Deconstructing the Heterogeneity of Surface-Bound Catalysts: Rutile

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Deconstructing the Heterogeneity of Surface-bound Catalysts: Rutile Surface Structure Affects Molecular Properties Carmella Calabrese, Heather Vanselous, and Poul Bering Petersen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09782 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

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The Journal of Physical Chemistry

Deconstructing the Heterogeneity of Surface-bound Catalysts: Rutile Surface Structure Affects Molecular Properties Carmella Calabrese, Heather Vanselous, and Poul B. Petersen* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States

ACS Paragon Plus Environment

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ABSTRACT. Catalytic interfaces involving surface-bound molecular catalysts often exhibit a large structural heterogeneity from uncontrolled variation in surface morphology. Conventional spectroscopic techniques typically average over these different structural motifs within the sample making it difficult to link the underlying surface morphology to the properties of the immobilized catalyst. Here we present the first direct comparison of the vibrational dynamics of a CO2 reduction catalyst bound to two different single-crystalline TiO2 surfaces, rutile (001) and (110), probed with transient surface-specific sum-frequency generation spectroscopy. We find that the change in surface structure between crystallographic faces alters both the vibrational frequency and relaxation time of the symmetric carbonyl stretching mode of the catalyst, with (001) displaying a lower frequency and longer relaxation time. This results from a change in the catalyst electronic structure and indicates that the molecular properties of the catalyst, likely including the catalytic properties, depend on the specific TiO2 surface to which it is bound. The comparison of the molecular properties on these two single crystal surfaces is an essential step toward understanding how semiconductor surface structure influences catalyst behavior and identifying optimal surface structures for improved catalytic performance.

KEYWORDS. CO2 catalysis, surface structure, vibrational relaxation, TiO2, rutile, time-resolved sum-frequency generation

1. Introduction Converting environmental pollutants into useful products such as chemical fuels, preferably through processes driven by renewable energy sources, would make a significant impact on leading environmental issues we face today. One system currently being explored is the photochemical (through direct solar energy) or electrochemical (utilizing photovoltaic devices)


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reduction of the greenhouse gas carbon dioxide (CO2). Homogeneous catalysis employing metalbased molecular catalysts features high efficiency and tunability for these processes but is complicated by low solubility and difficulty in isolating chemical species. Immobilizing the catalysts on a nanostructured inorganic framework produces a high surface area heterogeneous catalytic system, promoting high throughput with facile catalyst recovery.1–5 However, covalently anchoring to a semiconductor surface can dramatically change the properties of the catalyst. In such heterogeneous systems, it is difficult to distinguish and understand the limiting factors, thereby complicating efforts to optimize the efficiency of the catalytic system. Comparable to heterogeneous catalysis, dye-sensitized solar cells take advantage of the large surface area of colloidal TiO2 to effectively harvest solar radiation.6–9 Commercial TiO2 nanocrystalline powders are typically 80-90% anatase, with the remainder comprised of the more thermodynamically stable rutile form. It has been demonstrated that the surface structure of TiO2 influences chemical reactivity10 and the efficiency of dye-sensitized solar cells.11,12 By making fully functioning solar cells containing single-crystalline TiO2 surfaces sensitized with the ruthenium-based dye, cis-di(thiocyanato)-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II), Spitler and Parkinson observed a 10-fold difference in the efficiency of the solar cells depending on the TiO2 surface, with rutile (100) displaying the highest efficiency out of those studied.11 Among rutile surfaces, (110) is the most thermodynamically stable crystal face, while (001) is the higher energy surface and most often used for electrochemistry.13 The impact of the rutile surface structure on surface-bound catalysts is not well understood. Comparing different single-crystalline surfaces is needed to gain a deeper understanding of how the TiO2 surface structure affects the molecular properties of surfacebound catalysts and resolve the heterogeneity contained within nanocrystalline TiO2. In the


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present study, we focus on rutile (001) and (110) single crystal surfaces, shown schematically in Figure 1. The morphology of these two surfaces is quite different. The (001) surface (Figure 1, top) displays double rows of oxygens alternating with single rows of 5-coordinate titanium. In contrast, the (110) surface (Figure 1, bottom) exhibits rows of bridging oxygen atoms connected to 6-coordinate titanium atoms that alternate with 5-coordinate titanium atoms. Lower coordination Ti possess less electron density than the neighboring higher coordination Ti atoms.14 Presumably, a large catalyst will bind to and interact with these two surfaces differently. As such, it is imperative to discern the impact of surface morphology on the covalently attached catalyst in order to further our understanding of complex heterogeneous systems, which consist of many disparate exposed crystal facets.

Figure 1. Side view of rutile (001) and (110) surfaces; titanium atoms are represented in blue and oxygen in red. The (001) plane contains alternating double rows of oxygen and single rows of 5-coordinate titanium. However, (110) has rows of bridging oxygens bound to 6-coordinate titanium which alternate with 5-coordinate titanium. A few recent studies have used bulk two-dimensional infrared (2D-IR) spectroscopy to investigate how interfaces affect the vibrational dynamics of catalysts. A heterogeneous CO2 reduction system based on a rhenium tricarbonyl complex bound to nanocrystalline TiO2 thin


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films were studied by Zanni and co-workers using ground and exited state 2D-IR spectroscopy. Upon UV excitation, they found several different carbonyl species, each displaying distinct electron injection rates.15 While the initial report attributed the variation to multiple TiO2 adsorption sites, further investigations have pointed to aggregation of the catalyst playing a major role.16,17 Fayer and co-workers studied a rhenium tricarbonyl catalyst tethered to selfassembled monolayers with 2D-IR spectroscopy and found that the surface-bound molecules displayed different vibrational dynamics than in bulk solution and these depended on the local solvent environment.18–20 In this study we use surface-specific spectroscopy to examine catalytic monolayers on single-crystalline rutile surfaces. Vibrational sum-frequency generation spectroscopy (SFG) is a powerful tool for probing interfacial structure21–25 and dynamics.26–33 We focus on a rhenium tricarbonyl CO2 reduction catalyst: Re(4,4’-dicarboxyl-2,2’-bipyridine)(CO)3Cl, abbreviated as ReCO3. The structure and bulk linear absorption spectrum are shown in Figure 2. This catalyst is the dicarboxy version of the parent Re(2,2’-bipyridine)(CO)3Cl CO2 reduction catalyst which, along with others containing varying bipyridine (bpy) based ligands, has been studied extensively in the bulk34–42 and is similar to the catalyst studied by others using ultrafast spectroscopy.15–20,43,44 Lian and coworkers studied the same catalyst that is the focus of the present investigation with vibrational SFG spectroscopy. They determined the orientation of the catalyst on rutile (001) in good agreement with theoretical calculations,45,46 as well as the vibrational relaxation dynamics of the catalyst on rutile (110).47 The vibrational relaxation was found to be somewhat slower on single crystal TiO2 as compared to the catalyst adsorbed on a nanoporous TiO2 film or in bulk dimethylformamide. They also studied the catalyst immobilized


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on gold through a self-assembled monolayer, finding that the relaxation dynamics on gold were nearly identical to those in bulk dimethylformamide.47,48

Figure 2. ReCO3 structure and absorbance spectrum in ethanol (black line) shown with the intensity spectrum of the IR laser pulse used in the present study (shaded). In this study we make the first direct comparison of the vibrational dynamics of the catalyst on two different single crystal rutile surfaces: (001) and (110). Where previous studies have compared the dynamics of ReCO3 on single-crystalline rutile (110) to that of nanocrystalline TiO2,47 we investigate the influence of different single crystal surface structures on the molecular properties of ReCO3. Understanding the catalyst-surface relationship using single-crystalline surfaces will provide insight on the more complex heterogeneous systems. Isotope studies have shown that all three carbonyls are exchanged during catalysis.36 One proposed mechanism for CO2 photocatalytic reduction2 involves a metal-to-ligand charge transfer, CO2 attachment at the Re center, and subsequent CO ligand release. The degree of πbackbonding between the Re atom and CO ligand influences the binding strength of the CO ligands and thus the catalytic rate. The CO stretch frequency reports on the π-backbonding, with a lower vibrational frequency indicating stronger backbonding and correspondingly a less labile CO ligand. This information is relevant for optimizing the catalyst performance on TiO2 nanostructures with controlled surface structure, such as nanocrystals49 and nanorods.50 Such


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structures allow for tailoring the exposed nanoparticle surfaces in addition to providing a high surface area, and thus potentially increasing the efficiency of heterogeneous systems further if a higher performing surface can be identified.

2. Experimental Synthesis of ReCO3. Re(4,4’-dicarboxyl-2,2’-bipyridine)(CO)3Cl, referred to as ReCO3 in this study, was synthesized according to literature procedures51 and stored in the dark at room temperature. Re(CO)5Cl was purchased from Aldrich Chemical Co., and 4,4′-dicarboxy-2,2′bipyridine (dcbpy) was purchased from Alfa Aesar. Briefly, equimolar amounts of Re(CO)5Cl and dcbpy were refluxed in hot toluene and methanol for 1 hour. The transparent reaction mixture changed from colorless to bright orange within 10 min. Unreacted solid white starting material was precipitated out by placing the reaction mixture in a freezer (-30°C) over 1 hour. The reaction mixture was then filtered, and the orange filtrate was collected and dried to yield solid orange ReCO3. Substrate preparation and sensitization. All glassware was soaked in Nochromix (Godax Laboratories, Inc.) solution for 30 min and rinsed with ultrapure water (Millipore MilliQ, 18.2 MΩ•cm, ≤ 5 ppb total organic carbon) until neutral pH was achieved. Single crystal rutile (001) from MTI Corporation and (110) substrates from SurfaceNet GmbH were cleaned according to a modified literature procedure.46,52 The substrates were sonicated in piranha solution (3:1 H2SO4: 30% H2O2) for 1 hour, then rinsed with ultrapure water until reaching neutral pH. Warning: piranha solution reacts violently with organic compounds and should be handled with extreme caution. Cleaned substrates were soaked in 1 M NaOH for 5 min, followed by ultrapure water rinse, then soaked in 1 M HCl for 5 min. The soaking substrates were covered with a PELCO quartz substrate disc (Ted Pella, Inc.) and exposed to UV radiation for 10 min using a UVO


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cleaner (Jelight Company Inc., Model 42). The substrates were rinsed with ethanol and immediately immersed in saturated ethanolic solution of ReCO3. Surface functionalization of the catalyst occurred overnight in a dark environment. The sensitized crystals were then rinsed thoroughly with ethanol to remove physisorbed catalyst. Fresh samples were prepared each day in order to ensure sample integrity. Static and transient SFG experiments. Infrared (IR) and near-infrared (NIR) pulses for the SFG experiment were produced from the output of a Ti:Sapphire amplifier (Coherent Legend Elite Duo) seeded by a Ti:Sapphire oscillator (Coherent Micra-5). The regenerative amplifier provided 800 nm output pulses with energy of 5 mJ per pulse and time duration of 25 fs at a repetition rate of 1 kHz. The amplifier output was divided into three arms of energies 3 mJ, 0.75 mJ, and 1.25 mJ, which are compressed independently. The 1.25 mJ arm was not employed in the present experiment. A diagram of the IR pump – SFG probe experimental setup, as described below, is shown in Figure 3.

Figure 3. Left panel: Schematic of the IR pump – SFG probe experimental setup. Abbreviations: BS, beam splitter; λ/2, half waveplate; pol, linear polarizer; F-P, Fabry-Perot etalon; NB, narrow


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band filter; ND, variable neutral density filter; L, lens; PS, periscope; LP, long pass filter; SP, short pass filter. Right panel: Cross-correlation of the three incident beams on gold. To create the narrow-band NIR upconversion pulse, a Fabry-Perot etalon (TecOptics, Inc.) was used to spectrally filter the 0.75 mJ arm of the amplifier output to a full width at half maximum (FWHM) bandwidth of 0.6 nm (10 cm-1) centered at 792.6 nm. A 10 nm wide bandpass filter (Thorlabs, FBH800-10), angle-tuned to optimize throughput, was used to eliminate undesired higher and lower order Fabry-Perot transmission fringes. The 3 mJ arm of the amplifier output was used to pump a commercial optical parametric amplifier (Coherent OPerA Solo), which produced tunable mid IR pulses by difference frequency generation between the signal and idler pulses in a AgGaS2 crystal. For the present experiment, the IR pulse was centered at 2075 cm-1 with FWHM bandwidth of ~250 cm-1 (shown in Figure 2). The mid IR pulse was separated from signal and idler using a longwave-pass filter (Spectrogon, LP-3300 nm). The IR was split into probe and pump pulses by a Ge-coated KBr beam splitter (FTIR.com, BSP-945-1410). One-third of the IR beam was reflected from the beam splitter and used for the SFG probe along with the narrow-band NIR while the transmitted two-thirds of IR served as the pump. Each of the three beams incident on the sample (i.e. narrow-band NIR, probe IR, and pump IR) passed through separate motorized linear translation stages (Newport, XMS100) controlled by a single motion controller (Newport, XPS) via LabView programing. The sample was mounted on an XYZ linear translation stage (Newport, 460A-XYZ). The narrow-band NIR and probe IR pulses were overlapped temporally to maximize SFG signal, and the pump IR pulse delay was varied with respect to the probe pulse pair. The probe IR, narrow-band NIR, and pump IR beams were spatially overlapped at the sample at angles of 40, 50, and 57°, respectively, with


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respect to the surface normal. Optimal spatial overlap of the three incident beams was achieved by monitoring the third-order signal of a gold reference sample arising from the combination of IR pump, IR probe, and narrowband NIR (termed IIV). Temporal characterization of this third order signal produced a cross-correlation characterizing the IR pump-probe overlap (shown in Figure 3) with FWHM of 120 fs. The three incident beams were focused using lenses of the following types: ZnSe plano convex lens (Janos Tech., f = 200 mm) for pump IR, CaF2 plano convex lens (Thorlabs, LA5012, f = 150 mm) for probe IR, and N-BK7 plano convex lens (Thorlabs, LA1484-A, f = 300 mm) for NIR. The pump beam was set to p-polarization, and SFG was collected in the ppp (SFG, narrow-band NIR, and IR probe, respectively) polarization combination for all spectra obtained. The probe IR and pump IR energies were attenuated to 1.2 µJ and 4 µJ per pulse, respectively, through the combination of a tunable zero-order half-wave phase retardation plate (Alphalas GmbH) followed by a ZnSe wire grid linear polarizer (Specac). The upconversion beam was attenuated to 1 µJ using a variable neutral density filter (Newport, 100FS02DV.2), and its polarization was controlled using a true zero-order half-wave plate (Castech, λ/2 @ 800 nm) followed by a wire grid linear polarizer (Edmund Optics). Both IR beams were focused at the sample plane, while the NIR beam was focused slightly after the sample plane to prevent photodamage. Additionally, sample degradation was prevented by continuously moving the sample position in the longitudinal dimension with a LabView-controlled motorized linear actuator (Newport, NewStep Series). Higher upconversion beam fluence was seen to degrade the sample over time. The SFG signal was collimated by an achromatic lens (Thorlabs, AC254-150-A) and spatially isolated using an iris, which blocked SFG arising from the combination of IR pump and


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narrowband NIR. The third-order IIV signal was blocked with a 650 nm longpass filter (Andover Co., 650FH90-25). A 750 nm shortpass filter (Thorlabs, FES0750) was used to block scattered NIR light before the signal was focused into a polychromator (Princeton Instruments, Acton SP2500). The SFG signal was dispersed by a diffraction grating (150 grooves/mm for transient and 600 grooves/mm for static spectra, blazed at 500 nm) and imaged onto a liquid nitrogencooled CCD (Princeton Instruments, Model 7509-0001, 1340 x 400 pixels). Based on the design scheme of Ghosh et al.,30 alternating pumped and unpumped SFG spectra were measured simultaneously in the following manner: prior to detection, the SFG beam was reflected off a galvanometric mirror (Thorlabs, GVS011) rotating about a single axis at a frequency of 500 Hz and controlled by a sinusoidal signal from a function generator (Wavetek, Model 39A) synced with the amplifier. Also synced with the amplifier was an optical chopper (New Focus, 3501) located in the pump beam, which blocked every other pump pulse. Thus, pumped and unpumped spectra were spatially separated on the CCD chip and collected by binning corresponding regions of interest. Time-resolved SFG spectra were generated by exciting the carbonyl stretches of surfacebound ReCO3 using the infrared pump pulse and probing the SFG signal while varying the time delay between pump and probe pulses. Changes in the signal due to the influence of the pump pulse, i.e. reduced signal due to ground state bleach and induced signal at the overtone frequency, are observed by calculating the difference between pumped and unpumped SFG signal (∆SFG). A single scan of the transient SFG data was acquired as 5 s integrations of the CCD in 150 fs time steps from –5 to 10 ps and 1 ps time steps from 10 to 50 ps. To account for drifts in the laser intensity during each scan and over the course of the entire experiment, the pump-induced change in the SFG intensity (∆SFG) was normalized to the integrated SFG


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intensity without pump before averaging individual scans: 8 time scans for rutile (110) and 7 scans for rutile (001) were averaged and analyzed.

3. Results and Discussion Static SFG spectra of ReCO3 covalently bound to the (110) and (001) single-crystalline surfaces of rutile TiO2 are shown in Figure 4. Both spectra are comprised of one resonant peak, corresponding to the a’(1) in-phase symmetric stretch of the metal-bound carbonyl groups, which is sensitive to the local electronic environment of the molecule. The lower frequency peak at ~1900 cm-1 due to a combination of the antisymmetric a” stretch and the out-of-phase a’(2) symmetric stretch is not SFG active. The compound covalently binds to the titanium atoms on the surface through the carboxylic acid groups. It is unknown whether the molecule preferentially binds to the 5- or 6-coordinate Ti atoms present on (110), or whether the catalyst binds in a monodentate (through one carboxylic acid group) or bidentate (through both carboxylic acid groups) fashion. However, one can expect that the precise binding geometry would not be identical on the two rutile surfaces due to their different morphologies. There is a notable shift in the peak frequency between the two surfaces implying a difference in the electronic structure of the bound ReCO3 catalyst when immobilized on rutile (110) as compared to rutile (001). Metals interact with CO ligands through π-backbonding in accordance with ligand field theory, whereby electrons are shared between the metal center and carbon of the CO ligand. As π-backbonding strength increases, the strength of the Re-C bond increases, thereby weakening the C≡O bond and lowering the CO vibrational stretch frequency. It can therefore be concluded that the metal backbonding is stronger when ReCO3 is attached to the (001) surface compared to the (110) surface. Since π-backbonding transfers electron density from the metal center to the


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metal-ligand bond, the Re center when bound to (001) has a lower electron density than when bonded to (110) caused by the interaction with the surface—either through differences in the density of states of the two surfaces or changes in the binding geometry. The weaker πbackbonding on (110) would indicate a weaker binding of the CO ligands on (110) compared to (001) and potentially a higher catalytic activity.

Figure 4. Static SFG spectra of ReCO3 monolayer covalently bound to the (001), centered at 2048 cm-1, and (110), centered at 2059 cm-1, surfaces of rutile TiO2 shown in red and blue, respectively. To gain further insight into the influence of the different TiO2 surfaces on ReCO3, we study the vibrational dynamics using transient SFG. The SFG intensity I generated by the sample is given by:21,53

∝ , ,

(1)

where is the second-order susceptibility of the interface, is the near-IR upconversion frequency, is the IR frequency, and

=

+

. In static SFG experiments, only the

ground state of the vibrational modes under consideration is populated. The nonlinear susceptibility of the surface is given by:

∝ 〈〉


(2)

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where is the number density of interfacial molecules and 〈〉 is the orientationally averaged molecular second-order susceptibility, also called the hyperpolarizability. For the case of vibrationally resonant SFG, the hyperpolarizability is a sum-over-states of the product of the dipole and Raman transition moments of the relevant vibrational modes:54 = −

1 2ħ − − !"

(3)

where is the contribution from nonresonant pathways. Here 0 denotes the vibrational ground state, $ denotes the final state, is the transition frequency, and " is the linewidth of each vibrational transition. In transient SFG, population existing in multiple vibrational energy levels is accounted for by summing over all individual populated states %:

∝ 〈 & 〉. &

(4)

The hyperpolarizability is then given by: & = −

& ( ) − )&& 1 2ħ & − − !"& *(

(5)

where the summation is taken over all possible higher lying final states $ > ,, and ) and )&& are the populations of the final and initial states, respectively. The resulting general expression for the nonlinear susceptibility of the surface is:

= +

& *&

.& ) − )&& & − − !"&

(6)

where is the nonresonant susceptibility and .& the amplitude of a given vibrational mode. In some cases completely off-resonance conditions cannot be obtained, and the nonresonant contribution can contain a small phase. This can be implemented by adding a phase factor to the


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nonresonant susceptibility: / 01 . In the present study, the nonresonant contribution is negligible, and only a single vibrational mode is SFG active. Transient SFG experiments monitor the change in signal induced by pump excitation. Without the pump beam, only the ground state is populated at room temperature, giving rise to

the unpumped response 2 :

2 =

. . − − !"

(7)

The pump pulse moves some population from the ground vibrational state to the first excited

state. Therefore, the pumped SFG response 3 contains contributions from both the % = 0; $ = 1 and % = 1; $ = 2 transitions:

3 =

. )

− ) . )

− . − − !" − − !"

(8)

To reduce the number of free variables, we assume the amplitude of the overtone is twice the ground state, i.e. . = 2. , as is the case in the harmonic approximation. The population densities of the ground and first excited vibrational states are a function of the time delay 4 between the pump and probe pulses with the total population density equal to unity, i.e. ) 4 + )

4 = 1. This simplifies equation 8 to:

3 , 4 =

. 51 − 2)

46 2. )

4 + . − − !" − − !"

(9)

Accordingly, the pump induced change in SFG intensity ∆89: is given by:

∆89: , 4 = 89:3 , 4 − 89:2 ∝ ; 3 , 4; − ; 2 ; .

(10)

Substituting equations 7 and 9 into equation 10 and expanding yields an expression for ∆89: which can be further simplified; since the pump-induced population change is small, i.e.


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)

4 ≪ 1, terms containing )

4 as a factor are negligible and can be eliminated. Therefore, the simplified expression for ∆89: is given by: ∆89: , 4 4. 5" " + − − − " − − 6 (11) ∝ )

4. 5" + − 6 5" + − 6 Using equation 11, we perform a global two-dimensional fit of the pump-induced change in the SFG spectrum as a function of the pump – probe time delay. Here we assume that the vibrational frequencies and the linewidths do not vary with the pump – probe time delay, and the excited state population )

4 decays monoexponentially: )

4 = )

0/ >?/A .

(12)

Our model is similar to the one derived by Lian and co-workers for fitting IR pump – SFG probe measurements of ReCO3 on rutile (110).47 In their model a biexponential decay of the excited state population was incorporated, as a few hundred fs component was observed in the bulk. Within signal-to-noise, the inclusion of a biexponential decay did not significantly improve our fit. The transient SFG data along with the fits and residuals are shown in Figure 5, and the resulting fit parameters are shown in Table 1 for each rutile surface. Both the transient SFG data and the fit clearly show distinct vibrational dynamics of the ground state for the two surfaces. A bleach due to the resonant IR pump removing population from the vibrational ground state can be seen as a negative feature at 2048 cm-1 and 2059 cm-1 for the catalyst on rutile (001) and (110), respectively. Additionally, a positive feature at 2023 cm-1 and 2027 cm-1 for (001) and (110) due to the induced absorption arising from the new population in the excited vibrational state is observed. The excited-state population relaxes back to the ground state as the time delay between pump and probe is increased.


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Figure 5. Comparison of transient SFG data, fits, and residuals of ReCO3 bound to the (001) and (110) surfaces of rutile TiO2. Table 1. Fitting parameters of the transient SFG data. rutile surface ω10 (cm-1)

ω21 (cm-1)

Γ10 (cm-1)

Γ21 (cm-1)

τ (ps)

(001)

2048 ± 1

2023 ± 2

13

12

26 ± 3

(110)

2059 ± 3

2037 ± 4

17

17

18 ± 1

The good agreement between the transient data and model can be visualized by taking slices along the time and frequency axes. Figure 6 shows the transient spectra at different time delays with the corresponding global fit traces; Figure 7 compares the relaxation dynamics of the two surfaces by displaying the recovery of the measured ground state bleach along with the global fit result at the peak of the fundamental transition. The fitted relaxation times are 26 and 18 ps for the ReCO3 catalyst on rutile (001) and (110) surfaces, respectively.


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Figure 6. ∆SFG spectra showing data (squares) and fits (lines) averaged between the indicated time delay intervals.

Figure 7. Normalized ∆SFG time traces showing data (symbols) and fits (lines). Each data point represents the average of three neighboring frequency points. The difference in the vibrational frequencies and relaxation times reflects a change in the electronic structure of the catalyst as modified by the specific underlying TiO2 surface. ReCO3 is covalently bound to the rutile surface through the carboxylate groups on the dcbpy ligand.


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Carboxylate groups can bind to oxide surfaces via several different motifs. Calculations of the catalyst structure on rutile (001) have shown that the binding on this particular surface orients the symmetric carbonyl stretch mode approximately perpendicular to the surface in agreement with SFG experiments.46 The binding structure on rutile (110) has not been reported in the literature but is likely different than that on rutile (001) due to the variation in atomic ordering, coordination, and electron density of the titanium atoms on the underlying surfaces, as discussed above. Typically, stronger π-backbonding leads to faster vibrational relaxation due to the increased coupling to low frequency modes resulting from a tighter binding of the CO ligand to the metal center.18 We observed that the vibrational relaxation is faster for ReCO3 on rutile (110) despite having a weaker backbonding interaction. The increased vibrational relaxation dynamics for the (110) surface could result from an increased packing density due to a lower binding order compared to (001) or an effect from the orientational angle of the catalyst on the surface. Lian and coworkers45 determined that the orientation of the CO ligands of the catalyst on rutile (001) was perpendicular to the surface. The orientation is presumably different on (110), bringing the CO ligand closer to the surface and potentially providing a faster relaxation pathway to the TiO2 surface. Due to the heterogeneous nature of colloidal TiO2, a distribution of surface structures, binding motifs, and catalyst aggregates occur, making it difficult to distinguish between the different effects. Furthermore, this complicates making direct comparisons between experiment and theoretical calculations, as the exact surface structure is not known. Catalyst-functionalized single-crystalline surfaces offer a platform for systematically studying the effect of the TiO2 surface structure on the catalyst properties and allows for direct comparison of the results to theoretical calculations on the known surface structure.


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While this study focuses on the vibrational frequency and dynamics of a catalyst bonded to single-crystalline surfaces, these report on the electronic structure of the catalyst. This means that the TiO2 facet dependence we observe in the vibrational properties can be extended to the catalytic properties, i.e. that the catalytic properties of the molecular catalyst depend on the specific crystal facet of TiO2 to which the catalyst is bonded. Expanding the present transient SFG studies to involve electronic excitation will directly probe the electronic state dependence of the catalyst on the TiO2 surface structure.

4. Conclusions In this article we employ time-resolved IR pump – SFG probe experiments to explore the vibrational frequency and relaxation rate of a surface bound catalyst as a function of the exposed rutile crystal structure. The differences observed illustrate that the electronic structure of the catalyst is altered at the two rutile surfaces, impacting both the static and dynamic properties of the catalyst. The vibrational difference is due to the electronic structure of the catalyst being directly affected by the surface density of states of rutile, by the carboxylic acid binding motif to the surface, and/or by the packing density of the catalyst on the surface. As the CO2 reduction reaction involves losing a carbonyl ligand, the electronic structure and dynamics of these ligands will have a prominent effect on the catalytic properties of the system. Investigating catalysts on single-crystalline surfaces allows for a systematic study of the influence of the TiO2 surface structure on the properties of surface bound catalysts. This approach is applicable for not only CO2 reduction catalysts, but any heterogeneous system containing numerous domains, such as many other electro- and photo-catalysts as well as dye sensitized solar cells. With the ability to synthesize faceted nanocrystals and nanorods, it is possible to create high-surface area substrates of a desired surface structure. In addition to


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understanding fundamental semiconductor-molecule interactions, deriving the structure-function relationship between semiconductor surface structures and catalyst or dye properties is essential for optimizing the efficiency of these systems.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +001 (607) 255-4303. Funding Sources American Chemical Society Petroleum Research Fund (Award Number 53053-DNI6). National Science Foundation (NSF CAREER Award: CHE-1151079). Arnold and Mable Beckman Foundation (Young Investigator Award). NSF MRSEC program (DMR-1120296). National Science Foundation (Grant ECCS-0335765). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research under Award Number 53053-DNI6. We further acknowledge support from the National Science Foundation (NSF CAREER Award: CHE1151079) and the Arnold and Mable Beckman Foundation through a Young Investigator Award. This work made use of the Cornell Center for Materials Research Shared Facilities which are supported through the NSF MRSEC program (DMR-1120296), and the Cornell NanoScale


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Deconstructing the Heterogeneity of Surface-Bound Catalysts: Rutile

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