Quantum Dots in Visible Light - ACS Publications - American

Article pubs.acs.org/JPCC

Photocatalytic Activity of g‑C3N4 Quantum Dots in Visible Light: Effect of Physicochemical Modifications Arkamita Bandyopadhyay,† Dibyajyoti Ghosh,‡ Nisheal M. Kaley,§ and Swapan K. Pati*,†,§ †

New Chemistry Unit, ‡Chemistry and Physics of Materials Unit, and §Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, Karnataka 560064, India S Supporting Information *

ABSTRACT: Graphitic C3N4 (g-C3N4), a newly emerging layered semiconductor, has shown its promising performance for use as a photocatalyst in the hydrogen evolution reaction and as an active layer of solar cells. However, the unfavorable wide band gap seriously restricts its efficiency in this regard. To overcome the limitations, in the present study, we have explored several ways, such as modifying size, substrate, functionalization, and doping of hydrogen-passivated g-C3N4 quantum dots (QDs). Performing extensive density functional theory based calculations, we find that, unlike pristine QDs, proper modification of the electronic nature of the QDs can lead to efficient visible or nearinfrared (NIR) light response, making them better functional materials toward solar cell or photocatalytic applications. Interestingly, our studies further suggest that the modified passivated QDs are better catalysts than pristine ones used so far in the H2 evolution reaction. Also, the range of the optical gap these QDs display makes them appropriate for solar cell applications.

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INTRODUCTION

Despite all these fascinating properties, pure g-C3N4 has several limitations to be used as an efficient photocatalyst. Poor electrical conductivity and inefficient absorption of visible light due to its wide band gap nature strongly affect the performance of g-C3N4 as an active material for solar energy conversion.21 To overcome these limitations, various researchers have modified the pristine g-C3N4 sheets in several ways. In this regard, chemical doping, especially introducing P and S atoms in g-C3N4, has been already proved to be very successful experimentally.21−23 Experimentally, introducing dopant atoms into the g-C3N4 network is proved to be very cost-effective. For example, a P-doped g-C3N4 sheet can be formed using the very low cost and environmentally friendly precursors hexachlorocyclotriphosphazene and guanidiniumhydrochloride,24 and similarly, S-doped g-C3N4 sheet can be formed via a singlestep reaction using trithiocyanuric acid.25 Fundamentally, these dopant atoms give rise to midgap states in g-C3N4 as well as increase electrical conductivity resulting in effective photocatalytic activity.26−29 For example, Ye et al. and Ran et al. have demonstrated a P-doped g-C3N4 sheet as very efficient for the CO2 reduction reaction30 and photocatalytic H2 production31 under visible light, respectively. Another efficient way to modify the electronic nature of the g-C3N4 sheet is to use a suitable substrate, such as graphene, hBN, etc. The buckled nature of pure g-C3N4 is confirmed by the scanning electron microscopy (SEM) image,20 and this buckling leads to the curtailing of the potential number of

Efficient conversion of solar to chemical energy by solar cells and utilization of solar energy in various photocatalytic processes have been premeditated to be some of the best solutions to the energy crisis and environmental pollutions these days.1,2 Particularly, photocatalytic conversion of water to clean fuel H2 in the presence of a semiconductor photocatalyst plays an important role in this aspect as it involves easily available and nontoxic energy sources.3 Consequently, the search for efficient semiconductor materials for the water splitting reaction using solar energy has become one of the “hot topics” of material science research in recent days.4−7 A suitable semiconductor photocatalyst for the H2 evolution reaction should possess such a band gap that it can absorb light in the visible region of the solar spectrum.7 More importantly, the reduction and oxidation potentials of water should fall within the valence band maxima and conduction band minima of the semiconductor, which should also be stable in the presence of water for the reaction to proceed.8 Taking these factors into consideration, researchers have focused on finding effective and nontoxic transition metal free catalysts, and in the past few years, graphitic-carbon nitride (gC3N4)9 has brought a revolution in this field.8,10−12 Monolayer g-C3N4 is a heptazine unit based two-dimensional (2D) sheet, which is found to be the most stable allotrope of carbon nitride and is very stable under ambient temperature and pressure.11,13 Also, g-C3N4 is inexpensive, easy to synthesize, and possesses high surface area.14,15 Furthermore, due to its appropriate band gap (2.7 eV),16 it has been used as a catalyst in water splitting and CO2 photoreduction reactions.8,17−20 © 2017 American Chemical Society

Received: November 16, 2016 Revised: December 26, 2016 Published: January 4, 2017 1982

DOI: 10.1021/acs.jpcc.6b11520 J. Phys. Chem. C 2017, 121, 1982−1989

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The Journal of Physical Chemistry C

atom are relaxed until 0.0001 hartree/Bohr. For these systems, to calculate the optical properties, the SIESTA package55 with PBE/DFT-D256 has been used. Note that we have considered the absorption coefficient values perpendicular to the g-C3N4 plane to capture the stacking effect. Further details of the methods are mentioned in the Supporting Information. We have calculated the adsorption energy of the QDs with respect to their corresponding most stable phase (for example, for g-C3N4 QDs the buckled phase is the most stable phase), using the following formula:

catalytically active sites in g-C3N4 sheets. In the presence of substrate, the g-C3N4 sheet gets modified structurally as well as electronically; for example, effective charge transfer and increased electrical conductivity can be achieved in graphenesupported g-C3N4.14,32 Experimentally, it was found that graphene g-C3N4 composites are very effective catalysts for oxygen reduction reaction and these systems have good durability toward CO poisoning or electrolysis.33 One more interesting way to increase the number of catalytically active sites in g-C3N4 is to generate zerodimensional (0D) quantum dots (QD) from the 2D sheet.34 g-C3N4 QDs possess more numbers of catalytically active sites like pyridinic N, graphitic N, and edge amine groups.35 g-C3N4 QDs can be synthesized from the sheets efficiently via hydrothermal reactions.36 Furthermore, QDs with an absorption range covering the visible to near-infrared (NIR) regions of the solar energy find applications in photodetectors, sensors, bioimaging, and most importantly, solar cells (because of efficient IR absorption).37,38 Recently, experimental studies have shown that a nanocomposite of g-C3N4 QD and substrate displays enhanced visible light photocatalytic performance.39 Despite all these interesting findings, systematic study to improve the performance of g-C 3N 4 QDs by various modifications is still missing. Thus, in this present study, we have presented a systematic, in-depth exploration of structural, electronic, and optical properties of modified g-C3N4 QDs. Note that our QDs are nothing but the nanoflakes which are referred to in some literatures.40,41 However, we are terming these systems as QDs in this manuscript following some previous studies.35,42−44 We have considered hydrogenpassivated g-C3N4 QDs to find their applicability as a heterogeneous catalyst in the H2 evolution reaction and as a material that can be used in solar cell applications. Here, we have considered modification of the size, substrate, functionalization, and doping in the g-C3N4 QDs to modulate their optoelectronic properties. Interestingly, these modifications evidently alter the electronic nature of the QDs, leading to the realization of efficient visible or NIR light response. That, in turn, makes these QDs better functional materials toward solar cell or photocatalytic applications.

Eads = Ecomposite − EC3N4QD − Esubstrate

Finally, to verify the robustness of our results and to quantify it, we have calculated the highest occupied molecular orbital− lowest unoccupied molecular orbital (HOMO−LUMO, H−L) gap and electronic absorption spectra using the time-dependent density functional theory (TD-DFT) method, as implemented in the Gaussian 09 package using PBE and HSE06 functionals along with the 6-311+g(d) basis set. The details are given in Supporting Information and Tables S2 and S3.

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RESULTS AND DISCUSSION g-C3N4 QDs. At first, to explore the size effect in these systems, we have considered QDs with increasing number of heptazine units, starting from three (smallest QD, i.e., s-QD), to four (medium QD, i.e., m-QD), to the biggest of 12 heptazine units containing QD (biggest QD, i.e., b-QD). When HSE06 functional based DFT calculations are performed, it evidently appears that the H−L gap of the g-C3N4 QDs decreases with the increment in size of the QDs and it ranges between 3.09 and 2.76 eV (see Figure 1). Due to the quantum

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COMPUTATIONAL DETAILS The geometries of all small monolayer or supported g-C3N4 systems were optimized by density functional theory (DFT) as implemented in the Gaussian 09 program package.45 All the calculations are performed using the HSE06 hybrid functional46,47 as it gives accurate electronic structure for most of the semiconductors.48 For all the atoms, we have used 6-31g(d) and 6-311+g(d) basis sets to optimize the geometry and to calculate electronic structure, respectively. For the bigger bilayer and monolayer systems, we have used DFT with the PBE-GGA exchange-correlation functional, including Grimme’s empirical DFT-D2 dispersion correction,49 as implemented in the QUICKSTEP module of the CP2K package.50,51 We have used Goedecker−Teter−Hutter (GTH) pseudopotentials.52,53 Double-ζ valence polarized basis sets, optimized for the GTH pseudopotentials (DZVP−MOLOPT− SR−GTH), are used for the present calculations. We have considered a 20 Å vacuum in each side of the system to avoid any spurious interactions. An energy cutoff value of 320 Ry is used to construct the plane-waves in all the calculations. Using the highly efficient BFGS method,54 we have performed geometry optimizations where the interatomic forces on each

Figure 1. H−L gaps of different g-C3N4 QDs. Inset: g-C3N4 QDs of three different representative sizes, where cyan, blue, and silver balls are C, N, and H atoms, respectively (we have considered QDs other than these three sizes also, e.g., six or nine heptazine unit containing QDs). The pink star denotes the band gap of 2D g-C3N4 (ref 16).

confinement effect in smaller QDs, the H−L gap becomes larger than the band gap value of the bulk. With increment of the size of the QDs, the H−L gap slowly decreases, leading to the band gap found in bulk g-C3N4 (2.7 eV, see Figure 1).16 Note that the higher H−L gap of these g-C3N4 QDs prominently shows the absorbance of energy higher than the visible light range (i.e., about 400−700 nm). Consequently, these QDs are quite inefficient toward the visible light response. In the following part of the paper, we systematically explore various modifications in these pristine g-C3N4 QDs to make 1983


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As shown in Table 1, our calculations depict negative adsorption energy for all the systems, implicating a stable composite formation. It should be noted that the relative geometry between the supporting QD and the g-C3N4 QD is very important to describe the interactions accurately. Thus, we have considered the most stable stacking pattern found in previous studies for the bilayer nanocomposites.32,59 Moreover, as can be seen in Table 1, the composite formation generates a considerable amount of interlayer charge transfer (CT) in the GQD- and BNQD-supported systems. Note that here we have calculated the CT using Mülliken population analysis. Note that, for the g-C3N4 QD + GQD composite, the CT direction is from GQD to g-C3N4 QD, whereas the opposite trend can be found in g-C3N4 QD + BNQD. Fundamentally, carbon 2pz orbitals in graphene possess a delocalized π-electron cloud, which can be donated easily to g-C3N4 QD, whereas h-BN, being a CT compound,61 is more effective as an electron acceptor.43 This CT, as well as the π-stacking interaction between the sp2-hybridized C−N network and GQD, makes gC3N4 QD + GQD composites stable. Furthermore, higher adsorption energies for g-C3N4 QD + GQD than g-C3N4 QD + BNQD (see Table 1) appear due to the π-stacking interaction, which is only present between the sp2-hybridized C network of g-C3N4 QD and GQD. Importantly, irrespective of size and exchange correlation used, substrates introduce a huge change in the electronic properties of the composite QDs, especially, in the H−L gap of g-C3N4 QD + GQD (see Table 1); these changes do not occur due to the change in buckling, as g-C3N4 QDs show almost the same H−L gap, irrespective of buckling. As can be noted in Table 1, g-C3N4 QD + GQD shows a smaller spin-polarized H−L gap than both its parent QDs. Moreover, the composite is stable in the antiferromagnetic (AFM) ground state, similar to pure GQDs. Careful investigations show that, due to the spinpolarized CT in the system,43 the composite exhibits a spinpolarized H−L gap (see Table 1), whereas for g-C3N4 QD + BNQD, we do not find much variation in the H−L gap, as BNQD itself has a very large H−L gap, and thus, CT in this case does not impose significant modifications in the electronic

them suitable for photocatalytic as well as solar cell applications. Effect of Substrate. Lone-pair electron repulsion between nitrogen atoms of heptazine units corrugates g-C3N4 sheets, leading to deformation of the 2D planar network.57 This reduces the effective surface area, resulting in decrement of the potential number of catalytically active sites (accessible pyridinic or graphitic N sites).58 Use of suitable substrates to reduce the buckling of g-C3N4 is one of the well-demonstrated ways.59,60 Following that, we have considered a bilayer of gC3N4 QDs as well as other quantum dots of 2D layered materials, namely, parallelogram-shaped zigzag QDs of graphene (G) and hexagonal boron nitride (h-BN) QDs, under the g-C3N4 QDs (i.e., formation of g-C3N4 QD + GQD and g-C3N4 QD + BNQD composites) to eradicate the formation of buckled g-C3N4 QDs. To consider the effect of substrates in our systems, we have investigated g-C3N4 QDs stacked with different substrates using both the HSE06 functional (applied only for s-QDs) and PBE/ DFT-D2 (for QDs of experimentally reliable sizes,34 like b-QDs as well as another QD with nine heptazine units, see Figure S2 in the Supporting Information). We find that all the single and bilayer QDs show buckling in their optimized structure (see Figure 2a and Figure S1), whereas GQD- or BNQD-supported g-C3N4 QDs are relatively planar (see Figure 2b and Figure S3i).

Figure 2. Top and side views of (a) bilayer b-C3N4 QDs and (b) bC3N4 QD + GQD.

Table 1. CT, Adsorption Energy, and H−L Gaps in Different g-C3N4 QDs Calculated with PBE/DFT-D2a H−L gap (eV) g-C3N4 QDs bilayer b-QD b-QD from bilayer composite b-QD + GQD b-QD from GQD composite GQD b-QD + BNQD BNQD A-C3N4 QD + D-GQD S−C3N4 QDed + GQD S−C3N4 QDed S−C3N4 QDmid + GQD S−C3N4 QDmid P−C3N4 QDed + GQD P−C3N4 QDed P−C3N4 QDmid + GQD P−C3N4 QDmid a

charge transfer (e) 0.00

adsorption energy (eV/atom)

α-spin

β-spin

−0.025

1.58 1.73 0.38 1.7 0.57 1.79 3.90 0.06 0.13 0.22 0.40 0.99 0.05 0.41 0.39 1.07

1.58 1.73 0.61 1.7 0.57 1.79 3.90 0.08 0.57 1.89 0.22 1.14 0.57 0.62 0.34 1.23

−0.09

−0.05

0.12

0.00

−1.13 −0.03

−0.07 −0.27

−0.10

−0.27

−0.35

−0.27

−0.08

−0.27

Other QDs are reported in Table S1. 1984


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Figure 3. (a) Optical absorption spectra and (a′) the wave function plot corresponding to the low-energy transition in g-C3N4 QD + GQD composite, where the low-energy transition is marked by a blue star in panel a; (b) optical absorption spectra of A-C3N4 QD + D-GQD.

GQD to modified g-C3N4 QD is found and A-C3N4 QD + DGQD shows the maximum amount of CT as well as the lowest H−L gap. Enhanced CT also gives more stability to the composite systems (see Table 1 and Table S1). On the other hand, generating an electron-rich g-C3N4 QD shows a decrement in CT. We have functionalized g-C3N4 QD with hydroxyl groups (OH), which is also experimentally realizable,34 and found that indeed CT decreases in the composite (see Table S1). Due to enhancement of CT in A-C3N4 QD + D-GQD light gets absorbed over a huge range of wavelengths (see Figure 3b), starting from the UV to the IR. Note that the CT peak (∼0.3 eV) becomes more prominent for these systems. Thus, electronic and optical properties of the g-C3N4 QD + GQD composite can be modified easily using controlled functionalization of the QDs, making them more suitable for solar energy absorption. Effect of Doping. Another effective way to tune the wide band gap of g-C3N4 QDs is controlled doping of the QDs with suitable foreign nonmetal atoms,22,23,31,68,69 especially by ndoping the QDs with sulfur and phosphorus atoms,21 because of their similarity in atomic radii (S and P atoms have a little bigger radii than N or C atoms). Different characterization techniques, such as X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning tunneling microscopy (STM), etc., have already been used to characterize these doped g-C3N4 systems, and they give clear indication of the doped network formation.22,23 Experimental as well as computational studies have already shown significant modification of the band structure of 2D g-C3N4 as these n-type dopant atoms introduce prominent midgap states in the energy spectrum. As an example, Kong et al. have found that dopant S atoms contribute their 2p orbitals to the valence band of doped g-C3N4,28 resulting in band gap reduction. Looking at these promising results, we also focus on the effect of this elemental doping toward the CT, H−L gap, and optical absorption of the single-layer as well as graphene-stacked g-C3N4 QD systems. Note that, experimentally, S and P atoms generally replace parental (pyridinic or graphitic) N22 and C atoms,23 respectively. The electronegativity difference between S and C or N and P allows the S−C or N−P bonds to be quite stable. As shown in Figure 4, there can be two possibilities of doping sites for both the dopant atoms., i.e., one is at the edge of the pore [named as S/P−C3N4QDed (see Figure S4) and S/P− C3N4QDed + GQD, Figure 4, parts a and c] or at the middle of the melon unit [named as S/P−C3N4 QDmid (see Figure S4)

property of the composite as the CT peak also appears in a higher energy range than in g-C3N4 QD + GQD. Modification in electronic properties of the nanocomposites directly affects the optical properties of the systems. Appearance of a lower H−L gap in the g-C3N4 QD + GQD composite leads to the absorbance of light of lower wavelength than that of the parent QDs. When the optical absorption spectra of the composite as well as its constituents are plotted in Figure 3, it is quite evident that indeed the composite absorbs at a much lower energy range than pure g-C3N4 QD. Focusing on the low-energy peak at 0.4 eV for the g-C3N4 QD + GQD composite, the transition is CT in nature and appears due to transfer of charge from GQD to g-C3N4 QD. In fact, this composite absorbs in the visible and IR range, which is very important for high-efficiency solar cells as well as visible light photocatalytic applications.37,62,63 Interestingly, already Liu et al. have experimentally shown that carbon nanodots dispersed on a g-C3N4 sheet serve as an excellent photocatalyst in solar water splitting. 64 Also, Chen et al. have found that incorporation of g-C3N4 QDs in bulk heterojunction polymer solar cells leads to many fold efficiency enhancements.65 Looking at the experiments and our computational findings, we further predict that g-C3N4 QD + GQD can also be used as a high-efficiency photocatalyst for H2 evolution reactions. Effect of Functionalization. As the previous section evidently demonstrates improved performances of QDs due to CT from GQD to g-C3N4 QD, we further explore these systems by modifying the amount of CT in their composites. In this regard, chemical modification can provide a convenient and facile way to enhance the control over the CT and consequently over photophysical properties of the composite QDs. In this regard, controlled functionalization of graphene66 or g-C3N467 with electron donor or acceptor functional groups can be achieved easily. Thus, following to the direction of CT, the edge atoms of g-C3N4 QDs and GQDs have been passivated by equal numbers of electron acceptor (A) and donor (D) groups, respectively. Particularly, we have functionalized the g-C3N4 QDs with carboxyl groups (COOH or A) (this has been already achieved experimentally67), whereas GQDs are functionalized with amine groups (NH2 or D). These can form different kinds of composites, namely, A-C3N4 QD + GQD, C3N4 QD + D-GQD, and A-C3N4 QD + D-GQD (see Figure S3ii). These functionalizations increase the electron-deficient nature of g-C3N4 QD and electron-rich nature of GQD. Hence, as shown in Table 1 and Table S1 (see Supporting Information), an increment in CT from modified 1985


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dopant atoms in the 2D network. The doped systems, where dopant atoms show less out-of-plane movement, also become more stable. These results remain the same both for single-layer and composite systems. Doping also leads to charge redistribution in the systems. The electronegativity values of the atoms are in this order: N > S ≈ C > P. Thus, S or P doping in the systems leads to charge transfer from the dopants to the N (or C) atoms present near the dopant sites. Calculating the Mülliken charges on the atoms, we find that there is a visible amount of charge accumulation on the C3N4 moiety near the dopant sites, where the dopant atoms become electron-deficient. Charge accumulation on the N atoms near dopant sites (in the range of 0.2− 0.3 e) also becomes effective for catalytic activity of the systems. The electrostatic potential (ESP) plot in Figure 5e also confirms the charge redistribution. Further, as shown in Table 1, midgap states, which appear due to the n-type doping, get involved in interlayer CT between the doped g-C3N4 QD and GQD, resulting in increased stability (increased from −0.025 eV/atom in gC3N4 QD + GQD to −0.27 eV/atom in doped-C3N4 QD + GQD) of these composites relative to corresponding pristine systems. The presence of the midgap states affects the electronic properties of the doped systems also. These midgap states generally appear because of the singly occupied molecular orbital (SOMO), arising due to the S or P extra electron. The midgap states are mainly composed of the S or P 3pz orbitals. The systems become magnetic because of the presence of these electrons. Plotting the SOMO of these doped systems, we find that these orbitals are mainly located on the melon unit where the dopant atom is present (see Figure 5). This again demonstrates the possibility of charge redistribution from the dopant atom to the nearby sites. The presence of these SOMO levels creates spin-polarized H−L gaps in the systems. Thus, not only the composites, but the single-layer QDs also possess spin-polarized H−L gap after doping (see Table S1). Importantly, as shown in Table 1, these SOMOs tune the wide H−L gap of the pristine systems up to a large extent. Along with n-doping in g-C3N4, we also dope the GQD with nitrogen atoms to increase the electron-rich nature of it (see Figure S6 and Table S1). Evidently, as can be seen in Table S1, increasing the number of the dopant N atoms indeed facilitates the CT from doped GQD to g-C3N4 QD, resulting in stable composite formation. Focusing on the optical properties, most importantly, we find the appearance of low-energy optical transitions in these doped

Figure 4. Top view of (a) S−C3N4 QDed + GQD, (b) S−C3N4 QDmid + GQD, (c) P−C3N4 QDed + GQD, and (d) P−C3N4 QDmid + GQD; panels a′−d′ are their corresponding zoomed side views. Yellow and orange balls are S and P atoms, respectively.

and S/P−C3N4 QDmid + GQD, Figure 4, parts b and d]. For both the cases, S and P atoms get incorporated in the sp2hybridized network of g-C3N4 in spite of their bigger radii than those of N or C atoms. This leads to the structural distortion in g-C3N4 (see Figure 4a′−d′). But this distortion is quite local, and the network away from the doping site remains in the parent structure (details for the single-layer systems are given in Supporting Information). Calculating the out-of-plane movement of dopants in the doped QDs we find that, in S−C3N4 QDed + GQD, the S atom moves away from the GQD side by ∼0.72 Å, whereas in S− C3N4 QDmid + GQD the S atom shifts toward the GQD by ∼1.17 Å. For P-doped QDs we find that, in P−C3N4 QDed + GQD, the P atom moves toward the GQD by ∼0.76 Å, but in P−C3N4 QDmid + GQD the P atom remains almost in the plane with very little distortion (see Figure 4a′−d′). In the case of S doping, the S−C3N4 QDed + GQD becomes energetically more stable by 0.87 eV (with respect to total energy of the two structures) than S−C3N4 QDmid + GQD, whereas P doping shows the opposite trend, i.e., P−C3N4 QDmid gets stabilized by 0.57 eV relative to the other one. This can be visibly related to the out-of-plane movement of the

Figure 5. (a) SOMO of S−C3N4 QDed, (b) SOMO of S−C3N4 QDmid, (c) SOMO of P−C3N4 QDed, and (d) SOMO of P−C3N4 QDmid. (e) Electrostatic potential (ESP) plot of S−C3N4 QDed; the red parts indicate electron-rich and the blue part indicates electron-deficient sites. 1986


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Figure 6. Optical absorption spectra of (a) S−C3N4 QD + GQD composites and (b) P−C3N4 QD + GQD composites.

the H2 evolution reaction under visible light. Recent experimental studies have also shown that S-doped g-C3N4 2D systems indeed can be used as better photocatalysts for hydrogen evolution and carbon dioxide reduction reactions.27−29 These doped QDs become very good photocatalysts because of the presence of the optically active SOMO level inside the wide H−L gap of pristine g-C3N4 QD. The 3pz orbital of the S or P atoms provides a suitable state for the photoactivity of the doped QDs. Also, the charge redistribution from the dopant atoms to the nearby N atoms provides convenient sites for the reaction to happen.

g-C3N4 QD + GQD composites (see Figure 6). In these systems, not only does the interlayer CT peak appear, but also the doped g-C3N4 QDs themselves introduce distinct lowenergy peaks in the spectrum (see Figure S5, parts a and b). These peaks are the result of the charge redistribution (i.e., intralayer charge transfer) in the systems (see Figure S5c). This suggests the importance of the single-layer as well as composite-doped systems toward enhanced visible light response. Recent experimental reports also support our computational findings as doped g-C3N4 sheets were found to absorb in the visible energy region of the solar spectrum.23,27 Photocatalytic Activity of g-C3N4 QDs in Visible Light H2 Production. We have finally calculated the HOMO and LUMO energies of the systems using HSE06 functionals to check if the absolute values of the reduction and oxidation levels for H2O are located inside the optical gap of the systems. Experimentally, μeH+/H2 is equal to −4.44 V and μhO2/H2O = −5.67 eV.8,70 Thus, VO2/H2O − VH+/H2 is 1.23 V, and our system should have an H−L gap equal to or slightly greater than 1.23 eV for the material to be active for photocatalytic H2 evolution reaction. We find that the pure g-C3N4 QDs and the doped gC3N4 QDs can be used as catalysts in H2 production (see Figure 7). Out of all the systems, S−C3N4 QDmid is found to be the best catalyst in this reaction as it has the H−L gap very near to 1.23 eV for one spin channel and the HOMO and LUMO levels are arranged in a very favorable manner for the reaction to take place. Also, these doped QDs absorb in the visible range of the solar spectrum (see Figure S7, TD-DFT plot of the QDs) and, hence, can be used as an effective photocatalyst for

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CONCLUSIONS In conclusion, performing systematic DFT calculations on gC3N4 QDs and modified QDs, we have studied electronic, optical, and photocatalytic properties of the QDs. The experimental methods used by which these g-C3N4 QDs are prepared can also be used to prepare functionalized or doped gC3N4 QDs. We present a methodical study on the g-C3N4 QDs or their functionalized and doped variants. Our calculations reveal that pristine g-C3N4 QDs are not very effective as a photocatalyst in the H2 evolution reaction under visible light, as they do not absorb light in this energy range. Also, because of their high H−L gap, they cannot be useful in solar cell applications. In this work, we have incorporated some physicochemical changes in the g-C3N4 QDs to improve their visible light response. Evidently, by generating nanocomposites of g-C3N4 QDs and GQDs, we can modify the interlayer CT in the system, causing the material to absorb in the IR range. This makes the composite applicable in solar cells. The charge transfer from GQD to g-C3N4 QDs can be modified by functionalizing one or both of the QDs. Electron donor and acceptor group functionalization on the edges of the graphene and g-C3N4 QD, respectively, can further modify the H−L gap and optical absorption of the systems. Doping nonmetals into the QD network has also been studied, and we find that S or P atom doped QDs show prominent absorption peaks at the visible and IR range of the solar spectrum, confirming their capability to absorb visible light. Interestingly, for the doped gC3N4 QDs, reduction and oxidation levels for H2O are located inside the H−L gap of the systems, establishing their ability toward photocatalytic water splitting under visible light. With these exciting computational results and some recent experimental findings, we conclude that these QDs will find promising applications as a better solar energy harvesting system for various reactions.

Figure 7. HOMO and LUMO energy levels of (1) m-C3N4 QD, (2) β-spin of s-S−C3N4 QDmid, (3) β-spin of m-S−C3N4 QDmid, (4) β-spin of s-S−C3N4 QDed, (5) β-spin of m-S−C3N4 QDed, (6) α-spin of s-P− C3N4 QDed, (7) α-spin of m-P−C3N4 QDed, (8) α-spin of s-P−C3N4 QDmid, and (9) α-spin of m-P−C3N4 QDmid with respect to μeH+/H2 and μhO2/H2O. 1987


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11520. Discussion about computational details, details on electronic properties of g-C3N4 QDs on BN support, with OH functionalization, on N-doped GQD, and single-layer P/S-QDs, PBE and HSE06 H−L gap and optical transition comparison using Gaussian09 software (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Swapan K. Pati: 0000-0002-5124-7455 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.B. acknowledges UGC, India for a senior research fellowship, and S.K.P. thanks DST, Government of India for research grants.

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REFERENCES

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Quantum Dots in Visible Light - ACS Publications - American

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