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Exploring the hidden catalyst from boron pnictide family for HER and OER
international journal of hydrogen energy xxx (xxxx) xxx
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Exploring the hidden catalyst from boron pnictide family for HER and OER Trupti K. Gajaria a, Basant Roondhe a, Shweta D. Dabhi b, Prafulla K. Jha a,* a
Department of Physics, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, 390002, Gujarat, India b P. D. Patel Institute of Applied Science, Charotar University of Science and Technology, CHARUSAT Campus, Changa, 388421, Gujarat, India
highlights
graphical abstract
Nanostructured boron phosphide for HER and OER catalyst. Indirect to direct bandgap transition under confinement effect. Modulation of catalytic activity by means of substitutional doping. Toxic and expensive metal-free HER catalyst.
article info
abstract
Article history:
A systematic investigation of catalytic activity of boron phosphide nanowire (BP NW) to-
Received 15 April 2019
wards over-all water-splitting reaction has been performed by evaluating the hydrogen
Received in revised form
evolution reaction (HER) and oxygen evolution reaction (OER) activities. Intended to the
8 September 2019
mentioned aim, we have utilized Kohn-Sham formulated extensively popular ab initio
Accepted 12 September 2019
method based on density functional theory (DFT). The structural and electronic properties
Available online xxx
of the BP NW are computed and compared with its bulk phase. We observe dramatic indirect to direct bandgap transition with pronounced energy gap after introducing two-
Keywords:
dimensional confinement that is akin to the other reported III-V NWs. The calculated
Boron phosphide nanowire
partial density of states with van Hove singularity also confirms the same. Owing to its
Density functional theory (DFT)
moderate bandgap value, the applicability of the BP NW as an HER/OER catalyst is assessed
Hydrogen evolution reaction
by computing the site dependent HER/OER activities. Our computation on Gibbs free energy
Oxygen evolution reaction
for the case of hydrogen adsorption with 1.19 eV magnitude gives better results; whereas in case of OER, the results with higher magnitude of Gibbs energy implicate over binding of oxygen with adsorbent thus revealing non-feasible desorption of oxygen from adsorbent. Significant perturbation in electronic states of NW under hydrogen adsorption confirms high sensitivity of BP NW for hydrogen adsorption. Further, the effect of substitutional doping on HER and OER activities suggests that the doped NW shows poor HER activity in
* Corresponding author. E-mail address: [email protected] (P.K. Jha). https://doi.org/10.1016/j.ijhydene.2019.09.107 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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contrast to the site-dependent better OER activity in case of Ga doped BP NW. The present BP NW shows potential as an HER catalyst owing to its lower adsorption and Gibbs free energies (1.07 and 0.84 eV), as compared to previously conventionally utilized III-V NWs. Henceforth, we believe that the present study would serve as a blueprint for the researchers to design and develop toxic and/or metal-free catalyst that can be utilized for efficient water-reduction. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Extensive efforts are being made by researchers worldwide to overcome two major concerns; first one to bring down the ever-growing energy crises and the other one is pollution control. The eco-friendly pathways for generating and storing clean and green energy are being developed by means of exploring novel materials possessing exotic properties like high mechanical strength [1], tuneable electronic properties with wide range of bandgaps [2] and cost-effective production rates [3]. Available literature on III-V compounds from the modern periodic table suggests that the nitrides and arsenides have been studied extensively due to their versatile optoelectronic and piezoelectric properties [4,5]. However, the IIIphosphides have received scant attention. Similar to the conventional III-V compounds, the phosphides with aluminium, gallium and indium as cations possess direct bandgap in their bulk cubic phase [6]; while nano-structuring them alters the nature and magnitude of the bandgap in some of the phosphides arising due to distinct geometric alignment and hexagonal symmetry [7,8].The reported electronic properties of similar nanostructures suggest their probable applicability in water splitting and catalysis [9e15]. The successful fabrication of Wurtzite (WZ) gallium phosphide nanowires (NWs) possessing direct electronic bandgap with 2.1 eV is reported by Assali et al. [8]. The authors have also demonstrated similar trend for AlP. Apart from the generalized optoelectronic, photovoltaic and nanoelectronic applications, very scares studies are found on III-V compounds showing their probable applicability as water-splitting catalysts under reduced dimensions [16e21]. In recent past, the demonstration of gallium phosphide nanowire as an efficient water reducer was presented by Anthony et al. [16]. The authors have studied the GaP in nano regime so as to reduce the production cost with efficient electronic bandgap engineering, that can be useful for electrochemical water-splitting. Further, the authors claim reduction in electrical resistance followed by enhancement in the current-voltage profiles og GaP NW due to multistep platinum deposition. An interesting study on another III-V compound, InGaN is reported by Sheng et al. [17], showing the importance of indium concentration on the photo-electrochemical water oxidation efficiency. They report 25% efficiency in solar to hydrogen generation with high magnitude of current densities due to admired electronic bandgap of 1.7 eV. The GaAs compound generally studied for optoelectronics [22], photovoltaics [23] and nanoelectronics [24] based applications has
been experimentally studied under reduced dimensions as a photoanode for photo-electrochemical water oxidation by Zeng et al. [18]. They have addressed one of the major issues of corrosion caused by ambience around the catalyst by coating the nanowire surface with nickel oxide (NiO) layers. The system InGaN has also been studied under heterostructure geometry by Kibria et al. [19]. The authors report over-all watersplitting using multiband InGaN/GaN based nanowire heterostructures that are not only environmentally benign but also act as multipurpose photo-catalyst. The unsaturated dangling bonds present at the surface of the NW are responsible for false edge states, so have to be treated carefully for accurate estimation of electronic transport through the material. The effect of the surface passivation on electrochemical watersplitting efficiency is demonstrated by Varadhan et al. [20]. It is proved that the surface treated GaN NWs show photon to current efficiency of 18.3%, whereas, the untreated NWs possess only 8.1% efficiency with significantly low electrochemical stability of less than 4 hours. GaN based NWs are also utilized as cross-linkers between MoS2 and Si for fabricating heterostructure photo-electrode [21]. The authors report first ever noble metal free Si based photo-electrode that produces benchmarking photo-current of 40 mA/cm2 at 0 V. Last but not the least, we found a report on the locus of our study-BP, which in its cubic zinc blende bulk phase has been studied by Shi et al. [25] as a metal free photo-catalyst for hydrogen evolution. Also, being one of the robust materials, the authors have presented the corrosion free property of bulk BP as an active HER catalyst. Compared with the above reports, the BP in bulk phase [25] is found to be simplest metalfree compound that shows good HER activity. However, the BP (cubic compound that prefers to get crystalized in hexagonal phase under confined dimensions) at nanoscale has not been reported for over-all water-splitting activity. Our major goal in the present study is to address the applicability of nanostructured boron phosphide (BP) as an active hydrogen/oxygen evolution reaction (HER/OER) based metal-free active catalyst. Although, the availability of boron and phosphorous in earth’s crust is not abundant, yet can be sufficiently utilized under reduced dimensions to cut down the material need and production cost. The reduction in material dimensions yields one more advantage of engineering the material properties according to the application. With regard to the above concerns, we have first studied the structural and electronic properties of WZ BP in NW form, since its other colleagues from III-V group possess WZ geometry under reduced dimensions, while cubic structure is more stable in bulk phase [26,27]. Further computations on BP
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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NW are carried out to explore its capacity as an efficient catalyst yielding good hydrogen evolution reaction (HER) and/ or oxygen evolution reaction (OER) activities. In addition, improvement in the catalytic activity of the BP NW is explored by means of doping the NW with other group-III elements. Two major advantages of nanostructured BP, one being its corrosion-free nature with second advantage of low material consumption in-contrast to its bulk form are prime motivations for the present study. The study is organized in following order: the detailed methodology adopted for present study is explained in Computational details section followed by the obtained results with respective discussion in Results and discussion section. The brief summary of the study is presented in Conclusions section.
Computational details In the present study, we have computed the structural and electronic properties of BP NW under the framework of firstprinciples density functional theory (DFT) [28] as implemented in plane wave pseudopotential simulation package [29]. For computation of the proposed properties of the BP NW, we first optimized the Wurtzite (WZ) boron phosphide (BP) in bulk phase by minimizing the total energy of the system selfconsistently employing quasi-Newton Broyden-FletcherGoldfarb-Shanno algorithm. The atomic species were described as a combination of frozen cores and contributing valence electronic orbitals using well-suited local density approximated (LDA) pseudopotentials [30] with PerdewZungar parameterization [31]. The system parameters were optimized by converging the total energy of the system with respect to cell shape, cell size and ionic positions [32,33]. The kinetic energy and charge density cut-offs for wavefunction of 1630 and 16300 eV for bulk, and 680 and 6800 eV for nanowire unit cells were sufficient to satisfy the convergence threshold criteria of 108 eV and 103 eV/atom for energy and force respectively. Integration of Brillouin zone along the highsymmetry points was done using Monkhorst-Pack scheme [34] based dense k-mesh grids of 15 15 x 10 for bulk and 1 1 x 10 for nanowire unite cells. After successfully optimizing the bulk unit cell of the WZ BP, the same structure was utilized to make 5 5 x 1 supercell, and the NW was constructed from it by cutting the supercell along 0001 direction keeping the desired diameter and geometry in reference. The next step was to assess and modify the unsatisfied valencies of the surface atoms that give rise to surface dangling bond effect. The reports suggest, the proper saturation of the surface dangling bonds is necessary to remove the excess edge states that result in weak prediction of the electronic band structures [35]. To saturate the excess dangling bonds, the pseudo hydrogen adatoms were masked on the NW surface through passivation process and the finally obtained NW was utilized for optimizing final structural geometry. After successfully optimizing the structural parameters of the BP NW, we performed electronic band structure calculations to get insight to the electronic transport through the NW. Further, the contribution of individual atomic species to the electronic transport is verified by computing partial density of states (PDOS) of the
3
system. To analyse the hydrogen evolution reaction (HER) activity of BP NW, the adsorption and Gibbs free energies of the NW are computed by adsorbing hydrogen on different sites of the NW surface and the magnitudes of adsorption and Gibbs free energy were evaluated using the equations given below. H=ONW
NW EH=O ENW ads ¼ EComplex E
(1)
H=ONW
The terms EComplex ; ENW and EH=O represent the energies of the hydrogen/oxygen adsorbed pristine/doped NW, pristine/ doped NW without adsorption and half of the energy of isolated hydrogen/oxygen molecule. DGH=O ¼ DEH=O þ DEZPE TDSH=O
(2)
The term DEZPE which lies within the range 0.0e0.04 eV [36e38] for hydrogen and is nearly zero for oxygen [38], is the zero point energy difference of hydrogen/oxygen in adsorbed state and gas phase, and the third term DSH=O is the difference in the entropy of hydrogen/oxygen under adsorbed state and gas phase, and under experimental conditions, the value of TDSH=O is about 0.4 eV for hydrogen [37,38], and for oxygen, the entropies of adsorbed and gas phases are 0 and 0.631 eV respectively [37]. Considering the reported magnitudes, the terms DEZPE and TDSH=O reduce to 0.24 or 0.33 for the case of hydrogen or oxygen adsorption respectively. The effect of hydrogen/oxygen adsorption on electronic transport properties is also assessed by re-computing the PDOS of the NW after subjecting to hydrogen/oxygen adsorption on different sites. From optimizing the structure to computation of the proposed properties, the van der Waals dispersion correction term (DFT-D2) [39] was included for better prediction of the adsorption energy. The inclusion of van der Waals correction improves the binding in the case of low dimensional boron based III-V semiconductors [40]. Furthermore, for understanding the effect of substitution defect/doping on the electronic and adsorption response of the BP NW, we have re-computed the aforementioned properties after doping the NW with Al and Ga atoms individually.
Results and discussion Structural properties As mentioned in above section, for constructing the boron phosphide nanowire (BP NW) in 0001 direction, we first constructed and optimized the bulk unit cell of WZ BP crystal under self-consistent total energy minimization process. The optimized lattice parameters, bond lengths and anion-cation angles of the bulk unit cell are listed in Table 1. It can be observed from Table 1 that the present LDA predicted structural parameters for bulk BP are in good agreement with the previous reports [41]. After successfully optimizing the bulk BP, we constructed BP NW (see Computational details section) and re-optimized the initial parameters like kinetic energy cut-off, charge density cut-off, k-mesh by minimizing the total energy of the system, within the mentioned threshold criteria.
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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Table 1 e Structural parameters of WZ BP in bulk and NW configurations. System
BP (bulk) BP (NW)
Bond Length ( A)
No. of Atoms
Lattice Parameter
B/P/H
C ( A)
BeP
BeH
PeH
BePeB
PeBeP
2/2 13/13/18
5.225 5.257
1.94 1.93
e 1.21
e 1.41
109.2 109.9
109.2 104.7
The final step was to keep the initial parameters fixed to finally compute the structural parameters, which were obtained by totally relaxing the NW with respect to its cell shape, size and ionic positions. The computed structural parameters of BP NW enlisted in Table 1 show deviation in cation-anion distances and angles. In the case of bulk BP, the cationanion angles are same and consistent in all directions; whereas in the case of BP NW, the cation-anion angles show deviation of 5 . This might be the result of inclusion of hydrogen adatoms on the NW surface that re-assemble themselves and the surface atoms in such a way that the total energy of the system is minimized. The top and side views of optimized BP NW are presented in Fig. 1. As it can be viewed from Fig. 1(a), the hydrogen adatoms have settled in a buckled geometry so as the surface energy of the system gets minimized. For assessing the stability of the NW, we have computed the formation energy of the BP NW utilizing the equation given below: ¼ ENW F
ENW Tot mmB nmP lmH mþnþl
(3) NW
where, the terms ENW F and ETot represent the formation and total energies of NW and mB; mP and mH are chemical potentials of the boron, phosphorous and hydrogen with their respective number of atoms symbolized as m, n and l present in the NW unit cell. The computed formation energy of the NW is found to be 5.61 eV; wherein the negative sign and low magnitude
Angle
Formation Energy (eV) e 5.61
indicate that the NW is stable and the formation is exothermic in nature.
Electronic properties The electronic band structure of the system gives insight to the pathways of undergoing electronic transport in the Brillouin zone picture, and the density of states gives an account on the occupied and unoccupied electronic states of the system along with the spatial distribution of individual electronic orbitals of the constituent atoms. We have computed the above mentioned properties for understanding the electronic picture of the system, and the corresponding numeric results are tabulated in Table 2 with the pictorial results depicted in Fig. 2. Fig. 2(a) clearly indicates that the locations of the valence band maxima (VBM) and conduction band minima (CBM) are located at the G point-the centre of the BZ, which reveals direct nature of the electronic band gap, similar to the report on WZ GaP NW [8], but is in contrast to other NWs from the same family [26,40]. The gap in present case is 2.96 eV (see Table 2), which is higher than the bulk BP. Further, it should be noted that the nature of the electronic band gap is indirect in the case of bulk phase that transforms into direct type on introducing nano-structuring like other compounds [8,42]. These results are in accordance with the so called quantum confinement effect. The term effective mass is related to the dynamic mass of the charge carrier, which strongly depends on the position of the charge carrier and the considered direction of the carrier transport in the crystal. Considering the above two facts, we have tried to understand the dimension dependent carrier effective mass, and for this, the electron effective mass for bulk and NW systems in 0001 direction is computed (see Table 2) by extracting the edge states from the respective electronic band structures and fitting them with second order polynomial. From the computed values of effective masses, it can be concluded that subjected to gigantic modifications in the transport properties of the system, the electron effective mass show rapid enhancement due to confined geometry. Further, substitution of other group-III elements in place of boron,
Table 2 e Electronic band gap ðEg Þ, electron effective mass ðm*e Þand nature of band gap of Boron Phosphide in bulk and NW configurations. System
Fig. 1 e Top (a) and Side (b) views of pristine BP NW.
BP-bulk BP-NW BP-Al NW BP-Ga NW
Eg (eV)
m*e (me)
Nature
0.84 2.96 2.81 2.87
0.41 9.74 4.37 5.04
Indirect Direct Direct Direct
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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Fig. 2 e (a) Electronic Band Structure with corresponding density of states (DOS) and (b) partial electronic density of states (PDOS) of BP NW.
results in reduction of band gap from 2.96 eV for pristine NW to 2.81 eV and 2.87 eV for Al and Ga doped BP NWs respectively, keeping the direct bandgap nature unaltered. The PDOS plot presented in Fig. 2(b) shows the individual atomic orbital contribution to the electronic properties in form of the characteristic van Hove singularities. As observed in the figure, the 3p-orbital of the phosphorous atom dominates in the CBM regime, whereas its contribution to the VBM regime is almost equal to that of boron atom’s 2p-orbital. The significant contribution of 3s-orbital of phosphorous atom is observed in CBM regime. Further, the orientation of Fermi energy located near the VBM regime replicates the p-type semiconducting nature of the BP NW.
Catalytic activity of BP NW After understanding the structural properties and electronic transport of BP NW, we now focus on our prime goal of the present study. As mentioned earlier, the water-splitting process is sub-divided in two reactions, generally known as HER and OER. The catalytic activity of BP NW for over-all watersplitting is assessed by means of studying the HER and OER activities of NW under hydrogen and oxygen adsorption. The
strength of the adsorption is measured by means of computing the adsorption energy and corresponding Gibbs free energy of the system utilizing equations (1) and (2) (see Computational details).
Hydrogen evolution reaction (HER) activity As mentioned, hydrogen is the cleanest source of energy and one of the best ways to produce/obtain clean energy is to break the water molecules in to its constituents: hydrogen and oxygen molecules. The so-called process of watersplitting is the green and clean way for producing hydrogen that can be further utilized as a fuel. To carry out the HER activity at ambient conditions, the time and production rate are not feasible; and to overcome this, the materials that can act as catalysts are utilized, which just modifies the reaction rate without participating in the reaction. The conventionally utilized catalyst for this purpose is made up of the expensive metals like Pt, Pd, Au etc., which are not feasible at larger scale owing to their production expenditure. To overcome this disadvantage, we need to design/engineer materials with required catalytic activity with low production rates. For this purpose, BP acquiring much more portion of earth’s crust than the conventional
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expensive metals is being studied for assessing its performance as HER catalyst. To assess the HER activity of BP NW, we have adsorbed the H atom (green coloured atom) on two distinct sites of the NW surface. The top and side views of the hydrogen adsorbed BP NW are presented in Fig. 3(a, b). As can be seen from the figure, two distinct bridge sites of the NW surface are selected for studying the HER activity. The interaction of hydrogen with NW surface was analysed by fully relaxing the NW after adsorption of hydrogen; the final geometry of the NW subjected to hydrogen adsorption is depicted in Fig. 4, wherein Fig. 4(a), (b) and (c), (d) are the top and side views of NW with hydrogen adsorbed on site-A and site-B respectively. Also, the modifications in the cation-anion distances and angles subjected to hydrogen adsorption are tabulated in Table 3. For better HER activity, it is desired to have a catalyst that does not over bind the hydrogen so as to make desorption step possible, else the target of energy generation cannot be achieved. As it can be observed from Fig. 4 (a, b) and Table 3, while hydrogen is introduced on site-A of the NW surface, it gets adsorbed on the surface of the NW through chemisorption process and strongly binds with the surface cation. The chemisorption of hydrogen causes perturbation in the NW surface geometry, which can be clearly seen in Fig. 4(a, b). The observed BeP bond length near site-A which was 1.93 A in case of pristine NW, gets elongated to 2.89 A and the corresponding angles BePeB and PeBeP are modified to 119.6 and 83 from 109.9 to 104.7 , respectively. The corresponding computed adsorption and Gibbs free energies of the system are found to be 1.4 and 1.19 eV, respectively which
Fig. 3 e Top and side views of BP NW with hydrogen adsorbed on two distinct surface sites (a, b) Site-A and (c, d) Site-B.
Fig. 4 e Top and side views of BP NW subjected to hydrogen adsorption on Site-A (a, b) and Site-B (c, d), respectively.
replicates that the HER activity of BP NW is within the feasible range. However, in the second case, wherein the hydrogen is adsorbed near the concave surface of the BP NW, we found totally different adsorption mechanism. The B-site of the NW is found to be more suitable for adsorption due to physisorbed hydrogen on the NW surface (see Fig. 4(c, d)) without any chemical bond formation between the adsorbent and adsorbate. There is almost negligible perturbation in the surface geometry of the NW as compared to the A-site adsorption. This observation can be confirmed by the anion-cation distances and angular magnitudes enlisted in Table 3, which are almost similar to that of the pristine NW. Furthermore, the energies of adsorption and Gibbs function replicate better HER activity of BP NW in this case 1.07 and 0.83 eV, respectively. Henceforth, we conclude that the Bsite is the most preferential site for hydrogen adsorption keeping the geometry and morphology of the adsorbent unperturbed with desirable magnitudes of adsorption and Gibbs free energies. To understand the modifications in the electronic transport of BP NW subjected to site dependent hydrogen adsorption, we have computed the partial electronic density of states (PDOS). The graphical comparison of computed PDOS for pristine and H-adsorbed BP NW is depicted in Fig. 5. As it can be clearly observed from Fig. 5(a), (b) and (c) that the pristine NW possesses finite electronic bandgap with moderate value of 2.96 eV. However, when hydrogen is introduced in the
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Table 3 e Computed bond lengths and angles of BP NW subjected to hydrogen adsorption at different sites of NW surface. Bond Length ( A)
Configuration BeP Site-A Site-B
2.89/1.94 2.11/1.95
BeH 1.23 1.21
D ( A)
Angle PeH 1.42 1.43
PeBeP
83 /106.4 106.9 /105.4
BePeB
H-NW
119.6 /110.4 103.3 /110.4
1.22 1.55
Fig. 5 e Partial density of states (PDOS) of BP NW in (a) pristine, and H-adsorbed on (b) Site-A, and (c) Site-B, respectively.
system at A-site (see Fig. 4(a , b)), the VBM and CBM states get markedly modified and excess states at Fermi level are observed because of chemisorbed hydrogen. The modification in the hybridized states of BP NW results in metallic nature of the adsorbent. On the other hand, the adsorption of hydrogen at B-site results in much more pronounced electronic states at Fermi level showing enhancement in metallic character of the adsorbent. After unravelling the complete picture on hydrogen adsorption on two distinct sites of BP NW surface, we tried to understand the effect of substitutional doping on the hydrogen adsorption properties. The results on adsorption
energy and Gibbs free energy (see Supplementary Material Table S2) show that introducing Al and Ga atoms on B site of the NW, the magnitude of the adsorption and Gibbs function increases showing less compatibility of BP NW as HER catalyst under doped environment.
Oxygen evolution reaction (OER) activity For estimating over-all catalytic activity of BP NW for watersplitting, besides HER activity, we have also computed the oxygen evolution reaction (OER) activity of NW by adsorbing oxygen on two different sites of the BP NW surface (see Fig. 6). Similar to the hydrogen adsorbed NW, the adsorption and
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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Fig. 6 e Top and side views of BP NW before adsorbing oxygen atom on site-A (a, b) and site-B (c, d) respectively.
Gibbs free energies in this case were computed using equations (1) and (2), respectively and, the corresponding values with modified bond-lengths and angles are presented in Table 4. Fig. 7 represents the top and side views of the optimized geometries of BP NW with oxygen adsorbed on two distinct sites. We can observe that after adsorption, the oxygen atom in both cases migrates towards the NW surface and finally under chemisorption process, gets attached with boron atom in case of A-site and phosphorous atom in case of B-site by displacing the surface hydrogen atom. The computed adsorption and Gibbs free energies of BP NW under oxygen adsorption on A-site are 8.26 and 7.93 eV and 8.31 and 7.98 eV for B-site, respectively. These magnitudes are too negative and show strong bonding between oxygen and adsorbent. Further, the angles between so-formed oxygen centred geometry is 112.9 (BeOeH) and 109.2 (PeOeH) for sites A and B respectively. It is noteworthy that the cation-anion bond lengths in both cases are slightly modified as compared to pristine NW, but the cation-anion angle in case of B-site shows significant increase. Further, the observed OeH bond
Fig. 7 e Top and side views of BP NW subjected to oxygen adsorption on Site-A (a, b) and Site-B (c, d) respectively.
for A- and B-sites respectively, lengths are 0.97 and 0.98 A which are in agreement with the standard OeH bond length (0.98 A) [43]. The effect of strong bonding between oxygen and the adsorbent on the electronic transport of the system is analysed by computing partial density of states (PDOS). The PDOS plots of BP NW with oxygen adsorbed on site-A (Fig. 8(b)) and site-B (Fig. 8(c)) show similar semiconducting behaviour as pristine BP NW, with shift in electronic levels that further modify the electronic band gap from 2.98 eV (pristine) to 2.72 eV (site-A) and 2.77 eV (site-B). The clear splitting of oxygen electronic level into its bonding and anti-bonding states due to chemisorption (see Fig. 8(b) and (c)) supports our calculated values of adsorption energy and confirms the presence of strong bonding between oxygen with the NW. Further, the markable dominance of oxygen bonded atom’s porbital (boron in case of site-A, and phosphorous in case of site-B) in the CBM regime indicates that the key-role is played by oxygen atom in the electronic transport.
Table 4 e Computed bond lengths and angles of BP NW subjected to oxygen adsorption on different sites of NW surface. Bond Length ( A)
Configuration
Site-A Site-B
D ( A)
Angle
BeP
BeH
PeH
PeBeP
BePeB
BeOeH/PeOeH
O-NW
OeH
1.94 1.91
1.21 1.22
1.41 1.41
103.3 109.8
110.7 110.2
112.9 109.2
1.42 1.61
0.97 0.99
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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Fig. 8 e Partial density of states of BP NW in (a) pristine and O adsorbed on (b) Site-A and (c) Site-B respectively.
The shifting of Fermi level form VBM to CBM regime shows dramatic reversal of the electronic nature of the BP NW from p-type (pristine NW) to n-type (O-adsorbed NW) due to addition of excess electrons to the system. From the results of the adsorption energy and electronic states, we conclude BP NW not to be a good catalyst for water oxidation, as it over binds the oxygen atom making desorption procedure non-feasible and complicated. Further, similar to hydrogen adsorption case, we have also studied the effect on oxygen adsorption properties of BP NW by means of Al and Ga doping. Surprisingly, in this case Al doped NW, the results does not show any considerable modulation in adsorption profile (see Supplementary Table S2), with contrast to the Ga doped NW, in which the adsorption of oxygen on site-B of the NW results in dramatic decrease (~35%) in the adsorption and Gibbs free energies of the NW. These results indicate improvement in OER activity of BP NW can be achieved by means of increase in Ga concentration and surface site-engineering.
Conclusions In summary, the first-principles density functional theory based computations of structural and electronic properties of newly designed WZ boron phosphide nanowire have been performed and the results are compared with computed parameters in its bulk counterpart to assess the confinement effect. Our structural parameters in bulk phase agree well with the available data, whereas due to lack of availability of experimental or theoretical data on BP NW, we could not provide comparative results. We observe indirect to direct electronic bandgap transition with enhanced gap magnitude of BP subjected to strong quantum confinement effect. The assessment of site dependent HER and OER activities of BP NW has been carried out, and we have found the concave site-B, to be the most preferential site for hydrogen adsorption, owing to its better adsorption and Gibbs free energies. However, the results on OER activity show strong
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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chemisorption of oxygen on adsorbent with too negative energies of adsorption and Gibbs function. Further, the results on electronic transport under hydrogen and oxygen adsorption show distinct behaviour. The semiconducting nature of pristine NW transforms to metallic with hydrogen adsorption, whereas, it remains unaltered in case of oxygen adsorption with decreased magnitude of band gap from 2.98 eV (pristine) to 2.72 (A-site) and 2.77 eV (B-site). The magnitude of the adsorption and Gibbs energies under hydrogen adsorption, gives confidence to fabricate and utilize BP NW as an active HER catalyst for hydrogen production. Our partial density of states plots support the computed results and also provide clear pictorial explanation into the underlying electronic transport subjected to hydrogen/oxygen adsorption. It is also suggested that further reduction in the electronic bandgap of the BP NW can boost the catalytic activity of the BP NW. For band gap modulation and to understand the role of substitutional doping of the BP NW on its HER and OER activities, one of the boron atoms of the NW surface was substituted with Al and Ga atoms independently. These results suggest increase in adsorption and Gibbs free energies of the doped NWs indicating poor HER activity while in the case of oxygen adsorption, the Ga doped NW when adsorbed with oxygen on site-B, shows dramatic reduction in the adsorption and Gibbs free energies, indicating towards the pathway for improvement in OER activity of NW by means of increasing Ga concentration and NW surface site-engineering.
Acknowledgement Authors are thankful to the Department of Science and Technology, Govt. of India for financial assistance. One of the author TKG acknowledges Department of Science and Technology(DST), New Delhi, Government of India for financial assistance (Grant No.: SR/WOS-A/PM-95/2016).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.107.
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Available online at www.sciencedirect.com
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Exploring the hidden catalyst from boron pnictide family for HER and OER Trupti K. Gajaria a, Basant Roondhe a, Shweta D. Dabhi b, Prafulla K. Jha a,* a
Department of Physics, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, 390002, Gujarat, India b P. D. Patel Institute of Applied Science, Charotar University of Science and Technology, CHARUSAT Campus, Changa, 388421, Gujarat, India
highlights
graphical abstract
Nanostructured boron phosphide for HER and OER catalyst. Indirect to direct bandgap transition under confinement effect. Modulation of catalytic activity by means of substitutional doping. Toxic and expensive metal-free HER catalyst.
article info
abstract
Article history:
A systematic investigation of catalytic activity of boron phosphide nanowire (BP NW) to-
Received 15 April 2019
wards over-all water-splitting reaction has been performed by evaluating the hydrogen
Received in revised form
evolution reaction (HER) and oxygen evolution reaction (OER) activities. Intended to the
8 September 2019
mentioned aim, we have utilized Kohn-Sham formulated extensively popular ab initio
Accepted 12 September 2019
method based on density functional theory (DFT). The structural and electronic properties
Available online xxx
of the BP NW are computed and compared with its bulk phase. We observe dramatic indirect to direct bandgap transition with pronounced energy gap after introducing two-
Keywords:
dimensional confinement that is akin to the other reported III-V NWs. The calculated
Boron phosphide nanowire
partial density of states with van Hove singularity also confirms the same. Owing to its
Density functional theory (DFT)
moderate bandgap value, the applicability of the BP NW as an HER/OER catalyst is assessed
Hydrogen evolution reaction
by computing the site dependent HER/OER activities. Our computation on Gibbs free energy
Oxygen evolution reaction
for the case of hydrogen adsorption with 1.19 eV magnitude gives better results; whereas in case of OER, the results with higher magnitude of Gibbs energy implicate over binding of oxygen with adsorbent thus revealing non-feasible desorption of oxygen from adsorbent. Significant perturbation in electronic states of NW under hydrogen adsorption confirms high sensitivity of BP NW for hydrogen adsorption. Further, the effect of substitutional doping on HER and OER activities suggests that the doped NW shows poor HER activity in
* Corresponding author. E-mail address: [email protected] (P.K. Jha). https://doi.org/10.1016/j.ijhydene.2019.09.107 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
2
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contrast to the site-dependent better OER activity in case of Ga doped BP NW. The present BP NW shows potential as an HER catalyst owing to its lower adsorption and Gibbs free energies (1.07 and 0.84 eV), as compared to previously conventionally utilized III-V NWs. Henceforth, we believe that the present study would serve as a blueprint for the researchers to design and develop toxic and/or metal-free catalyst that can be utilized for efficient water-reduction. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Extensive efforts are being made by researchers worldwide to overcome two major concerns; first one to bring down the ever-growing energy crises and the other one is pollution control. The eco-friendly pathways for generating and storing clean and green energy are being developed by means of exploring novel materials possessing exotic properties like high mechanical strength [1], tuneable electronic properties with wide range of bandgaps [2] and cost-effective production rates [3]. Available literature on III-V compounds from the modern periodic table suggests that the nitrides and arsenides have been studied extensively due to their versatile optoelectronic and piezoelectric properties [4,5]. However, the IIIphosphides have received scant attention. Similar to the conventional III-V compounds, the phosphides with aluminium, gallium and indium as cations possess direct bandgap in their bulk cubic phase [6]; while nano-structuring them alters the nature and magnitude of the bandgap in some of the phosphides arising due to distinct geometric alignment and hexagonal symmetry [7,8].The reported electronic properties of similar nanostructures suggest their probable applicability in water splitting and catalysis [9e15]. The successful fabrication of Wurtzite (WZ) gallium phosphide nanowires (NWs) possessing direct electronic bandgap with 2.1 eV is reported by Assali et al. [8]. The authors have also demonstrated similar trend for AlP. Apart from the generalized optoelectronic, photovoltaic and nanoelectronic applications, very scares studies are found on III-V compounds showing their probable applicability as water-splitting catalysts under reduced dimensions [16e21]. In recent past, the demonstration of gallium phosphide nanowire as an efficient water reducer was presented by Anthony et al. [16]. The authors have studied the GaP in nano regime so as to reduce the production cost with efficient electronic bandgap engineering, that can be useful for electrochemical water-splitting. Further, the authors claim reduction in electrical resistance followed by enhancement in the current-voltage profiles og GaP NW due to multistep platinum deposition. An interesting study on another III-V compound, InGaN is reported by Sheng et al. [17], showing the importance of indium concentration on the photo-electrochemical water oxidation efficiency. They report 25% efficiency in solar to hydrogen generation with high magnitude of current densities due to admired electronic bandgap of 1.7 eV. The GaAs compound generally studied for optoelectronics [22], photovoltaics [23] and nanoelectronics [24] based applications has
been experimentally studied under reduced dimensions as a photoanode for photo-electrochemical water oxidation by Zeng et al. [18]. They have addressed one of the major issues of corrosion caused by ambience around the catalyst by coating the nanowire surface with nickel oxide (NiO) layers. The system InGaN has also been studied under heterostructure geometry by Kibria et al. [19]. The authors report over-all watersplitting using multiband InGaN/GaN based nanowire heterostructures that are not only environmentally benign but also act as multipurpose photo-catalyst. The unsaturated dangling bonds present at the surface of the NW are responsible for false edge states, so have to be treated carefully for accurate estimation of electronic transport through the material. The effect of the surface passivation on electrochemical watersplitting efficiency is demonstrated by Varadhan et al. [20]. It is proved that the surface treated GaN NWs show photon to current efficiency of 18.3%, whereas, the untreated NWs possess only 8.1% efficiency with significantly low electrochemical stability of less than 4 hours. GaN based NWs are also utilized as cross-linkers between MoS2 and Si for fabricating heterostructure photo-electrode [21]. The authors report first ever noble metal free Si based photo-electrode that produces benchmarking photo-current of 40 mA/cm2 at 0 V. Last but not the least, we found a report on the locus of our study-BP, which in its cubic zinc blende bulk phase has been studied by Shi et al. [25] as a metal free photo-catalyst for hydrogen evolution. Also, being one of the robust materials, the authors have presented the corrosion free property of bulk BP as an active HER catalyst. Compared with the above reports, the BP in bulk phase [25] is found to be simplest metalfree compound that shows good HER activity. However, the BP (cubic compound that prefers to get crystalized in hexagonal phase under confined dimensions) at nanoscale has not been reported for over-all water-splitting activity. Our major goal in the present study is to address the applicability of nanostructured boron phosphide (BP) as an active hydrogen/oxygen evolution reaction (HER/OER) based metal-free active catalyst. Although, the availability of boron and phosphorous in earth’s crust is not abundant, yet can be sufficiently utilized under reduced dimensions to cut down the material need and production cost. The reduction in material dimensions yields one more advantage of engineering the material properties according to the application. With regard to the above concerns, we have first studied the structural and electronic properties of WZ BP in NW form, since its other colleagues from III-V group possess WZ geometry under reduced dimensions, while cubic structure is more stable in bulk phase [26,27]. Further computations on BP
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NW are carried out to explore its capacity as an efficient catalyst yielding good hydrogen evolution reaction (HER) and/ or oxygen evolution reaction (OER) activities. In addition, improvement in the catalytic activity of the BP NW is explored by means of doping the NW with other group-III elements. Two major advantages of nanostructured BP, one being its corrosion-free nature with second advantage of low material consumption in-contrast to its bulk form are prime motivations for the present study. The study is organized in following order: the detailed methodology adopted for present study is explained in Computational details section followed by the obtained results with respective discussion in Results and discussion section. The brief summary of the study is presented in Conclusions section.
Computational details In the present study, we have computed the structural and electronic properties of BP NW under the framework of firstprinciples density functional theory (DFT) [28] as implemented in plane wave pseudopotential simulation package [29]. For computation of the proposed properties of the BP NW, we first optimized the Wurtzite (WZ) boron phosphide (BP) in bulk phase by minimizing the total energy of the system selfconsistently employing quasi-Newton Broyden-FletcherGoldfarb-Shanno algorithm. The atomic species were described as a combination of frozen cores and contributing valence electronic orbitals using well-suited local density approximated (LDA) pseudopotentials [30] with PerdewZungar parameterization [31]. The system parameters were optimized by converging the total energy of the system with respect to cell shape, cell size and ionic positions [32,33]. The kinetic energy and charge density cut-offs for wavefunction of 1630 and 16300 eV for bulk, and 680 and 6800 eV for nanowire unit cells were sufficient to satisfy the convergence threshold criteria of 108 eV and 103 eV/atom for energy and force respectively. Integration of Brillouin zone along the highsymmetry points was done using Monkhorst-Pack scheme [34] based dense k-mesh grids of 15 15 x 10 for bulk and 1 1 x 10 for nanowire unite cells. After successfully optimizing the bulk unit cell of the WZ BP, the same structure was utilized to make 5 5 x 1 supercell, and the NW was constructed from it by cutting the supercell along 0001 direction keeping the desired diameter and geometry in reference. The next step was to assess and modify the unsatisfied valencies of the surface atoms that give rise to surface dangling bond effect. The reports suggest, the proper saturation of the surface dangling bonds is necessary to remove the excess edge states that result in weak prediction of the electronic band structures [35]. To saturate the excess dangling bonds, the pseudo hydrogen adatoms were masked on the NW surface through passivation process and the finally obtained NW was utilized for optimizing final structural geometry. After successfully optimizing the structural parameters of the BP NW, we performed electronic band structure calculations to get insight to the electronic transport through the NW. Further, the contribution of individual atomic species to the electronic transport is verified by computing partial density of states (PDOS) of the
3
system. To analyse the hydrogen evolution reaction (HER) activity of BP NW, the adsorption and Gibbs free energies of the NW are computed by adsorbing hydrogen on different sites of the NW surface and the magnitudes of adsorption and Gibbs free energy were evaluated using the equations given below. H=ONW
NW EH=O ENW ads ¼ EComplex E
(1)
H=ONW
The terms EComplex ; ENW and EH=O represent the energies of the hydrogen/oxygen adsorbed pristine/doped NW, pristine/ doped NW without adsorption and half of the energy of isolated hydrogen/oxygen molecule. DGH=O ¼ DEH=O þ DEZPE TDSH=O
(2)
The term DEZPE which lies within the range 0.0e0.04 eV [36e38] for hydrogen and is nearly zero for oxygen [38], is the zero point energy difference of hydrogen/oxygen in adsorbed state and gas phase, and the third term DSH=O is the difference in the entropy of hydrogen/oxygen under adsorbed state and gas phase, and under experimental conditions, the value of TDSH=O is about 0.4 eV for hydrogen [37,38], and for oxygen, the entropies of adsorbed and gas phases are 0 and 0.631 eV respectively [37]. Considering the reported magnitudes, the terms DEZPE and TDSH=O reduce to 0.24 or 0.33 for the case of hydrogen or oxygen adsorption respectively. The effect of hydrogen/oxygen adsorption on electronic transport properties is also assessed by re-computing the PDOS of the NW after subjecting to hydrogen/oxygen adsorption on different sites. From optimizing the structure to computation of the proposed properties, the van der Waals dispersion correction term (DFT-D2) [39] was included for better prediction of the adsorption energy. The inclusion of van der Waals correction improves the binding in the case of low dimensional boron based III-V semiconductors [40]. Furthermore, for understanding the effect of substitution defect/doping on the electronic and adsorption response of the BP NW, we have re-computed the aforementioned properties after doping the NW with Al and Ga atoms individually.
Results and discussion Structural properties As mentioned in above section, for constructing the boron phosphide nanowire (BP NW) in 0001 direction, we first constructed and optimized the bulk unit cell of WZ BP crystal under self-consistent total energy minimization process. The optimized lattice parameters, bond lengths and anion-cation angles of the bulk unit cell are listed in Table 1. It can be observed from Table 1 that the present LDA predicted structural parameters for bulk BP are in good agreement with the previous reports [41]. After successfully optimizing the bulk BP, we constructed BP NW (see Computational details section) and re-optimized the initial parameters like kinetic energy cut-off, charge density cut-off, k-mesh by minimizing the total energy of the system, within the mentioned threshold criteria.
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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Table 1 e Structural parameters of WZ BP in bulk and NW configurations. System
BP (bulk) BP (NW)
Bond Length ( A)
No. of Atoms
Lattice Parameter
B/P/H
C ( A)
BeP
BeH
PeH
BePeB
PeBeP
2/2 13/13/18
5.225 5.257
1.94 1.93
e 1.21
e 1.41
109.2 109.9
109.2 104.7
The final step was to keep the initial parameters fixed to finally compute the structural parameters, which were obtained by totally relaxing the NW with respect to its cell shape, size and ionic positions. The computed structural parameters of BP NW enlisted in Table 1 show deviation in cation-anion distances and angles. In the case of bulk BP, the cationanion angles are same and consistent in all directions; whereas in the case of BP NW, the cation-anion angles show deviation of 5 . This might be the result of inclusion of hydrogen adatoms on the NW surface that re-assemble themselves and the surface atoms in such a way that the total energy of the system is minimized. The top and side views of optimized BP NW are presented in Fig. 1. As it can be viewed from Fig. 1(a), the hydrogen adatoms have settled in a buckled geometry so as the surface energy of the system gets minimized. For assessing the stability of the NW, we have computed the formation energy of the BP NW utilizing the equation given below: ¼ ENW F
ENW Tot mmB nmP lmH mþnþl
(3) NW
where, the terms ENW F and ETot represent the formation and total energies of NW and mB; mP and mH are chemical potentials of the boron, phosphorous and hydrogen with their respective number of atoms symbolized as m, n and l present in the NW unit cell. The computed formation energy of the NW is found to be 5.61 eV; wherein the negative sign and low magnitude
Angle
Formation Energy (eV) e 5.61
indicate that the NW is stable and the formation is exothermic in nature.
Electronic properties The electronic band structure of the system gives insight to the pathways of undergoing electronic transport in the Brillouin zone picture, and the density of states gives an account on the occupied and unoccupied electronic states of the system along with the spatial distribution of individual electronic orbitals of the constituent atoms. We have computed the above mentioned properties for understanding the electronic picture of the system, and the corresponding numeric results are tabulated in Table 2 with the pictorial results depicted in Fig. 2. Fig. 2(a) clearly indicates that the locations of the valence band maxima (VBM) and conduction band minima (CBM) are located at the G point-the centre of the BZ, which reveals direct nature of the electronic band gap, similar to the report on WZ GaP NW [8], but is in contrast to other NWs from the same family [26,40]. The gap in present case is 2.96 eV (see Table 2), which is higher than the bulk BP. Further, it should be noted that the nature of the electronic band gap is indirect in the case of bulk phase that transforms into direct type on introducing nano-structuring like other compounds [8,42]. These results are in accordance with the so called quantum confinement effect. The term effective mass is related to the dynamic mass of the charge carrier, which strongly depends on the position of the charge carrier and the considered direction of the carrier transport in the crystal. Considering the above two facts, we have tried to understand the dimension dependent carrier effective mass, and for this, the electron effective mass for bulk and NW systems in 0001 direction is computed (see Table 2) by extracting the edge states from the respective electronic band structures and fitting them with second order polynomial. From the computed values of effective masses, it can be concluded that subjected to gigantic modifications in the transport properties of the system, the electron effective mass show rapid enhancement due to confined geometry. Further, substitution of other group-III elements in place of boron,
Table 2 e Electronic band gap ðEg Þ, electron effective mass ðm*e Þand nature of band gap of Boron Phosphide in bulk and NW configurations. System
Fig. 1 e Top (a) and Side (b) views of pristine BP NW.
BP-bulk BP-NW BP-Al NW BP-Ga NW
Eg (eV)
m*e (me)
Nature
0.84 2.96 2.81 2.87
0.41 9.74 4.37 5.04
Indirect Direct Direct Direct
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5
Fig. 2 e (a) Electronic Band Structure with corresponding density of states (DOS) and (b) partial electronic density of states (PDOS) of BP NW.
results in reduction of band gap from 2.96 eV for pristine NW to 2.81 eV and 2.87 eV for Al and Ga doped BP NWs respectively, keeping the direct bandgap nature unaltered. The PDOS plot presented in Fig. 2(b) shows the individual atomic orbital contribution to the electronic properties in form of the characteristic van Hove singularities. As observed in the figure, the 3p-orbital of the phosphorous atom dominates in the CBM regime, whereas its contribution to the VBM regime is almost equal to that of boron atom’s 2p-orbital. The significant contribution of 3s-orbital of phosphorous atom is observed in CBM regime. Further, the orientation of Fermi energy located near the VBM regime replicates the p-type semiconducting nature of the BP NW.
Catalytic activity of BP NW After understanding the structural properties and electronic transport of BP NW, we now focus on our prime goal of the present study. As mentioned earlier, the water-splitting process is sub-divided in two reactions, generally known as HER and OER. The catalytic activity of BP NW for over-all watersplitting is assessed by means of studying the HER and OER activities of NW under hydrogen and oxygen adsorption. The
strength of the adsorption is measured by means of computing the adsorption energy and corresponding Gibbs free energy of the system utilizing equations (1) and (2) (see Computational details).
Hydrogen evolution reaction (HER) activity As mentioned, hydrogen is the cleanest source of energy and one of the best ways to produce/obtain clean energy is to break the water molecules in to its constituents: hydrogen and oxygen molecules. The so-called process of watersplitting is the green and clean way for producing hydrogen that can be further utilized as a fuel. To carry out the HER activity at ambient conditions, the time and production rate are not feasible; and to overcome this, the materials that can act as catalysts are utilized, which just modifies the reaction rate without participating in the reaction. The conventionally utilized catalyst for this purpose is made up of the expensive metals like Pt, Pd, Au etc., which are not feasible at larger scale owing to their production expenditure. To overcome this disadvantage, we need to design/engineer materials with required catalytic activity with low production rates. For this purpose, BP acquiring much more portion of earth’s crust than the conventional
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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expensive metals is being studied for assessing its performance as HER catalyst. To assess the HER activity of BP NW, we have adsorbed the H atom (green coloured atom) on two distinct sites of the NW surface. The top and side views of the hydrogen adsorbed BP NW are presented in Fig. 3(a, b). As can be seen from the figure, two distinct bridge sites of the NW surface are selected for studying the HER activity. The interaction of hydrogen with NW surface was analysed by fully relaxing the NW after adsorption of hydrogen; the final geometry of the NW subjected to hydrogen adsorption is depicted in Fig. 4, wherein Fig. 4(a), (b) and (c), (d) are the top and side views of NW with hydrogen adsorbed on site-A and site-B respectively. Also, the modifications in the cation-anion distances and angles subjected to hydrogen adsorption are tabulated in Table 3. For better HER activity, it is desired to have a catalyst that does not over bind the hydrogen so as to make desorption step possible, else the target of energy generation cannot be achieved. As it can be observed from Fig. 4 (a, b) and Table 3, while hydrogen is introduced on site-A of the NW surface, it gets adsorbed on the surface of the NW through chemisorption process and strongly binds with the surface cation. The chemisorption of hydrogen causes perturbation in the NW surface geometry, which can be clearly seen in Fig. 4(a, b). The observed BeP bond length near site-A which was 1.93 A in case of pristine NW, gets elongated to 2.89 A and the corresponding angles BePeB and PeBeP are modified to 119.6 and 83 from 109.9 to 104.7 , respectively. The corresponding computed adsorption and Gibbs free energies of the system are found to be 1.4 and 1.19 eV, respectively which
Fig. 3 e Top and side views of BP NW with hydrogen adsorbed on two distinct surface sites (a, b) Site-A and (c, d) Site-B.
Fig. 4 e Top and side views of BP NW subjected to hydrogen adsorption on Site-A (a, b) and Site-B (c, d), respectively.
replicates that the HER activity of BP NW is within the feasible range. However, in the second case, wherein the hydrogen is adsorbed near the concave surface of the BP NW, we found totally different adsorption mechanism. The B-site of the NW is found to be more suitable for adsorption due to physisorbed hydrogen on the NW surface (see Fig. 4(c, d)) without any chemical bond formation between the adsorbent and adsorbate. There is almost negligible perturbation in the surface geometry of the NW as compared to the A-site adsorption. This observation can be confirmed by the anion-cation distances and angular magnitudes enlisted in Table 3, which are almost similar to that of the pristine NW. Furthermore, the energies of adsorption and Gibbs function replicate better HER activity of BP NW in this case 1.07 and 0.83 eV, respectively. Henceforth, we conclude that the Bsite is the most preferential site for hydrogen adsorption keeping the geometry and morphology of the adsorbent unperturbed with desirable magnitudes of adsorption and Gibbs free energies. To understand the modifications in the electronic transport of BP NW subjected to site dependent hydrogen adsorption, we have computed the partial electronic density of states (PDOS). The graphical comparison of computed PDOS for pristine and H-adsorbed BP NW is depicted in Fig. 5. As it can be clearly observed from Fig. 5(a), (b) and (c) that the pristine NW possesses finite electronic bandgap with moderate value of 2.96 eV. However, when hydrogen is introduced in the
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Table 3 e Computed bond lengths and angles of BP NW subjected to hydrogen adsorption at different sites of NW surface. Bond Length ( A)
Configuration BeP Site-A Site-B
2.89/1.94 2.11/1.95
BeH 1.23 1.21
D ( A)
Angle PeH 1.42 1.43
PeBeP
83 /106.4 106.9 /105.4
BePeB
H-NW
119.6 /110.4 103.3 /110.4
1.22 1.55
Fig. 5 e Partial density of states (PDOS) of BP NW in (a) pristine, and H-adsorbed on (b) Site-A, and (c) Site-B, respectively.
system at A-site (see Fig. 4(a , b)), the VBM and CBM states get markedly modified and excess states at Fermi level are observed because of chemisorbed hydrogen. The modification in the hybridized states of BP NW results in metallic nature of the adsorbent. On the other hand, the adsorption of hydrogen at B-site results in much more pronounced electronic states at Fermi level showing enhancement in metallic character of the adsorbent. After unravelling the complete picture on hydrogen adsorption on two distinct sites of BP NW surface, we tried to understand the effect of substitutional doping on the hydrogen adsorption properties. The results on adsorption
energy and Gibbs free energy (see Supplementary Material Table S2) show that introducing Al and Ga atoms on B site of the NW, the magnitude of the adsorption and Gibbs function increases showing less compatibility of BP NW as HER catalyst under doped environment.
Oxygen evolution reaction (OER) activity For estimating over-all catalytic activity of BP NW for watersplitting, besides HER activity, we have also computed the oxygen evolution reaction (OER) activity of NW by adsorbing oxygen on two different sites of the BP NW surface (see Fig. 6). Similar to the hydrogen adsorbed NW, the adsorption and
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Fig. 6 e Top and side views of BP NW before adsorbing oxygen atom on site-A (a, b) and site-B (c, d) respectively.
Gibbs free energies in this case were computed using equations (1) and (2), respectively and, the corresponding values with modified bond-lengths and angles are presented in Table 4. Fig. 7 represents the top and side views of the optimized geometries of BP NW with oxygen adsorbed on two distinct sites. We can observe that after adsorption, the oxygen atom in both cases migrates towards the NW surface and finally under chemisorption process, gets attached with boron atom in case of A-site and phosphorous atom in case of B-site by displacing the surface hydrogen atom. The computed adsorption and Gibbs free energies of BP NW under oxygen adsorption on A-site are 8.26 and 7.93 eV and 8.31 and 7.98 eV for B-site, respectively. These magnitudes are too negative and show strong bonding between oxygen and adsorbent. Further, the angles between so-formed oxygen centred geometry is 112.9 (BeOeH) and 109.2 (PeOeH) for sites A and B respectively. It is noteworthy that the cation-anion bond lengths in both cases are slightly modified as compared to pristine NW, but the cation-anion angle in case of B-site shows significant increase. Further, the observed OeH bond
Fig. 7 e Top and side views of BP NW subjected to oxygen adsorption on Site-A (a, b) and Site-B (c, d) respectively.
for A- and B-sites respectively, lengths are 0.97 and 0.98 A which are in agreement with the standard OeH bond length (0.98 A) [43]. The effect of strong bonding between oxygen and the adsorbent on the electronic transport of the system is analysed by computing partial density of states (PDOS). The PDOS plots of BP NW with oxygen adsorbed on site-A (Fig. 8(b)) and site-B (Fig. 8(c)) show similar semiconducting behaviour as pristine BP NW, with shift in electronic levels that further modify the electronic band gap from 2.98 eV (pristine) to 2.72 eV (site-A) and 2.77 eV (site-B). The clear splitting of oxygen electronic level into its bonding and anti-bonding states due to chemisorption (see Fig. 8(b) and (c)) supports our calculated values of adsorption energy and confirms the presence of strong bonding between oxygen with the NW. Further, the markable dominance of oxygen bonded atom’s porbital (boron in case of site-A, and phosphorous in case of site-B) in the CBM regime indicates that the key-role is played by oxygen atom in the electronic transport.
Table 4 e Computed bond lengths and angles of BP NW subjected to oxygen adsorption on different sites of NW surface. Bond Length ( A)
Configuration
Site-A Site-B
D ( A)
Angle
BeP
BeH
PeH
PeBeP
BePeB
BeOeH/PeOeH
O-NW
OeH
1.94 1.91
1.21 1.22
1.41 1.41
103.3 109.8
110.7 110.2
112.9 109.2
1.42 1.61
0.97 0.99
Please cite this article as: Gajaria TK et al., Exploring the hidden catalyst from boron pnictide family for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.107
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Fig. 8 e Partial density of states of BP NW in (a) pristine and O adsorbed on (b) Site-A and (c) Site-B respectively.
The shifting of Fermi level form VBM to CBM regime shows dramatic reversal of the electronic nature of the BP NW from p-type (pristine NW) to n-type (O-adsorbed NW) due to addition of excess electrons to the system. From the results of the adsorption energy and electronic states, we conclude BP NW not to be a good catalyst for water oxidation, as it over binds the oxygen atom making desorption procedure non-feasible and complicated. Further, similar to hydrogen adsorption case, we have also studied the effect on oxygen adsorption properties of BP NW by means of Al and Ga doping. Surprisingly, in this case Al doped NW, the results does not show any considerable modulation in adsorption profile (see Supplementary Table S2), with contrast to the Ga doped NW, in which the adsorption of oxygen on site-B of the NW results in dramatic decrease (~35%) in the adsorption and Gibbs free energies of the NW. These results indicate improvement in OER activity of BP NW can be achieved by means of increase in Ga concentration and surface site-engineering.
Conclusions In summary, the first-principles density functional theory based computations of structural and electronic properties of newly designed WZ boron phosphide nanowire have been performed and the results are compared with computed parameters in its bulk counterpart to assess the confinement effect. Our structural parameters in bulk phase agree well with the available data, whereas due to lack of availability of experimental or theoretical data on BP NW, we could not provide comparative results. We observe indirect to direct electronic bandgap transition with enhanced gap magnitude of BP subjected to strong quantum confinement effect. The assessment of site dependent HER and OER activities of BP NW has been carried out, and we have found the concave site-B, to be the most preferential site for hydrogen adsorption, owing to its better adsorption and Gibbs free energies. However, the results on OER activity show strong
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chemisorption of oxygen on adsorbent with too negative energies of adsorption and Gibbs function. Further, the results on electronic transport under hydrogen and oxygen adsorption show distinct behaviour. The semiconducting nature of pristine NW transforms to metallic with hydrogen adsorption, whereas, it remains unaltered in case of oxygen adsorption with decreased magnitude of band gap from 2.98 eV (pristine) to 2.72 (A-site) and 2.77 eV (B-site). The magnitude of the adsorption and Gibbs energies under hydrogen adsorption, gives confidence to fabricate and utilize BP NW as an active HER catalyst for hydrogen production. Our partial density of states plots support the computed results and also provide clear pictorial explanation into the underlying electronic transport subjected to hydrogen/oxygen adsorption. It is also suggested that further reduction in the electronic bandgap of the BP NW can boost the catalytic activity of the BP NW. For band gap modulation and to understand the role of substitutional doping of the BP NW on its HER and OER activities, one of the boron atoms of the NW surface was substituted with Al and Ga atoms independently. These results suggest increase in adsorption and Gibbs free energies of the doped NWs indicating poor HER activity while in the case of oxygen adsorption, the Ga doped NW when adsorbed with oxygen on site-B, shows dramatic reduction in the adsorption and Gibbs free energies, indicating towards the pathway for improvement in OER activity of NW by means of increasing Ga concentration and NW surface site-engineering.
Acknowledgement Authors are thankful to the Department of Science and Technology, Govt. of India for financial assistance. One of the author TKG acknowledges Department of Science and Technology(DST), New Delhi, Government of India for financial assistance (Grant No.: SR/WOS-A/PM-95/2016).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.107.
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