J. Phys. Chem. C 2010, 114, 18139–18145
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Controlled Co(II) Doping of Zinc Oxide Nanocrystals D. Scott Bohle* and Carla J. Spina Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West, Montreal, H3A 2K6, Quebec, Canada ReceiVed: September 2, 2010
Dopants are non-native atoms commonly used to modify the properties of bulk semiconductors. In this paper we demonstrate that by controlling the addition of cobalt(II) to growing zinc oxide nanocrystals (ZnO NCs) it is possible to modulate the resulting properties. We show that the environment of cobalt may be controlled by varying the synthetic conditions, mainly through varying the time of dopant-precursor addition and concentration. These conditions prove critical to the resulting Co(II) configuration, which affects both the luminescent and photocatalytic properties of the ZnO NCs. Presynthetic doping with 2% Co(II) results in a 98% quenching of the visible emission of ZnO, whereas the same quantity doped post synthesis results in only a 60% quenching. The environment of cobalt in the ZnO wurtzite lattice is identified through UV-vis spectroscopy. The wurtzite structure of the ZnO lattice for all nanocrystalline species is confirmed through X-ray diffraction patterns obtained from a synchrotron radiation source. Postsynthetically doped Co(II) in ZnO NC is demonstrated to have potential applications as an “on-off” sensor, as exemplified with nitric oxide. Introduction Doping inorganic semiconductors is frequently used to alter their electrical and photophysical properties. Research into these low-impurity metal oxides has led to major developments in spintronics, dilute magnetic semiconductors (DMS), optical devices, and photocatalysis.1-8 Doping of zinc oxide nanocrystals (ZnO NCs) is known to alter the photoluminescent, photocatalytic, and magnetic properties of the nanocrystalline semicondutor.4,5,9-13 However, there has been limited research into the effect of the location, or environment, of the dopants on the resulting properties. A facile method to prepare ZnO NCs is through hydrolysis and condensation of zinc acetate and tetramethylammonium hydroxide in a dimethyl sulfoxide (DMSO) solution, eq 1.
Zn(O2CCH3)2 + 2NMe4OH f ZnO + H2O + 2NMe4+O2CCH3-
(1)
Growth of ZnO NCs may be monitored via the band gap absorption feature in the UV-vis spectra, Supporting Information Figure S1.14 With the gradual addition of base, this band shifts to lower energy as NC size increases and greater absorbency with increasing NC concentration. Nucleation of ZnO nanoparticles occurs around the addition of 0.4 equiv of OH-, as verified by the initial observation of green ZnO trap fluorescence and the appearance of the absorption edge in the UV-vis spectrum at about 333 nm associated with the first excitonic transition of ZnO. It is this band edge that can be monitored to follow the growth of ZnO NCs in solution. Further addition of base results in an increased intensity of the band edge, indicating more nucleation, and a bathochromic shift, indicating the growth of the average size of the NCs.12,15 Excitation of ZnO NCs at energies equivalent to or greater than the band gap results in fluorescence, and two major * To whom correspondence should be addressed.
emissions may be observed: a UV, or band gap fluorescence, and visible or deep trap fluorescence, Supporting Information Figure S2.14 The UV band or band gap emission results from radiative recombination of an excited electron in the conduction band (eCB-) with the remaining hole (hVB+) in the valence band. The deep trap luminescence is not as well-defined. It is less dependent on NC size, has a longer lifetime, and may be attributed to more localized states of energies deep within the band gap.16,17 These localized states may either be found in the bulk or at the surface of a semiconductor, and their energies are mainly determined by the chemical nature of the trap and its surroundings. The longer lifetime of the visible emission suggests efficient separation of photoinduced charge carriers, favoring the capture of photoinduced electrons by adsorbed surface species, which is a positive photocatalytic attribute for the potential oxidation of organic compounds in solution.18-22 The many advantages of ZnO NCs photoluminescent (PL) properties include surface sensitivity, “on-off” switchability, nanometer spatial resolution, and submillisecond temporal resolution, all of which are important in sensor devices.2,6,23-25 Transition metal doping of ZnO NCs is known to alter their electronic properties. Dopants are also theorized to both cause and quench the visible or defect emission.1,2,15,26 High-spin (S ) 3/2) 3d7 Co(II) in a tetrahedral crystal field acts as a deep trap in II-VI semiconductors, where excitonic quenching proceeds via rapid nonradiative relaxation through Co(II). These allowed transitions for tetrahedral Co(II) are 4A2(F) f 2E(G) (660 nm), 4A2(F) f 4T1(P) (606 nm), and 4A2(F) f 2T1(G) (568 nm).9,10,12 The physicochemical effects of transition metals doping in ZnO NCs, however, are not easily predicted due to the rich defect chemistry of ZnO. Consequently, determining the exact influence of dopants remains an important theme for NC research.20,27-31 In this report we demonstrate that we are able to direct the spatial geometry of Co(II) doped in ZnO NCs through control of synthetic conditions resulting in marked alteration in the physicochemical properties of the NCs. In our previous work we investigated the origin of the deep trap or visible PL of ZnO using dioxygen to probe the surface
10.1021/jp108391e 2010 American Chemical Society Published on Web 10/05/2010
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Bohle and Spina
Figure 1. (A) Trace of the relative intensity of the visible emission (λem ) 530 nm) of ZnO NCs with various concentrations of Co(II) doping post ripening (solid) and doped during synthesis after 0.8 equiv of base (open). (B) Resulting visible emission (b, λem ) 530 nm) intensity of Zn(98.5%)Co(II)(1.5%)O NCs Co(II)1.5% when Co(II)(1.5%) is added at various times during the NC synthesis; for comparison, the trace (0, λmax) of undoped growing ZnO NCs. Total time of batch NC synthesis: 90 min.
Figure 2. X-ray diffraction patterns obtained from a synchrotron radiation source, λ ) 0.69854 Å, for ZnO NCs doped with Co(II) (A) 1.5% internal or during synthesis and (B) external or postsynthesis doping with various quantities of Co(II).
of ZnO NCs and its relationship with the visible emission.32 In this paper we examine the consequence of varying the quantity and time of Co(II) doping on the structural, PL, photocatalytic (PC), and electronic properties of ZnO NCs. The effects of cobalt doping during synthesis or internally (Zn100-x%Cox%O) versus post synthesis or externally (ZnO/Cox%) are reported, and the results from these studies aid in the understanding of the relationship between structure, PL, and PC activity. This makes it possible to realize some control over the electronic and structural features of ZnO NCs. Results The synthetic procedure for ZnO NCs was adapted from a previous method.12 The NCs were grown via the steady addition of a base, tetramethylammonium hydroxide, to a solution of zinc acetate in dimethyl sulfoxide.12 The effect of Co(II) doping during synthesis (open, O) versus Co(II) added post synthesis/ ripening (solid, b) on the visible emission (λem ) 530 nm) of ZnO NCs is depicted in Figure 1A. In this figure Co(II) added during synthesis is observed to be a more efficient quencher of the visible emission than Co(II) added post synthesis. When Co(II) is added during synthesis the addition of only 2% Co(II), with respect to total zinc, results in a 98% decrease in the visible emission, where postsynthetic addition of 2% Co(II) results in only 60% quenching Figure 1A. Interpreting these quenching properties will be taken up in the Discussion section. The slow, controlled growth of ZnO NCs is useful for investigation the effect of varying the time of doping Co(II) during the synthesis. Addition of base to the zinc solution takes
place over 1.5 h; at set times during this synthesis we examined the effect of Co(II) addition, Figure 1B. It is clear that the time of Co(II) addition influences the final PL intensity of the Zn(98.5%)Co(II)(1.5%)O NCs, where optimal quenching was determined to coincide with Co(II) addition at time 40 min. This optimized time and conditions of Co(II) addition were utilized for all subsequent “during synthesis” Co(II) doping experiments. Variable Co(II) dopant concentrations were also investigated, Figure 1A. The crystalline structure of the ZnO NCs after doping was verified by synchrotron X-ray powder diffraction, Figure 2, parts A and B, respectively, for internally and externally doped NCs. Coordination geometry of the doped Co(II) was established via UV-vis spectroscopy, Figure 3. In solution, the “free” ligand geometry of solvated Co(O2CCH3)2 is pseudooctahedral, with allowed 4T1g(F) f 4T1g(P) and 4T1g(F) f 4A2g ligand field transitions, Figure 3A, λmax ) 555 nm. When Co(O2CCH3)2 is added during ZnO synthesis the appearance of Co(II) in tetrahedral geometry is observed: allowed 4A2(F) f 2E(G), 4A2(F) f 4T1(P), and 4A2(F) f 2T1(G) ligand field transitions, Figure 3A, light gray, λmax ) 606 nm. Finally, when Co(II) is added to ZnO NCs post synthesis, Figure 3A, dark gray, the presence of another species of intermediate energy, λmax ) 574 nm, is noted. This intermediate species has unique properties as it is reversibly bound to the surface and reversibly quenches the visible emission of ZnO NCs, Figure 3B. The spatial location of this postsynthesis-doped Co(II) as compared to during synthesis doping is interpreted as being externally bound versus internalized in the metal oxide NC, respectively.11,14 Thus the absorption maximum of externalized Co(II) occurs at
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Figure 3. (A) Absorption spectra of the Co(O2CCH3)2 in DMSO solution (black), doped post synthesis (dark gray), and doped during ZnO synthesis (light gray). (B) Trace of the visible emission of ZnO NCs (λem ) 530 nm) upon the addition of Co(II) and subsequent dilution of Co(II) with ZnO, demonstrating the reversibility of the Co(II) bound to the ZnO NC surface.
Figure 4. Comparison of the photoluminescent spectra of (A) ZnO, (B) internally doped Zn98.5%Co1.5%O, and (C) externally doped ZnO/Co20% under the effect of various gaseous environments: aerobic (black), anaerobic (dark gray), and upon the addition 30 s of NO(g) bubbling into the anaerobic sealed system.
higher energy (574 nm) than the internalized tetra-oxo Co(II) (606 nm). The coordination geometry of Co(II) added post synthesis is proposed to be pseudotetrahedral, bound to the ZnO NC surface with acetate terminal ligation.12 UV-vis spectroscopy is then established as a useful tool in conjunction with other analytical techniques to assess the incorporation of cobalt into the lattice, by monitoring the transition from an octahedral to tetrahedral geometry.12,13 The distinct absorption λmax ) 606 nm, 4T1(P) f 4A2 transition is indicative of internally doped Co(II). Doping alters the PL properties of ZnO NCs, and thus doped NCs have potential as switchable sensors, present in an “on” or “off” state. Doping with Co(II) both internally or externally quenches the deep-trap, or visible luminescence in ZnO nanocrystals to varying degrees of an “off” state of emission. By altering the electronic structure of the “off”, paramagnetic ZnO/ Co(II), through interaction of Co(II) with a paramagnetic radical species, we can in principle effectively turn “on” the fluorescence of the ZnO NC. This “on-off” switchability was tested with nitric oxide (NO) radical, a species which has biological relevance and is known to interact rapidly and reversibly with Co(II) complexes.33-36 We observe that Co(II) doping, both internally and externally, results in an “off” PL state. In this “off” state, the system is NO responsive. For comparison, we observed the addition of NO to undoped, internally, and externally Co(II)-doped ZnO NCs, under anaerobic conditions, Figure 4. Undoped ZnO NCs respond to nitric oxide much in the same way as to dioxygen, resulting in an increase in the intensity of the visible emission and a corresponding quenching of the UV emission, Figure 4A. Internally doped Zn98.5%Co1.5%O NCs, however, have minimal photoluminescent change upon NO exposure, Figure 4B, though the observed small change is greater than the response when they are titrated with O2(g). The
photoluminescent response of externally doped ZnO/Co20% NCs is much more reactive to NO, where an inversion of the intensities of the UV and visible emission is observed, similar to the response observed for undoped ZnO, which restores the visible emission to near-maximum intensity, Figure 4C. Little change was observed in the photoluminescent spectrum of externally doped ZnO/Co20% NCs upon exposure to O2, indicative that this is a specific reactivity of surface-bound Co(II) with NO(g) and that this results in photoluminescent restoration. Discussion Zinc oxide nanocrystal synthesis employed a sol-gel procedure developed by Schwartz et al.,12 where ZnO NCs are prepared through a hydrolysis and condensation in a DMSO solution where the Zn(II) and OH- are the only nonspectator ions. Acetate may have a catalytic role as a growth stabilizer and regulator of ZnO NCs interactions, eq 1.12 During ZnO NC synthesis nucleation is known to occur at ∼0.4 equiv of OH-, as indicated by UV-vis and PL spectroscopy.12 It has been shown that prior to nucleation, cobalt is not significantly incorporated into the ZnO NC lattice, and only after nucleation does Co(II) incorporate into the ZnO lattice, via an intermediate species, Figure 3A.12 This intermediate species (574 nm) forms during the growth of the NC and is analogous to the species we observe in our postsynthetic Co(II)-doped ZnO NCs, Figure 3A, where subsequent additions of base yield complete incorporation of Co(II), which is incorporated into the tetrahedral ZnO wurtzite lattice sites. This incorporation of Co(II) into the wurtzite lattice is denoted Znx-100%Cox%O and is verified by UV-vis spectroscopy, λmax ) 606 nm.12,37 This lower energy band has a defined phonon structure, where direct excitation is not possible, indicative of strongly localized, internally doped
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Figure 5. Previously proposed defects energy levels (black) (ref 14) and energy levels of dopant Co(II) (red) (ref 44) found within the band gap in ZnO NCs. Band gap energy of ZnO Eg ) 3.37 eV; relative energy levels of the conduction and valence band are in relation to vacuum.
Co(II) levels in the core of the nanocrystal.9,11 The presence of Co(II) in solution during ZnO NC growth has also been suggested to inhibit the nucleation and growth of the NCs.38 Although in our doping experiments we have verified the NC size through a variety of techniques, we observe no significant changes in ZnO NC radius when Co(II) doping was made during NC synthesis, Supporting Information Figure S3. Nor is there any evidence for the type of phase separation39 of zinc and cobalt oxides as is seen when solvothermal methods are used to dope the zinc oxides.40 To set the context for our discussion of the quenching dynamics of Co(II) internally versus externally, a brief review of the processes of electron recombination available to ZnO NCs is useful. The visible emission (λem = 530 nm) of ZnO NCs may not be excited independently from the UV emission. All radiative recombination processes in ZnO NCs originate from the initial excitation, with energy greater than Eg, of an electron from the valence band to the conduction band inside the semiconductor, Figure 5. The band gap, or UV, emission occurs through the radiative recombination of the excited electron from the conduction band (CB) to the valence band (VB), where the visible emission originates from a lower energy defect site on or in the ZnO NC. The photophysical properties of ZnO NCs are very sensitive to surface effects where photoluminescence may be used as a probe of electron-hole processes. Surface-adsorbed species can provide valuable information about the nature of the excited states and the mechanisms of photocatalytic reactions and photoluminescent properties.41 Exchange mechanisms responsible for magnetic ordering may be mediated by an “F” center (or bound magnetic polaron); an electron trapped by an oxygen vacancy is an example of this. The trapped electron is theorized to occupy an extended orbital state, overlapping the d shells of adjacent transition metal atoms. The radius of the overlapping orbital is related to the Bohr radius and dielectric constant of the material. For ZnO, the radius is on the order of 0.5 nm.42 In doped materials there are two interacting subsystems, the delocalized conduction band electrons (eCB-)-valence band holes (hVB+) and the random system of doped species.43 Quenching occurs through an electron-transfer mechanism, when in the lattice or on the surface, Co(II) acts as an alternative pathway to nonradiative recombination of the excited electron-hole pair. The position of the unoccupied d levels have recently been
Bohle and Spina calculated and experimentally measured,44 results which resolve some of the original uncertainty of the position and the electronic band contribution of Co(II).45 The energy levels introduced by Co(II) vary with concentration of Co(II) within the ZnO structure and, as shown above, with spatial location in the ZnO NC.45 Variations in the band gap energy (Eg) and charge-transfer energy (ECT) with increasing quantities of Co(II) are due to raising the valence band with the occupied 3d energy levels of Co(II) to the top of the valence band which hybridize with bulk ZnO states.46,47 The energy of the acceptor-type levels relative to the conduction band, however, do not vary with concentration of Co(II).45 The effect of internal versus external Co(II) on the fluorescent quenching may in part be attributed to the effective quenching radius around ZnO NCs, as demonstrated by the observed variations in ZnO PL with the addition of Co(II) at different stages during ZnO synthesis. The relationship between size and effective quenching radius of NCs has been demonstrated previously, where quenching of CdSe NCs by TEMPO has been shown to be dependent on NC radius.48 Within the wurtzite lattice, the minimum distance between Co(II) and an oxygen vacancy (VO) has been calculated to be 0.494 nm, which is within the theoretically calculated range of Co(II) orbital overlap.42,46 Internally doped Co(II) is therefore a more effective quencher through proximity to the ZnO core. This is exemplified through our observations of the relationship between the time of Co(II) addition to ZnO synthesis and the effective quenching, Figure 1B. Through these quenching studies it was demonstrated that internal Co(II) quenches statically and external Co(II) quenches though a dynamic mechanism (Ka ) 35 700 M-1), Supporting Information Figures S4 and S5. This binding constant is based upon the quenching of postsynthetic-doped cobalt, where maximum quenching is observed at around ZnO/Co30% interpreted as near-complete surface coverage. The nature of externally versus internally bound Co(II) differs; as such, the electronic structure, quenching dynamics, as well as the adsorption properties also may differ.14 We demonstrate here that the influence of surface or external Co(II) on ZnO PL may clearly be differentiated from that of internally doped Co(II), and we go on to show that the properties of externally bound Co(II) may be utilized in sensory applications and, potentially, photocatalytic and electronic devices. We have established previously that the addition of oxygen to deoxygenated ZnO NCs results in the restoration of the visible emission with a loss of the UV emission, Figure 4A.32 The effect of NO in facilitating the visible emission is thought to proceed by similar mechanisms as to oxygen.32 The addition of paramagnetic Co(II) to the ZnO NC system, as mentioned above, may be utilized as an “on-off” switch for photoluminescence, where the paramagnetic Co(II) high-spin (S ) 3/2) 3d7, may undergo a spin coupling upon complexation with NO to form diamagnetic Co(I)-NO d8, restoring the fluorescence of the ZnO NCs. Internal doping of Co(II) was initially investigated; however, no significant effect on the PL spectrum is observed upon the addition of NO, Figure 4B External Co(II) doping, under an inert atmosphere, results in an initial quenching of both UV and visible ZnO PL and a slight blue-shift in the visible emission. Exposure to NO results in an immediate and significant inversion of the spectrum, Figure 4C; full visible PL restoration was inhibited due to the formation of a secondary species in solution. This result is proposed to be due to a redistribution of exciton recombination within the NC; the visible recombination pathway increases and the band-edge pathway decreases, where restoration of the visible emission
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Figure 6. Proposed mechanism of visible emission restoration upon the addition of NO to externally doped ZnO/Co20%, either via (a) reduction of Co(II) f Co(I) upon coordination of NO producing a diamagnetic d8 Co(I)NO dopant center or (b) exchange of the Co(II) dopant with NO on the ZnO NC surface.
TABLE 1: Comparison of Internally Doped Co(II), Added during Synthesis of ZnO NCs, and External Co(II), Added Post Synthesis of ZnO NCs external Co(II)
ref
geometry allowed transition (nm) ligands
characteristic
tetrahedral 660, 606, 568
internal Co(II)
pseudotetrahedral 620, 574, 525
9, 12 9, 12
tetra-oxo
12
quenching mechanism binding constant I/Io at 2% Co(II) EPR signal
static
acetate, hydroxy, and oxo dynamic
0.98 g⊥ ) 2.2669, g| ) 2.2212
Ka ) 35 700 M-1 0.60
semiconductor materials. As we have shown in previous work, extrinsic defects significantly influence the properties of ZnO NCs.14 In this paper we have chosen to further investigate the effect of the spatial position and quantity of these extrinsic dopants in and on the ZnO NCs as controlled through synthetic manipulation. Through spectroscopic techniques we are able to verify the geometries and interpret the spatial geometry of Co(II) within ZnO NCs, allowing for a distinction between internal and external doping. No migration of Co(II) from within the wurtzite lattice to the exterior of the NC or vice versa is observed within the time scale of this experiment. From these batch studies it was found that the geometry of Co(II) doping within the ZnO NC lattice influences the photoluminescent properties and reactivity of the NCs. Optimal PL quenching is correlated with the end of nucleation of undoped ZnO NCs, which indicates that the ideal time of Co(II) addition is related to the growth process of ZnO NCs. Applying the unique characteristics of postsynthetic doping, we demonstrated the potential application for NO sensing. With this study we move one step closer toward controlling the properties of ZnO NCs and, ultimately, their applications.
9
Experimental Section of externally doped ZnO/Co(II) NCs by nitric oxide was observed. This redistribution is attributed to one of two mechanisms, Figure 6: the direct reduction of Co(II) f Co(I) by NO, path a, resulting in a diamagnetic Co(I) nitrosyl complex, or displacement of Co(II) on the surface to form a ZnO-NO adduct, path b, shown previously to facilitate fluorescence on anaerobic ZnO NCs, Figure 4A. By examining the binding affinities, reduction potentials, and quenching mechanisms, Table 1, the potential pathways were assessed. As the binding affinity of oxygen to ZnO is greater than that of NO, the exchange of oxygen and external Co(II) doped on the surface would be thought to be more favorable than NO. However, as exposure of the externally doped Co(II) system to oxygen yields less of a response than toward NO, this suggests the first mechanism of visible emission restoration is most probable, Figure 6. Co(II) doping of ZnO NCs has been shown to have significant effects on the ZnO NC properties depending on the spatial orientation of Co(II) within the lattice. Below, we review the properties of presynthetic-doped Co(II), versus postsyntheticdoped Co(II), Table 1. Identification of internal and external Co(II) doping has been established here primarily by UV-vis absorption spectroscopy, and previously though magnetic susceptibility measurements.9 These distinguishable characteristics of Co(II) doping internally versus externally have great potential as demonstrated above, in the “on-off” sensing of nitric oxide in solution. Spatial control of doped transition metals within the NC lattice presents the semiconductor material community with another tool toward regulation of the physical and chemical properties of ZnO. Conclusions Dopants are well-known to control or modify the properties of bulk semiconductors. In nanomaterials, obtaining materials of similar characteristics using dopants has proven to be a greater challenge than first anticipated. Preparation of doped NCs for DMS, catalysis, or sensory devices does not always result in materials with the desired properties. A major contribution toward the unpredictability and irreproducibility associated with these doped materials is the quantity and type of defects inherent in the NC and their contribution to the characteristics of the
Chemicals and solvents were of reagent grade and used without further purification, except the solvents employed in the inert atmosphere box and the trimethylsilylchloride, which were distilled before use. All reagents or glassware that were to be used in the inert atmosphere box were placed under vacuum for 24 h and then transferred, under vacuum, and stored in the inert atmosphere box for later use. Deoxygenation of solutions and solvents was performed by either distillation or freeze-thawing. The ZnO reaction mixtures were assumed to have gone to completion, and the solutions were used directly in further reactions with concentrations between 8 and 10 µM. 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 ∼360 nm, and a visible peak at ∼530 nm, the latter of which is dominant in aerated solution. The water employed in all preparations was purified by a Milli-Q system resulting in a resistivity of >18 MΩ. Absorption spectra of samples were obtained with an HP 8453 UV-vis system. Fluorescent spectra were recorded using a FluoroMax 2 (ISA) Jobin Yvon-SPEX spectrofluorometer, with a constant excitation wavelength of 337 nm. Infrared spectroscopy was obtained with the ABB Bomem MB series. Irradiation experiments were carried out using a mercury arc lamp 295 nm emission max, Oriel Instruments series 66033. Particle sizes were verified by transmission electron microscopy (TEM) on a JEOL JEM-2011; X-ray diffraction was measured at room temperature with λ ) 0.69854 Å on Beamline X-3B at the National Synchrotron Light Source at the Department of Energy’s Brookhaven National Laboratory. UV-vis absorption spectroscopy was measured on an HP-8453 diode array spectrophotometer. Samples for TEM were prepared by transferring a drop of a colloidal suspension to a carbon-coated copper mesh slide followed by removal of excess fluid and drying in a dust-free environment overnight: particle size determined by TEM, r ) 3.8 ( 0.7 nm. Solid samples for XRD analysis were isolated though precipitation of ZnO NCs from solution via the addition of hexanes to a 24 h ripened solution of ZnO NCs. Sample Preparation. Standard Preparation of ZnO Nanocrystals in DMSO. To a small flask, 2.5 mL of DMSO and 250 µL of Zn(O2CCH3)2 · 2H2O (0.101 M in DMSO, 25 µmol) were
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added with stirring at room temperature. To the rapidly stirring solution, a total of 81 µL of [N(CH3)4]OH · 5H2O (0.556 M in EtOH, 45 µmol) was added slowly in 9 µmol aliquots over 1.5 h. Once the addition was complete, the solution was left stirring overnight to complete ripening of the nanocrystals. This same synthesis was alternately prepared in an inert atmosphere box (under N2(g)) to afford deoxygenated ZnO NCs. Addition of Co(II) during ZnO NC Synthesis. Time Dependence of Co(II) Addition. Co(II)(O2CCH3)2 · 4H2O (0.05 M, 7.5 µL, DMSO) was added at various times during the synthesis to obtain a series of Zn98.5%Co1.5%O. To a small flask, 2.5 mL of DMSO and 250 µL of Zn(O2CCH3)2 · 2H2O (0.101 M in DMSO, 25 µmol) were added with stirring at room temperature. To the rapidly stirring solution, a total of 81 µL of [N(CH3)4]OH · 5H2O (0.556 M in EtOH, 45 µmol) was added in aliquots of 9 µL over 1.5 h, where Co(II) was added just prior to the additions of base. Once the addition was complete, the solution was left stirring overnight to complete ripening of the nanocrystals. Quantity Dependence of Co(II). Depending on the desired percentage of Zn(100-x%)Co(x%)O in the nanocrystals, Co(II)(O2CCH3)2 · 4H2O (0.05 M, DMSO 2.5-15 µL) was added to achieve x ) 0.5-3%. To a small flask, 2.5 mL of DMSO and 250 µL of Zn(O2CCH3)2 · 2H2O (0.101 M in DMSO, 25 µmol) were added with stirring at room temperature. To the rapidly stirring solution, a total of 81 µL of [N(CH3)4]OH · 5H2O (0.556 M in EtOH, 45 µmol) was added slowly in 9 µmol aliquots; just prior to the fourth addition Co(II)(O2CCH3)2 · 4H2O (0.05 M, DMSO 2.5-15 µL) was added into the stirring solution. The remainder of the aliquots of base were made so that the additions were complete over 1.5 h. Once the addition was complete, the solution was left stirring overnight to complete ripening of the nanocrystals. This same synthesis was alternately prepared in an inert atmosphere box (under N2(g)) to afford deoxygenated Zn(100-x%)Co(x%)O NCs. Addition of Co(II) Post ZnO NC Synthesis. Quantity Dependence of Co(II). Co(II)(O2CCH3)2 · 4H2O (0.05 M, 4.5-450 µL, DMSO) was added at post synthesis to obtain a series of ZnO-Cox%, x ) 0.5-50%. Into a small flask, 5 mL of the ripened ZnO NC solution (9.2 mM, DMSO) was added. To this rapidly stirring solution, aliquots of Co(II)(O2CCH3)2 · 4H2O (9-450 µL, 0.05 M, in DMSO) were added. The solutions were left to stir overnight. The final concentrations of Co(II) present on the surface of the ZnO NCs were 0.5%, 1%, 5%, 10%, and 50% with respect to theoretical ZnO. This same synthesis was alternately prepared in an inert atmosphere box (under N2(g)) to afford deoxygenated ZnO-Cox% NCs. Reaction of Nitric Oxide or Oxygen with Deoxygenated ZnO, Zn(98.5%)Co(II)(1.5%)O, and ZnO/Co20% Nanocrystals. A Schlenk quartz 1 cm fluorescent cuvette fitted with a Teflon J-Young valve and vacuum adapter was evacuated under vacuum and transferred to an inert (N2) atmosphere box. Into the cuvette, degassed EtOH (2.5 mL) and deoxygenated ZnO, Zn(98.5%)Co(II)(1.5%)O, or ZnO/Co20% (2.3 µmol, DMSO) was added via microsyringe under N2 atmosphere. The cuvette was then sealed and transferred out of the box. Fluorescent and UV-vis spectra were obtained, and the cuvette was placed under a flow of nitrogen. NO(g) or O2(g) were then bubbled slowly through the solution for 10 s, and a spectra was obtained. Subsequent 10 s exposures of either NO(g) or O2(g) were completed until no further spectroscopic changes were observed. Acknowledgment. We acknowledge the NSERC and CRC for generous support of this research in the form of discovery and a Canadian Research Chair for D.S.B., and the DOE for
Bohle and Spina the use of the synchrotron at the National Synchrotron Light Source, and to the CIHR for Chemical Biology Fellowship for C.J.S. Supporting Information Available: Additional figures and supporting data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Stoehr, M.; Juillaguet, S.; Kyaw, T. M.; Wen, J. G. Phys. Status Solidi C 2007, 4, 1432. (2) Schmidt-Mende, L.; MacManus-Driscoll, J. L. Mater. Today 2007, 10, 40. (3) Klingshirn, C. Phys. Status Solidi B 2007, 244, 3027. (4) Kouyate, D.; Ronfard-Haret, J. C.; Valat, P.; Kossanyi, J.; Mammel, U.; Oelkrug, D. J. Lumin. 1990, 46, 329. (5) Casey, P. S.; Rossouw, C. J.; Boskovic, S.; Lawrence, K. A.; Turney, T. W. Superlattices Microstruct. 2005, 39, 97. (6) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301/1. (7) Pearton, S. J.; Norton, D. P.; Ivill, M. P.; Hebard, A. F.; Chen, W. M.; Buyanova, I. A.; Zavada, J. M. Mater. Res. Soc. Symp. Proc. 2007, 999E, k03–k04. (8) Ekambaram, S.; Iikubo, Y.; Kudo, A. J. Alloys Compd. 2007, 433, 237. (9) Lommens, P.; Loncke, F.; Smet, P. F.; Callens, F.; Poelman, D.; Vrielinck, H.; Hens, Z. Chem. Mater. 2007, 19, 5576. (10) Hays, J.; Reddy, K. M.; Graces, N. Y.; Engelhard, M. H.; Shutthanandan, V.; Luo, M.; Xu, C.; Giles, N. C.; Wang, C.; Thevuthasan, S.; Punnoose, A. J. Phys.: Condens. Matter 2007, 19, 266203/1. (11) Volbers, N.; Zhou, H.; Knies, C.; Pfisterer, D.; Sann, J.; Hofmann, D. M.; Meyer, B. K. Appl. Phys. A: Mater. Sci. Process. 2007, 88, 153. (12) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205. (13) Hasuike, N.; Deguchi, R.; Katoh, H.; Kisoda, K.; Nishio, K.; Isshiki, T.; Harima, H. J. Phys.: Condens. Matter 2007, 19, 365223/1. (14) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2009, 131, 4397. (15) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789. (16) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Lumin. 2000, 87-89, 454. (17) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 4355. (18) Baran, W.; Makowski, A.; Wardas, W. Dyes Pigm. 2008, 76, 226. (19) Comparelli, R.; Fanizza, E.; Curri, M. L.; Cozzoli, P. D.; Mascolo, G.; Agostiano, A. Appl. Catal., B 2005, 60, 1. (20) Jing, L.; Yuan, F.; Hou, H.; Xin, B.; Cai, W.; Fu, H. Sci. China, Ser. B: Chem. 2005, 48, 25. (21) Richard, C.; Boule, P. Sol. Energy Mater. Sol. Cells 1995, 38, 431. (22) Sato, S.; Kadowaki, T. J. Catal. 1987, 106, 295. (23) Look, D. C. J. Electron. Mater. 2006, 35, 1299. (24) Kumar, S. A.; Chen, S.-M. Anal. Lett. 2008, 41, 141. (25) Wang, Z. L. Appl. Phys. A: Mater. Sci. Process. 2007, 88, 7. (26) Eustis, S.; Meier, D. C.; Beversluis, M. R.; Nikoobakht, B. ACS Nano 2008, 2, 368. (27) Georgekutty, R.; Seery, M. K.; Pillai, S. C. J. Phys. Chem. C 2008, 112, 13563. (28) Wang, L.; Chang, L.; Zhao, B.; Yuan, Z.; Shao, G.; Zheng, W. Inorg. Chem. 2008, 47, 1443. (29) Xu, F.; Yuan, Z. Y.; Du, G. H.; Halasa, M.; Su, B. L. Appl. Phys. A: Mater. Sci. Process. 2007, 86, 181. (30) Zhang, Y.; Mu, J. J. Colloid Interface Sci. 2007, 309, 478. (31) Zheng, Y.; Chen, C.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K.; Zhu, J.; Zhu, Y. Inorg. Chem. 2007, 46, 6675. (32) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2007, 129, 12380. (33) Flechtner, K.; Kretschmann, A.; Steinrueck, H.-P.; Gottfried, J. M. J. Am. Chem. Soc. 2007, 129, 12110. (34) Jezowska-Trzebiatowska, B.; Gerega, K.; Vogt, A. Inorg. Chim. Acta 1978, 31, 183. (35) Padden, K. M.; Krebs, J. F.; MacBeth, C. E.; Scarrow, R. C.; Borovik, A. S. J. Am. Chem. Soc. 2001, 123, 1072. (36) Shioya, T.; Swager, T. M. Chem. Commun. (Cambridge, U. K.) 2002, 1364. (37) Liu, X.-C.; Shi, E.-W.; Chen, Z.-Z.; Chen, B.-Y.; Huang, W.; Song, L.-X.; Zhou, K.-J.; Cui, M.-Q.; Xie, Z.; He, B.; Wei, S.-Q. J. Alloys Compd. 2008, 463, 435.
Controlled Co(II) Doping of ZnO Nanocrystals (38) Bryan, J. D.; Schwartz, D. A.; Gamelin, D. R. J. Nanosci. Nanotechnol. 2005, 5, 1472. (39) Wang, X.; Xu, J.; Yu, X.; Xue, K. Appl. Phys. Lett. 2007, 91, 031908. (40) Wang, X.; Xu, J.; Zhang, B.; Yu, H.; Wang, J.; Zhang, X.; Yu, J.; Li, Q. AdV. Mater. (Weinheim, Ger.) 2006, 18, 2476. (41) Anpo, M.; Kubokawa, Y. J. Phys. Chem. 1984, 88, 5556. (42) Prater, J. T.; Ramachandran, S.; Tiwari, A.; Narayan, J. J. Electron. Mater. 2006, 35, 852. (43) Pearton, S. J.; Norton, D. P.; Ivill, M. P.; Hebard, A. F.; Zavada, J. M.; Chen, W. M.; Buyanova, I. A. IEEE Trans. Electron DeVices 2007, 54, 1040.
J. Phys. Chem. C, Vol. 114, No. 42, 2010 18145 (44) Wang, X.; Song, F.; Chen, Q.; Wang, T.; Wang, J.; Liu, P.; Shen, M.; Wan, J.; Wang, G.; Xu, J.-B. J. Am. Chem. Soc. 2010, 132, 6492. (45) Guo, S.; Zhang, X.; Huang, Y.; Li, Y.; Du, Z. Chem. Phys. Lett. 2008, 459, 82. (46) Patterson, C. H. Phys. ReV. B: Condens. Matter Mater. Phys. 2006, 74, 144432/1. (47) Jaramillo, T. F.; Baeck, S.-H.; Kleiman-Shwarsctein, A.; Choi, K.S.; Stucky, G. D.; McFarland, E. W. J. Comb. Chem. 2005, 7, 264. (48) Scaiano, J. C.; Laferriere, M.; Galian, R. E.; Maurel, V.; Billone, P. Phys. Status Solidi A 2006, 203, 1337.
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