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CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity
Accepted Manuscript Title: Direct Z-scheme TiO2 /CdS hierarchical photocatalyst for enhanced photocatalytic H2 -production activity Authors: Aiyun Meng, Bicheng Zhu, Bo Zhong, Liuyang Zhang, Bei Cheng PII: DOI: Reference:
S0169-4332(17)31672-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.028 APSUSC 36228
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Received date: Revised date: Accepted date:
19-5-2017 1-6-2017 4-6-2017
Please cite this article as: Aiyun Meng, Bicheng Zhu, Bo Zhong, Liuyang Zhang, Bei Cheng, Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity
Aiyun Meng, Bicheng Zhu, Bo Zhong, Liuyang Zhang, Bei Cheng*
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China.
E-mail: [email protected]
TOC
1
Highlights
SILAR method was used to prepare TiO2/CdS hierarchical microspheres.
A direct Z-scheme photocatalytic mechanism was proposed.
The TiO2/CdS composite photocatalyst with improved photocatalytic activity.
Efficient electron-hole separation was achieved.
Abstract Photocatalytic H2 evolution, which utilizes solar energy via water splitting, is a promising route to deal with concerns about energy and environment. Herein, a direct Z-scheme TiO2/CdS binary hierarchical photocatalyst was fabricated via a successive ionic layer adsorption and reaction (SILAR) technique, and photocatalytic H2 production was measured afterwards. The as-prepared TiO2/CdS hybrid photocatalyst exhibited noticeably promoted photocatalytic H2-production activity of 51.4 μmol h-1. The enhancement of photocatalytic activity was ascribed to the hierarchical structure, as well as the efficient charge separation and migration from TiO2 nanosheets to CdS nanoparticles (NPs) at their tight contact interfaces. Moreover, the direct Z-scheme photocatalytic reaction mechanism was demonstrated to elucidate the improved photocatalytic
performance
of
TiO2/CdS
composite
photocatalyst.
The
photoluminescence (PL) analysis of hydroxyl radicals were conducted to provide clues for the direct Z-scheme mechanism. This work provides a facile route for the construction
of
redox
mediator-free
Z-scheme
photocatalytic water splitting.
2
photocatalytic
system
for
Keywords: TiO2; CdS; Direct Z-scheme; Hierarchical photocatalyst; Photocatalytic H2-production
1. Introduction As a green and promising technology, photocatalysis, which can convert solar energy into sustainable chemical energy, has been extensively explored to solve the worsening energy crisis. Especially, photocatalytic H2-production via water splitting using solar energy has been recognized as a potential method owing to the clean and renewable hydrogen energy [1–8]. To promote photocatalytic hydrogen production activity, the heterostructure designs and fabrication of semiconductor photocatalysts have attracted much attention [9–14]. Amongst them, Z-scheme photocatalysts have been implemented due to their improved separation efficiency of photogenerated charge carriers and stronger redox capacity relative to the conventional heterojunction-type photocatalysts [15–24]. In the previous studied Z-scheme photocatalytic systems, electron mediators play a significant role in promoting electron transfer, facilitating charge separation and inhibiting charge recombination between two semiconductor catalysts [25]. Common electron mediators are mainly solid-state noble metals (such as Au, Ag), redox ionic couples (such as IO3-/I-, Fe3+/Fe2+) and rGO [21,26–31]. For example, Tada et. al developed a Z-scheme CdS/Au/TiO2 photocatalyst, in which Au acted as the electron conductor and facilitated the vectorial transfer of photoinduced electrons from TiO2 CB (conduction band) to CdS VB (valence band) [32]. Abe et al. utilized IO3-/I- to transfer electrons between rutile TiO2 and Pt-loaded anatase TiO2 to generate H2 and 3
O2 under illumination of UV light [26]. However, the ternary Z-scheme systems with electron-mediators suffer from many shortcomings, such as expensiveness, low stability, reverse reactions for water splitting, strong visible light absorption, and operability only in solution systems for redox ionic couples [31]. In view of the above reasons, studying the all-solid-state direct Z-scheme photosynthetic system is highly desirable [33]. Recently, several direct Z-scheme photocatalytic systems without electron mediators have been constructed, such as rutile TiO2 with g-C3N4 quantum dots [34], SnS2 hexagonal nanoplates decorated irregular BiOBr nanosheet [35], CdS/Co9S8 hollow cube [20], g-C3N4 nansheets/oxygen vacancy-rich ZnO [36] and Ti0.91O2/CdS hollow spheres [37]. These direct Z-scheme systems exhibit strong electron reducibility and hole oxidizability, leading to superior photocatalytic performance. However, it still remains a great challenge to construct efficient direct Z-scheme photocatalyst with high photocatalytic performance. Thus, further investigation is needed. As a typical and efficient photocatalyst, TiO2 has been widely investigated [38–40]. On one hand, morphology optimizations such as forming hierarchical structure, which enhance light harvesting, increase the surface areas and provide more reaction sites, are carried out [41]. On the other hand, component optimizations such as coupling semiconductors with narrower band gap materials, which can absorb visible light, are performed. With a narrow band gap of 2.4 eV, CdS has been used to composite with TiO2 due to its visible-light response [42]. The combination of TiO2 4
and CdS not only enhances light capture but also introduces new active sites and restrains charge recombination [37,43–45]. Various synthetic techniques such as hydrothermal method, sol–gel method and calcination method have been used to prepare the TiO2/CdS composite nanostructures. Amongst them, SILAR method has also been explored because it is economical and convenient for large-area deposition [46]. In this work, we constructed a direct Z-scheme TiO2/CdS hybrid hierarchical photocatalyst via a simple SILAR method. TiO2 hierarchical microspheres composed of spiny nanosheets were used as support; the abundant pore structures are beneficial for the multiple light reflections and scattering inside the pore channels. CdS nanoparticles (NPs) were interspersed on the surface of TiO2 microspheres with electrostatic attraction. The intimate contact between TiO2 microspheres and CdS NPs created easier electron transfer. The obtained TiO2/CdS photocatalyst was further utilized for photocatalytic water splitting under illumination. The effect of the CdS NPs on the morphology, light absorption and photocatalytic activity of TiO2 microspheres was investigated. Ultimately, based on the analysis of hydroxyl radicals and photocurrent response, a direct Z-scheme mechanism was proposed to elucidate the improved photoactivity of TiO2/CdS binary photocatalytic system.
2. Experimental part 2.1. Preparation of TiO2 nanosheet microspheres All chemicals were of analytical grade and employed without further treatment. 5
1.0 mL of tetrabutyl titanate (TBOT) was added into 30 mL of glacial acetic acid dropwise, and then the mixture was stirred for 30 min and put into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and treated at 140 oC for 12 h. After that, the white precipitation was rinsed with deionized water and ethanol, dried in the oven at 80 oC in air atmosphere. Finally, the product was heated at 500 oC for 2 h to facilitate crystallization and remove organics. The as-prepared pure TiO2 nanosheet microsphere sample was marked as T.
2.2. Preparation of TiO2/CdS composite microspheres The TiO2/CdS composite microspheres were prepared by SILAR method. A single SILAR deposition cycle was illustrated in Fig. 1. In a typical process, 0.05 M (CH3COO)2Cd aqueous solution and 0.05 M Na2S aqueous solution were firstly prepared. Then the as-prepared TiO2 microspheres (T) were dispersed in (CH3COO)2Cd solution for 5 minutes until the Cd2+ ions were adsorbed on the nanosheets of TiO2 microspheres. Then the loose part of adsorbed Cd2+ ions was removed by rinsing with deionized water. Further, the TiO2 microspheres adsorbed with Cd2+ were immersed in Na2S solution for 5 minutes, thus the S2- ions were adsorbed and reacted with Cd2+ ions to form CdS NPs during the immersion process. By rinsing with deionized water, the redundant S2- ions were removed, leaving the CdS NPs on TiO2 surface. Thus, single SILAR deposition cycle was completed, the obtained sample was marked as TC3. By changing the concentration of (CH3COO)2Cd and Na2S solution, the composite samples with different content of 6
CdS were obtained and denoted as TC1, TC2, TC3, TC4 and TC5, respectively. The concentrations of (CH3COO)2Cd and Na2S solution and corresponding weight ratio of CdS NPs were demonstrated in Table 1. For comparison, CdS NPs were prepared by mixing Na2S solution (0.5 M, 50 mL) and (CH3COO)2Cd solution (0.5 M, 50 mL) under continuous stirring.
2.3. Characterization X-ray diffraction (XRD) patterns were obtained by a Rigaku X-ray diffractometer with Cu Kα radiation. Morphology is measured by field-emission scanning electron microscopy (JSM-7500, JEOL) with an Oxford energy-dispersive X-ray spectroscopy. Transmission electron microscopy (TEM) and high resolution TEM analysis were performed using JEM-2100F electron microscope (JEOL, Japan). X-ray photoelectron spectra (XPS) were obtained on an ultra-high-vacuum VG ESCALAB 210 electron spectrometer. All binding energies were referenced to the C 1s peak of the surface adventitious carbon at 284.8 eV. The weight ratios of CdS relative to TiO2 were measured by inductively coupled plasma–optical emission spectra (Prodigy 7, USA). The light absorption was measured by an UV–vis spectrophotometer (UV-2550, SHIMADZU). N2 adsorption-desorption isotherms were recorded on an ASAP 2020 nitrogen adsorption apparatus (Micromeritics, USA). The hydroxyl radicals were investigated on a F-7000 fluorescence spectrophotometer (Hitachi, Japan). Coumarin was used as a probe molecule because coumarin can react with ·OH to generate fluorescent product, 7-hydroxycoumarin (7HC). In a typical 7
experiment, 50 mg of samples were suspended into 20 mL of coumarin aqueous solution (1 × 10-3 mol L-1). After 1 h illumination of UV light, the suspension was filtered, and the PL intensities at 456 nm excited by 332 nm light were measured. The PL intensity can reflect the amount of hydroxyl radicals.
2.4. Photocatalytic activity and photoelectrochemical test The photocatalytic hydrogen evolution activity was tested in a 100 mL Pyrex flask. The light source was a 350 W Xenon lamp (Changzhou Siyu Science Co. Ltd, China). In a typical process, 0.05 g of catalyst was dispersed into 80 mL methanol/water solution (25% methanol in volume). Before irradiation, N2 was pumped into the suspension to remove air. After irradiation for 1 h, 0.4 mL gas was collected from the reactor and detected by gas chromatograph (GC-14C, Shimadzu, Japan, TCD detector, N2 carrier). The photoelectrochemical measurement was conducted on a CHI660C electrochemical analyzer (Chenhua Instrument, Shanghai) using a three-electrode system. A 2 × 1.5 cm clean FTO glass was coated with samples and used as working electrode. Pt foil and Ag/AgCl served as counter electrode and reference electrode, respectively. Electrolyte was 0.5 M Na2SO4 aqueous solution during the test. A 3 W UV-LED (365 nm) was used as the light source. Linear sweep voltammetry (LSV) was conducted with a voltage scanning speed of 10 mV s-1. The UV-LED light was switched on and off at regular intervals of 10 s.
8
2.5. DFT calculations Density function theory (DFT) calculations were conducted on a CASTEP Package. The Perdew-Burke-Ernzerhof (PBE) of the generalized gradient approximation (GGA) was used as the exchange-correlation function. An energy cutoff of 400 eV and the Monkhorst–Pack k-point mesh of 4 × 4 × 1 for both TiO2 and CdS models were used to perform geometry optimizations. The energy and force convergence criterions were set as 1.0 × 10–5 eV/atom and 0.03 eV/Å, respectively.
3. Results and discussion 3.1. Morphology and phase structure Fig. 1 illustrates the SILAR process for the fabrication of TiO2/CdS microspheres. During the preparation process, the TiO2 microspheres were negatively charged in neutral solution (pH = 7) because the isoelectric point of TiO2 is 6.2 [47]. Therefore, when adding TiO2 microspheres into (CH3COO)2Cd solution, the Cd2+ ions with positive charges can be adhered onto the surface of TiO2 due to electric attraction. Next, after immersing the sample into Na2S solution, the S2- ions with negative charges can be adsorbed and reacted with Cd2+ ions, forming CdS NPs. Thus, CdS NPs were uniformly deposited onto TiO2 surface by SILAR method. Table 1 lists the concentrations of reactant solution, the weight ratios of CdS NPs and corresponding sample names.
Among the obtained TiO2/CdS composite samples, TC3 exhibited the highest 9
photocatalytic performance (which will be discussed in section 3.5). Therefore, we take TC3 as a typical sample to measure the morphology, structure and chemical composition. Firstly, electron microscope was performed to demonstrate the morphology and microstructure of sample T, C and TC3. Fig. 2a presents a hierarchical flower-like microspheric TiO2 with 2~3 μm in diameter. The enlarged FESEM image (Fig. 2b) shows that the TiO2 microsphere is assembled by spiny mesoporous nanosheets, which is germinated from the central of the microsphere. Fig. 2c shows the SEM image of pure CdS prepared by precipitation method. As can be seen, pure CdS is composed of aggregated nanoparticles with uniform sizes of ca. 30 nm. Fig. 2d shows the morphology of TC3 hierarchical photocatalyst. The composite photocatalyst also exhibits a microspheric shape with 2~3 μm in diameter. The surface of the composite sample becomes rough, and the thorns are weakened due to the deposition of CdS NPs. Further observation shows that some NPs are interspersed on the surface of TiO2 spiny nanosheets (Fig. 2e). EDS mapping images (Fig. 2f) further show the existence of Ti, O, Cd and S elements, indicating the uniform distribution of CdS NPs on TiO2 microspheres. The thorn-like structure can also be seen in Fig. 2g. The lattice fringes of TC3 were shown in the enlarged HRTEM in Fig. 2h. The lattice spacing of 0.35 nm and 0.20 nm corresponds to anatase TiO2 (101) crystal facet and cubic CdS (220) crystal facet, respectively, indicating that the intimate integration of TiO2 and CdS. Fig. 2l shows the XRD patterns to indentify the phase structure of samples. The XRD pattern of pure TiO2 demonstrated characteristic diffraction peaks of anatase 10
phase (JCPDS, No. 21-1272). As comparison, the XRD pattern of pure CdS NPs was also analyzed. The diffraction peaks can be assigned to the pure cubic phase of CdS (JCPDS, No. 10-0454). Compared with standard PDF card, three main peaks which located at 26.5o, 43.9o and 52.1o broadened obviously, indicating that the deposited CdS particles are of nanoscale level, which is in accordance with SEM images. For sample TC3, all diffraction peaks can be attributed to anatase TiO2. No peaks of CdS was found due to its low content, poor crystallinity and high dispersion. No peak shift was observed, excluding the possibility of CdS incorporation into TiO2 lattice.
3.2. BET surface area and pore size distribution Fig. 3 displays the N2 adsorption-desorption isotherms and the corresponding pore
size
distribution
curves
of
T,
C
and
TC3.
According
to
the
Brunauer–Deming–Deming–Teller (BDDT) classification, the isotherms of T and TC3 are of type IV, implying the presence of mesopores (2–50 nm) [48]. The shapes of hysteresis loops of T and TC3 are of type H3, indicating the presence of slitlike pores
which
are
formed by the aggregation of TiO2
nanosheets.
The
adsorption-desorption isotherm of C is also of type IV, and the hysteresis loop can be classified as type H2, suggesting the presence of ink-bottle-like mesopores formed by uniform CdS NPs [49]. The above analysis is consistent with the FESEM results. Further observations demonstrate that the sorption isotherm of sample C shifts downward compared with sample T and TC3 in all range of relative pressure, suggesting a smaller specific surface area and pore volume of pure CdS. The pore size 11
distribution (inset in Fig. 3) indicated that sample T and TC3 exhibits a wide pore size distribution from 2 to >100 nm, suggesting that the mesopores and macropores coexist. In contrast, sample C exhibits a narrow pore size distribution in the range of 2–10 nm, indicating smaller pore size of pure CdS. This is also confirmed by the data in Table 2. Moreover, the specific surface area of TC3 (84 m2/g) is almost same as that of T (81 m2/g), indicating that CdS NPs on TiO2 surface are small and well-dispersed, which has little effect on the BET surface area of TC3 composite photocatalyst. The larger BET surface area, together with hierarchical structure of TC3, is crucial for the adsorption and migration of reactant and product molecules.
3.3. Light absorption Fig. 4 presents the DRS spectra of T, TC3 and C. For sample T, a significant absorption increase at wavelength of ca. 390 nm was observed, which agreed with the intrinsic band-gap energy of 3.2 eV. For sample C, an absorption onset occured at 550 nm, which corresponded to the band-gap energy of 2.3 eV of cubic phase CdS [15]. Compared with the pure TiO2, TC3 showed stronger photoabsorption in visible light range (400~500 nm). The enhancement in visible light region of sample TC3 stemmed from the successfully deposited CdS NPs on TiO2 nanosheets.
3.4. Work function and XPS analysis The work function of material is vital in investigating the charge transfer at interface [50]. Herein, the crystal structures of TiO2 (101) and CdS (110) were 12
constructed in Fig. 5a and 5c. The work function of the TiO2 (101) and CdS (110) were obtained by equation 1:
EVAC EF
(1)
where Evac is the electrostatic potential of vacuum level, and EF means the energy of Fermi level. The calculated work functions of TiO2 (101) and CdS (110) surface (Fig. 5b and 5d) were 7.1 eV and 5.8 eV, respectively, which coincides with the previous reports [51,52]. This indicated that the Fermi level of TiO2 (101) surface was more positive than that of CdS (110) surface. When CdS NPs contact with TiO2 surface, the electrons at the TiO2/CdS interface would rearrange and migrate from CdS to TiO2, leaving holes on the CdS. Then a depletion layer can be formed on CdS surface, while an accumulation layer can be formed on TiO2 surface, leading to the formation of internal electric field at the interface. The internal electric field, directing from the CdS surface to the TiO2 surface, restrained the further migration of electrons. Finally, the two Fermi levels reach the same level and the mobility of electron from CdS to TiO2 reaches equilibrium.
XPS measurement was conducted to confirm the chemical composition and elemental chemical states of T, C and TC3. Ti, O, C, Cd and S were detected in sample TC3 (not shown), indicating that TiO2 and CdS were successfully composited. Fig. 6 presents the comparison of the high-resolution XPS spectra of Ti 2p, O 1s, Cd 3d and S 2p in sample T, C and TC3. The Ti 2p peaks of sample T at 458.6 and 464.3 eV can be attributed to Ti 2p3/2 and Ti 2p1/2, respectively [44]. Two similar peaks at 13
458.3 and 464.0 eV were detected in the Ti 2p spectrum of TC3, which shifted towards the lower energy region by 0.3 eV relative to that of sample T. The O 1s spectrum of sample T located at 529.8 eV, which can be attributed to lattice oxygen (Ti−O−Ti). The corresponding O 1s peaks of TC3 located at 529.5 eV, which also shifted towards the lower binding energy by 0.3 eV. Another peak of O 1s at 531.4 eV was assigned to surface hydroxyl oxygen (Ti−OH) [53]. The Cd 3d spectrum of pure CdS demonstrated two obvious peaks at 405.0 and 411.7 eV, which were assigned to Cd 3d7/2 and Cd 3d5/2 [15]. The corresponding Cd 3d peaks of sample TC3 located at 404.3 and 411.0 eV. In contrast with that of Ti 2p and O 1s, the binding energies of Cd 3d shifted upward by 0.7 eV. The S 2p spectrum of pure CdS shows two clear peaks at 160.8 and 161.9 eV, which can be assigned to the S 2p3/2 and S 2p1/2 [54]. Moreover, the S 2p binding energy of TC3 was found to shift to the higher energy region. The opposite shift of binding energies suggests that the electrons of TC3 tend to flow from CdS to TiO2, which is due to the different position of Fermi level (see work function analysis). When CdS NPs were deposited on TiO2 microspheres, intimate contact between CdS and TiO2 was formed. Since the Fermi level of CdS was more negative than TiO2, the electrons from CdS would migrate to TiO2 until the Fermi level of TiO2 and CdS reach the same position. The shift of the binding energies indicates the interfacial intimate contact between TiO2 and CdS, which makes it possible to transfer photogenerated electrons between TiO2 and CdS without a redox mediator during the photocatalytic reaction [55]. Therefore, the close contact is beneficial for constructing direct Z-scheme TiO2/CdS binary composite photocatalyst. 14
3.5. Photocatalytic performance Photocatalytic H2 production activities of samples were measured under illumination of a 350 W Xenon lamp. H2 production was detected with the existence of both photocatalyst and irradiation. Fig. 7a shows the comparison of hydrogen yield of prepared samples loaded with different amounts of CdS NPs. Pure TiO2 and CdS exhibited negligible photocatalytic performance due to the rapid recombination of photoinduced charge carriers. With increment proportion of CdS, the H2 yields of TiO2/CdS composite photocatalyst had a remarkable enhancement. With optimal CdS content of 4.4 wt%, the sample TC3 reached the highest photocatalytic activity of 51.4 μmol h-1, which exceeded that of pure TiO2 by 28 times. The promoted photocatalytic performance was due to the following reasons: 1) The hierarchical structure of TiO2 microspheres was beneficial for the multiple light reflection and scattering, thus enhancing the light harvest; 2) The addition of CdS NPs broadened the light absorption region to the visible light region, which were proved by the DRS spectra in Fig. 4; 3) The combination and intimate contact of TiO2 and CdS facilitated the separation and transfer of photogenerated electrons and holes on the interface of two photocatalysts. The mechanism will be further investigated in the following discussion. Moreover, after three cycles, the photocatalytic activity of sample TC3 did not exhibit obvious decrease (Fig. 7b), which means that the hybrid photocatalyst is stable under successive photocatalytic reactions. When further increasing the content of CdS, the photocatalytic H2 production activities decreased, which is due to the following reasons: 1) excess CdS NPs shielded TiO2 from light harvesting, thus TiO2 15
cannot be excited efficiently by the ultraviolet light; 2) excessive CdS NPs resulted in a decline of surface active sites of TiO2; 3) excess CdS NPs led to the increase of particle size, thus resulting in the deterioration of photocatalytic properties of CdS particles. Therefore, a suitable amount of CdS NPs is significant for the optimization of TiO2/CdS composite photocatalyst.
3.6. Photocatalytic mechanism Two possible mechanisms to explain the enhanced photocatalytic activities and the improved charge separation are illustrated in Fig. 8. In the traditional heterojunction-type photocatalytic system, the electrons in the VB of CdS and TiO2 are excited to the CB of CdS and TiO2 under solar light, respectively. Then the photogenerated electrons in CdS CB with more negative energy potential transfer to TiO2 CB, while the photogenerated holes in TiO2 VB with more positive energy potential migrate to CdS VB. Therefore, the photoinduced electrons and holes are accumulated in TiO2 CB and CdS VB, respectively, resulting in the separation of photoinduced charge carriers. In the direct Z-scheme photocatalytic system, both TiO2 and CdS can generate electrons and holes under light excitation. Then the photogenerated electrons in TiO2 CB transfer and combine with the photogenerated holes in CdS VB, leaving the electrons in CdS CB and the holes in TiO 2 VB, thus maintaining the strong electron reducibility of CdS CB and strong hole oxidizability of TiO2 VB. Finally, the accumulated electrons in CdS CB react with H2O and produce H2. 16
To determine the more likely photocatalytic mechanism of TiO2/CdS composite catalyst, the hydroxyl radicals (·OH) generated from the surfaces of pure TiO2, pure CdS and TC3 were investigated by the photoluminescence analysis with coumarin as a probe molecule. It was observed that all of the PL peak intensities of T, C and TC3 increase with the increasing irradiation time (Fig. 9a and 9b), indicating that hydroxyl radicals are generated under light excitation. Meanwhile, the band edge positions of TiO2 and CdS were illustrated in Fig. 11.
O2 e O2
OH h
H 2O h
vb vb
OH OH H
E = – 0.33 V
(2)
E = 1.99 V
(3)
E = 2.34 V
(4)
As shown in Fig. 11 and equations 2 – 4, for sample C, the oxidation potential of OH-/·OH (1.99 V) and H2O/·OH (2.34 V) is more positive than the VB potential of CdS (1.88 V) [56,57]. Therefore, the holes in pure CdS are unable to oxidize OH- or H2O into ·OH and ·OH should not have been detected. However, In Fig. 9b, pure CdS also generated a little amount of hydroxyl radicals (·OH) with weak PL signals. This is attributed to the indirect generation of ·OH originated from ·O2-, which is produced due to the more negative CB potential of CdS than the reduction potential of O2/·O2[18]. For sample T, the VB potential of TiO2 is more positive than that of OH-/·OH and H2O/·OH, thus the photogenerated holes in TiO2 VB can react with OH- or H2O and ·OH can be directly generated, further causing the increased PL signals. For sample TC3, a higher PL peak signal than sample T was detected. These results strongly imply that TC3 complies with the direct Z-scheme reaction mechanism rather than the conventional heterojunction-type photocatalytic mechanism. If TC3 17
conforms
the
heterojunction-type mechanism,
the photogenerated electrons
accumulated in TiO2 CB have no ability to reduce the dissolved O2 into ·O2-, meanwhile, the photogenerated holes accumulated in CdS VB cannot oxidize OH- and H2O to ·OH, thus neither ·OH nor ·O2- could be detected.
For the purpose of investigating the separation rate of photoexcited charge carriers, the photocurrent–potential curves of T, C and TC3 were obtained with a voltage scanning speed of 10 mV s-1. As shown in Fig. 10, TC3 sample possesses the maximum photocurrent density compared to T and C samples, implying higher charge separation efficiency. This result supports the point that a direct Z-scheme mechanism contributes to separating the photoinduced electrons and holes, further increasing the photocatalytic efficiency.
Considering the above analysis of calculated work functions, XPS results, hydroxyl radicals and photocurrent–potential curves, a direct Z-scheme photocatalytic mechanism of TiO2/CdS hybrid photocatalyst was proposed and illustrated in Fig. 11. When CdS NPs were deposited on TiO2 surface, a tight contact was formed. Then the electrons on the CdS tend to transfer to TiO2 via the intimate contact interface due to the more negative Fermi level of CdS, leaving holes on the CdS. The electron diffusion continued until the Fermi levels are equilibrated [58–60]. As a result, an internal electric field at the interface of TiO2/CdS was formed, and the internal electric field directed from CdS to TiO2 (as shown in work function analysis). During the 18
photocatalytic process, the internal electric field restrained the migration of photogenerated electrons from CdS CB to TiO2 CB and photogenerated holes from TiO2 VB to CdS VB; In contrast, the electron transfer along the Z direction from TiO 2 CB to CdS VB was accelerated, leaving the photoexcited electrons in the CdS CB and photoexcited holes in the TiO2 VB. Thus, the construction of direct Z-scheme photocatalyst benefits the separation of photoexcited charge carriers at the TiO2/CdS interface. Moreover, the strong electron reducibility of CdS and strong hole oxidizability of TiO2 are maintained, which further improves the photocatalytic H2-evolution activity.
4. Conclusion In summary, we demonstrated an electron mediator-free TiO2/CdS binary Z-scheme system for photocatalytic H2 evolution by constructing a hierarchical structure consisting of TiO2 spiny nanosheet microspheres and CdS NPs via a SILAR method. The binary TiO2/CdS photocatalyst reaches the highest H2-production rate of 51.4 μmol h-1 under UV irradiation with 4.4 wt% CdS content. The remarkable enhancement is attributed to: 1) Hierarchical structures consisting of TiO2 nanosheets microspheres and CdS NPs, enhancing light harvest, increasing the BET surface areas and providing more active sites; 2) Intimate interfacial contact between TiO2 and CdS, facilitating the electron migration from TiO2 CB to CdS VB; 3) A direct Z-scheme structure, suppressing the electron-hole recombination of TiO2 and CdS and maintaining the strong electron reducibility in CdS VB. Our work provides a facile 19
route for constructing the artificial direct Z-scheme photocatalytic system without a redox mediator for photocatalytic water splitting.
Acknowledgement This work was supported by the NSFC (51320105001, 51372190, 21573170 and 21433007), 973 program (2013CB632402), the Natural Science Foundation of Hubei Province (2015CFA001), Innovative Research Funds of SKLWUT (2015-ZD-1) and the Fundamental Research Funds for the Central Universities (2016-YB-005).
20
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29
Fig. 1. Schematic description for the fabrication of TiO2/CdS photocatalyst via SILAR method.
30
Fig. 2. FESEM (a) and enlarged FESEM image (b) of sample T. FESEM (c) of sample C. FESEM (d), enlarged FESEM image (e), EDS mapping (f), TEM (g) and HRTEM (h) of sample TC3. XRD patterns (l) of T, C and TC3.
31
Fig. 3. Nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of T (black line), C (blue line) and TC3 (red line) composite photocatalyst.
32
Fig. 4. UV-vis diffuse reflectance spectra of T (black line), C (blue line) and TC3 (red line).
33
Fig. 5. Constructed TiO2 (101) and CdS (220) crystal structure: (a) red and gray balls stand for oxygen and titanium atoms, respectively; (c) rose red and yellow balls stand for cadmium and sulphur atoms, respectively. Calculated work functions of T (b) and C (d): The blue and red dashed lines represent the vacuum and Fermi levels, respectively.
34
Fig. 6. Comparison of high-resolution XPS spectra of Ti 2s (a), O 1s (b), Cd 3d (c) and S 2p (d).
35
Fig. 7. (a) Comparison of the photocatalytic hydrogen production rates of TiO2/CdS photocatalyst loaded with different amounts of CdS in methanol aqueous solution under Xenon irradiation for 1 h; (b) Cyclic H2-evolution curves of sample TC3.
36
Fig. 8. Schematic diagram of two mechanisms to explain charge separation: conventional heterojunction-type (a) and direct Z-scheme mechanism (b).
37
Fig. 9. (a) PL spectral changes of sample TC3 under irradiation in the presence of coumarin; (b) PL signal intensity at 456 nm of sample T, C and TC3 against illumination time.
38
Fig. 10. Photocurrent–potential curves of T, C and TC3 samples.
39
Fig. 11. The potential positions of TiO2 and CdS band edges and schematic illustration of direct Z-scheme photocatalytic mechanism for TiO2/CdS photocatalyst.
40
Table 1. The concentrations of (CH3COO)2Cd and Na2S aqueous solution for preparing TiO2/CdS composite photocatalysts, the weight ratios of CdS NPs in the TiO2/CdS composite photocatalysts and the corresponding photocatalytic activities. Sample C1
C2
WCdS (ICP-AES)
Activity (μmol·h-1)
T
0
0
0%
1.8
TC1
0.005
0.005 0.9%
9.7
TC2
0.01
0.01
1.9%
27.2
TC3
0.05
0.05
4.4%
51.4
TC4
0.1
0.1
4.8%
29.1
TC5
0.5
0.5
5.6%
11.4
C
0.5
0.5
100%
0
Notes: C1: the concentration of (CH3COO)2Cd; C2: concentration of Na2S; WCdS: the weight ratio of CdS relative to TiO2.
41
Table 2. Comparison of the specific surface area, pore volume and pore size of T, C and TC3.
Sample
SBET (m2/g)
Pore Volume (cm3/g) Pore Size (nm)
T
81
0.3
14.7
TC3
84
0.3
15.1
C
50
0.1
4.4
42
S0169-4332(17)31672-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.028 APSUSC 36228
To appear in:
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Received date: Revised date: Accepted date:
19-5-2017 1-6-2017 4-6-2017
Please cite this article as: Aiyun Meng, Bicheng Zhu, Bo Zhong, Liuyang Zhang, Bei Cheng, Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity
Aiyun Meng, Bicheng Zhu, Bo Zhong, Liuyang Zhang, Bei Cheng*
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China.
E-mail: [email protected]
TOC
1
Highlights
SILAR method was used to prepare TiO2/CdS hierarchical microspheres.
A direct Z-scheme photocatalytic mechanism was proposed.
The TiO2/CdS composite photocatalyst with improved photocatalytic activity.
Efficient electron-hole separation was achieved.
Abstract Photocatalytic H2 evolution, which utilizes solar energy via water splitting, is a promising route to deal with concerns about energy and environment. Herein, a direct Z-scheme TiO2/CdS binary hierarchical photocatalyst was fabricated via a successive ionic layer adsorption and reaction (SILAR) technique, and photocatalytic H2 production was measured afterwards. The as-prepared TiO2/CdS hybrid photocatalyst exhibited noticeably promoted photocatalytic H2-production activity of 51.4 μmol h-1. The enhancement of photocatalytic activity was ascribed to the hierarchical structure, as well as the efficient charge separation and migration from TiO2 nanosheets to CdS nanoparticles (NPs) at their tight contact interfaces. Moreover, the direct Z-scheme photocatalytic reaction mechanism was demonstrated to elucidate the improved photocatalytic
performance
of
TiO2/CdS
composite
photocatalyst.
The
photoluminescence (PL) analysis of hydroxyl radicals were conducted to provide clues for the direct Z-scheme mechanism. This work provides a facile route for the construction
of
redox
mediator-free
Z-scheme
photocatalytic water splitting.
2
photocatalytic
system
for
Keywords: TiO2; CdS; Direct Z-scheme; Hierarchical photocatalyst; Photocatalytic H2-production
1. Introduction As a green and promising technology, photocatalysis, which can convert solar energy into sustainable chemical energy, has been extensively explored to solve the worsening energy crisis. Especially, photocatalytic H2-production via water splitting using solar energy has been recognized as a potential method owing to the clean and renewable hydrogen energy [1–8]. To promote photocatalytic hydrogen production activity, the heterostructure designs and fabrication of semiconductor photocatalysts have attracted much attention [9–14]. Amongst them, Z-scheme photocatalysts have been implemented due to their improved separation efficiency of photogenerated charge carriers and stronger redox capacity relative to the conventional heterojunction-type photocatalysts [15–24]. In the previous studied Z-scheme photocatalytic systems, electron mediators play a significant role in promoting electron transfer, facilitating charge separation and inhibiting charge recombination between two semiconductor catalysts [25]. Common electron mediators are mainly solid-state noble metals (such as Au, Ag), redox ionic couples (such as IO3-/I-, Fe3+/Fe2+) and rGO [21,26–31]. For example, Tada et. al developed a Z-scheme CdS/Au/TiO2 photocatalyst, in which Au acted as the electron conductor and facilitated the vectorial transfer of photoinduced electrons from TiO2 CB (conduction band) to CdS VB (valence band) [32]. Abe et al. utilized IO3-/I- to transfer electrons between rutile TiO2 and Pt-loaded anatase TiO2 to generate H2 and 3
O2 under illumination of UV light [26]. However, the ternary Z-scheme systems with electron-mediators suffer from many shortcomings, such as expensiveness, low stability, reverse reactions for water splitting, strong visible light absorption, and operability only in solution systems for redox ionic couples [31]. In view of the above reasons, studying the all-solid-state direct Z-scheme photosynthetic system is highly desirable [33]. Recently, several direct Z-scheme photocatalytic systems without electron mediators have been constructed, such as rutile TiO2 with g-C3N4 quantum dots [34], SnS2 hexagonal nanoplates decorated irregular BiOBr nanosheet [35], CdS/Co9S8 hollow cube [20], g-C3N4 nansheets/oxygen vacancy-rich ZnO [36] and Ti0.91O2/CdS hollow spheres [37]. These direct Z-scheme systems exhibit strong electron reducibility and hole oxidizability, leading to superior photocatalytic performance. However, it still remains a great challenge to construct efficient direct Z-scheme photocatalyst with high photocatalytic performance. Thus, further investigation is needed. As a typical and efficient photocatalyst, TiO2 has been widely investigated [38–40]. On one hand, morphology optimizations such as forming hierarchical structure, which enhance light harvesting, increase the surface areas and provide more reaction sites, are carried out [41]. On the other hand, component optimizations such as coupling semiconductors with narrower band gap materials, which can absorb visible light, are performed. With a narrow band gap of 2.4 eV, CdS has been used to composite with TiO2 due to its visible-light response [42]. The combination of TiO2 4
and CdS not only enhances light capture but also introduces new active sites and restrains charge recombination [37,43–45]. Various synthetic techniques such as hydrothermal method, sol–gel method and calcination method have been used to prepare the TiO2/CdS composite nanostructures. Amongst them, SILAR method has also been explored because it is economical and convenient for large-area deposition [46]. In this work, we constructed a direct Z-scheme TiO2/CdS hybrid hierarchical photocatalyst via a simple SILAR method. TiO2 hierarchical microspheres composed of spiny nanosheets were used as support; the abundant pore structures are beneficial for the multiple light reflections and scattering inside the pore channels. CdS nanoparticles (NPs) were interspersed on the surface of TiO2 microspheres with electrostatic attraction. The intimate contact between TiO2 microspheres and CdS NPs created easier electron transfer. The obtained TiO2/CdS photocatalyst was further utilized for photocatalytic water splitting under illumination. The effect of the CdS NPs on the morphology, light absorption and photocatalytic activity of TiO2 microspheres was investigated. Ultimately, based on the analysis of hydroxyl radicals and photocurrent response, a direct Z-scheme mechanism was proposed to elucidate the improved photoactivity of TiO2/CdS binary photocatalytic system.
2. Experimental part 2.1. Preparation of TiO2 nanosheet microspheres All chemicals were of analytical grade and employed without further treatment. 5
1.0 mL of tetrabutyl titanate (TBOT) was added into 30 mL of glacial acetic acid dropwise, and then the mixture was stirred for 30 min and put into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and treated at 140 oC for 12 h. After that, the white precipitation was rinsed with deionized water and ethanol, dried in the oven at 80 oC in air atmosphere. Finally, the product was heated at 500 oC for 2 h to facilitate crystallization and remove organics. The as-prepared pure TiO2 nanosheet microsphere sample was marked as T.
2.2. Preparation of TiO2/CdS composite microspheres The TiO2/CdS composite microspheres were prepared by SILAR method. A single SILAR deposition cycle was illustrated in Fig. 1. In a typical process, 0.05 M (CH3COO)2Cd aqueous solution and 0.05 M Na2S aqueous solution were firstly prepared. Then the as-prepared TiO2 microspheres (T) were dispersed in (CH3COO)2Cd solution for 5 minutes until the Cd2+ ions were adsorbed on the nanosheets of TiO2 microspheres. Then the loose part of adsorbed Cd2+ ions was removed by rinsing with deionized water. Further, the TiO2 microspheres adsorbed with Cd2+ were immersed in Na2S solution for 5 minutes, thus the S2- ions were adsorbed and reacted with Cd2+ ions to form CdS NPs during the immersion process. By rinsing with deionized water, the redundant S2- ions were removed, leaving the CdS NPs on TiO2 surface. Thus, single SILAR deposition cycle was completed, the obtained sample was marked as TC3. By changing the concentration of (CH3COO)2Cd and Na2S solution, the composite samples with different content of 6
CdS were obtained and denoted as TC1, TC2, TC3, TC4 and TC5, respectively. The concentrations of (CH3COO)2Cd and Na2S solution and corresponding weight ratio of CdS NPs were demonstrated in Table 1. For comparison, CdS NPs were prepared by mixing Na2S solution (0.5 M, 50 mL) and (CH3COO)2Cd solution (0.5 M, 50 mL) under continuous stirring.
2.3. Characterization X-ray diffraction (XRD) patterns were obtained by a Rigaku X-ray diffractometer with Cu Kα radiation. Morphology is measured by field-emission scanning electron microscopy (JSM-7500, JEOL) with an Oxford energy-dispersive X-ray spectroscopy. Transmission electron microscopy (TEM) and high resolution TEM analysis were performed using JEM-2100F electron microscope (JEOL, Japan). X-ray photoelectron spectra (XPS) were obtained on an ultra-high-vacuum VG ESCALAB 210 electron spectrometer. All binding energies were referenced to the C 1s peak of the surface adventitious carbon at 284.8 eV. The weight ratios of CdS relative to TiO2 were measured by inductively coupled plasma–optical emission spectra (Prodigy 7, USA). The light absorption was measured by an UV–vis spectrophotometer (UV-2550, SHIMADZU). N2 adsorption-desorption isotherms were recorded on an ASAP 2020 nitrogen adsorption apparatus (Micromeritics, USA). The hydroxyl radicals were investigated on a F-7000 fluorescence spectrophotometer (Hitachi, Japan). Coumarin was used as a probe molecule because coumarin can react with ·OH to generate fluorescent product, 7-hydroxycoumarin (7HC). In a typical 7
experiment, 50 mg of samples were suspended into 20 mL of coumarin aqueous solution (1 × 10-3 mol L-1). After 1 h illumination of UV light, the suspension was filtered, and the PL intensities at 456 nm excited by 332 nm light were measured. The PL intensity can reflect the amount of hydroxyl radicals.
2.4. Photocatalytic activity and photoelectrochemical test The photocatalytic hydrogen evolution activity was tested in a 100 mL Pyrex flask. The light source was a 350 W Xenon lamp (Changzhou Siyu Science Co. Ltd, China). In a typical process, 0.05 g of catalyst was dispersed into 80 mL methanol/water solution (25% methanol in volume). Before irradiation, N2 was pumped into the suspension to remove air. After irradiation for 1 h, 0.4 mL gas was collected from the reactor and detected by gas chromatograph (GC-14C, Shimadzu, Japan, TCD detector, N2 carrier). The photoelectrochemical measurement was conducted on a CHI660C electrochemical analyzer (Chenhua Instrument, Shanghai) using a three-electrode system. A 2 × 1.5 cm clean FTO glass was coated with samples and used as working electrode. Pt foil and Ag/AgCl served as counter electrode and reference electrode, respectively. Electrolyte was 0.5 M Na2SO4 aqueous solution during the test. A 3 W UV-LED (365 nm) was used as the light source. Linear sweep voltammetry (LSV) was conducted with a voltage scanning speed of 10 mV s-1. The UV-LED light was switched on and off at regular intervals of 10 s.
8
2.5. DFT calculations Density function theory (DFT) calculations were conducted on a CASTEP Package. The Perdew-Burke-Ernzerhof (PBE) of the generalized gradient approximation (GGA) was used as the exchange-correlation function. An energy cutoff of 400 eV and the Monkhorst–Pack k-point mesh of 4 × 4 × 1 for both TiO2 and CdS models were used to perform geometry optimizations. The energy and force convergence criterions were set as 1.0 × 10–5 eV/atom and 0.03 eV/Å, respectively.
3. Results and discussion 3.1. Morphology and phase structure Fig. 1 illustrates the SILAR process for the fabrication of TiO2/CdS microspheres. During the preparation process, the TiO2 microspheres were negatively charged in neutral solution (pH = 7) because the isoelectric point of TiO2 is 6.2 [47]. Therefore, when adding TiO2 microspheres into (CH3COO)2Cd solution, the Cd2+ ions with positive charges can be adhered onto the surface of TiO2 due to electric attraction. Next, after immersing the sample into Na2S solution, the S2- ions with negative charges can be adsorbed and reacted with Cd2+ ions, forming CdS NPs. Thus, CdS NPs were uniformly deposited onto TiO2 surface by SILAR method. Table 1 lists the concentrations of reactant solution, the weight ratios of CdS NPs and corresponding sample names.
Among the obtained TiO2/CdS composite samples, TC3 exhibited the highest 9
photocatalytic performance (which will be discussed in section 3.5). Therefore, we take TC3 as a typical sample to measure the morphology, structure and chemical composition. Firstly, electron microscope was performed to demonstrate the morphology and microstructure of sample T, C and TC3. Fig. 2a presents a hierarchical flower-like microspheric TiO2 with 2~3 μm in diameter. The enlarged FESEM image (Fig. 2b) shows that the TiO2 microsphere is assembled by spiny mesoporous nanosheets, which is germinated from the central of the microsphere. Fig. 2c shows the SEM image of pure CdS prepared by precipitation method. As can be seen, pure CdS is composed of aggregated nanoparticles with uniform sizes of ca. 30 nm. Fig. 2d shows the morphology of TC3 hierarchical photocatalyst. The composite photocatalyst also exhibits a microspheric shape with 2~3 μm in diameter. The surface of the composite sample becomes rough, and the thorns are weakened due to the deposition of CdS NPs. Further observation shows that some NPs are interspersed on the surface of TiO2 spiny nanosheets (Fig. 2e). EDS mapping images (Fig. 2f) further show the existence of Ti, O, Cd and S elements, indicating the uniform distribution of CdS NPs on TiO2 microspheres. The thorn-like structure can also be seen in Fig. 2g. The lattice fringes of TC3 were shown in the enlarged HRTEM in Fig. 2h. The lattice spacing of 0.35 nm and 0.20 nm corresponds to anatase TiO2 (101) crystal facet and cubic CdS (220) crystal facet, respectively, indicating that the intimate integration of TiO2 and CdS. Fig. 2l shows the XRD patterns to indentify the phase structure of samples. The XRD pattern of pure TiO2 demonstrated characteristic diffraction peaks of anatase 10
phase (JCPDS, No. 21-1272). As comparison, the XRD pattern of pure CdS NPs was also analyzed. The diffraction peaks can be assigned to the pure cubic phase of CdS (JCPDS, No. 10-0454). Compared with standard PDF card, three main peaks which located at 26.5o, 43.9o and 52.1o broadened obviously, indicating that the deposited CdS particles are of nanoscale level, which is in accordance with SEM images. For sample TC3, all diffraction peaks can be attributed to anatase TiO2. No peaks of CdS was found due to its low content, poor crystallinity and high dispersion. No peak shift was observed, excluding the possibility of CdS incorporation into TiO2 lattice.
3.2. BET surface area and pore size distribution Fig. 3 displays the N2 adsorption-desorption isotherms and the corresponding pore
size
distribution
curves
of
T,
C
and
TC3.
According
to
the
Brunauer–Deming–Deming–Teller (BDDT) classification, the isotherms of T and TC3 are of type IV, implying the presence of mesopores (2–50 nm) [48]. The shapes of hysteresis loops of T and TC3 are of type H3, indicating the presence of slitlike pores
which
are
formed by the aggregation of TiO2
nanosheets.
The
adsorption-desorption isotherm of C is also of type IV, and the hysteresis loop can be classified as type H2, suggesting the presence of ink-bottle-like mesopores formed by uniform CdS NPs [49]. The above analysis is consistent with the FESEM results. Further observations demonstrate that the sorption isotherm of sample C shifts downward compared with sample T and TC3 in all range of relative pressure, suggesting a smaller specific surface area and pore volume of pure CdS. The pore size 11
distribution (inset in Fig. 3) indicated that sample T and TC3 exhibits a wide pore size distribution from 2 to >100 nm, suggesting that the mesopores and macropores coexist. In contrast, sample C exhibits a narrow pore size distribution in the range of 2–10 nm, indicating smaller pore size of pure CdS. This is also confirmed by the data in Table 2. Moreover, the specific surface area of TC3 (84 m2/g) is almost same as that of T (81 m2/g), indicating that CdS NPs on TiO2 surface are small and well-dispersed, which has little effect on the BET surface area of TC3 composite photocatalyst. The larger BET surface area, together with hierarchical structure of TC3, is crucial for the adsorption and migration of reactant and product molecules.
3.3. Light absorption Fig. 4 presents the DRS spectra of T, TC3 and C. For sample T, a significant absorption increase at wavelength of ca. 390 nm was observed, which agreed with the intrinsic band-gap energy of 3.2 eV. For sample C, an absorption onset occured at 550 nm, which corresponded to the band-gap energy of 2.3 eV of cubic phase CdS [15]. Compared with the pure TiO2, TC3 showed stronger photoabsorption in visible light range (400~500 nm). The enhancement in visible light region of sample TC3 stemmed from the successfully deposited CdS NPs on TiO2 nanosheets.
3.4. Work function and XPS analysis The work function of material is vital in investigating the charge transfer at interface [50]. Herein, the crystal structures of TiO2 (101) and CdS (110) were 12
constructed in Fig. 5a and 5c. The work function of the TiO2 (101) and CdS (110) were obtained by equation 1:
EVAC EF
(1)
where Evac is the electrostatic potential of vacuum level, and EF means the energy of Fermi level. The calculated work functions of TiO2 (101) and CdS (110) surface (Fig. 5b and 5d) were 7.1 eV and 5.8 eV, respectively, which coincides with the previous reports [51,52]. This indicated that the Fermi level of TiO2 (101) surface was more positive than that of CdS (110) surface. When CdS NPs contact with TiO2 surface, the electrons at the TiO2/CdS interface would rearrange and migrate from CdS to TiO2, leaving holes on the CdS. Then a depletion layer can be formed on CdS surface, while an accumulation layer can be formed on TiO2 surface, leading to the formation of internal electric field at the interface. The internal electric field, directing from the CdS surface to the TiO2 surface, restrained the further migration of electrons. Finally, the two Fermi levels reach the same level and the mobility of electron from CdS to TiO2 reaches equilibrium.
XPS measurement was conducted to confirm the chemical composition and elemental chemical states of T, C and TC3. Ti, O, C, Cd and S were detected in sample TC3 (not shown), indicating that TiO2 and CdS were successfully composited. Fig. 6 presents the comparison of the high-resolution XPS spectra of Ti 2p, O 1s, Cd 3d and S 2p in sample T, C and TC3. The Ti 2p peaks of sample T at 458.6 and 464.3 eV can be attributed to Ti 2p3/2 and Ti 2p1/2, respectively [44]. Two similar peaks at 13
458.3 and 464.0 eV were detected in the Ti 2p spectrum of TC3, which shifted towards the lower energy region by 0.3 eV relative to that of sample T. The O 1s spectrum of sample T located at 529.8 eV, which can be attributed to lattice oxygen (Ti−O−Ti). The corresponding O 1s peaks of TC3 located at 529.5 eV, which also shifted towards the lower binding energy by 0.3 eV. Another peak of O 1s at 531.4 eV was assigned to surface hydroxyl oxygen (Ti−OH) [53]. The Cd 3d spectrum of pure CdS demonstrated two obvious peaks at 405.0 and 411.7 eV, which were assigned to Cd 3d7/2 and Cd 3d5/2 [15]. The corresponding Cd 3d peaks of sample TC3 located at 404.3 and 411.0 eV. In contrast with that of Ti 2p and O 1s, the binding energies of Cd 3d shifted upward by 0.7 eV. The S 2p spectrum of pure CdS shows two clear peaks at 160.8 and 161.9 eV, which can be assigned to the S 2p3/2 and S 2p1/2 [54]. Moreover, the S 2p binding energy of TC3 was found to shift to the higher energy region. The opposite shift of binding energies suggests that the electrons of TC3 tend to flow from CdS to TiO2, which is due to the different position of Fermi level (see work function analysis). When CdS NPs were deposited on TiO2 microspheres, intimate contact between CdS and TiO2 was formed. Since the Fermi level of CdS was more negative than TiO2, the electrons from CdS would migrate to TiO2 until the Fermi level of TiO2 and CdS reach the same position. The shift of the binding energies indicates the interfacial intimate contact between TiO2 and CdS, which makes it possible to transfer photogenerated electrons between TiO2 and CdS without a redox mediator during the photocatalytic reaction [55]. Therefore, the close contact is beneficial for constructing direct Z-scheme TiO2/CdS binary composite photocatalyst. 14
3.5. Photocatalytic performance Photocatalytic H2 production activities of samples were measured under illumination of a 350 W Xenon lamp. H2 production was detected with the existence of both photocatalyst and irradiation. Fig. 7a shows the comparison of hydrogen yield of prepared samples loaded with different amounts of CdS NPs. Pure TiO2 and CdS exhibited negligible photocatalytic performance due to the rapid recombination of photoinduced charge carriers. With increment proportion of CdS, the H2 yields of TiO2/CdS composite photocatalyst had a remarkable enhancement. With optimal CdS content of 4.4 wt%, the sample TC3 reached the highest photocatalytic activity of 51.4 μmol h-1, which exceeded that of pure TiO2 by 28 times. The promoted photocatalytic performance was due to the following reasons: 1) The hierarchical structure of TiO2 microspheres was beneficial for the multiple light reflection and scattering, thus enhancing the light harvest; 2) The addition of CdS NPs broadened the light absorption region to the visible light region, which were proved by the DRS spectra in Fig. 4; 3) The combination and intimate contact of TiO2 and CdS facilitated the separation and transfer of photogenerated electrons and holes on the interface of two photocatalysts. The mechanism will be further investigated in the following discussion. Moreover, after three cycles, the photocatalytic activity of sample TC3 did not exhibit obvious decrease (Fig. 7b), which means that the hybrid photocatalyst is stable under successive photocatalytic reactions. When further increasing the content of CdS, the photocatalytic H2 production activities decreased, which is due to the following reasons: 1) excess CdS NPs shielded TiO2 from light harvesting, thus TiO2 15
cannot be excited efficiently by the ultraviolet light; 2) excessive CdS NPs resulted in a decline of surface active sites of TiO2; 3) excess CdS NPs led to the increase of particle size, thus resulting in the deterioration of photocatalytic properties of CdS particles. Therefore, a suitable amount of CdS NPs is significant for the optimization of TiO2/CdS composite photocatalyst.
3.6. Photocatalytic mechanism Two possible mechanisms to explain the enhanced photocatalytic activities and the improved charge separation are illustrated in Fig. 8. In the traditional heterojunction-type photocatalytic system, the electrons in the VB of CdS and TiO2 are excited to the CB of CdS and TiO2 under solar light, respectively. Then the photogenerated electrons in CdS CB with more negative energy potential transfer to TiO2 CB, while the photogenerated holes in TiO2 VB with more positive energy potential migrate to CdS VB. Therefore, the photoinduced electrons and holes are accumulated in TiO2 CB and CdS VB, respectively, resulting in the separation of photoinduced charge carriers. In the direct Z-scheme photocatalytic system, both TiO2 and CdS can generate electrons and holes under light excitation. Then the photogenerated electrons in TiO2 CB transfer and combine with the photogenerated holes in CdS VB, leaving the electrons in CdS CB and the holes in TiO 2 VB, thus maintaining the strong electron reducibility of CdS CB and strong hole oxidizability of TiO2 VB. Finally, the accumulated electrons in CdS CB react with H2O and produce H2. 16
To determine the more likely photocatalytic mechanism of TiO2/CdS composite catalyst, the hydroxyl radicals (·OH) generated from the surfaces of pure TiO2, pure CdS and TC3 were investigated by the photoluminescence analysis with coumarin as a probe molecule. It was observed that all of the PL peak intensities of T, C and TC3 increase with the increasing irradiation time (Fig. 9a and 9b), indicating that hydroxyl radicals are generated under light excitation. Meanwhile, the band edge positions of TiO2 and CdS were illustrated in Fig. 11.
O2 e O2
OH h
H 2O h
vb vb
OH OH H
E = – 0.33 V
(2)
E = 1.99 V
(3)
E = 2.34 V
(4)
As shown in Fig. 11 and equations 2 – 4, for sample C, the oxidation potential of OH-/·OH (1.99 V) and H2O/·OH (2.34 V) is more positive than the VB potential of CdS (1.88 V) [56,57]. Therefore, the holes in pure CdS are unable to oxidize OH- or H2O into ·OH and ·OH should not have been detected. However, In Fig. 9b, pure CdS also generated a little amount of hydroxyl radicals (·OH) with weak PL signals. This is attributed to the indirect generation of ·OH originated from ·O2-, which is produced due to the more negative CB potential of CdS than the reduction potential of O2/·O2[18]. For sample T, the VB potential of TiO2 is more positive than that of OH-/·OH and H2O/·OH, thus the photogenerated holes in TiO2 VB can react with OH- or H2O and ·OH can be directly generated, further causing the increased PL signals. For sample TC3, a higher PL peak signal than sample T was detected. These results strongly imply that TC3 complies with the direct Z-scheme reaction mechanism rather than the conventional heterojunction-type photocatalytic mechanism. If TC3 17
conforms
the
heterojunction-type mechanism,
the photogenerated electrons
accumulated in TiO2 CB have no ability to reduce the dissolved O2 into ·O2-, meanwhile, the photogenerated holes accumulated in CdS VB cannot oxidize OH- and H2O to ·OH, thus neither ·OH nor ·O2- could be detected.
For the purpose of investigating the separation rate of photoexcited charge carriers, the photocurrent–potential curves of T, C and TC3 were obtained with a voltage scanning speed of 10 mV s-1. As shown in Fig. 10, TC3 sample possesses the maximum photocurrent density compared to T and C samples, implying higher charge separation efficiency. This result supports the point that a direct Z-scheme mechanism contributes to separating the photoinduced electrons and holes, further increasing the photocatalytic efficiency.
Considering the above analysis of calculated work functions, XPS results, hydroxyl radicals and photocurrent–potential curves, a direct Z-scheme photocatalytic mechanism of TiO2/CdS hybrid photocatalyst was proposed and illustrated in Fig. 11. When CdS NPs were deposited on TiO2 surface, a tight contact was formed. Then the electrons on the CdS tend to transfer to TiO2 via the intimate contact interface due to the more negative Fermi level of CdS, leaving holes on the CdS. The electron diffusion continued until the Fermi levels are equilibrated [58–60]. As a result, an internal electric field at the interface of TiO2/CdS was formed, and the internal electric field directed from CdS to TiO2 (as shown in work function analysis). During the 18
photocatalytic process, the internal electric field restrained the migration of photogenerated electrons from CdS CB to TiO2 CB and photogenerated holes from TiO2 VB to CdS VB; In contrast, the electron transfer along the Z direction from TiO 2 CB to CdS VB was accelerated, leaving the photoexcited electrons in the CdS CB and photoexcited holes in the TiO2 VB. Thus, the construction of direct Z-scheme photocatalyst benefits the separation of photoexcited charge carriers at the TiO2/CdS interface. Moreover, the strong electron reducibility of CdS and strong hole oxidizability of TiO2 are maintained, which further improves the photocatalytic H2-evolution activity.
4. Conclusion In summary, we demonstrated an electron mediator-free TiO2/CdS binary Z-scheme system for photocatalytic H2 evolution by constructing a hierarchical structure consisting of TiO2 spiny nanosheet microspheres and CdS NPs via a SILAR method. The binary TiO2/CdS photocatalyst reaches the highest H2-production rate of 51.4 μmol h-1 under UV irradiation with 4.4 wt% CdS content. The remarkable enhancement is attributed to: 1) Hierarchical structures consisting of TiO2 nanosheets microspheres and CdS NPs, enhancing light harvest, increasing the BET surface areas and providing more active sites; 2) Intimate interfacial contact between TiO2 and CdS, facilitating the electron migration from TiO2 CB to CdS VB; 3) A direct Z-scheme structure, suppressing the electron-hole recombination of TiO2 and CdS and maintaining the strong electron reducibility in CdS VB. Our work provides a facile 19
route for constructing the artificial direct Z-scheme photocatalytic system without a redox mediator for photocatalytic water splitting.
Acknowledgement This work was supported by the NSFC (51320105001, 51372190, 21573170 and 21433007), 973 program (2013CB632402), the Natural Science Foundation of Hubei Province (2015CFA001), Innovative Research Funds of SKLWUT (2015-ZD-1) and the Fundamental Research Funds for the Central Universities (2016-YB-005).
20
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Fig. 1. Schematic description for the fabrication of TiO2/CdS photocatalyst via SILAR method.
30
Fig. 2. FESEM (a) and enlarged FESEM image (b) of sample T. FESEM (c) of sample C. FESEM (d), enlarged FESEM image (e), EDS mapping (f), TEM (g) and HRTEM (h) of sample TC3. XRD patterns (l) of T, C and TC3.
31
Fig. 3. Nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of T (black line), C (blue line) and TC3 (red line) composite photocatalyst.
32
Fig. 4. UV-vis diffuse reflectance spectra of T (black line), C (blue line) and TC3 (red line).
33
Fig. 5. Constructed TiO2 (101) and CdS (220) crystal structure: (a) red and gray balls stand for oxygen and titanium atoms, respectively; (c) rose red and yellow balls stand for cadmium and sulphur atoms, respectively. Calculated work functions of T (b) and C (d): The blue and red dashed lines represent the vacuum and Fermi levels, respectively.
34
Fig. 6. Comparison of high-resolution XPS spectra of Ti 2s (a), O 1s (b), Cd 3d (c) and S 2p (d).
35
Fig. 7. (a) Comparison of the photocatalytic hydrogen production rates of TiO2/CdS photocatalyst loaded with different amounts of CdS in methanol aqueous solution under Xenon irradiation for 1 h; (b) Cyclic H2-evolution curves of sample TC3.
36
Fig. 8. Schematic diagram of two mechanisms to explain charge separation: conventional heterojunction-type (a) and direct Z-scheme mechanism (b).
37
Fig. 9. (a) PL spectral changes of sample TC3 under irradiation in the presence of coumarin; (b) PL signal intensity at 456 nm of sample T, C and TC3 against illumination time.
38
Fig. 10. Photocurrent–potential curves of T, C and TC3 samples.
39
Fig. 11. The potential positions of TiO2 and CdS band edges and schematic illustration of direct Z-scheme photocatalytic mechanism for TiO2/CdS photocatalyst.
40
Table 1. The concentrations of (CH3COO)2Cd and Na2S aqueous solution for preparing TiO2/CdS composite photocatalysts, the weight ratios of CdS NPs in the TiO2/CdS composite photocatalysts and the corresponding photocatalytic activities. Sample C1
C2
WCdS (ICP-AES)
Activity (μmol·h-1)
T
0
0
0%
1.8
TC1
0.005
0.005 0.9%
9.7
TC2
0.01
0.01
1.9%
27.2
TC3
0.05
0.05
4.4%
51.4
TC4
0.1
0.1
4.8%
29.1
TC5
0.5
0.5
5.6%
11.4
C
0.5
0.5
100%
0
Notes: C1: the concentration of (CH3COO)2Cd; C2: concentration of Na2S; WCdS: the weight ratio of CdS relative to TiO2.
41
Table 2. Comparison of the specific surface area, pore volume and pore size of T, C and TC3.
Sample
SBET (m2/g)
Pore Volume (cm3/g) Pore Size (nm)
T
81
0.3
14.7
TC3
84
0.3
15.1
C
50
0.1
4.4
42