Photocurrent Switching Effects in TiO2 Modified with Ruthenium

ARTICLE pubs.acs.org/JPCC

Photocurrent Switching Effects in TiO2 Modified with Ruthenium Polypyridine Complexes Marek F. Oszajca,† Keri L. McCall,‡ Neil Robertson,*,‡ and Konrad Szacizowski*,†,§ †

Wydziaz Chemii, Uniwerstytet Jagiello nski, ul. Ingardena 3, 30-060 Krakow, Poland School of Chemistry and EaStChem, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K. § Wydziaz Metali Nie_zelaznych,Akademia Gorniczo-Hutnicza, al. Mickiewicza 30, 30-059 Krakow, Poland ‡

bS Supporting Information ABSTRACT: Selected ruthenium complexes [Ru(trpy)L], [Ru(bpy)2L], and [Ru(dcbpy)2L] were chemisorbed onto the surface of nanocrystalline titanium dioxide. The ligands (L) were Cl ions or cyanodithioimidocarbonate (C2N2S2), so the type of linkage with the TiO2 surface was different in each case. The resulting materials were sensitive to visible light. Photoelectrochemical studies revealed significant, potential-dependent photosensitization in the 300650 nm window. In addition, the direction of the photocurrent could be switched from cathodic to anodic and vice versa by application of adequate external potential. Further photoelectrochemical, spectroscopic and theoretical studies allowed prediction of the possible mechanism of current switching. The photoelectrochemical properties of those materials allow construction of optoelectronic ternary logic devices.

’ INTRODUCTION In recent years, a new aspect of chemistry, related to molecular devices and machines, has emerged. Conceptually, these units are powered by chemical, photochemical, and electrochemical inputs.1 On the other hand, the omnipresent demand for clean energy sources has driven significant developments in solar energy conversion and has been a central topic in various technological innovations and scientific discoveries in semiconductor technologies. The well known example of dye-sensitized solar cells (DSSCs) utilizes a semiconducting support surface modified with a photosensitizer.2,3 Materials used in DSSCs could also play a significant role in information processing, since they are sensitive to light, which is one of the key inputs in molecular devices. A promising way of creating nanosized electronics is the bottom-up approach, using molecular building blocks to achieve functional devices.1,49 Widebandgap semiconductors are one of the most suitable materials for construction of nanoscale devices, due to their stability and versatility.10,11 The discovery of the photoelectrochemical photocurrent switching (PEPS) effect opens a new field in nanoelectronics.12 Until now, wide-bandgap semiconductors have found applications in photocatalysis,1317 photovoltaic cells,1820 and electrochromic displays.21 Moreover, there have been reports of applications of nanocrystalline semiconductors as logic gates and optoelectronic switches.11,2233 The most successful wide-bandgap semiconductor in terms of application in nanosized devices is nanocrystalline titanium dioxide. Its main advantages come from low price, photocorrosion r 2011 American Chemical Society

resistance, chemical inertness, and lack of toxicity. The main disadvantage of titanium dioxide is its relatively large bandgap (3.03.2 eV, depending on the TiO2 polymorph34), which limits its photosensitivity range. The resolution of this problem lies in photosensitizing the semiconductor toward visible light. One way to achieve this is bulk doping TiO2 with various elements (e.g., N,3540 C,4145 S,40,43,46 Br,47 I,48 V,45,49,50 Cr,51,52 Ni,52,53 Pt,5257 Pb,58 or rare earth metals52,59,60). These defects, introduced into crystal structure, disturb the local potential energy, which is the part of the Hamiltonian operator in the Schr€odinger equation. It results in formation of intrabandgap electronic states that are near the conduction or valence band edges of the semiconductor (Figure 1a). The presence of new electronic states allows the material to absorb at lower energies in the visible part of the spectrum. Unfortunately, this method suffers from the efficient recombination of photogenerated charges because the defects induced by the foreign atoms serve as recombination centers.61 Another way to achieve visible light sensitization is surface modification of TiO2, performed by adsorbing various molecules that absorb visible light, through formation of covalent or ionic bonds between the semiconductor surface and the molecule. These systems show negligible electronic coupling between molecular surface species and semiconductor bands. Photosensitizers are usually based on organic dyetransition metal Received: February 23, 2011 Revised: May 6, 2011 Published: May 26, 2011 12187

dx.doi.org/10.1021/jp201805t | J. Phys. Chem. C 2011, 115, 12187–12195

The Journal of Physical Chemistry C

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Scheme 1. Investigated Ruthenium Complexes

Figure 1. Types of photosensitization: (a) bulk doping and formation of new energy levels; (b) photosensitization of the semiconductor surface with an organic or inorganic chromophore; (c) generation of a pair of surface states associated with a chromogenic ligand or transition metal complex.

complexes.2,3 In most cases, the indirect electron injection mechanism (photoinduced electron transfer) described by the Sakata-Hashimoto-Hiramoto model (Figure 1b) occurs.62,63 However, in some cases, the photosensitizer strongly interacts with the semiconductor surface, and electronic coupling is observed (Figure 1c). These interactions allow new optical transitions, which have been observed in cyanoferrate25,29,64 and catechol65,66 systems, and result in direct electron injection (optical electron transfer) from the photosensitizer into the conduction band of the semiconductor. Electronic coupling facilitates population of the conduction band, but at the same time provides a bonding framework for efficient pathways of charge carrier recombination.67 Therefore, such systems are usually characterized by low incident-photonto-current efficiency (IPCE) values, which disables their application in DSSCs. However, they are well suited for the construction of optoelectronic switches and other devices.12 Polypyridyl ruthenium complexes are widely known for their photochemical properties. They are so far the best photosensitizing compounds used in DSSCs.20,6874 These complexes absorb light in most of the visible spectrum and often reach IPCE higher than 80%. Furthermore, ruthenium complexes are relatively stable in their ground and excited states and rarely undergo decomposition during redox processes on the photoelectrode. These properties are being applied in DSSCs as mentioned above; however, they could also be used in optoelectronic logic devices; for example, molecular switches. The HOMO/LUMO gaps in these complexes can be tuned by changing the ligands, which has already been reported.7579 This is of high importance, because this factor affects the complex's ability to donate or accept (or both) electrons from the HOMO and into the LUMO respectively, one of the key factors in organic electronic devices.80 Despite the extensive study of Ru complexes as sensitizers in DSSCs, however, they have to date received only negligible attention in the wider study of photoelectrochemical switches and devices.24 This paper presents preparation of nanocrystalline TiO2 with immobilized ruthenium complexes along with their electrochemical, photoelectrochemical, and spectroscopic properties. Experimental data are supplemented with quantum-chemical investigations. Although the ruthenium-based dyes studied in this paper are not the most common in DSSC research, they offer a similar coordination sphere of the central metal ion and a variety of bonding modes to the surface of titanium dioxide.

’ RESULTS AND DISCUSSION

were investigated (Scheme 1). The type of linkage to the titanium dioxide surface was different in each case. [Ru(trpy)Cl3] and [Ru(bpy)2Cl2] (complexes I and II in Scheme 1, respectively) bond to the TiO2 surface via replacement of the chloride ligand with oxide from the semiconductor surface hydroxyl groups and formation of a TiORu bridge. [Ru(bpy)2(C2N2S2)] (complex III in Scheme 1) bonds to the TiO2 surface via replacing surface hydroxyl groups with nitrogen from the nitrile group of the C2N2S2 ligand. Thus formed, the TiNtC bond framework should resemble that observed in the case of penta- and hexacyanoferrates chemisorbed onto TiO2.22 The maximum absorption of the complex III deposited on the TiO2 is 6 times smaller in comparison with the maximum absorbance of the carboxy derivative [Ru(dcbpy)2(C2N2S2)] (complex IV in Scheme 1). This observation indicates that the cyano group is capable of reasonably strong interaction with the TiO2 surface. Therefore, it suggests that for the complex IV, the cyano group may be interacting with the TiO2, in addition to the predominant binding through the acid groups that forms ester bonds.81 This complex, however, gives materials of much stronger coloration than the others, which may indicate much more efficient attachment of the ruthenium complex to the semiconducting surface. Electronic absorption spectra of modified titania samples along with parent complexes are shown in Figure 2. The onsets of visible light absorption are localized in the 700800 nm window. The main metal-to-ligand charge transfer (MLCT) absorption bands in all the studied complexes are hypsochromically shifted by ∼50 nm (Figure 2, Table 1). The hypsochromic shift of the MLCT bands observed in each case can be, in turn, attibuted to the difference in polarity as well as electron donor/ acceptor and acid/base properties of the environment.82,83 Highly asymmetric iron and ruthenium polypyridine complexes usually show intense solvatochromic effects due to interactions of the excited state electric dipole with the solvent environment and accompanying ions. Usually, increasing acceptor number (AN), which is defined as the chemical shifts of the 31P NMR signal of triethylphosphineoxide dissolved in the respective Lewis acids normalized to AN = 100 for SbCl5, results in a significant hypsochromic shift of the metal-to-metal charge transfer (MMCT) band according to eq 1:82,84

Interaction of Ruthenium Complexes with TiO2. Three ruthenium bipyridine complexes and one terpyridine complex

vac νsolv max ¼ νmax þ 90AN

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ð1Þ

dx.doi.org/10.1021/jp201805t |J. Phys. Chem. C 2011, 115, 12187–12195


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Figure 2. Visible absorption spectra of the photosensitizers in DMF (left) and transformed diffuse reflectance spectra of titanium dioxide modified with the corresponding ruthenium complexes (right). UV parts of the spectra are omitted for the sake of clarity.

Table 1. Parameters of the Lowest Electronic Transitions and Redox Characteristics of the Ruthenium Complexes in Solution and at the TiO2 Surface compound

λmax/nma

λmax/nmb

E1/2/Vc

EPEPS/Vd

ΔE/Ve

ANf

I

482

450

0.576

0.570

0.06

32.9

II

551

535

0.658

0.460

0.198

24.9

III

537

485

0.632

0.385

0.247

41.1

IV

583

510

0.655

0.410

0.245

46.1

a

In acetonitrile. b Upon adsorption at TiO2 surface from DMF solution. c Ru3þ/2þ, in acetonitrile, vs NHE. d Average photocurrent switching potential vs NHE; compare to Figure 6. e E1/2 EPEPS f Apparent surface acceptor number.

where νsolv max is the absorption maximum in the presence of the solvent, νvac max is the absorption maximum in vacuum, and AN is the acceptor number of the solvent (for acetonitrile ANMeCN = 18.9).82 This also allows the estimation of the vacuum spectrum if the AN of the solvent is known. solv νvac max ¼ νmax 90AN

ð2Þ

If one can assume that the shift of the MMCT band is associated only with outersphere interactions between the ruthenium complex and its environment, one can calculate the AN of the new medium (i.e., the surface of titania) according to eq 3, ANsurf ¼

vac vsurf vsurf vsolv max vmax max þ 90ANMeCN ¼ max 90 90

ð3Þ

where νsurf max is the absorption maximum on the surface. For complexes III and IV, it yields values of 41.1 and 46.2, respectively, which is in agreement with the spectral properties of [Ru(bpy)2(CN)2] interacting with various cations in a variety of solvents.82 Interestingly, complexes I and II suffer much weaker spectral shifts than expected. This fact may originate from the changes in the first coordination sphere or more complex interactions with semiconducting surface. These two complexes form strong covalent bonds via oxide bridges. Titanium dioxide as an n-type semiconductor can also play the role of electron donor. Strong bonding via short bridges may compensate the acceptor character of Ti4þ centers due to electron-donating properties of the bulk semiconductor. Quantum Chemical Description of Materials. The assumed model of photosensitizerTiO2 surface system uses simple

Figure 3. Frontier HOMO and LUMO for I@TiO2, II@TiO2, III@TiO2, and IV@TiO2.

tetrahydroxytitanium as a simplified model of TiO2 bulk. Although it does not represent semiconductor bands, it reproduces the bonding between the semiconductor surface and the metal complex and provides basic information on electronic coupling within the system together with reliable spectroscopic estimates.67,85 The location of the metal complexTiO2 bond formation was chosen from preliminary geometry optimizing calculations. The binding site presenting the lowest energy was taken as optimal. In the IV@TiO2 case, the assumed TiO2 bulk model is slightly more complicated. In real conditions, these complexes bind to TiO2 via both carboxylate groups. Therefore, we used two trihydroxytitanium moieties bound with a bridging oxygen to approximate the bulk model. DFT calculations are useful in determining mechanisms of electron transfer in the materials examined. Selected frontier molecular orbitals (MOs) of the calculated systems are shown in Figure S1 and S2, and the HOMO and LUMO of complexes bound with model TiO2 bulk are shown in Figure 3. The character of the selected MOs is described in Table S3. For all ruthenium complexes, the HOMO orbitals mainly consist of ruthenium dxy, dxz and chlorine or sulfur pz and πCN for complexes I, II or III, IV, respectively. In addition, for complexes I, II, and IV, the HOMO orbital has a contribution from polypyridyl π*. The HOMOLUMO gap is smallest for II and increases with increasing aromatic framework of the ligandI or donor properties of coligandsIII, IV (cf. Figure S3). These results are consistent with those obtained at the Becke three parameters hybrid exchange and the PerdewWang 1991 correlation functionals (B3PW91) level of theory within the valence double-ζ (VDZ) basis set.81 The qualitative description of frontier molecular orbitals is further 12189



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Table 2. Normalized Mulliken Population Analysis of TiO2 Modified by the Ruthenium Complexes Shown in Percents I@TiO2 fragment ruthenium

II@TiO2

III@TiO2

IV@TiO2

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO 5.87

49.19

4.30

53.16

6.15

54.57

3.13

24.88

bridge ligand

3.92

0.03

18.74

0.55

38.49

1.52

0.04

0.54

titanium moiety

2.23

0.05

3.44

0.11

0.01

0.30

0.02

3.30

95.63

24.66

93.19

6.93

95.05

75.06

remaining ruthenium complex ligands HOMOLUMO energy gap (eV)

44.66 2.86

2.82

3.32

90.30 2.39

Figure 4. Photocurrent amplitude as a function of the photoelectrode potential and incident light wavelength recorded for photoelectrodes made of materials I, II, III, and IV (a, b, c, and d, respectively). Red and blue areas correspond to cathodic and anodic photocurrent, respectively; gray areas represent zero net photocurrent.

supported with quantitative Mulliken population analysis of model complexes (Table 2).86 In the case of I@TiO2, the electron density for the HOMO is localized mainly on ruthenium (49.19%) and terpyridine ligand (44.66%), whereas for the LUMO, almost all electron density is injected onto the terpirydyne ligand. A similar trend for the LUMO is observed for II@TiO2 and III@TiO2, but in these cases, there is a significant contribution from the bridging ligand (18.74% and 38.49% for the HOMO, respectively) at the expense of lowering the contribution from the bipyridine ligands (24.66% and 6.93%, respectively). The data obtained for IV@TiO2 describe the HOMO orbital to be

localized on ruthenium (24.88%) and the bipyridine ligands (75.06%). Again, for the LUMO, the electron density occupies bipyridine ligands (90.30%), but in this case, a small contribution from the titanium moiety is observed (3.30%). This could suggest the possibility of electronic coupling, but still, the effect would be very weak in comparison with the SakataHashimotoHiramoto indirect electron injection mechanism.62,63 In summary, the DFT calculations for all complexes and materials derived by adding TiO2 model groups show similar trends. The HOMO orbitals are localized on ruthenium and donor ligands, and the LUMO, on the polypyridine ligands. 12190


The Journal of Physical Chemistry C Photoelectrochemical Properties. All the materials based on surface modified titanium dioxide were studied for their ability to generate photocurrent as a function of electrode potential and incident photon energy. The recorded data allowed preparation of three-dimensional diagrams presented in Figure 4. All of them show the photocurrent switching effect. The characteristics of the switching, however, are different in each case. In the case of I@TiO2, photocurrent switching occurs at ∼400 mV vs Ag/AgCl (Figure 6a). Interestingly, despite quite low photocurrent values, the photosensitization range is impressive. Cathodic currents are observed until 400 mV and 650 nm incident light wavelength. The anodic photocurrent occurs only above 400 mV and at a narrow wavelength range (350450 nm). Similar results are observed in the III@TiO2 case, where also mainly cathodic photocurrents occur (Figure 4c), and its spectral range resembles that for neat titanium dioxide. Therefore, it can be concluded that oxidized ruthenium species cannot photosensitize TiO2. The photosensitization range is smaller than in the case of I@TiO2 (up to 600 nm). Photocurrent switching occurs within a 150200 mV potential window. Quite different photocurrent characteristics are observed for the II@TiO2 material (Figure 4b). Photocurrent switching occurs within the 180240 mV potential window, and photosensitization is observed within both the anodic and cathodic regimes. On

Figure 5. Redox potentials of titanium dioxide VB and CB relative to those of the ground and lowest excited state of various ruthenium(II) complexes.

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anodic polarization, it reaches 530 nm, whereas on cathodic polarization, it extends up to 600 nm. Finally, the IV@TiO2 material exhibits some similarities to the III@TiO2 sample (Figure 4d). Current switching occurs sharply at ∼200 mV. Furthermore, a spectacular photosensitization range for cathodic photocurrents, reaching 760 nm, can be observed. This material also shows anodic photocurrents within the 200600 mV potential window and up to 600 nm. The anodic photocurrent profile indicates that photocurrent switching is not concomitant with photosensitization switching, because anodic photocurrents are generated on visible excitation only within a narrow potential window. The electronic spectra of all the complexes deposited at the surface of titanium dioxide do not indicate any electronic coupling between the surface and the ruthenium species, unlike the previously reported photosensitizers containing the CN moiety.25,64,8790 In addition, DFT calculations for all the ruthenium species and rutheniumtitanium systems clearly indicate that there is negligible electronic coupling between the ruthenium and titanium centers in all the cases. Therefore, all the systems should behave according to the SakataHashimoto Hiramoto model and undergo photoinduced electron transfer from the excited ruthenium centers into the conduction band of titanium dioxide. The redox potentials of the ruthenium complexes in their excited states were estimated from the solid state absorption spectra (cf. Figure 2b). Thus calculated values indicate that the photoinduced electron transfer from excited complexes to the conduction band of TiO2 is thermodynamically allowed (Figure 5). Indeed, photosensitization toward visible light is observed within a wide potential range and ceases on electrochemical oxidation of the surface species (cf. Figure 4). At photoelectrode potentials higher than the E1/2, there is no photosensitization, and the materials behave like neat titanium dioxide; that is, they generate exclusively anodic photocurrent within the TiO2 absorption (Figure 6a). Residual anodic photocurrents in the visible range are a result of incomplete oxidation of surface complexes. Upon electrochemical reduction of the surface species (Figure 6b), photosensitization is associated with efficient photoinduced electron transfer from the excited ruthenium complex to the conduction band of titanium dioxide; therefore, cathodic photocurrents are observed in a wider spectral range. At

Figure 6. Mechanism of photocurrent switching in Ru-modified TiO2: anodic photocurrent generation at E > E1/2 (a), anodic photocurrent generation at potentials E1/2 > E > EPEPS (b), and cathodic photocurrent generation at potentials E < EPEPS (c). Dashed lines represent processes associated exclusively with excitation within fundamental transition in TiO2. 12191


The Journal of Physical Chemistry C sufficiently low potentials, electron transfer from titanium dioxide crystallites to the ITO support becomes inefficient due to energetic constraints, and conduction band electrons are rather transferred to the external electron acceptor; for example, molecular oxygen dissolved in the electrolyte. This process generates cathodic photocurrents within a wide spectral range (Figure 6c). Interestingly, photocurrent switching potential (EPEPS, cf. Table 1) does not correlate clearly with the redox potential of the surface complex (E1/2, cf. Table 1). This effect cannot be simply attributed to the change in redox potential upon binding to the TiO2 surface, because in the case of weak electronic coupling, the potential shift should be negligible.25,91 On the other hand, a more significant shift in the redox potential may be induced by changes in the coordination sphere. Interestingly, in the case of the I@TiO2 material, these two potentials are identical (within the experimental error), despite the changes in the coordination sphere of the ruthenium ion. In this case, photocurrent switching occurs at the same potential as the photosensitization switching, which must be associated with the surface complex oxidation. In the case of II@TiO2, III@TiO2, and IV@TiO2 materials, the difference between the redox potential and the switching potential of 200250 mV and photosensitization is observed both in the cathodic and anodic regimes. It indicates that the photocurrent switching process does not involve the redox transformation of the surface species, but the photocathodic and photoanodic processes compete due to some other processes. This effect may be associated with a Schottky barrier at the ITO/ molecule/TiO2 junction. Therefore, a more complex structure (instead of a simple ITO/TiO2 junction) should be considered because the TiO2 nanoparticles are chemically modified prior to deposition onto conducting support. The molecular interlayer between TiO2 and ITO should significantly modify the height of the Schottky barrier via dipole interactions.92,93 In this way, the surface molecules should control the height of the Schottky barrier. High potential difference (EPEPS E1/2), observed in the case of II, III, and IV, indicates a low energy barrier because at moderate cathodic polarization, anodic photocurrents are still observed. In contrast, complex I induces a much higher barrier, thus already at the potential corresponding to the electrochemical reduction of ruthenium complex, anodic photocurrents are effectively inhibited.

’ CONCLUSIONS Semiconducting materials prepared by chemisorption of several ruthenium polypyridine complexes onto a titanium dioxide surface show interesting photoelectrochemical properties. First of all, they show efficient photosensitization toward visible light within the whole absorption spectra of the complexes. Furthermore, the polarity of the photocurrent can be switched from anodic to cathodic and vice versa on application of appropriate potentials. Electrochemical processes involving the ruthenium surface species are responsible for switching the photosensitization on and off, and subtle potential variations applied upon complex reduction may generate energy barriers responsible for reversal of photocurrent polarity. The latter process strongly depends on the structure of the surface complex and is most pronounced in the case of the terpyridine complex. The photocurrent switching that is observed in this paper is another example of the PEPS effect, already observed in similar systems

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(TiO2 modified with redox-active iron species). Interestingly, this is the first case of photocurrent switching in an electroactive system with negligible electronic coupling between the semiconductor’s bands and surface molecular species. Furthermore, the systems studied here offer a new tool for the design of optoelectronic switching systems based on ITO/TiO2 junctions: the height of the Schottky barrier can be efficiently modulated via careful selection of ligands. The system presented here offers the first photoelectrochemical three-state switch, as photocurrent polarity and photosensitization can be independently controlled by photoelectrode potential. These states can be defines as (i) cathodic photocurrent on visible excitation; (ii) null photocurrent at visible excitation; and finally, (iii) anodic photocurrent at visible excitation. This opens new possibilities for optoelectronic logic devices based on surface-engineered wide-band-gap semiconductors. The hybrid systems described here can, in principle, be used in ternary optoelectronic logic devices. In this case, the balanced ternary logic would be the system of choice because the logic values of 1, 0, and 1 can be easily assigned to cathodic, null, and anodic photocurrents, respectively. Current research shows also that application of flexible PET substrates instead of glass slides could be beneficial for fabrication large area devices due to facile processing and low cost.

’ EXPERIMENTAL SECTION Materials. TiO2 (Degussa P25, ∼70% anatase, 30% rutile; 50 m2 g1) was used to prepare porous electrodes. Barium sulfate was supplied by Riedel-de Haen, and DMF, by Fluka. [Ru(bpy)2Cl2] (complex I) was supplied by Aldrich. All other chemicals were supplied by Aldrich. Synthesis of [Ru(trpy)Cl3] (complex II). Terpyridine (117 mg, 0.5 mmol) was stirred with absolute ethanol (50 mL) with gentle heating until dissolution. RuCI3 3 3H2O (131 mg, 0.5 mmol) was added, and the solution was refluxed for 3 h with stirring. After the mixture was cooled, the brown precipitate was filtered, washed with ethanol and diethyl ether, and air-dried. Yield: 180 mg, 80%.94 Synthesis of [Ru(bpy)2C2S2N2] (complex III). [Ru(bpy)2Cl2] (100 mg, 0.2 mmol) was dissolved in methanol (10 mL), and silver nitrate (71 mg, 0.4 mmol) in water (1 mL) was added. The mixture was refluxed for 1 h under N2 and in reduced light, yielding a scarlet red solution and a white precipitate. The precipitate was removed by centrifugation, and to the filtrate was added K2C2N2S2 (31 mg, 0.2 mmol) in water (1 mL). The mixture was stirred overnight under N2 and reduced light. The red precipitate was collected, washed with methanol, and dried in vacuum. Yield: 49 mg, 46%.81 Synthesis of [Ru(dcbpy)2C2S2N2] (complex IV). [Ru(dcbpy)2Cl2] (100 mg, 0.12 mmol) was dissolved in methanol (10 mL), and silver nitrate (44 mg, 0.24 mmol) in water (1 mL) was added. The mixture was refluxed for 1 h under N2 and in reduced light, yielding a deep red solution and a white precipitate. The precipitate was removed by centrifugation, and to the filtrate was added 0.1 M aq KOH (3 mL). The mixture was stirred for 10 min, followed by the addition of K2C2N2S2 (30 mg, 0.2 mmol) in water (1 mL). The mixture was stirred overnight under N2 and reduced light. The product was precipitated with 1 M HCl (1 mL), yielding a dark red precipitate. Yield: 73 mg, 80%. The crude product was further purified via a Sephadex LH-20 column using water as the eluent.81 12192


The Journal of Physical Chemistry C Instrumentation. Diffuse reflectance spectra were recorded on a Lambda 12 (PerkinElmer, U.S.A.) spectrophotometer equipped with a 50 mm integration sphere. Barium sulfate of spectral purity was used as a reference material. Photoelectrochemical measurements were performed on a BAS CV-50W electrochemical analyzer (Bioanalytical Systems, U.S.A.), and an aqueous 0.1 M KNO3 solution was used as an electrolyte throughout all the experiments. Working electrodes were prepared on indium tin oxide (ITO)-covered polyethylene terephthalate (PET) film (Aldrich) with a surface resistivity of 35 Ω/sq. Films were deposited from aqueous suspensions of semiconducting powders via a cast-coating technique. The semiconductor-modified nanopowder was suspended in distilled water, ultrasonicated, cast onto clean ITO surfaces, and dried with a stream of warm air. Measurements were performed immediately after preparation of electrodes. Thus obtained layers exhibited sufficient stability for photoelectrochemical measurements. To avoid the influence of the semiconductor layer thickness, the electrodes were illuminated through the conducting support. In this way, only the particles attached directly to the electrode contribute significantly to photocurrent generation, and the contribution of the bulk of the film is negligible.22 Furthermore, those films were stable against dye desorption in aqueous electrolyte solutions. Photocurrents were recorded using a classical three-electrode setup with a platinum wire counter electrode and a Ag/AgCl reference electrode. A highpressure xenon lamp XBO150 (Osram, Germany) equipped with a computer-controlled monochromator and shutter was used in all photocurrent measurements. Photocurrent action spectra were recorded using pulsed illumination of the photoelectrodes at potentiostatic conditions. Photoelectrodes were conditioned at measurement potentials for 20 s. Theoretical Calculations. Theoretical modeling was performed with Gaussian 03 rev D.01 (Gaussian, Inc.).95 Geometry optimization was done using the Becke’s generalized gradient approximation method combined with the HartreeFock exchange functional (B1B95)96 with the double-ζ quality basis set with effective core potentials and including relativistic effects for heavy atoms.97 Molecular orbitals were computed using the same theory level and tight convergence criteria. Mulliken population analysis was conducted with the Aomix software package.86 Ruthenium complex modified TiO2 was divided into four parts: ruthenium atom; bridging ligand; titanium moiety; and remaining ruthenium comlex ligands, which do not take part in chemisorption onto the TiO2 surface. The Mulliken population was assigned to each part and normalized to 100%.

’ ASSOCIATED CONTENT

bS

Supporting Information. Supporting information includes two figures with frontier molecular orbitals of [Ru(trpy)Cl3], [Ru(bpy)2Cl2], [Ru(bpy)2(C2N2S2)], and [Ru(dcbpy)2(C2N2S2)] as calculated at the B1B95/SDD level of theory; and frontier molecular orbitals of [Ru(trpy)Cl2OTi(OH)3], [Ru(bpy)2(O)2Ti(OH)2], [Ru(bpy)2(C2N2S2) Ti(OH)3]þ, and [Ru(dcbpy)2(C2N2S2)Ti(OH)3OTi(OH)3] as calculated at the B1B95/SDD level of theory; and one table, character of selected MO of examined materials and energy of HOMOLUMO (HL) gap. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mails: [email protected], [email protected], nrobert1@staffmail.ed.ac.uk.

’ ACKNOWLEDGMENT Financial support from the Polish Ministry of Science and Higher Education (Grants Nos. 1609/B/H03/2009/36 and 649/N-GDRE-GAMAS/2010/0), National Centre for Research and Development (Grants Nos. NCBiR/ENIAC-2009-1/1/ 2010 and Era-Chemistry 60 303) and European Nanoelectronics Initiative Advisory Council ENIAC (Grant No. 120122) are gratefully acknowledged. M.O. thanks the Foundation for Polish Science for the MPD Programme fellowship cofinanced by the EU European Regional Development Fund. N.R. thanks The Royal Society of Edinburgh International Exchange Programme for a mobility grant and the EPSRC Supergen Excitonic Solar Cells Consortium for financial support. ’ REFERENCES (1) Balzani, V.; Credi, A.; Venturi, M. Processing energy and signals by molecular and supramolecular systems. Chem.—Eur. J. 2008, 14, 26–39. (2) Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells. Chem. Rev. 2010, 110, 6664–6688. (3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595–6663. (4) Maruccio, G.; Cingolani, R.; Rinaldi, R. Projecting the nanoworld: concepts, results and perspectives of molecular electronics. J. Mater. Chem. 2004, 14, 542–554. (5) Balzani, V.; Credi, A.; Venturi, M. The bottom-up approach to molecular level devices and machines. Chem.—Eur. J. 2002, 8, 5525–5532. (6) Ma, R.-M.; Dai, L.; Huo, H.-B.; Xu, W.-J.; Qin, G. G. HighPerformance Logic Circuits Constructed on Single CdS Nanowires. Nano Lett. 2007, 7, 3300–3304. (7) Szacizowski, K. Digital Information Processing in Molecular Systems. Chem. Rev. 2008, 108, 3481–3548. (8) Hoenlein, W.; Kreupl, F.; Duesberg, G. S.; Graham, A. P.; Liebau, M.; Seidel, R.; Unger, E. Carbon Nanotubes for Microelectronics: Status and Future Prospects. Mater. Sci. Eng., C 2003, 23, 663–669. (9) Hoenlein, W.; Duesberg, G. S.; Graham, A. P.; Kreupl, F.; Liebau, M.; Palmer, W.; Seidel, R.; Unger, E. Nanoelectronics beyond Silicon. Microelectr. Eng. 2006, 83, 619–623. (10) Ashkenasy, C.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Molecular Engineering of Semiconductor Surfaces and Devices. Acc. Chem. Res. 2002, 35, 121–128. (11) Szacizowski, K.; Macyk, W. Photoelectrochemical Photocurrent Switching Effect: A New Platform for Molecular Logic Devices. Chimia 2007, 61, 831–834. (12) Gawe-da, S.; Podborska, A.; Macyk, W.; Szacizowski, K. Nanoscale Optoelectronic Switches and Logic Devices. Nanoscale 2009, 1, 299–316. (13) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735–758. (14) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53–229. (15) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Seminconductor Electrode. Nature 1972, 238, 37–38. (16) Shaban, Y. A.; Khan, S. U. M. Visible Light Active Carbon Modified n-TiO2 for Efficient Hydrogen Production by Photoelectrochemical Splitting of Water. Int. J. Hydrogen Energy 2008, 33, 1118–1126. 12193


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