Porous single crystals (PSCs) at the macroscale are considered a novel type of inorganic porous material that combines the characteristics of single crystals and porosity [1]. PSCs feature internal pores capable of carrying fluid phases in a 3-dimensional interconnected porous structure, while the continuous framework acts as the solid phase, exhibiting a long-range ordered single-crystal state in the lattice [2]. The traditional concept of bulk single-crystal materials generally refers to solid-phase materials with dimensions reaching centimeters or larger, characterized by the ordered arrangement and oriented repetitive combination of components such as atoms, ions, and molecules in 3-dimensional space [3,4]. Due to the high symmetry and ordered structure of single crystals, it is commonly recognized that the performance of materials can be substantially enhanced when they are in a single-crystal state [5]. Porous materials are typically materials with pores that are directionally designed to achieve specific functions [6,7]. Catalytic materials with porous structures can not only reduce effective mass density but also increase specific surface area, thereby expanding the capacity of catalytic reactions [8,9]. Customizing pores in bulk single crystals is a promising approach to creating novel porous materials. This material is characterized by the organic integration of nano-single-crystal frameworks and pore structures while maintaining crystal structure integrity independent of grain boundaries Therefore, this offers invaluable opportunities to enhance the overall performance of materials by altering their physical characteristics by leveraging the unique properties of crystals and manipulating customized growth conditions [10].

The high stability and catalytic activity of TiO₂ make it widely recognized as a crucial catalyst, leading to substantial research interest in its applications such as gas-phase catalysis, energy conversion, and organic pollutant removal [11,12]. Rutile-phase TiO₂ (R-TiO₂), the most stable structure in titanium dioxide, exhibits strong catalytic activity, high stability, good resistance to poisoning, and low cost, making it one of the most promising heterogeneous catalysts with great development potential [13]. However, the majority of reported R-TiO₂ exists in nano-powder form and their short-range ordered lattice structure limits their potential applications in catalysis [14]. To address this limitation and enhance its catalytic efficacy substantially, PSC R-TiO₂ monoliths have been developed. These monoliths combine porous materials with single crystals to create homogeneous 3-dimensional pathways and notably improve catalysis activity [15,16].

The utilization of oxide scaffolds with oxygen defects in the metal–oxide interface system holds great promise as a modification method in metal catalysts, enhancing lattice oxygen activation during the reaction [17–19]. For instance, the utilization of Pt supported on Al₂O₃ enables to complete oxidation of CO at low temperatures due to the synergistic effect between Pt/Al₂O₃ sites [20,21]. The metal–oxide interface system between Au and TiO₂ support was investigated by Yuan et al. [22] using in situ spherical aberration-corrected transmission electron microscopy (Cs-STEM). At the atomic scale, the presence of the Au/TiO₂ interface system can effectively enhance the catalytic performance of TiO₂ during CO oxidation reaction. However, the development of loaded precious metal catalysts has been somewhat constrained by the drawbacks of their high cost and limited reserves. The utilization of transition metals holds substantial potential in the modification of catalysts [23–25]. The field of catalysis has witnessed substantial attention toward transition metal Ni, with scholars exploring the loading techniques to enhance its catalytic activity in reactions such as volatile organic compound oxidation [26] and alkylphosphine compound hydrogenation [27]. The interaction between transition metal Ni and catalyst promotes its advantages in terms of catalytic oxidation activity and stability. By employing nickel as the catalyst instead of precious metals to create a metal–oxide interface system between Ni clusters and TiO₂, it is possible to prepare a highly active and stable catalyst for CO oxidation reaction.

In this work, we fabricated the metal–oxide interface system by growing centimeter-scale PSC R-TiO₂ monoliths and depositing atomic-layer Ni clusters on its long-range ordered lattice surface using atomic layer deposition (ALD) techniques. The charge transfer between Ni and the surface lattice oxygen sites on R-TiO₂ at the Ni cluster/TiO₂ interface system promotes the chemical adsorption and activation of CO molecules while simultaneously activating adjacent surface lattice oxygen in TiO₂ due to distortion caused by Ni clusters/TiO₂ at the interface. This dual activation provides an advantage for efficient and durable CO oxidation reaction. We evaluated the performance of PSC TiO₂ loading with Ni cluster (PSC Ni/TiO₂) catalyst, which exhibited complete oxidation of CO at low temperature (80 °C) with substantial catalytic stability during 200 h of continuous reaction. The creation of PSC R-TiO₂ and design of this interface system provide a new pathway for synthesizing and applying large-pore single-crystal materials.

The growth of KTiOPO₄ (KTP) single crystals is achieved using a molten-salt growth method that utilizes molybdates as the flux [28]. The synthesis and growth process of PSC R-TiO₂ single crystal and experimental setup are illustrated in Fig. 1A and Fig. S1. In brief, we cultivate KTP single crystals and divide them into substrates measuring 10 mm on each side and having a thickness of 0.5 mm. After the polishing process, we subjected the KTP with (100) facet to an O₂/Ar atmosphere ranging from 50 to 400 torr, with a controlled flow rate of 50 to 200 sccm (standard cubic centimeter per minute), while maintaining a temperature range of 900 to 1,000 °C. This approach aimed to cultivate porous TiO₂ single crystals through a high-temperature lattice reconstruction technique, as illustrated in Fig. S2. The K and P atoms migrate away from the lattice structure of KTP, causing the left Ti–O structures to undergo a reconstructive process and form an ordered lattice. As a result, PSC R-TiO₂ monoliths are formed, possessing dimensions comparable to the substrates. The KTP eventually undergoes a transformation into centimeter-scale single crystals of porous R-TiO₂. The lattice channels in KTP facilitate the removal of P/K, thereby promoting the growth of PSC R-TiO₂ monoliths.

The reconstruction of the Ti–O framework results in approximately 70.0% porosity within the porous architectures. Due to the different lattice constants of substrate and epitaxial layer, lattice mismatch will occur during the growth of TiO₂ on the KTP substrate. The lattice constants of the substrate are denoted as a _s, while those of the epitaxial layer are represented by a _f. The resulting lattice mismatch δ can be calculated using Eq. 1 [29]:

A reduction in the lattice mismatch rate leads to a decrease in the lattice strain grown on the substrate, thereby enhancing the degree of orientation correlation [30]. After calculation, the lattice mismatch ratio of (100) facet KTP single crystal corresponding to R-TiO₂ is found to be as low as 1.24%. Therefore, the (100) facet single-crystal KTP with the lowest lattice mismatch rate is an ideal substrate for the preparation of R-TiO₂. The lattice structure of R-TiO₂ prepared on various crystal facets was characterized using x-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), as illustrated in Figs. S3 to S5. The TiO₂ synthesized using a-axis oriented KTP exhibits enhanced crystallinity and uniform porosity, as evidenced by the observations. Due to its minimal lattice mismatch rate, the a-axis oriented KTP selectively undergoes transformation into R-TiO₂ at elevated temperatures while minimizing the occurrence of lattice defects [31]. The prepared materials were characterized by XRD to determine their material phase and the orientation of crystal facet. The XRD pattern of the synthesized KTP closely matches the diffraction peak of the corresponding crystal surface of the standard material, as illustrated in Fig. 1B to D, indicating exceptional crystallinity of the precursor KTP crystal prepared. The diffraction peak of porous TiO₂ single crystal is precisely matched with the characteristic peak of known rutile type (110) facet TiO₂ crystal (PDF#88-1174). The successful preparation of centimeter-scale rutile phase single-crystal TiO₂ with high crystallinity has been demonstrated through the utilization of KTP single crystal as the mother crystal and employing a high-temperature lattice reconstruction technique.

The SEM is utilized to analyze the microstructure and morphology of the prepared samples. The prepared PSC R-TiO₂ exhibits uniform 3-dimensional pores while maintaining its single-crystal property, thereby substantially enhancing the specific surface area and providing more adsorption sites for improved performance. The porous structure displays a uniform distribution of macropores, as shown in Fig. 1E, and Fig. S7 illustrates the PSC TiO₂ grown from different crystal facets of KTP, revealing consistent presence of uniform porous structures. However, considering that the R-TiO₂ obtained from the (100) facet of KTP theoretically exhibits minimal defects, we opted to employ R-TiO₂ prepared from the (100) facet of KTP for subsequent tests. We utilize x-ray photoelectron spectroscopy (XPS) for the examination of surface chemistry states linked to the PSC R-TiO₂ structure illustrated in Fig. 1F and G. The Ti 2p spectrum of PSC R-TiO₂ exhibits 2 distinct peaks at 458.4 and 464.1 eV, corresponding to the Ti 2p_3/2 (TiO₂) and Ti 2p_1/2 (TiO₂) states, respectively. This observation supports the prevalence of titanium element in an oxidation state of +4. The detailed O 1s spectrum exhibits prominent peaks at 530.3 and 532.4 eV, corresponding to surface lattice oxygen (O_lat) and surface adsorbed oxygen (O_ads), respectively [32].

The presence of abundant lattice oxygen in the PSC R-TiO₂ prepared by us suggests its potential for constructing the metal–oxide interface system. The aforementioned condition provides a favorable environment for subsequent loading of Ni clusters. In Fig. S8, we employed focused ion beam SEM (FIB-SEM) and scanning TEM (STEM) to investigate the internal structure images of PSC R-TiO₂. The structure of PSC R-TiO₂, as depicted in Fig. 2A and B, exhibits not only surface pores with uniformity but also internal monomer pores that are uniformly distributed. The presence of interconnected pores within the porous framework substantially enhances its specific surface area, thereby enabling it to function as a porous material that offers abundant active sites and creates favorable conditions for subsequent construction of the metal–oxide interface system onto its surface. The elemental mapping of PSC R-TiO₂, as shown in Fig. 2C and Fig. S9, provides evidence for the composition of its porous framework being TiO₂. At the same time, it is observable in Fig. 2D and E that the prepared PSC R-TiO₂ has excellent single-crystal property. The selected area electron diffraction (SAED) pattern of the internal framework of PSC TiO₂ exhibits a characteristic rutile structure, as evidenced by the inset image showing a single crystal of TiO₂, which is consistent with the XRD analysis. The periodic arrangement of atoms along the (110) and (121) directions is clearly observed in Fig. 2D. The calculated d-spacing of 0.34 nm corresponds well to the (110) facet. The atomic arrangement of rutile TiO₂ on the high-resolution TEM (HRTEM) diagram in Fig. 2E is simulated, and its distribution exhibits a high level of agreement with that of standard rutile TiO₂ crystals (PDF#88-1174). The electron energy loss spectra (EELS) of Ti and O are presented in Fig. 2F and G, where the peaks observed at 460, 465, and 533 eV correspond to the Ti L₃ edge, Ti L₂ edge, and O K edge of the standard TiO₂ film, respectively [33,34]. The theoretical model of PSC R-TiO₂, represented by Fig. 2H, exhibits the charge distribution of its cross section as calculated through simulation in Fig. 2I.

We employed the ALD technique to deposit Ni clusters onto PSC R-TiO₂ monolith, as illustrated in Fig. 3, thereby establishing a metal oxide interface system. Following multiple cycles through the ALD system, we transferred the sample into a porcelain boat and placed it in a heating furnace for reduction under a reductive atmosphere for 3 h to ensure complete conversion of all Ni species into Ni in metallic state. Through precise control of the exposure time, we successfully achieved homogeneous dispersion of nanoscale Ni clusters. The presence of these Ni clusters facilitates CO adsorption, while the enhanced activation of lattice oxygen can be attributed to the dynamic structure at the interface. The SEM and STEM images (Fig. 4A and B) demonstrate that the PSC Ni/TiO₂ maintains the porous characteristics of TiO₂, with the sample still exhibiting interconnected 3-dimensional pores after undergoing the ALD method. The Cs-corrected STEM (Cs-STEM) images presented in Fig. 4C to E reveal the presence of uniformly dispersed Ni clusters on the surface of PSC R-TiO₂ monoliths. Furthermore, these images highlight the spatial arrangement of Ni clusters, indicating that the decoration with Ni effectively enhances the metal–oxide interface system. Consequently, specific regions with enhanced catalytic efficiency and activation of lattice oxygen at the interfaces are observed. Through statistical analysis of Cs-STEM images (Figs. S12 and S13) at lower magnification, it can be concluded that the majority of Ni clusters fall within the size range of 2 to 3 nm, with an average particle size of ~2.66 nm. Within this particle size range, the risk of Ni cluster crystallization is minimal. The proportion of Ni clusters larger than 5 nm is extremely low, accounting for only approximately ~3.55% of the total content. Therefore, by precisely controlling the exposure time in the ALD system, we are able to confine the size of Ni clusters within a controllable range while minimizing their degree of crystallization.

The EELS results for PSC Ni/TiO₂ in Fig. 4F and G exhibit peaks similar to those of PSC R-TiO₂ at 460 and 465 eV. Additionally, a peak is observed at 855 eV, which corresponds to the L₃ edge of Ni [35]. This observation demonstrates the successful deposition of Ni clusters onto the surface of PSC R-TiO₂. The x-ray absorption fine structure (XAFS) and x-ray absorption near edge structure (XANES) are illustrated in Fig. 5A [36,37]. Furthermore, the comparable edge energy observed between PSC Ni/TiO₂ and conventional Ni foil indicates a favorable metallic state [11,38–40], aligning with the XPS results.

To gain a deeper understanding of the local structure of the PSC Ni/TiO₂ monoliths, systematic Ni K-edge extended XAFS (EXAFS) data were analyzed using the Athena-Artemis software, as shown in Fig. 5B and C and Fig. S14. The Fourier-transformed EXAFS (FT-EXAFS) spectra of the Ni/TiO₂ interface system exhibit a prominent peak at 2.22 Å, primarily attributed to the Ni–Ni shell [41–43]. The results of our study indicate that the Ni clusters in PSC Ni/TiO₂ predominantly exist in a metallic state rather than as NiO. The WT-EXAFS diagram effectively demonstrates the distinct characteristics of standard Ni foil and PSC Ni/TiO₂, as depicted in Fig. 5C. The highest signal peak (8.2 Å⁻¹; 2.2 Å) clearly indicates the presence of a Ni–Ni shell, while the minor signal peak (11.4 Å⁻¹; 2.8 Å) corresponds to the existence of a Ni–Ti shell. This provides compelling evidence for the interaction between Ni clusters and TiO₂ in PSC Ni/TiO₂. The fitting results of EXAFS are shown in Table S1.

The ALD deposition facilitates the formation of clustered Ni species, which are evenly distributed within the PSC R-TiO₂ crystals. Figure 5D is the XPS map of Ni clusters deposited on PSC R-TiO₂ by ALD, which shows the chemical valence states of Ni in PSC Ni/TiO₂ monoliths. Among them, 855.5 and 873.1 eV belong to Ni 2p_3/2 and Ni 2p_1/2, respectively, which is similar to the peak value of standard Ni [44]. This indicates that Ni deposited in PSC R-TiO₂ mainly exists in the form of zero-valent Ni. The relationship between the surface lattice oxygen exchange coefficient and the surface Ni cluster load content is illustrated in Fig. 5E. The PSC exhibits a surface oxygen exchange coefficient approximately 24.7% higher than that of the nonporous single crystal (NSC) TiO₂. Moreover, the introduction of the PSC Ni/TiO₂ interface system leads to an additional increase of about 129.3% in the surface oxygen exchange coefficient. The significant increase in the oxygen exchange coefficient demonstrates that the local Ni/TiO₂ interface system can greatly enhance the efficiency of oxygen transfer, thereby facilitating the oxidation reaction. The Raman map of the prepared crystal is shown in Fig. S15, where the peaks at 114.7, 445.2, and 610.9 cm⁻¹ correspond to the B_1g, E_g, and A_1g modes of PSC R-TiO₂, respectively [45]. Notably, the vibration characteristics of PSC R-TiO₂ at these specific peaks remain unaffected after loading Ni clusters, thereby providing further evidence for the preservation of rutile structure in PSC Ni/TiO₂. The state density of the sample is illustrated in Figs. S16 and S17. Before constructing the Ni/TiO₂ interface system, the state density of PSC R-TiO₂ near the Fermi level approaches zero, whereas after incorporating Ni clusters, Ti and Ni substantially contribute electrons near the Fermi level, thereby greatly enhancing its electrical conductivity and catalytic properties as well as sample activity. By calculating the difference between Ni/TiO₂ and TiO₂, we have generated a differential charge density diagram for PSC R-TiO₂ and PSC Ni/TiO₂, which is presented in Fig. 5F. The differential charges of PSC R-TiO₂ and PSC Ni/TiO₂, as calculated by DFT electron transfer within the interface system of Ni/TiO₂, occur from Ni to TiO₂ predominantly toward oxygen vacancy sites on the surface.

This confirms that Ni clusters facilitate oxygen vacancy formation in Ni/TiO₂ interface systems. Investigation of the CO trapping capability of PSC Ni/TiO₂ was conducted using in situ Fourier transform infrared spectroscopy (FTIR), as depicted in Fig. 5G and H. During the temperature transition from 30 to 90 °C, we observed the peaks of 2,063 cm⁻¹, which is the absorption peak of CO captured by Ni clusters. The absorption peak of gas-phase CO and gas-phase CO₂ are 2,170 and 2,360 cm⁻¹, respectively [46]. The adsorption of CO occurs on Ni clusters, followed by its catalytic conversion to CO₂. The results demonstrate an upward trend in the signal peak of CO adsorption by Ni clusters in PSC Ni/TiO₂ with increasing temperature, as illustrated in Fig. 5G and H. The simultaneous increase in temperature is accompanied by a noticeable upward trend in the signal peak of gas CO₂. The observation provides compelling evidence for the exceptional catalytic oxidation performance of PSC Ni/TiO₂. The transition state of CO oxidation at the PSC Ni/TiO₂ interface was determined using density functional theory (DFT), as illustrated in Fig. 5I. The adsorption energy of CO on the Ni/TiO₂ interface structure is approximately 2.32 eV, while the energy barrier for the reaction between lattice oxygen connected with the Ni cluster and CO to form carbon dioxide is around 0.77 eV, indicating a high likelihood for CO oxidation and interaction with lattice oxygen at the interface. The catalytic performance of PSC Ni/TiO₂ with (110) facet for CO oxidation in air is shown in Fig. 6A and Fig. S18. At low temperature ranging from 30 to 140 °C, PSC R-TiO₂ without Ni exhibits low enhancement on the CO oxidation reaction. At high temperature of 300 °C, it exhibits discernible catalytic activity, with a conversion rate of ~68.18% observed for the oxidation of CO. In contrast, the PSC R-TiO₂ with the Ni/TiO₂ interface system exhibits superior performance by achieving a CO conversion rate up to ~6.62% at near room temperature and complete CO oxidation at 80 °C. Additionally, testing the sample loading with 0.60 wt % Ni did not notably enhance its performance (complete CO oxidation at 75 °C). Furthermore, we conducted additional tests on samples with higher loads; however, results indicated minimal improvement in catalytic oxidation performance upon increasing loading capacity. TiO₂ supported by varying Ni contents exhibits a consistent enhancement trend at low temperatures, with the catalytic CO oxidation performance gradually improving as the temperature increases. Furthermore, we introduced additional data of Ni/TiO₂ with higher content. When the Ni cluster content of PSC Ni/TiO₂ exceeds 0.48 wt %, the catalytic oxidation performance of PSC Ni/TiO₂ has no marked improvement.

The durability test results of PSC Ni/TiO₂ containing 0.48 wt % Ni are presented in Fig. 6B, demonstrating the remarkable capability of PSC Ni/TiO₂ to maintain a consistent conversion rate of 100% even after continuous operation at 80 °C for 200 h. Figure 6C demonstrates the dynamics of CO oxidation at 0 to 200 h using PSC Ni/TiO₂ loading with 0.48 wt % Ni. Remarkably, the catalysis activity of PSC Ni/TiO₂ loading with 0.48 wt % Ni shows negligible deterioration after multiple operational cycles. The results demonstrate that our Ni cluster/TiO₂ interface system effectively activates the surface lattice oxygen, leading to a dual activation of both the surface lattice oxygen and CO molecules. The incorporation of the Ni cluster/TiO₂ interface system substantially enhances the catalytic performance of PSC R-TiO₂ for CO oxidation while maintaining long-term stability during prolonged reaction. The characteristics of PSC Ni/TiO₂ after a 200-h CO oxidation reaction are illustrated in Fig. 6D and F and Fig. S19. The SEM, Cs-HRTEM, SAED, and XRD analyses reveal that the PSC Ni/TiO₂ monoliths retain excellent monocrystalline properties even after prolonged reaction time. The XPS spectra results presented in Fig. S19B to D demonstrate that the valence states of Ti and O elements in PSC Ni/TiO₂ remain unchanged before and after the CO oxidation reaction of 200 h, with only a minor fraction (5.99%) of Ni being converted to its divalent state. The aforementioned characteristics suggest that PSC Ni/TiO₂ exhibits excellent structural and chemical stability.

In conclusion, we have successfully fabricated 1-cm-scale PSC rutile TiO₂ monoliths using a solid–solid phase transformation strategy. The single-crystalline solid phase framework with long-range ordered lattice provides structural advantages for creating the well-defined interface system, while the 3-dimensional interconnected pores enhance permeation and species diffusion. We constructed the metal–oxide interface by uniformly confining atomic-layer Ni clusters on the lattice surface of PSC TiO₂, which enhanced the catalytic oxidation performance of CO. We have confirmed that the PSC Ni/TiO₂ catalyst can efficiently capture CO and activate lattice oxygen at temperatures close to temperature, exhibiting complete catalytic oxidation performance of CO at ~80 °C, while maintaining excellent catalytic activity and stability even after continuous operation for 200 h. The current work serves as a reference for the development of other advanced ternary solid catalysts.