Research Article www.acsami.org
First-Principle Framework for Total Charging Energies in Electrocatalytic Materials and Charge-Responsive Molecular Binding at Gas−Surface Interfaces Xin Tan, Hassan A. Tahini, Prasenjit Seal, and Sean C. Smith* Integrated Materials Design Centre (IMDC), School of Chemical Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *
ABSTRACT: Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. This phenomenon offers the possibility of using voltage to manipulate the strength of the binding interaction with the target gas molecule and thereby circumvent thermochemistry constraints, which inhibit achieving both efficient binding and facile release of important targets such as CO2 and H2. Stability analysis of such charge-induced molecular adsorption has been beyond the reach of existing first-principle approaches. Here, we draw on concepts from semiconductor physics and density functional theory to develop a first principle theoretical approach that allows calculation of the change in total energy of the supercell due to charging. Coupled with the calculated adsorption energy of gas molecules at any given charge, this allows a complete description of the energetics of the charge-induced molecular adsorption process. Using CO2 molecular adsorption onto negatively charged h-BN (wide-gap semiconductor) and g-C4N3 (half metal) as example cases, our analysis reveals that - while adsorption is exothermic after charge is introduced the overall adsorption processes are not intrinsically spontaneous due to the energetic cost of charging the materials. The energies needed to overcome the barriers of these processes are 2.10 and 0.43 eV for h-BN and g-C4N3, respectively. This first principle approach opens up new pathways for a more complete description of charge-induced and electrocatalytic processes. KEYWORDS: electrocatalytic materials, charge-responsive molecular binding, gas−surface interfaces, charging energy, density functional theory
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INTRODUCTION Charging of electrocatalytic materials can be an efficient method for modifying or modulating molecular interactions and reactivity at heterogeneous interfaces, including gas capture and separation processes and electrochemical and electrocatalytic reactions.1−13 Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. For instance, density functional theory (DFT) calculations have revealed that gas phase CO2 molecules undergo weak physisorption (through van der Waals interactions) on hexagonal boron nitride (h-BN) in the absence of charging. However, when a certain amount of excess electrons is added to h-BN, the strength of adsorption is dramatically enhanced.3 Similar charge-responsive molecular binding phenomena have also been reported for electrocatalytically switchable CO2 capture on pyridinic N-doped carbon nanotubes or graphenes,4 stacked h-BN and graphene (hybrid BN/G) nanosheets,5 and graphitic carbon nitride (g-C4N3) nanosheets.6 Moreover, our recent studies have shown that charge modulation can also facilitate H2 molecular adsorption on graphitic carbon nitride (g-C4N3 and g-C3N4) nanosheets,7 and H2 separation using graphdiyne as membrane.8 These © 2016 American Chemical Society
phenomena raise the possibility of designing charge-controlled switchable capture or storage schemes for these important targets. Typically, these studies have focused on the free energy change and the kinetics of reactions at the heterogeneous interfaces for a given fixed charge state, and then compared with that of neutral case.2−7 In detail, excess electrons are manually added into the simulation supercell, the adsorption energies and energy barriers for the capture/release processes in the presence of the given charge are calculated and then the results compared with the neutral case. Following this procedure where charge is arbitrarily added to or subtracted from the supercell, the capture/release processes in these electrocatalytic materials are generally predicted to be exothermic, with small or negligible energy barriers. The capture/release of the target gas molecules are hence predicted to occur spontaneously once excess electrons are introduced or removed, with the implication that these processes could in Received: February 19, 2016 Accepted: April 12, 2016 Published: April 12, 2016 10897
DOI: 10.1021/acsami.6b02117 ACS Appl. Mater. Interfaces 2016, 8, 10897−10903
ACS Applied Materials & Interfaces
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principle be controlled and reversed by switching on/off the charging voltage. However, charging of electrocatalytic materials, especially for a wide-gap semiconductor, inevitably requires energy input, which we call “charging energy” in this paper. We expect the charging energy for a poorly conductive material (such as, for example, the wide-gap semiconductor hBN) should be much higher than that of a good conductive material. Our position in this paper is that, because the total energy of adsorption is typically calculated (as above) for a system comprising the electrocatalytic material and the target gas molecules in the supercell, and the material must be charged to impart the enhanced binding effect, then a more informative treatment of the effect should include the charging energy as a component of the overall binding energy. Charging energy should be included in the total energy to more completely characterize the free energy change and the kinetics of the charge-induced molecular adsorption at the interface. To date, an appropriate first-principle method to compute the charging energy for the electrocatalytic nanomaterial has been lacking and there has been no way to identify what adding a certain amount of electrons into the simulation model corresponds to in terms of a real electrocatalytic system. Hence, there is a fundamental lack of theoretical understanding of how to describe charge-induced molecular adsorption with first principle modeling. Here, we develop a general first-principle theoretical approach that allows calculation of the charging energy for electrocatalytic materials, which is then integrated with the calculated adsorption energy of gas molecules to completely describe the energetics of the charge-induced molecular adsorption processes. Taking CO2 molecular adsorption on negatively charged h-BN (wide-gap semiconductor) and g-C4N3 (half metal) as examples, we predict that the charge-induced adsorption is not intrinsically spontaneous for either material, because these materials do not spontaneously self-charge due to the energy cost for that process. Further, we quantify the voltage needed, in the case of perfect electrical connectivity with the circuit, to make the process spontaneous.
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Research Article
THEORY
In the previous studies of the charge-induced molecular adsorption at electrocatalytic material interfaces, the adsorption energy of gas molecules, Eads(q), for a given charge q (q is number of excess electrons) is calculated in terms of DFT total energies as2−7
Eads(q) = E TM − gas(q) − E TM(q) − E Tgas(0) EM−gas T
EM T
(1)
Egas T
(q), (q), and (0) are the total energies of the where charged electrocatalytic material with adsorbed gas; the electrocatalytic material alone at the given charge q; and isolated neutral gas molecule, respectively. Here, Eads(q) is expressed relative to the total energies of isolated electrocatalytic material for a given charge q and isolated neutral gas molecule. One of the challenges here is that total energies from periodic DFT calculations cannot be directly compared as one changes the amount of charge in the simulation supercell. Total energy comparisons are valid for the system of the same number of electrons; hence, one can predict the adsorption energy of gas molecules for a given charge. However, this comparison cannot be made directly across different charge states of the supercell, which has prevented a complete description of the energetics of the charge-induced molecular adsorption process. In other words, for heterogeneous electrocatalytic molecular adsorption there has been no known way to predict an energy profile analogous to a “minimum energy reaction pathway” that is typically used to characterize the energetics and kinetics of chemical reactions, because one could not make comparisons of energies across different charge states of the materials. To frame the adsorption energy in a way that directly addresses the issues raised above, we define the adsorption energy of gas molecules for a given charge q as follows 0 Eads (q) = ETM − gas(q) − E TM(0) − E Tgas(0)
EM−gas T
EM T
(2)
Egas T
(q), (0), and (0) are respectively the total where energies of charged electrocatalytic material with adsorbed gas; isolated neutral electrocatalytic material; and isolated neutral gas molecule. As with the previous definition of eq 1, a more negative adsorption energy indicates a stronger binding of gas molecule to electrocatalytic material. However, in contrast to eq 1, we now reference the adsorption energy to the total energies of the neutral material and gas molecule. This of course demands that we formulate a correct approach to compare the total energies across different charge states of the system. Here, we introduce the charging energy for an electrocatalytic material, EM C (q), which is the total energy difference between the charged material and the neutral material:
METHODS
ECM(q) = E TM(q) − E TM(0)
Our DFT calculations employed the linear combination of atomic orbital and spin-unrestricted method implemented in Dmol3 package.14 The generalized gradient approximation (GGA) in Perdew− Burke−Ernzerhof (PBE) functional form15 together with an allelectron double numerical basis set with polarization function (DNP) were adopted. Since the standard PBE functional is incapable of giving an accurate description of weak interactions, we adopted a DFT+D (D stands for dispersion) approach with the Grimme’s vdW correction in our computations.16 The real-space global cutoff radius was set to be 4.1 Å. Here, we employed 4 × 4, 4 × 4, 2 × 1, and 2 × 2 supercells for h-BN, graphene, g-C3N4, and g-C4N3, respectively, with periodic boundary conditions in the x-y plane (see Figure S1a−d). The vacuum space was set to larger than 20 Å in the z direction to avoid interactions between periodic images. In geometry optimizations, all the atomic coordinates were fully relaxed up to the residual atomic forces smaller than 0.001 Ha/Å, and the total energy was converged to 1 × 10−5 Ha. For graphene, h-BN and g-C4N3, the Brillouin zone integration was performed on a (6 × 6 × 1) Monkhorst−Pack k-point mesh.17 The Brillouin zone integration was performed on a (4 × 8 × 1) Monkhorst−Pack k-point mesh for g-C3N4. The electron distribution and transfer mechanism of CO2 adsorption on electrocatalytic materials are determined using the Mulliken method.18
EM T (q)
(3)
EM T (0)
and are the total energies of the electrocatalytic where material for a given charge q and the neutral case, respectively. Replacing eq 3 in eq 2, the adsorption energy of gas molecules for a given charge q is now expressed as 0 Eads (q) = [ETM − gas(q) − E TM(q) − E Tgas(0)] + ECM(q)
= Eads(q) + ECM(q)
(4)
(q), then (q) − which provides a more If we can calculate complete analysis of the overall charging + adsorption energetics−can be obtained in terms of DFT total energies. Our formulation of the charging energy EM C (q) is based on the recognition that the total energy change due to an incremental change in the charge of the supercell is comprised of a charging term and a relaxation term. The charging term at fixed geometry, EM C_F(q), is described by the shift of the Fermi level of the system, while the relaxation term EM C_G (q) is simply the total energy change due to the geometry relaxation induced by the addition (or removal) of electrons. If the charge state changes from qi to qj, the charging energy is written as follows EM C
E0ads
ECM(qi → qj) = ECM_F(qi → qj) + ECM_G(qi → qj) 10898
(5)
DOI: 10.1021/acsami.6b02117 ACS Appl. Mater. Interfaces 2016, 8, 10897−10903
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RESULTS AND DISCUSSION To exemplify the implementation of the above method, we consider as examples four different monolayer 2D materials with contrasting band structure and conductivity:21−24 h-BN, graphene, g-C3N4 and g-C4N3 (Figure S1). On the basis of our computations, we find that, for flat h-BN and graphene, the optimized geometries of the neutral and the charged cases are the same. However, for buckled g-C3N4 and reconstructed gC4N3, the optimized geometries of the neutral and charged cases are different (Figure S2). Charge-Dependent Fermi Levels. Clearly, a fundamental quantity that must be calculated in order to implement eqs 11 and 12 is the charge-dependent Fermi level for the material at any given optimized structure along its charging pathway. This is evaluated by computing the work function for the charged material at the prescribed geometry, implementing the option within DMol3 to localize the necessary screening charge onto the slab. This latter option allows for a well-defined vacuum energy to be determined when moving a test charge away from the slab into the vacuum space of the supercell. This approach allows one to determine the variation of the system Fermi level as a function of charge added or subtracted from the supercell. Figure 1 shows the structurally relaxed, charge-dependent
where M(qj)
ECM_G(qi → qj) = E T
M(qi)
(qj) − E T
(qj)
(6)
Note that in this paper, EM(x) T (y) denotes the total energy of the material at a given charge y with the geometry fixed at the optimized j) i) (qj) and EM(q (qj) are structure for charge state x. Thus, in eq 6, EM(q T T the total energies of the material at a given fixed charge qj with the geometries set at the optimized structures for charge states qj and qi, respectively. Next, we focus our attention to determine EM C_F (qi → qj) in eq 5. Inspired by the idea that for a system of N electrons (N is an integer) in an external field v(r), it has been proved in the Kohn−Sham (KS) ∂E (N )
scheme that ∂vN = ϵf ,19,20 where Ev (N) is the total electronic energy and ϵf is KS frontier orbital energy, with f being the highest occupied molecular orbital (HOMO) if N is approached from N − n, or the lowest unoccupied molecular orbital (LUMO) if N is approached from N + n. Here, n is a fractional number and 0 < n < 1. Similarly, we can define
M(qi)
∂E T
M(qi)
(q ′)
= EF
∂q ′
(qi) (q′), where EM (q′) is F
the Fermi level energy of the material for a given charge q′ (qi ≤ q′ ≤ qj) with geometry fixed at the optimized structure for charge state qi. In principle, the Fermi level energy is expressed in absolute terms relative to the vacuum energy, but since we are calculating here the charging energy starting from the neutral material, we write this differential relationship relative to the Fermi level of the neutral materialEM F (0): M(qi)
∂E T
(q′)
Research Article
M(qi)
(q′) − E FM (0)
= EF
∂q′
(7)
Integrating eq 7, EM C_F (qi → qj) is now expressed as
∫q
ECM_F(qi → qj) =
qj
M(qi)
(q′) − E FM (0)]dq′
[E F
(8)
i
Thus, replacing relations 8 and 6 in 5 we obtain the charging energy from qi to qj as ECM(qi → qj) =
qj
∫q
Figure 1. Excess charge density as a function of EF or V (vs standard hydrogen electrode, SHE) of h-BN, g-C3N4, graphene, and g-C4N3. The gray dashed line denotes the zero-charge neutral case, and its intersection with each of the charging plots identifies the voltage of the intrinsic Fermi level for the material (indicated by the arrows).
M(qi)
(q′) − E FM(0)]dq′
[E F
i
M(qj)
+ (E T
M(qi)
(qj) − E T
(qj))
(9)
If we consider the charging process is broken into m steps from neutral to the charge state q, the charging energy can be written as follows:
Fermi levels of the four 2D materials computed in this way, expressed relative to the Standard Hydrogen Electrode voltage (−4.4 eV relative to the vacuum energy). In the case of perfect electrical connectivity to the circuit, this would also indicate the net charge imparted onto the materials in response to a charging voltage (V), because the system Fermi level (EF) will equilibrate to the voltage under such conditions. Charging h-BN and Graphene. First, we examine the charging energies for h-BN and graphene, in which the contribution of the relaxation term EM C_G (q) due to the geometry change is zero since the optimized geometries of the neutral and the charged materials are the same. Figure 2 shows the calculated charging energies of charging h-BN and graphene from neutral to charge density of 4.56 × 1014 and 4.76 × 1014 e−/cm2, respectively, by using different sized charging steps (i.e., different integration stepsizes). Clearly, as the charging increments decrease in size, the calculated charging energies are converged both for h-BN and graphene. When the integral intervals are smaller than about 2.85 × 1013 e−/cm2, the errors of the calculated charging energies of charging h-BN and
m−1
ECM (q) =
∑ ECM (qk → qk + 1) (q0 = 0 and qm = q) k=0
(10)
In the end, replacing 10 in 4, the adsorption energy of gas molecules on the electrocatalytic material for a given charge q could be expressed in terms of DFT total energies: m−1 0 Eads (q) = Eads(q) +
∑ ECM(qk → qk + 1) (q0 = 0
and qm = q)
k=0
(11) where ECM(qk → qk + 1) =
∫q
qk + 1
M(qk)
(q′) − E FM(0)]dq′
[E F
k
M(qk + 1)
+ (E T
M(qk)
(qk + 1) − E T
(qk + 1))
(12) 10899
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Figure 2. Calculated charging energies of charging (a) h-BN and (b) graphene from neutral to the charge density of 4.56 × 1014 and 4.76 × 1014 e−/cm2, respectively, by using different integral intervals (the unit is e−/cm2).
Figure 3. Calculated charging energies for (a) g-C3N4 and (b) g-C4N3 from neutral to the charge density of 4.80 × 1014 and 4.92 × 1014 e−/ cm2, respectively, by considering different number of steps m for the charging process and different fixed geometry structures (M (qi) or M (qj)) for calculating EM C_F (qi → qj) in each charging step (qi → qj).
graphene are less than 1.5 and 0.4%, respectively. In this paper, we use charging increments smaller than 1.5 × 1013 e−/cm2 (corresponding to 0.125 incremental additions of electrons for these four material supercells) to ensure the converged results. Importantly, we find that the charging energy of charging h-BN is much larger than that for graphene, for example, the charging energies for h-BN and graphene at the charge density of about 4.56−4.76 × 1014 e−/cm2 are 9.62 and 5.54 eV, respectively. This indicates the charging process of h-BN costs more energy and is more difficult than that of graphene. Considering h-BN is a wide-gap semiconductor21 while graphene is a zero-gap semiconductor,22 our calculation results are in accord with the expectation that charging material is more facile as the conductivity of the material increases in correlation with their electronic spectral density (see Figure S1e, f). Charging g-C3N4 and g-C4N3. Next, we implement our proposed method to calculate the charging energies for g-C3N4 and g-C4N3. In contrast to the cases of h-BN and graphene, introducing excess electrons for these two graphitic carbon nitride materials leads to significant changes in the optimized structures. Thus, the relaxation term EM C_G (q) is now nonnegligible and must be accounted for. Figure 3 shows the calculated charging energies of charging g-C3N4 and g-C4N3 from neutral to the charge density of 4.80 × 1014 and 4.92 × 1014 e−/cm2, respectively. Here, we consider four different charging pathways, which have different number of steps m for the charging process and use different fixed geometry structures M for calculating EC_F (qi → qj) in each charging step (charge state changes from qi to qj). The charging energies calculated by using different charging pathways are almost the same, with the errors smaller than 1.6 and 5.5% for g-C3N4 and g-C4N3, respectively. These indicate that the calculated charging energy is independent of the simulated charging pathway, with the
implication that we can choose the most simple and convenient charging pathway to obtain the charging energy in practical applications. We find that the charging energy for the semiconductor g-C3N4 is much larger than that for the halfmetal g-C4N3. For example, the charging energies for g-C3N4 and g-C4N3 at the charge density of about 4.80−4.92 × 1014 e−/cm2 are 4.36 and 1.09 eV, respectively. These indicate that charging of the semiconductor g-C3N423 costs more energy and is more difficult than for the conductive g-C4N3,24 in accordance with the expectation that charging a material is more facile as the conductivity of the material increases in correlation with its electronic spectral density (see Figure S1g, h). Adsorption Energies for Charge-Induced Molecular Adsorption. We now shift our attention to integrate charging energy with the adsorption energy of gas molecules in order to completely describe the energetics of the charge-induced molecular adsorption process. Previous predictive computational studies3−7 have found that charge-induced molecular adsorption can occur for gas phase CO2 molecules on several electrocatalytic materials, such as h-BN, pyridinic N-doped carbon nanotubes or graphenes, g-C4N3 and so on. These studies concluded (without considering the charging energies) that the CO2 capture/release processes at these electrocatalytic material interfaces are exothermic and spontaneous at room temperature with negligible energy barriers. Since the strength of CO2 adsorption is dramatically enhanced when a certain density of excess electrons is added to these adsorbents, but not when electrons are removed, we focus on negative charging in this paper. 10900
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Figure 4. Top (upper) and side (lower) views of the most stable structures of two CO2 molecules on h-BN at (a) zero, (b) moderate, and (c) high charge densities, and of four CO2 molecules on g-C4N3 at (d) zero, (e) moderate, and (f) high charge densities. The light magenta, blue, gray, and red balls represent B, N, C, and O atoms, respectively, and distances between the C atoms of CO2 and the B(N) atoms of h-BN (g-C4N3) are shown in the figures. 0 Eads (q) = E1(q) + E 2(q)
Considering the remarkable difference of electronic spectral density of these electrocatalytic materials, we expect the free energy change and the kinetics of the CO2 capture/release processes on these materials should be quite different if the charging energy is taken into account. Our calculations indicate that in fact pristine graphene does not display charge-induced binding of CO2, hence we do not consider graphene further here. Although g-C3N4 displays charge-responsive CO2 binding that is qualitatively similar to g-C4N3 as charge is added to the supercell, it is less favorable as a candidate material for electrocatalytic capture due to the significantly greater charging energies caused by its bandgap. To demonstrate the approach and show the contrasts caused by conductive versus semiconducting behavior, we therefore implement the method described above to investigate the charge-induced CO 2 molecular adsorption on wide-gap semiconductor h-BN and half metal g-C4N3 as contrasting examples. As demonstrated above, the charging energy is independent of the simulated charging pathway, hence we are at liberty to choose the most simple and convenient charging pathway to obtain the charging energy in practical applications. Here, we choose the fourth charging pathway in Figure 3, i.e., using m = 1 we charge the material in a single step at the optimized neutral geometry in order to calculate the charging energy M EM C_F(q) contribution to EC (q). Thus, the adsorption energy of CO2 molecules at electrocatalytic material interface, i.e., eq 11, can be rewritten in a simple way as follows:
(13)
E1(q) = Eads(q) = E TM − gas(q) − E TM(q)(q) − E Tgas(0) (14)
E 2(q) = ECM (q) =
∫0
q
[E FM(0)(q′) − E FM (0)]dq′
+ [E TM(q)(q) − E TM(0)(q)]
(15)
In this definition, E1(q) is the adsorption energy of CO2 molecules at a given charge q without considering the charging energy. E2(q) is the charging energy for isolated electrocatalytic materials calculated using m = 1. On the basis of our computations, we find that for the charge density smaller than 5.0 × 1014 e−/cm2, the maximum number of CO2 molecules that can be adsorbed on (4 × 4) h-BN and (2 × 2) g-C4N3 supercells are two and four, respectively. In order to investigate the charge-induced molecular adsorption on these two electrocatalytic materials, we put two CO2 molecules on (4 × 4) h-BN supercell and four CO2 molecules on (2 × 2) g-C4N3 supercell to calculate the adsorption energies of CO2 molecules at electrocatalytic material interfaces. The most stable structures of CO2 molecules on h-BN and g-C4N3 at zero, moderate and high charge densities are shown in Figure 4. For the neutral case (Figure 4a, d), all the CO2 molecules are weakly physisorbed both on h-BN and g-C4N3. As the charge 10901
DOI: 10.1021/acsami.6b02117 ACS Appl. Mater. Interfaces 2016, 8, 10897−10903
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stage of the charging process, where majority of CO2 molecules are physisorbed on electrocatalytic materials and E2(q) is dominant, and then deceases where majority of CO2 molecules are chemisorbed on electrocatalytic materials and E1(q) is dominant. For the high charge density cases, all the CO2 molecules are chemisorbed on electrocatalytic materials, and E0ads(q) is dramatically larger than for the neutral case. For example, E0ads(q) − E0ads(0) are −2.00 and −1.72 eV for h-BN with charge density of 4.56 × 1014 e−/cm2 and g-C4N3 with charge density of 4.92 × 1014 e−/cm2, respectively. These indicate that the charge-induced CO 2 adsorptions are exothermic both for h-BN and g-C4N3. We note that this adsorption energetics corresponds to the maximum coverage case: for lower coverage of CO2 on the surface, which would be modeled by larger supercells−the absolute magnitude of E1(q) becomes progressively smaller compared with the charging energy E2(q), such that in the limit of very low coverage the overall energy cost is simply the material charging term. The apparent energy barriers for the CO2 adsorption processes are 2.10 eV on h-BN and 0.43 eV on g-C4N3, corresponding to charge densities of 3.3 × 1014 e−/cm2 and 2.0 × 1014 e−/cm2, respectively. This indicates that we need to provide some energy both for the CO2 capture and the release processes - these processes can be controlled and reversed by modulating the charging voltage. In the case of ohmic connection with the circuit, the charging voltages needed to achieve these charge levels and thereby push the adsorption process over the barrier to become spontaneous can be read off from the charging curves in Figure 1. This gives nominal threshold voltages of 2.5 V for h-BN and 0.4 V for g-C4N3. Importantly, considering the large energy barrier of the CO2 adsorption on h-BN, we expect that the conductive g-C4N3 offers a more economically viable sorbent material for electrocatalytically switchable CO2 capture. It should also be noted that imperfect electrical connectivity for semiconductors can lead to much higher effective voltages being needed to charge the materials; hence the effective threshold voltage for the charge-induced capture of CO2 on the wide bandgap h-BN could in fact be much higher than the estimate above.
density becomes moderate (Figure 4b, e), half of the physisorbed CO2 molecules change from physisorption into chemisorption, and significant amounts of excess electrons transfer from electrocatalytic materials to chemisorbed CO2 molecules (see Figure S3). When the charge densities of electrocatalytic materials are high (Figure 4c, f), all the CO2 molecules are chemisorbed both on h-BN and g-C4N3, and more excess electrons transfer from electrocatalytic materials to chemisorbed CO2 molecules (see Figure S3). More detailed information on CO2 molecules on h-BN and g-C4N3 with different charge densities are shown in the refs 3 and 6. In Figure 5, we show the calculated E1(q), E2(q), and E0ads(q) of CO2 molecules on h-BN and g-C4N3 with different charge
Figure 5. Calculated E1(q), E2(q), and E0ads (q) of CO2 molecules on (a) h-BN and (b) g-C4N3 with different charge densities. The yellow, green, and cyan regions in (a) denote 0, 1, and 2 CO2 chemisorption. The yellow, green and cyan regions in (b) denote 0, 2, and 4 CO2 chemisorption. The energy barriers of the charge-induced CO2 adsorption on these two electrocatalytic materials are shown in the figures. The empty symbols in a denote the calculation data that we only put one CO2 molecule on h-BN.25
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CONCLUSIONS In summary, we have developed a general first-principle theoretical approach that allows calculation of the chargingenergy component of the total supercell energy for charging of electrocatalytic materials and thereby facilitates a more complete accounting for the energetics of charge-induced molecular adsorption at interfaces. Examples are given from studies of the charge-induced CO2 adsorption on h-BN and gC4N3, which have significantly different conductivity. Contrary to the previous studies, our results show that the chargeinduced CO2 adsorptions are not spontaneous for either h-BN or g-C4N3. Although both of the adsorption processes become exothermic when sufficient charge density is imparted to the material, the energies needed to overcome the barriers of these processes at saturation coverage are 2.10 eV for h-BN and 0.43 eV g-C4N3. Our results are in accord with the expectation that charging material is more facile as the conductivity of the material increases in correlation with its electronic spectral density, and suggest that the conductive g-C4N3 is an economically viable sorbent material for electrocatalytically switchable CO2 capture.
densities. Generally, the tendencies of E1(q), E2(q), and E0ads(q) of CO2 molecules are quite similar both for h-BN (Figure 5a) and g-C4N3 (Figure 5b). On the one hand, E2(q) increases constantly from neutral to high charge density more than 4.5 × 1014 e−/cm2 because of the constant increase of material’s Fermi level during the charging process. Note that the E2(q) of h-BN are much larger than that of g-C4N3 because of the conductivity of half metal g-C4N3 is much better than that of wide-gap semiconductor h-BN. On the other hand, E1(q) are almost unchanged in CO2 physisorption region, but increase markedly (with negative sign) starting from the chemisorption region. When the charge densities are high and all the CO2 molecules are chemisorbed on electrocatalytic materials, E1(q) is much larger than that of the neutral case. Because E0ads (q) is the sum of E1(q) and E2(q), E0ads (q) increases at the initial 10902
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ACS Applied Materials & Interfaces
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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/acsami.6b02117. The geometries and the calculated band structures of neutral adsorbents, the optimized geometries of the neutral and the charged adsorbents, and the electron distributions of CO2 molecules on adsorbents (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was undertaken with the assistance of resources provided by the National Computing Infrastructure (NCI) facility at the Australian National University; allocated through both the National Computational Merit Allocation Scheme supported by the Australian Government and the Australian Research Council grant LE120100181 (“Enhanced merit-based access and support at the new NCI petascale supercomputing facility, 2012-2015).
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DOI: 10.1021/acsami.6b02117 ACS Appl. Mater. Interfaces 2016, 8, 10897−10903
