Step-by-Step Growth of HKUST-1 on Functionalized TiO 2 Surface: An Efﬁcient Material for CO 2 Capture and Solar Photoreduction

: The present study reports on a simple preparation strategy of a hybrid catalyst, TiO 2 /HKUST-1, containing TiO 2 anatase nanoparticles (NPs) with tailored morphology and photocatalytic activity coupled with a porous metal-organic framework (MOF), namely HKUST-1, as an advanced material for the CO 2 photocatalytic reduction. In detail, TiO 2 /HKUST-1 catalyst was prepared via an easy slow-diffusion method combined with a step-by-step self-assembly at room temperature. The growth of crystalline HKUST-1 onto titania surface was achieved by functionalizing TiO 2 nanocrystals, with phosphoesanoic acid (PHA), namely TiO 2 -PHA, which provides an intimate contact between MOF and TiO 2 . The presence of a crystalline and porous shell of HKUST-1 on the TiO 2 surfaces was assessed by a combination of analytical and spectroscopic techniques. TiO 2 /HKUST-1 nanocomposite showed a signiﬁcant efﬁciency in reducing CO 2 to CH 4 under solar light irradiation, much higher than those of the single components. The role of MOF to improve the photoreduction process under visible light was evidenced and attributed either to the relevant amount of CO 2 captured into the HKUST-1 porous architecture or to the hybrid structure of the material, which affords enhanced visible light absorption and allows an effective electron injection from TiO 2 -PHA to HKUST-1, responsible for the photochemical reduction of CO 2 .


Introduction
The significant increase in the CO 2 level in the past several decades is a matter of great concern [1,2]. While there are considerable investments in developing methods to reduce CO 2 emissions, it is apparent that its atmospheric concentration will continue to monotonically while HKUST-1 for its ability in favoring high CO 2 adsorption as well as controlled release kinetics [39], so that MOF pores may behave like "nano-reactors", in which the substrate is confined near to the TiO 2 surface.
In detail, we have prepared, by a cheap and easy to scale-up soft-chemistry method, TiO 2 nanocrystals having tailored structural and morphological features, with exposed {101} and {010} facets which are known to be active in photo-reduction reactions [40,41]. TiO 2 NPs were functionalized with an asymmetric organic linker, namely 6-phosphohexanoic acid (PHA), suitable to covalently interact with the oxide surface by Ti-O-P bond, as well as to selectively bind copper metal ions by carboxylate functionality. The growth of HKUST-1 on PHA-modified TiO 2 NPs (TiO 2 -PHA) was obtained using a slow-diffusion method combined with a step-by-step self-assembly approach [42]. After complexation of Cu 2+ metal ions by the carboxylic groups, the metal-carboxylate centers react with the BTC ligand activating the crystalline growth of the HKUST-1 structure on the oxide surface.
This synthetic strategy, based on the TiO 2 functionalization with an organic linker, greatly (i) facilitates NPs dispersion, preventing undesirable agglomeration phenomena, (ii) promotes the crystalline growth of MOF layers on TiO 2 surface and (iii) favors an intimate contact between TiO 2 and MOF, which makes easier, in principle, the electron transfer at the hybrid interface.
TiO 2 /HKUST-1 nanocomposite was characterized by a combination of analytical and spectroscopic techniques. After determining the structural and morphological features, the CO 2 adsorption aptitude and the photocatalytic performances of the developed hybrid material were examined. CO 2 photoreduction tests were performed under sunlight, in ambient conditions and in a heterogeneous gas/solid set-up, in order to simulate the conditions under which CO 2 capture and fixation proceed in a single step, while most of the existent studies describe gas/liquid systems. Based on the photocatalytic results, a model for explaining the possible charge transfer between HKUST-1 and TiO 2 -PHA in TiO 2 /HKUST-1 have been proposed, in connection with the peculiar structure and morphology of hybrid material.

Results and Discussion
TiO 2 /HKUST-1 catalyst was prepared via a simple slow-diffusion method combined with a step-by-step self-assembly at room temperature. Firstly, the solvothermal synthesis of shape-controlled anatase nanocrystals was performed according to a previously reported procedure [43] (Scheme 1, STEP 1) by reaction of the titanium (IV) butoxide (TB), in the presence of oleic acid (OA) and oleylamine (OM). After removing the residual amounts of capping agents by using tetramethylammonium hydroxide (TMAH) [44] (STEP 2), the pre-synthetized TiO 2 NPs have been functionalized with PHA (STEP 3), able to covalently interact with the oxide surface by the phosphonic group, as well as to bind copper metal ions by carboxylic acid, promoting the grown of HKUST-1.  [39], so that MOF pores may behave like "nano-reactors", in which the substrate is confined near to the TiO2 surface.
In detail, we have prepared, by a cheap and easy to scale-up soft-chemistry method, TiO2 nanocrystals having tailored structural and morphological features, with exposed {101} and {010} facets which are known to be active in photo-reduction reactions [40,41]. TiO2 NPs were functionalized with an asymmetric organic linker, namely 6-phosphohexanoic acid (PHA), suitable to covalently interact with the oxide surface by Ti-O-P bond, as well as to selectively bind copper metal ions by carboxylate functionality. The growth of HKUST-1 on PHA-modified TiO2 NPs (TiO2-PHA) was obtained using a slow-diffusion method combined with a step-by-step self-assembly approach [42]. After complexation of Cu 2+ metal ions by the carboxylic groups, the metal-carboxylate centers react with the BTC ligand activating the crystalline growth of the HKUST-1 structure on the oxide surface.
This synthetic strategy, based on the TiO2 functionalization with an organic linker, greatly (i) facilitates NPs dispersion, preventing undesirable agglomeration phenomena, (ii) promotes the crystalline growth of MOF layers on TiO2 surface and (iii) favors an intimate contact between TiO2 and MOF, which makes easier, in principle, the electron transfer at the hybrid interface.
TiO2/HKUST-1 nanocomposite was characterized by a combination of analytical and spectroscopic techniques. After determining the structural and morphological features, the CO2 adsorption aptitude and the photocatalytic performances of the developed hybrid material were examined. CO2 photoreduction tests were performed under sunlight, in ambient conditions and in a heterogeneous gas/solid set-up, in order to simulate the conditions under which CO2 capture and fixation proceed in a single step, while most of the existent studies describe gas/liquid systems. Based on the photocatalytic results, a model for explaining the possible charge transfer between HKUST-1 and TiO2-PHA in TiO2/HKUST-1 have been proposed, in connection with the peculiar structure and morphology of hybrid material.

Results and Discussion
TiO2/HKUST-1 catalyst was prepared via a simple slow-diffusion method combined with a stepby-step self-assembly at room temperature. Firstly, the solvothermal synthesis of shape-controlled anatase nanocrystals was performed according to a previously reported procedure [43] (Scheme 1, STEP 1) by reaction of the titanium (IV) butoxide (TB), in the presence of oleic acid (OA) and oleylamine (OM). After removing the residual amounts of capping agents by using tetramethylammonium hydroxide (TMAH) [44] (STEP 2), the pre-synthetized TiO2 NPs have been functionalized with PHA (STEP 3), able to covalently interact with the oxide surface by the phosphonic group, as well as to bind copper metal ions by carboxylic acid, promoting the grown of HKUST-1. Scheme 1. Schematic synthesis of TiO2 nanoparticles (NPs) with controlled rhombic elongated (RE) morphology (STEP 1 and 2) and functionalization with 6-phosphohexanoic acid (PHA) of TiO2 NPs (TiO2-PHA, STEP 3).
After that, TiO2/HKUST-1 was prepared by a step-by-step self-assembly at low temperature (40 °C) (Scheme 2). In detail, the copper centers of the copper acetate (AcCu(II)) can interact with the carboxyl groups of TiO2-PHA and react with the carboxyl groups of H3BTC ligands.  After that, TiO 2 /HKUST-1 was prepared by a step-by-step self-assembly at low temperature (40 • C) (Scheme 2). In detail, the copper centers of the copper acetate (AcCu(II)) can interact with the carboxyl groups of TiO 2 -PHA and react with the carboxyl groups of H 3 BTC ligands.

Spectroscopic and Morphological Characterization of TiO2-PHA and TiO2/HKUST-1
The characterization of TiO2-PHA and TiO2/HKUST-1 was performed in order to demonstrate: (i) the effective functionalization of TiO2 NPs by PHA phosphonic groups and consequently (ii) the crystalline growth of the HKUST-1 structure on the oxide surface.
This suggests that both TiO2 NPs functionalization with specific carboxylate end-group of PHA and the use of step-by-step self-assembly synthesis promote the crystalline growth of MOF porous material, even under mild conditions. In detail, in the first step of HKUST-1 synthesis, PHA carboxylic groups allow the complexation of Cu 2+ metal ions anchoring them onto the oxide surface. In the second step, the copper centres covalently bond to the BTC ligands enabling the growing of HKUST-1 on the TiO2 surface and the control of its crystallographic orientations [46].
The observed HKUST-1 crystallinity represents a key feature to impart desired functionality to the final hybrid material, such as improved adsorption ability and consequent enhanced catalytic activity [47].
Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) spectroscopy was performed in order to investigate the TiO2-MOF hybrid catalyst ( Figure 2).
Specifically, the spectrum of PHA shows the following main bands (Figure 2a, grey line): the intense carboxyl stretching vibration at 1710 cm −1 ; the P=O stretching vibration at 1220 cm −1 and the methylene C-H bending at 1309 and 1213 cm −1 (very weak).
After titania functionalization, the spectrum of TiO2-PHA (Figure 2a,b, red line) still displays the characteristic absorption bands at 1715 cm −1 deriving from COOH of the PHA, instead the PHA band at 1220 cm −1 , attributed to the P=O stretching vibration, disappears (red line in Figure 2b). TiO2-PHA spectrum also shows the vibrations at 995 cm −1 and at 1150 cm −1 , attributable to the symmetric stretching of P-O-Ti and P-CH2 bonds, respectively [48]. These results suggest that phosphoryl groups of PHA interact with TiO2 surfaces. Scheme 2. Synthesis of the TiO 2 /HKUST-1 by using TiO 2 -PHA, benzene-1,3,5-tricarboxylic acid (H 3 BTC) and copper acetate (AcCu(II) as starting materials. Green, gray, and red spheres represent Cu, C, and O atoms, respectively; H atoms have been omitted for clarity. The sky-blue line represents HKUST-1 shells onto TiO 2 -PHA.

Spectroscopic and Morphological Characterization of TiO 2 -PHA and TiO 2 /HKUST-1
The characterization of TiO 2 -PHA and TiO 2 /HKUST-1 was performed in order to demonstrate: (i) the effective functionalization of TiO 2 NPs by PHA phosphonic groups and consequently (ii) the crystalline growth of the HKUST-1 structure on the oxide surface.
The crystal structure of the materials was checked by powder X-ray diffraction (PXRD) analysis ( Figure 1). PXRD patterns of TiO 2 /HKUST-1 sample (Figure 1a) showed the superimposed pattern of HKUST-1 (Figure 1c) [31] and anatase TiO 2 diffraction peaks [45] (Figure 1b This suggests that both TiO 2 NPs functionalization with specific carboxylate end-group of PHA and the use of step-by-step self-assembly synthesis promote the crystalline growth of MOF porous material, even under mild conditions. In detail, in the first step of HKUST-1 synthesis, PHA carboxylic groups allow the complexation of Cu 2+ metal ions anchoring them onto the oxide surface. In the second step, the copper centres covalently bond to the BTC ligands enabling the growing of HKUST-1 on the TiO 2 surface and the control of its crystallographic orientations [46].
The observed HKUST-1 crystallinity represents a key feature to impart desired functionality to the final hybrid material, such as improved adsorption ability and consequent enhanced catalytic activity [47].
Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) spectroscopy was performed in order to investigate the TiO 2 -MOF hybrid catalyst ( Figure 2).
Specifically, the spectrum of PHA shows the following main bands (Figure 2a, grey line): the intense carboxyl stretching vibration at 1710 cm −1 ; the P=O stretching vibration at 1220 cm −1 and the methylene C-H bending at 1309 and 1213 cm −1 (very weak).
After titania functionalization, the spectrum of TiO 2 -PHA (Figure 2a,b, red line) still displays the characteristic absorption bands at 1715 cm −1 deriving from COOH of the PHA, instead the PHA band at 1220 cm −1 , attributed to the P=O stretching vibration, disappears (red line in Figure 2b). TiO 2 -PHA spectrum also shows the vibrations at 995 cm −1 and at 1150 cm −1 , attributable to the symmetric stretching of P-O-Ti and P-CH 2 bonds, respectively [48]. These results suggest that phosphoryl groups of PHA interact with TiO 2 surfaces.   The spectra of TiO2/HKUST-1 ( Figure 2b, blue line) and pure HKUST-1 ( Figure 2b, sky blue line) look rather similar and show characteristics analogous to those reported in the literature [49]. The bands at 1623 and 1550 cm −1 and at 1440 and 1364 cm −1 , corresponding to the asymmetric and symmetric stretching vibrations of the BTC carboxylate groups, confirm that MOF coating TiO2 in the composite maintains the structural features of HKUST-1.
In addition, the band centred at 1715 cm −1 , corresponding to the carbonyl stretching in pure TiO2-PHA, shifts to lower energy in the TiO2/HKUST-1 spectrum, evidencing between 1540 and 1650 cm −1 the antisymmetric mode of the chelated carboxyl group. This supports the deprotonation of carboxylic acid group and coordination with copper, confirming the effective role of PHA as grafting group for HKUST-1.
The thermal behavior of the hybrid material was assessed by thermogravimetric analysis (TGA). Figure 3 shows TGA curves obtained for both as-prepared shape-controlled TiO2 (black line) and TiO2-PHA (red line) nanocrystals. In the as-prepared sample (black curve), a small weight loss (3%) beginning at nearly 30 °C and continuing until 250 °C is detectable, which can be ascribed to physisorbed solvents removal. A more relevant weight loss is observable instead in TiO2-PHA in a wide temperature range, from 230 to 400 °C, attributable to the thermal degradation of the PHA. The total amount of PHA grafted onto TiO2 (11.2 wt. %) was evaluated by the net weight loss of TiO2-PHA between 150 and 400 °C, i.e., considering the total weight loss with exclusion of that associated to TiO2 (i.e., 3 wt. %).
TGA analysis was also performed on TiO2/HKUST-1 (blue line), in order to determine the real amount of HKUST-1 in the hybrid material. The loading of HKUST-1 (73% wt. %) was estimated from the net weight loss of TiO2/HKUST-1 between 150 and 400 °C, i.e., considering the total weight loss with the exclusion of that associated to pure PHA.
The morphological features of both TiO2 NPs and hybrid TiO2/HKUST-1 materials were investigated by TEM microscopy.
RE TiO2 sample exhibits well-formed rhombic elongated nanocrystals with estimated size of 16.5 nm in width and 45-60 nm in length (Table S1 in Supplementary Materials), and with mainly exposed {101} and {010} crystal facets (Figure 4a,b), according to the results reported by Dihn et al. [50].
In TiO2/HKUST-1 hybrid composite (Figure 4c,d), TiO2 NPs maintain their peculiar anisotropic morphology and appear organized in nanometric aggregates (Figure 4c). At a higher magnification (Figure 4d), the presence of HKUST-1 shells uniformly grown onto the TiO2 surfaces and intimately connecting the NPs can be observed (yellow arrows in d). The uniform distribution of HKUST-1 around TiO2 crystals into the hybrid material is also relevant; this is expected from the use of functionalized TiO2 NPs.  [49]. The bands at 1623 and 1550 cm −1 and at 1440 and 1364 cm −1 , corresponding to the asymmetric and symmetric stretching vibrations of the BTC carboxylate groups, confirm that MOF coating TiO 2 in the composite maintains the structural features of HKUST-1.
In addition, the band centred at 1715 cm −1 , corresponding to the carbonyl stretching in pure TiO 2 -PHA, shifts to lower energy in the TiO 2 /HKUST-1 spectrum, evidencing between 1540 and 1650 cm −1 the antisymmetric mode of the chelated carboxyl group. This supports the deprotonation of carboxylic acid group and coordination with copper, confirming the effective role of PHA as grafting group for HKUST-1.
The thermal behavior of the hybrid material was assessed by thermogravimetric analysis (TGA). Figure 3 shows TGA curves obtained for both as-prepared shape-controlled TiO 2 (black line) and TiO 2 -PHA (red line) nanocrystals. In the as-prepared sample (black curve), a small weight loss (~3%) beginning at nearly 30 • C and continuing until 250 • C is detectable, which can be ascribed to physisorbed solvents removal. A more relevant weight loss is observable instead in TiO 2 -PHA in a wide temperature range, from 230 to 400 • C, attributable to the thermal degradation of the PHA. The total amount of PHA grafted onto TiO 2 (11.2 wt. %) was evaluated by the net weight loss of TiO 2 -PHA between 150 and 400 • C, i.e., considering the total weight loss with exclusion of that associated to TiO 2 (i.e., 3 wt. %).
TGA analysis was also performed on TiO 2 /HKUST-1 (blue line), in order to determine the real amount of HKUST-1 in the hybrid material. The loading of HKUST-1 (73% wt. %) was estimated from the net weight loss of TiO 2 /HKUST-1 between 150 and 400 • C, i.e., considering the total weight loss with the exclusion of that associated to pure PHA.
The morphological features of both TiO 2 NPs and hybrid TiO 2 /HKUST-1 materials were investigated by TEM microscopy.
RE TiO 2 sample exhibits well-formed rhombic elongated nanocrystals with estimated size of 16.5 nm in width and 45-60 nm in length (Table S1 in Supplementary Materials), and with mainly exposed {101} and {010} crystal facets (Figure 4a,b), according to the results reported by Dihn et al. [50].
In TiO 2 /HKUST-1 hybrid composite (Figure 4c,d), TiO 2 NPs maintain their peculiar anisotropic morphology and appear organized in nanometric aggregates (Figure 4c). At a higher magnification (Figure 4d), the presence of HKUST-1 shells uniformly grown onto the TiO 2 surfaces and intimately connecting the NPs can be observed (yellow arrows in d). The uniform distribution of HKUST-1 around TiO 2 crystals into the hybrid material is also relevant; this is expected from the use of functionalized TiO 2 NPs. Selected area electron diffraction (SAED) analysis was in agreement with XRD and highresolution TEM (HRTEM) data, confirming again the presence of TiO2 anatase phase in both TiO2-PHA and TiO2/HKUST-1 samples and of crystalline HKUST-1 in the hybrid system, as revealed by lattice fringes analysis ( Figure S1). Finally, STEM-EDS analysis reveal the presence of Ti and Cu with a ratio close to the theoretical 1:1 ( Figure S2).
In summary, TEM investigation indicates the successful formation of the hybrid structure and supports close interaction between the RE NPs and HKUST-1.  Selected area electron diffraction (SAED) analysis was in agreement with XRD and high-resolution TEM (HRTEM) data, confirming again the presence of TiO 2 anatase phase in both TiO 2 -PHA and TiO 2 /HKUST-1 samples and of crystalline HKUST-1 in the hybrid system, as revealed by lattice fringes analysis ( Figure S1). Finally, STEM-EDS analysis reveal the presence of Ti and Cu with a ratio close to the theoretical 1:1 ( Figure S2).
In summary, TEM investigation indicates the successful formation of the hybrid structure and supports close interaction between the RE NPs and HKUST-1. Selected area electron diffraction (SAED) analysis was in agreement with XRD and highresolution TEM (HRTEM) data, confirming again the presence of TiO2 anatase phase in both TiO2-PHA and TiO2/HKUST-1 samples and of crystalline HKUST-1 in the hybrid system, as revealed by lattice fringes analysis ( Figure S1). Finally, STEM-EDS analysis reveal the presence of Ti and Cu with a ratio close to the theoretical 1:1 ( Figure S2).
In summary, TEM investigation indicates the successful formation of the hybrid structure and supports close interaction between the RE NPs and HKUST-1.   As mentioned above, the coupling of HKUST-1 to TiO 2 NPs was aimed at: (i) favouring the CO 2 access to the catalytic sites by improving the porosity necessary to capture CO 2 ; (ii) activating TiO 2 photocatalytic response in the visible region.
Hence, the porosity of the TiO 2 /HKUST-1 material was evaluated by nitrogen adsorption-desorption isotherms at 77 K (Figure 5a). The product exhibits a type IV isotherm with H1 hysteresis, which is typical of ordered mesoporous materials with uniform cylindrical pores [51]. The BET surface area resulted 349 m 2 /g and the BJH pore-size distribution (Harkins-Jura approximation) shows the presence of mesopores with pore widths ca. 4 nm. The total pores volume corresponds to 0.25 cm 3 /g. The specific surface area and total pore volumes of the bare TiO 2 NPs resulted instead 170 m 2 /g and 0.21 cm 3 /g respectively (Table S1).
In addition to a high specific surface area, TiO 2 /HKUST-1 also showed a significant adsorption CO 2 capacity at room temperature (303 K) and atmospheric pressure (Figure 5b), reaching a percentage of CO 2 adsorbed of 10.16 wt. % (2.3 mmol/g). As expected, this amount is about three times lower than that reported for unsupported HKUST-1 under the same conditions (7.23 mmol/g) [52]. However, the CO 2 uptake is rather similar to that determined for other Cu-based MOFs with even higher BET surface area [53][54][55]. As mentioned above, the coupling of HKUST-1 to TiO2 NPs was aimed at: (i) favouring the CO2 access to the catalytic sites by improving the porosity necessary to capture CO2; (ii) activating TiO2 photocatalytic response in the visible region.
Hence, the porosity of the TiO2/HKUST-1 material was evaluated by nitrogen adsorptiondesorption isotherms at 77 K (Figure 5a). The product exhibits a type IV isotherm with H1 hysteresis, which is typical of ordered mesoporous materials with uniform cylindrical pores [51]. The BET surface area resulted 349 m²/g and the BJH pore-size distribution (Harkins-Jura approximation) shows the presence of mesopores with pore widths ca. 4 nm. The total pores volume corresponds to 0.25 cm 3 /g. The specific surface area and total pore volumes of the bare TiO2 NPs resulted instead 170 m²/g and 0.21 cm 3 /g respectively (Table S1).
In addition to a high specific surface area, TiO2/HKUST-1 also showed a significant adsorption CO2 capacity at room temperature (303 K) and atmospheric pressure (Figure 5b), reaching a percentage of CO2 adsorbed of 10.16 wt. % (2.3 mmol/g). As expected, this amount is about three times lower than that reported for unsupported HKUST-1 under the same conditions (7.23 mmol/g) [52]. However, the CO2 uptake is rather similar to that determined for other Cu-based MOFs with even higher BET surface area [53][54][55]. The diffuse reflectance ultraviolet-visible (DR-UV/Vis) spectra ( Figure 6) were acquired for pure HKUST-1, TiO2-PHA and TiO2/HKUST-1, in order to study their optical absorption properties.
As expected, TiO2-PHA (red line) absorbs mainly UV light while HKUST-1 (sky blue line) absorption extends over the visible range [49]. Notably, TiO2/HKUST-1 (blue line) composite also shows a red shift of the optical absorption toward the visible range compared to TiO2-PHA.
Based on the reflectance spectra ( Figure S3 The enhanced absorption in the visible range of TiO2/HKUST-1 compared to TiO2, along with the smaller absorption edge value, provides a suitable platform for sensitizing TiO2, making the obtained hybrid material an active catalyst under solar light. In order to understand the electronic changes responsible for the absorption modification, the TiO2/HKUST-1, TiO2-PHA, and pristine TiO2 and HKUST-1 samples were investigated X-ray photoelectron spectroscopy (XPS).
The survey spectra of TiO2/HKUST-1 confirmed the presence of Cu, Ti, O, and C elements ( Figures S4). The evolution of the Cu 2p, Ti 2p and O 1s core levels by XPS spectra (Figures 7 and S5) allowed us to reveal the electronic structure of these levels in TiO2/HKUST-1. The diffuse reflectance ultraviolet-visible (DR-UV/Vis) spectra ( Figure 6) were acquired for pure HKUST-1, TiO 2 -PHA and TiO 2 /HKUST-1, in order to study their optical absorption properties.
As expected, TiO 2 -PHA (red line) absorbs mainly UV light while HKUST-1 (sky blue line) absorption extends over the visible range [49]. Notably, TiO 2 /HKUST-1 (blue line) composite also shows a red shift of the optical absorption toward the visible range compared to TiO 2 -PHA.
Based on the reflectance spectra ( Figure S3 The enhanced absorption in the visible range of TiO 2 /HKUST-1 compared to TiO 2 , along with the smaller absorption edge value, provides a suitable platform for sensitizing TiO 2 , making the obtained hybrid material an active catalyst under solar light. In order to understand the electronic changes responsible for the absorption modification, the TiO 2 /HKUST-1, TiO 2 -PHA, and pristine TiO 2 and HKUST-1 samples were investigated X-ray photoelectron spectroscopy (XPS). The survey spectra of TiO 2 /HKUST-1 confirmed the presence of Cu, Ti, O, and C elements ( Figure  S4). The evolution of the Cu 2p, Ti 2p and O 1s core levels by XPS spectra (Figure 7 and Figure S5) allowed us to reveal the electronic structure of these levels in TiO 2 /HKUST-1. The Cu 2p3/2 XPS spectra of HKUST-1 and TiO2/HKUST-1 (Figure 7a) show peaks with similar lineshapes, suggesting that the copper chemical environment in the hybrid material is analogous to that of pristine HKUST-1. The main peak of Cu 2p3/2 was observed at 935.0 eV and it was deconvoluted into two components at 935.0 eV and 932.8 eV, originating from Cu(II) (blue peak in Figure 7a) and Cu(I) (sky blue peak in Figure 7a). By supposing a homogeneous stoichiometry, XPS indicates that Cu(I) percentage with respect to the total copper amount is ~11% in HKUST-1. This amount increases up to 17% in the final hybrid material [56].
The Ti 2p core level spectra of TiO2, TiO2-PHA and TiO2/HKUST-1 samples are shown in Figure  7b. The Ti 2p3/2 binding energy (BE) of pure TiO2 sample was found at 458.5 eV, corresponding to the Ti(IV) state (green peak in Figure 7b), with a small fraction of reduced Ti(III) at 457.9 (yellow peak in Figure 7b) [57], indicating the presence of Ti(III) defects originated during the solvothermal synthesis.  The Cu 2p3/2 XPS spectra of HKUST-1 and TiO2/HKUST-1 (Figure 7a) show peaks with similar lineshapes, suggesting that the copper chemical environment in the hybrid material is analogous to that of pristine HKUST-1. The main peak of Cu 2p3/2 was observed at 935.0 eV and it was deconvoluted into two components at 935.0 eV and 932.8 eV, originating from Cu(II) (blue peak in Figure 7a) and Cu(I) (sky blue peak in Figure 7a). By supposing a homogeneous stoichiometry, XPS indicates that Cu(I) percentage with respect to the total copper amount is ~11% in HKUST-1. This amount increases up to 17% in the final hybrid material [56].
The Ti 2p core level spectra of TiO2, TiO2-PHA and TiO2/HKUST-1 samples are shown in Figure  7b. The Ti 2p3/2 binding energy (BE) of pure TiO2 sample was found at 458.5 eV, corresponding to the Ti(IV) state (green peak in Figure 7b), with a small fraction of reduced Ti(III) at 457.9 (yellow peak in Figure 7b) [57], indicating the presence of Ti(III) defects originated during the solvothermal synthesis. The Cu 2p 3/2 XPS spectra of HKUST-1 and TiO 2 /HKUST-1 (Figure 7a) show peaks with similar lineshapes, suggesting that the copper chemical environment in the hybrid material is analogous to that of pristine HKUST-1. The main peak of Cu 2p 3/2 was observed at 935.0 eV and it was deconvoluted into two components at 935.0 eV and 932.8 eV, originating from Cu(II) (blue peak in Figure 7a) and Cu(I) (sky blue peak in Figure 7a). By supposing a homogeneous stoichiometry, XPS indicates that Cu(I) percentage with respect to the total copper amount is~11% in HKUST-1. This amount increases up to 17% in the final hybrid material [56].
The Ti 2p core level spectra of TiO 2 , TiO 2 -PHA and TiO 2 /HKUST-1 samples are shown in Figure 7b. The Ti 2p 3/2 binding energy (BE) of pure TiO 2 sample was found at 458.5 eV, corresponding to the Ti(IV) state (green peak in Figure 7b), with a small fraction of reduced Ti(III) at 457.9 (yellow peak in Figure 7b) [57], indicating the presence of Ti(III) defects originated during the solvothermal synthesis.
The Ti 2p 3/2 level of TiO 2 -PHA presents a broader line shape (458.2 eV) compared to pure TiO 2 , indicating a higher amount of Ti(III) defects possibly generated after TiO 2 functionalization with PHA. It is well known that the concentration of Ti(III) strongly depends on the chemistry at the TiO 2 surface [58], where the presence of capping molecules, such as carboxylic or phosphonic acid [59,60], may cause the atomic oxygen diffusion, away from the lattice sites, reducing Ti(IV) to Ti(III) at the vacancy sites. This suggest that in TiO 2 -PHA, phosphoesanoic acid induces a higher amount of Ti(III) sites at the TiO 2 surface, as confirmed by XPS analysis (Figure 7b).
In the case of TiO 2 /HKUST-1, the Ti 2p peaks become even broader and shifts to higher BE compared to TiO 2 -PHA. The fitting of the major band indicates the presence of three different components: Ti(IV) (green peak), Ti(III) species (yellow peak) and an additional Ti(IV) 2p peak at 459.3 eV (red peak). The appearance of additional Ti(IV) centers and the increase of Cu(I) atomic percentage in the final hybrid material (Figure 7a) suggests a partial oxidation of the Ti(III) species of TiO 2 -PHA by the Cu(II) centers of HKUST-1. This hypothesis envisages the existence of an electron transfer between TiO 2 and HKUST-1 favoured by the intimate contact between titania and MOF anchored on its surface.

Photocatalytic Activity
The photocatalytic activity of TiO 2 /HKUST-1 was evaluated in the CO 2 degradation. In accordance with previous studies reporting the gas phase CO 2 photoreduction [61][62][63][64], the main products were CH 4 and traces of CO [65][66][67][68]. We assume the detected hydrocarbons are unambiguously formed from CO 2 as carbon source, since the photocatalytic processes were performed after a peculiar cleaning procedure, in accordance with Mei et al. [69]. CO 2 photoreduction in the presence of TiO 2 -PHA NPs (Table S2) produced a very low concentration of CH 4 , equal to 0.69 µM (corresponding to 0.28 µmol g −1 ) after 6 h of irradiation.
The tests performed in the presence of bare HKUST-1 sample (Table S3) yielded also very low amounts of CH 4 (0.42 µmol g −1 ). In the presence of TiO 2 /HKUST-1 hybrid material, a significant generation of CH 4 was observed (Table S4), indicating that methane derived from CO 2 reduction and the catalytic ability of TiO 2 /HKUST-1 is by the photocatalytic process affected during photocatalysis. A total amount of 2.63 µM CH 4 , corresponding to 1.05 µmol g −1 , was produced after 6 h of irradiation without any significant morphological and structural changes of TiO 2 /HKUST-1 (see Figure S6) after the second photocatalytic run.
As H 2 could be obtained as by-product during the CO 2 photocatalytic reduction, its presence was checked during the runs, but it was not detected. Figure 8 reports the comparison among the evolution of CH 4 concentration along irradiation time for TiO 2 -PHA, HKUST-1 sample and TiO 2 /HKUST-1 hybrid system (during the first and the second test). While a small amount of CH 4 was formed by using the bare samples, a relevant CH 4 production was observed for TiO 2 /HKUST-1. For this latter sample, the amount of products obtained during the run carried out after the cleaning procedure, was comparable to those measured during the first run. The higher activity of the hybrid sample suggests the occurrence of a synergistic effect between RE NPs, which are known to be active in photoreduction reactions, and the MOF grafted onto TiO 2 surface. This finding is in accordance with the XPS, FTIR and TEM results. may hinder the contact between the catalytic active sites and CO2 target molecules. In addition, in the presence of commercial TiO2 P25, we observed only a negligible CH4 production. These results support the efficacy of our synthetic approach and highlight the important role of PHA functionalization in order to guarantee a close contact between TiO2 NPs and MOF, which delivers to TiO2/HKUST-1 remarkable photoreduction properties.

Proposed Photocatalytic Pathway
Given the analysis above, a possible mechanism for the enhanced photocatalytic CO2 reduction over the synthetized TiO2/HKUST-1 hybrid material was proposed.
To better describe the electronic band structures of TiO2-PHA NPs and HKUST-1 at the hybrid interface in TiO2/HKUST-1, the relative energy of the conduction band (CB) and valence band (VB) versus normal hydrogen electrode (NHE) of both pristine HKUST-1 and TiO2-PHA were calculated, according to Schoonen et al. [70,71], by the empirical equations: where EVB and ECB are the CB and VB potentials, respectively. Moreover, Eg is the band gap of the semiconductor and E e is the energy of free electrons vs. hydrogen (4.5 eV) [72]. Finally, χ is the electronegativity of semiconductor and it was calculated by the following equation: where , n, and N are the electronegativity of the constituent atom, the number of species, and the total number of atoms in the compound, respectively [73]. The superscripts r, s, p and q refer to the numbers of the atoms 1, 2, n −1 , and n, respectively, in the molecule where (r + s + … + p + q = N).
Although this method cannot give absolute values because the structural factors are neglected, it may provide a rough estimation of the relative energy of CB and VB versus normal hydrogen electrode (NHE).
For HKUST-1, the values of Eg and χ values were 2.7 and 6.17 eV and consequently, ECB and EVB result were 3.02 and 0.32 eV while for TiO2-PHA Eg and χ were 3.15 and 5.84 eV, and its ECB and EVB as −0.24 and 2.92 eV, respectively, versus NHE, in line with the reported experimental data [74]. Finally, both the TiO 2 + HKUST-1 sample, obtained by mixing un-functionalized TiO 2 NPs and HKUST-1, and the commercial TiO 2 sample P25, as reference materials, were tested under the same experimental conditions. The TiO 2 + HKUST-1 sample displayed a negligible photoactivity, producing low amounts of CH 4 and CO, in line with the results obtained by using the single components as photocatalysts. This is probably due to: (i) the presence of bare TiO 2 NPs, which are not reactive under solar light; and (ii) the high agglomeration degree of TiO 2 NPs in TiO 2 /HKUST-1 may hinder the contact between the catalytic active sites and CO 2 target molecules. In addition, in the presence of commercial TiO 2 P25, we observed only a negligible CH 4 production.
These results support the efficacy of our synthetic approach and highlight the important role of PHA functionalization in order to guarantee a close contact between TiO 2 NPs and MOF, which delivers to TiO 2 /HKUST-1 remarkable photoreduction properties.

Proposed Photocatalytic Pathway
Given the analysis above, a possible mechanism for the enhanced photocatalytic CO 2 reduction over the synthetized TiO 2 /HKUST-1 hybrid material was proposed.
To better describe the electronic band structures of TiO 2 -PHA NPs and HKUST-1 at the hybrid interface in TiO 2 /HKUST-1, the relative energy of the conduction band (CB) and valence band (VB) versus normal hydrogen electrode (NHE) of both pristine HKUST-1 and TiO 2 -PHA were calculated, according to Schoonen et al. [70,71], by the empirical equations: where E VB and E CB are the CB and VB potentials, respectively. Moreover, E g is the band gap of the semiconductor and E e is the energy of free electrons vs. hydrogen (~4.5 eV) [72]. Finally, χ is the electronegativity of semiconductor and it was calculated by the following equation: where χ n , n, and N are the electronegativity of the constituent atom, the number of species, and the total number of atoms in the compound, respectively [73]. The superscripts r, s, p and q refer to the numbers of the atoms 1, 2, n −1 , and n, respectively, in the molecule where (r + s + . . . + p + q = N).
Although this method cannot give absolute values because the structural factors are neglected, it may provide a rough estimation of the relative energy of CB and VB versus normal hydrogen electrode (NHE).
For HKUST-1, the values of E g and χ values were 2.7 and 6.17 eV and consequently, E CB and E VB result were 3.02 and 0.32 eV while for TiO 2 -PHA E g and χ were 3.15 and 5.84 eV, and its E CB and E VB as −0.24 and 2.92 eV, respectively, versus NHE, in line with the reported experimental data [74].
With regard to TiO 2 -PHA, the presence of the electron trapping sites, associated to Ti(III) sites, as indicated by XPS analysis, was also considered. The energy levels associated to those electron trapping sites could range between 0.1 and 1 eV lower than the anatase CB [75,76].
By considering the values calculated for E CB and E VB of TiO 2 -PHA and HKUST-1, a scheme of the energy levels at the TiO 2 -PHA/HKUST-1 interface in TiO 2 /HKUST-1 can be proposed which suggests a pathway for the CO 2 photoreduction process (Figure 9). as indicated by XPS analysis, was also considered. The energy levels associated to those electron trapping sites could range between 0.1 and 1 eV lower than the anatase CB [75,76].
By considering the values calculated for ECB and EVB of TiO2-PHA and HKUST-1, a scheme of the energy levels at the TiO2-PHA/HKUST-1 interface in TiO2/HKUST-1 can be proposed which suggests a pathway for the CO2 photoreduction process (Figure 9).
It can be observed that the energy levels of Ti(III) are located at values higher than the CB of HKUST-1. Thus, upon UV-Vis irradiation of TiO2/HKUST-1, electrons photogenerated from TiO2-PHA VB can be trapped into Ti(III) centers and easily injected into HKUST-1 CB, thanks to the intimate contact between titania and HKUST-1. This seems to indicate the key role of Ti(III) defective sites of TiO2-PHA as donor of electrons [77], which can be transferred to HKUST-1 and used in the photocatalytic CO2 reduction.
Besides, photogenerated holes can migrate toward the VB of TiO2 and contribute to the water decomposition to OH• and H + [78]. The whole process highly hinders the electron-hole recombination [79,80], boosting the photoreduction activity of TiO2/HKUST-1.
Finally, considering the significant amount of copper in TiO2/HKUST-1, we cannot exclude its involvement in the photocatalytic reaction. Indeed, the beneficial effect of Cu toward the CO2 photoreduction is well-known [61,64]. In the present case, the improved TiO2 photoactivity may be connected to the simultaneous presence in TiO2/HKUST-1 of Cu(II) and Cu(I) species, as revealed by XPS analysis. According to Slamet et al. [81], Cu(II) can be easily reduced by photoexcited electrons to Cu(I), while this latter species, in the presence of H + or O2, can be re-oxidized giving Cu(II) centers and electrons. This redox cycle may also contribute to enhance the photoreduction performance of TiO2/HKUST-1 hybrid material. In order to provide a further experimental probe of the suggested mechanism, Electron Spin Resonance (EPR) investigation was tentatively performed on both TiO2/HKUST-1 hybrid material and TiO2 + HKUST-1 mixture. The results evidenced the presence of a broad and intense signal It can be observed that the energy levels of Ti(III) are located at values higher than the CB of HKUST-1. Thus, upon UV-Vis irradiation of TiO 2 /HKUST-1, electrons photogenerated from TiO 2 -PHA VB can be trapped into Ti(III) centers and easily injected into HKUST-1 CB, thanks to the intimate contact between titania and HKUST-1. This seems to indicate the key role of Ti(III) defective sites of TiO 2 -PHA as donor of electrons [77], which can be transferred to HKUST-1 and used in the photocatalytic CO 2 reduction.
Besides, photogenerated holes can migrate toward the VB of TiO 2 and contribute to the water decomposition to OH• and H + [78]. The whole process highly hinders the electron-hole recombination [79,80], boosting the photoreduction activity of TiO 2 /HKUST-1.
Finally, considering the significant amount of copper in TiO 2 /HKUST-1, we cannot exclude its involvement in the photocatalytic reaction. Indeed, the beneficial effect of Cu toward the CO 2 photoreduction is well-known [61,64]. In the present case, the improved TiO 2 photoactivity may be connected to the simultaneous presence in TiO 2 /HKUST-1 of Cu(II) and Cu(I) species, as revealed by XPS analysis. According to Slamet et al. [81], Cu(II) can be easily reduced by photoexcited electrons to Cu(I), while this latter species, in the presence of H + or O 2 , can be re-oxidized giving Cu(II) centers and electrons. This redox cycle may also contribute to enhance the photoreduction performance of TiO 2 /HKUST-1 hybrid material.
In order to provide a further experimental probe of the suggested mechanism, Electron Spin Resonance (EPR) investigation was tentatively performed on both TiO 2 /HKUST-1 hybrid material and TiO 2 + HKUST-1 mixture. The results evidenced the presence of a broad and intense signal related to Cu 2+ species, which are very abundant in HKUST-1. This feature appears almost unaffected by the irradiation and, due to its intensity and broadness, did not allow to discriminate the possible presence of other resonances (e.g., those related to Ti 3+ or O − /O 2 − species of titania).

Synthesis of Shape-Controlled TiO 2 NPs and Functionalization with PHA
In a typical experiment, TB (44 mmol, 15.0 g) was added to a mixture containing 88 mmol of OA (25.0 g) and 132 mmol (35.3 g) of OM in 25 mL of absolute EtOH. The obtained mixture was stirred for 15 min and then transferred into a 400 mL Teflon-lined stainless-steel autoclave containing 85 mL of absolute EtOH and 3.5 mL of Milli-Q water. The system was then heated at 180 • C and kept at this temperature for 18 h. After decantation, the TiO 2 powder was recovered from the autoclave, washed with EtOH several times, filtered and finally dried in vacuum (p < 10 −2 mbar) at room temperature. Then, the TiO 2 NPs were dispersed by ultrasound in EtOH solution and then stirred at room temperature for 3 days in the presence of a suitable amount of TMAH (molar ratio TMAH/TiO 2 = 25/1). After decantation, the TiO 2 powder was washed with EtOH several times, filtered and finally dried at 80 • C for 12 h (up to 87% yield). 660 mg of the washed TiO 2 NPs were dispersed by ultrasound in EtOH/water solution (4/1 v/v) and then functionalized with 540 mg of PHA. The solution was stirred for 24 h at reflux (Scheme 1, STEP 3), then the TiO 2 -PHA NPs were collected by centrifugation and the powders were washed several times with EtOH and dried in an oven at 80 • C for 12 h.

Synthesis of TiO 2 /HKUST-1 Hybrid Catalyst
In a typical reaction, a fixed amount of TiO 2 -PHA (300 mg) was firstly dispersed and sonicated for 10 min in an EtOH/H 2 O 1:1 solution. Simultaneously, two solutions with the precursors AcCu(II) and H 3 BTC have been prepared. In the first, 355 mg AcCu(II) were dissolved in 20 mL of EtOH/H 2 O 1:1 solution at 45 • C, while 222 mg of H 3 BTC were dispersed in 10 mL of EtOH/H 2 O to obtain the second solution. After that, the two solutions were mixed with the TiO 2 -PHA suspension prepared before. The suspension was stirred for 30 min at 45 • C and then the product was collected by centrifugation and the powders were re-dispersed in the EtOH/H 2 O solution. This suspension was mixed again with the AcCu(II) and H 3 BTC solutions. The process was repeated three times in order to obtain a material with a higher percentage of MOF. Finally, the TiO 2 /HKUST-1 powders obtained were washed 3 times with EtOH/H 2 O 1:1 and dried at 80 • C in an oven for one night.
Notably, the photocatalyst was carefully treated to remove possible organic contaminants. To eliminate the solvent and other organic agents from the pores of the TiO 2 /HKUST-1 hybrid materials, the powders were treated in vacuum at 150 • C for one night in a Büchi.
In order to highlight the photocatalytic performance of TiO 2 /HKUST-1, the following reference materials were also prepared: (i) pristine HKUST-1 and (ii) a composite material constituted of not-functionalized TiO 2 and HKUST-1, obtained by the same procedure described before and labeled as TiO 2 +HKUST-1.

Characterization of TiO 2 NPs, HKUST-1 and TiO 2 /HKUST-1
Analyses of the crystalline materials for phase identification were performed by PXRD. PXRD data were collected on a Rigaku Miniflex 600 diffractometer in reflectance Bragg-Brentano geometry with graphite monochromatized Cu-Kα radiation (λ = 1.5406 Å) at 600 W (40 kV, 15 mA) power. Samples were mounted on a zero-background silicon sample holder by dropping powders from a spatula and then gently leveling the sample surface with a razor blade. Samples were not ground before PXRD measurements. Scan rates were 1 • /min with 0.02 • angular steps in the 2θ range 2-65 • . Data were analysed with PDXL2 software (Rigaku, Tokyo, Japan) and Qualx2 [82].
To quantitatively assess the PHA and MOF grafting onto TiO 2 surfaces, thermogravimetric analysis (TGA) measurements were carried out. TGA thermograms were collected by a Mettler Toledo TGA/DSC1 STARe System, at a constant gas flow (50 cm 3 min −1 ). The sample powders were heated in air from 30 to 1000 • C. The thermal profile was the following: 30-150 • C at 2 • C min −1 ; dwell at 150 • C for 120 min; 150-1000 • C at 5 • C min −1 .
Morphological characterization by HRTEM and SAED of bare TiO 2 NPs and TiO 2 /HKUST-1 powders were performed on a ZEISS LIBRA200FE EFTEM. Elemental composition was evaluated by STEM-EDS analysis (Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectrometry Oxford INCA Energy TEM 200, Oxford Instruments, Abingdon-on-Thames, U.K.) of representative grains. The powders were suspended in isopropyl alcohol, sonicated and deposited onto a holey carbon film supported TEM grids. For STEM-EDS analysis holey-carbon molybdenum TEM grids were used. Samples were analyzed after overnight drying.
Low-pressure N 2 adsorption isotherm on HKUST-1 NPs were recorded by a Micromeritics ASAP2020 apparatus. The specific surface area (SSABET, BET method) was measured after evacuation of the samples at 120 • C for 12 h. A liquid N 2 bath was used for measurements at 77 K.
The CO 2 sorption was evaluated at room temperature (293 K) and controlled CO 2 pressure (p CO 2 from 5 to 920 mmHg) by Micromeritics ASAP 2020 after evacuation at 120 • C for 12 h.
DR-UV/Vis spectra of carefully ground powders were recorded in the 800-200 nm range with a UV Lambda 900 PerkinElmer spectrophotometer (PerkinElmer, Waltham, MA, USA), equipped with a diffuse reflectance accessory Praying Mantis sampling kit (Harrick Scientific Products, Pleasantville, NY, USA). A Spectralon disk was used as reference material.
The surface chemical composition of the TiO 2, TiO 2 -PHA NPs, HKUST-1 and TiO 2 /HKUST-1 powders was investigated by XPS. Analysis was performed on the as-prepared powders samples, fixing them on the sample holder using carbon tape. The XPS spectra were acquired in ultrahigh vacuum (base pressure:~4 × 10 −10 mbar) at room temperature in normal emission geometry using a conventional Mg X-ray source (hν = 1253.6 eV) and a hemispherical electron energy analyzer (total energy resolution~0.8 eV). Due to charging effects, all BEs are calibrated by fixing the C 1s BE of atmospheric contamination at 284.6 eV [83]. The standard deviation for the BEs values was~±0.2 eV. Survey scans were obtained in the 0-1100 eV range. Detailed scans were recorded for the O 1s, Ti 2p, and Cu 2p regions. To individuate all the possible differences between the samples, XPS spectra were reproduced by fitting the experimental data using a Shirley background and several Doniach-Sunjich components [84], corresponding to different oxidation states and chemical environments [85]. The fitting parameters have been fixed following Kaushik [86] and taking into account the energy resolution used in the measurements (~0.8 eV).
The EPR investigation was performed by a Bruker EMX spectrometer operating at the X-band frequency and equipped with an Oxford cryostat. The spectra of TiO 2 /HKUST-1 hybrid material and TiO 2 + HKUST-1 mixture were carried out at 130 • C in vacuum conditions (p < 10 −5 mbar), before and after UV-Vis irradiation, directly inside the EPR cavity.

Photocatalytic CO 2 Reduction
The photocatalytic CO 2 reduction was carried out in a batch cylindrical gas-solid reactor (V = 120 mL) containing 0.3 g of powder distributed as a thin layer. The system was irradiated from the top with a solar light simulating lamp (1500 W high pressure Xe lamp) inside a SOLARBOX (CO.FO.ME.GRA.). The reaction temperature was 60 • C.
The possible presence of products deriving from C impurities was checked by means of the standardized procedure reported by Mei et al. [69]. In the first step, after prolonged purging with water-saturated He, the photocatalyst was irradiated for about 2 h in contact with Helium in water vapor, (i.e., in the absence of CO 2 ). Then the system was completely purged again with Helium, to remove all carbon-containing species in the gas phase. The cleaning procedure was repeated until carbon contaminations were removed from the sample surface. After this so-called cleaning step, the photocatalytic tests were performed. In detail, the system was saturated with wet CO 2 . 500 µL of the gaseous mixture were withdrawn from the reactor for analyses at fixed irradiation times by using a gas-tight microsyringe. The evolution of CH 4 and CO was followed by a HP 6890 Series GC equipped with a packed column GC 60/80 Carboxen-1000 and a TCD detector, whilst the concentration of the organic species was measured by a GC-2010 Shimadzu gas chromatograph equipped with a Phenomenex Zebron Wax-plus column (30 m × 0.32 µm × 0.53 µm) and a flame ionization detector, using He as the carrier gas. Each photocatalyst was tested three times, to check the reproducibility of the photocatalytic runs, in terms of CO 2 (in the cleaning step), CH 4 and CO production.

Conclusions
In summary, a novel hybrid TiO 2 /HKUST-1 photocatalyst was successfully obtained via a slow-diffusion method combined with step-by-step self-assembly approach at room temperature, starting from TiO 2 NPs functionalized with an asymmetric organic linker. Our synthetic approach promotes the crystalline growth of MOF layers on titania surface and favors an intimate contact between HKUST-1 and TiO 2 -PHA.
The close contact between two hybrid components is a key point for expressing the remarkable performance of the hybrid material. In fact, this allows not only a high CO 2 uptake into the porous MOF structure but also, under solar light irradiation, favors an improved photoreduction activity of TiO 2 /HKUST-1 compared to that of pure TiO 2 -PHA and HKUST-1 samples.
The remarkable photoreduction ability of the material was related to the improved visible light absorption and to an effective electron injection from TiO 2 -PHA to HKUST-1 involving both photogenerated electrons and those trapped in Ti(III) centers of TiO 2 -PHA. This indicates that in the hybrid TiO 2 /HKUST-1 photocatalyst, also the presence of electrons deriving from a redox active role of TiO 2 -PHA can be exploited in the photocatalytic reduction of CO 2 . Moreover, the performance of the material was confirmed by subsequent photocatalytic runs carried out with the same sample.
We expect that our synthetic approach will enable the possibility to fabricate a wider range of hybrid porous photocatalytic materials, namely TiO 2 /MOF, with a designable MOF shell, suitable for a variety of energy and environmental applications. Particularly, the structure, composition, and function of the MOF shell could also be judiciously tailored by choosing different framework building blocks, i.e., metal ion and polyfunctional organic likers.