from core-shell to yolk-shell: keeping the intimately ... · sunlight and beyond visible light...

ISSN 1998-0124 CN 11-5974O4

2020 13(4) 1162ndash1170 httpsdoiorg101007s12274-020-2766-0

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From core-shell to yolk-shell Keeping the intimately contactedinterface for plasmonic metalsemiconductor nanorods towardenhanced near-infrared photoelectrochemical performance Xiaodong Wan Jia Liu () Dong Wang Yuemei Li Hongzhi Wang Rongrong Pan Erhuan Zhang Xiuming ZhangXinyuan Li and Jiatao Zhang ()

Beijing Key Laboratory of Construction-Tailorable Advanced Functional Materials and Green Applications Experimental Center of AdvancedMaterials School of Materials Science amp Engineering Beijing Institute of Technology Beijing 100081 China copy Tsinghua University Press and Springer-Verlag GmbH Germany part of Springer Nature 2020 Received 16 January 2020 Revised 3 March 2020 Accepted 21 March 2020

ABSTRACT Here we report a synthetic strategy for controllable construction of yolk-shell and core-shell plasmonic metalsemiconductor hybrid nanocrystals through modulating the kinetics of sulfurization reaction followed by cation exchange The yielded yolk-shell structured products feature exceptional crystallinity and more importantly the intimately adjoined and sharp interface between plasmonic metal and semiconductor which facilitates efficient charge carrier communications between them By exploiting the system composed of Au nanorods and p-type PbS as a demonstration we show that the AuPbS yolk-shell nanorods manifest notable improvement in visible and near infrared light absorption compared to the AuPbS core-shell nanorods as well as hollow PbS nanorods Moreover the photocathode constituted by AuPbS yolk-shell nanorods affords the highest photoelectrochemical activities both under simulated sunlight and λ gt 700 nm light irradiation The superior performance of AuPbS yolk-shell nanorods is considered arising from the combination of the favorable structural advantages of yolk-shell configuration and the surface plasmon resonance enhancement effect We envision that the reported synthetic strategy can offer a valuable means to create hybrid nanocrystals with desirable structures and functions that enable to harness the photogenerated charge carriers including the plasmonic hot holes in wide-range solar-to-fuel conversion

KEYWORDS yolk-shell cation exchange surface plasmon resonance core-shell solar-to-fuel conversion

1 Introduction Direct solar energy conversion to storable chemical fuels based on photocatalytic or photoelectrochemical (PEC) reactions provides a promising route to maintain the sustainability of human society by reducing the reliance on fossil fuels [1ndash6] The behaviors of photogenerated charge carriers including their generation separation migration and surface reaction are the key factors that dominate the efficiency of the solar-to-fuel conversion process How to steer the photogenerated charge carriers and maximize their utilization efficiency in each step is therefore of fundamental importance [7ndash9] To this end structural engineering of the semiconductors have been intensively investigated [10ndash15] In particular the construction of yolk-shell nanostructures offers an ideal platform for rational regulation of the photogenerated charge carriers This is because the yolk-shell structure can result in the multiscattering effect to enhance light absorption shorten the diffusion distance of charge carriers to suppress their recombination and offer sufficient active sites to promote the surface reactions [13] These com-pelling advantages have intrigued the rapid development of the synthetic methodologies for construction of various yolk-shell nanostructures with a high level of precision and complexity both in structures and compositional combinations [16ndash25]

Among the yielded materials the plasmonic metalsemi-conductor yolk-shell hybrid nanocrystals which integrate semiconductors with metal nanocrystals (such as Au) that display unique surface plasmon resonance (SPR) are especially attractive for improving the utilization of photogenerated charge carriers [18ndash22] For example the AuTiO2 yolk-shell nanostructures have shown excellent performance in photocatalytic dye degradation alcohol oxidation hydrogen evolution and CO2 reduction reactions [20ndash22] largely beneficial from the SPR- induced electromagnetic field enhancement effect andor the resonant energy transfer effect [26ndash29] In accompany with the large progresses made for the metalmetal oxide combinations very recently a few successes have also been achieved for the creation of plasmonic yolk-shell nanostructures constituted by metal and other types of semiconductors such as chalcogenides [18 19 30] However in most of the reported plasmonic yolk- shell nanostructures the cores are entirely isolated from the shells Such indirect contact between the plasmonic metal and semiconductor results in a substantial obstacle for their mutual charge carrier transfer that has been widely manifested in the metalsemiconductor core-shell nanostructures [26ndash28 31ndash34] Moreover the current studies mainly focus on exploiting the SPR-induced hot electron injection under visiblenear-infrared (NIR) light irradiation [35 36] while the plasmonic hot holes

Address correspondence to Jiatao Zhang zhangjtbiteducn Jia Liu liujia86biteducn

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which are generated concomitantly with the hot electrons have received much less attention This is partly because the fabrication of yolk-shell systems composed of plasmonic metal and p-type semiconductor that are capable of harvesting plasmonic hot holes has seldom been reported [37 38] Research efforts are highly required in this regard to provide a paradigm for demonstrating the superiority of plasmonic metalsemiconductor yolk-shell structures in hot holes utilization additional to the hot electrons

In our previous study we found that the aqueous cation exchange performed on AgAg2S core-shell nanoprisms can lead to oxidative etching of the Ag core accompanied with the phase transformation (Ag2S to MS M = Cd and Zn) in the semiconductor shell [39] These observations were rationalized by the co-presence of the oxygensoft base ligand (such as tri-n- butylphosphine abbreviated as TBP) pair and the vacancies in the shell matrix which made the Ag core accessible to TBP Herein we further report that by manipulating the reaction kinetics of the sulfurization process before the cation exchange step the controllable construction of AuPbS yolk-shell nanorods (AuPbS Y-S) can be achieved based on the formation of a Ag intermediate layer that serves as the sacrificial template Owing to the NIR light absorption capability of the Au nanorods the p-type conductivity of the PbS shell and the exceptional heterojunction formed between Au and PbS the yielded AuPbS Y-S provide a good prototype to explore how to efficiently collect the plasmonic hot holes for promoting solar-to-fuel conversion under the irradiation of NIR light Our results showed that in comparison with the AuPbS core-shell nanorods (AuPbS C-S) and the hollow PbS nanorods in the absence of Au the photocathode assembled by the AuPbS Y-S displayed much enhanced PEC performance both under AM 15 G simulated sunlight and beyond visible light irradiation (λ gt 700 nm) which can be ascribed to the synergism arising from the favorable yolk-shell architecture and the SPR enhancement effect

2 Results and discussion As illustrated in Scheme 1 the synthesis starts from the epitaxial growth of an Ag layer around the surface of the Au nanorods (70 plusmn 5 nm in length and 10 plusmn 5 nm in width Figs S1 and S2 in the Electronic Supplementary Material (ESM)) Then controlled sulfurization and Pb2+-for-Ag+ cation exchange were sequentially conducted on the resultant AuAg core-shell nanorods in an aqueous environment The key during these processes is to modulate the sulfurization step via adjusting the amount of the sulfurization agent (an aqueous mixture of S and Na2S in our study) More specifically it was found that when the reaction system was supplied with a sufficient amount of sulfurization agent (such as 100 μL) the whole Ag overlayer was converted to Ag2S which affords a well-defined region for subsequent Pb2+-for-Ag+ cation exchange to form the PbS shell and finally the AuPbS C-S were produced However when using a low

amount of sulfurization agent (for example 10 μL) only the peripheral domain of the Ag overlayer could be transformed into Ag2S giving birth to an AuAgAg2S core-shell-shell structure The residual Ag layer sandwiched between the Au core and Ag2S shell could be removed by the TBP-mediated oxidative etching simultaneously during the Pb2+-for-Ag+ cation exchange procedure resulting in a void within the formed AuPbS products in other words yielding the AuPbS Y-S products

The images in Fig 1 exhibit the structural and morphology characterizations of the formed products featuring good monodispersity in shapes and sizes As for the partial sulfurization- resulted samples a large hollow cavity can be observed for each individual particle indicative of the formation of a yolk-shell structure (Figs 1(a) and 1(c)) Interestingly in these samples the rod-like cores are all deviated from the center of the ellipsoid-shaped particles with one side in direct contact with the shell and the other side exposed to the void In stark contrast it can be seen that the samples originating from complete sulfurization exhibit a typical core-shell structure where the rod-like cores are entirely surrounded by a shell (Figs 1(b) and 1(d)) Moreover according to the enlarged scanning electron microscopy (SEM) image shown in the inset of Fig 1(e) many of the yolk-shell structured samples possess a notable hole on the shell surface implying the good accessibility of the core to the environmental medium However the core- shell samples were found enclosed by an intact shell structure (Fig 1(f))

From Fig 2 the high resolution transmission electron microscopy (HRTEM) images reveal that both of the samples are highly crystalline (Figs 2(a)ndash2(e)) In the shell region of the yolk-shell sample the identified lattice spacing of 0295 nm corresponds to the (200) crystal plane of the cubic phase PbS Remarkably a closely adjoined and atomically sharp interface was observed between the core and shell at their junction (Fig 2(c)) which suggests that efficient charge carrier communication can be achieved even in such yolk-shell structured material [31] This is superior to many of the reported plasmonic yolk-shell structures that lack the essential interface between plasmonic metal and semiconductor to enable the SPR-induced hot carrier injection As shown in Fig 2(d) the core-shell structured sample was also demonstrated with an atomically intimate and clean interface and the lattice spacing of 0345 nm observed in the shell region can be assigned to the (111) facet of cubic PbS The corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images of each sample disclosed that Au is located in the core while the atoms of Pb and S are evenly distributed across the shell (Figs 2(f) and 2(g)) Moreover the atomic ratio between Pb and S was determined at ~ 11 by means of the EDS elemental analysis (Figs S3 and S4 in the ESM) The above results indicate that irrespective of the structural difference the two resultant samples have similar compositional properties with the core and shell regions composed of Au and PbS correspondingly This is consistent with the X-ray diffraction

Scheme 1 Schematic illustration of the synthetic routes for AuPbS Y-S and AuPbS C-S Modulating the amount of the sulfur precursor in the sulfurization step is critical to determining the structural forms of the products obtained after cation exchange

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Figure 1 (a) annd (b) The TEM images (c) and (d) STEM images and (e) and (f) SEM images for AuPbS Y-S ((a) (c) (e)) and AuPbS C-S ((b) (d) (f)) correspondingly The insets in (e) and (f) are SEM images with a higher magnification

(XRD) patterns given in Fig 2(h) where two series of peaks were detected which can be assigned to the face-centered-cubic (fcc) Au (PDF 04-0784) and the cubic PbS (PDF 05-0592) respectively

To verify the key factors governing the structure of the final products we further carried out a series of comparison experiments by changing the amount of sulfurization agent and the thickness of the Ag overlayer (Figs S5 and S6 in the ESM) The results showed that the hollowing degree in the sample after cation exchange was negatively correlated with the former and positively related to the later In specific when the addition amount of sulfurization agent was increased from 7 to 20 μL the hollow cavity within the AuPbS nanocrystals began to disappear resulting in the core-shell structure (Fig S5 in the ESM) Meanwhile by improving the amount of the Ag precursor from 200 to 300 μL in the aim of increasing the thickness of the Ag overlayer the AuPbS Y-S with a larger void was achieved (Fig S6 in the ESM) and this can be explained by the enlarged portion of the residual Ag left between Au and Ag2S after the sulfurization process Therefore it is rational to presume that the kinetics of the sulfurization reaction is critical to determining whether the AuPbS nanorods adopt a yolk-shell structure or a core-shell structure This assumption was consolidated by further investigating the intermediates obtained after the sulfurization process As shown in Figs 3(a) 3(b) and Fig S7 in the ESM the HRTEM image EDS line scanning profiles and elemental mapping images of the intermediates derived from partial sulfurization uncovered the AuAgAg2S core-shell-shell

Figure 2 (a)ndash(e) The HRTEM images for AuPbS Y-S ((a) (c) (e)) and AuPbS C-S ((b) (d)) correspondingly (c) and (e) are the HRTEM images with higher magnification of the regions denoted by the yellow square box and the red square box in panel (a) respectively ((f) (g)) The STEM images and corresponding EDS elemental maps for AuPbS Y-S (g) and AuPbS C-S (f) respectively (h) The XRD patterns for AuPbS Y-S and AuPbS C-S

structure as expected In contrast the complete-sulfurization resultant intermediates were characterized by the AuAg2S core-shell structure (Fig S8 in the ESM) On these bases Fig 3(c) depicts the mechanism underlying the evolution from the AuAg core-shell nanorods to the AuPbS Y-S (1) During the sulfurization step the exterior layers of the Ag shell in AuAg core-shell nanorods were converted to Ag2S by reacting with the sulfurization agent leaving the inner layers of Ag shell undistributed due to the sluggish sulfurization kinetics (2) In the following cation exchange step TBP is considered playing dual decisive roles Firstly it is used to provide the ther-modynamic driving force for initiating the exchange reaction between Ag+ and Pb2+ cations [40ndash42] Explicitly on the basis of the Pearsonrsquos hard and soft acids and bases (HSAB) theory TBP a soft base preferentially binds to the Ag+ cation a soft acid as Ag+-TBP coordination compound to promote the outward diffusion of Ag+ from the Ag2S matrix and the replacement

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of Ag+ by Pb2+ cations that possess reduced softness Secondly the Ag intermediate layer can be in-situ removed by TPBO2 where the cation vacancies (or interstitial sites) in the Ag2S shell generated during cation exchange provide a pathway for TBP to access to the Ag atoms and induce their oxidative etching [39] It should be mentioned that during this process the pair of cetyltrimethylammonium bromide (CTAB which is used to inhibit the aggregation of the nanorods) and oxygen dissolved in solution probably can accelerate the etching of the Ag crystalline layer [39 42ndash44] Taken together the controlled sulfurization combined with the subsequent cation exchange and its initiated oxidative etching can be utilized to justify the formation of the AuPbS Y-S starting from the AuAg core-shell nanorods According to our previously studies [45] the deviation of the Au nanorod cores from the center of the final AuPbS Y-S products was possibly caused by the release of the interfacial strain during the sulfurization and cation exchange processes in consideration of the notable lattice mismatch between the metal and semiconductor components in the hybrid nanocrystals

The upper-limit of solar-to-fuel conversion efficiency is governed by the light harvesting capability of the photocatalysts Therefore extending the light-responsive range and increasing the effective optical path length of incident light to enhance the light absorption of semiconductors are importantly significant [10 21 46] On one hand integration of Au nanorods with semiconductors is propitious to expand their light-responsive range owing to the remarkable character of the Au nanorods which display broad SPR absorption covering a large portion of solar spectrum especially in visible and NIR regions [47] In our study the Au nanorods with an aspect ratio of 71 showed a notable longitudinal SPR band at 890 nm (Fig 4) After coated with a PbS shell the longitudinal SPR band of the resulting AuPbS C-S was red-shifted toward 1240 nm as a result of the varied refractive index of the local dielectric environment surrounding Au surface On the other hand it has been reported that construction of unique yolk-shell structures can greatly increase the length of light-path through semiconductors and therefore improving their light utilization efficiency [10 13 21] This is profited from the light-scattering effect where

the hollow void within the yolk-shell particles allows multiple reflection and scattering of the incident light leading to secondary absorption of the scattered light To validate the favorable effect of the yolk-shell structure in light harvesting we quantitatively compared the optical absorption properties of the two AuPbS samples in different structural forms at the same particular concentration [46] From Fig 4 it can be seen that the absorption curve of AuPbS Y-S is obviously upward shifted compared to its core-shell counterpart across the whole measured wavelength range from 450 to 1300 nm demonstrating the markedly enhanced light harvesting ability endowed by the yolk-shell structure These results hint that when the incident light penetrates the shell of the AuPbS Y-S it could undergo continuous scattering inside the hollow void exciting additional charge carriers both on the plasmonic Au core and the PbS shell The slight blue shift of the longitudinal SPR band observed for the AuPbS Y-S than AuPbS C-S likely reflects the partial exposure of the Au nanorods to the aqueous solvent in the yolk-shell sample (because of the lower refractive index of water than PbS) in agreement with the SEM observations as above

Figure 4 The optical absorption spectra for AuPbS Y-S AuPbS C-S and Au nanorods collected at the same particular concentration The schematic drawing shows the multiple reflection of the light within the AuPbS Y-S that is inapplicable to AuPbS C-S

Figure 3 (a) The HRTEM image and (b) EDS line scan of an individual AuAgAg2S (c) Schematic illustration of the mechanism underlying the evolution from AuAg core-shell nanorods to AuPbS Y-S

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discussed (inset in Fig 1(e)) Moreover by further quantitatively comparing the absorption curve of AuPbS Y-S with the linear addition of the adsorption curves corresponding to hollow PbS nanorods (which were prepared by removing the Au cores from the AuPbS C-S under hydrothermal condition [18] (Fig S9 in the ESM) and Au nanorods (Fig S10 in the ESM) we presume that the multiscattering of light in the yolk-shell structure can strengthen the SPR absorption of the Au nanorods In one word our results show that the integration of Au and PbS into a yolk-shell configuration can bring forth favorable synergism between the SPR effect and the light scattering effect concurrently extending the light absorption range and the light-path length to result in exceptional light harvesting behavior

As demonstrated by many research groups the plasmonic metal in direct contact with a n-type semiconductor can inject hot electrons into the conduction band of the semiconductor and contribute to the photocatalytic reduction reactions [32 47ndash49] However comparatively fewer studies have been reported concerning the capture and conversion of the hot holes which are formed in accompany with the hot electrons during surface plasmon decay and are supposed to be ldquohotterrdquo than the hot electrons [37 38 50ndash52] In our case the p-type conductivity of the PbS shells offers a desirable condition to collect the hot holes from the adjoining Au nanorods and to further make use of them in the solar-to-fuel conversion process [53] As exhibited in Fig S11 in the ESM the results of the open-circuit potential measurements confirmed the p-type conductivity of the AuPbS Y-S and AuPbS C-S [54] Under this scenario

theoretically the hot holes generated by the SPR of Au nanorods hold an opportunity to inject into the valence band of PbS shell and participate in the oxidation reaction

The PEC studies were performed in an electrolyte containing 05 M Na2SO4 using a three-electrode configuration with AuPbS nanorods assembled as the working electrode a platinum plate counter electrode and a saturated silver chloride electrode (AgAgCl) as the reference electrode Figure 5(a) exhibits the dependence plot of photocurrent density as a function of potential (IndashV curves) for AuPbS Y-S and AuPbS C-S under a chopped light source with simulated sunlight (AM 15G 100 mWcm2) The two samples both displayed cathodic photocurrents where a steer increase in the photocurrent toward negative direction was initiated upon illumination and instantaneously reverted to the initial stage when the illumination was turned off substantiating the p-type conductivity of the photoelectrode materials One can see that the PEC photocurrent of the AuPbS Y-S photocathode was evidently higher than that of the AuPbS C-S photocathode More importantly under a chopped light source with wavelength longer than 700 nm (λ gt 700 nm) the AuPbS Y-S photocathode still afforded remarkable PEC response and attained a photocurrent density of 382 μAcm2 at minus02 V vs the AgAgCl electrode (Fig 5(b)) However the PEC response of the AuPbS C-S photocathode was substantially lower at λ gt 700 nm with a photocurrent density of 75 μAcm2 achieved at minus02 V vs the AgAgCl electrode only one-fifth relative to the yolk-shell structured electrode The photocurrent densityminustime (Iminust) curves measured at a fixed bias under simulated sunlight and λ gt 700 nm

Figure 5 (a) and (b) The photocurrent density-potential curves of AuPbS Y-S and AuPbS C-S photocathodes under AM 15G simulated sunlight illumination (a) and under λ gt 700 nm light illumination (b) (c) and (d) The photocurrent densityndashtime curves of AuPbS Y-S and AuPbS C-S photocathodesunder AM 15G simulated sunlight illumination at a bias of minus005 V vs AgAgCl (c) and under λ gt 700 nm light illumination at a bias of minus005 V vs AgAgCl (d) (e) The HC-STH conversion efficiency of AuPbS Y-S and AuPbS C-S photocathodes (f) The EIS Nyquist plots for AuPbS Y-S and AuPbS C-S photocathodes under AM 15G simulated sunlight illumination

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are given in Figs 5(c) and 5(d) respectively The results are in good agreement with the IminusV plots demonstrating the higher PEC activity of the AuPbS Y-S photocathode relative to its core-shell equivalent in particular beyond the visible light region (λ gt 700 nm) Furthermore the half-cell solar-to-hydrogen conversion efficiency (HC-STH) of the two differently structured AuPbS photoelectrodes was estimated using the following equation

HC-STH = (Jp V)Jlight where Jp is the photocurrent density (mAmiddotcmminus2) at the measured bias V is the applied bias potential vs the reversible hydrogen electrode (RHE) and Jlight is the irradiance intensity of 100 mWmiddotcmndash2 (AM 15G) The results exhibited in Fig 5(e) uncovered that the AuPbS Y-S photocathode achieved the highest conversion efficiency of 004 at a bias of 038 V vs RHE notably improved relative to the AuPbS C-S electrode (001 at a potential of 038 V vs RHE) Considering the major variation between the two electrode materials lies in their structural divergence the superior PEC performance of AuPbS Y-S clearly signifies the positive impacts associated with the yolk-shell configuration As aforementioned the benefits of yolk-shell structure in promoting solar-to-fuel conversion include the enhanced light scattering the reduced diffusion distance of charge carriers and the abundant surface active sites etc [10ndash13] In our study aside from the enhanced light harvesting (Fig 4) the excellent PEC performance afforded by the yolk-shell structured photocathode could additionally attributable to the depressed charge recombination and the accelerated surface reaction taking into account that the exposure of the inner surface of PbS shell to solvent can lead to shortened charge- transfer distance and enlarged surface area correspondingly According to Fig 5(f) the electrochemical impedance spec-troscopy (EIS) Nyquist plots collected under illumination demonstrated that the charge transfer through the electrode electrolyte interface was indeed more favorable in the AuPbS Y-S electrode than the AuPbS C-S electrode Moreover the PEC activities of the AuPbS Y-S photocathode were compared

with the hollow PbS nanorod photocathode in terms of their Indasht curves As shown in Fig S12 in the ESM the presence of Au nanorods obviously improved the PEC response both under simulated sunlight and λ gt 700 nm irradiation suggesting the significant contribution arising from the SPR effect of the Au nanorods

The PEC water oxidation performance of the different photocathodes was analyzed at an external bias of minus015 V vs AgAgCl under AM 15G irradiation via an on-line chromato-graphy As presented Fig 6(a) during the 6 hours of continuous irradiation the AuPbS Y-S photocathode exhibited an evident improvement in oxygen evolution compared to the C-S photo-cathode However hydrogen gases were not detected in both cases To identify the PEC reduction products in our system we performed electron spin resonance (ESR) measurements for the AuPbS Y-S using 55-dimethyl-1-pyrroline-N-oxide (DMPO) as the probe molecule [55] The results are given in Fig 6(b) from which one can see that in sharp contrast to the indiscernible signal in dark condition the ESR signal with an intensity ratio of 1111 characteristic of the superoxide radicals (O2

bullminus) is displayed under simulated sunlight illumination Meanwhile as given in Figs 6(c) 6(d) and Fig S13 in the ESM the X-ray photoelectron spectroscopy (XPS) spectra of the photocathode material composed by AuPbS Y-S showed no noticeable changes before and after the PEC assay In particular the Pb 4f72 peaks were explicitly retained ruling out the possibility that the Pb2+ ions in the shell matrix were reduced by the photogenerated electrons In view of the above results we infer that in our system the photogenerated electrons were principally consumed by the in-situ formed oxygen molecules through one-electron reduction resulting in the formation of the O2

bullminus species Previous investigations have demonstrated that the plasmonic hot electrons energetically favor the transfer from the Fermi level of Au to the 2π-state of O2 to generate O2

bullminus [38] Such charge transfer path can be pertinent to our results where the hot electrons generated in the Au nanorods may inject into the LUMO level of the oxygen molecules adsorbed on the Au surface and produce the O2

bullminus radicals At

Figure 6 (a) The time course of oxygen evolution for AuPbS Y-S and AuPbS C-S photocathodes under AM 15G simulated sunlight illumination at a bias of ndash015 V vs AgAgCl (b) The ESR spectra obtained via mixing DMPO with AuPbS Y-S in methanol before (black curve) and after irradiation by AM 15G simulated sunlight for 1 min (red curve) (c) and (d) Comparison of the Pb 4f (c) and S 2p (d) XPS spectra for AuPbS Y-S photocathode before and after the PEC oxygen evolution measurement

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the same time the hot holes in the Au nanorods might be delivered to the Pt counter electrode under the drive of external bias to participate in the water oxidation reaction The function of the charge carriers generated by the interband transition of PbS was also determined in PEC oxygen evolution based on the photocathode assembled by hollow PbS nanorods and the results shown in Fig S14 in the ESM strongly substantiated the remarkable enhancement effect correlated with the plasmonic Au nanorods It is noteworthy that due to the spectral overlap between the Au nanorod SPR and the PbS absorbance the existence of the Au nanorods probably brings synergistic plasmonic effects involving the electromagnetic field enhan-cement (ie SPR-mediated enhancement in local electromagnetic field surrounding the plasmonic metal which contributes to the local generation of electron-hole pairs in the nearby semiconductor) andor the resonant energy transfer (ie the electromagnetic field-mediated plasmonic energy transfer in the form of a resonant energy transfer process) mechanisms additional to the contribution made by the hot charge carriers [28 29 56ndash58]

3 Conclusions In summary we have developed a cation exchange-mediated strategy for controllable construction of yolk-shell and core-shell metalsemiconductor nanocrystals by using the combination of plasmonic Au nanorods and p-type PbS as a representative The kinetics of the sulfurization step prior to cation exchange was demonstrated to be the knob governing the structural forms of the products obtained after cation exchange By systematically comparing the absorption property and PEC performance of the AuPbS Y-S with those of the AuPbS C-S and the hollow PbS nanorods we showed that the synergism between the structural benefits of the yolk-shell configuration and the SPR of plasmonic metal provides a viable tool for regulating the behaviors of photogenerated charge carriers in solar-to-fuel conversion process It should be highlighted that beneficial from the strong absorption throughout the visible and NIR regions the photocathode assembled by the AuPbS Y-S displayed excellent PEC activities even under the illumination of light with wavelength longer than 700 nm (λ gt 700 nm) Moreover the p-type conductivity of the PbS shell and its seamless contact with the Au nanorod in the AuPbS Y-S are able to constitute a good paradigm to investigate the hot hole collection in sustainable energy development

4 Experimental All chemicals were of analytical grade and were used as receivedwithout further purification in this study

41 Synthesis of Au nanorods

The Au nanorods were prepared following the method reported by Murray with slight modifications [59] To prepare the seed solution of gold nanorods 10 mL of 01 M CTAB and 0025 mL of 01 M HAuCl4 aqueous solutions were mixed in a 25 mL round-bottomed flask Then 006 mL of 01 M fresh NaBH4 solution was injected to the above mixture under vigorous stirring for 2 min and the resulting solution was aged at room temperature for 60 min For the preparation of the growth solution 36 g of CTAB and 49 g of benzyldimethylhexade-cylammonium chloride (BDAC) were dissolved in 100 mL of deionized water followed by the addition of 05 mL of 01 M HAuCl4 and 10 mL of 001 M AgNO3 aqueous solutions under stirring Then 056 mL of 01 M ascorbic acid (AA) was introduced into the resultant mixture Subsequently 100 μL of the seed

solution was added into the growth solution and the mixture was aged at room temperature for 12 h The Au nanorods colloids were obtained by centrifugation at 8000 rpm for 10 min and were washed three times with deionized water

42 Synthesis of AuPbS Y-S AuPbS C-S and hollow

PbS nanorods

8 mL of the prepared Au nanorods colloidal solution and 2 mL of 05 M CTAB aqueous solution were mixed in a centrifuge tube then 02 mL of 001 M AgNO3 solution 5 mL of 01 M AA solution and 5 mL of 01 M NaOH solution were sequentially dropped into the tube under magnetic stirring The resulting mixture was aged for 2 h at room temperature to give birth to the AuAg core-shell nanorods colloids which were collected by centrifugation washed thoroughly with deionized water and re-dispersed in 10 mL of deionized water With regard to the AuPbS Y-S colloids the sulfurization procedure was performed by adding a desired volume (lower than 20 μL such as 10 μL) of sulfur precursor solution (32 mg of sulfur powder and 1404 mg of Na2S were dissolved in 117 mL of deionized water by ultrasonication until the color of the solution was changed to light yellow Then the mixture was reacted at 80 degC for 12 h) into the prepared AuAg core-shell nanorods colloidal suspension The resulting particles were washed with deionized water and re-dispersed in 10 mL of 50 mM CTAB aqueous solution Thereafter the cation exchange step was carried out by sequentially adding 1 mL of 5 mgmiddotmLminus1 Pb(NO3)2 aqueous solution and 50 μL of TBP into the above yielded suspension under stirring and the mixture was aged for 1 hour at 60 degC The yellow-green precipitates were collected through centrifugation and were washed with deionized water The AuPbS C-S were synthesized by following the similar pro-cedures except that in the sulfurization step a larger amount (higher than 20 μL such as 100 μL) of sulfur precursor solution was introduced into the synthetic system The hollow PbS nanorods were prepared using the same method as AuPbS C-S except that during the cation exchange step 150 μL of TBP was exploited and the reaction was performed under hydrothermal condition at 120 degC for 4 h [12]

43 Characterizations

The TEM images were obtained by HITACHI H-7650 electron microscopy operating at 80 kV The HRTEM images and EDS elemental mapping analysis were collected on an FEI Tecnai G2 F30 S-Twin transmission electron microscopy operating at 200 kV equipped with X-ray energy-dispersive spectroscopy detector SEM images were obtained based on a Hitachi FESEM 4800 microscopic instrument Vis-NIR spectra were recorded using Shimadzu UV3600 spectrophotometer XRD analysis was performed using Bruker D8 multiply crystals X-ray diffractometer (5deg per min) The X-ray photoelectron spectroscopy (XPS) analysis was conducted on a PerkinElmer Physics PHI 5300 spectrometer

44 PEC measurements

The PEC measurements were performed using a standard three-electrode potentiostat system on Instruments760D electrochemical workstation (Chenghua Shanghai China) with a working electrode a Pt counter electrode and a AgAgCl reference electrode (saturated KCl) The potential conversion formula between reversible hydrogen electrode (RHE) and AgAgCl is as follows

E(RHE) = E(AgAgCl) + 00591 pH + 0197 The working electrode was prepared by depositing the sample

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colloidal suspension onto a fluorine-doped tin oxide (FTO) substrate (1 cm times 2 cm) Specifically 10 mg of sample crystals were dispersed in 1 mL of deionized water The formed uniform suspension was deposited onto the FTO substrate by the spray coating method with the surface area of the sample exposed to the electrolyte fixed at 1 cm2 Before the PEC measurements the obtained sampleFTO photocathode was annealed in a N2

atmosphere at 300 degC for 4 h to strengthen the contact between the sample and the substrate [60 61] An aqueous solution containing 05 M Na2SO4 (pH = 68) was used as the electrolyte The working electrode was illuminated from the front side with a 300 W Xe lamp (FX300 Beijing Perfectlight Technology) equipped with an AM 15 solar simulation filter (100 mWcm2) or an optical filter (PLS-CUT 700 λ gt 700 nm) The EIS Nyquist plots were collected under light illumination with the frequency ranging from 100 kHz to 1 Hz and the modulation amplitude of 5 mV The PEC oxygen evolution assay was examined in a Pyrex reaction cell connected to a closed gas circulation and evacuation system (Labsolar 6A Beijing Perfectlight Technology) The reaction cell was maintained at 25 degC by a flow of cooling water bath during the reaction The amount of evolved O2 was analyzed by a gas chromatograph (Agilent 7890B GC system) equiped with a thermal conductivity detector (TCD) and a molecular sieve 5A column

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos 51702016 51631001 21801015 51902023 and 51872030) the Fundamental Research Funds for the Central Universities (No 2017CX01003) and the Beijing Institute of Technology Research Fund Program for Young Scholars The characterization results were supported by Beijing Zhongkebaice Technology Service Co Ltd

Electronic Supplementary Material Supplementary material (additional TEM images HRTEM images STEM images EDS elemental analysis results optical absorption spectra open- circuit potential measurement results photocurrent density-time plots XPS spectra and PEC oxygen evolution curves for the samples) is available in the online version of this article at httpsdoiorg101007s12274-020-2766-0

References [1] Montoya J H Seitz L C Chakthranont P Vojvodic A Jaramillo

T F Noslashrskov J K Materials for solar fuels and chemicals Nat Mater 2017 16 70ndash81

[2] Kim D Sakimoto K K Hong D Yang P D Artificial photosynthesis for sustainable fuel and chemical production Angew Chem Int Ed 2015 54 3259ndash3266

[3] Maeda K Mallouk T E Two-dimensional metal oxide Nanosheetsas building blocks for artificial photosynthetic assemblies Bull Chem Soc Jpn 2019 92 38ndash54

[4] Hu C Li M Y Qiu J S Sun Y P Design and fabrication of carbon dots for energy conversion and storage Chem Soc Rev 2019 48 2315ndash2337

[5] Roy N Suzuki N Terashima C Fujishima A Recent improvements in the production of solar fuels From CO2 reduction to water splitting and artificial photosynthesis Bull Chem Soc Jpn 2019 92 178ndash192

[6] Jena A K Kulkarni A Miyasaka T Halide PerovskitePhotovoltaics Background status and future prospects Chem Rev 2019 119 3036ndash3103

[7] Wang Z Li C Domen K Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting Chem Soc Rev 2019 48 2109ndash2125

[8] Chen S S Takata TDomen K Particulate photocatalysts for overall water splitting Nat Rev Mater 2017 2 17050

[9] Bai S Jiang J Zhang Q Xiong Y J Steering charge kinetics in photocatalysis Intersection of materials syntheses characterization techniques and theoretical simulations Chem Soc Rev 2015 44 2893ndash2939

[10] Xiao M Wang Z L Lyu M Luo B Wang S C Liu G Cheng H M Wang L Z Hollow nanostructures for photocatalysis Advantages and challenges Adv Mater 2019 31 1801369

[11] Liu X Q Iocozzia J Wang Y Cui X Chen Y H Zhao S Q Li Z Lin Z Q Noblemetal-metal oxide nanohybrids with tailored nanostructures for efficient solar energy conversion photocatalysis and environmental remediation Energy Environ Sci 2017 10 402ndash434

[12] Abe H Liu J Ariga K Catalytic nanoarchitectonics for environmentally compatible energy generation Mater Today 2016 19 12ndash18

[13] Li A Zhu W J Li C C Wang T Gong J L Rational design of yolk-shell nanostructures for photocatalysis Chem Soc Rev 2019 48 1874ndash1907

[14] Tian H Liang J Liu J Nanoengineeringcarbon spheres as nanoreactorsfor sustainable energy applications Adv Mater 2019 31 1903886

[15] Tian H Liu X Y Dong L BRen X M Liu H Price C A H Li Y Wang G X Yang Q H Liu J Enhanced hydrogenation performance over hollow structured Co-CoOxN-C capsules Adv Sci 2019 6 1900807

[16] Liu J Qiao S Z Chen J S Lou X W Xing X R Lu G Q YolkShell nanoparticles New platforms for nanoreactors drug delivery and lithium-ion batteries Chem Commun 2011 47 12578ndash12591

[17] Wang M W Boyjoo Y Pan J Wang S B Liu J Advanced yolk-shell nanoparticles as nanoreactors for energy conversion Chin J Catal 2017 38 970ndash990

[18] Feng J W Liu J Cheng X Y Liu J J Xu M Zhang J T Hydrothermal cation exchange enabled gradual evolution of AuZnS- AgAuS yolk-shell nanocrystalsand their visible light photocatalytic applications Adv Sci 2018 5 1700376

[19] Chiu Y H Naghadeh S B Lindley S A Lai T H Kuo M Y Chang K D Zhang J Z Hsu Y J Yolk-shell nanostructures as an emerging photocatalyst paradigm for solar hydrogen generation Nano Energy 2019 62 289ndash298

[20] Li A Zhang P Chang X X Cai W T Wang T Gong J L Gold nanorodTiO2 yolk-shell nanostructures for visible-light-driven photocatalytic oxidation of benzyl alcohol Small 2015 11 1892ndash 1899

[21] Shi X W Lou Z Z Zhang P Fujitsuka M Majima T 3D-array of Au-TiO2 yolk-shell as plasmonicphotocatalyst boosting multiscattering with enhanced hydrogen evolution ACS Appl Mater Interfaces 2016 8 31738ndash31745

[22] Tu W G Zhou Y Li H J Li P Zou Z G AuTiO2 yolk-shell hollow spheres for plasmon-induced photocatalytic reduction of CO2 to solar fuel via local electrochemical field Nanoscale 2015 7 14232ndash14236

[23] Zhang N Fu X Z Xu Y J A Facile and green approach to synthesize PtCeO2 nanocomposite with tunable core-shell and yolk-shell structure and its application as a visible light photocatalyst J Mater Chem 2011 21 8152ndash8158

[24] You F F Wan J W Qi J Mao D Yang N L Zhang Q H Gu L Wang D Lattice distortion in hollow multi-shelled structures for efficient visible-light CO2 reduction with a SnS2SnO2 junction Angew Chem Int Ed 2020 132 731ndash734

[25] Tian H Huang F Zhu Y H Liu S M Han Y Jaroniec M Yang Q H Liu H Y Lu G Q M Liu J The development of yolk-shell-structured PdampZnOCarbonsubmicroreactors with high selectivity and stability Adv Funct Mater 2018 28 1801737

[26] Wang M Y Ye M D Iocozzia J Lin C J Lin Z Q Plasmon- mediated solar energy conversion via photocatalysis in noble metal semiconductor composites Adv Sci 2016 3 1600024

[27] Jiang R B Li B X Fang C H Wang J F Metalsemiconductor hybrid nanostructures for plasmon-enhanced applications Adv Mater 2014 26 5274ndash5309

[28] Zhang P Wang T Gong J L Mechanistic understanding of the Plasmonic enhancement for solar water splitting Adv Mater 2015 27 5328ndash5342

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[29] Linic S Christopher P Ingram D B Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy Nat Mater 2011 10 911ndash921

[30] Lee S U Jung H Wi D H Hong J W Sung J Choi S I Han S W Metal-semiconductor yolk-shell heteronanostructures for plasmon-enhanced photocatalytic hydrogen evolution J Mater Chem A2018 6 4068ndash4078

[31] Liu J Feng J W Gui J Chen T Xu M Wang H Z Dong H F Chen H L Li X W Wang L et al MetalSemiconductor core-shell nanocrystals with atomically organized interfaces for efficient hot electron-mediated photocatalysis Nano Energy 2018 48 44ndash52

[32] Jung H Song J Lee S Lee Y W Wi D H Goo B S Han S W Hierarchical metal-semiconductor-graphene ternary heteronano-structures for plasmon-enhanced wide-range visible-light photocatalysis J Mater Chem A2019 7 15831ndash15840

[33] Patra B K Khilari S Pradhan D Pradhan N Hybrid dot-disk Au-CuInS2 nanostructures as active photocathode for efficient evolution of hydrogen from water Chem Mater 2016 28 4358ndash4366

[34] Patra B K Khilari S Bera A Mehetor S K Pradhan D Pradhan N Chemically filled and Au-coupled BiSbS3 nanorodheterostructures for photoelectrocatalysis Chem Mater 2017 29 1116ndash1126

[35] Elbanna O Kim S Fujitsuka M Majima T TiO2 mesocrystals composited with gold nanorods for highly efficient visible-NIR- photocatalytic hydrogen production Nano Energy 2017 35 1ndash8

[36] Yang H Wang Z H Zheng Y Y He L Q Zhan C Lu X H Tian Z Q Fang P P Tong Y X Tunable wavelength enhanced photoelectrochemicalcells from surface Plasmon resonance J Am Chem Soc 2016 138 16204ndash16207

[37] DuChene J S Tagliabue G Welch A J Cheng W H Atwater H A Hot hole collection and photoelectrochemical CO2 reduction with plasmonicAup-GaNphotocathodes Nano Lett 2018 18 2545ndash2550

[38] Peng T H Miao J J Gao Z S Zhang L J Gao Y Fan C H Li D Reactivating catalytic surface Insights into the role of hot holes in Plasmoniccatalysis Small 2018 14 1703510

[39] Zhang E H Liu J Ji M W Wang H Z Wan X D Rong H P Chen W X Liu J J Xu M Zhang J T Hollow anisotropic semiconductor Nanoprisms with highly crystalline frameworks for high-efficiency photoelectrochemical water splitting J Mater Chem A2019 7 8061ndash8072

[40] De Trizio L Manna L Forging colloidal nanostructures via Cationexchange reactions Chem Rev 2016 116 10852ndash10887

[41] Beberwyck B J Surendranath Y Alivisatos A P Cationexchange A versatile tool for Nanomaterialssynthesis J Phys Chem C 2013 117 19759ndash19770

[42] Tsung C K Kou X S Shi Q H Zhang J PYeung M H Wang J F Stucky G D Selective shortening of single-crystalline gold Nanorods by mild oxidation J Am Chem Soc 2006 128 5352ndash5353

[43] Wiley B Herricks T Sun Y G Xia Y N Polyolsynthesis of silver nanoparticles Use of chloride and oxygen to promote the formation of single-crystal truncated cubes and tetrahedrons Nano Lett 2004 4 1733ndash1739

[44] Long R Zhou S Wiley B J Xiong Y J Oxidative etching for controlled synthesis of metal Nanocrystals Atomic addition and subtraction Chem Soc Rev 2014 43 6288ndash6310

[45] Zhao Q Ji M WQian H M Dai B S Weng L Gui J Zhang J T Ouyang M Zhu H S Controlling structural symmetry of a hybrid nanostructure and its effect on efficient Photocatalytichydrogen evolution Adv Mater 2014 26 1387ndash1392

[46] Lien D H Dong Z H Retamal J R D Wang H P Wei T C Wang D He J H Cui Y Resonance-enhanced absorption in hollow Nanoshellspheres with omnidirectional detection and high Responsivity and speed Adv Mater 2018 30 1801972

[47] Ni W H Kou X S Yang Z Wang J F Tailoring longitudinal surface plasmon wavelengths scattering and absorption cross sectionsof Gold Nanorods ACS Nano 2008 2 677ndash686

[48] Wu K F Rodriguez-Cordoba W E Yang Y Lian T Q Plasmon- induced hot electron transfer from the Au Tip to CdSrod in CdS-Au Nanoheterostructures Nano Lett 2013 13 5255ndash5263

[49] Ma X C Dai Y Yu L Huang B B New basic insights into the low hot electron injection efficiency of gold-nanoparticle-photosensitized titanium dioxide ACS Appl Mater Interfaces 2014 6 12388ndash12394

[50] Govorov A O Zhang H Gunrsquoko Y K Theory of photoinjection of hot plasmonic carriers from metal nanostructures into semiconductors and surface molecules J Phys Chem C2013 117 16616ndash16631

[51] Wang S Y Gao Y Y Miao S Liu T F Mu L C Li R G Fan F T Li C Positioning the water oxidation reaction sites in plasmonicphotocatalysts J Am Chem Soc 2017 139 11771ndash11778

[52] Li H Qin F Yang Z P Cui X M Wang J F Zhang L Z New reaction pathway induced by plasmon for selective benzyl alcohol oxidation on BiOClpossessing oxygen vacancies J Am Chem Soc 2017 139 3513ndash3521

[53] Bai S Li X Y Kong Q Long R Wang C M Jiang J Xiong Y J Toward enhanced photocatalytic oxygen evolution Synergetic utilization of plasmonic effect and schottky junction via interfacing facet selection Adv Mater 2015 27 3444ndash3452

[54] Pan R R Liu J Li Y M Li X Y Zhang E H Di Q M Su M Y Zhang J T Electronic doping-enabled transition from n- to p-type Conductivity over AuCdS core-shell nanocrystals toward unassisted photoelectrochemical water splitting J Mater Chem A 2019 7 23038ndash23045

[55] Yuan Q C Liu D Zhang N Ye W Ju H X Shi L Long R Zhu J F Xiong Y J Noble-metal-free Janus-like structures by Cationexchange for Z-Scheme photocatalytic water splitting under broadband light irradiation Angew Chem Int Ed 2017 56 4206ndash 4210

[56] Cushing S K Li J T Meng F K Senty T R Suri S Zhi M J Li M Bristow A D Wu N Q Photocatalyticactivity enhanced by plasmonic resonant energy transfer from metal to semiconductor J Am Chem Soc 2012 134 15033ndash15041

[57] Yu X J Liu F Z Bi J L Wang B Yang S C Improving the plasmonic efficiency of the Au nanorod-semiconductor photocatalysis toward water reduction by constructing a unique hot-dog nanostructure Nano Energy 2017 33 469ndash475

[58] Yu X J Bi J L Yang G Tao H Z Yang S C Synergistic effect induced high photothermal performance of Au NanorodCu7S4yolk- shell nanooctahedron particles J Phys Chem C 2016 120 24533ndash 24541

[59] Ye X C Zheng C Chen J Gao Y Z Murray C B Using binary surfactant mixtures to simultaneously improve the dimensional Tunability and monodispersity in the seeded growth of gold Nanorods Nano Lett 2013 13 765ndash771

[60] Wang Z L Wang L Z Photoelectrode for water splitting Materials fabrication and characterization Sci China Mater 2018 61 806ndash821

[61] Li Y M Liu J Li X Y Wan X D Pan R R Rong H P Liu J J Chen W X Zhang J T Evolution of hollow CuInS2 nanododecahedrons via kirkendall effect driven by Cation exchange for efficient solar water splitting ACS Appl Mater Interfaces 2019 11 27170ndash27177

Nano Res 2020 13(4) 1162ndash1170


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which are generated concomitantly with the hot electrons have received much less attention This is partly because the fabrication of yolk-shell systems composed of plasmonic metal and p-type semiconductor that are capable of harvesting plasmonic hot holes has seldom been reported [37 38] Research efforts are highly required in this regard to provide a paradigm for demonstrating the superiority of plasmonic metalsemiconductor yolk-shell structures in hot holes utilization additional to the hot electrons

In our previous study we found that the aqueous cation exchange performed on AgAg2S core-shell nanoprisms can lead to oxidative etching of the Ag core accompanied with the phase transformation (Ag2S to MS M = Cd and Zn) in the semiconductor shell [39] These observations were rationalized by the co-presence of the oxygensoft base ligand (such as tri-n- butylphosphine abbreviated as TBP) pair and the vacancies in the shell matrix which made the Ag core accessible to TBP Herein we further report that by manipulating the reaction kinetics of the sulfurization process before the cation exchange step the controllable construction of AuPbS yolk-shell nanorods (AuPbS Y-S) can be achieved based on the formation of a Ag intermediate layer that serves as the sacrificial template Owing to the NIR light absorption capability of the Au nanorods the p-type conductivity of the PbS shell and the exceptional heterojunction formed between Au and PbS the yielded AuPbS Y-S provide a good prototype to explore how to efficiently collect the plasmonic hot holes for promoting solar-to-fuel conversion under the irradiation of NIR light Our results showed that in comparison with the AuPbS core-shell nanorods (AuPbS C-S) and the hollow PbS nanorods in the absence of Au the photocathode assembled by the AuPbS Y-S displayed much enhanced PEC performance both under AM 15 G simulated sunlight and beyond visible light irradiation (λ gt 700 nm) which can be ascribed to the synergism arising from the favorable yolk-shell architecture and the SPR enhancement effect

2 Results and discussion As illustrated in Scheme 1 the synthesis starts from the epitaxial growth of an Ag layer around the surface of the Au nanorods (70 plusmn 5 nm in length and 10 plusmn 5 nm in width Figs S1 and S2 in the Electronic Supplementary Material (ESM)) Then controlled sulfurization and Pb2+-for-Ag+ cation exchange were sequentially conducted on the resultant AuAg core-shell nanorods in an aqueous environment The key during these processes is to modulate the sulfurization step via adjusting the amount of the sulfurization agent (an aqueous mixture of S and Na2S in our study) More specifically it was found that when the reaction system was supplied with a sufficient amount of sulfurization agent (such as 100 μL) the whole Ag overlayer was converted to Ag2S which affords a well-defined region for subsequent Pb2+-for-Ag+ cation exchange to form the PbS shell and finally the AuPbS C-S were produced However when using a low

amount of sulfurization agent (for example 10 μL) only the peripheral domain of the Ag overlayer could be transformed into Ag2S giving birth to an AuAgAg2S core-shell-shell structure The residual Ag layer sandwiched between the Au core and Ag2S shell could be removed by the TBP-mediated oxidative etching simultaneously during the Pb2+-for-Ag+ cation exchange procedure resulting in a void within the formed AuPbS products in other words yielding the AuPbS Y-S products

The images in Fig 1 exhibit the structural and morphology characterizations of the formed products featuring good monodispersity in shapes and sizes As for the partial sulfurization- resulted samples a large hollow cavity can be observed for each individual particle indicative of the formation of a yolk-shell structure (Figs 1(a) and 1(c)) Interestingly in these samples the rod-like cores are all deviated from the center of the ellipsoid-shaped particles with one side in direct contact with the shell and the other side exposed to the void In stark contrast it can be seen that the samples originating from complete sulfurization exhibit a typical core-shell structure where the rod-like cores are entirely surrounded by a shell (Figs 1(b) and 1(d)) Moreover according to the enlarged scanning electron microscopy (SEM) image shown in the inset of Fig 1(e) many of the yolk-shell structured samples possess a notable hole on the shell surface implying the good accessibility of the core to the environmental medium However the core- shell samples were found enclosed by an intact shell structure (Fig 1(f))

From Fig 2 the high resolution transmission electron microscopy (HRTEM) images reveal that both of the samples are highly crystalline (Figs 2(a)ndash2(e)) In the shell region of the yolk-shell sample the identified lattice spacing of 0295 nm corresponds to the (200) crystal plane of the cubic phase PbS Remarkably a closely adjoined and atomically sharp interface was observed between the core and shell at their junction (Fig 2(c)) which suggests that efficient charge carrier communication can be achieved even in such yolk-shell structured material [31] This is superior to many of the reported plasmonic yolk-shell structures that lack the essential interface between plasmonic metal and semiconductor to enable the SPR-induced hot carrier injection As shown in Fig 2(d) the core-shell structured sample was also demonstrated with an atomically intimate and clean interface and the lattice spacing of 0345 nm observed in the shell region can be assigned to the (111) facet of cubic PbS The corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images of each sample disclosed that Au is located in the core while the atoms of Pb and S are evenly distributed across the shell (Figs 2(f) and 2(g)) Moreover the atomic ratio between Pb and S was determined at ~ 11 by means of the EDS elemental analysis (Figs S3 and S4 in the ESM) The above results indicate that irrespective of the structural difference the two resultant samples have similar compositional properties with the core and shell regions composed of Au and PbS correspondingly This is consistent with the X-ray diffraction

Scheme 1 Schematic illustration of the synthetic routes for AuPbS Y-S and AuPbS C-S Modulating the amount of the sulfur precursor in the sulfurization step is critical to determining the structural forms of the products obtained after cation exchange

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from core-shell to yolk-shell: keeping the intimately ... · sunlight and beyond visible light...

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