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Highly Efficient Photocatalytic Hydrogen Evolution in Ternary Hybrid TiO2/CuO/Cu Thoroughly Mesoporous Nanofibers Huilin Hou, Ming-Hui Shang, Fengmei Gao, Lin Wang, Qiao Liu, Jinju Zheng, Zuobao Yang, and Weiyou Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06644 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Highly Efficient Photocatalytic Hydrogen Evolution in Ternary Hybrid TiO2/CuO/Cu Thoroughly Mesoporous Nanofibers Huilin Hou, Minghui Shang, Fengmei Gao, Lin Wang, Qiao Liu, Jinju Zheng, Zuobao Yang and Weiyou Yang∗ Institute of Materials, Ningbo University of Technology, Ningbo City, 315016, P.R. China.

∗

Corresponding author. E-mail: [email protected] (W. Yang)

Tel: +86-574-87080966, Fax: +86-574-87081221. 1 ACS Paragon Plus Environment


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ABSTRACT Development of novel hybrid photocatalysts with high efficiency and durability for photocatalytic hydrogen generation is highly desired but still remains a grand challenge currently. In the present work, we reported the exploration of ternary hybrid TiO2/CuO/Cu thoroughly mesoporous nanofibers via a foaming-assisted electrospinning technique. It is found that, by adjusting the Cu contents in the solutions, the unitary (TiO2), binary (TiO2/CuO, TiO2/Cu) and ternary (TiO2/CuO/Cu) mesoporous products can be obtained, enabling the growth of TiO2/CuO/Cu ternary hybrids in a tailored manner. The photocatalytic behavior of the as-synthesized products as well as P25 was evaluated in terms of their hydrogen evolution efficiency for the photodecomposition water under Xe lamp irradiation. The results showed that the ternary TiO2/CuO/Cu thoroughly mesoporous nanofibers exhibit a robust stability and the most efficient photocatalytic H2 evolution with the highest release rate of ~851.3 µmolg-1·h-1, which was profoundly enhanced for more than 3.5 times with respect to those of the pristine TiO2 counterparts and commercial P25, respectively, suggesting their promising applications in clean energy production.

Key Words: TiO2/CuO/Cu, Electrospinning, Mesoporous, nanofibers, photocatalytic hydrogen evolution

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1. Introduction Photocatalytic water splitting for hydrogen production by utilizing solar energy is considered as one of the promising strategies for resolving global energy and environmental problems, owing to its renewable energy production with no reliance on fossil fuels and no carbon dioxide emission.1-4 Titanium dioxide (TiO2), as one of the most representative photocatalysts, is used widely because of its excellent chemical resistance, low cost, nontoxicity, availability, and long-term stability against the photochemical corrosion in aggressive aqueous environments.5-8 However, the conventional TiO2 material has the intrinsic shortcomings: i) low specific surface areas; ii) easy aggregation; iii) the usually fast electron-hole recombination; iv) large band-gap barrier resulted low photocatalytic efficiency.9,10 To overcome these hindrances, many effective techniques are applied to pursue the satisfied structures and/or modify the TiO2 by incorporating other co-catalytic materials.11-14 Typically, one-dimensional (1D) mesoporous nanofibers could be an ideal candidate, since they possess low aggregation tendency, high porosity and large specific surface area, which can promote the charge and mass transfer for enhanced photocatalytic activities,15-18 especially for the heterostructureed counterparts.19-21 Introduction of CuO into the TiO2 matrix is recognized as a cost-effective and emerging strategies, which could not only help the full utilization of the solar light energy, but also render a built-in electric field at the interface within the hybrid, which profoundly facilitate the separation of photo generated electron and hole pairs.22,23 Additionally, metallic Cu modified TiO2 can promote the electron transfer and e–h separation, leading the enhanced photocatalytic activities.24, 25 That is, the exploration of CuO and/or Cu modified TiO2 mesoporous hybrid nanofibers could be a promising strategy to offer the photocatalysts with innovative physicochemical properties.



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Electrospinning is a versatile, compatible, low cost and productive technique for generating 1D nanofibers in various materials system with controllable morphologies.26-29 By virtue of this technique assisted by the subsequent air calcination, TiO2/CuO nanofibers have been successfully fabricated to meet the requirements of outstanding photocatalysts,30 which could endow the synergy of favorable photocatalytic reactions such as good contact of the heterojunctions, long lifetimes of photogenerated charge carriers and 1D structure for efficient charge transfer.31-33 However, to date, there is little work devoted to the investigation on ternary TiO2/CuO/Cu mesoporous nanofibers.

Herein, we report the exploration of TiO2/CuO/Cu thoroughly mesoporous nanofibers via electrospinning technique. The diisopropyl azodiformate (DIPA) used as the foaming agents was added in to the initial spinning solutions and homogeneous boxed throughout the precursor (tetrabutyl titanate (TBOT), polyvinylpyrrolidone (PVP)) and C4H6CuO4·H2O to create the porous structures. The obtained 1D ternary TiO2/CuO/Cu mesoporous nanostructures exhibited a significantly enhanced photocatalytic activities, as compared to the binary hybrids (i.e. TiO2/CuO and TiO2/Cu), unitary photocatalysts ( i.e. TiO2) and the commercial P25 products.

2. Experimental Procedure 1) Materials Polyvinylpyrrolidone (PVP, MW≈1300000), butyl titanate (TBOT), diisopropyl azodiformate (DIPA), absolute ethyl alcohol, acetic acid, copper(II) acetate monohydrate (C4H6CuO4·H2O), methanol and deionized water were all purchased from Aladdin. All Chemicals were used directly without further purification. 4 ACS Paragon Plus Environment

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2) Preparation of bare TiO2 and CuO and/or Cu modified TiO2 thoroughly mesoporous nanofibers Both pristine TiO2 and CuO and/or Cu modified mesoporous nanofibers were synthesized according to the foaming agent assisted electrospinning method as reported in our previous work.34 In a typical procedure, 6.5 wt% of PVP, 30 wt% of TBOT and 10 wt% of foamer (DIPA) were added into a solution comprising ethanol and acetic acid (3:1 in volume) with stirring vigorously for 12 h to obtain a homogenous precursor solution for the fabrication of pristine TiO2. The precursor solution for the fabrication of CuO and/or Cu modified TiO2 thoroughly mesoporous nanofibers was performed by adding 2 wt% of C4H6CuO4·H2O into the above mixture with further magnetically stirring for 2 h. For comparison, another blank experiment without the introduction of DIPA in the initial solutions. To disclose the effect of incorporated Cu on the growth of the hybrid fibers, two solution samples are also prepared with the Cu sources decreased to 1 wt% and otherwise increased to 3 wt%. The details for these five initial solutions were shown in Table 1, and the resultant products were referred to Sample A-E, respectively. Then, the as-prepared solutions were transformed into a plastic syringe with a stainless steel nozzle, which was sized in ~0.2 mm in diameter and used as the anode for electrospinning. The tip of the stainless steel nozzle was placed in the front of a metal cathode (collector) with a fixed distance of 20 cm. An electrical potential of 20 kV was applied for electrospinning of the precursor fibers. Subsequently, the precursor fibers were heated up in a conventional tube furnace to the desired temperature of 550 °C with a heating rate of 1 °C min−1, and maintained there for 2 h in air, followed by furnace cool to ambient temperature.

3) Characterization 5 ACS Paragon Plus Environment


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The obtained products were characterized with X-ray powder diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ=1.5406 Å), field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan), and high-resolution transmission electron microscopy (HRTEM, JEM-2010F, JEOL, Japan) equipped with energy dispersive X-ray spectroscopy (EDX, Quantax-STEM, Bruker, Germany). The porous properties of the as-prepared mesoporous nanofibers were characterized using N2 adsorption at 77k on a specific surface area and porosity analyzer (Micromeritics, ASAP 2020HD88, USA). The elemental compositions and chemical states of the hybrid fibers were studied by X-ray photoelectron spectroscopy (Shimadzu, AXIS ULTRA DLD, Japan). The diffuse reflectance absorption spectra of the products were recorded on a UV-visible spectrophotometer (UV-3900, Hitachi, Japan) equipped with an integrated sphere attachment.

4) Photocatalytic Activity Measurements The photocatalytic reaction is performed in an inner-irradiation quartz annular reactor with a 300W Xenon lamp (CEL, HUL300), a vacuum pump, a gas collection, a recirculation pump and a water-cooled condenser. The as-synthesized samples (0.1 g) were suspended in deionized water and methanol mixed solutions (40 mL, 3:1 in volumn) by an ultrasonic oscillator, respectively. Then the mixture was transferred into the reactor and deaerated by the vacuum pump. The Xenon lamp was utilized as a light source, and the cooling water was circulated through a cylindrical Pyrex jacket located around the light source to maintain the reaction temperature. The reactor was sealed with ambient air during irradiation, and the hydrogen evolution were monitored by an online gas chromatography (GC, 7900) equipped with a Porapak-Q column, high-purity nitrogen carrier and a thermal conductivity detector (TCD). In order to investigate the stability and recyclability, the products were reused for 3 6 ACS Paragon Plus Environment

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cycles. For comparison, Degussa P25 was commercially available, and used directly for the photocatalytic H2 generation.

3. Results and discussion The electrospun hybrid precursor nanofibers of Sample C are firstly observed by SEM, which are shown in Figure S1 (Supporting Information). They appear smooth without any obvious pores with a mean diameter of ~300 nm. Figure 1(a-d) display the typical SEM images of the corresponding calcined fibers under different magnifications. It seems that all the precursor fibers of Sample C have been completely converted into mesoporous nanofibers, disclosing their high purity in morphology (Figure 1(a-b)). The diameter of the products is slightly increased to ~400 nm, which could be due to the gas release from the foaming agents during the calcination process, causing the fiber in somewhat expanding. Closer observations of the fiber bodies represent that they have rough surfaces with numerous irregular-shaped pores within the fibers, which are typically sized in ~25 nm. Their representative cross-section view of the mesoporous nanofibers (Figure 1(d)) discloses that the fibers possess a thoroughly mesoporous structure throughout the entire bodies. Figure 1(e) shows the typical XRD pattern of the as-fabricated mesoporous nanofibers, which well matches the phase of anatase TiO2 (JCPDS: No. 21-1272).35 The close-up XRD pattern (Figure 1(f)) in the region of 2θ = 30°~45° suggests that, besides the mayor phase of anatase TiO2, some low intensity diffraction peaks are also detected (i.e., 2θ=35.6°, 38.8° and 43.2°), which orderly corresponds to the diffractions of (002) and (111) crystal faces of CuO (JCPDS, No. 48-1548) and (111) of Cu (JCPDS, No. 04-0836), respectively.24,36 These results verify that the as-fabricated thoroughly mesoporous fibers are ternary phases, which consist anatase TiO2, CuO and Cu. 7 ACS Paragon Plus Environment


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Figure 2(a) presents a typical TEM image of the obtained fiber under a low magnification, suggesting that the fiber possesses a thoroughly mesoporous structure based on the different contrast throughout the fiber. The corresponding EDX spectrum (Figure 2(b)) recorded from the marked area of A in Figure 2(a) (also see the element line scanning spectra in Figure S2, Supporting Information) implies the coexistence of Ti, Cu and Cu in the hybrids. It is notable that the Cu elements have a uniform spatial distribution within the TiO2 mesoporous matrix (Figure 2 (c)). The corresponding selective area electron diffraction (SAED) pattern (Figure 2 (d)) taken from the marked area of B in (a) exhibits the typical diffraction rings, which is suggestive of their polycrystalline nature. Figure 2(e) and Figure S3 (Supporting Information) shows the representative HRTEM images recorded form the marked area of C in (a), showing three sets of lattice fringes. The left side area with the d-space of 0.357 nm matches that of the (101) plane of anatase TiO2. Figure 2(f) and (g) are the enlarged images corresponding to the marked areas of D and E in (d), respectively. The measured two spacings in (f) with ~0.232 and 0.208 nm respond to the d-distances of (111) plane of CuO and (111) plane of Cu, respectively. Figure 2(e) also means that the Cu is typically inlaid in the CuO matrix.

More details concerning the elemental compositions and chemical states of the mesoporous hybrid nanofibers were further investigated by X-ray photoelectron spectroscopy (XPS). Figure 3(a) shows the representative XPS survey spectrum of Sample C, revealing that the existence of Ti, O, Cu and C elements within the fibers, which is in consistent with the EDX spectrum. The presence of C 1s can be ascribed to the adventitious carbon-based contaminant from the XPS instrument itself. The high resolution XPS spectra of Ti 2p, O 1s and Cu 2p are given in Figure 3(b-d), respectively. The peaks in Ti 8 ACS Paragon Plus Environment

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2p spectrum (Figure 3(b)) shows the peaks centered at ca. 458.1 and 463.7 eV, which are ascribed to the Ti 2p

3/2

and Ti 2p

1/2,

respectively, suggesting the existence of Ti4+ oxidation state.37 Figure 3(c)

provides the high resolution XPS spectra of O 1s. The peak at the binding energy of ca. 529.3 is assigned to Ti-O and Cu-O species, whereas the one at a higher binding energy of ca. 531.2 eV indicates the surface contamination by hydroxides and carbonate from the atmosphere.30 Therefore, the O is considered as coming from metal oxides (TiO2 and CuO) and adsorbed oxygen such as the surface hydroxyl species and carbonate species. As shown in Figure 3(d), the main peaks at ca. 933.4 and 953.2 eV, along with the presence of their characteristic shakeup satellite peaks at ca. 941.3 and 961.5 eV, respectively, verify the existence of CuO. 38 Besides the peaks belong to CuO, the detected two minor peaks centered at ca. 931.2 and 951.1 eV are possibly attributed to other copper species (i.e., Cu2O or metallic Cu). They should be derived from the metallic Cu, 24, 39 since no any Cu2O has been observed according to the characterizations of XRD and HRTEM. Accordingly, it means that the as-fabricated thoroughly mesoporous fibers should be ternary TiO2/CuO/Cu heterojunctions, which is schematically illustrated in Figure 3(e).

In order to account for the growth mechanism and achieve the tailored fabrication of the TiO2/CuO/Cu thoroughly mesoporous hybrid nanofibers, another four comparative experiments are carried out by adjusting the compositions of the initial solutions with other similar experimental conditions (see Table 1 for details). Figure 4 shows the SEM, TEM and XRD characterizations of the obtained samples, respectively. It seems like that only pristine anatase TiO2 mesoporous nanofibers (Figure 4(a1-a3)) of Sample A can be obtained. Meanwhile, the as-fabricated products of Sample B (i.e., without the foaming agent introduce) present the ordinary smooth TiO2/Cu mesoporous nanofibers 9 ACS Paragon Plus Environment


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(Figure 4(b1-b3)). As compared between Sample B and C, it implies that the introduced foaming agents can not only create the mesopores throughout the fiber body, but also play a profound effect on the evolution of the phase compositions. To further verify this point, the Cu sources in the initial solutions are reduced to 1 wt% (Sample D, Figure 4(c1-c3)) and otherwise increased to 3 wt% (Sample E, Figure 4(d1-d3)). The experimental results demonstrate that both obtained samples are mesoporous fibers. However, the XRD patterns (Figure 4(c3 and d3)) disclose that they possess different phases with TiO2/CuO and TiO2/CuO/Cu for Sample D and E, respectively. According to the experimental results of Sample B~E, it discloses that when the foaming agents is absent (i.e., Sample B) and the introduced Cu sources in the initial solutions is low enough (i.e., Sample D), only binary hybrid products can be obtained, suggesting that a higher content of Cu sources (i.e., Sample C and E) favors the formation of ternary TiO2/CuO/Cu hybrids.

Figure 5(a-b) show the nitrogen adsorption-desorption isotherms (Figure 5(a)) and pore size distribution curves (Figure 5(b)) of Sample A~E. Except for Sample B, the other four samples exhibit the typical type IV adsorption isotherms with hysteresis loops according to BDDT classification, indicating the formation of mesoporous fibers.40, 41 Their structural details are summarized in Table 2. It shows that the pristine TiO2 products of Sample A possess the highest BET surface area (45.3 m2/g) than the other products, which can be attributed to the increscent pore diameter by the foamer complexing of Cu2+ in the Cu and/or CuO incorporated TiO2 hybrids. However, the hybrid mesoporous products of Sample C and D possess higher pore volumes than the pristine counterpart, implying a more complex mesoporosity structure of the hybrid fibers. The UV-visible absorption spectra were employed to track the change of light absorbance characteristics of the as-prepared products. As shown in Figure 5(c), 10 ACS Paragon Plus Environment

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there is only a steep absorption edge at the UV region and scarcely absorption in the visible-light region for the pristine TiO2 of Sample A. However, after Cu species incorporated to the TiO2 matrix, the hybrids exhibit an additional broad absorption band at 400-800 nm, whose intensities are enhanced with the increase of Cu sources introduced within the solutions, suggesting their narrowed bandgap and improved photo-induced electrons-holes generation. The smaller bandgap can be attributed to the transition from the valence band of CuO to the conduction band of TiO2, which are both in good contact as witnessed by the HRTEM image in Figure 2(e). The difference among the UV-visible absorption spectra of the samples could be resulted from the various surface areas and porosity volumes, as evidenced by the BET specific surface area and N2 adsorption-desorption isotherm characterizations (Figure 5(a-b) and Table 2).

The photocatalytic H2 production activities over the as-prepared five products as well as P25 were evaluated by using methanol as sacrificial agents and irradiation under a 300 W Xenon arc lamp. Figure 6(a) plots the amounts of hydrogen evolved from the aqueous suspensions over the six photocatalysts according to the observed GC analysis patterns (Figure S5, Supporting Information), and the corresponding average hydrogen production rates are presented in Figure 6(b). The results suggest that the hydrogen evolution rate of the pristine TiO2 mesoporous nanofibers of Sample A (ca. 231.7 µmolg-1h-1) is higher than that of P25 (ca. 200.5 µmolg-1h-1), and lower than those of Sample B-E. It is notable that the TiO2/CuO/Cu ternary thoroughly mesoporous products of Sample C (ca. 851.3 µmolg-1h-1) and D (ca. 550.3 µmolg-1h-1) exhibit the significantly enhanced photocatalytic activities as compared to those of the binary products of Sample B (ca. 251.2 µmolg-1h-1) and E (ca. 368.3 µmolg-1h-1). That is, based our experiments, the as-prepared TiO2/CuO/Cu ternary thoroughly 11 ACS Paragon Plus Environment


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mesoporous hybrid photocatalysts present a profoundly enhanced photocatalytic H2 production, whose H2 release rate could be enhanced for more than ~3.5 times with respect to those of the pristine TiO2 thoroughly mesoporous fibers and commercial P25, respectively. To show their stability and reusability, these six photocatalysts are recovered and reused for photocatalytic H2 production under the same condition. As shown in Figure 6(c), it seems that there is nearly no loss of H2 evolution rate for Sample A-D, whereas the photocatalytic ability of P25 has been evidently declined after 3 cycles, indicating their robust stability and reusability. To further investigate their reusability for long-term photocatalytic application, as an example of Sample C, the structure and phases of the TiO2/CuO/Cu photocatalysts after the photocatalytic reactions for 3 cycles are investigated, as shown in Figure S6 (Supporting Information). It suggests that their structures and phases are almost identical before and after the photocatalytic reactions, verifying that the TiO2/CuO/Cu photocatalysts hold a satisfied stability.

In order to understand the enhanced and stable photocatalytic ability of the ternary TiO2/CuO/Cu thoroughly mesoporous nanofibers, a proposed diagram and their band energy structures are schematically illustrated in Figure 7(a) and (b), respectively. According to the semiconductor photocatalytic theory, the total hydrogen production is mainly governed by following issues. The first is the capacity of adsorption reactants and desorption product. The second is the amount of excited electrons in the water/photocatalyst interface.1 Thus, the obtained thoroughly mesoporous structure with satisfied specific surface area is beneficial for the first one, which possesses sufficient reactive sites to improve its photocatalytic efficiency.16,34,42 Additionally, the steady 1D geometry could limit the aggregation of the photocatalysts, which allows them to be long-term serviced.30 More importantly, in the present TiO2/CuO/Cu ternary system, TiO2 and CuO are both introduced to create electrons from 12 ACS Paragon Plus Environment

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their valence bands (VB) to conduction bands (CB) under the Xe lamp irradiation. Due to the fact that the position of the CuO conduction band is much more positive than that of TiO2, the photo-generated electrons should be transferred from TiO2 to the CuO conduction, while the holes would be accumulated at the VB of the TiO2 and CuO. In this way, the recombination of electrons and holes can be avoided. Unfortunately, the photoexcited electrons in CuO cannot directly contribute to the hydrogen production, since the CB edge position of CuO is more positive than the H+/H2 potential (NHE). However, as the continuous accumulation of the excess electrons in the CB of CuO, it would cause a negative shift in its Fermi level, behaving higher electron availability for the interfacial transfer to H+ in solution, which thus facilitate the generation of H2. This endows the CuO with required overvoltage for proton reduction to be worked as the hydrogen formation site. Hence, the CuO in this composite nanofiber acts dual roles to enhance the photocatalytic H2 production: i) promote the electron transfer from TiO2 to CuO in the heterojunctions for the enhancement of the charge separation; ii) the cocatalyst behaviors by offering the reduction sites for H2 production. Meanwhile, the physicochemical properties of the ternary hybrids would be further improved by the introduction of Cu0, which could act not only as the reducing centers by trapping excited electrons in the CB of TiO2,24 but also as the unique conductive electron transport “highway”. With respect to the Fermi level of Cu lower than that of p-type CuO, the electrons on the CB of CuO can further migrate to Cu until the systems is equilibration.43,44 That is to say, the presence of Cu prolongs the lifetime and accelerated the transfer speed of photo-generated carriers, conferring the ternary photocatalyst with enhanced photocatalytic efficiency. Overall, the constructed thoroughly mesoporous TiO2/CuO/Cu ternary hybrid photocatalysts can bring the synergistic effect such as suppressed charge recombination, improved interfacial charge transfer, increased photocatalytic reaction centers, as well as the favorable 1D thoroughly mesoporous nanostructures, which consequently make a 13 ACS Paragon Plus Environment


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significant enhancement on their photocatalytic performances with excellent efficiency and robust stabilities.

4. Conclusions In summary, we have reported the exploration of ternary hybrid TiO2/CuO/Cu thoroughly mesoporous nanofibers via a foaming-assisted electrospinning strategy. The introduced Cu contents within the initial solutions play a profound effect on the growth of the ternary hybrid fibers, including the phase compositions, porosities, specific surface areas and light harvest. The ternary TiO2/CuO/Cu thoroughly mesoporous system exhibits a uncompromising stability and a prominent photocatalytic H2 evolution efficiency of ~851.3 µmolg-1·h-1, which could be significantly enhanced for more than ~3.5 times as compared to those of the pristine TiO2 counterparts and commercial P25. We mainly attribute the enhanced photocatalytic behaviors to the constructed heterojunctions among the TiO2, CuO and Cu interfaces, which could favor the strengthened space separation and transfer of the photo-generated charge carriers. It is expected that current work might provide a novel and facile strategy for exploring superior photocatalysts to be used in photocatalytic clean energy production.

Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC, Grant No. 51372122, 51372123 and 51572133), Postdoctoral Science Foundation of China (No. 2015M581966) and Natural Science Foundation of Ningbo Municipal Government (Grant No. 2016A610102 and 2016A610108).


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Supporting Information The typical SEM images, element scanning patterns, TEM images, XRD patterns, and GC patterns of the obtained products are available at the Website (XXXXXXXXX).



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Table 1. The details of five solutions used for electrospinning polymer precursor fibers Sample A B C D E

PVP (wt%) 6.5 6.5 6.5 6.5 6.5

TBOT ( wt%) 30 30 30 30 30

Cu (Ac)2 (wt%) 0 2 2 1 3


Former (wt%) 10 0 10 10 10

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Figure 1. (a-b) Typical SEM images of the calcined products of Sample C under low magnifications. (c-d) Representative SEM images under high magnifications of the calcined products of Sample C. (e) A representative XRD pattern of the calcined products of Sample C. (f) A corresponding close-up XRD pattern (30°～45°) of (e).



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Figure 2. TEM characterization of the hybrid nanofibers of Sample C. (a) A Typical TEM image of a single mesoporous nanofiber under low magnification. (b) A Typical EDX pattern recorded from the marked area of A in (a). (c) The element mapping of Cu within a single fiber. (d) The corresponding SAED pattern recorded from the marked area of B in (a). (e) A typical HRTEM image recorded from the marked area of C in (a). (g-f) Enlarged images recorded from the marked areas of D and E in (e), respectively.


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Figure 3. Typical XPS spectra of TiO2/CuO/Cu mesoporous nanofibers of Sample C. (a) survey spectrum, (b) Ti 2p, (c) O 1s and (d) Cu 2p. (e) A schematic illustration of the atomic structure of constructed TiO2-CuO-Cu heterojunction.



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Figure 4. Typical SEM images, TEM images and XRD patterns of the calcined products of Sample A (a1-c1), Sample B (b1-b3), Sample D (c1-c3) and Sample E (d1-d3), respectively.


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Figure 5. (a) Typical N2 adsorption and desorption isotherms of the calcined products of Sample A-E. (b) The corresponding pore size distribution of Sample A-E. (c) UV-vis diffuse reflectance absorption spectra of Sample A-E.



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Table 2. Structural parameters of Sample A-E derived from the nitrogen adsorption–desorption isotherms

a

Samples

SBET (m2/g)a

A B C D E

45.3 11.3 19.6 24.8 13.6

Pore volume (cm3/g)b 0.10 0.04 0.11 0.13 0.08

Average pore size (nm)b 10.6 6.1 34.8. 24.4 35.7

The BET specific surface area was determined by multipoint BET method using the adsorption data. b The pore volume and average pore size were determined by nitrogen adsorption volume.


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Figure 6. (a) The H2 evolution over the photocatalysts of Sample A-E and p25 under different irradiation times. (b) The corresponding average H2 evolution rates of Sample A-E. (c) The reusability for photocatalytic H2 generation over the photocatalysts of Sample A-E and p25.



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Figure 7. (a) Schematically illustrated mechanism for the possible photocatalytic mechanism of the H2 generation over ternary TiO2/CuO/Cu thoroughly mesoporous nanofibers under xenon lamp irradiation. (b) Schematic illustration of the energy band structures of TiO2/CuO/Cu nanofibers.


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