Photocatalytic H2 Generation Efficiencies of TiO2 Nanotube-based

Aug 24, 2017 - TiO2 nanotube-based heterostructures grafted with ZnO nanorods, Ag nanoparticles, or Pd nanodendrites were synthesized for photocatalyt...
0 downloads 5 Views 5MB Size
Download PDF
Article pubs.acs.org/JPCC

Photocatalytic H2 Generation Efficiencies of TiO2 Nanotube-based Heterostructures Grafted with ZnO Nanorods, Ag Nanoparticles, or Pd Nanodendrites Yi-Ching Huang,† Shou-Yi Chang,*,‡ and Jih-Mirn Jehng§ †

Metrology Analysis Division, National Nano Device Laboratories, Hsinchu 30078, Taiwan Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan § Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan ‡

S Supporting Information *

ABSTRACT: TiO2 nanotube-based heterostructures grafted with ZnO nanorods, Ag nanoparticles, or Pd nanodendrites were synthesized for photocatalytic H2O/CH3OH splitting and H2 generation. Compared with P25 TiO2 nanoparticles and bare TiO2 nanotubes, these heterostructures, in particular, the one grafted with Pd nanodendrites, were found to present a markedly enhanced photocatalytic H2 generation efficiency (net H2 generation rate ∼143 μmol/h). Rather than the surface area of the photocatalysts, the lifetime (separation) of photogenerated carriers and, in particular, the surface plasmon resonance-stimulated carrier excitation dominated the number of total effective carriers. A power relationship with an exponent of 0.2 between the H2 generation rate and the number of total photogenerated carriers was determined, which suggests that the number of effective carriers or the efficiency in H2O/CH3OH splitting might decay in an exponential way. electron trappers)16−21 have afterward been developed to improve their photocatalytic activity. For heterostructures, in addition to the effects of large surface area, an inhibited electron−hole recombination (extended carrier lifetime) and the strong surface plasmon resonance (SPR) on metal surfaces (hot spots) will also enhance surrounding electric fields for stimulating the excitation of electrons-holes in adjacent semiconductors and will induce a local heating effect for accelerating surface chemical reactions.27−31 Moreover, the SPR effect will expand the light absorption range of photocatalyses to visible wavelengths.15,32 The photocatalytic H2 generation of bare semiconductors (e.g., commercial P25) and doped/dye-sensitized semiconductors has been widely investigated in recent years.12,33,34 However, the correlation of photocatalytic H2 generation efficiency to the photoexcitation of carriers by various heterostructures has not been thoroughly examined. Hence in this study, three TiO2−NT-based heterostructures were prepared (syntheses developed in our previous studies):16,17 grafted with ZnO nanorods (NRs, denoted as sample TiO2− NT/ZnO−NR) to investigate the effect of other semiconductor material with a different energy bandgap, ZnO NRs and Ag

1. INTRODUCTION Hydrogen, a clean recyclable fuel, can be produced by splitting H2O and is a high potential sustainable energy. However, the high electric potential of 1.6 to 1.9 V required for the uphill H2O splitting reaction limits the effectiveness of H2 generation via a direct electrolysis route.1,2 Since the 1970s, semiconductor photocatalysts (e.g., TiO2) that are conventionally adopted for pollutant degradations have been discovered to assist in H2O splitting under ultraviolet (UV) light exposure, attributed to the photoexcited electron-holes.3−7 Since then, photocatalytic H2O splitting, particularly under the exposure of visible solar light, to harvest ample clean H2 energy as an “artificial photosynthesis” has thus attracted great attention.8−13 More efficient photocatalysts14−21 and hole-sacrificial additives (e.g., S2−/SO32−, I3−, or methanol (MeOH))22−26 have been intensively developed to increase the efficiency of H2 generation. TiO2-based photocataysts in the form of nanoparticles (NPs, e.g., P25) or nanotubes (NTs) have been early developed for applications to pollutant degradations, gas sensing, surfaceenhanced Raman scattering, and so on.3−7 Bare semiconductor NPs or NTs, however, yield a low photocatalytic activity due to the easy recombination of photoexcited electron−hole pairs.4 Surface modifications, minor dopings, and, in particular, the syntheses of heterostructures decorated with other nanosized semiconductors (e.g., SnO2 or ZnO for electron−hole separation)14−16 or noble metals (e.g., Au, Ag, Pt, or Pd as © XXXX American Chemical Society

Received: June 13, 2017 Revised: August 8, 2017

A

DOI: 10.1021/acs.jpcc.7b05806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. SEM images of TiO2 NT-based nano/heterostructures (samples preparation referred to the synthesis methods developed in previous studies):16,17 (a) bare TiO2 NTs (sample TiO2−NT); (b,c) TiO2 NTs grafted with ZnO NRs (sample TiO2−NT/ZnO−NR); (d,e) TiO2 NTs grafted with ZnO NRs/Ag NPs (sample TiO2−NT/ZnO−NR/Ag−NP); and (f−h) TiO2 NTs grafted with Pd NDs (sample TiO2−NT/Pd−ND) at top surface and tube openings. Notes: NT: nanotube, NR: nanorod, NP: nanoparticle, ND: nanodendrite.

Figure 2. H2 generation amounts in a blank experiment without a photocatalyst and catalyzed experiments in the presence of TiO2 NPs or TiO2 NTbased nano/heterostructures: (a) gross amount and (b) net amount.

NPs (sample TiO2−NT/ZnO−NR/Ag−NP) to study the contribution of noble metal particles with a strong SPR effect, or Pd nanodendrites (NDs, sample TiO2−NT/Pd−ND) to examine the influence of geometry of noble metal on the SPR effect, as seen in the scanning electron microscopic (SEM) images in Figure 1. They were further used for photocatalytic H2O/MeOH splitting and H2 generation. Photocatalytic H2 generation with commercial P25 TiO2 NPs (sample TiO2−NP) and bare TiO2 NTs (sample TiO2−NT) was also tested for comparison. Importantly, the numbers of total photogenerated carriers (in consideration of the surface areas and light absorption of the heterostructures, the lifetimes of electron− hole pairs, and the SPR effect) were estimated. The relationship between the H2 generation rates and the numbers of carriers was examined.

ZnO−NR: anatase TiO2 NTs grafted with ZnO NRs (rod diameter ∼30 nm) by immersing TiO2 NTs in a ZnO synthesis solution consisting of Zn(NO3)2 and (CH2)6N4 at 70 °C;16 (4) TiO2−NT/ZnO−NR/Ag−NP: anatase TiO2 NTs grafted with ZnO NRs and Ag NPs (particle size ∼30 nm, spacing ∼20 nm) by sputtering an Ag layer on TiO2−NT/ZnO−NR and annealing the sample at 300 °C for Ag spheroidization;16 and (5) TiO2−NT/Pd−ND: anatase TiO2 NTs grafted with Pd NDs (arm length ∼ several micrometers, thickness ∼100 nm; Pd NPs also covered the surface of TiO2 NTs, size ∼30 nm, spacing ∼6 nm) by immersing TiO2−NT/ZnO−NR in a sensitization solution of SnCl2·2H2O, HCl, and H2O and in an activation solution of PdCl2, HCl, and H2O at 75 °C in sequence.17 According to the previous study,17 when TiO2− NT/ZnO−NR was immersed in the sensitization solution containing SnCl2 and HCl, the ZnO NRs would dissolve into the solution, and the Sn2+ ions and Zn2+ ions would adsorb the TiO2 NT surfaces, which would change the potential (electric field) distribution adjacent to the surface of the TiO2 NTs. When the sample was subsequently immersed in the activation solution containing PdCl2, the Sn2+ ions would oxidize to Sn4+ ions, and the Pd2+ ions would be reduced to grow Pd dendrites (along with some Pd and SnO2 NPs). During the growth of dendrites, the Zn2+ ions and Sn4+ ions were incorporated into Pd, leading to the formation of (Zn,Sn)-doped Pd dendrites.

2. EXPERIMENTAL SECTION Five TiO2-based nano/heterostructure samples for photocatalytic H2O splitting and H2 generation tests were prepared, as shown in the SEM (JEOL JSM-6700F) images in Figure 1, including (1) TiO2−NP: commercial P25 TiO2 NPs (Aeroxide, Germany; particle size ∼25 nm); (2) TiO2−NT: bare anatase TiO2 NTs (inner/outer diameters ∼190/250 nm), obtained by anodizing Ti foils in an electrolyte containing C4H10O3 (diethylene glycol, (HOCH2CH2)2O), NH4F, and H2O and then annealing the sample at 450 °C in air;16 (3) TiO2−NT/ B

DOI: 10.1021/acs.jpcc.7b05806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C The photocatalytic activities of the samples for H2O/MeOH splitting and H2 generation were measured using gas chromatography (GC, China Chromatography 9800; packed column: Porapak Q 2.25 m, carrier gas: argon, current: 60 mA). The amount of H2 was measured when an H2O/MeOH solution (2:1, in a quartz tube, at 50 °C)23 was exposed to simulated light (AM 1.5; from a 300 W Xe lamp, maximum intensity of 203.3 mW/cm2 at wavelength of 365 nm) in the presence of 1 mg P25 TiO2 NPs or ∼1 × 1 cm2 TiO2 NT-based nano/heterostructures (∼1 mg). One c.c. gas was taken from the headspace of the quartz tube using an injector and was injected into the GC for the analysis of H2 (the carrier gas, argon, was used only in the GC). A blank sample without the presence of photocatalyst was tested for calibration. Repeated tests were conducted (in a new H2O/MeOH solution) using recycled samples. The light absorption spectra of the samples were detected by UV−vis spectrometry (PerkinElmer Lambda 800, with the integrating sphere) at 200−800 nm in a diffuse reflectance mode. The surface areas of the samples (1 × 1 cm2, including the Ti plate) were measured at −196 °C using a surface area and porosity analyzer (Micromeritics ASAP 2020) by the Brunauer−Emmett−Teller (BET) method in the relative pressure range of 0.03 to 0.2.

sample

surface 460 530 750 980 1420

gross

net

460 230 450 680 1120

100.6 118.6 145.0 177.2 207.9 243.7

0 18.0 44.4 76.5 107.3 143.1

(1)

Conduction Band: (2)

Valence Band: H 2O + h+ → ·OH + H+

(6)

HCOOH → CO2 + H 2

(7)

(8)

(9)

It is noted that in the blank experiment a certain amount of H2 was generated as well (increasing with time), owing to the splitting of a part of MeOH (required energy ∼0.7 eV) under UV-light exposure.22 According to the literature,22 without the presence of catalysts, the direct splitting of MeOH to generate H2 (CH3OH → HCHO + H2) requires a high activation energy of 64.1 kJ/mol, but the splitting of H2O/MeOH mixture to generate H2 (CH3OH + H2O → CO2 + 3H2) requires a relatively low activation energy of only 16.1 kJ/mol. It is hence expected that, when exposed to the strong UV light from the high-power Xe lamp, the H2O/MeOH mixture might very possibly be rapidly split to yield the high H2 generation rate in the blank experiment. However, it is clear that the net (gross − blank) H2 generation rate was obviously raised in the presence of the photocatalysts. The net H2 generation rate of TiO2−NP, 18 μmol/h, is close to the reported value of commercial P25 TiO2 NPs, 17 μmol/h,24−26 confirming the accuracy of measurements. The TiO2 NT-based heterostructures, in particular, sample TiO2−NT/Pd−ND, present a markedly superior photocatalytic activity to the bare TiO2 nanostructures: The net H2 generation rate, 143.1 μmol/h, is eight times that of sample TiO2−NP, attributed to several factors that will be addressed below. In repeated experiments using recycled heterostructure samples, the same levels of net H2 generation rates were detected, which additionally suggests a high recyclability and reusability of the heterostructure catalysts. By examining the specific surface areas and solution contact areas of the TiO2 NT-based nano/heterostructures, as listed in Table 1 (the specific surface areas were measured by using the BET method and calibrated by excluding the surface area of Ti plate and the surface roughness of TiO2 NTs; the solution contact areas exclude the outer surface areas of the NTs by assuming that liquid reaches only the inside but not the outside of the NTs; see Supporting Information 1 for detailed estimations), a fact is reflected: Surface area (the number of reaction sites) is not the only parameter to determine the H2 generation efficiency because obviously sample TiO2−NP has a middle level of surface area but yields the worst efficiency. Alternatively, the effectiveness of photocatalysts in carrier generation involves the SPR-induced expansion of light absorption range, the SPR-stimulated excitation of electron− hole pairs,15,29−32 and the heterostructure-assisted separation (extension of lifetime) of excited pairs. From the absorbance spectra in Figure 3, the expansion and red shift of light absorption range of the TiO2 NT-based heterostructures to visible wavelengths is clear. The energy bandgaps Ea were calculated using the popular equation Ea = 1240/λa (λa is the wavelength of absorption edge)23 and were found to decrease from 3.26 eV for the TiO2 NTs to 3.10 eV for the heterostructures. The light absorbance of the heterostructures

lists the H2 generation rates in a blank experiment without a photocatalyst and in catalyzed experiments in the presence of P25 TiO2 NPs or TiO2 NT-based nano/heterostructures. When semiconducting TiO2 (or ZnO) receives light of enough energy,3 electron−hole pairs will be excited to split H2O/ MeOH for H2 generation4,22 following the anticipated chemical reactions given below

2H+ + 2e− → H 2

HCHO + H 2O → HCOOH + H 2

H 2O + CH3OH + e− + h+ → CO2 + 3H 2

a See the Supporting Information 1 for detailed estimations of specific surface areas and solution contact areas. Note: “cm2/mg” is used as the unit of specific area for consistency with the units of other items; 1 cm2/mg = 0.1 m2/g.

TiO2 (or ZnO) + hν → TiO2 (or ZnO) + e− − h+

(5)

Overall:

H2 generation rate (μmol/h)

contact

·CH 2OH → HCHO + H+ + e−

H 2O + CH3OH + h+ → 2H+ + CO2 + 2H 2 + e−

Table 1. Specific Surface Areas and Solution Contact Areas of TiO2 NPs and TiO2 NT-based Nano/heterostructures and H2 Generation Rates in a Blank Experiment without a Photocatalyst and Catalyzed Experiments in the Presence of TiO2 NPs or TiO2 NT-based Nano/heterostructuresa

blank TiO2−NP TiO2−NT TiO2−NT/ZnO−NR TiO2−NT/ZnO−NR/Ag−NP TiO2−NT/Pd−ND

(4)

Net:

3. RESULTS AND DISCUSSION Figure 2 presents the gross and net amounts of H2 generation as an H2O/MeOH solution was exposed to light, and Table 1

area (cm2/mg)

CH3OH + ·OH → ·CH 2OH + H 2O

(3) C

DOI: 10.1021/acs.jpcc.7b05806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. Logarithmic plot of net H2 generation rate versus the number of total photoexcited carriers with TiO2 NPs and TiO2 NTbased nano/heterostructures (for TiO2−NP: including the number of carriers in a rutile phase, insufficient for H2O splitting and H2 generation). See Supporting Information 2 for estimations of the numbers of total photogenerated carriers.

Figure 3. Light absorption spectra of TiO2 NT-based nano/ heterostructures.

and, in particular, sample TiO2−NT/Pd−ND also markedly arose in the visible range, both of which are owing to an SPR effect and the diffuse reflection of light on the rough surface of the heterostructures.29,35−37 In general, the carrier generation rates by the semiconductors (TiO2 and ZnO) of the present nano/heterostructures upon light exposure can be given as G = αN0e−αx (/cm3s; x: light exposure depth; assuming a photon generates an electron−hole pair) by introducing the number of photons, N0 = 3 × 1017/cm2 s (at the wavelength of light, λ = 365 nm) and the light absorption coefficients of the semiconductors, α (/cm; given by literature).38 By further introducing the exposure areas, A (cm2), and carrier lifetimes, τ (s; by literature), as listed in Table 2, the numbers of photogenerated carriers are estimated as N = ∫ x00GτA dx = ∫ x00αN0e−ατA dx (x0: maximum light exposure depth ∼ the radius or half thickness of the TiO2 or ZnO).38 In consideration of the SPR enhancement factors of metal nanostructures for stimulating carrier excitations, |E(x)|2 ∝ e−2ksp″ x (|E(x)|: the SPR-induced electric field, k″sp: the imaginary wavevector; both are suggested by literature),30,31 the numbers of total photogenerated carriers are estimated to be N′e−−h+ = N × |E (x)|2, as presented in Table 2 and Figure 4 (see Supporting Information 2 for detailed estimations). Clearly, from the logarithmic plot of net H2 generation rates versus total numbers of carriers for the TiO2 NPs and the TiO2 NT-based nano/heterostructures given in Figure 4, a power relationship RH2 = k[N′e−−h+]m with an exponent m = 0.2 is determined that implies two facts. First, a proper heterostructural design of photocatalysts undoubtedly yields a high carrier excitation/sustention capability and thus a high H2 generation rate. As listed in Table 2 and schematically illustrated in Figure 5, the short lifetime of carriers and their

transfer to the rutile phase (as a passive electron sink owing to the lower conduction band than that of the anatase phase) with low H+ reduction potential39,40 result in the worst photocatalytic H2 generation efficiency of TiO2 NPs because the conduction band of the rutile phase is below the electric potential required for H2O splitting; however, it has also been reported that the electrons on the conduction band of the rutile phase may further transfer to the surface of the anatase phase via the low-energy “lattice-trapping sites” in the anatase phase.41 Nevertheless, owing to the insufficient surface potential of the anatase phase,41 the electrons are expected to not be capable of H+ reduction and H2 generation. By grafting ZnO NRs onto TiO2 NTs, light exposure and absorption will increase, and the lifetime of carriers will be much extended (by three times for electron−hole separation),42 leading to an increased number of effective carriers. With the further decoration of Ag NPs or Pd NDs, although the exposure area might decrease due to light shielding, a very strong SPR effect of the metal nanostructures30,43 is believed to significantly enhance the photoexcitation of carriers. Second, the exponent of the power relationship, that is, the reaction order, m, of 0.2 (rather than 1) suggests a complex reaction (rather than an elementary reaction)33 and two possible causes, which may be a reference for future studies. (1) The actual number of total effective carriers is much less than the estimated number due to the ineffectiveness of the photocatalysts or the loss of generated carriers in the solution. (2) One electron−hole pair is unable to split one H2O/CH3OH or to yield three H2 molecules (reaction 9). The number of total effective carriers or the efficiency in H2 O/CH 3OH splitting may decay in an exponential way. In addition, the redox reaction kinetics

Table 2. Carrier-Photoexcitation Efficiency Factors of TiO2 NPs and TiO2 NT-based Nano/Heterostructuresa sample factor

A

α

τ

SPR

TiO2−NP TiO2−NT TiO2−NT/ZnO−NR TiO2−NT/ZnO−NR/Ag−NP TiO2−NT/Pd−ND

0.9 1.0 1.4 1.2 0.5

0.8 1.0 1.0/2.2 1.0/2.2 1.0

0.3 1.0 3.3 6.1 2.1

1.0 1.0 1.0 122/368 581

N′e−−h+ 2.9 3.9 2.0 7.1 2.3

× × × × ×

1011 1012 1013 1014 1015

A: light exposure area, α: light absorption coefficients of semiconductors, τ: carrier lifetimes, SPR: SPR enhancement factors, N′e−−h+: numbers of carriers. See Supporting Information 2 for detailed estimations of the total numbers of carriers.

a

D

DOI: 10.1021/acs.jpcc.7b05806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 5. Schematic illustrations of energy bands and carrier transfers in TiO2 NPs and TiO2 NT-based nano/heterostructures: (a) energy bands in TiO2−NP, (b) energy bands in TiO2−NT/ZnO−NR, (c) energy bands in TiO2−NT/Ag−NP, (d) energy bands in ZnO−NR/Ag−NP, (e) energy bands in TiO2−NT/Pd−ND, (f) carrier transfers in TiO2−NT/ZnO−NR, (g) carrier transfers in TiO2−NT/ZnO−NR/Ag−NP, and (h) carrier transfers in TiO2−NT/Pd−ND.

■

would significantly influence the H2 generation rate and might be different with and without metal catalysts. Further investigations of the reaction kinetics are of great importance in the quantitative estimation of the power relationship between the H2 generation rate and the number of charge carriers.

4. CONCLUSIONS Commercial P25 TiO2 NPs, bare TiO2 NTs, and the TiO2− NT-based heterostructures grafted with ZnO NRs, ZnO NRs and Ag NPs, or Pd NDs were prepared for photocatalytic H2O/ CH3OH splitting and H2 generation. The heterostructures, in particular, the one grafted with Pd NDs, showed a much higher H2 generation rate than the TiO2 NPs and the bare TiO2 NTs. Rather than the surface area of the photocatalysts, the lifetime (separation) of carriers and, in particular, the SPR-stimulated carrier excitation dominate the final number of total effective carriers. A power relationship with a reaction order of 0.2 between the efficiency of H2 generation and the number of photogenerated carriers is determined, which implies an exponential decay in the number of effective carriers or H2O/ CH3OH splitting efficiency.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05806. Estimations of specific surface areas and solution contact areas and estimations of the number of total photogenerated carriers (electron−hole pairs) (PDF)

■

REFERENCES

(1) On Solar Hydrogen & Nanotechnology; Vayssieres, L., Ed.; John Wiley & Sons (Asia): Singapore, 2009. (2) Bowker, M. Sustainable hydrogen production by the application of ambient temperature photocatalysis. Green Chem. 2011, 13, 2235− 2246. (3) Diebold, U. The surface science of titanium dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (4) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced reactivity of titanium dioxide. Prog. Solid State Chem. 2004, 32, 33−177. (5) Allam, N. K.; Shankar, K.; Grimes, C. A. Photoelectrochemical and water photoelectrolysis properties of ordered TiO2 nanotubes fabricated by Ti anodization in fluoride-free HCl electrolytes. J. Mater. Chem. 2008, 18, 2341−2348. (6) Ghicov, A.; Schmuki, P. Self-ordering electrochemistry: a review on growth and functionality of TiO2 nanotubes and other self-aligned MO(x) structures. Chem. Commun. 2009, 20, 2791−2808. (7) Musumeci, A.; Gosztola, D.; Schiller, T.; Dimitrijevic, N. M.; Mujica, V.; Martin, D.; Rajh, T. SERS of semiconducting nanoparticles (TiO2 hybrid composites). J. Am. Chem. Soc. 2009, 131, 6040−6041. (8) Kato, H.; Asakura, K.; Kudo, A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J. Am. Chem. Soc. 2003, 125, 3082−3089. (9) Kudo, A.; Niishiro, R.; Iwase, A.; Kato, H. Effects of doping of metal cations on morphology, activity, and visible light response of photocatalysts. Chem. Phys. 2007, 339, 104−110. (10) Ritterskamp, P.; Kuklya, A.; Wustkamp, M. A.; Kerpen, K.; Weidenthaler, C.; Demuth, M. A titanium disilicide derived semiconducting catalyst for water splitting under solar radiation-reversible storage of oxygen and hydrogen. Angew. Chem., Int. Ed. 2007, 46, 7770−7774. (11) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253−278. (12) Hernández-Alonso, M. D.; Fresno, F.; Suárez, S.; Coronado, J. M. Development of alternative photocatalysts to TiO2: Challenges and opportunities. Energy Environ. Sci. 2009, 2, 1231−1257. (13) Miseki, Y.; Kato, H.; Kudo, A. Water splitting into H2 and O2 over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure. Energy Environ. Sci. 2009, 2, 306−314. (14) Zou, C. W.; Yan, X. D.; Han, J.; Chen, R. Q.; Bian, J. M.; Haemmerle, E.; Gao, W. Preparation and enhanced photoluminescence property of ordered ZnO/TiO2 bottlebrush nanostructures. Chem. Phys. Lett. 2009, 476, 84−88. (15) Jung, S.; Shuford, K. L.; Park, S. Optical property of a colloidal solution of platinum and palladium nanorods: Localized surface plasmon resonance. J. Phys. Chem. C 2011, 115, 19049−19053. (16) Huang, Y. C.; Chang, S. Y.; Lin, C. F.; Tseng, W. J. Synthesis of ZnO nanorod grafted TiO2 nanotube 3-D arrayed heterostructure as

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Shou-Yi Chang: 0000-0003-0595-4383 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support provided for this research by the Ministry of Science and Technology, Taiwan, under Grant No. MOST 102-2221-E-007-150-MY3. E

DOI: 10.1021/acs.jpcc.7b05806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C supporting platform for nanoparticle deposition. J. Mater. Chem. 2011, 21, 14056−14061. (17) Huang, Y. C.; Chang, S. Y.; Huang, L.; Lin, C. F. Growing metal trees on tubular semiconductor land: TiO2/(Zn,Sn)Pd heterostructures with high SERS and photocatalytic activity. J. Mater. Chem. A 2014, 2, 8456−8464. (18) Liu, R.; Sen, A. Controlled synthesis of heterogeneous metaltitania nanostructures and their applications. J. Am. Chem. Soc. 2012, 134, 17505−17512. (19) Gallo, A.; Marelli, M.; Psaro, R.; Gombac, V.; Montini, T.; Fornasiero, P.; Pievo, R.; Dal Santo, V. Bimetallic Au−Pt/TiO2 photocatalysts active under UV-A and simulated sunlight for H2 production from ethanol. Green Chem. 2012, 14, 330−333. (20) Ye, M.; Gong, J.; Lai, Y.; Lin, C.; Lin, Z. High-efficiency photoelectrocatalytic hydrogen generation enabled by palladium quantum dots-sensitized TiO2 nanotube arrays. J. Am. Chem. Soc. 2012, 134, 15720−15723. (21) Mao, L.; Wang, Y.; Zhong, Y.; Ning, J.; Hu, Y. Microwaveassisted deposition of metal sulfide/oxide nanocrystals onto a 3D hierarchical flower-like TiO2 nanostructure with improved photocatalytic activity. J. Mater. Chem. A 2013, 1, 8101−8104. (22) Choi, H.-J.; Kang, M. Hydrogen production from methanol/ water decomposition in a liquid photosystem using the anatase and rutile forms of Cu-TiO2. Int. J. Hydrogen Energy 2007, 32, 3841−3848. (23) Kitano, M.; Hara, M. Heterogeneous photocatalytic cleavage of water. J. Mater. Chem. 2010, 20, 627−641. (24) Wang, Q.; An, N.; Bai, Y.; Hang, H.; Li, J.; Lu, X.; Liu, Y.; Wang, F.; Li, Z.; Lei, Z. High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2. Int. J. Hydrogen Energy 2013, 38, 10739−10745. (25) Wang, W.; Liu, S.; Nie, L.; Cheng, B.; Yu, J. Enhanced photocatalytic H2-production activity of TiO2 using Ni(NO3)2 as an additive. Phys. Chem. Chem. Phys. 2013, 15, 12033−12039. (26) Pulido Melián, E.; González Díaz, O.; Ortega Méndez, A.; López, C. R.; Nereida Suárez, M.; Doña Rodríguez, J. M.; Navío, J. A.; Fernández Hevia, D.; Pérez Peña, J. Efficient and affordable hydrogen production by water photo-splitting using TiO2-based photocatalysts. Int. J. Hydrogen Energy 2013, 38, 2144−2155. (27) Ingram, D. B.; Linic, S. Water splitting on composite plasmonicmetal/semiconductor photoelectrodes: evidence for selective plasmoninduced formation of charge carriers near the semiconductor surface. J. Am. Chem. Soc. 2011, 133, 5202−5205. (28) Zhang, L.; Mohamed, H. H.; Dillert, R.; Bahnemann, D. Kinetics and mechanisms of charge transfer processes in photocatalytic systems: A review. J. Photochem. Photobiol., C 2012, 13, 263−276. (29) Zhang, X.; Chen, Y. L.; Liu, R. S.; Tsai, D. P. Plasmonic photocatalysis. Rep. Prog. Phys. 2013, 76, 046401. (30) Hou, W.; Cronin, S. B. A review of surface plasmon resonanceenhanced photocatalysis. Adv. Funct. Mater. 2013, 23, 1612−1619. (31) Chiu, N.-F.; Tu, Y.-C.; Huang, T.-Y. Enhanced sensitivity of anti-symmetrically structured surface plasmon resonance sensors with zinc oxide intermediate layers. Sensors 2014, 14, 170−187. (32) Seh, Z. W.; Liu, S.; Low, M.; Zhang, S. Y.; Liu, Z.; Mlayah, A.; Han, M. Y. Janus Au-TiO2 photocatalysts with strong localization of plasmonic near-fields for efficient visible-light hydrogen generation. Adv. Mater. 2012, 24, 2310−2314. (33) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503− 6570. (34) Montes-Navajas, P.; Serra, M.; Corma, A.; Garcia, H. Contrasting photocatalytic activity of commercial TiO2 samples for hydrogen generation. Catal. Today 2014, 225, 52−54. (35) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated absorption and scattering properties of gold nanoparticles of different size, shape, and composition: Applications in biological imaging and biomedicine. J. Phys. Chem. B 2006, 110, 7238−7248. (36) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Review of some interesting surface plasmon resonance-enhanced properties of

noble metal nanoparticles and their applications to biosystems. Plasmonics 2007, 2, 107−118. (37) Zhang, N.; Liu, S.; Fu, X.; Xu, Y.-J. Synthesis of M@TiO2 (M = Au, Pd, Pt) core−shell nanocomposites with tunable photoreactivity. J. Phys. Chem. C 2011, 115, 9136−9145. (38) Van Zeghbroeck, B. V. Principles of Semiconductor Devices and Heterojunctions; Prentice Hall: Upper Saddle River, NJ, 2009. (39) Gaya, U. I.; Abdullah, A. H. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems. J. Photochem. Photobiol., C 2008, 9, 1−12. (40) Liu, G.; Hoivik, N.; Wang, K.; Jakobsen, H. Engineering TiO2 nanomaterials for CO2 conversion/solar fuels. Sol. Energy Mater. Sol. Cells 2012, 105, 53−68. (41) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. Explaining the enhanced photocatalytic activity of degussa P25 mixed-phase TiO2 Using EPR. J. Phys. Chem. B 2003, 107, 4545−4549. (42) Ren, J.; Que, W.; Yin, X.; He, Y.; Asif Javed, H. M. Novel fabrication of TiO2/ZnO nanotube array heterojunction for dyesensitized solar cells. RSC Adv. 2014, 4, 7454−7460. (43) Thomas, R.; Swathi, R. S. Organization of metal nanoparticles for surface-enhanced spectroscopy: A difference in size matters. J. Phys. Chem. C 2012, 116, 21982−21991.

F

DOI: 10.1021/acs.jpcc.7b05806 J. Phys. Chem. C XXXX, XXX, XXX−XXX