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DOI: 10.1039/C5RA17024A (Paper) RSC Adv., 2015, 5, 87496-87503

Electronic structure and optical properties of Zn(OH)2: LDA+U calculations and intense yellow luminescence†

Mingsong Wang*a, Lingxia Jianga, Eui Jung Kim*b and Sung Hong Hahnc
aSchool of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China. E-mail: wangms@mail.ujs.edu.cn
bDepartment of Chemical Engineering, University of Ulsan, Ulsan 680-749, South Korea. E-mail: ejkim@ulsan.ac.kr
cDepartment of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, South Korea

Received 23rd August 2015 , Accepted 8th October 2015

First published on 9th October 2015

Abstract

The electronic structure of Zn(OH)2 has been studied by first-principles calculations using the local density approximation + Hubbard U (LDA+U) scheme. Based on the LDA+U calculations of ZnO and Zn(OH)2, a principle for the correct assignment of the U values has been established. The assigned U values should assure an appropriate overlap of the Zn 3d and O 2p states. Both theoretical calculations and experimental investigations have determined an energy band gap of 5.65 eV for Zn(OH)2. We also report abnormal yellow luminescence of Zn(OH)2 resembling that of ZnO. Characterizations of cathodoluminescence, UV-Vis absorption and X-ray photoelectron spectroscopy reveal that ZnO of several atomic layers in thickness covers the surface of Zn(OH)2, which gives rise to the intense yellow luminescence observed for Zn(OH)2.

1. Introduction

Zinc hydroxide (Zn(OH)2) is an amphoteric compound, which normally crystallizes in wülfingite ε-Zn(OH)2 with orthorhombic structure (space group P212121) under ambient conditions.1 This material readily dehydrates in air or under hydro(solvo)thermal conditions at elevated temperatures and transforms into ZnO.2–5 Recent reports show that ε-Zn(OH)2 could be an intermediate during the growth of ZnO in alkaline solution.6–8 Several mechanisms, such as the dissolution of Zn(OH)2 and the reprecipitation of ZnO, in situ crystallization, and solid–solid topotactic transition by lattice dehydration, have been proposed for the phase transformation from Zn(OH)2 to ZnO in solution.9–11 The topotactic transition from Zn(OH)2 to ZnO indicates that the two materials are structurally correlated. Although ZnO is intensively studied as a wide band gap (3.37 eV) semiconductor,12 yet very little is known about the electronic structure and optical properties of Zn(OH)2.

During the past several decades the first-principles density-functional theory (DFT) based on the local density approximation (LDA) and the generalized gradient approximation (GGA) has been employed to investigate the electronic structure of wide band gap transition metal oxides.13–17 An extension of the LDA method, i.e., the LDA+U method, where the Hubbard parameter, U, is an on-site Coulomb repulsion parameter that incorporates part of the electron correlation absent in LDA, can reproduce the electronic energy bands accurately.18–20 However, the U parameters for the d electrons (Ud) of transition metal and the p electrons (Up) of oxygen are mostly empirically assigned.19–21 Thus, it is necessary to examine the predictive ability of the LDA+U method by applying the same U parameters to calculate the electronic structures of structurally different compounds that consist of the same transition metal, e.g., ZnO and Zn(OH)2.

In this work, combined computational and experimental investigations on the electronic structure and optical properties of Zn(OH)2 have been conducted. LDA+U calculations of the electronic structure of ZnO and Zn(OH)2 have been systematically performed based on the identical U values. The energy band gap of Zn(OH)2 predicted by the LDA+U calculation is in excellent agreement with the experimental results of 5.65 eV. Prepared by a simple coprecipitation method, Zn(OH)2 is observed to emit intense yellow luminescence resembling that of ZnO under ultraviolet (UV) light excitation. A detailed discussion on the origin of the abnormal yellow luminescence from Zn(OH)2 has been presented.

2. Methodology

2.1. Computational approach

The DFT calculations were carried out with the GGA method using the Perdew–Burke–Ernzerhof (PBE) exchange correlation potential implemented in the CASTEP code.22 The ion–electron interaction was modeled by the ultrasoft pseudopotential in the Vanderbilt form with valence electron configurations of 4s23d10 for Zn, 2s22p4 for O, and 1s1 for H. The electron wave functions were expanded through a plane wave basis set and the cutoff energy was selected as 340 eV. For k-point sampling, Monkhorst–Pack meshes of 3 × 3 × 2 and 3 × 3 × 4 yielding grid spacing less than 0.05 Å−1 in the first Brillouin zone were employed for the 3 × 3 × 2 supercell of ZnO and 2 × 2 × 1 supercell of Zn(OH)2, respectively. The convergence threshold for self-consistent iteration was set at 5 × 10−6 eV per atom, and the lattice constants and all atomic positions for each supercell were fully relaxed until the maximal force on each atom was less than 0.01 eV Å−1, the internal stress was below 0.02 GPa, and the displacement of each atom was below 5 × 10−4 Å. The LDA+U calculations were then performed after the geometry optimization of ZnO and Zn(OH)2. To reproduce the electronic band gap (Eg) of ZnO consistent with its experimental value, both Ud for Zn 3d and Up for O 2p were considered in the calculations. For example, Ud was varied at fixed Up until an Eg of 3.37 eV was obtained for bulk ZnO (Fig. S1†). In this way, five groups of (Up, Ud) values were determined, which were further employed for the calculation of the electronic structure of Zn(OH)2.

2.2. Experimental details

Zn(OH)2 was prepared by a coprecipitation method. Typically, 6.64 g Zn(NO3)2·6H2O and 7.68 g NaOH were dissolved in 120 and 80 mL deionized water, respectively. The two solutions were then mixed at 0 °C under constant stirring. The solution became turbid in 5 min, indicating the precipitation of Zn(OH)2 crystals. After a 10 min reaction, a white precipitate was harvested by centrifugation and subsequently washed with deionized water for several times, and finally dried at 60 °C overnight. Structural characterization of the precipitate was carried out using an X-ray diffractometer (XRD, Bruker Axs D8 Advance) with Cu Kα radiation. Raman spectra were obtained at room temperature from a laser Raman spectrometer (SPEX 1403) with a He–Ne laser at an excitation wavelength of 632.8 nm. The thermogravimetric (TG) analysis was conducted on a TA Q50 instrument (Universal Analysis 2000) under N2/air flow of 60/40 mL min−1 at a heating rate of 10 °C min−1. The optical absorbance of the precipitates was measured by using a double beam UV-Vis spectrophotometer (Shimadzu UV2550). Photoluminescence (PL) spectra were recorded at room temperature by exciting the samples with a 325 nm He–Cd laser at an output power of 30 mW (IK3301R-G) as well as a 150 W Xe lamp (Horiba Jobin Yvon fluorometer) at an excitation wavelength of 353 nm. Room temperature cathodoluminescence (CL) measurements were carried out in situ in a scanning electron microscope (SEM, Sirion200) integrated with a Gatan Mono CL system. Chemical compositions were analyzed by using an X-ray photoelectron spectroscope (XPS, Thermo ESCALAB 250). A monochromated Al Kα source (1483.6 eV) was used with the analyzer mode set at constant pass energy of 20 eV. All the peak energies in the spectra were referenced with respect to the C 1s peak (284.8 eV) arising from adventitious carbons.

3. Results and discussion

3.1. Electronic structure of Zn(OH)2 by LDA+U calculations

Fig. 1 presents the structure model of a 2 × 2 × 1 ε-Zn(OH)2 supercell employed in the calculation. As illustrated in Fig. 1, each zinc ion is surrounded by four hydroxyl ions, while each hydroxyl ion is coordinated with two zinc ions. Geometry optimization of ZnO and Zn(OH)2 is performed prior to the LDA+U calculations. The optimized cell parameters are shown in Table S1.† The deviation in cell volume between the calculated and experimental values is less than 1%, indicating that our calculations are reliable. Mulliken population analysis shows that Zn becomes more positively charged, while O sees a slight increase in its negative charge in Zn(OH)2 when compared with their charges in ZnO (Table S2†). This means that Zn contributes more electrons to O in Zn(OH)2, indicating an increased Coulomb attraction between Zn and O. The deceased O–Zn bond populations in Zn(OH)2 further confirm this fact, although some of the O–Zn bond lengths are larger than those in ZnO (Table S3†). Note also that in Zn(OH)2 the O–H bonds between adjacent OH ions are highly ionized (bond population 0.09), while those within the OH ions are predominantly covalent (bond population 0.61) (Table S3†).
image file: c5ra17024a-f1.tif
Fig. 1 Structure model of a 2 × 2 × 1 ε-Zn(OH)2 supercell. Red, gray, and blue spheres represent O, Zn, and H atoms, respectively.

To systematically investigate the LDA+U approach, five groups of (Up, Ud) parameters, i.e., (6.50, 13.60), (7.00, 9.80), (7.50, 7.32), (8.00, 5.57) and (8.50, 4.26), are determined to reproduce an Eg of 3.37 eV for ZnO. Although these U parameters can reproduce the correct Eg for ZnO (the DOSs in the VBM and conduction band (CB) are nearly the same), the DOSs in the lower valance band vary with the U values (Fig. 2). More specifically, the states in the range of −16 to −14 eV and −10 to −3 eV are shifted to the higher energy side with increasing Up or decreasing Ud values. To understand these changes, the DOSs for ZnO are calculated at fixed Up (Ud). As shown in Fig. S2a,† the DOSs (except those in the VBM) are uniformly shifted up with increasing Up. However, a shift toward lower energy in the range of −9 to −3 eV and a shift up in the CB are observed at higher Ud (Fig. S2b†). Thus, the variation of the DOSs in the lower valance band in Fig. 2 can be understood as follows: the shift of O 2s states from −16 to −14 eV is mainly ascribed to the increased Up values, while the dual effect of increasing Up and decreasing Ud results in a shift up of the hybridization states of Zn 3d and O 2p in the range of −10 to −3 eV. On the other hand, the opposite effect of increasing Up and decreasing Ud makes the states in the CB unchanged. As a consequence, the five groups of (Up, Ud) parameters result in the same Eg (3.37 eV) for ZnO.


image file: c5ra17024a-f2.tif
Fig. 2 DOSs for ZnO calculated with LDA+U using various U parameters.

It is well known that the DFT calculations based on LDA and GGA underestimate the band gap of many transition metal oxides, due to the artificially large p−d coupling pushing up the valence band maximum (VBM).23 Here, the dependence of the p−d coupling on the U parameters is investigated. The hybridization between Zn 3d and O 2p as a function of the U values is illustrated in terms of the electron density contour maps and the DOSs. As is easily seen in Fig. 3a–e, the electron density of Zn 3d decreases with decreasing Ud, while that of O 2p increases with increasing Up. Furthermore, the p−d coupling is enhanced at higher Up and lower Ud values as the increased overlap of the two states can be identified in the electron density contour maps as well as the DOSs (Fig. 3). Thus, it seems that there exists optimal U values correctly describing the p−d coupling. Considering the atomic populations for O 2p (5.11e) and Zn 3d (9.98e) (Table S2†), the electron density of Zn 3d should be nearly twice as large as that of O 2p. Therefore, Up = 7.0 and Ud = 9.8 should provide more accurate description of the electronic states of O 2p and Zn 3d, respectively, as judged from their relative electron density in the electron density contour maps in Fig. 3b.


image file: c5ra17024a-f3.tif
Fig. 3 Electron density contour maps and the corresponding DOSs showing the hybridization of Zn 3d and O 2p for ZnO calculated with LDA+U using various U parameters: (a) Up = 6.5, Ud = 13.6; (b) Up = 7.0, Ud = 9.8; (c) Up = 7.5, Ud = 7.32; (d) Up = 8.0, Ud = 5.57; (e) Up = 8.5, Ud = 4.26.

Now we apply the five groups of (Up, Ud) parameters to predict the Eg of Zn(OH)2. The calculated Eg of Zn(OH)2 as a function of (Up, Ud) values is plotted in Fig. 4. The Eg of Zn(OH)2 is found to increase from 5.589 to 5.721 eV with increasing (decreasing) Up (Ud). This expanding in Eg is ascribed to a slight shift of the CB as observed in the calculated DOSs (Fig. S3†). The other variations in the DOSs for Zn(OH)2 with different U values are similar to those for ZnO (Fig. 2 and S3†). As discussed above, considering the atomic populations of O 2p (5.17e) and Zn 3d (9.99e) for Zn(OH)2 (Table S2†), Up = 7.0 and Ud = 9.8 should reproduce a more accurate Eg (5.645 eV) because of their better description of the electronic states. As is shown below, the calculated Eg is in excellent agreement with the experimental one of 5.65 eV. Thus, a principle can be set to the assignment of the U parameters in the LDA+U calculations. Beyond the empirical adjustment of the U values to achieve a correct Eg, which is currently employed in most LDA+U calculations,19–21 the employed Up (Ud) values should keep the relative electron density of O 2p and Zn 3d consistent with their atomic populations so as to achieve an appropriate overlap of the two states.


image file: c5ra17024a-f4.tif
Fig. 4 Determined Ud + Up values for the reproduction of Eg of 3.37 eV for ZnO (a) and their employment for the calculation of Eg for Zn(OH)2 (b).

Fig. 5 presents the calculated band structure and DOSs of Zn(OH)2 with Up = 7.0 and Ud = 9.8. A direct Eg of 5.645 eV is obtained at the G point (Fig. 5a). It is seen from the partial DOSs shown in Fig. 5b that the Zn sp3 hybridization states contribute dominantly to the CB, while the VBM mainly consists of O 2p states. The hybridization states of Zn 3d and O 2p as well as H 1s and O 2s are responsible for the lower VB ranging from −7.3 to −5.5 eV and −15.5 to −14 eV, respectively.


image file: c5ra17024a-f5.tif
Fig. 5 Band structure (a) and DOSs (b) of Zn(OH)2 calculated with LDA+U (Up = 7.0, Ud = 9.8).

3.2. Yellow luminescence from Zn(OH)2

Fig. 6 shows the typical XRD pattern of the as-prepared Zn(OH)2. All diffraction peaks are consistent with those in the simulated pattern, suggesting that the as-prepared Zn(OH)2 possesses a wülfingite structure. Note that no impurity phase such as ZnO can be identified in Fig. 6. Raman measurement is further carried out to investigate the phase purity of the as-prepared Zn(OH)2. In Fig. 7 all Raman bands are ascribed to the characteristic modes of ε-Zn(OH)2.24 Two strong Raman bands at 367 and 381 cm−1 are attributed to the symmetric Zn–O stretching modes of the ZnO4 tetrahedron in Zn(OH)2, while two weak bands at 479 and 720 cm−1 are assigned to the translational modes and the OH librations, respectively.24 As shown in Fig. 7, no characteristic modes of ZnO could be found in the Raman spectra of Zn(OH)2. Thus, both XRD and Raman measurements confirm a pure phase of the as-prepared Zn(OH)2.
image file: c5ra17024a-f6.tif
Fig. 6 XRD pattern of the as-prepared Zn(OH)2. The simulated XRD pattern according to ICSD #50447 for ε-Zn(OH)2 is included for comparison.

image file: c5ra17024a-f7.tif
Fig. 7 Raman spectra of the as-prepared Zn(OH)2 and ZnO. ZnO is prepared by annealing Zn(OH)2 in air at 600 °C for 1 h.

TG analysis of the as-prepared Zn(OH)2 is plotted in Fig. 8. A steep weight loss of 16.5% is observed from 119 to 153 °C, which is followed by a slow weight loss of 1.6% until 600 °C. The former is obviously due to the decomposition of Zn(OH)2. However, the weight loss of 16.5% is smaller than the theoretical values of 18.1% predicted by the chemical formula. A total weight loss of 18.1% is reached at 600 °C, indicating that the thermal decomposition of Zn(OH)2 is not completed until 600 °C.25 Unfortunately, the exact mechanism of the slow decomposition process at temperatures higher than 153 °C is still not clear at the present time.


image file: c5ra17024a-f8.tif
Fig. 8 TG curve of the as-prepared Zn(OH)2.

The above characterizations indicate that a pure Zn(OH)2 is synthesized. Below we will discuss its optical properties. Fig. 9 depicts the PL spectra of Zn(OH)2. The PL spectra of ZnO obtained by thermal decomposition of Zn(OH)2 is also included for comparison. As seen in Fig. 9, a weak UV emission at 387 nm and an intense yellow luminescence centered at 588 nm are generated from Zn(OH)2 under the excitation of a 325 nm He–Cd laser. This yellow luminescence is strong enough to be visible to the naked eyes. The two emission bands of Zn(OH)2 are nearly identical to those of ZnO, except that the yellow luminescence is less intense for ZnO. The UV band is known as the near-band-edge emission of ZnO, while the yellow luminescence is attributed to the deep-level emission from oxygen interstitials (Oi), where the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the Oi centers gives out yellow light.26,27 We note that yellow luminescence is frequently observed from ZnO grown in an aqueous solution,28 implying an excess of oxygen in the solution-grown ZnO.29 Thus, it is demonstrated that Zn(OH)2 amazingly replicates the PL properties of ZnO. To corroborate this observation, the PL spectra of Zn(OH)2 excited by an Xe lamp are also taken. As illustrated in Fig. 10, an intense yellow luminescence is generated with the band peaked at 570 nm. Several sharp peaks ranging from 441 to 494 nm also appear, which should arise from the Xe lamp30,31 rather than the emissions of ZnO.29,32–35 A UV peak at 390 nm may be due to the near-band-edge emission of ZnO. Again, a typical PL of ZnO is observed for Zn(OH)2. The inset photograph of Fig. 10 undoubtedly confirms the yellow luminescence from Zn(OH)2.


image file: c5ra17024a-f9.tif
Fig. 9 PL spectra of the as-prepared Zn(OH)2 and ZnO. ZnO is prepared by annealing Zn(OH)2 in air at 600 °C for 1 h. A 325 nm He–Cd laser is used as the excitation source.

image file: c5ra17024a-f10.tif
Fig. 10 PL spectra of the as-prepared Zn(OH)2 under the excitation of a Xe lamp. The excitation wavelength is 353 nm. The inset shows the photograph of yellow luminescence from Zn(OH)2 under UV irradiation.

CL measurement is further performed to study the origin of the luminescence from Zn(OH)2. In Fig. 11b, the CL is observed to distribute uniformly across the Zn(OH)2 crystals. A close look at Fig. 11a and b reveals that no CL is generated from the defective holes on the surface of the Zn(OH)2 microcrystals. This implies that the CL is from the surface of Zn(OH)2. The CL spectra shown in Fig. 11c, however, are quite different from the PL data. A broad violet emission centered at 423 nm is observed, which can be resolved into three bands at 410, 444 and 500 nm. Violet emission has been occasionally observed from ZnO.36 The visible emission from ZnO may shift with the excitation wavelength.37 Provided that the CL from Zn(OH)2 is also related to ZnO, the difference between the CL and PL spectra might be due to the different excitation source employed.


image file: c5ra17024a-f11.tif
Fig. 11 SEM image (a) and the corresponding CL image (b) and CL spectra (c) of the as-prepared Zn(OH)2. The enclosed area in (a) and (b) indicates no CL is generated from the defective holes.

The optical properties of the as-prepared Zn(OH)2 are investigated in terms of UV-Vis absorption. Interestingly, two sharp absorption edges with peaks located at 219.6 and 368.6 nm are observed in Fig. 12, which are assigned to the band edge absorption of Zn(OH)2 and ZnO, respectively. The corresponding Eg determined by the optical absorption is 5.65 and 3.37 eV for Zn(OH)2 and ZnO, respectively. Note that the experimentally determined Eg for Zn(OH)2 and ZnO agrees quite well with the theoretically calculated values. The double absorption edges were previously observed for the layered Zn(OH)2.38 The authors did observe a ZnO-induced absorption edge at 370 nm, but they did not realize that the absorption edge at around 220 nm is due to Zn(OH)2.38 The absorption spectrum in Fig. 12 provides strong evidence that the structurally confirmed “pure” Zn(OH)2 does contain ZnO. The possible explanations are as follows: the detection depth for characterizations such as XRD and Raman is rather deep such that they can only obtain the bulk phase information. Instead, optical characterizations such as PL, CL and UV-Vis absorption is very sensitive to the surface so that a lot of surface information can be obtained. As revealed by the CL image in Fig. 11b, the CL is from the surface of Zn(OH)2. Thus, we can reasonably assume that a thin layer of ZnO is formed on the surface of Zn(OH)2.


image file: c5ra17024a-f12.tif
Fig. 12 UV-Vis absorption spectrum of the as-prepared Zn(OH)2.

To confirm the existence of ZnO outer layer, the surface chemistry of Zn(OH)2 is studied by XPS. Fig. 13 shows the XPS spectra of Zn 2p3/2 core level for the as-prepared Zn(OH)2 and ZnO. The Zn 2p3/2 core level obtained for Zn(OH)2 is quite broad, which can be deconvoluted into two components located at 1020.8 and 1023.0 eV. The high-energy peak at 1023.0 eV is ascribed to the Zn2+ in the hydroxide.39,40 The low-energy peak at 1020.8 eV is consistent with the 1021.1 eV for ZnO, thus it should be associated with the Zn2+ in the wurtzite ZnO.41,42 The sampling depth for Zn 2p3/2 is estimated to be about 3.5 nm.43 Therefore, the thickness of ZnO layer on Zn(OH)2 is determined to be ca. 1.5 nm based on the two decomposed band areas of the Zn 2p3/2 spectra for Zn(OH)2. Being covered with several atomic layers of ZnO, Zn(OH)2 works efficiently as ZnO emitting intense yellow luminescence (Fig. 9 and 10). Earlier works showed that the green emission of ZnO is strongly correlated with surface,43,44 and so is the yellow luminescence from Zn(OH)2. Our findings herein indicate a possible application of Zn(OH)2 as a phosphor, and the characteristic surface emission may be exploited to design new phosphors by simply coating a thin layer of luminescent material.


image file: c5ra17024a-f13.tif
Fig. 13 Zn 2p3/2 XPS spectra of the as-prepared Zn(OH)2 and ZnO. ZnO is prepared by annealing Zn(OH)2 in air at 600 °C for 1 h.

However, it is not clear how ZnO is formed on the surface of Zn(OH)2. Our previous work showed both ZnO and Zn(OH)2 can be grown from alkaline solution at room temperature by controlling the pH value, but the two materials have quite different morphologies.6 Since the preparation conditions are far below the decomposition temperature (≥119 °C, Fig. 8) of Zn(OH)2, the formation of ZnO by thermal decomposition can be excluded. We have proposed a mechanism of lattice dehydration between adjacent hydroxyl ions to explain the solid phase transition from Zn(OH)2 to ZnO.11 The surface of Zn(OH)2 contains many dangling OH− ions, which are energetically higher than those in the bulk due to a lack of full coordination of the ions. These OH− ions, therefore, might dehydrate between adjacent ones to increase coordination and form ZnO.

4. Conclusions

We have probed into the electronic structure and optical properties of Zn(OH)2 by means of both theoretical calculations and experimental investigations. LDA+U calculations have been explored with systematically adjusted Up (Ud) values for ZnO and Zn(OH)2. A principle is established for the correct description the electronic states by LDA+U, i.e., the U parameters are tuned so that an appropriate overlap of the O 2p and Zn 3d orbitals is reached. With this principle, Up and Ud are determined as 7.0 and 9.8, respectively. Our theoretical simulation accurately predicts an experimentally consistent Eg of 3.370 and 5.645 eV for ZnO and Zn(OH)2, respectively. Interestingly, the yellow luminescence of ZnO is observed for Zn(OH)2. Although the XRD and Raman spectra do not provide any information about ZnO, the UV-Vis spectra reveal the coexistence of ZnO with Zn(OH)2. CL and XPS analyses probe a thin layer of ZnO covering the surface of Zn(OH)2, which is responsible for the intense yellow luminescence generated.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant no. 51002066).

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Footnote

† Electronic supplementary information (ESI) available: Calculated lattice constants, atomic and bond populations, energy band gap and density of states of ZnO and Zn(OH)2. See DOI: 10.1039/c5ra17024a
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