Development of efficient bi-functional g-C₃N₄@MOF heterojunctions for water splitting

Abstract

Herein we report the development of highly efficient heterojunctions by combining n-type g-C₃N₄ and MOFs as bi-functional photoelectrocatalysts towards the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 have been synthesized via in situ incorporation of pre-synthesized g-C₃N₄ nanoparticles into MIL-125(Ti) and UiO-66. Bare MIL-125(Ti) and UiO-66 are also prepared for comparison. All the synthesized samples have been characterized by Powder X-ray Diffraction analysis, Fourier Transform Infrared Spectroscopic analysis, Scanning Electron Microscopic analysis, Energy Dispersive X-ray Spectrometry and UV-Vis Spectroscopic analysis. Cyclic voltammetry and linear sweep voltammetry studies have been carried out for all samples which indicates that under visible light exposure the g-C₃N₄@MIL-125(Ti)/NF heterojunction achieved a current density of 10 mA cm⁻² at just 86 and 173 mV overpotential for the HER and OER, respectively. Moreover, all the synthesized samples display significant stability and generate a constant current density up to 1000 cyles during water electrolysis performed at a constant applied potential 1.5 V.

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Introduction

In order to address the need for renewable and safe energy conversion and storage, water splitting into hydrogen and oxygen is considered to be one of the most significant and efficient technologies.¹ Water splitting comprises two half-reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.² A high over-potential is required to split water into hydrogen (HER) and oxygen (OER). In particular, the OER is considered a serious bottleneck to facilitate effective water splitting due to its high over-potential and poor kinetics, which are mainly due to multi-step proton coupled electron transfer reactions.^3–5 State of the art catalysts for water splitting are noble metal-based, and among them RuO₂ and IrO₂ based materials are used for the OER and Pt-based catalysts are used for the HER.^6–8 However, their extreme shortage and high costs have seriously diminished their commercial application.^9,10 Therefore a substantial amount of work has been done on the design and development of highly effective, noble metal free bi-functional catalysts for the HER and the OER, for example, heteroatom (N, P, S, Fe, Co, etc.) doped carbon materials, metal and nitrogen doped carbon (M–N–C), transition metal oxides, etc.^11–13 In particular, M–N–C catalysts have arisen as a promising alternative to noble metal based catalysts due to their unique structural properties of metal coordinated surface nitrogen.¹⁴

These materials can be synthesized using various methods. One of the various methods for the preparation of M–N–C catalysts is the formation of hetero junctions between metal organic frameworks (MOFs) and g-C₃N₄ by means of the in situ incorporation.¹⁵ MOFs are porous crystalline materials which are metabolized by the coordination of metal ion or metal clusters with organic moiety. In MOFs, both metal nodes and organic connectors are classified as isolated quantum dots and light absorption antenna, respectively.^16–19 Both metal nodes and organic connectors take part in photocatalytic reactions. Metal nodes can be directly stimulated by light irradiation or activated by an organic link.²⁰

Recently, metal free graphene based mesoporous carbon has proven to be cost effective and robust catalyst. Graphitic carbon nitride (g-C₃N₄) is a metal free n-type semiconductor and has been revealed to be a valuable catalyst under visible light due to its narrow band gap, excellent stability, fast charge transfer and easy availability of raw material for its synthesis.^21–25 However, g-C₃N₄ has one limitation of high recombination of the photo induced e⁻/h⁺ pair which adversely affects its catalytic activity.²⁶ Though both MOFs and g-C₃N₄ have been recorded as effective catalysts, their performance still seems to be poor and researchers are working on a number of techniques to increase their performance. Among other methods, heterojunction production using n-type g-C₃N₄ and MOFs is a successful tool for reducing the rate of recombination of photo induced electron hole pairs and increasing photocatalytic activity under visible light.²⁷ Numerous excellent heterojunctions have been reported which delivers 10 mA cm⁻² current density for HER and OER at low overpotential such as CuO@UiO-66/NF (ƞ₁₀ = 220 mV HER), CuO@MIL-125(Ti)/NF (ƞ₁₀ = 186 and 353 mV for HER and OER, respectively), MoS₂/g-C₃N₄ and Cl–CuO/g-C₃N₄.^28–31

In view of these findings, highly efficient heterojunctions have been developed by combining n-type g-C₃N₄ and MOFs (MIL-125(Ti) and UiO-66) via in situ incorporation. It has been found that g-C₃N₄@MIL-125(Ti)/NF heterojunction has effectively reduces the band gap, increases the absorption of visible light, reduces the rate of recombination of electron hole pair and under visible light exposure delivers 10 mA cm⁻² at just 86 and 173 mV overpotential for HER and OER respectively.

Experimental

Materials

The chemicals used for synthesis of materials are urea (NH₂CONH₂, 99.00%); 1, 4-benzendicarboxylic acid (H₂BDC, 99%); titanium isopropoxide (Ti[OCH(CH₃)₂]₄, 99%); zirconium(IV) chloride (ZrCl₄, 99.99%); N,N-dimethylformamide (DMF, 99.8%) and methanol (CH₃OH, 99.9%).

Synthesis of g-C₃N₄

g-C₃N₄ was prepared by pyrolysis of urea.³² In a typical reaction, 10 g of urea was taken in a covered crucible and put in Muffle furnace at 550 °C for 3 h. After that, the crucible was cooled to room temperature and yellow colored precipitates were obtained, which were stored for further process (Fig. 1).

Fig. 1 A systematic representation of synthesis of g-C₃N₄@MIL-125(Ti).

Development of g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 heterojunction

Solvothermal process was used for the development of g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 heterojunction via in situ introduction of pre-synthesized g-C₃N₄ into host MOFs. For g-C₃N₄@MIL-125(Ti), 15 mmol (2.62 g) of 1,4-benzendicarboxylic acid was dissolved in 50 mL solution of N,N-dimethylformamide and methanol (45 + 5 mL, respectively), followed by a dropwise addition of 10 mL DMF suspension of 0.2 g g-C₃N₄ under constant stirring. After that, 0.45 mmol (0.13 g) of titanium isopropoxide was introduced to the above mixture and stirred for 1 hour. The mixture was then moved to Teflon lined stainless steel autoclave and heated in an oven at 150 °C for 16 hours. At the end, the precipitates were obtained, isolated by centrifugation, washed with DMF and methanol, and dried in an oven at 100 °C under vacuum.

g-C₃N₄@UiO-66 was also synthesized via above mentioned solvothermal by using zirconium(IV) chloride instead of titanium isopropoxide. For comparison bare MIL-125(Ti) and UiO-66 was also prepared by following same procedure without addition of g-C₃N₄.

Fabrication of working electrode

To study the HER and OER, working electrodes were fabricated by pasting slurry of photocatalytic material on 1 cm² pieces of nickel foam (NF). Slurry was prepared by sonicating 0.10 g of synthesized material in 1 mL distilled water. The slurry was uniformly pasted on cleaned piece of NF and dried to fabricate working electrodes.

Characterization of synthesized materials

The crystal structure of synthesized specimens was studied by taking powder XRD patterns on Shimadzu XRD diffractometer over a 2θ range of 5°–80°. Infrared spectra were measured by using Nicolet Nexus 870 spectrometer in the range of 4000–400 cm⁻¹. Raman spectra of all synthesized materials were measured by using Jobin-Yvon LabRAM HR800 Raman spectrometer with helium neon laser (532.02 nm) in the range of 100–2000 cm⁻¹. The morphology and composition were studied by using Philips XL30 Environmental SEM attached with Oxford Instrument Inca 500 Energy Dispersive X-ray spectrometer (EDX). UV-Visible spectroscopy was performed via Shimadzu UV-2600 spectrophotometer and UV-Visible spectra were recorded between 200 and 900 nm.

Hydrogen and oxygen evolution reaction studies

Photoelectrochemical tests for HER and OER were carried out by using a traditional three electrode device consisting of a working electrode, Pt as a counter electrode and Ag/AgCl (3 M KCl) as a reference electrode. 1.0 M KOH solution was used as electrolyte. Photoelectrochemical tests were performed through a Potentiostat/Galvanostat (Uniscan instruments 3100) at room temperature under dark and in the presence of sunlight. Sunlight was used as a source of visible energy. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted for both HER and OER at 10 and 1 mV s⁻¹, respectively. All potential data collected were transformed to reversible hydrogen electrode (RHE) by using given equation.

E_RHE = E_Ag/AgCl + 0.197 + 0.059 pH (1)

Overpotential (ƞ) for HER and OER was found through following equations

HER ƞ = 0 − E_RHE (2)

OER ƞ = E_RHE − 1.23 (3)

Long term stability of some selected synthesized catalysts was examined by means of 1000 repeated CV sweeps and chronoamperometric tests at a steady applied voltage of 1.5 V in the presence of visible light for 600 minutes.

Result and discussion

Powder XRD analysis

The powder XRD patterns of g-C₃N₄, MIL-125(Ti), UiO-66, g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 are shown in Fig. 2. The powder XRD pattern of g-C₃N₄ contains a well-defined diffraction peak at about 27.45° 2θ which is indexed to aromatic conjugated system of g-C₃N₄ and matched well with previously reported patterns and JCPDS card number 85-1526 (ref. 33). The powder XRD pattern of bare MIL-125(Ti) matched well with simulated pattern. Similarly, the PXRD pattern of UiO-66 is matched well with the simulated pattern. However, after the incorporation of the g-C₃N₄, all the diffraction peaks in the g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 are matched well with simulated patterns. The incorporation of g-C₃N₄ in MOFs does not influence the crystal structure of host MOFs and MOFs maintain their integrity and dominance. The characteristic diffraction peaks of incorporated g-C₃N₄ are overlaid by MOFs. However an additional peak at 27.45° 2θ is also observed in the PXRD pattern of C₃N₄@MIL-125(Ti) which may corresponds to g-C₃N₄. On careful analysis it has been observed that a small peak around 27.50° 2θ is also present in the PXRD pattern of MIL-125(Ti). It is possible that the peak near 27.45° 2θ overlie with the peak for g-C₃N₄ which results in the increase in intensity of this peak.

Fig. 2 Powder XRD patterns of g-C₃N₄, MIL-125(Ti), UiO-66, g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 in comparison with simulated patterns of MIL-125(Ti) and UiO-66.

FTIR and Raman analysis

Fig. 3(a) shows the FTIR spectra of all synthesized materials. FTIR spectrum of g-C₃N₄ contains a distinct peak at 810 cm⁻¹ which belongs to triazine ring of g-C₃N₄. While, the peaks between 1200 cm⁻¹ and 1650 cm⁻¹ (1233, 1317, 1402, 1553 and 1635 cm⁻¹) are due to typical stretching modes of CN heterocycles.^34,35 FTIR spectra of both MIL-125(Ti) and g-C₃N₄@MIL-125(Ti) contain characteristic vibrational peaks of carboxylate group between 1300 and 1700 cm⁻¹ and O–Ti–O vibrations between 400 and 800 cm⁻¹ (ref. 36).

Fig. 3 (a) FTIR spectra and (b) Raman spectra of g-C₃N₄, MIL-125(Ti), UiO-66, g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66.

Similarly, FTIR spectra of UiO-66 and g-C₃N₄@UiO-66 contain vibrational peaks corresponding to carboxylate group between 1300 and 1700 cm⁻¹ and vibrational peaks of –OH, –CH of organic linker and Zr-(OC) at 743, 663 and 555 cm⁻¹ respectively. Fig. 3(b) shows the Raman spectra of all synthesized samples. Small broad characteristic peaks are found at 588, 760, 944, 1051, 1172, cm⁻¹ in the Raman spectra of g-C₃N₄ are attributed to the different types of ring breathing modes of triazine.³⁷ Raman spectra of MIL-125(Ti), UiO-66, g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 consist on five typical vibrational bands at 1615, 1446, 1143, 863 and 635 cm⁻¹ corresponding to stretching and bending vibrations of carboxylate group and C=C, C–H vibrations of benzene ring of organic ligand. However, it is observed that in both FTIR and Raman spectra of g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66, vibrational peaks of g-C₃N₄ are not observed which indicates that the host MOFs dominate the g-C₃N₄ and both of them are close to each other due to physical and electrostatic interactions.

Morphological and compositional analysis

Morphological and compositional analysis of all synthesized samples have been carried out by SEM and SEM based EDX which are shown in Fig. 4(a–e). Fig. 4(a) shows the SEM image of g-C₃N₄ which indicates that the g-C₃N₄ is grown in spherical form with agglomeration. MIL-125(Ti) and g-C₃N₄@MIL-125(Ti) have grown in cubic crystalline form and uniformly distributed which are represented in Fig. 4(b and c). While, UiO-66 and g-C₃N₄@UiO-66 have grown in irregular block shaped crystals and are uniformly distributed as shown in Fig. 4(d and e). Some variations in size have been observed after incorporation of g-C₃N₄ in MIL-125(Ti) and UiO-66. Fig. S1† shows the EDX images of all synthesized samples. EDX image of g-C₃N₄ consists of peaks of both C and N as shown in Fig. S1(a).† While, EDX images of MIL-125(Ti), g-C₃N₄@MIL-125(Ti), UiO-66 and g-C₃N₄@UiO-66 contain basic diffraction peaks of MIL-125(Ti) (Ti, C and O) and UiO-66 (Zr, C and O) as well as diffraction peaks of g-C₃N₄ respectively which are represented in Fig. S1(b–e).† To further investigate the distribution of incorporated g-C₃N₄ in MOFs, the elemental mapping is performed. Mix elemental maps of g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 reveal that g-C₃N₄ is successfully incorporated and uniformly distributed as represented in Fig. S2.†

Fig. 4 SEM images of (a) g-C₃N₄; (b) MIL-125(Ti); (c) g-C₃N₄@MIL-125(Ti); (d) UiO-66 and (e) g-C₃N₄@UiO-66.

Optical properties

The UV-Vis absorption spectra of g-C₃N₄, pure MOFs and their heterojunctions g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 are represented in Fig. 5(a). The g-C₃N₄ exhibits an absorption edge upto 456 nm due to π → π* transition of conjugated triazine units.³⁸ In pure MIL-125(Ti) absorption edge is up to 390 nm due to ligand-to-metal charge transfer while in g-C₃N₄@MIL-125(Ti) it extends to 467 nm. Similarly, in pure UiO-66 absorption edge is up to 328 nm while in g-C₃N₄@UiO-66 it extends to 450 nm. It indicates that incorporation of g-C₃N₄ in MOFs, affects the optical properties positively and brings the absorption of light from UV to visible region. It improves the utilization of solar energy, produces more electron–hole pair and increases the charge separation via heterojunction formation between g-C₃N₃ and central metallic cluster of MOFs. The band gap shown in Fig. 5(b) is determined from the plots of (αhν)² versus photon energy (hν). The band gap is 2.85, 3.33, 2.90, 3.97 and 3.74 eV and for g-C₃N₄, MIL-125(Ti), g-C₃N₄@MIL-125(Ti), UiO-66 and g-C₃N₄@UiO-66 respectively. It shows that incorporated materials have band gaps in between of precursor's, so they have hybrid nature and can decrease the photo driving force for water splitting into hydrogen and oxygen via heterojunction formation.

Fig. 5 (a) UV-Visible spectra and (b) full range plot of (αhν)² verses hν of g-C₃N₄, MIL-125(Ti), UiO-66, g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66.

Photoelectrochemical hydrogen evolution reaction analysis

All the prepared samples are used for the study of hydrogen evolution reaction (HER) via photoelectrochemical water splitting. Photoelectrochemical studies are performed via cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometric measurements. Firstly, CV is performed both in dark as well as in presence of visible light within potential range −0.2 to 0.2 V vs. RHE at 10 mV s⁻¹ scan rate as shown in Fig. 6(a). CV curves show almost zero current density generation under dark because no HER observed. While, remarkable generation of current density is observed under visible light illumination due to HER. It can also be seen that incorporated materials show greater current density generation as compared to bare MOFs and g-C₃N₄. Further, photoelectrochemical activity towards HER is analyzed via LSV within potential range −0.2 to 0.2 V vs. RHE at 1 mV s⁻¹ scan rate as shown in Fig. 6(b). Like CV, LSV is performed both under dark and in presence of visible light. In dark, no HER activity is observed and therefore, almost zero current density is generated. However, under solar light illumination, HER activity is observed and tremendous hydrogen gas bubbling is observed at surface of working electrode.

Fig. 6 (a) CV and (b) LSV curves of g-C₃N₄/NF, MIL-125(Ti)/NF, g-C₃N₄@MIL-125-(Ti)/NF, UiO-66/NF and g-C₃N₄/UiO-66/NF respectively.

CV curves show that, the maximum photoelectrochemical activity towards HER is shown by g-C₃N₄@MIL-125(Ti)/NF and it generated −12.83 mA cm⁻² current density at RHE potential −0.20 V as compared to MIL-125(Ti) (−4.81 mA cm⁻²), UiO-66/NF (−5.55 mA cm ⁻²), g-C₃N₄@UiO-66/NF (−8.88 mA cm⁻²) and g-C₃N₄/NF (−5.14 mA cm⁻²). Like CV, LSV curves also reveal that among all the synthesized materials, g-C₃N₄@MIL-125(Ti) exhibits maximum HER activity and delivers maximum current density −25.8 mA cm⁻² at 206 mV overpotential as compared to g-C₃N₄@UiO-66 (−14.7 mA cm⁻²), g-C₃N₄ (−3.36 mA cm⁻²), MIL-125(Ti) (−6.45 mA cm⁻²) and UiO-66 (−2.94 mA cm⁻²). These results indicate that incorporation of g-C₃N₄ has promotionary effected over photocatalytic activity of MOF's and therefore, incorporated materials have better HER activity as compared to individual materials due to heterojunction development between g-C₃N₄ and MOFs.

The maximum HER activity is shown by g-C₃N₄@MIL-125(Ti) and it delivers −10 mA cm⁻² current density at just 86 mV overpotential as compared to g-C₃N₄@UiO-66 (η₁₀ = 186 mV) and previously reported g-C₃N₄ and MOFs based HER catalysts. A brief comparison of HER activity of g-C₃N₄@MIL-125(Ti) with different reported g-C₃N₄@MIL-125(Ti) with different reported C₃N₄-based, transition metal based and MOFs catalysts is given in Table 1. From CV and LSV curves, it is evaluated that g-C₃N₄@MIL-125(Ti) exhibited better and enhanced HER activity as compared to all other synthesized materials due to heterojunction development.

Table 1 Comparison of HER activity of g-C₃N₄@MIL-125(Ti) with different reported C₃N₄-based, transition metal based and MOFs catalysts

Catalyst	Current density (mA cm^−2)	Overpotential (ƞ) at corresponding current density (mV)	Reference
C3N4/FTO: fluorine-doped tin oxide	10	300	39
N-GMT: nitrogen-doped graphene microtubes	10	464	40
CoMn-LDH@g-C3N4	50	448	41
Cu2O/g-C3N4	12.8	148.7	42
C3N4–CNT–CF	10	131	43
g-C3N4 QDs	10	208	44
g-C3N4-graphene hybrids	10	207	45
Sulfur-doped g-C3N4	10	145	46
Fe0.2Co0.8Se2/g-C3N4	20	83	47
UiO-66/MoS2	10	129	48
MoP@PC	10	153	49
CuO@UiO-66/NF	10	220	29
CuO@NH2-UiO-66/NF	10	166	50
CuO@NH2-MIL-125(Ti)/NF	5	146	28
g-C3N4@UiO-66/NF	10	186	This work
g-C3N4@MIL-125(Ti)	10	86	This work

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Photoelectrochemical oxygen evolution traction analysis

Further, all the prepared samples are used for oxygen evolution (OER) analysis. OER measurements are performed by using linear sweep voltammetry (LSV) in dark as well as in the presence of visible light, represented in Fig. 7.

Fig. 7 LSV curves of g-C₃N₄/NF, MIL-125(Ti)/NF, g-C₃N₄@MIL-125-(Ti)/NF, UiO-66/NF and g-C₃N₄/UiO-66/NF respectively.

Fig. 7 shows that in dark, negligible current density is generated due to very less activity. While under illumination, all the synthesized materials show better OER activity. Among all the synthesized materials, g-C₃N₄@MIL-125(Ti)/NF shows maximum OER activity and it shows lowest onset OER potential and delivers 10 mA cm⁻² current density at just 173 mV overpotential. It is lower than 310 mV for g-C₃N₄@UiO-66/NF and some of the previously reported OER catalysts, as given in Table 2. It has been found that g-C₃N₄@MIL-125(Ti)/NF generated maximum current density 32.50 mA cm⁻² at 236 mV overpotential as compared to g-C₃N₄@UiO-66 (28.58 mA cm⁻²), g-C₃N₄ (3.19 mA cm⁻²), MIL-125(Ti) (9 mA cm⁻²) and UiO-66 (8.56 mA cm⁻²) at same overpotential. Like HER, it has been observed that incorporation of g-C₃N₄ has promotionary effect for OER activity. So, incorporation of g-C₃N₄ increases the photocatalytic activity of MOFs towards both HER and OER, thus synthesized materials show bifunctional photocatalytic activity.

Table 2 Comparison of OER activity of g-C₃N₄@MIL-125(Ti) with different reported C₃N₄-based, transition metal based and MOFs catalysts

Catalyst	Current density (mA cm^−2)	Overpotential (ƞ) at corresponding current density (mV)	Reference
CoWO4	10	810	51
CoMoO4	10	765	51
21 wt% WCoMoO4	10	680	51
CoWO4/GC	10	388	52
CoWO4/Ni	10	336	52
Co0.5Mn0.5WO4	10	400	53
FeCoNiOx	10	203	54
CS-NiFe0.10Cr0.10 on Cu	10	200	55
NiCo2O4/NiO	10	360	56
N–NiFeOOH	10	320	57
Co0.5Fe0.125Mn0.375WO4	10	460	58
Fe0.2Co0.8Se2/g-C3N4	10	230	47
CuO@NH2–UiO-66/NF	10	283	50
UiO-66/MoS2	10	180	48
CoMn-LDH@g-C3N4	40	350	41
CoOx/UiO-66-3000	10	283	59
g-C3N4@MIL-125(Ti)	10	173	This work

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Stability of catalysts and proposed photocatalytic mechanism

The stability of working electrode is very important factor for photoelectrochemical studies. The stability of electrode is another important factor and it was studied for the sample g-C₃N₄@MIL-125(Ti) by continuous 1000 CV cycles at 100 mV s⁻¹ scan rate. It was observed that 1^st and 1000^th CV cycles almost overlapped with each other and there was very negligible degradation in current density. It indicated that these working electrodes were quite stable towards HER, represented by Fig. S3 in ESI.† Furthermore the chronoamperometric measurements of g-C₃N₄, g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 under visible light illumination at constant applied potential 1.5 V vs. RHE and the results are represented by Fig. S4 in ESI.† The chronoamperometric results show that the synthesized samples are generating almost constant current density without slight degradation. It represents that synthesized materials are quite stable for long term utilization. Among all the synthesized samples, g-C₃N₄@MIL-125(Ti) shows better photoelectrocatalytic activity towards both HER and OER as compared to all other synthesized samples. It is well known that the water splitting into H₂ and O₂ is an energetic reaction which needs a potential of 1.23 eV.³ Therefore, it is necessary to design the photocatalysts with the suitable edges of the conduction band (CB) and valence band (VB) for water splitting. The redox potential of H⁺/H₂ is 0.0 V vs. normal hydrogen electrode (NHE). Thus the bottom level of the CB should be more negative than 0.0 V. On the other hand the redox potential of O₂/H₂O is 1.23 V vs. NHE. Thus the top level of the VB should be more positive than the 1.23 V.⁶⁰ A possible mechanism is suggested for this system, represented by Fig. 8.

Fig. 8 Proposed mechanism for HER and OER under visible light illumination.

The conduction band (CB) and valence band (VB) of semiconductor are calculated by using following equations.

E_VB = X − E_e + 0.5 E_g (4)

E_CB = E_VB − E_g (5)

Here, X = absolute electronegativity of g-C₃N₄ (4.67 eV) and MIL-125(Ti) (5.69 eV), E_e = energy of free electron on hydrogen scale (4.5 eV). The calculated E_CB and E_VB of g-C₃N₄ are −1.25 and 1.59 eV, respectively. While, for MIL-125(Ti), the calculated E_CB and E_VB are −0.48 eVand 2.86 eV, respectively. This band alignment suggests the development of type-II heterojunction at interface between g-C₃N₄ and MIL-125(Ti), represented by Fig. 8. The pure g-C₃N₄ can be used for the generation of H₂ but the yield will be very less because the conduction band of g-C₃N₄ (−1.25 eV) is away from redox potential of H⁺/H₂ (0.0 V vs. NHE).⁶¹ Under visible light illumination, electron from valence band of g-C₃N₄ is excited to conduction band. The conduction band of g-C₃N₄ is much negative than the CB of MIL-125(Ti) so electrons easily move from CB of g-C₃N₄ to CB of MIL-125(Ti). This electronic transfer occurs via heterojunctional structure between g-C₃N₄ and titanium-oxo metallic cluster of MIL-125(Ti) via, resulting in the reduction of H₂O into H₂ at the CB of MIL-125(Ti).⁶⁰ On the other hand, the holes (h⁺) left in valence band of g-C₃N₄. The valence band of g-C₃N₄ (1.59 eV) is more positive than the redox potential of O₂/H₂O is 1.23 V vs. NHE. So the holes in the balance band of g-C₃N₄ are used for oxidation of water into oxygen gas. Thus, heterojunction development between g-C₃N₄ and central metallic cluster of MIL-125(Ti) promoted the charge separation, increased the absorption of visible light and enhanced the photocatalytic activity.

Conclusions

In this work, MIL-125(Ti) and UiO-66 frameworks and their Heterojunctions g-C₃N₄@MIL-125(Ti) and g-C₃N₄@UiO-66 have been synthesized via in situ incorporation of g-C₃N₄ using one-step solvothermal method. The synthesis of samples was confirmed by PXRD, FTIR, SEM and ED-XRF analysis. It has been observed that incorporation of g-C₃N₄ does not disturb the crystal structure of MIL-125(Ti) and UiO-66. This indicates that this method is useful for incorporation of nanoparticles into MOFs and it maintains its integrity which has increased the charge separation and absorption of visible light. Hydrogen evolution reaction and oxygen evolution reactions are studied by cyclic voltammetry and linear sweep voltammetry. Among the synthesized heterojunctions, the g-C₃N₄@MIL-125(Ti) heterojunction delivered 10 mA cm⁻² current density at just 86 mV and 173 mV overpotential for HER and OER respectively with significant stability up to 1000 s at 1.5 V applied potential under visible light exposure. This implies that the current strategy of incorporation of nanoparticles into MOFs is highly fertile for the enhancement of electrocatalytic activity of MOFs towards water splitting.

Author contributions

All the authors contributed equally to the article.

Conflicts of interest

There are no conflicts of interests to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra05594e

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