Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications.

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Engineering BiOX (X ¼ Cl, Br, I) nanostructures for highly efficient photocatalytic applications Cite this: Nanoscale, 2014, 6, 2009

Hefeng Cheng,a Baibiao Huang*a and Ying Daib Heterogeneous photocatalysis that employs photo-excited semiconductor materials to reduce water and oxidize toxic pollutants upon solar light irradiation holds great prospects for renewable energy substitutes and environmental protection. To utilize solar light effectively, the quest for highly active photocatalysts working under visible light has always been the research focus. Layered BiOX (X ¼ Cl, Br, I) are a kind of newly exploited efficient photocatalysts, and their light response can be tuned from UV to visible light Received 17th October 2013 Accepted 18th November 2013

range. The properties of semiconductors are dependent on their morphologies and compositions as well as structures, and this also offers the guidelines for design of highly-efficient photocatalysts. In this

DOI: 10.1039/c3nr05529a

review, recent advances and emerging strategies in tailoring BiOX (X ¼ Cl, Br, I) nanostructures to boost

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their photocatalytic properties are surveyed.

1. Introduction With the huge consumption of traditional fossil fuels in the past several centuries, including coal, crude oil and natural gas, human beings are now confronting a great energy crisis and environmental pollution issues. Semiconductor photocatalysis, which could utilize the abundant solar energy for water splitting into H2 gas,1–3 harmful pollutant decomposition,4,5 selective organic transformations6,7 and CO2 conversion to energybearing carbon fuel sources,8–10 has been regarded as an efficient, green and promising solution to energy replacement and environmental decontamination. The conventional TiO2 photocatalyst,11–16 however, is conned by its wide band gap (e.g., 3.0 eV for rutile and 3.2 eV for anatase) and responds only to UV light that accounts for less than 5% of sunlight energy. In order to take full advantage of the abundant solar energy or indoor illumination, it is of crucial signicance to develop visible-lightdriven photocatalysts with high efficiency. In efforts to exploit novel photocatalyst systems working under visible light, it has been revealed that orbitals of some p-block metals with a d10 conguration,3,17–19 such as Ag 4d in Ag(I), Sn 5s in Sn(II) and Bi 6s in Bi(III), could hybridize O 2p levels to form a new preferable hybridized valence band (VB), thus narrowing the band gap to harvest visible light. In terms of low toxicity and earth abundance, bismuth-based materials are more appropriate candidates; on the other hand, bismuthbased semiconductors (e.g., Bi2O3,20,21 CaBi2O4,22 Bi2WO6,23–26 BiVO4,27–31 Bi4Ti3O12,32,33 Bi2O2CO334,35 and BiOIO336) have shown efficient photocatalytic performances in waste water

purication and harmful pollutant removal. Hence, considerable attention has been drawn to the bismuth-based semiconductors, which could be endowed with strong visible light absorption and excellent photocatalytic activity. Bismuth oxyhalides BiOX (X ¼ Cl, Br, I), as another category of Bi-based semiconductors, are of immense importance for their outstanding optical and electrical properties and have exhibited promising applications in pharmaceuticals,37 pigments38 and catalysts,39 as well as gas sensors.40 All BiOX (X ¼ Cl, Br, I) compounds are crystallized in a tetragonal matlockite structure. Taking BiOCl for example, as illustrated in Fig. 1, BiOX (X ¼ Cl, Br, I) are characterized by the layered structure that are composed of [Bi2O2] slabs interleaved with double halogen atom slabs along the [001] direction. Inspiringly, Zhang et al.41 have recently reported the exceptional photocatalytic property of BiOCl, which exhibited superior activity to commercial P25 towards methyl orange (MO) dye degradation under UV irradiation. It is noted that with increasing atomic numbers, the band gaps of BiOX (X ¼ Cl, Br, I) become narrower

a

State Key Lab of Crystal Materials, Shandong University, Jinan 250100, China. E-mail: [email protected]; Fax: +86-531-88365969; Tel: +86-531-88364449

b

School of Physics, Shandong University, Jinan 250100, China

This journal is © The Royal Society of Chemistry 2014

Fig. 1

The schematic diagram of (a) unit cell and (b) crystal structure of

BiOCl.

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Review

for BiOCl (3.2 eV), BiOBr (2.7 eV) and BiOI (1.7 eV), which could allow one to maximize their photocatalytic activities in a large range. Since this pioneering work, exponential interests have been focused on the robust syntheses of BiOX (X ¼ Cl, Br, I) nano/micro-structures to boost their photocatalytic activities.42–82 It has become of great importance and necessity to summarize the design, preparation, and corresponding photocatalytic applications of BiOX-related nanomaterials in the last few years. Nevertheless, reviews outlining the utilization of such new-type ternary semiconductor nanostructures for heterogeneous photocatalysis have not been available. In this review, we aim to ll this gap and cover the recent advances in BiOX (X ¼ Cl, Br, I) photocatalysts to give an insight into the structure– property correlation. In addition, we will emphasize the engineering of BiOX (X ¼ Cl, Br, I) nanomaterials for efficient photocatalytic applications, which can be classied into microstructure modulation, heterologous hybridization and structural design. In this context, some BiOX (X ¼ Cl, Br, I) nanomaterials83–92 that are absent from photocatalytic applications will not be discussed thereinaer.

Due to their highly anisotropic layered structures, BiOX (X ¼ Cl, Br and I) tend to grow into nanoplates/sheets with 2D features. As a result, the realization of 1D bismuth oxyhalide nanostructures is usually resorted to the hard templates, which can be easily removed by subsequent thermal or chemical treatments.42–44 For example, Liu et al.42 adopted the electrospinning method to synthesize BiOCl nanobers, as shown in Fig. 2. Aer thermal treatments of the polyacrylonitrile (PAN) template at 500 C for 10 h, BiOCl nanobers with diameters ranging from 80 to 140 nm were obtained. Interestingly, the asprepared BiOCl nanobers showed high activity towards Rhodamine B (RhB) degradation under UV irradiation, which was about 3 times faster than that of Bi2O3 nanobers obtained in the same way. In addition to PAN, some other templates involving activated carbon bers (ACFs)43 and anodic aluminium oxide (AAO)44 have also been employed to prepare BiOCl nanobers/nanowire arrays, which displayed efficient photocatalytic performance in the degradation of organic dyes.

2. Microstructure modulation of BiOX photocatalysts

In the past few years, 2D nanomaterials, such as graphene, transition metal dichalcogenides and layered double hydroxides (LDHs), have received great attention for their exotic physical/chemical features and promising applications in a variety of elds.105–108 Intrinsically, such 2D nanostructures arise from their lamellar structures, which are built by the interlaminated weak van der Waals bonds or electrostatic forces. Similarly, the layered structure renders BiOX (X ¼ Cl, Br, I) prone to the intrinsic 2D nanostructures, involving nanoplates, nanosheets and nanoakes. The formed intra-electric eld between [Bi2O2] layers and halogen atom layers could accelerate the transfer of the photo-induced carriers and enhance the photocatalytic activity of BiOX (X ¼ Cl, Br, I).41 To date, numerous synthetic methodologies have been exploited for the preparation of 2D BiOX nanomaterials, such as hydrolysis,41,45–48 hydrothermal/solvothermal synthesis,49–51 and thermal annealing.52 For instance, by selective addition of the mineralizing agent NaOH, Zhang's group49 has recently prepared 2D BiOCl nanosheets with predominantly exposed {001} and {010} facets, respectively, by hydrothermal synthesis. Interestingly, BiOCl nanosheets with exposed {001} facets displayed higher UV-induced photocatalytic degradation of MO dye, whilst the counterpart with exposed {010} facets exhibited higher degradation activity under visible light. As shown in Fig. 3, on one hand, the generated internal electric eld along the [001] direction is more favorable for direct semiconductor photoexcitation under UV irradiation, which was also conrmed by the higher photocurrent of {001} facets than that of {010} facets from the transient photocurrent responses. On the other hand, compared to {001} facets, the larger surface area and open channel feature of {010} facets facilitate the adsorption of dye molecules, thus leading to its better indirect dye photosensitization performance under visible light irradiation. Such a facet-dependent photocatalytic property in BiOX (X ¼ Cl, Br, I) was also studied through density functional theory (DFT) computations.109 The halogen X-terminated {001} facets possess

Owing to the strong correlation between the physical/chemical properties and the microstructure (i.e., shape, size, surface area and dimensionality) of the materials, rational synthesis of novel nano- or micro-architecture has always been of great importance for both scientic research and industrial applications.93–95 With respect to their bulk counterparts, nanomaterials that are dened as materials with lengths in the range of 1–100 nm in at least one direction, usually possess exceptional optical and electrical properties derived from their quantized sizes.93 Furthermore, nanomaterials are endowed with a higher surface-to-volume ratio and more active sites, which will facilitate the separation of the photo-generated carriers and then improve the photocatalytic efficiency of BiOX (X ¼ Cl, Br, I) semiconductors. Recently, a variety of BiOX (X ¼ Cl, Br, I) nano-/microstructures, including one dimensional (1D) nanorods/wires,42–44 two dimensional (2D) nanoplates/ sheets,41,45–52 and three dimensional (3D) hierarchical architectures53–82 as well as supported thin lms96–102 have been synthesized to maximize their photocatalytic applications. The relevant parts will be discussed in detail in the following sections (Table 1). 2.1

1D templated nano-bers/wires

1D nanostructures, namely, are classied to the materials with thickness and width in the nanoscale range, while the length can be several micrometers or longer. The prolonged length scale could enable the 1D nanomaterials to contact the macroscopic world for various measurements.103,104 In addition, the high aspect ratio of 1D nanostructured semiconductors also facilitates the fast separation of the photoinduced electrons and holes, which is favorable for highly efficient photocatalytic reactions.

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2.2

2D intrinsic nanostructures


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Table 1

Nanoscale Recent studies on BiOX (X ¼ Cl, Br, I) nanostructures and their corresponding photocatalytic applications

Samples

Synthetic procedures

Photocatalytic applications

Ref.

1D template nanostructures BiOCl nanobers BiOCl bers BiOCl nanowire arrays

Electro-spinning Solvothermal synthesis (ethanol) Sol–gel

Completely RhB degraded within 60 min (UV) 75% MO mineralization in 110 min (UV) Almost 100% RhB degradation within 130 min (UV)

42 43 44

2D intrinsic nanostructures BiOCl plates BiOCl nanosheets

Hydrolysis Hydrolysis

41 45

BiOBr lamellas BiOX (X ¼ Cl, Br, I) nanosheets

Hydrolysis Hydrolysis

BiOCl nanosheets BiOCl nanoplates

Hydrothermal synthesis Mannitol-assisted hydrothermal synthesis Hydrothermal synthesis Thermal annealing

Completely MO degradation within 10 min (UV) About 2 times higher RhB photodegradation rate than that on P25 (UV) Completely RhB degraded within 30 min (Vis) 95.9% sodium pentachlorophenate (Na-PCP) degradation on BiOI within 1 h following BiOI > BiOBr > BiOCl (Xe-lamp) About 99% MO degradation within 45 min (UV) Completely RhB disappearance within 8 min (Vis)

BiOBr lamellar BiOI nanosheets

3D assembled hierarchical architectures BiOX (X ¼ Cl, Br, I) Solvothermal synthesis (EG) nanoplate microspheres BiOX (X ¼ Cl, Br, I) Solvothermal synthesis hierarchical architectures (2-methoxyethanol, EG) BiOCl porous nanospheres Solvothermal synthesis (EG) BiOCl nano-owers Solvothermal synthesis (pyridine) BiOCl hierarchical architectures Solvothermal synthesis (EG) BiOCl hierarchical self-assemblies Hydrothermal synthesis BiOCl micro-owers Hydrothermal synthesis (glycerol + H2O) BiOBr microspheres Solvothermal synthesis (EG) BiOBr nanoplate microspheres BiOBr microspheres

Solvothermal synthesis (EG) Solvothermal synthesis (EG)

BiOBr hollow microspheres

BiOBr microspheres

Solvothermal synthesis (2-methoxythanol) Solvothermal synthesis (EG) Solvothermal synthesis (EG) Microwave-assisted solvothermal synthesis (DEG) Solvothermal synthesis (TEG)

BiOBr 3D microspheres

Solvothermal synthesis (ethanol)

BiOBr microspheres BiOBr mesoporous microspheres

Solvothermal synthesis (isopropanol + EG) Solvothermal synthesis (ethanol)

BiOI hierarchical structures BiOI hollow microspheres BiOI micro-owers BiOCl nano-owers BiOCl sub-microcrystals BiOI micro-owers BiOI microspheres

Hydrothermal synthesis Solvothermal synthesis (EG) Solvothermal synthesis (EG) Hydrolysis Hydrolysis Direct precipitation Direct precipitation

BiOCl hierarchical owers

Sonochemical route

BiOBr fullerene-like eggshells

Ultrasound reaction and heating synthesis Reuxing method

BiOBr micro-owers BiOBr porous nanospheres BiOBr microspheres

BiOCl 3D desert roses


46 47 49 50

96% MO degraded within 120 min (Vis) 7 times higher photoactivity than that on irregular BiOI (Vis)

51 52

80% MO degraded on BiOI within 3 h with the order BiOI > BiOBr > BiOCl (Vis) Completely MO degradation on BiOI within 60 min with the order BiOI > BiOBr > BiOCl (Vis) Almost 100% RhB degraded within 2 h (Vis) Completely MO degradation within 10 min (UV) Completely degrade RhB within 60 min (UV) 90.2% RhB degraded within 50 min (UV) 99.3% RhB degraded within 15 min (Vis)

53

Higher MO degradation than that on BiOBr bulk plates (Vis) Nearly 30% NO removal within 10 min (Vis) 100% tetrabromobisphenol A decomposed within 15 min under simulated sunlight 100% RhB degraded in 15 min and 90% Cr(VI) reduced in 20 min (Vis) 92.5% MB degraded for 4.5 h (Vis) RhB completely degraded within 105 min (Vis) 99% phenol decomposed within 80 min (UV) 90% Micrococcus lylae inactivated aer 6 h under uorescent light Toluene conversion rate 2-fold larger than that on P25 under simulated sunlight 95% RhB degraded within 40 min (Vis) Nearly 100% bisphenol A degradation within 90 min under simulated sunlight MO completely degraded within 50 min (Vis) 92% MO degraded within 3 h (Vis) 80% RhB degraded within 4 h (Vis) Completely RhB degradation within 50 min (Vis) 99.5% RhB decomposed within 75 min (UV) 100% RhB degraded within 2 h (Vis) 94% tetracycline hydrochloride decomposed within 2 h (Vis) 90% MO degraded aer 1 h under simulated sunlight Over 95% RhB degraded within 25 min (Vis) Completely RhB degraded within 20 min (Vis)

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

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Table 1

Review

(Contd. )

Samples

Synthetic procedures

Photocatalytic applications

Ref.

BiOI microspheres BiOCl 3D owers

Chemical bath Solution oxidation process

97% phenol decomposed within 4 h (Vis) 100% RhB degraded within 80 min (Vis)

81 82

BiOCl nanoakes lm

Hydrolysis

BiOCl lms BiOI thin lms BiOI ake array

Hydrolysis Chemical vapor transport Successive ionic layer adsorption and reaction Spray pyrolysis

Comparable MO degradation rate and better durability than that on P25 lm (UV) Completely MO degradation within 150 min at 2.0 V voltage bias (UV) Durable superhydrophobicity when modied with FDTS (UV) 100% RhB degradation within 2 h (UV) 90% RhB degradation within 2 h (Xe-lamp) Maximum IPCE of about 4% under simulated sunlight

96

BiOCl thin lm

Solvothermal synthesis (methanol + EG) Electrochemical route

99 100 101

Maximum IPCE of over 20% (Vis)

102

Supported thin lms BiOCl nanosheet arrays

BiOI nanoplatelet lms

97 98

the band gap. This nding reveals the insight into the facetdetermined photocatalysis of BiOX (X ¼ Cl, Br, I), and indirectly explains the superior photocatalytic performance of BiOX (X ¼ Cl, Br, I) nanosheets with higher percentage of {001} facets than those with lower ones.45,46,52

2.3 SEM images of the prepared (A) PAN/BiCl3 nanofibers and (B) BiOCl nanofibers.42 Reprinted with permission from Elsevier. Fig. 2

Fig. 3 (a) Crystal structure of BiOCl. (b) Model showing the direction of the internal electric field in each of the BiOCl nanosheets. (c) Photocurrent responses of the BiOCl nanosheets in 0.5 M Na2SO4 aqueous solutions under UV-vis irradiation.49 Reprinted with permission from American Chemical Society.

thermodynamic stability and could separate photo-generated electron–hole pairs efficiently, whereas BiX-terminated {110} and other facets with surface O vacancies are detrimental to the carrier separation due to the formation of deep defect levels in

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3D assembled hierarchical architectures

As an essential part of nanotechnology, self-assembly of lowdimensional (e.g., 1D nanorods, 2D nanosheets) building blocks into their higher-order (3D) multifunctional superstructures has been paid more and more attention and plays a great role in material synthesis and device engineering.110–114 In comparison with 1D and 2D nanostructures, 3D hierarchical nano-/microstructures are more attractive for solar energy storage and conversion, which integrate the features of the nanoscale building units and their assembled architectures.112–114 Furthermore, 3D architectures could endow the BiOX (X ¼ Cl, Br, I) semiconductors with improved light harvesting, shortened diffusion pathways, faster interfacial charge separation and more reactive sites, thus enhancing their photocatalytic efficiencies. Among the methodological syntheses of the 3D BiOX (X ¼ Cl, Br, I) hierarchical assemblies, hydro-/solvothermal routes are denitely the most robust,53–72 which are usually carried out at critical conditions of water or other organic solvents. In 2008, Zhang et al.53 adopted a generalized solvothermal process in the presence of ethylene glycol (EG) to prepare BiOX (X ¼ Cl, Br, I) hierarchical microspheres, which are composed of 2D nanoplates by self-assembly. The band gaps of the resulting BiOX (X ¼ Cl, Br, I) samples are calculated to be 3.22, 2.64, and 1.77 eV for BiOCl, BiOBr, and BiOI, respectively. Evaluated by MO dye solution degradation under visible light irradiation, the BiOI sample exhibited the best photocatalytic performance with the order of BiOI > BiOBr > BiOCl. Almost at the same time, Tang et al.60 also prepared 3D microspherical BiOBr architectures assembled by nanosheets through EG-assisted solvothermal synthesis. With the band gap of 2.54 eV, the BiOBr


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architectures showed higher photocatalytic activity for MO decomposition under visible light irradiation than the BiOBr bulk plates. In the microstructure modulation of BiOX (X ¼ Cl, Br, I) nano-/microstructures, the realization of hierarchical architectures with hollow voids are more fascinating when derived from their better penetrability and higher light utilization. Recently, our group63 has developed a mini-emulsion mediated solvothermal route to synthesize uniform BiOBr hollow microspheres in the presence of 2-methoxyethanol solvent. With diameters in the range of 1–2 mm and shell thickness of about 100 nm, the BiOBr hollow microspheres are built by numerous interlaced 2D nanosheets. As illustrated in Fig. 4, the 1-hexadecyl-3-methylimidazolium bromide ionic liquid ([C16Mim]Br IL) not only performs as a Br source, but also gives rise to colloidal mini-emulsions, conrmed by the observed Tyndall effect of the precursor suspension. The reaction takes place at its phase interface of the mini-emulsion rather than in the emulsion itself, thus maintaining the dimensions of the micelles. Under visible light irradiation, such BiOBr hollow microspheres displayed superior photocatalytic activity in degradation of RhB dye and reduction of CrVI ions to the samples with micro-ower shape. Using 1-butyl-3-methylimidazoliumiodine ([Bmim]I) IL as the reactive templates and I source, Xia and co-workers72 prepared BiOI hollow microspheres by the EG-assisted solvothermal method. Under visible light irradiation, such 3D BiOI hollow microspheres exhibited higher photocatalytic activity towards MO degradation than that of 2D BiOI nanoplates. Besides the halide ion-containing ILs,54,55,63,65,67,72 surfactants such as poly(vinylpyrrolidone) (PVP)58 and hexadecyltrimethylammonium bromide (CTAB)61,62,64,66,68,69 have also been employed to tailor the selfassembly process of the BiOX (X ¼ Cl, Br, I) hierarchical architectures, and in particular CTAB could act as reactable template to provide Br ions for BiOBr.

In addition to the hydro-/solvothermal syntheses, other synthetic protocols including hydrolysis,74,75 direct precipitation,76,77 sonochemical route,78,79 reuxing method,80 chemical bath81 and solution oxidation process,82 are also employed to obtain the ordered superstructures of BiOX (X ¼ Cl, Br, I) semiconductors. For example, Xiong and co-workers82 reported a rapid in situ oxidation process to fabricate 3D ower-like BiOCl hierarchical nanostructures by reacting metallic Bi nanospheres and FeCl3 aqueous solution at room temperature. As presented in Fig. 5, in the presence of Cl ions, the redox potential of Bi species could be reduced from +0.308 V (Bi3+/Bi vs. SHE) to +0.16 V (BiOCl/Bi). Therefore, the high redox potential of Fe3+ (E(Fe3+/Fe2+) ¼ +0.771 V) could oxidize the surface of Bi nanospheres into the nal 3D BiOCl hierarchical nanostructures. Compared to the commercial BiOCl sample, such obtained ower-like BiOCl nanostructures displayed much better RhB photodegradation activity and higher photoelectric conversion performance.

Fig. 4 The schematic formation process of the BiOBr hollow microspheres by the mini-emulsion mediated solvothermal route.63 Reprinted with permission from Wiley-VCH.

Fig. 5 Schematic illustration of the fabrication of flower-like BiOCl hierarchical nanostructures by an in situ oxidation process.82 Reprinted with permission from Wiley-VCH.


2.4

Supported thin lms

As one important category of the 2D nanostructures, BiOX thin lms on solid substrates have recently attracted much interest owing to their easy reclamation and long-term use.96–100 Mu et al.96 prepared vertically aligned BiOCl nanosheet arrays on the uorine-doped tin oxide (FTO) substrate by a solvothermal method. They found that BiOCl nanosheet arrays showed a comparable MO degradation rate and superior durability to P25 lm under UV irradiation. Moreover, the construction of supported BiOX lms opens up the possibility to investigate their surface wettability. Recently, Li et al.98 have reported the fabrication of a BiOCl nanoake lm perpendicular to the stainless steel (SS) substrate by the hydrolysis of BiCl3 in ethanol solution. When modied with the 1H,1H,2H,2H,-peruorodecyltrichlorosilane (FDTS), the BiOCl lm exhibited excellent superhydrophobicity and the water contact angle could reach as high as 169 . To solve the energy crisis, photovoltaic systems based on semiconductor materials have paved the path for the construction of new alternative photoelectrochemical cells (PEC). Among the BiOX (X ¼ Cl, Br and I) series, BiOI is undoubtedly the most promising candidate for solar cell materials due to its low cost, strong visible light absorption and lower band gap (ca. 1.7 eV). Zhang et al.101 reported the synthesis

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of BiOI ake arrays on FTO glass by successive ionic layer adsorption and reaction (SILAR) method. The prepared BiOI nanoake array solar cell showed highly enhanced photovoltaic performance than the annealed TiO2/FTO one, and could reach the maximum incident photo-to current conversion efficiency (IPCE) value of 4%. Very recently, Mullins et al.102 have synthesized BiOI nanoplatelet photoelectrodes by spray pyrolysis on FTO substrate. As displayed in Fig. 6, the thickness of BiOI nanoplate lms is approximately 1 mm. Intriguingly, the BiOI lm exhibits n-type conductivity, and its maximum IPCE exceeds over 20% in the visible range for the oxidation of I to I3 at 0.4 V vs. Ag/AgCl in acetonitrile. Such ndings could expand the photovoltaic application of BiOX nanomaterials and allow their potential use in photoelectrochemistry.

3. Hybridization of BiOX photocatalysts with other materials Apart from the effect of size and shape, the catalytic property can also be tuned by varying the composition and support of a nanocatalyst. For heterogeneous photocatalysis, due to their intrinsic features such as wide band gap or photo-induced corrosion, single component catalysts (e.g., TiO2, CdS) are incapacitated for large scale practical applications. Therefore, in order to tackle these issues and fulll the required demands, hybrid photocatalysts, which are usually built by one or more active components and a functional support, such as CdS–Au– TiO2,115 Ag/AgCl,116 and TiO2/graphene,117 have been well developed. Such hybrid photocatalysts integrate the synergistic effects of the individual species,118,119 which could endow the composite systems with increased light harvesting, prolonged lifetime of carriers, enhanced catalytic performance as well as higher chemical stability. Generally, the BiOX-based (X ¼ Cl, Br, I) hybrid photocatalysts can be divided into three categories according to the type of newly added species (Table 2): (1) semiconductor/BiOX hybrids,120–152 (2) metal/BiOX hybrids,153–160 and (3) sensitized BiOX hybrids.161–170 3.1

Semiconductor/BiOX composites

In contrast to one individual semiconductor photocatalyst, semiconductor composites are more intriguing for their interfacial heterostructures, which are formed at their junctures and

Fig. 6 (a) SEM image of BiOI film deposited on FTO-coated glass substrates. (b) IPCE spectrum recorded at 0.4 V vs. Ag/AgCl for the BiOCl film with a peak light intensity of 476 mW cm2.102 Reprinted with permission from American Chemical Society.

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have an important effect on their photocatalytic performances. In terms of the band positions, there are usually three types of semiconductor heterojunctions: straddling gap (type I), staggered gap (type II) and broken gap (type III), as presented in Fig. 7. Among them, semiconductor composites with the staggered gap (type II) are the most studied in the eld of heterogeneous photocatalysis.3,12 In this case, the photoinduced electrons and holes are easily separated between the two semiconductors via effective interfacial charge transfer, thereby boosting the photocatalytic performance of the semiconductor composites. Restricted by its wide band gap (ca. 3.46 eV), BiOCl works only under UV light. Hence, by coupling with semiconductors with suitable band gaps, the as-formed BiOCl/narrow band gap (NBG) semiconductor composites could expand their photoabsorption into visible light range. By chemical etching of commercial Bi2O3 with hydrochloric acid (HCl), Lee et al.120 prepared BiOCl/Bi2O3 heterojunction structure. In this hybrid system, BiOCl works as the main photocatalyst, which shows strong oxidation ability towards gaseous 2-propanol and aqueous 1,4-terephthalic acid decomposition, while Bi2O3 seems to perform as the sensitizer to absorb visible light. Due to the formed heterojunction structure between them, the photogenerated carriers can be separated efficiently and participate in the subsequent photocatalytic oxidation process, leading to the superior photocatalytic activity of the BiOCl/Bi2O3 composites to that of P25. Besides this, other binary or ternary composites, such as WO3/BiOCl,121 Fe3O4/BiOCl,122 NaBiO3/BiOCl,123 WO3/ BiOCl/Bi2O3,124 BiOCl/BiNbO4/TiO2,125 BiOCl/bismuth oxyhydrate,126 BiOCl/BiOBr,127 and BiOI/BiOCl128,129 have been prepared and turned out to be efficient visible light-driven photocatalysts. For NBG semiconductors, quantum connement occurs obviously when their sizes reduce to several to tens of nanometers.171 In this condition, the band structure of a semiconductor becomes discrete, along with the band gap broadening to harvest solar light, which may allow one to tailor the light absorption by combination with BiOX (X ¼ Cl, Br, I) semiconductors. As expected, our group has designed and prepared a novel efficient Bi2S3 nanocrystal (NC) sensitized BiOCl hybrid photocatalyst by anion exchange.130 The anion exchange takes place between BiOCl architectures and sulfurcontaining sources (i.e., thiourea, L-cysteine and thiacetamide) in aqueous solution, which is based on the solubility difference of BiOCl (Ksp ¼ 1.8 1031) relative to Bi2S3 (Ksp ¼ 1.0 1097). Moreover, derived from the reactivity variance of the different sulfur sources, Bi2S3 NCs with tunable sizes (3.1–8.5 nm) could be obtained by an ion exchange reaction, thus regulating the light absorption of the Bi2S3/BiOCl hybrid across the visible light region. Evaluated by the decomposition of 2,4-dichlorophenol (2,4-DCP) solution under visible light irradiation, the as-prepared Bi2S3/BiOCl hybrid exhibited highly efficient photocatalytic activity and was dependent on the size of the Bi2S3 NCs. As illustrated in Fig. 8, the formed heterostructure by ion exchange favors the interfacial charge transfer, which is further conrmed by the band gap calculations. When the size of the Bi2S3 NCs decreases, quantum connement makes the


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Table 2

Nanoscale Recent studies on BiOX-based hybrids (X ¼ Cl, Br, I) and their photocatalytic applications

Hybrid photocatalysts

Synthetic procedure

Photocatalytic results

Ref.

Semiconductors composites BiOCl/Bi2O3 WO3/BiOCl Fe3O4/BiOCl NaBiO3/BiOCl

Chemical etching Impregnation Phase-transfer Chemical etching

5.7 times 2-propanol decomposition rate that of P25 (Vis) 40 times higher RhB degradation rate than that of P25 (Vis) 100% RhB degradation within 40 min (Vis) Higher RhB photodegradation rate than that of pure NaBiO3 and BiOCl samples (Vis) 1.3 times 2-propanol decomposition rate that of N-doped TiO2 (Vis) Higher RhB degradation rate than that of BiOCl and BiNbO4 (Vis) 5 times RhB removal rate larger than that on P25 (Vis) Higher RhB degradation rate than that of single BiOCl and BiOBr (Vis) Higher MO and RhB degradation rate than that of single BiOCl and BiOI (Vis) 4 times bisphenol-A degradation rate that of pure BiOI (Vis) 13 times 2,4-DCP decomposition rate faster than that of N-doped P25 (Vis) Higher RhB degradation rate than that of single BiOCl, Bi2S3 and P25 (Vis) 81.9% MO degraded within 5 h (Vis) 3 times phenol decomposition rate higher than that of BiOI (Vis) About 3 times 2,4-DCP decomposition rate faster than that of AgI/BiOI synthesized by chemical bath method (Vis) 7.8 and 3.0 times higher than that of bare BiOI and AgI, respectively (Vis) 100% RhB degradation within 30 min (Vis) Almost 100% RhB degradation within 40 min (Vis) 10.7 times RhB removal rate higher than that on P25 (Vis) Higher MO degradation rate than that of single BiOI and BiOBr (Vis) 1.5- and 48.9-fold RhB degradation rate faster than those over BiOBr and C3N4, respectively (Vis) 87% RhB degraded within 30 min (Vis) 95% MO degradation within 2 h (Vis) 3 times MO degradation rate faster than that of BiOI lm (Vis) Enhanced MO degradation rate than ZnO and BiOI (Vis) 86% MO degradation within 4 h (Vis) 9.8 and 11.1 times Rh 6G degradation higher than that of BiOI and ZnTiO3, respectively (Vis) Enhanced MO degradation than their individual counterparts (Vis) 8 times higher RhB degradation rate than that of BiOI (Vis) 3.9 times phenol decomposition rate that of Bi2O3 (Vis) Highly enhanced RhB degradation and phenol decomposition than Bi4Ti3O12 (Vis) About 4.2 and 5.3 times faster than that of C3N4 and BiOI, respectively. (Vis) 100% MO degraded within 40 min (Vis)

120 121 122 123

WO3/BiOCl/Bi2O3 BiOCl/BiNbO4/TiO2 BiOCl/bismuth oxyhydrate BiOCl/BiOBr

Chemical etching and incipient wetness In situ precipitation Hydrothermal synthesis Solvothermal synthesis

BiOCl/BiOI

Hydrothermal synthesis

BiOI/BiOCl Bi2S3/BiOCl

Solvothermal synthesis Ion exchange

Bi2S3/BiOCl

Ion exchange

Bi2S3/BiOI AgI/BiOI AgI/BiOI

Ion exchange Chemical bath Ion exchange

AgI/BiOI

Ion exchange

AgBr/BiOBr BiOBr/Bi2WO6 BiOBr/bismuth oxyhydrate BiOI/BiOBr BiOBr/C3N4

Co-precipitation Hydrothermal synthesis Hydrothermal synthesis Deposition–precipitation Deposition–precipitation

g-C3N4/BiOBr BiOI/TiO2 BiOI/TiO2 ZnO/BiOI ZnWO4/BiOI BiOI/ZnTiO3

Solvothermal synthesis So-chemical method Impregnating–hydroxylation Chemical bath Chemical bath Precipitation–deposition

BiOI/(BiO)2CO3 Bi2O2CO3/BiOI BiOI/Bi2O3 Bi4Ti3O12/BiOI C3N4/BiOI

Chemical etching Co-precipitation Chemical etching Successive ionic layer adsorption and reaction Chemical bath

BiOBr/ZnFe2O4

Precipitation–deposition

Metal/Semiconductor hybrids Pt/BiOI Bi/BiOCl Ag/BiOI Ag/Ti-doped BiOBr

Ag/BiOBr Ag/AgBr/BiOBr Ag/AgCl/BiOCl Ag/AgX/BiOX (X ¼ Cl, Br)

Photodeposition UV light-induced chemical reduction Photodeposition Chemical reduction; photoreduction; solvothermal reduction Precipitation–deposition Ion exchange and photoreduction Chemical bath Chemical bath


124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152

90% acid orange II degradation within 1 h (Vis) 4.8 times MO degradation rate that of BiOCl (UV)

153 154

4 times larger acid orange II degradation rate than that of BiOI (Vis) 3 times RhB degradation rate that of the Ti-doped BiOBr (Vis)

155 156

Decreased RhB degradation rate than that of primitive BiOBr (Vis) 56 times MO degradation rate faster than that of N-doped P25 (Vis)

157 158

Completely degrade RhB within 30 min (Vis) Ag/AgCl/BiOCl: 12 times that of Ag/AgCl; Ag/AgBr/BiOBr: 7 times that of Ag/AgBr (Vis)

159 160

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(Contd. )

Hybrid photocatalysts

Synthetic procedure


Co-catalyst or sensitizer modied hybrids Graphene/BiOCl Solvothermal synthesis BiOBr/graphene BiOBr/graphene BiOBr/graphene

BiOI/graphene BiOI/MWCNT BiOI/[Bmim]I IL MnOx/BiOI BiOCl/phthalocyanine copper

Solvothermal synthesis Solvothermal synthesis Microwave-assisted solvothermal synthesis Hydrothermal synthesis Solvothermal synthesis Chemical bath Photo-deposition Liquid loading

Bin(Tu)xCl3n/BiOCl

Hydrolysis

Photocatalytic results

Ref.

2 times larger methylbenzene degradation rate than that on BiOCl (UV) 2 times NO removal rate that of BiOBr (Vis) 3 times RhB degradation rate higher than that of BiOBr (Vis) Almost 100% MO degraded within 80 min (Vis)

161

6 times higher MO degradation rate than that of BiOI (Vis) Complete AOII degradation within 3 h (Vis) Enhanced MO degradation rate than that on BiOI (Vis) 78.8% RhB degradation within 30 min (Vis) 76 times photocurrent intensity higher than that of bare BiOCl electrode under simulated sunlight 13 times higher RhB degradation rate than that on BiOCl (Vis)

Fig. 7 Schematic diagram of three types of semiconductor heterostructures.

Fig. 8 Schematic illustration of band energy positions and the charge transfer process of the Bi2S3 NCs/BiOCl hybrid under visible light irradiation.130 Reprinted with permission from the Royal Society of Chemistry.

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165 166 167 168 169 170

Br, I)130–132 and even other Bi-based semiconductors with Bi2S3 NCs modication, which allows tunable solar light harvesting and photocatalytic efficiency enhancement. In view of highly efficient photocatalysts working under visible light, the semiconductor composites constructed by two visible light responsive semiconductors with staggered band potentials sounds more promising and meaningful. Our group has rst developed the novel nanostructured AgI/BiOI semiconductor composites by a chemical bath method.133 Compared to BiOI (1.73 eV) and AgI (2.8 eV), the as-prepared AgI/BiOI composites displayed greatly enhanced photocatalytic activities towards MO dye degradation and phenol decomposition under visible light irradiation. According to the band gap calculations, AgI and BiOI have staggered band potentials, in accordance with the type II heterostructure, which favors the interfacial charge transfer. Meanwhile, as the CB edge of AgI (0.55 eV) is more negative than that of BiOI (0.58 eV), the stability of AgI is also ensured by the interfacial transfer of the photo-induced electrons from AgI to BiOI, preventing its combination with the interstitial Ag+ to engender metallic Ag0. This work illustrates that AgI/BiOI composites could be used as stable and efficient visible-light-driven photocatalysts. Yet, precipitation of AgI NPs on BiOI nanoplates inevitably brings about poor dispersity and loose contact between them, which goes against the separation of the photo-generated carriers. Very currently, we have optimized the synthetic procedure and prepared 3D hierarchical AgI/BiOI hybrids, which consist of AgI NPs uniformly anchored on the BiOI nanosheets.134 To fulll this target, a facile ion exchange reaction in EG between BiOI hierarchical microspheres and AgNO3 was adopted, as presented in eqn (1) below. BiOI + Ag+ / AgI + BiO+

Bi2S3/BiOCl hybrids absorb less visible light; while the CB of Bi2S3 moves to more negative potentials to result in a larger CB energy difference between BiOCl and Bi2S3, which favors the electron transfer. Consequently, the optimized case is observed for the Bi2S3/BiOCl hybrid with Bi2S3 size of about 4.7 nm, which shows the best photocatalytic performance. This synthetic strategy could be generally applied to BiOX (X ¼ Cl,

162 163 164

(1)

With the addition of PVP surfactant, the sizes of AgI NPs were tailored ranging from 55 to 16 nm. As shown in Fig. 9, the AgI/BiOI hierarchical hybrids displayed much higher photocatalytic performances with respect to AgI, BiOI and AgI/BiOI composites in decomposition of 2,4-DCP under visible light irradiation. Furthermore, smaller AgI NPs are more active than


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Fig. 10 Schematic diagrams for (A) energy bands of p-BiOI and n-TiO2

before contact and (B) the formation of a p–n junction and its energy band diagram at equilibrium and transfer of photoinduced electrons from p-BiOI to n-TiO2 under visible-light irradiation.143 Reprinted with permission from American Chemical Society.

(a) Photocatalytic decomposition of 2,4-DCP solution over different samples under visible light irradiation; (b) the photocatalytic efficiencies of the AgI/BiOI hybrids versus the particle size of AgI under visible illumination for 1 h; (c) scheme of the charge transfer process of the AgI/BiOI hybrids exposed to visible light.134 Reprinted with permission from the Royal Society of Chemistry. Fig. 9

larger ones, thus leading to the size-dependent photocatalytic properties of the AgI/BiOI hybrids. The closer interfacial contact and more surface reactive sites were considered to be associated with the highly efficient photocatalytic activity of AgI/BiOI hierarchical hybrids. In addition to the above-mentioned shapeand size-dependence, the relative amount of AgI in AgI/BiOI hybrids also plays an important role in determining their photocatalytic performances and the optimal value is observed at 70.4%.135 Other binary semiconductor composites including AgBr/BiOBr,136 BiOBr/Bi2WO6,137 BiOBr/bismuth oxyhydrate,138 BiOI/BiOBr139 and BiOBr/C3N4,140,141 have also been reported to exhibit enhanced visible light photocatalytic activities for harmful organics removal. Apart from type II heterostructure photocatalysts, another important category of semiconductor composites is p–n junction photocatalysts. The p–n junction is formed at the juncture between n-type and p-type materials when they are placed in contact with each other. Because the electron concentration in the n-type material is much higher than that in the p-type material, the electrons will migrate to diffuse across the junction to the p-type material. Correspondingly, holes will diffuse from the p-type to the n-type material. As a consequence, the formed local electrostatic eld in p–n junction photocatalysts is favorable for the separation of the photo-induced electrons and holes, thus resulting in their enhanced photocatalytic activities.172,173 As is known, BiOX (X ¼ Cl, Br, I) exhibits the PEC properties of p-type semiconductors,174 which offers the opportunity to construct p–n junction photocatalysts. In particular, BiOI with the smallest band gap (1.7 eV) is more attractive and could serve as an efficient visible light sensitizer for n-type semiconductors with wide band gaps. So far, a number of BiOI-based p–n junction photocatalysts, which involves BiOI/TiO2,142,143 ZnO/BiOI,144


ZnWO4/BiOI145 and BiOI/ZnTiO3,146 BiOI/(BiO)2CO3,147,148 BiOI/ Bi2O3,149 Bi4Ti3O12/BiOI150 and C3N4/BiOI,151 have been prepared and displayed highly enhanced visible light photocatalytic activities. Taking the BiOI/TiO2 photocatalyst142,143 for example, the schematic band diagram is illustrated in Fig. 10. Before contact, the CB edge and Fermi level of BiOI are lower than those of TiO2. Aer contact, the Fermi level of BiOI moves up while the Fermi level of TiO2 moves down until the nal equilibrium state is formed, along with the corresponding band edges shi. As a result, the p-type BiOI/n-type TiO2 junction is formed, and the CB edge of BiOI is higher than that of TiO2. Upon visible light irradiation, the excited photoelectrons on the CB of p-type BiOI would migrate to that of n-type TiO2, while leaving holes on the VB of BiOI. Thus, the light-induced electron–hole pairs would be effectively separated by the formed interfacial p–n junction, and leads to the highly enhanced photocatalytic activity towards MO degradation and PEC response of BiOI/TiO2 semiconductor composites. Different from using p-type BiOI as the visible light absorbing agent, Kong and co-workers152 have employed an n-type ZnFe2O4 (1.67 eV) to sensitize p-type BiOBr (2.88 eV). The as-prepared BiOBr/ZnFe2O4 photocatalyst exhibited enhanced degradation of RhB dye solutions under visible light, which is attributed to the interfacial p–n junction that facilitates the charge transfer. From these results, it demonstrates that the photocatalytic efficiency of the semiconductor composites can also be dramatically improved by the formation of a p–n junction, which is crucial for the design and exploitation of highly efficient photocatalyst systems. 3.2

Metal/BiOX hybrids

To inhibit the recombination of the photo-induced carriers, one alternative is to load noble metal nanoparticles (e.g., Au, Ag, Pt) on the surface of semiconductors, which could favor the interfacial charge transfer and promote the photocatalytic performances of the metal/semiconductor hybrids.11,175 Because BiOX (X ¼ Cl, Br, I) exhibit p-type conductivity, their Fermi levels are generally lower than that of noble metals. Aer contact, the holes of BiOX will migrate to the noble metal until their Fermi levels are pulled to the same position, and as a consequence the Schottky

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Fig. 11 Schematic diagram illustrating two types of charge transfer process for metal/p-type semiconductor hybrid photocatalysts: (a) metal as the electron trap center and electrons from excited p-type semiconductor to metal, (b) electron transfer from excited noble metal due to LSPR to p-type semiconductor.

barrier for holes at the interface will be formed, as shown in Fig. 11a. When BiOX (X ¼ Cl, Br, I) semiconductors are excited by light with energy above their band gap (Eg) values, the photogenerated electrons would accumulate on the metal NPs that serve as trap centers, leaving holes on the BiOX semiconductor, so that the interfacial charge transfer substantially promotes the photocatalytic performances of the metal/BiOX hybrids. For example, Yu et al.153 deposited Pt NPs on the BiOI nanoplates and found the Pt/BiOI hybrid showed higher visible light photocatalytic degradation performance towards acid orange II than pure BiOI. By using a UV-induced chemical reduction route, Liu and co-workers154 prepared a Bi/BiOCl hybrid photocatalyst, which displayed much higher MO degradation performance and transient photocurrent responses compared to the pure BiOCl sample. The presence of metallic Pt or Bi NPs is crucial for the photocatalytic activity enhancement of the hybrids, which perform as the electron reservoir and thus accelerate the interfacial charge transfer between metal and BiOX semiconductors. On the other hand, the interaction between noble metal NPs (i.e. Au and Ag) and incident light with considerable energy could give rise to the collective oscillation of free electrons, which is known as localized surface plasmon resonance (LSPR).176–178 As presented in Fig. 11b, upon exposure to visible light, the electrons would transfer from the photo-excited noble metal NPs to the conduction band of BiOX (X ¼ Cl, Br, I) semiconductors, leaving holes in the noble metals. This electron transfer path is opposite to the case mentioned above, in which the BiOX semiconductor is excited by light and the photo-induced electrons on the CB migrate to the noble metal NPs. In fact, when the metal/semiconductor hybrid is comprised of a plasmonic noble metal and a visible-light-sensitive semiconductor, both of the electron transfer processes are present and competitive. Thus, under visible light irradiation, enhanced photocatalytic performance is found on Ag/BiOI155 and Ag/Ti-doped BiOBr photocatalysts,156 while the contrary decreased effect is also observed on the Ag/BiOBr photocatalyst.157

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Fig. 12 (a) The synthetic strategy of the Ag/AgBr/BiOBr hybrid. Typical SEM images of (b) BiOBr, and (c) Ag/AgBr/BiOBr hybrid photocatalyst.158 Reprinted with permission from the Royal Society of Chemistry.

To develop highly efficient photocatalysts working under visible light, it usually involves the integration of the heterostructured semiconductor/semiconductor composites and plasmonic metal/semiconductor hybrids. Recently, our group has reported a novel Ag/AgBr/BiOBr hybrid photocatalyst, which displayed highly efficient photocatalytic activities in the sterilization of pathogenic organism and degradation of organic dye.158 As shown in Fig. 12, such three-component hybrid was prepared through in situ ion exchange reaction between hierarchical BiOBr microspheres and Ag+ out of AgNO3 in EG solution, as shown in eqn (2). BiOBr + Ag+ / AgBr + BiO+

(2)

The AgBr NPs were then partially reduced to metallic Ag0 upon visible light illumination, and the resulting Ag/AgBr NPs were 20–50 nm in size and uniformly anchored on BiOBr microspheres. Under visible light irradiation, the Ag/AgBr/ BiOBr hybrid could deactivate the Escherichia coli (E. coli) bacteria in less than 10 min and bleach the MO dye in only 4 min, which is superior to N-doped P25 by a factor of 56. The plasmon-induced oscillation processes of Ag0 NPs upon light irradiation enable the Ag/AgBr/BiOBr hybrid to harvest light in the vis-NIR region and transfer the interfacial photo-excited electrons efficiently, thus ensuring the excellent activity and good stability of the hybrid. Followed by this work, Ag/AgCl/ BiOCl hybrids159,160 were also prepared and showed enhanced visible-light photocatalytic activity. 3.3

Co-catalyst or sensitizer modied hybrids

To expedite the separation of the photo-induced carriers on a semiconductor upon light excitation, proper co-catalysts (e.g.,


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4. Structural design of BiOX photocatalysts

Fig. 13 Schematic illustration of the visible light photocatalytic enhancement of BiOBr/graphene nanocomposites.162 Reprinted with permission from American Chemical Society.

In addition to the microstructure modication and heterostructure hybridization, structural design also plays a crucial role in determining the performances of the photocatalysts. By virtue of the structure–property correlation, the optical and catalytic properties of one inorganic semiconductor material is dependent on their structures, which involves crystal, electronic, phase and surface structures. Therefore, one can boost or maximize the photocatalytic performances of the photocatalysts by structural design, such as doping, solid solution formation, and composition tailoring.

4.1 NiOx, RuO2 and graphene)2,3,29 are usually introduced to promote its corresponding photocatalytic performances. The co-catalyst could perform as reaction sites, and accelerate the interfacial charge transfer between it and the light-harvesting semiconductor. Therefore, one can tailor the photocatalytic performances of BiOX (X ¼ Cl, Br, I) semiconductors by loading proper co-catalysts. For instance, graphene, which is a new carbon material with excellent electron mobility, has been employed for hybridization with BiOX (X ¼ Cl, Br, I) and showed rather improved photocatalytic efficiencies.161–165 Ai and co-workers162 have synthesized BiOBr/graphene hybrids through a solvothermal route using graphene oxide (GO), bismuth nitrite, and CTAB as the precursors. As shown in Fig. 13, BiOBr nanoplates with hundreds of nanometers are dispersed randomly on the 2D graphene sheet scaffold. Evaluated by the removal of gaseous NO under visible light irradiation, the as-prepared BiOBr/graphene hybrid displays a 2 times faster rate than that of BiOBr. The strong chemical bonding between BiOBr and graphene is considered to facilitate the fast transportation of photogenerated electrons from BiOBr to graphene, thus inhibiting the unwanted recombination and leading to its enhanced photocatalytic activity. In addition to graphene, multi-walled carbon nanotubes (MWCNT)166 and [Bmim]I IL167 as electron-trap species, while MnOx168 as hole-trap species could also be used to separate the photo-excited carriers and enhance the visible light photocatalytic activity of the BiOI semiconductor. Analogous to TiO2, BiOCl is also a WBG semiconductor that only responds to UV light. Aside from coupling with NBG semiconductors, an alternative to harvesting visible light absorption of the BiOCl semiconductor is modication with photosensitizers.169,170 For example, Wang et al.169 employed phthalocyanine copper (CuPc) to photosensitize BiOCl via liquid loading. The prepared BiOCl/CuPc has strong visible light absorption (500–800 nm) and displays 76 times photocurrent intensity higher than that of a bare BiOCl electrode under simulated solar light. Interestingly, in a methanol– H2O–RhB system, overall water splitting was realized on the BiOCl/CuPc hybrid. This may provide a prospective method for solar energy storage and conversion, though the activity is very low.


Doping or defect introduction

The optical property of one material is dependent on its underlying electronic and band structures, which could be tuned by doping of foreign atoms or introduction of defects.12,179 On the basis of the DFT calculations on the electronic structures of BiOX (X ¼ Cl, Br, I),180,181 Bi 6p orbitals dominate the CB, while both O 2p and X np (n ¼ 3, 4, and 5 for X ¼ Cl, Br, and I, respectively) orbitals contribute to the VB. Moreover, as the atomic number of X increases, the density peak of X np states in the valence band moves towards the VB top, leading to the decreasing band gap from BiOCl to BiOI. It is desirable that aer doping, the merits of the BiOX (X ¼ Cl, Br, I) host materials are maintained and favorable changes in its electronic structure could be engendered. To date, metal (e.g., Mn, Fe, Ti)182–185 and non-metal (e.g., I)186–188 ions have been reported to dope into BiOX nanomaterials. For example, because of the similar ionic radius of Mn2+ (0.080 nm) with respect to Bi3+ (0.096 nm), Mn ions can be incorporated into BiOCl.182 With red shi in its absorption edge, the Mn-doped BiOCl (2.75 eV) product displayed enhanced photocatalytic performance towards malachite green (MG) degradation under halogen lamp irradiation. In contrast to

Fig. 14 (a) Gradual color changes of the as-prepared BiOIx powders. (b) UV-vis diffuse reflectance spectra of self-doped BiOIx powders.186 Reprinted with permission from American Chemical Society.

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metal ion doping, non-metal anionic ion doping could effectively form a new valence band to engineer the band gaps of BiOX materials. Zhang et al.186 demonstrated that self-doping of iodine in BiOI could tune its electronic and band structures, which was also conrmed from DFT calculations. As shown in Fig. 14, with increasing iodine content, the color of the products changes from orange to deep red. In the meantime, the UV-vis diffuse reectance spectra of the BiOIx samples red shi accordingly. The optimal case was found for BiOI1.5, which showed the best photocatalytic performance for MO degradation and NO removal under visible light irradiation. The introduction of vacancy (e.g., oxygen vacancy, metal atom vacancy) under reducing atmosphere or thermal conditions could also cause the variation of electronic structure, charge transport and surface property of the semiconductor. When exposed to UV irradiation with the protection of Ar gas, the color of BiOCl can be changed from white to black, as displayed in Fig. 15.189 The black BiOCl product is endowed with visible light response due to the formation of oxygen vacancies. The oxygen vacancies could yield the defect state below the CB of BiOCl, which perform as active electron traps and facilitate the separation of photo-induced carriers, and thus lead to the highly enhanced photocatalytic efficiency of the black BiOCl. In addition to oxygen vacancy, metal atom vacancy also plays an important role in the tuning of band structure and electronic structure, and further photocatalytic activity of the semiconductor material. Very recently, Xie and co-workers190 have prepared two types of BiOCl samples by hydrothermal synthesis: BiOCl nanoplates (thickness of about 30 nm) and ultrathin BiOCl nanosheets (thickness of 2.7 nm). With the thickness of the nanosheets decreasing to the atomic scale, the predominant defects change from isolated V000 Bi to triple associ$$ 000 ates V000 V V , which is conrmed by the positron annihilation Bi O Bi $$ 000 spectra. The presence of triple associates V000 V V Bi O Bi brings the ultrathin BiOCl nanosheets with effective separation of

UV-vis diffuse reflectance spectra of the BiOCl samples. (B) Band structure model and photoreaction process on black BiOCl.189 Reprinted with permission from the Royal Society of Chemistry. Fig. 15

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electron–hole pairs and more reductive photo-excited electrons. Compared to BiOCl nanoplates, the ultrathin BiOCl nanosheets display signicantly improved solar-driven photocatalytic activity in RhB degradation.

4.2

Solid solutions

Different from doping of foreign atoms, which leads to isolated energy states below CB or above VB, the formation of solid solution could allow one to tune the band gaps of photocatalysts continuously.3 In solid solution photocatalysts, their band gaps can be narrowed by lowering the level of CB and/or liing the level of VB, thereby tailoring the electronic structures to maximize their photocatalytic activities. For BiOX (X ¼ Cl, Br, I) semiconductors, it has been reported that there is unlimited solubility among them,191 and this offers the possibility to form BiOX–BiOY (X, Y ¼ Cl, Br, I) solid solutions. Furthermore, theoretical calculations illustrate that the alloying effect in the solid solutions of BiOXs could greatly reduce the electron–hole recombination.192 Therefore, the photocatalytic performances of BiOX (X ¼ Cl, Br, I) semiconductors can be promoted by the construction of solid solution photocatalysts. Until now, a number of solid solution photocatalysts (BiOCl1xBrx, BiOCl1xIx and BiOBr1xIx)193–200 have been prepared by various methods. In 2007, Wang et al.193 used a sochemical method to synthesize a series of BiOClxI1x solid solution photocatalysts. As presented in Fig. 16, the BiOCl1xIx solid solutions display intense absorption in the visible light region and their band gaps can be tuned in the range of 2.31– 1.98 eV with varying the x value from 0.2 to 0.8. Moreover, the as-prepared BiOCl1xIx solid solutions exhibit enhanced visible light photocatalytic activities with respect to the single BiOCl and BiOI. Under the competition between visible light absorption and VB potential, the highest photocatalytic activity is observed for the BiOCl0.2I0.8 sample. Later, they also reported the BiOBr1xIx solid solution photocatalysts,194 which also showed promoted visible light photocatalytic performances towards MO degradation with tunable band gaps. Recently, our group196 has prepared BiOCl1xBrx solid solution photocatalysts

Fig. 16 UV-vis diffuse reflectance spectra of BiOCl1xIx photocatalysts. The inset shows the band gaps (Eg) of the samples at different x.193 Reprinted with permission from Elsevier.


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by the hydrothermal method. With the x value in the range of 0.2–0.8, the band gaps of BiOCl1xBrx solid solutions change from 3.05 to 2.77 eV. Evaluated by RhB degradation and 2-propanol decomposition under visible light irradiation, BiOCl1xBrx solid solutions display enhanced photocatalytic efficiencies and the best case is found at BiOCl0.5Br0.5. From DFT calculations, it is demonstrated that the cation vacancies VBi dominate the trapping center for photo-generated carriers. 4.3

Other bismuth oxyhalides

The composition and phase structure of semiconductor materials have great inuences on their optical, electronic and photocatalytic properties. The original bismuth oxyhalides belong to the Sill´ en family with a formula of [Bi2O2][Xm] or [Bi3O4+n][Xm] (X ¼ F, Cl, Br, I; m ¼ 1–3), where the [Bi2O2] or [Bi3O4+n] layers interlace with single, double or even triple halogen layers.201 Besides BiOX (X ¼ Cl, Br, I), through altering elements or compositions some other bismuth oxyhalides, such as BiOF,202 Bi3O4Cl,203 Bi12O17Cl2,204 Na0.5Bi1.5O2Cl,205 CaBiO2Cl,206 PbBiO2Cl,207,208 Bi3O4Br,209 PbBiO2Br,210 Bi5O7I211–213 and Bi7O9I3,214,215 have been prepared and shown considerable photocatalytic activities towards organic pollutant decontamination. For example, Bi3O4Cl (2.79 eV) as a novel efficient photocatalyst was obtained by a solid-state reaction between Bi2O3 and BiOCl.203 As illustrated in Fig. 17, Bi3O4Cl is crystallized in a layered structure with [Bi3O4] slabs alternating with [Cl] slabs along the c-axis. In contrast to BiOCl, red shi in the light absorption of Bi3O4Cl is observed due to its hybridized VB of Cl 3p, O 2p and Bi 6s orbitals. To understand the effect of band structure and VB potential on the photocatalytic efficiency, Xiao et al.212 prepared a series of oxygen-rich bismuth oxyhalides (i.e. BiOCl, Bi3O4Cl, BiOBr, Bi3O4Br, Bi4O5Br2, BiOI, Bi5O7I, Bi7O9I3) by a hydrothermal route. They found that different compositions of O : X (X ¼ Cl, Br, I) ratio could tailor the VB composition and level of the bismuth oxyhalides, which was further conrmed by DFT calculations. The prepared O-rich BiOX compounds exhibit efficient visible-light-driven photocatalytic activities in the degradation of bisphenol-A. In particular, owing to their narrower band gaps and stronger oxidation ability, Bi7O9I3 and Bi4O5I2 demonstrate superior activity to other products.

Fig. 17 (a) The layered structure and (b) UV-vis diffuse reflectance

spectrum of Bi3O4Cl.203 Reprinted with permission from American Chemical Society.


Nanoscale

Extension of bismuth oxyhalides leads to the Sill´ en–Aurivillius intergrowths [Bi2O2][An1BnO3n+1][Bi2O2][Xm], in which the Aurivillius family [Bi2O2][An1BnO3n+1] alternates with the Sill´ en family [Bi2O2][Xm].216 Recently, these combined bismuth oxyhalides, such as Bi4NbO8Cl217 and Bi4TaO8Cl,218 have also been reported to be novel efficient photocatalysts. For instance, Lin et al.217 have prepared Bi4NbO8Cl (2.38 eV) by a solid state reaction and observed its high photocatalytic performance in MO degradation under visible light irradiation. The formula of Bi4NbO8Cl can be denoted as [Bi2O2][NbO4][Bi2O2][Cl], where 2D [Bi2O2], [NbO4] and [Cl] slabs are stacked in sequence along the c-axis to form the layered structure. The polarizing elds in NbO6 and BiO8 local structures, as well as the internal electrical elds between [Bi2O2] and [Cl] slabs are believed to promote the separation of electrons and holes.

5.

Summary and outlook

As a novel kind of bismuth-based layered semiconductor, BiOX (X ¼ Cl, Br, I) have recently been of exceptional interest for their considerable photocatalytic activities. To boost the visible-lightdriven photocatalytic performances of BiOX (X ¼ Cl, Br, I) photocatalysts, numerous efforts have been done to expand the light-response and expedite charge-transfer. Generally, three dominant strategies have been adopted: (i) multi-length scale microstructure modulation in the form of 1D templated nanobers, 2D intrinsic nanosheets, 3D assembled hierarchical superstructure and supported thin lms; (ii) favorable hybridization with semiconductors, metals, and co-catalysts/sensitizers; (iii) rational structural design by doping, solid solution formation and composition tailoring. Notably, engineering BiOX (X ¼ Cl, Br, I) nanostructures for efficient photocatalysis usually requires the synergistic effect of several systems. For instance, a three-component Ag/AgBr/BiOBr hybrid158 combines the merits of a plasmonic Ag/AgBr photocatalyst and AgBr/ BiOBr composite photocatalyst, exhibiting greatly enhanced visible light photocatalytic activities for the sterilization of pathogenic organisms and degradation of organic dyes. Despite much progress, the study of BiOX (X ¼ Cl, Br, I) photocatalysts is still in its early stages, and additional challenges need to be addressed in future research. Conned by their CB levels, BiOX (X ¼ Cl, Br, I) fail to fulll the requirements to split water into H2. To date, most of the reports concerning BiOX photocatalysts are focused on the decomposition of organic pollutants. Other applications, such as CO2 reduction and selective organic transformation, should be extended utilizing BiOX photocatalysts. Since the photocatalytic performance of a semiconductor is dependent on its light-responsive range and carrier-separation capacity, rational design of the BiOX-based complex architecture systems (e.g., three-dimensional ordered inverse opals,219 non-symmetric Janus metal/semiconductor structure,220 metal/ insulator/semiconductor221) that meet the requirements is crucial for constituting highly efficient and robust heterogeneous photocatalysts. For example, the periodic structures in inverse opals could enhance its light harvesting by multiple scattering, and the unintermitted pore tunnels would accelerate

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the separation of carriers, thereby making it a promising candidate for next generation photocatalysis. Moreover, with the aid of voltage bias, it is fascinating to study the photoelectrocatalysis of BiOX, which facilitates the charge separation and offers new opportunities to enhance their catalytic properties. In the development of novel photocatalytic nanostructures, chemical transformation of the existing material into another demonstrates an effective and versatile synthetic strategy. As the crystal structure of BiOX (X ¼ Cl, Br, I) is built by the interlacing of [Bi2O2] slabs with double halogen slabs to form a layered structure, such a strong ionic feature could allow ion exchange reactions or lattice-directed topotactic transformations between BiOX precursors and incoming species. By adopting this method, a variety of well-dened nanostructures, such as Bi2WO6 hollow microspheres,222 Bi2S3 super structured networks,223–225 and Bi2E3 (E ¼ S, Se, Te) core–shell microspheres226 can be obtained. The advent of nanoscience and nanotechnology has brought great guidance and protocols to develop highly efficient semiconductor photocatalysts. With ever-growing in-depth investigations, it is anticipated that BiOX (X ¼ Cl, Br, I)-based photocatalysts will be more efficient and nd important applications in practical environmental purications.

Acknowledgements This work was nancially supported by the National Basic Research Program of China (973 program, no. 2013CB632401) and the National Natural Science Foundation of China (no. 21333006, 11374190, 51321091 and 21007031).

Notes and references 1 2 3 4 5 6

7 8 9 10 11 12 13

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