Metal paddlewheels as sensors for detection of SO2 gas: a DFT study

The sensing properties of the paddlewheel-type metal dimer (M-BTC: M = Fe, Ni, Cu, Zn, and Pd) toward SO2 gas were theoretically investigated using density functional theory (DFT) at the M06-L level of theory. Single-point calculations were carried at the M06 functional to correct the energetic properties and HOMO–LUMO energy gap. The O-bound adsorption complex of SO2 on the metal center of paddlewheels is found to be thermodynamically favorable than the S-bound ones. The trend of adsorption energy of SO2 on the metal center is in the order Pd-BTC < Cu-BTC < Ni-BTC < Zn-BTC < Fe-BTC. Among these five paddlewheels, the Ni-BTC and Zn-BTC paddlewheels are highly sensitive toward SO2 gas as compared to the other systems. The density of states reveals that the adsorption process significantly reduces the LUMO of the system to lower energies, enhancing the conductivity of the system. From DFT results, the energy gap of Ni-BTC and Zn-BTC is significantly reduced by 18.0 and 41.0% after the adsorption of SO2 on the metal center. These results suggest the great potential of Ni-, and Zn-BTC paddlewheels as SO2 sensors.


Introduction
After the industrial revolution, the air pollutants which are mainly released from various processes from petrochemical industries, coking plants, waste treatment units, etc., turn into a global concern. Harmful gases such as sulfur-containing gases, nitrous oxide (N 2 O), and carbon dioxide (CO 2 ) are principal contributors to effect on the environment, such as acid rain, global warming, and climate change. Various approaches have been proposed to reduce the unprecedented levels of these harmful gases (Peter 2018;Zhang et al. 2020). Sulfur dioxide (SO 2 ), which is an irritant colorless gas, is regarded as one of the most harmful gases. The burning of fossil fuels containing sulfur such as coal, oil, and diesel from vehicles, power plants, and industrial factories is the primary source of SO 2 emission. To control the emission of SO 2 effectively, SO 2 sensors are necessary to monitor and track the presence of SO 2 in the environment. Therefore, great efforts have been devoted to finding out gas-sensing materials for SO 2 detection.
Various materials such as metal oxides (Ji et al. 2019;Zhou et al. 2019), metal sulfides Zhang et al. 2017), and carbon-based nanomaterials (Kumar et al. 2017;Shen et al. 2013) have been employed as sensor materials for SO 2 gas. Among materials for sensing applications, metal-organic frameworks (MOFs) have attracted great interest in applications not only gas adsorption/separation, catalysis but also sensing applications (Bien et al. 2019;Chernikova et al. 2018;Furukawa et al. 2013;Kokcam-Demir et al. 2020). Pohle et al. (2011) investigated the utilization of Cu-BTC as a gas sensor based on work function changes. The experimental results revealed that Cu-BTC on the Au back electrode was sensitive toward NH 3 , H 2 S, ethanol, acetone gases at the temperature of 80 °C. Several MOF materials were tested as gas sensors for detecting gases (Achmann et al. 2009). Among studied MOFs (Al-BDC, Fe-BTC, Cu-BTC materials), Fe-BTC is identified to be an appropriate sensor for hydrophilic gases as water, methanol, and ethanol. However, Fe-BTC did not respond to O 2 , CO 2 , C 3 H 8 , NO, and H 2 gases. Recently, a composite of Ni-BTC and OH-functionalized single-walled carbon nanotubes (OH-SWNTs) was synthesized to test the sensing performance for sulfur dioxide (SO 2 ) detection (Ingle et al. 2020). The composite sensor exhibits selectivity to SO 2 gas as compared to other tested analytes as CH 4 , CO, and C 2 H 2 gases. From the experimental study, it was found that the composite has a detection limit of 4 ppm with a response time and a recovery time of 5 and 10 s, respectively. Here, inspired by the Ni-BTC-based sensor, the gas sensitivity of the paddlewheel-type di-metal center unit can be modified by adjusting the type of metal as similar as done for the improvement of MOFs in catalysis. The sensing performance of metal-paddlewheel clusters has not yet been demonstrated from the theoretical viewpoint. Based on DFT calculations, these results could provide deeper insight into the interaction between gases on the metal center of paddlewheel units and reveal some clues for metal-paddlewheelbased gas sensor design.
In this study, a theoretical investigation was employed to get insights into the potential ability of metal-paddlewheel units as SO 2 gas sensors. Five metal-paddlewheel units, namely, Fe-BTC, Ni-BTC, Cu-BTC, Zn-BTC, and Pd-BTC were selected to study the effect of metal properties on SO 2 adsorption. The adsorption energies, charge transfer, and electronic properties were calculated to investigate the sensitivity of these paddlewheels to detecting SO 2 gas. The results of this study could be beneficial for developing metalpaddlewheel-based sensors for detecting this harmful gas.

Methodology
The adsorption of SO 2 gas on the metal center of paddlewheel units was theoretically investigated using M06-L functional (Zhao and Truhlar 2006, 2007. The M 2 (BTC) 4 cluster, which was obtained from the experimental single-crystal diffraction (XRD) data (Chui et al. 1999), was adopted as the active center of the paddlewheel-based MOFs. The dangling bonds at the terminal -COO groups were saturated with hydrogen atoms as done in previous studies (Ketrat et al. 2017;Maihom et al. 2019;Sirijaraensre 2019Sirijaraensre , 2021Verma et al. 2013). Five clusters of paddlewheel units namely Fe-, Ni-, Cu-, Zn-, and Pd-BTC units were selected to investigate the adsorption property and sensing performance for SO 2 gas. For computations, the M06-L functional, which has been widely applied to study the interaction of molecules on the metal center of metal-organic frameworks (MOFs) (Grissom et al. 2019;Injongkol et al. 2017;Li et al. 2019;Paluka et al. 2020;Sirijaraensre 2019Sirijaraensre , 2021Verma et al. 2013;Yadnum et al. 2013), was used with the combination of 6-31G(d,p) for non-metal atoms and the Stuttgart-Dresden ECP (SDD) for transition metal atoms, respectively. The electronic ground states of M-BTC (M = Fe, Ni, Cu, Zn, and Pd) are nonet, triplet, triplet, singlet, and singlet spin states, respectively. For geometry optimization, convergence criteria, which are gradients of maximum force, RMS force, maximum displacement, and RMS displacement, are 0.000450, 0.000300, 0.001800, and 0.001200, respectively. For energy calculations, the SCF convergence criterion of 10 -8 a.u. along with ultrafine integration grid for the integrals was used for all calculations. During optimization, adsorbing molecules and M 2 (BTC) 4 cluster were allowed to relax which terminating hydrogen atoms were kept fixed. To obtain a more reliable width of the energy gap, single-point calculations were performed using the M06/6-311G(d,p) + SDD level. Reported adsorption energies at the M06 level were corrected by the counterpoise method to correct the adsorption energy for the basis set superposition error (BSSE) (Boys and Bernardi 2006;Simon et al. 1996). The adsorption energy (E ads ) is calculated from the following equation: Atomic charges were analyzed by natural bond orbital (NBO) population analysis (Weinhold 2012). All calculations were done by using the Gaussian 09 program (Frisch et al. 2009). The plots of the density of states (DOS) were obtained using the GaussSum program to reveal the change of electronic structures of adsorption complexes (O'Boyle et al. 2008).

Results and discussion
From the experiment, the energy gap (E g ) of Cu-BTC MOF was experimentally reported to be 3.6 eV (Gu et al. 2015). Based on the optimized structure of the Cu-BTC cluster, the energy gap based on the M06-L calculation is predicted to be 1.21 eV which is significantly lower than the experimental value. To obtain a more accurate prediction of the energy gap, single-point calculations were performed on M06-L-optimized structures by using hybrid M06 functional at the combination of 6-311G(d,p) + SDD ECP basis sets. The M06-calculated energy gap of the Cu-BTC cluster was predicted to be 3.55 eV which is consistent with the experimental observation (Gu et al. 2015). Therefore, all reported values in this work were obtained by using the M06//M06-L level of theory. The M06 functional predicts the energy gap of M06-L-optimized Fe-BTC, Ni-BTC, Zn-BTC, and Pd-BTC clusters to be 2.33 eV, 3.73 eV, 5.63 eV, and 3.21 eV, respectively.
Optimized structures of adsorption complexes are shown in Figs. 1 and 2. The adsorption of SO 2 molecules on the metal site of five metal-paddlewheel units was investigated by the M06//M06-L level of theory. For SO 2 adsorption, there are two possible configurations of the adsorbed sulfur dioxide (SO 2 ) for interacting with the metal center of metal-paddlewheel units. One is the interaction between the SO 2 molecule on the metal center of paddlewheel units via a terminal oxygen atom (O-bound complex). The other is the interaction via the central sulfur atom of the SO 2 molecule (S-bound complex). For the former configuration, the distance between the SO 2 oxygen atom and the metal center of paddlewheel units increases from 1.947 Å (Fe-BTC) to 2.648 Å (Pd-BTC). After the adsorption, two S-O bonds become inequivalent in which an S-O bond of SO 2 molecule elongates from 1.463 Å to 1.536, 1.484, 1.478, 1.495, and 1.476 Å for the adsorption on Fe-BTC, Ni-BTC, Cu-BTC, Zn-BTC, and Pd-BTC paddlewheels, respectively. The adsorption energy for SO 2 on the metal center in the O-bound complex increases as follows: Pd-BTC (-2.90 kcal mol −1 ) < Cu-BTC (-6.66 kcal mol −1 ) < Ni-BTC (-12.01 kcal mol −1 ) < Zn-BTC (-13.60 kcal mol −1 ) < Fe-BTC (-19.60 kcal mol −1 ). For the S-bound configuration, the SO 2 molecule was attached to the metal center of paddlewheels via the lone pair centered on the sulfur atom. This mode of adsorption was not found for the adsorption of the Cu-BTC paddlewheel. Starting from the adsorption on the Cu of Cu-BTC via its sulfur atom, the molecule was optimized to bind with BTC ligands via hydrogen bonds, as shown in Fig. 2. The adsorption energy is predicted to be -4.16 kcal/mol, which is smaller than the adsorption of SO 2 via its oxygen atom. For the other cases, the S-O bonds of adsorbed SO 2 are insignificantly distorted from the isolated values for the adsorption on the metal center of paddlewheel units. Much weaker adsorption energies ranging from − 0.68 to -4.42 kcal/mol were found for the adsorption of SO 2 molecules on the Fe-, Ni-, Zn-, and Pd-BTC paddlewheels. Among five metal paddlewheels, Pd-BTC weakly interacts with the SO 2 molecule both in O-bound or S-bound configurations. These findings agree with the previous studies of the adsorption of SO 2 on the coordinatively unsaturated metal center of MOFs (-21.77 for Mg-MOF-74 (Tan et al. 2017), -17.22 kcal mol -1 for Zn-MOF-74 (Tan et al. 2017), and -15.79 kcal mol -1 for Zn(BDC)(ted) 0.5 (Tan et al. 2013)), in which the SO 2 molecule adsorbs on the metal center via its oxygen atom. From calculated results, Fe-BTC paddlewheel has relatively high adsorption energy for SO 2 molecules both in O-bound and in S-bound configurations, indicating that SO 2 molecule is captured facilely by Fe-BTC paddlewheel.

Electronic properties of SO 2 adsorption complexes
Attempts have been made to investigate the change of electronic properties of metal paddlewheels after the adsorption of SO 2 molecule. Based on the adsorption energies between the O-bound and S-bound complexes, the O-bound complex is energetically preferred for the adsorption on the metal-paddlewheel clusters. From NBO analysis, the NBO charge of the metal center is predicted to be + 1.122e, + 1.03 7e, + 0.996e, + 1.277e, and + 0.711e for Fe(II), Ni(II), Cu(II), Zn(II), and Pd(II) cations in the paddlewheel clusters, respectively. NBO analysis reveals the amount of electron transfer and the orbital interaction between the adsorbing molecule and the metal center of metal-paddlewheel units. The results from the NBO analysis are tabulated in Table 1. Excepting the adsorption on the Fe-BTC, the charge of the adsorbed SO 2 molecule changes from zero to + 0.024e, + 0. 080e, + 0.127e, and + 0.096e for the adsorption on Pd-BTC, Cu-BTC, Ni-BTC, and Zn-BTC, respectively. The NBO results indicate that the SO 2 gas molecule acts as an electron donor molecule for the adsorption on the Pd-, Cu, Ni-, and Zn-BTC paddlewheels. For Fe-BTC, the HOMO and LUMO orbitals of Fe-BTC are localized on the metal centers of Fe-BTC. Fe centers of Fe-BTC can either provide electrons from the metal center or accept electrons. The direction of electron transfer between the Fe-BTC to the adsorbed SO 2 molecule is attributed to the Lewis character of Fe in the paddlewheel. From the FMO diagram, the |LUMO Fe-BTC -HOMO SO2 | gap and the |LUMO SO2 -HOMO Fe-BTC | is predicted to be 5.73 eV and 2.47 eV, respectively. It was found that the gap of |LUMO Fe-BTC -HOMO SO2 | is larger than the |LUMO SO2 -HOMO Fe-BTC | gap. Therefore, it might be expected that the electron transfer from the Fe center to the SO 2 molecule is the preferable route. NBO analysis showed that 0.247e was transferred from Fe-BTC to the adsorbed SO 2 molecule, suggesting that SO 2 acts as an electron acceptor molecule. Furthermore, the spin density localized on atoms of adsorbed SO 2 is increased as compared with the isolated SO 2 molecule. For S-bound configuration, the charge of adsorbed SO 2 molecule becomes positive after the adsorption on the Fe-, Ni-, Zn-and Pd-BTC paddlewheels, indicating the SO 2 molecule behaves as a nucleophile for donating an electron to the LUMO orbital of the paddlewheel. As a result, the net charge of SO 2 is + 0.118e, + 0.2 89e, + 0.153e, and + 0.036e for the adsorption on Fe-, Ni-, Zn-, and Pd-BTC paddlewheels, respectively. The atomic charge of metal is changed in accordance with the direction of electron transfer between SO 2 molecule and metal paddlewheels as shown in Table 1. Excepting the O-bound complex on the Fe-BTC paddlewheel, the adsorption of SO 2 on the paddlewheels results in a decrease of positive charge of the metal center. From NBO population analysis, it was found that there is no correlation between the adsorption energy of SO 2 and the amount of electron transfer between the SO 2 molecule and the substrate. From the frontier molecular orbitals (FMO) viewpoint (cf. Fig. 3), the highest occupied molecular orbital (HOMO) of SO 2 shows the participation of all three atoms of the SO 2 molecule. Either O atom or S atom of SO 2 molecule can act as an electron donor center to the available unoccupied orbital of the metal center. The adsorption energy of the S-bound configuration on M-BTC paddlewheels (M = Fe, Ni, Cu, and Zn) is much smaller than the adsorption energy of the O-bound configuration in the corresponding systems. NBO population analysis reveals that the NBO charge of oxygen atoms and the sulfur atom of SO 2 molecule is -0.758e and + 1.517e, respectively. From the electrostatic viewpoint, the metal center having a higher charge density tends to have a stronger interaction with a negative charge of the SO 2 oxygen atom. Therefore, the adsorption energy of SO 2 adsorbed on Ni-BTC, Zn-BTC, and Fe-BTC is significantly greater than that on Cu-BTC and Pd-BTC. On the other hand, the more positive charge the metal center, the weaker the SO 2 via its sulfur atom is adsorbed on the metal center. The weak interaction between the metal center and the SO 2 molecule in the S-bound configuration is due mainly to the electrostatic repulsion between the positive charge of metal and the positive charge of the SO 2 sulfur atom. It could be summarized that the interaction between the SO 2 molecule and the metal cluster is mainly governed by the electrostatic interactions, corresponding with the adsorption of styrene oxide on metal-paddlewheel clusters as reported previously (Sirijaraensre 2019(Sirijaraensre , 2021.

SO 2 sensing properties of M-BTC paddlewheels (M = Fe, Ni, Cu, Zn, and Pd)
The DOS analysis was used to provide more detailed data of electronic structure and the contribution of atomic orbitals to the chemical bonds. The DOS plots for the non-interacting system of five metal paddlewheels and SO 2 -adsorption complex in the O-bound configuration on five metal paddlewheels are illustrated in Figs. 4, 5, 6, 7, and 8. After the adsorption of SO 2 on the metal center of paddlewheels, the huge change in PDOS of adsorbed SO 2 happens for the adsorption on the Fe-, Ni, Cu, and Zn-BTC paddlewheels. For the adsorption on Fe-BTC, the peak at the energy of -6.16 eV, which is the HOMO state of the adsorption complex, shows the strong hybridization between the Fe d-orbitals and the orbitals of the SO 2 molecule. As a result, the peak of the energy at -3.60 eV in spin β, which is a π* antibonding orbital of SO 2 molecule, is reduced by half Fig. 3 The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of isolated SO 2 molecule at M06/6-311G(d,p)//M06-L/6-31G(d,p) level of theory     as compared to that of the non-interacting system, indicating that the electrons of the Fe-BTC paddlewheel partially fill in the LUMO orbital of SO 2 molecule (cf. Fig. 4). This leads to the elongation of the S-O bonds. The Wiberg bond order of S-O bonds is changed from 1.55 in the isolated molecule to 1.10 and 1.42, respectively. The result shows that a S-O bond of adsorbed SO 2 has a single bond character. Excepting the Fe-BTC system, the SO 2 adsorption introduces an impurity state, which becomes the LUMO state of the adsorption complex. Besides the electrostatic interaction between the Ni center of Ni-BTC and the oxygen atom of SO 2 , it was found that there is a σ-donation interaction from the SO 2 HOMO orbital to the empty Ni-3d orbital as shown in Fig. 5. The hybridization between two orbitals appears in the DOS pot at the energy of -12.44 eV. As a result, the Wiberg bond order of S-O bonds is reduced to be 1.38 and 1.61, respectively.
Unlike the SO 2 -adsorption on Fe-BTC and Ni-BTC, the overlap of SO 2 orbitals and d-orbitals of Zn and Cu cations creates a small hybridization at the energy of − 16.2 eV and − 12.3 eV, suggesting the small electron transfer exists between HOMO orbital of SO 2 molecule and unoccupied d-orbital of these paddlewheels (cf. Figs. 6, 7). The PDOS analysis is consistent with the quality of electron transfer between two fragments as shown in Fig. 9. Most of the electron density difference is observed at the adsorbed SO 2 fragment greater than the bond between the SO 2 oxygen atom and the metal center of paddlewheels. In the case of Zn-BTC system, the more polarization of adsorbed SO 2 molecule is mainly induced by the Zn(II) cation in the paddlewheel unit. These results suggest that the interaction between SO 2 and Zn-BTC in the O-bound configuration is more ionic than that found in the Fe-BTC system. The attachment of SO 2 to the Zn center of the Zn-BTC paddlewheel not only results in the distortion of the SO 2 molecule but also leads to modification of the TDOS of the system as compared to that of the non-interacting system. For the adsorption of SO 2 on the Pd-BTC (cf. Fig. 8), the PDOS plots for the s-, p-, d-orbitals of the metal atom of the non-interacting system and the SO 2 adsorption complex are almost identical, indicating that there is no strong orbital interaction between the adsorbed molecule and the metal center.
The change in the energy difference between the HOMO energy and the LUMO energy (E g ), affected by the adsorption of the targeted molecule is used to be a proper indicator to predict the gas-sensing performance of materials as described in the following equation (Behmagham et al. 2016;He et al. 2020;Peyghan et al. 2013): From the equation, a slight change in energy gap (E g ) results in a significant change in electronic conductivity (σ) of a system. Based on the calculated adsorption (2) exp −E g ∕k B T energies, the trend of energies for the SO 2 adsorption on the metal center via O-bound configuration is Pd-BTC < Cu-BTC < Ni-BTC < Zn-BTC < Fe-BTC, respectively. Even though, Fe-BTC has the strongest interaction with the SO 2 molecule, the E g of the Fe-BTC paddlewheel adsorbed by the SO 2 molecule is insignificantly changed by -0.02 eV (-0.8%) as compared to the E g of isolated Fe-BTC paddlewheel (cf. Fig. 4). Similarly, the E g of Cu-BTC after the adsorption of the SO 2 molecule on the Cu center is also identical to that of the isolated Cu-BTC paddlewheel (cf. Fig. 6). The calculated results indicate that Cu-BTC and Fe-BTC paddlewheels are not appropriate metal-paddlewheel-based centers for SO 2 sensors. Because of the strong interaction with the SO 2 molecule, it suggests that Fe-BTC is one of the candidate metal paddlewheels as an SO 2 adsorbent. From the population analysis, the E g of Ni-BTC and Zn-BTC paddlewheels is changed after adsorbing SO 2 gas. The E g is decreased by -0.17 eV (-5.3%), -0.67 eV (-18.0%), and -2.31 eV (-41.0%) for the adsorption of SO 2 on Pd-BTC, Ni-BTC, and Zn-BTC paddlewheels, respectively. Because the adsorption effect Fig. 9 Plots of the electron density difference (Δρ) isosurface with the isovalue of 0.002 e a.u. −3 for the SO 2 adsorption on the metal center of paddlewheels where Δρ = ρ(complex) -ρ(paddlewheel)ρ(SO 2 ). The electron accumulation and depletion regions are in green and blue colors, respectively of SO 2 on Pd-BTC is very weak as compared to that on Ni-BTC and Zn-BTC systems. Therefore, this effect has a small influence on the conductivity of the system. These calculated results showed that Ni-BTC and Zn-BTC paddlewheels display sensitivity to SO 2 gas as sensors.

Conclusion
Five metal paddlewheels namely, Fe-BTC, Ni-BTC, Cu-BTC, Zn-BTC, and Pd-BTC were selected to investigate the adsorption of SO 2 gas and the sensing performance by using the M06//M06-L level of theory. Interactions between the SO 2 molecule and the metal center of paddlewheels were determined through adsorption energies, structural parameters, and electronic structures. Excepting the adsorption on the Pd-BTC, the adsorption of SO 2 in the O-bound configuration is preferable rather than the S-bound configuration. The trend of adsorption energies of O-bound configuration is Fe-BTC > Zn-BTC > Ni-BTC > Cu-BTC > Pd-BTC. Even though, Fe-BTC is found to be an effective center for SO 2 capture, the electronic properties of Fe-BTC are slightly sensitive to this gas. Therefore, the Fe-BTC paddlewheel is not a candidate for the SO 2 -sensor devices. Among selected paddlewheels, Ni-BTC and Zn-BTC exhibit excellent sensitivity to detect sulfur dioxide (SO 2 ). The chemisorption of SO 2 gas on the metal center of Ni-BTC and Zn-BTC influences the electronic structure of paddlewheels. As a result, the E g significantly reduces about 18.0 and 41.0% upon the SO 2 adsorption process on the Ni-BTC and Zn-BTC paddlewheels, respectively. The calculated results will open the way for improving the SO 2 sensor based on metal paddlewheels.