Compensation of Acceptors in ZnO Nanocrystals by Adsorption of

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Compensation of Acceptors in ZnO Nanocrystals by Adsorption of Formic Acid Samuel T. Teklemichael and Matthew D. McCluskey* Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164-2814, United States ABSTRACT: Zinc oxide (ZnO) nanocrystals have potential applications in optoelectronic devices and are widely used as catalysts for a variety of chemical processes. Using infrared (IR) and photoluminescence (PL) spectroscopies, we provide evidence that ZnO nanocrystals contain hydrogenic acceptors and propose an energy-level scheme to explain the observations. IR absorption peaks at liquid-helium temperatures, which correspond to electronic transitions of the acceptor, disappeared after exposure to formic acid (HCOOH) vapor. This observation is consistent with electrical compensation of the acceptor by the formate ion. The energy level of the formate ion is estimated to be ∼0.9 eV above the valence-band maximum. Room temperature IR measurements show that the formate species are adsorbed on the surface of ZnO nanocrystals. A broad PL peak centered at 3.2 eV, for samples exposed to HCOOH, is attributed to an exciton bound to a formate species. carbonate (NaHCO3, 99.998%) were reacted at 200 °C for 3 h in an open-air furnace. Transmission electron microscopy showed that the particles have an average diameter of ∼20 nm.17 The nanopowder was pressed into pellets 7 mm in diameter with a thickness of 0.25 mm. The sample was exposed to HCOOH vapor for 15 min at room temperature. IR spectra were obtained with a Bomem DA8 Fourier transform IR spectrometer with a KBr beamsplitter. HgCdTe and InSb detectors were employed for the spectral ranges of 500−5000 cm−1 and 1800−7000 cm−1, respectively. Low temperature (10 K) measurements were performed in an attached closed-cycle liquid-helium cryostat. PL experiments utilized a JY-Horiba FluoroLog-3 spectrometer consisting of double-grating excitation and emission monochromators (1200 grooves mm−1 grating) and R928P photomultiplier tube (PMT). The PL spectra were obtained with a double monochromator and 450 W xenon CW lamp as an excitation source and an instrumental correction for the wavelengthdependent PMT response, grating efficiencies, and the variation in the output intensity from the lamp. The IR and PL spectral resolutions were 2 cm−1 and 1−3 nm, respectively.

1. INTRODUCTION Zinc oxide (ZnO), a direct wide band gap (Eg = 3.3 eV) semiconductor with a large exciton binding energy (60 meV),1 has attracted recent interest as a material for optoelectronic devices. ZnO has been used in many practical applications such as transparent conducting oxide layers2 in solar cells.3 Nanoscale materials, owing to their high surface-to-volume ratio, are sensitive to the environment4 and are useful for gas sensing applications. Along with TiO2, ZnO nanostructures have also received renewed interest as candidates for dyesensitized electrochemical devices,5 where adsorption of carboxylic acid groups is responsible for the dye surface contact. Surface catalysis on oxides is an important industrial process.6 Specifically, adsorption of formic acid is a key reaction in commercial methanol synthesis.7 Challenges in producing reliable p-type doping have become a bottleneck for ZnO to find widespread use in optoelectronic applications.8 Recent theoretical9,10 and experimental11 results showed that nitrogen, once thought to be a hydrogenic acceptor, is actually a deep acceptor. Our previous work provided evidence that ZnO nanocrystals contain hydrogenic acceptors as-grown.12 Although the identity of the acceptor is not known, EPR results suggested that it is a zinc vacancy related complex.13,14 The EPR experiments did not show any evidence for oxygen vacancies in our nanocrystals.12 Using infrared (IR) and photoluminescence (PL) measurements, we have developed a unified model for the acceptor and intragap surface states of ZnO nanocrystals.15 In this paper, by adsorption of formic acid (HCOOH) on ZnO nanocrystals, we provide further experimental evidence for this model.

3. RESULTS AND DISCUSSION Previous work12 showed a low temperature (10 K) series of IR absorption peaks in the energy range of 0.425−0.457 eV for the as-grown ZnO nanocrystals. The peaks are characteristic of a hydrogenic acceptor spectrum with a hole binding energy of 0.46 eV. These peaks were assigned to transitions to excited states originating from the A and B valence bands. The acceptor was observed in ZnO nanocrystals grown with lithium or sodium precursors,12 which suggests that it is not due to an

2. EXPERIMENTAL DETAILS ZnO nanocrystals were synthesized by a solid-state pyrolytic reaction process.16−18 In this method, zinc acetate dihydrate [Zn(CH3COO)2·2H2O, 99.999%] and sodium hydrogen © 2012 American Chemical Society

Received: April 12, 2012 Revised: July 16, 2012 Published: July 23, 2012 17248

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bidentate. Crook et al.,26 using high resolution electron energy loss spectroscopy (HREELS), indicated that the formate adopts a bidentate and monodentate-type configuration on the (101̅0) and (0001̅)-O surfaces, respectively. Theoretical studies (ab initio calculations) on the ZnO (1010̅ ) surface suggest that bridging represents the most stable adsorption mode.27 Figure 3 shows room temperature IR absorption spectra for our ZnO nanocrystals exposed to formic acid. The IR

alkali dopant. The 0.46 eV acceptor level was also observed in low-temperature PL spectra.15 Figure 1a shows an IR transmission spectrum of the acceptor peaks for the as-grown ZnO nanocrystals at 10 K. A quadratic

Figure 1. Low temperature (10 K) IR transmission spectra of ZnO nanocrystals (a) before and (b) after exposure to formic acid (HCOOH). The disappearance of the peaks indicates compensation of acceptors by formate ions. Spectra are shifted vertically for clarity.

baseline was subtracted from the transmission spectrum. The peaks disappear when the sample is exposed to HCOOH vapor (Figure 1b). This observation is consistent with our claim that the peaks are due to neutral acceptors. We propose that HCOOH gives electrons to the nanoparticles and, thus, compensates the observed acceptors. It has been reported that formate species serve as effective electron donors (hole scavengers) on ZnO quantum dots.19 Studies have shown that HCOOH can be adsorbed dissociatively on the ZnO surface at room temperature, resulting in a formate anion (HCOO−) and hydroxyl species.20−22 Three geometrical structures (unidentate, bidentate, and bridging) have been suggested for the types of coordination of the formate anion, as shown in Figure 2.20−22 In ZnO, a surface

Figure 3. Room temperature IR absorption spectra of ZnO nanocrystals after exposure to (a) formic acid (HCOOH) and (b) deuterated formic acid (DCOOD). Spectra are shifted vertically for clarity.

absorption spectra were calculated using Absorbance = log10(IR/I), where IR and I are the transmission spectra for the as-grown and formic acid exposed samples, respectively. The sample exposed to HCOOH, Figure 3a, shows two main features of IR formate absorption bands (characteristic for carboxylic groups coordinated to a metal) at 1320 and 1640 cm−1. The absorption strength of these modes was large such that the IR absorbance peaks were saturated. These peaks are attributed to symmetric νs(COO−) and asymmetric νas(COO−) stretch modes, respectively.23 The magnitude of the splitting of their frequencies (Δas−s = 320 cm−1) suggests that the formate structure is unidentate (Figure 2b). The peaks at 2892 and 790 cm−1 are assigned to the ν(CH) stretch mode and the δ(CO2−) bending mode, respectively.28 To confirm the adsorption of formic acid, we investigated the isotope dependence of the IR spectrum. Figure 3b shows the absorption spectrum for ZnO nanocrystals after exposure to deuterated formic acid (DCOOD). The peak at 2171 cm−1 is assigned to the ν(CD) stretch mode.28 This peak has the anticipated isotopic frequency shift from the ν(CH) stretch mode. The COO− bands formed from DCOOD are identical to those formed from HCOOH. The peak at 1023 cm−1 has been observed previously25 and has been assigned to the δ(CD) bending mode.28 Figure 4 shows low temperature (10 K) PL spectra of ZnO nanocrystals, under an excitation wavelength of 325 nm, before and after exposure to HCOOH. Peaks in the near-band gap range of 3.18−3.35 eV have previously been attributed to bound excitons and their phonon replicas.15 The emission peak at 2.97 eV was attributed to the transition of a free electron to the neutral acceptor. This acceptor peak is no longer evident after exposure to formic acid. Instead, a broad PL emission centered at 3.2 eV appeared (Figure 4b). We attribute this

Figure 2. Possible geometrical structures for the formate anion on the surface of ZnO.

zinc ion interacts equally with two oxygen atoms of the formate anion in the bidentate structure, whereas in the unidentate form, it interacts with one oxygen atom. In the bridging structure, two oxygen atoms of the formate anion interact with two zinc ions. The structures can be identified based on the comparison of the splitting of the COO symmetric and asymmetric stretch modes (Δas−s). In general, Δas−s is larger than 200 cm−1 for a unidentate-type structure and smaller than 110 cm−1 for a bidentate-type structure. For a bridging-type structure Δas−s is expected to lie between 140 and 200 cm−1.23,24 Using IR spectroscopy, Noto et al.25 have assigned the structure of the formate ion adsorbed on a ZnO surface as 17249

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previously proposed15 acceptor and intragap surface states. According to this unified model, a band of surface states lies 0.38 eV above the valence-band (VB) maximum. We refer to these states as the surface valence band. Similarly, there is a surface conduction band (CB) that extends ∼1 eV into the gap. This model explains the observation of a broad red emission band centered around 675 nm (1.84 eV), which is attributed to surface recombination.34 In the present work, exposing the sample to formic acid did not shift the red emission spectrum of ZnO nanocrystals (not shown), consistent with the deep formate level within the gap. The only unoccupied level that can accept an electron from the formate anion is that of the acceptor. All other unoccupied levels are higher than the formate level.

4. SUMMARY In conclusion, we have presented experimental evidence that confirms the previously observed12 hydrogenic acceptors on ZnO nanocrystals using IR and PL spectroscopy and developed an energy-level scheme to explain the observations. The disappearance of the low temperature IR transition lines after exposure to formic acid indicates that formate ions compensate the acceptors. The energy level of HCOO− is estimated to be ∼0.9 eV above the valence-band maximum. Room temperature IR measurements show that formic acid is adsorbed on the surface of ZnO nanocrystals as a formate species with a unidentate structure. The broad PL emission at 3.2 eV for samples exposed to HCOOH is attributed to an exciton trapped by a surface formate species.

Figure 4. Low temperature (10 K) PL emission spectra of ZnO nanocrystals at an excitation wavelength of 325 nm, (a) as-grown and (b) after exposure to HCOOH. The spectra are normalized to the same maximum intensity.

broad peak to an exciton bound to a formate species adsorbed on the surface of ZnO nanocrystals. While the details of this interaction are not known, an increase in the PL intensity after exposure to HCOOH, in the UV and visible (green) region, has been attributed to surface modifications accompanying formate decomposition.29 It has also been reported that the green emission30 correlates with hydroxides on the surface of ZnO nanocrystals.31 However, the observation of the broad PL emission around 3.2 eV after exposure to HCOOH, to our knowledge, has not been reported previously. It is possible that sodium on the nanocrystal surface could affect the PL (and IR) properties. The standard redox potential of HCOO− in contact with TiO2 was reported to be 1.9 V versus NHE32 or 2.53 V with respect to the conduction-band minimum. Assuming the conduction band edges of ZnO and TiO2 are roughly the same33 and taking the band gap of ZnO to be 3.43 eV, one can calculate that the formate ion level is deep, ∼0.9 eV above the valence-band maximum. Figure 5 depicts the energy level diagram of ZnO showing the formate level along with the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DOE Grant No. DE-FG0207ER46386. Partial support (M.D.M.) was also provided by NSF Grant No. DMR-1004804.

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REFERENCES

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Figure 5. Schematic energy-level diagram for ZnO showing the electronic level of the formate ion, along with the acceptor level and intragap surface states. Energies are in eV above the valence-band maximum. 17250

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