Facet-, composition- and wavelength-dependent photocatalysis of Ag₂MoO₄

Abstract

Faceted β-Ag₂MoO₄ microcrystals are prepared by controlled nucleation and growth in diethylene glycol (DEG) or dimethylsulfoxide (DMSO). Both serve as solvents for the liquid-phase synthesis and surface-active agents for the formation of faceted microcrystals. Due to its reducing properties, truncated β-Ag₂MoO₄@Ag octahedra are obtained in DEG. The synthesis in DMSO allows avoiding the formation of elemental silver and results in β-Ag₂MoO₄ cubes and cuboctahedra. Due to its band gap of 3.2 eV, photocatalytic activation of β-Ag₂MoO₄ is only possible under UV-light. To enable β-Ag₂MoO₄ for absorption of visible light, silver-coated β-Ag₂MoO₄@Ag and Ag₂(Mo_0.95Cr_0.05)O₄ with partial substitution of [MoO₄]²⁻ by [CrO₄]²⁻ were prepared, too. The photocatalytic activity of all the faceted microcrystals (truncated octahedra, cubes, cuboctahedra) and compositions (β-Ag₂MoO₄, β-Ag₂MoO₄@Ag, β-Ag₂(Mo_0.95Cr_0.05)O₄) is compared with regard to the photocatalytic decomposition of rhodamine B and the influence of the respective faceting, composition and wavelength.

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Introduction

Nanosized molybdenum and tungsten oxides have proven to be interesting with regard to a wide range of properties, including catalysis, applicability in high-power batteries and sensing, and luminescence.¹ Beside the binary MoO₃ and WO₃,² ternary molybdate and tungstate phases are intensely studied as well (e.g., CaWO₄, CdMoO₄, CoMoO₄).³ With regard to their properties, photocatalysis can be considered as particularly important. Special attention has been recently paid to molybdates/tungstates with closed-shell and lone-pair containing metal cations like Pb(II), Sn(II) or Bi(III) due to their unique electronic and structural features mediated by the “non-bonding” electron pairs.^3f,4 In this regard, especially, bismuth molybdate and bismuth tungstate have been intensely studied.⁵ For efficient photocatalysis, moreover, faceted microcrystals have turned out to be most promising.⁶

Aiming at molybdates/tungstates with closed-shell cations, silver molybdate has been also classified to be an catalytically interesting material.⁷ The cubic β-Ag₂MoO₄ exhibits a direct band gap of 3.2 eV,⁸ which is too high in energy for visible-light activation, whereas a band gap of 1.26 eV was reported for the tetragonal α-Ag₂MoO₄ phase.⁹ However, α-Ag₂MoO₄ is metastable and was yet only described twice as a nanomaterial.^9,10 The thermodynamically most stable β-Ag₂MoO₄ is already available with different morphologies, including spheres, rods, cylinders as well as octahedra and cube-like shapes.¹¹ To this concern, microemulsions,^11b hydrothermal and solvothermal synthesis were applied with water, methanol or ethanol heated in autoclaves up to temperatures of 180 °C.^8,11a,c–e Shape control and formation of specific facets were predominately initiated by high-molecular-weight surface-active agents like as polyvinylpyrrolidone (PVP) or 2,3-bis(2-pyridyl)pyrazine (BPP).^10,11a,c–e Despite of successful shape control, such high-molecular-weight stabilizers suffer from the necessity of their removal after the synthesis, since the photocatalytically active surface is otherwise blocked by the stabilizer. Motivated by the great interest in photocatalytic nanomaterials, we have already studied niobates, molybdates and tungstates such as Au@Nb@H_xK_1−xNbO₃ nanopeapods,¹² CuMoO₄ nanoparticles,¹³ or faceted β-SnWO₄.¹⁴ In this study, we report on a controlled liquid-phase synthesis of faceted β-Ag₂MoO₄ microcrystals, the chromate-driven red-shift of the light absorption as well as on the facet-, composition- and wavelength-depending photocatalysis.

Experimental

Material synthesis

General aspects. Silver nitrate (AgNO₃, p.a., VWR), potassium molybdate (K₂MoO₄, 98%, ABCR), potassium chromate (K₂CrO₄, 99%, ABCR), diethylene glycol (DEG, C₄H₁₀O₃, 99%, Alfa Aesar) and dimethylsulfoxide (DMSO, C₂H₆OS, 99.5%, Sigma-Aldrich) were used as purchased. Zwitterionic rhodamine B (RhB, C₂₈H₃₁ClN₂O₃, 99%, Acros Organics) was used as model dye for photocatalytic dye degradation without further purification.

DEG-/DMSO-mediated synthesis of β-Ag₂MoO₄ microcrystals. K₂MoO₄ (0.5 mmol) was dissolved in 50 mL of the liquid phase (DEG or DMSO/H₂O, Table 1). In the case of chromate substitution, 0.025 mmol of K₂MoO₄ were exchanged by K₂CrO₄. After dissolution of K₂MoO₄, an aqueous solution of 1 mmol of AgNO₃ in 2 mL of deionized water was injected into the molybdate solution. The temperature of injection was adjusted to 0, 50 or 80 °C (Table 1) with an ice bath or a heating mantle. Subsequent to AgNO₃ injection, a suspension was instantaneously obtained and allowed to come to room temperature within 1.0 or 2.5 hours (Table 1). For the heated samples, the round-bottomed flask was left inside the heating mantle for slow cooling (2.5 hours), whereas the flask was immediately removed from the heating mantle for fast cooling (1.0 hours). The resulting suspensions were centrifuged and the solid purified by repeated resuspension and centrifugation in/from water, which was performed three times. Finally, the β-Ag₂MoO₄ samples were dried at room temperature.

Table 1 Parameter of synthesis to obtain faceted β-Ag₂MoO₄ microcrystals

Morphology	Type of solvent	Temperature of injection/°C	Time of cooling/h	Composition
				Sample color
Spheres	DEG	80	1.0	β-Ag2MoO4@Ag
				Greyish
Truncated octahedra	DEG	80	2.5	β-Ag2MoO4@Ag
				Greyish
Spheres	DMSO/H2O	0–80	1.0–2.5	β-Ag2MoO4
	20 : 80 to 80 : 20			Colorless
Cubes	DMSO/H2O	0	2.5	β-Ag2MoO4
	85 : 15			Colorless
Cuboctahedra	DMSO/H2O	50	2.5	β-Ag2MoO4
	85 : 15			Colorless
Cuboctahedra	DMSO/H2O	50	2.5	Ag2(Mo0.95Cr0.05)O4
	85 : 15			Yellow

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Analytical methods

Field emission scanning electron microscopy (FESEM, Zeiss Supra 40VP equipped with energy dispersive X-ray spectrometer, EDXS) was used to determine the morphology of the as-prepared faceted β-Ag₂MoO₄ microcrystals. The microcrystals were suspended in ethanol, dropped and dried on sliced pieces of a silicon wafer (5 × 5 mm).

Crystallinity and chemical composition of the faceted β-Ag₂MoO₄ microcrystals were examined by XRD (Stoe STADI-MP diffractometer, equipped with Ge-monochromator and Cu-K_α1 radiation, 40 kV, 40 mA). About 10 mg of the microcrystals were deposited onto acetate foil and fixed with scotch tape.

To determine the zeta potential of microcrystal β-Ag₂MoO₄ suspensions, a Zetasizer Nano ZS from Malvern Instruments was used. About 1 mg of the microcrystals was suspended in water and ultrasonicated for 5 min prior to the analysis.

UV-Vis spectra of β-Ag₂MoO₄ microcrystals were recorded on a UV 2700 spectrometer (Shimadzu). 4–8 mg of the microcrystals were mixed with 100–120 mg of dried BaSO₄ (spectroscopic grade) and measured against dried BaSO₄ as a reference. The determination of the band gap was conducted based on the Kubelka–Munk formalism and Tauc plots.

Fourier-transform infrared (FT-IR) spectroscopy was performed with a Bruker Vertex 70 FT IR spectrometer. To this concern, the microcrystals were grounded with dried KBr (1–2 mg with 400 mg KBr) and pressed for 15 min under 50 kN.

Volumetric nitrogen sorption measurements (BEL BELSORP-max) were carried out at 77 K with nitrogen as analyte. According to the BET formalism (BET: Brunauer–Emmett–Teller) the specific surface area was deduced. Prior to the analysis, the microcrystalline samples were dried at 120 °C in vacuum.

Photocatalytic measurements

The photocatalytic dye degradation was performed with a Shimadzu Excalibur measuring head linked to the aforementioned UV-Vis spectrometer. For measurement, the quartz flask contained an aqueous suspension of the respective faceted β-Ag₂MoO₄ microcrystals (20 mg in 110 mL; 0.18 g L⁻¹) and RhB (0.20 mg L⁻¹). This suspension was continuously mixed with a magnetic stirrer. For photocatalytic degradation, a LOT Class ABA solar simulator (type LS0805) was applied that emits a spectrum of AM 1.5 G solar light (100 mW cm⁻²). Beside illumination with simulated sunlight, cut-off filters (Horiba, <350 nm and <400 nm) were applied to cut-off UV or UV and blue light. Prior to illumination, the suspensions were magnetically stirred in the dark for 1 h to ensure homogenous distribution of the photocatalyst and the dye. All experiments were carried out at room temperature and in air. The course of the RhB degradation was continuously monitored by comparing the intensity of the strongest dye absorption (λ_RhB = 554 nm) with the initial absorption at the beginning of the experiment. The obtained transmission-time plots were fitted by linear or allometric fits. For reasons of clarity, only every tenth raw data point is shown.

Results and discussion

Morphology control

Based on our knowhow on the synthesis of molybdate and tungstate nanoparticles in high-boiling alcohols (so-called polyol synthesis)^13,15 and the realization of faceted microcrystals of β-SnWO₄,¹⁴ we have tried to prepare Ag₂MoO₄ nanoparticles via a polyol-mediated approach. For this purpose, diethylene glycol (DEG) was chosen (Fig. 1a). In general, the advantage of DEG as a water-comparable but high-boiling solvent relates to its moderately coordinating properties, so that additional high-molecular-weight surface-active agents are not required. K₂MoO₄ was selected as starting material containing the required tetrahedral [MoO₄]²⁻ anion. Here, it needs to be noticed that starting materials containing octahedral (MoO₆) building units (e.g. (NH₄)₆Mo₇O₂₄) show inappropriate solubility and often result in incomplete reactions with several by-products.

Fig. 1 Synthesis of β-Ag₂MoO₄@Ag in diethylene glycol (DEG): (a) scheme of synthesis; (b) SEM image of microspheres after fast cooling; (c) photo of powder sample; (d) SEM image of truncated octahedra after slow cooling with surface-allocated Ag(0) nanoparticles stemming from DEG-driven reduction; (e) selected truncated octahedron with silver spots.

For the synthesis, a concentrated aqueous solution of AgNO₃ was injected at a temperature high enough for the crystallization of β-Ag₂MoO₄ (≥80 °C) but low enough to avoid instantaneous DEG-driven reduction of Ag⁺ (<100 °C) (Table 1, compare Fig. 5a). Fast cooling (1.0 hour) indeed results in highly crystalline β-Ag₂MoO₄ but predominately yields non-faceted, spherical particles with diameters of 200–300 nm (Fig. 1b). Slow cooling (2.5 hours), in contrast, supports facet formation and results in truncated β-Ag₂MoO₄ octahedra with edge lengths of 100 nm (Fig. 1d). For both the spherical particles and the truncated octahedra, however, certain formation of silver could not be avoided. Its formation is qualitatively indicated by the greyish color of the samples (Fig. 1c) and can be quantified by UV-Vis spectra displaying the characteristic plasmon-resonance absorption of nanosized silver (Table 1, compare Fig. 5b).¹⁶ Moreover, silver nanoparticles, 32 ± 13 nm in diameter, are clearly visible on SEM images as bright spots on the surface of the β-Ag₂MoO₄ microcrystals (Fig. 1e). Since visible light could be absorbed via the plasmon-resonance absorption, these Ag(0) nanoparticle-coated β-Ag₂MoO₄ microcrystals (Ag₂MoO₄@Ag) nevertheless be relevant for photocatalysis.

Aiming at pure, faceted β-Ag₂MoO₄ microcrystals without elemental silver, we have modified the synthesis. In order to maintain the polarity of the solvent in regard of the solubility of the starting materials but to avoid the reducing properties of DEG, we have selected a mixture of dimethylsulfoxide (DMSO) and water as alternative liquid phase (Fig. 2a). Highly crystalline β-Ag₂MoO₄ was indeed obtained upon injection of aqueous AgNO₃ into a solution of KMoO₄ in DMSO/water at a temperature of 0 to 50 °C (Table 1). The shape of the resulting Ag₂MoO₄ turned out to be highly dependent on the DMSO : water ratio. Thus, more-or-less spherical microcrystals were formed at high water contents (Table 1, Fig. 2b and c). Optimal conditions for the formation of well-faceted microcrystals including cuboctahedra and cubes were observed for a DMSO : H₂O ratio of 85 : 15 (Table 1 and Fig. 2d). In addition to the formation of specific crystal facets, the size of the microcrystals was significantly reduced from >5 μm to 1–2 μm upon decreasing the amount of water (Fig. 2d). The water content is limited to a minimum of 15% due to the poor solubility of K₂MoO₄. Finally, it should be noticed that the formation of elemental silver was prevented with these experimental conditions of the DMSO-mediated synthesis. Thus, the β-Ag₂MoO₄ microcrystals are colorless and do not show any plasmon-resonance absorption (Table 1 and Fig. 2e, compare Fig. 5b). They also do not show Ag(0)-related spots on electron-microscopy images (Fig. 2b–d). Small Ag(0)-related spots only appear on electron-microscopy images after certain period of exposition under the electron beam (compare Fig. 4).

Fig. 2 Synthesis of β-Ag₂MoO₄ in dimethylsulfoxide (DMSO): (a) scheme of synthesis; (b–d) SEM images of microparticles obtained at different DMSO : H₂O ratios. Spots of Ag(0) arise due to electron-beam irradiation; (e) photo of powder sample.

To obtain cuboctahedra and cubes – beside the aforementioned water content – the temperature for particle nucleation turned out to be relevant as well (Table 1 and Fig. 3a). Thus, injecting aqueous AgNO₃ at 0 °C results in β-Ag₂MoO₄ cubes (Fig. 3b), whereas β-Ag₂MoO₄ cuboctahedra were formed after AgNO₃ injection at 50 °C (Fig. 3c). Thereof, the cubes expose {100} crystal facets only, whereas the cuboctahedra show mixed {100} and {111} crystal facets. Again, the β-Ag₂MoO₄ microcrystals are colorless and do not show any plasmon-resonance absorption (Table 1 and Fig. 3d, compare Fig. 5b), and they also do not show Ag(0)-related spots on electron-microscopy images (Fig. 3b,c) if the exposition under the electron beam was not too long (Fig. 4).

Fig. 3 Synthesis of faceted β-Ag₂MoO₄ in dimethylsulfoxide (DMSO): (a) scheme of synthesis; (b) SEM image of cubes after AgNO₃ injection at 0 °C; (c) SEM image of cuboctahedra after AgNO₃ injection at 50 °C (both with DMSO : H₂O = 85 : 15). Spots of Ag(0) arise under irradiation with the electron beam; (d) photo of powder sample.

Fig. 4 Evolution of Ag(0)-related spots, up to 10 ± 4 nm in diameter, due to electron-beam-driven reduction on the surface of β-Ag₂MoO₄ cubes after 60 seconds.

Crystallinity and optical properties

All faceted β-Ag₂MoO₄ microcrystals – independent of their respective synthesis in DEG or DMSO – are highly crystalline (Fig. 5a). X-ray powder diffraction (XRD) displays all characteristic Bragg peaks of β-Ag₂MoO₄.¹⁷ In fact, this finding is expected since faceted shapes need to be single crystalline for facet formation. The optical properties were characterized by UV-Vis spectroscopy. Accordingly, the β-Ag₂MoO₄ samples show a steep absorption edge between 330 and 370 nm (Fig. 5b), which can be related to the valence band-to-transition band excitation. Based on Tauc plots, the absorption of DMSO-made β-Ag₂MoO₄ can be ascribed to a direct band gap with an energy of 3.2 eV, which is well in agreement with literature data (3.14 eV).⁸ For the DEG-made β-Ag₂MoO₄, the band gap is slightly reduced to 3.0 eV. In contrast to β-Ag₂MoO₄ made in DMSO, moreover, those samples prepared in DEG show a weak and broad plasmon-resonance absorption at 420 to 750 nm due to the presence of Ag(0) nanoparticles (Fig. 5b). In regard of photocatalysis, these β-Ag₂MoO₄@Ag microcrystals can be promising in terms of visible-light absorption as already outlined by Li et al.^11d

Despite of a potential plasmon-resonance-driven visible-light absorption of β-Ag₂MoO₄@Ag, we intended to evaluate other options to enable β-Ag₂MoO₄ for visible-light absorption in absence of Ag(0) nanoparticles. To this concern, partial substitution of [MoO₄]²⁻ by [CrO₄]²⁻ seemed promising. On the one hand, [CrO₄]²⁻ is known for its bright yellow to orange color indicating light absorption in the blue spectral regime. Moreover, both complex anions are of similar size as indicated by solid solution series Ag₂(Cr,Mo)O₄ with complete substitution of [MoO₄]²⁻ by [CrO₄]²⁻ and vice versa.¹⁸ On the other hand, the blue-light absorption of [CrO₄]²⁻ relates to a ligand-to-metal charge-transfer (LMCT) of a finite complex anion, which – in the case of partial substitution – is not necessarily affecting the band gap of the infinite lattice of the molybdate. Pure Ag₂CrO₄, finally, was reported to have a reddish black color, which is less optimal for photocatalytic application.^11b

As a result of the aforementioned considerations, we intended a substitution of 5 mol% of chromate in β-Ag₂MoO₄. Ag₂(Mo_0.95Cr_0.05)O₄ was prepared similar to β-Ag₂MoO₄ and, for instance, results in cuboctahedra with similar size and shape as observed for pure β-Ag₂MoO₄. In contrast to colorless β-Ag₂MoO₄, Ag₂(Mo_0.95Cr_0.05)O₄ indeed exhibits a bright yellow color (Fig. 5b). UV-Vis spectra show a steep absorption for Ag₂(Mo_0.95Cr_0.05)O₄, which is now red-shifted (470–570 nm) in comparison to the β-Ag₂MoO₄ cuboctahedra (Fig. 5b). The band gap of Ag₂(Mo_0.95Cr_0.05)O₄ was deduced to 2.1 eV, which lays in between of the values of β-Ag₂MoO₄ (3.14 eV)¹⁸ and Ag₂CrO₄ (1.75 eV).^11b,19 This finding points to an active role of chromate for the band gap of Ag₂(Mo_0.95Cr_0.05)O₄.

Fig. 5 Crystallinity and optical absorption of the as-prepared β-Ag₂MoO₄ microcrystals: (a) XRD (with bulk β-Ag₂MoO₄, ICSD no. 00-070-1497 as a reference) and (b) UV-Vis spectra of truncated Ag₂MoO₄@Ag octahedra (made in DEG; blue; plasmon-resonance absorption indicated by blue star), Ag₂MoO₄ cubes (made in DMSO; red), Ag₂MoO₄ cuboctahedra (made in DMSO; violet), Ag₂(Mo_0.95Cr_0.05)O₄ cuboctahedra (made in DMSO; green) with photos of the respective powder samples.

Photocatalytic examination

For a comparative evaluation of the photocatalytic activity of the different β-Ag₂MoO₄ microcrystals in dependence of their respective surface faceting and chemical composition, the respective experiments were performed with comparable conditions. Thus, identical concentrations of the respective β-Ag₂MoO₄ microcrystals (0.18 g L⁻¹) were suspended in water, and identical concentrations of zwitterionic rhodamine B (RhB, 2 mg L⁻¹, 4.18 μmol L⁻¹) were added. RhB was selected, on the one hand, since its absorption (500–600 nm) is different from the absorption of β-Ag₂MoO₄ (<500 nm). On the other hand, the adhesion of the zwitterionic RhB on the photocatalyst surface is independent from eventual charging of the respective surface of the photocatalyst. Additional criteria for the photocatalytic performance of the β-Ag₂MoO₄ microcrystals were examined as well (Table 2). First of all, the crystallinity and size of the β-Ag₂MoO₄ microcrystals (about 0.5 μm) can be considered as very comparable. Nitrogen sorption analysis also indicated the specific surface area to be similar (Table 2). Finally, the zeta potential was quantified to evaluate a potential influence of surface charging. As a result, all β-Ag₂MoO₄ microcrystals show negative charging ranging from −33 to −43 mV at pH = 7 (Table 2). Finally, any photoluminescence as potential competitive loss process to photoluminescence could be excluded (ESI: Fig. S1†). Taken together, several material parameters – including concentrations, size, crystallinity, surface area and surface charging – are similar, so that a direct correlation of the photocatalytic activity to the type of surface faceting and chemical composition is possible.

Table 2 Comparison of the relevant material properties and the photocatalytic activity of the as-prepared β-Ag₂MoO₄ microcrystals

Composition (synthesis)	Morphology with exposed facets	Zeta potential/mV	Specific surface area/m^2 g^−1	Band gap/eV	Photo-catalytic activity
Ag2MoO4@Ag (DEG)	Truncated octahedra {111} + {100}	−43	1	3.0	Poor
Ag2MoO4 (DMSO)	Cubes {100}	−37	2	3.2	Medium
Ag2MoO4 (DMSO)	Cuboctahedra {111} + {100}	−33	1	3.2	High
Ag2(Mo,Cr)O4 (DMSO)	Cuboctahedra {111} + {100}	−35	1	2.1	Medium

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To examine the photocatalytic activity of the β-Ag₂MoO₄ microcrystals, the absorption of RhB was continuously monitored at λ_RhB = 554 nm under artificial sunlight (AM 1.5 G solar light, 100 mW cm⁻²) (Fig. 6 and 7). First of all, the autophotolysis of RhB (in absence of β-Ag₂MoO₄) was verified and showed no considerable effect within the significance of the experiment. Thereafter, the different β-Ag₂MoO₄ microcrystals were studied and evidence a significantly higher photocatalytic activity as compared to the autophotolysis of RhB. It should also be noticed that the size and shape of the faceted microcrystals was maintained subsequent to the photocatalytic reaction (ESI: Fig. S2†).

With simulated sunlight, the β-Ag₂MoO₄ cuboctahedra made in DMSO show the highest photocatalytic activity, which is about twice to thrice as high as for all other faceted microcrystals (Fig. 6). The β-Ag₂MoO₄@Ag truncated octahedra made in DEG show the lowest activity, followed by β-Ag₂MoO₄ cubes made in DMSO. In regard of the surface faceting, the photocatalytic activity of {111} crystal facets is obviously higher as compared to {100} crystal facets (Fig. 6). Such situation is well known for metal oxides such as Cu₂O, γ-Al₂O₃ or Co₃O₄ and was related to the higher surface energy of the {111} crystal facets.²⁰ Since cubes exhibit only {100} facets, they show the lowest photocatalytic activity. The β-Ag₂MoO₄ cuboctahedra, having the highest area of {111} crystal facets, consequently exhibit the highest photocatalytic activity.

Fig. 6 Photocatalytic evaluation of β-Ag₂MoO₄ cuboctahedra and cubes as well as truncated β-Ag₂MoO₄@Ag octahedra depending on the type of surface facets (c₀(RhB): 0.18 g L⁻¹; c(β-Ag₂MoO₄): 0.20 mg L⁻¹; autophotolysis of RhB as a reference, illumination with simulated sunlight; every tenth raw data point shown).

Whereas the aforementioned β-Ag₂MoO₄ cubes and cuboctahedra only become photoactive upon absorption of UV photons (<390 nm due to a band gap at 3.2 eV), the truncated β-Ag₂MoO₄@Ag octahedra and Ag₂(Mo_0.95Cr_0.05)O₄ cuboctahedra are suitable for visible-light-driven photocatalysis, in principle. However, β-Ag₂MoO₄@Ag already under simulated sunlight with high-energy UV photons absorbed by β-Ag₂MoO₄@Ag shows low performance (Fig. 6). The low activity of β-Ag₂MoO₄@Ag that we observed is in contrast to former studies suggesting a considerable effect of Ag(0) particles on the photocatalytic activity of β-Ag₂MoO₄, which was claimed to be driven by plasmon-resonance absorption even at low wavelength (<420 nm)^11d or with significantly larger Ag(0) nanoparticles (80–100 nm).²¹ Due to its low performance, β-Ag₂MoO₄@Ag was not examined further in our study.

A comparison of the β-Ag₂MoO₄ cuboctahedra as the most active photocatalyst under simulated sunlight (Fig. 6 and 7) and the Ag₂(Mo_0.95Cr_0.05)O₄ cuboctahedra (Fig. 7), finally, shows an increased relative photocatalytic activity of Ag₂(Mo_0.95Cr_0.05)O₄ the longer the wavelength of the irradiated light is. Thus, illumination only with visible light >400 nm results in a higher photocatalytic activity of the Ag₂(Mo_0.95Cr_0.05)O₄ cuboctahedra as compared to the β-Ag₂MoO₄ cuboctahedra (Fig. 7). Such effect of a chromate-shifted optical absorption of a molybdate-based photocatalyst is shown for the first time and offers an additional option for visible-light-driven photocatalysis of metal molybdates and tungstates.

Fig. 7 Photocatalytic evaluation of β-Ag₂MoO₄ and Ag₂(Mo,Cr)O₄ cuboctahedra depending on the wavelength of illumination with simulated sunlight as well as with cut-off filters for UV light (cut-off <350/<400 nm, c₀(RhB): 0.18 g L⁻¹; c(β-Ag₂MoO₄): 0.20 mg L⁻¹; autophotolysis of RhB as a reference; every tenth raw data point shown).

Conclusions

Faceted β-Ag₂MoO₄ microcrystals are prepared by controlled nucleation and growth in diethylene glycol (DEG) and dimethylsulfoxide (DMSO). Both serve as solvents for the liquid-phase synthesis and surface-active agents for the formation of faceted microcrystals. Due to the reducing properties, truncated β-Ag₂MoO₄@Ag octahedra are obtained in DEG. The synthesis in DMSO allows avoiding the formation of elemental silver and results in β-Ag₂MoO₄ cubes and cuboctahedra. Whereas the colorless β-Ag₂MoO₄ (band gap: 3.2 eV) can be only activated by UV-light, β-Ag₂MoO₄@Ag (with Ag(0)-based plasmon-resonance absorption) as well as β-Ag₂(Mo_0.95Cr_0.05)O₄ (with partial substitution of [MoO₄]²⁻ by [CrO₄]²⁻) are suitable for visible-light excitation.

Based on the realized faceting and chemical composition – truncated octahedra, cubes, cuboctahedra, β-Ag₂MoO₄, β-Ag₂MoO₄@Ag, β-Ag₂(Mo_0.95Cr_0.05)O₄ – the photocatalytic properties were exemplarily monitored for the degradation of rhodamine B. As a result, the β-Ag₂MoO₄ cuboctahedra showed the highest photocatalytic activity under simulated sunlight, which could be related to their {111} crystal facets that turned out to be more active than {100} crystal facets. β-Ag₂MoO₄@Ag showed the lowest photocatalytical activity at all. The photocatalytic activity of Ag₂(Mo_0.95Cr_0.05)O₄ cuboctahedra turned out to be the higher in comparison to β-Ag₂MoO₄ cuboctahedra the longer the wavelength of the irradiated light is. The facet-, composition- and wavelength-depending photocatalytic activity generally contributes to the fundamental understanding of photocatalysis. The partial incorporation of chromate into molybdate-based photocatalysts and the resulting shift of the optical absorption are shown for the first time and can be of general interest for visible-light-driven photocatalysis of metal molybdates and tungstates.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Deutsche Forschungsgemeinschaft (DFG) for funding of equipment. Moreover, L. W. acknowledges the Studienstiftung des deutschen Volkes for scholarship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Data related to photoluminescence and shape of catalyst before and after photocatalysis. See DOI: 10.1039/d0ra02953j

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