Journal of Photochemistry and Photobiology A: Chemistry 347 (2017) 98–104

Synthesis and characterization of PbS/ZnO thin film for photocatalytichydrogen production

Omar A. Carrasco-Jaima,d, O. Ceballos-Sanchezb,c,*, Leticia M. Torres-Martíneza,Edgar Moctezumad, Christian Gómez-Solísa

aUniversidad Autónoma de Nuevo León, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N Ciudad Universitaria, SanNicolás de los Garza, Nuevo León, C.P. 66455, MéxicobConacyt � Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N CiudadUniversitaria, San Nicolás de los Garza, Nuevo León, C.P. 66455, MéxicocDepartamento de Ingeniería de Proyectos, Universidad de Guadalajara CUCEI, Zapopan, Jalisco 45100, Méxicod Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Manuel Nava # 6, San Luis Potosí, S.L.P. C.P. 78290, México

A R T I C L E I N F O

Article history:Received 10 March 2017Received in revised form 10 June 2017Accepted 17 July 2017Available online 20 July 2017

Keywords:PbS/ZnO thin filmsPhotocatalysisHydrogen production

A B S T R A C T

In this work, PbS/ZnO thin film was designed in order to assess the photocatalytic hydrogen productionfrom the water splitting. A p-type PbS thin film was prepared by chemical bath deposition onto a glasssubstrate, followed by an n-type ZnO thin film deposition by RF magnetron sputtering. The structural andmorphological characterization was carried out by grazing angle x-ray diffraction (GA-XRD), and atomicforce microscopy (AFM), respectively, while the optical properties were studied using ultraviolet-visiblespectroscopy (UV–Vis) and spectroscopic ellipsometry (SE). The photocatalytic activity was evaluated byquantifying the hydrogen production under UV light irradiation using gas chromatography. The resultsshowed a higher photocatalytic hydrogen production of PbS/ZnO thin film (7.38 mmol cm�2h�1)compared to PbS (3.35 mmol cm�2h�1) and ZnO (2.45 mmol cm�2h�1) thin films. Linear sweepvoltammetry (LSV) test showed the synergistic effect between PbS and ZnO thin films for the generationand transport of the charge carriers.

© 2017 Elsevier B.V. All rights reserved.

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Journal of Photochemistry and Photobiology A:Chemistry

journal home page : www.elsevier .com/ locat e/ jphotochem

1. Introduction

One of the main challenges facing humanity is the generation ofalternative clean and renewable fuels that allowing to meet thefuture energy needs. About 85% of our energy consumption isprovided by fossil fuels, which will not be able to keep up with theincreasing energy demand. The impact on the environment is animportant concern associated with the use of fossil fuels.Moreover, the contribution of greenhouse gasses to globalwarming, in particular, carbon dioxide (CO2), is forcing us toexplore new alternative energies sources. Hydrogen is a renewable,clean and environmentally friendly energy source that hasreceived considerable attention to solving the global energy crisis[1–4]. In this context, different processes have been used to

* Corresponding author at: Conacyt-Universidad Autónoma de Nuevo León,Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av.Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León, C.P.66455, México.

E-mail address: ceballos0516@gmail.com (O. Ceballos-Sanchez).

http://dx.doi.org/10.1016/j.jphotochem.2017.07.0161010-6030/© 2017 Elsevier B.V. All rights reserved.

produce hydrogen gas, such as electrolysis, thermochemical andphotobiological conversion. Photocatalytic water splitting is apromising and economically method to be used in the hydrogenproduction compared with the conventional methods [5,6]. In thiscontext, numerous efforts have been made to develop efficientphotocatalysts capable of splitting the water molecule undervisible light. Some simple oxides as ZnO, TiO2, WO3 and others [7–9], have been modified to improve its light absorption ability byintroducing of impurities into crystalline structure, morphologicalmodifications or the design of semiconductor heterojunctions [10].It has been reported that the heterojunction semiconductorsshowed a higher performance in photovoltaic and photoelectro-chemical cells compared to a single semiconductor [11–13]. Thedesign of this structure-type offers different advantages, since thedoping level as well the conduction and valence bands offset at theinterface can be controlled [14,15]. Under this configuration, thecharge carriers generated in one semiconductor are transferred tothe other one, decreasing the electron-hole pair recombination. Inparticular, the PbS/ZnO heterojunction has been widely used inphotovoltaic devices, in which the ZnO film is used as transparent

Fig. 1. GA-XRD patterns of the PbS, ZnO, and PbS/ZnO thin films deposited on glasssubstrates.

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layer and PbS film as absorbent layer [16,17]. The PbS film plays animportant role in the electron injection to ZnO film, when theconduction band level of PbS is above the electron affinity of ZnO.In general, the performance of a heterojunction is determined bythe interface quality (structure and composition), since theformation of defects or non-favorable chemical species affectthe charge carrier transport.

A few reports about the PbS/ZnO thin film have been made forphotocatalytic applications [18,19]. Hence, in this work, PbS/ZnOthin film was prepared by the combination of RF magnetronsputtering and chemical bath deposition techniques in order toevaluate its photocatalytic activity for hydrogen production viawater splitting. The photocatalytic test showed that the PbS/ZnOthin film exhibits a higher hydrogen production in comparisonwith the PbS and ZnO films by themselves. This is mainlyassociated with the effective separation of the charge carrierand their transport to the surface due to suitable positions ofenergy bands. The quality of the PbS/ZnO interface plays animportant role in the transport of this charge carrier from the PbSto ZnO surface.

2. Materials and methods

PbS thin film was deposited by chemical bath depositiontechnique (CBD). The deposition was done in a reactive solutionprepared in a 100 mL beaker containing lead acetate 0.07 M, lacticacid 0.07 M used as a complexing agent and thiourea 0.05 M assulfur source. The pH of the solution was adjusted to 10.5 by addingammonium hydroxide. Before the immersion of glass substrateinto the growth solution, it was washed ultrasonically withdeionized water (DIW) and absolute ethanol, respectively for15 min. Cleaned substrate was placed vertically in the depositionbath at 80 �C for 2 h. In this process, PbS deposition takes place dueto a chemical reaction in which controlled sulfur and lead ionsrelease are involved. First, dissociation of lactic acid � metalcomplex initiates a slow release of Pb2+ ions. At the same time, arelease of S2� ions starts due to thermally decomposition ofthiourea. When ions react, tend to form a uniform deposit onto thesubstrate. The general reaction is described as follows [20,21]:

Pb CH3COOð Þ2 þ CS NH2ð Þ2 þ 2OH�

! PbS þ 2 CH3COOð Þ� þ CH2N2 þ 2H2O ð1ÞAfter deposition, the substrate was removed from the solution,

rinsed with DIW, absolute ethanol and finally dried in air.ZnO thin film was deposited by RF magnetron sputtering

(13.56 MHz) onto PbS thin film previously prepared by CBD. A highpurity ZnO target (99.999%) of 2-inch diameter was used. Beforedeposition process, the chamber was evacuated until up a basepressure of 6.5 �10�6 Torr. A working pressure of 1.5 �10�2 Torrand an argon flow of 15 sccm were maintained during thedeposition process. A pre-sputtering process of 10 min was carriedout before deposition process begins to remove any contaminationfrom the target surface. The ZnO film was deposited at roomtemperature for two hours keeping a power of 80 W.

The structural properties of the PbS, ZnO and PbS/ZnO thin filmswere determined by grazing angle X-ray diffraction (GA-XRD) atlow incidence angle (u = 0.5) using a D8 Advance Brukerdiffractometer with Cu Ka radiation (l = 1.5418 Å). The topographyof the thin films was measured by atomic force microscopy (AFM)in contact mode. The root mean square (RMS) of the thin films wasdetermined from AFM images. Transmittance spectra wereobtained using a Cary 5000 UV–Vis-NIR spectrophotometer. Thethickness of the thin films was determined by spectroscopicellipsometry (SE) measurements. The ellipsometric angles psi (C)and delta (D), which determine the polarization state of light, were

measured using a Horiba Jobin Yvon UVISEL ellipsometer. The SEmeasurements were collected in the energy range of 0.6 to 4.7 eV(in steps of 0.05 eV) at an incident angle of 70�.

The photoelectrochemical performance of PbS, ZnO and PbS/ZnO thin films was evaluated through linear sweep voltammetry(LSV) under 254 nm UV light using an AUTOLAB PGSTAT 302Npotentiostat-galvanostat with a conventional three electrodessetup. For these electrochemical purposes, the thin films weredeposited onto indium tin oxide (ITO) substrates in the same wayas in the glass substrates (Sigma Aldrich, 15–25 V/sq) and used asworking electrode (with an effective area of 2.25 cm2); Ag/AgCl andPt wire as reference and counter electrodes, respectively. Theelectrolyte was 0.5 M Na2SO4 solution. Mott-Schottky (M-S) plotswere obtained by electrochemical impedance spectroscopy (EIS) inorder to calculate the flat band potential (Efb) of the films in therange from �0.2 to 0.4 V vs NHE at 1000 Hz frequency.

Photocatalytic hydrogen production of the PbS, ZnO and PbS/ZnO thin films was done with a UV pen-ray lamp (254 nm) asirradiation source. The area of the as-deposited films used in everysingle test was around 10 cm2. Experiments were carried out atroom temperature using 200 mL of DIW in a batch-type reactor.Nitrogen gas was bubbled into the reactor for 10 min before thereaction started. The hydrogen amount produced was determinedby a sampling process every 30 min during 3 h, using a ShimadzuGC-2014 chromatograph with TCD detector.

3. Results and discussion

3.1. Structure and surface morphology

Fig. 1 shows the GA-XRD patterns of the PbS, ZnO, and PbS/ZnOthin films. For PbS film, the peaks at 2u = 25.87�, 29.95�, 42.79�,50.70�, 53.23� and 62.11� are related to (111), (200), (220), (311),(222) and (400) planes, which are consistent with cubic structureof galena (JCPDS 01-077-0244) [22]. In the case of ZnO thin film, astrong peak at 2u = 34.01� associated to (002) plane is observed,which indicates a preferential growth along the c-axis of thehexagonal structure (JCPDS 01-0891397) [23]. The (002) plane ofthe ZnO thin film is the most thermodynamically favorable growthplane because of its low surface energy and the most denselypacked in the wurtzite structure [24]. Other peaks correspondingto (102) and (103) planes are also present with very low intensity

Fig. 3. Optical transmittance spectra of the PbS, ZnO, and PbS/ZnO thin films.

100 O.A. Carrasco-Jaim et al. / Journal of Photochemistry and Photobiology A: Chemistry 347 (2017) 98–104

compared to (002) plane. Finally, the GA-XRD pattern of the PbS/ZnO thin film shows clearly the characteristic peaks of both PbSand ZnO thin films, without additional phases as PbO, PbSO4 orZnS. The average crystallite sizes were calculated by Scherer’sformula [23,25], using the (200) and (002) peaks of the PbS andZnO GA-XRD patterns, respectively.

D ¼ Klbcosu

ð2Þ

Where D is the size of crystallites, l is the wavelength of Cu Karadiation (1.5418 A), K is correlation factor (0.94), b is FWHM of(200) and (002) peaks and u is Bragg’s diffraction angle. Theaverage crystallite sizes were found to be around 6.5 nm and4.9 nm for PbS and ZnO thin films, respectively.

The topography of the PbS/ZnO thin film was characterized byAFM as shown in Fig. 2, with a scanning area of 2 mm x 2 mm. Ascan be seen in Fig. 2a, the surface of the PbS thin film deposited byCBD shows irregular grains, which are randomly distributed on thesurface of the film. Conversely, ZnO thin film deposited by RFmagnetron sputtering shows a uniform surface with a lowersuperficial roughness (Fig. 2b). The RMS values obtained for PbSand ZnO films were found to be around 10.2 nm and 5.1 nm,respectively. When ZnO film is deposited onto PbS film, the ZnOfilm tends to growth with the morphological characteristics of thePbS film, increasing the superficial roughness up to 17.1 nm (seeFig. 2c). Some reports have shown that the superficial roughnessfavors the photocatalytic activity due to the formation of moreactive sites [26,27]. It is well known that the growth of a thin filmdepends strongly on the surface characteristics of the substrate[28]. So, it is likely that during the growth of the ZnO thin film, an

Fig. 2. 2D AFM images for (a) PbS, (b) ZnO, and (c) ZnO/PbS thin fi

amorphous interface is formed between PbS and ZnO thin films asconsequence of the difference between the crystal structures [29].

3.2. Optical properties

Fig. 3 illustrates the transmittance spectra as a function of thewavelength for PbS, ZnO, and PbS/ZnO thin films in the region of300–2000 nm. It was observed that ZnO thin film spectrum showsa maximum transmittance of around 90% in the visible light

lms, respectively. The scanning area for all images is 2 � 2 mm.

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wavelength range. PbS thin film shows low transmittance in theUV–Vis region increasing toward the infrared region. For the caseof PbS/ZnO thin film, it is observed a slight increase in thetransmission spectrum for the visible light wavelengths range,might be due to the interaction between both PbS and ZnO thinfilms [30]. Thus, ZnO thin film acts as transparent layer in the PbS/ZnO thin film, allowing the light absorption of PbS thin film.

By building an ellipsometric representative model of the PbS/ZnO thin film, it was possible to know the thickness and band gapenergy for the PbS and ZnO films. The proposed model was madeconsidering the PbS/ZnO system onto glass substrate. The PbS/ZnOinterface layer and the superficial roughness (considered as anexternal layer) was represented by a two-phase system describedwith a Bruggeman’s effective medium approximation (EMA). Inboth cases, the volume fraction of the phases mixture wasremained 50-50 during the fitting process. For describing theoptical response of the PbS and ZnO thin films, it was used theTauc-Lorentz’s expression (TL) with two oscillators [31]. Fig. 4shows the spectral dependence of the ellipsometric angles psi (C)and delta (D) (which represent the polarization state of light)against the photon energy (dots). Also, the best fitting (solid lines)that reproduce the experimental data of the PbS/ZnO thin films ispresented. As can be seen, the theoretical model reproducesclosely the experimental data according to ellipsometric modelmentioned above.

According to SE results, PbS and ZnO thin films can berepresented properly considering the TL expression. The obtainedthickness for each film in the PbS/ZnO thin film was around118.3 nm (PbS) and 146.4 nm (ZnO), while the thickness of theinterface layer was 29.4 nm. The thickness of this interfacial layer isconsistent if we consider that there is an intermixing of bothmaterials. The fundamental energy band gap (Eg) of the PbS thinfilm was found to be 0.44 eV, while for the ZnO film is 3.01 eV [32–36]. However, there are some reports that have shown Eg values forPbS thin films above 0.44 eV calculated from transmittancespectrum using Tauc’s plots. For this consideration, similarcalculation was done (Supplementary information) obtaining asimilar value with those previous reports (�1.6 eV) [37–39]. Thus,it is probably that this energy value obtained from transmittancespectrum correspond to another electronic transition type near toband gap [40]. From SE previous reports, it is possible to evidencethis discrepancy since the complex dielectric function of the PbSthin film has shown two critical points above of the fundamentalenergy band gap [41,42]. These critical points are related to thedominant fundamental reflectivity features in the 300–900 nm

Fig. 4. Experimental ellipsometry spectra (dots) along with the best fitting (solidline) obtained by reproducing the spectral response of the PbS/ZnO thin films onglass substrate.

spectrum range, where they take place at S(E1) and D(E2) pointsof the Brillouin zone for PbS crystal.

3.3. Photoelectrochemical measurements

Fig. 5 shows the LSV test for the PbS, ZnO and PbS/ZnO thin filmsunder UV light irradiation (254 nm). As can be seen, at 0 VNHE (thetheoretical potential necessary for water reduction [5]), thegenerated cathodic photocurrent for the ZnO thin film is negligiblecompared with the cathodic photocurrent of PbS/ZnO thin film,which was significantly enhanced by the synergy between the PbSand ZnO thin films to generate the charge carriers for reduction ofwater molecule. Additionally, the photocurrent of PbS/ZnO thinfilm at the onset potential (0.2 V vs NHE) was improved respect toPbS and ZnO thin film, which means that the electron-holetransport to the surface was improved [13,43]. This observation isconsistent with the fact that PbS/ZnO thin film dispose of a widerange of the light spectrum, as was observed in the transmittancespectra, Fig. 3.

The linear variation of C�2 against the applied potential can berepresented using Mott-Schottky plots (M-S):[34,44]:

1

C2 ¼ 2

qA2NDee0

!E � Ef b �

kTq

� �ð3Þ

where C is the interfacial capacitance, e is the dielectric constant, e0is the permittivity of free space, ND is the carrier concentration, Efbis the flat band potential, K is the Boltzmann’s constant, T is theabsolute temperature, and q is the elementary electron charge.Fig. 6 shows the Mott-Schottky plots obtained for a) PbS and b) ZnOthin films. The positive slope of M-S plot for ZnO thin film confirmsthe n-type semiconductor characteristic, while negative slope ofPbS thin film is related to p-type semiconductor. The flat bandpotential is given by the intersect with the axis potential. Thus, theEfb values obtained for PbS and ZnO thin films were 0.31 V (vs. NHE)and �0.35 V (vs. NHE), respectively. These values can be assigned asthe energy of the conduction band (ECB) for the n-typesemiconductor and the energy of the valence band (EVB) for thep-type semiconductor [44]. The relationship between Eg, EVB andECB is given by the following equation [45,46]:

EVB ¼ Eg þ ECB ð4ÞThe determination of valence and conduction bands of both

films were done and summarized in Table 1. It can be observed thatthe potential of the photogenerated electron of PbS thin film(-0.13 V vs NHE) is lower enough to drive the hydrogen evolution

Fig. 5. LSV test of the PbS, ZnO and PbS/ZnO thin films under UV light irradiation(254 nm).

Fig. 6. Mott-Schottky plots of a) PbS, and b) ZnO thin films, with respect to thenormal hydrogen electrode (ENHE= 0 V).

Fig. 7. a) Photocatalytic activity of the PbS, ZnO, and PbS/ZnO thin films for thehydrogen production. b) Proposed mechanism for the charge transfer in PbS/ZnOthin film using a UV light irradiation.

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reaction (H+! H2, E� = 0 V vs NHE), as well the potential of ZnO thinfilm (-0.35 V vs NHE). On the other hand, the photogenerated holepotential of ZnO thin film (+2.66 V vs NHE) is higher than wateroxidation potential (H2O ! O2, E� = +1.23 V vs NHE), but remainslow for PbS thin film (+0.31 V vs NHE). Note that the ZnO thin filmexhibits both, oxidation and reduction behavior while PbS thin filmonly for reduction. Therefore, the combination of both semi-conductors can be more efficient for photocatalytic water splittingdue to their suitable band positions.

3.4. Photocatalytic hydrogen production

Fig. 7a shows the photocatalytic hydrogen production of PbS/ZnO thin film. As expected, when PbS and ZnO thin films werecombined the amount of hydrogen produced was distinctlyimproved to � 22 mmol cm�2, corresponding to QE of 1.09%, incomparison to PbS (� 10 mmol cm�2) and ZnO (� 7 mmol cm�2),respectively. All results are summarized in Table 1.

Table 1Band positions and photocatalytic activity for the PbS, ZnO, and PbS/ZnO thin films.

Sample Eg (eV) ECB (V) EVB (V) H

PbS 0.44 �0.13 0.31 10ZnO 3.01 �0.35 2.66 7.PbS/ZnO – – – 2

The apparent quantum efficiency (QE) was determinedtheoretically by quantifying the amount of hydrogen generated(NH2 ) at a given photon flux (Nhv) using the formula [47]:

QE %ð Þ ¼ 2NH2

Nhvð5Þ

The improved photocatalytic activity of the PbS/ZnO thin filmfor the hydrogen production could be explained by their photo-catalytic mechanism based on the band structure analysis.Considering that conduction band of ZnO thin film (-0.35 V vsNHE) is more negative than conduction band of PbS thin film(-0.13 V vs NHE), the electron transfer was expected to be from ZnOto PbS thin film due to its lower potential. As previous mentioned,there are electronic transitions above Eg of PbS thin film that havebeen related with the charge carrier generation in the photo-catalytic process. It has been demonstrated that high energyelectrons could be thermodynamically excited by light with

2 amount (mmol cm�2) H2 rate (mmol cm�2h�1) QE (%)

.06 3.35 0.4935 2.45 0.362.16 7.38 1.09

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wavelength shorter than band gap [40]. Interestingly, it isconsistent with the results obtained by Tauc’s plots (Supplemen-tary information), in which other transitions near to fundamentalEg were observed. Therefore, in Fig. 7b is shown a proposedmechanism in which the photocatalytic process for the hydrogengeneration are carried out by this way.

Under UV light irradiation (254 nm), photons are absorbed bythe PbS thin film which is favored for transfer of high-energyelectrons from (EVB) to levels above (ECB). Then, the photo-generated electrons (e�) are transferred from these level to theconduction band of the ZnO. As result, the photogenerated chargecarriers are separated by the built-in electric field near to the PbS/ZnO interface, where electrons are drifting to ZnO thin film andholes to PbS thin film decreasing the probability of chargerecombination leading to enhance the photocatalytic activity [48].Therefore, a higher number of electrons are available on thesurface of the PbS/ZnO thin film to reduce H+ ions and producehydrogen gas. Additionally, the influence of the thin film surfacecan be related to this improvement of the photocatalytic activity.When electrons finally reach the surface of PbS/ZnO thin film, a lotof H+ ions are available to be reduced to H2 due to the increasingnumber of active sites created by the rough surface. It should bementioned that part of the UV light can also be absorbed by theZnO film (because the energy of the irradiated light is greater thatits band gap). However, when ZnO thin film is irradiated with UVlight, the measured photocurrent is negligible compared to PbS/ZnO thin film, as can be seen in Fig. 5. Then, it can be assumed thatthe contribution of ZnO thin film is only for the carrier transport asa transparent film.

4. Conclusions

In conclusion, PbS/ZnO thin film showed a higher photo-catalytic activity for hydrogen production under UV light irradia-tion (7.38 mmol cm�2h�1) compared to PbS (3.35 mmol cm�2h�1)and ZnO (2.45 mmol cm�2h�1) thin films. XRD results confirmedthe deposition of PbS and ZnO thin films without additionalphases. By SE it was observed the formation of an interface layercreated between PbS and ZnO thin films of around 24 nm. Theimprovement in the photocatalytic activity is attributed to thesynergy between PbS and ZnO thin films by the combination oftheir suitable band positions, leading to an enhanced in theseparation of photogenerated charge carriers near to PbS/ZnOinterface. The higher superficial roughness could act as active sites,increasing the efficiency in the reduction of H+ ions to producehydrogen gas.

Acknowledgments

This research was supported by CONACYT (CB-2014-237049,INFRA-2015-252753, PDCPN-2015-01-487), PRODEP (Integraciónde Redes Temáticas 103.5/15/14156) and FIC-UANL (PAIFIC-2015and PAICYT-2015 projects). Omar A. Carrasco-Jaim acknowledgesCONACYT for the PhD scholarship No. 514302/287841. Authorswant to thank CIMAV Monterrey for AFM images.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jphotochem.2017.07.016.

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