Enhanced Photo-Electrocatalytic Hydrogen Generation in Graphene

Jun 27, 2019 - Development of atomic layers and their heterostructures opened new avenues in developing novel photocatalysts of enhanced light-matter ...
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Enhanced Photo-Electrocatalytic Hydrogen Generation in Graphene/hBN van der Waals Structures Sumit Bawari,† Sourav Pal,†,‡ Shubhadeep Pal,† Jagannath Mondal,*,† and Tharangattu N. Narayanan*,† †

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Tata Institute of Fundamental ResearchHyderabad, Sy. No. 36/P, Gopanapally Village, Serilingampally Mandal, Hyderabad 500107, India ‡ Indian Institute of Science, Education, and ResearchKolkata, Kolkata, West Bengal 741246, India S Supporting Information *

ABSTRACT: Development of atomic layers and their heterostructures opened new avenues in developing novel photocatalysts for enhanced light−matter interaction. Graphene−hexagonal boron nitride (G/hBN) van der Waals structures were recently predicted for interfacial exciton generation upon visible light excitation, and also, the authors have shown their van der Waals stacking-induced electrocatalytic activity. Here, an enhanced photo-electrocatalytic hydrogen evolution reaction (HER) is demonstrated upon white light illumination on samples where graphene and hBN are stacked via van der Waals interaction. The individual layers of graphene and hBN-modified electrodes have no lightinduced effects along with a “poor” electrocatalytic HER activity. The origin of such lightinduced activity is probed via density-functional-theory-based calculations. The catalytic activity of such heterostructures is found to be further enhanced by nanostructuring, where the coupling between individual defective graphene layers is ensured via direct carbon− carbon coupling through the Suzuki reaction. With band-engineering-induced inherent activity enhancement and nanostructuring-enabled enhanced active sites, we were able to develop a G/hBN structure that has an exceptional photo-electrocatalytic activity.

1. INTRODUCTION Graphene−hexagonal boron nitride (G/hBN) van der Waals heterostructures have received considerable attention from researchers due to their close lattice constants (mismatch ∼1.8%) and their incommensurate stacking-induced tunable electronic properties via relative rotation.1 Furthermore, it has also been shown that G/hBN stacking can bring tunable dipoles at the interface, and hence, it brings the possibility of electrostatic doping in layers.2 Gao et al. have shown that mixing hBN and graphene in different proportions can have intermediate electronic transitions (between high-energy Frenkel excitations of hBN at 6.1 eV and metallic graphene), indicating the formation of sub-band states having several possible transitions in the visible range.2 This makes G/hBN a promising tunable band gap material for the generation of photogenerated carriers. Moreover, terahertz pump−probe spectroscopic studies showed the formation of long-lifetime dipoles (∼10 ps) at the interface, which are also found to be tunable with the number of graphene−hBN interfaces.3 Although the formation of photogenerated excitons has been shown in the past, their separation and use in chemical reactions have not been demonstrated so far. Recently, hBN domain-doped graphene structures were demonstrated for their photocatalytic hydrogen generation, where the graphene and hBN domains are in-plane covalently linked.4 In the present study, we demonstrate the application of turbostratic (random) G/hBN van der Waals stacks for photo-electro© 2019 American Chemical Society

catalytic hydrogen generation, which is a much simpler (less complex in terms of both the structure and synthesis protocol) and scalable system. The formation of possible light-absorbing states is shown through density functional theory (DFT)-based calculations, and a mechanistic insight is provided via classical molecular dynamics (MD)-based simulations. Additionally, another avenue to enhance catalysis, i.e., nanostructuring, is also explored by coupling individual oxidized graphene sheets through a linker to enhance the overall charge transfer.5 Recently, the authors have shown that G/hBN van der Waals solids can exhibit enhanced electrochemical hydrogen evolution reaction (HER), whereas individual G and hBN sheets are catalytically inactive.6 It was shown that the hBN on G (via van der Waals interaction) can influence the charge density on graphene, very similar to an in-plane-doped system, and hence can affect the electrocatalytic properties of G.6 The H3O+ adsorption centers and H2 desorption centers in such G/ hBN stacks were mapped using MD studies and through DFT analyses, and it was found that the carbon centers with an hBN shadow are the most active centers for HER.7 In these works, the possibility of charge transfer from these identified locations was further verified using DFT-based calculations. The combined MD and DFT studies showed that carbon centers Received: March 1, 2019 Revised: June 7, 2019 Published: June 27, 2019 17249

DOI: 10.1021/acs.jpcc.9b01996 J. Phys. Chem. C 2019, 123, 17249−17254

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The Journal of Physical Chemistry C in graphene under the hBN shadow are the most plausible HER centers and that a charge transfer between the graphene− hBN layers also occurs.6,7 In this context, the current work probes the possibilities of a band-gap-induced charge transfer having visible light absorption from the graphene−hBN structures. This gives rise to the possibilities of exciton formation and separation of the formed exciton into an electron and a hole, to transfer an electron to the adsorbed proton. A combined approach using both DFT and MD (with recently published data7) is used to calculate the band gap formation and for hydronium ion adsorption studies. The experimental studies are conducted using defective graphene (reduced graphene oxide (rGO) as well as shear-exfoliated graphene (SEG)) and exfoliated hBNstacked samples. Nanostructuring is expected to enhance the catalytic activity of an interface,5 and so the catalytic effect of the graphene interface is enhanced via covalent linkage formation between rGO sheets using the Suzuki coupling reaction. The effect of hBN stacking is apparent in photoelectrocatalytic studies.

Figure 1. Schematic of the Suzuki coupling process; graphene oxide (GO) is chlorinated via SOCl2, after which GO−Cl sheets are linked through a phenyl group via the Suzuki coupling reaction.

rGO is carried out by heating in ethylene glycol (1 mg/mL) at elevated temperatures (180 °C) for 24 h. A schematic of the cGO/hBN synthesis is shown in Figure 1. The shear-exfoliated graphene (SEG) is prepared using a high shear mixer (Silverson model) for 1 h in dimethylformamide (DMF) at a concentration of 1 mg/mL. 2.3. Structural Characterization. Fourier transform infrared (FTIR) spectra are recorded in the transmittance mode using a Bruker (model: α) spectrometer. A Renishaw inVia micro-Raman spectrometer with a 532 nm laser is used (50× objective lens and 10 s acquisition at 1% intensity) for Raman analyses. A UV−visible (Jasco V-670) spectrophotometer is used in the transmission mode, with the sample dissolved in dimethylformamide (DMF). 2.4. Photo-Electrocatalytic Studies. A Biologic SP-300 potentiostat is used for electrochemical studies such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements. A schematic is shown in Figure S1, consisting of a three-electrode system: a counter electrode (platinum/graphite), reference electrode (Ag/AgCl), and working electrode (glassy carbon electrode (GCE)). GCE is modified with different materials via drop-coating. H2SO4 (0.5 M) is used as the electrolyte. The GCE is modified with different inks of rGO, cGO, rGO/hBN, and cGO/hBN. The ink (4 mg/mL) of each sample is prepared via sonication in DMF, and 3 μL of the same is used for drop-coating. LSVs are performed at a scan rate of 10 mV/s, which are preceded by an electrode activation process using cyclic voltammetry for 30 cycles at 50 mV/s scan rate. A Lelesil 250 W xenon lamp source (range ∼370−730 nm) is used for the visible light excitation. The temperature of the electrolyte is monitored using the thermocouple (as shown in Figure S1), and it is ensured that the electrolyte stays at room temperature during the entire photocatalytic experiment. EIS measurements are carried out in the frequency range of 0.01 Hz to 7 MHz with a 10 mV sine wave amplitude, and the Nyquist plots are fitted with equivalent Randle’s circuit using the EC-Lab fitting module.

2. METHODS 2.1. DFT Studies. The SIESTA 4.0 package is used for all ab initio calculations.8 A stacked heterostructure of hBN (two atoms in a supercell) stacked over graphene (two atoms in a supercell) is studied in three types of stacking arrangements, AA (B and N coinciding with carbon), AB (B coinciding with carbon), and AB′ (N coinciding with carbon), in a hexagonal cell. As the primary interaction is via van der Waals interactions, pseudopotentials generated using the normconserving Troullier−Martins scheme incorporating the van der Waals “vdW-DF2” correction in the existing generalized gradient approximation Perdew−Burke−Ernzerhof exchange correlation functional are used.9−11 The energy cutoff for the real space grid is set at 525 Ry. Variable-cell relaxation is carried out through the Broyden method till the maximum force on each relaxed atom is less than 0.00001 hartree bohr−1.12 For electronic structure calculation, a 32 × 32 × 2 kpoint sampling of the Brillouin zone is carried out using the Monkhorst−Pack scheme.13 Spin-polarized basis sets are used for all self-consistent calculations. MD simulation details are provided in our previous work.7 2.2. Synthesis. Bulk graphite powder (Sigma-Aldrich) was oxidized using the improved Hummer’s method to obtain graphene oxide (GO).14 To synthesize cross-linked GO sheets, a similar method proposed by the authors to cross-link carbon nanotubes was attempted, which is shown in Figure 1.15 For this, GO (140 mg) was chlorinated with thionyl chloride (SOCl2) (100 mL) with 5 mL of 1,2 dichlorobenzene as the phase-mixing reagent, and it is refluxed at 100 °C for 1 day. The GO−Cl sheets are coupled through the Suzuki coupling reaction where benzenediboronic acid (20 mg) acts as a linker between GO−Cl (30 mg) with a catalyst tetrakis-phosphopalladium [Pd(PPh3)4] (50 mg) and base cesium carbonate [CsCO3] (160 mg) in ∼200 mL of toluene and refluxed at 100 °C in a nitrogen environment for 5 days. The thoroughly washed (methanol followed by water multiple times) sample is used for further analyses, and it is named c-GO (coupled GO). The shear exfoliation method is used to prepare hBN sheets from their bulk crystals.6 Exfoliated hBN (in different weight ratios) is added to the GO−Cl sheets in the coupling reaction for the development of hBN-stacked c-GO, henceforth called c-GO/hBN. Chemical reduction of GO for the production of

3. RESULTS AND DISCUSSION Graphene and hBN are known to form commensurate heterostructures when one is stacked over the other.16 Here, we first calculate, using DFT, the band structure for 17250

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eV, respectively, for AA, AB, and AB′. To absorb light in the visible region (1.7−3.1 eV), the nitrogen (VB)−carbon (CB) transition with an energy gap in the range of 1.46−1.7 eV barely grazes the visible region. Since DFT is well known to underestimate band gaps, the real band gap can be significantly higher, which can be captured by GW calculations.17 Recently, Aggoune et al. have also computed the possibility of light absorption in G/hBN stacks.18 It was shown that a nitrogen (VB)−carbon (CB) transition for three-layer hBN/one-layer graphene occurs at 1.9 and 2.5 eV for differently stacked systems (Table 1). It is apparent from these studies that a stacked heterostructure of G/hBN has the potential to absorb light in the visible region. To probe the predicted band gap modulations, UV−visible spectroscopic investigations are conducted. The optical band gap can be calculated by extrapolating straight lines from the (αhυ)2 vs hυ plots (Figure 3A,B), where α is the absorbance

commensurate AA-, AB-, and AB′-type [Figure 1 (insets)] stacked graphene and hBN. The computed band structure shows the band contribution from carbon (green), boron (pink), and nitrogen (blue), as shown in Figure 2. The stacking

Figure 2. Computed band diagrams of G/hBN structures (bilayer) in (A) AA, (B) AB, and (C) AB′ configurations, with labeled band contributions. Energy transitions from nitrogen (VB) to carbon (CB) (red arrow) and from carbon (VB) to boron (CB) are calculated.

of hBN on G induces a band gap of ∼0.1 eV, which is too small for visible light absorption (Figure 2A, inset). Apart from this small band in carbon, many other possible states are formed that independently have major contributions from the orbitals of either carbon, boron, or nitrogen. For light absorption, two transitions are possible, as shown in Figure 2, from a valence band (VB) of nitrogen to a conduction band (CB) of carbon or from a CB of carbon to a VB of boron. Here, a direct transition between a VB of nitrogen and a CB of carbon shows gaps of 1.52, 1.46, and 1.7 eV, respectively, for AA, AB, and AB′. On the other hand, a direct transition between a VB of carbon and a CB of boron shows gaps of 3.36, 3.50, and 3.21

Figure 3. Band gaps calculated for (A) rGO and (B) rGO/hBN by the plot between energy (hυ) and an absorption function (αhυ)2 through UV−vis spectroscopy [(αhυ)2 = hυ − Eg]. Band gaps calculated (values) through the intercept are specified. (C) Schematic of exciton formation between graphene (carbon) and hBN (nitrogen). (D) Band gap calculation for SEG/hBN similar to (A). (E) Photoelectrocatalytic HER LSVs of rGO/hBN. (F) Photo-electrocatalytic HER LSVs of SEG/hBN.

and υ is the corresponding frequency.2 The intercept on the xaxis gives the value of the band gap. The systems used for the present study are rGO/hBN and SEG/hBN. rGO itself has a

Table 1. Comparison of Band Gap Values Reported (Theoretical and Experimental) for Different Graphene/hBN Systems system considered

method

band gap value (eV)

refs

three-layer hBN/G SEG/hBN single-layer G/hBN SEG/hBN rGO/hBN

DFT-D2 + GW UV−vis spectroscopy DFT-VDW UV−vis spectroscopy UV−vis spectroscopy

1.9, 2.5 2.27 (3:1), 2.69 (1:1) 1.46−1.7 2.2 (1:1) 2.7 (1:1)

18 2 this study this study this study

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The Journal of Physical Chemistry C transition corresponding to π−π* absorption at around 3.6 eV (Figure 3A). The incorporation of hBN induces another transition in the mid-region at 2.7 eV (Figure 3B), which falls in the visible region. This shows that the graphene/hBN interface is capable of absorbing light in the visible region. The band gap values obtained from previous studies and this study, through computational and experimental methods, are shown in Table 1. One can see that there is a good agreement with different studies, and a band gap in the visible region at around 1.5−2.5 eV is predicted through ab initio studies. The experimental measurements show a band gap between 2.2 and 2.7 eV for rGO/hBN samples. Hence, the UV−vis spectroscopy study shows that rGO/hBN is capable of absorbing visible light, wherein upon absorption, it can generate an electron−hole pair (Figure 3C). A similar band gap opening is shown in the SEG/hBN sample, too (Figure 3D). The DFT study shows (Figure 2) that light absorption occurs between the graphene carbon and hBN nitrogen orbitals. Hence, the excitons will be generated, where electrons will be generated at graphene and holes on the hBN surface. It is found from our previous report that the carbon sites on graphene are the active centers for hydrogen reduction.6 The generation of an exciton on G/hBN due to visible light incidence generates electrons on the graphene surface that can benefit reduction of H+. The rGO/hBN sample shows visible light absorption at around 2.7 eV in UV−Vis spectroscopy. To probe whether the generated excitons can participate in reactions, photo-electrochemical HER studies are performed on rGO/hBN. In an electrolyte of 0.5 M H2SO4, the HER LSVs for the rGO/hBN sample without light and with 30 min (stabilized) of light exposure are studied (Figure 3E). rGO/hBN shows an HER onset potential of ∼−400 mV, which reduces to ∼−300 mV after light irradiation, with a significant increment in current (from −1.3 to −11.1 mA/cm2 at −500 mV) (Figure 3E). A similar light response is seen in the case of SEG/hBN, too, which also shows a band gap in the visible region, consistent with the results of Gao et al.2 Photoinduced HER is also seen in SEG/hBN, though at higher overpotentials, upon the light “on” condition (Figure 3F, where the onset shifted to low values and the current densities got enhanced upon irradiation), and the HER LSV is found to be going back to the previous values while switching off the lamp. These results indicate that stacking induced the photo-electrocatalytic activity of rGO/hBN and SEG/hBN, where the sheets individually have no light-induced HER activity. To further increase the photo-electrocatalytic activity of rGO/hBN, a nanostructuring approach is adopted where rGO sheets are coupled to each other via a benzene molecule through Suzuki coupling (cGO), as explained in the Methods section. Nanostructuring is expected to enhance the catalytic performance via an enhanced number of active sites5 and hence help in the overall H3O+ reduction on the system. Suzuki-coupled carbon nanotubes coupled through a benzene linker were recently shown to be an efficient HER and CO2 reduction catalyst.15 The carbon atoms near the linker, due to charge transfer, were shown to be the active sites for reduction of H3O+ and CO2 molecules in the two different studies.15,19 A similar mechanism of charge transfer from graphene (rGO) to the linker (C6H4) is also expected to occur, and the structures of cGO and cGO/hBN are shown in Figure 1. The FTIR spectra of rGO and cGO are compared in Figure S2. It is found that major peaks in cGO at ∼1200 cm−1 are not found in rGO. These peaks are recognized as C−H in-plane bending

vibrations that arise due to benzene C−H bonds,15 which proves linking via benzene. Peaks in the 700−900 cm−1 region correspond to C−Cl (residual from chlorination)15 or out-ofplane C−H bending vibrations. The FTIR analyses of cGO/ hBN and rGO/hBN show that a major peak corresponding to BN vibrations is found at ∼1380 cm−1, indicating the presence of hBN in these samples. The C−C bond vibration at 1630 cm−1 in rGO shifts to 1615 cm−1, confirming a greater strain in the C−C lattice, which is further proven using Raman spectroscopy. The micro-Raman spectra of GO, rGO/hBN (1:1), and cGO/hBN (1:1) are shown in Figure S3, indicating the presence of G and D peaks of graphene at 1605 and 1350 cm−1, respectively (Figure S3A).6 The hBN peak (in-plane bond vibration) is clearly visible at 1364 cm−1, which is found in both hBN-incorporated samples. A closer look at the G peak indicates (Figure S3B) a slight red shift in cGO, whereas GO and rGO show identical peaks. This indicates the presence of a higher strain in the graphitic lattice due to cross-coupling, as evidenced in FTIR. The defect density in cGO is found to be higher than that in rGO, indicated by a higher D peak, as can be seen in Figure S3C. Nanostructuring via benzene has previously been shown to enhance the HER and CO2 catalysis on carbon nanotubes.15,19 However, a nanostructuring attempt in our previous work using hemiacetal linkage between hydroxyl groups of GO is found not to enhance the activity, due to the poor charge transfer capabilities of the linker.6 This has been addressed here via a direct covalent linkage of rGO through Suzuki coupling. Scanning electron microscopy (SEM) images show the spongy nature of cGO/hBN sheets, whereas separated hBN-decorated individual rGO sheets appear in the SEM image of rGO/hBN (Figure S4). Photo-electrocatalytic studies of rGO, rGO/hBN, cGO, and cGO/hBN are compared in Figure 4A,B. Figure 4A,B shows LSVs of different working electrodes with (Figure 4B) and without (Figure 4A) white light exposure. It is reported that individual GO/rGO structures are not highly active for HER.20 Similarly, cGO does not have any light-induced activity (Figure

Figure 4. (A) LSVs of rGO, rGO/hBN, cGO, and cGO/hBN. (B) LSVs of cGO and cGO/hBN with and without light, compared with Pt/C. (C) EIS of cGO/hBN 3:1 with fits at −100 mV potential and 10 mV sine wave amplitude in the frequency range of 10 mHz to 7 MHz. (D) Histogram of adsorbed H3O+ ions across the graphene/ hBN surface, overlaid with the structure of graphene/hBN. 17252

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activity of the heterostructures can also contribute to the current densities (background) at these above-mentioned potentials. Hence, one cannot expect sudden changes in the current density values (where the current (density) acquisition time also matters) from these measurements in light on and off conditions, as reported before,21 whereas considerable enhancement and decrease in the current densities can be observed with and without light. However, no such change in the current response was observed in both cGO (or rGO) and hBN with light, in agreement with LSV measurements, indicating the photoresponse coming from these van der Waals hybrids. These results along with those in Figures 3F and S5 establish the role of light-induced HER in graphene− hBN van der Waals structures. So far, we have observed that although the defected graphene (rGO or cGO) lattice itself has no light absorption or enhanced catalytic activity, by stacking with hBN, the entire system gains a band gap in the visible region and hence the photocatalytic HER properties are enhanced. Although from DFT analyses it is clear that the G/hBN interface formation is the source of light absorption, with the nanostructuring approach alone, cGO does not have enhanced HER activity. This suggests that hBN also plays other roles that affect the overall HER. Polarization of the graphene lattice is one such effect, and hBN is also known to affect ion dynamics around it.7 An efficient catalyst for HER should be able to adsorb H3O+ ions effectively. For a graphene/hBN interface, at adsorption distance (∼0.3 nm) from the graphene surface, we plot a histogram of the sites visited by the H3O+ ion in 100 000 snapshots of the entire simulation (Figure 4D). While most locations on the graphene surface show low occupation, the hBN edge shows considerably higher occupation, at least 5 times higher than that of graphene. This edge behavior is seen only on one particular edge, the Boron-zigzag edge. This high adsorption of H3O+ ions on the hBN edges shows the catalytic effect of hBN on both cGO and rGO surfaces, whereas rGO and cGO surfaces themselves are not catalytically active.

4B). The electrocatalytic activity of cGO/hBN is found to be much better than that of rGO/hBN (Figure 4A), indicating the role of nanostructuring along with the inherent activity of graphene−hBN stacking, as discussed in our previous work.6 However, nanostructured cGO alone does not seem to have any catalytic HER activity (very high overpotential). With the incorporation of hBN (stacking), a low overpotential is found in cGO/hBN in the absence of light (HER onset potential of ∼−100 mV). In the presence of light, the onset potential for cGO/hBN is further reduced to ∼−50 mV. cGO/hBN also has light-induced activity as one can see the lowering of the onset potential and enhanced current density in the presence of light (Figure 4B). The experiments with light and dark are repeated several times to ensure the stability of the catalyst. Our previous studies show that graphene does not have any lightinduced HER catalytic effect,20 and here, it is found that rGO also does not have any photoinduced carrier generation. cGO/ hBN shows a current density increment from 1.7 to 4.1 mA/ cm2, with and without white light incidence (at −300 mV). The possibilities of a residual catalyst (Pd) or other metals in the sample and their role in the HER are also tested via poisoning of the metallic sites using −SCN groups, as reported by us previously,15 and such effects are subsequently ruled out. Furthermore, it has to be noted that cGO HER activity is found to be similar to that of rGO (Figure 4A), suggesting the absence of metal-induced or residual-unwashed-impurityinduced activities in cGO/hBN. The HER activity of benchmarked Pt/C is also shown in Figure 4. The observed enhanced HER photo-electrocatalytic activity of cGO/hBN is further verified using the Nyquist plots (fitted to Randle’s circuit, Figure 4C inset) from EIS spectroscopy, and the charge transfer resistances (Rct) in the absence and presence of white light are found to be 3536 and 740 Ω, respectively, indicating a drastic enhancement in the HER activity of cGO/hBN in the presence of light (Figure 4C). cGO/hBN shows an enhancement in current when irradiated by a white light source. During this process, the temperature of the solution is monitored to ensure that no thermal effects contribute to the observed HER, and no temperature change was observed during the entire process. The white light source was kept in a cooling jacket, and the electrode setup was far apart from this source. The photocurrent response is further studied using a 500 nm filter between the light source and the cell (as shown in Figure S1). Figure S5 shows the cyclic voltammogram in the dark condition (after the 20th cycle of electrode activation after which it is found to be stable), and an LSV in the dark condition is taken after that to confirm that a stable catalytic performance in the dark condition is achieved. The spectral response shown in Figure S1 shows a smaller power for the 500 nm irradiation, and the LSV for 500 nm irradiation is found to be having a less current density (HER response) than for the white light one, as shown in Figure S5. These studies show that the temperature effects on the observed phenomenon are null, and the enhanced activity upon irradiation is majorly due to the photon-induced activity. Furthermore, chronoamperometry studies on cGO/hBN and rGO/hBN, conducted at −300 mV (in cGO/hBN) and −450 mV (in rGO/hBN), are shown in Figure S6. It is clear from the figure that during white light irradiation (after stabilization of dark current), the current densities of cGO/ hBN and rGO/hBN increase substantially. It has to be noted that the white light source used here takes ∼10 min to reach the maximum intensity, and the intrinsic electrocatalytic

4. CONCLUSIONS Visible-light-induced enhanced hydrogen generation is demonstrated in graphene−hBN van der Waals structures. Although the possibilities of exciton generation in graphene− hBN van der Waals structures were predicted, here, the hydrogen generation is demonstrated with those generated electrons in a graphene lattice. Graphene and its derivatives such as rGO and cGO are shown for their inactiveness toward photoinduced electrocatalytic HER, whereas the hBN stacking drastically modified the HER performance with and without light. The enhanced photoabsorption in G/hBN is theoretically shown due to the formation of sub-band states, which allow the transition from a nitrogen orbital to a carbon orbital in the visible range and the optical band gap calculations. MD simulations shed light on the catalytic activity enhancement on both rGO and cGO, as the hBN edges favor H3O+ ion adsorption. Finally, through band engineering, nanostructuring, and vdW stacking, we are able to create an HER catalyst that has an exceptional photo-electrocatalytic activity than any other so-far-reported graphene−hBN systems. Demonstration of the development of a simple metal-free photoresponsive electrocatalyst based on a graphene/hBN hybrid for hydrogen production can open up new avenues in catalyst design, where it combines both the concepts of enhancing the inherent 17253

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(12) Johnson, D. D. Modified Broyden’s method for accelerating convergence in self-consistent calculations. Phys. Rev. B 1988, 38, 12807. (13) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188. (14) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z. Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806−4814. (15) Pal, S.; Sahoo, M.; Veettil, V. T.; Tadi, K. K.; Ghosh, A.; Satyam, P.; Biroju, R. K.; Ajayan, P. M.; Nayak, S. K.; Narayanan, T. N. Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering. ACS Catal. 2017, 7, 2676−2684. (16) Woods, C. R.; Britnell, L.; Eckmann, A.; Ma, R. S.; Lu, J. C.; Guo, H. M.; Lin, X.; Yu, G. L.; Cao, Y.; Gorbachev, R. V.; et al. Commensurate−Incommensurate Transition in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2014, 10, 451−456. (17) Chan, M. K. Y.; Ceder, G. Efficient Band Gap Prediction for Solids. Phys. Rev. Lett. 2010, 105, No. 196403. (18) Aggoune, W.; Cocchi, C.; Nabok, D.; Rezouali, K.; Belkhir, M. A.; Draxl, C. Enhanced Light-Matter Interaction in Graphene/h-BN van Der Waals Heterostructures. J. Phys. Chem. Lett. 2017, 8, 1464− 1471. (19) Pal, S.; Narayanaru, S.; Kundu, B.; Sahoo, M. R.; Bawari, S.; Rao, D. K.; Nayak, S. K.; Pal, A. J.; Narayanan, T. N. Mechanistic Insight into the Formate Production via CO2 Reduction in C-C Coupled Carbon Nanotube Molecular Junctions. J. Phys. Chem. C 2018, 122, 23385−23392. (20) Biroju, R. K.; Das, D.; Sharma, R.; Pal, S.; Mawlong, L. P. L.; Bhorkar, K.; Giri, P. K.; Singh, A. K.; Narayanan, T. N. Hydrogen Evolution Reaction Activity of Graphene−MoS2 van Der Waals Heterostructures. ACS Energy Lett. 2017, 2, 1355−1361. (21) Biroju, R. K.; Pal, S.; Sharma, R.; Giri, P. K.; Narayanan, T. N. Stacking Sequence Dependent Photo-Electrocatalytic Performance of CVD Grown MoS2/Graphene van der Waals Solids. Nanotechnology 2017, 28, No. 085101.

activity of a material and enabling more active catalytic sites via nanostructuring.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b01996. FTIR and Raman spectra and SEM characterization of rGO and cGO composites; photo-electrocatalysis measurement setup; catalytic response of cGO/hBN (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.M.). *E-mail: [email protected] or [email protected] (T.N.N.). ORCID

Jagannath Mondal: 0000-0003-1090-5199 Tharangattu N. Narayanan: 0000-0002-5201-7539 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Tata Institute of Fundamental Research, Hyderabad, India, for the financial support. REFERENCES

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DOI: 10.1021/acs.jpcc.9b01996 J. Phys. Chem. C 2019, 123, 17249−17254