Layered Molybdenum (Meta)phosphate for Photoreduction of Hexavalent Chromium and Degradation of Methylene Blue under Sunlight Radiance.

Noble metal, semiconductor, or metal-free nanomaterials have shown promising applicability as potential photocatalyst materials. A one-step process has been established for the synthesis of layered molybdenum (meta)phosphate [MoO2(PO3)2] using a solvothermal method. The nanopowders were characterized by X-ray diffraction (XRD), UV-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), surface area analysis (Brunauer-Emmett-Teller (BET)), electron spin resonance (ESR), and high-resolution transmission electron microscopy (HRTEM). Through this study, we demonstrate the use of MoO2(PO3)2 as a photocatalyst for wastewater treatment. The photoreduction of toxic Cr6+ to Cr3+ by layered molybdenum (meta)phosphate is investigated using formic acid as a scavenger. This catalyst has also been used for photodegrading organic dyes like methylene blue. MoO2(PO3)2 has been shown to complete photoreduction of toxic Cr6+ to Cr3+ in 6 min and achieved 78% degradation efficiency for methylene blue in 36 min. The reactive species trapping experiments revealed that the key active species like O2 •-, •OH, and h+ can exist and play an important role in methylene blue photodegradation.

Introduction

For decades, population growth, urbanization, and industrialization have resulted in energy depletion and environmental pollution. Many effluents discharged by industries into water resources endanger aquatic life and the lives of downstream users. These effluents are classified as inorganic or organic pollutants. Hexavalent chromium is the most toxic heavy metal cation due to its cytotoxic, mutagenic, and carcinogenic properties.[1] Because of its ability to produce responsive oxygen species in cells, it is extremely toxic to all faunae.[2] Chromium exists in Cr6+ to Cr3+ oxidation states, with Cr6+ being more toxic and causing more carcinogenic effects than trivalent chromium.[3] Chromium alloys are corrosion inhibitors and are useful in metal plating, leather tanning, and pigment manufacturing.[4] Cr6+ is released in water and soil matters without any pretreatment from these industries, and it is nonbiodegradable and remains in the environment for a long time, causing a harmful effect.[5] It is possible to reduce it from hexavalent to trivalent using a suitable photocatalyst and then remove it using a precipitating agent such as sodium hydroxide.[6] Organic cationic methylene blue, methyl orange, and rhodamine-B were also released into the environment from the dye industries, affecting human health.[7] When methylene blue is released into water resources, it can cause eutrophication, perturbations, and harm to the water ecosystem.[8,9]

According to a literature survey, UV–visible light sources and natural sunlight are suitable for the removal of toxic pollutants.[10,11] For the remediation of these pollutants, different methods such as chemical oxidation, extraction, adsorption, photocatalytic, and biochemical processes were used.[12] Metals, a semiconducting material, and their composites are currently being investigated as photocatalysts for converting them into less-polluting materials. Pd nanoparticles are used for the catalytic reduction of Cr6+ in the presence of formic acid.[13] Several semiconductor materials, including CdS/CulnS2,[14] ZnO,[15] and Cd0.5Zn0.5S@ZIF-8,[16] have been reported for the photoreduction of Cr6+. Recently, MoSe2/ZnO/ZnSe[17] hybrid materials demonstrated complete Cr6+ to Cr3+ reduction. Carbon materials like carbon/palladium nanocomposites[18] and activated carbon-supported ZnO[19] nanocomposites have also been used to reduce Cr6+. Sedimentation, chemical precipitation, adsorption, biological membranes, and ion-exchange processes have all been used to remove this harmful dye. These methods proved time-consuming and expensive, and the dye removal required a complicated process.[20] Semiconducting materials, such as Fe-ZnO,[21] TiO2,[22] and ZnO/CdO,[23] were found to be useful for methylene blue degradation. Carbon materials as well as semiconductors such as DyVO4/g-C3N4I[24] and C-doped TiO2[25] have also been shown to degrade methylene blue. TiO2 is the most extensively studied compound among those listed above due to its lack of toxicity, chemical stability, low cost, and superior catalytic properties.[26] The main impediment to TiO2 commercialization is its band gap, absorption in only ultraviolet (UV) light, low recyclability of TiO2 particles, and low adsorption capacity for hydrophobic pollutants.[27] To overcome the weaknesses of titanium oxide, numerous materials for photocatalytic applications have been tested.[28]

Molybdenum has several oxidation states, and its phosphate exists in different forms such as mono (PO43–), di-or (pyro P2O74–), meta (PO33–), oxyphosphate, and their combinations.[29] Transition-metal phosphates, which include zirconium and titanium phosphates, are another class of photocatalysts.[30] Bismuth oxychloride-modified titanium phosphate nanoplates have also been used to degrade organic pollutants.[31] A chitosan-zirconium phosphate nanomaterial has been reported for chromium and dye remediation.[32] Molybdenum polyphosphates such as (MoO2)2P2O7[29] and (MoO2)2P2O7[33] have recently been studied for battery applications in the literature. Chiweshe et al. reported the molybdenum (meta)phosphate phase[34] using a fusion method. In other studies, the spectroscopic and magnetic properties of[35] M(PO3)3 (where M = Mo, Cr) were investigated. To date, there have been few reports of Mo(VI)-containing phosphates, and further investigation is still an intellectual and scientific curiosity. The phosphite [(PO3)3–] present in molybdenum phosphate, like [(PO4)3–],[36] may have a strong bonding nature with water and chemical redox property to the generated electrons and holes.

From the above discussion, it is clear that there is an urgent need for the rapid reduction of Cr(VI) from wastewater and conversion to Cr(III) to protect the environment from Cr(VI)-related toxicities. Despite extensive research on the development of a new photocatalytic system, competent, low-cost, metal-free, and environmentally benign compounds with superior photocatalytic activity must be revealed. Molybdenum phosphate can serve dual purposes such as photoreduction and less evoked toxicity. Molybdenum phosphates are cheap and nontoxic. The layered molybdenum (meta)phosphate material was synthesized in one pot with the one-step solvothermal process in ethylene glycol (EG) at 198 °C. It was also investigated as a photocatalyst for photoreduction of Cr6+ to Cr3+ and methylene blue degradation under natural sunlight radiance.

Experimental Section

Materials

Hexaammonium molybdate ((NH4)6Mo7O24·4H2O, 98%), triphenylphosphine (C18H15P, 98%), ethylene glycol (EG, C2H6O2, 98%), ethylene diammonium tetraacetic acid disodium salt (EDTA·2Na, 98%), and isopropanol (C3H8O) were purchased from S. D. Fine Chemicals Pvt. Ltd. Methanol (CH3OH, 99%) and potassium dichromate (K2Cr2O7, 99.5%) were purchased from SRL Pvt. Ltd. Methylene blue was from Loba Chemie Pvt. Ltd (95%), and formic acid (HCOOH) was from Merck India. Dimethyl sulfoxide (DMSO, 99.7%) and ascorbic acid were acquired from Sigma-Aldrich Ltd. All of the above chemicals were used as received.

Synthesis of Layered Molybdenum (Meta)phosphate [MoO2(PO3)2]

Using hexaammonium molybdate [(NH4)6Mo7O24·4H2O] and triphenylphosphine (C18H15P) in EG under a nitrogen atmosphere, layered molybdenum (meta)phosphate was synthesized. In a 250 mL double-necked round-bottom flask, 10 mL of EG was refluxed in an inert atmosphere. Presonicated (5 min) 1.00 g of (NH4)6Mo7O24·4H2O in 10 mL of EG was added with a syringe to this preheated EG solution, and the solution turned lemon in color. Then, presonicated (5 min) 1.48 g of C18H15P in 10 mL of EG was added dropwise to the round-bottom flask using a syringe. The color of the solution changed from lemon to pale yellow. The reaction mixture was refluxed for 2 h under an inert atmosphere with vigorous stirring. The brown-color product was cooled, centrifuged at 4800 ppm, washed with methanol, and dried naturally.

Characterizations

MoO2(PO3)2 with a 2θ value ranging from 8 to 90° was studied using an Xpert PRO PANalytical X-ray diffractometer (XRD) with Cu K radiation. The vibrational modes in MoO2(PO3)2 were investigated using a Perkin Elmer Spectrum One FTIR spectrophotometer. A Perkin-Elmer LS 55 Luminescence spectrometer was used to measure the photoluminescence of MoO2(PO3)2. Using a JEOL JSM-840 field emission scanning electron microscope (FE-SEM) with a 20 kV operating voltage, a textural study of elemental mapping and energy-dispersive X-ray analysis (EDAX) was performed. On a Philips-CM 200 with an operating voltage of 20–200 kV, high-resolution transmission electron microscopy (HRTEM) imaging was obtained. The composition was studied using X-ray photoelectron spectroscopy (XPS) with an AXIS Supra spectrometer (Kratos Analytical Ltd, U.K.) having an Al Kα source (hν = 1486.6 eV). Absorption spectroscopy, photoreduction, photodegradation, and trapping experiments were performed on a UV–vis spectrophotometer (UV-1800 PC Shimadzu) in the wavelength scan limit of 200–800 nm with a fast scan speed and a slit of 1 nm. The surface area of MoO2(PO3)2 was recorded using the Brunauer–Emmett–Teller (BET) technique with the help of the SMART SORB 93, Smart Instruments Co. Pvt. Ltd, India. The electron spin resonance (ESR) analysis was performed on the ESR-JES-FA200 ESR spectrometer.

Photoreduction Experiment of Cr6+ to Cr3+

The photoreduction of Cr6+ to Cr3+ ions in solution was accomplished using the multilayered molybdenum (meta)phosphate material that was synthesized. To begin, a 100 ppm hexavalent chromium solution in distilled water was made using potassium dichromate (K2Cr2O7) powder. The photocatalytic reduction performance of MoO2(PO3)2 was investigated, with formic acid acting as a hole scavenger.[37] To test the photocatalytic activity, 20 mg of the catalyst, 40 mL of 100 ppm K2Cr2O7 solution, and 4 mL of formic acid were sonicated to disperse the catalyst and then left in the dark for 30 min to achieve adsorption–desorption equilibrium. For the photoreduction study in the UV–visible spectrophotometer, a 3 mL aliquot was taken at regular intervals. At the end of each cycle, a centrifuge was used to recover the catalyst from the solution.

Photodegradation of Methylene Blue (MB)

The performance of the layered molybdenum (meta)phosphate material in the degradation of methylene blue has also been studied. In a 100 mL beaker, an aqueous solution of methylene blue (40 mL, 10 ppm) and 20 mg of MoO2(PO3)2 were used.[38] The mixture was sonicated to ensure uniform dispersion of the catalyst, and it was kept in the dark for 30 min to maintain adsorption–desorption equilibrium. A 3 mL aliquot was placed in a cuvette and tested for photodegradation using a UV–visible spectrophotometer. The catalyst was then recovered by washing it with distilled water and drying it in a hot air oven set to 60 °C.

Results and Discussion

An X-ray diffraction pattern was used to examine the brown material. The XRD pattern of MoO2(PO3)2 with an orthorhombic structure is shown in Figure (ICDD: 01-074-1389).[34] The diffraction peaks can be assigned to the orthorhombic phase of MoO2(PO3)2. Noteworthy Miller indices are observed at (100), (110), (210), (021), (220), (002), (112), etc. The presence of broad peaks in XRD implies the presence of smaller particles. Debye–Scherrer’s formula was used to calculate the average particle size: D = Kλ/β cos θ, where D denotes the average crystallite size (nm), K denotes the dimensionless shape factor with a value of 0.89, λ denotes the X-ray diffraction wavelength, β stands for the full width at half-maximum, and θ is the Bragg angle. The average particle size using the diffraction peak (100) was found to be 7.72 nm. Figure a depicts a low-resolution scanning electron image of the layered MoO2(PO3)2 material. The layered material’s morphology was found to be granularly stacked one on top of the other, similar to grains. Figure b depicts the layered material morphology after the first cycle of Cr(VI) photoreduction. In the IR spectrum, the peak due to (P–Oint) is observed at 995 cm–1, which is in good agreement with the reported value (Figure S1).[35] Energy-dispersive spectrometry (EDS) elemental mapping was accomplished for the elements Mo, O, and P, which were distributed uniformly (Figure ).

Figure 1

X-ray diffraction pattern of orthorhombic layered molybdenum (meta)phosphate [MoO2(PO3)2].

Figure 2

SEM images (a) and (b) of MoO2(PO3)2, their elemental mapping images, and EDAX.

HRTEM was used to study the morphology of the MoO2(PO3)2 nanoscale layered material. The layered structure of MoO2(PO3)2 is depicted in Figure a. Figure b,c shows that the atomic lattices in the HRTEM image are crystalline. The value for d-spacing of the layered MoO2(PO3)2 nanoscale material was found to be 0.25 nm, which is attributed to the (220) plane.

Figure 3

(a) TEM image, (b) HRTEM image, and (c) HRTEM image of layered molybdenum (meta)phosphate.

For the photoreduction of hexavalent chromium, a UV–visible spectrophotometer was used. The UV–visible spectrum of MoO2(PO3)2 in isopropanol is shown in Figure a. Figure b represents the Tauc plot, which was obtained by plotting the αhν versus hν. In Tauc’s relation, α was calculated with the help of an equation, i.e., α = 4πA/λ [where A is the absorbance and λ is the wavelength]. In this graph, the optical band gap was calculated by extrapolation of the curves on the X-axis (energy axis). The calculated optical band gap for the obtained layered MoO2(PO3)2 was found to be 3.31 eV. The photoluminescence of MoO2(PO3)2 was recorded in isopropanol and was found to be a broad spectrum at 470 nm with an excitation wavelength of 330 nm (Figure ). The ESR activity of MoO2(PO3)2 was investigated using the ESR spectrum to check for the presence of unpaired electrons (Figure S3).[35,39]

Figure 4

(a) UV–visible spectrum of molybdenum (meta)phosphate in isopropanol and (b) Tauc’s plot.

Figure 5

PL spectrum of MoO2(PO3)2 in isopropanol (excitation wavelength = 330 nm).

The elemental composition analysis of the Mo, P, and O elements of MoO2(PO3)2 was performed using XPS. Figure a shows the molybdenum 3d3/2 and 3d5/2 at 231.44 and 228.27 eV.[40] For oxygen 1s, a peak was observed at 529.42 eV (Figure b), and for phosphorous 2p, a peak was observed at 136.79 eV (Figure c). Figure d shows the overall survey spectrum of MoO2(PO3)2.

Figure 6

XPS spectra of (a) Mo 3d3/2 and 3d5/2, (b) O 1s, (c) P 2p, and (d) wide spectrum of MoO2(PO3)2.

Photoreduction of Cr6+ to Cr3+

The primary goal of this research is to use a nontoxic, less expensive, and highly stable material as a photocatalyst. The surface area of MoO2(PO3)2 nanocomposites was determined to be 15.34 m2/g.[41] MoO2(PO3)2 acts as a photocatalyst due to its adequate surface area and absorption in the UV–visible region in the presence of sunlight. We used a layered nanoscale MoO2(PO3)2 material as a photocatalyst in this study, which demonstrated the complete reduction efficiency of chromium solution in the presence of natural sunlight. Figure A depicts the photoreduction of Cr6+ to Cr3+ using the layered MoO2(PO3)2 nanoscale material with methanoic acid acting as a hole scavenger. The entire reduction of Cr6+ ions from the solution was observed in 6 min. A rapid decrease in peak intensity around 349.50 nm indicates the conversion of toxic Cr6+ to less toxic Cr3+ by MoO2(PO3)2.[18] The addition of sodium hydroxide solution resulted in the green-colored precipitate of Cr(OH)3, indicating that the entire toxic form of chromium was converted to trivalent chromium. The X-ray diffraction pattern of used MoO2(PO3)2 after the first cycle was studied to check the stability of the catalyst, and it was found to be similar to the bare MoO2(PO3)2 (Figure S2). The kinetics of the reaction was investigated by plotting a graph of the ratio of the chromium concentration at time t (Ct) to the original concentration (C0) (i.e., Ct/C0), as shown in Figure C. When the reactant is scarce, the improved Langmuir–Hinshelwood expression is transformed into the form ln(C0/Ct) = kt(38) (where k = rate constant). The kinetics of a pseudo-first-order reaction was confirmed by plotting ln(C0/Ct) versus time, as shown in the inset image in Figure C. The photocatalytic degradation efficiency of the MoO2(PO3)2 catalyst was calculated as (A0 – A)/A0 × 100% (A0 and A = absorbance of the chromium solution at time = 0 and 6 min, respectively).[21] The photocatalyst’s stability and reusability are demonstrated in Figure E. We conducted chromium reduction in the UV–visible cabinet to compare the effects of sunlight and the UV–visible reactor. The reduction of Cr6+ to Cr3+ in the UV–visible cabinet is depicted in Figure S4. It took 9 min to complete, which is comparable to natural sunlight.

Figure 7

(A) Photoreduction of Cr6+ to Cr3+. (B) Photodegradation of methylene blue. (C) Plot of (a) Ct/C0 and (b) ln C0/Ct (inset image) versus indication time for Cr6+ to Cr3+ reduction. (D) Plot of (a) Ct/C0 and (b) ln C0/Ct (inset image) versus indication time for methylene blue degradation. (E) and (F) Reduction efficiency (%) for Cr6+ to Cr3+ reduction and for methylene blue degradation.

The chemical potential values for the donor and acceptor species are critical for understanding the ability and mechanism of Cr6+ reduction. As a result, using eqs and 2, the band-gap (Eg) value and Mulliken’s electronegativity are used to calculate the valence (EVB) and conduction band (ECB) potentials.[37]

In the above equations, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), Eg is the band gap (3.31 eV) of MoO2(PO3)2, which was determined by the extrapolation line on the energy axis in the Tauc plot, and χ is the electronegativity of Mo, O, and P elements. The number of atoms is represented by a, b,... while A, B,.. are the ions present in the compound. From eqs and 2, the calculated conduction band (ECB) potential and the valence band (EVB) potential are 0.7136 and 3.7536 V versus the standard hydrogen electrode (SHE). In a valence band, the reaction between generated holes and OH– gives rise to the active •OH radical. Here, the reduction potential value for Cr2O72–/Cr3+ (+1.33 eV) is detected in between the valence and conduction band potential of MoO2(PO3)2. All of the above conditions are favorable for an enhancement of the reduction of the Cr6+ to Cr3+ event.[42] With the observed experimental conditions mentioned above and reported, the conversion of the Cr6+ to Cr3+ mechanism is as shown below.[43] The UV–visible light absorption property of MoO2(PO3)2 leads to charge separation (eq ), and methanoic acid aids in lowering the recombination of holes and electrons to form CO2 and H+ (eq ).[43] The role of formic acid was examined in its absence for the photoreduction of Cr6+ to Cr3+ (Figure S5). In the absence of formic acid, the reduction of Cr6+ to Cr3+ did not occur within the time frame specified in Figure A. To inspect the role of formic acid, ethanol was used as a hole scavenger. Figure S6 shows that negligible degradation was observed, indicating that formic acid is a better hole scavenger than ethanol.[44]

Electrons present in the conduction band could react with Cr6+ to get Cr3+ (eqs and 8).[43]Figure depicts a plausible schematic illustration of the Cr6+ to Cr3+ mechanism.

Figure 8

Plausible mechanism for photoreduction of Cr6+ to Cr3+ using MoO2(PO3)2 under natural sunlight irradiation.

We attempted to use MoO2(PO3)2 for representative MB dye degradation after achieving promising results in the chromium reduction experiment. Figure B depicts the UV–visible spectra of methylene blue dye photodegradation in the presence of natural sunlight. The absorption wavelength peak at 664 nm caused by MB was greatly reduced after 36 min, indicating methylene blue dye degradation.[37] The 95% photodegradation of 10 ppm of methylene blue was achieved in 36 min under sunlight.[21] The order of photodegradation of methylene blue was studied using a modified Langmuir–Hinshelwood equation (Figure D(a)). The graph of ln(C0/Ct) versus solar light exposure time t in Figure D(b) shows that the degradation follows pseudo-first-order kinetics. The catalyst’s stability was tested over four consecutive cycles. Figure F shows a histogram of the percent degradation of MB over four cycles, i.e., 78, 77, 76, and 74%.

One of the reported plausible mechanisms is discussed further.[37] Under natural sunlight (eq ) illumination, holes and electrons are generated during photocatalytic degradation processes.

The oxidation of the adsorbed water molecule on the catalyst produces •OH radicals at the valence band (eqs to 12).

Photogenerated electrons react with adsorbed oxygen at the conduction band potential, resulting in superoxide anion radicals (O2•–), and H2O2 is formed in the presence of adsorbed oxygen and hydrogen (eqs and 14). Furthermore, H2O2 reacts with electrons in the conduction band, resulting in •OH radicals (eq ). Figure depicts a plausible schematic illustration of the MB mechanism.

Figure 9

Plausible mechanism for the photodegradation of MB using MoO2(PO3)2 under natural sunlight irradiation.

According to the literature, nanomaterials such as metal oxides, nanocomposites, and metal phosphate are beneficial for the photodegradation of methylene blue. The representative examples were tabulated (Table ) with the amount of catalyst used, the concentration of MB solution used, the reduction time, and the source of light.

Table 1

Comparison of Catalytic Degradation of MB with Different Catalysts

catalyst used	amount of catalyst	MB concentration	photodegradation time (min)	source of light	refs
CA/TPNC	100 mg	2 × 10–5 M	150	sunlight	(45)
CeO2/g-C3N4	0.05 g	10 mg L–1	180	UV-light	(46)
Mn3O4/ZnO/Eu2O3	15 mg	5 ppm	150	sunlight	(47)
Ag3PO4/NC	0.018 g	37.5 ppm	105	sunlight	(48)
CTHS	5 mg	10 mM	30	sunlight	(49)
MoO2(PO3)2	20 mg	10 ppm	36	sunlight	present work

To understand the mechanism, it is essential to comprehend the generated radicals and their roles in the degradation of MB. The reactive species trapping experiment was carried out to provide evidence for the existence of the main active species responsible for photocatalytic degradation of MB using MoO2(PO3)2 (Figure ). Typically, EDTA·2Na was used for h+, ascorbic acid for superoxide anion radicals O2•–, and dimethyl sulfoxide (DMSO) for •OH radicals as scavengers in the MB solution. The addition of EDTA·2Na (2 mL, 1 mM) as a hole scavenger reduced the photodegradation of methylene blue when exposed to sunlight.[36]Figure S7 depicts the photocatalytic degradation of MB by MoO2(PO3)2 in the presence of EDTA·2Na as a hole scavenger. In the presence of ascorbic acid, MB photodegradation was significantly reduced. Photocatalytic degradation of MB by MoO2(PO3)2 in the presence of DMSO as a •OH scavenger is shown in Figure S8. When DMSO is added, OH is the major active species contributing to the photocatalytic behavior of MoO2(PO3)2. It also implies that the active species in the oxidation of the adsorbed dye is the •OH radical.[50] These results are in good agreement with Bi(PO4).[36] The HO2• radical may also act as an active radical in several reactions during the process. In addition to ascorbic acid (2 mL, 1 mM), the majority of •OH radicals (Figure S9) may be converted into HO2• (eqs and 17).[36]

Figure 10

Plots of photogenerated carriers trapping in the system of photodegradation of methylene blue on MoO2(PO3)2.

As a result, if HO2• aids in the oxidation of methylene blue, the degradation rate would be faster in the case of ascorbic acid, which is not observed during the process (addition of the O2•– scavenger), indicating that O2•– is important in the photodegradation process.[36] Furthermore, the reaction of O2•– with ascorbic acid in the presence of a proton generates H2O2 and ascorbate free radicals (eq ). It would also aid in the production of •OH radicals, as mentioned in eq .[51] No photocatalytic reduction of Cr6+ to Cr3+ by MoO2(PO3)2 in the absence of sunlight was observed (Figure S10), indicating that MoO2(PO3)2 can act as a stable, cheaper, and noble-metal-free photocatalyst.

Conclusions

In this article, layered MoO2(PO3)2 was synthesized in a single step using the solvothermal method. We used layered molybdenum (meta)phosphate as a photocatalyst material for the first time. In PL spectroscopy, a broad spectrum at 470 nm was observed. The obtained material is ESR-active, so unpaired electrons may be present in it. Under natural sunlight, 20 mg of photocatalyst Cr6+ was reduced to Cr3+ in 6 min under optimal operating conditions such as 4 mL of formic acid. In the photoreduction of toxic chromium, formic acid was used as a hole scavenger. Furthermore, under natural sunlight, 78% photodegradation of methylene blue dye (40 mL, 10 ppm MB, and 20 mg of catalyst) was observed. By performing reactive species trapping experiments, the existence of O2•–, •OH, and h+ species, as well as the leading role of O2•– species, was confirmed. O2•– plays an important role in the fast degradation of methylene blue. The layered molybdenum (meta)phosphate photocatalyst is stable, inexpensive, and reusable.