Influences of Partial Destruction of Ti‐MOFs on Photo(electro)catalytic H₂ Evolution by Dominating Role of Charge Carrier Trapping over Surface Area

Abstract

The design of water‐stable photo and electrocatalysts of metal–organic frameworks (MOFs) for its promising catalytic applications at long‐term irradiations or persisted current loads is extremely necessary but still remains as challenging. A limited number of reports on Ti‐MOF‐based catalysts for water splitting are only available to explain and understand the correlation between the nature of materials and MOFs array. Herein, spherical Ti‐MOFs and corresponding partially annealed hollow core–shell Ti‐MOFs (Ti‐MOF/D) are designed and the correlation with their photo(electro)catalytic water splitting performance is evaluated. The switchable valence state of Ti for the Ti‐MOF as a function of molecular bonding is the possible reason behind the observed photocatalytic hydrogen generation and light‐harvesting ability of the system. Besides, the defect state, solid core–shell mesoporous structure, and active sites of Ti‐MOF help to trap the charge carriers and the reduction of the recombination process. This phenomenon is absent for hollow core–shells Ti‐MOF/D spheres due to the rigid TiO2 outer surface although there is a contradiction in surface area with Ti‐MOF. Considering the diversity of Ti‐MOF and Ti‐MOF/D, further novel research can be designed using this way to manipulate their properties as per the requirements.

Tuning and Sustainment of High Surface Area of Nano CeO2-Anchored NiMOF Flake Heterojunctions for Efficient Photocatalytic Hydrogen Generation

“…Further, the XPS studies (Figure ) after the photocatalytic activity illustrates the presence of Ni 2+ and Ni 3+ states prominently at the binding energies at 855.5 and 873.2 eV in the Ni 2p spectrum as already mentioned . The chance of possible molecular interactions with the ligand field during irradiation can be the reason for the existence of Ni in excess NiMOF with variable oxidation states within the composite . Anyway, this provides inevitable evidence for the observed hydrogen generation activity, as depicted in the mechanism proposed next to this section.…”

“…61 The chance of possible molecular interactions with the ligand field during irradiation can be the reason for the existence of Ni in excess NiMOF with variable oxidation states within the composite. 62 Anyway, this provides inevitable evidence for the observed hydrogen generation activity, as depicted in the mechanism proposed next to this section. These switchable valence states in Ni provide additional catalytic centers during irradiation.…”

“…Thus, the photogenerated electrons at the highest occupied molecular orbital (HOMO) of NiMOF are available for the proton reduction generating hydrogen. Also, the heterojunction rich in NiMOF contributes to hydrogen production at the Ni catalytic centers due to the switchable valence states of Ni in Ni 2+ /Ni 3+ states as seen from XPS (Figure ) due to the existence of molecular interactions . Hence, the study deliberately focused on tuning the ratio of NiMOF and CeO 2 in the heterojunction with excess NiMOF in the whole catalyst.…”

“…Also, the heterojunction rich in NiMOF contributes to hydrogen production at the Ni catalytic centers due to the switchable valence states of Ni in Ni 2+ /Ni 3+ states as seen from XPS (Figure 9) due to the existence of molecular interactions. 62 Hence, the study deliberately focused on tuning the ratio of NiMOF and CeO 2 in the heterojunction with excess NiMOF in the whole catalyst. Then, the holes at the VB of CeO 2 can undergo oxidation as well as can engage in Ce 3+ / Ce 4+ transitions as reported elsewhere.…”

Fluorinated-TiO2/Mn0.2Cd0.8S S-Scheme Heterojunction with Rich Sulfur Vacancies for Photocatalytic Hydrogen Production

“…As illustrated in Figure d, the peak at 684.64 eV is identified from F 1s in the F-TiO 2 composite, which is assigned to the Ti–F species on the surface of TiO 2 . Meanwhile, as for Ti, the two peaks of F-TiO 2 at 459.05 and 464.79 eV are equivalent to Ti 2p 3/2 and Ti 2p 1/2 , respectively (Figure e). − For S, the S 2p signal peak can be divided into two characteristic peaks at 161.40 and 162.51 eV, corresponding to S 2p 3/2 and S 2p 1/2 orbital signals, respectively, indicating the existence of S 2– in the pure MCS (Figure f) . As shown in Figure g, the two characteristic peaks of Cd 3d with binding energies of 405.15 and 411.93 eV correspond to Cd 3d 5/2 and Cd 3d 3/2 , respectively.…”