Influence of Optically Rectified Electric Fields on the Plasmonic

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C: Physical Processes in Nanomaterials and Nanostructures

Influence of Optically Rectified Electric Fields on the Plasmonic Photocatalysis of 4-Nitrothiophenol and 4-Aminothiophenol to 4,4-Dimercaptoazobenzene Darby A. Nelson, and Zachary D. Schultz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00662 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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The Journal of Physical Chemistry

Influence of Optically Rectified Electric Fields on the Plasmonic Photocatalysis of 4-Nitrothiophenol and 4-Aminothiophenol to 4,4-Dimercaptoazobenzene

Darby A. Nelson and Zachary D. Schultz*

Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

*corresponding author email: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 The excitation of plasmon resonances in nanostructured materials has been shown to drive catalytic processes. The ability to tune plasmon resonances across the solar spectrum has sparked interest in plasmonic catalysis for many applications. Results indicate that nanostructures with tight junctions can generate direct current (DC) electric fields, arising from optical rectification (OR). The impact of OR generated DC electric fields on plasmonic catalysis is not known. In this work, we use a mixed monolayer of p-mercaptobenzonitrile (MBN) and either 4-nitrothiophenol (NTP) or 4aminothiophenol (ATP) to correlate the DC field with surface catalytic activity. The DC electric field strength is measured using a vibrational Stark reporter (MBN). The catalytic activity is assessed by monitoring the formation of 4,4-dimercaptoazobenzene (DMAB) and loss of NTP/ATP using changes in the observed surface-enhanced Raman spectrum. Our data shows that, at relatively low laser powers, optical rectification modulates the plasmonic catalysis.


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Introduction Surface plasmon resonances have been shown to drive catalytic processes.1 Recent studies have attempted to utilize the electric fields, which originate from plasmonic nanostructures, for a broad range of applications. Some of which include water splitting, CO2 reduction, energy saving window coatings, and very recently drug delivery systems.2-9 Hot electrons are believed to drive these catalytic reactions. However, recent evidence indicates DC electric fields originating from optical excitation of plasmon resonances, may provide another mechanism for catalysis.10,11 Localized surface plasmon resonances (LSPRs) are produced through the absorption of light at the resonant wavelength of a metal nanomaterial. LSPR generation creates a confined electric field consisting of oscillating excited electrons near the metal surface.12-14 Large enhancements of the electric fields are reported between nanoparticle aggregates, known as ‘hot spots’.13,15 Relaxation of the electrons excited by LSPRs occurs through a few mechanisms that lead to physical effects such as heating, hot electrons, and optical signal generation.13,14,16,17 The electric fields are also associated with optical signal enhancements, such as surface enhanced Raman spectroscopy (SERS).14,16,18 Electron relaxation via thermal relaxation can produce sizable temperature increases on the surface of nanoparticles.19 The relaxation forming hot electrons can promote reactions near nanostructure surfaces. This mechanism has received the most attention for plasmonic catalysis.13,17,20,21 In light of increasing applications for plasmonic catalysis, much research has gone into understanding the mechanisms involved, such as the dimerization to form 4,4dimercaptoazobenzene (DMAB). Huang et al. showed plasmons have the ability to



facilitate the dimerization of 4-aminothiophenol (ATP) to DMAB when ATP is adsorbed onto a plasmonic surface under laser excitation. At the time, Huang et al. attributed the catalytic process to be induced through a local heating effect from the hot spots of the confined LSPRs.22 Using tip-enhanced Raman spectroscopy, temperature was shown not to increase during the plasmonically driven reaction suggesting local heating does not induce catalysis.23 This result was further supported by work performed by Navalon et al.20 Later research has shown catalysis occurs through hot electron generation.23-30 Other factors including applied potential and atmospheric changes, such as a saturated oxygen or hydrogen environments, have been shown to influence reactivity.22,28,31-36 Hot electron generation is the most agreed upon mechanism in the literature today. The dimerization of ATP and 4-nitrothiophenol (NTP) to DMAB has been extensively studied. Osawa et al. were the first to report spectral changes of ATP on a silver SERS active substrate.31 They attributed these new Raman bands to the b2 modes of ATP which arose from a charge transfer mechanism between the surface and the adsorbed molecule. These new bands were voltage and excitation wavelength dependent. Huang et al. were the first to show that a photocatalytic reaction, induced by laser excitation, was occurring on the surface rather than the excitation of the nonsymmetric modes of ATP.22 They showed spectroscopic evidence, and confirmed with mass spectrometry, that a new species (DMAB) was created on the surface through an oxidation reaction from the dimerization of two ATP molecules. Kim et al. quickly followed this up by reporting DMAB could also be produced from the photo reduction of NTP, even though they still attributed the DMAB bands to the b2 modes of ATP.37 Since these studies, much work has been done, experimentally as well as theoretically, to


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understand the reduction and oxidation mechanisms of NTP and ATP to DMAB.36,38-43 These reactions provide an accessible model system to investigate other factors affecting the reactivity. One factor that has yet to be investigated is the DC fields arising from optical rectification on plasmonic nanostructures. The intense optical fields arising from LSPR excitation can drive nonlinear optical phenomena, such as OR.10,11,44,45 OR is a second order nonlinear process in which an optical, alternating current, electric field (eg. a plasmon) is rectified to a DC electric field.46 Recently, these DC electric fields have been detected using vibrational Stark probes.10,47,48 The vibrational Stark effect is the perturbation of the electrons by an external electric field.49 The electric field disturbance causes shifts in the vibrational energies of specific bonds; the most studied are the carbon-nitrogen and the carbon-oxygen bonds.50-52 Changes in the nitrile stretch frequency can be correlated to the external electric field through the Stark tuning coefficient. This approach has been used in a variety of systems including: biological media, proteins, and understanding charges around DNA.53-57 Many groups have used the vibrational Stark effect to measure the strength of electric fields on plasmonic surfaces.10,47,48 Specifically, Apkarian et al. correlated the shifting of CO vibrational frequencies to the gap distance dependent electric field between nano-spheres.10 Marr et al. reported a sizable nitrile Stark shift (130 cm-1) between a plasmonic atomic force microscope tip and a roughened gold surface.11,48 Reports indicate p-mercaptobenzonitrile (MBN) is an effective Stark probe to measure the local electric field environment on heterogeneous nanostructured silver surfaces.58 The combination of theory and experiment demonstrated that Stark tuning and bias charging in



plasmonic junctions contribute to vibrational energy shifts.59,60 The intense fields provided from the excitation of plasmonic junctions promote nonlinear optical processes, and indirectly induce Stark shifts.11,44 The impact of OR on plasmonic catalysis has not been investigated. The DC bias attributed to OR is similar to the electric field applied at an electrode under electrochemical control, which can control the reaction rate. In this manuscript, we use Stark shifts from MBN to correlate the bias obtained from OR to electrochemical control on a nanostructured gold surface. Previous work has shown that nitrile stretch frequency exhibits Stark tuning effects with applied electrochemical potential.61 The trend in the nitrile frequency can be used to monitor the nitrile stark shift that we have previously reported to arise from optical rectification of plasmon induced electric fields.11,58 Using mixed monolayers of MBN and either ATP or NTP, we use the observed Raman scattering to track the local surface potential through the vibrational Stark effect as well as simultaneously monitor the photocatalytic conversion of NTP, or ATP, to DMAB. By combining a Stark reporter with a catalytic reporter in a mixed monolayer on a gold plasmonic surface we can correlate the local electric field strength to the photocatalytic activity and gain understanding of plasmonic catalysis.

Experimental Materials and Reagents Anodized aluminum oxide (AAO) filters with 0.1 µm pores (Anodisc 13, Whatman) were purchased from GE Life Sciences (Massachusetts, USA). Copper sheets (alloy 110) were purchased from Grainger (Indiana, USA). Orotemp 24 RTU,


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0.25 Troy/quart, gold electroplating solution was purchased from Technic, Inc. (Rhode Island, USA). Silver shot (99.999%), ethanol (90%), 4-nitrobenzenethiol (80%, Technical grade), 4-aminothiophenol (97%), and sodium perchlorate (98%) were obtained from Sigma Aldrich (Missouri, USA) and used without modification. The Notre Dame Chemical Synthesis Core synthesized p-mercaptobenzonitrile (>98% purity, verified by NMR) (Indiana, USA).

Substrate Preparation Silver was thermally evaporated onto AAO filters as previously reported.62 Gold was electrodeposited onto the filter using a commercial plating solution. Triple pulse voltammetry, controlled with a CHI660D potentiostat (CH Instruments), was used for the gold deposition with pulse voltages of -1 V, 0 V, and 0 V for 0.1 seconds per pulse. Each deposition run contained 3000 cycles and six runs were done per substrate, totaling 90 minutes of deposition time. After electroplating, the filter was removed by soaking the substrate in 0.1 M NaOH overnight. The substrate was fixed to a copper sheet to form the functional electrode. The copper sheets were soaked in NoChromix for 10 minutes to remove lacquer coating before use. The substrate was then allowed to dry on the copper sheet where capillary forces seal the substrate to the copper as drying takes place. After drying is complete the substrate is glued to the copper sheet with PURE Ice nail polish. Electroplating additives were removed from the surface by performing five consecutive oxidationreduction cycles in 0.1 M NaOH (CV, -0.9 V to 0.75 V).



Monolayer Preparation Clean substrates were soaked in a 0.01 M ethanolic solution of NTP or ATP and MBN for 24 hours to create a mixed monolayer. For nitrile Stark shift control experiments, substrates were soaked only in MBN. For photocatalytic activity experiments, mixed monolayers of NTP and MBN or ATP and MBN were adsorbed to the substrate surface.

Electrochemical Measurements Electrochemical measurements were performed with a CHI660D potentiostat. The nanostructured substrate was used as the working electrode with a Ag/AgCl reference electrode and platinum wire counter electrode. A 0.1 M NaClO4 solution was used as the electrolyte in all experiments.

Raman Measurements Raman spectra were acquired using a Renishaw inVia Raman microscope with a 632.8 nm HeNe laser (Thorlabs). When looking at DMAB dimerization the scattering between 919-1464 cm-1 was collected. For CN studies, the scattering between 19122374 cm-1 was collected. Spectral analysis was performed using MATLAB with a modified peak-fitting script from Mathworks. Spectra were fit with a single Gaussian, with the optimal fit being that with the smallest percent root mean squared (RMS) error. In general, 15 iterations were sufficient to minimize the RMS error. Intensities of bands associated with each species were normalized to the 1076 cm-1 band which is present in all spectra.


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Before Laser Exposure CN

NO2

NO2

S

S

S

After Laser Exposure

NH2

NH2

CN

CN

S

S

S

S

CN

S

Gold Nanorod Surface Electrode Support

a)

1000

1200

1400

1600

1800

2000

2200

b)

c) d) e)

1000

1200

1400

1600

-1

Raman Shift (cm )

Figure 1. The schematic illustrates the monolayers used. Top Left: the MBN (black) / NTP (blue) and MBN / ATP (red) mixed monolayers prepared are illustrated. Top right: After laser excitation of the sample, DMAB (green) is seen on the surface. Reference SERS spectra are shown of all surface species observed in the experiment. The species and identifying mode for each spectrum are: a) MBN, 2230 cm-1; b) NTP, 1337 cm-1; c) DMAB from NTP, 1142 and 1442 cm-1; d) ATP, e) DMAB from ATP. The band at 1076 cm-1 is present at a constant intensity in all spectra and is used for normalization.



Results To correlate local electric field strengths with surface reactivity, a mixed monolayer of MBN and NTP or ATP was formed on the gold nanorod surface as depicted in Figure 1. Reference spectra of the species and key vibrational bands used in this research are shown below the depiction. The nitrile stretch moded from MBN (~2230 cm-1) provides a probe of the local surface potential. The band used to track NTP is the symmetric stretching motion of the NO2 group (1337 cm-1). The bands used to track the formation of DMAB are the N=N stretch (1442 cm-1) or the C-N bond stretch (1142 cm-1). The spectra were normalized using the in-plane CH bending mode of the aromatic ring band at 1076 cm-1. Figure 2 is a dark field hyperspectral image of a bare gold nanorod substrate. The variety of colors in the image illustrates the surface heterogeneity and possibility of many different plasmonic environments on the surface. The variation of MBN vibrational frequencies (vida supra) indicates that many different plasmonic field environments exist and these differing local environments can be monitored with SERS. To calibrate the change in nitrile stretching frequency to a surface potential, spectra were acquired from MBN on a surface under electrochemical control. Figure 3 shows the shift in the nitrile stretch as the applied electrochemical potential is swept from 0 V to -1 V. The nitrile stretching frequency is observed at lower energy as the potential applied to the surface becomes more negative. From the change in peak frequency versus the applied potential, a Stark tuning coefficient of 8.7 ± 0.4 cm-1/V is determined by fitting the linear portion of the data between 0 V and -0.85 V. It is worth noting that the applied electrochemical potential reflects only the surface charge


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(voltage, V), while the Stark shift arises from the electric field (voltage/distance, V/m) experienced by the bond. An exact determination of the electric field would require a more precise determination of the Stark tuning coefficient; however, the observed electrochemical tuning effect agrees with previous reports in the literature. 61,63



10 µm

Figure 2. A true color dark-field scattering image of gold nanorod substrate. The different colors are indicative of heterogeneous plasmon resonances on the substrate.

Figure 3. The CN stretching frequency of MBN shows a linear shift in frequency with applied electrochemical potential (vs. Ag/AgCl). The inset depicts one data set used to determine the change in lineshape of the CN band with potential.


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Previous studies have shown that laser illumination of plasmonic nanostructures results in optical rectification, which gives rise to DC bias on the nanostructure surface.11,44-46,58,60 Figure 4 shows the change in the nitrile stretching frequency with increasing laser power. The error bars represent the average shift observed from triplicate measurements on different spots in both air and water. These trials were performed in the absence of a bias potential, indicating the induced Stark shift arises from the optically rectified DC field produced between plasmonic junctions on the surface. The measured nitrile Stark tuning coefficients observed in air and water are -0.4 ± 0.2 cm-1/mW and -0.3 ± 0.1 cm-1/mW respectively; and are statistically equivalent at the 90% confidence range. Correlating the laser induced Stark shift with the electrochemical measurement in Figure 3, indicates the surface develops an increasingly negative charge with increased laser power. Additionally, the surface potential offset corresponding to the rectified field with the 0.65 mW laser power used in Figure 3 corresponds to an average offset of only -0.02 V. Both Figures 3 and 4 represent ensemble measurements, and that localized fields may be much stronger. The similarity in the observed range over which the CN stretch is observed provides a qualitative indicator of surface charge generated by the optically rectified field. It is well known that changes in hydrogen bonding can alter the nitrile stretching frequency; however, the similarity in tuning coefficients in both air and water suggest the change observed here arises from the changes in surface potential.64-66 Additionally, heating resulting from absorption by the plasmon resonance can also affect molecules on the nanoparticles. Heating effects have been shown to depend strongly on the thermal conductivity of surrounding fluid (air or water).19 The similarity in



the shifts observed here with laser power suggest optical rectification of the plasmonic field is responsible for the frequency change rather than temperature.


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Figure 4. The CN stretching frequency of MBN observed on a plasmonically active gold nanorod substrate with increasing laser power in air (black line) and water (green line). The change in frequency is statistically the same in both environments. Inset shows a sample of the change in lineshape with increasing laser power from 0.05 mW to 6.14 mW.



To examine how this optically rectified field influences surface reactivity, we compared the electrochemical and photocatalytic conversion of NTP and ATP into DMAB. Figure 5 depicts electrochemical reduction of NTP and oxidation of ATP into DMAB. The spectra were acquired using a low laser power (3 µW) to minimize the likelihood of inducing the photoreaction on the surface. Figure 5A shows representative spectra acquired at different electrochemical potentials. Raman bands attributable to DMAB are observed at positive potentials and persist until the surface is made sufficiently negative that the surface species appear to be reduced. Figure 5B starts with the reduced molecule, ATP, and shows a constant spectrum until the applied potential is made sufficiently positive to promote the oxidation to DMAB. In Figure 5C, intensity of the 1142 cm-1 band in both experiments is plotted against the applied electrochemical potential. The error bars represent experiments on different SERS substrates (n=3 for ATP, n=4 for NTP). Our data reproduces observations in the literature concerning electrochemical reduction of NTP to ATP through a DMAB intermediate from 0 V to -1 V.22,31-35 The spectra at -1 V and -0.9 V, for NTP and ATP, respectively, are the same, indicating the fully reduced amine is the surface species in both cases.


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A

B

C

Figure 5. Representative SERS spectra from noted electrochemical potentials with surface species A) NTP and B) ATP. C) Electrochemical reduction of NTP (blue dots), and oxidation of ATP (red dots), to DMAB (1142 cm-1) monitored by the SERS intensity of their respective bands.



To our knowledge, Figure 5 shows the first experimental evidence of the electrochemical oxidation of ATP to DMAB at positive potentials. This experimental result agrees with theoretical evidence that high positive potentials can oxidize ATP to DMAB.22,41 Figure 6 shows the increase in Raman bands attributable to DMAB as a function of time and laser power in air. In Figure 6A, the DMAB bands at 1142 and 1442 cm-1 increased over time at a fixed laser fluence of 0.16 mW. In Figure 6B, the increase in the intensity of the 1442 cm-1 band (normalized to the 1076 cm-1 band) over time, is plotted for different laser powers. The intensity of the bands attributed to DMAB increase with laser power before leveling off in intensity at longer times. This trend suggests more rapid conversion of NTP to DMAB is associated with increased laser power, which is in agreement with previous reports.37,67-70


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A

0 sec 120 sec

B

Figure 6. A) Representative SERS spectra of NTP / MBN mixed monolayer over a time period of 0 to 120 seconds; arrows highlight decrease in NTP and increase in DMAB over time. Inset shows CN stretching region of MBN from same spectra is unchanged. B) Differences in the incident power alter the photocatalytic reduction of NTP to DMAB over time. The incident powers are: 0.05 mW (blue), 0.11 mW (red), 0.16 mW (green), 0.24 mW (pink), 0.34 mW (black).



By combining MBN moieties into the adsorbed monolayer, we can monitor the surface potential during these reactions. The inset in Figure 6A shows multiple spectra of the MBN vibrational frequency throughout a single DMAB generation time trial (0 – 120 sec). No change in peak shape (intensity, vibrational frequency, or width) is observed with time, indicating that surface charging occurs on a faster timescale than the photo reduction. The lack of change in nitrile stretch also indicates that the nitrile is not participating in the formation of DMAB. Therefore, the only reaction taking place on the nanorod surface is the dimerization of NTP (or ATP, vida infra) to DMAB. The mixed monolayer is prepared from equal molar mixtures and the presence of the MBN is not observed to inhibit the dimerization reaction. The CN stretching frequency observed in the mixed monolayers is identical in frequency and width to pure monolayers of MBN, further suggesting no interaction between MBN and the ATP/NTP molecules. In Figure 6, the variance in the final DMAB intensities observed may reflect some level of non-ideal mixing in the monolayer; however, sufficient ATP and NTP molecules are adjacent for the reaction to proceed. To examine the effects of heterogeneity in the plasmonic environment and the resulting optically rectified field on photocatalytic conversion to DMAB, a series of time dependent measurements were obtained at different locations on the nanostructured surface. A mixed monolayer of either NTP or ATP and MBN was adsorbed onto the nanorod surface. The laser power was fixed at 0.2 mW based on the results in Figure 5. In Figure 7 the initial (first 45 s) rate of change in the DMAB bands is plotted versus the frequency of the MBN stretch. Multiple trials were performed for both NTP and ATP. A linear trend is observed in the rate of DMAB conversion with respect to the MBN


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stretching frequency. With ATP, there is a small increase in rate corresponding to an increase in the MBN stretch frequency. The rate of NTP conversion into DMAB decreases as the MBN stretch increases. In Figures 3 and 4, a higher MBN stretch corresponds to a more positive surface potential. This result is consistent with more facile oxidation at positive potentials. The negative slope for the reduction rate of NTP to DMAB is consistent with the inhibition of the reduction reaction at positive potentials.



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NTP

DMAB

ATP

DMAB

Figure 7. The rates of photo oxidation of ATP to DMAB (green) and photo reduction of NTP to DMAB (black) are shown to vary with the observed CN stretch frequency, which reports the optically rectified surface potential. The DMAB formation rates are determined from a linear fit to the initial increase in bands associated with DMAB (inset).


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DISCUSSION The results in this manuscript improve our knowledge of plasmonic catalysis by understanding: the correlation between electrochemical and photogenerated DC potentials on catalytic activity, the effect of laser power on the mechanism, and the possible uses for OR generated potentials for photocatalytic efficiency improvements. The controlled electrochemical environment provides a reference to understand the optically rectified surface potential and its role in catalysis. Previous studies, along with our research, show that applied negative potentials can reduce NTP to ATP through a DMAB intermediate on plasmonic surfaces. Along with this, our research demonstrates ATP can be dimerized to DMAB through the application of positive potentials. Little is known about the impact optically rectified DC fields have on the photocatalytic reduction of NTP, or oxidation of ATP, and whether that impact mimics the effects of electrochemical potentials. Utilizing the ability to monitor local surface environments, our results show that reactivity varies across heterogeneous plasmonic surfaces and is modulated by optically rectified potentials. Our data shows that more negative local surface potentials promote reduction reactions, while more positive local surface potentials increase the rate of oxidation reactions. These results support the idea that surface potentials generated from OR imitate applied electrochemical potentials. Literature on both NTP and ATP dimerization to DMAB, as well as our power dependence study of NTP reduction to DMAB, show these photocatalytic reactions are governed by hot electron generation mechanism.37,67-70 This theory arises from the result of higher laser powers inducing more DMAB production on the surface.



Increasing laser power creates a higher number of electrons with sufficient energy to induce a photoreaction on the nanomaterial surface. However, our data alludes to the possibility that hot electrons are not the only factor impacting these photocatalytic reactions and that optical rectification plays a role as well. The availability of hot electrons to perform catalysis appears to be modulated by the OR potential on the surface. This trend is evident in Figure 7, where the reduction of NTP is observed at a slower rate with increasing surface potential. To understand the impact of OR on plasmonic catalysis, it is worth considering the laser powers used in most experiments. The average laser powers used in previously mentioned studies range from 1x106 mW/cm2 to 1x108 mW/cm2 (0.01 mW to 0.3 mW for our laser spot size).22,37,67,69,70 Looking at Figure 4, these powers are on the low end of powers inducing any surface optical rectification effects. At these low powers, the modulating OR effects shown in Figure 7 are believed to play a very limited role in the photoreactions on the surface. However, results from this manuscript show that using much higher laser intensities (7x108 mW/cm2) generates a far larger OR effect that could possibly impact the catalytic mechanism. When considering the local electric field, the impact of hotspots must also be considered, as intense local fields may not be evident in the ensemble SERS signal. By engineering hotspots, quite intense local fields can be observed, such as the ~130 cm-1 Stark shift observed between a TERS tip and a nanostructured surface.11 Lasers are believed to be too soft an excitation technique to induce the full reduction of NTP to ATP. This is based on previous theoretical research reporting a 3.32 eV energy barrier being too much energy for laser excitation alone to provide (633


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nm, ~ 2 eV).38 However, our data implies there is a laser power capable of inducing a large enough negative surface potential to cause full reduction of NTP to ATP without the DMAB intermediate, as seen in previous electrochemical experiments.41 Previous literature, as well as our data, reports this potential to be -0.8 V. Linear fits in Figure 4 equate a surface potential of -0.8 V (2226 cm-1) to a laser power of 2x109 mW/cm2 (~10 mW in our experimental setup); which is an attainable laser power, but one often associated with surface degradation in plasmonic experiments. It is interesting to consider if hotspots may provide sufficient electric field confinement to establish a local electric field sufficient for the full photo reduction of NTP to ATP. These results suggest surface structures that amplify nonlinear phenomena, such as the OR effect, may improve the efficiency of plasmonic catalysis. Second order nonlinear processes are observed in materials that lack inversion symmetry. Electrochemical deposition can produce anti-symmetric nanostructures, through sequential deposition of materials to make layers, which break symmetry inherently at the interface between the layers. This can be done through coating a plasmonic nanosurface with a thin film of another metal. This thin metal film provides two services: breaking symmetry and accepting/donating electrons to further promote reaction progression. It should be noted that nanojunctions have been reported necessary for OR in plasmonic materials; therefore, isolated single material nanorods are not expected to induce optical rectification or be influenced by these rectified fields.



Conclusion In this manuscript, we studied the impact optically rectified DC fields have on plasmonic catalysis. Our results show agreement between applied electrochemical potential and surface charge obtained from optical rectification. We showed that potentials generated through OR have a modulating impact on plasmonic surface reactivity at low laser powers. We also speculate that the impact of OR on the photocatalytic mechanism may increase at high electric fields, such as in hotspots. This speculation stems from the result that increasing laser power generates an increasingly negative potential at the nanostructure surface. In conclusion, results from this manuscript provide evidence that surface properties that promote second order processes influence photocatalytic efficiencies.

Acknowledgment This work was supported by the National Science Foundation award CHE-1507287 and CHE-1830994.

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TOC Graphic


Influence of Optically Rectified Electric Fields on the Plasmonic

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