One-Pot Synthesis of Pd-Based Ternary Pd@CdS@TiO2 Nanoclusters via a Solvothermal Route and Their Catalytic Reduction Efficiency toward Toxic Hexavalent Chromium.

In this work, we report the synthesis of Pd-based ternary Pd@CdS@TiO2 nanocomposites using molecular precursors. This method is facile, less time-consuming, and cost-effective. This catalyst is prepared within 2 h by a solvothermal route using molecular precursors. Information about the phase, morphologies, elemental mapping, and composition of the nanocomposites was obtained using various characterization techniques. The catalytic activity of the as-prepared Pd-based ternary Pd@CdS@TiO2 nanocomposites exhibits effective reduction efficiency for the conversion of toxic Cr(VI) to Cr(III) using formic acid as a reducing agent within 5-7 min. To the best of our knowledge, this is the first report on Pd-based ternary Pd@CdS@TiO2 nanocomposites prepared by a solvothermal route and used as catalysts toward the reduction of hexavalent chromium at room temperature.

Introduction

Nowadays, the synthesis of nanocomposites and their applications in selective organic transformations from wastewater have been investigated extensively. Hexavalent chromium Cr(VI) is the most hazardous pollutant found in soil, wastewater, and ground water.[1] Various pigment manufacturing, leather tanning, metal finishing, and electroplating industries make use of hexavalent chromium, Cr(VI). The effluents of such industries contain a large amount of hexavalent chromium Cr(VI) which is nonbiodegradable and exists for a longer period in the environment. Chromium is a heavy metal having variable oxidation states, with two valence states, hexavalent chromium Cr(VI) and trivalent chromium Cr(III). Cr(III) is less harmful as compared to Cr(VI).[2,3] Because Cr(VI) has strong oxidizing properties, it is more toxic and contains carcinogenic species as compared to Cr(III) species. Because of high toxicity of Cr(VI) at very low concentration, it has the ability to cause hazardous threats to human and other living organisms such as it can increase the risk of DNA mutation and lung cancer by chronic inhalation.[1,4,5]

Various semiconductor catalysts such as TiO2,[6−8] NiO@TiO2,[9] ZnO@TiO2,[10] ZnO,[11] Fe3O4@graphene,[12] Pd@SiO2–NH2,[2] ZnO–TiO2–OCNT,[13] and CdS/RGO[14] have been reported for the reduction of Cr(VI) to Cr(III). Recently, the use of formic acid (HCOOH) as a reducing agent for the reduction of Cr(VI) to Cr(III) has been reported because of its simplicity and efficiency.[5,15−18] In general mechanism, HCOOH undergoes a dehydrogenation decomposition process to generate CO2 and H2 (HCOOH → CO2 + H2). This H2 adsorbs on the surface of nanocomposites, which is mainly responsible for the reduction of Cr(VI) to Cr(III) (Cr2O72– + 8H+ + 3H2 → 2Cr3+ + 7H2O).[2,5,19−25] Generally, noble and transition metals Pd, Pt, Ru, Ag, and so forth are active catalysts for many catalytic hydrogenolyses and catalytic reduction of heavy metal pollutants. Although Pd is a high-cost metal, it has many more uses in electrocatalysis, C–C coupling reactions, and other catalytic reductions. Many researchers have focused on the fabrication of Pd-based nanocomposites with low-cost transition metals, which provide high efficiency in catalytic sites. According to World Health Organization (WHO), hexavalent chromium Cr(VI) is the most hazardous pollutant, hence there is a need to develop practical and cost-effective methodologies for reductive transformation of Cr(VI) to Cr(III). The Pd-based catalyst has high efficiency for the reduction of Cr(VI).[2,15−18,26,27]

So far, various methods such as chemical reduction, ion exchange, photocatalysis, and adsorption have been used for the reduction of Cr(VI) to Cr(III). For example, modified TiO2-mediated photocatalytic reduction of Cr(VI) to Cr(III) has been reported by Acharya et al.[28] However, the advantage of a chemical reduction method that has been used in the present study is that there is no need of external source of light and that reduction can be carried out at room temperature (RT) using a simple reducing agent.

Herein, we report a facile and efficient route for the synthesis of binary CdS@TiO2 nanocomposites and Pd-based ternary Pd@CdS@TiO2 nanocomposites by a solvothermal route using CdCl2(4-MebenztsczH)2 (precursor I), CdI2(4-MebenztsczH)2 (precursor II) (where 4-MebenztsczH = 4-methylbenzaldehyde thiosemicarbazone), and titanium isopropoxide (Ti(OC3H7)4) as molecular precursors. As these molecular precursors contain both metallic and nonmetallic parts, they act as good precursors for the synthesis of semiconductor nanoparticles (NPs). The use of such molecular precursors provides several advantages such as minimization of expensive chemicals, low toxicity, limited or no prereactions, and so forth.[29,30]

The catalytic reductive efficiency has been further studied using as-prepared Pd-based ternary Pd@CdS@TiO2 nanocomposites (TNC-I obtained from precursor I and TNC-II obtained from precursor II) for the reduction of toxic hexavalent Cr(VI) to Cr(III). For comparison, the catalytic reduction of Cr(VI) has also been studied using binary CdS@TiO2 nanocomposites (BNC-I obtained from precursor I and BNC-II obtained from precursor II). Two different precursors, that is, (CdCl2(4-MebenztsczH)2 and CdI2(4-MebenztsczH)2), were used to prepare the composites. This was carried out in order to find out whether two different precursors result into different composites, which may result into different catalytic activities.

Results and Discussion

The characterization of the precursors was done by elemental analysis and 1H and 13C {1H} nuclear magnetic resonance (NMR) (Figures S1 and S2). The IR spectra of precursor I, precursor II, and ligand are shown in Figure S3. The peaks observed at 3433 and 3259 cm–1 (for precursor I) and 3395 and 3287 cm–1 (for precursor II) are attributed to νNH asymmetric and symmetric vibrations, respectively. The bands at 3162 and 3188 cm–1 (for precursor I and precursor II, respectively) are observed because of νNH. The bands at 1536 and 1526 cm–1 (for precursor I and precursor II, respectively) are assigned to νC=N. The bands at 946 and 955 cm–1 are attributed to νC=S for precursor I and precursor II, respectively. The bands observed for νC=N in the case of precursor I and precursor II are shifted to lower wavenumber as compared to the band observed for the ligand (1594 cm–1).

In the X-ray diffraction (XRD) patterns, the sharp and intense diffraction peaks observed (Figure ) show successful formation of Pd-based ternary Pd@CdS@TiO2 nanocomposites and binary CdS@TiO2 nanocomposites (Figure S4). They match with anatase TiO2 (JCPDS file no. 00-021-01272) and hexagonal CdS (JCPDS file no. 00-041-1049). The additional peak observed at 2θ = 39.4° for TNC-I and 2θ = 39.8° for TNC-II confirmed the successful loading of Pd on the surface of CdS@TiO2 nanocomposites,[31,32] and it was observed that the crystallinity is retained after the loading of Pd as shown in Figure . The average particle size was calculated using the Scherrer formula,[33] which was found to be 10.26 nm (TNC-I), 7.31 nm (TNC-II), 30.5 nm (BNC-I), and 13.3 nm (BNC-II) for as-prepared nanocomposites.

Figure 1

XRD patterns of as-prepared nanocomposites obtained from TNC-I (I) and TNC-II (II).

In order to examine the morphology and elemental mapping of as-prepared Pd-based ternary nanocomposites, we have carried out high-resolution transmission electron microscopy (HRTEM) (Figure S5), TEM, scanning electron microscopy (SEM), and SEM–energy-dispersive X-ray spectroscopy (EDS) elemental mapping studies. TEM images are shown in Figure . From Figure Ia,IIa, spherical morphologies of the nanoclusters (TNC-I and TNC-II) can be seen. Figure Ib,IIb shows selected-area electron diffraction (SAED) patterns revealing the polycrystallinity of these materials. This was further confirmed by SEM and SEM–EDS elemental mapping. The SEM images show a spherical shaped morphology for as-prepared Pd-based ternary nanoclusters (TNC-I and TNC-II) (Figure a,b). The elemental mapping analysis of as-prepared Pd-based ternary nanoclusters confirmed that palladium (Pd) is evenly distributed on the surface of CdS@TiO2. In addition, the EDS elemental mapping of as-prepared Pd-based ternary nanoclusters shows the even elemental distribution of Ti, O, S, Cd, and Pd as given in Figure a,b for TNC-I and TNC-II, respectively. Their corresponding EDS spectra are shown in Figure S6a,b. The SEM images of BNC-I and BNC-II are shown in Figure S7, which confirm the successful formation of binary nanocomposites.

Figure 2

(a) TEM images and (b) their corresponding SAED patterns of TNC-I and TNC-II.

Figure 3

(a) SEM images and their elemental mapping of TNC-I. (b) SEM images and their elemental mapping of TNC-II.

The formation of binary and Pd-based ternary nanoclusters was further studied by Fourier transform infrared (FTIR) analysis. The peaks observed at 602 and 609 cm–1 for BNC-I and BNC-II, respectively, as shown in Figure S8 and 599 and 627 cm–1 for TNC-I and TNC-II, respectively, as shown in Figure confirm the stretching vibration bands of Cd–S.[34,35] Along with that, the peaks observed in the vicinity of 400–800 cm–1 in both the nanocomposites correspond to the stretching vibration of Ti–O–Ti in TiO2.[36,37] This was further confirmed by Raman analysis.

Figure 4

FTIR spectra of as-prepared TNC-I (I) and TNC-II (II).

Raman spectra of the as-prepared Pd-based ternary nanoclusters have been recorded with a 532 nm laser as the excitation source to investigate the various phases present in the as-prepared Pd-based nanoclusters Figure . CdS shows Raman vibrational bands at 263.7 and 565.6 cm–1 for TNC-I and 269.5 and 584.6 cm–1 for TNC-II, and the weak bands at 353.9 cm–1 for TNC-I and 354.7 cm–1 for TNC-II are assigned to the multiphonon scattering in CdS.[38−40] The presence of TiO2 can be confirmed by the vibration peaks observed around 388–390, 484–499, and 611–613 cm–1 (for TNC-I and TNC-II).[41−45] There is no intense Raman vibrational peak observed around 640 cm–1 (for TNC-I and TNC-II), which confirms the presence of Pd only. This is well supported by XRD results.[44,45]

Figure 5

Raman spectra of as-prepared TNC-I (I) and TNC-II (II).

The presence of Pd on CdS@TiO2 nanoclusters was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Figures and 7 show the XPS spectra of as-prepared nanoclusters TNC-I and TNC-II, which give the evidence of the presence of Cd, S, Ti, and O. It provides two states for cadmium (Cd 3d5/2 and Cd 3d3/2), two states for sulfur (S 2p3/2 and S 2p1/2), two states for titanium (Ti 2p3/2 and Ti 2p1/2), and one state for oxygen (O 1s). The binding energies located at 405 and 412 eV are attributed to Cd 3d5/2 and Cd 3d3/2 states of cadmium as shown in Figure (TNC-I). The peak located at 407 eV for Cd 3d5/2 and the peak located at 413 eV for Cd 3d3/2 are shown in Figure (TNC-II). These observations suggest that cadmium is in the Cd2+ chemical state.[46−49] The binding energy of sulfur is 161 eV for S 2p3/2 and 163 eV for S 2p1/2 for TNC-I (Figure ). For the material obtained from TNC-II, the binding energy for S 2p3/2 is 163 eV as shown in Figure . This suggests that sulfur is in the S2– chemical state.[50,51] Similarly, the peaks obtained from TNC-I and TNC-II located around 459–460 eV for Ti 2p3/2 and 464–466 eV for Ti 2p1/2 (Figures and 7) indicate that titanium is in the chemical state of Ti4+ and the oxygen shows binding energies of 530 and 532 eV as shown in Figure and 533 eV as shown in Figure , corresponding to the oxygen O 1s peak. This is due to Ti–O in TiO2 and hydroxyl groups.[52−57] Further, palladium was confirmed in the states of Pd 3d5/2 and Pd 3d3/2 with the binding energies of 336 eV (Pd 3d5/2) and 342 eV (Pd 3d3/2) as shown in Figure (TNC-I) and 338 eV (Pd 3d5/2) and 344 eV (Pd 3d3/2) as shown in Figure (TNC-II). The peak positions were attributed to the Pd 3d level, which is well matched with the previous literature.[58−60]

Figure 6

XPS spectra of as-prepared TNC-I.

Figure 7

XPS spectra of as-prepared TNC-II.

The optical properties observed by UV–visible diffuse reflectance spectroscopy (UV-DRS) were found to be enhanced toward the visible region because of the addition of Pd, that is, Pd@CdS@TiO2, as shown in Figure . Their corresponding band gaps were found to be 2.09 and 2.10 eV for the materials obtained from TNC-I and TNC-II, respectively (inset Figure ). They were calculated using Tauc’s plot.[61,62] For comparison, similar band gap calculations were carried for binary CdS@TiO2 nanocomposites. Their band gaps were found to be 2.30 and 2.28 eV for the materials obtained from BNC-I and BNC-II, respectively. The absorption spectra and corresponding Tauc’s plots of binary nanocomposites are given in Figure S9.

Figure 8

UV-DRS spectra of as-prepared TNC-I (I) and TNC-II (II); the inset shows their corresponding band gaps calculated by Tauc’s plot.

The photoluminescence (PL) spectra of binary and ternary nanocomposites are shown in Figure . The transfer and fate of photogenerated carriers are mainly investigated by PL spectra. The Pd-based ternary nanocomposites show lower intensities when compared with that of the binary nanocomposites. This phenomenon takes place because the Pd metal in ternary nanocomposites has higher Fermi energy level, which increases the electron-transfer reaction and leads to the prolonged electron–hole pair recombination.[63,64]

Figure 9

PL spectra of as-prepared TNC-I (a), TNC-II (b), BNC-I (c), and BNC-II (d).

The catalytic efficiency of the as-prepared Pd-based ternary nanoclusters was evaluated at RT for the reduction of highly toxic Cr(VI) to Cr(III). It was monitored by UV–vis spectra with the absorption peak observed at 349 nm for Cr(VI) as shown in Figure . Yu et al.[3] suggested that Pd provides large active sites and large surface for the enhancement of catalytic reduction of Cr(VI). It was observed that in the presence of Pd-based ternary nanoclusters, the absorption peaks of Cr(VI) decrease rapidly. Further, the peaks disappear completely, indicating the reduction of Cr(VI) into Cr(III) by HCOOH which was used as a reducing agent within 5 min for TNC-I and 7 min for TNC-II. It reveals that the as-prepared Pd-based ternary nanoclusters are more active and show greater catalytic reductive efficiency for Cr(VI). The recyclability of the catalysts TNC-I and TNC-II has also been performed. It is found that up to three cycles, the catalysts are stable and have good efficiency for the reduction of Cr(VI) (Figure S10). In the literature, it is reported that HCOOH gets adsorbed on the surface of the catalyst reducing itself into CO2 and H2. This plays a very important role in the catalytic reduction of Cr(VI).[22,23] During the reduction process, successive color change from yellow to colorless is observed, which indicates successful reduction of Cr(VI) to Cr(III) (Figure A). This was further confirmed by the addition of excess of NaOH to the final solution. The color of solution changed to green after addition of excess NaOH because of the formation of hexahydroxochromate(III), which clearly gives the evidence for the presence of Cr(III).[2] For comparison, reduction of Cr(VI) to Cr(III) using only HCOOH as a reducing agent (i.e., without the catalyst) (Figure S11) and also only binary CdS@TiO2 nanocomposites as a catalyst (Figure S12) was carried out. It was observed that even after 70 min of reaction time, no complete reduction of Cr(VI) takes place. The binary nanocomposites (BNC-I and BNC-II) show less efficiency for the reduction of Cr(VI) as compared to TNC-I and TNC-II nanoclusters because of the synergistic effect of Pd on the surface of TNCs. The plots of C/Co versus irradiation time for the catalytic reduction of Cr(VI) to Cr(III) in the presence as well as in the absence of the catalyst are shown in Figure B, and for comparison, reduction of Cr(VI) to Cr(III) was studied in the presence of the catalyst in the dark (Figure S13), where Co is the initial concentration and C is the final concentration at each time interval. The kinetic plots and rate constants calculated for the reduction of Cr(VI) to Cr(III) with binary and ternary nanocatalysts are of pseudo-first-order (Figure S14). It was observed that the rate constant k values of TNCs and BNCs are 0.433 (TNC-I), 0.516 (TNC-II), 0.015 (BNC-I), and 0.0218 (BNC-II). The higher k values demonstrate the enhancement of catalytic activity for the reduction of Cr(VI). TNC-I exhibits 28 times higher activity than BNC-I and TNC-II exhibits 24 times higher activity than BNC-II. From Figure S13, it can be seen that there is not much reduction in the dark. The catalytic reduction of Cr(VI) to Cr(III) is compared with the other catalysts reported in the literature (Table ). It has been observed that the catalytic reduction efficiency of TNC-I and TNC-II nanoclusters reported in this work is remarkably better compared to those reported in the literature.

Figure 10

Catalytic reduction of Cr(VI) to Cr(III) using TNC-I and TNC-II at RT.

Figure 11

(A) Photographs of Cr(VI) to Cr(III) reduction: (a) Cr(VI) solution, (b) Cr(III) solution, and (c) after addition of excess NaOH solution and (B) plots of C/Co vs irradiation time for the catalytic reduction of Cr(VI) to Cr(III): (a) TNC-I, (b) TNC-II, (c) BNC-I, (d) BNC-II, and without the catalyst.

Table 1

Catalytic Efficiency of Various Catalysts Reported in the Literature along with the Present Work for the Reduction of Cr(VI) to Cr(III)a

catalyst	conditions	time (min)	k (min–1)	refs
TiO2 (MT-1)	[M] = 0.2 mM (10 mg/L) [cat.] = 50 mL (1 g/L)	30	0.079	(65)
AgCl@Ag CS-NCs	[M] = 0.2 mM (10 mg/L) [cat.] = 20 mg	8	0.125	(66)
Pd NPs@Pro-ESM	[M] = 20 mM [cat.] = 15 mg	26	0.133	(4)
ZnO	[M] = 1.5 mM (75 mg/L) [cat.] = 2 g/L	60	0.044	(11)
BCN-BMO(Q)	[M] = 0.2 mM (10 mg/L) [cat.] = 50 mg	20	0.147	(67)
Ag/SnO2/NiO	[M] = 0.2 mM (10 mg/L) [cat.] = 1g/L	60	0.023	(68)
Fe-GCN	[M] = 0.4 mM (20 mg/L) [cat.] = 30 mg	120	0.021	(69)
Pd-NWWs	[M] = 0.8 mM [cat.] = 2 g/L	15	0.282	(5)
(H2bpp)6{Fe[Mo6O12 (OH)3(HPO4)2H2PO4)2]2}2·11H2O	[M] = 0.44 mM [cat.] = 10 mg	180	0.0013	(70)
TNC-I	[M] = 3 mM (150 mg/L) [cat.] = 5 mg	5	0.4339	this work
TNC-II	[M] = 3 mM (150 mg/L) [cat.] = 5 mg	7	0.5163	this work

[M] = chromium concentration, [cat.] = amount of the catalyst used.

The mechanism of catalytic activity of the materials can be explained as shown in Figure and Scheme . Although TiO2 has been extensively used as a catalyst because it is less toxic, less expensive, and highly abundant and has excellent photochemical stability, it has a limitation of having a wide band gap due to which it absorbs UV radiation only.[28] To be an effective catalyst, it is necessary that a material should absorb sunlight effectively and there should be sufficient electron–hole pair separation so that the electrons are available for reduction.

Figure 12

Schematic possible mechanism of the catalytic reduction of Cr(VI) to Cr(III) over TNCs.

Scheme 1

Schematic Representation of HCOOH as a Hole Scavenger

Therefore, in order to facilitate the absorption of the sunlight and increase electron–hole pair separation, combination of a narrow band gap semiconductor such as CdS with a wide band gap semiconductor such as TiO2 is carried out. In this, TiO2 has more positive conduction band than that of the CdS conduction band. Therefore, the energetic electrons of the CdS conduction band transfer to the conduction band of TiO2. These electrons are then easily transported to the palladium surface and then subsequently used for reduction. The noble metal can act as a trap in the catalytic process to boost the electron–hole pair separation. Thus, the introduction of a noble metal (Pd) on the surface of BNCs shows the enhancement in the interfacial charge transfer as well as charge separation efficiency, followed by an increase in the catalytic reductive efficiency of Cr(VI) to Cr(III). The proposed design of TNCs that empower the efficient CdS → TiO2 → Pd pathway for the vectorial electron transfer is illustrated in Figure , which is also supported by PL studies.[71−73]

The catalytic reduction is further enhanced by scavenging holes present in the valence band of CdS using HCOOH as a scavenger (Scheme ).[74] This further hinders the charge carrier recombination rate, thereby increasing the catalytic performance. Thus, HCOOH acts not only as a reducing agent but also as a hole scavenger. Because of this, the materials used in this work show significantly improved catalytic activity.

Conclusions

In conclusion, we report the synthesis and characterization of Pd-based Pd@CdS@TiO2 ternary nanoclusters by a solvothermal route using molecular precursors. The as-prepared TNCs show more catalytic efficiency to reduce highly toxic Cr(VI) to Cr(III) using HCOOH as a reducing agent at RT. Pd doped on the surface of BNCs activates the catalysts, hence it was observed that TNCs prepared from precursor I and precursor II show more efficiency than BNCs. From PL studies, it can be seen that TNCs show lower intensity as compared to BNCs, which results in slow electron–hole pair recombination and enhances the catalytic reduction of Cr(VI). HCOOH also acts as a scavenging agent for holes, thereby further enhancing the reduction of Cr(VI) to Cr(III). Thus, the environmental remediation of highly toxic Cr(VI) has been carried out with the help of environmental friendly, cost-effective, and highly efficient catalysts.

Experimental Details

Materials

All the solvents and metal salts used were of analytical grade and were used without further purification. Cadmium chloride (CdCl2, 95.0%), cadmium iodide (CdI2, 99.0%), potassium dichromate (K2Cr2O7, 99.99%), HCOOH (85.0%), palladium chloride (PdCl2, 99.0%), sodium borohydride (NaBH4, 98.0%), and ethylene glycol (EG, C2H6O2, 99.0%) were purchased from S.D. Fine Chemicals Limited. Ti(OC3H7)4 (98.0%) and 4-methylbenzaldehyde (C8H8O, 99.0%) were obtained from Sigma-Aldrich. Deionized water was used throughout the experiment.

Synthesis of Precursors

The precursors were prepared according to the previous study reported by our group.[75]

Preparation of CdCl2(4-MebenztsczH)2 Precursor I

A total of 1.696 g (4.387 mmol) of 4-MebenztsczH dissolved in 15 mL of dry methanol and 0.804 g (4.386 mmol) of CdCl2 dissolved in 20 mL of dry tetrahydrofuran (THF) were taken in separate round-bottom flasks. The former solution was added to the latter under inert atmosphere and allowed to stir for 48 h. A white colored product was obtained. It was separated by evaporating the solvent under vacuum. It was then washed with cyclohexane (3 × 5 mL), followed by n-hexane (3 × 5 mL) to remove impurities. Finally, the free white solid was obtained and characterized by elemental analysis, NMR, and FTIR (Yield: 2.389 g, 95.56%, mp 188 °C).

Elemental analysis % found (cal.): C 36.91 (37.93), H 3.90 (3.53), N 13.96 (14.74), S 12.94 (11.25), Cl 12.38 (12.44), and Cd 20.82 (19.72). 1H NMR (δ in ppm): 2.3 (s, 3H, −CH3), 7.2–8.0 (m, 7H, −C6H5 + NH2), 11.3 (s, 1H, −NH−) (Figure S1a). 13C{1H} NMR (δ in ppm): 177.4 (>C=S), 142.6 (>C=N), 139.9, 131.3, 129.2, 127.2 (aromatic carbons); 21.3 (−CH3) (Figure S1b). IR: 3433, 3259 cm–1 (νNH), 3162 cm–1 (νNH), 1536 cm–1 (νC=N), 946 cm–1 (νC=S) (Figure S3a).

Preparation of CdI2(4-MebenztsczH)2 Precursor II

In a round-bottom flask, 1.027 g (2.656 mmol) of 4-MebenztsczH was dissolved in 15 mL of dry methanol. In another round-bottom flask, 0.973 g (2.657 mmol) of CdI2 was dissolved in 20 mL of dry THF. These solutions were then mixed together and then allowed to stir for 48 h. A white product obtained was separated by evaporating the solvent under vacuum. It was then washed with cyclohexane (3 × 5 mL), followed by n-hexane (3 × 5 mL) to remove any impurities present. Finally, the free white solid was obtained. It was characterized by elemental analysis, NMR, and FTIR (Yield: 2.286 g, 91.44%, mp 162 °C).

Elemental analysis % found (cal.): C 29.37 (28.71), H 3.03 (2.67), N 11.04 (11.16), and Cd 15.93 (14.96). 1H NMR (δ in ppm): 2.3 (s, 3H, −CH3), 7.1–8.1 (m, 7H, −C6H5 + NH2), 11.4 (s, 1H, −NH−) (Figure S2a). 13C{1H} NMR (δ in ppm): 177.3 (>C=S), 142.6 (>C=N), 139.7, 131.0, 129.5, 127.4 (aromatic carbons), 20.7 (−CH3) (Figure S2b). IR: 3395, 3287 cm–1 (νNH), 3188 cm–1 (νN–H), 1526 cm–1 (νC=N), 955 cm–1 (νC=S) (Figure S3b).

Further, these molecular precursors (precursors I and II) were used for the preparation of binary CdS@TiO2 and ternary Pd@CdS@TiO2 nanocomposites.

One-Pot Synthesis of Nanocomposites

Synthesis of Binary CdS@TiO2 Nanocomposites (BNC-I and BNC-II)

Binary CdS@TiO2 nanocomposites using precursor I and precursor II were prepared by a solvothermal route.

In a typical synthesis, Ti(OC3H7)4 (1 mL) and CdCl2(4-MebenztsczH)2 (200 mg) (precursor I) were taken in a round-bottom flask containing 50 mL of EG. The mixture was sonicated for 30 min, and further it was allowed to stir and reflux for 2 h at 200 °C under nitrogen atmosphere. The reaction mixture was allowed to cool to RT. The yellowish product obtained was washed and centrifuged 4–5 times with methanol to remove the excess of EG. The final product obtained was dried under vacuum and characterized.

Similarly, binary CdS@TiO2 (BNC-II) using CdI2(4-MebenztsczH)2 (precursor II) was prepared and characterized.

Synthesis of Pd-Based Ternary Pd@CdS@TiO2 Nanocomposites (TNC-I and TNC-II)

Pd-based Pd@CdS@TiO2 (from precursor I) and Pd@CdS@TiO2 (from precursor II) ternary nanocomposites were prepared by a solvothermal route using molecular precursors as follows.

In this synthesis, 1 mL of titanium isopropoxide and 200 mg of CdCl2(4-MebenztsczH)2 (precursor I) were taken in 50 mL of EG. The mixture was sonicated for 30 min. After sonication, the solution was allowed to stir and reflux. To this solution, 20 mg of PdCl2 was added, followed by the addition of 50 mg of NaBH4 under the refluxing condition. The final reaction mixture was refluxed for 2 h at 200 °C under nitrogen atmosphere. Then the reaction mixture was allowed to cool to RT. The blackish green product obtained was washed and centrifuged 4–5 times with methanol to remove the excess of EG and any other impurities present. The final product was dried under vacuum.

Similarly, Pd-based Pd@CdS@TiO2 (TNC-II) was prepared using CdI2(4-MebenztsczH)2 (precursor II) and characterized (Scheme ).

Scheme 2

Schematic Representation of Synthesis of Pd-Based Ternary Pd@CdS@TiO2 Nanocomposites Using Precursor I (X = Cl) and Precursor II (X = I)

Catalytic Dichromate Reduction

The catalytic reduction of aqueous dichromate solution was further carried out using as-prepared binary CdS@TiO2 (BNC-I and BNC-II) and Pd-based ternary Pd@CdS@TiO2 nanocomposites (TNC-I and TNC-II) for the catalytic reduction of Cr(VI) to Cr(III) according to the reported method.[76] Typically, 5 mg of as-prepared Pd-based nanocatalysts in the mixture of potassium dichromate (K2Cr2O7, 150 ppm, 15 mL), HCOOH (85%, 1.5 mL), and deionized water (24 mL) was taken in a 50 mL beaker, and the mixture was vigorously stirred using a magnetic stirrer. During the reaction, the aliquots of 3 mL of aqueous solutions were withdrawn at each predetermined time interval for checking the catalytic reduction efficiency of the catalysts for the reduction of Cr(VI) to Cr(III) by measuring the absorbance. A UV-2450 PC Shimadzu UV–vis spectrophotometer was used for this purpose. For comparison, the measurements were also carried out without the catalyst under similar experimental conditions.

Material Characterization

FTIR spectroscopy was recorded on a PerkinElmer FTIR spectrometer using KBr pellets in the range of 400–4000 cm–1. A Thermo Finnigan, Italy FLASH EA 1112 series analyzer was used for elemental analysis. The 1H and 13C{1H} NMR spectra were recorded in DMSO-d6 on a Bruker AVANCE 300 spectrophotometer. The internal standard, tetramethylsilane, was used for 1H and 13C{1H} NMR spectra. The UV–vis spectra were recorded on a UV-2450 PC Shimadzu UV–visible spectrophotometer. Powder XRD was carried out on an XRD-7000 Shimadzu X-ray diffractometer with Cu Kα radiation at λ = 0.154060 nm. The Raman spectra were recorded on a Kaiser Optical Systems Inc. (KOSI) laser Raman spectrometer. The morphologies and EDS were observed using JEOL JSM-7600 FEG-SEM with an operating voltage of 0.1–30 kV. TEM and SAED were recorded on PHILIPS, CM 200 with an operating voltage between 20 and 200 kV. The XPS recording was carried out on an X-ray photoelectron spectrometer (AXIS Supra) Kratos Analytical, UK (SHIMADZU group) using (Al Kα) 600 W X-ray source, 1486.6 eV. The UV-DRS spectra were recorded on a UV-2450 PC Shimadzu UV–visible spectrophotometer using barium sulfate (BaSO4) as the standard. A PerkinElmer LS 55 fluorescence spectrometer was used for recording PL spectra.