Preparation of solution processed photodetectors comprised of two-dimensional tin(ii) sulfide nanosheet thin films assembled via the Langmuir–Blodgett method

We report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir–Blodgett method. The method we propose can coat a variety of substrates including paper, Si/SiO₂ and flexible polymer allowing for a potentially wide range of applications in future optoelectronic devices.

Norton et al. report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir–Blodgett method.

Two-dimensional (2D) materials are condensed matter solids formed of crystalline atomic layers held together via weak van der Waals forces.¹ They have a wide range of applications including use as channel materials in transistors,² absorber layers in solar cells,³ light emission,⁴ energy storage⁵ and drug delivery⁶ among others. 2D materials often have different properties from their bulk counterparts such as increased strength⁷ and electrical conductivity.⁸ 2D semiconductors may exhibit a change in electronic states from confinement in 1D.⁹ Thin films are often required for the creation of devices from nanomaterials for practical applications and can often be made into flexible devices such as thin film solar cells¹⁰ or photodetectors.^11,12 Thin film solar cells in particular have several advantages over conventional solar cells including lower materials consumption and are lightweight, yet have the potential for high power conversion efficiency.¹⁰Many of the two-dimensional materials produced thus far have been derived from mechanical exfoliation, where Scotch tape or an equivalent is manually used to remove single crystalline layers from a bulk van der Waals solid followed by transfer to a substrate. Whilst this method in general produces extremely high quality crystalline atomic layers,¹³ and is therefore often used to produce prototype devices, it inherently lacks scalabilty. In order to address the problem of mass manufacture of two dimensional materials, liquid phase exfoliation (LPE) was introduced as a cost effective method for producing two dimensional nanomaterials¹⁴ with the possibility of 100 L scales being produced and production rates up to 5.3 g h⁻¹ demonstrated by Coleman et al. with both NMP and aqueous surfactant solutions utilised.¹⁵ This method also does not require the high temperatures needed for methods such as CVD¹⁶ or transfer between the growth and final substrates. Liquid phase exfoliated nanomaterials are also directly processable from solution.¹⁵ Furthermore, LPE has been shown to be effective for the production of a wide range of 2D materials such as graphene,¹⁵ transition metal dichalcogenides¹⁷ and monochalcogenides such as SnSe.¹⁸Tin(ii) sulfide (SnS) is a van der Waals solid with a puckered ab structure consisting of alternating Sn and S atoms, and is isostructural and isoelectronic with black phosphorus.¹⁹ The bulk material has attracted interest due to its indirect band gap energy of 1.07 eV,²⁰ similar to bulk silicon at 1.14 eV. This band gap energy for SnS is useful for applications such as photodetection²¹ and due to its higher theoretical Shockley–Queisser efficiency limit (24%) for solar cells.²² The liquid phase exfoliation method established by Coleman et al. enables nanosheets to be separated from the bulk into solution utilising matching surface energies of the material and solvent.²³ Liquid phase exfoliation of SnS was first reported by Lewis et al. it was established that as layer number reduced, band gap energy increased, and by tuning layer number the onset of photon absorption can be tuned over the near infrared²³ to visible range.²⁴ Overall, LPE is capable of creating large quantities of nanosheets, with potential for industrial scale production. Liquid phase exfoliated SnS has, for example, recently been used in the creation of photoelectrochemical systems with strong stability under both acidic and alkali conditions.²⁵ Many of the functional devices produced thus far have been derived from micromechanical exfoliation and manual nanomanipulation. A far more elegant solution to producing functional devices is to assemble them from solution, for example Kelly et al. recently reported a transistor based on exfoliated WSe₂ nanosheets.²The Langmuir–Blodgett method involves the use of a trough with a layer of water and controllable barriers to compress the film. Nanomaterials in solution are added to the surface of the water and spread evenly to reduce their surface energy,²⁶ often by using a low surface tension spreading solvent such as chloroform.²⁷ The surface pressure is measured as the film is compressed with the substrate being withdrawn when the film becomes solid.²⁸ The Langmuir–Blodgett method has the advantages of large area deposition and improved control of the film at the nanoscale in comparison to vacuum filtration as well as the advantage of requiring no volatile solvents in comparison to liquid–liquid assembly methods. The use of movable barriers also allows for greater film compression.²⁶This method has been used to assemble large scale films of exfoliated MoS₂ by Zhang et al. MoS₂ was exfoliated using n-butyl lithium followed by solvent exchange. MoS₂ was deposited onto the water surface using a 1 : 1 mix of DMF and dichloroethane. Substrates up to 130 cm² were coated with a surface coverage of 85–95%.²⁶ Collapse mechanisms of MoS₂ Langmuir films have also been studied²⁹ alongside MoS₂ deposition on the surface of water with an upper hexane layer.³⁰ Graphene films have also been prepared using the Langmuir–Blodgett method.³¹ The Langmuir–Blodgett method has been used for the assembly of organo-clay hybrid films via the coating of octadecylammonium chloride in a 4 : 1 chloroform : ethanol solution onto a 2D nanoclay liquid phase exfoliated film using an electrospray method.³² A solvent mix of chloroform and NMP has also been utilised for the deposition of nanosheet films.³³ Recently the Langmuir–Blodget method has been used for the assembly of unmodified clay nanosheets,³⁴ Ti₃C₂Tx MXene nanosheet films for the removal of Cr(vi) and methyl orange from an aqueous environment³⁵ as well as for the growth of rGO wrapped nanostructures for use in electrocatalysts.³⁶Given the chemical similarity of the basal planes of inorganic 2D materials, we hypothesised that the assembly of group IV–VI nanomaterials such as SnS should also be possible at the air water interface. Due to their interesting semiconducting and properties described, it should also be possible to produce prototype optoelectronic devices from a fully solution processed pathway. In this paper we now communicate a methodology to assemble thin films comprised of 2D SnS nanosheets using the Langmuir–Blodgett technique (Scheme 1a). We report the use of these films in simple photodetectors. This represents a scalable methodology to produce fully solution processed devices based on 2D materials.Open in a separate windowScheme 1Preparation of SnS nanosheet thin films via the Langmuir–Blodgett method. (a) Cartoon of Langmuir–Blodgett film preparation. (b) Image of Langmuir–Blodgett trough with compressed SnS film. (c) Surface pressure profile during film compression. (d) Image of sample prepared on Si/SiO₂ substrate with edges masked (scale bar 1.5 cm). Scheme 1(a) shows the step by step process of film preparation. Firstly, bulk SnS is broken down by liquid phase exfoliation from the bulk material to produce a stable dispersion of crystalline nanosheets. Characterisation of the exfoliated nanomaterials was undertaken using atomic force and electron microscopy yielding average sheet dimensions of 23.9 nm height × 224 nm longest side length (Fig. S1^†). The nanosheets were then deposited onto the water air interface. The film is then compressed whilst an immersed substrate is withdrawn, leading to the creation of a densely packed nanosheet film. Scheme 1(b) shows that SnS can be successfully deposited on the water–air interface via the addition of chloroform as a spreading solvent, as shown previously with other Langmuir based films.²⁷Scheme 1(a) shows a z-type deposition of SnS as the hydrophilic glass and Si with a 300 nm oxide layer is withdrawn through the film at 1 atm pressure. The film compression occurred at a rate of 5.88 cm² s⁻¹. No further treatments were performed to change the hydrophilicity of the substrates, the oxide layer present was sufficient to provide hydrophilicity to the substrate.³⁷Scheme 1(c) shows a gradual increase in surface pressure as the area was decreased from 1175 cm² to 298 cm² before a sharp increase in pressure, indicating the film has reached full compression. The sharp increase in surface pressure during compression is common in Langmuir–Blodgett assembled films of nanomaterials.³⁸ In response to compression the surface pressure profile in Scheme 1(c) rises rapidly until it reaches a maximum due to the size of the sheets and the potential difficulty in sliding over each other compared to polymers or smaller nanomaterials. Scheme 1(d) shows that the film is capable of being coated onto Si/SiO₂ with a mask defining the areas covered.We characterised the resulting structural and electronic properties of the thin film of SnS nanosheets deposited via the Langmuir–Blodgett method using a range of techniques. Fig. 1(a) shows a height profile AFM image of a film edge with an average on-film roughness (R_a) of 31.9 nm and an average film thickness of 78.6 nm (Fig S3^† provides an additional film profile). Previous work on Langmuir–Blodgett deposition has produced thinner films. The use of high centrifugation speeds yielded 7 nm thick films for a single deposition³¹ whilst the use of lithium ion intercalation before exfoliation enabled film thicknesses of under 2 nm per layer to be realised.²⁶ The average film thickness is above the average sheet thickness, suggesting that the film is made up of overlapping flake multilayers. However, the thickness of the films is significantly lower than those grown via chemical bath deposition (e.g. 290 nm (ref. 39)) indicating that thinner films can be produced compared to chemical bath methods, and potentially at a much lower cost than methods such as CVD. Images of the film morphology in plan view SEM (Fig. 1(b)) suggest no notable alignment of the nanosheets in the lateral dimension as the film is formed and deposited (see Fig S4^† for statistical analysis of sheet angle measurement). The coverage of the film is 94.6% as determined by image thresholding using imagej software to determine the area left uncovered. This gives a coverage of 0.0142 gm⁻² as calculated from average thickness, SnS density and % coverage of the substrate. Preliminary SEM results also suggest that the Langmuir–Blodgett method is effective at coating SnS onto a variety of substrates including polyolefin films (Parafilm®), aluminium foil and paper (Fig S6^†). We also probed the structure of the thin films by powder X-ray diffraction (XRD). After exfoliation and film assembly, the diffraction peak associated with the (400) of SnS is still the most intense reflection but is characterised by a much larger FWHM compared to that of bulk SnS under the same recording conditions (0.442° ± 18.5% compared to 0.175° ± 5%). This indicates a successful breakdown of the crystal structure and thinning of the material in the (400) plane during exfoliation due to the reduction in long range order⁴⁰ (reflections for bulk SnS are assigned to orthorhombic SnS and indexed in Fig S2^†). The lack of any additional peaks indicates that there has not been any significant degradation of the material to the corresponding oxide which is in agreement with previous works.^24,25 The reflections at 88° and 94° are unlikely to be from crystalline silicon⁴¹ due to the thick oxide layer and low angle of incidence used. We tentatively ascribe these peaks to the 3,0,−3 and 3,2,4 peaks for SnS.⁴¹ However a confident assignment of this reflection requires further studies.Open in a separate windowFig. 1Structural characterisation of SnS nanosheet thin films assembled by the Langmuir-Bllodgett method. (a) AFM image of LB assembled SnS film edge. Inset film profile, scale bar = 10 μm. (b) SEM image of LB assembled film on Si/SiO₂ at 3 kV using secondary electron imaging, scale bar = 1 μm. (c) XRD pattern of coated film and bulk SnS powder, (additional peaks labelled in Fig. S2^†). (d) Raman spectra and for bulk and Langmuir–Blodgett assembled SnS nanosheets. (e) UV-Vis spectra of SnS suspension and deposited SnS film on glass (f) Tauc plot of SnS solution and film.We also characterised the optical properties of the nanosheet thin films using Raman and UV-Vis-NIR absorption spectroscopy. No shifts in the Raman peak positions B₃g, Ag and B₃u from bulk SnS to Langmuir–Blodgett film were observed. The broad feature at around 300 cm⁻¹ for the LB film may potentially be due to SnS₂ and Sn₂S₃ impurities.⁴² It is predicted that due to the lower density compared to SnS⁴³ the impurities may increase in concentration compared to the bulk after centrifugation. These impurities may have significant effects on the efficiency of the devices produced.⁴⁴A shift in peak positions is typically observed in nanomaterials which exhibit quantum confinement,⁴⁵ this occurs at 14 nm for SnS.⁴⁶Fig. 1(e) shows a UV-Vis spectra from which the absorption coefficients at fixed wavelengths may be obtained, for 350 nm, 405 nm, 450 nm, 500 nm, 600 nm and 800 nm the values obtained were: 2.26 × 10⁵ cm⁻¹, 2.21 × 10⁵ cm⁻¹, 2.16 × 10⁵ cm⁻¹, 2.04 × 10⁵ cm⁻¹, 1.67 × 10⁵ cm⁻¹ and 1.05 × 10⁵ cm⁻¹ respectively, this matches well to the absorption coefficients of SnS in literature (greater than 10⁴ cm⁻¹).⁴⁷ It also suggests there may be a greater response at shorter wavelengths. Fig. 1(f) shows a band gap of 0.92 eV for the exfoliated SnS in NMP which is below the expected value of 1.07 eV (ref. 20) although lies within the reasonable error introduced by the use of Tauc plots.⁴⁸ The band gap also matches well with SnS exfoliated in NMP in previous work.²⁴ The band gap of the film appears to change from nanosheet suspension to film in 1(f). This has been observed previously for Langmuir–Blodgett⁴⁹ and other deposited films. It has also been observed that apparent decreases in band gap may occur due to the presence of scattering artefacts within films of nanoscale objects.⁵⁰We then produced simple prototype photodetectors via the printing of Ag nanoparticles to form interdigitated electrodes on top of the SnS nanosheet film. Additionally, SnS films were deposited onto lithographically defined Au interdigitated electrodes for characterisation and referencing to the printed devices.Previously SnS photodetectors have been created via methods such as electron beam deposition,⁵¹ thermal evaporation⁵² and chemical bath deposition.⁵³ The Langmuir–Blodgett method allows SnS to be directly processed into a film from a liquid phase exfoliated solution, allowing them to be produced cheaply and with the potential for scalability.Inset to Fig. 2(a) is an image of an interdigitated Ag electrode SnS photodetector device with an area of 6.4 × 10⁻⁵ m². The electrodes can be clearly identified with an average spacing of 99 μm, and an average RMS edge roughness value of 1.89 μm (determined for individual contact lines using the imageJ ‘analyze_stripes’ plugin⁵⁴ (Fig S7^†)). Fig. 2(a) shows an increase in the slope of the I–V curve in the third quadrant indicating a reduction in resistance under 1 sun illumination (1000 W m⁻²) with the AM1.5 spectrum. No short circuit current under illumination was observed indicating that the device functions as a photoconductor. The non-linear response upon negative biasing is due to initial trap filling which once equilibrium has been reached results in linear device operation. Previously it has been shown that silver diffusion into SnS has an interstitial doping effect, neutralising defect states and lowering the film resistivity.^55,56 It is also possible that the Ag ink morphology and the concentration of nanoparticles in the ink may play an effect on the device properties.⁵⁷ A resistivity of 2.85 × 10⁶ Ω sq⁻¹ was obtained for the device which is significantly higher than SnS films prepared by physical vapour deposition (250 Ω sq⁻¹),⁵⁸ likely due to poor carrier mobility between flakes.Open in a separate windowFig. 2(a) IV curves of printed contacts SnS device under darkness and AM1.5 illumination with inset photograph of pseudo Langmuir–Blodgett device with printed Ag contacts scale bar 5 mm. (b) Device under +40 V bias under fixed darkness/illumination cycle. Fig. 2(b) indicates that a clear response is present under illumination when an external bias is applied (giving a field strength of 0.4 V μm⁻¹). Closer inspection shows a fast and slow decay component following the illumination being blocked. This biexponential decay indicates the capture of trapped carriers and the presence of trap states within the device.^59,60 This again supports the photoconductive nature of the device operation with a rise time of ∼0.22 s and a fall time of ∼2.83 s,⁶¹ both being longer than the shutter closing/opening time of 3.7 ms (which was considered negligible). The rise time is the time taken to get from 10% to 90% of the light current with the fall time being the time taken from 90% of the light current to 10%.Previous work performed by Jiang et al. has shown a slow fall time in Ag/SnS photoconductor devices arising from carrier trapping.⁶² Similarly, in our devices the large rise time may also be due to the presence of a high trap density which must be filled upon light exposure.The mean dark current is 2.78 × 10⁻¹⁰ A with a standard deviation of 2.02 × 10⁻¹¹ A. The mean light current was found to be 3.92 × 10⁻¹⁰ A with a standard deviation of 4.03 × 10⁻¹¹ A. A poor signal to noise ratio appears to be present within the device, possibly due to the large number of SnS nanosheets involved in charge carrier transit, leading to a low signal, hence a low signal to noise ratio. The noise could be reduced via surface passivation⁶³ or the use of a diode like structure to reduce leakage current under reverse bias.⁶⁴ A low responsivity of 2.00 × 10⁻⁹ A W⁻¹ ± 1.5 × 10⁻¹⁰ A W⁻¹ was found for energies above the band gap energy of 0.6 eV for the deposited film.The low responsivity may be due to poor bridging between individual SnS nanosheets and the poor transport of holes between adjacent flakes (hopping) relative to the higher mobility within each flake.⁶⁵ There are potentially hundreds of nanosheets between the contacts as determined by the average length obtained (Fig S1^†). To confirm that the optical response was due to the presence of the SnS a reference device was tested (without SnS deposition, Fig S8^†) with no photoresponse observed. Despite the low responsivity, it is notable that the SnS devices fabricated are one of the few examples of a thin film photodetector device based on 2D materials requiring only solution processing at ambient temperature and atmospheric pressure.To demonstrate that the observed behaviour originates from the photoresponse of the SnS flakes a second device was fabricated by pseudo Langmuir–Blodgett deposition on to lithographically defined Au interdigitated electrodes (15 μm separation) on fused silica (inset Fig. 3(b)). This enabled us to remove any effect of photoinduced Ag migration from the observed behaviour as well as eliminating the issue of potential printing irregularities. Fig. 3(a) shows that the devices display a similar photoresponse to the devices with printed Ag electrodes when exposed to modulated AM1.5 illumination. The dark current remains similar at ∼0.3 nA, though during illumination the current is higher (0.7 nA vs. 0.4 nA). This increase directly correlates to the higher electric field strength (0.66 V μm⁻¹vs. 0.4 V μm⁻¹) between the interdigitated electrodes. The responsivity of the device was determined to be 1.79 × 10⁻⁸ A W⁻¹, with a photoresponse rise and fall time of 0.77 s and 0.85 s respectively. The responsivity is lower than for photodetectors prepared by Guo et al.⁶⁶ Improvements to the device to improve the responsivity could include methods to improve the lateral size of nanosheets such as intercalation.⁶⁷ Other routes to improve the device may include doping^68,69 or a change in architecture to a phototransistor type device.⁷⁰ The removal of potential SnS₂ and Sn₂S₃ impurities via methods such as annealing at 500 °C, 500 mbar pressure under argon or the use of higher quality starting material may also be a key route to improve the efficiency of the device.⁴²Open in a separate windowFig. 3(a) Device under 30 s off, 30 s on solar simulator illumination at 1 sun and 10 V bias (b) IV curves under darkness and 350 nm illumination with inset optical microscopy image of contacts (c) monochromatic illumination responses under 10 V bias mapped onto UV-Vis transmission spectra (d) device response under fixed 10 V bias under 350 nm and 405 nm monochromatic illumination.It is also noticeable that the level of noise present in Fig. 3(a) is reduced compared to that in Fig. 2(b), indicating that the Ag electrodes themselves (in addition to the SnS sheets) also affect the performance.When exciting using AM1.5 illumination it is possible that thermal effects may be present which could give rise to the observed behaviour.In order to demonstrate a true photoresponse monochromatic illumination was used to determine if illumination energies above the band gap generated a photocurrent response in the device. Fig. 3(b) shows a small response under 350 nm (3.54 eV) illumination. (IV curves for other wavelengths are available in Fig. S9^†). Fig. 3(c) shows an increased response for 350 nm wavelength as determined via the IV curves. This increased response is likely due to increased absorption as shown in the UV-Vis spectra (Fig. 1e), the signal at longer wavelengths is difficult to observe due to the low responsivity. A higher response at lower wavelength has been observed previously for SnS.⁵³ Fig. 3(d) shows that an increase in current is present under 350 nm and 405 nm illumination which can be cycled on and off. A rise and fall time of 1.09 of 1.44 seconds respectively was observed for 405 nm illumination. A light/dark current ratio of 1.03 was obtained under 405 nm. To account for noise the on and off section had their current averaged using origin software. A drift in current during measurement was observed, this was considered as the reason for the significant difference between the dark current for 350 nm and 405 nm. To further reduce noise surface passivation may also be used to improve the device properties.⁶³ Alternatively, an increase in bias voltage or an increase in monochromatic illumination intensity may improve the signal: noise ratio though may risk damage to the device. A magnified off/on cycle for 405 nm is shown in Fig. S10.^†In conclusion, we report here a methodology for the assembly of 2D SnS nanosheets into thin films using the Langmuir–Blodgett method, and the testing of the films as prototype all-solution processed photodetectors. Tin(ii) sulfide was successfully exfoliated with an average sheet thickness of 33 nm with the average longest side length of 224 nm. A nanosheet based film was coated onto a variety of substrates via the Langmuir–Blodgett method with the addition of chloroform as a spreading solvent. The films were found to be polycrystalline with an average thickness of 78.6 nm with a high surface coverage up to 94.6% for an Si/SiO₂ substrate. The films were found to be semiconductive with the ability to respond to light under bias as shown by AM1.5 and monochromatic illumination. Proof-of-concept photodetectors have been successfully produced. It was also confirmed that the response was due to the photoresponse as opposed to a heating effect. This deposition method could potentially be used to create a variety of SnS films using different exfoliated nanosheet sizes separated via cascade centrifugation as well as the potential for future flexible photodetector devices. Despite the low responsivity, large rise and fall times further work could allow the gain to be optimised. We also note that the use of the Langmuir–Blodgett trough is an easily scalable technology and could provide coatings over very large area substrates not only for photodetectors but for other devices such as thin film solar cells.     

One-step hydrothermal synthesis of Ni–Fe–P/graphene nanosheet composites with excellent electromagnetic wave absorption properties

Ni–Fe–P nanoparticles/graphene nanosheet (Ni–Fe–P/GNs) composites were successfully synthesized by a simple one-step hydrothermal method. Specifically, Ni²⁺ and Fe²⁺ were reduced by using milder sodium hypophosphite as a reducing agent in aqueous solution. SEM and TEM images show that a large number of Ni–Fe–P nanoscale microspheres are uniformly deposited on graphene nanosheets (GNs). At the thickness of 3.9 mm, the minimum reflection loss (RL) of Ni–Fe–P/GNs reaches −50.5 dB at 5.3 GHz. In addition, Ni–Fe–P/GNs exhibit a maximum absorption bandwidth of 5.0 GHz (13.0–18.0 GHz) at the thickness of 1.6 mm. The significant electromagnetic absorption characteristics of the Ni–Fe–P/GN composites can be attributed to the addition of magnetic particles to tune the dielectric properties of graphene to achieve good impedance matching. Therefore, Ni–Fe–P/GN is expected to be an attractive candidate for an electromagnetic wave absorber.

Ni–Fe–P nanoparticle/graphene nanosheet composites synthesized by a one-step hydrothermal method have excellent performance in the field of electromagnetic wave absorption, with a minimum reflection loss of −50.5 dB and a maximum effective absorption bandwidth of 5 GHz.     

One-step synthesis of red-emitting carbon dots via a solvothermal method and its application in the detection of methylene blue

The synthesis of carbon dots (CDs) with long wavelengths, particularly the red-emitting ones, has always been the focus of researchers, and a carbon source is critical in this process. In this study, we report the synthesis of red-emitting CDs (CD-tetra) via a one-step solvothermal method with 1,2,4,5-benzenetetramine tetrahydrochloride as a novel carbon source and ethanol as a solvent, and the quantum yield (QY) of CDs is as high as 30.2%. Middle chromatography isolated gel (MCI Gel) column was used to obtain R-CDs, O-CDs and Y-CDs with emission wavelengths at 619, 608 and 554 nm, respectively. It was discovered that these CDs exhibited great differences in their particle sizes and elemental compositions. Moreover, the fluorescence of the CD-tetra could be efficiently quenched using methylene blue (MB). Under optimal conditions, a linear relationship between the decreased fluorescence intensity of the CD-tetra and the concentration of MB was established in the range of 0.05–9.5 μM. The limit of detection (LOD) is 10 nM, suggesting a promising assay for the detection of MB.

Red-emitting CDs was synthesized via a one-step solvothermal method with 1,2,4,5-benzenetetramine tetrahydrochloride as a novel carbon source and ethanol as a solvent. The luminescence mechanism of CDs was studied by MCI gel column chromatography.     

One-step fabrication of carbonaceous solid acid derived from lignosulfonate for the synthesis of biobased furan derivatives

An eco-friendly and low-cost lignosulfonate-based acidic carbonaceous catalyst (LS-SO₃H) was effectively fabricated using the sulfite pulping by-product of sodium lignosulfonate as a precursor by facile one-step simultaneous carbonization and sulfonation, and employed for the synthesis of promising biofuel furan derivatives from biorenewable feedstocks. The catalyst preparation conditions significantly affected the preparation and properties of LS-SO₃H. A relatively high catalyst preparation yield (40.4%) with strong –SO₃H density (1.33 mmol g⁻¹) were achieved when the lignosulfonate was treated in concentrated H₂SO₄ solution at 120 °C for 6 h. The preparation yield of LS-SO₃H was nearly twice as much as that of one-step prepared catalyst using alkaline lignin (another technical lignin from pulping) as a precursor. The as-prepared LS-SO₃H had similar textural characteristics to the frequently-used two-step prepared carbonaceous catalyst involving pyrolysis carbonization and sulfonation. LS-SO₃H was found to show good catalytic activity for the synthesis of 5-ethoxymethylfurfural (EMF) in ethanol medium, affording around 86%, 57% and 47% yields from 5-hydroxymethylfurfural (HMF), fructose and inulin, respectively. Also, a high HMF yield of 83% could be obtained from fructose when DMSO was replaced as reaction medium. The used LS-SO₃H was readily recovered by filtration, and remained active in recycle runs.

An easy-prepared and bio-supported lignosulfonate-based acidic carbonaceous catalyst was developed for the synthesis of promising furan biofuels from biorenewable feedstocks.     

One-step synthesis of hydrophobic clinoptilolite modified by silanization for the degradation of crystal violet dye in aqueous solution

Hydrophobic clinoptilolite (CP) was successfully synthesized via a silanization method using methyltriethoxysilane (MTS) or diethoxydimethylsilane (DMTS) as silane coupling agents. The structural and textural properties of the resultant hydrophobic CP were characterized using various methods. The effect of the amount of MTS or DMTS additive on the induction (nucleation) and growth of CP were also investigated, and the apparent activation energy values for induction and growth periods were calculated, suggesting that the induction period is kinetically controlled, while the rapid growth process is thermodynamically controlled. Meanwhile, DMTS modification enhanced the hydrophobicity of CP compared with its MTS-modified counterpart and pure CP. Finally, various ZnO-supported CPs were used as photocatalysts for the removal of crystal violet from aqueous solution, demonstrating that ZnO/hydrophobic CP has the largest adsorption capacity and best removal performance. These results suggest that hydrophobic CP, as an adsorbent or support, has the most potential for applications in separation and catalysis.

One-step synthesis of hydrophobic CPs was demonstrated, in which the kinetically-controlled induction period and thermodynamically-based rapid growth process were elucidated.     

Theoretical investigations of a new two-dimensional semiconducting boron–carbon–nitrogen structure

A new two-dimensional boron–carbon–nitrogen (BCN) structure is predicted and is theoretically investigated based on density functional theory. The BCN structure belongs to the space group C222, and is composed of twelve B, twelve C and twelve N atoms per orthorhombic cell (named oC-B₁₂C₁₂N₁₂). It consists of small hollow spheres with two hexagons per sphere. The dynamical, thermal and mechanical stabilities of oC-B₁₂C₁₂N₁₂ are respectively evaluated by phonon spectroscopy, ab initio molecular dynamics calculations and elastic constant measurements. The simulated in-plane stiffness and Poisson ratio display anisotropic features. The band structure shows that oC-B₁₂C₁₂N₁₂ is a direct semiconductor with a gap of 2.72 eV (GW). oC-B₁₂C₁₂N₁₂ has an absorption range from the visible light spectrum to the ultraviolet. Therefore, due to its small direct band gap and optical absorption, oC-B₁₂C₁₂N₁₂ may be a good candidate for electronic and optical applications.

A predicted 2D BCN structure has a direct band gap and is a good candidate for electronic and optical applications.     

Straightforward synthesis of quinazolin-4(3H)-ones via visible light-induced condensation cyclization

A green, simple and efficient method is developed for the synthesis of quinazolin-4(3H)-ones via visible light-induced condensation cyclization of 2-aminobenzamides and aldehydes under visible light irradiation. The reaction proceeds using fluorescein as a photocatalyst in the presence of TBHP without the need for a metal catalyst. In addition, this reaction tolerates a broad scope of substrates and could afford a variety of desirable products in good to excellent yields. Thus, the present synthetic method provides a straightforward strategy for the synthesis of quinazolin-4(3H)-ones.

Visible light was used as a readily available and renewable clean energy source for the green and metal catalyst free synthesis of quinazolin-4(3H)-ones. High and excellent yields of the desired products were obtained with good functional group tolerance.

In recent years, synthesis of nitrogen-containing heterocycles has drawn considerable attention due to their widespread occurrence in natural and synthetic organic molecules.¹ Among them, quinazolin-4(3H)-ones are common core structures found in a large number of natural products and synthetic drugs showing a broad range of biological and therapeutic activities (Fig. 1). For example, Pegamine, isolated from Peganum harmala, exhibits cytotoxic activity.² Afloqualone is a centrally acting muscle relaxant useful in the management of various conditions, including cerebral palsy, cervical spondylosis, and multiple sclerosis.³ Bouchardatine could significantly reduce lipid accumulation, and mainly inhibited early differentiation of adipocytes through proliferation inhibition and cell cycle arrest in a dose-dependent manner.⁴ Idelalisib have been shown to exhibit a broad spectrum of antimicrobial, antitumor, antifungal and cytotoxic activities.⁵ Ispinesib is one of the most potent kinesin spindle protein (KSP) inhibitors and is currently in clinical trials for cancer treatment.⁶ Sclerotigenin, isolated from organic extracts of the sclerotia of penicillium sclerotigenum, is responsible for most of the antiinsectan activity against crop pests.⁷ Besides, a number of quinazolin-4(3H)-ones have been synthesized to provide synthetic drugs and to design more effective medicines.^8,9Open in a separate windowFig. 1Representing natural and synthetic molecules containing quinazolin-4(3H)-one moieties.Owing to their pharmacological importance, considerable attention has been devoted to the development of simple and efficient methods for their construction (Scheme 1). Typically, quinazolin-4(3H)-ones are synthesized by acid or base-catalyzed condensation of amides with alcohols/aldehydes.^10,11 In some cases, an excess of hazardous oxidants, for instance KMnO₄, I₂, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), are employed in the reaction, which are significant limitations in this method.^12–14 Over the past decades, various metal-based catalysts such as copper-catalyzed cyclization of 2-halobenzoic acids with amidines, palladium-catalyzed benzylic C–H amidation with benzyl alcohols and vanadium-catalyzed redox condensation of benzamides with alcohols or aldehydes have been reported for the synthesis of quinazolin-4(3H)-ones.^15–17 Although these approaches result in an excellent formation of the product, most of them are suffering from its own limitations such as corrosive or non-benign acid/base catalyst, hazardous oxidants, precious metal-based catalysts, complexity in work-up and relatively harsher reaction conditions. Therefore, development of a green, simple and efficient synthetic approach for the preparation of quinazolin-4(3H)-ones from inexpensive and easily available starting materials under relatively mild conditions is desirable.Open in a separate windowScheme 1Our work for synthesis of quinazolin-4(3H)-ones.Visible light-induced reaction, which is widely recognized as an attractive “green synthesis pathway” in organic synthesis, has become a fast-developing research area in the past decades.^18–21 In spite of simple operation and mild reaction conditions, the common transition metals (e.g., iridium and ruthenium) employed as photocatalysts are usually expensive, toxic and not easily available. Nevertheless, organic dyes, which have shown similar photocatalytic activity in some reactions, could be more attractive candidates than the common transition metals, because they are usually relatively cheap, less toxic and accessible.²² In particular, fluorescein has been very recently studied as photocatalyst due to its cheap and commercially available characteristics.²³In this study, we focus our attention on developing a straightforward method to prepare quinazolin-4(3H)-ones with 2-aminobenzamides and aldehydes using fluorescein as photocatalyst via visible light-induced condensation cyclization, in which the synthesis of quinazolin-4(3H)-ones would proceed in good to excellent yields under mild conditions without the need for metal catalyst (Scheme 1). This method lays a solid foundation for the synthesis of quinazolin-4(3H)-ones. Moreover, this visible light-induced strategy has great potential in the synthesis of other types of useful organic molecules.At the initial stage of investigation, 2-aminobenzamide (1a) and benzaldehyde (2a) were chosen as the model substrates under blue LED irradiation, and a series of reaction conditions including photocatalysts, oxidants, solvents and reaction time were optimized. Initially, the reaction was performed in the presence of 10 mol% fluorescein as photocatalyst and the desired product, 2-phenylquinazolin-4(3H)-one (3aa), was obtained in 89% yield (

Enhancement of nitrogen self-doped nanocarbons electrocatalyst via tune-up solution plasma synthesis

The development of a metal-free carbon based electrocatalyst for the oxygen reduction reaction (ORR) is an essential issue for energy conversion systems. Herein, we suggest a tune-up solution plasma (SP) synthesis based on a simple one-step and cost-effective method to fabricate nitrogen self-doped graphitic carbon nanosheets (NGS) as an electrocatalyst. This novel strategy using a low-pass filter circuit provides plasma stability and energy control during discharge in pyridine, determining the graphitic structure of nanocarbons doped with nitrogen. Notably, NGS have a relatively high surface area (621 m² g⁻¹), and high contents of nitrogen bonded as pyridinic-N and pyrrolic-N of 55.5 and 21.3%, respectively. As an efficient metal-free electrocatalyst, NGS exhibit a high onset potential (−0.18 V vs. Ag/AgCl) and a 3.8 transferred electron pathway for ORR in alkaline solution, as well as better long-term durability (4% current decrease after 10 000 s of operation) than commercial Pt/C (22% current drop). From this point of view, the nitrogen self-doped graphitic carbon nanosheet material synthesized using the tune-up SP system is a promising catalyst for the ORR, as an alternative to a Pt catalyst for application in energy conversion devices.

The synthesized nitrogen self-doped graphitic carbon nanosheets material using the tune-up SP system is a promising catalyst for the ORR, as an alternative to Pt catalyst for energy conversion device application.     

Sorting and decoration of semiconducting single-walled carbon nanotubes via the quaternization reaction

A study for the selective separation and functionalization of alcohol-soluble semiconducting single-walled carbon nanotubes (sc-SWCNTs) is carried out by polymer main-chain engineering. Introducing tertiary amine groups endows the functionalized sc-SWCNTs with alcohol-soluble properties and introducing the pyrimidine rings allows to increase the selective purity of sc-SWCNTs. In this study, a series of poly[(9,9-dioctylfluorene)-2,7-(9,9-bis(3′-(N,N-dimethylamino)propyl)-fluorene)]_m-alt-[2-methylpyrimidine-2,7-(9,9-dioctylfluorene)]_n (PFPy) are used for the selective dispersion of semiconducting single-walled carbon nanotubes, where n and m are the composition ratio of the copolymer. When m = n, the effective isolation of sc-SWCNTs with purity greater than 99% is achieved. The alcohol-soluble sc-SWCNTs with a diameter in the range of 1.1–1.4 nm are obtained through designing reasonable molecular structure. Moreover, the particular preference of PFPy (m = n) for sc-SWCNTs was studied via density functional theory (DFT) calculations and it was proved to be a promising method for the separation and functionalization of sc-SWCNTs.

An effective route to decorate and selectively separate alcohol-soluble sc-SWCNTs has been proposed.     

In situ production of a two-dimensional molybdenum disulfide/graphene hybrid nanosheet anode for lithium-ion batteries

A solvent-free, low-cost, high-yield and scalable single-step ball milling process is developed to construct 2D MoS₂/graphene hybrid electrodes for lithium-ion batteries. Electron microscopy investigation reveals that the obtained hybrid electrodes consist of numerous nanosheets of MoS₂ and graphene which are randomly distributed. The MoS₂/graphene hybrid anodes exhibit excellent cycling stability with high reversible capacities (442 mA h g⁻¹ for MoS₂/graphene (40 h); 553 mA h g⁻¹ for MoS₂/graphene (20 h); 342 mA h g⁻¹ for MoS₂/graphene (10 h)) at a high current rate of 250 mA g⁻¹ after 100 cycles, whereas the pristine MoS₂ electrode shows huge capacity fading with a retention of 37 mA h g⁻¹ at 250 mA g⁻¹ current after 100 cycles. The incorporation of graphene into MoS₂ has an extraordinary effect on its electrochemical performance. This work emphasises the importance of the construction of the 2D MoS₂/graphene hybrid structure to prevent capacity fading issues with the MoS₂ anode in lithium-ion batteries.

A solvent-free, low-cost, high-yield and scalable single-step ball milling process is developed to construct 2D MoS₂/graphene hybrid electrodes for lithium-ion batteries.     

Electrochemical synthesis of quinazolinone via I2-catalyzed tandem oxidation in aqueous solution

The development of protocols for synthesizing quinazolinones using biocompatible catalysts in aqueous medium will help to resolve the difficulties of using green and sustainable chemistry for their synthesis. Herein, using I₂ in coordination with electrochemical synthesis induced a C–H oxidation reaction which is reported when using water as the environmentally friendly solvent to access a broad range of quinazolinones at room temperature. The reaction mechanism strongly showed that I₂ cooperates electrochemically promoted the oxidation of alcohols, then effectively cyclizing amides to various quinazolinones.

The development of protocols for synthesizing quinazolinones using biocompatible catalysts in aqueous medium will help to resolve the difficulties of using green and sustainable chemistry for their synthesis.

The N-heterocycles are key core structures that form the basis of many pharmaceutical, agrochemical and natural products.¹ Among them, quinazolinones are an important motif in several biologically relevant pharmacophores,² such as methaqualone which is famous for its effective sedative and hypnotic effects, luotonin A which is a quinazolinone alkaloid with anti-inflammatory effects, and erlotinib which is an anti-tumour agent, and all these compounds contain a quinazoline bond in their backbone (Fig. 1).³Open in a separate windowFig. 1Bioactive compounds containing quinazolinone skeleton.Due to their advantageous structures quinazolinones have been widely explored in numerous syntheses.⁴ The classical method involves condensation of aldehydes and o-aminobenzamides to give aminal intermediates, which then undergo oxidation to yield the final quinazolinone product.⁵ Another strategy is to use more benign and readily available alcohols as starting materials.⁶ The reaction takes place through a two-step oxidation pathway, where the alcohols are first oxidized to aldehydes, followed by coupling with o-aminobenzamides. The catalyst needs to demonstrate high activity and selectivity as the reaction involves dehydrogenation of both the C–H and N–H bonds in one pot. In 2018, Sarma and co-workers⁷ demonstrated that a magnetically recoverable iron oxide-carbon dot nanocomposite was an effective catalyst for cyclooxidative tandem synthesis of quinazolinones in aqueous medium using alcohols as starting materials. Furthermore, annulation reactions of o-aminoaryl acids may be the most employed strategies, which include the condensation of o-aminoaryl acids with amides, nitriles, or acid derivatives plus a nitrogen source.⁸ Moreover, the synthesis of quinazolinone involving transition metal catalysed reactions of o-haloarylamides with nitriles, or amines⁹ and reaction of o-halogenated aryl acid with amides¹⁰ have been explored (Scheme 1).Open in a separate windowScheme 1Methods for the synthesis of quinazolinones.Although the above approaches solved a lot of practical problems, there are still some limitations such as long reaction time, high temperature and by-products. Hence, development of greener, atom economic, synthetic approaches for the preparation of quinazolinones from inexpensive and easily available starting materials under relatively mild conditions is desirable. On one hand, electrochemical-induced direct functionalization has gained significant attention from the synthetic chemistry community due to it being environmentally friendly, and requiring mild conditions, and low-energy irradiation.¹¹ With electrons as the oxidizing/reducing agent, organic electrosynthesis could offer appropriate alternatives to traditional oxidation or reduction reactions. For example, Zhao and co-workers¹² reported an efficient electrochemical-induced C–H methylthiolation of electron-rich aromatics via a three-component cross-coupling strategy. On the other hand, the non-metallic oxidant, iodine, catalysed C–H oxidation has attracted great interest in recent times due to its low toxicity and because it is inexpensive compared with transition metal catalysts.¹³ Therefore, the combination of electrochemical catalysis and non-metallic oxidant iodine is a very feasible means of organic synthesis and does not require use of the historical large doses of iodine. In this research the possibility of combining the two in a one-pot reaction was explored, thus avoiding the isolation of either aldehyde or amine intermediates leading to quinazolinones formed from alcohols as starting materials.Furthermore, water as a reaction medium is generally considered as an inexpensive, safe, and environmentally benign alternative to organic solvents.¹⁴ Recently, Muthaiah and co-workers¹⁵ demonstrated a catalyst system for the dehydrogenative oxidation of alcohols to carbonyl compounds and dehydrogenative lactonization of diols in water catalyzed by a water-soluble bifunctional iridium complex. In continuation of our work to develop new organic transformations,^16,11f herein, it is demonstrated that I₂ is an efficient catalyst for a novel, one-pot electrochemical-induced tandem reaction in aqueous solution.The investigation was initially begun by selecting o-aminobenzamide 1a and benzyl alcohol 2a as the model substrate to optimize the reaction conditions shown in 17 It was also observed that the reduction in current also leads to a reduction in yield ( EntryVariations from the standard conditionsYield^b (%) 1 None 92 20.1 mmol I₂423MeCN/H₂O (v/v = 1 : 1) as solvent734In the absence of I₂Trace5In the absence of NaOHTrace6CuI instead of I₂197TBAI instead of I₂268KI instead of I₂429Cs₂CO₃ instead of NaOH3110KOH instead of NaOH7311No currentTrace12Addition of Bu₄NPF₆ as electrolyte92130.4 mmol I₂ instead of current28141.0 mmol I₂ instead of current3015C(+)/Pt(−)6416Pt(+)/Cu(−)161740 mA71Open in a separate window^aStandard conditions: undivided cell, Pt anode, Pt cathode, 1a (0.5 mmol), 2a (0.6 mmol), I₂ (0.2 mmol), NaOH (2 mmol), H₂O (3 mL), I = 80 mA at room temperature (r.t.) for 6 h.^bIsolated yield.The optimized conditions were then applied to various substrates to extend the scope of the method ( Open in a separate window^aStandard Conditions: undivided cell, Pt anode, Pt cathode, 1a (0.5 mmol), 2a (0.6 mmol), I₂ (0.2 mmol), NaOH (2 mmol) in H₂O (3 mL), I = 80 mA at room temperature for 6 h.^bIsolated yield.Subsequently, the substrates of o-aminobenzamide were also expanded (Scheme 2, which gave the desired product 3aa with an 83% yield (1.47 g). This reveals that the new procedure has significant advantages over many current methods for further practical applications.Open in a separate windowScheme 2Gram-scale experiment.To exclude the possibility of other reaction pathways, some control experiments were performed (Scheme 3). Firstly, benzyl alcohol 2a effectively gave benzaldehyde 4a using aerobic oxidation reactions under standard conditions. In contrast, the absence of either base or current lead to the production of trace amounts of benzaldehyde, furthermore, the lack of catalyst lead to the formation of 5a (Scheme 3 a and b) with a yield of 43%. Nevertheless, the yield of 4a was reduced to 90% when nitrogen was substituted for air (Scheme 3c). Then, the reaction of 1a and 4a under the standard conditions effectively gave 3aa with a high yield of 93% (Scheme 3d), whereas the reaction of o-aminobenzamide under a nitrogen atmosphere, resulted in 3aa with a yield of 68% and 32% (Scheme 3e and f).Open in a separate windowScheme 3Control experiments of alcohol oxidation and synthesis of 3aa.To gain an understanding of the reaction mechanism, cyclic voltammetry (CV) was then conducted (Fig. 2). Using H₂O as the solvent, a glass electrode as the working electrode, platinum wire as the opposite electrode, and SCE as the reference electrode with a 0.1 V s⁻¹ scanning rate. By comparing curves c, f and g, it was observed that the onset potential of the I₂ oxidation shifted from 1.57 to 1.07 V vs. SCE in the presence of substrate 2a, which was more than that in the presence of NaOH, indicating that I₂ was more likely to react with b than disproportionated with NaOH (Fig. 2, curves c, f and g). However, a reduction peak appears in b at 0.43 V, and the reduction peak of I₂ was 0.69 V, indicating that I⁻ is still carrying out the reduction reaction in the presence of b (Fig. 2, curve c and g). Interestingly, the addition of 1a and NaOH further decreases the onset potential of I₂ complex oxidation to 0.99 V vs. SCE (Fig. 2, curve e). In contrast, the oxidation potential increased when NaOH was added individually, and this may be evidence that NaOH does not interact with 2a and I₂ (Fig. 2, curve d).Open in a separate windowFig. 2Cyclic voltammetry measurements were performed at room temperature with standard three electrode systems.By considering the whole of the experimental findings, a plausible mechanistic pathway for the formation of compound 3aa is outlined in Scheme 4. First of all, benzyl alcohol 2a is first oxidized to benzaldehyde 4a by I₂, which is generated in situ on the anode.^6a,18 The iodine undergoes cathodic reduction by regenerating the iodide ion for the catalytic cycle. Next, 4a can readily react with 1a to obtain the imine D, imine D is then converted to E in the presence of a base, and thus E oxidizes and dehydrogenates to give the desired product 3aa.¹⁹ Finally, H⁺ removed from E combines with O₂ to form H₂O at the cathode.²⁰Open in a separate windowScheme 4Proposed mechanism for this transformation.Lots of nuclear nitrogen heterocycles, such as pteridine which is a widely existing aromatic compound, similar to quinazolines, which are tyrosine kinase inhibitors, which have good specificity and inhibitory activity on tumour cells. It is thought that many fluorine-containing drugs have been widely used in clinical situations. The introduction of fluorine atoms and fluorine-containing groups into drugs can improve the clinical efficacy of drugs and reduce their side effects. Based on this, the electrocatalytic system was applied to the synthesis of the structure of the trexine and the synthesis of gefitinib analogues. For example, N-(4-methoxyphenyl)-6-(2,2,2-trifluoroethoxy)pteridin-4-amine A3 was synthesized from 3-amino-6-(2,2,2-trifluoroethoxy) pyrazine-2-carboxamide with a yield of 56% (see Scheme 5). The compounds were then tested using the MTT assay and the results are shown in Fig. 3. The A3 inhibited human non-small-cell lung carcinoma cells, A549, human colon cancer cells, HCT-116, and human gastric cancer cells, SGC-7901 to different degrees. As shown in Fig. 3, the results showed that the IC50 value of A3 in HCT116 cells (IC50 = 14.79 μg mL⁻¹ in A549 cells, IC50 = 34.52 μg mL⁻¹ in HCT116 cells, IC50 = 36.44 μg mL⁻¹ in SCG-7901 cells) was just surpassed by that of gefitinib.Open in a separate windowScheme 5Synthesis of N-(4-methoxyphenyl)-6-(2,2,2-trifluoroethoxy) pteridin-4-amine (A3).Open in a separate windowFig. 3 In vitro inhibitory data of target compounds against A549, HCT-116 and SGC-7901 cell line.     

One-step sulfuration synthesis of hierarchical NiCo2S4@NiCo2S4 nanotube/nanosheet arrays on carbon cloth as advanced electrodes for high-performance flexible solid-state hybrid supercapacitors

To obtain high-performance hybrid supercapacitors (HSCs), a new class of battery-type electrode materials with hierarchical core/shell structure, high conductivity and rich porosity are needed. Herein, we propose a facile one-step sulfuration approach to achieve the fabrication of hierarchical NiCo₂S₄@NiCo₂S₄ hybrid nanotube/nanosheet arrays (NTSAs) on carbon cloth, by taking hydrothermally grown Ni–Co precursor@Ni–Co precursor nanowire/nanosheet arrays (NWSAs) as the starting templates. The optimized electrode of NiCo₂S₄@NiCo₂S₄ hybrid NTSAs demonstrates an enhanced areal capacity of 245 μA h cm⁻² at 2 mA cm⁻² with outstanding rate capability (73% from 2 to 20 mA cm⁻²) and cycling stability (86% at 10 mA cm⁻² over 3000 cycles). In addition, flexible solid-state HSC devices are assembled by using NiCo₂S₄@NiCo₂S₄ hybrid NTSAs and activated carbon as the positive and negative electrodes, respectively, which manifest a maximum volumetric energy density of 1.03 mW h cm⁻³ at a power density of 11.4 mW cm⁻³, with excellent cycling stability. Our work indicates the feasibility of designing and fabricating core/shell structured metal sulfides through such a facile one-step sulfuration process and the great potential of these materials as advanced electrodes for high-performance HSC devices.

One-step sulfuration synthesis of NiCo₂S₄@NiCo₂S₄ core–shell arrays on carbon cloth.     

One-step green synthesis of 2D Ag-dendrite-embedded biopolymer hydrogel beads as a catalytic reactor

Silver (Ag) nanocrystals with a dendritic structure have attracted intensive attention because of their unique structural properties, which include abundant sharp corners and edges that provide a large number of active atoms. However, the synthesis of Ag dendrites via a simple and environmentally friendly method under ambient conditions remains a challenge. In this paper, we report a simple water-based green method for the production of biopolymer hydrogel beads embedded with Ag dendrites without using an additional reducing agent, stabilizer, or crosslinking agent. The obtained Ag dendrites exhibit a unique two-dimensional (2D) structure rather than a conventional three-dimensional structure because Ag⁺ ions are reduced on the surface of the solid-phase hydrogel beads and grow into crystals. Reasonable mechanisms explaining the formation of the nanocomposite hydrogel beads and the formation of 2D Ag dendrites in the hydrogel are proposed on the basis of our observations and results. The hydrogel beads embedded the 2D Ag dendrites were used as an environmentally friendly catalytic reactor, and their catalytic performance was evaluated by adopting the reduction of 4-nitrophenol to 4-aminophenol with NaBH₄ as a model reaction.

Alginate hydrogel beads embedded with 2D Ag dendrites were synthesized by simply adding aqueous alginate droplets to an aqueous AgNO₃ solution.     

Highly uniform monolayer graphene synthesis via a facile pretreatment of copper catalyst substrates using an ammonium persulfate solution

The demand for large-area, high-quality synthesis of graphene with chemical vapor deposition (CVD) has increased for the realization of next-generation transparent and flexible optoelectronic applications. In conventional CVD processes, various synthesis parameters can strongly affect the quality of the resultant graphene. In particular, surface engineering of a copper catalyst substrate is one of the most promising pathways for achieving high-quality graphene with excellent reproducibility. For this purpose, simple wet chemical etching of a catalyst substrate without toxic fume byproducts or metal ion residues is desired. Here, we suggest a facile method for preparing a pretreated copper catalyst substrate for highly uniform, large-area CVD graphene growth. This pretreatment method involves a wet copper etchant, ammonium persulfate (APS) solution, and gentle ultrasonication (100 W), which do not produce unwanted or toxic fume byproducts during their reaction. Moreover, this approach does not leave metal ion residue on the cleaned copper substrates that serves as residual nucleation sites and leads to multilayer graphene growth. To evaluate the quality of the synthesized monolayer graphene on the cleaned copper catalyst substrates, we used various characterization techniques, such as Raman spectroscopy and sheet resistance, optical transmittance, and FET characterization.

A facile method for preparing a pretreated copper catalyst substrate for highly uniform, large-area CVD graphene growth is proposed.     

Rational synthesis of a hierarchical Mo2C/C nanosheet composite with enhanced lithium storage properties

Transition metal carbides have been studied extensively as anode materials for lithium-ion batteries (LIBs), but they suffer from sluggish lithium reaction kinetics and large volume expansion. Herein, a hierarchical Mo₂C/C nanosheet composite has been synthesized through a rational pyrolysis strategy, and evaluated as an anode material with enhanced lithium storage properties for LIBs. In the hierarchical Mo₂C/C nanosheet composite, large numbers of Mo₂C nanosheets with a thickness of 40–100 nm are uniformly anchored onto/into carbon nanosheet matrices. This unique hierarchical architecture can provide favorable ion and electron transport pathways and alleviate the volume change of Mo₂C during cycling. As a consequence, the hierarchical Mo₂C/C nanosheet composite exhibits high-performance lithium storage with a reversible capacity of up to 868.6 mA h g⁻¹ after 300 cycles at a current density of 0.2 A g⁻¹, as well as a high rate capacity of 541.8 mA h g⁻¹ even at 5.0 A g⁻¹. More importantly, this hierarchical composite demonstrates impressive cyclability with a capacity retention efficiency of 122.1% over 5000 successive cycles at 5.0 A g⁻¹, which surpasses the cycling properties of most other Mo₂C-based materials reported to date.

The hierarchical Mo₂C/C nanosheet composite exhibits excellent cyclability owing to its structural and kinetic advantages.     

Water-based synthesis of photocrosslinked hyaluronic acid/polyvinyl alcohol membranes via electrospinning

Electrospinning is a versatile and low-cost technique widely used in the manufacture of nanofibrous polymeric membranes applied in different areas, especially in bioengineering. Hyaluronic acid (HA) is a biocompatible natural polymer, but it has rheological characteristics that make the electrospinning process difficult. Thus, its association with another polymer such as poly(vinyl alcohol) (PVA) is an alternative, as PVA has good rheological properties for electrospinning. Based on this, the aim of this work was to produce, by the conventional electrospinning method, cross-linked HA/PVA membranes free from organic solvent with a low degradation rate in PBS 7.4 solution after the photocrosslinking process and without using any organic solvent. The results showed that the electrospinning occurred effectively for all conditions tested, but the best result for complete cross-linking only occurred with 15 and 30% crosslinker, which was evidenced by infrared spectroscopy. The addition of crosslinker favored the stability of the electrospinning jet, especially for 30% crosslinker concentration. The membranes did not show cytotoxicity even after the cross-linking process, which indicates that the material has potential as a drug delivery device.

Homogeneous nanofibers and non-cytotoxic HA/PVA membranes were produced by conventional electrospinning method followed by photocrosslinking process, without using any organic solvent. The membranes showed great potential for biomedical applications.     

Room-temperature preparation of a chiral covalent organic framework for the selective adsorption of amino acid enantiomers

Herein, we have reported the facile room-temperature synthesis of a chiral covalent organic framework (CCOF) for the enantioselective adsorption of amino acids. The prepared CCOF provides various stereoscopic interactions with amino acids for highly selective adsorption of their enantiomers.

A chiral COF CTzDa was synthesized at room temperature for the selective enantioselective adsorption of amino acids.

Chirality is one of the most common properties of natural compounds including proteins, polysaccharides, nucleic acids and enzymes, and it plays an extremely important role in life activities.^1,2 However, the selective recognition and interaction of their enantiomers with organisms make a huge difference in activity, toxicity, adsorption, transfer, metabolism and elimination. Therefore, the exploration of efficient ways to obtain pure enantiomers becomes more and more urgent; however, this is highly challenging owing to the dramatic similarity of the physicochemical properties of two enantiomers.^3,4 To date, various chiral separation techniques have been proposed such as chromatography,^5,6 crystallization^7,8 and extraction.^9,10 Adsorption separation based on porous materials has shown advantages due to their strong chiral recognition ability, long-term stability, and less complexity.¹¹The exploration of chiral-functionalized porous materials as adsorbents for the highly efficient resolution of enantiomers has received extensive attention; these materials include metal–organic frameworks,^12,13 porous organic cages,^14,15 metal–organic cages^16,17 and composite porous materials.¹⁸ However, the type of adsorbents for enantioselective adsorption was far more enough due to its challengeable preparation. As a consequence, it is necessary to design and prepare more new adsorbents with excellent stability and rapid kinetics for the selective adsorption of enantiomers.Covalent organic frameworks (COFs)^19,20 are crystalline organic porous materials with broad applications in diverse fields including chromatography separation,^21,22 heterogeneous catalysis,^23,24 fluorescence sensing^25,26 and optoelectronic materials.^27,28 The large surface area, excellent stability and the number of duplicate ordered units of COFs allow numerous interactions between the host and guests, such as hydrogen bonding, π–π interactions, hydrophobic interactions and molecular sieving, indicating COFs as a convenient platform for enantioselective adsorption. Chiral covalent organic frameworks (CCOFs) have been explored as the stationary phase in chiral chromatography and as catalysts in asymmetric catalysis.^29–31 However, the application of CCOFs as adsorbents for selective adsorption has been rarely reported.Here, we have reported the design and room-temperature (RT) synthesis of a CCOF, CTzDa, via the post-modification of the COF TzDa for the selective adsorption of the enantiomers of amino acids (AAs). TzDa consisting of 4,4′,4''-(1,3,5-triazine-2,4,6-triyl)trianiline (Tz) and 1,4-dihydroxyterephthalaldehyde (Da) was chosen as the platform for the preparation of chiral COF due to its high stability, easy synthesis and abundant active groups (–OH).³²d-Camphoric acid was converted to its acid chloride to react with the hydroxyl group of TzDa for obtaining CTzDa. The application of CTzDa as the adsorbent for the chiral separation of AAs was further investigated via detailed experimental characterizations and computational modeling. This work shows high potential of chiral COFs as adsorbents in enantioselective adsorption.The COFs used as adsorbents should possess great stability, high crystallinity and large surface areas. Moreover, the introduction of a chiral environment into the COF structure via a post-modification strategy is a widely accessible way to prepare CCOFs. In this work, TzDa, which possessed a highly ordered and stable structure with abundant active groups (–OH) for further modification, was chosen as the COF platform for preparing CCOF. As shown in Fig. 1, we synthesized TzDa by condensing Tz and Da at RT instead of high temperature and pressure (Fig. S1, ESI^†).Open in a separate windowFig. 1Room-temperature synthesis: (a) TzDa; (b) CTzDa. d-Camphor acid chloride (d-cam-ClO) (Fig. S2 and S3, ESI^†) prepared from d-camphor acid was then applied to react with hydroxyl groups to introduce the chiral moiety into the channel of TzDa for preparing CTzDa.The Fourier transform infrared (FTIR) spectra of TzDa show the C N peak at 1665 cm⁻¹ along with the disappearance of the peaks for the C O and NH₂ bonds for the starting materials, indicating the successful condensation of Tz and Da (Fig. S4, ESI^†). Compared with TzDa, CTzDa exhibited additional peaks at 1805 cm⁻¹, 1741 cm⁻¹ and 1259 cm⁻¹ for the C O bond of the carboxyl group and C O and C–O bonds of the ester group, respectively, but no peaks for the C O bond of acid chloride (Fig. 2a and S5, ESI^†). The result reveals the successful grafting of the chiral d-camphoric acid moiety on TzDa. The modification ratio of d-camphoric acid on TzDA was calculated to be 41% using a toluidine blue O (TBO) dye assay (ESI^†).Open in a separate windowFig. 2(a) FTIR spectra of TaDa and CTzDa. (b) PXRD patterns of TaDa and CTzDa. (c) Zeta potential of TzDa and CTzDa. (d) PXRD patterns of CTzDa after immersing in various solvents.The powder X-ray diffraction (PXRD) pattern of TzDa prepared via the RT approach not only matched well with the simulated PXRD pattern, but also showed all the characteristic peaks of TzDa obtained with the solvothermal approach, indicating the formation of the reported ordered structure of TzDa (Fig. S6, ESI^†). All the PXRD peaks of TzDa remained after modification with d-cam-ClO, indicating no change in the crystal structure. The coupling reaction of camphoric acid with its hydroxyl group prevents the formation of intramolecular hydrogen bonds between the hydroxyl groups (Da) on formaldehyde (Tz), which results in a decrease in the crystallinity of the synthesized CTzDa (Fig. 2b and S7, ESI^†).The grafting of d-cam made the zeta potential of COF more negative from −8.2 mV (TzDa) to −47.3 mV (CTaDa) due to the introduction of the hydroxyl group of d-cam (Fig. 2c; Table S1, ESI^†). There was no variation in the PXRD patterns and FTIR spectra of CTzDa after immersing in various solvents including tetrahydrofuran (THF), acetonitrile (ACN), dimethyl formamide (DMF), water, 0.1 M HCl and 0.1 M NaOH for 1 day, demonstrating the high chemical stability of CTzDa (Fig. 2d and S8, ESI^†). The prepared CTzDa also had high thermal stability up to 200 °C (Fig. S9, ESI^†).The transmission electron microscopy (TEM) images show a layer-like structure for both TzDa and CTzDa and no obvious change in morphology after the grafting of d-camphor acid onto TzDa (Fig. S10, ESI^†). The scanning electron microscopy (SEM) images indicate that the surface of CTzDa is rougher than that of TzDa (Fig. S11, ESI^†). The Brunauer–Emmett–Teller (BET) surface area and the pore size of TzDa were calculated to be 1380 m² g⁻¹ and 3.2 nm, respectively, while those of CTzDa decreased to 403 m² g⁻¹ and 1.8 nm, respectively, due to the introduction of d-camphor acid (Fig. S12 and Table S2, ESI^†).The introduction of a chiral moiety caused various stereoscopic interactions in the COF, which could improve the enantioselective ability of CTzDa. Thus, we employed the synthesized porous material CTzDa for the selective adsorption of chiral AAs (tryptophan (Trp), histidine (His), aspartic acid (Asp) and serine (Ser)). The effect of the concentration of AAs on the adsorption capacity indicated the appropriate concentrations of AAs in adsorption (Fig. S13, ESI^†). The effect of pH on the AAs adsorption showed that the adsorption process was favorable near the isoelectric point (Fig. S14, ESI^†). In comparison with TzDa, CTzDa exhibited obviously higher enantioselectivity and adsorption capacity to l-AAs than d-AAs (Fig. 3 and S15, ESI^†).Open in a separate windowFig. 3Time-dependent enantioselective adsorption of AAs on CTzDa at 293 K: (a) d-Trp and l-Trp (50 mg L⁻¹); (b) d-His and l-His (20 mg L⁻¹); (c) d-Asp and l-Asp (red, 20 mg L⁻¹); (d) d-Ser and l-Ser (20 mg L⁻¹).We further investigated the kinetics and adsorption isotherms of AAs on CTzDa. The time-dependent adsorption capacity (q_t) of AAs at three initial concentrations at 293 K showed that the adsorption equilibrium of AAs on CTzDa was achieved within 30 min, indicating the rapid adsorption of AAs on CTzDa (Fig. S16, ESI^†). The adsorption followed the pseudo-second-order kinetic model rather than the pseudo-first-order kinetic model (Fig. S17 and S18, ESI^†). The larger k₂ values of d-AAs than those of l-AAs indicate different interactions of CTzDa with d-AAs and l-AAs (Table S3, ESI^†).^33,34The adsorption isotherms were evaluated in an initial concentration range of 10–100 mg L⁻¹ at four different temperatures (20–50 °C) (Fig. S19, ESI^†). The adsorption isotherms of AAs could be better described by the Langmuir model than the Freundlich model (Table S4, ESI^†), indicating monolayer adsorption of AAs on CTzDa. The calculated maximum adsorption capacities (q_m) of l-AAs were higher than those of d-AAs, indicating the selective adsorption of chiral AAs on CTzDa. The adsorption enantioselectivity values of CTzDa were 4.20, 2.59, 2.60 and 1.62 for the enantiomers of Trp, His, Asp and Ser, respectively (Table S5, ESI^†). Compared with previous adsorbents, the developed CTzDa exhibited higher enantioselectivity (Table S6, ESI^†), showing the great potential of CTzDa as an adsorbent in the enantioselective adsorption of AAs.Efficient desorption and reusability are essential for adsorbents. Different types of eluents were used for the desorption of AAs from CTzDa at 60 °C under ultrasonication for 5 min (Fig. S20^†). The results showed that organic solvents were not favourable for AA desorption. The adsorbed AAs could be well desorbed from CTzDa with water (pH = 4 or 8) (Fig. S20, ESI^†) due to the increase in the hydrophilicity of AAs.³⁵ After five adsorption–desorption cycles, CTzDa exhibited no significant decrease in adsorption capacity, indicating the good reusability of CTzDa for the adsorption of AAs (Fig. S21, ESI^†). There was no obvious change in the PXRD pattern and FTIR spectra after five adsorption–desorption cycles, suggesting that CTzDa was stable during adsorption and desorption (Fig. S22, ESI^†).The adsorption thermodynamics was assessed by the change in Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) (Fig. S23, S24 and Table S7, ESI^†). The negative ΔG value indicated that the adsorption of AAs on CTzDa was thermodynamically spontaneous. The negative ΔH value suggested the presence of an exothermic process, which was related to the decrease in adsorption capacity at high temperatures. The negative ΔS value demonstrated the AAs lost freedom during the adsorption process.AutoDock Vina (ADVina) was used to perform docking calculations.^36,37 The calculated binding energy (BE, kcal mol⁻¹) represents the generated energy in adsorption (Table S8, ESI^†). The existing interaction modes between CTzDa and AAs are shown in Fig. 4. The binding interactions between AAs and the building unit mainly included π–π interactions, C–H⋯π interactions and H-bonds, but the strengths related to the stereoscopic interactions were different, which originally resulted in distinct adsorptions. For Trp, the carboxyl and amino groups of l-Trp could both form hydrogen bonds with CTzDa, while the different stereoscopic positions of d-Trp led to only carboxyl group forming aromatic H-bonds with CTzDa (Fig. 4a). The carboxyl group of l-His or l-Ser formed hydrogen bonds with CTzDa. On the contrary, the corresponding hydrogen bond of d-His or d-Ser between the carboxyl group and CTzDa was absent due to the large distance (Fig. 4b and d). The hydrogen bond length between l-Asp and CTzDa (1.95 Å) was shorter than that between d-Asp and CTzDa (2.61 Å) (Fig. 4c). The above-mentioned different stereoscopic interactions made the BE between the main framework and racemic AAs follow the order l-AAs > d-AAs, indicating the stronger adsorption of l-AAs than that of d-AAs on CTzDa (Table S8^†). The K_L/K_D ratios were 1.97, 1.66, 1.18 and 1.40 for Trp, His, Aps and Ser, respectively. K_L/K_D > 1 also indicated that CTzDa exhibited stronger adsorption of l-AAs than that of d-AAs.Open in a separate windowFig. 4Molecular docking modes between CTzDa and AAs: (a) Trp; (b) His; (c) Asp; (d) Ser. The receptor COF unit is displayed with thin stick style by marking C in yellow, O in red, N in blue and H in white. The AAs are displayed with thick stick style by marking C in green, O in red, N in blue and H in white. Blue, green and yellow dotted lines represent the π–π interaction, n–π interaction and hydrogen bond between CTzDa and AAs, respectively. Thin figure represents the distance of atoms.In summary, we have designed and synthesised the chiral COF CTzDa through introducing a chiral selector (d-cam) in the COF TzDa at room temperature in a facile manner. The prepared CTzDa showed good stability in various solvents, which was favourable for adsorption. CTzDa also exhibited rapid kinetics and high selectivity for the adsorption separation of the enantiomers of amino acids. Docking calculations showed that the difference in the stereoscopic hydrogen bonds between l-AAs and d-AAs is the key interaction for the enantioselective adsorption of AAs on CTzDa. This work provides a facile strategy for highly selective adsorption of AA enantiomers. Further research will focus on the potential of CTzDa in the chiral chromatographic separation of AAs.     

Aqueous solution photocatalytic synthesis of p-anisaldehyde by using graphite-like carbon nitride photocatalysts obtained via the hard-templating route

Graphite-like carbon nitride (GCN)-based materials were developed via the hard-templating route, using dicyandiamide as the GCN precursor and silica templates. That resulted in urchin-like GCN (GCN-UL), 3D ordered macroporous GCN (GCN-OM) and mesoporous GCN (GCN-MP). The introduction of silica templates during GCN synthesis produced physical defects on its surface, as confirmed by SEM analysis, increasing their specific surface area. A high amount of nitrogen vacancies is present in modified catalysts (revealed by XPS measurements), which can be related to an increase in the reactive sites available to catalyse redox reactions. The textural and morphological modifications induced in GCN an enhanced light absorption capacity and reduced electron/hole recombination rate, contributing to its improved photocatalytic performance. In the photocatalytic conversion of p-anisyl alcohol to p-anisaldehyde in deoxygenated aqueous solutions under UV-LED irradiation, the GCN-UL was the best photocatalyst reaching 60% yield at 64% conversion for p-anisaldehyde production after 240 min of reaction. Under oxygenated conditions (air), the process efficiency was increased to 79% yield at 92% conversion only after 90 min reaction. The GCN-based photocatalyst kept its performance when using visible-LED radiation under air atmosphere. Trapping of photogenerated holes and radicals by selective scavengers showed that under deoxygenated conditions, holes played the primary role in the p-anisaldehyde synthesis. Under oxygenated conditions, the process is governed by the effect of reactive oxygen species, namely superoxide radicals, with a significant contribution from holes.

Enhanced photocatalytic activity of graphite-like carbon nitride catalysts obtained through hard-templating for a sustainable synthesis of p-anisaldehyde.     

Theoretical study of a p–n homojunction SiGe field-effect transistor via covalent functionalization

p–n homojunctions are superior to p–n heterojunctions in constructing nanoscale functional devices, owing to the excellent crystallographic alignment. We tune the electronic properties of monolayer siligene (SiGe) into p/n-type via the covalent functionalization of electrophilic/nucleophilic dopants, using ab initio quantum transport calculations. It is found that the n-type doping effect of K atoms is stronger than that of benzyl viologen (BV) molecule on the surface of SiGe monolayer, owing to the strong covalent interaction. Both of p-type 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-adsorbed and n-type 4 K-adsorbed SiGe systems show enhanced optical absorption in the infrared region, indicating their promising applications in infrared optoelectronic devices. By spatially adsorbing F4TCNQ molecule and K atoms on the source and drain leads, respectively, we designed a p–n homojunction SiGe field-effect transistor (FET). It is predicted that the built F4TCNQ-4K/SiGe FET can meet the requirements for high-performance (the high current density) and low-power (low subthreshold swing (SS)) applications, according to the International Technology Roadmap for Semiconductors in 2028. The present study gains some key insights into the importance of surface functionalization in constructing p–n homojunction electronic and optoelectronic devices based on monolayer SiGe.

p–n homojunctions are superior to p–n heterojunctions in constructing nanoscale functional devices, owing to the excellent crystallographic alignment.     

Cu/TCH-pr@SBA-15 nano-composite: a new organometallic catalyst for facile three-component synthesis of 4-arylidene-isoxazolidinones

A copper complex supported on SBA-15 nanoparticles (Cu/TCH-pr@SBA-15) was synthesized by the post-synthesis modification of nano-mesoporous silica with 3-chloropropyltriethoxysilane (CPTES) and thiocarbohydrazide (TCH) and subsequent metal–ligand coordination with Cu(ii). These nanocomposites were thoroughly characterized by FT-IR spectroscopy, TEM, FE-SEM, EDX, atomic absorption spectroscopy and N₂ adsorption–desorption (BET) studies. Then, a solvent-free method was developed for the three-component synthesis of 4-arylidene-isoxazolidinones via condensation of hydroxylamine hydrochloride, ethyl acetoacetate and various aromatic aldehydes using Cu/TCH-pr@SBA-15 as a highly efficient nanocatalyst. This new economic and eco-friendly methodology has remarkable advantages such as excellent yields, a shorter reaction time, an easy purification procedure, simplicity, green conditions, solvent-free conditions, and recoverability of the nanocatalyst.

A copper complex supported on SBA-15 nanoparticles (Cu/TCH-pr@SBA-15) was synthesized by the post-synthesis modification of mesoporous silica with (OEt)₃Si-(CH₂)₃Cl and thiocarbohydrazide (TCH) and subsequent metal–ligand coordination with Cu(ii).