Effective Charge Carrier Utilization in Photocatalytic Conversions

Apr 14, 2016 - Among various types of new energy resources, solar energy has been considered as one of the most promising choices, since it is clean, ...
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Effective Charge Carrier Utilization in Photocatalytic Conversions Peng Zhang, Tuo Wang, Xiaoxia Chang, and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China CONSPECTUS: Continuous efforts have been devoted to searching for sustainable energy resources to alleviate the upcoming energy crises. Among various types of new energy resources, solar energy has been considered as one of the most promising choices, since it is clean, sustainable, and safe. Moreover, solar energy is the most abundant renewable energy, with a total power of 173 000 terawatts striking Earth continuously. Conversion of solar energy into chemical energy, which could potentially provide continuous and flexible energy supplies, has been investigated extensively. However, the conversion efficiency is still relatively low since complicated physical, electrical, and chemical processes are involved. Therefore, carefully designed photocatalysts with a wide absorption range of solar illumination, a high conductivity for charge carriers, a small number of recombination centers, and fast surface reaction kinetics are required to achieve a high activity. This Account describes our recent efforts to enhance the utilization of charge carriers for semiconductor photocatalysts toward efficient solar-to-chemical energy conversion. During photocatalytic reactions, photogenerated electrons and holes are involved in complex processes to convert solar energy into chemical energy. The initial step is the generation of charge carriers in semiconductor photocatalysts, which could be enhanced by extending the light absorption range. Integration of plasmonic materials and introduction of self-dopants have been proved to be effective methods to improve the light absorption ability of photocatalysts to produce larger amounts of photogenerated charge carriers. Subsequently, the photogenerated electrons and holes migrate to the surface. Therefore, acceleration of the transport process can result in enhanced solar energy conversion efficiency. Different strategies such as morphology control and conductivity improvement have been demonstrated to achieve this goal. Fine-tuning of the morphology of nanostructured photocatalysts can reduce the migration distance of charge carriers. Improving the conductivity of photocatalysts by using graphitic materials can also improve the transport of charge carriers. Upon charge carrier migration, electrons and holes also tend to recombine. The suppression of recombination can be achieved by constructing heterojunctions that enhance charge separation in the photocatalysts. Surface states acting as recombination centers should also be removed to improve the photocatalytic efficiency. Moreover, surface reactions, which are the core chemical processes during the solar energy conversion, can be enhanced by applying cocatalysts as well as suppressing side reactions. All of these strategies have been proved to be essential for enhancing the activities of semiconductor photocatalysts. It is hoped that delicate manipulation of photogenerated charge carriers in semiconductor photocatalysts will hold the key to effective solar-to-chemical energy conversion.

1. INTRODUCTION Developing sustainable energy resources is one of the most urgent tasks for human beings since the increasing energy demand is in drastic conflict with the limited global fossil fuel storage. Among various types of sustainable energy resources, solar energy is considered to be promising because of its inexhaustibility, universality, high capacity, and environmental benignancy.1,2 However, the solar irradiation in nature is decentralized, fluctuant, and intermittent. Therefore, effective utilization of solar energy in a clean, economical, and convenient way remains a grand challege.1 Solar energy could be utilized primarily through photothermal, photovoltaic, and photocatalytic approaches. The photocatalytic conversion of solar energy into chemical energy, by means of artificial photosynthesis, photodegradation of dyes, and photocatalytic chemical synthesis, could realize the application of solar energy in a variety of fields.3−6 During such photocatalytic processes, different photocatalysts are usually applied to achieve high efficiencies, among which © XXXX American Chemical Society

semiconductor materials have been investigated extensively since they possess outstanding chemical, physical, and electrical properties. Complicated processes are involved in semiconductor-based photocatalytic reactions.7 Electrons and holes are generated in the bulk of the semiconductor photocatalyst under irradiation with solar light (step (i) in Scheme 1). Then the electrons and holes migrate to the surface of the photocatalyst (step (ii) in Scheme 1). Meanwhile, electrons and holes tend to recombine and release the energy in the form of light or heat (step (iii) in Scheme 1). Electrons and holes that reach the surface would take part in reduction and oxidation reactions to complete the energy conversion process (step (iv) in Scheme 1). Since the semiconductor-based photocatalytic conversion of solar energy is realized by such complex photo-, electro-, and chemical processes, simultaneous promotion of the generation, migraReceived: January 20, 2016

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DOI: 10.1021/acs.accounts.6b00036 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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2. ENHANCING THE GENERATION OF CHARGE CARRIERS The photogenerated charge carriers play a crucial role in photocatalytic reactions. Enhancing the generation of charge carriers in semiconductor photocatalysts is essential to improve their photocatalytic performance. Extending the light absorption range of some outstanding semiconductor photocatalysts with large band gaps (e.g., TiO2 and ZnO) is an important approach to convert more photons into excited electrons and holes. Integration of plasmonic metals such as Au and Ag, which exhibit visible-light plasmonic excitations, is an effective method to extend the light absorption range of semiconductor photocatalysts with large band gaps. The plasmonic enhancement mechanism mainly includes (i) light absorption and scattering, (ii) hot electron injection, and (iii) plasmon-induced resonance energy transfer.9,10 Upon loading with plasmonic nanoparticles, semiconductor photocatalysts can absorb and utilize more light (especially in the visible range), extract hot electrons from the plasmonic metals, and generate electron− hole pairs much more readily. Consequently, the generation of charge carriers is enhanced, leading to a higher solar energy conversion efficiency. For example, we loaded Au nanoparticles onto the tips of one-dimensional (1-D) ZnO nanopencil arrays (Figure 1a,b), which showed a much higher photocurrent density compared with the bare ZnO nanopencil arrays, especially under visible-light irradiation.11 From the UV−vis absorption spectra, an additional absorption peak located at ∼530 nm was observed for the Au-loaded ZnO nanopencils, indicating that the surface plasmon resonance of Au nanoparticles could extend the light absorption range of the ZnO nanopencils. Moreover, the plasmon energy transfer process of

Scheme 1. Schematic Illustration of Different Steps during Semiconductor-Based Photocatalytic Solar Energy Conversion: (i) Generation of Electron−Hole Pairs under Solar Irradiation; (ii) Migration of Electrons and Holes to the Surface Active Sites; (iii) Recombination of Unreacted Electrons and Holes; (iv) Surface Reduction and Oxidation Reactions

tion, and reaction of charge carriers is highly desirable to enhance the activity of semiconductor photocatalysts. Moreover, suppressing the unfavorable recombination of electrons and holes is also of great importance.8 Therefore, taking full advantage of photogenerated charge carriers is the key for improving the activity of photocatalysts. Different strategies, including extending the light absorption range, controlling the morphology, enhancing the transport of charge carriers, constructing heterojunctions, and promoting surface reaction kinetics, have been developed to obtain highly active photocatalysts for effective solar-to-chemical energy conversion.

Figure 1. (a) Schematic illustration and (b) transmission electron microscopy (TEM) image of Au-loaded 1-D ZnO nanopencils. (c) Schematic illustration and (d) TEM image of Au-loaded 1-D branched TiO2 nanorods. Adapted with permission from refs 11 and 12. Copyright 2014 and 2013 Royal Society of Chemistry, respectively. B

DOI: 10.1021/acs.accounts.6b00036 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. (a) Schematic illustration and (b−d) corresponding TEM images of the synthesis procedures for the Au nanorod@TiO2 yolk−shell photocatalysts. The yellow, blue, and red colors in (a) represent Au, SiO2, and TiO2 materials, respectively. The aspect ratios of the Au nanorods in (b), (c), and (d) are 2.4, 4.2, and 5.4, respectively. Reproduced with permission from ref 13. Copyright 2015 Wiley.

the Au nanoparticles prolonged the lifetime of the charge carriers, which also accounted for the enhanced activity. We also integrated Au nanoparticles onto the tips of 1-D branched TiO2 nanorod arrays (Figure 1b,c), which exhibited a photoconversion efficiency of ∼1.27% for water oxidation at the low bias of 0.50 V vs reversible hydrogen electron (RHE) under 100 mW cm−2 irradiation (AM 1.5G).12 Upon loading of Au nanoparticles, the branched TiO2 nanorod arrays showed improved activity under visible-light irradiation. The carrier density of the Au-loaded branched TiO2 nanorods was nearly 6 times higher than that of the pristine branched TiO2 nanorods according to the Mott−Schottky plots, which proved that the integration of plasmonic metals can effectively increase the charge carrier density of semiconductor photocatalysts. By tuning of the morphology of the plasmonic materials, the light absorption range of the composite photocatalysts could be further extended to the infrared region. We conducted a multitemplating method to synthesize Au nanorod@TiO2 yolk−shell photocatalysts (Figure 2), which showed additional light absorption peaks at larger wavelengths as a result of the plasmonic oscillations along the longitudinal direction of the Au nanorods (Figure 3 inset).13 Moreover, the light absorption peaks could be tuned by adjusting the aspect ratio of the Au nanorods. We found that the Au nanorod@TiO2 yolk−shell photocatalyst with a medium aspect ratio exhibited the highest activity for the photooxidation of benzyl alcohol to benzaldehyde (1130 μmol gcat−1 benzaldehyde was generated over 16 h, as shown in Figure 3). A balance between the absorption of high-energy irradiation and the exposure of highly active surface was achieved by the medium-aspect-ratio Au nanorods, which resulted in the optimized activity. As another important approach to extend the light absorption range of semiconductor photocatalysts, doping

Figure 3. (a−c) Activities for photooxidation of benzyl alcohol into benzaldehyde over different Au nanorod@TiO2 yolk−shell photocatalysts. The aspect ratios of the Au nanorods in (a), (b), and (c) are 4.2, 2.4, and 5.4, respectively. (d, e) Control experiments with Au nanoparticle@TiO2 yolk−shell and TiO2 hollow-sphere photocatalysts. (f) Control experiment without photocatalyst. The experiments in (a−f) were carried out under visible−infrared irradiation (λ > 420 nm). (g) Control experiment without photocatalyst under dark conditions. The inset shows the light absorption spectra of different photocatalysts. Adapted with permission from ref 13. Copyright 2015 Wiley.

with external elements to introduce inter-band-gap energy levels has been shown to be effective.14 However, external dopants would also become recombination centers, which might lower the overall efficiency of charge carrier utilization. Recently, self-doping has been demonstrated to extend the light C

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Figure 4. Photocatalytic H2 production over platinized TiO2 photocatalysts (sub-10 nm Ti3+-doped rutile TiO2 (red), Ti3+-doped rutile TiO2 (blue), and pure rutile TiO2 (green)) under (a) UV light (320 nm < λ < 400 nm, ca. 83 mW cm−2), (b) visible light (400 nm < λ < 780 nm, ca. 80 mW cm−2), and (c) sunlight (AM 1.5 irradiation, ca. 100 mW cm−2). (d) Cycling tests of sub-10 nm Ti3+-doped rutile TiO2 photocatalyst under visible light. (e) Visible-light photocatalytic activity of sub-10 nm Ti3+-doped rutile TiO2 photocatalyst after long-term storage. Pt (1 wt %) was photodeposited in situ on the surface of the samples as the cocatalyst, and 10 mL of methanol was used as the sacrificial reagent for water splitting. Adapted with permission from ref 17. Copyright 2015 Nature Publishing Group.

Figure 5. (a) Diffuse-reflectance UV−vis spectra of sub-10 nm Ti3+-doped rutile TiO2, Ti3+-doped rutile TiO2, and pure rutile TiO2. (b) Band energy diagram of pure rutile TiO2 and sub-10 nm Ti3+-doped rutile TiO2 demonstrated by DFT calculations. Adapted with permission from ref 17. Copyright 2015 Nature Publishing Group.

absorption range of semiconductor photocatalysts with fewer induced recombination centers.15 Exfoliation of layered tungstic acid was conducted to prepare WO3 nanosheets.16 With the subsequent introduction of oxygen vacancies by post-treatment under vacuum or a hydrogen atmosphere, the as-prepared WO3 nanosheets became substoichiometric and showed enhanced performance in both photocurrent response and photocatalytic water oxidation compared with pristine WO3. The oxygen vacancies, acting as self-dopants, could also induce surface plasmon resonance, which extended the light harvesting range

to the near-infrared region and also promoted the light harvesting in the UV and visible regions. We also synthesized sub-10 nm rutile TiO2 with a large number of surface/ subsurface defects (in favor of Ti3+ self-dopant), which showed the state-of-the-art activity among TiO2-based photocatalysts for visible-light-driven water splitting (932 mmol h−1 g−1 H2 with 1 wt % Pt cocatalyst under 80 mW cm−2 irradiation at 400 nm < λ < 780 nm) with methanol as a sacrificial reagent (Figure 4).17 The existence of Ti3+ self-dopant greatly improved the visible-light absorption of TiO2 (Figure 5a) by shifting the top D

DOI: 10.1021/acs.accounts.6b00036 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research of the valence band (VB) upward for band-gap narrowing, which was confirmed by VB X-ray photoelectron spectroscopy (XPS) experiments and density functional theory (DFT) calculations (Figure 5b).

nanorods reduced the migration distance to the surface for the photogenerated holes. Meanwhile, electrons were rapidly transferred to the counter electrode because of the high conductivities of both the well-crystallized branches and the nanorods. In addition to nanorod structures, nanotubes with a thin wall, which further decreases the migration distance of charge carriers, are promising 1-D structures. We developed a facile and general template-free approach to synthesize graphitic C3N4 (g-C3N4) nanotubes (Figure 6c) by direct heating of melamine powder.22 The obtained g-C3N4 showed enhanced photocatalytic activities compared with bulk g-C3N4, resulting from the fast transfer of charge carriers to the surface. We also found that TiO2 photocatalysts with hollow (hemi)spherical structures (Figure 6d) exhibit superior performance for solar energy conversion compared with spherical TiO2 photocatalysts.23,24 The thin shell of the hollow (hemi)spheres reduces the migration distance of charge carriers. Moreover, the high surface area of hollow (hemi)spheres provides abundant reaction sites to promote the photocatalytic activity.25 In addition to reducing the migration distance of charge carriers, improving the conductivity of semiconductor photocatalysts can also accelerate the transport of charge carriers. In this scenario, the charge carriers can migrate to the surface reaction sites within a shorter time and are more likely to take part in surface reactions rather than recombination. To enhance the transport of charge carriers in semiconductor photocatalysts, integration of graphitic materials has been proved to be effective. We constructed a reduced graphene oxide (rGO)/ BiVO4 composite photocatalyst through an evaporationinduced self-assembly process (Figure 7a−c).26 The obtained rGO/BiVO4 photocatalyst showed enhanced activity, with a reaction rate constant for photocatalytic degradation of methylene blue that was 2.14 times higher than that of BiVO4. The formation of well-defined rGO/BiVO4 interfaces and the high conductivity of rGO accounted for the improved photocatalytic activity. Moreover, the ultrathin nature of the

3. ACCELARATING THE TRANSPORT OF CHARGE CARRIERS The efficiency of solar energy utilization can be enhanced by accelerating the transport of charge carriers in semiconductor photocatalysts, such as TiO2, Fe2O3, BiVO4, and Ta3N5, which have low conductivities. Different strategies have been applied to promote this process, among which reducing the size of the photocatalysts to nanoscale to decrease the required diffusion distance for the charge carriers is an effective approach.18 Among various nanostructures of semiconductor photocatalysts, the 1-D morphology provides large surface area, sufficient length to absorb incident light, and a shortened diffusion distance for minority carriers, which are all desired properties for accelerating the transport and the utilization of charge carriers.19 We demonstrated that 1-D ZnO nanorod arrays (Figure 6a) showed enhanced performance toward solar water splitting

Figure 6. Scanning electron microscopy (SEM) images of (a) ZnO nanopencil arrays, (b) branched TiO2 nanorod arrays, (c) g-C3N4 nanotubes, and (d) TiO2 nanocups. The inset of (b) shows a TEM image of a single branched TiO2 nanorod. Adapted with permission from refs 20−23. Copyright 2014 Elsevier, 2013 The PCCP Owner Societies, and 2014 and 2013 Royal Society of Chemistry, respectively.

compared with planar ZnO films. When conical tips with high aspect ratios were further grown on the top of these nanorods, forming ZnO nanopencil arrays, a nearly doubled photocurrent density was obtained at 1.6 V vs RHE under 100 mW cm−2 irradiation (AM 1.5G).20 Moreover, the activity of 1-D photoelectrodes could be further improved by constructing hierarchical nanostructures. Branched TiO2 nanorod arrays with a hierarchical 1-D configuration (Figure 6b) showed higher activity than TiO2 nanorod arrays.21 The tiny branches (with diameters ranging from 10 to 20 nm) on the TiO2

Figure 7. SEM images of (a) as-synthesized BiVO4 with a smooth surface, (b) rGO/BiVO4 with the rGO layer uniformly attached on BiVO4 and forming a wrinkled surface, as illustrated by the inset scheme. (c, d) TEM images of rGO/BiVO4 exhibiting a tight contact between BiVO4 and the rGO layer of about 2 nm. Reproduced from ref 26. Copyright 2014 American Chemical Society. E

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Figure 8. (a) SEM image, (b) TEM image, and (c) schematic diagram of energy band structures and the charge separation mechanism of the Ag3PO4/BiVO4 heterojunction photocatalyst. (d) SEM image, (e) TEM image, and (f) schematic diagram of energy band structures and the charge separation mechanism of the g-C3N4/BiVO4 heterojunction photocatalyst. Adapted with permission from refs 31 and 32. Copyright 2013 and 2014 Wiley, respectively.

of electron−hole pairs and enhanced activities.30 However, recombination centers could also be induced at the heterojunction. Thus, a finely controlled interface between the different components of the heterojunction is desirable. We selectively deposited Ag3PO4 on the highly active (040) facets of truncated bipyramidal monoclinic BiVO4 by an in situ deposition method (Figure 8a,b).31 A homotype heterojunction, considered as a junction composed of semiconductors with similar electronic band structures, was built between visible-light-responsive Ag3PO4 and BiVO4 photocatalysts. This heterojunction photocatalyst showed high catalytic activity for the photodegradation of methylene blue as a result of the enhanced separation of charge carriers and the effective utilization of visible light (Figure 8c). A simple annealing approach was employed to load g-C3N4 nanoislands onto the surface of coralline BiVO4, forming heterojunction photocatalysts with intimate contact (Figure 8d,e).32 Control experiments using metal salts as the electron detector showed that upon irradiation with solar light, the photogenerated electrons in the g-C3N4 were transferred to the surface of BiVO4, while holes traveled in the reverse direction (Figure 8f). Therefore, the charge separation efficiency was greatly enhanced to reach high photocatalytic activity. In addition to traditional heterojunction photocatalysts, other types of junctions, such as p−n junctions, phase junctions, and facet junctions, also show promotive effects on solar energy conversion by semiconductor photocatalysts. Atomic layer deposition (ALD) was utilized by Wang’s group to grow a thin layer of p-type hematite (α-Fe2O3) on the surface of n-type hematite.33 The photogenerated charge carriers could be separated effectively by the built-in electric field created by the p−n junction. This electric field resulted in an enhanced photocurrent density (especially at low applied bias) and a 200 mV cathodic shift of the onset potential (Figure 9a,b). For TiO2 photocatalysts, the surface contents of rutile and anatase

rGO (