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The search for renewable and clean energy sources is a key aspect for sustainable development as energy consumption has continuously increased over the years concomitantly with environmental concerns caused by the use of fossil fuels. Semiconductor materials have great potential for acting as photocatalysts for solar fuel production, a potential energy source able to solve both energy and environmental concerns. Among the studied semiconductor materials, those based on niobium pentacation are still shallowly explored, although the number of publications and patents on Nb(V)-based photocatalysts has increased in the last years. A large variety of Nb(V)-based materials exhibit suitable electronic/morphological properties for light-driving reactions. Not only the extensive group of Nb2O5 polymorphs is explored, but also many types of layered niobates, mixed oxides, and Nb(V)-doped semiconductors. Therefore, the aim of this manuscript is to provide a review of the latest developments of niobium based photocatalysts for energy conversion into fuels, more specifically, CO2 reduction to hydrocarbons or H2 evolution from water. Additionally, the main strategies for improving the photocatalytic performance of niobium-based materials are discussed.

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Immobilization of Re­(I) CO2 reduction photocatalysts on metal oxide surfaces is an interesting approach to improve their stability and recyclability. In this work, we describe the photocatalytic activity of two Re­(I) complexes (fac-[Re­(NN)­(CO)3(Cl)], NN = 4,4'-dicarboxylic acid-2,2'-bipyridine, 1, or 5,6-dione-1,10-phenantroline, 2) on the surface of hexaniobate nanoscrolls. After adsorption, the turnover number for CO production (TONCO) in DMF/TEOA of 1 was increased from 9 to 58, which is 20% higher than that observed on TiO2, being among the highest reported values for a Re­(I)-based photocatalyst under visible light irradiation without any sensitizer. The complex 2 is inactive in solution under visible-light irradiation, but it has a TONCO of 35 when immobilized on hexaniobate nanoscrolls. Transient absorption spectroscopy studies reveal that the slow back-electron transfer and the higher reducing power of the hexaniobate conduction-band electrons play a major role for the photocatalytic process. The results provide new insights concerning the role of the metal oxide substrate on Re­(I)-based molecular systems for CO2 reduction.

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The preparation of lamellar nanostructures through exfoliation of stacked niobates is an interesting approach to the development of photocatalysts for energy conversion and environmental remediation. These materials exhibit a rich surface chemistry and several nanocomposites can be produced through intercalation or impregnation of suitable precursors. In this work, the influence of the physico-chemical preparation conditions on the photocatalytic activity of Pt-hexaniobate nanocomposites was investigated aiming at the establishment of the main factors that control their photoreactivities. Modification of hexaniobate layers were carried out by adsorption and impregnation methods, using [Pt(NH3)4]Cl2 (Pt1) and H2PtCl6 (Pt2), respectively. The addition of platinum precursors (1% wt.) were performed in the presence of the exfoliating agent tert-butylammonium hydroxide, sNb, or after acidic precipitation followed by resuspension in plain water, eNb. All samples were submitted to photoirradiation to reduce the platinum precursors and the effect of a previous thermal treatment was also evaluated. It was observed that H2 evolution from aqueous methanol solutions is more favored on hexaniobate nanosheets (eNb-Pt1 and eNb-Pt2) instead of scrolled layers (sNb-Pt1 and sNb-Pt2), independent on the platinum precursor. Moreover, residual tert-butylammonium can act as hole scavenger and decrease the degradation rates for methanol oxidation in sNb samples. The curled layers observed for sNb samples seem to favor the photodegradation of cationic species, such as methylene blue. Thermal treatment at 500 °C leads to morphological changes with a decrease of the specific surface area due to restacking of the individual layers along with some curling. As a result, the H2 evolution rates strongly decreases in relation to the non-sintered samples, suggesting that the ‘soft’ photoreduction of platinum precursors is the best method for preparation of these photocatalysts. The correlations between the preparation conditions and the photocatalytic activity for different photoreactions can allow the development of optimized materials for specific applications.

Layered niobates are well-known photocatalysts for H2 evolution with a rich surface chemistry. Their photoactivity, however, is limited by their wide band gap energy (∼3.5 eV) and partial deactivation due to surface poisoning by photogenerated H2O2. In this way, a surface modification method able to induce novel electronic processes without changing the bulk properties can improve their performance. In this work, the surface of exfoliated hexaniobate (K4–x H x Nb6O17) layers was modified by grafting with metal ions such as Co­(II) and Fe­(III) and their photocatalytic properties were fully investigated. Morphological characterization showed that grafting ions are attached to the niobate surface forming amorphous clusters. These species induce an additional absorption feature in the UV-A region, which is attributed to an interfacial charge transfer from the niobate valence band to the metal ion centers. Enhanced UV-driven photoactivity was observed for 0.1% grafted samples, especially for those modified with Co­(II) ions, while smaller H2 evolution rates are observed as the concentration of the grafting ions increases. When Pt was added to the photocatalyst, the H2 evolution rate for the 0.1% Co-grafted sample in plain water was 70% higher than that observed for the nongrafted Pt-hexaniobate. Full characterization by electron paramagnetic resonance, transient absorption spectroscopy, and photoelectrochemical measurements reveals that grafted ions can work as both electron and hole acceptors. In the presence of Pt as a preferential electron acceptor, Co­(II) ions act as hole acceptors forming Co­(III) centers, favoring the formation of OH• radicals from water and avoiding surface poisoning. At higher Co­(II) concentrations and in the absence of Pt clusters, electrons are trapped at the Co centers, decreasing the H2 evolution rate. Thus, grafted Co­(II) ions act as active redox shuttles in the hexaniobate sheets, contributing to more efficient charge separation.

The interaction of light with semiconducting materials becomes the center of a wide range of technologies, such as photocatalysis. This technology has recently attracted increasing attention due to its prospective uses in green energy and environmental remediation. The characterization of the electronic structure of the semiconductors is essential to a deep understanding of the photocatalytic process since they influence and govern the photocatalytic activity by the formation of reactive radical species. Electron paramagnetic resonance (EPR) spectroscopy is a unique analytical tool that can be employed to monitor the photoinduced phenomena occurring in the solid and liquid phases and provides precise insights into the dynamic and reactivity of the photocatalyst under different experimental conditions. This review focus on the application of EPR in the observation of paramagnetic centers formed upon irradiation of titanium dioxide and niobium oxide photocatalysts. TiO2 and Nb2O5 are very well-known semiconductors that have been widely used for photocatalytic applications. A large number of experimental results on both materials offer a reliable platform to illustrate the contribution of the EPR studies on heterogeneous photocatalysis, particularly in monitoring the photogenerated charge carriers, trap states, and surface charge transfer steps. A detailed overview of EPR-spin trapping techniques in mechanistic studies to follow the nature of the photogenerated species in suspension during the photocatalytic process is presented. The role of the electron donors or the electron acceptors and their effect on the photocatalytic process in the solid or the liquid phase are highlighted.