CdS QDs-Decorated Self-Doped γ-Bi2MoO6: A Sustainable and Versatile Photocatalyst toward Photoreduction of Cr(VI) and Degradation of Phenol.

In this work, CdS quantum dots (QDs)-sensitized self-doped Bi2MoO6 has been synthesized using glucose as reducing agent by hydrothermal method, followed by in situ deposition of the QDs. The synthesized catalyst has been employed to reduce toxic Cr(VI) and degrade phenol from the aqueous solution. The structural, optical, and electrochemical characterizations are performed using X-ray diffraction, UV-vis diffuse reflection, photoluminescence (PL), scanning electron microscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The optical properties were precisely investigated by calculating the Urbach energy, PL, and photoluminescence excitation spectra. The orderly distribution of QDs is confirmed from the correlation between full width at half-maximum of PL spectra, Urbach energy, and TEM analysis. The versatile photocatalytic activity has been tested toward Cr(VI) reduction and degradation of phenol. 3% CdS QDs-sensitized self-doped Bi2MoO6 showed highest activity, i.e., 97 and 47.5% toward reduction of Cr(VI) and degradation of phenol under solar light. The reduction of Cr(VI) by the catalyst is supported by the kinetics and determination of the pHPZC value. In addition to this, the photostability and reusability test showed that the catalyst can be reused up to five cycles without diminishing its activity.

Introduction

Bi2MoO6 is an active and simplest member of the Aurivillius oxide family of layered perovskites that has attracted increasing attention in the field of photocatalysis because of its luminescence, photocatalytic properties, low band gap energy, layered structure, and suitable band edge potential.[1−3] Additionally, its resistivity toward corrosion, mesoporosity, low cost, outstanding chemical stability, and attractive physicochemical properties also contribute to its improved photocatalytic activity. The Aurivillius oxide (γ-Bi2MoO6) possesses a distinctive layered structure with perovskite wedges of MoO6 octahedra sandwiched between alternating (Bi2O2)2+ layers.[4] It should be mentioned here that depending on the proportions of bismuth and molybdenum present bismuth molybdate (BMO) may exist in a variety of phases, among which the Bi2Mo3O12 or α-phase,[5] the Bi2Mo2O9 or β-phase,[6] and the Bi2MoO6 or γ-phase are commonly studied.[7] The ratio of Bi to Mo in α-, β-, and γ-phases are 2/3, 2/2, and 2/1, respectively. Of these phases, γ-phase is known to possess more mobile oxygen, whereas α- and β-phases provide more absorption sites for hydrocarbons. However, the consensus is that mixture of both γ-phase and α- or β-phase shows better photocatalytic activity than any single phase of bismuth molybdate due to the synergy effect[8] that takes place in the mixture. This modification of the individual γ-phase is performed to optimize its photocatalytic activity, viz., to overcome its inherent bottlenecks, which include low efficiency toward utilization of visible light, fast recombination and transport of photogenerated charge carriers, modest surface area, less active sites, and poor selectivity toward redox reactions. Apart from the synergic mixture of interphase composites, the photocatalytic activity has also been improved by making composite with noble metal (Ag, Pt). Yuan et al.[9] decorated a Bi2MoO6 microsphere uniformly with Ag nanoparticles for the photodegradation of methyl orange, methylene blue, and rhodamine B. Ag here acts as electron sink, which facilitates charge transfer via the Schottky barrier at the interface of the noble metal and Bi2MoO6. Zhang and co-workers[10] synthesized ultrathin Bi2MoO6 nanoplates loaded with Pt nanoparticles as co-catalyst and demonstrated their superior photocatalytic performance toward aerobic selective oxidation of benzyl alcohol. Understanding the significance of surface plasmon resonance (SPR) and sidestepping the high cost of noble metals, Zhao et al.[11] synthesized Bi co-catalyst loaded on Bi2MoO6 microspheres nanohybrid inspiring by the fact that Bi also exhibits the noble metal behavior, i.e., direct plasmonic resonance and the matched band edge positions with Bi2MoO6.

To improve the visible light absorption ability, the effective separation of the electron–hole pairs, and the photocatalytic activity of Bi/Bi2MoO6, in this work, CdS quantum dots (QDs) have been incorporated into the reaction system to activate the substrate and effectively lower the activation energy of the desired reaction because of their quantum confinement effect, high dispersity, large absorption coefficient, ultrasmall particle size, narrow band gap of around 2.29 eV, and the wavelength- and size-dependent photoluminescence (PL) emission.[12]

In this work, we have designed solar light-driven CdS QDs-sensitized self-doped Bi2MoO6 for the reduction of Cr(VI) and degradation of phenol. Detailed investigation of the prepared materials like phase purity, crystal structure, optical and photoluminescence properties, and effect of point of zero charge (PZC) was carried out. The prepared material exhibit the synergistic effect of Bi and CdS QDs for better photocatalytic activity toward fast reduction of Cr(VI) and phenol degradation. The mechanisms of both Cr(VI) and degradation of phenol have been studied in detail. The mechanistic pathway is well explained by employing the scavenger test and the involvement of active species, which is established from the respective confirmatory tests.

Experimental Section

Synthesis of Bismuth Molybdate (BMO) and Self-Doped Bismuth Molybdate (BBMO)

For the synthesis of BMO, the typical procedure involves the dissolution of 0.242 g of Bi(NO3)3·5H2O and 0.06 g of Na2MoO4·2H2O in 10 mL of ethylene glycol separately. The two solutions were mixed and dropwise added to 30 mL of isopropanol, followed by stirring for 20 min. Then, the prepared solution was transferred to a stainless steel autoclave and heated at 160 °C for 24 h, followed by cooling to room temperature. The BMO sample was collected by centrifugation and washed several times with water and once with ethanol and finally dried at 80 °C for few hours.[13]

To synthesize BBMO, 5 mL of ethylene glycol (EG; Merck) containing 1.6866 g of Bi(NO3)3·5H2O (Merck) marked as solution A and 5 mL of EG containing 0.4210 g of Na2MoO4·2H2O marked as solution B were prepared. Solution B was then added dropwise to solution A with continuous stirring. To this mixture, 20 mL of EG was added and stirred to get a clear solution. A small amount of glucose (0.616 g; Merck) was added to get suspension with strong stirring. Then, the solution was transferred to a 100 mL stainless steel Teflon-lined autoclave and heated at 160 °C for 20 h. After the hydrothermal treatment, the product was collected by centrifugation and subsequent washing with water and ethanol two times each. The deep gray Bi/Bi2MoO6 was dried overnight at 60 °C and symbolized as BBMO.[11]

Synthesis of CdS QDs-Sensitized BBMO

To prepare CdS QDs-sensitized Bi/Bi2MoO6 (CBBMO), particular amount of BBMO was taken and the procedure to deposit required weight percentage of CdS QDs was followed according to our previous work.[12] Briefly, appropriate amount of BBMO was added to a 75 mM Cd(NO3)2·2H2O solution. Requisite amount of thioglycolic acid (TGA) was added to the solution so that the mole ratio of 1:2 was maintained between TGA and CdS QDs, and the pH of the solution was adjusted to 10.5 with 1 M NaOH. After this, Na2S was added and the prepared solution was stirred at 65 °C for 30 min. Then, after 90 min aging, the prepared sample was collected by centrifugation, followed by three times washing with water and once with ethanol. Finally, the composite was dried in a vacuum desiccator.

Experimental Protocol for Reduction of Cr(VI) and Phenol Degradation

For the reduction of Cr(VI), the aqueous solution of K2Cr2O7 was used. A 20 mL volume of 100 ppm hexavalent Cr solution was taken with 0.02 g of catalysts to test the photocatalytic activity. To check the effect of pH on the photocatalytic reduction process, the pH of the solution was maintained at 2, 4, 6, and 8. Then, the suspension was stirred under dark condition for 30 min to attain the adsorption and desorption equilibrium before irradiation under solar light (100 000 lx). At different time intervals, the catalysts were separated by centrifugation and the supernatant was used for colorimetric analysis at 540 nm by 1,5-diphenylcarbazide (DPC) method using a JASCO V-750 UV−Vis spectrophotometer.[14] Additionally, the remained Cr in the sample after the experiment was also determined.[15]

Photocatalytic degradation of phenol was carried out by taking 20 mg of catalyst with 20 mL of 10 ppm phenol. Then, the suspension was stirred under dark condition for 30 min to attain the adsorption and desorption equilibrium before irradiation of solar light. Then, the pH of phenol solution was maintained at 2, 4, 6, and 8. After the photocatalytic experiment, the catalyst was extracted from the phenol solution by centrifugation and the residue was directly analyzed by a JASCO V-750 UV−Vis spectrophotometer.

Analytical Characterization

The phase purity and crystal structure of the CdS QDs-sensitized self-doped bismuth molybdate samples were analyzed by a Rigaku Miniflex instrument using Cu Kα radiation (λ = 1.54 Å) in the 2θ range of 20–80°. A JASCO V-750 UV−Vis spectrophotometer was used to measure the diffuse reflection UV–vis (DRUV–vis) spectra in the wavelength range of 200–800 nm. The photoluminescence property of the prepared samples was analyzed by a JASCO-FP-8300 fluorescence spectrometer. The emission and excitation spectra were taken to explain the uniform distribution of QDs on the surface of BBMO. Scanning electron microscopy (SEM) was carried out using ZEISS SUPRA 55. The microsphere structure of BBMO and ultrasmall particle size of CdS QDs were investigated by a TEM-JEOL-2010- 200 kV instrument. The mode of molecular vibration was determined by a FT/IT-4600 Fourier transform infrared spectrometer in the range of 4000–400 cm–1. X-ray photoelectron spectroscopy (XPS) was performed with a VG Microtech Multilab ESCA 3000 spectrometer using a nonmonochromatized Mg Kα X-ray source. The C 1s peak was used for binding energy correction, which arises from adventitious source. The electrochemical study of the prepared samples was carried out by a multichannel Ivium potentiostat to know the photoelectrochemical properties. This was carried out in a Pyrex electrochemical cell using the synthesized catalysts as the working electrode, platinum sheet as the counter electrode, and Ag/AgCl as the reference electrode. The electrolyte utilized here to analyze the study was 0.1 M Na2SO4, and a 300 W Xe lamp was used for light irradiation with a 400 nm cutoff filter.

Results and Discussion

Phase Structure

Figure a shows the X-ray diffraction (XRD) pattern describing the crystal structure and phase purity of the as-prepared photocatalysts. The XRD pattern of Bi/Bi2MoO6 (BBMO) consists of diffraction peaks for both orthorhombic Bi2MoO6 (JCPDS no. 21-0102) and rhombohedral Bi (JCPDS no. 44-1246). Several diffraction peaks at 2θ values of 27.01, 31.2, 44.2, 53.6, 53.8, and 55.7° are indexed as (131), (200), (202), (062), (133), and (191) crystal planes of Bi2MoO6. In addition to these, diffraction peaks at 27.8, 37.5, 39.2, and 45.6° are observed corresponding to the (012), (104), (110), and (006) planes in prepared composite materials, which confirms the occurrence of Bi in metallic state. The XRD result confirmed the formation of well crystalline materials without any impurities. In addition to this, no characteristic diffraction peaks of CdS QDs are observed, which is attributed to low CdS QDs content in the prepared samples.[12] As the XRD peaks of Bi2MoO6 and metallic Bi are overlapped in the range of 27–28 and 44–46°, the corresponding planes are deconvoluted as presented in Figure c,d. It represents the existence of metallic Bi peaks at 27.8 and 45.6° corresponding to the (012) and (006) planes. With gradual increase in the loading amount of CdS QDs in Bi/Bi2MoO6, there is shifting of peaks to higher angles owing to the synergistic interaction between CdS QDs and Bi/Bi2MoO6.[16] Additionally, the loading amount of CdS QDs could inhibit the crystal growth of Bi/Bi2MoO6, resulting in a weaker and broader peak.[17]

Figure 1

(a) XRD pattern and (b) enlarged view of pure Bi/Bi2MoO6 and the synthesized composites, Bi/Bi2MoO6/CdS QDs; (c, d) enlarged view of deconvoluted XRD pattern showing peak position of both Bi and Bi2MoO6.

Optical Studies

The optical absorption properties of the as-prepared samples were investigated by UV–vis diffuse reflection spectroscopy (DRS) in the range of 200–800 nm. Figure a depicts that photoabsorption of pure BMO, BBMO, and CBBMO composites reflects in both UV and visible regions with strong light absorption between 300 and 700 nm. Its absorption range is also extended to near-IR region, which is contributed to light scattering and SPR effects of Bi metal.[11] It is also observed from Figure a that the absorbance spectra of BBMO and CBBMO consist of one strong and one small shoulder band in the UV region. The small shoulder band around 220–330 nm is attributed to the interband transition from valence band (consists of O 2p orbitals) to conduction band, which is made up of Mo 4d orbitals and Bi 6p orbitals.[5] In addition to this, CdS QDs have high absorbance value that shows strong absorption up to 580 nm and the steep shape is due to band gap transitions.[12] Moreover, no distinct maxima is found, rather a plateau-shaped spectrum is observed, which is attributed to different electronic structure. With increasing the loading amount of CdS QDs, the optical absorption shifted toward higher wavelength, which played a vital role in photocatalytic activity. This broad absorption band is attributed to uniform particle size distribution, and the red shift absorbance is attributed to the present localized states.[12] Also the observed red shift is consistent with the change in color of photocatalysts from deep gray to dark green. Consensus is that, on account of SPR effect, Bi-modified nanocomposites show improved light absorption, which is one of the important characteristics of enhanced photocatalytic activity. The direct band gaps of CdS QDs and BMO are calculated by the equation αhυ = K(hυ – Eg)1/2, where the symbols bear their usual designations values, that is, “α” is the absorption coefficient, hυ is the photon energy, K is a constant, and Eg is the optical band gap energy. It should be mentioned here that absorption coefficient (α) can be replaced by absorbance (A) because the concentration (c) and the optical path length (l) are invariable in the present situation.

Figure 2

UV–vis DRS of BMO, BBMO, CdS QDs, and CBBMO composites; (b) Urbach energy of prepared samples; (c) band gap of CdS QDs; and (d) band gap of bismuth molybdate.

Calculation of Urbach Energy

Urbach tail gives information about defects, disorder, band structure, and electron–phonon interaction in almost all amorphous semiconductors, degenerately doped crystalline materials, and in many ionic crystals in the weak absorption or low-energy side of the optical spectra. In Figure b, the absorption coefficient follows an exponential change, and this type of exponential decay is known as Urbach tail, which follows eq .where α0 is a constant, E is the incident energy, and EU is the Urbach energy.

Taking natural logarithm on both sides, we haveFrom eq , the reciprocal of the slope of this exponential graph plotted between ln α and incident energy (E) determines the Urbach energy value (EU). Additionally, EU is an expected explanation for density of states, especially in low-dimensional semiconductors in a nonvacated environment.[18−20] If the width of the exponential tail is large, there is more density of defect states in the material, which indicates fast recombination of charge carriers.[18] So, to delay the recombination of charge carriers and improve the photocatalytic application, it is important to find a material having small Urbach energy (EU). Table shows the calculated Urbach energy and band gap values of all of the prepared samples. From Table and Figure b, it is observed that CBBMO-3 has the lowest EU value among the prepared composites, which is an indication of containing less density of states and have undergone delayed recombination of photoexcited charge carriers. This characteristic property is also in well agreement with the PL behavior of CBBMO-3, which is discussed in the next section.

Table 1

Band Gap, Full Width at Half-Maximum (FWHM), and Urbach Energy of Prepared BBMO Samples

catalyst	band gap (eV)	FWHM of PL peak (nm)	Urbach energy (eV)
CBBMO-1	2.70	45.83	3.71
CBBMO-3	2.65	44.99	2.56
CBBMO-5	2.51	45.74	2.80

Photoluminescence Behavior

To provide evidence about the transfer and degree of recombination of photoexcited charge carriers in semiconductors, PL spectroscopy is proved to be an effective method. As shown in Figure a, the prepared neat material and all of the composites show a strong emission peak at ca. 415 nm and a small peak at 468 nm. The former peak is due to defect-related luminescence (blue luminescence), and the latter is because of band gap transition. To acquire more information about the optoelectronic properties of the as-synthesized samples, photoluminescence (PL) emission and photoluminescence excitation (PLE) spectroscopy is performed in detail. In addition to this, it is very clear from Figure a that CBBMO-3 shows the lowest PL intensity compared to the other made composites, which is an indication of delayed recombination of excited charge carriers and better photocatalytic activity of the concerned sample. Figure b shows a comparison study between FWHM of PL emission peak calculated from Figure a and Urbach energy calculated from absorption coefficient in the previous section. It is clearly observed that both the factors follow a similar trend with increasing loading amount of CdS QDs, which indicates that the source of foundation of both absorption and emission ascends from similar states.[21] The FWHM and EU values of the prepared composites follow the following order CBBMO-1 > CBBMO-5 > CBBMO-3. All of the values of calculated band gap, FWHM from PL peak, and Urbach energy are compiled in Table . It is observed from Table that CBBMO-3 shows the least FWHM and Urbach energy compared to the CBBMO composites, which indicates the low disorder among these. Uniform distribution of QDs on BBMO surface is analyzed by varying different excitation wavelengths in support of wavelength-dependent PL behavior of QDs.

Figure 3

(a) Photoluminescence spectra of BMO, BBMO, CdS QDs, and the prepared composites at excitation wavelength of 330 nm; (b) comparison of change in FWHM of PL emission peak and Urbach energy; (c) normalized PL spectra of CBBMO-3 at excitation wavelengths of 320, 325, 330, 335, and 340 nm; and (d) normalized PLE spectra of CBBMO-3 at emission wavelengths of 410, 415, 420, 425, and 430 nm.

Figure c,d represents PL and PLE spectra of CBBMO-3 at different excitation and emission wavelengths, respectively. The observed data are presented in Table .

Table 2

Extracted Data from Figure c,da

fixed excitation	observed emission	fixed emission	observed excitation
320	401.46	410	325.54
325	408.89	415	329.98
330	415.32	420	333.94
335	419.76	425	338.16
340	424.74	430	341.37

Note: All of the given data in this table are in nanometer scale.

In Figure c, when the excitation wavelengths are varied in the range of 320–340 nm at an interval of 5 nm, it has been observed that the emission spectra are gradually and periodically shifted toward higher wavelength (401.46–424.74 nm). On the contrary, when emission wavelengths are varied in the range of 410–430 nm at the same interval, PLE spectra are exhibited at the wavelength nearly same as that of the fixed excitation wavelength, as mentioned in Figure c. From this analysis, the Stokes shift is calculated to be 84 nm (Figure S1), which indicates the weak self-absorption and low energy loss of the composites.[22] Because the broad symmetric emission peaks are not at constant wavelength when excited with various excitation wavelengths, it can be suggested that the CBBMO composites are not with a monodisperse set of CdS QDs.

SEM

The structure and morphology of the BBMO and all of the prepared composites were investigated by field emission scanning electron microscopy (FESEM). As shown in Figure a,b, the microsphere morphology of BBMO is clearly revealed with average particle size of 1.5 μm and also with some interspaced nanoparticles of the same. The interspaced nanoparticles are formed because of the reducing agent taken (glucose). During the synthesis, although the amount of glucose taken for the purpose of formation of microsphere morphology of BBMO, still it has inhibited somehow the anisotropic development of BBMO.[11]Figure c–h represents the distinctive SEM images of all CBBMO composites, where the deposition of CdS QDs on the surface of BBMO is clearly visible, which confirms the formation of the QDs. In Figure c,d (magnified image), the deposition of the QDs is less noticeable compared to other SEM images of CBBMO composites as a result of its small loading amount (1 wt %). The deposition of CdS QDs on the BBMO surface is more prominent in CBBMO-3 and CBBMO-5, as shown in Figure e–h. It is observed from the SEM images of all of the CBBMO composites (Figure c–h) that irregular spherical and apparent agglomerated QDs have partially covered the BBMO surface.

Figure 4

FESEM images and enlarged views of (a, b) BBMO; (c, d) CBBMO-1; (e, f) CBBMO-3; and (g, h) CBBMO-5.

Transmission Electron Microscopy (TEM)

The microsphere morphology of BBMO and ultrasmall particle size of the as-prepared CdS QDs are clearly visible from the high-resolution transmission electron microscopy (HRTEM) images. Figure a represents the TEM image and Figure b represents the HRTEM image of microspherical BBMO, which is in resemblance with the corresponding FESEM analysis. The d spacing value mentioned in the figure corresponds to the (131) and (012) planes of Bi2MoO6 and Bi, respectively. The ultrasmall size of CdS QDs is confirmed from Figure c,d, which is representative of about 3.5 nm CdS QDs. The inset of Figure c clearly reveals the lattice spacing of CdS QDs, which corresponds to (111) crystal plane (0.33 nm). All of these obtained values of d spacing are in well coincidence with the XRD analysis. The TEM image of composite CBBMO-3 is represented in Figure e. The coexistence of Bi2MoO6, Bi, and CdS QDs is evidenced from the HRTEM image (Figure f).

Figure 5

TEM and HRTEM images of (a, b) BBMO; (c, d) CdS QDs; and (e, f) CBBMO-3.

Fourier Transform Infrared (FTIR) Spectroscopy

To explore the chemical composition and mode of molecular vibration, FTIR spectra of BBMO and CBBMO catalysts were analyzed in the range of 4000–400 cm–1, which is demonstrated in Figure . The peak at 448 cm–1 is the representative peak for Bi–O stretching vibrational modes, and the band at 584 cm–1 is the bending vibration mode of MoO6 octahedron.[23] The peaks positioned at 796 and 842 cm–1 can be correspondingly ascribed to the symmetric and asymmetric Mo–O stretching vibration modes of the corner sharing MoO6 octahedron.[24] The peak retained at 2360 cm–1 is attributed to the antisymmetrical stretching mode of carbon dioxide.[25] Peaks centered at 1620 and 3415 cm–1 correspond to H–O–H bending and O–H stretching vibration modes of absorbed free water.[26] As the prepared QDs are capped with thioglycolic acid (TGA), the C–O stretching is observed in the range of 990–1150 and the peaks at 400–700 cm–1 are attributed to Cd–S stretching.[27] It should be mentioned here that the S–H stretching at 2550–2970 cm–1 is absent in TGA-capped CdS QDs, which is due to the covalent bonding between thiols and Cd atom of CdS QDs.[28] The FTIR spectra for pure TGA and TGA-capped CdS QDs are given in Figure S2.

Figure 6

FTIR spectra of BBMO and the prepared composites.

Chemical Composition

The XPS analysis represents the surface chemical composition of CBBMO-3 composite. The presence of constituent elements, i.e., Bi, Mo, O, Cd, and S, is confirmed from the survey scan (Figure a). As shown in Figure b, two high-resolution and strong peaks are centered at 159.2 and 164.5 eV, which are due to 4f7/2 and 4f5/2, respectively, of Bi3+. In addition to these two peaks, other two peaks at 156.7 and 162.4 eV are obtained, which confirms the metallic state of Bi.[11]Figure c reveals that peaks at 232.5 and 235. 6 eV correspond to 3d5/2 and 3d3/2 of Mo6+ oxidation state in CBBMO-3.[29] The symmetric peak at 530.5 eV (O 1s) is attributed to Bi–O chemical bonding. Two peaks at 405.3 and 412.2 eV are ascribed to Cd5/2 and Cd3/2, respectively, and confirm the +2 state of Cd (Figure e). The presence of S at the junction is also proved from Figure f.[30]

Figure 7

XPS images of (a) survey scan of CBBMO-3, (b) Bi 4f, (c) Mo 3d, (d) O 1s, (e) Cd 3d, and (f) S 2p.

Photoelectrochemical Analysis

Mott–Schottky (MS) analysis, linear sweep voltammetry, and electrochemical impedance spectroscopy of BBMO and CBBMO composites were investigated to conclude the high performance toward electrochemical properties. Generally, the band edge position as well as the type of semiconductor, the photocurrent response, the charge carrier separation or transport, etc. are being examined from these above-mentioned analysis.

Mott–Schottky Analysis

To check the photocatalytic activity of the prepared composite toward degradation of phenol and reduction of hexavalent chromium, favorable band alignment is necessary for the flow of charge carriers and formation of the active radicals. To evaluate experimentally the band edge positions, Mott–Schottky analysis is carried out, and the Mott–Schottky plots of bare bismuth molybdate (BMO) and CdS QDs are given in Figure a,b.

Figure 8

Mott–Schottky plots of (a) BMO and (b) CdS QDs.

The graph is plotted by taking inverse of the square of measured space charge capacitance (CSC) in unit of Farad (F) as ordinate and the applied potential (Vapp) in volt (V) as abscissa using the following equation (eq ).where Vfb is the flat band potential, which is calculated by the extrapolation of the graph and intersection point of abscissa, and k, T, e, ND, ε, ε0, and A are the Boltzmann constant in J K–1, absolute temperature in K, electronic charge in C, donor density in cm3, semiconductor dielectric constant, dielectric constant in vacuum, and area in cm2, respectively. The slope of this equation can also be helpful in calculating the value of ND if the values of ε and A are known. The positive slope of the plot reveals the obvious n-type behavior for both BMO and CdS QDs, and the intersection point is independent of applied frequency. It is very well known that the Mott–Schottky plot is representative of the flat band potential of the semiconductor. Negative flat band potential of −0.86 and −0.63 V versus Ag/AgCl is found for both BMO and CdS QDs. The flat band potential in NHE scale is calculated to be −0.31 and −0.08 V for BMO and CdS QDs, respectively. As the conduction band of n-type semiconductors is 0.1 V more negative than the flat band potential,[31] the CB edge potential of BMO and CdS are at −0.40 and −0.18 V (vs NHE). Hence, the valence band potential of BMO and CdS QDs is calculated to be +2.08 and +2.11 V, respectively.

Linear Sweep Voltammetry

The typical n-type behavior is represented here from the increasing anodic current density, which has been increased with applied positive potential. As shown in Figure a, under dark condition, BBMO shows 0.14 mA/cm2 of photocurrent density, and there is enhancement of the photocurrent value in case of CBBMO composites. With the increase of loading content of CdS QDs from CBBMO-1 to CBBMO-3, there is a steady enhancement of photocurrent density, but after that, 5 wt % loading amount of CdS QDs inhibits the increment of photocurrent generation.

Figure 9

J–E curves under (a) dark and (b) light conditions of BBMO and CBBMO composites coated on FTO measured in 0.1 M Na2SO4.

Under dark condition, maximum 0.58 mA/cm2 of current was generated by CBBMO-3 composite. But the CBBMO-3 photoanode under front-side photoillumination could generate 0.8 mA/cm2 of current at the applied potential.

Electrochemical Impedance Spectroscopy

Figure shows the Nyquist plot (Zimg vs Zreal), which is a measure of intrinsic electrical properties, especially conductivity. The idea about conductivity or charge-transfer resistance can be easily best evaluated from the radius of the semicircle. The smaller the radius of semicircle, the better is the conductivity of the semiconductor, which is CBBMO-3 in our case. This is also well supported by the catalytic activity, which will be discussed later. The values of charge-transfer resistance of the prepared samples follow the order: CBBMO-3 (50.00 Ω) < CBBMO-5 (51.22 Ω) < CBBMO-1 (52.34 Ω) < BBMO (58.74 Ω). The plot was fitted with the circuit model, which consists of Rs, Rct, Ws, and CPE, where Rs is the ohmic series resistance (charge-transfer resistance and contact resistance at electrolyte/electrode), Rct is the electron-transfer resistance for the reaction, Ws is the Warburg resistance, and CPE is the constant phase element.

Figure 10

Nyquist plot of BBMO and CBBMO composites in the frequency range of 100–95 000 Hz.

Determination of Point of Zero Charge

The surface charge is a key point in the reduction of Cr(VI) as Cr species in anionic form get attracted by the catalyst surface. For this, the catalyst surface should be positively charged and hence to determine the surface charge of catalyst, drift method[15] is used. This method is useful to calculate the point of zero charge (PZC) of prepared samples. The typical procedure involves the preparation of 0.005 M NaCl aqueous solution, followed by boiling to remove dissolved CO2. The pH of the solution was maintained at 2, 4, 6, and 8 by 0.5 M HCl and 0.5 M NaOH. In the next step, 20 mL of the above-prepared solution with 0.02 g of CBBMO-3 was stirred for 24 h to attain the adsorption–desorption equilibrium. Then, the final pH of a required solution is plotted against the pH of the solution before treatment. This graph (Figure ) results a pH value of pHinitial = pHfinal, which is called the point of zero charge (pHPZC). The consensus is that if the pH is less than the pHPZC, the surface of catalyst is positively charged, and if the pH succeeds pHPZC, the surface becomes negatively charged.[15]

Figure 11

Drift method of calculation of pHPZC of CBBMO-3.

Photocatalytic Activity

Photocatalytic Reduction of Cr(VI)

Due to the wide-range light absorption ability and photostability of CBBMO, it was utilized to reduce toxic Cr(VI). Its photocatalytic nature was due to certain control experimental conditions. During the reaction, there is no significant reduction of Cr(VI) in the absence of either light or catalyst (Figure a), which confirms the admirable stability of K2Cr2O7 solution. As shown in Figure a, 20 and 82% reduction was observed by neat BBMO and CdS QDs, respectively. Interestingly, the rate of reduction was greatly enhanced after loading of CdS QDs on BBMO. It was observed that by the end of the reaction period, CBBMO-3 could reduce 97% of hexavalent Cr in 1 h. The Cr remediation efficiencies of the other prepared composites are 86, 97, and 90% over CBBMO-1, CBBMO-3, and CBBMO-5, respectively. This result shows the best photocatalytic activity of CBBMO-3 toward the photocatalytic reduction of hexavalent Cr in comparison to the other reported work (Table ). The spectral changes during the removal of Cr(VI) over all prepared catalysts were tested at various intervals of time and are represented in Figure b.

Figure 12

(a) Photocatalytic reduction of Cr(VI); (b) spectral changes during reduction of Cr(VI) over CBBMO-3 at various intervals of time; and (c) spectral changes during reduction of Cr(VI) at different pHs.

Table 3

Comparison Study on Reduction of Cr(VI) and Phenol Degradation over other Photocatalysts

catalyst	concentration of Cr(VI) solution/phenol	light source	reaction time (min)	pH	result (%)	ref
α-MnO2@RGO	10 mg/L Cr	visible light	120	2	97	(14)
P-doped porous ultrathin g-C3N4 nanosheets	20 mg/L Cr	visible light	120	2.13	75	(32)
N, S co-doped CeO2	50 mg/L Cr	visible light	120	2	93	(33)
Ag2S QDs/SnS2	50 mg/L Cr	visible light	60		57	(34)
Bi/Bi2MoO6	100 mg/L Cr	visible light	60	2	97	this study
Au-Pd/rGO	0.5 mM phenol	sunlight	300	6.9	94.45	(35)
TiO2 nanotube array	20 mg/L phenol	UV light	400		75	(36)
Pt and Na2CO3 on TiO2	0.43 mM phenol	UV light	60	8	60	(37)
Bi/Bi2MoO6/CdS QDs	10 ppm phenol	solar light	60	6	47	this study

Effect of pH on Cr(VI) Reduction

pH plays a very essential role in the reduction of hexavalent Cr; hence, to investigate its influence, a series of experiments were performed at pHs 2, 4, 6, and 8. The pictorial representation is shown in Figure c, which indicates that the suitable pH value is 2. On this basis, the entire experiment toward photocatalytic removal of Cr(VI) was carried out at pH 2. The lower pH value is maintained as reaction condition because, at this pH, Cr is predominantly present in anionic form, i.e., as Cr2O72–, and it is quite less than the pHPZC value (pHPZC = 6.8). As mentioned earlier, if the pH is less than pHPZC, the CBBMO surface is positively charged, resulting in effective interaction between catalyst surface and Cr2O72–, which is a suitable condition for the removal of hexavalent Cr. The reduction of Cr(VI) to Cr(III) proceeds as per the following equation.From this equation, it is clear that due to plethora of protons available the photocatalytic activity is encouraged at lower pH.

Confirmation of Cr(VI) to Cr(III)

After the experiment, Cr(VI) was reduced to Cr(III), and this was analyzed by KMnO4. To verify the existence of nontoxic Cr(III), KMnO4 solution was used to oxidize Cr(III) back to Cr(VI), which was confirmed colorimetrically by the DPC method.[15] In a typical experiment, 0.02 M KMnO4 aqueous solution was prepared, 0.1 mL of which was added to 1.9 mL of the residual solution after irradiated with sunlight for 1 h, and the same DPC method was used for the detection of concentration for Cr(VI). From the above experiment, it is clear that Cr(VI) is reduced to Cr(III).

Detection of Active Species

As it is known that electrons in CB of catalysts are the active species for Cr(VI) reduction, to prove this, methanol can be used as hole scavenger, which reacts with the photoinduced holes in the VB of the catalysts to achieve fast photocatalytic reduction of Cr(VI). The typical procedure involves addition of various amounts of CH3OH and citric acid to the acidic Cr(VI) solution at pH 2.

Figure a depicts that the addition of methanol decreases the photoreduction time and makes it a fast photocatalytic removal of hexavalent Cr. In addition to methanol, 1 mL of aqueous citric acid solution (1 g/10 mL) was also used to completely reduce Cr(VI) within 40 min. Complete reduction of Cr(VI) was observed in 20 min when both methanol (2 mL) and citric acid (1 mL) were in the reaction medium. This suggests that the main active species may be electron; this statement has also been proved by taking various amounts of AgNO3 and dimethyl sulfoxide (DMSO) as electron scavenger, as shown in Figure b. As shown in the figure, 0.2 mmol AgNO3 shows higher electron-trapping capacity than 0.3 and 0.4 mmol AgNO3. But, it was observed that the electron-trapping ability of DMSO is higher compared to AgNO3. It was studied experimentally that 4 mM DMSO is sufficient compared to 3 and 5 mM DMSO to scavenge the photoexcited electrons. Still some reduction occurred, which will be discussed in Section .

Figure 13

Control experimental conditions involving different (a) hole and (b) electron scavengers.

Kinetics Followed by CBBMO-3 in Cr(VI) Reduction

Zero-order kinetics (Figure ) was followed by CBBMO for the reduction of Cr(VI), which was concluded by fitting the experimental data with zero-, first-, and second-order kinetics model equations (eqs –7). The obtained fitting results, i.e., slope (rate constant, k), coefficient of determination, R2, and standard error, are tabulated in Table .Here, C0 and C are concentrations of Cr(VI) at time = 0 and t, respectively, in mg/L, and k0, k1, and k2 are zero-, first-, and second-order rate constants in mg/L min, min–1, and L/mg min, respectively.

Figure 14

Zero-order kinetics followed by reduction of Cr(VI) over CBBMO-3.

Table 4

Fitted Results of Cr(VI) Reduction over CBBMO-3 at pH 2

catalysts	CdS QDs	BBMO	CBBMO-1	CBBMO-3	CBBMO-5
zero order R2	0.89	0.93	0.95	0.96	0.97
slope (k0)	0.003	0.012	0.013	0.015	0.014
standard error	0.001	0.001	0.001	0.001	0.001
first order R2	0.87	0.75	0.753	0.72	0.77
slope (k1)	0.003	0.025	0.029	0.05	0.03
standard error	0.006	0.007	0.008	0.015	0.008
second order R2	0.84	0.54	0.52	0.41	0.5
slope (k2)	0.0038	0.064	0.087	0.442	0.126
standard error	0.028	0.026	0.03	0.022	0.056

Photocatalytic Degradation of Phenol

The versatile photocatalytic activity of CBBMO samples was investigated by degrading 10 ppm phenol as model organic environmental pollutant. Figure a shows spectral presentation of phenol degradation under solar light irradiation, and it was measured by a UV–vis spectrometer at ca. 269 nm. The effect of pH was also checked in this process, and pH 6 was found to be a suitable condition for the degradation. The degradation activity follows the sequence as pH 6 > pH 2 > pH 4 > pH 8 (Figure b). So, pH 6 was taken as the optimum pH condition to carry out further experiment, and 47.5% phenol was degraded in the presence of CBBMO-3 under solar light in 60 min.

Figure 15

(a) Absorption spectra of prepared catalysts in phenol degradation; (b) comparison of rate of degradation of phenol by CBBMO-3 at different pHs; (c) second-order kinetics observed in the degradation process; and (d) histogram representing the comparison of second-order rate constants of prepared samples at pH 6.

The kinetics through which the phenol degradation mechanism was performed is shown in Figure c. The kinetics process involves 0.02 g of CBBMO-3 at pH 6, and the same procedure was surveyed to investigate the kinetics. Results showed that it is the second-order kinetics followed by the degradation process. The symbols in this equation bear their usual meaning. Figure d indicates the second-order rate constants; when examined, it was observed that CBBMO-3 possesses a higher rate constant of 0.014 L/mg min, which is 14 and 7 times higher than that of the CBBMO-1 (0.001 L/mg min) and CBBMO-5 (0.002 L/mg min) composites, respectively. The conclusion drawn from the observation is that CBBMO-3 catalyst accelerates the photodegradation of 10 ppm phenol compared to all other prepared samples.

Mechanism Involved in Cr(VI) Reduction and Phenol Degradation

All of the above experimental procedure directs the following acceptable mechanistic pathway for the reduction of Cr(VI) and degradation of phenol, which is embodied in Scheme . CdS QDs and Bi in CBBMO-3 act here as the absorption center due to their high absorption capacity and SPR property, respectively. In addition to this, the obtained Brunauer–Emmett–Teller (BET) surface area data of the as-prepared samples (Table S1 of SI) and their respective photocatalytic activities signify that surface area is not the whole criteria in deciding the efficiency of photocatalysts.[38] Rather than charge separation efficiency, light absorption capacity, band gap, band edge potential, active sites, and photostability are key factors in determining the photocatalytic efficiency of the material.

Scheme 1

Schematic Presentation of Reduction of Cr(VI) and Degradation of Phenol over CBBMO-3

Upon solar light irradiation, photoinduced electrons and holes get separated. As a result of this, holes are in VB, whereas electrons are in CB of BBMO and CdS QDs. As per the CB edge positions of involved semiconductors, electrons migrate from CB of both BMO and CdS QDs to Fermi level of Bi (−0.17 V),[11] meanwhile holes transfer from VB of BMO to CdS QDs and eventually oxidize the hole scavenger (methanol, citric acid) to CO2, H2O, and other mineralized products. At the surface of Bi, some migrated electrons are utilized to reduce Cr(VI) as per eq and some electrons are engaged in the production of superoxide radicals (O2–), which indirectly reduce Cr(VI) to Cr(III) through a two-step process.[39] As E0 (O2/•O2) = −0.046 eV versus NHE,[39] which is less negative than the CB potential value of BMO, CdS QDs, and Bi, electrons are transferred to generate O2–. This mechanism is proposed because even in the presence of DMSO (e– scavenger), CBBMO-3 could reduce 60% Cr(VI); hence, it is supposed that •O2– is also participating in Cr(VI) reduction. The generation of •O2– is confirmed from the NBT (nitroblue tetrazolium chloride) test.[40] In CBBMO-3, less •O2– are formed compared to the neat materials, which is confirmed from the high absorbance value from Figure a. So, partial involvement of O2– is confirmed from this NBT test.

Figure 16

Absoption spectra of (a) NBT in neat and CBBMO-3 sample and (b) fluorescence spectra of BBMO and CBBMO-3 in basic solution of 5 × 10–5 M terephthalic acid.

From the scavenger test of phenol, it was concluded that the major active species behind this is OH and the minor species is O2–. In neutral and basic medium, O2– formation is favorable, whereas the formation of H2O is favorable in acidic medium. Both the species are responsible for the generation of OH radical.[35] The confirmation of production of these radicals is graphically presented in Figure a,b. The formation and role of O2– is similar in this regard, as explained previously. The formation of OH radical might be proposed in two ways. One is direct formation from holes at VB of CdS QDs because E0 (OH–/OH) = 1.99 versus NHE[32] and the other is indirect formation via H2O2, as proposed in the following equations.The resultant degradation reaction is phenol + O2– + OH → degradation

Stability and Reusability of CBBMO Composite

For the practical application (wastewater remediation) of this catalyst, the stability must be retained so that it can be reused several times. Figure shows the repeated use of this catalyst up to five cycles of Cr(VI) reduction and phenol degradation. Average reduction and degradation of Cr(VI) and phenol of 95.76 and 47% are maintained, respectively, for five cycles. CBBMO-3 obtained after Cr(VI) reduction was characterized by FTIR analysis, and it is very prominent from Figure a that there is shifting of peak toward lower wavenumber. This is an indication of good interaction between CBBMO-3 and Cr(VI). Even after five cycles of treatment in Cr(VI), the characteristic peaks still persist. Additionally, UV-DRS spectra (Figure b) are also provided here, which conclude little change in absorption region after reduction of Cr(VI).

Figure 17

Cr(VI) reduction and phenol degradation efficiency versus reaction cycles of CBBMO-3.

Figure 18

(a) FTIR spectra and (b) UV-DRS spectra of CBBMO-3 before and after reduction of Cr(VI) at pH 2.

Conclusions

A systematic methodology has been followed to evaluate the versatile photocatalytic efficiency of the prepared composite (CBBMO-3) for Cr(VI) reduction and phenol degradation. Comparative study of using different hole scavengers has also been investigated, which significantly reduces the reaction time to achieve the target. The zero- and second-order kinetics are followed by reduction of Cr species and degradation of phenol, respectively. Cr(VI) removal and phenol degradation efficiencies are found to be 97 and 47.5%, respectively, over CBBMO-3. From the scavenger test, electron is found to be the main key active species, O2– is partially involved in the reduction of Cr(VI), and the lion’s share goes to both O2– and OH in the degradation of phenol. The present work suggests that low-cost Bi acts as a substitute for noble metals and CdS QDs greatly enhance the photocatalytic activity. This work possibly provides industrial applications toward removal of Cr(VI) and degradation of phenol.