Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting.

Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV-visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality.

We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO4) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO4 films are smooth and compact, possessing remarkably high structural quality across complete 2'' wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO4, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO4, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.

Metastable Ta2N3 with highly tunable electrical conductivity via oxygen incorporation.

The binary Ta-N chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach - by which the carrier concentration is reduced by the oxygen impurity - offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.

Indirect bandgap, optoelectronic properties, and photoelectrochemical characteristics of high-purity Ta3N5 photoelectrodes.

The (opto)electronic properties of Ta3N5 photoelectrodes are often dominated by defects, such as oxygen impurities, nitrogen vacancies, and low-valent Ta cations, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport. Here, we explore the role of ammonia annealing following direct reactive magnetron sputtering of tantalum nitride thin films, achieving near-ideal stoichiometry, with significantly reduced native defect and oxygen impurity concentrations. By analyzing structural, optical, and photoelectrochemical properties as a function of ammonia annealing temperature, we provide new insights into the basic semiconductor properties of Ta3N5, as well as the role of defects on its optoelectronic characteristics. Both the crystallinity and material quality improve up to 940 °C, due to elimination of oxygen impurities. Even higher annealing temperatures cause material decomposition and introduce additional disorder within the Ta3N5 lattice, leading to reduced photoelectrochemical performance. Overall, the high material quality enables us to unambiguously identify the nature of the Ta3N5 bandgap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. The compact morphology, low defect content, and high optoelectronic quality of these films provide a basis for further optimization of photoanodes and may open up further application opportunities beyond photoelectrochemical energy conversion.

Nanoscale Heterogeneities and Composition-Reactivity Relationships in Copper Vanadate Photoanodes.

The photoelectrochemical performance of thin film photoelectrodes can be impacted by deviations from the stoichiometric composition, both at the macroscale and at the nanoscale. This issue is especially pronounced for the class of ternary compounds that are currently investigated for simultaneously achieving the optoelectronic characteristics and chemical stability required for solar fuel generation. Here, we combine macroscopic photoelectrochemical testing with atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) to reveal relationships between photoelectrochemical activity, nanoscale morphology, and local chemical composition in copper vanadate (CVO) thin films as a model system. For films with varying Cu/(Cu + V) ratios around the ideal stoichiometry of stoiberite Cu5V2O10, AFM resolves submicrometer morphology variations, which correlate with variations of the Cu content resolved by STXM. Both stoichiometric and Cu-deficient films exhibit a clear photoresponse, which indicates electronic tolerance to reduced Cu content. While both films exhibit homogeneous O and V content, they are also characterized by local regions of Cu enrichment and depletion that extend beyond individual grains. By contrast, Cu-rich photoelectrodes exhibit a tendency toward CuO secondary phase formation and a significantly reduced photoelectrochemical activity, indicating a significantly poor electronic tolerance to Cu-enrichment. These findings highlight that the average film composition at the macroscale is insufficient for defining structure-function relationships in complex ternary compounds. Rather, correlating microscopic variations in chemical composition to macroscopic photoelectrochemical performance provides insights into photocatalytic activity and stability that are otherwise not apparent from pure macroscopic characterization.

Composition-Dependent Functionality of Copper Vanadate Photoanodes.

To understand functional roles of constituent elements in ternary metal oxide photoanodes, essential photoelectrochemical (PEC) properties are systematically analyzed on a series of copper vanadate compounds with different Cu:V elemental ratios. Homogeneous and highly continuous thin films of ß-Cu2V2O7, γ-Cu3V2O8, Cu11V6O26, and Cu5V2O10 are grown via reactive co-sputtering and their performance characteristics for the light-driven oxygen evolution reaction are evaluated. All four compounds have similar bandgaps in the range of 1.83-2.03 eV, though Cu-rich phases exhibit stronger optical absorption and higher charge separation efficiencies. Transient photocurrent analysis reveals a reduction of surface catalytic activity with increasing Cu:V elemental ratio due to competitive charge recombination at Cu-related surface states. This comprehensive analysis of PEC functionalities-including photon absorption, carrier separation, and heterogeneous charge transfer-informs strategies for improving PEC activity in the copper vanadate materials system and provides insights that may aid discovery, design, and engineering of new photoelectrode materials.

Structurally Deformed MoS2 for Electrochemically Stable, Thermally Resistant, and Highly Efficient Hydrogen Evolution Reaction.

The emerging molybdenum disulfide (MoS2 ) offers intriguing possibilities for realizing a transformative new catalyst for driving the hydrogen evolution reaction (HER). However, the trade-off between catalytic activity and long-term stability represents a formidable challenge and has not been extensively addressed. This study reports that metastable and temperature-sensitive chemically exfoliated MoS2 (ce-MoS2 ) can be made into electrochemically stable (5000 cycles), and thermally robust (300 °C) while maintaining synthetic scalability and excellent catalytic activity through physical-transformation into 3D structurally deformed nanostructures. The dimensional transition enabled by a high throughput electrohydrodynamic process provides highly accessible, and electrochemically active surface area and facilitates efficient transport across various interfaces. Meanwhile, the hierarchically strained morphology is found to improve electronic coupling between active sites and current collecting substrates without the need for selective engineering the electronically heterogeneous interfaces. Specifically, the synergistic combination of high strain load stemmed from capillarity-induced-self-crumpling and sulfur (S) vacancies intrinsic to chemical exfoliation enables simultaneous modulation of active site density and intrinsic HER activity regardless of continuous operation or elevated temperature. These results provide new insights into how catalytic activity, electrochemical-, and thermal stability can be concurrently enhanced through the physical transformation that is reminiscent of nature, in which properties of biological materials emerge from evolved dimensional transitions.

Operando spectroscopic analysis of an amorphous cobalt sulfide hydrogen evolution electrocatalyst.

The generation of chemical fuel in the form of molecular H2 via the electrolysis of water is regarded to be a promising approach to convert incident solar power into an energy storage medium. Highly efficient and cost-effective catalysts are required to make such an approach practical on a large scale. Recently, a number of amorphous hydrogen evolution reaction (HER) catalysts have emerged that show promise in terms of scalability and reactivity, yet remain poorly understood. In this work, we utilize Raman spectroscopy and X-ray absorption spectroscopy (XAS) as a tool to elucidate the structure and function of an amorphous cobalt sulfide (CoSx) catalyst. Ex situ measurements reveal that the as-deposited CoSx catalyst is composed of small clusters in which the cobalt is surrounded by both sulfur and oxygen. Operando experiments, performed while the CoSx is catalyzing the HER, yield a molecular model in which cobalt is in an octahedral CoS2-like state where the cobalt center is predominantly surrounded by a first shell of sulfur atoms, which, in turn, are preferentially exposed to electrolyte relative to bulk CoS2. We surmise that these CoS2-like clusters form under cathodic polarization and expose a high density of catalytically active sulfur sites for the HER.

Charge carrier dynamics of photoexcited Co3O4 in methanol: extending high harmonic transient absorption spectroscopy to liquid environments.

Charge carrier dynamics in Co3O4 thin films are observed using high harmonic generation transient absorption spectroscopy at the Co M2,3 edge. Results reveal that photoexcited Co3O4 decays to the ground state in 600 ± 40 ps in liquid methanol compared to 1.9 ± 0.3 ns in vacuum. Kinetic analysis suggests that surface-mediated relaxation of photoexcited Co3O4 may be the result of hole transfer from Co3O4 followed by carrier recombination at the Co3O4-methanol interface.

Recent experimental advances on excited-state intramolecular proton coupled electron transfer reaction.

Proton-coupled electron transfer reactions form the basis of many important chemical processes including much of the energy conversion that occurs within living cells. However, much of the physical chemistry that underlies these reaction mechanisms remains poorly understood. In this Account, we report on recent progress in the understanding of excited-state intramolecular proton-coupled electron transfer (PCET) reactions. The strategic design and synthesis of various types of PCET molecules, along with steady-state and femtosecond time-resolved spectroscopy, have uncovered the mechanisms of several excited-state PCET reactions in solution. These experimental advancements correlate well with current theoretical models, in which the proton has quantum motion with a high probability of tunneling. In addition, the rate of proton transfer is commonly incorporated within the rate of rearrangement of solvent molecules. As a result, the reaction activation free energy is essentially governed by the solvent reorganization because the charge redistribution is considered based on a solvent polarity-induced barrier instead of the height of the proton migration barrier. In accord with this theoretical basis, we can rationalize the observation that the proton transfer for many excited-state PCET systems occurs during the solvent relaxation time scale of 1-10 ps: the highly exergonic reaction takes place before the system reaches its equilibrium polarization. Also, we have used various derivatives of proton transfer molecules, especially those of 3-hydroxyflavone to clearly demonstrate how researchers can tune the dynamics of excited-state PCET through changes in the magnitude or direction of the dipole vector within the reaction. Subsequently, using 2-(2'-hydroxyphenyl)benzoxazole as the parent model, we then report on methods for the development of an ideal system for probing PCET reaction. Because future biomedical applications of such systems will likely occur in aqueous environments, we discuss various 7-azaindole analogues, for which proton transfer requires the assistance of protic solvent molecules. These results provide a unique contrast to the ubiquitous studies on the dynamic solvent effects of PCET molecules that undergo intrinsic intramolecular proton motion.

[Comparison of treatment with micro endoscopic discectomy and posterior lumbar interbody fusion using single and double B-Twin expandable spinal spacer].

OBJECTIVE: To compare the therapeutic effect of posterior lumbar interbody fusion by single and double B-Twin expandable spinal spacer with micro endoscopic discectomy (MED) for lumbar intervertebral disc protrusion accompanying degenerative instability. METHODS: From March 2006 to May 2008, 45 patients with lumbar intervertebral disc protrusion accompanying degenerative instability were admitted and managed with posterior lumbar interbody fusion by B-Twin expandable spinal spacer with MED. The patients were randomly assigned to treatment with single B-Twin (Single group, n = 24) or double B-Twin (Double group, n = 21). There were 16 males and 8 females, with an average age of 45.5 years (43 - 60 years) in Single group; 13 males and 8 females, with an average age of 43.7 years (44-61 years) in Double group. All the cases suffered from only one level disc protrusion, L(3-4) 2 cases, L(4-5) 29 cases and L5-S1 14 cases. Clinical outcomes were evaluated with surgical time, blood loss, visual analogue scale (VAS) scores preoperatively, 1, 3, 6 month postoperatively. Oswestry disability questionnaire (ODI) of the preoperative, 1 month postoperative, and latest follow-up and the disk space heights. RESULTS: Forty three patients were followed-up for 1 to 3 years after surgery. The mean surgical time of Double group was longer than Single group [(152 ± 32) min vs. (91 ± 15) min, P < 0.01]. The average blood loss in Double group was more than that in Single group [(146 ± 73) ml vs. (95 ± 58) ml, P < 0.01]. The mean time of hospital stay in Single group was similar to that in Double group [(11.0 ± 3.2) d vs. (10.9 ± 3.3) d, P > 0.05]. Both groups could keep the disk space heights till the last follow-up [(7.7 ± 1.8) mm vs. (8.5 ± 1.7) mm]. In the 6 months follow-up post operation, the VAS score decreased from (8.1 ± 1.8) to (2.0 ± 1.0) in Single group, and (8.1 ± 1.9) to (2.1 ± 1.0) in Double group. At the last follow-up, the ODI decreased from (36 ± 7)% to (10 ± 4)% in Single group and (37 ± 6)% to (9 ± 4)% in Double group, but there was no significant difference between the two groups (P > 0.05). All the cases achieved fusion at the last follow-up, 3 patients in Single group and 2 patients in Double group suffered from intractable low back pain. One of the fins broke in one patient without any uncomfortable feeling. CONCLUSIONS: Compared with the management of lumbar intervertebral disc protrusion accompanying degenerative instability by double B-Twin expandable spinal spacer with micro endoscopic discectomy, the single B-twin can get similar clinical outcomes, but shorter surgical time, less blood loss and less medical costs.

Emissive Pt(II) complexes bearing both cyclometalated ligand and 2-pyridyl hexafluoropropoxide ancillary chelate.

A new series of mono-cyclometalated Pt(II) complexes 1-4 with chelating 2-pyridyl hexafluoropropoxide as the ancillary ligand were synthesized. Single crystal X-ray diffraction studies were examined, giving evidence for the occurrence of pi pi stacking between the cyclometalated ligands, but a lack of intermolecular Pt ... Pt interaction. Among these complexes, the benzo[h]quinoline analogue 2a shows the greatest degree of pi pi stacking, which is also confirmed by the observation of additional, large Stokes shifted emission attributed to the aggregated counterparts in solid thin film. All these Pt(II) complexes are highly emissive in solid state, whereas except for 4-phenylquinazoline analogues 4a and 4b, complexes 1-3 in solution are subject to dominant radiationless deactivation induced, in part, by the solvent collision to the square planar Pt(II) framework. Electroluminescent OLEDs employing 2-phenylpyridine analogue 1b as the dopants were fabricated, rendering satisfactory performance data suited for future improvement.

Cyano analogues of 7-azaindole: probing excited-state charge-coupled proton transfer reactions in protic solvents.

The interplay between excited-state charge and proton transfer reactions in protic solvents is investigated in a series of 7-azaindole (7AI) derivatives: 3-cyano-7-azaindole (3CNAI), 5-cyano-7-azaindole (5CNAI), 3,5-dicyano-7-azaindole (3,5CNAI) and dicyanoethenyl-7-azaindole (DiCNAI). Similar to 7AI, 3CNAI and 3,5CNAI undergo methanol catalyzed excited-state double proton transfer (ESDPT), resulting in dual (normal and proton transfer) emission. Conversely, ESDPT is prohibited for 5CNAI and DiCNAI in methanol, as supported by a unique normal emission with high quantum efficiency. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The results conclude that significant excited-state charge transfer (ESCT) takes place for both 5CNAI and DiCNAI. The charge-transfer specie possesses a different dipole moment from that of the proton-transfer tautomer species. Upon reaching the equilibrium polarization, there exists a solvent-polarity induced barrier during the proton-transfer tautomerization, and ESDPT is prohibited for 5CNAI and DiCNAI during the excited-state lifespan. The result is remarkably different from 7AI, which is also unique among most excited-state charge/proton transfer coupled systems studied to date.

Novel oxazabicycles as a new class of photochromic colorants.

A new photochromic colorant with an oxazabicyclic moiety has been synthesized by an efficient method. It turns pale red upon UV irradiation and undergoes reverse reaction while being heated. This work may open an exciting new avenue for future development of the photochromic dyes with novel molecular structures.

Blue-emitting platinum(II) complexes bearing both pyridylpyrazolate chelate and bridging pyrazolate ligands: synthesis, structures, and photophysical properties.

A new Pt(II) dichloride complex [Pt(fppzH)Cl2] (1), in which fppzH = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, was prepared by the treatment of a pyridylpyrazole chelate fppzH with K2PtCl4 in aqueous HCl solution. Complex 1 could further react with its parent pyrazole (pzH), 3,5-dimethylpyrazole (dmpzH), or 3,5-di-tert-butylpyrazole (dbpzH) to afford the monometallic [Pt(fppz)(pzH)Cl] (2), [Pt(fppz)(dmpzH)Cl] (3), [Pt(fppz)(dmpzH)2]Cl (4), or two structural isomers with formula [Pt(fppz)(dbpzH)Cl] (5a,b). Single-crystal X-ray diffraction studies of 2, 4, and 5a,b revealed a square planar Pt(II) framework, among which a strong interligand hydrogen bonding occurred between fppz and pzH ligands in 2. This interligand H-bonding is replaced by dual N-H...Cl interaction in 4 and both intermolecular N-H...O (with THF solvate) and N-H...Cl interaction in 5a,b, respectively; the latter are attributed to the bulky tert-butyl substituents that force the dbpzH ligand to adopt the perpendicular arrangement. Furthermore, complex 2 underwent rapid deprotonation in basic media to afford two isomeric complexes with formula [Pt(fppz)(mu-pz)]2 (6a,b), which are related to each other according to the spatial orientation of the fppz chelates, i.e., trans- and cis-isomerism. Similar reaction exerted on 3 afforded isomers 7a,b. Both 6a,b (7a,b) are essentially nonemissive in room-temperature fluid state but afford strong blue phosphorescence in solid state prepared via either vacuum-deposited thin film or 77 K CH2Cl2 matrix. As also supported by the computational approaches, the nature of emission has been assigned to be ligand-centered triplet pipi* mixed with certain metal-to-ligand charge-transfer character.