Chemical Engineering Journal 255 (2014) 394–402

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Chemical Engineering Journal

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Cu2O/BiVO4 heterostructures: synthesis and application in simultaneousphotocatalytic oxidation of organic dyes and reduction of Cr(VI) undervisible light

http://dx.doi.org/10.1016/j.cej.2014.06.0311385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 731 88821171.E-mail address: sf_yin@hnu.edu.cn (S.-F. Yin).

1 These authors contributed equally to this work.

Qing Yuan a,1, Lang Chen a,1, Miao Xiong a, Jie He a, Sheng-Lian Luo a, Chak-Tong Au a,b, Shuang-Feng Yin a,⇑a State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, Chinab Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong

h i g h l i g h t s

� Cu2O/BiVO4 photocatalyst wassynthesized under mild conditions.� It shows high catalytic activity for the

simultaneous removal of dyes andCr(VI).� The degradations of cationic and/or

anionic dyes operate under neutralcondition.� A synergistic photoreduction–

oxidation mechanism is proposed.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 March 2014Received in revised form 9 June 2014Accepted 10 June 2014Available online 20 June 2014

Keywords:PhotocatalysisBismuth vanadateCuprous oxideHeterojunctionCr(VI)

a b s t r a c t

We synthesized Cu2O/BiVO4 composites by growing Cu2O nanoparticles on BiVO4 under mild condition.The optimized composite shows high photocatalytic efficiency in the simultaneous oxidation of organicdyes and reduction of Cr(VI) in neutral media. The XPS results confirm that the Cr(VI) adsorbed on Cu2O/BiVO4 was completely reduced to Cr(III). The photocatalyst can be used for the degradation of cationic oranionic dyes as well as a mixture of them under visible light irradiation. The photocatalytic activity of thecomposite can be ascribed to the heterojunctions between Cu2O and BiVO4, which facilitate the separa-tion of photogenerated electrons and holes. The work demonstrates that the as-synthesized Cu2O/BiVO4

composite is a promising photocatalyst for the treatment of wastewater that contains organic dyes andCr(VI) ions.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Arising from industrial processes, toxic (carcinogenic and muta-genic) hexavalent chromium (Cr(VI)) is commonly found in waste-water [1,2]. The reduction of Cr(VI) to less mobile and less toxicCr(III) is a key procedure in wastewater treatment [3]. Recently,the photocatalytic reduction of Cr(VI) to Cr(III) is considered to

be efficient [4–7]. However, the photocatalysts only work underacidic condition, making the processes difficult to handle. To avoidproblems such as secondary pollution and catalyst corrosion, it isnecessary to develop photocatalysts that can efficiently reduceCr(VI) under neutral condition.

For the treatment of wastewater, the degradation of organicdyes by a photocatalytic approach is found to be applicable [8].In photooxidation, dyes are oxidized by photogenerated holes[9,10]. In wastewater treatment, the presence of different kindsof pollutants is common [11]. In a case such as the coexistence ofCr(VI) and organic dyes, it is highly desirable to reduce Cr(VI)

Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402 395

and to oxidize dyes simultaneously using a single photocatalyst.Such an idea was investigated over TiO2-based photocatalysts inthe past years [10,12–14]. Yu et al. [10] reported simultaneous oxi-dation of RY15 and reduction of Cr(VI) at TiO2-BDD heterojunctionsunder UV illumination. Kyung et al. [15] and Sun et al. [16] inves-tigated the photoinduced dye oxidation and Cr(VI) reduction inacidic suspension using TiO2 under visible-light illumination.Nonetheless, TiO2 can only be excited under UV irradiation andthe quantum yield is low. Furthermore, the photocatalysts devel-oped so far function only in an acidic medium and the use of a sac-rificial agent is a necessity. To be practical for the simultaneousreduction of Cr(VI) and oxidation of organic dyes, it is imperativeto design photocatalysts that work in neutral media under visiblelight without the need of a sacrificial agent.

Bismuth-based compounds are cheap and nontoxic. Due to theirunique electronic structures, they are efficient photocatalysts [17–19]. With a valence band edge of ca. +2.4 V (vs. NHE) and excellentability for visible light absorption (Eg = 2.4 eV), the n-type mono-clinic bismuth vanadate (m-BiVO4) is used for the oxidation oforganic dyes under visible-light irradiation. Furthermore, the con-duction band edge of m-BiVO4 (ca. 0 V vs. NHE) is more negativethan that of Cr(VI)/Cr(III) (Eh = 1.33 vs. NHE) [20]. It is envisagedthat one can achieve simultaneous photocatalytic reduction ofCr(VI) and oxidation of organic dyes over m-BiVO4. However, it isknown that the activity of BiVO4 is usually not satisfactory dueto the high recombination rate of photogenerated electrons andholes. One of the ways to improve the separation of charge carriersis to combine BiVO4 with a semiconductor of appropriate bandposition [21,22]. With successful cases such as Ag/BiVO4 [19],Bi2O3/BiVO4 [21], and Bi2O2CO3/BiOI [22] in mind, we turn ourattention to composites that can be used for simultaneous photo-redox reactions. Cu2O is a p-type semiconductor with a direct bandgap of 2.0 eV [23,24]. The conduction band edge of Cu2O is muchhigher than that of BiVO4. At Cu2O/BiVO4 heterojunctions, thereis easy transfer of photoelectrons from the conduction band ofCu2O to that of BiVO4, consequently enhancing the separation ofcharge carriers and promoting the photocatalytic activity of BiVO4.

In this study, Cu2O/BiVO4 composites were fabricated by a two-step method. Their photocatalytic efficiency towards the simulta-neous reduction of Cr(VI) and oxidation of organic dyes in a neutralmedium under visible light irradiation was evaluated. The Cu2O/BiVO4 composites were tested for the photodegradation of cationicand anionic dyes, individually as well as in the form of a mixture.Through the determination of major active species during thephotocatalytic process, we establish the mechanism of thesynchronized actions of photoreduction/oxidation. To the best ofour knowledge, investigation of this kind over Cu2O/BiVO4

composites has never been reported.

Table 1Description of dyes.

Name The molecularformulae

The maximum absorptionwavelength (kmax)(nm)

Methylene blue (MB) C16H18ClN3S�3H2O 664Rhodamine B (RhB) C28H31ClN2O3 552Crystal violet (CV) C25H30N3Cl�9H2O 590Methyl orange (MO) C14H14N3SO3Na 463

2. Experimental

2.1. Synthesis

All reagents were of analytical grade and commercially avail-able. They were used without further purification. The Cu2O/BiVO4

composites were prepared by a two-step method. First, BiVO4 wasprepared through a homogeneous co-precipitation process (HCP)reported elsewhere [25]. Second, with constant low-speed stirring,a designated amount of as-prepared BiVO4 was added into a mixedsolution (80 mL) of 0.5 g PVP and 0.1 mmol (17 mg) CuCl2�H2O,while the pH was maintained at 8.5 (by adding 0.01 M sodiumhydroxide dropwise). Finally, 8 mL of 0.1 M hydrazine hydrate(N2H4�H2O) was added with constant stirring to the above solution.The resulting solution was heated to 45 �C and kept at this temper-ature for 1 h with constant stirring. The precipitate was recovered

by centrifugation, washed with water and absolute ethyl alcohol,and dried at 60 �C. The harvested composites with Cu2O mole per-centage of 2.5%, 5% and 7.5% are denoted hereinafter as 2.5%-Cu2O/BiVO4, 5%-Cu2O/BiVO4 and 7.5%-Cu2O/BiVO4, accordingly.

2.2. Characterization

The Cu2O, BiVO4, and Cu2O/BiVO4 samples in the form of pow-der were collected and characterized by powder X-ray diffraction(XRD) on a Brüker Automatic Diffractometer (Bruker D8 Advance)with mono-chromatized Cu-Ka radiation (k = 0.15406 nm) at a set-ting of 40 kV and 80 mA. The scanning rate was 0.02� s�1 and thescanning range was 10–80� (2h). The micro- and nano-structuresas well as the morphology of as-prepared samples were examinedusing a field emission scanning electron microscope (FE-SEM) (Hit-achi S-4800). Images of transmission electron microscopy (TEM)and high-resolution transmission electron microscopy (HRTEM)were taken over a JEM-3010F transmission electron microscopeat an accelerating voltage of 200 kV. UV–vis diffuse reflectancespectra (UV–vis DRS) of samples were obtained over a UV–visspectrophotometer (Cary 100) using BaSO4 as reference. X-ray pho-toelectron spectroscopy (XPS) was used to determine the Bi4f, V2p,Cu2p, Cr2p and O1s binding energies (BEs) of surface bismuth,vanadium, copper, chromium and oxygen species, using Mg-Ka(hm = 1253.6 eV) as excitation source (XPS, SSX-100, Mg-Ka). ThepH of solutions was measured using OHAUS STARTER 2100/3C.The element analysis was carried out by ICP-AES (IRIS1000). Thephotoluminescence (PL) spectra were recorded using a HitachiF-7000 fluorescence spectrophotometer.

2.3. Catalyst evaluation

The photoreduction of Cr(VI) and the photocatalytic degrada-tion of methylene blue (MB), rhodamine B (RhB), crystal violet(CV), methyl orange (MO) as well as their mixtures were adoptedto evaluate the photocatalytic activities of the as-synthesizedCu2O/BiVO4 samples. The molecular formulae and the lambdamaxima of dyes are given in Table 1. Typically, 50 mg photocata-lyst was added to a 100 mL aqueous solution of MB, RhB, CV orMO (2 � 10�5 mol/L) in a homemade Pyrex glass vessel. When itwas the simultaneous degradation of organic dyes and reductionof Cr(VI), 100 mg photocatalyst was added into a homemade Pyrexglass vessel containing 50 mL aqueous solution of organic dye(2 � 10�5 mol/L) and Cr(VI) (3 � 10�5 mol/L). Before illumination,the suspension was stirred (using a magnetic stirrer) for 30 minin the dark to establish adsorption–desorption equilibrium. After-wards, the contents were exposed to visible light (k P 400 nm)originated from a 300 W Xe lamp with a 400 nm cutoff-filter. Thedistance between the light and the liquid surface was 25 cm. Thecontent was sampled (about 4 mL) at selected intervals. Withthe catalyst removed by centrifugation (10000 r/min for 3 min),the concentration of residual dye in the solution was determinedover a Cary-100 UV–vis spectrophotometer.

Cr(VI) concentrations were measured using the 1,5-diphenylc-arbazide (DPC) colorimetric method [16,26] by monitoring the

Table 2The mole percentage of Cu2O in the composites.

Theoretical mole percentage(%)

Actual mole percentage(%)

2.5 1.35.0 3.07.5 5.2

396 Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402

purple complex at 540 nm on a UV–vis spectrophotometer. Thespecific operation for measurement is as follows: In a 10 mL volu-metric flask, 1 mL of sample was mixed with 9 mL of 0.2 M H2SO4.Then 0.2 mL of freshly prepared 0.25% (w/v) DPC in acetone wasadded to the volumetric flask. The mixture was then shaken forabout 15–30 s and allowed to stand for 10–15 min (for full colordevelopment). The red–violet to purple color was measured andthe absorbance at 540 nm was denoted as A1i (i represents differ-ent reaction time intervals). For comparison, 1 mL sample and 9 mL0.2 M H2SO4 solution were mixed and the measured result at540 nm was denoted as A10i. Therefore, the accurate concentrationof Cr(VI) should be A1i�A10i. The concentration of total Cr(VI) atdifferent time intervals (Ci) was evaluated by combining the resid-ual in the supernatant. The efficiency of total Cr(VI) reduction(gCr(VI)) is calculated from the following equations:

gCrðVIÞ ¼ ðC0 � CiÞC0

� 100% ð1Þ

C0 ¼ A10 � A100 ð2Þ

Ci ¼ A1i � A10i ð3Þ

where C0 and Ci represent the initial concentration of Cr(VI) andtemporal concentration from the sum of solution and catalystsurface, respectively. In the present study, gCr(VI) represents theefficiency. The degradation of organic dyes was monitored byspectrophotometric analysis on a Cary 100 UV–vis spectrophotom-eter at the wavelength of maximum absorption (MB: 664 nm; RhB:553 nm; CV: 583 nm; MO: 464 nm).

(111)

(e) Cu2O

2.4. Analysis of hydroxyl radicals (OH�)

Hydroxyl radicals (OH�) produced by the photocatalysts undervisible light irradiation were measured by the fluorescence methodusing terephthalic acid (TA) as a probe molecule. TA readily reactswith OH� to produce the highly fluorescent product, 2-hydroxyte-rephthalic acid (TAOH). The fluorescence intensity of TAOH isproportional to the amount of OH� produced on the surface of phot-ocatalysts. The maximum emission intensity in fluorescencespectra was recorded at 425 nm (using 315 nm excitation) [27,28].We dispersed 5 mg Cu2O/BiVO4 in 30 mL of 5� 10�4 mol/L TAaqueous solution that was diluted by NaOH aqueous solution(2 � 10�3 mol/L). The resulting suspension was then exposed tovisible light irradiation. At regular intervals, 3 mL of the suspensionwas collected and centrifuged to measure the maximum fluores-cence emission intensity with an excitation wavelength of315 nm. The approach relies on the fluorescence signal at 425 nmof the hydroxylation of terephathic acid with OH� generated atthe Cu2O/BiVO4 interface.

20 40 60 80

(d)

(c)

(b)

7.5%-Cu2O/BiVO4

5%-Cu2O/BiVO4

Inte

nsit

y (a

.u.)

2θ (theta)

BiVO4

2.5%-Cu2O/BiVO4

(a)

(111)

Fig. 1. XRD patterns of BiVO4, Cu2O, and (2.5%, 5%, 7.5%)-Cu2O/BiVO4 composites.

3. Results and discussion

3.1. Characterization results

3.1.1. CompositionThe actual percentages of Cu2O in the prepared catalysts were

obtained by ICP analysis. As shown in Table 2, the actual amountof Cu2O is not quite in agreement with the nominal values. Theactual percentages of Cu2O are roughly half of the theoretical ones,and it increases with the rise of the amount of Cu source. Thediscrepancy could be due to the loss of Cu2O nanoparticles duringthe centrifugation process.

3.1.2. Phase structureFig. 1 shows the XRD patterns of Cu2O, BiVO4 and Cu2O/BiVO4

samples. The XRD patterns in Fig. 1(a) and (e) display sharp peakscharacteristic of monoclinic BiVO4 and cubic Cu2O (BiVO4: JCPDScard No. 14-0688; Cu2O: 05-0667), respectively, indicating thatthese samples show high degree of crystallinity. There is no detec-tion of peaks ascribable to impurities, implying that the final prod-ucts are phase pure. A weak peak due to the (111) planes of Cu2Oappears at 36.4� in the XRD patterns of 7.5%-Cu2O/BiVO4 composite(Fig. 1(d)). As demonstrated in Table 2, the actual amount of Cu2Oin the 2.5%-Cu2O/BiVO4 and 5%-Cu2O/BiVO4 composite are lowerthan that in the 7.5%-Cu2O/BiVO4 composite. There is no detectionof signals attributable to the (111) planes of Cu2O over the2.5%-Cu2O/BiVO4 and 5%-Cu2O/BiVO4 composites (Fig. 1(b) and(c)), showing Cu2O is highly dispersed on BiVO4.

3.1.3. Surface compositionFig. 2 shows the XPS spectra of 2.5%-Cu2O/BiVO4. The relevant

peak position was calibrated against the C1s signal of contaminantcarbon at a binding energy of 284.6 eV. Fig. 2(a) is a typical surveyspectrum, and only Bi, V, O, Cu and trace amount of C are detected.Fig. 2(b)–(e) are the high-resolution spectra of Bi4f, V2p, Cu2p andO1s. The strong peaks at 164.4 and 159.0 eV (Fig. 2(b)) correspond-ing to Bi4f7/2 and Bi4f5/2 are characteristics of Bi3+ [29], and theweak peaks at 157.1 and 162.4 eV can be ascribed to the Bi4f7/2

and Bi4f5/2 signals of metallic Bi [30], indicating that the Bi speciesin the composite exist mainly as Bi3+, and there is a trace amount ofmetallic Bi0. As reported by Liu et al., Bi3+ can be reduced to Bi [30],a trace amount of metallic Bi0 may originate from the reducing ofBi3+ by hydrazine hydrate during the preparation of photocatalyst.The V2p peak can be fitted with two peaks at 523.5 and 515.9 eV,which are assignable to V2p1/2 and V2p3/2 signals, respectively(Fig. 2(c)) [31]. The peaks located at 952.0 and 932.3 eV can beattributed to the Cu2p1/2 and Cu2p3/2 peaks of Cu2O (Fig. 2(d))[32]. The peaks located at 529.6 and 530.9 eV (Fig. 2(e)) can beassigned to the oxygen species of lattice oxygen of layer-structured

1200 1000 800 600 400 200 0

Inte

nsity

(a.u

.)

Binding Energy (eV)

Survey

Bi5dC1s

Bi4f

Bi4dCu2p

O1s V2p

Bi4d

(a)

168 166 164 162 160 158 156 154

Inte

nsity

(a.u

)

Binding Energy (eV)

Bi4f164.4 eV

159.0 eV

162.4 eV

157.1 eV

(b)

528 525 522 519 516 513 510

Inte

nsity

(a.u

.)

Binding Energy (eV)

V2p

523.5 eV

515.9 eV (c)

965 960 955 950 945 940 935 930 925

Inte

nsity

(a.u

.)

Binding Energy (eV)

Cu2p932.3 eV

952.0 eV

(d)

534 533 532 531 530 529 528 527

Inte

nsity

(a.u

.)

Binding Energy (eV)

O1s 529.6 eV

530.9 eV531.6 eV

(e)

Fig. 2. XPS spectra of Cu2O/BiVO4 samples: (a) survey scan; (b) Bi4f; (c) V2p; (d) Cu2p; and (e) O1s spectra.

Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402 397

Bi2O22+ and Cu2O, respectively [33,34]. The O1s peak at binding

energy of 531.6 eV may come from adsorbed H2O or surface hydro-xyl group [35]. Based on the XRD and XPS results, it is reasonable todeduce that the as-prepared composites are composed of Cu2O andBiVO4.

3.1.4. Morphological structureThe morphology of as-prepared BiVO4, Cu2O and Cu2O/BiVO4 as

studied by scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) are shown in Fig. S1 (see the Support-ing information, SI). Pure BiVO4 (Fig. S1(a), SI) shows irregularnanosheet with smooth surface, and the edge length of the nano-sheets ranges from 540 to 600 nm, while the edge thickness isabout 100 nm. Fig. S1(b), SI shows the SEM image of Cu2O nanopar-ticles, and one can see that the particle size of Cu2O is 20–200 nm[36]. Fig. S1(c), SI is the SEM image of 2.5%-Cu2O/BiVO4. The surfaceof 2.5%-Cu2O/BiVO4 is similar to that of BiVO4. Fig. (S1(d)–S1(f)), SIcollected at different regions of the HRTEM images of 2.5%-Cu2O/BiVO4 clearly shows ordered lattice fringes with inter-planar spac-ing of 0.304, 0.257 and 0.236 nm in accord with the (121), (002)and (220) planes of monoclinic BiVO4, respectively (JCPDS card

No. 14-0688). The lattice fringes with d spacings of 0.248, 0.302and 0.177 nm are ascribed to the (111), (110) and (211) planesof cuprite Cu2O, respectively (JCPDS card No. 05-0667). The HRTEMimage confirms the creation of heterojunction between Cu2O andBiVO4.

3.1.5. Optical propertiesUV–visible diffuse reflectance spectroscopy (UV–vis DRS) was

used to study the light absorption properties of Cu2O, BiVO4, and(2.5%, 5%, 7.5%)-Cu2O/BiVO4 samples. From Fig. 3, one can see thatall the samples absorb visible light. What is more, the Cu2O showsstronger absorption intensity than BiVO4 in both the ultravioletand visible light region. The absorbance of the Cu2O/BiVO4 com-posite increases with increasing Cu2O content, plausibly a resultof Cu2O modification of the fundamental process for electron–holepair formation upon irradiation as suggested by Abdulkarem et al.[23].

We calculated the value of band gap energies of the as-preparedBiVO4 and Cu2O/BVO4 using the formula (ahm)n = A(hm�Eg), wherea, h, m, Eg, and A are the absorption coefficient, Plank constant, lightfrequency, band gap, and a constant [21,22]. The as-obtained Eg

200 300 400 500 600 700 8000.0

0.4

0.8

1.2

2.5%-Cu2O/BiVO4 5%-Cu2O/BiVO4 7.5%-Cu2O/BiVO4 BiVO4 Cu2O

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Fig. 3. UV–vis absorption property of BiVO4, Cu2O and (2.5%, 5%, 7.5%)-Cu2O/BiVO4

composites.

398 Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402

values are 2.4, 2.4, 2.4 and 2.3 eV for BVO4, 2.5%-Cu2O/BVO4,5%-Cu2O/BVO4 and 7.5%-Cu2O/BVO4, respectively. It is calculatedthat the band gap of Cu2O is 2.2 eV (Fig. S2, SI). It is apparent thatwith the introduction of Cu2O to BiVO4, there is no change inEg, indicating that the light absorption properties of BiVO4 andCu2O/BVO4 are rather similar. The results further confirm thatthe samples can be excited by visible light.

3.2. Photocatalytic properties

3.2.1. Degradation of organic dyesFig. 4 shows the photodegradation of MB over Cu2O, BiVO4 and

Cu2O/BiVO4 composites. Herein C is the concentration of MB at aparticular irradiation time while C0 is the initial concentration.After irradiation for 75 min, MB is completely decolorized overthe Cu2O/BiVO4 composites. The degradation rate of MB over thecatalysts can be arranged in the order of 2.5%-Cu2O/BiVO4 >5%-Cu2O/BiVO4 > BiVO4 > 7.5%-Cu2O/BiVO4 > Cu2O. The higher activ-ity of 2.5%-Cu2O/BiVO4 and 5%-Cu2O/BiVO4 as compared with thesingle components is ascribed to the following reasons: (i) Theintroduction of Cu2O into BiVO4 results in better harvest of inci-dent light; and (ii) the presence of Cu2O/BiVO4 heterojunctionspromotes the separation of charge carriers as well as the diffusionof electrons and holes to the surface of catalyst. Nonetheless,7.5%-Cu2O/BiVO4 shows activity even lower than that of BiVO4.This could be owing to the excessive amount of Cu2O that acts as

0 30 60 90 120 1500.0

0.2

0.4

0.6

0.8

1.0

(g) (f) (e)

(d) (c)

(b)

C/C

0

Irradiation time (min)

(a)

Fig. 4. Photocatalytic degradation of MB over the as-prepared samples: (a) noirradiation; (b) Cu2O; (c) no catalyst; (d) 7.5%-Cu2O/BiVO4; (e) BiVO4; (f) 5%-Cu2O/BiVO4; (g) 2.5%-Cu2O/BiVO4. (The error bars reflect the standard deviations.)

a kind of recombination center rather than an electron pathway[37], and promotes the recombination of electrons and holes inBiVO4. We hence take the optimal Cu2O loading on BiVO4 to be2.5%.

It is well known that there are many kinds of organic dyes inindustrial wastewater and they may cause long-term ill effectson environment and human health [11,22]. In general, they canbe divided into cationic and anionic dyes. As reported, photocata-lysts always show degradation selectivity towards cationic or anio-nic dyes [11,38]. To investigate the wide application of the Cu2O/BiVO4 composite, we selected CV and RhB (cationic dyes) as wellas MO (anionic dye) as degradation targets. Under reaction condi-tion similar to that of MB degradation, nearly 100%, 91% and 73% ofthe initial CV, RhB and MO are removed after irradiation of 120 min(Fig. 5). The results demonstrate that the as-synthesized photocat-alyst can be applied for the degradation of a wide range of dyesunder visible light irradiation.

In real practice, it is common to find wastewater that containsmore than one kind of dyes [22,38]. It is hence meaningful toexamine the photocatalytic efficiency of Cu2O/BiVO4 in the treat-ment of wastewater that contains different dyes. As shown inFig. 6, a mixture containing two (MB and CV) or three (MB, MOand CV) kinds of dyes can be efficiently treated in the presenceof Cu2O/BiVO4 within a short period of time.

It was reported that the active species in photodegradationprocesses could vary with the choice of catalysts [17]. The activespecies in the photocatalytic process using Cu2O/BiVO4 could beidentified using tert-butyl alcohol (TBA) as hydroxyl radical (�OH)scavenger [17,39,40], and triethanolamine (TEOA) [17] or ethy-lenediamine tetraacetic acid (EDTA) as hole scavenger [41]. Inthe present work, we used EDTA as hole and TBA as �OH scavenger.Before irradiation, the scavenger (10 mmol/L) was added to the MB(or RhB) solutions together with the catalyst. As depicted in Fig. 7(and Fig. S3, SI), with the addition of EDTA and TBA, the degrada-tion efficiency of MB decreases from 93% to 14% and from 93% to85%, respectively (whereas the degradation efficiency of RhBdecreases from 98% to 17% and from 98% to 99%, respectively).The results indicate that holes are the main active species in thedecomposition of MB over Cu2O/BiVO4 while �OH only plays arelatively small role in the process.

The formation of OH� on the surface of photocatalysts was fur-ther investigated by the fluorescence technique. The PL emissionspectrum excited at 315 nm of terephthalic acid solution was mea-sured under visible-light irradiation. Fig. 8(a) shows the changes ofPL spectra of terephthalic acid solution with irradiation time. It canbe seen that the fluorescence intensity increases gradually with

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

Cu2O/BiVO4-RhB

BiVO4-RhB

Cu2O/BiVO4-MO

BiVO4-MO

Cu2O/BiVO4-CV

BiVO4-CV

C/C

0

Irradiation time (min)

Fig. 5. Photocatalytic degradation of rhodamine B (RhB), methyl orange (MO) andcrystal violet (CV) over 2.5%-Cu2O/BiVO4 and BiVO4.

200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Before Adsorption 0 min 30 min 45 min 60 min 75 min

MB

CV(a)

200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Before Adsorption 0 min 30 min 45 min 60 min

MB

CV

MO

(b)

Fig. 6. UV–vis absorption spectra of a mixture of organic dyes with respect to irradiation time over 2.5%-Cu2O/BiVO4.

0 30 60 900.0

0.2

0.4

0.6

0.8

1.0

no scavenger EDTA TBAC

/C0

Irradiation time (min)

Fig. 7. Photocatalytic degradation of MB over Cu2O/BiVO4 samples in the presenceof scavengers.

Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402 399

irradiation time, and no fluorescence intensity is observed in theabsence of visible light or Cu2O/BiVO4 photocatalysts, indicatingOH� is formed during the photocatalytic process under visible lightirradiation. Fig. 8(b) shows almost linear increase of fluorescenceintensity of TAOH against illumination time. Consequently, weconclude that OH� radicals formed at the Cu2O/BiVO4 interfaceare proportional to the light illumination time. We obtained theformation rate of OH� radicals based on the slope of the plot shownin Fig. 8(b). The result suggests that the fluorescence signal is onlycaused by the reaction of terephathic acid with OH� formed at the

400 450 500 550 600

PL In

tens

ity (a

.u.)

Wavelength (nm)

75 min 60 min 45 min 30 min 0 min no catalyst

Analysis of OH • radicals (a)

Inte

nsity

(a.u

.)

Fig. 8. (a) Fluorescence spectral changes measured during illumination of 2.5%-Cu2O/BiV(b) fluorescence intensity at 425 nm against illumination time for TAOH (2-hydroxytere

interface of the Cu2O/BiVO4 photocatalysts during visible-lightirradiation. Generally, the greater the formation rate of OH� radi-cals, the higher is the separation efficiency of electron-hole pairs.Hence, the photocatalytic activity has a positive correlation withthe formation rate of OH� radicals.

3.2.2. Degradation of organic dyes and Cr(VI)It was reported that the main active species in the process of

photocatalytic reduction of Cr(VI) are electrons [41–43]. As dis-cussed above, the main active species for MB degradation onCu2O/BiVO4 are holes. Thus we deduce that the photoreductionof Cr(VI) and photoxidation of MB on Cu2O/BiVO4 can proceed effi-ciently at the same time in such a situation. As shown in Fig. 9, theremoval of MB (Fig. 9(a)) and Cr(VI) (Fig. 9(b)) in the dark can beascribed to the adsorption of them on the surface of Cu2O/BiVO4.Moreover, both MB and Cr(VI) are slowly removed under visiblelight even in the absence of Cu2O/BiVO4, in agreement with reportsthat MB undergoes photolysis without a catalyst [44,45]. The pho-tolysis involves steps shown as Eqs. (4)–(6). It is clear that with theexcitation of dye to dye⁄, there is release of photogenerated elec-trons. The dye⁄ is oxidized by O2 to an intermediate that degradesaccording to Eq. (5). Meanwhile the released electrons are quicklytaken up by Cr(VI) to give Cr(III) (Eq. (6)).

Dye�!hvDye� þ e� ð4Þ

Dye� þ O2 ! intermediates! degraded products ð5Þ

CrðVIÞ þ e� ! CrðIIIÞ ð6Þ

0 15 30 45 60 75

0.0

0.2

0.4

0.6

0.8

1.0

Irradiation time (min)

Fluorescence Intensity of TAOH at 425 nm (b)

O4 photocatalysts in a basic solution of terephthalic acid (excitation at 315 nm) andphthalic acid).

0 30 60 900.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Irradiation time (min)

Cu2O/BiVO4 no irradiation no catalyst

(a)

0 30 60 900.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Irradiation time (min)

Cu2O/BiVO4 no irradiation no catalyst

(b)

Fig. 9. Simultaneous photocatalytic removal of (a) MB (2 � 10�5 mol/L) and (b) Cr(VI) (3 � 10�5 mol/L) over 2.5%-Cu2O/BiVO4 under visible irradiation.

400 Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402

In the presence of Cu2O/BiVO4, however, there is markedenhancement of MB oxidation and Cr(VI) reduction. It is apparentthat besides the photolysis process, there are photoredox processesthat are catalyzed by the Cu2O/BiVO4 photocatalyst. The photocat-alytic processes are:

Cu2O=BiVO4�!hv

hþ þ e� ð7Þ

hþ þH2OðOH�Þ ! �OHþHþ ð8Þ

Dyeþ hþð�OHÞ ! intermediates! degraded products ð9Þ

First, Cu2O/BiVO4 is excited under visible light and there is theproduction of charge carriers (Eq. (7)). Then the holes react withthe adsorbed water (or OH-) to produce �OH that can oxidize dyemolecules (Eqs. (8) and (9)). In the meantime, there is the directreduction of Cr(VI) to Cr(III) by the photogenerated electrons(Eq. (6)). As depicted in the UV–vis spectra of the mixture, all thecharacteristic peaks of Cr(VI) and MB decrease gradually and disap-pear finally (Fig. S4, SI). Kyung et al. [15] reported that dyes cancoordinate with Cr(VI) to form a complex intermediate. Weemployed the UV–vis technique to monitor the process of photo-catalytic reaction, and did not detect any new adsorption thatcan possibly related to any intermediate (Fig. S4(a), SI). The resultsindicate the reaction of MB with Cr(VI) does not take place in thesolution, further confirming the domination of the photoredoxprocesses. It is worth emphasizing that the degradation processesare conducted in neutral media, and acid adjustment is not required.

It is known that the pH of reacting solution has an significanteffect on the photodegradation of MB and reduction of Cr(VI).We conducted a study on the influence of pH using 1 mol/LHNO3 and 1 mol/L NaOH to adjust the pH to 3.01 and 11.04,

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Irradiation time (min)

no irradiation no catalyst Cu2O/BiVO4

(a)

Fig. 10. Simultaneous photocatalytic removal of (a) RhB (1 � 10�5 mol/L) and (

respectively. Fig. S5, (SI) shows that MB and Cr(VI) cannot beremoved at the same time in acidic or alkaline media, and simulta-neous removal of them only occur effectively under neutral condi-tion. The results further indicate that the processes for thedegradation of dyes and the reduction Cr(VI) can occur at the sametime under the set condition in our experiment.

To see whether Cu2O/BiVO4 can function in other combinationsof dye and Cr(IV), we adopted a solution of RhB (1 � 10�5 mol/L)and Cr(VI) (8 � 10�5 mol/L). Fig. 10 shows that the removal rateof RhB and Cr(VI) are negligible in the absence of light and catalyst.Since the photolysis rate of RhB is much lower than that of MB, thephotoreduction of Cr(VI) in the RhB/Cr(VI) case is significantlylower than that of the MB/Cr(VI) case, plausibly due to the smalleramount of electrons released in the former. In the presence ofCu2O/BiVO4, there is near complete removal of both RhB(Fig. 10(a)) and Cr(VI) (Fig. 10(b)) in 120 min. It is reckoned thatthe degradation of RhB and Cr(VI) undergoes reactions similar tothose of the degradation of MB and Cr(VI). Hence the simultaneousreduction of Cr(VI) and oxidation of organic dyes on Cu2O/BiVO4 ismade possible without any additives due to the synchronizedactions of Cr(VI) being an electron acceptor and organic dye(s)being an electron donator. The processes can be considered astypical for simultaneous photoredox reactions of this kind.

Besides having the reduction of Cr in the solution measured bythe 1,5-diphenylcarbazide colorimetric method [16,26], we moni-tored the product of Cr(VI) reduction by analyzing the valence stateof Cr that was adsorbed on the photocatalyst. The Cu2O/BiVO4 wascollected after the photocatalytic reaction and the surface compo-sition was characterized by means of XPS. From Fig. 11(a), one cansee that besides Cu, Bi, V, O and C species, there is Cr element. Fromthe high-resolution spectrum of Cr2p, there are peaks at 576.2 and

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Irradiation time (min)

no irradiation no catalyst Cu2O/BiVO4

(b)

b) Cr(VI) (8 � 10�5 mol/L) over 2.5%-Cu2O/BiVO4 under visible irradiation.

1200 1000 800 600 400 200 0

Inte

nsity

(a.u

.)

Binding Energy (eV)

Survey

Bi5d

Bi4f

C1s

Cu2p

O1s V2pCr2pBi4dBi4d

(a)

594 591 588 585 582 579 576 573

Inte

nsity

(a.u

.)

Binding Energy (eV)

Cr2p

586.5 eV

576.2 eV

(b)

Fig. 11. XPS spectra of Cu2O/BiVO4 samples after reaction: (a) survey scan and (b) Cr2p spectra.

Scheme 1. Band gap structures of Cu2O and BiVO4, and the possible process for theseparation of charge carriers.

Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402 401

586.5 eV (Fig. 11(b)) ascribable to Cr2p3/2 and Cr2p1/2 of Cr(III).Since there is no detection of the Cr(VI) peaks [46], it is consideredthat there is complete conversion of Cr(VI) to Cr(III).

4. Quantum efficiency for dyes degradation

The quantum efficiency for degradation is defined as the ratio ofthe reaction rate c to the photonic flux u, i.e. the ratio of the num-ber of molecules transformed per second to the number of incidentefficient photons per second [47]. The equation is shown as thefollowing:

q ¼ ru

ð10Þ

The rate of disappearance of MB was found to be0.0457 lmol L�1 min�1 for k P 400 nm (Fig. S6, SI). The corre-sponding efficient photonic flux u was 1.21 � 10�3 mol pho-tons s�1 and the resulting quantum efficiency for dyesdegradation was q = c/u = 0.099%.

5. Possible mechanism for the photocatalytic actions

It is necessary to find out the band structures of the componentsof a composite in order to clarify the mechanism of enhanced pho-tocatalytic performance over the composite. The valance band (VB)and conduction band (CB) potentials of a semiconductor arecalculated using the following empirical equations [48]:

EVB ¼ X þ 0:5Eg � Eg ð11Þ

ECB ¼ EVB � Eg ð12Þ

X ¼ ½xðAÞaxðBÞbxðCÞcxðDÞd�1

aþbþcþd ð13Þ

where EVB is the VB edge potential, X is the electronegativity ofsemiconductor (a,b,c and d are the atomic number of compounds),Ee is the energy of free electrons vs hydrogen (4.5 eV), Eg is theband-gap energy of semiconductor, and ECB is the CB edge potential.Based on the band gaps of Cu2O and BiVO4, the valance and conduc-tion band potentials of Cu2O and BiVO4 were calculated usingEqs. (11)–(13) and the results are illustrated in Scheme 1. The CBpotential of Cu2O is �0.28 eV, more negative than that of BiVO4

(0.46 eV). Hence there is diffusion of electrons from the CB of Cu2Oto BiVO4. At the same time, there is transfer of holes from the VB ofBiVO4 to Cu2O because the valance band potential of the former ismore positive than that of the latter. Thus the photogenerated elec-trons and holes are efficiently separated at the heterojunctions.The separated electrons reduce the adsorbed Cr(VI) to Cr(III), and

the organic dyes are simultaneously oxidized by the separatedholes.

Commonly, the process for the transfer of charge carriers isstudied indirectly by means of PL or photocurrent analysis. Ahigher PL intensity indicates lower charge separation efficiency.To examine the migration, transfer and recombination of thephotogenerated electron-hole pairs at the Cu2O/BiVO4 heterojunc-tions, we measured the photoluminescence (PL) spectra of thephotocatalysts. The room temperature PL emission spectra of pureBiVO4, 2.5%-Cu2O/BiVO4 monitored at an excitation wavelength of325 nm are shown in Fig. S7, SI. A peak at 525 nm was observedover BiVO4. The PL of 2.5%-Cu2O/BiVO4 is lower than that of pureBiVO4. The results indicate that the photogenerated charge carrierscan be efficiently separated on the photocatalyst, further confirm-ing the mechanism for the separation of charge carriers.

6. Conclusions

Cu2O/BiVO4. composites with different Cu2O contents were pre-pared and characterized. They show much higher photocatalyticactivity than Cu2O or BiVO4 for the degradation of MB in neutralmedia under visible light irradiation, and the optimal amount ofCu2O is 2.5%. It was found that the introduction of Cu2O effectivelyinhibits the recombination of photogenerated electrons and holes,leading to the enhanced photocatalytic activity. It was demon-strated that the photocatalyst is effective for the removal of bothcationic and anionic dyes as well as a mixture of them. This isthe first report on the use of Cu2O/BiVO4 for simultaneous removalof organic dyes and Cr(VI) in a neutral medium. A synergistic pho-toreduction-oxidation mechanism is proposed. The results widenup the approaches for the treatment of wastewater in real practice.

402 Q. Yuan et al. / Chemical Engineering Journal 255 (2014) 394–402

Acknowledgements

This project was financially supported by NSFC (Grant Nos.U1162109, 21273067 and J1210040), Program for ChangjiangScholars and Innovative Research Team in University (IRT1238),and the Fundamental Research Funds for the Central Universities.C.T. Au thanks the HNU for an adjunct professorship.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2014.06.031.

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