Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37

Synthesis of AMoO4 (A = Ca, Sr, Ba) photocatalysts and their potentialapplication for hydrogen evolution and the degradation of tetracyclinein water

Ali M. Huerta-Floresa,b, I. Juárez-Ramíreza, Leticia M. Torres-Martíneza,*,J. Edgar Carrera-Crespoa, T. Gómez-Bustamantea,c, O. Sarabia-Ramosa,d

aUniversidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N, CiudadUniversitaria, San Nicolás de los Garza, Nuevo León, C.P. 66455, Mexicob Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Manuel Nava #6, San Luis Potosí, S.L.P. 78290, Mexicoc Facultad de Ciencias Químicas, División de Ciencias Exactas y Naturales, Universidad Estatal de Sonora, Boulevard Luis Encinas y Rosales, S/N, Col. Centro,Hermosillo, Sonora, C.P. 83000, Mexicod Facultad de Ciencias Químico Biológicas, Universidad Autónoma de Sinaloa, Calzada de las Américas Norte 2771, Burócrata, Culiacán de Rosales, Sinaloa,C.P. 80030, Mexico

A R T I C L E I N F O

Article history:Received 28 October 2017Received in revised form 15 December 2017Accepted 18 December 2017Available online 19 December 2017

Keywords:MolybdatesScheelite structureHydrogen evolutionTetracycline degradation

A B S T R A C T

Alkaline earth metal molybdates AMoO4 (A = Ca, Sr, Ba) were synthesized by the traditional solid-statereaction. The structural, morphological, textural and optical properties of metal molybdates wereobtained by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmet-Teller (BET)method, diffuse reflectance UV–vis spectroscopy (UV–vis DRS), and photoluminescence spectroscopy(PL). Electrochemical characterization through OCP and EIS measurements was performed to elucidatethe photoresponse and electrical properties of metal molybdates. Energy band diagrams obtained bytheoretical calculations and experimental data revealed that the synthesized molybdates presentconvenient band positions to be used in photocatalytic processes. For this reason, these compounds weretested as catalysts in photoinduced processes such as hydrogen production and tetracycline degradation.Accordingly to results, the studied molybdates were able to produce hydrogen (�240 mmol g�1 h�1)under UV light irradiation and also these materials provoked the degradation of tetracycline (85% in purewater and 97% with the addition of H2O2). In conclusion, CaMoO4, SrMoO4, and BaMoO4 are suitablephotocatalysts for water splitting and degradation of organic compounds.

© 2017 Published by Elsevier B.V.

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Journal of Photochemistry and Photobiology A:Chemistry

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1. Introduction

The evaluation of materials with potential photocatalyticproperties for water splitting and the degradation of organiccompounds has attracted considerable attention in the last years.This is because of the interest to use clean fuels to decrease the airpollution [1], as well as the removal of complex organic moleculesfrom wastewater that conventional treatment methods cannoteliminate [2]. Regarding the above mentioned, the splitting ofwater using semiconductor materials with high photocatalyticactivity under UV and visible light illumination is a good way togenerate H2, a recyclable clean fuel and potential energetic vector[3,4]. Also, these materials can be used to catalyze the degradation

* Corresponding author.E-mail address: leticia.torresgr@uanl.edu.mx (L.M. Torres-Martínez).

https://doi.org/10.1016/j.jphotochem.2017.12.0291010-6030/© 2017 Published by Elsevier B.V.

of tetracycline from wastewater, which is an emerging pollutantdifficult to be degraded using conventional treatment processes[5,6].

Scheelite-structured molybdates are some of the materials thathave been investigated in recent years due to their attractiveproperties and potential application as photocatalysts [7]. In fact,PbMoO4 has shown a high photocatalytic activity in the degrada-tion of indigo carmine, orange G, rhodamine B, methyl orange,salicylic acid, ciprofloxacin, and tetracycline, as well as hydrogenproduction using ethanol as sacrificial agent, in aqueous solutionsunder UV and UV–vis light irradiation [8,9]. Particularly, Scheelite-structured compounds that have shown a suitable photocatalyticperformance in the photodegradation of methyl blue and methylorange [7],10,11], are the alkaline earth molybdates with theformula AMoO4 (A = Ca, Sr, Ba). From these compounds, onlySrMoO4 has been evaluated as an electrocatalyst in acid water forhydrogen evolution [12]. Moreover, AMoO4-based compounds,

30 A.M. Huerta-Flores et al. / Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37

such as Ln3+-doped CaMoO4 and the silver dispersed BaMoO4

doped with Er3+, Yb3+, and K+, have demonstrated a convenientphotocatalytic activity for antibacterial applications as well asdegradation of rhodamine B and ibuprofen [13–15]. Although thesealkaline earth molybdates have been extensively evaluatedfor different applications such as solid-state phosphors, cryogenicscintillation detectors, Li-ion batteries, oxygen sensing devices,optical detectors, and laser-host materials, among others[12,16–18], their application as materials for photocatalytic watersplitting and degradation of emerging water contaminants, likepharmaceuticals and anabolics, have been scarcely studied. For thisreason, in this work, it is proposed by the first time the evaluationof Scheelite-structured alkaline earth molybdates in the photo-catalytic degradation of tetracycline and hydrogen productionfrom pure water without the use of sacrificial agents. We preparedthe family of alkaline-earth molybdates AMoO4 (A = Ca, Sr, Ba)through solid-state method. Also, the structural, morphological,textural, optical and electrochemical properties of these materialsare integrally discussed and correlated with their photocatalyticactivity.

2. Experimental

2.1. Synthesis of the photocatalysts

The alkaline-earth molybdates were prepared through a solid-state method. Firstly, equimolar quantities of metal carbonates(99% Sigma-Aldrich) and MoO3 (99.5%, Sigma-Aldrich) were mixedand homogenized in an agate mortar. Then, the powder wastransferred to an alumina crucible and a thermal treatment of800 �C for 10 h was applied, using a heating rate of 3 �C per minute.

2.2. Characterization of samples

The structural properties of the samples were analyzed by X-raydiffraction, in a Bruker D8 Advance diffractometer (CuKaradiation(l = 1.5406 Å), from 10 to 70� in 2u angle). The crystallite size wasestimated by the Scherrer equation: L = kl/bcos(u), considering Las the crystallite size, k as the Scherrer constant (0.89), l is thewavelength of the X-ray radiation (0.15418 nm), and b is the fullwidth at half maximum (FWHM). The morphological analysis ofthe samples was performed in a scanning electron microscope(SEM-JEOL, 6490LV) in the secondary electrons mode. The opticalproperties were studied in a UV–vis NIR spectrophotometer (Cary5000) coupled with an integration sphere for the measurement ofdiffuse reflectance, using BaSO4 as standard. The band gap (Eg) wascalculated extrapolating the absorption onset to the x-axis of theplot and introducing the wavelength in the equation: Eg = 1240/l.To study the recombination process in the semiconductorpowders, a photoluminescence analysis was performed in afluorescence spectrophotometer Agilent Cary Eclipse, using anexcitation wavelength of 254 nm and a scanning speed of1000 nm/min. The textural properties of molybdates were deter-mined in a BELSORP mini II (BEL Japan), degassing the samples 3 hat 300 �C. The Brunauer-Emmet-Teller (BET) method wasemployed to estimate the surface area of the materials.The(photo)electrochemical characterization and electrochemical im-pedance spectroscopy (EIS) measurements were performed usinga potentiostat/galvanostat AUTOLAB model PGSTAT302N,equipped with a Frequency Response Analyzer (FRA) module. Tocarry out these analyses, the photocatalysts were supported onIn2O3/Au/Ag coated PET film (Delta Technologies, PF-65IN). Briefly,50 mg of the synthesized alkaline-earth molybdate was mixedwith 30 mL of Nafion (LIQUion, 5% wt.) and 0.5 mL of isopropylalcohol. The mixture was homogenized by sonication during15 min and deposited on the conductor substrate through drop-

casting method, then it was dried at room temperature. Thiselectrode with an exposed area to the electrolyte of 0.5 cm2, wasused as the working electrode, a Pt wire and an Ag/AgCl (3 M, KCl)were used as counter-electrode and the reference electrode,respectively. Photoelectrochemical measurements were done in aconventional three-electrode cell, and a pen-ray lamp was used asillumination source (UVP, 254 nm, and 4400 mW/cm2). An aqueoussolution of 0.5 M Na2SO4 was used as the electrolyte. The EIScharacterizations were carried out under dark conditions, with ACperturbation of � 10 mV. The Mott-Schotkky plots were obtainedthrough potentiodynamic EIS, at a frequency of 1000 Hz in apotential window with no faradaic currents. To obtain Nyquistplots, a potentiostatic EIS was carried out at OCP potential in afrequency range of 100 kHz to 1 Hz.

2.2.1. Photocatalytic testsThe activity of the materials for the photocatalytic hydrogen

evolution was studied in a 250 mL glass reactor with an innerquartz tube. The powder (0.1 g) was dispersed in deionized waterunder stirring. The sample was deaerated bubbling nitrogen during15 min. After that, the reactor was sealed and the irradiation timebegan, using a pen-ray lamp (UVP, 254 nm and 4400 mW/cm2). Thehydrogen evolved was monitored online every 30 min during 3 h ina Thermo Scientific gas chromatograph equipment with a thermalconductivity detector (TDC) and a fused silica capillary column(30 m long, 0.53 mm width); N2 gas was used as carrier. On theother hand, the degradation of organic compounds in water withthe prepared molybdates as catalysts was studied using theantibiotic tetracycline as model molecule. For this test, 200 mL of atetracycline solution (10 ppm) was placed in a glass reactorprovided with an internal quartz tube, where a pen-ray lamp (UVP,254 nm and 4400 mW/cm2) is placed (Fig. S1, Supportinginformation). Then, 0.1 g of the photocatalyst was added to thesolution under stirring. The reaction was left in darkness during 1 hto study the adsorption process, and after that, the lamp wasturned on. Finally, 10 mL aliquots were taken every 30 min during4 h, and then centrifuged and analyzed in a UV–vis spectropho-tometer (Perkin Elmer Lambda 25) to determine the absorbance ofthe samples at 375 nm. Additional experiments were performed tostudy the effect of H2O2 on the degradation efficiency oftetracycline over the metal molybdates, for this, H2O2

(100 mg/L) was incorporated in the reaction media. The otherconditions of the experiments were identical to the describedpreviously.

3. Results and discussion

3.1. X-ray diffraction

XRD patterns of the AMO4 (A = Ca, Sr, Ba) samples prepared bysolid-state method are presented in Fig. 1. Through this analysis, itwas corroborated the formation of pure isostructural phases ofalkaline earth molybdates with Scheelite structure AMX4; exhibit-ing well-defined peaks, characteristic of crystalline materials. Allsamples crystallized in a body-centered tetragonal phase withspace group I41/a, associated to the JCPDS 01-085-0585, 01-085-0809 and 08-0455 cards, for CaMoO4, SrMoO4, and BaMoO4,respectively. No preferential orientation was observed in thesamples. The increase in the ionic radii of metals (Ca2+ = 0.112 nm,Sr2+ = 0.125 nm, Ba2+ = 0.142 nm) generates a change in the chargedensity in MO8 polyhedral in the Scheelite structure, causing anincrease in the lattice parameters and a shifting of the peakstoward smaller angles [19].

The crystallite size was calculated using the Scherrer equationand the values obtained are shown in Table 1. The tendency of thecrystallite size of the samples is as follows: BaMoO4

Table 1Summary of the structural and physicochemical properties of the photocatalysts studied in this work.

Material Eg (eV) ECB (eV) SBET (m2/g) Crystallite size (nm) H2 evolution (mmol/g h) % Tetracycline degradation

In pure water With addition of H2O2

CaMoO4 3.54 �0.8 � 1 219 237 84 91SrMoO4 3.84 �1.0 197 140 79 88BaMoO4 3.97 �0.9 245 246 85 97

Fig. 1. XRD diffraction patterns of CaMoO4, SrMoO4, and BaMoO4.

A.M. Huerta-Flores et al. / Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37 31

(245 nm) > CaMoO4 (219 nm) > SrMoO4 (197 nm). These resultssuggest that BaMoO4 exhibited high crystallinity, which isfavorable for an efficient transport of charges to the surface ofthe metal molybdate for better photocatalytic redox reactions.

3.2. Scanning electron microscopy

Fig. 2 (a–c) shows the SEM images of CaMoO4, SrMoO4, andBaMoO4, synthesized by solid-state reaction. All materials presentgrains of regular size; a homogeneous grain size distributionpromotes good optical and photocatalytic properties [19]. The sizeof the CaMoO4 grains is similar to the observed in SrMoO4, around1 mm, while BaMoO4 grains are larger than 1.5 mm, even somegrains of 2.3 mm can be appreciated. As discussed in Section 3.1, theanalysis of the XRD patterns revealed a decrease in the volume ofthe cell with a reduction in the ionic radii. Theoretically, the grainsize of the samples would exhibit the following tendency:BaMoO4> CaMoO4> SrMoO4 [20]. This was corroborated in the

Fig. 2. SEM images of (a) CaMoO4,

SEM images of the samples synthesized in this work, also similarbehavior has been reported in other works [20].

The morphology of the samples is characteristic of non-poroushigh crystalline materials obtained by the traditional solid-statemethod. The uniformity of the shades and shapes observed in theSEM images corroborate the formation of the pure phases, as it wasconfirmed by the XRD analysis. In a photocatalytic process,particles of small and regular size are preferred, mainly consider-ing the transport path of the photogenerated charges from the bulkto the surface of the photocatalyst, where the redox reactionsinvolved occur.

3.3. UV–vis diffuse reflectance spectroscopy

Fig. 3 presents the absorption spectra of (a) CaMoO4, (b)SrMoO4, and (c) BaMoO4, respectively. It can be appreciated thatthe samples exhibit a considerable absorption below 300 nm.Reports in the literature about the optical properties of the

(b) SrMoO4, and (c) BaMoO4.

Fig. 3. UV–vis diffuse reflectance spectra of (a) CaMoO4, (b) SrMoO4, and (c)BaMoO4.

Fig. 4. Nitrogen physisorption isotherms of CaMoO4, SrMoO4, and BaMoO4.

32 A.M. Huerta-Flores et al. / Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37

alkaline-earth molybdates suggest that this band can beattributed to electronic transitions in the MoO4

2� complexand the creation of an excitonic state in A2+ (A = Ca, Sr, Ba)[20,21]. Accordingly to this result, these semiconductors can beexcited only by UV light to produce electron and hole pairs.Through the UV–vis spectra, the band gap (Eg) values werecalculated (see Table 1). Results showed that Eg values exhibitthe following tendency: BaMoO4 (3.97 eV) > SrMoO4

(3.84 eV) > CaMoO4 (3.54 eV), which corresponds to the reportedin the literature for these materials [20]. As can be appreciated

in Fig. 3 (a), the absorption of BaMoO4 is higher than theobserved in SrMoO4 and CaMoO4 under UV light. This behavioris favorable for the photocatalytic processes since a strongerabsorption is necessary to generate a higher amount of electron-hole pairs to carry out the redox reactions. In this case, BaMoO4

can absorb more photons compared to SrMoO4 and CaMoO4, andit could exhibit better photocatalytic activity.

3.4. Physisorption analysis

Fig. 4 shows the adsorption-desorption isotherms of thecalcium, strontium and barium molybdates prepared in this work.The materials showed characteristic type II isotherm, which iscommonly observed in non-porous materials prepared at hightemperatures by solid-state reaction. From these isotherms, it ispredictable low surface areas in all the samples. In this case, resultspresented in Table 1 showed that surface areas were close to1 m2 g�1. No significant differences were observed for thisparameter calculated in the materials, which is due to the similarcondition of annealing (800 �C) employed in the synthesis.

3.5. Photoluminescence analysis

To study the recombination process in the molybdate-basedphotocatalysts, a photoluminescence analysis was performed.Fig. 5 shows the emission spectra of the alkaline-earth molybdatesexcited under 254 nm. The photoluminescence emission isintrinsically related to the crystalline structure of the material.De Azevedo-Marques et. al., suggest that the PL emission isadjudicated to the MoO4

2� complex, where cations Ca2+, Sr2+, andBa2+ acts as lattice modifying agents [20], affecting the photo-luminescence property directly.

In our case, the highest emission intensity was observed for theSrMoO4 sample, followed by CaMoO4 and BaMoO4. As it is wellknown, a high PL intensity is related to a high recombination of thephotogenerated charges, which limits the efficiency of thephotocatalytic process. Therefore, accordingly to PL results,BaMoO4 could be presenting the lowest electron-hole pairrecombination; it is expected that this photocatalyst exhibitshigher catalytic efficiency than the other molybdates. This result isdirectly related with the crystallinity observed in the samples,BaMoO4 exhibits higher crystallinity than SrMoO4 and CaMoO4,implying a better crystal growth in this material, and a reduced

Fig. 5. PL spectra of CaMoO4, SrMoO4, and BaMoO4 excited under 254 nm.

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amount of defects, which promote the electron-hole recombina-tion. A greater crystallinity improves the charge transport in thecatalyst and also enhances the photocatalytic activity.

3.6. Electrochemical characterization

In order to elucidate the photoresponse of the three alkalineearth molybdates synthesized, the (photo)electrochemicalcharacterization was performed, supporting the materials ona conductive PET substrate (see Experimental section). Fig. 6ashows the variation of the OCP vs. time, where it is observedthat these materials exhibited good stability in the aqueoussolution under dark conditions. Additionally, it is appreciatedthat the OCP value is kept practically constant, showing CaMoO4

and SrMoO4 a similar OCP value and BaMoO4 a more negativevalue. Considering that in the dark the OCP is governed by thefilm/solution interface [22]; this behavior could be associatedwith the particle size since the calcium and strontiummolybdates present particles with a similar size and the bariummolybdate showed the highest particle size, around 2 mm(Fig. 2). On the other hand, a displacement of the OCP towardsmore negative potentials is exhibited for the three materialsafter UV light irradiation. This result indicates an electronaccumulation in their conduction band, a typical behavior of the

Fig. 6. (a) Time variation of the open circuit potential (OCP) in the dark and under UV lighmolybdates.

n-type semiconductors [22,23]. After that, CaMoO4 and SrMoO4

samples reached a maximum negative potential under illumina-tion, and then the OCP value tends to potentials that are morepositive. The SrMoO4 sample is recovering at a value near to theobtained in the dark (Fig. 6a), evidencing a higher recombinationof the photogenerated charge carriers in this sample, comparedto CaMoO4, as was indicated in the photoluminescence analyses(Fig. 5). BaMoO4 sample, which did not show an OCP potentialdecay, exhibited the lowest PL emission. When the UV-lightillumination was interrupted, the accumulated electrons in theconduction band are recombined with the holes in the valenceband or with the species in the solution, causing an OCPvariation towards more positive potentials [22]. To clarify if therapid OCP variation is associated with a fast transfer to thesolution or a high recombination of the charge carriers after theillumination was blocked, a characterization by potentiostaticEIS analysis was carried out. The Nyquist plots obtained byCaMoO4 and SrMoO4 samples showed a similar arc radii,whereas SrMoO4 sample exhibited the largest arc radii(Fig. 6b). This result indicates that the charge transport andtransference resistance are higher in SrMoO4, favoring the chargecarriers recombination and, therefore, diminishing its photo-catalytic activity [24,25].

3.7. Theoretical and experimental calculation of the energy bandpositions

Through the theoretical equation described in the experi-mental section, the values of the conduction and valence bandwere estimated (Fig. 7). Also, a potentiodynamic EIS was carriedout to obtain the energy band positions of the materials froman experimental method. The Mott-Schottky plots obtained bythis electrochemical technique are presented in Fig. 8, where allthe materials exhibited a positive slope in the linear region ofthe Csc

�2 vs. E, a typical behavior of n-type semiconductors [22],as it was determined by OCP measurements under illumination(Fig. 6a). The flat band potential (Efb) was determined from theintersection of this slope with the x-axis in y = 0, and thenconsidered as the conduction band (CB) in all the studiedmolybdates, since they are n-type semiconductors [23]. Theexperimental energy band diagrams were constructed from theestimation of the CB by EIS, as well as calculating the valenceband (VB) value by adding to CB the band gap value determinedfrom the UV–vis diffuse reflectance spectra (Fig. 3). In Fig. 7 arepresented the proposed theoretical and experimental energy

t and, (b) Nyquist plots obtained at OCP vs. Ag/AgCl in the dark, for the alkaline earth

Fig. 7. (a) Theoretical and (b) experimental energy band position diagrams of the molybdate-based photocatalysts studied in this work.

34 A.M. Huerta-Flores et al. / Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37

band position diagrams of the alkaline-earth molybdate photo-catalysts studied in this work. The trend shown in the energyband positions is similar, with the CaMoO4 obtaining the morepositive BC value. As can be observed in both diagrams, all thesemiconductors exhibited a suitable conduction and valenceband position to perform the reduction and oxidation of water,respectively. Also, the band conduction position of these threemolybdates is convenient to form the superoxide radicals [26],which participate along with the holes in the photocatalyticdegradation of the tetracycline [27]. These results indicate thatthe alkaline earth molybdates can be used as photocatalysts for

Fig. 8. Mott-Schottky plots obtained through potentiodynamic EIS at 1000 Hz, for the alkflat band potential.

the hydrogen evolution reaction and the photodegradation oftetracycline.

3.8. Photocatalytic activity

The activity for hydrogen evolution under UV light ofthe molybdate-based catalysts prepared in this work is showedin Fig. 9(a). Results demonstrated that the synthesized materialsare active catalysts for hydrogen evolution from water splittingreaction, without the use of sacrificial agents. The catalystsexhibited stable activity during 3 h. The average hydrogen

aline-earth molybdates. The traced line shows the slope considered to estimate the

Fig. 9. (a) Hydrogen evolution of molybdate-based photocatalysts under UV light. (b) Average hydrogen evolution.

A.M. Huerta-Flores et al. / Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37 35

evolution rate is summarized in Fig. 9(b) and Table 1. The activity ofthe molybdates showed the following tendency: BaMoO4

(246 mmol g�1h�1) > CaMoO4 (237 mmol g�1h�1) > SrMoO4

(140 mmol g�1h�1). On the other hand, the materials were alsotested in the degradation of tetracycline, due to the suitableposition of the conduction and valence bands to produce holes andoxygen active species, which play a main role in the decompositionmechanism of tetracycline molecule [27]. The curves of degrada-tion are shown in Fig.10 (a), where it can be appreciated the changein the concentration with the time of irradiation; the photolysis oftetracycline under UV light is also presented. Results showed that24% of tetracycline was degraded by only the effect of the light after3 h. Additionally, with the incorporation of the catalysts, more than79% of degradation was achieved. These values are summarized inTable 1. BaMoO4 showed the highest percentage of degradation(85%) followed by CaMoO4 (84%) and SrMoO4 (79%). Moreover, anadditional source of oxygen was added in the media (H2O2) toevaluate the effect on the degradation efficiency of tetracycline.These results are shown in Fig. 10 (b). As can be observed in thisfigure, the degradation efficiency was improved with the additionof H2O2, due to the beneficial effect of this electron acceptorthrough (i) the additional production of �OH and (ii) the reductionof the recombination of the electron-hole pairs in the surface of thephotocatalysts, since holes are consumed by H2O2 to produce O2

and H+ [28]. Degradation efficiencies of 91, 88 and 97% wereachieved in three hours of reaction for CaMoO4, SrMoO4, andBaMoO4, respectively, increasing the activity of the materials morethan 10%. A summary of these results is shown in Fig. 11.

Fig. 10. Photocatalytic degradation of tetracycline using calcium, strontium an

The highest activity exhibited by BaMoO4 is in agreement withthe behavior predicted from the crystallinity (XRD) and electron-hole recombination (electrochemical and PL analysis). BaMoO4

exhibited the highest photocatalytic activity because it is thesample with the highest crystallite size and the lowest recombi-nation. According to these results, in the case of the evaluatedmolybdates, other parameters such as the grain size, surface areaand the potential of the conduction band, were not determinantfactors, since BaMoO4, the material with the highest crystallinityand the lowest charge carrier recombination showed the bestphotocatalytic efficiency. Based on the energy band positiondiagrams and the photocatalytic activity reached for the molyb-dates, the possible reaction mechanism for the hydrogen evolutionand tetracycline degradation is shown in Fig. 12.

4. Conclusions

It is reported by the first time the evaluation of Scheelite-structured alkaline earth molybdates AMoO4 (A = Ca, Sr, Ba)prepared by solid-state reaction in photocatalytic processes. Theenergy band diagrams of these compounds indicate that allmolybdates are suitable for the hydrogen evolution and generationof active species for tetracycline degradation. Moreover, the resultsobtained in both photocatalytic processes revealed that CaMoO4,SrMoO4, and BaMoO4, exhibited a considerable photoactivityunder UV-light irradiation, using aqueous solutions withoutsacrificial agents. Particularly, BaMoO4 exhibits higher crystallinityand lower photoluminescence emission, related to a diminished

d barium molybdates in (a) pure water and (b) with the addition of H2O2.

Fig. 11. Summary of the photocatalytic degradation efficiency (%) of tetracycline over the metal molybdates in pure water and with the addition of H2O2.

Fig. 12. Proposed mechanism of the photocatalytic hydrogen evolution and tetracycline degradation over AMoO4 (A = Ca, Sr, Ba) materials.

36 A.M. Huerta-Flores et al. / Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 29–37

electron-hole recombination and enhanced the charge transport,which improves the photocatalytic activity. This boosts the rangeof applications of these materials and makes them potentialcandidates to be used in water splitting and emerging pollutantsdegradation on a large scale.

Acknowledgments

The authors would like to thank CONACYT (CB-256795-2016,CB-2014-237049, INFRA-2015-252753, PN-2015-01-105, PN-2015-01-487, PN-2015-01-610, NRF-2016-278729, and PhD Scholarship386267), SEP (PROFOCIE-2014-19-MSU0011T-1, PRODEP-103.5/15/14156), UANL (PAICYT 2015), and FIC-UANL (PAIFIC 2015-5).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at https://doi.org/10.1016/j.jphotochem.2017.12.029.

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