Journal of Photochemistry & Photobiology A: Chemistryfic.uanl.mx/ftp/MDV/MCOIA/Categoría 4. Resultados y... · 2018-06-07 · solution (0.05M of SnCl 2·2H 2O, Fermont™ 98.2%,

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Journal of Photochemistry & Photobiology A: Chemistry

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

SnS-AuPd thin films for hydrogen production under solar light simulation

Sergio D. López-Martíneza, Isaías Juárez-Ramíreza,⁎, Leticia M. Torres-Martíneza, Pravin Babarb,Abhishek Lokhandeb, Jin Hyeok Kimb

aUniversidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N, Ciudad Universitaria, SanNicolás de los Garza, Nuevo León, C.P. 66455, MexicobOptoelectronic Convergence Research Center, Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-Dong, Buk-Gu, Gwangju500-757, South Korea

A R T I C L E I N F O

Keywords:SnSThin filmsHydrogenPhotocatalysisAuPd co-catalysts

A B S T R A C T

In this work, SnS films were obtained by the SILAR method at 25 and 50 cycles. These films were used by the firsttime in the photocatalysis process to produce H2 under simulated sunlight. Additionally, AuPd was deposited byphysical vapor deposition method on SnS films, which served as co-catalyst to increase the photocatalytic ac-tivity of the films. Characterization was done by X-ray diffraction (XRD), where it was found that SnS crystallizesin an orthorhombic crystal structure. By scanning electron microscopy (SEM) it was appreciated a homogeneousdistribution in SnS 25 cycles film, while SnS 50 cycles film showed a greater density of particles deposited on thesubstrate. Band gap energy (UV-vis) for SnS 25 cycles and SnS 50 cycles films were close to 2.0 and 1.25 eV,respectively. Photoluminescence (PL) analysis showed that the emission spectra intensity of films decreases dueto the presence of co-catalyst because of the lower electron-hole pair recombination. Both, SnS 25 cycles and SnS50 cycles films, were able to produce H2 under solar light simulation (91 and 32 μmol/m2 after 3-h irradiation).This activity was increased when AuPd was deposited as co-catalyst on SnS films; around 8 and 6 times, for SnS-AuPd 25 cycles and SnS-AuPd 50 cycles films, respectively (728 and 175 μmol/m2 after 3-h irradiation).Therefore, it can be assumed that AuPd co-catalyst favors the transfer charge and decrease the electron-hole pairrecombination to promote and enhance the photocatalytic H2 production.

1. Introduction

The use of energy from fossil fuels has boosted the development andquality of life of society for many years, but consequently it has causednegative impacts on the environment, so it is imperative to take ad-vantage of renewable sources such as sunlight as a clean energy source.Particularly, hydrogen is an energetic vector of interest because it has ahigher energy content than traditional fuels, which can be obtainedthrough the process of photocatalysis using solar light and a semi-conductor material [1,2]. Among the photocatalysts employed, themost studied semiconductor is TiO2, however, its efficiency to producehydrogen is still low under visible light, hence that some research fo-cuses on finding more active materials [3,4].

Currently, metal sulfide materials have been attracted attention fortheir variety of applications, such as active material in optoelectronicdevices solar cells [5], lithium batteries, and environmental monitoringsensors [6]; also it has been used as a photocatalyst in the degradationof organic compounds [7]. These semiconductors have importantphysicochemical properties such as a high absorption coefficient

(> 104 cm−1), present a band gap ranging from 1.3–1.8 eV dependingon the synthesis method, and use of abundant chemical elements[8–10]. Among the metal sulfide materials, SnS is considered a p-typesemiconductor, with a high free-carrier concentration (1015 cm−3),which makes it an attractive material to be used in various oxide-re-duction systems [11]. This metal sulfide has an orthorhombic crystalstructure with the following cell parameters a= 4.148 Å, b= 11.480 Åand c=4.177 Å [12]. Each atom of Sn binds to 6 atoms of S, throughvan der Waals type bonds [13].

Generally, SnS films can be obtained by different methods such aschemical spray pyrolysis [14], DC magnetron sputtering [15], co -evaporation [16], spin-coating [17], electron beam evaporation [18],deposition by chemical bath [19], and successive ionic layer adsorptionand reaction (SILAR) [20]. But, sometimes it is difficult to obtain thepure phase due to the oxidation states of Sn that provokes the presenceof other phases [21]. Among these deposit methods, SILAR is a simple,not expensive and easy method that allows a control over the synthesisof the material [22]. This process consists in growing thin films of thematerial by means of immersions in cationic and anionic solutions,

https://doi.org/10.1016/j.jphotochem.2018.04.033Received 10 January 2018; Received in revised form 6 April 2018; Accepted 14 April 2018

⁎ Corresponding author.E-mail address: [email protected] (I. Juárez-Ramírez).

Journal of Photochemistry & Photobiology A: Chemistry 361 (2018) 19–24

Available online 02 May 20181010-6030/ © 2018 Elsevier B.V. All rights reserved.

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making rinses after each immersion in the precursor solution [23]. Theabsorption of the material at the surface of the substrate is due to thevan der Waals forces presented on the solution ions and the surface ofthe substrate. Among the factors that affect this process are con-centration of solution, temperature of solution, pressure, etc. [24].

On the other hand, with the aim of increasing the photocatalyticactivity of materials, it has been reported the formation of hetero-structures between a semiconductor material and a metal (co-catalyst),which favors the activity of the material by decreasing the re-combination rate of the electron-hole pair. This process allows thepresence of a great number of active sites for the existence of a favor-able electrical interaction between both materials, favoring the trans-ference of photogenerated electrons from the semiconductor towardsthe co-catalyst [25].

The most used metals as co-catalysts are Au, Pd, Pt, Ni, Ru, and soon [26,27]. The presence of the co-catalyst induces a charge transferthat takes place in the interface of both materials, speeding, thetransport of the electrons of a conduction band to another, diminishingthe possibility of its recombination [28]. Taking into consideration theabove mentioned and considering that until now, there are not reportsabout the photocatalytic activity of SnS thin films prepared by SILARmethod. In this paper, we present the results of the SnS thin films de-posited onto glass substrate as well as their characterization in order todetermine the effects of different cycles on the structural, optical andmorphology properties. It is discussed also the photocatalytic results ofpure SnS films prepared by SILAR method and impregnated with AuPdas co-catalyst for the hydrogen production from the water splittingreaction.

2. Experimental

2.1. Synthesis thin films

Glass substrates were washed with soap in an ultrasonic bath for10min, and then rinsed with solution 1:1 of distilled water and ethanolfor 10min; finally rinsed only with distilled water and dried to be usedin the deposit by SILAR.

SnS thin films of different thickness were prepared by SILAR methodonto glass substrates. Firstly, the substrate was immersed in the cationicsolution (0.05M of SnCl2·2H2O, Fermont™ 98.2%, dissolved in 5ml ofhydrochloric acid) and then, it was rinsed in desionized water before tobe immersed in an anionic solution (0.05M of Na2S·9H2O, DEQ™ 99%).After this step, the glass substrate was immersed in desionized wateragain. The time of immersion in the solutions precursor was 10 s eachone, while for the water rinsed was 3 s; the precursor solutions werechanged every 25 cycles. Finally, in order to obtain films with differentthickness glass substrates were immersed during 25, and 50 cycles. Onthe other hand, AuPd co-catalyst was deposited on SnS thin films byphysical Vapor Deposition (PVD) using a Denton Vacuum Desk IV froman Au-Pd target (60 wt.% Au and 40wt.% Pd) during 30 s.

2.2. Characterization of films

X-ray powder diffraction (XRD) was used to identify the phase of thethin films, each thin film was scanned using a Bruker, Model D8Advance diffractometer with CuKα radiation (λ=1.5406 Å), the dif-fraction intensity as a function of the diffraction angle (2θ) was mea-sured between 10° to 70°, using a step of 0.01. X-ray Rietveld refine-ment was carried out to determine the cell parameters of the films. Thesurface morphologies of the films were analyzed by scanning electronmicroscopy (SEM), and for the elemental quantification was used en-ergy dispersive X-ray spectroscopy (EDS) attached to the SEM equip-ment JEOL 6490LV. The film thickness was estimated using a profil-ometer Alpha-Step D-600. The absorption properties were measuredusing a spectrophotometer Cary 5000. The Eg value of the SnS thin filmswas calculated from the UV–vis absorption spectra using the Tauc

relation:

= −αhv A hv E( ) ( )ng

Where α is the absorption coefficient, h is the Plank constant, A is aconstant that is independent of the photon energy, n=2, for the directtransitions. A fluorescence spectrophotometer Agilent Cary Eclipse withan excitation wavelength of 254 nm was used for the photo-luminescence spectra of the thin films at room temperature; excitationand emission slit width were both of 5 nm.

2.3. Photocatalytic evaluation

The tests for hydrogen production were realized in a glass reactorcontaining the SnS thin film of 20× 40×1mm, 200ml of desionizedwater, and a solar light simulator was used as irradiation source (ABBclass with AM 1.5 G filter, Xe lamp of 450W, and 100mW/cm2),sample was irradiated with a distance of 10 cm. Previous to the reac-tion, the reactor was deareated during 30min. Finally, the hydrogengenerated was detected by a TCD Gas Chromatograph equipment andanalyzed each 30min during 3 h.

3. Results and discussion

3.1. Structural characterization

The XRD patterns of SnS and SnS-AuPd thin films deposited usingSILAR method are showed in Fig. 1. The peaks in the patterns wereidentified and correspond to SnS with orthorhombic crystal structure(JCPDS: 03-065-3875). Main peaks diffract at 2θ=26.368°, 31.138°and 31.350°, which are related to the crystallographic planes (021),(040) and (111), respectively. All thin films, SnS and SnS-AuPd ana-lyzed, showed a similar XRD pattern. In addition, the figure shows thatSnS 50 cycles and SnS-AuPd 50 cycles films, have more crystallinity,showing sharper peaks at 30.47 and 31.53 than the rest of the films. Thepresence of AuPd was not detected due to the low concentration of thedeposit.

An analysis by Rietveld was made to each one of the SnS films inorder to determine the experimental lattice parameters. The Table 1shows the results obtained by the Rietveld method, where it is observedthat the cell parameters for SnS 25 cycles film are slightly higher thanthose of the SnS 50 cycles film. It is assumed that Sn-S bonds in films at25 cycles cause distortion of the SnS octahedral. Also, the obtainedvalues of crystal size indicate that SnS 25 cycles film has a greatercrystal size, while SnS 50 cycles film has the smallest crystal size. Ac-cording to the refinement residual factor, it can be concluded that both,SnS 25 cycles and SnS 50 cycles films, showed good correlation be-tween the theoretical and the experimental data.

Fig. 1. XRD patterns of SnS and SnS-AuPd thin films grown on glass by SILARmethod obtained at room temperature.

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All the films prepared by the SILAR method at different cyclesshowed very good adherence and uniformity on the substrate glass.According to the X-ray analysis, it was determined that SnS filmsshowed the same crystalline phase, orthorhombic crystal structure. TheRietveld analysis corroborates that the pure phase was obtained withsmall variations in their cell parameters according to the number ofcycles used.

3.2. SEM analysis and EDS measurements

Fig. 2 shows the SEM images of the deposited films. It is observedthat both, SnS 25 cycles and SnS-AuPd 25 cycles films, presented welldefined grains, which are homogeneously distributed on the surface ofthe glass substrate; particle size is below 1 μm. Whereas SnS 50 cyclesand SnS-AuPd 50 cycles films, presented a greater density of distributedparticles on the surface of the substrate. It is appreciated that particlesize is increasing as number of cycles is increased; particle size is above1 μm in some areas. On the other hand, Table 2 shows the average EDSresults found for the SnS and SnS-AuPd films. The values indicate thatthe present elements Sn and S in the thin films, SnS 25 cycles and SnS50 cycles, are very close to the stoichiometric ratio. In addition, by EDSanalysis it was possible to detect the presence of AuPd; a higher con-centration of Au (1.73%) and Pd (1.80%) in SnS-AuPd 25 cycles film

was detected. While for SnS-AuPd 50 cycles film the concentration ofAu and Pd was lower than expected, around 0.47% of Au and 0.56% ofPd. This is because the EDS analysis was done in punctual zones ofsurface of SnS and surface of SnS 25 cycles is more homogeneous thansurface of SnS 50 cycles, allowing a better deposit of AuPd.

The above SEM images results indicated that in the case of SnS 25cycles and SnS-AuPd 25 cycles, homogeneous films are present.Whereas in SnS 50 cycles and SnS-AuPd 50 cycles films, it is observedthat there is a greater density of particles distributed on the surface ofthe substrate. On the other hand, in all cases the composition of thefilms, detected by EDS, corresponds to the SnS compound, which is inagree with the previous X-ray diffraction results.

Table 1Structural parameters of SnS thin films.

Sample a (Å) b (Å) c (Å) Cell volume (Å3) Crystallite Size (nm) Refinement residual factor (Rwp)

SnS theoricala 4.148 11.480 4.177 198.9 – –SnS 25 cycles 4.019 (± 7) 10.676 (± 4) 3.981 (± 9) 170.88 11.2 6.544SnS 50 cycles 4.007 (± 2) 10.687 (± 1) 3.966 (± 3) 169.86 8.8 9.955

aSnS crystallizes in an orthorhombic structure, with the space group Cmcm [12].

Fig. 2. SEM images of SnS and SnS-AuPd thin films deposited on glass: a) SnS 25 cycles, b) SnS 50 cycles, c) SnS-AuPd 25 cycles and d) SnS-AuPd 50 cycles.

Table 2Composition of SnS and SnS-AuPd films obtained from EDS analysis.

Sample Sn (%) S (%) Au (%) Pd (%) Sn/S

SnS 25 cycles 50.23 49.77 – – 1.009SnS 50 cycles 50.03 49.97 – – 1.001SnS-AuPd 25 cycles 50.25 46.22 1.73 1.80 1.087SnS-AuPd 50 cycles 49.90 49.07 0.47 0.56 1.016

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3.3. Film thickness

Table 3 presents the average values of the thicknesses in the SnSfilms calculated using the profilometer. Accordingly, to the values ob-tained, it can be observed that the thickness increases up to 5 timeswhen increasing the number of cycles, which is in agreement with theobserved in the SEM micrographs. SnS 50 cycles film has greater den-sity of material distributed on the surface than SnS 25 cycles film,which favored the increase in thickness.

3.4. Optical properties

The energy band values of each of the films prepared in this workwere obtained from Tauc plot, extrapolating the linear part of the curvetowards the E (eV) axis. This calculation was done using the absorptionradiation spectra of SnS and SnS-AuPd films, see Fig. 3. In addition, it isalso showed the radiation emission spectra of solar light simulator (ABBclass with AM 1.5 G filter, Xe lamp of 450W, and 100mW/cm2). It wasfound that the Eg values of SnS 50 cycles and SnS-AuPd 50 cycles filmsoscillate around 1.25 eV, while for SnS 25 cycles and SnS-AuPd 25cycles films, the band gap is displaced to higher values. This is becausethe number of cycles increase the film thickness, then the grain sizecould be increasing and the Eg value decrease, as Y. Yücel et al. [5]reported it, which is associated with the development in grain size andcrystallinity. In our case, thickness increased considerably for SnS 50cycles, but crystallite size was almost similar in both, SnS 25 cycles andSnS 50 cycles thin films. Table 4 shows the Eg values of the SnS and SnS-AuPd films at different cycles.

On the other hand, the photoluminescence spectra of the SnS andSnS-AuPd films excited at 254 nm are presented in Fig. 4. The PLemission peaks observed at 758 nm, indicate that intensity of SnS 25cycles and SnS 50 cycles films, without co-catalyst, could be relatedwith a greater electron-hole pair recombination than SnS-AuPd 25 cy-cles and SnS-AuPd 50 cycles films. This is because the presence of AuPdfavors the charge transfer of the photogenerated electrons and de-creases the electron-hole pair recombination. Therefore, it is expectedthat SnS-AuPd films can promote and enhance the photocatalytic ac-tivity for H2 production.

3.5. Band structure calculation

The values obtained from the theoretical calculation of the bandstructure are shown in Fig. 5. According to the procedure performed byDas et al. [29], the conduction band can be calculated through thefollowing equation:

= ℵ − +E A B E E( ) 1/2BC a b g 0

Where, EBC is the conduction band edge potential, ℵ A B( )a b is theelectronegativity of the semiconductor, which is the geometric mean ofthe electronegativity of Sn and S atoms, Eg is the band gap of SnS thinfilms, E0 is the scale factor, related to the free electrons on the normalhydrogen scale; taken as −4.5 eV. In addition, the electronegativityvalues used were 4.3 eV and 6.22 eV for Sn and S, respectively. In thiscase, for SnS 25 cycles film, it is observed that the conduction band isabove the most negative potential of the reduction potential of the H+/H2. In addition, the values of the valence band are present at a morepositive potential than the oxidation potential of O2/H2O. It is alsoobserved that for SnS 50 cycles film; the conduction band is slightlyclose to the band of the reduction potential and its valence band morepositive than the water oxidation band. Accordingly to these theoreticalresults, SnS 25 cycles film will present better thermodynamic condi-tions to perform the water splitting reaction.

3.6. Photocatalytic hydrogen production

The photocatalytic activity of SnS and SnS-AuPd thin films by thefirst time under solar simulator is showed in Fig. 6a and b. SnS-AuPd 25cycles film shows the highest production of H2 with 728 μmol/m2, thisvalue is 8 times higher than that obtained by SnS 25 cycles film(91 μmol H2/m2 after 3-h irradiation), see Fig. 6a. For the case of SnS-AuPd 50 cycles film, the photocatalytic activity is increased 6 timeswith respect to the film without co-catalyst (175 μmol H2/m2 vs32 μmol H2/m2, respectively). However, it is less than the result ob-tained for SnS-AuPd 25 cycles, which is due to the greater thickness ofthe film. This causes that the photogenerated charges take a longer way

Table 3Average thickness of SnS films.

Sample Thickness (nm)

SnS 25 cycles 83SnS 50 cycles 415

Fig. 3. a) Absorption radiation spectra of SnS and SnS-AuPd films. b) Radiation emission spectra of solar light simulator (ABB class with AM 1.5 G filter, Xe lamp of450W, and 100mW/cm2).

Table 4Eg values of SnS films prepared by SILAR method at differentcycles.

Sample Band gap, Eg (eV)

SnS 25 cycles 1.88SnS 50 cycles 1.25SnS-AuPd 25 cycles 2.0SnS-AuPd 50 cycles 1.25

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to reach the surface, provoking the electron-hole pair recombination,which decreases the photocatalytic activity. Accordingly to the aboveresults, the deposit of AuPd could be facilitating the transport of elec-trons and avoid the electron-hole pair recombination. This situationfavors the photocatalytic activity. On the other hand, the rate of the H2

production is showed in Fig. 6b, where it is observed that SnS-AuPd 25cycles film reaches the highest rate, around 313 μmol of H2/m2 h.

In Table 5, it is showed a comparison for hydrogen production ratewith different photocatalytic and photoelectrochemical systems usingTi based films as active material. According to results, SnS and SnS-

AuPd thin films prepared by SILAR method in this work presentedbetter activity than other photocatalyst films. All films prepared herecould be applied as semiconductor films in efficient photocatalytichydrogen production devices because it can promote and enhance thephotocatalytic activity for H2 production under solar light simulation.

4. Conclusions

It is reported by the first time the hydrogen production under si-mulated sunlight of SnS thin films obtained by SILAR method. SnS filmswere obtained in pure form and crystallizes in an orthorhombic crystalstructure with space group Cmcm. Both, SnS 25 cycles and 50 cyclesthin films, were able to produce H2 under solar light simulation (91 and32 μmol/m2 after 3-h of irradiation). This result is in good agreementwith their theoretical calculation of the band structure diagram. Thephotocatalytic activity was increased around 8 and 6 times, for SnS-AuPd 25 cycles and SnS-AuPd 50 cycles films, respectively. By photo-luminescence analysis it was determined that improved activity is be-cause of the presence of AuPd, which favors the charge transfer of thephotogenerated electrons and decreases the electron-hole pair re-combination. In conclusion, SnS-AuPd films prepared in this work canpromote and enhance the photocatalytic activity for H2 productionunder solar light simulation.

Fig. 4. Photoluminescence spectra of SnS films and SnS-AuPd on glass for 254 nm photon excitation.

Fig. 5. Theoretical band structure diagram of SnS thin films.

Fig. 6. a) Hydrogen production from water splitting using SnS and SnS-AuPd with different cycles. b) Rate of hydrogen production using SnS and SnS-AuPd thinfilms.

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Acknowledgements

Authors want to thank to CONACYT for the financial support of thisresearch through the scholarship of Sergio D. López-Martínez, No305469 and support projects CB-2015-256645, CB-2015-237049,PCPDN 2015-105, NRF-2016-278729 and SEP (PROFOCIE-2014-19-MSU0011T-1). Also thanks to the personal of the Ecomaterials andEnergy Department at Civil Engineering Faculty at UniversidadAutónoma de Nuevo León, México. As well as the OptoelectronicsConvergence Research Center of Department of Materials Science andEngineering at Chonnam National University, South Korea.

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Table 5Comparison of photocatalytic and photoelectrochemical systems using Ti based films as active material.

Photocatalyst Film Hydrogen production Experimental conditions Lamp type Reference

TiO2-Pt 23 μmol/h Photocatalytic process 500W Xe arc [30]Methanol as sacrificial agent

Ti/TNT/N-I-P/NaTaO3 8.73 μmol/cm2 h Photocatalytic process 300W pressure Hg [31]Water solution

TiO2/BiVO4/Co-Pi 7.31 μmol/cm2 h Photoelectrochemical process 300W Xe [32]0.1M Na2SO3 solution

CuO-doped TiO2 13.64 μmol/cm2 h Photoelectrochemical process Solar light simulator [33]1.0mol/L of NaOH 300W Xe

TiO2 79 μmol/m2 h Photocatalytic process 300W Xe with an ultraviolet band–pass filter cut at 420 nm [34]Water solution

SnS-AuPd 25 cycles 313 μmol/m2 h Photocatalytic process Solar light simulator This workWater solution 450W Xe

SnS 25 cycles 24 μmol/m2 h Photocatalytic process Solar light simulator This workWater solution 450W Xe

S.D. López-Martínez et al. Journal of Photochemistry & Photobiology A: Chemistry 361 (2018) 19–24

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