In the past decade, several kinds of conducting polymers, including polyaniline (PANI), [ 88–107 ] P3HT, [ 108–115 ] polypyr-role (PPy), [ 116–128 ] PEDOT, [ 129–133 ] and others, [ 134–147 ] have been used in the preparation of photocatalysts based on polymer/semiconductor nanocrystal nanocomposites. With suitable energy levels, conducting polymers can sensitize semiconductor nanocrystals to improve the light harvesting effi ciency as well as promote effective charge separation to increase the carrier utilization effi ciency. Both of these two merits are benefi cial for photocatalytic applications. Semi-conductor photocatalysts are usually in the form of powder, which possess large surface areas when dispersed in the reac-tion solureac-tion. There remain, however, several drawbacks affecting the practical use of powdery semiconductor catalysts: (i) time-consuming procedures for retracting photo-catalysts from the reaction solution, and the inevitable loss of photocatalysts; [ 148 ] (ii) the dust of powdery photocatalyst
is a risk factor to human health; [ 149 ] (iii) photocatalyst aggre-gation decreases the total surface area during the photocata-lytic reaction, and decreases the photocataphotocata-lytic activity. [ 150 ] An effective strategy to solve these problems is to immo-bilize the photocatalysts on a polymeric support. The ideal polymeric support should satisfy several features: good adhe-sion with the photocatalyst; [ 151 ] a large specifi c surface area;
high adsorption capability toward the reaction species; high chemical inertness; and good mechanical stability. Several methods have been developed to immobilize photocatalysts on polymeric supports, including electrospinning, [ 152–155 ] sol–gel methods, [ 156–163 ] atomic layer deposition, [ 164–166 ] sol-vent-casting process, [ 167,168 ] hydrothermal methods, [ 169–172 ] solvothermal methods, [ 173 ] solution polymerization, [ 174–176 ] ion exchange, [ 150,177–181 ] and impregnation. [ 149,182,183 ] Most of these methods can be performed at relatively low tempera-ture to avoid damage to the polymeric supports.
3.1. Polymer-Sensitized TiO2-Based Composite Photocatalysts
TiO 2 is the most widely studied photocatalyst because of its chemical stability, nontoxicity, and relatively low cost. Never-theless, the large bandgap of TiO 2 limits its light absorption to only 5% of the solar spectrum. Coupling with a conducting polymer having a narrow band gap is an effective means of extending the light absorption spectrum of TiO 2 and, thereby, improving the photocatalytic activity. Luo et al. [ 140 ] modifi ed TiO 2 nanoparticles with conjugated derivatives of polyisoprene (CDPIP) using a delicate bromine addition/
dehydrobromination approach. The as-prepared TiO 2 /CDPIP composite photocatalyst exhibited notable photocatalytic performance under illumination with visible light; Figure 7 a
Figure 7. a) HRTEM images of i) TiO 2 and ii) TiO 2 /CDPIP. b) Relative band structures of TiO 2 /CDPIP and the proposed CT processes for MO degradation.
c) Photocatalytic degradation of MO in the presence of P-25 TiO 2 and TiO 2 /CDPIP at various TiO 2 /CDPIP ratios. d) Recycling tests on TiO 2 /CDPIP for MO degradation. Reproduced with permission. [ 140 ] Copyright 2012, American Chemical Society.
displays the high resolution TEM (HRTEM) of i) TiO 2 and ii) TiO 2 /CDPIP. Figure 7 b presents the possible lytic mechanism for the enhanced visible light photocata-lytic activity of TiO 2 /CDPIP nanocomposites. CDPIP readily absorbs visible light to induce a π–π* transition and produce electron/hole pairs. Because of the band offset, the excited state electrons in the LUMO of CDPIP (–3.45 eV) readily transferred to the conduction band of TiO 2 (–4.0 eV) to achieve effi cient charge separation. Figure 7 c shows the pho-tocatalytic experiments revealing that TiO 2 /CDPIP nanocom-posites exhibited signifi cantly higher photocatalytic activities for the degradation of methyl orange (MO) than TiO 2 under irradiation with visible light. The rate constant of MO deg-radation was the highest ( k = 0.196 h −1 ) at the condition of TiO 2 /CDPIP (10:1). A greater number of CDPIP molecules attached to the surfaces of the TiO 2 nanoparticles resulted in more visible light being absorbed to produce more elec-tron/hole pairs. When the content of CDPIP on the TiO 2 surface exceeded a critical value, however, injection of elec-trons from the LUMO of CDPIP into the conduction band of TiO 2 became diffi cult because the electrons photogen-erated at the outermost layer needed to travel a long distance to arrive at the TiO 2 surface. As a result, the photocatalytic activity of TiO 2 /CDPIP fi rst increased and then decreased upon increasing the content of CDPIP. Figure 7 d shows the photocatalytic activity of TiO 2 /CDPIP (10:1) composite maintained 80% for the fi rst cycling run, indicating that the photocatalytic stability of the investigated nanocomposites is high and similar to those for TiO 2 /PPy and TiO 2 /P3HT nanocomposites.
Subsequently, a surfactant-directed in situ chemical polymerization method was used to synthesize PPy-dec-orated Ag-TiO 2 nanofi bers (PPy-Ag-TiO 2 ). [ 125 ] Figure 8 a shows the as-synthesized PPy-Ag-TiO 2 nanofi bers exhibited remarkable photocatalytic performance for the decomposi-tion of gaseous acetone under illuminadecomposi-tion with visible light.
Figure 8 b shows the relatively low PL intensity of PPy-Ag-TiO 2 , indicating substantially low rate of recombination of
the charge carriers. Figure 8 c presents the photocurrents with an order of PPy-Ag-TiO 2 > PPy-TiO 2 > Ag-TiO 2 > pure TiO 2 . PL spectra and photocurrent measurement both sug-gested the occurrence of more effective charge carrier sepa-ration for PPy-Ag-TiO 2 . Figure 8 d shows the PPy-Ag-TiO 2 nanofi bers with signifi cantly enhanced photocatalytic activity.
Such enhancement was due to the improved visible light harvesting, rapid CT, and a low probability of electron/hole recombination, based on the synergistic effect of TiO 2 , Ag, and PPy. When the PPy-Ag-TiO 2 nanofi bers were illuminated under visible light, electrons were excited from the HOMO to the LUMO of PPy, leaving holes behind in the HOMO of PPy. The excited state electrons were readily injected into the conduction band of TiO 2 and further injected into the Fermi level of Ag. Figure 8 e shows the recycling test of gas-eous acetone degradation for PPy-Ag-TiO 2 nanofi bers. The photocatalytic activity has no decreasing because of the large length-to-diameter ratios of the nanofi ber structures.
3.2. Polymer-Sensitized ZnO-Based Composite Photocatalysts
Although TiO 2 is the most widely employed photocatalyst, ZnO appears to be a suitable alternative in some applica-tions. Compared with TiO 2 , ZnO can absorb a larger portion of the UV spectrum, and it exhibits superior photocatalytic properties. [ 184–186 ] Coupling ZnO with a narrow band gap conducting polymer is also an effective means of promoting charge carrier separation and improving the utilization of solar light. A facile chemisorption method together with a cold plasma treatment technique was applied to prepare the PANI-hybridized ZnO photocatalyst. [ 107 ] Figure 9 a shows the PANI was uniformly coated on the ZnO surface with an inti-mate contact. Interestingly, the coated PANI acted coopera-tively with deliberately introduced defects of ZnO (oxygen vacancy, V o , and interstitial zinc, Zn i ) to enhance the resulting photocatalytic effi ciency. Figure 9 b shows the degradation of MO photocatalyzed by pure ZnO (Z), defective ZnO (Z-D),
Figure 8. a) Relative band structures of PPy-Ag-TiO 2 and the proposed CT processes. b) PL emission spectra and c) photocurrent transient responses for i) TiO 2 , ii) Ag-TiO 2 , iii) PPy-TiO 2 , and iv) PPy-Ag-TiO 2 . d) Photocatalytic activities of various samples toward gaseous acetone degradation under visible irradiation. e) Recycling tests for PPy-Ag-TiO 2 in acetone degradation. Reproduced with permission. [ 125 ] Copyright 2013, American Chemical Society.
PANI-hybridized ZnO (Z-P), and PANI-hybridized defective ZnO (Z-D-P). The Z-D-P composite photocatalyst exhib-ited signifi cantly enhanced photocatalytic activity, with a rate constant for MO degradation 2.5 times higher than that of pure ZnO. Figure 9 c shows that the electrochemical imped-ance spectroscopy (EIS) study revealed that both defects and PANI led to more effi cient charge separation and faster interfacial CT. A synergistic effect between PANI and defects was proposed as a probable mechanism to account for the enhanced photocatalytic activity. Figure 9 d-i displays a sche-matic diagram describing the well match energy band of PANI and ZnO. Under illumination with light, the photogen-erated holes transferred from ZnO toward the PANI mon-olayer, while the photoexcited electrons from both ZnO and PANI could be trapped by V o at the surface (Figure 9 d-ii).
The surfi cial V o acted as trapping sites for photoexcited electron and also as active sites for reaction species, both of which are benefi cial for photocatalytic reactions.
Consequently, signifi cantly enhanced pho-tocatalytic performance was achieved by the Z-D-P nanocomposite photocatalyst.
3.3. Other Types of Polymer-Sensitized Composite Photocatalysts
In addition to TiO 2 and ZnO, many other semiconductors with notable photo-catalytic properties have been studied intensively, including CdS, Si, and C 3 N 4 . CdS is one of the most popular visible-light-driven photocatalysts because it has
band gap energy of 2.5 eV. Furthermore, its conduction band at relatively negative potential (–1.0 versus NHE) offers CdS good photocatalytic activities. The signifi cantly enhanced photocatalytic activity of PANI-CdS composite photocata-lysts for the evolution of hydrogen was demonstrated. [ 102 ] A series of nanocomposite catalysts, denoted as PANI-CdS-1, PANI-CdS-2, PANI-CdS-3, PANI-CdS-4 and PANI-CdS-5, represent the molar ratio of PANI and CdS fi xing at 0.5, 0.7, 1, 1.5, and 2. Figure 10 a shows PANI-CdS-1 with a much higher rate of hydrogen production than pure CdS and other various photocatalysts. Figure 10 b exhibits the PANI-CdS-1 was considerably stable during the photocatalytic process; no appreciable decay in the rate of hydrogen evolution could be observed after 20 h of light irradiation. Two reasons may be responsible for the remarkable photocatalytic properties
Figure 10. a) Rates of hydrogen evolution over CdS and PANI-CdS composite photocatalysts, with different PANI contents, under irradiation with visible light. b) Recycling tests for hydrogen evolution over PANI-CdS-1. Reproduced with permission. [ 102 ] Copyright 2012 International Association for Hydrogen Energy.
Figure 9. a) HRTEM image of Z-D-P; inset: amorphous edge of the coated PANI. b) Photocatalytic degradation of MO over various samples under UV irradiation; inset: corresponding rate constants of MO degradation. c) EIS Nyquist plots for various samples under irradiation with UV light. d-i) Relative band structures of PANI/ZnO and the proposed CT processes for MO degradation. ii) Schematic diagram illustrating the synergistic effect between defects and PANI. Reproduced with permission. [ 107 ] Copyright 2014, American Chemical Society.
of PANI-CdS: i) the photoexcited electrons transfer from PANI to CdS, giving rise to an increased amount of elec-trons available for hydrogen evolution; ii) PANI effectively prevents the agglomeration of CdS particles, which makes electron transfer from PANI to CdS become easy. However, the hydrogen evolution rate of PANI-CdS nanocomposite photocatalyst gradually decreased with the increasing PANI amount. The excess loading of PANI reduced the content of CdS and decreased photocatalytic activity.
Crystalline Si is very applicable for photoelectrochemical water splitting because its band gap (1.12 eV) is an ideal match for the solar spectrum. [ 132 ] Coating with a thin layer of conducting polymer has been proven to effectively stabi-lize Si-based photoelectrodes and prevent photocorrosion.
Figure 11 a shows i) the SEM image of Si/PEDOT core/shell nanowire arrays and ii) HRTEM image of the Si/PEDOT interface. Prior to PEDOT polymerization, the Si nanowires were modifi ed with (3-aminopropyl)triethoxysilane (APS) to improve the PEDOT adhesion; an amorphous PEDOT shell formed on the crystalline Si nanowire core. The PEDOT layer performed three roles: as a multifunctional coating to prevent photocorrosion of the Si nanowires, to collect photo-generated holes, and to catalyze the water oxidation reac-tion. Figure 11 b depicts the CT mechanism of a Si/PEDOT composite photoanode during photoelectrochemical water splitting. The incident light is absorbed mostly by the Si nanowires, due to the high transparency of PEDOT over the visible light range, to generate electron/hole pairs. The pho-togenerated electrons move along the Si nanowires to the Pt
electrode via the external circuit to evolve hydrogen, while the photogenerated holes transfer to the PEDOT shell where they take part in the water oxidation reactions. Figure 11 c-i shows the current density versus bias potential characteristics ( J – V curves) for pure Si and Si/PEDOT electrodes recorded in 1 m KOH in the dark and under light illumination (AM 1.5G, 100 mW cm -2 ). The J – V curve of pure Si showed a pro-nounced electrochemical oxidation of Si from –0.5 to 0.0 V (vs SCE). Both Si/PEDOT and Si-APS/PEDOT electrodes exhibited signifi cant photoresponse, indicating that PEDOT layer effectively collected photogenerated holes to carry out the water oxidation reaction. Figure 11 c-ii shows the pho-tocurrent of pure Si nanowires decayed quickly because Si nanowires were completely corroded in few minutes. On the contrast, the photocurrent decay of Si/PEDOT and Si-APS/
PEDOT composite electrodes was greatly deferred, indi-cating that the PEDOT layer protected the Si nanowires from photocorrosion.
3.4. Polymer-Immobilized TiO2-Based Composite Photocatalysts
Zeng et al. [ 158 ] prepared TiO 2 /cellulose nanocomposite fi lms using a sol–gel method through the hydrolysis of pre-cursor TiO 2 sol solutions in regenerated cellulose (RC) fi lms.
Figure 12 a shows the porous RC substrate providing cavities for the growth and immobilization of TiO 2 nanoparticles through electrostatic and hydrogen bonding interactions.
Figure 11. a-i) SEM and ii) HRTEM images of Si/PEDOT nanowires. b) Relative band structures of Si/PEDOT and the proposed CT processes for photoelectrochemical water splitting. c-i) J – V curves of various photoanodes recorded in 1 M KOH in the dark and under light illumination (AM 1.5 G, 100 mW cm −2 ) and ii) J – t curves of different photoelectrodes recorded at a fi xed potential of +0.5 V (vs SCE). Reproduced with permission. [ 132 ] Copyright 2013, Royal Chemical Society.
Figure 12 b shows these TiO 2 /RC composite fi lms exhibited notable photocatalytic performance for the degradation of phenol under irradiation with weak UV light; their photocat-alytic activities were even comparable to that of anatase TiO 2 nanoparticles under the same experimental conditions, sug-gesting that the structure and properties of the TiO 2 nanopar-ticles were protected effectively. Moreover, Figure 12 c shows the TiO 2 /RC composite fi lms had good mechanical proper-ties, allowing them to be used as recyclable catalysts for the photodegradation of organic pollutants.
3.5. Polymer-Immobilized ZnO-Based Composite Photocatalysts
An atomic layer deposition (ALD) approach was used to either decorate nylon nanofi bers with ZnO nanoparticles or to coat nylon nanofi bers with a thin layer of ZnO, depending on the ALD parameters. [ 165 ] When highly dense ZnO nano-particles were decorated upon the nylon nanofi bers, they exhibited higher effi ciency for the photocatalytic decom-position of Rhodamine B (RhB), due to their large surface area. In addition to the ALD approach, the hydrothermal method is another practical method for immobilizing ZnO on polymeric supports. A combination of electrospinning and hydrothermal methods was employed for the preparation of hierarchical composites featuring oriented ZnO nanowires grown around electrospun nanofi bers. [ 171 ] The electrospun nanofi bers were composed of cellulose acetate (CeAc), poly-vinyl acetate (PVAc), and polyethylene glycol (PEG). These hierarchical composites were highly effective photocatalysts, as evidenced by the almost complete removal of the test pollutant, methylene blue, after 30 min of UV irradiation.
Because the ZnO nanowires were grown on the CeAc/PVAc/
PEG polymeric supports, the need for separation of the pho-tocatalyst from the reaction solution was eliminated.
3.6. Other Types of Polymer-Immobilized Composite Photocatalysts
Figure 13 a shows that D201, a macroporous resin of poly-styrene-divinylbenzene matrix, was adopted as a support to fabricate D201-CdS composite photocatalysts using an ion exchange process. [ 179 ] Figure 13 b displays the as-obtained D201-CdS beads were yellow, characteristic of the light absorption of CdS. Figure 13 c exhibits the composite pho-tocatalysts exhibited excellent photocatalytic activity for the degradation of RhB under irradiation with visible light.
Furthermore, the photocorrosion of CdS was inhibited after immobilization within the D201 matrix. Figure 13 d shows the D201-CdS composite photocatalysts employed for repeated use without noticeable loss in effi ciency; in addition, they could be separated readily from the reaction solutions through simple fi ltration, suggesting their use as promising photocatalysts for practical applications in environmental remediation.