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Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-9096-y

Hybrid SrZrO3-MOF heterostructure: surface assembly and photocatalytic performance for hydrogen evolution and degradation of indigo carmine dye

Luis A. Alfonso‑Herrera1,2 · Ali M. Huerta‑Flores1 · Leticia M. Torres‑Martínez1 · J. M. Rivera‑Villanueva2 · Daniel Julián Ramírez‑Herrera2

Received: 8 January 2018 / Accepted: 12 April 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractIn this work, we developed a novel heterostructure based on the coupling of a metal organic framework (MOF LEEL-037) with an inorganic semiconductor (SrZrO3) for two photocatalytic applications: hydrogen evolution from water splitting and the degradation of indigo carmine dye. A complete study of the structural, morphological, textural, optical, electronic, and electrochemical properties of MOF, SrZrO3 and the heterostructure is presented through X-ray diffraction, scanning electron microscopy, UV–Vis diffuse reflectance spectroscopy (UV–Vis), photoluminescence spectroscopy and electrochemical impedance spectroscopy, evaluating the effect of these parameters on the catalytic performance of the materials. The het-erostructure formation was studied by transmission electron microscopy, varying the loading of LEEL-037 from 0.5 to 5%. Microscope images corroborate the effective dispersion of LEEL-037 and the appropriate contact between the metal organic framework and SrZrO3 particles. It was found that the photocatalytic activity of SrZrO3 under UV light was significantly enhanced with the incorporation of MOF LEEL-037, which enhances the charge separation and transport, leading to an improved photocatalytic performance. After 1 h of reaction, the heterostructure with the optimal amount of LEEL-037 (5%) exhibited a hydrogen evolution of 66.9 µmol, corresponding to 6 times the activity of pure SrZrO3 (11.2 µmol). LEEL-037 exhibited an activity of 34.1 µmol, but the rate of hydrogen production was not constant. The stability and efficiency of the charge transference in the bare semiconductors and the heterostructure were studied through photoluminescence and elec-trochemical analysis, which demonstrated the suitable band coupling between SrZrO3 with MOF LEEL-037, the enhanced charge separation and injection from one semiconductor to another, and the reduction in the recombination of the electron–hole pairs. These studies and the integral correlation of the properties of the materials allowed to establish the path of the photogenerated charges and to propose the photocatalytic mechanisms involved in the reactions. The photocatalysts were also evaluated for the degradation of indigo carmine, where the highest dye degradation (69%) was exhibited by the heterostruc-ture. Based on our results, we propose the use of the heterostructure SrZrO3-5% MOF LEEL-037, obtained by an easy and low cost method, as a suitable new photocatalytic material for environmental and energy applications, highlighting at the same time the promising properties of metal–organic frameworks for their coupling with a variety of inorganic semiconductors.

Luis A. Alfonso-Herrera and Ali M. Huerta-Flores have contributed equally to this work.

* Leticia M. Torres-Martínez lettorresg@yahoo.com

* J. M. Rivera-Villanueva chemax7@yahoo.com

1 Departamento de Ecomateriales y Energía, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo

León, UANL, Av. Universidad S/N Ciudad Universitaria, C.P. 64455 San Nicolás de los Garza, Nuevo León, Mexico

2 Facultad de Ciencias Químicas, Universidad Veracruzana, UV, Prolongación Oriente 6, No. 1009, Colonia Rafael Alvarado, C.P. 94340 Orizaba, Veracruz, Mexico

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1 Introduction

In the present time, two of the most important challenges for humanity are the production of clean energy and the decontamination of sources of water. Photocatalysis is a technology with the potential to provide a solution for both problems, using a semiconductor and solar light to cata-lyze the splitting of the water molecule into oxygen and hydrogen [1], which can be used as clean fuels with high energetic power [2], and to degrade organic compounds in water [3], such as indigo carmine [4, 5]. Some of the most used photocatalysts include inorganic semiconductors such as TiO2, ZnO, titanates, tantalates, among others [6–8]. One of the main problems associated with photocatalysis is the efficiency loss due to recombination processes [9–12]. A strategy to reduce the recombination involves the con-struction of heterostructures, which are mainly based on the coupling of two or more inorganic semiconductors. In particular, heterostructures developed through metal dop-ing, and carbon-based components, such as graphene, have been used recently for heavy metal removal [13], the pho-todegradation of anionic and cationic organic pollutants [3] and antibacterial activity [14], obtaining photocatalytic efficiencies higher to 93%. The higher activities in these novel photocatalysts were attributed to an improved light absorption, a higher amount of oxygen vacancies and a better charge separation and diminished recombination in the interface of the heterostructures.

Moreover, innovative hybrid organic–inorganic hetero-structures have been developed recently for photocatalytic applications [15–17], though there are not many reports of their study.

SrZrO3 is an inorganic semiconductor with a perovs-kite-type structure that has shown interesting properties for its application in the photoinduced hydrogen evolution [18]. However, in the literature, there are no reports for the use of this perovskite for the degradation of organic dyes. SrZrO3 has been synthesized successfully by differ-ent methods, such as hydrothermal [19] and the solid-state reaction, ultrasound, and molten salt processes [20]. For photocatalytic hydrogen evolution application, this mate-rial was coupled with MoS2, promoting a hydrogen evolu-tion rate of 1.95 mmol h−1, compared to 5.31 mmol h−1 in the bare zirconate [21], using a solution of Na2S 0.35 M with Na2SO3 0.25 M as a sacrificial agent. In order to promote the separation of charge carriers and enhance its photocatalytic activity, in other work strontium zirconate was modified with inorganic co-catalysts, including FeO, CoO, NiO, and CuO, increasing the hydrogen evolution rate of SrZrO3 up to 28 times [22].

Metal–organic frameworks are coordination polymers formed by the coordination bonds between a metallic atom

(clusters) and organic molecules (linkers). This new kind of compounds exhibit interesting properties such as high crystallinity, high surface area, and controllable porous structure. Some of the applications of MOF include fuel storage, photoinduced hydrogen evolution, and photodeg-radation of dyes [23–25]. Metal–organic frameworks have been used in conjunction with different materials, showing promising results in photocatalytic water splitting [26, 27] and the degradation of organic compounds in water [28, 29]. However, there are not many reports of the use of metal organic frameworks coupled with inorganic oxides for this applications. Herein, we developed a novel het-erostructure based on the coupling of SrZrO3 with a metal organic framework (LEEL-037), for enhanced photocata-lytic hydrogen evolution and the degradation of indigo carmine dye. To the best of our knowledge, this is the first report on the photocatalytic activity of a hybrid hetero-structure using a MOF coupled to SrZrO3. This is also the first report on the photocatalytic activity of LEEL-037, which was synthesized by our research group in October of 2015 and deposited in the Cambridge structural database in the same year [30]. A complete characterization of the semiconductors and the heterostructure is presented and photocatalytic mechanisms based on photoluminescence and photoelectrochemical measurements are discussed.

2 Experimental section

2.1 Synthesis of metal–organic framework (LEEL‑037)

LEEL-037 was synthesized by the solvothermal method. The precursors 1,2-di(4-pyridyl)ethylene (0.05 g), 1,2,4,5-ben-zene tetracarboxylic acid (0.069 g), and cobalt(II) nitrate hexahydrate were dissolved in 5 mL of dimethylformamide (DMF); then, they were mixed and placed in a liner reactor. The mixture was transferred into an electric furnace and it was thermally treated at 90 °C for 3 days. The crystals obtained were filtered, washed with chloroform, and dried at 90 °C for 24 h.

2.2 Synthesis of SrZrO3

SrZrO3 was synthesized by the sol–gel method. Ethyl cel-lulose (5.4 g), strontium acetate (4.5 g), and zirconium(IV) butoxide (10.5 g) were dissolved in 200 mL of ethanol. The mixture was stirred during 12 h at 80 °C. The formed gel was dried at 100 °C for 24 h and placed in an alumina crucible, moved into an electric furnace and thermally treated in air at 800 °C for 8 h with a heating rate of 3 °C min−1.

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2.3 Preparation of SrZrO3‑LEEL‑037 heterostructure

SrZrO3-LEEL-037 heterostructures were prepared by a mechanical grinding method, mixing different proportions of LEEL-037 (0.5, 1, 2 and 5%) with SrZrO3 in an agate mortar, using acetone as a lubricant.

2.4 Characterization of the samples

The structure of the prepared materials was analyzed by X-ray diffraction, using a Bruker D8 diffractometer [40 kV and 40 mA with CuKα radiation (λ = 1.5406 Å)]. The analy-sis was performed from 10° to 70° angle, with a step of 0.05° and a counting time of 0.5 s per step. X-ray diffrac-tion pattern of LEEL-037 was simulated using the software mercury 3.7.

The crystallite size of the samples was calculated using the Scherrer equation: L = κλ/βcos(θ), where L = crystallite size, κ = 0.89 (Scherrer constant), λ = 0.15418 nm (wave-length of the X-ray radiation), β = is the full width at half maximum (FWHM) of the diffraction peak at 2θ, and θ = dif-fraction angle of the more intense peak of the pattern [31, 32]. The morphology and grain size of the samples were studied in a scanning electron microscope JEOL 6490LV (high vacuum and 20 kV of voltage), in a mode of second-ary electrons. Elemental quantification of the samples was performed by energy dispersive X-ray spectroscopy (EDS). The absorption properties of the samples were studied using a spectrophotometer (Cary 5000) coupled with an integra-tion sphere for diffuse reflectance measurements. A BaSO4 standard of 100% reflectance was used as a reference. Band gaps of the materials were obtained using the Kubelka Munk function and the equation Eg = 1240/λ, where λ is the wave-length (nm) of the exciting light [33, 34]. Recombination of charges in the materials was analyzed at room tempera-ture using a fluorescence spectrophotometer (Agilent Cary Eclipse) with an excitation wavelength of 254 nm for SrZrO3 and 290 nm for LEEL-037 (corresponding to the main absorption bands of the materials), using a scanning speed of 1000 nm/min. The width of excitation and emission slit were both 5 nm. Stability of the samples was determined in dark-ness and under UV light by measurements of open circuit potential (OCP), and the charge transference was studied by electrochemical impedance spectroscopy. The experiments were performed in a potentiostat–galvanostat (MetrohmAu-tolab), using a conventional three-electrode photoelectro-chemical quartz cell with a 0.5 M Na2SO4 aqueous solu-tion. In order to obtain information about the stability and oxidation state of samples, XPS analysis was carried out in an XPS-Auger Perkin Elmer electron spectrometer, model PHY 560, using a monochromatic Al Ka (hv = 1486.7 eV) X-ray source. The functional groups of the materials synthe-sized were identified by Fourier Transform infrared (FT-IR)

spectroscopy using a Perkin Elmer, FT-IR Spectrometer (model Spectrum 100) within a range of 4000–400 cm−1.

2.5 Photocatalytic tests

2.5.1 Hydrogen evolution

Photocatalytic hydrogen production over the three materials was evaluated in a 250 mL Pyrex reactor, dispersing 0.1 g of sample in 200 mL of deionized water and irradiating the reactor with a UVP pen-ray lamp (254 nm and 4400 µW/cm2). The evolved hydrogen was analyzed by gas chroma-tography using an online Thermo Scientific gas chromato-graph equipped with a thermal conductivity detector (TCD) and a fused silica capillary column.

2.5.2 Degradation of indigo carmine

The activity of the materials for the degradation of indigo carmine was evaluated under UV light, irradiating the reac-tor with the same UV lamp used for the hydrogen evolution test. For the photocatalytic test, 0.1 g of material was dis-persed in 200 mL of a solution 30 ppm of indigo carmine and placed in a glass reactor. The mixture was maintained under dark during 1 h until reaching the adsorption equi-librium. Then, the illumination was turned on, and samples were taken every 30 min during 4 h. The concentration of indigo carmine in the samples were determined by measure-ment of the absorbance at 611 nm through UV–Vis, using a Perkin Elmer spectrophotometer Lambda 35.

3 Results and discussion

To facilitate the discussion, SrZrO3 will be referred as SZO, and the metalorganic framework catena-(bis(µ-benzene-1,2,4,5-tetracarboxylic)-tris(µ-4-(2-(pyridine-4-yl)vinyl)pyridine)-tetra-cobalt N,N-dimethylformamide solvate) as LEEL-037. As in the photocatalytic tests, the heterostructure with 5% of LEEL-037 showed the highest efficiency, the representative analysis is presented of LEEL-037, SrZrO3 and SrZrO3-LEEL-037 5%.

3.1 X‑ray diffraction

Figure 1 shows the XRD patterns of SZO prepared by the sol–gel method, as well as the patterns of the heterostruc-tures formed with different proportions of LEEL-037 (0, 1, 2 and 5%). In the pattern of SZO, it can be observed that the main peaks correspond to the orthorhombic phase (JCPDS: 44-0161), with Pbnm 62 space group. The peaks are sharp and defined, which indicate high crystallinity. In the case of LEEL-037 (Fig. 2), the crystalline system found was

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triclinic. In the patterns of the heterostructures, it was not possible to identify the diffraction peaks corresponding to LEEL-037. This could be attributed to the low amount of the metal–organic framework in the heterostructure. The crystal-lite size of the samples was calculated through the Scherrer equation, and the results are shown in Table 1. LEEL-037

showed the highest crystallite size, due to the method employed for the preparation of the metalorganic framework, which involves the slow growth of crystals through solvo-thermal synthesis [35, 36]. The crystallite size calculated in the heterostructures was very similar to the observed in the strontium zirconate, due to the higher amount of this mate-rial in the composites.

3.2 Scanning electron microscopy

Morphology and elemental analysis of the samples were studied using SEM–EDS analysis (Fig. 3). The grain size exhibited by SZO, Fig. 3a (0.1 µm) is smaller than the observed in LEEL-037, Fig. 3b (7.7 µm). The size of the grains observed in SZO is smaller than the reported in the literature [19] and this is attributed to the sol–gel method used for the synthesis of SZO [37]. The grain size can influ-ence other properties, such as band gap and the surface area. In the heterostructures, this parameter was not influenced by the process of coupling. The morphology and grain size of SZO was not altered by the presence of LEEL-037. In the SEM image of the heterostructure (Fig. 3c), it is possible to observe small particles of SZO coupled to largest particles of LEEL-037.

Through EDS analysis, the elements present in LEEL-037 (C, O, and Co) were determined. Nitrogen and

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SZO JCPDS 00-044-0161 Orthorhombic Pbnm (62)

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SZO 5% LEEL - 037

Fig. 1 XRD patterns of SZO and the heterostructures formed with LEEL 037-SZO in different proportions (0.5, 1, 2 and 5%)

Fig. 2 a Structure and b XRD pattern of LEEL-037

Table 1 Characteristics and properties of SZO, LEEL-037, and heterostructure

Sample LEEL-037 SrZrO3 SrZrO3-5% LEEL-033

Band gap 2.8 eV 5.3 eV 5.3 eVAverage crystallite size (equation of Scherrer)

(XRD)664.4 nm 32.6 nm 32.6 nm

Grain size (SEM) 7.7 µm 0.1 µm –H2 production (after 1 h of reaction) 34.1 µmol 11.2 µmol 66.9 µmolDegradation of indigo carmine after 4 h 31% 51% 69%

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hydrogen were not detected due to the low atomic weight of these elements. In SZO, strontium, zirconium, and oxygen were observed. In the heterostructure SZO-LEEL 037, cobalt was found, indicating the presence of LEEL-037 in the heterostructure.

3.3 Transmission electronic microscopy

In the characterization by TEM two phases were identi-fied Fig. 4. The phase with the higher particle size is attributed to LEEL-037, and the phase with the smaller particles to SZO. As it is possible to observe in this Fig-ure, there is an intimate contact between the two photo-catalysts, possibly this contact is the responsible of the interaction between pi bonds of SZO and aromatics rings of LEEL-037 observed in the IR characterization, and the adequate transferences of charges observed in the photo-luminescence spectra.

Fig. 3 SEM and EDS analysis of a LEEL-037, b SZO, and c the heterostructure SZO-LEEL-037

Fig. 4 TEM image of the heterostructure SZO-LEEL 037 (5%)

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3.4 UV–Vis diffuse reflectance spectroscopy

The diffuse reflectance spectra of the prepared materials are presented in Fig. 5. SZO presented an absorption onset close to 250 nm, and it corresponds to the characteristic spectra reported in other works for this compound [18, 19]. On the other hand, LEEL-037 presented two signals: (1) one at a wavelength below to 400 nm; this peak is attributed to the transition of an electron from the organic linker to the metallic atom [38] and (2) the signal observed close to 600 nm, which is related with a transition d–d of the cobalt centers [39]. The presence of LEEL-037 in the surface of SZO in the heterostructures is evidenced in the absorption spectra, where the absorption peaks of LEEL-037 can be detected and become more intense when the concentration of LEEL-037 increases.

The band gap of the materials was calculated using the Kulbelka Munk function. The results are shown in Table 1. The high band gap of SZO obtained in this work (5.3 eV) is similar to the reported in other studies (5.2 eV) [40, 41]. No significant change in the band gap value of the hetero-structures was observed, compared to pure SZO, due to the small amount of LEEL-037 incorporated. The band gap of the metal organic framework is strongly influenced by the conjugation of organic linkers, functional groups and electronegativity of the metallic atom center [42], for this reason, the band gap of LEEL-037 is similar to the exhib-ited by MOF MIL-53 (Fe) (2.7 eV) [34]. The electronega-tivity of the cluster in this MOF (1.8) is very similar to the presented in LEEL-037, which has in its units cobalt (1.9).

3.5 FT IR spectroscopy

The FT-IR spectra of LEEL-037 and its ligands are com-pared in Fig. 6a. In this Figure, it is possible to observe the characteristics bands of the functional groups. Carboxylic acid appears in the frequencies of 1673.9 cm−1 (bond C=O). O–H group (present at 3522 and 3122.4 cm−1 in the linker spectra) disappear in in the spectra of LEEL-037, instead, new bands are observed in the frequencies of 1606, 1490 and 1383 cm−1, corresponding to the carboxylate group [43]. This confirmed the presence of the coordination bond Co–O. In the case of the linker 1,2-Di(4-pyridyl) ethylene, it is pos-sible to observe a band in the frequency of 1597.5 cm−1. This band is attributed to the vibration of the bond C=N. In LEEL-037 this band is observed in the frequency of 1563.3 cm−1, suggesting the presence of the bond Co–N.

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In the case of SZO, (Fig. 6b) it was found a band in the frequency of 857 and 1455 cm−1, attributed to the bond Zr–O [44, 45] and the bond Zr=O. In the heterostruc-ture (Fig. 6b) no bands corresponding to LEEL-037 were observed, due to the low concentration of the metal organic compound. However, it was possible to observe a distortion in the band at 1455 cm−1, corresponding to the bond Zr=O. This distortion is attributed to an interaction of the π bond of SZO and the aromatic rings of LEEL-037. In accordance with this, the band in the frequency of 857 cm−1, related to the sigma bond, is not affected by the presence of LEEL-037. This interaction is probably the responsible of a good transference of charges observed in the photoluminescence spectra.

3.6 Photoluminescence spectroscopy

To investigate the efficiency of the separation, migration, and use of the photogenerated charges, a photolumines-cence study was carried out. Figure 7 shows the emission spectra of the samples. The intensity of the emission in the photoluminescence spectra is related to the recombination of photogenerated charges [46]. Therefore, a low intensity is attributed to a lower recombination, which promotes a higher photocatalytic efficiency in the sample [46]. The het-erostructure SZO-LEEL-037 (5%) Fig. 7b showed the lowest recombination of charges, indicating an efficient transfer-ence of charge from SZO to LEEL-037. This result predicts an improved photocatalytic activity in the heterostructure compared to the pure SZO.

In the SZO emission spectra, the main signal is close to 425 nm, and it is attributed to the transference of one

electron from the 2p orbital of oxygen to a 4d orbital of zircon Fig. 8a [18]. Moreover, the small signal at 345 nm is assigned to the transition of electrons in intermediate levels of energy between the conduction band and the valence band. The emission peaks in LEEL-037 spectra are attributed to the transition of electrons from the energy levels of HOMO and LUMO Fig. 8b [39].

3.7 Electrochemical characterization

3.7.1 Open circuit potential (OCP)

Open circuit potential was used to investigate the stability of the samples. The analysis showed that all the materials are stable in water under dark during the time of analysis Fig. 9. When the samples were illuminated with UV radia-tion it was found that all the compounds show a p-type conductivity, indicating an accumulation of holes [47].

After the irradiation of UV light, LEEL-037 (Fig. 10a) does not recover its original potential and shows a more positive value, related to the oxidation of organics link-ers by the accumulated holes. The instability showed by LEEL-037 and the heterostructure predicts a loss in their photocatalytic activity after some reaction time. SZO presented stability under UV irradiation, showing a recu-peration to its original OCP value Fig. 10b, therefore, it is expected a constant photocatalytic activity in this mate-rial. Also, it was determined that the heterostructure is not stable under UV irradiation (Fig. 10c). In this Figure, it is observed a reduction process, which could be related to the cobalt atoms in LEEL-037.

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Fig. 7 Photoluminescence spectra of a LEEL-037, b SZO and heterostructure SZO-LEEL-037 (5%)

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3.7.2 Electrochemical impedance spectroscopy (EIS)

The resistance to charge transference from the electrode to the electrolyte was investigated by EIS. The conductivity in LEEL-037 is related to the charge transference from ligand to metal, metal to a ligand or the π–π* transition of aromat-ics ligands [48].

After the irradiation under UV light, LEEL-037 Fig. 11 a does not exhibit an increase in their conductivity due to the poor mobility of charges, which is characteristic of metal–organic frameworks [49]. This behavior is caused by the poor overlap in frontier orbitals and electronic states

[50]. On the other hand, the conductivity in SZO and het-erostructure improved (Fig. 11 b and c). This is attributed to the formation of pairs electron–hole.

In the heterostructure, a higher resistance is observed in the dark Fig. 11d, possibly due to the poor mobility of charges from SZO to LEEL-037. The electrons could be trapped in the empty orbitals of cobalt atoms, causing its reduction and the accumulation of holes (characteristic of a semiconductor p-type) as suggested in the OCP experiments.

3.8 X‑ray photoelectron spectroscopy

XPS analysis was carried out to obtain information about the stability of the heterostructure. For this, the heterostructure showing the highest photocatalytic activity (SZO-LEEL-037 5%) was analyzed before and after to the photocatalytic test.

Through XPS analysis it was possible to study the oxida-tion state of Co atoms in LEEL-037 coupled to SZO. Signals found at 780.3 and 796.2 eV are attributed to the presence of Co2+ (Fig. 12a). The peaks found in 784.4 and 802.4 eV are characteristic satellites produced by the excitation and relaxation of unpaired valence electrons [51, 52].

As it is possible to see in Fig. 12b, after the photocatalytic reaction, the valence of the chemical species in SZO did not exhibit a change, confirming its stability. In this spectrum, the peak found at 133.1 eV is characteristic of Sr 3d, and peak present at 183.7 eV corresponds to Zr 3d [22].

After the reaction, it was not observed the presence of cobalt Fig. 12 b. To explain this, it was proposed the decom-position of LEEL-037 in species soluble in water. The holes photogenerated by SZO or aromatics rings in the structure

Fig. 8 Basic mechanism of excitation and emission under UV radiation of a SZO and b LEEL-037

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of LEEL-037 oxidize the atoms from Co2+ to Co3+, and the change of valence in clusters of metal–organic frame-works caused the collapse of the structure. The signals of C at 286.6 eV, identified as C 1s, is attributed to adventitious carbon from XPS instrument [22] and not to the structure or wastes from LEEL-037.

3.9 Photocatalytic activity

The SrZrO3 has exhibited a suitable photocatalytic activ-ity for hydrogen evolution and degradation of organic com-pounds in our previous works [20, 22]. This material is a very stable oxide, with appropriate conduction and valence bands to perform the photocatalytic water reduction and oxidation reactions. For these reasons, we use it as a base semiconductor. A limitation in its photocatalytic activity is due to the high recombination of charges. As a strategy to reduce this process and improve the catalytic efficiency,

we developed a hybrid heterostructure based on SrZrO3 coupled to the metal organic framework LEEL 037. MOF LEEL 037 exhibits high light absorption and photogenerates a high number of charges, and if the charges are not used in the redox reactions with the adsorbed species or trans-ferred to another semiconductor, the holes can oxidize the organic molecule. Therefore, a hybrid coupling is the best strategy to take advantage of the properties offered by both materials: the stability and adequate electronic band struc-ture of SrZrO3, and the light absorption and active sites of MOF LEEL 037. The appropriate electronic band coupling is expected to promote an enhanced charge separation and transference in the interface of the heterostructure, dimin-ishing the recombination and increasing the photocatalytic activity. In the following sections, we present the results obtained from the evaluation of this heterostructure for the photocatalytic hydrogen evolution and the degradation of indigo carmine dye.

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Fig. 10 On–off experiments of the materials: a LEEL-037, b SZO and c the heterostructure SZO-LEEL 037 (5%)

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3.9.1 Hydrogen production

The photocatalytic activity of the samples was evaluated under UV light. The results of the experiments are shown in Fig. 13 and summarized in Table 1. The activity reported in this table corresponds to the showed by these materials in 1 h of reaction.

SZO phase exhibits the lowest photocatalytic activity (11.2 µmol), however, its hydrogen production is constant for almost 3 h. This is consistent with the open circuit poten-tial measurements, where the stability of this compound is observed.

On the other side, LEEL-037 exhibits higher hydrogen production than SZO (34.1 µmol). This is attributed to the higher crystallinity of this material. After 2 h of reaction, LEEL-037 showed a notable decrease in the hydrogen evolu-tion, suggesting instability to UV light. This was confirmed by XPS and OCP measurements, which show an oxidation

process possibly associated with the organic linkers. It has been reported that redox process on metal–organic frame-works can lead to the collapse of their structure [38], which could explain the instability of LEEL-037. Additionally, it is possible that water causes changes in the crystalline struc-ture of LEEL-037, similar to the reported on MOF-5 [53].

The highest photocatalytic activity was presented by the heterostructure SZO-LEEL-037 (5%) (66.9 µmol). This was attributed to a decrease in the recombination of electrons and holes generated, as it was observed in the photolumi-nescence spectra. Loses in the photocatalytic activity of the heterostructure after 1 h is attributed to reduction process on Co atom observed by experiments of open circuit potential. As it was discussed previously, redox process causes the collapse of metal–organic framework structure.

Table 2 shows a comparison of the photocatalytic hydro-gen evolution of the heterostructure developed in this work, the pure SrZrO3 synthesized by several methods and other

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(c) (d)

-Z´´

Ω

Z´Ω

Fig. 11 Comparison of the behavior in darkness and under UV light for a LEEL-037, b SZO and c the heterostructure SZO-LEEL 037 (5%); and d Nyquist plots of the materials in darkness

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composites and heterostructures based on SrZrO3 for this application. As it is possible to observe, the coupling with LEEL 037 produces a higher amount of hydrogen that all the materials, excepting for the heterostructure with 0.05% MoS2 and 1% CuO. The activity in SrZrO3-5% LEEL 037 is high and competitive, compared to similar materials and the improvement in the catalytic performance is attributed to an appropriate charge separation and transport. CuO enhances the electrical conductivity of SrZrO3 and reduces the recombination, increasing the photocatalytic activity in a higher rate than with the presence of LEEL 037, due prob-ably to the better electrical conductivity of CuO compared to the metal–organic framework. A direct comparison with the MoS2 heterostructure is difficult, due to a different light source is employed and the use of a sacrifice agent, there-fore, the activity reported of SrZrO3 coupled to MoS2 is higher.

3.9.2 Mechanism of the photocatalytic hydrogen evolution on SZO

In SZO, after the irradiation with UV light, electrons from the conduction band formed by the orbitals 2p of oxygen are excited and transferred to the conduction band, formed by the orbitals 4d of zirconium. Holes in the valence band oxi-dize the molecule of water to O2 and H+, then, the electron in the conduction band reduces the H+ ions to H2 Fig. 14.

3.9.3 Mechanism of the photocatalytic hydrogen evolution on LEEL-037

After the irradiation of LEEL-037 by UV light, an electron in the aromatics rings of the linkers is transferred to clusters of Co2+. In the process, the cluster could be reducing. Oxida-tion of water occurs on organic linkers producing O2 and H+. H+ is reduced to H2 in the cobalt clusters Fig. 15.

3.9.4 Mechanism of the photocatalytic hydrogen evolution on the heterostructure SZO-LEEL-037

LEEL-037 and SZO show suitable absorption of UV light, therefore, both semiconductors are able to produce electrons and holes under the irradiation with UV light. Considering that LEEL-037 has a more positive value of its valence band, holes on SZO will be transferred to LEEL-037. The trans-ference of holes to LEEL-037 enhances the charge separa-tion and reduces the recombination process, in agreement with photoluminescence results. Oxidation of water occurs in LEEL-037 and the reduction of H+ in SZO. In the pro-cess, Co2+ in the structure of LEEL-037 could be oxidized to Co3+, collapsing the MOF structure (Fig. 16).

3.9.5 Degradation of indigo carmine dye

The photocatalytic activity of the materials for the degra-dation of indigo is shown in Fig. 17. SZO shows a higher percent of degradation after 4 h compared to LEEL-037 (51 vs. 31%), possibly due to the saturation of the structure of LEEL-037 by indigo carmine that decreases the surface con-tact of the material. It has been reported that metalorganic frameworks usually show a great capacity to adsorb organic compounds [54, 55].

As in the case of the hydrogen evolution, the most effi-cient photocatalyst corresponds to the heterostructure, showing a 69% of degradation of indigo carmine. This was attributed to an efficient transference of charges in the interface of the heterostructure. However, the enhancement of the photocatalytic activity is lower than the observed in the hydrogen evolution. This could be due to the saturation of the surface of LEEL-037, similar to the observed in the pure MOF. In the case of the photocatalytic degradation,

Binding energy (eV) 1200 100 0 800 600 400 200 0

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Fig. 12 XPS a spectra and b survey of the heterostructure SZO-LEEL-037 5% before and after the reaction

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Fig. 13 Photocatalytic hydrogen evolution of LEEL-037, SZO, and heterostructure SZO-LEEL 037 (5%)

Table 2 Summary of the reported activities for photocatalytic hydrogen evolution of SrZrO3 and related heterostructures

Photocatalyst Synthesis method Reaction media H2 evolution (µmol/h)

Irradiation source References

SrZrO3 Solid state Pure water 10 254 nm UV lamp 4400 µW [20]Molten salts 6Ultrasound 7Hydrothermal Na2S 0.35 M Na2SO3 0.25 M 1950 UV Hg lamp 100 W [21]Sol–gel Pure water 11 254 nm UV lamp 4400 µW This work

SrZrO3-0.05% MoS2 Hydrothermal Na2S 0.35 M Na2SO3 0.25 M 5310 UV Hg lamp 100 W [21]SrZrO3-1% CuO Solid state Pure water 233 254 nm UV lamp 4400 µW [22]SrZrO3-2% NiO 21SrZrO3-1% FeO 15SrZrO3-5% CoO 11SrZrO3-5% LEEL-037 Sol–gel 67 This work

Fig. 14 Basic mechanism of the photocatalytic hydrogen evolu-tion over SZO

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Fig. 15 Basic mechanism of the photocatalytic hydrogen evolu-tion over LEEL-037

Fig. 16 Mechanism of the photocatalytic hydrogen evolu-tion over the heterostructure SZO-LEEL-037

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% Degradaon of indigo carmine (a) (b)

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Photolyisis SZO LEEL 037 SZO-LEEL 037 5%

LEEL-037

SZO-LEEL-037 5%

Fig. 17 a Photocatalytic degradation of indigo carmine on SZO, LEEL-037, and the heterostructure. b Summary of the photocatalytic activity of the materials for the degradation of indigo carmine

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the heterostructure showed higher stability, suggesting that the charges are transferred from LEEL-037 to indigo carmine, avoiding the changes in the valence of Co, and maintaining the structure of the metalorganic framework. The degradation efficiency of SrZrO3, LEEL-037, and the heterostructure SrZrO3-5% LEEL 037 with the time is pre-sented in Fig. 18.

Table 3 presents the degradation efficiency of indigo carmine using the materials developed in this work, com-pared to the efficiency exhibited by recent photocatalysts reported in literature used for the degradation of related organic molecules (methylene blue, indigo carmine, safranine-o, and tetracycline). The degradation efficien-cies oscillate around 60–97%. Though the degradation efficiency of SrZrO3-5% LEEL 037 (67%) is not the high-est, it is competitive compared to similar photocatalysts.

3.9.6 Mechanism of the photocatalytic degradation of indigo carmine on the heterostructure

The mechanism proposed for the degradation of indigo carmine via oxidation is based according to the reactions proposed by several authors [56, 57]. When UV light is irra-diated on the heterostructure, both semiconductors generate electron–hole pairs, then, holes in SZO are transferred to LEEL-037, avoiding the recombination process and enhanc-ing the photocatalytic activity.

The electrons accumulated in SZO react with the oxygen dissolved in water producing O2

− radicals, which attack the molecule of indigo carmine, generating different intermedi-ates such as isatin sulfonic acid and 2-amine-5 sulfo-benzoic acid, to finally produce simple molecules such as H2O and CO2. It is suggested that holes in LEEL-037 are transferred to indigo carmine contributing to its degradation and avoid-ing the change in the valence state of Co Fig. 19.

4 Conclusions

In this work, for the first time, we report the preparation of a hybrid heterostructure SZO-Metal-organic framework (LEEL-037) as a suitable photocatalyst for hydrogen evo-lution and the degradation of indigo carmine dye in pure water under UV light. An integrated study of the structural, morphological, textural and electrochemical properties of the materials is presented and discussed according to its photocatalytic activity. The photocatalytic activity for hydrogen evolution of SZO was enhanced 6 times in the heterostructure (66.9 µmol), and 1.4 times for the degra-dation of indigo carmine (69%). This was attributed to an efficient charge transference that decreases the recombina-tion of electron–hole pairs and improves the utilization of photogenerated charges. However, according to the studies performed in this work, the hybrid heterostructure presents instability, related to the collapse of LEEL-037 structure due

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ienc

y (%

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SZO-LEEL 037 5% SZO LEEL 037 5%

Fig. 18 Degradation efficiency over SZO, LEEL-037, and the hetero-structure SZO-LEEL 037 5%

Table 3 Photocatalytic activity for the degradation of several organic molecules using the materials developed in this work and recent photocata-lysts

Photocatalyst Synthesis method Model degrada-tion molecule

Degradation efficiency %

Irradiation source References

NiO-I Thermal decomposition of the coor-dinated metal precursors

Methylene blue 93 254 nm UV lamp 4400 µW [58]NiO-II 97CuO 97 [59]TiO2 Hydrothermal 96 [60]TiO2 Sol–gel Safranin-O 60 [61]Au–TiO2 Precipitation-depositation method 97SrZrO3–Sb2O3 Impregnation Tetracycline 70 [62]SrZrO3-5% LEEL 033 Impregnation Indigo carmine 69 This work

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to different redox processes. For this reason, further studies are needed to improve the stability of LEEL-037, which is a promising metalorganic framework that can be prepared by low cost and simple methods for its coupling with inorganic semiconductors.

5 Supplementary material

Electronic Supplementary Information (ESI) available: CIF files containing tables of crystallographic parameters, atomic coordinates, anisotropic thermal parameters, bond distances, bond angles, as well as a list of structure factors have been deposited with the Cambridge Crystallographic Data Cen-tre (CCDC no. 1434259 for MOF LEEL-037). Copies of this information may be obtained free of charge from the director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/data_reque st/cif).

Acknowledgements The authors would like to thank CONACYT (CB-256795-2016, CB-2014-237049, INFRA-2015-252753, PN-2015-01-487, NRF-2016-278729, and PhD Scholarship 386267), SEP (PRO-FOCIE-2014-19-MSU0011T-1, PRODEP-103.5/15/14156), UANL (PAICYT 2015), and FIC-UANL (PAIFIC 2015-5).

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