1,721,059 research outputs found
Surface modification and interfacial analysis for photoelectrode-driven water splitting
No full textHydrogen as energy carrier is an appealing alternative to traditional fossil fuels. Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is potentially feasible for sustainable and higher conversion of solar energy into a storable hydrogen supply. Although the conversion efficiency has been improved during the past decades, the accessible photoelectrodes still suffer from poor carrier disassociation and/or slow surface kinetics, as well as restricted solar spectrum utilization to varying degrees. All of these restrictions severely limit the applications of PEC technology for commercial hydrogen production. In this thesis, surface modifications were implemented on both photoanodes and photocathodes to enhance the water spliting efficiency, with particular emphasis on the interfacial interactions between electrode/electrolyte and each microconstituent of heterojunctions. Catalysts for oxygen evolution reactions (OER) are essential components of photoanodes for PEC water oxidation. In chapter 2, nanostructured metal boride MB (M = Co, Fe) catalysts synthesized by a Sn/SnCl2-redox-assisted solid-state method were integrated with WO3 thin films to construct heterojunction photoanodes. MB decorated WO3 photoanodes demonstrated improved charge carrier density and prolonged carrier lifetimes, which were ascribed to the improved separation and migration of photogenerated electrons and holes. In transient chronoamperometry (CA) experiments, CoB and FeB augmented the photocurrent density of WO3 photoanodes from 0.53 to 0.83 and 0.85 mA cm–2, respectively, being equivalent to an approximate 1.6-fold improvement. The deposited MB catalyst evolved in situ into a core-shell structure, i.e., a MB core wrapped by a metal oxide shell (MB@MO). The CoB@CoOx nanostructure also outperformed cobalt borate (Co-Bi) and cobalt hydroxide (Co(OH)x) counterparts in photocurrent enhancement when paired with WO3 thin film photoanodes. These results highlight the importance of the semiconductor/catalyst interface in photoelectrodes and their reliance on material combinations. Quaternary metal oxynitride-based photoanodes with high transparency are promising candidates to achieve the required solar-to-hydrogen (STH) efficiency in tandem PEC cells. In chapter 3, atomic layer deposition (ALD) was adopted to produce GaN current collector layers on SiC as transparent and conductive substrates to support oxynitride thin films. On GaN/SiC substrates, thin film SrTaO2N was grown using a generalized route that included spin-coating, pyrolysis, and ammonolysis. The possibility of transmitting visible light to the back side is elevated by the high transparency to exceed 60% in almost the entire solar spectrum. Upon simulated sunlight illumination, the integrated SrTaO2N/GaN/SiC photoanodes gave a photocurrent onset potential at around –0.4 V with respect to reversible hydrogen electrode (VRHE) in 0.1 M NaOH electrolyte, which is much lower than previously reported values and has the potential to achieve satisfactory STH efficiency in tandem cell. This paves the way to the development of high-efficient hierarchically nanostructured tandem PEC cells, and this research illustrates the feasibility of using ALD in fabricating transparent and conductive substrates for semi-transparent quaternary metal oxynitride photoanodes. Conjugated polymers featuring semiconducting properties are thriving as viable alternatives to existing inorganic semiconductors for PEC water splitting. In chapter 4, semi-transparent poly(4-alkylthiazole) films with dissimilar trialkylsilyloxymethyl (R3SiOCH2) side chains (PTzTNB, R = n-butyl; PTzTHX, R = n-hexyl) were used to modify NiO thin films to fabricate polymer−inorganic heterojunction photocathodes. The formed gradient energy structure at the hybrid interface enabled spatial disassociation of photogenerated charge carriers, which manifest in an enhanced PEC performance. In particular, as compared with bare NiO and PTzTHX, the PTzTHX-deposited composite photocathode improved photocurrent density by 6- and 2-fold at 0 VRHE, respectively. Due to the prolonged lifetime, increased concentration, and reduced recombination of photocharges, the augmented water reduction behavior also reflected in the significant anodic shift of the onset potential under simulated one sun illumination. The photoabsorption and stability were also revised enormously upon surface functionalization with poly(4-alkylthiazole) layers. Poly(4-alkylthiazole)s were confirmed to be a competent alternative to the known inorganic semiconductor materials and the significance of interfacial band edge alignment was featured in this chapter for construcing polymer−inorganics hydrid photoelectrodes.Water reduction using core-shell quantum dots (QDs) sensitized photocathodes has seldom been implemented. In chapter 5, core-shell ZnSe@CdS and CdS@ZnSe QDs were used to sensitize porous p-type NiO photocathodes for proton reduction. Compared with pristine NiO photocathodes, the QDs decorated heterojunctions exhibited significant improvement in water-reduction efficiency, with a 3.8- and 3.2-fold improvement for ZnSe@CdS/NiO and CdS@ZnSe/NiO systems, respectively. The origins of these suprisingly high efficiencies were carefully elaborated by means of the Mott–Schottky technique, electrochemical impedance spectroscopy (EIS) and quantification of separation efficiencies with respect to photocharges. Carrier dynamics analysis revealed the improved water-reduction capacity stemming from the synergetic effects, which specifically reflected in the mitigated charge transfer resistance and increased carrier density. This chapter highlighted the importance of positive synergistic effects in core-shell QDs-based multi-heterojunctions. In summary, these surface modification methodologies were verified to be effective for enhancing water splitting efficiency, and the interfacial kinetic research is critical in constructing practical multijunction photoelectrode-driven PEC devices
Direct Solar Energy-Mediated Synthesis of Tertiary Benzylic Alcohols Using a Metal-Free Heterogeneous Photocatalyst
No full textDirect hydroxylation via the functionalization of tertiary benzylic C(sp(3))-H bonds is of great significance for obtaining tertiary alcohols, which find wide applications in pharmaceuticals as well as in fine chemical industries. However, current synthetic procedures use toxic reagents, and therefore, the development of a sustainable strategy for the synthesis of tertiary benzylic alcohols is highly desirable. To solve this problem, herein, we report a metal-free heterogeneous photocatalyst to synthesize the hydroxylated products using oxygen as the key reagent. Various benzylic substrates were employed into our mild reaction conditions to afford the desirable products in good to excellent yields. More importantly, the gram-scale reaction was achieved via harvesting direct solar energy and exhibited high quantity of the product. The high stability of the catalyst was proved via recycling the catalyst and spectroscopic analyses. Finally, a possible mechanism was proposed based on electron paramagnetic resonance and other experimental evidence.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Surface modification of photoelectrodes for photoelectrochemical water splitting
No full textUtilizing solar energy to produce clean and renewable energy is an attractive route to solve the two major restriction factors i.e. the growing pollution of environment and energy shortage. A photoelectrochemical (PEC) cell, which can absorb sufficiently solar energy to split abundant water into the energy carrier hydrogen, has proven to be an encouraging technology to address both the energy demand and environment problems of water and air pollution. The semiconducting photoelectrodes and the cocatalysts serving as the two main components of PEC devices are indispensable to achieve sufficiently water splitting efficiency. Keeping in line with these goals, different semiconducting photoabsorbers and cocatalysts have been investigated in this thesis.In chapter 2, the ternary metal oxide CuWO4 with a band gap of 2.2–2.4 eV has been prepared as a thin film photoanode for PEC water oxidation and the performance has been augmented by a facile post-annealing under nitrogen atmosphere at 623 K for 3.5 h. The post-treated CuWO4 exhibits a photocurrent of 80 μA cm–2 at 1.23 V versus reversible hydrogen electrode (RHE) under Air Mass 1.5 Global (AM 1.5G) illumination in a phosphate buffer electrolyte at pH 7, almost three times higher when comparing to the pristine photoanode. Physical characterization indicates that post-annealing of the CuWO4 thin films in a N2 atmosphere does not introduce nitrogen into the crystal structure, thus no bathochromic shift is observed. The greater concentration of oxygen vacancies, which can improve charge carrier separation and reduce the charge transfer resistance, has been proposed the reasons of superior PEC water oxidation performance. In chapter 3, a quaternary oxynitride nanowire SrTaO2N thin film has been used as the core photoabsorber to construct a core-shell structure with functional overlayers for PEC water oxidation. The perovskite-related oxynitride structure is obtained by converting the hydrothermally grown oxide precursor on tantalum substrate via nitridation. The performance of the as-prepared nanowire SrTaO2N has been enhanced by the deposition of three functional overlayers. The first TiOx layer can protect the oxynitride from photocorrosion and suppress the recombination of charge carrier at the surface. A hole-storage layer Ni(OH)x can decrease the dark-current, leading to a significantly improved extraction of photogenerated holes to the electrode-electrolyte surface. The cocatalyst cobalt phosphate layer can increase the photocurrent up to 0.27 mA cm–2 at 1.23 V versus RHE under AM 1.5G illumination. The common dark current in case of oxynitride photoanodes grown on metallic substrates has been minimized almost to zero. In chapter 4, the tailoring of the surface properties of quaternary tantalum-based oxynitrides ATa(O,N)3 is critical to obtain efficient hole extraction. A cubic CaTaO2N particle-based photoanode has been altered by acidic treatment for PEC water oxidation. The acidic etching effect has been addressed by means of complementary physical characterization techniques, such as X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, 1H and 14N solid-state nuclear magnetic resonance (NMR) spectroscopy, and electron microscopy. Combining with PEC measurements, solid-state NMR indicates that the restructured surface displays a meaningfully higher concentration of terminating OH groups. The deposition of the cocatalyst nickel borate on the etched surface yields a higher percentual upsurge of photocurrent in comparison to pristine CaTaO2N. The work in this chapter highlights the application of solid-state NMR spectroscopy for understanding the semiconductor-catalyst interface in photochemical devices. In chapter 5, a trivalent iron-only layered oxyhydroxide mössbauerite has been investigated as cocatalyst for PEC water oxidation by coupling with a WO3 semiconductor photoanode. By combining Mott-Schottky analysis and UV-Vis diffuse reflectance spectroscopy, the band edge positions of mössbauerite have been determined. Mössbauerite is identified to be a n-type semiconductor with a flat band potential of 0.34 V versus RHE. However, the bare mössbauerite does not produce noticeable photocurrent during water oxidation. By constructing a type-II heterojunction with WO3 thin films photoanode, the charge carrier separation is amended and a photocurrent of up to 1.22 mA cm–2 at 1.23 V versus RHE is achieved. In chapter 6, the monodisperse spherical alloy FePt and pure Pt nanocrystals as cocatalysts have been used to modify p-WSe2 single-crystal photocathodes. The photocurrents of – 0.27 and – 4.0 mA cm– 2 at 0 V versus RHE, which are 7.4 and 15 times higher compared to pristine WSe2 single crystal, are achieved for the hydrogen evolution reaction (HER) after the modification with Pt or FePt, respectively. The density functional theory computations reveal that the water adsorption and thus enhanced H2O dissociation are preferential on FePt in comparison to Pt. The edge sites of both Pt and FePt are the preferential sites for hydrogen production because of a more negative adsorption energy than on the (111) and (100) facets. The size of the Pt nanocrystal within the range of Pt55 (1.1 nm) and Pt147 (1.6 nm) does not significantly influence the mechanism for the HER, as reveled by computational results
Synthesis and (photo-)electrochemical properties of nitrogen- and carbon-based materials
No full textDeveloping long-term sustainable, renewable and clean energy and aiding in a series of environmental problems while sustaining the growth of human requirements are tremendous challenges. To address these, potential solutions include utilizing photoelectrochemical (PEC) energy conversion technologies to produce hydrogen from water upon harnessing solar, as well as using electrochemical technologies to remove nitrite and nitrate contaminants by reducing them to ammonia. The metal carbodiimide and nitrogen-doped carbon (NDC) materials, as nitrogen- and carbon-based (N,C-based) material components, are employed as electrodes and/or cocatalysts. These materials combine strategies of improving the (photo-)electrochemical efficiencies for application in energy conversion and environmental purification. The synthesis and (photo-)electrochemical properties of N,C-based materials have been investigated in this thesis. In chapter 2, the tin oxide-carbodiimide Sn2O(NCN) is demonstrated as a prospective semiconductor material with a favorable band gap of 2.1 eV. The PEC properties of Sn2O(NCN) and the application of Sn2O(NCN) to couple with CuWO4 thin films to form a heterojunction for PEC water oxidation were investigated. Mott–Schottky measurements reveal that Sn2O(NCN) is an n-type semiconductor with a flat-band potential of −0.03 V vs. reversible hydrogen electrode (RHE). The position of its valence band edge is suitable for PEC water oxidation. Sn2O(NCN) increases the photocurrent density of CuWO4 thin films from 32 μA cm−2 to 59 μA cm−2 at 1.23 V vs. RHE in 0.1 M phosphate buffer (pH 7.0) under backlight AM 1.5G illumination. This upsurge in photocurrent density originates in a synergistic effect between the oxide and oxide carbodiimide, because the heterojunction photoanode exhibits a higher photocurrent density than the sum of its individual components. Structural analyses by means of powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS) reveal that Sn2O(NCN) forms a core–shell structure Sn2O(NCN)@SnPOx during the PEC water oxidation in phosphate buffer. This electrochemical activation is similar to the behavior of Mn(NCN) but different from Co(NCN). In chapter 3, bismuth oxide-carbodiimide Bi2O2(NCN) is demonstrated as a novel mixed-anion semiconductor and structurally related to bismuth oxides and bismuth oxysulfides. Given the structural versatility of these layered structures and the favorable band positions of Bi2O2(NCN), its photochemical properties for PEC water oxidation were investigated. Although Bi2O2(NCN) as a single photoabsorber employed as a photoanode does not generate a noticeable photocurrent density, the fabrication of heterojunctions with WO3 thin-film electrode shows an upsurge of photocurrent density from 0.9 mA cm−2 to 1.1 mA cm−2 at 1.23 V vs. RHE under AM 1.5G illumination in phosphate electrolyte (pH 7.0). Mechanistic analysis and structural analyses employing PXRD, scanning electron microscopy (SEM), XPS, and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM EDX) indicate that Bi2O2(NCN) transforms during operating conditions in situ to a core−shell structure Bi2O2(NCN)@BiPOx. In comparison to WO3/BiPO4, the in situ electrochemical electrolyte-activated WO3/Bi2O2(NCN) photoanode shows a higher photocurrent density owing to superior charge separation across the oxide/oxide-carbodiimide interface layer. Changing the electrolyte from phosphate to sulfate results in a lower photocurrent density. This reveals that the electrolyte determines the surface chemistry and mediates the PEC activity of the metal oxide-carbodiimide. A similar trend could be observed for CuWO4 thin-film electrode instead of WO3 thin-film electrode. These results show the potential of metal oxide-carbodiimides as relatively novel representatives of mixed-anion compounds and shed light on the importance of control over the surface chemistry to enable the in situ activation. In chapter 4, metal-free NDC is considered as a promising functional material for Green Chemistry, however, the structural determination of the atomic positions of nitrogen remains challenging. Directly-excited 15N solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful tool for determining such positions in NDC at natural 15N isotope abundance. Herein, a green approach for the synthesis of NDC using cellulose as a precursor was used, and the electrocatalytic properties and atomic structures of the related catalyst were investigated. NDC(NH3) was obtained by the nitrification of cellulose with HNO3 followed by annealing at 800 °C under NH3/H2 atmosphere. It contained 6.5 wt.% of N and had a surface area of 557 m2 g−1, and 15N ssNMR spectroscopy provided evidence for graphitic N besides regular pyrrolic and pyridinic N. This structural determination allowed to probe the role of graphitic N in electrocatalytic reactions, such as the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and nitrite reduction reaction (NO2RR). The NDC(NH3) catalyst exhibited higher electrocatalytic activities in the OER and HER under alkaline conditions and higher activity for NO2RR in comparison with a reference catalyst NDC(N2), which was prepared by the carbonization of HNO3-treated cellulose and annealing at 800 °C under N2. The electrocatalytic selectivity for NO2RR of NDC(NH3) catalyst is directly related to the graphitic N functions. Complementary structural analyses by means of 13C and 1H ssNMR, SEM, TEM, PXRD, XPS, Raman spectroscopy, and low-temperature N2 adsorption provided solid support to the findings. The results reveal that directly-excited 15N ssNMR spectroscopy at natural 15N abundance is generally capable of providing information on NDC materials if relaxation properties are favorable. It is expected that this approach can be applied to a wide range of solids with an intermediate concentration of N atoms. In chapter 5, electrochemical nitrate reduction into recyclable ammonia is one of the most promoting strategies to tackle nitrate degradation. As promising candidates of electrocatalysts for this strategy, tailored metal-free cellulose-derived NDC materials were prepared from renewable and sustainable cellulose with a combination of HNO3 treatment and carbonization under NH3/H2 atmosphere at 500 °C, 600 °C, 700 °C, 800 °C and 900 °C, respectively. Among the five tailored cellulose-derived NDC materials, the NDC material which was carbonized at 800 °C (NDC-800) showed the highest electrochemical performance, exhibiting 73.1% NH4+ selectivity and nearly 100% NO3− reduction efficiency when prolonging to 48 h CA for the nitrate reduction reaction (NO3RR) performed at the optimal potential of −1.5 V vs. Ag/AgCl in a 0.1 M Na2SO4 with 100 ppm NO3− electrolyte. Complementary structural analyses utilizing SEM, TEM, PXRD, XPS, Raman spectroscopy and low-temperature N2 adsorption provided solid supports. These results show the potential of NDC-800 as a metal-free cellulose-derived NDC material, which offers an appealing and supplementary alternative to efficient electrochemical nitrate reduction to ammonia toward the sustainable development of energy and environment. In chapter 6, the previous chapters of this thesis were concluded and a brief outlook on this area of research was given
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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