Electrocatalytic oxidation of water by the immobilized [CuII(l-ala)(Phen)(H2O)]+ complex on a self-assembled NCS− modified gold electrode

Copper(ii) complex [Cu^II(l-ala)(Phen)(H₂O)]⁺ (l-ala = l-phenylalanine, phen = phenanthroline) was immobilized over a self-assembled NCS⁻ modified gold electrode for the electrocatalytic oxidation of water. This surface anchored molecular complex can catalyze water oxidation reaction at a remarkably low overpotential of 327 mV with a current density of 0.5 mA cm⁻² at neutral pH.

Copper(ii) complex [Cu^II(l-ala)(Phen)(H₂O)]⁺ (l-ala = l-phenylalanine, phen = phenanthroline) was immobilized over a self-assembled NCS⁻ modified gold electrode for the electrocatalytic oxidation of water.

Oxidation of water is the key step for natural or artificial photosynthesis processes. In green plants, oxidation of water occurs at the Mn₄CaO₅ active site of the oxygen-evolving enzyme of photosystem II.¹ Nature converts water to dioxygen very easily, but by artificial means this is highly challenging due to its highly demanding thermodynamic and kinetic nature.² To overcome these problems several water oxidation catalysts have been developed. Despite much progress on water-oxidation catalysts, major improvements are necessary in several areas like lowering of the overpotential, increasing catalyst stability, enhancing efficiency and reducing the overall cost. In recent years, various homogeneous catalysts and electrocatalysts for the water oxidation reaction based on earth abundant metals such as manganese,^3,4 iron,^5,6 cobalt,^7,8 nickel,^9,10 and copper^11,12 have been reported. Among them copper is an attractive choice due to its high abundance, rich coordination chemistry¹³ and vital role in bio-mimetic oxygen chemistry.¹⁴ Numerous copper complexes have been used as homogeneous water oxidation electrocatalysts. Mayer and co-workers reported for the first time a homogeneous copper catalyst [(bpy)Cu(μ-OH)]₂(OAc)₂ that could electrochemically oxidize water in basic media at an overpotential 750 mV with a turnover frequency of 100 s⁻¹.¹⁵ Several copper complexes of the polyamide ligand have been shown their catalytic water oxidation property.^16,17 F. Chen et al. reported a water soluble copper complex [L–Cu^II–OAc]⁻ (where OAc = acetate, L = N,N′-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate) could electrocatalyze water oxidation to evolve O₂ in 0.1 M carbonate buffer (pH 10) with an onset overpotential of 650 mV.¹⁸ P. G. Barros and co-workers reported a copper(ii)-complex with redox non-innocent tetradentate amidate acyclic ligands that can electrocatalyze the water oxidation process at pH 11.5 with overpotential of 700 mV and as the electron-donating capacity at the aromatic ring increases, the overpotential notably reduced down to around 170 mV.¹⁹ Apart from these mononuclear copper catalysts which were operated in basic conditions, an oxidatively stable dinuclear copper-based catalyst, [Cu₂(BPMAN)(μ-OH)]³⁺ (BPMAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine) has been reported, which efficiently catalyzed water oxidation at a neutral pH without decomposition during long-term electrolysis.²⁰ On the other hand, several copper species such as Cu^II-Gly,²¹ Cu(ii)-1,2-ethylenediamine²² and [Cu^II(TPA)H₂O]²⁺ (ref. 23) have been used as precursors for active heterogeneous copper oxide formation through electrodeposition in alkaline media and catalyzed the water oxidation process. A. Prevedello et al. reported a copper(ii) species with tetraaza macrocyclic ligand that can acts as heterogeneous (active copper oxide layer) and homogeneous (molecular species) electrocatalyst in alkaline (pH = 9–12) and neutral media, respectively.²⁴ All of these known water oxidation catalysts have some practical problems, for example molecule based homogeneous catalysts are prone to decomposition under moderate applied potentials. To overcome these issues, some alternative strategies have been adopted such as grafting of molecular catalysts onto an electrode surface via covalent attachment or by anchoring on the functionalized self-assembled monolayer modified electrode. Placing the catalyst at an interface reduces the amount of catalyst needed and may enhance the rate. Few reports are available on surface anchored molecular complexes of Ru,²⁵ Ir,²⁶ V,²⁷ Ce²⁸ that could efficiently electrocatalyze the water oxidation reaction, are durable for a long time and have an overpotential that is quite low (250–350 mV) compared to the homogeneous system. Still earth abundant metal containing molecular complex modified electrode systems are scarce.In the present article we have fabricated a gold electrode using copper(ii)-complex, [Cu(l-phe)(Phen)(H₂O)](ClO₄) and NH₄SCN through two step self-assembly process (Scheme 1) and characterized by spectral, electrochemical and microscopic process. The modified electrode is stable and efficiently oxidizes water to oxygen in neutral pH medium.Open in a separate windowScheme 1Electrode modification using self-assembly process.The stepwise modification of the gold electrode was monitored by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) (Fig. 1a and b). CV of 0.5 mM [Fe(CN)₆]^3−/4− (redox probe) in 0.1 M PBS (pH 7.0) at bare gold electrode shows a quasi-reversible couple (ΔE = 80 mV) with an anodic current density of 2.7 mA cm⁻². After modification with NCS⁻ on gold electrode surface, an irreversible redox couple for [Fe(CN)₆]^3−/4− was obtained with significant decrease (around 2.5 mA cm⁻²) of anodic current density. This result supports the formation of self-assembled layer of NCS⁻ which retards the electron transfer process between electrode and the probe molecule. After immobilization of [Cu(l-phe)(phen)(H₂O)]⁺ complex on NCS⁻ modified gold electrode, the anodic current density was around 0.9 mA cm⁻² indicates the electronic communication between Au electrode and [Fe(CN)₆]^3−/4− through Cu^II-complex and in-turn confirms the proper modification. EIS study clearly shows that the charge transfer resistance (R_ct) of Cu^II-complex–SCN–Au electrode is less than SCN–Au electrode and support the CV results.Open in a separate windowFig. 1Overlaid cyclic voltammogram (a) and Nyquist plot (−Z′′ versus Z′) obtained from EIS experiment (b) of 0.5 mM K₄[Fe(CN)₆] in 0.1 M PBS (pH 7.0) at bare Au (brown curve), NCS-Au (red curve) and [Cu(l-phe)(phen)(H₂O)]–SCN–Au (green curve).In order to confirm the fabrication of Cu^II-complex on SCN–Au modified electrode a comparative CV was taken for bare Au and [Cu(l-phe)(phen)(H₂O)]–SCN–Au in 0.1 M PBS (Fig. S1^†). An anodic peak at +0.84 V versus RHE was obtained and is due to the Cu^II/III oxidation couple^29–31 and proves the presence of Cu^II-complex over SCN–Au electrode.With varying concentration of Cu^II-complex from 1.0 mM to 5.0 mM both the anodic and cathodic current were increased (Fig. S2a^†). A plot of anodic current versus concentration of Cu^II-complex gives a linear regression equation I_pa (μA) = 0.221C (mM) + 1.511 (R² = 0.998) and is shown in Fig. S2b.^† This observation also certifies a successful immobilization of Cu^II-complex on self-assembled NCS⁻ modified gold electrode. From the Fig. S3,^† it can be seen that the current density increases linearly with the increasing square root of scan rate, following the linear regression equation, J (mA cm⁻²) = 0.0063 √ν (mV s⁻¹) + 0.0247 (R² = 0.996), thereby, indicating a diffusion controlled electron transfer process at [Cu(l-phe)(phen)(H₂O)]–SCN–Au modified electrode.³²The modification process was also confirmed by using FE-SEM and EDX analysis. FE-SEM image (Fig. S4a and b^†) shows the surface morphology of bare and copper complex modified gold electrode. Bare gold shows a smooth surface morphology whereas the Cu^II-complex–SCN–Au electrode shows nearly smooth surface and supports the film formation.The EDX analysis (Fig. S4c^†) and elemental mapping images (Fig. S5^†) confirms the presence of Cu, N, O and S elements and supports the proper immobilization of Cu^II-complex on self-assembled NCS⁻ modified gold electrode.For further confirmation, FTIR Spectra of SCN–Au and Cu^II-complex–SCN–Au electrodes were recorded in the frequency range 450–4000 cm⁻¹ (Fig. S6^†). A sharp and broad peak at 2059 cm⁻¹, a weak peak at 795 cm⁻¹ and a peak at 480 cm⁻¹ are assigned as the ν(CN), ν(CS) and δ(NCS), respectively and confirms that NCS⁻ adsorbed on Au electrode surface and coordinated through N atom.³³ After immobilization of Cu(ii) complex over SCN–Au electrode surface the characteristic peaks for CN, CS and NCS are shifted to 2136 cm⁻¹, 781 cm⁻¹ and 476 cm⁻¹, respectively and supports the bond formation between Au-NCS and Cu(ii)-complex. Coordination of sulphur with copper of Cu(ii)-complex is confirmed by the presence of new bands at around 575 cm⁻¹ which is also assignable to Cu–N bond stretching for the Cu(ii)-complex.³⁴ Another indication for (Cu-S) bond is the presence of weak band at 456 cm⁻¹.³⁵ IR bands at around 3053 cm⁻¹, 1653 cm⁻¹ and 1595 cm⁻¹ are due to of CH₂, COO⁻ and NH₂ group, respectively³⁶ and supports the presence of l-ala in the Cu(ii)-complex. The peaks for O–H, N–H and C–H are observed at 3434, 2937 and 3343 cm⁻¹, respectively. From the IR data it can be conclude that the gold electrode was properly modified with NCS⁻ and [Cu(l-phe)(phen)(H₂O)]⁺.The electrocatalytic activity of the bare and modified gold electrodes for water oxidation were investigated using linear sweep voltammetry (LSV) in 0.1 M PBS at pH 7.0 in the potential window +0.6 to +2.0 V versus RHE (Fig. 2a). Oxidation of water was observed at higher potential ∼ +1.83 V versus RHE with current density 0.02 mA cm⁻² and 0.01 mA cm⁻² at bare and NCS⁻ modified gold electrode, respectively. The anodic peak potential is shifted towards less positive potential +1.58 V versus RHE and at the same time current height is increased to 0.54 mA cm⁻² when Cu^II-complex–SCN–Au electrode was used as working electrode. These results establish the electrocatalytic activity of Cu^II-complex–SCN–Au modified electrode towards the oxidation of water.³⁷ The oxidation of water by the Cu^II-complex–SCN modified gold electrode shows remarkably low overpotential of around 327 mV at J = 0.5 mA cm⁻² and onset overpotential of around 120 mV (J = 0.1 mA cm⁻²) in neutral PBS and the obtained result is comparable or in some cases quite better than the reported homogeneous Cu(ii)-complex based systems or heterogeneous copper oxide films, copper foil etc. (Table S1^†).Open in a separate windowFig. 2Overlaid LSV obtained at bare Au, SCN–Au and Cu^II-complex-NCS-Au electrode in 0.1 M PBS solution (pH 7.0) (a), overlaid LSV obtained at Cu²⁺ ion–SCN–Au electrode and Cu^II-complex–SCN–Au electrode in 0.1 M PBS solution (pH 7.0) (b).The electrocatalytic activity towards water oxidation at Cu^II-complex and Cu²⁺ ion immobilized SCN–Au electrodes was studied and shown in Fig. 2b. The LSV result shows that the anodic peak current is quite high (∼0.32 mA cm⁻²) and peak potential is less positive (∼0.15 V) in case of Cu^II-complex–SCN–Au than Cu²⁺–SCN–Au electrode suggest that the electrocatalytic activity towards water oxidation is higher in case of Cu^II-complex than Cu²⁺-ion modified electrode.To confirm that the anodic peak at +1.58 V is solely due to the oxidation of water, LSV was performed using [Cu(l-phe)(phen)(H₂O)]–SCN–Au electrode in the potential range of +0.6 to +2.0 V versus RHE in ultrapure CH₃CN containing 0.1 M tetrabutylammonium perchlorate [Bu₄N][ClO₄] (pH = 7.0). No anodic peak was observed (Fig. S7a^†), but upon addition of water a distinguished oxidative peak was appeared at +1.58 V versus RHE which proves that the water oxidation reaction taking place at the Cu(ii)-complex modified electrode surface. It was also observed that with increasing water concentration (0.1–0.5 M) the anodic peak current was increased linearly (Fig. S7b^†) which also confirms that the peak appeared at +1.58 V versus RHE is only due to the oxidation of water.³⁸Linear sweep voltammetry was carried out at low scan rate 5 mV s⁻¹ in the applied potential range 260 mV to 280 mV in 0.1 M PBS at pH 7.0. The plot of logJ versus η (overpotential) produces a Tafel slope of 49 mV dec⁻¹ (Fig. S8^†) which indicates an excellent catalytic activity of the Cu^II-complex-NCS-Au electrode towards the oxidation of water.³⁹ Fig. 3a shows the LSV at different scan rate ranging from 20–100 mV using [Cu(l-phe)(phen)(H₂O)]–SCN–Au modified electrode in PBS solution (pH 7.0). A plot of normalized catalytic current (i/ν_1/2) versus scan rates (ν) (Fig. 3b) gives an inverse relationship. This result indicates that a rate-limiting chemical step taking place prior to quick electron transfer to the electrode.⁴⁰ It also confirms that the chemical rate determining step of the catalytic process is likely to be the O–O bond formation step.⁴¹Open in a separate windowFig. 3Overlaid LSV at Cu^II-complex–SCN–Au electrode in 0.1 M PBS (pH 7.0) with increasing scan rate in the range of 20–100 mV s⁻¹ (a). A plot of normalized catalytic current versus scan rates (b).Fig. S9^† illustrates the LSVs at Cu^II-complex–SCN–Au electrode with varying pH (6.0, 6.5, 7.0, 7.5, 8.0) of phosphate buffer solution. The anodic peak potentials were shifted towards less positive potential with increasing pH of the medium. The oxidation peak potential varies linearly with pH of the medium (Fig. S9b^†) and follows the linear regression equation E_pa (V) = −0.123pH + 2.502 (R² = 0.997).The slope of 0.123 V per pH shift indicating that 1e⁻/2H⁺ couple is involved in Cu^II-complex electrocatalyzed water oxidation reaction.⁴² The influence of pH on the peak current density (J) of water oxidation at the modified gold electrode (Fig. S10^†) revealed that the J values were increased linearly up to 7.0 and then slowly decreased and thereafter decreased sharply. This observation indicates that pH 7.0 is the most effective pH for the oxidation of water by the Cu^II-complex modified electrode.A plausible mechanism for the water oxidation over Cu^II-complex modified gold electrode is given in Scheme S1.^† In the proposed mechanism, the catalytically active [Cu^III(H₂O)]⁺ complex on the gold electrode surface is formed after anodic oxidation of [Cu^II(H₂O)] at +0.84 V versus RHE. Once formed, [Cu^III(H₂O)]⁺ oxidized H₂O to O₂ at +1.55 V versus RHE in neutral pH.^41–43The stability and oxygen generation capability of the Cu^II-complex–SCN–Au electrode was investigated using controlled potential electrolysis (CPE) at +1.58 V versus RHE using in 0.1 M PBS (pH 7.0) (Fig. 4a). The CPE experiment shows that the current density (J) rapidly declines to around 0.02 and 0.38 mA cm⁻² within 20 seconds for the bare and the Cu^II-complex–SCN modified gold electrode, respectively and thereafter the current remains stable over the entire period of electrolysis.⁴⁴ The obtained result agrees the high stability of the modified Au electrode during electrolysis. The stability of the complex modified electrode was also checked by using chronopotentiometry experiment for 60 minutes at a fixed current density of 0.51 mA cm⁻² (Fig. S11^†). A stable potential was obtained during two hour long electrolysis. This result also supports the high stability of Cu^II-complex–SCN modified electrode.Open in a separate windowFig. 4Bulk electrolysis with (green curve) and without (brown curve) the Cu^II-complex catalyst over the gold electrode in 0.1 M PBS at 1.58 V versus RHE (a). O₂ evolution during controlled potential electrolysis of water at Cu^II-complex–SCN–Au electrode. Dotted line denotes the theoretical O₂ evolution with 100% efficiency (b).During controlled potential electrolysis the generated oxygen in the head space of the gas tight electrochemical cell was monitored using a fluorescent probe. The concentration of the evolved oxygen was increased nearly linear fashion (Fig. 4b). It gives around 17 μmol of oxygen within 30 minutes of electrolysis with a Faraday efficiency of 96%. The theoretical yield of generated oxygen was calculated by assuming that the obtained current is due to four electron oxidation of water.⁴⁵The long term stability of the Cu(ii)-complex modified electrode was explored by LSV measurement in 0.1 M PBS at 15 days intervals (Fig. S12^†) and almost similar LSV responses were obtained after 15^th and 30^th day (relative standard deviation was 0.03%). Thus, it can be concluded that the electrocatalytic activity of the Cu^II-complex-NCS modified electrode does not suffer from dissolution and remained active.³⁷ For further confirmation of stability of the catalyst, long term bulk electrolysis experiment was performed at +1.58 V for around eight hours (Fig. S13^†) and a constant catalytic current was obtained over the entire period of electrolysis. This observation also establishes the robustness of the system. To check the surface morphology and durability of the modified electrode, FE-SEM and EDX analysis was done after the bulk electrolysis experiment (Fig. S14^†). No considerable change of surface morphology was observed by comparing with the SEM image of the electrode surface before electrolysis (Fig. S4b^†) and the presence of different elements such as Cu, N, O and S in the EDX spectrum (Fig. S14b^†) support the stability of the catalyst.In summary, [Cu(l-phe)(phen)(H₂O)]–SCN–Au electrode was developed, characterized and applied for the electrocatalytic oxidation of water in neutral pH. The newly developed electrode was able to oxidize water at an impressively low overpotential 327 mV with a current density of 0.54 mA cm⁻². Tafel slope of 49 mV dec⁻¹ indicates excellent catalytic activity of the Cu^II-complex towards water oxidation reaction. During 30 minutes of electrolysis, 17 μmol of oxygen was produced with a Faraday efficiency of 96%. The electrode material was chip and the modified electrode was highly stable, reusable and may help for the development of commercial water oxidation catalysts.     

Electrocatalytic oxidation of water at a polyoxometalate nanoparticle modified gold electrode

Spherical polyoxometalate nanoparticles, [HPMo]NPs, were synthesized from a very well known Keggin-type polyoxometalate [H₃PMo₁₂O₄₀] in the presence of sodium dodecyl sulphate (SDS) and polyvinyl pyrrolidine (PVP) in aqueous medium and characterized by UV-Vis spectroscopy and Transmission Electron Microscopy (TEM). The [HPMo]NPs were used to modify a gold working electrode and they were characterized by SEM, EDX, elemental mapping, cyclic voltammetry and electrochemical impedance spectroscopy and applied for the electrocatalytic oxidation of water in a phosphate buffer solution at neutral pH. The modified electrode showed excellent electrocatalytic activity towards oxidation of water at an impressively low overpotential ∼350 mV with a high current density of around 1.7 mA cm⁻², good stability under exhaustive electrolysis conditions and also showed long term stability.

Polyoxometalate nanoparticles, [H₃PMo₁₂O₄₀]NPs, modified gold electrode showed excellent electrocatalytic activity towards water oxidation reaction at an overpotential of 350 mV with a current density of 1.7 mA cm⁻² in neutral pH medium.

To fulfill the worldwide clean energy demand, development of renewable and inexpensive energy sources is the most challenging task in the present era. In this endeavor, photo and electro catalytic splitting of water to generate molecular oxygen and hydrogen is a promising technique.¹ Molecular hydrogen is a clean and environmentally friendly fuel with high gravimetric energy density.² The water splitting process consists of two steps, 2H₂O → O₂ + 4H⁺ + 4e⁻ and 2H⁺ + 2e⁻ → H₂. Between them the water oxidation step is the energy controlling step in the overall water splitting process and is frequently an obstacle in the whole water splitting process due to its high overpotential and slow reaction kinetics.³ Extensive efforts have been devoted to make an efficient water oxidation catalyst for lowering the energy consumption and accelerating the kinetics of the reaction.To date a large number of polyoxometalates (POMs) have been extensively utilized as homogeneous as well as heterogeneous water oxidation catalysts (WOC''s).^4–8 To enhance the catalytic activity and stability of POMs different strategies have been taken, for instances, incorporation of transition metal ions in the POM structure,^9,10 immobilization of POM on appropriate surface such as carbon nanotube, graphene or TiO₂ nanoparticles etc.,^11–13 encapsulation of POM inside a cavity of metal organic frame work⁷etc.Catalytic activity of POMs can also be increased if POM molecules will be assembled into a defined shape and size such as in nanoparticles dimension. POM nanoparticles can be synthesised different ways such as microemulsion mediated POM nanoparticles formation¹⁴ or, self-assemble polymer or cationic micelles or substate for synthesizing nanostructured POM.^15,16In the present communication we have described the synthesis and characterization of spherical polyoxometalate nanoparticles by using the Keggin-type polyoxometalate, [H₃PMo₁₂O₄₀] (HPMo) and sodium dodecyl sulfate (SDS) surfactant where POMs are at the surface and SDS at the core and the [HPMo]NPs were utilized for electrode modification to study the electrocatalytic water oxidation reaction at neutral pH. The POM nanoparticles modified gold electrode can efficiently oxidized water to oxygen at low overpotential (350 mV) with high current density (1.7 mA cm⁻²).UV-visible spectra of [HPMo] nanoparticles (Fig. S1^†) shows a broad band at 225 nm, owing to the O → Mo charge transfer transitions.¹⁷ Deferent corresponding bond stretching frequencies of [HPMo] nanoparticles were investigated by FT-IR spectroscopy using KBr pallets. Stretching frequencies at 1063, 954, 824 and 754 cm⁻¹ were observed owing to the vibration modes ν_{P O}, ν_{Mo O} and ν_Mo–O–Mo, respectively.¹⁷To investigate the morphology and size of the [HPMo] nanoparticles, TEM was taken. Fig. 1 shows the TEM images of [HPMo] nanoparticles indicating that the size of the nanoparticles is in the range of 20–30 nm. Each particle has a hollow space confirms that micelle directed [HPMo] nanoparticles was formed.Open in a separate windowFig. 1TEM image and cartoon of [HPMo] nanoparticles.The electrode modification was characterized by comparing the SEM images of the bare and [HPMo] nanoparticles modified gold electrodes. Fig. S2a^† shows a smooth surface morphology for bare gold electrode whereas the modified electrode shows almost smooth surface (Fig. S2b^†) due to layer formation by the [HPMo] nanoparticles. This observation indicates that the gold electrode was properly modified. The EDX spectrum of the modified gold electrode (Fig. S2c and d^†) shows the presence of C, N, O, P, S, Mo and Au elements which confirms the formation of [HPMo] layer on Au electrode. Elemental mapping also supports the presence of C, N, P, O, S, Au, Mo and Na and confirms the proper modification of gold electrode by the [HPMo] nanoparticles (Fig. S3a–h^†). To characterize the modified electrode electrochemically, a comparative cyclic voltammetry of the redox probe [Fe(CN)]₆^3−/4− (0.5 mM) were carried out at bare and [HPMo]NPs modified gold electrodes in 0.1 M PBS (at pH 7.0) (Fig. S4^†). The cyclic voltammograms shows a cathodic peak current (I_pc) at ∼75 μA and ∼140 μA when bare and [HPMo]NPs modified Au-electrode were used, respectively. Increase current density obtained at [HPMo] nanoparticles modified electrode indicates an enhanced electronic communication between the probe and gold electrode. This observation is also supported by the electrochemical impedance spectroscopy. Nyquist plot shows that the charge transfer resistance (R_ct) is ∼0.42 × 10⁵ Ω at [HPMo] nanoparticles modified gold electrode which is almost half than obtained at bare gold electrode (R_ct = 0.80 × 10⁵ Ω). This result also supports the higher electron transfer ability of [HPMo]NPs–Au than the bare Au electrode (Fig. S5^†). Cyclic voltammograms of bare and modified gold electrodes (Fig. S6^†) in 0.1 M PBS at pH 7 also supports the successful immobilization of the nanoparticles on the gold electrode surface, as the peak current associated with the [HPMo]NPs modified electrode obtained a higher value than that associated with the bare electrode. Linear response of the peak current values to the increasing scan rate as seen in the voltammograms at different scan rates (Fig. S7^†) proclaims the electrochemical stability of the surface bound [HPMo]NPs/Au modified electrode. Fig. 2 displays the linear sweep voltammogram (LSV) of 0.1 M PBS solution (pH 7.0) at bare Au, [HPMo]–Au and [HPMo]NPs–Au electrode. An anodic peak was observed at +1.58 V versus RHE with a current density of 1.7 mA cm⁻² at [HPMo]NPs modified electrode. No such anodic peak was observed in this potential window (0.2 to 2.0 V versus RHE) when bare Au or [HPMo]–Au was used.Open in a separate windowFig. 2Overlaid LSV in 0.1 M PBS (pH 7.0) obtained at bare, [HPMo] and [HPMo]NPs modified gold electrode.This observation suggest that the [HPMo]NPs modified electrode can electrochemically oxidized water to oxygen, 2H₂O → O₂ + 4H⁺ + 4e⁻. In non-aqueous media like in CH₃CN no such anodic peak was observed. When the water is added to the solution of CH₃CN containing 0.1 M [Bu₄N][ClO₄] (pH = 7.0), the oxidative peak appeared at the same potential, +1.58 mV versus RHE (Fig. 3a). Initially, the anodic peak current density increased with increasing water concentration (up to 0.5 M) and after that the current density remained constant (Fig. 3b) as expected for surface attached species.¹⁸ The anodic peak current increased with increasing scan rate in the lower range (20–100 mV s⁻¹), following the linear regression equation J_pa (mA cm⁻²) = 0.0076ν (mV s⁻¹) + 0.943 (R² = 0.9995) (Fig. S8^†), which indicates that the electrode process is surface-controlled. However, at higher scan rate (200–700 mV s⁻¹), the peak current varies linearly with square root of scan rate and followed the linear regression equation J_pa (mA cm⁻²) = 0.3556ν^1/2 (mV s⁻¹) − 2.6553 (R² = 0.9993) which supports diffusion controlled water oxidation (Fig. S9^†). Therefore, it may be concluded that the electrocatalytic oxidation of water on [HPMo]NPS–Au electrode is the mixture of adsorption and diffusion controlled process and is dependent on scan rate.¹⁹Open in a separate windowFig. 3(a) LSV obtained from [HPMo]NPs–Au electrode with increasing amount of [water] (0.0 to 0.5 M). (b) A plot of anodic current density versus [H₂O].The effect of pH on the electrocatalytic oxidation of water was investigated using LSV in the pH range of 5.0–9.0 of PBS at a scan rate of 100 mV s⁻¹ and is presented in Fig. S10.^†With increasing pH of the medium the current density for water oxidation increases. Although the water oxidation performance of [HPMo]NPs–Au electrode is better at higher pH, but the stability of the system is decreases. So, pH 7.0 was chosen as working condition for the entire water oxidation experiment. On the other hand the anodic peak potentials were shifted towards less positive potential with increasing pH following the linear regression equation E_pa (V) = −0.0592pH + 1.9852 (R² = 0.9949). The slope of 59 mV per pH unit indicates that equal number of protons and electrons are involved in the electrode reaction process.²⁰Controlled potential electrolysis (CPE) was performed in a stirred 0.1 M PBS solution (pH = 7.0) at +1.58 V versus RHE in a gas tight electrochemical cell using bare Au, [HPMo]–Au and [HPMo]NPs–Au electrode and overlaid CPE curves are shown in Fig. S11.^† A steady-state current of 1.0 A cm⁻² was obtained for up to 30 minutes at [HPMo]NPs–Au electrode, whereas a very small amount of current (0.2 A cm⁻²) was obtained during the electrolysis of water at bare Au and [HPMo]–Au electrodes in similar condition. During the electrolysis of water (35 minutes) around 17 μM oxygen (Fig. S12^†) was produced (the amount of oxygen formed during the electrolysis was measured using fluorescent probe) with a Faraday efficiency of around 90%. Fig. 4a shows the chronoamperograms with increasing overpotential in 0.1 M PBS (pH 7.0) at [HPMo]NPs–Au electrode. The Tafel plots remained good linearity indicating a good electrical conductivity retained with the [HPMo]NPs. The Tafel slope (Fig. 4b) obtained for the [HPMo]NPs is 63 mV dec⁻¹ indicates excellent electrocatalytic activity of the [HPMo] nanoparticles. On the other hand, the Tafel slope obtained for [HPMo] modified electrode is quite high (around 597 mV dec⁻¹) and this high value indicates the inactivity of phosphomolybdic acid [HPMo] towards water oxidation under similar condition.²¹Open in a separate windowFig. 4(a) Current density versus time plot as the applied potential is stepped from 230 to 250 mV. (b) Tafel plot of [HPMo]NPs–Au electrode in 0.1 M PBS (pH 7.0) at different applied potentials. The slope of the graph is around 63 mV per decade.Comparison of different polyoxometalates, Co-oxides and Ir-oxides modified electrodes for the oxidation of water

Porous Pt–NiOx nanostructures with ultrasmall building blocks and enhanced electrocatalytic activity for the ethanol oxidation reaction

Oxidized species on surfaces would significantly improve the electrocatalytic activity of Pt-based materials. Constructing three-dimensional porous structures would endow the catalysts with good stability. Here, we report a simple strategy to synthesize porous Pt–NiO_x nanostructures composed of ultrasmall (about 3.0 nm) building blocks in an ethanol–water solvent. Structure and component analysis revealed that the as-prepared material consisted of interconnected Pt nanocrystals and amorphous NiO_x species. The formation mechanism investigation revealed that the preformed amorphous compounds were vital for the construction of porous structure. In the ethanol oxidation reaction, Pt–NiO_x/C exhibited current densities of 0.50 mA cm_Pt⁻² at 0.45 V (vs. SCE), which were 16.7 times higher than that of a commercial Pt/C catalyst. Potentiostatic tests showed that Pt–NiO_x/C had much higher current and better tolerance towards CO poisoning than the Pt/C catalyst under 0.45 V (vs. SCE). In addition, the NiO_x species on the surface also outperformed an alloyed Ni component in the test. These results indicate that the Pt–NiO_x porous nanomaterial is promising for use in direct ethanol fuel cells.

A porous Pt–NiO_x nanomaterial was constructed by a simple strategy to achieve excellent ethanol oxidation catalyst performance.     

Electrochemical oxidation of resorcinol: mechanistic insights from experimental and computational studies

This work investigates the mechanisms of resorcinol oxidation by density functional theory (DFT) calculation and cyclic voltammetry measurements. Complementary data from experimental and computational studies provide new insights into the reaction mechanisms. At both macro- and micro-electrodes, cyclic voltammetry of resorcinol is chemically and electrochemically irreversible over the whole pH range (1–14). Resorcinol molecules undergo a 1H⁺ 1e⁻ oxidation at pH < pK_a1 and a 1e⁻ oxidation at pH > pK_a2 to form radicals. The radicals then readily react to form dimers/polymers deposited on the electrode surface. All of the experimental findings are consistent with the proposed mechanisms, including the apparent transfer coefficient (β) of 0.6 ± 0.1, the slope of the peak potential (E_p) against pH of −54 mV pH⁻¹, the peak-shaped responses at micro-electrodes, and the fouling of the electrodes upon the oxidation of resorcinol. DFT calculation of the reaction energy of elementary steps and the eigenvalues of the highest occupied molecular orbital (HOMO) of the radical intermediates confirms that the (1H⁺) 1e⁻ oxidation is the energetically favorable pathway. In addition to mechanistic insights, an electrochemical sensor is developed for resorcinol detection at microelectrodes in low ionic strength samples with the sensitivity of 123 ± 4 nA μM⁻¹ and the limit of detection (3 s_B m⁻¹) of 0.03 μM.

Resorcinol oxidation mechanism was investigated by DFT calculation and cyclic voltammetry experiments at macro- and micro-electrodes (1 ≤ pH ≤ 14).     

Oxygen release from metal oxide for repeated hydrogen regeneration by proton irradiation with polyvinylpyrrolidone

In this study, we investigated the reduction of a 3D microporous NiO_x structure, used as a metal oxide catalyst, by proton irradiation with polyvinylpyrrolidone (PVP) for hydrogen regeneration. In general, the reduction process for hydrogen regeneration requires high temperatures (1000–4000 °C) to release saturated oxygen from the metal oxide catalyst. Proton irradiation with PVP could regenerate abundant oxygen vacancies by releasing the oxygen attached to NiO_x at room temperature. The 3D microporous NiO_x structure provided the maximum hydrogen generation rate of ∼4.2 μmol min⁻¹ g⁻¹ with the total amount of generated hydrogen being ∼460 μmol g⁻¹ even in the repetitive thermochemical cycle; these results are similar to the initial hydrogen generation data. Therefore, continuous regeneration of hydrogen from the oxygen-reduced 3D microporous NiO_x structure was possible. It is expected that the high thermal energy, which is the major problem associated with hydrogen regeneration through the conventional heat treatment method, would be resolved in future using such a method.

The reduction of a 3D microporous NiO_x structure, used as a metal oxide catalyst, was performed by proton irradiation with polyvinylpyrrolidone (PVP) for hydrogen regeneration.     

Mechanism of N,N-dimethylformamide electrochemical oxidation using a Ti/RuO2–IrO2 electrode

The compound N,N-dimethylformamide (DMF) is a widely used industrial chemical and a common environmental contaminant that has been found to be harmful to human health. In this study, electrochemical oxidation was adopted for the degradation of DMF. The effects of four kinds of electrodes on the removal rates of DMF and total organic carbon were compared, and based on the result, the Ti/RuO₂–IrO₂ electrode was selected as the operating electrode. The effects of three independent factors (current density, pH, and NaCl proportion) on the DMF degradation were investigated through single-factor experiments, and the experimental results were optimized by response surface methodology. The optimal experimental conditions were obtained as follows: current density = 47 mA cm⁻², pH = 5.5, and NaCl proportion = 15%. The electrochemical oxidation of 50 mg L⁻¹ DMF was performed under the optimal conditions; the degradation rate was 97.2% after 7 h, and the reaction followed the pseudo-first-order kinetic model. The degradation products under optimal conditions and chlorine-free conditions were analyzed, and four degradation pathways were proposed. The DMF degradation was more thorough under optimal conditions.

DEMS as an emerging technology was used to investigate the degradation mechanism of DMF.     

Facile synthesis of polypyrrole nanofiber (PPyNF)/NiOx composites by a microwave method and application in supercapacitors

In this study, polypyrrole nanofiber (PPyNF)/NiO_x composites were synthesized by a simple and fast microwave method. The samples were characterized using differential scanning calorimetry and thermal gravimetric analysis (DSC/TGA), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Furthermore, the synthesized PPyNF/NiO_x nanocomposites were electrochemically characterized using galvanostatic charge–discharge, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques. They showed the highest specific capacitance of 657 F g⁻¹ at 0.5 A g⁻¹, demonstrating their potential application in supercapacitors.

In this study, polypyrrole nanofiber (PPyNF)/NiO_x composites were synthesized by a simple and fast microwave method.     

Efficient charge separation and transfer of a TaON/BiVO4 heterojunction for photoelectrochemical water splitting

The separation and transfer of photogenerated electron–hole pairs in semiconductors is the key point for photoelectrochemical (PEC) water splitting. Here, an ideal TaON/BiVO₄ heterojunction electrode was fabricated via a simple hydrothermal method. As BiVO₄ and TaON were in well contact with each other, high performance TaON/BiVO₄ heterojunction photoanodes were constructed. The photocurrent of the 2-TaON/BiVO₄ electrode reached 2.6 mA cm⁻² at 1.23 V vs. RHE, which is 1.75 times as that of the bare BiVO₄. TaON improves the PEC performance by simultaneously promoting the photo-generated charge separation and surface reaction transfer. When a Co-Pi co-catalyst was integrated onto the surface of the 2-TaON/BiVO₄ electrode, the surface water oxidation kinetics further improved, and a highly efficient photocurrent density of 3.6 mA cm⁻² was achieved at 1.23 V vs. RHE. The largest half-cell solar energy conversion efficiency for Co-Pi/TaON/BiVO₄ was 1.19% at 0.69 V vs. RHE, corresponding to 6 times that of bare BiVO₄ (0.19% at 0.95 V vs. RHE). This study provides an available strategy to develop photoelectrochemical water splitting of BiVO₄-based photoanodes.

TaON/BiVO₄ heterojunction electrodes exhibited significant enhancement in the photoelectrochemical water oxidation.     

A stable LnMOF as a highly efficient and selective luminescent sensor for detecting malachite green in water and real samples

A series of isostructural 3D lanthanide metal–organic frameworks (LnMOFs), with the formula n(H₃O)[Ln(L)(H₂O)]_n·nH₂O (Ln = Gd 1, Eu 2 and Tb 3, H₄L = 3,5,3′,5′-oxytetrabenzoic acid), have been successfully synthesized by solvothermal reactions. Single-crystal X-ray diffraction analysis reveals that 1–3 are constructed from wave-like Ln–carboxylate chains which are further connected by the ligands to form 3D channel-type frameworks. Further experiments suggest that 3 is thermally stable up to 322 °C and exhibits outstanding chemical stability in aqueous solutions with the pH ranging from 3 to 11. Significantly, 3 can be utilized for the first time to detect malachite green (a synthetic antibiotic to cure saprolegniasis) in aqueous media even in the presence of other interfering antibiotics, with a high sensitivity (K_sv = 8.33 × 10⁴ M⁻¹), low detection limit (DL = 0.25 μM) and good recyclability. On a more practical note, we found that the luminescence intensity of 3 showed almost no response to pH changes (pH 3–11), allowing steady sensing in real samples such as river water, simulated human serum and urine with satisfactory recoveries and RSD.

A Tb-MOF with chemical stability was synthesized. It showed excellent luminescent sensing for MG in river water, simulated serum and urine.     

A ruthenium oxide and iridium oxide coated titanium electrode for pH measurement

This paper reports the pH sensing capability of a ruthenium oxide (RuO₂) and iridium oxide (IrO₂) coated titanium (ROIOT) electrode. The characterization results indicated that the ROIOT electrode had a cracked morphology. The RuO₂ and IrO₂ particles were decorated on the surface of the electrode. The ROIOT electrode showed near-Nernstian sensitivity of −50.8 mV pH⁻¹, with a wide detection range of pH 2–12. The response time was 4.0–13.5 s, which was fast and very sensitive to the pH change. The ROIOT electrode also demonstrated great detection reversibility and stability in various pH conditions. In the long-term experiment of 30 d, potential measurements using the ROIOT electrode had a minor fluctuation of 1.5 mV d⁻¹. The practical application of the ROIOT electrode was demonstrated by measuring the pH values of various buffer solutions and complex samples. With the advantages of low cost and simple production, it is believed that the ROIOT electrode could be a promising candidate for use as a sensing material for pH sensor development.

A ruthenium oxide and iridium oxide coated titanium electrode could be a good pH sensing material candidate.     

Oxidative functionalization of aliphatic and aromatic amino acid derivatives with H2O2 catalyzed by a nonheme imine based iron complex

The oxidation of a series of N-acetyl amino acid methyl esters with H₂O₂ catalyzed by a very simple iminopyridine iron(ii) complex 1 easily obtainable in situ by self-assembly of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf)₂ was investigated. Oxidation of protected aliphatic amino acids occurs at the α-C–H bond exclusively (N-AcAlaOMe) or in competition with the side-chain functionalization (N-AcValOMe and N-AcLeuOMe). N-AcProOMe is smoothly and cleanly oxidized with high regioselectivity affording exclusively C-5 oxidation products. Remarkably, complex 1 is also able to catalyze the oxidation of the aromatic N-AcPheOMe. A marked preference for the aromatic ring hydroxylation over Cα–H and benzylic C–H oxidation was observed, leading to the clean formation of tyrosine and its phenolic isomers.

Amino acid derivatives are oxidized by the 1/H₂O₂ system. A marked preference for the aromatic over Cα–H and benzylic C–H oxidation is observed with phenylalanine.     

Nanostructured RuO2–Co3O4@RuCo-EO with low Ru loading as a high-efficiency electrochemical oxygen evolution catalyst

Electrochemical water splitting technology is considered to be the most reliable method for converting renewable energy such as wind and solar energy into hydrogen. Here, a nanostructured RuO₂/Co₃O₄–RuCo-EO electrode is designed via magnetron sputtering combined with electrochemical oxidation for the oxygen evolution reaction (OER) in an alkaline medium. The optimized RuO₂/Co₃O₄–RuCo-EO electrode with a Ru loading of 0.064 mg cm⁻² exhibits excellent electrocatalytic performance with a low overpotential of 220 mV at the current density of 10 mA cm⁻² and a low Tafel slope of 59.9 mV dec⁻¹ for the OER. Compared with RuO₂ prepared by thermal decomposition, its overpotential is reduced by 82 mV. Meanwhile, compared with RuO₂ prepared by magnetron sputtering, the overpotential is also reduced by 74 mV. Furthermore, compared with the RuO₂/Ru with core–shell structure (η = 244 mV), the overpotential is still decreased by 24 mV. Therefore, the RuO₂/Co₃O₄–RuCo-EO electrode has excellent OER activity. There are two reasons for the improvement of the OER activity. On the one hand, the core–shell structure is conducive to electron transport, and on the other hand, the addition of Co adjusts the electronic structure of Ru.

The optimized RuO₂/Co₃O₄–RuCo-EO electrode with Ru loading of 0.064 mg cm⁻² exhibits the excellent oxygen evolution activity with an overpotential of 220 mV at the current density of 10 mA cm⁻² and a Tafel slope of 59.9 mV dec⁻¹.     

Theoretical hydrogen bonding calculations and proton conduction for Eu(iii)-based metal–organic framework

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation. The investigation of proton conduction shows that the conductivity of Eu(iii)-MOF obtained at 353 K and 98% RH is 3.5 × 10⁻³ S cm⁻¹, comparable to most of the Ln(iii)-MOF based proton conductors.

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation.

In recent years, with the aggravation of environmental pollution and growing depletion of petroleum, coal and other traditional fossil energy, the demand to exploit alternative cleaner energy is increasingly urgent. Compared to the dispersion of several developed new energy sources, such as solar energy, wind, geothermal heat, and so on,¹ the proton exchange membrane fuel cell (PEMFC) is recognized as a promising energy conversion system.² As an important component in PEMFC, the proton exchange membrane (PEM) directly affects the transmission efficiency of protons between electrodes.³ Currently, Nafion has been widely used as a PEM in commerce, and shows a conductivity higher than 10⁻¹ S cm⁻¹.⁴ However, the large-scale applications of Nafion are limited due to their high costs, narrow working conditions (low temperature and high relative humidity), amorphous nature, etc.⁵ To overcome these limitations, several types of proton-conducting materials have been explored over the past decade.⁶ Among them, MOF materials were employed as ideal platforms to regulate proton conductivity owing to their high crystallinity, tunable structure and tailorable functionality. The crystallographically defined structure is also conductive to the deeply analysis of proton transport path and mechanism,⁷ furthermore, due to the visual structure of MOF, Density Functional Theory (DFT) is recently used to analyse the factors that affect proton conduction from a theoretical perspective, thus providing strong support for the experimental results.⁸ Multi-carboxylate ligands usually exhibit versatile coordination modes and strong complexing ability to metal ions. Moreover, the hydrophilic –COOH groups not only donate protons but also facilitate the formation of continuous hydrogen bond channel with water molecules. Simultaneously, selecting lanthanide metal ions as nodes in the construction of MOF, more water molecules tend to be bound by Ln(iii) ions, leading to an increase in the concentration of proton carrier, which would be beneficial for the effective proton transport. Therefore, the carboxylate-bridged Ln(iii)-MOF are good candidates for proton conduction.⁹ Currently, there are several proton conductive MOF materials, such as {H[(N(Me)₄)₂][Gd₃(NIPA)₆]}·3H₂O (σ = 7.17 × 10⁻² S cm⁻¹, 75 °C, 98% RH),¹⁰ Na₂[Eu(SBBA)₂(FA)]·0.375DMF·0.4H₂O (σ = 2.91 × 10⁻² S cm⁻¹, 90 °C, 90% RH)¹¹ and {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n (σ = 2.57 × 10⁻² S cm⁻¹, 60 °C, 98% RH)¹² that showing ultra-high conductivities (>10⁻² S cm⁻¹). These superprotonic conductors provided advantageous supports for the assembly strategies involved Ln(iii) ion and carboxylate ligand. In this work, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H₆TTHA) and Eu(NO₃)₃·6H₂O were assembled at 140 °C for 72 h through solvothermal reaction to afforded colourless crystals, namely {[Eu₂(TTHA)(H₂O)₄]·9H₂O}_n (1). This complex has been previously reported by Wu and co-workers.¹³ In their work, the thermal stability and fluorescence properties of 1 were mainly focused. Research suggested that the complex 1 maintained structural stability until 400 °C and demonstrated strong fluorescent emission with high quantum yields (Φ > 70%), treating as a good candidate for light applications. To the best of our knowledge, MOFs usually exhibit a variety of potential applications for their structural diversity.¹⁴ For different researchers, their concerns about the applications of MOF may vary, but it is the continuously exploration and excavation of different performance that will enrich their potentials and meet them in different fields of the applications. Through careful structural analysis, we found that there is a rare infinite water cluster ((H₂O)_n) existing in the crystal structure of 1 (Fig. S1^†), (H₂O)_n further interacts with –COO⁻ groups to form an abundant hydrogen bond network (Fig. S2 and Table S1^†). The stability of (H₂O)_n as well as more complex hydrogen bond formed between (H₂O)_n and –COO⁻ groups has been confirmed by the density functional theory (DFT) calculations. The advantageous structural features including high concentration of water molecules and stable hydration channel provide the possibility to realize high proton conductivity of 1. Therefore, the proton conductivities of 1 under varying conditions were investigated in detail.Complex 1 crystallizes in the monoclinic space group C2/c, with the asymmetric building unit composed of two Eu(iii) ions, one [TTHA]⁶⁻ anion, four coordination water molecules and nine lattice water molecules. The Eu(iii) atom is distorted enneahedron coordinated by seven carboxylate oxygen atoms and two water molecules (Fig. 1a and Table S2^†). The bond length of Eu–O is in the range of 2.374(5)–2.606(5) Å (Table S3^†), comparable to that of the Eu(iii) complex reported in the literature.¹⁵ The coordination mode of [TTHA]⁶⁻ can be described as μ₆-η²η¹η¹η¹η¹η¹η¹η¹η¹η¹η¹η². In the complex 1, the adjacent metal ions were connected through O–C–O and μ–O bridging, forming a dimer, [Eu1]₂. The dimer acts as a linker and connects with four [TTHA]⁶⁻ (Fig. 1b). Furthermore, the [TTHA]⁶⁻ anions coordinate with [Eu1]₂ through six flexible arms in different directions, leading to the formation of a three-dimensional network structure, where the cavities with regular size of 8.356 × 10.678 Ų are left (Fig. 1c and Fig. S3^†). The topological representation of the network of 1 was analysed by using TOPOS software.¹⁶ As shown in Fig. 1d, the Eu(iii) ions are connected to four [TTHA]⁶⁻, which can be considered as 4-connected nodes. And the [TTHA]⁶⁻ anions were also viewed as 4-connected nodes for their connections with four Eu(iii) ions. So, the whole 3D structure was described as a 4,4-c net with an extended Schläfli symbol of {4²,8⁴}.Open in a separate windowFig. 1Coordination mode of the [TTHA]⁶⁻ in 1 showing [EuO₉] enneahedron (a). The dimer, [Eu1]₂, formed by O–C–O and μ–O bridging, connects with four [TTHA]⁶⁻ (b). The 3D structure of 1 formed by the coordination of Eu(iii) and [TTHA]⁶⁻ as well as water molecules (c). Topological representation of the network of 1 (d). Symmetry codes (i: 1.5 − x, 1.5 − y, 1 − z; ii: 0.5 + x, 1.5 − y, −0.5 + z; iii: x, 2 − y, −0.5 + z; iv: 2 − x, 2 − y, 1 − z; v: 2 − x, y, 0.5 − z; vi: 1 − x, y, 1.5 − z; vii: 0.5 + x, 0.5 + y, z).In 1, the theoretical hydrogen bonding calculations of (H₂O)_n and complex cluster were performed using the Gaussian 09 program. All the structures were obtained from the analysis of XRD results and the hydrogen atoms are optimized. We calculated the energy at DFT level by means of B3LYP-D3.^17a As polarity of molecule has great influence on intermolecular hydrogen bonding,^17b hydrogen bond-forming orbitals require larger space occupation.^17c Thus, diffuse and polarization functions augmented split valence 6-311+G(d,p) basis set is used. The binding energy (E_binding) is calculated as the difference between the energy of hydrogen-bonded cluster and the summation of the energies of each component monomer: E_binding = E_tol − ∑N_iE_iE_tol and E_i are energy of hydrogen-bonded cluster and each individual component monomer, respectively. A hydrogen-bonded cluster is more stable if interaction energy is more negative compared to other hydrogen-bonded configurations. With the help of density functional theory (DFT), we calculate the binding energies (E_binding) to compare the stability of systems. The binding energy of water cluster and complex cluster is −619.65 and −710.34 kcal mol⁻¹ (Fig. 2), respectively, indicating the complex cluster system is more stable.Open in a separate windowFig. 2The structures of water cluster and complex cluster. Oxygen, hydrogen, carbon, nitrogen atoms are marked by red, white, cyan, blue, respectively.The PXRD patterns of 1 were shown in Fig. S4.^† It was found that the diffraction peaks of powder sample are in good agreement with the simulated data from single-crystal diffraction, showing the high purity of the synthesized sample. The IR spectrum of 1 exhibits a strong peak at 3422 cm⁻¹, which corresponds to the stretching vibration of water molecules.^18a The absorption peaks appeared at 1551 cm⁻¹ and 1400 cm⁻¹ are attributed to the antisymmetric stretching of –COO⁻ groups^18b (Fig. S5^†). The water adsorption property of 1 was investigated at 25 °C by DVS Intrinsic Plus. Before the measurement, the sample was treated under 0% RH for 6 h (Fig. S6^†). Water adsorption and desorption isotherms of the fully dehydrated sample were shown in Fig. S7.^† The adsorption process in the RH range of 0–95% can be divided into three stages. In the initial stage (0–10%), the adsorption of water molecules increased rapidly, which can be attributed to the hydrogen bond interaction between carboxylic acid oxygen atoms and water molecules. Then the water adsorption increased slowly at 10–70% RH, corresponding to the formation of water clusters. Another abrupt increase of water adsorption was found when the RH is above 70%, illustrating that enough energy is needed for the water clusters to exist in the cavity of the crystal.¹⁹ Clearly, large hysteresis was observed in the adsorption–desorption isotherms, this phenomenon was caused by the strong hydrophilic of –COO⁻ groups in 1.²⁰ Furthermore, the structural integrity of the sample after adsorption/desorption cycle was confirmed by PXRD (Fig. S4^†).Based on the previous structural analysis, the proton conduction of 1 was evaluated by the alternating-current (AC) impedance analyses. The Nyquist plots of 1 obtained at different temperature and relative humidity are shown in Fig. 3a and b and Fig. 3d. The resistance is estimated from the intercept of spikes or arcs on the Z′ axis, and the conductivity (σ) is calculated by the equation of σ = l/(A·R), where l, A and R represent the sample thickness, surface area and resistance, respectively. It was found that there are two different modes observed from the impedance spectroscopies under lower relative humidity (60–90% RH), a partial arc at high frequency component can be attributed to the grain interior contribution, while a characteristic spur at low frequency component illustrates that partial-blocking electrode response allows limited diffusion.²¹ So, the only spikes displayed in the Nyquist spectra at 98% RH and 293–353 K suggest that high temperature and high relative humidity are more favourable for the proton conduction. From the temperature-dependent measurements under 98% RH, significantly, the conductivity of 1 increases gradually from 1.34 × 10⁻⁴ S cm⁻¹ at 293 K to 3.5 × 10⁻³ S cm⁻¹ at 353 K (Fig. 3c and Table S4^†). The increasing conductivity can be attributed to the important role of water molecule. The high concentration of water molecules act as carriers and transmit in the form of H⁺(H₂O)_n, and the mobility of H⁺(H₂O)_n accelerates with the rising temperature. Moreover, the higher acidity of water molecules at higher temperature is more conducive to the improvement of proton conductivity. The relative humidity dependence measured at 298 K indicated that the conductivity of 1 presented significant positive correlations with the humidity changes. The conductivity is 1.42 × 10⁻⁵ S cm⁻¹ at 60% RH and increases to be 1.63 × 10⁻⁴ S cm⁻¹ at 98% RH (Fig. 3e and Table S5^†). This can be explained by the ability of (H₂O)_n to bind water molecules and strong hydrophilic of –COO⁻ group that has been confirmed by the water adsorption process, especially when the RH is above 60%. For water-mediated proton conductors, the lower RH usually results in the insufficient of transport media and further affects the diffusion of protons. At present, the theoretical simulations (e.g. aMS-EVB3)²² and activation energy (E_a)^23–27 are the main methods to analysis the proton conduction mechanism. Compared with the theoretical calculations, the judgment rule with E_a is more straightforward. Here, the E_a of 1 determined from the linear fit of ln(σT) vs. 1000/T is 0.44 eV (Fig. 3f), which reveals that the proton transfer in 1 follows a typical vehicle mechanism.¹² Further evaluate the long-term stability of 1, the time-dependent proton conductivity has been conducted, indicating negligible decline of proton conductivities even lasted 12 h (Fig. 4, S8 and Table S6^†). The sample of 1 after property measurements was collected and characterized by PXRD to examine any structural change, and the PXRD spectrum shows structural integrity even at high temperature and high relative humidity environment (Fig. S4^†). The long-term stable proton conductivities of 1 can be attributed to the robust hydrogen bonding channel that has been confirmed by the DFT calculations. In recent years, the proton conductive carboxylate-based MOF have been systematic reviewed by G. Li’ group,⁹ it was found that the complex 1 shows higher conductivity of 3.5 × 10⁻³ S cm⁻¹ under 353 K and 98% RH when compared to the Ln(iii)-MOF materials, such as [Me₂NH₂][Eu(ox)₂(H₂O)]·3H₂O (σ = 2.73 × 10⁻³ S cm⁻¹, 95% RH, 55 °C),²³ {[Gd(ma)(ox)(H₂O)]_n·3H₂O} (σ = 4.7 × 10⁻⁴ S cm⁻¹, 95% RH, 80 °C),²⁴ (N₂H₅)[Nd₂(ox)₄(N₂H₅)]·4H₂O (σ = 2.7 × 10⁻³ S cm⁻¹, 100% RH, 25 °C),²⁵ {[SmK(BPDSDC)(DMF)(H₂O)]·x(solvent)}_n (σ = 1.11 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁶ [Nd(mpca)₂Nd(H₂O)₆Mo(CN)₈]·nH₂O (σ = 2.8 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁷ MFM-550(M) and MFM-555(M) (M = La, Ce, Nd, Sm, Gd, Ho) (σ = 1.46 × 10⁻⁶ to 2.97 × 10⁻⁴ S cm⁻¹, 99% RH, 20 °C)²⁸ as well as other conductive materials showing lower conductivities in the range of 10⁻⁹ to 10⁻⁵ S cm⁻¹.⁹ However, the conductivity of 1 is inferior to those Ln(iii)-MOFs with conductivities higher than 10⁻² S cm⁻¹ × ^10–12 In recent years, another two H₆TTHA-derived MOF and CP, {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n¹² and {[Co₃(H₃TTHA)₂(4,4′-bipy)₅(H₂O)₈]·12H₂O}_n^19b have been previously reported by our group, which show highest proton conductivities of 2.57 × 10⁻² S cm⁻¹ at 60 °C and 8.79 × 10⁻⁴ S cm⁻¹ at 80 °C under 98% RH, respectively. The noticeable performance difference between these two complexes and 1 was analysed based on the visual structures. The higher conductivity of 1 when compared to the Co(ii) complex can be attributed to the concentration of water molecules, 23.25% for 1 and 15.92% for the Co(ii) complex. The high concentration of proton carrier in 1 promotes the transfer of protons. Although the water molecular concentration of 1 is higher than that of the Tb(iii) complex, however, the coordination numbers of Ln(iii) ions in the two compounds are different, eight for the Tb(iii) ion and nine for the Eu(iii) ion, respectively. The coordination sites are obviously not satiated, especially for the Tb(iii) complex, the lower coordination number may prone to chelate more water molecules under high relative humidity, leading to the formation of more consecutive hydration channel with TTHA⁶⁻ anions and water molecules, thus accelerating the proton transport. In contrast, the molecular structure of the Eu(iii) compound contains nearly a quarter of water molecules, these water molecules have almost filled the pores, so the smaller pore structure is difficult to accommodate more adsorbed water molecules.Open in a separate windowFig. 3Nyquist plots for proton conductivity of 1 (98% RH) at 293–313 K (a) and 318–353 K (b). Plot of log(σ) vs. T for 1 in the temperature range of 293–353 K (c). Plots of the impedance plane for 1 at different relative humidities and 298 K (d). Humidity dependence of the proton conductivity at 298 K (e). Arrhenius plot of 1 at 98% RH (f).Open in a separate windowFig. 4Time-dependent proton conductivity of 1 at 343 K and 98% RH.In conclusions, a water-mediated proton-conducting Eu(iii)-MOF has been synthesized, displaying a 3D network structure with high concentration water molecules and –COO⁻ groups as well as abundant H-bond networks. Interesting, there is an infinite water cluster of (H₂O)_n existing in the crystal structure of Eu(iii)-MOF, which is rare in the H₆TTHA-derived complexes and even other reported MOF/CPs. Based on this, the density functional theory was conducted to evaluate the stability of water cluster and complex cluster. As expected, the calculated binding energies indicate that the more stable system was formed by (H₂O)_n and –COO⁻ groups, which provides a favourable guarantee for proton conduction. The advantageous structural features of Eu(iii)-MOF result in the realization of comparable proton conductivity of 3.5 × 10⁻³ S cm⁻¹ at 353 K and 98% RH and long-term stability at least 12 h. Additionally, the factors affecting the electrical conductivity of several H₆TTHA-derived MOF/CPs have been compared and analysed from the visual structures, and the structure-activity relationship of such compounds was also summarized, which will provide guidance to design novel crystalline superprotonic conductors assembled from multi-carboxylate.     

In situ electro-organic synthesis of hydroquinone using anisole on MWCNT/Nafion modified electrode surface and its heterogeneous electrocatalytic reduction of toxic Cr(vi) species

Owing to its electro-inactive character, anisole (phenylmethyl ether, PhOCH₃) and its related derivatives have been used as electrolytes in electrochemistry. Herein, we report a simple one-step electro-organic conversion of PhOCH₃ to hydroquinone (HQ) on a pristine-MWCNT–Nafion modified electrode glassy carbon electrode surface, GCE/Nf–MWCNT@HQ, in pH 2 KCl–HCl solution within 15 min of working time. The chemically modified electrode showed a highly redox-active and well-defined signal at an apparent standard electrode potential, E^o′ = 0.45 V vs. Ag/AgCl (A2/C2) with a surface excess value, Γ_HQ = 2.1 × 10⁻⁹ mol cm⁻². The formation of surface-confined HQ is confirmed by collective physicochemical and spectroscopic characterizations using TEM, UV-Vis, Raman, FTIR, NMR and GC-MS techniques and with several control experiments. Consent about the mechanism, the 2.1% of intrinsic iron present in the pristine-MWCNT is involved for specific complexation with oxygen donor organic molecule (PhOCH₃) and hydroxylation in presence of H₂O₂ (nucleophilic attack) for HQ-product formation. The GCE/Nf–MWCNT@HQ showed an excellent heterogeneous-electrocatalytic reduction of Cr(vi) species in acidic solution with a linear calibration plot in a range, 5–500 ppm at an applied potential, 0.4 V vs. Ag/AgCl with a detection limit, 230 ppb (S/N = 3; amperometric i–t). As a proof of concept, selective detection of toxic Cr(vi) content in the tannery-waste water has been demonstrated with a recovery value ∼100%.

MWCNT-surface confined Hydroquinone/quinone redox system is prepared in situ method and used for mediated reduction of Cr(vi) species     

A weaker donor shows higher oxidation state upon aggregation

The charge-transfer between TTFs and I₂ shows that the stronger donor TTF1 is in a cation radical state and the weaker donor TTF2 is neutral in solution, whereas TTF1 exists as a cation radical and TTF2 is dicationic in complexes. The dicationic and neutral states of TTF2 are reversible upon aggregation and solvation.

A weaker donor is dicationic but a stronger donor appears as a cation radical in their CT complexes with iodine.

Charge-transfer (CT) between an electron donor and acceptor plays the pivotal role in supramolecular assembly and creation of conducting materials. There remains a challenge in CT, that is, whether a weaker donor could show a positively charged state higher than a stronger donor through the CT with the same acceptor.Iodine (I₂) can serve as an acceptor to prepare CT complexes. The CT complex perylene–iodine is one of the earliest organic conductors.¹ Upon gaining one electron from a donor molecule, iodine would form polyiodides,² which show diverse structures and have received growing interest in supramolecular architectures and materials science.^3,4 Tetrathiafulvalene (TTF) is an electron donor with three reversible states, (TTF)⁰, (TTF)⁺˙, and (TTF)²⁺.⁵ TTF derivatives (TTFs) have been widely employed as building blocks for functional materials.⁶ The CT complexes of I₂ and TTFs can be prepared by mixing these two species.⁷ Because I₂ is not a strong acceptor, TTFs are mainly in the cation radical or partially charged state in CT complexes.⁸ Ar-S-TTFs are derived from TTF by decorating four arylthio groups onto the peripheral positions (Scheme 1). Ar-S-TTFs can adjust their geometry and electronic state to adapt to a guest molecule,⁹ and they form CT complexes with various acceptors such as fullerene,¹⁰ heteropoly acid,¹¹ and CuBr₂.¹²Open in a separate windowScheme 1Chemical structures of the Ar-S-TTFs reported herein, along with their first (E_1/2¹) and second (E_1/2²) redox potentials in CH₂Cl₂ recorded versus SCE.The structures of polyiodides depend on the nature of the counter cations,^3b and Ar-S-TTFs can modulate the geometry and electronic state according to the guest. Therefore, the CT complex containing these two flexible components seems promising. Being continuous study on Ar-S-TTFs, herein we report the CT between Ar-S-TTFs (TTF1 and TTF2) and I₂. It is found that a weaker donor TTF2 carries the positive charge higher than a stronger donor TTF1 in their CT complexes with I₂. Meanwhile, the iodine atoms form polyiodides with different structures in CT complexes, i.e., the infinite covalent chain of [(I_n)⁻]_∞ in TTF2 complex and 2-D network comprised of (I₃)⁻ and I₂ in TTF1 complex.Electrochemical analysis shows that both TTF1 and TTF2 have two reversible redox potentials. The first redox potential (E_1/2¹) of TTF2 (0.66 V vs. SCE in CH₂Cl₂) is higher than that of TTF1 (0.58 V), and the second redox potentials (E_1/2²) show similar tendency (Scheme 1). Therefore, as donor molecule, TTF2 is weaker than TTF1. Both donors display weak absorption band at 400–500 nm due to the intramolecular CT transition,⁹ whereas the cation radicals of them show broad absorption at 650–1100 nm.¹¹ For example, electrochemical oxidation of TTF1 under constant potential of 0.75 V results in an absorption band in this region as proved by the spectroelectrochemical study (Fig. 1a).Open in a separate windowFig. 1(a) Spectroelectrochemistry of TTF1 in CH₂Cl₂ (c = 5 × 10⁻⁴ mol L⁻¹); (b) UV-Vis absorption spectra and (c) ESR spectra of TTF1 and TTF2 upon adding 3 equivalents of I₂ in CH₂Cl₂ (c = 1 × 10⁻⁵ mol L⁻¹).By mixing TTF1 and I₂ in CH₂Cl₂, an absorption band appears at 650–1100 nm (Fig. 1b), which is identical to that observed in the spectroelectrochemistry. The mixture of TTF1 and I₂ in CH₂Cl₂ shows ESR signal with g = 2.006 (Fig. 1c). Therefore, the CT occurs between TTF1 and I₂ in CH₂Cl₂ solution, and TTF1 is at the cation radical state. While CT occurs between TTF1 and I₂ in CH₂Cl₂, the thin layer chromatography reveals that the neutral TTF1 remains in solution even though excess I₂ is added (>3 equiv.); this means I₂ cannot completely transform TTF1 into cation radical. On the other hand, there is no CT between TTF2 and I₂ in CH₂Cl₂ solution, because the absorbance of (TTF2)⁺˙ is not observed (Fig. 1b) and the mixture of TTF2 and I₂ is ESR inactive (Fig. 1c).Although TTF1 and TTF2 exhibit the different behaviors upon mixing with I₂ in CH₂Cl₂, they both afford CT complexes with I₂. The CT complexes are obtained as black block-like single crystals by evaporating the CH₂Cl₂/n-hexane (v/v, 1 : 1) solution of mixture of TTF1 (or TTF2) and I₂ at room temperature. The compositions of complexes are determined on the basis of single crystal structure analyses to be (TTF1)·(I₃)·(I₂) and (TTF2)·(I₅)·(I₂).(TTF1)·(I₃)·(I₂) crystallizes in the P$\bar{1}$ space group. There are one TTF1 molecule and three pairs of iodine atoms (I1–I2, I3–I4, and I5–I6) in the asymmetric unit. The I3 and I5 locate on the inversion centres. The bond length of central C C (bond a in Scheme 2) on TTF moiety can be used to estimate the charge on TTFs,⁷i.e., 1.34 Å, 1.39 Å, and 1.45 Å respectively for (TTF)⁰, (TTF)⁺˙, and (TTF)²⁺. Referring Fig. 2a, the central C C bond length in TTF1 is 1.39 Å, same to that in (TTF)⁺˙.⁷ The site charge (ρ) on TTF moiety also can be estimated via an empirical formula ρ = 6.347 − 7.436δ,¹³ where δ = (b + c) − (a + d), and a, b, c, and d are bond lengths (Scheme 2). The calculated δ-value of TTF1 is 0.721 Å, which gives the site charge on TTF1 to be +1. The iodine atoms (I1–I6) form three tightly connected units, [I1–I2], [I4–I3–I4], and [I6–I5–I6] (Fig. 2c). The I1–I2 bond length (2.74 Å) is identical to that of neutral I₂ (2.74 Å), and the I–I bond lengths (2.91–2.93 Å) in both [I4–I3–I4] and [I6–I5–I6] are very close to that of triiodide (2.90 Å).¹⁴ Therefore, the [I4–I3–I4] and [I6–I5–I6] units are intrinsic (I₃)⁻. These results indicate that TTF1 is at cation radical state in complex, which is reasonable according to the formation of (TTF1)⁺˙ in solution by mixing TTF1 and I₂.Open in a separate windowScheme 2The bonds (a–d) on Ar-S-TTFs for the estimation of charge ρ.Open in a separate windowFig. 2Crystal structure of complex (TTF1)·(I₃)·(I₂): (a) top view of molecule TTF1 with the central C C bond length shown in unit of Å; (b) TTF1 dimer with atomic short contacts shown in dashed lines (green for S⋯S and grey for C⋯S); (c) anion sheets composed of (I₃)⁻ and I₂ with the I–I bond length and I⋯I contacts (purple dashed lines) shown; (d) packing structure viewed along the longitudinal axis of TTF1 dimer with the I⋯I contacts shown in grey dashed lines.The TTF1 molecules are dimerized in complex (Fig. 2b). Within a dimer, there are S⋯S contacts (3.45–3.53 Å) between TTF cores, and C⋯S contacts (3.42–3.46 Å) between the peripheral sulfur atoms and the phenyls. Meanwhile, the (I₃)⁻ anions and neutral I₂ together form the two-dimensional (2-D) sheet via multiple I⋯I contacts (3.32–3.96 Å). The 2-D sheet is not flat but shows a zig–zag shape along the b-axis direction (Fig. 2d). The dimers of TTF1 are sandwiched by the neighbouring 2-D anion sheets. There are I⋯S contacts (3.69–3.78 Å) between the anion sheets and TTF1 dimers. This type of 2-D polyiodide framework is rare in the CT complexes of TTFs and I₂.¹⁵(TTF2)·(I₅)·(I₂) crystallizes in the C2/c space group. The asymmetric unit contains half of TTF2, three tightly connected iodine atoms (I1, I2, I3) with I3 on the 2-fold screw axis, and one isolated iodine atom (I4) at the general position. Referring Fig. 3a, the central C C bond length (1.45 Å) on TTF moiety is close to that observed in the dicationic salts of Ar-S-TTFs (1.42 Å).¹² The calculated δ value of TTF2 is 0.573 Å, giving the site charge on TTF2 to be +2. These results firmly prove that TTF2 is dicationic in complex, against the neutral state of TTF2 by mixing it with I₂ in CH₂Cl₂. As shown in Fig. 3b, the I4–I4 bond length (2.79 Å) is close to that of I₂ (2.73 Å), thus the (I4)₂ is a neutral I₂. The I1, I2, and I3 atoms form an infinite chain with a periodicity of –[I1–I2–I3–I2–I1]–. Regarding the charge on TTF2, a periodic unit [I1–I2–I3–I2–I1] has a charge of −2. The interatomic distances in [I1–I2–I3–I2–I1] unit vary from 3.04 Å to 3.19 Å, almost identical to those in the infinite polymeric [(I_n)⁻]_∞ (3.02–3.20 Å).^3a Therefore, the present polyiodide chain also would be a [(I_n)⁻]_∞ polymer, and all the iodine atoms in [(I_n)⁻]_∞ are partially charged.^3a The [(I_n)⁻]_∞ chains are connected by (I4)₂ through the I⋯I contacts (3.42 Å) to form a ladder-like structure. The TTF cores and peripheral aryls on TTF2 molecules together form a channel along the longitudinal axis of TTF2 (Fig. 3c), and the channel grows through the C⋯S contacts (3.34–3.48 Å) between the peripheral sulfur atoms and the phenyls. The [(I_n)⁻]_∞ chains penetrate into the channel. It is worth noting that [(I_n)⁻]_∞ chain has not been observed in the complexes comprised of TTFs and polyiodide.Open in a separate windowFig. 3Crystal structures of complex (TTF2)·(I₅)·(I₂): (a) top view of molecule TTF2 with the central C C bond length shown in unit of Å; (b) the (I₃)⁻ anion chain with the I–I bond lengths and I⋯I contacts (purple dashed lines) shown; (c) packing structure projected along the longitudinal axis of the TTF moiety.The charged states of TTF1/TTF2 in CT complexes are further proved by the spectroscopic studies. (TTF1)·(I₃)·(I₂) shows a ESR signal with g = 2.009 and (TTF2)·(I₅)·(I₂) is ESR inactive (Fig. 4a). This is consistent with crystallographic study, i.e., TTF1 and TTF2 are respectively at cation radical and dicationic states. The UV-Vis absorption spectra of both complexes in solid state are distinct from those of neutral TTF1 and TTF2 (Fig. 4b). (TTF1)·(I₃)·(I₂) shows two absorption bands at the low energy region. The band at 800–950 nm that belonging to absorbance of (TTF1)⁺˙. The band at 950–1400 nm ascribable to intermolecular CT transition between the TTF1 cation radicals in a dimer, i.e., (TTF1)⁺˙ + (TTF1)⁺˙ → (TTF1)²⁺ + (TTF1)⁰.^8a The (TTF2)·(I₅)·(I₂) displays very broad absorption at 500–1400 nm, which is distinct from (TTF1)·(I₃)·(I₂).Open in a separate windowFig. 4(a) ESR spectra for the crystalline complexes of (TTF1)·(I₃)·(I₂) and (TTF2)·(I₅)·(I₂); UV-Vis absorption spectra of (TTF1)·(I₃)·(I₂) and (TTF2)·(I₅)·(I₂) in the (b) solid state, and (c) CH₂Cl₂ solution (c = 10⁻⁵ mol L⁻¹) after standing under inert atmosphere for 30 min and/or 24 h.As aforementioned, TTF2 is neutral upon mixing with I₂ in CH₂Cl₂, whereas it is dicationic in (TTF2)·(I₅)·(I₂). Moreover, TTF2 is a donor weaker than TTF1, but it shows higher oxidation state in complex. This is against to the criteria for CT between TTF and acceptor, say, the charge on TTF in CT complex depends on the oxidation potential (E^ox_D) of TTF and the reduction potential (E^red_A) of acceptor.¹⁶ The TTF would be neutral, cation radical, and partially charged under the condition of E^ox_D − E^red_A > 0.34 V, E^ox_D − E^red_A < −0.02 V, and −0.02 V < E^ox_D − E^red_A < 0.34 V, respectively. In the present case, the E^red_A of TTF2 is 0.69 V and the E^red_A of I₂ is 0.53 V (Fig. S4 in ESI^†). Therefore, TTF2 would be partially charged in CT complex. One may concern that the increment of charge transfer degree between I₂ and TTF2 in (TTF2)·(I₅)·(I₂) would be attributed to the aggregation of donor and acceptor.In this regard, the absorption spectra of complexes are studied by dissolving them in CH₂Cl₂. (TTF1)·(I₃)·(I₂) shows characteristic absorbance of (TTF1)⁺˙ in CH₂Cl₂ (Fig. 4c), therefore the charged state of TTF1 remain the same in solution and CT complex. On the other hand, the charge on TTF2 is distinctly variated by dissolving (TTF2)·(I₅)·(I₂) in CH₂Cl₂. The TTF2 is reduced from (TTF2)²⁺ to (TTF2)⁺˙ in 30 min as proved by an absorption band at 700–1050 nm. And, the (TTF2)⁺˙ disappears to give neutral TTF2 when the solution is kept for 24 h under inert atmosphere. This means that the retro CT occurs from [(I_n)⁻]_∞ to (TTF2)²⁺ upon dissociation of (TTF2)·(I₅)·(I₂), and both anionic and cationic components return to the neutral state. Moreover, the absorbance of (TTF2)·(I₅)·(I₂) can be restored by evaporating the solution to gain solid complex. This process, exchanging the dicationic and neutral states of TTF2, is thus reversible upon aggregation and solvation of complex as shown in Scheme 3. These results prove that the dicationic state of TTF2 in CT complex comes from the aggregation of donor and acceptor.Open in a separate windowScheme 3Reversible process upon aggregation and solvation of (TTF2)·(I₅)·(I₂)In summary, the CT between TTF1/TTF2 and I₂ is studied in both solution and solid state. The stronger donor TTF1 turns into cation radical and the weaker donor TTF2 remains neutral upon mixing with I₂ in solution. On the other hand, TTF2 shows an oxidation state (dicationic) higher than that of TTF1 (cation radical) in their CT complexes, which is unusual for CT between TTFs and acceptors. The high oxidation state of TTF2 in complex is due to the aggregation of donor and acceptor. The dicationic and neutral states of TTF2 are reversible upon aggregation and solvation of CT complex. Moreover, the structures of polyiodides in CT complexes can be finely tuned by varying the aryls on Ar-S-TTFs, to give infinite [(I_n)⁻]_∞ and 2-D network comprised of (I₃)⁻ and I₂. Along with previous report, this work further indicates that Ar-S-TTFs show unique feature, i.e., self-modulation of electronic states and molecular geometries according to guest molecules.     

Chemically and thermally activated persulfate for theophylline degradation and application to pharmaceutical factory effluent

Degradation of PPCPs by AOPs has gained major interest in the past decade. In this work, theophylline (TP) oxidation was studied in thermally (TAP) and chemically (CAP) activated persulfate systems, separately and in combination (TCAP). For [TP]₀ = 10 mg L⁻¹, (i) TAP resulted in 60% TP degradation at [PS]₀ = 5 mM and T = 60 °C after 60 min of reaction and (ii) CAP showed slight degradation at room temperature; however, (iii) TCAP resulted in complete TP degradation for [PS]₀ = [Fe²⁺]₀ = 2 mM at T = 60 °C following a pseudo-first order reaction rate with calculated k_obs = 5.6 (±0.4) × 10⁻² min⁻¹. In the TCAP system, the [PS]₀ : [Fe²⁺]₀ ratio of 1 : 1 presented the best results. A positive correlation was obtained between the TP degradation rate and increasing temperature and [PS]₀, and a negative correlation was obtained with increasing pH. Both chloride and humic acid inhibited the degradation process, while nitrates enhanced it. TP dissolved in spring, sea and waste water simulating real effluents showed lower degradation rates than in DI water. Waste water caused the highest inhibition (k_obs = 2.6 (±0.6) × 10⁻⁴ min⁻¹). Finally, the TCAP system was tested on a real factory effluent highly charged with TP, e.g. [TP]₀ = 160 mg L⁻¹, with successful degradation under the conditions of 60 °C and [PS]₀ = [Fe²⁺]₀ = 50 mM.

Chemically activated persulfate in heated medium showed synergistic effect toward full degradation of theophylline in industrial factory effluents. This makes such AOP a well-adapted technology to treat highly concentrated hazardous pharmaceuticals.     

Photocatalysis,photoinduced enhanced anti-bacterial functions and development of a selective m-tolyl hydrazine sensor based on mixed Ag·NiMn2O4 nanomaterials

In this work, a tri-metal based nanocomposite was synthesized and characterized. A detailed investigation of the photocatalytic dye degradation efficiency of the nanocomposite under visible light showed promising results in a wide pH range, both acidic and basic medium. Studies on anti-bacterial activity against seven pathogenic bacteria, including both Gram positive and Gram negative species, were conducted in the presence and absence of light and compared with the standard antibiotic gentamicin. The minimum inhibitory concentration (MIC) values of Ag·NiMn₂O₄ against multidrug-resistant (MDR) pathogens ranged from 0.008 to 0.65 μg μL⁻¹, while the minimum bactericidal concentration (MBC) was found to be 0.0016 μg μL⁻¹. The nanomaterial, Ag·NiMn₂O₄ was deposited onto the surface of a glassy carbon electrode (GCE; 0.0316 cm²) as a thin film to fabricate the chemical sensor probe. The proposed sensor showed linear current (vs. concentration) response to m-THyd (m-tolyl hydrazine) from 1.0 pM to 0.01 mM, which is denoted as the linear dynamic range (LDR). The estimated sensitivity and detection limit of the m-THyd sensor were found to be 47.275 μA μM⁻¹ cm⁻² and 0.97 ± 0.05 pM, respectively. As a potential sensor, it is reliable due to its good reproducibility, rapid response, higher sensitivity, working stability for long duration and efficiency in the analysis of real environmental samples.

Photocatalytic dye degradation efficiency of Ag·NiMn₂O₄ at pH 4 was 91%; at pH 9, 77% and 95% in presence of H₂O₂ and at pH 7, 50%. Assembled Ag·NiMn₂O₄ nanomaterials/binder/GCE, as m-THyd sensor showed considerable sensitivity, DL, LDR, response time, reproducibility etc.     

Supramolecular interaction of sanguinarine dye with sulfobutylether-β-cyclodextrin: modulation of the photophysical properties and antibacterial activity

The noncovalent host–guest interaction of sanguinarine (SGR), a benzophenanthridine alkaloid, with a nontoxic, water soluble sulfobutylether-β-cyclodextrin (SBE₇βCD, commercially available as Captisol) macrocyclic host has been investigated using ground-state optical absorption, and steady-state and time-resolved fluorescence measurements. The pH-dependent changes in the absorbance of the dye at 327 nm showed a pK_a value of 7.5, which has been shifted to 8.1 in the presence of SBE₇βCD. The changes in the pK_a values, absorption and fluorescence spectra, and fluorescence lifetime values of these two forms of SG with SBE₇βCD indicate complex formation between them. The cationic form shows 3 times higher interaction towards SEB₇βCD (K = 1.2 × 10⁴ M⁻¹) as compared to the neutral form (K = 3.9 × 10³ M⁻¹) which leads to a moderate upward pK_a shift (pK_a values of SGR shifted by more than 0.6 units). The subsequent fluorescence “turn off” was demonstrated to be responsive to chemical stimuli, such as metal ions (Ca²⁺ ions). Upon addition of Ca²⁺ ions, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence “turn on”. Apart from improving the stability under ambient light conditions, the upward pK_a shift of SGR in the presence of SBE₇βCD results in increasing the antibacterial activity of the SBE₇βCD:SGR complex compared to that of the free dye towards four pathogenic micro-organisms at the physiological pH range. This work further compares SGR interaction with parent β-cyclodextrin.

The noncovalent host-guest interaction of sanguinarine (SGR) with a nontoxic, water soluble sulfobutylether-beta-cyclodextrin macrocyclic host modulates the photophysical properties, improves the photostability and antibacterial activity of SGR.     

Synergetic effect in water treatment with mesoporous TiO2/BDD hybrid electrode

Boron-doped diamond (BDD) electrodes have a wide potential window and can produce ozone by water electrolysis at high voltage. Though ozone has strong oxidative power (standard oxidation potential: 2.07 V vs. NHE), it cannot decompose certain types of recalcitrant organic matter completely. We developed an advanced oxidation process (AOP), in which hydroxy radicals with stronger oxidative power (standard oxidation potential: 2.85 V vs. NHE) are formed using a combination of ozone, photocatalyst, and UV. In this study, we fabricated a mesoporous TiO₂/BDD hybrid electrode and examined its potential for AOPs. A synergetic effect between electrochemical water treatment and photocatalytic water treatment was observed with the hybrid electrode that did not occur with the BDD electrode.

A mesoporous TiO₂/BDD hybrid electrode showed a synergetic effect between electrochemical water treatment and photocatalytic water treatment.