Viologen-modified platinum clusters acting as an efficient catalyst in photocatalytic hydrogen evolution

A highly efficient photocatalytic system for hydrogen evolution with dihydronicotinamide coenzyme (NADH) as a sacrificial agent in an aqueous solution has been constructed by using water-soluble platinum clusters functionalized with methyl viologen-alkanethiol (MVA2+) and a simple electron-donor dyad, 9-mesityl-10-methylacridinium ion (Acr+-Mes), which is capable of fast photoinduced electron transfer but extremely slow back electron transfer. The mean diameter of the platinum core was determined as R(CORE) = 1.9 nm with a standard deviation sigma = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogen-evolution rate of the photocatalytic system with MVA2+-modified platinum clusters (MVA2+-PtC) is 10 times faster than the photocatalytic system with the mixture of the same amount of MVA2+ and platinum clusters as that of MVA2+-PtC under otherwise the same experimental conditions. The radical cation of NADH has been successfully detected by laser flash photolysis experiments. The decay of the absorbance due to NAD*, produced by the deprotonation from NADH*+, coincides with the appearance of the absorption band due to Acr*-Mes. This indicates electron transfer from NAD* to Acr+-Mes to give Acr*-Mes, which undergoes the electron-transfer reduction of MVA2+-PtC, leading to the efficient hydrogen evolution.     

Photocatalytic hydrogen evolution over mesoporous TiO2/metal nanocomposites

Na⁺ complex with the dibenzo-18-crown-6 ester was used as a template to synthesize mesoporous titanium dioxide with the specific surface area 130–140 m²/g, pore diameter 5–9 nm and anatase content 70–90%. The mesoporous TiO₂ samples prepared were found to have photocatalytic activity in Cu^II, Ni^II and Ag^I reduction by aliphatic alcohols. The resulting metal–semiconductor nanostructures have remarkable photocatalytic activity in hydrogen evolution from water–alcohol mixtures, their efficiency being 50–60% greater than that of the metal-containing nano-composites based on TiO₂ Degussa P25.The effects of the thermal treatment of mesoporous TiO₂ upon its photocatalytic activity in hydrogen production were studied. The anatase content and pore size were found to be the basic parameters determining the photoreaction rate. The growth of the quantum yield of hydrogen evolution from TiO₂/Ag⁰ to TiO₂/Ni⁰ to TiO₂/Cu⁰ was interpreted in terms of differences in the electronic interaction between metal nanoparticles and the semiconductor surface. It was found that there is an optimal metal concentration range where the quantum yield of hydrogen production is maximal. A decrease in the photoreaction rate at further increment in the metal content was supposed to be connected with the enlargement of metal nanoparticles and deterioration of the intimate electron interaction between the components of the metal–semiconductor nanocomposites.     

Solvent-induced hydrogen tunnelling in ascorbate proton-coupled electron transfers

The over-the-barrier proton-coupled electron transfer interaction of an ascorbate monoanion with a hexacyanoferrate(III) ion in water entered into a tunnelling regime in water-1,4-dioxane, water-ethanol and water-acetonitrile mixed solvents.     

Reversing electron transfer in a covalent triazine framework for efficient photocatalytic hydrogen evolution

Covalent triazine-based frameworks (CTFs) have emerged as some of the most important materials for photocatalytic water splitting. However, development of CTF-based photocatalytic systems with non-platinum cocatalysts for highly efficient hydrogen evolution still remains a challenge. Herein, we demonstrated, for the first time, a one-step phosphidation strategy for simultaneously achieving phosphorus atom bonding with the benzene rings of CTFs and the anchoring of well-defined dicobalt phosphide (Co₂P) nanocrystals (∼7 nm). The hydrogen evolution activities of CTFs were significantly enhanced under simulated solar-light (7.6 mmol h⁻¹ g⁻¹), more than 20 times higher than that of the CTF/Co₂P composite. Both comparative experiments and in situ X-ray photoelectron spectroscopy reveal that the strong interfacial P–C bonding and the anchoring of the Co₂P cocatalyst reverse the charge transfer direction from triazine to benzene rings, promote charge separation, and accelerate hydrogen evolution. Thus, the rational anchoring of transition-metal phosphides on conjugated polymers should be a promising approach for developing highly efficient photocatalysts for hydrogen evolution.

Reversing the electron transfer in a covalent triazine-based framework by Co₂P anchoring achieved highly efficient photocatalytic hydrogen evolution from water splitting.

Photocatalytic water splitting into hydrogen fuels has been considered as a promising technique for converting solar energy into chemical energy.^1–3 To achieve this target, it is necessary to design and construct photocatalysts with high solar-to-hydrogen (STH) conversion efficiency.^4,5 Recently, covalent triazine-based frameworks (CTFs),^6,7 as a new class of conjugated polymer materials, have attracted significant attention in the photocatalytic water splitting field owing to their visible-light response, organized architecture, adjustable pore-size, and controllable functionalization.^8–12 However, owing to the high charge recombination and fewer active sites for the hydrogen evolution reaction (HER), the photocatalytic activities of pristine CTFs are very low. To overcome this drawback, noble-metal platinum (Pt) is generally required in CTF-based photocatalytic systems as the HER cocatalyst for promoting charge separation and catalyzing hydrogen generation.^13–15 However, the high cost and scarcity of metallic Pt greatly limit the large-scale commercial applications. Thus, it is highly desirable to explore economical materials as noble-metal substitutes for achieving comparable or even superior hydrogen evolution activities.Recently, transition-metal phosphides (TMPs) have attracted intensive attention as HER cocatalysts for photocatalytic water splitting, owing to their unique structural and electronic properties.^16–20 Up to now, a variety of semiconductor materials, including metal oxides,^21,22 metal sulfides,^23,24 g-C₃N₄,^25,26 MOFs,^27,28etc., decorated with TMPs have been extensively investigated, and the hydrogen generation activity in some reports is even higher than that of Pt cocatalysts. However, related studies about the decoration of TMP cocatalysts on CTF photocatalysts for highly efficient hydrogen generation have not been reported so far. Taking into account the molecular structure of CTFs, the nitrogen sites in the triazine frameworks could easily coordinate with transition metal ions to form nitrogen–metal interactions.^29–31 In contrast, the coordination of TMPs with CTFs is relatively difficult owing to the high electronegativity of P sites which could draw electrons from metal atoms.¹⁷ Moreover, most reported TMPs were fabricated directly from precursor metal salts, oxides, etc.,^32–35 and the resultant large-dimensions usually lead to very limited contact-interface with CTFs, or even a physical mixture form. Accordingly, the charge transfer between TMPs and CTFs is significantly restrained. Thus, effective anchoring of TMPs on CTFs for achieving highly efficient photocatalytic hydrogen evolution still remains a great challenge.Herein, a one-step phosphidation strategy has been developed to achieve phosphorus bonding with the benzene rings of CTFs and the anchoring of well-defined dicobalt phosphide (Co₂P) nanocrystals (∼7 nm). The photocatalytic results clearly reveal that an excellent hydrogen evolution activity (7.6 mmol h⁻¹ g⁻¹) is achieved, which is much higher than that of the CTF/Co₂P composite (0.37 mmol h⁻¹ g⁻¹). More detailed studies confirm that the phosphidation strategy could effectively facilitate the interfacial bonding between CTFs and Co₂P nanocrystals. More importantly, the SI-XPS results clearly suggest that the charge transfer direction in CTFs is completely reversed, and the photo-generated electrons efficiently transferred from the triazine rings to P-bonded benzene rings, where the Co₂P cocatalyst attracted electrons through the interfacial P–C bonds for efficient hydrogen evolution. To our knowledge, this is the first report on incorporating TMPs on a CTF photocatalyst for enhancing the hydrogen evolution activity. Fig. 1A shows the basic procedures for the preparation of CTF polymer anchored Co₂P nanocrystals. Briefly, the CTFs with adsorbed cobalt ions were directly phosphatized by thermal decomposition of NaH₂PO₂ under an Ar atmosphere, which could simultaneously achieve P atom bonding with benzene rings and the anchoring of the Co₂P cocatalyst (marked as P-CTF-Co₂P). Fig. 1B shows the typical transmission electron microscopy (TEM) image of the obtained sample, clearly revealing that well-defined Co₂P nanocrystals with an average diameter of 5∼9 nm were uniformly dispersed on the CTF surface. Furthermore, the high-resolution TEM (HR-TEM) image (Fig. 1C) of the formed nanocrystals exhibited two lattice fringes with d-spacing values of 0.20 and 0.21 nm, respectively, which could be well indexed to the (211) and (121) planes of orthorhombic Co₂P crystals.^36,37Fig. 1D shows the X-ray diffraction (XRD) pattern of the obtained P-CTF-Co₂P sample. For comparison, the pristine CTF, and Co₂P nanocrystals have also been studied. It can be clearly seen that for the P-CTF-Co₂P sample, except for the two broad diffraction peaks at 7.3° and 26.1° attributed to the in-plane (100) facets and interlayer (001) stacking of CTFs,^38,39 the other XRD peaks well matched with those of the Co₂P crystals,^40,41 confirming the successful incorporation of the Co₂P cocatalyst in the CTFs. Furthermore, energy-dispersive X-ray (EDX) elemental mapping (Fig. 1E) clearly reveals the uniform distribution of C, N, Co and P elements in the whole detection region, further confirming the uniform dispersion of Co₂P on CTF polymers.Open in a separate windowFig. 1(A) Basic procedures and the ideal structure scheme for fabricating P-CTF-Co₂P catalysts; (B) TEM image and (C) HR-TEM image of the P-CTF-Co₂P catalyst; (D) XRD patterns of CTFs, Co₂P and P-CTF-Co₂P catalysts; (E) EDX elemental mapping images of the P-CTF-Co₂P catalyst.To further explore the P-bonding sites, high-resolution X-ray photoelectron spectroscopy (XPS) was performed on both P-CTF-Co₂P and pristine CTFs. As shown in Fig. 2A, for pristine CTF samples, the C 1s peak could be well fitted into two peaks located at 284.8 and 286.8 eV, which could be assigned to C–C C and N–C N bonds,^38,39,42 respectively. Notably, after the phosphidation treatment, a new peak at 285.3 eV attributed to P–C bonds was detected in the P-CTF-Co₂P sample,^43–45 and the C–C C to N–C N ratio was significantly decreased from 10.2 (pristine CTFs) to 7.1 (P-CTF-Co₂P). In contrast, compared with the N 1s spectrum of the pristine CTF sample, no evident change of peak shape and intensity could be detected in the P-CTF-Co₂P sample (Fig. 2B). These results clearly reveal that after the phosphidation treatment, P atoms should mainly bond with the carbon sites of the benzene rings in CTFs instead of the triazine rings.^46,47 To further confirm this inference, XPS studies on phosphatized CTFs without Co₂P nanocrystals (marked as P-CTFs) were also performed. It can be clearly seen from Fig. S9^† that similar changes for the C 1s and N 1s peaks of P-CTF-Co₂P (Fig. 2A and B) are observed in the P-CTF sample. Moreover, in both P-CTF-Co₂P and P-CTF samples, an evident P 2p peak corresponding to P–C bonds could be observed (Fig. 2C),⁴⁸ further indicating the bonding of P atoms with C atoms on the benzene rings in CTFs after the phosphidation treatment. However, note that the binding energy (BE) position of P–C peaks in P-CTF-Co₂P (133.6 eV) is slightly higher than that of P-CTF (133.2 eV), which should be attributed to the anchoring with Co₂P nanocrystals. In addition to the XPS investigations, solid-state cross-polarization magic angle spinning carbon-13 and phosphorus-31 nuclear magnetic resonance (¹³C-NMR and ³¹P-NMR) spectroscopy studies were further performed to explore the bonding sites of phosphorus atoms in the P-CTF (Fig. S10^†). The Fourier-transformed infrared (FTIR) spectrum was also employed (Fig. 2D). It can be clearly observed that the typical FTIR peaks of the triazine frameworks and the stretching vibrations of carbon–nitrogen (C–N) (1521 cm⁻¹ and 1354 cm⁻¹) in P-CTF-Co₂P are consistent with those of pristine CTFs.^38,39 However, the peaks at 1672 cm⁻¹ and 1014 cm⁻¹ corresponding to the stretching vibrations of C C and C–H in benzene rings were evidently decreased in P-CTF-Co₂P compared with those of pristine CTFs.^49,50 Combining the results of XPS, NMR, and FTIR, it can be concluded that in the obtained P-CTF-Co₂P sample, the carbon sites of the benzene rings in the CTFs were partially bonded with P-atoms, which should anchor Co₂P through P–Co bonding.Open in a separate windowFig. 2High-resolution XPS spectra of (A) C 1, (B) N 1s and (C) P 2p in CTF, P-CTF and P-CTF-Co₂P samples (the insets show the molecular structures); (D) Fourier transform infrared spectroscopy (FT-IR) of CTF and P-CTF-Co₂P samples.Furthermore, the hydrogen evolution activities of the P-CTF-Co₂P (2 wt% Co₂P) sample were evaluated under simulated solar light irradiation. For comparison, the CTF/Co₂P composite and pristine CTFs were also measured under the same conditions. As shown in Fig. 3A, the P-CTF-Co₂P sample exhibits a much higher H₂ evolution activity (15.2 mmol g⁻¹) than the CTF/Co₂P composite (0.7 mmol g⁻¹) at 2 h, while no evident hydrogen generation could be detected in the pristine CTF sample. To further explore the crucial roles of interfacial P-bonding in CTFs and the anchoring of the Co₂P cocatalyst, the hydrogen evolution rates of various samples were calculated and their comparison is shown in Fig. 3B. Obviously, compared with the excellent activity of P-CTF-Co₂P (7.6 mmol h⁻¹ g⁻¹), CTF/Co₂P and P-CTF/Co₂P only exhibit very low hydrogen evolution rates of 0.37 and 0.38 mmol h⁻¹ g⁻¹, respectively, while no evident activity could be detected for CTFs, Co₂P, and P-CTF samples. The above results clearly confirm that the significant enhancement of the hydrogen evolution activity of the P-CTF-Co₂P sample should be mainly attributed to the P-bonding and the anchoring of the Co₂P cocatalyst. Furthermore, the photocatalytic performances of CTFs decorated with Pt nanoparticles were also studied (the inset of Fig. 3B, Fig. S15^†), as Pt is generally recognized as the most active cocatalyst for hydrogen generation. Amazingly, the hydrogen production rates of the Pt cocatalyst at different amounts (1∼5 wt%) were all lower than that of the P-CTF-Co₂P sample, indicating that the rational bonding of the Co₂P cocatalyst on CTFs should be a promising strategy for achieving highly efficient photocatalytic hydrogen evolution.Open in a separate windowFig. 3(A) Photocatalytic H₂ evolution tests of the as-prepared photocatalysts; (B) comparative presentation of the hydrogen evolution rates; (C) ultraviolet-visible diffuse reflectance spectrum of the P-CTF-Co₂P photocatalyst (red solid line) and wavelength-dependent hydrogen production activities of the P-CTF-Co₂P photocatalyst within 2 h; light source: a 300 W Xe lamp equipped with various cut-off filters. (D) The cycling photocatalytic tests of P-CTF-Co₂P and the CTF/Co₂P composite; (E) I–t curves of various catalysts at −0.3 V (vs. SCE); (F) electrochemical impedance spectroscopy under light of the various catalysts. Measurements were conducted in 0.2 mol L⁻¹ Na₂SO₄ electrolyte solution under AM 1.5 G illumination.Furthermore, the relationship between wavelength and the hydrogen evolution activity of the P-CTF-Co₂P photocatalyst was studied and is shown in Fig. 3C. With increasing the wavelength from full-spectrum light irradiation (λ > 300) to 500 nm, the hydrogen evolution rate significantly decreased from 7.6 to 0.06 mmol h⁻¹ g⁻¹, which is generally consistent with their absorption spectrum. The highest value of the apparent quantum yields (AQYs) attained is 31.8% at 365 nm. Moreover, the photocatalytic stability and durability of P-CTF-Co₂P and CTF/Co₂P were examined by cycling experiments. As shown in Fig. 3D, the CTF/Co₂P composite demonstrated relatively poor stability, and there is no evident activity after only one cycling experiment, resulting from the incompact combination between CTFs and Co₂P (Fig. S5^†). In contrast, the P-CTF-Co₂P sample exhibits relatively high stability, and no evident inactivation was detected during the whole test. These results further confirm that in addition to enhancing activities, the interfacial P–C bonds and the anchoring of the Co₂P cocatalyst could also effectively promote the hydrogen evolution stability. To further confirm the efficient charge separation and electron transfer in P-CTF-Co₂P, photoelectrochemical (PEC) tests were performed. As shown in Fig. 3E, the amperometric I–t curves clearly reveal that P-CTF-Co₂P exhibits higher photocurrent density than pristine CTFs and CTF/Co₂P samples, confirming its more efficient charge separation capability. Moreover, electrochemical impedance spectroscopy (EIS) was also performed to explore the interfacial charge transfer process.^51,52 As shown in Fig. 3F, the P-CTF-Co₂P sample with the smallest arc radius revealed remarkably increased interfacial charge transport efficiency. These PEC results clearly demonstrate the highly efficient photogenerated charge separation and transfer in the P-CTF-Co₂P photocatalyst, which are highly consistent with the above photocatalytic hydrogen evolution results.The photo-induced charge separation and transfer in the excited state are crucial processes for determining the photocatalytic activity. Herein, we have demonstrated an in situ irradiation X-ray photoelectron spectroscopy (SI-XPS) technique for exploring the intrinsic charge separation and transfer mechanisms between Co₂P and CTFs. As shown in Fig. S20 and S21,^† no evident binding energy (BE) shift could be observed in both pristine CTF and Co₂P samples, confirming their relatively poor charge separation capability. Amazingly, for the P-CTF-Co₂P sample (Fig. 4), distinct variations for C 1s, N 1s, P 2p, and Co 2p peaks were detected in the excited state. More specifically, the C 1s and N 1s peaks in the CTFs shifted towards the high BE region by 0.3 eV accompanied by the broadening of peak shape (Fig. 4B and C), while the P 2p and Co 2p peaks of the Co₂P cocatalyst shifted towards the low BE direction by 0.3 and 0.4 eV (Fig. 4D and E), respectively.Interestingly, as shown in Fig. 4D, the P 2p peaks attributed to interfacial P–C bonds exhibited no evident BE change under dark and light irradiation. On the basis of the above SI-XPS results, it can be concluded that under light irradiation, the photo-excited electrons effectively transferred from CTFs to Co₂P through the P-bonding sites as well as the interfacial P–C bonds, leading to electron enrichment in Co₂P nanocrystals and hole enrichment in CTFs. To further confirm the crucial roles of interfacial P-bonding in CTFs and the anchoring of the Co₂P cocatalyst in promoting charge separation, SI-XPS studies for the CTF/Co₂P composite were also conducted. As shown in Fig. S22,^† no new peak or shape change of C 1s, N 1s, P 2p, and Co 2p peaks could be detected in CTF/Co₂P compared with pristine CTFs and Co₂P in the ground state, indicating no interfacial bonding in the CTF/Co₂P composite. Furthermore, in the excited state, no evident BE shifts and shape change could be observed, indicating the poor charge-separation capability of CTF/Co₂P, which is highly consistent with its fairly low hydrogen evolution activity. These SI-XPS results further confirm the crucial roles of interfacial P-bonding in CTFs and the anchoring of the Co₂P cocatalyst in efficiently promoting charge separation and enhancing the photocatalytic activity.Open in a separate windowFig. 4(A) Schematic illustration of the SI-XPS technique for direct observation of electron transfer in the excited state. The (B) C 1s, (C) N 1s, (D) P 2p and (E) Co 2p of SI-XPS spectra in the P-CTF-Co₂P sample tested under dark and light illumination.On the basis of the above results, it can be concluded that the P-bonding and Co₂P-cocatalyst anchoring should reverse the photo-induced electron transfer direction from the triazine to benzene rings in CTFs. To further confirm this speculation, the photocatalytic behavior of various CTF-based photocatalysts was studied and their electron transfer directions in excitation states have been proposed (Fig. 5A and B). First, when Co²⁺ ions were further bonded with the N sites of the triazine rings in the P-CTF-Co₂P sample (marked as P-CTF-Co₂P-Co), the hydrogen evolution activity remarkably increased from 7.6 up to 8.4 mmol h⁻¹ g⁻¹. This result clearly reveals that the Co–N bonding in P-CTF-Co₂P could further promote charge separation due to the effective hole trapping on Co sites. In contrast, when the Pt cocatalyst was decorated on the P-CTF-Co₂P sample (marked as P-CTF-Co₂P–Pt) to form Pt–N coordination, the photocatalytic activity significantly decreased to 4.6 mmol h⁻¹ g⁻¹, mainly resulting from the electron transfer competition between Pt and Co₂P. Furthermore, the decoration of the single Pt cocatalyst on CTFs promoted the photo-induced electron transfer from the benzene rings to triazine rings, and the photocatalytic activity achieved was up to 3.8 mmol h⁻¹ g⁻¹. Furthermore, in situ FTIR spectroscopy was performed for exploring the intermediate products in the photocatalytic process of the P-CTF-Co₂P sample with co-adsorption of H₂O vapor (Fig. S27^†), which also demonstrated the direction of electron transfer combined with the in situ XPS results. Accordingly, a possible mechanism has been proposed for clarifying the hydrogen evolution activities of the P-CTF-Co₂P photocatalyst (Fig. 5B). Owing to the P-bonding and the anchoring of Co₂P nanocrystals, the photo-generated electrons could effectively transfer from the triazine rings to benzene rings, where the electrons are trapped by the Co₂P cocatalyst through the interfacial P–C bonds for the hydrogen evolution reaction. Simultaneously, the holes left behind in the CTFs participated in oxidation reactions. More importantly, this mechanism may provide a new insight for understanding the crucial roles of interfacial P-bonding in CTFs and their bonding with Co₂P in promoting the charge separation for efficient hydrogen evolution.Open in a separate windowFig. 5(A) Photocatalytic hydrogen evolution activities of various samples and the scheme of electron-transfer direction in the excited state. (B) The ideal structural illustration of the interfacial bonding and charge transfer of the P-CTF-Co₂P photocatalyst for hydrogen evolution.     

Pt nanoparticles@photoactive metal-organic frameworks: efficient hydrogen evolution via synergistic photoexcitation and electron injection

Pt nanoparticles of 2-3 nm and 5-6 nm in diameter were loaded into stable, porous, and phosphorescent metal-organic frameworks (MOFs 1 and 2) built from [Ir(ppy)(2)(bpy)](+)-derived dicarboxylate ligands (L(1) and L(2)) and Zr(6)(μ(3)-O)(4)(μ(3)-OH)(4)(carboxylate)(12) secondary building units, via MOF-mediated photoreduction of K(2)PtCl(4). The resulting Pt@MOF assemblies serve as effective photocatalysts for hydrogen evolution by synergistic photoexcitation of the MOF frameworks and electron injection into the Pt nanoparticles. Pt@2 gave a turnover number of 7000, approximately five times the value afforded by the homogeneous control, and could be readily recycled and reused.     

Electrocatalysis of the hydrogen evolution reaction by nanocomposites of poly(amidoamine)-encapsulated platinum nanoparticles and phosphotungstic acid

Poly(amidoamine) dendrimer (Generation-4) encapsulated platinum nanoparticles (PtNP-PAMAM) were prepared and used to fabricate nanocomposites with Keggin-type phosphotungstic acid (PW₁₂O₄₀³⁻) using a layer by layer electrostatic assembly technique. Indium tin oxide (ITO) electrodes, which were first modified with a monolayer of 3-aminopropyl triethoxysilane (3-APTES), were used as substrates for assembly of the PW₁₂O₄₀³⁻ monolayer. Nanocomposites were then fabricated by depositing PtNP-PAMAM on the monolayer of PW₁₂O₄₀³⁻. The amount of PtNP-PAMAM deposited was controlled by using different concentrations of PtNP-PAMAM diluted in 0.1 M H₂SO₄ solution. The hydrogen evolution reaction (HER) was used to test electrocatalytic activities of these nanocomposite modified electrodes. Modification of ITO|3-APTES with PW₁₂O₄₀³⁻ |PtNP-PAMAM showed significantly higher electrocatalytic activities toward the HER than electrodes modified with either PW₁₂O₄₀³⁻ or PtNP-PAMAM alone. The electrocatalytic activities were found to depend on the composition of PtNP-PAMAM and PW₁₂O₄₀³⁻ on electrode surfaces, which was attributed to an interaction between these species. Heat treatment of ITO|3-APTES|PW₁₂O₄₀³⁻ |PtNP-PAMAM electrodes at 200 °C produced significantly higher electrocatalytic activities, which supported the suggestion of an interaction. Presented at the 4th Baltic conference on Electrochemistry, Griefswald, March 13.−16., 2005.     

Design of efficient electrocatalysts for hydrogen evolution reaction based on 2D MXenes

The growing energy concern all over the world has recognized hydrogen energy as the most promising renewable energy sources.Recently,electrocatalytic hydrogen evolution reaction(HER)by water splitting has been extensively studied with a focus on developing efficient electrocatalysts that can afford HER at overpotential with minimum power consumption.The two-dimensional transition metal carbides and nitride,also known as MXenes,are becoming the rising star in developing efficient electrocatalysts for HER,owing to their integrated chemical and electronic properties,e.g.,metallic conductivity,variety of redox-active transition metals,high hydrophilicity,and tunable surface functionalities.In this review,the recent progress about the fundamental understanding and materials engineering of MXenes-based electrocatalysts is summarized in concern with two aspects:i)the regulation of the intrinsic properties of MXenes,which include the composition,surface functionality,and defects;and ii)MXenes-based composites for HER process.In the end,we summarize the present challenges concerning the efficiency of MXenes-based HER electrocatalysts and propose the directions of future research efforts.     

Bimolecular electron transfers that follow a Sandros-Boltzmann dependence on free energy

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(?+) + B ? A + B(?+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)*) by neutral donors (P(+)* + D → P(?) + D(?+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(?)/D(?+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(?-) + D(?+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(?-)/D(?+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(?-)/D(?+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(?-)/D(?+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ? exciplex ? A(?-)/D(?+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ～+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.     

污染物甲胺为电子给体可见光下Pt/ZnIn2S4光催化制氢

用水热法制备了ZnIn2S4固溶体,并通过XRD和UV-vis漫反射光谱进行了表征.研究了一甲胺、二甲胺和三甲胺为给电子体,在Pt/ZnIn2S4上的可见光光催化制氢及自身的降解反应.3种甲胺都能显著提高光催化分解水制氢活性,同时自身得到很好的降解.电子给体的放氢活性顺序为:TMADMAMMA.通过红外衰减全反射(ATR-IR)法检测电子给体在ZnIn2S4表面的吸附行为,吸附强度大小依次为MMADMATMA.光催化活性与分子结构和在催化剂表面的吸附行为有关.3种污染物浓度对放氢活性的影响都符合Langmuir-Hinshelwood动力学模型.讨论了可能的化学反应机理.     

Metallopolymer nanocomposites based on platinum nanoparticles for chemical fuel cells

Platinum nanoparticles were synthesized by radiation chemical reduction in solutions of reverse micelles for the modification of Nafion films. The effect of the degree of solubilization and sizes of water pools of micelles on nanoparticle formation was shown. The platinum nanoparticle size in the polymer matrix does not exceed 10 nm.     

Photocatalytic hydrogen evolution and Rifampicinum destruction over carbon-modified TiO2

Research on Chemical Intermediates - Carbon-modified TiO2 using a sol–gel method (TiO2/CSG) and mechanochemical treatment (TiO2/CMC) was obtained. X-ray powder diffraction (XRD), scanning...     

Photocatalytic hydrogen evolution from water on SiC under visible light irradiation

A kind of green SiC fine powder was characterized by XRD and UV-Vis diffuse reflectance, and studied in the photocatalytic splitting of water. The results showed that the green SiC fine powder can absorb visible light and split water with the formation of hydrogen under visible light irradiation. The activity is affected significantly by the initial pH of solutions and the types of cheap reagents, where the addition of OH⁻ or S²⁻ leads to a remarkable increase in the activity.     

Singlet energy transfer in porphyrin-based donor-bridge-acceptor systems: interaction between bridge length and bridge energy

Singlet excitation energy transfer is governed by two donor-acceptor interactions, the Coulombic and exchange interactions giving rise to the F?rster and Dexter mechanisms, respectively, for singlet energy transfer. In transfer between colliding molecules or between a donor (D) and acceptor (A) connected in donor-bridge-acceptor (D-B-A) system by an inert spacer (B), the distinction between these two mechanisms is quite clear. However, in D-B-A systems connected by a pi-conjugated bridge, the exchange interaction between the donor and acceptor is mediated by the virtual low-lying excited states (unoccupied orbitals) of that bridge and, as a consequence, becomes much more long-range in character. Thus, the clear distinction to the Coulombic mechanism is lost. This so-called superexchange mechanism for singlet energy transfer has been shown to make a significant contribution to the energy transfer rates in several D-B-A systems, and its D-A distance as well as D-B energy gap dependencies have been studied. We here demonstrate that in a series of oligo-p-phenyleneethynylene (OPE) bridged porphyrin-based D-B-A systems with varying D-A distances the F?rster and through-bond (superexchange) mechanisms both make considerable contributions to the observed singlet energy transfer rates. The donor is either a zinc porphyrin or a zinc porphyrin with a pyridine ligand, and the acceptor is a free base porphyrin. By comparison to a homologous series where only the D-B energy gaps varies, a separation between the two energy transfer mechanisms was possible and, moreover, an interplay between distance and energy gap dependencies was noted. The distance dependence was shown to be approximately exponential with an attenuation factor beta=0.20 A-1. If the effect of the varying D-B energy gaps in the OPE series was taken into account, a slightly higher beta-value was obtained. Ground-state absorption, steady-state, and time-resolved emission spectroscopy were used. The experimental study is accompanied by time-dependent density functional theory (TD-DFT) calculations of the electronic coupling, and the experimental and theoretical results are in excellent qualitative agreement (same distance dependence).     

单分散Ni簇锚定在CN上用于高效光催化析氢

利用太阳光在常温常压下驱动光催化反应高效进行是解决人类面临的能源、环境问题从而实现绿色化学的理想方案之一.然而,兼顾效率、成本和稳定性的高性能光催化体系的研究依然存在巨大的挑战.石墨氮化碳(g-C3N4)基光催化剂由于高稳定性、无毒无害和适合的能带结构,在光催化制氢方面存在巨大潜力.然而,表面的慢反应速率导致了光生电子...     

Highly efficient electrocatalytic hydrogen evolution reaction on carbonized porous conducting polymers

A rational design of an efficient and inexpensive electrocatalyst for water splitting still remains a challenge. Porous conducting polymers are attractive materials which not only provide a high surface area for electrocatalysis but also absorb light which can be harnessed in photoelectrocatalysis. Here, a novel and inexpensive electrochemical approach is developed to obtain nanoporous conducting copolymers with tunable light absorbance and porosity. By fine-tuning the copolymer composition and upon heat treatment, an excellent electrocatalytic hydrogen evolution reaction (HER) was achieved in alkaline solution with an overpotential of just 77 mV to obtain a current density of 10 mA cm⁻². Such an overpotential is remarkably low compared with other reported values for polymers in an alkaline medium. The nanoporous copolymer developed here shows a great promise of using metal-free electrocatalysts and brings about new avenues for exploitation of these porous conducting polymers.

     

Interplay between barrier width and height in electron tunneling: photoinduced electron transfer in porphyrin-based donor-bridge-acceptor systems

The rate of electron tunneling in molecular donor-bridge-acceptor (D-B-A) systems is determined both by the tunneling barrier width and height, that is, both by the distance between the donor and acceptor as well as by the energy gap between the donor and bridge moieties. These factors are therefore important to control when designing functional electron transfer systems, such as constructs for photovoltaics, artificial photosynthesis, and molecular scale electronics. In this paper we have investigated a set of D-B-A systems in which the distance and the energy difference between the donor and bridge states (DeltaEDB) are systematically varied. Zinc(II) and gold(III) porphyrins were chosen as electron donor and acceptor because of their suitable driving force for photoinduced electron transfer (-0.9 eV in butyronitrile) and well-characterized photophysics. We have previously shown, in accordance with the superexchange mechanism for electron transfer, that the electron transfer rate is proportional to the inverse of DeltaEDB in a series of zinc/gold porphyrin D-B-A systems with bridges of constant edge to edge distance (19.6 A) and varying DeltaEDB (3900-17 600 cm(-1)). Here, we use the same donor and acceptor but the bridge is shortened or extended giving a set of oligo-p-phenyleneethynylene bridges (OPE) with four different edge to edge distances ranging from 12.7 to 33.4 A. These two sets of D-B-A systems-ZnP-RB-AuP+ and ZnP-nB-AuP+-have one bridge in common, and hence, for the first time both the distance and DeltaEDB dependence of electron transfer can be studied simultaneously in a systematic way.     

Noble metal doped graphene nanocomposites and its study of photocatalytic hydrogen evolution

This work reports the deposition of platinum (Pt) nanoparticles on the surface of graphene nanosheet by a simple approach, using a microwave-assisted method. The photocatalytic activity has been investigated for hydrogen evolution. The hydrogen evolutions were attributed to graphene, due to its high photoelectron transport properties, and the Pt nanoparticles attached on the surface of graphene sheet, which act as reaction centers for H₂ evolution. The “as-prepared” composites were characterized by Brunauer Emmett Teller (BET) surface area measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectra (DRS). This work highlights the potential application of graphene-based materials in the field of energy conversion.     

Energy and electron transfers in photosensitive chitosan

Novel photosensitive chitosan was synthesized. The modified chitosan contains photoactive anthracene chromophore moieties. Because of the presence of anthracene chromophores, the polymer absorbs light in the UV-vis spectral region. Electronically excited polymeric chromophores could participate in energy and electron transfer processes to the suitable acceptor molecules. The photosensitive chitosan developed herein could could act as an efficient photosensitizer and lead to the application of the environmentally friendly photocatalytic system for an efficient degradation of a wide range of pollutants.     

基于绿色氢科学理念构筑从低碳制氢到高效储氢的氢能体系

本文从低碳制氢和高效储氢的角度思考及探讨氢能体系绿色化发展过程中的关键科学问题.提出＂绿色氢科学＂理念与＂低碳制氢,高效储氢＂技术发展路线图,以期通过相关科学问题的认识,来构建具有高度原子经济性及可持续性的绿色氢能体系.