Light-Harvesting Metal-Organic Frameworks (MOFs) La-PTC for Photocatalytic Dyes Degradation

A novel porous metal organic framework, La-PTC was synthesized by solvothermal method using a perylene3,4,9,10-tetracarboxylate ligand and lanthanum metal ion. The FTIR analysis showed that La-PTC has a different structure with PTCDA and Na4PTC. The La-PTC MOF has high crystallinity, bandgap energy of 2.21 eV with a maximum absorption area at 561 nm. A rod shape structure of La-PTC has been obtained with the surface area of 22.2364 m2.g−1 and classified into mesoporous material. The La-PTC was relative stable up to 376.93 °C. The LaPTC can degrade 64.76% of MO within ca. 240 min under visible light irradiation with the amount of 30 mg LaPTC. The addition of H2O2 improved the photocatalytic activity of La-PTC with degradation efficiency of 67.02%, 70.00%, and 99.60% for MB, RhB, and MO, respectively. This study presents the fabrication of the light-harvesting metal organic framework, La-PTC and its potential in dyes degradation. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
Photocatalysis technology has appealed to interest for inexpensive material and environmentally friendly nature in carrying out dyecontaining wastewater. Nowadays, several semiconductor materials are applied as photocatalysts for the efficient elimination of dyes in wastewater. Currently, semiconductors, such as: TiO2 [1], ZnO2-WO3 [2], and other metal oxides, are exploited as photocatalysts for photocatalytic dyes degradation purposes. Nevertheless, in practice, their application is greatly limited due to the application under ultraviolet light irradiation [3], limited adsorption capacity and difficult separation process. Therefore, the fabrication of a material which can cope with this limitation is being important.
Metal-organic frameworks (MOFs), a class of porous crystalline material, which are designed by metal ion or cluster metal ion coordinated organic linkers [4], have enhancing interest due to their unique properties and tunable structure. The MOFs have been applied in many applications including adsorption, separation and photocatalysis [5]. The MOFs are potential materials for the removal of dyes contaminant in wastewater [6] due to the characteristics which applicable in water treatment, such as: large surface area, abundant active sites, high stabil-ity in water, acid, base, tunable structure, and pore size [7].
Lanthanide metal ions with high versatile coordination modes are widely adopted for the fabrication of MOFs. Xia and co-workers in 2017 reported that Ln-MOFs was synthesized from an anthracene-coordinated organic linker with Ce, Tb and Dy metal ion exhibit highly rhodamine B (RhB) degradation under visible light irradiation in the presence of H2O2 [8].
Other MOFs such as lanthanum-based metalorganic frameworks (La-MOFs) linked by aromatic dicarboxylic acid ligand [9] and lanthanide-based metal-organic frameworks (Ln-MOFs) with chalcone dicarboxylic acid ligand [10] also has been reported for the degradation of RhB. Nevertheless, there have been limited studies concerned with anionic dye degradation by MOFs, particularly La-MOFs. Therefore, this research report the visible light responsive lanthanum-based MOF which has been fabricated using perylene-3,4,9,10-tetracarboxylate (PTC) as the ligand. To the best of our knowledge, PTC has never been used as the ligand for the La-MOFs. The lanthanum-based metal-organic frameworks linked by perylene-3,4,9,10-tetracarboxylate (La-PTC) was synthesized using a solvothermal method with N,Ndimethylformamide (DMF) as the solvent. The synthesized La-PTC then be used as the photocatalyst for cationic (methylene blue (MB), Rhodamin B (RhB)) and anionic (methyl orange (MO)) dye degradation under visible light irradiation.
The scanning electron microscopy (SEM) images were obtained on a FEI Quanta 650. The specific surface area was measured using the Brunauer-Emmett-Teller (BET) method on a Micromeritics TriStar II Surface Area and Porosity. Thermogravimetric analysis (TGA) was measured by TGA Q50 V20.13 Build 39 at the rate of heating was 10 ºC min −1 under air atmosphere.
2.2 Preparation of Sodium Perylene-3,4,9,10tetracarboxylate (Na4PTC) PTCDA (0.5 g, 1.27 mmol) were dissolved in distilled water (50 mL) on a beaker glass. NaOH (0.356 g, 8.9 mmol) were added to the mixture while stirring vigorously at 300 rpm for 1 hour. The greenish yellow solution was obtained and filtered. Then, excess ethanol was added to obtain a yellow sodium perylene-3,4,9,10-tetracarboxylate (Na4PTC) precipitate. The yellow bulk powder of Na4PTC were collected by filtration, washed with ethanol until a pH of 7 is reached and dried at room temperature overnight.

Preparation of La-PTC
The La-PTC was synthesized by using a solvothermal method. Typically, La(NO3)3.6H2O (866 mg, 2 mmol) as the metal source and Na4PTC (508 mg, 1 mmol) as the organic ligand were dissolved in a mixture of DMF and water (5:1 volume ratio, respectively) and magnetically stirred at 300 rpm for 60 minutes. Then, the solution was transferred to the Teflon autoclave. The Teflon autoclave was placed in the oven for 24 hours at 170 ºC and allowed to cool to room temperature overnight. The orange crystal La-PTC obtained was collected by filtration and washed with DMF and distilled water to eliminate the impurities. The bulk compound of La-PTC then was dried at 70 °C overnight.

Photocatalytic Degradation of Dyes Experiment
Photocatalytic activity of La-PTC for MB/RhB/MO degradation was investigated with the 250 watts mercury lamp as the visible light source. La-PTC (30 mg) was dispersed in 50 mL of MB/RhB/MO solution (10 mg/L). The suspension was magnetically stirred at 300 rpm and irradiated by the mercury lamp for 240 minutes. In addition, a fixed amount of H2O2 30%, CH3OH and tert-butyl alcohol (TBA) were added into the system in order to determine the role of the electron-hole scavenger.
Then, 2.0 mL of the suspension was taken out at different time intervals and centrifugation. The concentration of MB, RhB, and MO were determined using a UV-Vis spectrophotometer at 665 nm, 555 nm and 465 nm, respectively. The several control experiments were performed under dark conditions and without La-PTC at the same condition. The degradation efficiency of MB/RhB/MO was calculated by using the equation (1): C0 and Ct assert the initial and final concentration of MB/RhB/MO solution, respectively.

Results and Discussion
A yellow bulk compound of Na4PTC was obtained from the base hydrolysis of PTCDA. The functional groups of Na4PTC were investigated by FTIR in the range 4000-400 cm −1 (Figure 1).
The strong peak at ca. 1772 cm −1 and 1730 cm −1 are related to the asymmetric and symmetric C=O stretching vibration of the cyclic anhydride group in PTCDA. The peak at ca. 1234 cm −1 due to vibration of C−CO−O−CO−C cyclic anhydride group in PTCDA does not appear in the Na4PTC spectrum. In addition, the new peak at ca. 1633 cm −1 and 1423 cm −1 formed which are identified as asymmetric and symmetric stretching vibration (−COO) group of the carboxylate ion functional group. This indicated that PTCDA was converted to carboxylate ion.
The FTIR spectrum ( Figure 1) also showed that La-PTC has different band absorption with Na4PTC. The sharp peaks at ca. 1633 cm −1 and 1423 cm −1 in Na4PTC were shifted to 1538 cm −1 and 1435 cm −1 , respectively. This indicated that La-PTC was formed and all the carboxylate ion group from Na4PTC were completely deprotonated with La 3+ ion and −COO functional group in PTCDA was coordinated with La 3+ .
The La-PTC optical properties were measured by a UV-Vis spectrophotometer in order to ensure the band gap energy of the La-PTC. The band gap energy was calculated using the Kubelka-Munk (eq. 2) and the Tauc plot (eq. 3) as shown below: where Eg is the band gap energy (eV), h is Planck's constant, ʋ is the frequency of light, A is the absorption constant and  = F(R) is the absorption coefficient, ½ is electronic transtition number for direct allowed transtition material, respectively. A plot of [F(R)hʋ] 1/2 vs hʋ can be achieved using the y value of zero. The Eg was acquired by calculated the x-axis intercept of an extrapolated tangential line from the linearity of the curve. As shown in Figure 3, the main optical absorption band of La-PTC was around 561 nm, or the Eg was estimated to be 2.21 eV by using linear equation of y = 686.88x-1551.3 (R 2 = 0.9955). La-PTC show broad-range visible light absorption due to the presence of highly conju-) of perylene structure. The Eg of perylene tetracarboxylate ligand-based La-MOF was lower than 2,6-naphthalene dicarboxylic acid ligand-based La-MOF that is 3.10 eV [11]. The constriction band gap energy oc--* than 2,6-naphthalene dicarboxylic acid. In the lap of orbitals, which will reduce the gap ener-  gy between the neighboring orbitals and leads to a bathochromic shift in the absorption spectrum.
On the other hand, in the conjugated bonding system, when an electron absorbs photons at the right wavelength, the electron will be normalized to a higher energy level. Commonly, the electron transition occurs in π to antibonding π -*) electron. A conjugated system with conjugated double bonds of less than eight members can only absorb the energy around the ultraviolet area, so they will appear colorless. With the addition of a double bond, the system will absorb photons from the wider wavelength area. For example, the naphthacene (four rings) has maximum absorption at 470 nm, while pentacene (five rings) has a maximum absorption at 575 nm. Figure 4 showed that the morphology of La-PTC has a rod shape structure with various lengths and diameters. The structure of La-PTC has a good homogeneity as indicated by the crystal structure having almost the same morphology at each observation point. Meanwhile, the EDS characterization showed the elements of the La-PTC which consisted of carbon, oxygen and lanthanum with amounts of 51.8%, 28.3%, and 19.9%, respectively.
La-PTC has a type II adsorption isotherm as shown from the BET analysis in Figure 5a. The presence of an initial curve pattern indicated the occurrence of monolayer or multilayer adsorption on the material. The isotherm graph did not show a sharp increment in adsorption, so it is indicated that when the entire surface of the monolayer is covered by adsorbate, multilayer adsorption occurs. At the beginning of the curve, there was a very sharp increment. It is indicated that monolayer adsorption has oc-curred overall. Whereas, if there is no sharp increment at the beginning of the curve, multilayer adsorption begins when the entire surface of the monolayer is covered by adsorbate [12].
La-PTC has a surface area of 22.2364 m 2 .g −1 . The surface area is relatively smaller when compared to the surface area of perylene based MOF based on different metals which is 36.228 m 2 .g −1 for Dy-MOF; 51.853 m 2 .g −1 for Sm-MOF [13]; and 67.779 m 2 .g −1 for Ni-MOF [14]. In addition, the pore size distribution of La-PTC was found at the range of 2-25 nm (Figure 5b). It is assumed that La-PTC was a mesoporous material group, which has the advantage of being a photo-catalyst that can provide a short distance to reduce the recombination of photo-excited electron-hole [15].
Thermogravimetric analysis was carried out to ensure the temperature at which the material tends to stabilize and begin to decompose and what compounds are produced when the materials were decomposed. The TGA analysis exhibits the La-PTC material relatively stable at temperatures of 0 to 376.39 °C. It suggested that the La-PTC material was stable at high temperatures.
The methylene blue (MB), rhodamin B (RhB) and methyl orange (MO) photocatalytic degradation were investigated in order to determine the efficiency of La-PTC as photocatalysts in MB, RhB and MO degradation under visible irradiation. Several control experiments were designed to clarify the properties of photocatalytic reactions. MB, 25.55% for RhB and 16.71% for MO. The photolysis experiment shows the MB, RhB, and MO are slightly stable for 240 minutes of irradiation with and without addition of H2O2. La-PTC shows no photocatalytic activity to degrade MB and RhB, but it degraded MO with a degradation efficiency of 64.74% for 240 minutes irradiation. It is indicated that the di-rect photocatalytic degradation of MB and RhB by La-PTC was less energetic favorable. Figure 7a showed the conduction band (CB) of La-PTC (-1.60 V) which is less negative than the LUMO of MB (-0.25 eV) and RhB (-1.00 eV). On the other hand, the valence band of La-PTC (+0.61 eV) [16] is less positive than HO-MO of MB (+1.61 eV) and RhB (+1.10 eV) and oxidation potential (H2O/·OH) (+2.32 eV), therefore the photoexicet electron-hole pairs will be recombined and the formation of radical species does not occur and inhibit the degradation of methylene blue and rhodamin B [17]. Meanwhile, the HOMO of MO is less positive (+0.14 eV) than the valence band of La-PTC and the LUMO of MO (-1.87 eV) is more negative than the conduction band of La-PTC. The degradation of MO took place via the photosensitization degradation pathway [18]. The photon irradiated the system, MO molecule was photogenerated to the LUMO level, generating  abundant of the photogenerated electrons at the LUMO level and holes at the HOMO level. These electrons can be an injection from the MO molecule to the conduction band (CB) of La-PTC, facilitating the generation of intermediate reactive species for starting dye degradation ( Figure 7b). Then, the photo-catalytic activity of MOF La-PTC was optimized by adjusting the amount of H2O2 for the MB, RhB and MO degradation. As shown in Figure 6, the appearance of H2O2 elevated the dyes degradation efficiency by La-PTC about 67.02% for MB, 77.00% for RhB and 99.60% for MO. The replenishment of H2O2 would be an advantage for photocatalytic activity by generating a highly reactive intermediate of hydroxyl free radical [19]. In general, the photocatalytic activity of MOF La-PTC is associated with the electron charge transfer from the photoexcited organic ligand to the metal (LMCT) within MOFs accounts for their photocatalytic activity [20]. When La-PTC was irradiated with photons, the photoexcited electrons are transferred from the valence band (VB) to the conduction band (CB), accompanied by the formation of positive-charged holes (h+) in the VB [21]. H2O2 will react with photoexcited electrons to form •OH radical and hinder photoexcited electron-hole pairs recombination. Simultaneously, electrons react with oxygen to form superoxide radicals (•O2 − ) [22] and h + react with H2O or OH − form •OH radical will also have a strong ability to oxidize dyes molecules.
The influence of La-PTC and H2O2 concentration on MB and MO photocatalytic degradation then were evaluated [23]. As shown in Figure 8, a furthermore increase of H2O2 induces a decrease of degradation of MB and MO, suggesting the increasing H2O2 concentration destructed La-PTC active sites and decrease the photocatalytic performance. Abramovic and coworkers in 2015 reported that the additional excess of H2O2 as an electron scavenger will generate a hydroperoxyl (•HO2) radical then react strongly with hydroxyl radical (•OH) to form oxygen and water, causes the concentration of hydroxyl radical in the system was decreasing. Therefore, the subsequent photocatalytic MB/RhB/MO degradation was carried out in which H2O2 0.2 M was added [24].
To explain the main contributor species in the reaction, several studies verified the change of the degradation efficiency of MB/MO in the presence of different scavengers such as H2O2 as electron (e − ) scavenger, methanol as hole (h + ) scavenger and tert-butyl alcohol (TBA) as the ·OH scavenger [25]. Figure 8a and 8b exhibit on the photodegradation of MB and RhB, the activity increased in the presence of H2O2 and CH3OH than without the presence of H2O2 and CH3OH. This happened because the addition of H2O2 and CH3OH can trigger the formation of the radical species, hence decreasing the rate of electron-hole recombination. When the TBA was adding in the system, the degradation of MB was slightly decrease due to the limited •OH formed. Accordingly •OH radical, •O2 − radical, photo-excited electron and holes were the contributor in MB and RhB degradation with the major contributor were hole and •OH [26]. Whilst the photo-degradation of MO was slightly enhanced with the addition of H2O2 and decreasing when the addition of CH3OH and TBA were applied (Figure 8c). This represented that the holes and •OH radicals were major species that play a role in MO degradation. To assess the reusability of the La-PTC, recycling activities were carried out. La-PTC shows high photocatalytic activity for MB and RhB degradation during four cycle reactions with degradation efficiency of 77% for MB and 87% for RhB, and MO degradation during three cycle reactions with degradation efficiency of 62.57% with the presence of H2O2 in the system.

Conclusions
The La-PTC MOF was synthesized by a solvothermal method using a perylene-3,4,9,10tetracarboxylate ligand and lanthanum metal ion. La-PTC has a broad-range visible light absorption with the main optical absorption band of La-PTC was around 561 nm, high photocatalytic degradation activity of MO with the degradation efficiency of up to 99.60% in the presence of H2O2. H2O2-assisted MB/RhB/MO study suggested that adding the H2O2 into the system enhanced the photo-catalytic activity by quickly generating radical reactive species of hydroxyl. Hydroxyl species and holes are the contributor species in MO degradation. Meanwhile, electrons, superoxide radicals, hydroxyl radicals and holes are the contributor species in MB and RhB degradation, with hydroxyl radicals and holes as the main contributors.