Optical and quantitative detection of cobalt ion using graphitic carbon nitride-based chemosensor for hydrometallurgy of waste lithium-ion batteries.

A hydrometallurgy is one of the most important techniques for recycling waste LIBs, where identifying the exact composition of the metal-leached solution is critical in controlling the metal extraction efficiency and the stoichiometry of the regenerated product. In this study, we report a simple and selective optical detection of high-concentrated Co2+ using a graphitic carbon nitride (g-CN)-based fluorescent chemosensor. g-CN is prepared by molten salt synthesis using dicyandiamide (DCDA) and LiI/KI. The mass ratio of LiI/KI to DCDA modifies the resulting g-CN (CNI) in terms of in-plane molecular distances of base sites including cyano functional groups (─CN) and fluorescent emission wavelength via nucleophilic substitution. The fluorescent sensing performance of CNIs is evaluated through photoluminescence (PL) emission spectroscopy in a broad Co2+ concentration range (10-4-100 M). The correlation between the surface exposure of hidden nitrogen pots (base sites) and PL intensity change is achieved where the linear relationship between the PL quenching and the logarithm of Co2+ concentration in the analyte solution is well established with the regression of 0.9959. This study will provide the design principle of the chemosensor suitable for the fast and accurate optical detection of Co2+ present in a broad concentration range for hydrometallurgy for the recycling of waste LIBs.

Solar-driven enhanced chemical adsorption and interfacial evaporation using porous graphene-based spherical composites.

Solar-energy-driven water purification is a promising technology for obtaining clean water during the current global climate crisis. Solar absorbers with high light absorption capacity and efficient energy conversion are critical components of solar-driven water evaporation and purification systems. Herein, we demonstrate that porous reduced graphene oxide (rGO)-based composite spheres facilitate efficient water evaporation and effective organic pollutant adsorption from water. Most solar light (>99% for 1 mm thick composites) is absorbed by the porous rGO-based composite spheres floating on water and is subsequently converted into heat, which is efficiently transferred to water at the air-water interface. Evaporation efficiency via energy conversion by the floating sphere composites reaches ∼74%. The increase in surface temperature of water also contributes to improving the adsorption capacity of the rGO-based composite spheres for organic pollutants. Furthermore, the composites can effectively block ultraviolet radiation, preventing the chemical reaction of water pollutants into harmful components.

Macroscopic graphitic carbon nitride monolith for efficient hydrogen production by photocatalytic reforming of glucose under sunlight.

Large particulate photocatalysts allow efficient recovery or installation into the substrate, while limiting possible light-catalyst interaction or mass/charge-transfer. In this study, we developed monodisperse organic single-crystal monoliths with controllable dimensions in the range of 10-100 µm. These were prepared on a 10-g scale by a solution-processed molecular cooperative assembly between melamine (M) and trithiocyanuric acid (TCA) and then transformed into the corresponding g-CN (MTCA-CN) by thermal polycondensation. Molecular precursors that are tightly bound in the crystal undergo polycondensation without losing their macroscopic properties depending on the dimensions of MTCA, thereby changing the microstructure, electronic structure, and photocatalytic activity. Such dimensional tunability enables the fulfillment of various catalytic requirements such as particle size, light absorption, charge separation, band edge potential, and mass transfer. As a proof-of-concept, it was shown that MTCA-CN is tailored to have a high rate of evolution of hydrogen (3.19 µmol/h) from glucose via photoreforming under AM1.5G by using MTCA-100 crystals, leading to the formation of g-CN with the more positive highest occupied molecular orbital (HOMO) level. This study highlights the possibility of developing photocatalysts for practical use and obtaining value-added products (VAPs) without losing the photocatalytic activity relevant for wastewater treatment.

Synthesis and NOx removal performance of anatase S-TiO2/g-CN heterojunction formed from dye wastewater sludge.

In this study, sludges generated from Ti-based flocculation of dye wastewater were used to retrieve photoactive titania (S-TiO2). It was heterojunctioned with graphitic carbon nitride (g-CN) to augment photoactivity under UV/visible light irradiance. Later the as-prepared samples were utilized to remove nitrogen oxides (NOx) in the atmospheric condition through photocatalysis. Heterojunction between S-TiO2 and g-CN was prepared through facile calcination (@550 °C) of S-TiO2 and melamine mix. Advanced sample characterization was carried out and documented extensively. Successful heterojunction was confirmed from the assessment of morphological and optical attributes of the samples. Finally, the prepared samples' level of photoactivity was assessed through photooxidation of NOx under both UV and visible light irradiance. Enhanced photoactivity was observed in the prepared samples irrespective of the light types. After 1 h of UV/visible light-based photooxidation, the best sample STC4 was found to remove 15.18% and 9.16% of atmospheric NO, respectively. In STC4, the mixing ratio of S-TiO2, to melamine was maintained as 1:3. Moreover, the optical bandgap of STC4 was found as 2.65 eV, where for S-TiO2, it was 2.83 eV. Hence, the restrained rate of photogenerated charge recombination and tailored energy bandgap of the as-prepared samples were the primary factors for enhancing photoactivity.

Hierarchical Porous, N-Containing Carbon Supports for High Loading Sulfur Cathodes.

The lithium-polysulfide (LiPS) dissolution from the cathode to the organic electrolyte is the main challenge for high-energy-density lithium-sulfur batteries (LSBs). Herein, we present a multi-functional porous carbon, melamine cyanurate (MCA)-glucose-derived carbon (MGC), with superior porosity, electrical conductivity, and polysulfide affinity as an efficient sulfur support to mitigate the shuttle effect. MGC is prepared via a reactive templating approach, wherein the organic MCA crystals are utilized as the pore-/micro-structure-directing agent and nitrogen source. The homogeneous coating of spherical MCA crystal particles with glucose followed by carbonization at 600 °C leads to the formation of hierarchical porous hollow carbon spheres with abundant pyridinic N-functional groups without losing their microstructural ordering. Moreover, MGC enables facile penetration and intensive anchoring of LiPS, especially under high loading sulfur conditions. Consequently, the MGC cathode exhibited a high areal capacity of 5.79 mAh cm-2 at 1 mA cm-2 and high loading sulfur of 6.0 mg cm-2 with a minor capacity decay rate of 0.18% per cycle for 100 cycles.

Boron Nitride Nanotube-Based Separator for High-Performance Lithium-Sulfur Batteries.

To prevent global warming, ESS development is in progress along with the development of electric vehicles and renewable energy. However, the state-of-the-art technology, i.e., lithium-ion batteries, has reached its limitation, and thus the need for high-performance batteries with improved energy and power density is increasing. Lithium-sulfur batteries (LSBs) are attracting enormous attention because of their high theoretical energy density. However, there are technical barriers to its commercialization such as the formation of dendrites on the anode and the shuttle effect of the cathode. To resolve these issues, a boron nitride nanotube (BNNT)-based separator is developed. The BNNT is physically purified so that the purified BNNT (p-BNNT) has a homogeneous pore structure because of random stacking and partial charge on the surface due to the difference of electronegativity between B and N. Compared to the conventional polypropylene (PP) separator, the p-BNNT loaded PP separator prevents the dendrite formation on the Li metal anode, facilitates the ion transfer through the separator, and alleviates the shuttle effect at the cathode. With these effects, the p-BNNT loaded PP separators enable the LSB cells to achieve a specific capacity of 1429 mAh/g, and long-term stability over 200 cycles.

Facile synthesis and characterization of anatase TiO2/g-CN composites for enhanced photoactivity under UV-visible spectrum.

For the purpose of atmospheric NO removal, anatase TiO2/g-CN photocatalytic composites were prepared by using a facile template-free calcination route in atmospheric conditions. Considerably fiscal NP400 and laboratory-grade melamine were used as the precursor of the composites. Additionally, samples were prepared with different wt. ratios of TiO2 and melamine by using two distinct calcination temperatures (550 °C/600 °C). The morphological attributes of the composites were assessed with X-ray diffraction, scanning and transmission electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. Additionally, the optical traits were evaluated and compared using UV-visible diffuse reflectance spectroscopy and photoluminescence analysis. Finally, the photodegradation potentials for atmospheric NO by using the as-prepared composites were assessed under both UV and visible light irradiation. All the composites showed superior NO oxidation compared to NP400 and bulk g-CN. For the composites prepared by using the calcination temperature of 550 °C, the maximum NO removal was observed when the NP400 to melamine ratio was 1:2, irrespective of the utilized light irradiation type. Whereas for increased calcination temperature (600 °C), the maximum NO removal was observed at the precursor mix ratio of 1:3 (NP400:melamine). Successfully narrowed energy bandgaps were perceived in the as-prepared composites. Moreover, a subsequent drop in NO2 generation during NO oxidation was observed under both UV and visible light irradiation. Interestingly, higher calcination temperature during the synthesis of the catalysts has shown a significant drop in NO2 generation during the photodegradation of NO.

Microporous Carbon Nanoparticles for Lithium-Sulfur Batteries.

Rechargeable lithium-sulfur batteries (LSBs) are emerging as some of the most promising next-generation battery alternatives to state-of-the-art lithium-ion batteries (LIBs) due to their high gravimetric energy density, being inexpensive, and having an abundance of elemental sulfur (S8). However, one main, well-known drawback of LSBs is the so-called polysulfide shuttling, where the polysulfide dissolves into organic electrolytes from sulfur host materials. Numerous studies have shown the ability of porous carbon as a sulfur host material. Porous carbon can significantly impede polysulfide shuttling and mitigate the insulating passivation layers, such as Li2S, owing to its intrinsic high electrical conductivity. This work suggests a scalable and straightforward one-step synthesis method to prepare a unique interconnected microporous and mesoporous carbon framework via salt templating with a eutectic mixture of LiI and KI at 800 °C in an inert atmosphere. The synthesis step used environmentally friendly water as a washing solvent to remove salt from the carbon-salt mixture. When employed as a sulfur host material, the electrode exhibited an excellent capacity of 780 mAh g-1 at 500 mA g-1 and a sulfur loading mass of 2 mg cm-2 with a minor capacity loss of 0.36% per cycle for 100 cycles. This synthesis method of a unique porous carbon structure could provide a new avenue for the development of an electrode with a high retention capacity and high accommodated sulfur for electrochemical energy storage applications.

Rechargeable Intermetallic Calcium-Lithium-O2 Batteries.

The growing demand for rechargeable batteries with high energy density has triggered research on batteries based on polyvalent cations such as Ca2+ , Mg2+ , Al3+ , and Y3+ . Ca is, in particular, a promising anode material as an alternative to Li because of its mechanical strength (ρ=1.55 g cm-3 ), safety in terms of thermal runaway (m.p.=839 °C), earth-abundance (world production of 150 million tons of gypsum in 2012), high specific charge capacity (1.340 mAh g-1 or 2.077 mAh cm-3 ), and standard reduction potential (-2.87 V vs. normal hydrogen electrode, NHE) comparable to that of Li. As with Mg, the practical application of Ca in rechargeable batteries with organic liquid electrolytes has been hindered by the passivation layer resulting from undesirable reactions between metallic Ca and electrolytes, which precludes the possibility of reversible plating of any metal cations on Ca electrodes. Here, a battery system based on intermetallic CaLi2 anodes was developed. Li was used as a host for Ca through the formation of an intermetallic compound, which simultaneously enabled 1) the assembly of a rechargeable battery system with Ca anodes and liquid organic electrolytes and 2) coupling these with an earth-abundant, high-energy-density air cathode without special passivation agents. This strategy is simple and broadly applicable to the other polyvalent cations listed above, opening a new avenue to further engineer the electrode materials required for practical, efficient electrochemical energy-storage systems.

Polyimide-Coated Glass Microfiber as Polysulfide Perm-Selective Separator for High-Performance Lithium-Sulphur Batteries.

Although numerous research efforts have been made for the last two decades, the chronic problems of lithium-sulphur batteries (LSBs), i.e., polysulfide shuttling of active sulphur material and surface passivation of the lithium metal anode, still impede their practical application. In order to mitigate these issues, we utilized polyimide functionalized glass microfibers (PI-GF) as a functional separator. The water-soluble precursor enabled the formation of a homogenous thin coating on the surface of the glass microfiber (GF) membrane with the potential to scale and fine-tune: the PI-GF was prepared by simple dipping of commercial GF into an aqueous solution of poly(amic acid), (PAA), followed by thermal imidization. We found that a tiny amount of polyimide (PI) of 0.5 wt.% is more than enough to endow the GF separator with useful capabilities, both retarding polysulfide migration. Combined with a free-standing microporous carbon cloth-sulphur composite cathode, the PI-GF-based LSB cell exhibits a stable cycling over 120 cycles at a current density of 1 mA/cm2 and an areal sulphur loading of 2 mgS/cm2 with only a marginal capacity loss of 0.099%/cycle. This corresponds to an improvement in cycle stability by 200%, specific capacity by 16.4%, and capacity loss per cycle by 45% as compared to those of the cell without PI coating. Our study revealed that a simple but synergistic combination of porous carbon supporting material and functional separator enabled us to achieve high-performance LSBs, but could also pave the way for the development of practical LSBs using the commercially viable method without using complicated synthesis or harmful and expensive chemicals.

Molecular Cooperative Assembly-Mediated Synthesis of Ultra-High-Performance Hard Carbon Anodes for Dual-Carbon Sodium Hybrid Capacitors.

Although sodium hybrid capacitors (NHCs) have emerged as one of the most promising next-generation energy storage systems, further advancement is delayed primarily by the absence of high-performance battery-type anodes. Herein, we report a nature-inspired synthesis route to prepare hard carbon anodes with high capacity, rate capability, and cycle stability for dual-carbon NHCs. Shape- and size-controllable crystal aggregates of inexpensive triazine molecules are utilized as reactive templates that perform triple duties of structure-directing agent, porogen, and nitrogen source. This enables the fine control of microstructure/morphology/composition and thereby electrochemical reactions toward Na-ion. The resulting hard carbon optimized in terms of lateral size, interlayer spacing, and surface affinity of graphene-like layers achieves a specific capacity of ∼380 mAh/g after 100 cycles at a current density of 250 mA/g mainly via intercalation, the current record of hard carbons. Combined with a commercial microporous carbon fiber cathode, the full cell is able to deliver a volumetric energy density of 2.89 mWh/cm3 and a volumetric power density of 160 mW/cm3, outperforming NHCs based on inorganic Na-ion anode materials. More importantly, such performance could not only be retained for 10000 cycles (4.5 F/cm3 at 10 mA/cm3) with 0.000 028 6% loss per cycle at >97% Coulombic efficiency but also successfully transferred to flexible pouch cells without significant performance loss after 300 bending cycles or during wrapping at a 10R condition. Simple preparation of hard carbon anodes using organic crystal reactive templates, therefore, demonstrates great potential for the manufacture of high-performance flexible NHCs using only carbon electrode materials.

Shape-controlled assemblies of graphitic carbon nitride polymer for efficient sterilization therapies of water microbial contamination via 2D g-C3N4 under visible light illumination.

Bacterial pathogens of water origin have potential public threats thus suggesting the need of developing efficient and sustainable water disinfection strategies from waterborne pathogens. We set out to synthesize different controlled morphologies of graphitic carbon nitride (g-C3N4) polymer, evaluate their comparative effects on the generation of reactive oxygen species (ROS), and investigate potential applications in water purification systems. Characterization of the synthesized microstructures of g-C3N4, such as melamine-cyanuric acid (MCA)-based rosette-type, rod-type, 2D hexagonal, and 3D cubic mesoporous silica was accomplished using Fourier transform infrared (FT-IR), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The microbial inhibitory potential of 2D g-C3N4 photocatalyst against waterborne Escherichia coli, Staphylococcus aureus, and Salmonella typhimurium was evaluated based on the effective activity of 2D g-C3N4 upon visible light excitations. The microbicidal efficiency of 2D g-C3N4 was evident within 30 min of visible light exposure via direct interaction, while other microstructures of g-C3N4 demonstrated only slight antimicrobial effects after 120 min, with insufficient ROS generation. The antimicrobial and ROS-generating effects of 2D g-C3N4 depended on the type and surface area of the synthesized 2D g-C3N4 material. Considering its availability and excellent disinfection activity, 2D g-C3N4 obtained from simple and convenient facile synthesis is a promising solar-driven photocatalyst for clearing microbial contamination from water.

Salt-templated three-dimensional porous carbon for electrochemical determination of gallic acid.

We report an electrochemical sensor based on three-dimensional porous amorphous carbon (3DPAC) for the sensitive and selective determination of gallic acid (GA). The tailor-made carbon was prepared via salt-templating in which the organic molecular precursor, i.e., glucose, was simply ground and carbonized with a eutectic mixture of LiBr and KBr at 800 °C in an inert atmosphere. Salt removal from the carbon-salt mixture with water yielded 3DPAC with a hierarchical porous structure and oxygen-containing functional groups. When employed as an electrochemical sensor, 3DPAC exhibited remarkable sensitivity (0.1045 µA pM-1 cm-2) with a lower detection limit of 0.434 pM at a signal-to-noise ratio of 3 and a linear response up to 1-150 pM for determination of GA. Under optimized test conditions, 3DPAC showed a superior peak current response for GA as compared to the glassy carbon electrode. In addition, ascorbic, uric, and caffeic acids did not interfere with the voltammetric detection of GA in terms of selectivity, stability, and repeatability. We envision that 3DPAC can provide a promising platform for the development of electrochemical sensors.

Visible-light-driven dynamic cancer therapy and imaging using graphitic carbon nitride nanoparticles.

Organic graphitic carbon nitride nanoparticles (NP-g-CN), less than 30 nm in size, were synthesized and evaluated for photodynamic therapy (PDT) and cell imaging applications. NP-g-CN particles were prepared through an intercalation process using a rod-like melamine-cyanuric acid adduct (MCA) as the molecular precursor and a eutectic mixture of LiCl-KCl (45:55 wt%) as the reaction medium for polycondensation. The nano-dimensional NP-g-CN penetrated the malignant tumor cells with minimal hindrance and effectively generated reactive oxygen species (ROS) under visible light irradiation, which could ablate cancer cells. When excited by visible light irradiation (λ > 420 nm), NP-g-CN introduced to HeLa and cos-7 cells generated a significant amount of ROS and killed the cancerous cells selectively. The cytotoxicity of NP-g-CN was manipulated by altering the light irradiation and the BP-g-CN caused more damage to the cancer cells than normal cells at low concentrations. As a potential non-toxic organic nanomaterial, the synthesized NP-g-CN are biocompatible with less cytotoxicity than toxic inorganic materials. The combined effects of the high efficacy of ROS generation under visible light irradiation, low toxicity, and bio-compatibility highlight the potential of NP-g-CN for PDT and imaging without further modification.

A Polysulfide-Infiltrated Carbon Cloth Cathode for High-Performance Flexible Lithium-Sulfur Batteries.

For practical application of lithium-sulfur batteries (LSBs), it is crucial to develop sulfur cathodes with high areal capacity and cycle stability in a simple and inexpensive manner. In this study, a carbon cloth infiltrated with a sulfur-containing electrolyte solution (CC-S) was utilized as an additive-free, flexible, high-sulfur-loading cathode. A freestanding carbon cloth performed double duty as a current collector and a sulfur-supporting/trapping material. The active material in the form of Li2S6 dissolved in a 1 M LiTFSI-DOL/DME solution was simply infiltrated into the carbon cloth (CC) during cell fabrication, and its optimal loading amount was found to be in a range between 2 and 10 mg/cm² via electrochemical characterization. It was found that the interwoven carbon microfibers retained structural integrity against volume expansion/contraction and that the embedded uniform micropores enabled a high loading and an efficient trapping of sulfur species during cycling. The LSB coin cell employing the CC-S electrode with an areal sulfur loading of 6 mg/cm² exhibited a high areal capacity of 4.3 and 3.2 mAh/cm² at C/10 for 145 cycles and C/3 for 200 cycles, respectively, with minor capacity loss (<0.03%/cycle). More importantly, such high performance could also be realized in flexible pouch cells with dimensions of 2 cm × 6 cm before and after 300 bending cycles. Simple and inexpensive preparation of sulfur cathodes using CC-S electrodes, therefore, has great potential for the manufacture of high-performance flexible LSBs.

Solid Suspension Flow Batteries Using Earth Abundant Materials.

The technical features of solid-electrode batteries (e.g., high energy density, relatively low capital cost ($/kWh)) and flow batteries (e.g., long cycle life, design flexibility) are highly complementary. It is therefore extremely desirable to integrate their advantages into a single storage device for large-scale energy storage applications where lifetime cost ($/kW-h/cycle) is an extremely important parameter. Here, we demonstrate a non-Li-based-flow battery concept that replaces the aqueous solution of redox-active molecules found in typical redox flow batteries with suspensions of hydrophilic carbon particles ("solid suspension electrodes") coated with earth-abundant redox-active metals. The solid suspension electrodes charge by depositing earth-abundant redox-active metals onto the carbon particle suspension, which are then stripped during discharge operation. The electrical contact to the solid suspension electrodes is fed through fixed redox-inert hydrophobic carbon current collectors through "contact charge transfer" mechanism. The hydrophobicity of the current collectors prevents direct plating of redox-active metals onto their surfaces. The above concept was successfully used to demonstrate several non-Li-based battery chemistries including zinc-copper, zinc-manganese oxide, zinc-bromine, and zinc-sulfur, providing a pathway for potential applications in medium and large-scale electrical energy storage.

Polyimide dendrimers containing multiple electron donor-acceptor units and their unique photophysical properties.

A high-yielding synthesis of a series of polyimide dendrimers, including decacyclene- and perylene-containing dendrimer D6, in which two types of polyimide dyes are present, is reported. In these constructs, the branching unit is represented by trisphenylamine, and the solubilizing chains by N-9-heptadecanyl-substituted perylene diimides. The photophysical properties of the dendrimers have been studied by absorption, steady-state, and time-resolved emission spectroscopy and pump-probe transient absorption spectroscopy. Photoinduced charge-separated (CS) states are formed on the femtosecond timescale upon visible excitation. In particular, in D6, two different CS states can be formed, involving different subunits that decays independently with different lifetimes (ca. 10-100 ps).

Sulfur-functionalized mesoporous carbons as sulfur hosts in Li-S batteries: increasing the affinity of polysulfide intermediates to enhance performance.

The Li-S system offers a tantalizing battery for electric vehicles and renewable energy storage due to its high theoretical capacity of 1675 mAh g(-1) and its employment of abundant and available materials. One major challenge in this system stems from the formation of soluble polysulfides during the reduction of S8, the active cathode material, during discharge. The ability to deploy this system hinges on the ability to control the behavior of these polysulfides by containing them in the cathode and allowing for further redox. Here, we exploit the high surface areas and good electrical conductivity of mesoporous carbons (MC) to achieve high sulfur utilization while functionalizing the MC with sulfur (S-MC) in order to modify the surface chemistry and attract polysulfides to the carbon material. S-MC materials show enhanced capacity and cyclability trending as a function of sulfur functionality, specifically a 50% enhancement in discharge capacity is observed at high cycles (60-100 cycles). Impedance spectroscopy suggests that the S-MC materials exhibit a lower charge-transfer resistance compared with MC materials which allows for more efficient electrochemistry with species in solution at the cathode. Isothermal titration calorimetry shows that the change in surface chemistry from unfunctionalized to S-functionalized carbons results in an increased affinity of the polysulfide intermediates for the S-MC materials, which is the likely cause for enhanced cyclability.

High quality reduced graphene oxide through repairing with multi-layered graphene ball nanostructures.

We present a simple and up-scalable method to produce highly repaired graphene oxide with a large surface area, by introducing spherical multi-layered graphene balls with empty interiors. These graphene balls are prepared via chemical vapor deposition (CVD) of Ni particles on the surface of the graphene oxides (GO). Transmission electron microscopy and Raman spectroscopy results reveal that defects in the GO surfaces are well repaired during the CVD process, with the help of nickel nanoparticles attached to the functional groups of the GO surface, further resulting in a high electrical conductivity of 18,620 S/m. In addition, the graphene balls on the GO surface effectively prevent restacking of the GO layers, thus providing a large surface area of 527 m(2)/g. Two electrode supercapacitor cells using this highly conductive graphene material demonstrate ideal electrical double layer capacitive behavior, due to the effective use of the outstanding electric conductivity and the large surface area.

Three-dimensional macroscopic assemblies of low-dimensional carbon nitrides for enhanced hydrogen evolution.

Simple organic cooperative assembly of triazine molecules leads to three-dimensional macroscopic assemblies of low-dimensional graphitic carbon nitrides (g-CNs), for example, nanoparticles, nanotubes, and nanosheets. The approach enables the characterization of the cooperative properties and photocatalytic activities of low-dimensional g-CN materials in hydrogen evolution reactions from water under visible light.