Studies on an argon laser-induced photopolymerization employing both mono- and bischromophoric hemicyanine dye–borate complex as a photoinitiator: Part II

Several photoredox pairs containing dichromophoric hemicyanine dyes have been evaluated as novel photoinitiators for free radical polymerization induced with an argon-ion laser irradiation. The tested photoredox couples are the pairs composed of the hemicyanine dye cations acting as electron acceptors and n-butyltriphenyl borate anions being the electron donors. The photoinitiating abilities of the series of dimmeric dichromophoric stilbazolium borates; 1,2-; 1,3-; and 1,4-bis-[4-(p-N,N-dialkylaminostyryl)pyridinyl]xylene di-n-butyltriphenylborates, were compared to the photochemistry of structurally related, monochromophoric styrylpyridinium borates. The obtained results clearly documented that the dicationic photoinitiators exhibit a marked increase in the photoinitiation ability compared to the initiators consisting of a single charged hemicyanine dye.Our studies revealed also that the rate of photopolymerization depends on ΔG_el of electron transfer between borate anion and styrylpyridinium cation. The latter value was estimated for all series of styrylpyridinium borate salts. The relationship between the rate of polymerization and the free energy of activation shows the dependence predicted by the classical theory of electron transfer.     

The three-component radical photoinitiating systems comprising thiacarbocyanine dye,n-butyltriphenylborate salt and N-alkoxypyridinium salt or 1,3,5-triazine derivative

Three-component systems, which contain a light-absorbing species (thiacarbocyanine dye), an electron donor (typically n-butyltriphenylborate salt), and a third component (usually an N-alkoxypyridinium salt or 1,3,5-triazine derivative, respectively), have been applied as the efficient, visible-light-sensitive photoinitiators. The kinetic studies of free radical polymerization reveal a significant increase in polymerization rate with addition of a third component to the photoinitiating system. Although three-component systems have been consistently found to be faster and efficient than their two-component counterparts, these systems are not well understood and a number of distinct mechanisms have been reported in the literature.     

Ln2+ stabilization in strontium borates

The effect of crystal structure on the stability of Ln²⁺ ions in strontium borates has been studied by luminescence spectroscopy. The results indicate that the stability of Ln²⁺ in strontium borates depends on the nature of the borate anions surrounding it and the effectiveness of the compensation of the charge on Ln³⁺, which can be quantified by the formal volume per oxygen atom, V _O, in the structure of the borate.     

Importance of Volume Ratio in Photonic Effects of Lanthanide‐Doped LaPO4 Nanocrystals

Experimental variation of the volume ratio (filling factor: i.e., volume of nanoparticles (NPs) compared with that of medium) of nanocomposite materials with doped lanthanide ions demonstrates that it has a significant affect upon local field effects. Lanthanum orthophosphate NPs are doped with Eu³⁺ and/or Tb³⁺ and immersed in organic solvents and lead borate glasses for Tb^{3+ 5}D₄ lifetime measurements. For media with a refractive index (n_med) less than that of LaPO₄ (n_np = 1.79), the ⁵D₄ emission decay rate increases with increasing volume ratio of the NPs, whereas for n_med > 1.79, the decay rate decreases with increasing volume ratio. Fitting with the model of Pukhov provides an estimation of the radiative lifetime of ⁵D₄ and the quantum yield. Energy transfer (ET) from Tb³⁺ to Eu³⁺ occurs in co‐doped LaPO₄ NPs with excitation into a Tb³⁺ absorption band. The ET rate is independent on n_med and the energy transfer efficiency decreases with an increase in n_med. The behavior of ET rate with regard to the local field is consistent with the Dexter, but not Förster, equation for ET rate involving the electric dipole–electric dipole mechanism. This has consequences when using the spectroscopic ruler approach to measure distances between donor–acceptor chromophores.     

A novel shortened electrospun nanofiber modified with a ‘concentrated’ polymer brush

Abstract

We report the fabrication of shortened electrospun polymer fibers with a well-defined concentrated polymer brush. We first prepared electrospun nanofibers from a random copolymer of styrene and 4-vinylbenzyl 2-bromopropionate, with number-average molecular weight M_n=105 200 and weight-average molecular weight M_w=296 700 (M_w/M_n=2.82). The fibers had a diameter of 593±74 nm and contained initiating sites for surface-initiated atom transfer radical polymerization (SI-ATRP). Then, SI-ATRP of hydrophilic styrene sodium sulfonate (SSNa) was carried out in the presence of a free initiator and the hydrophobic fibers. Gel permeation chromatography confirmed that M_n and M_w/M_n values were almost the same for free polymers and graft polymers. M_n agreed well with the theoretical prediction, and M_w/M_n was relatively low (<1.3) in all the examined cases, indicating that this polymerization proceeded in a living manner. Using the values of the graft amount measured by Fourier transform infrared spectroscopy, the surface area, and M_n, we calculated the graft density σ as 0.22 chains nm^?2. This value was nearly equal to the density obtained on silicon wafers (σ=0.24 chains nm^?2), which is categorized into the concentrated brush regime. Finally, we mechanically cut the fibers with a concentrated poly(SSNa) brush by a homogenizer. With increasing cutting time, the fiber length became shorter and more homogenous (11±17 μm after 3 h). The shortened fibers exhibited excellent water dispersibility owing to the hydrophilic poly(SSNa) brush layer.     

Rapid and Controllable Synthesis of Nanocrystallized Nickel‐Cobalt Boride Electrode Materials via a Mircoimpinging Stream Reaction for High Performance Supercapacitors

Nickel‐cobalt borides (denoted as NCBs) have been considered as a promising candidate for aqueous supercapacitors due to their high capacitive performances. However, most reported NCBs are amorphous that results in slow electron transfer and even structure collapse during cycling. In this work, a nanocrystallized NCBs‐based supercapacitor is successfully designed via a facile and practical microimpinging stream reactor (MISR) technique, composed of a nanocrystallized NCB core to facilitate the charge transfer, and a tightly contacted Ni‐Co borates/metaborates (NCB_i) shell which is helpful for OH^? adsorption. These merits endow NCB@NCB_i a large specific capacity of 966 C g^?1 (capacitance of 2415 F g^?1) at 1 A g^?1 and good rate capability (633.2 C g^?1 at 30 A g^?1), as well as a very high energy density of 74.3 Wh kg^?1 in an asymmetric supercapacitor device. More interestingly, it is found that a gradual in situ conversion of core NCBs to nanocrystallized Ni‐Co (oxy)‐hydroxides inwardly takes place during the cycles, which continuously offers large specific capacity due to more electron transfer in the redox reaction processes. Meanwhile, the electron deficient state of boron in metal‐borates shells can make it easier to accept electrons and thus promote ionic conduction.     

Lasing from Mechanically Exfoliated 2D Homologous Ruddlesden–Popper Perovskite Engineered by Inorganic Layer Thickness

2D Ruddlesden–Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA)₂(MA)_n?1Pb_nI_3n+1 RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large‐n RPPs (n ≥ 3) in the spectral range of 620–680 nm but not from small‐n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n‐engineered lasing behaviors are attributed to the stronger Auger recombination and exciton–phonon interaction as a result of the enhanced quantum confinement in the smaller‐n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low‐threshold, substrate‐free, and multicolor 2D semiconductor microlasers.     

Hierarchical NiO@N‐Doped Carbon Microspheres with Ultrathin Nanosheet Subunits as Excellent Photocatalysts for Hydrogen Evolution

Achieving highly efficient hierarchical photocatalysts for hydrogen evolution is always challenging. Herein, hierarchical mesoporous NiO@N‐doped carbon microspheres (HNINC) are successfully fabricated with ultrathin nanosheet subunits as high‐performance photocatalysts for hydrogen evolution. The unique architecture of N‐doped carbon layers and hierarchical mesoporous structures from HNINC could effectively facilitate the separation and transfer of photo‐induced electron–hole pairs and afford rich active sites for photocatalytic reactions, leading to a significantly higher H₂ production rate than NiO deposited with platinum. Density functional theory calculations reveal that the migration path of the photo‐generated electron transfer is from Ni 3d and O 2p hybrid states of NiO to the C 2p state of graphite, while the photo‐generated holes locate at Ni 4s and Ni 4p hybrid states of NiO, which is beneficial to improve the separation of photo‐generated electron–hole pairs. Gibbs free energy of the intermediate state for hydrogen evolution reaction is calculated to provide a fundamental understanding of the high H₂ production rate of HNINC. This research sheds light on developing novel photocatalysts for efficient hydrogen evolution.     

New Insight into Procedure of Interface Electron Transfer through Cascade System with Enhanced Photocatalytic Activity

Recombination of photogenerated electron–hole pairs is extremely limited in the practical application of photocatalysis toward solving the energy crisis and environmental pollution. A rational design of the cascade system (i.e., rGO/Bi₂WO₆/Au, and ternary composites) with highly efficient charge carrier separation is successfully constructed. As expected, the integrated system (rGO/Bi₂WO₆/Au) shows enhanced photocatalytic activity compared to bare Bi₂WO₆ and other binary composites, and it is proved in multiple electron transfer (MET) behavior, namely a cooperative electron transfer (ET) cascade effect. Simultaneously, UV–vis/scanning electrochemical microscopy is used to directly identify MET kinetic information through an in situ probe scanning technique, where the “fast” and “slow” heterogeneous ET rate constants (K_eff) of corresponding photocatalysts on the different interfaces are found, which further reveals that the MET behavior is the prime source for enhanced photocatalytic activity. This work not only offers a new insight to study catalytic performance during photocatalysis and electrocatalysis systems, but also opens up a new avenue to design highly efficient catalysts in photocatalytic CO₂ conversion to useful chemicals and photovoltaic devices.     

Colloidal Single‐Layer Photocatalysts for Methanol‐Storable Solar H2 Fuel

Molecular surfactants are widely used to control low‐dimensional morphologies, including 2D nanomaterials in colloidal chemical synthesis, but it is still highly challenging to accurately control single‐layer growth for 2D materials. A scalable stacking‐hinderable strategy to not only enable exclusive single‐layer growth mode for transition metal dichalcogenides (TMDs) selectively sandwiched by surfactant molecules but also retain sandwiched single‐layer TMDs' photoredox activities is developed. The single‐layer growth mechanism is well explained by theoretical calculation. Three types of single‐layer TMDs, including MoS₂, WS₂, and ReS₂, are successfully synthesized and demonstrated in solar H₂ fuel production from hydrogen‐stored liquid carrier—methanol. Such H₂ fuel production from single‐layer MoS₂ nanosheets is CO_x‐free and reliably workable under room temperature and normal pressure with the generation rate reaching ≈617 µmole g^?1 h^?1 and excellent photoredox endurability. This strategy opens up the feasible avenue to develop methanol‐storable solar H₂ fuel with facile chemical rebonding actualized by 2D single‐layer photocatalysts.     

Computer-Assisted Search for Nonlinear Optical Crystals

Based on anionic group theory, a computer-assisted material design system (CAMDS) has been developed. This method has proved to be a highly efficient means of discovering nonlinear optical crystals. In this method, important optical properties of the target compounds (borates, for example), such as the d_ij coefficients, refractive indices, and energy bandgap, are calculated so that a prior evaluation can be made before experiments. The results have given a meaningful guide to ensuing experiments, which have led to our discoveries of KBBF (KBe₂BO₃F₂) and SBBO (Sr₂Be₂B₂O₇) in the past several years, followed by other members of the SBBO family in recent years. On the other hand, this system can also be used to evaluate the d_ij coefficients of the borate nonlinear optical (NLO) crystals discovered recently whose d_ij coefficients have not been determined experimentally.     

Luminescence of Eu3+ and Eu2+ in the new BaLnB9O16 borates (Ln = rare earth)

The luminescence of Eu³⁺ in the borates BaLnB₉O₁₆ (Ln = La, Gd and Y) has been investigated. Under UV excitation the Eu³⁺-activated lanthanum and gadolinium compounds show a bright red luminescence at room temperature, whereas the Eu³⁺-activated yttrium borate is characterized by a red-orange luminescence which indicates a change in the symmetry of the rare earth ion sites. As a consequence of the high energy of the charge transfer band the Eu³⁺ emission has a high efficiency under 253.7 nm excitation, a characteristic favorable for lighting applications. The sensitization of the Eu³⁺ emission by Bi³⁺ has been examined. Energy transfer from Bi³⁺ to Eu³⁺ is observed, but in presence of bismuth the efficiency of excitation through the Eu³⁺ charge transfer band is reduced. The luminescence of Eu²⁺ has also been studied. Eu²⁺-activated BaGdB₉O₁₆ shows an intense blue emission band under UV excitation.     

π‐Conjugated Polymer–Fullerene Covalent Hybrids via Ambient Conditions Diels–Alder Ligation

The established ability of graphitic carbon‐nanomaterials to undergo ambient condition Diels–Alder reactions with cyclopentadienyl (Cp) groups is herein employed to prepare fullerene‐polythiophene covalent hybrids with improved electron transfer and film forming characteristics. A novel precisely designed polythiophene (M _n 9.8 kD, ? 1.4) with 17 mol% of Cp‐groups bearing repeat unit is prepared via Grignard metathesis polymerization. The UV/Vis absorption and fluorescence (λ_ex 450 nm) characteristics of polythiophene with pendant Cp‐groups (λ_max 447 nm, λ_e‐max 576 nm) are comparable to the reference poly(3‐hexylthiophene) (λ_max 450 nm, λ_e‐max 576 nm). The novel polythiophene with pendant Cp‐groups is capable of producing solvent‐stable free‐standing polythiophene films, and non‐solvent assisted self‐assemblies resulting in solvent‐stable nanoporous‐microstructures. ¹H‐NMR spectroscopy reveals an efficient reaction of the pendant Cp‐groups with C₆₀. The UV/Vis spectroscopic analyses of solution and thin films of the covalent and physical hybrids disclose closer donor‐acceptor packing in the case of covalent hybrids. AFM images evidence that the covalent hybrids form smooth films with finer lamellar‐organization. The effect is particularly remarkable in the case of poorly soluble C₆₀. A significant enhancement in photo‐voltage is observed for all devices constituted of covalent hybrids, highlighting novel avenues to developing efficient electron donor‐acceptor combinations for light harvesting systems.     

Polymerization and structure of poly(sulphur nitride) prepared under high pressure

Poly(sulphur nitride)was prepared by spontaneous solid state polymerization of disulphur dinitride crystals immersed in perfluoro(methyl cyclohexane). The polymerization was carried out at hydrostatic pressures of up to 3.5 kbar in order to eliminate the lattice strain due to the mismatch in the monomer and the polymer lattice in the polymerization direction.It was observed that the pressure had no marked effect on the polymerization rate.The structure of the (SN) _x samples polymerized at high hydrostatic and at atmospheric pressure was examined by electron microscopy and electron diffraction techniques.They were complemented by the measurements of (SN) _x densities and electrical conductivities in the direction along and across the polymer chains. The results show that there is no significant difference in the structure and in the density of the high and the low pressure polymerized (SN) _x . A modest increase in the electrical conductivity parallel to the polymer chains observed in the high pressure polymerized (SN) _x is explained by a pressure induced increase in the average chain length. It is concluded that the long range lattice strain originating from the monomer-polymer lattice mismatch in the polymerization direction is less important for the solid state polymerization of (SN) _x than the nearest-neighbour interactions in the direction across the chain.     

Microstructural and chemical variation of TiO2 electrodes in DSSCs after ethanol vapour treatment

TiO₂ based dye-sensitized solar cells (DSSCs) have great potential to solve many energy challenges, however, their low energy conversion rate is still a barrier for further applications. Ethanol vapour post-treatment can improve the DSSC's conversion efficiency without changing its architecture, and a stable 2–3% improvement was found in our experiments. Microstructural and chemical factors were investigated using scanning electron microscopy and analytical electron microscopy on treated and untreated electrodes. The vapour treatment improved the porosity and surface-to-volume ratio of the TiO₂ particles, decreased electron transport loss between TiO₂ and fluorine doped tin oxide, and increased hydroxyl sites on the TiO₂ particle's surface. The modification therefore enhanced the dye uptake and dye–TiO₂ coupling, and it reduced the energy loss during the carrier transfer.     

Rose bengal-sensitized nanocrystalline ceria photoanode for dye-sensitized solar cell application

For efficient charge injection and transportation, wide bandgap nanostructured metal oxide semiconductors with dye adsorption surface and higher electron mobility are essential properties for photoanode in dye-sensitized solar cells (DSSCs). TiO₂-based DSSCs are well established and so far have demonstrated maximum power conversion efficiency when sensitized with ruthenium-based dyes. Quest for new materials and/or methods is continuous process in scientific investigation, for getting desired comparative results. The conduction band (CB) position of CeO₂ photoanode lies below lowest unoccupied molecular orbital level (LUMO) of rose bengal (RB) dye. Due to this, faster electron transfer from LUMO level of RB dye to CB of CeO₂ is facilitated. Recombination rate of electrons is less in CeO₂ photoanode than that of TiO₂ photoanode. Hence, the lifetime of electrons is more in CeO₂ photoanode. Therefore, we have replaced TiO₂ by ceria (CeO₂) and expensive ruthenium-based dye by a low cost RB dye. In this study, we have synthesized CeO₂ nanoparticles. X-ray diffraction (XRD) analysis confirms the formation of CeO₂ with particle size ～7 nm by Scherrer formula. The bandgap of 2.93 eV is calculated using UV–visible absorption data. The scanning electron microscopy (SEM) images show formation of porous structure of photoanode, which is useful for dye adsorption. The energy dispersive spectroscopy is in confirmation with XRD results, confirming the presence of Ce and O in the ratio of 1:2. UV–visible absorption under diffused reflectance spectra of dye-loaded photoanode confirms the successful dye loading. UV–visible transmission spectrum of CeO₂ photoanode confirms the transparency of photoanode in visible region. The electrochemical impedance spectroscopy analysis confirms less recombination rate and more electron lifetime in RB-sensitized CeO₂ than TiO₂ photoanode. We found that CeO₂ also showed with considerable difference between dark and light DSSCs performance, when loaded with RB dye. The working mechanism of solar cells with fluorine-doped tin oxide (FTO)/CeO₂/RB dye/carbon-coated FTO is discussed. These solar cells show V _OC ～360 mV, J _SC ～0.25 mA cm ^?2 and fill factor ～63% with efficiency of 0.23%. These results are better as compared to costly ruthenium dye-sensitized CeO₂ photoanode.     

Experimental study of the parametric interaction between space-charge waves in thin-film GaAs-based semiconductor structures

An experiment on the effect of low-frequency pumping on the output spectrum of a thin-film n-GaAs semiconductor structure with electron drift is reported in the 3-cm and 8-mm wavelength intervals. It is found that the presence of a low-frequency pump signal, whose power greatly exceeds that of a test signal, considerably enhances microwave transfer within two frequency bands separated by Δf=f _s−f _p, where f _s is the frequency of a weak test signal and f _p is the frequency of the pump signal (f _p<f _s). The transfer enhancement is evidence of the existence of effective parametric coupling between space-charge waves in the drift electron stream. This effect confirms the conclusions of previous theoretic studies.     

Anatase Mesoporous TiO2 Nanofibers with High Surface Area for Solid‐State Dye‐Sensitized Solar Cells

Mesoporous nanofibers (NFs) with a high surface area of 112 m²/g have been prepared by electrospinning technique. The structures of mesoporous NFs and regular NFs are characterized and compared through scanning electron microscope (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and selected area electron diffraction (SAED) studies. Using mesoporous TiO₂ NFs as the photoelectrode, solid‐state dye‐sensitized solar cells (SDSCs) have been fabricated employing D131 as the sensitizer and P3HT as the hole transporting material to yield an energy conversion efficiency (η) of 1.82%. A J_sc of 3.979 mA cm^?2 is obtained for mesoporous NF‐based devices, which is 3‐fold higher than that (0.973 mA cm^?2) for regular NF‐based devices fabricated under the same condition (η = 0.42%). Incident photon‐to‐current conversion efficiency (IPCE) and dye‐desorption test demonstrate that the increase in J_sc is mainly due to greatly improved dye adsorption for mesoporous NFs as compared to that for regular NFs. In addition, intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) measurements indicate that the mesopores on NF surface have very minor effects on charge transport and collection. Initial aging test proves good stability of the fabricated devices, which indicates the promise of mesoporous NFs as photoelectrode for low‐cost SDSCs.     

Nanogap Engineered Plasmon‐Enhancement in Photocatalytic Solar Hydrogen Conversion

Graphitic carbon nitride modified with plasmonic Ag@SiO₂ core–shell nanoparticles (g‐C₃N₄/Ag@SiO₂) are proposed for enhanced photocatalytic solar hydrogen evolution under visible light. Nanosized gaps between the plasmonic Ag nanoparticles (NPs) and g‐C₃N₄ are created and precisely modulated to be 8, 12, 17, and 21 nm by coating SiO₂ shells on the Ag NPs. The optimized photocatalytic hydrogen production activity for g‐C₃N₄/Ag@SiO₂ is achieved with a nanogap of 12 nm (11.4 μmol h⁻¹) to be more than twice as high as that of pure g‐C₃N₄ (5.6 μmol h⁻¹). The plasmon resonance energy transfer (PRET) effect of Ag NPs is innovatively proved from a physical view on polymer semiconductors for photoredox catalysis. The PRET effect favors the charge carrier separation by inducing electron–hole pairs efficiently formed in the near‐surface region of g‐C₃N₄. Furthermore, via engineering the width of the nanogap, the PRET and energy‐loss Förster resonance energy transfer processes are perfectly balanced, resulting in considerable enhancement of photocatalytic hydrogen production activity over the g‐C₃N₄/Ag@SiO₂ plasmonic photocatalyst.     

Monodisperse micron-sized polymethylmethacrylate particles having a crosslinked network structure. V

Highly monodisperse micron-sized polymethylmethacrylate (PMMA) particles crosslinked with carboxylic group-containing urethane acrylates (CUA) were produced by simple dispersion polymerization in methanol. In proper condition, CUA employed as a crosslinker had an excellent ability to achieve the monodisperse PMMA particles to the concentration of about 10 wt%. This arose from the fact that CUA formed monomer-swellable primary particles due to its structurally long tetramethylene oxide groups in the molecule. The influence of the concentrations of the initiator and CUA on the particle diameter (D _n), particle number density (N _p), and polymerization rate (R _p) was found to obey the following approximate relationship, D _n [initiator]^0.41[CUA]^–0.06, N _p [initiator]^–1.22 [CUA]^0.21, and R _p [initiator]^0.34±0.03, respectively. The power law dependence of D _n and N _p on the initiator concentration coincided well with that of linear polymers in the literature. Especially, in this study, it was found that CUA did not have a serious influence on the nucleation. However, the dependence of R _p on the initiator concentration was observed to be higher than that of linear polymers, suggesting that uniquely, the solution polymerization process competed with the heterogeneous polymerization during polymerization, because of the crosslinked network structure of the primary particles.