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Energy Level Alignment of a Zinc(II) Tetraphenylporphyrin Dye Adsorbed onto TiO2(110) and ZnO(112j0) Surfaces Sylvie Rangan,*,† Senia Katalinic,† Ryan Thorpe,† Robert Allen Bartynski,† Jonathan Rochford,‡ and Elena Galoppini‡ Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers UniVersity, 136 Frelinghuysen Road, Piscataway, New Jersey 08854, and Chemistry Department, Rutgers UniVersity, 73 Warren Street, Newark, New Jersey 07012 ReceiVed: September 28, 2009; ReVised Manuscript ReceiVed: October 19, 2009
The electronic structure of the Zn(II)-5-(3,5-dicarboxyphenyl)-10,15,20-triphenylporphyrin dye (ZnTPP-Ipa), chemisorbed onto ZnO(112j0) and TiO2(110) single-crystal surfaces, has been investigated by means of density functional theory (DFT) and by electron spectroscopy methods in an ultra-high-vacuum environment. The core levels (Ti 2p and Zn 2p) as well as the valence band have been probed using X-ray and ultraviolet photoemission spectroscopies, whereas the conduction band has been evaluated from inverse photoemission spectroscopy. The calculated density of states for the gas phase molecule compares well to the experimentally determined electronic structure, allowing both a simple understanding of the adsorbate electronic properties and a direct determination of the ZnTPP-Ipa frontier orbitals with respect to the substrates’ band edges. Introduction Since the breakthrough work of O’Regan and Gra¨tzel, dyesensitized solar cells (DSSCs) have been shown to convert solar light into electricity with promising efficiencies and have attracted considerable interest in the fundamental aspect of their operation.1 At the heart of the device is a thin oxide film, composed of a network of nanocrystalline TiO2 particles, deposited on a transparent conducting substrate and sintered so as to establish an effective conduction path. This nanoporous structure is then sensitized with an organic dye molecule and infiltrated with an electrolyte, which, in turn, makes contact to a counter electrode. As the band gap of TiO2 is over 3 eV, only ultraviolet radiation directly produces electron-hole pairs in the native material. However, with the appropriate alignment of the electronic levels, sensitization by a chemisorbed dye molecule capable of harvesting photons of energy smaller than the TiO2 band gap, enables efficient absorption across a large fraction of the solar spectrum. Figure 1a schematically represents the ground-state energy alignment between the three main components of DSSCs: a wide band gap semiconductor substrate, a dye molecule chemisorbed onto this substrate, and an electrolyte in contact with the dye. Focusing on the dye/oxide interface, a light-harvesting dye molecule can absorb a photon from within the visible region of the solar spectrum, resulting in a photoexcitation that can, in the simplest terms, be thought of as the elevation of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the dye (Figure 1a, step 1). If the LUMO is degenerate with the substrate conduction band, the excited electron can transfer to the substrate (Figure 1a, step 2) and participate in current flow across the cell. The resulting singly occupied HOMO of the oxidized dye can then be filled via an appropriately chosen electrolyte * To whom correspondence should be addressed. E-mail: rangan@ physics.rutgers.edu. † Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers University. ‡ Chemistry Department, Rutgers University.
Figure 1. (a) Schematic energy level alignment in a DSSC semiconductor/dye/electrolyte system: (step 1) photoinduced electron excitation from the dye HOMO to the dye LUMO, (step 2) electron transfer to the semiconductor, and (step 3) dye regeneration via electron transfer from the electrolyte into the dye HOMO. (b) Idealized geometry of the ZnTPP-Ipa dye considered in this study.
whose redox potential is above the dye HOMO level, thus regenerating the ground-state dye molecule (Figure 1a, step 3). Metalloporphyrins play an essential role in, for example, the light-harvesting antennae and the reaction center of the photosystem II complex2 and, therefore, are attractive candidates for photoinduced electron-transfer mediators in DSSCs. Among metalloporphyrins, zinc tetraphenylporphyrin (ZnTPP) derivatives have been found to have similar electron injection and charge recombination properties as N3 dye, the standard ruthenium-containing dye used for DSSCs3 while exhibiting reasonable performances using either nanostructured TiO2 or ZnO as substrates.4,5 Moreover, this class of molecules includes efficient dyes and are also useful models to study interfacial electronic processes.6,7 In particular, porphyrin arrays have been proposed as light harvesters on planar surfaces with the advantages of extending the visible-region absorption capabilities and allowing efficient electron transfer to the interface.4,8,9
10.1021/jp909320f 2010 American Chemical Society Published on Web 11/10/2009
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Figure 2. Representative molecules considered in this study: (a) ZnTPP-Ipa, (b) ZnP, (c) ZnTPP, and (d) Ipa.
Anchoring of the ZnTPP to TiO2 and ZnO can be achieved through carboxylic acid functionalization of the phenyl groups. Several binding modes have been reported for carboxylic groups on oxide surfaces, ranging from chemisorption to hydrogenbonding-derived adsorption.4,10,11 The main binding modes observed for this class of molecules are a chelating and/or a bidentate bridging of the carboxylate on the surface.12 In this study, in an effort to reduce the possible adsorption configurations on the surface, we have chosen the ZnTPP-Ipa molecule (Figure 2a), shown in an idealized schematic diagram in Figure 1b. From simple geometrical considerations, the molecule is expected to bond with a large angle between the plane of the porphyrin ring and the plane of the substrate surface.4,13 The electronic properties of the ZnTPPs are generally understood in terms of the Gouterman four orbitals model, which states that the degeneracy of the two eg LUMOs and of the two nearly degenerated a2u and a1u HOMOs causes mixing between the two 1(a2ueg) and 1(a1ue g) electronic states via configuration interaction, resulting in a two-band optical absorption spectrum: an intense B (or Soret band) in the near UV region and a weaker Q band at lower energy.14 To date, experimental investigations of the molecule-nanoparticle, particularly porphyrin-metal oxide, interface have been predominantly carried out in a condensed phase environment using optical spectroscopic methods (UV-vis absorption, fluorescence emission) and electrochemical techniques.3,4,12,13 Theoretical analysis of these measurements uses a complex convolution of occupied and unoccupied states that have been calculated using density functional theory (DFT) and time-dependent DFT studies.15-18 For a more direct probing of the unoccupied electronic structure, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has provided information on the electronic structure modification (mostly from strong π* levels) upon metal insertion or complexation with gaseous species.19-21 However, owing to relaxation of the electronic levels that can occur upon core hole creation, this technique does not reflect the ground state of the system.22 In this paper, we report on a combined direct (XPS and UPS) and inverse photoemission (IPS) spectroscopy study, performed in a single ultra-high-vacuum (UHV) chamber, of the ZnTPPIpa molecule (Figure 2a) adsorbed on the ZnO(112j0) and TiO2(110) surfaces, directly probing both occupied and unoccupied electronic states. In particular, we determine the alignment of the HOMO and LUMO levels of the adsorbed ZnTPPIpa dye with respect to the band edges of the semiconductor substrate. In addition, we have performed DFT calculations of the gas-phase ZnTPP-Ipa molecule to aid interpretation of the experimental results. Numerous factors, such as the nature of
the substrate (nanoparticles vs single crystal),1 the solvent and electrolyte choice,23 the pH,24 and other additives, can cause the real DSSC environment to deviate from an idealized dye/ single-crystal substrate system.25 However, using a UHV approach, important fundamental parameters can be explored with a high degree of control of the surface properties, allowing direct access to the electronic structure and a fundamental understanding of energy level alignment in dye/semiconductors systems. From this perspective, this work provides a direct experimental measurement of quantities closely related to the ground-state electronic properties of a ZnTPP-Ipa dye chemisorbed onto ZnO(112j0) and TiO2(110) single-crystal substrates. Techniques Spectroscopic Methods. The measurements presented here were obtained using a single ultra-high-vacuum experimental chamber that housed instrumentation for X-ray and ultraviolet photoemission spectroscopies (XPS and UPS) as well as inverse photoemission (IPS) spectroscopy described in detail elsewhere.26 The base pressure of the chamber was less than 5 × 10-10 Torr. Valence band photoelectrons were excited using a Leybold-Heraeus helium discharge photon source (HeII: 40.8 eV), and core levels were probed using the nonmonochromatized Al KR line of a SPECS XR50 dual anode source. The energy analysis of the emitted electrons was performed in an angleintegrated mode using a double pass Phi 15-255G cylindrical mirror analyzer (CMA). The axes of the photon sources and the CMA formed a 90° angle, and the sample normal was oriented midway between the two. Inverse photoemission spectra were obtained using a grating spectrometer, described in detail elsewhere,27 which was mounted on the same experimental chamber. Briefly, a wellcollimated, monoenergetic electron beam (primary energy Ep ) 20.3 eV in this study) was directed toward the sample along the surface normal. The electrons couple to high-lying unoccupied states and a subset relax via a direct optical transition to low-lying unoccupied states in the conduction band, emitting a photon in the process. The photons were dispersed by a concave spherical diffraction grating and detected by a microchannel plate with a position-sensitive resistive anode encoder. With this approach, the intensity of photons as a function of photon energy reflects the density of unoccupied states in the conduction band. In the photoemission and inverse photoemission spectra, the valence band maximum (VBM) and the conduction band minimum (CBM) are both measured with respect to the Fermi level of a gold sample in contact with the oxide substrate. The overall energy resolutions (fwhm) for the UPS and IPS spectra are estimated to be 0.3 and 0.6 eV, respectively.
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TABLE 1: Characteristic Calculated Interatomic Distances (in Å) Compared to Reported Experimental Values for the ZnTTP Molecule and Averaged for a D4h Symmetry. The Atom Labeling Corresponds to the One of Figure 2 dZn-N dN-C1 dC1-C2 dC2-C3 dC4-C5
ZnP
ZnTPP
ZnTPP exptl29
2.038 1.375 1.445 1.363 1.395
2.037 1.378 1.446 1.361 1.405
2.042 1.374 1.425 1.374 1.414
The IPS measurements presented in this work have been taken while keeping a small sample current (0.5-1 µA) for a beam size of 1 mm2 and by sampling several spots on a large 1 cm2 sample with short beam exposure (3-10 min). For such electron doses, no beam damage was observed during IPS nor in subsequent UPS and XPS spectra. Sample Preparation. The rutile TiO2 sample was a commercially produced single crystal from MTI corporation, cut to within 0.5° of the (110) plane. The ZnO sample was an epitaxial film CVD-grown on an Al2O3 substrate with the c axis of the film lying in the plane and the (112j0) surface exposed. Both samples were degassed and prepared in UHV through several cycles of 1 keV Ar + ion sputtering (while maintaining a maximum sample current of 2 µA) and annealing in UHV at 900 K for TiO2 and 800 K for ZnO. The cleanliness of the surfaces was checked using XPS, and the surface geometry was assessed by low-energy electron diffraction. Following an ex situ 20 min sensitization in a solution of dye in anhydrous ethanol and rinsing with anhydrous ethanol to prevent dye accumulation, the sample was reintroduced into the UHV analysis chamber. The synthesis details of the ZnTPP-Ipa can be found elsewhere.13 Computational. Electronic structure calculations were performed with the GAMESS(US) software package using Becke3Lee-Yang-Parr (B3LYP) three parameter DFT theory.28 Geometries of local minima on the potential energy surface were calculated with a 6-31G* basis set for nitrogen, carbon, and zinc. The H ligands were described by a 6-31G basis. The density of states was obtained as a sum of the individual electronic states convoluted with a 1 eV full width at halfmaximum Gaussian function. Results and Discussion Calculated Electronic Structure of ZnTPP Derivatives. Porphyrins, in particular, Zn(II) porphyrin (ZnP) (as shown in Figure 2b) or ZnTPP (Figure 2c), have been the subject of numerous theoretical studies.16-18 However, to obtain a comprehensive comparison of the electronic structure of these molecules to the present experimental results, we have performed calculations of the electronic structure of the ZnTPP molecules and (un)functionalized phenyls in order to simulate in a simple way the electronic DOS of the ZnTPP-Ipa molecule. Geometry Optimization. The calculated geometries of the ZnP and ZnTPP molecules (shown in Figure 2b,c, respectively), optimized in the D4h symmetry, are in good agreement with previous experimental and theoretical studies of these molecules.16-18 Some characteristic interatomic distances are given in Table 1 and are compared to previous X-ray diffraction data, averaged here for a D4h symmetry.29 In particular, the critical Zn-N distance is well-described. For comparison, the electronic structures of a simple phenyl (Ph) and an isophtalic acid molecule (Ipa in Figure 2d) have been calculated using the same scheme.
ZnTPP-Ipa Electronic Structure. The calculated DOS for ZnP and ZnTPP, as well as the shapes of their Ko¨hn-Sham HOMO and LUMO orbitals, are compared Figure 3. The frontier orbitals obtained for ZnP are fully delocalized over the entire ring and are similar to those reported in earlier work.14-18 The molecular orbital energy positions are shown by the red bars in the figure, and the frontier orbital energies are tabulated in Table 2. The two nearly degenerate HOMOs are separated by 0.01 eV; the lowest orbital being of a2u symmetry is found at -5.12 eV and the highest of a1u symmetry is found at -5.11 eV (note that these energies are referenced to the zero of the calculation). The two LUMOs of eg symmetry are degenerate and found at -2.04 eV. The ZnP DOS, obtained as a sum of 1 eV fwhm Gaussian function centered on the calculated molecular orbital energy positions, is shown as the red curve in Figure 3. The shapes of the four frontier orbitals of ZnTPP are reported in Figure 3 and display an interesting feature of the metallo-TPP molecule family. Because the meso-phenyl rings are perpendicular to the main porphyrin ring, there is little or no conjugation between the ZnP ring and the phenyls. As a consequence, the HOMOs and LUMOs have almost no weight on the meso-phenyl groups and are quite similar to the HOMOs and LUMOs found for ZnP. From Table 2 a small difference that can be noted is that the order of the two HOMOs is reversed in this case, leading to an a1u symmetry HOMO-1 at -5.02 eV and an a2u symmetry HOMO at -4.95 eV. The LUMO and LUMO+1 levels are degenerate in energy and found at -2.00 eV. The similarities between the frontier orbitals of the two molecules can obviously be found when comparing the HOMOs and LUMOs regions of the ZnTPP calculated orbital energies and DOS (green bars and curve in Figure 3) to those of ZnP. Notable differences appear only roughly 2 eV above the LUMO levels and 2 eV below the HOMO levels, corresponding to molecular levels that are mostly centered on the meso-phenyl groups (and noted as phenyls in Figure 3). A direct consequence of this electronic structure is that the optical absorption properties of the ZnTPP are expected to depend mainly on the ZnP macrocycle. This observation can, in fact, be extrapolated to all ZnTPP derivatives: as the meso-phenyl groups are unconjugated with the main ZnP ring, the functionalization of the phenyl groups will not modify drastically the optical absorption properties. The particular electronic structure of these molecules raises the question as to whether we can conclude from the previous remarks that the ZnP and the phenyls (or functionalized phenyls) can be considered as fully independent moieties in ZnTPP derivatives. To test this hypothesis, we have also optimized a phenyl (Ph) and isophtalic acid (Ipa). The corresponding most relevant molecular orbital energies are reported in the Table 2. The simple phenyl has two degenerate HOMOs and LUMOs found, respectively, at -6.60 and +0.19 eV. Compared to the ZnTPP phenyl-related molecular orbitals, the occupied states of the simple phenyl match its ZnTPP counterparts. In the unoccupied states, the phenyl-related states of the ZnTPP are found slightly lower than the LUMOs of the simple phenyl. Figure 4 compares the calculated DOS for ZnP (in red), Ph (blue curve), Ipa (black curve), and ZnTPP (continuous green curve). For comparison, the figure includes the DOS that is obtained assuming a perfect decoupling between the main ZnP ring and the meso-phenyl, that is, the sum (DOS(ZnP) + 4 DOS(Ph)) (green dotted curve) superimposed to the DOS of the full ZnTPP molecule. A few differences appear when comparing the full ZnTPP DOS to the sum of the separated DOSs, but to first order, the electronic structure of the ZnTPP molecule can be under-
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Figure 3. Calculated density of states for the ZnP and the ZnTPP molecules. The four frontier orbitals of the two molecules are also shown and their symmetry indicated.
TABLE 2: Characteristic Frontier Molecular Orbital Energies (in eV) Obtained for ZnP, ZnTPP, Ph, and Ipa ZnP frontier orbitals
LUMO LUMO HOMO HOMO+1
ZnP
ZnTPP
-2.04 (eg) -2.04 (eg) -5.11 (a1u) -5.12 (a2u)
0.02 0.02 -2.00 (eg) -2.00 (eg) -4.95 (a2u) -5.02 (a1u) -6.63 -6.63
Ph
Ipa
0.19 0.19
-1.21 -1.66
-6.60 -6.60
-7.34 -7.38
stood as the superposition of several independent moieties. This result is in good agreement with previous experimental observations for a series of Zn(II) tetraphenylporphyrins functionalized with rigid-rod anchoring groups.13 We believe that this observation could be useful for understanding the electronic structure of other ZnTPP derivatives, leading to simpler DOS models for these large molecules. Similarly, the DOS of the ZnTPP-Ipa molecule can be approximated by the sum (DOS(ZnP) + 3 DOS(Ph) + DOS(Ipa)) (red dotted curve). Compared to a simple ZnTPP, a few differences are expected for the DOS of the ZnTPP-Ipa considered here, mostly due to the presence of the two carboxylic groups on one of the meso-phenyls. As illustrated in Table 2 and in Figure 4 (black curve), the electronic levels of the Ipa molecule are pulled down in energy with respect to those of the unfunctionalized phenyl ring. This can be easily explained by a lesser screening of the nuclei of the phenyl group due to the addition of the two COOH electron-withdrawing groups. However, because the ZnTPP-Ipa molecule contains only one functionalized meso-phenyl, the total DOS is found only slightly modified with respect to the DOS of the ZnTPP molecule. Experimental Spectra Substrate Band Edges. Great care has been taken in the preparation of the two substrates in UHV. The quality of the
Figure 4. Calculated DOS of Ipa (black), Ph (blue), ZnP (red), and ZnTPP (green) molecules. The ZnTPP DOS (green) can be directly compared to the estimated DOS for ZnTPP using a sum (DOS(ZnP) + 4 DOS(Ph)) (dotted green) and to the DOS calculated for ZnTPPIpa as (DOS(ZnP) + 3 DOS(Ph) + DOS(Ipa)) (dotted red).
surfaces was assessed by LEED, which showed bright, sharp spots on a low background. The absence of surface contamination as well as the oxidation states of the metal ions (Ti4+ and Zn2+) has been confirmed using XPS for each substrate. Charging effects, which can occur when performing electron spectroscopies on a large band gap substrate, were not observed on any of the samples. Because the values found in the literature for the TiO2 or ZnO energy band gaps span a wide range, mainly dependent on the measurement techniques and surface prepara-
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Figure 6. Valence and conduction bands (obtained using, respectively, UPS and IPS) measured on the clean ZnO(112j0) substrate (black curve) and the sensitized substrate (continuous red curves). The difference between the sensitized surface and the clean surface spectra (dotted red curve) is representative of the molecular contribution to the VB and CB spectra.
Figure 5. Valence band measured in UPS and conduction band measured in IPS of the two wide band gap substrates considered in this study: (a) ZnO(112j0) and (b) TiO2(110). The energy band gap of the substrates is deduced using a linear fit of the valence and conduction band edges to the background of the spectra.
tion, it is necessary to define the clean substrates spectroscopic results in a consistent way.30-33 Figure 5 shows the VB spectra obtained in UPS and the CB spectra obtained in IPS from the clean ZnO(112j0) and TiO2(110) substrates (the zero of energy refers to the Fermi level of the system). The main valence band feature in the ZnO(112j0) spectrum of Figure 5a is primarily derived from O 2p states and is characterized by a strong edge that can be linearly fitted and extrapolated to meet the spectrum background at -3.2 eV. A strong Zn 3d level is also observed at a binding energy of -10.7 eV.34 The conduction band is composed of a mixture of Zn 4 s and 4p states, whose cross section in IPS is much lower than the O 2p cross section in UPS. The CB can, however, be fitted linearly and extrapolated to meet the background level at 0.4 eV, giving a measured energy gap value of 3.6 eV. For the TiO2(110) surface, the valence and conduction band spectra are shown Figure 5b. Similar to ZnO, the valence band of TiO2 is mainly of O 2p character, and the high kinetic energy edge can be fitted linearly, giving a threshold of -3.4 eV.35 The conduction band is characterized by strong Ti 3d states, resulting in a strong and sharp edge that can be fitted linearly, giving a threshold of 0.2 eV. The energy gap measured on the TiO2(110) surface is found immediately to be 3.6 eV, identical to what was measured on the ZnO(112j0) surface. It should be noted that the gaps measured here are closely related to the energy difference between the valence band maximum (VBM) and conduction band minimum (CBM) in the ground state. As expected from these substrates, both TiO2 and ZnO are found to be n-doped due to the creation upon annealing of bulk oxygen vacancies.35
The energy gap values obtained from these measurements are larger than the typical value of ∼3.0 eV commonly found in the literature.30,31 Several factors contribute to this discrepancy. First, the ∼3.0 eV value is typically obtained using UV-vis optical absorption. As a result, the exciton binding energy may lead to an underestimation of ground-state VBM-CBM gap. Indeed, similar results using a combination of UPS and IPS, but with a lower energy resolution than the present work, have been reported for TiO2(110).32,33 A second consideration is that the lower band gaps reported in the literature could originate from the existence of band edge states that have weak intensity in UPS and IPS employed in this study. An indication of such features can be observed at the bottom of both the sharp VB and the CB edges of the substrates, as seen in Figure 5a,b. It is possible that these weak band edge states could give a significant intensity contribution to the onset of a UV-visible absorption spectrum. Nevertheless, extrapolation of the strong edges in the measured spectral features provides the most robust measurement of band edge energies to which we can reference the observed molecular levels. Molecular Electronic Levels. The UPS and IPS spectra of the ZnO(112j0) surface before (continuous black curve) and after sensitization (continuous red curve) with the ZnTPP-Ipa molecule are reported in Figure 6. Upon sensitization, both the UPS and the IPS spectra are strongly modified, displaying sharp molecular features. Moreover, the characteristics of the substrate electronic structure (the strong Zn 3d shallow core level, for example) show significant attenuation.36 Figure 7 contains the equivalent measurements obtained from the clean (continuous black curve) and subsequently sensitized (continuous red curve) TiO2(110) surfaces. Similarly, the UPS and IPS spectra of the sensitized surfaces have lost most of the TiO2 substrate signature and display well-defined molecular states. Because there are no sharp and well-defined substrate features in the sensitized UPS and IPS experimental spectra, it is important to have a common substrate reference before and after sensitization to investigate possible energy band bending upon molecular adsorption. For example, upward surface band bending has been reported upon adsorption of O2 onto a nominally clean UHV-prepared TiO2(110) surface.35 Core-level spectra from each substrate (Zn 2p and Ti 2p) have been
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Figure 7. Valence and conduction bands (obtained using, respectively, UPS and IPS) measured on the clean TiO2(110) substrate (black curve) and the sensitized substrate (continuous red curves). The difference between the sensitized surface and the clean surface spectra (dotted red curve) is representative of the molecular contribution to the VB and CB spectra.
measured and are reported in the insets of Figures 6 and 7, respectively. The core levels of the clean substrates (black curves) are unchanged after sensitization (red curves), indicating no major chemical alteration of the oxide surface and no band bending upon adsorption of the dye. Having established the absence of band bending, we directly subtract the clean surface spectrum from the dye-covered surface spectrum to obtain the spectrum of the adsorbed molecule. The ZnTPP-Ipa contribution to the electronic structure is shown as the dotted red curves in Figures 6 and 7 for both substrates.37 Strikingly, the ZnTPPIpa portion of the measured electronic structure both in the occupied and in the unoccupied states appears independent of the oxide onto which the molecule is chemisorbed. As a first observation, the UPS results presented here resemble UPS measurements of a ZnTPP multilayer on Si(111) reported previously by Castellarin-Cudia et al.21 Apart from differences in the peaks’ intensity, mostly due to photoemission cross section (hν ) 91 eV as compared to 40.8 eV in the present work), the main features of the ZnTPP thin film UPS spectrum are also found in the spectra of adsorbed ZnTPP-Ipa from the present work. The origin of these similarities can be traced to the nature of bonding to the surface. As the molecule probed by UPS in a multilayer are far from the underlying surface, it not surprising that a multilayer of ZnTPP molecules retains most of the electronic properties of a ZnTPP molecule in the gas phase. For the chemisorbed ZnTPP-Ipa studied here, as the meso-Ipa ring that binds to the surface can be considered as decoupled from the rest of the molecule, only a small perturbation of the gas phase electronic structure of ZnTPP-Ipa will occur upon chemisorption. Although the ground-state DOS calculated in the gas phase for both molecules is found to be very similar to the experimental results reported here, excitedstates spectroscopic studies have shown that appreciable exciton coupling does exist for this class of molecule chemisorbed to TiO2 and ZnO surfaces due to H aggregation as a result of close packing of the porphyrin molecules at the surface.13 Molecule-Substrate Energy Level Alignment. A more detailed interpretation of both the UPS and the IPS spectra is facilitated by a comparison to the DOS calculated for the ZnTPP-Ipa molecule, as shown in Figure 8. The top panel of
Rangan et al.
Figure 8. Experimental molecular DOS of the ZnTPP-Ipa adsorbed on ZnO(112j0) compared to the simulated DOS of the gas phase molecule. The calculated band gap has been increased by 1 eV and the DOS aligned with the experimental spectrum. The positions of strong electronic features are indicated above the experimental spectrum, as well as the main character of the calculated DOS. Note that the cross section effects are not taken into account in the calculated DOS.
Figure 8 shows the difference spectrum of Figure 6, where the energy positions of the main features are noted. The indicated energies of the experimentally measured unoccupied states have been obtained by fitting the spectrum with three Gaussian functions. The bottom panel of Figure 8 is the DOS obtained by taking the sum DOS(ZnP) + 3 DOS(Ph) + DOS(Ipa). As DFT methods tend to underestimate energy gaps, the HOMOLUMO gap of the calculated DOS has been artificially increased by 1 eV, using the so-called “scissor operation” so that the calculated HOMOs-LUMOs centroids better match the experimental ones. Furthermore, for a better comparison of the calculated DOS with the experimental data, both energy scales have been aligned using the experimental Fermi level as zero of energy. Strong similarities are observed between the experimental and calculated spectra, both in the occupied and in the unoccupied states, despite the fact that the effects of the photoemission or inverse photoemission cross section are not taken into account in the calculated DOS. It is clear that the two peaks measured, respectively, in UPS and in IPS at -1.7 and 2.4 eV can be attributed, respectively, to the centroids of the HOMOs and LUMOs of the ZnTPP-Ipa molecule. Furthermore, the peaks found at -3.9 and 4.2 eV can be attributed to the phenyl groups. The Zn 3d states are much weaker than what was found in the VB measurements of a ZnTPP multilayer on Si(111), but that is the result of photoemission cross section effects.21 To first order, the measured occupied and unoccupied DOS of the adsorbed ZnTPP-Ipa molecule appears well-described by the calculated DOS of the gas phase molecule. This seems to favor the idea that, but for the anchoring carboxylic acid group, the ZnTPP-Ipa molecule interacts weakly with the oxide substrate, likely with a large angle between the porphyrin ring and the plane of the surface. Moreover, a substantial lack of coupling between the meso-phenyl groups and the main porphyrin ring,
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TABLE 3: Semiconductor Band Edges and Molecular Frontier Orbital Positions (in eV)
occupied states unoccupied states energy gap
ZnO(112j0)
TiO2(110)
edge
edge
edge
estimated edge
centroid
-3.2 ( 0.1 0.4 ( 0.1 3.6 ( 0.2
-3.4 ( 0.1 0.2 ( 0.1 3.6 ( 0.2
-0.9 ( 0.1 0.5 ( 0.1 1.4 ( 0.2
-1.0 ( 0.1 0.7 ( 0.1 1.7 ( 0.2
-1.7 ( 0.1 2.4 ( 0.1 4.1 ( 0.2
expected from simple geometrical considerations, gives rise to only minor changes in the adsorbate DOS with respect to the DOS of the individual molecule. It has to be noted that, from one sample to another, small variations of peak intensities (while the general shape of the UPS and IPS spectra remains) and energy positions ((0.1 eV) have been observed while measuring sensitized surfaces produced in the same way. These differences are most likely the result of small amounts of atmospheric contamination during exposure to air before sensitization. Taking this into account, there is no marked difference in the energy level alignment of the ZnTPP-Ipa onto the two different TiO2(110) and ZnO(112j0) oxide surfaces. The results obtained here can be compared to other attempts to determine the unoccupied levels of ZnTPP. Cudia et al.21 have also performed interesting NEXAFS measurements both at the C 1s and at the N 1s edges on a ZnTPP multilayer. Because of the symmetry selection rules applying to NEXAFS transitions, the N 1s edge spectrum is mostly representative of the main ZnP part of the ZnTPP molecule, whereas the C 1s edge spectrum contains components both from the ZnP part and from the phenyl groups. Thus, the LUMOs levels are, in principle, visible from both the N 1s edge and the C 1s edge, whereas the phenyl-related unoccupied orbitals are accessible only from the C 1s edge. From the C 1s NEXAFS spectrum, the energy spacing between the LUMOs absorption peak and the phenyl-related absorption peaks is found to be roughly 1.5 eV. This compares well with the energy difference of 1.8 eV that we measure using IPS, particularly since a possible relaxation of the unoccupied levels upon core hole creation may occur in NEXAFS.22 In principle, once the positions of the frontier orbitals in the experimental spectra are established, the alignment of the ZnTPP-Ipa HOMO and LUMO levels with respect to the VB and CB edges of the TiO2(110) and ZnO(112j0) substrates can be determined directly. However, before discussing this alignment in more detail, it is important to point out that, owing to the finite energy width of the spectral features, different methods of analyzing the spectra give rise to different quoted values of the molecular level position and, in particular, the HOMOLUMO gap. One approach to the analysis is to use a linear fit of the highenergy edge of the HOMOs and the low-energy edge of the LUMOs and quote the energy where the fits intersect the background level. This may be interpreted as the minimum possible HOMO-LUMO energy separation. Using this method, which we refer to as the edge method in Table 3, the ionization potential and electron affinity levels are found at -0.9 and 0.5 eV, respectively, resulting in a 1.4 eV HOMOs-LUMOs gap for the molecule. As expected for a light harvester, the molecular HOMO-LUMO gap appears much smaller than the gap of the oxide surfaces. A weakness of this approach, however, is that several factors contribute to the width of the HOMO and LUMO spectral features, including the energy resolution of the instrumentation (whose fwhm contribution is estimated to 0.3 eV in UPS and 0.6 eV in IPS), the intrinsic width of the level (related to the lifetime of the final state), and possible inhomogeneities, such as inequivalent bonding sites at the semiconductor surface.
ZnTPP-Ipa
Because the edges of the spectral features are highly dependent upon their width, this method gives an underestimate of the energy gap separating the ZnTPP-Ipa frontier orbitals. It is possible to improve this approach slightly by assuming that the instrumental resolution has a Gaussian form and removing it from the nearly Gaussian-shaped molecular frontier orbitals obtained in UPS and IPS spectra by deconvolution. We refer to this as the estimated edge approach in Table 3. With this method, an estimation of the HOMO and LUMO edges are found at -1.0 ( 0.1 eV and 0.7 ( 0.1 eV, respectively, giving an energy gap of 1.7 ( 0.2 eV for the molecule. A second approach is to fit the main experimental features with Gaussian or Voight functions and then quote the centroid of these functions as the HOMO and LUMO energies. Applying this approach to the spectra of Figures 6 and 7, the best fits give HOMO and LUMO positions of -1.7 and 2.4 eV, respectively. As we are quoting centroids of these functions, the error due to the widths of the spectral features is, in principle, eliminated, and a HOMOs-LUMOs gap of 4.1 eV is obtained (shown in Table 3). However, as VB and CB spectra are typically broad and difficult to fit due to the superposition of multiple electronic states (particularly for the LUMOs here), the centroid positions contain errors which, in the case of the frontier orbitals, we estimate to be ∼0.1 eV. Alternatively, a method that uses the most information at hand is to use the DOS calculated for ZnTPP-Ipa in gas phase. Assuming that the adsorbates retain most of its gas phase properties and that the theoretical model is precise in its description of the molecular electronic structure, the frontier orbital positions can be determined using their calculated positions, adjusted to the experimental DOS. Using this approach, the HOMO and the LUMO centroids are found, respectively, at -1.7 and 2.3 eV, leading to an energy gap of 4.0 eV. In the discussion that follows, as the second and third method give similar results and are insensitive to the peak widths, we have chosen to use the fitted values for the HOMO and LUMO centroids (shown in Table 3), resulting in the alignment of these levels with respect to the VB and CB edges of the TiO2(110) and ZnO(112j0) substrates that is schematically represented in the lower panel of Figure 8. We note that, although deconvolution of the instrumental resolution gives a reasonable result for sharp, well-defined molecular spectral features, this approach is problematic for line shapes that are more complicated, as is the case for the substrate valence and conduction bands. Thus, the error on the band edge positions is estimated to ( 0.1 eV, comparable with the error on the band edges determination defined earlier. Both the HOMO-1/HOMO levels are found well inside the substrate band gap, on average 1.6 ( 0.1 eV above the VB edge, and the LUMOs levels are found around 1.9 ( 0.1 eV above the CB edge. On both surfaces, the experimental HOMOs-LUMOs gap, measured from the two centroids, is found to be 4.1 eV.38 This value is much larger than the optical absorption threshold reported for ZnTPP-Ipa adsorbed on ZnO and TiO2 nanoparticles (2.00 eV) or for ZnTPP-Ipa in solution (2.07 eV).13 This difference can originate from several factors.
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First, in the presence of inhomogeneous or lifetime broadening, the minimum energy for photon absorption to occur would be related to the high- and low-energy edges of the occupied and unoccupied molecular DOS, respectively. This would correspond to the edge values in Table 3 that are smaller than the HOMO-LUMO centroid-to-centroid distance (although, again, subject to greater uncertainty). Optical absorption thresholds also can occur at energies significantly smaller than the HOMOLUMO gap due to the creation of excitons.39,40 To avoid the exciton binding energy contribution, the energy gap can, in principle, be obtained from the difference between the ionization energy and the affinity level of the dye. From the first oxidation and first reduction potentials measured using cyclic voltammetry for ZnTPP-Ipa in solution, an energy gap of 2.18 eV is reported by Rochford et al.,13 still much smaller than the 4.1 eV measured in the present work. However, discrepancies between cyclic voltammetry and UPS are well-known in the literature.41-43 Underestimations of the ionization potential ranging from 0.5 to 1 eV have been reported for organics when cyclic voltammetry results are compared to gas phase ionization potentials, thin films UPS data, scanning tunnel spectroscopy, or theoretical calculations. It, however, is not clear whether this is an effect of solvation or whether it is intrinsic to electrochemical techniques.43,44 The essential conclusion from the above considerations is that one must exercise extreme caution when comparing energy gaps or other aspects of the electronic structure obtained from different experimental techniques (UPS, IPS, optical or electrochemical methods) and in different media (gas phase, monolayer, thin films, or solution phase). Using a combination of a HOMO position of a dye molecule measured in UPS, with an optical energy gap, is a commonly used method to evaluate the position of a dye LUMO level. This can lead, however, to errors as large as several electronvolts on the position of the molecular orbitals with respect to the substrate band edges. The approach taken in this work contrasts with other literature results by showing a comprehensive study of a well-defined monolayer of dye chemisorbed onto single-crystal oxide surfaces, probed both in the occupied and in the unoccupied states in the same UHV system, allowing a direct evaluation of the molecular level and substrate band edge positions, providing a solid basis for theoretical computations.
Rangan et al. energy alignment of the molecular levels with respect to the band edges of the oxide surface. Direct comparison with the results obtained here to, for example, the optical absorption properties of these dye molecules in DSSCs, should be approached with caution as description of an absorption spectrum requires a complex convolution of the ground-state and excited-state densities, accompanied by the creation of an excitonic states. However, information such as that presented here, which is closely related to the ground-state electronic structure of the adsorbate system, will facilitate a more direct understanding of molecular interactions with the surface or the molecule-molecule interactions. In this study, because of the high level of decoupling between the functionalized phenyl linked to the surface and the rest of the ZnTPP-Ipa molecule, the measured DOS is essentially identical to the gas phase DOS. This might not be the case when the porphyrin macrocycle is functionalized or for molecules, such as catechol or perylenes, which have been extensively studied as model sensitizers and that contain delocalized π systems that are conjugated with the surface-anchoring groups. That is why, in conjunction with more traditional optical spectroscopies and electrochemical methods, UPS and IPS spectroscopic techniques with the help of ab initio models will allow a more straightforward grasp of sensitizers bonding at the surface of wide band gap semiconductors and of its consequences on energy level alignment. Ongoing experiments are exploring the effects of selective functionalization of the meso-phenyls as well as anchoring group substitution in ZnTPPs, with emphasis on the consequences on the adsorption geometry and molecular conjugation to the substrates. The far too ideal single crystals are also being compared to nanostructured TiO2 and ZnO. This should provide crucial information on the effect of local disorder or defect sites on the bonding and on the electronic structure on a dye/ semiconductor interface. Acknowledgment. The authors would like to thank Y. Lu for providing the ZnO(112j0) epitaxial films. E.G. thanks the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy for funding through Grant No. DE-DE-FC02-01ER15256. References and Notes
Conclusion In this work, the electronic structure of the Zn(II)-5-(3,5dicarboxyphenyl)-10,15,20-triphenylporphyrin dye (ZnTPP-Ipa), chemisorbed onto ZnO(112j0) and TiO2(110) single-crystal substrates, has been determined experimentally using both photoemission and inverse photoemission spectroscopies in a single UHV system. In addition, the electronic DOS of this molecule was calculated within the DFT framework (B3LYP, 6-31G* basis sets). The calculations indicate that the DOS of a ZnTPP derivative can be simulated by the sum of the ZnP macrocycle DOS with the DOS of the functionalized phenyls. Furthermore, the measured electronic structure compares well with the DOS calculated for the gas phase ZnTPP-Ipa dye. The similarity between the measurements of the adsorbed molecule and the calculations of the free ZnTPP-Ipa molecule is attributed to a lack of significant conjugation between the porphyrin macrocycle and the phenyl moieties and that only one of the latter contains the surface-anchoring groups. These results suggest that a combination of an experimental and a theoretical approach allows one to obtain a fundamental understanding of the ZnTPP-Ipa electronic structure and the determination of the
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J. Phys. Chem. C, Vol. 114, No. 2, 2010 1147 (33) Purdie, D.; Reihl, B.; Thomas, A.; Hardman, P.; Muryn, C.; Thornton, G. J. Phys. IV 1994, 04, C9-163C9-166. (34) Solomon, E. I.; Henrich, V. E. Surf. Sci. Spectra 1998, 5, 186. (35) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (36) The weak sensitivity to the surface is reasonable in terms of escape depths of the photoelectrons (with a kinetic energy of 40 eV) during UPS and of the incoming electrons (20 eV) during IPS, considering that a ZnTPP molecule has a diameter of roughly 20 Å. (37) In the absence of a formal normalization scheme between the sensitized and pristine substrates, the substraction of the clean surfaces’ contribution from the VB and CB spectra has been optimized to obtain similar molecular contribution on both substrates. (38) Looking back at the UHV work done by Castellarin-Cudia et al. on the ZnTPP multilayer on Si(111),21 it appears that the authors, on the basis of the calculated position of ZnTPP molecular levels from an earlier work16 and of small dichroic effects observed in their VB spectra, have attributed the HOMO level to a weak feature 1.25 eV below the measured Fermi level. If the origin of this weak peak is unclear, when comparing the shape of their VB to the shape of our calculated DOS, the HOMO level should have been attributed to the peak situated 2.5 eV above the Fermi level, implying a HOMO-LUMO gap for ZnTPP of at least 2.5 eV.21 (39) Hill, I. G.; Kahn, A.; Soos, Z. G.; Jr, R. A. P. Chem. Phys. Lett. 2000, 327, 181. (40) Hwang, J.; Wan, A.; Kahn, A. Mater. Sci. Eng., R 2009, 64, 1. (41) Schlettwein, D.; Armstrong, N. R. J. Phys. Chem. 1994, 98, 11771. (42) Kadish, K. M., Smith, K. M., Guilard, R., Eds. Phtalocyanines: Spectroscopic and Electrochemical Characterization; Academic Press: New York, 1999; Vol. 16. (43) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2002, 106, 996. (44) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Org. Electron. 2005, 6, 11.
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