Advances in Materials Science and Engineering
Volume 2015 (2015), Article ID 934587, 22 pages
http://dx.doi.org/10.1155/2015/934587

Review Article

Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites

Muhammad Ali Johar,¹ Rana Arslan Afzal,¹ Abdulrahman Ali Alazba,¹ and Umair Manzoor1,2

¹Alamoudi Water Research Chair, King Saud University, P.O. Box 2460, 11451 Riyadh, Saudi Arabia
²Centre for Micro & Nano Devices, Department of Physics, COMSATS Institute of Information Technology, 44000 Islamabad, Pakistan

Received 20 August 2015; Revised 12 October 2015; Accepted 13 October 2015

Academic Editor: Filippo Giannazzo

Copyright © 2015 Muhammad Ali Johar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Nanocomposites have a great potential to work as efficient, multifunctional materials for energy conversion and photoelectrochemical reactions. Nanocomposites may reveal more improved photocatalysis by implying the improvements of their electronic and structural properties than pure photocatalyst. This paper presents the recent work carried out on photoelectrochemical reactions using the composite materials of ZnO with CdS, ZnO with SnO₂, ZnO with TiO₂, ZnO with Ag₂S, and ZnO with graphene and graphene oxide. The photocatalytic efficiency mainly depends upon the light harvesting span of a material, lifetime of photogenerated electron-hole pair, and reactive sites available in the photocatalyst. We reviewed the UV-Vis absorption spectrum of nanocomposite and photodegradation reported by the same material and how photodegradation depends upon the factors described above. Finally the improvement in the absorption band edge of nanocomposite material is discussed.

1. Introduction

A humongous amount of water pollutants is discharged into the environment by the industries on daily basis which causes many hefty problems for humans, amphibious environments, and microorganisms [1–11]. The main sources of the water pollutants are fertilizers [12–14], microorganisms [15–18], application of pesticides and chemicals to soils [19–25], sewage [26–29] and wastewater [30–32], septic tanks [33–36], underground storage and tube leakages [37], atmospheric deposition [38–41], industrial waste which usually contains sulphur [42], asbestos, lead, mercury, nitrates and phosphates, oils, textile dyes, and so forth. These water pollutants cause the death of aquatic animals [43–49], disruption of food chains, different human diseases [50–59], destruction of ecosystems, and so on.

To decontaminate the contaminated water, researchers have taken many steps and have suggested many pollutants remediation techniques. One method is to treat the wastewater on site by the treatment plants, as it has a great potential [60–63]. There are a variety of water treatment processes like chemical, physical, and biological techniques, but each has its limitations for the application, cost, and effectiveness point of view. The pollutants are being transferred to solid phase from liquid phase by physical techniques like adsorption, precipitation, or air stripping; hence the pollutants are not destroyed. Chemical oxidation may be slow to moderate in the rate and selective or rapid but nonselective, hence generating oxidant cost. When the feed is inhibitory or toxic to bioculture, the limitation of biological oxidation takes place. Rest of the techniques are limited due to oxidative potential, economics, or tendency to farm harmful byproducts [64, 65]. Due to these limitations there may be offered an effective particular process which may be the combination of the available techniques in such a way to exploit their individual strength, thus an appropriate solution obtained within the economic constraints. Nowadays the most appropriate techniques for the water treatment are advanced oxidation processes (AOPs) which have very fewer limitations [66–70]. Among AOPs, heterogeneous photocatalysis is a tertiary water treatment process and has attracted the interest of researchers due to its ability to completely decompose the target pollutants [71–73].

There is a great potential for the mitigation of the toxic chemicals from the polluted water by photocatalytic degradation using nanostructured semiconductors [74, 75]. Currently, the hot issue among the most important challenges faced by science researchers for clean energy, pollutant-free water and air is designing new materials for the maximal harvesting of solar radiation. An extensive work has been carried out on ZnO and TiO₂ for the application of photocatalysis and photovoltaic cells due to their advantage of high stability against photocorrosion, suitable bandgap, and good photovoltaic and photocatalysis efficiencies [76–82].

The photocatalytic behavior of the nanocomposites varies with morphologies [83–93]. For ZnO, the difference in photocatalytic behavior occurs due to polar planes, surface areas, and oxygen vacancies. Xu et al. synthesized different morphologies of ZnO by solvothermal method and used them as photocatalyst for the degradation of phenol [83]. They suggested that NPs and nanoflowers exhibited enhanced photodegradation results compared to nanorods, nanotubes, nanoflowers, and hour-glass-like ZnO spheres. Liu et al. prepared TiO₂ nanostructures with different morphologies like NPs, nanorods, and microspheres via hydrothermal route and applied them for the photodegradation of phenol [87]. They observed excellent photodegradation results when nanorods were used as photocatalyst.

Although ZnO has been studied since 1935, new techniques and advance equipment make it possible to explore its remarkable properties [94]. ZnO is now considered to be the future material for various optoelectronics devices and sensors and as a catalyst. The characteristic of ZnO as photocatalyst becomes more prominent due to the enhanced photocatalytic efficiency of ZnO in the pure and doped forms and as a physical mixture. The figure of merits of doped and undoped ZnO nanomaterials is high carrier mobility, environmental sustainability, high photocatalytic efficiency, facile, simple tailoring of structures, nontoxicity, low cost for massive synthesis, and so forth.

2. ZnO Properties and Crystal Structure

ZnO occurs as a white powder. ZnO is an amphoteric oxide. ZnO is II-VI compound semiconductor whose iconicity lies at the borderline between ionic and covalent semiconductors. ZnO has three crystal structures, cubic zinc blende, cubic rocksalt, and hexagonal wurtzite, as shown in Figure 1. Hexagonal wurtzite structure is most common as it is most stable at ambient conditions; rocksalt can be formed at relatively high pressure, approximately 10 GPa, and a large volume decreases about 17% [95], while zinc blende can only be synthesized from cubic substrates [96]. Wurtzite and hexagonal ZnO have two crystal lattice parameters,  Å and  Å, and ratio is 1.60. A wide range of novel structures has been grown of ZnO by changing growth conditions. The main objective of this review is to appraise the recent research of one-dimensional ZnO hierarchical nanostructures used in photodegradation of water pollutants.

(a)

(b)

(c)

Figure 1: Stick and ball representation of different ZnO crystal structures: (a) cubic rocksalt, (b) cubic zinc blende, and (c) hexagonal wurtzite. The shaded gray and black spheres denote Zn and O atoms, respectively [96].

3. Photocatalysis

Photocatalysis was first reported in 1839 [97]. However, boom took place in the field of heterogeneous photocatalysis after an article reported by Fujishima and Honda in 1972. They reported photo-assisted catalysis of water on irradiation on TiO₂ with photons of energy greater than the bandgap of TiO₂ semiconductor [98]. Figure 2 illustrates the underlying science of photocatalysis of a pure semiconductor. As the photon with an equivalent or more energy hits the surface of the semiconductor, an electron jumps from the conduction band to valance band, thus creating an electron-hole pair. These photo-induced electrons and holes move separately to the surface of the semiconductor and react with O₂ and involved in the dye solutions. This leads to the formation of hydroxyl radicals (), superoxide radical anions (), and hydroperoxyl radicals () [99].

Figure 2: Schematics of principle of photocatalysis.

Several scenarios have been implemented to improve the harvesting spectrum to improve photo-assisted catalysis [100–109]. One of them is to synthesize 1D nanostructures. Researchers have fabricated different morphologies of nanostructures like NPs, NWs, nanoneedles, nanobelts, nanocombs, and flower-like nanostructures as shown in Figure 3. Doping of transition metal has limited success [110–117]. The addition of second metal oxide has also been used to enhance the light harvesting spectrum of ZnO.

Figure 3: Electron microscopy images of common morphologies of ZnO 1D nanostructures reported in literature. (a) Nanorods with sharp tips [229]. (b) Nanobelt. The inset clearly shows a rectangular belt-shape width [230]. (c) Nanocombs with secondary arms [229]. (d) More complex mixed morphologies [231] and (e) long nanowires [232]. Interestingly some of these morphologies may be considered as 2D structures but conventionally they come under 1D category [233].

3.1. Photocatalysis by Pure ZnO Nanostructures

ZnO is a promising material for the degradation of water pollutants. Lu et al. synthesized ZnO dense nanosheets-built network and applied it for the degradation of methyl orange [118]. They observed high photocatalytic activity due to high surface to volume ratio. Wang et al. synthesized ZnO NPs, NWs, and nanorods in the ionic solution at low temperature [119]. They used them for the photodegradation of RhB and showed size/shape dependent photocatalytic activity. Yan et al. grew films of ZnO nanoneedles, NPs, and flower-like structures and used them for decomposition of methyl blue under the principle of photocatalysis [120]. The efficiency of photocatalysis depends upon harvested region of the solar spectrum by ZnO nanostructures and the lifetime of the generated electron-hole pair. As ZnO is a wide bandgap semiconductor and its bandgap is in UV region, thus it can only harvest UV region. UV light is just 5% of the solar spectrum [121]. To improve efficiency, the first step is to harvest larger spectrum of sunlight so that more electron-hole pairs can be generated. The second step is to improve efficiency of photon to electron conversion. The third step is to increase the lifetime of photogenerated electron-hole pair.

3.2. Photocatalysis by ZnO Nanocomposite

A narrow bandgap metal oxide is doped in ZnO which increases the range of a sensitization process (SP). Primarily SP is limited by the relative positions of the conduction bands of the wide and narrow bandgap semiconductors and also by the nature of the interfaces in the system [122, 123]. The former factor can be controlled by tuning bandgap of sensitizer and also by choosing the appropriate material [124]. In order to ease the facile electron transfer the creation of heterojunction or favorable interface is still a challenge. Currently, a lot of researchers have reported their attempts to create the efficient heterojunctions for CdS-ZnO [125–127], thus improving efficiency. One way to improve the photocatalytic efficiency of the photovoltaic cell is to synthesize the one-dimensional (1D) nanomaterials of ZnO. One-dimensional nanomaterials have better crystallinity and may provide the more direct path for the transfer of electron and will decrease the charge recombination, thus increasing the efficiency [79–82, 128, 129]. Another possibility to enhance efficiency is to increase the photon to electron ratio of photocatalyst and one can achieve this by introducing the light scattering by light scatterers into photocatalyst. Cao et al. described a new technique to improve photoconversion efficiency by using ZnO submicrometer spheres as photocatalyst film. For light scattering they used polydisperse ZnO aggregates, while to achieve the higher adsorption of dye molecules in the photocatalyst film the increased surface area and necessary mesoporous structure were provided by the compositive monocrystalline ZnO [130, 131].

Bulk ZnO is wide bandgap semiconductor, thus a low efficient harvester of sunlight which is the disadvantage of ZnO for the use of visible light. To increase the efficiency of wide bandgap semiconductors for the visible light harvesting, different narrow bandgap semiconductors have been introduced as photosensitizers like InAs [132], CdS [133–135], CdSe [136–140], InP [141], PbS, and so forth. The nanoheterostructures which are also called combined nanocrystals show improved property which is distinct from that of any component in them. The mutual transfer of photogenerated charge carriers of nanomaterials of different semiconductors can enhance the photocatalytic efficiency [142–144]. On the basis of electron transfer process for two or more desirable semiconductors where photogenerated electrons can flow from one semiconductor with a higher CBM to the other with a lower CBM, is of great importance in better realization of photocatalytic degradation of organic pollutants [145–147]. It has many advantages like the oxidation and reduction processes taking place at different sites. There are also some particular advantages of using the narrow bandgap semiconductors: due to the quantum size effect one can harvest the required bandwidth of optical spectrum by tailoring the particle size; one can achieve the longer charge carrier separation by decreasing electron-hole pair recombination due to charge injection from narrow bandgap semiconductor to wide bandgap semiconductor [148].

Our work is focused on the review of photocatalytic properties of composite nanostructures of ZnO with CdS [81, 126, 127, 145, 148–166], TiO₂ [167–177], SnO [178], SnO₂ [179–194], CdSe [195], In₂O₃ [196], PbS, GaAs, Gas, CuO [197, 198], WO3 [199], graphene [200–215], Ag₂S [216–218], and so forth. The absorption band edge comparision of UV-Vis results of different ZnO nanocomposites are given in Table 1.

Table 1: Composites described here for light harvesting and photocatalytic activity.

Composite
Nanocomposite type
Year of publication
UV-Vis absorption range/edge (nm)
Reference
Remarks

ZnO/CdS
Nanospheres
August 2011
480
[148]

ZnO-CdS
Core-shell nanorods
August 2010
540
[150]

CdS@ZnO
Nanourchins
December 2012
512
[151]
Enhanced efficiency due to specific morphology which increased reactive area.

CdS-ZnO
CdS NPs on ZnO disk and CdS NPs on ZnO nanorods
August 2011
550
[153]
Metallic features of both polar surfaces provide more feasible path for charge transfer between ZnO and CdS, thus enhancing PC performance.

ZnO/CdS
ZnO/CdS core-shell nanorods
October 2012
480
[154]
CdS3 showed superior absorption; the photocatalytic efficiency was better due to ZnO and CdS3 favorable synergetic effect.

ZnO/CdS
Flower-like ZnO modified by CdS NPs
July 2011
500
[155]
ZnO/CdS nanoheterostructures exhibit superior PC activities due to increased photoresponding range and increased charge separation rate.

ZnO/CdS
CdS NPs/ZnO NWs
March 2009
550
[156]

ZnO-CdS@Cd
Rod-like Cd core and a ZnO-CdS heterostructural shell
December 2012
570
[157]

ZnO/TiO₂
Composite nanofibers
February 2010
386.5
[168]
Superior PC activity of ZnO/TiO₂ composite nanofibers. The reason behind that was superior light harvesting capacity and better quantum efficiency.

ZnO/TiO₂
Nanoscale coupled oxides
June 2010
460
[176]
Better UV-Vis absorption for ZnO/TiO₂ approximately band edge at 460 nm. Enhanced photocatalytic activity for coupled ZnO/TiO₂ due to bonded heterostructures, thus increasing quantum efficiency.

ZnO-SnO₂
Nanoporous ZnO-SnO₂ heterojunction
June 2012
390
[182]
Nanoporous heterojunction of ZnO-SnO₂ exhibited excellent photocatalytic behavior although UV-Vis band edge was not higher than ZnO.

ZnO/SnO₂
Nanofibers
May 2010
396
[189]
Mesoporous ZnO/SnO₂ nanofibers were synthesized with Sn % content from 25, 33, and 50% and then calcinated at different temperatures. UV-Vis absorption spectroscopy was done and band edges were at about 390 nm. Photodegradation was better for the sample with molar ratio of Zn : Sn 2 : 1 and calcinated at 500°C.

ZnO-SnO₂
Hollow spheres and hierarchical nanosheets
November 2007
390
[190]
Higher photocatalytic efficiency due to increased life time of photogenerated electron-hole pair and also the nanosheets provided the favorable condition for the transfer of electron-hole to the surface.

Mn-ZnO/graphene
NPs
April 2014
600
[205]
Enhanced photocatalysis was observed for 3% Mn-ZnO/graphene nanocomposite and UV-Vis DRS showed better results for Mn-ZnO/graphene.

(GO/ZnO)
GO/ZnO nanorods hybrid
November 2014
600
[206]
The synergic effect between GO and ZnO was responsible for an improved photogenerated carrier separation. 3% GO/ZnO showed superior photocatalytic activity.

ZnO/Ag₂S
Core-shell nanorods
August 2014
700
[216]
The absorption peak also shifted to 470 nm from 374 nm, while overall absorbance spectrum was broadened up to 700 nm. The photodegradation results were also much better than ZnO nanorods.

ZnO/Ag₂S
CSNPs
August 2015
550
[217]
Visible region exhibits the main peak around 550 nm. A huge difference in efficiency of photocatalytic degradation was observed and ZnO/Ag₂S CSNPs showed tremendous results.

ZnO/Ag₂S
NPs
June 2012
500
[218]
Photodegradation experiment was carried out under sunlight with nearly constant flux. NPs of ZnO/Ag₂S showed better performance than bare ZnO NPs, commercial ZnO, P25, and TiO₂ Merck.

The most explored composite material with ZnO for photocatalysis is CdS. CdS has been used as sensitizer. After CdS sensitization, there was clear absorption of visible light. With the increase of CdS loading (from 10% to 40%), there was continuous red shift of absorption edges. The results indicated that, with CdS as photosensitizer for Ba0.9Zn0.1TiO₃, there was better harvesting of solar light [151]. A clear absorption of visible light by using CdS as sensitizer was reported by Zou et al. [133]. One can see in Figure 4(a) that UV-Vis absorption spectrum of CdS is covering most of the region, which is the reason to use CdS as sensitizer ZnO photocatalyst.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 4: (a) UV-Vis absorbance spectra of as-prepared photocatalyst [133]. (b) UV-Vis absorption spectra of ZnO nanospheres and ZnO nanoparticle films. (c) UV-Vis absorption spectra of the ITO/ZnO electrode dipped in the reaction solution for different times [148]. (d) UV-Vis absorption spectra of ZnO, the ZnO-CdS core-shell nanorods ((ZnO)₁-(CdS)x, , 0.2, 0.3, 0.4, 0.5, and 0.6, are denoted as 101, 102, 103, 104, 105, and 106, resp.), and CdS [150]. (e) ZnO/CdS electron transfer process. (f) ZnO precursor. (Inset: the bandgap of ZnO/CdS nanoheterostructure is estimated from the absorption edge [155].)

3.2.1. ZnO/CdS Nanocomposite Photocatalysis

Bandgap of bulk CdS is 2.40 eV at room temperature. CdS has higher electron affinity than ZnO. The bandgap diagram of CdS-ZnO composite is shown in Figure 4(e). According to Anderson’s model, between CdS and ZnO a type II model is formed. As the visible light is radiated on the CdS-ZnO composite, the electron is generated in the conduction band of CdS and it jumps to the conduction band of ZnO by ballistic diffusion [219]. The time required for electron to be transferred from conduction band of CdS to conduction band of ZnO is 18 picoseconds which is less than the lifetime of electron in CdS [220].

Shen et al. synthesized ZnO/CdS hierarchical nanospheres. They first synthesized ZnO nanospheres by hydrolysis of zinc salt under ultrasound irradiation. Then CdS nanocrystals were grown on ZnO nanospheres selectively. UV-Vis absorption spectroscopy of ZnO nanomaterials suggested that the peak at around 370 nm for both films was due to the bandgap of ZnO nanostructures as shown in Figure 4(b) [148]. An extra peak at around 420 nm was also observed by Shen et al. for ZnO nanospheres film; its cause was the light scattering which was due to large secondary colloidal spheres. As Figure 4(c) depicts, a red shift from 370 nm (curve (B), ZnO) to 480 nm (curves (D)–(F)) was observed by increasing the dipping time, and the absorption intensity was increased gradually. The science behind this phenomenon is the quantization size effect which caused the longer wavelength due to large particle size. Hence the light absorption and charge separation were significantly enhanced [148].

As compared to pure ZnO, the optical absorption edges of the ZnO-CdS core-shell nanorods are extended into visible light range at about 540 nm approximately as shown in Figure 4(d). It was found that the absorption edge of hydrothermally synthesized (ZnO)x-(CdS)y core-shell nanorods is not sensitive to the increased amount of CdS after the ratio of CdS to ZnO is larger than 0.2 : 1 [150]. Barpuzary et al. synthesized CdS@Al₂O₃ and CdS@ZnO nanourchins-like structure by hydrothermal route using autoclave. They grew CdS NWs on oxide core and found enhanced photocatalytic results. There were two sharp absorption steps for CdS@ZnO photocatalyst: one at ca. 380 nm is for ZnO and the other at ca. 512 nm is for CdS. The apparent quantum yield (AQY) of 8% for CdS NWs has been enhanced up to 11% and 15% by growing hierarchically over Al₂O₃ and ZnO, respectively [151]. Wang et al. fabricated the CdS NPs on ZnO disks and CdS NPs on ZnO nanorods by hydrothermal technique. The percentage of polar facets of ZnO was controlled by the concentration of NaOH. Both the polar surfaces (0001) and () behaved like metals while rest of the surfaces behaved like semiconductors. These metallic facets provided a more feasible path for the transfer of charges between ZnO and CdS. This feature contributed mainly to enhancing the photocatalytic activity by shifting the absorption edge to 550 nm [153]. Khanchandani et al. prepared CdS coated ZnO nanorods by surface functionalization route. They fabricated ZnO nanorods of 100 nm and CdS as shell with variable shell thickness (10–30 nm). UV-Vis spectrum shows that after CdS coating the band edge had a red shift. The sample with CdS coating of 30 nm (CdS₃) showed more superior absorption, which may lead to enhanced visible light degradation efficiency [154].

Li and Wang synthesized flower-like heterostructures of ZnO/CdS by a facile two-step precipitation method. The flower-like nanostructures of ZnO were modified by CdS NPs and successfully applied them in the photocatalytic degradation of RhB [155]. They suggested that the characteristic absorption of RhB at 553 nm decreased rapidly with extension of the exposure time. The peak completely disappeared after about 90 minutes showing efficient and enhanced results for the ZnO/CdS nanoheterostructures. For ZnO/CdS nanoheterostructures, the photoresponse range was extended and more light energy was utilized than pure ZnO under the simulated sunlight irradiation. UV-Vis absorption spectroscopy showed enhanced absorption with ZnO/CdS nanoheterostructures absorption up to 500 nm approximately as in Figure 4(f). More importantly, the enhanced photocatalytic efficiency of ZnO/CdS nanoheterostructures is mainly due to the inhibition of electron-hole pair recombination by a charge transfer process in ZnO-CdS heterostructures [155]. Tak et al. fabricated CdS NPs/ZnO NWs heterostructures array; ZnO NWs arrays were vertically aligned. They reported that a bare ZnO nanowire array absorbed only the light of the wavelength less than 400 nm. However, CdS NPs deposition increased the light absorption limit up to 550 nm. They investigated photoelectrochemical cell performances of CdS NPs/ZnO NWs photoanodes prepared at different deposition conditions. It was also suggested that enhanced photocurrent of the CdS NP/ZnO NW heterostructures was due to their higher visible light absorption capability and charge carrier transfer efficiency [156]. Wang et al. synthesized a rod-like ZnO-CdS@Cd heterostructure in which Cd was core and ZnO-CdS was heterostructural shell. They were grown by chemical method which consisted of two steps: replacement and sulfurization reactions. The absorption edge was increased to 570 nm and photocatalytic activity was improved due to Z-Scheme and the shortened charge carrier transport length in thin ZnO-CdS heterostructural shell and due to efficient charge carrier transport channel provided by Cd [157].

3.2.2. ZnO/TiO₂ Nanocomposite Photocatalysis

Bandgap of TiO₂ is 3.2 eV. As there is a strong effect of optical properties on photocatalytic performance, the relationship between bandgap energy, particle size, and performance is well described for TiO₂ [121]. ZnO-TiO₂ nanocomposite is a potential material for high efficiency photocatalyst because TiO₂ has high reactivity and ZnO has large binding energy. TiO₂ is also preferred due to its resistance to photocorrosion and low toxicity [221]. They both have lower prices as well.

Liu et al. fabricated composite nanofibers of TiO₂/ZnO by electrospinning. UV-Vis absorption spectroscopy proved that the ZnO/TiO₂ composite nanostructures were superior with respect to light harvesting range. They fabricated ZnO/TiO₂ composite nanostructures with different weight percentages of ZnO. TiO₂/ZnO-2 15.8% showed better UV-Vis results and also in the photocatalytic degradation of RhB as shown in Figures 5(a) and 5(b). The reason behind this is the increased quantum efficiency of the system due to coupling effect of TiO₂ and ZnO in grain-like composite NPs. Because of this, efficient charge separation increased the lifetime of electron-hole pair and reduced its recombination in the composite nanofibers [168].

(a)

(b)

(c)

(d)

(e)

Figure 5: (a) UV-Vis diffuse reflectance spectra of nanofibers: (A) TiO₂/ZnO-1, (B) TiO₂/ZnO-2, (C) TiO₂/ZnO-3, and (D) pure TiO2 [168]. (b) Photocatalytic degradation of RhB in an aqueous solution in the presence of nanofibers [168]. (c) Effect of ZnO nanoflowers, pristine titanium oxide nanofibers (TiO₂), titanium oxide nanofibers incorporating ZnO NPs (ZnO doped TiO₂), and the newly introduced ZnO/titanium oxide nanostructure on the photocatalytic degradation of RhB dye [170]. (d) Ultraviolet-visible absorption spectra of pure ZnO, TiO₂, and ZnO/TiO₂ [176]. (e) Photocatalytic degradation of MO by various photocatalysts.

ZnO NPs doped TiO₂ nanofibers were synthesized by electrospinning followed be hydrothermal process. As shown in Figure 5(c) that ZnO-TiO₂ hierarchical nanostructures eliminated the methyl red blue less than 90 min and RhB before 105 min, even the other nanostructures did not remove any pollutant even after 3 hours. Again the incorporation of ZnO NPs in TiO₂ nanofibers enhanced the photocatalytic activity to a certain extent [170]. ZnO-TiO₂ nanocomposites were prepared by distribution of TiO₂ NPs over ZnO nanorods and their original structure was well preserved as reported by Chen et al. [173]. The higher donor density for the nanocomposite electrode was also reported in the same article. Coupled ZnO-TiO₂ nanocomposite was used for the photocatalytic degradation of active methylene blue as model reaction. It was clear that the photocatalytic degradation results of coupled nanocomposite were better than individual ZnO nanorods or TiO₂ NPs [173]. Zhang et al. synthesized ZnO/TiO₂ photocatalyst by two-step method, the homogeneous hydrolysis and low temperature crystallization [176]. UV-Vis absorption results revealed that ZnO/TiO₂ coupled oxides are better light harvesting photocatalysts than ZnO and TiO₂ individually as shown in Figure 5(d). Degradation of MO was evaluated by photocatalysis and MO was easily degraded under UV irradiation by using coupled ZnO/TiO₂ and highest photocatalytic activity was observed, which is shown in Figure 5(e). The reason behind enhanced photocatalytic activity of coupled ZnO/TiO₂ was bonded surface heterostructure which increased lifetime of photogenerated electron-hole pair, thus increasing quantum efficiency [176]. Rakkesh and Balakumar synthesized ZnO/TiO₂ core-shell nanostructures by wet chemical method [222]. Core-shell nanostructures exhibited excellent optical properties and their spectrum was up to visible light wavelength. They used them for the degradation of acridine orange under sunlight irradiation. ZnO/TiO₂ core-shell nanostructures showed higher photocatalytic activity than ZnO and TiO₂ nanostructures.

3.2.3. ZnO/SnO₂ Nanocomposites Photocatalysis

SnO₂ is a wide direct bandgap semiconductor and its bandgap at room temperature is 3.7 eV [223]. It is a rutile structure and six oxygen atoms surround one tin atom in an octahedral way. The conduction band of ZnO is higher than the conduction band of SnO₂, so the conduction band of SnO₂ acts like a sink for the photogenerated electrons [178]. Holes will be injected in opposite direction. The recombination rate will be slow; thus more carriers will be available to produce free radicals by interfacial charge transfer [178].

Zhang et al. synthesized one-dimensional ZnO-SnO₂ nanofibers by combining sol-gel process and electrospinning technique. In Figure 6(a) UV-Vis absorption spectra are shown for SnO₂, ZnO, and ZnO-SnO₂ nanofibers. The overall absorption spectrum for ZnO-SnO₂ was better than the rest of the two. The reason for high photocatalytic activity was the heterojunction between ZnO and SnO₂ which could enhance the separation of photogenerated electrons and holes. The material is also recyclable as the recycles photocatalytic activity is shown in Figure 6(b) [180]. Uddin et al. fabricated nanoporous ZnO-SnO₂ heterojunction by two-step method: first, the fabrication of nanosized SnO₂ particles by homogeneous precipitation along with hydrothermal treatment and, second, their reaction with zinc acetate followed by calcination at 500°C. UV-Vis diffused reflectance showed average results and band edge for ZnO-SnO₂ was at 390 nm as shown in Figure 6(c), but good photocatalytic activity was observed for the same heterojunction due to enhanced separation of photogenerated electrons and holes as shown in Figure 6(d). So it is potential material for photocatalytic applications [182]. Zheng et al. fabricated SnO₂/ZnO heterojunction photocatalyst by simple two-step solvothermal method. The samples of ZnO/SnO₂, SnO₂, and ZnO were prepared and were applied for the photodegradation of methyl orange. The photocatalytic activity of nanostructured ZnO/SnO₂ heterojunction photocatalyst was found to be superior than others and even better than the standard Degussa P25 as exhibited by the graphs shown in Figure 6(e). Two main reasons were reported by the author for enhanced photodegradation of MB by nanostructured ZnO/SnO₂ heterojunction photocatalyst. The first was the higher Brunauer-Emmett-Teller (BET) surface area. The second was the improvement of separation of photogenerated electron-hole pair due to promotion of interfacial charge transfer kinetics between SnO₂ and ZnO semiconductors by the SnO₂-ZnO heterojunction [183].

(a)

(b)

(c)

(d)

(e)

Figure 6: (a) UV-Vis diffuse reflectance spectra of the electrospun nanofibers: (A) ZnO nanofibers, (B) ZnO-SnO₂ nanofibers, and (C) SnO₂ nanofibers [180]. (b) Photocatalytic activity of the ZnO-SnO₂ nanofibers for RB degradation with three times of cycling uses [180]. (c) UV-Vis diffuse reflectance spectra (a) and plots of () versus Wavelength (nm) (A) SnO₂, (B) SnO₂-ZnO, and (C) ZnO. (d) ln () as a function of the irradiation time for calcined SnO₂ (triangle), SnO₂-ZnO (square), and ZnO (circle) photocatalysts [182]. (e) Photodegradation of MO by the as-synthesized samples: (A) SnO₂, (B) ZnO, and (C) SnO₂/ZnO nanocatalyst. The commercial TiO₂ (Degussa P25) is used as a photocatalytic reference [183].

Liu et al. synthesized mesoporous ZnO/SnO₂ composite nanofibers by electrospinning technique. Samples were calcinated at 700°C and most superior absorbance was exhibited by Zn₂SnO₄ (ZS) with 50% Sn content with absorption edge at 396.3 nm, followed by Zn₂SnO₄ (Z₂S) of Sn content 33% with absorption edge of 393.2 nm. The ZnO/SnO₂ composite nanofibers showed higher photocatalytic activity than pure ZnO and SnO₂ nanofibers. This was attributed to its high surface areas, high efficiency in the light utilization, and high efficient separation of photogenerated electron-hole pairs (shown in the following section). For the same Sn content, as the calcinations temperature increases the photocatalytic activity decreases. The reason for lower photocatalytic activity was the reduction in the surface area of ZnO/SnO₂ nanofibers. Also, SnO₂ content ratio is important because if the SnO₂ mole percent decreases from 25% the photocatalytic activity will decrease. If the mole percent approaches 50% the photocatalytic activity will also decrease further because ZnO active sites will be surrounded by SnO₂, which may behave like isolation between ZnO and oxygen-containing species [189]. ZnO-SnO₂ hollow spheres and hierarchical nanosheets were successfully synthesized, using hydrothermal method. Although the absorption band edge of ZnO-SnO₂ nanostructures was at 390 nm, less than ZnO, ZnO-SnO₂ nanostructures showed superior photocatalytic degradation efficiency compared to ZnO nanorods or SnO₂ nanomaterials alone. The reasons described for the higher photocatalytic activity were the increased lifetime of photogenerated electron-hole pair and also the nanosheets were in favor for the transfer of electrons and holes generated inside the crystal to the surface [190].

Li and Liu synthesized core-shell and coupled particles of ZnO/SnO₂ via successive precipitation and coprecipitation methods, respectively [224]. They applied both of them for the photodegradation of MO. The photocatalytic activity of core-shell particles was higher and the reported reason by the authors was the increase of charge separation efficiency [224]. Core-shell micropyramids of ZnO/SnO₂ have also enhanced optical properties [225].

3.2.4. ZnO/Graphene Nanocomposites Photocatalysis

Graphene consists of two-dimensional sheets of carbon atoms and carbon atoms are arranged in a hexagonal structure. Graphene has magnificent electrical conductivity and good mechanical properties [207]. The morphology of G-ZnO composites can enhance the photocatalytic efficiency. G-ZnO composite thin films were synthesized using the electrostatic spray deposition technique. G-ZnO thin films were composite of nanoplatelets of ZnO and graphene. G-ZnO thin films of different weight percentage in the films were annealed at different temperatures. G-ZnO was used for the photodegradation of methyl blue. G-ZnO thin film with 0.1% weight percentage was annealed at 300°C suggesting better photocatalytic degradation of MB than rest of the samples. The reason described by Joshi et al. for the better performance was the reduced charge recombination due to introduction of graphene [202]. Worajittiphon et al. synthesized amine-functionalized graphene nanoplatelets decorated with ZnO NPs using hydrothermal method. RhB was used to evaluate the photocatalytic properties of nanostructures. Enhanced UV-Vis absorption spectrum band edge was up to 400 nm as shown in Figure 7(a). Excellent photodegradation results were observed for 5 wt.% f-GNP/ZnO as suggested in Figure 7(b). The reason behind good photodegradation was the increased specific surface area of reactive sites, increased light harvesting span, and the increased lifetime of photogenerated electron-hole pair or suppressed charge carrier recombination [204].

(a)

(b)

(c)

(d)

Figure 7: (a) UV-Vis absorption spectra of catalysts [204], (b) photocatalytic RhB degradation [204], (c) UV-Vis absorption spectra of ZnO, ZnO/graphene, 1% Mn ZnO/graphene, 3% Mn-ZnO/graphene, and 5% Mn-ZnO/graphene and composites [205], and (d) the photocatalytic degradation of MB in the presence of ZnO, Mn-ZnO NPs, and Mn-ZnO/graphene nanocomposites under visible light irradiation [205].

Mn doped ZnO/graphene nanocomposites were synthesized by Ahmad et al. using facile single-step solvothermal method [205]. A red shift was observed in the band edge absorption for ZnO/graphene nanocomposites as shown in Figure 7(c), while overall better performance was observed after doping of Mn in ZnO/graphene nanocomposites. To evaluate the photodegradation effects of synthesized nanocomposites MB was used. There were two phases of pollutants degradation: the first was the adsorption, for which the 5% Mn-ZnO/graphene showed better performance but during photodegradation 3% Mn-ZnO/graphene showed superior photocatalytic activity and 90% MB was degraded within one hour with this nanocomposite as shown in Figure 7(d). The responsible factors held by the author for this enhanced photocatalysis were improved adsorption of dyes, improvement in charge separation, enhanced visible light absorption, efficient electron transfer, the produced hydroxyl radicals, improved adsorption of dyes, and large surface area of contact between Mn-ZnO and graphene [205].

Dai et al. synthesized GO/ZnO nanorods hybrid via facile hydrothermal process. UV-Vis DRS band edge for GO/ZnO was at 391 nm and baseline extended to 600 nm due to GO nanosheets. Methyl blue was used to evaluate the photocatalytic activity of nanomaterials. LEDs with wavelength of 375 nm were used for irradiation for photodegradation. 3% GO/ZnO hybrid showed better photocatalytic activity than rest of the composites. The reason behind superior photocatalytic activity was larger surface area and low recombination rate of photogenerated electron-hole pair [206]. Optical properties of core-shell ZnO/graphene nanoparticles are far better than ZnO [226]. Bu et al. synthesized graphene/ZnO composite with quasi-core-shell structure by one-step wet chemical method [227]. UV-Vis absorption spectroscopy reveals that core-shell composite material exhibited peaks in visible region and was found to be better photocatalyst material than ZnO. The reported reason by the authors was the establishment of an effective electric field between graphene coating layer and ZnO [227].

3.2.5. ZnO/Ag₂S Nanocomposite Photocatalysis

Bandgap of Ag₂S is 1.1 eV [228]. Due to low bandgap energy, Ag₂S can absorb a broad solar spectrum. Band alignment diagram of ZnO and Ag₂S is shown in Figure 8(a). ZnO/Ag₂S core-shell nanocomposites comprise high efficiency for light harvesting; the conduction band offset between ZnO and Ag₂S is small which promotes efficient charge carrier separation of core-shell interface [216]. Khanchandani et al. used ZnO/Ag₂S and ZnO/CdS core-shell nanostructures as photocatalyst for the degradation of MB. The light harvesting spectrum of ZnO/Ag₂S core-shell nanostructures was better and the absorption peak shifted to 470 nm from 374 nm as compared to ZnO nanorods, which is shown in Figure 8(b). ZnO/Ag₂S core-shell nanostructures exhibited better photodegradation results as shown in Figure 8(c). Sadollahkhani et al. synthesized ZnO/Ag₂S core-shell nanoparticles (CSNPs) by chemical approach at relatively low temperature around 60°C [217]. CSNPs were photocatalyst for the degradation of Eriochrome Black T dye. UV-Vis absorption spectroscopy showed excellent results for ZnO/Ag₂S CSNPs and absorption spectrum was broadened as shown in Figure 8(d). Photodegradation results of ZnO/Ag₂S CSNPs were far better than ZnO NPs as depicted in Figure 8(e). Subash et al. synthesized ZnO/Ag₂S nanoparticles by sol-gel method [218]. They applied them as photocatalyst for the degradation of acid black 1 (AB 1). The experiment was carried out under sunlight and light flux was nearly constant. From the diffuse reflectance spectra of the bare ZnO and ZnO/Ag₂S NPs, it was shown that introduction of Ag₂S decreased the bandgap of NPs. Photoluminescence spectroscopy reveals that ZnO/Ag₂S NPs also have a main peak in visible range along with a peak lying ultraviolet range. NPs of ZnO/Ag₂S revealed excellent results of photodegradation and results were better than bare ZnO NPs, TiO₂-P25, commercial ZnO, and TiO₂ Merck [218], which are shown in Figure 8(f). The above discussion suggested that Ag₂S improves the light harvesting spectrum along with better photocatalysis results.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 8: (a) Band alignment of ZnO and Ag₂S [218]. (b) Optical absorption spectra of uncoated ZnO nanorods and ZnO/Ag₂S and ZnO/CdS core-shell nanostructures [216]. (c) Photocatalytic performances of uncoated ZnO nanorods and ZnO/Ag₂S and ZnO/CdS core-shell nanostructures for the degradation of MB solution under illumination of visible light [216]. (d) UV-Vis absorption spectra [217]. (e) Photodegradation. The plot of versus reaction time at pH 4 [217]. (f) Photodegradability of AB 1; [AB 1] = 2 × 10−4 M; catalyst suspended = 4 g L⁻¹, pH = 9; airflow rate = 8.1 mL s⁻¹ [218].

4. Conclusions

ZnO nanocomposites with CdS, TiO₂, SnO₂, graphene, and Ag₂S have been studied for the photocatalytic activities. It was found that ternary nanocomposites should be used for enhanced photocatalysis as they provide efficient heterojunction and better sensitization. Apparent quantum yield should be increased by increasing lifetime of photogenerated electron-hole pair and reactive surface area. One more characterization is also suggested which may provide the lifetime of photogenerated electron-hole pair. For large reactive surface area, the scientists and researchers should determine the appropriate percentage of ZnO with other compounds, which should be such an optimum point, where light harvesting capability should be better along higher photocatalytic activity, so dopants may also be added in the nanocomposite to achieve the same. In this way, the underlying science will be at its best level and ball will be in the court of material engineers for industrial reactor. It is essential to evaluate the recycled photocatalytic degradation efficiency, which is one of the most important parameters towards the device design.

Abbreviations

ZnO: | Zinc oxide
CBM: | Conduction band minimum
MO: | Methylene orange
RhB: | Rhodamine B
G-ZnO: | Graphene-zinc oxide
GO: | Graphene oxide
NWs: | Nanowires
NPs: | Nanoparticles
CSNPs: | Core-shell nanoparticles
MB: | Methylene blue
AB 1: | Acid black 1.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This Project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award no. 2451.

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