Self-Sensitized Photocatalytic Degradation of Colorless Organic

Article pubs.acs.org/JPCC

Self-Sensitized Photocatalytic Degradation of Colorless Organic Pollutants Attached to Rutile NanorodsExperimental and Theoretical DFT+D Studies Kezhen Qi,† Filip Zasada,*,† Witold Piskorz,† Paulina Indyka,† Joanna Gryboś,† Mateusz Trochowski,† Marta Buchalska,† Marcin Kobielusz,† Wojciech Macyk,*,† and Zbigniew Sojka† †

Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland S Supporting Information *

ABSTRACT: Utilization of visible light by photosensitization of semiconductor photocatalysts via surface attachment of small colorless organic pollutants (COP) is an effective way to stimulate their photocatalytic degradation. Herein, by means of the spectroscopic, photoelectrochemical, spectroelectrochemical, and photocatalytic studies combined with the DFT+D molecular modeling, we show how disubstituted benzene derivatives, like catechol (CAT), salicylic acid (SAL), phthalic acid (PTA), and terephthalic acid (TPA), can tune the photocatalytic properties of rutile nanorods with the dominant (110) termination. We elucidated in a systematic way the COP ligand-binding configurations, the alignment of energy levels, and the charge-transfer pathways from the organic admolecules to the titania substrate. The pDOS structures of the COP@r-TiO2(110) assemblies were interpreted in terms of electronic interactions between the titania photocatalyst and the COP adspecies. It was shown that the appearance of additional states within the band gap and in the conduction band allows for a one-step HOMO → CB ligand to metal charge transfer and a twostep HOMO → LUMO → CB sensitization. Screening of the photocatalytic performance of the COP@r-TiO2 samples revealed that the self-degradation efficiency gauged by the initial rate constant varies in the following order: catechol (−OH, −OH; 0.024 min−1) > salicylate (−OH, −COOH; 0.013 min−1) > phthalates (−COOH, −COOH; 0.005 and 0.004 min−1 for PTA and TPA, respectively), showing a beneficial role of hydroxyl functionalities at an early stage of degradation process. It was found that the higher activity of the OH-bearing catechol and salicylate adspecies was associated with the direct HOMO → CB electron-transfer pathway operating in the visible light. The two-step HOMO → LUMO → CB mechanism (requiring UV light) characteristic of carboxyl-bearing functionalities, despite favorable energy level alignment and coupling, is less efficient due to low density of the electronic states at the top of the conduction band, and low flux of the solar radiation in that energy region. The in situ diffuse reflectance spectroscopic (DRS) measurements revealed that at early stages of the photocatalytic degradation the aromatic rings of the COP moieties are readily photohydroxylated, fostering the visible light utilization via the HOMO → CB electron transfer route. Such latent autocatalytic hydroxylation processes are relevant for photocatalytic degradation of those pollutants that originally do not exhibit hydroxyl functionalities provided that a photogenerated hole is localized at the organic moiety.

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INTRODUCTION

photocatalysts. Band gap modifications, resulting from either tuning the valence and conduction edge levels or by introducing the empty or occupied states within the band gap, are of a primary importance in chemical systems that utilize light for various photocatalytic purposes. Another astute approach involves fabrication of appropriate heterostructures that play a crucial role in the titania photocatalyst amelioration, as it has been recently reviewed.14 The resultant variants of band alignment at the interface include a type I scheme, where the band energy levels of one material straddle those of the other resulting in the transfer of both holes and electrons to the

Wide-band-gap semiconductors, such as titania, are promising materials for a wide range of applications in environmental photocatalysis1−4 and energy conversion.5,6 Their importance results from the unique electronic properties that depend mostly on the favorable energies of the band gap edges. Yet, the photoactivity of TiO2 is limited by its rather large intrinsic band gap of ∼ 3.0 eV, making this oxide capable of absorbing the ultraviolet light only. This fact lowers the efficiency of the solar light harvesting and disfavors photocatalytic degradation of pollutants such as volatile organic compounds (VOC) or watersoluble organic compounds (WSOC). Band gap engineering and appropriate sensitization by bulk doping7−9 and surface functionalization10−13 are then the key issues for more effective application of various titania polymorphs as the proficient © XXXX American Chemical Society

Received: November 9, 2015 Revised: February 15, 2016

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DOI: 10.1021/acs.jpcc.5b10983 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C narrower band gap material. In the type II heterostructures, the band energy levels are staggered (or broken−type III), making the charge separation energetically more favorable.15 Functionalization of semiconductor surfaces with organic dyes has attracted much attention because the ensuing hybrid interfaces allow us to control more effectively not only the light harvesting but also the electron injection from the photoexcited dye LUMO state to the semiconductor conduction band and the charge recombination as well.16,17 Despite the undeniable progress made in this field recently, understanding the detailed mechanism of those processes at the molecular level is still a subject of extensive experimental and theoretical investigations.18 Typically, the efforts toward improving the sunlightinduced activity are focused at increasing dye extinction coefficient, shifting its absorption toward longer wavelengths to improve the compatibility with the solar light and a better energetic matching between the electron donor and acceptor levels. In contrast to organic dyes, which are intentionally designed to exhibit good light-harvesting properties due to their high extinction coefficient, small aromatic pollutants are usually colorless. Their efficient photodegradation depends essentially on a direct excitation of the photocatalyst (UV-light) or a selfsensitization effect,19 made possible even in the visible region by favorable band alignment and appropriate frontier orbital coupling to the conduction band states of the photocatalyst.20 It has been argued that formation of surface charge-transfer complexes plays a beneficial role in pushing the photocatalytic degradation of colorless organic pollutants (COP) toward the visible region.21 However, until now, this issue has been rather scarcely studied. Among various wide-band-gap semiconductor materials, rutile TiO2 (r-TiO2) is a common substrate for the design of tailored heterointerfaces.22−24 The main exposed (110) facet of the r-TiO2 crystals is not only very stable but also an excellent template with protruding dangling bonds apt for interacting efficiently with organic compounds such as alcohols (e.g., methanol),25 phenols (e.g., 4-chlorophenol),26 or aromatic diols (e.g., catechol).27,28 These organic molecules with different functional groups were used to exploit the site-specific adsorption chemistry of the r-TiO2(110) surface.29 The density functional theory (DFT) modeling has been widely applied to explore the interactions between organic admolecules and oxide surfaces.30−32 For instance, Selloni et al. studied self-organization of catechol on rutile (110)29 and anatase (101)26,33 using DFT calculations supported by STM imaging. They found that the most energetically favorable bonding takes place in a bridging fashion involving two titanium atoms. DFT calculations with inclusion of dispersion forces revealed, in turn, the key importance of the van der Waals interactions for ordering aromatic admolecules on the rutile surface.34,35 Calzolari et al. have used DFT calculations to study sensitization of the ZnO (10−10) surface by adsorbed catechol and found that the alignment of energy levels originates from the simultaneous interplay between the conjugation of the adsorbed molecule and the electron donor/acceptor capability of the specific anchoring groups of the adsorbed molecule.36 Thomas et al. have investigated by means of DFT modeling p-aminobenzoic acid (pABA) interaction with the anatase-TiO2(101) surface. The observed red shift after pABA adsorption has been attributed to the presence of the highest occupied molecular orbitals within the TiO2 band gap region.37

The interaction between small benzene derivatives and the titania surface depends not only on the structure of the exposed facets but also on the chemical nature of the functional groups acting as anchoring moieties. Among them, hydroxyl (−OH) and carboxyl (−COOH) groups belong to the most common anchors.38 Herein, we combine photocurrent, spectroelectrochemical, and photodegradation measurements with DFT modeling to study the self-sensitization of r-TiO2 nanorods with selected simple COP benzene derivatives, such as catechol, salicylic acid, phthalic acid, and terephthalic acid. These molecules contain hydroxyl and carboxyl functionalities in different positions of the aromatic ring, which are commonly formed during oxidation of aromatic pollutants. The goal was to elucidate the impact of the anchoring diversity imposed by the −COOH and −OH groups on the electronic structure modification of the rutile nanorod photocatalyst, alignment of energy levels, and the photodegradation pathways of these prototypical colorless pollutants.

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EXPERIMENTAL SECTION Sample Preparation and Characterization. Synthesis. For synthesis of rutile nanorods, 0.2 mL butyl titanate (TBOT) was dropped into 16 mL of hydrochloric acid solution (composed of 4 mL of 36.5% HCl, 12 mL of water, and 2 mg of cetyltrimethylammonium bromide). Then the mixture was transferred to a 20 mL Teflon-lined steel autoclave and heated at 180 °C for 24 h. Then the autoclave was cooled to room temperature. The resulting product was separated by centrifugation, washed several times with absolute ethanol and distilled water, and finally dried at 60 °C in air for 5 h. The synthesized material was additionally calcined at 450 °C for 3 h to produce a well-crystalline r-TiO2 material. The surface modification of the rutile samples with organic molecules was achieved by impregnation of r-TiO2 (50 mg) with 5 mL of the 0.1 mol·dm−3 of COP solution in methanol. Catechol (CAT), salicylic acid (SAL), phthalic acid (PTA), and terephthalic acid (TPA) were used as exemplary COP molecules for surface modification. The suspension of TiO2 in the COP solution was treated with ultrasound for 5 min and left overnight. Then the powders were centrifuged, washed 3 times with water, and dried in the oven at 50 °C. Characterization of Materials. The phase composition of the samples was verified by X-ray diffraction (XRD) (Rigaku D/max 2500 V/PC, Cu Kα radiation, λ = 1.5406 Å). UV−vis diffuse reflectance spectra of the synthesized materials were recorded using PerkinElmer Lambda 12 spectrophotometer equipped with an integrating sphere of 5 cm in diameter. Raman spectra were recorded using a Renishaw, in Via instrument, operating with the 785 nm excitation laser source. Transmission electron microscopy (TEM) imaging was carried out by means of a Tecnai Osiris microscope (FEI) operating at 200 kV. Prior to TEM analysis the samples were ultrasonically dispersed in methanol on a holey carbon film supported on a copper grid (400 mesh). The grid was dried for 45 min, and then surface contaminations were removed by plasma-cleaning (Solarus Gatan 950). The simulations of TEM images were carried out using JEMS simulation package.39 According to the experimental conditions of imaging, the Cs and Cc coefficients, equal to 1.2 μm, were used. Two perpendicular families of diffraction spots, located on the diffraction pattern 3.26 nm−1 and 3.02 nm−1 away from the central spot, were assigned to the (001) and (01̅1) planes, respectively, revealing a [110] orientation of the specimen. In order to determine the values B


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The Journal of Physical Chemistry C

the band gap, we employed the DFT+U approach46 with the Hubbard parameter set to U = 3.0 eV for all Ti ions. For modeling the interaction between the hydrocarbon admolecules and the oxide substrate, a semiempirical dispersion term, parametrized by Grimme,47 was added to the quantummechanical energies and gradients (DFT+D). A number of tests of the TiO2 bulk properties were initially performed to verify the accuracy of the applied calculation scheme (variation of the cutoff energy and the k-point set). The standard Monkhorst−Pack48 grid (4 × 4 × 3 sampling mesh for bulk calculations and 3 × 3 × 2 for slab calculations) with the cutoff energy of 450 eV and the Methfessel−Paxton49 smearing parameter σ = 0.1 eV were used. For solving the Kohn−Sham equations, the SCF convergence criterion was set to energy change of 10−5 eV between two successive iterations. Geometry optimization was performed until the changes in the forces acting upon the ions were smaller than 0.001 eV/Å per atom. Bulk titania unit cell was obtained by optimization of the experimental tetragonal (D4h14-P42/mnm) rutile (2 × 2 × 4) unit cell (a = b = 9.16 Å, and c = 11.80 Å) containing 96 ions (Ti32O64). The optimal cell volume was calculated from the E/ V fit (Birch−Murnaghan equation of state50) with full optimization of all internal degrees of freedom (with an error SAL > PTA > TPA, and is well correlated with the EHOMO(COP) − EF(TiO2) energetic distance (SI, Figure S6). It shows that an extensive charge rearrangement accompanies the COP-titania surface interaction. The extent of the charge transfer is essentially controlled by the nature of the binding group, and is favored by the hydroxyl functionalities. The resultant surface dipole is the highest for PTA, since the angular top-on configuration moves the carboxylic oxygen atoms not involved in the binding back F


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molecular orbitals with a contribution to DOS from the organic admolecule larger than 3%. These are states located around −3 eV (b1), −1 eV (b3), and 1 eV (b4). Both frontier orbitals of the CAT molecule were shifted upward in an uneven way (HOMO by 0.3 eV and LUMO by 0.6 eV). Because in the bound state these orbitals formally are no longer frontier orbitals of the COP molecule, in order to emphasize their derivation they are labeled “HOMO” and “LUMO” hereafter. Moreover, in the middle of the conduction band, a new state with an appreciable contribution of the catechol ligand appeared. The electron density contours corresponding to the “HOMO” (b1) and “LUMO” (b4) orbitals of the bare CAT are still strongly localized on the ligand and are very similar to their parent shape at distal arrangement (Figure 3a). The electron density contours at the very bottom of the conduction band consists essentially of titanium 3d states of the t2g manifold (b2), whereas the top of the valence band is dominated by the catechol “HOMO−1”. A strong π-type bonding between the CAT admolecule and the substrate is clearly visible by accumulation of the charge density between the catechol oxygen and the surface titanium atoms with the resultant bond order equal to 0.57. The calculated sizable charge transfer between the admolecule and the substrate is equal to 0.50 |e|. It is responsible for the appearance of a marked absorption band the in the UV−vis spectra at 450 nm (vide inf ra). On the other hand, the “LUMO” states are significantly coupled to the continuum of the conduction band d-states, which shall favor electron injection form the photoexcited COP admolecule to deep of the rutile substrate. However, a low density of titania acceptor states in this region is rather inauspicious for the efficient interfacial electron transfer (vide inf ra). SAL@r-TiO2. In the case of the r-TiO2(110) surface functionalized with salicylic acid, the type II energy-level alignment (Figure 4a and 4b) is quite similar to that of the catechol system. In a distal configuration, the strongly localized highest occupied band (a1 in Figure 4a), distanced from the CB minimum by 0.25 eV, has mainly the HOMO character of the parent salicylic acid (cf. Figure S6b, SI). The band corresponding to the HOMO−1 orbital of the admolecule is

reported value of 1.57 eV obtained with cluster model of the (110) rutile termination.67 The band edge energy levels of the rutile support and the HOMO−LUMO energy levels of the COP admolecules for all investigated samples are collected in Table 3. Table 3. Band Edge Energy Levels (VBM and CBM) for Bare and COP Covered (110) Surface of r-TiO2, and the Frontier Energy Levels of COP Admolecules (COPHOMO and COPLUMO)a

a

eV

bare TiO2

CAT@ TiO2

SAL@ TiO2

PTA@ TiO2

TPA@ TiO2

VBM CBM EFermi COPHOMO COPLUMO

−3.17 −2.01 −2.85 − −

−4.17 −3.31 −3.36 −3.43 0.92

−4.38 −3.46 −3.82 −3.99 −0.42

−4.36 −3.48 −4.26 −4.46 0.41

−4.76 −3.63 −4.38 −4.45 0.18

All values correspond to the Γ point.

CAT@r-TiO2. In Figure 3a and 3b, the partial density of states (pDOS) diagrams are presented, showing alignment of the adsorbed catechol energy levels with respect to the band structure of the TiO2 (110) surface for distal and proximal arrangements, respectively. In both diagrams, the green and the red lines denote the density of states for the rutile oxygen and titanium ions, respectively, whereas the navy blue contours indicate the pDOS contribution of the catechol admolecule. The gradient gray bars epitomize the width of the conduction and valence bands of the TiO2 substrate. The positioning of the HOMO/LUMO energy levels of the catechol molecule with respect to the edges of the rutile surface conduction and the valence bands before attachment (Figure 3a), reveals a staggered (type II) band alignment. The HOMO level (a1) is situated in the band gap just below CB (0.1 eV), the HOMO− 1 at the top of VB, whereas the LUMO (a2) is placed ∼0.4 eV above the conduction band. Upon attachment, the type II alignment is essentially preserved. Considering the energetic window of interest defined by the band gap and the conduction band regions, following the literature, we report only the

Figure 3. Partial DOS of the r-TiO2(110) surface slab model calculated for the catechol molecule at distal (a) and proximal (b) arrangements showing the corresponding energy alignments. (a1) and (a2) are the charge density contours of the HOMO and LUMO orbitals of the catechol molecule in the distal configuration, whereas (b1−b4) refer to the charge density contours corresponding to the most important states of the proximal arrangement: (b1) “HOMO” of the admolecule, (b2) bottom of the titania CB, (b3) a CB state with an appreciable (>3%) contribution of catechol, (b4) “LUMO” of the admolecule. On the DOS plots, the green and red lines refer to titanium and oxygen of TiO2 states, respectively, whereas the blue color denotes the molecule contribution. Partial charge density contours are color coded to reveal if they are occupied (blue) or empty (yellow). G


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Figure 4. Partial DOS of the r-TiO2(110) surface slab model calculated for the salicylic acid molecule at distal (a) and proximal (b) arrangements showing the corresponding energy alignments. (a1) and (a2) are the charge density contours of the HOMO and LUMO orbitals of the salicylic acid molecule in the distal configuration, whereas (b1−b4) refer to the charge density contours corresponding to the most important states of the proximal arrangement: (b1) “HOMO” of the admolecule, (b2) bottom of the titania CB, (b3) “LUMO” of the admolecule. On the DOS plots, the green and red lines refer to titanium and oxygen of TiO2 states, respectively, whereas the blue color denotes the molecule contribution. Partial charge density contours are color coded to reveal if they are occupied (blue) or empty (yellow).

Figure 5. Partial DOS of the r-TiO2(110) surface slab model calculated for the phthalic acid molecule at distal (a) and proximal (b) arrangements showing the corresponding energy alignments. (a1) and (a2) are the charge density contours of the HOMO and LUMO orbitals of the phthalic acid molecule in the distal configuration, whereas (b1−b4) refer to the charge density contours corresponding to the most important states of the proximal arrangement: (b1) “HOMO” of the admolecule, (b2) bottom of the titania CB, (b3) a CB state with an appreciable (>3%) contribution of phthalic acid, (b4) “LUMO” of the admolecule. On the DOS plots, the green and red lines refer to titanium and oxygen of TiO2 states, respectively, whereas the blue color denotes the molecule contribution. Partial charge density contours are color coded to reveal if they are occupied (blue) or empty (yellow).

the oxidized dye can be expected, in comparison to deeply localized bulk states.69 On the other hand, localization of the “HOMO” orbital in the band gap, just below the CB edge, facilitates significant charge transfer between the SAL admolecule and the rutile substrate, revealed by the appearance of a sizable light absorption in the visible region (Figure 8, vide inf ra). The calculated partial charge on the SAL adspecies equal to 0.37 |e| and the total bond order of 0.54 are in line with such significant charge transfer. PTA@r-TiO2. The electronic structure of the phthalic acid adsorbed on the r-TiO2(110) in a distal setting with HOMO (a1) and LUMO (a2) embracing the titania conduction band (Figure 5a) resembles those previously found for the CAT or SAL molecules. Yet, in contrast to CAT and SAL, upon the ligation the HOMO state (b1) is significantly stabilized (by 0.9 eV) and placed on the top of VB (0.05 eV below the edge), whereas there are no PTA related states at the bottom of CB

positioned 0.2 eV above the VB edge, whereas the LUMO state (a2) is situated on the top of the conduction band. Binding of the SAL molecule moves the “HOMO” level (b1) by 0.2 eV below the CB minimum (Figure 4b) and shifts the LUMO orbital (b3) down by 0.15 eV. As expected, the bottom of the CB is composed mainly of Ti d states (b2), whereas due to stabilization of the HOMO−1 state upon ligation, the VB maximum is dominated by the oxygen 2p states of rutile. The mixing of “LUMO” and “HOMO” with the substrate states is rather small, implying a weak coupling. Although it hinders undesirable charge recombination with a hole center localized on “HOMO”, such situation is less favorable for photoelectron injection via “LUMO” channel, suggesting a nonadiabatic mechanism of the interfacial electron transfer. Following de Angelis,68 it can be therefore conceived that the photoelectron injection should correspond to surface CB states localized close to “LUMO”, for which a faster undesirable recombination with H


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Figure 6. Partial DOS of the r-TiO2(110) surface slab model calculated for the terephthalic acid molecule at distal (a) and proximal (b) arrangements showing the corresponding energy alignments. (a1) and (a2) are the charge density contours of the HOMO and LUMO orbitals of the phthalic acid molecule in the distal configuration, whereas (b1−b4) refer to the charge density contours corresponding to the most important states of the proximal arrangement: (b1) “HOMO” of the admolecule, (b2) bottom of the titania CB, (b3) a CB state with an appreciable (>3%) contribution of terephthalic acid, (b4) “LUMO” of the admolecule. On the DOS plots, the green and red lines refer to titanium and oxygen of TiO2 states, respectively, whereas the blue color denotes the molecule contribution. Partial charge density contours are color coded to reveal if they are occupied (blue) or empty (yellow).

anchoring groups of the COP admolecules has more influences on the relative position of the “HOMO” than on the “LUMO” states, with respect to the CB and VB edges. The “HOMO” position can be systematically shifted from the CB to VB edge through replacement of the −OH anchors by the −COOH ones. Charge Transfer. The calculated DOS structures for the COP@r-TiO2 photocatalysts can be used to assess the efficiency of possible electron transfer along the A and B pathways. In order to favor a fast injection of the photoelectron along the two-step pathway A, the COP donor level should be well-immersed in the semiconductor manifold of the unoccupied CB acceptor states, preferably in the region of the high density of states.70,71 Following the literature, two properties related to the molecular structure of the COP admolecules, namely, the electron injection energy, ELUMO, and the percentage of “LUMO” localized on the anchoring moiety, are crucial.67,68 For all investigated molecules the high “LUMO”-CB gap is adequate for a pronounced thermodynamic driving force for the fast electron transfer. The dependence of the injection efficiency on the “LUMO” energy with respect to CB can be accounted for within the Newns−Anderson model.52,53 The center of the projected rDOS distribution corresponds to the energy of the effective “LUMO” of the COP adspecies, which epitomizes the excited electron injection state, was evaluated as the position of the fitted Lorentzian peak (Figure 7). Within this model, the “LUMO” broadening, Γ, gives an estimation of the electron transfer: τ = 1/Γ. The calculated values, equal to 2.0, 2.7, 2.9, and 3.6 fs and the peak positions with respect to EF are 4.67, 4.86, 4.48, and 4.35 eV, for CAT, PTA, TPA, and SAL adspecies, respectively, show that all COP molecules exhibit rather similar rates of the electron transfer. The only remarkable difference is a more favorable immersion of the PTA “LUMO” in the conduction band of the rutile nanorods in comparison to the other COP admolecules. It may be thus expected that for all investigated COP molecules in their adsorbed state the “LUMO” energetic position with respect to the CB manifold is favorable for a fast photoelectron injection. There is also no substantial influence of the anchoring groups (−OH, −COOH) on the electron

(b2). An extended pDOS features that can be traced back to “LUMO” (b4) appeared in the upper part of the conduction band, together with localized state with a sizable PTA contribution (b3) placed lower by 0.45 eV with respect to b4. As a result, there are no new localized states in the band gap. The charge transfer and the total bond order of the PTA-TiO2 assembly are equal to 0.31 |e| and 0.41, respectively. TPA@r-TiO2. The DOS structure of the r-TiO2 covered by the plane-on adsorbed TPA molecules in distal placement is presented in Figure 6a. The HOMO state is located on the top of VB (a1), and the LUMO above the middle of the conduction band (a2). There are also the same localized states with TPA contribution at the upper part of CB. The binding of TPA does not influence the position of “HOMO” appreciably (Figure 6b), and its shape reveals rather weak coupling with the titania VB orbitals (b1). Together with the titanium t2g states (b2), they constitute the VB and CB edges. The TPA related “LUMO” state, b4, is shifted to the upper part of the conduction band. There are also empty states with significant contribution of TPA, located in the upper part of the CB (exemplified by b3). The electron density repartition shows that they are quite well coupled with the corresponding acceptor levels of the conduction band. The side-on attachment results in a considerably smaller charge transfer (0.20 |e|) as a result of the multiplicity of the TPA-TiO2 bonds in the largest bond order (0.63), in comparison to other examined COP molecules (Table 3). In summary, the results of electronic structure analysis showed that the “HOMO” levels are shifted to lower energies with the increasing −COOH to −OH ratio, changing thereby their positions dramatically from the level close to the CB minimum to the top of VB. The surface band gap narrows in the opposite direction, from 1.13 eV for TPA@r-TiO2 (nearly the same as in bare TiO2) to 0.78 eV for CAT@r-TiO2. This accounts quite well for the observed enhancement of the light absorption of the visible region (vide inf ra). The difference between the energies of the “HOMO” and the CB levels is lower for CAT@r-TiO2 than for SAL@r-TiO2, and it implies a bathochromic shift of the absorption onset observed with the decreasing −COOH to −OH ratio. The nature of the I


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Figure 8. Diffuse reflectance spectra of bare and modified rutile nanorods together with the picture of the samples suspended in water (inset).

role in the photosensitization of the rutile nanorods in the visible region. The highest effect was observed in the presence of two −OH groups (CAT), a medium one for the mixed −COOH/−OH case (SAL), whereas for dicarboxylic molecules (PTA and TPA), no effect was observed essentially. Spectroelectrochemical Measurements. Determination of the redox properties of the COP@r-TiO2 photocatalysts was performed according to the method developed recently.74 The platinum electrode covered with the tested material was subjected to a slow potential sweep (0.5 mV·s−1). The simultaneously measured changes of the reflectance signal at 780 nm, transformed into the Kubelka−Munk (K−M) function, enabled determination of the potential at which TiO2 was reduced (Figure 9). In this way, it is possible to probe

Figure 7. Lorentzian fitting of the pDOS corresponding to the approximated “LUMO” states of the COP admolecules, showing their position with respect to schematically outlined DOS profile of the COP@TiO2 assembly.

transfer rate. Furthermore, for all the COP species the VB is dominated by the “HOMO” orbital of the admolecule, which is beneficial for charge separation. As a result, we can expect some UV sensitization effect along the two-step pathway A, but its overall efficiency should be reduced by low intensity of the solar light in the energy window above 3 eV, where this process is expected to take place. However, formation of the new donor states in the band gap associated with the “HOMO” level in the case of the CAT and SAL admolecules (Figure 3 and 4) opens the route for a parallel visible light sensitization via a direct injection of the photoelectron according to ligand (“HOMO” of COP) to metal (CB of TiO2) electron transfer (one step pathway B). The corresponding absorption bands were indeed observed in UV−vis spectra (vide inf ra). Inspection of Figure 3b shows that for CAT species, a unique localization of “HOMO” just below the CB and HOMO−1 above the VB levels are especially beneficial for this photosensitization channel. UV−vis Absorption. In order to study the light absorption of the functionalized r-TiO2 nanorods, the UV−vis spectra were measured (Figure 8). The bare nanorods show a significant absorption of UV light (λ < 400 nm), due to a direct VB to CB excitation of TiO2. The CAT@r-TiO2 sample shows the highest absorption of the visible light, extending from 400 nm to ca. 600 nm, and the dark orange color of this sample is the most intense (see inset in Figure 8). Absorption of visible light by the SAL@r-TiO2 sample (light yellow) is less pronounced as the tail of the absorption extends to ca. 550 nm. All other samples remained practically white upon the functionalization. These results are in agreement with the calculated DOS structure and with those reported previously.72,73 A brief inspection of the Figure 8 reveals that the −COOH/−OH ratio plays a crucial

Figure 9. Normalized spectral changes at 780 nm (represented by changes of the Kubelka−Munk function) recorded upon the potential sweep from −0.6 to −1.5 V vs Ag/AgCl electrode.

the density of states of the photocatalyst, and the higher it is, the steeper slope of the K−M curve is observed. An increase of the light absorption below −1.0 V results from the reduction of the energy states localized at the bottom of the conduction band. They can essentially be identified with the position of the b2 states in Figures 3−6, which are crucial for the driving the dioxygen reduction. For SAL@r-TiO2 and PTA@r-TiO2, the edge of the conduction band is shifted toward lower energies, implying the lower efficiency of O2•− formation.75 CAT@rJ


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Figure 10. Photocurrent as a function of the photoelectrode potential (vs Ag/AgCl) and incident light wavelength recorded for photoelectrodes covered with (a) bare r-TiO2, (b) CAT@r-TiO2, (c) SAL@r-TiO2, (d) PTA@r-TiO2, and (e) TPA@r-TiO2. The red and blue areas correspond to the anodic and cathodic photocurrents, respectively, whereas the white areas represent a zero net photocurrent.

in the case of TPA@r-TiO2 is quite small (compare Figures 2 and 9). Photocatalytic Tests. Photocatalytic degradation of colorless organic pollutants was tested by monitoring the total organic carbon (TOC) content in the COP@TiO2 assembly irradiated with UV−vis light (full light of XBO lamp). The initial decomposition rate constants were 0.024, 0.013, 0.005, and 0.004 min−1 for CAT, SAL, PTA, and TPA, respectively, as derived from the degradation curves shown in Figure 11. Two classes of the pollutant molecules can be distinguished here, depending on the photodegradation rate constant. Molecules containing hydroxyl anchoring group (CAT, SAL) degrade much faster than those with the carboxyl attachment only

TiO2 and TPA@r-TiO2 samples exhibit the band edge position close to that of r-TiO2, making the photoelectron transfer faster. Yet, these changes are not very significant in the case of rutile. The appearance of the shoulder for CAT@r-TiO2 at −1.2 V corresponds to an empty acceptor state separated from the conduction band by strong interaction with catechol ligand. Photocurrent Measurements. The UV−vis-induced activity of the samples was examined by photoelectrochemical measurements. The results shown in Figures 10a−e reveal significant differences among the investigated samples in the generated photocurrent as the function of the applied potential and the wavelength of the incident light. A pronounced photocurrent induced by the ultraviolet light is observed for all materials. In the case of r-TiO2 modified with organic molecules a cathodic to anodic photocurrent switching appears at various potential range. Photocurrents generated by the incident visible light are higher for the electrodes covered with modified r-TiO2 compared to that of bare r-TiO2. The most pronounced sensitization effect was observed for CAT@r-TiO2 (Figure 10b); however, the visible-light-induced photoactivity of SAL@r-TiO2 (Figure 10c) is also significant when compared to its photoactivity induced by light of 400 nm. This observation remains in a good agreement with the results of the DFT calculations. Because the “HOMO” orbitals of the catechol and salicylic acids are situated within the bandgap of r-TiO2, these modifiers act as the photosensitizers of r-TiO2. The photocurrent map recorded for TPA@r-TiO2 material (Figure 10e) resembles that of r-TiO2. Although the “HOMO” of TPA is also localized within the bandgap, the experimental coverage of this modifier at the (110) facet is significantly lower than the adsorption of other molecules (vide supra), and therefore, the influence of terephthalic acid on photoelectrochemical properties of r-TiO2 is less significant. Lower values of measured photocurrents are very likely the consequence of a higher hydrophobicity of TPA@r-TiO2 material, which stems from the unique plane-on adsorption. In this mode, both polar groups are anchored with the surface, whereas the aromatic moiety is exposed to the solution. Also the charge density flow between the organic molecule and the r-TiO2 crystal (ΔqB = 0.20 |e|) is less efficient than in the case of other molecules, and therefore, the photosensitization effect

Figure 11. Self-decomposition of COP substrates: relative changes of total organic carbon content as the function of irradiation time (irradiation: full light of xenon lamp). K


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formed, similar to that of catechol that was found to be beneficial for the visible photosensitization of the system. Upon the prolonged irradiation, the photodegradation rates become comparable (Figure 11), because the sensitization effect ceases due to gradual deterioration of the COP admolecules into smaller fragments absorbing UV only and finally to CO2 and H2O end-products. To the best of our knowledge, the nature of such photocatalytic self-degradation in the case of the colorless organic compounds has not been elucidated until now using jointly experimental and theoretical approaches.

(PTA, TPA). These results show also that the photosensitization of the rutile nanorod catalyst via the direct pathway B is much more efficient than the two step process that involves the pathway A. It should be noted that the influence of the sensitization on the photocatalytic decomposition rate is most pronounced at the beginning (early stage) of the degradation process when all pollutant molecules are not fragmented significantly yet and thus can act as the selfphotosensitizers. In order to elucidate the fate of the COP adspecies upon irradiation, we investigated photoinduced chemical changes in the SAL@r-TiO2 sample (taken as an example) by in situ DRS measurements. The resultant differential spectra are presented in Figure 12. As a reference, we added the differential spectra of

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CONCLUSIONS By combining several photoelectrochemical and spectroelectrochemical techniques with the DFT molecular modeling we probed the electronic nature of self-sensitization of rutile nanorods by small colorless organic pollutants, providing a comprehensive background for mechanistic understanding of their photodegradation. We selected disubstituted benzene derivatives with −OH and −COOH groups (i.e., catechol, salicylic acid, phthalic acid, and terephthalic acid) as model colorless pollutants. The role of the anchoring groups (−OH and −COOH) was explained in terms of one-step (HOMO → CB) and two-step (HOMO → LUMO → CB) photosensitization mechanisms. It was shown that the attachment by hydroxyl groups triggers the one-step pathway via a direct ligand to metal charge transfer (CAT, SAL). The presence of carboxyl anchors, in turn, favors the two step sensitization involving intramolecular excitation followed by the photoelectron injection from LUMO to CB. The carboxyl anchors locate the LUMO state at the top of the conduction band making the two step pathway less efficient due to the low overlap with the solar spectrum (far UV), despite the fast electron transfer (2−4 fs). The efficiency of self-sensitized degradation of disubstituted benzene derivatives decreases in the order: (−OH, −OH) > (−OH, −COOH) > (−COOH, − COOH), in an excellent agreement with the theoretical and experimental predictions. As the result the molecular understanding of photocatalytic degradation of small pollutants showing a crucial role of the anchoring and photogenerated hydroxyl groups in the self-sensitization process was accounted for with an unprecedented comprehensive way. It was shown that at early stages of the photocatalytic degradation process the aromatic rings of the COP moieties are readily photohydroxylated, fostering the visible light utilization. Such autocatalytic hydroxylation processes can also be relevant for photocatalytic degradation of those pollutants that originally do not exhibit hydroxyl functionalities.

Figure 12. Differential UV−vis spectra of SAL@r-TiO2 in the solid state recorded for different photoreaction times (up to 60 min) (a) together with reference differential spectra of r-TiO2 functionalized with various organic molecules containing two hydroxyl groups: 2,3dihydroxybenzoic acid (2,3-DHB), 2,5-dihydroxybenzoic acid (2,5DHB), and catechol (CAT) with the subtracted SAL@r-TiO2 (for the sake of a direct comparison) (b).

r-TiO2 functionalized with various organic molecules containing two hydroxyl groups, such as 2,3-dihydroxybenzoic acid (2,3-DHB), 2,5-dihydroxybenzoic acid (2,5-DHB), and catechol (CAT) with the subtracted SAL@r-TiO2 (for the sake of a direct comparison). Gradual development of a broad pronounced band in the range of 400−800 nm, characteristic for dihydroxybenzene derivatives, reveals a progressive insertion of the photogenerated hydroxyl groups into the aromatic ring of the SAL admolecules during the photocatalytic reaction. As it was discussed above, this explains the observed bathochromic shift of the absorption with the increasing −OH/ −COOH ratio. As a result, the “HOMO” position is shifted toward the conduction band edge, which is beneficial for more effective utilization of the visible range of the solar light, via ligand (“HOMO” of COP) to metal (CB of TiO2) electron transfer. This observation reveals also an autocatalytic stage of COP self-degradation that results from specific tuning of the band alignment between the photohydroxylated COP and the rutile moieties of the photocatalyst. It promotes localization of the hole on the organic component facilitating immediate reaction with water, [COP−H]•+ + OH− → COP−OH + H•. As a result, a doubly hydroxylated aromatic ring system is

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10983. TEM image simulation conditions; relaxation of the bare (110) surface of rutile TiO2; XRD, Raman, and FTIR spectra for unmodified and modified r-TiO2 nanorods; calculations of pollutants coverage; the details of the electronic structure for the set of gas phase COP molecules; starting geometries for COP deposition; Bader charge (qB) vs the EHOMO(COP) − EF(TiO2) plot; DOS diagrams for bulk and bare surface titania (PDF) L


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +48 12 663 20 73. *E-mail: [email protected]. Tel.: +48 12 663 22 22. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support from the Foundation for Polish Science within the TEAM/2012-9/4 project, cofinanced by the EU European Regional Development Fund, is highly acknowledged. A part of the work (photocatalytic tests) was financed by the National Science Centre, Poland, within the grant number 2011/01/B/ ST5/00920. M.B. acknowledges the financial support by FP7 EU project 4G-PHOTOCAT (309636). The authors want to dedicate this paper to professor Elio Giamello from University of Torino to honor his 65th anniversary.

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ABBREVIATIONS BG, band gap; CAT, catechol; CB, conduction band; COP, colorless organic pollutants; DFT, density functionals theory; DFT+D, density functionals theory with inclusion of dispersion forces; DFT+U, density functionals theory with Hubbard correction; EF, Fermi Energy; SI, Supporting Information; GGA, General Gradient Approximation; ITO, indium tin oxide; PAW, Plane Augmented Wave; PTA, phthalic acid; r-TiO2, rutile phase of TiO2; SAL, salicylic acid; TEM, transmission electron microscopy; TPA, terephthalic acid; VB, valence band; XC, eXchange-Correlation

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Self-Sensitized Photocatalytic Degradation of Colorless Organic

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