Hydrothermal synthesis of ZnO quantum dot/KNb3O8 nanosheet photocatalysts for reducing carbon dioxide to methanol

ZnO quantum dots and KNb3O8 nanosheets were synthesized by a two-step hydrothermal method for the photocatalytic reduction of CO2 to methanol where isopropanol is simultaneously oxidized to acetone . The as-prepared photocatalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and UV–vis absorption spectroscopy (UV–vis). The photocatalytic activity of the materials was evaluated by formation rate of methanol under UV light irradiation at ambient temperature and pressure. The methanol formation rate of pure KNb3O8 nanosheets was found to be 1257.21 μmol/g/h, and after deposition of 2 wt % ZnO quantum dots on the surface of KNb3O8 nanosheets, the methanol production rate was found to increase to 1539.77 μmol/g/h. Thus, the ZnO quantum dots obviously prompted separation of charge carriers, which was explained by a proposed mechanism for this photocatalytic reaction.


Introduction
In recent years, the rapidly increasing rate of carbon dioxide emissions into the global atmosphere has received significant attention as it is considered to be the primary greenhouse gas that has led to global warming and even threatens the existence and development of human society [1]. Under the pressure of severe environmental issues, researchers are seeking solutions to reduce CO 2 emissions or to utilize the emitted CO 2 . Among various problem-solving approaches, photocatalytic reduction of CO 2 is most promising for converting CO 2 into useful organics without any footprint, similar to what occurs in natural photosynthesis. Until now, the organics produced by such artificial photosynthesis include methane [2], formaldehyde [3], methanol [4], methyl formate [5], among others.
Alkaline niobates, which are great potential photocatalysts, have been developed in virtue of their excellent nonlinear optical, piezoelectric, ferroelectric, ionic conductivity, selectiveion exchange and photocatalytic properties [6][7][8][9]. Zhang et al. prepared K 4 Nb 6 O 17 with a sheet-like nanostructure by hydrothermal synthesis and found its photocatalytic activity for degrading acidic red G to be much higher than that of the commercial Degussa P25 [10]. Zhou et al. synthesized porous K 4 Nb 6 O 17 microspheres with large surface area via a homogeneous precipitation method, which showed not only two times higher photocatalytic activity than that of the commercial Degussa P25 (the titanium dioxide particle diameter is about 25 nm) in the degradation of rhodamine B, but also a higher generating rate of H 2 at 3.0 mmol/h under UV-light irradiation [11]. Song prepared nanocrystals of ferroelectric NaNbO 3 phase by a hydrothermal method using Nb 2 O 5 and NaOH as precursors in one step, where a lower concentration of Nb 2 O 5 was favorable to form NaNbO 3 nanorods and nanoplates, while a lower concentration of NaOH facilitated formation of NaNbO 3 cubes [12]. Shi and his co-workers synthesized polymeric g-C 3 N 4 coupled with NaNbO 3 nanowires for enhancing photocatalytic reduction of CO 2 into renewable fuel [13]. KNb 3 O 8 , a well-known layered niobate, has an orthorhombic crystal structure that consists of negatively charged sheets and is linked by NbO 6 octahedral units and K + ions between the sheets [14]. Its unique microstructure has aroused researchers' interest. Traditionally, KNb 3 O 8 was synthesized by a solid-state reaction or alkoxide methods at more than 800 °C [15][16][17], which make the particle size uncontrollable. It is well-known that the morphology, size and crystal structure of the photocatalyst are crucial properties to control its photocatalytic activity [18]. Francesco and his team [19] synthesized rod-like KNb 3 O 8 by using KCl, K 2 CO 3 , and Nb 2 O 5 by calcination at 800 °C for 3 h. Zhan et al. [20] prepared the single-crystalline KNb 3 O 8 nanobelts via a molten salt method and investigated its photocatalytic performance in degradation of methyl orange under UV irradiation.
To the best of our knowledge, there have been few reports about the photocatalytic reduction of CO 2 using KNb 3 O 8 nanosheets. In this paper, we describe the synthesis of composite photocatalysts comprised of ZnO quantum dots and KNb 3 O 8 nanosheets, produced by hydrothermal synthesis. The as-prepared photocatalysts were tested by reducing CO 2 in isopropanol to methanol under UV-light irradiation.
ZnO quantum dots were loaded onto KNb 3 O 8 nanosheets by a two-step hydrothermal method. Typically, 0.2 g of Nb 2 O 5 powder in 40 mL of aqueous alkaline solution containing 1-4 mol/L KOH were heated to 180 °C for 48 h in a 75 mL sealed Teflon-lined autoclave [21]. After cooling to room temperature naturally, a clear solution was obtained. The pH of the solution was gradual adjusted with diluted HCl solution to 5-6 under stirring and a white slurry was obtained. A given amount of obtained ZnO quantum dots [22] were added into this white slurry under magnetic stirring. Then the slurry was poured into a 75 mL Teflon-lined autoclave and heated to 200 °C and kept for 48 h, where the precipitate was collected by centrifuging the mixture, then washed with ethanol and deionized water three times, and finally dried at 65 °C for 12 h for further characterization.

Catalyst characterization
The crystal structure of the synthesized materials were characterized with a powder X-ray diffraction instrument (XRD, D/Max-2500, Rigaku). The morphology and microstructure were examined by scanning electron microscopy (FE-SEM, JEOL-JSM 700F). The chemical composition was characterized using energy dispersive X-ray spectroscopy (EDS). TEM and HRTEM images were obtained with a Tecnai G2 F20 instrument. The UV-vis diffuse reflectance spectra were captured by a Shimadzu UV-2550 UV-vis spectrophotometer using BaSO 4 as a reference in the wavelength region of 200-800 nm. Photoluminescence (PL) spectra were measured at room temperature on a Renishaw 1000 Raman system using a 325 nm laser.

Photocatalytic reaction
The photocatalytic reduction of CO 2 to methanol was carried out at ambient conditions in a quartz container and batch operated slurry reactor with cooling jacket. 20 mg of the catalyst was added to 20 mL of isopropanol under magnetic stirring. High purity CO 2 was bubbled through the isopropanol solution at a rate of 200 mL/min for 30 min to eliminate air and to saturate the solution before irradiation. Then the reactor was illuminated by a 250 W high-pressure mercury lamp with peak wavelength of 365 nm for 10 h. After the reaction, the transparent solution was centrifugally separated from the slurry and analyzed by a gas chromatograph (SCION 456-GC). The reference experiments were carried out in the absence of photocatalyst or laser irradiation, and there were no products formed.

Catalyst characterization
The two-step hydrothermal process for synthesizing KNb 3 O 8 nanosheets was influenced by several factors. Among them, the concentration of the KOH solution, which is presented by the pH, is the critical factor [23]. Figure 1 shows the powder XRD patterns of the as-synthesized particles with different KOH concentrations. The Nb 2 O 5 could not completely convert to KNb 3 O 8 under lower KOH concentration during the hydrothermal treatment [24]. Only when the KOH concentration was increased to 2 mol/L was a pure KNb 3 O 8 phase obtained. All diffraction peaks of the samples can be readily compared to JCPDS#75-2182, thus no impurity peaks were found. With an increase in the concentration of KOH, the diffraction peaks become gradually strong. A large amount of nanosheets with length larger than 1 μm, ≈700 nm in width and <10 nm thickness were obtained.   [25]. The lattice fringe spacing of 0.34 nm was observed in Figure 5, which is ascribed to the (031) plane of KNb 3 O 8 . Additionally, the elemental EDS analysis showed the sample to be composed of K, Nb, Zn and O in Figure 6.
The UV-vis diffuse reflectance spectra for different amounts of ZnO quantum dots loaded onto KNb 3 O 8 nanosheets are illustrated in Figure 7. The steep absorption edge of the pure KNb 3 O 8 nanosheets occurred around 343 nm under UV light owing to its large intrinsic band gap. When increasing the ZnO quantum dot loading, the sample displayed an absorption threshold at 376 nm. Compared with the pure KNb 3 O 8 nanosheets, we found a significant red-shift of the absorption edge, which could attribute to the enhancement observed by the ZnO quantum dots upon visible-light absorption. The optical band gap (E g ) of semiconductors could be estimated from the Kublka-Munk transformation. The bang gap value of pure KNb 3 O 8 nanosheets was estimated to be 3.46 eV, which is in agreement with the previous reports [26]. After the ZnO quantum dots were loaded, the bang gap value reduced to 3.12 ev.

Photocatalytic reduction of CO 2
The photocatalytic activity of the as-synthesized samples towards the photocatalytic reduction of CO 2 in isopropanol is shown in Figure 8. The pure KNb 3 O 8 nanosheets demonstrated the lowest yield of methanol, which may be due to the unique structure of the distorted NbO 6 octahedron and the large external surface area (37.639 m 2 /g ) of the nanosheets. Compared with pure KNb 3 O 8 nanosheets, the yield of methanol was improved until a ZnO concentration of 2 wt % was reached. The highest yield of methanol was 1539.77 μmol/g/h for the 2 wt % ZnO quantum dots. This high yield can be ascribed to the heterojunction between the KNb 3 O 8 nanosheets and the ZnO     quantum dots, inhibiting the efficient recombination of photogenerated electron-hole pairs. However, with a superfluous amount of ZnO quantum dots, the photocatalytic activity became worse due to the aggregation of ZnO quantum dots which gradually form a recombination center for photogenerated electron-hole pairs.
In addition, The stability of the ZnO quantum dots and KNb 3 O 8 nanosheets after the photocatalytic reduction of CO 2 was evaluated by reusing them after each batch without any treatment. In Figure 9, the composite photocatalysts of ZnO quantum dots and KNb 3 O 8 nanosheets were recycled three times. This shows that the activity of the materials slowly declines but the formation rate of methanol was maintained at around 1500 μmol/g/h.
Photoluminescence (PL) spectroscopy was used to trace the migration and capture of photogenerated electrons and holes from the as-synthesized samples. As is known, the higher the emission peak intensity, the more efficient the recombination of photogenerated carriers, which may limit the photocatalytic performance [26]. In Figure 10, KNb 3 O 8 nanosheets excited high emission peaks. However, after 2 wt % ZnO quantum dots were loaded onto the nanosheets, the emission intensity evidently decreased, which indicates that the photogenerated charge carriers are efficiently separated and the lifetime of free carriers is expected to be prolonged. In addition, heterostructures act as an excellent electron transport platform to facilitate the electron-hole transfer [27], efficiently enhancing the photocatalytic reduction of CO 2 .

Reaction mechanism
The mechanism of photocatalytic reduction of CO 2 to methanol on ZnO quantum dots loaded onto KNb 3 O 8 nanosheets under UV irradiation is shown in Figure 11. Under UV radiation, the catalyst can create photogenerated electron-hole pairs. The conduction band and the valence band of the  catalyst were calculated by the empirical equations [13]: where X is the absolute electronegativity of the semiconductor, E c is the energy of the free electrons under standard hydrogen electrode potential (4.5 eV) and E g is the band gap energy. The results indicated that the conduction band of pure ZnO quantum dots (E CB = −1.02 V vs NHE at pH 7, the same below) [25,28] and pure KNb 3 O 8 nanosheets (E CB = −0.94V) are more negative than the position of the reduction of CO 2 to methanol (E CO2/CH3OH = −0.38 V) [29], the valence band of pure ZnO quantum dots (E VB = 1.6 V) and pure KNb 3 O 8 nanosheets (E VB = 2.52 V) which are more positive than that of the isopropanol oxidation to acetone (E C3H8O/C3H6O = 0.49 V) [30], which made the reduction of CO 2 to methanol and the oxidation of isopropanol to acetone possible. Under UV irradiation, photoinduced electrons in the conduction band (CB) of KNb 3 O 8 migrate to the valence band (VB) of ZnO quantum dots and combine with the holes to efficiently prevent photogenerated electron-hole pairs from vast and fast recombination. Therefore, the lifetime of free carriers is prolonged and the photocatalytic performance could be enhanced.
The photocatalytic reaction mechanism is as follows.

Conclusion
In this experiment, ZnO quantum dots were loaded onto KNb 3 O 8 nanosheets via a two-step hydrothermal method. After evaluating the as-synthesized samples for the photocatalytic reduction of CO 2 to methanol in isopropanol under UV-light irradiation, we discovered that the methanol yield was evidently improved by the presence of ZnO quantum dots (as compared to pure KNb 3 O 8 nanosheets). The ZnO quantum dots form a heterojunction with KNb 3 O 8 nanosheets to efficiently separate the photogenerated electron-hole pairs and inhibit their recombination while prolonging the lifetime of free carriers. The 2 wt % ZnO quantum dots loaded onto KNb 3 O 8 nanosheets showed superior photocatalytic activity with 1539.77 μmol/g/h of methanol yield. Furthermore, the mechanism of photocatalytic reduction of CO 2 is photogenerated electrons and holes to be consumed for producing methanol and acetone separately.