Composite Ferroelectric and Plasmonic Particles for Hot Charge

Subscriber access provided by University of Sunderland

Article

Composite Ferroelectric and Plasmonic Particles for Hot Charge Separation and Photocatalytic Hydrogen Gas Production Nathalia Ortiz, Brandon Zoellner, Vineet Kumar, Tara Janelli, Shuli Tang, Paul A. Maggard, and Gufeng Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00772 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on September 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Composite Ferroelectric and Plasmonic Particles for Hot Charge Separation and Photocatalytic Hydrogen Gas Production

Nathalia Ortiz, Brandon Zoellner, Vineet Kumar, Tara Janelli, Shuli Tang, Paul A. Maggard,* and Gufeng Wang*

Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204

*Corresponding Authors: Paul Maggard, email: [email protected] Gufeng Wang, email: [email protected] Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204

Keywords: Pt-end-capped gold nanorods; ferroelectrics; surface plasmon resonance; hot electrons; photocatalysts; H2 production

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Plasmonic nanoparticles are excellent light absorbers for harvesting solar energy, resulting in hot electrons that can be utilized in photocatalytic hydrogen production. However, the hot electrons generated in localized surface plasmon resonance process have a very short lifetime and are challenging to be used efficiently. Herein, using near IR light irradiation, we show that by combining gold nanorods (AuNRs) with ferroelectric PbTiO3 particles that possess a large remanent electric dipole moment, hot charges generated on plasmonic particles can be injected into ferroelectric materials and drive the photocatalysis reaction. Compared to metallic Pt-endcapped AuNRs, the efficiency of using hot electrons for photocatalytic reactions is enhanced for the composite catalyst, which improves the light-to-chemical energy conversion efficiencies by about one order of magnitude for the same amount of plasmonic particles being used.

Introduction Molecular hydrogen is an ideal chemical energy carrier that is clean, has a high energy density, and can be conveniently transported and stored.1-6 Using solar energy to produce hydrogen gas from water remains one of the most attractive yet challenging reactions in converting solar energy into chemical energy. To achieve hydrogen gas production from sunlight, photocatalyts are required. In general, a photocatalytic reaction consists of three steps: First, photoexcitation which leads to the generation of electron (e-) and hole (h+) pairs. Second, the electrons and holes must be separated and then diffuse to the catalyst surfaces. Third, the electrons and holes react with adsorbed electron acceptors and donors, respectively, to complete the photocatalytic reaction. Hence, the photocatalytic efficiency is predominantly affected by three main factors: amount of light absorption, charge separation efficiency, and surface reaction rates.


Page 2 of 35

Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Semiconductors and semiconductor-based hybrid systems have been extensively studied and used as photocatalysts in hydrogen and/or oxygen production.7-8 The advantages of these systems include low cost, environmentally friendly nature, and excellent stability in aqueous solutions.9-10 However, the main limitations of semiconductor-based photocatalysts, e.g., TiO2 and Fe2O3, are poor light absorption and fast recombination of charge carriers (i.e., at the ns time scale). In spite of efforts including doping,11 sensitization,12 and surface treatment,13 their practical efficiency in converting solar energy to chemical energy remains low (< 1%), while a minimum solar-to-hydrogen efficiency of ~10% is required for any system to be commercially viable.14 Plasmonic particles have recently been employed as a component of photocatalytic systems due to their localized surface plasmon resonance (SPR), which can be described as the collective oscillation of conduction band electrons excited by light. At the SPR wavelength, the absorption cross-section of the particles is greatly enhanced, allowing efficient light harvesting in the visible to the near IR range. This is very important because the photon flux of the sun light is fairly low, at the ~100 mW/cm2 scale. Thus, using metallic plasmonic nanoparticles as antennas can greatly boost the efficiency of light absorption. More importantly, high-energy hot electrons are generated in the SPR process,15-16 which can participate in the chemical reaction on the particle surface.17-28 Herein, hot electrons refer to the excited electrons with energies above the Fermi level of the metal particle.25,

28

There are many studies showing that for plasmonic

particle-based catalysts, the excitation of their SPR greatly enhances their catalytic activities.17-28 While how this happens is still under intense investigation and likely to vary case-by-case, it is generally believed that in many reactions, the SPR enhances the chemical reactions not merely through a photothermal effect17-19 but involves hot charge injection from the particle surface to



the adsorbates directly or indirectly, inducing the chemical reactions.25,

Page 4 of 35

28-29

Especially, the

excited electron gas in the nanoparticle initially would adiabatically establish equilibrium with a mean temperature of several thousands of degrees. Thus, high energy hot electrons (i.e., photon energy much larger than that at the SPR frequency) are produced in the SPR process even when the particle is excited by low energy photons.30 The generation of high energy hot electrons makes it possible for many catalytic reactions with high activation energy barriers to proceed.

Scheme 1. Scheme of the proposed plasmonic-ferroelectric composite particle for hydrogen production. Ox: oxidant, e.g., H+ or a sacrificial oxidant in a model redox reaction; Red: reductant, e.g. H2O, sacrificial agent, or other reductant in a model redox reaction; EF: Fermi level.

However, hot electrons in metallic particles have a very short lifetime in the femtosecond to picosecond time scale, which hinders their efficient use in driving chemical reactions. The optimal use of the hot charge carriers generated in a SPR process is currently under intensive investigation.31 In this study, we hypothesize that by combining plasmonic nanoparticles with 4 ACS Paragon Plus Environment

Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ferroelectric (FE) particles that have a large remanent electric field, hot electrons generated in the plasmonic particles can be efficiently injected into the ferroelectric material and their lifetime extended by orders of magnitude (Scheme 1).

Scheme 2. Energy diagram and band structures of the ferroelectric and plasmonic composite particles before and after contact. (A) Energy diagram of normal n-type semiconductor and metal before contact. (B) Band bending at the ferroelectrics-solvent interfaces. (C) and (D) Band bending after contact with a metal. Metal in contact with (C) the positively polarized surface or (D) the negatively polarized surface of the ferroelectrics, respectively. EF: Fermi level. S: semiconductor. M: metal. CB: conduction band. VB: valence band. BH: Barrier height.

Ferroelectric semiconductors, e.g. PbTiO3, have been extensively studied for their spontaneous electric polarization that can be reversed by an external field.

32-35

Because of the

spontaneous polarization, they are characterized with a polarization-dependent band bending, which lies close to the ferroelectric surface and promotes electron-hole pair separation. This is an 5 ACS Paragon Plus Environment


attractive feature for photocatalytic reactions such as water splitting and significant promise has been shown for their photocatalytic activity.36 However, for example, PbTiO3 has a bandgap that falls at the edge of the visible range at ~2.7 eV thus does not absorb light with a wavelength longer than ~460 nm. By joining plasmonic particles with ferroelectric particles, visible to near IR light can be used to generate hot electrons, which can then be injected into the ferroelectric particles and their lifetime potentially greatly extended. Hot electron injection from plasmonic particles to semiconductors has been observed and the hybrid materials have been used in photovoltaic devices24, 37-40 and photocatalysts.21, 25, 41-42 However, the challenge is that the injection efficiency is low for conventional semiconductor materials.24, 31, 43 Current efforts to improve the hot electron injection efficiency include tuning the Schottky barrier on the metal-semiconductor interface,44 designing composites structure so that the hot electrons have sufficient momentum to cross the interface,45 and chemically modifying the surfaces so that efficient electronic coupling between the plasmonic particle and the semiconductor occurs.46 The use of an internally polarized ferroelectric particle would be an ideal approach to enhance this charge injection (Scheme 2) as well as the charge separation (Scheme 1). Here, as an exemplary system, lead titanate (PbTiO3) is selected as the ferroelectric material because of its large, stable ferroelectric polarization and tunable conduction band energy that can be further used to facilitate charge injection, transport, and separation.36, 47-51 The band structure of PbTiO3 has been well characterized in the literature and by our groups as well. Scheme 2A shows the band structure of PbTiO3, which can be viewed as a n-type semiconductor. The bandgap EBG of nano- or micro-PbTiO3 particles is ~2.7 eV52 and the electron affinity χ for PbTiO3 is reported to be ~3.5 eV.53-54 Its Fermi level is near the bottom of the conduction band.55 Specifically for ferroelectrics like PbTiO3, its band will bend at the


Page 6 of 35

Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface, with the negatively polarized surface bent upward while the positively polarized side bent downward (Scheme 2B).56-57 This band bending can either promote or demote charge transfer across the interface when they are in contact with a metal, e.g., Au-FE interface. When Au (work function ФM = 5.1 eV) is in contact of a normal semiconductor with a similar electron affinity χ that of PbTiO3, their Fermi levels will align and the semiconductor bands will bend, leading to a Schottky energy barrier.57 However, if the normal semiconductor is replaced by ferroelectric PbTiO3, the band will further be bent, with the negatively polarized surface further bent upward (an even higher energy barrier) and the positively polarized surface bent downward (a lower energy barrier) (Scheme 2CD). It is conjectured that the lower energy barrier would favor the charge injection from Au to FE particles, leading to a higher efficiency in charge injections. In addition, it is also proposed that the remanent electric field in the FE particles will trap the injected electrons at the domain boundary,58-59 which prevents them from flowing back to the metal (being quenched) and extends their lifetimes (Scheme 1). The internal electric field of PbTiO3 is challenging to measure directly but similar ferroelectric material was estimated theoretically to be on the order of magnitude of ~107 V/m,60 orders of magnitude larger than the 2 electric field of a coherent light source ( I = 1 / 2εcE Max , where ε is permittivity, c is the speed of

light). Thus, charge injection from the plasmonic particle to the ferroelectric particle can be facilitated by the remanent internal electric field. It is worth mentioning that while this project was ongoing, a research article was published, which shows that SPR-excited charges can be transferred from gold nanoparticle arrays to a Pb(Ti,Zr)O3 (PZT) film using electrochemical methods; the photocurrent can be enhanced by nearly one order of magnitude by tuning the polarization of the PZT film with respect to the solution.49 However, photocatalytic reactivity has not been demonstrated so far



using this new hybrid material. Here, with near IR light irradiation that excites the plasmonic particle only, we show that charge injection does occur by using two different photocatalytically driven reactions, i.e., Pt photo-reduction and hydrogen gas generation respectively, on the ferroelectric particle surfaces. The efficiency of using hot electrons generated on plasmonic particles is enhanced by nearly one order of magnitude in hydrogen production for these composite catalysts as compared to the metallic Pt-AuNR catalysts. Compared to other metalsemiconductor composite catalysts,61-62, which usually have a reported incident-photon-tocurrent conversion efficiency (IPCE) of 0.01~0.1% characterized with electrochemical methods, our catalyst has a higher photon efficiency (0.28% for near IR light) for hydrogen production although it is an unfavorable comparison. Thus, the new composite plasmonic-ferroelectric particles represent a key photocatalytic system that can achieve high efficiency in light absorption, electron-hole generation, and charge separation.

Results and Discussion AuNRs and Pt-end-capped AuNRs synthesis. To generate hot electrons, AuNRs and Pt-endcapped AuNRs were used as the plasmonic nanoparticles. AuNRs and AuNR-based particles are especially useful because: (1) AuNRs are efficient in harvesting solar energies because of their large absorption coefficient.47-48,

63-65

(2) Most importantly, AuNRs are excellent in producing

high energy hot electrons from low energy visible-to-infrared photons through a fast charge carrier re-equilibrium process.28,

66

This is especially important because semiconductor-based

photocatalysts typically requires a band gap > 3.0 eV to efficiently catalyze the water splitting reaction due to many energy loss processes,14 while most of the solar irradiation is below this threshold. Thus, the use of AuNRs allows the utilization of the energy deemed “less useful” and


Page 8 of 35

Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increases the attainable solar to chemical energy conversion efficiency. (3) AuNRs are photostable. We recently showed that using 5-formyl salicylic acid as the organic modifier and by tuning the pH, Pt-end-capped AuNRs (Pt-AuNRs) with different aspect ratios could be synthesized.67 In this study, the “long AuNRs” (aspect ratio ~5) were mainly used, which has an SPR wavelength of 850 nm when unmodified and 920 nm after Pt-end-capping (Figure S1). The SPR wavelength better matches the light source output at 976 nm. The “regular AuNRs” (aspect ratio ~4, SPR 810 nm for unmodified particles, Figure S2), and Au nanospheres with a diameter of 40 nm (SPR 532 nm, Figure S3) were used as controls. While the AuNRs themselves can absorb light and generate hot charge carriers, the motivation for investigating the Pt-AuNRs in addition to AuNRs were two-fold: (1) to compare the hydrogen gas production efficiency for Pt-AuNRs and composite Pt-AuNRs/ferroelectric particles; and (2) to achieve an optimal match so that the AuNRs and the ferroelectric particles will have enhanced surface contact to facilitate the charge injection and separation. At a Pt:Au ratio of 1:10, the nanorod ends are slightly expanded and less protected by the ligands,67 which will provide specific contact points between the particles. To demonstrate the charge injection from plasmonic particles to the ferroelectric oxide, and to exclude the possibility that the excited electrons are generated by excitation of the ferroelectric particles directly, near IR light irradiation at 976 nm was utilized, which has a significantly lower photon energy than the band gap of the PbTiO3 at ~460 nm. The AuNRs (as well as Pt-AuNRs) were then deposited on PbTiO3 particles through wet impregnation method. The highly-faceted PbTiO3 particles were synthesized in the form of either nano-particles (125~500 nm, NaCl flux) or micro-particles (1 ~ 5 µm, PbO flux).36 They



are named as nano-ferroelectric (nano-FE) and micro-ferroelectric (micro-FE) particles, respectively. The resulting particles were characterized using scanning electron microscopy (in Figure S4 and in Figures 1-3 together with impregnated AuNRs). Their crystalline structures were characterized by powder X-ray diffraction (Figure S5), which are consistent with expected XRD pattern. Their bandgaps were characterized using UV-VIS diffuse reflectance spectroscopy (~2.7 eV, Figure S6), consistent with literature values.52 The BET surface areas of two typical samples were measured. They were 0.94 m2/g-1 for the nano-FE (250 nm in size) and 0.092 m2/g1

for the micro-FE particles(2 µm), respectively. These values are again consistent with the

literature reported values.70

Charge injection from AuNRs to PbTiO3: Photochemical surface reduction of Pt using near IR light. The fundamental hypothesis is that the hot electrons generated on plasmonic particles will be injected into ferroelectric particles owing to their permanent electric polarization. To test whether charge injection occurs, the photon-induced reduction of Pt(IV) onto the composite AuNR-FE particles was performed. The AuNR particles (no Pt) were used and impregnated onto bare micro-FE PbTiO3 particles to avoid confusion with photo-reduced Pt. Before impregnation, the AuNRs were cleaned 3 times to remove excessive ligands on the particles. The suspension of the composite particles was then exposed to 976 nm near IR light in the presence of aqueous H2PtCl6 and methanol as the sacrificial agent. The near IR light was from a laser expanded to have an irradiation area of ~1.0 cm2, giving a power density of 2.0 W/cm2.


Page 10 of 35

Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. SEM images of the AuNR/FE particles after the photon-driven reduction of Pt islands onto their surfaces under irradiation by 976 nm light. (A) PbTiO3 particles impregnated with AuNRs. Scale bar: 500 nm. (B) Control experiments for PbTiO3 particles with no AuNRs. Scale bar: 1 µm. (C) Enlarged areas (1) and (2) in (A). (1) and (2) are two examples where Pt islands emerged near the deposited AuNRs after photochemical reduction. (1) shows an isolated AuNR and (2) shows a cluster of AuNRs. Scale bar: 100 nm. EDX scanning shows that a small area including AuNRs and surrounding Pt islands contains Pb: 60.8 ± 0.4%, Ti: 14.5 ± 0.2%, O: 18.9 ± 0.3%, Au: 3.5 ± 0.2%, and Pt: 1.4 ± 0.2%, respectively. As a contrast, the control PbTiO3 particles contain Pb: 65.8 ± 0.3%, Ti: 16.2 ± 0.2%, and O: 16.3 ± 0.2%, respectively.

After irradiation under 976 nm light for 30 minutes, the particles were then characterized by SEM. As shown in the SEM images, Pt islands emerged on FE particle surface, with most of them located within a distance of ~100 nm from the AuNRs (Figure 1). The presence of metal Pt was confirmed using EDX scanning. This is strong supporting evidence that the hot electrons 11 ACS Paragon Plus Environment


were injected from plasmonic particles to the FE particles, which drove the photochemical reduction of the Pt at the surfaces. Again, 976 nm near IR light has a photon energy well below the bandgap of the PbTiO3, so that excited electrons were not from the light absorption within the ferroelectric. As a control, under the same conditions, no Pt islands were found to deposit on the FE particle surface (Figure 1B) in the absence of AuNRs. These experiments confirm the hypothesis that charge injection takes place from the AuNR to FE particles.

Photocatalytic H2 production: Enhanced rates for composite AuNR and FE particles. The photocatalytic activities of the particles were investigated under irradiation in aqueous solutions for the reduction of water to H2 for the composite plasmonic-ferroelectric particles. AuNRs alone are poor catalysts although they are excellent light absorbers. Our collaborative efforts have demonstrated that that by end-capping AuNRs with Pt, they become highly active for the production of hydrogen gas even when being illuminated with near IR photons.67 The initial H2 production rate (in the first 10 min) was measured to be ~0.25 µmol/min for ~0.4 µmol total Au (or, a photon efficiency of 0.050%), which compares very favorably among all AuNP-based metallic nanoparticles.68 This shows that they are efficient in absorbing low energy photons and converting them to hot electrons that can drive hydrogen production. The composite Pt-AuNR and ferroelectric particles were similarly investigated for their photocatalytic activity for hydrogen gas production and compared to that for the Pt-AuNRs alone. The ferroelectric particles were first deposited with Pt islands (1 wt%) to serve as surface co-catalysts using a photochemical reduction reaction under white light irradiation (namely FEPt, Figure S7). Importantly, the cocatalyst islands serve to lower the kinetic barrier (i.e.,


Page 12 of 35

Page 13 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


overpotential) for hydrogen formation. The composite particles were synthesized using the wet impregnation methods and annealed together at a low temperature of 80 oC for surface attachment of the particles. The Pt-AuNRs with an aspect ratio of 5 and Pt:Au = 1:10 were used. The mass ratio between the AuNRs and FE particles were kept constant throughout this study. Figures 2A and 2B show that the Pt-AuNRs were distributed evenly over nano-FE (average size ~125 nm) and micro-FE (average size ~1 µm) particle surfaces. The average loading of the AuNRs on the FE particles were consistent with the expected values (AuNR:FE ~0.60 for 125 nm nano-FE and ~280 for 1 µm micro-FE, respectively). Since the AuNRs are relatively sparsely deposited on the FE particle surface, the total “effective FE surface area”, i.e., the area beneath the AuNR and in its vicinity that the hot electrons can diffuse to (~ 100 nm), is nearly a constant so that the hydrogen production initial rate can be compared directly.

Figure 2. Photocatalytic H2 gas production on composite Pt-AuNR/FE-Pt particles. (A and B) SEM image of Pt-ferroelectric particles impregnated with Pt-AuNRs. (A) NaCl Fluxed PbTiO3 particles; dimension: ~125 nm. Scale bar: 500 nm. (B) NaCl Fluxed PbTiO3 particles; dimension: ~ 1 µm. Scale bar: 1 µm. (C) Hydrogen production on composite Pt-AuNR/micro-



FE-Pt and Pt-AuNR/nano-FE-Pt particles. As a comparison, hydrogen gas production on PtAuNRs and Nano-FE-Pt particles are also shown.

The composite plasmonic and ferroelectric particles, i.e., using either nano-FE or microFE particles, were suspended in a solution containing methanol for H2 production and tested for their photocatalytic rates under 976 nm near IR light irradiation. Both the nano-FE and micro-FE composite particles showed significant amount of hydrogen production after light exposure, which was confirmed using gas chromatography. The other half of the reaction is the oxidation of the methanol to formaldehyde and/or formic acid, which was confirmed using Tollen’s test.69 No significant amount of CO2 was detected among the product gases using gas chromatography. The Pt-AuNR/nano-FE-Pt particles gave a H2 production initial rate of 1.41 ± 0.40 µmol/min for a total of 12 runs using FE particles made from 3 different batches (Figure 2C). This corresponds to an initial photon efficiency of 0.28%. In each experiment, a sample containing 16.0 mg FE, 0.80 µmol Pt on nano-FE, 0.40 µmol Au, and 0.040 µmol Pt on AuNRs were used. This initial rate is compared to that of the pure metallic Pt-AuNR particles (0.25 µmol/min, Figure 2C), where the total Au in both solutions (0.40 µmol Au) was kept constant in order to compare the efficiency of using the hot electrons generated from the AuNR particles. This result shows that the initial production rate was enhanced by a factor of 5.6, almost an order of magnitude when the FE particles are present. Similarly, the Pt-AuNR/micro-FE-Pt particles also show an enhanced H2 initial production rate of 0.88 ± 0.27 µmol/min for 7 different experiments using different batches of FE particles (Figure 2C). Thus, the initial production rate is enhanced by 3.5 times for the PtAuNR/micro-FE-Pt composite particles. The initial rate for the micro-FE is lower than nano-FE composite particles, which will be discussed later.


Page 14 of 35

Page 15 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Note that in the composite particles, there was more Pt present over the composite particles’ surfaces compared to Pt-AuNRs. However, this is desired so that the photocatalytic rates are not limited by the inherent surface activities of the ferroelectric oxide or the AuNR’s, but rather, only by the efficiency of generation of hot electrons and the charge injection between the AuNR and the FE particle. The experimental design confirms a scheme that can distribute these hot electrons onto multiple cocatalyst islands that are exposed to the solution. Thus, their lifetimes presumably can be extended and the solar to hydrogen conversion efficiency is observed to increase in this system design. The photon-driven catalytic activity of the composite particles generally decreased over time and was lost after ~60 min. The possible reasons can be inferred from the observation of the sample solution: the composite particles aggregated overtime; the AuNRs also detached from the FE particles and aggregated. These effects require further planned investigations. As a control experiment, the nano-FE-Pt particles (no AuNRs or Pt-AuNRs) were also tested under 976 nm irradiation. Surprisingly, the Pt-nano-FE particles also produced hydrogen gas, but at a much smaller initial rate and the particles lost activity quickly. It is possible that the heating effect of the laser can also promote electrons from the valence band to the conduction band, which can thermally drive the reaction. However, this process is very inefficient and the hydrogen production initial rate is low. Thus, together with the controls, the two photocatalytic reactions show that AuNRs can inject hot electrons to FE particles upon irradiation; the excited electrons can participate in chemical reactions, e.g., producing H2. The enhancement of the hydrogen production initial rate supports the hypothesis of charge injection because of the permanent polarization inside the ferroelectric particle. Although in our current experiments, the contact between the plasmonic



and the ferroelectric particles were not controlled in terms of the crystal facts, it is still not surprising to see this enhancement. One would envision that ~half of the AuNRs will deposit onto a surface with a polarization that favors the charge injection while the other half unfavorable (Scheme 2). If the favorable combination enhances the overall initial production rate by many times, the net effect would still be significant despite that the rest of the nanoparticles are curbed or even nulled.

SPR involvement in hot electron generation. To further support that the hot electrons are generated on the plasmonic nanoparticles and being transported to the FE particle surface to participate reaction, the Au nanoparticles were used with different aspect ratios without the Ptend-capping. The photocatalytic activities were investigated for the nano-FE-Pt (average size 500 nm) impregnated with long AuNRs, short AuNRs, and spheres (40 nm in diameter) using 976 nm near IR light irradiation in aqueous solutions, as described above. To have a fair comparison, FE particles from the same batch were used.


Page 16 of 35

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Photocatalytic H2 gas production on composite AuNP/FE-Pt nanoparticles. (A, B, C) SEM images of the composite particles. NaCl fluxed FE particle size: ~500 nm. (A) With 40 nm Au nanospheres. Scale bar: 500 nm. (B) With regular AuNRs. Scale bar: 1 µm. (C) With long AuNRs. Scale bar: 300 nm. (D) Hydrogen production as a function of time for the respective types of composite particles. Shown in Figures 3A-C are SEM images of the composite particles with three different shapes for the gold nanoparticles, i.e., 40 nm spheres, regular nanorods and long nanorods. Their photocatalytic activities for hydrogen production as a function of time is plotted in Figure 3D. All of these composite particles showed activity. The initial reaction rates were 0.71 ± 0.20, 0.46 ± 0.28, 0.09 ± 0.04 µmol H2 molecules/min for the same amount of Au and FE particles in the first ~10 min for long AuNRs, regular AuNRs, and 40 nm Au nanospheres, respectively. The standard deviation was obtained from 2~3 replicates. The trend of the reaction rates is consistent with the absorbance of these particles at 976 nm: from the long AuNRs to regular AuNRs, the absorbance decreased by almost half and the initial production rate also reduced by almost half; for the Au nanospheres, the absorbance at 976 nm is almost negligible and the initial production rate also becomes ~10 times smaller. These experimental results strongly support that the particle SPR is involved in the light absorption and hydrogen gas generation, and that the hydrogen production happens predominantly on the FE particle surface rather than on AuNR ends.

Composites using AuNRs or Pt-AuNRs? We have shown that composite particles prepared from both AuNRs and Pt-AuNRs (i.e., with and without the Pt end capping functionality) can greatly enhance the H2 production initial rate as compared to metallic particles only. Generally, composites using Pt-AuNRs have an initial production rate higher than those using AuNRs. The Pt-AuNR/nano-FE-Pt particles gave a H2 production initial rate of 1.41 ± 0.40 µmol/min (12 runs



using different FE particles) while the AuNR/nano-FE-Pt particles gave an initial rate of 0.81 ± 0.37 µmol/min (7 runs using different FE particles). Since there is variability between the FE particles made from different batches, to have a direct and fair comparison, Figure 4 shows one example that both AuNR- and Pt-AuNRcomposites were prepared with the same batch of nano-FE particles. The initial production rate was 1.69 ± 0.33 µmol H2/min for Pt-AuNR/FE-Pt and 0.88 ± 0.49 µmol H2/min for AuNR/FE-Pt composites, respectively. The standard deviation was from 3~4 replicated runs. The hydrogen gas production initial rate is ~two times higher for the Pt-AuNRs composites. The higher initial production rate is apparently not solely caused by the extra Pt on AuNR ends, which is only ~5% of the total Pt in the composites. More likely, Pt serves as a bridge between the AuNRs and the FE particles, thereby reducing the energy barrier for charge transfer. Thus, the Pt end-capping of the AuNRs is a key factor in the fully optimized composite particle system.

Figure 4. Comparison of the photocatalytic H2 gas production on Pt-AuNR/FE-Pt versus AuNR/FE-Pt particles. Nano-FE: Prepared in a NaCl flux with sizes of ~ 250 nm.

Nano-FE or micro-FE particles? The FE particles showed variability in their sizes according to the flux reaction involving a PbO or a NaCl molten-salt. Though, the particles prepared in the


Page 18 of 35

Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NaCl flux were in the nanometer range (125~500 nm) while the particles prepared in the PbO flux were in the micrometer range (1~ 5 µm). Generally, the nano-FE composites showed a higher reactivity (1.41 ± 0.40 µmol/min, 12 runs with different FE particles) than the micro-FE composites (0.88 ± 0.27 µmol/min, 7 runs using different FE particles). Note again, the mass ratio between AuNRs and FE particles were kept constant in this comparison. Since the AuNRs were relatively sparsely deposited on FE particle surface, total FE surface area should not affect the hydrogen production rate significantly. The possible reason for this difference may be caused by the exposed crystallographic surface facets of the FE particles, which may show different activities in electron trapping and chemical reactions. Further investigations are planned to determine the impact of the particle surface properties, surface areas, as well as the loadings of AuNRs onto the FE particles.



Figure 5. Photocatalytic H2 gas production on composite Pt-AuNR/nano-FE-Pt particles under 976 nm light. Nano-FE: PbTiO3 nanoparticles prepared within a NaCl with a size of ~250 nm. (A) Reaction kinetics as a function of irradiation power; (B) Initial reaction rate as a function of irradiation power.

Initial reaction rate as a function of irradiation power. The above results demonstrate that the composite particles made of Pt-AuNRs and nano-FE-Pt particles exhibit high photocatalytic activities for the production of molecular hydrogen. To have a better understanding how SPRgenerated hot electrons are involved in the photocatalytic reaction, the hydrogen production initial rate was measured as a function of the laser irradiation power on Pt-AuNRs/nano-FE-Pt particles. Figure 5A shows the reaction rates at different irradiation levels (0.25, 0.98, 1.6, and 20 ACS Paragon Plus Environment

Page 20 of 35

Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 W/cm2). As the laser power increased, the initial reaction rate increased from 0.14 ±0.04, 0.58 ± 0.21, 1.02 ± 0.26, to 1.69 ± 0.33 µmol H2 molecules/min. The standard deviation was obtained from 2-4 replicates. Thus, the initial reaction rate follows a non-linear relationship with respect to the irradiation power (Figure 5B). A quadratic function initial rate = 2.93 × 10-7 I2 + 2.23 × 10-4 I + 3.13 × 10-2 can satisfactorily fit the data (R2 = 0.992), where I is the irradiation intensity. Interestingly, this observed quadratic relationship is in direct contrast to the linear initial rate ~ power relationship for the same hydrogen reduction reaction observed on Pt-AuNRs surface. In that case, it was suggested the chemical reaction, either the interfacial transfer of hot electrons from the nanoparticle to the reactant molecule or some photo-activated steps along the chemical reaction pathways, is the rate-limiting step for the hydrogen gas production; the surface hot electron concentration on Pt-AuNRs is linearly proportional to the photon flux under our experimental conditions. The second conclusion is, however, unexpected because the instantaneous temperature of electron gas in metal particle upon light absorption is dominated by the photon influx and increases linearly with respect to the irradiation laser power. The total number of hot electrons exceeding the Fermi level is linear. However, the total number of “high energy” hot electrons exceeding a large threshold value would increase non-linearly with respect to the temperature, or the photon flux, according to Fermi-Dirac distribution: f (E ) =

1

(1)

exp[( E − E F ) / k B T ] + 1

where f is the probability an energy level is filled; EF is the Fermi level; kB is the Boltzmann constant; T is the temperature. The linear rate~power relationship may be caused by the low activation energy of the hydrogen gas production reaction in the presence of methanol. Thus,



even very low energy hot electrons can participate in the chemical reaction, giving a linear reaction initial rate ~ light intensity relationship. Here, the non-linear relationship possibly suggests that the metal-semiconductor Schottky barrier plays an important role in selecting the “high energy” hot electrons to pass through the interface. That is, only when the hot electrons have an energy higher than the interfacial energy barrier can they be transferred to FE particle and participate in the chemical reaction. This energy barrier process will make the population of the hot electrons that participate in chemical reaction follow a non-linear relationship, whereas in the earlier case (Pt-AuNRs), hot electrons can directly diffuse to the metal surface so that the hot electron population will be linearly proportional to the light intensity.

Photocatalytic H2 production under Sunlight. All the experiments described above have been performed under near IR light irradiation at 976 nm. This wavelength was purposely selected in order to exclude the possibility that hot electrons are being generated from bandgap excitation within the FE particles directly. Thus, at 976 nm, all of the electrons participating in the chemical reactions are injected from the plasmonic AuNRs. However, light in the entire UV-VIS-near IR spectrum can be used to generate hot electrons in plasmonic particles, as well as in the FE particles, and used to drive the photocatalytic reaction. To demonstrate this, the photocatalytic activity of the composite PtAuNR/nano-FE-Pt particles was tested under natural sun light in September, Raleigh, North Carolina, USA. Figure 6 shows H2 gas production yield as a function of time. The experiments were carried out around noon, where the sun irradiance was measured to be ~245 mW/cm2 orthogonal to the ray direction. Considering the incident angle of the solar rays, the irradiation


Page 22 of 35

Page 23 of 35

power on the sample was on the order of magnitude of ~100 mW/cm2. At this irradiation level, the hydrogen gas was produced with an initial rate of 0.54 µmol/min, which corresponds to a photon efficiency of 2.2% for natural solar light. This photon efficiency is much large than that using near IR light (0.28%), possibly because there are many high energy photons that can excite the FE particles directly and catalyze the chemical reaction. These experiments confirm that the nanoparticles are practically useful in producing H2 gas from natural sun light.

9

H2 (µmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

3 Solar (100 mW/cm2) Pt-AuNR/nano-FE-Pt

0 0

12

24

36

48

Time (min) Figure 6. Photocatalytic H2 gas production kinetics on composite Pt-AuNR/nano-FE-Pt particles under natural sun light. Nano-FE: PbTiO3 nanoparticles prepared within a NaCl flux with sizes of ~ 250 nm. Average solar irradiation on the sample: ~100 mW/cm2.

Conclusions In summary, these investigations have demonstrated that by combining Pt-end-cappedAuNR and ferroelectric particles PbTiO3, an effective photocatalyst design can be obtained for hydrogen gas production using light. The large permanent electric polarization of the FE particles can be utilized to extract hot electrons generated on a nearby plasmonic particle that is in contact with FE particle. Effective charge separation within the particle photocatalyst can be achieved, which can presumably extend the lifetime of hot electrons by orders of magnitude and improves their efficiency in driving the chemical reaction. The outcome of this study is an 23 ACS Paragon Plus Environment


effective photocatalyst system that takes advantage of a new fundamental mechanism that effectively separate electrons and holes generated in metal SPR process. The current design shows that the efficiency of using hot electrons generated on plasmonic particles can be improved by ~ 5 times compared to metal-only plasmonic catalysts. Compared to other metalsemiconductor catalysts,61-62 which usually have a reported incident-photon-to-current conversion efficiency (IPCE) of 0.01~0.1% using electrochemical methods, our plasmonicferroelectric composite catalyst has a higher photon efficiency (0.28% for near IR light) although it is already an unfavorable comparison. It can be envisioned that with the optimization of the composite particle system, there are great potentials to further improve the efficiency using this new route.

EXPERIMENTAL SECTION Chemicals. HAuCl4·3H2O (99%), NaBH4 (99%), AgNO3 (99%), H2PtCl4 (>99%), ascorbic acid (99%), CTAB (99%), 5-formyl-salicilic acid (5-FSA), hydrochloric acid (HCl, concentrated), Methanol (99%) were purchased from Sigma Aldrich and used as received. PbO (99.99%) and TiO2 (Anatase, 99.9%) were obtained from Alfa Aesar, Ward Hill, MA.

AuNP synthesis. Preparation of Au seeds: AuNRs were synthesized using the seed-mediated method following our procedure previously reported.67 In a typical seed synthesis, 5.0 mL of 0.50 mM HAuCl4 solution was gently mixed with 5.0 mL of 0.20 M CTAB solution. Then, 600 µL of 10 mM freshly prepared NaBH4 solution was introduced to the solution and mixed at drastic stirring for 2.0 minutes. The final solution was left undisturbed for 30 minutes prior to use. 24 ACS Paragon Plus Environment

Page 24 of 35

Page 25 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Preparation of regular AuNRs (aspect ratio ~4): A growth solution containing 10 mM 5-FSA and 50 mM CTAB was prepared (100 mL). Then, 1.92 mL of 10.0 mM AgNO3 was added. After keeping the mixture undisturbed for 15 minutes, 100 mL of 1.0 mM HAuCl4 was mixed with the growth solution at medium stirring speed for 90 min. Then, at drastic stirring condition, 512 µL of 0.10 M ascorbic acid was added to the solution within 30 seconds. At the same stirring speed, 320 µL previously synthesized gold seeds was added and the solution was kept being stirred for an extra 30 s. The final solution was left undisturbed for 12 hours at room temperature. The final total Au concentration was ~0.50 mM. Preparation of long AuNRs (aspect ratio ~5). Long AuNRs were synthesized following the same procedure for the regular AuNRs except that 2.0 mL of 1.0 M HCl was added dropwise with stirring after HAuCl4 has been introduced. The final total Au concentration was ~0.50 mM. Preparation of Pt-End-Capped-AuNRs (Pt-AuNR; Pt:Au = 1:10). To selectively coat Pt on the ends of AuNRs with regular and higher aspect ratios, the same procedure was adopted as reported in our published work.67 Briefly, 10 mL of fresh regular or long AuNRs solution was used. For Pt loading at a Pt:Au = 1:10, 40.4 µL of 0.010 M HCl was added to the Au solution at 35 °C under mild stirring for 5.0 minutes, followed by the slow addition of 50 µL of 0.01 M H2PtCl6 in 5.0 min under mild stirring. Finally, 287 µL of freshly made 0.10 M ascorbic acid was added dropwise. The final solution was left undisturbed at 35 °C for 8 hours. The solution color changed from light reddish pink to light purplish, indicating the reaction had taken place. The final total Au concentration was ~0.50 mM. Preparation of Au spheres: Au spheres capped with CTAB were synthesized using the same seeds for AuNR synthesis. In a typical Au sphere synthesis, 10 mL of 0.50 mM HAuCl4 solution was mixed with 10 mL of 0.20 M CTAB solution for 15 minutes. 1.0 mL of 6.0 mM



cold NaBH4 solution was then added under drastic stirring in 2.0 minutes. The final solution was allowed to age for 48 hours. The as synthesized AuNP were 40 nm in diameter.

Preparation of PbTiO3. The flux synthesis of PbTiO3 was performed by combining a stoichiometric mixture of PbO and TiO2. The mixture was ground in acetone for 30 min before the addition of salt flux. For nano ferroelectric particles: NaCl salt flux was introduced in a flux-to-reactant molar ratio of 1:1; for micro ferroelectric particles: PbO flux was added in a flux-to-reactant molar ratio of 1:1. After grinding, the reactant mixture was placed inside alumina crucibles and heated to 1000 ˚C inside a box furnace in air for a reaction time of 1.0 h. The crucibles were cooled to room temperature inside the furnace. The resulting powder was first washed with deionized water to remove the flux, then washed briefly in 1.0 M HNO3 to remove excess PbO flux, and finally dried overnight in an oven at 800 ˚C. Fine homogeneous yellow powders of PbTiO3 were obtained in high purity, as judged from powder X-ray diffraction. The FE particles showed variability in their sizes and H2 production rates for each batch of synthesis. The nano-FE particles varied from 125 ~ 500 nm and the micro-FE particles varied from 1 ~ 5 µm, respectively. Each batch of FE particles were characterized using SEM. An appropriate comparison of photocatalytic rates was only performed when the same batch of particles was used.

Pt deposition on PbTiO3 through photo-reduction using white light (Pt-PbTiO3). The ferroelectric particles were coated with 1% Pt islands (by weight) as surface co-catalysts using photochemical reduction. In a typical experiment, 300 mg of ferroelectric material was


Page 26 of 35

Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


suspended in 15 mL solution containing 16% methanol (v/v) and 15.0 µmol of Pt salt (H2PtCl6). The photo-reduction was carried out using an outer-irradiation type fused-silica reaction cell. The cell was irradiated for 4.0 hours with a 400 W Xe Arc lamp. The lamp output was 280 ~ 950 nm with a power density of 800 mW/cm2. The solution was stirred throughout the entire irradiation time. The final product was washed three times with DI water and re-suspended to a total volume of 15.0 mL of DI water. (Final concentrations: 20 mg/mL PbTiO3; total Pt concentration ~1.0 mM.)

AuNRs (or Pt-AuNR) impregnation onto PbTiO3 (or Pt-PbTiO3). The AuNRs (either with or without Pt-end capping) solution was pre-cleaned 2~3 times using DI water to remove excessive ligands and resuspended in the same amount of solution. 7.5 mL of the prepared PbTiO3 or Pt-PbTiO3 suspension was mixed with 7.5 mL of clean AuNR solution. The solution was then placed in a glass vial and heated to 80 ˚C, under constant stirring, overnight. The mixture dried up completely with the AuNRs adsorbed on PbTiO3 or Pt-PbTiO3 surface. The solids were stored for future use or immediately resuspended in DI water for H2 production. For H2 production, the solid was resuspended in 7.5 mL DI water and a part of the suspension was used in hydrogen production. (Concentrations: 20 mg/mL PbTiO3; Pt on PbTiO3 ~1.0 mM; total Au ~0.50 mM; Pt on AuNR ~0.050 mM.)

Photo-reduction of Pt on AuNR/FE composite using 976 nm light.



To observe the charge injection from AuNRs to PbTiO3, the 976 nm light photo-reduction was carried out on AuNR-PbTiO3 composite. The AuNR was first impregnated onto PbTiO3 using the same procedure for impregnating AuNRs onto Pt-PbTiO3. The photo-reduction was then carried out using an outer-irradiation type fused-silica reaction cell. 50 mg of AuNR-PbTiO3 composite was added to 2.5 mL binary mixture containing 1.0 mg/mL Pt salt and 400 µL methanol. The cell was irradiated for 1.0 hour with a continuous wave, 976 nm laser (MDL-III, Dragon lasers). The total output was set at 2000 mW with an irradiation area of 1.0 cm2 (~1/5 of the solution was illuminated). (Concentrations: 20 mg/mL PbTiO3; total Pt ~1.0 mM; total Au ~0.5 mM; Pt on AuNR ~0.05 mM. Note Pt was not completely reduced under this condition; a significant amount of Pt was still in the solution.)

Electron microscopy. Scanning electron microscopy (SEM) images were collected using an FEI Verios 460L field emission SEM (FESEM).

BET measurement. BET surface area analyses were performed for two typical samples using a Quantachrome ChemBET Pulsar TPR/TPD Quantachrome, Boynton Beach, FL). Surface areas of both the nano (~250 nm in size) and micro (2.0 µm in size) PbTiO3 powder samples were determined to be 0.94 and 0.092 m2/g-1, respectively. These numbers are consistent with the literature reported values.70

Photocatalytic Hydrogen Production.


Page 28 of 35

Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In photocatalytic H2 production experiments, the impregnated Pt-AuNR/FE-Pt or AuNR/FE-Pt particles were re-suspended in DI water before use (Concentrations: 20 mg/mL PbTiO3; Pt on PbTiO3 ~1.0 mM; total Au ~0.50 mM). First, 800 µL composite particle solution was mixed with 500 µL of methanol and 1.2 mL DI water to form a 2.5 mL solution. The solution was then degassed by bubbling with nitrogen gas for 15 minutes. The solution was finally transferred to a quartz cell for irradiation. The quartz cell was connected to a thin, L-shaped tubing. The inside of the cell and tubing were purged with nitrogen for 15 minutes. A water droplet was used to seal the whole reaction chamber. The volume change was read from the movement of the droplet in the horizontal segment of the L-shaped tubing. After the photoreaction, the volume of the produced gas was re-read after the whole system was re-equilibrated at room temperature. The reported gas production volume was corrected for the temperature change during reaction. (Total materials used in the reaction: 16 mg ferroelectric material, Pt on FE 0.8 µmol, ~0.4 µmol Au, Pt on AuNRs ~0.04 µmol). A continuous wave, 976 nm laser (MDL-III, Dragon lasers) was used in hydrogen gas production. The total output was set at desired power (2000 mW, 1575 mW, 975 mW, or 250 mW). The irradiation area was ~1 cm2. 1/5 of the solution was illuminated.

Gas chromatography of the photoreaction products. All gas chromatograms were taken using a SRI 8610C gas chromatograph with a 2.0 meter silica gel-packed column (1.0 mm internal diameter and 1/16th outer diameter), a thermal conductivity detector, and nitrogen as the carrier gas.

ASSOCIATED CONTENT



Supporting Information. 7 supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org. Author contributions. B.Z. and T.J. synthesized the FE particles. N.O. synthesized the Au particles. N.O., B.Z., and S.T. did the hydrogen production experiments. P.M. and G.W. proposed the research idea and guided the whole research project. N.O., P.M., and G.W. wrote the manuscript.

ACKNOWLEDGMENTS This work was supported by the Chemistry Department, North Carolina State University. N.O. acknowledges the Graduate School for a GAANS NEEM fellowship. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

REFERENCES 1. Navarro, R. M.; del Valle, F.; de la Mano, J. A. V.; Alvarez-Galvan, M. C.; Fierro, J. L. G., Photocatalytic Water Splitting under Visible Light: Concept and Catalysts Development. In Photocatalytic Technologies, DeLasa, H. I.; Rosales, B. S., Eds. 2009, pp 111-143. 2. Sivula, K.; Le Formal, F.; Graetzel, M., Solar Water Splitting: Progress Using Hematite (AlphaFe2o3) Photoelectrodes. Chemsuschem 2011, 4 (4), 432-449. 3. Yuan, Y. P.; Ruan, L. W.; Barber, J.; Loo, S. C. J.; Xue, C., Hetero-Nanostructured Suspended Photocatalysts for Solar-to-Fuel Conversion. Energy & Environmental Science 2014, 7 (12), 3934-3951. 4. Li, X.; Yu, J. G.; Low, J. X.; Fang, Y. P.; Xiao, J.; Chen, X. B., Engineering Heterogeneous Semiconductors for Solar Water Splitting. J. Mater. Chem. A 2015, 3 (6), 2485-2534. 5. Ingram, D. B.; Linic, S., Water Splitting on Composite Plasmonic-Metal/Semiconductor Photoelectrodes: Evidence for Selective Plasmon-Induced Formation of Charge Carriers near the Semiconductor Surface. J. Am. Chem.Soc. 2011, 133 (14), 5202-5205.


Page 30 of 35

Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K., Photocatalyst Releasing Hydrogen from Water - Enhancing Catalytic Performance Holds Promise for Hydrogen Production by Water Splitting in Sunlight. Nature 2006, 440 (7082), 295-295. 7. Kawai, T.; Sakata, T., Photocatalytic Hydrogen-Production from Liquid Methanol and Water. J. Chem. Soc. Chem. Comm. 1980, (15), 694-695. 8. Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W., Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95 (1), 69-96. 9. Carp, O.; Huisman, C. L.; Reller, A., Photoinduced Reactivity of Titanium Dioxide. Prog. Solid State Chem. 2004, 32 (1-2), 33-177. 10. Kim, S. B.; Hong, S. C., Kinetic Study for Photocatalytic Degradation of Volatile Organic Compounds in Air Using Thin Film Tio(2) Photocatalyst. Appl. Cat. B - Environ. 2002, 35 (4), 305-315. 11. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y., Visible-Light Photocatalysis in NitrogenDoped Titanium Oxides. Science 2001, 293 (5528), 269-271. 12. Odobel, F.; Pellegrin, Y., Recent Advances in the Sensitization of Wide-Band-Gap Nanostructured P-Type Semiconductors. Photovoltaic and Photocatalytic Applications. J. Phys. Chem. Lett. 2013, 4 (15), 2551-2564. 13. Ma, X.; Dai, Y.; Guo, M.; Huang, B., Insights into the Role of Surface Distortion in Promoting the Separation and Transfer of Photogenerated Carriers in Anatase Tio2. J. Phys. Chem. C 2013, 117 (46), 24496-24502. 14. Hans-Joachim Lewerenz, L. P., Photoelectrochemical Water Splitting: Materials, Processes and Architectures. Royal Society of Chemistry: 2013. 15. Major, T. A.; Lo, S. S.; Yu, K.; Hartland, G. V., Time-Resolved Studies of the Acoustic Vibrational Modes of Metal and Semiconductor Nano-Objects. J. Phys. Chem. Lett. 2014, 5 (5), 866-874. 16. Brandt, N. C.; Keller, E. L.; Frontiera, R. R., Ultrafast Surface-Enhanced Raman Probing of the Role of Hot Electrons in Plasmon-Driven Chemistry. J. Phys. Chem. Lett. 2016, 7 (16), 3179-3185. 17. Adleman, J. R.; Boyd, D. A.; Goodwin, D. G.; Psaltis, D., Heterogenous Catalysis Mediated by Plasmon Heating. Nano Lett. 2009, 9 (12), 4417-4423. 18. Fasciani, C.; Alejo, C. J. B.; Grenier, M.; Netto-Ferreira, J. C.; Scaiano, J. C., High-Temperature Organic Reactions at Room Temperature Using Plasmon Excitation: Decomposition of Dicumyl Peroxide. Organ. Lett. 2011, 13 (2), 204-207. 19. Bueno Alejo, C. J.; Fasciani, C.; Grenier, M.; Netto-Ferreira, J. C.; Scaiano, J. C., Reduction of Resazurin to Resorufin Catalyzed by Gold Nanoparticles: Dramatic Reaction Acceleration by Laser or Led Plasmon Excitation. Catal. Sci. Technol. 2011, 1, 1506-1511. 20. Liu, Z. W.; Hou, W. B.; Pavaskar, P.; Aykol, M.; Cronin, S. B., Plasmon Resonant Enhancement of Photocatalytic Water Splitting under Visible Illumination. Nano Lett. 2011, 11 (3), 1111-1116. 21. Christopher, P.; Xin, H. L.; Linic, S., Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nature Chem. 2011, 3 (6), 467-472. 22. Warren, S. C.; Thimsen, E., Plasmonic Solar Water Splitting. Energy & Environmental Science 2012, 5 (1), 5133-5146. 23. Hoggard, A.; Wang, L.-Y.; Ma, L.; Fang, Y.; You, G.; Olson, J.; Liu, Z.; Chang, W.-S.; Ajayan, P. M.; Link, S., Using the Plasmon Linewidth to Calculate the Time and Efficiency of Electron Transfer between Gold Nanorods and Graphene. Acs Nano 2013, 7 (12), 11209-11217. 24. Mubeen, S.; Lee, J.; Singh, N.; Kraemer, S.; Stucky, G. D.; Moskovits, M., An Autonomous Photosynthetic Device in Which All Charge Carriers Derive from Surface Plasmons. Nature Nanotech. 2013, 8 (4), 247-251. 25. Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J., Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H-2 on Au. Nano Lett. 2013, 13 (1), 240-247. 31 ACS Paragon Plus Environment


26. Ha, J. W.; Ruberu, T. P. A.; Han, R.; Dong, B.; Vela, J.; Fang, N., Super-Resolution Mapping of Photogenerated Electron and Hole Separation in Single Metal-Semiconductor Nanocatalysts. J. Am. Ceram. Soc. 2014, 136 (4), 1398-1408. 27. Marchuk, K.; Willets, K. A., Localized Surface Plasmons and Hot Electrons. Chem. Phys. 2014, 445, 95-104. 28. Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M., Photochemical Transformations on Plasmonic Metal Nanoparticles. Nature Mater. 2015, 14 (6), 567-576. 29. Haes, A. J.; Zou, S.; Zhao, J.; Schatz, G. C.; Van Duyne, R. P., Localized Surface Plasmon Resonance Spectroscopy near Molecular Resonances. J. Am. Chem.Soc. 2006, 128 (33), 10905-10914. 30. Moskovits, M., The Case for Plasmon-Derived Hot Carrier Devices. Nature Nanotech. 2015, 10 (1), 6-8. 31. Ma, X. C.; Dai, Y.; Yu, L.; Huang, B. B., Energy Transfer in Plasmonic Photocatalytic Composites. Light - Sci. & Appl. 2016, 5. 32. Bonnell, D. A.; Kalinin, S. V., Local Polarization, Charge Compensation, and Chemical Interactions on Ferroelectric Surfaces: A Route toward New Nanostructures. In Ferroelectric Thin Films X, Gilbert, S. R.; TrolierMcKinstry, S.; Miyasaka, Y.; Wouters, D. J., Eds. 2002, pp 317-328. 33. Kalinin, S. V.; Bonnell, D. A., Local Potential and Polarization Screening on Ferroelectric Surfaces. Phys. Rev. B 2001, 63 (12). 34. Streiffer, S. K.; Eastman, J. A.; Fong, D. D.; Thompson, C.; Munkholm, A.; Murty, M. V. R.; Auciello, O.; Bai, G. R.; Stephenson, G. B., Observation of Nanoscale 180 Degrees Stripe Domains in Ferroelectric Pbtio3 Thin Films. Phys. Rev. Lett. 2002, 89 (6). 35. Cai, Z. Y.; Xing, X. R.; Yu, R. B.; Sun, X. Y., Morphology-Controlled Synthesis of Lead Titanate Powders. Inorg. Chem. 2007, 46 (18), 7423-7427. 36. Arney, D.; Watkins, T.; Maggard, P. A., Effects of Particle Surface Areas and Microstructures on Photocatalytic H-2 and O-2 Production over Pbtio3. J. Am. Ceram. Soc. 2011, 94 (5), 1483-1489. 37. Atwater, H. A.; Polman, A., Plasmonics for Improved Photovoltaic Devices. Nature Mater. 2010, 9 (3), 205-213. 38. Clavero, C., Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nature Photonics 2014, 8 (2), 95-103. 39. Linic, S.; Christopher, P.; Ingram, D. B., Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nature Mater. 2011, 10 (12), 911-921. 40. Thomann, I.; Pinaud, B. A.; Chen, Z.; Clemens, B. M.; Jaramillo, T. F.; Brongersma, M. L., Plasmon Enhanced Solar-to-Fuel Energy Conversion. Nano Lett. 2011, 11 (8), 3440-3446. 41. Marimuthu, A.; Zhang, J.; Linic, S., Tuning Selectivity in Propylene Epoxidation by Plasmon Mediated Photo-Switching of Cu Oxidation State. Science 2013, 339 (6127), 1590-1593. 42. Zhao, G.; Kozuka, H.; Yoko, T., Sol-Gel Preparation and Photoelectrochemical Properties of Tio2 Films Containing Au and Ag Metal Particles. Thin Solid Films 1996, 277 (1-2), 147-154. 43. Leenheer, A. J.; Narang, P.; Lewis, N. S.; Atwater, H. A., Solar Energy Conversion Via Hot Electron Internal Photoemission in Metallic Nanostructures: Efficiency Estimates. J. Appl. Phys. 2014, 115 (13). 44. Li, J. T.; Cushing, S. K.; Zheng, P.; Senty, T.; Meng, F. K.; Bristow, A. D.; Manivannan, A.; Wu, N. Q., Solar Hydrogen Generation by a Cds-Au-Tio2 Sandwich Nanorod Array Enhanced with Au Nanoparticle as Electron Relay and Plasmonic Photosensitizer. J. Am. Chem.Soc. 2014, 136 (23), 84388449. 45. Giugni, A.; Torre, B.; Toma, A.; Francardi, M.; Malerba, M.; Alabastri, A.; Zaccaria, R. P.; Stockman, M. I.; Di Fabrizio, E., Hot-Electron Nanoscopy Using Adiabatic Compression of Surface Plasmons. Nature Nanotech. 2013, 8 (11), 845-852. 46. Tamura, T.; Ishibashi, S.; Terakura, K.; Weng, H. M., First-Principles Study of the Rectifying Properties of Pt/Tio2 Interface. Phys. Rev. B 2009, 80 (19). 32 ACS Paragon Plus Environment

Page 32 of 35

Page 33 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47. Kakekhani, A.; Ismail-Beigi, S.; Altman, E. I., Ferroelectrics: A Pathway to Switchable Surface Chemistry and Catalysis. Surf. Sci. 2016, 650, 302-316. 48. Kakekhani, A.; Ismail-Beigi, S., Ferroelectric Oxide Surface Chemistry: Water Splitting Via Pyroelectricity. J. Mater. Chem. A 2016, 4 (14), 5235-5246. 49. Wang, Z. J.; Cao, D. W.; Wen, L. Y.; Xu, R.; Obergfell, M.; Mi, Y.; Zhan, Z. B.; Nasori, N.; Demsar, J.; Lei, Y., Manipulation of Charge Transfer and Transport in Plasmonic-Ferroelectric Hybrids for Photoelectrochemical Applications. Nature Comm. 2016, 7. 50. Cao, D.; Wang, C.; Zheng, F.; Fang, L.; Dong, W.; Shen, M., Understanding the Nature of Remnant Polarization Enhancement, Coercive Voltage Offset and Time-Dependent Photocurrent in Ferroelectric Films Irradiated by Ultraviolet Light. J. Mater. Chem. 2012, 22 (25), 12592-12598. 51. Qin, M.; Yao, K.; Liang, Y. C.; Gan, B. K., Stability of Photovoltage and Trap of Light-Induced Charges in Ferroelectric Wo3-Doped (Pb0.97la0.03)(Zr0.52ti0.48)O-3 Thin Films. Appl. Phys. Lett. 2007, 91 (9). 52. Reddy, K. H.; Parida, K., Fabrication, Characterization, and Photoelectrochemical Properties of Cu-Doped Pbtio3 and Its Hydrogen Production Activity. ChemCatChem 2013, 5 (12), 3812-3820. 53. Suriyaprakash, J.; Xu, Y. B.; Zhu, Y. L.; Yang, L. X.; Tang, Y. L.; Wang, Y. J.; Li, S.; Ma, X. L., Designing of Metallic Nanocrystals Embedded in Non-Stoichiometric Perovskite Nanomaterial and Its Surface-Electronic Characteristics. Sci. Rep. 2017, 7 (1), 8343. 54. Pintilie, L.; Vrejoiu, I.; Hesse, D.; Alexe, M., The Influence of the Top-Contact Metal on the Ferroelectric Properties of Epitaxial Ferroelectric Pb(Zr0.2ti0.8)O3 Thin Films. J. Appl. Phys. 2008, 104 (11), 114101. 55. Li, L.; Zhang, Y. L.; Schultz, A. M.; Liu, X.; Salvador, P. A.; Rohrer, G. S., Visible Light Photochemical Activity of Heterostructured Pbtio3-Tio2 Core-Shell Particles. Cat. Sci. Technol. 2012, 2 (9), 1945-1952. 56. Schultz, A. M.; Zhang, Y.; Salvador, P. A.; Rohrer, G. S., Effect of Crystal and Domain Orientation on the Visible-Light Photochemical Reduction of Ag on Bifeo3. ACS Appl. Mater. Interf. 2011, 3 (5), 15621567. 57. Zhang, Z.; Yates, J. T., Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces. Chem. Rev. 2012, 112 (10), 5520-5551. 58. Sluka, T.; Tagantsev, A. K.; Bednyakov, P.; Setter, N., Free-Electron Gas at Charged Domain Walls in Insulating Batio3. Nature Comm. 2013, 4. 59. Wu, W. D.; Horibe, Y.; Lee, N.; Cheong, S. W.; Guest, J. R., Conduction of Topologically Protected Charged Ferroelectric Domain Walls. Phys. Rev. Lett. 2012, 108 (6). 60. Kathan-Galipeau, K.; Wu, P. P.; Li, Y. L.; Chen, L. Q.; Soukiassian, A.; Xi, X. X.; Schlom, D. G.; Bonnell, D. A., Quantification of Internal Electric Fields and Local Polarization in Ferroelectric Superlattices. Acs Nano 2011, 5 (1), 640-646. 61. Pu, Y. C.; Wang, G. M.; Chang, K. D.; Ling, Y. C.; Lin, Y. K.; Fitzmorris, B. C.; Liu, C. M.; Lu, X. H.; Tong, Y. X.; Zhang, J. Z.; Hsu, Y. J.; Li, Y., Au Nanostructure-Decorated Tio2 Nanowires Exhibiting Photoactivity across Entire Uv-Visible Region for Photoelectrochemical Water Splitting. Nano Lett. 2013, 13 (8), 3817-3823. 62. Robatjazi, H.; Bahauddin, S. M.; Doiron, C.; Thomann, I., Direct Plasmon-Driven Photoelectrocatalysis. Nano Lett. 2015, 15 (9), 6155-6161. 63. Hartland, G. V., Optical Studies of Dynamics in Noble Metal Nanostructures. Chem. Rev. 2011, 111 (6), 3858-3887. 64. Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; Vicky, D.-N.; Kang, Y.; Engheta, N.; Kagan, C. R.; Murray, C. B., Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives. Acs Nano 2012, 6 (3), 2804-2817.



65. Jing, H.; Zhang, Q.; Large, N.; Yu, C.; Blom, D. A.; Nordlander, P.; Wang, H., Tunable Plasmonic Nanoparticles with Catalytically Active High-Index Facets. Nano Lett. 2014, 14 (6), 3674-3682. 66. Ortiz, N.; Zoellner, B.; Hong, S. J.; Hoang, K.; Ji, Y.; Liu, Y.; Maggard, P. M.; Wang, G. F., Harnessing Hot Electrons from near Ir Light for Hydrogen Production Using Pt-End-Capped-Aunrs. ACS Appl. Mater. Interf. 2017, 9, 25962–25969. 67. Ortiz, N.; Hong, S. J.; Fonseca, F.; Liu, Y.; Wang, G., Anisotropic Overgrowth of Palladium on Gold Nanorods in the Presence of Salicylic Acid Family Additives. J. Phys. Chem. C 2017, 121 (3), 1876-1883. 68. Zheng, Z.; Tachikawa, T.; Majima, T., Single-Particle Study of Pt-Modified Au Nanorods for Plasmon-Enhanced Hydrogen Generation in Visible to near-Infrared Region. J. Am. Chem.Soc. 2014, 136 (19), 6870-6873. 69. Zoellner, B.; Gordon, E.; Maggard, P. A., A Small Bandgap Semiconductor, P-Type Mnv2o6, Active for Photocatalytic Hydrogen and Oxygen Production. Dalton. Trans. 2017, 46 10657-10664. 70. Arney, D.; Watkins, T.; Maggard Paul, A., Effects of Particle Surface Areas and Microstructures on Photocatalytic H2 and O2 Production over Pbtio3. J. Am. Ceram. Soc. 2010, 94 (5), 1483-1489.


Page 34 of 35

Page 35 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Composite Ferroelectric and Plasmonic Particles for Hot Charge

Recommend Documents