Identification of intrinsic hydrogen impurities in ZnO with 1H solid-state nuclear magnetic resonance spectroscopy

Chemical Physics Letters 627 (2015) 7–12

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Identification of intrinsic hydrogen impurities in ZnO with 1 H solid-state nuclear magnetic resonance spectroscopy Meng Wang a , Guiyun Yu a,b , Wenxu Ji a , Lei Li a , Weiping Ding a , Luming Peng a,∗ a b

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China School of Chemical and Biological Engineering, Yancheng Institute of Technology, Yancheng 224051, China

a r t i c l e

i n f o

Article history: Received 27 December 2014 In final form 16 March 2015 Available online 21 March 2015

a b s t r a c t 1 H solid-state nuclear magnetic resonance (NMR) spectroscopy was used to study intrinsic hydrogen (H) impurities in zinc oxide (ZnO), which play a key role in its n-type conductivity. Two 1 H signals were resolved, and assigned for the first time, to interstitial H species (Hi ) and H species substituting for oxygen ions (HO ), with the help from the model compound zinc hydroxide (␧-Zn(OH)2 ) and 1 H → 17 O double resonance NMR spectroscopy. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide (ZnO) has attracted a lot of research attention as a promising wide-gap semiconductor [1–4]. Hydrogen (H) is ubiquitous in inorganic materials and H in ZnO has been investigated extensively in the past decade, since the intrinsic hydrogen impurities (also known as unintentionally doped hydrogen species) may act as a shallow donor which contributes to the n-type conductivity of ZnO [5–11]. Experimental results obtained by using a variety of techniques, including muon spin rotation [12,13], electron paramagnetic resonance (EPR) [14], solid-state nuclear magnetic resonance (NMR) [15–18], photoluminescence measurements [19], infrared spectroscopy [20,21], Hall-effect measurements [22], photoconductivity studies [23] and Raman scattering, [24,25] have shown that different hydrogen defects which can induce shallow donor states may exist in ZnO, however, the detailed information on the nature of these hydrogen species is not clear and has been under debate for the past ten years [26,27]. For example, firstprinciple calculations suggest that both interstitial H species (Hi ) which is bound to an oxygen ion and H species substituting for oxygen (HO ) which forms hydrogen multicenter bonds can act as shallow donors [5,6], while these results are questioned by Singh and others [28–30]. Among the experimental methods mentioned above, 1 H solid-state NMR spectroscopy represents a powerful and quantitative technique that can detect and distinguish hydrogen species in different local environments [31–35]. In previous studies the peak at 0–2 ppm was assigned to sorbed water and hydroxyl

∗ Corresponding author. E-mail address: [email protected] (L. Peng). http://dx.doi.org/10.1016/j.cplett.2015.03.024 0009-2614/© 2015 Elsevier B.V. All rights reserved.

groups [15], however, sorbed water on solids usually resonates at a much higher frequency (4–8 ppm) [32,33]. Although the 1 H NMR shift of hydroxyl sites in hydrogen bonding can be more negative [32], a peak at 0–2 ppm would mean that the O H. . .O distance is in the range of 0.302–0.310 nm, which is very large and a situation like this is rare [33,36,37]. In addition, no information on the structure of the lattice defect sites was given. Herein, we identified the nature of intrinsic hydrogen impurities in ZnO by using 1 H solid-state NMR spectroscopy. We show that 1 H NMR spectra of the model compound, as well as 1 H–17 O double resonance solid-state NMR data can be used to confirm the spectral assignment. Hi and HO in ZnO, for the first time, can be distinguished according to the 1 H NMR shift. We find Hi and HO behave differently under thermal treatment in bulk ZnO, as well as in ZnO nanoparticles.

2. Materials and methods 2.1. Sample preparations To identify the H species bound to O in ZnO, we prepared ␧-Zn(OH)2 as a model compound for 1 H solid-state NMR spectroscopy, by mixing Zn(NO3 )2 and NaOH solution following previous work [38]. Typically, 25 mL of 0.3 M Zn(NO3 )2 (Sinopharm Chemical Reagent Co., Ltd.) solution was added dropwise into 25 mL of 1.6 M NaOH (Sigma Aldrich 99%) solution. The mixture was kept at 313 K for 10 h before the precipitates were filtered, washed three times with water and finally vacuum-dried at room temperature to obtain ␧-Zn(OH)2 . The sample of ␧-Zn(OD)2 were prepared in the same way by using D2 O (99.9% 2 H, Sigma-Aldrich) as the solvent.

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Bulk ZnO was obtained from Sinopharm Chemical Reagent Co., Ltd. Since the semiconductive properties of bulk ZnO and nanoparticles are different [39], we also prepared ZnO nanoparticles by mixing (C2 H5 )2 Zn (Sigma Aldrich 99%) and H2 O. 18 ␮L H2 O was added into 1 mL 1 M (C2 H5 )2 Zn solution in n-hexane at room temperature with stirring under N2 atmosphere to initiate a hydrolysis reaction. (C2 H5 )2 Zn + H2 O → ZnO + 2C2 H6 The sample was then powdered and vacuum dried, then heated to 773 K gently to remove the residual organic fragments which were attached to the oxide and obtain phase pure ZnO nanoparticles. The 17 O-enriched ZnO nanoparticles were synthesized in the same way by using H2 17 O (70%, Cambridge Isotope Laboratories) as the reactant. 2.2. Characterization X-ray diffraction (XRD) analysis was performed on a Philips ˚ X’Pro X-ray diffractometer using Cu K␣ irradiation ( = 1.541, 84 A) operated at 40 kV and 40 mA at 298 K. The data were collected between 5◦ and 80◦ (2). The morphology of ␧-Zn(OH)2 and nanosized ZnO were characterized using a Hitachi S-4800 scanning electron microscope (SEM) and a JEOL JEM-2010 transmission electron microscope (TEM), respectively. Thermal gravimetric analysis (TGA) was recorded on Netzsch STA 449C under air atmosphere with a heating rate of 10 K/min. Fourier transform infrared (FT-IR) spectra were collected on a Bruker Vertex 70 spectrophotometer in the range of 4000–400 cm−1 at room temperature. 2.3. NMR spectroscopy All NMR experiments were carried out at ambient temperature. and 17 O NMR spectra were acquired with a Bruker Avance III spectrometer equipped with an 89 mm wide-bore 9.4 T superconducting magnet in 3.2 mm or 4 mm rotors at Larmor frequencies of 400.1 and 54.2 MHz, respectively. The chemical shifts for 1 H and 17 O were referenced to TMS and H O at 0 ppm, respectively. Prior 2 to each 1 H MAS NMR experiment for ZnO, the sample was put in a glass tube, which was later connected to a vacuum line and heated at different temperature under vacuum. The samples were then allowed to cool down to room temperature and packed into NMR rotors in a dry N2 glove box. Spin counting techniques were used to estimate the low-level H concentrations existing in these systems. The total hydrogen concentrations were determined via integrating the spectral intensities ranging from 120 to −110 ppm and comparing to an external spin counting standard adamantane. The signals are corrected for different sample masses. The background NMR signals were measured using the same parameters on an empty rotor and subtracted from the spectra. 1H

3. Results and discussion The XRD pattern of the prepared Zn(OH)2 can be indexed as orthorhombic ␧-Zn(OH)2 (JCPDS 38-0385), and the SEM images show uniform octahedral-like particles with sizes of about 50 ␮m (Figure S1). The XRD peaks of ZnO nanoparticles match well with the bulk ZnO (JCPDS 36–1451), however, the widths are much broader (Figure S2a), suggesting a much smaller particle size. High resolution transmission electron microscopy (HRTEM) images show small particle sizes of around 15 nm (Figure S2b), which is consistent with the particle sizes obtained according to the full width at half-maximum (0.5–0.6◦ ) of the three strongest diffraction peaks of ZnO nanoparticles at 31.7◦ , 34.4◦ and 36.3◦ by using Scherrer equation.

Bulk and nanosized ZnO, as well as ␧-Zn(OH)2 were further investigated by thermal analysis (Figure S3). TG/DTG curves for ␧Zn(OH)2 show a mass loss of more than 16% at 375–475 K, while additional mass loss of less than 2% occurs at a temperature of 475–1073 K. The first and major loss in weight can be ascribed to the release of water in the reaction of Zn(OH)2 → ZnO + H2 O. Although the total mass losses for bulk ZnO and ZnO nanoparticles are small (1–3%) at a temperature range from room temperature (RT) to 1100 K, the major losses were observed at around 525 K and 405 K, respectively, which can be attributed to the small amount of adsorbed water. At an even higher temperature, the mass losses are tiny (≤1–2%) for both bulk ZnO and ZnO nanoparticles, suggesting subtle changes in the structure of ZnO during thermal treatment. 1 H magic angle spinning (MAS) NMR spectra of ␧-Zn(OH) are 2 shown in Figure 1. A broad peak at a range of approximately 17 to −7 ppm along with a sharp resonance at 4.6 ppm can be observed for ␧-Zn(OH)2 . Careful examinations of the broad resonance reveal that the peak maximum is at 4.9 ppm. When water (H2 O) is added to ␧-Zn(OH)2 , the sharp peak at 4.6 ppm becomes much stronger, suggesting this resonance is due to sorbed water (wet ␧-Zn(OH)2 in Figure 1a and b). The broad resonance at 4.9 ppm can then be tentatively assigned to hydroxyl groups in ␧-Zn(OH)2 . The large linewidth of the peak is associated with large 1 H–1 H homonuclear dipolar coupling in ␧-Zn(OH)2 , which is often observed in the spectra of metal hydroxides [40,41] and consistent with the presence of large spinning sideband manifolds (␧-Zn(OH)2 in Figure 1c). Deuteration has often been used to dilute 1 H and increase the spectra resolution [41]. The FT-IR data of deuterated ␧-Zn(OH)2 (Figure S4) as well as NMR spin counting results suggest D has been successfully introduced to Zn(OH)2 (see additional discussion in the supporting information). 1 H NMR spectrum of ␧-Zn(OD)2 shows a relatively sharp resonance at 4.9 ppm (␧-Zn(OD)2 in Figure 1a and b), confirming that 1 H–1 H homonuclear coupling is significantly reduced in the deuterated sample. Therefore, the broad peak at 4.9 ppm is assigned to hydroxyl groups in ␧-Zn(OH)2 . The sharp peak at 4.6 ppm reappears when adding water (H2 O) to the ␧-Zn(OD)2 (wet Zn(OD)2 in Figure 1a and b), proving that the resonance at ∼4.6 ppm arises from sorbed water. There is a relationship ‘ıiso (ppm) = 79.05 − 0.255d(O H· · ·O) (pm)’ between the hydrogen bond length (d(O H· · ·O)) and the 1 H chemical shift (ıiso ). [34,42] According to the d(O H· · ·O) distance (∼280 ppm) [43] in Zn(OH)2 , the calculated chemical shift in Zn(OH)2 is about ∼7 ppm, while 1 H chemical shift for Mg(OH)2 is less than 1 ppm based on the calculation [44]. Therefore, according to the crystal structure and previous report, the peak with relatively high shift (∼4.9 ppm) should arise from hydroxyl groups. 1 H MAS NMR spectra of bulk ZnO and ZnO nanoparticles acquired at ambient temperature are shown in Figure 2. A relatively broad high frequency resonance centered at 4.2 ppm and a set of low frequency resonances at 0–2 ppm can be observed for as-obtained bulk ZnO (top of Figure 2a). As-prepared ZnO nanoparticles show similar spectrum (top of Figure 2b), however, the intensity of the high frequency peak at 4.2 ppm is much stronger than the low frequency peaks. In addition, the spectral intensities of nanosized ZnO are significantly stronger than that of bulk ZnO, as indicated by the much higher signal/noise ratio, implying a much higher water content and/or more H associated defect sites in nanosized ZnO. After exposing the samples to vacuum, the intensities of the broad component centered at 4.2 ppm decrease significantly for both bulk ZnO and ZnO nanoparticles (more than 50% for bulk and 80% for nanoparticles), indicating the removal of a large quantity of sorbed water. Now the intensity of the peak at 4.2 ppm is much weaker than the other low frequency peaks (0–2 ppm) in nanosized ZnO, while this peak still dominates the spectrum of bulk ZnO. Please note that the spectral intensities in Figure 2a are scaled 3 times larger than that of Figure 2b, thus the intensities of the

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Figure 1. 1 H single-pulse MAS NMR spectra for ␧-Zn(OH)2 (red) and ␧-Zn(OD)2 (black). The spectra are shown in the chemical shift range of (a) 17 to −7 ppm, (b) 5.8 to 3.6 ppm, and (c) 110 to −100 ppm. Spinning speed, 14 kHz; recycle delay, 1200 s; ‘*’ denotes spinning sidebands. (For interpretation of the references to color in this text, the reader is referred to the web version of the article.)

Figure 2. (a) Ambient temperature 1 H MAS NMR spectra of bulk ZnO exposed to air at RT, bulk ZnO exposed to vacuum at RT and different temperatures as well as bulk ZnO heated at 773 K then exposed to air. (b) 1 H MAS NMR spectra of as-prepared ZnO nanoparticles at RT and ZnO nanoparticles exposed to vacuum at RT and different temperatures. The inset shows the entire spectrum of as-prepared ZnO nanoparticles at RT. Note the spectral intensities in (a) are scaled 3 times larger than that of (b).

resonance at 4.2 ppm are comparable for bulk and nanosized ZnO samples after exposure to vacuum at RT (also see Table 1). In combination with the 1 H NMR results of the model compound ␧-Zn(OH)2 , which prove that the chemical shifts for sorbed water and hydroxyl groups in this material are very similar and in the frequency range of

4–5 ppm, the resonance centered at 4.2 ppm seen in the ZnO samples can be assigned to the water and possibly surface hydroxyl groups in ZnO. After a thermal treatment at 373–573 K, both the intensities of the resonances at around 4.2 ppm and 0–2 ppm in bulk ZnO further decrease, indicating the dehydroxylation in the

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Table 1 Summary of the data obtained from integration and deconvolution of the 1 H MAS NMR spectra of bulk ZnO and ZnO nanoparticles (Figure 2). The intensity of the peaks in the frequency range of 120 to −110 ppm of bulk ZnO at room temperature after vacuum treatment is defined as 100. Temp.a

Bulk ZnO

ZnO nanoparticles

Intensityb

Ratioc

Total amountd (1018 /g)

Intensityb

Ratioc

Total amountd (1018 /g)

100.0 65.1 58.5 17.3 10.7 8.6 3.2

4.1 2.9 2.7 2.2 1.0 0.8 0.4

250 (25) 185 (19) 145 (15) 45 (5) 25 (3) 20 (2) 8 (1)

240.0 185.2 121.0 48.0 11.6

0.8 0.6 0.4 0.3 0.3

600 (60) 465 (50) 300 (30) 120 (12) 30 (3)

(K) RT 373 473 573 773 873 1073 a

The temperature that the sample was heated at under vacuum before cooling down to RT for NMR measurements. The relative intensity of the peaks in the frequency range of 12 to −8 ppm. c The intensity ratio of the two sets of peaks I(4.2 ppm)/I(0–2 ppm). d The total amount of H extracted from 1 H NMR signals in the frequency range of 120 to −110 ppm. The uncertainties of the values for the total amount of H are shown in the parenthesis. b

samples. This is in agreement with the major weight loss shown in TG profile in this temperature range (Figure S3). The intensity ratio of the two sets of peaks I(4.2 ppm)/I(0–2 ppm), however, decrease from 4.1 at RT (in vacuum), to 2.2 at 373 K and 1.0 at 573 K, implying the species associated with the high frequency peak (4.2 ppm) are less stable than the species arising from the resonances at 0–2 ppm (Table 1). Similar observation is seen in the data for ZnO nanoparticles, while the intensity ratio of the two sets of peaks I(4.2 ppm)/I(0–2 ppm) is very different from the ratio of bulk ZnO after being heated at the same temperature. Because residual water and surface hydroxyl groups in bulk ZnO and ZnO nanoparticles after being heated at 573 and 473 K, respectively, can be neglected (Figure S3), the remaining peak at 4.2 ppm observed at a temperature higher than those should arise from a different species other than sorbed water. Thus, this peak can be tentatively assigned to interstitial Hi which is believed to be bound to oxygen and have an environment comparable to hydroxyl groups [8,9]. The decrease in intensities of the peaks at 0–2 ppm occurs more significantly at a temperature higher than 773 K for both bulk ZnO and ZnO nanoparticles, coinciding with the elimination of HO (H trapped in an oxygen vacancy) in ZnO [5,6,10], therefore, these low frequency resonances at 0–2 ppm can be tentatively assigned to HO . Both Hi and HO are possibly H+ as suggested by calculations [6]. This assignment is also consistent with the fact that HO is thermally more stable than Hi [10]. The ratio of In the range of 0–2 ppm, up to 4 peaks may be observed in the spectra of bulk ZnO and less (2 or 3) peaks are visible for ZnO nanoparticles. However, no attempts were made to assign them in this work and we believe these peaks arise from HO species with slightly different local environments. The spectra of bulk and nanosized ZnO change differently under thermal treatment and the different relative amounts of Hi and HO observed in the two samples (Table 1) suggest these species should contribute differently to the properties of ZnO. Careful spin counting NMR experiment results show that the H concentration (Hi and HO ) in bulk ZnO heated at above 573 K is about 1018 cm−3 , which is in general agreement with the doping limit of H in ZnO [9,11]. Therefore, both sorbed water and Hi , can contribute to the signal at 4.2 ppm. At a relatively low temperature, i.e., less than 573 and 473 K for bulk and nanosized ZnO, respectively, the peak at 4.2 ppm mainly arises from sorbed water, and possibly surface hydroxyl groups. At a higher temperature, the peak is mainly originated from Hi species. To confirm the spectral assignments, we further investigated H–O proximity of nanosized ZnO after thermal treatment at high temperature by 1 H–17 O transfer of population in Double Resonance (TRAPDOR) NMR method [45]. A non-zero TRAPDOR fraction, defined as (1 − S/S0 ), where S and S0 are the intensities of the double resonance and control spectra, respectively, will be observed in case the 1 H and 17 O nuclei are close in proximity. Typical TRAPDOR NMR

spectra are shown in Figure 3a, in which the intensity of the resonance at around 4.2 ppm decreases significantly (50 ± 3%) in the double resonance experiment, while there is much smaller change (12 ± 1%) for the intensities of the peaks at 0–2 ppm, indicating the former arises from hydrogen species a lot closer to oxygen than the latter. When the 17 O irradiation time is as short as one rotor period (0.2 ms), a TRAPDOR fraction of 29 ± 2% for the peak at 4.2 ppm can be observed, while no TRAPDOR fraction can be seen for the peaks at 0–2 ppm. At such short 17 O irradiation time, only the 17 O ions in the first coordination shell (i.e., 17 O bound to H) are close enough to have an impact. Thus this data confirms our assignments that the peak at 4.2 ppm arises from Hi (Figure 3b). By comparing the 17 O NMR spectra of enriched ZnO nanoparticles and natural abundance ZnO (Figure S5), the molar percentage of 17 O in ZnO nanoparticles can be determined as 61 ± 5%. Therefore, the corrected TRAPDOR fraction is approximately 48% (29%/61%) at an 17 O irradiation time of 0.2 ms when taking account of the 17 O molar percentage. This value is smaller than that of O–H in Bronsted acid sites in zeolite HY (∼100%) [46], which should be associated with higher mobility of Hi in ZnO [11,47] and/or longer H–O distance. With a longer 17 O irradiation time, both resonances show significant TRAPDOR effects (Figure 3b), due to the 17 O ions further away from hydrogen species (i.e., in the second/third coordination shell). The TRAPDOR fractions of the peak at 4.2 ppm are systematically larger than the peaks at 0–2 ppm, consistent with our spectral assignments. The 17 O NMR spectrum of ZnO nanoparticles (Figure S5) shows a relatively broad and featureless signal which can be simulated with a single site. We have also measured the 17 O–1 H REDOR NMR data of the sample and no REDOR effect is observed, indicating the H concentration in the sample is too small, which is consistent with our spin counting results. It is worth noting that after the thermal treatment all of the samples were packed in rotors in N2 glove box prior to 1 H NMR measurement to prevent the effects of moisture in the air. The loss of more than 50% of the signal for the samples exposed to vaccum at RT indicate severe impact from sorbed water. The 1 H NMR spectrum of the bulk ZnO sample heated at 773 K then exposed to moisture in the air at (bottom of Figure 2a) shows that the peak at 4.2 ppm is much stronger while the peaks at 0–2 ppm have similar intensities compared to the sample prior to wetting. Again this result confirms the peak at 4.2 ppm for the ZnO sample after high temperature treatment should arise from the water/hydroxyl groups on the surface of wetted ZnO and the peak at 0–2 ppm is due to HO , which is not affected by surface wetting. Our assignments are in contrast to the assignments in previous 1 H NMR studies, where the peak at 0–2 ppm was assigned to water/surface hydroxyl groups [15–17]. Again, it is unlikely that the peak from water appears at such low frequency according to our experimental data and literature

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Figure 3. (a) Ambient temperature 1 H–17 O TRAPDOR NMR spectra of 17 O enriched ZnO nanoparticles after being heated at 773 K. 17 O irradiation time: 0.6 ms (b) Plot of the 1 H–17 O TRAPDOR fractions. Spinning speed, 5 kHz; recycle delays, 60 s. The difference spectrum is obtained by subtracting the double resonance spectrum from the control spectrum. ‘*’ Denotes spinning sidebands.

[31,34,42], and care must be taken when dealing with metal oxides [18] and interpretations of the 1 H NMR data. 4. Conclusions 1 H solid-state NMR spectroscopy was applied to investigate and distinguish different intrinsic hydrogen impurities including Hi and HO in bulk and nanosized ZnO. 1 H NMR of the model compound Zn(OH)2 and the samples with different thermal treatment, as well as and 1 H–17 O double resonance (TRAPDOR) solid-state NMR data were used to confirm the spectral assignments. The intrinsic hydrogen impurities, including interstitial Hi as well as HO in the vacancies, can be distinguished in 1 H NMR and quantitatively measured. The amounts of Hi and HO species are found to vary upon thermal treatment in both samples. Since intrinsic hydrogen impurities are associated with the n-type conductivity of ZnO, different thermal behaviors observed for Hi and HO imply that the two hydrogen species can be tuned to improve the properties of ZnO. The approach presented in this letter provides new information of intrinsic hydrogen impurities in ZnO. Since a low concentration of H species may appear in a variety of technologically important oxides and play a very important role in controlling their functions [48], we expect the approach can be extended to other oxides to extract more information and help rational design of improved functional materials.

Acknowledgements This work was supported by the National Basic Research Program of China (2013CB934800), the National Natural Science Foundation of China (NSFC) (20903056, 21222302), NSFC–Royal Society Joint Program (21111130201), Program for New Century Excellent Talents in University (NCET-10-0483), the Fundamental Research Funds for the Central Universities (1124020512) and National Science Fund for Talent Training in Basic Science (J1103310). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.03.024.

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