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Materials Science & Engineering B

journal homepage: www.elsevier.com/locate/mseb

Sonochemical synthesis of ZnO nanoparticles and its use as photocatalyst inH2 generation

E. Luévano-Hipólitoa, L.M. Torres-Martínezb,⁎

a CONACYT, Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455 San Nicolás delos Garza, NL, Mexicob Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455 San Nicolás de los Garza,NL, Mexico

A R T I C L E I N F O

Keywords:UltrasoundZnOHydrogen productionOxygen vacancies

A B S T R A C T

ZnO was synthesized by a sonochemical method and it was used as photocatalyst for hydrogen generation. Thephysical properties of ZnO that had a major influence in the H2 production were high surface area and lowparticle size. Furthermore, the effect of the oxygen vacancies in H2 production was evaluated through theanalysis of the ratio of the intensities of the (0 0 2) and (1 0 0) planes, and it was observed that when this ratiowas higher than 1, a low photocatalytic activity was obtained. This fact was evidenced by the analysis of HRTEMimages, which showed that ZnO sample with the highest H2 production presented a higher amount of (1 0 0)crystal planes and a lower amount of (0 0 2), which confirms the relation observed by XRD. A lower amount of(0 0 2) planes was related to a higher amount of oxygen vacancies on ZnO surface. Thus, when ZnO was used as aphotocatalyst in H2 production, the presence of oxygen vacancies in its surface had a detrimental effect in theactivity due to the photocorrosion phenomenon in ZnO. The highest photocatalytic hydrogen production was107 μmol g−1 h−1. The solar-to-hydrogen efficiencies obtained were higher (0.65%) than the reported values forZnO and other simple oxides in these applications.

1. Introduction

In the scientific literature, several oxides have been investigated forphotocatalytic water splitting, such as TiO2-Pt, NiO-SrTiO3, Sr2Ta2O7,LiTaO3, NaTaO3, MgTa2O6, RuO2-CaIn2O4, and ZnO [1,2]. There are afew reports in which the use of ZnO as a catalyst has been proposedbecause of its numerous advantages such as low cost, low toxicity,thermal and chemical stability. In addition, it has a band gap that isrelatively enough to capture solar light to activate it. However, it isrecommended to take into account the ZnO photocorrosion underUV–Vis irradiation. Different strategies have been proposed in order toincrease the photocatalytic activity of ZnO for example, Viswanathet al. synthesized ZnO by means of a wet etching method that providesa high concentration of defects, which allows the presence of moreactive sites for the generation of 350 μmol g−1 h−1 of hydrogen byusing Na-EDTA as a sacrificial agent [3]. Table 1 shows a summary ofsome scientific publications, which reported the use of ZnO as a catalystin the photocatalytic H2 production [3–13]. As shown in Table 1, themajority of scientific works employed a co-catalyst and a sacrificialagent to increase the photocatalytic activity of ZnO. The main co-cat-alysts that have been proposed in this area are TiO2, CuO, Ga, ZnS,

Ag2S, CdS, and laminar compounds such as InFeO3(ZnO)m [4–11].Regarding the sacrificial agents, methanol is the most widely used,followed by ethanol, and sodium sulfite. In those reports, high tem-perature (≥500 °C) was required to obtain the ZnO phase. The highestH2 production reported in the literature is 11250 μmol g−1 h−1 using alaminar compound, of formula InFe3(ZnO)m that was prepared by asolid-state reaction at high temperatures (1350 °C). The photocatalyticH2 production was high due to the addition of methanol as a sacrificialagent [13]. In addition, in the majority of the scientific works, the useof high power (> 300 W) lamps is proposed, which requires a highamount of energy to work, thereby reducing the solar-to-hydrogen(STH) efficiency.

There are other alternatives to increase the photocatalytic activityof ZnO, such as the design of experiments without the use of sacrificialagents and co-catalysts, which involve complicated synthesis methods,long reaction times, high calcinations temperature, and thus high pro-duction cost. The full factorial design allows studying the effect of eachfactor on the response variable, as well as the interactions between thefactors on the response variable. The use of factorial design has a lot ofadvantages, such as greater precision in estimating the overall factoreffects, the interaction between the factors studied, and additional

http://dx.doi.org/10.1016/j.mseb.2017.09.023Received 12 June 2017; Received in revised form 21 September 2017; Accepted 28 September 2017

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

Materials Science & Engineering B 226 (2017) 223–233

0921-5107/ © 2017 Elsevier B.V. All rights reserved.

MARK

factors can help to extend the validity of the conclusions derived [14].On the other hand, the photocatalytic activity of photocatalysts is clo-sely related to their synthesis methods. For this reason, since the lastdecade, the ultrasound irradiation has been considered a convenienttool to fabricate photocatalysts with novel nanostructures and adequatephysical and chemical properties. Ultrasound irradiation inducesacoustic cavitation, which generates unique physicochemical condi-tions (500 °C, and 100 MPa) in a fast rate of heat conduction(≫1 × 101 °C s−1) [15,16]. These unique conditions provided by ul-trasound irradiation have been applied to synthesize nanostructuredZnO for different applications [17–19]. However, it has not been re-ported for the preparation of ZnO for photocatalytic hydrogen gen-eration (see Table 1).

In recent years, the thin film technology has been proposed byseveral researchers for the fabrication of ZnO with adequate physicaland chemical properties to generate photocatalytic hydrogen. For ex-ample, Kumar and collaborators studied the carrier mobility of thinfilms of Al-doped ZnO prepared by sputtering, and they found thathighly c-axis-oriented films with uniformed sizes promoted high con-ductivity [20]. The same research group investigated the effect ofpressure and power density variations during the deposition of Al-doped ZnO thin films and correlated the preparation parameters withthe electrical properties of films [21].

In the present work, the photocatalytic activity of ZnO prepared bymeans of the so-gel method assisted with ultrasound irradiation andobtained at 100 °C was investigated for hydrogen production withoutthe addition of sacrificial agents. The advantage of the synthesismethod proposed was related to the use of low power of ultrasound(< 150 W) and low reaction times in comparison with previous reportsin the literature, which have reported the use of ultrasonic probes of750 W [22,23]. In addition, we proposed the use of an organic pre-cursor to avoid the inclusion of chlorine and nitrate ions that are dif-ficult to remove before 100 °C. Furthermore, the experimental condi-tions during the synthesis were modified in order to find the optimalconditions to get a ZnO photocatalyst with the adequate properties toproduce H2 from water splitting.

2. Experimental

2.1. Synthesis

ZnO oxide was synthesized by the sol-gel method assisted with ul-trasound radiation. This method implies the preparation of two solu-tions. In the first one, zinc acetate (Zn(O2CCH3)2·2H2O) (99% Fermont)was dissolved in 50 mL of ethyl alcohol at room temperature. A secondsolution 0.2 M of NaOH (99% Fermont) was dissolved in deionizedwater using a conventional ultrasound bath for 5 min. The second so-lution was added dropwise into the zinc acetate solution with vigorousstirring at room temperature. The resulting white suspension was put ina cylindrical glass container of 70 mL. The container was fixed in ametal base support and an ultrasonic probe (Hielscher’s UP200Ht) wasimmersed in the center. The suspension was exposed to different ul-trasonic times from 0 to 60 min. For further information of the ex-perimental set-up used, please see the Supplementary Fig. S1. Thetemperature at the end of the ultrasound treatment in the resultingmixture was 70 °C. The resulting mixtures obtained were centrifuged toremove the solvent. The obtained powders were washed with deionizedwater and ethanol several times in order to remove the by-productsgenerated during the synthesis process. Finally, the powders were driedat 100 °C.

The experiments were designed using the software Minitab™, whichprovides a randomized order to make the synthesis of eight samplesderived from a full 23 factorial design. The factors or variables modifiedwere the concentration of zinc precursor, the power of the ultrasonicprocessor, and the time of the sonochemical treatment. An additionalexperiment was performed using the Optimizer tool of the softwareemployed. Table 2 summarized the experimental conditions used toprepare ZnO.

In order to investigate the possible photo-corrosion of ZnO, threeconsecutive photocatalytic experiments were performed using the re-cycled sample. After the experiment, the ZnO was characterized by XRDand FTIR to investigate its photostability after the photocatalytic re-action. Also, ZnO samples were calcined at different temperatures(200 °C, 300 °C, 400 °C, and 500 °C) during 12 h in order to investigatetheir photocatalytic activity in H2 production.

Table 1Summary of the use of ZnO as catalyst in photocatalytic H2 production.

Photocatalyst Synthesis method Thermal treatment H2 production (µmol g−1 h−1) Experimental conditions Ref.

ZnO Wet etching 400 °C/4h 350 Hg lamp of 450 W.Water + Na-EDTA

3

10%ZnO-90%TiO2 Sol-gel 500 °C/5h 2600 UV lamp of 254 nm. Ethanol/Water

4

90%ZnO-10% CuO Hydrothermal 500 °C/1h 493 Hg lamp of 400 W.Methanol/Water

5

ZnO-Ga Hydrothermal 130 °C/24 h 24 UV lamp of 150 W.Methanol/Water

6

ZnO-GaP1−xNx Solid state reaction in N2 atmosphere 750 °C/14 h 36 Xenon lamp of 300 W.Water

7

Ag2S-ZnO-ZnS Thin films by hydrothermal and chemicalbath

95 °C/6h 1500 µmol g−1 cm−2 UV and Vis light.Water

8

ZnO-ZnS Precipitation 500 °C 6015 Xenon lamp of 300 W.Water + Na2S + Na2SO3

9

ZnO-CdS Photodeposition 500 °C 150 Xenon lamp of 300 W.Water + Na2S + Na2SO3

10

ZnO-CdS Impregnation Not specified 1000 Xenon lamp of 450 W.Water + Na2S + Na2SO3

11

ZnO-Pt-Cd1-yZnyS Impregnation 600 °C/2h 35 Xenon lamp of 450 W.Water + Na2S + Na2SO3

12

InFeO3(ZnO) Solid state reaction 1350 °C/12 h 11250 Hg lamp of 400 W.Methanol/Water

13

ZnO In this work, ZnO was applied as photocatalyst without the use of co-catalyst neither sacrificial agents. For comparative purposes, the STH (%) efficiency wascalculated.

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2.2. Characterization

The structural characterization was carried out by X-ray powderdiffraction using a Bruker D8 Advance diffractometer with Cu Kα ra-diation (40 kV, 30 mA). A typical run was made using a scan rate of0.05° over 0.5 s. From this data, the crystallite size was obtained usingthe Debye’s-Scherrer equation, using Al2O3 corundum as reference, inorder to know the instrument broadening.

The morphology of the samples was analyzed by scanning electronmicroscopy using a JEOL 6490 LV. The optical properties of the sampleswere analyzed between 200 and 800 nm using a UV–Vis NIR spectro-photometer (Cary 5000) coupled with an integration sphere for diffusereflectance measurements. The band gap energy was calculated usingthe Kubelka-Munk function considering a direct transfer of charge inZnO samples. From the Eg values of ZnO, their energy band positionswere evaluated for each sample according to the original proposal byButler and Ginley [24].

The BET surface area measurements were carried out by N2 ad-sorption-desorption isotherms by means of a Bel-Japan Minisorp IIsurface area and pore size analyzer. The N2 adsorption-desorption iso-therms were evaluated at −196 °C after a pretreatment of the samplesat 150 °C for 24 h. The surface groups in ZnO were studied by FTIRusing a Perkin Elmer FTIR/FIR Frontier with ATR accessory in a rangeof 500–4000 cm−1. A Thermo scientific XRD Raman microscope wasemployed to measure the Raman spectra of ZnO samples. All spectrawere taken in the backscattering configuration at room temperature.

The surface study was performed with X-ray photoelectron spec-troscopy (XPS) using a Thermo Scientific K-Alpha XPS instrument withmonochromatized Al Kα radiation (hν= 1486.68 eV). Binding energies(B.E) of all the peaks were corrected using C 1 s energy at 284.5 eVcorresponding to adventitious carbon in addition to the charge com-pensation by the flood gun associated with the spectrometer. Theidentification of crystal planes in ZnO structure was performed by high-resolution transmission electron microscopy using a FEI Titan G280–300 microscopes with an accelerating voltage of 300 kV.

The linear sweep voltammetry was performed using a three-elec-trode cell with Na2SO4 0.5 M as an electrolyte, Ag/AgCl as a referenceelectrode, and Pt as a counter electrode. Prior to the experiments, thesystem was purged with argon for 15 min. The electrochemical ex-periments were carried out using a potentiostat AUTOLABPGSTAT302 N connected to a computer running the software NOVA for dataacquisition. The ZnO samples (working electrode) were fixed in coppertape of an area of 1 × 1 cm2.

2.3. Photocatalytic activity

The photocatalytic reactions for H2 production were carried out in acylindrical Pyrex batch reactor at room temperature. The mass of thephotocatalyst used in each experiment was fixed in 0.1 g and it wasdispersed in 200 mL of deionized water under vigorous stirring. Thesuspension was bubbled with N2 in order to remove the dissolved

oxygen in the reaction medium for 12 min, then the suspension wasirradiated in the center of the photocatalytic reactor by introducing aquartz tube that contains a UV Pen Ray lamp of 254 nm with an irra-diance of 4401 μW·cm−2, according to the provider. The hydrogenproduced was monitored every 30 min using a gas chromatographShimadzu GC-2014 equipped with a thermal conductivity detector(TCD). In order to assess the hydrogen produced of a water splittingreaction under solar or artificial light, a term called solar-to-hydrogen(STH) conversion efficiency was calculated [2]. The definition of theSTH conversion efficiency is shown in equation 1. The error estimationof %STH fluctuated between 3–5% regarding with the variation of theproduction of mmol of hydrogen per second.

=

∗

∗

( )STH

mmol H s

Power lamp Area

( / ) 237 KJmole2

(1)

3. Results and discussion

3.1. Characterization of ZnO

The synthesis of ZnO was carried out by the sol-gel method assistedwith ultrasound irradiation employing as precursors, zinc acetate andsodium hydroxide. The preparation of ZnO involves a consecutive re-action series in basic medium to produce a colloidal gel of zinc hy-droxide, Zn(OH)22- Eq. (2). At the same time, when the mixture wasexposed to ultrasound irradiation, the medium promotes the formationof radicals as OH% and H% Eq. (3) due to the extreme punctual condi-tions created during the implosion of the bubbles by the acoustic ca-vitation phenomenon [25]. Zn(OH)22- reacts with the hydroxyl ions toform the growth unit Zn(OH)42- Eq. (4). Then, Zn(OH)4-2 is converted toZnO by the action of hydroxyl and the extreme punctual conditions ofpressure and temperature that are present in the ultrasonic treatmentEq. (5). On the other hand, Yu et al. has proposed the formation of H2O2

Eq. (6) during the sonochemical treatment, which can react with zinchydroxide Eq. (4) and promote a faster formation of ZnO Eq. (7) [26].

+ → + +Zn CH COO H O NaOH Zn OH CH COONa H O( ) ·2 2 ( ) 2 23 2 2 2 3 2

(2)

%⎯ →⎯⎯⎯⎯ +H O OH HUS

2))) · (3)

Zn(OH)2+OH%→Zn(OH)42− (4)

→ +−Zn OH ZnO H O( )4

2 Δ2 (5)

OH%+OH%→H2 O2 (6)

+ → +Zn OH H O ZnO H O( ) 22 2 2 2 (7)

In order to verify possible residual OH groups in ZnO, an analysis byFTIR was performed in two representative samples (ZnO-2 and ZnO-4)and the results are shown in Supplementary Fig. S2. The presence ofOH- groups was detected in 3369 cm−1, and additional bands asso-ciated with residual carbon were detected in 1569, 1475, and1397 cm−1. The bands that appear in lower wavenumbers are asso-ciated with the presence of crystalline ZnO in the samples. The varia-tion of the band area associated with the presence of hydroxyl groupsseems not to change between the ZnO samples.

The effect of the content of hydroxyl groups in ZnO surface canaffect the photocatalytic activity, since these groups can capture thephotogenerated hole (h+) producing the hydroxyl radical, which ishighly reactive. Usually, an increase in the hydroxyl content on thephotocatalyst surface enhances the photocatalytic activity, especially inthe photooxidation of different organic and inorganic compounds[27,28].Fig. 1 shows the X-ray diffraction patterns of ZnO obtained bydifferent experimental conditions. All the diffraction lines correspond

Table 2Design of experiments and physical properties of ZnO samples.

Sample Zn(O2CCH3)2(M)

US time(min)

US power(W)

Crystallite size(nm)

I

I(002)

(100)

ZnO-1 0.06 30 100 35 1.05ZnO-2 0.06 30 150 44 0.95ZnO-3 0.6 30 100 44 1.16ZnO-4 0.6 60 150 32 1.10ZnO-5 0.6 30 150 33 1.19ZnO-6 0.06 60 150 41 0.98ZnO-7 0.06 60 100 49 0.99ZnO-8 0.6 60 100 32 1.11ZnO-O 0.01 30 150 38 0.94

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with the hexagonal ZnO polymorph wurtzite according to the JCPDScard number 36–1451. From this data, the crystallite size values of ZnOsamples were calculated using the Scherrer equation, which fluctuatebetween 32 and 49 nm and they are summarized in Table 2. Althoughthe crystallite sizes were similar, some differences were observedamong them. The lowest value of crystallite sizes (< 35 nm) wasreached using high concentration of zinc acetate and high power ofultrasound. In addition, the ratio of the intensity of (0 0 2) and (1 0 0)crystalline planes was studied to analyze a preferential orientation in(0 0 2) that is associated with the creation of oxygen vacancies in ZnO(Table 2). According to the results obtained, a clear tendency of thisratio was not found. Although, it was observed that the samples pre-pared at high concentration of zinc acetate (0.2 M) showed higher ra-tios (> 1.1) of the intensities. In the scientific literature, several re-searchers have associated this ratio with the photocatalytic activity,which will be discussed in Section 3.2 of photocatalytic hydrogenproduction [29–33]. This is related to the polar nature of the (0 0 2)plane, which preferentially adsorbs O2 and OH- ions, which eventuallyproduce radicals with high oxidizing power that play an important rolein the degradation of organic compounds. In photocatalytic H2 gen-eration, the presence of oxygen vacancies in the photocatalyst promoteshigh efficiencies, which is related to an improvement: i) in light ab-sorption, and ii) in an efficient charge transfer by means of the gen-eration of inter-bandgap states in the semiconductor [34].

The morphology of the ZnO is shown in Fig. 2. In general, the ZnOparticles showed a sphere-like morphology at this level of resolution(10,000×). It had a tendency to form big agglomerates composed ofsmall particles on their surface in the samples prepared under thelowest concentration of zinc precursor. From SEM images, the averageparticle size of ZnO particles was estimated and the results are sum-marized in Table 3. It is important to note that the sample ZnO-2showed the closest distribution between the samples studied, which ischaracteristic of a homogenous morphology (Supplementary Fig. S3). Inparticular, an analysis performed using the average particle size as aresponse in the statistical software showed that short time of ultrasoundexposure promotes the development of low particle sizes. Also, it wasfound that it is necessary to increase the power in order to promote thedeagglomeration of the particles.

The optical properties of the ZnO samples were investigated byUV–Vis diffuse reflectance spectroscopy. From the reflectance data, theremission function of Kubelka-Munk was calculated in order to knowthe band gap energies (Eg), which are listed in Table 3. The Eg of theZnO samples fluctuated between 3.18 and 3.24 eV, which is in agree-ment with previous reports in the literature [35–37].

From the Eg data, the position of the conduction (CB) and valenceband (VB) of ZnO samples was estimated and it is shown in Fig. 3. It canbe seen that all the samples prepared have the ability to reduce H+ toH2 from a thermodynamic standpoint. An analysis made with the valuesof the CB of ZnO samples showed that the factor that has the main effectin developing more negative values of CB, was the concentration of zincprecursor. In particular, when the concentration of acetate was de-creased, it promoted the development of more negative potentials tocarry out the H2 production.

Table 3 shows the values of the specific surface area of the ZnOsamples prepared by a sonochemical method. The surface area valueswere similar among them; however, the samples prepared with thelowest concentration of zinc precursor showed higher surface areas.Also, the increment in the power of the US treatment has a positiveeffect in increasing the surface area. Regarding the time of US exposure,shorter times (30 min) promote an efficient dispersion of the particlespromoting higher values of surface area. When the time of US exposurewas increased from 30 to 60 min, the particles tended to agglomeratewith each other to develop lower surface area values. The results ob-tained are in agreement with previous reports in literature, in which theagglomeration between the particles becomes notable after longer timeof exposure, which defeats the original purpose of the sonochemicaltreatment [38,39]. Also, this effect is more pronounced when a probe isused instead of a sonochemical bath. One cause for the agglomerationof the particles at this condition is that ultrasonic waves impart vi-bratory motion to the particles, in which the smaller ones are stronglyinfluenced, and thus the increased frequency in collisions promotestheir agglomeration.

The highest surface area obtained was 37 m2 g−1, which is higherthan some of the previous reports for ZnO. On the other hand, ac-cording to the desorption profile, the isotherms can be classified as typeII, which is characteristic of non-porous materials, which are shown inSupplementary Fig. S4.

3.2. Study of the photocorrosion of ZnO

Another aspect to consider in the evaluation of photocatalytic re-actions is the photocorrosion that can experiment the catalyst duringthe reaction. In this sense, previous studies have been reported thephotocorrosion of ZnO by the action of the photogenerated holes (h+)during the activation of the catalyst as is shown in Eq. (8).

+ → ++ +ZnO Zn Oh 0.52

2 (8)

For this purpose, the photocatalytic activity of ZnO-2 sample wasselected in order to evaluate it three consecutive times, and the resultsobtained are shown in Fig. 4a. After a second evaluation, the photo-catalytic activity was almost constant; however, from the third ex-periment a significant decrease in H2 generation was observed. Afterstudying the XRD analysis, we observed a gradual increase in the in-tensity of (0 0 2) plane (Fig. 4b), which is related to defects in ZnO. Fora clear analysis, the ratio of the (0 0 2) and (1 0 0) planes was shown inthe secondary axis in Fig. 4a, which confirms the inverse relationshipbetween the photocatalytic activity and this ratio related to the creationof defects in ZnO. An additional analysis was performed by FTIR aftereach experiment (see Fig. 5). In comparison to the reference sample, theFTIR spectra of the ZnO after the second and third runs showed twoadditional bands in 1162 and 1151 cm−1, which is related to thebending mode of Zn-OH. The peak area of this signal increased aftereach experiment. The formation of this group confirms the photo-corrosion of ZnO, which had a detrimental effect in the photocatalyticactivity of the ZnO-2 sample.

In order to study if other properties changed after the photocatalyticexperiment, the morphology of ZnO-2 particles was analyzed after thephotocatalytic experiment, and the results obtained are shown inSupplementary Fig. S5. In general, no significant change was observedin the morphology. However, after analyzing the size of some particles,

Fig. 1. X-ray powder diffraction patterns of ZnO prepared according with Table 2.

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this property was observed to tend to develop slightly higher values,and the distribution tended to be wider in comparison to the originalsample. These results agree with previous reports, in which differentauthors have observed a slight increase in the particle size of photo-catalyst after the photocatalytic reaction [40,41].

To prevent this phenomenon, some authors have reported the ad-dition of sacrificial agent during the photocatalytic evaluation, such asNa2S, Na2SO4, or as was demonstrated in this work by means of thedecrease of defects on the ZnO surface. Thus, if ZnO is chosen for theproduction of energetic vectors by photocatalysis, it is necessary toselect an optimal synthesis method that provides both high crystallinityand surface area.

4.3. Photocatalytic hydrogen production

The hydrogen production via heterogeneous photocatalysis wasinvestigated in a batch reactor under N2 atmosphere at room tem-perature. Fig. 6 shows the hydrogen production using the eight ZnOsamples. ZnO samples (1, 2, 6, and 7) prepared under low concentrationof zinc precursor showed high photocatalytic activity, producing morethan 28 μmol g−1 h−1. In the best case, ZnO-2 sample had the highestphotocatalytic activity generating 70.8 μmol g−1 h−1 of H2, which was141 times higher than the lowest result (0.5 μmol g−1 h−1) with theZnO-4 sample. After performing an analysis of the physical propertiesobtained by these ZnO samples, the best result can be correlated withfour factors: i) low particle size, and ii) high surface area, iii) a ratioaround 1 of I(0 0 2)/I(1 0 0) associated with a low amount of oxygen

Fig. 2. SEM images of the ZnO samples.

Table 3Physical and chemical properties of ZnO samples.

Sample Crystallitesize (nm)

Particlesize(nm)

Eg (eV) I

I(002)

(100)

as(m2

g−1)

H2

production(μmolg−1h−1)

STH (%)

ZnO-1 35 156 3.2 1.05 30 30.4 0.188ZnO-2 44 122 3.2 0.95 37 70.8 0.437ZnO-3 44 160 3.2 1.16 23 1.5 0.009ZnO-4 32 187 3.1 1.10 20 0.5 0.003ZnO-5 33 191 3.1 1.19 25 7.0 0.044ZnO-6 41 170 3.2 0.98 32 36.9 0.227ZnO-7 49 300 3.2 0.99 28 28.5 0.176ZnO-8 32 245 3.1 1.11 21 1.1 0.006

ZnO-O 38 119 3.2 0.94 38 107.1 0.657

Fig. 3. Energy band positions suggested for ZnO samples.

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vacancies in ZnO, and iv) the photocorrosion phenomenon in ZnO.The ZnO-2 sample had the lowest particle size (122 nm) with a

small distribution of the size of these particles, which is characteristic ofa homogeneous particle size. In heterogeneous photocatalysis, a lowparticle size is associated with a high number of surface sites available

to participate in the reaction. In this sense, the specific surface area ofthe samples with the highest (ZnO-2) and lowest (ZnO-4) photocatalyticactivity were 37 m2·g−1, and 20 m2·g−1, respectively. A complementaryanalysis performed in their adsorption isotherms showed differences inthe pore size related with to the hysteresis loop (see Supplementary Fig.S4a). In the ZnO-4 sample, the hysteresis loop was narrow, which ischaracteristic of non-porous materials. On the other hand, the ZnO-2sample had a higher hysteresis loop, which can be related to a morecomplex network of porosity consisting of an average pore diameter of3.8 nm, according to the BJH analysis (onset Supplementary Fig. S4a).In this sense, high porosity can promote an increment of the active sitesto carry out the reduction of H+ to H2. Thus, according to the resultsobtained a high surface area with high pore diameters favors the H2

production using ZnO as a photocatalyst.Another, ZnO property that can affect the photocatalytic activity is

the ratio of the intensities of (0 0 2) and (1 0 0) planes. When this ratiois higher than 1, several authors have proposed the presence of oxygenvacancies in ZnO [42,43], that promote the increase of H2 productiondue to an improvement of light absorption and the creation of interbandgap states that avoid the recombination of the electron and the hole.However, our results showed an opposite relation. In general, thesamples with ratios less than 1.0, which are associated with a loweramount of oxygen vacancies promote the formation of a higher amountof H2 (see Table 3). For an additional study of this property, an analysisby high resolution transmission electron microscopy (HRTEM) andRaman spectroscopy was performed. For this purpose, the samples withthe highest (ZnO-2) and lowest (ZnO-4) photocatalytic activity wereanalyzed by HRTEM in order to confirm the presence of the (1 0 0) and(2 0 0) planes in both samples. The HRTEM images of Fig. 7a,b showsthat the crystal planes (1 0 0) were preferably present in the samplewith the highest photocatalytic activity for hydrogen generation (ZnO-2). By the contrary, the analysis carried out in the particles of thesample with the lowest photocatalytic activity (ZnO-4) showed a pre-ferential orientation in the (0 0 2) facet (Fig. 7c,d), which confirms therelation observed by XRD. Also, the analysis of the HRTEM imagesrevealed the presence of (1 0 1) plane in both samples. The above re-sults are of considerable importance since the activity of photocatalystscan be influenced by the orientation of their crystals, and the same timeit can be related with the presence of a higher number of oxygen va-cancies. Thus, TEM analysis introduces a differentiating element be-tween the ZnO samples that can be used for establishing the origin oftheir different photocatalytic activities, beyond other physical proper-ties of the material as surface area and particle size.

For comparative purposes, Fig. 8 shows the Raman spectra of theZnO-2 and ZnO-4 samples. It is important to note the measurement ofZnO-4 sample was difficult due to its high luminescence, thus due to thehigh intensity of its spectra, this was shown as secondary axis in Fig. 8.This effect can easily be seen in a broad range of wavenumbers, in theOnset of Fig. 8. The band associated with oxygen vacancies appears in

Fig. 4. a. Evaluation of the photocorrosion inZnO-2 sample, and b. X-ray powder diffraction ofZnO-2 after three consecutives.

Fig. 5. FTIR spectra of the ZnO after the photocatalytic test.

Fig. 6. Photocatalytic hydrogen production after 3 h of continuous irradiation.

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585 cm−1 [44], and its intensity in the ZnO-2 spectra is significantlylower than in ZnO-4. Other bands associated with the wurtzite structureappear in 437 and 383 cm−1. In addition a signal appeared in334 cm−1, which is associated with a multiple phonon scattering pro-cess. In summary, from this characterization we assumed that in thesample with the lowest photocatalytic activity (ZnO-4) a high re-combination of the photogenerated pair occurred, which can be asso-ciated with its higher number of oxygen vacancies and high lumines-cence. Also, it is assumed that this kind of defects promoted thephotocorrosion phenomenon in ZnO.

To obtain information of the composition of ZnO samples and to

determine the chemical oxidation states, a XPS analysis was performedto two representative samples: ZnO-2 (Fig. 9 a,b) and ZnO-4 (Fig. 9 c,d).The elemental composition of the samples is summarized in Table 4.The compositional analysis of the ZnO samples indicated that they arenon-stoichiometric, corroborated in the deconvolution of the O1s peaksand the Zn/O ratios. The Zn/O ratio was higher in the ZnO-4 sample,and this fact can be attributed to the presence of oxygen vacancies.Fig. 9a and c shows the Zn 2p3/2 spectra, with a single peak centered at1021 eV, indicative of the presence of zinc in Zn2+ oxidation state. Theoxygen O1s spectrum was fitted into two peaks (O1 and O2). The O1peak (529.6 eV) corresponds with O2- ions in the wurtzite structure ofZnO. The O2 peak is associated to O2- ions in the oxygen deficient re-gions of the ZnO lattice. The intensity of this peak is related to theconcentration of oxygen vacancies [45,46]. As it can be observed inFig. 9, the intensity of this peak was slightly higher in the ZnO-4sample. Thus, we assume that this sample exhibits a higher con-centration of oxygen vacancies, which is in agreement with the resultsobserved in the analysis by Raman spectroscopy.

In order to obtain information of the residual carbon of the pre-cursors employed in the synthesis of ZnO, the C1s spectrum of thesamples was obtained. Three main peaks were observed at 284.5 eV,286.2 eV and 288.2 eV, which were attributed to CeC/CeH, CeO, andC] O, respectively. These signals confirm the presence of minor tracesof residual carbon in the samples. According to the C1s spectrum, theZnO-2 sample exhibited a higher amount of residual carbon, whichcould be positively influencing the photocatalytic performance of thematerial (Fig. 10)

The effect of the content of hydroxyl groups (OH-) in ZnO surfacecan also affect the photocatalytic hydrogen generation (see FTIR data inFig. 5) since hydroxyl groups can be oxidized by the photogeneratedholes to produce hydroxyl radicals (OH%) Eq. (9) that eventually

Fig. 7. HRTEM images of the a. b. ZnO-2, and c. d. ZnO-4samples.

Fig. 8. Raman spectra of some representative ZnO samples.

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decomposed to O2 and protons (H+), which will be available to reactwith the photogenerated electrons to form H2. In addition, the presenceof an adequate amount of hydroxyl groups over ZnO surface can be veryconvenient since the holes generated during its activation will be re-acting with OH- instead ZnO, which will eventually prevent its photo-corrosion Eq. (8)

OH-+h+→OH· (9)

Additionally, a linear sweep voltammetry (LSV) was performed in

Fig. 9. XPS spectra of the Zn 2p3/2 and O 1s for the samples a, b. ZnO-2 and c, d. ZnO-4.

Table 4Atomic concentration of oxygen and zinc elements on the surface of ZnO.

Sample/Composition Zn (%) O (%) Zn/O

ZnO-2 18.85 18.72 1.007ZnO-4 37.98 34.66 1.095

Fig. 10. XPS spectra of the C1s for the samples a. ZnO-2 and b. ZnO-4.

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order to know the amount of charge carriers in the ZnO-2 and ZnO-4samples, which are responsible for the reduction of H+ to H2. Thesevalues were obtained from the slope of the linear part of theSupplementary Fig. S6a and were 3 × 1012 cm−3 and 2 × 1012 cm−3

for the ZnO-2 and ZnO-4 samples, respectively. The values obtainedconfirm the higher photocatalytic activity of ZnO-2 due to a highdensity of charge carriers per unit of active surface area with respect toZnO-4. Similarly the Nyquist plot, which was associated with chargetransfer resistance and the separation of the electron and the hole wereinvestigated. As it can be seen in Supplementary Fig. S6b, a larger ra-dius was observed in the ZnO-4 sample, which can be related to highercharge transfer resistance. In contrast, the sample with the highestphotocatalytic activity ZnO-2 showed a lower radius related to a higherconductivity and a higher charge transfer.

In regard to solar-to-hydrogen efficiency (%), the highest efficiencyreached was 0.43% with the ZnO-2 sample (see Table 3). In the pho-tocatalytic H2 production, the STH efficiencies of some simple oxidessuch as: Fe2O3 (15%), WO3 monoclinic (6%), and TiO2 anatase (1%)have been reported [47]. In the case of ZnO, the STH efficiency only hasbeen reported for the photoelectrochemical H2 production. The re-ported values have been 0.10% for ZnO nanoparticles [48], 0.28% forCu-doped ZnO [49], and 0.38% for core-shell mixtures of ZnO-ZnS[50]. The values obtained in this work are higher than the values re-ported, which confirmed the ability of ZnO to carry out the split ofwater.

3.4. Analysis of the design of experiments

As it was mentioned in previous sections, low concentration of zincprecursor, low times of the ultrasound exposure and higher power ofultrasound promote the best conditions to prepare a ZnO photocatalystwith the ideal properties to carry out an effective production of H2. Thesoftware employed proposed a mathematical model in order to predictthe H2 production Eq. (10).

= − + ∗ + ∗ + ∗ − ∗ ∗ − ∗ ∗

− ∗ ∗ + ∗ ∗ ∗

H C t P C t C P

t P C t P

367.7 505.4 6.773 4.751 9.321 6.744

0.06982 0.095932

(10)

Where H2 represents the hydrogen production in μmol·g−1, C is theconcentration of zinc acetate in molarity, t is the time in min, and P isthe power of the ultrasound in Watts. The production of H2 obtained

from equation 10 was close to the experimental data (Fig. 11). The errorpercentage was less than 5% in all the cases. Thus, using the modelobtained we can assume that if we decrease C, the hydrogen productiontends to increase. Using the response optimizer tool in the statisticalsoftware, it suggests the following experimental conditions to prepareZnO: 0.01 M, 30 min of US, and 150 W (ZnO-O). Theoretically, underthese experimental conditions, the H2 production might be230 μmol·g−1, however when the ZnO was prepared under the condi-tions suggested (C = 0.01 M, t = 30 min, and P = 150 W) the pro-duction was higher than the expected value (320.6 μmol·g−1) (Fig. 12)due to the extrapolation of the data. Also, it is important to discuss thelimitations of equation 10. In this sense, the minimum concentration ofZnO is 0.01 M, otherwise the amount of the product (ZnO) will be verylow, and there will not be mass enough to carry out the photocatalyticreaction. Also the power of the microwave oven is limited to 150 W.

Fig. 11. Experimental and predicted values of photocatalytichydrogen production.

Fig. 12. Photocatalytic hydrogen production for 3 h using the ZnO sample optimized.

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3.5. Effect of the calcination temperature in the photocatalytic activity

In order to investigate the effect of the calcination temperature onthe photocatalytic activity, the ZnO-2 sample was calcined at 200, 300,400 and 500 °C for 12 h. Fig. 13 shows that as long as the calcinationtemperature increases, the hydrogen production decreases until< 5μmol g−1 h−1. The low photocatalytic activity obtained at higher cal-cination temperature of ZnO can be related to a decrease in the surfacearea, in spite of the increase in its crystallinity. As it was previouslymentioned, one factor that played an important role in reaching a highH2 production is the surface area due to the improvement of a chargeseparation and the diffusion process between the species involved. Asthe calcination temperature increased, the surface area decreased,which has a detrimental effect in the activity due to a limited diffusionprocess between the species involved in the photocatalytic process. Ananalysis performed in the N2 isotherms of each sample showed a col-lapse in the pore size due to the decrease in the size of the hysteresisloop (Supplementary Fig. S4b). From these experiments, we can con-clude that the surface area played the most important role in the pho-tocatalytic production of H2 using ZnO prepared by sol-gel assisted withultrasound irradiation.

3.6. Effect of the source of irradiation in photocatalytic hydrogen generation

In order to evaluate the photocatalytic activity of ZnO-2 sampleunder different sources of irradiation and study how the material will

be performed under real application, an additional experiment wascarried out using a xenon lamp to simulate the solar spectrum (UV andVisible light), and the results are shown in Fig. 14a. In general, thephotocatalytic activity decreased 42% using a xenon lamp as irradiationsource in comparison to the experiment performed using a UV-lamp asirradiation source. Also, it is very important to note that the irradianceof the xenon lamp doubled that of the UV lamp, thus if the %STH wasestimated, the solar-to-hydrogen efficiency under this experimentalcondition was 0.12%, which was lower than the value obtained with alow irradiance UV lamp (0.43%).

Fig. 14b shows the emission spectrum of the lamps used, and in asecondary axis the absorption spectra of ZnO-2. As can be seen inFig. 14b, only the UV lamp provided enough energy to promote theexcitation of the electron in VB to CB. However, the xenon lamp pro-vided a slight amount of UV lamp, which allowed it to excite the ma-terial and promote the generation of a higher number of electrons andholes to start the photocatalytic reaction.

4. Conclusions

The photocatalytic production of hydrogen was performed using asustainable process without sacrificial agents or a co-catalyst at roomtemperature. The synthesis of ZnO was performed by the sol-gel methodassisted with ultrasound irradiation to avoid high temperatures andlong reaction times during the synthesis. The most important physicalproperties of ZnO that affect the photocatalytic H2 production were: i)high surface area, ii) low particle size, iii) a low amount of oxygenvacancies related to a decrease in the ratio of the intensities of (0 0 2)and (1 0 0), and iv) the photocorrosion phenomenon. The low particlesize was associated with a high number of surface sites available toparticipate in the reaction, and thus a high surface area. An additionalanalysis in the N2 isotherms of the ZnO samples showed that the samplewith high hysteresis loop resulted in higher pore diameter and a highphotocatalytic activity, which can be related to a porous network thatcan promote an increment of the active sites to carry out the reductionof H+ to H2. The importance of the surface area was evidenced whenthe ZnO samples was calcined at higher temperature.

In addition, we propose that the presence of oxygen vacancies de-creases the photocatalytic activity for H2 production in ZnO from ananalysis of the ratio of the intensities of (0 0 2) and (1 0 0) planes. Inthis sense, when these intensities were in the same proportion (around1.0), the efficiencies for H2 generation were higher than those with aratio about 1.05. The presence of oxygen vacancies in the sample withthe highest ratio and lowest photocatalytic activity was confirmed byRaman spectroscopy, in which a high recombination of the photo-generated pair occurred despite of its higher number of oxygen va-cancies. Also, it is assumed that this kind of defects promoted the

Fig. 13. Effect of calcination temperature of ZnO in the photocatalytic H2 productionafter 3 h of reaction.

Fig. 14. a. Comparison of the photocatalytic activity of ZnO-2 sample using UV and UV–Vis lamp as irradiation source, and b. Lamp emission and ZnO absorption in 200–900 nm.

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photocorrosion phenomenon in ZnO.A mathematical model was proposed to estimate the photocatalytic

H2 production, which allowed us to perform the optimization of thesynthesis to produce a photocatalyst with the highest H2 production(107 μmol g−1 h−1) and STH% (0.65%) without using co-catalyst norsacrificial agents.

Acknowledgments

The authors wish to thank CONACYT for financial support for thisresearch through the following projects: Cátedras 1060, CB-2014-237049, INFR-2015-251936, PAICYT 2015, SEP PIFI 2014, and SEPIntegración de redes temáticas de colaboración académica 103.5/15/14156. Also, the authors want to thank PhD Marco Antonio GarzaNavarro for its valuable help in HRTEM analysis.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in theonline version, at http://dx.doi.org/10.1016/j.mseb.2017.09.023.

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