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A novel Cs2O–Bi2O3–TiO2–ZnO heterostructure with direct Z-Scheme for efficient photocatalytic water splitting
Ceramics International 45 (2019) 23756–23764
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A novel Cs2O–Bi2O3–TiO2–ZnO heterostructure with direct Z-Scheme for efficient photocatalytic water splitting
T
Q.A. Drmosha,∗, A. Hezamb, M.K. Hossainc, M. Qamara, Z.H. Yamania,d, K. Byrappae a
Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia Center for Materials Science and Technology, University of Mysore, Vijana Bhavana, Manasagangothiri, Mysuru, 570006, India c Center of Research Excellence in Renewable Energy (CoRERE), King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia d Physics Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia e Adichunchanagiri University, N.H.75, B. G. Nagara, Mandya District, 571448, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Z-Scheme Hydrogen Heterostructure Water splitting Photocatalysis
Efficient production of hydrogen through photocatalytic water splitting requires innovative catalyst design. One of the potential ways to enhance the photocatalyst's efficiency is to prepare metal oxide heterostructures. Herein, we report fabrication of a novel heterostructure consisting of oxides of cesium, bismuth, titanium and zinc (Cs2O–Bi2O3–TiO2–ZnO) by a simple solution combustion method. Diffusion pathway of charge carriers in the as-synthesized Cs2O–Bi2O3–TiO2–ZnO is investigated by high resolution Valence Band X-Ray Photoelectron Spectroscopy (VB-XPS) and electron spin resonance studies. On the basis of analytical findings and redox potential values of each photocatalytic component, a plausible mechanism is proposed showing the formation and diffusion of charge carrier. The results suggest that the charge carrier migration follows the heterojunction approach among TiO2, Bi2O3, and ZnO and the Z-scheme approach between each of them and Cs2O. The performance of the as-obtained catalytic system is investigated by quantifying the photocatalytic formation of oxygen and hydrogen gases, as well as photoelectrochemical water oxidation under simulated sunlight irradiation. In addition, the apparent quantum efficiency is estimated as high as 1.56% at 420 nm. The high apparent quantum efficiency is primarily ascribed to the synergetic effect between Z-scheme and heterojunction operative in the as-prepared heterostructure.
1. Introduction Solar irradiation is the most abundant source of energy, although huge challenges put it in the backyard to find the solution for utilizing this untapped energy [1,2]. On the other hand, hydrogen (H2) production through water splitting has emerged into new dimensions as a clean energy [3–5]. Water, as a source of H2, can be used by photocatalytic water splitting technology and, thus, H2 production in presence of solar irradiation and water can revolutionize the modern energy demand [6–10]. Consequently, the photocatalysis process for H2 production has been considered as a new pathway to face challenges in growing energy demands. A wide variety of photocatalysts based on metal oxide semiconductors, such as TiO2, WO3, ZrO2, nO2, CeO2, and ZnO, have been utilized as promising photocatalyst to generate H2 [11–17]. However, heterojunction photocatalysts consisting of two or more such oxides enhance the underneath photocatalytic process as well as properties which are highly desired for designing efficient
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system [18–25]. It is, therefore, indispensable to develop new class of efficient photocatalysts [24–26]. Largely, there are three processes involved in photocatalysis which govern the overall performance: the excitation, the bulk diffusion and the surface transfer of charge carriers [27,28]. Thus, inherent semiconducting and chemical properties, crystallinity and surface characteristics are primary ingredients for any efficient photocatalyst. A heterojunction created by two or more materials tend to improve charge separation and overcome the limit of single-component photocatalysts. To the extent, when two or more semiconductors with suitable band edge positions are combined, the photocatalytic process can be dramatically enhanced [29–31]. Recently, great attention has been paid to constructing indirect and direct Z‐scheme photocatalysts for water splitting, degradation of pollutants, and CO2 reduction due to their effectiveness to optimize the oxidation and reduction ability of the photocatalytic system and for spatially separating photoexcited electron–hole pairs [32–41]. In our previous works [42], we reported the synergetic effect of
Corresponding author. E-mail address: [email protected] (Q.A. Drmosh).
https://doi.org/10.1016/j.ceramint.2019.08.092 Received 20 July 2019; Received in revised form 7 August 2019; Accepted 9 August 2019 Available online 09 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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heterojunction between heterojunction and Z-scheme using Cs2O–Bi2O3–ZnO heterostructure. The apparent quantum efficiency of the optimized Cs2O–Bi2O3–ZnO heterostructure reached up to 0.92% at 420 nm. To further improve the synergetic effect between heterojunction and Z-scheme and thus photocatalytic efficiency, we demonstrate the construction of quaternary metal oxide heterostructure (Cs2O–Bi2O3–TiO2–ZnO). VB-XPS and Tauc plot analysis indicated that the incorporation of Cs2O as a photosensitizer was appropriate to introduce a direct Z-scheme system in the heterostructure. High resolution XPS and ESR studies revealed the Z-scheme charge carrier migration pathway in the Cs2O–Bi2O3–TiO2–ZnO. We find that the charge separation has been greatly improved leading to the enhancement of overall photocatalytic water spitting in the presence of fabricated heterostructure. In the as-prepared multicomponent system, the apparent quantum efficiency has reached up to 1.56% at 420 nm (approximately 50% higher than that of the previous studies). A plausible mechanism is suggested showing the charge carrier separation and the high redox potential of such heterostructures. 2. Experimental 2.1. Fabrication of Cs2O–Bi2O3–TiO2–ZnO heterostructure The targeted Cs2O–Bi2O3–TiO2–ZnO heterostructure was prepared by modifying the synthesis procedure reported in our previous study [42]. Briefly, 100 ml of 1.18 M Bismuth nitrate pentahydrate (Sima Aldrich, 98%) was prepared using 1.2 M nitric acid as a solvent (termed hereafter as BS). Another 100 ml aqueous solution of 0.82 M zinc nitrate hexahydrate (Alfa Aesar, 99%) (termed hereafter as ZS) was prepared as a stock solution. Then, 20 mol% titanium dioxide (Merck, 99.7%) was added to each flask and magnetically stirred for 40 min. Afterwards, different amount of cesium nitrate (Alfa Aesar, 99.9%), was added to each flask to obtain 0, 2.5, 5, 7.5, and 10 mol% Cs2O, and labeled as TZBC0, TZBC25, CBZ50, TZBC75, and TZBC100, respectively. In all samples, the Bi: Zn: Ti ratio was estimated to be 1:0.7:0.2. Enough amount of sucrose was added to each conical flask in a way that the ratio of the total reducing valencies to the total oxidizing valencies of the fuel and the oxidizer was equal to unity. Each mixture was magnetically stirred for 40 min at 70 °C followed by heating on a hot plate at 320 °C. Initially the mixture boiled and underwent rapid dehydration and foaming followed by igniting and generating a large amount of gas and at the end very fine orange powder was obtained. Pristine ZnO, Bi2O3, TiO2, and Cs2O were synthesized flowing the same procedure. Cs2O was annealed at 180 °C in argon atmosphere for 1 h to remove Cs2CO3 which might be formed by the reacting of the CO2 with Cs2O. After the annealing, the Cs2O powder was stored in closed quartz ampoules filled with argon. 2.2. Characterization The morphology and the elemental analysis of the samples were investigated by field emission scanning electron microscopy (FESEM, LYRA-3 TESCAN), transmission electron microscopy (TEM, JEM2100HR), and energy-dispersive X-ray (EDX) spectroscopy. X-ray diffraction (XRD) analysis was carried out to investigate the crystallography of the samples on a powder diffractometer (XRD, Rigaku) with Cu Kα radiation (= 1.5414 Å). The optical absorption was investigated using diffuse reflectance spectrophotometer (UV–Vis DRS, Thermo Scientific). The chemical compositions were analyzed by X-ray photoelectron spectrometer (XPS, USA, Thermo ESCALAB 250Xi). Electron Spin Resonance (ESR) spectra were obtained using a JEOL JES-FA200 ESR spectrometer (140 K, 0.998 mW, 9064 MHz, X-band). The specific surface area, and pore volume were measured by N2 adsorption at 77 K using a Micromeritics Tristar II Plus analyzer. The actual amount of Cs in the samples was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES).
2.3. Evaluation of photocatalytic performance The photocatalytic water splitting experiments were carried out in a 135 ml quartz reactor under simulated solar irradiation (Xenon lamp, AM 1.5G, 100 mW cm−2). 5 mg of the photocatalyst was dispersed in 50 ml of ultrapure water without any sacrificial agent or noble metal. Prior to the reaction, the suspension was magnetically stirred for 30 min in dark under nitrogen purging to remove the dissolved O2. The amount of evolved H2 and O2 was quantified at regular intervals using a gas chromatograph (Shimadzu GC-2014) equipped with TCD detector. Nitrogen was used as a carrier gas. 2.4. Evaluation of photoelectrochemical performance A CHI 608e electrochemical analyzer was used to measure the photocurrent generated in the presence of as-prepared photoelectrodes. The working electrodes were prepared by spin coating the photocatalytic samples on fluorine-doped tin oxide (FTO) glass. The preparation of the working electrodes was carried out as follows. 5 mg of the sample was dispersed in 2 ml isopropanol and 4 μl Nafion solution. The suspension was subjected to ultrasonication for 15 min. Then, the obtained suspension was coated on 1 cm × 2 cm cleaned FTO glass using a Milman SPN 2000 spin coater. Platinum (Pt) wire and saturated silver/silver chloride (Ag/AgCl) electrode were used as a counter and a reference electrode respectively. Prior to the measurements, the electrolyte (0.5 M Na2SO4) was purged with argon for 20 min. The working electrodes were irradiated by a 250 W xenon lamp (100 mW cm−2) and the photocurrent densities were measured at 0 V. 3. Results and discussion 3.1. Topographic, optical, and elemental analyses The morphology of as-prepared heterostructure was determined by FESEM as shown in Fig. 1. Fig. 1(a) shows a low magnification image comprising individual Cs2O, Bi2O3, TiO2, and ZnO nanoparticles. A further high magnification image, as shown in Fig. 1(b), confirmed that some of the particles were small in size and spherical in shape co-existing with nanosheets and hexagonal nanostructures. As the prepared photocatalytic system consists of four different components, the nanoparticles with different morphology were speculated to be of different oxides. Further structural details of these individual nanoparticles were obtained as shown in Fig. 1(d). Since backscattered electron (BSE) image intensity is proportional to the atomic number of the targeted element, an image was taken for the same position of interest Fig. 1(c) shows a BSE image of the same. It is noteworthy that nanosheet-like structure showed the highest intensity of ca. 2.5e4 cnts along with others such as ca. 1.5e4, ca. 0.8e4 and ca.0.7e4 cnts as mentioned in Fig. 1(c). BSE image intensities at specific four positions, i.e. ca. 2.5e4, ca. 1.5e4, ca. 0.8e4 and ca.0.7e4 cnts correspond to the atomic number of Bi (83), Cs (55), Zn (30) and Ti (22), respectively. Further topographic information was obtained by TEM as shown in Fig. 2. A typical TEM image showing the distribution, size and morphology of Cs2O–Bi2O3–TiO2–ZnO heterostructure is presented in Fig. 2(a). A high resolution TEM image is shown in Fig. 2(b). It was observed that different lattice fringes with interlayer spacing of 0.34, 0.32, 0.52, and 0.35 nm related to the (012) plane of Cs2O, the (121), plane of Bi2O3, the (002) plane of ZnO, and the (101) plane of anatase TiO2 as shown in Fig. 2(c). Selected area electron diffraction (SAED) patterns is presented in Fig. 2(d), which shows the characteristic features of oxides. The SAED pattern of the Cs2O–Bi2O3–TiO2–ZnO heterostructure showed diffuse fringes due to the presence of different oxide nanoparticles as was observed by microscopic analyses. Elemental analysis was performed by SEM-aided EDX to confirm and verify the composition of individual constituent nanoparticles. The results are shown in Fig. 3. Fig. 3(a) shows the sample position of
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Fig. 3. FESEM-aided EDX (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure, (b) entire spectrum and (c) Atomic percentage and weight percentage of the individual elements.
Fig. 1. FESEM images of (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure, (b) high magnification image of the same with distinct nanoparticle shapes, (c) BSE image of the same corresponding to different atomic number of Bi, Cs, Zn and Ti; inset: numbers represent to detector intensity due to backscattered electrons and (d) specific nanostructures of constituent oxides as mentioned in insets of Fig. 1b.
Fig. 2. TEM images of (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure, (b) high magnification image showing different nanoparticles, (c) further high resolution image showing the lattice fringes of different oxides, and (d) SAED pattern of the same.
interest as square for EDX measurements. The EDX signature spectrum as shown in Fig. 3(b) confirmed the coexistence of the oxides of Bi, Cs, Zn and Ti. Atomic percentage and weight percentage of the individual elements are given in Fig. 3(c)
We carried out further in-depth investigation to confirm the nature of individual 4 sites of interest that refer to oxides of Bi, Cs, Zn, and Ti by SEM-aided EDX and elemental mapping as shown in Fig. 4. Fig. 4(a) represents Cs2O–Bi2O3–TiO2–ZnO heterostructure that possesses four sites of interest. It is to be noted that these oxides coexist together and not that straight forward to distinguish by EDX. A typical EDX spectrum for a site considered to be Cs2O is shown in Fig. 4(b) that indicated all the peaks from oxides instead of only Cs2O. This is expected because of coexistence of individual metal oxides. EDX spectra of other sites of interest showed the same trend (data not shown here). However, the percentage of element for individual sites of interest as shown in Fig. 4(c) revealed that spectrum 2, spectrum 3, spectrum 4 and spectrum 5 as mentioned in Fig. 4(a) indeed represent Cs2O (4.18%), Bi2O3 (54.29%), ZnO (47.95%), and TiO2 (10.26%) respectively. Elemental mappings for individual elements are shown in Fig. 4(e and f). Keeping the SEM image of interest as shown in Fig. 4(d), it is speculated that oxides of Bi, Cs, Zn, and Ti coexist and distributed homogenously. Slight differences in distribution of particular elements are marked by circles and ovals in corresponding images. Absorption property of Cs2O–Bi2O3–TiO2–ZnO heterostructure was investigated, and the impact of Cs2O amount on the absorption behavior of as-prepared photocatalytic system was determined. It was observed that the variation in Cs2O amount has prominent impact on the absorption intensity, particularly in the absorption range between 200 and 400 nm. Tauc plot was constructed to understand the optical band gap position of different samples. The results are shown in inset of Fig. 5(a). It was evident that the optical band gap remains approximately at 3.1 eV with addition of different amount of Cs2O. Fig. 5(b) displays the nitrogen adsorption-desorption isotherms of the TZBC75 sample. The Brunauer-Emmett-Teller (BET) analysis showed that the BET surface area and the pore volume are 6.7 m2/g and 0.041 cm³/g, respectively. The crystal structures of Cs2O–Bi2O3–TiO2–ZnO (TZBC) containing two different amounts of Cs2O (TZBC0 and TZBC75), and individual oxides (Cs2O, Bi2O3, TiO2 and ZnO) were investigated by XRD and compared with pristine Cs2O, TiO2, ZnO, and Bi2S3. The diffractions are shown in Fig. 6. XRD peaks of the heterostructures (TZBC0 and TZBC75) were found to be broadened and weakened compared to those observed in individual oxides. High resolution XPS was employed to analyze the surface composition and chemical elements. Fig. 7(a–d) correspond to spectra of Cs3d, Zn2p, Ti2p, and Bi4f of TZBC75, respectively. The high-resolution spectrum of Cs3d, Fig. 7(a), indicated two peaks centered at 725.06 (3d5/2) and 739.00 eV (3d3/2). These binding energy values are lower than those of bare Cs2O, suggesting the variation in valence electron density after heterojunction formation. The high-resolution spectrum of Zn2p orbital (Fig. 7(b)) presented two peaks corresponding to Zn2p3/2 and Zn2p1/2 with a splitting width of 23.0 eV. The Ti2p core level spectrum was decomposed into two peaks located at 457.23, and
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Fig. 4. SEM-aided EDX and elemental mapping (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure that include 4 sites of interest, (b) entire spectrum for one site, (c) normalized percentage of element for specific four sites of interest, (d) SEM image of interest used for mapping and (e)–(i) percentage of elements distribution for Bi, O, Zn, Cs and Ti respectively.
463.12 eV confirming the presence of Ti4+ in anatase titanium (Fig. 7(c)). Furthermore, deconvolution of Bi4f core level spectrum (as shown in Fig. 7(d)) displayed two peaks with binding energies of 158.8 and 164.1 eV. These peaks are attributed to Bi 4f7/2 and Bi 4f5/2,
respectively. The spin orbit splitting of 5.3 eV between these two peaks demonstrates the Bi3+ oxidation state of Bi in Bi2O3. In contrast to Cs2p, Bi4f exhibits a slight positive shift in its binding energy compared to that of pure Bi2O3.
Fig. 5. (a) UV–vis absorption of Cs2O–Bi2O3–TiO2–ZnO heterostructures with different mol% of Cs2O. Inset: Tauc plot showing the optical band gap for such heterostructures and (b) nitrogen adsorption/desorption isotherms for TZBC75 nanocomposite. 23759
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findings confirm the role of Cs2O as photosensitizer; no water splitting was observed in the presence of TZBC0 sample, which consists of only Bi2O3–TiO2–ZnO. However, the presence and the amount of Cs2O has significant impact on the photocatalytic activity of Bi2O3–TiO2–ZnO heterostructure. A clear correlation between Cs2O and photocatalytic performance (or amount of formed gases, such as H2 and O2), was discerned; the performance increases with increasing amount of Cs2O nanoparticles and reaches maximum values (H2 = 255 μmol g−1, O2 = 127.5 μmol g−1) when the amount of Cs2O is 7.5 mol% (sample TZBC75). A further increase in Cs2O amount (sample TZBC100) led to decrease in the performance. This is related to higher surface coverage of heterostructure by Cs2O, which is likely to reduce the accessibility of the surface-active sites of the Bi2O3–ZnO–TiO2 heterostructure. Since the photocatalytic sample TZBC 75 exhibited the highest performance, the photocatalytic stability of this sample was investigated by carrying out three recycling measurements (for 30 h) under simulated-sunlight irradiation. As shown in Fig. 8(b), H2 and O2 production increases gradually in each cycle and the photocatalytic performance was found to be the same in the three repeated cycles, indicating its excellent reusability and stability attribute. The apparent quantum efficiency of H2 evolution by the optimized Cs2O–Bi2O3–ZnO–TiO2 heterostructure (TZBC 75) measured at 420 nm to be 1.56%. 3.3. Possible theoretical photocatalytic mechanism Fig. 6. XRD patters of Cs2O–Bi2O3–TiO2–ZnO heterostructures (TZBC) with two different concentrations of Cs (named as TZBC0 and TZBC75) and individual oxides (Cs2O, Bi2O3, TiO2 and ZnO).
The actual amounts of Cs estimated by ICP-AEs for TZBC0, TZBC725, TZBC50, TZBC75, and TZBC100 are 0, 0.68, 0.12, 0.20, and 0.27 wt%, respectively. 3.2. Photocatalytic activity The photocatalytic activity of the as-prepared samples was investigated by monitoring the formation of H2 and O2 during water splitting under solar simulator. The results are shown in Fig. 8(a). As stated above, Cs2O was used as a photosensitizer. The photocatalytic
On the basis of the above finding results, the improvement in the photocatalytic activity could be ascribed by the synergetic effect between hetrojunction and formation of direct Z-scheme which in turn leads not only promote the separation of photo-generated electrons and holes, but also keep high redox potential. To understand the charge migration pathway in the Cs2O–Bi2O3–TiO2–ZnO photocatalytic system, valence and conduction band edge positions of Bi2O3, TiO2, ZnO and Cs2O are illustrated in Fig. 9. The valence and conduction bands of the nanostructured Bi2O3, TiO2, ZnO and Cs2O were calculated using the following formula: EVB = X- Ee – 0.5 Eg ECB = Eg – EVB where ECB and EVB are the conduction and valence band potential,
Fig. 7. (a)–(d) XPS high resolution spectra of Cs 3d, Zn2p, Ti2p, and Bi4f of TZBC75 specimen respectively. 23760
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Fig. 8. (a) The photocatalytic H2 and O2 evolution of the Cs2O–Bi2O3–TiO2–ZnO heterostructure with different concentrations of Cs2O under solar simulator, and (b) stability experiments for the overall water splitting.
respectively. X is the electronegativity of the semiconductor. The X values for Bi2O3, TiO2, ZnO and Cs2O are 5.99 eV, 5.81, 5.79 eV, and 3.89 eV, respectively. Ee is the energy of free electrons with the H2 scale (4.5 eV versus NHE) and Eg is the band gap of the semiconductor. On the basis of previous literature including our study [42,43], the band gaps of Bi2O3, TiO2, ZnO and Cs2O are 2.77, 3.4, 3.02 and 1.92 eV respectively. Thus, the conduction band potential of Bi2O3, TiO2, ZnO and Cs2O are determined to be −0.12, −0.39, −0.28 and −1.50 eV respectively, while the respective valence bands are calculated to be 2.88, 3.01, 1.92 and 0.42 eV. On the basis of these findings, it was confirmed that the band edges of Bi2O3, TiO2, and ZnO are quite different than that of Cs2O. The Cs2O possesses much higher conduction band potential and its valence band remains in close proximity of the conduction band of Bi2O3, TiO2, and ZnO. Due to the n-p heterojunction between n-type ZnO and p-type TiO2, the electrons of ZnO can migrate to the conduction band of TiO2 and the holes of TiO2 can transfer to the valence band of ZnO. Since the valence band of Bi2O3 is located at slightly positive value than the valence band of TiO2, the holes generated in the valence band of Bi2O3 can diffuse to the valence band of the TiO2 and then to the valence band of the ZnO. These charge separation processes are similar to that of conventional heterojunction photocatalytic system. Although, this charge separation process is strong enough to oxidize and produce oxygen from water, the conduction band of Bi2O3 is positive and hence unable to reduce H+ to generate H• or H2. However, the conduction band edge of Cs2O is more negative and its valence band is less positive than that of Bi2O3. Under simulated sunlight irradiation, valence electrons of TiO2, ZnO, Cs2O and Bi2O3 absorb appropriate photonic energy, and excited to their conduction bands. On
the basis of band gap positions of Bi2O3 and Cs2O, the separation processes of photogenerated electron-hole pairs are likely to occur in two possible ways; (1) heterojunction or (2) Z-scheme. According to the heterojunction mechanism, the electrons in the conduction band of Cs2O will be transferred to the conduction band of Bi2O3, which is the lowest conduction band among the four metal oxides TiO2, Cs2O, Bi2O3, and ZnO. Simultaneously, the holes in the valence band of Bi2O3 will drift to the valence band of Cs2O. However, the accumulated electrons in the conduction band of Bi2O3 cannot produce H2 due to its lower conduction band position compared to H+/H2 potential [44]. Therefore, it seems reasonable to infer that the charge separation process and ensuing enhancement in the overall water splitting in the presence of Cs2O–Bi2O3 is not driven by heterojunction model. We, therefore, made an attempt to rationalize the charge separation through Z-scheme model. Both Bi2O3 and Cs2O can be excited under simulated sunlight irradiation to generate electron-hole pairs, and then the electrons of Bi2O3 can migrate to the valence band of Cs2O and recombine with the holes. The electrons in the valence band of Cs2O are further excited under irradiation to its conduction band leading to efficient separation of the photoexcited charge carriers. The carriers flow between Bi2O3 and Cs2O continues till charge equilibrium is attained. As a result, the net charge accumulation causes the creation of an electric field at the Bi2O3 and Cs2O interface. The direction of the internal electric field is from Cs2O surface to Bi2O3 surface, which in turn accelerates the transfer of photogenerated electrons from the conduction band of Bi2O3 to the valance band of Cs2O and restrain the photogenerated electrons in the conduction band of Cs2O to migrate into the conduction band of Bi2O3. The same process occurs between Cs2O
Fig. 9. Charge transfer pathway and the mechanism of overall water splitting. 23761
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Fig. 10. (a) Transient photocurrent responses TZBC0, TZBC25, TZBC50, TZBC75 and TZBC100 simulated sunlight irradiation in 0.5 M Na2SO4 aqueous solution, and (b) photoluminescence spectra of TZBC0, TZBC25, TZBC50, TZBC75, and TZBC100 with excitation of 200 nm.
and both ZnO and TiO2 under simulated sunlight irradiation. In this way, the photogenerated electrons in the conduction band of Cs2O which have more negative potential would lead to superior photocatalytic efficiency of the Cs2O–Bi2O3–TiO2–ZnO heterostructure for water splitting reactions. 3.4. Evidence of the mechanism 3.4.1. Photoelectrochemical analysis (PEC) The photoelectrochemical properties is commonly considered to be efficient evidence to demonstrate the electron−hole pair separation and transfer in the composite photocatalysts [45]. The photoelectrocatalytic performance was studied by monitoring the photooxidation of water. The photocurrent was recorded for three on−off light illumination cycles. As can be observed in Fig. 10(a), photocatalytic samples are incapable of producing current in the absence of light. However, the photocurrent increased instantaneously upon illumination, and reached to a maximum value. The photocurrent recorded in the presence of TZBC25, TZBC50, TZBC75, and TZBC100 heterostructures was much higher than that of TZBC0, confirming the enhanced photocatalytic performance of Bi2O3–ZnO–TiO2 heterostructure when combined with Cs2O. Furthermore, the photoelectrocatalytic performance trend was found to be similar to that of photocatalytic trend; oxidation of water increases with an increase in the amount of Cs2O up to certain limit, and a further increase causes decrease in the photocurrent. Sample TZBC75 exhibited the highest photocurrent density with the value as high as 5 μA cm−2, which is almost nine folds higher than that of TZBC0. The improved photoelectrocatalytic performance of the TZBC25, TZBC50, TZBC75, and TZBC100 heterostructures samples is attributed to the enhanced charge separation compared with TZBC0. 3.4.2. Photoluminescence emission spectra To verify the above proposed theoretical mechanism for Cs2O–Bi2O3–TiO2–ZnO photocatalysts system, steady-state photoluminescence (PL) analysis was carried out to gain further insights about the spatial charge separation capability of the as-prepared photocatalytic samples. In general, a higher PL intensity indicates a higher recombination rate of photogenerated electron−hole pairs and lower PL intensity correlated with a higher separation efficiency of photogenerated electron hole pairs. The results presented in Fig. 10(b) show that the Cs2O mol% amount enhanced the transmission efficiency of electron–hole pairs, and thus reduced the recombination rate of the photogenerated charge carriers. The emission peak intensity of the TZBC75 is lower than the other samples, indicating the higher charge separation efficiency. This finding is consistent and supports with photocatalytic and the photoelectrocatalytic trend, in which the TZBC75 exhibited the highest performance.
3.4.3. Valance band x ray photoelectron spectroscopy (VB-XPS) VB-XPS combined with UV–vis diffuse reflection spectroscopic (UV–Vis DRS) analysis can be used to confirm the direct Z-Scheme photocatalytic mechanism by confirming the suitable Z-scheme band edges/maximum−minimum, which match with the redox potential/ level of water. The XPS spectra were obtained using Mg Kalpha XPS with low pass energy and about 0.8 eV resolution. The intersection of the linear extrapolations of the leading edge and the background of the XPS spectrum is the flat valance bands maximum. Fig. 11 showed that the values determined to be to be 2.9, 0.4, 3.2 and 2.2 eV for Bi2O3, Cs2O, TiO2 and ZnO respectively which is in a good agreement with the theoretical values calculated in section 3.2.
3.4.4. Electron spin resonance (ESR) ESR spectroscopy was carried out to find the reactive species in the presence of as-synthesized multicomponent system. 5,5-Dimethyl-Lpyrroline N-oxide (DMPO), which reacts with the superoxide anion radicals (•O2−) and hydroxyl radicals (•OH) to produce DMPO−•O2 and DMPO−•OH in methanol and aqueous media respectively, was added in the solution to, [46,47]. The ESR signal corresponding to DMPO−•O2 collected in the presence of TZBC75 and TZBC0 is shown in Fig. 12(a). The strongest DMPO−•O2 signal was obtained when TZBC75 heterostructure was used as photocatalyst, while for TZBC0, the signal was negligible. These results clearly demonstrate the enhanced activity of the TZBC75 heterostructure to produce •O2− radicals than that of TZBC0, revealing that •O2− radical was preferably generated by TZBC75 under light irradiation. In the case of the TZBC0, the charge migration follows the heterojunction model leading to the accumulation of electrons in the conduction band of Bi2O3 which do not have enough potential to produce •O2−. In contrast to TZBC0, the charge migration in the TZBC75 follows the Z-scheme model, leading to the accumulation of the photogenerated electrons in the conduction band
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Fig. 11. XPS valence band spectra of the pure Bi2S3, Cs2O, ZnO, and TiO2.
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Fig. 12. ESR spectra of (a) DMPO−•O2, and (b) DMPO−•OH in methanol and aqueous solution for the TZBC75 and TZBC0 under solar irradiation.
of Cs2O, which has enough redox potential to produce •O2−. On the other hand, the photoexcited holes are accumulated in the conduction band of ZnO for both TZBC0 and TZBC75 which have enough redox potential to produce •OH, thus the DMPO−•OH signal was detected for both (Fig. 12(b)). The higher signal intensity of DMPO−•OH in the presence of TZBC75 is attributed to the higher light absorption as compare to TZBC0. This was confirmed by UV–vis spectroscopy. The results strongly suggest that the charge transfer mechanism follows both the heterojunction and the Z-scheme model at the same time. 4. Conclusions A multicomponent photocatalytic nanoassembly comprising Cs2O, Bi2O3, TiO2, and ZnO was synthesized through a solution combustion method. The method is simple with potential to scale up. Both the photocatalytic and photoelectrocatalytic performance suggested that Cs2O can be utilized as an effective photosensitizer to activate multicomponent structures. By varying the content of Cs2O in the heterostructure, it was found that Cs2O–Bi2O3–TiO2–ZnO with 7.5 mol% exhibited the highest photocatalytic activity for overall water splitting. The optimal H2 and oxygen production rates were 255 μmol g−1 h−1 and 127.5 μmol. g−1 h−1, respectively in the absence of any cocatalyst and sacrificial agent under solar simulator irradiation. The improved performance of Cs2O–Bi2O3–TiO2–ZnO was correlated to the synergetic effect of Z-scheme and heterojunction charge carrier separation which was confirmed by various analytical techniques. The study introduces for the first time the synergetic effect between Z-scheme and heterojunction charge migration transfer pathways and highlights its impact in overall water splitting. Acknowledgment The authors would like to acknowledge the support provided by the Deanship of Scientific Research at King Fahd University of Petroleum and Minerals for funding this work through project No. DF181021. The authors would like to acknowledge Center of Excellence in Nanotechnology (CENT), and Center of Research Excellence in Renewable Energy (CoRERE), King Fahd University of Petroleum and Minerals (KFUPM). References [1] O. Ellabban, H. Abu-Rub, F. Blaabjerg, Renewable energy resources: current status, future prospects and their enabling technology, Renew. Sustain. Energy Rev. 39 (2014) 748–764. [2] N. Kannan, D. Vakeesan, Solar energy for future world: a review, Renew. Sustain. Energy Rev. 62 (2016) 1092–1105. [3] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Photocatalyst releasing hydrogen from water, Nature 440 (2006) 295. [4] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets, J. Phys. Chem. C 114 (2010) 13118–13125.
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A novel Cs2O–Bi2O3–TiO2–ZnO heterostructure with direct Z-Scheme for efficient photocatalytic water splitting
T
Q.A. Drmosha,∗, A. Hezamb, M.K. Hossainc, M. Qamara, Z.H. Yamania,d, K. Byrappae a
Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia Center for Materials Science and Technology, University of Mysore, Vijana Bhavana, Manasagangothiri, Mysuru, 570006, India c Center of Research Excellence in Renewable Energy (CoRERE), King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia d Physics Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia e Adichunchanagiri University, N.H.75, B. G. Nagara, Mandya District, 571448, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Z-Scheme Hydrogen Heterostructure Water splitting Photocatalysis
Efficient production of hydrogen through photocatalytic water splitting requires innovative catalyst design. One of the potential ways to enhance the photocatalyst's efficiency is to prepare metal oxide heterostructures. Herein, we report fabrication of a novel heterostructure consisting of oxides of cesium, bismuth, titanium and zinc (Cs2O–Bi2O3–TiO2–ZnO) by a simple solution combustion method. Diffusion pathway of charge carriers in the as-synthesized Cs2O–Bi2O3–TiO2–ZnO is investigated by high resolution Valence Band X-Ray Photoelectron Spectroscopy (VB-XPS) and electron spin resonance studies. On the basis of analytical findings and redox potential values of each photocatalytic component, a plausible mechanism is proposed showing the formation and diffusion of charge carrier. The results suggest that the charge carrier migration follows the heterojunction approach among TiO2, Bi2O3, and ZnO and the Z-scheme approach between each of them and Cs2O. The performance of the as-obtained catalytic system is investigated by quantifying the photocatalytic formation of oxygen and hydrogen gases, as well as photoelectrochemical water oxidation under simulated sunlight irradiation. In addition, the apparent quantum efficiency is estimated as high as 1.56% at 420 nm. The high apparent quantum efficiency is primarily ascribed to the synergetic effect between Z-scheme and heterojunction operative in the as-prepared heterostructure.
1. Introduction Solar irradiation is the most abundant source of energy, although huge challenges put it in the backyard to find the solution for utilizing this untapped energy [1,2]. On the other hand, hydrogen (H2) production through water splitting has emerged into new dimensions as a clean energy [3–5]. Water, as a source of H2, can be used by photocatalytic water splitting technology and, thus, H2 production in presence of solar irradiation and water can revolutionize the modern energy demand [6–10]. Consequently, the photocatalysis process for H2 production has been considered as a new pathway to face challenges in growing energy demands. A wide variety of photocatalysts based on metal oxide semiconductors, such as TiO2, WO3, ZrO2, nO2, CeO2, and ZnO, have been utilized as promising photocatalyst to generate H2 [11–17]. However, heterojunction photocatalysts consisting of two or more such oxides enhance the underneath photocatalytic process as well as properties which are highly desired for designing efficient
∗
system [18–25]. It is, therefore, indispensable to develop new class of efficient photocatalysts [24–26]. Largely, there are three processes involved in photocatalysis which govern the overall performance: the excitation, the bulk diffusion and the surface transfer of charge carriers [27,28]. Thus, inherent semiconducting and chemical properties, crystallinity and surface characteristics are primary ingredients for any efficient photocatalyst. A heterojunction created by two or more materials tend to improve charge separation and overcome the limit of single-component photocatalysts. To the extent, when two or more semiconductors with suitable band edge positions are combined, the photocatalytic process can be dramatically enhanced [29–31]. Recently, great attention has been paid to constructing indirect and direct Z‐scheme photocatalysts for water splitting, degradation of pollutants, and CO2 reduction due to their effectiveness to optimize the oxidation and reduction ability of the photocatalytic system and for spatially separating photoexcited electron–hole pairs [32–41]. In our previous works [42], we reported the synergetic effect of
Corresponding author. E-mail address: [email protected] (Q.A. Drmosh).
https://doi.org/10.1016/j.ceramint.2019.08.092 Received 20 July 2019; Received in revised form 7 August 2019; Accepted 9 August 2019 Available online 09 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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heterojunction between heterojunction and Z-scheme using Cs2O–Bi2O3–ZnO heterostructure. The apparent quantum efficiency of the optimized Cs2O–Bi2O3–ZnO heterostructure reached up to 0.92% at 420 nm. To further improve the synergetic effect between heterojunction and Z-scheme and thus photocatalytic efficiency, we demonstrate the construction of quaternary metal oxide heterostructure (Cs2O–Bi2O3–TiO2–ZnO). VB-XPS and Tauc plot analysis indicated that the incorporation of Cs2O as a photosensitizer was appropriate to introduce a direct Z-scheme system in the heterostructure. High resolution XPS and ESR studies revealed the Z-scheme charge carrier migration pathway in the Cs2O–Bi2O3–TiO2–ZnO. We find that the charge separation has been greatly improved leading to the enhancement of overall photocatalytic water spitting in the presence of fabricated heterostructure. In the as-prepared multicomponent system, the apparent quantum efficiency has reached up to 1.56% at 420 nm (approximately 50% higher than that of the previous studies). A plausible mechanism is suggested showing the charge carrier separation and the high redox potential of such heterostructures. 2. Experimental 2.1. Fabrication of Cs2O–Bi2O3–TiO2–ZnO heterostructure The targeted Cs2O–Bi2O3–TiO2–ZnO heterostructure was prepared by modifying the synthesis procedure reported in our previous study [42]. Briefly, 100 ml of 1.18 M Bismuth nitrate pentahydrate (Sima Aldrich, 98%) was prepared using 1.2 M nitric acid as a solvent (termed hereafter as BS). Another 100 ml aqueous solution of 0.82 M zinc nitrate hexahydrate (Alfa Aesar, 99%) (termed hereafter as ZS) was prepared as a stock solution. Then, 20 mol% titanium dioxide (Merck, 99.7%) was added to each flask and magnetically stirred for 40 min. Afterwards, different amount of cesium nitrate (Alfa Aesar, 99.9%), was added to each flask to obtain 0, 2.5, 5, 7.5, and 10 mol% Cs2O, and labeled as TZBC0, TZBC25, CBZ50, TZBC75, and TZBC100, respectively. In all samples, the Bi: Zn: Ti ratio was estimated to be 1:0.7:0.2. Enough amount of sucrose was added to each conical flask in a way that the ratio of the total reducing valencies to the total oxidizing valencies of the fuel and the oxidizer was equal to unity. Each mixture was magnetically stirred for 40 min at 70 °C followed by heating on a hot plate at 320 °C. Initially the mixture boiled and underwent rapid dehydration and foaming followed by igniting and generating a large amount of gas and at the end very fine orange powder was obtained. Pristine ZnO, Bi2O3, TiO2, and Cs2O were synthesized flowing the same procedure. Cs2O was annealed at 180 °C in argon atmosphere for 1 h to remove Cs2CO3 which might be formed by the reacting of the CO2 with Cs2O. After the annealing, the Cs2O powder was stored in closed quartz ampoules filled with argon. 2.2. Characterization The morphology and the elemental analysis of the samples were investigated by field emission scanning electron microscopy (FESEM, LYRA-3 TESCAN), transmission electron microscopy (TEM, JEM2100HR), and energy-dispersive X-ray (EDX) spectroscopy. X-ray diffraction (XRD) analysis was carried out to investigate the crystallography of the samples on a powder diffractometer (XRD, Rigaku) with Cu Kα radiation (= 1.5414 Å). The optical absorption was investigated using diffuse reflectance spectrophotometer (UV–Vis DRS, Thermo Scientific). The chemical compositions were analyzed by X-ray photoelectron spectrometer (XPS, USA, Thermo ESCALAB 250Xi). Electron Spin Resonance (ESR) spectra were obtained using a JEOL JES-FA200 ESR spectrometer (140 K, 0.998 mW, 9064 MHz, X-band). The specific surface area, and pore volume were measured by N2 adsorption at 77 K using a Micromeritics Tristar II Plus analyzer. The actual amount of Cs in the samples was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES).
2.3. Evaluation of photocatalytic performance The photocatalytic water splitting experiments were carried out in a 135 ml quartz reactor under simulated solar irradiation (Xenon lamp, AM 1.5G, 100 mW cm−2). 5 mg of the photocatalyst was dispersed in 50 ml of ultrapure water without any sacrificial agent or noble metal. Prior to the reaction, the suspension was magnetically stirred for 30 min in dark under nitrogen purging to remove the dissolved O2. The amount of evolved H2 and O2 was quantified at regular intervals using a gas chromatograph (Shimadzu GC-2014) equipped with TCD detector. Nitrogen was used as a carrier gas. 2.4. Evaluation of photoelectrochemical performance A CHI 608e electrochemical analyzer was used to measure the photocurrent generated in the presence of as-prepared photoelectrodes. The working electrodes were prepared by spin coating the photocatalytic samples on fluorine-doped tin oxide (FTO) glass. The preparation of the working electrodes was carried out as follows. 5 mg of the sample was dispersed in 2 ml isopropanol and 4 μl Nafion solution. The suspension was subjected to ultrasonication for 15 min. Then, the obtained suspension was coated on 1 cm × 2 cm cleaned FTO glass using a Milman SPN 2000 spin coater. Platinum (Pt) wire and saturated silver/silver chloride (Ag/AgCl) electrode were used as a counter and a reference electrode respectively. Prior to the measurements, the electrolyte (0.5 M Na2SO4) was purged with argon for 20 min. The working electrodes were irradiated by a 250 W xenon lamp (100 mW cm−2) and the photocurrent densities were measured at 0 V. 3. Results and discussion 3.1. Topographic, optical, and elemental analyses The morphology of as-prepared heterostructure was determined by FESEM as shown in Fig. 1. Fig. 1(a) shows a low magnification image comprising individual Cs2O, Bi2O3, TiO2, and ZnO nanoparticles. A further high magnification image, as shown in Fig. 1(b), confirmed that some of the particles were small in size and spherical in shape co-existing with nanosheets and hexagonal nanostructures. As the prepared photocatalytic system consists of four different components, the nanoparticles with different morphology were speculated to be of different oxides. Further structural details of these individual nanoparticles were obtained as shown in Fig. 1(d). Since backscattered electron (BSE) image intensity is proportional to the atomic number of the targeted element, an image was taken for the same position of interest Fig. 1(c) shows a BSE image of the same. It is noteworthy that nanosheet-like structure showed the highest intensity of ca. 2.5e4 cnts along with others such as ca. 1.5e4, ca. 0.8e4 and ca.0.7e4 cnts as mentioned in Fig. 1(c). BSE image intensities at specific four positions, i.e. ca. 2.5e4, ca. 1.5e4, ca. 0.8e4 and ca.0.7e4 cnts correspond to the atomic number of Bi (83), Cs (55), Zn (30) and Ti (22), respectively. Further topographic information was obtained by TEM as shown in Fig. 2. A typical TEM image showing the distribution, size and morphology of Cs2O–Bi2O3–TiO2–ZnO heterostructure is presented in Fig. 2(a). A high resolution TEM image is shown in Fig. 2(b). It was observed that different lattice fringes with interlayer spacing of 0.34, 0.32, 0.52, and 0.35 nm related to the (012) plane of Cs2O, the (121), plane of Bi2O3, the (002) plane of ZnO, and the (101) plane of anatase TiO2 as shown in Fig. 2(c). Selected area electron diffraction (SAED) patterns is presented in Fig. 2(d), which shows the characteristic features of oxides. The SAED pattern of the Cs2O–Bi2O3–TiO2–ZnO heterostructure showed diffuse fringes due to the presence of different oxide nanoparticles as was observed by microscopic analyses. Elemental analysis was performed by SEM-aided EDX to confirm and verify the composition of individual constituent nanoparticles. The results are shown in Fig. 3. Fig. 3(a) shows the sample position of
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Fig. 3. FESEM-aided EDX (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure, (b) entire spectrum and (c) Atomic percentage and weight percentage of the individual elements.
Fig. 1. FESEM images of (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure, (b) high magnification image of the same with distinct nanoparticle shapes, (c) BSE image of the same corresponding to different atomic number of Bi, Cs, Zn and Ti; inset: numbers represent to detector intensity due to backscattered electrons and (d) specific nanostructures of constituent oxides as mentioned in insets of Fig. 1b.
Fig. 2. TEM images of (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure, (b) high magnification image showing different nanoparticles, (c) further high resolution image showing the lattice fringes of different oxides, and (d) SAED pattern of the same.
interest as square for EDX measurements. The EDX signature spectrum as shown in Fig. 3(b) confirmed the coexistence of the oxides of Bi, Cs, Zn and Ti. Atomic percentage and weight percentage of the individual elements are given in Fig. 3(c)
We carried out further in-depth investigation to confirm the nature of individual 4 sites of interest that refer to oxides of Bi, Cs, Zn, and Ti by SEM-aided EDX and elemental mapping as shown in Fig. 4. Fig. 4(a) represents Cs2O–Bi2O3–TiO2–ZnO heterostructure that possesses four sites of interest. It is to be noted that these oxides coexist together and not that straight forward to distinguish by EDX. A typical EDX spectrum for a site considered to be Cs2O is shown in Fig. 4(b) that indicated all the peaks from oxides instead of only Cs2O. This is expected because of coexistence of individual metal oxides. EDX spectra of other sites of interest showed the same trend (data not shown here). However, the percentage of element for individual sites of interest as shown in Fig. 4(c) revealed that spectrum 2, spectrum 3, spectrum 4 and spectrum 5 as mentioned in Fig. 4(a) indeed represent Cs2O (4.18%), Bi2O3 (54.29%), ZnO (47.95%), and TiO2 (10.26%) respectively. Elemental mappings for individual elements are shown in Fig. 4(e and f). Keeping the SEM image of interest as shown in Fig. 4(d), it is speculated that oxides of Bi, Cs, Zn, and Ti coexist and distributed homogenously. Slight differences in distribution of particular elements are marked by circles and ovals in corresponding images. Absorption property of Cs2O–Bi2O3–TiO2–ZnO heterostructure was investigated, and the impact of Cs2O amount on the absorption behavior of as-prepared photocatalytic system was determined. It was observed that the variation in Cs2O amount has prominent impact on the absorption intensity, particularly in the absorption range between 200 and 400 nm. Tauc plot was constructed to understand the optical band gap position of different samples. The results are shown in inset of Fig. 5(a). It was evident that the optical band gap remains approximately at 3.1 eV with addition of different amount of Cs2O. Fig. 5(b) displays the nitrogen adsorption-desorption isotherms of the TZBC75 sample. The Brunauer-Emmett-Teller (BET) analysis showed that the BET surface area and the pore volume are 6.7 m2/g and 0.041 cm³/g, respectively. The crystal structures of Cs2O–Bi2O3–TiO2–ZnO (TZBC) containing two different amounts of Cs2O (TZBC0 and TZBC75), and individual oxides (Cs2O, Bi2O3, TiO2 and ZnO) were investigated by XRD and compared with pristine Cs2O, TiO2, ZnO, and Bi2S3. The diffractions are shown in Fig. 6. XRD peaks of the heterostructures (TZBC0 and TZBC75) were found to be broadened and weakened compared to those observed in individual oxides. High resolution XPS was employed to analyze the surface composition and chemical elements. Fig. 7(a–d) correspond to spectra of Cs3d, Zn2p, Ti2p, and Bi4f of TZBC75, respectively. The high-resolution spectrum of Cs3d, Fig. 7(a), indicated two peaks centered at 725.06 (3d5/2) and 739.00 eV (3d3/2). These binding energy values are lower than those of bare Cs2O, suggesting the variation in valence electron density after heterojunction formation. The high-resolution spectrum of Zn2p orbital (Fig. 7(b)) presented two peaks corresponding to Zn2p3/2 and Zn2p1/2 with a splitting width of 23.0 eV. The Ti2p core level spectrum was decomposed into two peaks located at 457.23, and
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Fig. 4. SEM-aided EDX and elemental mapping (a) Cs2O–Bi2O3–TiO2–ZnO heterostructure that include 4 sites of interest, (b) entire spectrum for one site, (c) normalized percentage of element for specific four sites of interest, (d) SEM image of interest used for mapping and (e)–(i) percentage of elements distribution for Bi, O, Zn, Cs and Ti respectively.
463.12 eV confirming the presence of Ti4+ in anatase titanium (Fig. 7(c)). Furthermore, deconvolution of Bi4f core level spectrum (as shown in Fig. 7(d)) displayed two peaks with binding energies of 158.8 and 164.1 eV. These peaks are attributed to Bi 4f7/2 and Bi 4f5/2,
respectively. The spin orbit splitting of 5.3 eV between these two peaks demonstrates the Bi3+ oxidation state of Bi in Bi2O3. In contrast to Cs2p, Bi4f exhibits a slight positive shift in its binding energy compared to that of pure Bi2O3.
Fig. 5. (a) UV–vis absorption of Cs2O–Bi2O3–TiO2–ZnO heterostructures with different mol% of Cs2O. Inset: Tauc plot showing the optical band gap for such heterostructures and (b) nitrogen adsorption/desorption isotherms for TZBC75 nanocomposite. 23759
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findings confirm the role of Cs2O as photosensitizer; no water splitting was observed in the presence of TZBC0 sample, which consists of only Bi2O3–TiO2–ZnO. However, the presence and the amount of Cs2O has significant impact on the photocatalytic activity of Bi2O3–TiO2–ZnO heterostructure. A clear correlation between Cs2O and photocatalytic performance (or amount of formed gases, such as H2 and O2), was discerned; the performance increases with increasing amount of Cs2O nanoparticles and reaches maximum values (H2 = 255 μmol g−1, O2 = 127.5 μmol g−1) when the amount of Cs2O is 7.5 mol% (sample TZBC75). A further increase in Cs2O amount (sample TZBC100) led to decrease in the performance. This is related to higher surface coverage of heterostructure by Cs2O, which is likely to reduce the accessibility of the surface-active sites of the Bi2O3–ZnO–TiO2 heterostructure. Since the photocatalytic sample TZBC 75 exhibited the highest performance, the photocatalytic stability of this sample was investigated by carrying out three recycling measurements (for 30 h) under simulated-sunlight irradiation. As shown in Fig. 8(b), H2 and O2 production increases gradually in each cycle and the photocatalytic performance was found to be the same in the three repeated cycles, indicating its excellent reusability and stability attribute. The apparent quantum efficiency of H2 evolution by the optimized Cs2O–Bi2O3–ZnO–TiO2 heterostructure (TZBC 75) measured at 420 nm to be 1.56%. 3.3. Possible theoretical photocatalytic mechanism Fig. 6. XRD patters of Cs2O–Bi2O3–TiO2–ZnO heterostructures (TZBC) with two different concentrations of Cs (named as TZBC0 and TZBC75) and individual oxides (Cs2O, Bi2O3, TiO2 and ZnO).
The actual amounts of Cs estimated by ICP-AEs for TZBC0, TZBC725, TZBC50, TZBC75, and TZBC100 are 0, 0.68, 0.12, 0.20, and 0.27 wt%, respectively. 3.2. Photocatalytic activity The photocatalytic activity of the as-prepared samples was investigated by monitoring the formation of H2 and O2 during water splitting under solar simulator. The results are shown in Fig. 8(a). As stated above, Cs2O was used as a photosensitizer. The photocatalytic
On the basis of the above finding results, the improvement in the photocatalytic activity could be ascribed by the synergetic effect between hetrojunction and formation of direct Z-scheme which in turn leads not only promote the separation of photo-generated electrons and holes, but also keep high redox potential. To understand the charge migration pathway in the Cs2O–Bi2O3–TiO2–ZnO photocatalytic system, valence and conduction band edge positions of Bi2O3, TiO2, ZnO and Cs2O are illustrated in Fig. 9. The valence and conduction bands of the nanostructured Bi2O3, TiO2, ZnO and Cs2O were calculated using the following formula: EVB = X- Ee – 0.5 Eg ECB = Eg – EVB where ECB and EVB are the conduction and valence band potential,
Fig. 7. (a)–(d) XPS high resolution spectra of Cs 3d, Zn2p, Ti2p, and Bi4f of TZBC75 specimen respectively. 23760
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Fig. 8. (a) The photocatalytic H2 and O2 evolution of the Cs2O–Bi2O3–TiO2–ZnO heterostructure with different concentrations of Cs2O under solar simulator, and (b) stability experiments for the overall water splitting.
respectively. X is the electronegativity of the semiconductor. The X values for Bi2O3, TiO2, ZnO and Cs2O are 5.99 eV, 5.81, 5.79 eV, and 3.89 eV, respectively. Ee is the energy of free electrons with the H2 scale (4.5 eV versus NHE) and Eg is the band gap of the semiconductor. On the basis of previous literature including our study [42,43], the band gaps of Bi2O3, TiO2, ZnO and Cs2O are 2.77, 3.4, 3.02 and 1.92 eV respectively. Thus, the conduction band potential of Bi2O3, TiO2, ZnO and Cs2O are determined to be −0.12, −0.39, −0.28 and −1.50 eV respectively, while the respective valence bands are calculated to be 2.88, 3.01, 1.92 and 0.42 eV. On the basis of these findings, it was confirmed that the band edges of Bi2O3, TiO2, and ZnO are quite different than that of Cs2O. The Cs2O possesses much higher conduction band potential and its valence band remains in close proximity of the conduction band of Bi2O3, TiO2, and ZnO. Due to the n-p heterojunction between n-type ZnO and p-type TiO2, the electrons of ZnO can migrate to the conduction band of TiO2 and the holes of TiO2 can transfer to the valence band of ZnO. Since the valence band of Bi2O3 is located at slightly positive value than the valence band of TiO2, the holes generated in the valence band of Bi2O3 can diffuse to the valence band of the TiO2 and then to the valence band of the ZnO. These charge separation processes are similar to that of conventional heterojunction photocatalytic system. Although, this charge separation process is strong enough to oxidize and produce oxygen from water, the conduction band of Bi2O3 is positive and hence unable to reduce H+ to generate H• or H2. However, the conduction band edge of Cs2O is more negative and its valence band is less positive than that of Bi2O3. Under simulated sunlight irradiation, valence electrons of TiO2, ZnO, Cs2O and Bi2O3 absorb appropriate photonic energy, and excited to their conduction bands. On
the basis of band gap positions of Bi2O3 and Cs2O, the separation processes of photogenerated electron-hole pairs are likely to occur in two possible ways; (1) heterojunction or (2) Z-scheme. According to the heterojunction mechanism, the electrons in the conduction band of Cs2O will be transferred to the conduction band of Bi2O3, which is the lowest conduction band among the four metal oxides TiO2, Cs2O, Bi2O3, and ZnO. Simultaneously, the holes in the valence band of Bi2O3 will drift to the valence band of Cs2O. However, the accumulated electrons in the conduction band of Bi2O3 cannot produce H2 due to its lower conduction band position compared to H+/H2 potential [44]. Therefore, it seems reasonable to infer that the charge separation process and ensuing enhancement in the overall water splitting in the presence of Cs2O–Bi2O3 is not driven by heterojunction model. We, therefore, made an attempt to rationalize the charge separation through Z-scheme model. Both Bi2O3 and Cs2O can be excited under simulated sunlight irradiation to generate electron-hole pairs, and then the electrons of Bi2O3 can migrate to the valence band of Cs2O and recombine with the holes. The electrons in the valence band of Cs2O are further excited under irradiation to its conduction band leading to efficient separation of the photoexcited charge carriers. The carriers flow between Bi2O3 and Cs2O continues till charge equilibrium is attained. As a result, the net charge accumulation causes the creation of an electric field at the Bi2O3 and Cs2O interface. The direction of the internal electric field is from Cs2O surface to Bi2O3 surface, which in turn accelerates the transfer of photogenerated electrons from the conduction band of Bi2O3 to the valance band of Cs2O and restrain the photogenerated electrons in the conduction band of Cs2O to migrate into the conduction band of Bi2O3. The same process occurs between Cs2O
Fig. 9. Charge transfer pathway and the mechanism of overall water splitting. 23761
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Fig. 10. (a) Transient photocurrent responses TZBC0, TZBC25, TZBC50, TZBC75 and TZBC100 simulated sunlight irradiation in 0.5 M Na2SO4 aqueous solution, and (b) photoluminescence spectra of TZBC0, TZBC25, TZBC50, TZBC75, and TZBC100 with excitation of 200 nm.
and both ZnO and TiO2 under simulated sunlight irradiation. In this way, the photogenerated electrons in the conduction band of Cs2O which have more negative potential would lead to superior photocatalytic efficiency of the Cs2O–Bi2O3–TiO2–ZnO heterostructure for water splitting reactions. 3.4. Evidence of the mechanism 3.4.1. Photoelectrochemical analysis (PEC) The photoelectrochemical properties is commonly considered to be efficient evidence to demonstrate the electron−hole pair separation and transfer in the composite photocatalysts [45]. The photoelectrocatalytic performance was studied by monitoring the photooxidation of water. The photocurrent was recorded for three on−off light illumination cycles. As can be observed in Fig. 10(a), photocatalytic samples are incapable of producing current in the absence of light. However, the photocurrent increased instantaneously upon illumination, and reached to a maximum value. The photocurrent recorded in the presence of TZBC25, TZBC50, TZBC75, and TZBC100 heterostructures was much higher than that of TZBC0, confirming the enhanced photocatalytic performance of Bi2O3–ZnO–TiO2 heterostructure when combined with Cs2O. Furthermore, the photoelectrocatalytic performance trend was found to be similar to that of photocatalytic trend; oxidation of water increases with an increase in the amount of Cs2O up to certain limit, and a further increase causes decrease in the photocurrent. Sample TZBC75 exhibited the highest photocurrent density with the value as high as 5 μA cm−2, which is almost nine folds higher than that of TZBC0. The improved photoelectrocatalytic performance of the TZBC25, TZBC50, TZBC75, and TZBC100 heterostructures samples is attributed to the enhanced charge separation compared with TZBC0. 3.4.2. Photoluminescence emission spectra To verify the above proposed theoretical mechanism for Cs2O–Bi2O3–TiO2–ZnO photocatalysts system, steady-state photoluminescence (PL) analysis was carried out to gain further insights about the spatial charge separation capability of the as-prepared photocatalytic samples. In general, a higher PL intensity indicates a higher recombination rate of photogenerated electron−hole pairs and lower PL intensity correlated with a higher separation efficiency of photogenerated electron hole pairs. The results presented in Fig. 10(b) show that the Cs2O mol% amount enhanced the transmission efficiency of electron–hole pairs, and thus reduced the recombination rate of the photogenerated charge carriers. The emission peak intensity of the TZBC75 is lower than the other samples, indicating the higher charge separation efficiency. This finding is consistent and supports with photocatalytic and the photoelectrocatalytic trend, in which the TZBC75 exhibited the highest performance.
3.4.3. Valance band x ray photoelectron spectroscopy (VB-XPS) VB-XPS combined with UV–vis diffuse reflection spectroscopic (UV–Vis DRS) analysis can be used to confirm the direct Z-Scheme photocatalytic mechanism by confirming the suitable Z-scheme band edges/maximum−minimum, which match with the redox potential/ level of water. The XPS spectra were obtained using Mg Kalpha XPS with low pass energy and about 0.8 eV resolution. The intersection of the linear extrapolations of the leading edge and the background of the XPS spectrum is the flat valance bands maximum. Fig. 11 showed that the values determined to be to be 2.9, 0.4, 3.2 and 2.2 eV for Bi2O3, Cs2O, TiO2 and ZnO respectively which is in a good agreement with the theoretical values calculated in section 3.2.
3.4.4. Electron spin resonance (ESR) ESR spectroscopy was carried out to find the reactive species in the presence of as-synthesized multicomponent system. 5,5-Dimethyl-Lpyrroline N-oxide (DMPO), which reacts with the superoxide anion radicals (•O2−) and hydroxyl radicals (•OH) to produce DMPO−•O2 and DMPO−•OH in methanol and aqueous media respectively, was added in the solution to, [46,47]. The ESR signal corresponding to DMPO−•O2 collected in the presence of TZBC75 and TZBC0 is shown in Fig. 12(a). The strongest DMPO−•O2 signal was obtained when TZBC75 heterostructure was used as photocatalyst, while for TZBC0, the signal was negligible. These results clearly demonstrate the enhanced activity of the TZBC75 heterostructure to produce •O2− radicals than that of TZBC0, revealing that •O2− radical was preferably generated by TZBC75 under light irradiation. In the case of the TZBC0, the charge migration follows the heterojunction model leading to the accumulation of electrons in the conduction band of Bi2O3 which do not have enough potential to produce •O2−. In contrast to TZBC0, the charge migration in the TZBC75 follows the Z-scheme model, leading to the accumulation of the photogenerated electrons in the conduction band
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Fig. 11. XPS valence band spectra of the pure Bi2S3, Cs2O, ZnO, and TiO2.
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Fig. 12. ESR spectra of (a) DMPO−•O2, and (b) DMPO−•OH in methanol and aqueous solution for the TZBC75 and TZBC0 under solar irradiation.
of Cs2O, which has enough redox potential to produce •O2−. On the other hand, the photoexcited holes are accumulated in the conduction band of ZnO for both TZBC0 and TZBC75 which have enough redox potential to produce •OH, thus the DMPO−•OH signal was detected for both (Fig. 12(b)). The higher signal intensity of DMPO−•OH in the presence of TZBC75 is attributed to the higher light absorption as compare to TZBC0. This was confirmed by UV–vis spectroscopy. The results strongly suggest that the charge transfer mechanism follows both the heterojunction and the Z-scheme model at the same time. 4. Conclusions A multicomponent photocatalytic nanoassembly comprising Cs2O, Bi2O3, TiO2, and ZnO was synthesized through a solution combustion method. The method is simple with potential to scale up. Both the photocatalytic and photoelectrocatalytic performance suggested that Cs2O can be utilized as an effective photosensitizer to activate multicomponent structures. By varying the content of Cs2O in the heterostructure, it was found that Cs2O–Bi2O3–TiO2–ZnO with 7.5 mol% exhibited the highest photocatalytic activity for overall water splitting. The optimal H2 and oxygen production rates were 255 μmol g−1 h−1 and 127.5 μmol. g−1 h−1, respectively in the absence of any cocatalyst and sacrificial agent under solar simulator irradiation. The improved performance of Cs2O–Bi2O3–TiO2–ZnO was correlated to the synergetic effect of Z-scheme and heterojunction charge carrier separation which was confirmed by various analytical techniques. The study introduces for the first time the synergetic effect between Z-scheme and heterojunction charge migration transfer pathways and highlights its impact in overall water splitting. Acknowledgment The authors would like to acknowledge the support provided by the Deanship of Scientific Research at King Fahd University of Petroleum and Minerals for funding this work through project No. DF181021. The authors would like to acknowledge Center of Excellence in Nanotechnology (CENT), and Center of Research Excellence in Renewable Energy (CoRERE), King Fahd University of Petroleum and Minerals (KFUPM). References [1] O. Ellabban, H. Abu-Rub, F. Blaabjerg, Renewable energy resources: current status, future prospects and their enabling technology, Renew. Sustain. Energy Rev. 39 (2014) 748–764. [2] N. Kannan, D. Vakeesan, Solar energy for future world: a review, Renew. Sustain. Energy Rev. 62 (2016) 1092–1105. [3] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Photocatalyst releasing hydrogen from water, Nature 440 (2006) 295. [4] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets, J. Phys. Chem. C 114 (2010) 13118–13125.
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