Extending the visible light absorption of NH2-UiO-66 through diazotization reaction for photocatalytic chromium (VI) reduction

The optical properties of NH2-UiO-66 as a visible light–active metal organic framework was further enhanced through the diazotization reaction with π-conjugated 1-naphthol reagent. Diffuse reflectance UV–Vis spectrum of diazotized MOF, named as Azo-UiO-66, exhibited a significant red shift compared to unfunctionalized NH2-UiO-66 due to the formation of diazo compound. Also, Tauc calculations indicated considerable decrease in band gap energy from 2.68 to 1.7 eV, resulting in improvement of visible light harvesting. Furthermore, other physicochemical techniques, e.g., X-ray diffraction (XRD), N2 adsorption–desorption analysis, thermogravimetric analysis (TGA), energ-dispersive X-ray (EDX), and CHN elemental analyses demonstrated the successful MOF diazotization with 1-naphthol and preservation of NH2-UiO-66 framework upon post-modification process. The reduction of hexavalent chromium, Cr(VI), as a serious contaminant in wastewater to less toxic Cr(III) was performed over prepared photocatalyst, which demonstrated the positive role of ligand functionalization and enhancement of visible light absorption on overall photocatalytic performance of Azo-UiO-66.


Introduction
Water contamination has become a serious problem in almost all societies and attracted more considerations of the researchers in purification of polluted water (Buaisha et al. 2020). Chromium (VI) as one of toxic heavy metals in wastewater is extremely harmful for human health, since it can simply penetrate through skin and respiration and results in drastic damage to kidney and liver (Wang et al. 2021a). Among various techniques such as adsorption, precipitation, and membrane separation, photocatalytic reduction of Cr(VI) to very low toxic Cr(III) is the more appropriate method to treat wastewater comprising Cr(VI) Zhao et al. 2021). In general, technologies that deal with solar energy like photocatalysis are of a great importance especially in environmental remediation, since solar energy is abundant, sustainable, and clean Wu et al. 2020). Therefore, utilizing efficient photocatalysts with energy levels suitable for Cr(VI) reduction is beneficial in wastewater treatment.
Porous and crystalline metal organic frameworks (MOFs) have been widely utilized in several research areas such as gas storage and separation (Bonneau et al. 2020), drug delivery (Abánades Lázaro et al. 2020), supercapacitor (Zheng et al. 2020), heterogeneous catalysis (Mortazavi et al. 2020), and photocatalysis (Kong et al. 2021;Zhang et al. 2022) owing to their specific features like adjustable pore size, large surface area, and high thermal and chemical resistance (Mortazavi et al. 2021;Wang et al. 2021b). Moreover, MOFs have been emerged as powerful photocatalysts especially for removal of water contaminants due to their semiconducting behavior, in which the organic ligands can behave as lightharvesting antennas to activate the inorganic metal cluster through the charge transfer from ligands to metal sites (Hu et al. 2019;Zhang et al. 2019).
One of the benefits of using MOFs as photocatalysts is the easy tuning of band gap energies by using different metal units and organic ligands. MOFs with amine functionalities exhibited good performance in photocatalytic wastewater treatment compared to amine-free MOFs with the same metal sites due to the reduced band gap energies and enhanced visible light harvesting (Carson et al. 2013;Zhang et al. 2019). Among them, NH 2 -UiO-66 metal organic framework has received considerable interest because of its high stability in liquid medium (Lin et al. 2022;Ling et al. 2018;Zhao et al. 2019). However, the visible light absorption of these kinds of MOFs are limited to maximum 450 nm. Therefore, the optical properties of these MOFs needs to be further improved for maximizing visible light harvesting and, hence, boosting the photocatalytic activity in wastewater treatment. Herein, the post-synthetic modification (PSM) was utilized to modify the amino groups of NH 2 -UiO-66 with π-conjugated 1-naphthol through diazotization reaction to prepare Azo-UiO-66 photocatalyst, in which the color of MOF was changed from yellow to deep red. According to literature, diazo-containing MOFs have been used for some photocatalytic applications such as alcohol oxidation (Nasalevich et al. 2013), dye degradation (Kirchon et al. 2020;Otal et al. 2016), and Cr(VI) reduction (Fu et al. 2021(Fu et al. , 2023, but in the best of our knowledge, no reports were found on using these kinds of visible light extended UiO-type MOFs for Cr(VI) reduction. Therefore, Cr(VI) reduction experiment was performed over Azo-UiO-66 photocatalyst, and exhibited higher photocatalytic performance in Cr(VI) reduction compared to parent NH 2 -UiO-66 due to promoted visible light absorption.

Preparation of NH 2 -UiO-66
NH 2 -UiO-66 was synthesized via solvothermal method reported in literature (Sun et al. 2013). Briefly, ZrCl 4 (0.048 g, 0.2 mmol) was dissolved completely in anhydrous DMF (12 mL), and 2-aminoterephthalic acid (0.037 g, 0.2 mmol) was added to the solution, followed by addition of water (38 μL). The obtained solution was stirred for another 10 min and finally transferred to 20-mL Teflon-lined steel autoclave and heated at 120 °C for 24 h in an oven. After naturally cooled to room temperature, the obtained yellow powder was rinsed twice with DMF and soxhlet washed with methanol to remove unreacted zirconium salt and organic ligand. The pure product was then thermally dried at 100 °C to remove the entrapped solvent molecules. The yield of 45% was achieved for activated NH 2 -UiO-66 based on the initial weight of raw materials.

Synthesis of Azo-UiO-66
In order to synthesis Azo-UiO-66 photocatalyst, 0.2 g of NH 2 -UiO-66 was dispersed in 60 mL of distilled water, and NaNO 2 (1.2 mmol) was added to the mixture, followed by immediate but slow addition of HCl 0.03 M (120 mL). After stirring for 1 h, the color of mixture was changed from pale yellow to pale orange. The container was kept in an ice bath during the whole synthetic steps. Afterwards, 5 mL of acetic acid containing 1-naphthol (2.4 mmol) was added drop by drop to the container, and the color of mixture turned into deep red during 3-h stirring time (Nasalevich et al. 2013). The prepared Azo-UiO-66 was centrifuged, rinsed with distilled water, and then soxhlet washed with ethanol for 24 h and dried at 100 °C.

Characterization
The X-ray diffraction (XRD) was conducted with employing PW1730 instrument (PILIPS Company, Netherlands) with copper radiation (Kα at λ = 1.54 Å), and the step size and time per step of 0.05° and 1 s, respectively. The N 2 adsorption-desorption technique was performed using BEL-SORP MINI II apparatus (BEL Company) connected with a BEL PREP VAC II degassing instrument. Before analysis, the samples were degassed in vacuum oven at 100 °C for removal of adsorbed molecules. Also, specific surface area was determined with Brunauer-Emmett-Teller (BET) method. The thermogravimetric analysis (TGA) was performed by TGA Q600 instrument (TA Company, USA) under Ar flow. Carbon and nitrogen amounts of photocatalysts were detected with employing Perkin Elmer 2400 SERIES II and Thermo Finnigan (Flash 1112 Series EA) CHN Analyzer. Field emission scanning electron microscopy (FESEM) was carried out by TESCAN MIRA III instrument joined with energy-dispersive X-ray spectroscopy (EDX). The optical analysis of materials were examined by a UV-Vis diffuse reflectance spectroscopy (UV-2600, Shimadzu) using a standard material such as BaSO 4 . Moreover, Shimadzu UV-2450 UV-Vis spectrometer was utilized to acquire the UV-Vis absorption spectra of liquid samples. The Mott-Schottky experiment was conducted by PGSTAT302N-High Performance in 0.2 M Na 2 SO 4 solution as electrolyte at frequency of 500 Hz and voltage amplitude of 10 mV in dark condition. A glassy carbon, a saturated calomel electrode (SCE), and a platinum disk were employed as working, reference, and counter electrodes, respectively. The glassy carbon was modified prior to electrochemical measurement as follow: first, the powdered photocatalyst (1 mg) was sonicated in ethanol (100 μL) for 5 min. Afterward, the well-dispersed mixture (5 μL) was dripped on the surface of electrode, followed by drying under IR lamp. Eventually, Nafion solution (2 μL) was added on the electrode surface and dried similarly. The electrochemical impedance spectroscopy (EIS) was also performed with the same instrument and method in the frequency range of 0.01-100,000 Hz at open circuit potential, with voltage amplitude of 0.1 V.

Photocatalytic reduction of Cr(VI)
In a typical method, 0.01 g of photocatalyst was added to 30 mL of K 2 Cr 2 O 7 solution (10 ppm) at pH = 2, and stirred at dark for half an hour to reach adsorption-desorption equilibrium. Afterward, the photocatalytic reduction of Cr(VI) was initiated by turning on the LED lamps with general power of 60 W. The samples were periodically collected, centrifuged to isolate the photocatalyst, and the concentration of Cr(VI) was analyzed by UV-Vis spectroscopy using diphenylcarbazide method (Idris et al. 2011).

Preparation and characterization of materials
The schematic procedure for diazotization reaction between amino groups of NH 2 -UiO-66 and 1-naphthol is clearly shown in Scheme 1. In general, diazotization is occurred between primary amines and nitrous acid to produce diazonium salts, which are key intermediates especially for synthesis of diazo compounds. In the first step, the addition of NaNO 2 and HCl to aqueous dispersion of MOF leads to in situ formation of nitrous acid (HNO 2 ) and subsequent generation of diazonium salt. Since the diazotization is an exothermic process, the reaction temperature should be maintained around 0-5 °C in order to prohibit the diazonium salt decomposition and eliminate the formation of other byproducts. In the final step, the electrophilic aromatic substitution between the diazonium compound and 1-naphthol as an electron-rich organic substrate leads to diazo-coupling reaction and production of Azo-UiO-66 photocatalyst with deep red color (Chakraborty 2014, Hosseini and Masteri-Farahani 2021, Kouklovsky 2005, Roos and Roos 2015. In order to demonstrate the changes of chemical bonds after diazotization reaction, FT-IR spectroscopy was carried out and the obtained spectra for NH 2 -UiO-66 and Azo-UiO-66 are depicted in Fig. 1. NH 2 -UiO-66 exhibits a broad peak at 3420 cm −1 related to O-H and N-H stretching vibrations of surface adsorbed water molecules and amino groups of organic ligand. The stretching vibrational peaks of C-O and C = O have been shifted from 1387 and 1569 cm −1 for NH 2 -UiO-66 to 1312 and 1580 cm −1 , respectively, owing to the conjugation of diazo bond (N = N) with aromatic ring of benzene (Fu et al. 2021, Wen andGuo 2018). On the other hand, the diazo bond itself exhibits a weak vibrational peak in the range of ~ 1500-1550 cm −1 , which the strengthened peak at 1502 cm −1 in Azo-UiO-66 could be due to the presence of N = N bond (Bartošová et al. 2017;Zimmermann et al. 1993). Also, the peak located at 1256 cm −1 refers to the C-N stretching vibration of NH 2 -UiO-66, while the intensity of this peak has been lowered and converted to two individual peaks, revealing the presence of two C-N bonds in Azo-UiO-66 (Rodríguez et al. 2017;Subudhi et al. 2020).
The XRD technique was applied to confirm the high stability of NH 2 -UiO-66 during diazotization reaction. As depicted in Fig. 2a, the XRD pattern of synthesized NH 2 -UiO-66 is in compliance with simulated one (Øien et al. 2014), suggesting the formation of three dimensional Scheme 1 Preparation of Azo-UiO-66 photocatalyst and highly crystalline metal organic framework. After diazotization with 1-naphthol, no alteration was observed in MOF structure and crystallinity, showing the acceptable resistance of NH 2 -UiO-66 framework against deformation in diazotization reaction condition.
To demonstrate the variations in specific surface area and pore volume after diazotization reaction, nitrogen adsorption-desorption technique was employed. According to Fig. 2b, the NH 2 -UiO-66 isotherm corresponds to type I isotherms refereeing to Brunauer-Deming-Deming-Teller (BDDT) categorization which is the characteristics of microporous materials ). The MOF microporosity was maintained during diazotization reaction by observing type I isotherm for Azo-UiO-66 photocatalyst. However, the N 2 gas uptake was reduced evidently as a result of micropores occupation with 1-naphthol. The presence of hysteresis loop in NH 2 -UiO-66 and Azo-UiO-66 isotherms at higher pressures arises from pore accumulation . Moreover, the specific surface area was measured with utilizing Brunauer-Emmett-Teller (BET) calculation, which the obtained data are shown in Table 1. The dramatic decrease in BET surface area from 880 to 492 m 2 /g for diazotized MOF along with reduction in total pore volume from 0.547 to 0.486 cm 3 /g are mainly related to the insertion of 1-naphthol into the NH 2 -UiO-66 micropores via formation of diazo compound.
Thermogravimetric analysis is a useful technique to investigate the thermal stability of solid materials. Accordingly, the TG analysis was conducted for both NH 2 -UiO-66 and  Fig. 2c. Two main weight loss steps can be observed in TGA profile of NH 2 -UiO-66. The first step involves the release of entrapped solvent molecules and surface adsorbed water molecules up to 120 °C with weight loss of 27%. This clearly shows the absence of DMF molecules in the micropores of NH 2 -UiO-66 due to the successful solvent exchange with methanol. In the next step, about 39% of weight loss especially in the range of 350-700 °C can be observed which is related to decomposition of NH 2 -UiO-66 framework by losing organic ligands (Kim et al. 2018). Similarly, the TGA profile of Azo-UiO-66 exhibits two weight loss steps. First, about 13% weight loss can be seen below 120 °C related to the release of solvent and adsorbed water molecules. This amount of solvent removal is much lower than NH 2 -UiO-66, resulting from the MOF pore occupation with 1-naphthol, which is in accordance with the reduced total pore volume of Azo-UiO-66 (Table 1). In the second step, almost 47% weight loss is occurred at higher temperature especially between 350 and 700 °C, corresponding to photocatalyst framework decomposition. Since the bonded 1-naphthol is an organic moiety and decomposed above 350 °C, the observed increase in weight loss percentage from 39% for NH 2 -UiO-66 to 47% for Azo-UiO-66 indicates the successful incorporation of 1-naphthol in MOF structure through diazotization reaction. Figure 2d illustrates the EDX elemental analysis for prepared photocatalysts. Both NH 2 -UiO-66 and Azo-UiO-66 photocatalysts contain zirconium, oxygen, carbon, and nitrogen which are well observed in their EDX spectra. The results confirm the presence of all MOF constitutional elements in final Azo-UiO-66 photocatalyst. Also, CHN elemental analysis was further conducted to show the accurate amount of carbon and nitrogen and monitor the changes of these elements upon diazotization reaction. On the basis of the data given in Table 1, the amount of carbon for Azo-UiO-66 was found more than NH 2 -UiO-66 due to the insertion of 1-naphthol with high carbon content in MOF structure, while the amount of nitrogen was slightly reduced. This observation is due to this fact that the number of carbon added to MOF structure is more than the number of inserted nitrogen. Therefore, the increase in carbon weight results to reduction in the weight percentage of nitrogen.
To realize the morphology and particle size of Azo-UiO-66 photocatalyst, field emission scanning electron microscopy was performed, and typical images are shown in Fig. 3a and 3b, respectively. According to these images, the photocatalyst exhibits well-shaped structure with almost uniform particle size of about 90-110 nm (the inset of Fig. 3a).
The optical properties of NH 2 -UiO-66 and Azo-UiO-66 photocatalysts were investigated with employing UV-Vis diffuse reflectance spectroscopy in order to demonstrate the effect of ligand functionalization on the enhancement of visible light absorption. As shown in Fig. 4a, NH 2 -UiO-66 only exhibits a shoulder in visible region at 400-450 nm, which corresponds to the observed yellow color of MOF (the inset of Fig. 4a) (Otal et al. 2016). Upon diazo-coupling with 1-naphthol, the maximum absorption peak was red shifted, which almost included all visible region from 400 to 700 nm. This observation is in complete agreement with the MOF apparent color change from yellow to deep red, indicating that the modified MOF ligand could efficiently harvest visible light. To find out the changes in band gap energies, Tauc equation (αhʋ) 1/n = B(hʋ-E g ) was employed in which α, h, ʋ, B, and E g refers to absorption coefficient, Plank constant, light frequency, a constant, and band gap energy, respectively (Makuła et al. 2018). Also, n represents for the electronic transition type, which is assumed to be 1/2 for direct allowed transitions Hosseini et al. 2022). According to the Tauc plots depicted in Fig. 4b, the band gap energy has been significantly decreased from 2.68 to 1.7 eV for Azo-UiO-66, mainly due to the formation of diazo compound in MOF structure, resulting in the facilitation of visible light absorption.

Photocatalytic activity
The Cr(VI) reduction was performed to assess the photocatalytic properties of Azo-UiO-66 with extended visible light absorption compared to primitive NH 2 -UiO-66, and the corresponding results are demonstrated in Fig. 5. The photo-reduction of Cr(VI) in the absence of photocatalyst was found only to be 11% at 150 min, indicating the high resistance of Cr(VI) conversion to reduced chromium species (Fig. 5a). By addition of NH 2 -UiO-66 to the Cr(VI) solution, the photo-reduction of Cr(VI) was improved up to 78% due to photocatalytic activity of parent MOF. But, after diazotization with 1-naphthol and improvement of visible light absorption, Azo-UiO-66 exhibited outstanding performance in photocatalytic reduction of Cr(VI) (95%), resulting from efficient visible light harvesting of azo-naphthol antenna and photo-induced electron transfer to Zr-O cluster, and hence, acceleration of Cr(VI) conversion to reduced chromium species during 150-min visible light irradiation. Moreover, a similar experiment was carried out in the presence of Azo-UiO-66 photocatalyst in dark condition, but, no Cr(VI) reduction was observed, which further confirmed that both photocatalyst and light are two crucial factors for leading photocatalytic Cr(VI) reduction. Figure 5b reveals the reduction of Cr(VI) absorbance peak at 540 nm over time, arising from the decrease in Cr(VI) concentration in the presence of Azo-UiO-66 photocatalyst. The inset picture of Fig. 5b demonstrates the real photograph of withdrawn samples at different times, showing that the colorless solution is obtained after 150 min, proving the almost complete conversion of Cr(VI) to Cr(III). The kinetic behavior of prepared photocatalysts for Cr(VI) reduction was further investigated to understand the type and kinetic values for each reduction system. In this regard, the zero-order, pseudo-first-order, and pseudo-second-order kinetic plots were achieved using the C 0 -C = k 0 .t, Ln(C 0 /C) = k 1 .t, and (1/C)-(1/C 0 ) = k 2 .t kinetic formulas Ma et al. 2020), respectively, as shown in Fig. 6. Also, the kinetic data and R 2 values are given in Table 2. When comparing the three kinetic models for both NH 2 -UiO-66 and Azo-UiO-66 photocatalysts (Fig. 6a-c), it was concluded that the photocatalytic reduction of Cr(VI) in the presence of these photocatalysts tend to follow zero-order kinetic model by observing the linear curve with high R 2 values ( Fig. 6a and Table 2). Therefore, the Cr(VI) reduction in the presence of these photocatalysts follow Eley-Rideal mechanism, which means that only Cr(VI) moiety as a single reactant is physically adsorbed on the surface of photocatalyst and then accepts the electrons from the photo-excited photocatalysts, whereas the adsorption of Cr(VI) is not considered as rate determining step . The resulted kinetic constant for Cr(VI) reduction over Azo-UiO-66 photocatalyst (0.0041 mg.L −1 .min −1 ) was found to be 1.28 and 10.25 times greater than pristine NH 2 -UiO-66 and the blank one, respectively (Fig. 6d), showing the positive role of extending the light absorption to visible region on acceleration of Cr(VI) reduction.
The influence of initial Cr(VI) concentration and photocatalyst amount on photocatalytic performance of Cr(VI) reduction were also assessed, and the corresponding plots are illustrated in Figs. 7a and 7b, respectively. By increasing the concentration of Cr(VI) solution from 10 ppm to higher concentrations of 20 ppm and 30 ppm, it was observed that the efficiency of Cr(VI) reduction decreased to 75% and 55% within 150-min visible light irradiation in the presence of Azo-UiO-66 photocatalyst, respectively, which can be due to the fact that the extra amount of hexavalent chromium species in solution may cover the surface of photocatalyst and hinder the fast Cr(VI) reduction (Wei et al. 2021). Moreover, the effect of Azo-UiO-66 amount on efficiency of Cr(VI) Fig. 6 a The zero-order, b pseudo-first-order, c pseudosecond-order kinetic curves, and d zero-order kinetic constants for Cr(VI) reduction in the presence of photocatalysts. Photocatalytic condition: Cr(VI) solution (10 ppm, 30 mL) and photocatalyst (0.01 g) Table 2 The kinetic constants of photocatalytic Cr(VI) reduction

Photocatalyst
Zero-order pseudo-first-order pseudo-second-order k 0 (mg.L −1 .min −1 ) R 2 k 1 (min −1 ) R 2 k 2 (L.mg −1 .min −1 ) reduction was further evaluated by using different dosages of photocatalyst. As expected, by increasing the photocatalyst amount from 0.002 to 0.005 g and 0.010 g, the photo-reduction of hexavalent chromium enhanced from 46 to 86% and 95%, respectively. This highly suggests that the presence of higher amounts of photocatalyst in reaction medium would provide more active sites for Cr(VI) reduction . Generally, when a photocatalyst is excited upon light irradiation, oxidative holes and reductive electrons are generated, which the produced electrons are capable to reduce Cr(VI) to lower toxic species. According to literature (Du et al. 2019;Wang et al. 2021a;Zhao et al. 2021), some hole scavengers can be consumed by holes, leading to suppression of electron-hole recombination and acceleration of electron transfer from conduction band (CB) of photocatalyst to Cr(VI) moiety. Therefore, to examine the effect of hole scavengers on reduction behavior, citric acid, oxalic acid, phenol and ethanol as well-known hole scavengers were added to photocatalytic reaction mixture containing Azo-UiO-66 photocatalyst. As depicted in Fig. 8, the photocatalytic reduction of Cr(VI) was enhanced dramatically up to 100% at shorter reaction time while using these four types of hole scavengers. This clearly reveals that the photo-generated electrons are long-lived enough to accelerate reduction process when the photo-generated holes are captured by some organic compounds.
Besides UV-Vis diffuse reflectance spectroscopy (Fig. 4) which well demonstrated the extension of visible light absorption of Azo-UiO-66 and hence improvement of photocatalytic activity, the other important analysis to prove the observed high photocatalytic performance is electrochemical impedance spectroscopy (EIS) which reveals the separation of photo-generated charge carriers in prepared materials. As depicted in Fig. 9a, the radius of curvature for Azo-UiO-66 is much smaller than NH 2 -UiO-66, indicating that introduction of 1-naphthol to NH 2 -UiO-66 framework via diazotization reaction has a positive effect on suppressing the recombination of photo-generated electron-hole, and subsequently, the long-lived electrons can efficiently reduce Cr(VI).
The Mott-Schottky analysis was conducted to estimate the CB position of prepared photocatalysts. According to Mott-Schottky diagrams shown in Fig. 9b, NH 2 -UiO-66 and Azo-UiO-66 photocatalysts are considered as n-type semiconductors due to the positive slope in corresponding diagrams. To achieve the flat band potential of NH 2 -UiO-66, the curve was extrapolated, and as a result, − 0.82 V vs. SCE was obtained. Since, the flat band potential of most n-type semiconductors are assumed to be 0.1 V greater than the conduction band, − 0.67 V vs. NHE was obtained for the conduction band of NH 2 -UiO-66 (Subudhi et al. 2020). Therefore, the valence band (VB) position of MOF was calculated to be 2.01 V vs. NHE using the E g = E VB -E CB equation. With the same method, the CB and VB positions of Azo-UiO-66 were determined as − 0.58 and 1.12 V vs. NHE, respectively, indicating that the insertion of 1-naphthol in MOF structure via The probable photocatalytic mechanism for Cr(VI) reduction over NH 2 -UiO-66 and Azo-UiO-66 photocatalysts is schematically revealed in Scheme 2. The photocatalysts can be excited easily upon visible light irradiation due to their suitable band gap energies, followed by separation of electron (e − ) and hole (h + ) charge carriers (Eq. 1). According to literature, the potential level of Cr(VI)/Cr(V) was estimated to be + 0.55 V vs. NHE at pH = 2 (Huang et al. 2017;Hussain et al. 2021;Testa et al. 2004), which is more positive than the conduction levels of these photocatalysts. Therefore, the electron transfer from the CB of photocatalysts to potential level of Cr(VI) can be easily occurred and finally Cr(III) as less toxic and environmentally friendly moiety is produced (Eq. 2). As illustrated, the acidic hydrogen plays a vital role in facilitation of Cr(VI) conversion in acidic solution (Wang et al. 2021a). It is worth noting that both reduction in band gap energy and conduction band level of Azo-UiO-66 photocatalyst compared to NH 2 -UiO-66 have accelerated the Cr(VI) reduction via enhancement of visible light absorption and facile electron transfer from photocatalyst to Cr(VI) moiety, respectively. Moreover, as described earlier, the small organic compounds in Cr(VI) solution as hole scavengers would react with generated holes and consequently, carbon Mott-Schottky plots of prepared photocatalysts Scheme 2 The probable photocatalytic mechanism for Cr(VI) reduction over NH 2 -UiO-66 and Azo-UiO-66 photocatalysts Fig. 10 a The reusability of Azo-UiO-66 in photocatalytic reduction of Cr(VI) and b XRD patterns of fresh and reused Azo-UiO-66 photocatalyst dioxide and water as degraded products can be formed Du et al. 2019;Guo et al. 2022).
The Azo-UiO-66 reusability and stability were also investigated during four photocatalytic cycles under the same circumstances, and the data are shown in Fig. 10a. After each cycle, the photocatalyst was washed with methanol to remove adsorbed chromium species and thermally activated in oven. It is well evident from Fig. 10a that the efficiency of 4th reused photocatalyst is almost identical to the fresh one with only 3% reduction in overall efficiency. Moreover, no change in crystallinity was observed in the XRD pattern of reused photocatalyst (Fig. 10b), revealing the high stability of Azo-UiO-66 framework during photocatalytic Cr(VI) reduction.
To show the good efficiency of Azo-UiO-66 photocatalyst and its real applicability in Cr(VI) removal, a simple comparison was made with other similar reported works based on MOF materials which are listed in Table 3. Considering the most important factors in Cr(VI) reduction such as photocatalyst amount, Cr(VI) concentration and volume, pH of solution, time and light source, the photocatalytic efficiency of Azo-UiO-66 was found comparable with previous works with almost the same or higher reduction percentage. However, the present work provides more convenient and cost-effective photocatalytic set-up using commercially available LED lamps compared to more expensive Xe lamps used in other works. (1)

Conclusion
In summary, the diazotization reaction between NH 2 -UiO-66 and 1-naphthol resulted in the formation of Azo-UiO-66 photocatalyst with deep red color. UV-Vis diffuse reflectance spectroscopy approved the optical changes of NH 2 -UiO-66 through diazotization reaction by observing the extension of MOF absorption peak to visible region. According to Tauc plot measurements, the band gap energies of NH 2 -UiO-66 and Azo-UiO-66 photocatalysts were calculated to be 2.68 and 1.7 eV, respectively, which indicated the impact of ligand functionalization on lowering the band gap energy and enhancement of visible light harvesting. On the other hand, the CB and VB levels of photocatalysts were determined using Mott-Schottky method, which indicated that the diazotization reaction had noticeable effect on lowering and increasing CB and VB levels of parent MOF, respectively. Considering the obtained energy levels of photocatalysts, photocatalytic Cr(VI) reduction with suitable reduction potential was carried out. As expected, Azo-UiO-66 photocatalyst was found more efficient than NH 2 -UiO-66 with Cr(VI) removal of 95% in 150-min visible light irradiation, mainly due to its extended visible light absorption. Moreover, Azo-UiO-66 photocatalyst was found stable and reusable up to four photocatalytic cycles with no remarkable loss of activity.
Acknowledgements The authors gratefully acknowledge financial support from University of Tehran and the Iran National Science Foundation, Iran (INSF) [Grant No. 99004825].
Author contribution Mahdiyeh-Sadat Hosseini has participated in analysis, interpretation of the data, and writing the manuscript. Alireza Abbasi and Majid Masteri-Farahani has participated in conception and design, and approval of the final version.
Data availability Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

Declarations
Ethical approval Not applicable.  (Liang et al. 2015) Consent to participate Not applicable.

Competing interests
The authors declare no competing interests.