fe3O4 nanoparticles with photothermic properties can also promote the photocatalytic reaction through utilizing the NIR irradiation of sunlight. (Figure 8) During the reaction under visible light using Fe3O4-CdS-Au trimer as catalyst, it is found that the temperature of solution has obvious influence upon the hydrogen production, which doubles as the temperature being raised from 15 °C to 65 °C. Here, the temperature was controlled by the cooperation of a heating plate and a condenser with flow of cold methanol. In fact, the reaction solution containing the photothermic material (such as Fe3O4) 35-37

will be effectively heated under illumination even without the thermal supply of heating plate. In

order to quantify the photothermic effect of different catalysts, we measure the temperature of solution after the same period of illumination. When CdS-Au and Fe3O4-CdS-Au catalysts are suspended in the reaction solution, its temperature increases to 20.3 °C and 22.8 °C after 4-hour irradiation of visible light, and the temperature increases to 38.3 °C and 44.8 °C after 4-hour irradiation of Vis-NIR light. It suggests that Fe3O4 effectively improve the photothermic capabilities of catalysts, and the using of Vis-NIR irradiation instead of visible light will be benefit to transfer of thermal energy to reaction, as proved by a larger raise of temperature. The temperature dependence of reaction rate and the photothermic effect of Fe3O4 component - 11 -



suggest that Fe3O4 may contribute to the acceleration of hydrogen production as a localized micro-heater in reaction. Here, the water reductions were catalyzed by two photocatalysts (CdS-Au and Fe3O4-CdS-Au) under the illumination of NIR light, visible light and Vis-NIR light respectively to clarify the function of Fe3O4. (Figure 8) Firstly, the coincidence of monochromic AQY and absorption of CdS-Au in Figure 6 has reached the conclusion that CdS-Au is the active component for photocatalysis and Fe3O4 is inactive to the production of photoelectrons. It well explains that CdS-Au photocatalyst works under the illumination of visible light or Vis-NIR light but not with pure NIR light, and the introduction of Fe3O4 does not change the applicable illumination conditions at all. Secondly, through the comparison of hydrogen production for these two catalysts, one can find that the H2 produced under visible light increased from 47.7 µmol to 52.2 µmol when CdS-Au catalyst was replaced by Fe3O4-CdS-Au. On the other hand, the H2 produced under visible light increased from 55.9 µmol to 74.5 µmol when CdS-Au catalyst was replaced by Fe3O4-CdS-Au. The 9.4% of increase in hydrogen production was amplified to 33.3% when the reaction was performed under Vis-NIR light instead of visible light. All these results proved that Fe3O4 component can accelerate hydrogen production through photothermic effect, although itself has no photocatalytic activity. Thirdly, the introduction of Fe3O4 to CdS-Au catalyst can increase the hydrogen production from 52.2 µmol to 74.5 µmol when the illumination changes from visible light to Vis-NIR light. The 42.7% of increase in hydrogen production suggests that the Fe3O4-CdS-Au, compared to CdS-Au, has greater potential in the practical photocatalytic reaction under sunlight illumination, where energy in the infrared range can also be utilized.

Conclusion In summary, we have developed a functional modular assembly method to synthesize Fe3O4-CdS-Au trimer nanoparticles and its derivatives serving as magnetically separable catalysts for photocatalytic water-splitting. (Figure 9) The strategy of functional modular assembly provided an effect and flexible way to replace the semiconductor (CdS, ZnS, CdSe) and metal (Au, Pt, Ni) component in the trimer, which produce abundant photocatalysts for the investigation of correlation between structure and activity. Based on a group of water-splitting reactions and comparison of the catalyst activities, it was found that semiconductor acted as an active component for photocatalysis - 12 -


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and the metal nanoparticle acts as a co-catalyst for enhancement of charge separation and surface reduction, while the Fe3O4 component gives these trimer nanoparticles the ability to be separated in a magnetic field as well as the ability to utilize NIR light to promote the H2 production with photothermic assistance. Among all nanocatalysts, Fe3O4-CdS-Pt trimers possess the highest photocatalytic activity and Fe3O4-CdS-Au trimer has the best performance when the activity and long-term stability are both evaluated. The current material system reveals that the functional modular assembly has good flexibility in tuning the structure and chemical composition within a hybrid nanostructure, which will have great potential in the synthesis of functional nanomaterials in a broader field.

Acknowledgements This work is supported by the National Key Research and Development Program of China (2016YFB0701103), National Natural Science Foundation of China (21471058, 21671067), and Shuguang Program supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (15SG21).

Supporting Information Synthesis and characterization of all dimer and trimer heterostructure nanoparticles, the procedures of photocatalytic water reduction and the size control of Au on Fe3O4-CdS-Au are supplied as Supporting Information.

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Figures and Captions

Figure 1. (a) Synthesis of Fe3O4-CdS-Au trimer nanoparticles through sulfuration of Fe3O4-Ag dimer, in-situ ion-exchange and selective deposition of Au on CdS. TEM images of (b, c) cubic Fe3O4-Ag dimers, (d, e) Fe3O4-Ag2S dimers, (f, g) Fe3O4-CdS dimers and (h, i) Fe3O4-CdS-Au trimers.

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Figure 2. (a) XRD patters and (b) UV-Vis absorption spectrum of Fe3O4 nanocubes, Fe3O4-Ag dimers, Fe3O4-Ag2S dimers, Fe3O4-CdS dimers and Fe3O4-CdS-Au trimers in n-hexane.

Figure 3. (a - c) TEM images and (d) the number of Au domains of Fe3O4-CdS-Au trimer nanoparticles with different Au loading and their (e) photocatalytic activity. - 19 -



Figure 4. TEM images of (a) Fe3O4-CdS-Au, (b) Fe3O4-CdS-Pt and (c) Fe3O4-CdS-Ni trimer nanoparticles as well as their (d) photocatalytic activity and (e) time-resolved fluorescence decay spectra.

Figure 5. TEM images of (a) Fe3O4-CdS, (b) Fe3O4-ZnS and (c) Fe3O4-CdSe dimers as well as their (d) UV-Vis absorption spectrum and (e) photocatalytic activity after deposition of Au domains.

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Figure 6. Wavelength-dependent AQY of water splitting catalyzed by Fe3O4-CdS-Au and the UV-Vis absorption spectrum of Fe3O4-CdS-Au trimers and CdS-Au dimers.

Figure 7. (a) Magnetic hysteresis loops of Fe3O4 nanocrystals and Fe3O4-CdS-Au trimers. (b) The magnetic separation of Fe3O4-CdS-Au trimer in aqueous solution in a field of ~ 600 Gauss. (c) Evolution of H2 in water splitting catalyzed by Fe3O4-CdS-Au trimer in 5 successive reactions. - 21 -



Figure 8. (a) Influence of reaction temperature upon the H2 production under visible light. (b) Comparison of H2 production catalyzed by CdS-Au and Fe3O4-CdS-Au under irradiation of NIR, visible and Vis-NIR light. (c, d) Temperature raise of reaction solution caused by the illumination of visible and Vis-NIR light in the presence of CdS-Au and Fe3O4-CdS-Au catalysts.

Figure 9. Schematic illustration of photoinduced charge separation in CdS-Au heterostructures under irradiation of visible light and the photothermic effect of Fe3O4 under illumination of NIR light.

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