Improving Sensing of Sulfur-Containing Gas Molecules with ZnO

Oct 17, 2017 - School of Chemical and Molecular Biosciences, The University of ... *E-mail: [email protected]., *E-mail: [email protected]. ... ZnO...
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Article Cite This: J. Phys. Chem. C 2017, 121, 24365-24375

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Improving Sensing of Sulfur-Containing Gas Molecules with ZnO Monolayers by Implanting Dopants and Defects Tanveer Hussain,*,† Marlies Hankel,*,† and Debra J. Searles†,‡ †

Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane QLD 4072, Australia ‡ School of Chemical and Molecular Biosciences, The University of Queensland, Brisbane QLD 4072, Australia

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ABSTRACT: Two-dimensional ZnO materials are proposed for use in nanosensors and their ability to adsorb and detect toxic H2S and SO2 gases are compared. Graphene-like, two-dimensional ZnO monolayer (ZnO-ML) materials are considered that are doped with B, C, N, or S atoms or have Zn or O vacancies. In its pristine form, a ZnO-ML binds the two gases weakly, with binding energies of −0.33 eV and −0.67 eV for H2S and SO2, respectively. However, the presence of defects or the substitution of a Zn or O atom with heteroatoms was found to result in significant increases in the adsorption energy, resulting in a binding energy of up to −3.67 for H2S on a ZnO-ML with a Zn vacancy and −5.15 eV for SO2 on a C-doped ZnOML. The H2S molecule is observed to undergo dissociative adsorption on these substituted monolayers, which makes the materials unsuitable as reusable H2S sensors. However, SO2 does not dissociate in any of the cases studied. On SO2 adsorption, significant changes in the conductivity of the ZnO-ML that has an O vacancy occurs, observed as a reduction in the band gap. We also find a reduction in the band gap for Sdoped ZnO when SO2 is adsorbed. In both cases, this is coupled with a value of the adsorption energy of about −1 eV, making them suitable for a reusable sensor for SO2.

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tions.23−31 However, despite it being able to be produced in various nanostructures, the existence of ultrathin ZnO-ML is rather new and less explored on both the experimental and theoretical sides. There are only few theoretical studies on the applications of graphene-like ZnO-ML.20,21,32−40 Topsakal et al.32 studied the electronic and magnetic properties of twodimensional ZnO-ML along with armchair and zigzag ZnO nanoribbons. The ML, bilayer, and the nanoribbons of various orientations were found to be promising for several electronic and magnetic applications and different from graphene and BNsheets. First-principles calculations were performed by Schmidt et al.33 who reported the magnetic coupling of ZnO-ML upon Co substitution at the Zn site. Substitution of a Zn atom by Co in the single ZnO-ML resulted in ferromagnetic behavior, and its nature differs from that due to Co doping of bulk ZnO. Guo et al.34 investigated the effects of substitution of an O atom by nonmetals B, C and N in a ZnO-ML using density functional theory (DFT) calculations. It was found that B or C substitution in a 72-atom supercell resulted in a ferromagnetic half-metal, whereas N substitution resulted in a ferromagnetic semiconductor. A number of other theoretical reports35−38 discuss the electronic and magnetic properties of ZnO-ML and

INTRODUCTION Two-dimensional (2D) nanostructures have recently attracted interest due to their promise in various basic and applied scientific endeavors. The successful exfoliation of graphene, a one-atom thick honeycomb structure, 1−3 enabled new applications of 2D materials to be proposed and tested. Due to its distinctive properties, graphene has numerous applications, including spintronics, energy storage, photovoltaics, and gas sensing.4−10 The presence of Dirac charge carriers and their high surface area per mass have meant that graphene-based materials have been considered as gas sensing materials. However, issues such as weak graphene-adsorbent interactions with some gases and the lack of a band gap in its pristine form put limitations on this.11,12 Rapid evolution in technology has made it possible to consider 2D monolayers (ML) other than graphene, which also have a large surface area but have an intrinsic band gap. The recently synthesized, ultrathin single layer of ZnO (ZnO-ML), which has a graphene-like structure, is one of them.13,14 Zinc oxide (ZnO) is a material that has been extensively studied in its various morphologies (bulk, thin films, nano rods, nanowires, nanosheets, etc.).15−19 Thus, over a period of time, it has found its application in several fields including gas sensing.20−22 The semiconducting ZnO has features including high charge mobility, cost effectiveness, safety and stability, which are considered desirable for gas sensing applica© 2017 American Chemical Society

Received: May 21, 2017 Revised: October 5, 2017 Published: October 17, 2017 24365

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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Here E(ML + H2S/SO2) represents the total energy of the ML loaded with H2S or SO2, E(ML) the energy of the ML, and E(H2S/SO2) the energy of the gas molecule being adsorbed. ML could be the pristine ZnO-ML, ML with a Zn or O vacancy or a ZnOX-ML. In accordance with this definition, the more negative the value of Eads is, the stronger the binding.

two very recent investigations consider this ultrathin material for gas sensing. Zhang et al.20 investigated the H2S sensing performance of metal doped ZnO-ML using DFT calculations. They find that H2S only weakly binds to the pristine ML and with single atom defects. In the Pd-doped system, the authors find a significant change in the electric conductivity on the adsorption of H2S, indicating its suitability as a sensor. Meng et al.21 employed DFT calculations to investigate the effects of gas concentration, number of layers, and presence of hetero layers on the sensing of a range of different gases including H2S and SO2. They find that ZnO-ML weakly interacts with H2S and find that SO2 chemisorbs onto the ZnO-ML. However, they find no changes to the conductivity of the ZnO-ML on the adsorption of these two gases. Here, the sensing capability of a series of defective ZnO-ML materials for the two toxic and corrosive sulfur-containing gases, H2S and SO2, has been investigated. Although there are many changes in properties that can be measured in a sensor, here we particularly discuss the change from a semiconductor to a conductor, or vice versa, as this is particularly simple to detect. This work considers three situations for the adsorption of these gases: (i) on pristine ZnO-ML, (ii) on defective ZnOML with Zn or O vacancies, and (iii) on a ZnO-ML with an O atom substituted by B, C, N, or S. The substituted materials have empirical formula Zn16O15X. For simplicity, we will represent these structures as ZnOX in all cases. ZnOB, ZnOC, ZnON, and ZnOS structures have been prepared by substituting an O atom with a B, C, N, and S atom, respectively. In this work, we have only considered p-doped materials, formed by substituting the “O” atom of ZnO-ML with B, C, N, and S. The choice of these materials is based on the fact that such systems have already been studied and were found to form stable monolayers.

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RESULTS AND DISCUSSION We will first discuss the adsorption of H2S and SO2 on the pristine ZnO-ML. After this, we will show how the adsorption properties, such as binding energy and electronic structure, change when vacancies or heteroatoms are introduced. In this work, the changes to the calculated band gap for the material will be used to determine if the material is suitable as a sensor or not. Such changes could be the change from a semiconductor to a conductor (or zero band gap semiconductor) or vice versa or even a significant change in band gap while the nature of the semiconductor is not changed. It is well-known that band gaps are underestimated using GGA calculations. While GGA underestimates the band gap, if they are observed at this level of theory, a significant change in band gap is still expected experimentally. So the suitability of the material as a sensor can be evaluated using the level of theory used here. Pristine ZnO-ML. The optimized structure of ZnO-ML is shown in Figure 1. The Zn−O bond lengths are 1.90 Å, and the

COMPUTATIONAL DETAILS

Spin-polarized DFT calculations have been performed throughout this project using the VASP code.41−43 We have used the generalized gradient approximation (GGA) of Perdew-Burke and Ernzerhof for the exchange-correlation functional and projector augmented wave method (PAW) to treat electron− ion interactions.44,45 For weakly interacting systems where the van der Waals forces dominate, the GGA-based DFT calculations do not give accurate predictions of the adsorption energies. Therefore, the current calculations have been performed by considering van der Waals corrections (DFTD2) by Grimme as implemented in VASP.46 Our model ZnOML consists of a 4 × 4 × 1 supercell having 32 atoms (Zn = 16, O = 16) in a periodic cell and a vacuum space of 15 Å above the ZnO-ML that is formed, to avoid the unwanted interaction between the periodic images of the layers. An energy cutoff criterion of 500 eV is used for the plane wave basis set. Sampling of the Brillouin zone has been carried out using a Monkhorst−Pack scheme with 5 × 5 × 1 k-points for structural optimization and 7 × 7 × 1 k-points for density of states calculations.47 All the structures have been optimized until all forces become less than 0.01 eV/ Å. The charge analysis has been performed using a Bader analysis.48 The adsorption energies, Eads, of H2S or SO2 on pristine or defective ZnO-ML can be calculated by

Figure 1. Optimized structure of ZnO-ML. Gray and red balls represents Zn and O atoms, respectively.

Zn−O−Zn bond angles are 120°. This is in accordance with previous studies.26 The band gap value obtained here for ZnOML is 1.65 eV, making it a semiconductor, as shown in the density of states in Figure 2. This value is consistent with published data using a similar level of theory; however, a much more accurate value of 3.25 eV using a higher level of theory has also been reported previously.34 Figure S1 shows that the top of the valence band of ZnO is dominated by the Zn(d) and O(p) orbitals. As mentioned above, we considered the sensing of selected gases on three types of the ZnO-ML surface. First, we will describe the adsorption of H2S and SO2 on the pristine ZnOML sheet. In order to find the preferential binding configuration, we initially situate the gas molecules on each of four sites: above the Zn atom, above the O atom, above the bond between the Zn and O atoms, and above the sixmembered ring. In addition, two orientations of each gas were considered. For H2S, we considered the molecule oriented so that the H atom is pointing toward the ML and with the S atom is pointing toward the ML. For SO2, molecular orientations

Eads = E(ML + H 2S/SO2 ) − E(ML) − E(H 2S/SO2 ) (1) 24366

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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Figure 2. Total DOS of pristine ZnO-ML. The energy shown is that relative to the Fermi energy, E − Ef.

Figure 3. TDOS of pristine ZnO-ML and H2S and SO2 on pristine ZnO-ML. The energy shown is that relative to the Fermi energy, E − Ef.

with van der Waals corrections and show that it is necessary to employ methods such as DFT-D2. The adsorption energy of H2S is much smaller than the one found by Zhang et al.20 of −0.54 eV. This can be explained by the fact that they used a LDA functional which is known to overbind, whereas GGA+D2 has been used here. Meng et al.21 found binding energies of approximately −0.38 eV and −0.77 eV (taken from Figure 1a from ref 21) for H2S and SO2, respectively, which are similar to the results we obtained. This method used by Meng et al.21 was also similar to ours. Figure 3 depicts the total density of states (TDOS) for the adsorption of H2S and SO2 on ZnO-ML. It can be seen that there is no change in the band gap for both adsorptions

with the O atom is pointing toward the ML and the S atom is pointing toward the ML were considered. Minimum energy structures starting from these configurations were calculated, and the lowest energy structure selected. Consideration of the adsorption energies indicates that both gases are weakly adsorbed on pristine ZnO-ML with adsorption energies of −0.328 eV and −0.671 eV for H2S and SO2, respectively (these values are typical for strong physisorption). To show the importance of van der Waals corrections, we have also calculated the adsorption of both gases on the pristine ZnOML without van der Waals corrections. The magnitudes of the adsorption energies are less than 0.01 and 0.06 eV for H2S and SO2, respectively. These energies differ significantly from those 24367

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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Figure 4. Total density of states (DOS) for ZnOO‑vac and ZnOO‑vac-H2S and ZnOO‑vac-SO2. The energy shown is that relative to the Fermi energy, E − Ef .

This is mainly due to the spin down Zn(d) and O(p) orbitals now populated at the bottom of the conduction band right at the Fermi level (see Figure S4). The gas molecules were then introduced at various adsorption sites, and they were found to bind most strongly at the sites where defects were introduced (Zn or O atoms removed). We will first discuss the interaction of both gases with the ZnO-ML with an O vacancy. Here Figure 4 shows that introducing an O vacancy increases the band gap but the change is small. The H2S molecule binds to the substrate near the vacant site with an adsorption energy of −0.329 eV for the O vacancy. This shows that an O vacancy does not change the binding of H2S compared to the case of the pristine membrane. This finding is in accordance with the results by Zhang et al.,20 where they also found that introducing an O vacancy did not affect the H2S adsorption energy. There is also no change in the band gap, see Figure 4, which means the existence of an Ovacancy in the ZnO-ML did not improve the suitability of the material as a sensor for H2S. When an O vacancy was present, the calculated adsorption energy of SO2 was found to be −1.09 eV, which is higher than the one for pristine ZnO-ML. The ZnO-ML with an O vacancy is a semiconductor, see Figure 4. On adsorption of SO2, there is a significant change in electronic properties with the conducting band shifting downward very close to the Fermi level. This shift is due to the p orbitals of the SO2 molecule interacting with the O(p), Zn(d), and Zn(s) orbitals. This shows that ZnOO‑vac-SO2 is a semiconductor with a small band gap. The reversible adsorption energy coupled with the measurable change in electronic properties suggests that ZnO with an O vacancy could be suitable as a reversible SO2 sensor. We will now discuss the adsorption of H2S and SO2 on ZnOML with a Zn vacancy. In the case of H2S, the binding energy is now much higher, −3.67 eV, compared to the pristine ZnOML. The incident molecule sits in-plane on the defect, with a small distance of 1.0 Å to the neighboring O atoms of ZnOML. The large adsorption energy is the result of a significant

compared to the pristine sheet. This is in accordance with the results by Zhang et al.20 and Meng et al.21 who also showed that there is no significant change in the band structure and no change in the band gap. This suggests that the pristine ZnOML is not suitable for sensing of these two gas molecules. ZnO-ML with Vacancies. For an efficient reusable sensor, the adsorption energies of the incident gases should be those corresponding to strong physisorption or weak chemisorption to ensure that the properties of the material change on adsorption but that the gas molecules can desorb again. In some cases, weak adsorption has led to a detectable change in the conductivity of a material, making it useful as a sensor.48 However, recently Tawfik et al. proposed that for an ideal system, adsorption/desorption energies of incident gases should be around −1 eV.49 Due to the unchanged electronic structure of the pristine ZnO-ML and the weak adsorption energies for the adsorption of H2S and SO2, we will now investigate if defective ZnO-ML is more effective as a sensor for these two gases. The defects considered here are Zn and O vacancies that are obtained by removing a single Zn or O atom from a 4 × 4 × 1 supercell, resulting in a relatively small defect concentration of 3.125% of the atoms. The creation of these vacancies changes the structures of the ZnO-ML with the modification in the Zn−O bond length and Zn−O−Zn bond angle. In the case of the Zn vacancy, the Zn−O bond length reduces to 1.82 Å with an increase in Zn−O−Zn bond angle from 120° to 133°. The opposite effect is observed on creation of an O vacancy with an increase in the Zn−O bond length to 1.94 Å and a reduction in the Zn−O−Zn bond angle to 117°. However, the planar structure of ZnO-ML is preserved after structural optimization. We also find that in both cases there is a change in the electronic properties (TDOS) of the systems as can be seen in Figure S2, which shows the total DOS for both vacancy cases and the pristine ZnO-ML. For the O-vacancy, there is only a small change in the band gap. For the Znvacancy there is a more significant change, with the material now being a magnetic semiconductor with a zero band gap. 24368

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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The Journal of Physical Chemistry C change in the H2S structure. One H−S bond is broken (the distance between the atoms is now 1.98 Å, which is considerably longer than the usual H−S bond length of 1.34 Å). However, the other H−S bond-length remained at 1.34 Å. The H−S−H bond angle increased from 92° to 101°. The S− H moiety was bound to one of the O atoms near the Zn vacancy, and the cleaved H atom was bound to one of the other neighboring O. This structural change can be seen in the electron density plot in Figure 5a.

The dissociation of H2S observed here contrasts to the results by Zhang et al.20 They found that H2S does not dissociate and weakly binds to a ZnO-ML that has a Zn-vacancy with a just slightly smaller adsorption energy than the pristine ZnO-ML. These different results could be due to the different methods and functionals used and, in particular, because they used LDA without van der Waals corrections, whereas here GGA with van der Waals corrections is used. It could also be due to the different initial positions tested for the H2S molecule. In the case where the H2S does not dissociate, the binding is weaker and similar to that observed for the pristine ZnO-ML. However, as dissociation has been observed, the material is unsuitable as a sensor for H2S. Figure 6 shows that the adsorption of H2S has a significant effect on the electronic properties. The material changes from a magnetic zero band gap semiconductor to a semiconductor. However, the high adsorption energy for H2S on the Zn vacancy and its dissociation makes the ZnO-ML with a Zn-vacancy unsuitable as a reusable sensor for H2S. The strong binding and a change from a zero band gap magnetic semiconductor to a nonmagnetic semiconductor with a band gap of over 1 eV, see Figure 6, does point to a possible application to capture H2S or as a single use sensor. The binding energy of SO2 on ZnO-ML with a Zn vacancy is also much higher than for the pristine ZnO-ML, −3.64 eV. However, unlike the case for H2S, SO2 does not dissociate but remains intact. Figure 6 shows that ZnO-ML with a Zn vacancy is a magnetic semiconductor with a zero band gap. With SO2 adsorption, the TDOS shows that the material becomes a semiconductor with a small band gap. The adsorption energy for SO2 on Zn vacancy is high and therefore not easily reversible so, just like for H2S, one could consider this material for capture or a single-use sensor for SO2. ZnO-ML with Heteroatom Substitution. As another strategy to enhance the adsorption energy of H2S and SO2 on the ZnO-ML and facilitate a significant change in electronic

Figure 5. Electron density of H2S adsorption on (a) ZnOZn‑Vac, (b) ZnOB, (c) ZnOC, and (d) ZnON. Gray, red, white, yellow, blue, green, and brown balls represents Zn, O, H, S, N, B, and C atoms. The gray clouds represent the electron density which is plotted at 0.3 e−/ Å3.

Figure 6. Total density of states (DOS) for ZnOZn‑vac, ZnOZn‑vac-H2S, and ZnOZn‑vac-SO2. The energy shown is that relative to the Fermi energy, E − Ef. 24369

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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Figure 7. Total density of states (DOS) for ZnO, ZnOB, ZnOC, ZnON, and ZnOS. The energy shown is that relative to the Fermi energy, E − Ef.

There are some differences. One is that the vacuum above the monolayer is set to 15 Å here and to 10 Å in ref 34. We also used a smaller supercell with 32 atoms compared to 72 and denser k-point mesh, 7 × 7 × 1 compared to 3 × 3 × 1, for the calculation of the DOS. The k-point mesh could have an influence on the intensity and resolution of peaks in the DOS, so this might explain the observed difference. The effects of the substituents on the structure and adsorption energy are discussed below for each of these elements. H2S Adsorption. In ZnOB (B substituted ZnO-ML), the Zn−B bond length and Zn−B−Zn bond angle have been found to be 2.0 Å and 120°, respectively. When exposed to H2S, the H2S chemisorbs near the B atom with an adsorption energy of −3.27 eV. On adsorption, the H2S dissociates. One of the H atoms detaches from the H2S and binds to the B atom with a bond length of 1.24 Å. The other H remains bound to the S with a bond length of 1.36 Å, which is a little longer than the H−S bond length in H2S. The S of H2S binds with the dopant B with a bond length of 1.8 Å. The electron density plot in Figure 5b clearly shows the dissociated H2S molecule and new bonds on the ZnOB-ML. In the optimized structure, the B atom is 1.4 Å out of the ZnO plane. To determine the charge transfer, which is caused by the difference of electronegativities, we employed a Bader analysis. The B atom has the lowest electronegativity of all the elements in the system, and it loses almost half of its charge to the H2S molecule. Each of the H atoms receive almost 1.1 e− of the charge, primarily coming from the ZnOB monolayer. The optimized structure of ZnOB-H2S is given in Figure 8. The adsorption of H2S on ZnOB changes the magnetic conducting material into a magnetic semiconducting material as shown in Figure 9. The change in the electronic properties is explained in Figure S13, which shows that the larger contributions from the B(p), O(p), and Zn(d) orbitals around the Fermi level have now nearly disappeared. This is consistent with the significant charge transfer from the monolayer and the B atom to the H2S molecule. While there is a significant change

properties, we considered the substitution of O by B, C, N, and S. The choice of these elements is based on the fact that they have comparable size to O and have already been studied for other purposes in the literature.25,26 Our DFT-D2 calculations revealed that the substitution of one O atom with B, C, N, or S in ZnO-ML gives a stable structure, similarly to the results shown by Guo et al.26 The changes in the interaction of the gases with the material and their electronic structure are discussed below. Substitution of one O atom in some of the cases results in significant changes in the electronic structure of the ZnO-ML semiconductor, see Figure 7. With S substitution, the material remains a semiconductor with no real change to the band gap. Substitution by B changes the monolayer into a magnetic conductor with spin down states appearing across the Fermi level. This is similar to the substitution by C, where the material is now also a magnetic conductor with (spin down) states appearing across the Fermi level. Substitution by N makes the monolayer magnetic as well but it remains a semiconductor. Figure S9 shows that the changes in the DOS for ZnOB is due to the appearance of spin down states across the Fermi level in the B(p), O(p), and Zn(d) orbitals and smaller contributions from the Zn(p) and Zn(s) orbitals. One can also see the appearance of spin up and spin down states in the conduction band mainly from the B(p) orbital. This is similar to the finding by Guo et al.,35 who also showed that main contributions across the Fermi level are from the B(p), O(p), and Zn(d) orbitals. The case for ZnOC is similar to spin down states across the Fermi level from the Zn(d), O(p), and C(p) orbitals, see Figure S10. Again, this is in accordance with the results from ref 35. For ZnON, our results differ from those in ref 34. The current results show that the ZnON is a magnetic semiconductor with a small band gap for the spin down DOS, while Guo et al. report a nonmagnetic semiconductor. The PDOS in Figure S11 shows that spin down states appear just above the Fermi level with contributions from the N(p) orbital and much smaller contributions from the O(p) and Zn(d) orbitals. Guo et al. used the same level of theory and software. 24370

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H2S molecule dissociates and HS binds strongly to the C of ZnOC sheet with an adsorption energy of −3.23 eV. This can be seen in the electron density plot in Figure 5c. One of the H atoms from H2S binds to the C in ZnOC with a bond length of 1.11 Å. The other H remains bound to the S atom with a 1.37 Å bond length. The S atom of HS binds with the C dopant in ZnOC with a bond length of 1.81 Å. The structure of the ZnOC changes considerably after exposure to H2S, with elongation of the Zn−C bond length to 1.99 Å and reduction in the Zn−C−Zn bond angle to 110°. The lowest energy configuration of ZnOC-H2S is shown in Figure 8. In this case, both C and S have almost the same electronegativity and there is hardly any charge transfer. However, there is a significant change in the electronic properties in case of H2S adsorption on ZnOC as shown in total density of states in Figure 9. The magnetic conducting ZnOC turns into a nonmagnetic semiconductor upon the adsorption of H2S with a significant band gap. Figure S14 shows that there is overlap of the H2S(s) and H2S(p) orbitals with the C(p) orbitals just below the Fermi level consistent with the bonding of the dissociated molecule. However, the contributions to the spin down states around the Fermi level from the C(p), O(p), and Zn(d) have now disappeared. Similar to the case for ZnOB, the dissociation for the molecule make this material unsuitable as a reusable sensor even with the significant change in electronic properties. When a N atom substitutes an O atom in the ZnO-ML, the ZnON has an optimized structure with a Zn−N bond length of 1.91 Å and a Zn−N−Zn bond angle of 120°, which is very similar to the pristine ZnO-ML. However, the adsorption of H2S causes structural variations, and ZnON rearranges in such a way that substituted N has elongated bonds of 1.95 Å each with two of its neighboring Zn atoms and 2.10 Å with a third Zn. The average bond angle between Zn and N is reduced to 115°. Like in the previous cases, H2S dissociates, as shown in the electron density plot Figure 5d, but unlike the cases of B

Figure 8. Optimized structure of B, C, N, and S substituted ZnO-ML loaded with a H2S molecule. Gray, red, green, orange, blue, yellow, and pink balls represent Zn, O, B, C, N, S, and H atoms, respectively.

in the electronic properties, the large adsorption energy and the dissociation of the molecule make ZnOB unsuitable as a reversible sensor for H2S. Substitution of an O atom by a C atom in ZnO-ML resulted in a ZnOC sheet with optimized Zn−C bond length of 1.93 Å and Zn−C−Zn bond angle of 120°. As was seen for ZnOB, the

Figure 9. Total density of states (DOS) for ZnOB-H2S, ZnOC-H2S, ZnON-H2S, and ZnOS-H2S. The energy shown is that relative to the Fermi energy, E − Ef. 24371

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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The Journal of Physical Chemistry C and C substitution, here the HS binds with the Zn atom rather than the dopant N in ZnON. One H atom binds to the N atom with a bond length of 1.03 Å, and the other H remained attached to the S atom with a bond length of 1.35 Å. The adsorption energy of H2S in this case is −1.35 eV, which is significantly smaller than the adsorption energy of H2S on ZnOB or ZnOC. A Bader charge analysis and the electron density plot (Figure 5d) suggest that the H atoms of the H2S have different charge states, and one of them makes a bond with the substituted N atom in ZnON, whereas the other H atom remains attached to the S atom and the later bound to the Zn of ZnO-ML. The N atom has a charge of approximately 1.9 e−, and the H atom attached to the N has lost its electron while the H atom attached to S has a charge of 1.3 e−. The optimized structure of ZnON-H2S is shown in Figure 8. Figure 9 shows that the magnetic semiconductor ZnON-ML remains a magnetic semiconductor but now with a zero band gap when H2S is adsorbed. Figure S15 shows a substantial overlap of the H2S(p) orbitals with the Zn(d) orbitals just below the Fermi level, which is consistent with the binding of the HS fragment with one of the Zn atoms. The spin down states just above the Fermi level are resulting from the H2S(p), Zn(d), and O(p) orbitals. Finally, we describe the adsorption of H2S on S-substituted ZnO (ZnOS). The adsorption energy of H2S has been calculated as −1.04 eV. The most stable configuration of H2S is shown in Figure 8. It is evident from Figure 8 and the low adsorption energy that in the case of S substitution, the H2S does not dissociate like in the previous substitution cases. While the adsorption energy of H2S is in the right range there is no real change in the total DOS, see Figure 9. The material remains a semiconductor with a similar band gap, and therefore, there is no change in the conducting character of the material, which suggests that this material will not sense H2S, based on measurements of conductivity. SO2 Adsorption. We will now look at the adsorption of SO2 on the substituted materials. In the case of SO2 binding on ZnOB, the adsorption energy and adsorption distance have been found to be −2.38 eV and 1.75 Å, respectively. Upon structural optimization, the S−O bond length elongates from 1.43 to 1.46 Å for both of the O atoms in the SO2 molecule with an O−S−O bond angle of 119°. The preferred configuration of SO2 is the one where the S atom binds over the B atom on the ZnOB sheet as shown in Figure 10. Since the O atoms are more electronegative than Zn, B, and S, charge is transferred from the ZnOB sheet to the SO2 molecule and each O atom acquires 0.73e− of this transferred charge primarily coming from the B atom. The electron density plot of SO2 with ZnOB is shown in Figure 11b. This shows clearly that, unlike H2S, SO2 does not dissociate while binding to the monolayer. Figure 12 shows that the magnetic conducting ZnOB-ML changes to a magnetic semiconductor with a small band gap. Figure S17 shows that the spin down states just above the Fermi level and the spin up state just below the Fermi level have contributions from the B(p), O(p) and Zn(d) orbitals. While we observe a significant change in the electronic properties the adsorption energy is strong and the adsorbent difficult to remove. For ZnOC, the adsorption of SO2 takes place through the S atom that is located above the substituted C in ZnOC at a small distance of 1.62 Å with large adsorption energy of −5.15 eV. This is the strongest binding among all the studied configurations. The C atom binds to the neighboring Zn

Figure 10. Optimized structure of B, C, N, and S substituted ZnO-ML loaded with SO2 molecule. Gray, red, green, brown, orange, yellow, and pink balls represent Zn, O, B, C, N, S, and H atoms, respectively.

Figure 11. Electron density of SO2 adsorption on (a) ZnOZn‑Vac, (b) ZnOB, (c) ZnOC, and (d) ZnON. Gray, red, white, yellow, blue, green, and brown balls represents Zn, O, H, S, N, B, and C atoms. The gray clouds represent the electron density which is plotted at 0.3 e−Å3.

atoms in the ZnOC monolayer with a bond length of 2.03 Å and Zn−C−Zn bond angle of 115°. Upon adsorption of SO2 on ZnOC, there is slight change in the geometry of the SO2 molecule with an elongated S−O bond length of 1.45 Å and reduced O−S−O bond angle of 117°. The top and side views of the optimized structure can be seen in Figure 10. When SO2 is exposed to ZnOC, charge is transferred from the S to O atoms within the SO2 molecule, according to Bader analysis. There is also considerable charge transfer between the substituted C of ZnOC and the S atom. Similarly to the adsorption of H2S, the adsorption of SO2 also led from a magnetic conductor to a nonmagnetic semiconducting ZnOC− 24372

DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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The Journal of Physical Chemistry C

Figure 12. Total density of states of ZnOB-SO2, ZnOC−SO2 and ZnON-SO2, and ZnOS-SO2. The energy shown is that relative to the Fermi energy, E − Ef.

SO2(p) states appearing just above the Fermi level, see Figure S20. ZnOS could therefore be a suitable sensor for SO2 as the adsorption energy of −0.88 eV is close to 1 eV reported to be good for reversible sensing and the change in the band gap on adsorption of SO2.

SO2 structure with a significant band gap as shown in Figure 12. This is due to the spin down states from the C(p), O(p), and Zn(d) disappearing around the Fermi level, see Figure S18. The electron density plot of SO2 on ZnOC is given in Figure 11c. Carbon substitution results in a significant change in adsorption energies for both sulfur-containing molecules. Also, for both, we see a marked change in the electronic properties. H2S breaks on adsorption and SO2 stays intact, but the adsorption energy is high, therefore these results indicate that this material could be suitable for capture or a nonreversible sensor of these Scontaining molecules. For SO2 adsorption on ZnON, the most favorable SO2 adsorption geometry is shown in Figure 10. The S atom of SO2 sits at the top of the N of the ZnON, and there is a slight elongation in S−O bond length from 1.43 to 1.48 Å and reduction in O−S−O bond angle from 119° to 118°. The ZnON-ML also changes with an increase in the Zn−N bond lengths to 2.0 Å and a much-reduced Zn−N−Zn bond angle of 114°. The SO2 adsorption energy and adsorption distance have been calculated to be −1.44 eV and 1.66 Å, respectively. The bonding of SO2 on ZnON can be seen in the electron density plot in Figure 11d. The S atom is the least electronegative atom in this system and loses most of its charge to the O atoms within the SO2 molecule as well as to the N atom in the ZnON sheet. The resulting ZnON-SO2 is a magnetic semiconductor with a small band gap, see Figure 12 and Figure S19. While we find a significant change in adsorption energy, there is no significant change in the electronic properties on adsorption of SO2; it remains a magnetic semiconductor with a small band gap. Finally, we describe the adsorption of SO2 on S-substituted ZnO (ZnOS). The adsorption energy of SO2 has been calculated as −0.88 eV. The most stable configuration of SO2 on ZnOS is shown in Figure 10. ZnOS shows a change in its electronic properties on adsorption of SO2, as shown in total density of states in Figure 12 where it can be seen that it is still a semiconductor but now with a small band gap. This is due to

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DISCUSSION Above, we discussed the different membranes and their possible suitability as a sensor for H2S and SO2. We will now summarize the different findings. Table 1 shows all the adsorption energies for the different cases as well as if a change in conductivity was Table 1. Adsorption Energies (eV) of H2S and SO2 on All Materials Studieda

ZnO-H2S ZnO−SO2 ZnO-O-vacH2S ZnO-O-vacSO2b ZnO-Zn-vacH2S ZnO-Zn-vacSO2 ZnOB-H2S ZnOB-SO2 ZnOC-H2S ZnOC−SO2 ZnON-H2S ZnON-SO2 ZnOS-H2S ZnOS-SO2b

binding energy (eV)

dissociation

change in conductivity

−0.328 −0.671 −0.329

no no no

no no no

yes yes yes

−1.09b

nob

yesb

yesb

−3.67

yes

yes

no

−3.64

no

yes

no

−3.27 −2.38 −3.23 −5.15 −1.35 −1.44 −1.04 −0.88b

yes no yes no yes no no nob

yes yes yes yes yes no no yesb

no no no no no yes yes yesb

reversible

a It is also indicated whether or no H2S dissociates, and if a change in conductivity was observed. bIndicates the materials which our DFT calculations suggest are the most suitable for reversible gas sensors.

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DOI: 10.1021/acs.jpcc.7b04923 J. Phys. Chem. C 2017, 121, 24365−24375

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The Journal of Physical Chemistry C observed or not and if the H2S dissociates or not. It also shows if the adsorption of the SO2 or H2S molecule is reversible or not. From Table 1, one can see that none of the ZnO-ML and defective ZnO-ML materials is suitable as a reversible sensor for H2S. In some cases, this is due to the fact that H2S dissociates. This would result in the blockage of the active sites and the poisoning of the sensor material. While one could consider single use sensors of H2S or capture, due to the small percentage of the defects within the material, the ability to capture a large amount of H2S would be small. In other cases, adsorption of H2S does not result in a change in conductivity of the material. For SO2, we also either find a large adsorption energy (< −2 eV) or no change in conductivity in most cases. However, there are two cases where the adsorption energy is calculated to be around −1 eV, and a change in conductivity is observed. These are ZnO with a O vacancy and ZnOS. From our results, we conclude that these two materials could be suitable as a reversible sensor for SO2.

Debra J. Searles: 0000-0003-1346-8318 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Queensland for support of this project through the UQ Postdoctoral Fellowship Scheme. We would also like to thank the Australian Research Council for support of this project through the LIEF program. This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme supported by the Australian Government, support from the Queensland Cyber Infrastructure Foundation (QCIF), and the University of Queensland Research Computing Centre. It was also supported by resources provided by The Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia.

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CONCLUSION Spin-polarized, van der Waals corrected first-principles DFT were performed to study the structural and electronic properties of pure, defective, and substituted ZnO-ML and their changes after adsorption of H2S and SO2. It was observed that both the gases showed weak binding (with a magnitude