Communication via Electron and Energy Transfer Between Zinc Oxide Nano Particles and Organic Ads or Bates

8/4/2019 Communication via Electron and Energy Transfer Between Zinc Oxide Nano Particles and Organic Ads or Bates

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Communication via Electron and Energy Transfer between Zinc Oxide Nanoparticles and

Organic Adsorbates

Renata Marczak, Fabian Werner, Jan-Frederik Gnichwitz, Andreas Hirsch,*,

Dirk M. Guldi,*, and Wolfgang Peukert*,

Institute of Particle Technology, Friedrich-Alexander-UniVersity Erlangen-Nuremberg, Cauerstrasse 4,91058 Erlangen, Germany, Department of Chemistry and Pharmacy, Friedrich-Alexander-UniVersity

Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany, and Institute of Organic Chemistry,Friedrich-Alexander-UniVersity Erlangen-Nuremberg, Henkestrasse 42, 91054 Erlangen, Germany

ReceiVed: December 5, 2008; ReVised Manuscript ReceiVed: January 13, 2009

Stable ZnO nanoparticles suitable for further surface functionalization were synthesized in the liquid phasefrom homogeneous ethanolic solutions of the precursors lithium hydroxide and zinc acetate. It was foundthat the growth of the particles was governed by temperature as well as the presence of the reaction byproductlithium acetate during the aging process. In particular, the reaction could be almost completely arrested byremoval of this byproduct. The washing consisted of repeated precipitation of the ZnO particles by additionof alkanes such as heptane, removal of the supernatant, and redispersion in ethanol. Furthermore, the surfaceof the colloidal ZnO nanoparticles was successfully modified by catechol-anchoring group containing dye

molecules, i.e., 5-(N-(3,4-dihydroxyphenethyl)-2-phenoxyacetamide)-10,15,20-(p-tert-butyltriphenyl)porphy-rinatozinc (DOPAZ) and 5-(3,4-dihydroxy-N-phenylbenzamide)-10,15,20-tris(4-tert-butylphenyl)porphyrina-tozinc (CAMIZ), for the study of photochemical properties. Thermogravimetric analysis proved the stabilityof the catechol anchor groups. Steady-state absorption spectroscopy as well as steady-state and time-resolvedemission studies confirmed the electronic communication between the ZnO nanoparticles in their excitedstate and both of the porphyrins. More than 96% emission quenching of ZnO can be achieved by addition ofthe porphyrins, proving that the visible emission of the ZnO is caused by surface states, since only the surfaceof the particles was altered by the grafting experiments. Moreover, with increasing porphyrin concentrationsthe lifetimes changed from 46.0 to 15.3 ns. The shortened lifetimes prompt a new deactivation pathway,namely, through the electronic coupling of the porphyrins to the ZnO nanoparticle. Assuming that the decreasein lifetime is entirely due to electron transfer to the porphyrins, a rate constant of 0.35 108 s-1 could bedetermined for this process. When testing the excited state of the porphyrin in comparative assays betweenZnO and Al2O3, we conclude a similar electron transfer deactivation.

Introduction

Zinc oxide is an attractive material for a broad range ofelectronic, optical, and piezoelectric applications due to its directband gap and excellent thermal, chemical, and structuralproperties.1 For example, ZnO has been suggested for use inapplication such as solar cells,2-5 light emitting diodes,6

transparent electrodes,7 sensors,8,9 and many other devices.10,11

The various applications of ZnO nanoparticles are due tosensitivity to surrounding environments and superior lumines-cence and photoelectric properties.

Different synthesis routes have been developed, includingsolid-vapor phase thermal sublimation,12 spray pyrolysis,13,14

RF plasma synthesis,15 sonochemical or microwave-assistedsynthesis,16,17 and hydrothermal processing.18,19 However, wet-chemical synthesis of ZnO is an area of particular interest, since

it provides a low-temperature, economical way to producevarious ZnO nanostructures.20-24 Growth from solution typicallystarts with nucleation and is followed by growth of the nucleiuntil the metal cation concentration reaches the solubility ofthe oxide. Then particle aging proceeds in the mother liquors.Synthesis conditions such as temperature,25 overall concentrationof the precursors,21,23 water concentration,26 ions present insolution,27,28 and solvents29 have been shown to influence particlenucleation and growth. Therefore, the investigation of ZnOnanoparticles and the controlled synthesis thereof is of greatinterest due to their wide range of potential applications.

A very interesting example of an application are solar cellsbased on nanocrystalline metal oxides, i.e., TiO2 and ZnO,developed by Gratzel and co-workers.2,4 This devicessdye-sensitized solar cells (DSSCs)sare based on photoelectrochemi-cal dye-sensitized mesoporous metal-oxide electrodes and havebeen the subject of intense interest for years. This is due to thefact that they combine potentially low cost with mediumperformance as an alternative to traditional photovoltaic devices.Interfacial electron transfer reactions are reported on TiO230-34

and ZnO5,35-37 in the fast and ultrafast time regimes and areinvestigated to understand the mechanism which ultimately willassist in improving the solar light conversion efficiency. Despitethe considerable interest in these materials, many questions are

* Corresponding authors. Phone +49 9131 8529400; Fax +49 9131 8529402; E-mail [email protected] (W.P.). Phone +49 91318527341; Fax +49 9131 85-28307; E-mail [email protected](D.M.G.). Phone +49 9131 85-22537; Fax +49 9131 85-26864; [email protected] (A.H.).

Institute of Particle Technology, Friedrich-Alexander-University Er-langen-Nuremberg.

Department of Chemistry and Pharmacy, Friedrich-Alexander-Univer-sity Erlangen-Nuremberg.

Institute of Organic Chemistry, Friedrich-Alexander-University Erlan-gen-Nuremberg.

J. Phys. Chem. C 2009, 113, 46694678 4669

10.1021/jp810696h CCC: $40.75 2009 American Chemical SocietyPublished on Web 02/23/2009


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to be answered with regard to their fundamental properties, suchas charge carrier dynamics, electronic transport, and energy leveldistribution.5,36,38

There are various ways to anchor organic and/or inorganicdyes onto metal oxides host surfaces: (I) covalent attachmentby anchoring groups, (II) electrostatic interactions, via ionexchange, ion-pairing, or donor-acceptor interactions, (III)

hydrophobic interactions leading to self-assembly of long alkylchains, (IV) hydrogen bonding, (V) van der Waals, and (VI)physical entrapment inside the pores or cavities of hosts suchas cyclodextrins, micelles, etc. However, the covalent bondingis the most stable form of attachment. The covalent attachmentis realized by a variety of anchoring groups with differentaffinities to the several metal-oxide surfaces.39,40 Typically thebest anchoring groups for metal oxides are phosphonic acids(P(O)(OH)2), followed by carboxylic acids (COOH) and theirderivatives. The main disadvantages of these anchoring groupsare, however, their poor solubility in common organic solventsand their ability to dissolve some of the metal oxides, especiallyZnO. To overcome these limitations, silanes, ethers, acetylac-

etonate, and salicylates have been explored, but their instabilityagainst water, acids, and bases hampers their applications.In this paper, we report the introduction of catechol-anchoring

groups (3,4-dihydroxybenzo compounds) for grafting porphyrinsonto ZnO nanoparticles. There are only a few examples41-43

where catechol functionalities are used for grafting functionalmolecules onto metal-oxide surfaces although it is known thatcatechol forms very stable metal complexes and that theanchoring group is relatively stable and soluble. Porphyrins areknown to facilitate a good control over the reactions takingplace in a DSSC.44 Based on a simple colloidal method tosynthesize ZnO nanoparticles, we present a suitable newapproach for further derivatization of their surface. The particleswere prepared from zinc acetate dihydrate in ethanolic solution

under basic conditions. To learn more about the growthmechanism, we employed in situ UV-vis absorption spectros-

copy and dynamic light scattering (DLS) to quantify bothparticle size and energy band gap as they vary during the courseof time in order to study the influence of aging conditions onthe growth of ZnO nanocrystals. The synthesis of the catechol-anchoring group containing molecules, i.e. 5-(N-(3,4-dihydro-xyphenethyl)-2-phenoxyacetamide)-10,15,20-( p-tert-butyltri-phenyl)porphyrinatozinc, DOPAZ, and 5-(3,4-dihydroxy-N-

phenylbenzamide)-10,15,20-tris(4- tert-butylphenyl)porphyrina-tozinc, CAMIZ, is reported as well. In particular, we useddopamine in the case of DOPAZ as the effective anchor coupledvia a modified Steglich coupling reaction with a porphyrin corefollowed by the metalation of the porphyrin with zinc (Scheme1a). The second molecule, i.e., CAMIZ, was synthesized witha 3,4-dihydroxybenzoic acid derivative to shorten the distancebetween porphyrin and anchoring group (Scheme 1b). Bothmolecules are very well soluble in the most common organicsolvents. Because their synthesis includes an amide coupling,purification on silica gel is needed. This is, however, a crucialpoint of the synthesis, since they may anchor to silica, whichleads to a certain loss in yield in product. Nevertheless, the yields

are quite good despite this enduring problem.Moreover, the surface functionalization of the ZnO nanoc-rystals with organic molecules, namely, DOPAZ and CAMIZ(Scheme 2), enabled examining the interactions between ZnOnanoparticles and electroactive porphyrins by using steady-stateabsorption and emission spectroscopies as well as time-resolvedemission spectroscopy.

Experimental Section

Synthesis of ZnO Nanoparticles. All chemicals wereanalytical grade reagents purchased from commercial sourcesand used without further purification. Colloidal ZnO nanopar-ticles were prepared by hydrolyzing zinc acetate dihydrate in

basic ethanol solution. The overall preparation procedure wasadapted from Spanhel, Anderson,22 and Meulenkamp20 with a

SCHEME 1: (a) Synthesis of 5-(N-(3,4-Dihydroxyphenethyl)-2-phenoxyacetamide)-10,15,20-(p-tert-butyltriphenyl)-porphyrinatozinc, DOPAZ, and (b) Synthesis of 5-(3,4-Dihydroxy-N-phenylbenzamide)-10,15,20-tris(4-tert-butylphenyl)-porphyrinatozinc, CAMIZ

4670 J. Phys. Chem. C, Vol. 113, No. 11, 2009 Marczak et al.


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few modifications. A 2.19 g (0.01 mol) sample of zinc acetatedihydrate was dissolved in 100 mL of boiling ethanol atatmospheric pressure and cooled down to the synthesis tem-perature. A white powder of anhydrous zinc acetate precipitatedclose to room temperature.20 A 0.34 g (0.014 mol) sample oflithium hydroxide was dissolved in 100 mL of boiling ethanoland cooled to the synthesis temperature. Then, the lithiumhydroxide solution was added dropwise to the zinc acetatesolution under vigorous stirring. The reaction mixture becametransparent after addition of about 1/3 volume of the lithiumhydroxide solution. The ZnO colloid was stored at e0 C toprevent rapid particle growth. In order to remove the reactionbyproduct, lithium acetate, the ZnO suspension was washed byrepeated flocculation of ZnO affected by addition ofn-heptane.

The supernatant was separated from the ZnO white precipitateby centrifugation and decantation. For colloidal characterization,

the ZnO flocculates were redispersed in ethanol. In order toobtain the powder, the ZnO flocculates were dried undernitrogen for about 5 min.

Particle Characterization. Particle size distribution of ZnOnanoparticle suspensions were determined via dynamic lightscattering (DLS) by using a Malvern Nano ZS Instrument witha 633 nm red laser. For each sample, 10 measurements wereperformed and the average value per sample was calculated.Optical properties of the nanoparticles were determined from

UV-vis absorption spectra recorded using a Cary 100 ScanSpectrometer (Varian) with a 10 mm path length cuvette.Structural analysis of the ZnO nanoparticles was performed ina D8 Advance (Bruker AXS) X-ray diffractometer (XRD) usingCu KR radiation (0.154 nm). The measurement was in the rangeof 20 e 2 e 70. HRTEM images were obtained using aPhilips CM 300 UltraTwin microscope with the particlesdeposited on a standard copper grid supported carbon film. FTIRspectra were recorded on a Varian Excalibur Spectrometer FTS3100 with a resolution of 2 cm-1 using the Easy Diffusereflectance accessory. The samples were packed into a smallsample cup. A reproducible sample surface was achieved bysmoothing with a razor blade.

Synthesis of the Dye Molecules. 5-(N-(3,4-Dihydroxyphen-ethyl)-2-phenoxyacetamide)-10,15,20-(p-tert-butyltriphenyl)por-

phyrin. A solution of 500 mg of porphyrin 1 (Scheme 1)45 (0.55mmol) and 200 mL formic acid was stirred for 4 h to obtainthe deprotected acid. Progress of the reaction was followed viaTLC. The solvent was removed on a rotary evaporator,transferred into toluene, and evaporated twice to remove anyresidual formic acid. The product was finally dried underreduced pressure. The dried product was dissolved in DMF at0 C, and after addition of 315 mg of EDC (1.65 mmol), 222mg of HOBT (1.65 mmol), and 200 mg of DMAP (1.65 mmol),the solution was allowed to stir for 1 h at 0 C. Subsequently155 mg of 3-hydroxytyramine hydrochloride (dopamine) (0.825mmol) was added to the solution, and the mixture was stirredat room temperature for 48 h. The solvent was removed on arotary evaporator, and the residue was purified with columnchromatography on silica gel with a mixture of dichloromethane/methanol (19:1) as eluent. The product was obtained bycrystallization in pentane. Yield: 410 mg (75%). 1H NMR (400MHz, 25 C, THF-d8): ) -2.66 (s, 2H), 1.60 (s, 27H), 2.76(t, 3J ) 7.3 Hz, 2H), 3.53 (dd, 3J ) 6.5 Hz, 3J ) 14.1 Hz, 2H),4.70 (s, 2H), 6.55 (dd, 4J ) 2.1 Hz, 3J ) 7.9 Hz, 1H), 6.66 (d,3J ) 7.9 Hz, 1H), 6.71 (d, 4J ) 2.0 Hz, 1H), 7.36 (d, 3J ) 8.7Hz, 2H), 7.52 (t, 3J ) 5.8 Hz, 1H), 7.71 (s, 1H), 7.82 (m, 7H),8.14 (m, 8H), 8.82 (d, 3J ) 4.0 Hz, 8H) ppm. 13C NMR (100.5MHz, 25 C, THF-d8): ) 31.87, 35.45, 36.22, 41.52, 68.88,114.07, 115.92, 116.52, 120.44, 120.52, 120.96, 124.51, 131.61,

131.76, 135.26, 136.27, 136.48, 140.42, 144.40, 146.48, 151.41,159.18, 168.10 ppm. UV/vis (ethanol): max (log ) 417 (5.26),519 (4.58), 551 (4.51), 593 (4.42), 646 nm (4.36). MS (FAB):m/z (%) 993 [M]+.

5-(N-(3,4-Dihydroxyphenethyl)-2-phenoxyacetamide)-10,15,20-

(p-tert-butyltriphenyl)porphyrinatozinc (DOPAZ). An amountof 110 mg of 5-(N-(3,4-dihydroxyphenethyl)-2-phenoxyaceta-mide)-10,15,20-(p-tert-butyltriphenyl)porphyrin (0.11 mmol)was dissolved in 100 mL of THF, and 80 mg of zinc acetate(0.44 mmol) was added. The mixture was heated to reflux overa period of 4 h. The reaction was monitored by TLC by thedisappearance of the free base porphyrin. The solution wasconcentrated on a rotary evaporator, precipitated with water,

and finally filtered. The dried product was obtained as a purplered solid in very good yield (113 mg, 98%). 1H NMR (400

SCHEME 2: (a) 5-(N-(3,4-Dihydroxyphenethyl)-2-phe-noxyacetamide)-10,15,20-(p-tert-butyltriphenyl)porphyri-natozinc, DOPAZ, and (b) 5-(3,4-Dihydroxy-N-phenylbenzamide)-10,15,20-tris(4-tert-butylphenyl)porphyrinatozinc, CAMIZ, Molecules ontothe ZnO Nanoparticle Surface

Synthesis, Characterization of ZnO Nanoparticles J. Phys. Chem. C, Vol. 113, No. 11, 2009 4671


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MHz, 25 C, THF-d8): ) 1.62 (s, 27H), 2.76 (t, 3J ) 7.3 Hz,2H), 3.54 (dd, 3J ) 6.5 Hz, 3J ) 14.1 Hz, 2H), 4.71 (s, 2H),6.56 (dd, 4J ) 2.1 Hz, 3J ) 7.9 Hz, 1H), 6.67 (d, 3J ) 7.9 Hz,1H), 6.71 (d, 4J ) 2.0 Hz, 1H), 7.34 (d, 3J ) 8.7 Hz, 2H), 7.54(t, 3J ) 5.8 Hz, 1H), 7.72 (s, 1H), 7.80 (d, 3J ) 8.3 Hz, 6H),7.84 (s, 1H), 8.12 (m, 8H), 8.85 (m, 8H) ppm. 13C NMR (100.5MHz, 25 C, THF-d8): ) 31.93, 35.41, 36.25, 41.52, 68.91,113.67, 116.51, 120.54, 120.86, 121.45, 124.09, 131.99, 132.19,132.27, 135.34, 136.39, 137.82, 141.69, 144.38, 146.48, 150.83,

151.13, 151.24, 158.78, 168.17 ppm. UV/vis (ethanol): max (log) 424 (5.23), 557 (3.95), 598 nm (3.67). MS (FAB): m/z (%)1053 [M]+.

5-(N,2,2-Triphenylbenzo[d][1,3]dioxole-5-carboxamide)-

10,15,20-tris(4-tert-butylphenyl)porphyrin. An amount of 100mgof3,4-diphenylmethylenedioxyprotocatechuicacid46(0.31mmol)was dissolved in 200 mL of dichloromethane at 0 C. Then130 mg of DCC (0.62 mmol) and 85 mg of HOBT (0.62 mmol)were added, and the solution was stirred for 1 h at 0 C. Afterthat period 250 mg of 5-(4-aminophenyl)-10,15,20-tris(4-tert-butylphenyl)porphyrin 2 (Scheme 1)47 (0.62 mmol) was addedto the solution, and the mixture was stirred at room temperaturefor 96 h. The solvent was removed on a rotary evaporator, and

the residue was purified by column chromatography on silicagel with dichloromethane as eluent. The yield was 120 mg(35%). 1H NMR (400 MHz, 25 C, CDCl3): ) -2.76 (s, 2H),1.61 (s, 27H), 7.04 (d, 3J ) 7.9 Hz, 1H) 7.42-7.65 (m, 12H),7.76 (d, 3J ) 8.3 Hz, 6H), 8.00 (d, 3J ) 8.5 Hz, 2H), 8.03 (s,1H), 8.14 (m, 6H), 8.22 (d, 3J ) 8.5 Hz, 2H), 8.87 (m, 8H)ppm. 13C NMR (100.5 MHz, 25 C, CDCl3): ) 31.63, 34.84,108.05, 108.51, 118.31, 120.30, 121.86, 123.66, 126.34, 128.47,129.49, 129.34, 131.11, 134.54, 135.33, 137.75, 138.51, 139.25,139.73, 145.91, 150.52, 150.56, 165.47 ppm. UV/vis (ethanol):max (log ) 417 (5.07), 520 (4.31), 557 (4.28), 596 (4.18), 648nm (4.17). MS (FAB): m/z (%) 1099 [M]+.

5-(3,4-Dihydroxy-N-phenylbenzamide)-10,15,20-tris(4-tert-

butylphenyl)porphyrin. An amount of 100 mg of 5-(N,2,2-triphenylbenzo[d][1,3]dioxole-5-carboxamide)-10,15,20-tris(4-tert-butylphenyl)porphyrin was dissolved in 100 mL of formicacid. The solution was stirred further for 5 h. The progress ofthe reaction was monitored via TLC. After completion of thereaction, the solvent was removed by rotary evaporation. Themixture was transferred into toluene and evaporated twice toremove any residual formic acid. The final product is obtainedby recrystallization in toluene overnight at 0 C with a yield of83 mg (98%). 1H NMR (400 MHz, 25 C, THF-d8): ) -2.66 (s, 2H), 1.61 (s, 27H), 6.86 (d, 3J ) 8.2 Hz, 1H), 7.49 (dd,4J ) 2.1 Hz, 3J ) 8.2 Hz, 1H), 7.56 (d, 4J ) 2.1 Hz, 1H), 7.82(d, 3J ) 8.2 Hz, 6H), 8.15 (m, 8H,), 8.25 (d, 3J ) 8.5 Hz, 2H),8.51 (s, 1H), 8.67 (s, 1H), 8.83 (m, 6H), 8.91 (d, 3J ) 4.6 Hz,2H), 9.59 (s, 1H) ppm. 13C NMR (100.5 MHz, 25 C, THF-d8): ) 31.94, 35.51, 115.42, 115.98, 118.78, 120.40, 120.99,124.58, 128.18, 131.89, 135.36, 135.72, 137.78, 140.52, 141.14,146.29, 150.07, 151.43, 166.33 ppm. UV/vis (ethanol): max (log) 417 (5.03), 518 (4.50), 551 (4.49), 596 (4.43), 647 nm (4.36).MS (MALDI-TOF): m/z (%) 934 [M]+.

5-(3,4-Dihydroxy-N-phenylbenzamide)-10,15,20-tris (4-tert-

butylphenyl)porphyrinatozinc (CAMIZ). An amount of 80 mgof 5-(3,4-dihydroxy-N-phenylbenzamide)-10,15,20-tris(4-tert-butylphenyl)porphyrin (0.09 mmol) was dissolved in 50 mL ofTHF, and 65 mg of zinc acetate (0.36 mmol) was added. Themixture was heated to reflux over a period of 4 h. The reactionwas monitored by TLC by the disappearance of the free base

porphyrin. The solution was concentrated on a rotary evaporator,precipitated with water, and finally filtered. The dried product

was obtained as a purple red solid in nearly quantitative yield(81 mg, 95%). 1H NMR (400 MHz, 25 C, THF-d8): ) 1.62(s, 27H), 6.86 (d, 3J ) 8.2 Hz, 1H), 7.48 (dd, 4J ) 2.1 Hz, 3J) 8.2 Hz, 1H), 7.55 (d, 4J ) 2.1 Hz, 1H), 7.81 (d, 3J ) 8.2 Hz,6H), 8.14 (m, 8H), 8.22 (d, 3J ) 8.7 Hz, 2H), 8.44 (s, 1H),8.60 (s, 1H), 8.86 (m, 6H), 8.92 (d, 3J ) 4.7 Hz, 2H), 9.52 (s,1H) ppm. 13C NMR (100.5 MHz, 25 C, THF-d8): ) 30.65,35.52, 115.42, 115.98, 118.78, 120.40, 120.99, 124.58, 128.18,131.78, 131.99, 135.36, 135.72, 137.78, 140.52, 141.14, 146.29,

150.07, 150.74, 151.15, 151.43, 166.33 ppm. UV/vis (ethanol):max (log ) 424 (5.08), 559 (4.06), 601 nm (3.98). MS (FAB):m/z (%) 995 [M]+.

Spectroscopic Characterization. In order to investigate theinteraction between ZnO nanoparticles and dye molecules, theethanolic solutions of ZnO nanoparticles and different concen-tration of porphyrins were mixed and sonicated for 15 min.Thermogravimetric measurements were carried out on a NetzschSTA 409 CD. The powder of the samples was obtained bydrying the concentrated colloid at 30 C in vacuum for ca. 5 h.A small amount of the powders (about 15-20 mg) was heatedfrom room temperature to 1000 C under a helium flow of 80mL min-1. The UV-vis spectra were recorded with a Perkin-Elmer Lambda 2 spectrophotometer. Steady-state fluorescencestudies were carried out with a Fluoromax 3 (Horiba) instrument.Fluorescence lifetimes were measured with a Laser Strobefluorescence lifetime spectrometer (Photon Technology Inter-national) with 337-nm laser pulses from a nitrogen laserequipped with a stroboscopic detector.

Results and Discussion

ZnO Nanoparticles Synthesis. ZnO nanoparticles wereformed via precipitation in ethanolic solution. The zinc acetateand lithium hydroxide solutions in ethanol were mixed atdifferent temperatures under vigorous stirring. In order to followthe progress of oxide formation, the suspensions of the nano-

particles were aged in their mother liquors and the crystal growthwas monitored by UV-vis spectroscopy and DLS. The influenceof nanocrystal size on the electronic structure of semiconductingmaterial is represented by the band gap increasing withdecreasing of the particle sizes, which is attributed to thequantum confinement effect. ZnO shows this effect for particlessmaller than 8 nm.25,48,49 Hence, the measurement of absorptionspectra provides a convenient way to investigate particle growth.Figure 1 shows the absorption spectra of the ZnO suspensionrecorded immediately after the preparation (3 min) and per-formed in 60 min intervals for a total of 14 h. Just after thezinc acetate was mixed with the lithium hydroxide, the absor-bance spectrum shows a well-defined exciton peak at 300 nm,

which suggests a very fast nucleation. In addition, a markedred shift in the absorption edge was observed during the earlyformation stagesan indication of fast crystal growth. Theseabsorption measurements also allow gathering insight into thecrystal size distribution. In fact, the sharp excitonic peak in theabsorption spectra, as seen in the case of small nanocrystals (atshorter times), is indeed indicative of a narrow size distributionof the corresponding nanoparticles in the sample. For the largerparticles, on the other hand, sharp excitonic features are absentin the absorption spectra (at longer times). Instead only a broadand featureless absorption edge is registered. The latter is dueto the fact that a number of exciton peaks appear at differentenergies corresponding to different sized nanocrystals, whichare superimposed onto each other. As a matter of fact, we may

expect that the nanocrystals reveal a rather broadened sizedistribution as the aging time increases. Independent confirma-



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tion for this hypothesis was deduced from measurements thatfocused on the number-weighted size distributions recordedusing DLS at different times during the aging of the particles(Figure 2). Here, the longer the aging proceeds, the largerparticles with broader size distributions were detected.

The influence of the aging temperature, i.e., 10, 25, and 35

C, on the particle growth was also examined by using UV-

visabsorption spectroscopy and DLS. For each of the absorptionspectra, the average band gap energies were calculated fromthe absorption onset (Figure 3) and correlated with the differentaging times. The higher the aging temperature, the smaller theenergy band gap observed at the same time. Thus, larger ZnOnanocrystals were formed. Hydrodynamic diameter changes thatwere monitored during the aging by DLS (Figure 4) furtherconfirm this. These results imply that the particle size kept onevolving, even when their suspensions were stored at lowtemperatures. Moreover, higher aging temperature leads to lowercolloidal stability. At a temperature of 35 C, about 7 h afterthe start of the synthesis, the transparent ZnO suspension turnedwhite with the simultaneous increase in hydrodynamic diameter

(Figure 4). Such behavior is indicative of particle coagulation.Additionally, a marked decrease in the corresponding band gap

energy is observed (Figure 3). Important in this context is thatthe energy band gap gradually shifts toward the value for themacrocrystalline ZnO (3.34 eV)50 as the size of the particlesgrows.

The ability to obtain various stable particle sizes is based onthe phenomena that the growth of the particles is governed notonly by aging temperature and time but also by the presence ofthe reaction byproduct, lithium acetate, during the aging process.The reaction could be almost completely stopped by removing

this byproduct and, in turn, providing unique control over theexact particle size. This process, so-called washing, consistedof reversible flocculation of the ZnO nanoparticles by additionof n-heptane.20 After centrifugation and removal of the super-natant, the white ZnO flocculates were either redispersed inethanol to obtain a transparent colloid or dried under nitrogenfor about 5 min to get a white powder. The powder could bestored and redispersed even months after its synthesis. To verifythis restriction on crystal growth, the ZnO flocculation wasenforced after 4 h of aging, and the optical, compositional, andstructural characterizations were carried out on the washed ZnOnanocrystal solution as well as the redispersed powder. Nosignificant difference was registered.

Figure 1. Room temperature absorption spectra recorded at differentaging times of ZnO nanocrystals in ethanolic suspension. Red-shift inthe absorption onset was observed between interval time 0 and 60 min.

Figure 2. Number size distributions recorded by DLS at different agingtimes of the ZnO crystals in ethanolic suspension at 25 C. The sizedistribution broadening was observed between interval time 0 and 60min.

Figure 3. Calculated band gap energies as a function of aging timefor the ZnO nanoparticles in ethanolic suspension at 10 C (triangles),25 C (circles), and 35 C (squares). The band gap energy can be tunedcontinuously between 3.3 and 3.9 eV by choosing the reaction timeand temperature.

Figure 4. Particle aging in the ZnO ethanolic suspension at 10 C(triangles), 25 C (circles), and 35 C (squares) monitored by DLS.



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ZnO Nanoparticles Optical, Compositional, and Struc-

tural Characterizations. Figure 5 shows the steady-stateabsorption and emission spectra recorded for the suspensioncontaining the washed ZnO nanoparticles. The absorption onsetof 354 nm corresponds to a band gap energy of 3.5 eV. Theemission spectrum consists of a relatively narrow emission bandin the UV region at 355 nm and an intense broad emission band,which is observed in the visible region of the spectrum andthat maximizes at 530 nm. The UV emission is due to direct

recombination of photogenerated charge carriers, i.e., electronsin the conduction band and holes in the valence band (excitonemission).22,51-55 The width of the exciton emission correspondsto an inhomogeneous broadening of the particle size distribution.The visible emission, on the other hand, relates to the defectsin ZnO and has been extensively examined.51-60 Peaks observedin the range from 494 to 582 nm are attributed to oxygenvacancies.55,58-60 Thus, in our study the emission at 530 nm(2.34 eV) originates from oxygen vacanciessa fact that couldbe rationalized by insufficient oxidation conditions during thenanoparticles synthesis.

The experimental number size distribution of the washed ZnOnanoparticles remained in a narrow range, as shown in Figure6, with a mean hydrodynamic diameter of 4.90 and 4.0 nmobtained by DLS and from HRTEM image analysis, respec-tively. These results are in accordance with the size distributionconstructed from conversion of the absorption spectrum (Figure5 solid line) by an algorithm developed by Peukert et al.61 Thisalgorithm converts the measured absorption spectra into thesingle particle contributions using the bulk absorption coefficientdetermined by Bergstrom62 and the tight binding model (TBM)63

to correlate the measured wavelengths with distinct particlesizes.

Structural analysis of the sample was performed by usingX-ray diffraction. In the XRD pattern of the ZnO nanoparticlespresented in Figure 7, no impurity peaks are observed. All thediffraction peaks are well assigned to the standard hexagonal

phase of ZnO with a wurtzite structure reported in JCPDS cardno. 36-1451 (a ) 3.249 , c ) 5.206 ). The broadness of theXRD peaks reveals the nanocrystalline nature and size of theZnO nanocrystals. An average crystallite size of 4.7 nm wascalculated by using the Debye-Scherrers equation64 coincidingwith the previously described mean particles size. The crystallinenature of the ZnO nanoparticles is also evidenced in the HRTEMimages (Figure 8) that gives rise to lattice features. Moreover,it is seen that the nanocrystals are highly monodisperse.

As it was reported before,56 the ZnO particle surface is notbare but is stabilized with acetate groups, which originate fromthe precursor materials and are adsorbed on the surface of thecrystals. Acetate ions that are bound to the ZnO surface areeffectively preventing coagulation of colloidal ZnO nanopar-ticles. The presence of acetate groups on the ZnO particle surface

is confirmed by infrared spectroscopy (Figure 9) showing bandsat about 1585 and 1415 cm-1 and at around 1343 cm-1.56 Thesebands correspond to CdO stretching (1585 cm-1) and C-Ostretching (1415 cm-1). The smaller band at 1343 cm-1 is dueto the weakly bound acetic acid molecules. The intensive bandbelow 500 cm-1 is attributed to the vibrational modes ofZn-O.65,66

ZnO Nanoparticles Surface Functionalization. To examinethe capability for the surface functionalization of the ZnOsurface, the washed ZnO particles were mixed with solutionsof dye molecules, i.e., DOPAZ and CAMIZ. The bond stabilityof the bond between dye and nanoparticles was probed withTGA-MS. The results are summarized in Figure 10. A weightloss of about 13% is discernible in the range of 300-350 C in

the case of not-functionalized ZnO as well as for the dye-sensitized nanoparticles due to the loss of acetate groups

Figure 5. Room temperature steady-state absorption (solid line) andemission (dotted line) spectra recorded for the washed ZnO nanocrystalsin ethanolic suspension. Excitation wavelength was 330 nm.

Figure 6. Number size distributions of the washed ZnO nanoparticlesethanolic suspension obtained by DLS (black line), image analysis ofHRTEM (gray columns), and inversion of the UV-vis spectrum (Figure5a) based on the TBM (blue line).

Figure 7. X-ray powder pattern of ZnO nanoparticles. All peakscorrespond to the ZnO wurtzite structure. The broadness of the XRDpeaks reveals the nanocrystalline nature of the ZnO powders. Theaveragecrystallitesizewascalculatedtobe4.7nmusingDebye -Scherrersequation.



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adsorbed on the surface. This trend documents that carboxylicacids are relatively labile, what renders their use as anchoringgroup to metal oxides rather limiting. In the range of 600-780C, an additional weight loss takes place when the dyes arefunctionalized onto the nanoparticles. In particular, an additional

15% and 30% were noted for CAMIZ and DOPAZ grafted ontoZnO, respectively. Implicit is that the relatively flexible anchor-ing group of DOPAZ containing an ethyl spacer allows a higherdye coverage onto the surface, when compared to the rigidanchoring group of CAMIZ. However, the catechol group isbound very strongly to ZnO surfaces and is stable up to 600C, exceeding the stability of carboxylic anchor groups by far.

Electronic Communication. Next, the interactions of ZnOnanoparticles with the dye molecules, i.e., DOPAZ and CAMIZ,were tested in titration experiments by using steady-stateabsorption and emission spectroscopy. In these cases the ZnOnanoparticles concentrations were kept constant (1.5 10-4

M, based on the Zn2+ concentration), while those of the dyemolecules were varied incrementally between 0 and 2.25 10-6

M and 0 and 4.49 10-6 M for DOPAZ and CAMIZ,respectively. Figures 11a and b show the UV-vis absorption

spectra of ethanolic suspensions of the ZnO in the absence andin the presence of the dye molecules. The peak at 330 nmcorresponds to the absorption of ZnO nanoparticles, whereasthe strong feature at 424 nm and weaker features at 550 and600 nm are attributed to the porphyrins Soret and Q-bands,respectively. A broad absorption of the dye molecules is alsoobserved in the UV range.

Figure 8. High resolution transmission electron microscopy image of

ZnO nanocrystals.

Figure 9. Infrared spectrum of ZnO nanoparticles covered with acetategroups.

Figure 10. TGA curves of pure ZnO nanoparticles (solid line), ZnOnanoparticles grafted with CAMIZ (dashed line), and ZnO nanoparticlesgrafted with DOPAZ (dotted line) in the range of 25-1000 C over5 h.

Figure 11. Room temperature absorption spectra of ZnO in ethanolin the presence of different concentrations of (a) DOPAZ (0-2.25 M)and (b) CAMIZ (0-4.49 M).



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As we described before, the visible emission from ZnOcolloids arises from oxygen vacancies. In turn, the 530 nmemissionsupon photoexcitation at 330 nmsemerged as a usefulprobe to monitor charge transfer processes at the ZnO interface,thus to investigate the nanoparticles interaction with dyemolecules. To see whether the ZnO emission is sensitive to thepresence of the dye molecules, we monitored the ZnO emissionat different concentrations of DOPAZ and CAMIZ (Figures 12aand b). For both of the porphyrins, the ZnO emission intensity

decreases with increasing porphyrin concentrations. More than97% emission quenching of ZnO was achieved when adding2.25 M of DOPAZ, whereas the quenching is more than 96%when adding 4.49 M of CAMIZ.

When the porphyrin molecules are adsorbed onto ZnOnanoparticles with perpendicular or parallel orientation relativeto the surface of the particle, they occupy an area of about 0.25or 1.96 nm2, respectively. Thus, to form a monolayer, themaximum number of the adsorbed molecules per nanoparticlesis estimated to be about 120 and 15, respectively. In order todeactivate the excited state of ZnO nanoparticles quantitatively,the number of needed DOPAZ and CAMIZ molecules pernanoparticle is 35 and 67, respectively. This corresponds to aporphyrin density of about 1 molecule nm-2 and 2 molecule

nm-2. Aggregates have been ruled out since the correspondingshifts in the Soret band have not been observed (Figures 11a

and b). Thus, it is safe to assume a standing orientation of

the porphyrins relative to the nanoparticle surface and a graftingthrough the catechol group.

In summary, our emission quenching results prove theelectronic communication between the ZnO nanoparticles intheir excited state and both of the porphyrins. Furthermore, itsuggests that the visible emission of the ZnO is caused bysurface states, since only the surface of the particles is alteredby these grafting experiments.

If indeed the observed emission quenching of ZnO arises fromthe charge transfer interaction with the dye molecules, enhanceddecay rates of the visible emission with increased concentrationof the porphyrins should be observed. In order to confirm thisassumption, the samples were analyzed by time-resolved

fluorescence spectroscopy. The experiments on the lifetimemeasurements were carried out by exciting ZnO nanoparticlesat 355 nm and monitoring the visible emission at 530 nm. Figure13 displays the emission decay profiles of ZnO nanoparticlesin the presence of different quencher concentrations. All theemission decays could be best fitted by a biexponential fittingfunction in the studied time range, and the decay componentswere used to determine the average lifetimes.67 With increasingporphyrin concentrations, the lifetimes change from 46.0 nsdown to 15.3 ns. The shortened lifetimes prompt to a newdeactivation pathway, which is now available for the deactiva-tion of the excited state.

In the absence of the dye molecules, the ZnO emission decay

reflects the charge carrier recombination via radiative andnonradiative processes.51-60,67 In the presence of the porphyrins,an additional pathway is introduced, namely, through theelectronic coupling of the porphyrins to the ZnO nanoparticles.If the decrease in lifetime is entirely due to electron transfer tothe porphyrins, a rate constant of 0.35 108 s-1 could bedetermined for this process.67

Finally, in the context of excited-state interactions, we haveadded fluorescence experiments that focus on the DOPAZemission rather than on that of ZnO. In particular, porphyrinswere grafted to ZnO and Al2O3, and the porphyrin fluorescencewas tested as a function of conduction band energy, that is,ZnO versus Al2O3. Notable, higher conduction band energiesin Al2O3srelative to that of ZnOsrender an electron transfer

deactivation of the porphyrin singlet excited-state thermody-namically unfeasible. The corresponding fluorescence spectra

Figure 12. Room temperature fluorescence spectra of ZnO nanopar-ticles in ethanol, exhibiting an optical absorption of 0.1 at the

wavelength of photoexcitation of 330 nm, in the presence of differentconcentrations of (a) DOPAZ (0-2.25 M) and (b) CAMIZ (0-4.49M).

Figure 13. Normalized fluorescence lifetime measurements for thevisible band of ZnO nanoparticles in the presence of different CAMIZconcentrations: (a) 0, (b) 0.57, and (c) 1.60 M.



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of the ZnO and Al2O3 films are gathered in Figure 14. The

porphyrin emission when grafted to Al2O3 is nearly unchangedrelative to what is seen in reference experiments. In starkcontrast, the porphyrin emissionsexciting into the Soret bandat 424 nm sis nearly quantitatively quenched in the ZnO-sensitized films. Taking the aforementioned into account, weconclude charge injection that evolves from the photoexcitedporphrins with an injection efficiency of about 90%.

Conclusions

The synthesis of ZnO nanoparticles in ethanol by using asimple colloidal method was presented. It was shown that thegrowth of the particles was governed by temperature as well asthe presence of the reaction byproduct lithium acetate during

the aging process. This process could be almost completelystopped by removal of this byproduct by repeated flocculationof the ZnO particles by addition of n-heptane. Those ZnOnanoparticles were shown to be suitable for further derivatizationof their surface. The catechol-functionalized zinc porphyrinscould be used as support architectures to anchor the ZnOnanoparticles. Steady-state absorption spectroscopy as well assteady-state and time-resolved emission studies confirmed aninteraction between the ZnO and dye molecules and electroninjection from the dye molecules into the ZnO. The resultspresented in this study open the way toward the design ofordered ZnO-based nanostructures that can harvest light energyefficiently.

Acknowledgment. The authors gratefully acknowledge thefunding of the German Research Council (DFG), which, withinthe framework of its Excellence Initiative, supports the Clusterof Excellence Engineering of Advanced Materials (www.eam.uni-erlangen.de) at the University of Erlangen-Nuremberg.The authors want to thank Christoph Dotzer for the TGA-MSmeasurements and Dr. Robin Klupp Taylor for the HRTEMmeasurements.

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