Brookite TiO2quasi nanocubes decorated with Cu nanoclusters for enhanced photocatalytic hydrogen production activity

Materials Today Chemistry 1-2 (2016) 23e31

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Brookite TiO2 quasi nanocubes decorated with Cu nanoclusters for enhanced photocatalytic hydrogen production activity Jinyan Liu a, Jingpeng Jin a, Jiang Luo a, Xiaolan Li b, Ling Zan a, Tianyou Peng a, * a b

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China Research Department of High Voltage Engineering, China Electric Power Research Institute, Wuhan 430074, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 September 2016 Received in revised form 6 October 2016 Accepted 7 October 2016

Brookite TiO2 quasi nanocubes (BTN) decorated with various Cu nanoclusters (CueNCs) contents (hereafter referred to as Cu/BTN composite) were synthesized via a facile chemical reduction process by NaBH4. The obtained products and its Cu's existential state were characterized by X-ray diffraction, UV eVis diffuse reflectance absorption spectroscopy, electron microscopy, X-ray photoelectron spectroscopy and X-ray fluorescence spectroscopy. It was found that the introduction of CueNCs with small size of ~1 e2 nm on BTN surfaces can improve the photocatalytic H2 production activity, and the maximum photoactivity (225 mmol h
1) for H2 production over 1.0 mol% Cu/BTN composite is similar to that (220 mmol h
1) of the benchmark photocatalyst (P25) under the optimum photoreaction conditions, which is 5.2 times higher than that (42.5 mmol h
1) of the BTN alone. This significant enhancement in the photoactivity of BTN is deemed to result from the metallic CueNCs with high surface area and dispersion, which favour the co-catalyst functions to cause an effective photogenerated carrier separation in space and an improvement in the photocatalytic activity and stability for H2 production. The present results not only demonstrate the brookite TiO2 would be a potential effective photocatalyst for H2 production, but also provide an inexpensive, efficient and stable means of enhancing light-to-hydrogen energy conversion by using metallic Cu nanoclusters alternative to the commonly used noble metal co-catalyst. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Brookite titania Copper nanocluster Photocatalyst H2 production Co-catalyst

1. Introduction Photocatalytic solar-to-hydrogen energy conversion process has been regarded as a promising route to solve the increasingly serious energy shortage since the pioneering work on water splitting over a TiO2 electrodes was first reported in 1972 [1]. Up till now, the majority of researches in this field emphasizes on the development of heterogeneous photocatalysts with higher efficiency [2e4]. Amongst those photocatalysts developed, TiO2 is the most extensively investigated UV-light-responsive catalyst because of its exceptional properties such as high chemical stability, low cost, easy availability, and suitable band alignment to water redox potentials [5]. Among the three TiO2 polymorphs that exist in nature, rutile is thermodynamically stable form, while anatase and brookite are metastable ones and can be transformed to rutile with heat treatment [6]. In addition, anatase, rutile and their mixed crystal have been comprehensively studied and applied in the field

* Corresponding author. E-mail address: [email protected] (T. Peng). http://dx.doi.org/10.1016/j.mtchem.2016.10.006 2468-5194/© 2016 Elsevier Ltd. All rights reserved.

of photocatalysis [2,7], while brookite is rarely investigated because the majority of attempts for the pure brookite TiO2 often obtained binary and ternary mixtures of brookite with anatase and/or rutile due to the thermodynamic instability of brookite [8e10]. Nevertheless, several studies have indicated that brookite TiO2 has the highest bandgap energy (Eg) among the three TiO2 polymorphs, which can contribute to the most negative conduction band (CB) level because they have similar valence band (VB) levels due to the same elementary composition [11e14]. This readily suggests that brookite should exhibit a better photoactivity than anatase and rutile [15e18]. As early as in 2007, nanocrystals with three TiO2 polymorphs were synthesized by controlling hydrothermal conditions, and the obtained brookite nanoplates exhibited the highest activity for methyl orange degradation under the same surface area [8]. Lin and co-workers [15] reported that brookite single-crystal nanosheets surrounded with four {210}, two {101}, and two {201} exposed facets exhibited outstanding activity for organic contaminant degradation due to the exposure of high-energy crystal facets and the effective suppression of the charge recombination caused by

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J. Liu et al. / Materials Today Chemistry 1-2 (2016) 23e31

these facets acting as oxidative and reductive sites, respectively. Recently, high-quality brookite TiO2 quasi nanocubes (BTN) were synthesized through a hydrothermal method in our group [19], and its Ag-loaded products showed significantly enhanced photocatalytic CO2-reduction activity and selectivity for CH4 generation as compared to BTN alone [20]. Although the above investigations showed that brookite TiO2 would be promising in the field of photocatalysis, the photoactivity of unmodified one is usually compromised, like the extensively used anatase TiO2, mainly due to the rapid photogenerated charge recombination [21]. To overcome the rapid charge recombination, several strategies have been developed to modify TiO2 for enhancing the photoactivity [2e4]. One of the most popular approaches is to load metal or metal oxide co-catalyst acting as electron acceptor for effectively retarding the charge recombination [22e28]. Compared to the noble metals (such as Pt and Rh) that can catalyze the backward reaction even though they are very good promoters of H2 production, low-cost transition metals and/or metal oxides are more attractive and promising co-catalyst since they do not promote the backward reaction [4,26,27]. In particular, Cu would be the most prospective candidates for low-cost photocatalytic applications due to its advantages such as abundance, non-toxicity and large work function (4.65 eV) [21,29], and a possible formation of Schottky junction between Cu and TiO2 to enhance the separation efficiency of photogenerated charge carriers [21,28], and thus various Cu species such as Cu [4,30], Cu2O [31], CuO [32], CuOx [33], and Cu(OH)2 [34] have been loaded on anatase TiO2 to promote the photoactivity. Herein, high-quality brookite TiO2 quasi nanocubes (BTN) with four {210} and two {001} exposed crystal facets as well as mean particle size of ~50 nm, which was synthesized through a hydrothermal method according to our previous report [19], was selected as the photocatalyst subject since there is no report on brookite TiO2 applied in the field of photocatalytic H2 production so far to the best of our knowledge. A series of BTN decorated with Cu nanoclusters (CueNCs) with small size of ~1e2 nm were synthesized using chemical reduction method. The effects of variations in CueNCs contents on the crystal structure, morphology, spectral absorption, stability and photoactivity of Cu/BTN composites (referred to as Cu/BTN) were investigated systematically. A significantly enhanced photocatalytic activity and stability for H2 production as compared with the pristine BTN were both achieved in the present study. It is believed that this study provides some insight into non-noble metallic Cu and brookite TiO2 for promising low-cost and high-performance photocatalytic H2 production application. 2. Experimental 2.1. Material preparation All chemical reagents used are analytical grade and without further purification. P25 (TiO2, Degussa) was obtained from a commercial source. Brookite TiO2 quasi nanocubes (BTN) were prepared according to our previous report [19]. Typically, 15 mmol TiCl4 was dropped into 40 g ice water in a Teflon autoclave, then 5.0 g urea was added under stirring. After the urea was dissolved, 5.0 mL sodium lactate solution (60%) was mixed into the Teflon autoclave drop by drop until the solution distributed uniformly. Eventually, the Teflon autoclave was sealed and hydrothermally treated in an oven at 200  C for 20 h. The obtained precipitate was washed with distilled water and alcohol for several times respectively and dried in vacuum at 70  C for 12 h. Finally, the obtained white power was treated in air for 3 h at 500  C with as heat rate of 2  C min
1.

Cu-loading on BTN was conducted by chemical reduction process. Typically, the as-prepared BTN (240 mg) was dispersed in 50 mL distilled water, and then 0.30 mL CuCl2 (0.1 M) was mixed under stirring. After the suspension dispersed uniformly, 0.1 g NaBH4 was added for the Cu2þ reduction. The suspension after stirring for 3 h was centrifuged and washed with water and alcohol for several times, and then dried in vacuum at 70  C overnight to obtain 1.0 mol% Cu-loaded brookite TiO2 (denoted as 1.0% Cu/BTN). For comparison, different Cu contents (0.5 mol%, 0.8 mol%, 1.5 mol%, 2.0 mol%, 5.0 mol%, 10 mol%, and 20 mol%) in the composite were synthesized by varying the addition amount of CuCl2 solution.

2.2. Material characterization X-ray powder diffraction (XRD) pattern was performed on Miniflex 600 X-ray diffractometer with Cu Ka irradiation (l ¼ 0.154 nm) at 40 kV and 15 mA and a scan rate of 4 min
1 in the range of 10  2q  60 . The morphology was investigated by using ZeissSigma field emission scanning electron microscope (FESEM). The high-resolution transmission electron microscopy (HRTEM) observation was conducted on a JEOL JEM 2100F electron microscope working at 200 kV. UVevis diffuse reflectance absorption spectra (DRS) were obtained with a Shimadzu UV-3600 UVeviseNIR spectrophotometer equipped with an integrating sphere with BaSO4 as the reference. X-ray photoelectron spectra (XPS) were recorded on a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al Ka). Photoluminescence (PL) spectra were obtained on a Hitachi F4600 fluorometer. The contents of various elements in sample were detected by using Bruker S4 Pioneer X-ray fluorescence (XRF) spectrometer with Rh target without standard sample. For obtaining the photocurrent, a working electrode (Pt wire) was immersed in 50 mL aqueous suspension containing catalyst (25 mg), NaOH (1.9 g) and methyl viologen (6.0 mg), and a saturated Ag/AgCl electrode and a Pt gauze electrode were used as a reference and a counter electrode, respectively. Before irradiation, the suspension was continuously purged by N2 to remove O2, and then illuminated by light from a 300 W Xe-lamp. The working electrode was held at þ0.5 V vs. Ag/AgCl by using a CHI 618 workstation to collect the photocurrent-time curve.

2.3. Photoactivity measurement The photocatalytic H2 production reaction was carried out in a closed outer-irradiation photoreactor (pyrex glass) with a total volume of 75 mL. Typically, the photocatalyst was uniformly dispersed in 10 mL of a sacrificial reagent solution by sonicating. If necessary, the pH value of the photocatalyst aqueous suspension, which was determined by a FE20/EL20 model pH meter (MettlerToledo Instruments Co. Ltd.), was adjusted by using HCl solution. A 300 W Xe-lamp (PLS-SXE300, Beijing Trusttech Co. Ltd, China) was employed as a light source. Before the light irradiation, the photoreactor containing the photocatalyst and the sacrificial reagent was sonicated for several minutes to let the photocatalyst disperse uniformly, and then thoroughly degassed to remove air completely. The amount of H2 evolved was determined by a gas chromatograph (GC, SP-6800A, TCD, 5 Å molecular sieve columns and Ar carrier). The long-term (20 h) photocatalytic experiment under the optimal photoreaction conditions was conducted as follow: The photocatalyst after the first run of 5 h light irradiation was recycled for the next run through centrifugation and washing with water for several times, and then drying at 70  C under vacuum condition.

J. Liu et al. / Materials Today Chemistry 1-2 (2016) 23e31

Fig. 1. XRD patterns of the pristine BTN and its Cu-loaded products (Cu/BTN) with different Cu contents.

3. Results and discussion 3.1. Microstructure and composition analyses Fig. 1 depicts the XRD patterns of the pristine brookite TiO2 quasi nanocubes (BTN) and its Cu-loaded products (Cu/BTN) with

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different Cu contents. All samples are well-conformed to the orthorhombic brookite TiO2 (JCPDS PDF 29-1360), and the diffraction peaks at 2q ¼ 25.3 , 25.7, 30.8 , 36.2 , 37.3 , 40.1, 42.4 , 46.0 , 48.0 , and 49.1 can be attributed to the reflection of the (210), (111), (211), (102), (021), (202), (221), (302), (321), and (312) planes of brookite [19,20], respectively. No characteristic peak for any other TiO2 crystal phase can be observed from these XRD patterns, indicating that all products have high crystallinity and phase purity, and the present post-loading of Cu species through chemical reduction process cannot obviously change the crystal phase of BTN. In addition, no diffraction peak ascribable to Cu, Cu2O or CuO can be observed from the XRD patterns of those Cu/BTN composites with a Cu content of less than 2.0%, probably due to the low content and the high dispersion of Cu species on BTN surfaces. Moreover, no evident shift can be observed in those diffraction peak positions of brookite, suggesting that the deposited Cu species did not incorporate into the brookite TiO2 lattice, and are probably attached on the BTN surfaces. Once the Cu-loading content reaches 5.0%, Cu/ BTN composites show the diffraction peaks at 2q ¼ 43.3 and 50.7 attributable to the reflection of (111) and (200) planes of cubic metallic Cu (JCPDS PDF 04-0836), respectively. The intensities of those diffraction peaks attributable to cubic Cu become higher with enhancing Cu content, indicating the increasing sizes of the metallic Cu species. Moreover, no sign for any Cu oxides even for 20% Cu/BTN indicates that Cu2þ ions are completely reduced during the NaBH4 reduction process.

Fig. 2. FESEM images of the pristine BTN and its Cu-loaded products (Cu-BTN) with different Cu contents. The pristine BTN (a), 0.5% Cu/BTN (b), 2.0% Cu/BTN (c), 5.0% Cu/BTN (d), 10% Cu/BTN (e), and 20% Cu/BTN (f). The scale bar is 100 nm.

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Fig. 3. FESEM image (a) of 0.5% Cu/BTN composite and its Ti (b) as well as Cu (c) element mapping.

FESEM images (Fig. 2) indicate that most of the pristine BTN particles have quasi nanocube-like morphology and smooth surfaces, which have relatively uniform particle sizes in the range of 15e90 nm with a mean size of ~50 nm and occasionally smaller irregular particles loaded on those nanocubes' surfaces (Fig. 2a), which is similar to our previous report [19]. After loading with Cu species, the nanocube-like morphology and surface structures of BTN particles can be maintained (Fig. 2bef). Although no evident nanoparticle ascribable to the loaded metallic Cu species can be observed from those products with Cu content less than 5.0% (Fig. 2b and c), some small bright points can be observed on the BTN's surfaces when the Cu content is higher than 5.0%, which might be related to the loaded Cu species (Fig. 2d). More Cu species with larger sizes are uniformly dispersed on the BTNs when Cu content reaches 10% (Fig. 2e) and 20% (Fig. 2f). The existence of Cu species in 0.5% Cu/BTN can be observed from the element mapping shown in Fig. 3, whereby the Cu element is uniformly dispersed in the observed FESEM region, indicating its existence and uniform distribution on the BTN's surfaces.

The above observations can be further confirmed by the TEM images shown in Fig. 4. The pristine BTNs have regular quasi nanocube-like shapes with relatively smooth surfaces (Fig. 4a), in accordance with the above FESEM observation. Although the morphology of BTN particles is not changed after loading with Cu species, coarser surfaces can be observed on the BTN particles (Fig. 4b and c), which are related to the loaded metallic Cu nanoclusters (CueNCs). HRTEM images exhibit that the BTN particles in 1.0% Cu/BTN composite were closely decorated by a small quantity of CueNCs with small size of ~1e2 nm (Fig. 4d). The CueNCs were too small to obtain clear lattice fringe by HRTEM images, and the low crystallinity of CueNCs also caused the coarser surfaces of Cu/ BTN as compared with the pristine BTN (Fig. 4b). HRTEM images (Fig. 4e and f) show the loaded CueNCs have slightly increased particle sizes and denser distribution on BTN surfaces with enhancing the Cu-loading content, but the sizes of those CudNCs are still smaller than 3.0 nm even for 5% Cu/BTN and 10% Cu/BTN. This phenomenon is in good accordance with the results of XRD

Fig. 4. TEM and HRTEM images of the pristine BTN (a), 1.0% Cu/BTN (b, d), 5.0% Cu/BTN (c, e), and 10% Cu/BTN (f) composites.

J. Liu et al. / Materials Today Chemistry 1-2 (2016) 23e31

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Fig. 5. XPS spectra of 1.0% Cu/BTN composite and its recycled product after 20 h photoreactions in 10 vol% TEOA aqueous suspension (pH4.70).

patterns that the diffraction peak intensity of Cu in Cu/BTN composites are slightly increased with enhancing the Cu contents. The coupling between the two components can be observed from the HRTEM image shown in Fig. 4f. An interplanar distance of 0.216 nm can be determined from the decorated quasi-spherical particle, which corresponds to the (111) lattice of metallic Cu fcc structure [35], and the nanocube-like region has an interplanar distance of 0.352 nm that matched well to (210) planes of brookite [19,20]. It can be conjectured that a strong interaction between CueNCs and brookite TiO2 occurs, and those small CueNCs with high surface area and dispersivity can offer a shorter charge transfer pathway for quick electron trapping [35], which may improve the photoactivity for H2 production. To further confirm the existential state of Cu species on BTN surfaces, X-ray photoelectron spectrum (XPS) analyses are conducted and shown in Fig. 5. The survey XPS spectrum (Fig. 5a) indicates that 1.0% Cu/BTN contains Cu, Ti and O elements, and its high resolution Ti 2p XPS spectrum (Fig. 5b) shows two peaks at binding energy of 458.5 and 464.3 eV, which can be ascribable to Ti 2p3/2 and Ti 2p1/2, respectively. These binding energy values are very close to the reported ones [20,36], indicating that the valence state of Ti is þ4. The O 1s XPS spectrum (Fig. 5c) can be deconvolved as two peaks at 531.9 and 529.8 eV, which can be ascribable to TieOH and oxygen anions in brookite TiO2 lattice (TieOeTi) [20,37], respectively. The Cu 2p spectrum (Fig. 5d) of 1.0% Cu/BTN shows two peaks at 932.2 and 952.0 eV, which match with the binding energy values of Cu 2p3/2 and Cu 2p1/2 of Cu0 [35,37,38], respectively. The small shoulder at ~934.2 eV on the higher binding energy side of Cu 2p3/2 might imply the presence of a very small

amount of Cu2O [4], while the unobvious peak at ~943.0 eV can be ascribed to the satellite peak characteristic peak of Cu2þ (Fig. 5d), indicating the possibility of existence of finite CuO [4,35]. Possibly, the presences of CuI and CuII from a thin layer on the metallic CueNCs surfaces, which likely forms during the sample transfer [35,39]. Nevertheless, the two intense peaks at 932.2 and 952.0 eV can be observed from Fig. 5d, confirming that 1.0% Cu/BTN is mainly contained metallic Cu species rather than Cu2O or CuO [4,35]. Zou and co-workers [38] synthesized Cu-loaded rutile TiO2 and confirmed the present of Cu and CuO by XPS, whereby the Cu components loaded on the TiO2 nanosheets are metallic Cu with small amount of CuO due to the oxidation of metallic Cu during the XPS measurement. The elemental analysis results of X-ray fluorescence (XRF) are listed in Table 1. As can be seen, the Cu content in various Cu/BTN composites is approximate to the corresponding initial addition amount, suggesting that the Cu2þ can be completely reduced to metallic Cu nanoclusters with very finite loss during the present chemical reduction process. Moreover, the pristine BTN has Ti:O atom ratio of 1:2, equals to the theoretical stoichiometric ratio of TiO2. All Cu/BTN composites with different Cu contents have a Ti:O atom ratio similar to the pristine BTN as shown in Table 1, indicating that the addition of metallic Cu species has limited influence on the chemical composition of BTN. In addition, the oxygen content in the Cu/BTN composites is increased when enhancing the Cu-loading contents to larger than 2.0%, implying that the higher Cu loading would lead to the partial oxidization of those CueNCs, which is in good accordance with the above XPS analyses results.

Table 1 Element compositions of BTN and its Cu-loaded products containing different Cu contents detected by XRF spectrometer. Sample

Pristine BTN

0.5% Cu/BTN

1.0% Cu/BTN

2.0% Cu/BTN

5.0% Cu/BTN

10% Cu/BTN

20% Cu/BTN

Cu/mol% Ti/mol% O/mol% Ti:O atom ratio

e 1.25 2.50 1:2.00

0.45 1.24 2.49 1:2.00

0.98 1.24 2.48 1:2.00

1.90 1.23 2.47 1:2.01

4.87 1.19 2.43 1:2.04

9.70 1.14 2.37 1:2.08

18.47 1.09 2.31 1:2.10

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Fig. 6. UVevis diffuse reflectance absorption spectra (DRS) of Cu/BTN composites with different Cu contents.

3.2. Absorption spectrum analyses Fig. 6 depicts the UVevis diffuse reflectance spectra (DRS) of the pristine BTN and its Cu-loaded products (Cu/BTN). As can be seen, the pristine BTN exhibits a strong absorption band on the edge of 379 nm with the corresponding Eg value of ~3.27 eV, which is consistent with the previous reports [19,20]. Although the pristine BTN has almost no absorption in the region of 400e800 nm, all Cu/ BTN composites showed an elevating broad background in this region with enhancing Cu content. Different from the Cu/BTN composites with Cu content of less than 2.0% that just show broad elevated background but with no great change in the absorption edge, those Cu/BTN composites with higher Cu contents exhibit unobvious Cu plasmon extinction in the range of 400e600 nm with a broad peak at ~430 nm (Fig. 6), which shows a red-shift trend with variation in Cu content from 5.0% to 20%. As to why there is no obvious Cu plasmon peak for those Cu/BTN composites with lower Cu contents, the much smaller CueNCs might be easier to suffer from the surface oxidation because any slight surface oxidation is detrimental to the surface plasmon properties [40]. Furthermore, it

was reported that the formation of surface Cu oxide will show up as an absorption peak in l > 600 nm region [40]. Notably, the present Cu/BTN composites exhibit increasing peaks of such surface Cu oxide absorption with enhancing the Cu content from 0.5% to 20%, indicating the conversion to Cu oxide of CueNCs in the present Cu/ BTNs composites. Usually, surface plasmon resonance in metallic nanoparticles involves dipolar oscillations of the free electrons in the metallic CB near the Fermi level, and this phenomenon has predicted the linear dependency of plasmon peak position on the average particle size, subject to internal scattering limits [40]. Therefore, it can be conjectured that the present CueNCs in Cu/BTN composites gradually grow up with enhancing the Cu contents followed by a deposition mechanism as follows: nucleation and reduction of Cu occurs when NaBH4 solution is added to the BTN suspension, forming unstable Cu0 monomers that closely attach themselves to BTN surfaces. A low Cu2þ concentration benefits the formation of discrete CueNCs, while a high Cu2þ concentration leads to more Cu0 monomers deposited on BTNs. With further enhancing Cu2þ concentration, the Cu2þ ions can be directly reduced on the deposited CueNCs to promote the growth of CueNCs. HRTEM images (Fig. 4e and f) show the loaded CueNCs have slightly increased particle sizes and denser distribution on BTN surfaces with enhancing the Cu-loading content even though the particle sizes are still smaller than 3.0 nm in 5% Cu/BTN and 10% Cu/BTN. It indicates that the present chemical reduction process is beneficial for the formation of Cu nanoclusters on BTN surfaces.

3.3. Effects of photoreaction conditions on the photoactivity of Cu/ BTN composites Photocatalytic H2 production reaction conditions are firstly optimized by using an aqueous suspension containing photocatalyst (10 mg) and 10 vol% TEOA solution (10 mL) under Xelamp full-spectrum irradiation. Fig. 7a depicts the effect of Culoading content on the photoactivity for H2 production over BTNs.

Fig. 7. Effects of photoreaction conditions on the photoactivity for H2 production over Cu/BTN composites. Cu-loading contents (a), sacrificial reagents (10 vol% TEOA, 10 vol% MeOH, 10 mM EDTA, 50 mM AA, and water) (b), pH values of the photoreaction system (c), photocatalyst dosages (d). Conditions: 1.0% Cu-loading, 10 mg catalyst, 10 mL 10 vol% TEOA aqueous solution, if otherwise stated.

J. Liu et al. / Materials Today Chemistry 1-2 (2016) 23e31

As can be seen, the pristine BTN only shows a photocatalytic H2 production activity of 17.7 mmol h
1, which then increases from 43.8 to 90.0 mmol h
1 with enhancing the Cu content from 0.5% to 1.0%. Once the Cu content is larger than 1.0%, the photoactivity shows a slight decreasing trend, and thus a maximum photocatalytic H2 production activity (90 mmol h
1) is obtained over 1.0% Cu/BTN. This enhancement in photoactivity with enhancing the Culoading content can be attributed to the loaded CueNCs acting as co-catalyst on BTNs, which favors the more effective separation of the photogenerated carriers; while excessive Cu loading could result in CueNCs growing and denser distribution on BTN surfaces (Fig. 5), which in turn weaken the above co-catalyst functions. Also, the denser distribution of CueNCs on BTN surfaces would absorb and scatter the incident light, which can cause the Cu/BTN composites unable to be excited effectively, and hence the reduced activity. Consequently, the 1.0% Cu/BTN provides a better activity as compared to the other Cu/BTN composites possibly because this particular case with the small CueNCs has high surface area and dispersion to offer a shorter charge transfer pathway for quick electron trapping [35], and therefore the following studies are focused on 1.0% Cu/BTN. It has been reported that the activity can be efficiently affected by various electron donors [41]. Fig. 7b depicts the activity of 1.0% Cu/BTN in the presence of the commonly used sacrificial reagents such as triethanolamine (TEOA), methanol (MeOH), disodium ethylenediamine tetraacetic acid (EDTA), and ascorbic acid (AA) under light irradiation. As can be seen, the addition of TEOA and MeOH as electron donors can improve the activity, and TEOA solution exhibits the highest H2 production activity (90 mmol h
1). However, AA and EDTA solution lead to an obvious decrease in the activity, and AA as electron donor even gives trace H2 production. Usually, the differences in reaction rate, decomposition route and products of sacrificial reagent can influence the H2 production activity [42]. What's more, different sacrificial reagents added can lead to different pH values of the photocatalyst suspension, and thus affect the contact ability with photocatalyst and the charge transfer for the photoreactions. Therefore, the effects of pH values of TEOA solution on the H2 production activity over 1.0% Cu/BTN were investigated and shown in Fig. 7c. The initial pH value of TEOA solution in the present photoreaction system is 10.50, in which 1.0% Cu/BTN exhibits an activity of 90 mmol h
1. When the pH value of photoreaction system was changed from 10.50 to 7.00 using HCl solution, the activity only shows limited fluctuation, indicating that Cu/BTN is less sensitive to the pH value in an alkaline condition. Once the TEOA solution was adjusted to acidity, the activity improved greatly, and 1.0% Cu/BTN exhibits the maximum H2 production activity (211 mmol h
1) when pH was adjusted to 4.70, which is ~2.3 times higher than that (90 mmol h
1) of initial pH value. Further adjusting the solution to stronger acidity, the activity dropped rapidly. This phenomenon might be due to following reasons: (1) The surface properties and adsorption capability of the photocatalyst (such as the eOH concentration and the surface charge density) and the shift of the bandgap energy depend on the change in pH values; (2) pH value influences the redox potential of Hþ/H2 and the Fermi level of Cu; (3) Metallic CudNCs may become unstable in either strong acidic or strong alkaline reaction condition. The effect of 1.0% Cu/BTN additive amount on the activity for H2 production is shown in Fig. 7d. As can be seen, the H2 production activity increases twice (from 107 to 211 mmol h
1) when the additive amount was enhanced from 5 to 10 mg, and then slightly increases to 225 mmol h
1 with enhancing the additive amount to 15 mg. Once the Cu/BTN additive amount exceeds 15 mg, the activity again decreases to 195 mmol h
1. Namely, 15 mg photocatalyst in the present photoreaction system can give a maximum H2

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production activity (225 mmol h
1). The possible reason may be that insufficient photocatalyst is incapable of utilizing the incident photons fully and efficiently, and the H2 production activity increases with enhancing the photocatalyst amount; while only a small portion of photocatalyst near the photoreactor wall can completely adsorb the incident light even at higher catalyst amount in the present outer-irradiation-type photoreactor with suspension system. Therefore, an optimal photoreaction conditions for the present Cu/BTN composites should be 15 mg photocatalyst with 1.0% CueNCs dispersing in TEOA aqueous suspension (pH4.70). 3.4. Photocatalytic activity and stability analyses of Cu/BTN composites Comparative experiment results (Fig. 8) of 1.0% Cu/BTN, 1.0% Cu/ P25, and 1.0% Pt/BTN under the optimal reaction condition indicate that 1.0% Cu/BTN exhibits an activity of 225 mmol h
1, which is similar to that (220 mmol h
1) of the benchmark P25 but slightly lower than that (293 mmol h
1) of 1.0% Pt/BTN. Anyway, the maximum activity of 225 mmol h
1 over 1.0% Cu/BTN is 5.2 times higher than that (42.5 mmol h
1) of the BTN alone. These results not only demonstrate the brookite TiO2 would be a potential effective catalyst for H2 production, but also provides an inexpensive and high-efficient means of enhancing H2 production by using metallic Cu nanoclusters alternative to the commonly used noble metal cocatalyst. Fig. 9 shows the relatively steady activity of 1.0% Cu/BTN under an optimal photoreaction conditions such as 15 mg photocatalyst in 10 mL of 10 vol% TEOA aqueous solution (pH4.70), which was investigated in four consecutive runs of accumulatively 20 h under UVevis light irradiation with the sacrificial reagent solution periodically being replaced in each run. As can be seen, no noticeable decrease in the activity for H2 production can be observed for the present photocatalyst after 20 h irradiation. The slight drop in the H2 production activity may be due to the consumption of the sacrificial reagent in the reaction mixture over the time, since the concentrations of sacrificial reagent largely affected the performance of H2 production. The stable H2 production rate in acid solution (pH4.70) indicates that the present Cu/BTN composites have considerable stability during the photoreaction processes, which can be confirmed by the following XPS results of the recycled 1.0% Cu/BTN after 20 h photoreaction shown in Fig. 5. The survey XPS spectrum (Fig. 5a) of the recycled 1.0% Cu/BTN is essentially similar to that of the original one, and there is no obvious deviation for the locations of the main elements. Moreover, both of the high resolution Ti 2p XPS spectra (Fig. 5b) for 1.0% Cu/

Fig. 8. Comparison of the photoactivity for H2 production over the pristine BTN, 1.0% Cu/BTN, 1.0% Cu/P25, and 1.0% Pt/BTN. Conditions: 15 mg photocatalyst, 10 mL 10 vol% TEOA aqueous solution (pH4.70), if otherwise stated.

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Fig. 9. Typical time course for H2 production over 1.0% Cu/BTN composite under 300 W Xe-lamp irradiation. Conditions: 15 mg photo-catalysts in 10 mL 10 vol% TEOA aqueous solution (pH4.70).

BTN and its recycled one have similar doublet Ti4þ peaks, indicating the stable electron structure of BTN [20]. Although the O 1s XPS spectra (Fig. 5c) seem to change a lot in shape after 20 h irradiation, the positions of XPS peaks for the recycled one remain at the binding energies of 532.0 and 529.9 eV, which can be related to the hydroxyl adsorbed on the surface and TieO bonds in brookite TiO2, respectively. The increase of hydroxyl peak at 532.0 eV can be ascribed to the hydroxyl adsorption on BTN surfaces during the photoreaction process. As shown in Fig. 5d, both the unobvious satellite peak characteristic peak of Cu2þ at ~943.0 eV and the small shoulder at ~934.2 eV for the original 1.0% Cu/BTN disappeared after 20 h irradiation (Fig. 5d), indicating those finite oxidized species on CueNCs can be removed and CueNCs are not easy to be oxidized by those photoinduced holes of BTN during the photoreaction process. Nevertheless, no evident drop in the H2 production amount can be observed during the recycling runs as shown in Fig. 9. It can be concluded that those metallic CueNCs decorated on BTN surfaces have considerable photochemical stability, those loaded CueNCs but not its oxidized species are responsive to the enhanced H2 production performance of brookite TiO2. 3.5. Discussion on the photocatalytic mechanism of Cu/BTN composites Usually, the acceptance of the photogenerated charge by a metal co-catalyst is strongly related to the size of the metal nanoparticles because the interfacial electronic states relative to the CB position are a function of the size of the metal nanoparticles [35]. In the present Cu/BTN composites, the size and density of CueNCs on BTN surfaces are affected by the Cu-loading contents as-mentioned above, and the particular case in 0.1% Cu/BTN provides small Cu nanoclusters with high surface area and dispersion, which can offer a shorter charge transfer pathway for quick electron trapping of BTN, and then causing an enhanced H2 production activity. In addition, metallic Cu acting as co-catalyst has a more positively leveled work function (ca. 4.65 eV) compared to the CB level (4.26 eV) of brookite TiO2 [34], indicating that the charge transfer from the brookite TiO2 to the CueNCs is thermodynamically favorable. Therefore, a possible mechanism for the enhanced H2 production over Cu/BTN can be proposed as follows. In an initial step of charge carrier generation, BTN decorated with CueNCs is excited by light irradiation; photons with higher energy than the bandgap of BTN generate electron-hole pairs on Cu/BTN. Next, those CueNCs closely contacted with BTN surfaces act as the electron acceptor and suppress the photoexcited carrier recombination due to enhanced charge separation at the Cu/BTN

interface. Finally, the H2 is more effectively performed on the CueNCs decorated BTN surfaces due to the transfer of electrons from the CueNCs to the adsorbed proton/water. At the surfaces of Cu/BTN composite, the photogenerated holes may react with the adsorbed water to produce oxygen, but this reaction is usually difficult due to the more complex water oxidization reaction, and the accumulation of holes on BTN surfaces may lead to an increase in surface charge recombination and thus decreasing the product formation after certain period of irradiation time. After addition of electron sacrificial reagent such as TEOA, the photocatalytic stability for H2 production can be obtained as shown in Fig. 9. The above assumption on the charge transfer trend can be validated by the photoluminescence (PL) spectra of the pristine BTN and 1.0% Cu/BTN shown in Fig. 10. As can be seen, those emission peaks in the range of 350e550 nm can be attributed to the bandgap excitation of brookite TiO2, and a significant PL quenching is observed after loading with 1.0% CueNCs. It can be mainly due to the fact that the electrons are excited from the VB to the CB of brookite TiO2 and then transfer to CueNCs, which prevents the direct recombination of those photoexcited carriers., and resulting in a lower PL intensity as compared to BTN alone. Also, the photocurrent can be used to evaluate the separation efficiency of the photoexcited charge carriers since it is stemmed from the photogenerated electrons in the CB of photocatalyst under light irradiation. Transient photocurrent responses (Fig. 11) of the pristine BTN and 1.0% Cu/BTN were recorded for several on-off cycles under Xe-lamp full spectrum irradiation. Upon light illumination, the photocurrent sharply increases to a steady state, and then returns to its dark current state once the light is turned off. Cu/BTN shows higher photocurrents than the pristine BTN, revealing that the photogenerated charge of Cu/BTN can be more efficiently separated to retard the charge recombination as compared to the BTN alone, and thus causing the improved activity for H2 production over Cu/BTN. Apparently, those small sized CueNCs closely decorated on BTN particles can not only act as efficient and stable co-catalyst to increase the electron acceptance and transport rates in the Cu/BTN composite, but also provide high surface area and dispersion to offer a shorter charge transfer pathway for quick electron trapping of BTN, and then the photogenerated carriers recombination can be efficiently suppressed. The enhanced activity and the proposed mechanism of CueNCs decorated BTN provide an opportunity for future development of high-efficient photocatalyst based on brookite TiO2, which has seldom reported previously [20]. Moreover, the present metallic Cu nanoclusters are shown to be a promising alternative to the commonly used noble metal cocatalyst to provide an inexpensive, efficient and stable means of

Fig. 10. Photoluminescence (PL) spectra of the pristine BTN and 1.0% Cu/BTN composite under excitation wavelength of 290 nm.

J. Liu et al. / Materials Today Chemistry 1-2 (2016) 23e31

31

References

Fig. 11. Photocurrent-time curves of the pristine BTN and 1.0% Cu/BTN composite in NaOH solution under 300 W Xe-lamp irradiation.

enhancing light-to-hydrogen semiconductors.

energy

conversion

over

4. Conclusion A series of brookite TiO2 quasi nanocubes (mean size of ~50 nm) decorated with Cu nanoclusters (CueNCs) are synthesized by using a facile chemical reduction process. The size and density of CueNCs on BTN surfaces are affected by the Cu-loading contents, and 0.1% Cu/BTN composite provides Cu nanoclusters with small size of ~1e2 nm, high surface area and dispersion, and then exhibits the maximum H2 production activity (225 mmol h
1) under optimal photoreaction condition, which is not only similar to that (220 mmol h
1) of the benchmark photocatalyst (P25), but also 5.2 times higher than that (42.5 mmol h
1) of the BTN alone. This significant enhancement in the activity of BTN is deemed to result from the metallic CueNCs with small size, high surface area and dispersion, which favor the co-catalyst functions to offer a shorter charge transfer pathway for quick electron trapping of BTN, and thus causing an effective charge separation in space and an improvement in the photocatalytic activity and stability for H2 production. The present results not only demonstrate the brookite TiO2 would be a potential effective photocatalyst for H2 production, but also provide an inexpensive, efficient and stable means of enhancing light-to-hydrogen energy conversion by using metallic Cu nanoclusters alternative to the commonly used noble metal cocatalyst.

Acknowledgements The present work is financially supported by the Natural Science Foundation of China (21573166, 21271146, 20973128, and 20871096), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (SBK2015020824), China.

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