Plasma-Enhanced Synthesis of Poly(allylamine)-Encapsulated

Article pubs.acs.org/JPCC

Plasma-Enhanced Synthesis of Poly(allylamine)-Encapsulated Ruthenium Dye-Sensitized Titania Photocatalysts Angela Kruth,* Antje Quade, Volker Brüser, and Klaus-Dieter Weltmann Leibniz Institute for Plasma Science and Technology (INP), Felix-Hausdorff-Str. 2, 17489 Greifswald, Germany ABSTRACT: Photocatalysis for sustainable hydrogen production from water is becoming one of the core challenges for an efficient conversion of solar energy into a clean and renewable fuel, enabling the growth of a carbon-free hydrogen economy. In dye-sensitized TiO2, performances were obtained to be unstable over time due to weak covalent bonding of the molecule at the surface leading to detachment of the dye from the TiO2 surface and dye self-aggregation, with recombination of charge carriers at the dye/TiO2 interface being the second major factor for efficiency losses. Our strategy for improvement of the stability of the dye/TiO2 catalyst is the encapsulation of the assembly by a plasmapolymerized allylamine layer, providing a chemically and mechanically stable nanoconfinement of the dye in close proximity of the titania surface. The TiO2-layer was synthesized by a plasma-enhanced DC magnetron sputtering process on FTO substrates, resulting in homogeneous porous semiconductor layers. The stability of the PPAAm layer on the sputtered TiO2-layer as well as catalyst performance with regard to impairment of the charge transfer between the photoactive sites and the electrolyte was investigated; further XPS, electron microscopy, and UV/vis spectroscopy, as well as photoelectrochemical studies, were carried out for characterization of the new catalyst structure.

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INTRODUCTION Photocatalysis is one of the main technologies for meeting the current and future demand for energy, while managing the environmental consequences of energy production and consumption. So, for instance, solar photoelectrochemical (PEC) water splitting devices use sunlight for the electrolysis of water to produce high purity hydrogen, reducing urban air pollution as well as lowering the dependence on foreign oil. New materials concepts based on nanoparticular structures of wide-band gap semiconductors are the key for the technological breakthrough of photocatalysis. TiO2 remains as the most effective candidate material due to its favorable band structure, high photocatalytic activity, high corrosion stability, nontoxicity, and low cost.1 However, because the light absorption capability of TiO2 is limited to the UV range, it needs to be extended into the visible light region for efficient sun light conversion. Visible light activity can be achieved by surface sensitization with dye adsorbers containing d6 transition metals, for instance, Ru(II) or Ir(III) compounds, such as ruthenium 2,2′-bipyridine-4,4′-dicarboxylate)2 (NCS)2, widely known as N3, or bis(2-phenylpyridin)-(4,4′-dicarboxy-2,2′-bipyridin)iridium(III)-hexafluorophosphate, respectively. This popular concept is successfully being employed in dye-sensitized solar cells, DSSC,2 but is also being investigated with regard to H2 production and photodegradation of pollutants.3 One of the main challenges is the stability of anchoring of the dye molecule at the TiO2 surface. Usually, an ester bond is being employed between carboxylic anchor groups attached to the dye and hydroxyl groups at the TiO2 surface. Although electronic coupling is highly favorable, the ester bond readily undergoes hydrolysis under ambient conditions leading to rapid degradation of the photocatalytic performance. Another anchor group that is often employed for attachment of the sensitizer to © 2013 American Chemical Society

the TiO2 surface is the phosphonic acid group. The stability of the bond is greatly improved, however, the kinetics of electron injection was found to be significantly reduced by about 50%.4,5 The synthesis of stable amide links has also been shown to be highly successful with regard to stability of dye attachment.6 For this, a propylamine silane-coupling reagent containing amino groups was deposited onto the surface of the TiO2 prior to dye impregnation leading to coupling of the carboxylic acid group of the dye with the amino groups of the silane coupling agent. Inhibition of electron transport was, however, also observed for the new material, leading to lower photocatalytic activities for the hydrogen evolution reaction. For visible light photocatalysis reactions, sensitization can be expected to be most efficient if all electrons that are injected from the excited dye into the conduction band of TiO2 are transferred to the electron-accepting species such as protons or water in solar water splitting. Surface recombination between the injected electrons and the oxidized dye is another main drawback for the use of dye-sensitized titania, especially when colloidal solutions of isolated semiconductor nanoparticles are being employed in slurry reactors. The use of thin semiconductor films has the advantage that recombination processes may be minimized, first, by arranging the nanoparticles in close proximity and, furthermore, by the possibility to apply an external polarizing bias to the semiconductor in order to scavenge electrons from the photoanode. The latter is considered to be one of the most promising approaches for water splitting, with the external bias being applied to a PEC cell by a PV array.7 In dye-sensitized TiO2, detachment and Received: November 30, 2012 Revised: January 30, 2013 Published: January 31, 2013 3804

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agglomeration of the dye may occur, associated with poor binding of the dye at the surface.8 New research strategies are focused on the development of functional polymer coatings for the catalyst structure to minimize dye aggregation. Approaches employed materials such as polysaccharide, the ionic surfactant sodium di-2-ethaylhexylsulfocuccinate, dendritic side chains and cyclodextrin.9−12 A DSSC based on cyclodextrin-encapsulated dye-sensitized TiO2 was observed to exhibited excellent stability under light soaking over a period of 1000 h and an attractive overall conversion efficiency of 7.4%.12 A further challenge for the application of dye-sensitized TiO2 is the photodegradation of the dye molecule. This was shown to be significantly inhibited by coating of the TiO2 nanostructure with polymers such as polyvinyl alcohol, polystyrene sulfonate, and polyethylene glycol, whereby the latter showed the best result.13 Polymers containing functional groups may be especially suited for dye encapsulation as they are able to form stable links with the functionality of the dye sensitizer and the TiO2 surface. So, for instance, for the N3 molecule containing functional carboxylic acid groups, amino-functionalized polymers may be highly suitable for encapsulation because of the formation of the highly stable amide bond between the dye molecule and the polymer. Because amino-functionalized polymer coatings have already numerous applications in biomedicine and bioengineering, for instance, for immobilization of biologically active molecules, such as peptides or proteins at titanium implant materials, coating procedures are already well-established techniques,14,15 among them, plasma-enhanced polymerization and deposition of poly(allylamine), PPAAm, is one of the most successful procedures because the resulting films were found to exhibit excellent adhesion and high stability in aqueous solutions, with high densities of amino groups. PPAAm deposition processes typically involve low-pressure pulsed radio frequency or microwave plasma-enhanced chemical vapor deposition (PECVD) methods, but also by plasmaenhanced DC magnetron sputtering deposition (PMD).16−22 Technologies based on plasma methods are increasingly applied in materials synthesis because in a plasma environment, unique nonequilibrium states of reactive species are present, opening up novel routes for surface modification and providing new opportunities in materials synthesis that can often not be obtained with conventional methods, producing large-scale uniform coatings with strong adhesion of the deposited films.23 Recently, we carried out a study where Ru dye-sensitized TiO2 nanoparticles were coated by PECVD with PPAAm and tested as photoactive component in a water reduction catalyst system for the hydrogen reduction reaction.30 A significant enhancement of long-term stability of the titania/dye assembly was achieved. In this work, PPAAm has been deposited onto nanostructured dye sensitized TiO2 that was synthesized on transparent conductive glass (TCO) by a PMD process. Photosensitization was carried out prior to the polymer encapsulation with the Ru(II) polypyridyl complex N3, as one of the most efficient sensitizers exhibiting terminal carboxylic acid functionality, Figure 1. The effect of the PPAAm encapsulation on some of the properties of the catalyst structure was investigated and is discussed in this paper.

Figure 1. Chemical structure of N3 dye for photosensitization of titania.

substrates were cut into 2.5 × 2.5 mm pieces and thoroughly cleaned following a procedure involving ultrasonic cleaning in ethanol and acetone as well as soaking in 30% HNO3, with rinsing in double deionized (DD) water (Mulli-Q Integral 3 System, 18.3 MΩ) in between cleaning steps and afterward. The substrates were then dried in a furnace at 60 °C and a cellulose acetate was subsequently applied as a removable mask to the corners of the specimen in order to realize efficient current collection from the FTO electrode during electrochemical measurements. The remaining geometric area of the unmasked TiO2-layer was 5 cm2. After another drying step at 60 °C, samples were immediately transferred to the PMD reactor. The DC magnetron sputtering process was carried out in an O2/N2/Ar containing plasma at a pressure of 3 Pa and a DC magnetron power of 5.3 kW, using a Ti target (Ti-133, Bekaert Advanced Costings NV, Belgium). A solid state zirconia oxygen sensor (ZIROX Sensoren and Elektronik GmbH, Greifswald, Germany) was integrated for process control of sputtering magnetron power as a function of oxygen partial pressure during the reactive sputtering process. Oxygen pressure was reported to be critical for the formation of the anatase phase in PVD processes.24 Prior to the deposition process, all samples were subjected to a surface treatment in oxygen at 8 kW magnetron power for 5 min in order to remove any remaining solvents and surface impurities. After TiO2 deposition, samples were annealed in O2 gas at 400 °C for 1 h, with a heating rate of 10 °C/min and at an oxygen flow rate of 30 mL/min. Photosensitization with Dye. The synthesized TiO2/ FTO substrates were loaded with ruthenium(II) dye, that is, N3 (ruthenium(II)-2,2′-bipyridine-4,4′-dicarboxylate) 2 (NCS)2). For chemical impregnation, a 0.3 mM solution of the dye in absolute ethanol was prepared at room temperature and continuously stirred for 24 h to ensure complete dissolution of the dye. Samples were then soaked in the dye solution for another 24 h and briefly rinsed in DI water to remove any remaining excess dye and subsequently dried at 60 °C prior to further modification. For PPAAm-coated TiO2/ FTO samples, great care was taken to commence the dye adsorption step immediately after PPAAm deposition in order to minimize the loss of amino group functionality through oxidation in ambient air. Deposition of Plasma-Polymerized Poly(allylamine) and Investigation of Layer Stability. Plasma-polymerized allylamine was deposited either directly onto the TiO2 surface or onto dye-sensitized photocatalyst layers. The polymer deposition was carried out using a low pressure pulsed microwave discharge plasma polymerization process, Figure 2. Alternating cycles of continuous microwave and pulsed regimes were applied at 2.45 GHz, 500 W, and 50 Pa in 50 sccm allylamine/50 sccm Ar. The deposition time was varied from 240 to 1920 s, resulting in layer thicknesses varying from 6 to

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EXPERIMENTAL SECTION Synthesis of the TiO2/FTO Substrates. TiO2 layers were deposited onto fluorine-doped tin oxide (FTO, SOLARONIX, TCO 22-7) used as a transparent conductive oxide (TCO) by a reactive magnetron sputtering process. First, the FTO 3805

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and evaluation of density. For this, 4-trifluoromethylbenzaldehyde, TFBA, Sigma-Aldrich, was applied as derivatization agent, with one primary amino groups being marked by three fluorine groups, according to eq 1.19

Figure 2. Schematics of the PECVD process for deposition of plasmapolymerized allylamine.

The initial amino group density of the sample was calculated from the peak area corresponding to the CF3 bond in the C 1s spectrum. For chemical derivatization, samples were placed in a gas-tight glass chamber immediately after deposition of the PPAAm layer and exposed to vapors of 0.5 mL of TFBA at a temperature of 37 °C for 2 h. For quantification, an average was calculated from data measured in three different samples spots. Photoelectrochemical Studies. Photoelectrochemical measurements were performed using a Zahner Controlled Intensity Modulated Photo Spectroscopy System (CIMPS) with an Im6e electrochemical workstation, an XPOT external potentiostat (both Zahner Elektrik GmbH) and an LED array. The photoelectrical cell (PEC), Figure 3, consisted of a Teflon-

50 nm of the PPAAm encapsulation. A detailed description of the plasma process is given elsewhere.17 The stability of the PPAAm coating was investigated by the means of XPS by analyzing and comparing quantitative elemental compositions before and after exposure to solvent or water. Grazing Incidence X-ray Diffraction (GIXRD), Scanning Electron Microscopy (SEM), UV/vis Spectroscopy, and Profilometry. GIXRD was carried out in order to investigate the crystallographic structure of the sputtered and tempered TiO2 layer using a Siemens D5000 AXS Diffractometer with Cu Kα radiation 40 kV, 40 mA. Measurements were performed at a constant incident angle ω of 0.5° relative to the sample surface, over a range of 2θ from 23 to 58°, with a step width of 0.02° and data collection time of 3 s per step. The microstructures of the uncoated and PPAAm-coated TiO2/FTO substrates were investigated using a JEOL JSM 5800 low vacuum scanning electron microscope, with a SEI detector and an acceleration voltages of 1−5 kV and typical working distances of 3−6 mm. UV/vis absorption properties of the catalyst structures were obtained using a PerkinElmer Lambda UV/vis 850 spectrophotometer, with measurements performed over a spectral range of 250 to 850 nm. For this transmission, measurements were carried out under normal incidence, with air as reference. Layer thicknesses of the TiO2 semiconductor layer as well as the PPAAm polymer layers were determined using a Dektak 3ST Profilometer. For these measurements, layers were deposited onto flat glass substrates under the same experimental conditions as for depositions on FTO in order to measure a nominal layer thickness. X-ray Photon Spectroscopy (XPS). The chemical composition of the uncoated and PPAAm-coated FTO/TiO2 substrates, the stability of the polymer in water and ethanol as well as the density of the amino groups at the surface of the plasma-polymerized allylamine were determined by XPS (Axis Ultra DLD, Kratos, Manchester, U.K.). For XPS measurements layers were deposited onto flat glass substrates under the same experimental conditions as for depositions on FTO. The spectra were recorded by means of monochromatic Al Kα excitation (1,486.6 eV) with a medium magnification (field of view 2) lens mode and by selecting the slot mode, providing an analysis area of approximately 250 μm in diameter. A pass energy of 80 eV was used for estimating the chemical elemental composition and 10 eV for the high energy resolution of the Ti 2p region and C 1s peaks to investigate chemical functional groups. Charge neutralization was implemented by low energy electron injected into the magnetic field of the lens from a filament located directly atop the sample. Data acquisition and processing were carried out using CASAXPS software, version 2.14dev29 (Casa Software Ltd., U.K.). To quantify the density of the amino groups at the surface of the PPAAm, chemical derivatization was required for labeling of the primary amines

Figure 3. Zahner PEC showing (1) gas inlet/outlet, (2) Pt ring counterelectrode, (3) Si diode, and (4) quartz window.25

based gas-tight single cell compartment with a quartz window of 20 mm diameter that was arranged opposite the sample that was employed as working electrode, a Pt-ring counter electrode, and an Ag/AgCl reference electrode. An O-ring of 20 mm diameter was pressed against the surface of the working electrode for sealing of the electrolyte inside the cell compartment, limiting the active sample area to 6 cm2. A Si photosensor was in close approximation of the quartz window for measurement of the incident light power, with the sensor output feeding into the additional control loop of the Zahner CIMPS system for stability control of light intensity and modulation.25 For photoelectrochemical measurements, a 0.1 M KCl solution with pH 5.7 was employed as electrolyte. Incident current photon efficiencies (IPCEs) were typically measured over a range of 430 to 650 nm using a tunable high power LED light source (TLS, Zahner Elektric GmbH), with an applied current of 300 mA and a modulation amplitude of 100 mA at a frequency of 10 Hz. During the measurement, the sample potential was fixed at the dark open cell potential, which was typically around 250 mV, and data points were collected in steps of 5 nm. Measurements of photovoltage were performed 3806

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Figure 4. Microstructures of (a) the uncoated FTO-substrate and (b) a reactively sputtered TiO2 layer after tempering at 400 °C at magnification 100000×.

at λ = 530 nm, using a high power LED array with a spectral half width of 30 nm and a maximum output of light flux of 160 lm (GRR01, Zahner Elektrik GmbH). Data analysis was performed using the Thales Software package (Zahner Elektrik GmbH) and all data were corrected for sample geometry and for the voltage offset arising from the reference electrode employed.

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and 55° 2θ can be assigned to (103), (004), (112) and (105), (211).26 No evidence of the presence of other crystalline titania phases such as rutile, brookite or suboxides such as Ti3O5, Ti4O7, or Ti2O3 was observable in the XRD diffractogram. The surface composition of the synthesized layers was observed from the XPS surface scan and found to consist of C, O, and Ti. Because the information depth of the experimental technique is ≤10 nm, the absence of elements in the glass substrate suggests that a continuous TiO2 layer is formed on top of the FTO substrate. Figure 6 shows the highly resolved

RESULTS AND DISCUSSION

Characterization of the TiO2/FTO Layer. Figure 4a shows the microstructure of the FTO substrate prior to the TiO2 layer deposition. Crystallites with a grain size of 50 to 300 nm are apparent, forming a highly dense layer. Figure 4b shows the microstructure of the sample after sputtering deposition and tempering of a TiO2 layer with a nominal thickness of 280 nm. Nanoclusters of 100 to 500 nm in size were observed, consisting of agglomerated TiO2 nanoparticles of 10 to 20 nm diameter. The layer was, however, observed to be porous allowing for a high surface area of the photocatalyst layer. GIXRD patterns are shown in Figure 5. A broad pattern with low intensity peaks was observed suggesting that the agglomerates are composed of irregular grown polycrystallites. The measured peak positions at 2θ = 25 and 48° correspond to the fundamental (101) and (200) reflections in the standard diffractogram for the anatase phase (JCPDS 84−1286), respectively, where as the triplet and doublet at around 38

Figure 6. XPS highly resolved measured Ti 2p spectrum in reactively sputtered TiO2 layer after tempering at 400 °C.

measured Ti 2p spectrum, with peaks appearing at binding energies of 459.1 and 464.8 eV, which are assigned to Ti4+ 2p3/2 and Ti4+ 2p1/2 states in TiO2.27 All titania was found to be present in oxidation state +4 and there was no evidence of reduction of Ti4+ that would indicate oxygen-deficiency, the formation of other oxides or presence of metallic Ti, in accordance with the results from GIXRD. An eventual presence of oxygen vacancies at the TiO2 surface was reported to be of disadvantage with regard to photocatalytic properties of the material since such defects may provide electron traps and thus inhibit electron transport in the system.28 Investigation of PPAAm-Coated FTO/TiO2 Layers. For the investigation of its homogeneity and stability in water, a 20 nm layer of microwave plasma-polymerized allylamine was applied as a coating on top of the TiO2 layer. In Figure 7, the elemental fractions, obtained by XPS, are shown for three

Figure 5. GIXRD pattern for reactively sputtered TiO2 layer after tempering at 400 °C. 3807

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Figure 7. XPS elemental fraction for (a) the uncoated TiO2 layer, (b) the TiO2 layer coated with 20 nm of PPAAm, and (c) the polymercoated sample after exposure to H2O.

Figure 8. XPS C1s high res: estimation of amino group density in PPAAm layer.

samples: (a) the uncoated TiO2 layer, (b) the TiO2 layer coated with PPAAm, and (c) the polymer-coated sample after exposure to stirred DI water over a period of 24 h. This period of time has been chosen based on a previous study on plasma polymerized allylamin film, where film degradation was investigated over a similar rinsing period.17 As described earlier, the spectrum for the noncoated TiO2 layer, Figure 7a, consists of peaks attributable to the elements in the semiconductor layer itself (Ti, O), whereas the presence of C is most likely resulting from contamination of the uncoated sample to atmospheric hydrocarbons during exposure to ambient air. Such high degrees of carbon contaminations are typically observed at surfaces that are exposed to laboratory air or plastic storage containers, with main components ascribing to C−H and C−C bonds.29 For the PPAAm-coated TiO2 layer, significantly increased relative amounts for C and N, that is, 73 and 22 at %, respectively, were observed, Figure 7b, due the formation of the polymeric layer. The absence of Ti in the elemental scan suggests that the layer was continuous without pores or pinholes across the area investigated and confirms that the layer was thicker than the penetration depth of the XPS measurement. After exposure to DD H2O over a period of 24 h, a small concentration of Ti was detected at the surface, Figure 7c, suggesting that partial detachment of the film occurred in water, although no quantitative delamination was observed. Minor delamination maybe a result of the high surface roughness of the underlying TiO2 layer, where some of the PPAAm domains may not be anchored to the TiO2 surface. In a previous report on the stability of PPAAm films during sonication in an aqueous environment, outstanding stability of the film was observed with no compositional changes occurring over the time period investigated.17 The amino group content of the PPAAm coating was determined via derivatization of the amino groups with TFBA, as described in the Experimental Section. The C 1s peak for the derivatized sample is shown in Figure 8, consisting of contributions of aliphatic C−C/C−H, C−N, C− O, CN, CN, N−CO, COO, and CF3 bonds at 285.0, 285.7, 286.7, 288.0, 289, and 292.9 eV, respectively. From the relative area of the CF3 peak at a bond energy of 292.9 eV, that is, 2.5%, the contents of primary amino groups at the surface of the initial sample can be estimated to a value of 2−3%. This result is in good agreement with the NH2 content found for PPAAm layers in earlier studies published by B. Finke et al.19

SEM and UV/vis absorption measurements were carried out on the TiO2 layer on FTO glass with different thicknesses of the polymeric coating. Figure 9 shows the microstructures of TiO2 layers that were coated with PPAAm layers of (a) 6 nm and (b) 50 nm thicknesses. It is apparent that at a very thin coating of 6 nm, no visible coverage of the porous microstructure of the TiO2 layer occurs since microstructural features are retained. It is likely that the polymer is located within pores and cracks between the TiO2 agglomerated nanostructure. At a thicker coating of PPAAm of 50 nm, however, a close to continuous polymeric layer was formed on top of the TiO2 layer, although a number of nanosized pinholes was present, partially exposing the TiO2 microstructure, Figure 9b. UV/vis transmittance data are shown for the uncoated TiO2 layer as well as for three samples with different coatings of PPAAm, that is, 6, 20, and 50 nm, Figure 10. For all samples, a band gap energy of about 3.2 eV was calculated which is in good agreement with the literature value for the band gap energy in anatase.30 Interestingly, transmittance across the visible light range was found to increase with layer thickness of the PPAAm. At a thickness of 6 nm, the increase of transparency was only marginal, and it became more significant at higher thicknesses of PPAAm of 20 and 50 nm. A change of absorption properties of titania is commonly a result of alternation of the Ti valence state at the titania surface. In studies of the electronic band structure of titania nanosheets, absorption enhancement within the visible wavelength range was observed due to reduction Ti4+ to Ti3+, which was attributed to intraband transitions by electrons occupying the conduction band states.31 Although in sputtered TiO2 layers, Ti3+ is likely to be present induced by the formation of oxygen vacancies, Vö, in the oxygen sublattice during the sputtering process, subsequent annealing in oxygen was observed to result in all titanium being present in oxidation state +4 by our XPS/ GIXRD studies. Partial reduction of Ti4+ to Ti3+ might indeed occur during the PPAAm deposition process as a possible charge compensation mechanism for carbon or nitrogen anion doping. This process is, however, expected to lead to an enhancement of visible light absorption due to constitution of mid gap energy levels by the incorporation of carbon.32,33 PPAAm-coated TiO2 powders have indeed shown such absorption enhancement as compared to uncoated powders, possibly be a result of carbon doping of the titania surface.34 It 3808

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Figure 9. SEM images for PPAAm-coated TiO2 layers on FTO with nominal thicknesses of (a) 6 and (b) 50 nm. Magnification 40000×.

Figure 10. UV/vis transmittance for uncoated and PPAAm-coated TiO2 catalyst layers.

is possible that in TiO2 layers, an eventual increase of absorption due to C- or N-doping arising from the PPAAm deposition process is largely dominated by another process. High nano-roughness of the porous TiO2 surface was previously shown to result in effective light scattering and lower electron transfer resistance.35 Because the refractive index for polyallylamine (nd = 1.436) is close to that found for porous sputtered TiO2 (nd ∼ 1.9 for a porosity of 36%, with nd decreasing with increasing porosity of TiO237), the absorption decrease that is observed for PPAAm-coated TiO2 layers might therefore indeed be due to a microstructural effect and decrease of diffuse scattering at the TiO2/air interface with increasing continuity of the polymer layer and associated increasing length of the TiO2/PPAAm interface. The spectral photocurrent responses over a visible light range are shown for uncoated different photocatalyst layers in Figure 11a, with IPCE values plotted as a function of excitation wavelength. It is apparent that although IPCEs are almost similar, a small improvement of photocurrent is observed at short wavelengths between 430 and 450 nm, as well as a marginal increase of current up to 540 nm for the PPAAmcoated sample as compared to the uncoated sample. This could be an indication that carbon or nitrogen doping of the TiO2 surface might have occurred during the PPAAm deposition process, leading to enhancement of photocatalytic activity of the TiO2 surface within the visible light range. Characterization of the PPAAm-Encapsulated DyeSensitized Photocatalyst Assembly. The TiO2 layer was coated with a microwave plasma-polymerized allyl amine film of 50 nm thickness immediately after the dye impregnation step,

Figure 11. Comparison of IPCE values for (a) bare and PPAAmencapsulated TiO2 catalyst layers and (b) bare and dye-sensitized catalyst structures with a 50 nm PPAAm encapsulation.

giving rise to the catalyst nanostructure that is shown schematically in Figure 12. In this arrangement, the dye is directly adsorbed at the titania surface and assumed to be anchored to the OH groups of the TiO2 via the usual ester bond. Additional stability may be provided by the PPAAm nanoconfinement containing amino groups that can interact with the carboxyl groups of the Ru dye but also with the hydroxyl groups at the titania surface to form a stable catalyst assembly. For the FTO/TiO2 substrate used for this study, PPAAm encapsulation of 50 nm was observed to fully cover the TiO2 surface, Figure 9b, although the coating was observed to still contain a number of nanosized pinholes and therefore some porosity. 3809

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Figure 12. Schematic of dye-sensitized catalyst structure with PPAAm forming an encapsulation for the dye-sensitized TiO2 surface.

Spectral IPCE values are compared in Figure 11b for the two dye-sensitized structures, that is, the uncoated structure and the assembly with a 50 nm PPAAm encapsulation. Because the Ru complex N3 is known to display a broad absorption band at around 530 nm,38 an enhancement of photoefficiency may be expected at these wavelengths, given that regeneration and electron injection is not compromised by other processes. An enhancement of photoefficiency within the wavelengths region up to 580 nm is apparent for both catalyst structures in Figure 11b. At shorter visible wavelengths up to 450 nm, the PPAAmcoated sample shows increased IPCEs as compared to the uncoated dye-sensitized layer, in accordance with the results for the TiO2 layers without dye sensitizer, with a comparable magnitude of increase, Figure 11a. A marginal decrease of photocurrent compared to the uncoated dye-sensitized sample was, however, apparent at higher wavelengths ranging from 450 to 580 nm. This could possibly be a result of less light being absorbed by the catalyst due to a decrease of diffuse scattering, as discussed earlier in the paper. The open cell potential of the photoelectrode, EOC, was measured for the uncoated and the PPAAm-coated catalyst structures under 530 nm irradiation over a power range of 0 to 60 Wm2−. From the variation of open cell voltages, EOC, Figure 13a, it is apparent that the induced photovoltage is slightly smaller for the PPAAm-coated sample as compared for the uncoated catalyst over the power range investigated. An increase of series resistance in the polymer-coated structure could possibly lead to a decreased EOC and photocurrent. The change of EOC with increasing light power was, however, observed to be greater for the PPAAm-coated sample. The irradiation dependence data of ΔEOC may be modeled by the diode equation, where ΔEOC is related to the natural logarithm of the flux of electrons into the semiconductor, Iinj, divided by the summation of recombination rate constants, kcr, of the electrons, n, to the acceptor, Aj, at any temperature, T, eq 2.39 ΔEOC = i ×

⎡ Iinj ⎤ ⎛ kT ⎞ ⎥ ⎜ ⎟ × ln⎢ ⎝ e ⎠ ⎢⎣ n ∑j kcrA j ⎥⎦

Figure 13. Irradiation dependence of (a) the open cell potential, EOC, and (b) the logarithms of the change of the open cell potential, ΔEOC, of the photoelectrode and under visible light illumination for different samples: (i) plain TiO2, (ii) dye-sensitized TiO2, and (iii) PPAAmencapsultated dye-sensitized TiO2.

samples, respectively. This suggests that a significant larger decrease of EOC may be expected at higher irradiation for the PPAAm sample, that is, an additional 59 mV per decade of irradiation. Such an increase in EOC is usually associated with a decrease in charge recombination and mass transport limitations within the catalyst structure.41

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CONCLUSIONS PPAAm-encapsulated, dye-sensitized TiO2 catalyst layers were successfully synthesized by the means of a combination of plasma-enhanced magnetron sputtering and chemical vapor depositions processes. It was anticipated that the Ru dye sensitizer is stabilized at the titania surface by the PPAAm nanoconfinement without obstruction of the electronic transport processes in the photocatalyst assembly. The magnetron sputtered titania layers were found to exhibit the anatase structure, with high porosity and all titanium in oxidation state +4. Good homogeneity, stability, and fairly good adhesion of the PPAAm layer on the titania layer were observed. From UV/ vis measurements, diffuse scattering of visible light may be slightly reduced, depending on the thickness of the PPAAm coating, although improvement of the visible light photocurrent efficiency was obtained at low wavelengths up to 450 nm, possibly due to carbon or nitrogen doping of the titania. The observed photocurrents and photovoltages show no evidence of a significant obstruction of the electron transport in the new catalyst structure. Although the open cell potential is reduced in the PPAAm-coated sample as compared to the uncoated sample within the range of light intensities investigated, the encapsulated sample showed higher ideality factors, suggesting more favorable recombination rates in the polymer-coated structure at higher light intensities.

(2)

The ideality factor, i, represents the deviation of the measured photopotential from nonideality due to potential drops at the semiconductor/electrolyte interface and by the electrolyte and is typically found to range from values of 2−3 for dye-sensitized TiO2 in solar cell application.40 The logarithmic irradiation dependence of ΔEOC was plotted for both assemblies as a function of irradiation power in Figure 13b. Because no redox mediator was employed, the predominant electron acceptor is the oxidized sensitizer itself. The ideality factors were calculated to be i ∼ 1.6 for the PPAAm/dye/TiO2/FTO and i ∼ 0.6 for the dye/TiO2/FTO 3810

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the BMBF for funding this work as part of the L2H network of the “Spitzenforschung & Innovation in den Neuen Bundesländern” initiative. The authors would like to thank Urte Kellner for the PPAAm deposition, Daniel Köpp for synthesis of the TiO2/FTO substrates, Uwe Lindemann for measurement of the layer thicknesses, Christian Walter and Dr. Jan Schäfer for the SEM work, and Anja Albrecht for XRD measurements.

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dx.doi.org/10.1021/jp311787k | J. Phys. Chem. C 2013, 117, 3804−3811