Multicompound inverse opal structures with gold nanoparticles for visible light photocatalytic activity

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DSpace https://erepo.uef.fi

Rinnakkaistallenteet Luonnontieteiden ja metsätieteiden tiedekunta

2020

Multicompound inverse opal structures with gold nanoparticles for visible light photocatalytic activity

Pham, Khai

Elsevier BV

Tieteelliset aikakauslehtiartikkelit

© 2020 Published by Elsevier Ltd.

CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/

http://dx.doi.org/10.1016/j.matdes.2020.108886

https://erepo.uef.fi/handle/123456789/23991

Downloaded from University of Eastern Finland's eRepository

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Multicompound inverse opal structures with gold nanoparticles for visible light photocatalytic activity

Khai Pham ⁎ , Filipp Temerov, Jarkko J. Saarinen ⁎

Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland

H I G H L I G H T S

• Multicompound (TiO2, SiO2, ZrO2) inverse opals (IOs) with gold nanoparticles displayed enhanced photocatalytic activity.

•Photocatalytic activity was increased by 206% under UVA light with AuNP doped TiO2-SiO2IO.

•Visible light photocatalytic activity was observed in multicompound TiO2-ZrO2

IO and TiO2IO structures with AuNPs.

G R A P H I C A L A B S T R A C T

a b s t r a c t a r t i c l e i n f o

Article history:

Received 19 March 2020

Received in revised form 11 June 2020 Accepted 12 June 2020

Available online 13 June 2020

Keywords:

Multicompound inverse opal Photocatalytic activity Gold nanoparticles TiO2

TiO2-SiO2

TiO2-ZrO2

Multicompound inverse opal (IO) structures from titanium dioxide‑silicon dioxide (TiO2-SiO2, TSIO), titanium dioxide‑zirconium dioxide (TiO2-ZrO2, TZIO), and titanium dioxide (TIO) structures were synthesized using the self-convective method. Gold nanoparticles (AuNPs) were deposited into synthesized multicompound IO structures by simply immersing samples in solution with AuNPs. Our results show that highly ordered IO structures were fabricated. The photocatalytic activity of multicompound IO structures without and with AuNPs was exam- ined under UVA and visible light excitation using an in-house built gas-phase reactor. The highest photocatalytic activity under UVA illumination was observed for TSIO with AuNPs that increased the activity by 206% and 125%

compared to the reference TIO structure without and with AuNPs, respectively. Additionally, photocatalytic activity was also observed under visible light excitation with AuNP deposited TZIO and TIO structures.

creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

World requires highly efﬁcient photocatalytic materials, for example, for elimination of polluted environment [1]. Recently, nanoporous semiconducting materials (TiO2, ZnO, SnO2, and CeO2) have been inves- tigated extensively due to their stability, low-cost, and non-toxicity as well as their potential to remediate harmful compounds [2]. Titanium dioxide (TiO2) in anatase crystalline form has been found more photocatalytically active than the other crystalline forms that has different chemical properties, higher catalytic reactivity, and surface acidity

based on the different crystalline surface planes [3–6]. Additionally TiO2has good photocatalytic activity, low cost, high stability in various chemical environments, good biocompatibility, and high oxidizing photogenerated holes [7]. Therefore, nanocrystalline TiO2in anatase form has been studied for several applications such as in waste water treatment [8], photocatalytic hydrogen production [9], self-cleaning concrete [10], air purification [11], and off-set printing [12]. However, due to a large inherent bandgap of 3.2 eV (387 nm), the electron-hole pairs can only be excited by UVA light (that covers only 3–5% of solar energy spectrum is in the UVA range). This bottleneck imposes significant limitations for practical applications of TiO₂in industrial scale, and much effort has been dedicated for the modification of TiO2for visible light activation [13].

⁎ Corresponding authors.

E-mail addresses:khai.pham@uef.ﬁ(K. Pham),jarkko.j.saarinen@uef.ﬁ(J.J. Saarinen).

https://doi.org/10.1016/j.matdes.2020.108886

Contents lists available atScienceDirect

Materials and Design

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m a t d e s

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For visible light activity several methods have been suggested such as optimizing the crystalline structure [14], optimizing the particle size [15], mixing with metal oxide nanocomposites, [16] and addition of dopants [17]. For optimizing the crystalline structure, inverse opal structure is well-known for having a large surface area with more active catalytic sites and faster electron transfer, for example, for absorbing pollutant molecules [19]. In addition, light absorption into IO can be improved due to a photonic band gap (PBG) [18]. Synthesis of multicompound such as TiO2/ZrO2, TiO2/Fe3O4, and TiO2/ZnO inverse opal structures with improved photocatalytic activity have been dem- onstrated to increase charge separation efﬁciency and extend the pho- toexcitation energy [20–22]. As an example, addition of ZrO2into TiO2

layers improved the photocatalytic activity with improved phase stability compared to pure TiO2[23]. Addition of TiO2into SiO2induced a sig- niﬁcantly higher acidicity than in bulk TiO2[24]. Metal or non-metal atoms can also be doped into the structure to expand the absorption spectra of semiconductor into the visible range. Noble metal nanoparticles (NPs) (cation-doped) can also reduce the recombination rate of electron-hole pairs (EHPs) with metal NPs acting as an electron or hole trap [25]. Furthermore, noble metal NPs exhibit localized surface plasmon resonance (LSPR), which can be excited by visible light and cascaded into the adjacent semiconductor material for visible light activated photocatalysis [26].

IO structures have been fabricated by various methods such as atomic layer deposition [27], chemical vapor deposition [28], electrochemical deposition [29], and sol-gel method [30]. Sol-gel method has been widely used for the fabrication of IO structures due to low fabrication temperature, high purity and uniformity, easy control of the precursor added volume, and reaction conditions as well as simple preparation process. However, efficiency of thefilling process is rather low since capillary forces drive the sol particles onto the outermost layer of the substrate. For improved quality of the structure, it is important to repeat the infiltration process with adequate precursor concentration [31].

In this study, single- and multicompound high-quality and crack- free IO structures were fabricated through three steps:first, self- assembly of monodisperse spheres of silica or polymer, then infiltration of the assembled template with precursor solutions, andfinally calcination for removal of the used template [32]. Photocatalytic activity of all synthesized multicompound IO structures was measured using a recently developed in-house built gas-phase reactor instead of liquid- phase reactor [33]. Typical liquid phase reactors are based on a color change of optical dyes such as methylene blue. However, the dyes may also be bleached by the used irradiation itself, which can generate an erroneous result [34]. Additionally, when the sample is immersed in a liquid phase, the NPs may detach from the sample surface and transfer into the solution. The fabricated IO structures can also be detached from the glass substrate [35]. The gas-phase detection removes all mechanical stresses from the multicompound IO structures.

This paper summarizes the photocatalytic activity and synthesis of gold nanoparticle (AuNP) functionalized multicompound TiO2(TIO), TiO2-SiO2(TSIO), and TiO2-ZiO2(TZIO) inverse opal structures. The developed multicompound IO structures are versatile as they possess a high degree of freedom for tailored photocatalytic response by e.g.

controllable pore size, constituent material mixtures, and the used dopants. Here the photocatalytic response was tailored using the multicompound IO structures as binary mixtures, which can reduce the crystallite size compared to pure TiO₂structures reducing recombination rate of photogenerated electron-hole pairs and thus enhancing photocatalytic activity. In addition, functionalizing with AuNPs broadens the applications of multicompound IO into visible light photocatalysis, solar cells, and optical ﬁlters [36]. The AuNPs were deposited on the walls and cavities of IO structures by vertically immersing the glass substrates containing the IO structure into the AuNPs solution. The photocatalytic activity characterization was conducted using the in-house built gas-phase photoreactor under

both UVA and visible light activation by monitoring C2H2oxidation into CO2.

2. Materials and methods 2.1. Materials

Titanium (IV) isopropoxide (TTIP, 97%, Sigma-Aldrich), zirconium (IV) propoxide (70 wt% in 1-propanol, Sigma-Aldrich), and tetraethyl orthosilicate (TEOS, Sigma-Aldrich,≥98%) were used as precursors for the fabrication of TIO, ZrO2, SiO2, TZIO and TSIO structures. Hydrochloric acid (37%, VWR chemicals) was used as catalyst and ethanol (99.5%, Etax AA) was used as solvent in the preparation of precursors. For the preparation of polystyrene, styrene (99%, Acros Organics), ammonium persulfate (APS, 98%, Sigma-Aldrich), and sodium dodecyl sulfate (SDS as a capping agent, 98.5%, Sigma-Aldrich) were used. Gold (III) chloride trihydrate (HAuCl4·3H2O, ≥99.9%, Alfa Aesar), and sodium citrate (TSC,≥99%, Sigma-Aldrich) were used to synthesize gold nanoparticles (AuNPs). Glass plates (Thermo Scientiﬁc) were used as a substrate for the fabrication of the IO structures.

2.2. Synthesis of polystyrene (PS) spheres

Negatively charged PS spheres with a diameter of 250 nm (PS250) and 400 nm (PS400) were prepared. First, the seed solution of 200 nm PS spheres was prepared by a free-radical emulsion polymerization, 200 mg of SDS and 160 mL of deionized water were put into a rounded bottomﬂask and heated to 70 °C while stirred at 500 rpm and under nitrogen atmosphere. Subsequently, 20 g of styrene was added into the this solution at 70 °C and stirred for 30 min, then 20 mL of APS solution (200 mg of APS dissolved in 20 mL of deionized water) was slowly dropped into the reactor. The reaction then was left at that condition for 20 h. 121.5 mL or 30.4 mL of 200 nm seed solution were diluted with 70 mL or 150 mL in aﬂask to make PS250 and PS400 solutions.

In the next step, 25 mg of SDS was poured into the solution and heated to 70 °C under nitrogenﬂow. After stabilizing at 70 °C, 20 mL solution of 28 mg APS and deionized water was added. Finally, styrene (11.8 g or 21.6 g) was carefully poured to the reactor solution and the reaction was maintained for 20 h.

2.3. Synthesis of inverse opal structures

Inverse opal structures were synthesized by the templatefilling method as shown inFig. 1. The precursors were fabricated by the sol- gel approach. Briefly, monodisperse PS spheres were uniformly organized into an opal photonic crystal on the surface of microscope slide glass cleaned by distilled water through self-assembly method. The precursor solution wasfilled into the template by dipping method; in the final step, calcination was used to remove the used template.

2.3.1. Synthesis of polystyrene colloidal crystal thinﬁlm

Polystyrene (PS) colloidal crystal thinﬁlms on glass microscope slides were synthesized using the vertical deposition method. A 0.6 mL of 400 nm PS, which was diluted in 25 mL of distilled water, was ultrasonicated for 30 min. After that, the glass microscope slides cleaned by distilled water were then vertically immersed in the colloidal suspension. Beakers were then placed in an oven at 65 °C for 48 h to remove the water. Finally, the colloidal suspension coated glass substrate was kept in an oven close to the glass transition temperature of PS at 100 °C for 2 h to enhance the connectivity between the PS spheres resulting in improved opal quality and durability during the IO synthesis.

2.3.2. Synthesis of multicompound TIO, TZIO, and TSIO

In the preparation of TIO, TiO2precursor was fabricated using sol-gel approach. A solution of titanium precursor was produced by mixing

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5 mL of titanium isoproxide (TTIP) into 47 mL of ethanol stirred at 1000 rpm followed by addition of 2 mL of 37% HCl and 2 mL of water into the mixture. The mixture was continuously blended for at least 30 min until a transparent, homogenous solution appeared. For the colloidal crystal infiltration stage, the prepared PS colloidal crystals were used as a template, and 5 drops of precursor solution were introduced onto the substrate (PS spheres on glass). This infiltration step was repeated twice to increase thefilling efficiency. The sample was left in air for 24 h for hydrolyzing and drying purposes. Finally, the template was removed by calcinating the sample in the air at 500 °C with the ramp rate was 2 °C/min, then held at 500 °C for 3 h.

For the synthesis of multicompound TZIO, a solution of the zirconium precursor was prepared by mixing 5 mL of zirconium (IV) propoxide into 40 mL of ethanol stirred at 1000 rpm followed by addition of 2 mL of 37% HCl and 2 mL of water into the mixture. The mixture of titanium and zirconium was prepared by mixing two precursor solutions of TiO2and ZrO2prepared as described above with a ratio of 1:1. It was then blended for 30 min and used for inﬁltration step. The dipping process and calcinating process were the same as in the case for TIO.

The process of the TSIO formation was conducted in two steps. At the first step, 100μL of silica precursor was prepared by mixing 1 mL of TEOS into 1.5 mL of ethanol stirred at 1000 rpm followed by the addition of 1 mL of 0.1 M HCl and 600μL of PS 400 nm into 25 mL of deionized water. It was then sonicated for 30 min before the glass microscope slides were immersed into the solution. In the second step, after colloidal crystal thinfilms on glass microscope slides had dried, 5 drops of TiO2precursor were infiltrated onto the sample surface. This process was also repeated 2 times with 2 h break between each infiltration.

The sample was left in the air for 24 h for hydrolyzing and drying purposes. Finally, the template was removed by calcinating the sample in air at 500 °C with a ramp rate of 2 °C/min followed by a hold at 500 °C for 3 h.

Synthesis of ZrO2IO (ZIO) was similar to the synthesis of TIO but dif- fered only in the used precursor. A solution of zirconium precursor was prepared by mixing 5 mL of zirconium (IV) propoxide into 40 mL of ethanol stirred at 1000 rpm followed by addition of 2 mL of 37% HCl and 2 mL of H2O into the mixture.

2.4. Synthesis of AuNPs

AuNPs were synthesized using a hydrothermal reduction method [37]. 5 mL aqueous solution of HAuCl4·3H2O (0.1%,) was diluted in 50 mL deionized water and boiled. Then 2 mL of sodium citrate 1%

was dropped into the mixture and kept stirring for 15 min. Finally, the solution was left for cooling down to ambient temperature that resulted in the AuNPs formation.

2.5. Deposition of AuNPs into IO structures

3 mL of the prepared aqueous AuNPs was diluted in deionized water (20 mL) and ultrasonicated for 30 min. After ultrasonication, the glass substrates containing IO structures were vertically immersed into the AuNPs solution and put in the oven at 65 °C to remove the solvent.

The doped IO sample was then ready for characterization as shown in Fig. 2.

2.6. Characterization methods

The morphological information of the prepared material surface was assessed using aﬁeld-emission scanning electron microscope (FE-SEM, Hitachi S-4800) at 5 kV accelerating voltage with a working distance of 8 mm. The composition of the prepared materials was also characterized using the energy dispersive X-ray spectroscopy (EDS) tools of the FE-SEM with Noran system Six (NSS) software.

UV/Vis/NIR spectrometer (PerkinElmer Lambda 900) was used to measure the AuNPs absorption spectrum. The scanning transmission electron microscope (STEM, Hitachi S-4800) was used to characterize the shape and size of AuNPs. The STEM sample was prepared by drying a diluted solution of AuNPs on a copper grid coated with a Lacey carbonﬁlm.

IO samples doped with AuNPs were characterized using FE-SEM and STEM operation mode in the used FE-SEM. The absorption spectrum of the samples was collected by UV/Vis/NIR spectrometer and compared with the spectrum of non-doped reference samples and spectrum of AuNPs in the original solution.

2.7. Photocatalytic activity measurement

The photodegradation of acetylene (C2H2) into carbon dioxide (CO2) was used to characterize the photocatalytic activity of multicompound IO structures. The reaction was conducted using an in-house built gas- phase reactor (details are given in Ref. [33]). The sample was placed in the reactor chamber in which C2H2together with technical air were ﬁlled with rates of 80 mL/min and 100 mL/min, respectively. The IO samples were excited by the high intensity UVA light (450 W, peak emission at 365 nm, High Power Xenon Light Sources). Acetylene was Fig. 1.Schematic illustration for the fabrication of IOs.

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then oxidized into CO2on IO surface. The concentration of CO2produc- tion was monitored using the CO2detector (Vaisala GMP343) inside the reactor chamber and transferred to computer via the Vaisala MI70 transmitter.

For the assessment of photocatalytic activity of samples under visible light, the same reactor was used but with visible light. This wasﬁl- tered from the UVA light source (details above) using a silica cut-off ﬁlter to remove all UV wavelengths.

3. Results and discussion

3.1. Characterization of TIO, ZIO, and SIO structures

A sol-gel approach was carried out for the synthesis of IO structures.

Fig. 3shows FE-SEM images of the synthesized single compound TIO, ZIO, and SIO structures, respectively. Smooth sample surfaces were achieved by varying concentration and amount of precursor added onto the PS template. The SEM images reveal that all fabricated IOs had a well-defined, hexagonal close-packed (HCP) array with minor cracks. Structures in a relatively large area of more than 4μm contained cracks that were caused by shrinkage during the calcination process that may also decrease the photocatalytic activity [38]. Capillary forces during infiltration of the template or drying process can lead to additional cracking of such mechanically fragile templates. The smoothest surface and least cracks were observed on the SIO sample presented in Fig. 3(c). This is probably due to the hydrophilic properties of SiO2in contact with opalfilm [39].Fig. 3(b) shows that the quality of ZIO was lowest with a rather thin IO wall, and the shape of the opal was not con- sistent over the whole observed surface. The main reason for weak pore wall connection ZIO was the crystalline structure changes at different temperature that was converted from monoclinic at ambient temperature to cubic and tetragonal at higher temperatures. The change in volume caused brittle IO structure to break when cooling from high

temperatures [40]. For TIO structure presented inFig. 3(a), the opal size was rather uniform, and no cracks were observed with a 1μm scale bar. However, at lower magniﬁcations many cracks were observed both in TIO and ZIO samples. This may be followed by the condensation and decomposition of the precursor as well as the interaction between precursor and PS spheres during the inﬁltration and calcination processes that can result in a collapse of the template. The condensation and decomposition occurred most strongly with ZrO₂precursor de- creasing the quality of ZIO. The cracks were unavoidable with random stacking of the close packed planes [41].

To strengthen self-assembled templates, partial sintering [42] or growth of necks between the spheres [43] have been applied. Therefore, samples after self-assembly of the PS spheres were heated to 100 °C for 2 h for making an interconnection between spheres. This also improved the connection between the assembled PS spheres and glass substrate.

3.2. Characterization of multicompound TSIO and TZIO structures High quality SIO was employed to synthesize two multicompound IO structures. The morphology and composition of the fabricated multicompound titania-silica TSIO and titania-zirconia TZIO structures are presented in Fig. 4. SEM images confirm that uniform multicompound IO structures were achieved without any precursor overlayers. The pores were well-organized in the HCP array with less cracks than in the case of single compound IOs. TSIO sample had a uniform pore size and pore wall thickness as shown inFig. 4(a). In this case, SiO₂plays an important role in determining the IO structure of the un- dersurface layer increasing also adhesion onto the glass substrate. This is known to improve quality of TSIO multicompound structure compared to the TIO structure [44]. From the corresponding EDS spectra in Fig. 4, the peaks at 0.5 and 4.5 keV confirmed the presence of TiO2in anatase crystalline form, while the existence of Si was confirmed by a peak at 1.8 keV. Additional peaks at 0.1, 0.3, 1.1, and 2.3 keV can be assigned

Fig. 3.FE-SEM images of (a) TIO, (b) ZIO, and (c) SIO structures.

Fig. 2.Schematic illustration for deposition of AuNPs into IOs.

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to the presence of Si, C, Na and S, respectively. These impurities were present due to the fabrication process and the used reagents. In TZIO sample, the IO structure had a lower quality compared to TIO due to the presence of zirconia as explained earlier. The peaks at 0.5 and 4.5 keV conﬁrmed the presence of TiO2in anatase crystalline form whereas the presence of ZrO2was conﬁrmed by the peak around 2 keV.

3.3. Characterization of AuNP doped IO structures

AuNPs having a diameter of 12 ± 2 nm were prepared in deionized water and imaged using STEM as shown inFig. 5(a). The round shape of synthesized AuNPs was confirmed by the STEM image. The crystalline form of AuNPs was confirmed by the EDS spectrum presented inFig. 5 (b). A typical optical peak at 2.2 keV confirmed a successful formation of AuNPs [50] whereas the other peaks can be assigned for the supporting copper grid for the AuNPs. The round shape of AuNPs was also confirmed by a single peak in the UV–Vis spectrum since gold nanorods would display two distinct peaks instead of a single one [49]. A single SPR absorption peak of the AuNPs in deionized water was observed around 520 nm in the visible range as shown inFig. 5(c) that depends on the particle size [47]. The sharp absorption peak confirmed also a low polydispersity of formed AuNPs. Larger particles have a larger optical cross-section and scatter more light that results in the absorption spectrum to shift towards longer wavelengths (red shift), while smaller size particles tend to shift the absorption peak towards lower wavelengths (blue shift) [48]. The size of the AuNPs can be controlled by changing the temperature, the concentration of substances, and pH of the solution [45,46]. The particle size distribution chart in Fig. 5 (d) confirms that no capping agent was used during AuNPs preparation i.e. at low temperature small size nanoparticles were aggregated to form larger particles with diameters ranging from 13 to 16 nm.

The AuNPs were deposited into IO structures by simply dipping the structures into the solution with AuNPs. The morphology and size of AuNPs in the different IO substrates was observed using SEM to be similar with the inﬁltrated particles. However, clusters with diameter of 60–70 nm were occasionally observed.Fig. 6shows SEM, STEM, and

UV–Vis results of AuNP doped multicompound IO structures. The highest number of AuNPs was observed in TSIO with 90 particles / μm²and the lowest number in TZIO with 60 particles /μm². STEM images conﬁrmed the round shape of AuNPs after depositing into the IO structures and the opal shape of the fabricated materials. AuNPs were not observed in STEM image of the TZIO sample due to the thickness of the STEM sample. SPR of AuNPs increases light absorption into the structure by coupling light absorption into the visible range. In the UV–vis absorption spectrum of Au-TIO, the absorption peak of AuNPs was shifted to longer wavelength (about 540 nm). This may be due to formation of larger particles by aggregation of small particles shown inFig. 6(a). The absorption peaks of AuNPs in Au-TSIO and Au-TZIO samples were not shifted as the particle size was unchanged inFigs. 6 (b) and (c).

3.4. Photocatalytic activity of multicompound IO structures

Photocatalytic activity of multicompound IO structures as such and with AuNPs was evaluated by the rate of CO2production through the photodegradation of C2H2in the presence of both UVA and visible light. AuNPs can act as electron capturing agent promoting separation of photo-generated holes and electrons. The size of the used nanoparticles has an inﬂuence on the photocatalytic activity of the semiconductor [51]. The intrinsic photocatalytic reaction in TiO₂occurs when a light quantum with energy equivalent or higher than TiO2 bandgap is absorbed. [52]. This will excite an electron from valence band (VB) into the conduction band (CB) and thus generate an electron-hole pair. Electron paramagnetic resonance (EPR) studies have conﬁrmed that electrons and holes get separated from each other and trap at different sites [53]. The VB holes conduct the oxidation reaction of donor molecules and water molecules that resulted in hydroxyl radical forma- tions followed by their interaction with acetylene molecules. The CB electrons interact with oxygen resulting in superoxide ion formation, which then conduct the reduction and oxidation reactions to break down acetylene molecules attached onto the catalyst surface [54].

Fig. 4.FE-SEM images and EDS spectra of (a) TSIO and (b) TZIO structures.

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Under the UVA light, electrons from VB of TIO can excited and moved to its CB and left a hole (h⁺), where the oxidation of C2H2take place. The detailed mechanism for the reaction under UVA light is presented by * in the Eqs.(1⁎)–(7⁎)below. The electron can be transferred to AuNPs to produce superoxide anion radicals (O₂^·-). In contrast, under the visible light given by Eqs.(1)–(7)(note no * with visible light), the

excited electron can be transferred from the excited AuNPs to TiO2

whereas the other processes (4–6) remain the same:

MeNPsþhν→MeNPs ð1Þ

TiO2þhν→TiO2 ð1⁎Þ

Fig. 5.(a) STEM image, (b) EDS spectrum, (c) UV–Vis absorption spectrum, and (d) particle size distribution of AuNPs.

Fig. 6.FE-SEM, STEM, and UV–Vis results for (a) Au-TIO, (b) Au-TSIO, and (c) Au-TZIO structures.

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MeNPsþTiO2→MeNPs^þ h^þ

þTiO2ð Þe⁻ ð2Þ MeNPsþTiO2→MeNPs eð Þ þ⁻ TiO2þ h^þ

ð2⁎Þ TiO2ð Þ þe⁻ O2→TiO2ð Þ þe⁻ O2− ð3Þ MeNPsðe⁻Þ þO2→MeNPsðe⁻Þ þO2 ð3⁎Þ

O2þH^þ→H2O ð4Þ

H2 Oþ H^þþ e⁻→ H2 O2 ð5Þ

H2O2þe⁻→OHþ ⁻OH ð6Þ

MeNPs^þ h^þ

þ2OHþ2C2H2þ2O2→MeNPsþ2CO2þ2H2O ð7Þ TiO2þ h^þ

þ2OHþ2C2H2þ2O2→TiO2þ2CO2þ2H2O ð7⁎Þ A schematicﬁgure of Au-TZIO multicompound structure with the corresponding energy band diagrams is presented inFig. 7. ZrO₂has a larger bandgap (around 5 eV) than TiO2[55]. When both catalysts are excited under UVA excitation, the electrons from the CB of ZrO2

automatically transfer into the CB of TiO₂ that can improve the photocatalytic response, especially when coupled with plasmonic AuNP excitation. Therefore, the photocatalytic activity of TZIO multicompound is induced similar to pure TIO structures but with enhanced electron hole separation. Gold nanoparticles with approx- imately 10–20 nm diameter have been reported to display a sharp absorption peak around 520 nm [56] that is in good agreement with our observed particle size distribution presented inFig. 5(c).

For AuNP doped IO structures, photon absorption into the metal NPs can excite plasmonic electron oscillations that can transfer as hot electrons into the conduction band (CB) of the adjacent semiconductor via the formed Schottky barrier inducing extra electrons in the CB of the semiconductor. For Au-TiO2interface, a typical Schottky barrier height varies from 0.23 eV [57] to 1.0 eV [58] that depends on the crystalline structure and sample morphology.

SiO2 does not have photocatalytic activity due to a very large bandgap of 8.9 eV, but it was added to improve the quality of the IO

structure as clearly seen inFig. 6(b). For TSIO multicompound structure, the energy bands are widely separated due to large bandgap difference and SiO₂does not contribute directly to photocatalytic response as in the case of ZrO2. However, addition of SiO2was reported to reduce TiO2surface roughness [59] in agreement with our observations. Re- duced surface roughness can lower excitation light scattering and improve the light incoupling into the IO structure.

The CO2production rate for each sample is presented as an average of three measurements inFig. 8both under UVA and visible light excitation. The photocatalytic activities of AuNP doped IO structures were compared with the undoped IO structures. For multicompound IOs without AuNPs under the UVA activation presented inFig. 8(a), the highest photocatalytic activity was observed for TSIO (2.14 ppm / min) with 8.3% increase compared to TIO (1.98 ppm /min) whereas TZIO showed a decrease of 24% (1.51 ppm / min). The situation changed drastically upon deposition of AuNPs as the multicompound IO structures had signiﬁcantly higher photocatalytic activities compared to pure TIO with AuNPs. The highest activity was observed for Au-TSIO (6.04 ppm / min) having 125% higher activity than Au-TIO (2.68 ppm / min). Similarly Au-TZIO showed an increase of 101% (5.41 ppm / min) compared to Au-TIO. Au-TSIO and Au-TZIO displayed an increase of 206% and 174% compared to the reference TIO structure without AUNPs.

The observed enhancement in photocatalytic activity for AuNPs deposited multicompound IO structures may originate from several factors. First, multicompound IO structures can alter the crystallite size compared to pure TiO2anatase structures. For example, dispersion of ZrO2crystallites into the TiO2can prolong carrier lifetime and inhibit EHP recombination [60]. Crystallite size has also an effect on the photocatalytic activity. Smaller crystallite sizes can increase the photocatalytic activity due to a higher number of active catalytic sites and increased charge carrier reactivity. However, the grain boundaries may also increase recombination of photogenerated holes and electrons [61].

Therefore, the crystallite grain size is a delicate balance of combining the beneﬁts of higher reactivity with recombination rate. It has been veriﬁed that addition of both SiO2and ZrO2can inhibit growth of TiO2

colloidal particle size resulting in a smaller TiO2crystallite size veriﬁed by X-ray diffraction analysis [59,62]. Secondly, both SiO2and ZrO2elec- tron charges can be conﬁned into TiO2area due to a higher bandgap of SiO2and ZrO2in multicompound IO structures. Depending on the amount of SiO2or ZrO2on TiO2surface, emission wavelengths can decrease and quantum yield increase, which can increase photocatalytic activity [63]. Addition of small amounts of SiO2or ZrO2into TiO2can also inhibit phase transformation from anatase to rutile i.e. dopants can improve phase stability compared to pure TiO2[23].

Visible light excited photocatalytic activity was measured for AuNP functionalized multicompound IO structures as shown inFig. 8(b).

Here the excitation time was increased to 50 min as the signal levels were signiﬁcantly lower for visible excitation compared to the UVA excitation. No photocatalytic activity was observed for Au-TSIO structure whereas Au-TIO and Au-TZIO produced 9.0 ppm (0.18 ppm / min) and 10.4 ppm (0.21 ppm /min) of CO2, respectively. Visible light can only couple via plasmonic excitation from the AuNPs. The addition of SiO2

into TIO sample improved the pore quality and wall thickness as well as smoothness of the surface [44]. However, due to a very large bandgap of SiO2, the photocatalytic activity of TSIO only comes from TiO2. Addi- tion of SiO2can also reduce the active sites and thus the amount of photogenerated EHPs. These factors can decrease the photocatalytic activity of the sample.

In general, the inﬂuence of SiO2in the photocatalytic performance of TSIO sample is complicated and multifactorial [55]. When doped with AuNPs, the photocatalytic activity of Au-TSIO was signiﬁcantly improved under UVA excitation as shown inFig 8(a). This may be due to a larger surface area and better thermal stability of SiO2[64]. However, under visible excitation, the photocatalytic activity of Au-TSIO sample was completely lost. This may be connected to the observed changes Fig. 7.Schematic diagram for the enhanced photocatalytic activity in AuNP-TZIO structure.

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in the UV–Vis absorption spectra shown inFig. 6, and we plan to return to this issue in detail in a future communication.

As a summary, we have developed multicompound TiO2, SiO2, and ZrO2multicompound IO structures with increased photocatalytic activity that can be signiﬁcantly enhanced by incorporating AuNPs into the structure. AuNPs also enabled visible light activated photocatalysis.

Similar results have been observed in recent literature, for example, with graphene coated Pt [65], PtRu [66], and graphic carbon nitride Au [67] functionalized TiO2structures that have been developed for enhanced visible light driven photocatalytic reactions such as CO2conver- sion into CH₄. Porous TiO₂ composites with large surface area comparable to the developed multicompound IO structures have been studied in several recent designs for photocatalytic degradation of or- ganic compounds from glyphosate by titania-silica aerogels [68] to or- ganic dyes such as methyl orange by forest-like nanostructured TiO2

ﬁlms with AuNPs [69] and methylene blue by graphene functionalized TiO2IO structures [70]. These examples highlight the versatility and potential of the developed multicompound IO designs having a large surface area with tailorable properties.

4. Conclusions

High quality multicompound TiO2(TIO), TiO2-SiO2(TSIO) and TiO2- ZrO2(TZIO) inverse opal structures over a large area were fabricated using a convective self-assembly method that were functionalized with AuNPs for enhanced photocatalytic activity. TSIO structure displayed a 8.3% increase in the photocatalytic activity compared to pure TIO structure under UVA excitation. However, deposition of AuNPs signiﬁcantly increased the photocatalytic activity by increasing the lifetime of the photogenerated electron-hole pairs. The highest photocatalytic activity was observed for Au-TSIO sample that displayed 206% and 125% increase of activity compared to the reference TIO structure without and with AuNPs, respectively. Furthermore, AuNP doped TIO and TZIO samples displayed photocatalytic activity also in the visible excitation.

The developed multicompound IO structures can be tailored accord- ing to the demands of the application simply by changing the pore size, ratio of constituent materials, and AuNP loading. It is believed that the results of this paper can broaden the application of fabricated materials with enhanced performance in theﬁeld of solar-driven photocatalysis, differential drug release, solar cells, and opticalﬁlters.

Credit author statement

All authors contributed to the manuscript planning and preparation.

KPham: measurements, data handling, writing the original manuscript.

FTemerov: supervision of the measurements, and editing the manuscript. JJSaarinen: project administration and supervision, funding ac- quisition, andﬁnal editing and review of the manuscript.

Data availability

The authors declare that the main data supporting theﬁndings and conclusions of this study are available within the article. Original and additional data is available from the corresponding author upon request.

Declaration of Competing Interest

The authors declare that there are no known conﬂicts of interest as- sociated with this publication.

Acknowledgements

KP would like to thank the UEF Department of Chemistry for the research grant. FT wishes to thank the Finnish Cultural Founda- tion for a research grant. JJS acknowledges the Faculty of Science and Forestry at the University of Eastern Finland for theﬁnancial support (grant no. 579/2017) and the Academy of Finland Flagship for Photonics Research and Innovation (PREIN, decision no.

320166).

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