Ligand-Triggered Tunable Charge Transfer toward Multifarious

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C: Energy Conversion and Storage; Energy and Charge Transport

Ligand-Triggered Tunable Charge Transfer toward Multifarious Photoreduction Catalysis Tao Li, Yubing Li, Xiaocheng Dai, Ming-Hui Huang, Yunhui He, Guangcan Xiao, and Fang-Xing Xiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11363 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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The Journal of Physical Chemistry

Ligand-Triggered Tunable Charge Transfer Toward Multifarious Photoreduction Catalysis Tao Li,a Yu-Bing Li, a Xiao-Cheng Dai, a Ming-Hui Huang, a Yunhui He,b Guangcan Xiao,b FangXing Xiaoa* a. College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China. b. Instrumental Measurement and Analysis Center, Fuzhou University, Fuzhou, 350002, People’s Republic of China. E-mail: [email protected]

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Abstract Metal/semiconductor heterostructures have been arousing persistent interest on account of prominent roles of metal nanocrystals (NCs) in modulating charge transfer in photocatalysis. It has been generally accepted that ligands capped on the tailor-made metal NCs surface should be removed for exposing more active sites and the interface between metal and semiconductor should be clean enough to facilitate charge flow. Whether the ligands of metal NCs would definitely deteriorate charge separation/transfer and what is its correlation with photoactivity? Inspired by this motivation, herein, interface configuration between metal and semiconductor were exquisitely designed by a ligand-triggered electrostatic self-assembly strategy, wherein tailor-made intrinsically positively charged Pd NCs capped with hierarchically branched ligands (DMAP) and negatively charged surface-modified CdS nanowires (NWs) were judiciously utilized as the building blocks. Significantly, spontaneous and monodispersed immobilization of Pd@DMAP on the CdS NWs was readily initiated by DMAP ligands, which endows self-assembled Pd@DMAP/CdS NWs heterostructure with conspicuously enhanced and versatile photoreduction performances in comparison with CdS NWs toward anaerobic photoreduction of aromatic nitro compounds, photocatalytic hydrogen generation and photoreduction of heavy metal ions under visible light irradiation, owing to the crucial role of Pd@DMAP for efficaciously capturing electrons without the inhibition of hierarchical ligand structure and interfacial integration mode. More intriguingly, interfacial distance between Pd@DMAP and CdS NWs was finely mediated to achieve tunable charge transport. Finally, ligand-involved photocatalytic mechanism was elucidated. It is anticipated that our work could shed new insight on the role of ligand in modulating the directional charge transfer over metal/semiconductor photocatalytic system for substantial solar energy conversion. 2


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1. Introduction In the past few decades, enormous attention has been devoted to developing high-efficiency solar energy harvesting/conversion techniques to ameliorate the increasingly serious energy depletion since solar energy has been regarded as a green, inexhaustible, and abundant energy source without exerting detrimental impact on the environment. As one of the most intriguing techniques that can efficiently utilize solar energy, semiconductor-based photocatalysis has been attracting burgeoning interest owing to its fascinating solar-to-chemical conversion efficiency and thus recent years has witnessed its wide-spread exploration in a myriad of fields such as photocatalytic water splitting, environmental remediation, photocatalytic organic transformation to fine chemicals, and photocatalytic CO2/N2 reduction.1-6 Among the plethora of semiconductor-based photocatalysts with diverse nanostructures, one-dimensional (1D) semiconductors such as nanorods, nanowires, and nanotubes have received tremendous attention on account of unique structural advantages in comparison with bulk or nanoparticulate counterparts including 1) higher aspect ratio and larger specific surface area; 2) excellent light absorption and propagation; 3) efficient vectorial charge separation/transfer along the 1D nanostructures. Nevertheless, developments of highly efficient 1D

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semiconductor photocatalysts are retarded by the short lifetime of photoinduced charge carriers and sluggish charge transport kinetic, thereby leading to unfavorable solar conversion efficiency. To surmount these obstacles, diverse strategies have been explored, such as metal or non-metal elements doping,7,8 noble metal deposition,9,10 photosensitization with narrow bandgap semiconductors,11,12 integration with carbon materials (e.g., carbon nanotubes, graphene, etc.) or construction of heterojunctions with other semiconductors with suitable energy level alignment.13,14 Of particular note, coupling 1D semiconductor with metal has been established as an efficacious strategy to boost the photoactivities of semiconductors in terms of the important roles of metals as electron reservoirs for capturing electrons photoexcited from adjacent semiconductors owing to the formation of Schottky barrier at the interface.15-23 On the other hand, metals have also been ascertained to play pivotal roles as plasmonic light-harvesting antennas in triggering the production of hot charge carriers for plasmon-induced photocatalysis.24 An overview of previous works on metal/semiconductor photocatalysts which were prepared by depositing tailor-made metal nanocrystals (NCs) on the semiconductor reveals that interface between metal and semiconductor normally should be clean enough to boost electrons flow from semiconductor to metal.25-27 Therefore, metal/semiconductor nanocomposites in previous works are generally calcined at high temperature to remove the ligands to maximally expose the active sites on the metal NCs surface and meanwhile to achieve intimate interfacial integration with semiconductors substrate. Inspired by this, we wonder the correlation of metalsemiconductor interface configuration with the charge transfer pathway and we also wonder whether ligands capped on the metals surface definitely deteriorate the charge transfer efficiency leading to poor photoactivity.

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Thus far, a large variety of noble metals NCs (e.g., Au, Ag, Pd, Pt, etc.) have been extensively investigated for wide-spread photocatalytic applications,28-30 among which Pd has received the most tremendous attention since nanosized Pd-loaded semiconductors have been evidenced to act as highly active photocatalysts for versatile photocatalytic reactions, such as environmental remediation, organic transformation and H2 generation, markedly surpassing other metal counterparts.31-33 Nonetheless, it should be stressed that construction of Pd/semiconductor heterostructures is primarily confined to conventional approaches such as sol-gel, deposition-precipitation, dipping-annealing or photo-deposition method,34-37 most of which normally involves relatively complex synthetic procedures and, more importantly, it is virtually difficult to realize perfect monodisperse distribution of Pd on the semiconductor along with intimate interfacial contact. Moreover, rational construction of well-defined Pd/semiconductor heterostructures with tunable interface configuration via spontaneous electrostatic self-assembly strategy at ambient conditions has not yet been reported. Consequently, it is highly desirable to develop a facile and green approach to construct high-efficiency Pd/semiconductor photocatalysts for substantial solar energy conversion. With these considerations, herein, a progressive surface ligand-triggered electrostatic self-assembly strategy has been developed to construct Pd NCs decorated CdS nanowires (NWs) heterostructures with variable interface configurations, for which tailor-made positively charged 4-dimethylaminopyridine (DMAP)-capped Pd NCs (Pd@DMAP) and negatively charged mercaptoacetic acid (MAA)-modified CdS NWs were selected as the building blocks. Significantly, Pd@DMAP can be uniformly and intimately anchored on the CdS NWs with the aid of DMAP ligands under pronounced electrostatic interaction at ambient conditions. More intriguingly, it was unleashed that spatially hierarchical architecture of DMAP ligand and interface configuration between Pd@DMAP and CdS NWs did not influence smooth electrons 5



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transfer from CdS to Pd NCs, which affords conspicuously enhanced and versatile photoreduction performances of Pd@DMAP/CdS NWs heterostructures toward anaerobic photoreduction of aromatic nitro compounds, photocatalytic hydrogen production, and photoreduction of heavy metal ions under visible light irradiation remarkably outperforming pristine CdS NWs counterpart under the same conditions. Furthermore, interface configuration engineering-induced charge transfer mechanism between Pd@DMAP and CdS NWs were determined and the origins accounting for the significantly boosted photoreduction performances of Pd@DMAP/CdS NWs heterostructure were ascertained by a series of controlled experiments.

2. Experimental section 2.1 Materials Deionized water (DI H2O, Millipore, 18.2 MΩ·cm resistivity), sodium diethyldithiocarbamate trihydrate (C5H10NNaS2·3H2O), cadmium chloride (CdCl2·2.5H2O), ethylenediamine (C2H8N2), mercaptoacetic acid (C2H4O2S), sodium tetrachloropalladate (Na2PdCl4), tetraoctylammonium bromide (TOAB), toluene, sodium borohydride (NaBH4), mercaptoacetic acid (MAA), 4dimethylaminopyridine (DMAP), sulfuric acid (H2SO4), sodium hydroxide (NaOH), sodium sulfate (Na2SO4), ammonium formate (NH4HCO2), 3-nitroaniline (3-NA), 2-nitroaniline (2-NA), 1-chloro-4nitrobenzene, 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), 4-nitroanisole, and 4-nitrotoluene, were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). All the materials were used as received without further purification. 2.2 Preparation of CdS NWs 1.124 g of cadmium diethyldithiocarbamate (Cd(S2CNEt2)2), prepared by precipitation from a stoichiometric mixture of sodium diethyldithiocarbamate trihydrate (C5H10NNaS2·3H2O) and 6


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cadmium chloride (CdCl2·2.5H2O) in DI H2O, was added into a Teflon-lined autoclave with a capacity of 50 mL. Afterwards, the autoclave was filled with 40 mL of ethylenediamine to 80 % of the total volume and maintained at 453 K for 24 h. After cooling to room temperature, a yellowish precipitate was collected and washed with absolute ethanol and DI H2O to remove the residue of organic solvents.38 The final products were dried in an oven at 333 K for 12 h. 2.3 Preparation of negatively charged CdS NWs 0.1 g of CdS NWs was first dispersed in 100 mL of DI H2O by sonication for 30 min, in which 9 mL of 1.0 mol L-1 mercaptoacetic acid (MAA) was added under vigorous stirring for 2 h at room temperature. Finally, MAA-modified CdS NWs were sufficiently rinsed with ethanol to wash away any remaining MAA moiety and dried at 333 K in an oven. 2.4 Preparation of Pd@DMAP Preparation of Pd@DMAP was referred to a previously published work.39 Prior to experiment, all the glassware was thoroughly washed with aqua (3:1 in volume for HCl and HNO3) for 12 h and DI H2O. A 30 mM Na2PdCl4 aqueous solution (30 mL) was added to a 25 mM TOAB solution in toluene (80 mL). Transfer of the platinum salt to the toluene phase was seen in a few seconds. A freshly prepared 0.4 M NaBH4 (25 mL) aqueous solution was added to the above mixture under vigorous stirring. After 30 min, the two phases were separated and the toluene phase was subsequently washed with 0.1 M H2SO4, 0.1 M NaOH, and DI H2O for three times, and dried with anhydrous Na2SO4. Afterwards, an aqueous 0.1 M 4-dimethylaminopyridine (DMAP) solution (80 mL) was added to aliquots (80 mL) of the as-prepared nanoparticle mixtures. Direct phase transfer across the organic/aqueous interface was completed within 2 h with no stirring or agitation required. Finally,

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Pd-dissolved water phase was separated by a separatory funnel and Pd@DMAP aqueous solution was thus obtained. 2.5 Self-assembly of Pd-CdS NWs heterostructures Pd-CdS NWs heterostructures with different weight percentage of Pd were fabricated by an electrostatic self-assembly method at ambient conditions, as illustrated in Scheme 1. Specifically, intrinsically positively charged Pd@DMAP aqueous solution (1.2 mg mL−1) was diluted to 0.1 mg mL−1, after which different amount of Pd@DMAP aqueous suspension (0.1 mg mL−1, pH=10) was added dropwise to the negatively charged CdS NWs aqueous dispersion (1 mg mL-1, pH=10) under vigorous stirring for 2 h at ambient conditions. The weight percentage of Pd@DMAP NPs in the heterostructures was controlled to be 1, 3 and 5% by adding the corresponding volumes of 10, 30, and 50 mL, respectively. Finally, the mixture was centrifuged and dried in an oven at 333 K. 2.6 Characterization Zeta potential (ξ) measurements were performed by dynamic light scattering analysis (ZetasizerNano ZS-90). Crystal structure was studied by X-ray diffraction (XRD, X’Pert Pro MPD, Philips, Holland) using Cu Kα as the radiation source under 40 kV and 40 mA. Morphologies of samples were probed by field-emission scanning electron microscopy (FESEM, Supra55, Carl Zeiss, Germany) equipped with an energy-dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) and high-resolution (HR) TEM images were collected on a JEOL-2010 with an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were recorded on a TJ27030A infrared spectrophotometer (Tianjin, China). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a photoelectron spectrometer (Escalab 250, Thermo Scientific, America), where binding energy (B.E.) of the elements was calibrated by the B.E. of carbon (284.60 eV). UV-vis 8


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diffuse reflectance spectra (DRS) (Varian Cary 500 UV-vis spectrophotometer, Varian, America) were obtained using BaSO4 as the reflectance background ranging from 250 to 800 nm. BrunauerEmmett-Teller (BET) specific surface areas were determined on a Quantachrome Autosorb-1-CTCD automated gas sorption analyzer. Photoluminescence (PL) spectra were collected on a Varian Cary Eclipse spectrometer. 2.7 Photocatalytic performances 2.7.1 Photocatalytic reduction of 4-nitroaniline (4-NA) For anaerobic photocatalytic reduction of 4-nitroaniline (4-NA) under an inert atmosphere (N2), a 300 W Xe lamp (PLS-SXE300D, Beijing Perfect Light co. LTD, China) equipped with a 420 nm cutoff filter (λ>420 nm) was used as the light source and kept 15 cm away from the glass reactor. 10 mg of catalyst and 40 mg of ammonium formate (NH4HCO2) were mixed with 30 mL of 4-NA aqueous solution (20 mg L-1) in a glass reactor under N2 bubbling. Before light irradiation, the suspension was stirred and kept in dark for 1 h to achieve the adsorption-desorption equilibrium between the photocatalyst and reactants. After that, the suspension was irradiated with visible light (λ>420 nm). At different time interval (0, 1, 2, 3, 4 and 5 min), 3 mL of the sample solution was collected, centrifuged (12000 rpm) and analyzed on a UV-Vis spectrophotometer (Thermal Fisher). Photoreduction of other aromatic nitro compounds was also carried out under the same conditions. 2.7.2 Photocatalytic hydrogen evolution Photocatalytic hydrogen evolution measurements were conducted on an online photocatalytic water splitting system at ambient temperature using a 300 W Xe lamp (PLS-SXE300D, Beijing Perfect Light co. LTD, China) equipped with an optical cutoff filter (λ>420 nm) to eliminate the UV light. Specifically, 10 mg of catalyst was dispersed in 5 mL of DI H2O containing 0.5 mL of lactic acid as 9



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the sacrificial reagent and stirred throughout the reaction. Hydrogen evolution was analyzed on an online gas chromatograph (Shimadzu GC-8A, argon as a carrier gas and MS-5A column). Photocatalytic activities were evaluated based on the H2 evolution amount in the first 2 h of the reaction.

2.7.3 Photoreduction of heavy metal ions The photocatalytic activity of the samples was evaluated by the photoreduction of Cr2O72- under visible light irradiation (λ>420 nm). A 300 W Xe lamp (PLS-SXE300D, Beijing Perfect Light co. LTD, China) equipped with a 420 nm cut-off filter was used as the light source and kept 15 cm away from the glass reactor. 10 mg of photocatalyst and 40 mg of ammonium formate (NH4HCO2) were mixed with 30 mL of potassium dichromate aqueous solution (50 mg L-1) in a glass reactor. Before light irradiation, the suspension was stirred and kept in dark for 1 h to achieve the adsorption-desorption equilibrium between the photocatalyst and reactants. After that, the suspension was irradiated with visible light. At different time interval (0, 5, 10, 15, 20 and 25 min), 3 mL of the sample solution was collected, centrifuged (12000 rpm) and analyzed on a UV-Vis spectrophotometer (Thermal Fisher). 2.8 Photoelectrochemical (PEC) measurements PEC measurements were performed on electrochemical workstation (CHI660E, CHI Shanghai, Inc., and Interface 1000 E, Gamary, America). Electrochemical setup was composed of conventional three electrodes, a single compartment quartz cell containing 100 mL of Na2SO4 (0.5 M) aqueous solution and a potentiostat. A platinum sheet (1 cm × 1 cm) electrode was used as the counter electrode and Ag/AgCl electrode as the reference electrode. The working electrode was prepared on fluorine-dope tin oxide (FTO) glass that was cleaned by sonication in ethanol for 30 min and dried at 353 K. The boundary of FTO glass was protected using Scotch tape. The 5 mg sample was 10


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dispersed in 0.5 mL of ethyl alcohol absolute by sonication to get slurry. The slurry was spread onto the pretreated FTO glass. After air drying, the Scotch tape was unstuck, and the uncoated part of the electrode was isolated with nail polish. The exposed area of the working electrode was 1 cm2. Finally, the working electrode were vertically dipped into the electrolyte and irradiated with visible light (λ>420 nm) (FX300, Beijing Perfect Light co. LTD, China).

3. Results and discussion

Scheme 1. Schematic illustration for electrostatic self-assembly of Pd-CdS NWs heterostructures. Scheme 1 illustrates the spontaneous ligand-triggered self-assembly process for fabricating Pd-CdS NWs heterostructures by mediating the intrinsic surface charge properties of CdS NWs and Pd NPs building blocks. The accessibility of this process is validated by the zeta potential results. As shown in Fig. S1a, zeta potentials of MAA-modified CdS NWs (MAA-CdS NWs) aqueous solution under varying pH conditions (pH = 2, 4, 6, 8, 10, 12) were determined to be ca. -21.46, -13.62, -20.38, 18.7, -19.32, and -45.2 mV, respectively. Apparently, MAA-CdS NWs substrate is featured by negatively charged surface, which is mainly ascribed to the fact that CdS NWs are capped by a large amount of MAA molecules that afford plentiful carboxyl functional groups on the surface and thus, deprotonation of these carboxyl groups endows CdS NWs with pronounced negatively charged

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surfaces.40 On the other hand, Pd@DMAP is characteristic of positively charged surface over a wide pH (2-12) profile, which is reflected by its zeta potential results (Fig. S1b). This is because the DMAP molecules capped on the Pd NPs are able to form a labile donor-acceptor complex with the surface Pd atoms via the endocyclic nitrogen atoms, where surface charge arises from partial protonation of the exocyclic nitrogen atoms, thereby giving rise to a positively charged surface.41 Hence, when these two assembly units are mixed together in an aqueous phase at ambient conditions, positively charged Pd@DMAP NPs can be spontaneously attracted by the negatively charged MAA-CdS NWs under substantial electrostatic attractive interaction, which establishes a solid foundation for selfassembly construction of Pd-CdS NWs heterostructures. Noteworthily, surface linkers of DMAP capped on the Pd were not removed but rather kept in the final Pd-CdS NWs nanocomposites to probe the influence of interface configuration on the charge transfer.

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3.1 Characterizations of Pd-CdS NWs heterostructure

Fig. 1. (a) XRD patterns of pristine CdS NWs and Pd-CdS NWs heterostructures with different weight percentage of Pd NPs, (b) Raman spectra of pristine CdS NWs and 3 wt% Pd-CdS NWs heterostructure and (c) DRS results of pristine CdS NWs and Pd-CdS NWs with different loading percentage of Pd@DMAP (1, 3, 5 %) heterostructures together with the corresponding (d) plots transformed by Kubelka-Munk function vs. the energy of light. The inset shows the graphs of the samples: left: CdS NWs, right: 3 wt% Pd-CdS NWs. Structure characterizations of nanomaterials were firstly systematically explored to validate the self-assembly efficiency in fabricating Pd-CdS NWs heterostructures. Crystal phase of the different samples are determined by X-ray diffraction (XRD) patterns. As shown in Fig. 1a, all the samples display good crystallinity and the characteristic peaks at 2θ values of 24.8, 26.6, 28.2, 36.6, 43.7, 13



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47.9, 50.9, 51.8, 52.8, 66.8, 69.2, 70.9, 72.3 and 75.4° can be accurately indexed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (203), (210), (211), (114) and (105) crystal planes of greenockite structure CdS with a hexagonal phase (JCPDS card no. 41-1049), respectively. With regard to the XRD patterns of Pd-CdS NWs heterostructures with varying Pd NPs loading percentages, diffraction peaks of Pd are too weak to be detected due to relatively low loading amount and ultra-small size of Pd NPs or possibly, Pd peaks were shielded by the substantial peaks of CdS substrate. Raman spectra were utilized to further investigate the phases of these samples (Fig. 1b). It is observed that vibration peaks located at 306 and 610 cm-1 in the Raman spectra of CdS NWs and Pd-CdS NWs heterostructure correspond to the 1 LO (longitudinal optical) and 2 LO phonon modes of hexagonal CdS and the result is consistent with XRD results.42,43 Although no Pd peaks were observed in the Raman spectrum of Pd-CdS NWs heterostructure, deposition of Pd NPs on the CdS NWs can be verified by other characterization tools which will be demonstrated in the latter part. Notably, characteristic vibration modes at 806, 1538 and 1627 cm-1 in the FTIR spectrum of Pd-CdS NWs heterostructure (Fig. S2) are attributed to the functional groups of DMAP ligands capped on the Pd NPs surface, which is in faithful agreement with the FTIR spectrum of pure Pd@DMAP, strongly indicating Pd NPs have been successfully electrostatically attached on the surface of CdS NWs. Optical properties of CdS NWs and Pd-CdS NWs heterostructures with different weight percentages of Pd@DMAP were probed by UV-Vis diffuse reflectance spectra (DRS). As shown in Fig. 1c, both CdS NWs and Pd-CdS NWs heterostructures exhibit a characteristic absorption band edge at ca. 520 nm in the visible region, owing predominantly to the bandgap photoexcitation of CdS substrate. Apparently, Pd-CdS NWs heterostructures showed remarkably enhanced light 14


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absorption in the visible region (520-800 nm) as compared with pristine CdS NWs, among which 3 wt% Pd-CdS NWs heterostructure demonstrates the most pronounced light absorption intensity, suggesting uniform distribution of Pd NPs on the CdS NWs through electrostatic interaction was beneficial for boosting the light absorption of CdS. The different light absorption between CdS NWs and Pd-CdS NWs can also be reflected by the color change of the sample from yellow to dark green (Fig. 1c, inset). As displayed in Fig. 1d, bandgap energies (Eg) of CdS NWs and Pd-CdS NWs heterostructures can be roughly identified by the following formula:44 αhν = A(hν - Eg)n/2 where α, hν, and A denote the absorption coefficient, photon energy, and constant, respectively. The n value depends on the optical transition type of the semiconductors, e.g., n = 1 for direct transition and n = 4 for indirect transition. Herein, CdS is a direct transition semiconductor with n = 1. Based on the above formula, calculated Eg values of CdS NWs and Pd-CdS NWs (1, 3, 5 %) heterostructures are approximately the same, which vary from 2.33 to 2.4 eV, indicating Pd NPs decoration did not cause substantial Eg variation for CdS NWs substrate.

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Fig. 2. FESEM images of (a) pristine CdS NWs and (b) 3 wt% Pd-CdS NWs. TEM images of (c) pristine CdS NWs, (d & e) 3 wt% Pd-CdS NWs heterostructures, (f) HRTEM, (g) the TEM-EDS and (h-k) elemental mapping results of 3wt% Pd-CdS NWs heterostructure. Microstructure and morphologies of CdS NWs, Pd NPs and 3 wt% Pd-CdS NWs heterostructures were explored by field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and high-resolution TEM (HRTEM). FESEM and TEM images in Fig. 2(a-c) and Fig. S3 reveal that pristine CdS NWs possess a rather smooth surface with length of several micrometers and size of ca. 70 nm. Such well-defined nanostructure of CdS NWs is favorable to provide excellent charge separation/transfer efficiency in terms of structural merits of 1D semiconductors such as enhanced light scattering/absorption and efficient vectorial charge transfer along the 1D framework.45 Actually, it is difficult to directly differentiate Pd NPs on the CdS NWs surface in the FESEM images owing to the ultra-small size of Pd@DMAP. However, TEM images of Pd-CdS NWs in Fig. 2(d & e) and Fig. S4 demonstrate the uniform and intimate attachment of Pd 16


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NPs (ca. 2.8 nm) on the CdS NWs without agglomeration, suggesting the electrostatic self-assembly strategy developed in our work is efficient in fabricating Pd-CdS NWs heterostructures. Note that average diameter of Pd NPs in the nanocomposite is analogous to that in pure Pd@DMAP (Fig. S5), indicative of the size of Pd was not changed during the self-assembly process. Close interfacial integration of Pd@DMAP with CdS NWs can be confirmed by the HRTEM image (Fig. 2f), wherein the lattice fringe of 0.34 nm can be accurately indexed to the (002) plane of hexagonal CdS and the spacing of 0.225 nm corresponds to the (111) crystallographic plane of face-centered cubic (fcc) Pd.46 Alternatively, monodispersed distribution of Pd@DMAP on the CdS NWs substrate in a broader vision in the nanocomposite can be evidenced by the elemental mapping results in Fig. 2 (h-k) and Fig. S6 which exhibit clear Pd, Cd, and S signals. Consistently, energy dispersive spectroscopy (EDS) result corroborates successful self-assembly of Pd-CdS NWs heterostructures.

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Fig. 3. (a) Survey spectra and high-resolution XPS spectra of (b) Cd 3d, (c) S 2p for (I) 3 wt% PdCdS NWs heterostructure and (II) pristine CdS NWs, and (d) high-resolution Pd 3d spectrum of 3 wt% Pd-CdS NWs heterostructure. Composition and elemental chemical states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS). Survey spectrum of Pd-CdS NWs heterostructure (Fig. 3a) shows the appearance of Cd, S and Pd core-elements (Table S2), which agrees with EDS result. Highresolution Cd 3d spectrum of Pd-CdS NWs heterostructure (Fig. 3b) shows two peaks assignable to Cd 3d5/2 and Cd 3d3/2 at 404.9 and 411.6 eV, which correspond to Cd2+ specie.47,48 Consistently, high-resolution S 2p spectra of pristine CdS NWs and Pd-CdS NWs heterostructure in Fig. 3c are attributed to S2- specie.49 Moreover, it is interesting to find that binding energies (B.E.) of Cd 3d in Pd-CdS NWs heterostructure are positively shifted by 0.75 and 0.7 eV for Cd 3d5/2 and Cd 3d3/2 as compared to pristine CdS NWs, which is ascribed to the pronounced electronic interaction- induced charge transfer between CdS NWs and Pd NPs.15,48,50,51 Similarly, B.E. of S 2p3/2 and S 2p1/2 in the high-resolution S 2p spectra of Pd-CdS NWs heterostructure are also positively shifted by 0.95 and 0.9 eV in comparison with pristine CdS NWs (Fig. 3c). Fig. 3d shows the high-resolution Pd 3d spectrum of Pd-CdS NWs heterostructure, in which two peaks located at 335.7 and 340.7 eV corresponding to Pd 3d5/2 and Pd 3d3/2 are assigned to metallic Pd0,52 further substantiating successful immobilization of Pd@DMAP on the CdS NWs. As a result, XPS results indicate that chemical states of the elements in Pd-CdS NWs heterostructure were retained after self-assembly.

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3.2 Photoreduction performances of Pd-CdS NWs

Fig. 4. (a) Photocatalytic activities of pristine CdS NWs and Pd-CdS NWs heterostructures with varying loading of Pd@DMAP (1, 3, 5 %) toward reduction of 4-NA under visible light irradiation (λ>420 nm) with the addition of ammonium formate as quencher for photogenerated holes and N2 purge under ambient conditions, (b) comparison on the conversation and yield of different samples toward photoreduction of 4-NA, and (c-i) photoreduction of a series of nitroaromatic compounds including 3-nitroaniline (3-NA), 1-chloro-4-nitrobenzene, 4-nitrophenol (4-NP), 4-nitroanisole, 4nitrotoluene, 2-nitroaniline (2-NA) and 2-nitrophenol (2-NP) over pristine CdS NWs and 3 wt% Pd-

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CdS NWs heterostructure under the same conditions, along with corresponding (j) photoreduction reaction model under the current experimental conditions. Photocatalytic performances of Pd-CdS NWs heterostructures were evaluated by anaerobic photoreduction of 4-nitroaniline (4-NA) to corresponding 4-phenylenediamine (4-PDA) in an aqueous phase under visible light irradiation (λ>420 nm) with the addition of ammonium formate as quencher for photogenerated holes and N2 purge under ambient conditions. UV-vis light absorption spectra were used to monitor the conversion of 4-NA to 4-PDA with irradiation time. According to the UV-visible absorption spectrum of 4-NA (Fig. S7), it is apparent that absorption peak intensity at 380 nm corresponding to 4-NA gradually decreases and concomitantly, two new absorption peaks at 300 and 240 nm corresponding to 4-PDA appear and their peak intensity increases with prolonging the irradiation time,24,53 suggesting progressive conversion of 4-NA to 4-PDA under visible light irradiation. Blank experiments show that no activity is observed in the absence of catalyst or light irradiation (Fig. S8) under the same experimental conditions, suggesting it is indeed a lightdriven photocatalytic process. As seen in Fig. 4a, photocatalytic performance of Pd-CdS NWs heterostructures increases with increasing the weight percentage of Pd@DMAP from 1 to 3 % and then decreases upon further increasing the percentage to 5 %, based on which the optimal weight percentage of Pd@DMAP was determined to be 3 wt%. The result indicates a synergistic effect between CdS NWs and Pd@DMAP for optimally improving the photoactivity. Excess loading percentage of Pd@DMAP on the CdS NWs matrix is detrimental to the photocatalytic performance of Pd-CdS NWs since light absorption of CdS NWs might be substantially shielded or surface active sites of CdS NWs might be occupied by excess Pd@DMAP, which has been evidenced in previous

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works on metal/heterostructures photocatalysts.54,55 Moreover, over-deposition of Pd@DMAP might also lead to agglomeration of Pd NPs, thereby seriously reducing the active site number on the CdS NWs.56,57 Fig. 4b demonstrates the conversion and yield of photoreduction of 4-NA to 4-PDA over different samples, from which it is apparent to see that photoreduction of 4-NA to 4-PDA was nearly completed within 5 min over 3 wt% Pd-CdS NWs heterostructure under visible light irradiation, far surpassing pristine CdS NWs counterpart. Noteworthily, pure Pd@DMAP with the same amount to that in 3 wt% Pd-CdS NWs heterostructure demonstrate negligible photoreduction performance and the result persuasively corroborates the significantly enhanced photocatalytic performance of PdCdS NWs heterostructure is originated from the cooperativity of CdS NWs and Pd@DMAP, especially the charge transfer between them. More intriguingly, analogous results were observed in photocatalytic reduction of other nitroaromatic compounds to corresponding amino organics, included 3-NA, 1-chloro-4-nitrobenzene, 4-NP, 4-nitroanisole, 4-nitrotoluene, 2-NA and 2-NP over CdS NWs and 3 wt% Pd-CdS NWs under the same experimental conditions (Fig. 4c-i), for which 3 wt% Pd-CdS NWs always demonstrates the markedly enhanced photocatalytic performances in comparison with pristine CdS NWs. This undoubtedly highlights the pivotal role of Pd@DMAP in contributing to the conspicuously enhanced photocatalytic performances of Pd-CdS NWs heterostructure. Action spectra of Pd-CdS NWs heterostructures under different monochromatic light irradiation was probed to determine the wavelength region contributing to the conspicuously boosted photoactivities. As manifested in Fig. S9, a substantial peak ranging from 400 to 550 nm was seen in the action spectrum of 3 wt% Pd-CdS NWs heterostructures toward photoreduction of 4-NA, which confirms the excellent visible-light-responsive photoactivity of 3 wt% Pd-CdS NWs heterostructure results primarily from the substantial light absorption of CdS NWs substrate. It 21



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should be emphasized that photoreduction of aromatic nitro compounds over different photocatalysts was performed in an inert atmosphere with nitrogen bubbling to retard any electron acceptor in current reaction system and simultaneously, ammonium formate was added to capture the holes produced during the photocatalytic reaction process, thus featuring typical photoelectronsinvolved photoreduction reactions (Fig. 4j).

Fig. 5. (a) Control experiments with the addition of K2S2O8 as electron scavenger and performed in an O2 atmosphere, (b) cyclic photoreduction of 4-NA over 3 wt% Pd-CdS NWs heterostructure, (c) photocatalytic hydrogen evolution and (d) photoreduction of Cr2O72− over pristine CdS NWs and 3 wt% Pd-CdS NWs heterostructure under visible light irradiation (λ>420 nm). Control experiments were performed to disclose the predominant role of electrons in current reaction system. To this end, photoreduction performance of 3 wt% Pd-CdS NWs heterostructure 22


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was evaluated in the presence of electron scavenger (K2S2O8) or in an O2 atmosphere. As displayed in Fig. 5a, photocatalytic performance of 3 wt% Pd-CdS NWs heterostructure was considerably reduced once K2S2O8 was added into the reaction system and the result verifies electron is the predominant driving force to proceed the reaction. Besides, apparently, photoactivity of 3 wt% PdCdS NWs heterostructure in O2 atmosphere was inferior to that in N2 atmosphere with other experimental conditions unchanged. In this regard, we believe Pd@DMAP plays a crucial role as electron reservoir in capturing electrons photoexcited from CdS NWs and this will be elaborated in the latter part. Stability of the photocatalysts was probed and it is a paramount sector of photocatalyst for future practical applications. Fig. 5b shows that negligible deactivation in photoreduction performance of 3 wt% Pd-CdS NWs heterostructure was observed with 95 % photoactivity retained even after ten successive cyclic reactions. Consistently, XPS results of 3 wt% Pd-CdS NWs heterostructure (Fig. S10) before and after cyclic photocatalytic reactions are similar, indicating elemental chemical states of the samples were not changed and thus additionally verifying the favorable photostability of 3 wt% Pd-CdS NWs heterostructure.58 Apart from the photocatalytic reaction toward aromatic nitro compounds reduction, other electrondriven photocatalytic reactions including photocatalytic hydrogen evolution and photoreduction of Cr2O72− ions were also carried out to gain more deep understanding on the universal role of Pd@DMAP in modulating the charge transfer process. As reflected by Fig. 5c, 3 wt% Pd-CdS NWs heterostructure demonstrates remarkably enhanced H2 evolution rate of 7.5 mmol·g-1·h-1 which is 9.8 times larger than blank CdS NWs (0.76 mmol·g-1·h-1) under visible light irradiation (λ>420 nm) for 2 h. As well, more significantly enhanced photocatalytic performance of 3 wt% Pd-CdS NWs heterostructure relative to blank CdS NWs was also observed in the photoreduction of Cr2O72− ions 23



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to Cr (III) oxo species under visible light irradiation (λ>420 nm). Alternatively, as mirrored in Fig. S11, photoreduction activity of 3 wt% Pd-CdS NWs heterostructure is strongly inhibited when K2S2O8 is added to the reaction system. The result once again substantiates the crucial impact of Pd@DMAP on boosting the versatile photoreduction performances of Pd-CdS NWs heterostructure by capturing electrons photoexcited from CdS NWs. More significantly, apparently, hierarchical surface ligand (DMAP) of Pd did not influence the charge transfer from CdS to Pd, thus affording versatile and high-efficiency photoreduction performances of Pd-CdS NWs heterostructures. To reveal the reasons accounting for the significantly enhanced photoreduction performances of 3 wt% Pd-CdS NWs heterostructure, specific surface area and porosity of pristine CdS NWs and 3 wt% Pd-CdS NWs heterostructure were explored. As shown in Fig. S12, nitrogen adsorptiondesorption isotherms of pristine CdS NWs and 3 wt% Pd-CdS NWs heterostructure are ascribed to type III isotherm according to IUPAC classification.59 This type of isotherm is the normal form of isotherm obtained with a mesopore adsorbent.59 Specific surface area and pore volume of 3 wt% Pd-CdS NWs heterostructure were determined to be 9.1252 m2 g−1 and 12.76593 cm3 g−1, which are similar to those of blank CdS NWs (i.e., 6.9930 m2 g−1 and 17.68200 cm3 g−1), as summarized in Table S3. This implies analogous adsorption capability of 3 wt% Pd-CdS NWs heterostructure to pristine CdS NWs. Based on which, decisive factor that contributes to the significantly enhanced and versatile photocatalytic performances of 3 wt% Pd-CdS NWs heterostructure cannot be ascribed to the difference in specific surface area but rather remarkably boosted separation of photoexcited electron-hole pairs, which will be further corroborated by the following photoelectrochemical (PEC) and photoluminescence (PL) results.

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3.3 PEC performances of Pd-CdS NWs heterostructure

Fig. 6. (a) LSV results with a scan rate of 2 mV s−1, (b) on-off transient photocurrent responses (I-t), (c) M-S plots, (d) EIS Nyquist plots, (e) open circuit potential decay and (f) electron lifetime of blank CdS NWs and 3 wt% Pd-CdS NWs heterostructure under visible light irradiation (λ>420 nm) in an aqueous Na2SO4 solution (0.5 M, pH=7). PEC performances of CdS NWs and 3 wt% Pd-CdS NWs heterostructure were evaluated under visible light irradiation (λ>420 nm) to probe the separation of photogenerated charge carriers over 25



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photocatalysts.60 Fig. 6a shows the linear-sweep voltammograms (LSV) results of CdS NWs and 3 wt% Pd-CdS NWs heterostructure as a function of applied bias under visible light irradiation. It was found that photocurrent density of the samples increases with augmenting the forward bias, indicative of typical n-type semiconductor behavior. Obviously, 3 wt% Pd-CdS NWs heterostructure demonstrates substantially enhanced photocurrent in comparison with blank CdS NWs across the entire potential profile, while only weak photocurrents were observed in the dark arising from the photoresponse of FTO substrate. It has been well-established that photocurrent is produced due to the diffusion of photoelectrons to the back contact and meanwhile holes are captured by the hole acceptors in the electrolyte.61 As such, improved photocurrent indicates a more effective charge separation efficiency and longer lifetime of photogenerated electron-hole pairs over Pd-CdS NWs heterostructure as compared with blank CdS NWs. Fig. 6b displays the on-off transient photocurrent responses of the samples under chopped visible light illumination (λ>420 nm), from which it is clearly seen that Pd@DMAP deposition remarkably improves the photocurrent of Pd-CdS NWs heterostructure that exhibits at least four-fold enhancement as compared with blank CdS NWs, verifying the separation efficiency of photogenerated charge carriers over 3 wt% Pd-CdS NWs is much higher than pristine CdS NWs.62 Alternatively, Mott-Schottky (M-S) measurement offers a very simple approach to determine the charge carrier density of the semiconductor electrode.63 As revealed in Fig. 6c, slope of the linear part of the M-S plot for 3 wt% Pd-CdS NWs heterostructure is smaller than CdS NWs, implying its higher charge carrier density than blank CdS NWs. Charge carrier density (ND) of the semiconductor photoelectrodes can be determined by the following equation:64

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()

d

1

-1

C2 2 ND = [ ] εε0e0 dV

( )

where ε denotes the dielectric constant of the semiconductor (εCdS=8.9),65 ε0 denotes the permittivity of a vacuum (8.86 × 10−12 F m−1), e0 is the electronic charge unit (1.6 × 10−19 C), and V is the potential applied on the electrode. Based on which, ND values of blank CdS NWs and 3 wt% PdCdS NWs heterostructure were calculated to be 5.69 × 1028 and 10.8 × 1028 cm−3, respectively. Apparently, ND of 3 wt% Pd-CdS NWs heterostructure is almost 1.9 times larger than blank CdS NWs, which signifies much larger charge carrier density of 3 wt% Pd-CdS NWs than CdS NWs, thereby leading to more enhanced photoactivities aforementioned. Electrochemical impedance spectroscopy (EIS), as a powerful tool to study the charge transfer process at the interface between electrode and electrolyte, was utilized to probe the separation efficiency of charge carriers.66 The semicircle in the Nyquist plots conveys information on charge transfer process with the diameter of the semicircle corresponding to the charge transfer resistance.67 Fig. 6d displays the EIS results of different samples, in which EIS Nyquist plot of Pd-CdS NWs heterostructure exhibits substantially smaller semicircle arc radius than blank CdS NWs in the high-frequency region, indicating charge separation efficiency of 3 wt% Pd-CdS NWs is higher than CdS NWs and this corroborates the crucial role of Pd@DMAP in capturing photoelectrons for suppressing the recombination of photogenerated electron-hole pairs. Furthermore, electron lifetime of the semiconductor was estimated via the decay profile of open circuit voltage by turning off the light irradiation at a steady state and subsequently, monitoring the photovoltage decay with time (Fig. 6e & f). It is worth noting that 3 wt% Pd-CdS NWs heterostructure exhibits much more prolonged electron lifetime and larger

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photovoltage than blank CdS NWs, which corroborates the more efficient separation of photogenerated electrons-holes pairs over 3 wt% Pd-CdS NWs heterostructure. The more efficient charge separation efficiency of 3 wt% Pd-CdS NWs heterostructure than CdS NWs has also been confirmed by PL results. It has been well-accepted that lower PL intensity of semiconductor suggests longer lifetime of photogenerated charge carriers and reduced recombination rate.68 As shown in Fig. 7a, PL spectrum of CdS NWs displays strong emission intensity originating from the intrinsic recombination of photoinduced charge carriers. On the contrary, 3wt% Pd-CdS NWs heterostructure exhibits remarkably lower PL intensity in comparison with CdS NWs, indicating recombination of charge carriers over Pd-CdS NWs heterostructure is greatly suppressed. Note that PL results are in faithful agreement with photocatalytic performances and PEC results.

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3.4 Influence of interface configuration on the charge transfer

Fig. 7. (a) PL spectra of pristine CdS NWs and 3 wt% Pd-CdS NWs heterostructure with an excitation wavelength of 350 nm, (b) photoreduction performances of 3 wt% Pd-CdS NWs heterostructure with and without MAA modification on the CdS NWs surface, (c) photoreduction performances of 3 wt% Pd-CdS NWs heterostructure after calcination for different time in N2 atmosphere at 300 oC, and (d-f) photoreduction performances of 3 wt% Pd-CdS NWs heterostructures by replacing MAA with PSS. The three insets show the corresponding electron transfer models were displayed in the insets. 29



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As disclosed in Scheme 1, Pd-CdS NWs heterostructures are prepared by a ligand-triggered selfassembly strategy based on the pronounced electrostatic interaction between negatively charged MAA-modified CdS NWs and positively charged Pd@DMAP NPs. The DMAP ligands capped on the Pd NPs surface and surface charge mediator (i.e., MAA) grafted on the CdS NWs are indispensable for spontaneous directional self-assembly of Pd@DMAP on the CdS NWs. To corroborate this speculation, a series of control experiments were performed. As displayed in Fig. 7b, photoreduction activity of Pd-CdS NWs heterostructure consisting of CdS NWs without MAA modification and positively charged Pd@DMAP was substantially decreased, suggesting MAA indeed plays a pivotal role in initiating the electrostatic attraction between Pd@DMAP and CdS NWs. Only trace amount of Pd NPs can be deposited on the CdS NWs due to rather weak electrostatic interaction between Pd@DMAP and CdS NWs without MAA modification, thus leading to inferior photoactivity. When negatively charged Pd capped by citrate ions (Pd@Citrate) rather than DMAP were utilized as the building blocks for similar self-assembly, Pd@Citrate could hardly be deposited on the CdS NWs owing to the strong electrostatic repulsion interaction between negatively charged Pd@Citrate and CdS@MAA (Fig. S13), additionally highlighting the crucial role of DMAP ligands for initiating the spontaneous electrostatic self-assembly in fabricating Pd-CdS NWs heterostructures. Although unique interface configuration consisting of hierarchical DMAP ligands on the Pd surface and MAA molecules on the CdS NWs endows Pd-CdS NWs heterostructure with highly efficient photoreduction activity, more efficient charge transfer efficiency can be expected if the molecular distance in the interfacial domain of assembly units is finely modulated. To validate this speculation, photoactivities of Pd-CdS NWs heterostructure after calcination in N2 atmosphere were probed to 30


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clarify the correlation of molecular distance between DMAP and MAA with charge transfer efficiency. Fig. 7c exhibits the photoreduction performances of 3 wt% Pd-CdS NWs heterostructure after calcination in N2 for different time, from which it is clear to see that photocatalytic performances of 3 wt% Pd-CdS NWs after calcination increase as calcination time increases and this is mainly due to gradual removal of DMAP ligands and MAA molecules in the interfacial region, hence reducing the molecular distance between Pd and CdS NWs and giving rise to more intimate interfacial integration. In this regard, calcined samples afford more efficient charge transport efficiency as a result of markedly reduced charge transfer distance, thereby resulting in remarkably more enhanced photocatalytic performance in comparison with pristine sample. Moreover, it should be noted that the longer calcination time the better photoactivity of the calcined sample since the organic moieties in the interfacial region can be more completely decomposed once elevating the calcination time and this helps to further reduce the charge transfer distance. To substantiate this speculation, length of surface charge mediator (MAA) grafted on the semiconductor was tuned to unveil its influence on the photocatalytic performance of Pd-CdS NWs heterostructure. As revealed in Fig. 7d, no photoactivity is observed over Pd-CdS NWs heterostructure wherein MAA molecules grafted on the CdS NWs surface were replaced with long-chain macromolecular poly (sodium 4-styrenesulfonate) (CdS@PSS) which also endows CdS NWs substrate with a pronounced negatively charged surface.69 Analogous results were also observed in the photocatalytic hydrogen production and photoreduction of Cr6+ ions over Pd-CdS@PSS heterostructure. The inferior photoactivities of PdCdS@PSS are mainly ascribed to the shielding effect caused by the ultra-long molecular chain of PSS which conspicuously exceeds the electron transfer mean free path, therefore making direct electrons transfer from CdS to Pd rather difficult and leading to rapid recombination of charge 31



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carriers. That is, the longer molecular chain, the lower charge transfer efficiency from CdS NWs to Pd NPs. Consequently, rational design of interface configuration between metal and semiconductor should be carefully considered in order to develop high-efficiency photocatalysts. 3.5 Photocatalytic mechanism

Scheme 2. Schematic illumination depicting the photoreduction mechanism of Pd-CdS NWs heterostructures. Based on the above systematic investigation, a clear and reasonable photocatalytic mechanism of Pd-CdS NWs heterostructures can thus be proposed. As illustrated in Scheme 2, photo-induced charge carriers were instantly produced upon band-gap-photoexcitation of CdS NWs under visible light irradiation, thus generating electrons in the CB and holes in the valence band (VB) of CdS matrix. Band bending occurs on account of the lower Fermi level of Pd NPs compared with that of CdS along with the formation of Schottky barrier in the interfacial region. In this regard, electrons in the CB of CdS can spontaneously and smoothly migrate to Pd NPs by directly passing through the interfacial domain consisting of DMAP ligands and MAA molecules, which effectively restrains the recombination of photogenerated charge carriers. Subsequently, electrons captured by Pd@DMAP

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NPs are involved in the versatile photoreduction reactions to either directly reduce water to hydrogen or reduce Cr2O72− ions to Cr (III) oxo species on account of their favorable energy level alignment. With regard to photocatalytic reduction of aromatic nitro compounds, it should be pointed out that the holes in the VB band of CdS are completely quenched by the scavenger (ammonium formate) in a N2-bubbling aqueous phase which assures all the photoelectrons trapped by Pd@DMAP participate in the reduction of nitroaromatics to corresponding amino derivatives. When molecule distance between DMAP and MAA was finely modulated via either removing them by annealing or involving long-chain polyelectrolyte (PSS) to reduce or increase the molecule distance between Pd and CdS, tunable charge transfer in the interfacial region was achieved.

4. Conclusions In summary, tunable modulation of spatial charge transfer via interface configuration engineering has been achieved on the Pd-CdS NWs heterostructures which were progressively designed by a facile, green, and efficient ligand-triggered self-assembly strategy based on the pronounced electrostatic interaction between tailor-made positively charged Pd@DMAP and negatively charged MAA-modified CdS NWs matrix. It was intriguing to find that Pd@DMAP can be spontaneously and uniformly tethered on the CdS NWs generating well-defined heterostructure. Impressively, Pd-CdS NWs heterostructures exhibited significantly enhanced and multifarious photocatalytic reduction performances toward anaerobic reduction of nitroaromatics, photoreduction of heavy metal ions (Cr2O72−) and photocatalytic hydrogen production under visible light irradiation as compared with CdS NWs, predominantly ascribing to the pivotal role of Pd@DMAP as electron reservoir for efficaciously capturing electrons photoexcited from CdS NWs matrix without the restriction of

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surface ligands in the interfacial domain, thereby remarkably prolonging the lifetime of photoinduced charge carriers. More significantly, detailed influence of interface configuration on the electron transfer from CdS to Pd during the photoreduction process was unraveled. It can be concluded that ligands capped on Pd did not affect the interfacial charge transfer but rather benefit pinpoint deposition of Pd on the CdS NWs, which is distinct from conventional opinion that ligands capped on the metal surface normally limits the exposure of surface active sites for accessing the catalytic reactions. Moreover, charge transfer efficiency between Pd and CdS NWs can be finely tuned by varying the interfacial distance. As a consequence, rational interface design between metal nanocrystals and semiconductor must be rationally considered for developing a large variety of high-efficiency metal/semiconductor photocatalysts. It is hoped that our work could provide new insights on the role of surface ligands capped on the metal nanocrystals in mediating charge carriers transfer and simultaneously, open up a new avenue to achieve tunable modulation of spatial charge transfer via intelligent interface configuration engineering for solar energy conversion.

Supporting Information Zeta potentials of CdS@MAA NWs and Pd@DMAP, FTIR spectra of pristine CdS NWs, CdS@MAA, Pd@DMAP and Pd-CdS NWs, FESEM images of CdS NWs and 3 wt% Pd-CdS NWs, TEM images of 3% Pd-CdS NWs, TEM images of Pd@DMAP NPs, FESEM-EDS and elemental mapping results of 3 wt% Pd-CdS NWs, relative percentage of elements for 3 wt% Pd-CdS NWs, chemical bond species vs. B.E. for different samples, UV-vis absorption spectra of 4-NA over 3 wt% Pd-CdS NWs, blank experiments for reduction of 4-NA, 4-NA reduction under different monochromatic light irradiation, XPS spectra of 3 wt% Pd-CdS NWs before and after cyclic photocatalytic reactions, controlled experiments of photoreduction of Cr2O72−, nitrogen adsorption-desorption isotherms of CdS NWs and 3 wt% Pd-CdS NWs, specific surface 34


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area, pore volume and pore size of CdS NWs and 3 wt% Pd-CdS NWs, photocatalytic reduction rate constant (k) of blank CdS NWs and 3 wt% Pd-CdS NWs, photographs of Pd@DMAP-CdS@MAA NWs and Pd@Citrate-CdS NWs, molecular structure of MAA, DMAP and PSS, and experimental details for synthesis of Pd@Citrate-CdS NWs and Pd@DMAP-CdS NWs@PSS NWs.

Acknowledgements The support by the award Program for Minjiang scholar professorship is greatly acknowledged. This work was financially supported by the National Natural Science Foundation of China (No. 21703038).

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