Three-dimensional MoS2–CdS–γ-TaON hollow composites have been successfully synthesized by anchoring MoS2–CdS nanocrystals on the surfaces of γ-TaON hollow spheres via a two-step ion-exchange route with assistance from a hydrothermal process. Even without the noble-metal cocatalyst, the as-prepared MoS2–CdS–γ-TaON hollow structure with 1 wt% MoS2/CdS cocatalyst (0.2 wt% MoS2) decorated on its surface produces a high photocatalytic hydrogen production rate of 628.5 μmol h−1.

Hierarchical tantalum-based oxide and (oxy)nitride with hollow urchin-like nanostructures have been synthesized for the first time by an in situ self-assembly wet-chemical route in addition with post-thermal nitridation. Notably, a single-phase metastable γ-TaON with hollow urchin-like spheres among the tantalum (oxy)nitrides was obtained during the phase transformation process from an orthorhombic Ta2O5 to a typical monoclinic β-TaON, corresponding the order of phase formation: Ta2O5 → γ-TaON → β-TaON → Ta3N5. The combined effect of the crystal and electronic structures and hierarchical morphology on the tunable photocatalytic and photoelectrochemical performances of the tantalum-based photocatalysts was systematically investigated. Efficient photocatalytic hydrogen production as high as 381.6 μmol h−1 with an apparent quantum efficiency of 9.5% under 420 nm irradiation (about 47.5 times higher than that of the conventional TaON) was achieved over this metastable γ-TaON architecture. Furthermore, the hierarchical γ-TaON photoanode exhibited a photocurrent density of ∼1.4 mA cm−2 at 0.8 V vs. SCE in Na2SO4 solution under visible light irradiation. This excellent photocatalytic activity is ascribed to the unique urchin-like nanostructure with large specific surface area, the metastable crystal structure and appropriate electronic structure as well as the efficient charge carrier separation.

Ta3N5 nanorod arrays were fabricated by nitridation of fluorine-containing tantalum oxide (F–Ta2O5) nanorod arrays grown in situ on Ta substrates by a one-pot vapour-phase hydrothermal induced self-assembly technique. In this protocol, the in situ generation and the morphology of arrays elaborately adjusted by reaction time, play a vital role in the formation of the F–Ta2O5 nanorod arrays and a highly conductive interlayer between the nanorods and the substrate. Due to the shape anisotropy, ordered hierarchical structure and high surface area, a high photoelectrochemical activity was achieved by the optimum Ta3N5 nanorod photoelectrode with a photocurrent density of 1.22 mA cm−2 under AM 1.5G irradiation at 1.23 V vs. RHE (reversible hydrogen electrode). Furthermore, a higher and more stable photocurrent was demonstrated by combining the highly active Ta3N5 nanorods with stable Co3O4/Co(OH)2 (Co3O4/Co(II)) bilayer catalysts when compared with that demonstrated for Co(II)/Ta3N5 and Co3O4/Ta3N5 photoelectrodes, exhibiting that not only is the onset potential negatively shifted but also the photocurrent and the stability are significantly improved, which is correlated to an order of magnitude reduction in the resistance to charge transfer at the Ta3N5/H2O interface. Specifically, about 92% of the initial stable photocurrent remains after long-term irradiation at 1.23 V vs. RHE. At 1.23 V vs. RHE, the photocurrent density of Co3O4/Co(II)/Ta3N5 arrays reached 3.64 mA cm−2 under AM 1.5G simulated sunlight at 100 mW cm−2, and a maximum IPCE of 39.5% was achieved at 440 nm. This combination of catalytic activity, stability, and conformal decoration makes this a promising approach to improve the photoelectrochemical performance of photoanodes in the general field of energy conversion.

Noble metal modification has been demonstrated to be an efficient strategy to improve the photocatalytic performance of TaON photocatalysts. However, the previous studies about the noble metal modification are only restricted to the surface loading of metallic noble metal, investigations have seldom been focused on the simultaneously deposited and doped X/TaXzON (X = Pt, Ru) photocatalyst. In this work, single-phase Ta1−zXzO1+zN1−z (X = Pt, Ru, 0 < Z ≤ 0.001) (as TaXzON) and heterostructured Xδ/Ta1−z+δXz−δO1+z−δN1−z+δ (X = Pt, Ru, 0.001 < Z ≤ 0.016) (as Xδ/TaXz−δON) photocatalysts were synthesized by a sol–gel method in combination with a post-treatment nitridation process. The chemical states and form of noble metals in the as-prepared TaXzON and Xδ/TaXz−δON samples were characterized by XPS and HRTEM. In a few noble metal doping sol–gel processes, the visible-light photocatalytic activity of single-phase TaPtzON (Z = 0.0005) and TaRuzON (Z = 0.0005) showed a rate of H2 production at 51 and 90 μmol h−1 higher than that of the pristine TaON. With the increase of noble metal content, the photocatalytic activity of heterostructured Ptδ/TaPtz−δON (Z = 0.004) and Ruδ/TaRuz−δON (Z = 0.004) exhibited a rate of H2 evolution at 198 and 258 μmol h−1 which was about 28 and 6 times than the TaON loaded noble metal by photodeposition, respectively. Moreover, further photodeposition of noble metal was performed at the heterostructured materials. It was found that the highest photocatalytic activity of the catalyst was achieved based on Xδ/TaXz−δON (X = Pt, Ru, Z = 0.002). This enhanced photocatalytic activity is mainly ascribed not only to the doping effect leading to formation of an isolated energy level contributing to more visible-light absorption but also to the well-distributed noble metal on the surface of the photocatalysts with strong interaction with TaON resulting in photoinduced interfacial charge transfer. This work may provide some insight into the smart design of novel and high-efficiency photocatalytic materials.

Single-crystalline niobium oxide fluoride (Nb3O7F) hierarchical nanostructures are firstly prepared via a facile hydrothermal method without using any template or surfactant. The results of scanning electron microscopy and transmission electron microscopy showed that the hierarchical morphology of Nb3O7F could be effectively controlled by adjusting the reaction time. Ultraviolet-visible spectra showed that such nanostructures have a narrow absorption peak at around 400 nm compared to Nb2O5. Based on the first-principles plane-wave ultrasoft pseudo potential (USPP) method, the crystal structures of Nb3O7F was optimally calculated for the total density of states (TDOS) and the partial density of states (PDOS) of Nb, O and F atoms. According to the observations of architectures formation, a possible growth mechanism was proposed to explain the transformation of nanoparticles to hierarchical nanostructures via an Ostwald ripening mechanism followed by self-assembly. In particular, the excellent photocatalytic activity of the Nb3O7F hierarchical nanostructures was confirmed by photodegradation of methylene blue, methyl orange and rhodamine B molecules.

Photocatalytic oxygen evolution with a high efficiency was achieved using tantalum nitride (Ta3N5) quantum dots (QDs) coupled TaON hollow spheres (Ta3N5–TaON). TaON hollow spheres coupled with the surface enriched Ta3N5 QDs were prepared by an in situ chemical reduction route in ammonia solution at −45 °C and were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, UV-vis diffuse reflectance spectra and photoluminescence spectra. The Ta3N5–TaON composites containing 4 mol% Ta3N5 QDs showed a high rate of O2 production at 208.2 μmol h−1 with an apparent quantum efficiency of 67% under 420 nm light. The rate of oxygen formation of the Ta3N5–TaON heterojunction was 3.3 times higher than that of the pristine TaON hollow spheres. Furthermore, relative photoelectrochemical properties of Ta3N5–TaON composite photoelectrodes were investigated. The resulting 4 mol% Ta3N5–TaON heterojunction films exhibited a photocurrent of ca. 2.7 mA cm−2 under visible light irradiation at 1.0 V vs. SCE in Na2SO4 solution. This excellent photocatalytic activity is ascribed to the Ta3N5 QDs that alter the energy levels of the conduction and valence bands in the coupled semiconductor system and the slow recombination of photogenerated electron–hole pairs. Moreover, the Ta3N5–TaON composite exhibited strong durability which could be attributed to the inhibition of Ta3N5 QDs leaching owing to its strong interaction with TaON.

Large-scale hydrogen production through water splitting using photocatalysts with solar energy can potentially produce clean fuel from renewable resources. In this work, photocatalytic evolution of H2 with a high efficiency was achieved using graphene oxide (GO) nanosheets decorated with CdS sensitized TaON core–shell composites (GO–CdS@TaON). The CdS@TaON core–shell nanocomposites were prepared by an ion-exchange route with assistance from a hydrothermal process on GO as the support. The TaON core–shell composites containing 1 wt% CdS nanocrystals showed a high rate of H2-production at 306 μmol h−1 with an apparent quantum efficiency (QE) of 15% under 420 nm monochromatic light. The rate of hydrogen formation was 68 times faster in comparison with the rate observed on pure TaON. The rate was further increased to 633 μmol h−1 with a high quantum efficiency of 31% when the GO–CdS@TaON hybrid composite was coupled with 1 wt% of graphene oxide and 0.4 wt% of Pt (about 141 times higher than that of the pristine TaON). This high photocatalytic H2-production activity is ascribed firstly to the presence of CdS nanocrystals that alter the energy levels of the conduction and valence bands in the coupled semiconductor system; secondly to the involvement of graphene oxide that serves as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from CdS@TaON composites. This investigation can open up new possibilities for the development of highly efficient TaON-based photocatalysts that utilize visible light as an energy source.

Large-scale hydrogen production through water splitting using photocatalysts with solar energy can potentially produce clean fuel from renewable resources. In this work, photocatalytic hydrogen evolution with a high efficiency was achieved using CdS nanocrystal decorated CdLa2S4 microspheres (CdS/CdLa2S4) successfully prepared by a two-step hydrothermal process. The obtained CdS/CdLa2S4 composite was characterized by X-ray diffraction (XRD), electron microscopy (EM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance absorption spectroscopy (DRS), and photoluminescence spectroscopy (PL). XRD demonstrated that highly crystalline hexagonal CdS was obtained in CdS/CdLa2S4. EM results revealed that CdLa2S4 microspheres assembled from a large number of nanoprisms, were intimately enwrapped by the surrounding CdS nanocrystals with a particle size below 20 nm. This unique architecture resulted in the appropriate dispersion of CdS nanocrystals and intimate multipoint contacts between the CdS nanocrystals and CdLa2S4, which led to significant enhancement of charge separation in CdS/CdLa2S4. Especially, the CdLa2S4 microspheres decorated with 3 wt% CdS nanocrystals containing 0.4 wt% of Pt showed a high rate of H2-production at 2.25 mmol h−1 with an apparent quantum efficiency of 54% under 420 nm monochromatic light. The rate of hydrogen evolution from water splitting was 9 times faster in comparison with the rate observed on pure CdLa2S4, which is ascribed to the presence of CdS nanocrystals that alter the energy levels of the conduction and valence bands in the coupled semiconductor system.

Large-scale hydrogen production through water splitting using photocatalysts with solar energy can potentially produce clean fuel from renewable resources. In this work, photocatalytic evolution of H2 with a high efficiency was achieved using graphene oxide (GO) nanosheets decorated with CdS sensitized TaON core–shell composites (GO–CdS@TaON). The CdS@TaON core–shell nanocomposites were prepared by an ion-exchange route with assistance from a hydrothermal process on GO as the support. The TaON core–shell composites containing 1 wt% CdS nanocrystals showed a high rate of H2-production at 306 μmol h−1 with an apparent quantum efficiency (QE) of 15% under 420 nm monochromatic light. The rate of hydrogen formation was 68 times faster in comparison with the rate observed on pure TaON. The rate was further increased to 633 μmol h−1 with a high quantum efficiency of 31% when the GO–CdS@TaON hybrid composite was coupled with 1 wt% of graphene oxide and 0.4 wt% of Pt (about 141 times higher than that of the pristine TaON). This high photocatalytic H2-production activity is ascribed firstly to the presence of CdS nanocrystals that alter the energy levels of the conduction and valence bands in the coupled semiconductor system; secondly to the involvement of graphene oxide that serves as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from CdS@TaON composites. This investigation can open up new possibilities for the development of highly efficient TaON-based photocatalysts that utilize visible light as an energy source.

Single-crystalline niobium oxide fluoride (Nb3O7F) hierarchical nanostructures are firstly prepared via a facile hydrothermal method without using any template or surfactant. The results of scanning electron microscopy and transmission electron microscopy showed that the hierarchical morphology of Nb3O7F could be effectively controlled by adjusting the reaction time. Ultraviolet-visible spectra showed that such nanostructures have a narrow absorption peak at around 400 nm compared to Nb2O5. Based on the first-principles plane-wave ultrasoft pseudo potential (USPP) method, the crystal structures of Nb3O7F was optimally calculated for the total density of states (TDOS) and the partial density of states (PDOS) of Nb, O and F atoms. According to the observations of architectures formation, a possible growth mechanism was proposed to explain the transformation of nanoparticles to hierarchical nanostructures via an Ostwald ripening mechanism followed by self-assembly. In particular, the excellent photocatalytic activity of the Nb3O7F hierarchical nanostructures was confirmed by photodegradation of methylene blue, methyl orange and rhodamine B molecules.

Three-dimensional MoS2–CdS–γ-TaON hollow composites have been successfully synthesized by anchoring MoS2–CdS nanocrystals on the surfaces of γ-TaON hollow spheres via a two-step ion-exchange route with assistance from a hydrothermal process. Even without the noble-metal cocatalyst, the as-prepared MoS2–CdS–γ-TaON hollow structure with 1 wt% MoS2/CdS cocatalyst (0.2 wt% MoS2) decorated on its surface produces a high photocatalytic hydrogen production rate of 628.5 μmol h−1.

Noble metal modification has been demonstrated to be an efficient strategy to improve the photocatalytic performance of TaON photocatalysts. However, the previous studies about the noble metal modification are only restricted to the surface loading of metallic noble metal, investigations have seldom been focused on the simultaneously deposited and doped X/TaXzON (X = Pt, Ru) photocatalyst. In this work, single-phase Ta1−zXzO1+zN1−z (X = Pt, Ru, 0 < Z ≤ 0.001) (as TaXzON) and heterostructured Xδ/Ta1−z+δXz−δO1+z−δN1−z+δ (X = Pt, Ru, 0.001 < Z ≤ 0.016) (as Xδ/TaXz−δON) photocatalysts were synthesized by a sol–gel method in combination with a post-treatment nitridation process. The chemical states and form of noble metals in the as-prepared TaXzON and Xδ/TaXz−δON samples were characterized by XPS and HRTEM. In a few noble metal doping sol–gel processes, the visible-light photocatalytic activity of single-phase TaPtzON (Z = 0.0005) and TaRuzON (Z = 0.0005) showed a rate of H2 production at 51 and 90 μmol h−1 higher than that of the pristine TaON. With the increase of noble metal content, the photocatalytic activity of heterostructured Ptδ/TaPtz−δON (Z = 0.004) and Ruδ/TaRuz−δON (Z = 0.004) exhibited a rate of H2 evolution at 198 and 258 μmol h−1 which was about 28 and 6 times than the TaON loaded noble metal by photodeposition, respectively. Moreover, further photodeposition of noble metal was performed at the heterostructured materials. It was found that the highest photocatalytic activity of the catalyst was achieved based on Xδ/TaXz−δON (X = Pt, Ru, Z = 0.002). This enhanced photocatalytic activity is mainly ascribed not only to the doping effect leading to formation of an isolated energy level contributing to more visible-light absorption but also to the well-distributed noble metal on the surface of the photocatalysts with strong interaction with TaON resulting in photoinduced interfacial charge transfer. This work may provide some insight into the smart design of novel and high-efficiency photocatalytic materials.

Photocatalytic oxygen evolution with a high efficiency was achieved using tantalum nitride (Ta3N5) quantum dots (QDs) coupled TaON hollow spheres (Ta3N5–TaON). TaON hollow spheres coupled with the surface enriched Ta3N5 QDs were prepared by an in situ chemical reduction route in ammonia solution at −45 °C and were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, UV-vis diffuse reflectance spectra and photoluminescence spectra. The Ta3N5–TaON composites containing 4 mol% Ta3N5 QDs showed a high rate of O2 production at 208.2 μmol h−1 with an apparent quantum efficiency of 67% under 420 nm light. The rate of oxygen formation of the Ta3N5–TaON heterojunction was 3.3 times higher than that of the pristine TaON hollow spheres. Furthermore, relative photoelectrochemical properties of Ta3N5–TaON composite photoelectrodes were investigated. The resulting 4 mol% Ta3N5–TaON heterojunction films exhibited a photocurrent of ca. 2.7 mA cm−2 under visible light irradiation at 1.0 V vs. SCE in Na2SO4 solution. This excellent photocatalytic activity is ascribed to the Ta3N5 QDs that alter the energy levels of the conduction and valence bands in the coupled semiconductor system and the slow recombination of photogenerated electron–hole pairs. Moreover, the Ta3N5–TaON composite exhibited strong durability which could be attributed to the inhibition of Ta3N5 QDs leaching owing to its strong interaction with TaON.