quaterpyridinium Complexes at TiO2 Particles - American Chemical

Dec 8, 2010 - ... of Chemistry, Kansas State UniVersity, 213 CBC Building, Manhattan, .... The influence of a sudden change in light conditions was...
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J. Phys. Chem. C 2010, 114, 22763–22772

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A New Heterogeneous Photocathode Based on Ruthenium(II)quaterpyridinium Complexes at TiO2 Particles Nils Baumann,†,‡ Pubudu S. Gamage,‡ Thilani N. Samarakoon,‡ Jim Hodgson,‡ Ju¨rgen Janek,§ and Stefan H. Bossmann*,‡ Institute of Physical Chemistry, Justus Liebig UniVersity, Heinrich-Buff Ring 58, D-35392 Giessen, Germany, and Department of Chemistry, Kansas State UniVersity, 213 CBC Building, Manhattan, Kansas 66506-0401, United States ReceiVed: October 7, 2010; ReVised Manuscript ReceiVed: NoVember 7, 2010

We have studied a new type of heterogeneous composite electrode featuring a ruthenium(II)-quaterpyridinium complex (Ru3PHOS) bearing six phosphonate groups for strong chemisorption on TiO2. Ru3PHOS@TiO2 has then been deposited on microscopy glass slides using Ag-doped epoxy glue. Nanomolar amounts of palladium have been electrodeposited onto the Ru3PHOS@TiO2. To our surprise, we have observed the deposition of carbon needles onto the composite material when (photo)electrochemically reducing bicarbonate from aqueous buffers. 1. Introduction Given the vast amount of energy being delivered to earth in the form of sunlight, it is more than apparent that solar energy is a very realistic chance that we have of finding a clean and renewable energy source.1 Investments that will lead to the development of technologies capable of reducing or even reversing some or many of the predicted catastrophic events, which are commonly labeled “global warming” (i.e., the reduction of global harvests or the rising of the sea level), will pay off very well!2 However, conceptual breakthroughs have to be made to turn sunlight into a practicable solution to meet the compelling need for clean, abundant sources of energy. Biomimetic artificial photosynthetic systems possess a series of distinct advantages when compared to natural systems:3 (a) they contain fewer components, (b) therefore, the assembly process is much easier, (c) inorganic components can be integrated, leading to higher stabilities, and (d) experimental conditions can be tuned toward the absorption of a higher incident photon rate than plants usually tolerate. On the other hand, the separation of the reductive and the oxidative steps during the evolution of photosynthetic bacteria and eventually green plants principally allows higher photoconversion yields, because short-circuiting can be avoided.4 Since the same observations have been made in artificial photosynthesis systems, separating the reductive and the oxidative steps of the photoelectrochemical cells for water and CO2-reduction will have a far better prospect for reaching high photoconversion yields,4,5 leading coincidentally to the elimination of sacrificial donors. Another big challenge is the preparation of electrodes from inexpensive materials to harvest solar energy at a cost that is competitive with existing technologies. We have prepared a composite electrode consisting of commercially available titanium dioxide (Fisher T315) to which one of two ruthenium(II)quaterpyridinium complexes (featuring terminal carboxylic acid (RuC2COOH6) or phosphonate groups * Corresponding author. Fax: (785) 532-6666. E-mail: [email protected]. † Present address: Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, UT 84112-0850. ‡ Kansas State University. § Justus Liebig University.

(Ru3PHOS)) was adsorbed. This material then has been attached to glass plates by using commercial epoxy glue featuring silver rods of 1-5 µm in length as conductive elements.8 The last step of the electrodes’ preparation was the electrochemical in situ reduction of palladium(II) and the consequential deposition onto the ruthenium(II)complex@TiO2 composite electrodes. We have determined the (photo)electrochemical properties of these inexpensive electrodes by employing differential pulse voltammetry (DPV). Their morphology was determined by using high-resolution scanning electron microscopy (HRSEM). Fourier-transform infrared spectroscopy (FTIR) and UV/vis spectroscopy in combination with energy-dispersive X-ray spectroscopy (EDX) were employed to detect the formation of organic substances/carbon at the surface of the working electrode. Interestingly, our novel composite electrodes have been able to form hydrogen and graphite-like carbon material that has been deposited onto the electrode’s surface from aqueous bicarbonate solutions. 2. Experimental Methods 2.1. Synthesis of the Ruthenium(II)quaterpyridinium Complexes RuC2COOH and Ru3PHOS. Detailed synthetic procedures for the two ruthenium complexes are described in the Supporting Information, and the structures are shown in Figure 1. 2.2. UV/Vis Absorption and Luminescence Spectroscopy. Both experiments were carried out in 4.0 mL quartz cuvettes (Helma) using a spectrofluorometer (Fluoromax2) with dual monochromators and a diode array UV/vis absorption spectrometer (HP 8453). 2.3. Electrochemical Characterization of the Ru(II) Complexes. RuC2COOH and Ru3PHOS (1.0 × 10-5 M each) were investigated in an aqueous buffer (0.50 M KCl, 0.050 M phosphate buffer, pH ) 7.0). Differential pulse voltammetry (DPV,7 CHI model 600B series) employing a three-point setup was used to determine the redox potentials of the complexes. Pt-working and counter electrodes were used; the reference electrode was Ag/AgCl (+0.227 V vs SHE (standard hydrogen electrode)). A potential range from -0.80 to 1.80 V (vs Ag/ AgCl corresponding to -1.027 to 1.573 vs SHE) was applied.

10.1021/jp109634v  2010 American Chemical Society Published on Web 12/08/2010

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Figure 1. Chemical structures of tris(N,N′′′-bis(2-carboxyethyl)-4,4′: 2,2′′:4′′,4′′′-quaterpyridine-N′N′′-dium-N′,N′′′) ruthenium(II)octachloride (RuC2COOH) and tris(N,N′′′-bis(2-propyl-phosphonic acid)-4,4′:2,2′′: 4′′,4′′′-quaterpyridine-N′N′′-dium-N′,N′′′) ruthenium(II) octachloride (Ru3PHOS). The complexes have been used as chlorides in the experiments reported here.

Figure 2. (Photo)electrochemical cell and measurement setup.

2.4. Preparation of the Composite Electrodes. 1.0 g of TiO2 (Fisher T315, particle size, 100-300 nm; BET surface, 8.8 m2 g-1) was suspended in 5.0 mL of a solution of RuC2COOH or Ru3PHOS (c(Ru complex) ) 1.0 × 10-2 M) in MeOH (Aldrich, ACS-grade) at 300 K, and stirred continuously for 72 h. The RuC2COOH@TiO2 or Ru3PHOS@TiO2 (nano)particles were then separated by centrifugation and collected. The RuC2COOH@TiO2 or Ru3PHOS@TiO2 (nano)particles have been dried at 365 K (air atmosphere) for at least 5 days to facilitate chemisorption and then stored under these conditions. After being dried, the RuC2COOH@TiO2 or Ru3PHOS@TiO2 (nano)particles have been resuspended in 4.0 mL of MeOH and precipitated two times from MeOH. The second MeOH wash solution was free of ruthenium(II)quaterpyridinium complex, as UV/vis indicated. Langmuir-adsorption isotherms of RuC2COOH@TiO2 or Ru3PHOS@TiO2 at TiO2 have been recorded under the same experimental conditions before the drying procedure.9,10 The amount of adsorbed ruthenium(II)quaterpyridinium complexes has been determined by using UV/vis absorption spectroscopy (RuC2COOH: λmax (3MLCT) ) 490 nm, ε (M-1 cm-1) ) 18 700; Ru3PHOS: λmax (3MLCT) ) 498 nm, ε (M-1 cm-1) ) 18 750). A microscopy glass plate (Fisher) was immersed for 1 d in a solution of KF (1.0 M) in H2O, pH ) 3.0 (by adding conc. HCl) to roughen the surface and then thoroughly washed in H2O (three times) and MeOH. The glass slide was dried at 365 K for at least 12 h. Silver-epoxy glue (silver conductive epoxy, MG Chemicals #8331) was deposited onto one side of the roughened glass plate by a spin-casting procedure (described in detail in ref 11), the glass slide was subjected to 1000 rpm for 300 s. The glass slide was then brought in contact with RuC2COOH@TiO2 or Ru3PHOS@TiO2 on a filter paper at 300 K. After 24 h, the surplus material was tapped off, and the composite electrode was thoroughly washed with MeOH (two times) and then with water (three times), dried at 365 K, and then stored under argon at room temperature. 2.5. Preparation of the Pd-Modified Electrodes. An electrode (Ru3PHOS@TiO2 on glass plate) was mounted into the measurement cell (see Figure 2), and the cell was filled with a 1.0 × 10-7 M PdCl2 water solution, with 0.20 M NaCl and 0.050 M NaH2PO4 as buffer. Beforehand, a DPV of the Pdbuffer solution was recorded with a Pt working electrode to

determine the reduction peaks of Pd. In this solution, an amperometric i-t scan (17 000 s run time) was run at the prior determined reduction potential (0.43 V vs Ag/AgCl, corresponding to 0.20 V vs SHE) to deposit the Pd. DPV before and after the Pd deposition was employed to compare the influence. 2.6. REM and EDX Characterization of the Electrodes. Used electrodes were investigated with high-resolution scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) (both: Leo Gemini 982, 5 and 10 kV, respectively) to determine the structure of the deposit and examine any impurities. 2.7. Electrochemical Characterization of the Composite Electrodes. The novel composite electrodes were investigated with DPV by placing them in a specially designed glass cell. Figure 2 shows the position of the three electrodes together with important geometrical parameters. The composite electrodes were placed on a perforated glass to permit the solution to be stirred from underneath by means of a magnetic stirring rod. The RuC2COOH@TiO2 or Ru3PHOS@TiO2 particles on the silver epoxy layer were contacted with a copper wire. An Ag wire was used as counter electrode, brought closely to the surface. As reference electrode, an Ag/AgCl electrode was employed. We have measured the volume resistivity of the electrode at 300 K to be 2.5 ( 0.25 Ω cm, which is almost one order of magnitude higher than the volume resistivity of the silver epoxy glue (0.38 Ω cm).12 The (photo)electrochemical cell was filled with 0.50 M NaHCO3 in bidistilled water. 0.50 M NaH2PO4 and 0.25 M KCl were added (the resulting starting pH was 7.6). DPV was run in a potential range of -1.0 V to +1.0 V (vs Ag/AgCl). Note that raising the redox potential to voltages above +1.2 V (vs Ag/AgCl) led to the oxidative destruction of the composite electrodes, as indicated by the green color (Ru(III)) of the resulting solution. For all long-term experiments, an amperometric i-t scan was recorded at the highest reduction peak, previously determined by two DPV scans. 2.8. Performance Tests of the Composite Electrodes. The performance of the composite electrodes under incident UV/ vis light, employing a mercury-high pressure arc lamp (125 W, Philips) as polychromatic light source, was determined by DPV. Because of the presence of a quartz window in the photoelectrochemical cell, the UV-cutoff occurred at approximately 220

Photocathode Based on Ru3PHOS Complexes nm (50% intensity). The main lines of the Hg-lamp are λ ) 253.7, 365.4, 404.7, 435.8, 546.1, and 578.2 nm, but a pronounced emission continuum under high pressure conditions exists as well.13 The electrochemical behavior of the composite electrode under continuous irradiation was compared to the electrode characteristics in the dark. For that purpose, the measurement cell was covered with aluminum foil and measured in a dark room. Long-term measurements were performed by exciting the electrode for 60 000 s with Hg-lamp light (cutoff filter: quartz13). The influence of a sudden change in light conditions was investigated by turning off the light source and continuing the measurement in darkness after 60 000 s for another 17 400 s, during an amperometric i-t scan. The influence of differently vigorous stirred solutions on the electrode performance was investigated by controlling the speed of the magnetic stirring bar and measuring DPV scans. Relation of performance change due to light and stirring was studied by stirring the solution while simultaneously irradiating it by means of a Hg-medium pressure lamp (quartz filter). These measurements have been compared to the DPV scans that have been recorded in the dark. Long-term studies were carried out by applying a DPV scan in five stirring conditions: none, slight, intermediate, fast, and very fast. DPV was compared after each cycle, consisting of DPVs consecutively taken at all five stirring conditions. Fisher Scientific Isotemp magnetic stirrers have been used to achieve well-defined stirring rates. 2.9. Determination of the Incident Light Power (W cm-2). The power of the incident light in the window between 220 and 600 nm, divided by the area of the quartz window of the photoelectrochemical cell, has been determined by ferrioxalate actinometry to be 0.62 ( 0.04 W cm-2, as described in ref 14. 2.10. Analysis of Gaseous Reaction Products by Using Gas Chromatography. Gas samples were taken out by a Hamilton syringe at different time intervals and analyzed by gas chromatography (GOW-MAC, Series 580). A typical GC trace obtained after 8000 s of continuous irradiation is shown in the Supporting Information. Two peaks appear in the GC spectra, and the peaks were identified as H2 and N2 (as well as H2O when analyzing aqueous samples) using calibration gas mixtures. We did not find evidence for the formation of methane or any other hydrocarbons. The upper bound of the H2 concentration was determined to be (2 ( 0.8) × 10-6 M in the gas phase, indicating that its formation is not the main pathway of reductive chemistry at the heterogeneous (photo)cathodes. The concentration of H2 in the aqueous phase was slightly lower. We have estimated that the total charge flowing through the electrode was 48 C (As) in 8000 s. Two electrons are required for the formation of H2: 48 C/(2 × 96 485 C mol-1) equals approximately 2.5 × 10-4 mol of H2, which is several orders of magnitude higher than the experimentally determined hydrogen concentration, considering that the volume of the reactor is 6.5 mL. 3. Results and Discussion 3.1. Ruthenium(II)quaterpyridinium Complexes at TiO2: Photochemical Reactivity. Ruthenium(II)quaterpyridinium complexes exhibit superior reactivity as sensitizer relay assemblies (SRA’s) in sacrificial systems for water and hydrogencarbonate reductions,15 while harvesting the ultraviolet and most of the visible fraction of the incident solar spectrum. Contrary to the function principles of a dye-sensitized solar cell,16,17 the electrons required for the photochemical reduction of the SRA’s, which

J. Phys. Chem. C, Vol. 114, No. 51, 2010 22765 SCHEME 1: (A) (Photo)reductive Electron Transfer from the Composite Electrode to the Adsorbed Redox-Couple Ru(II) Complex/Ru(I) Complex;a (B) Dye-Sensitized Electron Transfer from an Electronically Excited Ruthenium(II) Complex to the Composite Electrode (According to Ref 14)

a When excited, the ruthenium(II)quaterpyridinium complex is populating its electronically excited 3MLCT state, which leads to an increase in its redox potential (Ru(II)complex*/Ru(I)complex) and, consequently, a higher current (cpx, complex; vb, valence band; cb, conduction band).

is the first step of the complex series of chemical reactions required for the reduction of carbon dioxide, will be provided by the cathode of the (photo)electrochemical cell. This requires that the working potential of the (photo)cathode is enabling the reduction of the TiO2-adsorbed ruthenium(II)quaterpyridinium complexes to ruthenium(I)quaterpyridinium complexes. When excited, both complexes form long-lived 3MLCT-states (metalto-ligand charge transfer) featuring excited-state lifetimes of the order of approximately 400 ns.15 This permits photochemically enhanced electron-transfer reactions from the composite electrode to the ruthenium(II)quaterpyridinium complexes (Scheme 1). As a consequence, the photoredox current is expected to be higher during excitation. In the presence of palladium(0), which was electrochemically deposited from palladium(II), the (photo)electrochemically reduced ruthenium(I)complexes undergo electron transfer to the palladium nanoparticles present at the surface of RuC2COOH@ TiO2 and Ru3PHOS@TiO2. This is a very efficient pathway, as we have established using platinum catalysts during the 1990s.18,19 It is our mechanistic paradigm that the reduction of carbon dioxide and water occurs then at the palladium nanocatalyst. However, our work presented here was not able to prove this unambiguously. We cannot exclude that some reactions occur at the TiO2 interface or that a minor fraction of ruthenium(0) nanocatalyst is formed from the decomposition of the (thermodynamically and kinetically) very stable ruthenium complexes.20 3.2. Light Absorption by the Ruthenium Sensitizers. In Figure 3, the UV/vis absorption spectra of RuC2COOH and Ru3PHOS (2.40 × 10-5 M in MeOH) and RuC2COOH@TiO2 and Ru3PHOS@TiO2 (2.40 × 10-5 M and 0.0020 g L-1 TiO2) dispersed in methanol) are shown. Adsorption of RuC2COOH and Ru3PHOS at TiO2 leads to spectral broadening. However,

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Figure 3. Left: UV/vis absorption spectra of RuC2COOH and Ru3PHOS (2.40 × 10-5 M in MeOH). Right: UV/vis extinction (extinction ) absorption + scattering) spectra of RuC2COOH@TiO2 and Ru3PHOS@TiO2 (2.40 × 10-5 M and 0.0020 g L-1 TiO2) in MeOH.

Figure 4. Steady-state excitation (blue) and luminescence spectra (red) of RuC2COOH (left, λmax ) 684 nm) and of Ru3PHOS (right, λmax ) 674 nm); c ) 1.25 × 10-6 M in MeOH.

it must be noted that in the presence of nanoscopic TiO2 scattering occurs, which has a broadening effect as well! 3.3. Excitation/Luminescence of the Ruthenium(II)quaterpyridinium Sensitizers. In Figure 4, the excitation and luminescence spectra of RuC2COOH and Ru3PHOS (2.40 × 10-5 M in MeOH) are shown. It is apparent that both complexes are able to convert energy from a broad region of the solar spectrum (up to 650 nm) into electrical energy through photoelectron transfer processes at the TiO2 interface. It should be noted that no luminescence was detected from RuC2COOH@ TiO2 and Ru3PHOS@TiO2. This can be indicative of either a strong electronic coupling between the adsorbed ruthenium complexes and the TiO2 nanoparticles21 and/or an effective homoenergy transfer20 occurring within the layer of TiO2adsorbed ruthenium complexes. 3.4. DPV Characterization of the Ru Complexes. Reduction peaks of the redox couples RuII/RuI and RuIII/RuII were well discernible for both RuC2COOH and Ru3PHOS, as shown in Figure 5. Slight shifts of the peaks for the two complexes can be attributed to structural differences of their quaterpyridinium ligands, especially the presence of acidic protons in RuC2COOH.

The excited-state potentials reported in Table 1 have been calculated according to the following equations:20 3+/2+ E*Ru ) E0Ru3+/2+ - ∆E0-0 0

(1)

2+/1+ E*Ru ) E0Ru2+/1+ + ∆E0-0 0

(2)

3.5. Preparation of the Composite Electrodes. RuC2COOH and Ru3PHOS have each been adsorbed on TiO2 (Fisher T315, particle size: 100-300 nm) from methanol, and the Ru complex doped nanoparticles have then been glued onto glass plates using conductive silver epoxy glue. The preparation procedure is described in detail in the Experimental Methods. Before being firmly attached to the glass surface, RuC2COOH@TiO2 and Ru3PHOS@TiO2 have been dried at 365 K (air atmosphere) for at least 5 days to facilitate chemisorption. The underlying mechanistic picture has been developed by Michael Gra¨tzel and co-workers:22 Both Ru(II)complexes become physisorbed at the TiO2 interface first, and then a slower chemisorption process occurs. The latter can be expedited by heating.

Photocathode Based on Ru3PHOS Complexes

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Figure 5. DPV measurements (employing a three-point setup) of RuC2COOH and Ru3PHOS (1.0 × 10-5 M each) in aqueous buffer (0.50 M KCl, 0.050 M phosphate buffer, pH ) 7.0) (vs Ag/AgCl reference electrode).

TABLE 1: Ground-State and Excited-State (3MLCT) Redox Potentials of RuC2COOH and Ru3PHOSa complex

E0 Ru3+/2+ V (vs SHE)

E0 Ru2+/1+ V (vs SHE)

∆E0-0 eV

E0* Ru2+/3+ V (vs SHE)

E0* Ru2+/1+ V (vs SHE)

RuC2COOH Ru3PHOS

1.571 1.525

-0.445 -0.423

2.20 2.25

-0.629 -0.725

1.755 1.525

: Standard potential for the Ru3+/Ru2+ redox transition in aqueous KCl (0.50 M) at pH ) 7.0 (0.050 M phosphate buffer). E0 Ru : Standard potential for the Ru2+/Ru1+ redox transition in aqueous KCl (0.50 M) at pH ) 7.0 (0.050 M phosphate buffer). ∆E0-0: Excited-state energy of the 3MLCT states (eV), determined from the onset of the 3MLCT luminescence bands at room temperature. E0* Ru3+/2+: Standard potential for the Ru3+*/Ru2+ redox transition occurring from the 3MLCT state in aqueous KCl (0.50 M) at pH ) 7.0 (0.050 M phosphate buffer). E0* Ru2+/1+: Standard potential for the Ru2+*/Ru1+ redox transition occurring from the 3MLCT state in aqueous KCl (0.50 M) at pH ) 7.0 (0.050 M phosphate buffer). a

E0 Ru 2+/1+

3+/2+

Langmuir-adsorption isotherms of RuC2COOH@TiO2 or Ru3PHOS@TiO2 at TiO2 have been recorded. The Langmuir isotherm9,10 (eq 3), although originally developed for the adsorption on surfaces featuring discrete adsorption sites, is a simple and straightforward method to quantify and compare the adsorption of both ruthenium(II)quaterpyridinium complexes at TiO2.

cads )

cmaxKc 1 + Kc

(3)

where cabs is the concentration of Ru complex that is adsorbed at TiO2, cmax is the maximal concentration of Ru complex that is adsorbed at TiO2, c is the concentration that is added to the heterogeneous system (all concentrations are in mol L-1 g-1), and K is the adsorption constant (g L-1 mol-1).

c cads

)

c cmax

1 + Kcmax

(4)

The Langmuir linear regression method was proposed by Irvin Langmuir in 1918:10 The plot of cads versus c yields a slope of (cmax)-1 and an intercept of (Kcmax)-1. The results are summarized in Figure 6 and Table 2. As Figure 6 indicates, the Langmuir model of adsorption can be applied here. The mechanistic paradigm is that the ruthenium(II)quaterpyridinium complexes form a monolayer at the surface of the TiO2 nanoparticles. We have approximated the space demand of the surface-adsorbed complexes by using Chemdraw 3D. Considering that one RuC2COOH complex is

Figure 6. Adsorption of RuC2COOH and Ru3PHOS at TiO2-T315 from methanol according to the Langmuir linear regression method.

blocking 7.36 × 10-18 m2 and that the BET surface of TiO2T315 is 8.8 m2 g-1, approximately 82% of the surface is covered after 24 h adsorption from methanol. For Ru3PHOS (space demand 1.03 × 10-17 m2), we have estimated a surface coverage of 72%. We are aware that the physical conditions of recording a BET isotherm (N2 at 77 K) and of the Langmuir isotherm reported here are quite different. However, the data indicate that our measurements are meaningful. It should be noted that the adsorption constant K for RuC2COOH on TiO2 of 8.50 ( 0.06 × 104 (g L-1 mol-1) is comparable to the adsorption constant reported by Michael Gra¨tzel and co-workers for the carboxylate complex N3 (5 × 104 M-1).21 Ru3PHOS adsorbs more strongly

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TABLE 2: Maximal Concentrations That Are Adsorbed by TiO2 (cmax) and Adsorption Constants K from Methanol Solution cmax (mol L g-1) RuC2COOH Ru3PHOS

K (g L-1 mol-1)) -6

(1.62 ( 0.08) × 10 (1.10 ( 0.06) × 10-6

(8.50 ( 0.06) × 104 (1.52 ( 0.07) × 106

at TiO2 than RuC2COOH. This is in agreement with findings from Bernasek and co-workers, who have reported strong adsorption behavior of phosphonates at oxidized titanium.23 3.6. Characterization of the Composite Electrodes by HRSEM. The characterization of the composite electrodes by HRSEM has demonstrated that the novel materials are indeed heterogeneous. Although the TiO2 nanoparticles of T315 are only 100-300 nm in size, the adsorption process of the ruthenium(II)quaterpyridinium complexes leads to the formation of clusters that are between 5 and 150 µm in diameter. These clusters are then glued onto the underlying glass surface, as well as partially onto each other from the glass side (backside). We are showing here a series of the HRSEM images obtained for Ru3PHOS@TiO2/Ag(epoxy)/glass. However, there were no principle differences in morphology between RuC2COOH@ TiO2/Ag(epoxy)/glass and Ru3PHOS@TiO2/Ag(epoxy)/glass. The big Ru3PHOS@TiO2 cluster from the lower middle of Figure 7 has been enlarged and is shown in Figure 8. The material is clearly sponge-like and featuring a large surface area, which is advantageous for heterogeneous (photo)electrodes. It is noteworthy that bigger palladium (nano)particles could not be discerned. Because we have made sure by using DPV that the palladium in the aqueous phase has indeed been consumed during the electrochemical in situ deposition of Pd(0) from Pd(II), we have concluded that the particle size of the resulting catalytic Pd nanoparticles is too small to be resolved by this instrument. 3.7. Electrochemical Characterization of the Electrodes. In general, all electrodes that have been prepared as described above showed the same peaks in contact with the bicarbonate buffer. These were reduction peaks of various carbonate reduction processes and the hydrogen reduction reaction, as summarized in Table 3. However, there were discernible differences in the observed peak heights. We attribute these changes to slight variations in the production of the heterogeneous electrodes, which has been performed in three consecutive manual procedures. The use of

Figure 7. Typical HRSEM image of a RuC2PHOS@TiO2/Ag(epoxy)/ glass composite electrode.

Figure 8. Typical HRSEM image of a Ru3PHOS@TiO2/Ag(epoxy)/ glass composite electrode.

TABLE 3: Various Possible Reduction Processes of Carbon Dioxide versus SHE at pH 7a reaction CO2 + 1e- f CO2· -

reference E0/V (vs SHE) ∼2

+

-

-0.61

+

-

-0.52

+

-

CO2 + 2H + 2e f HCOOH CO2 + 2H + 2e f CO + H2O CO2 + 4H + 4e f HCHO + H2O

-0.48

2H+ + 2e- f H2

-0.41

CO2 + 6H+ + 6e- f CH3OH + H2O

-0.38

+

-

-0.24

+

-

-0.20

CO2 + 8H + 8e f CH4 + 2H2O CO2 + 4H + 4e f C + 2H2O a

All potentials were taken from ref 4. Note that the potential shift between SHE and Ag/AgCl is -0.227 V in all experiments.

appropriate machinery for the production of the heterogeneous electrodes will be explored in future experiments. Whereas experimental factors, such as pH, ionic strength, and the concentration of deposited ruthenium(II)complexes at the titanium dioxide surfaces were kept strictly constant, the peak heights (and, to a much smaller extent, also the peak positions) in the DPVs have been dependent on the chosen potential prior to the electrochemical scans. At very low potentials (E < -1.0 V vs Ag/AgCl), the electrodes’ surface turned purple due to the formation of the Ru(I)quaterpyridinium complex, originating from the reduction of the Ru(II) metal centers. At very high potentials (E > 1.0 V vs Ag/AgCl), a green solid was formed at the bottom of the electrochemical reactor due to the oxidation of the Ru(II)quaterpyridinium complex to Ru(III), followed by the irreversible oxidation of the epoxy glue. Beyond these potentials, the electrodes could not be used any further, due to decomposition of the ruthenium(I) complexes. The accessible potential range extends from approximately +1.0 to -1.0 V, as shown in Figure 5. Even at those potentials, a minor fraction of the Ru(I)quaterpyridinium complex was formed at the low potential end of the window, which was visible as the characteristic purple color was intensifying with ongoing DPV studies. It is noteworthy that the electrochemical deposition of palladium(0) from palladium(II) (5.23 × 10-5 mol PdCl2, based on 1.26 × 10-4 mol total Pd, and an area of the copper wire of 1.41 × 10-4 m2 (d ) 1.5 mm, l ) 3 cm) and an electrode area of 1 cm2) does not significantly alter the electrochemical

Photocathode Based on Ru3PHOS Complexes

Figure 9. Differential pulse voltammogram (DPV) of a heterogeneous Ru3PHOS@TiO2/Ag(epoxy)/glass composite electrode before and after the electrochemical deposition of 5.23 × 10-5 mol of palladium in aqueous 0.20 M NaCl, 0.050 M NaH2PO4 buffer (vs Ag/AgCl).

Figure 10. Long-term experiments (constant potential V ) -0.115 V vs Ag/AgCl) employing the Ru3PHOS@TiO2 electrode in aqueous 0.50 M NaHCO3, 0.50 M NaH2PO4, and 0.25 M KCl buffer and optional irradiation with polychromatic light (Philips HPK 125, quartz window, distance: 5 cm). This solution has not been stirred.

properties of the heterogeneous Ru3PHOS@TiO2/Ag(epoxy)/ glass composite electrodes (Figure 9). In the redox window between -1.0 and +0.5 V (vs Ag/AgCl, corresponding to -1.227 to +0.273 V vs SHE), basically all redox processes summarized in Table 3 are possible. Note that the decomposition of carbon is most likely, because the redox potential for this process is only -0.20 V (vs SHE) and only four electrons are required, as compared to eight for the reduction of CO2 to methane. The presence of nanoscopic palladium(0) centers enhances the reductive current occurring between 0.0 and +0.4 V (vs Ag/AgCl). That is exactly the window in which the deposition of carbon should be the preferred redox process. 3.8. Photoelectrochemical Experiments. The effects of continuous irradiation on the electron transfer processes from the heterogeneous Ru3PHOS@TiO2/Ag(epoxy)/glass composite electrodes to the carbonate in the electrolyte solution have been determined by employing a mercury medium pressure lamp (Philips HPK 125, incident light energy in the wavelength range between 220 and 600 nm ) 0.62 ( 0.04 W cm-2, as determined by using a ferrioxalate actinometry as described in detail in ref 14). The distance between the light source and the electrode was 5.0 cm. As shown in Figure 10, the initial reductive photocurrent of the Ru3PHOS@TiO2/Ag(epoxy)/glass composite electrode was -1.24 × 10-4 A. However, a strong decrease of the cathodic photocurrent was observed within the first 800 s, until a stationary photocurrent of (-1.0 ( 0.5) × 10-6 A was reached. We attribute this finding to the rapid depletion of charge carriers

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Figure 11. Deposition of needles that are high in carbon content on a Ru3PHOS@TiO2/Ag(epoxy)/glass composite electrode after longterm irradiation in an aqueous buffered bicarbonate solution (0.50 M NaHCO3, 0.50 M NaH2PO4, and 0.25 M KCl).

located in the Nernst layer24 under continuous irradiation conditions. It is our hypothesis that bicarbonate ions react at the nanoscopic palladium centers to form the carbon deposits shown in Figure 11. It is noteworthy that this depletion of charge carriers only occurs when irradiated, most likely due to the photochemically enhanced reactivity of the system. Moreover, the solution was not stirred, and the absence of convection explains additionally why no more charge carriers were brought close to the electrode’s surface and the Nernst layer could not reform. A second hypothesis that would also explain the observed photoelectrochemical reactivity under irradiation is that the Ru3PHOS centers and/or the palladium centers become negatively polarized, and, therefore, charge repulsion between the heterogeneous catalyst’s surface and the bicarbonate (and other) anions occurs, limiting the amount of accessible bicarbonate anions near the surface. We will further investigate the chemical nature of the observed decrease in photoelectrochemical reactivity under continuous irradiation in our future research, because it, quite apparently, represents the biggest obstacle to high-efficiency removal of carbonate from water. After the light has been switched off, a remarkable recovery of the cathodic photocurrent to -5.2 × 10-5 A can be observed within 450 s, followed by its gradual decrease to -3.8 × 10-5 A. This observation is in agreement with both hypotheses. Again, no shift in reduction peak or relative peak height was observable when changing from irradiated to dark conditions. In Figure 11, “needles” are shown that are principally made from carbon and are the only discernible deposit on the electrodes’ surfaces during electrochemical and photoelectrochemical reduction of bicarbonate buffers. These needles are absent in the absence of bicarbonate buffer. The formation of carbon-rich material on the electrodes’ surface has been corroborated by reflective FT-IR spectroscopy and energy-disperse X-ray spectroscopy (see the Supporting Information). The appearance of the carbon material can be discerned by a significant “blackening” of the electrodes. 3.9. Comparison of Thermal and Photoinduced Reduction Efficiency as a Function of Stirring. As it can be seen in Figure 12, the reductive (photo)current increases as a function of the stirring speed. This experimental behavior has been observed for the thermal reduction of dissolved carbon dioxide/ bicarbonate by means of the heterogeneous electrodes, as well as for the photoelectrochemical reduction. DPVs for stirring

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Figure 12. Effect of stirring on the photoelectrochemical current (under continuous irradiation by means of a Hg-medium pressure lamp, incident power ) 0.62 ( 0.04 W cm-2). Stirrer speed: 0 rpm (blue line), 60 rpm (red), 120 rpm (yellow), 240 rpm (green), 480 rpm (brown).

Figure 14. Effect of stirring/irradiation on the cathodic reactions; stirring speed: (a) 60 rpm, (b) 240 rpm.

Figure 13. Plot of the extension of the Nernst diffusion layer, calculated according to eq 5 versus the experimentally determined reductive current at various stirring intensities.

comparisons under Hg-lamp light are shown in Figure 12. The results summarized in Figure 12 clearly show that the thickness of the Nernst layer, which can be influenced by stirring the aqueous buffer, is an important factor determining the photoelectrochemical reactivity. Fick’s first law applied to a stagnant (e.g., stirred) solution (eq 5) connects the measurable electrochemical current with the reciprocal extension of the quiescent layer δ (Nernst diffusion layer) in direct proximity of the electrode’s surface. Within δ the mass transport occurs only via diffusion. The extension of δ varies with the extent of stirring.

i ) nFRFAD

Cbulk δ

(5)

where i is the electrochemical current (A), n is the number of exchanged electrons (for carbon deposition from dissolved carbon dioxide, n ) 4), F is the Faraday constant (96 485 C mol-1), RF is the roughness factor (estimated to be 5), A is the geometric surface of the heterogeneous electrodes (1 cm2), D is the diffusion constant (2.2 × 10-5 cm2 s-1),25 and Cbulk is 0.50 M NaHCO3. We have calculated the geometric extension of the Nernst diffusion layer δ according to eq 5 (Figure 13). δ is ranging from 3.8 × 10-3 cm (no stirring) to 2.1 × 10-3 cm (maximal

stirring), which is in good agreement with the range reported in the literature.26,27 In Figure 14, the effect of stirring on the differences between the thermal and the photochemically enhanced cathodic processes is shown. The reductive current increased from -5.8 × 10-3 to -6.8 × 10-3 A or by a factor of 1.17 ( 0.05 at a redox potential of 0.09 V (vs Ag/Ag) at low stirring intensity (60 rpm) when switching from the thermal to the photoelectrochemical reduction process. At high stirring conditions (240 rpm), the reductive current increased from -8.4 × 10-3 to -9.2 × 10-3 A or by a factor of 1.09 ( 0.03 when switching from the thermal to the photoelectrochemical reduction process. This clearly indicates that the efficiency increase through photoelectrochemical processes is limited to approximately 10-20% and that mass transfer is more important than charge transfer for the observed (photo)cathodic processes. With increasing stirring speed, the difference between DPV in the dark and under irradiation became gradually insignificant in the window from 0 V to +1.0 V (vs Ag/AgCl), as can be seen in Figure 14. However, every investigated composite electrode did feature a higher photoelectrochemical than thermal current during the reduction of dissolved carbonate. The observed (photo)electrochemical reactivity can be explained by the existence of a thermal and an additional photochemical pathway of carbon dioxide reduction (Scheme 2). The overall efficiency of this process appears to be limited by the thermal conductivity of TiO2 (Fisher T315, approximately 1 × 10-7 S cm-1). It is our hypothesis that the small conductivity of TiO2 is also responsible for the relatively small contribution of the photochemical electron transfer pathway to the overall reduction of carbon dioxide to carbon. However, it should be noted that the charge injection in this process is opposite to the “conventional” oxidative pathway that is commonly found in solar cells.22 Long-term investigations of the electrodes, using the comparison of cycles of different stirring speeds, showed several interesting results. First, as already stated, with ongoing DPV scans of the heterogeneous composite electrodes, immersed in

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SCHEME 2: Electrochemical Reduction of Carbon Dioxide to Carbon Features a Thermal and a Photochemical Brancha

a

Depending on the mass transfer conditions to the electrodes, the photochemical electron transfer reactions add up to 17% of additional reactivity. However, the limited conductivity of TiO2 does prevent a larger contribution by the photochemical pathway.

exclude partial chemical decomposition of the employed ruthenium(II)quaterpyridinium complexes, but the IR spectra have clearly indicated that a large fraction of them (>90%) remains intact at the point when no more reductive (photo)electrochemistry has been observed. Furthermore, the consequent deposition of black organic material reduces the ability of the ruthenium(II)quaterpyridinium complexes to absorb the incident light, resulting in a shut-off of the photochemical electron transfer pathway after 45 ( 5 cycles. 4. Conclusion Figure 15. Shift of the peak reduction potential as a function of the number of consecutive DPV cycles performed. A DPV cycle consists of employing the electrode consecutively in DPVs at no, slight, intermediate, fast, and very fast stirring conditions.

NaHCO3 solution, a black solid deposited onto the working electrode. The formation of needles at the electrodes’ surfaces (observed in HRSEM) and the FT-IR spectra of these needles featuring characteristic peaks at 2958, 2917, and 2868 cm-1 (aliphatic C-H), 1728 cm-1 (CdO from oxidized graphite), 1605 cm-1 (skeletal vibrations of graphite domains), and 1580 cm-1 (graphite CdC stretching) support the hypothesis that a carbon-rich material not unlike partially oxidized graphite28 is deposited on the surface during a reduction cycle. This assumption was further strengthened by the large amount of deposit observed after several cycles (see Figure 12) and the recurrence of the black deposit after each DPV scan. The origin of the black solid from the electrodes or the epoxy-glue, in form of diffusion, was ruled out, because the copper wire that was used to contact the working electrode showed the same layer of black deposit. Second, the reduction peaks revealed an almost linear decrease of the peak potential, as seen in Figure 15. The investigated electrode showed a change in the peak reduction potential from 0.107 to-0.069 V (vs Ag/AgCl) within 19 DPV cycles. It is our hypothesis that the black deposit plays a decisive role in the observed electrochemical behavior. It appears to be at least partially poisoning the catalytic centers and causes, therefore, shifts to negative potentials to facilitate the further reduction of carbon dioxide. On the other hand, the growth of the carbon-rich materials proceeds in needles that extend into the third dimension. It cannot be excluded at this stage of our investigations that the carbon material is able to take up CO2 from the aqueous buffer, which would alleviate the effects from catalytic poisoning. After 80 ( 5 DPV cycles, the complete surface appears to be covered with deposited material, and no reductive current can be observed anymore. Note that we cannot

The electrochemical and photoelectrochemical behavior of novel heterogeneous composite cathodes has been investigated. The ruthenium(II)quaterpyridinium complex Ru3PHOS bearing phosphonate anchors has been chemisorbed at the surface of TiO2 (Fisher-T315). Ru3PHOS@TiO2 has then been deposited on glass slides using Ag-doped epoxy glue. (Photo)electron transfer from the cathodes to the Ru3PHOS complexes has been observed. Very small amounts of palladium have been deposited onto the Ru3PHOS@TiO2 particles via electrodeposition. The novel composite electrodes showed deposition of carbon needles at negative potentials in the presence of aqueous bicarbonate solutions. However, the photoelectrochemical reactivity quickly decreases either due to the depletion of bicarbonate from the Nernst diffusion layer or by negative polarization of the electrode’s surface because of photoenhanced electron transfer processes. Experiments varying the efficiency of stirring have indicated that mass transfer is a more important factor than the photoenhancement of the electrode’s reactivity with respect to the process of carbon deposition. The thickness of the Nernst diffusion layer has been determined to be 3.8 × 10-3 to 2.1 × 10-3 cm, which is comparable to classic redox electrodes. Future experiments will be concerned with the chemical nature of the reaction hindrance under continuous irradiation and with the optimization of the sensitizer and palladium loadings of the composite electrode. It should be noted that this composite electrode is easy to prepare and offers the opportunity of studying carbon-deposition processes from aqueous solution. Acknowledgment. S.H.B. thanks the ACS Petroleum Research Fund (grant no. 47077-AC10) for financial support for this research. This material is also based upon work supported by the National Science Foundation under Award No. EPS0903806 and matching support from the State of Kansas through Kansas Technology Enterprise Corp. N.B. acknowledges a scholarship from the Studienstiftung des Deutschen Volkes e.V. Supporting Information Available: Synthesis of the ruthenium(II)-quaterpyridinium complexes, the characterization

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