Amino-Functionalized Titanium Based Metal-Organic Framework for Photocatalytic Hydrogen Production.

Photocatalytic hydrogen production using stable metal-organic frameworks (MOFs), especially the titanium-based MOFs (Ti-MOFs) as photocatalysts is one of the most promising solutions to solve the energy crisis. However, due to the high reactivity and harsh synthetic conditions, only a limited number of Ti-MOFs have been reported so far. Herein, we synthesized a new amino-functionalized Ti-MOFs, named NH2-ZSTU-2 (ZSTU stands for Zhejiang Sci-Tech University), for photocatalytic hydrogen production under visible light irradiation. The NH2-ZSTU-2 was synthesized by a facile solvothermal method, composed of 2,4,6-tri(4-carboxyphenylphenyl)-aniline (NH2-BTB) triangular linker and infinite Ti-oxo chains. The structure and photoelectrochemical properties of NH2-ZSTU-2 were fully studied by powder X-ray diffraction, scanning electron microscope, nitro sorption isotherms, solid-state diffuse reflectance absorption spectra, and Mott-Schottky measurements, etc., which conclude that NH2-ZSTU-2 was favorable for photocatalytic hydrogen production. Benefitting from those structural features, NH2-ZSTU-2 showed steady hydrogen production rate under visible light irradiation with average photocatalytic H2 yields of 431.45 μmol·g-1·h-1 with triethanolamine and Pt as sacrificial agent and cocatalyst, respectively, which is almost 2.5 times higher than that of its counterpart ZSTU-2. The stability and proposed photocatalysis mechanism were also discussed. This work paves the way to design Ti-MOFs for photocatalysis.

1. Introduction

Photocatalytic hydrogen production from water using solar light as clean and sustainable energy is one of the most promising solutions to solve the energy crisis [1,2,3,4,5]. As a new kind of porous materials, metal-organic frameworks (MOFs) have been applied in many fields such as gas adsorption/storage/separation, sensor, drug delivery, batteries, electrocatalysis, photocatalysis, due to their ultrahigh surface area and void space, adjustable structure, tunable pore sizes, and modifiable internal surfaces [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. Since Mori et al. first reported that MOFs can achieve photocatalytic hydrogen production, a series of MOFs have been reported to show potential in this application [22,23,24,25,26,27,28,29]. However, most of those MOFs survived low stability during photocatalysis process.

Titanium-based MOFs (Ti-MOFs), as a kind of robust MOFs, are constructed by organic ligands and high valent Ti4+ ions, showing high chemical stability [9,30]. The high stability of Ti-MOFs can be explained by the Pearson’s hard-soft acid-base principle, in which carboxylate ligands can be seen as hard base, and the high valent Ti4+ ions as hard acid, thus, robust coordination bond between carboxylate ligand and Ti4+ ions are formed [7]. Moreover, titanium ions are preferred to form Ti-oxo clusters or infinite Ti-oxo chains/sheets, which will be coordinated with many ligands, further strengthening the stability of Ti-MOFs. However, due to the high reactivity and harsh synthetic conditions of titanium precursors, only a limited number of Ti-MOFs have been reported so far [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45].

Among the various semiconductors, TiO2 is the first example used for photocatalytic hydrogen production due to its light sensitive Ti ions [46]. Superior to TiO2, Ti-MOFs not only possess Ti-oxo clusters or Ti-oxo chains/sheets, but also have light harvested ligands, endowing them with promising photocatalytic activity [47]. Especially, the adjustable structures of Ti-MOFs make them efficiently utilize the solar light beyond ultraviolet region (accounts only 4%). Herein, we synthesized an amino functionalized Ti-MOF, named NH2-ZSTU-2 (ZSTU stands for Zhejiang Sci-Tech University), for photocatalytic hydrogen production. This MOF is composed of infinite Ti-oxo chains and amino functionalized ternary carboxylic acid ligands, which is isomorphic to ZSTU-2 (Figure 1). Compared with the counterpart ZSTU-2, NH2-ZSTU-2 showed a nearly 2.5 times higher photocatalytic hydrogen production activity with a rate of 431.45 μmol·g−1·h−1.

Figure 1

Crystal structures of the ZSTU-2 (a) and NH2-ZSTU-2 (b).

2. Results and Discussions

2.1. Structural Characterizations of Photocatalysts

The crystallinity of NH2-ZSTU-2 was improved by introducing acetic acid as the modulator, which can delay the crystallization speed of MOF, and finally obtain better crystallinity. The regular rod-shaped crystallites with diameter of approximately 50 nm and length of 150 nm were characterized by scanning electron microscope (SEM), which is isomorphic to ZSTU-2 (Figure 2). The size of NH2-ZSTU-2 is too small to directly determine the crystal structure using single-crystal diffraction measurements. Therefore, powder X-ray diffraction (PXRD) analysis was used to discovery the MOF structure. The PXRD pattern of NH2-ZSTU-2 is quite similar to ZSTU-2 (Figure S1), and we thus modeled the structure of NH2-ZSTU-2 using the framework of ZSTU-2 with installed amino group on BTB linkers, followed by structural optimization using material studio. Based on the structure model, Pawley refinement was performed on the PXRD data, and we obtained the unit cell parameters of a = 11. 7987 Å, b = 34.6036 Å, and c = 20.1266 Å, and α = β = γ = 90°, with agreement factors of R = 0.0720 and R = 0.0943 for NH2-ZSTU-2 (Figure 3), strongly supporting its validity. Detailed lattice parameters and atomic coordinates of NH2-ZSTU-2 are provided in Tables S1 and S2. Based on the structure of NH2-ZSTU-2 we obtained by Pawley refinement, every six titanium atoms form a secondary unit of Ti6(µ3-O)6(COO)6 through a bridge, while such a Ti6 cluster is interconnected on the c-axis by adjacent µ2-OH to form an infinite one-dimensional [Ti6(µ3-O)6(µ3-OH)6(COO)6]n chain of titanium-oxygen clusters. The 1D Ti-oxo chains were then extended by the triangular NH2-BTB linkers to form a 3D porous structure. The high porous structure of NH2-ZSTU-2 was further studied by nitrogen sorption isotherms (Figure 4). The calculated BET specific surface area from nitrogen sorption isotherms is about 604 m2/g, which is comparable to its counterparts ZSTU-2 (657 m2/g). Through the infrared (IR) spectrogram (Figure S2), we can find that the titanium oxide bonds had been formed in both NH2-ZSTU-2 and ZSTU-2, with the corresponding vibration band near 773 cm−1 [38]. Furthermore, IR vibration band at approximately 1430 cm−1, 1604 cm−1, 3459 cm−1 are associated with the C-O stretching vibration, the benzene ring skeleton vibration, and the stretching vibration of hydroxyl coordination, respectively. Compared with ZSTU-2, an extra vibration band near 3399 cm−1 of NH2-ZSTU-2 can be attributed to the uncoordinated amino group. In addition, we found that the IR peak at 3399 cm−1 was kept but 3459 cm−1 was decreased after heating the NH2-ZSTU-2 at 200 °C for 2 h under vacuum, which indicated that the absorbed water was vapored and the amino groups were retained after heating. In order to obtain the thermal stability of the MOFs, thermogravimetry analysis (TG) was further studied (Figure S3). Through the TG curves, we can conclude that both NH2-ZSTU-2 and ZSTU-2 can maintain their structure at about 400 °C. The weight loss at around 200 °C is mainly attributed to the loss of coordinated solvents in MOFs.

Figure 2

SEM images of ZSTU-2 (a,b) and NH2-ZSTU-2 (c,d).

Figure 3

The PXRD analysis of NH2-ZSTU-2 displaying the experimental pattern (black circles), refined pattern based on Pawley refinement (blue line), the difference plot (green line), the simulated plot (red line), and Bragg positions (pink).

Figure 4

Nitrogen sorption isotherms of ZSTU-2 and NH2-ZSTU-2.

As we know, the band structures determine thermodynamics of photocatalysts for photocatalytic hydrogen production. The band gaps of ZSTU-2 and NH2-ZSTU-2 were first studied by solid-state diffuse reflectance absorption spectra. As shown in Figure 5a, the light harvesting region of ZSTU-2 can only reach 450 nm, and the corresponding band gap calculated from Tauc plot is 3.29 eV (Figure 5b). To extend the absorption range of ZSTU-2 to visible region, amino functionalized NH2-BTB linkers were adopted to replace the H3BTB linkers during MOF synthesis. The light absorption region of the NH2-ZSTU-2 illustrated by solid-state diffuse reflectance absorption spectra can be largely extended to 700 nm (Figure 5c), and the band gap is only 2.24 eV (Figure 5d). For photocatalysts, the larger light-harvesting region and lower band gap mean that they can utilize more sunlight and achieve better photocatalytic hydrogen production performance. The conduction band positions of ZSTU-2 and NH2-ZSTU-2 were further determined by Mott–Schottky measurements. Positive slope in both Figure 5e,f indicated that both ZSTU-2 and NH2-ZSTU-2 are n-type semiconductors. The conduction band potentials of them were determined to be −0.68 eV and −0.66 eV, respectively. Then the valence band potentials of them were calculated to be 2.61 eV and 1.58 eV, respectively. The energy band diagram of ZSTU-2 and NH2-ZSTU-2 are shown in Figure S4. The introduction of amino groups in MOFs mainly shifts the valence band potential to a higher position and shows little impact on the conduction band potential. Based on the band structural information, we can conclude that both ZSTU-2 and NH2-ZSTU-2 were favorable for photocatalytic hydrogen production.

Figure 5

Band structural information of photocatalysts. The solid-state diffuse reflectance absorption spectra of ZSTU-2 (a) and NH2-ZSTU-2 (c); Tauc plots for ZSTU-2 (b) and NH2-ZSTU-2 (d), presenting band gap of MOFs calculated under the hypothesis that absorption follows: (αhν)2 = K (hν − Eg); Mott–Schottky plots of ZSTU-2 (e) and NH2-ZSTU-2 (f).

2.2. Photoelectrochemical Characterizations of Photocatalysts

The generation of separated electron-hole pairs was characterized by both transient photocurrent responses and electrochemical impedance spectroscopy (EIS) measurements. As shown in Figure 6a, ZSTU-2 showed low transient photocurrent response under visible light due to the narrow light-harvesting region. As expected, the transient photocurrent responses of NH2-ZSTU-2 increased dramatically, which indicated that a better photogenerated charge carries separation efficiency. The EIS of NH2-ZSTU-2 was further studied both with and without visible light irradiation. As shown in Figure 6b, compared with the dark state, dramatically decreased radius of the EIS curve under visible light irradiation indicated that a large number of separated electron-hole pairs were photogenerated in NH2-ZSTU-2 with visible light shining on.

Figure 6

Photoelectrochemical characterizations of photocatalysts. Transient photocurrent plots of NH2-ZSTU-2 and ZSTU-2 (a); EIS curves of NH2-ZSTU-2 under dark and visible light (b).

2.3. Photocatalytic Hydrogen Production of Photocatalysts

Before photocatalytic hydrogen production, both MOFs were loaded with Pt using a photo deposition method [48]. The Pt nanoparticles were successfully deposited in MOFs and characterized by TEM (Figure S5). The photocatalytic hydrogen production was then performed in TEOA/CH3CN/H2O mixed solvents under 300 W Xe lamp irradiation with a L42 light filter and triethanolamine (TEOA) as a sacrificial agent, and Pt as cocatalyst [48]. Before the photocatalytic reaction, the solution was degassed for 20 min to remove the dissolved O2 in solvent. The production of hydrogen was detected by an on-line GC with a TCD detector. As shown in Figure 7a, both Pt@ZSTU-2 and Pt@NH2-ZSTU-2 showed steady hydrogen production rate under visible light irradiation. The average photocatalytic H2 yields of Pt@ZSTU-2 and Pt@NH2-ZSTU-2 were 170.45 μmol·g−1·h−1 and 431.45 μmol·g−1·h−1, respectively. The almost 2.5 times enhanced photocatalytic hydrogen production rate of Pt@NH2-ZSTU-2 is mainly attributed to the enlarged light-harvested region. It should be noted that the cocatalyst Pt plays important role on photocatalytic hydrogen production. The hydrogen production rate of Pt@NH2-ZSTU-2 is also comparable to the state-of-the-art Ti-MOFs, such as PCN-416 (484 μmol·g−1·h−1), MIL-100(Ti) (42 μmol·g−1·h−1), MUV-10(Mn) (271 μmol·g−1·h−1), NH2-MIL-125 (367 μmol·g−1·h−1) [28,49,50,51]. The stability of Pt@NH2-ZSTU-2 during photocatalysis was studied by the recycle experiments, which indicated that Pt@NH2-ZSTU-2 is stable at least three cycles under visible light irradiation. The hydrogen evolution rates of the first, second and third cycles were 431.45 μmol·g−1·h−1, 421.50 μmol·g−1·h−1 and 420.71 μmol·g−1·h−1, respectively (Figure 7b). The retained PXRD patterns of the recycled Pt@NH2-ZSTU-2 also indicated that Pt@NH2-ZSTU-2 is stable (Figure S3).

Figure 7

Photocatalytic performance. (a) Time-dependent photocatalytic hydrogen production of Pt@ZSTU-2, Pt@NH2-ZSTU-2, and NH2-ZSTU-2 in triethanolamine/acetonitrile/water system under visible light irradiation; (b) recycle performance of Pt@NH2-ZSTU-2 under same condition.

A proposed, photocatalytic hydrogen evolution mechanism of Pt@NH2-ZSTU-2 is shown in Figure 8. Under visible light irradiation, NH2-BTB linkers absorb light and the generated photogenerated electrons then transfer to infinite Ti-oxo chains through LMCT mechanism, thus reducing Ti4+ to Ti3+, and the photogenerated electrons in NH2-ZSTU-2 conduction band transfer to Pt cocatalyst for reduction of water to produce H2 [16,26,35]. The holes in the valence band oxidize the sacrificial agent TEOA to TEOA+, constituting a complete REDOX reaction.

Figure 8

Proposed photocatalytic hydrogen production mechanism over NH2-ZSTU-2 under visible light irradiation.

3. Experimental

3.1. Synthesis of NH2-ZSTU-2

2,4,6-tris(4-carboxyphenyl)-aniline (NH2-BTB) (100 mg, 0.220 mmol) and ultra-dry DMF (5 mL) were first added into a 25 mL Teflon-lined stainless-steel autoclave, and then 100 μL glacial acetic acid was added dropwise. After sonication for 10 min, NH2-BTB was fully dissolved to obtain a yellow transparent solution, and then titanium tetraisopropanolate (Ti(i-Pro)4) (0.04 mL, 0.128 mmol) was added dropwise, and sonication was performed for 20 min to form a yellow slurry. The autoclave was then heated in an oven at 190 °C for 22 h. After cooling down, the yellow powder NH2-ZSTU-2 was obtained by centrifuging and washing with DMF and methanol for several times. At last, NH2-ZSTU-2 was dried in a vacuum oven at 60 °C for 12 h to remove the residual methanol. CHN element analysis data of NH2-ZSTU-2 had also been done with average weight ratio of 43.915:2.612:2.18. The chemical formula of NH2-ZSTU-2 was determined to be Ti6(μ3-O)6(μ2-OH)6(NH2-BTB)2 (DMF)0.3 based on element analysis and its structural information obtained from Pawley refinement of PXRD data.

3.2. Synthesis of Pt@NH2-ZSTU-2

Pt NPs were deposited in the NH2-ZSTU-2 using a photo deposition method [52]. First, NH2-ZSTU-2 (50 mg) was dispersed in a mixture of H2O (8 mL) and MeOH (13 mL) in a reaction vessel. After NH2-ZSTU-2 was fully dispersed in the mixture, 1 mL chloroplatinic acid hexahydrate aqueous solution (1.33 mg·mL−1) was then added and the system was vacuumed for 20 min to remove the air. The mixture was then irradiated with a 300 W Xe lamp without light filter for 4 h. The sample was then centrifuged and dried overnight in an oven at 100 °C, and resulted sample was labeled as Pt@NH2-ZSTU-2. The chlorine and Pt content in NH2-ZSTU-2 had been determined to be 1.25 wt% and 9.33 wt% by energy dispersive spectrometer (FESEM, JEOL, Japan).

3.3. Photoelectrochemical Measurementsz

Electrode Preparation: About 10 mg of photocatalyst was dispersed in 1 mL of isopropanol, and then 30 μL of naphthol solution (5% w/w in water) was added, and the mixture was sonicated for 2 h afterwards. The obtained dispersion was then dropped onto one side of a FTO glass with an area of 1 × 1 cm2 (total area of 1 × 3 cm2), and dried in air at 60 °C on a hotplate.

Photocurrent measurements were carried out on an electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., Shanghai, China) in a standard three-electrode system with photocatalysts-coated FTO as the working electrode, Pt net as the counter electrode, Ag/AgCl as the reference electrode, and 0.5 M Na2SO4 solution (pH ≈ 7.0) as the electrolyte. A 300 W Xe lamp with a L42 light filter was used as visible light source, and the photo-responsive signals of photocatalysts was then recorded with alternating 20 s light on/off. Mott–Schottky plots of those photocatalysts were also performed on the same workstation in a standard three-electrode system at frequencies of 500, 1000, 1500 HZ. EIS curves were obtained using the same workstation and photocatalysts-coated FTO was used as the working electrode.

3.4. Photocatalytic Hydrogen Production Experiments

The photocatalytic hydrogen production experiments were evaluated using a batch-type reaction system (Beijing Perfectlight Technology, Beijing, China) at ambient temperature irradiated by a 300 W Xe lamp equipped with a UV cut-off filter (>420 nm). The temperature of condensed circulating water for cooling down the solvent vapor was set to 1 °C. In a typical procedure, 50 mg sample was dispersed into 102 mL mixed solution of acetonitrile, triethanolamine (TEOA), and de-ionized water with volume ratio of 9:1:0.2, and then the suspension was vacuumed for 10 min to remove air. Hydrogen gas was measured by an on-line gas chromatography (GC) (Techcomp-GC7900, argon as a carrier gas) using a thermal conductivity detector (TCD). The production of hydrogen was quantified by a calibration plot to the internal hydrogen standard. For the recycle experiment, the procedure is as follows: after the first experiment test, the system was vacuumed to remove the produced hydrogen and then the second run was restarted the next day. Same procedure was carried out for the third run. In this way, we can avoid the loss of photocatalyst during recovery.

4. Conclusions

In this work, we had synthesized an amino-functionalized Ti-MOF, named NH2-ZSTU-2, for photocatalytic hydrogen production. The NH2-ZSTU-2 was synthesized by a facile solvothermal method, composed of 2,4,6-tri(4-carboxyphenylphenyl)-aniline (NH2-BTB) triangular linker and infinite Ti-oxo chains. The structure of NH2-ZSTU-2 was fully studied by PXRD, SEM, nitrogen sorption isotherms, etc. The band structural information was also obtained by using solid-state diffuse reflectance absorption spectra and Mott-Schottky measurements, which conclude that NH2-ZSTU-2 was favorable for photocatalytic hydrogen production. The generation of separated electron-hole pairs was also characterized by both transient photocurrent responses and electrochemical impedance spectroscopy (EIS) measurements, further showing the potential photocatalytic hydrogen production ability of NH2-ZSTU-2. Benefitting from those structural features, NH2-ZSTU-2 showed steady hydrogen production rate under visible light irradiation with average photocatalytic H2 yields of 431.45 μmol·g−1·h−1 with triethanolamine and Pt as sacrificial agent and cocatalyst, respectively, which is almost 2.5 times higher than that of its counterpart ZSTU-2. The stability and proposed photocatalysis mechanism were also discussed. This work paves the way to design Ti-MOFs for photocatalysis.