Advances on Theory and Experiments of the Energy Applications in Graphdiyne

Open AccessCCS ChemistryMINI REVIEWS8 Sep 2022Advances on Theory and Experiments of the Energy Applications in Graphdiyne Feng He and Yuliang Li Feng He *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 and Yuliang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202328 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail After years of development, graphdiyne (GDY) has demonstrated the characteristics of transformative materials in many fields and has promoted great progress in fundamental and applied research. In practice, some important new concepts have been proposed, such as natural surface charge distribution inhomogeneity, multicavity space limiting effect, incomplete charge transfer effect on the atomic level, alkyne–alkene conversion of a chemical bond, in situ induction of constrained growth, reversible transition from high to low valence state, and so on. These characteristics originating from the special electronic structure and chemical structure of GDY have rapidly promoted the development of GDY science in recent years and produced many exciting results in fundamental and applied science. Therefore, we systematically introduce the recent theoretical and experimental progress of GDY in terms of its new structural, electronic, mechanical, thermal, and optical properties and its promising applications in the energy fields of membrane sciences, catalysis, energy storage, and conversion. Specifically, the great breakthrough of GDY zero-valence atomic catalysts, quantum dots catalysts, and heterostructure catalysts for catalytic applications are discussed in detail. We believe this review will provide some significant guidelines for the design and development of GDY-based high-performance materials and devices in energy fields. Download figure Download PowerPoint Introduction As an emerging carbon material containing sp and sp2 hybridization, graphdiyne (GDY), first discovered by Li's team in 2010,1 has many outstanding characteristics. The appearance of GDY opens up the new research field of carbon materials and will gradually form new directions in the fields of chemistry, materials, and physics.2,3 With its rapid development in recent years, GDY has attracted more and more attention from scientists, demonstrating the great potential of transformative materials in the new fields. As one of the TOP10 frontiers in chemical and materials science in 2020 released by Clarivate and Chinese Academy of Sciences, GDY research has garnered high international attention. GDY shows many natural advantages including the in situ growth abilities on any substrate surface under low temperature and mild conditions, the inhomogeneous surface charge distribution, the rich alkyne bonds and high intrinsic activities, the high porosity and large specific-surface area, the wide interplanar distance and excellent chemical stability, and the high electrical conductivity and carrier mobility.4–12 These novel properties of GDY have inspired many new ideas and derived many new concepts and knowledge for materials research. The new concepts, phenomena, and properties initiated by GDY have changed scientists' understanding of traditional carbon materials. In recent years, the fundamental and applied research of GDY have expanded into the fields of self-assembly, in situ growth, catalysis, optoelectronics, environmental engineering, new materials, intelligent devices, new models of energy storage, and conversion technologies.13–18 For example, GDY-based materials have been widely applied in photoelectric conversion,19–22 photo-thermal conversion,23 electrochemical actuators,24 photo- and electrocatalysis,25–30 oil-water separation,31,32 biological detection,33,34 photoelectric detection,35 electrolyte-gated transistors,36 and artificial synapse.37 The previous reviews on GDY research mainly focus on the controlled synthesis and the versatile applications in experiments. It is crucially important for an in-depth understanding of GDY from a theoretical perspective of the new properties, new phenomena, and new concepts. In this mini review, the latest theoretical and experimental advances on the properties and applications of GDY over the past 5 years are systematically introduced. First, we introduce the recent discoveries on the novel properties of GDY including the structural, electronic, mechanical, thermal, and optical properties. Then, an overview is provided to summarize the newest theoretical insights derived from experiments on the applications of GDY in the energy fields of membrane sciences, catalysis, energy storage, and conversion. Specifically, the great breakthrough for the catalysis applications using GDY as a support to induce growth of zero-valence atomic catalysts (ACs), quantum dots (QDs) catalysts, and heterostructure catalysts are discussed in detail. Many new concepts such as natural surface-charge distribution inhomogeneity, multicavity space limiting effect, the incomplete charge-transfer effect on the atomic level, the alkyne–alkene conversion of the chemical bond, the "donor–acceptor" electron transfer stabilization mechanism, the in situ induction for constrained growth, and the reversible transition from high to low valence state are deeply elaborated by combining the theoretical and experimental studies. Finally, the opportunities and challenges of the fundamental and applied research of GDY are briefly discussed. We firmly believe that this review will certainly bridge the gap between the theoretical and experimental research and provide some significant guidelines for exploring the new application modes of GDY-based materials and devices in the energy fields. Intrinsic Properties of GDY Structural and electronic properties The intrinsic structural and electronic properties highlight the great potential applications of GDY in nanoelectronic devices and field-effect transistors. Using density functional theory (DFT), Shuai et al.38 calculated the Vienna ab-initio simulation package optimized lattice constant of the two-dimensional (2D) GDY monolayer (Figure 1a) as a0 = b0 = 9.48 Å at the equilibrium state. GDY is a semiconductor with a natural direct band gap of ∼0.46 eV (Figure 1b) at the Γ point of the Brillouin zone.39–41 The characteristics with intrinsic band gap is quite appropriate for the semiconductor and optoelectronic devices. Note that the generalized-gradient approximation or local-density approximation functional will underestimate the band gap of GDY without considering the self-energy from many-electron Coulomb interactions. The calculated band gap value of GDY can be noticeably increased to ∼1.22 eV42 at the hybrid functional (HSE06) level and to ∼1.10 eV43 at the GW theory level. The electronic transport properties of GDY were studied within a supercell sheet. The calculated in-plane electron mobility (μe) of GDY can reach 2 × 105 cm2 V−1s−1 at room temperature, comparable to that of graphene,44 whereas the hole mobility (μh) of GDY is about 2 × 104 cm2 V−1s−1, an order of magnitude lower than the electron mobility. The much larger electron mobility compared to hole mobility can be qualitatively explained by the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of GDY, where the number of nodes for the HOMO is greater than that for LUMO in either direction (Figure 1c), resulting in the hole is much strongly scattered by phonon. Figure 1 | (a) Schematic representation of GDY structure. (b) Band structure and DOSs of GDY at DFT level. (c) Γ-point degenerate HOMO and LUMO for GDY. Reprinted with permission from Long et al.38 Copyright 2011 American Chemistry Society. 2D and 3D band structures of GDY under (d–f) symmetrical biaxial tensile strain of 5%, 9%, and 15%, (g–i) uniaxial tensile strain of 5%, 9%, and 15% along the armchair direction, and (j–l) uniaxial tensile strain of 5%, 9%, and 19% along the zigzag direction. Reprinted with permission from Cui et al.46 Copyright 2013 Royal Society of Chemistry. Download figure Download PowerPoint Band gap engineering The relatively small band gap of GDY may limit its practical applications in electronic devices. The electronic structures of GDY can be effectively modified by applying strain41,45 because the π electron densities and the overlap of the pz orbitals that dominate the electronic structures around the Fermi surface are significantly affected by strain. Cui et al.46 systemically studied the strain-induced electronic structures of GDY using DFT calculations. Under the symmetrical biaxial tensile strain, the band gap of GDY gradually increases to 1.39 eV as the strain increases (Figure 1d–f). However, when applying the asymmetrical uniaxial tensile strain, the band gap will decrease continuously to nearly zero as the strain increases either along the armchair (Figure 1g–i) or zigzag direction (Figure 1j–l), forming Dirac cone-like electronic structures and inducing the properties transition of GDY from a semiconductor to a semimetal. This is because the uniaxial tensile strain destroys the geometrical symmetry of GDY, thereby lifting the energy bands degeneracy by upshifting the valence band maximum (VBM) and downshifting the conduction band minimum (CBM). In contrast, the electronic structures of GDY become more complex when applying the uniaxial tensile strain along the zigzag direction, where the band gaps will be opened again as the strain exceeds 17%. The electronic properties of GDY can be effectively modified by doping with metal or nonmetal atoms. The anchored metal atoms endow GDY with spin-polarized half-semiconductor properties,47 which are mainly derived from the charge transfer between metal atoms and GDY as well as the electron redistribution caused by the strong d–p orbital coupling. The nonmetal dopants with fewer electrons than C (such as B) and more electrons than C (such as N) will introduce additional holes and electrons into the valence band and conduction band of GDY, respectively. The B-doped and N-doped GDY thereby exhibit the p-type and n-type semiconductor characteristics with Fermi level below VBM and above CBM, respectively.48 The co-doping of BN (BN, pairs of B and N atoms) pairs can also significantly open the band gap of GDY, which increases gradually but abruptly as the concentration of BN pairs increase.49 The sudden change in band gap is derived from the heterogeneity of π-bonds between C-2pz orbitals in the diacetylenic chains (sp-C) and aromatic rings (sp2-C), which will be magnified when substituting different hybridized carbon atoms with the BN pairs. In addition, the band gap of GDY can be efficiently modified by employing functionalizations such as halogenation,50 depending on the positions (sp or sp2-C site), styles, and concentrations of the adsorbed halogen atoms. The tunable band gaps arise from the decreased density of states (DOSs) of vertically arranged pz orbitals near the Fermi level when increasing the concentration of halogen atoms. Second-order topological insulator properties The unique electronic properties endow the GDY monolayer with 2D second-order topological insulator (SOTI) features based on first-principles calculations.51 As known, the higher-order topological insulator (TI) is a new topological phase, which has recently inspired great research interest and is only theoretically predicted in a few three-dimensional (3D) materials. According to the definition of the conventional first-order TIs that feature the bulk boundary correspondence, the GDY material is topologically trivial due to the negligible spin–orbit coupling and time-reversal symmetry. However, the GDY monolayer is the first realistic 2D electronic second-order topology insulator. The band inversion appears at the Γ point between two doublet states for the electronic band structures of GDY, with an approximate chiral symmetry that derives from the sublattice symmetry structure. There exist the in-gap edge states with roughly symmetrical low-energy spectra at zero energy for the zigzag edge (Figure 2a,b) and armchair edge (Figure 2c,d) of GDY. Also, there are six degenerate states at zero energy (Figure 2e), and the charge accumulation of zero-energy states is well localized at the My-invariant corners of GDY (Figure 2f). Figure 2 | (a,b) Zigzag and (c,d) armchair edge of a semi-infinite GDY sheet with corresponding projected spectra. (e) Charge distribution for a hexagonal-shaped GDY nanodisk, which are localized at the corners. (f) The six zero-energy states in the corresponding energy spectrum. Reprinted with permission from Sheng et al.51 Copyright 2019 American Physical Society. (g) Discrete out-of-plane and (h) in-plane phonon levels of hexagonal GDY cluster with six corner states. (i) EPC between 42 electron states around the Fermi level and 32 out-of-plane phonon modes around the nontrivial gap. (j) Spatial distribution of the localized six phononic and electronic corner states. Reprinted with permission from Mu et al.52 Copyright 2022 American Chemistry Society. Download figure Download PowerPoint GDY monolayer was extended to a natural 2D phononic SOTI material in both out-of-plane and in-plane modes based on DFT calculations.52 The 2D phononic SOTI properties of GDY were identified by the following characteristics. There are six almost degenerate and six spatially localized corner states (red cube) inside the bulk gap in the discrete out-of-plane and in-plane phonon levels, respectively, which is spatially localized at six corners of the hexagonal GDY cluster, showing the in-gap topological corner state characters for 2D SOTI (Figure 2g,h). The electron–phonon coupling (EPC) between the six phononic and electronic topological corner states in GDY is distinctive, showing the unique interplay between the localized phononic and electronic topological corner states (Figure 2i,j). In addition, the EPC is obviously enhanced when the phononic and electronic states with even mirror symmetry of spatial distribution are localized at the same corner. The spatially localized corner phonon modes are useful for the phononic applications and phonon–photon coupling at nanoscale. Dimension dependent properties The stacking style, crystal structure, and electronic properties of 3D bulk GDY multilayers were identified by numerous theoretical and experimental studies.53–55 It is evidently confirmed that the synthesized GDY multilayers are exclusively matched with the ABC-type stacking style (Figure 3a). Theoretical calculations show that the ABC-stacked bulk GDY is a semiconductor with a direct band gap of 0.37 eV using the Perdew–Burke–Ernzerbof functional or 0.73 eV using the HSE06 functional, which is close to the optical absorption spectrum measured value of 0.64 eV.55 First-principles calculations predicted that the ABC-stacked bulk GDY was the first realistic 3D second-order topological nodal-line semimetal or real Chern insulator when applying the tensile strain along the z axis.56 The electronic structures of 3D GDY are different from those of 2D GDY. Although the band gap of bulk GDY (Figure 3b) is comparable to that of 2D GDY at the high-symmetry of Γ point, the conduction and valence bands in the low-energy bands actually cross at a pair of closed nodal rings around the high-symmetry of Z point (Figure 3c). According to the spectrum (Figure 3d,e), there exists a pair of drumhead surface bands bounded by the topological projected nodal rings for the bulk GDY. The Fermi surface of ABC-stacked GDY features the unique self-intersecting hourglass shape,57 which is distinct from the torus or pipe shape in the conventional nodal-line semimetals.58,59 Figure 3 | (a) Side view of crystal structure of 3D bulk GDY with ABC stacking. (b) 2D energy band structures and projected DOS (PDOS) along (c) the high symmetry points in Brillouin zone with two real nodal rings. (d) Spectrum for the sample with a tube-like geometry. The red color indicates the topological hinge band. (e) Projected spectrum for the (001) surface. Reprinted with permission from Chen et al.56 Copyright 2022 American Physical Society. (f) Schematic illustration of the synthesis procedures for GDY nanospheres. (g) Model of GDY540 (left) and the differential charge density of Pd13@GDY540 (right). Reprinted with permission from Yu et al.60 Copyright 2022 Wiley-VCH. Download figure Download PowerPoint Very recently, zero-dimensional GDY nanospheres were successfully produced by directly heating hexaethynylbenzene at 120 °C in the air without the use of any metal catalyst (Figure 3f).60 The corresponding theoretical model of single GDY nanosphere was built by He et al.60 (Figure 3g). The minimum GDY nanosphere is composed of 540 carbon atoms with an approximate diameter of 3.0 nm (denoted as GDY540), consistent with the measured one (3–5 nm) from transmission electron microscopy (TEM). Theoretical studies found that the GDY nanosphere acts as an electron acceptor that can alter the electronic structure of Pd13 nanocluster via the strong d–π interaction. Based on the differential charge density analysis, an electron is obviously transferred from the Pd13 nanocluster to GDY nanosphere (Figure 3g). Also, the Pd 4d band centers are significantly apart from the Fermi level for all bottom four Pd atoms that interact with alkyne bonds in the triangular cavity of Pd13@GDY540 compared with those of pure Pd(111), effectively weakening the adsorption ability for reactants, which is usually beneficial for catalysis according to the Sabatier principle.61,62 The electronic properties of one-dimensional GDY nanoribbons (GDNRs) were widely studied using first-principles calculations.63,64 GDNRs with either the armchair edge (AGDNRs) or the zigzag edge (ZGDNRs) show intrinsic semiconductor properties and high carrier mobility.64 The band gaps of GDNRs varied from 0.54 to 0.97 eV for AGDNRs and from 0.73 to 1.65 eV for ZGDNRs, respectively, which are larger than that of 2D GDY. The carrier mobility of GDNRs is in a range of 102–106 cm2 V−1s−1 at room temperature. Similar to the GDY monolayer, the electron mobility of GDNRs is always significantly larger than the hole mobility. Besides the edge morphologies, the band gaps and carrier mobility of ZGDNRs and AGDNRs are closely related to the widths of nanoribbons, that is, the band gaps will obviously decrease as the widths increase, whereas the trend is opposite for the carrier mobility. The band gaps of GDNRs can be controllably adjusted by applying strain,65 which is significantly enlarged to 2.92 eV. Moreover, the band gaps of AGDNRs will undergo the transition from a direct-gap semiconductor to an indirect-gap semiconductor when applying sufficient strain. Mechanical properties The unique mechanical properties of GDY attract the wide attention and interest of scientists. The inclusion of acetylenic linkages decreases the bonding number and planar density of the GDY structure, thereby reducing the rigidity of the GDY material. The unique mechanical properties make GDY much softer and facilitates its consideration for a wide variety of applications that need soft materials such as membrane separations. Whether under tensile or compressive strain, the strain energy increases monotonously with the applied strain, indicating the elastic deformation behavior of GDY. The fracture strain of GDY strongly depends on the loading direction, which is lower along the armchair direction than the zigzag direction. The unique mechanical properties facilitate the design of GDY-based advanced nanomechanical carbon materials with direction-dependent properties. The fracture behavior of GDY film was depicted by simulating the nanoindentation processes using molecular dynamics (MD) simulations (Figure 4a).66 When the indentation depth exceeds the critical value, the GDY film greatly expands and the bonds are broken in the order of acetylenic (–C≡C–), double (–C=C–), and single bonds (–C–C–) due to the more stable σ bond than π bond. To sustain further deformation of GDY film, the broken bonds can be partially recombined to form the asymmetrical lathy crack. The fracture behavior of GDY film is completely different from the symmetrical fracture of graphene film.67 Buehler et al.68 systematically studied the mechanical properties of extended GDYs with repeated acetylenic groups (n = 1–4) using MD simulations. There is a simple scaling law between the in-plane stiffness (Cn) and acetylenic groups (n) for extended GDYs: Cn = 166.3/[1 + 0.373 (n − 1)], where the in-plane stiffness is degraded gradually with the increase of acetylenic groups.69 According to the stress–strain response plots, the stress of graphene and extended GDYs increases almost linearly with the strain and then drops sharply during failure (Figure 4b). In contrast, the fracture behavior of graphene is brittle-like, while the responses for extended GDYs are more complex, which evidences a unique "two-tier" fracture mode with multiple stress peaks due to the mobility of acetylenic groups. Although the ultimate stress of GDYs is less than that of graphene, the modulus and strength of GDYs are comparable to the high performance of fibers, showing the potential applications in chemically selective filters, energy storage, or composites scaffolds. Figure 4 | (a) Evolution of lattice fracture behavior of GDY film. Reprinted with permission from Xiao et al.66 Copyright 2019 Elsevier. (b) Stress–strain plots for each model and the visualization of failure. Reprinted with permission from Cranford et al.68 Copyright 2012 Royal Society of Chemistry. (c) Maps of calculated electronic transport coefficients of GDY, such as Seebeck coefficient S, electrical conductivity σ, power factor S2σ, and ZT values as a function of temperature T and carrier concentration n. Reprinted with permission from Tan et al.70 Copyright 2015 Royal Society of Chemistry. Download figure Download PowerPoint Thermoelectric properties Thermoelectric materials are environmentally friendly and can directly realize the energy conversion between heat and electricity. The performance of a thermoelectric material is evaluated by its figure of merit, ZT = S2σT/(κe + κl), where σ is electrical conductivity, S is the Seebeck coefficient, and κe and κl are the electronic and lattice part of the thermal conductivity. Theoretical studies reveal that GDY is a promising thermoelectric candidate material due to the high electrical conductivity, large Seebeck coefficient, and low thermal conductivity (i.e., high thermal resistance). Tan et al.70 investigated the thermoelectric properties of 2D GDY using first-principles calculations. It was found that the calculated ZT values of GDY exhibit highlighted areas in a wide temperature range (almost 300–900 K) and a wide doping range (n = 0.2 × 1019− 8.0 × 1019 cm−3) (Figure 4c). MD simulations showed that the optimized ZT value of GDY at room temperature (300 K) can reach 3.0 for holes (p-type carrier) and 4.8 for electrons (n-type carrier) with a carrier concentration of 2.74 × 1011 cm−2 and 1.62 × 1011 cm−2, respectively.71 Such high ZT values exceed the reported values for traditional thermoelectric materials and meet the target (ZT = 3.0) for the commercial thermoelectric applications. Optical properties The attractive optical properties of GDY were widely studied.72,73 Electrochemiluminescence (ECL) technology is one powerful tool for ultrasensitive sensing and imaging. Mao et al.74 found that GDY shows a strong ECL emission with potassium persulfate (K2S2O8) as coreactant. Mechanistic study indicates that the ECL emission of GDY is generated through the lower-energy surface-state-transition mechanism rather than the band gap transition (Figure 5a), which is verified by regulating the surface state through GDY oxidation (Figure 5b). Interestingly, ECL is generated at 705 nm in the near infrared region with an ECL efficiency of 424% (Figure 5c). These findings indicate the emerging applications of GDY in photoelectrochemistry, light-emitting devices, and ECL sensing. Figure 5 | (a) Schematic illustration of the ECL emission of GDY. (b) ECL–time curves recorded at the GDY/GC electrode in 0.1 M PBS (pH 7.4) with 100 mM K2S2O8 at different electrolysis potentials. (c) Comparison of ECL intensity before and after acid oxidation for 2 h (GDYO-1) and 4 h (GDYO-2). Reprinted with permission from Gao et al.74 Copyright 2022 Wiley-VCH. (d) Total and partial DOS of [email protected] The purple dashed lines correspond to the HOMO and LUMO. (e) Simulated optical absorption spectra of [email protected] The oscillator strengths of the transitions are inserted. (f) Main electronic transitions in crucial excited states (Tc) of [email protected] Reprinted with permission from Li and Li.75 Copyright 2019 Royal Society of Chemistry. Download figure Download PowerPoint 2D materials with promising and tunable nonlinear optical (NLO) properties are desirable for the development of next-generation carbon-based optoelectronic nanodevices. Li and Li75 found that the NLO responses of GDY can be dramatically enhanced when adsorbing single alkali metal atoms (AM = Li, Na, K). These AMs energetically prefer to anchor in the large triangular pores of π-conjugated GDY via the van der Waals interactions and the intramolecular electron donor–acceptor (D–π–A) process. The adsorbed AMs extremely affect the electronic structures of GDY (Figure 5d). Moreover, the adsorption of AMs greatly increases the first hyperpolarizability (βtot) of GDY due to the strong alkalide characteristics. Especially, the K adsorbed GDY possesses the highest βtot value of 3.93 × 105 (arbitrary units), exhibiting the strong NLO properties. The high βtot values of AM adsorbed GDY can be understood by the crucial electronic transitions from time-dependent DFT calculations (Figure 5e). The electronic transitions maps visually reveal that the main electronic transitions in the crucial excited states mostly originate from the HOMO–2 → LUMO transitions of alpha orbitals (Figure 5f), which is mainly associated with out-of-plane π orbitals that make the dominant contributions to the βtot value. Energy Applications of GDY Owing to its novel structural, electronic, mechanical, thermal, and optical properties, in the past 5 years, GDY has shown great advantages and advances on the fundamental and applied research in the energy fields of membrane science, catalysis, new models of energy storage, and conversion technologies. Moreover, many new concepts and new systems are derived and established by combining the theoretical and experimental studies. GDY membrane for ion/gas separation The advantages such as natural triangular pores, superior pore uniformity, suitable van der Waals pore size (0.06 nm2), unprecedented pore density (2.5 × 1018 m−2), and high mechanical stability endow GDY great application potential as a superior membrane material for ion/gas separation.42 Moreover, GDY provides a unique platform to facilitate selective separation without constructing the pores through chemical functionalization. The pore size of GDY membrane is ∼0.54 nm (Figure 6a),76 which is suitable for the passage of targets with smaller diameter. Both experiments and theoretical calculations indicate that GDY has highly selective ion separation abilities for An3+/Ln3+, Th4+ /UO22+, and Cs+/Sr2+ in the nuclear fuel cycle (Figure 6a) due to the distinguished adsorption behaviors.77 That is, the An3+, Th4+, and Cs+ preferably adsorb on the special triangular structure of GDY, while the Ln3+, UO22+, and Sr2+ are not adsorbed by GDY at all