ToF-SIMS and XPS Studies of the Adsorption Characteristics of a
Zn-Porphyrin on TiO2
Manuela S. Killian,† Jan-Frederik Gnichwitz,‡ Andreas Hirsch,‡ Patrik Schmuki,† andJulia Kunze*,†
†Department of Materials Science and Engineering 4, Chair for Surface Science and Corrosion,Friedrich-Alexander-University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany, and‡Department of Chemistry and Pharmacy and Interdisciplinary Center of Molecular Materials (ICMM),Friedrich-Alexander-University of Erlangen-Nuremberg, Henkestrasse 42, 91054 Erlangen, Germany
Received August 27, 2009. Revised Manuscript Received September 22, 2009
Time-of-flight secondary ionmass spectrometry (ToF-SIMS) andX-ray photoelectron spectroscopy (XPS)were usedto study monolayers (ML) and thick films of porphyrin Zn-TESP (C67H75N5O5SiZn) attached to titanium dioxide(TiO2) substrates via silanization. Films on ideal hydroxyl-terminated silicon (SiO2) surfaces were used for comparison.ToF-SIMS and XPS spectra show that the type of adsorption varies depending on the thickness of the organic film, thepreparation temperature, and the adsorption time. We show that the intensity of a molecular peak at mass 1121.5 u inToF-SIMS can be used as a direct measure of the ratio of chemisorption/physisorption of Zn-TESP. On TiO2, theamount of chemisorbed porphyrin can be increased by increasing the reaction temperature and time during thesilanization process. On the SiO2 reference, only chemisorbed species were detected under all investigated preparationconditions. The present work thus not only gives information on the Zn-TESP linkage to TiO2 but provides a direct toolfor generally determining the type of adsorption of monolayers.
Introduction
Self-assembled monolayers (SAMs) are highly ordered struc-tures to which molecules can arrange upon adsorption on orreaction with a substrate. Because of the modularity of theorganic building blocks, SAMs can be used to alter the surfaceproperties of various kinds of substrates relatively independentlyof the characteristics of the bulk. The formation of self-assembledmonolayers (SAMs) on metal or metal oxide surfaces1 is com-monly employed in the fabrication of model surfaces with highlycontrolled chemical properties.
To date, the best-studied SAM systems are self-assembledalkanethioles on gold,1-6 alkyl phosphates and phosphonates onmetal oxide surfaces,1,7-9 and SAMs formed via silanization on
silicon and other oxides.10-16 The orientation of the SAMs on thesurface can in many cases determine the surface properties.17-19
Titanium dioxide20-22 modified by self-assembled monolayersis technologically of great interest due to the possibility to adjustsurface properties such as wetting behavior,23,24 protein adsorp-tion,25,26 cell interaction in biomedical applications,27,28 andcontrolled drug release.29-31
*Corresponding author. Current address: Technical University of Munich(TUM), Institute forAdvanced Study (IAS), PhysicsDepartment E19, James-Franck Strasse 1, D-85748 Garching, Germany. Phone: þ49 89 28912526.E-mail: [email protected].(1) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir
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SAMs consisting of porphyrin molecules provide ideal arche-types of 2D structures for fundamental studies on one hand, andthey can be useful in a host of technology applications on the otherhand. They are employed in photovoltaic devices32-37 and forspecific catalytical processes.38 Furthermore, porphyrins occupyan important role in natural processes because they are the mainbuilding blocks of hemoglobin, chlorophyll, vitamin B12, andvarious enzymes.39 They can be synthesized in a large variety andcan easily be manipulated by changing the central metal atom orby introducing different ligands into the porphyrin skeleton.40
In the present study, a porphyrin with a triethoxysilane anchorgroupwas synthesized prior to its assembly on the oxide substratesand examined with X-ray photoelectron spectroscopy (XPS) andtime-of-flight secondary ion mass spectrometry (ToF-SIMS).
Because of its high surface sensitivity, which lies in the mono-layer range,41-43 ToF-SIMS is a well-suited tool for the analysisof ultrathin organic layers. For example, Hagenhoff et al.observed the occurrence of defects inLangmuir-Blodgett films,44
Houssiau and co-workers investigated the formation of siloxanemonolayers on hydroxyl-terminated surfaces,45,46 and severalother SAM systems have been examined by ToF-SIMS.18,47-49
Surface-attached thin coatings of proteins can also be investigatedwith ToF-SIMS,50-52 and even attempts to determine their sur-face orientation have been undertaken.53
Porphyrins have only scarcely been examinedwith ToF-SIMS.For the current investigation, however, it is interesting that theywere reported to often clearly display a molecular peak in theirmass spectra.54-56
In this work, a zinc porphyrin containing a triethoxysilanefunctional group, Zn(II)-5-(tris-ethoxy-silane-propyl-amide-acet-ato-phenoxy)-10,15,20-(p-tert-butyl-triphenyl)-porphyrin (Zn-TESP, C67H75N5O5SiZn) with a mass of 1121.5 u, was studied
in regard to understand its adsorption behavior on native TiO2
layers on titanium metal and thin silicon dioxide on silicon forcomparison.
Experimental Section
Synthesis. Chemicals. All chemicals were purchased bychemical suppliers and used without further purification. Allanalytical reagent-grade solvents were purified by distillation.Dry solvents were prepared using customary literature proce-dures.57 Thin layer chromatography (TLC): Riedel-de Ha€ensilica gel F254 andMerck silica gel 60 F254. Detection: UV lampand iodine chamber. Flash chromatography (FC): Merck silicagel 60 (230-400mesh, 0.04-0.063 nm). Solvents were purified bydistillationprior to use.UV/vis spectroscopy: ShimadzuUV-3102PC UV/vis/NIR scanning spectrophotometer; absorption max-ima λmax are given in nanometers.Mass spectrometry:MicromassZabspec, FAB (LSIMS) mode, matrix 3-nitrobenzyl alcohol.NMR spectroscopy: JEOL JNM EX 400 and JEOL JNM GX400 and Bruker Avance 300. The chemical shifts are given in ppmrelative to TMS. The resonance multiplicities are indicated as s(singlet), d (doublet), t (triplet), q (quartet), quin (quintet), and m(multiplet), and nonresolved and broad resonances are indicatedas br. Elemental analysis (C,H,N): Succeeded by combustionandgas chromatographical analysis with an EA 1110CHNS analyzer(CE Instruments).
Synthesis of the Porphyrin Zn-TESP. 5-(Tris-ethoxy-si-lane-propyl-amide-acetato-phenoxy)-10,15,20-(p-tert-butyl-triphenyl)-porphyrin. A solution of 500 mg (0.55 mmol) of5-(p-tert-butyl-acetato-phenoxy)-10,15,20-(p-tert-butyl-triphe-nyl)-porphyrin58 in 200 mL of formic acid was stirred for 8 h toobtain the deprotected acid. The progress of the reaction wasfollowed via TLC. The solvent was removed on a rotary evapora-tor. Subsequently, the product was transferred to toluene andevaporated twice to remove any residual formic acid. The productwas finally dried under reduced pressure. The dried product wasdissolved in 50mLof dry dichloromethane at 0 �C. EDC (105mg,0.55 mmol) and 75 mg HOBT (0.55 mmol) were added, and thesolution was stirred for 1 h at 0 �C. After that period of time,0.2 mL of 3-aminopropyltriethoxysilane (APS) was added to thesolution, and themixture was stirred at room temperature for 48 h.The reaction mixture was purified directly by column chroma-tography on silica gel without concentration on a rotary evapora-tor to avoid any polymerization of the silane anchoring group. Amixture of dichloromethane/ethyl acetate (4:1) as the eluent wasused. The yield of the product after evaporation of the solventwas380mg (65%yield). 1HNMR (400MHz, 25 �C, CDCl3): δ-2.76(s, 2H, pyrr.-NH), 0.76 (m, 2H, CH2Si), 1.26 (t, 3J = 7.3, 9H,CH3CH2OSi), 1.61 (s, 27H, C(CH3)3), 1.81 (m, 2H, SiCH2CH2-CH2), 3.51 (dd, 3J = 6.9, 13.4, 2H), 3.88 (q, 3J = 7.02, 6H,SiOCH2CH3), 4.80 (s, 2H, CH2CdO) 7.02 (t, 3J = 5.9, 1H, CdONH), 7.32 (d, 3J=8.7 Hz, 2H,m-Ar-H), 7.77 (d, 3J=8.3 Hz,6H,m-Ar-H), 8.16 (m, 8H, o-Ar-H), 8.82 (d, 3J=4.6Hz, 2H,β-pyrr.-H), 8.88 (m, 6H, β-pyrr.-H). 13C NMR (100.5 MHz, 25�C,CDCl3): δ 7.67 (1C, CH2-Si), 18.27 (3C, SiOCH2CH3), 23.08(1C, CH2CH2CH2), 31.63 (9C, C(CH3)3), 34.83 (3C, C(CH3)3),41.39 (1C, NH-CH2), 58.50 (3C, Si-O-CH2-CH3), 67.71 (1C,CH2CdO), 112.97 (2C,m-Ar-C), 120.30 (3C,m-C), 120.38 (1C,m-C), 123.65 (6C, o-Ar-C), 131.08 (8C, β-pyrr.-C), 134.54 (6C,o-Ar-C), 135.88 (2C, m-Ar-C), 136.27 (1C, i-Ar-C), 139.23(3C, i-Ar-C), 150.58 (3C, p-Ar-C), 157.14 (1C, p-Ar-C), 168.19(1C, CdO). UV/vis (CH2Cl2) λmax (log ε): 419 (5.30), 516 (4.27),553 (4.06), 593 (3.83), 648 nm (3.82). EA calculated forC67H77N5O5Si (1060.4): C 75.88; H 7.32; N 6.60; found: C76.10, H 7.23, N 6.73%. MS (FAB): m/z (%) 1060 [M]þ.
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5-(Tris-ethoxy-silane-propyl-amide-acetato-phenoxy)-10,15,20-(p-tert-butyl-triphenyl)-porphyrinato-zinc. 5-(Tris-ethoxy-silane-propyl-amide-acetato-phenoxy)-10,15,20-(p-tert-butyl-triphenyl)-porphyrin (300 mg, 0.28 mmol) was dissolved in200 mL of THF, and 120 mg of zinc-acetate (0.72 mmol) wasadded. The mixture was heated to reflux overnight. The reactionwasmonitoredbyTLCby thedisappearance of the freeporphyrinbase. The solvent was removed on a rotary evaporator, and theresidue was dissolved again in dichloromethane and filtered toremove any unreacted zinc-acetate. Evaporation of the solventyielded 305 mg of product (95% yield). 1H NMR (400 MHz,25 �C, CDCl3): δ 0.57 (m, 2H, CH2Si), 1.23 (t, 3J = 7.3, 9H,CH3CH2OSi), 1.54 (m, 2H, SiCH2CH2CH2), 1.62 (s, 27H, C-(CH3)3), 2.93 (dd,
The yields of the coupling reaction are good (65%) whenethyldiaminoisopropylcarbodiimide (EDC) is used as a couplingagent. Dry solvents are used and acids and bases are avoided forthe reaction and during the purification on silica gel by columnchromatography to prevent the functionalization of the silica gelor the polymerization of the molecule.
The subsequent metalation with zinc works nearly quantita-tively. Scheme 1 shows the last steps in the synthesis of Zn-TESP.
Sample Preparation. Thin TiO2. Titanium foils (99.6%purity, Advent Ltd., 0.1 mm thickness, (1 � 1) cm2 pieces) wereultrasonically cleaned in ethanol (Merck) and deionized (DI)water, dip-etched in a solution consisting of 5 wt % HF, 50 wt% HNO3 (all Merck), and 45 wt % DI water, and thoroughlyrinsed with DI water (Millipore) to grow an oxide film of∼6 nmthickness. The oxide thickness was determined with XPS.
Thin SiO2. Si wafers (Si(100), both sides polished, p-doped,thickness 485 ( 25 μm, 10 nm oxide) were cut and cleaned inacetone, isopropanol, and DI water in an ultrasonic bath. Subse-quently, theywere dipped intoammoniumfluoride solution (40%,Merck) for 10 min in order to remove the native oxide, rinsedthoroughly, and immersed in piranha solution (1:3 H2O2 (30%)/H2SO4 (97%),Merck) for90min at 120 �Ctogrowadefinedoxidefilm of ∼2 nm thickness, as determined with XPS. The preparedwafers were thoroughly washed and stored in DI water.
Functionalization.Thepretreated sampleswere blowndry in anitrogen streamand immersed in a 0.05mMsolution ofZn-TESPin water-free toluene (Aldrich, 99.8% purity) for up to 24 h(1440 min) at 8 �C, room temperature (RT, 20 �C), and 70 �C.After self-assembly, the samples were rinsed with the solvent for15 min to remove loosely bound material and to receive amonolayer coating of Zn-TESP. The samples were dried at70 �C for 30 min and stored in the dark in a refrigerator. Forthe preparation of thick films, a droplet of Zn-TESP solution wasallowed to evaporate on the substrates at 70 �C. All samples werestored in glass vessels covered with Al foil to prevent contamina-tion. They are stable for more than 1 week of storage but wereintroduced into the UHV system quickly after preparation.
ToF-SIMS and XPS Analysis. Positive and negative staticSIMS measurements were performed on a Tof.SIMS 5 spectro-meter (ION-TOF, M€unster). The samples were irradiated with apulsed 25 keV Bi3
þ liquid-metal ion beam. Spectra were recordedin high mass-resolution mode (m/Δm > 8000 at 29Si). The beamwas electrodynamically bunched down to 25 ns to increase themass resolution and rastered over a 500 � 500 μm2 area. Theprimary ion dose density (PIDD) was kept at ∼5 � 1011 ions �cm-2, ensuring static conditions. Signals were identified using theaccurate mass as well as their isotopic pattern. In this article, onlythe positiveToF-SIMSspectra are presentedbecause they containa greater amount of information regarding our area of interest.
XPSmeasurements were conducted on a high-resolutionX-rayphotoelectron spectrometer (PHI 5600) using monochromoatedAl KR radiation (1486.6 eV, 300 W) for excitation. The bindingenergy of the target elements was determined at a pass energy of23.5 eV and a total energy resolution of<0.4 eV, and values wererecorded every 0.1 eV and at a takeoff angle of 45�with respect tothe surface normal. The binding energy of an external Au 4f7/2reference was used to correct the spectra for charging. The back-groundwas subtracted using the Shirley method in all spectra. Toobtain the molar fractions of each species, the peak areas of themeasured XPS spectra were corrected with the photoionizationcross sections of Scofield59 σ and the asymmetry parameterβ (orbital geometry),60 which are contained in the sensitivityfactors of the acquisition software (MultiPak V6.1A, June 16,1999, copyright Physical Electronics Inc., 1994-1999).
Results
Both XPS and ToF-SIMS were used for the analysis of theadsorption characteristics of the Zn-TESPmolecule. Its structureis shown in Scheme 1. Two different substrates;titanium with athin oxide layer and a thin silicon dioxide film on silicon;werecoatedwithZn-TESP.The uncoated surfaceswere first controlledwith XPS and ToF-SIMS to exclude contamination. They showno signals in the regions of interest discussed in this article. To
Scheme 1. Schematic Drawing of the Last Steps of the Zn-TESP Synthesis and Possible Fragmentation of Zn-TESP in ToF-SIMS
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investigate the temperature dependence of the SAM formation,61
monolayers were prepared at 70 �C, at room temperature (20 �C),and at 8 �C.
After self-assembly of the Zn-porphyrin, XPS revealed char-acteristic Zn 2p and N 1s signals indicative of metal-centeredporphyrins.62-65 The Zn 2p region shows two peaks, Zn 2p3/2 andZn 2p1/2. The evaluation of the data was performed with the Zn2p3/2 peak that is situated at ∼1022.6 eV in the case of Zn-TESPadsorbed on thin TiO2 (Figure 1a).
TheN 1s region of the same sample shows three distinct signals(Figure 1b) arising from the four nitrogen atoms of the porphyrinmacrocycle at lower binding energy (398.40 eV), one nitrogenincorporated into the amide bond interconnecting the spacergroup with the porphyrin skeleton (400.24 eV), and adsorbednitrogen (400.50 eV). The origin of this signal is discussed else-where.66-68
To get information on the adsorption kinetics, a time series ofthe adsorption of Zn-TESP at 8 �C and at 70 �C on thin TiO2 wasrecorded. Figure 2a shows the development of the N 1s peak withadsorption time. Mainly the peaks at 398.40 eV (nitrogen in theporphyrinmacrocycle inZn-TESP) and at 400.24 eV (amidebondin Zn-TESP) increase with increasing adsorption time. Theamount of adsorbed Zn-TESP, shown in atomic percent in
Figure 2b, increases to a saturation concentration of ∼1 atom% on the surface, which can be monitored by plotting the atomicfraction of Zn as a function of assembly time (Figure 2b).
The graph shows irregularities in the early adsorption kineticsand saturation after∼8 h of adsorption. The amount of adsorbedZn-TESP after 24 h is almost identical for the 8 and 70 �Cassembly temperatures.
Peak positions of the Si 2p and the Zn 2p3/2 signals have beendetermined to obtain information on the nature of the bondbetween the oxide substrate and theZn-TESPmolecules. They areplotted for 8 and 70 �C assembly temperatures as a function ofassembly time in Figure 3a,b.
On thin TiO2, the binding energy of the Si signal first shows anoscillation between 102.4 and 101.8 eV and stabilizes after∼3 h; itreaches a final value of (102.15 ( 0.02) eV in the case of a 70 �Cassembly temperature and (102.25 ( 0.02) eV in the case of an 8�Cassembly temperature. The binding energyof theZn2p3/2 peakincreases for both 8 and 70 �C assembly temperatures afterdisplaying an oscillation in the initial stages of adsorption. Thefinal values are (1022.63( 0.01) eV for 70 �Cand (1022.60( 0.01)eV for 8 �C.
The most characteristic fragments observed in the ToF-SIMSspectra are marked in the schematic drawing of Scheme 1. Thearomatic part of the molecule, consisting of the tetraphenylpor-phyrin ring including the ether moiety, produces a signal typicalfor Zn-TESP with positive as well as negative ion detection(Scheme 1, I). The porphyrin including the spacer group, withoutthe triethoxysilane headgroup (Scheme 1, II), the hydrolyzedmolecule (Scheme 1, III) and the intact Zn-TESPmolecule can beobserved in the mass spectra as well. These signals will be referredto in this article as denoted in italics: (1) mass 859.3 u (872.3 u):aromatic moiety (plus one CH2 group) of Zn-TESP (I); (2) mass957.3 u: porphyrin ring and spacer groupR (II); (3) mass 1037.4 u:hydrolyzedmoleculeR-Si(OH)3 (III); and (4)mass 1121.5 u: intactmolecule R-Si(OEt)3 (M
þ).
Figure 1. XPS spectra showing (a) the Zn 2p3/2 and (b) the N 1ssignals of Zn-TESP on thin TiO2 after 24 h of assembly at 8 �C.
Figure 2. XPS spectra showing the development with adsorptiontimeof (a) theN1s peakon thinTiO2 at 8 �Cand (b) the amount ofadsorbedZn in atomic percent on thinTiO2 at 8 �C (() and at 70 �C(4); the kinetics are represented by trend lines.
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A full positive ToF-SIMS spectrum of a thick film of Zn-TESPon TiO2 is shown in Figure 4. In the upper mass range, thepreviously described signals at 1121.5 u (intactmolecule:Mþ) andat 957.3 u (porphyrin ring and spacer group; isotopic patternsuggests overlap of IIþ and (IIþH)þ) are clearly visible, whereasthe signal from the hydrolyzed molecule at 1037.4 u is missing(inset in Figure 4).
At mass 859.3 u, the signal of the aromatic moiety of Zn-TESPis observed. The corrugated fragmentation pattern (100-900 u) isa consequence of the continuous loss of substituents and can beused as a “fingerprint” to identify porphyrins with ToF-SIMS.56
Under certain conditions (e.g., not water free), additional signalsare measured at mass 1037.4 u, which originate from the hydro-lyzed molecule. The only characteristic fragment in the lowermass range is found at 115 u and can be ascribed to the spacergroupof theporphyrinwith the sum formulaC5H9NO2. The peakassignment of all relevant identifiably signals is summarized inTable 1.
The distribution of Zn-TESP-related signals is likewise on allinvestigated substrates.
Themultiplets observed uponmeasuring the regions of interestwith higher resolution are attributed to isotopes of the centralmetal atom (64Zn (48.6%), 66Zn (27.9%), and 68Zn (18.8%)) and13C (1.1%). The detected isotopic patterns are in good agreementwith calculations performed with the ToF-SIMS 5 software(Figure 5).
The two following regionswill be in the center of interest for theevaluation of the ToF-SIMS data: the multiplets at mass 957.3 u(porphyrin and spacer group) and at mass 1121.5 u (intactmolecule). Figures 6 and 7 show high resolution ToF-SIMSspectra of these regions at 957.3 u and 1121.5 u for thin TiO2
and SiO2. SiO2 was used as a model substrate for comparison.Each panel shows four graphs depicting spectra of the
respective substrate covered with Zn-TESP that was allowed to
assemble at 70 �C, room temperature (20 �C), and 8 �C and asbulk material by evaporation of a droplet (from top to bottom).
The signal of the porphyrin ring with the spacer group at957.3 u is observed for all four samples on both substrates(Figures 6a and 7a). Signals originating from the intact porphyrinare clearly visible in the case of SAMs and bulk film on TiO2 butonly for the bulk film on SiO2 (Figures 6b and 7b). On allmonolayer samples, the signal at 957.3 u has a higher intensitythan the signal of the intact molecule.
The reaction temperature seems to have an influence in the caseof thin TiO2 grown on titanium in the investigated temperaturerange because the ToF-SIMS results reveal differences in thespectra recorded for the specimen functionalized at the threedifferent temperatures (70 �C, RT, 8 �C) (Figure 6). The intensityof the signal at mass 957.3 u decreases with decreasing functio-nalization temperature to approximately 80% of the signalintensity of the film assembled at 70 �C, and the bulk film givesonly a small signal. Conversely, the intensity of the signal of theintact molecule at 1121.5 u increases with decreasing reactiontemperature to approximately 2.6 times the intensity of the signaldetected at 70 �C. The spectrum of the thick Zn-TESP film showsa very intense signal in the range of the molecular peak whereasthe signal at 957.3 reaches a value of a maximum of 1.7% of thesignal at 1121.5 (by area). The SAM-covered SiO2 surface doesnot show the same behavior, and the peak intensities of themonolayer samples are independent and do not show any varia-tion with varying assembly temperature.
Figure 8 depicts the ToF-SIMS results on the adsorptionkinetics on thin TiO2 at 8 and 70 �C assembly temperatures.
The adsorption kinetics can bemonitored by plotting the ratiosof the peak areas at 957.3 u and 1121.5 u versus the adsorptiontime. At a 70 �C assembly temperature, this ratio increases,whereas it decreases slightly at 8 �C.
Discussion
Both XPS and ToF-SIMS reveal that the Zn-TESP moleculesare adsorbed on the surfaces after self-assembly. This adsorptionundergoes saturation; the amount of adsorbedZn-TESP does notchange further after ∼8 h of adsorption and shows comparablevalues for 8 and 70 �C assembly temperatures.
Because the molecule contains a headgroup comprising atriethoxysilane moiety, it can bind to the substrate via silaniza-tion if a sufficient degree of OH termination is available.9
Alternatively, the Zn-TESP can adopt a planar alignment witha distorted porphyrine macrocycle, as is usually observed fortetraphenylporphyrin layers on metal substrates.69,70 Thus, theadsorption of Zn-TESP can be covalent;binding via silaniza-tion;or noncovalent;physisorption in the upright position orvia the porphyrin ring. The nature of adsorption is thereforedependent on the reactivity of the substrate in terms of asilanization reaction.
The XPS and ToF-SIMS measurements presented hereprovide evidence that the nature of adsorption of Zn-TESP onTiO2 varies depending on adsorption time and temperature. XPSshows that the Si 2p peak position oscillates in the initial stagesof adsorption for both 8 and 70 �C assembly temperaturesand then stabilizes. The final value of the SAM adsorbed athigher temperature ((102.15 ( 0.02) eV) is slightly lower than
Figure 3. XPS spectra showing the development with adsorptiontime of the peakpositions of (a) Si 2p and (b) Zn 2p3/2 peaks at 8 �C( () and at 70 �C (4) on thin TiO2; the kinetics are represented bytrend lines.
(69) Buchner, F.; Comanici, K.; Jux, N.; Steinr€uck, H.-P.; Marbach, H. J. Phys.Chem. C 2007, 111, 13531–13538.
that of the SAM adsorbed at lower temperature ((102.25 ( 0.02)eV). The Zn 2p3/2 peak shifts to higher binding energieswith longer adsorption times. The final binding-energy valueof the SAM adsorbed at higher temperature is slightly higherthan that of the SAMadsorbed at lower temperature. These shiftsare most likely dominated by final-state effects due to theinteraction of the Si and the Zn photo ions with the under-lying Ti metal. Shifts to lower binding energies for the Si 2p peakand to higher binding energies for the Zn 2p3/2 peak are observedduring the adsorption process. This is most likely due to amarginally decreasing distance of Si and a clearly increasingdistance of Zn to the surface with increasing adsorptiontime, which may suggest that the molecule adopts, after an
undefined oscillatory behavior in the beginning, a more uprightposition throughout the process.71 Similar oscillating behavioris known from the literature for organosilane adsorption, where-by the surface coverage of the silane species oscillates as a func-tion of time. Such behavior has been observed with XPS andToF-SIMS in the past and was found to occur for several casesof organosilane adsorption onto hydroxyl-terminated surfaces
Figure 4. Positive secondary ion mass spectrum of a thick Zn-TESP film on TiO2.
Figure 5. Measured and calculated isotopic patterns of the mo-lecular peak of Zn-TESP.
Table 1. Peak Assignment for All Identifiable ToF-SIMS Spectra
such as native oxides of metals.45,46,72-74 Another explanation ofthe binding energy shifts could be a difference in packing density.However, because the total amount of Zn reaches approximatelythe same value after 24 h of adsorption (Figure 2b), this explana-tion is regarded as being unlikely.
It is noteworthy that a significant difference between the SAMsadsorbed at 8 �C and at 70 �C cannot be deduced from the XPSresults.
The ToF-SIMS results support the XPS findings and can evengive deeper insight into the systems from a molecular point ofview. The signal of the intact molecule R-Si(OEt)3 can beobserved only if no hydrolysis or covalent bond formation takesplace; in this case, the molecule must be physisorbed. Strongmolecular signals are observed on TiO2; their intensities decreasewith increasing functionalization temperature. Thus, TiO2 filmshave a lower reactivity and need higher activation energy forsilanization than SiO2 substrates. Multiplets at 957.3 u areattributed to fragmentation occurring at the connection of theheadgroup to the organic remainder of the molecule, (i.e., theporphyrin ring and the spacer group (957.3 u) form fragmentsupon ion bombardment in ToF-SIMS). These fragments (cf.Scheme 1, II) are reported to be an indicator of chemisorbedand possibly cross-linked Zn-TESP.47 Chain scission between theheadgroup and the organic remainder was also observed for UVirradiation of Zn-TESP (adsorbed at 70 �C for 24 h on TiO2),indicating a strong, covalent bondwith the substrate, which leadsto weakening of the Si-C bond.75 It has to be considered that the
Figure 6. Positive secondary ion mass spectra at different mass ranges (a, b) of Zn-TESP films on thin TiO2. (Top graphs) Zn-TESP SAMsassembledat 70 �C, (secondgraphs fromtop)Zn-TESPSAMsassembledatRT(20 �C), (thirdgraphs fromtop)Zn-TESPSAMsassembledat8 �C, and (bottom graphs) thick Zn-TESP film.
Figure 7. Positive secondary ion mass spectra at different mass ranges (a, b) of Zn-TESP films on thin SiO2. (Top graphs) Zn-TESP SAMsassembledat 70 �C, (secondgraphs fromtop)Zn-TESPSAMsassembledatRT(20 �C), (thirdgraphs fromtop)Zn-TESPSAMsassembledat8 �C, and (bottom graphs) thick Zn-TESP film.
Figure 8. Ratios of the summarized intensities of multiplets at957.3 u and 1121.5 u vs the functionalization time for SAMsassembled at 70 �C (4) and 8 �C ((); the kinetics are representedby trend lines.
(72) Quinton, J.; Thomsen, L.; Dastoor, P. Surf. Interface Anal. 1997, 25, 931–936.(73) Quinton, J. S.; Dastoor, P. C. Surf. Interface Anal. 2000, 30, 21–24.(74) Quinton, J. S.; Dastoor, P. C. Surf. Interface Anal. 2001, 32, 57–61.
(75) Schmidt-Stein, F.; Hahn, R.; Gnichwitz, J. F.; Song, Y. Y.; Shresta, N. K.;Hirsch, A.; Schmuki, P. Electrochem. Commun. 2009, doi: 10.1016/j.ele-com.2009.08.036, in press.
ToF-SIMS signal at 957.3 u can additionally originate from thefragmentation of the intactmolecule, as observedon the thick filmsamples. However, it reaches only 1.7% of the intensity of thesignal at 1121.5 u (by area) in the case of the thick films. In the caseof themonolayer samples, this amount is negligible. Chemisorbedspecies that detach from the surface and subsequently physisorbare expected to yield the most significant signal at 1037.4 u(hydrolyzed molecule). Consequently, it is very likely that thesignal at 957.3 u originates from covalently bound (via the silylgroups) Zn-TESP and that at 1121.5 u originates from physi-sorbed Zn-TESP.
Molecular peaks are never found for SAMs on SiO2, whichindicates that this surface has a very high reactivity towardsilanization. On SiO2, only the multiplet at 957.3 u is observed.It is therefore likely that a strong bond of the siloxane headgroupto the substrate is formed on thismodel surface either due to triplebinding to the substrate or to local cross-linking of the porphyrin,if sterically possible.76 Due to the absence of the signal of intactZn-TESP, we assume that our monolayer preparation method issuccessful.
ToF-SIMS can give direct information on the nature ofadsorption by providing data on the physisorbed and chemi-sorbed species separately in two different mass peaks. XPS showsonly a total signal that consists of a mixture of these twocomponents. Thus, ToF-SIMS is much more sensitive to thedetection of physisorbed species because it measures the respec-tive signals separately. Figure 8 clearly shows that the maindifference in the SAMs prepared at 8 and 70 �C comes from theratio of chemisorbed (957.3 u) to physisorbed (1121.5 u) species.Initially, at a reaction temperature of 70 �C, the molecules arechemisorbed as well as physisorbed, with the number of physi-sorbedZn-TESPmolecules increasing to a smaller extent than thenumber of chemisorbed molecules with increasing reaction time.In the case of samples prepared at 8 �C, at short reaction times,
smaller numbers of Zn-TESPmolecules are adsorbed (Figure 2b)and these are mainly chemisorbed.With increasing reaction time,the relative increase in the signal of physisorbed Zn-TESP exceedsthe relative increase in the signal of chemisorbed porphyrin. Thisdifference is not visible in the XPS spectra.
Conclusions
The results show that ToF-SIMS together with XPS can beused to identify the fraction of chemisorbed and physisorbedZn-TESP on TiO2 surfaces. The most characteristic signals ofZn-TESP SAMs originate from intact and surface-bound mole-cules. The nature of adsorption influences the generation ofsecondary ions, and the detected masses depend on the fractionof silanized molecules. Therefore, one can distinguish betweenchemisorbed and physisorbed organic molecules using the ToF-SIMS spectra. The signals at 957.3 u indicate chemisorption onthe respective substrate, and the intact molecule can be observedonly if the porphyrin is physisorbed. On SiO2 substrates, amaximum number of chemisorbed molecules is observed; it isassumed that the molecules form strong bonds to the surface andlocally cross-link once they have adsorbed. On TiO2, the amountof chemisorbed porphyrin can be increased by increasing thereaction temperature during silanization; it also increases withadsorption time during the silanization process. In contrast tothis, the molecules are always completely chemisorbed on theSiO2 model surface, independent of the preparation temperatureranging from 8 to 70 �C.
ToF-SIMS gives clear information on the nature of adsorptionof Zn-TESP on different oxide surfaces.
Acknowledgment. We thank the DFG and the Cluster ofExcellence “Engineering of Advanced Materials - HierarchicalStructure Formation for Functional Devices” for financial sup-port and Prof. Dr. Thomas Drewello, Dr. Michael Gottfried,Felix Schmidt-Stein, and Heike Krebs for helpful discussions andscientific assistance.(76) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367–4373.
