ZnO Nanoparticles Functionalized with Organic Acids: An

Sep 15, 2009 - Linnéa SelegÅrd , Volodymyr Khranovskyy , Fredrik Söderlind , Cecilia Vahlberg , Maria Ahrén , Per-Olov Käll , Rositza Yakimova and Kaj...
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J. Phys. Chem. C 2009, 113, 17332–17341

ZnO Nanoparticles Functionalized with Organic Acids: An Experimental and Quantum-Chemical Study Annika Lenz,‡ Linne´a Selega˚rd,† Fredrik So¨derlind,*,‡ Arvid Larsson,§ Per Olof Holtz,§ Kajsa Uvdal,† Lars Ojama¨e,‡ and Per-Olov Ka¨ll‡ DiVisions of Molecular Surface Physics and Nanoscience, Chemistry and Materials Science, Department of Physics, Chemistry and Biology (IFM), Linko¨ping UniVersity, SE-581 83 Linko¨ping ReceiVed: June 11, 2009; ReVised Manuscript ReceiVed: August 21, 2009

Electrochemical synthesis and physical characterization of ZnO nanoparticles functionalized with four different organic acids, three aromatic (benzoic, nicotinic, and trans-cinnamic acid) and one nonaromatic (formic acid), are reported. The functionalized nanoparticles have been characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-vis, and photoluminescence spectroscopy. The adsorption of the organic acids at ZnO nanoparticles was further analyzed and interpreted using quantum-chemical density-functional theory computations. Successful functionalization of the nanoparticles was confirmed experimentally by the measured splitting of the carboxylic group stretching vibrations as well as by the N(1s) and C(1s) peaks from XPS. From a comparison between computed and experimental IR spectra, a bridging mode adsorption geometry was inferred. PL spectra exhibited a remarkably stronger near band edge emission for nanoparticles functionalized with formic acid as compared to the larger aromatic acids. From the quantum-chemical computations, this was interpreted to be due to the absence of aromatic adsorbate or surface states in the band gap of ZnO, caused by the formation of a complete monolayer of HCOOH. In the UV-vis spectra, strong charge-transfer transitions were observed. 1. Introduction Zinc oxide has been extensively studied due to its interesting electrical, optical, and catalytic properties. The material is widely used commercially as, e.g., de-sulfuring agent for natural gases when used as hydrogen source in ammonia synthesis, white pigment in oil-based white dyes, UV-B radiation blocker in sun lotions, and antiseptic filler material in dental treatments.1,2 ZnO is a wide, direct band gap semiconductor with an Eg ≈ 3.3 eV (bulk state) exhibiting both a strong near band edge photoluminescence in the UV region and defect-related bands in the blue and green regions.3 ZnO is attractive for optoelectronic applications due to its favorable thermal behavior caused by the high exciton binding energy (∼60 meV), which accordingly survives to high temperatures. The most stable crystal structure of ZnO is the hexagonal wu¨rtzite polymorph (e.g., P63mc, No. 186), the unit cell parameters of which are a ) 3.249 Å and c ) 5.205 Å (PDF 70-8070). The past decade’s strong interest in nanoscience and nanotechnology has led to a renewed interest in ZnO-based materials, and several studies have been published on the synthesis and properties of various types of nanosized ZnO materials such as particles, rods, and thin films.4-8 Recently, nanowires of ZnO have become highly interesting for novel types of lasers.9 It has also been demonstrated that ZnO nanoparticles have interesting sensing properties for oxygen in field effect transistor-based gas sensors at increased temperatures (250-500 °C).10 ZnO has emerged in parallel with TiO2 as a prime candidate material for photovoltaics such as dye-sensitized,11,12 nanohy* To whom correspondence should be addressed: Ph. +46 (0)13 28 1384; Fax +46 (0)13 28 1399; e-mail [email protected]. † Division of Molecular Surface Physics and Nanoscience. ‡ Division of Chemistry. § Division of Materials Science.

brid,13 and polymer-based solar cells14 due to its optoelectronic and photocatalytic properties. The chemical synthesis of very small metal oxide nanoparticles (2-5 nm) makes it possible to achieve very large effective surface areas at reasonably low costs and at high production speed. The development of a dyesensitized light harvesting system will, however, depend on improved understanding of the energetic and structural interactions between the chemisorbed surface (dye) molecules and the oxide particles. Two critical questions are (i) how does energy (electrons, photons, phonons) transfer between the chemisorbed molecule and the metal oxide particle, and (ii) which is the coordination geometry of the chemisorbed molecules? These types of questions are not easily answered by sheer experimental approaches but are within the reach of quantum-chemical calculations. In a previous work, we have studied, by IR and X-ray photoelectron spectroscopy and by quantum-chemical computations, the chemisorption of various organic acids (e.g., formic acid) onto TiO2 nanoparticles.15-17 We have also reported the functionalization by mercaptopropyltrimethoxysilane (HS(CH2)3Si(OMe)3) on wide band gap semiconductor surfaces, e.g., SiC, ZnO, and GaN. The formation of chemisorbed monolayers of HS(CH2)3Si(OMe)3 was confirmed by X-ray photoelectron spectroscopy and atomic force microscopy, suggesting the molecules to be chemisorbed onto the surfaces via the silane group and with the free thiol groups oriented away from the surface.18 In this article, we report on the electrochemical synthesis and physical characterization of ZnO nanoparticles functionalized with organic acids: three aromatic (benzoic, nicotinic, and cinnamic acid) and one nonaromatic (formic acid). The structures of the acids are shown in Figure 1. Obtained synthesis products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy

10.1021/jp905481v CCC: $40.75  2009 American Chemical Society Published on Web 09/15/2009

ZnO Nanoparticles Functionalized with Organic Acids

Figure 1. Organic acids used as adsorbates in this study. From left to right: formic, benzoic, nicotinic, and trans-cinnamic acid.

(XPS), UV-vis, and photoluminescence (PL) spectroscopy. The interactions between the adsorbed organic acids and the ZnO surface were studied using ab initio quantum-chemical calculations. 2. Experimental Section Synthesis. ZnO nanoparticles were synthesized by the EDOC method (electrochemical deposition under oxidizing conditions), using a two-electrode electrochemical cell consisting of an anode of zinc metal (99.99%) and a cathode of stainless steel.19,20 The electrodes were immersed in a glass beaker containing an electrolyte of 0.1 M tetrabutylammonium bromide (TBAB) in 2-propanol. In order to obtain homogeneous nanoparticles, the current density through the solution was held at ca. 1 mA/cm2, while the electrode potential was allowed to vary freely (between 30 and 70 V). The nanosized metallic zinc clusters, which are believed to form on the cathode surface, are immediately oxidized by bubbling air through the solution. After several minutes of electrolysis, a white turbidity appeared in the electrolyte, and after 3-4 h, the electrolysis was stopped. The white precipitate was allowed to settle overnight and was then isolated by centrifuging and washed repeatedly with MeOH. In the electrolysis, TBAB acts both as charge carrier and capping molecule of the ZnO nanoparticles formed, preventing further growth of the particles. Functionalization of the ZnO particles was performed by adding the organic acid to the electrolysis bath from the beginning at a concentration of 20 mM, except for the formic acid where a lower concentration (9 mM) was used in order to avoid dissolution of the particles. Characterization. XRD measurements were performed on a Philips PW 1820 powder diffractometer using Cu KR1 radiation (λ ) 1.5406 Å, 40 kV, 40 mA). TEM studies were carried out with a FEI Tecnai G2 electron microscope operated at 200 kV. Samples for TEM analysis were prepared by dispersing the synthesis products in methanol, and a drop of the dispersion was placed on a copper grid supported holey amorphous carbon film. FT-IR spectra in transmission mode were collected with a Perkin-Elmer FTIR Spectrum 1000 spectrometer, using pressed KBr tablets. UV-vis characterization was performed on functionalized ZnO particles dispersed in ethanol. Prior to measurements, the samples were sonicated for 30 min. The measurements were done in a Hitachi U-2001 double-beam spectrophotometer, using quartz cuvettes. XPS measurements were performed on both benzoic acidand nicotinic acid-capped ZnO nanoparticles. The nanoparticles were dispersed in ethanol and spin-coated onto gold substrates. The gold was prepared by thermal evaporation onto silicon wafers according to the following procedure. Single crystal silicon (100) wafers were cut into 10 mm × 10 mm pieces and cleaned in a 5:1:1 mixture of deionized Millipore water (18.2 MΩ), 25% H2O2 and 30% NH3 heated to 80 °C for 5 min (TL-1 cleaning procedure). The silicon samples were then rinsed in deionized Millipore water prior to evaporation.

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17333 The gold thermal evaporation was done using a Balzers UMS 500 P system. The clean single crystal Si(100) wafers were first primed with a 25 Å layer of titanium with the evaporation rate of 2 Å/s, followed by 2000 Å Au with the evaporation rate 10 Å/s.21 The base pressure was 200 cm-1 a monodentate ligand is expected and for ∆ < 110 cm-1 a bidentate chelate. For a bridging ligand, ∆ is somewhere between 140 and 200 cm-1.32 For the acids in Table

ν(O-H) ν(Ar-H) ν(CdC) νas(COO-) ν(Ar ring) νs(COO-) Fw(HC)CH) δ(Ar-H)

formic acid

benzoic acid

nicotinic acid

cinnamic acid

∼3430

∼3425 3058-2880

∼3400 3080-2870

1589

1560, 1600 1490, 1447

1350

1395

1618 1600, 1565, 1438 1400

∼3400 3085-3025 1642 1562 1578, 1495

716-687

762-694

1406 974 770-684

1, the observed ∆ values are 239 cm-1 for the formic acid, 165/ 205 cm-1 for the benzoic acid, 218 cm-1 for the nicotinic acid, and 156 cm-1 for the cinnamic acid. The relationship between coordination geometry and observed IR vibrations is discussed in detail below. It cannot be excluded that small amounts of carboxylate-zinc(II) complexes are formed during functionalization of the particles. Such complexes, however, are most likely removed in the washing procedure. XPS. XPS measurements were performed on functionalized ZnO nanoparticles to investigate the elemental composition and quality of the core material and to confirm the presence of nicotinic acid and benzoic acid on the ZnO surface. The corelevel XPS Zn(2p) spectra of the nicotinic and benzoic acid capped particles are shown in Figure 5. Two distinct peaks are observed, corresponding to the spin-orbit split between the Zn(2p3/2) and Zn(2p1/2) levels with binding energy peak positions at 1021.1 and 1044.1 eV, respectively. The peak positions and split between the peaks are in good agreement with the oxidation state of Zn2+ in ZnO. The O(1s) and C(1s) spectra for benzoic acid-capped ZnO are shown in Figures 6 and 7, respectively. The O(1s) spectrum shows a broad asymmetric peak centered at about 531.1 eV. The main contribution to this peak originates from O2- in the ZnO lattice.33,34 There may be a contribution from loosely bound oxygen on the surface of ZnO nanocrystals, O2- ions in the oxygen-deficient regions,34,35 and also a small contribution from oxygen in hydroxyl36 and carboxyl groups.37,38 The relative ratio between Zn and O is ≈0.5. The C(1s) core-level spectrum shows an asymmetric binding energy peak positioned at 284.3 eV with a broad shoulder on the high binding energy side. The individual peaks of the

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Figure 5. Zn(2p) XPS core-level spectra of ZnO nanoparticles functionalized with (a) nicotinic acid and (b) benzoic acid. Figure 7. C(1s) XPS core-level spectrum of benzoic acid-capped ZnO nanoparticles.

Figure 6. O(1s) XPS core-level spectrum of benzoic acid-capped ZnO nanoparticles.

shoulder are not fully resolved but indicate the presence of electronegative neighboring atoms, in good agreement with the expected adjacency of carbonyl and carboxyl groups. A full deconvolution procedure cannot be done due to peak broadness, but a tentative deconvolution using peak positions from the literature for related molecular systems and molecular stoichiometry is given in Figure 7. The deconvolution resembles five peaks with a relative broad full width at half-maxima due to charging related to varying thickness of the nanoparticle layer after spin-coating on the gold substrate. The peak with a binding energy peak position at 284.3 eV is assigned to the five equivalent aromatic ring carbons.39,40 A slight shift to higher binding energy is expected for the aromatic carbon with COOH as nearest neighbor. The peak position is 284.5 eV in the deconvolution. The relative intensity ratio of these aromatic carbons is set to 5:1. The peak associated with carboxyl carbons is expected to be found at 289.1 eV. A shoulder is observed at this position. Furthermore, shake-up structure due to the presence of aromatic ring in benzoic acid is also excepted on the high binding energy side of the main peak. Shake-up structures for different aromatic compounds are well-known and have been presented in the literature.41

The binding energy positions are, after binding energy alignment to reference peaks, in good agreement with earlier XPS studies of tyrosine-terminated propanethiol adsorbed on gold36 and benzoic acid and nicotinic acid on rutile TiO2(110).38 The relative ratio between the carboxyl carbon peak and the sum of the aromatic ring carbon peaks is 0.15, which is close to 1/6 which is the case in benzoic acid. For comparison, it can be noted that synchrotron light-based XPS studies of carbamate chemisorbed on gold surfaces have been published earlier including high resolution C(1s) spectra with fully resolved peaks on the high energy binding side of the main peak corresponding to the presence of -C(dO)-O and -CdO functional groups.42 In short, the shoulder at the high binding energy side of the main C(1s) peak in the XPS spectrum for benzoic acid-capped ZnO shows the presence of a COO- group, indicating that functionalization of the nanoparticles is completed. This is consistent with the IR results. The N(1s) core-level spectra of nicotinic acid and benzoic acid-capped ZnO particles are shown in Figure 8. For the nicotinic acid-coated particles, a nitrogen peak with a maximum around 399.6 eV is clearly visible (Figure 8a) while no corresponding peak is observed for the particles with benzoic acid (Figure 8b). The tetrabutylammonium bromide (TBAB) is used as electrolyte in the ZnO synthesis and stabilizes the surface of the as-prepared nanoparticles. TBAB do contain nitrogen, and consequently, there is a N(1s) signal for the as-prepared TBAB-capped ZnO nanoparticles, as expected. The N(1s) spectrum of the benzoic acid-capped particles shows no nitrogen signal, which indicates a complete replacement of the TBAB molecules. In the case of the nicotinic acid-capped particles, a clear nitrogen signal is observed (see Figure 8), in good agreement with the nitrogen-containing aromatic ring structure, indicating the expected presence of nicotinic acid. The relatively low N(1s) binding energy peak position (399.6 eV) found for nicotinic acid coordinated to ZnO is in good agreement with earlier reported studies of monolayer nicotinic acid coordinated to TiO2.38 Thus, the latter observation strongly suggests that the TBAB molecule can be replaced by both the nicotinic and benzoic acids on the ZnO surface. In this study it is shown that cationic TBAB molecules attached to the nanoparticle surface can be replaced by anionic

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Figure 8. N(1s) XPS core-level spectra of ZnO nanoparticles capped with (a) nicotinic acid and (b) benzoic acid.

Figure 9. PL spectra of ZnO nanoparticles coated with formic, nicotinic, cinnamic, and benzoic acid. The inset shows the same spectra on an expanded intensity scale. All spectra were recorded at T ) 4 K, with an excitation wavelength of 266 nm and intensity P0 ≈ 5 W/cm2, and were normalized with respect to the peak intensity of the greenband luminescence.

carboxylates. Whether or not the molecular exchange could have induced surface changes of the particles affecting, e.g., the photoluminescence properties, has not been investigated. Photoluminescence. PL spectra collected at 4 K of ZnO nanoparticles coated with formic, nicotinic, cinnamic, and benzoic acid are shown in Figure 9. The most striking result is that the UV luminescence, centered around 3.38 eV, from the particles coated with formic acid is about 2 orders of a magnitude stronger than those of the other particles. The UV luminescence of colloidal ZnO is, analogous to that of bulk crystalline ZnO, known to be due to near band edge (NBE) emission.3,43 However, because of the inhomogeneous broadening in the studied samples, the emission cannot be attributed to any specific impurity. The green-band luminescence in the range 2.2-2.5 eV in ZnO is due to a deep-level emission (DLE). This transition has been suggested to be associated with a Zn vacancy,44 but there are also alternative assignments of the green luminescence being due to the O vacancy, based on e.g. optically detected magnetic resonance and positron annihilation experiments.45,46 The ratio between the NBE and the DLE is of the order of unity for the nicotinic, cinnamic, and benzoic acid-coated particles, whereas for the particles with formic acid, the NBE emission is 2 orders of a magnitude stronger than the DLE. As discussed below, this observation agrees with the theoretical

Lenz et al. calculations (see Figure 15), showing that the formic acid-capped particles in contrast to those with aromatic acids may have no molecular orbitals (either from the aromatic part of the adsorbates or from surface states) within the band gap of the core ZnO particle. Coating with formic acid would thus not result in depletion of carriers from the particles into deep surface states, as will be the case for the otherwise coated particles. Suppressed carrier depletion should enhance the emission intensity from the particles, as is indeed observed in the PL spectrum. In accordance with the high exciton binding energy in ZnO (∼60 meV), the NBE emission is strong at all temperatures from 4 K up to 300 K. Computational Results. In the quantum-chemical modeling formic acid was adsorbed in monodentate and bridging modes at the Zn10O10 cluster. The most stable forms found for each type can be seen in Figure 10a,b, and the corresponding energies are displayed in Table 2. A single molecule is seen from Table 2 to adsorb more strongly in the bridging configuration than in the monodentate (adsorption energies of 232 and 189 kJ/mol, respectively), in good agreement with previous theoretical studies.22,23 Since a completely bare surface is unlikely to ever exist in an experimental situation, some simple models for a solvated particle were considered. In the cleansing of the particles MeOH was used, and if there is less than a full coverage of the acid at the particle, the alcohol could adsorb as well. For simplicity, water rather than a specific alcohol was used to study solvent coadsorption (Figure 10d,e). The model therefore also represents a situation where also water could be present as contaminant. The process where one HCOOH molecule is adsorbed onto a hydrated surface was studied for two cases. The water molecule either is assumed to leave as vapor (case 1) or is absorbed by the surrounding liquid solvent (case 2). For the latter, the aqueous solvation is modeled by letting the leaving H2O monomer to end up at the center of a (H2O)20 cluster (representing the liquid) forming a (H2O)21 cluster, which results in an estimated hydration energy of -70 kJ/mol and hydration free energy of -1 kJ/mol for water. Monodentate mode adsorption implies one leaving water molecule, whereas bridging mode implies twice as many. As can be seen from Table 3, assuming that water leaves as vapor (case 1) results in that the monodentate mode would be energetically more favorable than the bridging mode since there is a larger energy penalty when two water molecules (about 237 kJ/mol) instead of one (∼143 kJ/mol) desorb and leave as gas phase molecules. But if Gibbs free energy is considered, the bridging mode is again the most stable. Bridging adsorption is also the favored mode when the leaving water molecule is assumed to be absorbed by the liquid solvent (case 2), with respect to both the electronic and Gibbs free energies. Covering the surface with a full monolayers of bridging or monodentate adsorbing molecules (Figure 10f,g) leads to very similar results (Table 3). The entries in Table 3 all refer to the case where a formic acid molecule from the gas phase reacts with the particle. The formic acid molecule could instead have been assumed to initially be in aqueous solution. From calculations on a formic acid molecule interacting with a 20 molecular water cluster, the energy and Gibbs free energy of hydration were estimated to be about -66 and -12 kJ/mol, respectively. This would change for example ∆G for the monodentate monolayer adsorption to -24 kJ/mol (from -36 kJ/mol in Table 3) and ∆G for the bridging monolayer adsorption to -35 kJ/mol (from -47 kJ/mol).

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Figure 10. A single formic acid molecule adsorbed on a Zn10O10 cluster (a) monodentate and (b) bridging. (c) A complete monolayer of water molecules on Zn10O10. A single formic acid adsorbed onto the hydrated cluster (d) monodentate and (e) bridging. A complete monolayer of formic acid molecules in (f) monodentate and (g) bridging configurations.

TABLE 2: Reactions Energies, ∆E, for Zn10O10 + Acid(g) f Acid@Zn10O10 (in kJ/mol) acid

coordination

∆E

formic acid formic acid benzoic acid nicotinic acid nicotinic acid nicotinic acid cinnamic acid

bridging monodentate bridging bridging monodentate N-coordinated bridging

-232 -189 -235 -235 -193 -159 -233

TABLE 3: Reactions Energies, ∆E, and Gibbs Free Energies, ∆G, per Formic Acid Adsorbed onto the Zn10O10 Cluster (in kJ/mol) ∆E surface state of product H 2O monodentate bridging monodentate monolayer bridging monolayer

bare

∆E

∆E

∆G

hydrated hydrated bare g l

∆G

∆G

hydrated hydrated g l

-189 -232 -152

-46 5 -45

-116 -134 -114

-139 -182 -93

-39 -43 -35

-40 -46 -36

-222

-6

-145

-168

-51

-47

The measured (experimental) and computed IR spectra for formic acid-capped ZnO particles are compared in Figure 11. In the calculated IR spectrum for formic acid adsorbed in the monodentate mode, as well as in the spectrum for one formic acid molecule adsorbed on a hydrated surface, absorption peaks are expected at 1000-1200 cm-1, which are not observed in the measured spectra. It appears that the computed spectrum for the bridging mode adsorption shows the best agreement with the experimental spectrum, in line with the results for the adsorption energies of a single molecule at the ZnO surface (Table 2). This holds also when full monolayers of the adsorbate are considered (Figure 11, bottom two graphs, and Figure 10f,g). Since the bridging coordination requires two surface zinc atoms be occupied, and the monodentate only one, twice as many molecules can be adsorbed in the latter configuration.23 However, the adsorption energy of a bridging mode adsorbant is less than twice of that of a monodentate one (Tables 2 and 3). This means that monodentate adsorption is thermodynamically favored (if there are enough acid molecules present). A

Figure 11. Comparison between observed (continuous line) and calculated (discrete peaks) IR spectra for ZnO nanoparticles functionalized with formic acid. Spectra from top to bottom: a single HCOOH molecule in monodentate and bridging configurations; a single molecule at a hydrated cluster in monodentate and bridging configurations; cluster completely covered with HCOOH molecules in monodentate and bridging configurations.

possible explanation why the IR spectrum seems to indicate bridging mode adsorption is that the first molecules adsorbing to the surface do so in the bridging configuration, until the surface is fully occupied. To adsorb the extra molecules to create a full monodentate layer requires that the already adsorbed molecules reorient themselves into less favorable geometries. If this is correct, it means that the formic acid capped particles are kinetically metastable. On the other hand, for the larger molecules the possibility of adsorbing more acids in the monodentate than the bridging configuration is probably of less relevance due to steric repulsion between the molecules which prevents close packing. The studied adsorption modes of benzoic acid, nicotinic acid, and cinnamic acid adsorbed on a Zn10O10 cluster are shown in Figure 12. In modeling the adsorption of benzoic and cinnamic acids, the bridging configuration was used. For nicotinic acid, both the bridging and monodentate configurations were considered. For the nicotinic acid, there is the possibility that a molecule from the solvent or cleansing liquid (in the synthesis

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Figure 12. Aromatic acids chemisorbed onto Zn10O10: (a) benzoic acid, (b) nicotinic acid (bridging mode), (c) cinnamic acid, (d) nicotinic acid (monodentate mode), (e) nicotinic acid plus one H2O molecule, and (f) nicotinic acid plus a second H-bonded nicotinic acid.

Figure 13. Observed (continuous line) and calculated (discrete peaks) IR spectra for ZnO functionalized with cinnamic, nicotinic, benzoic, and formic acid.

MeOH, in our model H2O) or a second layer of acid molecules forms an H-bond to the nitrogen atom, as illustrated in Figure 12e,f. Calculations were also made for the nicotinic acid coordinating the ZnO surface via its N atom, but as the adsorption energy was found to be considerably smaller than that for the carboxylic coordination (see Table 2), the N-bonded configurations are not further considered. Comparisons between observed and calculated IR spectra for the aromatic acids are shown in Figure 13 and for nicotinic acid under different coordination geometries in Figure 14. The agreement between observed and calculated IR spectra for the bridging mode configuration is in general seen to be satisfactory (cf. Figure 11). The observed and calculated (unscaled) symmetric and antisymmetric COO- stretches are listed in Table 4. The agreement between calculated and experimental splittings between the symmetric and antisymmetric peaks is found to be very good. (According to an often-used rule of thumb,32 splittings larger than 200 cm-1 should imply a monodentate adsorption mode, but the calculations show that also for bridging coordination modes the splitting can be well above 200 cm-1.) Electronic density-of-state (DOS) graphs for an acid molecule chemisorbed on Zn10O10 are shown in Figure 15 for the different

Lenz et al.

Figure 14. Observed (continuous line) and calculated (discrete peaks) IR spectra for nicotinic acid in the configurations: (a) bridging (as in Figure 12b), (b) bridging, double layer (Figure 12f), (c) bridging with H2O coordinated to N (Figure 12e), and (d) monodentate (Figure 12d).

acids. Also shown are the projected density-of-states (PDOS) graphs of the adsorbed acid molecules, where the molecular orbitals of the complex have been projected onto the basis functions belonging to the adsorbate. Both HOMO and LUMO are found to be located at the Zn10O10 cluster. For the pure formic acid, the lowest unoccupied orbital, which is foremost located on the adsorbate molecule (LUMOadsorbate), is ∼2 eV higher than the LUMO of the whole system, while for the aromatic acids LUMOadsorbate moves closer to LUMO (1.2, 0.9, and 0.6 eV for benzoic, nicotinic, and cinnamic acid, respectively). The DOS for the two cases when a single HCOOH molecule is adsorbed and when the ZnO particle is covered by a monolayer of formic acid are compared in Figure 16. The formation of a monolayer of formic acid molecules is seen to profoundly alter the DOS for the system as compared to when only a single molecule is adsorbed. The optimized structure of a totally covered cluster possesses a smaller amount of dangling unsaturated bonds as compared to the original Zn10O10 cluster. Saturation of the unsaturated bonds at the surface results in the removal of the low-lying conduction-band states of the singlemolecule adsorption. The LUMO of the total system moves to higher energies (∼1.1 eV) for the covered cluster, implying that LUMOadsorbate for the larger aromatic acids could be located within the band gap. It is this kind of unoccupied aromatic molecular surface states that could be responsible for the much lower PL intensity of the aromatic acid-capped particles compared to the formic acid-capped discussed in the previous section. Another possibility would be that formic acid more readily forms complete monolayers on the particles than the bulkier aromatic acids and therefore removes unsaturated surface states located in the band gap more efficiently. It is seen in Figure 16 that the energy of the edge of the valence band (HOMO) decreases when a monolayer is formed. The high-energy occupied states when only a single molecule is adsorbed can also be associated with ZnO surface states, which are removed (or whose energies are decreased) when all dangling bonds are saturated. The DOS of the aromatic acids in Figure 15 are probably affected by these dangling bonds, too. All occupied states of these adsorbates lie within the valence band, but one can infer from the orbital energies of the isolated

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TABLE 4: Observed and Calculated (Multiplied by a Scaling Factor of 0.977) Vibrational Frequencies νasCOO, νsCOO, and the Splitting (∆) between Them (cm-1) molecular system formic acid(g) (calc) formic acid(g) (exp25) formic acid@Zn10O10 (calc) (formic acid + 11H2O)@Zn10O10 (calc) (formic acid)6@Zn10O10 (calc) formic acid@Zn10O10 (calc) (formic acid + 12H2O)@Zn10O10 (calc) (formic acid)12@Zn10O10 (calc) formic acid@ZnO (exp) benzoic acid(g) (calc) benzoic acid(g) (exp47) benzoic acid@Zn10O10 (calc) benzoic acid@ZnO (exp) nicotinic acid(g) (calc) nicotinic acid@Zn10O10 (calc) nicotinic acid(2layer)@Zn10O10 (calc) nicotinic acid@ZnO (exp) cinnamic acid(g) (calc) cinnamic acid(g) (exp48) cinnamic acid@Zn10O10 (calc) cinnamic acid@ZnO (exp)

adsorption mode

νasCOO

bridging bridging bridging monolayer monodentate monodentate monodentate monolayer

bridging bridging bridging (layer 1, 2)

bridging

molecules and the DOS of the bare Zn10O10 cluster that the highest occupied molecular levels should lie close to or within the band gap of ZnO. In ref 24 a monolayer of isonicotinic

Figure 15. DOS for the bare Zn10O10 cluster and for the cluster with different adsorbates. PDOS for the acids are shown as dark filled areas. The vertical lines at the bottom of each graph indicates the MO levels of the adsorbate-ZnO system, whereas the vertical lines at the top shows the MO energy levels of the acid molecule in the gas phase.

Figure 16. DOS for the complex consisting of HCOOH in monodentate or bidentate modes adsorbed at the Zn10O10 cluster. PDOS for formic acid are shown as dark filled areas. The vertical lines at the bottom of each graph indicates the MO levels of the adsorbate-ZnO system, whereas the vertical lines at the top shows the MO energy levels of the acid molecule in the gas phase.

1778 1769 1606 1610 1568, 1575 1603 1636, 1589 1749 1752 1554, 1560, 1757 1596 1611, 1618 1743 1721 1549 1562

1636 1690

1602 1600 1720

νsCOO

∆

1108 1104 1351 1347 1332-1356 1359 1338 1292-1329 1350 1339 1383, 1086 1392 1395 1339, 1085 1488 1390, 1299 1400 1353, 1117 1290, 985 1393 1406

670 655 255 263 212-304 216 265 307-398 239 410 369, 666 162, 210 165, 205 418, 672 208 221, 421 218 390, 626 431, 766 156 156

acid on a ZnO surface was studied by periodic calculations, and it was indeed found that the highest occupied molecular energy levels were situated in the band gap.24 It can be noted in Figure 16 that at least for the monodentate mode the highest occupied molecular state for HCOOH lies in the band gap. Calculated UV-vis spectra for Zn10O10 with adsorbed acids are shown in Figure 17. The calculated band gap transition (HOMO to LUMO) is located at about 430 nm, which is somewhat larger than the observed band gap of ∼380 nm (as measured from the formic acid coated ZnO nanoparticles, where no interfering absorption could be identified from the HCOOH molecule). For wavelengths down to about 200 nm, transitions from a state located at the ZnO cluster to another cluster state are found to be the most abundant, but with some important exceptions. In the calculated absorbance spectrum for the benzoic acid-coated cluster, a large set of peaks is seen around 240 nm where the main three transitions occur at 241.7 nm (corresponding to a transition from a state located on the adsorbate to another adsorbate state, the strongest absorbance), 245.3 nm (from a ZnO state to another ZnO state), and 238.2 nm (from a ZnO state to an adsorbate state). Experimental values for the benzoic acid coated nanoparticles are 236, 272, and 278 nm, with similar intensities. For nicotinic

Figure 17. Calculated UV-vis absorption spectra for a Zn10O10 cluster with different adsorbates where one molecule is adsorbed. The continuous spectra were obtained by convoluting the intensities from the quantum-chemical calculations for the different transitions by Gaussian functions with a fwhm of 4 nm.

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Figure 18. Initial and final molecular orbital states responsible for the main absorption peaks in the calculated UV-vis spectra for benzoic acid, nicotinic acid, and cinnamic acid. The surfaces represent isoamplitude contour levels for the square of the absolute value of the molecular orbital.

acid, two larger peaks appear in the calculations at 246 and 226 nm. The transition at 246 nm is an adsorbate to adsorbate transition, while that at 226 nm is a transition from the ZnO cluster to the adsorbate. The measured absorbance gives three peaks at 257, 266, and 272 nm, respectively, similar to that of the benzoic acid-coated particles. In the calculated absorbance spectrum for cinnamic acid, a very strong peak appears at 290 nm dominated by a ZnO to adsorbate transition at this wavelength, and an adsorbate to adsorbate transition at 294 nm, while the experimental spectrum gives one strong peak at the lower value 266 nm. The transitions behind the main absorption peaks were further analyzed by visualizing the initial and final molecular orbitals that were the main participants in the excitation (see Figure 18). The initial states are as previously stated orbitals that are located on the ZnO cluster, whereas the final states are orbitals located on the adsorbate molecule. The excitation of an electron between these states corresponds to a charge transfer (CT) transition which implies a large displacement of electron density and, hence, the formation of a large transition dipole. This is the reason for the large absorption intensity observed. The abovementioned discrepancy between experimental and computed wavelengths for the dominant peak in the cinnamic acid spectrum may be a consequence of the tendency of common density functional methods to underestimate the excitation energies of CT transitions.49 4. Conclusions ZnO nanoparticles of high crystallinity (∼5 nm) were synthesized and functionalized by organic acids, three aromatic and one nonaromatic, by a single step EDOC method. Successful functionalization of the particles was confirmed both by the splitting between the symmetric and antisymmetric carboxyl group stretches (νsCOO-, νasCOO-) as observed by IR and by results obtained by XPS. For the latter, it can be noted that the strong N(1s) signal observed for the particles treated with nicotinic acid corroborated the functionalization of the nanoparticles with that acid. For the benzoic acid-capped nanoparticles, no corresponding signal is observed, showing that the tetrabutylammonium bromide (TBAB) molecules present in the electrolyte during the synthesis were replaced by the benzoic acid. The C(1s) spectrum for the benzoic acid-capped particles exhibited a broad feature at the high binding energy side, further evidencing the presence of the acid on the particles. Computations of the adsorption mode geometries and adsorption energies by quantum-chemical DFT calculations, together

Lenz et al. with comparison between experimental and computed IR spectra, indicate that the acids molecules adsorb to the ZnO nanoparticles mainly in bridging adsorption mode. The above splitting between the symmetric and antisymmetric stretch vibrations of the carboxyl group could be well reproduced. PL spectra collected at 4 K of the functionalized nanoparticles exhibited green-band luminescence in the range 2.2-2.5 eV, attributable to deep-level emissions (DLE). Interestingly, for the particles coated with the aromatic acids (nicotinic, cinnamic, and benzoic acid) the ratio between the near band edge emission (NBE) at 3.4 eV and the DLE is of the order of unity, while for the formic acid-coated particles the NBE is about 2 orders of a magnitude stronger than DLE. This observation can be explained by the computational result which shows that full monolayers of formic acid, in contrast to adsorbed aromatic acids in less than full coverage, has no empty molecular orbitals with energies in the band gap of the ZnO particle. Hence, coating with HCOOH will not result in depletion of carriers from the particles and therefore enhance the PL emission intensity. The computations imply that the main absorption peaks in UV-vis spectra originate from charge transfer transitions from the ZnO nanoparticle to the adsorbate molecules. Acknowledgment. Financial support from the Swedish Research Council, VR, the Centre in Nano Science and Technology at Linko¨ping University, CeNano, and computational resources from the Swedish National Supercomputer Centre, NSC, is acknowledged. We also thank Mr. Jani Tuoriniemi for his help with the initial experiments in this work. References and Notes (1) Serpone, N.; Dondi, D.; Albini, A. Inorg. Chim. Acta 2007, 360, 794. (2) Fukumura, T.; Hamada, Y.; Toyosaki, H.; Hasegawa, T.; Koinuma, H.; Kawasaki, M. Appl. Surf. Sci. 2004, 223, 62. (3) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789. ¨ zgur, U ¨ .; Alivov, Ya. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (4) O Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc¸, H. J. Appl. Phys. 2005, 98, 041301. (5) Klingshirn, C.; Hauschild, R.; Priller, H.; Decker, M.; Zeller, J.; Kalt, H. Superlattices Microstruct. 2005, 38, 209. (6) Willander, M.; Nur, O.; Lozovik, Y. E.; Al-Hilli, S. M.; Chiragwandi, Z.; Hu, Q. H.; Zhao, Q. X.; Klason, P. Microelectron. J. 2005, 36, 940. (7) Tang, Z. K.; Kawasaki, M.; Ohtomo, A.; Koituma, H.; Segawa, Y. J. Cryst. Growth 2006, 287, 169. (8) Xu, H.; Liu, X.; Cui, D.; Li, M.; Jiang, M. Sens. Actuators, B 2006, 114, 301. (9) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (10) Eriksson, J.; Khranovskyy, V.; So¨derlind, F.; Ka¨ll, P. O.; Yakimova, R.; Lloyd Spetz, A. Sens. Actuators, B 2009, 137, 94. (11) Snaith, H. J.; Schmidt-Mende, L. AdV. Mater. 2007, 19, 3187. (12) Westermark, K.; Rensmo, H.; Siegbahn, H.; Keis, K.; Hagfeldt, A.; Ojama¨e, L.; Persson, P. J. Phys. Chem. B 2002, 106, 10102. (13) Kumar, S.; Scholes, G. D. Microchim. Acta 2008, 160, 315. (14) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92, 715. (15) Ojama¨e, L.; Aulin, C.; Pedersen, H.; Ka¨ll, P. O. J. Colloid Interface Sci. 2006, 296, 71. (16) Nilsing, M.; Persson, P.; Lunell, S.; Ojama¨e, L. J. Phys. Chem. C 2007, 111, 12116. (17) Lenz, A.; Karlsson, M.; Ojama¨e, L. J. Phys.: Conf. Ser. 2008, 117, 012020. (18) Petoral, R. M., Jr.; Yazdi, G. R.; Lloyd Spetz, A.; Yakimova, R.; Uvdal, K. Appl. Phys. Lett. 2007, 90, 223904. (19) Natter, H.; Hempelmann, R. Electrochim. Acta 2003, 49, 51. (20) Dierstein, A.; Natter, H.; Meyer, F.; Stephan, H. O.; Kropf, Ch.; Hempelmann, R. Scr. Mater. 2001, 44, 2209. (21) Uvdal, K.; Bodo¨, P.; Liedberg, B. J. Colloid Interface Sci. 1992, 149, 162. (22) Persson, P.; Ojama¨e, L. Chem. Phys. Lett. 2000, 321, 302. (23) Persson, P.; Lunell, S.; Ojama¨e, L. Int. J. Quantum Chem. 2002, 89, 172.

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