Accepted Manuscript Title: Perovskite-based photocatalysts for organic contaminants removal: Current status and future perspectives Authors: Jiejing Kong, Ting Yang, Zebao Rui, Hongbing Ji PII: DOI: Reference:
S0920-5861(18)30848-4 https://doi.org/10.1016/j.cattod.2018.06.045 CATTOD 11534
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Catalysis Today
Received date: Revised date: Accepted date:
15-2-2018 7-6-2018 28-6-2018
Please cite this article as: Kong J, Yang T, Rui Z, Ji H, Perovskite-based photocatalysts for organic contaminants removal: Current status and future perspectives, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.06.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Perovskite-based photocatalysts for organic contaminants removal: Current status and future perspectives
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Jiejing Kong a, b, c, Ting Yang d, Zebao Rui a*, Hongbing Ji b,*
a
School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China
b
Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen University,
c
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Guangzhou 510275, China
School of Environmental Science and Engineering, Guangdong University of Technology,
Guangzhou 510006, China d
Materials Science and Engineering, School for Engineering of Matter, Transport and Energy,
Correspondence to the authors can be sent to Z.B. Rui (
[email protected])
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*
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Arizona State University, Tempe, AZ 85287,USA
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and H.B. Ji (
[email protected])
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Graphical abstract
1
Highlights
Perovskite-related photocatalysts for organic contaminants removal are reviewed. Design strategies of perovskite-related photocatalysts are highlighted.
Future perspectives of perovskite-related photocatalysts are proposed.
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TABLE OF CONTENTS
2. Perovskite and perovskite-related materials AxByOz
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1. Introduction
3. Design strategies of perovskite-related photocatalysts
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3.1 General strategies
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3.2.1 Ion doping/substitution
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3.2 Specific implementations
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3.1.2 Charge recombination suppression
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3.1.1 Bandgap engineering
3.2.2 Multi-component heterojunctions
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3.2.3 Micro-/nanostructural adjustment 4. Application for photocatalytic organic contaminants decomposition
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4.1 Single phase systems 4.2 Multiphase systems
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5. Concluding remarks and future perspectives
Abstract The development of new generation photocatalytic materials for the organic contaminants removal has been a research focus. Perovskite and perovskite-related 2
structures offer a broad scope in designing novel photocatalysts for this process. The present review summarizes and highlights the state-of-the-art progress of thirdgeneration photocatalysts perovskite and perovskite-related semiconductors, and their application for the photocatalytic decomposition of both waterborne and airborne organic contaminants. Special attention is paid to the design strategies for promoting
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the photocatalytic performance of perovskite and perovskite-related materials, including ion doping/substitution, noble metal decoration, heterojunctions formation
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and morphology regulation. Future perspectives of perovskite-related photocatalysts are also included in this review.
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Keywords: Perovskite-related; Photocatalytic oxidation; Organic contaminants;
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Design strategies; Review; Perspective
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1. Introduction
Human beings are now facing a tremendous set of environmental problems related
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to hazardous wastes, polluted groundwater, and toxic air contaminants. Among them, organic contaminants received particular attentions. Many industries based on textiles,
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printing, dyeing, food and cosmetics use dyes to color their products, leading to the discharge of large quantities of organic pollutants into the surrounding soil and water, which adversely affect the environment and human health [1]. Besides, the volatile
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organic contaminants (VOCs) emitting from the exhaust gases of traffic and petrochemicals are the most common outdoor air pollutants [2]. VOCs are also wellknown indoor pollutants, emitted from combustion byproducts, cooking, construction materials, furniture and consumer products. Considering with the toxicity and wide range existence of these organic contaminants [3], varies control methods have been 3
developed, including physical separation/transfer [4] and chemical degradation [5-9]. The advantages that make photocatalytic techniques superior to traditional methods are their ability to remove both waterborne and airborne organic contaminants in the range of ppb through a nonselective, room temperature and pressure, and cost-effective process without generating polycyclic compounds [10]. Photocatalytic process is
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composed of three basic steps: 1) photogeneration of charge carriers; 2) charge carrier
separation and diffusion to photocatalyst surface; and 3) redox reactions on the catalyst
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A
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surface, as illustrated in Fig. 1.
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Fig. 1. Illustration of a photocatalytic process for organic contaminants oxidation.
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Upon the absorption of a photon with sufficient energy equal to or greater than the bandgap (Eg) of the semiconductor catalyst, charge carriers generation and separation occur through promoting electrons (e-) transfer from the valence band (VB) to the
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conduction band (CB) along with the generation of holes (h+) in the VB of the photocatalyst [11]. The photoinduced h+ and e- are powerful oxidizing and reducing agents, respectively. The positive h+ can oxidize the pollutant directly or surface hydroxyl (OH-) /water (H2O) to produce •OH radicals with the VB maximum located at more positive oxidation potentials than the electrochemical potential of the desired 4
reaction (•OH /OH- = 1.89 eV, •OH/H2O = 2.72 eV at pH= 7) [12]. The photogenerated e- in the CB can reduce the adsorbed oxygen to produce superoxide radical (O2•−) when the CB minimum is located at a more negative potential than that of the reaction (O2 /O2•− = -0.33 eV; O2/HOO• = -0.05 eV, at pH=7) [13]. The as-generated oxidative species can oxidize the adsorbed organics into CO2 and H2O. This activation process
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of a semiconductor by light can be represented with the following steps [11],
photocatalyst + hυ → e- + h+ Charge-carrier trapping of e-:
Charge-carrier trapping of h+:
Electron–hole recombination:
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etr- + hVB+ (htr+) →heat
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Oxidation of hydroxyls:
(2)
(3)
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hVB+ → htr+
(1)
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eCB- → etr-
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Photoexcitation:
(4)
OH- + h+ →•OH
(5)
(O2)ads + e- → O2•-
(6)
O2•- + e- →HOO•
(7)
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Photogenerated e- scavenging:
Photodegradation of organic by •OH: (8)
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Organics + •OH → CO2 + H2O
Photodegradation of organic by h+: Organics + h+ → Intermediates/CO2 + H2O
(9)
Photodegradation of organic by O2•- / HOO•: Organics + O2•- / HOO• → CO2 + H2O 5
(10)
Here, etr- and htr+ respectively represent the surface-trapped VB electron and CB hole, O2•- is a superoxide radical and HOO• is a hydroperoxyl radical. As depicted in Eq. 4, the photogenerated e- can recombine with h+ in nanoseconds (ns) with the simultaneous dissipation of heat energy (Eq.6&7), which must be prevented as much as possible in a photocatalytic process [14].
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Photocatalysts are usually solid semiconductors which should meet the following requirements: (1) light, especially ultraviolet (UV) and/or visible light, absorption
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ability, (2) chemically and biologically inert and photo-stable, (3) low-cost and (4) nontoxic [15]. The semiconductors TiO2, ZnO and SnO2 can all act as photoactive
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materials for redox/charge-transfer processes due to their electronic structures which
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are characterized by a filled VB and an empty CB. They were extensively studied in
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photoassisted degradation and mineralization reactions of a large number of waterborne
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and airborne organic contaminants, as they fulfill all of the above requirements and exhibit adequate conversion efficiencies [10]. However, their application is still limited
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by their wide bandgap and only UV light responsive property [16]. This type of UVresponsive single metal oxide photocatalysts were referred as first-generation
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photocatalyst [17]. There is an urgent need for developing photocatalytic systems that operate effectively under visible light irradiation. Many researchers have been devoted
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to design semiconductor photocatalysts with maximum absorption thresholds (minimum bandgap) toward the visible region, which is the so-called second-generation
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photocatalysts [18]. One type of second-generation photocatalysts is the simple visiblelight-driven semiconductors such as WO3, Fe2O3 and Cu2O, which have been utilized as photocatalysts for their nontoxic nature, high photosensitivity, simple structure and preparation [19]. However, their pervasive application is restricted by their low quantum yield due to the rapid electron-hole recombination and poor stability. 6
Therefore, new and/or more effective visible-light photocatalysts are being sought with a view to meet the requirements of future environmental and energy technologies driven by solar energy. Suitable band engineering is needed in order to develop new photocatalysts for visible light applications [20]. The band tailoring of inorganic semiconductors can be
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undertaken using different approaches: (1) creation of discrete electronic levels
between the VB and CB (this is normally achieved by doping or co-doping in the case
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of oxides); (2) creation of a new valence band through the synthesis of novel
compounds; (3) formation of solid solutions exhibiting bandgap values intermediate
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between those of the parent materials. Considering the shortcomings of the second
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generation photocatalysts, e.g., the poor stability of the doped photocatalysts, novel
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binary metal oxide photocatalysts with the chemical formula AxByOz are being developed, which are called the third-generation photocatalysts [15], including
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perovskites (A3+B3+O3, A2+B4+O3), perovskite-related materials, A3+B5+O4 compounds
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with scheelite structures (such as tungstate, molybdate or vanadate) and even iron spinels (AB2O4). As one type of the third-generation photocatalysts, perovskite-like
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compounds, AxByOz, are stable structures, which form solid solutions with a range of metal ions to achieve the proper band engineering for photocatalytic applications [21].
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The availability of such a great variety of compounds and structures opens up a wide range of possibilities for visible-light-driven photocatalytic applications.
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With the increasing attentions to the organic contaminants removal with
photocatalytic technology, more researches have focused on the development of new generation photocatalytic materials in recent years. While the progress of photocatalytic oxidation of VOCs and other organic pollutants have been reviewed elsewhere [10, 22, 23], the present review will summarize and highlight the state-of-the-art progress of 7
third-generation photocatalysts perovskite and perovskite-related semiconductors, and their application for the photocatalytic decomposition of organic contaminants. Their design strategies for promoting the photocatalytic performance will be focused.
2. Perovskite and perovskite-related materials AxByOz
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The perovskite oxide structure with the general formula ABO3 is a frequently encountered structure in inorganic chemistry. This structure can accommodate most of
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the metallic ions in the periodic table together with a significant number of other anions.
An ideal perovskite structure has an ABO3 stoichiometry and a cubic crystal lattice with
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space group Pm3m, which is composed of a three-dimensional framework of corner-
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sharing BO6 octahedra (Fig. 2). A wide variety of compositions and constituent
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elements essentially keep the basic structure unchanged, and more than 90% of metal
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elements can be successfully incorporated into the perovskite lattice. The B-site cation is a transition metal element [24]. The A-site cation occupies the 12 coordinate position
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formed by the BO6 network and often consists of an alkaline-earth metal element or a rare earth element, and the properties of the perovskite are determined by the cations
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occupying its A- and B-site lattice [25].
Fig. 2. Ideal cubic perovskite ABO3 structure (cyan, BO6 units; yellow, A atom; red, O atoms) [15]. Copyright 2011, American Chemical Society. 8
For the geometrical configuration of ideal perovskite structures, the following relationship can be proposed: (rA + rO ) =
√2 a 2
= √2(rB + rO )
(11)
Although primitive cubic is the idealized structure, the differences in radii between both
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cations can in fact distort the structure. This normally involves tilting of the BO6 units (octahedral tilting). There is a tolerance factor (t) that describes the range of relative sizes within which the perovskite structure is stable [26]. (rA +rO )
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𝑡=
√2(rB +rO )
(12)
Herein, rA, rB and rO refer to the ionic radii of cations A, B and anion O, respectively.
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As an experimental fact, the structure will be cubic if 0.95 ≤ t ≤ 1.0. Compounds
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having tolerance factor in the range 0.75 ≤ t ≤ 0.95 are non-ferroelectric with distorted
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structure, while those with t ≥1.0 are ferroelectric. If t < 0.75, the compound does not
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crystallize into perovskite structure, but the ilmenite structure instead [27].
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Consequently, compared with the ideal cubic perovskite ABO3, perovskite-related structures arise from the loss of one or more of the symmetry operators in the basic
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cubic structure and exhibit lattice distortion to varying degrees, thereby resulting in the transformation of crystal phases in the following sequence: orthogonal, rhombohedral,
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tetragonal, monoclinic, and triclinic phase. As far as photocatalysis is concerned, lattice distortion has important impact on crystal field which changes the dipoles and
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electronic band structures, thereby influencing the behavior of photogenerated charge carriers, including excitation, transfer, and redox reaction, in the photocatalytic process [28]. In addition to the various crystal phases in perovskites-related structures just discussed, there are other perovskite-related compounds modified by the partial 9
doping/substitution of cations in A- and B-sites, as most metal elements in the periodic table are known to be stably located in perovskite structures. Such substitutions can alter the symmetry of the pristine structure and create cation or oxygen vacancies, which have a major effect on the electronic band structures and photocatalytic behaviors of these materials. Perovskite structures provide the stable basic framework
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and extensive space for the combination of elemental compositions and the construction of electronic band structures in photocatalysts [29]. Moreover, owing to the requirement
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of charge balance in A- and B-sites, the mixed oxidation states or unusual oxidation
states of metal cations are expected to be maintained in perovskite structures. With
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regard to photocatalysts, valence states of elements generally play a key role in
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determining the ability of light-response and the lifetime of photogenerated charge
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carriers. Accordingly, perovskite structures can be employed to guarantee the valence
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state control of metal cations in photocatalysts [30]. The A- or B-substituted sites are relatively dependent on the metal ionic radii. The metal ions substituted onto the B-site
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of perovskite can act as extrinsic acceptors, such as Fe3+ [31, 32], Co3+ [31, 33], Ni2+ [34], Cr3+ [35], Ca2+ [36], and Mg2+ [37], while those substituted onto the A-sites, e.g.
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Sm3+ [38], La3+ [39], Pr3+ [34], Nd3+ [40], and Mn2+ [41], can act as extrinsic donors. Moreover, some metal ions can be placed at both A- and B-sites such as Sn2+/4+ [42,
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43], Pb2+/4+ [42], Sb3+ [44], Er3+ [45], Dy3+ [46] and Ho3+ [46]. Some other relevant perovskite-related materials can be derived from the presence
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of anion excess (Dion-Jacobson phases), anion deficiency (brownmillerite, A2B2O5 as Sr2Fe2O5 or Ca2Fe2O5) or even the incorporation of other components into the structure (Ruddlesden-Popper and Aurivillius phases). Within this perovskite-related family, a number of layered variants of the perovskite structure are also known. The most common layered perovskites are the Dion-Jacobson phases (general formula An10
1BnO3n+1) as RbLaNb2O7, the Ruddlesden-Popper phases (general formula An+1BnO3n+1)
typified by Li2CaTa2O7 and the Aurivillius phases (general formula (An-1BnO3n+1)2‑ ) [15], where, in all cases, n represents the number of the perovskite-like layers. Among these perovskite-related classes, Aurivillius phases have been considered the most
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successful in providing significant photocatalytic results.
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Fig. 3. Representation of a Bi2WO6 Aurivillius phase (cyan, WO6 units; yellow, Bi
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atoms; red, O atoms) [15]. Copyright 2011, American Chemical Society. The Aurivillius phase consists of n perovskite-like layers (An-1BnO3n+1)2-
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sandwiched between fluorite-like A-oxygen sheets (A2O2)2+ [47], as depicted in Fig. 3. The A- and B-sites can accommodate a great variety of cations from (A) Na, K, Ca, Sr,
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Ba, Bi, etc. to (B) Fe, Cr, Ti, Ga, Nb, V, Mo, W, etc. Among these, Bi2WO6 is the simplest and the most frequently studied sample within this family for photocatalytic applications. In this material, the perovskite-like structure is defined by the WO6 units
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which form layers’ perpendicular to the [100] direction and are sandwiched between the (Bi2O2)2+ units [48]. Bi2WO6 is a promising visible-light-driven photocatalyst since it has a narrow bandgap with the interaction between 6s Bi and 2p O orbitals at the top of the VB [49]. Its high photocatalytic activity under visible light for organic 11
contaminant degradation, such as acetaldehyde (CH3CHO) [50], phenol [51], rhodamine B (RhB) [48] and methyl orange (MO) [52] has been reported. Another candidate with Aurivillius structure that has been reported in recent years is Bi2MoO6 [53]. Its crystal structure is similar to that described above but the W6+ cation is substituted by Mo6+ in the perovskite-like layers. Similarly, Bi2MoO6 with different
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morphology and structure, such as flower-like hollow spheres [54], cage-like hollow spheres [55], nanosheet-built frameworks [56] and nanotubes [57], possesses excellent
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visible-light-driven photocatalytic activity for degradation of organic contaminants.
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3. Design strategies of perovskite-related photocatalysts
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In general, the fundamental material design parameters that affect photocatalytic
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performance include 1) electronic structure; 2) surface structure and 3) crystal structure.
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Varies methods have been adopted for tailoring these structures aiming to enhance light absorption, extend light absorption range and suppress electron–hole recombination. In
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this section, the general bandgap engineering and charge recombination suppression strategies for perovskite and perovskite-related photocatalysts are first summarized.
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Then some specific implementations, including ion doping/substitution, noble metal
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decoration, heterojunctions formation and morphology regulation, are summarized and highlighted. These discussions are all based on their application or potential application for photodegradation of organic contaminants.
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3.1 General strategies 3.1.1 Bandgap engineering In UV-responsive photocatalyst, the bottom of CB, which consists mainly of empty transition metal d orbitals, is located at a potential slightly more negative than 0 eV, and the top of the VB, consisting of O 2p atomic orbitals, is more positive than 3 12
eV. As mentioned above, such a large bandgap makes the materials unable to harvest visible light, but most of these photocatalysts have sufficient potential to reduce O2 to O2•- and oxidize OH- to •OH. Donor-acceptor incorporation is one of the crucial strategies to engineer the bandgap (Eg). Using 4d and 5d cations as donors with higher atomic d orbital energies can guarantee that the CB maximum energy level is not
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lowered and high electron mobility is maintained [58]. However, in such doped photocatalysts, dopants not only act as visible light absorption centers with an
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absorption coefficient dependent on the density of dopants, but also as recombination sites for photogenerated charges. Incorporating 2p or 3p anions with higher atomic p
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orbital energies than that of O can raise the VB maximum [58], which results in a
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decrease of Eg without affecting the CB level, thus producing a visible-light-driven
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photocatalyst with band edge potentials suitable for photodegradation of organic
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contaminants. Besides, particle size and morphology of a photocatalyst, tailored by the preparation methods, can also affect its bandgap. Chen et al. [59] synthesized single-
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crystalline Bi4Ti3O12 nanosheets with rectangular shape and exposed {001} facets via a sol–gel hydrothermal process. It was found that the Eg of Bi4Ti3O12 nanosheet (2.9
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eV) was much narrower than that of conventionally calcined Bi4Ti3O12 powder (3.3 eV), which was mainly related to its small particle size and unique morphology [60]. Eg
3.7
3.6
3.1
2.8
2.6
3.2
4.7
3.2
2
oO 6
O 3 Nb
BiF
Ag
O2/O2
-
O2/HO2
3.4
3.4
3.3
2.7 2.5 3.1
2.7 2.4 OH/OH OH/H
2
3
O 2p
4
13
CdSnO3
4.1 3.4
3.2
O 3
Nb O 3
KN b
O 3
Na
aO 3 Ag Ta KT
TiO 3 Ba
iO 3 Zn TiO 3 Sr TiO 3
TiO 3
2.3
eO 3 Ag VO 3 Bi 2M
0 1
LiTaO3 NaTaO 3
NiT
-1
Eg/eV
Co
-2
Mg TiO 3 Ca TiO 3 Mn TiO 3 Fe TiO 3
Potential (eV vs. NHE)
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values and band positions of some typical perovskite are shown intuitively in Fig. 4.
-
O
Fig. 4. Eg values (in eV) and position of CB (up) and VB (down) for various perovskite semiconductors at pH = 7 vs. NHE. (Values are taken from references: MgTiO3 [61], CaTiO3 [62], MnTiO3 [63], FeTiO3 [64], CoTiO3 [65], NiTiO3 [65], ZnTiO3 [66], SrTiO3 [30], BaTiO3 [67], LiTaO3 [68], NaTaO3 [69], KTaO3 [70], AgTaO3 [71], NaNbO3 [72], KNbO3 [73], AgNbO3 [30], CdSnO3 [74], BiFeO3 [75], AgVO3 [76] and
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Bi2MoO6 [77]) 3.1.2 Charge recombination suppression
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Photogenerated charge recombination inhibits photocatalytic efficiency. Those
interband or surface states, in which holes are trapped to form recombination centers,
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are usually related to defects in the crystal structure or grain boundaries [78]. Improving
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the crystallinity of photocatalyst materials can reduce recombination probability, as the
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density of crystal defects is reduced with increasing crystallinity. Moreover, reducing
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the particle size of a photocatalyst shortens the diffusion pathway of charge carriers, leading to decreased recombination probability. Therefore, small particle size along
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with high crystallinity are advantageous for an increase in the probability of the photogenerated charge carriers reacting with organic contaminant molecules over
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recombining. Kato et al. [79] doped La in A-site of NaTaO3 to yield a reduction in the particle size as well as high crystallinity, resulting in 9 times higher photocatalytic
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activity than that of nondoped NaTaO3. Besides, the preparation methods have a strong effect on the morphological/crystal structure of perovskite materials, referring to
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crystallinity and particle size. The commonly used solid state reaction method requires annealing at high temperatures for a long time with frequent intermediate grindings, which makes difficulties in controlling the particle size. Several wet chemical methods were proposed to improve the homogeneity and crystallinity of perovskite materials at relatively low preparation temperatures, such as sol–gel and hydrothermal methods, 14
which can control the particle size and morphology flexibly [80]. These advanced preparation technologies are expected to be used for the development of highly active and visible-light-driven perovskite photocatalysts. In addition to regulating particle size and crystallinity via advanced preparation methods and ion doping to inhibit charge carrier recombination, other popular strategies include decoration with noble metal and
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formation of multi-component heterojunctions, which will be addressed in the
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following sections.
3.2 Specific implementations
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3.2.1 Ion doping/substitution
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From the initial perovskite structure, a double perovskite can be developed by the
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substitution of one of the original perovskite cations (A or B), which leads to either the
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AA’BO3 or ABB’O3 structures. This kind of suitable cationic substitution can alter the photophysical properties of the system and enhance visible light absorption.
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Doping/substitution of B-site elements
The undoped Ti-based perovskites always have wide bandgaps due to the large
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difference in electronegativity between Ti and O. The replacement of Ti by another element with higher electronegativity, such as Ni, is an effective approach to narrow
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bandgap. For example, LaNiO3, synthesized using a sol–gel combustion technique, exhibited a small Eg of 2.3 eV, suggesting potential visible-light-driven photocatalytic
A
activity [81]. Fei et al. examined the effect of partial substitution of Ti cations on the photocatalytic oxidation activity for Nile blue (NB) degradation over CeCoxTi1-xO3+δ, and observed significantly low bandgap (1.5~2.2 eV) as well as good visible photocatalytic activity over CeCo0.05Ti0.95O3.97 [33]. Borse et al.[82] reported similar situation for the Ba(M0.5Sn0.5)O3 system. The incorporation of Ti, V, Cr, Zr and Ce into 15
the Sn position was found to produce a clear improvement of the photocatalytic features, and a bandgap even lower than 2.5 eV was achieved in some cases. The presence of 3d, 4d, 4f or 6s orbitals, depending on the substitution, contributed to the drop of CB bottom, which in turn leaded to a narrowing of the bandgap of unmodified BaSnO3. A similar approach was adopted by Hur et al. for BaInO3 perovskite with the substitution of
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indium cations with electronegative non-transition metal cation Pb, and the participation of Pb 6s in the conduction band narrowed the Eg value of BaInO3 [42].
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Doping/substitution of A-site elements
Some researchers focused on reducing the Eg value and decreasing the particle size
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of perovskites via selective A-site doping. The undoped NaSbO3 was only active under UV-light irradiation due to the wide band gap (about 3.4 eV) [83]. Through the A-site
A
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replacement with Ag+, Kako et al. measured a narrow bandgap of 2.6 eV over AgSbO3
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[84]. Band structure calculation revealed that the conduction band minimum (CBM) and valence band maximum (VBM) of NaSbO3 were composed of Sb 5s and O 2p
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orbitals, respectively [83], whereas the CBM of AgSbO3 consisted of the hybridized Ag 5s and Sb 5s orbitals, and its VBM mainly consisted of the Ag 4d and O 2p orbitals,
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which was at a more negative level than O 2p orbitals [71]. Consequently, the Ag+ largely contributed to the bandgap narrowing through lifting up the VB. Kumar et al.
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[43] reported that the incorporation of Sn2+ on the A-site of the layered perovskite K2La2Ti3O10 made an obvious absorption edge red shift of approximately 100 nm from
A
that of the parent K2La2Ti3O10. The narrowing of the bandgap by Sn2+ doping was estimated to be 2.7 eV versus 3.6 eV for K2La2Ti3O10, which was related to the VB formation of Sn 5s and O 2p orbitals [43]. Li et al.[85] decreased the crystalline size of LaFeO3 through partial substitution of A-site La3+ in LaFeO3 with Ca2+, and an increase in the number of photogenerated e- was observed, as CaLa’ donor centers and oxygen 16
vacancies formed, leading to less charge recombination. Li et al.[86] studied the effect of the Ln element doping on the performance of La2Ti2O7 perovskite for photocatalytic MO degradation. Among the three doped perovskites La1.5Ln0.5Ti2O7 (Ln = Pr, Gd, Er), La1.5Gd0.5Ti2O7 exhibited the best photocatalytic activity, which was attributed to the half-filled f shell electronic configuration of Gd3+. Moreover, this particular
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characteristic of dopant ions could also promote charge transfer and separation through trapping electrons [86].
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Doping/substitution of both A- and B-site elements
Another doping structural possibility corresponds to quadruple perovskite
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structures, where both the A and B cations in the structure can be substituted by
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equivalent cations, resulting in perovskites with the empirical formula AA’BB’O3.
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Wang et al.[30] co-doped the Sr and Ti into AgNbO3 (Ag1-xSrx)(Nb1-xTix)O3 and
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attributed the improved visible light absorption and charge separation to the modulated band structure formed by a hybrid CB of the empty (Ti 3d + Nb 4d) orbitals and a
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hybrid VB of the occupied (O 2p + Ag 4d) orbitals (Fig. 5). A similar strategy was adopted by Comes et al. [87] for co-doping SrTiO3 with equal concentrations of La and
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Cr, and the as-prepared sample Sr1-xLaxTi1-xCrxO3 enhanced visible light absorption in epitaxial thin films while avoiding any compensating defects. Besides, in some cases,
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the B-site dopants could cause dual substitutions on both A- and B-site in a perovskite lattice. Su et al.[35] found the dual substitutions of a single Cr dopant on both A- and
A
B-site in NaTaO3 depending on the doping level of Cr. The undoped NaTaO3 had a bandgap of 4.1 eV (corresponding to a 300 nm absorption edge) with an excitation from O 2p to Ta 5d. Upon increasing the doping level of Cr3+, a systematic red shift of the absorption edge and the appearance of two broad absorptions in the range of 400~750 nm were observed, where in the range of 400~500 nm was ascribed to the charge 17
transfer from the 3d orbital of Cr3+ to the 5d orbital of Ta5+, and 550~750 nm corresponded to the d–d transition for Cr3+. The dual substitution yielded mid-gap levels between the CB and VB, which resulted in a decrease in the bandgap energies and
N
U
SC R
IP T
enhancement for visible light absorption.
A
Fig. 5. Schematic band structures of (a) AgNbO3, (b) (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3, and
ED
Doping/substitution of O-site
M
(c) SrTiO3 [30]. Copyright 2008, American Chemical Society.
Reducing Eg values of perovskite structured materials via selective O-site doping
PT
has also been widely studied. Nitrogen doping is known to shift the absorption edge toward higher wavelengths because of the contribution of N 2p orbitals to the VB
CC E
comprised of O 2p orbitals, thereby moving the VB upwards [43, 88]. After nitrogen doping, Eg of K2La2Ti3O10 was noticeable decreased from 3.9 to 3.4 eV [88]. Besides,
A
nitridation using urea resulted in not only nitrogen doping but also sensitization due to the presence of carbon nitride (CN) polymers, therefore decreased the bandgap of K2La2Ti3O10 to 2.9 eV and enhanced its visible light absorption [43]. In short, ion doping was proved as an effective method to alter the energy bands in perovskite structured photocatalysts and thus to change the photoresponse range. 18
Some concrete examples are summarized in Table 1.
photocatalysts Eg
Ion
Perovskite-related
/eV
doping
Eg
change
SC R
Perovskite
IP T
Table 1. Bandgap engineering by ion doping in different sites of perovskite
/eV
4.1
B-site
Fe-BaNbO3
BaNbO3 [31]
4.1
B-site
Co-BaNbO3
(Pb1/2Sr1/2)TiO3 [32]
3.7
B-site
(Pb1/2Sr1/2)(Ti1-xFex)O3
-0.2 -0.2
-3.9x
N
U
BaNbO3 [31]
1.4
B-site
-0.7
(δ = (3-2x)/3) (x > 0.1) LaFe0.875Mg0.125O3
-1.0
1.4
B-site
LaFe0.75Mg0.25O3
-1.3
2.4
B-site
Bi (Fe0.95Mn0.05)O3
+0.2
Ba(In1/2Nb1/2)O3 [42]
3.3
B-site
Ba(In1/3Sn1/3Nb1/3)O3
-0.3
Ba(In1/2Nb1/2)O3 [42]
3.3
B-site
Ba(In1/3Pb1/3Nb1/3)O3
-1.8
Ba(In1/2Ta1/2)O3 [42]
3.0
B-site
Ba(In1/3Sn1/3Ta1/3)O3
-0.1
Ba(In1/2Ta1/2)O3 [42]
3.0
B-site
Ba(In1/3Pb1/3Ta1/3)O3
-1.5
BaSnO3 [82]
3.4
B-site
Ba(Zr0.5Sn0.5)O3
-0.9
SrTiO3 [89]
3.2
B-site
SrTi1-xRhxO3
-1.5
CC E
BiFeO3 [41]
A
CeCoxTi1-xO3+δ
PT
LaFeO3 [37]
B-site
ED
LaFeO3 [37]
2.2
M
CeTiO4 [33]
A
(0 ≤ x ≤ 0.1)
(x = 0.005) SrTiO3 [89]
3.2
B-site
SrTi1-xRuxO3 (x = 0.005)
19
-1.3
SrTiO3 [89]
3.2
B-site
SrTi1-xIrxO3
-0.9
(x = 0.005) SrTiO3 [89]
3.2
B-site
SrTi1-xMnxO3
-0.5
(x = 0.005) Bi(Fe0.95Mn0.05)O3 [38]
2.6
A-site
(Bi1-xSmx)(Fe0.95Mn0.05)O3
+0.8x
3.2
A-site
Ba0.98La0.02SnO3−δ
-0.4
K2La2Ti3O10 [43]
3.6
A-site
(Sn1.0K0.2H0.9)La2Ti3O10
-1.0
SrTiO3 [90]
3.2
A-site
PdxSr1-xTiO3 (x = 0.2)
-0.2
AgNbO3 [30]
2.7
A/B
(Ag0.75Sr0.25)(Nb0.75Ti0.25)O3
+0.1
NaTaO3 [35]
4.1
A/B
Cr-NaTaO3
-1.0
SrTiO3 [87]
3.2
A/B
Sr1−xLaxTi1−xCrxO3
LaCoO3 [91]
2.1
A/B
La0.6Sr0.4Co0.8Fe0.2O3−δ
K2La2Ti3O10 [43]
3.6
O-site
N-K2La2Ti3O10
N
-0.1
K2La2Ti3O10 [43]
3.6
O-site
CN-K2La2Ti3O10
-0.7
K2La2Ti3O10 [88]
2.7
O-site
K2La2Ti3O10-xNx
-0.3
Sr2Nb2O7 [92]
2.9
O-site
Sr2Nb2O5N2
-0.7
SC R
U
A
ED
SrTiO3 [93]
IP T
BaSnO3 [39]
M
(0 ≤ x ≤ 0.1)
-0.9 -1.5
3.2
O-site
SrTiO2.75N0.125
-0.4
3.1
O-site
SrTiO2N
-1.0
NaTaO3 [69]
4.1
O-site
NaTaON2
-2.0
NaTaO3 [69]
4.1
A/O
Na0.25La0.75TaO1.5N1.5
-1.8
CC E
PT
SrTiO3 [94]
CHM: Composite-Hydroxide-mediated, OA-MBE: Oxygen-assisted molecular beam epitaxy
A
3.2.2 Multi-component heterojunctions Synergistic effect on photocatalytic performance can be achieved when different
materials are combined to form multi-component heterojunctions [22], mainly including multi-component solid solution and multi-component composites. The
20
components that are coupling with perovskite generally function as cocatalysts or photosensitizers. Multi-component solid solution When the mixing of two or more solid state constituents does not change the crystal structures, the resulting material is often referred as a solid solution. A key
IP T
advantage offered by solid solutions is that the properties can be precisely modulated in a systematic way. In principle, fine properties tuning over a wide range is possible
SC R
through the choice of “end” members and their concentrations. In addition, there is the possibility that unexpected properties may result beyond the simple average of the end
U
members’ properties [95]. Designing solid solutions among end-member materials with
N
analogous crystal structures but different electronic band structures has greatly
A
promoted the development of photocatalysts in recent years, because of the ease to
M
control their properties (e.g., band edge positions, charge carrier transport, and chemical stability) [95, 96]. On account of the highly susceptible substitution of different cations
ED
at both A- and B-sites of perovskite oxides, the derived solid solutions with versatile properties are noteworthy [95-97]. Due to the excellent photocatalytic performance of
PT
SrTiO3 and NaTaO3 under UV irradiation and the inability of the two materials to harness visible light, a series of SrTiO3- and NaTaO3-based solid-solution
CC E
photocatalysts, such as SrTiO3-BiFeO3 [95], SrTiO3–AgNbO3 [30, 97], NaTaO3– LaCoO3 [98], NaTaO3–LaCrO3 [99], and NaTaO3–NaBiO3 [100], were developed and
A
successfully applied to harvest visible light. Wang et al. [97] synthesized a series of solid solution semiconductors SrTiO3–AgNbO3 for the decomposition of organic pollutants under visible light irradiation. The mixed perovskites (AgNbO3)1-x(SrTiO3)x possessed a modulated energy band structure with their CB composed of the hybrid (Ti 3d + Nb 4d) orbitals and the VB constructed by the hybrid (O 2p + Ag 4d) orbitals. The 21
modulation of their band structure (bandgap energy, band edge positions, etc.) depended on the extent of orbital hybridization, as shown in Fig. 6. As a result of competition between the visible light absorption ability and the redox abilities, the highest visible light activity was realized over (AgNbO3)0.75(SrTiO3)0.25 [97]. Apparently, making solid solution oxide semiconductors with tunable electronic
IP T
structures is a feasible band engineering approach for the development of effective
M
A
N
U
SC R
visible-light-driven photocatalysts.
ED
Fig. 6. Band structures of AgNbO3, SrTiO3, and (AgNbO3)1-x(SrTiO3)x with x = 0.25
PT
and 0.95, respectively [97]. Copyright 2009, American Chemical Society.
In addition to solid solutions between two perovskite end-member materials, solid
CC E
solutions between end-member materials of one ABO3 and one ABX3 perovskite oxynitride, such as SrTiO3–LaTiO2N [101], BaZrO3–BaTaO2N [102], and CaZrO3–
A
CaTaO2N [103], were also designed by many research groups, aiming at lifting up VB and thus narrowing the bandgaps by incorporating N to take part in the formation of VB with the hybridization of N 2p and O 2p orbital. For example, the bandgap of (SrTiO3)1-x(LaTiO2N)x was reduced from 3.2 to 2.0 eV with increasing x from 0 to 0.3 [101]. Maeda et al. [102] reported a solid solution between BaZrO3 and BaTaO2N. With 22
increasing Zr/Ta ratio, the absorption edge shifted slightly to the shorter wavelengths, accompanied by a reduction of the background level in the wavelength range of 700~800 nm. The absorption edge shift was explained in terms of the difference in bandgap between BaTaO2N (ca. 1.7~1.8 eV) and BaZrO3 (ca. 4.8~4.9 eV). The lesspronounced background level observed in the solid solution materials compared to
IP T
BaTaO2N (Zr/Ta = 0) indicated that they had a lower defect density than BaTaO2N [102]. Besides, the analogous solid solution of CaZrO3-CaTaO2N was prepared,
SC R
demonstrating narrow bandgap, sufficient crystallization, enough pore volume, large
specific surface and well photocatalytic activity of (CaZrO3)0.2(CaTaO2N)0.8 [103].
U
Shortly, solid solution construction has been demonstrated as a powerful strategy for
N
designing perovskite-based photocatalysts, exhibiting the following merits: 1) The band
A
structures of photocatalysts can be optimized to mediate the competition between the
M
visible light absorption ability and the redox abilities; 2) The unfavorable valence states of multivalent metal cations and the lattice defect density can be depressed to reduce
ED
the recombination of photogenerated charge carriers. Multi-component composites
PT
Perovskite-based multi-component composites mainly include metal–perovskite composites [75, 104-106], semiconductor–perovskite composites (single metal oxide-
CC E
perovskite composites [107-109], perovskite-perovskite composites [110]), and dyeperovskite composites [111-113]. Husin et al. [104] designed LaxNa1-xTaO3
A
photocatalyst loaded with the classic Ni/NiO core/shell cocatalyst by reductionoxidation pretreatment, in which the existence of metallic Ni between the interface of NiO and LaxNa1-xTaO3 facilitated the transfer of electrons between both materials, and therefore enhanced the photocatalytic activity. Other cocatalysts, especially noble metal decoration, such as metallic Pt [105], Ag [106] and Au [75], acts not only as e23
sinks/reservoirs, spatially separating e- from photogenerated h+ in the semiconductor VB, but also as effective active sites for photocatalytic reduction reaction, thus serving as cocatalysts. Besides, coupling with Ag and Au would also extend the absorption to the visible region through the plasmonic effect and simultaneously accelerate the separation of photoexcited charge carriers through Schottky barriers [114]. Xu et al.
IP T
[106] synthesized Ag-decorated ATaO3 (A = K, Na) nanocubes and claimed that visible light was harvested by large Ag nanoparticles (10 nm) as surface plasmon resonance
SC R
(SPR) which was excited and then transferred to the ATaO3 (A = K, Na) CB. Meanwhile, the Ag SPR that enhanced the local electric field for ATaO3 (A = K, Na)
U
promoted charge separation under UV light irradiation. The small Ag nanoparticles (6 nm) on ATaO3 (A = K, Na) acted as an effective cocatalyst and then trapped
A
N
photogenerated e- which came from the CB of ATaO3 (A = K, Na). So under UV−vis light irradiation, synergetic and promotion effects on the separation of the
CC E
PT
ED
M
photogenerated electron−hole pairs were achieved, as illustrated in Fig. 7.
Fig. 7 Promotion effects of Ag particles on the photocatalytic activity of ATaO3 (A =
A
K, Na) under UV-vis light irradiation [106]. Copyright 2015, American Chemical Society. For semiconductor–perovskite composites, the formed heterojunction can be classified into three different types, as depicted in Fig. 8. In a type I heterojunction, the VB and CB of semiconductor B are respectively lower and higher than the 24
corresponding bands of semiconductor A. Since e- and h+ gain energy by moving down and up respectively, all charge carriers are accumulated on semiconductor A, which yields no improvement to charge carrier separation. Moreover, a redox reaction takes place on the semiconductor A with the lower redox potential, thereby significantly reducing the redox ability of the heterojunction photocatalyst. For example, the Fe3O4-
IP T
Bi2MoO6 composite belongs to this type of heterojunction [115]. A type II heterojunction, whose CB and VB levels of semiconductor B are higher than the
SC R
corresponding levels of semiconductor A, provides the optimum band positions for charge carrier transportation. Therefore, under light irradiation, photogenerated e- and
U
h+ are spatially separated from each other, reducing the recombination probability
N
significantly and increasing electron lifetimes, which can be proven by transient
A
spectroscopic techniques. However, similar to the type I heterojunction, the redox
M
ability of the type II heterojunction photocatalyst will be also reduced because the reduction reaction and the oxidation reaction take place on semiconductor A with lower
ED
reduction potential and on semiconductor B with lower oxidation potential, respectively. In the past several decades, enormous efforts have been made to prepare various type
PT
II heterojunction photocatalysts, such as TiO2-Bi2WO6,[50] TiO2-Bi2MoO6,[108] TiO2SrTiO3 [116] and so on, for enhancing their photocatalytic activity. Moreover, when
CC E
the heterogeneous photocatalyst combines p-type (B) and n-type (A) semiconductors, a p-n junction would form and the band architecture is the same as type II. However,
A
the electron–hole separation effciency in p–n heterojunction photocatalysts is faster than that of type-II heterojunction photocatalysts due to the synergy between the internal electric field and the band alignment in p–n interface. The architecture of type III heterojunctions is similar to that of type II semiconductors, only that the bandgaps do not overlap. Such arrangements of band positions are also called broken-gap 25
situations. The electron–hole migration and separation between the two semiconductors cannot occur for the type III heterojunction, making it unsuitable for enhancing the
SC R
IP T
separation of electron–hole pairs.
Fig. 8. Different types of semiconductor heterojunctions [22]. Copyright 2014, WileyVCH.
U
Heterojunctions can be formed by coupling other band-structure-matching
N
metal oxides with perovskite photocatalysts (type II) to accelerate the charge separation.
A
Kong et al. [116] synthesized SrTiO3 (STO) on TiO2 nanotubes (TN) by the in-situ
M
hydrothermal techniques and then simultaneously sensitized with carbon nitride
ED
polymer (CN) and doped with nitrogen (CN-STO/TN) for photocatalytic toluene mineralization. In comparison to the corresponding pristine semiconductor
PT
photocatalysts, CN-STO/TN simultaneously exhibited the following advantages: (1) the nanotube structure enhanced light-harvesting, charges transport and separation,
CC E
retained sufficient energy of the charges, facilitated the pollutant species transport and reactants enrichment by the confinement effect; (2) synergistic effect of CN
A
sensitization and N-doping in O-site extended the light response to the visible light region; (3) multichannel charges separation and transport among the multijunctions promoted the photogenerated carriers separation, as illustrated in Fig. 9. These beneficial factors leaded to the remarkably higher visible light photocatalytic performance of CN-STO/TN. 26
IP T
Fig. 9. Diagram for the band levels and the proposed electron-hole pairs separation of (a) STO/TiO2 (or STO/TN) and (b) CN-STO/TiO2 (or CN-STO/TN) heterojunctions
SC R
[116]. Copyright 2017, American Chemical Society.
Compared to the conventional heterojunction described above, p–n heterojunction
U
photocatalyst shows higher efficiency in accelerating the electron–hole migration
N
across the heterojunction through providing an additional electric feld. Kong et al. [117]
A
prepared hybrid CuxO/SrTiO3 by in-situ reduction of CuSO4 using ascorbic acid over
M
SrTiO3 at room temperature. CuxO is a p-type semiconductor with Fermi level close to
ED
VB, while SrTiO3 is n-type, whose Fermi level lies close to CB. When coupling p-type CuxO with n-type SrTiO3 in the intimate contact, their Fermi levels tended to align. At
PT
the same time, an inner electric field formed at the interface of p–n heterojunctions, and promoted the transfer and separation of electron–hole pairs efficiently for the surface
A
CC E
redox reaction, as present in Fig. 10.
27
IP T
SC R
Fig. 10. Mechanism of photocatalytic aromatic VOCs degradation and homeostasis over CuxO/SrTiO3 [117]. Copyright 2017, The Royal Society of Chemistry.
U
Heterojunctions can also be formed by coupling two perovskite-related
N
photocatalysts to suppress charge recombination. Both Bi4Ti3O12 and Bi2XO6 (X= Mo,
A
W) are typical Aurivillius oxides, which have been explored as visible-light-driven
M
photocatalysts [50, 59]. But their overall photocatalytic efficiencies are all low because
ED
of the rapid recombination of electron–hole pairs. Coupling Bi4Ti3O12 with Bi2XO6 (X = Mo, W) formed an effective photocatalyst because of their structural analogy [110].
PT
Bi2XO6 (X = Mo, W) and Bi4Ti3O12 are all consisted of [Bi2O2]2+ layers (X–Bi–O–Bi– X) sandwiched between two slabs of X ions, atoms or groups (where X = Bi2Ti3O102−,
CC E
MoO42− or WO42−). They can easily grow together to form heterostructures through ion exchange process. The relative CB and VB positions of Bi4Ti3O12 and Bi2XO6 facilitate the photogenerated carriers’ separation that the photoexcited electrons from CB of
A
Bi4Ti3O12 can transfer to the CB of Bi2XO6, and the holes can be transferred reversely from the VB of Bi2XO6 to that of Bi4Ti3O12, as shown in Fig. 11. Such a synergistic effect between Bi4Ti3O12 and Bi2XO6 with the modified electronic band structure enhanced charge transition, reduced recombination of carriers, and prolonged the charge lifetime [110]. 28
IP T
SC R
Fig. 11. Schematic of the band structures of Bi4Ti3O12/Bi2XO6 (X=Mo, W) heterostructures and possible electron–hole separations [110]. Copyright 2014, Elsevier
U
B.V.
N
The coupled semiconductors and perovskite photocatalysts can not only inhibit
A
electron–hole recombination, but also act as photo-sensitizers to harness low-energy
M
photons that could not be utilized by the perovskite host due to the wide bandgaps [107].
ED
A series of SrTiO3-based heterostructured photocatalysts (such as SrTiO3/Fe2O3 SrTiO3/BiFeO3 [107], CuxO/SrTiO3 [117], ZnFe2O4/SrTiO3 [118], and C3N4/SrTiO3
PT
[119]) have been developed and successfully applied to visible light response. Luo et al. [107] prepared composites of SrTiO3 with small bandgap metal oxides, including
CC E
Fe2O3 and BiFeO3. Both SrTiO3/Fe2O3 and SrTiO3/BiFeO3 systems exhibited visiblelight-driven photocatalytic activity, while pure SrTiO3 held little visible light activity due to the wide bandgap. In addition to semiconductors, dyes (such as porphyrinoids
A
[111, 112] and EosinY [113]) could also be employed as sensitizers to enhance light response. Upon light excitation, the adsorbed dye molecules injected electrons from their excited states into the CB of the wide bandgap perovskite semiconductor and then reacted with the oxygen adsorbed on the photocatalyst to superoxide radical. This 29
process makes the wide bandgap perovskite achieve visible light response. In short, constructing perovskite-based composites can not only accelerate the separation of photogenerated charge carriers by matching band structures in heterostructures, but also improve the utilization of incident light through photosensitization.
IP T
3.2.3 Micro-/nanostructural adjustment
Various preparation strategies have been reported to adjust the micro-
SC R
/nanostructures of perovskite-based photocatalysts [15], aiming at optimizing the photocatalytic process. The techniques used for the synthesis of nanostructured
U
perovskites include mainly solid state reaction and wet chemical methods. Solid state
N
method is the most commonly used one to prepare perovskite-based materials, but the
A
high-temperature process generally results in the as-prepared materials with defects,
M
large particle sizes and low surface areas, leading to the ultrafast electron–hole recombination and low activity [120]. In order to circumvent such limitations, a number
ED
of alternative preparation strategies have been attempted to reduce the calcination temperature, which is necessary for perovskite phase formation. The recently evolved
PT
high energy ball milling technique is popularly known to activate solid state reactions at low temperatures due to the increased surface area and the formation of different
CC E
crystal lattice defects in the milled powder particles [121]. To overcome the heterogeneity drawback of the solid state methods and achieve
A
nanocrystalline phase pure powders, several groups focused on wet chemistry methods including ultrasonic method [94], sol–gel method [122], precipitation method [123, 124], electrospinning technique [66, 125] and hydrothermal method [77, 108, 126]. Among these methods, hydrothermal synthesis is widely acknowledged as a powerful tool to tailor perovskite-based micro-/nanostructures with high crystallinity and purity, 30
few defects, small sizes, large surface areas, specific orientation and novel
N
U
SC R
IP T
morphologies [126].
A
Fig. 12. Shape evolution of Bi2WO6 micro-/nanostructures prepared with different pH
ED
M
values and/or surfactants [126]. Copyright 2011, Elsevier B.V.
Zhang et al. [126] successfully synthesized different shapes of Bi2WO6 including
nanoplates,
nanoparticles,
and
flower/sphere-like,
PT
nanostructures,
nest/tyre/helix-like and nanocage-like superstructures, through tuning the parameters in
CC E
hydrothermal processes such as pH value, surfactants and/or template, as summarized in Fig. 12. Apart from tailoring morphology, hydrothermal methods can also be used
A
for adjusting the crystal structure of photocatalyst, such as crystal orientation and exposed facets. Wang et al. [127] synthesized SrTiO3 single crystals enclosed with high-indexed {023} facets and {001} facets via a one-pot hydrothermal method. The ratio of {023} to {001} facets could be simply adjusted by tuning the NaOH and ethanolamine (EA) concentrations during the hydrothermal growth process. Li et al. 31
[128] employed hydrothermal method to prepare a large single crystal of monoclinic BiVO4 with two exposed facets {010} and {110} to explore its mechanism. They found that the spatial separation of photogenerated electrons and holes between {010} and {110} facets, evidenced by facet-selective photo-deposition of metals and oxides, were mainly owing to the difference in energy levels of these two facets. Since the
IP T
photocatalytic activity of the perovskite- related photocatalysts closely interrelates with
their morphology and crystalline structure, the photocatalytic performance of
SC R
perovskite-based photocatalysts can be adjusted by preparation strategies through controlling the physicochemical properties, such as crystallinity, crystal size, crystal
N
U
orientation, exposed facets, surface area and morphology.
A
4. Application for photocatalytic organic contaminants decomposition
M
As an effective and stable photocatalyst for solar energy harvesting and organic contaminants decomposition, several critical requirements must be satisfied [129]. First
ED
of all, the catalyst light harvesting antenna must have a large enough bandgap to provide energetic electrons (Eg ≥ 1.2 eV, typically >2.0 eV). At the same time, the
PT
semiconductor must have a small enough bandgap to allow for effective absorption
CC E
overlap with the solar spectrum (Eg ≤ 3.0 eV). Second, the catalyst is beneficial to drive the charge separation and transportation efficiently. Furthermore, the catalyst light harvesting antenna should be closely integrated with selected redox catalysts to allow
A
for effective utilization of photogenerated charges for desired photochemical reactions. Lastly, there should be a mechanism to protect the catalysts from direct photochemical reactions to ensure photocatalytic stability. Generally speaking, an individual unmodified perovskite material can hardly satisfy all these requirements, therefore
32
modification in single phase and heterogeneous photocatalysts have become the focus of recent researches.
4.1 Single Phase Systems Although the typical single phase perovskite-related materials exhibit low
IP T
photocatalytic activity for organic contaminants removal [130], they still can perform
well in photocatalytic applications under sunlight use upon suitable treatments [131],
SC R
such as hydroxide treatment, ion doping and structure alteration. Considering that photocatalysis is a typical surface process and the surface functionalization can regulate
U
the performance of the photocatalysts at no cost of the unique properties of the bulk
N
materials, Kong et al. [132] modified SrTiO3 with surface hydroxylation by a hydrothermal treatment. It was found that the enriched surface hydroxyl groups could
M
A
act as hole scavengers to form •OH, and promote the separation of electron-hole pairs. Meanwhile, the hydroxide ions shifted the surface energy band of SrTiO3 to a more
ED
negative level, which facilitated the reduction of the adsorbed oxygen to form O2•-, another important kind of oxidant. Therefore, greatly enhanced photocatalytic
PT
performance of toluene mineralization over the modified SrTiO3 (SrTiO3-OH) was obtained, that 500 ppm toluene was completely oxidized into CO2 after UV light
CC E
illumination (λ = 300~400 nm, 80 mW cm-2) for 6 h at ambient temperature, showing about 2.9 times larger than that for SrTiO3, 1.7 times higher than P25 [132] . Similar
A
strategies have been reported in other literatures [123, 124, 133]. Besides, ion doping in single phase perovskite materials is another popular technology to produce photocatalysts with adequate electronic characteristics for sunlight operation applications. Wang et al. [30] developed a novel highly active perovskite-type photocatalysts (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 for efficient decomposition of acetaldehyde 33
(CH3CHO) under visible-light irradiation. As this configuration is favorable for narrowing the bandgap and promoting charge carrier transportation, a good compatibility between the absorbance of visible light and the redox abilities was obtained in (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 with a modulated band structure, thus giving rise to a significantly enhanced photocatalytic activity for CH3CHO decomposition.
IP T
Upon visible light irradiation (λ > 400 nm), more than 80% of 500 ppm CH3CHO was decomposed into CO2 in the short linear stage. Apart from ion doping, structure
SC R
alteration is another method used to enhance the photocatalytic activity of individual perovskite materials. Marchelek et al. [134] reported that 64% of 100 ppm toluene
U
degraded into CO2 after UV irradiation (λ = 310~375 nm) for 1 h over KTaO3 with
N
cubic structure, showing higher activity than octahedral KTaO3, as the crystal structure
A
could affect the optical property. Li et al. [135] synthesized uniform erythrocyte-like Bi2WO6 hierarchical architecture via a facile hydrothermal process and reported that
M
99% of 10 mg L-1 Rhodamine B (RhB) could be removed after visible light illumination
ED
(λ > 400 nm) for 15 min over the erythrocyte-like Bi2WO6. This photocatalytic activity was obviously improved relative to that for the typical Bi2WO6 microspheres, as its
PT
erythrocyte-like unique hierarchical architecture exhibited high specific surface area and abundant mesoporous distribution, which provided more reaction active sites and
CC E
molecule transport channels. Moreover, the novel structure also facilitated charge carriers transfer and separation. Besides, the possible photocatalytic mechanism of RhB
A
decomposition has also been illuminated that a small part of photogenerated electrons are captured by the dissolved oxygen in the water to produce superoxide radicals O2•− degrading RhB molecules, and most of electrons react with extra H2O2 molecules in the water to yield abundant hydroxyl radicals •OH to remove RhB molecules. Meanwhile, the holes can also directly degrade RhB molecules. It should be pointed out that H2O2 34
here serves as an efficient electron scavenger, which can also facilitate separation and transfer of charge carriers to improve the photocatalytic activity. More state-of-the-art examples of the single phase perovskite-related photocatalysts application for organic contaminants removal are summarized in Table 2 and Table 3.
Eg
Conversion
mCatal
Light
(%)
(mg)
(λ/nm)
Organics (eV)
Ref.
SC R
Photocatalyst
IPA SrTiO3
3.3
λ>300
[136]
-
310~375
[134]
98%, 6 h
200
300~400
[132]
44%, 1 h
-
310~375
[134]
64%, 1 h
-
310~375
[134]
~100%, 3 h (100 μmol)
3.2
U
C 7H 8 SrTiO3
41%, 1 h
N
(100 ppm) C 7H 8 3.3
A
SrTiO3-OH
IP T
Table 2 Single phase systems for gaseous organic contaminants photodegradation.
100
M
(500 ppm) C 7H 8
Octahedral KTaO3
3.4
ED
(100 ppm) C 7H 8
Cubic KTaO3
3.3
CC E
AgNbO3
PT
(100 ppm)
(Ag0.75Sr0.25)(Nb0.75Ti0.25)O3
A
PbBi2Nb2O9
C 7H 8 2.7
15%
300
λ≥220
[137]
80%, 3 h
400
λ>400
[30]
7.5%, 5 day
300
λ≥420
[28]
8.7%, 5 day
300
λ≥420
[138]
19%, 3 h
200
400~700
[122]
(100 ppm) CH3CHO 2.8 (300 ppm) CH3CHO 2.9 (200 ppm) IPA
PbBi2Nb2O9
2.9 (200 ppm)
KLaTi2O6
3.4
C6H5Cl
35
KLaTi1-xNixO6
3.4
C6H5Cl
28%, 3 h
200
λ≥400
[122]
KLaTi1-xZnxO6
3.3
C6H5Cl
38%, 3 h
200
λ≥400
[122]
KLaTi1-xMnxO6
2.9
C6H5Cl
46%, 3 h
200
λ≥400
[122]
KLaTi1-xCoxO6
2.9
C6H5Cl
50%, 3 h
200
λ≥400
[122]
ZnSn(OH)6
4.1
66%
100
254
[139]
89%
100
254
C 6H 6 (250 ppm)
4.1
IP T
C 6H 6 MgSn(OH)6
(250 ppm)
4.7
21% (300 ppm) C 6H 6
Sr2Sb2O7
4.2
24%
U
(220 ppm) CH3CHO 2.6
75%, 5 h
2.8
M
HCHO Bi2WO6
300
254
[140]
300
254
[141]
300
λ>420
[50]
-
λ≥400
[142]
A
(100 ppm)
N
Bi2WO6
SC R
C 6H 6 Zn2GeO4
[139]
99%, 80 min
ED
0.28 mg L-1
Table 3. Single phase systems for liquid organic contaminants photodegradation.
PT
Eg
Photocatalyst
Degradation
mCatal
Light
(%)
(mg)
(λ/nm)
Organics
Ref.
(eV) 2.2
MO (5 mg L-1)
~100%, 7.5 h
300
λ≥400
[143]
BiFeO3
2.1
MB (10 mg L-1)
20%, 2.5 h
50
λ≥420
[109]
BiFeO3
2.1
RhB (6 mg L-1)
42%, 4 h
160
λ>420
[144]
BiFeO3
2.5
RhB (10 mg L-1)
18%, 7 h
-
λ≥400
[75]
BiFeO3
2.2
RhB (5 mg L-1)
55.1%, 7 h
Sunlight
[145]
BiFeO3
2.2
CR (0.1 g L-1)
7%, 2 h
-
λ≥400
[125]
Bi1-xBaxFeO3
2.2
CR (0.1 g L-1)
38%, 2 h
-
λ≥400
[125]
LaFeO3
2.5
MB (10 mg L-1)
55%, 1.5 h
50
λ≥400
[85]
A
CC E
BiFeO3
36
100
2.0
MB (10 mg L-1)
86%, 1.5 h
50
λ≥400
[85]
SrTiO3
3.1
MB (10 ppm)
45%, 3 h
100
λ>320
[94]
SrTiO3
3.2
MB (10 ppm)
35%, 80 min
100
λ>420
[146]
SrTiO3-xNx
2.9
MB (10 ppm)
85%, 80 min
100
λ>420
[146]
CaTiO3
3.2
MB (10 ppm)
30%, 3 h
100
λ>320
[94]
CaTiO3
3.6
As(III) (2 mg L-1)
99%,10 min
80
λ320
[94]
BaTiO3
3.0
RhB (10 ppm)
15%, 45 min
150
sunlight
[67]
ZnTiO3
3.1
RB (10 mg L-1)
18%, 4 h
LaNiO3
2.2
MO (10 mg L-1)
80%, 5 h
NaBiO3
2.6
MB (16 mg L-1)
99%, 10 min
AgNbO3
2.8
MB (10 ppm)
92%, 5 h
AgNb7O18
2.8
MB (10 mg L-1)
95%, 4 h
NaNb0.5Ta0.5O3
3.3
RhB (5 mg L-1)
Ba(In1/2Nb1/2)O3
3.3
4-CP
Ba(In1/3Sn1/3Nb1/3)O3
3.0
4-CP
[66]
400
λ>400
[81]
300
λ≥420
[148]
100
λ≥220
[137]
N
SC R λ>420
100
λ>420
[149]
99%, 1h
50
UV
[150]
76%, 4 h
-
UV-vis
[42]
68%, 4 h
-
UV-vis
[42]
M
A
U
50
1.5
4-CP
~100%, 1.5 h
-
UV-vis
[42]
3.0
4-CP
90%, 4 h
-
UV-vis
[42]
PT
Ba(In1/2Ta1/2)O3
ED
Ba(In1/3Pb1/3Nb1/3)O3
IP T
La1-xCaxFeO3
2.9
4-CP
76%, 4 h
-
UV-vis
[42]
Ba(In1/3Pb1/3Ta1/3)O3
1.5
4-CP
~100%, 2 h
-
UV-vis
[42]
Bi4Ti3O12
2.9
RhB (6 ppm)
90%, 1.5 h
80
375~475
[59]
Bi4Ti3O12
2.9
RhB (10 ppm)
92%, 1 h
50
λ>420
[151]
Bi4Ti3O12
3.1
MO (5 mg L-1)
~100%, 2 h
-
360
[152]
Bi4Ti3O12
3.2
Phenol (20 mg L-1)
99%, 2 h
50
UV
[153]
CeCoxTi1-xO3+δ
1.6
NB (30 ppm)
91%, 3 h
-
UV-vis
[33]
Bi2WO6
2.9
RhB (20 μM)
80%, 2 h
50
λ>420
[154]
Bi2WO6
2.6
RhB (10 μM)
58%, 5 h
100
λ>420
[50]
A
CC E
Ba(In1/3Sn1/3Ta1/3)O3
37
2.9
RhB (10 μM)
65%,3 h
80
λ≥420
[155]
Bi2WO6
2.8
RhB (10 mg L-1)
96%, 40 min
50
λ>400
[156]
Bi2WO6
2.8
Phenol (10 mg L-1)
90%, 40 min
50
λ>400
[156]
Bi2WO6
2.8
Phenol (20 mg L-1)
12%, 2 h
50
λ>420
[51]
Bi2WO6
2.8
MO (20 mg L-1)
60%, 40 min
20
λ≥420
[52]
Bi2WO6
2.7
MB (20 mg L-1)
30%, 2.5 h
25
λ>420
[157]
Bi2WO6
2.9
BPA (10 mg L-1)
70%, 80 min
50
Sunlight
[158]
Bi2WO6
2.8
DCP (30 mg L-1)
32%, 3 h
50
λ>420
[159]
Bi2WO6 nanocrystals
2.9
RhB (10 μM)
13%, 30 min
20
λ≥420
[48]
Bi2WO6 monolayers
2.7
RhB (10 μM)
98%, 25 min
Bi2WO6 microsphere
2.7
RhB (10 mg L-1)
99%, 35 min
2.7
RhB (10 mg L-1)
99%, 15 min
Bi2-xCdxWO6
2.6
RhB (20 μM)
99%, 40 min
N
Bi2MoO6
2.6
ARS (5 mg L-1)
Bi2MoO6
2.8
Bi2MoO6
2.6
SC R 20
λ≥420
[48]
50
λ>400
[135]
50
λ>400
[135]
100
Sunlight
[160]
U
erythrocyte-like
A
Bi2WO6
IP T
Bi2WO6
-
λ>420
[108]
MB (10 μM)
85%, 2 h
100
λ>420
[161]
MB
50%, 0.5 h
410
[162]
ED
M
55%, 4 h
-
2.6
MB (20 mg L-1)
82%, 4 h
60
Sunlight
[57]
2.7
RhB (10 mg L-1)
95%, 2 h
50
λ>420
[54]
2.6
RhB (10 μM)
~100%, 0.5 h
20
λ≥420
[56]
2.6
Phenol (20 mg L-1)
99%, 1.5 h
100
λ>420
[55]
Er3+/Bi2MoO6
2.6
RhB (10 μM)
~100, 50 min
50
λ≥420
[163]
La2Ti2O7
3.9
MO (10 μM)
55%, 1.5 h
150
λ420
[50]
73%, 3 h
10
λ400
[176]
A
430
400~780
96%, 6 h (1000 ppm)
CN-SrTiO3/TiO2
[134]
N
380
200
U
C7H8 SrTiO3/TiO2
SC R
(100 ppm)
IP T
C7H8 KTaO3/CdS/ MoS2
M
(1000 ppm) C7H8 CuOx/SrTiO3
600
ED
(500 ppm) TiO2@SrTiO3@Pt@Bi2O
C7H8
550
(500 ppm)
PT
3@Pt
CaFe2O4/PbBi2Nb1.9W0.1
CH3CHO
520
CC E
O9
540
BiVO4/RGO/Bi2O3
564
A
BiVO4/TiO2
BiVO4/g-C3N4
(200 ppm) C7H8 C7H8 (25 ppm) C7H8
570 (25 ppm)
42
Table 5. Heterogeneous systems for liquid organic contaminants photodegradation. Abs. Photocatalyst
Degradation
mCatal
Light
(%)
(mg)
(λ/nm)
Organics
Ref.
(nm) RhB Ag/BiFeO3
496
32%, 7 h
λ≥400
[75]
28%, 7 h
λ≥400
[75]
Sunlight
[145]
-1
(10 mg L )
496
IP T
RhB Au/BiFeO3
(10 mg L-1) RhB Ag@BiFeO3
620
79.3%, 7 h
RhB Ag/BaTiO3
620
~100%, 45 min
Sunlight
[67]
40
λ≥420
[167]
~100%, 45 min
100
λ>420
[168]
88%, 2 h
-
λ≥400
[177]
92%, 2 h
50
λ≥420
[178]
~100%, 2.5 h
50
λ≥420
[109]
94%, 3 h
200
λ≤400
[179]
92%, 3 h
200
λ≤400
[179]
~100%, 40 min
10
313
[180]
99%, 100 min
-
365
[181]
(10 ppm)
490
18%, 40 min
N
(10 μM)
600
M
(20 μM)
A
RhB Ag/AgCl/ Bi2MoO6
150
U
RhB Au/Bi2MoO6
100
SC R
(5 mg L-1)
MO Pt/Fe-Bi2Ti2O7
517
ED
(10 μM) Phenol
LaFeO3/Ag3PO4
760
PT
(20 mg L-1)
CC E
TiO2/ BiFeO3
A
ATiO2-BaTiO3
RTiO2-SrTiO3
MB
435
(10 mg L-1) Phenol
376 (50 mg L-1) Phenol 400 (50 mg L-1) RB
SrTiO3/TiO2
388 (10 mg L-1)
SrTiO3/TiO2
388
MB 43
(10 μg L-1) Phenol SrZrO3/Cu2O/Bi2O3
570
99%, 40 min
7.5
UV-Vis
[182]
96%, 60 min
7.5
UV-Vis
[182]
~100%, 40 min
300
λ≥420
[183]
~100%, 3 h
80
λ≥420
(20 M) Phenol BaZrO3/Cu2O/Bi2O3
570 (20 M)
550 (20 mg L-1) RhB
Bi2O3/ Bi2WO6
550
MO Bi2WO6/TiO2
460
[52]
100
λ>420
[50]
~100%, 1 h
50
λ>400
[184]
84%, 2 h
100
Sunlight
[185]
90%, 1.5 h
50
λ≥400
[110]
52%, 2 h
50
λ>420
[51]
57%, 2.5 h
25
λ>420
[157]
95%, 3 h
100
λ>420
[186]
98%, 3 h
50
λ>420
[159]
80%, 2 h
50
λ>420
[154]
~100%, 5 h
N
(10 μM)
415
Bi2WO6-TiO2/
M
(10 mg L-1)
A
RhB Bi2WO6/TiO2
20
U
RhB 459
[155]
λ≥420
96%, 40 min (20 mg L-1)
Bi2WO6/TiO2
SC R
(10 μM)
IP T
MO YBiO3/Bi2O3
RhB -
(10 μM)
ED
nickel foam
RB
Bi4Ti3O12/ Bi2WO6
443
PT
(10 ppm)
CC E
Bi2S3/ Bi2WO6
BiVO4/ Bi2WO6
A
Fe3O4@Bi2WO6
Phenol
800
(20 mg L-1) MB
600 (20 mg L-1) RhB 650 (10 μM) DCP
BiOI/Bi2WO6
599 (30 mg L-1)
Graphene/Bi2WO6
443
RhB
44
(20 μM) BPA Bi2WO6- RGO
450
90%, 80 min
50
Sunlight
[158]
50%, 40 min
40
λ≥420
[167]
~100%, 40 min
40
λ≥420
[167]
93%, 5 h
100
λ>420
(10 mg L-1) RhB RGO/ Bi2MoO6
490 (10 μM) RhB
RGO/ Bi2MoO6/Au
590
IP T
(10 μM) RB Bi2MoO6/TiO2
448
ARS Bi2MoO6/TiO2
477
λ>420
[108]
Fe3O4/Bi2MoO6
460
MB
-
410
[162]
100
λ>420
[115]
98%, 1.5 h
50
λ≥400
[110]
50%, 3 h
20
Vis-NIR
[187]
~100%, 2.5 h
50
λ≥400
[169]
~100%, 60 min
100
λ≥400
[188]
~100%, 40 min
100
λ≥400
[188]
~100%, 50 min
100
λ≥400
[188]
~100%, 60 min
100
λ≥400
[188]
95%, 0.5 h
U
470
RhB
N
92%, 3 h
A
(10 mg L-1)
459
M
RB Bi4Ti3O12/ Bi2MoO6
[77]
-
85%, 4 h (5 mg L-1)
Bi2MoO6/C
SC R
(10 mg L-1)
(10 ppm) SA 653
ED
Ca2Fe2O5/ZnFe2O4
(10 mg
L-1)
4-CP
500
PT
Pd/AgBr/BiVO4
CC E
InVO4-BiVO4
A
Au/InVO4-BiVO4
Ag/InVO4-BiVO4
(15 mg L-1) RhB
484
(15 mg L-1) RhB 488 (15 mg L-1) RhB 490 (15 mg L-1) RhB
Pd/InVO4-BiVO4
496 (15 mg L-1)
45
RhB Pt/InVO4-BiVO4
488
~100%, 70 min
100
λ≥400
[188]
(15 mg L-1) ATiO2: Anatase; RTiO2: Rutile; RGO: Reduced graphene oxide RhB/RB: Rhodamine B; MO: Methyl orange; MB: Methylene blue; SA: Salicylic acid; BPA:
IP T
Bisphenol A; DCP: 2, 4-dichlorophenol; 4-CP: 4-chlorophenol
5. Concluding remarks and future perspectives
SC R
Photocatalytic oxidation is an effective technology for organic contaminants removal. Perovskite and perovskite-related structures offer a broad scope in designing
novel photocatalysts for this process. The doping and substitution of A- and B-sites as
U
well as the O site are useful for reducing the bandgaps. Besides, forming
N
multicomponent heterojunctions is another promising strategy to improve
A
photocatalytic activity of perovskite photocatalysts through adjusting band structure for
M
promoting the solar energy harvest and charge carrier separation. By selectively decorating the surface of perovskite or perovskite-related photocatalysts with co-
ED
catalysts, more active heterojunctions can be formed through the directional charge
PT
transfer to the desired phase surface in the composite photocatalyst. Moreover, the performance of perovskite and perovskite-related photocatalysts can also be regulated
CC E
through the tailoring of their particle size, crystallinity and morphology. All these controllable physicochemical properties provide perovskite and perovskite-related materials great application potentials for organic contaminants removal, which deserve
A
further study in the future. 1. Doping foreign elements into large bandgap perovskite and perovskite-related photocatalysts is widely used to induce visible light absorption and to subsequently enable photocatalytic activity in the visible spectrum. Nonetheless, very limited knowledge is available about the adverse effects of dopants on the photophysical 46
properties of the compounds, e.g., the trade-off between the benefits derived from dopant-induced visible light activity and the loss of UV light activity, which should be properly evaluated in the future. Appropriate dopants that retain the beneficial properties of the host materials and inducing visible light response should be identified. Besides, designing complex perovskite compounds with suitable elements at A- and B-
IP T
sites to yield desired photocatalytic properties is a challenge, and computational design would help to shorten the selection process. Recent advances in computational tools
SC R
such as density functional theory (DFT) based band structure calculations are highly effective in designing and understanding novel material systems.
U
2. Multicomponent heterojunction formation is another effective way to promote the
N
photocatalytic performance of perovskite and perovskite-related photocatalysts.
A
Unfortunately, the molar/weight ratio among the components of a heterojunction is not
M
reported in many literatures, making a reliable comparison difficult. The simple photocatalytic activity prediction by considering the relative band positions of the
ED
components is not always reasonable. The difference in electron mobility in the components and the proportion of effective heterogeneous interface should also be
PT
considered. Experimental and theoretical study should be combined to reveal the interface interaction and interface optical microscopic mechanism of carrier mobility
CC E
to explore their influence on photocatalytic activity. 3. Due to the complexity of the structure, compositions and properties of perovskite and
A
perovskite-related materials, researchers need to pay much more attention to the materials characterization, particularly crystal phase analysis. One typical challenge lies in identifying the “impurity” phases or even amorphous phases and their functions in a photocatalyst, which is important for its precise design and reproducible synthesis. In addition, some perovskite compounds exhibit ferroelectric, ferromagnetic, or 47
piezoelectric effect, which may correlate with their photocatalytic performance and need to be further studied. In short, further understanding of the crystal and electronic structural factors behind photocatalytic activity is needed for the future development of effective visible-light-driven perovskites and perovskite-related photocatalysts for
IP T
organic contaminants removal.
Acknowledgments
SC R
Acknowledgments are given to the financial support by national natural science
foundation of China (NSFC no. 21776322, 21576298 and 21425627), NSFC-SINOPEC
U
Joint fund (U1663220), and Natural Science Foundation of Guangdong Province
A
CC E
PT
ED
M
A
N
(2014A030308012).
48
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SC R
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