TiO2 Photonic Crystals with Localized Surface Photothermal Effect

Subscriber access provided by University of Sunderland

Article

TiO2 Photonic Crystals with Localized Surface Photothermal Effect and Enhanced Photocatalytic CO2 Reduction Activity Jingxiang Low, Liuyang Zhang, Bicheng Zhu, Zhengyou Liu, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04150 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

TiO2 Photonic Crystals with Localized Surface Photothermal Effect and Enhanced Photocatalytic CO2 Reduction Activity

Jingxiang Low,† Liuyang Zhang,† Bicheng Zhu,† Zhengyou Liu,*,‡ Jiaguo Yu*,†

†

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China. E-mail: [email protected] ‡

Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education

and School of Physics and Technology, Wuhan University, Wuhan 430072, China. E-mail: [email protected]

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Photothermal effect through absorbing infrared light is an important strategy for various living beings to regulate their body temperature. Although utilization of this effect in modern human technologies such as photoconversion and catalysis is important, it is still in its infancy for the scientific research. Herein, we design and prepare TiO2 photonic crystals (TiO2 PCs) through template-free anodization– calcination method for photocatalytic solar fuel production. Encouragingly, slow photon effect was substantiated, contributing to enhanced light utilization ability. More importantly, we present the localized surface photothermal (LSPT) effect and explain how this heat arising from TiO2 PCs can be harnessed for photocatalytic reaction. Owing to its extraordinary physicochemical property and effective light utilization, photocatalytic CH4 production performance of TiO2 PCs is greater than that of commercial TiO2 and TiO2 nanotube arrays. Such photothermal effect deriving from unique photonic crystal structure has great potential in photoconversion application.

Keywords: TiO2, Papilio nireus, Photonic crystals, Photocatalysis, CO2 reduction

2


Page 2 of 31

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ INTRODUCTION Photocatalysis is of paramount importance because of its potential for addressing global energy demand and mitigating CO2 pollution.1,2 Developing efficient photocatalysts by utilizing solar energy and greenhouse gases (e.g. CO2) to produce solar fuels has attracted broad attention from scientific community.3 Various semiconductors have been studied and investigated to realize the industrial applications of the photocatalytic CO2 reduction.4,5 Among assorted semiconductor photocatalyst, TiO2 is regarded as one of the most promising photocatalysts because of its cheapness, nontoxicity and stability.6,7 However, photoconversion efficiency of TiO2 is still unsatisfactory even in the UV-region mainly because of its low light utilization and high charge recombination rate. Fundamentally, heat is regarded as a stimulus for photoconversion and catalytic reactions.8,9 Specifically, suitable heat source can significantly enhance their reaction rates.10,11 However, additional thermal source requires an external power source, inducing extra cost and complexity. Therefore, a heat-source free approach by effective trapping of the surrounding heat during these reactions is highly attractive.12 Since the breakthrough finding of photonic crystals reported by E. Yablonovitch13 and S. John in 1987,14 extensive attentions have been paid on the development of photonic crystals owning to their fascinating structures and exceptional optical properties.15 The advancements of photonic crystals provide a myriad of opportunities for numerous applications, including optical computing, telecommunication, solar cells and photocatalysis. Generally, photonic crystal is a periodic macroporous 3



structure which gives rise to photonic bandgap (PBG) in specific wavelength.16 Specifically, if the wavelength of incident light overlaps with PBG edges of the photonic crystals, photons in the photonic crystals will propagate and travel with very low group velocity, namely slow photon effect. This slow photon effect can increase effective optical path length and light utilization ability of semiconductor. Therefore, significant photoconversion efficiency enhancement can be found in the photonic crystal materials.17 On account of these facts, the trapping of infrared light (760 nm– 1200 nm), also known as heat radiation, is expected and achievable by photonic structure. Therefore, construction of semiconductor photonic crystal with infrared light absorption property can be a viable method for enhancing the photocatalytic performance of semiconductor. Herein, we implemented a template-free anodization–calcination method to synthesize TiO2 photonic crystals (TiO2 PCs). Such a photonic crystal structure endowed TiO2 PCs with particular photon energy/heat radiation capture ability. Namely, the TiO2 PCs exhibited slow photon and localized surface photothermal (LSPT) effect, enhancing light utilization and catalytic reaction rate, respectively. As a result, TiO2 PCs exceeded TiO2 nanotube arrays (TiO2 NTAs) greatly in photocatalytic CO2 reduction performance. This report presents a simple way for in-situ drawing heat from surroundings, for elevating the photocatalytic reaction activity through LSPT effect of photonic crystal.

4


Page 4 of 31

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ RESULTS AND DISCUSSION In this work, we implemented a simple top-down electrochemical anodization approach to construct multilayer photonic crystal structure using TiO2. In detail, the diameter and thickness of each layer can be modified by changing applied voltage during the sample preparation. This technique momentously simplified the preparation process for photonic crystal structures while retaining their unique optical properties. This simple preparation method is crucial and expected to be heuristic in the advancement of photonic crystal.

Figure 1. (a) X-ray diffraction (XRD) patterns for TiO2 NTAs and TiO2 PCs. (b,c) X-ray photoelectron spectroscopy (XPS) survey spectra (b) and Ti 2p XPS spectra (c) for TiO2 NTAs and TiO2 PCs.

5



XRD characterization was performed to determine the crystallite phases of TiO2 NTAs and TiO2 PCs. Figure 1a shows the comparison of their XRD patterns. Apparently, both of their diffraction peaks matched well with the anatase phase originating from TiO2 and metallic Ti phase originating from titanium foil.18 These results confirm that the TiO2 in both samples existed as anatase phase which has excellent photocatalytic activity. The chemical and electronic states of these two samples were examined through XPS characterization. Clearly, both of them contained Ti and O elements (Figure 1b). Meanwhile, Ti 2p XPS spectra of the TiO2 NTAs and TiO2 PCs exhibited two peaks at ca. 458.6 (Ti 2p3/2 of TiO2) and 464.4 eV (Ti 2p1/2 of TiO2) (Figure 1c). Small F peak can also be found on XPS spectra of both samples due to the residual F- ions introduced by the NH4F during the sample preparation. Obviously, no significant changes can be observed from the XPS spectra between TiO2 NTAs and TiO2 PCs, indicating their similar chemical and electronic properties.

6


Page 6 of 31

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a,b) Top view (a) and cross-sectional view (b) scanning electron microscope (SEM) images of TiO2 PCs. (c) Transmission electron microscope (TEM) image of TiO2 PCs. (d) Schematic showing the anodization mechanism of the TiO2 PCs.

To confirm the successful preparation of photonic crystals using TiO2, the SEM images of TiO2 PCs are presented in Figure 2a and b. Obviously, TiO2 PCs exhibited a typical macroporous photonic crystal structure, which was composed of periodically-arranged air cylinders with average diameter of ca. 100 nm (Figure 2a 7



and b). Moreover, surface pores of the TiO2 PCs were also distributed (on the periphery diagram) according to hexagonal lattice, further confirming their photonic crystal behavior. Furthermore, as shown in Figure 2b, several layers can be found, indicating that voltage supply of 20 V and 30 V can create two different layers of TiO2 nanotube arrays on the Ti foil, generating a multilayer photonic crystal structure. Specifically, voltage supply of 30 V and 20 V will create the layers with the thickness of ca. 250 nm and 200 nm, respectively. It can be expected that a total of 12 layers can be created during the 6 h preparation due to the alternation of voltage supply of 20 V and 30 V every 30 min. To have clear understanding on the morphology of the prepared samples, TEM characterization was carried out. As shown in the Figure 2c, TiO2 PCs exhibited a porous structure, in accordance with the SEM characterization. Notably, the edge of TiO2 PCs further reveals that TiO2 PCs were composed of two different layers. Both layers have nanotube structures with measured diameters of 80 nm and 100 nm, respectively, produced by alternating voltage supply. Therefore, it can be estimated that this structure is mainly composed of air (76% for the layer of 30 V and 64% for the layer of 20 V) (see Figure 2d), which is favorable for incident light to diffract and propagate.19 Moreover, it should be noted that the refractive index difference between interconnected TiO2 ( = 2.5) and air void (nair = 1.0) is sufficient to cause Bragg’s light scattering.20,21 Therefore, the photonic bandgap can be expected on the prepared TiO2 PCs.22

8


Page 8 of 31

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a,b) Cyclic voltammetry (CV) curves for TiO2 NTAs (a) and TiO2 PCs (b) measured at different scan rate. (c) Comparison of cyclic voltammetry curves of TiO2 NTAs and TiO2 PCs measured at a scan rate of 120 mV s-1. (d) Linear fitting of the capacitive current density of TiO2 NTAs and TiO2 PCs vs. scan rate.

Typically, the surface area of a film is expected to have a linear correlation with the electrochemical double layer capacitance (EDLC). The EDLC can normally be obtained via CV method. And the results are shown in Figure 3. Obviously, both TiO2 NTAs and TiO2 PCs exhibited similar CV curves, indicating that they have similar electrochemical properties (see Figure 3a-c). To compare the surface area on the TiO2 NTAs and TiO2 PCs, EDLCs were calculated based on the slopes of the curves (see Figure 3d). Namely, the central width of the current density range vs. scan rates was 9



plotted.23 Apparently, the capacitive current density of TiO2 PCs is larger than that of TiO2 NTAs, indicating it larger specific surface.

Figure 4. (a) UV-Vis absorption spectra for TiO2 NTAs and TiO2 PCs and (inset) the optical photograph of the TiO2 NTAs, TiO2 PCs and Papilio nireus. (b) Light absorption spectra of the TiO2 PCs with different incident light angles, showing the changes in the position of photonic stop band with incident light angles. (c,d) Infrared thermograms of the (c) TiO2 NTAs and (d) TiO2 PCs, and (e) their corresponding temperature change spectra along the reference line (white line in c and d). (f) Transient photocurrent responses of the TiO2 NTAs, infrared light irradiated TiO2 NTAs, TiO2 PCs and infrared light irradiated TiO2 PCs. 10


Page 10 of 31

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The optical behaviors of samples were investigated by UV-Vis spectrometer. Comparison of the results between TiO2 NTAs and TiO2 PCs is shown in Figure 4a. For both samples, intense absorption below ca. 378 nm was found attributed to the intrinsic bandgap absorption of pristine anatase.24,25 However, in the range of 400–800 nm, TiO2 PCs demonstrated more intense absorption than TiO2 NTAs, which is due to the layered photonic crystal structure. Moreover, it should be noted that photonic absorption edge of TiO2 PCs at a ca. 390 nm intercepted (see blue circle in Figure 4a) with the intrinsic light absorption of anatase TiO2, resulting in slow photon effect within the TiO2 PCs.26 Such slow photon effect can cause the light to transmit within the sample with reduced group velocity, or with longer lifetime, or with elongated optical path length, thereby improving the interaction between light and photocatalyst.27 Moreover, the PBG at different light ranges endows TiO2 PCs with different advantages. The light trapped at around 400 nm can be applied for increasing light utilization ability of TiO2 PCs. Meanwhile, the light trapped at around 760 nm (infrared light) can be applied for creating LSPT effect and raising the surface temperature of the TiO2 PCs. The increased surface temperature of the TiO2 is expected to accelerate the photocatalytic reaction. The inset of Figure 4a exhibits the colors of the TiO2 NTAs and TiO2 PCs. TiO2 NTAs showed grey color due to their smooth light absorption property at visible light range, while the prepared TiO2 PCs showed brilliant green color, suggesting oscillating absorption behavior thus indicating successful preparation of photonic crystal structure using TiO2. More 11



Page 12 of 31

interestingly, the color of the TiO2 PCs is similar to that of the butterfly Papilio nireus,28,29 indicating that their interactions with light are similar.

Table 1. Calculated D values of the TiO2 PCs at different incident light angles using wavelength of the most intense peaks of their light absorption spectra.

a

θa (o)

λmaxb (nm)

neffc

Dd (nm)

0

656

1.6

204.4

30

625

1.6

205.3

45

590

1.6

204.8

angle of light incidence on the sample; blight absorption wavelength; ceffective

refractive index; dperiod of the TiO2 PC structure

It is widely accepted that PBG of a photonic crystal can be evaluated by the modified Bragg’s law,30 as described in Equation 1 and 2, mλ = 2 ( − )/

(1) /

= ∗ + ∗ (1 − )

(2)

where λmax is the light absorption wavelength, m is the Bragg reflection order, D is the period of the TiO2 PC perpendicular to the layers, θ is the angle of light incidence on the sample, nTiO2 is the refractive index of the TiO2, nair is the refractive index of air and f is the percentage of the TiO2 in the sample. Therefore, by changing the angles, the origin of the multi-absorption peaks of TiO2 PCs can be confirmed using a fiber spectrophotometer (Figure 4b). When the incident light angle changed from 0o to 30o 12


Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and 45o, the absorption spectra of TiO2 PCs experienced a blue-shift (see also Table 1). This result reaffirms that the absorption band of TiO2 PCs at 400–800 nm is attributed to their photonic crystal structure. Moreover, for our sample, it is known that the values for f, nTiO2 and nair are 30%, 2.5 and 1.0, respectively. Therefore, the effective refractive index can be evaluated to be 1.6 according to Drude effective refractive index (see Equation 2). Since the D value is 450 nm when the incident angle is 0o, the λmax can be calculated to be 720 nm for m = 2, the second order Bragg reflection, which locates indeed in the absorption band of the experiment (Figure 4b). As mentioned above, thermal stimulation should be able to improve the photoconversion and catalytic efficiency. The PBG of the TiO2 at infrared range is expected to increase the surface temperature of TiO2 PCs. To authenticate this conjecture, we imaged the temperature distribution on TiO2 NTAs and PCs after 3-min infrared light irradiation (3 W 760 nm LED) in Figure 4c. As for TiO2 NTAs, no noticeable temperature change can be detected; while obvious temperature change can be observed on TiO2 PCs (Figure 4d). This distinction indicates TiO2 PCs, rather than TiO2 NTAs, have infrared light absorption ability. A more vivid image of the temperature changes along the centre of the irradiation light spot was plot and shown in Figure 4e. Likewise, no noticeable temperature change can be observed on TiO2 NTAs. Of note, the temperature of TiO2 PCs increased monotonically as it approached the light spot. The surface temperature increase of the TiO2 PCs can reach up to 2 oC. To expound, when the wavelength of infrared light matches with the PBG of TiO2 PCs, LSPT effect works. Therefore, the incident infrared light can be trapped within 13



the TiO2 PCs, producing LSPT on the TiO2 PCs to increase their surface temperature. The increased temperature is beneficial for reactant adsorption, desorption and catalytic reaction.31 To confirm the function of the LSPT effect on photogeneration efficiency of electron-hole pairs, we conducted electrochemical transient photocurrent response measurements. Figure 4f exhibits the transient photocurrent response for the TiO2 NTs and PCs. Generally, photocurrent generation process can be interpreted as follows. Upon irradiation, the electron-hole pairs were generated by TiO2 and separated by an external potential bias. Afterwards, photogenerated holes migrated to the photocatalyst/electrolyte interface, and photogenerated electrons transferred to the cathode (Ti substrate), producing photocurrents. For both samples, anodic photocurrent spikes can be found during the initial irradiation due to separation of the photogenerated electron-hole pair on prepared samples. Subsequently, the photocurrent density underwent a reduction before levelling off. This result suggests that the photogenerated holes is favourable to recombine with photogenerated electrons instead of being consumed by the reducing species in electrolyte. Then, the photocurrent values dropped steeply to zero once the light turned off. The higher photocurrent of TiO2 PCs, in comparison to TiO2 NTAs, was accounted for the well-inherited elegant photonic crystal structure as well as the light absorption ability, which can effectively utilize the incident photon by trapping the light.32,33 More interestingly, as for infrared light irradiated TiO2 PCs, it has slightly more intense photocurrent density than its non-irradiated counterpart; contrastively, as to infrared 14


Page 14 of 31

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


light irradiated TiO2 NTAs, hardly any change can be observed relative to its non-irradiated counterpart. This result states that the LSPT created on TiO2 PCs through the trapping of infrared light irradiation can accelerate the transfer and generation of photogenerated charge carrier, thereby enhancing the photogenerated electron–hole utilization efficiency.

Figure 5. (a) Gas chromatogram for the product of photocatalytic CO2 reduction test using TiO2 PCs. (b) Comparison of the photocatalytic CH4-production activity of P25, TiO2 NTAs and TiO2 PCs. (c,d) Gas chromatography-mass spectrometry (GC-MS) data of the CH4 produced during photocatalytic

12

CO2 (c) and

13

CO2 (d) reduction

using TiO2 PCs, where m/z represents the mass/charge number of ions.

The aforementioned results have proven that the prepared TiO2 PCs showed a typical photonic crystal structure and unique physicochemical properties. These 15



Page 16 of 31

superior properties are appreciable to the photocatalytic activity. Ample surface active sites are provided by the porous structure; while improved light absorption ability can be theoretically testified by the calculation. To correlate the experimental proof with calculation, the photocatalytic CO2 reduction was carried out using different samples. The main product for the reaction was methane and a small amount of methanol (see Figure 5a). The CH4 production rates of P25, TiO2 NTAs and TiO2 PCs are compared in Figure 5b. Surprisingly, TiO2 PCs exhibited superior photocatalytic CH4 production performance which was ca. 15 and 4 times greater than that of P25 and TiO2 NTAs, respectively. This result is mainly due to the slow photon effect of the TiO2 PCs. To be more specific, if the PBG falls in the intrinsic absorption range of the TiO2, the incident light will propagate at a very low group velocity, known as slow photon. This slow photon can be effectively absorbed and utilized by TiO2 for photocatalytic reaction. There has been enormous concern that the hydrocarbon fuels produced during photocatalytic CO2 reduction stem from the organic contaminants rather than the CO2.34 To determine the actual origin of these hydrocarbon products, isotope tracer analysis was performed for TiO2 PCs through a GC-MS.35 The obtained results are shown in Figure 5c and d. It can be found that the ratio of photocatalytic

13

13

CH4 to

12

CH4 of

CO2 reduction was much higher than that of photocatalytic

12

CO2

reduction. These results confirmed that CO2 was the carbon source for the produced hydrocarbon fuels in our experiment.

16


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) spectra of TiO2 PCs with a flow of CO2/H2O under adsorption condition (step i) and photocatalytic condition with 365 nm LED light irradiation (step ii) changing over time.

To elucidate this process, DRIFTS of TiO2 PCs and TiO2 NTAs has been carried out with the presence of CO2 and H2O. In a typical test, the reactant adsorption on prepared samples was firstly investigated by introducing CO2/water vapor mixture into the DRIFTS system for 60 mins under dark condition until the intensities of adsorption peaks remain unchanged (Figure 6, step i). Then, the LED light (3 W, 365 nm) was turned on and irradiated the prepared samples for 60 mins to determine intermediate products of the photocatalytic CO2 reduction reaction (Figure 6, step ii). The DRIFTS spectra of the adsorption and photocatalytic process using TiO2 PCs were recorded at every 20 min and shown in Figure 6. Obviously, both prepared 17



samples exhibited similar DRIFTS spectra. As shown in the DRIFTS spectra of step i, multiple peaks attributed to the carbonate species and H2O were manifested, indicating the adsorption of CO2 and H2O on surface of TiO2 PCs. Besides, photocatalytic mechanism of TiO2 PCs was further investigated through the DRIFTS spectra under 60-min UV LED irradiation (Figure 6, step ii). Obviously, the UV LED irradiation caused the gradual increase of the HCOO, HCHO and CH3O characteristic peaks, suggesting the conversion of CO2 into these hydrocarbon intermediates during the formation of CH4 and CH3OH. However, no CH4 can be detected by the DRIFTS because of its nonpolar properties and low affinity on TiO2 PCs. The in situ DRIFTS results indicate that the CO2 and H2O were initially adsorbed on the TiO2 PCs. Finally, these adsorbed CO2 and H2O were converted into HCOO, HCHO and CH3O before their transformation into the CH4 and CH3OH.

18


Page 18 of 31

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Optimized structures of CH4 adsorbed TiO2 systems at O1 (a), O2 (b) and Ti (c) sites, and CH3OH adsorbed TiO2 systems at O1 (d), O2 (e) and Ti (f) sites.

As evidenced by experimental results, the main product of the photocatalytic CO2 reduction using TiO2 PCs was methane. It is due to the fact that the reduction potential of the TiO2 (−0.5 V vs. normal hydrogen electrode (NHE) at pH = 7) is only sufficient to convert CO2 into CH4 and CH3OH (see Table S1). To examine why CH4 instead of CH3OH was the major product, density functional theory (DFT) simulation was performed using the CASTEP module of Materials Studio software. The geometrical structure of the optimized (101) surface of anatase TiO2 is illustrated in Figure 7. Basically, three adsorption sites can be found on the (101) surface of anatase TiO2 including one Ti site and two O sites. Therefore, the adsorption models of CH4 and CH3OH molecules on these adsorption sites of (101) surface of anatase TiO2 are illustrated (see Figure 7a–f) to evaluate their desorption energy. Typically, the desorption energy of an adsorbate on the (001) surface of TiO2 is defined by following 19



Equation 3, Edes = E(TiO2) + E(M) – E(M-TiO2)

(3)

where M is the adsorbate molecule, E(TiO2), E(M) and E(M-TiO2) are the energies of anatase (101) surface, M and the total energy of the adsorption system, respectively. Generally, lower desorption energy favors desorption from the TiO2 surface after the CO2 reduction reaction. The desorption energies of CH4 and CH3OH molecules at different adsorption sites on TiO2 are shown in Figure 7a–f. It is obvious that, for all cases, the desorption energy of CH4 was smaller than that of the CH3OH. This result indicates that the CH4 is easier to be desorbed from the TiO2 surface in comparison with CH3OH. Therefore, it is not surprising that the CH4 is the main product for the photocatalytic CO2 reduction using TiO2 PCs.

20


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. (a–c) Optical photograph (a), cross-sectional view SEM image (b) and top view SEM image (c) of Papilio nireus wing scales. (d–f) Optical photograph (d), cross-sectional view SEM image (e) and top view SEM image (f) of TiO2 PCs. (g) Comparison of optical pathway of TiO2 NTAs and TiO2 PCs. (h) Schematic illustration of the enhancement mechanism for the photocatalytic CO2 reduction on 21



the TiO2 PCs. Image b and c are reprinted with the permission from Ref. 28. Copyright 2005, American Association for the Advancement of Science.

On the grounds of above findings, the upgrading reasons can be proposed and summarized. Notably, the physical and light utilization behaviors of this TiO2 PCs are stunningly similar to those of Papilio nireus butterfly wing scale. Firstly, both TiO2 PCs and Papilio nireus butterfly wing scale show multilayer porous structures on their surface (see Figure 8a-f). This structure not only endows them with large specific surface area, but also renders them with superior photonic crystal properties. Secondly, their light absorption abilities at around 400 nm are enhanced through the slow photon effects given by their photonic structure. Specifically, when the edge of the PBG of photonic crystal structure matches with the intrinsic light absorption of TiO2 or Papilio nireus’s fluorophore, the light in the photonic crystal (also named as slow photons) will travel with very low group velocity (Figure 8g). The absorption of such slow photon on TiO2 and Papilio nireus becomes effective. Thirdly, the presence of LSPT effect on their surface by absorbing infrared light (also known as heat radiation) for surface heating. This increase in surface temperature is important for accelerating the adsorption/desorption of the reactant on the TiO2 PCs. Meanwhile, it can help the butterfly to regulate and increase its wing temperature when needed. By combining these three advantages (Figure 8h), persuasive proof for the photocatalytic activity enhancement is provided.

22


Page 22 of 31

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ CONCLUSIONS After billion years of development, nature has created innumerable phenomena with respective functions, which inspires human to mimic these phenomena for different applications. Photothermal effect through absorbing infrared light is an important strategy for various living beings to regulate their body temperature. In this work, we design and prepare TiO2 PCs through a simple anodization-calcination method, to borrow this unique phenomenon and try to use it in photocatalytic reaction through the LSPT effect. In detail, the LSPT effect allows TiO2 PCs to absorb heat radiation at infrared light range for accelerating the reactant adsorption and boosting the electron– hole separation. The collaboration of unique LSPT effect and the extraordinary optical properties of TiO2 PCs contributes to remarkably enhanced photocatalytic CO2 reduction performance. It can be foreseen that this simple method for utilization of photothermal is not only important in photocatalysis, but also promising on the development of other research fields including catalysis, solar cell and photoelectrochemistry.

23



■ Supporting Information Experimental section; Experimental setup and spectral distribution; Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was funded by NSFC (21433007, 51320105001 and U1705251), Self-determined and Innovative Research Funds of SKLWUT (2017-ZD-4 and 2016-KF-17) and the Natural Science Foundation of Hubei Province of China (No. 2015CFA001).

■ REFERENCES (1) Zhang, T.; Low, J. X.; Koh, K.; Yu, J. G.; Asefa, T. Mesoporous TiO2 comprising small, highly crystalline nanoparticles for efficient CO2 reduction by H2O. ACS Sustain. Chem. Eng. 2018, 6, 531-540, DOI 10.1021/acssuschemeng.7b02827. (2) Zhang, X.; Yang, J.; Cai, T.; Zuo, G.; Tang, C. TiO2 nanosheets decorated with B4C nanoparticles as photocatalysts for solar fuel production under visible light 24


Page 24 of 31

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


irradiation. Appl. Surf. Sci. 2018, 443, 558-566, DOI 10.1016/j.apsusc.2018.02.275. (3) Xu, D. F.; Cheng, B.; Wang, W. K.; Jiang, C. J.; Yu, J. G. Ag2CrO4/g-C3N4/graphene oxide ternary nanocomposite Z-scheme photocatalyst with enhanced CO2 reduction activity. Appl. Catal. B 2018, 231, 368-380, DOI 10.1016/j.apcatb.2018.03.036. (4) Murugesan, P.; Narayanan, S.; Manickam, M.; Murugesan, P. K.; Subbiah, R. A direct Z-scheme plasmonic AgCl@g-C3N4 heterojunction photocatalyst with superior visible light CO2 reduction in aqueous medium. Appl. Surf. Sci. 2018, 450, 516-526, DOI 10.1016/j.apsusc.2018.04.111. (5) Xu, F. Y.; Zhang, J. J.; Zhu, B. C.; Yu, J. G.; Xu, J. S. CuInS2 sensitized TiO2 hybrid nanofibers for improved photocatalytic CO2 reduction. Appl. Catal. B 2018, 230, 194-202, DOI 10.1016/j.apcatb.2018.02.042. (6) Edelmannova, M.; Lin, K. Y.; Wu, J. C. S.; Troppova, I.; Capek, L.; Koci, K. Photocatalytic hydrogenation and reduction of CO2 over CuO/ TiO2 photocatalysts. Appl. Surf. Sci. 2018, 454, 313-318, DOI 10.1016/j.apsusc.2018.05.123. (7) Kong, L. N.; Zhang, X. T.; Wang, C. H.; Xu, J. P.; Du, X. W.; Li, L. Ti3+ defect mediated g-C3N4/TiO2 Z-scheme system for enhanced photocatalytic redox performance. Appl. Surf. Sci. 2018, 448, 288-296, DOI 10.1016/j.apsusc.2018.04.011. (8) Tian, J. L.; Zhang, W.; Fang, X. T.; Liu, Q. L.; Gu, J. J.; Deng, T.; Wang, Y. H.; Zhang, D. Coupling of plasmon and 3D antireflection quasi-photonic crystal structure for enhancement infrared absorption. J. Mater. Chem. C 2015, 3, 1672-1679, DOI 10.1039/c4tc02519a. 25



Page 26 of 31

(9) Low, J. X.; Zhang, L. Y.; Tong, T.; Shen, B. J.; Yu, J. G. TiO2/MXene Ti3C2 composite with excellent photocatalytic CO2 reduction activity. J. Catal. 2018, 361, 255-266, DOI 10.1016/j.jcat.2018.03.009. (10) Low, J. X.; Jiang, C.; Cheng, B.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J. G. A review of direct Z-scheme photocatalysts. Small Methods 2017, 1, 1700080, DOI 10.1002/smtd.201700080. (11) Yang, D.; Yang, G. X.; Yang, P. P.; Lv, R. C.; Gai, S. L.; Li, C. X.; He, F.; Lin, J. Assembly of Au plasmonic photothermal agent and iron oxide nanoparticles on ultrathin black phosphorus for targeted photothermal and photodynamic cancer therapy. Adv. Funct. Mater. 2017, 27, 14, DOI 10.1002/adfm.201700371. (12) Meng, X. G.; Wang, T.; Liu, L. Q.; Ouyang, S. X.; Li, P.; Hu, H. L.; Kako, T.; Iwai, H.; Tanaka, A.; Ye, J. H. Photothermal conversion of CO2 into CH4 with H2 over group VIII nanocatalysts: An alternative approach for solar fuel production. Angew. Chem., Int. Ed. 2014, 53, 11478-11482, DOI 10.1002/anie.201404953. (13) Yablonovitch, E. Inhibited spontaneous emission in solid-state physics and electronics.

Phys.

Rev.

Lett.

1987,

58,

2059-2062,

DOI

10.1103/PhysRevLett.58.2059. (14) John, S. Strong localization of photons in certain disordered dielectric superlattices. Phys. Rev. Lett. 1987, 58, 2486-2489, DOI 10.1103/physrevlett.58.2486. (15) Chen, S. L.; Wang, A. J.; Hu, C. T.; Dai, C.; Benziger, J. B. Enhanced photocatalytic performance of nanocrystalline TiO2 membrane by both slow photons and stop-band reflection of photonic crystals. AlChE J. 2012, 58, 568-572, DOI 26


Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.1002/aic.12712. (16) Zhang, X.; Liu, Y.; Lee, S. T.; Yang, S. H.; Kang, Z. H. Coupling surface plasmon resonance of gold nanoparticles with slow-photon-effect of TiO2 photonic crystals for synergistically enhanced photoelectrochemical water splitting. Energy Environ. Sci. 2014, 7, 1409-1419, DOI 10.1039/c3ee43278e. (17) Curti, M.; Schneider, J.; Bahnemann, D. W.; Mendive, C. B. Inverse opal photonic crystals as a strategy to improve photocatalysis: Underexplored questions. J. Phys. Chem. Lett. 2015, 6, 3903-3910, DOI 10.1021/acs.jpclett.5b01353. (18) Wei, Y.; Wu, X.; Zhao, Y.; Wang, L.; Zhao, Z.; Huang, X.; Liu, J.; Li, J. Efficient photocatalysts of TiO2 nanocrystals-supported PtRu alloy nanoparticles for CO2 reduction with H2O: Synergistic effect of Pt-Ru. Appl. Catal. B 2018, 236, 445-457, DOI 10.1016/j.apcatb.2018.05.043. (19) Joannopoulos, J. D.; Johnson, S. G.; Winn, J. N.; Meade, R. D. Photonic crystals: molding the flow of light. Princeton university press Princeton, 2011. (20) Chiarello, G. L.; Zuliani, A.; Ceresoli, D.; Martinazzo, R.; Selli, E. Exploiting the photonic crystal properties of TiO2 nanotube arrays to enhance photocatalytic hydrogen

production.

ACS

Catalysis

2016,

6,

1345-1353,

DOI

10.1021/acscatal.5b02817. (21) Passoni, L.; Criante, L.; Fumagalli, F.; Scotognella, F.; Lanzani, G.; Di Fonzo, F. Self-assembled hierarchical nanostructures for high-efficiency porous photonic crystals. ACS Nano 2014, 8, 12167-12174, DOI 10.1021/nn5037202. (22) Liu, J.; Zhao, H.; Wu, M.; Van der Schueren, B.; Li, Y.; Deparis, O.; Ye, J.; 27



Page 28 of 31

Ozin, G. A.; Hasan, T.; Su, B. L. Slow photons for photocatalysis and photovoltaics. Adv. Mater. 2017, 29, 1605349, DOI 10.1002/adma.201605349. (23) Gao, D. Q.; Zhang, J. Y.; Wang, T. T.; Xiao, W.; Tao, K.; Xue, D. S.; Ding, J. Metallic Ni3N nanosheets with exposed active surface sites for efficient hydrogen evolution. J. Mater. Chem. A 2016, 4, 17363-17369, DOI 10.1039/c6ta07883d. (24) Akple, M. S.; Low, J.; Liu, S. W.; Cheng, B.; Yu, J. G.; Ho, W. K. Fabrication and enhanced CO2 reduction performance of N-self-doped TiO2 microsheet photocatalyst by bi-cocatalyst modification. J. CO2 Util. 2016, 16, 442-449, DOI 10.1016/j.jcou.2016.10.009. (25)

Meng,

A.;

Zhang,

L.;

Cheng,

B.;

Yu,

J.

TiO2–MnOx–Pt

hybrid

multiheterojunction film photocatalyst with enhanced photocatalytic CO2-reduction activity. ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b02552. (26) Jiao, J.; Wei, Y.; Zhao, Y.; Zhao, Z.; Duan, A.; Liu, J.; Pang, Y.; Li, J.; Jiang, G.; Wang, Y. AuPd/3DOM-TiO2 catalysts for photocatalytic reduction of CO2: High efficient separation of photogenerated charge carriers. Appl. Catal. B 2017, 209, 228-239, DOI 10.1016/j.apcatb.2017.02.076. (27) Low, J. X.; Qiu, S. Q.; Xu, D. F.; Jiang, C. J.; Cheng, B. Direct evidence and enhancement of surface plasmon resonance effect on Ag-loaded TiO2 nanotube arrays for photocatalytic CO2 reduction. Appl. Surf. Sci. 2018, 434, 423-432, DOI 10.1016/j.apsusc.2017.10.194 (28) Vukusic, P.; Hooper, I. Directionally controlled fluorescence emission in butterflies. Science 2005, 310, 1151-1151, DOI 10.1126/science.1116612. 28


Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Van Hooijdonk, E.; Vandenbem, C.; Berthier, S.; Vigneron, J. P. Bi-functional photonic structure in the Papilio nireus (Papilionidae): modeling by scattering-matrix optical

simulations.

Opt.

Express

2012,

20,

22001-22011,

DOI

10.1364/OE.20.022001. (30) Li, R. H.; Faustini, M.; Boissiere, C.; Grosso, D. Water capillary condensation effect on the photocatalytic activity of porous TiO2 in air. J. Phys. Chem. C 2014, 118, 17710-17716, DOI 10.1021/jp5046468. (31) Low, J. X.; Yu, J. G.; Ho, W. K. Graphene-based photocatalysts for CO2 reduction to solar fuel. J. Phys. Chem. Lett. 2015, 6, 4244-4251, DOI 10.1021/acs.jpclett.5b01610. (32) He, J.; Chen, D.; Li, Y.; Shao, J.; Xie, J.; Sun, Y.; Yan, Z.; Wang, J. Diatom-templated TiO2 with enhanced photocatalytic activity: biomimetics of photonic

crystals.

Appl.

Phys.

A

2013,

113,

327-332,

DOI

10.1007/s00339-013-7970-2. (33) Zheng, X.; Meng, S.; Chen, J.; Wang, J.; Xian, J.; Shao, Y.; Fu, X.; Li, D. Titanium dioxide photonic crystals with enhanced photocatalytic activity: Matching photonic band gaps of TiO2 to the absorption peaks of dyes. J. Phys. Chem. C 2013, 117, 21263-21273, DOI 10.1021/jp404519j. (34) Prajapati, P. K.; Kumar, A.; Jain, S. L. First photocatalytic synthesis of cyclic carbonates from CO2 and epoxides using CoPc/TiO2 hybrid under mild conditions. ACS Sustain. Chem. Eng. 2018, 6, 7799-7809, DOI 10.1021/acssuschemeng.8b00755. (35) Meng, A. Y.; Wu, S.; Cheng, B.; Yu, J. G.; Xu, J. S. Hierarchical TiO2/Ni(OH)2 29



composite fibers with enhanced photocatalytic CO2 reduction performance. J. Mater. Chem. A 2018, 6, 4729-4736, DOI 10.1039/c7ta10073f.

30


Page 30 of 31

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ Graphical Abstract

The collaboration of unique LSPT effect and the extraordinary optical properties of TiO2 PCs contributes to remarkably enhanced photocatalytic CO2 reduction performance.

31


TiO2 Photonic Crystals with Localized Surface Photothermal Effect

Recommend Documents