Journal of Photochemistry and Photobiology A: Chemistry 345 (2017) 21–26

Novel visible light-driven PbMoO4/g-C3N4 hybrid composite withenhanced photocatalytic performance

D.B. Hernández-Urestia,*, D. Sanchez-Martinezb, L.M. Torres-Martinezb

aUniversidad Autónoma de Nuevo León, CICFIM – Facultad de Ciencias Físico Matemáticas, Av. Universidad S/N, Cd. Universitaria, 66455 San Nicolás de losGarza, N. L., MexicobUniversidad Autónoma de Nuevo León, Facultad de Ingeniería Civil – Departamento de Ecomateriales y Energía, Cd. Universitaria, 66455 San Nicolás de losGarza, N. L., Mexico

A R T I C L E I N F O

Article history:Received 24 December 2016Received in revised form 20 April 2017Accepted 10 May 2017Available online 11 May 2017

Keywords:CiprofloxacinPbMoO4/g-C3N4

PhotocatalysisHybrid composite

A B S T R A C T

Novel visible-light-driven PbMoO4/g-C3N4 hybrid composites were synthesized with loadings 0, 20, 50,80 and 100 wt.% of PbMoO4 by sonochemical method. The PbMoO4/g-C3N4 hybrid composites werecharacterized by X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (DRS), photo-luminescence (PL), scanning electron microscopy (SEM) and the BET method. The photocatalytic activityof the PbMoO4/g-C3N4 hybrid composites were evaluated using ciprofloxacin as pharmaceuticalpollutant model. The as-prepared PbMoO4/g-C3N4 (50/50 wt.%) improved the photocatalytic activity thanthe pure g-C3N4 and PbMoO4 nanoparticles. A photocatalytic mechanism and the kinetics of PbMoO4/g-C3N4 hybrid composites were also proposed. The photoactivity of the PbMoO4/g-C3N4 (50/50 wt.%) hybridcomposite was found to increase by 11 times than of the pure g-C3N4. The total organic carbon (TOC)analysis of samples irradiated revealed that mineralization of organic compounds by the action ofPbMoO4/C3N4 was feasible in CIP (70%) of simulated sunlight irradiation.

© 2017 Elsevier B.V. All rights reserved.

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Journal of Photochemistry and Photobiology A:Chemistry

journal homepa ge: www.elsev ier .com/ locate / jphotochem

1. Introduction

Photocatalysis has attracted close attention as a promisingenvironmental remediation compared to conventional wastewatertreatment [1–4]. Semiconductors such as TiO2 [5–7], PbMoO4 [8–10], ZnO [11–13] have been studied extensively by manyresearches. However, these photocatalyst are seriously problemswith the solar spectra due to low solar energy conversion efficiencydue to limited to ultraviolet visible (UV) light that is less than 4% ofsolar irradiance at the Earth's surface. Therefore, is necessary todevelop an efficient photocatalysts with be activated under visiblelight as WO3 semiconductor [14,15]. Recently, novel polymericsemiconductor graphitic carbon nitride (g-C3N4) was gain consid-erable attention due to its potential in photocatalysis reactions, anappropriate band gap (2.8 eV) as photocatalyst under visible lightand high chemical stability. Our previous work demonstrated thatg-C3N4 could be degrade of several pharmaceutical compoundsunder UV–Vis irradiation [16]. Others applications reported of thispolymeric photocatalyst we can find the bacterial disinfection[17,18], elimination of organic pollutants [19,20], reduction of CO2

* Corresponding author.E-mail address: ing.dianahdz@gmail.com (D.B. Hernández-Uresti).

http://dx.doi.org/10.1016/j.jphotochem.2017.05.0131010-6030/© 2017 Elsevier B.V. All rights reserved.

to hydrocarbon fuels [21,22], the hydrogen and oxygen production[23,24] and selective synthesis of organic compounds [25,26].However, g-C3N4 has a low efficiency in the separation ofphotogenerated electron–hole pairs causing a low photocatalyticactivity [27]. Consequently, the synthesis of hybrid composites bycoupling polymer g-C3N4 with inorganic semiconductors could begenerated an efficiency photocatalyst with higher photoactivityactivated under visible light irradiation. In this study, we provide ahybrid composite consisting of g-C3N4 and PbMoO4 (PbMoO4/C3N4) for the photodegradation of ciprofloxacin under UV–Visiblelight irradiation.

2. Experimental

2.1. Preparation of powder

The preparation of the g-C3N4 photocatalyst which wasreported previously [16]. 4 g of melamine was annealed at500 �C for 4 h in a covered crucible. Then, the g-C3N4 was obtainedwith yield was approximately 50%. PbMoO4 was prepared by asonochemical method: 0.005 mol of Pb(NO3)2 (99% purity, SigmaAldrich) and 0.005 mol of H2MoO4 (85% purity, Sigma Aldrich),were separately dissolved in distilled water. Then, the twosolutions were mixed to give a white precipitate. The pH of

Fig. 1. X-ray diffraction patterns (a), the crystalline size and lattice strain (b) of thePbMoO4/g-C3N4 composites.

22 D.B. Hernández-Uresti et al. / Journal of Photochemistry and Photobiology A: Chemistry 345 (2017) 21–26

resulting solution was adjusted to 4 with HNO3 using pH meter andthen with vigorous stirring during 10 min. Afterwards, the solutionwas transferred to ultrasonic bath during 2 h at room temperaturewith ultrasonic frequency was set at 37 kHz. The resulting powderwas washed several times with water and ethanol, dried at 70 �Cfor overnight.

The g-C3N4/PbMoO4 hybrid composites were prepared by an insitu sonochemical method. The precursor for PbMoO4 was mixedwith a g-C3N4 at different stoichiometric, and the Pb2+ ion can beon the surface of g-C3N4 under constant stirring at roomtemperature. Then, H2MoO4 solution was added, Pb2+ ions wereconverted into PbMoO4 nanoparticles on the surface of g-C3N4. Inthis way, the different PbMoO4 (wt.%)/g-C3N4 photocatalysts (wt. 0,20, 50, 80, 100) were obtained, respectively.

2.2. Materials characterization

PbMoO4/g-C3N4 powder was characterized by X-ray powderdiffraction using a Bruker D8 advanced diffractometer with CuKaradiation (l = 1.5418 Å) coupled with a Vantec high speed detectorand Ni filters. The diffraction data were registered between 10� and60� (2u) with a scanning rate of 0.05� s�1. The crystallite size wascalculated by X-ray line broadening analysis using the Scherrerequation. Nitrogen adsorption was measured at evaluated at�196 �C after a pretreatment at 50 �C for 12 h using a Bel-JapanMinisorp II Surface Area & Pore Size analyzer. The specific surfacearea (BET) were calculated according to the nitrogen adsorption–desorption isotherm. Diffuse reflectance measurements (DRS)were carried out using a UV–Vis spectrophotometer modelEvolution 220 Thermo Sci. equipped with an integrating sphereattachment. The analysis range was from 350 to 600 nm, and BaSO4

was used as a reflectance standard. For the estimation of the bandgap value from the UV–Vis spectra, a straight line was extrapolatedfrom the absorption curve to the abscissa axis. The surfacemicrostructures of the hybrid composites were examined by usinga FEI Nova 200 Nano-SEM scanning electron microscope operatedat low vacuum.

2.3. Photodegradation of ciprofloxacin

The photocatalytic activity of PbMoO4/g-C3N4 samples wasevaluated by the degradation reaction under UV–Vis lightirradiation of ciprofloxacin (CIP) as a model of pharmaceuticalpollutant. The concentration of CIP was set in 10 mg L�1 with acharacteristic absorption band at 270 nm. The degradation of CIPwas carried out in a batch-type reactor consisted in a borosilicateglass beaker with a volume of 200 mL, surrounded with a coldwater jacket to maintain the reaction temperature at roomtemperature. A Xenon lamp of 35 W (6000 K) was used as UV–Vis irradiation source [8]. The intensity of UV radiation (l < 390nm) was 1380 mW cm�2. The powder samples (0.2 g) were thenplaced in the reaction vessel with 200 mL of the contaminantsolution was dissolved in distilled water and the mixture wasplaced in an ultrasonic bath during 1 min to remove aggregates.Then, it was left under dark conditions for 1 h to achieve theadsorption–desorption equilibrium of the pollutant on the photo-catalyst surface. Afterwards, the Xe lamp was turned on. During thereaction, the aliquots were taken at irradiation time intervals andthen separated through centrifugation. Also, several scavengeragents were added for obtain an estimate of the contribution ofhigh radicals oxidizing species (hROS) generated during thephotodegradation of ciprofloxacin. In Table 2 described the dosageof the scavenger agent added to the reaction solution. Themineralization degree was measuring by analyzing the totalorganic carbon content (TOC) after 96 h of irradiation. In this case,the initial concentration of the pharmaceutical compound was

established in 50 mg L�1. The aliquots were analyzed in a ShimadzuVSCN8 TOC analyzer.

3. Results and discussion

3.1. Samples characterization

The XRD patterns no other impurity could be observed in thepure g-C3N4 shows two broad peaks at 2u = 13.2� and 27.3�, whichare in good agreement with the hexagonal phase (JCPDS 87-1526),as shown in Fig. 1a. PbMoO4 pattern (sample Pb) shows the crystalstructure in good agreement with the characteristic peaks ofstandard tetragonal scheelite structure of PbMoO4 with spacegroup I41/a, according to JCPDS card (No. 01-071-4910). ThePbMoO4/g-C3N4 composites (samples Pb20, Pb50 and Pb80)cannot be seen the intensities of two diffraction peaks of g-C3N4

due to peaks become weaker with the contents of PbMoO4. Theresults showed that the photocatalysts are crystalline withoutsecondary phases or impurities. Fig. 1a shows an increase in thecrystallinity degree when the amount of Pb decreases, could beattributed to a g-C3N4 exfoliation in water during the PbMoO4

synthesis on the g-C3N4 surface, this exfoliation allows to increasethe crystallinity degree as reported in previously in the literature[28]. The average crystallite size was calculated using Debye–Scherrer's equation:

D ¼ Klb cos u

ð1Þ

Table 2Scavenger agents using during the photocatalytic tests.

Scavenger agent hROS quenched Scavenger dosage

Isopropanol �OH 0.02 MCatalase H2O2 935,000 U L�1

Benzoquinone �O2�� 0.02 M

KI h+ 0.02 M

D.B. Hernández-Uresti et al. / Journal of Photochemistry and Photobiology A: Chemistry 345 (2017) 21–26 23

where D is the crystalline size, K is the shape factor, l is thewavelength of CuKa radiation, b is the line broadening at half themaximum intensity and u is the angle of the maximum intensity.The lattice strain in composites due to crystal imperfection anddistortion was calculated using the equation:

e ¼ b4 tan u

ð2Þ

Crystallite size and lattice strain of the hybrid composites werecalculated by the X-ray line broadening performed on thediffraction (112) of PbMoO4 lattice. Therefore, crystallite sizeand lattice strain were calculated by Eqs. (1) and (2) for the hybridcomposites, as is shown in Table 1. The results showed that thesharpening of the peaks of the Pb50 composite results from thedecreasing of crystallite size and from the increasing of latticestrain, as seen in Fig. 1b. Accordingly, Pb50 is the sample with ahigh degree of crystallinity as corroborated with the X-raydiffraction patterns (Fig. 1a) with greater intensity in comparisonto the others samples. Fig. 2 show SEM images of the PbMoO4, g-C3N4 and hybrid composites. The g-C3N4 surface morphology(Fig. 2a) seemed to be irregular as sheets type. It is easy to see fromFig. 2b, the PbMoO4 particles spread on the g-C3N4 surface in thePb20 composite. Instead, the Pb50 sample (Fig. 2c) the PbMoO4

particles appreciates with slightly agglomeration to form needle-like morphology observed on the g-C3N4 surface and almost allparticles were in direct contact on the surface. It is evident thatPb80 (Fig. 2d) composite shows a significant PbMoO4 particlesagglomeration and totally covered the g-C3N4 surface. In Fig. 2e,PbMoO4 sample had a particle size of 80–100 nm, and a seed-likemorphology with ovoid grain shapes. With increasing PbMoO4

content in the hybrid composites, the area covered by the PbMoO4

particles increased on the g-C3N4 surface. The optical properties ofthe hybrid composites were examined by UV–Vis diffuse reflec-tance spectroscopy (DRS) (Fig. 3a). The tendency of the resultsshows in Fig. 3b, demonstrate when the amount of PbMoO4

decreases, the BET area increases to a maximum point (samplePb20) and, conversely, the Eg decreases to a minimum point(sample Pb50). So, the interaction between g-C3N4 and PbMoO4

occurs. The PbMoO4 and g-C3N4 samples shows absorptionwavelength at 397 nm (in UV light absorption) and 455 nm (Vislight absorption), respectively. These values can be assigned to theintrinsic band gap around 3.11 eV for PbMoO4 and 2.8 eV for g-C3N4, which is agrees with the literature [16]. The g-C3N4/PbMoO4

composites decreased the UV light absorption, due to theabsorption wavelength had increased from 397 to around450 nm. This is consistent with the pale yellow color of thesamples may be attributed to the interaction between the PbMoO4

and g-C3N4 therefore these hybrid composites can be excited undervisible-light irradiation. The band edge position of CB and VB of thesemiconductor can be calculated by the following formulas:

ECB ¼ X � Ee � 0:5Eq ð3Þ

EVB ¼ ECB þ Eg ð4Þwhere ECB and EVB is the conduction and valence band edgepotentials; X is the Mulliken electronegativity of the

Table 1Physical properties of the hybrid composites, PbMoO4 and g-C3N4.

Sample Band gap (eV) Surface area (m2 g�1) Crys

g-C3N4 2.80 4.80 –

Pb20 2.74 5.80 12.1

Pb50 2.69 4.40 10.5

Pb80 2.79 2.11 11.1

Pb 3.11 1.30 11.2

semiconductor, which is the geometric mean of the electro-negativities of the constituent atom; Ee is the energy of freeelectrons on the hydrogen scale (about 4.5 eV) and Eg is the bandgap energy of the semiconductor. By calculation, the estimated thevalues for ECB are (�1.22) and (�0.10) for g-C3N4 and PbMoO4,respectively. The EVB the values are (+1.58) for g-C3N4 and (+3.01)for PbMoO4, which is consistent with the reported in literature[16]. The Brunauer–Emmett–Teller method was used to calculatethe surface area, the results are shown in Table 1. Compared withPbMoO4, the surface area of the Pb20, Pb50 and Pb80 compositeshas a tendency but is increased slightly, as seen in Fig. 3b. However,is not significant difference in the BET area value among the hybridcomposites.

3.2. Photocatalytic degradation of pharmaceutical compounds

In order to investigate the photocatalytic activity of thePbMoO4, g-C3N4 and the hybrid composites, CIP was selected asmodel molecule of antibiotic pollutants. Ciprofloxacin is a broad-spectrum antibacterial agent widely used for treating bacterialinfections [29]. The CIP degradation under UV–Visible lightirradiation in the presence of the synthesized g-C3N4, PbMoO4

and the hybrid composites are depicted in Fig. 4a. The photolysis isthe absorption test in the absence of catalysts with irradiation, itshows that no significant change in the CIP concentration during2 h. The g-C3N4 photocatalytic activity shows the lowest degrada-tion reaching only 50% with a half time (t1/2) of 135 min during 2 hof UV–Vis irradiation. In the case of PbMoO4 (Pb sample), thephotodegradation increased to 75% and reduced the t1/2 to 30 min.A similar behavior was observed in the degradation of Pb80composite, where only 77% of the initial concentration decreasedafter 2 h of irradiation. The CIP concentration was drasticallyreduced using Pb50 sample as photocatalyst, the CIP degraded toalmost 100% degraded after 2 h with UV–Vis irradiation. Therefore,the concentration of CIP for the g-C3N4, PbMoO4 and hybridcompounds gradually decreased with the increase of the PbMoO4

content until obtaining a maximum value (in Pb50). Then, thesaturation of PbMoO4 occurs on the g-C3N4 surface, limiting theinteraction between them. On other hand, the reaction rates(Fig. 4b) were estimated according to the Langmuir–Hinshelwoodmodel for a pseudo-first order reaction, which were used to standfor the photocatalytic activity in order to evaluate the influencewith the crystallinity degree and the narrow band gap. The relativelow band gap energy obtained for Pb50 leads to an importantabsorption in the UV–Vis spectrum [30]. The reaction rates aredirectly proportional to the crystallinity degree (similar behavior)while it is inversely proportional to the crystallite size, indicating

talline size (nm) Lattice strain (%) PbMoO4 content (wt.%)

– 01.547 201.578 501.562 801.559 100

Fig. 2. SEM images of the g-C3N4 (a), Pb20 (b), Pb50 (c), Pb80 (d) and PbMoO4 (e) photocatalysts.

Fig. 3. UV–Vis diffuse reflectance spectra (a), BET area and Eg tendency (b) in thePbMoO4/g-C3N4 composites.

24 D.B. Hernández-Uresti et al. / Journal of Photochemistry and Photobiology A: Chemistry 345 (2017) 21–26

that crystallinity degree plays an important role on the CIPphotodegradation increasing 2.5 and 11 times more than of thepure PbMoO4 and g-C3N4, respectively. Thus, smaller crystallitesize usually has larger lattice strain values so have less defectsleading to increase photocatalytic activity. These defects in thesemiconductor promote electron–hole recombination, which isthe principal limitation in photocatalytic reactions. Consequently,the increment of the photoactivity could be attributed to the highcrystallinity degree, lower crystalline size and an optimuminteraction between PbMoO4 and g-C3N4, which accelerates theinterfacial charge transfer and reduces the electrons–holes pairrecombination.

3.3. Roles of oxidizing species

During CIP degradation, some high reactive oxidizing species(hROS) are generated on the basis of active holes and electronspairs generated at the irradiated semiconductor interface, which isessential for understand the photocatalysis mechanism. First, theholes (h+) are hROS generated by photocatalyst, which are capableof oxidizing organic contaminants into the wastewater. OthershROS produced during the photodegradation are H2O2, O2

�� and�OH. In order to determine the role of the hROS, several scavengeragents were employed during the photocatalytic tests, as seen inFig. 5. The effect of alcohols, such as isopropanol, on thephotocatalytic rate has commonly been used to free hydroxylradical (�OH) scavenger [31]. Isopropanol effect on the Pb50sample for the CIP degradation was inhibited around 30% toimplying that �OH radicals are not principal radicals involved in thephotodegradation process. To confirm the role of superoxide anionand H2O2 radicals in the degradation mechanism, the CIPdegradation was carried out in the presence of benzoquinoneand catalase as a radical scavenger, respectively. However, thepresence of benzoquinone and catalase provides a lower degrada-tion of 40% and 60% respect to the Pb50 sample without radicalscavenger. These results suggested that principal contribution forthe CIP degradation is not of hROS formed by electrons in the CB ofthe semiconductor. Iodide ions (KI) can be used as active h+ radical

Fig. 4. Evolution of the concentration of the ciprofloxacin (a) and the apparent first-order constants (b) using the PbMoO4/g-C3N4 composites as catalysts.

D.B. Hernández-Uresti et al. / Journal of Photochemistry and Photobiology A: Chemistry 345 (2017) 21–26 25

scavenger to determine the role of the direct oxidation reactionbetween the holes generated on the semiconductor and theorganic pollutants [32]. It was found that KI has a remarkable

Fig. 5. Effect using different radical scavengers on the efficiency of CIP degradation.

influence on the CIP degradation with an inhibition percent around95%. Therefore, h+ formed in the VB, are essential factor in theremoval CIP photodegradation. Electron–hole separation processaccording to the band gap structures of PbMoO4 and g-C3N4, candescribed in Fig. 6. Under UV–Vis light irradiation, the photo-generated electrons in CB of g-C3N4 would transfer with theelectrons in CB of PbMoO4 at the interface of the composite, whilesome electrons migrate to PbMoO4 surface as act as scavenger of O2

to yield �O2��. The values of the band edge potentials corroborate

that accumulate electrons in the CB (�0.10 eV) of PbMoO4 cannotreduce O2 to yield �O2

�� (�0.33 eV), nevertheless could be generateH2O2 (+0.68 eV). The predominant mechanism in the CB of thecomposite is the H2O2, as was corroborated with the radicalscavenger experiments. These experiments showed a high influ-ence in the photocatalytic activity using the catalase as H2O2

scavenger than the benzoquinone as �O2�� scavenger. At the same

time, the active holes (h+) in the VB of PbMoO4 would migrate tothe VB of g-C3N4, while the some holes in the VB of PbMoO4 wouldreact with water to form the �OH radicals and other holes willdirectly oxidize the organic compounds. In this case, the activeholes in the VB (+1.58 eV) of g-C3N4 cannot generate �OH(+2.40 eV), while the VB of the PbMoO4 could be possible.However, the results in the radical scavenger experimentsdemonstrated that h+ (using KI as scavenger) are the majorreactive species in comparison to the �OH radicals (usingisopropanol as scavenger) for CIP photodegradation. Consequently,the possible mechanism reaction is the holes directly oxidize of theciprofloxacin using PbMoO4/g-C3N4 as photocatalyst.

3.4. Ciprofloxacin mineralization

Fig. 7 shows the mineralization degree using TOC (total organiccarbon) analysis the three principal samples: PbMoO4, Pb50 and g-C3N4 during 96 h under UV–Vis light irradiation. Mineralizationdegree was much lower using g-C3N4 as photocatalyst than whenused the Pb50 composite. The value of TOC content using g-C3N4

and PbMoO4 as photocatalyst is 42% and 48%, respectively. TOCexperiments with PbMoO4 sample shows a quick reduction duringthe first 24 h of irradiation, while g-C3N4 took place more slowly.Otherwise, TOC removed with Pb50 composite was graduallydecreasing with the reaction time increment until 70%, would beattributed to addition of a hydroxyl radical and the holes directattack in the breakdown of their quinolone ring structures, asprevious studies have reported [33].

4. Conclusions

Novel PbMoO4/g-C3N4 composite were prepared successfullywith high crystallinity, narrow-band-gap and adequate BET area

Fig. 6. Schematic diagram of the proposed photocatalytic mechanism for the CIPdegradation.

Fig. 7. Evolution of the mineralization degrees of the ciprofloxacin during itsdegradation during the photodegradation in the presence of the PbMoO4/g-C3N4

hybrid composites.

26 D.B. Hernández-Uresti et al. / Journal of Photochemistry and Photobiology A: Chemistry 345 (2017) 21–26

via sonochemical method. Ciprofloxacin degradation in aqueoussolution was successfully evaluated using the PbMoO4/g-C3N4

under UV–Vis irradiation. The increased photocatalytic activitymay be attributed to a less defects, which promote electron–holerecombination, due to a high crystallinity and a better absorptionof UV–Vis spectrum by narrow band gap. Afterwards, some chargetrapping species were used to determine the possible photo-catalytic process, showing that the photogenerated holes on thePbMoO4 are the main oxidizing species responsible in the CIPdegradation that was corroborated by the values of PbMoO4 and g-C3N4 of band edge potentials. However, g-C3N4 as an indispensablecomponent because is the responsible of better absorption in thevisible spectrum and could be increase the crystallinity degree so isdetermined as the best composition is 50–50 of each one. Finally,mineralization degree was feasible in order to validate theremediation of the pharmaceutical pollutant.

Acknowledgements

We wish to thank to the Universidad Autónoma de Nuevo León(UANL) for the financial support to the Project PAICYT 2015 andCONACYT for supports the Project No. 220792.

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