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Hollow Mesoporous Plasmonic Nanoshells for Enhanced Solar Vapor Generation Marcin Stefan Zielinski, Jae-Woo Choi, Thomas La Grange, Miguel A Modestino, Seyyed Mohammad Hosseini Hashemi, Ye Pu, Susanne Birkhold, Jeffrey Hubbell, and Demetri Psaltis Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b03901 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016
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Hollow Mesoporous Plasmonic Nanoshells for Enhanced Solar Vapor Generation Marcin S. Zielinski,1 Jae-Woo Choi,1 Thomas LaGrange,2 Miguel Modestino,1 Seyyed Mohammad Hosseini Hashemi,1 Ye Pu,1* Susanne Birkhold,1† Jeffrey A. Hubbell,3,4 and Demetri Psaltis1
1
Laboratory of Optics (LO), School of Engineering (STI), École Polytechnique Fédérale de
Lausanne (EPFL), CH-1015 Lausanne, Switzerland 2
Interdisciplinary Center for Electron Microscopy (CIME), School of Basic Sciences (SB),
École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 3
Institute of Bioengineering (IBI) and Institute of Chemical Sciences and Engineering
(ISIC), School of Life Sciences, École Polytechnique Fédérale de Lausanne (EPFL), CH1015 Lausanne, Switzerland 4
Institute for Molecular Engineering, University of Chicago, Chicago, IL, 60637 USA
KEYWORDS: composite nanoshell, plasmonics, thermal cavity, mesoporosity, solar-vapor, steam nanobubble
ABSTRACT: In the past decade, nanomaterials have made their way into a variety of technologies in solar energy, enhancing their performance by taking advantage of the phenomena inherent to the nanoscale. Recent examples exploit plasmonic core/shell nanoparticles to achieve efficient direct steam generation, showing great promise of such
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nanoparticles as a useful material for solar applications. In this paper, we demonstrate a novel technique for fabricating bimetallic hollow mesoporous plasmonic nanoshells that yield a higher solar vapor generation rate compared with their solid-core counterparts. Based on a combination of nanomasking and incomplete galvanic replacement, the hollow plasmonic nanoshells can be fabricated with tunable absorption and minimized scattering. When exposed to sun light, each hollow nanoshell generates vapor bubbles simultaneously from the interior and exterior. The vapor nucleating from the interior expands and diffuses through the pores and combines with the bubble formed on the outer wall. The lack of a solid core significantly accelerates the initial vapor nucleation and the overall steam generation dynamics. More importantly, because the density of the hollow porous nanoshells is essentially equal to the surrounding host medium, these particles are much less prone to sedimentation, a problem that greatly limits the performance and implementation of standard nanoparticle dispersions.
Among “green” renewable energy sources, solar light is one of the most technologically challenging to harness and use at reasonably high conversion efficiencies and low processing costs.1 A complete solar-thermal device must be capable of harvesting light efficiently and converting it into usable or storable heat, a key factor difficult to achieve for a wide range of applications. Specific to solar-vapor generation, most of the current technologies rely on absorbing the solar energy through macroscopic cavity or vessel surfaces and transferring the heat into the surrounding liquid volume and therefore suffer from energy loss by heat dissipation from absorbing surfaces to the bulk fluid, requiring high optical concentration levels to overcome these inefficiencies.2, 3 Several interesting strategies have been recently proposed to overcome the demand for a high optical concentration, e.g. directional light selectivity of confined infrared (IR) radiation, reflected back into the solar
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light absorber by spectrally selective vessel inner walls.4, 5 Such a device can reduce the radiative losses from a black body absorber by up to 75%.6 Another recent example introduces a broadband absorbing double layer of exfoliated graphite and highly porous carbon foam, which when placed at the top surface of water provides solar-to-steam conversion efficiency as high as 85% due to a well-engineered porous absorber, implemented to thermally insulate the vicinity of the foam surfaces from the liquid volume underneath.7 The advent of nanotechnology opened up a new avenue in solar-thermal applications. Instead of the macroscale approaches mentioned above, a concept of highly porous surfaces of nanoparticles (NPs), in a volumetric dispersion, can be implemented to locally enhance generation of steam without a heat loss into the entire bulk volume. This can be achieved by taking advantage of plasmonic effects in metallic colloidal NPs that generate heat under exposure to light.8-12 Similar NP systems have been used to for light-to-thermal energy conversion in multiple processes, including cancer thermal therapy,13 studies of phase transition of ice,14 or photochemical transformations.15,
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Upon solar irradiation, the NPs
convert the solar energy into heat, leading to vapor envelopes that nucleate around the heated NPs. The initial vapor envelopes further expand into nanobubbles, which can coalesce and produce micron scale vapor bubbles that can be released at the liquid-air interface.17 One of the key factors in this process is the poor thermal conductivity of vapor, which reduces heat losses to the surrounding medium and achieves a high steam generation rate while only nucleating bubbles at the NP surface vicinity.18 A recent demonstration19 of solar vapor generation in water using colloidal SiO2/Au core/shell NPs showed that a remarkable 80% of the converted energy from light went into the evaporation process, achieving an overall energy efficiency of 24% at an irradiation level of 106 W m-2, or 1000× solar irradiation at air mass (AM) 1.5.
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In this paper, we introduce a nanoparticle engineering approach that combines the plasmonic properties of metallic NPs and the extended active surface provided by mesoporous thermal cavity walls to boost the solar vapor generation rates. To the best of our knowledge, hollow plasmonic nanoshells (NSs) have not been implemented in this area, possibly due to the difficulties in the fabrication of such structures with controllable properties. Often, the diffusion transport across the shell is inefficient due to a poor porosity control. The preservation of the NS shape is also challenging, especially when a thin walled structure is desired.20, 21 Here we describe in detail the fabrication and characterization of Ag/Au alloy NSs, which exhibit a hollow, mesoporous plasmonic nanoshell (HMP-NS) architecture. Such a structure was obtained using a controlled nanomasking technique,22 followed by an incomplete galvanic replacement reaction.20, 23, 24 The synthesized HMP-NSs consist of a mesoporous composite shell architecture and a void interior, where the pore size has a bimodal distribution that enables different functionalities. The smaller pores extend the cavity aspect ratio with their size and number depending on the material, wall thickness, and processing conditions,25 while the larger pores (referred to as “pinholes” for clarity) obtained by nanomasking with SiO2 beads enable higher vapor bubble diffusion and heat transport rates across the shell barrier. Within the proposed synthesis method, the size of the larger pores can be tuned by varying the diameter of the SiO2 nanomasks. Similar to the process reported in SiO2/Au core/shell NPs,17-19 when the HMP-NSs are heated by solar irradiation, ballistic heat transfer26, 27 takes place in their nanoscale vicinity, which could initiate a phase transition of the surrounding aqueous media into a steam envelope. The key difference, however, is the lack of a solid core, which is replaced by a hollow thermal cavity, thereby allowing water vapor to nucleate on both the inner and outer walls of the NS. We show that the structure and the composition of the Ag/Au alloy promote absorption and suppress scattering in a broad spectral range, with a resonance tunability
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resulting from different geometry and material compositions28,
29
. Furthermore, our
experiments reveal that the shell thickness and the surface ligands of the HMP-NS play a vital role in solar vapor generation. NPs composed of metallic shells are typically prepared by depositing the desired metal onto a dielectric structural template nanoparticle, such as silica30-34 or polymer beads,35, 36
by means of seed mediated growth.37, 38 Alternative preparation methods use the absorption
of metal cations on coordinated sites of linking peptide molecules,39,
40
which provide a
sufficient density of nucleation spots to obtain a complete coverage of a sacrificial core. All these methods are straightforward and provide good control of a shell thickness, though the polycrystallinity of the obtained structures makes the scaffold fragile and prone to collapse. This represents a significant challenge when etching the nanoparticle interior to obtain a hollow structure. Such polycrystalline scaffold composed of poorly connected metallic domains is mainly supported by interactions with anchoring molecules pre-adsorbed onto a dielectric nanoparticle core,30, 37, 38 therefore, subsequent removal of the core may completely disintegrate the shell structure, especially when deposited on a small spherical core with a high-curvature surface.28 Additionally, the core removal requires a wet chemical etching with difficult to control process parameters,34 or calcination procedures,38,
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which may cause
further damage to the polycrystalline shell and result in an incomplete coverage, structural defects, fragility and drastically lower the yield of the final product. In order to overcome these problems, a metallic shell must balance the required amount of porosity that provides the necessary transport of chemical species and a mechanically robust structure to prevent collapse after the template core is removed. Nanomasking technique22 in combination with a partial Ag galvanic replacement reaction20, 42 allowed us to obtain hollow mesoporous Ag/Au alloy NSs with high level of
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control over their pore size and shell thickness. A schematic of the synthesis principle is presented in Figure 1A. Samples corresponding to each intermediate processing step were pre-characterized by scanning electron microscopy (SEM). Two particle samples were prepared, 5-nm- and 10-nm-thick HMP-NSs, following a similar synthesis (more specific details can be found in the Methods section). First, an aqueous solution containing 193.7 ±21.2 nm spherical Ag nanoparticles stabilized with trisodium citrate (Na3C6H5O7) was vigorously stirred and mixed dropwise with a solution containing an excess of amine terminated SiO2 beads having a diameter of 23.7 ±2.5 nm. The SiO2 particles covalently bind to the Ag core surface (Figure 1B) and act as masking sites for pinholes. In the second step, the solution was heated up either to 70 °C and to 100 °C in order to obtain 10-nm- |and 5-nmthick alloy shells, respectively. As described by the redox reaction in Figure 1A, exposed unmasked silver atoms at the core surface are rapidly oxidized after the injection of HAuCl4 gold precursor, leading to the formation of a thin Ag/Au bimetallic shell, indicated by a rapid color change in the solution from yellow to light purple. In the standard galvanic replacement process,20 the amount of Au precursor is kept at a stoichiometric ratio of 1:3 with respect to Ag and Ag+ cations continuously diffuse via the Kirkendall effect through an incomplete bimetallic shell until the whole Ag core is consumed,43 and a crystalline metallic Au shell structure is formed due to Ostwald ripening.44 As the injected amount of HAuCl4 was stoichiometrically limiting, the Ag core was only partially oxidized resulting in a thin Ag/Au bimetallic core/shell NP usually containing one open pore, consistent with previous reports on similar systems.23,
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After one hour of reaction, NPs were cooled down to ambient
temperature, followed by deposition of thiol-terminated polyethylene glycol (PEG) molecules to enhance NP colloidal stability. Following this step, the SiO2 beads were removed by either a chemical etching process with hydrofluoric acid (HF), or by increasing the pH of the solution in order to deprotonate their anchoring amine groups. The removal of the SiO2 beads
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that produces pinholes improves shell penetration and disintegration of the Ag core. In the final product, most of the SiO2 masking beads were removed from hollow NSs, as determined through X-ray energy dispersive spectroscopy (EDS) shown in Figure S1. However, a residue of unbound SiO2 beads was detected in the sample solutions using dynamic light scattering (DLS), as shown in Figure S1B. Figure 1C and 1D show SEM images of 10-nm-thick bimetallic shells deposited on an Ag core before and after removing the nanomasks. The Ag core was dissolved in the last step using low concentration nitric acid (HNO3) while stirring at 40 °C for two hours until the solution color changed to light grey. A SEM micrograph of the obtained HMP-NS is presented in Figure 1E, which when deposited on a hydrophilic silicon substrate appear to be well dispersed and homogenously distributed on the substrate (Figure 1F). After the last synthesis step, NSs were centrifuged and rinsed several times with water, then redispersed in an ultrasound bath and stored in the dark at 4 °C for further use and characterization. The visible-near infrared (VIS-NIR) spectra normalized with the surface plasmon resonance (SPR) peaks is shown in Figure S2A, which demonstrates the evolution of their optical properties during the synthesis process.
Figure 1. (A) Schematic illustration of the fabrication of HMP-NSs combining nanomasking and galvanic replacement. (B) – (E) Magnified SEM micrographs showing the evolution of nanoparticle morphology during the synthesis: (B) Ag core nanomasking with 20-nm SiO2 beads, examples of which are marked with the white arrows; (C) deposition of a 10-nm-thick
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Ag/Au alloy shell, where the white arrow marks an example of SiO2 beads clearly embedded till their midpoints; (D) pinholes formed by removal of the nanomasking beads; and (E) Ag/Au HMP-NS after etching the Ag core, with some SiO2 beads still remaining on the surface. Scale bars: 100 nm. (F) Low-magnification SEM micrograph showing homogenous NSs distribution. Scale bar: 1 µm. The hollow character of HMP-NSs was confirmed by means of bright-field transmission electron microscopy (TEM). Figure 2A and 2B show TEM micrographs of HMP-NS clusters with 5-nm- and 10-nm-thick walls, respectively. The dark contrast of the boundaries and bright contrast of nanoshell interiors suggest that the hollow cavities have a uniform wall thickness. The observed aggregation was due to the poor wetting of the particle solution onto the hydrophobic TEM carbon grid. This is observed neither on hydrophilic silicon substrates (Figure 1F) nor while the particles are dispersed in the solution as examined by DLS (Figure S2B). Among the completely hollow 10-nm-thick NSs sample, a fraction of semi-hollow NS was present in a moderate concentration. The inset in Figure 2B and Figure S3 show examples of such semi-hollow NSs, which are composed of a network of fine (~10 nm diameter) intertwined fibril-like structures which extend from their surfaces surrounding the hollow cavity. These intertwined fibrils have similar morphology to those found in nanoporous sponge-like Au thin films, formed by a dealloying process.46
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Figure 2. (A) TEM micrographs of 5-nm-thick HMP-NSs (B) TEM micrographs of 10-nmthick HMP-NSs. The hollow structure is visible. Scale bars: 100 nm. The inset in (B) shows TEM picture of a semi-hollow particle. Scale bar: 50 nm. (C) HAADF STEM images of 5nm-thick HMP-NSs. (D) HAADF STEM images of 10-nm-thick HMP-NSs. The pores and pinholes are revealed. Scale bars: 50 nm. (E) Statistical analysis of the mesoporosity represented by two histograms, obtained by analyzing pore (blue bars) and pinhole (red bars) areas using various HAADF STEM images of both types of NSs. The analysis does not take into account rarely occurring pores above 40-50 nm, resulting from the galvanic replacement mechanism and structural damage. The inset in (E) represents an average NS size distribution histogram, obtained by analyzing TEM and SEM images of 5-nm- and 10-nm-thick hollow NSs. The NS size distribution histogram, shown in the inset in Figure 2E, was obtained by analyzing multiple TEM and SEM images, indicates an average NP diameter of 209 ±3.5 nm (for both types of hollow NSs). We have examined the mesoporosity of both samples in details by analyzing multiple images obtained with high-angle annular dark-field (HAADF)
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scanning TEM (STEM), where the contrast mechanism is strongly sensitive to atomic number and density of the material, therefore pores appear as dark spots on the surface of the hollow nanoparticle. Figures 2C and 2D show examples of 5-nm- and 10-nm-thick HMP-NSs with numerous pores or pinholes, respectively. A large ~9.5-nm pinhole is marked by the white arrow in Figure 3A, surrounded by a ~25-nm-wide slightly ellipsoidal recess created by the removal of a SiO2 nanomask bead. For comparison, the yellow arrows point out two pores that are ~2.5 nm in diameter originating from the dealloying process. Diameters of the pores and pinholes were statistically analyzed by measuring their surfaces using an image processing software and assuming a spherical shape. Obtained results are plotted in Figure 2E, where the mean pore diameter is 3.3 ±0.3 nm. The average diameter of the pinholes is 11 ±0.2 nm, which is consistent with the contact surface area between the core and a 24-nm SiO2 masking beads. We have observed very few damaged NSs within both types of the hollow NSs samples, which suggests that their scaffolds are strong enough to resist collapse in the TEM vacuum, as well as long exposure to centrifugal spinning at 4000 rpm. The porous NS structure obtained by nanomasking allows for adequate shell permeability for solvents, such as water, or larger chemical molecules that can be trapped by smaller 2-4 nm pores.21, 22, 47 High magnification surface image of a 10-nm-thick hollow NS in Figure 3A shows changes in the HAADF contrast, having lamellae-like appearance with features sizes on the order of 2-5 nm. Since Ag-Au alloys do not form eutectic phases nor phase separate, it is proposed that the observed contrast intensity variations result from small shell thickness variations, compositional differences due to an incomplete alloying or the combination of both effects. Alternatively the observed intensity variations in the HAADF image may result from the excess Ag left from the incomplete chemical etching and core removal despite the prolonged etching time and use of an excess amount of HNO3, where one would expect complete shell
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dealloying. Selected area electron diffraction (SAED) and EDS data support the presence of an excess amount of Ag in the hollow NPs. Figure 3B shows the SAED pattern of a group of hollow NPs and Figure 3C is a plot of the rotational average intensity in the SAED pattern with reciprocal lattice vector. The peaks in the plot exhibit shoulders and splitting suggesting that the Ag/Au alloy is not compositionally homogenous. Fitting the diffraction spectrum with Gaussian profiles, each broad peak can be attributed to two distinguishable underlying peaks in the plot of Figure 3C corresponding to two different lattice constants: 408 pm and 405 pm determined from the linear regression fit of the peak positions. Though the precision of the selected area diffraction patterns is not sufficient to determine the absolute value of the lattice constants, the relative difference measured constants suggest that two different alloys exist with the prominent phase being Ag-rich. Though X-ray EDS images of both types of hollow NSs, presented in Figure S4, shows that Ag and Au are distributed throughout the hollow NSs, the Ag/Au material is not a compositionally homogenous solid-solution alloy. The plots of the normalized mass percentage across the marked line scans shown in Figure S4D and S4H show the change in the Ag/Au composition hollow NSs, which varies in regions by as much as 8% for each element across features of 5-10 nm in size. The intensity variations observed in the HAADF images in Figure S4 correlate well with the fluctuations in the composition that are also influenced by the presence of pores. Moreover, the EDS chemical analysis provides proof for the assumptions drawn from the measured SPR spectral response (Figure S2A), suggesting different Ag/Au alloy compositions between both types of hollow NSs. For the 5-nm-thick HMP-NSs the Ag/Au alloy composition is 52.5/47.5 ±8.3%. The thicker NSs are richer in Ag showing about 72.5/27.5 ±8.5% Ag/Au ratio, although the overall net amount of Au is higher than that of the 5-nm-thick NSs. A more detailed discussion about the EDS compositional analysis, including characterization of the semihollow particles is included in the Supporting Information.
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Figure 3. (A) High magnification HAADF picture of the surface of a 10-nm-thick NS, showing local intensity variations attributed to thickness and alloy compositional variations. The white arrow points to a pinhole obtained by nanomasking with a surrounding material recess after the SiO2 masking bead removal. Yellow arrows point out two small pores (20 nm scale bar). (B) X-ray diffraction of Ag/Au alloy lattice of a 10-nm-thick hollow NS. (C) Plot of the rotationally averaged intensity with reciprocal lattice vector of the diffraction pattern in (B). The microscopic structures as revealed in Figure 2 and 3, including the mesopores, the pinholes, and the domain boundaries, play an important role in the optical properties of the HMP-NS by creating a dense collection of electron scattering centers. These scattering centers induce strong randomness in the collective electron motion driven by the optical field and cause a rapid conversion from optical to thermal energy through optical absorption. The reduced coherence in the electron motion also results in a drastically suppressed scattering.
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Figure 4 shows the theoretical and experimental VIS-NIR spectra of their extinction, absorption, and scattering cross-sections. The air mass (AM) 1.5 solar irradiance is also plotted as a reference. In the Mie-theory48 calculations for the 5-nm- and 10-nm-thick HMPNS (the thin blue and green curves, respectively), the cross-sections were a weighted average based on the size distribution shown in Figure 2E. The calculation was based on a weighted mean values of the dielectric functions49 corresponding to the Ag/Au alloy compositions. An electron mean-free-path of 1 nm was used, which is in agreement with the findings with the electron microscopy and gave the best match to the measurement results. The experimental cross-sections of the unetched Ag/Au core-shells, the 5-nm-, and the 10-nm-thick HMP-NS (the thick red, blue, and green curves, respectively) were projected from the extinction spectrum measurements and the particle concentration obtained through DLS after
