photocatalysts for degradation of dyes in industrial effluents: … · 2019. 5. 8. · regarded as...

ISSN 1998-0124 CN 11-5974/O4

2019, 12(5): 955–972 https://doi.org/10.1007/s12274-019-2287-0

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Photocatalysts for degradation of dyes in industrial effluents: Opportunities and challenges Hassan Anwer1,§, Asad Mahmood1,§, Jechan Lee2,§, Ki-Hyun Kim1 (), Jae-Woo Park1 (), and Alex C. K. Yip3

1 Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-ro, Seoul 04763, Republic of Korea 2 Department of Environmental and Safety Engineering, Ajou University, Suwon 16499, Republic of Korea 3 Department of Chemical and Process Engineering, University of Canterbury, Christchurch 8041, New Zealand § Hassan Anwer, Asad Mahmood, and Jechan Lee contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 22 September 2018 / Revised: 27 December 2018 / Accepted: 3 January 2019

ABSTRACT Discharging dye contaminants into water is a major concern around the world. Among a variety of methods to treat dye-contaminated water, photocatalytic degradation has gained attention as a tool for treating the colored water. Herein, we review the recent advancements in photocatalysis for dye degradation in industrial effluents by categorizing photocatalyst materials into three generations. First generation photocatalysts are composed of single-component materials (e.g., TiO2, ZnO, and CdS), while second generation photocatalysts are composed of multiple components in a suspension (e.g., WO3/NiWO4, BiOI/ZnTiO3, and C3N4/Ag3VO4). Photocatalysts immobilized on solid substrates are regarded as third generation materials (e.g., FTO/WO3-ZnO, Steel/TiO2-WO3, and Glass/P-TiO2). Photocatalytic degradation mechanisms, factors affecting the dye degradation, and the lesser-debated uncertainties related to the photocatalysis are also discussed to offer better insights into environmental applications. Furthermore, quantum yields of different photocatalysts are calculated, and a performance evaluation method is proposed to compare photocatalyst systems for dye degradation. Finally, we discuss the present limitations of photocatalytic dye degradation for field applications and the future of the technology.

KEYWORDS photocatalyst, dye wastewater, degradation mechanism, performance evaluation

1 Introduction Recently, water contamination by dye discharges from various industries (e.g., paper making, textile dyeing, cosmetics, paints, food processing, etc.) has attracted significant attention due to hazards to public health and ecosystems [1]. The current global colorant market is 32 billion USD and is estimated to increase to 42 billion USD by 2021 [2]. The annual total production of synthetic dyes is over 700,000 tons [3, 4]. More than 15% of these synthetic colorants are discharged into water annually [5]. The dye contaminants in water are toxic, carcinogenic, and xenobiotic [6].

Dyes are classified based on chromophore structure into reactive dyes, solvent dyes, basic dyes, direct dyes, and vat dyes, as shown in Fig. 1. Only 47% of synthetic dyes are biodegradable [7]. Conventional wastewater treatment technologies (e.g., adsorption, coagulation/ flocculation, and precipitation) generally require a long operation time and produce secondary sludge, which is costly to dispose of [7]. Advanced oxidation processes (AOPs) are effective techniques to degrade organic compounds by producing reactive oxygen species (ROS). Hydroxyl (·OH) and superoxide (·O2

−) radicals are well known ROSs with oxidation potentials of 2.7 and −2.3 eV, respectively [8]. The oxidation potential of organic compounds varies from −1 to 2 eV. Due to the difference in potential between ROS and organic species, an organic species entering a reactor containing ROS will either gain or lose electrons immediately and transform into two or more smaller constituent parts [9]. The high redox potential of ROS provides the basis for the superior removal performance of photocatalytic systems. A comparison of the degradation potential of

Figure 1 Classification of dyes based on chromophore structure.

AOPs and of a conventional chemical treatment was conducted [10]. The chemical treatment eliminated 60% of the chemical oxygen demand (COD) from wastewater, while AOP removed 90% of COD. Moreover, AOP removed 95% of colored compounds, whereas the conventional ferrous sulfate treatment removed 49% of the colored compounds from dye wastewater. Some AOP technologies are listed in Table 1 along with their generated ROSs [11].

Address correspondence to Ki-Hyun Kim, [email protected]; Jae-Woo Park, [email protected]

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Among AOPs, photocatalysis has emerged as a strong candidate for practical field applications. The process does not rely on the use of any chemicals or gases that could increase the risk of secondary pollution. In addition, the oxidation capacity of the photocatalytic process is sufficient to break down resilient pollutants into their corresponding simpler forms [12]. The basic mechanism of photocatalytic degradation is illustrated in Fig. 2. Photocatalysis starts with the transport of contaminants from the surroundings to the photocatalyst surface. The dye compounds are first adsorbed onto the surface on which oxidation–reduction reactions occur. These redox reactions are forced by photogenerated electrons in the conduction band (CB) and holes in valence band (VB). The products are then desorbed from the surface and transported back into the fluid phase [13]. The reactions responsible for the photocatalytic dye degradation can be summarized as follows

Photocatalyst + hν → hVB+ + eCB (1)

hVB+ + eCB → Energy (heat) (2)

H2O + hVB+ → ·OH (hydroxyl radical) + H+ (3)

O2 + eCB → ·O2 (superoxide radical) (4) ·OH + Pollutant → Intermediates → H2O + CO2 (5) ·O2 + Pollutant → Intermediates → H2O + CO2 (6)

A number of studies have established the notion that photocatalysis is a robust and effective way to degrade colorants in water, as shown in Table 2 and Fig. 3 [14–22]. The photodecomposition of dyes available in previous literature is listed in Table 2. The corresponding percentage of each in the literature is also presented in Fig. 3. It is evident that thiazine dyes are the most researched group of colorants, which includes methylene blue. Xanthanes form the second highest-studied group of dyes, which are dominated by rhodamine B [23]. Note

Figure 2 Basic degradation mechanism of photocatalysis.

that azo dyes belong to the largest group of industrial colorants, representing more than 50% of all dyes and pigments. However, only 7% of the total publications addressed the remediation challenges associated with azo dye-contaminated wastewater [24].

Despite many efforts, the fundamental principles of how photocatalysis works are still not well understood [25]. The present article reviews different photocatalytic dye degradation technologies for treating wastewater. To this end, important photocatalytic

Table 1 Major AOP technologies and brief descriptions thereof [8]

Order AOP technology Oxidant hierarchy Advantages Disadvantages

1 O3 ·OH O3

Established technology for remediation. Supplementary disinfectant.

May require ozone off-gas treatment.

2 O3/H2O2 ·OH ·O2

−

O3

H2O2

Effective for high concentrations of MTBE in water. Supplementary disinfectant. Established technology for remediation. More effective than O3 or H2O2 alone.

Potential for bromate formation. May require removing excess H2O2. Established technology for remediation.

3 O3/UV ·OH ·O2

−

O3 UV

Supplementary disinfectant. More effective than O3 or UV alone. Generates more ·OH than H2O2/UV.

Energy and cost intensive. Potential for bromate formation. UV light penetration is inhibited by turbidity. UV lamp failure can potentially contaminate water.

4 H2O2/UV ·OH ·O2

−

UV H2O2

Bromate formation suppressed. Can oxidize more MTBE compared to UV or H2O2 alone.No off-gas treatment required.

UV light penetration is inhibited by turbidity. UV lamp failure can potentially contaminate water.Interfering compounds can absorb UV.

5 Electron beam ·OH eaq

·H H2O2

Bromate formation suppressed. Bromate might be reduced. Can help in disinfection. Minimal effect of turbidity. No off-gas treatment.

No full-scale application exists. Energy and cost intensive. Requires skilled professionals for operation.

6 Cavitation ·OH ·H

Minimal maintenance cost. Less energy usage than UV. No bromate formation. No off-gas treatment.

No full-scale application exists. Low efficiency and supplementary oxidants are required.

7 Photocatalyst/UV ·OH ·O2

−

UV HO2·

No bromate formation. Less energy consumption than other AOPs. Can utilize solar irradiation. No off-gas treatment.

Pre-treatment required. Loss of photocatalyst activity with usage. Performance might be pH dependent.

8 Fenton’s reaction ·OH ·O2

− No bromate formation. Less energy intensive than ozone and UV alone. No off-gas treatment needed.

Iron extraction system required. Works in low pH only. Higher maintenance costs.

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Table 2 Photodecomposition of dyes in wastewater [23]

Order Dye class UV light source

Visible light source

Percentage

1 Anthraquinone 238 390 1.3% 2 Azo dyes 1,285 2,006 6.8% 3 Natural dyes 187 303 1.0% 4 Thiazine 7,496 13,471 43.4% 5 Triarylmethanes 1,439 2,758 8.7% 6 Xanthanes 5,625 12,244 37.0% 7 Others 303 557 1.8%

Figure 3 Major segments indicating the number of publications on the photodecomposition of colorants in industrial effluents.

degradation mechanisms and operation parameters affecting dye degradation are discussed in detail. In addition, we highlight some doubts associated with photocatalytic phenomena that need further investigation. A clear understanding of the photocatalytic phenomena could help us design more active and selective photocatalysts for

more reliable and robust processes. Finally, we establish a perfor-mance evaluation method and assess the feasibility of various photocatalytic dye degradation systems for practical applications. Overall, this review will help provide a better understanding of photocatalysis for dye degradation in industrial wastewater and highlight the current challenges for commercializing the process.

2 Nanomaterials for photocatalysis A variety of photocatalysts (e.g., metals, metal oxides, semiconductors, carbon-based nanostructures, quantum dots, metal–organic frame-works (MOFs), magnetic cored dendrimer, and other materials) have been extensively studied for degradation of dyes in wastewater [26–32]. Among the various materials, we will focus on metal oxide photocatalysts classified into three generations, as shown in Fig. 4. Each generation of material will be discussed in detail in the following subsections.

2.1 First generation photocatalysts

First generation photocatalysts are single component oxides, sulfides, nitrides, and phosphates. These materials have a CB–VB pair and rely on electron–hole generation for their operation [33, 34]. The materials absorb UV or visible light, promoting electrons from the VB to the CB with the formation of holes that oxidize dye molecules. Titanium dioxide (TiO2), one of the most famous photocatalysts, was first investigated for the water splitting reaction [35]. Later, its applications have been extended to hydrogen production, solar cells, pollutant photooxidation, and dye removal in liquid and gaseous phases [36]. However, the TiO2-photocatalytic systems are operated only under UV irradiation (250–350 nm) due to their large band gap (~ 3.2 eV). Despite this limitation, TiO2 is widely used due to its non-toxic nature, high chemical stability, and ready availability [37]. Anionic dyes, cationic dyes, and dyes with different types of chromophores are degraded using TiO2. For example, basic cationic dyes with –NH3+ or NR2+ functional groups (e.g., basic violet 2, crystal violet, and Rhodamine B) are used for dyeing substrates with negatively charged end groups [38]. Malachite green, methylene blue, basic violet, and methyl red are well-known cationic dyes that are degraded

Figure 4 The three generations of photocatalysts.

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using TiO2 [39]. Basic dyes with a large molecular weight are susceptible to attack by ·OH and ·O2

− radicals generated by TiO2. Once the dye compounds are in proximity to the photocatalyst surface, they are decomposed by the high potential ROS on the surface. Note that TiO2 exists in two major phases (anatase and rutile). The anatase phase has a larger band gap (3.2 eV) than the rutile (3.0 eV); however, anatase is preferred due to its better adsorptive affinity for dyes. Additionally, anatase is also favorable to lower charge recombination rates and higher photocatalytic activity due to its highly crystalline nature [40, 41].

Other than TiO2, ZnO, ZrO2, ZnS, SnO2, and NiO have been considered as single component photocatalytic materials [42–45]. Among them, ZnO has been widely employed as a photocatalytic material for decolorization of wastewater, because of its chemical stability, low cost and larger light absorption spectrum than that of TiO2. The effect of defects in the ZnO lattice on the degradation of anionic dyes was investigated [46]. Methyl orange, acid orange, and acid red were degraded using ZnO under UV illumination (365 nm). The observed degradation rates for the dyes followed the order: acid red > acid orange > methyl orange. ZnO catalysts that had a number of higher defect states exhibited better dye adsorption but a low photocatalytic activity.

The drawback of the first generation photocatalysts like TiO2, ZnO, and ZrO2 is their large band gap (ranging from 3 to 5 eV) that requires UV irradiation for electron excitation. These materials consist of a single CB and a single VB. Multiple energy bands are absent, increasing the probability that electrons fall back into the VB without generating ROSs. As recombination of charge carriers reduces the generation of reactive species for dye degradation, the overall performance of the photocatalytic system decreases [47, 48]. In response to the drawback, doping has emerged as a potential solution leading to the development of second generation materials. A summary of TiO2 and ZnO mediated photocatalytic systems for dye degradation are given in Table 3.

2.2 Second generation photocatalysts

Heterojunction materials were developed to improve the photocatalytic performance of the first generation materials [49].

The multi-component photocatalytic systems showed significant improvement by suppressing charge carrier recombination [50]. In heterojunction photocatalysts, electrons are confined in the CB of one semiconductor, and the holes are confined in the VB of the other. This spatial distance between the charge carriers overwhelms the recombination, thereby generating active sites at which degradation of dye molecules occurs [51]. The second generation photocatalytic materials show light absorbance in the visible region (λ ≥ 420 nm) accompanied with lower band gap energies than first generation photocatalytic materials. Representative second generation photo-catalysts are summarized in Table 4. Among the second generation photocatalysts, graphitic carbon nitride (g-C3N4) has attracted attention due to its low cost and facile synthesis. Theoretically, the band gap energy of a visible light active photocatalyst should be less than 2.8 eV. The band gap energy of g-C3N4 is 2.65 eV, which makes it a visible light-active photocatalyst [52]. Thus, g-C3N4-based photocatalysts have been studied for dye degradation. A heterojunction composite of Ag3VO4/g-C3N4 was reported for the treatment of a textile effluent [53]. Performance of the composite was evaluated for degradation of three colorants: malachite green, basic fuchsine, and crystal violet. Under visible irradiation (420 nm < λ < 700 nm), 97% of malachite green was degraded in 1 h with the composite. The rates of degradation of basic fuchsine and crystal violet were slower than that of malachite green. Within 2.5 h, the composite degraded 96% of basic fuchsine and 75% of crystal violet. The high performance of the Ag3VO4/g-C3N4 photocatalyst was attributed to its optimal band edge positions. The potential of the CB of C3N4 was sufficiently high (−0.73 eV) to generate superoxide radicals (·O2

−). Similarly, the VB of Ag3VO4 (2.24 eV) has the potential to generate hydroxyl radicals (·OH). The high performance of the Ag3VO4/g-C3N4 photocatalyst was likely due to the formation of the reactive ROSs.

2.3 Third generation photocatalysts

Extra equipment and energy required for separating the second generation photocatalyst from the suspension systems increase the cost of the effluent treatment. Moreover, the separation process cannot completely remove the catalyst, leading to leaching of the

Table 3 First generation titania and zinc oxide photocatalysts for dye degradation

Order Photocatalyst Dyes Light source Time (min) Degradation (%) References 1 TiO2 Acid orange 7 Vis 60 75 [108] 2 TiO2 Acid orange 2 UV 120 98 [109] 3 TiO2 Methyl red UV 120 99 [110] 4 TiO2 Alizarin UV 120 99 [110] 5 TiO2 Crocein orange UV 120 98 [110] 6 TiO2 Congo red UV 120 95 [110] 7 TiO2 Methyl blue UV 120 98 [110] 8 TiO2 Fast green FCF UV 80 93 [111] 9 TiO2 Patent blue VF UV 80 93 [111]

10 TiO2 Reactive black 5 UV 300 64 [112] 11 ZnO Acid red 14 UV 210 70 [45] 12 ZnO Acid brown 14 Solar light 120 85 [113] 13 ZnO C.I. Yellow 23 UV 60 90 [114] 14 ZnO Rhodamine B UV 80 98 [115] 15 ZnO Methyl orange UV 120 98 [116] 16 ZnO Rhodamine 6G UV 180 98 [117] 17 ZnO Reactive yellow 17 UV 300 40 [118] 18 ZnO Reactive red 2 UV 60 49 [118] 19 ZnO Reactive blue 4 UV 180 29 [118] 20 ZnO Reactive red 4 Solar light 480 85 [118]

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catalyst into water [54]. Loss in photocatalytic activity of the recycled composite is another problem associated with separation techniques. For instance, membrane filtration requires high pressure or tem-perature that disintegrates the photocatalyst assembly. Metallic photocatalysts can attach to organic functional groups of membranes, which could lead to leaching problems [55]. Thus, studies have recently focused on immobilizing photocatalysts on different substrates or supports. The third generation photocatalyst materials provide a viable solution to post-separation problems. Representative third generation photocatalysts are listed in Table 5. Semiconductor, metallic nanocrystals, metal oxides, carbon/boron nitrides, and many other composites are deposited on different substrates for photocatalytic dye degradation. Two approaches used for immobilization of first and second generation photocatalysts onto different substrates are the binder-through and binder-less approaches [56]. The binder- through approach uses organic and inorganic binders for the formation of films on the catalyst surface. Using this approach, a wide range of first and second generation materials can be deposited onto substrates. However, deposition using binders leads to a decrease

in surface area and active sites for photooxidation of contaminants. The binder-through approach was used by Fengquan et al. for depositing a TiO2/Ag composite onto a fluorine-doped tin oxide (FTO) substrate [57]. The authors used colloidal silica binder and a spin coating technique for fabricating a fine film on the FTO glass. The photocatalytic activity of the composite was evaluated by monitoring a methylene blue degradation pattern under UV illumination. The composite showed good performance in the suspension form; however, the photocatalyst film needed a long operation time (3 h) to degrade 90% of the pollutant after deposition.

Organic binders were also used for immobilizing photocatalysts on catalyst support. The average binding strength of organic binders is higher than that of inorganic binders; however, the latter is more hydrophilic, which is essential for solid–liquid interactions. The binding strength of organic epoxy binder was seven times higher than that of inorganic sodium and potassium silicate binders for depositing a TiO2 photocatalyst on a glass substrate. However, the epoxy binder was more hydrophobic than the silicate binders,

Table 4 Second generation photocatalysts for dye degradation

Order Photocatalyst Dyes Light source Time (min) Degradation (%) References 1 WO3/NiWO4 Methylene blue UV 400 92 [119] 2 Co3O4-g-C3N4 Methyl orange Vis 180 99 [120] 3 Bi2O2CO3/BiOI Rhodamine B Vis 15 99 [121] 4 SnO2/g-C3N4 Rhodamine B Vis 80 99 [122] 5 Bi2S3/TiO2/RGO Methylene blue Vis 90 99 [123] 6 BiOI/ZnTiO3 Rhodamine 6G Vis 180 82 [124] 7 Bi2O2CO3/Bi2S3 Rhodamine B Vis 180 98 [125] 8 BiVO4/Ag3VO4 Rhodamine B Vis 20 96 [51] 9 BiVO4/Ag3VO4 Methylene blue Vis 40 70 [51]

10 BiVO4/Ag3VO4 Methyl red Vis 40 78 [51] 11 BiVO4/Ag3VO4 Methyl violet Vis 40 95 [51] 12 g-C3N4/Ag3VO4 Crystal violet Vis 150 75 [53] 13 g-C3N4/Ag3VO4 Basic fuchsin Vis 150 95 [53] 14 g-C3N4/Ag3VO4 Malachite green Vis 150 97 [53] 15 g-C3N4/BiOI Rhodamine B Vis 50 99 [126] 16 g-C3N4/BiOI Methylene blue Vis 60 99 [126] 17 BiOI/Ag3VO4 Basic fuchsin Vis 120 97 [127] 18 BiOI/Ag3VO4 Malachite green Vis 120 92 [127] 19 BiOI/Ag3VO4 Crystal violet Vis 120 85 [127] 20 BiPO4/C3N4 Methyl orange Vis 120 96 [128]

Table 5 Third generation materials for dye degradation

Order Photocatalyst Dyes Light source Time (min) Degradation (%) References 1 Steel/TiO2-WO3 Methylene blue UV 240 91 [129] 2 FTO/WO3-ZnO Methylene blue UV 60 82 [130] 3 Si/TiO2 Methylene blue UV 240 89 [131] 4 Al/ZnO Methyl orange UV 180 98 [132] 5 FTO/BiOBr Methyl orange Vis 300 48 [133] 6 Ni/TiO2 Methyl orange UV 120 85 [134] 7 TiO2 film Methyl orange UV 900 99 [135] 8 FTO/BiOCl-TiO2 Rhodamine B Vis 180 99 [60] 9 Bi/BiOCl Rhodamine B Vis 210 98 [136]

10 Ti/BiOCl Rhodamine B UV 120 98 [137] 11 Ti/BiOCl Rhodamine B Vis 180 94 [137] 12 Ce/TiO2 Basic blue UV 180 90 [138] 13 Glass/P-TiO2 Butyl benzyl phthalate Vis 240 99 [139]

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making it unsuitable for dye wastewater applications [58]. The organic binder was susceptible to photooxidation, undermining the long-term stability. Methyl cellulose was used as an organic binder to deposit a TiO2/Ag composite on a glass substrate [59]. The photocatalyst film degraded more than 90% of methyl orange in 150 min. However, no reusability test was performed to assess the stability of the photocatalyst film. The drawbacks of binder-through approaches (e.g., low surface area, engaged reactive sites, and decreased hydrophilicity) could be overcome using binder-less approaches. These methods (e.g., chemical vapor deposition, spray pyrolysis, electrophoretic deposition, and direct hydrolysis) use fewer chemical reagents and solvents. Photocatalyst films deposited by binder-less methods offer higher surface area and reaction sites, which are engaged with binders in binder-through deposition methods [56]. The binder-less method was used to deposit a BiOCl/TiO2 composite on FTO glass [60]. The composite film was used to degrade Rhodamine B dye under visible irradiation (λ > 420 nm). Dye molecules were adsorbed and degraded on the film surface, resulting in 99% dye removal in 3 h. Based on a degradation performance perspective, the binder-less methods are superior to the binder-through methods. However, deposition of multi-component heterojunction photocatalysts on substrates is challenging as it requires a multi-step synthesis approach. For instance, the use of chemical vapor or electrophoretic methods is recommended to achieve the deposition of a single photocatalyst in one cycle. Subsequently, two or more deposition cycles are required to fabricate a heterojunction based on these approaches. Similarly, hydrothermal deposition involves complex chemical routes and multiple steps to assemble a hetero-junction on a substrate. These limitations in turn offer opportunities for researchers to find better binder-less approaches that can practically overcome these undermining factors.

3 Photocatalytic degradation mechanisms The general mechanism of photocatalysis involves photo-absorption and excitation of electrons from the VB to the CB of a semiconductor material. Based on the band gap theory, researchers have proposed several mechanisms for electron–hole formation, their transfer across the valence, conduction, and forbidden energy bands, and their recombination [61]. Some well-accepted mechanisms for photocatalytic dye degradation are discussed in the following subsections.

3.1 Heterogeneous pairing

A heterogeneous combination of semiconductors (e.g., SnO2/ZnO, WO3/NiWO4, and Sb2S3/TiO2) was introduced to suppress electron– hole recombination for effective dye degradation [49]. The formation of multiple junctions in such materials changes the energy levels where electrons can reside. The proposed mechanism suggested that electrons shift from the CB of a material having a high Fermi level to the CB of a second material having a low Fermi level. By contrast, holes move from the VB of one material having a low Fermi level to the VB of a second material having a high Fermi level [62].

A heterogeneous junction between g-C3N4 and BiOClBr was synthesized and investigated for its degradation potential for the photooxidation of Rhodamine B [63]. Results showed that 95% of the dye was degraded in 1 h under visible light. The band gaps of g-C3N4 and BiOClBr are 2.67 and 2.14 eV, respectively. Under visible light, the excited electrons in the CB of g-C3N4 were transported into the CB of BiOClBr. On the other hand, the holes were transferred from the VB of BiOClBr to the VB of g-C3N4 (Fig. 5). This heterogeneous coupling separates charge carriers from the junctions to effectively degrade Rhodamine B. Other examples of semiconductor pairs that operate using this mechanism are listed in Table 6. All heterojunction composites showed a staggered band

Figure 5 Heterogeneous pairing between g-C3N4/BiOClBr for Rhodamine B degradation under visible irradiation (reprinted with permission from Ref. [63], © Elsevier B.V. 2014).

gap alignment. In this context, the VB of the first semiconductor material is at a higher potential than that of the second semiconductor material. On the other hand, the CB of the second material occupies a higher potential than that of the first material. Charge-carrier generation and separation by photocatalysts with straddled and broken band gaps were reported. The staggered type of band alignment is appropriate for charge transfer among the three types of band gap alignments [64]. Heterogeneous coupling with the staggered band gap was used for molecular fusion synthesis of a graphene quantum dot (GQD)/TiO2 composite [65]. The composite was activated in visible illumination (λ > 420 nm). The graphene quantum dots acted as photosensitizers and injected electrons into the CB of TiO2. Additionally, holes accumulated in the VB of GQDs. The ROS generated via charge carriers effectively degraded 95% of methyl orange in 2 h. However, TiO2 or GQDs without the heterojunction coupling degraded only 20% of methyl orange. This indicates the advantages of additional energy bands and charge separation in heterojunction coupling.

3.2 p–n junctions

A p–n junction between two semiconductors leading to charge carrier separation is described in Fig. 6. The VB and CB of silver phosphate (Ag3PO4) and cerium oxide (CeO2) have a straddling band gap alignment. The band edge positions are determined by Mulliken electronegativity theory, which is the geometric mean of the electronegativity of the constituent atoms. Once the heterojunction is formed between two semiconductors, the electron transfer occurs until equilibrium. This establishes an internal electric field across the p–n junction. These electrostatic forces result in effective separation of charge carriers, thereby reducing the recombination rate and improving the catalytic performance [66]. Well-known p–n junction-based degradation mechanisms are listed in Table 7. Most composite materials following the p–n junction mechanism are designed to operate under visible irradiation, which is useful for practical applications. As shown in Table 7, the band gap alignment in p–n junction photocatalysts is overlapping type, which changes after a connection is established between two materials. This band gap offset was studied using a BiOI/Bi2Sn2O7 nanocomposite [67]. The BiOI with a lower Fermi level acted as a p-type semiconductor, whereas Bi2Sn2O7 (with a higher Fermi level) acted as an n-type semiconductor. After a contact was established, the electrons moved from n-type to p-type. This raised the potential of the BiOI energy bands, resulting in the formation of a virtual staggered band alignment. At equilibrium, an electrostatic field was established, which prevented further crossing of electrons and holes across the photocatalyst junction. Under the conditions of visible irradiation

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(λ > 420 nm), the electrons moved from p- to n-type of CB, while holes are transferred from n- to p-type of VB. Rhodamine B molecules were adsorbed on the catalyst surface, where degradation of the dye occurs. The charge separation caused by the p–n junction in the BiOI/Bi2Sn2O7 composite resulted in 99% degradation of the

dye in 6 h. The dye degradation performance of the composite was superior to that of BiOI or Bi2Sn2O7. The increase in photocatalytic activity after the p–n junction formed between two semiconductor materials highlights the importance of the p–n junction for photocatalytic dye degradation.

Table 6 Heterogeneous pairing of photocatalyst materials, their degradation mechanisms, and applications

No. Heterogeneous system Application Light source Mechanism References1. SnO2-ZnO Methylene blue UV

[140]

2. WO3/NiWO4 Methylene blue UV

[119]

3. TiO2/GQDs Methyl orange Visible

[65]

4. Sb2S3/TiO2 p-hydroxyazobenzene Visible

[141]

5. g-C3N4/ZnTcPc Rhodamine B Visible

[142]

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Figure 6 p–n junction mechanism for contaminant degradation (reprinted with permission from Ref. [66], © Zhang, W. et al. 2016).

3.3 Z-scheme mechanism

The p–n junctions offer charge carrier separation by electrostatic

forces; however, the redox potential of the holes and electrons is reduced [68]. The Z-scheme mechanism describes changes in oxidation/reduction during the light reactions of photosynthesis, which can help solve the problem of reduced redox potential in p–n junctions [69]. The photoexcitation process in the Z-scheme occurs in two steps. First, electrons in the highest occupied molecular orbital (HOMO) of the first material are transferred to the HOMO of the second material. Electrons then move from the HOMO of the second material to the lowest unoccupied molecular orbital (LUMO) of the second material. As a result, holes are created in the HOMO of the first material. This naturally occurring phenomenon does not necessarily reduce the redox potential of holes and electrons. The reactive oxygen species generated at the HOMO and LUMO of semiconductors degrade the dye molecules. Representative photocatalytic systems with the Z-scheme dye degradation mechanism are summarized in Table 8. Like the p–n junction mechanism, most composite materials using the Z-scheme mechanism operate under visible irradiation. For instance, the Z-scheme dye degradation

Table 7 p–n junction degradation mechanisms

No. p–n junction Application Light source Mechanism References 1. CuO-BiVO4 Rhodamine B Visible

[143]

2. BiOI/Bi2Sn2O7 Rhodamine B Visible

[67]

3. Cu3SnS4/TiO2 Methyl orange Visible

[144]

4. BiOI/Bi4O5I2/Bi2O2CO3

Methylene blue Visible

[145]

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(Continued)

No. p–n junction Application Light source Mechanism References 5. TiO2/ZnO

Methyl orange Visible

[146]

Table 8 Photocatalyst systems with Z-scheme mechanisms

No. p–n junction Application Light source Mechanism References 1. g-C3N4/Ag2WO4

Methyl orange Visible

[147]

2. g-C3N4/RGO/Bi2MoO6 Rhodamine B Visible

[70]

3. MoO3–g-C3N4 Methyl orange Visible

[148]

4. SnO2−x–g-C3N4 Rhodamine B Visible

[122]

5. Ag3PO4–MoS2 Methylene blue Visible

[149]

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mechanism in a g-C3N4/reduced graphene oxide (RGO)/Bi2MoO6 photocatalyst was recently investigated [70]. The Z-scheme mechanism in the composite material was verified using scavenging tests. Electrons migrated to the CB of g-C3N4 under visible irradiation, while the holes were restricted to the VB of Bi2MoO6. The electron migration resulted in 97% degradation of Rhodamine B dye in 2 h, which was due to the electron mediation of graphene oxide and charge separation by the Z-scheme mechanism.

4 Key factors controlling dye degradation Performance of photocatalytic degradation of dyes depends on several parameters such as pH, light intensity, the ratio of catalyst to dye, and the presence of interfering compounds (e.g., bromides, anti- scaling agents, bicarbonates, phosphates, and radical scavengers) [71]. The effects of these factors on the dye degradation performance will be discussed in the following subsections.

4.1 Effect of pH

The pH of wastewater containing dye determines the electrostatic interaction between catalyst surfaces, substrates, and radicals during the degradation process. Surface charge and aggregation of the catalysts are associated with wastewater pH [71, 72]. Gaya et al. investigated the effect of pH on the photocatalytic oxidation capability of TiO2 [73]. They observed that TiO2 shows a strong oxidation performance at a pH of less than 2. However, the reaction rate decreased when pH became less than 2. The effect of pH on the decomposition of methyl orange was studied [74]. Table 9 shows the change in reaction rate as a function of pH. The first-order reaction rate was 25 times higher at pH 2 than at pH 9. The reaction rate decreased from 1.42 to 0.86 min−1 as the pH increased from 2 to 3. A rapid drop in the reaction rate was observed when pH further increased from 3 to 9, confirming the sensitivity of dye degradation to the pH of the reaction mixture.

4.2 Pollutant/photocatalyst adsorption

The adsorption of dyes on the photocatalyst surface is dependent on the electrostatic interactions and binding affinity between the dye molecule and the catalyst surface [5]. Moderate adsorption of dye molecules on the photocatalyst surface is beneficial for degradation performance based on the synergy between adsorption and photocatalysis. The adsorption of dye molecules onto the catalyst surface is an essential step for effective degradation. Composite materials with good adsorption properties utilize this synergy principle for simultaneous adsorption and degradation of dye molecules [75]. The synergy between the two mechanisms has been reported for methylene blue degradation [76]. Accordingly, graphene oxide incorporated with TiO2 resulted in a 50% enhancement in the dye degradation relative to TiO2 alone. Although adsorption of dye on the surface is important for photocatalytic dye degradation, it may also be disadvantageous beyond a certain limit. A very high adsorption of dye molecules on the catalyst surface suppresses the number of photons reaching surface-active sites. In addition, dye molecules may also act as sensitizers that can absorb electrons and

Table 9 First-rate constants of methyl orange degradation at different initial pHs [74]

Initial pH of reaction mixture Kapp. × 102 2.02 1.42 3.00 0.86 5.02 0.26 7.04 0.11 9.01 0.06

Normal experiment, pH = 6.3 0.15

scatter them in undesirable directions [77].

4.3 Effect of light intensity

Photocatalytic phenomena rely on the energy supplied by light quanta. Electron–hole pairs are generated in the CB and VB of a photocatalytic material when they receive photons with energy equal to or greater than the band gap of the material [78, 79]. The light intensity plays a critical role in photocatalytic dye removal [80]. The effect of light intensity on photocatalytic dye degradation has been extensively studied. As an example, the effect of light intensity on the degradation and decolorization of wastewater containing reactive yellow azo dye (RY14) is shown in Fig. 7 [81]. When the radiation intensity increased from 16 to 62 W, 33% more dye decolorization was observed. An increase in irradiation intensity led to improvements in the penetration of light and to an increase in the rate of generation of ROSs. However, the increase in light intensity improves photocatalytic performance only up to a certain limit. As shown in Fig. 7, dye removal increases significantly when irradiation power shifts from 16 to 32 W. A subsequent increase from 32 to 48 W yielded a small change in dye removal. After that, the dye removal becomes independent of the irradiation intensity. The effect of light intensity on kinetic rate constant in the degradation of Fast Green dye was investigated [82]. A linear rise in dye degradation rate was observed with an increase in the light intensity. The results showed that dye removal rate almost doubled as the light intensity increased from 10 to 70 mW·cm−2. In another study, a negative effect of light intensity on Congo Red dye degradation rate was observed when the light intensity varied from 50 to 90 J·cm−2 [83]. It was seen that the dye removal rate increased with the light intensity up to 80 J·cm−2. However, at 90 J·cm−2, the degradation rate began to drop which should be ascribed to the thermal effects associated with the rise in temperature of the dye contaminated solution.

4.4 Effect of photocatalyst loading

The mass of photocatalyst in a wastewater suspension has a significant effect on the reaction rate and total dye degradation capability. Lu et al. reported that the ratio of Rhodamine B removal increased with increasing loading of a CdS/graphene composite photocatalyst. The removal ratio increased from 49.1% to 84.5% with an increase in the photocatalyst loading from 200 to 1,800 mg·L−1 [84]. Increasing the photocatalyst loading generates more electron– hole pairs, resulting in higher and faster degradation of dye molecules. However, the turbidity of wastewater increases at very high con-centrations of composites, thereby increasing light scattering and reducing its penetration in the reaction mixture [31, 71]. Recently, the negative effect of composite dosage on degradation performance was reported [75]. Degradation of para-nitrophenol (PNP) was monitored against initial composite dosage concentrations of 0.05, 0.15, 0.25, 0.35, and 0.45 g·L−1. The best removal performance (97%) was achieved at a composite dosage of 0.25 g·L−1. The degradation

Figure 7 Effect of UV radiation intensity on the decolorization and degradation of RY14 (reprinted with permission from Ref. [81], © Elsevier 2006).

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capability dropped when composite concentration increased to 0.35 or 0.45 g·L−1. The decrease in removal performance was associated with reduced penetration and increased light scattering due to a high composite density per unit volume.

4.5 Effect of dye loading

The interaction between a photocatalyst substrate and a dye molecule is related to the nature of the functional groups of the dye molecules as well as their abundance in the reactor. The optimal concentration of dye molecules for a photocatalytic reaction is highly dependent on the kind of dye. In general, as the initial dye pollutant concentration increases, the photocatalytic degradation efficiency decreases because more dye molecules compete for limited active sites. Increasing photocatalyst loading leads to increased turbidity. Furthermore, a high dye concentration makes the dye itself absorb more light than a photocatalyst, thereby deteriorating photocatalytic degradation performance [85]. A higher initial con-centration of methyl orange made fewer photons reach the catalyst surface due to the reduction of light penetration [86]. The effect of the initial concentration of acid red dye on the photocatalytic degradation performance was also investigated. The degradation rate decreased when the dye concentration increased from 20 to 40 mg·L−1. The increase in dye loading prevented the photons from reaching the catalyst surface, inhibiting the generation of ROSs. It was reported that the decrease in dye removal efficiency is chiefly due to a lower concentration of ROSs in the photocatalytic reactor.

In addition to the factors discussed above, the presence of inorganic metal ions in dye-contaminated solution can also affect the photocatalyst’s performance. In this regard, depending upon the ionic nature of dye molecules, the metal ions may either accelerate or decelerate the dye degradation. Generally, metal ions are adsorbed on to the photocatalyst’s surface to make the catalyst electro-neutral or slightly positively charge. As this effect reduces the electrostatic repulsion for anionic dyes, they can be adsorbed and degraded readily in the presence of metal ions. On the contrary, a retarding effect can also be observed for cationic dyes due to the decrease in attraction between positively charged dye molecules and neutral/ slightly positive catalyst surface [87].

5 Uncertainties and challenges Even though researchers have made tremendous progress in the field of photocatalysis, some doubts remain about the process and operation of photocatalysts in real applications. Therefore, in this section, we attempt to highlight uncertainties and ambiguities regarding photocatalytic dye degradation.

5.1 Degradation mechanisms

A photocatalyst absorbs light photons to generate electrons and holes in its energy bands. The photon-derived electrons and holes react with available oxidants and reductants, respectively. Subsequently, the radicals generated from electron–hole pairs react with dye molecules to form H2O and CO2 [88]. Several mechanisms for electron–hole pair formation and separation have already been discussed. Nevertheless, there is no clear way to show how the photocatalytic reactions proceed in an actual reactor. Researchers have proposed different mechanisms for electron–hole transport across the heterojunctions in photocatalysts; however, the transport and degradation mechanism at photocatalytic active sites could be explained in more than one way [67, 70]. The degradation mechanism is usually studied via indirect methods such as scavenging, radical trapping, and electron spin resonance tests [89, 90]. However, a direct method is needed that could verify the charge migration pathway and identify actual degradation sites on the photocatalyst surface. Detailed and precise knowledge about the photocatalytic mechanism

will help to design better photocatalyst materials for dye degradation applications.

5.2 Intermediate species

Ideally, the photocatalytic reaction under UV light irradiation should yield carbon dioxide and water as final products [91]. The organic dyes have large structures that are broken into smaller segments during photocatalytic degradation. It is possible that the intermediates formed during this process could have more detrimental effects than the parent dye compound. However, the intermediates formed during photocatalysis are generally not monitored or studied. There was an effort to explore transitional products formed in dye degradation. Chen et al. identified intermediates with a different number of N-ethyl groups in the photocatalytic degradation of N,N,N,N-tetraethylsulforhodamine-B dye [92]. The intermediate products generated during the breakdown of dye molecules were not radically different from the parent dye molecules. However, it would be wrong to assume this safe scenario is the case for all other intermediates formed during dye degradation. The intermediates formed during photocatalytic degradation of dye can be more dangerous than the original dye. For instance, catechol is one of the by-products of phenol degradation via photooxidation. The possibility of experiencing hypertension and convulsions in animals from catechol are higher than for phenol [93, 94]. Therefore, intensive investigation is required to determine the nature of transitional products and the fate of the intermediate pollutants produced when treating the dye-containing wastewater.

5.3 Electron–hole recombination

Recombination of an electron and a hole in a photocatalyst, if it occurs, reduces the quantum efficiency of a photocatalytic reaction. The most common method to evaluate the recombination rate is photoluminescence analysis. In this analysis, it is assumed that the emission intensity of light from a photocatalyst is inversely proportional to the electron–hole recombination [95]. Photo-luminescence emission occurs when energy is released from radiative decay of an electron from the CB to the VB. However, if a photocatalyst has some component that scatters or absorbs light (e.g., reduced graphene oxide and quantum dots), the emission intensity could then be completely misleading [96]. There are other indirect methods to measure the electron–hole recombination, such as open circuit potentiometry and radical scavenging, but no absolute method is available for assessing the recombination. To overcome this deficiency, investigations are needed to find better methods that provide an accurate prediction of the electron–hole recombination rate.

5.4 Band gap

In a semiconducting material, electrons are excited from the VB to the CB when irradiated by photons having an energy level greater than their corresponding band gap. This inter-band excitation occurs among three bands: the CB, the forbidden band, and the VB. There are several approaches to find band gaps of different semiconductor materials. However, it is hard to find the accurate position of the VB and the CB and band gap energy, particularly in composite materials. Given that the selection of light source highly depends on the band gap energy, an ambiguous band gap position makes the selection of the light source (and light intensity) difficult. Choosing an inappropriate light source could lead to energy waste and add to the total cost of dye wastewater treatment.

5.5 Structure and dopant distribution

Doping refers to the introduction of impurities/foreign atoms in to the basic structure of a parent material that can improve photocatalytic performance for dye degradation. An accurate and

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precise understanding of the dopant distribution in the parent structure is crucial. Characterizations using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) only provide information about a small representative part of the bulk material. The composition and dopant distribution in a representative part of the sample could be different from the bulk phase. Hence, it is unsafe to assume that the bulk material will have same properties as the representative sample. This erroneous assumption demands extensive investigation for developing reliable methods for analyzing bulk materials. Similarly, the determination of the structure, density, and three-dimensional distribution of doped materials in the bulk phase is challenging.

6 Performance evaluation A wide range of colorants, including organic, inorganic, cationic, anionic, and amphoteric dyes, can be photocatalytically decomposed. Nevertheless, it is often hard to validate the performance of a photocatalyst by simply monitoring its performance of degrading a specific dye. The difficulty is because a photocatalyst showing an outstanding performance under a specific set of conditions might perform very poorly under different circumstances. The conditions affecting photocatalytic activity are highly dependent on the type of catalysts and kind of dye; thus, a direct comparison of different photocatalysts operating at different conditions is very difficult and not reasonable.

The quantum yield (QY) is important to photocatalytic reactions. QY quantifies the ability of a photocatalyst to absorb photons and shows how effectively the absorbed photons are used for photocatalytic degradation. The definition and calculation method for QY may vary between different application fields. For instance, the QY for solar-driven systems was described in terms of “electrical energy per order” or “electrical energy per unit mass” [97]. In an industrial approach, QY was also defined as the monitoring the amount of product obtained per unit of energy supplied [98]. Likewise, in this

research, the QY for the photocatalysts was calculated using the method reported previously [99, 100]

Decay rate (molecules per second)QYPhoton flux (photons per second)

= (7)

The QY for photocatalytic systems was also assessed through the evaluation of the degradation capacity between different nano-materials [101–103]. However, QY alone cannot be used as a standard metric to allow performance comparison between different photocatalytic systems, unless all operational parameters are used on parallel basis. Hence, we established an evaluation method that can be used to assess, quantify, and compare the performance of different photocatalysts. The method is based on operational parameters that are critical for selecting photocatalysts for practical applications. Recently, the validity of these parameters was investigated experimentally [75, 104]. The selection criteria included the quantity of dye-wastewater processed, catalyst mass, time, and energy con-sumption for photodegradation. Using these parameters at the same time, a figure of merit (FOM) was calculated as

1 1

FOMProduct obtained (L)

Catalyst dosage (g L ) Time (h) Energy consumption (Wh μmol )

=

⋅ ´ ´ ⋅

(8)

Chlorophyll, a natural photocatalyst with a QY of 0.04–0.06, was used as a benchmark for normalizing the QYs of all photocatalyst systems [105]. For easy interpretation, the highest obtained FOM was set to 100, and the conversion factor was used to calculate the FOMs for other photocatalyst systems. Important operational parameters, QYs, and modified FOMs of 85 different photocatalytic systems are listed in Table 10. As discussed earlier, the QY of photocatalyst systems presented only one aspect of the degradation potential. However, the FOM quantified the overall performance of all systems based on four selected parameters. Some photocatalytic

Table 10 Performance evaluation and FOM of photocatalyst systems

Order Photocatalyst Genera-tion

Dyes Light source

Time(h)

Energy consumption (Wh·μmol−1)

Product obtained

(L)

Catalyst dosage (g·L−1)

QY (mol·photon−1)

FOM Remarksa References

1 TiO2 1st Fast green FCF UV 1.3 92.5 0.25 1.0 1.89 × 10−5 5.18 Average [111] 2 TiO2 1st Patent blue VF UV 1.3 22.9 0.25 1.0 7.64 × 10−5 20.88 Good [111] 3 TiO2 1st Reactive black 5 UV 5.0 9.7 0.50 2.0 1.96 × 10−4 13.18 Good [112] 4 TiO2 1st Reactive yellow 145 UV 5.0 10.4 0.50 2.0 1.84 × 10−4 12.34 Good [112] 5 ZnO 1st Acid red 14 UV 3.5 74.9 0.05 0.6 3.50 × 10−5 0.81 Below average [45] 6 ZnO 1st Rhodamine B UV 1.3 81.5 0.10 2.0 2.34 × 10−5 1.18 Average [115] 7 ZnO 1st Methyl orange UV 2.0 25.1 0.04 1.3 7.28 × 10−5 1.63 Average [116] 8 ZnO 1st Rhodamine 6G UV 3.0 88.0 0.10 0.5 2.07 × 10−5 1.94 Average [117] 9 ZnS 1st Methylene blue UV 1.5 412.7 0.05 1.0 4.61 × 10−6 0.21 Below average [150]

10 ZnS 1st Methyl orange UV 1.0 43.6 0.08 1.0 6.00 × 10−5 4.39 Average [81] 11 ZnS 1st Bromophenol blue UV 3.0 618.4 0.10 2.5 3.08 × 10−6 0.06 Below average [151] 12 WO3 1st Malachite green Vis 5.8 373.3 0.03 0.2 3.24 × 10−6 0.23 Below average [152] 13 WO3 1st Rhodamine B Vis 0.2 1,774.1 0.03 1.3 6.82 × 10−7 0.19 Below average [153] 14 WO3 1st Methylene blue Vis 2.0 1,023.5 0.10 1.0 1.27 × 10−6 0.12 Below average [154] 15 WO3 1st Orange G Vis 2.0 1,447.6 0.10 1.0 9.02 × 10−7 0.09 Below average [154] 16 TiO2 1st Reactive black 5 UV 3.0 60.2 0.50 2.0 3.16 × 10−5 3.53 Average [155] 17 CdS 1st Reactive black 5 UV 3.0 54.2 0.50 2.0 3.50 × 10−5 3.93 Average [155] 18 CdS 1st Eosin red Vis 1.7 951.0 0.10 0.5 1.27 × 10−6 0.32 Below average [156] 19 CdS 1st Congo red Vis 2.0 1,211.6 0.10 0.5 9.98 × 10−7 0.21 Below average [156] 20 CdS 1st Methylene blue Vis 3.0 903.1 0.10 0.5 1.34 × 10−6 0.19 Below average [156] 21 CdS 1st Methylene blue UV 3.0 38.6 0.20 1.0 4.26 × 10−5 4.41 Average [157]

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(Continued)


Dyes Light source

Time (h)


Product obtained

(L)




22 CdS 1st Phenol red UV 3.0 47.3 0.20 1.0 3.48 × 10−5 3.60 Average [157] 23 CdS 1st Methyl red UV 3.0 35.1 0.20 1.0 4.68 × 10−5 4.85 Average [157] 24 CdS 1st Methylene blue Vis 6.0 533.1 0.60 0.1 2.26 × 10−6 4.79 Average [158] 25 ZnS 1st Methylene blue Vis 6.0 253.9 0.60 0.1 4.76 × 10−6 10.06 Good [158] 26 Fe-doped TiO2 2nd Methyl orange UV 1.3 103.6 0.05 1.0 1.89 × 10−5 0.99 Below average [159] 27 Fe-doped TiO2 2nd Methyl orange Vis 6.0 818.3 0.05 1.0 1.48 × 10−6 0.03 Below average [159] 28 Ag2CO3/C3N4 2nd Methyl orange Vis 0.2 62.0 0.05 1.0 1.95 × 10−5 12.36 Good [160] 29 Graphene/TiO2 2nd Methyl orange UV 0.3 34.6 0.08 0.6 3.52 × 10−5 28.13 Good [161] 30 Graphene/TiO2 2nd Methyl orange Vis 3.0 17.9 0.08 2.5 3.90 × 10−5 1.52 Average [161] 31 BN/CdS 2nd Rhodamine B Vis 1.5 14.1 0.16 0.6 1.13 × 10−4 30.68 Best [106] 32 RGO/Bi2MoO6 2nd Rhodamine B Vis 1.0 96.8 0.25 1.0 1.22 × 10−5 6.60 Average [70] 33 Ag3PO4/CeO2 2nd Rhodamine B Vis 1.0 65.3 0.25 0.4 1.85 × 10−5 24.45 Good [66] 34 Ta2O5/g-C3N4 2nd Rhodamine B Vis 2.0 299.0 0.05 1.0 4.04 × 10−6 0.21 Below average [162] 35 Graphene/TiO2 2nd Rhodamine B Vis 3.0 300.3 0.05 0.3 3.98 × 10−6 0.47 Below average [163] 36 RGO/TiO2 2nd Methylene blue UV 1.0 94.1 0.05 0.2 2.00 × 10−5 6.79 Average [164] 37 RGO/TiO2 2nd Methylene blue Vis 1.0 228.5 0.05 0.2 5.34 × 10−6 2.80 Average [164] 38 CdS/CoFe2O4 2nd Methylene blue Vis 2.0 85.3 0.25 0.4 1.42 × 10−5 9.36 Average [165] 39 Ni-doped TiO2 2nd Methylene blue Vis 3.0 575.1 0.20 1.2 2.10 × 10−6 0.25 Below average [166] 40 WO3/g-C3N4 2nd Methylene blue Vis 1.5 29.5 0.10 0.5 2.00 × 10−5 11.55 Good [107] 41 WO3/NiWO4 2nd Methylene blue UV 6.7 41.2 0.35 0.6 4.62 × 10−5 5.70 Average [119] 42 Co3O4-g-C3N4 2nd Methyl orange Vis 3.0 245.5 0.10 1.0 4.92 × 10−6 0.35 Below average [120] 43 Bi2O2CO3/BiOI 2nd Rhodamine B Vis 0.3 437.5 0.10 0.5 2.76 × 10−6 4.67 Average [121] 44 SnO2/g-C3N4 2nd Rhodamine B Vis 0.8 139.7 0.10 1.0 8.66 × 10−6 2.19 Average [122] 45 Bi2S3/TiO2/RGO 2nd Methylene blue Vis 1.5 39.3 0.50 0.4 3.08 × 10−5 54.24 Best [123] 46 BiOI/ZnTiO3 2nd Rhodamine 6G Vis 3.0 525.8 0.02 1.0 2.30 × 10−6 0.03 Below average [124] 47 Bi2O2CO3/Bi2S3 2nd Rhodamine B Vis 3.0 231.8 0.10 1.0 5.22 × 10−6 0.37 Below average [125] 48 BiVO4/Ag3VO4 2nd Rhodamine B Vis 0.3 41.6 0.10 1.0 2.90 × 10−5 18.41 Good [51] 49 BiVO4/Ag3VO4 2nd Methylene blue Vis 0.7 82.9 0.10 1.0 1.46 × 10−5 4.62 Average [51] 50 BiVO4/Ag3VO4 2nd Methyl red Vis 0.7 63.1 0.10 1.0 1.92 × 10−5 6.07 Average [51] 51 BiVO4/Ag3VO4 2nd Methyl violet Vis 0.7 72.5 0.10 1.0 1.67 × 10−5 5.29 Average [51] 52 BiPO4/mg-C3N4 2nd Methyl orange Vis 2.0 99.2 0.10 1.0 1.22 × 10−5 1.29 Average [128] 53 g-C3N4/Ag3VO4 2nd Basic fuchsin Vis 2.5 426.1 0.05 1.0 2.84 × 10−6 0.12 Below average [53] 54 g-C3N4/Ag3VO4 2nd Malachite green Vis 1.0 188.1 0.05 1.0 6.42 × 10−6 0.68 Below average [53] 55 g-C3N4/Ag3VO4 2nd Crystal violet Vis 2.5 680.0 0.05 1.0 1.78 × 10−6 0.08 Below average [53] 56 g-C3N4/BiOI 2nd Rhodamine B Vis 0.8 121.0 0.10 0.1 1.00 × 10−5 25.34 Good [126] 57 g-C3N4/BiOI 2nd Methylene blue Vis 1.0 96.9 0.10 0.2 1.25 × 10−5 13.18 Good [126] 58 BiOI/Ag3VO4 2nd Basic fuchsin Vis 2.0 267.1 0.05 1.0 4.52 × 10−6 0.24 Below average [127] 59 BiOI/Ag3VO4 2nd Malachite green Vis 2.0 317.3 0.05 1.0 3.82 × 10−6 0.20 Below average [127] 60 BiOI/Ag3VO4 2nd Crystal violet Vis 2.0 384.0 0.05 1.0 3.14 × 10−6 0.17 Below average [127] 61 C3N4/CdS 2nd Rhodamine B Vis 2.8 135.7 0.25 0.4 8.92 × 10−6 4.15 Average [167] 62 RGO/CdS 2nd Rhodamine B Vis 2.5 119.8 0.25 0.4 1.01 × 10−5 5.33 Average [167] 63 C3N4/CdS/RGO 2nd Rhodamine B Vis 1.0 47.9 0.25 0.4 2.52 × 10−5 33.33 Best [167] 64 C3N4/RGO 2nd Rhodamine B Vis 2.0 95.8 0.25 0.4 1.26 × 10−5 8.33 Average [167] 65 C3N4/CdS 2nd Rhodamine B UV 0.7 95.8 0.25 0.4 1.98 × 10−5 25.00 Good [167] 66 RGO/CdS 2nd Rhodamine B UV 0.7 95.8 0.25 0.4 1.98 × 10−5 25.00 Good [167] 67 C3N4/RGO 2nd Rhodamine B UV 0.5 71.9 0.25 0.4 2.64 × 10−5 44.44 Best [167] 68 C3N4/CdS/RGO 2nd Rhodamine B UV 0.3 47.9 0.25 0.4 3.96 × 10−5 100.00 Best [167] 69 ZnS/CdS 2nd Methylene blue Vis 3.0 148.1 0.60 0.1 8.16 × 10−6 34.50 Best [158] 70 SnO2−x/g-C3N4 2nd Rhodamine B Vis 0.7 111.8 0.10 1.0 1.08 × 10−5 3.43 Average [122] 71 SnO2−x/g-C3N4 2nd Methyl orange Vis 1.5 171.9 0.10 1.0 7.04 × 10−6 0.99 Below average [122] 72 SnO2−x/g-C3N4 2nd Methylene blue Vis 0.7 74.6 0.10 1.0 1.62 × 10−5 5.13 Average [122] 73 Steel/TiO2-WO3 3rd Methylene blue UV 4.0 12,794.0 0.01 3.0 1.58 × 10−7 0.00 Below average [129]

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systems exhibited higher QYs but lower FOM, indicating their deficiency in at least one of the four parameters selected for FOM. Photocatalytic systems were categorized in to four classes based on the FOM values: best, good, average, and below average. The percentage share of FOM classes in different generations of photocatalyst materials is shown in Fig. 8. The first generation materials failed to qualify for the best FOM class: 40% of the first generation materials exhibited a FOM less than one, which was classified as below average. Thirteen percent of the second generation materials were in the best category of FOMs. It is worth mentioning that the best materials contained RGO/- or carbon nitride (C3N4) in their hybrid structure. Therefore, it is safe to assume that RGO/- or C3N4 could play a pivotal role in designing potentially efficient photocatalyst materials for dye degradation. Better performance of these materials is associated with their high adsorption capacity that incorporates adsorption-photocatalysis synergy for improved dye degradation [106, 107]. Two photocatalytic systems with third generation materials showed the best FOMs. In both these systems, the support material was a conductive substrate that enhanced the electron mobility along the horizontal film surface. The increased carrier mobility suppressed the undesirable electron–hole recombination, which resulted in better overall performance. Third generation

Figure 8 Performance evaluation results for photocatalyst materials consisting of first, second, and third generation materials.

materials address post-separation problems, but most of the currently available systems are still unsuitable for field applications, and further research is required to advance this technology.

7 Conclusions and prospects We summarized the advancement in photocatalysis to remedy the dye-contaminated industrial effluent by categorizing the photocatalysts in to three generations. First generation photocatalysts proved to be inadequate for dye removal as they were limited by their large band gaps and interfacial charge recombination. With the progress in this research field, a better knowledge on diverse functional properties (e.g., the optoelectronic, physical, and band gap characteristics) opened a new avenue to develop a second generation of photocatalysts. Among photocatalysts, the second generation materials were studied most extensively due to their excellent charge separation, faster reaction kinetics, visible light utilization, and remarkable QYs for dye degradation. To expand the photocatalyst applications to industrial scale, the post separation problems associated with the first and second generation materials were addressed by the third generation photocatalysts. The low QYs of the third generation photocatalysts (e.g., due to reduced surface area and reaction sites after deposition) presented a new opportunity to develop or improve immobilization technologies.

Besides the design of photocatalysts, the underlying mechanisms that govern the degradation process are of immense importance. In this regard, the Z-scheme mechanism that replicates the natural degradation process of chlorophyll is ideal for designing photocatalysts for dye degradation. Moderate reusability and low catalyst dosage (100–200 g·m−3) are also advantageous to wastewater treatment. The ambiguities regarding process parameters, material design, and functioning in real wastewater create an impression of skepticism towards photocatalysts. However, the limitations and challenges that currently undermine the industrial applications of photocatalysts can be resolved. In conclusion, the third generation materials hold a tremendous potential for the future of dye wastewater treatment on an industrial scale.

Acknowledgements K. H. K. and J. W. P. acknowledge support made by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Plan-ning Grant Nos. 2016R1E1A1A01940995 and 2018R1A2A1A05023555, respectively.

(Continued)


Dyes Light source

Time (h)


Product obtained

(L)




74 FTO/WO3-ZnO 3rd Methylene blue UV 1.0 61.0 0.20 0.3 4.20 × 10−5 33.50 Best [130] 75 TiO2 3rd Methylene blue UV 0.8 103.0 0.05 1.0 1.76 × 10−5 1.49 Average [131] 76 Al/ZnO 3rd Methyl orange UV 3.0 3,131.0 0.02 0.5 5.80 × 10−7 0.01 Below average [132] 77 FTO/BiOBr 3rd Methyl orange Vis 5.0 4,088.0 0.01 4.0 3.60 × 10−7 0.00 Below average [133] 78 Ni/TiO2 3rd Methyl Orange UV 2.0 8.0 0.95 2.0 3.40 × 10−4 75.05 Best [134] 79 TiO2 3rd Methyl Orange UV 15.0 255.0 0.05 6.4 7.20 × 10−6 0.01 Below average [135] 80 FTO/BiOCl-TiO2 3rd Rhodamine B Vis 3.0 871.0 0.10 2.1 1.52 × 10−6 0.05 Below average [60] 81 Bi/BiOCl 3rd Rhodamine B Vis 3.5 593.0 0.10 0.6 2.60 × 10−6 0.21 Below average [136] 82 Ti/BiOCl 3rd Rhodamine B UV 2.0 43.0 0.05 8.8 6.20 × 10−5 0.17 Below average [137] 83 Ti/BiOCl 3rd Rhodamine B Vis 3.0 908.0 0.05 8.8 2.00 × 10−6 0.01 Below average [137] 84 Ce/TiO2 3rd Basic blue UV 3.0 26.0 0.08 7.5 9.20 × 10−5 0.35 Below average [138] 85 Glass/P-TiO2 3rd Benzyl butyl phthalate Vis 4.0 95.0 0.20 0.6 1.76 × 10−5 2.24 Average [139]

aFOM classification: best (100–30), good (30–10), average (10–1), below average (less than 1).

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