Photoexcited Porphyrins Functionalizing TiO2 and SnO2 Nanocrystals

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Photoexcited Porphyrins Functionalizing TiO and SnO Nanocrystals Salvatrice Millesi, Raffaella Lo Nigro, Marco Pedroni, Adolfo Speghini, and Antonino Gulino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06574 • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on September 24, 2015

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The Journal of Physical Chemistry

Photoexcited Porphyrins Functionalizing TiO2 and SnO2 Nanocrystals Salvatrice Millesi,

a

Raffaella Lo Nigro,b Marco Pedroni,c Adolfo Speghini,

c

and Antonino

Gulino* a a

Dipartimento di Scienze Chimiche, Università di Catania, and INSTM Udr of Catania, viale A.

Doria 6, 95125 Catania, Italy, Tel: +39 095 7385067, email:[email protected] b

Istituto per la Microelettronica e Microsistemi (IMM)- CNR, Strada VIII n 5, 95121 Catania,

Italy c

Dipartimento di Biotecnologie, Università di Verona and INSTM, UdR Verona, Strada Le

Grazie 15, I-37134 Verona, Italy

ACS Paragon Plus Environment

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ABSTRACT: Porphyrins and metallo-porphyrins on TiO2 and SnO2 play a leading role for the exploitation of solar energy. A way to increase the device efficiency may be the synthesis of well-organized porphyrin molecules, covalently anchored to the appropriate semiconductor surface. These covalent systems overcome the problem of loss of efficiency during the electron injection since the dye layer is directly in contact (covalent) with the semiconductor surface. In the present study we show the synthesis, structure and optical properties of a monolayer of a porphyrin and its copper complex covalently assembled

on TiO2 and SnO2 powders. The

porphyrin-functionalized TiO2 and SnO2 nanocrystals show luminescence quenching that is strongly dependent on the oxide substrates.


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INTRODUCTION The combination of organic molecules and conducting metal oxides has delivered many novel hybrid functional materials showing unique properties.1 Dye sensitized solar cells (DSSC) are photovoltaic systems that convert visible light to electrical energy.2-8 Porphyrins and/or metallo-porphyrins assembled on nanostructured semiconducting metal oxides such as TiO2 or SnO2 play a leading role for the conversion of solar energy to electricity and for other optical applications.9-19 In fact, SnO2 is a prototype oxide that adopts the tetragonal rutile structure (lattice parameters a= 4.737Å, c= 3.369 Å), belongs to the D144h space group and contains two formula units per cell. In the SnO2 there is a good level of metal-oxygen covalency that involves the diffuse Sn 5s and 5p atomic orbitals.20 This covalency, in turn, causes a band-gap of 3.6 eV and a large O 2p valence band width (10 eV).20 In TiO2 the band-gap is 3.1 eV and the O 2p valence band width is 6.3 eV.20 These differences are due to the fact that the conduction band states in TiO2 are Ti 3d in nature whilst in SnO2 involve the more diffuse Sn 5s and 5p levels. In this context, these low dimensional nanostructured inorganic materials have been recognized as building blocks for the next generation technology.21-22 The attractive properties displayed by these nanostructures are based on their atomic scale structures and can be tailored for appropriate physico-chemical properties.23 For example, low dimensional SnO2 nanowires and nanobelts are highly sensitive and efficient transducers of surface chemical processes into electrical signals.24-25 Also nanoscale geometrical size and shape strongly influence the electrontransport process.26 As anticipated above, porphyrins assembled on TiO2 or SnO2 play a leading role in the DSSC technology. If these dyes are physisorbed on the oxide surfaces, through-space electron


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interactions dominate.27 In contrast, if the dyes are covalently assembled on the oxide surfaces,2846

through-bond electron interactions dominate thus increasing the device efficiency.27

Therefore well-organized porphyrin molecules, covalently anchored to appropriate nanosized conducting inorganic oxides, are promising materials for the next generation of molecular-based DSSC devices. In this perspective we recently reported on a two steps approach to obtain a free base porphyrin monolayer covalently bonded to semiconducting SnO2 nanocrystals.47 In the present study now we report on the synthesis, characterization and optical properties of a free base 5,10,15,20-tetrakis(4-hydroxyphenil)-21H,23H-porphyne (H2THPP) or its copper complex (CuTHPP) covalently assembled on TiO2 and SnO2 nanocrystals. These three new systems were never reported before. The optical properties under investigation will be a strong indication of the utility of the present systems for the DSSC technology. In fact, to investigate the donor–acceptor effect between the porphyrin monolayers and the metal oxide grains we have compared the luminescence spectra of the different functional nanostructures presently synthesized and we announce that covalency between dye and semiconducting nanocrystals plays a fundamental role in the electron injection and in all cases the SnO2 functionalized nanocrystals disclose the best donor–acceptor behaviour. EXPERIMENTAL DETAILS Synthetic procedure: TiO2 and SnO2 white powders (Aldrich) were annealed overnight to 600°C in a furnace using recrystallized alumina crucibles and then, left to cool to room temperature and transferred in a glove-box under a N2 atmosphere (1ppm H2O, 1 ppm O2). At that point, the single TiO2 and SnO2 powders were dispersed at room temperature in two n-pentane solutions (100 mL per each) 0.2 : 100 v/v of the chemisorptive reagent, trichloro[4-(chloromethyl)phenyl]silane (siloxane), soon removed


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from the glove-box in sealed flasks and sonicated for 60 min (25°C) to afford a coupling agent (CA) monolayer on each crystal grain.47 Afterward, the powder-containing flasks were moved back into the glove-box and the powders were decanted and washed with npentane 10 times while stirring. Then, they were removed from the glovebox and heated up to 135°C for 15 min in an oven to complete the CA grafting. Subsequently, the powders were sonicated twice in pentane for 5 min (25°C) to remove any eventual remaining physisorbed CA, decanted and transferred again in the glovebox. Then they were immersed into stirred 3.5 × 10-4 M CH3CN 100 mL solutions of 5,10,15,20tetrakis(4-hydroxyphenil)-21H,23H-porphyne, H2THPP, removed from the glovebox in sealed flasks, and sonicated for 7 h while heating at 50 °C. Then, the flasks were removed from the sonicator and heated-up to 70°C while stirring on a hotplate for 120 h. Similarly, some silanized TiO2 and SnO2 powders were also reacted with a 4.6 x 10-4 M CH3CN solution of the tetrakis(4-hydroxyphenyl)porphyrin copper complex (CuTHPP). Lastly, each suspension of the porphyrin (or CuTHPP ) functionalized TiO2 and SnO2 was cooled to room temperature and subjected to multiple cycles of sonication/centrifugationfiltration until there was no UV-vis evidence of any residual porphyrin in the remaining solutions. Each sonication (10 min) was performed in CH3CN and the related centrifugation-filtration was achieved using a column (Sartorius Stedim Biotech) containing a molecular membrane having a 3000 MWCO molecular weight cut-off, to remove any residual unreacted porphyrin. The filtration column was positioned at a fixed 25° angle in the centrifuge and, when subjected to 5000 rpm, the internal membrane restrains particles having a MWCO ≥ 3000. Pale yellow-green or pink nanocrystals were obtained for H2THPP or CuTHPP functionalized oxides, respectively.


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Characterization of the functionalized oxides. X-ray photoelectron spectra (XPS) were measured at 45°, relative to the surface sample holder, with a PHI 5600 Multi Technique System (base pressure of the main chamber 3×10–10 Torr).48-52 Spectra were excited with Al-Kα radiation. Structures due to the Kα3,4 satellite radiations were subtracted from the spectra prior to data processing.49 XPS peak intensities were obtained after a Shirley background removal. Spectra calibration was achieved by fixing the main C 1s peak at 285.0 eV.48 Experimental uncertainties in binding energy (B.E.) lie within ± 0.4 eV. UV-vis measurements were carried out on a UV-vis V-650 Jasco spectrometer and the spectra were recorded with a 0.2 nm resolution at room temperature. Room temperature luminescence measurements were carried out using a Jasco FP 8200 spectrofluorimeter with an excitation radiation of 418 nm. The optical spectral resolution was 1 nm. The emission was recorded at 90° with respect to the exciting line beam. All sample weighing for UV-vis and luminescence measurements were performed using a Mettler Toledo M3 microbalance with 1 µg precision. The morphology of both SnO2 and TiO2 powders was observed by Transmission Electron Microscopy (TEM). Powder samples for the electron microscopy were analyzed on copper TEM grids coated with a thin amorphous carbon film using a JEOL transmission electron microscope, using an accelerating voltage of 200 kV. Infrared transmittance spectra were recorded using a Jasco FT/IR-430 spectrometer in the 4000-400 cm-1 scan range, with an instrumental resolution of 4 cm-1 using KBr/sample mixtures.


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RESULT AND DISCUSSIONS Preparation Procedure: The adopted synthetic procedure involved functionalization of TiO2

and

SnO2

powders

with

the

bi-functional

chemisorptive

trichloro[4-

(chloromethyl)phenyl]silane (siloxane) and further covalent anchoring of the porphyrins. In particular the synthetic method (Scheme 1) involves the sonication of the solutions during the powder silanization process since this reduces the presence of grain agglomerates. In addition, the sonication during the first 7 hours of reaction of the silanized grains with the porphyrin solutions is helpful in avoiding the formation of silanized oxide grain aggregates. Moreover, the sonication should partially prevent also some porphyrin aggregation due to the well-known stacking effect. Finally, the reaction time for the porphyrin grafting was much larger (120 h) than that typically used.47 To resume, we have synthesized three new functional systems: a monolayer of H2THPP on TiO2 and a monolayer of CuTHPP on TiO2 and SnO2.

Scheme 1. Reaction pathway for TiO2 and SnO2 nanoparticles functionalized with a monolayer of H2THPP or CuTHPP.


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X-rays Photoelectron Spectroscopy: The electronic structures of all these systems have been investigated using the XPS technique.48-54 We have already demonstrated the fundamental contribution of the XPS in the study of nanostructures. In fact, this technique provides information on the bonding states of the grafted molecules and permits the valuation of the surface elemental composition.48 Figure 1 shows the XP spectrum of the TiO2 after the H2THPP grafting (hereafter H2THPP_TiO2) in the Ti 2p binding energy region. The two well resolved peaks at 459.3 and 465.1 eV represent the spin-orbit components of the Ti 2p3/2,1/2 states, respectively.55-56 These values are about 0.8 eV at higher B.E. (vide infra) with respect to those we measured for pure TiO2 annealed overnight at 600°C under a continuous 100 sccm (standard cubic centimetres per minute) flux of oxygen (Ti 2p3/2 = 458.5, 2p1/2 = 464.3 eV), being the last values typical of pure titania.55

Figure 1: Al-Kα excited XPS of the H2THPP_TiO2 in the Ti 2p binding energy region. Structures due to Kα3,4 satellites were subtracted from the spectra.

Figure 2 shows the XP spectrum of the SnO2 after the H2THPP grafting (hereafter H2THPP_ SnO2) in the Sn 3d binding energy region. The two well resolved peaks at 487.1 and 495.5 eV represent the spin-orbit components of the Sn 3d5/2,3/2 states, respectively.47,57-58 Also in this case


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the observed values are at 0.8 eV higher B.E. (vide infra) with respect to those we measured for pure SnO2 annealed overnight at 600°C under a continuous 100 sccm flux of oxygen (Sn 3d5/2 = 486.3, 3d3/2 = 494.7 eV).

Figure 2: Al-Kα excited XPS of the H2THPP_SnO2 in the Sn 3d binding energy region. Structures due to Kα3,4 satellites were subtracted from the spectra.

In contrast to these observations, the XPS N 1s main peak of both the H2THPP_TiO2 and H2THPP_SnO2 (Figure 3) lies at 400.1-400.2 eV that is at lower B.E. (vide infra) with respect to that of the pure H2THPP (400.4 eV). In agreement with many previous reported data on similar systems, a peak shape consisting of two components in a 1:3 ratio is evident (~398 and ~400 eV).59-68 The observed B.E. is consistent with a system that has received some electron density.


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Figure 3. Al-Kα excited XPS of the H2THPP_SnO2 (black line) and H2THPP_TiO2 (red line) in the N 1s binding energy region. Structures due to Kα3,4 satellites were subtracted from the spectra.

A similar behaviour can be observed for both CuTHPP_TiO2 and CuTHPP_SnO2 systems that show Cu 2p states at 934.8 ± 0.1 (Cu 2p3/2) and 954.6 ± 0.1 eV (Cu 2p1/2) (Figure 4), whilst these states in the CuTHPP itself lie at 935.1 and 954.9 eV.59,62,69 In addition, two well resolved shakeup satellites peaked at 944.0 and 963.8 eV confirm the presence of the different final states upon the ionization of the 2p levels in the open-shell Cu(II) d9 system.70 In fact the main peaks correspond to 2p53d10H final states, whereas the satellite structures correspond to 2p53d9 final states (H denotes a hole on the ligand after a charge-transfer processes).59 Furthermore, the XPS atomic concentration analyses confirmed for both systems a Cu/N atomic ratio of 0.25 thus excluding any presence of unmetallated porphyrin.

Figure 4. Al-Kα excited XPS of the CuTHPP_TiO2 in the Cu 2p binding energy region. Structures due to Kα3,4 satellites were subtracted from the spectra.


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All these observations indicate that both TiO2 and SnO2 release electrons to the tetrapyrrole porphyrin core regardless of the fact that it is free base or Cu-complexed. If stoichiometric, both TiO2 and SnO2 have filled O 2p-based valence bands and empty Ti 3d or Sn 5s conduction bands. In contrast, native donor defects arising from nonstoichiometry, which in turn are strongly dependent on the synthetic procedure adopted, introduce carriers in the conduction bands.20,58 It is recognized that oxygen vacancies (Vo) represent the main kind of defect in both oxides. When any of these n-type semiconductors is exposed to porphyrin molecules, some surface electrons are transferred to the porphyrin molecules as it occurs for oxygen molecules that are captured by the surface electrons and become adsorbed oxygen ions (O2-, O- and O2-). Obviously, the electron affinity of the porphyrin has to be higher than the work function of the oxide.71 In this case the adsorbed molecules create an electron depletion layer on the oxide grain surface that continues to grow until the Fermi energy of the bands of the porphyrin equals that of the given oxide. Since the depletion layer increases the energy barrier and hence increases the electrical resistance of the given oxide, it is possible to use these semiconductors for sensing by electrical measurements.25 It is important to note that all the above description applies to porphyrin molecules in their electronic ground states deposited on semiconducting surfaces, and this findings have an important impact on the rationalization of the mutual electronic charge transfers between dye and oxide. In fact they behave differently when the dye is in the excited state (vide infra).

FTIR measurements. The FTIR spectra of H2THPP_TiO2 (Figure S1), CuTHPP_TiO2 (Figure S2) and CuTHPP_SnO2 (Figure S3) show the characteristic νCH aromatic stretching at 2856 and


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2923-2929 cm-1 due to the siloxane coupling layer, and the νasym Si-CH2 mode at 1122-1128 cm1

. Moreover they show all the expected porphyrin bands. In particular, the whole 1100-1230 cm-

1

region shows the typical phenyl and pyrrole group “ring breathings”. At 1260-1285 cm-1 there

is a band due to the C-O-C ether group mode, absent in the H2THPP, that confirms the porphyrin grafting on the substrate surface. The weak signal at 1200 cm-1 is typical of the phenolic C-O stretching. Furthermore the C-N stretching mode appears at 1381-1385 cm-1. The weak band in the 1466-1472 cm-1 region is assigned to conjugated C-C symmetric stretching, while the intense band observed at 1400-1402 cm-1 is consistent with the aromatic O-H stretching. The band at 1635-1640 cm-1 is due ring stretching of benzene derivatives.

Absorption Spectroscopy and Transmission Electron Microscopy: Figure 5 shows the UV-vis spectra of 0.12 mg (± 1µg) of H2THPP_SnO2 (black line) and of 0.12 mg of H2THPP_TiO2_(red line) in 25 mL of N-methyl-2-pyrrolidone (NMP). The Soret bands appear at 425.4 nm for both SnO2 and TiO2 oxides with an evident red shift (7.4 nm) with respect to the porphyrin free-base indicative of interactions between the metal oxide and the porphyrin.47,59,68 No UV-vis evidence of any residual porphyrin was observed in the solutions after the last centrifugation-filtration cycle thus confirming that no unreacted porphyrin was physisorbed on the investigated systems. The Brunauer, Emmett, Teller (BET) surface area of SnO2 (6.9 m2/g) is smaller than that of TiO2 (9 m2/g) nevertheless we observe at 425.4 nm a larger absorbance (1.5 x 10-2) for the H2THPP_SnO2 (vs. 1.3 10-2 for the H2THPP_TiO2). As a consequence, 3.4 x 1018 porphyrin molecules/g have been grafted on the H2THPP_SnO2 (ε of H2THPP = 572200 M-1 cm-1 at 418.0 nm) while only 3.0 x 1018 molecules/g have been grafted on the H2THPP_TiO2 being their density ratio (3.4 x 1018/3.0 x 1018) = 1.1 (Table 1).59-60


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Figure 5. UV-vis spectra of 0.12 mg of H2THPP_SnO2 (black line) and 0.12 mg of H2THPP_TiO2 (red line) in 25 mL of NMP. In order to understand why we observe for the H2THPP_SnO2 an absorbance larger than that for the H2THPP_TiO2, nevertheless the surface area of SnO2 is smaller than that of TiO2, we performed TEM measurements. TEM micrographs of bare SnO2 nanoparticles annealed at 600°C in recrystallized alumina crucibles and sonicated in CH3CN reveal a large quantity of spherical particles having a grain size distribution in the 30-60 nm range (Figure 6A-B)26,72 and only a few particles show bigger size of about 90 nm. The spherical particles are jointed each other forming quite linear aggregates having lengths in the 150-300 nm (Figure 6C). Also the grain surface seems very smooth.

Figure 6. TEM images of the SnO2 nanoparticles annealed overnight at 600°C.


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Single annealed and sonicated TiO2 nanoparticles show similar mean sizes of about 30 nm, however in this case they form no linear but bigger and more rounded aggregates, of about 200400 nm range (Figure 7A-B).26,72 These TiO2 aggregates are consistent with the anatase phase, as demonstrated by the electron diffraction. In particular, the electron diffraction pattern (Figure 7C) shows several spots lying in circles related to the following plane distances and reflection typical of the anatase phase: 3.52 Å (101), 2.37 Å (004), 2.33 Å (112), 1.89 Å (200), 1.70 Å (105) and 1.67 Å (211).

Figure 7. TEM images (A and B) of the TiO2 nanoparticles annealed overnight at 600°C. Electron diffraction pattern (C) indicates the presence of the anatase phase.

Therefore, TEM analysis can give a rationale for the UV-vis results. In fact, the TiO2 grains tend to agglomerate thus reducing the surface available for porphyrin functionalization. This agglomeration is much less pronounced for SnO2 crystal grains. Figure 8 shows the UV-vis spectra of 0.12 mg of CuTHPP_SnO2 (black line) and of 0.12 mg of CuTHPP_TiO2_(red line) in 25 mL of NMP.59,62,73 The Soret bands appear at 424.0 and 423.4 nm for the SnO2 and TiO2 systems, respectively with evident red shifts (6.0 and 5.4 nm, respectively) with respect to the Cu-porphyrin complex.47,59,68 Again we observe for the


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CuTHPP_SnO2 an absorbance (4.1 x 10-3) (ε of CuTHPP = 155500 M-1 cm-1 at 418 nm ) larger than that for the CuTHPP_TiO2 (3.0 x 10-3).59-60 As a consequence, 3.2 x 1018 molecules/g have been grafted on the CuTHPP_SnO2 while only 2.4 x 1018 molecules/g have been grafted on the CuTHPP_TiO2. From these observation it emerges that the ratio between the number of porphyrin molecules/g and the number of Cu-complexed porphyrin molecules/g grafted on TiO2 is 3.0 x 1018/2.4 x 1018 = 1.3. Noteworthy, also the ratio between the number of porphyrin molecules/g and the number of Cu-complexed porphyrin molecules/g grafted on SnO2 is similar to the above value being 3.4 x 1018/3.2 x 1018 = 1.1. It is clear that there must be a mechanism that increases the footprint of the Cu-complexed porphyrin molecules on oxide surfaces and, in a very close system, this behavior was interpreted in terms of increased face-to-face repulsions between porphyrin molecules due to the presence of Cu++ ions in the porphyrin core.62 In this case the ratio of the CuTHPP molecules/g grafted on SnO2 / the CuTHPP molecules/g grafted on TiO2 (3.2 x 1018 molecules/g)/( 2.4 x 1018 molecules/g) is 1.4 and this is due to the above specified tendency of TiO2 grains to agglomerate more than the SnO2 crystal grains. Therefore combined TEM measurements and UV-vis results were fundamental to give a rationale for the optical data.


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Figure 8. UV-vis spectra of 0.12 mg of CuTHPP_SnO2 (black line) and of 0.12 mg of CuTHPP_TiO2_(red line) in 25 mL of NMP.

Luminescence Spectroscopy: Figure 9 shows the luminescence spectra of H2THPP_SnO2 and of H2THPP_TiO2.53-54 The H2THPP_SnO2 luminescence is less intense than that observed for the H2THPP_TiO2 (ITi/ISn = 1.2, where ITi and ISn refer to the luminescence intensity observed at 654.0 nm for 0.12 mg of H2THPP_TiO2 and 0.12 mg of H2THPP_SnO2, respectively in 25 mL of CH3CN), whilst the absorbances showed an opposite trend (Figure 5). In practice there is a larger number of porphyrin molecules/g grafted on SnO2, as evidenced by the UV-vis measurements, but we observe a less intense porphyrin luminescence.

Figure 9. Luminescence spectra of 0.12 mg of H2THPP_SnO2 (black line), of 0.12 mg of H2THPP_TiO2 (red line) in 25 mL of CH3CN (λexc = 418 nm) and of a CH3CN solution of H2HPP (blue line) containing in 25 mL a number of porphyrin molecules identical to that found in H2THPP_TiO2 (1.91 x 10-8 M, Table 1).


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This behavior is further evident in the case of CuTHPP_SnO2 and of CuTHPP_TiO2 being the ITi/ISn = 1.9 (Figure 10) and reveals the role of central metal ions, that affect the overall porphyrin energetics, in the electron transfer processes.74 In fact there is a relevant decrease in the luminescence intensity of the CuTHPP_SnO2 as compared with the TiO2 analogue or, even better, with the luminescence observed for a CuTHPP solution having the same porphyrin concentration. This is an evidence of the fact that Cu modifies the oxidation potential (or LUMO) of the dye. We already noted (vide supra) that XPS had excluded any presence of unmetallated porphyrin on these systems.

Figure 10. Luminescence spectra of 0.12 mg of CuTHPP_SnO2 (black line), of 0.12 mg of CuTHPP_TiO2_(red line) in 25 mL of CH3CN (λexc = 418 nm) and of a CH3CN solution of CuTHPP (blue line) containing in 25 mL a number of Cu-porphyrin molecules identical to that found in CuTHPP_TiO2 (2.39 x 10-8 M, Table 1). In this context, the observed less intense porphyrin or Cu(II) porphyrin luminescence on the SnO2 substrate cannot be ascribed to a quenching effect due to porphyrin stacking interaction since in all cases the observed porphyrin or Cu-porphyrin footprints (Table 1) are large enough to rule out stacking effects.47


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_______________________________________________________ Table 1. Number of grafted molecules/g and molecule footprint.

N° mol / g 3.0 x 1018

Mol/L] 2.39 x 10-8

Footprint Å2] 300

H2THPP_SnO2 3.4 x 1018 CuTHPP_TiO2 2.4 x 1018

2.71 x 10-8 1.91 x 10-8

205 375

CuTHPP_SnO2 3.2 x 1018

2.55 x 10-8

219

System H2THPP_TiO2

____________________________________________________ The observed luminescence intensity trend can find an explanation by considering that the conduction band of the SnO2 is mainly Sn 5s in character and that of TiO2 is represented by the Ti 3d levels. In the simple metals the B.E. of the Sn 5s states is about 12 eV while the B.E. of the Ti 3d states is about 8 eV. In addition, also the Pauling electronegativity of Sn (1.8) is larger than that of Ti (1.5). Therefore, simple naïve chemical intuition suggests that the Sn 5s levels lie at lower (more negative) energy than the Ti 3d and, as a consequence, electron injection should be easier towards SnO2 than towards TiO2. This observation finds a support in the recent study of Brudvig and Schmuttenmaer who, using time resolved terahertz spectroscopy, investigated the energy levels and the photoexcited electron injection dynamics of some porphyrins and Cuporphyrins bound to TiO2 and SnO2 nanoparticles and arrived at the same conclusion.71 Generally, inclusion of copper into the porphyrin structure enhances intersystem crossing into the triplet state thus lowering the singlet emission. A triplet state is unlikely to lead to electron injection into the TiO2/SnO2 nanoparticles. In fact there is a definite decrease in the emission intensity on going from the porphyrin free-base to the copper complex. In our systems we evidenced a significant decrease in the luminescence intensity on passing from the porphyrin free


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base or Cu-porphyrin complex to their related covalently assembled monolayers on TiO2 or SnO2. This is an evidence of how important may be that the first layer of the dye were covalently assembled on the semiconducting oxide surfaces instead of to simple be physisorbed.75-77 To resume, porphyrins in the excited states inject electrons into conduction bands of the TiO2 and SnO2 nanoparticles and, the larger electron injection the lower luminescence intensity.47 As a control check of the above observation we measured the luminescence spectrum of a sample of CuTHPP physisorbed on SnO2 (CuTHPP-SnO2) in absence of the coupling agent, obtained using the same treatments and reaction conditions reported above for the covalent assembly, apart the experiments involving the formation of a siloxane coupling layer. This final suspension was subjected to cycles of sonication/centrifugation-filtration until the UV-vis absorbance of 0.14 mg of CuTHPP-SnO2 in 25 mL of NMP was similar to that obtained for the covalent parent system.

The related luminescence spectrum (Figure 11, black line) was

compared with a CH3CN solution of CuTHPP (Figure 11, blue line) containing a number of Cuporphyrin molecules identical to that found in CuTHPP-SnO2. It emerged that the luminescence intensity decrease of CuTHPP-SnO2 is much less pronounced with respect to the covalent system. This is a further indication of the importance of the covalently assembly for the electron injection.


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Figure 11. Luminescence spectra of 0.14 mg of CuTHPP-SnO2 (black line) in 25 mL of CH3CN (λexc = 418 nm) and of a CH3CN solution of CuTHPP (blue line) containing a number of Cu-porphyrin molecules identical to that found in CuTHPP-SnO2 (2.96 x 10-8 M).

CONCLUSION Porphyrins and/or metallo-porphyrins assembled on TiO2 or SnO2 play a leading role for the conversion of solar energy to electricity and for other optical applications. In the present study we reported on the synthesis, characterization and optical properties of the new monolayer of a free-base porphyrin or its copper complex covalently assembled

on TiO2 and SnO2

nanocrystals. We proposed an important synthesis pathway to obtain covalency between the dyes and the oxide nanocrystals and presented a unique collection of experimental data (XPS, IR, UV, TEM, PL) on these nanostructures. Since macroscopic properties are always related to the electronic structure, by fine measurements of the binding energies we demonstrated that in the ground state both TiO2 and SnO2 release electrons to the tetrapyrrole porphyrin core thus


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forming a depletion layer on the oxide grain surface. In contrast when excited at 418 nm porphyrins transfer electrons towards SnO2 and TiO2. The description of this behaviour has an important impact on the rationalization of the mutual electronic charge transfers between dye and oxide. The donor–acceptor properties between the porphyrin monolayers and the metal oxides have been investigated by studying their optical properties. The electron injection from the porphyrin or Cu-porphyrin into the conduction bands of SnO2 or TiO2 resulted in an evident decrease of the related luminescence. In these cases, the electron injection should occur mainly via chemical bonds. In fact, the porphyrins are covalently assembled on the oxide surfaces and through-bond electron interactions are possible. Therefore our study evidences that covalency between the dye and semiconducting nanocrystals plays a fundamental role in the electron injection and this is of relevance for DSSC. The overall results are consistent with the convenience of the present systems for the next generation of DSSC technology.

Supporting Information. FT-IR of H2THPP_TiO2 (Figure S1), CuTHPP_TiO2 (Figure S2) and CuTHPP_SnO2 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Prof. Antonino Gulino, Dipartimento di Scienze Chimiche, Università di Catania, and INSTM Udr of Catania, viale A. Doria 6, 95125 Catania, Italy, Phone: +39-095-7385067, Fax. +39-095580138, E-mail: [email protected]. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENT AG thanks the MIUR, the FIRB projects RINAME (RBAP114AMK) and University of Catania. A.S and M.P gratefully acknowledge the company “Performance in Lighting” (Colognola ai Colli, Verona, Italy), for financial support in the framework of the Joint Projects 2011 of the University of Verona, Italy.


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separation in a π-conjugated antenna polymer bound to TiO2. J. Phys. Chem. C 2014, 118, 28535-28541. (76) Schuetz, R.; Malhotra, S.; Thomas, I.; Strothkaemper, C.; Bartelt, A.; Schwarzburg, K.; Hannappel, T.; Fasting, C.; Eichberger, R. Dynamics of a covalently conjoined FRET dye ensemble for electron injection into ZnO nanorods. J. Phys. Chem. C 2014, 118, 93369345. (77) Natali, M.; Ravaglia, M.; Scandola, F.; Boixel, J.; Pellegrin, Y.; Blart, E.; Odobel, F. Long-Range Charge Separation in a Ferrocene−(Zinc Porphyrin) Naphthalenediimide Triad. Asymmetric Role of 1,2,3-Triazole Linkers. J. Phys. Chem. C 2013, 117, 19334−19345.


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Table of Contents Porphyrin-functionalized TCO nanocrystals show luminescence quenching that is strongly dependent on the oxide substrates.


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Photoexcited Porphyrins Functionalizing TiO2 and SnO2 Nanocrystals

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