Nanostructured Ti1‑xSxO2‑yNy Heterojunctions for Efficient Visible- Light-Induced Photocatalysis

Highly visible light active S, N co-doped anatase-rutile heterojunctions is reported for the first time. The formation of heterojunctions at a relatively low temperature and visible light activity are achieved through thiourea modification of the peroxo-titania complex. FT-IR spectroscopic studies indicated the formation of a Ti4+-thiourea complex upon reaction between peroxo-titania complex and thiourea. Decomposition of the Ti4+-thiourea complex and formation of visible light active S, N co-doped TiO2 heterojunctions are confirmed using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and UV/Vis spectroscopic studies. Existence of sulfur as sulfate ions (S6+) and nitrogen as lattice (N-Ti-N) and interstitial (Ti-N-O) species in heterojunctions are identified using X-ray photoelectron spectroscopy (XPS) and FT-IR spectroscopic techniques. UV-vis and valence band XPS studies of these S, N co-doped heterojunctions proved the fact that the formation of isolated S 3p, N 2p and П* N-O states between the valence and conduction bands are responsible for the visible light absorption. Titanium dioxide obtained from the un-doped peroxo-titania complex exists as pure anatase up to a calcination temperature as high as 900 °C. Whereas, thiourea modified samples are converted to S, N co-doped anatase-rutile heterojunctions at a temperature as low as 500 °C. The most active S, N co-doped heterojunction (0.2 SN-TiO2) obtained at 600 °C exhibits a two-fold and eight-fold increase in visible light photocatalytic activities in contrast to the control sample and the commercial photocatalyst Degussa P-25 respectively. It is proposed that the efficient electron-hole separation due to anatase to rutile electron transfer is responsible for the exceptionally high visible light photocatalytic activities of S, N co-doped heterojunctions

Visible-Light Activation of TiO2 Photocatalysts: Advances in Theory and Experiments

The remarkable achievement by Fujishima and Honda (1972) in the photo-electrochemical water splitting results in the extensive use of TiO2 nanomaterials for environmental purification and energy storage/conversion applications. Though there are many advantages for the TiO2 compared to other semiconductor photocatalysts, its band gap of 3.2 eV restrains application to the UV-region of the electromagnetic spectrum ≤387.5 nm. As a result, development of visible-light active titanium dioxide is one of the key challenges in the field of semiconductor photocatalysis. In this review, advances in the strategies for the visible light activation, origin of visible-light activity, and electronic structure of various visible-light active TiO2 photocatalysts are discussed in detail. It has also been shown that if appropriate models are used, the theoretical insights can successfully be employed to develop novel catalysts to enhance the photocatalytic performance in the visible region. Recent developments in theory and experiments in visible-light induced water splitting, degradation of environmental pollutants, water and air purification and antibacterial applications are also reviewed. Various strategies to identify appropriate dopants for improved visible-light absorption and electron–hole separation to enhance the photocatalytic activity are discussed in detail, and a number of recommendations are also presented

Understanding capacity fade in silicon based electrodes for lithium-ion batteries using three electrode cells and upper cut-off voltage studies

Commercial Li-ion batteries are typically cycled between 3.0 and 4.2 V. These voltages limits are chosen based on the characteristics of the cathode (e.g. lithium cobalt oxide) and anode (e.g. graphite). When alternative anode/cathode chemistries are studied the same cut-off voltages are often, mistakenly, used. Silicon (Si) based anodes are widely studied as a high capacity alternative to graphite for Lithium-ion batteries. When silicon-based anodes are paired with high capacity cathodes (e.g. Lithium Nickel Cobalt Aluminium Oxide; NCA) the cell typically suffers from rapid capacity fade. The purpose of this communication is to understand how the choice of upper cut-off voltage affects cell performance in Si/ NCA cells. A careful study of three-electrode cell data will show that capacity fade in Si/NCA cells is due to an ever-evolving silicon voltage profile that pushes the upper voltage at the cathode to >4.4 V (vs. Li/Liþ). This behaviour initially improves cycle efficiency, due to liberation of new lithium, but ultimately reduces cycling efficiency, resulting in rapid capacity fade

Ultrafast Nanocrystalline-TiO2(B)/Carbon Nanotube Hyperdispersion Prepared via Combined Ultracentrifugation and Hydrothermal Treatments for Hybrid Supercapacitors

Anisotropically grown (b-axis short) single-nano TiO2 (B), uniformly hyper-dispersed on the surface of multiwalled carbon nanotubes (MWCNT), was successfully synthesized via an in situ ultracentrifugation (UC) process coupled with a follow-up hydrothermal treatment. The uc-TiO2 (B)/MWCNT composite materials enable ultrafast Li(+) intercalation especially along the b-axis, resulting in a capacity of 235 mA h g(-1) per TiO2 (B) even at 300C (1C = 335 mA g(-1) )

Capacitive Organic Anode Based on Fluorinated-Contorted Hexabenzocoronene: Applicable to Lithium-Ion and Sodium-Ion Storage Cells

Conducting polymer-based organic electrochemical capacitor materials have attracted attention because of their highly conductive nature and highly reversible redox reactions on the surface of electrodes. However, owing to their poor stabilities in aprotic electrolytes, alternative organic electrochemical capacitive electrodes are being actively sought. Here, fluorine atoms are introduced into contorted hexabenzocoronene (cHBC) to achieve the first small-molecule-based organic capacitive energy-storage cells that operate at high current rates with satisfactory specific capacities of approximate to 160 mA h g(-1) and superior cycle capabilities (>400) without changing significantly. This high capacitive behavior in the P2(1)/c crystal phase of fluorinated cHBC (FcHBC) is caused mainly by the fluorine atoms at the end of each peripheral aromatic ring. Combined Monte Carlo simulations and density functional theory (DFT) calculations show that the most electronegative fluorine atoms accelerate ion diffusion on the surface to promote fast Li+ ion uptake and release by an applied current. Moreover, FcHBC has potential applications as the capacitive anode in Na-ion storage cells. The fast dynamics of its capacitive behavior allow it to deliver a specific capacity of 65 mA h g(-1) at a high current of 4000 mA g(-1)

Formation of an ion-free crystalline carbon nitride and its reversible intercalation with ionic species and molecular water

The development of processes to tune the properties of materials is essential for the progression of next-generation technologies for catalysis, optoelectronics and sustainability including energy harvesting and conversion. Layered carbon nitrides have also been identified as of significant interest within these fields of application. However, most carbon nitride materials studied to date have poor crystallinity and therefore their properties cannot be readily controlled or easily related to their molecular level or nanoscale structures. Here we report a process for forming a range of crystalline layered carbon nitrides with polytriazine imide (PTI) structures that can be interconverted by simple ion exchange processes, permitting the tunability of their optoelectronic and chemical properties. Notable outcomes of our work are (a) the creation of a crystalline, guest-ion-free PTI compound that (b) can be re-intercalated with ions or molecules using "soft chemistry" approaches. This includes the intercalation of HCl, demonstrating a new ambient pressure route to the layered PTI· HCl material that was previously only available by a high-pressure-high-temperature route (c). Our work also shows (d) that the intercalant-free (IF-) PTI material spontaneously absorbs up to 10 weight% H O from the ambient atmosphere and that this process is reversible, leading to potential applications for membranes and water capture in dry environments

Phenol-Catalyzed Discharge in the Aprotic Lithium-Oxygen Battery

Discharge in the lithium‐O2 battery is known to occur either by a solution mechanism, which enables high capacity and rates, or a surface mechanism, which passivates the electrode surface and limits performance. The development of strategies to promote solution‐phase discharge in stable electrolyte solutions is a central challenge for development of the lithium‐O2 battery. Here we show that the introduction of the protic additive phenol to ethers can promote a solution‐phase discharge mechanism. Phenol acts as a phase‐transfer catalyst, dissolving the product Li2O2, avoiding electrode passivation and forming large particles of Li2O2 product—vital requirements for high performance. As a result, we demonstrate capacities of over 9 mAh cm−2areal, which is a 35‐fold increase in capacity compared to without phenol. We show that the critical requirement is the strength of the conjugate base such that an equilibrium exists between protonation of the base and protonation of Li2O2

Design and performance of rechargeable sodium ion batteries, and symmetrical li-ion batteries with supercapacitor-like power density based upon polyoxovanadates

The polyanion Li7V15O36(CO3) is a nanosized molecular cluster (≈1 nm in size), that has the potential to form an open host framework with a higher surface-to-bulk ratio than conventional transition metal oxide electrode materials. Herein, practical rechargeable Na-ion batteries and symmetric Li-ion batteries are demonstrated based on the polyoxovanadate Li7V15O36(CO3). The vanadium centers in {V15O36(CO3)} do not all have the same VIV/V redox potentials, which permits symmetric devices to be created from this material that exhibit battery-like energy density and supercapacitor-like power density. An ultrahigh specific power of 51.5 kW kg−1 at 100 A g−1 and a specific energy of 125 W h kg−1 can be achieved, along with a long cycling life (>500 cycles). Moreover, electrochemical and theoretical studies reveal that {V15O36(CO3)} also allows the transport of large cations, like Na+, and that it can serve as the cathode material for rechargeable Na-ion batteries with a high specific capacity of 240 mA h g−1 and a specific energy of 390 W h kg−1 for the full Na-ion battery. Finally, the polyoxometalate material from these electrochemical energy storage devices can be easily extracted from spent electrodes by simple treatment with water, providing a potential route to recycling of the redox active material