Correction: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Correction for ‘Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell’ by Jingpei Huo et al., RSC Adv., 2019, 9, 6163–6168.

The authors regret that the funding information in the acknowledgements of the original article was incorrect. “Guangdong Natural Science Foundation of China (Grant No. 2018A030310350)” should be “Guangdong Natural Science Foundation of China (Grant No. 2018A1660001)”.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m⁻².

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m^−2.

It is well known that inkjet printing works as precise and versatile patterning method for printed electronics.^1,2 As for its advantages, it is already being exploited widespread for printing electronics.^3,4 Those merits are easily processed from solutions and conveniently used for air-stable electrodes.^5,6Light-emitting electrochemical cells (LECs) have emerged as an active layer,^7,8 arousing tremendous attention over the years.^9–11 Nevertheless, LECs are simpler than organic light-emitting diodes (OLEDs).^12,13 Due to its single layer architecture, low fabrication price and operating voltages, LECs are considered as a promising, next-generation, emissive thin-film technology.^14–16The most efficient device used to date for LECs are biscyclometalated iridium(iii) (Ir) complexes, because they have highly efficient and stable devices spanning the whole visible range.^17–19 However, avoiding the use of Ir is strongly desired because of its high cost and limited supply.^20–22 Till now, thiazole has worked as active component in a LECs system with a long-lived charge-separation molecule, without additional ions in its active layer.^23–26 As for its advantages, it can reduce recombination of charge carriers and facilitate carrier transfer.^27–29 More and more importance has been attached to thiazole compounds, especially bithiazole-based ones.^30–33 Therefore, these have aroused interest in the syntheses of bithiazoles.^34–36Based on our previous research,^37–41 the use of bithiazole ligands already explored by our group for photocatalytic technology has the advantage of an easy transformation of neutral complexes into charged ones by substitution on the N-atom of the thiazole moiety.^42,43 Furthermore, we demonstrate the fabrication of LEC devices by a combination of inkjet printing and spin coating, on A4 paper substrates for low cost, disposable and flexible conductive pattern.^44,45 Their corresponding material characteristics were closely investigated, a detailed report of the material properties of the resulting coatings, and an envisaged proof of concept application are disclosed in this paper. These fully printed devices demonstrate the potential upscaling of the fabrication of optoelectronic devices.Herein we report the design for a range of bithiazole derivatives 1a–1c (Fig. 1, their synthesis is shown in Scheme S1^†), and explore their properties. First of all, the UV-visible diffuse-reflectance (UV-vis DRS) spectra of those specimens at room temperature are displayed in Fig. 2 and the data are collected in Table S1.^† This enhancement was ascribed to the increase of aromatic rings, which has an intense absorption in the visible-light region. And their bandgap energies (E_g) are between 2.97–3.01 eV (Table S1^†), and are estimated from these absorption spectra according to the Kubelka–Munk method. Expanding the conjugated system and electron density with these donor groups, it can lead to a larger bathochromic shift of the absorption maximum.^46,47Open in a separate windowFig. 1Chemical structures of compounds 1a–1c described in this work.Open in a separate windowFig. 2UV-vis DRS spectra of 1a–1c.All of the three compounds are photoluminescent at room temperature, the relevant fluorescence peak maximum ranges from 360 to 398 nm (Fig. 3). They emit blue light when they are being excited. In agreement with Fig. 3, the presence of electron-releasing groups would shift the emission maximum. Among them, the most electron-donating carbazole unit would give the most red-shifted peak at 398 nm for 1c (Table S1^†). The PL quantum yield (ϕ_PL) studies show that both 1a and 1b are as high as 62 and 78%, while another one is 85% (Table S1^†). The data for their fluorescent quantum yields largely depends on interaction effect with molecules 1a–1c in the crystal packing. In this context, π–π intermolecular interactions can inhibit the fluorescence.⁴⁸Open in a separate windowFig. 3Typical PL spectra of 1a–1c.It may indicate that enlarging the conjugation length and electron density with these donor groups plays an important role in increasing the ϕ_PL.⁴⁹ Moreover, those with high ϕ_PL may be suitable for application as efficient light-emitting material in LECs.⁵⁰In addition, time-resolved measurements of the donor lifetime (τ) in 1a–1c were carried out, and their corresponding values are given in Table S1.^† The bithiazole derivatives 1a–1c showed long-lived singlet fluorescence lifetimes (τ_F), ranging between 1.04 and 9.54 ns (Table S1^†). This was on average more than double the fluorescence lifetime of the bithiazole starting material.⁵¹ These three samples possess relatively high decay time, as a result of their inhomogeneity.⁵² Compound 1c exhibited the longest fluorescence lifetime decay τ_F = 9.54 ns. The sensitivity of the emission to the polarity of the solvent is beneficial for an intramolecular charge transfer (ICT)-like emission.⁵³ This difference probably accounts for the high concentration of donor units in the bithiazole backbone, low rates of intersystem crossing to reactive triplet states.^48,54Consequently, the composite 1c was the best performing LEC and chosen for the below test. The EL spectra obtained for both devices were almost identical, with a main peak at 498 nm and other peak at 504 nm wavelength as illustrated in Fig. 4a. It may account for trapping, cavity, and self-absorption effects from within the LEC device 1c multilayer-structure. The spin-coated emission is quantified by the commission Internationale de L’Eclairage (CIE) coordinates of (0.28, 0.42), and a colour rendering index (CRI) of 65 (Table S2^†). While inkjet-printed light emission with CIE coordinates of (0.34, 0.43) and a CRI value of 83 was achieved for device 1c (Table S2^†). It exhibits good colour rendering indices (CRI > 70), this device exhibited a warm-white appearance, and the colour rendering is considered sufficient for indoor lighting applications.Open in a separate windowFig. 4Comparison of device characteristics with spin-cast and inkjet printer emitting layer for 1c: (a) electroluminescent spectra, (b) luminance vs. voltage, (c) current density vs. voltage, (d) efficiency vs. luminance behaviour.The time-dependent brightness and current density under constant biases of 2.9–3.3 V for device 1c are shown in Fig. 4b and c. Interestingly, the device with the inkjet-printed emitting layer (EML) produced a light output of around 3600 cd m⁻², whereas 4600 cd m⁻² was achieved with the spin-coated EML. In other words, the device with the inkjet printed EML obtained about 78.3% of brightness compared with the reference device. The differences between the two devices regarding current efficiency (J) and power efficiency (PE) were rather closed, as listed in the Table S2.^† A luminous efficiency much larger than 10 cd A⁻¹ was achieved for the inkjet-printed EML, which tended to be 73.9%, and is similar to the spin-coated emitting layer.The performance gap between the devices (Fig. 4d) can be attributed to differences in the height and surface roughness of the emitting layer.⁵⁵ Lateral sizing histograms (Fig. S11^†) show that the spin-coated EML possessed a thickness of 35 nm and a very smooth surface. The inferior performance of the inkjet-printed EML is caused by a less homogeneous surface morphology and an overall fatter layer.⁵⁶ The depth of the inkjet-printed EML ranged from 25 nm (pixel centre) to 35 nm (pixel edges). As a consequence, it is an inkjet deposition process and the related evaporation dynamics reduces the light output and efficiency, but also postpones the stability of the device in the system.^10,57We then shifted our attention to the turn-on kinetics, efficiency and long-term stability of the LEC device for 1c; the typical evolution of current density and light emission is presented in Fig. 5a and b, showing a slow turn-on followed by a progressive decay over time.Open in a separate windowFig. 5(a) Current density (closed symbols) and brightness (open symbols) versus time at 3 V for a device with the representative sample 1c; (b) EQE versus time at an applied voltage of 3 V.The build-up of the light output is synchronous with that of the current density. This time-delayed response is one of the striking features of the operation of an electrochemical cell and reflects the mechanism of device operation.²² The champion device in this set exhibited a peak power efficiency (PE) of 17.4 lm W⁻¹ at a luminance of 18 000 cd m⁻². Meanwhile, the current density began to decrease, possibly due to the electrochemical oxidation or electro corrosion that occurred on component 1c through accumulated electroinduced holes, and then it kept up a durative datum.⁵⁸ Apparently it exhibited outstanding long-term operation stability beyond 40 h (Fig. 5b).The time-dependent external quantum efficiency (EQE) of LEC for the representative substrate 1c are shown in Fig. 5b. It also exhibited similar temporal tendency in device efficiency. When a bias was applied on the LECs, the EQE quickly increased since balanced carrier injection was achieved by the formation of the doped layers.⁵⁹ After attaining the crest value, the device current was still rising while the EQE reduced little by little. It implied that both growing the doped layers and weakening of the EQE was resulting from exciton quenching near the bithiazlole core in persistently extended doped layers.⁶⁰ Doping-induced self-absorption was rather adaptable to the temporal roll-off in device efficiency. This configuration shows the best performance (EQE = 12.8%) compared to those devices prepared with the single compounds, which is given in Table S3.^† The reason may be that the steady-state recombination zone in 1c device was close to the central active layer.⁶¹ Besides it contains fluorenes on its side chains, constructing plane structure, benefiting from good electron injection abilities, leading to enhancing the EQE itself.⁶²The fabrication of inkjet-printed bithiazole interdigitated electrode (IDE) is illustrated in Fig. 6. All the prepared varieties of bithiazole-based ink have been found to be highly stable with nil or miniscule precipitation in over 180 days being stored on the shelf (Fig. 6a). After HCl treatment of the paper substrate, the prepared inks have filled within ink cartridges of a low-cost desktop printer Canon iP1188 (Fig. 6b). Fig. 6c shows the photograph of inkjet-printed conductive patterns.Open in a separate windowFig. 6(a) The representative ink based on sample 1c was stable for 180 days; (b) electrode printing using Canon iP1188 printer; (c) scale showing size of printed electrodes; (d) interdigitated electrode (inset shows small size printed pattern).In addition, the most attractive prospect of the bilayer device structure at this stage is the possibility for patterned emission for the creation of a static display. Fig. 7 presents a photograph of a small portion of a larger static display, with a resolution of 170 PPI, which repeatedly exhibits a message in the form of the word “LEC”. As shown in Fig. 7, the pixel array produced are fairly luminous. The EL spectra were collected from each pixel on the substrate, the emission peak wavelength was about 510 nm and the full width at half maximum was about 170 nm. The low variation in emission intensity in the different pixels implies a small variation in thickness of the solution-coated layers. More specifically, if we assume a minimum diameter of 20 μm for an inkjetted electrolyte droplet, and a smallest inter-droplet distance of 10 μm, we attain a pitch of 30 μm, which corresponds to a high display resolution of 850 PPI. Finally, we draw attention to the herein presented static-display LEC that comprises solely air-stabile materials, and that we routinely fabricate the bilayer stack under ambient atmosphere.Open in a separate windowFig. 7(a) The patterned light emission from a bilayer LEC, with the emission pattern defined by the selected positions of the ink jetted electrolyte droplets. (b) The droplet diameter and pitch were 50 and 150 μm, respectively, and the device was driven at V = 3 V. The scale bar measures 300 μm.     

Low temperature atomic layer deposition of zirconium oxide for inkjet printed transistor applications

We report the growth of zirconium oxide (ZrO₂) as a high-k gate dielectric for an inkjet-printed transistor using a low-temperature atomic layer deposition (ALD) from tetrakis(dimethylamido)zirconium (TDMAZr) and water precursors. All the samples are deposited at low-temperature ranges of 150–250 °C. The films are very uniform with RMS roughness less than 4% with respect to their thickness. The atomic force microscopy (AFM) shows a significant change in surface morphology from tapered posts to undulating mountain-like structures with several hundreds of ALD cycles. The results from X-ray diffraction (XRD) analysis exhibit an amorphous to the crystalline structure with temperature variation, which is independent of the thickness of the films. All our samples are hydrophilic as contact angles are less than 90°. The capacitance–voltage (C–V) and conductance–voltage (G_p/ω–V) characteristics of ZrO₂ dielectrics for silicon metal–oxide–semiconductor (MOS) capacitors are studied for different temperatures. For the n-type substrate MOS capacitors, the dielectric constants are estimated to be 7.5–11. Due to the low deposition temperature, a hydrophilic surface, and high k value, the ALD-ZrO₂ dielectric can be compatible for printed transistors. The processes of fabrication and characterization of inkjet-printed graphene transistors is demonstrated using the ZrO₂ dielectric. The possible solvents, surfactant, and the dielectric induced modifications in graphene flakes are demonstrated by Raman spectra. The graphene flakes spread uniformly on the ZrO₂ surface. The functional inkjet-printed graphene transistor characteristics are demonstrated to illustrate the field effect behavior with the ALD-ZrO₂ dielectric.

We report the growth of zirconium oxide (ZrO₂) as a high-k gate dielectric for an inkjet-printed transistor using a low-temperature atomic layer deposition (ALD) from tetrakis(dimethylamido)zirconium (TDMAZr) and water precursors.     

Molecularly engineered electroplex emission for an efficient near-infrared light-emitting electrochemical cell (NIR-LEC)

Electroplex emission is rarely seen in ruthenium polypyridyl complexes, and there have been no reports from light-emitting electrochemical cells (LECs) to date. Here, for the first time, near-infrared (NIR) emission via the electroplex mechanism in a LEC based on a new blend of ruthenium polypyridyl complexes is described. The key factor in the design of the new complexes is the 0.4 V decrease in the oxidation half-potential of Ru(ii)/Ru(iii) in [Ru(DPCO)(bpy)₂]ClO₄ (DPCO = diphenylcarbazone, bpy = 2,2 bipyridine), which is about one-third of the value for benchmark [Ru(bpy)₃](ClO₄)₂, as well as the long lifetime of excited states of 350–450 ns. The LEC based on the new blend with a narrow band gap (≈1.0 eV) of a Ru(DPCO) complex and Ru(bpy)₃²⁺ can produce an electroluminescence spectrum centred at about 700 nm, which extends to the NIR region with a high external quantum efficiency (EQE) of 0.93% at a very low turn-on voltage of 2.6 V. In particular, the very simple LEC structure was constructed from indium tin oxide (anode)/Ru(DPCO):Ru(bpy)₃²⁺/Ga:In (cathode), avoiding any polymer or transporting materials, as well as replacing Al or Au by a molten alloy cathode. This system has promising applications in the production of LECs via microcontact or inkjet printing.

Electroplex emission is rarely seen in ruthenium polypyridyl complexes, and there have been no reports from light-emitting electrochemical cells (LECs) to date. Here, near-infrared (NIR) emission via the electroplex mechanism in a LEC was reported.     

The effect of viscosity and surface tension on inkjet printed picoliter dots

In this study, we investigated the effect of liquid viscosity and surface tension for inkjet printing on porous cellulose sheets. We used five model liquids, representing the operational field of an industrial high speed inkjet printer, as specified by Ohnesorge- and Reynolds number. Drops with 30 pl and 120 pl drop size were jetted with a commercial HSI printhead. We printed on four uncoated papers representing the most relevant grades on the market in terms of hydrophobisation and surface treatment. We are presenting a quantitative analysis of viscosity and surface tension on the print outcome, evaluating dot size, liquid penetration (print through) and surface coverage of the printed dots. The most important finding is that for liquids within the jetting window the variation of the liquid viscosity typically has a 2–3 times higher impact on the print outcome than variation of the liquid surface tension. Increased viscosity in all cases reduces dot area, liquid penetration and liquid surface coverage. Surface tension plays a smaller role for liquid spreading and penetration, except for hydrophobised substrates, where both are reduced for higher surface tension. Interestingly, higher surface tension consistently increases liquid surface coverage for all papers and drop sizes. A detailed analysis on the competing effect of dot spreading and liquid penetration is presented, in terms of viscosity, surface tension and surface coverage of the liquid.

In this study, we investigated the effect of liquid viscosity and surface tension for inkjet printing on porous cellulose sheets.     

Retraction: Wavelength modulation of ZnO nanowire based organic light-emitting diodes with ultraviolet electroluminescence

Retraction of ‘Wavelength modulation of ZnO nanowire based organic light-emitting diodes with ultraviolet electroluminescence’ by Runze Chen et al., RSC Adv., 2020, 10, 23775–23781, DOI: 10.1039/D0RA04058D.

We, the named authors, hereby wholly retract this RSC Advances article due to a calibration error in the CVD tool used when fabricating the inverted UV-OLED devices. As a result, the subsequent data collected is not reliable and the conclusions of this paper are not supported.Signed: Runze Chen, Chuan Liu, Kyeiwaa Asare-Yeboah, Ziyang Zhang, Zhengran He and Yun LiuDate: 29^th November 2020Retraction endorsed by Laura Fisher, Executive Editor, RSC Advances     

Facile preparation of a tetraethylenepentamine-functionalized nano magnetic composite material and its adsorption mechanism to anions: competition or cooperation

A tetraethylenepentamine (TEPA)-functionalized nano-Fe₃O₄ magnetic composite material (nFe₃O₄@TEPA) was synthesized by a facile one-pot solvothermal method. It was characterized by elementary analysis (EA), powder X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). The results show that the nFe₃O₄@TEPA has an average size of ∼20 nm, with a saturation magnetization intensity of 48.2 emu g⁻¹. Its adsorption properties were investigated by adsorbing fluorine ions, phosphate, Cr(vi) and their co-existing water system. The adsorption performance was studied as a function of solution pH, initial concentration of ions, contact time and temperature for each ion. The adsorption of the multi-ion co-existing system was studied via batch tests, XPS and FTIR analyses. The effect of co-existing ions was studied through Box-Behnken Design (BBD) and response surface methodology (RSM). It can be deducted that the adsorption mechanism of an individual fluorine ion or phosphate was mainly related to electrostatic attraction, while that of Cr(vi) might be mainly related to electrostatic attraction and coordination interactions. For the fluorine ion and phosphate bi-component system, their adsorption was competitive via ion exchange. For the Cr(vi), fluorine ion and phosphate tri-component co-existing system, Cr(vi) took priority for adsorption and could replace the absorbed fluorine ion or phosphate by competitive reaction, but not vice versa.

A tetraethylenepentamine (TEPA)-functionalized nano-Fe₃O₄ magnetic composite material (nFe₃O₄@TEPA) was synthesized by a facile one-pot solvothermal method.     

Impact of inkjet printed ZnO electron transport layer on the characteristics of polymer solar cells

In this paper, we demonstrate that zinc oxide (ZnO) layers deposited by inkjet printing (IJP) can be successfully applied to the low-temperature fabrication of efficient inverted polymer solar cells (i-PSCs). The effects of ZnO layers deposited by IJP as electron transport layer (ETL) on the performance of i-PSCs based on PTB7-Th:PC₇₀BM active layers are investigated. The morphology of the ZnO-IJP layers was analysed by AFM, and compared to that of ZnO layers deposited by different techniques. The study shows that the morphology of the ZnO underlayer has a dramatic effect on the band structure and non-geminate recombination kinetics of the active layer deposited on top of it. Charge carrier and transient photovoltage measurements show that non-geminate recombination is governed by deep trap states in devices made from ZnO-IJP while trapping is less significant for other types of ZnO. The power conversion efficiency of the devices made from ZnO-IJP is mostly limited by their slightly lower J_SC, resulting from non-optimum photon conversion efficiency in the visible part of the solar spectrum. Despite these minor limitations their J–V characteristics compare very favourably with that of devices made from ZnO layer deposited using different techniques.

In this paper, we demonstrate that zinc oxide (ZnO) layers deposited by inkjet printing (IJP) can be successfully applied to the low-temperature fabrication of efficient inverted polymer solar cells (i-PSCs).     

Facile fabrication of self-assembled nanostructures of vertically aligned gold nanorods by using inkjet printing

We demonstrated that the vertically aligned gold nanorods (AuNRs) were quickly and easily formed by using inkjet printing when aqueous dispersion of AuNRs containing a small amount of ethylene glycol (EG) was employed as an ink. It was observed that the content of EG in water suppressed rapid drying and convection in the droplets, which is favorable for the formation of the nanostructures.

Slow evaporation of a droplet of water/ethylene glycol (EG) mixture allows the fabrication of vertically aligned gold nanorods using inkjet printing.

Self-assembly is a useful technique for improving the functionality of colloidal nanomaterials.^1–3 For example, when nanoparticles such as gold and silver, which exhibit localized surface plasmon resonance, form self-assembled nanostructures, the plasmon resonance will be further enhanced.^4–7 In recent years, nanostructures in which gold nanorods (AuNRs) are vertically aligned on a substrate have attracted particular attention.^8–11 In such nanostructures, strong plasmon resonance appears uniformly in the gaps between the particles. Therefore, they can be applied to photoluminescence enhancement^12–14 and reproducible surface-enhanced Raman scattering (SERS) sensing.^15–18However, strict fabrication conditions make it difficult to put the vertically aligned AuNRs into practical use. For preparation of vertically aligned AuNRs on a substrate, it is basically necessary to slowly evaporate a droplet of the dispersion of colloidal AuNRs for about a day while keeping the droplet under high humidity conditions.⁸ While effective, the fabrication process is very time-consuming. Although rapid fabrication is possible by using AuNRs dispersed in an organic solvent, a complicated process of ligand exchange is required.¹⁹ In addition, when drying a large-sized droplet on a substrate, it is difficult to precisely control the position where the structure will be formed. Thus, for practical use, a technique that enables rapid fabrication of vertically aligned AuNRs in any pattern at any positions on a substrate is absolutely necessary. One promising method is using inkjet printing technology (Fig. 1(a)), which has been used to make self-assemblies of nanomaterials such as polystyrene nanoparticles.²⁰Open in a separate windowFig. 1(a) Schematic illustrations of the vertically aligned AuNRs fabricated by inkjet printing. (b) The formation of the vertically aligned AuNRs in the droplet of the water/ethylene glycol (EG) mixture during evaporation.Inkjet printing has received a great deal of attention in recent years because it can accurately deposit materials at any location. It allows the printing pattern to be changed without the use of printing plates or photomasks, which leads to lower costs and lower material consumption. Therefore, it is widely used in the manufacture of electronic circuits,²¹ sensors,²² thin film transistors,²³ solar cells²⁴ and light emitting diodes.²⁵ If vertically aligned AuNRs can be rapidly prepared by using inkjet printing, a facile construction of chip-based platforms for biosensing and SERS sensing will be enabled. However, in the inkjet printing process, the solvent of the ink dries at a high speed, which is the opposite of the conditions for the formation of the vertically aligned AuNRs. This may be the reason why the inkjet printing has not been employed previously. Here, we attempted to fabricate vertically aligned AuNRs by inkjet printing using an aqueous dispersion of AuNRs mixed with a low-volatility solvent as an ink.Much research is still being done to elucidate the formation mechanism of vertically aligned AuNRs. It is known that van der Waals attraction, electrostatic repulsion, and depletion forces between the AuNRs affect the formation of the structure. It has also been confirmed that slow evaporation of the droplet is a particularly important factor.^18,26 Recently, Jung et al. have reported favorable conditions for the formation of vertically aligned nanorods by numerically solving the Smoluchowski coagulation equation under moving boundary conditions.²⁷ According to the report, the condition where the evaporation rate is slow at a sufficiently high concentration of nanorods in the droplet promotes the formation of the nanostructure. This finding suggests that the droplets do not need to evaporate slowly during the entire drying process. Since the concentration of nanorods increases as the droplets evaporate, the evaporation rate needs to be slow just before the droplets are completely evaporated. If this condition is satisfied, the formation of vertically aligned AuNRs can be expected even by using inkjet printing, in which the ejected fine droplets are quickly dried. Therefore, we proposed to use an aqueous dispersion of AuNRs containing a small amount of a low-volatility solvent. It was expected that the formation of the vertically aligned AuNRs would be assisted by the slow evaporation of the low-volatile solvent after the rapid evaporation of water (Fig. 1(b)).We used ethylene glycol (EG) as the low-volatility solvent. First, we observed the drying behavior of droplets of the mixed solution with an optical microscope. 0.4 M EG aq. was prepared and 0.5 μL of the solution was dropped onto a cleaned silicon substrate ((100) surface). The surface of the silicon substrate was cleaned by a UV ozone cleaner. The temperature was around 25 °C and the relative humidity was around 30%. Fig. 2(a) shows how the size of the droplet decreases due to the evaporation. The diameter of the droplets decreases rapidly in the first 6 minutes. Then, the decrease rate clearly slowed down. The drying behaviors of the EG/water mixture, pure water and pure EG droplets are shown in Movies S1, S2 and S3 (see ESI^†), respectively. Fig. 2(d) shows the change in the projected area of each droplet. The droplets of the EG/water mixture evaporated in the same way as pure water for the first 6 minutes (Fig. 2(b)). After that, the evaporation became very slow, similar to the evaporation behavior of pure EG (Fig. 2(c)). This suggests that in the evaporation of the EG/water mixture, the remaining EG slowly evaporates after the rapid evaporation of water. This behavior is expected to be a favorable condition for the formation of vertically aligned AuNRs.Open in a separate windowFig. 2Microscopic images of the drying behavior of (a) EG/water mixture, (b) water, and (c) EG. The scale bars are 500 μm. (d) The changes in the projected area during the evaporation for each droplet of EG/water mixture, water, and EG.In the process of evaporation, convection is driven in the droplet.²⁸ It has been reported that suppression of this convection facilitates the formation of vertically aligned AuNRs.¹⁸ Thus, we performed particle image velocimetry (PIV) analysis to observe the flow driven in the droplets on the substrate. Fig. 3(a) shows the vector field of the average flow velocity of tracer particles (fluorescent pigment, particle size 3–5 μm, JUJO Chemical Co., Ltd.) in a droplet of pure water. The frame rate was 50 fps and the measurement time was 110 seconds. The convection due to evaporation was driven from the edge to the center of the droplet. The motions of the tracer particles can also be observed in Movie S4.^†Open in a separate windowFig. 3The convections driven in the droplets for (a) water and (b) EG/water mixture during evaporation. The arrows show the average velocity of flow at each point analyzed by particle image velocimetry (PIV). The color bar indicates the magnitude of the velocity. The scale bars are 100 μm. Fig. 3(b) shows the vector field of the average flow in the droplet of 0.4 M EG aqueous solution. Although a flow appeared at the edge of the droplet, there was no clear flow near the center. As one can observe in Movie S5,^† convection initially carries particles throughout the droplet. However, once the contact line begins to retract due to evaporation the convection velocities approach zero. The particles showed chaotic movement around the center, which was not recognized as a flow in a specific direction detected by PIV. Faster evaporation of water at the air-liquid interface would be expected to cause local increase in the EG concentration, resulting in the suppression of the motion of the particles from the edge to the center. From these results, it was found that the EG in the droplet suppressed the convection, which is a favorable condition for the formation of vertically aligned AuNRs.With evidence to suggest its potential, we attempted to fabricate vertically aligned AuNRs by using inkjet printing. The AuNRs were synthesized and concentrated to about 3 nM by centrifugation.^29,30 The average length and width of AuNRs were about 76 ± 9.2 nm and 29 ± 4.1 nm, respectively. The average aspect ratio was about 2.6. The detailed synthetic procedure is described in ESI.^† The AuNRs were protected by cetyltrimethylammonium bromide (CTAB) and dispersed in water. The CTAB concentration was fixed at 3 mM for all AuNR dispersions used in this experiment. The EG concentration of each dispersion was adjusted to 0 M, 0.1 M, 0.4 M, and 1.0 M. We used electrostatic inkjet printing (Microjet FemtoJet-2000HB, MICROJET Corp., Fig. S1, see ESI^†), which is suitable for the fabrication of micropatterns. Line patterns (∼100 μm width, ∼200 μm interval) as shown in Fig. 4(a) were successfully prepared.Open in a separate windowFig. 4(a) An SEM image of line patterns of AuNRs assembly fabricated by inkjet printing on a silicon substrate. The scale bar is 200 μm. (b) The nanostructures of AuNRs resulting from varied EG concentration in the printing inks, 0 M, 0.1 M, 0.4 M, and 1.0 M. The scale bars are 200 nm. (c) Vertically aligned AuNRs fabricated by using 0.1 M EG and 0.4 M EG inks observed at an angle of 65°. The scale bars are 200 nm. Fig. 4(b) shows the nanostructures fabricated by inkjet printing of AuNR dispersions at each EG concentration observed by using scanning electron microscope (SEM). The zoomed-out images are shown in Fig. S2(a)–(d).^† For 0 M EG dispersion, random aggregation was formed. Vertically aligned AuNRs were observed for 0.1 M and 0.4 M EG dispersion. As shown in Fig. 4(c), when the sample is tilted, AuNRs standing perpendicular to the substrate are observed. This revealed that the addition of low concentration of EG can dramatically facilitate the formation of vertically aligned AuNRs. These results support our hypothesis based on the previous reports.²⁷ Furthermore, it was also confirmed that the vertically aligned AuNRs were formed when 10 μL droplets (0.1 M EG or 0.4 M EG) were cast and dried on a silicon substrate under atmospheric conditions (Fig. S3(a) and (b)^†). We have achieved the fabrication of vertically aligned AuNRs in a very simple method. In the case of 1.0 M EG dispersion, it should be noted that AuNRs were assembled parallel to the substrate. Similarly, vertically aligned AuNRs could not be observed in the case of 1.78 M EG dispersion (10 vol%) (Fig. S4^†). Further showing that when the EG concentration is too high, it becomes difficult to form vertically aligned structures.We investigated surface-enhanced Raman scattering (SERS) for vertically aligned AuNRs fabricated by using 0.4 M EG dispersion. The substrate was immersed in 1 nM benzyl butyl phthalate (BBP) ethanol solution for several seconds, then allowing it to dry completely in an ambient condition. BBP is a type of environmental pollutant. As shown in Fig. S5,^† a peak appeared at 1005 cm⁻¹, which is attributed to a β (CC) loop in-plane deformation mode.³¹ Although we have succeeded in detecting the trace amount of BBP, the sensitivity is lower than the previously reported BBP detection using vertically aligned AuNRs.¹⁶ It is considered that this is because the number of the domains where the AuNRs are vertically aligned is not enough for the printed area. It is expected that the sensitivity will be improved by drawing the pattern multiple times using inkjet printing.EG is known to have the effect of increasing the critical micelle concentration (CMC) of CTAB.³² Therefore, one would expect the addition of EG to the dispersion to destabilize the CTAB bilayers surrounding AuNRs, resulting in random aggregation. The peak wavelength of the absorbance of the dispersion changes significantly according to the aggregation of AuNRs.^33,34 Thus, the degree of aggregation can be estimated from the spectral shape. For AuNRs dispersions with different EG concentrations, absorbance spectra were measured by using a UV-Vis spectrometer (V-670, JASCO Corp.). Fig. 5(a) shows the absorbance spectra of the AuNRs dispersions with EG concentration of 0 to 1.0 M. There was almost no change in the spectral shape, suggesting that the AuNRs in the dispersions were stable.Open in a separate windowFig. 5(a) Absorbance spectrum of AuNRs dispersion for each EG concentration, 0, 0.1, 0.4, and 1.0 M. (b) Spectral changes in absorbance with increasing EG concentration, 0, 10, 25, 50,75, 90 vol%. The inset shows the peak wavelength corresponding to each EG concentration. (c) Decrease in the zeta potential value caused by increased EG concentration.However, in this experiment, the concentration of EG gradually increases as the water in the droplet evaporates at high speed. We also measured the absorbance of the AuNRs dispersions with high EG concentrations. As shown in Fig. 5(b), vol%. The peak wavelength was slightly redshifted (Fig. 5(b) inset). This is because the addition of EG increased the refractive index of the mixture solution. In principle, the peak wavelength of absorbance of AuNRs is redshifted almost linearly according to the refractive index of the dispersion.²⁹ For the dispersion used in this experiment, the peak wavelength of absorbance and the refractive index have an almost linear relationship in the range of EG concentration from 0 to 50 vol% (Fig. S6^†).On the other hand, for 75 vol% EG dispersion, the peak was blue-shifted, suggesting that AuNRs were slightly aggregated (Fig. 5(b) inset). As shown in Fig. 5(b), for 90 vol% EG dispersion, the spectral shape changed significantly, resulting in the loss of a clear peak. This indicates that the increase in EG concentration caused aggregation of AuNRs.Next, we investigated the changes in the zeta potentials of AuNRs due to the addition of EG to the dispersions by using electrophoretic light scattering measurement (Zetasizer ZS, Malvern Panalytical Co., Ltd.). As shown in Fig. 5(c), the zeta potential of AuNRs decreased as the EG concentration increased, which indicates that the AuNR dispersion became unstable. This result is consistent with the result of the absorbance measurement. Note that for 90 vol% EG aqueous mixture, it was impossible to measure the zeta potential because the dispersion did not contain enough particles due to the aggregation.From these results, it was found that the addition of a small amount of EG did not affect the stability of AuNRs in the dispersion, but aggregation occurred at a high concentration of EG. Sufficiently high concentration of AuNRs in the droplet is required for the formation of vertical structures,^16,18 which means that the AuNRs concentration relative to the amount of EG is important.When the concentration of AuNRs in the droplet is high, AuNRs are close to each other, while being electrostatically repelled by CTAB. This reduces orientational degrees of freedom of the AuNRs and their alignment is facilitated.^8,35 As the volume of the droplet decreases due to evaporation, the aggregation will be caused by the further increase in AuNRs concentration, resulting in the formation of vertically aligned AuNRs. At this time, the slow evaporation rate of the droplets allows the AuNRs to assemble slowly. The slow evaporation rate of EG further facilitates the formation of vertically aligned AuNRs.On the other hand, as shown in Fig. 5(b) and (c), when the concentration of EG is too high, the aggregation of AuNRs occur due to the decrease in electrostatic repulsion. In a droplet of a mixture of EG and water, the concentration of EG will gradually increase because the water evaporates more quickly. Therefore, when the EG concentration increases to around 90 vol%, the aggregation of AuNRs occurs regardless of the AuNRs concentration. In the droplet with low concentration of AuNRs at this point, the AuNRs are far apart from each other. Thus, AuNRs can be oriented freely, resulting in randomly oriented aggregation.In the process of droplet evaporation, both EG concentration and AuNRs concentration increase. Since the initial concentration of AuNRs is fixed in this experiment, the AuNRs concentration relative to the amount of EG becomes low when the initial concentration of EG is high. Therefore, it is probable that for EG concentrations of 1.0 M and 1.78 M, the AuNRs concentrations at the start of the aggregation were not high enough for the formation of vertically aligned AuNRs. For EG concentrations of 0.1 M and 0.4 M, the AuNRs concentrations were sufficiently high relative to the amount of EG. In addition, the EG causes slow evaporation when AuNRs start to aggregate. These two conditions, high AuNRs concentration and slow evaporation rate, are critical factors that facilitate the formation of vertically aligned AuNRs. In other words, there should be an optimum value of EG concentration for AuNRs alignment. Optimization of the initial AuNRs concentration will be required to precisely control the formation of vertically aligned AuNRs by using inkjet printing. In addition, in the future work, it is important to enable the fabrication of vertically aligned AuNRs with various aspect ratios because the aspect ratio can affect sensing performances.³⁶     

Electronic performance of printed PEDOT:PSS lines correlated to the physical and chemical properties of coated inkjet papers

PEDOT:PSS organic printed electronics chemical interactions with the ink-receiving layer (IRL) of monopolar inkjet paper substrates and coating color composition were evaluated through Raman spectroscopy mapping in Z (depth) and (XY) direction, Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDS). Other evaluated properties of the IRLs were pore size distribution (PSD), surface roughness, ink de-wetting, surface energy and the impact of such characteristics on the electronics performance of the printed layers. Resin-coated inkjet papers were compared to a multilayer coated paper substrate that also contained an IRL but did not contain the plastic polyethylene (PE) resin layer. This substrate showed better electronic performance (i.e., lower sheet resistance), which we attributed to the inert coating composition, higher surface roughness and higher polarity of the surface which influenced the de-wetting of the ink. The novelty is that this substrate was rougher and with somewhat lower printing quality but with better electronic performance and the advantage of not having PE in their composite structure, which favors recycling.

PEDOT:PSS ink chemical interactions with the coated surface of inkjet papers and their composition were evaluated through Raman, FTIR and EDS. Morphology of the pores and surface energy were also evaluated and how these impact sheet resistance.     

Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO₂ gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm⁻³). Using the EMI-BF₄ electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g⁻¹ (delivering a gravimetric energy density of 85.6 W h kg⁻¹), a volumetric capacitance of 94 F cm⁻³ (delivering a volumetric energy density of 44.7 W h L⁻¹), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF₄/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g⁻¹ with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

A supercapacitor electrode is developed with a free-standing graphene film by a facile two-step strategy. The graphene electrode achieved a gravimetric capacitance of 180 F g⁻¹ and a volumetric capacitance of 94 F cm⁻³.     

Facile production of quercetin nanoparticles using 3D printed centrifugal flow reactors

Drug nanocrystals are a delivery system comprised of an active pharmaceutical ingredient, with small amounts of a surface stabilizer. Despite offering simplicity in formulation, their manufacture can be a challenging endeavour; this is especially true when the production is performed using microfluidic devices. Although precipitation within microchannels can lead to issues such as clogging, microfluidics is an appealing manufacturing method as it provides fine control over mixing conditions. This allows production of nanoparticles with a narrower size distribution and greater reproducibility compared to batch methods. To generate microfluidic devices cost effectively, replica moulding techniques are considered the manufacturing standard. Due to its simplicity and relatively low cost, 3D printing has become prevalent at the laboratory scale, especially during iterative development of new devices. A challenge of microfluidic-based methods is that they require specialized equipment and multi-step procedures, making them less accessible to users with no previous experience. In a recent study we developed a 3D printed flow-through reactor, referred to as reactor-in-a-centrifuge (RIAC). It is a simple device designed to fit in a 50 mL tube and actuated using a laboratory centrifuge, which removes the need for specialized instrumentation. The manufacturing capabilities of the RIAC have been already proven, by reproducible production of liposomes and silver nanoparticles. The present work demonstrates the use of RIACs with a straight- and spiral-shaped channel architecture to produce quercetin nanocrystals, with therapeutically relevant size (190–302 nm) and very low size dispersity (polydispersity index, PDI < 0.1). The work focused on evaluating how changes in operational parameters (actuation speed) and formulation components (medium viscosity and stabilizer type), impacted on nanocrystal size and PDI. Under all tested conditions the obtained nanocrystals had a smaller size and narrower size distribution, when compared to those produced with alternative methods. The obtained quercetin nanosuspensions however showed limited stability, which should be addressed in future investigations. The simplicity of the RIAC makes it an appealing technology to research groups, especially in low-resource settings and without prior expertise in microfluidics.

A 3D printed reactor-in-a-centrifuge (RIAC) was developed to produce drug nanocrystals. Quercetin nanocrystals were manufactured at varying operational and formulation conditions, and had a small size (190–302 nm) and low size dispersity (PDI < 0.1).     

Facile one-pot solvothermal preparation of two-dimensional Ni-based metal–organic framework microsheets as a high-performance supercapacitor material

We report a facile one-pot solvothermal way to prepare two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) using only Ni precursor and ligand without any surfactant. The Ni-MOFms exhibit good specific capacities (91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively) and long-term stability in 5000 cycles when used for a supercapacitor electrode.

Two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) were synthesized via a facial one-pot solvothermal approach and exhibited good specific capacities and excellent long-term stability when used for a supercapacitor electrode.

With the continuous growth of energy demand worldwide, high-performance, environmental-friendly, and low-cost energy storage devices have attracted extensive research interest.^1–3 Among them, supercapacitors are considered most promising because of their high power density, long lifespan, and fast charging/discharging speed.^4–6 To date, numerous materials have been explored for fabricating supercapacitors. Carbon materials have been usually used for electrical double-layer capacitors (EDLCs), including carbon fibers, carbon nanotubes, carbon spheres, carbon aerogels, and graphene,^7–12 while conducting redox polymers and transition metal oxides/hydroxides are widely explored as active materials for pseudocapacitance and battery-type electrodes.^13–16Metal–organic frameworks (MOFs), a porous crystalline material composed of metal nodes and organic linkers, have been widely applied in versatile fields including chemical sensors, catalysis, separation, biomedicine, and gained more and more attention in the area of energy storage.^17–25 Recently, two-dimensional (2D) MOFs have aroused great interest as a new kind of 2D materials.^26,27 Compared with traditional bulk MOFs, 2D MOFs possess distinctive properties, such as short ion transport distances, abundant active sites, and high aspect ratios, making them exhibit better performance than their bulk counterparts.^28–32 Bottom-up methods are generally adopted to prepare 2D MOFs with the addition of surfactants to control the growth of MOFs in a specific direction.^33–35 However, the use of surfactants inevitably blocks part of the active sites at the expense of the performance of materials. Therefore, it is highly necessary to explore and develop a direct solvothermal synthesis of 2D MOFs with the advantages of additive-free, simple operation, and easy scale-up.Herein, we report a facile one-pot solvothermal method to synthesize 2D Ni-based MOF microsheets (denoted as Ni-MOFms) by treating nickel chloride hexahydrate (NiCl₂·6H₂O, the metal precursor) together with the trimesic acid (H₃BTC, the ligand) in a mixed solvent of N,N-dimethylformamide (DMF), ethanol (EtOH) and H₂O. During the whole preparation process, only Ni precursor and the ligand are used while no surfactant is added. When used as active materials for a supercapacitor electrode, the obtained Ni-MOFms displayed excellent reversibility and rate performance. It also exhibited specific capacities of 91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively. Besides, they showed a good cycling performance in 5000 cycles with about 70% of the specific capacity and almost 100% of the coulombic efficiency maintained.Morphologies of the Ni-MOFms were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1a and b, the Ni-MOFms were successfully fabricated via the facile one-pot solvothermal method with varying lateral sizes on the micron scale. Energy dispersive spectroscopy (EDS) mapping indicated that the obtained microsheets were mainly composed of C, O, and Ni. A trace amount of N was also observed, which could be attributed to the residual DMF in the mixed solvent (Fig. 1c). These elements were uniformly distributed throughout the whole microsheet. To measure the exact thickness of the Ni-MOFms, atomic force microscopy (AFM) was used. Fig. 1d showed that the thickness of the microsheet was about 58 nm. Considering the large lateral size, even such thickness could produce a relatively high aspect ratio, which is beneficial to the electrochemical performance.Open in a separate windowFig. 1(a) TEM image, (b) SEM image, (c) EDS mapping, and (d) AFM image and the corresponding height profile of the Ni-MOFms.The composition information of the Ni-MOFms was analyzed by X-ray diffraction (XRD) and the resulting diffraction pattern was shown in Fig. 2a. It was clear that the sample was a crystalline material. However, the exact structure was difficult to determine because no matching MOF structure has been found. Therefore, the structure of the Ni-MOFms was further confirmed by Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2b, there was a sharp peak at 1721 cm⁻¹ for H₃BTC, which could be ascribed to the stretching vibration of C O in the nonionized carboxyl group.³⁶ For the Ni-MOFms, the peak at this location disappeared while four new peaks appeared. Bands at 1634 and 1557 cm⁻¹ were related to the asymmetric stretching vibration of carboxylate ions (–COO⁻) and peaks at 1433 and 1371 cm⁻¹ were the characteristic peaks of the symmetric stretching vibration of –COO⁻.^37,38 All these changes indicate that the ligand interacted well with the metal precursor.Open in a separate windowFig. 2(a) XRD pattern of the Ni-MOFms. (b) FT-IR spectra of H₃BTC and the Ni-MOFms.The chemical status and surface composition of the Ni-MOFms were further examined by X-ray photoelectron spectroscopy (XPS). From Fig. S1a^† we could see that the Ni-MOFms were composed of C, O, Ni, and N, which was consistent with the result of EDS mapping. High-resolution spectra of C 1s, Ni 2p, O 1s, and N 1s were shown in Fig. S1b–e.^† Characteristic peaks of C 1s at 288.27, 286.50, 285.85, and 284.80 eV were related to O C–OH, C–O, C–C, and C C, respectively, suggesting the presence of H₃BTC (Fig. S1b^†).³⁹ The Ni 2p spectrum showed two peaks at 873.32 and 855.77 eV, which could be ascribed to Ni 2p_1/2 and Ni 2p_3/2, respectively, together with two satellite peaks at 879.26 and 861.05 eV, verifying the existence of Ni²⁺ (Fig. S1c^†).⁴⁰ In the O 1s region, bands positioned at 532.94 and 531.40 eV could be ascribed to the adsorbed H₂O molecules on the surface of Ni-MOFms and typical metal–oxygen bonds, respectively, further corroborating the coordination between H₃BTC and Ni²⁺ (Fig. S1d^†).³⁹ Finally, the high-resolution spectrum of N 1s was also analyzed (Fig. S1e^†). There were two main peaks at 400.18 and 402.21 eV that could be ascribed to neutral amine and charged nitrogen, respectively,⁴¹ further proving the residual DMF on the Ni-MOFms surface.To explore the crucial factors in the formation process of the Ni-MOFms, the reaction time and temperature, the solvent, the ligand addition amount, and the ligand type were studied. As shown in Fig. S2,^† different crystalline materials were obtained at different reaction times. With the increase of reaction time, the material gradually changed from sphere to sheet. The reaction temperature is another crucial factor. At 120 °C, the material was amorphous and spherical. When the temperature rose, the crystal formed and appeared as microsheets (Fig. S3^†). The effect of solvent was illustrated in Fig. S4.^† Microsheets could not be synthesized in DMF or DMF with a small amount of EtOH. In the mixed solvent of DMF and H₂O, crystals could be prepared, indicating the vital role of H₂O. However, spheres existed in the products. Only when a mixture of DMF, EtOH, and H₂O with a certain proportion was used as the solvent, the Ni-MOFms could be obtained. Furthermore, we investigated the effect of the ligand addition amount. From Fig. 1 and S5^† we can see that the Ni-MOFms crystals formed when the molar ratio of Ni precursor and H₃BTC was 1 : 2 (Fig. 1). We speculated that ligands could simultaneously act as regulators to adjust the morphology of materials, avoiding the use of additional surfactants. When the ligand was replaced with 2-methylimidazole (2-MI) or terephthalic acid (H₂BDC), flower-like crystals rather than microsheets were obtained (Fig. S6^†), indicating the importance of the ligand type. Taking the above factors into account, we could finally determine the suitable conditions for preparing the Ni-MOFms (see the experimental section in ESI^†).The potential application of the Ni-MOFms in supercapacitors was first tested by cyclic voltammetry (CV) in 3 M KOH between 0 and 0.4 V (vs. saturated calomel electrode, SCE). As can be seen from Fig. 3a, all CV curves had similar shapes and the peak currents improved gradually as the scan rate increased, suggesting the good capacitive behavior of the Ni-MOFms electrode.⁴² When the scan rate was as high as 150 mV s⁻¹, redox peaks could still be observed, which indicated the excellent rate performance and kinetic reversibility.⁴³ Besides, as the scan rate went up from 10 to 150 mV s⁻¹, the reduction and oxidation peaks moved towards negative and positive potential, respectively, demonstrating the electrode polarization at large scan rates.⁴⁴Open in a separate windowFig. 3Electrochemical measurements of the Ni-MOFms. (a) CV curves at different scan rates. (b) GCD curves at various current densities and (c) corresponding specific capacities. (d) The EIS Nyquist plot at the bias potential of 0.4 V and the equivalent circuit model with the fitted plots (the red dots).The galvanostatic charge–discharge (GCD) behavior was further investigated to assess the coulombic efficiency and the specific capacity of the Ni-MOFms (see the ESI^† for detailed calculation method).^45,46 As shown in Fig. 3b, the shape of GCD curves was highly symmetric during charging and discharging, indicating that the coulombic efficiency of Ni-MOFms was almost 100% at various current densities. The specific capacities of 91.4, 78.4, 71.4, 64.0, and 60.0 C g⁻¹ were achieved at current densities of 2, 4, 6, 8, and 10 A g⁻¹ (Fig. 3c), respectively, demonstrating the excellent rate capability with about 65.6% of the specific capacity maintained from 2 to 10 A g⁻¹. The specific capacity at 2 A g⁻¹ was comparable with or even superior to that of some MOF materials reported in the literatures (Table S1^†).^47–50The kinetics of the electroanalytical process was then investigated by electrochemical impedance spectroscopy (EIS). Fig. 3d showed the Nyquist plot of Ni-MOFms from 0.01 to 100000 Hz and the corresponding equivalent circuit model (inset) with the fitted plots. CPE was the constant phase element related to the double layer capacity.⁵¹ The equivalent series resistance was denoted by R_s and its value obtained from the x-axis intercept was about 2.1 Ω, indicating the low resistance of the solution.⁴³R_ct represented the charge-transfer resistance at the interface of the electrode and electrolyte.⁵² For Ni-MOFms, the value of R_ct was up to 147.1 Ω, which could be attributed to the poor conductivity of MOF materials.The long-term stability of Ni-MOFms was also explored by charging–discharging at 10 A g⁻¹ for 5000 consecutive cycles. From Fig. 4 we could see that the specific capacity retention remained about 70% after 5000 cycles and the coulombic efficiency was maintained at almost 100% throughout the whole process. Furthermore, the inset in Fig. 4 exhibited that the GCD curves of the last 10 cycles were the same as the first 10 cycles, indicating excellent cycling stability.Open in a separate windowFig. 4Cycle property of Ni-MOFms at 10 A g⁻¹. Inset: GCD curves of the first 10 cycles (left) and the last 10 cycles (right).     

Retraction: Facile fabrication of water-dispersible AgInS2 quantum dots and mesoporous AgInS2 nanospheres with visible photoluminescence

Retraction of ‘Facile fabrication of water-dispersible AgInS₂ quantum dots and mesoporous AgInS₂ nanospheres with visible photoluminescence’ by Hui Jin et al., RSC Adv., 2015, 5, 68287–68292. DOI: 10.1039/C5RA11545K

(1) The Royal Society of Chemistry has been notified by the Office of Academic Research, Qingdao University that the authorship, affiliations and acknowledgements of this paper are incorrect. They informed us that “Rijun Gui has confirmed that he independently completed the experimental research before he joined Qingdao University. Without their knowledge and prior notice, he signed the names of irrelevant researchers (Hui Jin, Zonghua Wang, Jianfei Xia, Min Yang, Feifei Zhang and Sai Bi) in his paper and added the Funding numbers of the irrelevant researchers in his papers without authorization. Rijun Gui confirmed that these irrelevant researchers did not participate in experimental researches reported in his paper and they did not provide financial support. They have no contribution to the paper.” They concluded that “the names of the irrelevant researchers without contribution are required to be deleted from the authorship of the paper, and their funding numbers without providing financial support are required to be deleted from the “Acknowledgements”.” The corrected authorship list and affiliations for this paper are as follows:Rijun Gui*^{a a}School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China, email: moc.361@nujiriugThe corrected Acknowledgements for this paper are as follows:This work was financially supported by the Postdoctoral Science Foundation of Shanghai (13R21413800).(2) The Royal Society of Chemistry hereby wholly retracts this RSC Advances article due to concerns with the reliability of the data in the published article.The TEM image in Fig. 2a contains duplications of the same particles within the image, which indicates that it has been manipulated.The TEM image in Fig. 2c duplicates data from another publication by Tan et al., but representing different materials.¹The TEM image in Fig. 2d duplicates data from another publication by Tan et al., but representing different materials.²Given the number and significance of the concerns about the validity of the data, the findings presented in this paper are no longer reliable.Rijun Gui requested to retract this article due to the incorrect authorship, but opposes the wording in this retraction notice.Signed: Laura Fisher, Executive Editor, RSC AdvancesDate: 21^st September 2020     

Facile synthesis of a two-dimensional layered Ni-MOF electrode material for high performance supercapacitors

Recently, various metal–organic framework (MOF)-based supercapacitors (SCs) have received much attention due to their porosity and well-defined structures. Yet poor conductivity and low capacitance in most MOF-based devices limit their wide application. As an electrode material, 2D MOFs exhibit a rapid electron transfer rate and high specific surface area due to their unique structures. In this work, a 2D layered Ni-MOF is synthesized through a simple solvothermal method and serves as an electrode material for SCs. Electrochemical studies show that the Ni-MOF exhibits low charge transfer resistance, excellent specific capacitance of 1668.7 F g⁻¹ at 2 A g⁻¹ and capacitance retention of 90.3% after 5000 cycles at 5 A g⁻¹. Moreover, Ni-MOF//AC asymmetric SCs are assembled. The device exhibits high specific capacitance of 161 F g⁻¹ at 0.2 A g⁻¹ and the energy density reached 57.29 W h kg⁻¹ at a power density of 160 W kg⁻¹. The high electrochemical performance can be ascribed to the inherent porosity of MOFs and the 2D layered structure.

A 2D Ni-MOF was synthesized by a hydrothermal method and used as an electrode for SCs.     

Facile synthesis of g-C3N4 with various morphologies for application in electrochemical detection

In the present study, g-C₃N₄ with various morphologies was successfully synthesized via a variety of facile in situ methods. The as-prepared products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD). The results obtained using square wave anodic stripping voltammetry (SWASV) showed that when g-C₃N₄ was applied as an electrochemical sensor, it exhibited excellent sensitivity and selectivity for the detection of heavy metal ions including Pb(ii), Cu(ii) and Hg(ii). Compared to nanoporous graphitic carbon nitride (npg-C₃N₄) and g-C₃N₄ nanosheet-modified glass carbon electrode (GCE), g-C₃N₄ successfully realized the individual and simultaneous detection of four target heavy ions for the first time. In particular, g-C₃N₄ displayed significant electrocatalytic activity towards Hg(ii) with a good sensitivity of 18.180 μA μM⁻¹ and 35.923 μA μM⁻¹ under the individual and simultaneous determination conditions, respectively. The sensitivity for simultaneous determination was almost 2 times that of the individual determination. Moreover, the fabricated electrochemical sensor showed good anti-interference, stability and repeatability; this indicated significant potential of the proposed materials for application in high-performance electrochemical sensors for the individual and simultaneous detection of heavy metal ions.

In the present study, g-C₃N₄ with various morphologies was successfully synthesized via a variety of facile in situ methods.     

Facile preparation of 3D porous agar-based heteroatom-doped carbon aerogels for high-energy density supercapacitors

The fabrication of heteroatom-doped porous carbon materials with high electrical conductivity and large specific surface area via an environmentally friendly route is critical and challenging. Herein, nitrogen and oxygen co-doped agar porous carbon (APC) was developed for supercapacitors via a one-step carbonization method with agar as the raw material and ammonia as the activator and nitrogen source. APC outperformed pectin porous carbon, tamarind porous carbon, and the previously reported carbon-based supercapacitors with a high capacitance retention of 72% even from 0.5 A g⁻¹ to 20 A g⁻¹ and excellent cycling stability in 6 M KOH solution (retained after 10 000 cycles) with a rate of over 98.5%. Furthermore, the APC electrode-based symmetric device exhibited an impressive energy density of 20.4 W h kg⁻¹ and an ultra-high power density of 449 W kg⁻¹ in 1 M Na₂SO₄ electrolyte together with excellent cycling stability (103.2% primary capacitance retentivity after 10 000 cycles). This study offers a novel method for the synthesis of nitrogen heteroatom-doped hierarchical porous carbon materials for performance-enhanced energy storage devices.

N and O co-doped agar porous carbon (APC) as electrode materials exhibit excellent performance. A respectable energy density of 20.4 W h kg⁻¹ and an ultra-high power density of 449 W kg⁻¹, as well as excellent cycle stability in 1 M Na₂SO₄ electrolyte.     

Facile preparation of low-cost HKUST-1 with lattice vacancies and high-efficiency adsorption for uranium

In this work, we prepared HKUST-1 and HKUST-1 with lattice vacancies (HLV) using benzoic acid (BA) as a low-cost modulator to replace part of the traditional trimesic acid ligand (H₃BTC). The structure and morphology of the products were characterized by FTIR, XRD, SEM and XPS. The adsorption performance of the products for uranium from aqueous solutions was investigated. The results showed that the sorption of U(vi) on HKUST-1 and HLV agreed with the Langmuir isotherm model (R_HKUST-1² = 0.9867 and R_HLV² = 0.9828) and the maximum adsorption capacity was 430.98 mg g⁻¹ and 424.88 mg g⁻¹, respectively. According to kinetics studies, the adsorption fitted better with a pseudo-second-order model (R_HKUST-1² = 1.0000 and R_HLV² = 0.9978). The as-prepared adsorbents were used for the removal of uranium from real water samples as well. The results showed that HLV with lower cost is a promising adsorbent for uranium from aqueous solutions.

HLV was prepared using benzoic acid with lower cost as the modulator to replace part of the traditional ligand H₃BTC for uranium removal.     

Facile preparation of hybrid porous polyanilines for highly efficient Cr(vi) removal

In the present work, leucoemeraldine-based hybrid porous polyanilines (LHPPs) have been synthesized by the Friedel–Crafts reaction of leucoemeraldine and octavinylsilsesquioxane (OVS) for Cr(vi) removal. The resulting LHPPs were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and N₂ adsorption–desorption. The findings indiated that the LHPPs were amorphous, with apparent surface areas (S_BET) in the range of 147 to 388 m² g⁻¹ and total volumes in the range of 0.13 to 0.44 cm³ g⁻¹. Cr(vi) removal experiments displayed that the LHPPs exhibited highly efficient Cr(vi) removal performance. The maximum Cr(vi) removal capacity of LHPP-1 was 990.1 mg g⁻¹ at 308 K and pH 1, which is higher than those of other reported polyaniline-based adsorbents. The adsorption process was a spontaneous, endothermic and chemical adsorption process. The adsorption behavior agreed well with Langmuir models and pseudo second-order equations. X-ray photoelectron spectroscopy and Fourier transformed infrared (FTIR) spectroscopy analysis revealed that the highly efficient Cr(vi) removal performance can be mainly attributed to the existence of numerous amine and imine groups on the surface of the LHPPs; these can function as adsorption active sites for Cr(vi) removal through electrostatic adsorption and reduction to Cr(iii) under acidic conditions. Moreover, the LHPPs exhibited excellent adsorption selectivity for Cr(vi) despite the presence of other metal ions (K⁺, Cu²⁺, Mn²⁺) and anions (NO₃⁻, SO₄²⁻). Therefore, the LHPPs have potential applications for Cr(vi) removal in industrial wastewater.

In the present work, leucoemeraldine-based hybrid porous polyanilines (LHPPs) have been synthesized by the Friedel–Crafts reaction of leucoemeraldine and octavinylsilsesquioxane (OVS) for Cr(vi) removal.     

Facile preparation of partially reduced graphite oxide nanosheets as a binder-free electrode for supercapacitors

Preparation of graphene (GR) based electrode materials with excellent capacitive properties is of great importance to supercapacitors. Herein, we report a facile approach to prepare partially reduced graphite oxide (PRG) nanosheets by reducing graphite oxide (GO) using commercial Cu₂O powder as a reduction agent, moreover, we demonstrate that the PRG nanosheets can act as building blocks for assembling hydrogels (PRGH) and flexible film (PRGF). The obtained PRGH and PRGF can be directly used as binder-free electrodes for supercapacitors and give high specific capacitance (292 and 273 F g⁻¹ at a current density of 0.5 A g⁻¹ in a three-electrode system, respectively) due to the existence of oxygen-containing functional groups in PRG nanosheets. PRG also gives excellent rate ability and cycle stability. This study suggests a facile pathway to produce GR-based materials with excellent capacitive properties and is meaningful for flexible supercapacitors.

Partial reduced graphite oxide nanosheets with excellent capacitive property have been prepared using commercial Cu₂O powders as reduction agent.