Letter Cite This: Nano Lett. 2018, 18, 4370−4376
pubs.acs.org/NanoLett
Plasmon-Enhanced Multicarrier Photocatalysis Firdoz Shaik,† Imanuel Peer,† Prashant K. Jain,‡ and Lilac Amirav*,† †
Schulich Faculty of Chemistry, Technion−Israel Institute of Technology, Haifa 32000, Israel Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
‡
Nano Lett. 2018.18:4370-4376. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 08/29/18. For personal use only.
S Supporting Information *
ABSTRACT: Conversion of solar energy into liquid fuel often relies on multielectron redox processes that include highly reactive intermediates, with back reaction routes that hinder the overall efficiency of the process. Here, we reveal that these undesirable reaction pathways can be minimized, rendering the photocatalytic reactions more efficient, when charge carriers are harvested from a multiexcitonic state of a semiconductor photocatalyst. A plasmonic antenna, comprising Au nanoprisms, was employed to accomplish feasible levels of multiple carrier excitations in semiconductor nanocrystal-based photocatalytic systems (CdSe@CdS core−shell quantum dots and CdSe@CdS seeded nanorods). The antenna’s near-field amplifies the otherwise inherently weak biexciton generation in the semiconductor. The twoelectron photoreduction of Pt and Pd metal precursors served as model reactions. In the presence of the plasmonic antenna, these photocatalyzed two-electron reactions exhibited enhanced yields and kinetics. This work uniquely relies on a nonlinear enhancement that has potential for large amplification of photocatalytic activity in the presence of a plasmonic near-field. KEYWORDS: Photocatalysis, surface plasmon resonance, multicarrier generation, semiconductor-metal nano hybrids, water splitting, CO2 reduction
C
Strong optical resonances and intense near-fields generated in plasmonic nanostructures are known to enhance a range of optical processes.9−18 In fact, plasmonic enhancement has been utilized for enhancing semiconductor photocatalysis.19−21 However, common manifestations of plasmon-enhanced photocatalysis originate from direct charge injection from the excited plasmonic-metal into the semiconductor22−26 or are attributed to linear near-field enhancement27−30 of visibleabsorption, typically in an otherwise UV-band gap semiconductor. Here, we uniquely rely on a nonlinear enhancement, which has potential for much more dramatic amplification in the presence of a plasmonic near-field. Whereas linear field enhancement scales as |Enf |2,31 where Enf represents the nearfield amplitude, nonlinear enhancement scales as |Enf |2n, where n is the order of the process. A classic example of large electromagnetic enhancement is surface-enhanced Raman scattering (SERS), cross sections of which scale as |Enf |4.32 Electromagnetic enhancement factors32 as high as 106 have been found, which give rise to dramatic amplification of SERS sensitivities.33 Analogously, the second-order process of biexciton generation is expected to be amplified in the
onversion of solar energy into liquid fuel often relies on a redox process that requires the injection of multiple photoinduced charge carriers into an adsorbate. For instance, the water oxidation half-reaction involves the injection of four photoinduced holes,1 and the photoconversion of CO2 to CH4 involves an 8e−−8H+ transfer.2 The intermediates of individual reaction steps of these multielectron processes typically involve highly reactive ions or radicals, which have a high probability of back reaction, causing a major bottleneck in efficient conversion. By transferring multiple charges, essentially simultaneously, to the adsorbate, it would be possible to reduce back-reactions, diminish undesirable reaction pathways, and enhance photocatalytic efficiency. We hypothesize that the generation of a multiexcitonic state in a semiconductor photocatalyst would make such simultaneous multielectron transfer feasible. Our findings presented here show that multielectron photocatalytic reactions indeed become more efficient when charge carriers are harvested from a multiexcitonic state of a semiconductor photocatalyst. The multiexciton state requires sequential filling of states via consecutive absorption of two photons, a process that has an inherently low cross-section. Hence, we harness plasmonic field enhancement for increasing the cross-section of multiphoton absorption and achieving multiexciton generation in the semiconductor3−8 at levels feasible for catalyzing a twoelectron redox reaction. © 2018 American Chemical Society
Received: April 7, 2018 Revised: June 15, 2018 Published: June 22, 2018 4370
DOI: 10.1021/acs.nanolett.8b01392 Nano Lett. 2018, 18, 4370−4376
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Nano Letters
Figure 1. Schematic illustration of the metal photostaining method, wherein photogenerated carriers drive the deposition of Pt on semiconductor nanostructures in the presence of a plasmonic antenna (top panel). TEM micrographs of CdSe@CdS QDs/SRs coupled/decoupled to the silicacoated Au nanoprisms (Au@SiO2) are presented before and after photoillumination in the presence of (CH3)2PtCOD. CdSe-CdS QDs coupled to Au@SiO2 before (A) and after photoillumination (B), HAADF image (C), and its corresponding magnified image (D). Free-standing QDs before illumination (E) and after photoillumination (F) (inset: HRTEM of a single QD), and its corresponding HAADF image (G) and HR-HAADF image (H). CdSe-CdS SRs coupled with Au@SiO2 before (I) and after photoillumination (J), HAADF image (K), and its corresponding magnified image (L). Free-standing SRs before photoillumination (M) and after photoillumination (N), its corresponding HAADF (O), and HRTEM images (P).
presence of a plasmonic near-field by a scale given by |Enf |4, with prospects for large enhancement factors. We employ a set of controlled plasmonic/semiconductor nanocrystal heterostructures as our model system. In these hybrid nanostructures, the semiconductor component serves as the photocatalyst, while the plasmonic component serves as a resonant near-field-generating antenna under visible-light excitation. The heterostructures are designed so as to ensure strong electrodynamic coupling between the plasmonic antenna and the semiconductor nanocrystal, while preventing any electronic transfer between the two components, by means of a several nanometer-thick insulating silica barrier.27,34−36 The metal and semiconductor nanoparticles are synthesized separately under their respective ideal conditions, and the hybrid is then assembled, as presented in the illustration in Figure S1.
Au nanoprisms were chosen as the plasmonic component because of their strong visible-region plasmon resonance, and the strong electromagnetic field hotspots known to exist at their sharp vertices,37−41 as well as for their chemical stability.42 The Au nanoprisms, with a typical average lateral dimension of 32 nm, were synthesized via seedless growth through oxidative etching, as described in the literature,43 and were coated with silica using a modified sol−gel method.36 For detailed description of the procedure used, the reader is referred to the Supporting Information (SI) section. The surface of the silica-coated nanoprisms (Au@SiO2) was then functionalized with N-[3-(trimethoxysilyl) propyl] ethylenediamine (TMPSEDA). The aminosilane coupling agent served as an anchor for attaching semiconductor photocatalyst nanostructures. Both CdSe@CdS core−shell quantum dots (QDs) and CdSe@CdS seeded nanorods (SRs) were examined. These 4371
DOI: 10.1021/acs.nanolett.8b01392 Nano Lett. 2018, 18, 4370−4376
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from TEM, we performed an elemental composition analysis by inductively coupled plasma atomic emission spectroscopy (ICP-OES). The measurement was performed on samples that were fully cleaned of residual metal salt by means of repetitive precipitation and redispersion, as detailed in the SI. The ICPOES analysis, presented in Figure S5, provides quantitative characterization of Pt photodeposition that can be translated to a relative reaction yield. Nearly 2.7 mg/L of Pt was detected in photoilluminated samples of the QDs coupled with Au@SiO2. The free-standing photoilluminated QDs themselves contained only negligible amount of Pt at, or below, the detection limit (under dilution factor of 20). Similarly, 3.4 mg/L of Pt was detected in photoilluminated samples of SRs coupled with Au@SiO2, whereas trace levels of Pt were detected for the freestanding SRs. These results demonstrate that, under certain reaction conditions, a two-electron reaction may progress at an enhanced level, perhaps even near-exclusively, in the presence of a plasmonic near-field. To explore the generality of such a plasmon-enhanced multielectron reaction, we further examined the case of Pd deposition using the precursor cis-dimethly-(N,N,N,N-tetramethylene-diamine) palladium(II) (TMEDAPdMe2). Similar to (CH3)2PtCOD, TMEDAPdMe2 requires two electrons for its reduction to metallic Pd.50 The semiconductor QDs and SRs in their free-standing form, as well as those coupled to the silica-coated Au nanoprisms, were illuminated in the presence of TMEDAPdMe2 under reaction conditions described in the SI. The postillumination nanostructures were examined under the electron microscope. The SRs coupled to the Au nanoprisms and the free-standing SRs both show Pd deposition, as seen in the TEM and HAADF micrographs in Figure 2A−D. However, the extent of metal deposition on the SRs coupled to the plasmonic antenna (Figure 2A,B) far exceeds that observed on the free-standing SRs (Figure 2C,D). Results for QDs are included in the SI, Figure S6. As was done in the case of Pt, visual examination by TEM was verified by a quantitative characterization of Pd metal deposition by ICP-OES. These results are presented in Figure 2E, for SRs as well as QDs. In general, Pd nanoparticles with an average size of ∼6 nm were found to be deposited on the surfaces of the SRs coupled to the plasmonic Au@SiO2 nanoprisms, and the total Pd measured by ICP-OES amounted to 43 mg/L. However, the Pd nanoparticles found on the freestanding SRs averaged ∼3 nm in diameter, 8-fold smaller in volume than the former case. The total amount of Pd, determined to be 7.3 mg/L, was correspondingly lower. A similar trend is obtained for the QDs. For QDs coupled with Au@SiO2 nanoprisms, deposited Pd nanoparticles averaged ∼4 nm, and the total Pd deposited amounted to 29 mg/L. However, for free-standing QDs, the Pd nanoparticles averaged only ∼2 nm, and the total Pd deposited amounted to 17 mg/L. These results confirm that the yield of the two-electron photoreduction of TMEDAPdMe2 is significantly enhanced in the presence of the plasmonic antenna. It is possible that metal photodeposition performed in the presence of the plasmonic enhancement has already reached saturation within the time duration employed for the experiments. In such a situation, the effective growth duration varies between the samples, and the measured plasmonic enhancement in the yield of metal photodeposition, as reflected in the nanoparticle deposit size, represents a lower limit. As for the case of Pt photodeposition, the favorable effect of the plasmonic nanostructure seems to be more pronounced
4 nm QDs and 40 nm long SRs were chosen as the semiconductor components because of their ability to support biexciton formation44,45 and their proven photocatalytic activity.46,47 Electron micrographs of CdSe@CdS QDs and SRs are presented in Figure 1, anchored to the Au@SiO2 (Figure 1A,I), or in their free-standing form (Figure 1E,M). The absorption spectra of the QDs, SRs, Au@SiO2, and Au@ SiO2-SRs are presented in Figure S4, showing the broad absorbance band of the Au@SiO2, which has spectral overlap with the excitonic peaks of the semiconductor nanostructures. Using these hybrid photocatalysts, the effect of the plasmonic near-field on the yield of the photocatalytic multielectron reaction was investigated. A two-electron photoreduction of a metal precursor was chosen as a model reaction. The outcome of this reaction is a metal nanoparticle photodeposited on the semiconductor photocatalyst. Because such a metal deposit can be detected and directly observed by electron microscopy, we effectively have a “photostaining” method for visualizing successful photoinduced charge harvesting from the photocatalyst.48,49 The nanoparticle deposit size serves as a surrogate measure of the reaction yield, which we characterize as a function of our photocatalytic reaction conditions. In particular, we measured the yield of metal photodeposition in the presence and absence of the plasmonic antenna. This approach is illustrated by the schematic in the top panel of Figure 1. Semiconductor QDs and SRs, both free-standing and those coupled to silica-coated Au nanoprisms, were tested for their photocatalytic activity. For the test, a colloidal solution of the photocatalyst was excited at 455 nm (using a light-emitting diode set to a power of 100 mW), for a duration of 8 h, in the presence of dissolved dimethyl (1,5-cyclooctadine)platinum(II) [(CH3)2PtCOD]. (CH3)2PtCOD was chosen as the metal precursor because it is known to require two electrons for its photoinduced reduction into metallic Pt.49,50 Other reaction conditions are detailed in the SI. Care was taken to maintain the reaction conditions identical for photocatalytic tests performed with and those performed without the plasmonic antenna. The nanostructures were examined postillumination under the electron microscope. The results, labeled as “after photoillumination”, are presented in Figure 1. Pt deposition is clearly observed on photoilluminated QDs and SRs that are coupled to the Au nanoprisms (Figure 1B,J). The metal deposits are visualized as bright white dots in the high-angle annular dark-field (HAADF) imaging micrographs (Figure 1C−D,K−L). Close examination reveals that Pt nanoparticles of an average size of ∼2 nm were grown on the surfaces of QDs coupled to silica-coated Au nanoprisms. However, no indication of Pt deposition was observed for the free-standing semiconductor nanostructures subject to similar photoillumination (Figure 1F,N), suggesting a small to negligible yield of the photoreduction reaction. This finding is more pronounced and better visualized with the SRs. Pt nanoparticles of an average size of ∼4 nm were observed to be deposited on the surface of photoilluminated SRs that are coupled to plasmonic nanoprisms, whereas no Pt deposition is found on freestanding SRs subject to similar photoillumination. Although TEM and HAADF images of the free-standing nanostructures do not show metal deposition (Figure 1F− H,N−P), micrographs alone cannot be regarded as unambiguous evidence for the lack of photoreaction. To verify findings 4372
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rather than a photocarrier-driven one, another control experiment was performed. Solutions of SRs, with dissolved metal precursors, were heated to 70 °C and maintained at that temperature under ambient illumination. Except for the lack of light illumination and elevated temperature, all other reaction conditions were identical to those of the photocatalytic reactions. Even after 8 h in the presence of the Pt(II) salt, no metal deposition was observed (Figure S7), and in the presence of Pd(II) salt, only scarce amount of ultrasmall Pd deposits were found. To further exclude the possibility of the photocatalytic activity being a result of localized heating of the gold and to exclude the direct-plasmon-mediated process,52,53 the hybrid photocatalysts were tested for photocatalytic activity under excitation of 505 and 530 nm light. These excitation wavelengths are closer to the plasmonic peak maximum of the Au nanoprisms, but in this spectral range, the light absorption of the semiconductor photocatalysts is significantly weaker. No metal photodeposition was obtained under these excitation conditions (Figure S8), proving that photocatalytic activity results from light absorption by the semiconductor component. Thus, our results consistently point to plasmonic enhancement of multielectron harvesting from the semiconductor. Comparing and contrasting the plasmonic effect on a twoelectron reaction with that on a one-electron reaction can ascertain the nonlinear mechanism of enhancement most unambiguously. Such an investigation was performed with the aid of two dye probes.54 One of the dyes was methylene blue, a common redox indicator with a characteristic absorbance band at ∼667 nm. This band is bleached when methylene blue (MB) undergoes reduction. This reduction reaction requires two electrons, similar to the case of (CH3)2Pt-COD and TMEDAPdMe2 precursors. The other dye chosen for this study was rhodamine 6G (R6G). This dye has a characteristic absorbance band at ∼530 nm, which is bleached when R6G undergoes conversion to its reduced form. In this case, however, the reduction is a one-electron reaction. The photocatalytic reduction of each dye was examined in the presence of both the free-standing semiconductor nanocrystals and the semiconductor nanocrystals coupled to the plasmonic nanoprisms. By monitoring the characteristic absorbance band of the dye as a function of photoreaction time, the kinetics of
Figure 2. TEM and HAADF micrographs showing photocatalytic Pd deposition on CdSe@CdS SRs that are (A,B) coupled to the silicacoated Au nanoprisms and (C,D) in their free-standing form. Scale bars are 10 nm. (E) Amount of Pd photodeposited on coupled and free-standing QDs/SRs, as determined by ICP-OES measurements.
when using SRs as the photocatalyst as compared to QDs. We attribute this difference to the improved ability of SRs to support biexciton formation.51 A control study confirmed that metal deposition is indeed a result of a light-induced reaction: no metal deposition was observed on nanostructures stirred in the dark, under otherwise similar conditions. To exclude the possibility of metal deposition being the result of a photothermal process,
Figure 3. Photocatalytic activity of CdSe@CdS QDs/SRs coupled/decoupled to the silica-coated Au nanoprism (Au@SiO2) for one-electron reduction of R6G (A) and two-electron reduction of MB (B). The plots show the relative decrease in the peak absorbance of the dye in the course of its photocatalytic reduction. 4373
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Figure 4. TEM and HAADF micrographs showing Pt photodeposition on QDs that are coupled to Au@SiO2, with different silica shell thickness. Average shell thickness of (A,D) 8, (B,E) 13, and (C,F) 60 nm, respectively. Scale bars are 20 nm. On the right, the amount of Pt photodeposited in each case, as determined by ICP-OES measurements, is plotted along with a guide to the eye. At the farthest spacing of 60 nm, the amount of Pt deposited is at or below 1 mg/L, the limit of detection (LOD).
photoreduction was measured. The results are presented in Figure 3. In the study with QDs, the kinetics of the one-electron (R6G) reduction reaction (Figure 3A, gray squares) appears not to be altered by the presence of the plasmonic near-field antenna (purple circles). In contrast, the kinetics of the twoelectron (MB) reduction reaction was found to be faster under photocatalysis by QDs coupled to the plasmonic nanoprisms as compared to photocatalysis by free-standing QDs (Figure 3B). A similar trend was also observed when SRs were used as the photocatalysts. There was no clear benefit of the plasmonic antenna to the kinetics of the one-electron reduction (Figure 3A). However, the two-electron reduction of MB displayed a striking acceleration in the presence of the plasmonic antenna. With SRs coupled to plasmonic nanoprisms, MB reduction was nearly complete even before the first absorbance spectrum could be measured, whereas MB reduction photocatalyzed by free-standing SRs took over 4 min (Figure 3B). It is instructive to determine the distance range over which the observed plasmonic enhancement effect is operative. After all, plasmonic near-fields are known to decay near-exponentially as a function of distance away from the plasmonic nanostructure surface, with a decay length that scales as the nanoparticle dimensions.35,55−57 In this study, the physical distance between the semiconductor photocatalyst (QDs) and Au nanoprisms was systematically varied by tuning the thickness of silica spacer from 8 to 13 nm, and also, in an extreme case, to 60 nm (Figure 4). As a function of this controlled lateral separation between the semiconductor nanostructure and the plasmonic antenna, the yield of the plasmon-enhanced Pt(II) photoreduction reaction was characterized in terms of the amount of Pt metal deposited. As expected from the known distance-decay of the plasmonic field, the amount of Pt photodeposited on the QD surfaces was found to decrease with increasing of the silica spacer thickness. This observed decrease in the photoreduction activity confirms that the presence of the silica layer alone cannot account for the observed multielectron redox activity. Rather, the Au nanoprism is the source of the enhancement. Going from a spacer thickness of 8 to 13 nm, the total amount of deposited Pt drops by nearly 1.5-fold. At the farthest spacing of 60 nm,
the amount of Pt deposited is at or below 1 mg/L, the limit of detection (LOD). The decrease in the yield of the two-electron photoreduction reaction with increasing distance away from the plasmonic antenna mirrors the known distance-decay of the near-field of the antenna.35,55−57 This observation further ascertains the critical role of the plasmonic antenna on the efficiency of multielectron harvesting. These findings motivate future theoretical work on plasmonic antenna effects on multiexciton generation. In plasmonic antenna/semiconductor hybrids as the ones here, near-field enhancement of photoexcitation, Purcell enhancement,58 radiation and reabsorption processes,59 and exciton− plasmon coupling60 are all expected to operate in close association with one another, and a rigorous theoretical model will need to include an interplay of all these phenomena. Conclusions. It is found that photocatalytic reactions that involve multiple electron transfer steps can become more efficient when multiple excited carriers are generated/ accumulated on the photocatalyst and transferred to an adsorbate simultaneously. A plasmonic antenna was employed to accomplish feasible levels of multiple carrier excitations in semiconductor nanocrystal-based photocatalytic systems. The antenna’s near-field amplifies the otherwise inherently weak biexciton generation in the semiconductor. In the presence of the plasmonic antenna, photocatalyzed two-electron reactions exhibited enhanced yields and kinetics. We envision that this hitherto unexplored effect of multicarrier excitations on semiconductor photocatalysis would be beneficial for promoting kinetically challenging reactions of high technological importance such as the solar-driven splitting of water into hydrogen and oxygen, and CO2 conversion to hydrocarbons. In addition, the nonlinear, distance-dependent enhancement of plasmonic near-fields may be exploited for driving redox chemistry in a highly spatially controlled manner.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b01392. 4374
DOI: 10.1021/acs.nanolett.8b01392 Nano Lett. 2018, 18, 4370−4376
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Detailed description of the synthetic procedures, photodeposition experiments, and control studies (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Firdoz Shaik: 0000-0002-6821-174X Prashant K. Jain: 0000-0002-7306-3972 Lilac Amirav: 0000-0002-0539-0488 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was carried out in the framework of Russell Berrie Nanotechnology Institute (RBNI) and the Nancy and Stephen Grand Technion Energy Program (GTEP). We acknowledge the generous support from the I-CORE Program of the Planning and Budgeting Committee, and The Israel Science Foundation (Grant No. 152/11). We thank Dr. Yaron Kauffmann for his assistance with HAADF and HRTEM characterization. P.K.J. acknowledges support by the National Science Foundation under Grant NSF CHE-1455011 for his work on concept development and cowriting of the manuscript. F.S. thanks the Schulich and GTEP postdoctoral fellowships for their support.
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DOI: 10.1021/acs.nanolett.8b01392 Nano Lett. 2018, 18, 4370−4376
Letter
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DOI: 10.1021/acs.nanolett.8b01392 Nano Lett. 2018, 18, 4370−4376
