Chelating the Surface of Zinc in Zinc Oxide Nanocrystals

J. Phys. Chem. C 2009, 113, 14435–14439

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Chelating the Surface of Zinc in Zinc Oxide Nanocrystals: Spectroscopic Characterization of ZnO Surface-Bound Eriochrome Black T and 8-Hydroxyquinoline D. Scott Bohle* and Carla J. Spina Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West, Montreal, H3A 2K6, Quebec, Canda ReceiVed: April 30, 2009; ReVised Manuscript ReceiVed: June 17, 2009

The surface binding of eriochrome black T (ErT) and 8-hydroxyquinoline (HQ) with zinc oxide nanocrystals is studied with UV-vis absorption and fluorescence spectroscopy. Stern-Volmer analysis of the quenching dynamics in neutral water leads to binding constants of 2.52(1) × 10-6 and 7.14(1) × 10-6 M-1 for the binding of HQ and ErT, respectively. ErT coordinates in a 1:1 tetrahedral geometry for about 10% of the surface zinc sites, while HQ occupies a similar coordination geometry for half of the available sites. A new transient species forms during the interaction of ErT with the zinc oxide nanocrystals which is characterized by an absorption at 680 nm. Introduction The surface of semiconducting zinc oxide (ZnO) plays a large role in its optical, catalytic, and electronic properties.1-12 In nanomaterials the importance of this surface increases tremendously due to the large surface to volume ratio, quantum confinement, and increasing relative quantity of the surfacerelated defects.2,11,13-17 Investigating the coordination geometries of zinc and oxygen on the surface of ZnO nanocrystals (NCs) would provide insight into surface related properties and effects of surface modification on the physiochemical properties of the nanomaterials. The surface structure of nanomaterials is difficult to assess, as most analytical methods represent an average of the surface properties.18 Modification, or coordination of surface sites with ligands of various, known geometries and observing the effects on absorption and fluorescence will extend our knowledge of the surface structure in relation to luminescent and electronic properties of ZnO NCs. Using fluorescent or chromic chelates with known preferred geometries, either Td or Oh, we monitored their coordination to the ZnO NC surface, and by UV-vis or PL spectroscopy, observed the effects of ZnO NCs. Two coordinating ligands were investigated: hydroxyquinoline (HQ) and eriochrome black T (ErT). Metal chelates of the conjugate base of HQ have recently attracted much attention due to their performance in organic light-emitting devices (OLEDs).19-22 Historically, since the 19th century, the structure and properties of metal coordination complexes of HQ, also known as 8-quinolinol or oxine complexes, have been investigated for a variety of metal species and are used in medicinal, agricultural, and analytical studies.23-27 Zinc coordination complexes of HQ include the octahedral dihydrate [Zn(II)(HQ)2(OH2)2] and its distorted tetrahedral anhydride [Zn(II)(HQ)2].21,28-30 Although no correlation of the ligand structure to the device properties has been established, HQ has been shown to facilitate electronic transition pathways for use in sensory and light emitting applications, as mentioned above.19,21-23,26,29 ErT is an azo dye introduced in the early 1940s. It is prevalent as a metal indicator dye due to its ability to form metal chromic chelates with a variety of alkali earth, transition, and main group metals including Ca, Mg, Mn, Cd, * To whom correspondence [email protected].

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Hg, Pb, Cu, Al, Zn, etc.27,31,32 Biochemically, metal complexes of these R-hydroxyazo compounds are important as models for metal-enzyme and biological metal-ion transport interactions. The Zn(II) chelate of ErT, is an effective fungicide.33 ErT has historically been used in the detection and separation of variety of metal coordination complexes of known geometries.25,27,31,34,35 ErT and HQ were chosen as ligands for zinc coordination complexes due to their known coordination geometires, spectroscopic absorption profile, and emission characteristics. Zinc complexes of both ErT and HQ have been previously investigated, their spectral properties are well-defined, and both are known to bind to zinc but not so strongly as to decompose the ZnO NCs. In this paper we utilize the spectroscopic properties of HQ-Zn(II) and ErT-Zn(II) to investigate the geometries and quantify of the surface features of ZnO NCs. Due to the rich spectroscopic nature of these complexes we are able to determine the surface geometries, quenching dynamics, and electron or energy transfer mechanisms. Herein we report the successful coordination complexation of ErT and HQ to the surface of ZnO. These bound chromophores influence the optical properties of ZnO NCs, and in the case of HQ, a new emission band was observed in the emission spectrum. Monochelate, tetrahedral geometries are determined for the species bound to a fraction of the ZnO surface, providing information on the available geometry and quantity of the sites on the ZnO NC surface. These results are critical in the analysis and design of sensory and catalytic applications for ZnO nanomaterials. Experimental Procedures Sample Preparation.36 Preparation of ZnO NCs. Colloidal nanocrystalline zinc oxide (ZnO NC) are prepared at room temperature by the slow addition of tetramethylamonium hydroxide pentahydrate solution in ethanol (45 µmol, 81 µL) over 1.5 h to a rapidly stirring solution of zinc acetate dihydrate (25 µmol, 250 µL in DMSO) in DMSO (2.5 mL). Once the addition is complete, the solution is sealed and left to stir overnight to complete ripening of the nanocrystals. As demonstrated by TEM and X-ray powder diffraction, Figure S7, the average ZnO NC radius is 2.72(22) nm. The effect of the pH of the tetramethyl ammonium hydroxide was also investigated by a simple titration with both the hydroxide salt in ethanolic solution and the Zn(OAc)2 salt in DMSO. Within the concentra-

10.1021/jp904014u CCC: $40.75  2009 American Chemical Society Published on Web 07/17/2009

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Figure 1. (a) UV-visible spectra of the addition of ZnO NCs (1.8 µmol, λabs ) 337 nm, λem ) 530 nm) to ErT (0.1 µmol, λabs ) 505 λem ) 399, 620 nm) in ethanol. (b) Photoluminescent spectrum of the addition of ErT (57 nmol) to ZnO NCs (46 nmol); for Stern-Volmer analysis, see Figure S6.

Figure 2. (a) UV-visible spectra of the addition of ZnO NCs (0.9 µmol, λabs ) 337 nm, λem ) 530 nm) to HQ (1.1 µmol, λabs ) 318 nm, λem ) 417 nm) in ethanol. (b) Photoluminescent spectrum of the addition of HQ (94 nmol) to ZnO NCs (46 nmol); for Stern-Volmer analysis, see Figure S6.

tion range which were used in the ZnO NC experiments, there was no significant effect of the tetramethylammonium hydroxide, or the zinc acetate. ErT or HQ Chelation to ZnO NCs. Photoluminescent Spectroscopy. Into a quartz cuvette ZnO NCs (45 µmol, 50 µL, DMSO) are added to ethanol (2.5 mL). To the solution of ZnO NCs, ErT (total: 57 µmol, 75 µL, ethanol) or HQ (94 µmol, 100 µL, ethanol) is slowly titrated into the ZnO solution. PL spectra were observed after each addition until a steady spectrum was obtained, and then subsequent additions of ErT or HQ are added until there is no longer any intensity changes in the visible emission of ZnO. Absorption Spectroscopy. Into a quartz cuvette ErT (0.11 mmol, 10 µL, ethanol) or HQ (0.5 mmol, 50 µL, ethanol) is diluted with ethanol (2.5 mL). To the ErT solution, ZnO NCs as prepared above (total: 1.8 mmol, 200 µL, DMSO) is slowly titrated into the ErT solution until no changes are observed. To the HQ solution (0.92 mmol, 100 µL, DMSO) is added and spectra are observed every 10 min until no further changes are observed. Physical Measurements. All chemicals and solvents were of reagent grade and used without further purification. ZnO NC reaction solutions were used directly in further reactions with concentrations of ∼9 µM. Absorption spectra of samples were obtained with a HP 8453x UV-visible system. Fluorescent spectra were recorded using a FluoroMax 2 (ISA) Jobin YvonSPEX spectrofluorometer, with a constant excitation wavelength of 337 nm. Unless otherwise stated, the onset band-edge absorption spectra of the ZnO nanoparticles was 337 nm, and the observed emission spectra contained two peaks, a UV peak at 355 nm, and a broad visible peak at ∼530 nm, the later of which is dominant in aerated solution.

Results With the addition of ZnO NCs to ErT (λErT ) 505 nm, Figure 1), we observe two different ErT species with separate maxima at λ1 ) 584 nm and λ2 ) 680 nm. Although, both of these species are observed upon initial addition of ZnO NCs to ErT, with higher ZnO concentration (1eq HQ (λem ) 433 nm), there is a simultaneous decrease in the fluorescence from the initial species (λem ) 554 nm), Figure S4. This emission band λem ) 433 nm is not observed upon the addition of HQ to ZnO NCs. Discussion ErT has found extensive use as a metal indicator in trace metal analysis in competition chelatometric titrations with ethylenediaminetetraacetic acid (EDTA). The dye itself is pH sensitive where a color change from red to blue corresponds to the ionization of one of the phenolic groups, and the second color change corresponds to the second ionization, Table 1. In various organic solvents ErT has been noted to exhibit different spectral features depending upon both the solvent characteristics and the concentration of ErT in solution.35 ErT belongs to the R-hydroxyazo category of dyes, these dyes undergo rapid intramolecular proton transfer between enol-azo and ketohydrazone. The spectral changes observed are attributed to this azo/hydrazone tautomerization, Figure S5. and hydrogen bonding of the hydroxyl groups with solvent molecules, where in acetone, ACN, and DMSO solutions the azo form is predominant, as well as in other solvents such as ethanol. The dye exhibits an acid-base equilibrium: H2ErT- T HErT-2 and at

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14437 higher ErT concentrations two main maxima may be observed at λabs ) 515 and 575 nm. The absorbance at higher energy corresponds to the hydrazone isomer. Previous research reports found an absorbance band at λabs = 675-680 nm or 680 nm in ethanol, pKs ) 18.9. This broad, lower-intensity band is not linearly dependent upon ErT concentration, and its extinction which levels out at higher concentrations, is attributed to the formation of a dianionic ErT-2 species.35 The eno-azo isomerization, in terms of chelation, translates to two effective electron-pair donor centers, oxygen and nitrogen, where -SO3- and -NO2 groups compete with -OH and -NdN- groups for complex formation, Figure 3.33 Metal complexation of ErT follows the general order: Fe < Co < Ni < Cu < Zn < Pb < Cd < Hg, loosely following the Irving-Williams order and an increase in metal ion radii.32 Upon single Zn(II) complexation there is a color change, (pH 8.5, λErT ) 615 nm f λZn-ErT ) 565 nm), where the stability of Zn-ErT has been determined, K ) 1010, and the formation constant of Zn-ErT complexes is Kf ) 5.18.27,35,37,38 Stability constants indicate a 1:1 metal/chelate complex, of tetrahedral geometry, is preferentially formed with Zn(II) chelation to ErT and there is a bathochromic shift in the UV-visible spectrum.31,33 In our investigations we are able to observe the formation of a Zn(II)-ErT complex (λ1 ) 584 nm), as well as a secondary species with λmax ) 680 nm. The identity of the first species has been proposed, based on spectroscopic data, to be a monoligated Zn-ErT complex of tetrahedral geometry, which we have verified with our own investigations of the complexation of ErT and labile Zn(II) in solution, Figure S1.27 Chelation of ErT to ZnO NCs results in the observation of this Zn-ErT complex (λ1 ) 584 nm), Figure 1, and this complex is calculated to chelate ∼10% of the total zinc from ZnO NCs. In terms of the ZnO NC surface, this translates to ∼40% of surface zinc atoms chelated as calculated from the radius, Figure S7. In the photoluminescent spectrum we observe a near-complete quenching of the visible emission of ZnO NCs, Figure 1, from the fluorescent quenching studies, a binding constant was determined, Ka ) 7.14 × 106 ( 5 × 105 M-1 where a static-type quenching mechanism is observed from the Stern-Volmer photoluminescent analysis, Figure S6. From the overlap observed for the ZnO NC emission and Zn-ErT absorption spectrum it is clear that there may be energy transfer from the excited ZnO NC complex to the Zn-ErT species, Figure S8a. HQ has a long history of use as a metal chelator, in particular for for medicinal, analytical, and optical applications.19-26,40 HQ, like ErT, is pH sensitive with a variety of protonated states with different spectral characteristics, the electronic transitions of HQ also exhibit dependence on the solvent choice, Table 2. Deprotonated HQ acts as a bidentate ligand through N and O-, shown above, forming a wide range of chelates with metal

Figure 3. Proposed coordination geometries of ErT to Zn(II) either monoligated in a tetrahedral geometry or in an octahedral geometry bound to two ErT ligands. Formation constants do not support the formation of the doubly bound Zn-ErT2 complex. Dashed lines represent the extended ErT structure.

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Figure 4. Chelation complexes of HQ with Zn(II) in solution. Possible chelation geometries involve mono or bis bidentate chelation in a tetrahedral complex or octahedral tris chelate coordination.

ions.41 The formation of such Mn+Ln complexes have been previously identified, and the stability of such bidentate metal complexes follows the Irving-Willimas order trend: Mn2+ < Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+.24,41 Many Zn(II) complexes of HQ have been previously prepareds Zn(HQ)2 · 2H2O has a trans-planar arrangement of the two HQ groups about the central atoms, and ZnHQ2 has a tetrahedral geometry.20,24,29 Thin-film ZnHQ2 absorption and emission spectra exhibit λabs ) 385 nm, λem ) 545 nm, and aqueous solution spectra of ZnHQ2 also have a visible emission at λem = 535 nm. These HQ-Zn complexes have a range of reported metal binding pK ) 8.1-24.7.20,23,26 The chelation HQ complexes on the ZnO NC surface were investigated. In the formation of HQ-metal complexes if the coordination number is more than twice the charge of the metal, as is the case for Zn(II), there may be additional ligands which give rise to other possible binding geometries, Figure 4.40 The changes in the absorption spectra observed upon the addition of HQ to ZnO NCs indicate the formation of a new species (λabs ) 385 nm) in solution, and this species is attributed to the singly bound HQ/Zn, 1:1 species. This species is formed during the addition of HQ to Zn(O2CCH3)2, Figure S9. The same complex (λabs ) 385 nm) is observed upon the addition of HQ to ZnO NCs in solution, Figure 1. In the fluorescent spectrum there is a new species (λem ) 550 nm), unrelated to both the free HQ (λem ) 417 nm) or to the visible emission of ZnO NCs (λem ) 530 nm). From the emission spectrum, this band is associated with the complex of λabs ) 385 nm, Figure S10. This new complex, as mentioned above, is attributed to the singly bound ZnHQ complex on the surface of ZnO. From fluorescent spectral analysis, we were able to determine the association constant of HQ to the ZnO surface addition of Ka ) 2.52 × 10-6 M-1, Figure S6, the percentage of available surface zinc bound to HQ was also calculated, considering the fraction of zinc on the surface of NCs (r ) 2.7 nm) the fraction of surface sites bound was determined to be ∼50%. The quenching mechanism here appears to be static from the Stern-Volmer analysis; however, the minimal spectral overlap suggests an alternate quenching mechanism than energy transfer, Figure S8, may operate here. Quenching is then proposed to be due to electron transfer, where as there is no observable decomposition of the chelate the electron transfer is not unidirectional,42 and the electron may be transferred to the chelate and then back to the nanoparticles through nonradiative recombination. Conclusions By understanding the features on the surface of ZnO NCs, which influence the observed luminescent changes, the sensory, catalytic, and electronic applications can be developed.1-12 In

Bohle and Spina this paper we have examined the binding character of two chromophoric chelating agents, HQ and ErT and their effect on the absorption and emission properties of ZnO NCs. The static binding of both of the chelates was inferred from quenching dynamics though Stern-Volmer analysis, and the binding constants determined to be Ka ) 2.52 × 10-6 and 7.14 × 10-6 M-1 for HQ and ErT, respectively. ErT was found to coordinate to the ZnO surface in a 1:1 Zn/ErT tetrahedral geometry binding to ∼10% of potential Zn(II) sites, where HQ was also found to bind in a 1:1 fashion of similar geometry calculated to bind to approximately half of available surface Zn(II) sites. Acknowledgment. We acknowledge the NSERC and CRC for generous support of this research in the form of discovery and a Canadian Research Chair for DSB, and the CIHR for Chemical Biology Fellowship for C.J.S. Supporting Information Available: Spectroscopic data, including absorption, photoluminescent, and Stern-Volmer analyses for the binding of HQ and ErT to ZnO NCs. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Baird, N. C.; Draper, A. M.; De Mayo, P. Can. J. Chem. 1988, 66, 1579–1588. (2) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2007, 129, 12380– 12381. (3) Carraway, E. R.; Hoffman, A. J.; Hoffmann, M. R. EnViron. Sci. Technol. 1994, 28, 786–793. (4) Chang, M.; Cao, X. L.; Zeng, H.; Zhang, L. Chem. Phys. Lett. 2007, 446, 370–373. (5) Fallert, J.; Hauschild, R.; Stelzl, F.; Urban, A.; Wissinger, M.; Zhou, H.; Klingshirn, C.; Kalt, H. J. Appl. Phys. 2007, 101, 073506/073501073506/073504. (6) Goepel, W.; Lampe, U. Phys. ReV. B: Condens. Matter 1980, 22, 6447–6462. (7) Hariharan, C. Appl. Catal., A 2006, 304, 55–61. (8) Kamat, P. V.; Patrick, B. J. Phys. Chem. 1992, 96, 6829–6834. (9) Lin, Y.; Wang, D.; Zhao, Q.; Li, Z.; Ma, Y.; Yang, M. Nanotechnology 2006, 17, 2110–2115. (10) Meyer, B. Phys. ReV. B: Condens. Matter Mater. Phys. 2004, 69, 045416/045411-045416/045410. (11) Ton-That, C.; Phillips, M. R.; Foley, M.; Moody, S. J.; Stampfl, A. P. J. Appl. Phys. Lett. 2008, 92, 261916/261911–261916/261913. (12) Woell, C. Prog. Surf. Sci. 2007, 82, 55–120. (13) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789–3798. (14) Norberg, N. S.; Gamelin, D. R. J. Phys. Chem. B 2005, 109, 20810– 20816. (15) Yadav, H. K.; Sreenivas, K.; Gupta, V.; Singh, S. P.; Katiyar, R. S. J. Mater. Res. 2007, 22, 2404–2409. (16) Djurisic Aleksandra, B.; Leung Yu, H. Small 2006, 2, 944–961. (17) Gu, Y.; Kuskovsky, I. L.; Yin, M.; O’Brien, S.; Neumark, G. F. Appl. Phys. Lett. 2004, 85, 3833–3835. (18) Zhang, P.; Xu, F.; Navrotsky, A.; Lee, J. S.; Kim, S.; Liu, J. Chem. Mater. 2007, 19, 5687–5693. (19) Burrows, P. E.; Sapochak, L. S.; McCarty, D. M.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 1994, 64, 2718–2720. (20) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson, M. L.; Schmidt, A.; Kippelen, B.; Padias, A. B.; Hall, H. K., Jr.; Peyghambarian, N.; Armstrong, N. R. Chem. Mater. 1996, 8, 344–351. (21) Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E. J. Am. Chem. Soc. 2002, 124, 6119–6125. (22) Sapochak, L. S.; Falkowitz, A.; Ferris, K. F.; Steinberg, S.; Burrows, P. E. J. Phys. Chem. B 2004, 108, 8558–8566. (23) Mahanand, D.; Houck, J. C. Clin. Chem. 1968, 14, 6–11. (24) Phillips, J. P. Chem. ReV. 1956, 56, 271–297. (25) Ringbom, A. Complexation in Analytical Chemistry: A Guide for the Critical Selection of Analytical Methods Based on Complexation Reactions; Interscience Publishers: New York, 1963; Vol. XVI. (26) Watanabe, S.; Frantz, W.; Trottier, D. Anal. Biochem. 1963, 5, 345– 359.

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