The Ni/NiOx particles were in situ photodeposited on MIL-101 metal organic frameworks as catalysts for boosting H2 generation from Erythrosin B dye sensitization under visible-light irradiation. The highest H2 production rate of 125 μmol h−1 was achieved from the system containing 5 wt. % Ni-loaded MIL-101 (20 mg) and 30 mg Erythrosin B dye. Moreover, the Ni/NiOx catalysts show excellent stability for long-term photocatalytic reaction. The enhancement on H2 generation is attributed to the efficient charge transfer from photoexcited dye to the Ni catalyst via MIL-101. Our results demonstrate that the economical Ni/NiOx particles are durable and active catalysts for photocatalytic H2 generation.

Solar hydrogen generation through water splitting over photocatalysts is an ideal route to convert solar energy into chemical fuel energy. Since the first report on photocatalytic water splitting in 1972, great endeavors have been devoted to exploring the hydrogen generation systems that always contain light-absorbent (photosensitizer or/and photocatalyst), metal-based catalyst, and sacrificial electron donor.1 The metal-based catalysts can not only serve as the electron sink to promote the separation of photogenerated electrons and holes but also act as the active sites to lower the H2 evolution overpotential.2 So far the noble metals, such as Pt, Pd, and Au, have been extensively used as hydrogen generation catalysts. Among them, Pt is the most active catalyst for hydrogen evolution reaction (HER).3 However, scale-up usages of these noble metals are questionable due to the high cost and scarcity in nature. For practical applications, economic catalysts with reasonable activities are substantially needed for solar hydrogen production.

Enlightened by the active sites in natural hydrogenases to promote hydrogen generation, the complexes of earth-abundant metals, such as Fe, Co, and Ni, have been attempted to catalyze water reduction.4 In general, these molecular complexes are unstable under light irradiation after long term usages. To transcend this problem, the nanoparticles of these metal compounds have been used as the alternative catalysts to replace their complexes.5 Note that the photocatalysts in these hydrogen generation systems are mainly confined to the metal oxides/sulfides.5 Taking consideration of the base material effect on hydrogen generation, it is very meaningful to investigate the loading of these cheap metals on novel photocatalysts.

Metal Organic Frameworks (MOFs) are a class of crystalline solids composed of metal ions or clusters coordinated to organic linkers,6 and have been used extensively in gas storage, molecular sensing, catalysis, and separation.7 Just like metal oxides in which the metal ions are linked by the O2− ions, organic linkers in MOFs function as that of O2− ions in metal oxides to link the metal centers. Therefore, from the viewpoint of energy band, the empty outer orbitals of metal centers in MOFs can form the conduction band while the outer orbitals of organic linkers contribute to compose the valence band. In principle, MOFs can be excited to produce electron-hole pairs under light irradiation.8 Recently, the photocatalytic water splitting for hydrogen production over MOFs has been reported by several groups.9 For example, Garcia and coworkers reported the hydrogen generation over UiO-66 MOFs under UV light irradiation.10 More recently, dye-sensitization strategy has been used to improve the hydrogen generation rate of MOFs.11 Up to now, various HER catalysts, such as noble metal Pt, platinum complex, and hydrogenase, have been employed to boost hydrogen generation over MOFs.12 However, loading of economical metal catalysts, such as Ni, Co, and Fe, on MOFs has not been explored yet for enhancing hydrogen generation.

Herein, we report the in situ photodeposition of Ni/NiOx nanoparticles on MIL-101 photocatalysts for hydrogen generation. Erythrosin B (denoted as ErB) dye was used as light absorbing antenna in the reaction system as ErB dye has been proved as a durable sensitizer for photocatalytic H2 production over MOFs in our recent work.11(a) The H2 generation system was denoted as Nix/101/Ey, in which 101 refers to the MIL-101 MOF, E refers to the ErB dye, x is weight percentage of the loaded Ni on MIL-101 based on the amount of nickel nitrate precursor, and y represents the amount (mg) of ErB dye in H2 generation system. It was found that the Ni/NiOx nanoparticles can act as efficient catalysts for hydrogen generation. The hybrid photocatalyst with optimum 5 wt. % Ni catalyst (Ni5/101/E30) shows a H2 generation rate of 125 μmol h−1. In addition, Ni/MIL-101 hybrids possess excellent stability for hydrogen generation. No obvious decrease on hydrogen generation rate occurs after 8 h reactions.

The base material MIL-101 was hydrothermally prepared according to the previous procedure,13 followed by in situ photodeposition of Ni nanoparticles in the solution containing nickel nitrate, triethanolamine (TEOA), and ErB dye for photocatalytic H2 generation. In Fig. S1, the narrow and sharp peaks reveal high crystallinity of the resultant MIL-101 products. All the diffraction peaks can be indexed to those of MIL-101, suggesting the single phase MIL-101 according to Fig. S1(A) in the supplementary material.14 Fig. S1(B) in the supplementary material14 shows that no peaks belonging to Ni and Ni-derivatives (NiOx) were detected, which might be ascribed to the low Ni content and the very small particle size. Moreover, the pattern of MIL-101 was kept unchanged upon Ni loading, indicating the excellent photostability of MIL-101 in water. Fig. S2 in the supplementary material14 shows the UV-Vis spectrum of the MIL-101 which exhibits strong absorption in the visible region with an absorption edge of ∼360 nm, corresponding to a band gap of ∼3.4 eV. The evident absorption peak at 440 nm should be caused by the d-d transitions in Cr ions in MIL-101. As shown in the Fig. S3 of the supplementary material,14 the UV-Vis spectrum of the suspension containing MIL-101 and ErB dye shows a mixed absorption feature of MIL-101 and ErB dye, and a red-shift of ErB absorption peak from 553 nm to 561 nm. This small red shift in ErB absorption peak might be induced by the interaction of dye with MIL-101, which is very essential for the charge transfer from photoexcited dyes to MIL-101.

The as-prepared MIL-101 MOF is octahedral microcrystals with an average edge length of 350 nm, as observed by scanning electron microscope (SEM, Fig. 1(a)). The octahedral morphology can be further visualized by the transmission electron microscope (TEM) observations. The surface of pristine MIL-101 is very smooth (Fig. 1(b)). When the reaction suspension was exposed under visible light (λ > 420 nm) irradiation, some nanoparticles with less than 20 nm in diameter were distributed uniformly on the surface of MIL-101. The Ni loading on MIL-101 surface can be visualized by TEM observation (Figs. 1(c) and S4 in the supplementary material14). The high-resolution TEM (HRTEM) image (Fig. 1(d)) of the small nanoparticle reveals a lattice fringe of 0.202 nm inside the particle, which can be indexed to the (011) plane of Ni. Notably, the lattice spacing outside the particle is 0.208 nm, which can be indexed to the (012) crystallographic planes of NiO. This observation suggests the oxidation of metallic Ni. We could not determine whether the Ni nanoparticles were oxidized during the reaction or in air before TEM sample preparation. Nevertheless, the TEM observation still can confirm the loading of Ni@NiOx particles upon MIL-101 during the H2 generation reactions. In addition, the interfaces between Ni@NiOx particles and MIL-101 can be observed. Such an intimate contact is very essential for charge carrier transfer in the following photocatalytic reactions.

FIG. 1.

(a) SEM image, (b) TEM image of pristine MIL-101, (c) TEM image, and (d) HRTEM image of Ni nanoparticles loaded MIL-101.

FIG. 1.

(a) SEM image, (b) TEM image of pristine MIL-101, (c) TEM image, and (d) HRTEM image of Ni nanoparticles loaded MIL-101.

Close modal

In a typical photocatalytic H2 generation test, the reaction suspension contains MIL-101 microcrystals, nickel nitrate for in situ deposition of Ni/NiOx nanoparticles on MIL-101, ErB dye as the sensitizer, and TEOA as the sacrificial electron donor. Control experiments show that without nickel nitrate, the pristine MIL-101 and ErB-sensitized MIL-101 are inactive for hydrogen generation under visible light irradiation (Fig. 2(a)). However, an evident enhancement on hydrogen generation rate was achieved when small amount of nickel nitrate was added into the reaction solution (Fig. 2(a), Ni1/101/E30). This result is attributed to the photogenerated Ni/NiOx nanoparticles on MIL-101, which can serve as the highly efficient catalysts for hydrogen evolution under light irradiation. We obtained 4.8 μmol H2 after 3 h reactions, corresponding to a hydrogen generation rate of 1.6 μmol h−1. The photocatalytic H2 generation rate was controlled by the added ErB dye amount (Fig. 2(b)). The highest H2 generation rate of 125 μmol h−1 was achieved when 30 mg ErB dye was involved in the reaction system (Ni5/101/E30). The result indicates that more adsorbed dyes on MOF result in more electron transfer from photoexcited ErB dye to MIL-101. Further increase of ErB dye amount results in a decreased H2 generation rate (Ni5/101/E40). The similar phenomenon has been observed in ErB sensitized UiO-66 MOF for H2 generation.11(a) The decreased H2 generation rate might be ascribed to the increased amount of non-adsorbed dyes, which serve as competitive light-absorbing molecules over adsorbed dyes, and thereby lead to a loss of light harvesting for H2 generation. Therefore, in the following experiments, the system containing 30 mg ErB dyes was used for the evaluation of photocatalytic H2 generation.

FIG. 2.

Photocatalytic H2 generation from (a) MIL-101, Ni0/101/E30, and Ni1/101/E30, and (b) the Ni5/101/Ex (x = 5, 10, 15, 25, 30, and 40) system.

FIG. 2.

Photocatalytic H2 generation from (a) MIL-101, Ni0/101/E30, and Ni1/101/E30, and (b) the Ni5/101/Ex (x = 5, 10, 15, 25, 30, and 40) system.

Close modal

As stated above, small amount of nickel loading on MIL-101 can result in an evidently improved H2 generation. In addition, the amount of Ni-loading has great effect on H2 generation rate (Fig. 3(a)). Further increasing the Ni-loading amount led to an increased hydrogen generation rate. The highest H2 generation rate of 125 μmol h−1 was achieved when 5 wt. % Ni/NiOx nanoparticles was loaded (Ni5/101/E30). Total amount of 980 μmol H2 was produced after 9 h reactions over optimum Ni loaded MIL-101. Further increasing Ni amount causes the decrease of H2 generation rate. The occurrence of an optimum Ni loading amount reveals the existence of the interfacial active sites near the peripheries of the Ni nanoparticles. The total peripheral length increases with larger number and size of Ni nanoparticles until the active domains overlap with each other. Therefore, further increase in Ni loading amount leads to the reduced peripheral length, and thereby the low activity. Comparatively, the average H2 generation rates were measured as 82 and 52 μmol h−1 over the MIL-101 loaded with 5 wt. % Ni(OH)2 and 5 wt. % NiO, respectively, under the same experimental condition. The Ni(OH)2 and NiO samples were prepared according to Yu’s work.15 This comparison further reveals the positive effect of the Ni/NiOx catalyst in boosting the H2 generation.

FIG. 3.

(a) Effect of Ni loading amount on photocatalytic H2 generation rate over Nix/101/E30 (x = 1, 3, 5, 8 and 10), and (b) long-term H2 generation over Ni5/101/E30.

FIG. 3.

(a) Effect of Ni loading amount on photocatalytic H2 generation rate over Nix/101/E30 (x = 1, 3, 5, 8 and 10), and (b) long-term H2 generation over Ni5/101/E30.

Close modal

It was well known that Ni particles can be easily oxidized to NiOx in water.5(a) Therefore, the photostability of Ni/MIL-101 will be a severe problem for long-term usages. After 8 h photoreactions, no loss on hydrogen generation rate was observed, demonstrating the excellent photostability of Ni particles as non-noble metal co-catalysts for H2 generation (Fig. 3(b)). However, H2 generation rate reaches a plateau after 16 h reaction. The loss in H2 generation rate might be caused by the aggregation of Ni particles. To reveal this assumption, Ni/MIL-101 hybrids at various reaction times were taken for further TEM observations, as shown in Fig. 4. After 8 h reaction for H2 generation, the size and morphology of MIL-101 MOF was kept unchanged. Moreover, the Ni particle size was kept unchanged after 8 h H2 generation reaction, showing the high photostability of Ni particles as a co-catalyst. However, after 10 h reaction for H2 generation, the morphology of MIL-101 becomes collapsed, as shown in Figs. 4(c) and 4(f). However, the XRD patterns before and after 10 h reaction for H2 generation are essentially the same (Fig. S514). The above results suggest that the aggregation of Ni particles is not the reason for decreased H2 generation rate since no evident change in Ni particle size occurs when the reaction time prolonged to 8 h. Therefore, the decrease on H2 generation rate should be caused by the collapsing of MIL-101 since MOFs are normally not stable in basic solution for long time.

FIG. 4.

TEM image of Ni loaded MIL-101 at various reaction times, (a) 6 h, (b) 8 h, and (c) 10 h with scale bars of 100 nm; images of (d)–(f) are the low-magnified images of (a)–(c), respectively, with scale bars of 200 nm.

FIG. 4.

TEM image of Ni loaded MIL-101 at various reaction times, (a) 6 h, (b) 8 h, and (c) 10 h with scale bars of 100 nm; images of (d)–(f) are the low-magnified images of (a)–(c), respectively, with scale bars of 200 nm.

Close modal

The chemical states of Ni nanoparticles at different reaction times (2, 6, and 10 h) during H2 generation were identified by X-ray photoelectron spectroscopy (XPS). Fig. S6 in the supplementary material14 shows the binding energies of Ni 2p3/2 and 2p1/2 at 855.7 and 873.9 eV after 2 h H2 generation, respectively. The XPS values are the characteristics of the oxidized Ni.16 The shake-up feature peaks at higher binding energy could be ascribed to the paramagnetic nickel oxide species. No peak at ∼752 eV was observed, indicating the absence of metallic Ni because it can be easily oxidized during the photocatalytic reactions. The Ni oxidization started from the surface, and led to the formation of a core-shell structure of Ni@NiOx (Fig. 1(d)). The intensity of the Ni 2p3/2 and 2p1/2 peaks decreased gradually with the prolonged reaction time. But no evident shift of these two peaks occurred, indicating the formed Ni@NiOx catalysts are stable in the H2 generation reactions.

The photoluminescence (PL) spectra were used to monitor the charge transfer pathway in this H2 generation system. As shown in Fig. 5, upon excitation at 450 nm, Erythrosin B dye shows a strong emission peak at ∼552 nm. An evident decrease of this PL peak occurs upon dispersing pristine MIL-101 microcrystals into the ErB solution, indicating the efficient charge transfer from photoexcited ErB dye to MIL-101 MOF. Notably, the PL peaks red-shifted to 560 nm, which should be caused by the interaction of ErB dye and MIL-101, in good agreement with UV-Vis absorption measurements. The effective interaction between dye and MIL-101 MOF is very essential for charge transfer. The intensity of this PL peak can be decreased further when Ni-loaded MIL-101 MOF was dispersed into the ErB solution because the accepted electrons by MOF can further transfer to Ni/NiOx nanoparticles, resulting in decreased PL intensity. The PL spectra clearly reveal the charge transfer from photoexcited dyes to MIL-101 MOF, and then to Ni/NiOx nanoparticles.

FIG. 5.

Steady-state photoluminescence spectra of ErB dye, ErB sensitized MIL-101, and ErB sensitized Ni-loaded MIL-101 system.

FIG. 5.

Steady-state photoluminescence spectra of ErB dye, ErB sensitized MIL-101, and ErB sensitized Ni-loaded MIL-101 system.

Close modal

The proposed mechanism for H2 generation in the present system is illustrated in Fig. S7 of the supplementary material.14 The excited photoelectrons in the LUMO state of ErB dye can transfer not only to the Ni/NiOx nanoparticles directly (pathway I) but also to the unsaturated Cr(III) sites (pathway II) in MIL-101.11(b) The injected electrons in MIL-101 can then transfer to the loaded Ni/NiOx nanoparticles. This electron migration pathway was evidenced by the decreased PL intensity of ErB in the presence of MIL-101 or Ni/NiOx loaded MIL-101. Meanwhile, the photoexcited electrons in MIL-101 by visible light can also transfer to the Ni/NiOx nanoparticles. Finally, the hydrogen evolution reaction occurred on the surface of Ni/NiOx nanoparticles. The excited ErB molecules return to the ground state by accepting the electrons from the sacrificial reagent TEOA.

In summary, we have demonstrated that the economical Ni/NiOx nanoparticles can act as active and durable co-catalysts for boosting H2 generation in photocatalytic reactions. Highly active H2 generation can be ascribed to the efficient charge transfer from photoexcited dye sensitizer to MIL-101 and then to the Ni/NiOx particles, where the hydrogen evolution reaction occurs. Taking the consideration of the low cost and easy operation of present H2 production system, this study provides a potential approach to develop highly efficient H2 generation systems employing non-noble metals as the catalysts.

The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51002001), Anhui Provincial Natural Science Foundation (No. 1508085ME105), the State “211 Project” of Anhui University, and AcRF-Tier2 (No. MOE2012-T2-2-041, ARC 5/13) from Ministry of Education Singapore.

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Supplementary Material