A new charge-neutral Ru(III) complex RuL(pic)3 (1) (H3L = 3,6-di-tert-butyl-9H-carbazole-1,8-dicarboxylic acid, pic = 4-picoline) was synthesized and fully characterized. This complex promoted chemical and photochemical water oxidation efficiently with turnover frequencies of 0.28 s−1 and 5 min−1, respectively. In particular, for photochemical water oxidation, complex 1 showed excellent stability and good activity. The X-ray crystal structure, electrochemical results, and the detection of the RuIV–OH intermediate by high-resolution mass spectrometry revealed that complex 1 exchanged its 4-picoline ligand with water at the RuIII state to form the authentic water oxidation catalyst. The kinetics studies suggested a reaction mechanism involving nucleophilic attack by a water molecule.

Adsorption state of catalyst on photoanode is an important factor on influencing the performance of dye-sensitized photoelectrochemical cells (DS-PECs) for water splitting. Photoanode TiO2(1 + 2) was assembled with Ru(bpy)3 phosphoric acid derivative (complex 1) as photosensitizer and complex 2 as water oxidation catalyst to compare with photoanode TiO2(1 + 3). The photocurrent density of photoanode TiO2(1 + 3) with catalyst 3 synthesized with only one end fixing on the surface of TiO2 is about four-fold of the photoanode assembled with catalyst 2 fixing with two claws on the surface of TiO2. The phenomenon should be caused by the littery arrangement and shorter distance of catalyst 2 from the active center of catalyst to TiO2 on the surface of semiconductor which led to lowly efficient electron transfer.Photoanodes TiO2(1 + 2) and TiO2(1 + 3) were assembled with complex 1 as the photosensitizer and complexes 2 and 3 as the water oxidation catalysts to investigate the impact of adsorption state of the catalysts on the performances of photoelectrochemical water splitting of DS-PECs.Download high-res image (148KB)Download full-size image

The oxygen evolution reaction (OER) is a key step in the overall water splitting process. Numerous electrocatalysts have been developed to lower the overpotential and accelerate the kinetics of the OER. In this work, a simple soaking and heating treatment was used to form a stable and efficient FexNi1−xOy/CP electrode. The electrode combined nickel and iron oxides on a commercial carbon paper were used for electrocatalytic water oxidation. The best FexNi1−xOy/CP electrode (Ni/Fe = 15/1) displayed a current density of 10 mA/cm2 at a low overpotential of 290 mV in 0.1 M KOH solution with a Tafel slope of 52 mV/dec. A higher current density of ∼50 mA/cm2 at the same overpotential and a lower Tafel slope of 43 mV/dec was obtained for this electrode in 1.0 M KOH solution. Excellent durability of the FexNi1−xOy/CP electrode in 1.0 M KOH solution was confirmed under a high current density of 136 mA/cm2 at an overpotential of 340 mV.A stable and efficient FexNi1−xOy/CP electrode was fabricated for OER and displayed a current density of ∼50 mA/cm2 at over potential of 290 mV and a Tafel slope of 43 mV/dec in 1.0 M KOH solution.Download high-res image (179KB)Download full-size image

Dye-sensitized photoelectrochemical cell (DS-PEC) is an especially attractive method to generate hydrogen via visible light driven water splitting. Electrolyte, an essential component of DS-PEC, plays a great role in determining the photoactivities of devices for water splitting. When using phosphate buffer (pH = 6.4) as electrolyte, the DS-PEC displayed much higher photoactivity than using 0.1 M Na2SO4 (pH = 6.4) as electrolyte. The insight is phosphate anion gathers together to form a negative electrostatic field on TiO2 surface, which increases the resistance in the TiO2/catalyst and electrolyte interface and validly reduces the charge recombination from TiO2 to the oxidized catalyst.Download high-res image (175KB)Download full-size imagePhosphate anions form a negative electrostatic field on TiO2 surface, which increases the resistance in TiO2/catalyst interface and validly reduces the charge recombination from TiO2 to oxidized catalyst.

•A novel [Ru(bpy)2(tpphz)](PF6)2 complex (PS1) was synthesized and characterized fully.•PS1 displayed a high photoactivity for water oxidation in a three-component system with Ru(bda)(isoq)2 as catalyst.•The π-π stacking supramolecular interaction promoted the molecular aggregation and valid electronic transmission.As we all know, the photosensitizer is the vital role in photocatalytic water oxidation system. In this paper, a novel [Ru(bpy)2(tpphz)](PF6)2 complex (PS1) was synthesized and characterized fully. And, it displayed a higher photoactivity (TON, 110) than [Ru(bpy)3](PF6)2 (PS2) (TON, 62) and [Ru(bpy)2(dcb)](PF6)2 (PS3) (TON, 82) for water oxidation in a three-component system with Ru(bda)(isoq)2 as catalyst. The main reason for the enhancement of photocatalytic activity should be that the π-π stacking supramolecular interaction between the π-tpphz ligand of the PS1 and the π-isoq ligand of the catalyst increased the intermolecular interaction between PS1 and catalyst, and promoted the molecular aggregation and accelerated valid electronic transmission. Nevertheless, a modest driving force of PS1 for photocatalysis water oxidation should be another contribution.A novel [Ru(bpy)2(tpphz)](PF6)2 complex (PS1) was synthesized and displayed a higher photoactivity than [Ru(bpy)3](PF6)2 and [Ru(bpy)2(dcb)](PF6)2 for water oxidation with Ru(bda)(isoq)2 as catalyst. The main reason for the attractive photocatalytic activity should be that the π-π stacking supramolecular interaction between the π-tpphz ligand of the PS1 and the π-isoq ligand of the catalyst increased the intermolecular interaction between PS1 and catalyst, and promoted the molecular aggregation and accelerated valid electronic transmission.Download high-res image (159KB)Download full-size image

Water splitting is deemed as an effective pathway for producing ideal clean energy, such as hydrogen. Here, a copper oxide film (Cu-Tris film) was prepared in-situ from a 0.2 M phosphate buffer solution (pH = 12.0) containing 1.0 mM Cu2+ and 2.0 mM Tris via controlled-potential electrodeposition. The Cu-Tris film showed a significantly low overpotential of 390 mV at a current density of 1.0 mA/cm2 for electrocatalytic water oxidation. Simultaneously, a considerably low Tafel slope of 41 mV/decade was achieved. This Cu-Tris film also exhibited a high and stable current density of ca. 7.5 mA/cm2 at 1.15 V vs. NHE for long-term electrocatalysis (10 h). These results demonstrated the superior performance of the developed Cu-Tris film, which should be attributed to the regulating effect of the five coordinated planar structure of the Cu-Tris complex precursor during the process of electrodeposition.

•A PMMA oligomer coating photoanode for light driven water splitting.•The PMMA overlayer was optimized as immersing 2 wt% PMMA in DCM for 15 s.•The DS-PEC displayed better long-term stability with the decorated photoanode.A PMMA (polymethly methacrylate) oligomer coating photoanode sensitized with the Ru(bpy)3 phosphoric acid derivative 1 as photosensitizer and complex 2 as water oxidation catalyst was assembled for light driven water splitting. A simple and effective method was optimized by immersing the sensitized photoanode into the DCM solution dissolving 2 wt% PMMA for 15 s. With the decorated photoanode as working electrode, the DS-PEC displayed much better long-term stability, the photocurrent density of which was found to be 0.57 mA/cm2 after 200 s sustaining light illumination and is four times as high as the undecorated one (0.15 mA/cm2).

An efficient dye (1) sensitized photoelectrochemical cell (DS-PEC) has been assembled with a silicon compound (3-chloropropyl)trimethoxy-silane (Si-Cl) decorated working electrode (WE) TiO2(1 + 2). The introduction of this Si-Cl molecule on photoanode leads to better performances on efficiency than untreated ones for light driven water splitting. The firm Si-O layer formed on TiO2 increased the resistance of the TiO2/catalyst interface which is assumed to decrease charge recombination from TiO2 to the oxidized catalyst 2. The work presented here provides an effective method to improve the performances of DS-PECs.Figure optionsDownload full-size imageDownload as PowerPoint slide

•Development of a dinuclear ruthenium complex for probing catalytic water oxidation•Kinetic studies revealed a first-order reaction mechanism.•The results implied a water nucleophilic attack pathway for OO bond formation.A new dinuclear ruthenium complex 1 (L1[Ru(bda)2(picoline)]2) based on Ru–bda (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) and dipyridyl xanthene (L1 = 4,5-dipyridyl-2,7-di-tert-butyl-9,9-dimethyl xanthene) ligand was synthesized to probe the catalytic oxidation of water. An oxygen evolution experiment displays a low catalytic water oxidation activity with a first-order reaction kinetic mechanism. The result indicates that the OO bond formation of the dinuclear catalyst 1 follows a water nucleophilic attack pathway rather than a radical coupling pathway. The most plausible interpretation is that the steric hindrance effects of the L1 and bda ligands lead to a disadvantage in forming the face-to-face configuration of the two active sites in a one dimer molecule.A new dinuclear ruthenium complex 1 was synthesized to probe the catalytic oxidation of water. An oxygen evolution experiment displays a low catalytic water oxidation activity with a first-order reaction kinetic mechanism. The result indicates that the OO bond formation of the dinuclear catalyst 1 follows a water nucleophilic attack pathway rather than a radical coupling pathway.

By using a supramolecular self-assembly method, a functional water splitting device based on a photoactive anode TiO2(1+2) has been successfully assembled with a molecular photosensitizer 1 and a molecular catalyst 2 connected by coordination of 1 and 2 with Zr4+ ions on the surface of nanostructured TiO2. On the basis of this photoanode in a three-electrode photoelectrochemical cell, a maximal incident photon to current conversion efficiency of 4.1% at ∼450 nm and a photocurrent density of ∼0.48 mA cm–2 were successfully obtainedKeywords: molecular catalyst; photoanode; visible light; water oxidation; water splitting

An efficient two-electrode molecular PEC was assembled, in which a photoanode was constructed using a co-adsorbed method with a molecular photosensitizer (PS) 1 and a molecular catalyst 2 on TiO2-sintered FTO electrode (TiO2(1 + 2)). Without applied bias against a reference electrode, the system achieves remarkable photocurrent densities and carries out light driven water oxidation as evidenced by Clark electrode measurements in solution. A photocurrent density of 70 μA/cm2 has been obtained within 10 s illumination time, and a TON of about 220 was obtained with a maximum turnover frequency (TOF) of ca. 4 min−1 within the initial 5 minutes illumination duration.Schematic illustration of the two-electrode PEC with a photoanode co-adsorbed with a PS 1 and a molecular catalyst 2 on nano-structured TiO2 and a passive Pt cathode.

Photoactive anodes consisting of Ru(bpy)3 type photosensitizer 1 and molecular catalysts 2 and 3 on nanostructured TiO2 have been assembled in functional devices for successful light driven water splitting. From their performance measurements we found that the photoanode TiO2(1 + 3) in which the molecular ruthenium catalyst and the phosphonate anchoring group are linked by a flexible –CH2CH2CH2– chain showed a significantly higher photocurrent density than the photoanode TiO2(1 + 2) with only –CH2– linkage. The possible reasons for the different water splitting performance of otherwise identical devices are discussed.

A molecular water oxidation catalyst (2) has been synthesized and immobilized together with a molecular photosensitizer (1) on nanostructured TiO2 particles on FTO conducting glass, forming a photoactive anode (TiO2(1+2)). By using the TiO2(1+2) as working electrode in a three-electrode photoelectrochemical cell (PEC), visible light driven water splitting has been successfully demonstrated in a phosphate buffer solution (pH 6.8), with oxygen and hydrogen bubbles evolved respectively from the working electrode and counter electrode. By applying 0.2 V external bias vs NHE, a high photocurrent density of more than 1.7 mA·cm–2 has been achieved. This value is higher than any PEC devices with molecular components reported in literature.

A new charge-neutral Ru(III) complex RuL(pic)3 (1) (H3L = 3,6-di-tert-butyl-9H-carbazole-1,8-dicarboxylic acid, pic = 4-picoline) was synthesized and fully characterized. This complex promoted chemical and photochemical water oxidation efficiently with turnover frequencies of 0.28 s−1 and 5 min−1, respectively. In particular, for photochemical water oxidation, complex 1 showed excellent stability and good activity. The X-ray crystal structure, electrochemical results, and the detection of the RuIV–OH intermediate by high-resolution mass spectrometry revealed that complex 1 exchanged its 4-picoline ligand with water at the RuIII state to form the authentic water oxidation catalyst. The kinetics studies suggested a reaction mechanism involving nucleophilic attack by a water molecule.

Two new mononuclear Ru complexes RuII(bipa)(pic)3 (1; H2bipa = 6-(1H-benzo[d]imidazol-2-yl)picolinic acid, pic = 4-picoline) and RuII(pbic)(pic)3 (2; H2pbic = 2-(pyridin-2-yl)-1H-benzo[d]imidazole-7-carboxylic acid, pic = 4-picoline) based on anionic ligands were successfully synthesized, and characterized using NMR spectroscopy, mass spectrometry, and X-ray crystallography. These catalysts showed high activities and stabilities in water oxidation in homogeneous systems with a high turnover number of 2100 and a turnover frequency of 0.21 s−1 for complex 1. The O–O band formation mechanism involved water nucleophilic attack. An active catalytic intermediate, i.e., RuIV–OH, was detected using high-resolution mass spectrometry.

Photoactive anodes consisting of Ru(bpy)3 type photosensitizer 1 and molecular catalysts 2 and 3 on nanostructured TiO2 have been assembled in functional devices for successful light driven water splitting. From their performance measurements we found that the photoanode TiO2(1 + 3) in which the molecular ruthenium catalyst and the phosphonate anchoring group are linked by a flexible –CH2CH2CH2– chain showed a significantly higher photocurrent density than the photoanode TiO2(1 + 2) with only –CH2– linkage. The possible reasons for the different water splitting performance of otherwise identical devices are discussed.