In Situ Generation of Copper Species Nanocrystals in TiO2

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In Situ Generation of Copper Species Nanocrystals in TiO2 Electrospun Nanofibers: A Multi-Heterojunction Photocatalyst for Highly-Efficient Water Reduction Kuichao Liu, Zhenyi Zhang, Na Lu, and Bin Dong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03361 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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ACS Sustainable Chemistry & Engineering

In Situ Generation of Copper Species Nanocrystals in TiO2 Electrospun Nanofibers: A Multi-Heterojunction Photocatalyst for Highly-Efficient Water Reduction Kuichao Liu, Zhenyi Zhang*, Na Lu, and Bin Dong* Key Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs Commission, Key Laboratory of Photosensitive Materials & Devices of Liaoning Province, School of Physics and Materials Engineering, Dalian Nationalities University, 18 Liaohe West Road, Dalian 116600, P. R. China Corresponding authors: [email protected]; Tel: +8641187658872; Fax: +8641187658872; [email protected]; Tel: +8641187556959; Fax: +8641187556959.

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ABSTRACT Engineering the multi-heterojunction in semiconductor photocatalysts have been recognized as one of promising ways to achieve highly-efficient photocatalytic solar-fuels generation, because the photoinduced hetero-interfacial charge transfer can greatly hinder the recombination process of charge-carrier in photocatalysts. In this work, we fabricated copper species nanocrystals/TiO2 multi-heterojunction photocatalysts through in-situ reduction of CuO nanocrystals in CuO/TiO2 electrospun nanofibers by hydrothermal method assisted by glucose. By changing the concentration of glucose, the composition ratio of copper species nanocrystals, including CuO, Cu2O, and Cu, can be adjusted in multi-heterojunction nanofibers. Upon simulated sunlight irradiation, the optimal copper species nanocrystals/TiO2 multi-heterojunction nanofibers exhibited the H2 evolution rate of ∼10.04 µmol h-1, which increase 17.3 times than bare TiO2 nanofibers (~0.57 µmol h-1).

KEYWORDS: Electrospinning, H2 generation, Photocatalysis, Multi-heterojunction, Nanofibers

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INTRODUCTION The rapid consumption of non-renewable fossil fuels is forcing people to find an efficient and sustainable energy. Hydrogen (H2), as a promising clean energy, has drawn dramatically increasing attention on account of its high energy storage and environment friendliness.[1-2] Since the electrochemical photolysis of water at the TiO2 electrode was firstly reported by Fujishima and Honda in 1972,[3] numerous efforts have been made to enhance photoabsorption efficiency and charge-carriers separation of TiO2 due to its large bandgap (3.20 eV for anatase; 3.00 eV for rutile) and poor quantum efficiency.[4-6] In recent years, nanosized multi-component photocatalysts have shown fascinating advantages in restraining the recombination of photoinduced electron-hole pairs based on an efficient charge transfer process, and therefore fulfill efficient photo-reduction and -oxidation reactions at spatially separated sites.[7-12] It is desirable to enhance the photocatalytic performance of TiO2 by building nanosized multi-component photocatalysts with a suitable component ratio.[13] Incorporating sensitizers, such as metals and transition metal oxides, with TiO2 nanostructures has been proved to be an effective tactic for broadening the absorption range and enhancing the efficiency of photocatalysis.[14-22] Among the transition metal oxides, copper species nanocrystal is considered to be one of the most suitable sensitizers due to the good catalytic property of copper (Cu)[23-27] and the narrow band gap of both cupric oxide (CuO)[5,8,14,18,28-30] and cuprous oxide (Cu2O).[23-24,31] The introduction of CuO (or Cu2O) into TiO2 nanostructures can not only extend the light absorption range, but also render a step-by-step energy band structure. The step-by-step energy band structure is beneficial to the efficient transfer of photoinduced charge carrier between each other, thus suppress their 3



recombination.[8,23,32] Additionally, the metallic Cu nanocrystal can enhance photocatalytic activities through boosting the separation and transfer of photoinduced electrons.[25,27] However, it’s still a challenge to fabricate a multi-heterojunction including CuO, Cu2O, Cu and TiO2 nano-components, simultaneously keep the stable structure and excellent efficiency of photocatalysis.[24,31] Herein, we fabricated CuO/TiO2 heterojunction nanofibers via a simple electrospinning process and the subsequent calcination process (Scheme 1). Following by an in-situ hydrothermal reduction technique, the above binary heterojunction photocatalyst of CuO/TiO2 nanofibers can be further transformed into a controllable quaternary heterojunction photocatalyst of Cu/Cu2O/CuO/TiO2 nanofibers. The suitable energy bands of copper species and TiO2 in this multi-heterojunction photocatalyst, greatly promote the efficient separation and migration of photoinduced charge carriers.[33] Combined with the low-energy photon absorption of CuxO (x=1 and 2) and the catalytic active-sites of Cu, the H2 generation rate of the as-synthesized Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers could reach ∼10.04 µmol h-1, which increase 17.3 times than bare TiO2 nanofibers (~0.57 µmol h-1) upon simulated sunlight irradiation with triethanolamine (TEOA) as electron donor.

EXPERIMENTAL SECTION Fabrication of CuO/TiO2 heterojunction nanofibers: CuO/TiO2 heterojunction nanofibers were synthesized through traditional electrospinning technique. Typically, 0.1 g of Cu(CH3COO)2·H2O and 1.0 ml of tetrabuty titanate (Ti(OC4H9)4) were successively added into a mixture solution of acid (2 ml) and ethanol (5 ml) dispersed by magnetic stirrer for 30 min. Then, 0.4g of poly (vinyl pyrrolidone) (PVP) 4


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powder (Mn=1300K) were dissolved in this mixture solution with continuously magnetic stirring overnight. Subsequently, the as-obtained precursor solution was decanted into an electrospining equipment as shown in Scheme 1. The applied voltage and the distance from the needle tip to aluminum foil collector were 10 kV and 10 cm, respectively. Finally, the collected composite nanofibers of PVP/Ti(OC4H9)4/Cu(CH3COO)2 were calcined in a muffle furnace under air atmosphere for 2 h at 500 oC with a rising rate of 2 oC min-1. Fabrication of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers: Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were fabricated though an in-situ hydrothermal reaction technique. 10 mg of CuO/TiO2 NFs were added to the glucose aqueous solution (the concentrations of glucose were 0.125 mg ml-1, 0.25 mg ml-1, 0.375 mg ml-1 and 0.5 mg ml-1, respectively) and stirred for 10 min. Subsequently, the mixture was transferred to a 25 ml Teflon-lined autoclave, and kept at 150 oC for 1 h in an electric oven. The product was centrifuged and washed with ethanol several times. After that, Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were obtained by drying in the oven at 60 oC for 4 h. Characterization: The X-ray diffraction (XRD) measurements of the fabricated photocatalysts were analyzed by X-ray diffractometer (Shimadzu XRD-6000). The surface morphologies of the fabricated photocatalysts were examined using a scanning electron microscope (SEM; Hitachi S-4800) and structures of the fabricated photocatalysts were examined by transmission electron microscopy (TEM; JEOL JEM-2100). The surface chemical composition of the fabricated photocatalysts were analysed by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out on a VG-ESCALAB LKII system with Mg Kα ADES 5



(hν =1253.6 eV) source at a residual gas pressure below 10-8 Pa. A UV-visible spectrophotometer (UV–Vis–NIR, Lambda 750, Perkin-Elmer, USA) was used to study the absorption spectra. Nitrogen adsorption/desorption isotherms were analyzed on an ASAP 2020. Photoluminescence (PL) spectra was detected using fluorescence spectrophotometer (Hitachi F-4600) equipped with a 150 W Xe lamp as the excited light source. Photocatalytic H2 generation: The photocatalytic activity tests have been carried out in a 35-ml quartz cylindrical reactor. In a typical manner, 5 mg of the as-prepared nanofibers and 10 ml aqueous solution containing of 15 vol% TEOA were put into the quartz cylindrical reactor. The quartz cylindrical reactor was sealed and purged with argon (Ar) gas for 15 min. Finally, the reactor loading with the suspension illumination under simulated sunlight irradiation by using a xenon arc lamp of intensity AM 1.5 sunlight (100 mW cm-2) (PLS-SXE300UV). The gas samples collecting from the upper quartz reactor space above the liquid, and the gas samples composition was analyzed periodically by a thermal conductivity detector (TCD) using gas chromatography (Beifen-Ruili Analytical Instrument, SP-3420A). Electrochemical and photoelectrochemical measurement: The photocurrent responses were analyzed in using a three-electrode system of electrochemical analyzer (CHI 660D, CH Instruments Inc.). Pt wire as the counter and Ag/AgCl as reference electrode during the process of photocurrent testing. For the working electrode construction, 5 mg of the prepared TiO2 nanofibers, CuO/TiO2 heterojunction nanofibers and Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were mixed with 0.1 ml ethanol to form a paste, then put the above paste over an In-doped SnO2 (ITO) glass (the 6


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effective area of 1 cm × 1cm) and followed put a drop of Nafion ethanol solution (1 wt.% ) on the surface. 0.1 M NaOH was used as the electrolyte, and the solution have to degas by purging N2 gas for 10 min before irradiation. Then, a 300-W Xe lamp (NBET, HSX-F300) equipped with a monochromator (NBET) was used to irradiated the as-prepared working electrode.

RESULTS AND DISCUSSION The Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were synthesized by a two-steps process: (1) the fabrication of CuO/TiO2 heterojunction nanofibers via an electrospinning process and the followed calcination treatment; (2) in-situ reduction of CuO/TiO2 heterojunction nanofibers into the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers via a glucose-assisted hydrothermal method (Scheme 1). During the hydrothermal process, the glucose could serve as a mild reducing agent to offer the electron for the reduction of CuO into Cu2O or Cu nanocrystals.[34-35] The possible process could be expressed as follows:

hydrotherm al Glucose + CuO      → Cu 2 O + Gluconic acid

(1)

hydrotherm al Glucose + Cu 2 O       → Cu + Gluconic acid

(2)

As observed the above equations, this hydrothermal process contains two redox reactions with slow dynamic behaviors. The reaction rate is related to the reaction time, the reaction temperature and the concentration of reducing agent (glucose). During the reaction, the glucose reduced the CuO into Cu2O and forming the gluconic acid. Meanwhile, the as-produced Cu2O can be further reduced into Cu by the residual glucose in the hydrothermal solution. A high concentration of glucose in the reaction solution would accelerate the reduction of Cu2+ into Cu0, when the reaction temperature and time were fixed at appropriate 7



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values. Thus, we could control the ratio of copper species nanocrystals in the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers by adjusting the concentration of glucose. When an appropriate reaction parameter was used during the hydrothermal process, the copper species including Cu, Cu2O, CuO could co-exist in the multi-heterojunction nanofibers. SEM and TEM images of CuO/TiO2 heterojunction nanofibers showed that the randomly deposited nanofibers appeared smooth surface, with a diameter of 200nm in and several micrometers in length (Figure 1A and B). The high-resolution TEM (HRTEM) image of CuO/TiO2 heterojunction nanofibers showed two kinds of lattice fringes with the interplanar distances at 0.25 and 0.35 nm, corresponding to d-spacings of the (002) plane of tenorite CuO and the (101) plane of anatase TiO2, respectively (Figure 1C). In the case of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers, the diameters had no change after hydrothermal treatment of CuO/TiO2 heterojunction nanofibers. However, the surface of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers became rough and appeared numerous irregular-shaped nanopores (Figure 1D and E). It suggested that after the hydrothermal treatment, the part of CuO nanoparticles in CuO/TiO2 heterojunction nanofibers were reduced to Cu2O or Cu nanoparticles, which induced the changes of lattice structures and grain sizes of copper species nanocrystals, resulting in the formation of nanopores in the multi-heterojunction

nanofibers.

The

HRTEM

image

of

Cu/Cu2O/CuO/TiO2

multi-heterojunction nanofibers indicated that the lattice fringes of 0.35, 0.23, 0.25, and 0.21 nm are attributed to the (101) plane of anatase TiO2, the (002) plane of tenorite CuO, the (111) plane of cuprite Cu2O, and the (111) plane of cubic Cu, respectively (Figure 1F). This 8


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confirmed the co-existence of Cu, Cu2O, CuO, TiO2 nanoparticles in the multi-heterojunction nanofibers. Meanwhile, the chemical-element mapping images of Ti, Cu, and O implied that the copper species nanocrystals and TiO2 nanocrystals co-existed in the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers (Figure 1G). The crystal structures of CuO/TiO2 heterojunction nanofibers and Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were investigated by XRD patterns. The diffraction peaks of CuO/TiO2 heterojunction nanofibers can be indexed to the tenorite CuO (JCPDS, no. 45-0937) and the anatase TiO2 (JCPDS, no. 21-1272), as is shown in Figure 2A. The diffraction peaks of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers are originated to the cubic Cu (JCPDS, no. 01-1241), the cuprite Cu2O (JCPDS, no. 05-0667), the tenorite CuO and the anatase TiO2. More details concerning the valence states of copper elemental in Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were further studied by XPS analysis (Figure 2B). The peaks with binding energies at 934.2, 955.1, 935.3, 957.2, 936.8, and 958.4 eV are attributed to the Cu+ 2p3/2, Cu+ 2p1/2, Cu0 2p3/2, Cu0 2p1/2, Cu2+ 2p3/2, and Cu2+ 2p1/2, respectively.[36] This further verifies the existence of CuO, Cu2O and Cu components in the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers. Meanwhile, the relative molar ratio of Cu/Cu2O/CuO in the optimal multi-heterojunction nanofibers were 1: 0.6: 0.53, which is calculated by XPS result (Equation S1 Supporting Information). Furthermore, the XRD patterns of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers obtained at different concentrations of glucose solution were also investigated (Figure S1 Supporting Information). The results clearly show that the intensities of diffraction peaks for the copper species nanocrystals are dependent on the concentrations of glucose during the hydrothermal process. 9



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It reveals that the composition ratio of copper species nanocrystals including CuO, Cu2O, and Cu can be adjusted in the multi-heterojunction nanofibers by changing the concentration of glucose. The optical properties of TiO2 nanofibers, CuO/TiO2 heterojunction nanofibers, and Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were evaluated through the UV-Vis absorption spectra. Note from Figure 3A that the TiO2 nanofibers show a narrow absorption band in UV light region on account of the wide energy band of anatase TiO2 (3.20 eV). After the addition of CuO hetero-component into TiO2 nanofibers, the light absorption of CuO/TiO2 heterojunction nanofibers obviously extend to the visible region because of the narrow bandgap of CuO. When the CuO hetero-component in the CuO/TiO2 heterojunction nanofibers was reduced by the hydrothermal reaction, the as-obtained Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers display a weakened light absorption in visible region for the existence of Cu nanocrystals that absorb the light in NIR region.[37-39] Furthermore, the Cu/Cu2O/CuO/TiO2 multi-heterojunction

nanofibers

fabricated

with

the

different

concentrations of glucose solution show slight differences in the absorption spectra (Figure S2 Supporting Information). This further indicates the changed composition ratio of copper species nanocrystals in the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers. Figure 3B presents the

nitrogen

adsorption-desorption

isotherms

of

the

Cu/Cu2O/CuO/TiO2

multi-heterojunction nanofibers, CuO/TiO2 heterojunction nanofibers and TiO2 nanofibers. The corresponding Brunauer-Emmett-Teller (BET) analyses revealed that the specific surface area of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers were 27.6 m2 g-1. This value is close to the specific surface areas of CuO/TiO2 heterojunction nanofibers (29.4 m2 g-1) and 10


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TiO2 nanofibers (28.2 m2 g-1), implying that this specific surface area contributes the same effect on the photocatalytic activities of the as-fabricated nanofibers. Photocatalytic activities of the nanofibers were assessed through the water reduction reaction under simulated sunlight irradiation (AM 1.5 with light density of 100 mW cm-2) by using TEOA as a scavenger.[40] Control experiments indicated that no H2 generation was detected without either light irradiation or photocatalysts. As shown in Figure 4A, the bare TiO2 nanofibers display a low H2 evolution rate of ~0.57 µmol h-1 due to its poor light harvesting property. After coupling copper species nanocrystals with the TiO2, the CuO/TiO2 binary heterojunction nanofibers present a slightly increase on the H2 evolution rate (1.31 µmol h-1). This can be attributed to the formation of binary heterojunction for boosting the separation of photoinduced charge carriers in CuO/TiO2 nanofibers.[8,30] When the CuO/TiO2 heterojunction nanofibers were treated by the hydrothermal reaction using glucose as reductant, the formed Cu/Cu2O/CuO/TiO2 quaternary heterojunction nanofibers exhibited significant enhancement on the photocatalytic activities for H2 evolution. It is demonstrated that a “step-by-step” energy band structure is built in the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers, which could not only extend the light harvesting range, but also enhance the charge-carrier separation and the surface-reactivity of nanofiber catalysts.[8,33] The highest rate of H2 evolution was achieved on the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers fabricated by the hydrothermal treatment with the glucose concentration of 0.25 mg ml-1 (Figure 4B). This rate increase 17.3 times than that of bare TiO2 nanofibers and even 7.66 times than CuO/TiO2 nanofibers. Furthermore, the apparent quantum efficiency (AQE) of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers at 365 nm 11



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is ~2.014%, which is 18.5 times higher than the corresponding value (~0.109%) obtained by the pure TiO2 nanofibers (Equation S2 See Supporting Information).[41] It is revealed that the optimal ratio of copper species hetero-components is obtained to greatly enhance the photocatalytic activity of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers for water reduction. Besides, the cyclability of the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers was also investigated. The result showed that after four cycling tests, the photocatalytic activity for H2 generation could still maintain the original activity as compared to the first cycle (Figure 5), verifying a good stability of the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers. When

copper

species

nanocrystals

and

TiO2

nanoparticles

formed

the

multi-heterojunction in the electrospun nanofiber matrix, the step-by-step energy band structure would rationally construct for charge carriers to efficiently separate and migrate, therefore enhancing the photocatalytic activity. The incident-photon-to-current-conversion efficiency (IPCE) measurement with different excitation wavelengths (Figure 6A) give further evidence at the above viewpoint. Its standard equation can be expressed as:

IPCE =

1240 Ι × 100% λJ light

(3)

Where I denotes the incident light wavelength; Jlight is the incident light power density, and I is the photocurrent density.[42-43] The bare TiO2 nanofibers show a low IPCE signal in UV region was caused by the fast recombination of photoinduced charge-carriers. After introducing the CuO hetero-component into the TiO2 nanofibers, the IPCE value of CuO/TiO2 heterojunction nanofibers slightly increases in UV region, which benefits from the heterojunction effect to promote the charge separation in the CuO/TiO2 heterojunction 12


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nanofibers.

The

IPCE

signal

at

UV

light

region

of

the

Cu/Cu2O/CuO/TiO2

multi-heterojunction nanofibers increase 57 times than that of the CuO/TiO2 binary heterojunction nanofibers, which further verifies the highly-efficient separation and migration of charge-carriers in the step-by-step energy band structure of Cu/Cu2O/CuO/TiO2 nanofibers. Meanwhile, the IPCE response region of the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers range from UV to near-IR light, implying the existence of new visible-light-active absorbers of such Cu2O nanocrystals and plasmonic Cu nanocrystals. A high and stable photon-to-electricity efficiency of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers under UV-IR light irradiation was detected by the transient photocurrent response tests with several on-off cycles of irradiation (Figure 6B).[44-45] Moreover, the charge-carriers dynamics process in the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers was further illustrated by the photoluminescence spectroscopy (PL). The excitation wavelength (290 nm) was used to irradiate the heterojunction nanofibers, so that induce the interband transitions of all the semiconductor nanoparticles. It could be observed that the PL intensity exhibited an obvious decrease after coupling copper species nanocrystals with the TiO2 nanofiber matrix (Figure S3 Supporting Information). Especially, the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers have the lowest PL intensity. These results indicated that the formed step-by-step energy band structure in the Cu/Cu2O/CuO/TiO2 nanofibers could hinder the recombination of photoinduced charge-carriers due to the existence of interfacial charge separation and migration. For better understanding of the enhancement of photocatalytic ability, the step-by-step energy band structure of CuO/TiO2 and Cu/Cu2O/CuO/TiO2 heterojunction nanofibers are 13



schematically

illustrated.

Upon

interband

excitation

of

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both

CuO

and

TiO2

hetero-components in the CuO/TiO2 heterojunction nanofibers, the electrons on the CB of TiO2 move to the CB of CuO, while the holes on the VB of TiO2 transfer to the VB of CuO due to the formation of Ⅰ type hetero-interface in the CuO/TiO2 heterojunction. In consequence, the electrons on the CB of CuO can reduce the H+ to produce H2, and the holes on the VB of CuO can achieve the oxidation process.[5,46] In the case of energy band structure of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers, the photoinduced electrons on the CB of Cu2O will transfer to TiO2, then to CuO, and accumulate on the Cu surface to reduce the H+ to produce H2 in the end (Scheme 2).[33,47] Notably, the Cu nanocrystal, as the electron storage center, plays a critical role in prolonging the lifetime and benefiting to the separation and migration of photoinduced charge-carriers, ultimately enhancing the performance of H2 generation.[8,26] Thus, the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers exhibit the higher efficiency and stability for H2 generation due to its high absorption efficiency and the step-by-step energy band structure that can greatly promote photoactive electron transfer.[8,23,33]

CONCLUSIONS In summary, the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers have been successfully fabricated via in situ reduction of CuO nanocrystals in the CuO/TiO2 heterojunction nanofibers. The construction of Cu/Cu2O/CuO/TiO2 multi-heterojunction in the electrospun nanofibers could not only extend the light absorption in visible light region, but also lead to a step-by-step energy band structure for greatly boosting photoactive electron transfer across the hetero-interface. Therefore, the Cu/Cu2O/CuO/TiO2 multi-heterojunction 14


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nanofibers exhibited a prominent photocatalytic activity for H2 generation with the rate of ∼10.04 µmol h-1, which increase 17.3 times than bare TiO2 nanofibers. It is expected that our present work might put forward a facile and effective method to prepare low-cost multi-heterojunction photocatalysts with highly photocatalytic efficiency.

ASSOCIATED CONTENT Supporting Information Molar ratios for the hetero-components in the heterojunction nanofibers; apparent quantum efficiency measurement; XRD patterns, UV-Vis absorption spectra and PL spectra of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI:

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos: 51772041, 11474046, 61775024, and 51402038), Natural Science Foundation of Liaoning Province (20170540190), the Program for Liaoning Excellent Talents in University (LNET) (Grant No. LR2015016), the Program for Dalian Excellent Talents (Grant No. 2016RQ069), the Science and Technique Foundation of Dalian (Grant Nos. 2014J11JH134 and 2015J12JH201), and the Fundamental Research Funds for Central Universities (Grant Nos. DC201502080203 and DC201502080304).

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Scheme 1 Schematic diagram for the synthesis process of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers.

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Scheme 2 Schematic diagrams showing the energy band structure and photoinduced charge-carriers transfer in (A) CuO/TiO2 heterojunction nanofibers and (B) Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers.

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Figure 1 (A) SEM image of CuO/TiO2 heterojunction nanofibers; (B) TEM image of CuO/TiO2 heterojunction nanofibers; (C) HRTEM image of CuO/TiO2 heterojunction nanofibers; (D) SEM image of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers; (E) TEM image of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers; (F) HRTEM image of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers; (G) Element mapping performed at a single Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofiber.

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Figure 2 (A) XRD patterns of (a) TiO2 nanofibers, (b) CuO/TiO2 heterojunction nanofibers, and (c) Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers; (B) XPS spectrum of Cu 2p core-level for the Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers.

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Figure 3 (A) UV-Vis absorption spectra of the (a) TiO2 nanofibers, (b) CuO/TiO2 heterojunction nanofibers, and (c) Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers. (B) Nitrogen adsorption-desorption isotherm of (a) TiO2 nanofibers，(b) CuO/TiO2 heterojunction nanofibers and (c) Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers.

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Figure 4 (A) Time-dependent H2 generation amount under AM 1.5 irradiation over (a) TiO2 nanofibers and (b-f) CuO/TiO2 heterojunction nanofibers obtained in different concentrations of glucose solution; (B) The corresponding H2 evolution rates of Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers obtained in different concentrations of glucose solution (c-f).

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Figure 5 Cycling test of photocatalytic H2 production for the optimal Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers.

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Figure 6 (A) measured IPCE spectra of (a) TiO2 nanofibers, (b) CuO/TiO2 heterojunction nanofibers, and (c) Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers recorded at the incident wavelength range from 300 to 800 nm at a potential of -0.2 V vs Ag/AgCl in an aqueous solution of 0.1M NaOH; (B) stable photocurrent test of (a) TiO2 nanofibers, (b) CuO/TiO2 heterojunction nanofibers, and (c) Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers under AM 1.5 sunlight irradiation.

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TOC

Synopsis A low-cost multi-heterojunction photocatalyst of Cu/Cu2O/CuO/TiO2 nanocomposite was developed via a glucose-assisted hydrothermal process, which exhibited a 17.3-fold enhancement on the photocatalytic H2 generation as compared to the pure TiO2 nanofibers.

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In Situ Generation of Copper Species Nanocrystals in TiO2

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