Covalently anchoring silver nanoclusters Ag44 on modified UiO-66-NH2 with Bi2S3 nanorods and MoS2 nanoparticles for exceptional solar wastewater treatment activity

For the first time, covalently anchoring size selected silver nanoclusters [Ag44(MNBA)30] on the Bi2S3@UiO-66-NH2 and MoS2@UiO-66-NH2 heterojunctions were constructed as novel photocatalysts for photodegradation of methylene blue (MB) dye. The anchoring of Ag44 on MoS2@UiO-66-NH2 and Bi2S3@UiO-66-NH2 heterojunctions extended the light absorption of UiO-66-NH2 to the visible region and improved the transfer and separation of photogenerated charge carriers through the heterojunctions with a unique band gap structure. The UV–Vis-NIR diffuse reflectance spectroscopic analysis confirmed that the optical absorption properties of the UiO-66-NH2 were shifted from the UV region at 379 nm to the visible region at ~ 705 nm after its doping with Bi2S3 nanorods and Ag44 nanoclusters (Bi2S3@UiO-66-NH-S-Ag44). The prepared Bi2S3@UiO-66-NH-S-Ag44 and MoS2@UiO-66-NH-S-Ag44 photocatalysts exhibited exceptional photocatalytic activity for visible light degradation of MB dye. The photocatalysts exhibited complete decolorization of the MB solution (50 ppm) within 90 and 120 min stirring under visible light irradiation, respectively. The supper photocatalytic performance and recycling efficiency of the prepared photocatalysts attributed to the covalent anchoring of the ultra-small silver clusters (Ag44) on the heterojunctions surface. The X-ray photoelectron spectroscopic analysis confirmed the charge of the silver clusters is zero. The disappearance of the N–H bending vibration peak of primary amines in the FTIR analysis of Bi2S3@UiO-66-NH-S-Ag44 confirmed the covalent anchoring of the protected silver nanoclusters on the UiO-66-NH2 surface via the condensation reaction. The Bi2S3@UiO-66-NH-S-Ag44 catalyst exhibited excellent recyclability efficiency more than five cycles without significant loss in activity, indicating their good potential for industrial applications. The texture properties, crystallinity, phase composition, particle size, and structural morphology of the prepared photocatalysts were investigated using adsorption–desorption N2 isotherms, X-ray diffraction (XRD), HR-TEM, and FE-SEM, respectively.


Synthesis of UiO-66-NH 2
UiO-66-NH 2 was synthesized as reported before by the Farha group 42 .Briefly, 10 mL conc.HCl was added dropwise over suspended ZrCl 4 (1.25 g) in 50 mL DMF, and then the solution was sonicated for 20 min until fully dissolved.2-Aminoterephthalic acid as linker (1.34 g) was dissolved in 100 mL DMF and added to the solution.The solution was sonicated for a further 20 min and then heated in an oven at 80 °C for 16-18 h.The resulting solid was filtered and washed with DMF (2 × 30 mL) and EtOH (2 × 30 mL).Finally, NH 2 -UiO-66 was dried in an oven at 70 °C overnight.The NH 2 -UiO-66 was used after activation at 150 °C for 12 h.
In-situ preparation of Bi 2 S 3 @UiO-66-NH 2 and MoS 2 @UiO-66-NH 2 heterojunctions The one-pot synthesis of the Bi 2 S 3 @NH 2 -UiO-66 and MoS 2 @NH 2 -UiO-66 is based on the same procedure used for the preparation of NH 2 -UiO-66 in section "Synthesis of UiO-66-NH 2 ".The Bi 2 S 3 and MoS 2 in DMF were added to the NH 2 -UiO-66 precursors, to immobilize the Bi 2 S 3 and MoS 2 inside the NH 2 -UiO-66 frameworks.After that, the precipitate was filtered and washed with DMF (2 × 30 mL) and EtOH (2 × 30 mL).Finally, the Bi 2 S 3 @UiO-66-NH 2 and MoS 2 @UiO-66-NH 2 were dried in an oven at 70 °C overnight (Fig. 1).The loading percentage of the metal chalcogenides (Bi 2 S 3 or MoS 2 ) was around 3%.  30 was synthesized as reported before 45 .Briefly, 9.91 mg of 5,5′-dithiobis(2-nitrobenzoic acid) (DTNBA) was stirred in 20 mL NaOH aqueous solution (1 M).The disulfide bond was cleaved which was indicated by the formation of a dark yellow solution from 5-mercapto-2-nitrobenzoic acid (MNBA).8.5 mg of AgNO 3 (50 mmol) was dissolved in 5 mL DI water and added to the MNBA solution.The color was changed from dark yellow to greenish yellow indicating the formation of an Ag-S complex.A fresh NaBH 4 solution (1 mg in 2 mL DI water) was then used to reduce the complex.The solution turned dark brown immediately and gradually changed to dark red under vigorous stirring for 4 h, indicating the formation of Ag 44 (SR) 30 NCs.The clusters were purified by repeated precipitation with 50% methanol followed by repeated centrifugation at 9000 rpm for 10 min and decantation of the supernatant until it became colorless.

Photocatalytic studies of the prepared photocatalysts
The photocatalytic degradation of methylene blue (MB) solution using visible light was carried out to evaluate the photocatalytic activity of the prepared photocatalysts.A 450 W medium-pressure mercury lamp with a < 420 nm UV cut-off filter was used as a visible light source for the photocatalytic experiments, the lamp was fixed 10 cm away from the reaction system, as used in our previous work 5,6,[46][47][48] .30 mg of the prepared photocatalysts were suspended in 50 mL of highly concentrated aqueous solution MB (50 ppm) under magnetic stirring.To establish an adsorption-desorption equilibrium, the reaction system was first kept in the dark for 60 min and then exposed to visible light for two hours.5 mL aliquots from each sample were taken at the desired time intervals, followed by centrifugation and filtration to remove the photocatalyst.The decolorization of the MB solution was evaluated by measuring the change in its characteristic optical absorbance using an Evolution 300 UV-Vis spectrophotometer 5,6 .To check the advantages of the prepared photocatalysts and their applicability to reuse 5,6 , the photodegradation reaction of the MB solution was achieved with Bi 2 S 3 @UiO-66-NH-S-Ag 44 photocatalyst.
Then the photocatalyst was collected at the end of the reaction and reused for a second cycle and the process repeated so on till five cycles keeping all other parameters constant.

Results and discussion
Atomically precise monodispersed thiol-protected silver nanoclusters [Ag 44 (MNBA) 30 ] were synthesized using 5-mercapto-2-nitrobenzoic acid as a protecting ligand (Fig. S1).The used method produced monodisperse and stable silver nanoclusters in aqueous solution for at least 9 months at room temperature under ambient conditions.Electrospray ionization mass spectrometry (ESI-MS) was used to determine the composition, size, and monodispersity of the clusters 45 .The silver nanoclusters [Ag 44 (MNBA) 30 ] showed at least five characteristic absorption peaks in the visible-NIR region with absorption maxima at 400, 480, 550, 650, and 850 nm (Fig. S2).

Characterization of the prepared photocatalysts
The crystallinity phase composition, texture properties, structure morphology, and particle size of the prepared photocatalysts were characterized by X-ray diffraction (XRD), adsorption-desorption N 2 isotherms, FE-SEM, and HR-TEM, respectively.The chemical structure and the stoichiometry and charge of the prepared photocatalysts were investigated by FT-IR and X-ray photoelectron spectroscopy (XPS), respectively.The UV-Vis diffuse reflectance spectroscopic analysis was used to investigate the optical absorption properties and the band-gap of the prepared photocatalysts 49 .
The textural properties of the prepared photocatalysts were investigated using the N 2 adsorption-desorption isotherms at 77 K, as shown in Fig. 2II.The specific surface area (S BET ) and the pore volume distribution of the prepared photocatalysts were determined using the Brunauer-Emmett-Teller (BET) equation and the Barrett-Joyner-Halenda (BJH) method 6,18 , respectively.The S BET of the pure NH 2 -UiO-66 is 1106 m 2 /g with a total pore volume of 0.40 cm 3 /g 18 .The specific surface area and pore volume of the NH 2 -UiO-66 were decreased after loading with Bi 2 S 3 or MoS 2 and Ag 44 as shown in Table 1.All of them showed isotherms belonging to type I with H4 hysteresis loops, corresponding to the IUPAC classification of the hysteresis loops, these materials have mesoporous pores 51 .However, the synthesized metal chalcogenides (Bi 2 S 3 and MoS 2 ) exhibited lower specific surface areas (Table 1).The surface areas of the prepared photocatalysts were measured by another method (T-method, S t ).The values of S t are equal S BET , which confirms the correct choice of the standard t-curves (Table 1).
FTIR analysis was carried out to verify the formation of linkages between the NH 2 -UiO-66 and the protected silver nanoclusters [Ag 44 (MNBA) 30 ] via a condensation reaction.Figure 3Ia shows the FTIR spectrum of the pristine NH 2 -UiO-66, where the absorption peaks at 3440 cm −1 and 1577 cm −1 are assigned to the amino N-H and carbonyl C=O groups, respectively 17 .The C-N stretching vibration modes show two absorption bands at around 1360 cm −1 and 1259 cm −1 .The absorption bands between 768 cm −1 and 572 cm −1 are assigned to the Zr-O modes.The C = C skeletal vibration of the benzene ring shows an absorption peak at 1566 cm −1 in NH 2 -UiO-66 17 .No significant differences were observed between the FTIR spectra of MoS 2 @UiO-66-NH-S-Ag 44 (Fig. 3Ib) and Bi 2 S 3 @UiO-66-NH-S-Ag 44 (Fig. 3Ic) in comparison to NH 2 -UiO-66, confirming the existence of NH 2 -UiO-66 in the photocatalysts, instead of the absences of the N-H bending vibration peak of primary amines that observed in the region 1650-1580 cm −1 as shown in Fig. 3II, due to the covalent anchoring of the protected silver clusters throw the condensation reaction between the NH 2 group of NH 2 -UiO-66 and the carboxylic group of the 5-mercapto-2-nitrobenzoic acid ligand.
The morphological characteristics of the Bi 2 S 3 @UiO-66-NH-S-Ag 44 and MoS 2 @UiO-66-NH-S-Ag 44 and the particle size of the loaded silver nanoclusters were identified by High resolution-transmission electron Table 1.Surface area data for the prepared photocatalysts.www.nature.com/scientificreports/microscopy (HR-TEM) (Fig. 4I,II), respectively.The octahedral morphology with very smooth crystals of the pristine NH 2 -UiO-66, as reported before in our previous work 18,19 does not appear in the case of Bi 2 S 3 @UiO-66-NH-S-Ag 44 (Fig. 4I) and MoS 2 @UiO-66-NH-S-Ag 44 (Fig. 4II), and the surface becomes rough, due to the complete encapsulation of the Bi 2 S 3 and MoS 2 by NH 2 -UiO-66.There are homogenous black dots inside the blue circle of the TEM images of Bi 2 S 3 @UiO-66-NH-S-Ag 44 and MoS 2 @UiO-66-NH-S-Ag 44 , which refer to the silver nanoclusters (Ag 44 ) with an average particle size of ~ 1.5 nm as shown in Fig. S1, Fig. 4I,II.The inset images in Fig. 4I,II refer to the crystallinity of the prepared Bi 2 S 3 @UiO-66-NH-S-Ag 44 and MoS 2 @UiO-66-NH-S-Ag 44 photocatalysts.The high magnification FE-SEM images of Bi 2 S 3 @UiO-66-NH-S-Ag 44 (Fig. 4III) and MoS 2 @ UiO-66-NH-S-Ag 44 (Fig. 4IV) indicate that the Bi 2 S 3 and MoS 2 appear as nanorods and nanoparticles over the surface and inside the cavities of the NH 2 -UiO-66 without apparent aggregation.
To estimate the valence state of the covalently anchoring silver nanoclusters and the elemental content of the prepared Bi 2 S 3 @UiO-66-NH-S-Ag 44 (Fig. 5) and MoS 2 @UiO-66-NH-S-Ag 44 (Fig. 6) photocatalysts, the X-ray photoelectron spectroscopy (XPS) measurements were conducted.Figure 5I displays a typical survey spectrum of the Bi 2 S 3 @UiO-66-NH-S-Ag 44 and confirms the existence of Bi 4f, 4d and 5d, Zr 3d, S 2s and 2p, C 1s, O 1s, N 1s, and Ag 3d.The Zr, C, and O elements show the strongest peaks in the survey spectrum, with a small peak of N indicating the crystal lattice of NH 2 -UiO-66.To indicate the presence of the Bi 2 S 3 in the prepared photocatalyst the high-resolution XPS spectra of Bi and S are analyzed separately, as shown in Fig. 5II,III, respectively.The binding energy peaks at 158.5 eV and 163.8 eV were ascribed for Bi 4f 7/2 and Bi 4f 5/2 (Fig. 5II) and the peaks at 161.9 eV and 163.12 eV observed for S 2p 3/2 and S 2p 1/2 transitions (Fig. 5III), respectively 1,17 .The chemical states of Bi and S were Bi 3+ and S 2− in the loaded Bi 2 S 3 , which are following the previous literature 1, 17 .The XPS analysis was used to determine the charge of the covalently anchoring Ag 44 nanoclusters in the Bi 2 S 3 @UiO-66-NH 2 photocatalyst.The XPS spectrum of Ag 3d shows two peaks at binding energy around 368 eV and 374 eV, corresponding to Ag 3d 5/2 and Ag 3d 3/2 , respectively (Fig. 5IV).This is a characteristic peaks for the metallic silver (Ag 0 ) 6 .This confirms the covalently anchoring silver nanoclusters (Ag 44 ) have zero charge 45 .
The MoS 2 @UiO-66-NH-S-Ag 44 photocatalyst shows the same XPS survey spectrum of the Bi 2 S 3 @UiO-66-NH-S-Ag 44 with the same elements, instead of replacing the Bi with the Mo element (Fig. 6I).The high-resolution XPS spectrum of the Mo element exhibited two binding energy peaks at 229.2 eV and 232.4 eV that can be assigned to the Mo 3d 5/2 and Mo 3d 3/2 52, respectively (Fig. 6II).The sulfur element in the MoS 2 @UiO-66-NH-S-Ag 44 exhibited the same two binding energy peaks at 161.9 eV and 163.1 eV (Fig. 6III).Figure 6IV exhibited two binding energy peaks at 368.1 eV and 374 eV that are corresponding to metallic silver (Ag 0 ).
To measure the optical response of the pure UiO-66-NH 2 , Bi 2 S 3 , and MoS 2 and the prepared photocatalysts (Bi 2 S 3 @UiO-66-NH-S-Ag 44 and MoS 2 @UiO-66-NH-S-Ag 44 ) the diffuse reflectance spectra (DRS) of the powder samples were recorded.The Kubelka-Munk (K-M) equation was used to correlate the absorbance of the samples with the diffuse reflectance 23 .Figure 7I exhibited the plot of the K-M function of the prepared photocatalysts depicting the K-M plots and Fig. 7II shows the corresponding (F(R)hν) 1/2 vs. hν plot for the calculation of effective indirect band gap of the prepared photocatalysts.
Figure 7Ia exhibited the UV-Vis-NIR diffuse reflectance spectrum of the pure UiO-66-NH 2 with two distinct peaks.The first peak centered at ~ 230 nm originated from the electron transition from the organic linker to the Zr-O cluster.The second strong peak centered at ~ 360 nm is attributed to the substitution of -NH 2 in the organic linker 24 .The pure Bi 2 S 3 shows a broad absorption peak that extended from UV to the near-infrared (NIR) region (300-1000 nm) and is centered at 705 nm (Fig. 7Ie), also the absorption range of the pure MoS 2 covers the whole UV and visible light region (Fig. 7Id), which indicates the prepared metal chalcogenides have an excellent optical response.Compared to NH 2 -UiO-66, Bi 2 S 3 @UiO-66-NH-S-Ag 44 (Fig. 7Ic) and MoS 2 @UiO-66-NH-S-Ag 44 (Fig. 7Ib) exhibit an extended absorption in the whole visible light region and near-infrared (NIR) region, due to the special electronic distribution for the covalently anchoring silver nanoclusters (Ag 44 ) and the doped optically active metal chalcogenides.The band gap values were calculated by the Kubelka-Munk from the extrapolation of the linear portion at (F(R)hν) 1/2 = 0 providing the effective indirect band gap of the prepared photocatalysts (Fig. 7II).The band gaps of the pure NH 2 -UiO-66 and Bi 2 S 3 were calculated to be 2.87 eV and 1.28 eV, respectively (Fig. 7II).

Photocatalytic activity of the prepared photocatalysts under visible light irradiation
Methylene blue (MB) dye was chosen as a pollutant model to investigate the adsorption and photocatalytic activities of the prepared photocatalysts.A UV-Vis spectrophotometer (Evolution 300) was used to evaluate  www.nature.com/scientificreports/ the photodegradation reaction.It is well known that the MB molecule is stable under visible light irradiation without photocatalyst 17 .
As shown in Fig. 8a, the NH 2 -UiO-66 sample shows the highest adsorption properties compared to the prepared MoS 2 and Bi 2 S 3 (Fig. 8b,c), respectively, due to its large surface area of 1106 m 2 /g and ordered nanosized channels, but the photodegradation activity of the NH 2 -UiO-66 is low due to its absorption properties is limited in the UV region, as confirmed by UV-Vis diffuse reflectance analysis (Fig. 7Ia).
The photocatalytic activity of the Bi 2 S 3 @UiO-66-NH 2 and MoS 2 @UiO-66-NH 2 heterojunctions is 56% and 49% in comparison to the pure UiO-66-NH 2 (18%), indicating the synergistic effect between UiO-66-NH 2 and Bi 2 S 3 and MoS 2 (Fig. 8e,d), respectively.Modification of UiO-66-NH 2 with metal chalcogenides semiconductors (Bi 2 S 3 and MoS 2 ) plays an important role in the photocatalytic degradation of MB dye (Fig. 8e,d).Due to the Bi 2 S 3 and MoS 2 delay the recombination of the electron-hole pairs, enhance the conduction of charge carriers of UiO-66-NH 2 and redshift the optical absorption of UiO-66-NH 2 from the UV region (379 nm) to visible region at ~ 705 nm, as confirmed by the UV-Vis diffuse reflectance spectroscopic analysis (Fig. 7).
The extremely photocatalytic activity of the prepared photocatalysts was attributed to their amazing optical absorption properties.The Bi 2 S 3 @UiO-66-NH 2 heterojunction absorbs in visible region at ~ 705 nm.The covalently anchoring zero charge silver clusters (Ag 44 ) enhanced this absorption as shown in Fig. 7, due to the silver clusters have special optical properties, where it have five characteristic absorption peaks in the visible-NIR region with absorption maxima at 400, 480, 550, 650, and 850 nm, as shown in Fig. S2.
The elemental trapping experiments have been performed to investigate the contribution of active species during the photocatalytic degradation of MB.P-benzoquinone (BQ), ethylene diamine tetraacetic acid disodium (EDTA-2Na) and isopropanol (IPA) were used as scavengers to quench the superoxide radical O 2 •− , photogenerated holes h + and free radical hydroxide •OH, respectively.As shown in Fig. 9I, the photodegradation of MB over Bi 2 S 3 @UiO-66-NH-S-Ag 44 photocatalyst without any scavenger reached 100% under visible irradiation for 90 min.The addition of IPA decreased the activity to 63%, while the degradation efficiency quenched to 92% and 81% when using p-BQ and EDTA-2Na, respectively.These results indicate that ⋅OH radical is the major active species for the MB degradation reaction.However, h + and O 2 •− have a minor effect on the photocatalytic process 8 .The covalently anchoring silver nanoclusters on the MoS 2 @UiO-66-NH 2 and Bi 2 S 3 @UiO-66-NH 2 heterojunctions are not only enhancing the photocatalytic activity but also the recyclability properties.Where, Bi 2 S 3 @ UiO-66-NH-S-Ag 44 photocatalyst shows a very good activity for at least five catalytic runs without any loss in the photocatalytic degradation of MB (Fig. 9II).This means, these photocatalysts possess high stability and may be reusable for at least 5 runs, showing a good potential for industrial applications.This recyclability efficiency confirms a very low silver clusters leaching from the MoS 2 @UiO-66-NH 2 and Bi 2 S 3 @UiO-66-NH 2 surface.That was confirmed by measuring the XPS analysis for the catalyst before and after each recycling run, where the percentage of silver clusters remains nearly constant.
In the light of the previous discussion, the possible mechanism for MB photodegradation over the Bi 2 S 3 @ UiO-66-NH-S-Ag 44 photocatalyst is shown in Fig. S3.Under visible light illumination, UiO-66-NH 2 modified by Bi 2 S 3 and Ag 44 was excited, leading to the generation of charge carriers.The photoexcited electrons on the conduction band (CB) of Bi 2 S 3 could transfer directly to the CB of the UiO-66-NH 2 .Meanwhile, the valence band (VB) of UiO-66-NH 2 (2.27) 53 is more positive than Bi 2 S 3 (1.48) 54, thus the photoexcited holes migrate in the reverse direction of electrons (Fig. S3).