Reactive Porphyrin Adsorption on TiO2

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Reactive Porphyrin Adsorption on TiO Anatase Particles: Solvent Assistance and the Effect of Water Addition Johannes Schneider, Thomas Berger, and Oliver Diwald ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00894 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

Reactive Porphyrin Adsorption on TiO2 Anatase Particles: Solvent Assistance and the Effect of Water Addition

Johannes Schneider, Thomas Berger, Oliver Diwald* [email protected]

Department of Chemistry and Physics of Materials, Paris Lodron University of Salzburg, Jakob-Haringer-Straße 2a, A-5020 Salzburg, Austria

Keywords: hybrid materials, porphyrin adsorption, optical properties, solvent assisted adsorption, nanoparticle functionalization

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Abstract The surface functionalization of metal oxide nanoparticles with complex organic molecules can lead to optoelectronically very different materials’ properties depending on whether adsorption occurs at the solid-gas or solid-liquid interface. Here, we report on two different approaches to decorate anatase TiO2 nanoparticle powders with 2H-tetraphenylporphyrin (2HTPP) molecules: i) porphyrin adsorption in dispersions of organic liquids and ii) gas phase functionalization where evaporated porphyrin molecules attach to dehydrated particle surfaces in the absence of solvent molecules. In the latter case, a bottom-up approach is pursued to explore both the impact of organic solvent molecules and the impact of spurious water on the surface chemistry of porphyrin-sensitized TiO2 nanoparticles. Vis Diffuse Reflectance and Photoluminescence emission spectroscopy provide clear evidence for the promotion of interfacial reorganization processes of the adsorbate species by co-adsorbed solvent molecules in liquids. Moreover, traces of spurious water were found to induce protonation-deprotonation reactions on the adsorbed porphyrins with a strong impact on the optical properties of the resulting hybrid materials.

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Introduction There is an increasing number of applications for functional hybrids to be used in photoanodes or sensors.1–6 Their successful implementation makes it necessary to establish a profound mechanistic

understanding

related to the chemical functionalization of

metal oxide

nanostructures with complex organic molecules. This is particularly true for materials processing for device applications where the organic molecules’ chemical and optical properties complement those of the metal oxide substrate, which acts as a semiconducting backbone, as a catalyst or just as a high surface area support. Mechanistically, adsorption is the key step for the interface functionalization of the solid. Subsequent surface chemistry, where chemical bonds are formed or broken and where ions are exchanged between the molecule and the metal oxide surface can provide additional means for the generation of chemical and optical functionalities. Porphyrins are a particular promising and interesting class of functional organic molecules for the optoelectronic functionalization of semiconducting nanostructures. In the porphyrin molecule, the nature of the central metal atoms and the potential side groups determine the molecule’s electronic and chemical coupling to the oxide substrate. Subsequent metalation and selfmetalation reactions on metal and metal oxide surfaces have been studied for a variety of materials systems in great detail. Related activities span the range from surface science studies on well-defined 2D substrates7–15 to spectroscopic and/ or photo-electrochemical investigations on technologically relevant high surface area systems.3,4,6,16–21 A recent combined X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunnel Microscopy (STM) study addressed the reactive adsorption of the free base tetraphenylporphyrin (2HTPP, Figure 1a) on TiO2 rutile (110) single crystal surfaces.22



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Figure 1: Molecular schemes of a) free base tetraphenylporphyrin (2HTPP), b) the protonated tetraphenylporphyrin diacid (4HTPP2+) and c) the titanyl tetraphenylporphyrin (TiOTPP). The study revealed that depending on temperature and coverage, a number of interrelated porphyrin species can form upon protonation and ion exchange between the metal oxide surface and the porphyrin molecules (Figure 1). During room temperature adsorption, the first monolayer of 2HTPP (Figure 1a) is protonated by residual protons of the metal oxide surface and transforms into the diacid form 4HTPP2+ of the porphyrin (Figure 1b). Unprotonated 2HTPP was found to metalate at T = 400 K forming titanyl tetraphenylporphyrin (TiOTPP). Protonated 4HTPP2+ species on the other hand, metalate at even higher temperatures (i.e. T = 550 K) leading to the full conversion of the free base form into the titanyl form (Figure 1c).22 For the preparation of dye-functionalized sensors, catalysts23 and solar cells, TiO2 nanostructures are usually functionalized via liquid phase adsorption20 in more or less controlled reaction environments. Organic solvents of variable chemical composition and with sometimes unspecified residual water concentrations are employed for such processes. The aim of the present study is to explore for anatase TiO2 nanoparticles, which previously have been established as a model system for photocatalysis24 or for bio-nano interaction studies25,26, how the details of the functionalization process impact on the optoelectronic properties of porphyrinsensitized TiO2 hybrid materials. For this purpose we compared two different functionalization approaches: (i) porphyrin adsorption in organic liquids and (ii) gas phase functionalization where evaporated porphyrin molecules adsorb at dehydrated particle surfaces. This approach enabled us to explore the impact of organic solvent molecules and spurious water on the surface 4 ACS Paragon Plus Environment

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chemistry of porphyrin-sensitized TiO2 nanoparticles. Using Vis Diffuse Reflectance (Vis-DR) and photoluminescence (PL) emission spectroscopy as analytical techniques we obtained clear evidence for porphyrin protonation-deprotonation reactions in the presence of adsorbed water molecules. The here reported insights underline, that for the preparation of organic-inorganic hybrids minute changes in the chemical processing conditions have a strong impact on their spectroscopic and, thus, on their electronic properties.

Experimental Section Materials Synthesis and Processing TiO2 nanoparticles were prepared by metal-organic chemical vapor synthesis (MO-CVS).27 The CVS reactor consists of a fused silica tube placed inside a cylindrical furnace and a preheating zone to evaporate the precursor (Ti(IV)isopropoxid, Sigma Aldrich, 99.999 %) at T = 393 K. Argon gas (5.0) transports the gaseous precursor from the preheating zone into the furnace where decomposition occurs at 1073 K. Stable process conditions are guaranteed by the spatial separation of the precursor evaporation and the reaction zone. Continuous pumping keeps the residence time of resulting nuclei within the reactor short and prevents substantial coarsening and coalescence. After production, the TiO2 nanoparticles are transferred into fused silica cells, which allow thermal activation of the powder in high vacuum (p < 10-5 mbar) as well as in O2 atmosphere. Heating the as-obtained TiO2 powders to 873 K in high vacuum (p < 10-5 mbar) using a rate of 10 K min-1 and subsequent exposure to molecular oxygen at this temperature leads to efficient carbon removal.28 The activated anatase TiO2 nanoparticles, representative TEM images and Xray diffraction pattern are provided in the Supporting Information Figures S1-S3,are characterized by a narrow particle size distribution with a maximum at 12 nm (Figure S2) .29 Porphyrin adsorption 5 ACS Paragon Plus Environment


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For liquid phase adsorption of porphyrins30, a defined amount of 2H-tetraphenylporphyrin (Porphyrin Systems, 98.0 %) was dissolved in anhydrous toluene (Sigma Aldrich, ≥ 99.8 %) and stirred in this solution for 24 h. Afterwards, approximately 400 mg of TiO2 nanoparticle powder was dispersed in 25 mL of the stock solution with a porphyrin concentration of c = 1 x 10-3 mol L1

and stirred for another period of 24 h. In addition to room temperature adsorption, we

performed experiments at temperatures close to the boiling point of toluene (383 K) using a reflux condenser. The dye solution was kept in the dark by wrapping the flask in aluminum foil to prevent photochemical reactions during the adsorption process. Subsequently, the suspension was centrifuged (8 min, 4000 min-1) and the solid fraction of TiO2 particles was separated from the liquid phase. Then, the TiO2 powder was washed for several times in fresh toluene to remove loosely bound porphyrin molecules from the particle surfaces. The number of adsorbed porphyrin molecules was estimated from the photometrically determined porphyrin concentration in the supernatant solution. For the spectroscopic powder studies the solid fraction was dried first using a membrane pump, and then transferred into a fused silica cell. The cell was pumped down to pressures below p = 5 x 10-6 mbar with a turbomolecular pump at room temperature to remove physisorbed toluene. Gas phase adsorption19 was performed in a dedicated fused silica cell19 which consists of a flask for the thermal annealing of the TiO2 powder, a cuvette for spectroscopic measurements and an adaptor, that holds the porphyrin crystals and connects the cell to a high vacuum pumping rack at the same time. After TiO2 powder activation, the porphyrin crystals were transferred to the flask and physically mixed with the TiO2 powder. Then, the temperature is raised to 550 K for 120 min. This set-up, which is described in Figure S4 of the Supporting Information, allows us to thermally activate the TiO2 nanoparticle powder, to adsorb the porphyrin and to perform the spectroscopic measurements without breaking the vacuum. After a first set of measurements the powder was transferred into toluene, to eliminate residual 2HTPP crystals via dissolution and solvent removal. 6 ACS Paragon Plus Environment

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In another set of experiments we subsequently dosed water to the powder functionalized via the gas phase approach and adjusted the exposure time to water vapor. Prior to all these experiments, the water (Millipore, specific resistance: 18.2 MΩ·cm at 298 K) was purified by several freeze-pump-thaw cycles to remove dissolved gases such as oxygen. Spectroscopy A Perkin Elmer Lambda 750 spectrometer was used to record Vis transmission spectra of porphyrin solutions. Vis Diffuse Reflectance (Vis-DR) spectra of TiO2 nanoparticle powders were acquired using an integrating sphere. After porphyrin adsorption in liquid toluene dispersion, the samples were transferred to an optical high vacuum cell which allows for measurements either at pressures of p < 5 x 10-6 mbar or in defined gas atmospheres. All Vis-DR spectra were acquired in the presence of 100 mbar O2 to eliminated false luminescence related contributions to the absorption spectrum. Photoluminescence emission spectra of the powders were measured in the same cuvette at p < 5 x 10-6 mbar using an Edinburgh Instruments FLS 980 spectrometer. All spectroscopic measurements were performed at room temperature.

Results We performed three different functionalization approaches on TiO2 nanoparticle powders of the anatase modification: i) gas phase functionalization at T = 550 K and porphyrin adsorption in toluene dispersion ii) at T = 293 K and iii) at T = 383 K (Figure 2). (It has to be noted that toluene is an aprotic solvent, where the influence of protons on adsorption and subsequent surface chemistry can be neglected.)



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Figure 2: Schematic outline of the porphyrin adsorption experiments via the gas phase (middle panel) or performed in liquid toluene at different temperatures (right panel). The gas phase functionalized samples (middle panel) were contacted with liquid toluene or with water vapor in a subsequent step. The digital photographs (bottom row) were taken after the different porphyrin adsorption steps. Porphyrin adsorption at reduced pressures For the interpretation of Vis DR spectra of porphyrin sensitized TiO2 nanoparticle powders, the transmission spectra of TiOTPP and 2HTPP in toluene solution are shown in Figure 3a and b as reference. They contain two key absorption features in the visible that are related to π → π* transitions: the intense Soret band (S0 → S2) in the range between 400 and 450 nm and the weaker Q-Bands (S0 → S1) between 500 nm and 700 nm which also involve different vibrational states (Q(0,0) and Q(0,1)).31,32 Number and position of the Q-bands indicate whether the adsorbed species correspond to the metalated form (Figure 3a) or to its free base (Figure 3b). Whereas the non-metalated form shows four Q-bands at 515 nm, 550 nm, 590 nm and 645 nm,


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the increased symmetry of the metalated molecule reduces the number of Q-bands from four to two at 550 nm and 590 nm with a contribution of smaller intensity at 515 nm. Gas phase adsorption at 550 K transforms the white powder of TiO2 nanocrystals into an inhomogeneous yellow to reddish-brown colored powder (Figure 2). Related Vis-DR spectra (Figure 3c) reveal an intense Soret-band at 440 nm and a weaker Q-band at 560 nm with an absorption shoulder at 620 nm.

Figure 3: Vis transmission spectra of a) TiOTPP and b) 2HTPP acquired in toluene solution. Vis Diffuse Reflectance spectra acquired on c) TiO2 nanoparticles after gas phase adsorption at 550 K for 120 min and d) after subsequent contact with liquid toluene. All spectra were normalized and an offset was added. The inset spectra in the right column were multiplied by the factor of 12 (a and b) and 4 (c and d) for better conspicuity. 9 ACS Paragon Plus Environment


As a result of sample contact with liquid toluene, the reddish-brown powder coloration of the entire sample appears homogeneous to the eye.* The associated spectral changes (Figure 3d) are the Soret-band shift from 440 to 420 nm and the shift of the two Q-bands from 560 and 620 nm to 550 and 590 nm, respectively. In addition to band shifts and the decrease of the bandwidth, we also observed the emergence of a Q-band at 650 nm. Comparison of the transmission spectrum of 2HTPP in toluene (Figure 3b) and the DR powder spectrum after porphyrin adsorption via the gas phase (Figure 3c) points to the formation of the metalated porphyrin (i.e. TiOTPP) upon adsorption on the TiO2 nanoparticles. Apart from the slight red shift of the bands, the powder spectrum is in fair agreement with the solution spectrum of TiOTPP (Figure 3a). In addition to the TiOTPP specific bands, a band at 650 nm arises after sample contact with liquid toluene and indicates the presence of 4HTPP2+ (for details see later Figure 5d). The decrease in width of TiOTPP-specific bands (from c to d in the left panel of Figure 3) indicates the solvent-assisted re-organization of the adsorbed porphyrin molecules on the TiO2 nanoparticle surfaces becoming effective during powder immersion into liquid toluene.

* Remaining 2HTPP crystals (i.e. unreacted starting material) dissolve and can be removed from the functionalized powder during this washing step. At room temperature re-adsorption of dissolved porphyrin on the TiO2 powder occurs only to a minor extent. As will be shown below, there is no evidence for significant 2HTPP adsorption and subsequent metalation reaction under such conditions 10 ACS Paragon Plus Environment

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Figure 4: Photoluminescence emission spectra of TiO2 anatase nanoparticles a) after gas phase adsorption of 2HTPP at 550 K for 120 min, b) after admission and consecutive removal of liquid toluene. Emission spectra of TiOTPP and 2HTPP in toluene solution are shown in Figure 4c and d for comparison. All spectra were normalized and plotted with an offset for the sake of clarity. The bands at 660 and 755 nm that are marked with an asterisk in Figure 4a are assigned to residual porphyrin crystals in the non-metalated form. (This assignment is supported by reference spectra acquired on polycrystalline porphyrin samples shown in Figure S5 of the Supporting Information). We performed complementary photoluminescence emission measurements using excitation light of λExc. = 425 nm. All spectral features in the PL emission spectrum of the TiO2 powder after gas phase adsorption (Figure 4a) originate from unreacted and unconsumed 2HTPP crystals (for a 11 ACS Paragon Plus Environment


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detailed explanation see the experimental section. The absence of a respective spectral contribution in the Vis diffuse reflectance spectrum (Figure 3c) is attributed to the lower sensitivity of this technique.) After subsequent sample contact with liquid toluene, new emission bands at 595 nm, 650 nm and 715 nm arise (Figure 4b). Comparison of the powder spectrum (Figure 4b) with reference emission spectra of TiOTPP and 2HTPP in toluene (Figure 4c and d) allows one to identify the chemical nature of the adsorbed porphyrin. The emission band at 595 nm arises from the metalated porphyrin (see Figure 4c for comparison), whereas the band at 720 nm is attributed to the non-metalated porphyrin (compare Figure 4d). The band at 650 nm is composed of contributions from both 2HTPP and TiOTPP. The absence of any type of a signal contribution from TiOTPP prior to sample contact with liquid toluene seems to be inconsistent with the Vis-DR data (Figure 3). This discrepancy is resolved by the presence of quenching effects associated with the adsorbed TiOTPP species and of so far unspecified origin. 33,34

Porphyrin adsorption via the gas phase is apparently associated with a restricted mobility of the porphyrin molecules and gives rise to their inhomogeneous distribution throughout the particle powder. The Soret and Q-bands in the corresponding spectra are broadened and red-shifted as compared to molecules dissolved in toluene (Figure 3). Corresponding spectroscopic effects arise from intermolecular interactions inside molecular aggregates with additional contributions of the non-metalated form that originate from molecules without contact with the particle surface. From this we conclude that heterogeneous nucleation of porphyrin molecules in combination with local buildup of excess concentrations thereof favors porphyrin aggregation in particular regions on the TiO2 nanoparticle powders. Importantly, the solvent assisted redistribution and reorganization of the adsorbed porphyrins at the TiO2 nanoparticle surfaces gives rise to higher porphyrin dispersion (Figure 3d) and – at a macroscopic level – to a more homogeneous sample coloration. It is moreover important to note that TiOTPP specific PL emission bands appear only after subsequent contact with toluene (Figure 4b). This observation clearly 12 ACS Paragon Plus Environment

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underlines that the reorganization of porphyrins at the solid-liquid interface results in adsorbate geometries, which enable a more efficient radiative deactivation of the photoexcited porphyrin state. Water adsorption and porphyrin protonation The sequence of experiments reported so far has addressed the adsorption of 2HTPP on dehydrated TiO2 surfaces and was performed under high vacuum conditions.28,29 Moreover, before and after 2HTPP adsorption using this vacuum based gas phase approach, the surface hydroxyl concentration is significantly below that of samples prepared by a conventional dye sensitization approach.35 Sample transfer into the dye solution under ambient conditions and outside a glove box is inevitably associated with adsorption of spurious amounts of water.36 We therefore investigated the influence of water adsorption on the optical absorption properties of the pre-adsorbed porphyrin molecules using the above described gas phase approach, where respective samples (Figure 5a) were exposed to water vapor (p(H2O) = 23 mbar) at room temperature and for different time intervals (Figure 5b). In a subsequent experiment, we studied the stability of associated spectral changes at different temperatures in the range between 293 and 523 K and in vacuum (Figure 5c).



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Figure 5: Vis spectra of a) TiO2 nanoparticles after gas phase adsorption of 2HTPP at 550 K and for 120 minutes and b) after subsequent adsorption of water vapor for different time intervals c) after subsequent vacuum annealing induced water desorption, d) Reference TiOTPP and 2HTPP transmission spectra in toluene solution and 4HTPP2+ prepared by adding small amounts of HCl to a methanolic 2HTPP solution. All spectra were normalized and an offset was added. The inset spectra in the right column were magnified.


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As a result of sample contact with water vapor (Figure 5b), the powder adopts a light beige coloration, an effect being similar to that observed after sample immersion in liquid toluene (Figure 3d). The associated spectral changes are characterized by a Soret-band shift (Figure 5b) in parallel to band shifts in the Q-band region to higher energies and a significant narrowing of TiOTPP specific bands upon water vapor adsorption. All these effects are explained by a hydration induced increase in the surface mobility of TPP molecules which favors the more uniform distribution of adsorbed porphyrin molecules over the particle ensemble. At the molecular scale, both water and toluene molecules that adsorb in the proximity of porphyrin molecules seem to generate an electronic environment that is comparable to the one in solution (i.e. in a bulk liquid). Water exposure for longer time intervals produces a new band at 665 nm. At the same time, the intensity ratios between the Q-bands i.e. the shape of the envelope, changes significantly: the band at 555 nm loses intensity while the intensity of the band at 665 nm, which is indicative of a protonation reaction to form 4HTPP2+ (see the corresponding reference spectrum in Figure 5d), gains in intensity. The band at 620 nm shifts to smaller wavelengths and loses relative intensity (Figure 5b). Increase of the sample temperature during continuous pumping (p < 5 x 10-6 mbar) reverses the spectral changes induced by water adsorption. The Soret-band shifts to 440 nm, i.e. to its original position, and the characteristics of the Q-band region (Figure 5c) prior to water adsorption become re-established. Earlier FT-IR spectroscopic measurements (see Figure S6 in the Supporting Information Section) addressing H2O adsorption on bare TiO2 nanoparticle surfaces revealed that resulting absorption patterns are rather unspecific with regard to the way of water addition (e.g. via the gas phase or as a liquid). On the basis of these exploratory experiments we did not expect additional relevant information from FT-IR measurements that complement those described along Figure 5. 15 ACS Paragon Plus Environment


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Porphyrin adsorption in dispersions of liquid toluene Liquid phase adsorption of 2HTPP at room temperature leads to a homogeneous light beige colored powder (Figure 2) and the supernatant solution shows essentially no change in color and intensity. This process produces in the Vis-DR spectrum (Figure 6a) a weak Soret-band at 420 nm and Q-bands of less intensity at 520 nm, 595 nm and 645 nm. All features are indicative of adsorbed 2HTPP molecules that remain attached to the TiO2 nanoparticle surfaces (compare Figure 6d). Liquid phase adsorption at 383 K (Figure 2), however, bleaches the supernatant solution and leads to a darker reddish-brown color of the particle powder. The corresponding spectrum (Figure 6b) exhibits TiOTPP specific features, such as a broad Soret-band with a maximum at 430 nm and Q-bands at 555 and 590 nm. (A small contribution at 645 nm originates from 4HTPP2+ residues.) Photometric control experiments reveal that in average less than one porphyrin molecule per particle remains adsorbed after room temperature adsorption. For the corresponding adsorption experiment carried out at 383 K, this number can be increased by two orders of magnitude. If one assumes a flat lying adsorption geometry, less than 50 % of a nanoparticle monolayer can be covered by porphyrins using this functionalization approach.


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Figure 6: Vis spectra of 2HTPP on TiO2 anatase nanoparticles after liquid phase adsorption (c = 1 x 10-3 mol L-1) a) at room temperature and b) at T = 383 K. Solution spectra of c) TiOTPP and d) 2HTPP in toluene as reference. All spectra were normalized and an offset was added. The inset spectra in the right column were magnified by the factor of 3 (a and b) and 12 (c and d) for better conspicuity. Complementary photoluminescence spectra with emission bands at 655 nm and 720 nm suggest that irrespective of the adsorption temperature only non-metalated porphyrin species (Figure 7a and b) contribute to the overall signal.



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Figure 7: Photoluminescence emission spectra of TiO2 nanoparticles after porphyrin functionalization in liquid toluene with 2HTPP (c = 1 x 10-3 mol L-1). The adsorption experiments were performed a) at room temperature and b) at 383 K. Emission spectra of c) TiOTPP and d) 2HTPP dissolved in toluene are shown for comparison. There are substantial discrepancies between the optical absorption data (Vis-DR spectra, Figure 6 b) and the photoluminescence emission fingerprint (Figure 7 b) observed on the sample where TiO2 nanoparticle functionalization was carried out at T = 383 K in liquid toluene. The emission feature at 595 nm shows almost negligible intensity in Figure 7b and therefore indicates that photoluminescence related to adsorbed TiOTPP is quenched. Interestingly, such PL quenching effects are not observed on TiO2 powders sensitized at high vacuum conditions


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and subsequently washed with toluene (Figure 4b). In that case we observed a TiOTPP specific emission band at 595 nm. Our results clearly show that details in the protocol of the sensitization process have a significant impact on the optical absorption and photoluminescence emission properties of the resulting hybrid material. The lack of a significant TiOTPP specific emission on samples sensitized in toluene and the impact of surface hydroxylation deserves further investigation. The degree of surface hydroxylation on samples functionalized in the liquid phase is expected to be typically much higher as compared to those samples, where porphyrin adsorption on dehydrated and partially dehydroxylated TiO2 surfaces was performed without breaking the high vacuum conditions. Importantly, very recent results from a combined atomic force and Scanning Tunneling Microscopy study of porphyrin adsorption on rutile TiO2 (110) surfaces revealed that surface hydroxylation significantly modifies the electronic coupling of the molecule with the semiconductor.37 However, one must not forget that nanoparticles exhibit substantially higher surface defect concentrations, and, therefore, exhibit a dramatically altered surface reactivity as compared to well-defined singles crystal surfaces.

Conclusion For the first time we compared the surface functionalization of TiO2 with porphyrins in the gas phase with a functionalization approach carried out in liquid organic solution. For this purpose, Vis-DR and photoluminescence emission spectroscopy enabled us to track porphyrin adsorption and subsequent surface chemistry such as the protonation and/ or self-metalation of the free base porphyrins. For the solution-based functionalization approach at T = 383 K we observed the self-metalation reaction of the free base porphyrin. In the corresponding gas phase experiment, porphyrin adsorption and a consecutive metalation reaction do occur on dehydrated and partially 19 ACS Paragon Plus Environment


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dehydroxylated TiO2 nanoparticle surfaces at T = 550 K. Characteristic differences in the spectral envelope with respect to the spectrum of TiOTPP in liquid toluene, i.e. bathochromic shifts of the Soret- and Q-bands, vanish after contacting the powder with liquid toluene. Thus, the co-adsorbed solvent molecules generate an electronic environment for the adsorbed porphyrins that is comparable to that in the bulk liquid. Complementary photoluminescence emission measurements on samples with surface adsorbed TiOTPP molecules were found to be exempt from TiOTPP-specific spectroscopic fingerprints. This effect is attributed to the efficient binding of the porphyrin to the surface of the metal oxide nanoparticle causing efficient photoluminescence quenching34 e.g. by charge transfer across the porphyrin - TiO2 interface.33 A H2O adsorption experiment addressed the potential impact of co-adsorbed water molecules on the porphyrins’ spectroscopic properties and revealed a reversible porphyrin protonation reaction at room temperature. This protonation step can be reversed upon application of high vacuum at T = 523 K. Our findings underline that for the production of dye functionalized metal oxide nanostructures small variations in processing parameters, such as compositional changes in the solvent or the intentional or non-intentional addition of water during functionalization, have a strong influence on the bonding, subsequent surface chemistry and, conclusively, on structure and electronic properties of the adsorbate. All these features are of key importance for the performance of the resulting hybrid nanostructures as a major component of redox-active inks, as parts of sensors and photoelectrodes.

Supporting Information Electron microscopy data and X-ray diffraction pattern of the TiO2 anatase particle powders used, set-up a fused silica cell employed for sample functionalization and spectroscopy, additional photoluminescence and FT-IR spectra; The Supporting Information is available free of charge on the ACS Publications website 20 ACS Paragon Plus Environment

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at DOI:

Acknowledgments This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Research Unit FOR 1878 “funCOS – Functional Molecular Structures on Complex Oxide Surfaces”.



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TOC graphic


Reactive Porphyrin Adsorption on TiO2

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