Coordination Structure Modulation in Group-VIB Metal Doped Ag3PO4 Augments Active Site Density for Electrocatalytic Conversion of N2 to NH3.

Doping is considered a promising material engineering strategy in electrochemical nitrogen reduction reaction (NRR), provided the role of the active site is rightly identified. This work concerns the doping of group VIB metal in Ag3PO4 to enhance the active site density, accompanied by d-p orbital mixing at the active site/N2 interface. Doping induces compressive strain in the Ag3PO4 lattice and inherently accompanies vacancy generation, the latter is quantified with positron annihilation lifetime studies (PALS). This eventually alters the metal d-electronic states relative to Fermi level and manipulate the active sites for NRR resulting into side-on N2 adsorption at the interface. The charge density deployment reveals Mo as the most efficient dopant, attaining a minimum NRR overpotential, as confirmed by the detailed kinetic study with the rotating ring disk electrode (RRDE) technique. In fact, the Pt ring of RRDE fails to detect N2H4, which is formed as a stable intermediate on the electrode surface, as identified from in-situ attenuated total reflectance-infrared (ATR-IR) spectroscopy. This advocates the complete conversion of N2 to NH3 on Mo/Ag3PO4-10 and the so-formed oxygen vacancies formed during doping act as proton scavengers suppressing hydrogen evolution reaction resulting into a Faradaic efficiency of 54.8% for NRR.

Seasonal impact on microbiological quality of drinking water in Solan City of Himachal Pradesh, India.

Water contamination with faecal matter is usually the main cause of microbial waterborne diseases. Such diseases are an alarming situation for small cities in developing countries like India. In this research, to check the microbiological status of drinking water in Solan, Himachal Pradesh (India), water samples were collected from baories/stepwells (n = 14), handpumps (n = 9), and the municipal water distribution system (MWDS) (n = 2) in alternative months of the year (covering three main seasons). In 6 months, 150 samples were collected, and they were all examined for the presence of total coliforms and other bacterial pathogens. The associations between the isolates' ecological and seasonal prevalence were also examined. The coliforms were detected by the Most Probable Number (MPN) method, whose range was noticed from the 2-540/100-ml MPN index. The colony forming unit (CFU) count for different samples at the base log 10 value ranged from 3.03 to 6.19. Different genera isolated and identified were Escherichia coli, Salmonella enteric subsp. enterica, Pseudomonas spp., Klebsiella spp., and Staphylococcus aureus. Overall, 74% of the isolates identified in water samples were from the Enterobacteriaceae family. E. coli was about 42.67% (n = 102), followed by Salmonella enterica subsp. enterica 20.92% (n = 50), Staphylococcus aureus 13.38% (n = 32), Pseudomonas spp. 12.55% (n = 30), and Klebsiella spp. 10.46% (n = 25) amongst the total of 239 isolates. The seasonal impact and the dependency of the occurrence of bacteria on one another were determined to be insignificant in the Spearman correlation test. These results showed that external factors (anthropogenic activities) are mainly responsible for the presence of these bacteria in water resources. The occurrence of bacterial isolates has been noticed in all water samples, irrespective of collecting site or season.

Nanosheets of In2S3/S-C3N4-Dots for Solar Water-Splitting in Saline Water.

Hydrogen generation from splitting of water under the photoelectrochemical (PEC) pathway is considered as the most promising strategy for covering the upcoming fuel crisis by taking care of all environmental issues. In this context, In2S3 can be explored as it is a visible light-active semiconductor with an appropriate band alignment with the water redox potential. Herein, In2S3 nanosheets are developed by the chemical method. The nanosheets of In2S3 absorb high visible light due to the manifold inside scattering and reflection. The PEC activity of In2S3 is enhanced because of the increase in the light absorbance of the materials. In the present work, at 1.18 V versus RHE in 3.5 wt % NaCl, a maximum 2.07 mA/cm2 photocurrent density can be achieved by In2S3 nanosheets. However, In2S3 suffers strongly due to photo-corrosion. To improve the efficacy of the In2S3 nanosheets in saline water, the charge-carrier transportation ability of In2S3 is aimed to increase by decorating S-C3N4-dots on In2S3. The heterostructure of type-II is developed by sensitization of S-C3N4-dots on In2S3. It increases both the transportation of charge carriers as well as separation. In the heterostructure, the transient decay time (τ) increases, which indicates a decrease in photogenerated charge-carrier recombination. S-C3N4-dots also act as an optical antenna and increase the range of visible light absorbance of In2S3. The heterostructure can generate ∼2.38-fold higher photocurrent density of 1.18 V versus RHE in 3.5 wt % NaCl. The photoconversion efficiency of the heterostructure is 0.88% at 0.95 V versus RHE. The nanosheets of In2S3 and In2S3/S-C3N4-dots are stable, and photocurrent density is measured up to 2700 s under continuous back-illumination conditions.

Doping of MoS2 by "Cu" and "V": An Efficient Strategy for the Enhancement of Hydrogen Evolution Activity.

To replace Pt-based compounds in the electrocatalytic hydrogen evolution reaction (HER), MoS2 has already been established as an efficient catalyst. The electrocatalytic activity of MoS2 is further improved by tuning the morphology and the electronic structure through doping, which helps the band energy position to be modified. Presently, thin sheets of MoS2 (MoS2-TSs) are synthesized via a microwave technique. Thin sheets of MoS2 can outperform nanosheets of MoS2 in the HER. Further, the efficiency of the thin sheets is improved by doping with different metals like Cu, V, Zn, Mn, Fe, Sn, etc. "Cu"- and "V"-doped MoS2-TSs are highly efficient for the HER. At a fixed potential of -0.588 V vs RHE, Cu-doped MoS2 (Cu-MoS2-TS), V-doped MoS2 (V-MoS2-TS), and MoS2-TS can generate current densities of 327.46, 308.45, and 127.82 mA/cm2, respectively. The electrochemically active surface area increases nearly 7.7-fold and 2.5-fold for Cu-MoS2-TS and V-MoS2-TS than for MoS2-TS, respectively. Cu-MoS2-TS shows exceptionally high electrocatalytic stability up to 140 h in an acidic medium (0.5 M H2SO4). First-principles calculations using density functional theory (DFT) are performed, which are well matched with the experimental observations. DFT calculations dictate that after doping with "V" and "Cu" both valance band maxima and conduction band minima are uplifted, which indicates the higher hydrogen-ion-reducing ability of M-MoS2-TS (M = Cu, V) compared to bare MoS2-TS.

Aggregates of Ni/Ni(OH)2/NiOOH Nanoworms on Carbon Cloth for Electrocatalytic Hydrogen Evolution.

The development of an efficient electrocatalyst for hydrogen evolution reaction (HER) is essential to facilitate the practical application of water splitting. Here, we aim to develop an electrocatalyst, Ni/Ni(OH)2/NiOOH, via electrodeposition technique on carbon cloth, which shows efficient activity and durability for HER in an alkaline medium. Phase purity and morphology of the electrodeposited catalyst are determined using powder X-ray diffraction and electron microscopic techniques. The compositional and thermal stability of the catalyst is checked using X-ray photoelectron spectroscopy and thermogravimetry analysis. Electrodeposited Ni/Ni(OH)2/NiOOH material is an efficient, stable, and low-cost electrocatalyst for hydrogen evolution reaction in a 1.0 M KOH medium. The catalyst exhibits remarkable performance, achieving a current density of 10 mA/cm2 at a potential of -0.045 V vs reversible hydrogen electrode (RHE), and the Tafel slope value is 99.6 mV/dec. The overall electrocatalytic water splitting mechanism using Ni/Ni(OH)2/NiOOH catalyst is well explained, where formation and desorption of OH- ion on the catalyst surface are significant at alkaline pH. The developed electrocatalyst shows significant durability up to 200 h in a negative potential window in a highly corrosive alkaline environment along with efficient activity. The electrocatalyst can generate 165.6 µmol of H2 in ∼145 min of reaction time with 81.5% faradic efficiency.

Type-II Heterostructure of ZnO and Carbon Dots Demonstrates Enhanced Photoanodic Performance in Photoelectrochemical Water Splitting.

Hydrogen evolution through ecofriendly photoelectrochemical (PEC) water splitting is considered to be one of the most cost-effective and desirable methods for meeting ever-growing energy demands. However, the low photoconversion efficiency limits the practical applicability of PEC water splitting. To develop an efficient photoelectrode, here the morphology of ZnO is tuned from 0D to 3D. It is observed that vertically grown 2D nanosheets outperform other morphologies in PEC water splitting by generating nearly 0.414 mA cm-2 at 0 V vs Ag/AgCl. Furthermore, these perpendicularly developed 2D nanosheets of ZnO are sensitized by metal-free carbon (C) dots to improve the photoconversion efficiency of ZnO. The prepared ZnO/C dots work as an effective photoanode, which can produce a 0.831 mA cm-2 photocurrent density upon application of 0 V vs Ag/AgCl under constant illumination, which is 2 times higher than that of bare ZnO. The enhanced PEC performance of ZnO/C dots is confirmed by the photoconversion efficiency (η). The ZnO/C dots exhibit a 2-fold-higher photoconversion efficiency (η) compared to that of ZnO. Additionally, the enhancement in PEC activity of ZnO/C dots is attributed to the higher carrier concentrations in the heterostructure. Bare ZnO has a 1.77 × 1020 cm-3 carrier density, which becomes 3.70 × 1020 cm-3 after sensitization with C dots. Enhanced carrier density successively leads to higher PEC water splitting efficiency. Band alignments of ZnO and C dots indicate the creation of the type-II heterostructure, which facilitates successful charge transportation among C dots and ZnO, producing a charge-carrier separation. Two-dimensional sheets of ZnO and ZnO/C dots exhibit appreciable stability under continuous illumination for 1 and 2 h, respectively.

2D Thin Sheet Heterostructures of MoS2 on MoSe2 as Efficient Electrocatalyst for Hydrogen Evolution Reaction in Wide pH Range.

Two-dimensional layered transition metal dichalcogenides, MoSe2 and MoS2, have drawn potential attention in the field of water splitting. Coupling of MoS2 and MoSe2 provides a sustainable route to improve the electrocatalytic activity for the hydrogen evolution reaction (HER). Here, the heterostructures of thin sheets (ts) of MoSe2 and MoS2 are combined to develop the MoSe2-ts@MoS2-ts heterostructure via multiple-step methodology. First, thin sheets of MoSe2 are synthesized following the stepwise hydrothermal method. After the successful synthesis of MoSe2-ts, MoS2-ts is synthesized on it to develop the heterostructure: MoSe2-ts@MoS2-ts. By tuning the amount of MoS2-ts and MoSe2-ts in the heterostructure separately, the optimum condition is obtained for HER. The unique heterostructure is efficient for HER under wide pH conditions like 1 M KOH, pH-7 phosphate buffer, 3.5% saline water, and finally 0.5 M H2SO4. MoSe2-ts@MoS2-ts can generate 10 mA/cm2 current density under the application of -0.186 V vs RHE with a low Tafel value of 71 mV/decade. The formation of the heterojunction plays an essential role in facilitating charge transportation. Furthermore, the heterostructure provides the more active sites for the adsorption of hydrogen to generate H2. An excess amount of any of the bare counter parts in the heterostructure leads to a decrease in electrocatalytic efficiency because of the lowered heterojuction formation. MoSe2-ts@MoS2-ts has very high stability during the electrocatalytic reaction, which is determined from 1000 consecutive cycles and a 24 h prolonged scan. MoSe2-ts@MoS2-ts can generate 147 µmol of H2 in ∼50 min of reaction time with 100% Faradaic efficiency.

Nanosheets of MoSe2@M (M = Pd and Rh) function as widespread pH tolerable hydrogen evolution catalyst.

In this present study we have developed method for the synthesis of MoSe2 nanosheets following a simple hydrothermal technique. Palladium (Pd) and rhodium (Rh) nanoparticles were decorated on the surface of MoSe2 following a simple wet-chemical route. Pd and Rh nanoparticles decorated MoSe2 were applied for hydrogen evolution reaction (HER) in different pH conditions like acidic (0.5 M H2SO4), neutral (pH-7 buffer) and in alkaline (1 M KOH) medium and 3.5 wt% of saline water. Pd and Rh decorated MoSe2 show efficient activity towards HER irrespective of the applied electrolyte. In 0.5 M H2SO4, MoSe2 can produce 10 mA/cm2 current density with applied potential of -0.256 V vs. RHE. Rh decorated MoSe2 shows more shift in the onset potential. Upon applied potential of -0.192 V vs. RHE, MoSe2/Rh can produce 10 mA/cm2 current density. MoSe2/Rh is electrocatalytically more active than MoSe2/Pd which is established from the calculated electrochemically active surface area (ECSA) value. Significantly lower (47 mV/decade) Tafel value is observed for MoSe2/Rh in 0.5 M H2SO4 which indicates the superior activity. MoSe2/Rh is more stable in neutral and alkaline medium compared to acidic medium and it can retain its own activity even after continuous 12 h reaction.