A p-Type Zinc-Based Metal–Organic Framework - Inorganic Chemistry

Article pubs.acs.org/IC

A p‑Type Zinc-Based Metal−Organic Framework Congcong Shang,* Romain Gautier, Tengfei Jiang, Eric Faulques, Camille Latouche, Michael Paris, Laurent Cario, Martine Bujoli-Doeuff,* and Stéphane Jobic Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44300 Nantes, France S Supporting Information *

ABSTRACT: An original concept for the property tuning of semiconductors is demonstrated by the synthesis of a p-type zinc oxide (ZnO)-like metal−organic framework (MOF), (ZnC2O3H2)n, which can be regarded as a possible alternative for ZnO, a natural n-type semiconductor. When small oxygen-rich organic linkers are introduced to the Zn−O system, oxygen vacancies and a deep valence-band maximum, the two obstacles for generating p-type behavior in ZnO, are restrained and raised, respectively. Further studies of this material on the doping and photoluminescence behaviors confirm its resemblance to metal oxides (MOs). This result answers the challenges of generating p-type behavior in an n-type-like system. This concept reveals that a new category of hybrid materials, with an embedded continuous metal−oxygen network, lies between the MOs and MOFs. It provides concrete support for the development of p-type hybrid semiconductors in the near future and, more importantly, the enrichment of tuning possibilities in inorganic semiconductors.

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INTRODUCTION Semiconductor materials surround human life in electronics, lighting, energy, and even home-use tools. Property tuning of those materials is usually the core of their researches for different applications, among which inversion between types n and p in most semiconductors is extremely desirable and complicated. Take the widely studied zinc oxide (ZnO),1 for example. Low formation energies of the oxygen vacancies and zinc interstitials lead to an n-type semiconductor, and the naturally presence of oxygen vacancies, as well as a deep valence-band maximum (VBM), prohibits its p-type dopability.2−6 So far, despite great efforts from researchers for years, a reproducible simple realization of p-type ZnO remains challenging.2−6 At the same time, p-type ZnO, or simply ptype materials, are highly desirable for the development of numerous devices (e.g., light-emitting diodes, transistors, lasers).7,8 Basically, zinc vacancies (or oxygen interstitials) are expected in a p-type ZnO,9 which means the O/Zn ratio needs to be inversed from less than 1 in its natural state (oxygen deficiency state) to greater than 1 in the zinc deficiency state. ZnO, most commonly found in the hexagonal wurtzite structure, has very low structural tunability and a deep VBM, which complicates the obtainment of a high concentration of zinc vacancies and, therefore, p-type behavior. This paper introduces the use of oxygen-rich small organic linkers to accomplish structural tuning of a Zn−O network within a metal−organic framework (MOF) structure, as presented in Figure 1, and, hence, to restrain oxygen vacancies and increase the O/Zn ratio. Furthermore, the presence of an organic linker can also modify the Zn−O coordination, the oxygen electronic configuration, and the contribution of O(2p) orbitals to the total density of states (DOS). Contrary to general studies focusing on MOFs © 2017 American Chemical Society

with large porosities and/or the intrinsic properties of organic linkers and metals,10−12 our work aims at achieving structural varieties and developing new metal oxide (MO)-like compounds that favor charge flow. In the following, a p-type zincbased MOF (Zn MOF) with an embedded continuous 3D inorganic network is studied. The observed p-type behavior of the Zn MOF is discussed in view of the achievability of our concept to obtain an oxygen-rich p-type Zn−O network. Moreover, its photoluminescence (PL) properties upon nitrogen doping are investigated as well to demonstrate its resemblance to ZnO.

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EXPERIMENTAL SECTION

Synthesis. Crystals and/or crystalline powders of the zinc glycolate (ZnC2O3H2)n, noted as Zn-GA, can be obtained reproducibly by heating a mixture of zinc salt and glycolic acid (H2GA) under solvothermal conditions. H2GA is the smallest hydroxyl acid that is nonharmful and commonly used in the cosmetics and food industries.13 The two compounds in this work, Zn-GA and N:ZnGA were synthesized in parallel under the same conditions. A total of 15 mL of an ethanol solution of Zn(CH3COO)2·2H2O (1.97 g, 9 mmol) in the case of Zn-GA, or Zn(NO3)2·6H2O (2.68 g, 9 mmol) in the case of N:Zn-GA, and H2GA (0.68 g, 9 mmol) were prepared in a 20 mL Teflon liner. The Teflon liners were then sealed in homemade autoclaves and treated at 160 °C for 16 h. The obtained products were filtered and washed by ethanol. After drying at 80 °C in an oven, around 1.2 g of white (Zn-GA) or brown (N:Zn-GA) powder was obtained. Several syntheses under the same conditions gave 97% yield on average. Characterizations. The crystal structure was determined from single-crystal X-ray diffraction (SXRD) data collected with a BrukerReceived: January 30, 2017 Published: May 22, 2017 6208

DOI: 10.1021/acs.inorgchem.7b00198 Inorg. Chem. 2017, 56, 6208−6213

Article

Inorganic Chemistry

Figure 1. Illustration of the embedded [Zn−O] network in p-type Zn MOF with the support of a small organic linker. When small oxygen-rich [−OCH2CO2−] moieties are introduced to n-type ZnO (left), a p-type Zn MOF (right) with a continuous 3D Zn−O skeleton is obtained. The schemes of ZnO and Zn MOF in the graph contain similar numbers of zinc atoms. The common elements in these compounds are all represented under the same scale.

environments, which is confirmed by the two specific 13C signals at 64.2 and 181.4 ppm observed on solid-state NMR spectra (Figure S2), similar to those of free H2GA at 62.4 and 180.7 ppm for −CH2OH and −CO2H carbon atoms, respectively.17 As shown in Figure 2b, the carbonyl oxygen (O1) is connected to only one zinc, while each deprotonated hydroxyl oxygen atom (O2 and O3) is coordinated to two adjacent zinc centers. All zinc sites are equivalent in a pseudotriangular-bipyramidal environment, constituted with five oxygen atoms from four different double-deprotonated H2GA linkers. The Zn−O bond lengths vary from 1.93 to 2.25 Å in Zn-GA, which covers the Zn−O bond length range in ZnO (1.97 and 1.99 Å).18 We believe the short Zn−O2 bond lengths (1.93−1.94 Å) reflect the strong π-donor nature of the alcoholate oxygen, which contributes to the rise of the VBM energy level. Theoretical calculations are ongoing to confirm our proposition. The pseudotriangular bipyramids share corners to form infinite 1D chains running along the c direction (Figure 2c). These chains are connected through apexes in the a and b directions, leading to tunnels along the a, b, and c axes, as shown in Figure 2a. According to TGA−DSC plots (Figure S3), the Zn-GA compound is stable up to 350 °C before its single-step decomposition to ZnO. Within the Zn-GA structure, a continuous 3D Zn−O network is formed without any interruption by an organic moiety, as presented in Figure S4a. Similar to the cornersharing polyhedra in the hexagonal wurtzite ZnO (Figure S4b), this embedded inorganic structure also consists of cornersharing polyhedra. Moreover, the shortest Zn−Zn distance of 3.40 Å is slightly longer than that of ZnO with 3.21 Å.18 As such, in Zn-GA, the presence of organic linkers contributes to the tuning of a continuous Zn−O crystalline structure, while the embedded Zn−O network might provide a carrier delocalization pathway and, hence, possible semiconducting properties.19 Unlike other MOFs with high porosity, the BET specific surface area of Zn-GA was found to be 11 m2 g−1, which indicates a nonporous microstructure.

Nonius Kappa CCD diffractometer (Mo Kα radiation). Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker AXS D8 diffractometer with Cu Kα radiation (λ = 0.15418 nm) at room temperature. {1H}13C CP-MAS (cross-polarization and magic angle spinning) NMR spectra were acquired on a Bruker 500 MHz Avance III spectrometer using a 4 mm CP-MAS probe. Thermogravimetric analysis and differential scanning calorimetry (TGA−DSC) measurements were conducted with a Jupiter STA 449 F3 instrument in air from room temperature to 600 °C with a heating rate of 2 K min−1. The Brunauer−Emmett−Teller (BET) specific surface area of the compound was measured by a Tristar II 3020 instrument from Micromeritics using N2 gas. Elemental analyses on Zn-GA materials for carbon, hydrogen, and nitrogen were effectuated using a FLASH 2000 analyzer from Thermo Scientific. PL properties were characterized by a Spex Fluorolog-3 spectrofluorometer from Jobin Yvon Instruments. IR spectroscopy analyses of products containing KBr pellets were done using a Bruker Vertex 70 Fourier transform infrared device. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Kratos Nova X-ray photoelectron spectrometer. Electrochemical impedance spectroscopy measurements of Zn-GA pellets, confined in epoxy glue, were performed with a Biologic Sciences Instruments SP-300 workstation. Calculations. Density functional theory (DFT) calculations were performed using the CASTEP code and ultrasoft pseudopotentials. A generalized gradient approximation with the Perdew−Burke− Ernzerhof functional14 was employed. Geometry optimization was carried out using the Broyden−Fletcher−Goldfarb−Shanno optimization scheme with a convergence threshold for a maximum force of 0.01 eV A−1. The cutoff energy was fixed at 380 eV. The Monkhorst−Pack grid was set at 5 × 5 × 4 for ZnO and 2 × 2 × 2 for Zn-GA. The bandgap underestimation of bulk ZnO was corrected by a scissor operator of Δ = 2.7 eV.15,16

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RESULTS AND DISCUSSION SXRD revealed the Zn-GA 3D structure as shown in Figure 2. Details about the structure are listed in Table S1. The simulated PXRD pattern from SXRD data corresponds perfectly to the experimental PXRD pattern of Zn-GA (Figure S1). The compound crystallizes in the space group I41/a, with [C2O3H2]2− ligands coordinated to Zn2+ cations leading to a 3D network (Figure 2a,b). The GA linkers are all in the same 6209

DOI: 10.1021/acs.inorgchem.7b00198 Inorg. Chem. 2017, 56, 6208−6213

Article

Inorganic Chemistry

O(2p) orbitals. This enhancement can be due to the decrease of the O−Zn coordination number from OZn4 in ZnO to OZn2 or OZn1 in Zn-GA and the integration of carboxylate20 and, more importantly, the alcoholate functions by the GA linker. As mentioned previously, the short Zn−O2 bond length could be a good indication for the π-donor nature enhancement of alcoholate oxygen. Unfortunately, simple alcoholate oxygen can hardly be found in reported MOFs. According to the XPS VB spectrum shown in Figure 3b, the difference between the Fermi level and the VB edge in Zn-GA is approximately 0.8 eV, much smaller compared to 3.5 eV for that in ZnO. Considering the optical band gap of 3.2 eV, the Fermi level of the Zn-GA MOF is much closer to its VB, which implies its p-type nature. The p-type behavior of Zn-GA is also confirmed by a clear negative slope in the Mott−Schottky plot (Figure 3c), obtained from electrochemical measurements of materials issued from different batches. The flat band potential slightly varies for different batches, but the tendency compared to the band structure of ZnO remains the same. The Fermi level, VBM, and conduction-band minimum (CBM) of Zn-GA calculated from the flat band potential, binding energy, and optical band gap are respectively −5.4, −6.2, and −3.0 eV. Compared to the VBM of ZnO at −7.5 eV,1 the VBM energy level of Zn-GA is 2.1 eV higher, which is in striking accordance with the DFT calculations presented in Figure 3a. As mentioned previously, one of the difficulties for p-type ZnO doping is the undesired generation of oxygen vacancies (and zinc interstitials) in intrinsic ZnO, which induces n-type conductivity.2 In the case of the Zn-GA MOF, oxygen atoms are not only bonded to zinc(2+) but also fixed on carbon atoms through C−O bonds owing to the presence of GA moieties. Moreover, the oxygen-rich nature of Zn-GA (O/Zn = 3) makes it apparently zinc-deficient compared to ZnO. Thus, oxygen deficiency is unlikely to happen in Zn-GA. Therefore, the two obstacles for obtaining p-type ZnO, the oxygen deficiency and deep energy level of VBM, are overcome by the concept of an embedded M−O 3D network described at the beginning of this work. Nevertheless, it is worth noting that the p-type behavior of Zn-GA must be correlated with the presence of defects that can hardly be detected. These defects could be missing linkers, missing metal centers,21,22 and maybe proton vacancies. Failure to account for these defects could explain the strong discrepancy between the observed and calculated optical band gap. The latter is commonly underestimated in DFT calculations. Investigations are in progress to discuss the nature of the potential defects and their impact on the optoelectrical properties. Furthermore, we obtained both nondoped Zn-GA, using a zinc acetate precursor, and nitrogen-doped Zn-GA, noted as N:Zn-GA, using a zinc nitrate precursor. White powder was recovered in the case of Zn-GA, whereas N:Zn-GA appears as a red-brown powder and needlelike crystals. In Figure S7, IR analyses of Zn-GA and N:Zn-GA, compared to zinc nitrate and H2GA precursors, did not give any conclusive suggestions on the presence or nature of nitrogen doping. A high-resolution XPS spectrum (deconvoluted peaks at 399.6 and 402.8 eV in Figure S8) and elemental analysis of N:Zn-GA (Table S2) show the presence of nitrogen with 0.51 mass %. The nitrogen peaks in the XPS spectrum ruled out the possibility for nitrate filling in the Zn-GA structure;23 however, the exact nature of the nitrogen doping in Zn-GA is still under investigation. Both Zn-GA and N:Zn-GA clearly show p-type behavior according to XPS VB spectra and Mott−Schottky plots

Figure 2. Structural representations of Zn-GA. (a) View of the Zn-GA structure along the c direction (blue polyhedra represent zinc-centered units). (b) Coordination environments of Zn2+ cations. (c) View of the Zn−O chain. Yellow, red, gray, and white balls represent zinc, oxygen, carbon, and hydrogen atoms, respectively.

DFT calculations on Zn-GA compared to ZnO are presented in Figure 3a. More details about the DOS of Zn-GA can be found in Figure S5. This shows that Zn-GA is a semiconductor with a calculated band gap of 3.9 eV, higher than the optical band gap of 3.2 eV measured on the absorption spectrum (Figure S6). This difference will be discussed hereafter. The total electronic DOS shows two bands at −22 to −18 eV and at −13 to −11 eV, which can be attributed respectively to O(2s) and mixed C(2s)−C(2p)−O(2p) orbitals. The valence band (VB) spreads between −9.3 and 0 eV and corresponds to mixed Zn(3d)−O(2p) states, with limited contribution from C(2p). The major contribution for the VBM was found to be O(2p) in Zn-GA, the same as that of ZnO.3,4 However, the O(2p) that contributes to the VB of Zn-GA is more dispersed than that of ZnO. When the DOS of ZnO and Zn-GA are aligned according to the O(2s) energy position, a rise of the VBM energy level for about 2.2 eV from ZnO to Zn-GA can be clearly observed in Figure 3a. This increase of the VBM energy is crucial for p-type dopability of the material. We believe the shift is a result of the π-donor nature enhancement of the 6210

DOI: 10.1021/acs.inorgchem.7b00198 Inorg. Chem. 2017, 56, 6208−6213

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Figure 3. DFT calculations and electrical and optical properties of Zn-GA. (a) DOS from periodic DFT calculations of the electronic band structure of ZnO (upper panel) and Zn-GA (lower panel). Orange and purple zones represent the VB and CB, respectively. Black solid lines correspond to the full DOS of the compounds. Red dotted lines are for the alignment of O(s) energy positions, and orange dotted lines are the VBM levels. (b) VB spectrum of Zn-GA compared to ZnO from XPS. (c) Mott−Schottky plot at 2.016 kHz in a 1 M acetonitrile solution of lithium perchlorate of ZnGA synthesized from a zinc acetate precursor.

nm, resembling closely those of free GA (Figure S12), can be attributed to the intrinsic PL of the H2GA linker. This phenomenon is similar to other reported MOFs with PL properties arising from organic linkers.24,25 In the case of N:ZnGA, the broad, intense emission band with only one maximum at 488 nm (Figure 4c) can be attributed to nitrogen doping. Localized nitrogen-dopant centers induce critical modification of the electronic structure of N:Zn-GA, promote strong electron−phonon interactions, and eventually result in a significant gain of broad-band white luminescence.23,26,27 To date, the most promising achievements for luminescent MOFs (or hybrid materials) involve either rare-earth elements or the selection of luminescence-inducing linkers.24,28−30 Defects31 or

(Figures S9 and S10). At room temperature, N:Zn-GA emits intense white light under UV irradiation and shows a green-towhite afterglow with a few seconds duration, while Zn-GA exhibits nearly no luminescence (Figure 4a,b). As shown in emission bands on PL spectra, when excited at 300 nm (Figure 4c, inset, and Figure S11), both materials show two emission bands with maxima at around 390 and 490 nm. The first emission band with a maximum at around 390 nm is attributed to the free exciton emission, which is characteristic for the ZnGA MOF. The second emission band was studied more closely while excited at 380 nm, as shown in Figure 4c. In the case of Zn-GA (red curve in Figure 4c), the weak emission band with three maxima located at 413, 440, and 472 6211

DOI: 10.1021/acs.inorgchem.7b00198 Inorg. Chem. 2017, 56, 6208−6213

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sets an example for the fundamental tuning of semiconductor properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00198. Crystal data, PXRD patterns, NMR, absorption, IR transmission, VB, and PL and normalized emission spectra, TGA−DSC plots, structural views, DOS plots, XPS, elemental analysis results, and Mott−Schottky plots (PDF) Accession Codes

CCDC 1404779 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Figure 4. PL images of (a) N:Zn-GA and (b) Zn-GA under 365 nm UV irradiation. (c) PL emission spectra of the two samples (Zn-GA in red and N:Zn-GA in black) with the excitation wavelength at 380 nm (and at 300 nm for the inset). The maximum positions in the spectra are indicated by short vertical lines.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.S.). *E-mail: [email protected] (M.B.-D.). ORCID

Congcong Shang: 0000-0001-5569-4108 Romain Gautier: 0000-0002-3042-0335 Camille Latouche: 0000-0002-3541-3417 Laurent Cario: 0000-0001-5720-4395 Stéphane Jobic: 0000-0002-1900-0030

common element-doping (e.g., nitrogen-doping)-induced luminescence is more observed in MOs.32 Accordingly, MO behavior is kept in the Zn-GA MOF through a continuous Zn− O network. The doping effect can also be observed in the absorbance spectra in Figure S6. The addition of different electronic states in the band gap of N:Zn-GA results in the modification of light absorption, which explains the color change of the powders from white in Zn-GA to red-brown upon nitrogen doping. This is a common phenomenon for doped MOs, including ZnO.23,33 Therefore, the PL and absorbance behavior of Zn-GA toward nitrogen doping demonstrates its resemblance to the MO.

Author Contributions

C.S. and M.B.-D. supervised this project; C.S. prepared the samples and acquired the data; R.G. realized crystal analysis and structural determination; T.J. performed electrochemical measurements; E.F. conducted the theoretical calculations; C.L. participated in the analysis of the theoretical calculations; M.P. carried out NMR analyses of the materials; C.S., M.B.-D., R.G., S.J., and L.C. analyzed and interpreted the results; C.S. wrote the manuscript while taking into account suggestions from all authors; all authors reviewed and approved the manuscript.

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CONCLUSION In conclusion, this study demonstrated the successful generation of p-type behavior in a ZnO-like hybrid Zn−O network, in either its natural state or its nitrogen-doped state. The carbon atoms from organic linkers are isolated from the Zn−O network, which preserves an M−O skeleton in the structure. The high O/Zn ratio of the Zn−O network and the increase of the VBM energy level due to π-donor enhancement of O(2p) in Zn-GA are believed to be the key aspects for the achievement of p-type behavior. The optical properties, electronic states, and PL behavior upon nitrogen doping of Zn-GA suggest its similarity to MOs. Future exploration of similar hybrid semiconductors can be of great interest,34 especially for the progress of areas that are limited by the lack of p-type semiconductors, such as electronics and photovoltaics.5,12,35,36 In the meantime, further study of Zn-GA in comparison with other p-type and semiconducting MOFs37,38 can be of great interest for the development of MOF-based semiconductors. Moreover, this material lies at the “interface” of MOs and MOFs, reveals a new family of hybrid semiconductor materials with metal−oxygen networks, and

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Denis Loquet for elemental analyses, Stephane Grolleau for thermal analyses, and Jonathan Hamon for help with XPS. E.F. and C.L. thank the CCIPL (Centre de Calculs Intensifs des Pays de la Loire) for resources. C.S., M.B.-D., L.C., and S.J. thank Solvay for financial support, and T.J., E.F., L.C., C.L., and S.J. equally thank the Agence Nationale de la Recherche (Grant-13-BS08-0010) for fundings.

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DOI: 10.1021/acs.inorgchem.7b00198 Inorg. Chem. 2017, 56, 6208−6213

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DOI: 10.1021/acs.inorgchem.7b00198 Inorg. Chem. 2017, 56, 6208−6213